Showing papers on "Vinyl acetate published in 2010"

Role of polymer chemistry in influencing crystal growth rates from amorphous felodipine

Abstract: The inhibition of crystallization from organic amorphous solids is currently of great interest in the pharmaceutical field, since the amorphous form of the drug can enhance drug delivery. Polymers have been found to be effective crystallization inhibitors for many organic glasses and supercooled liquids. The objective of this study was to investigate potential correlations between drug–polymer hydrogen bonding and crystal growth inhibition. Quench cooled samples of a model hydrophobic drug, felodipine, were prepared with various polymers: poly(vinylpyrrolidone) (PVP), hydroxypropyl methylcellulose acetate succinate (HPMCAS), poly(vinylpyrrolidone)/vinyl acetate (PVP/VA) and poly(vinyl acetate) (PVAc). Crystal growth rates as a function of temperature (70–110 °C) were measured using optical microscopy, in the presence and absence of 3% w/w polymer. Differential scanning calorimetry (DSC) was used to evaluate glass transition temperatures (Tg) and melting points. Infrared (IR) spectroscopy was used to probe drug–polymer hydrogen bonding interactions. The various polymers were found to inhibit the crystal growth to different degrees. The order of inhibition effectiveness was PVP > PVP/VA > HPMCAS > PVAc with PVP being the best inhibitor among the polymers used. The growth rates in the presence of the polymers were similar to those of the drug alone at high temperatures but showed a significant reduction as the temperature was reduced. The Tg's of the drug–polymer dispersions were not significantly different from that of the pure drug. The order of the strength/extent of drug–polymer hydrogen bonding interactions was PVP > PVP/VA > HPMCAS ≥ PVAc. Hence polymers which can form stronger/more extensive hydrogen bonds with the drug appear to be better crystallization inhibitors.

An investigation of the potential of ethylene vinyl acetate/polyethylene blends for use in recyclable high voltage cable insulation systems

Abstract: Ethylene vinyl acetate (EVA) co-polymers can potentially provide novel materials for inclusion into extruded high voltage cable systems, providing a degree of electrical conductivity whilst avoiding the dispersion problems associated with conventional particulate fillers or conducting polymers. Although a degree of conductivity can decrease the electrical breakdown performance, it can help to suppress the development of space charge and increase the tree initiation voltage leading to enhanced dielectric properties. In addition, novel two phase morphologies can be formulated leading to the ability to control key thermal and mechanical properties and the ability to tailor these to suit the application. In addition, one of the problems with conventional cross-linked polyethylene (XLPE) is that it cannot easily be recycled; therefore, in this time of increasing environmental awareness, it is prudent to begin investigations into alternative recyclable materials to replace XLPE in extruded cables for the medium to long term. The current article focuses on the crystallisation behaviour, morphology, mechanical and dielectric properties of a range of polymeric insulation systems based on an EVA co-polymer together with a high density polyethylene (HDPE) component. The morphology was controlled by choosing co-polymers containing different vinyl acetate contents together with appropriate crystallisation routes. The relationships between the morphology and the mechanical and dielectric properties were explored. Blends containing a low vinyl acetate content co-polymer combined with HDPE have significant potential to replace XLPE in cable systems and have the advantage of being easily recycled at the end of their service life.

Synthesis and Phase Behavior of CO2-Soluble Hydrocarbon Copolymer: Poly(vinyl acetate-alt-dibutyl maleate)

Abstract: We report the synthesis of a new hydrocarbon copolymer which is soluble in supercritical carbon dioxide. Poly(vinyl acetate-alt-dibutyl maleate) (PVAc-alt-PDBM) copolymer was synthesized by free-radical polymerization mediated by the RAFT technique. Liquid CO2 extraction was performed to remove the residual monomer and solvent from the final product. The solubility of the copolymers was measured in a variable volume view cell at temperatures between 25 and 75 °C. The phase behavior of the copolymer in CO2 was studied in terms of its molecular weight and concentration in the solvent. It was found that the copolymer shows good solubility in CO2 approaching that of perfluoropolyether (PFPE) and poly(dimethylsiloxane) monomethacrylate (PDMS-mMA).

