Effect of interfacial pretreatment on the properties of montmorillonite/poly(vinyl alcohol) nanocomposites

This work explores the factors that control the dispersion of exfoliated montmorillonite (MMT) in poly(vinyl alcohol) (PVOH) during solution blending and solvent evaporation. Nanocomposite films were prepared by solution blending of aqueous PVOH solutions with dilute suspensions of fully exfoliated MMT platelets (as confirmed by AFM). Dynamic light scattering (DLS) indicates that addition of MMT suspensions to PVOH solutions results in undesired particle aggregation and thus poor MMT dispersion in cast films (as evidenced by transmission electron microscopic images and gas permeation measurements). We believe that PVOH bridging induces MMT platelet aggregation. To counteract bridging aggregation, we explore the novel idea of pretreating the MMT surface with a small amount of compatible polymer prior to solution blending with PVOH. We hypothesize that “pretreating” the MMT platelet surfaces with adsorbed polymer in dilute suspensions will protect the platelets from bridging aggregation during solution blending and solvent evaporation. MMT/PVOH composite films have been prepared using low‐molecular‐weight PVOH as the pretreatment polymer; and low‐, medium‐, and high‐molecular‐weight PVOH as the matrix polymer. A PEO‐PPO‐PEO triblock copolymer (F108 from the Pluronics^® family) was also evaluated as the pretreatment polymer. DLS shows that pretreated MMT platelets are less susceptible to aggregation during blending with PVOH solutions. Results compare the crystalline structure, thermal properties, dynamic mechanical properties, gas permeability, and dissolution behavior of MMT/PVOH films incorporating untreated versus pretreated MMT. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41867.     

SBS/蒙脱土纳米复合材料耐热性能研究

采用溶液混合方法制备了苯乙烯/丁二烯三元嵌段共聚物（SBS）/有机蒙脱土（OMMT）纳米复合材料（NC）,对SBS/OMMT NC的热学性能进行了研究。结果表明,添加OMMT对于所有复合体系均提高了耐热性能,其中以SBS4402/OMMT-DK1B体系最为明显,热失重中心温度Tdc提高幅度达27℃;当OMMT用量为2.5质量份时,NC的Tdc最高,表明耐热性能最好,超过2.5质量份后,耐热性能下降;星型结构SBS耐热性能的提高明显高于线型SBS。     

Electron beam synthesis and characterization of poly(vinyl alcohol)/montmorillonite nanocomposites

Poly(vinyl alcohol) (PVA)/montmorillonite clay (MMT) nanocomposites in the form of films were prepared under the effect of electron beam irradiation. The PVA/MMT nanocomposites gels were characterized by X‐ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and mechanical measurements. The study showed that the appropriate dose of electron beam irradiation to achieve homogeneous nanocomposites films and highest gel formation was 20 kGy. The introduction of MMT (up to 4 wt %) results in improvement in tensile strength, elongation at break, and thermal stability of the PVA matrix. In addition, the intercalation of PVA with the MMT clay leads to an impressive improved water resistance, indicating that the clay is well dispersed within the polymer matrix. Meanwhile, it was proved that the intercalation has no effect on the metal uptake capability of PVA as determined by a method based on the color measurements. XRD patterns and SEM micrographs suggest the coexistence of exfoliated intercalated MMT layers over the studied MMT contents. The DSC thermograms showed clearly that the intercalation of PVA polymer with these levels of MMT has no influence on the melting transitions; however, the glass transition temperature (T_g) for PVA was completely disappeared, even at low levels of MMT clay. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1129–1138, 2006     

The investigation of thermal decomposition pathway and products of poly(vinyl alcohol) by TG‐FTIR

