Annealing

Poly(ethylene terephthalate) H Morphological Changes on Annealing R. C. ROBERTS Morphological changes which occur when partially crystalline poly(ethylene terephthalate) is annealed were aerected and followed quantitatively using differential scanning calorimetry. These changes appeared as an additional fusion endotherm at a temperature intermediate between the annealing temperature and the melting point. The magnitude of this endotherm increased with both annealing time an d temperature and was found to be associated with decreases in the original fusion endotherm and not with crystallization of amorphous material. These changes are discussed in relation to present concepts of the morphology o/ bulk crystallized poly(ethylene terephthalate).

WHEN amorphous poly(ethylene terephthalate), PET, is heated above its glass transition temperature spontaneous crystallization occurs in the solid state. The overall rate at which this occurs has been studied extensively as a function of temperature, polymer structure and molecular weight (see ref. 1 for earlier literature references). Crystallization occurs most rapidly in the temperature range 140--220°C with 50 per cent of the resulting crystallinSty developing in less than 30 seconds. The overall degree of crystallinity achieved~, however, depends on the annealing temperature and to attain the highest degree of crystallinity one must anneal at temperatures approaching the melting point. The availability of non-equilibrium techniques, such as differential thermal analysis (DTA), in which the sample is subjected to a rapid temperature scan, has enabled studies to be made of the intermediate stages of such annealing treatments. Thus Gray and Casey8 demonstrated that discontinuous crystallite size distributions could be induced in polymers by appropriate thermal treatments and that earlier' DTA multiple fusion endotherms could be explained on this basis. An exception here is trans-polyisoprene which exists in two different crystalline forms and shows corresponding fusion endotherms. Multiple fusion endotherms have since been reported for annealed samples of high and low density bulk crystallized polyethylenes5 and polyethylene single crystals6, 66 and 6 nylons 7,1°, polypropylene~, polypropylene oxide8, poly(ethylene terephthalatey and poly(4-methyl pentene-1) single crystals1~. Detailed quantitative measurements of these morphological changes on annealing are presented here for poly(ethylene terephthalate). EXPERIMENTAL

Poly(ethylene terephthalate) was the same sample as described previously~1. Annealing was carried out in the cell of a Perkin-Elmer model 1B differential scanning calorimeter. 5 to 15 mg samples of polymer were accurately weighed into a sample pan and the cell purged with nitrogen. After holding in the melt at 270°C for five minutes the temperature was lowered to 100°C 117

R. C. ROBERTS

at 64 deg. C/min in order to crystallize the samples in a standard way and then raised to the annealing temperature at 64 (leg. C/rain. After the required annealing the samples were again cooled to 100°C at 64 deg. C/min and finally scanned at 16 deg. C/min. Low angle X-ray patterns were recorded photographically with a Kratky low angle camera using 1 mm sample thickness. The camera entrance slit was I00/~m and the specimen to film distance 23.5 era. The films were measured by microdensitometer and the slit-smeared scattering curves thus obtained were unsmeared by the method of Schmidt and Hight n. No correction was made for the width of the primary beam or of the microdensitometer slit. RESULTS

(i) Effect of annealing temperature Differential scanning calorimetry (DSC) fusion endotherms of samples annealed for 16 hours at temperaturesbetween 135 ° and 220°C are shown in Figure 1. Besides the original melting endotherm of peak temperature (Tin), an additional endottierm ( T ' ) is seen at eemperatures 25-40 deg. C above the annealing temperature. These melting points and their associated heats of fusion, AH and AT-/', respectively, are given in Table 1. AH is seen to decrease with annealing whilst ~r-/, increases markedly. Thus the ovexall

(5)

I d.._.HH, dT

(

(21

0) I

1SO

I

180

I

I

I

200

I

220

~

J

I

240

I

260

T,oc

Figure 1--DSC fusion endo~hcrms of PET annealed for 16 h at (1)135°C (2)170°C (3)1850C (4)200°C (5)220°C