Preparation and characterizations of PVAc/P(VdF-HFP)-based polymer blend electrolytes

Abstract: A novel group of polymer blend electrolytes based on the mixture of poly(vinyl acetate) (PVAc), poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), and the lithium salt (LiClO4) are prepared by solvent casting technique. Ionic conductivity of the polymer blend electrolytes has been investigated by varying the PVAc and PVdF-HFP content in the polymer matrix. The maximum ionic conductivity has been obtained as 0.527 × 10−4 Scm−1 at 303 K for PVAc/PVdF-HFP ((25/75) wt.%)/LiClO4 (8 wt.%). The complex formations ascertained from XRD and FTIR spectroscopic techniques and the thermal behavior of the prepared samples has been performed by DSC analysis. The surface morphology and the surface roughness are studied using SEM and AFM scanning techniques respectively.

Heterocomplementary H-Bonding RAFT Agents as Tools for the Preparation of Supramolecular Miktoarm Star Copolymers

Abstract: Supramolecular miktoarm stars (AB2 type) composed of poly (methyl methacrylate)-polystyrene2 (PMMA-PS2), poly(isoprene)-polystyrene2 (PI-PS2), and poly(vinyl acetate)-polystyrene2 (PVAc-PS2) were successfully synthesized by assembling reversible addition−fragmentation chain transfer (RAFT)-polymerized chains bearing hydrogen-bonding heterocomplementary associating units. To this end, thymine and diaminopyridine-functionalized chain transfer agents were designed to efficiently mediate the polymerization of vinyl acetate, methyl methacrylate, isoprene, and styrene. The selective associations of the resulting hydrogen-bonding macromolecular building blocks PVAc/PS, PI/PS, and PMMA/PS were demonstrated by 1H NMR in CDCl3 solutions. Miktoarm stars formation in the bulk was also confirmed by transmission electronic microscopy.

Coverage Effects on the Palladium-Catalyzed Synthesis of Vinyl Acetate: Comparison between Theory and Experiment

Abstract: The high adsorbate coverages that form on the surfaces of many heterogeneous catalysts under steady-state conditions can significantly lower the activation energies for reactions that involve the coupling of two adsorbed intermediates while increasing those which result in adsorbate bond-breaking reactions. The influence of the surface coverage on the kinetics of metal-catalyzed reactions is often ignored in theoretical and even in some ultrahigh vacuum experimental studies. Herein, first principle density functional theoretical calculations are combined with experimental surface titration studies carried out over well-defined Pd(111) surfaces to explicitly examine the influence of coverage on the acetoxylation of ethylene to form vinyl acetate over Pd. The activation energies calculated for elementary steps in the Samanos and Moiseev pathways for vinyl acetate synthesis carried out on acetate-saturated palladium surfaces reveal that the reaction proceeds via the Samanos mechanism which is consistent with experimental results carried out on acetate-saturated Pd(111) surfaces. The rate-limiting step involves a β-hydride elimination from the adsorbed acetoxyethyl intermediate, which proceeds with an apparent calculated activation barrier of 53 kJ/mol which is in very good agreement with the experimental barrier of 55 ± 4 kJ/mol determined from kinetic measurements.

Effective Cobalt-Mediated Radical Coupling (CMRC) of Poly(vinyl acetate) and Poly(N-vinylpyrrolidone) (Co)polymer Precursors

Abstract: Cobalt-mediated radical coupling (CMRC) is successfully applied to poly(vinyl acetate) (PVAc) and poly(N-vinylpyrrolidone) (PNVP) precursors for the first time. The coupling process is based on addition of isoprene onto polymer chains preformed by controlled radical polymerization with cobalt complexes (CMRP). The extents of coupling were high (>90%) to moderate (75−80%) for PVAc and PNVP precursors, respectively. Effects of the length of the polymer precursors and conditions used in the polymerization step on the coupling efficiency are discussed. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) analyses conducted on the coupling products demonstrate the preferential insertion of two isoprene units in the final polymers. The CMRC mechanistic proposal, supported by DFT calculations, is based on this microstructure feature. Finally, illustration of the macromolecular engineering potential of this technique is given by the preparation of symmetrical PVAc-b-PNVP-b-PVAc triblock copolymers starting...

