MOORE
Content
The Crystallization of Polyethylene Terephthalate by Organic Liquids
W. R, MOOREand R. P. SHELDON
The development o[ crystallinity in essentially amorphous polyethylene terephthalate film following immersion in a range o[ organic liquids at 25°C ha~ been studied. The rate o[ development o[ crystallinity varies markedly with the liquid and although molecular size o[ the liquid may be a contributory [actor it is not the only one.~ Both the equitibrium degree of crystallinity and swelling o[ the film vary with the liquid and in a similar manner and it would seem that polarity, type and solubility parameter of the liquid are important [actors governing both equilibrium crystalllnity and swelling. It is suggested that solvation of the polymer occurs and that interaction o[ solvated polymer and liquid leads to conditions [avourable to crystalliza(ion.
THE ability of some organic liquids to induce crystallization of a potentially crystallizable but otherwise essentially amorphous polymer has been known for some time but little quantitative information is available. Spence1, using films of cellulose triacetate, found that the better the solvent the sharper was the X-ray diagram, suggesting a greater degree of crystallinity. Baker et al. 2 drew similm conclusions from work on cellulose triacetate and tributyrate. Kolb and Izard 3 showed that some organic and inorganic liquids would induce crystallization of polyethylene terephthalate at temperatures well below those at which rates of crystallization are appreciable in the absence of liquid. A recent study4 of the effects of a homologous series of methyl ketones on the crystallization of polyethylene terephthalate suggested that, if allowance were made for any absorbed liquid, apparent densities could be corrected to true densities which were independent of the amount of liquid absorbed. The rate of crystallization appeared to decrease with increasing size of the ketone. There was some evidence that not only the rate of crystallization but also the equilibrium degree of crystallinity might depend on the nature of the liquid. In a further study of factors affecting the crystallization of polyethylene terephth'alate in liquids this work has been extended to cover a range of organic liquids of differing chemical types and properties. EXPERIMENTAL The polyethylene terephthalate used was essentially amorphous film, 0.008 in. in thickness. Preliminary measurements showed it to have a density of l'340g/cm ~ at 25°C, corresponding to a degree of crystallinity of 4-2 per cent. The following liquids were used: hexane, carbon tetrachloride, ethanol, n-butanol, benzyl alcohol, acrylonitrile, dioxan, ethyl formate, m-cresol, aniline, nitromethane, nitroethane, acetic acid, dimethyl phthalate, benzene, toluene, nonyl methyl ketone, n-amyl methyl ketone, ethyl methyl ketone and acetone. The best available grades were further purified, dried by appropriate agents and fractionally distilled before use. 315
W. R. MOORE, A N D R. P. SHELDON
Known weights, approximately 0"07 g, of polymer were immersed in each of the liquids at 25 ° ~+0'01 °C for varying periods of time. Surface liquid was then removed by lightly blotting with filter paper and the sample stored in an evacuated vacuum desiccator for at least 24 hours. It was then weighed, all weighings being made with a semi-micro balance reading to 0-00001 g. With less volatile liquids such as dimethyl phthalate a surface washing with ethanol preceded blotting. After reweighing the density of the sample was determined by immersion in 10 cm3 of carbon tetrachloride at 25 ° +0"01°C and addition, with agitation, of dry ethanol from a micro burette until the sample neither rose nor sank in the mixture. The density of the sample was then obtained from a graph showing the variation of density with composition of the carbon tetrachloride-ethanol mixtures. Preliminary experiments showed that such mixtures did not induce crystallization, at least within 24 hours. The method was quite reproducible and densities could be estimated to 0-001. With m-cresol some solution of the polymer occurred and dissolved polymer was precipitated by addition of ethanol and recovered by centrifuging and drying in a vacuum oven at 50°C, the weight of recovered material being used to correct the apparent density of the undissolved material to the true density. The influence of this procedure on the results will be discussed later. Swelling of the polymer in the different liquids was obtained from the weight of liquid absorbed when density had reached its equilibrium value and expressed as a percentage of the original volume of the polymer. With volatile liquids such as acetone and benzene weighings were made as a function of time and followed by extrapolation to zero time to obtain the true weight. As pointed out previously* some liquid is retained by the polymer after evacuation and the density da obtained by the flotation method is not that of the polymer d~ but of the polymer plus retained liquid.
