# Influence of Shape-Forming Elements on Microstructure and Mechanical Properties in Coextruded Thermoplastic Composites

## TL;DR

This paper explores how shape-forming elements in coextrusion can improve the mechanical properties of polymer composites.

## Contribution

The introduction of shape-forming elements inspired by fractals to control interfacial architecture in coextruded composites.

## Key findings

- Shape-forming elements improve tensile modulus and stress at break with high correlation to draw ratio.
- Discrepancies between simulations and experiments were attributed to material incompatibility and moisture-induced defects.

## Abstract

The immiscibility of most polymers leads to poor interfacial adhesion in blends, a critical challenge that often limits the mechanical performance of polymer composites. This research introduces shape-forming elements (SFEs), a novel class of coextrusion dies designed to create additional geometric complexity and control over interfacial architecture. Specifically inspired by Julia Set and T-Square fractals, SFEs were simulated, prototyped, and found to be effective in coextrusion of different-colored polymer clays. The SFEs were employed to coextrude architected composites consisting of a liquid crystalline polymer (Vectra A950) and a cycloaliphatic polyamide (Trogamid CX7323). Mechanical testing revealed a strong positive correlation between the draw ratio and both the tensile modulus (adjusted R2 = 0.94) and tensile stress at break (adjusted R2 = 0.84). However, experimental cross-sections significantly differed from simulation results. These discrepancies were attributed to interfacial instabilities caused by material incompatibility between the two polymers and potential moisture-induced defects. This finding highlights critical challenges that arise during practical processing, emphasizing the importance of addressing polymer compatibility and moisture management to realize the full potential of SFEs in designing advanced polymer composites with targeted properties.