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Morphology of bulk crystallized trans-1,4-polybutadiene
Morphology of bulk crystallized trans1,4-polybutadiene S. FERN,~NDEZ BERMODI~Z, J. M a G. FATOU and F. CATALINA
Replicas of fracture surfaces of fractions of trans-l,4-polybutadiene, over the molecular weight range from 2 × 104 to 3.1 × 105, have been examined by electron microscopy. Striated, lamella type crystallites were observed and the sizes increased, slightly as the molecular weight increased, and they were very much smaller than the extended chain length. The crystallite interfacial free energy increased with molecular weight. INTRODUCTION
THE EXAMINATIONof surfaces or fracture surfaces of crystalline polymers by electron microscopy is a very interesting experimental technique which allows the analysis of the morphology in these systems and the direct measurement of the sizes of the crystallites formed from isothermally crystallized polymers from the melt. It has been established that under the proper conditions of crystallization, many polymers display lamellar structures TM. The step heights of these lamellae have been associated with the sizes of the crystallites. Anderson 5 observed in polyethylene crystallized from the melt three different types of lamellae: type I or regular lamellae, which are similar to solution grown lamellae, type II or narrow lamellae and type III lamellae which have step heights equal to fully extended chain lengths, with a crystallite structure similar to that reported by Bunn et al 6 for polytetrafluoroethylene. Mandelkern et al 7 have demonstrated that striated lamellae are observed in polyethylene for all molecular weights and, therefore, structures of this type are not indication that the crystallite sizes are comparable to the extended chain length. In a recent study, crystallization kinetics from the melt and the melting and transition temperatures of molecular weight fractions of trans-l,4polybutadiene have been analysed8,9. Because of the importance of the relationship between these parameters and the morphology of isothermally bulk-crystallized samples, we decided to investigate the fracture-surfaces by electron microscopy. Moreover, there is no information about the crystallite sizes and their relation with molecular weight in bulk crystallized fractions of trans-l,4-polybutadiene. In this work, we report the results in a molecular range of 20 000-310 000. EXPERIMENTAL
The molecular weight fractions utilized in this work were obtained from trans-1,4-polybutadiene, kindly supplied by Phillips Petroleum Co. 27
S. F. BERMI~IDI~Z, J. Ma G. FATOU AND F. CATALINA
The method of fractionation and characteIization has previously been described s. The viscosity average molecular weights of the fractions, were obtained from intrinsic viscosity measurements in benzene at 25°C and the molecular weight of the fractions selected for this work were 20 000, 129 000 238 000 and 310 000. The microstructure of these fractions was determined by infra-red in a Perkin Elmer 621 spectrophotometer, using the technique described by Silas et al 1°. The 1,4-trans content for the fraction of the lowest molecular weight was 92 % and for the other fractions was 97 %. Samples for study were prepared by moulding films 1 mm thick in a small aluminium mould; these films were then recovered in aluminium foil and after being completely melted in vacuum sealed tubes, were isothermally crystallized at 86°C for a sufficient length of time so that no further crystallization would occur at these temperatures. The time required for this was predetermined through the dilatometric experiments 8 and range from ten to twelve days, depending on the molecular weight of the fractions. After completion of the crystallization at 86°C, the tubes were cooled to room temperature over a period of 24 h. In another set of experiments, the fractions were quenched at 0°C, from the melted state. The densities of the specimens crystallized in these two manners were determined in a gradient column. The crystallinity was calculated from the wide-angle x-ray diffraction patterns, which were obtained in a North American Phillips Co. spectrometer with Ni filtered Cu-Ka radiation. Table 1 summarizes these data. Table 1 Te = 86°C
My 20 129 238 310
000 000 000 000
Te = 0 ° C
Density (g/cm z) Crystallinity % 0"9847 0"9708 0.9640 0.9640
56"8 60.0 65"0 64.7
Density
Crystallinity %
0.9760 0"9535 0.9522 0-9512
47 54 52 55
The specimens thus prepared were fractured after storage in liquid nitrogen. The freshly fractured surfaces were fastened to a slide with double back Scotch tape and oriented parallel to each other. A film (1 mm thick) of Triafol was deposited over the surfaces, with the contact face wetted with acetone. Following this operation, the polymer was removed and chromium was evaporated on the replica in high vacuum; the angle of shadow was 45 o, and after the deposition of the metal, a carbon replica was made in the usual manner. Following the deposition of the thin metal-carbon film, the Triafol was removed with acetone. The calibration was preformed with polystyrene latex particles (Dow Chem. 586). The particle size was 2.200/~. The replicas were picked up on naked cooper grids and examined with a Siemens Elmiskop 1 A electron microscope. Micrographs were made on 28
MORPHOLOGY OF BULK CRYSTALLIZED
TRANS--1,4-POLYBUTADIENE
Agfa-Gevaert, type Scientia 19D-50P plates, photographically enlarged as desired. The micrographs of the fracture's surface was measured with an optical microscope.
