Indian Journal of Textile Research

Vol. 7, September 1982, pp. 82-86

Evaluation of Crystallinity in Poly (Vinyl Alcohol) and Its Oxidation Productsby X-Ray Diffraction *

S D WADEKARt, H U MEHTA, A K KULSHRESHTHAt & N E DWELTZ

Ahmedabad Tex..tileIndustry's Research Association, Ahmedabad 380015

&

J J TRIVEDI

M G Science Institute, Ahmedabad 380009

Received 14 May 1982; accepted 24 July 1982

An X-ray method for evaluating the degree of crystallinity in poly(vinyl alcohol) is described. The method has been used forthe oharacterization of oxidized PVA samples. The results of X-ray characterization of PVA polymers are in conformity with

thei~ infrared spectroscopic evaluation.

r

Polyvinyl alcohol (PVA) has assumed increasingtechnological significance on account of its role intextile finish formulations I and use as a sizing agent 2

for multicomponent yarns. Most of the studies carriedout on the physico-chemical characterization of PVArelate to its hydrogen-bonding characteristics,solubility" stereo regularity and crystallinity. It isknown tQ.atthe stereoregularity of PVA chains doesnot lead to higher crystallinity in the product3 . Thephysical properties ofPV A, such as solubility in water,are likely to be influenced by its degree of crystallinity(Dq. Estimates of DC in PVA are generally madeusing theiinfrared spectroscopy4, as no suitable X-raymethod teems to have been reported in literature.

The present study was aimed at the development ofan X-raX diffraction technique for the estimation ofPVA polymer crystallinity and the application of thistechnique for the characterization of oxidized PVAsamples, which have been fractionated from solutionand represent the aggregates of insoluble highmolecular weight residues. Since the principles ofvarious X-ray methods for crystallinity evaluation arewell-known, a particular procedure can be adapted forobtaining the relative crystallinities of PVA samples.We have chosen the correlation methodS, which

provides a numerical comparison of an unknownsample with the most crystalline and most amorphous

.This work was carried out as a partial fulfilment for Ph. D. degreeby one of the authors (SDW).tPresent address: Cd279, Ranga Rasayan Ind., GIDC, Naroda,Ahmedabad.tPresent address: R&D Division, I.P.C.L., Vadodara 391346.

82

extremes (standards) that can be physically obtainedfor the polymer species in a laboratory. Thecorrelation method has been used widely for theestimation of DC in a variety of polymers andfibres6•7, copolymers8 and phase mixtures9. Some ofthe pioneering work, leading to the success of thismethod, was carried out earlier by Radhakrishnan andco-workers9 -II at ATIRA. Since the procedures forthe collection of polymer X-ray data and computationof DC have already been standardized in thelaboratories of ATIRA, the problem of estimating DCin various PVA samples reduces to that of developingtbe appropriate crystalline and amorphous standardsfor this particular polymer.

Materials and MethodsOut of the six commercially available samples,

ALCOTEX - 1017 (fully hydrolyzed grade) and PVA ­057 (partially hydrolyzed grade, containing 17% acetylgroups) were selected for X-ray analysis.

Hypochlorite oxidationl2•13 of PV A-ALCOTEX­1017 was oxidized at room temperature and at neutralpH (buffered with NaHC03), using 310 m moles ofavailable chlorine per PVA monomer unit as oxidantconcentration, for various durations (15 - 1440 min).The oxidized products were treated with dilute aceticacid, stirred to remove the residual chlorine and finallytreated with sodium thiosulphate solution. Distilledalcohol was then added to the reaction mixture

subjected to constant agitation. As a result, thepolymer got precipitated and was allowed to settledown. The precipitates were filtered through a sintered

~i

WADEKAR et at.: CRYSTALLINITY EVALUATION IN POLY (VINYL ALCOHOL)

150

Fig. I-Normalized X-ray diffractograms of crystalline andamorphous standards of PVA

50

-- Crystallin" Standard___ AmorphOUS Standard

POLY(VINYL ALCOHOL)

1101

20 30 40DIFFRACTION ANGLE (28),d"g

101

o10

8 501!::!-'«:l:a:oz

>­•...in

~ 100•...

~

(S-A)jand (C-A)jat each incremental position in 28. Itthen calculates the linear regression of (S-A) on (C-A)and the degree of crystallinity is defined by the slope ofthis regression line.

Results

The normalized X-ray diffractograms of crystallineand amorphous standards of PVA are shown in Fig. I.The crystalline peaks have been indexed on the basis ofan orthorhombic unit cell 1 5 for PVA and it is ratherconvenient for the present analysis, as the isotacticPVA in literature has the same crystalline structure assyndiotactic PVA 16-IS. X-ray diffraction photo­graphs of the crystalline and amorphous standards areshown in Fig. 2.