d, = (mp + m31(% + ~3
where m~ and ~ are the mass and volume of the polymer and mz and vz those of the liquid. Assuming, from previous work" and direct measurements of swelling in a density bottle, that the liquid is uncompressed, then
da = (roT,+ m3 / (m~/d~ + todd3
from which the density of the polymer d~ is given by
d,=d,/t(m,/rr~) (1 - d : / d3 + 11
RESULTS
Plots of density against time of immersion are shown in Figure 1. An equilibrium density is reached with each liquid and these densities are collected in Table 1 together with corresponding percentage crystaUinities x~ obtained from x,-- 100 (d~ -dA) / (de -dA) where dA is the density of the completely amorphous polymer, taken5 to be 1-335 and de that of the completely crystalline, taken 6 as 1"455. Values for 316
THE CRYSTALLIZATION OF POLYETHYLENE
TEREPHTHALATE
C"E 1.42 Ethyl forma,te
Acrylonitrile
~ Dioxan ,
--
s
o x
×o--e~ + l , "t" 24 h
x-
I
1'42[1"401~
1"38[~
Aniline .:.
~
12 Time Nitromethane Nitroethane ",, x ~
I 12 ~ ~
~o-
1.40[-
~ Benzene
Time ~
24 Toluene
h
D
I.--I-"
1'34g
I 1 1"401- Benzyl alcohol
Time ~x~ ~-'x"
I 2
I 3 day Dimethyl phthalate o-.
.x-
1'36i~
1 " 3 M I
I
I
I
0
I
2 Ti me
3
4
day
Figure / - - R a t e of crystallization of polyethylene terephthalate in
various liquid media at 25 °C
Table 1
Equilibrium values of density, swelling and degree of crystallinity
Liquid
Hexane Nonyl methyl ketone n-Amyl methyl ketone Carbon tetrachloride Toluene Benzene Ethyl methyl ketone Ethyl formate Acetone Dioxan Acrylonitri!e Dimethyl phthalate Nitroethane Benzyl alcohol n-Butanol Aniline m-Cresol Nitromethane Acetic acid Ethanol
5250 (cal/cma)~ 7.3 7"9 8-45 8"6 8"9 9"15 9" 15 9-40 9"75 I0"05 10"5 10-6 11"1 11-3 11 "4 11 "5 11 "9 12"6 12"9 13-2 317
Density
(g/cc) 1" 340 1 "340 1 "358 l "340 1"381 I "390 1 "395 1.390 1 "405 1 •396 1 "384 1 "385 1-389 1-396 1" 340 1.400 1 "413 1 "394 1 -390 1 "340
Swelling
(% vol.) Nil Nil 6"3 Nil 14'7 18"8 18"7 20.0 30"8 21-0 12-6 9"9 17-7 22 "3 Nil 36"8 125 14"3 13' 1 Nil
Crystallinity
(% vol.) 4' 2 4"2 19"2 4"2 38-1 45'8 50'0 45-8 58 "3 50-8 40"8 41 "7 45-0 50-8 4' 2 54-2 63 49-2 45"8 4"2
W. R. M O O R E A N D R. P. SHELDON
the ketones are somewhat lower than those previously reported" and the reason for this is not clear. As mentioned previously, some polymer, presumably of low molecular weight, was soluble in m-cresol. By taking into account the weights of precipitated and undissolved polymer and also a presumably proportional loss of m-cresol from the sample by extraction with ethanol, corrections were applied to the density and swelling of the undissolved polymer. Since the soluble fraction may behave differently from the insoluble and very low molecular weight polymer may not be precipitated by ethanol the corrected value given in Table 1 should be regarded as approximate. The development of crystallinity was always accompanied by increasing opacity of the film and, when absorbed solvent was removed, by increased brittleness. The latter effect has been previously noted 7 after prolonged heating in an aqueous dye bath at 100°C. Values of equilibrium swelling are also given in Table 1.
