Thickness dependence of free‐standing thin films

Below a critical thickness, of about 60 nm, the glass transition temperature of polystyrene (PS) films decreases with film thickness, as demonstrated using free‐standing films. A geometrical model is developed here describing this phenomenon in the case of ideal (Gaussian) chains. This model, which can be considered as an application of the free volume model, assumes that the decrease of the glass transition temperature from thick to ultrathin films is due to the modification of the interpenetration between neighboring chains. The theoretical curve deduced from the model is in excellent agreement with the PS experimental results, without using any adjustable parameters. From these results, it can be concluded that new chain motions, usually buried in bulk samples, are expressed by the presence of the surface. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 10–17, 2007     

Simulation of single‐ and two‐phase flows on sliding unstructured meshes using finite volume method

The paper presents an efficient finite volume method for unstructured grids with rotating sliding parts composed of arbitrary polyhedral elements for both single‐ and two‐phase flows. Mathematical model used in computations is based on the ensemble averaged conservation equations. These equations are solved for each phase and in case of single‐phase flow reduce to the transient Reynolds‐averaged Navier–Stokes (TRANS) equations. Transient flow induced by rotating impellers is thus resolved in time. The use of unstructured grids allows an easy and flexible meshing for the entire flow domain. Polyhedral cell volumes are created on the arbitrary mesh interface placed between rotating and static parts. Cells within the rotating parts move each time step and the new faces are created on the arbitrary interfaces only, while the rest of the domain remain ‘topologically’ unchanged. Implicit discretization scheme allows a wide range of time‐step sizes, which further reduce the computational effort. Special attention is given to the interpolation practices used for the reconstruction of the face quantities. Mass fluxes are recalculated at the beginning of each time step by using an interpolation scheme, which enhances the coupling between the pressure and velocity fields. The model has been implemented into the commercially available CFD code AVL SWIFT (AVL AST, SWIFT Manual 3.1, AVL List GmbH, Graz, Austria, 2002). Single‐phase flow in a mixing vessel stirred by a six‐bladed Rushton‐type turbine and two‐phase flow in aerated stirred vessel with the four‐blade Rushton impeller are simulated. The results are compared with the available experimental data, and good agreement is observed. The proposed algorithm is proved to be both stable and accurate for single‐phase as well as for the two‐phase flows calculations. Copyright 2004 John Wiley & Sons, Ltd.     

Synthesis and chiroptical properties of optically active poly(ethynylcarbazole) derivatives: Substituent effect on the helix formation

Novel chiral acetylene monomers containing carbazole, 2‐ethynyl‐9‐[(S)‐2‐methylbutoxycarbonyl]carbazole ( 1 ), 3‐ethynyl‐9‐[(S)‐2‐methylbutoxycarbonyl]carbazole ( 2 ), 2‐ethynyl‐9‐[(S)‐2‐methylbutyl]carbazole ( 3 ), and 2‐ethynyl‐9‐[(S)‐4‐methylhexyl]carbazole ( 4 ) were synthesized and polymerized with [(nbd)RhCl]₂? Et₃N. The corresponding polyacetylenes with number‐average molecular weights ranging from 68,700 to 310,000 were obtained in good yields. Poly( 1 ) exhibited a large specific rotation and an intense Cotton effect in toluene, indicating that it formed a helix with predominantly one‐handed screw sense, while the other three polymers showed no evidence for taking a helical structure. Poly( 1 ) largely decreased the CD intensity upon heating from ?10 to 60 °C. Poly( 1 ) showed a Cotton effect in film state in a manner similar to solution state. No chiral amplification was observed in the copolymerization of 1 with achiral 2‐ethynyl‐9‐tert‐butoxycarbonylcarbazole ( 5 ). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4450–4458, 2007     

Sterically stabilized emulsion polymerization of styrene: Pseudo‐semicontinuous approach

The sterically stabilized emulsion polymerization of styrene initiated by a water‐soluble initiator at different temperatures has been investigated. The rate of polymerization (R_p) versus conversion curve shows the two non‐stationary‐rate intervals typical for the polymerization proceeding under non‐stationary‐state conditions. The shape of the R_p versus conversion curve results from two opposite effects—the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two‐phase or three‐phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer‐swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the “highly monomer” swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40–50% conversion. The monomer droplets and/or large highly monomer‐swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804–820, 2003     

Determination of the rheological behavior of epoxy–amine thermosets by dynamic mechanical analysis: Isothermal methods versus nonisothermal methods

The reticulation process of an epoxy resin using an amine as a cure agent was studied at different temperatures and concentrations of the cure agent with dynamic mechanical thermal analysis. The study was performed under both isothermal and nonisothermal conditions, and a temperature–time–transformation diagram was obtained. The measurements from the two modes gave similar results, although the nonisothermal mode required fewer experiments. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1965–1977, 2003     

Reversible 5′-end biotinylation and affinity purification of synthetic RNA

A reversible biotinylation phosphoramidite was synthesized and incorporated onto the 5′-end of an oligoribonucleotide on a solid phase synthesizer. After cleavage and deprotection, the crude synthetic oligomer mixture was incubated with NeutrAvidin^® coated microspheres, and the failure sequences removed by washing with a buffer followed by treating the microspheres with tetrabutylammonium fluoride to give a high quality unmodified full-length oligoribonucleotide.     

