Microfibers within fibers: A review

Microfibers Within Fibers: A Review PAUL TUCKER and WALLER GEORGE

School of Textiles North Carolina State University

Raleigh, North Carolina

Literature relating to the fundamental nature of the fibrous state is reviewed with the goal of illustrating evidence for the existence of a possibly fundamental morphological unit, the elemental microfiber or fibril. Following a short summary of the rather well documented situation in natural fibers, the problem of the microfiber in synthetic fibers is probed in terms of phenomena in solution and bulk polymeric systems. The reviewed literature indicates the existence of unique, elemental microfibers in synthetic fibers is largely tenuous; however, there is wide evidence of the existence of complex elongated structures axially aligned within macrofibers which future work may resolve into a central microfiber. The structural complexity appears to simplify for transformations occurring in high shear fields. This review covers articles through 1971, and it is notable that in 1971 approximately 100 articles of high ralevance appeared in major journals.

INTRODUCTION he concept of the microfiber as an integral mor- T phological structure of natural fibers was enun-

ciated by Nehemiah Grew ( 1 ) in 1682. The observa- tional basis of his conclusion was thin at best, but with revelation of the fine structure of wool by Astbury, Woods and their students, the supposition seems definite. Macrofibers in nature are composed of microfibers on one or more levels and derive their properties in part from their microfibrillar character. The situation in man-made fiber is less well documented. It is the premise of this review that man-made fiber also is composed of a t least a rudi- mentary microfibrillar structure. If this can be established, one may in essence generalize the observations of Grew which say that to some degree all fiber is composed, on some fine scale, of microfibrillar elements and the macroscopic properties are in part derived from this morphology,

The fibrillar morphological state implies a knowledge of the fibril. A fibril is defined by Webster (2) as “one of the minute elongated elements that make up the structure of fibers of certain natural and synthetic materials (as textile fibers, wood, or fibrous proteins) and that are held to be made up ultimately of long chain molecules oriented in a bundle in one direction.” Alternately a textile authority ( 3 ) holds a fibril to be “one of the minute fibrous elements making up a fiber.” Recently, Hearle and Greer (4) state that conceptually a fibril is like a “micelle” ex- cept “fibrils are very much longer in one direction”; they consider micelles as “ordered regions with dimensions of the same order in all directions.”

Two major classes of observations are reported, “Fibrils in Natural Materials” and “Fibrils in Man-

Made Materials.” [ Compilations of Ogle, 1961 ( 5 ) are presented (see Table 1 ) and also a current tabulation prepared by the writers’ (see Table 2 ) . Some observations not discussed in this text are, however, included in the tabulation. Sikorski, 1963 ( 6 ) ) has presented elsewhere an extensive compilation of fibrillar observations.]

FIBRILS IN NATURAL MATERIALS

Natural Cellulose A brief critical review of the observations and

sizes of the elementary cellulose fibril has recently been presented by Cauldfield [ 19711 ( 7 ) who cites considerable evidence for an ubiquitous 35.4 diameter. An opposing view is that the true diameter is much larger (of the order of 150A) and the 35A is an artifact; the smaller indication is mainly obtained by negative staining techniques while the larger value is obtained by metal shadowing (8). From studies on bacterial cellulose, however, Franke and Ermen [1969] ( 9 ) conclude “. . . the concept of an elementary fibril with a cross-section of about 35 X 35A . . . cannot be further sustained.”

An interesting study of precipitated cellulose was conducted by Macchi and Palma (10) who found that fibrils showing the native cellulose structure (unit cell I ) can precipitate spontaneously from dilute solutions even if mercerized cotton (unit cell 11) is the material initially put into solution. The in vitro spontaneous formation of microfibrils with native organization is independent of the morpho-

* This latter tabulation does not include thin film (<ZOO0 A) and deformed single crystal observations even though other aspects of such studies are discussed elsewhere.

364 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1972, Vol. 12, No. 5

Micro fibers Within Fibers: A Review

Table 1. Fibrillar diameters, A [after Ogle, 1961 (511

Basic Fibril Microfibril Fibril Macrofibril

Cellulose and six derivatives 30-75

Six protein fibers

Polyacrylonitri le Four polyarnides

One polyester

(8) 30-100 (4) 50 30 (1)

Polyethylene -

~~

- 100-350,350-500 800-4000 (10) (7) (7)

100-250, 250-500 800-1200 36,000 (3) (3) (3) (1) 150 450 3000-4000 30,000-40,000

100-200,400-500 2000-5000 30,000-40,000 (1) (2) (1) (1)

100-200 400 1200-5000 30,000-40,000 (2) (1) (2) (1)

100-200,400-500 2000-3000 (2) (1) (1)

~~

Numbers in parentheses show the number of authors reporting.

logical nature of the starting material and any influence of matrix origin ( 10). Ruck and Pohle [ 19701 ( 11) attempted to show that the microfibril is not the result of accidental drying conditions in the living cell, but rather is a direct telegenic product of nature.

Bacterial cellulose microfibrils were produced in drops on Formvar film by Colvin and Beer ( 12). The exact mechanism of formation was not elucidated but the fibrils formed without contact with the bacterial cell surface and increased in mass by growth at one or both tips; there did not seem to be any amorphous intermediate (12, 13). These fibrils are reported to exceed 60,000A in length ( 9).

Varied aspects of cellulose fine structure, including possible artifacts, were discussed at a recent [ 19711 cellulose conference ( 14).

Natural Proteins The ultrastructure of protein fiber was reviewed in

1967 by Finean (15). He concluded that alpha keratin appears to be composed of dense microfibrils of about 70A diameter which are spaced about 1OOA apart; X-ray diffraction indicates a hexagonal or cylindrical packing. The individual microfibril of keratin appears to be constructed in an arrange- ment similar to that found in .cilia acd flagella in which a circle of nine pairs of fibrils surround a central pair. It should be noted that Woods [1967] (16) has questioned this “9 + 2” structure in wool. The keratin protofibrils which combine to form the microfibril are about 2011 in diameter and might be composed of either two- or three-stranded a-helices (16). Woods (16) has discussed some of the prob- lems of X-ray diffraction and microscopy of keratin fibers; he states the hexagonal packing often de- generates into whorles or layers and often macrofibrils are observed which do not contain microfibrils.

The protofibrils composing the l p diameter myofi- bril in muscle tissue seem quite varied and complex. Two sets of protofibrils in different bands have been distinguished; thick ones of about lOOA diameter and others of 50 to 60A diameter. The 100.4 fibrils display projections 30 to 40A in diameter (15).

Considerable uncertainty also exists about the sub- structure of flagella and cilia. In addition to the

common packing of the filaments in a “9 + 2” ar- rangement, “9 + 1” and “9 + 0 packings are also observed ( 15). In sectioned material, these filaments appear to have diameters on the order of 200 to 500A in diameter. Some shadowed preparations show filaments of much finer diameter that resemble the helical models of fibrils proposed for textile fibers (15).

Collagen can be seen as fibers of 100 to 2 0 0 ~ in diameter which can be subdivided to the ultimate protofibril which is probably composed of a three- stranded helical configuration of polypeptide chains

In addition to Finean’s text, a good review of keratin is provided by Lundgren and Ward (17) in a text edited by Borasky. Most of Lundgren and Ward‘s material is also included and extended by Dobb and Rogers [1968] (18). They distinguish between the mammalian keratins which consist of 75A diameter microfibrils and the feather and rep- tilian keratins which consist of 35A diameter microfibrils.

An interesting recent observation is that fibroin coagulated in silk glands of B. mori has a spherulitic morphology which is destroyed by drawing of the gel (19).

(15).

FIBRILS IN MAN-MADE MATERIALS Szabolcs and Szabolcs (20) in a recent briefer

review of microfibrils conclude “. , . it can be as- sumed that the microfibril is one of the smallest elements of the fiber structure, even though it is a very complicated system itself.”

In discussing the multifibrillar structure of polyacrylonitrile Ogle (5) provides an extensive review that is summarized in Table 1 with the dimensions in A and the numbers in parentheses showing the number of papers up to 1961 reporting the indicated result.

Sikorski [1963] ( 6 ) has discussed the lateral dimensions of fibrils, using studies of fiber fine structure that were conducted up until about ten years ago. He presents a table which lists the dimensions of microfibrils (protofibrils) as found by various authors.

Zaspinok et al. (21) discuss fibrils in triacetate

POLYMER ENGINEERING AND SCIFNCE, SEPTEMBER, 1972, Yo/. 12, No. 5 365

Table 2. Compilation of fibrillar data through 1971 (current authors)

Material

Transverse dimensions Length

A A Technique Authors

Regenerated Cel Iu lose Viscose Various reg. celluloses Tire cord

Fort isa n

Cellulose hydrate

Polynosic

Cellulose nitrate

Various native celluloses Native Cellulose

Cotton

Synthetic Materials Nylon 66

Fibrils parallel to

Fibrils parallel to

Fibrils perpendicular to

Fibrils perpendicular to

Nylon 6

draw axis

draw axis

draw axis

draw axis

Annealed @ 100°C Annealed @ 210°C

Polyethylene terephthalate

Polyoxymethylene

Poly(4-methypentane-1)

Poly(N-vinylcarbazole) Polytetra hydrof uran

Polyacrylonitrile Unoriented fiber

Oriented fiber

Structure perpendicular to draw direction

Polypropylene Annealed

Not annealed

Polyethylene

Annealed

Not annealed

32-40 35 x 20

16 100

40-48

150-200

1000

100

1000

18

35 x 20 27-33 8-100

1000

200 50

1000-1500 100-150

70 r 5

70-75

150 5

150-160

90-100 230-260 150

140-170 200-1000 200-300

50-175 Modal value

about 70

Modal value about 200

250 250

90-500

118-530

320-672

105 f 10

110,210,510

120,210,480

200

200-300 Few hundred A dia.

