Characterization of hydrogel blends of poly(vinyl pyrrolidone) and poly(vinyl alcohol-vinyl acetate)

Hydrogel blends were prepared from water-soluble polymers of poly(vinyl alcohol-vinyl acetate) and poly(vinyl pyrrolidone). The method of preparation was optimized and different compositions of blends were characterized. The effect of thermal treatment and the introduction of an aldehydic crosslinking agent in the blend was also studied. The swelling characteristics of the various compositions, their thermal behaviour and the state of water was examined. Mechanical properties of the hydrogels were determined and it was observed that blends containing glutaraldehyde produced materials with good mechanical integrity and high water contents.     

Release of dexamethasone from PLGA nanoparticles entrapped into dextran/poly(vinyl alcohol) hydrogels

Hydrogels based on blends of poly(vinyl alcohol) (PVA) with dextran were prepared by a physical cross-linking procedure and used as matrices for the entrapment of biodegradable nanoparticles loaded with dexamethasone. The nanoparticles were prepared, by a solvent evaporation technique, using biodegradable copolymers of poly(lactic acid)–poly(glycolic acid) (PLGA). Size, morphology and surface characteristics of the nanoparticles were evaluated by scanning electron microscopy. The mechanism of drug release from the nanoparticles entrapped into the PVA-based matrices was studied and compared to that from free nanoparticles. The effect of dextran on the in vitro release profile of dexamethasone from the hydrogels was investigated. The obtained results indicate that PLGA nanoparticles are able to release dexamethasone following a diffusion-controlled mechanism. The entrapment of the nanoparticles into the hydrogels affects only slightly this mechanism of drug release. In addition, dextran/PVA hydrogels release a higher amount of drug with respect to pure PVA hydrogels and by increasing dextran content in the hydrogels, the amount of drug released increases.     

Self-cross-linkable plastic-rubber blend system based on poly(vinyl chloride) and epoxidized natural rubber

Melt-mixed blends of poly(vinyl chloride) and epoxidized natural rubber become cross-linked during moulding at elevated temperatures in the absence of any cross-linking agent. Such a system of self-cross-linkable plastic-rubber blend is immiscible, as is evident from differential scanning calorimetric studies and dynamic mechanical analysis. Such blends were found to have good oil resistance, high abrasion resistance, high modulus but low resilience with moderate tensile and tear strength.     

Preparation and characterization of chitosan/poly(vinyl alcohol)/poly(vinyl pyrrolidone) electrospun fibers

Ultrafine fibers of chitosan/poly(vinyl alcohol)/poly(vinyl pyrrolidone) (CS/PVA/PVP) were prepared via electrospinning. The structure and morphology of CS/PVA/PVP ultrafine fibers was characterized by the Fourier transform infrared (FT-IR) spectroscope and scanning electron microscope (SEM). Furthermore, the effects of the concentration of PVA, PVP and the electrospinning voltage on the morphology of ultrafine fibers were investigated the the SEM. When the concentration of PVA was at the range of 30wt%–40wt%, ultrafine fibers could be obtained. The diameter distributions of ultrafine fibers decreased when the electrospinning voltage increased from 20 to 30 kV. The rough surface fibers could be obtained after etching with CHCl₃.     

Effect of chitosan and dextran on the properties of poly(vinyl alcohol) hydrogels

Hydrogels are three-dimensional polymeric networks very similar to biological tissues and potentially useful as drug delivery systems. Poly(vinyl alcohol)-based hydrogels containing different amounts of dextran or chitosan were prepared using the freezing–thawing method. Repeated freezing–thawing cycles of a poly(vinyl alcohol) (PVA) aqueous solution lead to the formation of crystallites which act as cross-linking sites, and a hydrogel with a high capacity to swell is obtained. The effects of the two different polysaccharides on the properties of the obtained materials were investigated by differential scanning calorimetry, dynamic mechanical analysis and scanning electron microscopy. In addition the release with time of poly(vinyl alcohol) in aqueous medium, was monitored and evaluated. On the basis of the obtained results it seems that the presence of dextran favors the crystallization process of PVA, allowing the formation of a more ordered and homogeneous structure. Instead, chitosan seems to perturb the formation of PVA crystallites leading to a material with a less regular structure. © 1999 Kluwer Academic Publishers     

