Dynamic thermogravimetric degradation of gamma radiolytically synthesized Ag–PVA nanocomposites

The degradation kinetics of gamma radiolytically synthesized Ag–PVA nanocomposites was investigated by thermogravimetric method under dynamic conditions (30–600 °C) in an inert atmosphere. Thermogravimetric analysis showed that thermal degradation of composites was a two-stage process for the lower amount of nanofiller and single-stage for the higher amount of nanofiller. The Vyazovkin model-free kinetics method was applied to calculate the activation energy (E_a) of the degradation process as a function of conversion and temperature. At a given degradation temperature, PVA as a host in nanocomposite presents lower reaction velocity, while its E_a is higher than that of pure PVA.     

Structure and compatibility of poly(vinyl alcohol)-silk fibroin (PVA/SA) blend films

The structure and compatibility of poly(vinyl alcohol)-silk fibroin (PVA/SF) blend films were analyzed by differential scanning calorimetry (DSC), thermomechanical (TMA) and thermogravimetric (TGA) analysis, x-ray diffractometry, and scanning (SEM) and transmission (TEM) electron microscopy. DSC curves of PVA/SF blend films showed a major endothermic peak at 220°C, along with a peak at 280°C. These endotherms were assigned to the thermal decomposition of the ordered PVA elements and to the thermal degradation of silk fibroin, respectively. The PVA/SF blends behaved in a manner intermediate to the pure components, as suggested by both contraction expansion and sample weight retention properties recorded by TMA and TGA measurements. The IR absorption spectra of the blends were identified as purely a composite of the absorption bands characteristic of both PVA and SF pure polymers. The X-ray diffraction patterns of PVA/SF blends showed overlapping spacing due to PVA and SF. A dispersed phase formed by spherical particles of 3–7 μm diameter was observed by SEM and TEM. All these findings suggest that PVA and SF are incompatible. © 1994 John Wiley & Sons, Inc.     

Thermal properties of pure and doped (polyvinyl-alcohol) PVA

Films ≈350 μm of poly(vinyl-alcohol) composites, containing copper (Cu), aluminium (Al) and iron (Fe), metallic powder very fine, were prepared by a casting method. Thermal conductivity, phonon velocity, mean free path and specific heat were studied. The pure sample of PVA has a lower values of thermal conductivity than that which are doped with metals. For all samples the thermal conductivityK increases up to a certain temperatureT _gg (120–160°C) and then decreases with temperature. The specific heat increase with temperature up to ≈120°C and above 120°C is nearly independent on temperature. The pure sample of PVA has small values of mean free path (L)≈0.2 Å at room temperature, but for PVA⁺ metalsL≈2.0 Å. The phonon velocity of pure PVA is larger than that of PVA containing metals.     

Enhanced thermal stability of poly(vinyl alcohol) in presence of melanin

A novel method was developed to enhance the thermal stability of PVA by using natural and synthetic melanins from oxidation of dopamine. Thermogravimetric (TG) curves indicated that the synthetic melanin changed the thermal degradation behaviors of PVA and largely improved the decomposed temperature by 80–110 °C in nitrogen when incorporation of synthetic melanin with low content (0.5–2 mass%). The thermal degradation kinetics suggested the activation energies of PVA/synthetic melanin blends were much higher than these of pure PVA. Isothermal TG curves conformed that the PVA/synthetic melanin blends exhibited more thermal stability than pure PVA. Moreover, the chemical structure changes of macromolecular after degradation were characterized by using fourier transform infrared and the results suggested that elimination reaction on the first degradation step did not took place for the PVA/synthetic melanin blends at 270 °C.     

Synthesis of nanosized zinc and magnesium chromites starting from PVA–metal nitrate solutions

In this article, we present a detailed study regarding the preparation of nanosized zinc and magnesium chromites starting from a 4% poly(vinyl)alcohol (PVA) aqueous solution and metal nitrates. The controlled thermal treatment of these solutions has permitted the isolation of an intermediary solid product, used as precursor of the preferred mixed oxides: zinc and magnesium chromites. The as-obtained precursors were characterized by FT-IR spectrometry and thermal analysis. FT-IR spectrometry has evidenced the disappearance of the NO₃ ^? anions at 140?°C, due to the redox interaction with PVA. The thermal decompositions of the synthesized precursors were different, as resulted from both thermal analysis and FT-IR spectrometry. Thus, while ZnCrPVA precursor decomposes up to 400?°C with formation of zinc chromite, the precursor MgCrPVA decomposes up to 500?°C, with formation of MgCrO₄ as intermediary amorphous phase. By thermal decomposition of MgCrO₄ at 500?°C, weakly crystallized MgCr₂O₄ powder is obtained. The obtained chromite powders consist of fine nanoparticles with diameters ranging from 10 to 30?nm at 500?°C; on raising the annealing temperature to 1000?°C, chromite particles become octahedral, with diameter up to 500?nm, but with no sign of sintering.     