3000

200-250 150,340,580

120,260,550

880-1150 180

3 0 0 - 4 0 0 237

270-380 927-1452 60,000

500 to 10,000 to 100,000

Elect. mic. Elect. mic. Calc. from reactivities Elect. rnic.

Elect. mic. Calc. from reactivities Elect. mic.

Elect. mic.

Elect. mic.

Elect. mic. Calc. from reactivities Elect. mic.

Elect. mic.

Elect. rnic.

Elect. mic.

SAXS

Elect. mic.

SAXS

SAXS

Elect. mic. SAXS Elect. mic. Elect. mic.

Elect. mic.

Elect. mic.

Elect. mic. SAXS

Calc. for density, surface, etc.

Calc. for density, surface, etc.

Elect. mic. and SAXS

SAXS

SAXS

Elect. mic.

Elect. mic. Elect. rnic.

Elect. mic.

SPXS

Heyn, 1971 (35) Manley, 1971 (36) Scallan, 1971 (37) Wlodarski and Balarzyk,

Heyn, 1971 (35) Scallan, 1971 (37) Mikhailov and Mikheleva,

Mikhailov and Mikheleva,

Wlodarski and Balarzyk,

Mikhailov and Mikheleva,

Manley, 1971 (36) Scallan, 1971 (37) Franke and Ermen,

Peterlin and Ingram,

1970 (38)

1970 (39)

1970 (39)

1970 (38)

1970 (39)

1969 (9)

1970 (29)

VanVeld et al., 1968 (40) VanVeld et al., 1968 (40) Crystal and Hansen,

Crystal and Hansen,

Crystal and Hansen,

Crystal and Hansen,

Gezalov, 1970 (42) Gezalov, 1970 (42) VanVeld et al., 1968 (40) VanVeld et al., 1968 (40) Vasilenko, et al., 1970 (43) Garber and Clark,

Hase and Geil, 1971 (45)

1968 (41)

1968 (41)

1968 (41)

1968 (41)

1970 (44)

Hase and Geil, 1971 (45)

Crystal, 1971 (46) Takayanagi et al., 1966 (47)

Craig et al., 1962 (48)

Craig et al., 1962 (48)

Hinrichsen and Orth, 1971 (49)

Takayanagi, et al.,

Takavanaai. et al.. 1966 (47)

1966 (4fi '

Balta-Calleia and Peterlin. 1970 (50)

Takesue et at., 1966 (51) Muzzy and Hansen, 1971 (52)

Crystal and Southern, 1971 (53)

Takayanagi et al., 1966 (47)


Microfibers Within Fibers: A Review

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1972, Vol. 12, No. 5 367

fibers but do not give their diameters. There appeared to be two types of fibrils, one type was non- uniform in width up to l p long and organized in a mesh type iietwork and the other type was the conventional fibril of about lop length normally visu- alized.

Observed on fracture surfaces of solid state polymerized trioxane are fibrils with diameters between 100 and several thousand angstroms ( 22).

An interesting study of polyethylene (PE ) with NMR was conducted by Lobodi-Cackovic et al. in 1969 (23). The NMR spectra for stretched PE had components of different width. They stated the broad component was caused by protons inside the crystalline part, the mean component was associated with the protons in the lateral fibril boundaries and the noncrystalline part, and the narrow component came from chain ends not included in the crystallites. The bulk- and melt-crystallized material showed no narrow component and high-pressure crystallized PE showed no mean component.

Yams formed of polypropylene-polyethylene ( PP/ PE) blends by Japanese workers in 1967 (24) show PP and PE fibrils isolated from each other and ab- normal behavior in mechanical properties was explained as due to slippage between the PP and PE fibrils.

Numerous patents also describe the production of microfibers or the modification of yarn properties through the extrusion of polymeric blends; repre- sentative ones are listed ( 25) .

MICROFIBRILLAR BUNDLES: NATURAL AND SYNTHETIC

Ogle ( 5 ) reviewed the literature with the goal of investigating size distributions of micro- and macrofibrils ( Table I ). He shows polyacrylonitrile fibril diameters to be approximately 3.5, 0.4, 0.045 and 0.005~ (50-4) with the average diameter of a molecular rod being about 5.5-4. He states that in more ordered structures a factor of nine seems to relate the diameters of many fibrils with each of the stablest fibrils being composed of approximately 61 of the next smallest.

Support for Ogle’s hexagonal thesis is often found in wool fibers; Woods (16) noted that when a microfibrillar texture is seen, the packing is fundamentally hexagonal. Heyn (26, 27), by correlating actual with theoretical small-angle X-ray intensity plots, also held cellulose elementary fibrils to be in a hexagonal close-packed pattern.

However, Manley (28) considers the cellulose protofibril diameter to be rectangular-10-4 by 35-4. Another contradiction to the hexagonal packing hy- pothesis is the observation of Peterlin and Ingram (29) that cellulose macrofibrils are composed of a great many microfibrils of varying width, which, as a rule, seem to be narrow, ribbon-like lamellae con- sisting of one single layer of elementary fibrils with the former having lateral dimensions of about 200A and the latter about 50A.

Other than hexagonal packing is also reported for synthetic materials. Hosemann (30, 31) discusses

ultrafibrils as being composed of mosaic blocks and has used his paracrystalline analysis to calculate the dimensions of ultrafibrils and their degree of im- perfection. In a detailed review of Hosemann’s work (32) it is reported that a substantial portion of polyethylene fibrils grouped into clusters of 4 and 16 and it was calculated that the relative numerical abun- dances of clusters of 1, 4 and 16 fibrils are, respec- tively, 19, 37 and 44 percent. Through his analysis, Hosemann ( 31 ) found polyethylene “ultrafibrils” to have a very uniform thickness of n X 90.7A where n = 1, 2 or 3; and for an annealed polyethylene, groups of 2 X 2, 3 X 3 and 4 x 4 “ultrafibrils” were found (33).

In a detailed study of fibrillar “bundling” in cotton, Dolmetsch and Dolmetsch (34) point out in 1969 that the large macrofibrils have “hardly been discussed for quite some time.” They state that for cotton, at least, there actually is little specific data as to the extent to which bundles of elementary fibrils can be considered as individual units. They show data for a cotton sample where there seemed to be a fairly uniform distribution of macrofibrillar widths ranging from 11 x 102A to 20 x 102A (34).

Colvin (13) in 1966 published an interesting observation on the in &TO (on an agar surface) growth of bundles of bacterial cellulose fibrils; a variable number of microfibrils, 20 to 50 or more, form the bundle and the ends of the relatively uniform length microfibrils are in close juxtaposition.

Ogle (5) does not think such observations of ribbon-like fibrils disagree with his close-packing hy- pothesis; he considers “the ribbon shapes as being caused by the bonding of adjacent rod-like fibrils together by crystallites in which the intermolecular forces are stronger in one plane than in another.”

Table 2 presents a compilation of recent fibrillar data ( 35-53).

EFFECT OF SHEAR ON MICROFIBER FORMATION

In Bulk Deformation has long been known to enhance the

rate of crystallization in polymeric materials and to yield in rubber X-ray diffraction patterns typical of a fibrous structure (54). Until recently, most of the work was conducted on cross-linked rubbers with emphasis given to bulk properties and X-ray diffraction patterns with scant attention to the morphology developed through such strain-induced crystallization. A typical study is that of Smith and Saylor in 1938 (55); Mandelkern in 1964 (56) reviewed these classical studies. Morphological deductions were obtained by Gent in 1954 (57) who found Avrami’s treatment of crystallization rates to be satisfied in unstretched rubber by spherical growth. In stretched rubber crystallization occurs more rapidly and the resulting Avrami constants indicated uniaxial growth-a result that can now be interpreted as a fibrillar structure in light of the more recent findings of Andrews (58) and Yeh (59).

Actually, the importance of strain to the development of fibrils had been recognized before Gent’s

Paul Tucker and Waller George

paper. In 1951, Ribi (60) concluded from studies on chitin, rayon and polyamides that “for the formation of a system of elementary fibrils with the character of a fibre, an external orienting force is necessary.”

The important effect of stress on the crystal morphology was noted by Thompson in 1949 (61) : “The entropy effect can also be seen: it leads to heavy nucleation and a much finer scale fibrillar structure under high stress, when considerable molecular order is produced by the stress, and the configurational entropy change on crystallizing is less.”

Keller in 1955 (62) introduced the important concept of “row structures” to describe lamellar crystal growth (normal to the stress direction) that was nucleated along lines parallel to the stress. Due to the frequent appearance of rows of nuclei, the “row structures” are now more often termed “row nucleated structures.” Keller observed that nucleation tends to occur along “cracks, edges and flow lines” and that nucleation can be so numerous as to cause steric interference between resulting degenerate spherulites which must now grow cylindrically rather than spherically from such row nuclei.

