Effects of interfaces on crystallization in semicrystalline polymer thin films

Polymer-air and polymer-polymer interfaces are ubiquitous in polymer samples. These interfaces can impact layer-by-layer processing, such as 3D printing, and the overall material properties. In this work, we apply molecular dynamics simulations to investigate the effects of polymer-air and polymer-polymer interfaces on crystallization in thin polyethylene films. We reveal that free surfaces, although only mildly affect the local orientational order and entanglement density, can significantly enhance crystal nucleation by promoting the local segmental dynamics. By applying another polymer layer to the well-crystallized film, mimicking layer-by-layer processing of polymeric materials, we show rapid primary nucleation near free surfaces and secondary nucleation near the semicrystalline interfaces. Using in silico tensile tests, we demonstrated that the co-crystallization between adjacent polymer layers enhances interfacial strength. Overall, our findings offer new insights into the crystallization behavior of thin films and multilayer polymer samples and could help optimize the layer-by-layer processing of semicrystalline polymers.