We present an efficient method for the creation of atomistic model structures of cross-linked polymer matrices. The method consists of preparation of a physical mixture of the monomer and the cross-linker molecules in the box followed by a single-step polymerization of the entire mixture. For this purpose, the simulated annealing algorithm is used to identify pairs of reacting atoms that are spatially close. The technique is used to create five structures of cross-linked epoxy as well as cross-linked epoxy-POSS (i.e., polyhedral oligomeric silsesquioxane) nanocomposite. The models so generated are characterized with respect to the density, volume-temperature behavior, and the detailed molecular structure. Our results show that incorporation of POSS particles (at 5 wt %) in the cross-linked epoxy resin leads to a weak tendency for lowering the coefficient of volume thermal expansion but does not cause a measurable change in the glass transition temperature.

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We studied the local chain dynamics as well as the dynamic heterogeneity in highly cross-linked epoxy near glass transition by molecular dynamics simulation. In previous work (Lin and Khare in Macromolecules2009424319, we had reported creation of fully relaxed atomistic structures of cross-linked epoxy; the glass transition temperature of these structures was also determined from volume−temperature behavior. The local chain dynamics in these structures is characterized by using molecular dynamics simulation in this work. Local translational mobility of cross-linked epoxy as measured by mean-squared displacement of atoms exhibited two subdiffusive regimes within the simulation time of 200 ns. Local orientational mobility was determined by monitoring the autocorrelation function (ACF) of a vector associated with the epoxy monomer. Time dependence of an order parameter based on this ACF showed that there was almost no orientational relaxation when the temperature was near or below glass transition temperature and the system cannot completely relax orientationally even at temperatures that are 120 K higher than the glass transition temperature. Furthermore, a time scale for the identification of glass transition in simulations was determined using the kinetic interpretation of glass transition. Dynamic heterogeneity was studied by identifying the mobile and immobile atoms in the system. Simulation results confirmed the existence of dynamic heterogeneity in the cross-linked system with the fraction of the immobile domains in the structures showing a rapid increase as the temperature dropped below the glass transition temperature. Results are also presented for the probability of percolation of the system by the immobile domains in the vicinity of glass transition.

We have investigated the properties of vapor-deposited glasses prepared from short polymer chains using molecular dynamics simulations. Vapor-deposited polymer glasses are found to have higher density and higher kinetic stability than ordinary glasses prepared by gradual cooling of the corresponding equilibrium liquid. In contrast to results for binary Lennard-Jones glasses, the deposition rate is found to play an important role in the stability of polymer vapor-deposited glasses. Glasses deposited at the slowest deposition rate and at the optimal substrate temperature are found to correspond to the ordinary glasses that one could hypothetically prepare by cooling the liquid at rates that are 4-5 orders of magnitude slower than those accessible in the current simulations. For intermediate-length polymer chains, the resulting vapor-deposited glasses are found to be highly anisotropic. For short chains, however, the glasses are isotropic, showing that structural anisotropy is not a necessary condition for formation of stable glasses by physical vapor deposition.

In this study, a novel synthetic route was developed to prepare polyimide-nanocrystalline-titania hybrid optical films with a relatively high titania content (up to 50 wt %) and thickness (20-30 lm) from soluble polyimides containing hydroxyl groups. Two series of newly soluble polyimides were synthesized from the hydroxy-substituted diamines with various commercial tetracarboxylic dianhydrides. The hydroxyl groups on the backbone of the polyimides could provide the organic-inorganic bonding and resulted in homogeneous hybrid solutions by controlling the mole ratio of titanium butoxide/hydroxyl group. AFM, SEM, TEM, and XRD results indicated the formation of well-dispersed nanocrystalline-titania. The flexible hybrid films could be successfully obtained and revealed relatively good surface planarity, thermal dimensional stability, tunable refractive index, and high optical transparency. A three-layer antireflection coating based on the hybrid films was prepared and showed a reflectance of less than 0.5% in the visible range indicated its potential optical applications.

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal Li x CoO 2 (x = 0.33, 0.43, 0.50, 0.53, 0.72, and 0.87) are reported. The room temperature optical absorption spectra for x values in the range of 0.33-0.72 exhibit three bands near 1.60, 3.35, and 5.20 eV. On the basis of firstprinciples calculations, the observed optical excitations were appropriately assigned. The chargetransfer absorption bands shift to higher energies in Li 0.87 CoO 2 because of symmetry-breakinginduced distortions of the hybridized Co-O orbitals with shifted oxygen 2p states. Furthermore, two Raman-active phonon modes, which display E g and A 1g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li 0.50 CoO 2 , large splitting of the E g mode and a discontinuous change in the frequency of the A 1g mode were observed. These results highlight the importance of spin-phonon coupling in Li 0.50 CoO 2 .

In this study, nickel-phosphorus ͑Ni-P͒ deposits were electroplated from the nickel sulfamate bath containing phosphorous acid using a pulse current, with emphasis on the effect of current density, duty cycle, and frequency of the pulse current. Experimental results show that both the deposit phosphorus content and current efficiency were substantially enhanced by employing the pulse current, preferentially at low duty cycles. The underlying difference for the dc and pulse currents on the effect of deposit phosphorus content and current efficiency can be explained by the detailed half-reactions relevant to incorporation of phosphorus into NiP alloys. Less variation in surface proton, Ni 2+ and H 3 PO 3 concentration due to the diffusion recovery during the time off of a pulse current is believed to play an important role in the improvement of the plating process.