High Molecular Weight PVA Research Articles

Lignin/PVA hydrogel with enhanced structural stability for cationic dye removal

In this study, a lignin-based hydrogel for wastewater treatment was prepared by incorporating kraft lignin (KL) into a poly (vinyl alcohol) (PVA) matrix. The underwater structural stability of the KL-PVA hydrogel was guaranteed through physicochemical crosslinking, involving freeze–thaw process and chemical crosslinking reaction. The KL-PVA hydrogel displayed superior compressive characteristics compared to the original PVA hydrogel. This improvement was attributed to the chemical crosslinking and the reinforcing effect of the incorporated KL microparticles. The incorporation of anionic KL microparticles into the PVA three-dimensional network structure enhanced the cationic methylene blue (MB) and crystal violet (CV) adsorption efficiency of the prepared KL-PVA hydrogel. The MB adsorption results were well explained by pseudo-2nd order kinetics model and Langmuir isotherm model. Electrostatic forces, hydrogen bonding and π-π stacking interactions were the main adsorption mechanisms between cationic dyes and KL surfaces, indicating the potential of KL-PVA hydrogel as an effective adsorption material. Moreover, regulating the molecular weight of PVA not only prevented lignin leakage from the KL-PVA hydrogel but also elevated the KL content within the hydrogel, consequently improving its dye removal performance. For KL-PVA hydrogel with high molecular weight PVA, the MB and CV adsorption capacities were 193.8 mg/g and 190.0 mg/g, respectively.

Preparation of well-defined Poly(Vinyl alcohol) by hydrolysis of Poly(Vinyl acetate) synthesized by RAFT suspension polymerization

Poly(vinyl alcohol) (PVA) is a widely used synthetic polymer that is made by hydrolysis of poly(vinyl acetate) (PVAc). A significant challenge in the production of well-defined PVA arises due to the high frequency of chain transfer reactions that occur in the radical polymerization of PVAc, which limits the molecular weight and leads to branching. Upon hydrolysis of PVAc, both acetate groups and branch points can be hydrolyzed, resulting in a PVA with a significantly lower molecular weight than the parent PVAc, making control over the molecular weight distribution of PVA difficult. In this work we demonstrate the production of PVAc with a well-defined and linear structure by reversible addition-fragmentation chain transfer (RAFT) polymerization under conditions in which chain transfer is reduced. PVAc samples were prepared by suspension polymerization using both conventional free-radical polymerization and RAFT polymerization at 50 °C. When using RAFT polymerization, upon hydrolysis there was a minimal decrease of the degree of polymerization and high molecular weight PVA of low dispersity was obtained. These results demonstrate the synthesis of well-defined PVA and offers a potential route to the production of PVAc and PVA products with uniform properties.

Synthesis of poly(vinyl alcohol) by blue light bismuth oxide photocatalysed RAFT. Evaluation of the impact of freeze/thaw cycling on ice recrystallisation inhibition.

Poly(vinyl alcohol), PVA, is the most potent polymeric ice recrystallisation inhibitor (IRI), mimicking a complex function of ice binding proteins. The IRI activity of PVA scales with its molecular weight and hence broad molecular weight distributions in free radical-derived PVAs lead to activity measurements dominated by small amounts of heavier fractions. Well-defined PVA can be prepared by thermally initiated RAFT/MADIX polymerization using xanthates by the polymerization of the less activated monomer vinyl acetate. The low conversions and molecular weights obtained during this approach, often requires feeding of additional initiator and bulk polymerization. Here we employ bismuth oxide photo-RAFT in solution, using blue light (450 nm), rather than previously reported white light, to obtain a library of PVA's. The use of blue light enabled quantitative conversion and acceptable dispersities. Purple light (380 nm) was also used, but asymmetric molecular weight distributions were obtained in some cases. High concentrations of high molecular weight PVA is known to form cryogels during freeze/thaw which has led to speculation this might limit the use of PVA in environments where the temperature cycles e.g. the construction industry. After 4 freeze/thaw cycles there was only small changes in observable IRI for all synthesised PVAs and two commercial standards. In an extended test, activity was retained after 100 freeze/thaw cycles, mitigating concerns that PVA could not be used in situations where freeze/thaw cycles occur. This work presents a convenient method to obtain well-defined PVAs for cryoscience studies compared to conventional thermal-RAFT and indicates that cryogelation concerns may not prevent their use.