RESULTS
The electron micrographs of the fracture surface replicas are similar to those reported for other crystalline polymers ~-7. The striated, banded type lamellar crystallite is the predominant structural feature and these structures correspond to the type III lamella, reported by Anderson in polyethylenes. Besides these striate structures, type I and II lamella are observed. (Figures Z-S).
Figure 1. Micrograph of replica of fracture surface of 1,4-transpolybutadiene. Mr/= 238 000; T~ -- 86°C.
29
S. F. BERMI~ID//Z, J. Ma G. FATOU AND F. CATALINA
Figure 2. Micrograph of replica of fracture surface of 1,4-transpolybutadiene. M r / = 310 000; Tc = 0°C
30
MORPHOLOGY OF BULK CRYSTALLIZEDTRANS--I,4-POLYBUTADIENE
Figure 3. Micrograph of replica of fracture surface of 1,4-transpolybutadiene. M r / = 20 000; Tc = 0°C. We have u n d e r t a k e n a n analysis o f the lamella thicknesses or step heights for the fracture surfaces of o u r samples crystallized at 0 a n d 86°C. More t h a n twenty m e a s u r e m e n t s were made o n each fracture surface, a n d the results o f these m e a s u r e m e n t s are s u m m a r i z e d in Table 2, where the average thicknesses are given.
Table 2 Mr~ Min 20000 129000 238000 310 000
130 168 188 135
Te = 86°C (A) Max Average 540 562 560 630
341 355 392 369
Min
O°C (A) Max
112 100 112 112
505 900 1.070 1.123
T,: :
Average 278 290 364 350
A plot o f the crystallite thickness as a f u n c t i o n of molecular weight is 31
S. F. BERMUD~Z, J. Ma G. FATOU AND F. CATALINA
1°'1
I
/
Y
Te= 86' C
I F1
I 10 ~
I F3
I I 2.10 s F 4
I I 3"1~F5
Mw Figure 4. Crystal]ite size against molecular weight for ],4-transpolybutadiene tractions.
given in Figure 4 for the samples crystallized at 86°C. The range of sizes is indicated by the lines. From this figure, we conclude that in the molecular weight range studied here, the crystallite size increases only slightly with molecular weight either in the samples crystallized at 86°C or in the ones crystallized at 0°C. However, smaller lamellae are present in the last conditions. In both cases the average lamella thickness is smaller compared to the extended chain length. Therefore, the banded lamellae are characteristic over a very wide molecular weight range and their existence is not indication that the crystallite sizes are comparable to the extended chain length. This conclusion has been previously reached by Mandelkern et a l l With the samples crystallized at 0°C, the predominant type is the thinner lamella. We can conclude that this occurs as a consequence of crystallization where the sizes of the crystallites are smaller. Similar results have been indicated by others 5,7. Moreover, the crystallite sizes reported in this work are higher than those reported by Takayanagi et a111,12in single crystals of trans- 1,4 polybutadiene. This thickness corresponds to 100 A. After heat treatment, there is a thickening of the lamella and they have suggested that the amorphous region, attached to the end surface of the crystal is dragged into the crystalline phase to form tight loops. It is clear that in our molecular range, a significant portion of the chain units cannot be assigned to the interior of crystallites. They have to be assigned to the interfacial regions in the 001 face or to the interzonal regions. Therefore, it would be expected a relatively large value for the crystallite interracial energies. The melting temperature of a crystallite of finite thickness is given byla,14 1 __1 Tm Tm °
R [ 2~ee AHu ~RTm
lln(X--~+l)k ~ x
]
(1)
In this equation, Tm ° represents the equilibrium melting temperature; Tm is the observed melting temperature and ~ee is the interfacial free energy per chain as it emerges from the 001 face of the crystallite. 32
MORPHOLOGY OF BULK CRYSTALLIZED
TRANS--1,4-POLYBUTADIENE
When x is large, equation (1) reduces to 1 Tm
l Tm°
2nee ~AHu Tm
(2)
