Equilibrium and Nonequilibrium Microphase ... - ACS Publications

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 28, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch004

4 Equilibrium and Nonequilibrium Microphase Structures of Interpenetrating Polymer Networks Yuri S. Lipatov Institute of Macromolecular Chemistry, Ukrainian Academy of Sciences, 253160, Kiev, Ukraine

Formulation of a new approach to the structure of interpenetrating polymer networks (IPNs) is the objective of this review of the author's experimental works. As a whole, IPNs are not thermodynamically equilibrium systems because, in the course of chemical reactions leading to gelation, simultaneous phase separation proceeds due to increased thermodynamic immiscibility. However, the phases that evolve preserve the inherent structure of the state of mixing at an earlier stage of the reaction, before the onset of phase separation. Thus, the IPN is considered to be in a state of quasi-equilibrium with the molecular level of mixing. As a consequence, the nonequilibrium microphase structure of the IPN is described as a microheterogeneous system with a lack of molecular mixing of the two constituent networks throughout the bulk, but with a limited level of forced molecular mixing in each of the phases.

PROPERTIES OF INTERPENETRATING POLYMER NETWORKS (IPNs) depend On both the thermodynamic miscibility of the constituent networks and the kinetic conditions of the cross-linking reaction. The principal work on IPNs has been done by Sperling (I) and Frisch et al. (2). Research has established that IPNs have a complex structure and morphology and frequendy exhibit dual-phase continuity (3). Thermodynamic considerations have yielded equa0065-2393/94/0239-0125$06.00/0 © 1994 American Chemical Society

Klempner et al.; Interpenetrating Polymer Networks Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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INTERPENETRATING POLYMER NETWORKS

tions that predict domain sizes in IPNs (4, 5), and the two-phase morphology was investigated by direct structural methods (6-9). A phenomenological theory was formulated for chemically quenched binary IPNs and the condi tions of their phase stability were elucidated by Binder and Frisch (10). A l l of these results support the conclusion that IPNs cannot be considered as systems with a molecular level of mixing of the two networks. With some exceptions, IPNs have a heterogeneous structure and are multiphase systems. The structural heterogeneity in IPNs is caused by the thermodynamic immis cibility of the networks that arises at a definite degree of conversion in the course of the chemical reactions of polymerization and cross-linking. The thermodynamic immiscibility leads to microphase separation that proceeds in accordance with the spinodal mechanism (11). This chapter provides a review of the critical papers produced in Kiev on the microphase structure of IPNs. The principal features of the mechanism and the kinetics of I P N formation and microphase separation will be discussed.

Development of Thermodynamic Immiscibility in IPNs The thermodynamic immiscibility of the constituent networks in an I P N begins at low degrees of conversion [both for full and semi-IPN (SIPN)]. For example, Figure 1 shows the phase diagrams of SIPNs based on a styrene-divinylbenzene copolymer and poly(butyl methacrylate). The regions that corre spond to the two-phase SIPN (hatched areas) are much larger than the regions of the one-phase state. In the phase diagram, heterogeneous regions are separated from the homogeneous regions by a binodal curve. In the course of SIPN formation, the polymerizing system passes from point A to point A which correspond to the initial mixture and to the SIPN, respec tively. Phase separation occurs after the border of the two-phase region (point 0) is passed. A n increase in the reaction temperature does not change the shape of the diagram; however, the area of the one-phase state slightly increases, which is typical for systems with an upper critical solution tempera ture. Analysis of the region of I P N compositions situated inside the triangle B B j C shows that polymerization of mixtures with poly(butyl methacrylate) (PB M A ) content above some critical value is accompanied by phase separa tion (12) almost from the very beginning. Figure 2 presents kinetic curves of the reaction of cross-linking polyurethane in the presence of poly(butyl methacrylate). The arrows indicate the onset time of phase separation. Microphase separation begins very early and depends on the kinetic condi tions. Characteristics of the system are given in Table I. 1 ?

These data show the dependence of phase separation time on the reaction conditions and composition of the system. From these data it follows that an interconnection exists between the kinetic conditions of reaction and



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Equilibrium and Nonequilibrium Microphase Structures

styrene/DVB

c

styrene/DVB

Styrène

Copolymer styrene/DVB

d

PBMA

Styrene

Copolymer styrene/DVB

PBMA

Figure 1. Phase diagrams of SIPN: styrene-divinylbenzene-poly(butyl methacrylate) network copolymer at temperatures of 333 (a), 343 (b), 353 (c), and 363(d) Κ (12).

the properties of the phase-separated system. A n absence of inflection points on the kinetic curves at the onset of microphase separation is noted. The lack of an inflection point often shows an absence of volume changes brought about by microphase separation, although the volume change itself does not affect the reaction rate. Such a situation may be possible only when phase separation leads to the appearance of two phases that have compositions very close to one another (a sign of spinodal decomposition). The existence of a definite correlation between the kinetic conditions of chemical reactions leading to I P N formation and the degree of microphase separation has been shown in many cases (13). Concurrently, a definite mutual influence of the reaction mixture components on the kinetics of formation of constituent networks in IPNs exists: the rate of cross-finking of each network is dependent on the network ratio (Figure 3). The conversion degree of the components that form the I P N at the onset of phase separation also depends on the cross-linking level of each network.



