Nanocomposites

The ability of block copolymers to form well-defined nanostructures has been exploited in the generation of nanocomposites by combining the copolymers with other nanoparticles that would not normally form nanostructures, for example silica, metals, or proteins. This allows the controlled preparation of multifunctional materials with customized properties, such as catalytic activity, optoelectrical, and magnetic properties. Recently the combination of nanoscopic clay with polyurethane in a selective solvent condition has led to the development of a high performance elastomer.

As well as nanotemplating, block copolymer structures have been observed to act as toughening agents when they assemble in thermosets. The modified thermosets experience an improvement in their mechanical properties because of toughening. This depends on the morphology adopted by the copolymers. Over moderate-to-high polymer concentrations the system behaves as expected for block copolymers placed in a solvent that is selective for one block.

The formation of nanostructured systems in cured blends of epoxy resin and a diblock copolymer was first reported by Hillmyer. Since that initial report, extensive investigation has been carried out by numerous research groups into epoxy/block copolymer blend systems. Two types of copolymers have been studied: unreactive and reactive modifiers. In the unreactive case, the nanoscale structures are formed in solution and fixed during cure. In this way a block copolymer self-assembles in the pre-cure stage via a block copolymer core (resinophobe) and a corona (resinophile) that are immiscible and miscible with the resin, respectively. As with block copolymers in aqueous solution, the self assembly in the pre-cured phase yields morphologies such as micelles and vesicles. The polymerization of the resin (typically an epoxy resin but the principles apply universally) causes the high molecular weight cross-linking mixture to become a poorer solvent. This can cause macrophase separation or a change in the particle morphology. If the solubilities are selected correctly, then the crosslinking of the resin causes debonding between the micelles and thermoset and this yields optimum toughness.

In the reactive scenario the epoxy miscible block is reactive towards the resin or the curing agent. Chemical linking through reactive corona molecules only increases the toughness of the materials if they remain brittle; that is; they have not reached a sufficiently high number density. The effect of poly(ethylene oxide)-b-poly(ethylene-alt-propylene), PEO-PEP block copolymers at low concentration on a Bisphenol A diglycidyl ether (BADGE) resin cured with methylene dianiline MDA. Spherical micelles were found to improve fracture toughness K_Ic by 25–35%. A vesicular morphology increased K_Ic by 45% even at half the concentration of the micelle forming systems. In a further paper unreactive poly(ethylene oxide)-b-poly(butadiene) PEO-PB copolymers are compared with epoxidised poly(isoprene)-b-poly(butadiene) ePI-PB and polymers with a reactive poly(methylacrylate-co-glycidyl methacrylate) P(MA-co-GMA) epoxy miscible block. The ePI-PB and reactive epoxy miscible P(MA-co-GMA) block copolymers form nanoscale structures which are chemically bonded to the resin after and before gelation of the epoxy resin, respectively. Once more, vesicles are the best at improving fracture mechanics. The vesicles in which the resinophilic block reacts with the resin after gelation provide higher toughness than unreactive vesicles. Differing findings have been obtained in a study using PEO-PEP and reactive block copolymers in partially brominated BADGE resins cured with phenol novolac. Spherical micelles were found to give considerably superior improvements in toughness than vesicles. Even greater enhancement has been found when wormlike micelles form ( 4× improvement in K_Ic; 3× with spherical micelles). Similar behavior when they studied PEO-PBO diblock copolymers in non-brominated BADGE + phenol novolac. Again, wormlike micelles show the greatest improvement in K_Ic ( 4×), followed by spherical micelles ( 2.5×) and vesicles ( 1.8×). It is suggested that the toughening observed with micelles may be due to cavitation processes. With wormlike micelles, scanning electron microscopy (SEM) indicates that worms bridging the crack are ‘pulled-out’.