Search results for "polymers"
showing 10 items of 3567 documents
In-situ nano-silica deposition and air-abrasion with Bioglass 45S5 or aluminium oxide: Effects on methacrylate bonding to yttria-tetragonal zirconia …
2015
This study aimed at assessing the bonding performance after water storage (24 h/6-month) of a methacrylate-based cement applied onto Yttria-Tetragonal Zirconia Polycrystal(Y-TZP) pre-treated using an innovative in-situ silica nanoparticle deposition method and/or air-abrasion with Bioglass 45S5 (BAG) or aluminium oxide (AlO). Pre-sintered blocks of Y-TZP were coated with nano-silica using an experimental in-situ deposition method or used as received (control/silica-free). Air-abrasion was performed with AlO or BAG particles both on silica-coated and control Y-TZP specimens. All the specimens were then analysed using SEM– EDX. Further specimens were treated as described above, bonded using a…
Surfactant-Free Emulsion Polymerization of Various Methacrylates: Towards Monodisperse Colloids for Polymer Opals
2004
A systematic investigation of the surfactant-free emulsion polymerization (SFEP) on three different methacrylates (MMA, tert-butyl methacrylate and 2,2,2-trifluoro ethyl methacrylate) is described to obtain monodisperse colloids for the preparation of artificial polymer opals. The experimental results are in agreement with a model, in which seed particles are formed very early during the polymerization process. The particles are formed very early during the polymerization process. The particles grow afterwards into the colloids until all monomer is consumed. This gives rise to a linear relationship between the volume of the colloids and the ratio of monomer to water in the reaction batch. T…
Carbanions on Tap – Living Anionic Polymerization in a Microstructured Reactor
2008
The paper describes the living anionic polymerization of styrenes to homo- and diblock copolymers in continuous flow, using a microstructured mixing set-up ("microreactor"). Reaction times and experimental effort are significantly reduced compared to classical batch methods that often require stringent reaction conditions and strict drying of the apparatus by "break-seal" and "high vacuum" techniques. In continuous flow, residual impurities can be removed by purging the reactor with monomer and initiator solution before polymer samples are collected at the device outlet on a scale of up to 200 g.h -1 . Facile molecular weight adjustment is achieved by variation of the flow rates of initiato…
Macromonomers on the basis of 2-phenyl-2-oxazoline
1988
By initiation of the ring-opening polymerization of 2-phenyl-2-oxazoline (1) with p-iodomethylstyrene a macromonomer with a styrene end-group (2) is obtained. The degree of polymerization was varied between 10 and 16. The radical copolymerization of 2 with styrene is described. 1 forms with methyl triflate a very stable oxazolinium salt which is a good initiator for the bulk polymerization of 1 at temperatures of 50–70°C. It was conclusively proved that under these conditions the polymerization proceeds by a living mechanism. By termination with N, N-dimethylaminopropylmeth-acrylamide a macromonomer (5) was obtained. 5 can be radically homopolymerized. This comb-like polymer 6 has the typic…
Synthesis of block copolymers with poly(methyl methacrylate): P(B-b-MMA), P(EB-b-MMA), P(S-b-B-b-MMA) and P(S-b-EB-b-MMA)
1993
Well-defined diblock copolymers poly(butadiene-b-methyl methacrylate) (=P(B-b-MMA)) and triblock copolymers poly(styrene-b-butadiene-b-methyl methacrylate) (=P(S-b-B-b-MMA)) have been prepared by sequential anionic polymerization in THF. The synthesis of P(B-b-MMA) and P(S-b-B-b-MMA) was most efficient in the presence of lithium alkoxides. By this method side reactions are suppressed and the polymerization can be performed at higher temperatures. The resulting triblock copolymers have narrow molecular weight distribution. The 1,2-PB midblock was quantitatively hydrogenated with tosylhydrazide to enhance thermal stability. Alternatively the hydrogenation can be performed at elevated pressure…
Acrylic thermoplastic elastomers and comb-shaped poly(methyl methacrylate) via the macromonomer technique
1996
Comb-shaped poly(methyl methacrylate) (PMMA) and poly(butyl acrylate) (PnBuA) grafted with PMMA were prepared by radical copolymerization of ω-methacryloyl-PMMA with MMA and nBuA, respectively. The comb-shaped PMMA is characterized with respect to radius of gyration by using gel permeation chromatography equipped with a multi-angle laser light scattering detector. The radical copolymerization of the macromonomer with nBuA in toluene follows complex kinetics. The dependence of the relative reactivity of the macromonomer on absolute concentration and on the ratio of comonomers may be explained by preferential solvation of comonomers by segments of their own kind (“bootstrap effect”) or even m…
Capillary Nematization of Semiflexible Polymers
2016
emiflexible polymers under good solvent conditions confined by two planar parallel repulsive walls are investigated for a wide range of monomer concentrations and distances between the walls, for a case where persistence length and contour length of the macromolecules are almost equal. Chain conformations and local nematic ordering near the walls are studied by both molecular dynamics methods and density functional theory, putting it in perspective with the recent work where the isotropic phase of semiflexible polymer solutions in the vicinity of a single repulsive wall in semi-infinite geometry is considered. Profiles of the total density of monomers as well as densities of end- and middle…
Flow properties of low density/linear low density polyethylenes
1986
Rheological data have been collected both in shear and non-isothermal elongational flow on three different types of blends, made from one low density polyethylene sample and three linear low density polyethylene samples. In addition to the flow curves, data are presented on the extrudate-swell phenomenon, on the instability arising in capillary flow and on the tensile behavior in the molten state.
The rheological behavior of HDPE/LDPE blends 4. Instability phenomena in capillary flow
1984
Experimental data, relative to instability phenomena in capillary flow, have been collected for HDPE/LDPE blends already characterized from a rheological viewpoint. The data comprise morphological observations as well as the determination of critical parameters as a function of the blend composition and of the processing conditions.
Rheology-morphology relationships in nylon 6/liquid-crystalline polymer blends
1990
Extrusion measurements have been carried out on blends of nylon 6 and a liquid-crystalline copolyesteramide (LCP). The flow curves at low temperature show a behavior similar to that of pure LCP with a rapid rise of the viscosity at low shear rates. At high shear rates the viscosity is lower than that for each of the two components. This minimum has been attributed to the lack of interactions between the two phases and to the formation of fibrils of the LCP phase. The SEM analysis shows, indeed, that fibrils of the LCP phase are produced in the convergent flow at the inlet of the capillary at high shear rates. These fibrils are lost during the flow in the long capillary.