128


120

240

360

T(min)

Figure 2. Kinetic curves of polyurethane formation in the presence of 15 wt% of PBMA. a is the conversion degree. For designations, see Table I. (Reproduced with permission from reference 13. Copyright 1990.)

Superposition of Chemical and Physical Kinetics Both processes that accompany I P N formation—chemical reaction of crosslinking and physical process of microphase separation-—proceed simultane ously in the time interval between the onset of phase separation and the gel point of each network. Such superposition results in a complicated I P N


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Table I. Physieochemical Parameters of Eight SIPNs Based on PU-PBMA (85:15)


Parameters Curing temperature, ± 1 (K) Catalyst mass fraction, ±0.001 Reaction rate constant, Χ Π Γ (kg/mol s) Phase separation onset, ± 1 (min) Conversion degree at phase separation, α Conversion degree at anamorphose inflection point, α, +0.01 Glass-transition temperature of PU-enriched phase, + KK) Glass-transition temperature of PBMA-enriched phase, ±100 Segregation degree (SD) 4

1

2

3

4

5

6

7

8

333

333

313

333

333

333

353

313

0.005 0.010

0.010

0.010

0.005 0.050 0.010

0.010

3.46

1.90

2.62

1.3

8.1

7.05

0.73

7.63

—

—

—

43

100

28

30

29

—

—

—

0.23

0.13

0.39

0.56

0.48

—

—

—

0.68

0.58

0.74

0.82

0.47

253

261

248

265

249

316 0.33

333 0.32

323 0.35

315 0.32

318 0.34

253

— —

251

— —

255

— —

NOTE: Data were taken from reference 3.

microphase structure. In this case, when curing occurs simultaneously with growth in conversion degree, the composition and molecular weights of the network fragments change with time. The onset of phase separation takes place when the thermodynamic interaction parameter, χ , reaches a critical value or, more generally stated, when the free energy of mixing becomes positive. The rate at which this critical value is achieved depends on the rates of two independent curing reactions. After reaching the critical value, χ continuously changes with increasing cross-link density. As a result, the thermodynamic immiscibility of the I P N components and the driving force for phase separation increase as well. Despite the nonequihbrium conditions for both chemical and physical processes and because of their superposition, the phase separation may be described as a spinodal decomposition. The experimental data show the corresponding relationships (11). Α Β

Α Β

Incomplete Phase Separation Both the chemical reactions and the phase separation proceed under nonequihbrium conditions. After some high degree of chemical conversion and cross-linking is reached, however, microphase separation is impeded and the system fixes at a nonequilibrium structure characterized by incomplete



130


60

180

300

T(min)

Figure 3. Kinetics of formation of simultaneous IPN: Curing polyurethane (curves 1-3) and polymerization of butyl methacrylate (curves 4-6) at various component ratios: 85:15 (curves 1 and 4); 75:25 (curves 2 and 5); 65:35 (curves 3 and 6). a is the degree of conversion of the components. phase separation. Thus completion of the I P N polymerization evolves in two phases. The separated system may be characterized by the segregation degree (II), which can be calculated from small-angle X-ray scattering (SAXS) data or from the viscoelastic functions (14). Figure 4 shows schematically the temperature dependence of the mechanical losses in a two-phase polymer system with differing degrees of component segregation (miscibility). Case a corresponds to a mixture of two components where the phases are clearly separated and each phase is characterized by its own glass-transition tempera ture (maximum tan δ); case e corresponds to a fully miscible system. From the curve parameters the segregation degree may be estimated easily as


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1.00

(a)

t


131

*>

«4 !

L—

φ 2 U) Φ T3 c g

c

(d)

I

(e)

j

_

0.51

»V« !

«Φ i_

CD Φ

0)

0.14

0

1

J\\ J I \

I

T

l

T

2

T

Figure 4. Temperature dependence of mechanical loss tan δ at various degrees of component segregation. (Reproduced with permission from reference 14. Copyright 1985 Blackwell Scientific Publications, Ltd.)

where the significance of α can be gleaned from Figure 4. As illustrated in Figure 4, l and l represent the temperature shift of the lower and upper glass transitions, respectively. The quantities h and h represent the intensi ties of the transitions above the plateau region between the transitions. The superscript zero refers to the pure components. The quantity L is the initial temperature difference between the peaks. The subscript m refers to the temperature and intensity of an intermediate mixed phase peak, if any. Equation 1 is a semiempirical equation to determine the extent of phase separation; it is a more sophisticated model than the model used earlier by Curtius et al. (15). With reference to Figure 4, l h = 0 for parts a, b, and c because there is no mixed phase peak. The quantity h l is finite only for part d. For part e, the equation does not apply because there is only one peak. Note that and l h are measures of the area under the peaks. When ZJL + l — L , a microheterogeneous morphology emerges. x

