Molecular Weight Dependence of Interdiffusion and Adhesion of Polymers at Short Contact Times

The autohesion and subsequent debonding of thin layers of three linear and monodisperse random copolymers of styrene-butadiene (SBR) with molecular weights varying between 30 and 75 times the average molecular weight between entanglements M_e were investigated using a carefully controlled tack adhesion testing device in conjunction with a fast camera setup over a range of contact times t_c (10 ms to 10 s) much shorter in comparison to the terminal relaxation times of the polymers. The evolution of the stress-strain curves and debonding mechanisms with increasing contact time was examined, and the work required to debond the layers was found to be strongly dependent on molecular weight at long contact times, but not at short contact times. We propose a cutoff contact time of 300 ms, corresponding to 10⁴ times the entanglement time τ_e after which molecular weight becomes important in controlling the interdiffusion process and the debonding mechanisms of the tack test. For contact times over 300 ms, the debonding energy plotted as a function of t_c normalized by the reptation time τ_rep, collapses onto a master curve. Below this threshold t_c, by comparing the adhesion of SBR on itself with the adhesion of SBR on glass, we also show that interdiffusion plays a part in adhesion of two identical polymer layers even at the shortest contact times, where the interdiffusion is controlled by the number of entanglements formed which scales with 1/√N.