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Properties of urethane elastomers—III. Depolarization currents from unsaturated polyurethanes based on 4,4′-diphenylmethane diisocyanate
Eur. Polym. J. Vol. 26, No. 5, pp. 571-573, 1990
0014-3057/90 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain
PROPERTIES OF URETHANE ELASTOMERS--III. DEPOLARIZATION CURRENTS FROM UNSATURATED POLYURETHANES BASED ON 4,4'-DIPHENYLMETHANE DIISOCYANATE 1. DIACONU, C. CIOBANU and CR. I. SIMIONESCU "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda, No. 41 A, 6600 Jassy, Romania (Received 11 April 1989; in revised form 17 July 1989) Abstract--The dielectric relaxation processes in polyurethanes based on 4,4'-diphenylmethane diisocyanate (MDI) and both saturated and unsaturated polyesters were studied in the range 240-410 K using thermally stimulated depolarization currents method (TSDC). The structure of the TSDC spectra and the location and intensity of the constituent peaks depend both on the urethane content and thermal history of the polymer. The relaxation processes have been associated with the molecular motions within domains with distinct hard segment concentrations.
concentration 10 3mol/g. For convenience, this series of polyurethanes will be referred to as unsaturated polyurethanes based MDI. Polymer films ca 0.1 mm thick were prepared by casting dilute dimethylformamide-polymer solutions on clean glass plates. After removing from the support, the films were washed with water and then dried in vacuum for 2 hr at 60~C. For annealing, the samples were further kept for 1 hr at 100°C. The TSDC measurements were performed on films provided with vacuum-evaporated silver electrodes of circular form (~b = 13 mm). Measurements were carried out in dry N2 at normal pressure using a device described elsewhere [3]. The thermal cycles of polarization and depolarization of samples consisted of the following steps. A sample with a polarization field Ep = 15 kV/cm was warmed up to the polarization temperature Tp (75°C for original samples and 100°C for annealed) which was maintained for a polarization time tp of 20 min.; after cooling to 200 K using liquid N2, gp was removed and the sample was short-circuited for 10min in order to eliminate the rapid depolarization currents; then the depolarization currents were recorded at a heating rate of 3.5 K/min.
INTRODUCTION In previous papers of this series, the dielectric relaxation processes in saturated [1] a n d mixed (saturated a n d u n s a t u r a t e d ) [2] elastomeric p o l y u r e t h a n e s based on 4,4'-dibenzyl diisocyanate ( D B D I ) were investigated in c o n n e c t i o n with chemical a n d m i c r o p h a s e structures using thermally stimulated depolarization currents ( T S D C ) a n d electron microscopy methods. The T S D C o b s e r v a t i o n s were explained in terms of the multiphase structure model. It was accepted that, besides soft a n d hard domains, the polyurethanes c o n t a i n certain intermediate rnicrophases with distinct h a r d segments c o n c e n t r a t i o n s situated between those c o r r e s p o n d i n g to the soft a n d h a r d domains. The c o n c e n t r a t i o n of h a r d segments mainly determines the molecular dielectric relaxation processes in polyurethanes. The purpose of the present p a p e r is to investigate the molecular dielectric relaxation processes in p o l y u r e t h a n e s based o n 4'-diphenyl m e t h a n e diisocyanate ( M D I ) a n d b o t h saturated a n d u n s a t u r a t e d polyester by studying the influence of thermal history a n d chemical structure on dielectric relaxation properties determined by the T S D C method.
RESULTS AND DISCUSSION Figure 1 presents the T S D C t h e r m o g r a m s of the original samples with different u r e t h a n e contents. The structure of the T S D C spectra a n d the location a n d intensity of the constituent peaks depend o n the u r e t h a n e concentration. Over the whole range of composition, the T S D C spectra exhibit three resolved peaks (#, ~, ~ ' ) a n d part of a high t e m p e r a t u r e peak (6). The lowest u r e t h a n e content sample, P U - S N 1,27, has a n additional small peak (~") between the and ~' peaks. A n increase in u r e t h a n e c o n t e n t decreases the intensities of the fl a n d ~ peaks a n d shifts the ~ peak to higher temperatures. The position of the fl peak seems to be unaffected, whereas the ~' peak moves to higher temperatures a n d increases in intensity. U r e t h a n e increase also b r o a d e n s the relaxation process.