2

x

2

m

m

m

2

m

2

2


132


Table II. Dependence of Segregation Degree on Reaction Temperature for IPN Based on Poly(butyl methacrylate) (PBMA) and a Copolymer of Styrene-Divinylbenzene


Component Ratio (PBMA.PS) 95:5 85:15 60:40

333 °C

353 °C

363 °C

0.46 0.30 0.15

0.51 0.32 0.20

0.65 0.43 0.40

In general, the segregation degree a determines the fraction of the system mass that is phase separated: for fully separated systems, α = 1; for miscible systems, α = 0. Thus, α is an effective measure of phase separation. This value strongly depends on the kinetic conditions (Tables I and II). The comparatively low degrees of segregation in most IPNs (0.3-0.6) shows that a large fraction of the system mass is preserved in the unseparated state and distributed between two phases. Both phases represent mixtures of both components; each phase is enriched by the opposite component. This state ment is illustrated in Figure 5. There are two distinguishable relaxation maxima that correspond to the two phases. The position of these maxima does not correspond to the transition temperatures of the pure components. Thus, the position of the two maxima depends on the reaction kinetics that govern phase separation. The dependence of the transitions on the kinetics of curing was observed for many systems. At the same time, it was found that the degree of phase separation is determined by the kinetic conditions and that increased reaction rates lead to a diminished degree of phase separation. A n increase in the reaction rate decreases the time for the onset of phase separation. As the reaction rate of I P N formation increases, the segregation degree decreases (16). In some cases this inverse relationship may lead to incorrect conclusions about the miscibility of the two networks, as judged by their mechanical behavior.

Nonequilibrium Structure of IPNs The microphase structure of IPNs may be described using the concept of the formation of various clusters: "physical" clusters due to phase separation of constituent networks under conditions of nonequilibrium phase transition and "chemical" clusters due to cross-linking. The real structure of an I P N should be considered as a multiphase structure that is determined by the coexistence of at least three "phases" (not in the true thermodynamic sense). Two phases are formed by the networks due to phase separation. Each phase may be considered as an independent I P N in which phase separation did not take place (the state of "forced" compatibility) (I J) and in which mixing near the molecular level is preserved. The composition of these two phases is deter-


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134


mined by the reaction rate and the temperature. Each phase has an average composition that does not correspond to the ratio in the I P N . The third "phase" is the nonequilibrium transition zone from one phase to another; its dimension is dependent on the conditions of phase separation. This zone may be called the mesophase and may be considered a nonequihbrium I P N phase because the molecular level of mixing is also preserved. At spinodal decompo sition conditions typically there is no sharp border between coexisting regions of phase separation. This transition zone may be chosen in such a way that its composition corresponds to the average composition of the I P N . The volume fraction of this zone depends on the conditions of I P N formation. The appearance of microregions of various composition constitutes the reason for the strong structural heterogeneity in IPNs. In general, the microphase structure of IPNs may be described as nonequilibrium. However, in such a nonequilibrium I P N one can still distin guish various microregions that can be described as quasi-equilibrium; that is, microregions with a near molecular level of mixing of the constituent compo nents. Only the thermodynamic stability of the IPNs is discussed here. The nonequilibrium state and thermodynamic instability of IPNs do not imply the instability of physical and mechanical properties because the relaxation time to establish true equilibrium is unattainable due to entanglements. A l l the ideas connected with the nonequihbrium multiphase structure of IPNs are in a good agreement with a comparatively low segregation degree in IPNs. In such a way the whole structure of IPNs may be presented as a mesophase matrix with embedded microphase regions that represent two evolved phases. Such a structure model coincides with the spinodal mecha nism of decomposition and many X-ray and electron microscopy studies ( I I , 17). Thus, two thermodynamic states in IPNs can be distinguished. The I P N as a whole is a nonequilibrium system due to incomplete phase separation and thermodynamic immiscibility of the constituent networks. Figure 6 shows the free energy of mixing for a polyurethane-poly(ethyl acrylate) ( P U - P E A ) I P N ; the positive values signify the lack of miscibility. However, the two phases may be considered as quasiequihbrium phases because they are the result of microphase separation and each phase preserves the composition that almost corresponds to the one-phase state; that is, the equihbrium state of molecular mixing. Because the level of mixing that was inherent to the state of mixing at earlier reaction stages is substantially preserved, which corresponds to the onset of phase separation, these phases are called quasiequilibrium phases. Thus, the nonequilibrium microphase structure of IPNs may be presented as a microheterogeneous two-phase system that lacks molecular mixing of two constituent networks throughout the bulk and with near molecular level of mixing in each phase and transition zone (mesophase).



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