DISCUSSION
Figure 1 shows that the rate of increase of density and hence of crystallinity varies markedly with the liquid. In some cases crystallization is rapid while in others it occurs at a rate convenient for measurement. Figure 1 also shows that although molecular size may be a factor affecting the rate of crystallization it is not the only one. Dioxan, ethyl formate, acrylonitrile, m-cresol, aniline, nitromethane and nitroethane induce much more rapid crystallization than benzene, toluene, acetic acid, benzyl alcohol and dimethyl phthalate. Kinetic studies of this induced crystallization will be discussed in a later communication and are not further considered here, but it may be noted that the rate of disappearance of available amorphous content at any given time seems to follow a first order equation. The equilibrium values of density and crystallinity also vary with the liquid. The non-polar hexane and carbon tetrachloride do not induce crystallinity and neither do ethanol and n-butanol. The polarizable aromatic hydrocarbons benzene and toluene do so, however, and so does benzyl alcohol. Other liquids inducing crystallinity are polar but there seems to be no obvious relationship between the equilibrium density of the polymer and the polarity of the liquid. Results for acetone and n-amyl methyl ketone and for nitromethane and nitroethane suggest that molecular size may be a factor with liquids of the same type but this is not general. Table 1 shows that, in general, the greater the equilibrium density the greater the swelling of the polymer although density and swelling are not linearly related. Swelling may be regarded as a measure of the interaction between polymer and liquid and the development of crystaUinity may depend on such interaction. Figures 2 and 3 show equilibrium density and swelling respectively as functions of the solubility parameter 3 of the liquid. 3 =[(L, - R T ) / V ] } where Le is the molar latent heat of vaporization of the liquid and V is molar volume, both at the absolute temperature T. Values of 3, which are given in Table 1, were taken from published values 8' 9 or calculated using values of Le obtained by use of the Hildebrand rule 1°. The difference 318
THE CRYSTALLIZATION OF POLYETHYLENE TEREPHTHALATE between the solubility parameters of a liquid and a not too polar amorphous polymer can be related to the heat change in their mixing and the interaction of polymer and liquid will therefore depend, in part, on the solubility parameter of the latter.
"E 1"42 ..90 1-40
-~ i.3B
C Q
Figure
1-36
2 - - Equilibrium density as a function of solubility parameter of liquid
7
oJ
8
I
I
I
I
12
9 10 11 Solubility parameter
(cal/cm3) ~h
13
120 I00
80
~C
6003
Figure 3--Equilibrium swel-
>o 4 0 20
0
ling as a function of solubility parameter of liquid
7
8
9 10 11 Solubility parameter
12
13
(cal/cm3)Y2
F i g u r e 2 shows two maxima at 3 values of approximately 9 7 and 12-0 and similar maxima are seen in F i g u r e 3. The first maximum is associated with liquids such as ketones and esters which may be regarded as basic in the Lewis sense. Benzene and toluene, which are associated with this region of the plots, can also be regarded as basic u. The second maximum is associated with liquids of acidic type such as m-cresol, acetic acid and nitromethane. The existence of two maxima may be a consequence of the presence of basic carbonyl groups in the polymer and acidic hydrogen atoms in CH~ groups adjacent to oxygen atoms. Giles et al. 1~ have suggested that ester groups in polyethylene terephthalate may act as proton donors. Liquids containing acidic groups should solvate basic polymer groups and those containing basic groups should solvate acidic ones. Alternatively, dipolar attraction may lead to solvation by both types of liquid.
