Challenges and solutions of environmental scanning electron microscopy characterisation of biomaterials: Application to hygro‐expansion of paper

Abstract

Most methodologies to measure the moisture-induced deformation (hygro-expansion) of paper microconstituents, including fibres and interfibre bonds, are low resolution or time-consuming. Hence, here, a novel method is proposed and validated to measure high-resolution full-field strain maps of paper microconstituents during hygro-expansion, based on environmental scanning electron microscopy (ESEM). To this end, a novel climate stage enables accurate control of the relative humidity (RH) near the specimen in the ESEM from 0%-100%. The fibre surface, which is decorated a priori with a microparticle pattern, is captured during RH change. Subsequently, correlating the fibre surface using a dedicated global digital image correlation algorithm enables high-resolution hygro-expansion strain maps. Method optimisation involved performing contrast enhancement, scan-correction to reduce ESEM artefacts and a background correction, resulting in a strain resolution of 6 Á 10 À4 . Method validation revealed that the fibres' crystallinity is affected by the electron beam, even for minimal invasive electron beam settings. Interestingly, however, the fibres consistently exhibit conventional hygro-expansion behaviour during the drying slopes. Using the optimised procedure, hygroexpansion characterisation of two interfibre bonds and four interfibre bond cross-sections revealed the competition between the low longitudinal and large transverse fibre hygro-expansion in the bonded area.