The following discussion on the “row nucleation” concept only pertains to such structures as derived from bulk materials and not dilute solution-dilute solution or “shish-kebab structures are discussed afterwards.

Even though the “row structure” concept was given some attention between 1955 and recently [Sim- mens (63) and Judge and Stein (64), for example], it was first shown in detail by Kobayashi with photo- micrographs, published in Geil’s 1963 book (65, p. 466).

Keller and Machin in 1967 (66), Keller and Hill [ne‘e Machin] in 1969 (67) and 1971 (68) published detailed reports on aspects of crystallization related to oriented textures, specifically in polyethylene thick film, and the more general problem of crystallization under stress. Lamellar crystal growth, normal to the stress direction, was found nucleated along lines parallel to the stress. For polyethylene, the alignment and twisting of the lamellae and the density of the nucleating lines were the principal stress-dependent variables. Crystallization under high stress pre- vented twisting of the lamellae leading to a “trans- crystalline” type growth, whereas crystallization under low stress enabled the lamellae to twist as they do in ringed spherulites (66). Crystallization of a quiescent melt occurs through a random distribution of seemingly independent nuclei but nucleation in a stressed system occurs along rows of contiguous nuclei (fibrils) which are aligned in the stress direction. In stirrer-induced crystallization, the existence of epitaxially nucleated lamellae appear for nuclei aligned along a primary “backbone” structure in analogy with the structure originally seen by Keller. The central fibrils are thought to be essentially of an extended chain character; the lamellae chain- folded (66).

Direct evidence of such stress-induced nucleus crystals was later obtained for cross-linked polyethylene (67) with X-ray diffraction patterns re-

corded at the appropriate temperature which demon- strated a two-stage crystallization. First, a smaller number of crystals (type I ) form which are c-axis oriented, followed by a second group of crystals (type 11) having a more complex textural pattern. Stress gives rise to lines of nucleating centers parallel to the stress direction from which lamellar growth can emanate (67). Increased stress produces more nucleating lines and hence a finer columnar texture; as a result the overall texture becomes in- creasingly fibrillar. Unsuccessful attempts were made to separate the fibrous nucleus, and electron micrographs were not conclusive because of the rarity of nuclei ( 67).

Keller and Hill (68) recently reconfirmed the above conclusions on thin and thick films through further studies on the morphology, calorimetry and stress relaxation of stress crystallized polyethylene.

Flory in 1962 (69) anticipated the discovery of Keller and Hill by stating that bundle nuclei generated through the influence of initial orientation may subsequently grow by lateral accretion of chains which might fold.

Garber and Clark (44, 70, 71) reported the first direct observation of the row nuclei predicted by Keller and Hill; in 1967 Clark had also predicted the existence of such fibrillar nuclei (72). Electron microscopy showed that what appeared to be rows of nuclei in blown film of polyoxymethylene actually were fibrils 200-300A in diameter and up to lop in length. Garber and Clark state that in industrial processes, morphologies often are dramatically different from the spherulitic structures obtained from relaxed melts and highly *‘row nucleated” rather than spherulitic forms can be obtained. The morphology of the blown polyoxymethylene film was that of being completely covered with the edges of protruding lamellae, all highly oriented in the extrusion direction-no evidence for spherulitic morphology was found. Lamellae epitaxially crystallized normal to these row nuclei and although the fibrous row nuclei comprise a negligible amount of the total mass, they determine whether the morphology will be spherulitic or oriented. Garber and Clark state “the actual formation of the [ p ] fibers is still a mystery and will not be considered here” ( 44).

In 1966 Andrews (58) obtained fibrils of about lOOA width in thin films of stretched rubber. He notes that “a fibrous texture may be obtained in a semicrystalline polymer either by allowing the melt to crystallize under a uniaxial stress or by ‘drawing’ the isotropic material at an environmental temperature lower than its crystalline melting point.” Two mechanisms are proposed by Andrews for the observed row nucleated structures: (1) the first ex- planation is that nuclei form along lines of high strain existing in the sample before crystallization but against this view he argues that fortuitous regions of higher than average strain are unlikely to be so narrowly defined, and ( 2 ) an alternative explana- tion is that strings of nuclei are self-propagating with the formation of one nucleus giving rise to a second close by such that the line joining the nuclei lies in



the direction of strain. The fibrils were about lOOA wide and had obvious internal periodic longitudinal discontinuities. Line broadening from electron diffraction indicated the crystalline regions were of dimensions 120-150A in the fibril axis direction and 100-120A in the transverse direction; these sizes are very close to those directly observed in the trans- mission electron microscope.

Yeh in 1969 (59) attempted the identification of the nature and formation of line nuclei in natural rubber. He found ball-like grains of about 40A diameter in undeformed thin films and fractured bulk samples; observation of such grains in “amorphous” materials might be expected since similar structures have been reported in other X-ray diffraction amorphous materials as is discussed by Yeh. He states “that crystallization of one grain may readily lead to crystallization of the grain that is connected to it in order to distribute the strain locally. This would then lead to the formation of ‘line’ nuclei.”

The reviewers do not see how Yeh‘s model can be general enough to apply to direct stressed crystallization from the melt as in fiber spinning, for example; in this case the existence of such requisite nodules before crystallization is unknown.

Other writers, such as the following three, have also recently presented review comments on row structures.

Quynn et al. (73, 74) report on fibers from several different polymers ( polypropylene, poly-3-methybu- tene-1, the acetal copolymer, Celcon, and others) that directly imparted a row structure upon crystallization from the melt under high stress. However, they were unable to present direct evidence for or against the existence of a thin central “thread” as the connecting spine of the row structure (74).

Collier et al. (75) comment on the fact that most commercial processes involve some degree of stress- ing upon the crystallizing polymer and provide a brief review on the topic. For a chlorinated poly- ether, they found that proper selection of compression molding conditions enabled the mechanical properties to be changed from brittle behavior to ductile through what appeared to be an increase in the degree of intercrystalline linkages.

Support for the contention that morphologies of commercial products are often mainly nonspherulitic is drawn from work by Frenkel’s Russian group on how polypropylene spherulitic development is influenced by conditions of spinnerette drawing (76). Small-angle light scattering showed the size of spherulites to be inversely proportional to the degree of spinnerette drawing and two supermolecular phases coexisted in the fibers: a spherulitic and an oriented phase. Extrusion at low melt temperatures and greater degrees of stretching from the spinnerette (conditions approaching commercial practice ) increased the volume fraction of the oriented phase.

Kleinent and Geil [ 19721 (77) recently discussed the production of row structures in thin films by a procedure similar to that of Andrews in the earlier discussion, namely, by annealing the strained amor-

phous polymer at a temperature between the glass transition and the crystalline melting transitions. From these studies on polycarbonate (PC), isotactic polymethyl methacrylate ( i-PMMA) and isotactic polystyrene (i-PS ) , Klement and Geil state that even though the niorphology resembles that of Keller and Machin (66), “there is evidence both for and against long fibrillar, possibly extended-chain crystals acting as nuclei”.

“The evidence for fibrillar nuclei is as follows.

1. The uniform perpendicular orientation of the lamellar morphology. Note, however, that this does not necessarily imply long extended chains, only nuclei with a common orientation.

2. The appearance of distinct ribbons of lamellae several microns long that appear to have nucleated along a common line. This morphology was observed frequently in i-PS, occasionally in i-PMMA, and seldom in PC.

“The evidence against fibrillar nuclei is as follows.

1. The nucleating centers were frequently arranged randomly over the films.

2. No central fibers were observed in any of the bright field micrographs, even in regions where the lamellae had grown or pulled apart.

3. No central fibers were observed in any of the numerous dark field micrographs. The dimension of diffracting regions in the fiber direction was never greater than the normal lamellar thickness ( 100-200A). Although a small number of nucleating fibers would con- tribute little to the total diffraction intensity, they should have appeared as intense as the lamellae in dark field when they were prop- erly oriented for diffraction. Since the fibers are the proposed nuclei, some should be in lattice register with the lamellae they have nucleated or at least have fiber orientation. Unfortunately, previous work on shish kebabs and row-nucleated structures is of no help, since, to our knowledge, no dark field studies were carried out on these structures.”

From the existence of two melt endotherms and changes in X-ray diffraction patterns, Clough interpreted the results in terms of fibrillar crystals in drawn, cross-linked high molecular weight polyethylene films ( 78).

Southern and Porter in 1969 (79, 80) reported on the orientation and pressure crystallization of high- density polyethylene in an Instron capillary rheom- eter. A translucent, highly oriented and pronounced fibrillar structure was found near the entrance to the capillary where the pressure and shear were greatest. The fibrils were oriented parallel to the flow axis and it was concluded very probable that an extended chain component was present in the oriented structure. Also reported was that the highly oriented crystalline polyethylene was transparent rather than turbid as normal (80, 81). It has more recently

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1972, Vol. 12, No. 5 369


( 1971 ) been reported that actually two distinct fibrous morphologies exist in this transparent material. An outer sheath composed of 3000A diameter fibers and the core composed of 200 to 250A diameter fibrils arranged in a ribbon-like texture (53). Electron diffraction indicated a high degree of c-axis molecular orientation parallel with fibrillar axes.