EXPERIMENTAL PROCEDURES
The polyurethanes were prepared from a mixture of saturated poly(ethylene adipate) and unsaturated poly(ethylene fumarate) polyesters in the mole ratio of 2: 1, the ethylene glycol. Using different ratios of MDI to ethylene glycol, polymers with urethane content ranging from 1.27 to 2.40 x 10-3 mol/g were obtained. The polyester had a M~ of 2000. The syntheses were performed in dimethylformamide at 60°C for 6 hr, giving polymers with Mw of ca 25,000. The dry polymer concentration obtained was 30%. Three samples with different urethane concentrations have been investigated, designated as PU-SN-I.27, PU-SN2.01 and PU-SN-2.40, where SN refers to saturated and unsaturated polyester and the number to the urethane 571
572
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Figures 2-4 show the effects of annealing on the samples with various urethane contents. By annealing the ct, ct' and ~" peaks decrease in intensity and shift to lower temperatures. The greatest change occurred for the ~ peak. The fl peak slightly decreased in intensity approximately at the same temperature. The annealing effects depend on the urethane content, being greater for the lower urethane content samples (see also Table 1). For PU-SN-2.01, an additional peak a" arises on annealing. The slight asymmetry of the ~t peak suggests that the ~" process might be overlapped by the process for the original sample. In the preceding paper of this series [1], the TSDC relaxation processes in unsaturated polyurethanes based on DBDI were analysed in connection with thermal history and chemical and microphase structures. Four relaxation processes fl, ct, ~(' and 6 were observed. The origin of these processes was explained in terms of the multiphase structure model [1-4]. The ct, ct' and 6 peaks were associated with the thermal transition (Tg) of the domains with various concentrations of hard segments; the fl peak was ascribed to the motion of the polyester carbonyl groups. The urethane content and thermal history dependences of the relaxation processes in polyurethanes based on DBDI were found to be roughly similar with those observed in polyurethanes based on MDI now studied; there are however some differences between the relaxation processes observed for these two urethane elastomers. The main one is that the ~" process was not found in polyurethanes based on DBDI (original or annealed samples). According to the multiphase structure approach, the existence of this peak indicates that the polyurethanes based on MDI contain an intermediate microphase with a distinct hard segments concentration situated between those corre-
-~
•
# ' b 4 • x
a
6
z
.
8
J,,
/
I, _ /~
I /
~=" V..
/o/ a'
I 400
T(K)
Fig. 3. TSDC spectra of PU-SN-2.01: (a) original film; and
(b) annealed film.
Fig. 1. TSDC spectra as a function of urthane content: (a) PU-SN-1.27; (b) PU-SN-2.01; and (c) PU-SN-2.40.
8
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sponding to the domains where ~ and a' originate. This microphase may exist only in annealed sample, in the case of PU-SN-2.01, or both original and annealed samples of PU-SN-I.27. At high urethane concentration, the ct" peak is probably overlapped by ct and/or ~' peaks, which are much wider (Fig. I) than those for the lower urethane content samples. The ct peaks of polyurethanes based on MDI or DBDI are located at around the same temperature, whereas ct' and 6 peaks temperatures are lower by 4-40 K and respectively higher by 1-16 K for polymer based on MDI. These observations suggest that the intermediate microphase from which a originates has lower hard segment concentration for polyurethanes based on MDI than polyurethanes based on DBDI, and the rigid domains are better ordered in those based on MDI. The fl peak temperature is higher by c a 10 K for polyurethanes based on MDI, showing that the motions of the carbonyl group are less hindered than polyurethanes based on DBDI. The ratio between the intensities of the ~ and ct' peaks, for original samples, are higher for the polyurethanes based on MDI, showing a diminution of the intermediate microphase corresponding to the ct' peak in respect of the soft microphase. However, for low urethane content samples, this diminution is accompanied by the appearance of a new intermediate microphase from which the ~" peak originates. On the basis of the assignments of relaxation processes in polyurethanes based on DBDI [1], it may be assumed that the ct, a", ct' and 5 peaks of polyurethanes based on MDI correspond to the thermal transitions (Tg) of the microphases with various segments concentrations. The hard segments concentration increases from domains from which the ct peak originates (soft domains) to those corresponding to 6 peak (hard domains). The ~t, ~" and a '
i4f3 Ji
x
tl,
t~ 250
300
350
T(K)
Fig. 2. TSDC spectra of PU-SN-I.27: (a) original film; and (b) annealed film.