Branched/Hyperbranched Copolyesters from Poly(vinyl alcohol) and Citric Acid as Delivery Agents and Tissue Regeneration Scaffolds

AbstractMultifunctional branched/hyperbranched copolymers from poly(vinyl alcohol) (PVA) and citric acid (CA) are synthesized by varying the mole compositions of PVA and CA and are used as a prospective vehicle for encapsulation and release of bioactive molecules and as a potential scaffold for cell adhesion and growth. The branched architecture is established from spectroscopy and rheological measurements. All the copolymers have shown a lower hydrodynamic size and viscosity than the linear, high molecular weight PVA because of spherical and more compact architecture. Importantly, the size of the highly branched copolymer is found independent of pH which proved that the branch ends are capped with OH groups. Lower viscosity at equivalent solid content, biocompatibility, high antibacterial property, and presence of adequate macromolecular voids make the branched/hyperbranched copolymers a potential platform for encapsulation and release of gentamicin and other bioactive molecules. The macromolecular voids and chain end functionality also promote adhesion and growth of differentiated primary cells as well as undifferentiated stem cells implying that the copolyester can also be used as a potential 2D/3D scaffold for tissue engineering applications.

Production of poly(vinyl alcohol)/Nafion® nanofibers and their stability assessment for the use in direct methanol fuel cells

The aim of this study is to investigate the electrospinning of Nafion® nanofibers with poly(vinyl alcohol) (PVA) as a carrier polymer and to assess the thermal and chemical stability of resultant PVA/Nafion® nanofibers for the use in direct methanol fuel cells, in simulated conditions. Bead-free PVA/Nafion® nanofibers were produced using higher molecular weight PVA. Resultant PVA and PVA/Nafion® nanofibers were stabilized using two different methods which are BTCA crosslinking and thermal stabilization, followed by sulfonation of the PVA part. FT-IR analysis demonstrated that the membranes were stabilized and sulfonated successfully. Thermal, water, methanol and oxidative stability of the membranes were tested in addition to ion-exchange capacity. Morphological changes in the structure were analyzed using SEM analysis. Thermally stabilized PVA/Nafion® nanofibrous membrane was found to be stable against water, methanol and oxidative effects. The nanofibrous structure was well preserved after treatments, while the other membranes became a film-like material. Thermal stability of the PVA/Nafion® nanofibrous membrane was similar to that of commercial Nafion® 115 membrane up to 200℃.

Solution properties and electrospinning of poly(galacturonic acid) nanofibers

Poly(galacturonic acid) (PGuA) is an important natural biopolymer, however its potential has not been realized due to its anionic nature and rigid structure, which limits its processability into fine films and fibres. This study aims at modifying the solution properties of PGuA in alkaline medium (aq. sodium hydroxide) to enable their conversion into electrospun nanofibers. Addition of anionic surfactants was found to play an important role in individualizing the PGuA chains that lead to formation of small spindle shaped fibers of length ranging from 2 to 10 μm and diameter from 287 to 997 nm. However, continuous fibers were not formed even at concentrations higher than the critical concentration. Addition of small amount (10–30%) of high molecular weight PVA resulted in formation of continuous fibers. Correlation of fiber diameters of PGuA/PVA with the rheological properties suggested a strong dependence of diameter with the elasticity of the blend solutions. Such PGuA based fibers may be utilized in various biomedical applications.

PVA-gelatin hydrogels formed using combined theta-gel and cryo-gel fabrication techniques

