Indian Journal of Textile ResearchVol. 1, June 1916, pp. 72-76

Evaluation of Crystallinity in Polyethylene TerephthalateFibre by X-Ray Diffraction

V. B. GUPTA & SATlSH KUMARTextile Technology Department, Indian Institute of Technology, New Delhi 110029

Received 19 May 1976; accepted 31 May 1976

Five techniques of determining the crystallinity of polyethylene terephthalate (PET) fibres (due to Farrow andPreston, Hermans and Weidinger, Statton, Bosley, and Dumbleton and Bowles), all based on diffraction study, areevaluated. For this purpose, a large number of samples were prepared from commercial PET yarn by heat-setting andtexturized under different conditions. These samples had high crystallite orientation, but widely different structures andmorphologies. The crystallinity of these samples was computed using the five procedures. It has been concluded thatthe absolute crystallinity of a particular sample, as determined by these procedures, may show considerable variations,but a comparison of the relative values of crystallinity for samples belonging to a particular set shows broadly similartrend within the set irrespective of the procedure adopted. This observation highlights the caution necessary in inter-preting data on crystallinity of textile fibres as determined by the X-ray diffraction method. Another important conclusionis that the procedures which make use of the total scattering curve appear to result in a parameter which is more clo-sely related to crystallinity than those which are based on intensity at one or two points on the intensity curve.

INthe characterization of semi-crystalline textilefibres, the concept of the degree of crystallinityis extremely useful. This is because a number of

important characteristics of the fibres, e.g. theirmechanical properties, dye-uptake, etc., have beenfound to be affected by their degree of crystallinity.

The crystallinity of PET fibres has been studiedextensively by X-ray diffraction+s. This is becauseof the commercial importance of the fibre and alsobecause wide variations in crystallinity are easilyaccomplished. In view of the important differencesin the various procedures used for evaluating theX-ray crystallinity of PET fibres, it was considereduseful to compute the crystallinities of a large numberof PET samples employing most widely used techni-ques and to see how they correlate.

Over 25 samples were prepared from the commer-cial multifilament PET yarn by subjecting it to twotypes of treatments which involved heat-setting it insilicone oil under different conditions and texturizingit on a false-twist machine with a subsequent heat-setting treatment in one case. Their crystallinity wasdetermined by X-ray diffraction using five differentcomputational procedures':". The differences in thevalues of crystallinity obtained at different tempe-ratures are discussed and comments are made on thecorrelations among them.

Experimental Procedure

Sample PreparationThe starting material was drawn multifilament

PET yarn 76/24/0, i.e. 76 denier, 24 filaments andzero twist, manufactured by J. K. Synthetics Ltd,Kota, Rajasthan.

The heat-set samples were prepared by heat-settingthe control yarn in a silicone oil bath for 30 minunder two conditions : (i) when the yarn was free to

72

shrink, and (ii) when it was clamped at a fixedlength. The temperatures of heat-setting were :80°, 125°, 145°, 176°, 200° and 220°C. A total of 12heat-set samples were thus obtained.

The texturized samples were prepared by processingthe control yarn on a false twist crimping unit. Duringcrimping, the heater temperature and contact periodwere varied, as reported elsewhere". The controlsample and one of the texturized samples (heatertemperature 230°C, contact period 0.66 see) wereheat-set at 193° for 15 min in silicone oil in clampedand unclamped conditions. A total of 12 sampleswere thus obtained.

The crystalline standard was obtained followingthe procedure suggested by Statton", This involvesannealing the control sample at 245°C for 65 hr inair in a small muffle furnace.

The amorphous standard was a "Mylar" film-an undrawn melt-cast film of PET supplied by DuPont Co., USA.

X-Ray Diffraction StudiesThe filaments were cut with a pair of scissors and

reduced to a fine powder. The powder (50 mg)was compressed tightly in a small rectangular orificein an aluminium holder under nominal hand pressure.The aluminium holder containing the sample wasmounted on the rotating stage of a Philips X-raydiffractometer. Nickel-filtered copper radiation col-limated with r divergence slit was incident on thesample, which was scanned at 1°/min in reflectionfrom 29 = 10-45°.

From the radial scans of intensity vs 29, the crys-tallinity was derived using the following procedures.

Farrow and Preston (F & P) procedure - In theprocedure suggested by Farrow and Preston', theradial scans for the randomized sample and for theamorphous standard are required for the calculation

GUPTA & SATISH KUMAR: CRYSTALLINITY IN POLYETHYLENE TEREPHTHALATE FIBRE

of percentage X-ray crystallinity. The amorphousscattering curve is constructed below the scatteringcurve for the unknown sample by proportionatelyreducing the scattering curve for the completelyamorphous sample until it matches the observed in-tensity at one or two particular diffraction angles.The degree of crystallinity (X) is then given byX = C/(C+A), where C is the integrated area of thecurve corresponding to the crystalline phase, andA, the area corresponding to the amorphous part.

In the present investigation, the scans were notobtained by microdensitometry of the X-ray diffrac-tion patterns, as done by Farrow and Preston, butby direct diffractometry.