Physico-chemical and mechanical characterization of hydrogels of poly (vinyl alcohol) and hyaluronic acid

Hydrogels are three-dimensional polymeric networks very similar to biological tissues and potentially useful as soft tissue substitutes and drug delivery systems. Many synthetic polymers can be used to make hydrogels: poly (vinyl alcohol) is widely employed to make hydrogels for biomedical applications. Improvements in the biocompatibility characteristics of synthetic materials could be achieved by the addition of biological macromolecules. The resulting materials named bioartificial polymeric materials could possess the good mechanical properties of the synthetic component and adequate biocompatibility due to the biological component. We have used poly (vinyl alcohol) to make hydrogels containing various amounts of hyaluronic acid. These bioartificial materials were studied to investigate the effect of the presence of the hyaluronic acid on the structural properties of the hydrogels. Thermal, mechanical, morphological and X-ray analyses were performed. A close correspondence between the network consistency and the degree of crystallinity developed in the matrix suggested that the hyaluronic acid, when its content is about 20%, could provide heterogeneous crystallization nuclei for poly (vinyl alcohol) thus increasing the crystallization degree, and consequently, the storage modulus.     

Preparation and characterization of electrospun core sheath nanofibers from multi-walled carbon nanotubes and poly(vinyl pyrrolidone)

Electrospinning is a versatile technique to prepare polymer fibers in nano to micrometer size ranges using very high electrostatic fields. Electrospun nanofibers with tunable porosity and high specific surface area have various applications, including chromatographic supports for protein separation, biomedical devices, tissue engineering and drug delivery matrices, and as key components in solar cells and supercapacitors. Unspinnable materials such as nanoparticles, nanorods, nanotubes or rigid conducting polymers can also be electrospun into fibers through co-axial electrospinning. In this study, we have prepared core-sheath nanofibers utilizing co-axial electrospinning. The core portion of these electrospun fibers consists of multi-walled carbon nanotubes and the sheath portion is poly(vinyl pyrrolidone) (PVP). Various morphologies were obtained by changing both core and sheath solution concentrations. The core-sheath nanofibers were characterized by scanning electron microscopy and transmission electron microscopy, to confirm core-sheath morphology, thermogravimetric analysis, and mechanical strength testing. The electrical conductivity of the surfaces of poly(vinyl pyrrolidone) fibers and poly(vinyl pyrrolidone)-multi-walled nanotube fibers were both 10(-15) S/m. The highest bulk conductivity observed for the poly(vinyl pyrrolidone)-multi-walled nanotube fibers was 1.2 x 10(-3) S/m.     

A simple hydrothermal route to fabrication of single-crystalline silver nanoplates using poly(vinyl pyrrolidone)

Single-crystalline silver nanoplates with an average in-plane dimension and thickness of about 700 and 25 nm, respectively, were fabricated on a large scale in aqueous poly(vinyl pyrrolidone)/AgNO₃ solution by a simple one-step hydrothermal method. In this process, poly (vinyl pyrrolidone) (PVP) acts as both reducing agent and shape-directing agent. SAED and XRD measurements reveal that the growth of silver nanoplates is mainly dominated by (111) facets.     

