Thermo-switchable pressure-sensitive adhesives with strong tunable adhesion towards substrate surfaces of different hydrophilicity

Abstract

Short-chain oligomeric poly(ethylene glycols), PEGs, are capable of forming supramolecular hydrogen-bonded stoichiometric network complexes with stimuli-responsive polymer such as N-substituted high molecular weight polyamide, poly(N-isopropyl acrylamide), PNIPAM, demonstrating Lower Critical Solution Temperature (LCST) in aqueous solutions. H-bonding occurs through interaction of PEG terminal hydroxyl groups with complementary amide groups in the PNIPAM monomer units. The complexes combine high cohesive strength (due to H-bond crosslinking of long-chain macromolecules) with large free volume (resulting from telechelic structure and essential length of PEG chains). Combination of these factors is the necessary condition of pressure sensitive adhesion. “Smart” pressure-sensitive adhesives (PSAs) adhere towards the hydrophilic substrates at room temperature and detach reversibly at the temperatures above LCST, while their behavior towards hydrophobic substrates follows opposite pattern. As contact angle measurements have shown, at room temperatures the thermo-switchable PSAs have low contact angle with water, which becomes large at temperatures above LCST. Under temperature elevation, the complex turns hydrophobic inside out and loses hydrophilicity, similarly to the complexes of another «smart» polyamide, poly-N-vinyl caprolactam (PVCL), with PEG-400. Evident correlation has been observed between the strength of hydrogen bonding and molecular structures of both the PVCL – PEG and PNIPAM – PEG PSA complexes and their adhesion.