Polymerization-Induced Phase Separation in Rubber-Toughened Amine-Cured Epoxy Resins: Tuning Morphology from the Nano- to Macro-scale

Abstract

Polymerization-induced phase separation enables fine control over thermoset network morphologies, yielding heterogeneous structures with domain sizes tunable over 1−100 nm. However, the controlled chain-growth polymerization techniques exclusively employed to regulate the morphology at these length scales are unsuitable for a majority of thermoset materials typically formed through step-growth mechanisms. By varying the composition of a binary curing agent mixture in a classic rubber-toughened epoxy thermoset, where the two curing agents are selected based on disparate compatibility with the rubber, we demonstrate facile tunability over morphology through a single compositional parameter. Indeed, this method yields morphologies spanning the nano-scale to the macro-scale, controlled by the relative reactivities and thermodynamic compatibility of the network components. We further demonstrate a profound connection between chain dynamics and microstructure in these materials, with the tunable morphology enabling exquisite variations in glass transition. In addition, previously unattainable control over tensile mechanical properties is realized, including atypical increase of elongation at failure while maintaining the modulus and ultimate strength.

Tuneable phase behaviour and glass transition via polymerization-induced phase separation in crosslinked step-growth polymers

“…47 It was previously determined that q x-link peaks ( d x-link = 1.38–1.57 nm) were associated with the crosslink lengths of the CA-crosslinked epoxy while q l and q m describe the complex length scale of the rubber and the strong nanoscale variations in composition between epoxy and rubber domains, with q m indicating a peak value and q l indicating the heterogeneity in this length scale. 39 For a given series of n at constant m , the longest wavelength, d l , of the nanoscale structure increased up to the value of n at which metastability is observed, for example, n = 0.4 in Fig. 2a, and then decreased at higher values.…”

“…A rubber loading of m = 0.1 was used to assess the new parameter's ability to tune morphology and glass transition in a rubber-toughened epoxy system as our previous work demonstrated values of m greater than 0.2 resulted in a rubber-rich continuous matrix and CA-rich dispersed domains. 39 This inversion of matrix/domain composition as compared with m r 0.1 yielded significant loss in mechanical properties. Transparent samples were observed at values of e below 0.5, where DER comprised the majority of the binary mixture, but opaque samples were produced at values of 0.5 and greater (Fig.…”

“…1a). 39 The system can be tuned through the parameters of rubber content and the composition of the CA mixture defined by the variables m and n , respectively. Here, increasing values of m indicate higher rubber content ( m = 0 is entirely epoxy + CA, m = 1 is entirely rubber), whereas increasing values of n indicate increased TETA content and decreased D230 content within the binary CA mixture ( n = 0 the CA is pure D230, n = 1 the CA is pure TETA).…”

“…Very recently, we introduced an elegantly simple approach to tune PIPS morphology over nanoscale to macroscale dimensions in rubber-toughened epoxy resin crosslinked with common diamine curing agents (CAs). 39 We showed that two different CAs can be selected to yield extremely disparate phase-separated length scales based on their chemical structure or reactivity. Then, the morphology can be conveniently tuned between these extremes by employing a binary mixture of the CAs, with their relative ratio dictating the final structure.…”

Analysis of the Chemical Distribution of Self-Assembled Microdomains with the Selective Localization of Amine-Functionalized Graphene Nanoplatelets by Optical Photothermal Infrared Microspectroscopy

“…Typically, engineering thermoplastics, such as polyethersulfone, , polysulfone, and polyetherimide (PEI), , display good compatibility with the epoxy oligomers but undergo gradual separation from the epoxy matrix during the polymerization to form a second phase. This so-called polymerization-induced phase separation (PIPS) offers a pathway for generating thermoset polymers with well-defined nanomicrostructures via the spontaneous segregation of otherwise miscible components upon an increase in the molecular weight of at least one of the components during polymerization. − For instance, Wang et al reported that the selective localization of multiwall carbon nanotubes (MWCNTs) in the cocontinuous phase structure in an epoxy/PEI system was able to improve the mechanical, thermal, and electrical properties, simultaneously. Among the increased comprehensive performance, the most significant enhancement is that the volume resistivity decreased from 3.29 × 10 15 to 3.86 × 10 6 Ω·m at 2.0 wt % MWCNTs due to the formation of a self-assembled filler network via PIPS.…”