Physical and biological stability of dehydro-thermally crosslinked collagen—poly(vinyl alcohol) blends

Dehydro-thermal treatments for 3, 24 and 72 h were used to crosslink blends of collagen and poly(vinyl alcohol) with various compositions. This crosslinking method increases the biological stabilityin vitro of collagen, as was established by an enzymatic test. When the poly(vinyl alcohol) content is not more than 20% the resistance of collagen to enzymatic digestion is not affected by the presence of the synthetic component. A higher content of poly(vinyl alcohol) produces a steric hindrance screening that enhances the resistance of collagen to the collagenase. Dehydro-thermal treatment performed for 24 and 72 h increases the crystallinity of poly(vinyl alcohol), thus reducing the solubility of this component of the blend. Calorimetric analysis was carried out by differential scanning calorimetry to investigate the structure and the thermal stability of the blends. Dehydro-thermal treatments carried out for 24 and 72 h induce high degrees of crosslinking in collagen and high crystallinity in poly(vinyl alcohol). The two components of the blend seem to create independent structures and the blend can show interpenetrating-network-like behaviour.     

Swelling and mechanical behaviors of carbon nanotube/poly(vinyl alcohol) hybrid hydrogels

In this paper, carbon nanotubes (CNTs) were added into poly(vinyl alcohol) (PVA) hydrogels to modify their mechanical properties. A series of CNT/PVA hybrid hydrogels were prepared by freezing/thawing method. The mechanical and swelling properties of all hybrid hydrogels are better than those of the original PVA hydrogel. Especially, for CNTP-0.5 specimen with 0.5% w/w CNTs, its tensile modulus, tensile strength and strain at break are increased by 78.2%, 94.3% and 12.7%, respectively. And its swelling behavior is different from that of the pure PVA hydrogel. Its final swelling ratios at room temperature and 310 K are increased by 35.7% and 44.9%, respectively.     

Thermomechanical properties of poly(vinyl alcohol) plasticized with varying ratios of sorbitol

The objective of this work is to study the thermal and mechanical properties of films based on blends of poly(vinyl alcohol) (PVA) with different weight percent of sorbitol. Solid-state PVA/sorbitol polymer membranes were prepared by a solution casting method. The characteristic properties of these polymer membranes were examined by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation methods and by Fourier Transform Infrared (FTIR) spectroscopy. It was found that the thermal properties (glass transition, T_g, melting point, T_m and decomposition temperature, T_d) for PVA blends showed a decrease proportional to the sorbitol concentrations. The hardness and elastic modulus obtained from nanoindentation test were also found to decrease with increase in plasticizer concentration. FTIR confirmed the reduction in hydrogen bonding between PVA chains in favour of formation new bonding between the plasticizer and the PVA chains.     

Poly(vinyl alcohol)/poly(acrylic acid)/TiO2/graphene oxide nanocomposite hydrogels for pH-sensitive photocatalytic degradation of organic pollutants

Poly(vinyl alcohol)/poly(acrylic acid)/TiO₂/graphene oxide nanocomposite hydrogels were prepared using radical polymerization and condensation reaction for the photocatalytic treatment of waste water. Graphene oxide was used as an additive to improve the photocatalytic activity of poly(vinyl alcohol)/poly(acrylic acid)/TiO₂ nanocomposite hydrogels. Both TiO₂ and graphene oxide were immobilized in poly(vinyl alcohol)/poly(acrylic acid) hydrogel matrix for an easier recovery after the waste water treatment. The photocatalytic activity of poly(vinyl alcohol)/poly(acrylic acid)/TiO₂/graphene oxide nanocomposite hydrogels was evaluated on the base of the degradation of pollutants by using UV spectrometer. The improved removal of pollutants was due to the two-step mechanism based on the adsorption of pollutants by nanocomposite hydrogel and the effective decomposition of pollutants by TiO₂ and graphene oxide. The highest swelling of nanocomposite hydrogel was observed at pH 10 indicating that poly(vinyl alcohol)/poly(acrylic acid)/TiO₂/graphene oxide nanocomposite hydrogels were suitable as a promising system for the treatment of basic waste water.     

Hydrogels of poly(vinyl alcohol) and collagen as new bioartificial materials

Poly(vinyl alcohol) was used to make hydrogels containing various amounts of collagen. These bioartificial materials, made of synthetic and biological polymers, were studied to investigate the effect of the presence of the collagen on the structural properties of the hydrogels. A comparison between thermal and morphological properties of collagen-containing hydrogels and hydrogels of pure poly(vinyl alcohol) was made.     