The value of ace can be calculated by means of equations (1) and (2). The quantities required must be known independently. The equilibrium melting temperature of the form I has been estimated to be 75°C. The fusion enthalpy for the form I, was obtained for the relation between the apparent enthalpy, calorimetrically measured, and the crystallinity of the samples 9. This heat of fusion corresponds to 28 -5z 1 cal/g (1510 ~ 54 cal/mol), for the form I, and 16 + 1 cal/g (865 + 54 cal/mol) for the form II. The heat of fusion for the form I is smaller than the values reported by others 1~-17, which correspond to a range of 2400--3300 cal/mol. The value for the form II, agrees with the values reported 16, but there is not any explanation for the contradictory results in the form I. However, Dainton et al TM by direct specific heat measurements estimated the heat for the transition I-II to be 830 cal/mol. It is quite clear that if the higher values of 2.400--3300 cal/mol were considered, this value corresponds to 2.200 cal/m01e, which is too high. For our values, AHu (I-II) corresponds to 645 cal/mol which agrees better with Dainton's results. The values of ~ec, calculated by means of equation (1) are tabulated in Table III. These values are given for both the average value and the largest value of ~. Table 3 MV
20000 129 000 238 000 310000
oec (erg/cm2) Average ~ Max
134 140 153 159
212 222 240 271
As the molecular weight increases there is an increase on the crystallite interfacial free energy. For the three higher molecular weight fractions the relative change is not as great as for the lowest molecular weight fraction. In the higher molecular weight region the crystallite sizes represent only a very small portion of the extended molecular length, and the interracial energies increase. These results agree with those reported for polyethylene 7. ACKNOWLEDGEMENT We wish to acknowledge our gratitude to Dr Silas for sending us the details of his infia-red technique and the standards to be used. 33 P-C
S. F. BERMUDI~Z~J. Ma G. FATOU AND F. CATALINA
Secci6n de Q~imica Fisica y de Polimeros, lnstituto de Pl6sticos ~: Caucho, Juan de la Cierva 3, Madrid 6,
Spain
(Received 28 July 1970)
REFERENCES 1 Eppe, R., Fischer, E. W., and Stuart, H. A. J. Polym. Sci. 1959, 34, 721 2 Keller, A. Makromol. Chem. 1959, 34, 1 3 Geil, P. H. J. Polym. Sci. 1960, 44, 449 4 Kampf, G. KolloidZ. 1960, 172, 507 5 Anderson, F. R. J. Appl. Phys. 1964, 35, 64 6 Bunn, C. W., Cobbold, A. J. and Palmer, R. P. J. Polym. Sci. 1958 38, 365 7 Mandelkern, L., Price, J. M., Gopalan, M. and Fatou, J. G. J. Polym. Sci. (A-2) 1966, 4, 385 8 Bermfidez, S. F., Fatou, J. G., and Royo, J. Anal. Iris. Quin. (in press) 9 Fatou, J. G. and Bermt~dez, S. F. (unpublished results) 10 Silas, R. S., Yates, J. and Thornton, V. Anal. Chem. 1959 31,529 11 Takayanagi, M., Imada, K., Nagai, A., Tatsumi, T. and Matsuo, T. J. Polym. Sci., (C-I) 1967, 6, 867 12 Tatsumi, T., Fukushima, T., Imada, K. and Takayanagi, M. J. MacromoL Sci. Phys. (B) 1967, 1,459 13 Flory, P. J. J. Chem. Phys. 1949, 17, 223 14 Fatou, J. G. and Mandelkern, L. J. Phys. Chem. 1965, 69, 417 15 Natta, G., Corradini, P., Porri, L. and Morero, Atti. Acad. Nazi. Lincei. Rend. 1956, 20, 728 16 Natta, G., Porri, L., Corradini, P. and Morero, D. La Chimica e L'Industrie 1958, 40, 362 17 Danusso, F. Polymer, Lond. 1967, 8, 302 18 Dainton, F. S., Evans, D. M., Hoare, F. E. and Meli~l,T. P. Polymer,Lond. 1962, 3, 297
34
Replicas of fracture surfaces of fractions of trans-l,4-polybutadiene, over the molecular weight range from 2 × 104 to 3.1 × 105, have been examined by electron microscopy. Striated, lamella type crystallites were observed and the sizes increased, slightly as the molecular weight increased, and they were very much smaller than the extended chain length. The crystallite interfacial free energy increased with molecular weight. INTRODUCTION
THE EXAMINATIONof surfaces or fracture surfaces of crystalline polymers by electron microscopy is a very interesting experimental technique which allows the analysis of the morphology in these systems and the direct measurement of the sizes of the crystallites formed from isothermally crystallized polymers from the melt. It has been established that under the proper conditions of crystallization, many polymers display lamellar structures TM. The step heights of these lamellae have been associated with the sizes of the crystallites. Anderson 5 observed in polyethylene crystallized from the melt three different types of lamellae: type I or regular lamellae, which are