用UV-Vis法研究聚乙烯醇水溶液的透明度

用紫外可见分光光度计(UV-vis)测试了聚乙烯醇(PVA)水溶液和盐溶液的透明度,研究了PVA醇解度、电解质和温度对PVA溶液透明度的影响.实验结果表明,UV-Vis法是一种简便可行的测定PVA溶液透明度的方法.     

Characterization of poly(vinyl alcohol) during the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as emulsifier

During the emulsion polymerization of vinyl acetate (VAc) using poly(vinyl alcohol) (PVA) as stabilizer and potassium persulfate as initiator, the VAc reacts with PVA forming PVA-graft-PVAc. When the grafted polymer reaches a critical size it becomes water-insoluble and precipitates from the aqueous phase contributing to the formation of polymer particles. Since particle formation and therefore the properties of the final latex will depend on the degree of grafting, it is important to quantify and to characterize the grafted PVA. In this work, the quantitative separation and characterization of the grafted water-insoluble PVA was carried out by a two-step selective solubilization of the PVAc latex, first with acetonitrile to separate PVAc homopolymer, followed by water to separate the water-soluble PVA from the remaining acetonitrile-insoluble material. After the separation, the water-soluble and water-insoluble PVA were characterized by Fourier Transform Infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance (NMR) analyses, from which the details of the PVA-graft-PVAc structure were obtained. © 1996 John Wiley & Sons, Inc.     

Influence of different degrees of hydrolysis of poly(vinyl alcohol) membrane on transport properties in pervaporation of IPA/water mixture

Crosslinked dense poly(vinyl alcohol) (PVA) membranes with different degrees of hydrolysis were prepared and used in sorption and pervaporation of isopropanol (IPA)/water mixtures. Partial flux of water permeation was increased with the water content in the liquid mixture, but the partial flux of IPA due to the coupling effect of sorption and permeation, had a maximum value. The degree of PVA hydrolysis and the feed temperature influenced the permeation flux and water selectivity due to crystallinity and the number of polar side groups in PVA. Thus the partial flux of water and IPA was inversely proportional to the degree of PVA hydrolysis and the selectivity of PVA for water was proportional to the hydrolysis level of PVA.     

Deliberately designed processes to physically tether the carboxyl groups of poly(pentacosadiynoic acid) to a poly(vinyl alcohol) glassy matrix to make poly(pentacosadiynoic acid) thermochromically reversible in the matrix

In this article, we demonstrate that by tethering carboxyl groups of poly(10,12-pentacosadiynoic acid) (PDA) to a poly(vinyl alcohol) (PVA) matrix, PDA, which is irreversible in its pure form, becomes reversible in the thermochromism. The tethering is realized by simple but deliberately designed processes: (1) Disperse the commercially available monomer 10,12-pentacosadiynoic acid (DA) nanocrystals in a PVA aqueous solution by the "NCCM" method invented in our laboratory. (2) Anneal and dry the mixture solution at a temperature higher than the melting point of pure DA crystal. (3) Polymerize the as-annealed DA/PVA blend films by UV irradiation. After the polymerization, PDA/PVA films with completely reversible thermochromism are obtained. The reversible PDA/PVA films can be easily dissolved in water, leading to water-dispersible nanoaggregates with the reversibility. Blends of PDA with other water-soluble polymers such as poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA) and poly(allyamine) (PAM), were prepared respectively, by the same processes and under the same conditions. It is found that all these nanocomposites are irreversible or partially reversible in the thermochromism; either the relatively low glassy transition temperature of the polymer matrix (in the case of PEO) or the partial ionization nature of the polymer (in the cases of PAA and PAM) is responsible for the irreversibility or the partial reversibility.     

