Phase morphology and mechanical properties of polyetherimide modified epoxy resins: A comparative study

Highlights

• Phase morphology of thermoplastic modified different epoxy resins was investigated.

• Functionality of epoxy had great impact on phase morphology of modified epoxy.

• Modified epoxy with different functionality presented different fracture toughness.

• Energy dissipation mechanisms of different modified epoxy resin were clarified.

Abstract

This paper presents a detailed study of the relationship between phase morphology and fracture toughness of two types of thermoplastic modified epoxy resins, a tetrafunctional and a bifunctional type. The reaction-induced phase separation of the thermoplastic (TP) during curing leads to distinct morphologies, depending on the modifier content and the functionality of the matrix. This, in turn, impacts the fracture toughness of modified epoxy resins. A transition from a particulate to a co-continuous morphology is observed at sufficiently high TP content (20 wt%) in both epoxy systems. Thermoplastic phases exhibit a significant level of plastic deformation during crack propagation followed by tearing or debonding, depending on the composition. This, in addition to the formation of shear bands in the matrix, is the most dominant energy dissipation mechanism. Higher viscosity and cross-link density of the tetrafunctional epoxy result in the formation of uniformly distributed submicron-sized thermoplastic particles, where the limited diffusion growth of thermoplastic phase, following the onset of phase separation, is further restricted.

Introduction

Epoxy resin is one of the most widely used matrix materials in fibre reinforced polymer (FRP) composites. This is predominantly due to its relatively high stiffness, superior compatibility and adhesion, and chemical resistance. Epoxy resins with higher functionalities such as tetraglycidyl diamino diphenyl methane (TGDDM) also exhibit high temperature performance [[1], [2], [3]]. However, the intrinsic brittleness of cured epoxy resins, arising from the high degree of cross-linking during curing, often leads to low resistance against crack formation and propagation. This is reflected in the commonly reported modest values of the in-plane and out-of-plane matrix-dominated properties of the resulting FRP composites, especially interlaminar fracture toughness, transverse strength, and interlaminar shear strength of a laminate [4,5]. Various approaches have been proposed to mitigate this brittleness. The inclusion/formation of microscale heterogeneities is widely reported as one of the most effective approaches. Matrix heterogeneities, or tougheners, in the form of micro- or nano- “inclusions” can induce various energy dissipation mechanisms thereby limiting the onset and propagation of cracks. “Inclusions” encompass wide range of materials from rigid to soft organic or inorganic materials in different sizes and shapes. They may be added to the resin either as pre-formed particles or form/precipitate during curing in an irreversible thermodynamic process often referred to as reaction-induced phase separation (RIPS). The latter, which is applied to polymeric modifiers, has been shown to be more effective in enhancing the fracture toughness of epoxies in comparison with systems where pre-formed particles were used [6]. In this case, polymer modifiers are initially dissolved in the epoxy resin forming a homogeneous solution. Modifier particles then phase-separate following the start of the curing reaction till the gelation of epoxy resin. Thermodynamic characteristics of the RIPS process, including the underlying mechanisms and phase morphology, have been widely studied [[7], [8], [9]].

Rubber tougheners, including reactive liquid monomers and pre-formed core-shell rubbers (CSR), are known to be able to provide the highest increase in fracture toughness of epoxy resins [[9], [10], [11]]. However, rubber tougheners can significantly compromise the mechanical properties and glass transition temperature (Tg) of epoxy resins. In one case, for instance, addition of a CSR toughener with 22 vol% resulted in a more than 25% loss in tensile strength of a diglycidylether of bisphenol A (DGEBA) epoxy resin [10]. Ratna [12] reported a 20% drop in the T_g of DGEBA epoxy upon the addition of 20 parts by weight (pbw) of epoxy rubber toughener. Moreover, in comparison with the bifunctional epoxy resins, such as DGEBA, rubber toughening of multifunctional epoxy resins such as TGDDM and triglycidyl-p-aminophenol (TGPAP), can be less effective. This could be explained by the fact that major energy dissipation mechanisms in rubber modified epoxy systems are shear banding of epoxy matrix and cavitation of rubber particles along the crack propagation path, both of which would greatly depend upon the ductility of the epoxy matrix [13,14]. Engineering thermoplastics on the other hand, have attracted considerable attention in view of their effectiveness in increasing the fracture toughness of epoxy resins, with no or limited effects on other mechanical properties and high temperature performance of the epoxy resins [[14], [15], [16]].

In thermoplastic (TP) modified epoxy blends, RIPS can result in different types of morphologies including particle/matrix, co-continuous and phase-inverted, depending on variables such as composition of the blend [17,18], molecular weight of thermoplastic [17,19], curing profile [[20], [21], [22]] and existence of other additives (such as graphene oxide [23] and SiO₂ [24,25]). This has been shown to have significant impact on the fracture toughness of the resulting blends [17,18,26]. Increasing the weight fraction of the TP modifier could turn a particle/matrix morphology into a co-continuous type or even a phase-inverted system. In some cases, it would even be possible to achieve various morphologies without changing the composition and just through modification of the curing conditions. Cho et al. [27] produced a phase-separated system with a co-continuous morphology at a relatively low modifier content by lowering the curing temperature and extending the curing time. In this case, a particulate morphology was realised at higher curing temperatures at similar modifier content. Our preliminary investigations revealed that besides the widely studied effects of the processing conditions on phase separation, the functionality of the resin also plays a major role in the phase separation process and hence the morphological characteristics of the resulting multiphase system. This would, in turn, have serious implications on the properties of the toughened resin including fracture behaviour. Despite of the importance of this matter, especially in consideration of their potential consequences on the properties of the resulting FRP composites, the literature available on the subject is quite scarce. Hodgkin et al. [28], after reviewing results obtained by other researchers, came to the conclusion that the thermoplastic toughening of epoxy resins with a higher number of functional groups would be more effective. Nevertheless, the reviewed studies employed different curing profiles, which may also affect the fracture toughness of modified epoxy systems.

A lack of sufficient information on the role of the matrix underscores the necessity of a systematic study on phase separation and morphological characteristics of TP modified epoxies with different epoxide content and their relationship with the resulting fracture behaviour. From an application perspective, epoxy resins with a higher number of functional groups, and hence higher cross-link density, exhibit superior mechanical and thermal properties following curing with a hardener [7,29]. The higher epoxide content of these resins often results in further limitations in ductility, making the TP tougheners a better choice since, as mentioned earlier, unlike rubber modifiers, the dominant toughening mechanism with engineering TPs would be less dependent on the matrix ductility.

In the present work, an engineering thermoplastic, polyetherimide (PEI), is used to modify two types of epoxy resins, namely diglycidylether of bisphenol A (DGEBA) and tetraglycidyl diamino diphenyl methane (TGDDM). DGEBA is a commonly-used epoxy resin in FRP composites and adhesives. TGDDM is a tetrafunctional epoxy used in applications requiring higher temperature performance and mechanical properties. The phase morphology and mechanical properties of the modified resin systems were analysed with a focus on the evolution of the phase morphology of modified epoxy blends and the underlying mechanisms. Mode-I fracture toughness of the cured systems was then measured and the fracture surfaces were examined to gain further insight into the energy dissipation mechanisms.