Thermally stable and soft pressure-sensitive adhesive for foldable electronics

Abstract

Soft pressure-sensitive adhesives (PSAs) are widely adopted in foldable electronics to integrate devices and to form multiple neutral planes (MNPs), that protect brittle films in the devices during folding. The mechanical properties and reliability of PSAs at the operating temperature of foldable devices are key factors for their practical application. Herein, we synthesize a thermally stable, yet soft PSA via initiated chemical vapor deposition (iCVD). The molecular network of the soft PSA is composed of a copolymer film of 2-ethylhexyl acrylate (EHA) and 2-hydroxyethylacrylate (HEA) monomers, whose composition is controlled systematically by tuning the input flow rate of each monomer during iCVD. As a result, the storage modulus of the synthesized PSA is lower than 1 MPa from −35 to 85 ℃ without any flow. The hard-coated laminate integrated by the synthesized PSA can be protected during bending at −35 ℃ by the MNPs, as the inserted PSA is still soft at such a low temperature. Moreover, the shear strength of the synthesized PSA does not degrade even after exposure to temperatures of −40 ℃ and 85℃ for 7 days. The extraordinary mechanical reliability and the durability against delamination is further confirmed through a folding fatigue test and a static folding test (holding fold state) under various operating temperatures (-40 ℃, 25 ℃, and 85 ℃).