118

POLY(ETHYLENE TEREPHTHALATE) II

effect of the formation of this low melting species is almost original sample degree of crystallinity.

to

double the

Table 1. Effect of annealing PET for 16 hours at different temperatures Annealing temperature °C

T',n

Tm oC

AH" cal / g

AH cal / g

AH + AH' cal / g

135 170 185 200 220

160 202 227 236 246

253 253 253 254 254

0"2 if3 5"2 8'0 14-4

8"9 9"0 8'2 6"8 1"7

9"1 9-3 13"4 14"8 16"1

°C

(ii) Effect of annealing time The rate of formation of the low melting form is seen to increase with annealing temperature. The effect of annealing time at a constant annealing temperature could therefore conveniently be studied at a temperature of 220°C. Samples were annealed for times between 1 and 2600 rain as previously described. DSC endotherms are shown in Figure 2 and the observed

(I0)

~) dH

(3)

J 220

240 Loc

260

240

260 T,oc

Figure 2 - - D S C fusion endotherms of PET annealed at 220°C

for (1) 0 (2) 5 (3) 30 (4) 60 (5) 90 (6) 180 (7) 240 (8) 400 (9) 1 000 (10) 2 600 minutes melting points and heats o f fusion a r e plotted as a function of l o g (time) in F i g u r e 3. I t can be seen t h a t whilst the m e l t i n g point (T'~) increases a l m o s t linearly with l o g (time) the heats of fusion follow a m o r e c o m p l e x sequence. T h e heat of fusion (AH) of the original f o r m decreases m o n o t o n i c a l l y to zero at times between 1 000 a n d 2 000 minutes. T h e rate of decrease is, 119

R. C. ROBERTS 260

250

o

240 230

3:

15 10

w o

10 o~

1.gl

15 10 .¢1

-1

5

103 ~ 102 Time, rain Figure 3--Effect of annealing time at 220°C on PET I T',., [] (AH+AH'), © AH', ~ AH °

1 101

however, initially greater than the increase in the heat of fusion (AH') of the low melting form so that the total observed heat of fusion (AH + AH') decreases slightly over the first 70 minutes annealing time. In the region 100 to 1 000 minutes annealing AH" and (AH + AH') increase linearly with log (time) consistent with a secondary crystallization mechanism. For times longer than 1 000 minutes the rate of increase in AH' appears to be slowing down as the polymer attains its maximum degree of crystallinity. Thermal degradation is also almost certainly beginning to affect the results at this stage. (iii) Isothermal melt crystallization If, instead of annealing in the solid state, the polymer is isothermally crystallized from the melt the same overall effect is seen. Thus, Figure 4 shows fusion endotherms of PET which has been held in the molt for five minutes, cooled at 64 deg. C/min to 220°C and held isothermally for various times, followed by cooling at 64 deg. C/min to 100°C and scanning at 16 deg. C/min. With a five minute annealing time a peak at 277°C appears and, as the annealing time is increased, the peak increases in size and moves to higher temperatures. The rate of increase is faster than that of the solid state annealed PET as can be seen from Table 2 which lists the relative heats and temperatures of fusion. Comparison with Figure 3 shows that the low temperature peak is not well resolved in the melt annealed 120

POLY(ETHYLENE TEREPHTHALATE) II

(6)

I

220

L

240

I

1

:

i

P

260

240 260 r, oc T,°C Figure 4--DSC fusion endotherms of PET crystallized from the melt isothermally at 2200C for (1) 5 (2) 15 (3) 60 (4) 200 (5) 300 (6) 450 minutes samples and hence some of the peak areas of Table 2 are only approximate; the total heat of fusion is, of course, accurately determined. The sample annealed for as little as 450 minutes shows only a single fusion endotherm Table 2. 220°C isothermal crystallization of PET from the melt Time rain

