Molecular insight on thermal welding of semicrystalline polymer interfaces

We study the thermal welding of semicrystalline polymers by means of molecular dynamics simulation. We prepare molten films composed of chains with 200 and 2000 coarse-grained monomers and allow them to interpenetrate at 450 K and 100 bar for varying amounts of time. Due to the presence of a free boundary in the films before contact, chain segments near the interface are found to be slightly parallel to it. We observe that this orientational anomaly and the depletion of interfacial entanglements persist for a very long time in the 2000 monomer system. The resulting configurations are quenched to STP conditions to obtain a semicrystalline structure. The crystallization process enhances the orientational anomaly observed in the molten state, indicating that the residual orientational order biases the direction of crystal growth. This phenomenon is unique to semicrystalline polymers and is expected to cause long-lived heterogeneities in the thermo-mechanical response of the interfacial region. We observe that under simple shear the interface strength increases with the degree of interpenetration and saturates to the bulk value when the entanglement network at the interface is fully re-established. Conversely, elongation at fixed cross-section leads to cavitation at about 10% strain. Interestingly, cavitation occurs most of the times away from the interface even at low degrees of interpenetration. We speculate that this behaviour may be partly due to the persistence of bond orientational order near the interface.