A generalized form of a semiquantitative method has been developed based on the multilinear least‐squares regression technique applied on the entire FTIR absorbance spectrum of a gaseous mixture to determine components concentration. Thermal degradation of poly(vinyl alcohol) samples with high, PVA(98), and low degree of hydrolysis, PVA(80), has been investigated by TG‐FTIR simultaneous analysis performed in an inert atmosphere. Analysis of gaseous products was carried out using a routine developed in Matlab and this routine returns the product concentration with a reasonable RMS error. The correlation coefficients of the original mixture spectrum with the mixed output were obtained at some specific peak temperatures using irAnalysis software. The first process is the loss of physically adsorbed water which followed by two main processes of thermal degradation. In spite of the similarity of evolved gaseous products, two samples showed some differences in components concentrations identified in the volatile mixture. Acetaldehyde has been identified as the main volatile product in the first thermal degradation step of PVA(98) and PVA(80). The second major degradation product of PVA(80) is acetic acid due to presence of more residual acetate group while 2‐butenal have been identified for PVA(98). Water was mainly produced in the first stage of thermal degradation of PVA(98) while it was identified in the first and second stages for PVA(80). This might be attributed to existence of a competition between water and residual acetate group for elimination that postpones the complete elimination of OH group to the second degradation stage. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42117.     

Preparation of UV reactive montmorillonite and characterization of its nanocomposites with poly(vinyl alcohol)

A cation‐exchanged montmorillonite clay is prepared as filler material with high dispersibility, that should act as Fe³⁺‐donor for photochemical crosslinking of PVA/Fe³⁺‐MMT materials with filler contents of up to 50 wt % with regard to the polymer matrix. Hence, no organic photoinitiators and hazardous compounds are utilized. This material may be considered environmentally benign and could be applied in the field of food packaging or for biomedical applications. Upon UV exposure of liquid PVA/Fe³⁺‐MMT dispersion samples, a significant change of the refractive index is determined and the absorption at 360 nm decreased. This indicates the transition of Fe³⁺ to Fe²⁺, which initiates a radical crosslinking mechanism. A homogenous distribution and parallel orientation of the modified clay particles is revealed by SEM measurements. The solubility behavior of the PVA/Fe³⁺‐MMT composite materials in deionized water, changes considerably due to the high filler content and UV‐induced crosslinking, resulting in gel contents exceeding 90 wt %. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermal decomposition characteristics of poly(ether imide) by TG/MS

Thermal degradation of poly(ether imide) (PEI) was studied by the combination of pyrolysis/gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetric analysis/mass spectrometry (TG/MS) techniques. The composition of evolved gases was determined by Py-GC/MS and the real-time formation curves were obtained through TG/MS. The thermal degradation mechanisms of PEI were resolved through TG/MS methods. The major pyrolysis mechanisms with the two-stage reaction regions were main chain random scission and carbonization. In the first stage pyrolysis, the decomposition of the hydrolyzed-imide, ether and isopropylene groups caused the evolution of CO₂ and phenol as major products accompanied by a chain transfer of carbonization to form partially carbonized solid residue. In the second stage pyrolysis, the decomposition of partially carbonized solid residue and the remaining imide group produced CO₂ as a major product along with benzene and small a amount of benzonitrile. Afterward, the chain transfer of carbonization dominated the decomposition of solid residue in higher temperatures to produce a high char yield. A kinetic model was proposed from the calculation of two flat regions in the activation energy curve. The theoretical pyrolysis curve from the proposed model was calculated and compared with the experimental curve, which were quite well matched.     

Preparation and characterization of graphene/poly(vinyl alcohol) nanocomposites

Graphene (GE)‐based nanocomposites are emerging as a new class of materials that hold promise for many applications. In this article, we present a general approach for the preparation of GE/poly(vinyl alcohol) (PVA) nanocomposites. The basic strategy involved the preparation of graphite oxide from graphite, complete exfoliation of graphite oxide into graphene oxide sheets, followed by reduction to GE nanosheets, and finally, the preparation of the GE/PVA nanocomposites by a simple solution‐mixing method. The synthesized products were characterized by X‐ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry, and differential scanning calorimetry analysis. The GE nanosheets were well dispersed in the PVA matrix, and the restacking of the GE sheets was effectively prevented. Because of the strong interfacial interaction between PVA and GE, which mainly resulted from the hydrogen‐bond interaction, together with the improvement in the PVA crystallinity, the mechanical properties and thermal stability of the nanocomposites were obviously improved. The tensile strength was increased from 23 MPa for PVA to 49.5 MPa for the nanocomposite with a 3.25 wt % GE loading. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effects of water content on the properties of starch/poly(ethylene–vinyl alcohol) blends