Unraveling Mechanistic Events in Solids-Stabilized Emulsion Polymerization by Monitoring the Concentration of Nanoparticles in the Water Phase

Abstract: The fate of nanoparticles used as stabilizers in solids-stabilized, or Pickering, emulsion polymerization or the formation of armored hybrid polymer latexes was studied. We showed that disk centrifugation can be used as a powerful quantitative tool to analyze and determine the concentration of nanoparticles M the water phase throughout solids-stabilized emulsion polymerizations. We performed a series of emulsion polymerizations using vinyl acetate, vinyl pivalate, methyl methacrylate, or butyl acrylate in presence of silica nanoparticles (Ludox TM-40, ca. 25 nm in diameter). The developed method to quantity the number of silica nanoparticles in the water phase proved to be an invaluable tool for determining key features of the polymerization process. The obtained concentration profiles versus monomer conversion explained the existence of limited coalescence of armored particles in the later stages of the solids-stabilized emulsion polymerization process of vinyl acetate, leading to nonspherical structures. Moreover, we demonstrated that the correlation of the measured number of silica nanoparticles present in the water phase with the average particle sizes of the latex particles provided excellent predictions for the coverage of the armored layer of nanoparticles on the surfaces of the polymer particles, corresponding to observed packing patterns.

Nitroxide-mediated photo-living radical polymerization of vinyl acetate

Abstract: The photoradical polymerization of vinyl acetate was performed using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate (BAI). The MTEMPO/BAI system using 2,2’-azobis(isobutyronitrile) or 2,2’-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator did not succeed in controlling the molecular weight and produced polymers that showed a bimodal gel permeation chromatography with the broad molecular weight distribution. On the other hand, the polymerization using 1-(cyano-1-methylethoxy)-4-methoxy-2,2,6,6-tetramethylpiperidine and BAI proceeded by the living mechanism based on linear increases in the first order time–conversion and conversion–molecular weight plots. The molecular weight distribution also increased with the increasing conversion due to cloudiness of the solution as the polymerization proceeded. It was found that the polymerization had a photolatency because the propagation stopped by interruption of the irradiation and was restarted by further irradiation.

Microstructure and Dynamics of Semicrystalline Poly(ethylene oxide)−Poly(vinyl acetate) Blends

Abstract: The microstructure and dynamics of semicrystalline, melt-miscible poly(ethylene oxide)/poly(vinyl acetate) (PEO/PVAc) blends were investigated using small-angle X-ray scattering (SAXS) and broadband dielectric relaxation spectroscopy, respectively. PEO/PVAc blends with selected compositions were crystallized, and SAXS was used to determine the location of the noncrystallizable PVAc in the structure. Values of the microstructural parameters indicate that little, if any, PVAc is incorporated into interlamellar regions under these crystallization conditions, but PVAc diffuses to interfibrillar regions during the crystallization process. For crystalline blends, a dielectric relaxation appears in the same location as the neat PEO α-process, indicating the presence of relatively mobile amorphous segments consisting almost entirely of PEO, in blends with compositions having as much as 50% PVAc. Considering the findings from the SAXS experiments, we attribute αPEO in the blends to the segmental process of the mob...

Kinetic resolution of a drug precursor by Burkholderia cepacia lipase immobilized by different methodologies on superparamagnetic nanoparticles

Abstract: Burkholderia cepacia lipase was immobilized on superparamagnetic nanoparticles using three different methodologies (adsorption, chemisorption with carboxibenzaldehyde and chemisorption with glutaraldehyde) and employed in the kinetic resolution of a chiral drug precursor, (RS)-2-bromo-1-(phenyl)ethanol, via enantioselective acetylation reaction. An excellent improvement of lipase catalytical performance was observed. Free B. cepacia lipase gave the ester (S)-2 with poor E-value 200. The effect of several reaction parameters in the kinetic resolution was studied. The best results for kinetic resolution were obtained using vinyl acetate as acetyl donor and toluene as solvent, typically yielding the ester in high enantiomeric excess (>99%) and E-value (E > 200). Of the three tested immobilization methods, chemisorption with glutaraldehyde was the best one in terms of temperature stability and yield product.

The thermal decarboxylation of alkali trichloroacetates in aprotic solvents. II†. Decarboxylation in the presence of olefins

Abstract: The thermal decarboxylation of alkali trichloroacetates in aprotic solvents in the presence of olefins has been investigated. The isolation of dichlorocyclopropanes in good yields demonstrates the formation of CCl2 as an intermediate. By this method, the reaction of base-sensitive acceptors such as allylchloride with CCl2 may be successfully accomplished, since it proceeds under essentially neutral conditions. Vinyl acetate gave, in addition to the cyclopropane derivative, 1-trichloromethylethyl acetate.

