Mechanism of pressure sensitive adhesion in nematic elastomers

TL;DR

This study uncovers that the high pressure-sensitive adhesion in nematic liquid crystal elastomers is due to the nematic phase's slow relaxation of stress and director orientation, with bonding strength reaching maximum after 24 hours.

Contribution

It demonstrates that the enhanced PSA is specifically linked to the nematic phase and reveals the time-dependent relaxation process responsible for adhesion strength.

Findings

Enhanced PSA occurs only in the nematic phase.

Bonding strength saturates after over 24 hours.

Cooling the pressed film reproduces maximum adhesion.

Abstract

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed 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 crosslinking density, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90-deg 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 hours to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in nematic domains after pressing against the surface. We confirm this mechanism by showing that a 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.