X-ray diffractograms of the control (unoxidizedsample) and various oxidized PVA samples are shownin Fig. 3. The X-ray crystallinities of the various PVAsamples are given in Table I. X-ray diffraction powderphotographs of the control and fully oxidized PVAsamples are shown in Fig. 4.

IR absorption spectra of the control and variousoxidized PVA samples (Fig. 5) show interestingdifferences between the polymers in the stereospecificregion (950 - 800 cm -1) as well as in the region of thecrystalline band at 1141cm -1. The band at 916 cm-1is specific for syndiotacticity, whereas the origin of theband at 849 cm -1 is attributed to the isotacticity in thepolymer. The values of the density ratio D916/ DS49 forthe control and the oxidized PVA samples are given inTable I. An important observation made from Fig. 5 isthat in the initial stages of oxidation (i.e. up to areaction period of 20 min), the intensity of absorptionband at 1141 cm -1 reduces, but in the final stages ofreaction this band becomes sharp.

Fig. 6 shows a semi-log plot of X-ray crystallinityand IR density ratio (which provides a measure oftacticity) as a function of oxidation reaction period.

funnel and washed successively with acidic alcohol­water (80:20) mixture, alcohol-water (80:20) mixtureand absolute alcohol, to remove the low molecularweight polymer fractions. They were then dried undervacuum and weighed. Unconsumed available chlorinewas estimated with respect to reaction period. Fromthis, the available chlorine consumed per PVA unitwas calculated as a function of reaction period.

Infrared measurements of polymer tacticity­Density ratio (D916/ DS49) was measured from spectraobtained on a Perkin-Elmer, Model 180, IR

spectrophotometer according to the proceduredescribed by Fujii et al. 14 , using the absorptions at 916cm -1 and 849 cm -1 . A value of density ratio greaterthan 0.30 indicates syndiotacticity, whereas a value lessthan 0.25 indicates isotacticity in the PVA polymer 15 .

For an atactic polymer, density ratio values lie in therange 0.25-0.30.

Preparation of standards-The amorphous stan­dard was prepared by dissolving PVA - 057 in waterand rapidly precipitating it by the addition of acetone.The precipitated material was allowed to dry in air.The crystalline standard was prepared by heatingALCOTEX - 1017 in triethylene glycol untildissolution occurred at 180°C. The solution wascooled slowly to room temperature, resulting in theprecipitation of crystalline PVA, which was filteredand washed with methanol. The crystalline PVAstandard thus obtained is insoluble in water and has adensity ratio value of 0.32.

Collection of X-ray intensity data-X-ray scans wereobtained by chart recording between 10 and 50°(28)angular range. CuKa X-rays were employed as incidentbeam and monochromatization of diffracted in­tensities was achieved electronically. The X-ray systemconsisted of a vertical Philips X-ray diffractometer set­up to collect transmitted intensities from any sampleusing a curved crystal focalizer accessory to reduce thebroadening caused by the instrument. A 4°divergenceslit and a 0.3 mm receiving slit were used. X-raydiffraction powder photographs of polymers weretaken using a cylindrical film type camera, keeping aspecimen-to-film distance of 3 em.

Computation of the degree of crystallinity- X-rayintensities from the polymer sample (S), crystallinestandard (C) and amorphous standard (A) were readfrom the chart scans at 0.25° increments in 28. Thesevalues formed the input for a computer programmewritten in FORTRAN IV and executed on an IBM 360computer. The programme carries out the normali­zation of diffraction data, so that the areas under thecurves between 10 and 50° (28) are the same for allsamples and then computes the differential intensities

83

INDIAN] TEXT RES, VOL. 7, SEPTEMBER 1982

Table I-Results of Hypochlorite Oxidation of PYA and Characterization of Crystallinity and Tacticity in OxidizedPoly(Vinyl Alcohol) Samples

Sample

ReactionWt of theIR densityX-rayCorrelationAvailableNo.

periodinsolubleratiocrystallinitycoefficient Cl2 consumedmin

polymerD9!6/DS49% per PYA unitgig

m mole

I(control)-0.33880.75

2

150.900.29460.58

3

300.850.26420.55

4

600.650.26290.43170

5

90 380.53192

6

1200.550.25290.432207

150 300.462478

180 -360.532479

3600.320.26420.6010

7200.250.30430.64II

14400.300.3]410.59

*Fraction insoluble in alcohol-water (80:20) mixture. I

50

GOmin

120min

~) ~ .Fig. 2-X-ray diffraction photographs of (a) crystalline standard of PVA and (b) amorphous standard of PVA

