Mechanism of Pressure-Sensitive Adhesion in Nematic Elastomers

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed that this is the cause of their anomalously high-pressure-sensitive adhesion (PSA). Here, we investigate the mechanism behind this enhanced PSA by first preparing thin adhesive tapes with LCE of varying cross-linking densities, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90° peel, lap shear, and probe tack tests. The study confirms that the enhanced PSA is only present in (and due to) the nematic phase of the elastomer, and the strength of bonding takes over 24 h to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in the nematic domains after pressing against the surface. We confirm this mechanism by showing that freshly pressed and annealed tape reaches the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film.