Establishing a universal dry etching methodology to unveil the nanoscale crystalline structure of fiber reinforced thermoplastic composites via scanning electron microscopy

Abstract

This study aims at the establishment of a universally applicable etching methodology to unveil the nanoscale crystalline structure of the matrix resin in fiber reinforced thermoplastic (FRTP) composites via scanning electron microscopy (SEM). The crystalline structure hierarchically consists of crystalline texture, spherulite and lamella. The details of these structures are key parameters to understand the relationship with the mechanical properties of the material for the advancement. During previous studies, two novel methodologies based on optical microscopy and micro-spectroscopy were developed via an innovative polishing strategy enabling to process FRTPs at a thickness of a few micrometers. Thereby, comprehensive information on both crystalline texture and spherulite was gained. A remaining challenge was the characterization of nanometer-scale lamella, which may be accomplished via etching-SEM. However, etching pretreatments are not commonly applied, as they require detailed knowledge, expertise on selecting the appropriate etchant and conditioning to avoid structural degradation. In the present study, a simple yet powerful strategy has been established based on plasma etching with a gas mixture comprising oxygen and argon. Five types of major matrix resins (PBT, PA6, PPS, PEEK and PE) were processed, successfully resulting in the distinct and universal exposure of the lamella topography by preferentially removing amorphous polymer chains, yet, with negligible crystalline structural changes. Using the developed treatment routine, the formation of nanoscale transcrystalline structures from fiber surfaces was notably unveiled. To complement this study, the relative etching rate of the amorphous zone with respect to the lamella was determined numerically at the range of sub-nanometers per sec.