Side-Chain Crystallization-Induced Switchable Adhesion Regulated by Heterogeneous Dynamics of Poly(alkyl acrylate)

Thermo-triggered adhesives based on side-chain crystalline polymers have gained considerable attention in recent years owing to the programmable and reversible switch between attaching and detaching. The variation of heterogeneous structure before and after the thermo-triggered transition was the key to controlling the smart behaviors; however, the intrinsic molecular-level transition mechanism was still missing. In this work, we used polyacrylate with alkyl side chains as a model thermo-triggered adhesive to explore the molecular-level mechanism therein. Low-field solid-state NMR was used to investigate the segmental dynamics variation before and after triggering. The detailed chemical structures of the corresponding polymer segments were unveiled by high-field solid-state NMR. Throughout the cooling process, it was found that the crystallization began from the mid part of the side chain, then along the direction to the backbone, and last, the end of the alkyl side chain. During this process, the segmental mobility showed similar behaviors along the side chains. The result of the corresponding tack experiment suggested three kinds of detaching behavior domains throughout the cooling process, separated by melting temperature (Tm) and crystallization temperature (Tc). A tentative model depending on the competition between cohesion energy and interfacial energy was proposed to help understand the mechanism therein and guide the design of high-performance thermo-triggered adhesives.