Hermans and Weidinger (H & W) procedure - Thebasic principle of this procedure- was adopted byWhochowicz and Jeziorny" to evolve a simplifiedprocedure for computing the crystallinity of PETfibres, which leads to the following expression forthe degree of crystallinity

CKX= CK + h

where X is the degree of crystallinity ; C, the areaof the curve corresponding to radiation diffractedby the crystalline regions; h, the height of the back-ground diffuse scattering at 21.5; and K, a constantfor a given system.

Statton's procedure - This procedure- does notattempt to estimate absolute crystallinity. The amor-phous standard and the crystalline standard are takenas standards of zero and 100 crystallinity index. Thepattern of an unknown sample is compared with thestandards by measuring the diffracted energy atseveral angles and interpolating the intensity of theunknown between the intensities of the standards.

Bosley's procedure - This procedure- enablesthe estimation of relative crystallinity. Bosley definescrystallinity index as the ratio of the intensity of (100)peak at 26 = 26°, which is proportional to theamount of crystalline phase, to the intensity at28 = 28S, which is shown to be proportional to themass of the specimen for a randomized sample.

In the present calculations, a slightly differentnormalizing procedure was used, since this gavebetter correlation with other methods. The crystal-linity (X) was taken to be

X = 1 - (J26/128·5)amorphous /(/26/12s.5)sampleDumbleton and Bowles (D & B) procedure - This

procedure" derives from Bosley's basic approach.These authors take the scattering at 28 = 14° as ameasure of the amount of amorphous material.Following Bosley", the intensity at 26 = 28S istaken to be-proportional to the mass of the sample.The degree of crystallinity (X) is defined as

X = 1 - (A/AlOo), where A is the ratio of theintensities at 14° and 28.5° for any given sample,and AlOO is the value of this ratio for the amorphoussample.

Results and DiscussionFor the samples prepared by heat-setting commercial

PET yarn in silicone oil in the relaxed condition,

the crystallinity, as computed by different procedures,is shown as a function of the heat-setting temperaturein Figs. 1-5. It is interesting to note that in allcases, the broad trend is the same, viz. crystallinity

80 r-----------------------------~

20- -CONTROL-- - - -- --

TEMPERATUREJ °cFig. 1 - F & P crystallinity as a function of heat-setting

temperature

so 120 160 200

80 r-----------------------------~

20 --CONTROL-- - ----- --j

80 120 160 2()() 24()

TEMPERAT(JRE , °cFig. 2 - H & W crystallinity as a function of heat-setting

temperature

80 o

240

,EMPERA7t//?E J "cFig. 3 - Statton's crystallinity index as a function or heat-

setting temperature

-- CONTROt---- ------

80 /20 160 200

73

INDIAN J. TEXT. RES., VOL. I, JUNE 1976

;:r•••~~-.&•.: 40 - - CONTROL. - - - - - - - - -~~~ 20

240

Fig. 4 - Crystallinity calculated by Bosley's method as a func-tion of heat-setting temperature

80 1.20 !GO 200

SO ~1--------------------------------;

TEMPE.RATlJRE

CONTROL --- - - - --

80 120 /60 ,200 240

•TEIHPEI?ATUI?E r cFig. 5 - D & B crystallinity as a function of heat-settig tem-

perature

100..--------------------------------.

-o'!-" 80s,

I-..~~--J 60'-J

~l:!~ 40

C\~~ 20

H & IV CRYSTAU..INITY,"IoFig. 6 - Correlation of F & P crystallinity with H & W crysta-llnity [0, Heat-set; x , textured and texrurized/heat-set: and,

eI' crystalline standard]

74

.20 60 80

increases with increase in heat-setting temperature ..Also, the crystallinity of the control sample is in almostall cases less than the crystallinity of the heat-setsamples.. The absolute values of crystallinity and theextents of increase are, however, different for diffe-rent methods ..

The correlation between different methods wasfound out by calculating the correlation coefficient.The correlations shown by the Farrow-Prestonmethod with the other four methods are presented inFigs. 6-9. The correlation coefficients indicatingcorrelations among different methods are given in

100r------------------- _

0;:--

A. 80I,

~ ,. •• O' 91-c'-J-.j 60~~~ 40Q.

~L....

20

o 20 40 80 100

STATTON CRYSTALLINITY, v;Fig. 7 - Correlation of F & P crystallinity with Statton's crys-tallinity index [0, heat-set; x , textured and texturized/heat-set;

and I!!I, crystalline standard]

'00 ~------------ ~

~o

r=

o4020 60 80

&JOSLE>' CNysrAi.LINITY, %Fig. 8 - Correlation of F & P crystallinity with crystallinitycomputed by Bosley's method [0, heat-set; x , textured and

texturized/heat-set; and L, crystalline standard]