Effect of different salts on PVAc/PVdF‐co‐HFP based polymer blend electrolytes

Abstract: Solid polymer electrolytes comprising poly(vinyl acetate) (PVAc), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP), low molecular weight plasticizer [Ethylene Carbonate (EC)] and different lithium salts (LiBF4, LiClO4, and LiCF3SO3) are prepared by solution casting technique. The electrolyte films are subjected to various characterization techniques such as XRD, FTIR, DSC, SEM and ac impedance analysis. Ionic conductivity is obtained as a function of frequency at various temperatures ranging from 302 to 363 K. The maximum room temperature ionic conductivity is found to be 1.18 × 10−3 S cm−1 for the film containing LiBF4 and the temperature dependent ionic conductivity values seem to obey VTF relation. Microstructure of the samples has been depicted by means of scanning electron microscope. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Effect of lithium salt concentration in PVAc/PMMA-based gel polymer electrolytes

Abstract: Hybrid solid polymer electrolyte films comprising of poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), LiClO4, and propylene carbonate are prepared by solution casting technique by varying the salt concentration. In this study, PVAc/PMMA polymer blend ratio is fixed as 25:75 on the basis of conductivity and mechanical stability of the film. X-ray diffraction, Fourier transform infrared impedance, thermogravimetry/differential thermal analysis and scanning electron microscopy studies are carried out for the polymer electrolytes. The maximum ionic conductivity is found to be 4.511 × 10−4 S cm−1 at 303 K for the plasticized polymer electrolyte with 8 wt.% of LiClO4. The ionic conductivity is found to decrease with an increase of LiClO4 concentration.

Cross-linked poly(ethylene vinyl acetate) (EVA)/low density polyethylene (LDPE)/metal hydroxides composites for wire and cable applications

Abstract: Formulations of chemically cross-linked poly(ethylene vinyl acetate) (EVA) and low density polyethylene (LDPE) blends containing metal hydroxides flame retardants such as aluminum hydroxide (ATH) and magnesium hydroxide (MH) were prepared. Comparison of both type of metal hydroxides in respect of their influence on flammability as well as mechanical, thermal, and electrical properties of EVA/LDPE composites is presented. Most of the investigated properties are better for composites containing MH in comparison with composites containing ATH. Influence of various EVA/LDPE ratios on investigated properties is presented as well. Importance of improving compatibility using compatibilizers to improve some of the investigated properties is described. Polyethylene grafted with maleic anhydride (PEgMA) was found to be better compatibilizer for ATH than vinyl silanes.

Polymer glass transitions switch electron transfer in individual molecules

Abstract: Essentially complete photoinduced electron transfer quenching of the fluorescence of a perylene−calixarene compound occurs in poly(methyl acrylate) and poly(vinyl acetate) above their glass transition temperatures (T-g), but the fluorescence is completely recovered upon cooling the polymer matrix to a few degrees below the T-g. The switching can be observed in an on/off fashion at the level of individual molecules.

Investigation of UV aging influences on the crystallization of ethylene-vinyl acetate copolymer via successive self-nucleation and annealing treatment

Abstract: Influences of UV aging on the crystallization of ethylene-vinyl acetate copolymer (EVA) were researched via successive self-nucleation and annealing (SSA) treatment. During the aging process, the polar vinyl acetate (VAc) units in the amorphous region were the most vulnerable structure. FTIR results demonstrated that, VAc units were initially attacked by the UV radiation, which further resulted in chain scission of molecules. Degradation expanded from the amorphous region to the crystal region gradually. Chain scission reaction freed crystallizable ethylene sequences from inter-/intra-molecular tanglement and confinement of neighboring VAc units. Although the crystallinity decreased after aging, newly freed crystallizable sequences preferentially arranged into more densely packed lamellae, leaving less residual fraction to arrange into loosely packed crystal region. WAXD patterns showed that the predominant orthorhombic crystal phase did not vary during aging. Simultaneously, re-arrangement in crystallization also resulted in the growth in lateral crystal size of EVA.

Investigation of free-radical copolymerization propagation kinetics of vinyl acetate and methyl methacrylate.