Facile synthesis of LDH nanoplates as reinforcing agents in PVA nanocomposites

In the present study, layered double hydroxide (LDH) nanoplates with high crystallinity and uniform size were facilely synthesized to act as reinforcing agents in polymer materials. The structure of the synthesized LDH nanoplates was characterized by X‐ray diffraction, Fourier transform infrared spectra, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy measurements. Subsequently, the LDH nanoplates were incorporated into poly(vinyl alcohol) (PVA) matrix as reinforcing agents based on a solution casting method. The LDH nanoplates were well dispersed in PVA matrix and formed strong interfacial interactions with PVA chains, leading to remarkable improvements of thermal stability, flame retardancy, and mechanical properties. With the incorporation of 1 wt% LDH nanoplates into PVA, the T_onset and T_50% increased by 11°C and 57°C, respectively. Moreover, the presence of LDH nanoplates decreases the decomposition rates of PVA and increases the amount of char residues. Compared with pure PVA, the peak heat release rate value of the PVA/5 wt% LDH nanocomposites is decreased by 52%. The tensile strength and the elongation at break increased by 71% and 187%, respectively, when incorporating with 3 wt% LDH nanoplates. Copyright © 2016 John Wiley & Sons, Ltd.     

Resistance of polyvinyl alcohol blends stabilized by sodium and ammonium salts of lignite humic acids against γ-irradiation

The dried blends containing sodium and ammonium salts of lignite humic acids (humates, 0.5–10% w/w) in polyvinyl alcohol (PVA) were exposed to high dosage of γ-irradiation in the range of doses 127–806 kGy. Resulted products were then tested for their stability using thermogravimetrical analysis. As a reference the non-treated blends were used since the pure PVA exposed to γ-irradiation very quickly lost its stability and resulting consistence did not allow the stability tests. Stabilities showed a strong concentration and counterion dependency. While sodium counterion caused mostly destabilization with increasing dose, the ammonium counterion acted in an opposite way. The tests carried out in a moisturizing container revealed the changes in water absorbing capacity of irradiated samples and allowed partial explanation of humate stabilizing effect. Generally, at lower concentration of a humate the increase was observed with an increase in the γ-irradiation dose and vice versa. The results confirmed the antioxidant and stabilizing effect of humic acids added to some synthetic polymers and their applicability in materials exposed to γ-irradiation.     

Dependence of structure of dehydrated PVA on the crystallinity of the original PVA

Poly(vinyl alcohol) (PVA) films having different crystallinities were prepared by elongating PVA films to different degrees and heating the films with and without elongation treatment at several temperatures between 60 and 200°C. Then, they were dehydrated by heating from 80°C to 230 or 330°C in hydrogen chloride gas. Infrared spectral measurements were made on the dehydrated PVA films obtained. Absorbances of the absorption bands due to several groups seen in the infrared spectra depended only on the density of original PVA films, regardless of its degree of elongation. From these dependences, it was found that the dehydration reaction progressed more easily in the crystalline region than in the amorphous region, that the aromatic rings with four or five adjacent hydrogen atoms were formed mainly in the amorphous region and those with two adjacent hydrogen atoms in the crystalline region, and that the aromatic rings with two isolated hydrogen atoms were formed in both the amorphous and crystalline regions. Relative concentrations of the groups of which the dehydrated PVA were composed were estimated. Comparison of the numbers of carbon atoms among the composing groups indicated that the main groups were the methylene group and the aromatic ring.     

聚乙烯醇与溶菌酶的相互作用及其对溶菌酶构象的影响

在生理条件下, 使用凝胶过滤色谱、荧光光谱法、差示扫描量热分析和傅里叶变换红外光谱法(FT-IR)研究了溶菌酶与聚乙烯醇(PVA)的相互作用. 结果表明PVA与溶菌酶结合形成复合物, 在它们的相互作用过程中, 溶菌酶酪氨酸的发射荧光部分被猝灭, 但是, 相互作用并没有改变酪氨酸的微环境; 差示扫描量热分析结果表明, 溶菌酶与PVA之间的相互作用没有破坏溶菌酶的高级结构; 进一步使用红外光谱法结合可增强分辨率的傅里叶去卷积技术和高斯曲线拟合技术共同用于对溶菌酶与PVA复合物冻干粉中溶菌酶酰胺I带的定量分析, 发现冻干粉溶菌酶分子中与分子间相互作用相关的β-折叠组分含量减少了, 但是, 用于衡量冻干状态蛋白质结构完整性的α-螺旋组分含量没有降低. 活性分析结果进一步确认, PVA与溶菌酶的相互作用没有破坏溶菌酶的三级结构.     