The physical properties of unmodified starch, poly(ethylene vinyl alcohol), glycerol, and water mixtures are reported. Thermal and melt-flow properties of the preprocessed, physically mixed materials were determined along with the tensile properties and morphology of injection-molded microtensile samples. Melt-flow properties were measured by a capillary rheometer, and the water content was varied from 4 to 18%. The morphology, rheology, and tensile properties are all highly related to the percentage of water present. A transition in the tensile properties and morphology of the blends was observed at approximately 11% moisture content. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Citric acid crosslinking of poly(vinyl alcohol)/starch/graphene nanocomposites for superior properties

Polyvinyl(alcohol)/starch/graphene nanocomposites with enhanced properties were prepared by solution mixing and casting process with the aid of glycerol as plasticizer and citric acid (CA) as crosslinker. The dispersion of graphene in water was made by sonication prior to mixing it with PVA/starch solution. The effect of varying the concentration of CA crosslinker in PVA/starch nanocomposite with 0.5 wt% of graphene was studied in detail. The structural changes, properties and morphologies were characterized by different techniques. The FTIR results revealed that the crosslinking reaction enhanced the interaction between the hydroxyl groups in PVA and/or starch and the oxygen-containing groups present on the graphene sheets. The mechanical properties were also improved by the crosslinking reaction and reinforcing with graphene. The formation of PVA crystal from solution was interrupted to a large extent by the interface at the amorphous zone of polymers and also the crosslinks between the PVA and starch polymer chains. The total crystallinity of the system was found to decrease with increase in degree of crosslinking. There was a marked increase in the thermal stability as the blend system was crosslinked with CA. CA crosslinking produced compact bulk morphology and improved the homogeneity between PVA and starch. The results of this study illustrate that citric acid can be an effective crosslinker and/or compatibilizer in PVA/starch/graphene nanocomposites for improving properties, and for this reason it is a candidate to replace non-biodegradable plastic films in food packaging sector.     

Revisiting the thermal relaxations of poly(vinyl alcohol)

The molecular dynamics of poly(vinyl alcohol) (PVA) were studied by dielectric spectroscopy and dynamic mechanical analysis in the 20–300°C range. The well-established plasticizing effect of water on the glass-transition temperature (T_g) of PVA was revisited. Improper water elimination analysis has led to a misinterpretation of thermal relaxations in PVA such that a depressed T_g for wet PVA films (ca. 40°C) has been assigned as a secondary β relaxation in a number of previous studies in the literature. In wet PVA samples, two different Vogel–Fulcher–Tammann behaviors separated by the moisture evaporation region (from 80 to 120°C) are observed in the low- (from 20 to 80°C) and high- (>120°C) temperature ranges. Previously, these two regions were erroneously assigned to two Arrhenius-type relaxations. However, once the moisture was properly eliminated, a single non-Arrhenius α relaxation was clearly observed. X-ray diffraction analysis revealed that the crystalline volume fraction was almost constant up to 80°C. However, the crystallinity increased approximately 11% when temperature increased to 180°C. A secondary β_c relaxation was observed at 140°C and was related to a change in the crystalline volume fraction, as previously reported. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

In situ FTIR spectroscopy study on the rapid dissolution process of modified poly(vinyl alcohol)

In this article, modified poly(vinyl alcohol) (m-PVA) has been synthesized via copolymerization of vinyl acetate in methanol by adding comonomers acrylic acid and butyl acrylate followed by saponification. This modified poly(vinyl alcohol) can be completely dissolved into water before 60 °C without agitation. The rapid dissolution process of m-PVA was investigated by temperature-dependent Fourier transform infrared (FTIR) spectroscopy from 20 °C to 85 °C. The results showed that the wavenumber ranges of m-PVA in the O-H stretching and bending bands beyond the measuring range of the spectrometer were much larger than those of common PVA. This demonstrated that m-PVA chains could interact with water molecules more strongly and form hydrogen bonds with the hydroxyl groups of water more easily. The introduction of carboxyl groups destroyed the chemical regularity of polymer chains, which led to the decreasing crystallinity degree and imperfect crystal structures. Both the lower crystallinity degree and imperfect crystal structures were crucial factors to the rapid dissolution of m-PVA. Moreover, traditional Fourier transform infrared (FTIR) spectra, ¹H nuclear magnetic resonance (¹H NMR), and differential scanning calorimetry (DSC) were also applied to study the m-PVA in this paper.     