Poly(vinyl alcohol) (PVA) is a synthetic, biocompatible polymer that has been widely studied for use in bioengineered tissue scaffolds due to its relatively high strength, creep resistance, water retention, and porous structure. However, PVA hydrogels traditionally exhibit low percent elongation and energy dissipation. PVA material and mechanical properties can be fine-tuned by controlling the physical, non-covalent crosslinks during hydrogel formation through various techniques; PVA scaffolds were modified with gelatin, a natural collagen derivative also capable of forming reversible hydrogen bonds. Blending in gelatin and poly(ethylene glycol) (PEG) with PVA prior to solidification formed a highly organized hydrogel with improved toughness and dynamic elasticity. Theta-gels were formed from the solidification of warm solutions and the phase separation of high molecular weight gelatin and PVA from a low molecular PEG porogen upon cooling. While PVA-gelatin hydrogels can be synthesized in this manner, the hydrogels exhibited low toughness with increased elasticity. Thus, theta-gels were additionally processed using cryo-gel fabrication techniques, which involved freezing theta-gels, lyophilizing and re-hydrating. The result was a stronger, more resilient material. We hypothesized that the increased formation of physical hydrogen bonds between the PVA and gelatin allowed for the combination of a stiffer material with energy dissipation characteristics. Rheological data suggested significant changes in the storage moduli of the new PVA-gelatin theta-cryo-gels. Elastic modulus, strain to failure, hysteresis and resilience were studied through uniaxial tension and dynamic mechanical analysis in compression.

Dehydration performance of double-network poly(vinyl alcohol) nanocomposite membranes (PVAs-DN)

Double-network poly(vinyl alcohol) nanocomposite membranes (PVAs-DN) were prepared from interpenetrating of two PVA networks. The sequential method was employed in which the first PVA network was completely established from crosslinking of high molecular weight PVA (HPVA) in the presence of silica nanospheres (SNSs) prior to a formation of the second network by the thermal crosslinking of low molecular weight PVA (LPVA). SNSs particle loading was varied at 1 and 3.5wt% which assured an even distribution of the particles in PVAs-DN matrix. Introducing of SNSs enhanced the crystallinity and mechanical properties of PVAs-DN as revealed by XRD and DMA analysis. Furthermore DMA result indicated that HPVA and LPVA networks in PVAs-DN membranes, despite their chemically identical structure, established a separate network and possessed its own relaxation. Effect of temperature on the sorption and the pervaporation was performed using 90wt% ethanol solution. The results showed that the permeability and the selectivity of water were pronouncedly improved in SNSs-filled PVAs-DN membranes compared with unfilled one and HPVA-SN membranes. Consequently thermodynamic parameters and Arrhenius activation parameters were determined.

Hydrogels reinforced with nanoclays with improved response rate

ABSTRACTTwo series of semiinterpenetrating networks (SIPN) based on linear hydrophilic poly(vinyl alcohol) (PVA) and thermo‐responsive poly(N‐isopropylacrylamide) (PNIPA), physically crosslinked with inorganic clay, are presented. The hydrogels with different crosslinking densities were prepared by varying the content of clay from 1 to 6 wt % and contained linear interpenetrant, PVA in the range of 0.5–1.5 wt %. The effect of clay content on swelling/deswelling behavior and phase transition in PNIPA gels, as well as the feasibility of reinforcing the gels with high molecular weight PVA, were analyzed. The thermal response of hydrogels, followed by DSC, confirmed that the insertion of hydrophilic PVA did not have a significant effect on the onset of the volume phase transition temperature, while the response was faster. The equilibrium degree of swelling of SIPNs and PNIPA hydrogels was in the range of 9–79 and decreased with increasing content of clay. The internal morphology and surface wettability of the hydrogels were investigated by scanning electron microscope analysis and contact angle measurements, respectively. The network structural parameters of the PNIPA and SIPN nanocomposites hydrogels, such as the average molecular weight between crosslinks, Mc, and effective crosslinking density, Ne, were determined by dynamic mechanical analysis. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44535.

Aqueous solution blow spinning of poly(vinyl alcohol) micro- and nanofibers

This work addresses the challenge to produce fibers from the water-soluble polymer poly(vinyl alcohol) (PVA), using solution blow spinning (SBS) with forced solvent evaporation at the point of fiber formation. PVA at two different molecular weights, with different degrees of acetylation, were successfully blow spun into nano- and micro-fiber membranes, across a range of concentrations in water (12–20w/v%). Fiber spinnability, morphology and size are correlated to precursor solution viscosity and PVA type. PVA with degree of polymerization 1100 and degree of hydrolysis 98–99% was relatively facile to spin into fibers. Difficulties in spinning high molecular weight PVA were overcome by introducing hot air at the point of fiber formation to force water evaporation. The procedure developed here opens SBS to other aqueous-based polymer and composite systems for environmentally benign fiber and membrane formation.