The kinetics of thermo-oxidative humic acids degradation studied by isoconversional methods

Humic acids represent a complicated mixture of miscellaneous molecules formed as a product of mostly microbial degradation of dead plant tissues and animal bodies. In this work, lignite humic acids were enriched by model compounds and the model-free method suggested by Šimon was used to evaluate their stability over the whole range of conversions during the first thermooxidative degradation step. The kinetic parameters obtained were used to predict the stability at 20 and 180°C, respectively, which served for the recognition of processes induced by heat and those naturally occurring at lower temperatures. Comparison of the conversion times brought a partial insight into the kinetics and consequently into the role of individual compounds in the thermooxidative degradation/stability of the secondary structure of humic acids. It has been demonstrated that aromatic compounds added to humic acids, except pyridine, increased stability of humic acids and intermediate chars. The same conclusion can be drawn for acetic and palmitic acids. Addition of glucose or ethanol decreased the overall humic stability; however, the char of the former showed the highest stability after 40% of degradation.     

PREPARATION AND CHARACTERIZATION OF PVA COATED MAGNETIC NANOPARTICLES*

Polyvinyl alcohol coated magnetic particles (PVA ferrofluids) have been synthesized by chemical co-precipitationof Fe(Ⅱ)/Fe(Ⅲ) salts in 1.5 mol/L NH_4OH solution at 70℃ in the presence of PVA. The resultant colloidal particles havecore-shell structures, in which the iron oxide crystallites form the cores and PVA chains form the shells. The hydrodynamicdiameter of the colloidal particles is in the range of 108 to 155 nm, which increases with increasing PVA concentration from5 wt% to 20 wt%. The size of the magnetic cores is ca. 5～10 nm, which is relatively independent of PVA concentration.Under transmission electron microscopic (TEM) examination, the magnetic cores exhibit somewhat irregular shapes varyingfrom spherical, oval, to cubic. Magnetometry measurement revealed that the PVA coated magnetic particles aresuperparamagnetic. The saturation magnetization of 5 wt% and 20 wt% PVA ferrofluids at 300 K is 54 and 49 emu/g.respectively. All the PVA ferrofluids exhibited excellent colloidal stability in pure water and phosphate buffer saline (PBS,pH=7.4). The ferrofluids can remain stable in above solutions for more than three months at 4℃.     

Manipulating the degradation rate of PVA nanoparticles by a novel chemical‐free method

The high water solubility of poly (vinyl alcohol) (PVA) is one of the challenging problems in its application. In order to rectify this problem, PVA needs to be crosslinked. Freeze‐thawing in solid state as a novel physical crosslinking method was employed for enhancement the stability of PVA nanoparticles in aqueous solutions during this study. PVA nanoparticles were successfully prepared by electrospraying and electrospray conditions were optimized in the view points of polymer concentration and solvent system. The morphology of nanoparticles was tailored from collapsed particles and mixture of particles/fibers to spherical particle by manipulating of polymer solution concentration and solvent system. After preparation of PVA nanoparticles in optimum condition, they were frozen at ?20°C and subsequently thawed at 25°C for different cycles of 1, 2, and 3. Field‐emission scanning electron microscope (FE‐SEM), Fourier‐transform infrared (FTIR), X‐ray diffraction (XRD), differential scanning calorimeter (DSC), and biodegradation were used to evaluate the effect of freeze‐thawing on properties of PVA nanoparticles. FE‐SEM showed the spherical morphology of the PVA nanoparticles with sizes ranging from 200 to 300 nm. The FTIR spectroscopy indicated that the crystallinity of PVA nanoparticles increases after freeze‐thawing process. Moreover, by increasing the number of cycles, degree of crystallinity of nanoparticles increases. The XRD and DSC analysis of PVA nanoparticles again demonstrated the increasing of crystallinity of nanoparticles after freeze‐thawing process. The biodegradation behavior of PVA nanoparticles after freeze‐thawing exhibited the decreasing of degradation rate by increasing the number of cycles. Our overall results present a solvent‐less and safe method for crosslinking of PVA nanoparticles in solid state, which make it suitable for biomedical applications.     