It should be noted that Southern and Porter and other authors writing on stirred solutions have interpreted the existence of higher crystalline melting temperatures than those normally observed as an indication of extended chains rather than folded chains. However, Kawai et al. (82) contended the backbone might not be of the extended chain form but that the observed higher melting points could result from strained tie molecules; a similar explana- tion has been proposed by Yeh (59).

Other workers have also reported row structures (49,83-87).

Several other papers cover novel aspects of this topic. One such paper is that by Stein and Prudhomme (88) who through light scattering investigated patterns of the origin of polyethylene transparency such as that previously discussed by Southern and Porter (80) ; the analysis was consistent with a row or “shish-kebab” structure. Row nucleation in pyrolyzed polyacrylonitrile was determined from X-ray diffraction by Tyson (89). Crystal’s (46) report on the crystalline morphology of poly( N- vinylcarbazole ) indicates to the reviewers that this polymer has a greater propensity for fibrillar formation than most other polymers, but poor mechanical properties are attributed to weak interfibrillar bonding. Binsbergen (90) found an oriented crystallization which could not be relaxed by heating a polypropylene melt up to a temperature where severe degradation occurred. This effect was considered due to the orientation of a strongly birefringent nucleating agent during shearing of the polymer melt.

Hermans (91) has done additional work on the discovery by Battista and Smith (92) that gels can be formed by the action of large shear stresses acting on aqueous suspensions of level-off DP cellulose. Some of the gel’s anomalous flow properties are discussed in terms of “liquid crystal” formation under the influence of shear forces and reference is made to the general tendency of cellulose microcrystals to aggregate side by side in dilute solution as reported by Marchessault et al. (93).

Recently ( 1970) the crystallization kinetics of molten polyethylene subjected to a constant shear stress were investigated theoretically and experi- mentally by Kobayashi and Nagaswawa (94). They stated that the crystallization rate depends on the entropy difference between the crystalline and molten state; constant shear stress in the melt decreases the entropy difference and causes an abrupt increase in the crystallization rate and a decrease in the thickness of folded chain crystals. The folded chain equations were corrected for shear influence but it was stated that the crystallization mechanism under a thermal gradient or during molecular orien-

tation, such as melt spinning, has not been elucidated. At high shear rates a c-axis orientation was observed and cleaved surfaces revealed some fibrous texture but mainly lamellar crystals perpendicular to the shear direction. Similar results were obtained by Haas and Maxwell (95) for polyethylene between heated glass slides under a polarizing microscope and their results were discussed in terms of nucleation lines formed by impurities and spherulites in the deforming melt. Haas and Maxwell gave a thermodynamic discussion on how orientation influences the crystallization mainly by decreasing the entropy change that occurred upon crystallization; in this manner the degree of supercooling was increased.

Wereta and Gogo in 1971 (96) discussed apparatus that was used to investigate the deformed poly- butene-1 melt; a strain history of the polymer during deformation also was obtained.

In Dilute Solutions Aspects of solution crystallization directly pertain

to fibrous morphologies. Long strings of platelets, termed “shish-kebabs” by Lindenmeyer according to Keller, were frequent accidental occurrences in the early studies of solution grown crystals (97). Kawai et at!. (98) state this “shish-kebab structure formed in sheared dilute solutions showed a remarkable similarity of that obtained previously for cross-linked polyethylene films drawn in the molten state and crystallized isothermally. Kawai et al. and Keller and Machin (66) conclude that both structures are analogous examples of the general texture obtained when crystallization occurs under molecular orientation.

Major morphological characteristics of these shish- kebab structures are (99) :

“1. Probably composed of an extended chain overgrowth of lamellae.

2. The backbone forms first and the fibrous crystallization is highly fractionating with the high molecular weight material precipitating first.

3. The folded-chain platelet epitaxial over- growths occur as the solution is cooled to room temperature.

4. The fibers can be obtained free of the platelets either by replacing the solution with pure solvent either before the folded lamellae can form or the platelets can be removed by high temperature washing.

5. Each individual backbone fibril is also found upon further investigation to have a shish-kebab morphology.

6. Ribbon-like threads of lateral dimensions less than lOOA seem to serve as the ultimate fibrous structure and backbone.”

This morphology has also been reviewed by Pennings (100, 101) who concluded that the sites along the backbone where lamellar overgrowth de- velops probably are determined by a random nucleation process (100). Most studies have been with polyethylene but a peculiar phenomenon en-

ENGINEERING AND SCIENCE, SEPTEMMR, 1972, Vol. 12, No. 5 370 POLYMER


countered with some polypropylene and ethylene- propylene shish-kebabs is that part of the fibrillar crystals cannot be dissolved even by exposing them for several days to boiling solvents ( 100).

Manley et al. have reported on isotactic polystyrene fibrous crystals (102, 103) but attempts to reveal the naked central thread as had been done with polyethylene were not successful ( 102). Rijke et al. ( 104) recently (1971) reviewed and presented additional observations on the fine features of polyethylene shish-kebabs. An extensive review in 1971 of polyethylene crystals also includes a review of oriented crystallization ( 105). Polycaproamide fibrils have been crystallized in a solution of glycerol by Ruscher and Schulz (106), while Yamaura et al. (107) have conducted detailed studies on such structures from polyvinyl alcohol solutions.

Pennings developed a model of “unrolling” polymer coils on the lateral growth faces to form the fibril and he estimated the work done by the hydro- dynamic force ( 108).

Pennings ( 101), Keller (log), Wikjord and M&- ley (110) and Harrison and Baer (111) commented on the close analogy of stirred solution crystals and the fibrous morphology often developed during certain polymerizations of ethylene by Ziegler-Natta catalysis. Such points are discussed by Iguchi et al. in a 1971 review of the formation of polymer crystals during polymerization ( 112).

The close relationship between shish-kebab structures and fibrils has been shown with smooth fibrillar polytrioxane which is converted to a shish- kebab structure upon annealing ( 22). Fibrils pulled between lamellae in drawn polyoxymethylene ( POM ) exhibit similar annealing behavior. A reveal- ing experiment on epitaxial growth of polyethylene (PE) on drawn POM surfaces showed that lamellar growths of polyethylene crystallized on the POM substrate with the longer axes of the lamellae perpendicular to the fiber axis of the POM but it was confirmed the molecular axes of the PE oriented parallel to the POM fiber axis (113). A similar epitaxial oriented growth occurred when POM was polymerized directly onto a drawn POM substrate (114). Peterlin (115) has reported that annealing of drawn polypropylene at 16OOC can result in recrystallization and formation of shish-kebab structures.

Another interesting class of structure is the intercrystalline link or bridging structure ( 116-120). Most of these structures have been formed in concentrated solutions of polyethylene-paraffin mixtures; William- son and Busse ( 121 ) sheared the latter type mixture and found fibrous crystals of over 200 microns in length and fibrous intercrystalline links of up to 1 micron in length.

MODELS OF MICROFIBERS IN DENSE MATERIAL

The two previous sections recount the complexity of secondary effects in the apparent structure of the microfibers produced from sheared dilute solutions and diluted polymer melts, both con-

fused with the effects of ever present lower molecular weight components in the source polymer. The major environmental parameter which controls the microfiber formation is the presence of a strong shear or extensional detormating field. This and temperature conditions favorable for transformation lead to the observed morphologies. We now turn to polymer systems which are dense and discuss the evidence for microfibers in cases where secondary transformations are essentially different in kind and in degree.

Early models for crystalline regions developed in the late 1920’s and early 1930’s were close to being models for fibrils but they either neglected showing how the discrete blocks or crystallites were connected to form the observed long fibrils [for example, the micelle model of Nageli (122) in about 1860, and Meyer and Mark (123) in 19291 or they did not explain the transverse boundaries of the fibrils as in the continuous crystalline model of Staudinger (124) in 1932.

The next development of a fringed-micelle concept by Abitz Gerngross and Herrmann [ 19301 ( 125) was originally intended as a model for crystalline regions in gelatin, but as applied to the oriented form of cellulose by Kratky and Mark [ 19371 (126), it is easy for one to visualize weak cleavage planes between the oriented fringed micelles. It is thus a suitable model for elementary fibrils, since earlier it was shown that crystallites were on the order of 50A in width, a typical width for many elemental fibrils. According to Krassig and Sippel (127), Frey- Wyssling (128) in 1938 was actually the first to use the fringed micelle theory to represent fibrils; however, Preston (129) in 1934 alluded to the fringed micelle and fringed fibril but did not present a full depiction.

Hearle ( 130) later (in 1958) modified the fringed micelle model, as then popularly understood, and renamed it the fringed fibril model; his depictions indicate the only basic differences between the latter and the fringed micelle involve ordered regions being on the order of lOOA in width rather than 50A as deduced earlier and all the molecules are no longer shown diverging at the same position. He considers the crystalline regions as continuous “fringed fibrils” in spite of the arguments of the earlier model derived from X-ray line broadening considerations. In 1971, Scallan (37) defined more clearly the case for the fringed micellar model and depicted it to scale for various cellulosic materials.

