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Calorimetric study of oligoethyleneglycoladipates and linear and crosslinked polyurethanes synthesized from them
266
v.P.
PRIVALKO et aZ. REFERENCES
1. E. Ya. DEVIRTS, L. S. ZHEREBKOVA and A. S. NOVIKOV, Kauchuk i rezina, No. 3, 14, 1967 2. R. PUMMERER and W. Gt~NDEL, Rubber Chem. and Technol. 2: 373, 1929 3. R. F. MARTEL and E. D. SMITH, Rubber Chem. and Technol. 35: 141, 1962 4. R. F. MARTEL and D. E. SMITH, Rubber Chem. and Technol. 34: 658, 1961 5. P. J. FLORY and J. REHNER, Industr. and Engng. Chem. 38: 500, 1946 6. J. BRUNI and LEIGER, Atti accadi Lincei, [6] 5: 823, 1927 7. G. SLOMP and J. G. LINDBERG, Analyt. Chem. 39: 60, 1967
CALORIMETRIC
STUDY
AND LINEAR
OF
OLIGOETHYLENEGLYCOLADIPATES
AND CROSSLINKED
SYNTHESIZED
FROM
POLYURETHANES THEM*
V. P. PRIVALKO, Y r . S. LII'ATOV and Y r . Y~. KERCHA Institute of the Chemistry of High Molecular Weight Compounds, Ukr.S.S.R. Academy of Sciences (Received 26 February 1968)
IT IS well known that the physical and mechanical properties of crosslinked polyurethanes are determined by their molecular structure. The alternation in the macromolecular chain of oligoester and isocyanate units of variable flexibility and the presence of chemical crosslinks and polar functional groups capable of physical reaction is reflected to a certain extent in the macroscopic properties of polyurethanes, such as crystallization, temperature and phase transition ranges, etc. Most of the papers previously published [1-6] have dealt only with linear polyurethanes. In the best known studies concerning crosslinked polyurethanes [7-13] only Muller et al. [8, 9] used quantitative calorimetry; however, the value of their results is considerably reduced because no report is given of the actual chemical structure of the polyurethanes. It is therefore of interest to study systems in which the chemical nature of components remaining unchanged, their individual physical parameters (chain flexibility, network density of physical and chemical bonds) vary successively. In this case from the variation of the thermodynamic properties of the system (heat capacity, enthalpy) the effect of these parameters on variation can be evaluated. For this purpose we carried out a calorimetric investigation of the systematic series, oligoester-linear polyurethane-crosslinked polyurethane elastomer, which made it possible to define more accurately and develop further the qualitative results obtained in a former study [13]. * Vysokomol. soyod. All: No. 1, 237-246, 1969.
Calorimetric study of oligoethyleneglycoladipates
267
EXPERIMENTAL T h e following w e r e u s e d for t h e i n v e s t i g a t i o n : 1) o l i g o m e r s o f e t h y l o n o g l y c o l a d i p a t e s * o f m o l e c u l a r w ~ g h t s 1000 a n d 2000 ( E A - 1 0 0 0 and EA-2000 respectively). Molecular weights were determined from the concentration of hydroxyl end groups; 2) o l i g o u r e t h a n e s f r o m E A - 1 0 0 0 a n d E A - 2 0 0 0 a n d 2 , 4 - t o l u y l e n e d i - i s o c y a n a t o ( T D I ) , OU-1000 and OU-2000 respectively. The molecular weights of OU-1000 and OU-2000 determ i n e d b y diffusion a n d t h e v i s c o s i t y m e t h o d were 4700 a n d 4400 r e s p e c t i v e l y ; 3) c r o s s l i n k e d p o l y u r e t h a n e e l a s t o m e r s (EL) f r o m E A - 1 0 0 0 a n d E A - 2 0 0 0 , c r o s s l i n k e d w i t h a m i x t u r e o f d i e t h y l e n o g l y c o l ( D E G ) a n d g l y c e r i n (G): a) E L - 1 o b t a i n e d f r o m E A - 1 0 0 0 a n d e r o s s l i n k e d w i t h a D E G - G m i x t u r e i n a 1 : 3 ratio. T h e m o l e c u l a r w e i g h t b e t w e e n a d j a c e n t n e t w o r k u n i t s (Ma) a t 25 ° d e t e r m i n e d f r o m s t r e s s r e l a x a t i o n m e a s u r e m e n t s w a s 3800; b) E L - 2 o b t a i n e d f r o m E A - 1 0 0 0 a n d e r o s s l i n k e d w i t h a D E G - G m i x t u r e i n a 1 : 1 r a t i o ; Mn-----5400; e) E L - 3 a n d E L - 4 o b t a i n e d f r o m E A - 2 0 0 0 a n d c r o s s l i n k e d w i t h a D E G - G m i x t u r e in a 1 : 1 r a t i o . Since t h e s e r u b b e r s a r e c a p a b l e of c r y s t a l l i z a t i o n , t h e m e t h o d o f d e t e r m i n i n g M n f r o m s t r e s s r e l a x a t i o n is u n s u i t a b l e i n t h i s case a n d t h e r e f o r e v a l u e s of M n for E L - 3 and EL-4 were not experimentally determined. F o r all p o l y u r e t h a n e s t h e N C O : O H r a t i o was 1 : 1.
T A B L E 1. E Q U A T I O N S ~ f ( T )
FOR L I N E A R SECTIONS OF T H E CURVE FOR SPECIFIC H E A T
OF OLI(]OESTERS, OLIGOURETHAN'ES AND CROSSLINKED P O L Y U R E T H A N E S
Initial specimen
Quenched specimen
Polymer
/ e q u a t i o n C~ = f ( T , ° K ) t e m p . r a n g e , °C
EA-1000 EA-2000 OU-1000 OU-2000 EL-3
EL-4
EL-1 EL-2
- - 0 " 0 1 2 + 1"49 × 10 -3 T 0 " 4 6 2 + 10 -4 T - - 0 . 1 8 6 + 2 . 1 7 × 10 -8 T 0 . 4 5 3 + 10 -4 T - - 0 " 1 2 5 + 1 " 5 X 10 -3 T 0"320 + 10 _4 T - - 0 . 1 0 3 + 1 . 5 1 × 10 -3 T 0.403+ 10-' T !I 0"195 + 10-* T T - - 0 " 1 1 6 + 1"5I Z 10 -3 0 " 3 2 5 + 10 -4 T 0 " 1 9 1 ~- 1 0 - 4 T
--0"117+ 0"315 + 0"190+ 0"306 + 0"190+ 0"314+
1"5× 10 -3 T 10 -4 T 10 -4 T 10 -4 T 10 -4 T 10 -4 T
equation
C~=f (T, °K)
--60-20 ' - - 0 " 0 1 2 + 1 " 4 9 × 10-3TI 0 " 4 6 2 + 1 0 -4 T 65-100 --70-20 60-100 0.512 --30-20 45-100 0"320 + 10 -4 T -- 3 0 - 2 0 0 " 3 7 6 + 10 _4 T 0"403 + 10 -4 T 50-100 - - 5 0 - --30 0 " 1 9 5 + 10 -4 T
temp. range, °C
--60-20 65-100 60-100 -- 3 0 - 1 0 0 -- 30-25 50-100 --50- --30
--20-10
50-100 --50- --30 --20-10 50-100 - - 5 0 - --20
0-100 - - 5 0 - --20 --
5-100
0"325 + 10 -4 T 0 " 1 9 1 + 1 0 -4 T
50-100 --50- --30
0 " 3 1 5 + 1 0 -4 0 " 1 9 0 + 10 -4 0 " 3 0 6 + 10 -4 0 " 1 9 0 + 1 0 -4 0 " 3 1 4 + 1 0 -4
-- 2 0 - 1 0 0 --50-20 0-100 --50- --20
T T T T T
--
5-100
* O l i g o m e r s o f e t h y l e n e g l y e o l a d i p a t e s a n d u r e t h a n e s were used, as d e s c r i b e d i n a n o t h e r p a p e r [15].