250
300
350
400
T(K)
Fig. 4. TSDC spectra of PU-SN-2.40: (a) original film; and (b) annealed film.
Properties of urethane elastomers--III
573
Table I. The dielectric relaxation characteristics of polyurethanes Peak temperatures (K) Apparent activation energy (eV) Polymer ~ a a" ~' 6 PU-SN-1.27 269 276 288 300 372* PU-SN-2.01 270 278 285* 304 398* PU-SN-2.40 270 285 -309 407* • From TSDC thermograms of the annealed samples. processes involve motions of the soft polyester segments and the 6 process is associated with the segmental motions within hard domains. The position and intensity of the fl peak for dry and soaked in water samples are about the same, as for polyurethanes based on D B D I [1], suggesting that the fl peak could be ascribed to the motion of the polyester carbonyl groups. The relaxation characteristics of the samples derived from T S D C spectra are presented in Table 1. The apparent activation energies were determined using the initial current rise method applied to the T S D C spectra obtained by partial heating of the polarized samples. By partial depolarization of the samples, single values of the apparent activation energy were found for the ~t, ct", ~' and 6 peaks and ranges of values for the fl peak. These findings suggest that the fl process arose from a dielectric
~ 0.52-1.34 0.56-1.40 0.60-1.50
• 2.05 2.20 2.32
~' 6 2.32 2.85* 2.40 2.90* 2.47 2.93*
relaxation distributed in activation energy, whereas ~, ~", ~' and 6 processes presumably originate from a distribution of the relaxation processes in natural frequency. REFERENCES
1. I. Diaconu, C. Ciobanu and Cr. I. Simionescu. Polym. Bull. 20, 195 (1988). 2. I. Diaconu, C. Ciobanu and Cr. I. Simionescu. Polvm. Bull. 21, 203 (1989). 3. I. Diaconu and S. Dumitrescu. Eur. Polym. J. 14, 971 (1978). 4. H. E. Carius, G. Pohl and G. Goering. 31st IUPAC Macromolecular Syrup. 1987, Abstracts, Microsymposium IV-V, pp. 147. 5. H. Goering, H. E. Carius and G. Pohl. 31st IUPAC Macromolecular Symp. 1987, Abstracts, Microsymposium IV-V, pp. 147.
0014-3057/90 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain
PROPERTIES OF URETHANE ELASTOMERS--III. DEPOLARIZATION CURRENTS FROM UNSATURATED POLYURETHANES BASED ON 4,4'-DIPHENYLMETHANE DIISOCYANATE 1. DIACONU, C. CIOBANU and CR. I. SIMIONESCU "Petru Poni" Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda, No. 41 A, 6600 Jassy, Romania (Received 11 April 1989; in revised form 17 July 1989) Abstract--The dielectric relaxation processes in polyurethanes based on 4,4'-diphenylmethane diisocyanate (MDI) and both saturated and unsaturated polyesters were studied in the range 240-410 K using thermally stimulated depolarization currents method (TSDC). The structure of the TSDC spectra and the location and intensity of the constituent peaks depend both on the urethane content and thermal history of the polymer. The relaxation processes have been associated with the molecular motions within domains with distinct hard segment concentrations.
concentration 10 3mol/g. For convenience, this series of polyurethanes will be referred to as unsaturated polyurethanes based MDI. Polymer films ca 0.1 mm thick were prepared by casting dilute dimethylformamide-polymer solutions on clean glass plates. After removing from the support, the films were washed with water and then dried in vacuum for 2 hr at 60~C. For annealing, the samples were further kept for 1 hr at 100°C. The TSDC measurements were performed on films provided with vacuum-evaporated silver electrodes of circular form (~b = 13 mm). Measurements were carried out in dry N2 at normal pressure using a device described elsewhere [3]. The thermal cycles of polarization and depolarization of samples consisted of the following steps. A sample with a polarization field Ep = 15 kV/cm was warmed up to the polarization temperature Tp (75°C for original samples and 100°C for annealed) which was maintained for a polarization time tp of 20 min.; after cooling to 200 K using liquid N2, gp was removed and the sample was short-circuited for 10min in order to eliminate the rapid depolarization currents; then the depolarization currents were recorded at a heating rate of 3.5 K/min.
