Nature of the Ring-Banded Spherulites in Blends

of Aromatic Poly(ether ketone)s

Jian Chen, Decai Yang*

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, P. R. ChinaFax: (þ86) 431 5262126; E-mail: dcyang@ns.ciac.jl.cn

Received: April 23, 2004; Revised: June 14, 2004; Accepted: June 17, 2004; DOI: 10.1002/marc.200400162

Keywords: blends; morphology; poly(ether ketone)s; ring-banded spherulites; rhythmic growth

Introduction

Ring-banded spherulites in some semicrystalline homo-

polymers, with a typical morphological feature of extinc-

tion rings under polarized light microscope, have been

widely investigated for nearly half a century. The classical

interpretation of the extinction bands in spherulites is that

they are caused by the periodic twisting of lamellae along

the radial growth direction of the spherulites.[1–6] However,

Keith and Padden,[7] based on the observation of the same

concentric rings in polyethylene spherulites under an un-

polarized configuration, proposed that the lamellar twisting

orientation cannot be the only cause for the observed

concentric ringed patterns in polymer spherulites, while the

concentric ringed pattern is a consequence of structural

discontinuities caused by rhythmic crystal growth. Recent-

ly, the rhythmic crystal growth of ring-banded spherulites in

some blends of crystalline and amorphous polymers

has been observed experimentally,[8,9] and explained by

simulation.[10]

In this communication, a ring-banded spherulite in liquid

crystalline poly(aryl ether ketone) (LC-PAEK) and poly-

(aryl ether ether ketone) (PEEK) blends is reported for the

first time, and the rhythmic radial growth of the ring-banded

spherulites is discussed.

Experimental Part

The poly(aryl ether ketone) (PEEK) used in this work was acommercial-grade product, with a number-average molecularweight of 22 000 and a polydispersity index (Mw=Mn) of 2.8.The liquid crystalline poly(aryl ether ketone) (LC-PAEK) witha lateral chloro-group, was synthesized by a nucleophilicsubstitution reaction of 4,40-biphenol and chlorohydroquinonewith 1,4-bis(p-fluorobenzoyl)benzene.[11,12] The number-average molecular weight is about 5 000 and the polydispersityindex is 1.4. The chemical structure of the molecular chain isindicated in Scheme 1.

Thin films of the LC-PAEK/PEEK blends were prepared bycasting a 2-wt.% solution of the blends in pentafluorophenol

Summary: The ring-banded spherulites in liquid crystallinepoly(aryl ether ketone) (LC-PAEK) and poly(aryl ether etherketone) (PEEK) blends with a higher content (>50%) of LC-PAEK are investigated by polarizing light microscopy (PLM)and atomic force microscopy (AFM) techniques. The resultsindicate that the light core and rings of the ring-bandedspherulites under PLM are mainly composed of an LC-PAEKphase, while the dark rings consist of coexisting phases ofPEEK and a small amount of LC-PAEK. The formation of thering-banded spherulites is attributable to structural disconti-nuity caused by a rhythmic radial growth.

PLM image of ring-banded spherulites in a 70:30 LC-PAEK/PEEK blend caused by rhythmic growth.

Macromol. Rapid Commun. 2004, 25, 1425–1428 DOI: 10.1002/marc.200400162 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication 1425

onto a glass slide. The nonisothermal crystallization of thefilms was performed by heating to 380 8C for 2 min and then bycooling to room temperature at a cooling rate of 10 8C �min�1.

The polarized light microscopy (PLM) observations ofthe thin films were carried out using a Leica DMLP micro-scope equipped with a hot stage and a CCD camera. The atomicforce microscopy (AFM) studies were performed with a SPA-HV300 with a SPI 3800N controller (Seiko InstrumentsIndustry Co., Ltd). Probes with a resonant frequency of 250–300 kHz and spring constants of 42 N �m�1 were used. A150mm scanner was selected, and the tapping mode was used toobtain height images.

Results and Discussion

It is known that the LC-PAEK films display a typical mosaic

texture,[13] a highly ordered liquid crystalline phase SE,

while PEEK exhibits a spherulitic morphology.[14,15] How-

ever, in nonisothermally crystallized films of the LC-PAEK/

PEEK blends with a higher content (>50%) of LC-PAEK, a

ring-banded spherulite is formed, which has never been

found in the two pure components. Especially in the 70:30

LC-PAEK/PEEK blend, well-developed spherulites with

concentric extinction rings are observed under PLM

(Figure 1a). The size of the perfect ring-banded spherulites

is greater than 500 mm and the band spacing is about 20 mm.

The ring-banded spherulites have been further investigated

by AFM. Figure 1b shows the AFM height image of the

nonisothermally crystallized film of the 70:30 LC-PAEK/

PEEK blend. The concentric dark and bright bands alter-

nating around the light core are also observed. The bright-

ness contrast of the alternating light and dark bands, which

represent the depth profiles of the ridges and valleys,

respectively, is attributed to height contrast. The vertical

distance between the lowest and highest point on the sample

surface (band height amplitude) ranges from 50 to 90 nm.

The ridges and valleys correspond, respectively, to the

bright and dark regions of the extinction rings seen under

the PLM.

Figure 2 contains the PLM images depicting the

evolution of the ring-banded spherulite in the 70:30 LC-

PAEK/PEEK blend heat treated at 380 8C for 2 min and then

cooled at a cooling rate of 10 8C �min�1. At about 294 8Cthe core of the spherulite is first discerned. With further cool-

ing, the spherulite continues to grow, showing concentric

rings around the core. The spherulitic growth front is nearly

circular, while the spherulitic boundary is wavy. Ultimately,

a well-developed ring-banded spherulite is formed in the

system at about 282.7 8C.

