An innovative sample preparation strategy to visualize the crystalline structure of fiber reinforced thermoplastic composites by polarized optical microscopy

Abstract

This study aims at an innovative sample preparation strategy for fiber reinforced thermoplastic (FRTP) composites analyzed via polarized optical microscopy to visualize the crystalline structure of the matrix resin. Investigating the structure is relevant for understanding the relationship with the mechanical properties. In general, polarized optical microscopy is a powerful technique for visualizing polymer crystalline structures at samples sliced into thin sections below 30 μm. However, analyzing FRTPs remains challenging in absence of suitable methods for the preparation of thin sections owing to the embedded fibers. Herein, the preparation of appropriately thin sections in a novel sequence of steps is shown that may generically be applied to FRTPs. Four different types of FRTPs with major matrix resins (GF/PBT, CF/PA6, CF/PEEK and GF/PP) were successfully processed to thickness of 5 μm or below without fiber loss, resin breakage or delamination between fiber and resin demonstrating the utility of the developed method, which is the significant outcome of the study. To the best of our knowledge, the subsequently obtained polarized optical microscopic images for the first time visualized the crystalline structure of the matrix resin revealing nucleation of crystals from the fiber surface and effects of mold flow on the crystallization behavior.Highlights Thinning of fiber reinforced thermoplastics to thickness of 5 μm or below. Thinning achieved despite fiber volume at industrial applications level. Sample preparation ensures ideal conditions for polarized optical microscopy. Visualization of crystalline structure of the matrix resin for the first time. Potential advancement for spectroscopic and electron microscopic analyses.