Rapid synthesis of high strength cellulose–poly(vinyl alcohol) (PVA) biocompatible composite films via microwave crosslinking

The present study describes microwave (MW)-assisted rapid synthesis of biocompatible poly(vinyl alcohol) (PVA) composite films that demonstrate synergy between reinforcement and crosslinking. Bacterial cellulose (5% w/w) nanowhiskers (reinforcement) and tartaric acid 35% (w/w) (crosslinker) are incorporated in PVA to prepare crosslinked cellulose–PVA composite films. The properties of thus prepared crosslinked cellulose–PVA composite films are compared with samples crosslinked with conventional hot air oven heating (CH). Crosslinking by both of the methods reduces water absorption of PVA by around an order of magnitude and improves its thermal stability. An increase in strength from 42 (PVA) to 172 MPa and 159 MPa for MW and CH crosslinked samples, respectively is also observed. Although composites prepared using MW and CH show similar properties, MW takes only 14 min compared to 2 h in case of CH. Notably, the prepared composites demonstrate hemocompatibility and cytocompatibility, and may also be explored for biomedical applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47393.     

Effect of SiC whisker on benzoxazine‐epoxy‐phenolic ternary systems: Microwave curing and thermomechanical characteristics

Microwave radiation at 2.45 GHz with variable power input was investigated as a tool to facilitate the curing reaction of benzoxazine‐epoxy‐phenolic molding compound i.e., BEP893. Dielectric filler for microwave coupling was silicon carbide whisker (SiC_w). Factors such as whisker loading and input irradiation power were found to have a profound effect on the microwave heating of the BEP893 particularly on the rate of temperature rise and maximum heating temperature. The SiC_w loading of 10% by weight with the microwave irradiation condition of 300 W for 10 min renders the ultimate curing of the molding compound. Significant reduction in processing time of the microwave cured sample compared with the conventional heat cured sample i.e., 150 min at 200°C using conventional heating is the key benefit of this technique. Mechanical properties of the microwave cured and conventional heat cured samples show similar characteristics with slightly lower T_g in the microwave cured samples. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Comparative study on the properties of cross-linked cellulose nanocrystals/chitosan film composites with conventional heating and microwave curing

Cross-linking of chitosan film composites was carried out by using conventional heating and microwave curing methods in this study. Non-cross-linked and glutaraldehyde (GA) cross-linked neat chitosan and cellulose nanocrystals (CNC)/chitosan film composites were cured by either conventional oven heating or microwave irradiation. Tensile strength and Young's modulus of chitosan composites were enhanced significantly by the addition of CNC and GA especially for the microwave-cured samples. The changes in chemical interaction of the chitosan film composites was determined by Fourier transform infrared (FTIR) spectroscopy. The microwave-cured GA-cross-linked chitosan film composites were more thermally stable than non-cross-linked and conventionally heated GA-cross-linked chitosan film composites due to the formation of a more stable structure between GA and chitosan. Nevertheless, the reduced antimicrobial efficacy of film composites against Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae was observed in cross-linked film composites compared with non-cross-linked composites.     

Crosslinked poly(vinyl alcohol) composite films with cellulose nanocrystals: Mechanical and thermal properties

In this work, poly(vinyl alcohol) (PVA) and cellulose nanocrystals (CNCs) were crosslinked using sodium tetraborate decahydrate (borax) to improve the mechanical and thermal properties of the neat PVA. The results showed that the CNCs affected the crystallization behavior of the crosslinked PVA. The crystallization temperature of the crosslinked PVA with CNCs increased considerably from ～152 to ～187 °C. The continuous improvement of the thermal stability was observed with the increasing content of CNCs in the crosslinked PVA films. Additionally, the strong interaction between the CNCs and PVA was theoretically estimated from the Young's modulus values of the composites. Thermodynamic mechanical testing revealed that the crosslinked PVA composite films with CNCs could bear higher loads at high temperature compared to the films without the CNCs. At 60 °C, 2.7 GPa was reported for the storage modulus of the crosslinked composites with 3 wt % of CNCs, twice as high as that for the crosslinked films without CNCs. Moreover, creep results were improved when CNCs were added in the crosslinked nanocomposites. The materials prepared in this work could broaden the opportunities for applications in a wide range of temperatures. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45710.     