INTRODUCTION In previous papers of this series, the dielectric relaxation processes in saturated [1] a n d mixed (saturated a n d u n s a t u r a t e d ) [2] elastomeric p o l y u r e t h a n e s based on 4,4'-dibenzyl diisocyanate ( D B D I ) were investigated in c o n n e c t i o n with chemical a n d m i c r o p h a s e structures using thermally stimulated depolarization currents ( T S D C ) a n d electron microscopy methods. The T S D C o b s e r v a t i o n s were explained in terms of the multiphase structure model. It was accepted that, besides soft a n d hard domains, the polyurethanes c o n t a i n certain intermediate rnicrophases with distinct h a r d segments c o n c e n t r a t i o n s situated between those c o r r e s p o n d i n g to the soft a n d h a r d domains. The c o n c e n t r a t i o n of h a r d segments mainly determines the molecular dielectric relaxation processes in polyurethanes. The purpose of the present p a p e r is to investigate the molecular dielectric relaxation processes in p o l y u r e t h a n e s based o n 4'-diphenyl m e t h a n e diisocyanate ( M D I ) a n d b o t h saturated a n d u n s a t u r a t e d polyester by studying the influence of thermal history a n d chemical structure on dielectric relaxation properties determined by the T S D C method.
RESULTS AND DISCUSSION Figure 1 presents the T S D C t h e r m o g r a m s of the original samples with different u r e t h a n e contents. The structure of the T S D C spectra a n d the location a n d intensity of the constituent peaks depend o n the u r e t h a n e concentration. Over the whole range of composition, the T S D C spectra exhibit three resolved peaks (#, ~, ~ ' ) a n d part of a high t e m p e r a t u r e peak (6). The lowest u r e t h a n e content sample, P U - S N 1,27, has a n additional small peak (~") between the and ~' peaks. A n increase in u r e t h a n e c o n t e n t decreases the intensities of the fl a n d ~ peaks a n d shifts the ~ peak to higher temperatures. The position of the fl peak seems to be unaffected, whereas the ~' peak moves to higher temperatures a n d increases in intensity. U r e t h a n e increase also b r o a d e n s the relaxation process.
EXPERIMENTAL PROCEDURES
The polyurethanes were prepared from a mixture of saturated poly(ethylene adipate) and unsaturated poly(ethylene fumarate) polyesters in the mole ratio of 2: 1, the ethylene glycol. Using different ratios of MDI to ethylene glycol, polymers with urethane content ranging from 1.27 to 2.40 x 10-3 mol/g were obtained. The polyester had a M~ of 2000. The syntheses were performed in dimethylformamide at 60°C for 6 hr, giving polymers with Mw of ca 25,000. The dry polymer concentration obtained was 30%. Three samples with different urethane concentrations have been investigated, designated as PU-SN-I.27, PU-SN2.01 and PU-SN-2.40, where SN refers to saturated and unsaturated polyester and the number to the urethane 571
572
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250
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Figures 2-4 show the effects of annealing on the samples with various urethane contents. By annealing the ct, ct' and ~" peaks decrease in intensity and shift to lower temperatures. The greatest change occurred for the ~ peak. The fl peak slightly decreased in intensity approximately at the same temperature. The annealing effects depend on the urethane content, being greater for the lower urethane content samples (see also Table 1). For PU-SN-2.01, an additional peak a" arises on annealing. The slight asymmetry of the ~t peak suggests that the ~" process might be overlapped by the process for the original sample. In the preceding paper of this series [1], the TSDC relaxation processes in unsaturated polyurethanes based on DBDI were analysed in connection with thermal history and chemical and microphase structures. Four relaxation processes fl, ct, ~(' and 6 were observed. The origin of these processes was explained in terms of the multiphase structure model [1-4]. The ct, ct' and 6 peaks were associated with the thermal transition (Tg) of the domains with various concentrations of hard segments; the fl peak was ascribed to the motion of the polyester carbonyl groups. The urethane content and thermal history dependences of the relaxation processes in polyurethanes based on DBDI were found to be roughly similar with those observed in polyurethanes based on MDI now studied; there are however some differences between the relaxation processes observed for these two urethane elastomers. The main one is that the ~" process was not found in polyurethanes based on DBDI (original or annealed samples). According to the multiphase structure approach, the existence of this peak indicates that the polyurethanes based on MDI contain an intermediate microphase with a distinct hard segments concentration situated between those corre-
-~
•
# ' b 4 • x
a
6
z
.
8
J,,
/
I, _ /~
I /
~=" V..
