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Heat capacity study of linear polyurethanes
HEAT CAPACITY STUDY OF LINEAR POLYURETHANES* Yu. K. GODOVSKII and Yu. S. L]:PATOV High Polymer Chemical Institute, U.S.S.R. Academy Sciences (Received 5 September 1966)
A r~ARG~, group amongst the considerable number of urethane-type polymers is formed by the linear polyurethanes (PI~), which can crystallize. The X-ray studies of the formation and decomposition of the crystal structure of the P U based on hexamethylene diisocyanate and various low molecular weight (m.w.) glycols led to the conclusion that the main feature of the crystal structure is the distinct anisotropy of the crystal lattice, which is due to the considerable n u m b e r of hydrogen bonds present [1-5]. The crystal structure therefore consists of a number of fiat lattices, in which adjacent macromolecules are regularly linked by hydrogen bridges, while those superimposed on each other form a three-dimensional network linked by Van I)er Waals forces. The different nature of the bonds inside the lattices and between them results in an anisotropic decomposition of the crystal lattice, in which the Van Der Waals forces are gradually disrupted first; this is followed by a relatively quick disruption of the hydrogen bonds. The same investigations also established t h a t linear P U easily form pseudo-crystalline structures which can rearrange themselves, under certain conditions, into a three-dimensional network. On this basis it is thought that the main features of the melting and crystallization processes of linear P U are determined by the hydrogen bonds. These should naturally also affect the nature of the changes of thermodynamic properties during the formation and decomposition of crystal lattices, and in particular the temperature dependence of heat capacity, which is the subject of this investigation. Two linear P U of the following structures were used in our work: [-- C001~H(CH~)4NHCO0(CH2)6-- ]n-- PU
I
[-- COONH(CH2).NHC00 (CH~)~O(CI-I~)2--]~--P U I I The above polymers were produced from hexamethylene diisocyanate and tetramethylene glycol (PIg I), or diethylene glycol ( P U I I ) . These were selected because detailed studies had been made on them by X-ray methods. The apparatus and the methods used to measure the heat capacity and the various heat effects had been described earlier [6, 7]. The sample weights w e r e * Vysokomol. soyed. A10: No. 1, 32-40, 1968. 34
Heat capacity study of linear polyurethanes
35
0"6-0-9 g. The heating gradients were 1 and 2°C/rain. The calorimeter was cooled to --60°C with liquid nitrogen. The samples were quenched b y immersion in liquid nitrogen. EXPERIMENTAL
Temperature dependence of the heat capacity of quenched samples. This is shown in Fig. la as the heat of melting, Cp, of tempered samples. Sample P U I was produced b y isothermal crystallization at 173°C after heating the melt to 200°C. Sample P U I I was produced b y slow cooling (at a rate of about 0.8 °C/min) of the melt from 150°C to room temperature. This was then heated to ll0°C and again cooled slowly to room temperature. The analysis showed that C~ changed in the studied range as follows: PU I
-- 5 0 4 1 0 ° Cp----0.422+1.5× 10 -3 t; 45--120 ° C p = 0 . 4 9 5 + 1 . 8 5 × 10 -3 t; 195--210 ° Cp=0.665.
P U II from - - 5 0 t o - - 5 ° C ~ - - 0 . 4 2 2 + 1 . 5 × 10 -3 t; 50--100 ° C ~ = 0 . 5 1 2 + 1 . 5 × 1 0 -a t; 140--160 ° C~-~0.623. in which t----temperature, °C; C~-~heat capacity in cal/g.degree. The heat capacity curve of P U I showed a rise in the range 10-40°C. This increase was greatly accelerated above 120°C compared with the temperature dependence, and it reached a peak which corresponded with the melting of the crystalline formations. The m.p. of P U I was 183°C. The heat capacity-temperature curve of P U I I showed two distinct sectors of increase, the first in the range from --5 to +10°C, the second from 30 to 45°C. There was fairly sharp rise of Cp above 100°C and, in contrast with P U I, the progress was fairly complex in the melting range; this was apparent in the shape of the two peaks. The first peak was at 130, the second at 132.5°C. These two peaks indicated the occurrence of at least 2 consecutive processes, each of which represented a certain heat effect (Fig. lb). This was particularly clear from the experimental melting thermogram. The curve was used to calculate the temperature dependence of enthMpy as shown in Fig. lc. The zero value of enthalpy was taken as corresponding to --50°C. The above experimental results were used to calculate the heat of melting of the two P U samples (these did not take into account the crystallinity of the samples). The found values were 20.7 cal/g for P U I and 19.8 cal/g for P U I I . As separate melting stages could not be identified in the case of P U II, the heat of melting given here is equivalent to both the stages. Effect of tempering on heat capacity. The study of P U I melt tempering showed that 7 mm dia. cylindrical samples could not be produced from the amorphous phase. This appeared to be due to a rate of crystallization much larger than the rate of heat transfer. The references made in the literature to
Yr. K. Goi)ovsmi and Yu. S. LIPATOV
86
