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The mechanism of the transition from a lamellar phase to a gyroid phase in an aqueous solution of a diblock copolymer has been studied by time-resolved synchrotron small-angle X-ray scattering. The transition occurs via a metastable perforated lamellar structure. The perforations initially have liquidlike ordering before developing hexagonal packing. The transient phase of irregularly perforated layers is revealed by the development of diffuse scattering peaks, just below the Bragg peaks of the lamellar structure. The diffuse scattering is modeled by Monte Carlo simulations of perforated layers. Following the formation of perforations, Bragg peaks characteristic of a hexagonal structure signal an ordering into a hexagonal lattice (with the concomitant loss of diffuse scattering). Computer simulations based on a dynamic density functional model reproduce these features. The hexagonal perforated lamellar phase is rapidly replaced by the gyroid phase. The domain spacing of the gyroid phase is larger than that of the perforated lamellar structure. The perforated lamellar and gyroid phases coexist for a defined period. The reverse transition from gyroid to lamellae occurs directly, with no transient or metastable intermediates.

Microdomain structures and phase transitions of binary and ternary mixtures of polystyrene−polyisoprene−polystyrene (PS−PI−PS) triblock copolymer Vector 4111 (containing 18% PS) and the corresponding homopolymers were investigated in thin films by atomic force microscopy and in bulk by small-angle X-ray scattering. The neat copolymer shows a hexagonally packed cylinder structure. The addition of PI homopolymer caused a transition from the cylindrical to a spherical microdomain structure via a biphasic intermediate state with coexisting cylindrical and spherical domains for both thin films and bulk. In contrast, addition of a small amount (14 wt %) of minority-component PS homopolymer in a mixture leads to a significant increase of the cylindrical microdomain curvature in thin films. The influence of the added homopolymer PS on the structure of bulk samples is discussed. Addition of equal amounts of both components together does not change the cylindrical structure for bulk or thin film samples but results in an aligned cylindrical microdomain structure observed for thin films. Interdomain distances decreased upon addition of small or moderate amounts (7−38 wt %) of either or both homopolymer.

This paper discusses the solid state and melt nanoscale structure of a series of novel poly(ethylene glycol) (PEG) hybrid di- and triblock copolymers, which contain amphiphilic β-strand peptide sequences. The block copolymers have been prepared via solid-phase synthesis, affording perfectly monodisperse peptide segments with a precisely defined α-amino acid sequence. Attenuated total reflection Fourier transform infrared spectroscopy and X-ray scattering experiments indicate that the self-assembly properties of the peptide sequences are retained upon conjugation to PEG and mediate the formation of an ordered superstructure consisting of alternating PEG layers and peptide domains with an highly organized antiparallel β-sheet structure. The results suggest that combination of biological structural motifs with synthetic polymers may be a versatile strategy for the development of novel self-assembled materials with complex internal structures and the potential to interface with biology.

SiC−C solid solution powders have been analyzed by X-ray diffraction (XRD) and magic-angle spinning nuclear magnetic resonance (MAS NMR). The XRD data reveal a previously unreported phase separation of the as-synthesized material into two cubic silicon carbide phases characterized by slightly different lattice parameters [4.35287(9) and 4.35841(6) Å]. The ²⁹Si MAS NMR spectra of the powders after different heat treatments (annealing in a vacuum and high-pressure sintering) show that the small excess of carbon (<1 at %) in the β-SiC crystal structure has a large influence on the ²⁹Si chemical shift resulting in a displacement of the ²⁹Si MAS NMR peak from −18.5 ppm to lower field. At least five nonequivalent silicon sites have been detected in Si1-xC1+x:  the solid solution formed by high-pressure sintering (4 GPa, 1800 °C) of the as-synthesized SiC−C powder. These sites are assigned to point (carbon antisite) defects in the cubic silicon carbide structure.