On the Structure of Star-Polymer Networks

TL;DR

This study uses the bond fluctuation model to analyze the structure and defects of star-polymer networks, revealing differences between single-type and bi-type systems and identifying cyclic defects with potential for detailed analysis.

Contribution

It introduces a detailed analysis of network defects and structure in star-polymer networks, highlighting differences between A and AB networks and deriving order parameters for defect characterization.

Findings

A networks are dominated by short dangling loops near overlap concentration.

AB networks form more perfect, amorphous structures with fewer dangling loops.

Cyclic defects in AB networks can be characterized by specific order parameters.

Abstract

Using the bond fluctuation model we study polymer networks obtained by endlinking of symmetric 4-arm star polymers. We consider two types of systems. Solutions of one type (A) of star polymers and solution of two types (A,B) of star polymer where A-type polymers can only crosslink with B-type polymers. We find that network defects in $A$ networks are dominated by short dangling loops close to overlap concentration $c^{*}$. $AB$ networks develop a more perfect network structure, since loop sizes involving an odd number of stars are impossible, and thus, the most frequent dangling loop with largest impact on the phantom modulus is absent. The analysis of the pair-correlation and scattering function reveals that there is an amorphous packing of $A$ and $B$ type stars with a homogenization of $A$ and $B$ concentrations upon cross-linking at intermediate length scales in contrast to the previously suggested crystalline like order of $A$ and $B$ components at $c^{*}$. This result is corroborated by the coincidence of the probabilities of the shortest loop structures (which is impossible upon the previously suggested packing of stars) in both types of networks. Furthermore, we derive the vector order parameters associated with the most frequent network structures based on the phantom model. In particular, for $AB$ networks we can show that there is a dominating cyclic defect with a clearly separated order parameter that could be used to analyze cyclic network defects.