V.P.
2 6 8
PRIVALKO e t a / .
Calorimetric investigations were carried out in an apparatus similar to that described previously [14]. The specimens weighed 0.15-0.20 g. Annealed EA-1000 and EA-2000 specimens were obtained by slow ~0"3-0"5 degrec/min) cooling of the calorimeter with the heater. " I n i t i a l " linear oligourethane and crosslinked elastomer samples were obtained by heating to 60 °, followed b y retention at 18-20 ° for 3 months. All polymer samples studied were quenched in liquid nitrogen after being rendered amorphous b y heating to 100 °. QUenched samples were then placed in a calorimetric unit, pre-cooled to 0 °, and the u n i t was cooled with liquid nitrogen to -- 100 ° at a rate of 3-4 degree/rain. The rate of heating was less t h a n 1"5 degree/min. Studies of specific heat are shown in Figs. 1-3. These Figures indicate that specific heat curves Ca (T) of the samples studied consist of several linear parts which correspond to ranges of glassy, high elastic, crystalline and plastic (molten) states of polymers and of parts with anomalous deviations from linearlty in the ranges of glass and phase transitions. Corresponding equations C~=f(T) for linear sections of the curves were calculated b y the method of least squares and are shown in Table 1.* Specific heat of ethyZeneglycoladipate oligomer~. Curves showing the specific heats of quenched a n d annealed EA-1000 and EA-2000 samples are shown in Fig. 1. This Figure indicates that the specific heats of annealed samples (curves 1) of both oligomers at temperatures ranging from low temperatures to temperatures corresponding to the inception of phase transition vary in a linear manner. Starting from 20 ° on curves 1 of both oligomers a rise is observed, which for EA-1000 passes through a point of infiexion at 40 ° and later passes into the melting peak at 50 ° with Ca----1.95. For EA-2000 this rise is completed as a endothermic peak at 40 ° with (7~=0"94 after which the specific heat suddenly decreases to C~--~0-76 at 43 ° and again increases changing into a second endothermic peak (melting) at 53 ° with ( 7 ~ 1 " 7 6 . The melting point of EA-2000 agrees with the value obtained for high molecular weight polyethyleneadipate [16]. Melting of annealed EA-1000 a n d EA-2000 specimens is complete at 60 °, after which the specific heat of the melts remains almost unchanged. The latent heats of melting of annealed EA-1000 and EA-2000 samples are 18.7 a n d 18.8 eal/g, respectively. The specific heat behaviour of quenched samples is characteristic. Figure l a shows (curve 2) that an EA-1000 sample could not be quenched, which is confirmed by the absence of a typical infiexion of curve 2 during glass-transition and b y the linear dependence of specific heat on temperature in the range of -- 60 to 5°; this coincides with the value obtained for an annealed sample. However, at 1~° on curve 2 an exothermic crystallization peak is observed, after which the specific heat continuously increases, passing into a melting peak at 45 ° with C ~ 1-98 and decreasing to C~-----0"493 at 55 °. During heating a quenched EA-2000 sample (Fig. lb, curve 2) a n endothermic discontinuity is observed in the specific heat in the temperature range of 62 ° to 54 °, dependent on the transition of the oligomer from the glassy to the high elastic state, and two exothermic crystallization peaks are observed at 16 and 20 °, after which the specific heat continuously increases to C~ = 2.06 at the melting point (46 °) and then decreases to C~=0"46 at 53 °. As with annealed samples, the specific heat of melts from quenched samples of both oligomers only slightly increases with temperature. The latent heats of crystallization and fusion could not be quantitatively evaluated owing to the superposition of these processes; however, the latent heat of melting an EA-2000 quenched sample is approximately 1.5 times higher than the overall latent heat of crystallization. Specific heat of linear o~igourethanea. Figure 2 shows the diagrams of the specific heats of oligourethanes. I t can be seen in this Figure that no glass-transition was observed in the initial samples and the specific heat increases in a linear m a n n e r from low temperatures to 20 °. Above 20 ° the specific heats of the initial samples increase evenly and pass into melting - -
- -
- -
* Specific heat values in the Tables and later in the text are given in cal/g/deg.
Calorimetric study of oligoethyloneglycoladipates
269
peaks at 38 ° (C~-----1'02) and 40 ° ( C p = l ' 6 6 ) for OU-1000 a n d OU-2000 respectively. The corresponding heat effects are 6"9 and 12.8 cal/g. Melting of OU-1000 and OU-2000 is completed at 43 and 49 ° respectively, after which the specific heat remains almost constant. I n quenched OU-1000 and OU-2000 samples (curves 2) at --36 a n d --46 ° respectively, discontinuities are observed in the specific heat, which are due to the transition from the glassy to the high elastic state. Above the glass-transition point (Ts) the specific heats of both quenched samples are linear (Table 1) up to 100°; however for OU-2000 a small m a x i m u m is observed at 40 ° with C~-----0"44. Above 50 ° the specific heats of initial a n d quenched sampies agree. Cp,coUg.deF'ee 2.o a f.5
f.o
i
I
-50
0
leo T,°C
5O
2.0
b 1.5 I? I'0
0"5
! I,
-50
0
50
100
r,'C
Fie. 1. Specific heat of EA-1000 (a), EA-2000 (b): / - - a n n e a l e d sample; 2 - - q u e n c h e d sample.
Specific hea~ of crosslinked polyurethanes. Figure 3a and b indicate t h a t EL-3 and EL-4 elastomers from OU-2000 are capable of crystallization, which is proved b y the presence of melting peaks for the initial samples (curve 1}. The temperatures and latent heats of fusion of initial EL-3 a n d EL-4 samples are 36 ° a n d 8"95 cal/g a n d 36 ° a n d 8-20 cal/g, respectively. The presence of small discontinuities in the specific heat (of the order of 0.02-0.03) in the range of T~ proves t h a t crystallization in these systems is incomplete. Curves 2 of the specific heat of quenched samples consist of linear sections before and after T~, which
270
V . P . PRIW~,XO et al.
for EL-3 and EL-4 are --27 and --29 ° respectively. EL-1 and ED-2 elastomers are incapable of crystallization, as shown by Fig. 3c-d. The curves of initial and quonched samples for these elastomers coincide and consist of linear parts which are separated by intervals of inflexions corresponding to Tg at --7 ° for EL-1 and --14 ° for EL-2. RESULTS T a b l e 2 shows values which characterize glass-transition in t h e specimens studied. This T a b l e indicates t h a t during t r a n s i t i o n f r o m pligoester to crosslinked elastomer, t h e Tg v a l u e r e g u l a r l y increases. I t is k n o w n [17] t h a t t h e Tg v a l u e of p o l y m e r s d e p e n d s on m o l e c u l a r weight, r e g u l a r i t y of structure, chain flexibility, presence a n d position o f functional groups, n e t w o r k dens i t y of chemical crosslinks, etc. Thus, increased Tu of linear oligourethanes m a y either b e due to a higher chain rigidity c o m p a r e d w i t h t h e initial polyester, d e t e r m i n e d b y the t h e r m o d y n a m i c flexibility, or to t h e presence in t h e chains of polar u r e t h a n e groups influencing kinetic chain flexibility (mobility). T h e t h e r m o d y n a m i c flexibility of oligoesters a n d oligourethanes m a y be comp a r e d s t a r t i n g f r o m t h e f a c t t h a t the f o r m a n d dimensions of m a c r o m o l e c u l e s in a 0-solvent a n d in the n o n - o r d e r e d (amorphous) s t a t e are e q u i v a l e n t [18-20]. Glass-transition.
