Showing papers by "Marc A. Hillmyer published in 1998"

Nanostructured Thermosets from Self-Assembled Amphiphilic Block Copolymer/Epoxy Resin Mixtures

Abstract: Thermoset materials containing ordered structures with ∼10-nm dimensions were prepared from a mixture of a low-molecular-weight poly(ethylene oxide)−poly(ethylene-alt-propylene) (PEO−PEP) diblock copolymer and a poly(Bisphenol-A-co-epichlorohydrin) epoxy resin that selectively mixes with the PEO block. The phase behavior of PEO−PEP/epoxy blends, with compositions spanning 10−93 wt % block copolymer, was investigated in the uncured state (without hardener) using small-angle X-ray scattering (SAXS) and dynamic mechanical spectroscopy. Without hardener, the phase behavior of block copolymer/epoxy blends was similar to model block copolymer/homopolymer blends and varied with changes in the blend composition and temperature. The following morphologies were observed with increasing epoxy concentration: lamellar, cubic bicontinuous, hexagonally packed cylinders, body-centered cubic packed spheres, and disordered micelles. Methylene dianiline, an aromatic amine hardener, was added to the blends, and the real-tim...

Transition mechanisms for complex ordered phases in block copolymer melts

Abstract: We describe microstructural aspects of phase transitions between the lamellar (L), perforated layer (PL), and gyroid (G) morphologies in diblock copolymer melts. Using small-angle scattering, dynamic mechanical spectroscopy, and transmission electron microscopy, we show that these transformations proceed through the nucleation and growth of the final phase, in contrast to recent calculations that assume evolution from a thermodynamically unstable initial state. Direct L → G transitions are suppressed by the high surface tension associated with L−G grain boundaries; the formation of the metastable PL structure under such conditions reflects the ease with which the L → PL transition can occur, compared to L → G. Similar effects dominate the G → L transition. Mismatches in spacings between epitaxially related lattice planes also influence relaxation kinetics; the P → LG transition rate depends strongly on the relative spacings of the PL [10] and G [211] planes, and the considerable discrepancy between the G ...

Complex phase behavior in aqueous solutions of poly(ethylene oxide)-poly(ethylethylene) block copolymers

Abstract: The many similarities in morphological behavior exhibited by diblock copolymer melts and lyotropic surfactant suspensions suggest the existence of common physical principles underlying these phenom...

Block copolymer self-diffusion in the gyroid and cylinder morphologies

Abstract: Forced Rayleigh scattering (FRS) and pulsed-field-gradient NMR have been used to measure the self-diffusion coefficient, D, of a poly(ethylene oxide)−poly(ethylethylene) diblock copolymer in the molten state. The copolymer contains 42% PEO by volume and has a total molecular weight of 4100 g/mol. Upon heating from room temperature the sample transforms from crystalline lamellae to hexagonal cylinders, and then to a bicontinuous cubic “gyroid” phase (with Ia3d space group symmetry), before finally disordering at 175 °C. FRS measurements were performed in the gyroid and cylinder phases, and NMR measurements in the gyroid and disordered states. Cylinder samples both with and without shear alignment were employed. A hysteresis loop permitted measurements of D in both cylinder and gyroid phases at the same temperature (60 °C). FRS decays from cylindrical samples were described by a sum of two exponentials. For the aligned samples, values of the diffusivity along (Dpar) and across (Dperp) the cylinders were ex...