# Rheological Behavior and Molecular Interactions in Concentrated Polycarbosilane Solutions in Linear and Cyclic Hydrocarbon Solvents

## TL;DR

This paper studies how different solvents affect the flow and structure of polycarbosilane solutions, important for making SiC fibers.

## Contribution

Identifies linear heptadecane as a promising solvent for polycarbosilane, enabling better fiber-spinning potential.

## Key findings

- Linear heptadecane enables thermostable, homogeneous PCS solutions with enhanced deformability.
- Solvent molecular geometry, not just solubility, governs PCS solution behavior in concentrated regimes.
- Bicyclic decalin and linear heptadecane show distinct rheological and viscoelastic responses in PCS solutions.

## Abstract

Concentrated solutions of polycarbosilane (PCS) are critically important for the development of continuous SiC precursor fibers, where solvent–polymer interactions govern rheology, viscoelastic stability, and spinnability. In this work, PCS solutions in two nonpolar hydrocarbon solvents with different molecular architectures as linear n-heptadecane and bicyclic decalin were systematically investigated over a wide concentration range, with emphasis on the semi-dilute entangled and concentrated regimes relevant to solution-based fiber spinning. A combined experimental approach involving steady and oscillatory rheometry and Fourier transform infrared (FTIR) spectroscopy was used to elucidate the influence of solvent structure on solvation, viscoelastic response, microstructural organization, and local intermolecular interactions. Despite similar dilute-solution interaction parameters, the concentrated regimes exhibit pronounced solvent-dependent differences in elasticity and flow behavior. For the first time, linear heptadecane is identified as a viable and technologically promising solvent for PCS, enabling the formation of thermostable homogeneous concentrated solutions with enhanced deformability. This behavior opens a realistic pathway toward a new solution-based fiber-spinning route based on elasticity-controlled processing. The results demonstrate that solvent molecular geometry governs the structure–rheology–processability relationship of concentrated PCS systems rather than solubility parameters alone, providing a new framework for solvent selection in SiC precursor fiber technologies.