Stiffness induced structures and morphological transitions in semiflexible polymers

TL;DR

This paper develops a unified theoretical framework using free-energy analysis to describe various morphologies and transitions of semiflexible polymers in poor solvents, integrating simulation and experimental insights.

Contribution

It introduces a coarse-grained, field-theoretic model that captures multiple polymer morphologies and their transitions within a single, analytic phase diagram.

Findings

Identifies phase boundaries between coil, globule, toroid, and rod conformations.

Derives analytic expressions for free energies of different morphologies.

Predicts a possible triple point involving globules, rods, and toroids.

Abstract

Semiflexible polymers in poor solvents exhibit a rich variety of collapsed morphologies, including globules, toroids, and rodlike bundles, arising from the competition between attractive interactions and chain stiffness. Computer simulations and experiments on stiff and conjugated polymers have revealed complex morphological crossovers, yet a unified theoretical description remains incomplete. Here we develop a coarse-grained, field-theoretic free-energy framework for linear polymers with variable stiffness that captures these morphologies and their transitions within a common description. The theory is built on three key ingredients: a density field describing monomer attraction and excluded-volume effects, a nematic order parameter accounting for orientational ordering in dense regions, and the bending rigidity of a worm-like chain. Using simple variational ansatzes for competing morphologies, we derive analytic expressions for their free energies and identify the boundaries separating coil, globule, toroidal, and rodlike conformational regimes as functions of the reduced attraction strength and the effective persistence length. The resulting phase-diagram topology provides a transparent free-energy-based framework for interpreting morphology diagrams observed in simulations and experiments on semiflexible polymers in poor solvents. We find the possibility of the existence of a triple point involving globules, rods and toroids.