Strong molecular weight effects of gate-insulating memory polymers in low-voltage organic nonvolatile memory transistors with outstanding retention characteristics

Organic nonvolatile memory transistors, featuring low-voltage operation (⩽5 V) and high retention characteristics (>10 000 cycles), are demonstrated by introducing high molecular weight poly(vinyl alcohol) (PVA) as a gate insulating layer. PVA polymers with four different molecular weights (9.5–166 kDa) are examined for organic memory devices with poly(3-hexylthiophene) channel layers. All devices show excellent p-type transistor behavior and strong hysteresis in the transfer curves, but the lower molecular weight PVA delivers the higher hole mobility and the wider memory window. This has been attributed to the higher ratio of hydroxyl group dipoles that align in the out-of-plane direction of the PVA layers, as supported by impedance spectroscopy (dielectric constants), polarized Fourier transform-infrared spectroscopy and synchrotron radiation grazing incidence X-ray diffraction measurements. However, outstanding retention characteristics (<4% current variation after 10 000 cycles) have been achieved with the higher molecular weight PVA (166 kDa) rather than the lower molecular weight PVA (9.5 kDa). Researchers have created an organic transistor that retains memory states for over 10,000 cycles with minimal, sub-5-volt power requirements. Flexible memory devices made from semiconducting plastics are significantly easier to fabricate than silicon-based materials. However, the high voltages usually needed to switch the on/off state of organic transistors are detrimental to device longevity. An international team led by Youngkyoo Kim from Kyungpook National University in Korea used insulating poly(vinyl alcohol) (PVA) films with high molecular weights to reduce the voltage demands of plastic circuits. The researchers inserted PVA films between a transparent electrode and a semiconducting polymer, and then fabricated the stack into a source/drain/gate transistor setup. The most stable data retention characteristics were achieved with the heaviest PVA films — a relationship the researchers attribute to hydroxyl dipole orientations at interfacial boundaries. Low-voltage (<5 V) organic memory transistors with outstanding retention stability (>10 000 cycles), which were achieved by employing both the P3HT channel layer and the PVA gate-insulating memory layer, were strongly influenced by the PVA molecular weights due to the different orientation of hydroxyl dipoles.

Influence of poly(vinyl alcohol) molecular weight on drop coalescence and breakage rate

The influence of poly(vinyl alcohol), PVA, molecular weight on drop breakage and coalescence in stirred liquid–liquid dispersion was considered. Three types of PVA of the same degree of hydrolysis 88% and different molecular weights in the range 13,000–186,000 were used in experiments. Molecular weight affects steric forces, interfacial tension and surface viscosity, which all have an influence on drop size distribution. PVA of lower molecular weight reduces the interfacial tension more effectively while the adsorption of high molecular weight PVA is slower and more complicated. Drop interfaces can remain partially mobile for polymer concentration as small as 0.001wt% or they can be significantly immobilized for PVA concentration c≥0.002wt%. Drop size distributions were predicted using multifractal breakage and coalescence models. Lost of stabilizing properties of PVA of highest molecular weight at high impeller speed was observed which can be explained by unfavorable molecule conformation in such conditions.

Molecular mechanism of poly(vinyl alcohol) mediated prevention of aggregation and stabilization of insulin in nanoparticles.

It is a challenge to formulate polymer based nanoparticles of therapeutic proteins as excipients and process conditions affect stability and structural integrity of the protein. Hence, understanding the protein stability and complex aggregation phenomena is an important area of research in therapeutic protein delivery. Herein we investigated the comparative role of three kinds of surfactant systems (Tween 20:Tween 80), small molecular weight poly(vinyl alcohol) (SMW-PVA), and high molecular weight PVA (HMW-PVA) in prevention of aggregation and stabilization of hexameric insulin in poly(lactide-co-glycolide) (PLGA) based nanoparticle formulation. The nanoparticles were prepared using solid-in-oil-in-water (S/O/W) emulsification method with one of the said surfactant system in inner aqueous phase. The thermal unfolding analysis of released insulin using circular dichroism (CD) indicated thermal stability of the hexameric form. Insulin aggregation monitored by differential scanning calorimetry (DSC) suggested the importance of nuclei formation for aggregation and its prevention by HMW-PVA. Additional guanidinium hydrochloride based equilibrium unfolding and in silico (molecular docking) studies suggested maximum stability of released insulin from formulation containing HMW-PVA (F3). Furthermore, in vivo studies of insulin loaded nanoparticle formulation (F3) in diabetic rats showed its bioactivity. In conclusion, our studies highlight the importance of C-terminal residues of insulin in structural integrity and suggest that the released insulin from formulation containing HMW-PVA in inner aqueous phase was conformationally and thermodynamically stable and bioactive in vivo.

