ABSTRACT
A new fabrication method for an ultrathin (500 nm thick) pressure-sensitive adhesive (PSA) was demonstrated by utilizing a series of in situ cross-linked viscoelastic copolymer films. Viscoelastic films composed of poly(2-hydroxyethyl acrylate- co-2-ethylhexyl acrylate) were synthesized successfully in a one-step manner by an initiated chemical vapor deposition (iCVD) process, where free-radical polymerization is triggered in the vapor phase either by heat or UV, or a combination of both. In particular, the photoinitiated chemical vapor deposition method generated a highly cross-linked polymer film, whereas cross-linking of the copolymer film was suppressed greatly in the conventional thermal iCVD method. A combination of thermal and photoinitiated chemical vapor deposition could regulate the cross-linking density of the copolymer films. We controlled the cross-linking density of the copolymer films to exhibit a viscoelastic property so that they would readily adhere to various kinds of substrates with only 500 nm thick copolymer PSA. The adhesion performance of the PSA was systematically optimized by tuning the copolymer composition as well as the cross-linking density, and consequently a high shear strength of more than 85.2 ± 5 N/cm2 was achieved despite the 500 nm thickness. In addition, the PSA was completely transparent. We expect that 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 optical transparency.
ABSTRACT
Demand of adhesives that are strong but ultrathin with high flexibility, optical transparency, and long-term stability has been rapidly growing recently. Here, we suggest a thermally curable, "sticky" nanoadhesive with outstanding adhesion strength accomplished by single-side deposition of the nanoadhesive on arbitrary substrates. The sticky nanoadhesive is composed of an ionic copolymer film generated from two acrylate monomers with tertiary amine and alkyl halide functionalities, formed by a solvent-free method, initiated chemical vapor deposition (iCVD). Because of the low glass transition temperature (Tg) of the copolymer (-9 °C), the ionic copolymer shows a viscoelastic behavior that makes the adhesive attachable to various substrates, regardless of the substrate materials. Moreover, the copolymer film is thermally curable via a cross-linking reaction between the alkyl halide and tertiary amine functionalities, which substantially increased the adhesion strength of the 500 nm thick nanoadhesive greater than 25 N/25 mm within 5 min of curing at 120 °C. The adhesive thickness can further be reduced to 50 nm to achieve greater than 35 N/25 mm within 30 min at 120 °C. The nanoadhesive layer can form uniform adhesion in a large area substrate (up to 130 × 100 mm2) with the deposition of the adhesive only on one side of the substrates to be laminated. Because of its ultrathin nature, the nanoadhesive is also optically transparent as well as highly flexible, which will play a critical role in fabrication and the lamination of future flexible/wearable devices.