production of 4,6-PU in the amorphous state must be to samples in the shape of thin films. The temperature dependence of the heat capacity of tempered P U I I samples is shown in Fig. 2. I n the range of temperatures from --50 to --5°C Cp varies according to the function C ~ = 0 . 4 2 2 ÷ 1 . 5 × 1 0 -a t, which" is identical with t h a t of an quenched sample. I n the range from --5 to +10°C the heat capacity quickly increases from 0.427 to 0.623 and remains constant at this value up to 20°C. This behaviour corresponds with a transition from the glasslike to the highly elastic state. The rapid decrease after 20°C is, no doubt, connected with the crystallization of macromoleeules which had become sufficiently flexible after transition to a rubber-like state. C~ became slightly smaller after the completion of crystallization, compared with the same temperatures of the annealed sample; from 85°C onwards it started to decrease and passed through a minimum at 103°C. The course of the heat capacity in the range 50-110°C permitted us to think t h a t crystallization was slow at the start in the case of the tempered sample, and became quite distinct from 85°C onwards. The crystal structure of P U I I started to decompose above ll0°C and the melting process had two stages. The value of the heat capacity after melting was again 0.623 and remained constant during the subsequent temperature elevation. Worth noting was the fact that the Cp was the same in the melt and in the highly elastic state; this is evidence of the intermolecular reactions being of the same type in both the states. The heats of transitions of the tempered samples are given below: Process: Heat, cal/g
Crystallization I 15.3
Crystallization II 2.2
Melting 19.8
The numeral I signifies a process taking place after transition into the highly elastic state, I I the crystallization in the range 85-110°C. Analysis showed that the heat of melting was larger by 1.3 cal/g than the sum of the crystallization heats. This difference also seems to be due to slow crystallization in the range 50-85°C. We also examined samples obtained by slow cooling of the melt from 150°C to room temperature in a thermostat, which was then switched off and allowed to cool down naturally. The cooling rate in the range 110-90°C, in which P U I I crystallized, was about 0.8°C/rain. The examination of the samples produced in this manner showed the temperature dependence of heat capacity to be practically identical with t h a t of the annealed sample in the range --50 to +50°C, but with that of a tempered sample above -{-55°C. It was thus impossible to eliminate the crystallization processes in the range 50-110°C by slow cooling of the melt. This could be achieved only by a second heating of a slowly cooled sample to 110°C and then slowly cooling it again. These conditions, which we called tempering above, yielded samples with a course of heat capacity as a function of tern-
I
IO0
I
150
200 ~°O
0
z
+ z]t, mV
75 I00
125
150 T,°C
i
f~ b.._ r
0
ca//g
-50
50
tOO
150
50
c
100
2
150
FIG. la. Temperature dependence of C~: 1--1)U I; 2 - - P U II. b--Thermogram of 1)U I I melting and crystallization. c--Temperature dependence of enthalpy: 1 - - P U I, 2 - - P U II.
I
50
0
I
a
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2.0-
Op,col~degree .g
zlh
200 F,°C
If
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Yu. K. GODOVSKIIand Yu. S. LIPAmov
$8
perature shown in Fig. la. We should also dwell on the ratio of the heat of melting to that of crystallization of samples produced b y cooling the melt in a thermostat. This condition showed an exothermic effect on the thermograms in the range 110-90°C; this corresponded to crystallization and amounted to 12.6 cal/g. The same quantity was also found on crystallization from the melt in isothermal conditions (these results form the subject of a separate paper). The heat of melting of such samples, determined immediately after cooling to room temperature, was 19.8 cal/g. The calculation of the crystallization heat from this in the heating range from 50-110°C gave a difference of 3.7 cal/g, which indicated the existence of fairly large heats of secondary crystallization in P U I I . X - r a y diffraction pictures were taken of the sample produced b y cooling from the melt, and this is shown in Fig. 3. This t y p e of scattering can be attributed more to a pseudo-crystalline than to a true crystalline state. It indicates the greater possibilities of secondary crystallization processes in this type of structure. We also wish to point out that a similar X-ray picture had been obtained also with a sample subjected to isothermal crystallization, followed b y slow cooling to room temperature.
Cp,:c ~//deqme.g ZO
I-5
?,q
?
/5o ~ °o F1G. 2
FIo. 3
FIG. 2. Temperature dependence of the C~ of annealed P U II. FIG. S. X-ray diffraction picture of P U I I .
Vitrification studies. The effect of the degree of crystallinity of P U I I on the quantity of heat involved in the transition range from the glass-like to the highly elastic state was of considerable interest as also was a detailed analysis of the G~ changes of an annealed sample in the range showing two of its peaks. The P U I I melt was cooled and was crystallized at 22°C for a certain time under iso-
Heat capacity study of linear polyurethanes
39
thermal conditions, and was then tempered again. This method made it possible to control the degree of crystallinity after vitrification of the tempered sample. Figure 4a shows the variation with temperature of the heat capacity of samples subjected to two temperings, and also of a chilled sample. A suitable w a y of analysing vitrification is to examine the C J T function during changing temperature.
07
~Z
I
o.3 -
\71I
I
I
~°C
cz/7-, 1o~,ccWdegpee23 23
1
21 19 17 15 250
I
I
270
2#0
I
3fJ
3~8 T,°K
FIG. 4. Temperature dependence: a--of C~ in the temperature range of PU II vitrification after double annealing; b--encraty at Q equal to: 1 - 0,2- 7.6, 3 - 9.5, d - 14.35, 5-16.3, 6 - - P U I.
