Toward microphase separation in epoxy systems containing PEO–PPO–PEO block copolymers by controlling cure conditions and molar ratios between blocks

Nanostructuring of thermosetting systems using the concept of templating and taking advantage of the self-assembling capability of block copolymers is an exciting way for designing new materials for nanotechnological applications. In this first part of the work, reactive blends based on stoichiometric amounts of a diglycidylether of bisphenol-A epoxy resin and 4,4′-diaminodiphenylmethane cure agent modified with three poly(ethylene oxide)-co-poly(propylene oxide)-co-poly(ethylene oxide) block copolymers were studied. Cure advancement of these systems was analyzed by differential scanning calorimetry. The experimental results show a delay of cure rate, which increases as copolymer content and PEO molar ratio in the block copolymer rise. Infrared spectroscopy shows that PEO block is mainly responsible of physical interactions between the hydroxyl groups of growing epoxy thermoset and ether bonds of block copolymer. These interactions are mainly responsible for the delaying of cure kinetics. The molar ratio between blocks also has a critical influence on the delaying of the cure rate. A mechanistic approach of cure kinetics allows us to relate the delay of cure as a consequence of block copolymer adding to physical interactions between components.     

Reaction-induced microphase separation in epoxy resin containing polystyrene-block-poly(ethylene oxide) alternating multiblock copolymer

Polystyrene-block-poly(ethylene oxide) alternating multiblock copolymer (PS-alt-PEO) was synthesized with the combination of atom transfer radical polymerization (ATRP) and Huisgen 1,3-dipolar cycloaddition (i.e., click chemistry). The copolymer has been characterized by means of Fourier transform infrared spectroscopy (FTIR), ¹H nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). The PS-alt-PEO alternating multiblock copolymer was incorporated into epoxy resin to investigate the behavior of reaction-induced microphase separation, which has been compared to the case of the thermosets containing PS-b-PEO diblock copolymer. The morphology of epoxy thermosets containing PS-alt-PEO alternating multiblock copolymer were investigated by means of atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) and the nanostructures were detected in all the thermosetting blends investigated. In marked contrast to the case of the thermosets containing PS-b-PEO diblock copolymer, the thermosets containing PS-alt-PEO multiblock copolymer displayed disordered nanostructures, which have been interpreted on the basis of the restriction of the alternating multiblock topology of the block on the formation of the nanostructures via reaction-induced microphase separation.     

Towards quantification of butadiene content in styrene-butadiene block copolymers and their blends with general purpose polystyrene (GPPS) and the relation between mechanical properties and NMR relaxation times

The properties of styrene-butadiene-styrene (SBS) block copolymers do not only depend on the butadiene content and the degree of polymerisation but also on their chain architecture. In this contribution we present the results of a low-field time domain (TD) NMR study in which the transverse relaxation behaviour of different SBS block copolymers was analysed and correlated with findings from mechanical testing on pure and blended materials and transmission electron microscopy data which provide information on the microphase separation.The results indicate that while a straightforward determination of the butadiene content as in blended materials like ABS is not possible for these materials, the TD-NMR results correlate quite well with the mechanical performance of blends from SBS block copolymers with general purpose polystyrene (GPPS), i.e. industrial grade homopolymer polystyrene. Temperature-dependent experiments on pure and blended materials revealed a slight reduction in the softening temperature of the GPPS fraction in the blends.