Effects of Hyperbranched Polyethylenimine on the Properties of Commercial Epoxy Resins for Composite Applications

TL;DR

This paper studies how adding hyperbranched amine to epoxy resins affects their structure and mechanical properties.

Contribution

The novel contribution is the discovery of how hyperbranched amines influence cure kinetics and network structure in epoxy resins.

Findings

Hyperbranched amine cures faster than aromatic amines, leading to a co-continuous network structure.

At 1% concentration, hyperbranched amine increases free volume but reduces mechanical strength and strain to failure.

Abstract

Hyperbranched polymers are commonly used as surface and
cure modifiers,
along with nanoparticles, of epoxy resins used in medical and infrastructure
applications; however, their impact on structure and performance of
the epoxy resins is not fully understood. Here, a commercial epoxy-aromatic
amine resin (EP-AA) system is blended with off-stoichiometric amounts
of hyperbranched amine (HA), and the cure kinetics, structure, polymer
dynamics, and mechanical properties of the different compositions
are evaluated. Cure kinetic studies indicate that the curing of the
hyperbranched amine is nearly complete prior to that of the aromatic
amines. At its highest concentration (1% by mass), the hyperbranched
amine is found to autocatalyze the epoxy and the aromatic amine cure,
but the bulky HA groups slow the diffusion kinetics during the final
stages of the cure process. This results in a heterogeneous, cocontinuous
network structure consisting of unreacted secondary amines that increase
the side-group/pendant contribution and increase the free volume within
the polymer network. The restricted cooperative segmental mobility
of the polymer network in the 1% by mass HA blend results in poor
energy dissipative properties, as evidenced by reduced strength and
strain to failure.

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