Dole et al. [8] suggested for it the name encraty and the symbol L. Figure 4b shows its temperature dependence. The term Q used in the legend to this Figure is the heat quantity in cal/g established before the second tempering. The Table contains the peak values of the encraty changes AL in the range of vitrification the glass temperature Tg, which was determined as that at which encraty had half its peak value, and also the 3Cp values, essential to a comparison of the experimental with the theoretical results. The whole of the experimental results shown make it clear that increasing crystallinity of the sample caused Tg to shift towards higher temperatures and the temperature range to broaden. The results for the annealed sample are given in the Table separately for the first and second temperature elevation on the tem-
Yu. K. GODOVSKII and YU. S. LIP)~TOV
4O
perature-heat capacity curve. The sample which was tempered only once, with Q--0, a n d considered completely amorphous, had its heat capacity changes during vitrification calculated per mole of chain units, which is in accordance with the Wunderlieh theory [9]. The Cp change during vitrification was 0.196 cal/degree, g in our case. The m.w. of the repeating unit of the P U I I macromolecule was 274.3. According to the Wunderlich theory [9], this segment contains 17 kinetic units. On this basis ~16.3×0.196~3.16 cal/degree.mole. This was obtained b y Wunderlieh on the basis of the Hirai and Eyring [10] hole theory of vitrification and agreed well with the value of 2.7 cal/degree.mole, obtained in the analysis of information on a fairly large number of monomers and polymers in the glass-like state.
AC'p-~M/17×AC~-~I.zIC~
Tg, AL AND Cp VALUES Polymer
Q' cal/g.
PU I PU II
AL, cal/degree2 × ,~Cp, × g × 10~ cal/degree.g
Tg
2.8 6.6 4.6 4.1 3'1 1"9 1.1
22
0 7.6 9"5 14.35 16.3
---
1 I
0 2
4 43
0"128 0"196 0"144 0"131 0.108 0.091 0.054
Figure 5 shows the heat capacity increments during transition of the sample from the glass- to the rubber-like state as a function of the degree of crystallinity, characterized b y the corresponding heat quantity. The response line is almost
ACp,cal/degrve.g 0.2~ . ~ . . . ~
o
5
1o
15a,~z,,j
FIG. 5. Dependence of ztC~ on Q for PU II.
ACp -----0,
linear. Assuming that this linear relationship can be extended to the intersect of the response line with the abscissa gives Q=32.8 cal/g. This value should correspond with that of a sample showing no heat capacity increment in
Heat capacity study of linear polyurethanes
41
the vitrification range. I t can be used to assess the crystalliuity in the temperature range 20-40°C. This assumption is based on two factors, i.e. preservation of linearity to a zero value of AC~and, that the latter should be zero for a completely crystalline sample. RESULTS
B y analysing the temperature dependence of the heat capacity of the studied polyurethanes, we found that the difference in the behaviour of P U I and P U I I is particularly clear in the transition ranges, especially as far as the dynamics of transition are concerned, without characterizing their quantitative values. The encraty changes during P U I vitrification are thus 2.8 encratic units (see Table) and the total value for the first and second peak was 3.0 in the case of P U II. The transition kinetics also differed substantially in this temperature range, and the same occurred during melting (see Fig. la). Let us now examine the transitions of P U I I in the range --5 to ~45°C. The displacement of Tg with increasing crystallinity at higher temperatures and the broadening of the range of vitrification point to a gradual structural change of the amorphous regions. I n addition to the completely amorphous regions, there appear to be some pseudo-amorphous regions, which are responsible for the indicated t y p e of transition. The fact that the first heat capacity increase lay in the same temperature range in the case of tempered and annealed samples indicated that non-crystalline regions were present in both. The somewhat unexpected appearance of the second peak on the annealed sample in the range 30-45°C was similar in nature to the first. We believe the appearance of the second peak to be due to extra mobility acquired b y the P U I I macromolecular segments. As P U I had only a single peak in approximately the same temperature range, it is natural to regard the two-peak nature of P U I I vitrification to be associated with the presence of ether oxygen in its macromolecules. This raises the question of whether the second peak will be evident when using samples having incomplete crystallization after tempering, or whether this is a feature of the annealed sample only. This could not be experimentally verified because of the crystallization which occurred in these samples in this particular temperature range of the second peak (see Fig. 4). The heat capacities after crystallization were smaller than in the annealed samples at the same temperatures, but we are inclined to explain this fact b y a slower crystallization. I t can be also explained, b y the absence of a second temperature increase on the curve of the annealed sample. We shall now examine the characteristics of the melting and crystallization processes. In the case of the annealed P U I a broad melting range is noticeable. At 130-160°C there is a relatively slow increase of heat capacity, followed b y a sharp peak. This behaviour-can be associated with the anisotropic nature of the crystal lattice; the first region of Cp increase is equivalent to a gradual disappearance of the Van Der Waals bonds, while the second is due to relatively fast disintegration of the hydrogen bonds which link the macromolecules into a flat network.