Physical stabilisation of electrospun poly(vinyl alcohol) nanofibres: comparative study on methanol and heat-based crosslinking

Methanol crosslinking and heat-treatment methods for physical crosslinking of electrospun poly(vinyl alcohol) (PVA) nanofibres were investigated to assess their stability in water. For this purpose, PVAs with low and high molecular weights were selected. Morphology of the crosslinked membranes was characterised by scanning electron microscopy. Crystallinity of the resultant crosslinked fibres were analysed by FT-IR and differential scanning calorimetry. It has been shown that physical crosslinking increases the crystallinity of the fibres. High molecular weight PVA nanofibres showed better stability and better preservation of nanofibrous structure. Stability of the crosslinked membranes was also tested by immersion into water at room temperature and boiling water. Combined methanol and heat treatments at different temperatures and exposure periods were also investigated. Treatment at 180 °C HMW PVA nanofibres for longer durations exhibited best results in terms of water stability, although it exhibited somewhat lower swelling ratios as compared to those subjected to only methanol treatment.

Freeze-Thawed Hybridized Preparation with Biomimetic Self-Assembly for a Polyvinyl Alcohol/Collagen Hydrogel Created for Meniscus Tissue Engineering

Freeze-thawed hybridized preparation and the biomimetic self-assembly technique were used to fabricate hydrogel as tissue engineered scaffolds for meniscus tissue. Because of the advantages of both techniques, they were hybridized together as an interesting preparation for hydrogel. Three molecular weights (high, medium, and low) of PVA were prepared in a biomimetic solution before formation into hydrogel by freeze-thawing. The most suitable molecular weight PVA for hydrogel formation was chosen to be mixed with collagen. PVA, PVA/collagen, and collagen were prepared in biomimetic solutions and freeze-thawed into hydrogels. The hydrogels were analyzed and characterized by FTIR, DSC, and SEM. FTIR characterization indicated that high molecular weight PVA formed molecular interaction better than the other molecular weights, and PVA molecules formed molecular interaction with collagen molecules via –OH and C=O groups. DSC characterization showed that the hybridized preparation of freeze-thawing and biomimetic self-assembly kept the characteristics of PVA and collagen. SEM analysis demonstrated that the morphological formation of PVA/collagen was hybridized during freeze-thawing and collagen self-assembly. The morphological structure was organized into a porous network structure. The porous structure showed a rough wall that was formed by the hybridized structure of the crystal domain dispersed in amorphous and collagen self-assembly.

Physical, mechanical, and transdermal diltiazem release analysis of nanosilica tailored various poly(vinyl alcohol) membranes

ABSTRACTHybrid membranes based on poly(vinyl alcohol) (PVA) of widely different molecular weights and ex situ nanosilica were synthesized and characterized as transdermal delivery device for Diltiazem hydrochloride. Investigations showed that change in PVA molecular weight strongly influenced physico‐mechanicals of the hybrids especially at low nanosilica content than at higher levels. As for example at 1 wt %, low molecular weight PVA induced finer dispersion of silica nanoparticles resulting into higher dry state crystallinity and mechanical strength but slightly lower biocompatibility as compared to high molecular weight PVA. Those variations in physico‐mechanicals finally affected Diltiazem retention and its elution from those membranes under physiological conditions. Low molecular weight PVA produced highest drug retention as well as slowest yet steady release than both high molecular weight PVA and neat PVA membranes. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2076–2086, 2013

Uniquely different PVA-xanthan gum irradiated membranes as transdermal diltiazem delivery device