Thiol–norbornene materials: Approaches to develop high Tg thiol–ene polymers

The ability to prepare high T_g low shrinkage thiol–ene materials is attractive for applications such as coatings and dental restoratives. However, thiol and nonacrylated vinyl materials typically consist of a flexible backbone, limiting the utility of these polymers. Hence, it is of importance to synthesize and investigate thiol and vinyl materials of varying backbone chemistry and stiffness. Here, we investigate the effect of backbone chemistry and functionality of norbornene resins on polymerization kinetics and glass transition temperature (T_g) for several thiol–norbornene materials. Results indicate that T_gs as high as 94 °C are achievable in thiol–norbornene resins of appropriately controlled chemistry. Furthermore, both the backbone chemistry and the norbornene moiety are important factors in the development of high T_g materials. In particular, as much as a 70 °C increase in T_g was observed in a norbornene–thiol specimen when compared with a sample prepared using allyl ether monomer of analogous backbone chemistry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5686–5696, 2007     

Microscopic Theory of Blue Phases Ⅰ and Ⅱ of Liquid Crystal

The microscopic theory of the blue phases of chiral liquid crystal is proposed. Beginning with the potential between two molecules, by using the cell model of liquid, applying statistical physical method, the distribution function and the free energy of the system are obtained. By using variational approach and zero-order approximation, the differential equation that the order parameter tensor of the blue phase can satisfy is obtained. Then we change the differential equation to the eigenequation problem in quantum mechanics. Considering the symmetry of the blue phases,the order parameter tensors of blue phases Ⅰ and Ⅱ can be made up of the eigenvectors. Our results are the same as the results of Ginzberg-Landau‘s phenomenological theory. The parameters in the order parameter tensors that we calculate in the located system are close to the predecessors‘ results.     

Immiscibility,upper critical solution temperature,and miscibility in blends of poly(vinyl ether)s with polyacrylics: Effects of pendant groups

Various phase behavior of blends of poly(vinyl ether)s with homologous acrylic polymers (polymethacrylates or polyacrylates) were examined using differential scanning calorimetry, optical microscopy (OM), and Fourier‐transformed infrared spectroscopy. Effects of varying the pendant groups of either of constituent polymers on the phase behavior of the blends were analyzed. A series of interestingly different phase behavior in the blends has been revealed in that as the pendant group in the acrylic polymer series gets longer, polymethacrylate/poly(vinyl methyl ether) (PVME) blends exhibit immiscibility, upper critical solution temperature (UCST), and miscibility, respectively. This study found that the true phase behavior of poly(propyl methacrylate)/PVME [and poly(isopropyl methacrylate)/PVME)] blend systems, though immiscible at ambient, actually displayed a rare UCST upon heating to higher temperatures. Similarly, as the methyl pendant group in PVE is lengthened to ethyl (i.e., PVME replaced by PVEE), phase behavior of its blends with series of polymethacrylates or polyacrylates changes correspondingly. Analyses and quantitative comparisons on four series of blends of PVE/acrylic polymer were performed to thoroughly understand the effects of pendant groups in either polyethers (PVE's) or acrylic polymers on the phase behavior of the blends of these two constituents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1521–1534, 2007     

Side‐chain crystallization behavior of graft copolymers consisting of an amorphous main chain and crystalline side chains. II. Poly(n‐hexyl methacrylate)‐graft‐poly(ethylene glycol)

The phase behavior and crystallization of graft copolymers consisting of poly(n‐hexyl methacrylate) (PHMA) as an amorphous main chain and poly(ethylene glycol) (PEG) as crystallizable side chains (HMAx with 15 ≤ x ≤ 73, where x represents the weight percentage of PEG) were investigated. Small‐angle X‐ray scattering profiles measured above the melting temperature of PEG suggested that a microdomain structure with segregated PHMA and PEG domains was formed in HMA40 and HMA46. This phase behavior was qualitatively described by a calculated phase diagram based on the mean‐field theory. Because of the segregation of PEG into microdomains, the crystallization temperature of the PEG side chains in HMAx was higher than that in poly(methyl acrylate)‐graft‐poly(ethylene glycol) having a similar value of x, which was considered to be in a disordered state above the melting temperature. In HMAx with x ≤ 40, PEG crystallization was strongly restricted, probably because the PEG microdomains were isolated in the PHMA matrix. As a result, the growth of PEG spherulite was not observed because the PEG crystallization occurred after vitrification of the PHMA segregated domains. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 129–137, 2007