Development of Interfacial Strength and Entanglements During Welding of Polymers

Thermal welding is a common means of joining polymer parts. Interfacial strength increases with welding time $t_w $ as polymer chains diffuse across the interface. The microscopic origin of this interfacial strength enhancement was investigated with large scale molecular simulations employing a coarse-grained bead-spring model. Polymer surfaces were held together at a temperature well above the glass transition temperature $T_g$. States at $t_w $ up to $10^9$ time steps were then quenched to a temperature below $T_g $ for mechanical tests. We test the interfacial strength by shearing the weld along a direction parallel to the interface. The maximum shear stress $\sigma _{\max} $before failure is used to characterize the interfacial strength. We find that $\sigma _{\max} $ increases as $t_w ^{1/4}$ before saturating to its bulk value. This agrees with previous experiments by a lap-joint shear method [1]. In addition, our analysis shows that the dominant shear failure mode changes from chain pull-out at the interface for small $t_w $, to chain scission for large $t_w $. We examine the average contour length $$ of chains that have diffused across the interface. As predicted by the reptation dynamics, $$ increases as $t_w ^{1/4}$. We also track the evolution of entanglements using the \textit{Primitive Path Analysis} (PPA) algorithm [2]. Our results show that the total number of interfacial entanglements is directly related to interfacial strength and also increases as $t_w ^{1/4}$. \\[4pt] [1] D. B. Kline, R. P. Wool,\textit{ Polym. Eng. Sci. }1988, 28(1), 52--57.\\[0pt] [2] R. Everaers \textit{et al}. , \textit{Science}, 303, 823-826, 2004