TABLE 2. GLASS TEMPERATURE AI~D INCREMENTS IN SPECIFIC HEAT DURING T H E GLASS-TRA~SITIOI~OF QUEI~'CHEDSAMPLES
Polymer
EA-2000 OU-1000 OU-2000 EL-4 EL-3 EL-2 EL-1
AC~, cal/g, deg
zITg,* °C
Tg?, °C
0"108 0"132 0"130 0"127 0"135 0"126 0"120
8 8 8 9 8 12 15
--57 --36 --46 --29 --27 --14 --
7
Ae~, cal/mole, deg
17"2 22"4 22"4 22"4 22"4 22"4 22 "4
1"86 2"96 2"91 2"85 3"02 2"82 2"69
* ATg is the glass-transitioninterval.
Tg is the temperatureat w h i c h of its maximum value.
the specific heat Increment In glass-transition
A~p reachedhalf
As a m e a s u r e of t h e r m o d y n a m i c chain flexibility p a r a m e t e r K 0 of t h e K u h n - M a r k H o u w i n k e q u a t i o n for p o l y m e r viscosity in 0-solvents can be used, since K 0 d e p e n d s only on t h e m a c r o m o l e c u l a r s t r u c t u r e a n d c o n f o r m a t i o n . I t c a n be e x p e c t e d t h a t t h e presence in the oligourethane chain of rigid i s o c y a n a t e units has a certain effect on the overall t h e r m o d y n a m i c chain flexibility, i.e. on t h e v a l u e of K0. T o d e t e r m i n e K0 we used V a n K r e v e l e n ' s empirical f o r m u l a [21]:
K . = (S,/M,t) a,
Calorimetric study of oligoethyleneglycoladipates
Cp, col/g.clegpee
271
~2
1"0
0"5
-50
I
1.5
5~
O
l'O
-50
/.
0
I,
50
100 7;,°C
50
100
13
T,'c
FIG. 2. Specific heat of OU- 1000 (a), OU-2000 (b): / - - i n i t i a l samples; 2--quenched samples; 3-5--samples o b t a i n e d b y slow cooling of the melt and retention at 18-20 ° for 12, 24 and 96 hr. respectively.
where M , is the molecular weight of one atom in the main chain, S , is a value dependent on the specific rigidity per one atom in the main chain. The numerica values of parameters M , , S , and K 0 calculated b y Van Krevelen's method for EA are Ko~- (50 × 1 0 - 2 / 4 . 1 4 ) a-~ 17.8 × 10 -4
and for OU K 0 : ( 5 1 . 2 × 10-2/4.15)a~ 18.8 × 10 -a. The K 0 value calculated for E A agrees exactly with the experimental value obtained previously [15], which proves the authenticity of the results. Since the K 0 values obtained for EA and OU are similar the thermodynamic flexibilities of EA and OU macromolecules are practically the same, hence it follows that the presence of the isocyanate unit has no effect on the thermodynamic flexibility of linear oligourethane chains in the amorphous (glassy) state. Thus, an increase in Tg on changing from EA to OU can be due only to reduction in the kinetic chain flexibility, which depends on the concentration of polar urethane groups. The dependence of T~ on urethane group concentration is shown b y straight line 1 in Fig. 4. Since in polyurethanes prepared from polyesters
272
V . P . PRIVAT,'rO et ca.
Cp, co~/.~,degpee
0"5 ~
//2,
r
]
-50
I
50
A
i'O
I00
• '
b
T'°C
0.5 ]
T
Z50
0
~0
I0o
-50
0
50
C
I00
d
-SO
7;,°c
50
0
~°C
iO0
r,°C
Fie, 3. Specific heat of cross|inked polyurethanes: a--EL-4; b--EL-3; c--EL-2; d--EL-I: /--initial samples; 2--quenched samples.
0
0 f O
l
f
5
I0
NCO concentPation,% 5 I0 I/'2Mn ,,I05
(f) ...__J
FIo. 4. Dependence of glass transition point T~ on the concentration of the NCO-group (1) and the degree of crosslinking (2).
Calorimetric study of oligoethyleneglycoladipates
273
intermolecular hydrogen bonds are formed between the •H groups of the urethane unit and the ester groups [22] it is obvious that the network of physicM bonds formed considerably restricts chain mobility, causing a corresponding increase in Tg. Similar results have been obtained previously [13]. Chemical crosslinks in elastomers also considerably reduce the segmental mobility of macromolecules, causing a further increase in Tg. It is natural that with an increase of three-dimensional network density (i.e. with a reduction of molecular weight between adjacent units), Tg should increase further. This is shown in Table 2, where Tg for EL-1 having a]/n of 3800 is higher than Tg for EL-2 for which M,~ is 5400. M n values for crystallizing EL-3 and EL-4, for these reasons, have not been determined experimentally; however they can be approximately evaluated from the inversely proportional relationship between the polyurethane network density and the length of the oligoester unit [10]. I t can therefore be assumed that since the ratio of DEG to G was the same for EL-2, EL-3 and EL-4, Mn for EL-3 and EL-4 was about twice as high as for EL-2, i.e. of the order of 10-11 × 10a. The dependence of Tg on the degree of crosslinking expressed by 1/2M n, is Shown by straight line 2 in Fig. 4. I t follows from Fig. 4 t h a t the network of physical bonds determined by the concentration of NCOgroups (straight line 1) has the same qualitative effect on Tg as the network of chemical bonds (straight line 2). In addition, three-dimensional network density also affects the width of the glass transition range. Table 2 indicates that with an increase in chemical network density the glass transition range widens. As glass transition is a relaxation process [17], an increase in the glass transition range on increasing the network density indicates a corresponding widening of the spectrum of relaxation times. It should be noted that Tg m a y also increase as a result of partial crystallization of the elastomer, which can be seen from corresponding parts of curves 1 and 2 in Fig. 3a and b. In this case the rigid crystalline sections also restrict chain mobility in the adjacent amorphous regions, thus increasing Tg [6]. Additional quantitative information on the process of glass transition can be obtained by comparing the specific heat drop during glass transition for different objects. According to Wunderlieh's law [23] the specific heat increment during glass transition AC~ calculated per mole of structural "beads" of the molecule is a constant value, which is 2.97 cal/mole/.degree. Table 2 illustrates AC'~ values for the polymer series studied ,which were derived by a formula given in an earlier paper [23]:
zG'= .zG where 11~ is the average molecular weight per "bead", AC~ is the specific heat decrease found in the experiment during glass transition. Table 2 indicates data for crosslinked polyurethanes, since crosslinking does not al~er the chemical structure of recurrent chain elements during transition from ohgourethanes to crosslinked elastomers. Table 2 shows that the ACp values for all the systems studied,
274
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Pm'VALKO et a/.
except for F~A-2000, are similar to the theoretical. A somewhat lower value of AC~' can be obtained for EA-2000 either because a certain mobility of vacancies is possible in EA, at low temperatures, or due to the partial crystallization of the polymer during quenching. In our case the second alternative is observed, since, as noted above, the heat of fusion of an EA-2000 quenched sample is higher than the heat of crystallization. It is interesting to note that the ratio of latent heats of fusion and crystallization of EA-2000 (about 1.5) corresponds to the ratio of theoretical specific heat increment and the value obtained by us during glass transition and can be a quantitative measure of the degree of crystallization of the sample [6, 24]. In accordance with theory [23], AG~' is proportional to the specific volume and the energy of formation of new vacancies and is inversely proportional to T~. It is known [25] that polymer crosslinking is accompanied by a reduction in specific volume. At the same time, Table 2 shows that during transition from oligoesters to crosslinked elastomers, AC~' remains unchanged and Tg increases. It can hence be concluded that the higher the network density the greater is the increase in the energy of formation of new vacancies caused by the presence in the polyurethane molecular structure of a network of physical and chemical bonds (see corresponding values of AC~ for EL-4, EL-3, EL-2 and EL-1 in Table 2). It is significant that the absolute value and the rate of specific heat increase at low temperatures is lower during transition from oligoesters to crosslinked rubbers (see Table 1). This may be due to a reduction in the number of oscillatory degrees of freedom of the system owing to the presence of marked molecular interaction between polar groups in oligourethanes and the network of chemical bonds in crbsslinked elastomers. Crystallization a~d melting. A comparison of curves showing the temperature dependence of the specific heat of EA-1000 and EA-2000 annealed and quenched samples (see Fig. 1) shows that the peak maxima of melting quenched samples of both oligomers are displaced somewhat towards lower temperatures. In addition, two exothermic peaks of crystallization are observed on curve 2 of the EA-2000 quenched sample (see Fig. lb). The results can be explained by the fact that aecordingto heat treatment, various low temperature (quenching) and high temperature (annealing) crystalline modifications are formed in EA-1000 and EA-2000 samples. This is in agreement with results given in another paper [26], in which a report was made on the polymorphism in oligoethyleneglycoladipate. The small peak area of crystallization of the high temperature form (at 16°) and the absence of any crystallization peak of the low temperature form from EA-1000 confirms the extremely high rate of crystallization, which cannot be completely suppressed even by quenching in liquid nitrogen. The complex dependence of 'the specific heats of both oligoesters at 20-40 ° (curves 1) is due to the transition and decomposition of supermolecular structures [23]. A comparison of Figs. 1 and 2 indicates that transition from EA to OU is accompanied by a noticeable reduction in crystanizability, which is confirmed
Calorimetric study of oligoethylcneglycoladipates
275
by a marked reduction of the latent heat of fusion for :initial samples (curves 1) and the absence of crystallization in quenched OU samples (curves 2). These oligourethanes can only crystallize over a period of time, OU-2000 crystallizing at a much higher rate than OU-1000, which is apparent from a comparison of the latent heats of fusion of samples retained for varying lengths of time at 20 ° (curves 3-5 in Fig. 2). Polymer crystallization may be divided into two stages [18]: formation of a metastable two-dimensional ordered state (with no variation in molecular interaction) and spontaneous transition to a final state with a threedimensional order, accompanied by increased molecular interaction. For polymers having no polar functional groups crystallizability is basically determined by thermodynamic chain flexibility and these stages of crystallization cannot therefore be experimentally distinguished. However, as indicated, the macromolecular properties of OU are determined by kinetic flexibility dependent on the concentration of urethane groups and not by thermodynamic flexibility. In this case intermolecular hydrogen bonds noticeably stabilize the-two-dimensional-ordered "pseudocrystalline" [2, 3, 6] state formed in the first stage of crystallization, as a result of which the second stage of crystallization takes place over a long period of time, months in some cases [8, 9]. As the concentration of urethane groups in OU-2000 is lower than in OU-1000, this explains the high crystallizability (i.e. shorter duration of the first stage of crystallization) of OU-2000. On the other hand, in spite of the fact that the density of cohesion energy in OU, owing to the presence of urethane groups, is higher than in corresponding EA, the temperatures and latent heats of fusion of OU are lower. This may be explained by a considerably greater damage to the crystalline lattice of OU caused by the molecular structural asymmetry of 2,4-TDI isomer units. For this reason, the latent heat of fusion of OTJ-1000 is much less than that of OU-2000. Therefore, as the concentration of 2,4-TDI units is higher in OTJ-1000 than in OU-2000, a much greater reduction could be expected in the melting point of OU-1000; however the difference between the melting points of EA and corresponding OU is about the same. The relatively high melting point of OU-1000 is, apparently, due to the low entropy of melting caused by the increased entropy in the crystalline state resulting from the disordering effect of 2,4-TDI units. Transition to crosslinked elastomers, as might be expected, is accompanied by a further reduction in the crystallizability as a result of steric hindrance of chain mobility by network units. Figure 3 shows that only EL-4 and EL-3, in which the network density is much lower than in EL-1 and EL-2, are able to crystallize. According to the theory of fusion of crosslinked polymers [27], crosslinking of statistical chains should reduce the melting point in proportion to the network density. A slight (of the order of 2°) reduction in melting point apparently indicates that the three-dimensional network density in EL-3 and EL-4 is so low that the exclusion of crosslinked chain sections from the crystalline lattice only slightly increases the entropy of fusion, which hardly influences the melting point.
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V.P.
PRIVALKO et aZ.
It has been pointed out that the absolute specific heats of melts of all the polymers studied hardly increase at temperatures up to 100 °. A similar effect was also observed for linear polyurethane [6] and polyoxypropylene [28] melts. Data available do not enable us to draw a simple conclusion concerning this effect, but it can be assumed that it is influenced b y the presence of intermolecular hydrogen bonds in polyester and polyurethane melts, since it is known that the specific heats of non-polar polymer, melts, in which only weak Van der Waals interaction is possible between the macromolecules, increase markedly with temperature. According to the theory of free volume [29], the specific heats of liquids are composed of "oscillatory" and "perforated" (due to the formation of new vacancies) parts. I f we assume in a first approximation that fusion involves the formation and enrichment of the melt with vacancies [30], the specific heat variation of melts of the polymers studied can be explained b y the r~duced effect of one of the components of overall specific heat, namely the vacancy effect. This proves an increased energy of formation of new vacancies, which we are inclined to attribute to the effect of hydrogen bonds. Consequently, in nonpolar polymer melts (polyethylene, polypropylene, etc.) the formation energy of vacancies should be considerably lower and the specific heat of the melts therefore increases with temperature. The authors are grateful to Yu. K. Godovskii for his assistance in introducing the methods of calorimetric investigation. CONCLUSIONS
(1) The specific heat of a systematic series of oligoethyleneglycoladipatelinear oligourethane-crosslinked polyurethane was studied a t temperatures of --50 to 100% (2) I t was found that the addition of isocyanato units to the polyester chain has no effect on the thermodynamic flexibility of chains in the amorphous state. (3) I t was established that with increase of the urethane group concentration and network density of chemical bonds in polymers of the series studied, crystallizability decreases, and the glass-transition point and glass-transition range increase. (4) An assumption was expressed that the presence of a network of physical and chemical bonds increases the energy of vacancy formation in the high elastic and highly plastic states. Translated by E. SEMER~. REF.ERENCES
1. H. G. KILIAN and E. JENCKEL, Z. Elektrochem. 63: 951, 1959 2. H. G. KILIAN, Kolloid. Z. 176: 49, 1961 3. B. V. VASIL'EV and O. G. TARAKANOV, Vysokomol. soyed. 6: 2189, 2193, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 12, 2423, 2427, 1964) 4. B. Ye. MYULLER, N. P. APUKHTINA and A. L. KLEBANSKII, Vysokomol. soyed. 6: 1330, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 7, 1410, 1964)
Calorimetric study of oligoethyloneglycoladipates
277
5. E. F. GUBANOV, A. G. SINAISKII, N. P. APUKHTINA and B. Ya. TEITEL'BAUM, Dokl. AN SSSR 163: 1151, 1965 6. Yu. K. GOLOVSKII and Yu. S. LIPATOV, Vysokomol. soyed. A10: 32, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 1, 34, 1968) 7. T. L. SMITH and A. B. MAGNUSSON, J. Polymer Sci. 42: 391, 1960 8. F. H. MULLER and H. MARTIN, Kolloid. Z. 171: 119, 1960 9. tI. MARTIN and F. H. MULLER, Kolloid. Z. 191: 1, 1963; J. Polymer Sci. C6: 83, 1963 10. A. DAMUSIS, W. ASHE and K. C. FRISCH, J. Appl. Polymer Sci. 9: 2965, 1965 11. K. SHIBAYAMA and M. KODAMA, J. Polymer Sci. 4: A-l, 83, 1966 12. B. V. VASIL'EV, O. G. TARAKANOV, A. I. DEMINA and A. I. SHIROBOKOVA, Vysokomol. soyed. A8: 938, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 1031, 1966) 13. Yu. S. LIPATOVA Yu. Yu. KERCHA, V. G. SINAYAVSKII and N. A. LIPATNIKOV, Vysokomol. soyed. A9: 1340, 1967 (Translated in Polymer Science U.S.S.R. 9: 1502, 1967) 14. Yu. K. GODOVSKII and Yu. P. BARSKII, Plastmassy, No. 7, 57, 1965 15. A. Ye. NESTEROV, Yu. S. LIPATOV, B. Ye. MYULLER and L. V. MOZZHUKHINA, Vysokomol. soyed. B1O: 900, 1968 (Translated in Polymer Sci. U.S.S.R.) 16. C. S. FULLER and C. L. ERICKSON, J. Amer. Chem. Soe. 59: 344, 1937 17. V. A. KARGIN and G. L. SLONIMSKII, Kratkie oeherki po fiziko-khimii polimerov (Brief Essay on the Physico-chemistry of Polymers). Izd. " K h i m i y a , " 1967 18. P. $. FLORY, Proc. Roy. Soc. A234: 60, 1956 19. G. V. SCHULZ, K. v. GUNNER and H. GERRENS, Z. Phys. Chem. 4: 192, 1965 20. F. BUECHE, B. J. KINZIG and C. J. COVEN, J, Polymer Sci. B3: 399, 1965 21. D. W. van KREVELEN and P. J. HOFTYZER, J. Appl. Polymer Sei. 11: 1409, 1967 22. Yu. M. BOYARCHUK, L. Ya. RKPOPORT, V. N. NIKITIN and N. P. APUKHTINA, Vysokomol. soyed. 7: 778, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 5, 859, 1965) 23. B. WUNDERLICH, J. Phys. Chem. 64: 1052, 1960 24. M. DOLE, Kolloid. Z. 165: 40, 1959 25. T. G. FOX and S. LOSHAEK, J. Polymer Sci. 15: 371, 1955 26. B. Ya. TEITEL'BAUM, N. A. PALIKHOV, L. I. MAKLAKOV, N. P. ANOSHINA, I. O. MURTAZINA and V. KOVALENKO, Vysokomol, soyed. A9: 1672, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 8, 1880, 1967) 27. P. J. FLORY, J. Amer. Chem. Soe. 78: 5222, 1956; L. MANDEL'KERN, Kristallizatsiya polimerov (Polymer Crystallization). 6th ed. Izd. " K h i m i y a " , 1966 28. Yu. K. GODOVSKII and G. L. SLONIMSKII, Vysokomol. soyed. A9: 845, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 4, 946, 1967) 29. N. H I R A I and H. EYRING, J. Polymer Sci. 37: 51, 1959 30. Ya. I. FRENKEL', Kineticheskaya teoriya zhidkostei (Kinetic Theory of Liquids). Izd. AN SSSR, 1959
v.P.
PRIVALKO et aZ. REFERENCES
1. E. Ya. DEVIRTS, L. S. ZHEREBKOVA and A. S. NOVIKOV, Kauchuk i rezina, No. 3, 14, 1967 2. R. PUMMERER and W. Gt~NDEL, Rubber Chem. and Technol. 2: 373, 1929 3. R. F. MARTEL and E. D. SMITH, Rubber Chem. and Technol. 35: 141, 1962 4. R. F. MARTEL and D. E. SMITH, Rubber Chem. and Technol. 34: 658, 1961 5. P. J. FLORY and J. REHNER, Industr. and Engng. Chem. 38: 500, 1946 6. J. BRUNI and LEIGER, Atti accadi Lincei, [6] 5: 823, 1927 7. G. SLOMP and J. G. LINDBERG, Analyt. Chem. 39: 60, 1967
CALORIMETRIC
STUDY
AND LINEAR
OF
OLIGOETHYLENEGLYCOLADIPATES
AND CROSSLINKED
SYNTHESIZED
FROM
POLYURETHANES THEM*
V. P. PRIVALKO, Y r . S. LII'ATOV and Y r . Y~. KERCHA Institute of the Chemistry of High Molecular Weight Compounds, Ukr.S.S.R. Academy of Sciences (Received 26 February 1968)
IT IS well known that the physical and mechanical properties of crosslinked polyurethanes are determined by their molecular structure. The alternation in the macromolecular chain of oligoester and isocyanate units of variable flexibility and the presence of chemical crosslinks and polar functional groups capable of physical reaction is reflected to a certain extent in the macroscopic properties of polyurethanes, such as crystallization, temperature and phase transition ranges, etc. Most of the papers previously published [1-6] have dealt only with linear polyurethanes. In the best known studies concerning crosslinked polyurethanes [7-13] only Muller et al. [8, 9] used quantitative calorimetry; however, the value of their results is considerably reduced because no report is given of the actual chemical structure of the polyurethanes. It is therefore of interest to study systems in which the chemical nature of components remaining unchanged, their individual physical parameters (chain flexibility, network density of physical and chemical bonds) vary successively. In this case from the variation of the thermodynamic properties of the system (heat capacity, enthalpy) the effect of these parameters on variation can be evaluated. For this purpose we carried out a calorimetric investigation of the systematic series, oligoester-linear polyurethane-crosslinked polyurethane elastomer, which made it possible to define more accurately and develop further the qualitative results obtained in a former study [13]. * Vysokomol. soyod. All: No. 1, 237-246, 1969.
Calorimetric study of oligoethyleneglycoladipates
267
EXPERIMENTAL T h e following w e r e u s e d for t h e i n v e s t i g a t i o n : 1) o l i g o m e r s o f e t h y l o n o g l y c o l a d i p a t e s * o f m o l e c u l a r w ~ g h t s 1000 a n d 2000 ( E A - 1 0 0 0 and EA-2000 respectively). Molecular weights were determined from the concentration of hydroxyl end groups; 2) o l i g o u r e t h a n e s f r o m E A - 1 0 0 0 a n d E A - 2 0 0 0 a n d 2 , 4 - t o l u y l e n e d i - i s o c y a n a t o ( T D I ) , OU-1000 and OU-2000 respectively. The molecular weights of OU-1000 and OU-2000 determ i n e d b y diffusion a n d t h e v i s c o s i t y m e t h o d were 4700 a n d 4400 r e s p e c t i v e l y ; 3) c r o s s l i n k e d p o l y u r e t h a n e e l a s t o m e r s (EL) f r o m E A - 1 0 0 0 a n d E A - 2 0 0 0 , c r o s s l i n k e d w i t h a m i x t u r e o f d i e t h y l e n o g l y c o l ( D E G ) a n d g l y c e r i n (G): a) E L - 1 o b t a i n e d f r o m E A - 1 0 0 0 a n d e r o s s l i n k e d w i t h a D E G - G m i x t u r e i n a 1 : 3 ratio. T h e m o l e c u l a r w e i g h t b e t w e e n a d j a c e n t n e t w o r k u n i t s (Ma) a t 25 ° d e t e r m i n e d f r o m s t r e s s r e l a x a t i o n m e a s u r e m e n t s w a s 3800; b) E L - 2 o b t a i n e d f r o m E A - 1 0 0 0 a n d e r o s s l i n k e d w i t h a D E G - G m i x t u r e i n a 1 : 1 r a t i o ; Mn-----5400; e) E L - 3 a n d E L - 4 o b t a i n e d f r o m E A - 2 0 0 0 a n d c r o s s l i n k e d w i t h a D E G - G m i x t u r e in a 1 : 1 r a t i o . Since t h e s e r u b b e r s a r e c a p a b l e of c r y s t a l l i z a t i o n , t h e m e t h o d o f d e t e r m i n i n g M n f r o m s t r e s s r e l a x a t i o n is u n s u i t a b l e i n t h i s case a n d t h e r e f o r e v a l u e s of M n for E L - 3 and EL-4 were not experimentally determined. F o r all p o l y u r e t h a n e s t h e N C O : O H r a t i o was 1 : 1.
T A B L E 1. E Q U A T I O N S ~ f ( T )
FOR L I N E A R SECTIONS OF T H E CURVE FOR SPECIFIC H E A T
OF OLI(]OESTERS, OLIGOURETHAN'ES AND CROSSLINKED P O L Y U R E T H A N E S
Initial specimen
Quenched specimen
Polymer
/ e q u a t i o n C~ = f ( T , ° K ) t e m p . r a n g e , °C
EA-1000 EA-2000 OU-1000 OU-2000 EL-3
EL-4
EL-1 EL-2
- - 0 " 0 1 2 + 1"49 × 10 -3 T 0 " 4 6 2 + 10 -4 T - - 0 . 1 8 6 + 2 . 1 7 × 10 -8 T 0 . 4 5 3 + 10 -4 T - - 0 " 1 2 5 + 1 " 5 X 10 -3 T 0"320 + 10 _4 T - - 0 . 1 0 3 + 1 . 5 1 × 10 -3 T 0.403+ 10-' T !I 0"195 + 10-* T T - - 0 " 1 1 6 + 1"5I Z 10 -3 0 " 3 2 5 + 10 -4 T 0 " 1 9 1 ~- 1 0 - 4 T
--0"117+ 0"315 + 0"190+ 0"306 + 0"190+ 0"314+
1"5× 10 -3 T 10 -4 T 10 -4 T 10 -4 T 10 -4 T 10 -4 T
equation
C~=f (T, °K)
--60-20 ' - - 0 " 0 1 2 + 1 " 4 9 × 10-3TI 0 " 4 6 2 + 1 0 -4 T 65-100 --70-20 60-100 0.512 --30-20 45-100 0"320 + 10 -4 T -- 3 0 - 2 0 0 " 3 7 6 + 10 _4 T 0"403 + 10 -4 T 50-100 - - 5 0 - --30 0 " 1 9 5 + 10 -4 T
temp. range, °C
--60-20 65-100 60-100 -- 3 0 - 1 0 0 -- 30-25 50-100 --50- --30
--20-10
50-100 --50- --30 --20-10 50-100 - - 5 0 - --20
0-100 - - 5 0 - --20 --
5-100
0"325 + 10 -4 T 0 " 1 9 1 + 1 0 -4 T
50-100 --50- --30
0 " 3 1 5 + 1 0 -4 0 " 1 9 0 + 10 -4 0 " 3 0 6 + 10 -4 0 " 1 9 0 + 1 0 -4 0 " 3 1 4 + 1 0 -4
-- 2 0 - 1 0 0 --50-20 0-100 --50- --20
T T T T T
--
5-100
* O l i g o m e r s o f e t h y l e n e g l y e o l a d i p a t e s a n d u r e t h a n e s were used, as d e s c r i b e d i n a n o t h e r p a p e r [15].
V.P.
2 6 8
PRIVALKO e t a / .
Calorimetric investigations were carried out in an apparatus similar to that described previously [14]. The specimens weighed 0.15-0.20 g. Annealed EA-1000 and EA-2000 specimens were obtained by slow ~0"3-0"5 degrec/min) cooling of the calorimeter with the heater. " I n i t i a l " linear oligourethane and crosslinked elastomer samples were obtained by heating to 60 °, followed b y retention at 18-20 ° for 3 months. All polymer samples studied were quenched in liquid nitrogen after being rendered amorphous b y heating to 100 °. QUenched samples were then placed in a calorimetric unit, pre-cooled to 0 °, and the u n i t was cooled with liquid nitrogen to -- 100 ° at a rate of 3-4 degree/rain. The rate of heating was less t h a n 1"5 degree/min. Studies of specific heat are shown in Figs. 1-3. These Figures indicate that specific heat curves Ca (T) of the samples studied consist of several linear parts which correspond to ranges of glassy, high elastic, crystalline and plastic (molten) states of polymers and of parts with anomalous deviations from linearlty in the ranges of glass and phase transitions. Corresponding equations C~=f(T) for linear sections of the curves were calculated b y the method of least squares and are shown in Table 1.* Specific heat of ethyZeneglycoladipate oligomer~. Curves showing the specific heats of quenched a n d annealed EA-1000 and EA-2000 samples are shown in Fig. 1. This Figure indicates that the specific heats of annealed samples (curves 1) of both oligomers at temperatures ranging from low temperatures to temperatures corresponding to the inception of phase transition vary in a linear manner. Starting from 20 ° on curves 1 of both oligomers a rise is observed, which for EA-1000 passes through a point of infiexion at 40 ° and later passes into the melting peak at 50 ° with Ca----1.95. For EA-2000 this rise is completed as a endothermic peak at 40 ° with (7~=0"94 after which the specific heat suddenly decreases to C~--~0-76 at 43 ° and again increases changing into a second endothermic peak (melting) at 53 ° with ( 7 ~ 1 " 7 6 . The melting point of EA-2000 agrees with the value obtained for high molecular weight polyethyleneadipate [16]. Melting of annealed EA-1000 a n d EA-2000 specimens is complete at 60 °, after which the specific heat of the melts remains almost unchanged. The latent heats of melting of annealed EA-1000 and EA-2000 samples are 18.7 a n d 18.8 eal/g, respectively. The specific heat behaviour of quenched samples is characteristic. Figure l a shows (curve 2) that an EA-1000 sample could not be quenched, which is confirmed by the absence of a typical infiexion of curve 2 during glass-transition and b y the linear dependence of specific heat on temperature in the range of -- 60 to 5°; this coincides with the value obtained for an annealed sample. However, at 1~° on curve 2 an exothermic crystallization peak is observed, after which the specific heat continuously increases, passing into a melting peak at 45 ° with C ~ 1-98 and decreasing to C~-----0"493 at 55 °. During heating a quenched EA-2000 sample (Fig. lb, curve 2) a n endothermic discontinuity is observed in the specific heat in the temperature range of 62 ° to 54 °, dependent on the transition of the oligomer from the glassy to the high elastic state, and two exothermic crystallization peaks are observed at 16 and 20 °, after which the specific heat continuously increases to C~ = 2.06 at the melting point (46 °) and then decreases to C~=0"46 at 53 °. As with annealed samples, the specific heat of melts from quenched samples of both oligomers only slightly increases with temperature. The latent heats of crystallization and fusion could not be quantitatively evaluated owing to the superposition of these processes; however, the latent heat of melting an EA-2000 quenched sample is approximately 1.5 times higher than the overall latent heat of crystallization. Specific heat of linear o~igourethanea. Figure 2 shows the diagrams of the specific heats of oligourethanes. I t can be seen in this Figure that no glass-transition was observed in the initial samples and the specific heat increases in a linear m a n n e r from low temperatures to 20 °. Above 20 ° the specific heats of the initial samples increase evenly and pass into melting - -
- -
- -
* Specific heat values in the Tables and later in the text are given in cal/g/deg.
Calorimetric study of oligoethyloneglycoladipates
269
peaks at 38 ° (C~-----1'02) and 40 ° ( C p = l ' 6 6 ) for OU-1000 a n d OU-2000 respectively. The corresponding heat effects are 6"9 and 12.8 cal/g. Melting of OU-1000 and OU-2000 is completed at 43 and 49 ° respectively, after which the specific heat remains almost constant. I n quenched OU-1000 and OU-2000 samples (curves 2) at --36 a n d --46 ° respectively, discontinuities are observed in the specific heat, which are due to the transition from the glassy to the high elastic state. Above the glass-transition point (Ts) the specific heats of both quenched samples are linear (Table 1) up to 100°; however for OU-2000 a small m a x i m u m is observed at 40 ° with C~-----0"44. Above 50 ° the specific heats of initial a n d quenched sampies agree. Cp,coUg.deF'ee 2.o a f.5
f.o
i
I
-50
0
leo T,°C
5O
2.0
b 1.5 I? I'0
0"5
! I,
-50
0
50
100
r,'C
Fie. 1. Specific heat of EA-1000 (a), EA-2000 (b): / - - a n n e a l e d sample; 2 - - q u e n c h e d sample.
Specific hea~ of crosslinked polyurethanes. Figure 3a and b indicate t h a t EL-3 and EL-4 elastomers from OU-2000 are capable of crystallization, which is proved b y the presence of melting peaks for the initial samples (curve 1}. The temperatures and latent heats of fusion of initial EL-3 a n d EL-4 samples are 36 ° a n d 8"95 cal/g a n d 36 ° a n d 8-20 cal/g, respectively. The presence of small discontinuities in the specific heat (of the order of 0.02-0.03) in the range of T~ proves t h a t crystallization in these systems is incomplete. Curves 2 of the specific heat of quenched samples consist of linear sections before and after T~, which
270
V . P . PRIW~,XO et al.
for EL-3 and EL-4 are --27 and --29 ° respectively. EL-1 and ED-2 elastomers are incapable of crystallization, as shown by Fig. 3c-d. The curves of initial and quonched samples for these elastomers coincide and consist of linear parts which are separated by intervals of inflexions corresponding to Tg at --7 ° for EL-1 and --14 ° for EL-2. RESULTS T a b l e 2 shows values which characterize glass-transition in t h e specimens studied. This T a b l e indicates t h a t during t r a n s i t i o n f r o m pligoester to crosslinked elastomer, t h e Tg v a l u e r e g u l a r l y increases. I t is k n o w n [17] t h a t t h e Tg v a l u e of p o l y m e r s d e p e n d s on m o l e c u l a r weight, r e g u l a r i t y of structure, chain flexibility, presence a n d position o f functional groups, n e t w o r k dens i t y of chemical crosslinks, etc. Thus, increased Tu of linear oligourethanes m a y either b e due to a higher chain rigidity c o m p a r e d w i t h t h e initial polyester, d e t e r m i n e d b y the t h e r m o d y n a m i c flexibility, or to t h e presence in t h e chains of polar u r e t h a n e groups influencing kinetic chain flexibility (mobility). T h e t h e r m o d y n a m i c flexibility of oligoesters a n d oligourethanes m a y be comp a r e d s t a r t i n g f r o m t h e f a c t t h a t the f o r m a n d dimensions of m a c r o m o l e c u l e s in a 0-solvent a n d in the n o n - o r d e r e d (amorphous) s t a t e are e q u i v a l e n t [18-20]. Glass-transition.
TABLE 2. GLASS TEMPERATURE AI~D INCREMENTS IN SPECIFIC HEAT DURING T H E GLASS-TRA~SITIOI~OF QUEI~'CHEDSAMPLES
Polymer
EA-2000 OU-1000 OU-2000 EL-4 EL-3 EL-2 EL-1
AC~, cal/g, deg
zITg,* °C
Tg?, °C
0"108 0"132 0"130 0"127 0"135 0"126 0"120
8 8 8 9 8 12 15
--57 --36 --46 --29 --27 --14 --
7
Ae~, cal/mole, deg
17"2 22"4 22"4 22"4 22"4 22"4 22 "4
1"86 2"96 2"91 2"85 3"02 2"82 2"69
* ATg is the glass-transitioninterval.
Tg is the temperatureat w h i c h of its maximum value.
the specific heat Increment In glass-transition
A~p reachedhalf
As a m e a s u r e of t h e r m o d y n a m i c chain flexibility p a r a m e t e r K 0 of t h e K u h n - M a r k H o u w i n k e q u a t i o n for p o l y m e r viscosity in 0-solvents can be used, since K 0 d e p e n d s only on t h e m a c r o m o l e c u l a r s t r u c t u r e a n d c o n f o r m a t i o n . I t c a n be e x p e c t e d t h a t t h e presence in the oligourethane chain of rigid i s o c y a n a t e units has a certain effect on the overall t h e r m o d y n a m i c chain flexibility, i.e. on t h e v a l u e of K0. T o d e t e r m i n e K0 we used V a n K r e v e l e n ' s empirical f o r m u l a [21]:
K . = (S,/M,t) a,
Calorimetric study of oligoethyleneglycoladipates
Cp, col/g.clegpee
271
~2
1"0
0"5
-50
I
1.5
5~
O
l'O
-50
/.
0
I,
50
100 7;,°C
50
100
13
T,'c
FIG. 2. Specific heat of OU- 1000 (a), OU-2000 (b): / - - i n i t i a l samples; 2--quenched samples; 3-5--samples o b t a i n e d b y slow cooling of the melt and retention at 18-20 ° for 12, 24 and 96 hr. respectively.
where M , is the molecular weight of one atom in the main chain, S , is a value dependent on the specific rigidity per one atom in the main chain. The numerica values of parameters M , , S , and K 0 calculated b y Van Krevelen's method for EA are Ko~- (50 × 1 0 - 2 / 4 . 1 4 ) a-~ 17.8 × 10 -4
and for OU K 0 : ( 5 1 . 2 × 10-2/4.15)a~ 18.8 × 10 -a. The K 0 value calculated for E A agrees exactly with the experimental value obtained previously [15], which proves the authenticity of the results. Since the K 0 values obtained for EA and OU are similar the thermodynamic flexibilities of EA and OU macromolecules are practically the same, hence it follows that the presence of the isocyanate unit has no effect on the thermodynamic flexibility of linear oligourethane chains in the amorphous (glassy) state. Thus, an increase in Tg on changing from EA to OU can be due only to reduction in the kinetic chain flexibility, which depends on the concentration of polar urethane groups. The dependence of T~ on urethane group concentration is shown b y straight line 1 in Fig. 4. Since in polyurethanes prepared from polyesters
272
V . P . PRIVAT,'rO et ca.
Cp, co~/.~,degpee
0"5 ~
//2,
r
]
-50
I
50
A
i'O
I00
• '
b
T'°C
0.5 ]
T
Z50
0
~0
I0o
-50
0
50
C
I00
d
-SO
7;,°c
50
0
~°C
iO0
r,°C
Fie, 3. Specific heat of cross|inked polyurethanes: a--EL-4; b--EL-3; c--EL-2; d--EL-I: /--initial samples; 2--quenched samples.
0
0 f O
l
f
5
I0
NCO concentPation,% 5 I0 I/'2Mn ,,I05
(f) ...__J
FIo. 4. Dependence of glass transition point T~ on the concentration of the NCO-group (1) and the degree of crosslinking (2).
Calorimetric study of oligoethyleneglycoladipates
273
intermolecular hydrogen bonds are formed between the •H groups of the urethane unit and the ester groups [22] it is obvious that the network of physicM bonds formed considerably restricts chain mobility, causing a corresponding increase in Tg. Similar results have been obtained previously [13]. Chemical crosslinks in elastomers also considerably reduce the segmental mobility of macromolecules, causing a further increase in Tg. It is natural that with an increase of three-dimensional network density (i.e. with a reduction of molecular weight between adjacent units), Tg should increase further. This is shown in Table 2, where Tg for EL-1 having a]/n of 3800 is higher than Tg for EL-2 for which M,~ is 5400. M n values for crystallizing EL-3 and EL-4, for these reasons, have not been determined experimentally; however they can be approximately evaluated from the inversely proportional relationship between the polyurethane network density and the length of the oligoester unit [10]. I t can therefore be assumed that since the ratio of DEG to G was the same for EL-2, EL-3 and EL-4, Mn for EL-3 and EL-4 was about twice as high as for EL-2, i.e. of the order of 10-11 × 10a. The dependence of Tg on the degree of crosslinking expressed by 1/2M n, is Shown by straight line 2 in Fig. 4. I t follows from Fig. 4 t h a t the network of physical bonds determined by the concentration of NCOgroups (straight line 1) has the same qualitative effect on Tg as the network of chemical bonds (straight line 2). In addition, three-dimensional network density also affects the width of the glass transition range. Table 2 indicates that with an increase in chemical network density the glass transition range widens. As glass transition is a relaxation process [17], an increase in the glass transition range on increasing the network density indicates a corresponding widening of the spectrum of relaxation times. It should be noted that Tg m a y also increase as a result of partial crystallization of the elastomer, which can be seen from corresponding parts of curves 1 and 2 in Fig. 3a and b. In this case the rigid crystalline sections also restrict chain mobility in the adjacent amorphous regions, thus increasing Tg [6]. Additional quantitative information on the process of glass transition can be obtained by comparing the specific heat drop during glass transition for different objects. According to Wunderlieh's law [23] the specific heat increment during glass transition AC~ calculated per mole of structural "beads" of the molecule is a constant value, which is 2.97 cal/mole/.degree. Table 2 illustrates AC'~ values for the polymer series studied ,which were derived by a formula given in an earlier paper [23]:
zG'= .zG where 11~ is the average molecular weight per "bead", AC~ is the specific heat decrease found in the experiment during glass transition. Table 2 indicates data for crosslinked polyurethanes, since crosslinking does not al~er the chemical structure of recurrent chain elements during transition from ohgourethanes to crosslinked elastomers. Table 2 shows that the ACp values for all the systems studied,
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except for F~A-2000, are similar to the theoretical. A somewhat lower value of AC~' can be obtained for EA-2000 either because a certain mobility of vacancies is possible in EA, at low temperatures, or due to the partial crystallization of the polymer during quenching. In our case the second alternative is observed, since, as noted above, the heat of fusion of an EA-2000 quenched sample is higher than the heat of crystallization. It is interesting to note that the ratio of latent heats of fusion and crystallization of EA-2000 (about 1.5) corresponds to the ratio of theoretical specific heat increment and the value obtained by us during glass transition and can be a quantitative measure of the degree of crystallization of the sample [6, 24]. In accordance with theory [23], AG~' is proportional to the specific volume and the energy of formation of new vacancies and is inversely proportional to T~. It is known [25] that polymer crosslinking is accompanied by a reduction in specific volume. At the same time, Table 2 shows that during transition from oligoesters to crosslinked elastomers, AC~' remains unchanged and Tg increases. It can hence be concluded that the higher the network density the greater is the increase in the energy of formation of new vacancies caused by the presence in the polyurethane molecular structure of a network of physical and chemical bonds (see corresponding values of AC~ for EL-4, EL-3, EL-2 and EL-1 in Table 2). It is significant that the absolute value and the rate of specific heat increase at low temperatures is lower during transition from oligoesters to crosslinked rubbers (see Table 1). This may be due to a reduction in the number of oscillatory degrees of freedom of the system owing to the presence of marked molecular interaction between polar groups in oligourethanes and the network of chemical bonds in crbsslinked elastomers. Crystallization a~d melting. A comparison of curves showing the temperature dependence of the specific heat of EA-1000 and EA-2000 annealed and quenched samples (see Fig. 1) shows that the peak maxima of melting quenched samples of both oligomers are displaced somewhat towards lower temperatures. In addition, two exothermic peaks of crystallization are observed on curve 2 of the EA-2000 quenched sample (see Fig. lb). The results can be explained by the fact that aecordingto heat treatment, various low temperature (quenching) and high temperature (annealing) crystalline modifications are formed in EA-1000 and EA-2000 samples. This is in agreement with results given in another paper [26], in which a report was made on the polymorphism in oligoethyleneglycoladipate. The small peak area of crystallization of the high temperature form (at 16°) and the absence of any crystallization peak of the low temperature form from EA-1000 confirms the extremely high rate of crystallization, which cannot be completely suppressed even by quenching in liquid nitrogen. The complex dependence of 'the specific heats of both oligoesters at 20-40 ° (curves 1) is due to the transition and decomposition of supermolecular structures [23]. A comparison of Figs. 1 and 2 indicates that transition from EA to OU is accompanied by a noticeable reduction in crystanizability, which is confirmed
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by a marked reduction of the latent heat of fusion for :initial samples (curves 1) and the absence of crystallization in quenched OU samples (curves 2). These oligourethanes can only crystallize over a period of time, OU-2000 crystallizing at a much higher rate than OU-1000, which is apparent from a comparison of the latent heats of fusion of samples retained for varying lengths of time at 20 ° (curves 3-5 in Fig. 2). Polymer crystallization may be divided into two stages [18]: formation of a metastable two-dimensional ordered state (with no variation in molecular interaction) and spontaneous transition to a final state with a threedimensional order, accompanied by increased molecular interaction. For polymers having no polar functional groups crystallizability is basically determined by thermodynamic chain flexibility and these stages of crystallization cannot therefore be experimentally distinguished. However, as indicated, the macromolecular properties of OU are determined by kinetic flexibility dependent on the concentration of urethane groups and not by thermodynamic flexibility. In this case intermolecular hydrogen bonds noticeably stabilize the-two-dimensional-ordered "pseudocrystalline" [2, 3, 6] state formed in the first stage of crystallization, as a result of which the second stage of crystallization takes place over a long period of time, months in some cases [8, 9]. As the concentration of urethane groups in OU-2000 is lower than in OU-1000, this explains the high crystallizability (i.e. shorter duration of the first stage of crystallization) of OU-2000. On the other hand, in spite of the fact that the density of cohesion energy in OU, owing to the presence of urethane groups, is higher than in corresponding EA, the temperatures and latent heats of fusion of OU are lower. This may be explained by a considerably greater damage to the crystalline lattice of OU caused by the molecular structural asymmetry of 2,4-TDI isomer units. For this reason, the latent heat of fusion of OTJ-1000 is much less than that of OU-2000. Therefore, as the concentration of 2,4-TDI units is higher in OTJ-1000 than in OU-2000, a much greater reduction could be expected in the melting point of OU-1000; however the difference between the melting points of EA and corresponding OU is about the same. The relatively high melting point of OU-1000 is, apparently, due to the low entropy of melting caused by the increased entropy in the crystalline state resulting from the disordering effect of 2,4-TDI units. Transition to crosslinked elastomers, as might be expected, is accompanied by a further reduction in the crystallizability as a result of steric hindrance of chain mobility by network units. Figure 3 shows that only EL-4 and EL-3, in which the network density is much lower than in EL-1 and EL-2, are able to crystallize. According to the theory of fusion of crosslinked polymers [27], crosslinking of statistical chains should reduce the melting point in proportion to the network density. A slight (of the order of 2°) reduction in melting point apparently indicates that the three-dimensional network density in EL-3 and EL-4 is so low that the exclusion of crosslinked chain sections from the crystalline lattice only slightly increases the entropy of fusion, which hardly influences the melting point.
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It has been pointed out that the absolute specific heats of melts of all the polymers studied hardly increase at temperatures up to 100 °. A similar effect was also observed for linear polyurethane [6] and polyoxypropylene [28] melts. Data available do not enable us to draw a simple conclusion concerning this effect, but it can be assumed that it is influenced b y the presence of intermolecular hydrogen bonds in polyester and polyurethane melts, since it is known that the specific heats of non-polar polymer, melts, in which only weak Van der Waals interaction is possible between the macromolecules, increase markedly with temperature. According to the theory of free volume [29], the specific heats of liquids are composed of "oscillatory" and "perforated" (due to the formation of new vacancies) parts. I f we assume in a first approximation that fusion involves the formation and enrichment of the melt with vacancies [30], the specific heat variation of melts of the polymers studied can be explained b y the r~duced effect of one of the components of overall specific heat, namely the vacancy effect. This proves an increased energy of formation of new vacancies, which we are inclined to attribute to the effect of hydrogen bonds. Consequently, in nonpolar polymer melts (polyethylene, polypropylene, etc.) the formation energy of vacancies should be considerably lower and the specific heat of the melts therefore increases with temperature. The authors are grateful to Yu. K. Godovskii for his assistance in introducing the methods of calorimetric investigation. CONCLUSIONS
(1) The specific heat of a systematic series of oligoethyleneglycoladipatelinear oligourethane-crosslinked polyurethane was studied a t temperatures of --50 to 100% (2) I t was found that the addition of isocyanato units to the polyester chain has no effect on the thermodynamic flexibility of chains in the amorphous state. (3) I t was established that with increase of the urethane group concentration and network density of chemical bonds in polymers of the series studied, crystallizability decreases, and the glass-transition point and glass-transition range increase. (4) An assumption was expressed that the presence of a network of physical and chemical bonds increases the energy of vacancy formation in the high elastic and highly plastic states. Translated by E. SEMER~. REF.ERENCES
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