Studies on the preparation of hydrolyzed starch-g-PAN (HSPAN)/PVA blend films--Effect of the reaction with epichlorohydrin

Absorbent hydrolyzed starch-g-PAN (HSPAN)/polyvinylalcohol (PVA) blend films crosslinked with epichlorohydrin (ECH) were prepared to overcome the phase separation and improve the mechanical properties of blend films. The absorbency of HSPAN/PVA blend films decreased with PVA contents due to the reduction of HSPAN contents which is known to play a major role in absorbing ability of the film. And also the absorbency decreased with the ECH contents due to the crosslinking reaction. As far as the solubility is concerned, it increased with PVA contents which is water soluble. But because of the crosslinking reaction between HSPAN and PVA by ECH, the solubility decreased with ECH contents. In the mechanical properties, the strength as well as the strain at break of the HSPAN/PVA blend films were improved very much if compared with HSPAN film, and those mechanical properties were improved even more by the reaction with ECH. The DSC thermograms of HSPAN films showed two major endothermic peaks at 32 and 128 °C, while those of PVA film showed two major endothermic peaks at 49 and 190 °C. In the non-crosslinked blend films, each four endothermic peak was apparent. But as the ECH content increased, both peaks at 128 and 190 °C disappeared and a new peak appeared at the intermediate temperature. In other words, the compatibility of blend films was increased by the reaction with ECH. Also, from the results of TGA, it was confirmed that the thermal degradation of blend films was decelerated by the crosslinking reaction.     

Silver Nanoparticle-Anchored Human Hair Kerateine/PEO/PVA Nanofibers for Antibacterial Application and Cell Proliferation

The main core of wound treatment is cell growth and anti-infection. To accelerate the proliferation of fibroblasts in the wound and prevent wound infections, various strategies have been tried. It remains a challenge to obtain good cell proliferation and antibacterial effects. Here, human hair kerateine (HHK)/poly(ethylene oxide) (PEO)/poly(vinyl alcohol) (PVA) nanofibers were prepared using cysteine-rich HHK, and then, silver nanoparticles (AgNPs) were in situ anchored in the sulfur-containing amino acid residues of HHK. After the ultrasonic degradation test, HHK/PEO/PVA nanofibrous mats treated with 0.005-M silver nitrate were selected due to their relatively complete structures. It was observed by TEM-EDS that the sulfur-containing amino acids in HHK were the main anchor points of AgNPs. The results of FTIR, XRD and the thermal analysis suggested that the hydrogen bonds between PEO and PVA were broken by HHK and, further, by AgNPs. AgNPs could act as a catalyst to promote the thermal degradation reaction of PVA, PEO and HHK, which was beneficial for silver recycling and medical waste treatment. The antibacterial properties of AgNP-HHK/PEO/PVA nanofibers were examined by the disk diffusion method, and it was observed that they had potential antibacterial capability against Gram-positive bacteria, Gram-negative bacteria and fungi. In addition, HHK in the nanofibrous mats significantly improved the cell proliferation of NIH3T3 cells. These results illustrated that the AgNP-HHK/PEO/PVA nanofibrous mats exhibited excellent antibacterial activity and the ability to promote the proliferation of fibroblasts, reaching our target applications.     

Influence of the nanoparticle type on the thermal decomposition of the green starch/poly(vinyl alcohol)/montmorillonite nanocomposites

In this study, some aspects concerning the thermal decomposition of starch/poly(vinyl alcohol) (PVA)/montmorillonite (MMT) nanocomposites with 2 wt% nanoclay, prepared by melt mixing method, were studied. For these loadings, the inorganic fillers are well dispersed through the PVA/starch matrix, i.e., the nanocomposites formed are mostly intercalated hybrids. The aim of this article is to establish the effect of the nanofiller nature on the thermal decomposition of the starch/PVA/MMT nanocomposites. The thermal behavior of the 50 wt% starch/50 wt% PVA blend and its nanocomposites with 2 wt% nanoclay has been investigated by thermogravimetric analysis coupled with Fourier transform-infrared spectroscopy and mass spectrometry (MS). The volatile compounds resulting during the thermal degradation were studied by in situ vapor phase FT-IR spectroscopy and MS technique under a controlled temperature/time program. Apart from the identification of the volatile compounds, some conclusions on the nanoclays effect on the degradation mechanism and formation of the volatile compounds in accordance with the previously developed general mechanisms for PVA and starch degradation have been formulated. The clay–PVA/starch nanocomposites show completely different degradation product distribution patterns, which may be attributed to the presence of the head-to-head structures and Si–O–C linkages formed between clay and blend components.     

