Toughening epoxy thermosets and their carbon fibre reinforced composites with electrospun nanofibres

Electrospun nanofibres have emerged as important fibrous materials for diverse applications. They have been shown excellent toughening results when they are applied as interlayer materials between carbon/epoxy laminas in the structural carbon fibre reinforced epoxy matrix composites. They also exhibit synergistic modification effects when they are combined with carbon nanofibres in the thermosetting polymer matrix. In this study, electrospun polyetherketone cardo (PEK-C) nanofibres were used in two ways: directly electrospun onto the surface of carbon fabric [1], and blended with epoxy resin in the form of PEK-C/VGCNF (vapour grown carbon nanofibre) composite nanofibres[2].The interlaminar fracture toughness, flexural properties and thermal mechanical properties of the modified systems were investigated.

Analysing ordered nanostructures in epoxy thermosets using small angle X-ray scattering techniques

Small angle X-ray scattering (SAXS) is essential for the morphological investigation of nanostructured systems as it is a bulk sampling technique and provides information about the overall distribution of the components in the system. In our study we have used SAXS to identify various ordered and disordered morphologies in block copolymer modified epoxy thermosets. We have used a reactive block copolymer and hydrogen bonding block copolymer to modify epoxy resin (ER) to see the effect of various blocks on the morphological changes.

Toughening epoxy thermosets with block ionomers : the role of phase domain size

Herein we report a novel approach to toughen epoxy thermosets using a block ionomer, i.e., sulfonated polystyrene-block-poly(ethylene-co-butylene)-block- polystyrene (SSEBS). SSEBS was synthesized by sulfonation of SEBS with 67 wt % polystyrene (PS). Phase morphology of the epoxy/SSEBS blends can be controlled at either nanometer or micrometer scale by simply adjusting the sulfonation degree of SSEBS. It has been found that there exists a critical degree of sulfonation (10.8 mol %) forming nanostructures in these epoxy/SSEBS blends. Above this critical value, macrophase separation can be avoided and only microphase separation occurs, yielding transparent nanostructured blends. All epoxy/SSEBS blends display increased fracture toughness compared to neat epoxy. But the toughening efficiency varies with the phase domain size, and their correlation has been established over a broad range of length scales from nanometers to a few micrometers. In the nanostructured blends with SSEBS of high sulfonation degrees, the fracture toughness decreases with decreasing size of the phase domains. In the macrophase-separated blends, only a slight improvement in toughness can be obtained with SSEBS of low sulfonation degrees. The epoxy blend with submicrometer phase domains in the range 0.05-1.0 μm containing SSEBS of a moderate degree of sulfonation (5.8 mol %) displays the maximum toughness. This study has clearly clarified the role of phase domain size on toughening efficiency in epoxy thermosets.

Nanostructure toughening of epoxy thermosets with block Ionomer complexes

This thesis addresses an important issue in polymer materials science, the toughening of thermosetting polymers. A novel approach has been developed, i.e., the use of block ionomers/complexes to promote compatibilization with thermosetting epoxies. The morphology and mechanical properties of the resulting nanostructured epoxies were intensively studied to establish structure-property correlation.

Nanostructures and nanoporosity in thermoset epoxy blends with an amphiphilic polyisoprene-block-poly(4-vinyl pyridine) reactive diblock copolymer

Nanostructured thermoset blends were prepared based on a bisphenol A-type epoxy resin and an amphiphilic reactive diblock copolymer, namely polyisoprene-block-poly(4-vinyl pyridine) (PI-P4VP). Infrared spectra revealed that the P4VP block of the diblock copolymer reacted with the epoxy monomer. However, the non-reactive hydrophobic PI block of the diblock copolymer formed a separate microphase on the nanoscale. Ozone treatment was used to create nanoporosity in nanostructured epoxy/PI-P4VP blends via selective removal of the PI microphase and lead to nanoporous epoxy thermosets; disordered nanopores with the average diameter of about 60 nm were uniformly distributed in the blend with 50 wt% PI-P4VP. Multi-scale phase separation with a distinctly different morphology was observed at the air/sample interface due to the interfacial effects, whereas only uniform microphase separated morphology at the nanoscale was found in the bulk of the blend.

High-performance composite from epoxy and glass fibers : morphology, mechanical, dynamic mechanical, and thermal analysis

Diglycidyl ether of bisphenol-A type epoxy resin cured with diamino diphenyl sulfone was used as the matrix for fiber-reinforced composites to get improved mechanical and thermal properties for the resulting composites. E-glass fiber was used for fiber reinforcement. The morphology, tensile, flexural, impact, dynamic mechanical, and thermal properties of the composites were analyzed. The tensile, flexural, and impact properties showed dramatic improvement with the addition of glass fibers. Dynamic mechanical analysis was performed to obtain the Tg of the cured matrix as well as the composites. The improved thermal stability of the composites was clear from the thermogravimetric analysis. Scanning electron micrographs were taken to understand the interfacial adhesion between the fiber and the matrix. The values of mechanical properties were compared with modified epoxy resin composite system. Predictive models were applied using various equations to compare the mechanical data obtained theoretically and experimentally.