42
Yu. K. GODOVSKIIand Yu. S. LIPATOV
A narrower melting range can be seen with the annealed PU II, but it also shows two peaks. It must be stressed that the 2 peaks in the melting range can also be found with other PU II samples, i.e. the annealed sample and the one produced by gradual cooling of the melt. This result is thus independent of the type of thermal treatment. We believe the presence of the second process during PU II melting to be the result of crystallization in the range 50-110°C, or more precisely 85-110°C. The total heat of crystallization at these temperatures is about 3.5 cal/g, and 2.2 cal/g of it is associated with the process in the range 80-110°C. The relationship between the heats of melting is such that the second stage consists of about 15-17% of the total, i.e. 3-3.4 cal/g. This is almost exactly the value of the heat of crystallization. The above experimental findings and the evaluation of the crystallization and melting of the annealed P U I I permit the following interpretation: a crystal structure is formed, after passage through the crystallization range, at 20-45°C; this is mainly due to the hydrogen bond formation in volume, i.e. a pseudo-crystalline structure is formed. The order gradually increases in the pseudocrystals above 50°C and ester bonds become ordered in the range 85-110°C. A gradual disintegration of the crystal lattice starts at the same time. The first peak on the melting curve corresponds with the disintegration of the main crystal mass, while the second peak is connected with the disintegration of the crystal regions formed by ether bonds. This scheme was fully confirmed formally by the X-ray studies of P U I I made by Vasil'ev and Tarakanov [4]. The diffraction curves of the annealed samples, obtained as a result of crystallization up to 70°C, indicate the presence of only pseudoerystalline structures. A further increase of temperature produced a great improvement of the order inside the pseudoCrystals and simultaneously resulted in the appearance of regular packing along the main axis of the macromoTecules. The good correlation between the results given in [4] and ours can be seen in the range 90-110°C, in which we assume ordering of the ether bonds. The melting- and crystallization processes in samples produced by gradual cooling of the melt are similar and the only difference is that the pseudoerystals form immediately on cooling the melt from 110-90°C (Fig. lb). The sum of the experimental findings permit the conclusion that the kinetics of transitions in the studied types of polyurethane strongly depend on the nature of the ether bonds. The difference in the nature of the formation and disintegration processes of the crystal structure and vitrification of P U I I , compared with PU I, is obviously the result of increase chain flexibility due to the introduction of addition of oxygen atoms into the main chain. The experimental findings on the heat capacity of linear polyurethanes also indicate that their characteristics are not due entirely to the presence of a large number of hydrogen bonds. The macromoleeular structure plays an important part, primarily the ratio of urethane to other macromolecular bonds.
Photoelasticity of three-dimensional copolymers of styrene
43
CONCLUSIONS
(1) T h e t e m p e r a t u r e d e p e n d e n c e of Cp of t w o linear p o l y u r e t h a n e s b a s e d on h e x a m e t h y l e n e d i i s o c y a n a t e a n d t e t r a m e t h y l e n e glycol, or d i e t h y l e n e glycol was s t u d i e d in t h e r a n g e f r o m -50 to 200°C; t h e i r characteristics a n d b e h a v i o u r were established, a n d t h e r a n g e s of t e m p e r a t u r e a n d h e a t of t r a n s i t i o n were determined. (2) T h e vitrification kinetics of a m o r p h o u s a n d crystalline P U b a s e d on die t h y l e n e glycol showed this process to p r o c e e d in t w o stages for t h e crystalline polymer. (3) T h e influence of t h e r m a l t r e a t m e n t conditions on t h e n a t u r e of t h e m e l t i n g a n d c r y s t a l l i z a t i o n processes .was i n v e s t i g a t e d . (4) I t was f o u n d t h a t t h e differences in t h e t r a n s i t i o n kinetics of t h e p o l y u r e thalms s t u d i e d were due to t h e different s t r u c t u r e s of t h e e t h e r b o n d s p r e s e n t in t h e m . Translated by K. A. ALLE~ REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
E. JENCKEL and E. KLEIN, Kolloid-Z. 118: 86, 1950 H. G. KILIAN and E. JENCKEL, IZolloid.-Z. 165: 25, 1959 H. G. KILIAN, Kolloid.-Z. 176: 49, 1961 B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soyed. 6: 2189, 1964 (Translated in Polymer Sei. U.S.S.R. 6: 12, 2423, 1964) B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soyed. 6: 2193, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 12, 2427, 1964) Yu. K. GODOVSKII and Yu. P. BARSKH, Plast. Massy 570. 7, p. 57, 1965 Yu. K. GODOVSKII, Dissertation, 1965 J. ABU-ISA, V. A. CRAWFORD, A. R. HALY and M. DOLE, J. Polymer Sci. C6: 149, 1964 B. WUNDERLICH, J. Phys. Chem. 64: 1052, 1960 N. HIRAI and H. EYRING, J. Appl. Phys. 29: 810, 1958
THE PHOTOELASTICITY OF THREE-DIMENSIONAL COPOLYMERS OF STYRENE* M. S. ZLOT~IKOV, I. A. ARBUZOVA a n d YE. V. KUVSItI~SKII High Molecular Weight Compounds Institute, U.S.S.R. Academy of Sciences (Received 26 September 1966)
DETERMI~ATI01~ of t h e birefringence of solutions of high m o l e c u l a r w e i g h t c o m p o u n d s , a n d of p o l y m e r films generally, involves a s t u d y of t h e c o n f o r m a t i o n al peculiarities of m a c r o m o l e c u l e s [1], a n d it is o n l y w i t h b u l k p o l y m e r s , b e c a u s e * Vysokomol. soyed. A10: No. 1, 41-45, 1968.