,oo.------------- ~GUPTA & SATISH KUMAR: CRYSTALLINITY IN POLYETHYLENE TEREPHfHALATE FIBRE

~•.80•...~'"-.j-.j 60

~~~ 40

~

"" 20

I"" • O· 74

00x

1< ~'" "Q E)8'",0

o 20 50 fOO

D.& B CRYSTALLINITY, v.Fig. 9 - Correlation of F & P crystallinity with D & B crysta-llinity [0, heat-set; x, texturised and texturized/heat-set; and

~, crystalline standard]

TABLE 1 - CORRELATION COEFFICIENTS AMONG DIFFERENTMETHODS

F & P H & W Statton Bosley D & Bmethod method method method method

F & PmethodH& W methodStatton methodBosley methodD & B method

10.900.910.850.74

10.880.640.62

10.790.71

10.50

Table 1. It is seen that the correlation is always posi-tive and in most cases, quite high. It can thus bestated that the different methods of measuring crys-tallinity are reasonably satisfactorily correlated.Thus, when any of these methods is used to determinethe crystallinity of a fibre, the basic parameter mea-sured is related to the crystalli ne content of the sample.

The frequency distribution for crystallinity ob-tained by the use of different methods is shown inFig. 10. The mean value for the population studiedis also indicated in each case. It is seen that the F &P method gives relatively low values for crystallinityfollowed by the methods of H & W, Statton, Bosleyand D & B in increasing order.

Bosley" has pointed out that Statton's method-is perhaps the most straightforward method andthat the philosophy of his method seems more rea-sonable compared to the other methods. Also, Sta-tton's insistence on the term "crystallinity index" isbased on sound logic. In view of this, it is reasonableto compare the values of crystallinity obtained byvarious methods with Statton's values, taking Sta-tton's crystallinity index as the reference point.

The data presented in Fig. IO show that the crys-talline standard has 100 % crystallinity index accord-ing to Statton's method, 90 % for D & B method,80 % for Bosley's method and about 74 % for F & Pmethod. If these methods gave the crystallinity values

c OMPurA rlo# PlfOCEDUJi>E

Fig. 10 - Frequency distribution of crystallinity values compu-ted by different methods [Arrows indicate the mean values;0, heat-set; x , textured and texturized/heat-set; and 0,

crystalline standard]

for other samples in correct proportion with referenceto Station's method, the mean values of crystallinityfor these methods should also have followed the sameorder as for the crystalline standard, irrespective ofwhether the methods gave crystallinity index or per-centage crsytallinity. However, as shown in Fig. 10,the order is different, viz. the mean values are higherthan those obtained in the case ofD & B and Bosley'smethods and lower in the case of Hand Wand F& P methods. Thus, it would appear that the methodssuggested by D & B and Bosley overestimate crysta-llinity.

The data presented in Table 1 show that correla-tion of Statton's method with the other four methodsgives the following values for the correlation coeffi-cient : F & P 0.91, H & W 0.88, Bosley 0.85 and D &B 0.71. This again shows that if Statton's method istaken as the yardstick, the F & P method gives themost realistic estimate of crystallinity, followed by themethods of H & W, Bosley and D & B.

It was stated earlier that the F & P method estimatescrystallinity from the integrated areas of the scatter-ing curves for the crystalline and amorphous phasesof the sample. Statton's method also uses the wholescattering curves for the highly crystalline, highlyamorphous and unknown samples. The H & Wmethod uses the area of the crystalline part and theheight of the amorphous part at a fixed point for thecomputation of crystallinity. The other two methods,viz. those of Bosley and Dumbleton and Bowles donot make use of the total scattered intensity curve.It would thus appear that methods which make useof the total information contained in the scatteredintensity curves obtained from X-ray diffractionanalysis perhaps give a more realistic estimate ofwhat is known as "crystallinity" in textile fibres.

AcknowledgementThe authors would like to acknowledge the contri-

bution of Shri O. P. Sharma in the preparation of theheat-set samples. The texturized samples were pre-pared at J.K. Synthetics Ltd, Kota and for this theyare grateful to Dr J. K. Nigam. They are indebtedto Dr N. B. Patil and Shri P. K. Chidambareshwaram

75

INDlAN J. TEXT. RFS., VOL. I, JUNE 1976

of Cotton Technological Research Laboratory,Matunga, Bombay for the diffractograms and toDr M. L. Gulrajani for providing the 'Mylar' filmsample.

References1. FARROW, G. & PRPsroN, D., Br. J. appl, Phys., 11 (1960),

353.

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. 2. HERMANS, P. H. & WEIDINGER, A., J. Po/ym. ss., 4 (]949).709; Makromol. Chem., 44/46 (1961), 24; 50 (1961), 98.

3. WHOCHOWICZ, A. & JEZIORNY, A., J. Polym. s«, 10(Pt A-2) (1972),1407-]4.

4. STATION, W.O., J. appl. Polym. Sci., 7 (1%3), 803.5. BoSLEY, D. E., J. appl, Polym. Sci., 8 (1964), ]52]-29.6. DUMBLETON, J. H. & BoWL~, B. B., J. Polym. s«, 4 (Pt

A-2) (1966), 951-58.7. GUPTA, V. B. & KUMAR, M., Text. Res. 1.,45 (1975), 382-88.