Numerous models other than the fringed-micelle have been developed for the cellulose fibril. Morgan (131) in 1954 rejected the fringed-micelle theory and considered the fibril as a continuous crystalline phase with the crystalline material being a tightly wound spiral which he considered the elementary fibril. Manley modified this view in 1964 by considering the spiral composed of folded chains and the spiral collapsed to form a tightly wound ribbon rectangular in cross section rather than circular (28, 36). Sup- port and models both for and against cellulosic molecular folding were presented at a recent (1971)

POLYMER ENGINERING AND SCIENCE, SEPTEMBER, 1972, Vol. 12, No. 5 37 1


cellulose conference so the issue is still unresolved

Before the acceptance of the polymer chain folding concept, Hess (132) in 1958 suggested that elementary fibrils consist of alternating bands of crystalline and amorphous regions with all rather parallel molecules going from one phase to the other. It now seems difficult to reconcile this view with Flory’s [1962] (69) calculations showing that most polymers have insufficient space in the disordered regions to accom- modate all the chains without some folding into the ordered regions. He notes that the spatial factors enforcing reentry of emerging chains become opera- tive only when the transverse dimensions of the crystal are large: this point might serve to restrict the lateral dimensions of crystallites if the crystallites are in series with amorphous regions as in the case for many models of fibrils.

As earlier stated for cellulose, some contro- versy still exists over the existence of chain folds in oriented polymers but there is more agreement that folding probably exists in oriented polymers that are allowed to shrink during annealing. For example, Heffelfinger and Lippert (133) in late 1971 state “we are unconvinced that the morphology responsible for the long period diffraction in highly oriented PET films consists of folded-chain lamellar domains.” J. Sikorski ( 134) noted that “folding in fibers has not been established, we deal in speculations.” In contrast, Prevorsek and Sibilia (135) found that an infrared absorption band attributed to regular chain folding remained in samples drawn to a maximum and they concluded that chain unfolding in PET fibers and films cannot be achieved by conventional drawing techniques.

Hearle (136) has modified his fringed-fibril model to allow folds between the crystalline regions. The model of Hosemann (137) is so general it can account for most all observed features. Prevorsek‘s (138) structural model also well satisfies most experimental data. One should not forget, however, as Scallan cautions (37), that all such models are based on limited quantitative data with too much flexibility left to the “scientist-turned-artist.’’

The reviewers feel that the coil-helix (C-H) transition might serve as a one-dimensional model for the “molecule-fibril” transition and conclusions from various C-H investigations should be applicable to the general formation of fibrils. Tsvetkov (139) in 1969 provides an extensive review of literature pertaining to intramolecular and supermolecular structure. He states that the random coil to helix transformation was thought until recently to be peculiar to molecules with secondary structures formed through hydrogen bond intramolecular interactions but this view has been shown incorrect by synthesis of lad- der polymers that had the crystal-like behavior and also highly ordered structures were formed solely as a result of main chain rigidity. Tsvetkov (139) also refers to the “melting” of the helices undergoing the C-H transition. An exhaustive review of the C-H transition and techniques for its study is provided

(14). (140). The H-C transition occurs in solid gelatin at high temperatures even in the absence of water, and is accompanied by hypercontraction and the “dis- appearance of mechanical properties characteristic of fibrillar structures” (141).

Such a structure might be termed a one-dimensional fibril and it could serve as an embryo for the three-dimensional fibrillar growth. An apparent dif- Bculty with this supposition, however, is that it seems to violate a thermodynamic law stated by Landau and Lifshitz (142) for one-dimensional systems: “It is impossible for different phases to exist in a one- dimensional system.” This point is elaborated on by Birshtein and Ptitsyn (143) who do not disagree with Landau and Lifshitz and they state that “Landau and Lifshitz examined a linear system formed of consecutive alternating regions of two different ‘phases’ and showed that these regions cannot have any desirable size, ie., the examined phases are not true phases in the thermodynamic sense.”

In support of the contention that a one-dimensional phase transition might occur, it should be noted that Poland and Scheraga (140) reprint 44 papers pertaining to the helix-coil problem and several of these claim that such a transition is possible.

With the above complexities in mind, the future may reveal a true one-dimensional transition in polymer molecules at least under certain circumstances.

FIBRIL FORMATION IN DRAWN POLYMER SOLIDS

Many solid polymeric materials, normally in film or fiber form, are cold or cool drawn after extrusion; this attenuation generally transforms a spherulitic into a fibrillar structure. Extensive investigations on the morphological changes with this type process have been conducted under the guidance of Peterlin; other major groups associated with this topic are those of Geil, Yeh, Samuels and Hansen.

Peterlin’s investigations on polyethylene ( 144, 145) and polypropylene (50, 146) have led to a generally accepted model of the spherulitic to fibrillar transformation. This partial reference listing is augmented in reviews contained in (115) and (147). Peterlin’s model as developed is based primarily on observations of high density polyethylene but has been shown to also apply to low density polyethylene (148) and to certain polypropylenes ( 115).

Peterlin and BaltBCalleja ( 146) state: “Macro- scopically, one hence observes in the neck a gradual transformation of the spherulitic structure into a fibrous structure although the individual ultramicro- scopic process is discontinuous as in the case of the microfibrils pulling out of single crystals.”

Sakaoku and Peterlin (115) summarize features of the model. The decisive step in the plastic deformation of a microspherulitic structure into a highly oriented fiber structure is the complete destruction of every lamella of the original structure through “micro-necking.’’ During deformation micro- necks occur at cracks which cut through a sufficient number of chain folds. “The chains do not rupture but bridge the gap, partially unfolding and tearing



off folded-chain blocks which are incorporated into microfibrils of the new structure. The unfolded- chain sections form intrafibrillar tie molecules which are mainly responsible for the high longitudinal strength of the new fiber structure” (115). The folded-chain blocks, with lateral dimensions between 100 and 400A, already exist in the original as the mosaic structure. Dark field electron microscopy of thin membranes of polyethylene and of polypropylene drawn in this manner showed isolated crystal blocks in the fibrils (115). The discrete blocks were all nearly the same size and did not seem to be very anisometric; however, the results were not ade- quate to yield a definitive statement about their shape, These results mean that along the fibril axis the crystalline blocks adjacent to those seen in the dark field micrograph must be mismatched as far as the orientation of the a and b axes is concerned. With deformation of isolated single crystals the thickness of the lamella directly determines the thickness of blocks incorporated into the microfibrils but with destruction of stacks of parallel lamella as occurs with bulk samples, small-angle X-ray long periods of the drawn material do not equal those of the undrawn as in the case with very thin layers on the order of less than 100019. Instead, the long period of the thicker drawn sample depends on the temperature of drawing, presumably due to poor heat dissipation which enables considerable chain mobil- ity to thicken the lamellae.

Lamellae located in different parts of a micro- spherulite receive different degrees of strain and compression (50) and as a consequence of nonuni- form strain some microfibrils may have an orientation quite different from the rest (115).

Meinel and Peterlin (144) report two major components in the total stress-strain curve to rupture for a spherulitic to fibrillar transformed material. The first is associated with the destruction of the originally microspherulitic structure and the second is related to deformation of the fiber structure which was generated from the destroyed microspherulites; this second deformation component is stated to be due mainly to the sliding motion of the created microfibrils. Strain hardening is explained as due to extending interfibrillar tie molecules which most likely were developed from tie molecules which connected adjacent lamellae in the undeformed, microspherulitic material. Slippage of fibrils has been proposed and discussed by several other authors (24, 149,150)-

Williams and Peterlin ( 145) recently investigated the transport properties of drawn polyethylene and concluded “the low sorption and the still smaller diffusion constant characterize the microfibril as a very impermeable structural element, sharply con- trasting with the rather permeable spherulitic structure of the original undrawn material.”

Even though the Peterlin model is widely accepted, a few workers have recently published dif- ficulty in rationalizing it with their results (53, 151) Crystal and Southern (53) state that a lamellar structure connected by tie molecules as is the model pro-

posed by Peterlin would not satisfy their electron diffraction results and that “only a well developed extended-chain crystal model which maintains a high degree of order and register across the aperture opening could account for the observed diffraction spots from the inner-core fibers.” It should be noted, however, that this material was not conventionally drawn but was polyethylene crystallized in a capillary viscometer. Singhania and Geil (151) state “this [Peterlin] model, however, is difficult to reconcile with the discrete (ca. l00A) diameter fibrils which are particularly evident in samples which have undergone micro-necking.”

The report that uniaxial compression of spherulitic polyethylene yields a fibrillar structure (152) might be of value for interpreting the compression effects at neck drawing as discussed above.

MLIZZY and Hansen (52) deformed 8 mm diameter spherulitic rods of polyethylene and found two deformation mechanisms; one was in concert with the macroscopic deformation and the other involved small craze-like disruptions within the spherulites. The elongated spherulites imparted a gross fibrous structure and the disruptions within spherulites .were oriented to create a fine, fibrillar structure. The authors state these dilations or crazes are probably similar to what Peterlin has labeled micro- necks.

Relative to the above point on spherulitic deformation, Hay and Keller (153) reported in detail on two different modes. One is a relatively uniform deformation of spherulites while the second mode is characterized by micro-necking or formation of a highly deformed region of a spherulite which grows at the expense of undeformed portions of the spherulite and they attribute formation of fibrils to the creation of voids between deformed spherulites after micro-necking. Their mechanism of fibrillar formation seems similar to that of Muzzy and Hansen.

