Modeling domain mixing in semi‐interpenetrating polymer networks composed of poly(vinyl chloride) and 5% to 15% of crosslinked thermosets

Abstract

AbstractThe nature of phase mixing in semi‐interpenetrating polymer networks (SIPNs) of poly(vinyl chloride) (PVC)/thermoset blends was studied by using both the glass transition temperature third power composition equation and DMTA modeling. From 5% to 15% by weight of an oligomeric MDI isocyanate or a low viscosity epoxy were blended separately with PVC to make two series of SIPNs. The DMTA‐derived Tg data were modeled by the Tg third power composition equation to characterize “energy interaction” features of PVC/thermoset phase mixing. Fitting experimental Tg values gave estimations of the characteristic parameters, K1 and K2 of the Tg power equation. K1 and K2 were each positive for the PVC/epoxy (K1 = 1.1, K2 = 7.2) and the PVC/isocyanate (K1 = 29.9, K2 = 38) blends, showing that binary hetero‐interactions (enthalpic effects) and conformational redistributions (entropic effects) during the binary hetero‐interactions both contributed favorably to phase mixing. Negative K1 ‐ K2 values for both groups of blends indicate entropic contributions predominate. The thermoset dilution produced a lightly crosslinked thermoset network, which was locked into the amorphous PVC, forming a mixed thermoset/PVC SIPN domain. Conformational redistributions of PVC and thermoset segments continue to occur within the developing SIPN domain before phase separation can occur. The combined series‐parallel Takayanagi coupling model, which assumed that a PVC phase and a mixed PVC/thermoset SIPN phase coexisted, was employed to describe the viscoelastic behavior of PVC/thermoset blends. Reasonable fits between the experimental DMTA plots and modeling predictions were found. The predictions were not sensitive to the degree of series‐parallel coupling. The modeled DMTA plots, like the experimental results, exhibited only a single E″ peak in the glass transition temperature range for both the PVC/PAPI and PVC/epoxy systems. Thus, very small PVC/thermoset SIPN domains (< 20 nm diameter) that are dispersed in separate domains of a PVC‐rich phase (despersive phase mixing characteristics) provide a reasonable explanation of the blends' phase structures.