Chemical Effects on the Ultimate Properties of Polymer Networks in the Glassy State
Highly crosslinked polymer networks have been prepared by crosslinking low molecular weight, reactive copolymers. The dependence of the ultimate properties of these networks in the glassy state on the molecular weight of the prepolymer, its functionality, the stoichiometry of the reactants and conversion of the reactive groups has been measured. It is shown that the ultimate properties are remarkably sensitive to slight variations in chemical composition and network topology. This dependence could not be rationalized in terms of network parameters or defects on a molecular scale (as is the case in the rubbery state) but might be explained if incoherent network structures were to form.
A statistical theory is developed based on configurational restrictions, which provides a general mechanism of formation of crosslink density inhomogeneities. The theory predicts that each intramolecular reaction enhances the probability that more intramolecular reactions take place, leading to crosslinked regions which are poorly connected with the matrix or with each other.
Accordingly, carbon replicas of fracture surfaces of the thermosets before and after etching with sulfuric-chromic acid have been prepared. Electron micrographs of these replicas indicate the presence of crosslink density inhomogeneities which are 100 to 150 Å in diameter; a size range correctly predicted by theory. Thus, the present theory provides a basis for an explanation of the sensitivity of the network properties to variations in chemical composition.
KeywordsCrosslinking Agent Crosslink Density Radial Distribution Function Crosslinking Reaction Tensile Modulus
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