In order to identify the material composition in both the

light (ridge) and dark (valley) regions of the ring-banded

spherulites, a solvent washing experiment was carried out,

using a 40:60 (w/w) mixture of p-chlorophenol and 1,1,2,2-

tetrachloroethane as washing solvent, which can only

dissolve LC-PAEK in the two components. Figure 3 shows

the PLM image of the etched ring-banded spherulites in a

70:30 LC-PAEK/PEEK blend. By carefully comparing

Figure 3 with Figure 1a, it is found that the light rings in

Figure 3 correspond to the dark rings in Figure 1a, while the

dark rings and the dark core in Figure 3 correspond to the

light rings and the light core in Figure 1a, respectively.

Clearly, the above results indicate that in Figure 1a the light

rings and core represent the LC-PAEK phase (which could

be washed out by the solvent, resulting in the dark rings and

core in Figure 3), while the dark rings are mainly the PEEK

phase (which could not be washed out, exhibiting bright

Scheme 1. The chemical structure of the liquid crystallinepoly(aryl ether ketone) (LC-PAEK) synthesized here.

Figure 1. (a) PLM image and (b) AFM height image of ring-banded spherulites in thin films of a 70:30 LC-PAEK/PEEK blendheat treated at 380 8C for 2 min and then cooled to roomtemperature at a cooling rate of 10 8C �min�1.

1426 J. Chen, D. Yang

Macromol. Rapid Commun. 2004, 25, 1425–1428 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

rings in Figure 3). Figure 4a shows an AFM height image of

etched thin films of the blend. Comparing Figure 4a with

Figure 1b, it is clear that the light core and rings (ridges) in

Figure 1b are etched away by the solvent and become the

dark core and rings in Figure 4a, while the dark rings

(valleys) in Figure 1b correspond to the light rings in

Figure 4a. This is in good agreement with the PLM results,

that is, the ridges in Figure 1b represent the LC-PAEK

phase, while the valleys correspond to the PEEK phase.

Figure 4b shows a magnified image of Figure 4a in the

location shown by the circle, to identify a more detailed

phase composition. After etching, almost no LC-PAEK can

be found in the dark band (the middle in Figure 4b), most of

which are washed out. In the light bands (the right-up

and left-down in Figure 4b), however, there are some dark

regions among the unwashed PEEK crystals, as marked by

the arrows, which are no doubt etched LC-PAEK. It implies

Figure 2. PLM images depicting the morphological evolution of the ring-banded spherulites in the 70:30 LC-PAEK/PEEK blend heat treated at 380 8C for 2min and then cooled at a cooling rate of 10 8C �min�1: (a) 294,(b) 288.1, (c) 285.4, and (d) 282.7 8C.

Figure 3. PLM image of the etched ring-banded spherulites inthe 70:30 LC-PAEK/PEEK blend.

Nature of the Ring-Banded Spherulites in Blends of Aromatic Poly(ether ketone)s 1427

Macromol. Rapid Commun. 2004, 25, 1425–1428 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

that the light bands in Figure 4a or the valleys in Figure 1b

are actually a coexisting phase of PEEK with a small

amount of LC-PAEK.

The above results indicate that there are two completely

different phase compositions for both the light and dark

regions (Figure 1a and 1b) in the ring-banded spherulites.

The light core and rings are mainly composed of an LC-

PAEK phase, while the dark rings consist of the coexisting

phase of PEEK and a small amount of LC-PAEK. Unam-

biguously, this kind of concentric ring (target) structure is

different from traditional banded spherulites, and a rhyth-

mic growth process is suggested. For the blending

system, the two components in the melting state should

be thermodynamically miscible on a molecular scale. Dur-

ing the cooling process, a highly ordered liquid-crystalline

phase (SE) of LC-PAEK with small dimension will form

first since the isotropic-to-liquid crystalline transition

temperature (about 303 8C) of LC-PAEK is higher than

the crystallization temperature (about 295 8C) of PEEK. It

should be pointed out that the SE phase is actually a solid-

state phase, which can act as a nucleus of the ring-banded

spherulites (Note: during the cooling process, the SE phase

of the LC-PAEK will transform into a crystalline phase very

quickly since the transition is almost a thermodynamic

equilibrium). At the same time, the PEEK molecular chains

are rejected into the core periphery, forming the PEEK-rich

regions around the LC-PAEK core (i.e., the first dark or

valley region). Repeating the above rhythmic growth

processes, that is, LC-PAEK phase transitions (including

both the isotropic-to-liquid crystalline and subsequent

liquid crystalline-to-crystalline transitions) and PEEK

being rejected simultaneously, the concentric ring or target

spherulites are produced in the blends.

Conclusion

The development of the ring-banded spherulites in the LC-

PAEK/PEEK blends is a rhythmic growth process, forming

crystalline LC-PAEK ridges (including core) and PEEK-

rich valleys, and thus, it is consistent with the structural

discontinuity model proposed by Padden and Keith[7] for

the polyethylene spherulites.

Acknowledgements: The financial support of the NationalNatural Science Foundation of China is gratefullyacknowledged. The authors wish to thank Prof. Z. W. Wu (inJilin University) for kindly providing the LC-PAEK samples.

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Figure 4. (a) AFM height image of the etched ring-bandedspherulites of the 70:30 LC-PAEK/PEEK blend and (b) amagnified image of (a) in the location shown by a circle.

1428 J. Chen, D. Yang

Macromol. Rapid Commun. 2004, 25, 1425–1428 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim