Macromolecules 1986, 19, 2589-2593
2589
Polyurethane-Polystyrene Interpenetrating Polymer Networks: Effect of Photopolymerization Temperature Bong Sup Kim and Doo Sung Lee* Department of Textile Engineering, Sung Kyun Kwan University, Suwon, Kyungki 170, Republic of Korea Sung Chul Kim Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Chongyangni, Seoul 131, Repubtic of Korea. Received March 11, 1986
ABSTRACT Interpenetratingpolymer networks (IPN’s)and linear blends of polyurethane-polystyrene were synthesized at low temperature. Polyurethane was polymerized thermally first, followed by the photopolymerization of polystyrene at low temperature (as low as 0 “C). The dynamic mechanical behavior, morphology, and thermal stability were analyzed to evaluate the effect of synthesis temperature and the effect of interlocking on the degree of intermixing of the component polymers. The dynamic mechanical behavior of polyurethane-polystyrene IPNs showed a gradual inward shift of the component Tis when the synthesis temperature was lowered, and the IPN synthesized at 0 O C showed a broad transition. The degree of intermixing of the components was lower in the linear blends than in the IPN’s. The polystyrene domain sizes of the IPN changed from about 2000 8, to 300 8, with decreasing synthesis temperature from 40 to 0 “C.
Introduction Interpenetrating polymer networks (IPN’s) have been prepared by three different methods: sequential polymerization,’ latex blending,2 and simultaneous polymerizat i ~ n . The ~ , ~simultaneous polymerization method involves mixing of monomers or low molecular weight prepolymers and cross-linking agents of the component polymers and polymerizing/cross-linking them simultaneously via noninterfering reaction mechanisms (e.g., free radical polymerization vs. step polymerization). As with other multipolymer systems, the incompatibility of the P N s arises from the low entropy of mixing obtained on blending the high molecular weight polymers. It is well-known that the interpenetration plays a significant role in enhancing the compatibility of the polymer components because physical interlocking prohibits phase separation when the molecular weight is built up during the simultaneous polymerization p r o ~ e s s . ~ The important factors in determining the morphology in IPN synthesis are the onset point of phase separation, the rate of phase separation, and the time of physical interlocking. The onset point of phase separation is the time when the Gibbs free energy of mixing becomes positive. The Gibbs free energy of mixing is determined by the conversion (or molecular weight), synthesis pressure, and synthesis temperature. The rate of phase separation is related to the mobility of the polymer chain and the medium viscosity of the reaction mixture and thus is indirectly related to the synthesis pressure and temperature. The time of physical interlocking or network formation is the time when both component polymers reach the gel point, and thus the phase domain size cannot increase much further beyond this point. The relative rate of the two competing kinetic processes of phase separation and network formation is the major factor of importance for control of the morphology of the final product. When the interlocking of the two component polymers occurs before the onset of phase separation, the interlocked state prohibits further phase separation, and the resulting simultaneous interpenetrating network (SIN) shows a homogeneous mixture of the two. When the interlocking occurs after the onset of phase separation, SINS with heterogeneous morphology are obtained where only partial interpenetration exists around the phase 0024-9297/86/2219-2589$01.50/0
boundaries of the dispersed phases. The domain size and the degree of partial interpenetration are dependent on the rate of phase separation and the timing of the interlocking. We were able to increase the degree of interpenetration in the incompatible polyurethane-poly(methy1 metha ~ r y l a t eand ) ~ polyurethane-polystyrene6-8 SIN’S by applying pressure during the simultaneous polymerization process and were able to obtain a transparent, nearly molecular level mixture of the two components. It appears that the synthesis pressure has several direct and indirect effects in determining the morphology of the SINS. At high synthesis pressure, the onset point of phase separation moves toward higher conversion and the mixture stays homogeneous a t high molecular weight. The synthesis pressure also affects the rate of phase separation by reducing the mobility of the polymer chain with reduced free volume. The combined effect of the increased compatibility and the reduced rate of phase separation increases the degree of mixing of the two component polymers at the time of interlocking. The synthesis temperature can be another factor enhancing the miscibility in multipolymer systems. The synthesis temperature, also, has direct and indirect effects on the onset point of phase separation, the rate of phase separation, and the time of interlocking. In general, the reaction mixture initially shows upper critical solution temperature (UCST) behavior. The onset point of phase separation, therefore, moves toward lower conversion with decreasing synthesis temperature. The rate of phase separation, however, is reduced due to the high medium viscosity of the reacting mixture at lower temperature as shown in Figure 1. In this study, IPN’s, semi-IPN’s (one of the components is linear), and linear blends (both components are linear) of polyurethane and polystyrene were prepared to study the effect of synthesis temperature and the cross-linked state on the phase separation and the network interlocking process.
Experimental Section Materials. Poly(tetramethy1eneether) glycol (PTMG, molecular weight 1045), 1,4-butanediol(1,4-BD),and trimethylolpropane (TMP) were degassed at 60 O C for about 4 h under vacuum. Styrene monomer was purified before use.O 4,4’0 1986 American Chemical Society
Macromolecules, Vol. 19, No. 10, 1986
2590 Kim, Lee, and Kim
SYNTHESIS TEMPERATURE(~C)
0.4t
A
20
0
40
0.3.c)
z
C,;
CONVERSION O F
2
U.2L
NETYORK I N T E R L O C K I N G VI;
VITRIFICATION LINE
c,
0.1-
CONVERSION
Figure 1. Schematic diagram showing the qualitative effect of the photopolymerization temperature on the phase separation and network interlocking process during the simultaneous polmerization of the component polymers: (1) mixture stays homogeneous at low conversion; (2) onset of phase separation; (3) phase separation process (rate is dependent on temperature); (4) time of network interlocking; (5) time-independent morphology; (6) vitrification of PS component (the conversion at which the Tgof the PS component mixture exceeds the reaction temperature).
Met hylenebis(phenyl isocyanate) (MDI), divinylbenzene (DVB, 55% purity), and benzoin were used without further purification. Synthesis. The isocyanate-terminated polyurethane prepolymer was prepared by reacting 1equiv of PTMG with 2 equiv of MDI at 60 OC. The reaction was carried out until the theoretical isocyanate content was reached as determined by the di-n-butylamine method. A mixture of 1,4BD and TMP in 1:l equivalent ratio was used as the cross-linking and chain-extending agent for the polyurethane (PU) network. DVB was used as the crosslinking agent for the polystyrene (PS) network, its composition being 2.5% by weight in styrene monomer. A thoroughly mixed and degassed mixture of PU prepolymer, TMP/1,4-BD mixture (amount being adjusted to give total NCO/OH ratio as l),PU catalyst tetramethylbutanediamine (TMBDA, 0.1 w t % of PU component mixture), styrene monomer, DVB, and benzoin (0.3 wt % of PS component mixture) was charged between two glass plates sealed with a rubber O-ring and kept at 40 "C for about 4 h for the polymerization of the component PU in the presence of styrene monomer mixture. Then this glass mold was inserted into a reaction jacket with a glass window, and the styrene monomer mixture was photopolymerized by exposing to UV light. The temperature of the reaction jacket was controlled by circulating a cooling medium, isopropyl alcohol. The photopolymerization of the styrene monomer mixture was carried out for 48 h at 0,20, and 40 O C . The PU component was allowed to react almost to form the network earlier with the intention of observing the effect of temperature mainly on the mobility of the polystyrene component to phase-separate during the photopolymerization. The resulting theoretical molecular weight between cross-link (M,) was 3300. The composition of the polyurethane and polystyrene was fixed at 50150 by weight. The linear blend and semi-IPN were formed by excluding the appropriate cross-linking agents (TMP for PU and DVB for PS) in the component polymer formulations. Samples prepared were coded as follows: the first letter denotes the polymer component (U for polyurethane and S for polystyrene), the second letter, C or L, denotes the polymer network (C = cross-linked, L = linear), and the third numeral denotes the weight percentage of the component polymer. The IF",two types of semi-IPN, and a linear blend were prepared at various temperatures. Samples were dried under vacuum at room temperature for 3 days before testing. The maximum percentage of the removed styrene monomer in the polystyrene phase was about 1.5% by weight. Dynamic Mechanical Analysis. The dynamic mechanical properties were measured on a DuPont 981-990 dynamic mechanical analyzer (DMA). The oscillation amplitude was 0.2 mm and the gap setting was 6.4 mm. The scanning rate was 5 OC/min over a temperature range of -50 to +130 O C . Rectangular (1.5 mm thick, 6 mm wide, 16 mm long) test specimens were prepared. Electron Microscopy. Transmission electron micrographs were obtained on a JEM 100 CX (JEOL) electron microscope.
01
-40
-20
0
20
40 TEMPERATURE
,
60
80
100
(OC)
Figure 2. Dissipation factor (tan 6) w. temperature of UC5OSC50 IPN's synthesized at various temperatures.
SYNTHESIS
TEMPERITURE(~C)
-
A
20
0
40
-20
0
0 . .2 1 O
, -40
20
40
60
I 80
100
TEMPERATURE(~C)
Figure 3. Dissipation fador (tan 6) vs. temperature of UL50SL50 linear blends synthesized at various temperatures.
The sample preparation technique used was based on Kato's osmium tetraoxide staining techniquelo and Matsuo's two-step sectioning method." Thermogravimetric Analysis. A DuPont Model 951 thermogravimetric analyzer was used to measure the thermal stability. The sample weight was about 10 mg, N2 flow rate was 120 cm3/min, and the heating rate was 20 OC/min.
Results and Discussion Dynamic Mechanical Behavior. T h e dynamic mechanical properties of the UC5OSC50 IPNs and UL5OSL50 linear b l e n d s synthesized u n d e r different photopolymerization temperatures for the PS polymerization are shown in Figures 2 and 3. IPNs prepared at 40 O C distinctly show the Tis of the PU and PS components. With low synthesis temperature, a gradual inward shift of t h e two T i s is noted. T h e y finally merge to form a broad transition where the PU transition appears as a shoulder when t h e synthesis temperature is set at 0 " C . As the polymerization of the styrene monomer proceeds and the molecular weight increases, the Gibbs free energy of mixing increases a n d t h e phase separation starts t o occur. As the conversion of t h e styrene monomer reaches the gel point, the phase separation is nearly stopped a n d fixed at that morphology due to the interlocking of the two cross-linked polymer networks. With low photosynthesis temperature, the chain mobility of the PS is reduced due to increased medium viscosity, a n d the rate of phase separation is lowered. T h e rate of network formation is slightly decreased at low photosynthesis temperature. T h u s t h e effects of t h e reduced ratio of t h e rate of phase separation to the rate of network formation increases the
Macromolecules, Vol. 19, No. 10, 1986
Effect of Synthesis Temperature for PU-PS IPN 2591
Table I T,and Polymer Composition (Calculated from the Fox Equation) of the PU- and PS-Dominant Phases" composition sample code
uc50sc50
synthesis temp, "C 0
low Tg, K
high Tg, K
20 40
287 281 268 269 263 291 271
345 349 357 342 353 363 342 346
UL5OSL50
0 20
40 UC5OSL50 UL5OSC50
0 0
PU-rich phase PU PS 0.76 0.84 0.87 0.86 0.93 0.70 0.84
PS-rich phase
PU
0.16 0.12 0.06 0.14 0.06
0.24 0.16 0.13 0.14 0.07 0.30 0.16
PS 0.84 0.88 0.94 0.86 0.94
0
1
0.16 0.13
0.84 0.87
" Tgof the homopolymer: UC100, 269 K; UL100, 258 K; SC100, 364 K; SL100,361 K. The-T,'s of the PS homopolymers synthesized at 0,20, and 40 "C are in the experime_ntal_errorrange. The number-average molecular weight (M,) of the linear polystyrene was from 100000 to 102000 and the polydispersity (M,/M,) was 4.36. The data were obtained by gel permeation chromatography (GPC).
0.3
0
uc50sc50
0
uc5osLso
A
ULSOSC50
A
ULSOSL50
0 uc50sc50 0 UCSOSLSO 0.30 z
A
UL5OSC50
A
ULSOSL50
0.2 L
c
0.1
20
40
0
SYNTHESIS TERPERATURE(OC)
O L -40
I
-20
,
0
40
20
TEMPERATURE
60
,
80
1
100
(OC)
Figure 5. Calculated mass fraction of PS in the PU-dominant phase in IPN's, semi-IPN's, and linear blends.
Figure 4. Dissipation factor (tan6 ) vs. temperature of the IPN, the semi-IPN's, and the linear blend synthesized at 0 "C (for PU5OPS50).
0 uc5oscso 0 UC5OSL50
A
OaJ
degree of mixing of the two component polymers a t the time of interlocking. The tan 6 vs. temperature plot of the linear blends synthesized under different temperature shows a similar trend, but its degree of inward shift due to the intermixing of the component polymer chains is lower than that of the IPNs due to the absence of the interlocking of cross-linked networks which prohibit further phase separation (Figure 3). The reason the degree of mixing of the linear blend increases with decreasing synthesis temperature is presumed to be the combined effect of the reduced rate of phase separation and the early stopping of phase separation due to the early vitrification a t low temperature. To observe the effect of the cross-link state, the tan 6 curves for the UC5OSL50 semi-I IPN (PU cross-linked/PS linear) and the UL5OSC50 semi-I1 IPN (PU linear/PS crosslinked) synthesized at 0 "C is compared to the IPN and linear blend synthesized at the same temperature (Figure
4). The shifts of the T i s of the PU-dominant phase (low
T,) and the PS-dominant phase (high T,) are listed in Table I. The higher degree of intermixing between the networks is shown as the inward shifting of the Tis of both the PU- and PS-dominant phases. The low Tg of the sample synthesized a t 0 "C is not detected because of difficulties in obtaining the maximum peak point in the DMA curve due to the very broad transition. This Tgshift is converted to the mass fraction of the PS and PU component polymers within each dominant phase by assuming that the Fox equation is valid in this system:12
-1--- w1 + -w2 Tg
Tg1
Tg2
2
0.1 0.2
i
A
UL~OSCSO
f
l
;
a3 . 0
cI-40
20
0
SYNTHESIS TEMPERATURE(OC)
Figure 6. Calculated mass fraction of PU in the PS-dominant phase in IPNs, semi-IPN's, and linear blends. where Tgl and Tg2represent homopolymer T i s of the PU and PS components and w1 and w z represent weight fractions. The resulting mass compositions within each dominant phase are shown in Table I and in Figures 5 and 6. When the IPN is synthesized a t 40 OC, there exists only partial interpenetration of the two component polymers, presumably because of the relatively high rate of phase separation as compared with the reaction rate. Thus, the IPNs have a two-phase structure with a PS-dominant phase and a PU-dominant phase. The degree of intermixing of the PS component in the PU-dominant phase is shown to be 16% by weight. As the synthesis temperature is decreased, the mass fraction of the PS component in the PU-dominant phase increases and reaches as high as above 30% due to the relatively low rate of phase separation. The linear blends have the lower degree of intermixing as expected due to the linear nature of the component polymers. A t the synthesis temperature of 0 "C, the degree of mixing of the UC5OSC50 IPN and UC5OSL50 semi-I IPN
2592 Kim, Lee, and Kim
Macromolecules, Vol. 19, No.10, 1986
I
-Lo
linear PS in semi-I1 IPN and linear blend in the degree of intermixing is also not expected. A t the low synthesis temperature of 0 "C, therefore, the degree of mixing is controlled by the linear or cross-linked nature of the PU component showing a low Tg.without regard to the cross-link state of the PS showing a high T,. The effect of decreasing synthesis temperature on the miscibility of PU-PS IPN and linear blend (Figures 5 and 6) shows a similar trend to the effect of increasing synthesis pressure which was previously reported!" From these results, we could expect that the miscible polymer blend can be prepared whenever the relative reaction rate is faster than the rate of phase separation. The dynamic Young's modulus vs. temperature plot (Figure 7) of the PU-PS I P N s shows two distinct 7s;' in the I P N s synthesized