Chain-exchange dynamics and kinetics by ATR–FTIR: Effects of shear stress and temperature on high density polyethylene flow

The chain exchange dynamics at a polymer–solid interface were studied using attenuated total reflectance Fourier transform infrared spectroscopy technique (ATR–FTIR) in a rectangular flow channel. The decrease in the deuterated polyethylene (d-PE) absorbance, previously deposited on ZnSe crystal, as a function of time was monitored as it was replaced by molten high-density polyethylene (HDPE). The experiments were carried out at 0.020, 0.023 and 0.028 MPa and at 260, 270 and 280 °C. The chains exchange dynamics are characterized by two flow regimes: the fast flow regime is described by the equation for convective-diffusive mass transfer. In the slower regime, the flowing chains are interacting with the chains physically adsorbed on the ZnSe crystal. The chain exchange was treated as a surface reaction that follows a first-order kinetic. The experimental data were fitting to a single exponential decay function of which parameters, time constant (ks), absorbance and time at the transition point among fast and slow dynamics were determined. From the kinetics model, it was demonstrated that the chain exchange is a transport-limited process in which, ks decreases with the increasing shear rate (γw) and temperature. It is an activated process with an activation energy near to flow activation energy. From the theoretical and experimental angular coefficients of the plot ks versus ( γ w − 1 / 3 ), we conclude that the chains diffusivity near the surface is less than the bulk diffusivity.