Steric stabilization of thermally responsive N-isopropylacrylamide particles by poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) was used as a steric stabilizer for the dispersion polymerization of cross-linked poly(N-isopropylacrylamide) (PNIPAM) in water. A series of reactions were carried out using PVA of varying molecular weight and degree of hydrolysis. Under appropriate conditions, PNIPAM particles of uniform and controllable size were produced using PVA as the stabilizer. The colloidal stability was investigated by measuring changes in particle size with temperature in aqueous suspensions of varying ionic strength. For comparison, parallel colloidal stability measurements were conducted on PNIPAM particles synthesized with low-molecular-weight ionic surfactants. PVA provides colloidal stability over a wide range of temperature and ionic strength, whereas particles produced with ionic surfactants flocculate in moderate ionic strength solutions upon collapse of the hydrogel as the temperature is increased. Experimental results and theoretical consideration indicate that sterically stabilized PNIPAM particles resulted from the grafting of PVA to the PNIPAM particle surface. The enhanced colloidal stability afforded by PVA allows the temperature-responsive PNIPAM particles to be used under physiological conditions where electrostatic stability is ineffective.     

Interaction of water with poly(vinyl methyl ether) in aqueous solution

Near-infrared, viscometric, and calorimetric measurements were made on aqueous poly(vinyl methyl ether) (PVME) solutions at temperatures between 15 and 43°C. We found a hydrogen-bonded structure of water around the polymer chain (a polymer-water complex), which is characterized by two distinct hydration numbers (i.e., 2.7 and 5.0 water molecules on each monomer unit of the chain) by analyzing the concentration dependence of endothermic enthalpies at a cloud point temperature, ca. 35°C. In particular, the 2.7 water-polymer complex has been suggested to be cooperatively formed by using data of the near-infrared (nir) absorption spectrum around 1930 nm. Furthermore, the peak-wavelength of the nir spectrum has been observed to change drastically at the cloud point when the temperature is raised. This can be interpreted as a cooperative collapse of the hydrogen-bonded water structure to free water, resulting in the aggregation of the polymer chains due to the exposure of their hydrophobic groups at the cloud point. © 1994 John Wiley & Sons, Inc.     

Acrylamide plasma-induced polymerization onto expanded poly(tetrafluoroethylene) membrane for aqueous alcohol mixture vapor permeation separation

To improve the vapor permeation performance of aqueous alcohol mixtures, acrylamide (AAm) plasma activation then post-graft polymerization onto an expanded poly(tetrafluoroethylene) (e-PTFE) membrane, e-PTFE-g-AAm, was synthesized in this study. The surface properties of the e-PTFE-g-AAm membrane were characterized using ATR/FTIR, SEM, AFM and the water surface contact angle. The degree of grafting increases with increasing polymerization temperature. A maximum value was obtained at 80 °C. The water contact angle of the pristine e-PTFE membrane and the e-PTFE-g-AAm membrane with a 21% grafting degree was 109.7° and 34.1°, respectively. Optimum vapor permeation performance was obtained using an e-PTFE-g-AAm membrane with a 21% grafting degree for a 90 wt.% aqueous ethanol solution, giving a permeate water concentration of 99.4% and a permeation rate of 648 g/m² h.     

Synthesis and functionalities of poly(N‐vinylalkylamide). XII. Synthesis and thermosensitive property of poly(vinylamine) copolymer prepared from poly(N‐vinylformamide‐co‐N‐vinylisobutyramide)

The differences in the polymerization abilities of N‐vinylformamide (NVF) and N‐vinylisobutyramide (NVIBA) and the synthesis of their copolymers were studied. The polymerization abilities were fairly good and quite similar to those of N‐vinyl‐ acetamide (NVA), a monomer in the same class as N‐vinylalkylamides. Since the monomer reactivity ratios were r₁ = 1.08 and r₂ = 0.92 (M₁ = NVF, M₂ = NVIBA), respectively, it is clear that the comonomers definitely were converted to random copolymers. The resulting copolymers poly(NVF‐co‐NVIBA) exhibited the cloud points sharply. The light transmittance profiles were the same as those for poly(NVIBA) although they increased from 39 °C for poly(NVIBA), with an increase in the corresponding hydrophilic NVF component. Our final objective was to produce a cloud point controlled polymer material with primary amino groups. To achieve this, we examined the hydrolysis of poly(NVF), poly(NVA), poly(NVIBA), and poly(NVF‐co‐NVIBA) to obtain poly(vinylamine) [poly(VAm)]. The hydrolytic cleavage of poly(NVF) and poly(NVA) was promoted by an increase in temperature. However, poly(NVIBA) was not cleaved appreciably. The hydrolysis of poly(NVF‐co‐NVIBA) was done under controlled conditions, and amino groups selectively were introduced to only one of two components of the copolymer. The cloud point of the hydrolyzed copolymer shifted to a higher temperature than that of the copolymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3674–3681, 2000     

