Rearrangement of Nanoporous Cavity Structures in Crystalline Syndiotactic Polystyrene Associated with Stress-Induced Phase Transition.

This work demonstrates the possibility of rearranging nanoporous cavity structures in crystalline syndiotactic polystyrene (s-PS) by applying external stresses. A molecular dynamics (MD) simulation was performed for the s-PS crystal with increasing or decreasing of each stress tensor component, starting from the nanoporous ε form. Upon uniaxial compression along the b-axis, the ε form was transformed into a lower density porous form, accompanied by significant elongation of the a-axis. The new form, named the S-I form, was stable only under the stress and was transformed into the γ form after release of the stress. The cavity structure was drastically changed by the transition. The straight cylindrical channels in the ε form, which may be used for the separation of organic solvents, were rearranged to narrower zigzag channels in the S-I form, which is suitable for the precise separation of gases. The molecular cavities disappeared after release of the stress, associated with the transition to the γ form.

Roles of the ether oxygen in hydration of tetrahydrofuran studied by IR, NMR, and DFT calculation methods.

We studied the concentration dependence of nu(C-H)'s in IR and (1)J(C,H) in NMR for binary water-tetrahydrofuran (THF) mixtures and found different trends for the two types of CH(2) groups in the five-membered ring. The changes of the nu(C-O) spectra showed that complexes of THF associated with water are formed, in which the number of water molecules increases with the water concentration. We suggested that hydration proceeds through the formation of 1:1, and 1:2 complexes of [THF:water] up to X(H(2)O) approximately 0.9, where X(H)((2))(O) is the mole fraction of the water in the mixtures. We carried out ab initio MO and DFT calculations to optimize the geometries of a THF dimer as a model of THF molecules in pure liquid, and 1:1 and 1:2 complexes of [THF:water] to simulate observed concentration dependence of nu(C-H)'s in IR and (1)J(C,H) in NMR. The changes of the calculated nu(C-H) spectra and (1)J(C,H) values for the optimized complexes are in agreement with those observed with varying X(H)((2))(O), supporting our proposal. From the vibrational and NBO analyses of the optimized complexes, the observed blue shift of nu(C-H)'s and the increase of (1)J(C,H) for the CH(2) groups neighboring to the ether oxygen were explained in terms of the changes in the stereoelectronic effect, resulting from HO-H...O< hydrogen bonding. The optimized 1:2-complex contains two weak C-H...OH(2) hydrogen bonds, and blue shift of nu(C-H)'s and increase of (1)J(C,H) were demonstrated from the same analyses of the complexes. This result of simulation also supports that the blue shift of nu(C-H)'s and increase of (1)J(C,H) observed for both the type of CH(2) groups at 0.6 X(H)((2))(O) < 0.9 are attributed to these interactions. On the basis of all these results, we propose that the formation of the 1:2-complex involving weak C-H...OH(2) hydrogen bonds is responsible dominantly for the hydrophobic hydration of THF.

Effect of encaged aromatic guests on the shape and connectivity of molecular cavity in crystalline polystyrene evaluated by molecular simulations.

The delta form of crystalline syndiotactic polystyrene is a clathrate molecular compound in which various aromatic molecules are encaged. We have investigated the size, shape, and connectivity of the molecular cavity in the crystal using a molecular dynamics simulation. The effects of the guest species on the cavity structure were investigated in detail. In order to systematically vary the guest structure, various aromatic guests, e.g., benzene, toluene, p-xylene, m-xylene, o and mesitylene were examined. The interstitial spaces between the guests and the polymer chains were analyzed by cluster analysis of the free volumes. The individual cavity volumes into which the guests are clathrated were also evaluated. It was found that the guest molecules can greatly affect not only the cavity size and shape but also the connectivity of the cavities. The transport of small molecules in the crystal is discussed in connection with the cavity structure.

Effects of pressure on the fragile nature of fluorozirconates studied by molecular dynamics simulations.

Molecular dynamics simulations for a multicomponent zirconium fluoride, ZBEN (55ZrF(4)-17BaF(2)-5EuF(3)-23NaF) have been carried out at various temperatures and pressures including those of the molten and glassy states. The effect of pressure on the fragile nature of ZBEN was investigated. The relaxation times were calculated from the decay curves of the intermediate scattering function. The fragile nature of ZBEN was reflected by the obvious non-Arrhenius behavior in the temperature dependence of the relaxation time. The so-called "Angell plot" of the relaxation time was represented by a single curve irrespective of applied pressure; the fragile nature changes negligibly with pressure. This seems to contradict the enhancement of the connectivity of ZrF(n) polyhedra under high pressure. The origin of the pressure effects is discussed in comparison with those for molecular liquids and covalent-network glasses.