319
W. R. MOORE AND R. P. SHELDON Swelling will occur in liquids whose solubility parameters do not differ too greatly from that of the solvated polymer and be a maximum when these solubility parameters are equal or nearly so. The solubility parameter of the solvated polymer will presumably vary with the solvating liquid but variations for the polymer solvated by liquids of similar type may not be large. It has been pointed out that in eases of polymers containing both acidic and basic groups there will be two ranges of solubility parameter within which swelling or solution can occur, corresponding to the polymer solvated by basic and acidic liquids respectively 13. Behaviour of this type has been observed with secondary cellulose acetate TM. Dependence of induced crystallinity on solubility parameter of liquid has also been noted for crystallizable polymethyl methacrylate 1~. This correlation of equilibrium density and swelling with solubility parameter of liquid should perhaps be regarded as a n approximation and other factors may be operative. A number of the liquids used--ethanol, butanol, benzyl alcohol, m-cresol and acetic acid--are known to be associated. Latent heats of vaporization will often include energy required to overcome forces causing association and it is perhaps doubtful to what extent solubility parameters obtained from such latent heats can be related to forces between associated molecules in the liquid state. Association might account for the lack of swelling and density increase in ethanol and butanol. The marked effect of benzyl alcohol, with almost the same solubility parameter as butanol, may be due to the influence of the phenyl group. The points for aniline in Figures 2 and 3 fall amongst those for acidic liquids although aniline is generally regarded as basic. There is some evidence, however, that the hydrogen atoms of the NH~ group can take part in hydrogen bonding to basic carbonyl groups 1~. Reasons for the correlation between swelling and development of crystallinity are not entirely clear. It is possible that the solvated polymer imbibes the solvating liquid in swelling and that the imbibed liquid acts as a plasticizer or molecular lubricant, reducing internal stresses and permitting movement of the polymer chains at the temperature of the experiments and the adoption of more favourable conditions for the development of crystallinity. Here it may be relevant to note that the amount of dimethyl phthalate absorbed at equilibrium reduced the glass temperature of the polymer, determined by a dilatometric method, from 67 ° to 62°C. The value of 62 ° refers, of course, to the polymer after crystallization. The glass temperature of the amorphous polymer containing dimethyl phthalate may be much lower. If this picture of the action of liquids is correct it follows that the actual degree of crystallinity attainable may also depend on such aspects of the nature of the polymer as its molecular weight and previous thermal history. This may be supported by the fact that although aniline and m-cresol both cause high degrees of swelling the equilibrium level of crystallinity is not very different from lhat resulting from the use of other liquids, such as acetone, which cause smaller degrees of swelling.
We are grateful to Imperial Chemical Industries Ltd, Plastics Division, for the gift of polyethylene terephthalate film.
320
THE CRYSTALLIZATION
OF POLYETHYLENE TEREPHTHALATE
P o l y m e r R e s e a r c h Laboratories, D e p a r t m e n t o f C h e m i c a l Technology, Institute o f Technology, Brad[ord, Y o r k s . ( R e c e i v e d M a r c h 1961)