T'~ °C

T,n °C

AH" cal/g

AH cal/g

~H" + AH cal/g

5 15 60 200 300 450 1 000

227 234 244 250 251 251 254

250 251 252 254 253 ---

0-8 2.0 3.7 ~-.~8.0 ,-~8-0 14.2 15.0

8.8 8.3 7.5 ~.~6.3 ,~,5.6 0.0 0-0

9.6 10-3 11.2 14.3 13.6 14-2 15.0

at the melting point of the original polymer and it would therefore seem that at the stage when T',, has increased to equal Tm the original P E T is completely converted into the new form. (iv) Annealing at temperatures approaching T,, This transformation is seen more clearly when the effect of annealing at higher temperatures is studied. A comparison of the effect of annealing P E T at 220 °, 230% 240 ° and 2 5 0 ° C for 3, 10 and 32 minutes at each temperature is shown in Table 3. Annealing for three minutes produces an endotherm some 6-8 deg. C above the annealing temperature, for ten minutes an endotherm s o m e 8-11 deg. C above the annealing temperature and for 32 minutes an endotherm 10-15 deg. C above the annealing temperature. 121

R . C . ROBERTS

T h e magnitude of these endotherms is seen to increase with both time and annealing temperature; thus. annealing at 240°C for as little as 32 minutes is sufficient to convert all the material to the new form and at an annealing temperature of 250°C the P E T was completely converted by only three minutes annealing. Table 3. Effect of annealing time and temperature on PET Annealing time min

Annealing temp. *C

T'm °C

T,n °C

AH" cal/g

AH cal/g

All" + AH cal/g

220 230

227 238

255 255

1"3 3-1

10"2 8"2

11"5 11"3

240 250

247 256

255 --

,~,5"0 9"5

,-.,5"9 0.0

10"9 9"5

220 230

231 241

255 255

2"1 4"1

8"5 7"3

10"6 11"4

240 250

249 258

255 --

,~o8-3 10"3

,--,4"0 0-0

12"3 10"3

220 230

235 244

255 255

3"9 5"9

7"6 6"2

I 1"5 12-1

240 250

253 260

---

13"2 11"6

0-0 0-0

13"2 11"6

I0

32

On annealing for longer periods of time (up to 4 000 minutes) at 250°C the melting point of the P E T was raised to 276°C, approaching that of 278°C determined by Hartley (see ref. 13) but lower than 284°C predicted by Taylor/3 from oligomer melting points and by I k e d a and Mitsuishi 1~f r o m crystallization studies. The observed heat of fusion, however, did not exceed 16-5 cal/g. (v) lrreversibility o f observed changes It is interesting to note that these morphological changes on annealing are not reversible as detected by DSC. Inspection of Table 4 shows that annealing for ten minutes at 240°C transforms 67 per cent of the crystalTable 4. Irreversibility of morphological changes Annealing conditions

T,n °C

AH cal / g

T-°C

AH" cal / g

2400C/10 mm 250°C/10 min 250°C/10 rain followed by 240°C[ 10 rain 250°C/120 min 250°C/120 min followed by 240°C/10 rain

255 258 259 264 264

4-0 10-3 11"3 12"8 13"5

249 -251 -251

,~,8-3 -1"7 -0-1

linity. Prior annealing for ten minutes at 250°C reduces this to 13 per cent and prior annealing for 120 minutes at 250°C to less than one per cent. 122

POLY(ETHYLENE TEREPHTHALATE) II

Table 3 shows that all the sample crystallinity is transformed by the 250°C annealing and hence the appearance of a low melting peak on further annealing at 240°C is associated with an increase in crystallinity and not reorganization of the material produced by the 250°C annealing. Thus as the overall degree of crystallinity is increased by annealing for longer times at 250°C the amount of amorphous material available for crystallization on further annealing at 240°C is decreased and hence AH'---->-0 for this process. (vi) Low angle X-ray measurements The low angle X-ray diffraction patterns of the amorphous PET show broad scattering corresponding to spacings of 100 to 300 A. Superimposed on this amorphous scattering annealed samples showed small shoulders or peaks at 120-130 A and 150-160 A and in addition, a peak or shoulder which varied between 2(K)--500 A but was of low intensity. DISCUSSION