a t 20 and 40 "C and one broad transition in the IPN synthesized a t low temperature ( 0 "C),which also shows an increased degree of intermixing of the component polymers when the IPN is synthesized a t low temperature. Morphology. The morphology via transmission electron microscopy of the I P N s also shows the synthesis temperature effect on the miscibility and phase structure (Figure 8). The PU phase is stained by osmium tetraoxide and appears black and the unstained PS phase appears white in the micrographs. The morphology of I P N s synthesized at 40 "C shows a somewhat cocontinuous structure of the PU and PS phases, and the domain size of the PS phase is in the range 2OC€-3000 A. As the synthesis temperature is decreased, the PS phase forms completely dispersed domains, and the PS phase domain size decreases as the intermixing increases. The IPN synthesized at 0 "C shows domain sizes in the range of hundreds of angstroms (about 200-300 A). A t higher magnification, the IPN synthesized at 0 "C reveals that it has undergone secondary phase separation in the PU-rich matrix phase.' The secondary phase separation in the PU-rich matrix phase is also shown in the IPN synthesized a t 20 "C, but the amount is smaller than that of the IPN synthesized at 0 O C . The T,of the PU phase is affected significantly by the small domain of the secondary phase-separated PS in the PU matrix, while the T of the PS phase is determined mainly by the primary passe-separated PS domain. Thermal Stability. The enhancement of the thermal stability of PU-PMMA" and PU-PS IPNs3 was reported, and it was presumed that the unzipped MMA or styrene
I -20
20
'0 ,S".l"Al""E
60
10
(00
P
Figure 7. Dynamic Young's modulus vs. temperature of the UC5OSC50 IPNs synthesized at varying temperatures. is almost equal, and that of the UL5OSC50 semi-I1 IPN and linear blend is also similar. In the case of IPN and semi-I IPN synthesized at 0 "C, it is considered that the phase separation is stopped mainly by the vitrification of the PS chain at low conversion. In other words, the vitrification of the polymerizing PS component mixture occurs earlier or at nearly the same conversion as the conversion that the network interlocking occurs in I P N s (Figure 1). Thus the differences in the cross-linked and linear PS in the degree of phase separation are not noted in this case. The vitrification is expected to occur a t higher conversion when the synthesis temperature is raised to 40 "C, and the network interlocking occurs earlier than the vitrification. Thus differences in the degree of intermixing between IPN and semi-I IPN are expected in this case. Gillham and Peng have illustrated the gelation-vitrification-chemical reaction ('I" of) epoxy systems, whose mechanism of polymerization is step polymerizati~n.'~The rate of polymerization of the styrene and epoxy after the vitrification might be somewhat different because of differences in polymerization mechanisms. When the PU component is made linear, phase separation occurs continuously throughout the polymerization process due to the mobile PU chain (T,= -15 "C by DMA). The difference of the cross-linked PS and the
(a)
(b)
Figure 8. Electron micrographs of UC5OSC50 IPNs synthesized at (a) 40, (b) 20, and
(C)
(c)
0 OC.
Macromolecules 1986, 19, 2593-2601
r
-x
60
-
40
-
I
* Iz
SYNTHESIS TEMPERATURE
Y
YI +
t I
"
UClOO
v
SClOO 0
w
Y 20
-
01
0
uC5OSC50 A
200
of the weight retention of the pure components (Figure 9). On the other hand, the PU-PS linear blends synthesized a t 0 "C and 40 "C show the approximately proportional average of the weight retention of the pure components (Figure 10). But the additional enhancement of the thermal stability expected due to the increased miscibility in IPN's and linear blend synthesized at low temperature is not observed. The thermal stability of the highly miscible homogeneous IPNs synthesized at high pressure was not increased in comparison with the heterogeneous I P N s synthesized a t atmospheric pressure.8
OC
4 0 OC
(0
2593
1
300
400
500
Acknowledgment. This work was supported by the Korea Research Foundation. We thank Dr. J. K. Yeo of Lucky Ltd. for help in the electron microscopy work.