Conformation of oligo(ethylene glycol) grafted poly(norbornene) in solutions: A small angle neutron scattering study

The conformation of newly synthesized amphiphilic poly(methoxyoligo(ethylene oxide) norbornenyl esters) macro-homopolymers in dilute solutions of toluene-d8 and D₂O was investigated by small angle neutron scattering (SANS). The macro-homopolymers consist of a polynorbornene (PNB) backbone with a degree of polymerization (DP) of 50, and each repeat unit has a grafted ethylene glycol (EG) side chain with an average DP of 6.6. The hydrophobic backbone and hydrophilic side chains interact differently with solvents of different polarity, which makes the polymer conformation very sensitive to the solvent quality. It was found that in a 0.5 wt.% toluene solution the polymers assume coil-like conformation and gradually contract and become more compact with increasing polymer concentration. In D₂O, the conformation of the polymers were studied at different concentrations: 0.1, 0.5, 1.0 and 2.0 wt.% and at different temperatures: 25, 44, 60 and 74 °C. The polymers are partially contracted in D₂O and their shape can be described by the form factor of a rigid cylinder. The second virial coefficient A₂ was extracted at three temperatures (25, 44 and 60 °C) and the theta point was estimated to be reached at ∼45 °C. The attractive interactions between the polymers in D₂O increase with temperature, which leads to the polymer-solvent phase separation at the cloud point temperature (CPT). The polymer conformation remains virtually temperature independent below the CPT and at 74 °C polymers collapse and form compact structures with water soluble side chains in the shell.     

Preparation and characterization of cationic poly vinyl alcohol with a low degree of substitution

The cationization of polymers has been regarded as an effective method to improve their performance for various applications. In this work, the cationization of poly vinyl alcohol (PVA) was investigated under different conditions, i.e., various glycidyl-trimethylammonium chloride (GTMAC) to PVA ratios, reaction temperatures, times, PVA and NaOH concentrations and solvent compositions. The results showed that the overall efficiency of the cationic modification was rather low, which was due to the hydrolysis of both GTMAC and cationic-modified PVA (CPVA) under the strong alkaline conditions employed. The results also showed that the optimum GTMAC/PVA ratio depended on the solvent composition. The cationization was confirmed by means of ¹H NMR and FTIR analyses. The maximum efficiency in water was obtained under the conditions of 95 °C, 1 h, 0.5 (mol) GTMAC/PVA ratio, and 5% (mol) NaOH concentration, while that in the ethanol/DMSO mixture (1.25 v/v) was obtained under the conditions of 70 °C, 1 h, 0.5 (mol) GTMAC/PVA ratio, and 5% (mol) NaOH concentration. Additionally, the interaction of CPVAs with a silicon wafer (as a substrate) was determined by employing an atomic force microscope (AFM) in water and air.     

Propeller-like multicomponent microstructures: Self-assemblies of nanoparticles of poly(vinyl alcohol)-coated Ag and/or Cu2O

Novel propeller-like multicomponent microstructures, which are actually self-assemblies of nanoparticles of poly(vinyl alcohol) (PVA)-coated Ag and/or Cu2O, were synthesized in aqueous solution of amphiphilic polyvinylacetone (PVKA) (ketalization degree D(H) = 0.549), via one-step in situ reduction of Ag+ and Cu2+ under gamma-ray irradiation, utilizing the low hydrolysis rate of PVKA in the dilute acidic solution. Herein, PVA chains are obtained from hydrolyzed PVKA. The reaction mechanism and the formation mechanism are proposed. The room temperature photoluminescence spectrum has also been applied to explore the optical property.     

