Synthesis of poly(vinyl pyrrolidone) nanohydrogels by the template-assisted ionizing radiation

Poly(vinyl pyrrolidone), PVP nanohydrogels were successfully synthesized by a nanotemplate-assisted ionizing radiation method. Steady-state ionizing radiation (γ-ray) was used to generate the gel-forming chemical crosslinking bonds on polymer molecules confined in the highly ordered cylindrical capillary well structure of the nanoporous membrane. The inter-molecular separation of PVP molecules by the capillary well promoted the inter- and intra-molecular carbon-centered PVP radical recombination reaction only within the same capillary pore upon irradiation and produced the nano-sized hydrogels. It was observed that size of the synthesized PVP nanohydrogel particles was determined by the diameter of nanocapillary pore.     

Alkaline solid polymer electrolyte membranes based on structurally modified PVA/PVP with improved alkali stability

Novel alkaline solid polymer electrolyte membranes that can conduct anions (OH⁻) have been prepared from poly(vinyl alcohol)/poly(vinylpyrrolidone) (PVA/PVP) by blending and chemical cross-linking, followed by doping in aqueous KOH solution. The physicochemical properties of these membranes have been studied in detail by FTIR, TG, and SEM analyses. The ionic conductivity was found to be greatly dependent on the concentration of KOH and the interpenetrated PVP in the PVA matrix. A maximum conductivity of up to 0.53 S cm⁻¹ at room temperature was achieved for PVA/PVP in a mass ratio of 1:0.5 after doping in 8 m aqueous KOH solution. The membrane showed perfect alkaline stability without losing its integrity even upon exposure to 10 m KOH solution at up to 120 °C. Scanning electron micrographs revealed a highly ordered microvoid structure uniformly dispersed on the membrane surface with a pore size of ca. 200 nm after heat-curing, which imparted the membrane with good liquid electrolyte (KOH) retention ability. FTIR spectra showed that these high ionic conductivities may be attributed to the presence of excess free KOH in the polymer matrix in addition to KOH bound to the polymer. Almost constant, highly stable, ionic conductivity while maintaining mechanical integrity was retained at room temperature for more than one month.     

Wear resistant UHMWPE with high toughness by high temperature melting and subsequent radiation cross-linking

The goal of this study was to create wear resistant ultra high molecular weight polyethylene (UHMWPE) with improved strength and toughness. It was previously demonstrated that high temperature melting (HTM) of UHMWPE at 280-320 °C improved its toughness without detrimentally affecting its wear resistance. We hypothesized that radiation cross-linking after high temperature melting could further improve the wear resistance of UHMWPE, and the loss in toughness by radiation cross-linking could be compensated by the improved toughness achieved by the high temperature melting prior to irradiation. In this work, we demonstrated that irradiation after HTM generated UHMWPE with improved toughness compared to the irradiated UHMWPEs without HTM, partly due to the low cross-link density of irradiated HTM UHMWPE. At a given cross-link density, irradiated HTM UHMWPEs showed higher wear resistance than irradiated UHMWPE. Therefore, successive HTM and radiation cross-linking strategy is promising to create UHMWPE materials with low wear and improved mechanical properties for total joint implants.     

Synthesis of the combined inter- and intra-crosslinked nanohydrogels by e-beam ionizing radiation

Thermally collapsed poly(vinylpyrrolidone) (PVP) polymer radicals preferably undergo intra-crosslinking reactions producing a nanogel structure rather than a macro-gel that would be caused by inter-crosslinking reactions. In the radiolysis of PVP aqueous solutions, the predominance of the intra-crosslinking reactions was achieved at elevated temperatures up to 77 °C. The dominance of intra-crosslinking under these conditions was confirmed by the molecular weight measurements using asymmetric field flow fractionation (AFFF). A higher yield of C-centered free radicals along the PVP chain at higher electron beam pulse repetition rate (300 Hz) enhances the intra-crosslinking reactions. This represents, mainly, the intra-crosslinking of the radicals of collapsed PVP molecules, along with a minor contribution of inter-crosslinking of the polymer dimers formed by transient occupation by two molecules in the same hydrodynamic volume and the increased diffusivity at the higher temperature.     

Molecular aggregation of poly(γ-phenacyl L-glutamate) in N,N-dimethylformamide

Summary Gel formation and the melting of solutions of poly (-phenacyl L-glutamate) in DMP were investigated up to 40 wt% of polymer concentration. Two thermally reversible transitions were observed around 24°C and 55°C. The transition temperatures were almost independent of the concentration. The transition at 24°C was considered to be due to the collapse of the order in side-chain structures. The transition from gel into Isotropie or cholesteric liquid-crystal phase occured in the 50–60°C range, above which the gel rigidity and an X-ray reflection of about 40 A suggesting a complex phase disappeared.     

Water sorption in cross-linked poly(vinyl alcohol) networks

Poly(vinyl alcohol) (PVA) networks of different cross-linking densities were prepared by reaction with hexamethylene diisocyanate in solution and casting. The dynamic-mechanical properties of PVA films have been investigated in the temperature range of −150 to +150 °C. Two relaxations processes labeled α and β in order of decreasing temperature were observed. The α-relaxation shifts to lower temperature and the average molecular weight between cross-links decreases with increasing cross-linking density. Isothermal sorption from vapor and liquid water allowed determination of the Flory-Huggins interaction parameter between water and the polymer chain segments, which decreased with the water activity in the hydrogel and increased with the cross-linking density as a consequence of the hydrophobic character of the cross-linking agent. The water diffusion coefficients, D, in the networks obtained by means of dynamic sorption experiments increased with increasing water activity. This behavior is interpreted in terms of plasticization of the polymer by water molecules.     