Oxidative thermal decomposition of poly(vinyl chloride) compositions

“Pure” poly(vinyl chloride) resin and four compositions containing poly(vinyl chloride) were subjected to oxidative thermal degradation in air at &400°C both in a quiescent and a flow system. The volatiles formed were identified and quantitatively determined on a gram-per-gram basis. Hydrogen chloride was the main product found. The nature and relative concentration of the produced organic chlorinated species appeared to be dependent not only on the poly(vinyl chloride) constituent but also on the other ingredients. All the compositions contained phthalate ester plasticizers. In the dynamic system, these distilled largely unchanged, whereas under static conditions transformation into phthalic anhydride occurred.     

Studies on the thermal analysis of modified poly(vinyl alcohol)

Poly (vinyl alcohol) (PVA) has been grafted with different monomers like acrylonitrile, methyl methacrylate and acrylamide. Also it has been modified with formaldehyde and benzaldehyde to obtain partially formylated and benzylated poly (vinyl alcohol) samples. These modified PVA samples were investigated by means of differential thermal analysis. It was shown that samples with a low degree of modification or moderate grafting retain the crystalline nature of PVA. The melting temperatures are affected differently. In the modified PVA samples the decomposition endotherms are shifted to higher temperatures and are reduced in intensity. This indicates an improved thermal stability of those samples.     

Fabrication and properties of clay-supported carbon nanotube/poly (vinyl alcohol) nanocomposites

Clay-supported carbon nanotubes (Cs-CNTs) were used as novel nanofillers to improve the thermal and mechanical properties of a polymer. Cs-CNT/poly (vinyl alcohol) (PVA) nanocomposite films were successfully fabricated, and their relative properties were investigated by using differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. Experimental results showed that the thermal stability and dynamic mechanical properties of PVA were remarkably enhanced by incorporating the Cs-CNTs into PVA matrix. The largest T_g difference of 14°C was obtained between pure PVA and PVA nanocomposite with 7 wt% Cs-CNTs. Moreover, the storage modulus of PVA was significantly improved by 133% at 50°C, when 7 wt% Cs-CNTs was added to PVA matrix. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

The influence of poly(vinyl alcohol) suspending agents on suspension poly(vinyl chloride) morphology

The requirements for PVC suspension resin have changed considerably in the last few years, so much so that few companies have products on their ranges that are more than 4 or 5 years old. The suspending agent has a crucial influence on the morphology of the resin, so the changes in resin characteristics have largely been achieved by changes in the suspending agent systems. After a brief review of the mechanism of PVC suspension polymerisation, the properties of polymers made using PVOH suspending agents are related to changes in the latter. The effect of variations in PVAc degree of hydrolysis and viscosity are related to changes in surface tension. Methods of achieving higher porosity by using low hydrolysis co-suspending agents are described. It is shown that higher bulk densities can be achieved by delayed addition of the PVOH. Levels of conjugated unsaturation and copolymer distributions are also shown to have important influences.     

Extremely oriented tunicin whiskers in poly(vinyl alcohol) nanocomposites

In order to utilize the excellent mechanical properties of cellulose whiskers (CWs) along their length, the present work was undertaken to embed CWs with highly oriented forms in a polymer matrix. Nanocomposite fibers were prepared using poly(vinyl alcohol) (PVA; degree of polymerization of 1500) as the matrix and a stable aqueous suspension of CWs extracted from tunicates as the reinforcing phase. Macroscopically homogeneous suspensions of PVA–CW were gel‐spun in a methanol coagulating bath. The as‐spun fibers included CWs oriented along the fiber axis and showed a significant increase in dynamic storage modulus. Hot drawing of the PVA–CW as‐spun fibers to their maximal draw ratio led to extremely high orientation of the CWs together with a drastic reduction in voids in the fiber matrix. Outstanding mechanical properties of the drawn composites were obtained by the incorporation of only a small amount (1 wt% of solid PVA content) of CWs. The stress transfer mechanism in the fibers was studied using an X‐ray diffraction technique by applying stress to the whole composite with in situ monitoring of stress on the incorporated CWs. The applied external stress was found to be translated efficiently to the incorporated CWs through the PVA matrix, suggesting strong interfacial bonding between filler and matrix. The strong interaction and efficient stress transfer between matrix and filler are suggested as the cause for the observed improvements in mechanical properties of the composites. Copyright © 2011 Society of Chemical Industry     