When PET is subjected to heat treatment above its glass transition temperature and below its melting point (Tin) morphological changes occur which can be detected as an additional fusion endotherm appearing at a temperature (T',~) up to 40 deg. C higher than the annealing temperature. The magnitude (AH') and melting point of this additional endotherm increase with treatment time at a particular temperature or with treatment temperature for a fixed annealing time. At the same time the magnitude of the original fusion endotherm (h/-/) decreases until as T~ ---->-T,~, AH ~ 0 and the sample is completely converted into the new form. The same effect is seen on isothermal crystallization from the melt although the rate of change is more rapid. This is further evidence for a reorganization mechanism in the solid state of the existing crystallites and not merely secondary crystallization effects. Isothermal crystallization from the melt will not involve such reorganizations to the same extent. The different nucleation or growth mechanism producing the low melting form ~ill proceed without prior melting of pre-formed crystallites and hence the overall growth rate of the low melting species will be faster. These changes are not reversible and further annealing at temperatures below the maximum annealing temperature only brings about crystallization of existing available amorphous polymer. Thus the phenomenon of multiple fusion endotherms produced by annealing will only be detected in samples having relatively imperfect crystallites which can rearrange to a more ordered form, not necessarily having a higher degree of crystallinity. Wide angle X-ray measurements 9,17detect no difference in crystal lattice on such annealing. Changes in crystallite size, however, do occur as indicated by narrowing of the diffraction peaks. PET crystallizes from the melt and in the solid state in the form of spherulites. The crystal morphology within such spherulites is not clear although lamellae of thickness about 100 A and 1 000-2 000 A in lateral dimension have been observed~.le in PET crystallized from the glass at 97 ° and 154°C. Low angle X-ray scattering of annealed PET has been studied in some detaiP6,~7. The ob123

R. C. ROBERTS

served long spacing is found to increase with annealing temperature but it decreases with time at any particular annealing temperature. The increase in spacing could be interpreted as an increase in thickness of lamellae with annealing temperature as is observed for other bulk crystallized polymersTM. The origin of the decrease of long period with time is more obscure but is thought to be associated with the incorporation of loose chain ends into the lamellae, ordering of folded chains and hence closer packing of the lamellae. Further evidence for a chain folded structure of the lamellae has been presented by Koenig and Hannon TM who associat~ an i.r. absorption band at 988 cm -1 with a fold conformation. Thus, although the evidence for a chain folded lamellar structure within the PET spherulite is by no means complete it is illustrative to consider the present results on the basis of this model. The initial crystallization of the PET brought about by cooling rapidly from the melt would be expected to produce imperfectly ordered spherulites and the overall morphology would be of 'fringed-micelle' type. The individual crystallites would be small, have large surface free energies and hence low melting points. Annealing would increase the perfection and size of these crystallites by the formation of thin lamellae and bring about further crystallization of amorphous polymer. The DSC results indicate that this occurs by the initial formation of crystallites having even larger surface free energies and hence lower melting points than the initial imperfect crystallites. This could be due to a smaller size or a large surface free energy of the folded ends of the lamellae. Further annealing would increase the thickness of these lamellae and decrease the total surface free energy until the ultimate melting point was reached when the surface energy would be negligible. The X-ray low angle measurements showed three spacings; the two at 120--130 A and 150--160 A being common to all crystalline samples and an additional spacing of much lower intensity which varied between 200 and 500 A. Identification of this spacing with the thickness (/) of a lamella and further identification of the DSC detected T',~ with the lamella melting point would lead to values of 100-200 ergs/cm 2 for the folded surface free energy, o-e, on application of Hoffman's ~° equation T" = T°~(1 2o-,/AH,/) -