A r~ARG~, group amongst the considerable number of urethane-type polymers is formed by the linear polyurethanes (PI~), which can crystallize. The X-ray studies of the formation and decomposition of the crystal structure of the P U based on hexamethylene diisocyanate and various low molecular weight (m.w.) glycols led to the conclusion that the main feature of the crystal structure is the distinct anisotropy of the crystal lattice, which is due to the considerable n u m b e r of hydrogen bonds present [1-5]. The crystal structure therefore consists of a number of fiat lattices, in which adjacent macromolecules are regularly linked by hydrogen bridges, while those superimposed on each other form a three-dimensional network linked by Van I)er Waals forces. The different nature of the bonds inside the lattices and between them results in an anisotropic decomposition of the crystal lattice, in which the Van Der Waals forces are gradually disrupted first; this is followed by a relatively quick disruption of the hydrogen bonds. The same investigations also established t h a t linear P U easily form pseudo-crystalline structures which can rearrange themselves, under certain conditions, into a three-dimensional network. On this basis it is thought that the main features of the melting and crystallization processes of linear P U are determined by the hydrogen bonds. These should naturally also affect the nature of the changes of thermodynamic properties during the formation and decomposition of crystal lattices, and in particular the temperature dependence of heat capacity, which is the subject of this investigation. Two linear P U of the following structures were used in our work: [-- C001~H(CH~)4NHCO0(CH2)6-- ]n-- PU
I
[-- COONH(CH2).NHC00 (CH~)~O(CI-I~)2--]~--P U I I The above polymers were produced from hexamethylene diisocyanate and tetramethylene glycol (PIg I), or diethylene glycol ( P U I I ) . These were selected because detailed studies had been made on them by X-ray methods. The apparatus and the methods used to measure the heat capacity and the various heat effects had been described earlier [6, 7]. The sample weights w e r e * Vysokomol. soyed. A10: No. 1, 32-40, 1968. 34
Heat capacity study of linear polyurethanes
35
0"6-0-9 g. The heating gradients were 1 and 2°C/rain. The calorimeter was cooled to --60°C with liquid nitrogen. The samples were quenched b y immersion in liquid nitrogen. EXPERIMENTAL
Temperature dependence of the heat capacity of quenched samples. This is shown in Fig. la as the heat of melting, Cp, of tempered samples. Sample P U I was produced b y isothermal crystallization at 173°C after heating the melt to 200°C. Sample P U I I was produced b y slow cooling (at a rate of about 0.8 °C/min) of the melt from 150°C to room temperature. This was then heated to ll0°C and again cooled slowly to room temperature. The analysis showed that C~ changed in the studied range as follows: PU I
-- 5 0 4 1 0 ° Cp----0.422+1.5× 10 -3 t; 45--120 ° C p = 0 . 4 9 5 + 1 . 8 5 × 10 -3 t; 195--210 ° Cp=0.665.
P U II from - - 5 0 t o - - 5 ° C ~ - - 0 . 4 2 2 + 1 . 5 × 10 -3 t; 50--100 ° C ~ = 0 . 5 1 2 + 1 . 5 × 1 0 -a t; 140--160 ° C~-~0.623. in which t----temperature, °C; C~-~heat capacity in cal/g.degree. The heat capacity curve of P U I showed a rise in the range 10-40°C. This increase was greatly accelerated above 120°C compared with the temperature dependence, and it reached a peak which corresponded with the melting of the crystalline formations. The m.p. of P U I was 183°C. The heat capacity-temperature curve of P U I I showed two distinct sectors of increase, the first in the range from --5 to +10°C, the second from 30 to 45°C. There was fairly sharp rise of Cp above 100°C and, in contrast with P U I, the progress was fairly complex in the melting range; this was apparent in the shape of the two peaks. The first peak was at 130, the second at 132.5°C. These two peaks indicated the occurrence of at least 2 consecutive processes, each of which represented a certain heat effect (Fig. lb). This was particularly clear from the experimental melting thermogram. The curve was used to calculate the temperature dependence of enthMpy as shown in Fig. lc. The zero value of enthalpy was taken as corresponding to --50°C. The above experimental results were used to calculate the heat of melting of the two P U samples (these did not take into account the crystallinity of the samples). The found values were 20.7 cal/g for P U I and 19.8 cal/g for P U I I . As separate melting stages could not be identified in the case of P U II, the heat of melting given here is equivalent to both the stages. Effect of tempering on heat capacity. The study of P U I melt tempering showed that 7 mm dia. cylindrical samples could not be produced from the amorphous phase. This appeared to be due to a rate of crystallization much larger than the rate of heat transfer. The references made in the literature to
Yr. K. Goi)ovsmi and Yu. S. LIPATOV
86