similar to solution grown lamellae, type II or narrow lamellae and type III lamellae which have step heights equal to fully extended chain lengths, with a crystallite structure similar to that reported by Bunn et al 6 for polytetrafluoroethylene. Mandelkern et al 7 have demonstrated that striated lamellae are observed in polyethylene for all molecular weights and, therefore, structures of this type are not indication that the crystallite sizes are comparable to the extended chain length. In a recent study, crystallization kinetics from the melt and the melting and transition temperatures of molecular weight fractions of trans-l,4polybutadiene have been analysed8,9. Because of the importance of the relationship between these parameters and the morphology of isothermally bulk-crystallized samples, we decided to investigate the fracture-surfaces by electron microscopy. Moreover, there is no information about the crystallite sizes and their relation with molecular weight in bulk crystallized fractions of trans-l,4-polybutadiene. In this work, we report the results in a molecular range of 20 000-310 000. EXPERIMENTAL
The molecular weight fractions utilized in this work were obtained from trans-1,4-polybutadiene, kindly supplied by Phillips Petroleum Co. 27
S. F. BERMI~IDI~Z, J. Ma G. FATOU AND F. CATALINA
The method of fractionation and characteIization has previously been described s. The viscosity average molecular weights of the fractions, were obtained from intrinsic viscosity measurements in benzene at 25°C and the molecular weight of the fractions selected for this work were 20 000, 129 000 238 000 and 310 000. The microstructure of these fractions was determined by infra-red in a Perkin Elmer 621 spectrophotometer, using the technique described by Silas et al 1°. The 1,4-trans content for the fraction of the lowest molecular weight was 92 % and for the other fractions was 97 %. Samples for study were prepared by moulding films 1 mm thick in a small aluminium mould; these films were then recovered in aluminium foil and after being completely melted in vacuum sealed tubes, were isothermally crystallized at 86°C for a sufficient length of time so that no further crystallization would occur at these temperatures. The time required for this was predetermined through the dilatometric experiments 8 and range from ten to twelve days, depending on the molecular weight of the fractions. After completion of the crystallization at 86°C, the tubes were cooled to room temperature over a period of 24 h. In another set of experiments, the fractions were quenched at 0°C, from the melted state. The densities of the specimens crystallized in these two manners were determined in a gradient column. The crystallinity was calculated from the wide-angle x-ray diffraction patterns, which were obtained in a North American Phillips Co. spectrometer with Ni filtered Cu-Ka radiation. Table 1 summarizes these data. Table 1 Te = 86°C
My 20 129 238 310
000 000 000 000
Te = 0 ° C
Density (g/cm z) Crystallinity % 0"9847 0"9708 0.9640 0.9640
56"8 60.0 65"0 64.7
Density
Crystallinity %
0.9760 0"9535 0.9522 0-9512
47 54 52 55
The specimens thus prepared were fractured after storage in liquid nitrogen. The freshly fractured surfaces were fastened to a slide with double back Scotch tape and oriented parallel to each other. A film (1 mm thick) of Triafol was deposited over the surfaces, with the contact face wetted with acetone. Following this operation, the polymer was removed and chromium was evaporated on the replica in high vacuum; the angle of shadow was 45 o, and after the deposition of the metal, a carbon replica was made in the usual manner. Following the deposition of the thin metal-carbon film, the Triafol was removed with acetone. The calibration was preformed with polystyrene latex particles (Dow Chem. 586). The particle size was 2.200/~. The replicas were picked up on naked cooper grids and examined with a Siemens Elmiskop 1 A electron microscope. Micrographs were made on 28
MORPHOLOGY OF BULK CRYSTALLIZED
TRANS--1,4-POLYBUTADIENE
Agfa-Gevaert, type Scientia 19D-50P plates, photographically enlarged as desired. The micrographs of the fracture's surface was measured with an optical microscope.
