جستجوی مقالات مرتبط با کلیدواژه « poly(vinyl acetate) » در نشریات گروه « فنی و مهندسی »

Abstract One of the important issues concerning the cement mixtures is the durability against several destructive factors such as chloride induced corrosion and sulfate attack, which can in some way result in concrete deterioration. In the current experimental study, the effects of polyethylene vinyl acetate and polyvinyl acetate on the durability of clacium aluminum cement based mixtures have been investigated and evaluated. In this study, the test results including compressive strength, permeable pores, rapid chloride migration coefficient and the compressive strength of sulfuric acid-treated mixtures have been reported. The results suggest that the polymers of ethylene vinyl acetate and polyvinylacetate can reduce the penetration rate of water and chloride ion migration in cement based mixtures and ethylene vinyl acetate outperformed the polyvinyl acetate in enhancing these properties. The polymers reduced compresssive strength noticeably at early ages, however at later ages the differences between the compressive strengths of the polymer modified and plain mixure were reduced, even at high percentages of incorporation (25%).

Hypothesis: Shape memory polymers (SMPs) are intelligent materials that can be set into temporary shape and return to their permanent (original) shape when an external stimulus is applied. Poly(vinyl acetate) (PVAc) is a low cost biopolymer. However, its use in biomedical applications, especially as an SMP, is limited due to its low modulus and strength. On the other hand, poly(lactic acid) (PLA) is a biodegradable polymer with robust structure. Hence, blending of these two polymers can improve mechanical properties as well as shape memory behavior of PVAc.
a series of shape memory materials were prepared through blending of PVAc and PLA through solution mixing method using chloroform as solvent.
Findings: Microstructure of the prepared samples studied by X-ray diffraction spectroscopy (XRD) and atomic force microscopy (AFM), indicated that the components of the blends were favorably compatible. Moreover, dynamic mechanical thermal analysis (DMTA) and tensile test showed that the blending of these compatible biopolymers led to improvement in tensile strength and modulus of PVAc. Finally, the shape memory experiments revealed that the PVAc/PLA blends exhibited improved shape memory behavior as compared with their parent polymers. For instance, by incorporation of 30 wt% PLA into PVAc, the shape recovery increased from 75.4 to 91.5%. The improvement in the shape recovery was assigned to higher stored elastic energy in the blends as compared with that in neat polymers, which provided a larger driving force for corresponding quick and almost complete shape recovery. This procedure may open an avenue to fabricate SMPs through a simple blending method to be applied in different biomedical areas.

Effect of colloidal nanosilica (SiO2) on the mechanical, thermal and rheological properties of poly(vinyl acetate) synthesized by in situ emulsion polymerization method was investigated. For this purpose, a poly(vinyl acetate) latex containing 1.5 wt% colloidal silica nanoparticles was produced and the results were compared with of a blank sample. The effect of nanoparticles on the shear strength of a blank and modified poly(vinyl acetate) was characterized by tensile test. The effect of nanoparticles on glass transition temperature (Tg), thermal stability and char yield of pristine poly(vinyl acetate) and its nanocomposite was evaluated by differential scanning calorimetric (DSC) and thermogravimetric analysis (TGA), respectively. The rheological behavior of the products was studied by rheometric mechanical spectrometry (RMS). Eventually, field emission scanning electron microscopy (FE-SEM) coupled with elemental mapping of X-ray spectroscopy (EDX) was used to study the morphology and elemental analysis of the nanocomposite. The results showed that the shear strength was improved by 11% with increasing 1.5 wt% colloidal silica nanoparticles into poly(vinyl acetate). Besides, with the addition of silica nanoparticles, Tg increased approximately 1°C due to creating more free volume between the polymer chains. The TGA results showed that the nanocomposite char yield increased by 3.8% at 800°C in comparison with the blank polymer char yield, suggesting a thermal stability improvement in the presence of colloidal silica nanoparticles as a result of molecular interactions. The results of RMS revealed the shear thinning behavior of the latexes. The FE-SEM-EDX results showed a uniform dispersion of nanoparticles throughout the poly(vinyl acetate) matrix.