MISCIBILITY, THERMAL STABILITY AND RETENTION OF PVP FOR CROSSLINKED PVA/PVP BLENDS

The thermal behavior, miscibility, crystallite conformation and thermal stability ofcrosslinked(CL-) PVA/PVP blends were studied by DSC and TG methods, respectively. DSCresults showed that in the blend, the crystallinity,T_m and T_c of PVA were obviously lower thanthose of pure PVA; the crystal growth changed from three dimensional to two dimensional andonly a single T_g was detected. These facts demonstrated that this crystalline and amorphousblend have good miscibility. TG curves showed that providing the quantity of K_2S_2O_8 added ismore than 3 wt%,in the blends PVA will form a stable CL-network, whose thermal degradationtemperature was near to that of PVP. But crosslinking reaction will not take place for PVP. Theprocesses of thermal degradation of CL-blends are based on combining both the thermaldegradation of PVP and that of PVA crosslinked with corresponding quantity of K_2S_2O_8 CL-agent, respectively. The UV measurements showed that 75 wt% of PVP may be remained in CL-blend hydrogelscrosslinked by adding (3--5 wt% )K_2S_2O_8. This is mainly due to the stable CL-network formed and the good compatibility and properentanglement between the composites in the CL-blends.     

Preparation of polypyrrole/polyvinylalcohol (PPy/PVA) composite foam electrode material

<正>Polypyrrole/polyvinylalcohol(PPy/PVA) foam was prepared by direct foam polymerization in water and then it was coated on the indium-tin oxide transparent conductive glass(ITO) to form conventional three-electrode cell.FTIR and UV-vis spectra were adopted to characterize the molecular structure and the absorption spectra of foam material,respectively.The porous structure of PPy/PVA foams and their photoelectric conversion behaviors were studied.The dimension of the pores is bigger than 100μm in diameter.Compared with the smooth film,the V_(oc) and I_(sc) of the foam film enhanced by 1.58-fold and 5.59-fold,respectively.     

A thermal degradation mechanism of polyvinyl alcohol/silica nanocomposites

The thermal degradation mechanism of a novel polyvinyl alcohol/silica (PVA/SiO₂) nanocomposite prepared with self-assembly and solution-compounding techniques is presented. Due to the presence of SiO₂ nanoparticles, the thermal degradation of the nanocomposite, compared to that of pure PVA, occurs at higher temperatures, requires more reaction activation energy (E), and possesses higher reaction order (n). The PVA/SiO₂ nanocomposite, similar to the pure PVA, thermally degrades as a two-step-degradation in the temperature ranges of 300-450 °C and 450-550 °C, respectively. However, the introduction of SiO₂ nanoparticles leads to a remarkable change in the degradation mechanism. The degradation products identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA) and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) suggests that the first degradation step of the nanocomposite mainly involves the elimination reactions of H₂O and residual acetate groups as well as quite a few chain-scission reactions. The second degradation step is dominated by chain-scission reactions and cyclization reactions, and continual elimination of residual acetate groups is also found in this step.     

Enzymatic degradation of some nanocomposites of poly(vinyl alcohol) with starch

A study of the enzymatic degradation of some montmorillonite-containing nanocomposites of poly(vinyl alcohol) with starch was based on the determinations of mass loss and the reducing sugars. The degraded residues have been examined by FT-IR spectroscopy and optical microscopy. It has been established that the nanoparticles hinder degradation, while the susceptibility to enzymatic degradation varies in the order: PVA/starch/nanocore > PVA/starch/Bentonite > PVA/starch/Peruvian clay.     

Comparative study for differentiation of aquatic humic-type organic constituents by capillary zone electrophoresis using polyvinyl alcohol-coated capillary

Capillary zone electrophoresis (CZE) with UV detection (254 nm) was applied to characterize aquatic dissolved humic matter (DHM) from different environmental sources (lake, river and sea waters, in all 15 different samples). A series of separation examples of DHMs using a polyvinyl alcohol (PVA)-coated silica open tubular capillary were carried out in a phosphate buffer (40 mM) as a background electrolyte at neutral acidity (pH 6.8). The separative power of electropherograms was reasonable and the reproducibility was above the mark. Each electropherogram was characteristic of the corresponding humic sample. Special functional fulvic and humic acids or their overall mixtures separated with XAD, DAX and DEAE sorbing solids as well as the original dissolved organic matter (DOM) were nicely differentiated according to their environmental sources. The PVA coating of open tubular silica capillaries seems to be very potential in electrophoretic characterization and separation of different humic solutes at neutral acidities with low sample concentrations thus permitting a workable technique, in a growing series of CZE studies, for better compared results from different studies.