Miscible blends from rigid poly(vinyl chloride) and epoxidized natural rubber

Miscible blends of rigid poly(vinyl chloride), PVC, and epoxidized natural rubber (ENR) having 50 mol % epoxidation level, are prepared in a Brabender Plasticorder by the melt-mixing technique. Changes in Brabender torque and temperature, density, dynamic mechanical properties and DSC thermograms of the samples are studied as a function of blend composition. The PVC-ENR blends behave as a compatible system as is evident from the singleT _g observed both in dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The moderate level broadening of theT _g zone in blends is due to microinhomogeneity, which may arise from the particle structures of PVC perturbing the molecular level mixing of PVC and ENR. Scanning electron microscopic studies were conducted on nitric acid-etched samples and the results showed continuous structures of blend components as well as the occurrence of solvent-induced cracks in high PVC blends.     

Preparation,characterization and release of amoxicillin from cellulose acetate and poly(vinyl pyrrolidone) coaxial electrospun fibrous membranes

Electrospinning is a method that has been used to prepare polymeric fibers, with diameters ranging from nanometers to a micrometer of polymers such as cellulose acetate (CA) and poly(vinyl pyrrolidone) (PVP), and to develop membranes with applications in microencapsulation, for controlled release of drugs and for chemical and biological sensors. This work shows the feasibility and optimal conditions for the preparation of fibrous composite membranes of cellulose acetate and poly(vinyl pyrrolidone), via electrospinning, and their morphology; FTIR, and mechanical characterization and the effect of pH on the release of amoxicillin were analyzed. Tubes of CA with diameters around 500 nm were obtained. It was found that the release behavior of the drug from these fibrous membranes was dependent on the pH of the medium. It was observed that the amount of amoxicillin released as a function of time for a pH equal to 7.2 was approximately three times higher than that observed for a pH equal to 3.0. This suggests a greater interaction of amoxicillin with components of the membrane at a pH equal to 3.0, most likely due to the formation of hydrogen bonds. These materials have potential application in gastrointestinal administration and for transdermal patches.     

Characterization of blends of poly(vinyl chloride) waste for building applications

In the present article we study the improvement of the performance of recycled poly(vinyl chloride) (PVC) coming from credit cards for its use in the fabrication of pipes and fittings in the hydrosanitary sector, in order to meet the requirements established in the international standards. With this purpose, styrene polymers from both virgin and recycled origin have been added to recycled PVC in different concentrations. Since these are recycled materials, the previous degradation of the recycled materials used has been taken into consideration. On the other hand, the compatibilization between polymers considered, which is the influential factor in the final properties of the blend, has been analysed by differential scanning calorimetry. The mechanical properties of the blends have been determined by tensile and impact test, and the thermal properties by Vicat softening temperature and dynamic mechanical analysis. Finally, facture surfaces of the different blends have been analysed by scanning electron microscopy. The results obtained show a generalized improvement in the properties for small quantities added from styrene polymers, which makes the use of these blends in the building industry a feasible option.     

Evaluation of poly(vinyl alcohol) hydrogels as a component of hybrid artificial tissues