Neck deformed polyethylene films have been drawn a second time perpendicular to the initial deformation; fibrils formed in each case were parallel to the last draw direction but it was unknown if recrystallization occurred ( 154).

Samuels has done extensive work on characteriz- ing the fibrillization of initially spherulitic isotactic polypropylene (i-PP) (155-157). He does not em- phasize fibrillar formation but his brief descriptions are similar to those of Peterlin. A most interesting discovery in i-PP by Samuels is that different fabrica- tion conditions used in film and fiber processes are apparently different paths by which a given average orientation is produced which he measures with Hermans’ orientation function, f . Samuels found that most physical properties, such as tensile recovery, have maxima at f values of 0.76 and this is the transformation region of spherulitic to microfibrillar structure (156, 157). Along with Muzzy and Hansen (52) for polyethylene, Samuels also has reported considerable void formation occurring during drawing. At low extensions the microvoids were oriented transverse to the fiber axis while at higher extensions the microvoids were extended parallel to the fiber

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1972, VoI. 12, No. 5 373


axis; interspherulitic yielding formed the macrovoids (seen in light dark field microscopy) and microvoids (order of 100-ZOOA) and such formation was complicated by and dependent on the previous heat setting history (155). Movement of voids during drawing is very complex as illustrated by the neck drawing of amorphous polycarbonate where crazes formed perpendicular to the fiber axis were carried through the neck virtually unchanged in size or shape

Gezovich and Geil (159) state that spherulites of the quenched form of polypropylene (smectic crystalline structure ) have been homogeneously deformed whereas slowly cooled films (containing a greater amount of monoclinic structure than does the quenched) show complete destruction of spherulitic order. Kargin and associates (160) have neck deformed polypropylene films up to six times in length with the original spherulitic boundaries still visible.

The present authors agree that the above discussions of spherulitic deformation can account for much fibrillation potential but it is not clear that all grossly microfibrillar structures must arise from the cold drawing of spherulitic morphology.

In a detailed study of quenched, drawn and annealed polypropylene, Wyckoff ( 161) found the lateral dimensions for the crystallites to be 50A initially and they could be transformed to 200A in width with annealing and recrystallization.

Tuichiev et al. (162) suggest a scheme for structural rearrangement in polyvinyl alcohol fibers with increasing degree of drawing. They believe greater drawing increases the number of intrafibrillar tie molecules between crystalline regions while decreasing the number of folded chains. There is a simul- taneous slight reduction of lateral dimensions of fibrils and an increase in density of inter- and intrafibrillar amorphous layers.

Vasilenko et al. (43) have studied structural and morphological changes in polyethylene terephthalate (PET) monofilaments during drawing at 70°C and 100°C. They found that differences in drawing temperature influenced the fine structure; at a drawing temperature of 70°C the microfibrils showed a characteristic internal globularization (“beady” fibrils) whereas the microfibrils in fibers drawn at 100°C are better oriented and show no beadiness.

Davis, Van Veld and Billica (40) found that PET fibers exhibited such a nodular structure after thermal relaxation but not before. Such relaxation sig- nificantly altered the fibrous texture. Morphological units contracted and became somewhat tilted with respect to the fiber axis and the internal boundaries which were sharply defined before relaxing usually broadened and became less defined; they interpret this as indicating the internal boundaries probably are not simply microcracks (40). Peterlin (20) reports observations of similar tilting of lamellae with polyethylene.

Klement and Geil (163, 164) in 1971 reported that a 250-5008 nodular structure was observed under a variety of conditions ih drawn thick and thin films

(158).

of PET. They state that oriented PET, heat set at 180 to 240°C, is composed of small (140A) iso- metric crystallites and they “found no evidence to support a platelet lamellar structure in drawn and annealed PET thin films.”

Heffelfinger and Lippert (133) recently (1971) reported an extensive study of X-ray low-angle scattering from oriented PET films and concluded there were three major sizes of structural domains: (1) 5-15A, the unit cell, ( 2 ) 100-150A, of uncertain morphology, and (3) large laminar domains of 2,000A X 10,000A average size. Their uniaxially deformed film did not show rotational symmetry in small angle X-ray scattering ( SAXS )-the three major directions yielded entirely different patterns. They suggest in- terpretations of fiber morphology based on SAXS effects may not be applicable to films (133).

Retractable interlamellar links have been observed in poly( 4-methylpentene-1) (P4MP1) by Hase and Geil (45, 165). A solid swelling agent was used to separate the interlamellar regions and reveal the links which retracted upon deswelling. Retraction of the fibrils seemed to also take a major role in the total sample shrinkage from high mechanically induced elongations. Hase and Geil believe there are at least two types of fibril structures. They state “although a ‘wavy lamella’ model appears to rea- sonably well describe the structure of the type usually seen in fully drawn polymers, the structure observed here for P4MP1 and in POM would appear to have a different and, as yet, unknown structure.” Hase and Geil also state these different links or fibrils have little lateral cohesion other than thin interconnecting bridging fibrils, their formation is not peculiar to the micro-necking process and upon annealing the fibrils do not develop striations as do most micro-neck formed fibrils.

A study by Frolov (166) might be useful in ex- plaining the recoverability of the fibrils reported by Hase and Geil. Polyvinyl alcohol film deformation due to the joint effect of solvent and static stress indicated the combined action causes disintegration of the crystalline phase without destruction of macromolecular structures (bundles, fibrils, etc.) and a direct transition to the highly elastic state (under the influence of stress and solvent) seemed “associated with deformation of the primary macromolecular structures, and not with deformation of the macromolecules as such, or their segments” ( 166).

Numerous other studies on deformed single crystals, and thick and thin films, have been conducted but will not be discussed here because of space limitations and because the results might not be directly applicable to bulk polymers (as with deformed single crystals) or there are no unique additional results that could be added to the above review. Such references are polyethylene ( 167, 168), polypropylene ( 159, 169-172), polyoxymethylene ( 173, 174,) polyurethane ( 175), polyvinylidene- fluoride ( 176), polyvinyl alcohol ( 177), polyethylene terephthalate ( 178), polycarbonate ( 77), isotactic polystyrene ( 77), isotactic polymethyl methacrylate (77), cellulose (35, 179) and gelatin (180).



ACKNOWLEGMENTS The authors wish to acknowledge stimulating cor-

respondence on the subject of this paper with Prof. J. Sikorski of Leeds University and numerous recent discussions with Dr. A. Peterlin of the Camille Drey- fus Laboratory, Research Triangle Institute, Durham, N. C. Dr. Harry Billica of E. I. duPont de Nemours and Co., Inc. through his careful discussions and revelatory experimental techniques has contributed substantially to our interest and understanding of microfibers in drawn textile fibers. We are further appreciative of the comments of unknown reviewers of our initial writing in this field.

1.

2.

3.

4. 5. 6.

7. 8. 9.

10.

11. 12.

13. 14. 15.

16. 17.

18.

19.

20.

21.

22.

23.

24. 25.

26. 27. 28. 29. 30. 31.

REFERENCES N. Grew, “The Anatomy of Plants,” Johnson Reprint Corp., New York and London (1965). P. Grove (Ed.), “Webster’s Third New International Dictionary,” G. & C. Merriam Co., Springfield, Mass. (1961). I. Wingate (Ed.), “Fairchild’s Dictionary of Textiles,” Fairchild Publications, Inc., New York (1967). J. Hearle and R. Greer, Textile Progress, 2 (4), 1 (1970). H. Ogle, ACS Polymer Preprints, 2, 169 ( 1961). J. Sikorski, in “Fiber Structure” (J. Hearle and R. Peters, Eds.), p. 269, The Textile Institute, Butter- worths, Manchester and London ( 1963). D. Cauldfield, Tex. Res. I., 41,267 (1971). J. Colvin, J. Cell. Biol., 17, 105 (1963). W. Franke and B. Ermen, Z. Naturforschg., 24b, 918 (1969). E. Macchi and A. Palma, Makromol. Chemie, 123, 286 (1969). H. Ruck and R. Pohle, Tex . Res. J., 40, 1048 (1970). J. Colvin and M. Beer, Canad. J. Microbiol., 6, 631 (1960). J. Colvin, 1. Poly. Sci., B-4, 747 ( 1966). E. Jahn (Ed.), J. Poly. Sci., C-36 ( 1971 ). J. Finean, “Engstrom-Finean Biological Ultrastructure,” Academic Press, New York ( 1967). H. Woods, 1. Poly. Sci., C-20, 37 (1967). R. Borasky (Ed.), “Ultrastructure of Protein Fibers,” Academic Press, New York ( 1963). W. Crewther (Ed.), “Symposium on Fibrous Proteins, Australia 1967, Plenum Press, New York (1968). K. Hirabayashi, H. Ishikawa and M. Kakudo, Sen-i Gakkaishi, 25, 440 (1969). H. Mark, S. Atlas and E. Cernia (Eds.), “Man-Made Fibers, Science and Technology,” Vol. 1, Interscience, New York (1967). G. Zaspinok, N. Zhegalova, B. Vasil’ev, N. Naimark and 0. Tarakanov, Polymer Science USSR, 11, 172 (1969). T. Amano, E. Fischer and G. Hinrichsen, J. Macromol.