These are much larger than previously1.,21 reported values of 40 ergs/cm ~ which have indicated a loosely folded end of the lamellae. Reported values of o-, for polyethylene of similar molecular weight, however, vary from 50-200 ergs/cm ~ and thus comparison of the difference in free energies of a polyethylene and PET fold is not possible. The author thanks Dr H. A. Long ]or the low angle X-ray measurements and Mrs E. Charles for experimental assistance with DSC. NOTE

ADDED

IN

PROOF

Further low angle X-ray measurements on samples showing two fusion endotherms have been carried out by Dr I. M. Ward, University of Bristol, using a Franks camera. Only a single low angle spacing was observed which w~s no~ resolved into two or more peaks. This would appear to be 124

POLY(ETHYLENE TEREPHTHALATE) II surprising in view of the distinct crystal size distributions indicated by DSC; although theoretical calculations by D r D. J. B l u n d e l l of this laboratory predict that only one low angle diffraction is likely t o be observed from such a system if r a n d o m l y dispersed. T h e r e is therefore at present some a m b i g u i t y over the low angle m e a s u r e m e n t s a n d further work is p l a n n e d to clarify the situation.

Imperial Chemical Industries Limited, Petrochemical & Polymer Laboratory, P.O. Box 11, The Heath, Runcorn, Cheshire. (Received Ianuary 1968) REFERENCES J. R. and BAER,E..L appl. Polym. Sci. 1966, 10, 1409 ~COBBS,W. H. and BURTON,R. L. J. Polym. Sci. 1953, 10, 275 3 GRAY,A. P. and CASEY,K. J. Polym. Sci. 1964, 2B, 381 4 CLAMPITT,B. H. Analyt. Chem. 1963, 35, 577 5 HOASHI,K. and MOCHIZ-OKI,T. Makromol. Chem. 1967, 100, 78 6 BAIR,H. E., SALOVEY,R. and HUSEBY,T. W. A.C.S. Preprints, 1966, 7, 430 YOSmMOTO,T. and MIYAGI,A. Kogyo Kagaku Zasshi, 1966, 69, 1771 a COOPER,W., EAVES, D. E. and VAUGHAN,G. Polymer, Lond. 1967, 8, 273 gKANETSUNA,H. and MAEDA,K. Kogyo Kagaku Zasshi, 1966, 69, 1784 10HYBART,F. J. and PEATT,J. O. ,l. appl. Polym. Sci. 1967, 11, 1449 11ROBEnTS,R. C. Polymer, Lond. 1969, 10, 113 12SCHMIDT,P. W. and HXGHT,R. Acta cryst., Camb. 1960, 13, 480 Crrc, B. and TAN CRETI,D. M. Acta cryst., Camb. 1965, 18, 1083 13TAYLOR,G. W. Polymer, Lond. 1962, 3, 543 14IKEDA,M. and MITSUtSm,Y. Kobunshi Kagaku, 1967, 24, 378 15MORROW,D. R., RICHARDSON,G. C., KLEINMAN,L. and WOODWARD,A. E. J. Polym. Sci. 1967, $ A2, 493 16YEH, G. S. Y. and GEIL, P. H..l. macromol. Sci. 1967, BI, 235 17ZACHMAN,H. G. and SCrlMIDT,G. F. Makromol. Chem. 1962, 52, 23 as GEIL, P. H. Polymer Single Crystals. Interscience: New Yorkl 1963 19KOENqG,J. L. and HANNON,M. J. 1. macromol. Sci. 1967, B1, 119 20LAUI~TZEN,J. I. and HOFFMAN,J. D..L Res. Nat. Bur. Stand. 1960, A64, 73 WUNDERUCH,B. 1. chem. Phys. 1958, 29, 1395 I COLLIER,

125