Microwave-irradiated tanning reaction of aluminum with collagen

Given the toxicity of chrome tanning in leather industry, ecologically compatible alternatives are intensively researched, in particular on the side of aluminum tanning. Herein, the tanning reaction of aluminum with collagen was investigated using microwave irradiation as a new heating method, and the traditional water bath heating (WBH) was used as control. Results from dynamic light scattering and very-positive differential scanning calorimetry (DSC) analysis reveal that, microwave heating led to larger particle size and increased main denaturing transition temperature of collagen clusters crosslinked by aluminum sulfate. The aluminum-tanned collagen products obtained under microwave and WBH methods were further compared and characterized using many techniques including circular dichroism, fluorescence spectroscopy, atomic force microscope, and Fourier transform infrared spectroscopy, DSC, and thermogravimetry. Results verified that both heating methods accelerate the crosslinking reaction of aluminum with collagen and increase the crosslinking degrees. The crosslinked products exhibited improved thermal stability, but no change was observed concerning the binding mode of aluminum with collagen. However, compared to WBH microwave heating has a more positive effect on the aluminum-tanning processes for the improvement of collagen crosslinking processes and product performances, probably due to both thermal and nonthermal effects of microwave irradiation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48682.     

Synthesis,characterization and efficacy of chemically crosslinked PVA hydrogels for dermal wound healing in experimental animals

In this study, we have investigated the efficacy of crosslinked polyvinyl alcohol (PVA) hydrogel as a wound dressing material, using rat as the animal model. The hydrogel was synthesized by chemical crosslinking of PVA with potassium persulphate and the crosslinking reaction parameters were optimized. The developed hydrogel was found to possess excellent mechanical properties, high water absorption capacity, gel content, and optimum water vapor transmission rate, indicating its ability to act as an effective wound dressing material. The inherent nontoxic characteristics of PVA remained unaltered after crosslinking. The in vitro diffusion studies of bovine serum albumin (BSA) as model protein, indicated a relatively slow release of protein resulting from its microencapsulation in the polymeric matrix. For in vivo studies, full‐thickness excision wounds (2 × 2 cm²) were made on the dorsal surface of rats. The hydrogel was applied on the wound and changed at regular intervals. For comparison of wound healing ability, a radiation crosslinked PVA‐based hydrogel, “HiZel” was used as a reference control. The wounds treated with PVA hydrogel healed faster as indicated by an increased rate of wound contraction (16.5 days versus 22.0 in control group). Treatment with “Hizel” led to increase in hydroxyproline in the wound tissue, whereas treatment with PVA hydrogel led to increase in both hydroxyproline as well as hexosamine. This probably provides added strength to the tissue, thereby indicating that PVA hydrogel had higher efficacy than “Hizel”. The results suggest that chemically crosslinked PVA hydrogel could be used as an effective wound dressing material. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Antioxidant,antifungal, water binding,and mechanical properties of poly(vinyl alcohol) film incorporated with essential oil as a potential wound dressing material

In this study, the properties of poly (vinyl alcohol) (PVA) films incorporated with Zataria multiflora essential oil (ZMO) as a potential antioxidant/antibacterial material was investigated. PVA films were prepared from PVA solutions (2% w/v) containing different concentrations of ZMO. Water solubility, moisture absorption, water swelling, and water vapor permeability for pure PVA films were 57 ± 1.1, 99 ± 3.2%, 337 ± 8%, and 0.453 ± 0.015 g mm/m² h, respectively. Incorporation of ZMO into PVA films caused a significant decrease in water swelling and moisture absorption and increase in solubility and water vapor permeability. Tensile strength, elastic modulus, and elongation at break for pure PVA films were 13.5 ± 0.61 MPa, 15.2 ± 0.8 MPa, and 216 ± 4%, respectively. Incorporation of ZMO into the PVA films caused a significant decrease in tensile strength and elastic modulus and increase in elongation at break of the films. Pure PVA film showed UV‐visible light absorbance ranging from 280 to 440 nm with maximum absorbance at 320 nm. Addition of ZMO caused a significant increase in light absorbance and opacity. PVA films exhibited no antioxidant and antifungal activities, whereas PVA/ZMO films exhibited excellent antioxidant and antifungal properties. Although the bioactivity PVA films were improved by the addition of ZMO, however, the mechanical properties and water binding capacity of the films were weaken slightly. Thus, ZMO emulsified in the ethanol not compatible with PVA matrix and more suitable emulsifier was needed in order to obtain strong film with higher mechanical properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40937.     

