Interpenetrating polymer networks (IPN's) have been synthesized by swelling a cross-linked rubbery polymer (I) with a second plastic monomer (II or III or II-co-III) plus initiator and cross-linking agent and polymerizing the second monomer in situ. IPN's have also been produced by inverting the order of preparation. According to the overall compositions, IPN's of elastomeric or leathery or plastic behavior have been obtained. Polymers employed were poly(ethyl acrylate) (I), polystyrene (II), and poly(methyl methacrylate) (III). Like most other types of polymer blends, IPN's exhibit a complex twophase morphology. The electron micrographs show a characteristic cellular structure of about 1000-A diameter simultaneously with a fine structure with phase domains of the order of 100 A. In the midrange leathery materials the cell walls are composed of the second network polymer. The fine structure is observed most clearly within the cell walls, and probably originates through a second, later phase separation as polymerization continues beyond the initial cellular formation stage. Increasing compatibility of the two polymers is attained as methyl methacrylate mers replace styrene mers in the plastic component. This leads to the disappearance of the cellular envelopes but retention of the fine 100-A domain structure. Inverting the sequence of preparation (swelling monomer I into network II or III) showed that the network synthesized first controls the morphology of the IPN's, comprizing the more continuous phase.
The physical and mechanical properties of poly(ethyl acrylate)-poly(styrene-comethyl methacrylate), PEA-P(S-co-MMA), interpenetrating polymer networks (IPN's) have been investigated. Dynamic mechanical spectroscopy measurements via Vibron instrumentation show how compatibility increases as MMA-mers replace S-mers in the IPN. Principally, two reasonably sharp glass transitions are replaced by one very broad continuous transition, which may be interpreted as a continuous range of transitions reflecting different local compositions. The extent of molecular mixing increases as the percent MMA increases, but a significant amount of phase separation remains even with the PEA-PMMA IPN's. Application of Bauer's theoretical treatment suggests that both phases exhibit some degree of continuity for most, if not all, of the materials investigated. Stress-strain and tensile data show that the work to break as well as the actual tensile values of the samples steadily increase as the amount of plastic component is increased in the elastomer-rich materials. These IPN's show many features common to the toughened blends and block copolymers. An analysis of the impact properties of IPN's in the light of Bragaw's theory of crack branching in blends confirms the widely found empirical fact that 7g of the rubber component must be at least 600 below test temperature to achieve significant impact improvement.
SynopsisThe creep behavior of a series of poly(ethy1 acry1ate)-poly(methy1 methacrylate) interpenetrating polymer networks was investigated. For comparison purposes, soMe stress relaxation data were included. Master curves containing a single broad transitibn covering approximately 20 decades of time were found for midrange compositions. qthough the time-temperature superposition principle and the WLF equation should not strictly apply, reasonable agreement was found over a large portion of shift factor versus temperature plots. Application of a modified Tobolsky-Aklonis-Dupre glass-rubber theory suggested that the breadth of the transition could be attributed to a near continuum of phase compositions in the material, each phase composition making its specific contribution to the relaxation spectrum. Whether or not these phase regions @re so small as to arise from random concentration fluctuations in an otherwise compatible polymer pair remains unknown.
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