REFERENCES 1 SPENCE, J. J. phys Chem. 1941, 45, 401 2 BAKER, W. O., FULLER, C. S. and PAPE, N. R. J. Amer. chem. Soc. 1942, 64, 776 KOLB, J. and IZARD, E. F. J. appl. Phys. 1949, 20, 571 a MOORE, W, R., RICHARDS, D. O. and. SHELDON, R. P, J. Text. lnst. 1960, 51, T438 5 THOMPSON, A. B. and WOODS, D. W. Nature, Lond. 1955, 176, 78 BUNN, C. W. in Fibres jrom Synthetic Polymers edited by R. HILL. Elsevier: Amsterdam, 1953 r SHELDON, R. P. Unpublished Results 8 MOORE, W. R., EPSTEIN. J. A., BROWN, A. M. and TIDSWELL B. M. J. Polym. Sci. 1957, 23, 23 9 BURRELL, H. lnterchemical Review, 1955, Spring, 3 ~°H1LDF.aR.'e~D, J. L." and SCOTT, R. L. Solubility of Non-Electrolytes, 3rd ed. Reinhold : New York, 1950 11 BARKER, J. A. and SMITFI, F. J. chem. Phys. 1954, 22, 375 15 ALLINGHAM, M. M., GILES, C. H. and NEUSTADTER, E. L. Disc. Faraday Soc. 1954, 16, 92; ARSHID, F. M., GILES, C. H. and JAIn, S. K. J. chem. Soc. 1956, 260, 1272 ~3 MOORE, W. R. J. Soc. Dy. Col. 1958, 74, 500 14 MOORE, W. R. and RUSSELL, J. J. Colloid Sci. 1953, 8, 243 ~5 Aus. Patent No. 36684 ~ MOORE, W. R. and RUSSELL, J. J. appl. Chem. 1954, 4, 369
321
W. R, MOOREand R. P. SHELDON
The development o[ crystallinity in essentially amorphous polyethylene terephthalate film following immersion in a range o[ organic liquids at 25°C ha~ been studied. The rate o[ development o[ crystallinity varies markedly with the liquid and although molecular size o[ the liquid may be a contributory [actor it is not the only one.~ Both the equitibrium degree of crystallinity and swelling o[ the film vary with the liquid and in a similar manner and it would seem that polarity, type and solubility parameter of the liquid are important [actors governing both equilibrium crystalllnity and swelling. It is suggested that solvation of the polymer occurs and that interaction o[ solvated polymer and liquid leads to conditions [avourable to crystalliza(ion.
THE ability of some organic liquids to induce crystallization of a potentially crystallizable but otherwise essentially amorphous polymer has been known for some time but little quantitative information is available. Spence1, using films of cellulose triacetate, found that the better the solvent the sharper was the X-ray diagram, suggesting a greater degree of crystallinity. Baker et al. 2 drew similm conclusions from work on cellulose triacetate and tributyrate. Kolb and Izard 3 showed that some organic and inorganic liquids would induce crystallization of polyethylene terephthalate at temperatures well below those at which rates of crystallization are appreciable in the absence of liquid. A recent study4 of the effects of a homologous series of methyl ketones on the crystallization of polyethylene terephthalate suggested that, if allowance were made for any absorbed liquid, apparent densities could be corrected to true densities which were independent of the amount of liquid absorbed. The rate of crystallization appeared to decrease with increasing size of the ketone. There was some evidence that not only the rate of crystallization but also the equilibrium degree of crystallinity might depend on the nature of the liquid. In a further study of factors affecting the crystallization of polyethylene terephth'alate in liquids this work has been extended to cover a range of organic liquids of differing chemical types and properties. EXPERIMENTAL The polyethylene terephthalate used was essentially amorphous film, 0.008 in. in thickness. Preliminary measurements showed it to have a density of l'340g/cm ~ at 25°C, corresponding to a degree of crystallinity of 4-2 per cent. The following liquids were used: hexane, carbon tetrachloride, ethanol, n-butanol, benzyl alcohol, acrylonitrile, dioxan, ethyl formate, m-cresol, aniline, nitromethane, nitroethane, acetic acid, dimethyl phthalate, benzene, toluene, nonyl methyl ketone, n-amyl methyl ketone, ethyl methyl ketone and acetone. The best available grades were further purified, dried by appropriate agents and fractionally distilled before use. 315
W. R. MOORE, A N D R. P. SHELDON
