Enhancing the Surface Mobility of Polymer Glasses by Inducing Flattened Chains at the Surface

Increased mobility of molecules at the surface of glassy materials enables cold welding and processing at temperatures below their glass-transition temperatures. However, deep penetration of surface polymer coils into the glassy bulk “locks” surface chains, slowing their diffusivity and compromising the interfacial bonding process. This study demonstrates a strategy to promote surface chain mobility by altering the ways in which the polymers spread out at the surface. This was achieved by adding a copolymer interfacial modifier, comprising a component with low surface free energy (γ) and one with the same chemistry as the polymer, e.g., poly(styrene-sta-pentafluorostyrene) [P(S-sta-PFS)]. The low-γ units (i.e., PFS) spathaceously adsorb onto the polystyrene (PS) film surface, driven by minimization of the free surface energy, generating two-dimensional, flattened chains with a reduced penetration depth at the surface, which alleviates the dynamic locking effect and lowers the energy barrier for surface chain diffusion. The enhanced surface diffusivity imparted by flattened P(S-sta-PFS) chains facilitated the welding of PS in its glassy state, achieving a bonding strength nearly twice that of naive PS glass. Our endeavor represents a step toward efficient cold processing of polymer materials.