Preparation and crystallization behavior of multiwalled carbon nanotubes/poly(vinyl alcohol) nanocomposites

The poly(vinyl alcohol) (PVA)‐based nanocomposites embedded with modified multiwalled carbon nanotubes (MWCNTs) were prepared. To enhance the interfacial interaction between MWCNTs and PVA, acid‐treated MWCNTs were grafted with PVA chains, compatibilizing MWCNTs and the matrix. The better dispersion of MWCNTs in PVA matrix was obtained by the introduction of MDI reaction bridges and then PVA molecules onto the surface of MWCNTs. Moreover, strong interaction between MWCNTs and PVA matrix was evidenced through the measurement results of the melting behavior, polarized Raman measurement, and nonisothermal crystallization behavior of the nanocomposites. Owing to the reinforcement of MWCNTs, the tensile strength and modulus of PVA nanocomposite containing 0.9 wt% MWCNTs were increased by 160.7 and 109.2%, respectively, compared to neat PVA. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Physico-mechanical properties of poly (vinyl alcohol), poly (vinyl alcohol)/boric acid,and poly (vinyl alcohol) nanocomposites incorporated with amino-functionalized and pristine halloysite nanotubes films

Eco-friendly poly(vinyl alcohol) (PVA), PVA/boric acid, PVA/halloysite nanotubes (HNTs), and PVA/amino-functionalized HNTs (APTES-HNTs) films were fabricated by a solution casting technique. The samples were characterized by fourier transform infrared, X-ray diffraction, differential scanning calorimetry, scanning electron microscope, and energy-dispersive spectroscopy. The characterization results proved the chemical and physical interactions between the PVA and different additives. The viscoelastic behavior of the films was evaluated by DMA and creep analysis. The storage modulus, loss factor, and both α_α and β_β transitions affected by APTES-HNTs as a potential filler to form effective cross-links. APTES-HNTs existence enhanced creep-recovery beyond expectations. Tensile and impact strength were measured to understand samples' mechanical stability. PVA/APTES-HNTs and PVA/boric acid showed more yield behavior after the elastic limit. Furthermore, the subsequent rupture and impact strength were increased significantly compared with neat PVA and PVA/HNTs. The viscoelastic and mechanical behaviors were linked to each other by the area under Tanδ curve and the work of rupture and impact strength, which their linear correlation coefficient is statistically significant at 95% confidence limits. It seems that the presence of APTES-HNTs provides new cross-links, which altered (improved) the physico-mechanical properties of PVA, offering a bionanocomposite suitable for further applications. From the literature, possible explanations are provided for these observations.     

Structural characteristics of biodegradable thermoplastic starch/poly(ethylene–vinyl alcohol) blends

To establish relationships among the blend composition, processing history, and the resultant properties of starch-based thermoplastics, three varieties of corn starch: (1) Waxy Maize, (2) Native Corn, and (3) high-amylose Hylon VII were extrusion-blended with poly(ethylene-vinyl alcohol) (EVOH) containing 56 mol % VOH. Wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the structural characteristics of the blends. All starches were destructurized upon compounding and a fine dispersion was achieved with EVOH. The Native Corn and Hylon VII blends were phase-separated and exhibited some miscibility between the polymer components as evident in EVOH melting-point depression, smaller domain sizes, lower contrast between phases in TEM, and increased resistance to moisture and enzyme-etching treatments. Starches containing amylose exhibited complexation and crystallinity in the starch fraction, although most of the crystallinity in the blends was due to the EVOH component. Waxy Maize blends were well phase-separated with larger domain sizes and underwent phase coarsening as a function of time in the melt. When subjecting the blends to capillary flow, orientation of both starch-rich and EVOH-rich domains was observed at various compositions, with the EVOH component undergoing significantly more orientation relative to starch as evident by the presence of EVOH-rich fibrils. Finally, EVOH was found to coat the surfaces of filaments produced from the blends even at rather high levels of starch (70%), which is expected to improve moisture sensitivity and slow down the initial rate of biodegradation. © 1995 John Wiley & Sons, Inc.