Known weights, approximately 0"07 g, of polymer were immersed in each of the liquids at 25 ° ~+0'01 °C for varying periods of time. Surface liquid was then removed by lightly blotting with filter paper and the sample stored in an evacuated vacuum desiccator for at least 24 hours. It was then weighed, all weighings being made with a semi-micro balance reading to 0-00001 g. With less volatile liquids such as dimethyl phthalate a surface washing with ethanol preceded blotting. After reweighing the density of the sample was determined by immersion in 10 cm3 of carbon tetrachloride at 25 ° +0"01°C and addition, with agitation, of dry ethanol from a micro burette until the sample neither rose nor sank in the mixture. The density of the sample was then obtained from a graph showing the variation of density with composition of the carbon tetrachloride-ethanol mixtures. Preliminary experiments showed that such mixtures did not induce crystallization, at least within 24 hours. The method was quite reproducible and densities could be estimated to 0-001. With m-cresol some solution of the polymer occurred and dissolved polymer was precipitated by addition of ethanol and recovered by centrifuging and drying in a vacuum oven at 50°C, the weight of recovered material being used to correct the apparent density of the undissolved material to the true density. The influence of this procedure on the results will be discussed later. Swelling of the polymer in the different liquids was obtained from the weight of liquid absorbed when density had reached its equilibrium value and expressed as a percentage of the original volume of the polymer. With volatile liquids such as acetone and benzene weighings were made as a function of time and followed by extrapolation to zero time to obtain the true weight. As pointed out previously* some liquid is retained by the polymer after evacuation and the density da obtained by the flotation method is not that of the polymer d~ but of the polymer plus retained liquid.
d, = (mp + m31(% + ~3
where m~ and ~ are the mass and volume of the polymer and mz and vz those of the liquid. Assuming, from previous work" and direct measurements of swelling in a density bottle, that the liquid is uncompressed, then
da = (roT,+ m3 / (m~/d~ + todd3
from which the density of the polymer d~ is given by
d,=d,/t(m,/rr~) (1 - d : / d3 + 11
RESULTS
Plots of density against time of immersion are shown in Figure 1. An equilibrium density is reached with each liquid and these densities are collected in Table 1 together with corresponding percentage crystaUinities x~ obtained from x,-- 100 (d~ -dA) / (de -dA) where dA is the density of the completely amorphous polymer, taken5 to be 1-335 and de that of the completely crystalline, taken 6 as 1"455. Values for 316
THE CRYSTALLIZATION OF POLYETHYLENE
TEREPHTHALATE
C"E 1.42 Ethyl forma,te
Acrylonitrile
~ Dioxan ,
--
s
o x
×o--e~ + l , "t" 24 h
x-
I
1'42[1"401~
1"38[~
Aniline .:.
~
12 Time Nitromethane Nitroethane ",, x ~
I 12 ~ ~
~o-
1.40[-
~ Benzene
Time ~
24 Toluene
h
D
I.--I-"
1'34g
I 1 1"401- Benzyl alcohol
Time ~x~ ~-'x"
I 2
I 3 day Dimethyl phthalate o-.
.x-
1'36i~
1 " 3 M I
I
I
I
0
I
2 Ti me
3
4
day
Figure / - - R a t e of crystallization of polyethylene terephthalate in
various liquid media at 25 °C
Table 1
Equilibrium values of density, swelling and degree of crystallinity
Liquid
Hexane Nonyl methyl ketone n-Amyl methyl ketone Carbon tetrachloride Toluene Benzene Ethyl methyl ketone Ethyl formate Acetone Dioxan Acrylonitri!e Dimethyl phthalate Nitroethane Benzyl alcohol n-Butanol Aniline m-Cresol Nitromethane Acetic acid Ethanol
5250 (cal/cma)~ 7.3 7"9 8-45 8"6 8"9 9"15 9" 15 9-40 9"75 I0"05 10"5 10-6 11"1 11-3 11 "4 11 "5 11 "9 12"6 12"9 13-2 317
Density
(g/cc) 1" 340 1 "340 1 "358 l "340 1"381 I "390 1 "395 1.390 1 "405 1 •396 1 "384 1 "385 1-389 1-396 1" 340 1.400 1 "413 1 "394 1 -390 1 "340