The stabilization and improvement of the soil is a branch of geotechnical engineering by which improve the engineering and geotechnical characteristic (same as strength, ductility and bearing capacity) of the problematic soils. In this study, Micronized Calcium Carbonate and a Polymer named as Poly Vinyl Acetate as the two types of additives were used to stabilize the silty sand soil, and their effects on modulus of elasticity were studied. For this purpose, after performance the index tests, the Micronized Calcium Carbonate and Poly Vinyl Acetate with different percentages of 1, 2, 3 and 4 were added to the soil and with curing times of 1, 7, 14 and 28 days, uniaxial compressive strength were done on stabilized samples. Then elastic modulus of soil in different conditions (different additives percentage and curing time) were determined. Besides, SEM (Scanning Electron Microscopy) imaging were performed on the three soil to investigate the structural characteristic changes of each sample before and after the stabilizing samples. According to the tests results, in general increasing the additives to a certain percentage in a constant curing time, results in the enhancement of the elastic modulus of the soil and by increasing curing time at a constant percentage of additives, elastic modulus will increase significantly. The results show that the effect of Poly Vinyl Acetate on the enhancement of the elastic modulus of silty-sand soil is more than the Micronized powder of Calcium Carbonate

Poly(vinyl acetate) is synthesized via solution polymerization, and then it is converted to poly(vinyl alcohol) by alkaline alcoholysis. The aim of the work study was to investigate statistically the influence of reaction variables in vinyl acetate polymerization, the conversion of this monomer to polymer, degree of branching of acetyl group in poly(vinyl acetate), as well as the molecular weight of poly(vinyl alcohol), using Taguchi experimental design approach. The reaction variables were polymerization time, molar ratio of initiator to monomer, and volume ratio of monomer to solvent. The statistical analysis of variance of the results revealed that all factors have significantly influenced the conversion and degree of branching. Volume ratio of monomer to solvent is the only factor affecting the molecular weight of poly(vinyl alcohol), and has the greatest influence on all responses. By increasing this ratio, the conversion, degree of branching of acetyl group in poly(vinyl acetate), and molecular weight of poly(vinyl alcohol) were increased.

Atom transfer radical polymerization of styrene (St) and methyl methacrylate (MMA) was performed at 90oC in the absence and presence of nanoclay (Cloisite 30B). Trichloromethyl-terminated poly(vinyl acetate) telomer and CuCl/ PMDETA were used as a macroinitiator and catalyst system, respectively. The experimental results showed that the atom transfer radical polymerization of St and MMA in the absence or presence of nanoclay proceeds via a controlled/living mode. It was observed that nanoclay significantly enhances the homopolymerization rate of MMA, which was attributed to the activated conjugated C=C bond of MMA monomer via interaction between the carbonyl group of MMA monomer and the hydroxyl moiety (Al-O-H) of nanoclay as well as the effect of nanoclay on the dynamic equilibrium between the active (macro) radicals and dormant species.Homopolymerization rate of St (a non-coordinative monomer with nanoclay) decreased slightly in the presence of nanoclay. This could be explained by insertion of a portion of macroinitiator into the clay galleries, where no sufficient St monomer exists due to the low compatibility or interaction of St monomer with nanoclay to react with the macroinitiator. The results obtained from XRD, TEM and TGA analyses were fully in agreement with the kinetic data. Structure of the poly(vinyl acetate)-bpolystyrene nanocomposite was found to be a combination of stacking layers and exfoliated structures while poly(vinyl acetate)-b-poly(methyl methacryale) nanocomposite had an exfoliated structure. This difference in the structure of nanocomposites was attributed to the different capability of the monomers (styrene and methyl methacrylate) to react with the hydroxyl moiety (Al-O-H) of nanoclay.