production of 4,6-PU in the amorphous state must be to samples in the shape of thin films. The temperature dependence of the heat capacity of tempered P U I I samples is shown in Fig. 2. I n the range of temperatures from --50 to --5°C Cp varies according to the function C ~ = 0 . 4 2 2 ÷ 1 . 5 × 1 0 -a t, which" is identical with t h a t of an quenched sample. I n the range from --5 to +10°C the heat capacity quickly increases from 0.427 to 0.623 and remains constant at this value up to 20°C. This behaviour corresponds with a transition from the glasslike to the highly elastic state. The rapid decrease after 20°C is, no doubt, connected with the crystallization of macromoleeules which had become sufficiently flexible after transition to a rubber-like state. C~ became slightly smaller after the completion of crystallization, compared with the same temperatures of the annealed sample; from 85°C onwards it started to decrease and passed through a minimum at 103°C. The course of the heat capacity in the range 50-110°C permitted us to think t h a t crystallization was slow at the start in the case of the tempered sample, and became quite distinct from 85°C onwards. The crystal structure of P U I I started to decompose above ll0°C and the melting process had two stages. The value of the heat capacity after melting was again 0.623 and remained constant during the subsequent temperature elevation. Worth noting was the fact that the Cp was the same in the melt and in the highly elastic state; this is evidence of the intermolecular reactions being of the same type in both the states. The heats of transitions of the tempered samples are given below: Process: Heat, cal/g
Crystallization I 15.3
Crystallization II 2.2
Melting 19.8
The numeral I signifies a process taking place after transition into the highly elastic state, I I the crystallization in the range 85-110°C. Analysis showed that the heat of melting was larger by 1.3 cal/g than the sum of the crystallization heats. This difference also seems to be due to slow crystallization in the range 50-85°C. We also examined samples obtained by slow cooling of the melt from 150°C to room temperature in a thermostat, which was then switched off and allowed to cool down naturally. The cooling rate in the range 110-90°C, in which P U I I crystallized, was about 0.8°C/rain. The examination of the samples produced in this manner showed the temperature dependence of heat capacity to be practically identical with t h a t of the annealed sample in the range --50 to +50°C, but with that of a tempered sample above -{-55°C. It was thus impossible to eliminate the crystallization processes in the range 50-110°C by slow cooling of the melt. This could be achieved only by a second heating of a slowly cooled sample to 110°C and then slowly cooling it again. These conditions, which we called tempering above, yielded samples with a course of heat capacity as a function of tern-
I
IO0
I
150
200 ~°O
0
z
+ z]t, mV
75 I00
125
150 T,°C
i
f~ b.._ r
0
ca//g
-50
50
tOO
150
50
c
100
2
150
FIG. la. Temperature dependence of C~: 1--1)U I; 2 - - P U II. b--Thermogram of 1)U I I melting and crystallization. c--Temperature dependence of enthalpy: 1 - - P U I, 2 - - P U II.
I
50
0
I
a
-50
/.0
2.0-
Op,col~degree .g
zlh
200 F,°C
If
#
~..
Yu. K. GODOVSKIIand Yu. S. LIPAmov
$8
perature shown in Fig. la. We should also dwell on the ratio of the heat of melting to that of crystallization of samples produced b y cooling the melt in a thermostat. This condition showed an exothermic effect on the thermograms in the range 110-90°C; this corresponded to crystallization and amounted to 12.6 cal/g. The same quantity was also found on crystallization from the melt in isothermal conditions (these results form the subject of a separate paper). The heat of melting of such samples, determined immediately after cooling to room temperature, was 19.8 cal/g. The calculation of the crystallization heat from this in the heating range from 50-110°C gave a difference of 3.7 cal/g, which indicated the existence of fairly large heats of secondary crystallization in P U I I . X - r a y diffraction pictures were taken of the sample produced b y cooling from the melt, and this is shown in Fig. 3. This t y p e of scattering can be attributed more to a pseudo-crystalline than to a true crystalline state. It indicates the greater possibilities of secondary crystallization processes in this type of structure. We also wish to point out that a similar X-ray picture had been obtained also with a sample subjected to isothermal crystallization, followed b y slow cooling to room temperature.
Cp,:c ~//deqme.g ZO
I-5
?,q
?
/5o ~ °o F1G. 2
FIo. 3
FIG. 2. Temperature dependence of the C~ of annealed P U II. FIG. S. X-ray diffraction picture of P U I I .
Vitrification studies. The effect of the degree of crystallinity of P U I I on the quantity of heat involved in the transition range from the glass-like to the highly elastic state was of considerable interest as also was a detailed analysis of the G~ changes of an annealed sample in the range showing two of its peaks. The P U I I melt was cooled and was crystallized at 22°C for a certain time under iso-
Heat capacity study of linear polyurethanes
39
thermal conditions, and was then tempered again. This method made it possible to control the degree of crystallinity after vitrification of the tempered sample. Figure 4a shows the variation with temperature of the heat capacity of samples subjected to two temperings, and also of a chilled sample. A suitable w a y of analysing vitrification is to examine the C J T function during changing temperature.
07
~Z
I
o.3 -
\71I
I
I
~°C
cz/7-, 1o~,ccWdegpee23 23
1
21 19 17 15 250
I
I
270
2#0
I
3fJ
3~8 T,°K
FIG. 4. Temperature dependence: a--of C~ in the temperature range of PU II vitrification after double annealing; b--encraty at Q equal to: 1 - 0,2- 7.6, 3 - 9.5, d - 14.35, 5-16.3, 6 - - P U I.