TEMPERATURE(OC)
Registry No. (1,4-BD).(PTMG)-(MDI).(TMP) (copolymer),
Figure 9. TGA thermograms for UC5OSC50 IPN's.
39281-41-9;(PS).(DVB) (copolymer), 9003-70-7.
References and Notes (1) Huelck, V.; Thomas, D. A.; Sperling, L. H. Macromolecules 1972,5, 340,348.
80-
x
;600 t
z
SYNTHESIS
t
SL100 0
01
I
200
,
I
300
I
I
400
I
5ao
TEMPERATURE(~C)
Figure 10. TGA thermograms for UL5OSL50 linear blends. monomer acted as the radical scavenger for the radicals produced from the PU degradation. The PU-PS IPNs that were prepared in this series show the enhancement of weight retention compared to the proportional average
(2) Klempner, D.;Frisch, H. L.; Frisch, K. C. J . Polym. Sci., Part A-2 1970,8, 921. (3) Kim, S. C.; Klempner, D.; Frisch, K. C.; Frisch, H. L.; Ghiradella, H. Polym. Eng. Sci. 1975, 25, 339. (4) Kim, S.C.; Klempner, D.; Frisch, K. C.; Radigan, W.; Frisch, H. L. Macromolecules 1976, 9, 258. (5) Lee, D.S.;Kim, S. C. Macromolecules 1984, 17, 268. (6) Lee, D.S.;Kim, S. C. Macromolecules 1984, 17, 2193. (7) Lee, D.S.;Kim, S. C. Macromollecules 1984, 17, 2222. (8) Lee, D.S.; Kim, S. C. Macromolecules 1985, 18, 2173. (9) Collins, E.A.;Bares, J.; Billmeyer, F. W., Jr. Experiments in Polymer Science; Wiley: New York, 1973. (10) Kato, K. J . Polym. Sci., Part B 1966, 4 , 35. (11) Matsuo, M.; Kwei, J. K.; Klempner, D.; Frisch, H. L. Polym. Eng. Sci. 1970, 10, 327. (12) Sperling, L.H. Interpenetrating Polymer Networks and Related Materials; Plenum: New York, 1981. (13) Peng, X.;Gillham, J. K. J. Appl. Polym. Sci. 1985, 30, 4685. (14) Kim, S.C.;Klempner, D.; Frisch, K. C.; Frisch, H. L. J . Appl. Polym. Sei. 1977, 21, 1289.
Translational Diffusion of Polystyrene Single Chains in Semidilute Solutions of Poly(methy1 methacrylate)/Benzene As Measured by Quasi-Elastic Light Scattering Naoko Numasawa, Kenji Kuwamoto, and Takuhei Nose* Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152,Japan. Received April 8, 1986
ABSTRACT The translational diffusion coefficient (D)of single polystyrene (PS) chains in isorefractive solutions of poly(methy1 methacrylate) (PMMA)/benzene was measured by quasi-elastic light scattering for a variety of molecular weight combinations of guest (MN) and host (Mp) polymers and matrix concentration (C) in the semidilute region. The radius of gyration R, of P S and the macroscopic viscosity q of the matrix solution were also measured. Results of D as a function of MN, Mp, and C strongly suggest crossover from reptational to Stokes-Einstein (S-E)type diffusion. The onset of reptational diffusion is around MN/MtN = Mp/Mcp= 4,where Mt is the molecular weight yielding the dimension of the static correlation length. The crossover curve separating the two regions has been determined on a (MN/McN)-(Mp/Mtp).map. In the S-E type region, D a flR;* holds approximately, but the ratio of R, to the hydrodynamic radius is 50-100% higher than that in pure benzene.
Introduction Self-diffusion of a single labeled chain in highly entangled polymer solutions has been of recent interest. Several modern techniques have been applied to measure the 0024-9297/86/2219-2593$01.50/0
self-diffusion coefficient: forced Rayleigh scattering (FRS),1-3pulsed field gradient NMR (PFGNMR),4-8dynamic light scattering with an optical labeling method (OL-DLS),*14 and others, as reviewed by Tirrell.l5 0 1986 American Chemical Society