J. Lobodi-Cackovic, R. Hosemann and W. Wilke, Kol- loid Z., 235, 1253 ( 1969). A. Nojiri, Kobunshi Kagaku, 24,250 ( 1967). Belgian Patent 65,743 to British Nylon Spinners, Ltd. ( 1965); Belgian Patent 686,401 to Imperial Chemical Industries, Ltd. (1967); British Patent 930,074 to Union Carbide Corp. (1963); U.S. Patent 3,235,644 to Philli s Petroleum Co. (1966); U.S. Patent 3,369,057 to AllieiChemical Corp. (1968); U S . Patent 3,546,063 to E. I. duPont de Nemours and Co. (1970); US. Patent 3,548,048 to Phillips Petroleum Co. ( 1970); U.S. Patent 3,549,734 to T. Yasuda and Y. Fujiwara, Japan ( 1970 ) . A. Heyn, 1. Appl. Phys., 26,519 (1955). A. Heyn, J. Appl. Phys., 26, 1113 (1955). R. Manley, Nature, 204, 1155 ( 1964 ) . A. Peterlin and P. Ingram, Tex. Res. J., 40, 345 (1970). R. Hosemann, 1. Appl. Phys., 34, 25 ( 1963). R. Hosemann, J. Poly. Sci., C-20, 1 (1967).

Sci.-Phys., B3, 209 ( 1969).

32. L. Alexander, “X-ray Diffraction Methods in Polymer Science,” John Wiley & Sons, Inc., New York ( 1969).

33. J. Lobodi-Cackovic, R. Hosemann and W. Wilke, Kolloid Z., 235, 1162 (1969).

34. H. Dolmetsch and H. Dolmetsch, Tex. Res. J., 39, 568 ( 1969).

35. A. Heyn, Kolloid Z., 244, 309 (1971). 36. R. Manley, J. Poly. Sci., A-2, 9, 1025 (1971). 37. A. Scallan, Tex. Res. J., 41, 647 (1971 ). 38. G. Wlodarski and E. Balcerzyk, Cellulose Chemistry

39. N. Mikhailov and G. Mikheleva, Polymer Science USSR,

40. R. Van Veld, G. Morris and H. Billica, J. Appl . Poly.

41. R. Crystal and D. Hansen, I. P o b . Sci., A-2, 6, 981

and Technology, 4,651 ( 1970).

12, 1898 ( 1970).

Sci., 12, 2907 (1968). . .

( 1968’).

USSR. 12.2027 ( 1970). 42. M. Gezalov, V. Kuksenko and A. Slutsker, Polymer Sci.

43. Z. Vaiilenko, N.‘ Mikhailov and V. Berestnev, Polymer

44. C. Garber and E. Clark, 1. Macromol. Sci.-Phys., B4,

45. Y. Hase and P. Geil, Polymer J., 2, 581 (1971). 46. R. Crystal, Macromolecules, 4, 379 ( 1971). 47. M. Takayanagi, K. Imada and T. Kajiyama, J. Poly. Sci.,

Sci. USSR, 12, 1645 (1970).

499 ( 1970).

C-15.263 1966). 48. J. C;aig,- J: xnudsen and V. Holland, Tex . Res. J., 32,

435 ( 1962 ). 49. G. Hinrichsen and H. Orth, Polymer Letters, 9, 529

50. F. BaltCCalleja and A. Peterlin, J. Macromol. Sci.-Phys.,

51. M. Takesue, Reports on Progress in Physics in Japan,

52. J. Muzzy and D. Hansen, Tex. Res. I., 41,436 (1971). 53. R. Crystal and J. Southern, J. Poly. Sci., A-2, 9, 1641

54. J. Katz and K. Bing, Z. Angew. Chem., 38, 439 (1925). 55. W. Smith and C. Saylor, J. Res. &’at. Bur. Standards,

56. L. Mandelkern, “Crystallization of Polymers”, McGraw-

57. A. Gent, Trans. Faraday SOC., 50,521 (1954). 58. E. Andrews, J. Poly. Sci., A-2, 5, 1235 (1967). 59. G. Yeh, Polymer Preprints, 10, 1020 (1969). 60. E. Ribi, Nature, 168, 1082 (1951). 61. A. Thompson, 1. Polymer Sci., 34, 741 (1959). 62. A. Keller, 1. Poly. Sci., 15, 31 (1955). 63. S. Simmens, J. Text . Inst., 46, T715 (1955). 64. J. Judge and R. Stein, 1. Appl. Phys., 32, 2357 (1961). 65. P. Geil, “Polymer Single Crystals”, Interscience Pub-

66. A. Keller and M. Machin, J. Macromol. Sci.-Phys., B1,

67. M. Hill and A. Keller, J . Macromol. Sci.-Phys., B3, 153

68. M. Hill and A. Keller, J. Macromol. Sci.-Phys., B5, 591

69. P. Flory, J. Amer. C h e m . SOC., 84, 2857 (1962). 70. C. Garber and E. Clark, ACS Polilmer Preprints, 10.

( 1971 ).

B4,519 ( 1970).

9,231 (1966).

(1971).

21,257 ( 1938).

Hill, New York ( 1964).

lishers, New York ( 1963).

41 (1967).

(1969).

(1971).

I -~ 1004 (1969).

( 1971 ) . 71. E. Clark and C. Garber, Inter. 1. Poly. Muter., 1, 31

72. E. Clark, SPE Journal, 23, 246 (1967). 73. R. Quynn, H. Brody, S. Sobering, I. Park, R. Foley,

H. Noether, W. Whitney, R. Pritchard, M. Sieminski, J. Hutchison, H. Wagner, K. Sakaoku and R. Comelius- sen, 1. Macromol. Sci.-Phys., B4, 953 (1970).

74. R. Quynn and H. Brody, J. Macromol. Sci.-Phtp., B5, 721 7 i971).

75. J. Collier, Poly. Eng. and Sci., 11,452 (1971). 76. T. I. Volkov. G. S. Farshvan, V. G. Baranov and S. Ya.

Frenkel, Poly. Sci., USSR,‘11; 119 (1969).

(1972). 77. J. Klement and P. Geil, 1. Macromol. Sci.-Phys., B6, 31

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER, 1972, Yo/. f2, No. 5 375


78. 79.

80.

81.

82.

83. 84 I

85. 86. 87.

88.

89,

90,

91 92

S. Clough, J. Appl . Poly. Sci., 15, 2141 (1971). J. Southern and R. Porter, Polymer Preprints, 10, 1028 (1969). 1. Southern and R. Porter, J. Macromol. Sci.-Phys., B4, 341 (1970). C. Desper, J. Southern, R. Ulrich and R. Porter, J. Appl . Phys., 41, 4284 (1970). T. Kawai, K. Ebara and H. Maeda, Kolloid Z., 229, 168 (1969). P. Lindenmeyer, SPE Trans., 4, 157 ( 1964). K. Kobavashi and T. Naeasawa. J. Poh . Sci., C-15, 163 (1966).’ F. Anderson, J. Poly. Sci., (2-8, 275 (1965). J. Dietl, Kunststofle, 59, 515 (1969). I. Hardin, D. Luch and G. Yeh, J. Poly. Sci., B-9, 771 (1971). R. Stein and R. Prud’homme, Polymer Letters, 9, 595 (1971). C. Tyson, Nature (London), Phys. Sci., 229, 121 ( 1971). F. Binsbergen, Ph.D. Thesis, Groninger, Netherlands (1969). J. Hermans, J. Poly. Sci., C-2, 129 (1963). 0. Battista and P. Smith, Ind. Eng. Chem., 54, 20 (1962).

”

93. R. Marchessault, F. Morehead and M. Koch, J. Colloid

94. K. Kobayashi and T. Nagaswawa, J. Macromol. Sci.-

95. T. Haas and B. Maxwell, Poly. Eng. and Sci., 9, 225

96. A. Wereta and C. Gogos, Poly. Eng. and Sci., 11, 19

97. A. Keller, Rep. Progr. Phys., 31, 623 ( 1968). 98. T. Kawai, Kolloid Z., 222, 1 (1968). 99. A. Keller, Kolloid Z., 231, 386 (1969).

S d . , 16, 345 ( 1961).

Phys., B4, 331 (1970).

(1969).

(1971).

100. A. Pennings, J. Mark and A. Kiel, Kolloid Z., 237, 336

101. A. Pennings, J. Mark and H. Booij, Kolloid Z., 236, 99

102. A. Wikjord, D. Page and R. Manley, J. Macromol. Sci.-

103. A. Wikjord and R. Manley, J. Macromol. Sci.-Phys., B4,

104. A. Rijke, J. Hunter and R. Flanagan, J. Poly. Sci., A-2, 9,

105. R. Fava, J. Poly Sci., D-5, 1 (1971). 106. C. Ruscher and E. Schulz, Faserforschung u n d Ter-

tiltechnik, 22, 260 ( 1971 ) . 107. K. Yamaura, S. Kinugasa and S. Matsuzawa, Kolloid

Z., 248,893, ( 1971). 108. “Characterization of Macromolecular Structure,” Proc.

of Conf. April 1967, Warrenton, USA (Published by Nat. Acad. Sci., Washington, D. C. 1968).