Dole et al. [8] suggested for it the name encraty and the symbol L. Figure 4b shows its temperature dependence. The term Q used in the legend to this Figure is the heat quantity in cal/g established before the second tempering. The Table contains the peak values of the encraty changes AL in the range of vitrification the glass temperature Tg, which was determined as that at which encraty had half its peak value, and also the 3Cp values, essential to a comparison of the experimental with the theoretical results. The whole of the experimental results shown make it clear that increasing crystallinity of the sample caused Tg to shift towards higher temperatures and the temperature range to broaden. The results for the annealed sample are given in the Table separately for the first and second temperature elevation on the tem-
Yu. K. GODOVSKII and YU. S. LIP)~TOV
4O
perature-heat capacity curve. The sample which was tempered only once, with Q--0, a n d considered completely amorphous, had its heat capacity changes during vitrification calculated per mole of chain units, which is in accordance with the Wunderlieh theory [9]. The Cp change during vitrification was 0.196 cal/degree, g in our case. The m.w. of the repeating unit of the P U I I macromolecule was 274.3. According to the Wunderlich theory [9], this segment contains 17 kinetic units. On this basis ~16.3×0.196~3.16 cal/degree.mole. This was obtained b y Wunderlieh on the basis of the Hirai and Eyring [10] hole theory of vitrification and agreed well with the value of 2.7 cal/degree.mole, obtained in the analysis of information on a fairly large number of monomers and polymers in the glass-like state.
AC'p-~M/17×AC~-~I.zIC~
Tg, AL AND Cp VALUES Polymer
Q' cal/g.
PU I PU II
AL, cal/degree2 × ,~Cp, × g × 10~ cal/degree.g
Tg
2.8 6.6 4.6 4.1 3'1 1"9 1.1
22
0 7.6 9"5 14.35 16.3
---
1 I
0 2
4 43
0"128 0"196 0"144 0"131 0.108 0.091 0.054
Figure 5 shows the heat capacity increments during transition of the sample from the glass- to the rubber-like state as a function of the degree of crystallinity, characterized b y the corresponding heat quantity. The response line is almost
ACp,cal/degrve.g 0.2~ . ~ . . . ~
o
5
1o
15a,~z,,j
FIG. 5. Dependence of ztC~ on Q for PU II.
ACp -----0,
linear. Assuming that this linear relationship can be extended to the intersect of the response line with the abscissa gives Q=32.8 cal/g. This value should correspond with that of a sample showing no heat capacity increment in
Heat capacity study of linear polyurethanes
41
the vitrification range. I t can be used to assess the crystalliuity in the temperature range 20-40°C. This assumption is based on two factors, i.e. preservation of linearity to a zero value of AC~and, that the latter should be zero for a completely crystalline sample. RESULTS
B y analysing the temperature dependence of the heat capacity of the studied polyurethanes, we found that the difference in the behaviour of P U I and P U I I is particularly clear in the transition ranges, especially as far as the dynamics of transition are concerned, without characterizing their quantitative values. The encraty changes during P U I vitrification are thus 2.8 encratic units (see Table) and the total value for the first and second peak was 3.0 in the case of P U II. The transition kinetics also differed substantially in this temperature range, and the same occurred during melting (see Fig. la). Let us now examine the transitions of P U I I in the range --5 to ~45°C. The displacement of Tg with increasing crystallinity at higher temperatures and the broadening of the range of vitrification point to a gradual structural change of the amorphous regions. I n addition to the completely amorphous regions, there appear to be some pseudo-amorphous regions, which are responsible for the indicated t y p e of transition. The fact that the first heat capacity increase lay in the same temperature range in the case of tempered and annealed samples indicated that non-crystalline regions were present in both. The somewhat unexpected appearance of the second peak on the annealed sample in the range 30-45°C was similar in nature to the first. We believe the appearance of the second peak to be due to extra mobility acquired b y the P U I I macromolecular segments. As P U I had only a single peak in approximately the same temperature range, it is natural to regard the two-peak nature of P U I I vitrification to be associated with the presence of ether oxygen in its macromolecules. This raises the question of whether the second peak will be evident when using samples having incomplete crystallization after tempering, or whether this is a feature of the annealed sample only. This could not be experimentally verified because of the crystallization which occurred in these samples in this particular temperature range of the second peak (see Fig. 4). The heat capacities after crystallization were smaller than in the annealed samples at the same temperatures, but we are inclined to explain this fact b y a slower crystallization. I t can be also explained, b y the absence of a second temperature increase on the curve of the annealed sample. We shall now examine the characteristics of the melting and crystallization processes. In the case of the annealed P U I a broad melting range is noticeable. At 130-160°C there is a relatively slow increase of heat capacity, followed b y a sharp peak. This behaviour-can be associated with the anisotropic nature of the crystal lattice; the first region of Cp increase is equivalent to a gradual disappearance of the Van Der Waals bonds, while the second is due to relatively fast disintegration of the hydrogen bonds which link the macromolecules into a flat network.