Effects of microwave heating on dielectric and piezoelectric properties of PZT ceramic tapes

Commercial lead zirconate titanate (PZT) perovskite powders were used to fabricate ceramic tape and then sintered by microwave and conventional methods. Both dielectric and piezoelectric properties of PZT ceramic tapes were studied in terms of sintering process. X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) show the PZT perovskite phase with smaller grain size and dense microstructure can be obtained at a lower sintering temperature by microwave process. It was also observed that shrinkage ratio and bulk density of the tapes sintered at 800 °C were obtained about 19% and 7.46 g/cm³ by the microwave heating method, respectively, that is corresponding to those values of sintered PZT tapes at 950 °C by conventional process. Moreover, the dielectric constant and maximum permittivity are increased about 30% as compared with conventional processing method. The experimental results demonstrated that the characteristics of the PZT tapes could be significantly improved by microwave heating method. These results demonstrate that such a simple approach can upswing the piezoelectric and dielectric properties of these tapes by using microwave process with a short heating time.     

Enhancement of the mechanical properties and thermostability of poly(vinyl alcohol) nanofibers by the incorporation of sodium chloride

The mechanical properties and thermostability of poly(vinyl alcohol) (PVA) nanofiber mats have been obviously improved by the incorporation of sodium chloride (NaCl). The tensile properties including tensile strength and modulus of membranes with an addition of 1.0 wt % NaCl increased from 2.51 to 4.22 MPa and 33.0 to 176.30 MPa, respectively, more than 160 and 700% of those of the electrospun pure PVA membranes. Moreover, thermogravimetric analysis showed that the initial decomposition temperature (T_i) and the half decomposing temperature (T_50%) of PVA nanofibers with the addition of NaCl were at least 26 and 59 °C higher than that of pure PVA nanofibers, respectively, indicating a strong interaction between the PVA and the salt ions. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45981.     

Synergism of microwave irradiation and enzyme catalysis in synthesis of isoniazid

Isoniazid is a useful antitubercular drug widely employed in combination therapy with rifampicin. The synthesis of isoniazid from ethyl isonicotinate and hydrazine hydrate was studied in non‐aqueous media via lipase‐catalyzed hydrazinolysis under both conventional heating and microwave irradiation by using different supported lipases. Among three different commercial lipases used, namely Novozym 435 (Candida antarctica lipase), Lipozyme RM IM (Rhizomucor miehei lipase) and Lipozyme TL IM (Thermomyces lanuginosus lipase), Novozym 435 was found to be the most effective, with conversion of 54% for equimolar concentrations at 50 °C in 4 h. The rate of reaction as well as final conversion increased synergistically under microwave irradiation in comparison with conventional heating, which showed 36.4% conversion, even after 24 h, for the control experiment. Effects of various process parameters such as speed of agitation, catalyst loading, substrate concentration, product concentration and temperature were studied. A kinetic model is also described. Copyright © 2007 Society of Chemical Industry     

Characterization of PVA/glycerin hydrogels made by <Emphasis Type="Italic">γ</Emphasis>-irradiation for advanced wound dressings

The aim of this study was to investigate the enhanced absorption property of PVA/Glycerin (PVA/Gly) hydrogel for advanced wound dressing. A simple crosslinking method was introduced to prepare the PVA/Gly hydrogels with the use of γ-irradiation. An absorption ratio and thermal properties of the PVA/Gly hydrogels can be controlled by varying the irradiation dose and weight ratio of the PVA/Gly. When the PVA/Gly content was 20/5 wt% and the irradiation dose at 25 kGy, the PVA/Gly hydrogels showed excellent absorption properties (>350%). These results imply that the PVA/Gly hydrogel is highly absorbent and converts wound exudates to the hydrogel matrices that create a moist and clean environment in the wound healing process. Therefore, the PVA/Gly hydrogel prepared by this method can be used as an advanced wound dressing.     