Discussion

Data presented in Table I give an overview of theeffect of hypochlorite oxidation on the variousphysico-chemical characteristics of PV A. As theduration of oxidation increases, the amount of fractionsoluble in 80:20 alcohol-water mixture increases and

that of the insoluble fraction decreases, as is evidentfrom the decrease in the weight of the oxidizedpolymer. Syndiotacticity in PV A favours a much fasterreaction rate 13. As a result, the IR density ratiodecreases from 0.33 (for the control PV A) to 0.29, 0.26and 0.25 in oxidized PV A samples obtained after 15,60and 120 min of reaction respectively. During thisperiod, the consumption of oxidant increasesprogresflively up to 247 m moles (out of 310 m molesinitially present). Thus, after ISO min of reaction, thereis practically no further oxidation, but the;)lmount ofoxidized PV A obtained\ decreases considerably withfurther increase in the reaction period (from ISO to

o10

>!::VIzw•...~>«a:,><0WN-'«::i:a:0z

0

20 30 40

DIFFRACTION ANGLE (26),deg

Fig. 3-Normalized X-ray diffractograms of control and oxidizedPYA samples

84

WADEKAR et a/.: CRYSTALLINITY EVALUATION IN POLY (VINYL ALCOHOL)

w ~Fig. 4-X-ray photographs of (a) control PVA and (b) fully oxidized PVA

Conclusion

During the hypochlorite oxidation of PVA, thedegradation of polymer chains occurs. Oxidation isvirtually completed after 150 min of reaction, butdepolymerization continues to occur with furtherincrease in the reaction period, reducing the yield ofoxidized PVA. During the reaction, the X-ray

It is observed from Table I that during theoxidation, the tacticity of PVA changes fromsyndiotactic (for the control PVA) to atactic (forsample Nos. 2 to 10)and again to syndiotactic(SampleNo. II). These tacticity changes go hand in hand withchanges in X-ray crystallinity during oxidation (Fig. 6).The X-ray data suggest a rapid initial decrystallizationof PVA with the crystallinity changing from 88 to 46%in the first IS min of oxidation. The crystallinity thenfurther decreases gradually to a value of about 30%when the oxidation reaction has virtually levelled offafter 150 min. On further increasing the reactionperiod, the crystallinity begins to show a slightincrease. This slight increase in crystallinity issignificant and is supported by the disappearance andreappearance of the 1141cm -1 crystalline band in theIR spectra of oxidized PVA samples (Fig. 5). Thisincrease in crystallinity can be understood in thefollowing terms. Beyond a reaction period of ISO min,the oxidation of PVA reaches a saturation limit (whichis determined by the availability of OH - groups in thepolymer) and depolymerisation of oxidized PVA ispreponderant. Also, the chain configuration is atactic.Both these factors favour crystallization, because it iswell known that (i) lower molecular weight PVA'scrystallize better than higher molecular weight PVA's,and (ii) syndiotacticity and atacticity inducecrystallization in PVA more than isotacticity19 -23.

>­>-;;;zUJ

0.3 ClClUJa:..a:"-

0.2~

~oX)

Cl

O.5~;;;

Cl

Q>-

0·4 ;i

JOmin

360min

720min

1440min

120min

60min

15 min

,-Control.

~ X-Ray Crystallinity-e- IR Oeonsity Ratio

WAVELENGTH, }J5 IS 7 a 9 10 12 15 20 302·5

---

2000 1600 1100 aoo 400

WAVENUMBER~ cm-t

Fig. 5-1 R spectra of control and oxidized PVA samples

102 103

OXIDATION REACTION PERIOD,min

Fig. 6-A semi-log plot of X-ray crystallinity and IR density ratioagainst reaction period

1440 min). This observation strongly suggests thepossibility of rapid depolymerization of oxidized PVAand very little increase in the extent of oxidationbeyond a reaction period of ISO min. The IR spectra ofoxidized PVA show strong absorptions at 1720 and1640 cm -1 , which can be attributed to the carbonylgroups and carboxylate ions ( - COO) respectively.

••~.zj 100~III>­a:u"- 60o~ 40a:'"UJCl 20

85

INDIAN 1 TEXT RES, VOL. 7, SEPTEMBER 1982

crystallinity and the tacticity of the polymer undergochanges. The X-ray diffraction method can be used forquantitative characterization of structural order inPV A and oxidized PVA. Both X-ray and IR methodsshow a similar trend while characterizing the oxidationproducts of PVA.

AcknowledgementThe authors are thankful to Dr 1. Radhakrishnan,

Director, ATIRA, for permission to publish this work.They express their appreciation to Shri N.R. Kotharifor the help in computer analysis of data and to ShriK.M. Patel for the help in collecting X-ray data.

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