109. A. Keller and F. Willmouth, Makromol. Chemie, 121, 42 (1969).

110. A. Wikjord and R. Manley, J. Macromol. Sci.-Phys., B2, 501 (1968).

111. I. Harrison and E. Baer, Polymer Letters, 9, 843 (1971).

( 19.70).

(1970).

Phys., B4,412 ( 1970).

397 ( 1970).

531 (1971).

112. M. Iguchi, H. Kanetsuna and T. Kawai, Brit. Poly. J., 3, 177 ( 1971 ).

113. K. Kobavashi and T. Takahashi. Kobushi Kaeaku, 28, v - -

178 ( 1971).

( 1971 ). 114. T. Takahashi and N. Ogata, Polymer Letters, 9, 895

115. K. Sakaoku and A. Peterlin, J. Poly. Sci., A-2, 9, 895

116. A. Keller, Phil. Mag., 2, 1171 (1957). 117. D. Basset, A. Keller and S. Mitsuhashi, J. Poly. Sci.,

118. 0. Spurr and W. Niegisch, J. Appl . Poly. Sci., 6, 585

119. R. Williamson and R. Novak, J. Poly. Sci., By 5, 147

120. H. Keith, F. Padden and R. Vadimsky, J. Appl . Phys.,

(1971).

A, 1,763 ( 1963 ) . (1962).

( 1967).

42,4585 ( 1971).

121. R. Williamson and W. Busse, J . Appl. Phys., 38, 4187 (1967).

122. C. Nageli, “Micellartheorie,” original papers reprinted as Ostwald‘s Klassiker, No. 227 (Edited by A. Frey), Leipzig ( 1929 ) .

123. H. Mark and K. Meyer, 2. Physik. Chem., B2, 115 ( 1929).

Verbindungen,” Springer-Verlag, Berlin ( 1932 ) . 18,754 ( 1930).

( 1937).

124. H. Staudinger, “Die Hochonolekularen Organischen

125. W. Abitz, 0. Gerngross and K. Herrmann, Naturwiss.,

126. 0. Kratky and H. Mark, 2. Physik. Chem., B26, 129

127. H. KrassiE and A. Sippel, Chemiefasern, 19, 510 (1969); _ - 19,612 (7969).

128. A. Frey-Wyssling, “Submikroskopische Morpholigie des Protoulasmas und Seiner Derivate,” Gerb. Borntrager, Berlin’ ( 1938).

(1934).

- 129.

130. J. Hearle, J. Poly. Sci., 28, 432 (1958). 131. L.Morgan, J. Appl . Poly. Sci., 4, 160 (1954). 132. K. H a s , E. Gutter and H. Mahl, Kolloid Z., 158, 115

133. C. Heffelfinger and E. Lippert, J. Appl . Poly. Sci., 15,

134. J. Sikorski, University of Leeds, England. Private com-

135. D. Prevorsek and 1. Sibilia, J. Macromol. Sci.-Phqs.,

R. Preston, Phil. Trans. of the Royal SOC., By 224, 131

( 1958).

2699 (1971).

munication (July 1971 ).

B5,617 (1971). “ 136. J. Hearle, J. Poly. Sci., C-20, 215 (1967). 137. R. Hosemann. Pohmer. 3. 349 ( 1962 ). 138. D. Prevorsek,’J. P h y . Sci.; C-32, 343 ( 1971). 139. V. Tsvetkov, Polymer Sci. U S S R , 11, 148 (1969). 140. D. Poland and H. Scheraga, “Theory of Helix-Coil

Transitions in Biopolymers”, Academic Press, New York (1970).

141. G. Burdygina, I. Fridman, P. Kozlov and V. Kargin, Polymer Sci. USSR, 11, 132 (1969).

142. L. Landau and E. Lifshitz, “Statistical Physics”, Claren- don Press, Oxford ( 1938).

143. T. Birshtein and 0. Ptitsyn, “Conformations of Macro- molecules”, Interscience Publishers, New York ( 1966).

144. G. Meinel and A. Peterlin, J. Poly Sci., A-2, 9, 67 (1971).

145. J. Williams and A. Peterlin, J. Poly. Sci., A-2, 9, 1483 (1971).

146. A. Peterlin and F. BaltA-Calleja, J. Appl . Phys., 40, 4238 ( 1968).

147. A. Peterlin, J. Material Sci., 6, 490 ( 1971). 148. A, Peterlin and G. Meinel, Makromolekulare Chemie,

149. K. O’Leary and P. Geil, J. Appl . Phys., 38, 4169 (1967). 150. R. Robertson, J. Poly. Sci., A-2, 9, 453 (1971). 151. A. Singhania and P. Geil, Makromolekulare Chemie,

152. V. Gerasimov and D. Tsvankin, Polymer Sci. USSR, 12,

153. I. Hay and A. Keller, Kolloid Z., 204, 43 (1965). 154. L. Nardareishvili and T. Magradze, Polymer Sci. USSR,

155. R. Samuels, J. Poly. Sci., A-2, 6, 2021 (1968). 156. R. Samuels, ACS Polymer Preprints, 10, 1218 (1969). 157. R. Samuels, J. Macromol. Sci.-Phys., B4, 701 (1970). 158. 0. Spurr and W. Niegisch, 1. Appl . Poly. Sci., 6, 585

142,227 ( 1971 ).

143,231 ( 1971 ).

2422 (1970).

10, 1395 (1968).

(1962). 159. D. Gezovich and P. Geil, Poly. Eng. and Sci., 8, 202

160. V. Selikhava, G. Markova and V. Kargin, Polymer Sci. ( 1968).

U S S R , 6,1246 ( 1964). 161. H. Wyckoff, J. Poly. Sci., 62, 83 (1962). 162. S. Tuichiev, N. Sultanov, B. Ginzburg and S. Frenkel,

163. J. Klement and P. Geil, J. Macromol. Sci.-Phys, B5, 505 Polymer Sci. USSR, 12,2298 ( 1970 ).

(1971).

376 POLYMER ENGlNEERlNG AND SCIENCE, SEPTEMBER, 1972, Vol. J2, No. 5


164. J. Klement and P. Geil, J. Macromol. Sci.-Phys., B5, 535

165. Y. Hase and P. Geil, Polymer J., 2, 560 (1971). 166. M. Frolov, Polymer Sci. USSR, 12, 1733 (1970). 167. H. Sasano and T . Kawai, Sen-i Gakkaishi, 27, 1 (1971). 168. G. Yeh and P. Geil, J . Macromol. Sci.-Phys., 32, 29

169. P. Cerra, D. Morrow and J. Sauer, J . Macromol Sci.-

170. M . Kojima, J . Poly. Sci., A-2, 6, 1255 (1968). 171. K. Matsumoto, K. Sato and R. Imamura, Sen-i Gakkaishi,

172. S. Kinoshita, F. Matsumoto, Y. Yamamoto and Y. Saito,

173. C. Garber and P. Geil, Makromolekulare Chemie, 113,

174. K. OLeary and P. Geil, Makromolekulare Chemie, 118,

175. V . Kuz’mina, Y. Pasechnik, L. Gul’ko, Y. Lipatov and

176. Y. Gal’perin, V. Mindrul and V . Smirnov, Polymer Sci.

177. M . Nagano and T. Yano, Kobunshi Kugaku, 27, 769

178. N. Yoshioka and H . Sato, Kobunshi Kagaku, 26, 846

(1971).

( 1968 ).

Phys., B3, 33 ( 1969).

27, 197 ( 1971).

Kobunshi Kagaku, 28,531 (1971).

251 (1968).

77, ( 1968).

L. Listrovaya, Polymer Sci. USSR, 12, 2182 (1970).

USSR, 12,2207 (1970).

(1970).

(1969).

179. S. Banduryan, S. Papkov and M. Iovleva, Polymer Sci.

180. G. Burdygina, I. Fridman, P. Kozlov and V. Kargin, USSR, 12,1571 (1970).

Polymer Sci. USSR, 11, 1030 (1969).

BIBLIOGRAPHY Consideration of relevant literature through 1971

was the major goal of this review. However, four important publications which have appeared in 1972 are listed below with a brief description of each. 1. A. Peterlin, Tex. Res. J . , 42, 20 (1972). A concise review

of Peterlin’s results and model for drawn spherulitic polymers.

2. D. Krueger and G. Yeh, J. Macromol. Sci.-Phys., B6, 431 ( 1972). Recent observations on the morphology and structure of polyethylene shish kebabs.

3. A. Keller and F. Willmouth, J. Macromol. Sci.-Phys., B6 493 ( 1972). Recent observations on macroscopic properties of polyethylene shish kebabs.

4. S. Matsuzawa, K. Yamaura, and A. Yanagisawa, Kolloid Z., 250, 20 ( 1972 ) . Additional paper on stress-induced crystallization of poly (vinyl alcohol) solutions that includes omitted references to stress-induced crystallization of synthetic polypeptide-and polyoxymethylene-solutions.

POLYMER ENGINEERING A N D SCIENCE, SEPTEMBER, 1972, Vol. 12, No. 5 377

Microfibers within fibers: A review

Documents

Transcript of Microfibers within fibers: A review