RESULTS
The electron micrographs of the fracture surface replicas are similar to those reported for other crystalline polymers ~-7. The striated, banded type lamellar crystallite is the predominant structural feature and these structures correspond to the type III lamella, reported by Anderson in polyethylenes. Besides these striate structures, type I and II lamella are observed. (Figures Z-S).
Figure 1. Micrograph of replica of fracture surface of 1,4-transpolybutadiene. Mr/= 238 000; T~ -- 86°C.
29
S. F. BERMI~ID//Z, J. Ma G. FATOU AND F. CATALINA
Figure 2. Micrograph of replica of fracture surface of 1,4-transpolybutadiene. M r / = 310 000; Tc = 0°C
30
MORPHOLOGY OF BULK CRYSTALLIZEDTRANS--I,4-POLYBUTADIENE
Figure 3. Micrograph of replica of fracture surface of 1,4-transpolybutadiene. M r / = 20 000; Tc = 0°C. We have u n d e r t a k e n a n analysis o f the lamella thicknesses or step heights for the fracture surfaces of o u r samples crystallized at 0 a n d 86°C. More t h a n twenty m e a s u r e m e n t s were made o n each fracture surface, a n d the results o f these m e a s u r e m e n t s are s u m m a r i z e d in Table 2, where the average thicknesses are given.
Table 2 Mr~ Min 20000 129000 238000 310 000
130 168 188 135
Te = 86°C (A) Max Average 540 562 560 630
341 355 392 369
Min
O°C (A) Max
112 100 112 112
505 900 1.070 1.123
T,: :
Average 278 290 364 350
A plot o f the crystallite thickness as a f u n c t i o n of molecular weight is 31
S. F. BERMUD~Z, J. Ma G. FATOU AND F. CATALINA
1°'1
I
/
Y
Te= 86' C
I F1
I 10 ~
I F3
I I 2.10 s F 4
I I 3"1~F5
Mw Figure 4. Crystal]ite size against molecular weight for ],4-transpolybutadiene tractions.
given in Figure 4 for the samples crystallized at 86°C. The range of sizes is indicated by the lines. From this figure, we conclude that in the molecular weight range studied here, the crystallite size increases only slightly with molecular weight either in the samples crystallized at 86°C or in the ones crystallized at 0°C. However, smaller lamellae are present in the last conditions. In both cases the average lamella thickness is smaller compared to the extended chain length. Therefore, the banded lamellae are characteristic over a very wide molecular weight range and their existence is not indication that the crystallite sizes are comparable to the extended chain length. This conclusion has been previously reached by Mandelkern et a l l With the samples crystallized at 0°C, the predominant type is the thinner lamella. We can conclude that this occurs as a consequence of crystallization where the sizes of the crystallites are smaller. Similar results have been indicated by others 5,7. Moreover, the crystallite sizes reported in this work are higher than those reported by Takayanagi et a111,12in single crystals of trans- 1,4 polybutadiene. This thickness corresponds to 100 A. After heat treatment, there is a thickening of the lamella and they have suggested that the amorphous region, attached to the end surface of the crystal is dragged into the crystalline phase to form tight loops. It is clear that in our molecular range, a significant portion of the chain units cannot be assigned to the interior of crystallites. They have to be assigned to the interfacial regions in the 001 face or to the interzonal regions. Therefore, it would be expected a relatively large value for the crystallite interracial energies. The melting temperature of a crystallite of finite thickness is given byla,14 1 __1 Tm Tm °
R [ 2~ee AHu ~RTm
lln(X--~+l)k ~ x
]
(1)
In this equation, Tm ° represents the equilibrium melting temperature; Tm is the observed melting temperature and ~ee is the interfacial free energy per chain as it emerges from the 001 face of the crystallite. 32