Thermal,mechanical, and surface characterization of starch–poly(vinyl alcohol) blends and borax‐crosslinked films

Starch–poly(vinyl alcohol) (PVA) blends with different compositions were prepared and crosslinked with borax by in situ and posttreatment methods. Various amounts of glycerol and poly(ethylene glycol) with a molecular weight of 400 were added to the formulations as plasticizers. The pure starch–PVA blends and the crosslinked blends were subjected to differential scanning calorimetry, thermogravimetry, and X‐ray photoelectron spectroscopic studies. Broido and Coats–Redfern equations were used to calculate the thermal decomposition kinetic parameters. The tensile strengths and elongation percentages of the films were also evaluated. The results suggested that the glass‐transition temperature (T_g) and the melting temperature strongly depended on the plasticizer concentration. The enthalpy relaxation phenomenon was dependent on the starch content in the pure blend. The crosslinked films showed higher stability and lower T_g's than pure PVA and starch–PVA blends, respectively. High‐resolution X‐ray photoelectron spectroscopy provided a method of differentiating the presence of various carbons associated with different environments in the films. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1313–1322, 2005     

UV crosslinking of Fe3+‐doped poly(vinyl alcohol)—Characterization of optical properties and swelling behavior

The effects of UV irradiation on iron(III)chloride doped poly(vinyl alcohol) (PVA) films, using a high molar mass polymer, have been studied. It has been found that the polymer is oxidized and crosslinked during UV irradiation. UV/VIS spectra reveal an exponential loss of absorption at λ = 360 nm, and the refractive index of the PVA:FeCl₃ films decreases significantly during UV irradiation (Δn^D = ?0.09). The effects of crosslinking have been studied using the sol–gel technique, which revealed high gel contents due to doping and UV‐exposure. Photolithographic patterning of doped PVA films using a medium‐pressure mercury‐vapor UV source has been carried out, leading to good contrast behavior before and after development in aqueous media. A photobleaching effect was observed, therefore the curing of thicker films is feasible. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Development of highly conductive hybrid Ni-biocarbon-based polyvinyl alcohol composites for microwave shielding properties

In this study a highly flexible microwave shielding material was fabricated by solution casting method utilizing Nickel and biocarbon particles in PVA matrix and characterized for mechanical, magnetic, and microwave shielding properties. The main aim of this study was to prove the significant role of magnetic particles in electromagnetic interference (EMI) shielding along with conductive particles. The results show that the addition of Ni-biocarbon hybrid particle increases the shielding properties up to 56.5 dB at 20 GHz. The magnetic permeability increased gradually with the inclusion of Ni particles with a highest magnetization, coercivity, and retentivity of 1250 E⁻⁶ emu, −9000 G, and 1100 E⁻⁶ emu. Similarly the mechanical results show that adding biocarbon enhances the composite's mechanical properties. A highest tensile strength, tear strength, elongation, and hardness are noted as 38, 168 MPa, 18.4%, and 36 Shore-D. Comparatively, the hardness and elongation% of composite designations contains 3 and 5 vol% of hybrid particles have increased by 9% and 26%, respectively, in comparison to composite containing only 5 vol% of biocarbon with PVA. Scanning electron microscope fractography indicates biocarbon particles reduce voids and improve adhesion. These flexible EMI shielding composites could be used in telecommunication and other wave transmitting devices in engineering applications.     

Preparation and characterization of PAM/SA tough hydrogels reinforced by IPN technique based on covalent/ionic crosslinking

In order to fabricate tough hydrogels with superior formability, polyacrylamide/sodium alginate (PAM/SA) interpenetrating polymer network (IPN) hydrogels were produced with ionically crosslinked SA interpenetrated in covalently crosslinked PAM. TGA results show that the heat resistance of PAM/SA IPN hydrogel is improved as compared to that of the individual component. Swelling studies indicate that increasing either chemical crosslinker content or ionic crosslinking via adding more N,N′‐methylenebisacrylamide (MBA) or SA results in lower ESR. It is concluded by tensile test that loosely crosslinked PAM coupled with tightly crosslinked SA improve mechanical strength for hydrogels based on covalent/ionic crosslinking. PAM/SA hydrogels via “one‐pot” method can form different complex shapes with mechanical properties comparable to conventional double network (DN) gels. The fracture strength of PAM_0.05/SA₂₀ reaches level of MPa, approaching 2.0 MPa. The work strives to provide method to tune mechanical and physical properties for hydrogels, which is hopefully to guide the design of hydrogel material with desirable properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41342.     