42
Yu. K. GODOVSKIIand Yu. S. LIPATOV
A narrower melting range can be seen with the annealed PU II, but it also shows two peaks. It must be stressed that the 2 peaks in the melting range can also be found with other PU II samples, i.e. the annealed sample and the one produced by gradual cooling of the melt. This result is thus independent of the type of thermal treatment. We believe the presence of the second process during PU II melting to be the result of crystallization in the range 50-110°C, or more precisely 85-110°C. The total heat of crystallization at these temperatures is about 3.5 cal/g, and 2.2 cal/g of it is associated with the process in the range 80-110°C. The relationship between the heats of melting is such that the second stage consists of about 15-17% of the total, i.e. 3-3.4 cal/g. This is almost exactly the value of the heat of crystallization. The above experimental findings and the evaluation of the crystallization and melting of the annealed P U I I permit the following interpretation: a crystal structure is formed, after passage through the crystallization range, at 20-45°C; this is mainly due to the hydrogen bond formation in volume, i.e. a pseudo-crystalline structure is formed. The order gradually increases in the pseudocrystals above 50°C and ester bonds become ordered in the range 85-110°C. A gradual disintegration of the crystal lattice starts at the same time. The first peak on the melting curve corresponds with the disintegration of the main crystal mass, while the second peak is connected with the disintegration of the crystal regions formed by ether bonds. This scheme was fully confirmed formally by the X-ray studies of P U I I made by Vasil'ev and Tarakanov [4]. The diffraction curves of the annealed samples, obtained as a result of crystallization up to 70°C, indicate the presence of only pseudoerystalline structures. A further increase of temperature produced a great improvement of the order inside the pseudoCrystals and simultaneously resulted in the appearance of regular packing along the main axis of the macromoTecules. The good correlation between the results given in [4] and ours can be seen in the range 90-110°C, in which we assume ordering of the ether bonds. The melting- and crystallization processes in samples produced by gradual cooling of the melt are similar and the only difference is that the pseudoerystals form immediately on cooling the melt from 110-90°C (Fig. lb). The sum of the experimental findings permit the conclusion that the kinetics of transitions in the studied types of polyurethane strongly depend on the nature of the ether bonds. The difference in the nature of the formation and disintegration processes of the crystal structure and vitrification of P U I I , compared with PU I, is obviously the result of increase chain flexibility due to the introduction of addition of oxygen atoms into the main chain. The experimental findings on the heat capacity of linear polyurethanes also indicate that their characteristics are not due entirely to the presence of a large number of hydrogen bonds. The macromoleeular structure plays an important part, primarily the ratio of urethane to other macromolecular bonds.
Photoelasticity of three-dimensional copolymers of styrene
43
CONCLUSIONS
(1) T h e t e m p e r a t u r e d e p e n d e n c e of Cp of t w o linear p o l y u r e t h a n e s b a s e d on h e x a m e t h y l e n e d i i s o c y a n a t e a n d t e t r a m e t h y l e n e glycol, or d i e t h y l e n e glycol was s t u d i e d in t h e r a n g e f r o m -50 to 200°C; t h e i r characteristics a n d b e h a v i o u r were established, a n d t h e r a n g e s of t e m p e r a t u r e a n d h e a t of t r a n s i t i o n were determined. (2) T h e vitrification kinetics of a m o r p h o u s a n d crystalline P U b a s e d on die t h y l e n e glycol showed this process to p r o c e e d in t w o stages for t h e crystalline polymer. (3) T h e influence of t h e r m a l t r e a t m e n t conditions on t h e n a t u r e of t h e m e l t i n g a n d c r y s t a l l i z a t i o n processes .was i n v e s t i g a t e d . (4) I t was f o u n d t h a t t h e differences in t h e t r a n s i t i o n kinetics of t h e p o l y u r e thalms s t u d i e d were due to t h e different s t r u c t u r e s of t h e e t h e r b o n d s p r e s e n t in t h e m . Translated by K. A. ALLE~ REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
E. JENCKEL and E. KLEIN, Kolloid-Z. 118: 86, 1950 H. G. KILIAN and E. JENCKEL, IZolloid.-Z. 165: 25, 1959 H. G. KILIAN, Kolloid.-Z. 176: 49, 1961 B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soyed. 6: 2189, 1964 (Translated in Polymer Sei. U.S.S.R. 6: 12, 2423, 1964) B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soyed. 6: 2193, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 12, 2427, 1964) Yu. K. GODOVSKII and Yu. P. BARSKH, Plast. Massy 570. 7, p. 57, 1965 Yu. K. GODOVSKII, Dissertation, 1965 J. ABU-ISA, V. A. CRAWFORD, A. R. HALY and M. DOLE, J. Polymer Sci. C6: 149, 1964 B. WUNDERLICH, J. Phys. Chem. 64: 1052, 1960 N. HIRAI and H. EYRING, J. Appl. Phys. 29: 810, 1958
THE PHOTOELASTICITY OF THREE-DIMENSIONAL COPOLYMERS OF STYRENE* M. S. ZLOT~IKOV, I. A. ARBUZOVA a n d YE. V. KUVSItI~SKII High Molecular Weight Compounds Institute, U.S.S.R. Academy of Sciences (Received 26 September 1966)
DETERMI~ATI01~ of t h e birefringence of solutions of high m o l e c u l a r w e i g h t c o m p o u n d s , a n d of p o l y m e r films generally, involves a s t u d y of t h e c o n f o r m a t i o n al peculiarities of m a c r o m o l e c u l e s [1], a n d it is o n l y w i t h b u l k p o l y m e r s , b e c a u s e * Vysokomol. soyed. A10: No. 1, 41-45, 1968.