A new route to nanostructured thermosets with block ionomer complexes

We report a novel approach to prepare nanostructured thermosets using block ionomer complexes. Neither block copolymer polystyrene-block-poly(ethylene-ran- butylene)-block-polystyrene (SEBS) nor block ionomer sulfonated SEBS (SSEBS) is miscible with diglycidyl ether of bisphenol A (DGEBA) type epoxy resin. It is thus surprising that the block ionomer complex of SSEBS with a tertiary amine-terminated poly(3-caprolactone) (PCL), denoted as SSEBS-c-PCL, can be used to prepare nanostructured epoxy thermosets. The block ionomer complex SSEBS-c-PCL is synthesized via neutralization of SSEBS with 3-dimethylamino- propylamine-terminated PCL. Sulfonation of SEBS yields the block ionomer SSEBS which is immiscible with epoxy. But the block ionomer complex SSEBS-c-PCL can be easily mixed with DGEBA. When the curing agent 4,4'-methylenedianiline (MDA) is added and the epoxy cures, the system retains the nanostructure. In cured epoxy thermosets containing up to 30 wt% SSEBS-c-PCL, the exclusion of the poly(ethylene-ran-butylene) (EB) phase forms spherical micro-domains surrounded by separated sulfonated polystyrene phase while the PCL side-chains of SSEBS-c-PCL are dissolved in the cured epoxy matrix. The spherical micro-domains are highly aggregated in the epoxy thermosets containing 40 and 50 wt% SSEBS-c-PCL. The existence of epoxy-miscible PCL side-chains in the block ionomer complex SSEBS-c-PCL avoids macro-phase separation. Hence, the block ionomer complex can act as an efficient modifier to achieve nanostructured epoxy thermosets.

Thermally flexible epoxy/cellulose blends mediated by an ionic liquid

Blends between the widely used thermoset resin, epoxy, and the most abundant organic material, natural cellulose are demonstrated for the first time. The blending modification induced by charge transfer complexes using a room temperature ionic liquid, leads to the formation of thermally flexible thermoset materials. The blend materials containing low concentrations of cellulose were optically transparent which indicates the miscibility at these compositions. We observed the existence of intermolecular hydrogen bonding between epoxy and cellulose in the presence of the ionic liquid, leading to partial miscibility between these two polymers. The addition of cellulose improves the tensile mechanical properties of epoxy. This study reveals the use of ionic liquids as a compatible processing medium to prepare epoxy thermosets modified with natural polymers.

Novel approach to trigger nanostructures in thermosets using competitive Hydrogen-Bonding-Induced Phase Separation (CHIPS)

A new route to prepare nanostructured thermosets by the utilization of intermolecular hydrogen-bonding interactions is demonstrated here. In this study, competitive hydrogen-bonding-induced microphase separation (CHIPS) in epoxy resin (ER) containing an amphiphilic block copolymer poly(ε-caprolactone)-block-poly(2-vinylpyridine) (PCL-b-P2VP) is investigated for the first time. The phase separation takes place due to the disparity in the hydrogen-bonding interactions in ER/P2VP and ER/PCL pairs leading to the formation of ordered nanostructures in the ER/block copolymer blends. SAXS and TEM results indicate that the hexagonally packed cylindrical morphology of neat PCL-b-P2VP block copolymer remains but becomes a core-shell structure at 10 wt % addition of ER, and changes to regular lamellae structures at 20-50 wt % then to disordered lamellae with 60 wt % ER. Wormlike structures are obtained in the blends with 70 wt % ER, followed by a completely homogeneous phase of ER/P2VP and ER/PCL. The formation of nanostructures and changes in morphologies depend on the relative strength of hydrogen-bonding interactions between each component block copolymer and the homopolymer. This versatile method to develop nanostructured thermosets, involving competitive hydrogen-bonding interactions, could be used for the fabrication of hierarchical and functional materials.

Reactive block copolymer modified thermosets : highly ordered nanostructures and improved properties

A highly ordered poly(dimethyl siloxane)-poly(glycidyl methacrylate) (PDMS-PGMA) reactive diblock copolymer was synthesized and used to modify bisphenol A-type epoxy resin (ER). The PDMS-PGMA block copolymer consisted of epoxy-miscible PGMA blocks and an epoxy-immiscible PDMS block. The PGMA reactive block of the block copolymer formed covalent bonds with cured epoxy and was involved in the network formation, and the PDMS block phase separated to give different ordered and disordered nanostructures at different blend compositions. The solvent cast PDMS-PGMA diblock copolymer showed ordered hexagonal cylindrical morphology. A highly ordered morphology consisting of hexagonal cylinders inside the lamellar morphology was observed in the cured PDMS-PGMA block copolymer. In the cured ER/PDMS-PGMA blends, a variety of morphologies including lamellar, cubic and worm-like and spherical nanostructures were detected depending on the blend composition. Moreover, the addition of this reactive diblock copolymer significantly increases the hydrophobicity and the glass transition temperature. It also improves the tensile strength and tensile ductility of the nanostructured thermosets at low diblock copolymer contents.

Individual dispersion of carbon nanotubes in epoxy via a novel dispersion-curing approach using ionic liquids

The effective dispersion of carbon nanotubes (CNTs) in a thermoset was achieved using ionic liquid as the dispersion-curing agent. We preferentially dispersed multiwalled carbon nanotubes (MWCNTs) down to individual tube levels in epoxy resin. Here the dispersion is ruled by the depletion of physical bundles within the MWCNT networks, for which molecular ordering of ionic liquids is considered responsible. The quantitative analyses using ultra small angle X-ray scattering (USAXS) confirmed the dispersion of individual MWCNTs in the matrix. The distance between the dispersed nanotubes was calculated at different nanotube loadings using the power law fitting of the USAXS data. The fine dispersion and subsequent curing, both controlled by ionic liquid, lead to composites with substantially enhanced fracture mechanical and thermomechanical properties with no reduction in thermal properties. Merging processing techniques of nanocomposites with ionic liquid for efficient dispersion of nanotubes and preferential curing of thermosets facilitates the development of new, high performance materials.