Abstract: The free-radical copolymerization propagation kinetics of vinyl acetate (VAc) and methyl methacrylate (MMA) at 50 degrees C were investigated through an experimental study combined with a computational analysis based on quantum chemistry. Copolymer composition data, obtained using pulsed laser polymerization followed by size exclusion chromatography (PLP-SEC) and proton nuclear magnetic resonance (NMR), were well represented by the terminal model using monomer reactivity ratios obtained with the computational approach (r(VAc) = 0.001 and r(MMA) = 27.9). Concerning the composition-averaged copolymerization propagation rate coefficient k(p,cop), the differences between the terminal model and the implicit penultimate unit effect (IPUE) model (s(MMA) = 0.544 and s(VAc) = 0.173) are small for VAc/MMA, with the terminal model sufficient to describe the experimental k(p,cop) data measured by PLP-SEC. Monomer and radical charge distributions determined computationally are used to explain the reactivity exhibited by the VAc/MMA system.

Highly Efficient Rhodium Catalysts for the Asymmetric Hydroformylation of Vinyl and Allyl Ethers using C1-Symmetrical Diphosphite Ligands

Abstract: Here, we describe the successful application of novel glucofuranose-derived 1,3-diphosphites in the rhodium-catalysed asymmetric hydroformylation of vinyl acetate, 2,5-dihydrofuran and 2,3-dihydrofuran. In the hydroformylation of vinyl acetate, total regioselectivity and high ee (up to 73%) were obtained. When 2,3- and 2,5-dihydrofuran were the substrates, total chemo- and regioselectivities were achieved together with ees up to 88%. These results correspond to the highest ee values reported to date in the asymmetric hydroformylation of these substrates. The HP-NMR studies of the [RhH(CO) 2 (L)] species (L=15 and 17) demonstrated that both ligands coordinate to the Rh centre in an eq-eq fashion. The complex [RhH(CO) 2 (15)] was detected as a single isomer with characteristic features of eq-eq coordination. However, the broadening of the corresponding signals indicated that this species is rapidly interchanging in solution. In contrast, complex [RhH(CO) 2 (17)] was detected as a mixture of two conformational isomers at low temperature due to the greater flexibility of the monocyclic backbone of this ligand.

Fire-retardant and fire-barrier poly(vinyl acetate) composites for sealant application

Abstract: Fire-retardant ceramifying poly(vinyl acetate) (PVAc) sealants have been prepared. The degradation of PVA was integrated with the action of the fire retardants to reduce flammable gases, produce carbonaceous char and convert the fillers into a self-supporting ceramic barrier. PVA is readily degraded by elimination of acetic acid, yielding a char that provides a transitory phase as the filler particles fuse into a ceramic mass. Acetic acid is eliminated at similar temperature to the release of water from magnesium hydroxide fire-retardant, thereby diluting flammable acetic acid. The residual oxide from the fire-retardant filler and structural filler are fused by a flux, zinc borate. The degradative and ceramifying processes were characterised using thermogravimetry, infrared spectroscopy, scanning electron microscopy and ceramic strength. Thermogravimetry of the composites was compared with additive mass loss curves calculated from the components. Deviations between the experimental and additive curves revealed interactions between the components in the composites. The modulus of the PVAc composites and the strength of their ceramic residues after combustion were determined.

Preparation and properties of xerogels obtained by ionic liquid incorporation during the immobilization of lipase by the sol-gel method

Abstract: Lipase from Pseudomonas fluorescens (Amano AK) has been immobilized by the sol–gel method using tetramethoxysilane and trimethoxysilanes with alkyl or aryl groups as precursors and ionic liquids as immobilization additives. Room temperature ionic liquids (ILs) based on imidazolium cations with different hydrophobicities and inorganic or organic anions were investigated. The biocatalytic activity in the acylation reaction of secondary aliphatic alcohols by vinyl acetate increased at lower polarity of the IL, being 58% higher in case of octyl than for ethyl substituent of the alkylimidazolium cation. The optimum IL/silane molar ratio in the immobilization mixture was around 0.2. Hydrophobic groups from both the silane precursor and IL converge to the optimal microenvironment for the enzyme action. ILs as reaction media led to higher activities as common organic solvents in the kinetic resolution of secondary alcohols, but the highest enantioselectivities (enantiomeric ratio E values > 50) have been observed in acetone and tetrahydrofuran. Structural investigation of the preparates by SEM–EDX and FT-IR has confirmed the partial confinement of the IL in the xerogel structure.