Controlled microphase separation in polyurethane acrylate-based optically clear pressure sensitive adhesives

Abstract

Constructing fit-for-purpose overall properties in optically clear pressure sensitive adhesives (OCAs) is a prerequisite for achieving high-quality displays and remains challenging. Herein, we report a series of OCAs based on UV-curable polyurethane acrylate (PUA) copolymers with varied diisocyanates and average acrylate functionalities in the PUA prepolymer to unveil the impact of microphase structure on the overall properties of OCAs. Both simulation and experimental results suggest that OCAs based on diisocyanates with higher symmetry and rigidity show stronger hard-phase interaction, which further enhances adhesion. While OCAs based on alicyclic diisocyanates exhibit better optical properties than those based on aromatic diisocyanates, which can attribute to the lower refractive index of alicyclic hard phase. An increase of average acrylate functionality reduces the hard segment stacking near the crosslinking point and promoting phase mixing, which is confirmed by SAXS and FTIR experiments. However, the haze value of the related OCA is abnormally increased. It is tentatively inferred that chemical crosslinking has an opposite effect: it decreases the degree of microphase separation (which decreases haze value) yet amplifies scattering resulting from refractive index mismatch of the microphase (which increases haze value). This work provides design insights for balancing the overall performance of OCAs and further enabling high-quality displays.