Semicrystalline poly(vinyl ether)s with high and phototunable glass transition temperature: application for thermally stable and reworkable adhesives

Selective poly(vinyl ether) upcycling via photooxidative degradation with visible light

Poly(vinyl ethers) (PVEs) have many applications, such as adhesives, lubricants, and anticorrosive agents, thanks to their elastic, nonirritating, and chemically inert properties. The recycling of PVEs remains largely underexplored, and current methods lack generality towards other polymer classes. Thus, the chemical upcycling of PVE into small molecule feedstocks would provide an alternative approach to combat these current issues. Here, we report a visible light-mediated method of upcycling poly(isobutyl vinyl ether) (PIBVE) into small molecules via photooxidative degradation using chlorine or bromine radicals. PIBVE can be degraded to low molecular weight oligomers within 2 h, producing good yields of alcohols, aldehydes, and carboxylic acids. Mechanistic studies suggest that hydrogen atom transfer (HAT) from the backbone or the side chain leads to small molecule generation via oxidative cleavages. Additionally, this protocol was applied to a copolymer of poly(methyl acrylate-co-isobutyl vinyl ether) to demonstrate the preference for the degradation of polymers bearing more electron-rich C-H bonds through a judicious choice of abstraction agent. Ultimately, we show that photooxidative degradation enables the selective chemical upcycling of PVEs as a method of plastic waste valorization.

Photohardenable Pressure-Sensitive Adhesives using Poly(methyl methacrylate) containing Liquid Crystal Plasticizers

Hardenable pressure-sensitive adhesives, which show pressure-sensitive adhesion state with weak adhesion strength in their initial semisolid state and general adhesion state with strong adhesion strength in their hardened state, are desirable in various industrial fields to improve efficiency of manufacturing and recycling products. Here we developed novel photohardenable pressure-sensitive adhesives triggered by photoplasticization of poly(methyl methacrylate) containing photoresponsive liquid crystal (nematic and smectic E) plasticizers at various ratios. It was found that photoplasticization, which is the photoinduced reduction of glass transition temperature and hardness of polymers, could be repeatedly induced by alternate irradiation with ultraviolet (UV) and visible (Vis) light in all mixtures, regardless of the phase structures of the photoresponsive plasticizers. Upon photoplasticization under UV-light irradiation, all mixtures exhibited glassy-to-rubbery transition to a pressure-sensitive adhesion state under appropriate conditions. Upon irradiation of the photoplasticized samples with Vis light, the samples recovered their initial hardened state, recovering the glassy nature with elastic moduli. The adhesion strength of the samples in the hardened state was significantly influenced by the phase structures of the plasticizers. When a photoresponsive plasticizer exhibited the smectic E phase, which is a highly ordered liquid-crystalline phase, the adhesion strength was remarkably larger than those of the case using the plasticizers showing nematic and crystalline phases. This result was reasonably explained in terms of the suppressed bleed-out of the photoresponsive plasticizers from the polymer and the good mechanical properties of the mixture stemming from the characteristics of the smectic E phase. Furthermore, through the reversibility of a photoplasticization process, we achieved a photoinduced reduction of the adhesion strength by UV irradiation of the samples in the hardened state. Photohardenable pressure-sensitive adhesives with reversibility has been developed using a commodity plastic just by adding the photoresponsive plasticizer showing the smectic E phase.