The structure and properties of the compositions of polyisocyanurates modified with low molecular weight rubber networks have been investigated by means of dynamical mechanical analysis (DMA), electron microscopy, and stress-relaxation experiments. The network compositions consist of two different polymeric networks. The first component (macrodiisocyanate based on low molecular weight polybutadiene or copolymers of tetrahydrofuran and propylene oxide) has bulky cross-linked points connected by short flexible chains. The second component (diphenylmethanediisocyanate) also has bulky cross-linked points of the same structure, but the linear fragments between them in this case are very small and rigid. These compositions result in the formation of the heterophase system. As a result, transparent samples were prepared, which differ from the mechanical properties of both the glassy and rubbery polymers. These materials have a modulus of elasticity (from lo3 to 10 MPa) that is usual for the transition zone between the glassy and rubbery states; nevertheless, these materials show elastic (and not viscoelastic) properties. For the materials investigated, the modulus is decreased not more than 10 times in the wide temperature interval from 200 to 500 K. A new state of the polymer, which differs from both the glassy and rubbery states, has been identified in the present case. 0 1995 John Wiley & Sons, Inc.
I NTRO DUCT1 0 NModification of rigid polymer networks with reactive rubber oligomers permits one to increase the impact resistance of polymers significantly. Thermodynamical analysis of phase transitions in different polymers, e.g., in epoxy-rubber systems,' shows that the thermodynamical compatibility of the components varies depending on the content of rubber during the polymerization process and, hence, phase separation occurs. The investigation of the rupture mechanics in the epoxy-rubber systems 2-5 reveal that the rupture effective energy depends on the rubbery phase particles' dimensions, which, in turn, depend on the compatibility of the components. The smallest dimensions of rubber particles (about 1000 * To whom correspondence should be addressed. A) have been reported in compositions with polar rubbers (e.g., butadieneacrylonytrile ) .2-5 Thus, one can change the phase separation and properties of a copolymer by changing the composition of the rubber oligomers.It is known that usual polymer networks in the glassy state have an elastic modulus about 2-3 X lo3 MPa, which is almost independent of the network's chemical structure. After the glass transition, the rubbery polymer has a modulus some orders lower (from 0.1 to 30-50 MPa for polymers with various cross-linked points density). Such a decrease in the polymer's modulus occurs in a very narrow temperature interval (20-30°C).It is also known that for both glassy and rubbery states a relaxation stress comes quickly to the quasiequilibrium state and the equilibrium stress urn for them is finite. For polymers in the transition zone, the relaxation process goes ...
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