This paper reports interesting differences between physical and mechanical properties of various membranes prepared from high and low molecular weight poly (vinyl alcohol) (PVA) and xanthan gum (XG) blends irradiated under low dose electron beam. The membranes were designed for sustained delivery of diltiazem hydrochloride through skin. Electron beam irradiation produced crosslinks and turned PVA into crystalline phase from its amorphous organization in the unirradiated state. PVA crystals were fibrillar at low XG content (1wt.%) when the molecular weight was high while similar orientation at higher XG content (5wt.%) when the molecular weight was low. Low molecular weight PVA–XG membranes showed equivalent physical properties under dry condition but wet-mechanical properties were superior for high molecular weight PVA–XG hybrids. Both of them showed slow and sustained diltiazem release but the later induced slightly slower release despite low drug encapsulation efficiency due to its better wet mechanical strength.

A transdermal diltiazem hydrochloride delivery device using multi-walled carbon nanotube/poly(vinyl alcohol) composites

Membranes with high strength and elasticity are of great demand in patch therapy. Similar membranes have been developed by combining carboxy-functionalized multiwalled carbon nanotube (c-MWCNT) with different poly(vinyl alcohol) (PVA) as potential diltiazem delivery device through aqueous mixing. High molecular weight PVA (PVAH) produced stronger interaction with c-MWCNT than low molecular weight PVA (PVAL) preferably at low concentration. Positive changes in favor of PVAH in infrared and solid state 13C nuclear magnetic resonance spectroscopy, wide angle X-ray scattering, thermal stability, morphology and dry and wet mechanical properties clearly demonstrate that. Fibrillar c-MWCNT array at 1wt.% in PVAH (PVAH/1) has drastically improved PVA crystalline cell dimension, tensile strength (201%) and elongation (196%) than neat PVAH whereas the similar improvement is much less (100% and 185%) in PVAL (PVAL/1) due to globular morphology. Instead, c-MWCNT performed better at 0.5wt.% in PVAL (PVAL/0.5). The kinetic data reflects better encapsulation and slower release by PVAH (5.87%) than PVAL (10.17%) due to greater interfacial interaction.

Interesting correlation between structure, physicomechanical, swelling and sustained transdermal release behavior of diltiazem hydrochloride in various poly(vinyl alcohol) hydrogel membranes

AbstractInteresting correlation between physicomechanical and in vitro release behavior of diltiazem hydrochloride has been obtained in physically treated poly(vinyl alcohol) (PVA) membranes of widely different molecular weights. Physical treatment could not raise crystallinity in dry state, but some of them show significant postswelled crystallinity and achieves elastomeric character. They also retain lot of equilibrium water, ideally suited for drug‐impregnated transdermal patches. In effect, low molecular weight PVA and dimethyl sulfoxide‐modified high molecular weight PVA membranes have shown acceptable release of diltiazem hydrochloride when compared with thermally treated high molecular weight PVA membrane. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Flow Instability of Polyvinyl alcohol)/Dimethyl Sulfoxide Solution under Steady Shear

The effects of solution temperature and molecular weight on the rheological responses of poly(vinyl alcohol) (PVA) semidilute solutions (13 g/dL- 19 g/dL) in dimethyl sulfoxide (DMSO) have been studied using PVA with two different number-average degrees of polymerization (Pn): 1700 and 4000. For the rheological experiments using dynamic shear, the PVA solution with a Pn of 1700 showed rather simple Newtonian fluid behaviour, whereas that with Pn of 4000 exhibited non-Newtonian shear thinning behaviour. Steady shear rheology experiments suggested that the lower Pn PVA solutions were Newtonian fluids, which is consistent with the dynamic shear rheology results. However, an abrupt decrease of shear viscosity at high shear rates was observed for the higher Pn PVA solutions at lower temperatures, such as 30 °C and 50 °C. At higher temperatures of 70 °C and 90 °C, the higher molecular weight PVA solutions exhibited simple shear thinning behaviour without any further complications of flow instability. The fact that temperature plays a critical role in controlling the observed flow instability of the PVA solutions in DMSO strongly suggests that there exist enhanced inter-chain interactions by hydrogen bonding of PVA chains beyond the chain entanglements, particularly for the higher Pn PVA semidilute solutions at low temperatures. Therefore, it is important to consider both chain entanglements and hydrogen bonding interactions in order to process PVA solutions without causing undesirable flow instability.