ABSTRACT
Interpenetrating polymer networks (IPN’s) are relatively novel combinations of two or more crosslinked polymer networks held together primarily by permanent entanglements (catenation) rather than mutual covalent bonds (grafting). IPN’s can be formed by sequential or simultaneous crosslinking of the component networks; in the latter case the resulting IPN’s are designated as SIN IPN’s. For weakly crosslinked rubbery networks with positive interaction parameter between the networks theory suggests that no sequentially formed IPN would be stable (to micro phase separation) over all network compositions but stable phases could occur for all IPN’s and pseudo-IPN’s (in which one network is not corsslinked). Many investigations of SIN-IPN’s have appeared in over more than a decade, carried out by a variety of groups, which we shall briefly summarize. Our principal focus though will be on the compatible crosslinked poly(2,6-dimethyl-l,4-phenylene oxide) (PPO)-poly-styrene (PS) and related IPN’s, the incompatible polyurethane – poly(methacrylate), polyurethane – epoxy and the three component, polyurethane – epoxy – poly(methacrylate) IPN’s and the stiff polydiacetylene and polydiacetylene-epoxy IPN’s studied recently by us. These studies included determination of the stress-strain curve and ultimate mechanical properties according to standard ASTM procedures. Dynamical mechanical properties were studied with a rheovibron. Glass temperatures were also obtained from differential scanning calorimetry. Micrographs were obtained both by transmission and scanning electron microscopy.
The PPO-PS IPN’s were single phase materials with a single glass transition temperature (Tg). The tensile strength to break curve as a function of PPO weight percent showed a well-defined maximum at about 75%. The PU-polymethacrylate IPN’s and PU-epoxy IPN’s exhibited a variety of morphologies and properties dependent on the type of polymer (e.g. NCO/OH ratio), molecular weight of precursors, presence of charge groups of either sign and the presence of intentional grafts between the component polymer networks. In general, decreasing molecular weight of prepolymers, presence of intentional grafts and/or charge groups of opposite charge results in enhanced homogeneity (due presumably to more interpenetration) and this in turn results in enhanced mechanical properties. Both single and two phased compositions of polydiacetylene – epoxy and two different polydiacetylenes could be 89prepared. These systems exhibited a number of unusual properties including Tg’s larger than those of either pure component.
