Geometry-Dependent Adhesion in Transparent, Monodomain Liquid Crystal Elastomers

TL;DR

This study explores how the alignment of liquid crystal elastomers affects their adhesion properties, revealing that geometry-dependent anisotropic adhesion can be tuned with temperature for smart adhesive applications.

Contribution

It demonstrates the influence of molecular alignment on adhesion strength in liquid crystal elastomers, providing a theoretical and experimental framework for tunable, transparent pressure-sensitive adhesives.

Findings

Planar alignment yields the strongest adhesion (0.67 Nmm-1).

Adhesion varies significantly with molecular orientation and temperature.

Adhesion is dominated by bulk properties, not surface effects.

Abstract

Elastomeric pressure-sensitive adhesives (PSAs) form adhesive bonds under light pressure. Liquid crystal elastomers (LCEs) are exciting PSA candidates as they can impart both anisotropy and temperature-dependence to adhesion, but the full potential of their anisotropic adhesion is unexplored. Here, identical side-chain LCEs, produced as transparent isotropic or nematic films are investigated; the latter aligned in homeotropic or planar geometries. Their room-temperature adhesion, determined through a 90-degree peel test, is consistent with theoretical predictions and strongest in a planar geometry (peeled parallel to the director) with adhesive force per unit length of 0.67 Nmm-1. In contrast, adhesion of the planar perpendicular, isotropic and homeotropic films is 62.5%, 38.5% and 23.0% lower, respectively. The surface contribution to adhesion is identical for all films, confirming that the variation in adhesion is determined solely by the bulk LCE alignment controlled during film preparation. A temperature-dependent adhesion factor is determined from 0 Celsius to 80 Celsius using dynamic mechanical analysis, and found to be in excellent agreement with the peel data at room temperature. Molecular relaxations active above the glass transition temperature are dominant in determining LCE adhesion. The results show that side-chain LCEs can function as transparent, tunable, broad-temperature smart PSAs