Investigation of nanostructures and properties of sulfonated poly(arylenethioethersulfone) copolymer as proton conducting materials by small angle neutron scattering

Sulfonated poly(arylenethioethersulfone) copolymer (SPTES-50), a promising candidate material for proton exchange membrane fuel cell (PEMFC), exhibited excellent thermal stability, high proton conductivity (135 mS/cm at 85 °C, 85% relative humidity), and electrochemical property. Small angle neutron scattering (SANS) of fully hydrated SPTES-50 membranes revealed the presence of embedded spherical nanodomains containing ionic group and water within the polymer membranes. The polydispersity of the nanoscale structure limited scattering contrast between the polymer backbone and sulfonated groups, and precluded analysis of intermediate and large scattering vectors in terms of the polymer-water interface structure. Inter-cluster correlations associated with the large extent of water absorption in the fully hydrated SPTES-50 membranes were accounted by Percus-Yevick liquid-like ordering of polydispersed hard sphere model with Schulz polydispersity approximation. Approximation of their low q upturn with an exponential decay results in a decay of −3 at 25 °C accounted for inter-cluster correlations which changed to a decay of −1.1 at 55 °C and 77 °C. This indicated a change in morphology upon increase of temperature such as to fractal morphology or an interconnected cylindrical network. The scattering patterns don't exhibit any further changes within examined range of q when the temperature increased from 55 °C to 77 °C. The number density of ionic clusters remained approximately constant (∼1.1818 × 10¹⁷ cm³), which indicated that additional water adsorbed by the polymer at the elevated temperature did not result in substantial coalescence of the clusters. Transmission electron microscopy (TEM) observation of the silver exchanged SPTES-50 membranes exhibited aggregates of Ag⁺ embedded within the dry membranes which can be approximated by isolated spheres.     

New highly proton-conducting membrane poly(vinylpyrrolidone)(PVP) modified poly(vinyl alcohol)/2-acrylamido-2-methyl-1-propanesulfonic acid (PVA-PAMPS) for low temperature direct methanol fuel cells (DMFCs)

A new type of chemically cross-linked polymer blend membranes consisting of poly(vinyl alcohol) (PVA), 2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) and poly(vinylpyrrolidone) (PVP) have been prepared and evaluated as proton conducting polymer electrolytes. The proton conductivity (σ) of the membranes was investigated as a function of cross-linking time, blending composition, water content and ion exchange capacity (IEC). Membranes were also characterized by FT-IR spectroscopy, thermogravimetric analysis (TGA), and the differential scanning calorimetry (DSC). Membrane swelling decreased with cross-linking time, accompanied by an improvement in mechanical properties and a small decrease in proton conductivity due to the reduced water absorption. The membranes attained 0.088 S cm⁻¹ of the proton conductivity and 1.63 mequiv g⁻¹ of IEC at 25±2 °C for a polymer composition PVA-PAMPS-PVP being 1:1:0.5 in mass, and a methanol permeability of 6.1×10⁻⁷ cm² s⁻¹, which showed a comparable proton conductivity to Nafion 117, but only one third of Nafion 117 methanol permeability under the same measuring conditions. The membranes displayed a relatively high oxidative durability without weight loss of the membranes (e.g. 100 h in 3% H₂O₂ solution and 20 h in 10% H₂O₂ solution at 60 °C). PVP, as a modifier, was found to play a crucial role in improving the above membrane performances.     

Thermally reversible cross-links in a healable polymer: Estimating the quantity,rate of formation,and effect on viscosity

The conversion behavior of 2MEP4FS, a polymer with thermally reversible Diels–Alder cross-links, is modeled. A processing method is developed to create small, homogeneous prepolymer samples. The glass transition temperature of the prepolymer is estimated using temperature modulated differential scanning calorimetry and equated with the conversion of the polymer. Comparing the measured energy with the literature and computational estimates, the fully cured polymer appears to have a large portion of its moieties unreacted. An autocatalytic model is considered to approximate the reaction rate of 2MEP4FS as a function of conversion and temperature. Outside of the fitted temperature range, the model underpredicts the reaction rates at room temperature and 100 °C. Manual mixing of the monomers is determined to be inadequate to obtain a maximum level of cross-linking. Viscosity measurements made at room temperature and elevated temperatures are correlated with the conversion of the prepolymer.     

Regime II–III transition in isotactic polybutene-1 tetragonal crystal growth

The lateral growth rate and growth shape morphology of isotactic polybutene-1 tetragonal crystals were investigated for crystallization from the melt at temperatures of 68–101 °C. The growth rate of tetragonal crystals shows supercooling dependence derived from the nucleation theory, and a regime II–III transition is observed at temperatures of 77–82.2 °C. The morphology of single crystals is rounded below temperatures of 77–85 °C, while the growth shape has a faceted morphology at a higher crystallization temperature of 100 °C. The kinetic roughening transition occurs between 77 and 85 °C. The regime II growth mode is observed above 82.2 °C and proceeds by multiple nucleation on the faceted growth front, while the regime III growth mode is observed below 77 °C and proceeds by rough surface growth on the kinetically roughened growth front. The observed regime II–III transition can therefore be explained by the morphology transition of crystal growth shape.