MORPHOLOGY OF BULK CRYSTALLIZED
TRANS--1,4-POLYBUTADIENE
When x is large, equation (1) reduces to 1 Tm
l Tm°
2nee ~AHu Tm
(2)
The value of ace can be calculated by means of equations (1) and (2). The quantities required must be known independently. The equilibrium melting temperature of the form I has been estimated to be 75°C. The fusion enthalpy for the form I, was obtained for the relation between the apparent enthalpy, calorimetrically measured, and the crystallinity of the samples 9. This heat of fusion corresponds to 28 -5z 1 cal/g (1510 ~ 54 cal/mol), for the form I, and 16 + 1 cal/g (865 + 54 cal/mol) for the form II. The heat of fusion for the form I is smaller than the values reported by others 1~-17, which correspond to a range of 2400--3300 cal/mol. The value for the form II, agrees with the values reported 16, but there is not any explanation for the contradictory results in the form I. However, Dainton et al TM by direct specific heat measurements estimated the heat for the transition I-II to be 830 cal/mol. It is quite clear that if the higher values of 2.400--3300 cal/mol were considered, this value corresponds to 2.200 cal/m01e, which is too high. For our values, AHu (I-II) corresponds to 645 cal/mol which agrees better with Dainton's results. The values of ~ec, calculated by means of equation (1) are tabulated in Table III. These values are given for both the average value and the largest value of ~. Table 3 MV
20000 129 000 238 000 310000
oec (erg/cm2) Average ~ Max
134 140 153 159
212 222 240 271
As the molecular weight increases there is an increase on the crystallite interfacial free energy. For the three higher molecular weight fractions the relative change is not as great as for the lowest molecular weight fraction. In the higher molecular weight region the crystallite sizes represent only a very small portion of the extended molecular length, and the interracial energies increase. These results agree with those reported for polyethylene 7. ACKNOWLEDGEMENT We wish to acknowledge our gratitude to Dr Silas for sending us the details of his infia-red technique and the standards to be used. 33 P-C
S. F. BERMUDI~Z~J. Ma G. FATOU AND F. CATALINA
Secci6n de Q~imica Fisica y de Polimeros, lnstituto de Pl6sticos ~: Caucho, Juan de la Cierva 3, Madrid 6,
Spain
(Received 28 July 1970)
REFERENCES 1 Eppe, R., Fischer, E. W., and Stuart, H. A. J. Polym. Sci. 1959, 34, 721 2 Keller, A. Makromol. Chem. 1959, 34, 1 3 Geil, P. H. J. Polym. Sci. 1960, 44, 449 4 Kampf, G. KolloidZ. 1960, 172, 507 5 Anderson, F. R. J. Appl. Phys. 1964, 35, 64 6 Bunn, C. W., Cobbold, A. J. and Palmer, R. P. J. Polym. Sci. 1958 38, 365 7 Mandelkern, L., Price, J. M., Gopalan, M. and Fatou, J. G. J. Polym. Sci. (A-2) 1966, 4, 385 8 Bermfidez, S. F., Fatou, J. G., and Royo, J. Anal. Iris. Quin. (in press) 9 Fatou, J. G. and Bermt~dez, S. F. (unpublished results) 10 Silas, R. S., Yates, J. and Thornton, V. Anal. Chem. 1959 31,529 11 Takayanagi, M., Imada, K., Nagai, A., Tatsumi, T. and Matsuo, T. J. Polym. Sci., (C-I) 1967, 6, 867 12 Tatsumi, T., Fukushima, T., Imada, K. and Takayanagi, M. J. MacromoL Sci. Phys. (B) 1967, 1,459 13 Flory, P. J. J. Chem. Phys. 1949, 17, 223 14 Fatou, J. G. and Mandelkern, L. J. Phys. Chem. 1965, 69, 417 15 Natta, G., Corradini, P., Porri, L. and Morero, Atti. Acad. Nazi. Lincei. Rend. 1956, 20, 728 16 Natta, G., Porri, L., Corradini, P. and Morero, D. La Chimica e L'Industrie 1958, 40, 362 17 Danusso, F. Polymer, Lond. 1967, 8, 302 18 Dainton, F. S., Evans, D. M., Hoare, F. E. and Meli~l,T. P. Polymer,Lond. 1962, 3, 297
34