Swelling
(% vol.) Nil Nil 6"3 Nil 14'7 18"8 18"7 20.0 30"8 21-0 12-6 9"9 17-7 22 "3 Nil 36"8 125 14"3 13' 1 Nil
Crystallinity
(% vol.) 4' 2 4"2 19"2 4"2 38-1 45'8 50'0 45-8 58 "3 50-8 40"8 41 "7 45-0 50-8 4' 2 54-2 63 49-2 45"8 4"2
W. R. M O O R E A N D R. P. SHELDON
the ketones are somewhat lower than those previously reported" and the reason for this is not clear. As mentioned previously, some polymer, presumably of low molecular weight, was soluble in m-cresol. By taking into account the weights of precipitated and undissolved polymer and also a presumably proportional loss of m-cresol from the sample by extraction with ethanol, corrections were applied to the density and swelling of the undissolved polymer. Since the soluble fraction may behave differently from the insoluble and very low molecular weight polymer may not be precipitated by ethanol the corrected value given in Table 1 should be regarded as approximate. The development of crystallinity was always accompanied by increasing opacity of the film and, when absorbed solvent was removed, by increased brittleness. The latter effect has been previously noted 7 after prolonged heating in an aqueous dye bath at 100°C. Values of equilibrium swelling are also given in Table 1.
DISCUSSION
Figure 1 shows that the rate of increase of density and hence of crystallinity varies markedly with the liquid. In some cases crystallization is rapid while in others it occurs at a rate convenient for measurement. Figure 1 also shows that although molecular size may be a factor affecting the rate of crystallization it is not the only one. Dioxan, ethyl formate, acrylonitrile, m-cresol, aniline, nitromethane and nitroethane induce much more rapid crystallization than benzene, toluene, acetic acid, benzyl alcohol and dimethyl phthalate. Kinetic studies of this induced crystallization will be discussed in a later communication and are not further considered here, but it may be noted that the rate of disappearance of available amorphous content at any given time seems to follow a first order equation. The equilibrium values of density and crystallinity also vary with the liquid. The non-polar hexane and carbon tetrachloride do not induce crystallinity and neither do ethanol and n-butanol. The polarizable aromatic hydrocarbons benzene and toluene do so, however, and so does benzyl alcohol. Other liquids inducing crystallinity are polar but there seems to be no obvious relationship between the equilibrium density of the polymer and the polarity of the liquid. Results for acetone and n-amyl methyl ketone and for nitromethane and nitroethane suggest that molecular size may be a factor with liquids of the same type but this is not general. Table 1 shows that, in general, the greater the equilibrium density the greater the swelling of the polymer although density and swelling are not linearly related. Swelling may be regarded as a measure of the interaction between polymer and liquid and the development of crystaUinity may depend on such interaction. Figures 2 and 3 show equilibrium density and swelling respectively as functions of the solubility parameter 3 of the liquid. 3 =[(L, - R T ) / V ] } where Le is the molar latent heat of vaporization of the liquid and V is molar volume, both at the absolute temperature T. Values of 3, which are given in Table 1, were taken from published values 8' 9 or calculated using values of Le obtained by use of the Hildebrand rule 1°. The difference 318
THE CRYSTALLIZATION OF POLYETHYLENE TEREPHTHALATE between the solubility parameters of a liquid and a not too polar amorphous polymer can be related to the heat change in their mixing and the interaction of polymer and liquid will therefore depend, in part, on the solubility parameter of the latter.
