Scaling behavior: Effect of precursor concentration and precursor molecular weight on the modulus and swelling of polymeric networks

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Abstract
End-linked poly(dimethylsiloxane) (PDMS) networks, containing a minimal number of defects, are prepared from vinyl-terminated PDMS chains in the presence of a theta (θ) solvent. Unreactive tri-methyl terminated PDMS of the same molecular-weight distribution as the reactive PDMS precursors were chosen as the θ solvent. Four precursor molecular weights ranging from 10 000 to 100 000 daltons were studied. The dependence of the initial modulus after cure, the dry modulus after solvent extraction, and the degree of equilibrium swelling on precursor concentration during cure were compared to recent scaling predictions. The experimental scaling exponents were found to be strong functions of the molecular weight of the precursor chains and their values approach the theoretical predictions for high-molecular-weight precursors. The relationship between the modulus of the fully swollen network and the degree of equilibrium swelling is found to be independent of precursor molecular weight and precursor concentration during cure and in better agreement with the corresponding, more robust, theoretical prediction.