Interfacial Effect in Supported Thin PET Films Covered with a Thin PPO Layer

Quantifying and Modeling the Crystallinity of Polymers Confined in Nanopores

We propose a methodology to characterize the crystalline content of interfacial polymer layers in systems confined at the nanoscale level in a 2D geometry. Based on the crystallinity data of a set of polymers, we introduce a simple model to describe the gradient in crystallinity introduced by confining polymer chains in nanopores. Our model underscores the pivotal role that interfaces play in crystallization and unequivocally contradicts the existence of interfacial "dead" layers where crystallization cannot take place. Further, we verified that the organization of crystals near the pore walls resembles the macromolecular architecture of adsorbed layers, hinting at a strong interplay between crystallization and adsorption.

Exploring the Molecular Origin for the Long-Range Propagation of the Substrate Effect in Unentangled Poly(methyl methacrylate) Films

The polymer/substrate interface plays a significant role in the dynamics of nanoconfined polymers because of its suppression on polymer mobility and its long-range propagation feature, while the molecular origin of the long-range substrate effect in unentangled polymer material is still ambiguous. Herein, we investigated the propagation distances of the substrate effect (h*) by a fluorinated tracer-labeled method of two unentangled polymer films supported on silicon substrates: linear and ring poly(methyl methacrylate) films with relatively low molecular weights. The results indicate that the value of h* has a molecular weight dependence of h*∝N (N is the degree of polymerization) in the unentangled polymer films, while h*∝N1/2 was presented as previously reported in the entangled films. A theoretical model, depending on the polymer/polymer intermolecular interaction, was proposed to describe the above long-range propagation behavior of the substrate effect and agrees with our experiment results very well. From the model, it revealed that the intermolecular friction determines the long-range propagation of the substrate effect in the unentangled system, but the intermolecular entanglement is the dominant role in entangled system. These results give us a deeper understanding of the long-range substrate effect.

The role of the molecular weight of the adsorbed layer on a substrate in the suppressed dynamics of supported thin polystyrene films

The adsorbed layer on a solid surface plays a crucial role in the dynamics of nanoconfinement polymer materials. However, the influence of the adsorbed layer is complex, and clarifying this influence on the dynamics of confined polymers remains a major challenge. In this paper, SiO₂-Si substrates with various thicknesses and adsorbed layers of PS with various molecular weights were used to reveal the effect of the adsorbed layer on the corresponding segmental dynamics of the supported thin PS films. Strongly suppressed segmental dynamics of thin PS films were observed for the films supported on thicker adsorbed layers or prepared using higher molecular weight. Neutron reflectivity revealed that the overlap region thickness between the adsorbed layer and the top overlayer increased with increasing thickness and molecular weight of the adsorbed layer, both of which correlate well with the distance over which the polystyrene dynamics were depressed by the adsorbed layer. The results show that the influencing distance of the adsorbed layer is related to the overlap zone formed between the adsorption layer and the upper thin film. The effect of the adsorbed layer molecular weight can be ascribed to the fact that large loops and long tails in the adsorbed layer result in stronger interpenetrations and entanglements between polymer chains in the adsorbed layer and in the overlayer, causing a stronger substrate effect and suppression of the segment dynamics of the supported thin PS films.

Lamellar crystal-dominated surfaces of polymer films achieved via melt stretching-induced free surface crystallization

Lamellar crystal-dominated (LCD) surfaces hold great superiority and broad prospects in polymer surface engineering. The key to this is avoiding the formation of an amorphous phase in the interlamellar region. Here we give a first report of achieving LCD surfaces of polyethylene films via melt stretching-induced free surface crystallization. We demonstrate that the resultant surface is constructed directly by orientated and edge-on lamellae within a surface depth of tens to hundreds of nanometers, while the normally existing amorphous phase is avoided. The crystallization-driven formation of the LCD surface has been ascribed to the heterogeneous chain dynamics of a melt free surface, that is, high chain mobility, low viscosity and loose chain entanglement, which facilitates the complete chain disentanglement during crystallization. In addition, we confirm that the surface morphology is controllable with respect to lamellar orientation, spacing and depth by changing the melt stretching strain or quenching the deformed melt. Meanwhile, owing to a possible kinetics competition between crystallization and chain disentanglement, the structural spacing of surface lamellae holds a positive correlation with the lamellar depth. Since free surface effects are immanent in polymer materials, the currently proposed melt processing strategy is demonstrated to be transferable to other semicrystalline polymers.