/o/ a'
I 400
T(K)
Fig. 3. TSDC spectra of PU-SN-2.01: (a) original film; and
(b) annealed film.
Fig. 1. TSDC spectra as a function of urthane content: (a) PU-SN-1.27; (b) PU-SN-2.01; and (c) PU-SN-2.40.
8
I 350
350
T(K)
3,
I 300
]~
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,7
/
sponding to the domains where ~ and a' originate. This microphase may exist only in annealed sample, in the case of PU-SN-2.01, or both original and annealed samples of PU-SN-I.27. At high urethane concentration, the ct" peak is probably overlapped by ct and/or ~' peaks, which are much wider (Fig. I) than those for the lower urethane content samples. The ct peaks of polyurethanes based on MDI or DBDI are located at around the same temperature, whereas ct' and 6 peaks temperatures are lower by 4-40 K and respectively higher by 1-16 K for polymer based on MDI. These observations suggest that the intermediate microphase from which a originates has lower hard segment concentration for polyurethanes based on MDI than polyurethanes based on DBDI, and the rigid domains are better ordered in those based on MDI. The fl peak temperature is higher by c a 10 K for polyurethanes based on MDI, showing that the motions of the carbonyl group are less hindered than polyurethanes based on DBDI. The ratio between the intensities of the ~ and ct' peaks, for original samples, are higher for the polyurethanes based on MDI, showing a diminution of the intermediate microphase corresponding to the ct' peak in respect of the soft microphase. However, for low urethane content samples, this diminution is accompanied by the appearance of a new intermediate microphase from which the ~" peak originates. On the basis of the assignments of relaxation processes in polyurethanes based on DBDI [1], it may be assumed that the ct, a", ct' and 5 peaks of polyurethanes based on MDI correspond to the thermal transitions (Tg) of the microphases with various segments concentrations. The hard segments concentration increases from domains from which the ct peak originates (soft domains) to those corresponding to 6 peak (hard domains). The ~t, ~" and a '
i4f3 Ji
x
tl,
t~ 250
300
350
T(K)
Fig. 2. TSDC spectra of PU-SN-I.27: (a) original film; and (b) annealed film.
250
300
350
400
T(K)
Fig. 4. TSDC spectra of PU-SN-2.40: (a) original film; and (b) annealed film.
Properties of urethane elastomers--III
573
Table I. The dielectric relaxation characteristics of polyurethanes Peak temperatures (K) Apparent activation energy (eV) Polymer ~ a a" ~' 6 PU-SN-1.27 269 276 288 300 372* PU-SN-2.01 270 278 285* 304 398* PU-SN-2.40 270 285 -309 407* • From TSDC thermograms of the annealed samples. processes involve motions of the soft polyester segments and the 6 process is associated with the segmental motions within hard domains. The position and intensity of the fl peak for dry and soaked in water samples are about the same, as for polyurethanes based on D B D I [1], suggesting that the fl peak could be ascribed to the motion of the polyester carbonyl groups. The relaxation characteristics of the samples derived from T S D C spectra are presented in Table 1. The apparent activation energies were determined using the initial current rise method applied to the T S D C spectra obtained by partial heating of the polarized samples. By partial depolarization of the samples, single values of the apparent activation energy were found for the ~t, ct", ~' and 6 peaks and ranges of values for the fl peak. These findings suggest that the fl process arose from a dielectric
~ 0.52-1.34 0.56-1.40 0.60-1.50
• 2.05 2.20 2.32
~' 6 2.32 2.85* 2.40 2.90* 2.47 2.93*
relaxation distributed in activation energy, whereas ~, ~", ~' and 6 processes presumably originate from a distribution of the relaxation processes in natural frequency. REFERENCES
1. I. Diaconu, C. Ciobanu and Cr. I. Simionescu. Polym. Bull. 20, 195 (1988). 2. I. Diaconu, C. Ciobanu and Cr. I. Simionescu. Polvm. Bull. 21, 203 (1989). 3. I. Diaconu and S. Dumitrescu. Eur. Polym. J. 14, 971 (1978). 4. H. E. Carius, G. Pohl and G. Goering. 31st IUPAC Macromolecular Syrup. 1987, Abstracts, Microsymposium IV-V, pp. 147. 5. H. Goering, H. E. Carius and G. Pohl. 31st IUPAC Macromolecular Symp. 1987, Abstracts, Microsymposium IV-V, pp. 147.