Hydrogels are three-dimensional polymeric networks very similar to biological tissues. Many synthetic polymers can be used in preparing hydrogels. Among them poly(vinyl alcohol) (PVA), physically crosslinked by repeated freeze-thawing cycles of polymer aqueous solutions, is widely employed to make hydrogels for biomedical applications. To increase the similarity between hydrogels and natural tissues and to obtain polymeric hybrid tissues, we attempted to incorporate 3T3 cells, from a mouse fibroblast cell line, into PVA hydrogels obtained by one freeze-thawing cycle using as a solvent complete culture medium. Hydrogels were also made using eight freeze-thawing cycles from PVA solutions prepared using as a solvent either complete culture medium or water. Cell adhesion experiments were performed by seeding 3T3 and human umbilical vein endothelial cells (HUVEC) on to the hydrogel surface. The effect of the solvent and of the different number of freeze-thawing cycles on the mechanical characteristics of the PVA hydrogels were investigated by dynamic-mechanical techniques. A scanning force microscope analysis of the hydrogel surface viscoelastic properties was also carried out. Our results show that PVA is not cytotoxic. Although PVA hydrogel surface characteristics do not seem to favour the adhesion of substrate-dependent cells, encouraging results were obtained with the 3T3 cells incorporation. DMA analysis indicates that the networks prepared by eight freeze-thawing cycles possess a mechanical consistency comparable, even slightly better, than the ones prepared by only one freeze-thawing cycle and used for the cell incorporation studies.     

Viscoelastic modelling and strain-rate behaviour of plasticized poly(vinyl chloride)

The effect of four different types of plasticizers and four strain-rates on the tensile behaviour of poly(vinyl chloride) (PVC) has been studied. di(2-ethylhexyl phthalate), benzyl butyl phthalate, epoxidized soyabean oil and chloroparaffin were mixed at different ratios and were used as plasticizers in concentration levels of up to 77% of the PVC weight. The plasticized and unplasticized PVC were processed into sheets by compression moulding. Tensile tests were conducted at different strain rates. It was found that tensile modulus increases with increasing strain rate while it decreases with increasing plasticizer concentration. The rate of variation of tensile modulus either as a function of strain rate and/or the plasticizer concentration was, in all cases, dependent on the mixing ratio of the different types of plasticizers. Assuming the material to be a linearly viscoelastic one, a simple viscoelastic model along with a least-squares-based computer procedure was applied which enabled us to fit the experimentally obtained curves with the respective theoretical predictions as well as to study the strain-rate effect on the relaxation spectrum, H(), of the material under consideration.     

Mechanical properties of poly(vinyl chloride)-poly(acrylonitrile-co-butadiene) blends with modulated structure

By solution-casting we prepared poly(vinyl chloride)-poly(acrylonitrile-co-butadiene) blends with a modulated structure (a co-continuous two-phase structure with periodic distance of a few micrometres or less). Mechanical properties of the film specimens were investigated by the uniaxial tensile test combined with small-angle light scattering. A 50: 50 plastic-rubber blend has a high initial modulus which is close to that of the rigid plastic. Nevertheless, it can deform to a large extension ratio up to 6. It cannot immediately recover to its original length after the stress-strain test, i.e. residual strain in the stress-strain curve persists. However, after release from the instrument the film gradually shrinks to its original length at room temperature. After strain recovery the film shows a stress-strain curve almost identical to the original. It seems that the stretched film healed simply by resting at room temperature. Based on the light-scattering studies, a helical spring model is derived for the two-phase system with modulated structure, exhibiting the characteristic mechanical properties mentioned above. The spring model interprets well the dependence of Young's modulus on the blend composition and the periodic distance of the modulated structure.     

Latex derived blends of poly(vinyl acetate) and natural rubber: thermal and mechanical properties

Poly(vinyl acetate) (PVAc) and natural rubber blends (NR) were prepared by low shear blending of the corresponding lattices. Thin films were cast using a doctor blade technique. SEM and DMA confirmed the essential immiscibility of the two polymers. Even when the poly(vinyl acetate) forms the matrix phase, it still contains domains encapsulated by a rubber phase where particles that resemble the original latex are visible. This incomplete droplet coalescence of the poly(vinyl acetate) is attributed to the fact that the rubber latex droplets were an order of magnitude smaller than the poly(vinyl acetate) latex droplets. Tensile testing revealed a nonlinear dependence of tensile strength and elongation on blend composition. Surprisingly good tensile yield strengths were obtained at intermediate to high PVAc contents. Thermogravimetric analysis of degradation in air and nitrogen atmospheres indicated independent degradation of the parent polymers.