Synthesis of viscoelastic polymers for ultrathin pressure-sensitive adhesive via initiated chemical vapor deposition

Abstract

A new fabrication method for ultrathin (500 nm-thick) pressure sensitive adhesive (PSA) was demonstrated by utilizing a series of in-situ cross-linked viscoelastic copolymer films. Viscoelastomer films comprised of poly(2-hydroxyethyl acrylate-co-2-ethylhexyl acrylate) [p(HEA-co-EHA)] were synthesized successfully in one-step manner by initiated chemical vapor deposition (iCVD) process, where the free-radical polymerization is triggered either by heat or ultraviolet (UV), or the combination of both in vapor phase. Surface growth polymerization of iCVD, which allows the polymer synthesis and deposition in one-step, enables ultra-thin and conformal coating even for viscoelastic materials. Especially, photo-initiated chemical vapor deposition (PiCVD) method generated a highly cross-linked polymer film, whereas the cross-linking of the copolymer film was substantially suppressed in conventional thermal iCVD method. The combination of thermal and photo-initiated chemical vapor deposition (TPiCVD) could regulate the cross-linking density of the copolymer films. The synthesized copolymer films showed a unique sticky property and readily adhered to various kinds of substrate materials, whose adhesion performance could be modulated systematically by tuning the copolymer composition and the cross-linking density. A high shear strength over 80 N/$cm^2$ was achieved only with 500 nm-thick copolymer PSA. The highly cross-linked PSA was totally transparent and exhibited an excellent thermal stability even at 200 ℃ with no apparent degradation of adhesion performance. The ultrathin PSAs developed in this work will be utilized widely for the realization of various soft electronic devices, which usually require strong adhesion, tunable viscoelastic properties, and optically transparency.