VISCOELASTIC EFFECTS IN THE COALESCENCE OF POLYMER PARTICLES

A study was made of the coalescence of linear acrylic resins over a much greater dynamic range than previously investigated. The rate of neck growth between a sphere and a flat slab of the same material was measured in situ by optical microscopy and found to exhibit a number of features typical of polymer viscoelastic response. Results are compared with independent measurements of the melt rheology (torsional creep compliance). An analytic approximation is proposed which allows the neck-growth kinetics to be predicted from the recoverable creep compliance and viscosity of the melt. It predicts qualitative trends consistent with the limited data on molecular weight, structure, and particle size variations. This analysis helps resolve some apparent contradictions among earlier studies and implies a qualitative difference in the coalescence mechanisms for large particles (> 5 mum) and colloids (<0.5 mum). In either case, the ultimate equilibration to homogeneous bulk mechanical properties requires times on the order of the terminal relaxation time, regardless of particle size. Evidence is presented that, in addition to the recognized effects of chain inter-diffusion, stress relaxation may play a role in the equilibration kinetics.