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Results from experimental studies of strain-crystallized polyethylenes, prepared in a simple shear couette rheometer, indicate a substantial increase in nucleation rate, a decrease in long period, as well as an increase in melting temperature with increasing shear. The changes appear to have the same thermodynamic origin in the reduction of the amount of melt entropy, Delta S', just prior to crystallization according to a strain-induced crystallization theory by Yeh and Hong. Consequently from the measured nucleation rate one can predict what the decrease in long period or the increase in melting temperature should be for a given strain-crystallized polyethylene. For example, the measured melting temperatures are indeed shown to be comparable to those predicted from nucleation rate measurements.

Hydrolytic polycondensation of metal alkoxides produces polymeric units consisting of an oxide network framed by hydroxyl and organic groups. The morphology of the polymerization as well as the structure of these materials is determined by the kinetics of hydrolysis polymerization reactions. By controlling the kinetics of these reactions it is possible to produce modified ceramic materials with altered properties. In this work, by using three representative systems, i.e., silica, titania and zirconia, the effect of hydrolytic polycondensation parameters on the resultant materials was investigated. These parameters include: molecular separation of species, their ratios, the reaction medium, the concentration of catalyst, the reaction time, the temperature and the type of alkyl group in the alkoxide. In the silica system, the parameters that affect the hydrolytic polycondensation of titanium alkoxides Ti(OR)4 were investigated. In the zirconia system, it has been shown that the alkyl groups and molecular separation during the hydrolytic condensation have particular significance in the modification of the monoclinic tetragonal transformation of the resultant ZrO2. Originator supplied keywords include: SOL, GEL, Polymers, Silane, Siloxanes, Zirconium oxide, Sintering, Solution, Transformation, Organometallic, Condensation, Crystallization, Titanium dioxide, Titanium oxide.

The synthesis and characterization of several copolymers based on the monomer pairs of 11-(4-methoxy-(4'-oxy)-alpha-methylstilbene)undecanyl methacrylate (4-11-MA) and 11-(4-oxy-(4'-methoxy)-alpha-methylstilbene)undecanyl methacrylate (4'-11-MA), 11-(4-methoxy-(4'-oxy)-alpha-methylstilbene)undecanyl acrylate (4-11-AC) and 11-(4-oxy-(4'-methoxy)-alpha-methylstilbene)undecanyl acrylate (4'11-AC), and 8-(4-methoxy-(4'-oxy)-alpha-methylstilbene(octyl acrylate (4-8-AC) nd 8-(4-methoxy-(4'-oxy)-alpha-methylstilbene(octyl acrylate (4-8-AC) and 8-(4-oxy-(4'methoxy)-alpha-methylstilbene)octyl acrylate (4-8'-AC) are described. In the case of polymers with poly-methacrylate backbones, the copolymerization of the two constitutional isomeric mesogenic monomers cancels the side chain crystallization tendency characteristic of their parent homopolymers. Subsequently, the copolymer presents a broader mesophase temperature range than any of the two parent homopolymers. In the case of polymers with polyacrylate backbones, copolymerization suppresses the tendency towards side chain crystallization and subsequently, transforms one or even two monotropic mesophases into enantiotropic mesophases.

The synthesis and characterization of several copolymers based on the monomer pairs of 11-(4-methoxy-(4'-oxy)-alpha-methylstilbene)undecanyl methacrylate (4-11-MA) and 11-(4-oxy-(4'-methoxy)-alpha-methylstilbene)undecanyl methacrylate (4'-11-MA), 11-(4-methoxy-(4'-oxy)-alpha-methylstilbene)undecanyl acrylate (4-11-AC) and 11-(4-oxy-(4'-methoxy)-alpha-methylstilbene)undecanyl acrylate (4'11-AC), and 8-(4-methoxy-(4'-oxy)-alpha-methylstilbene(octyl acrylate (4-8-AC) nd 8-(4-methoxy-(4'-oxy)-alpha-methylstilbene(octyl acrylate (4-8-AC) and 8-(4-oxy-(4'methoxy)-alpha-methylstilbene)octyl acrylate (4-8'-AC) are described. In the case of polymers with poly-methacrylate backbones, the copolymerization of the two constitutional isomeric mesogenic monomers cancels the side chain crystallization tendency characteristic of their parent homopolymers. Subsequently, the copolymer presents a broader mesophase temperature range than any of the two parent homopolymers. In the case of polymers with polyacrylate backbones, copolymerization suppresses the tendency towards side chain crystallization and subsequently, transforms one or even two monotropic mesophases into enantiotropic mesophases.

