Polymerization-induced shrinkage and dynamic thermal expansion behavior during network formation of polyurethanes

Abstract

Detailed knowledge and understanding of volume expansion properties is essential for various fields of application of composite materials. This holds in particular for the evolution of polymerization-induced shrinkage and the evolution of the statics and dynamics of the thermal volume expansion coefficient during the polymerization of multi-component adhesives. We here demonstrate that the recently introduced technique called “Temperature Modulated Optical Refractometry” is an excellent tool to simultaneously investigate in-situ both aspects of volume changes. The focus of the study lies amongst others on the impact of the network formation on molecular dynamics and the polymerization-induced glass transition as reflected by the dynamic thermal expansion coefficient as well as the correlation between molecular packing and thermal expansion in the course of the network formation of a vitrifying polyurethane adhesive. The dynamic thermal expansion coefficient in dependence of conversion shows the typical features of a polymerization-induced dynamic glass transition. Prior to percolation of the cross-linking polymer network, the static thermal expansion coefficient is independent of the polymerization-induced shrinkage coefficient. When approaching the polymerization-induced glass transition, dependence between both kinds of volume changes exists and gets increasingly pronounced.