"E 1"42 ..90 1-40
-~ i.3B
C Q
Figure
1-36
2 - - Equilibrium density as a function of solubility parameter of liquid
7
oJ
8
I
I
I
I
12
9 10 11 Solubility parameter
(cal/cm3) ~h
13
120 I00
80
~C
6003
Figure 3--Equilibrium swel-
>o 4 0 20
0
ling as a function of solubility parameter of liquid
7
8
9 10 11 Solubility parameter
12
13
(cal/cm3)Y2
F i g u r e 2 shows two maxima at 3 values of approximately 9 7 and 12-0 and similar maxima are seen in F i g u r e 3. The first maximum is associated with liquids such as ketones and esters which may be regarded as basic in the Lewis sense. Benzene and toluene, which are associated with this region of the plots, can also be regarded as basic u. The second maximum is associated with liquids of acidic type such as m-cresol, acetic acid and nitromethane. The existence of two maxima may be a consequence of the presence of basic carbonyl groups in the polymer and acidic hydrogen atoms in CH~ groups adjacent to oxygen atoms. Giles et al. 1~ have suggested that ester groups in polyethylene terephthalate may act as proton donors. Liquids containing acidic groups should solvate basic polymer groups and those containing basic groups should solvate acidic ones. Alternatively, dipolar attraction may lead to solvation by both types of liquid.
319
W. R. MOORE AND R. P. SHELDON Swelling will occur in liquids whose solubility parameters do not differ too greatly from that of the solvated polymer and be a maximum when these solubility parameters are equal or nearly so. The solubility parameter of the solvated polymer will presumably vary with the solvating liquid but variations for the polymer solvated by liquids of similar type may not be large. It has been pointed out that in eases of polymers containing both acidic and basic groups there will be two ranges of solubility parameter within which swelling or solution can occur, corresponding to the polymer solvated by basic and acidic liquids respectively 13. Behaviour of this type has been observed with secondary cellulose acetate TM. Dependence of induced crystallinity on solubility parameter of liquid has also been noted for crystallizable polymethyl methacrylate 1~. This correlation of equilibrium density and swelling with solubility parameter of liquid should perhaps be regarded as a n approximation and other factors may be operative. A number of the liquids used--ethanol, butanol, benzyl alcohol, m-cresol and acetic acid--are known to be associated. Latent heats of vaporization will often include energy required to overcome forces causing association and it is perhaps doubtful to what extent solubility parameters obtained from such latent heats can be related to forces between associated molecules in the liquid state. Association might account for the lack of swelling and density increase in ethanol and butanol. The marked effect of benzyl alcohol, with almost the same solubility parameter as butanol, may be due to the influence of the phenyl group. The points for aniline in Figures 2 and 3 fall amongst those for acidic liquids although aniline is generally regarded as basic. There is some evidence, however, that the hydrogen atoms of the NH~ group can take part in hydrogen bonding to basic carbonyl groups 1~. Reasons for the correlation between swelling and development of crystallinity are not entirely clear. It is possible that the solvated polymer imbibes the solvating liquid in swelling and that the imbibed liquid acts as a plasticizer or molecular lubricant, reducing internal stresses and permitting movement of the polymer chains at the temperature of the experiments and the adoption of more favourable conditions for the development of crystallinity. Here it may be relevant to note that the amount of dimethyl phthalate absorbed at equilibrium reduced the glass temperature of the polymer, determined by a dilatometric method, from 67 ° to 62°C. The value of 62 ° refers, of course, to the polymer after crystallization. The glass temperature of the amorphous polymer containing dimethyl phthalate may be much lower. If this picture of the action of liquids is correct it follows that the actual degree of crystallinity attainable may also depend on such aspects of the nature of the polymer as its molecular weight and previous thermal history. This may be supported by the fact that although aniline and m-cresol both cause high degrees of swelling the equilibrium level of crystallinity is not very different from lhat resulting from the use of other liquids, such as acetone, which cause smaller degrees of swelling.
We are grateful to Imperial Chemical Industries Ltd, Plastics Division, for the gift of polyethylene terephthalate film.
320
THE CRYSTALLIZATION
OF POLYETHYLENE TEREPHTHALATE
P o l y m e r R e s e a r c h Laboratories, D e p a r t m e n t o f C h e m i c a l Technology, Institute o f Technology, Brad[ord, Y o r k s . ( R e c e i v e d M a r c h 1961)
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