The usefulness of simultaneous polymerization and crystallization as a preparative tool for transition metal polymers is established; syntheses of both stable and metastable mixed-metal (M=Ni,Co) polymers are described. The metastable polymers show solid-state reactivity as well as room-temperature photochromism. (Author)

By means of multicanonical computer simulations, we investigate thermodynamic properties of the aggregation of interacting semiflexible polymers. We analyze a mesoscopic bead-stick model, where nonbonded monomers interact via Lennard-Jones forces. Aggregation turns out to be a process, in which the constituents experience strong structural fluctuations, similar to peptides in coupled folding-binding cluster formation processes. In contrast to a recently studied related proteinlike hydrophobic-polar heteropolymer model, aggregation and crystallization are separate processes for a homopolymer with the same small bending rigidity. Rather stiff semiflexible polymers form a liquid-crystal-like phase, as expected. In analogy to the heteropolymer study, we find that the first-order-like aggregation transition of the complexes is accompanied by strong system-size dependent hierarchical surface effects. In consequence, the polymer aggregation is a phase-separation process with entropy reduction.

By means of multicanonical computer simulations, we investigate thermodynamic properties of the aggregation of interacting semiflexible polymers. We analyze a mesoscopic bead-stick model, where nonbonded monomers interact via Lennard-Jones forces. Aggregation turns out to be a process, in which the constituents experience strong structural fluctuations, similar to peptides in coupled folding-binding cluster formation processes. In contrast to a recently studied related proteinlike hydrophobic-polar heteropolymer model, aggregation and crystallization are separate processes for a homopolymer with the same small bending rigidity. Rather stiff semiflexible polymers form a liquid-crystal-like phase, as expected. In analogy to the heteropolymer study, we find that the first-order-like aggregation transition of the complexes is accompanied by strong system-size dependent hierarchical surface effects. In consequence, the polymer aggregation is a phase-separation process with entropy reduction.

Investigating thermodynamic properties of liquid-solid transitions of flexible homopolymers with elastic bonds by means of multicanonical Monte Carlo simulations, we find crystalline conformations that resemble ground-state structures of Lennard-Jones clusters. This allows us to set up a structural classification scheme for finite-length flexible polymers and their freezing mechanism in analogy to atomic cluster formation. Crystals of polymers with "magic length" turn out to be perfectly icosahedral.

We map out the solid-state morphologies formed by model soft-pearl-necklace polymers as a function of bending stiffness $k_b$ spanning the range from fully flexible to rodlike chains. The ratio of Kuhn length to bead diameter ($l_K/r_0$) increases monotonically with increasing $k_b$ and yields a one-parameter model that relates chain shape to bulk morphology and yields insights into the packing of anisotropic particles. In the flexible limit, monomers occupy the sites of close-packed crystallites while chains retain random-walk-like order. In the rodlike limit, nematic chain ordering typical of lamellar precursors coexists with close-packing. At intermediate values of bending stiffness the competition between random-walk-like and nematic chain ordering produces glass-formation; the range of $k_b$ over which this occurs increases with the thermal cooling rate $|\dot{T}|$ implemented in our molecular dynamics simulations. Finally, values of $k_b$ between the glass-forming and rodlike ranges produce complex ordered phases such as close-packed spirals. Our results should prove useful for rational design of dense colloidal-polymer phases with desired morphologies.

Journal of Research of the National Bureau of Standards