Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles

Abstract

Model diglycidyl ether of bisphenol-A based epoxy resins containing well-dispersed 15 nm block copolymer (BCP) nanoparticles were prepared to study the effect of matrix crosslink density on their fracture behavior. The crosslink density of the model epoxies was varied via the controlled epoxy thermoset technology and estimated experimentally. As expected, it was found that the fracture toughness of the BCP-toughened epoxy is strongly influenced by the crosslink density of the epoxy matrix, with higher toughenability for lower crosslink density epoxies. Key operative toughening mechanisms of the above model BCP-toughened epoxies were found to be nanoparticle cavitation-induced matrix shear banding for the low crosslink density epoxies. The toughening effect from BCP nanoparticles was also compared with core-shell rubber-toughened epoxies having different levels of crosslink density. The usefulness of the present findings for designing toughened thermosetting materials with desirable properties is discussed.

Introduction

Rubber modification has been reported as an effective approach for toughening brittle epoxy thermosets since the beginning of the 1970s [1], [2]. Since then, significant work has been done to gain a better understanding of the structure–property relationship between the polymer matrix and the toughening agents for designing epoxy thermoset systems with desired properties [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Self-assembled amphiphilic block copolymers (BCP) are a new type of toughening agent that has been shown to greatly improve toughness without sacrificing other mechanical properties. In our previous work [20], [21] we identified the toughening mechanisms of a BCP-modified controlled epoxy thermoset (CET) system at a specific crosslink density. The major toughening mechanisms are BCP particle cavitation and subsequent matrix shear banding, which are analogous to those found in other rubber-toughened systems. But the BCP appears to be more effective than other conventional rubber particles, probably due to its much smaller sizes and unique morphology. In this paper we report the fracture behavior of BCP-toughened epoxy with variation in crosslink densities.

Although rubber modification has been recognized as an effective toughening approach, not all epoxy resins can be toughened to significant extents. Toughenability has been found to be related to thermoset crosslink density [22], [23], [24], [25], [26], [27], [28], [29], [30]. It is known that the crosslink density of a cured thermoset generally dictates its intrinsic ductility, simply because ductile deformation requires large-scale cooperative conformational arrangements of polymer backbones. Because matrix shear banding has been identified as the major energy dissipation process in rubber-toughened epoxies [5], [6], [9], [11], [20], it is not difficult to understand that modifying the matrix ductility can change its toughenability. The function of rubber particle cavitation is to relieve the triaxial stress at the crack tip and consequently to facilitate matrix shear banding. Thus, the thermoset matrix ductility plays an important role in enhancing fracture toughness of rubber-modified thermosets.

The present work is part of a larger effort to understand the fundamentals of structure–property relationships in a model epoxy system containing poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP-PEO) BCP nanoparticles. The toughening mechanisms and strain rate dependence of this modified epoxy system were reported earlier [20], [21]. In this study, attention will be placed on determining whether or not the BCP-modified epoxy thermosets with variation in crosslink densities will exhibit a similar toughening effect on fracture behavior as what has been observed in other rubber-toughened systems. The implication of the present findings for designing high performance thermosetting resins is discussed in detail.