Microwave‐irradiated ring‐opening polymerization of D,L‐lactide under atmosphere

Poly(D ,L ‐lactide) (PDLLA) was synthesized by microwave‐irradiated ring‐opening polymerization catalyzed by stannous octoate (Sn(Oct)₂) under atmosphere. The effects of heating medium, monomer purity, catalyst concentration, microwave irradiation time, and vacuum level were discussed. Under the appropriate conditions such as carborundum (SiC) as heating‐medium, 0.15% catalyst, lactide with purity above 99.9%, 450 W microwave power, 30 min irradiation time, and atmosphere, PDLLA with a viscosity–average molecular weight (M_η) over 2.0 × 10⁵ and a yield over 85% was obtained. The dismission of vacuum to ring‐opening polymerization of D ,L ‐lactide (DLLA) under microwave irradiation simplified the process greatly. The temperature under microwave irradiation and conventional heating was compared. The largely enhanced ring‐opening polymerization rate of DLLA under microwave irradiation was the coeffect of thermal effects and microwave effects. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2244–2247, 2006     

Microwave processing of SiC nanoparticles infused polymer composites: Comparison of thermal and mechanical properties

The goal of this study is to compare thermal and mechanical properties of an epoxy resin system reinforced with SiC nanoparticles using both conventional thermal curing and microwave irradiation techniques. The microwave curing technique has shown potential benefits in processing polymeric nanocomposites by reducing the curing time without compromising the thermo‐mechanical performances of the materials. It was observed from this investigation that, the curing time was drastically reduced to ～30 min for microwave curing instead of 12 h room temperature curing with additional 6 h post curing at 75°C. Ductile behavior was more pronounced for microwave curing technique while thermal curing showed brittle like behavior as revealed from flexural test. The maximum strain to failure was increased by 25–40% for microwave‐cured nanocomposites over the room temperature cured nanocomposites for the same loading of nanofillers. The glass transition temperature (T_g) also increased by ～14°C while curing under microwave irradiation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41708.     

Microwave Assisted Synthesis of Organic-Inorganic Polymer Hybrids

Summary Organic-inorganic polymer hybrids from various organic polymers such as poly(ethylene oxide) and poly(N-vinylpyrrolidone) were prepared with the assistance of microwave heating. With the 500 W, 2.45 GHz of microwave irradiation, the formation of polymer hybrids was accelerated. The obtained polymer hybrids were optically transparent and no phase separation was observed. From the SEM images and IR spectra of the obtained polymer hybrids, the composition and the properties of the polymer hybrids synthesized with microwave heating were found to be mostly the same as that synthesized with conventional heating.     

Biodegradable poly(vinyl alcohol)‐graft‐ poly(ε‐caprolactone) comb‐like polyester: Microwave synthesis and its characterization

Poly(vinyl alcohol)‐initiated microwave‐assisted ring opening polymerization of ε‐caprolactone in bulk was investigated, and a series of poly(vinyl alcohol)‐graft‐poly(ε‐caprolactone) (PVA‐g‐PCL) copolymers were prepared, with the degree of polymerization (DP) of PCL side chains and the degree of substitution (DS) of PVA by PCL being in the range of 3–24 and 0.35–0.89, respectively. The resultant comb‐like PVA‐g‐PCL copolymers were confirmed by means of FTIR, ¹H NMR, and viscometry measurement. The introduction of hydrophilic backbone resulted in the decrease in both melting point and crystallization property of the PVA‐g‐PCL copolymers comparing with linear PCL. With higher microwave power, the DP of PCL side chains and DS of PVA backbone were higher, and the polymerization reaction proceeded more rapidly. Both the DP and monomer conversion increased with irradiation time, while the DS increased first and then remained constant. With initiator in low concentration, the DP and DS were higher, while the monomer was converted more slowly. Microwaves dramatically improved the polymerization reaction in comparison of conventional heating method. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104, 3973–3979, 2007     

Influence of microwave sintering on electrical properties of BCTZ lead free piezoelectric ceramics

Barium titanate doped with calcium and zirconium (BCTZ) could be used at low temperature to replace lead based piezoelectric ceramics (PZT). The classical way to obtain BCTZ is the solid-state route coupled with conventional sintering, but this step is time-consuming. To reduce the duration of this process, microwave heating was used for sintering. It is a fast sintering method and the heating rate was around 200 °C/min in this study. Slightly better electrical properties with finer microstructures (d₃₃* = 706 pm/V, grain size about 42.1 ± 14.2 μm) were obtained for samples sintered by microwave heating during 50 min compared to the conventional sintering (d₃₃* = 622 pm/V, 22.6 ± 4.4 μm). The main result of this study is that by using microwave heating, the sintering step duration (including heating, dwell time and cooling) was drastically reduced: 1.5 h for microwave sintering against 12.5 h for conventional sintering.