Effect of the molecular weight of poly(vinyl alcohol) on the water stability of a syndiotactic poly(vinyl alcohol)/iodine complex film

To precisely identify the effect of the molecular weight of syndiotactic poly(vinyl alcohol) (s-PVA) on the water stability of a s-PVA/iodine complex film, we prepared four s-PVAs with similar syndiotactic diad (s-diad) contents of about 63% and with different number-average degrees of polymerization, P_n, of 900, 6,000, 10,000, and 17,000, respectively. The desorption behavior of iodine in the s-PVA/iodine complex film in water was investigated in relation to the solubility of s-PVA in water. The degree of solubility of a s-PVA film having different P_n in water at 80 °C was limited to about 0.3-10%, whereas the degrees of solubility of atactic PVA films with P_n of 6,000 and 10,000 were 100% at the same conditions. The degree of iodine desorption of the complex film decreased with increasing P_n of s-PVA. Especially, the degree of iodine desorption of a PVA drawn film having P_n of 17,000 was limited to 2%, regardless of soaking temperature from 40 to 80 °C. The desorption of iodine in water was strongly affected by the dissolution of PVA. In addition, the degree of iodine desorption of the drawn s-PVA/iodine film was larger than that of the undrawn one.     

Rheological and DSC changes in poly(vinyl alcohol) gels induced by immersion in water

Poly(vinyl alcohol) (PVA) gels were prepared by freezing and thawing aqueous solutions at temperatures from ?20 to 15°C. The temperature was varied periodically by use of a computer. The endothermic DSC peak was observed for the PVA gels at about 60°C for five specimens of different degrees of saponification (DS). Another endothermic peak was also observed in the range 67–80°C, and this peak shifted to higher temperature with increasing DS. These endothermic peaks shifted to lower temperature on immersion of the PVA gels in water. The dynamic Young's modulus E′ at room temperature was also decreased by immersion of PVA gels in water; E′ decreased monotonically with increasing temperature for PVA gels without immersion in water, while it increased up to a certain temperature and then decreased with increasing temperature for PVA gels in water. The X-ray diffraction showed a characteristic crystalline pattern for PVA gels of higher DS, and this peak was intensified by stretching the gel.     

Thermosensitive properties of a novel poly(methyl 2-acetamidoacrylate-co-methyl acrylate)

Copolymerizations of methyl 2-acetamidoacrylate (MAA) with methyl acrylate (MA) were carried out at 60 °C in chloroform. MAA-rich copolymers are soluble in water and MAA-poor copolymers insoluble. Among water-soluble copolymers obtained, only one (HP-77) which contains 77% of MAA units was thermosensitive. Thermal properties of HP-77 were investigated in the presence or absence of inorganic salts. The cloud point of aqueous HP-77 solution depended on polymer concentration: The cloud point decreased exponentially with an increasing concentration of the polymer. The cloud point of HP-77 was also affected significantly by the type and concentration of salts. The effectiveness of salts to reduce the cloud point is NaBr≈KBr<NaCl≈KCl<Na₂SO₄≈K₂SO₄. The salting-out coefficients were evaluated as 2.45 l/mol for sodium chloride and 14.56 l/mol for sodium sulfate, respectively, from the relationship (Setschenow's equation) between logarithm of the solubility of HP-77 and salt concentration. The salting-out coefficient of sodium sulfate is larger than that of sodium chloride.     

Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water

Changes occurring in the degree of crystallinity and lamellar thickness distribution of poly(vinyl alcohol) (PVA) samples during dissolution in water were investigated. PVA samples of three different molecular weights were crystallized by annealing at 90, 110, and 120°C. The initial degrees of crystallinity measured by differential scanning calorimetry (DSC) and by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) varied from 43 to 60% and the average lamellar thicknesses measured by DSC ranged from 50 to 400 Å. PVA dissolution was followed at 25, 35, and 45°C from 30 s up to 195 min. Lamellar thicknesses were determined as a function of dissolution time using DSC. There was an initial drastic decrease in the degree of crystallinity, which leveled off to a fairly constant value before reaching zero by the time the polymer dissolved completely. Increase in molecular weight led to lesser number of crystals, but with larger average lamellar thickness, which were more stable in the presence of water. Increase in crystallization temperature or decrease in dissolution temperature led to larger average lamellar thickness. Based on these findings, a dissolution mechanism involving unfolding of the polymer chains of the crystal was proposed. © 1996 John Wiley & Sons, Inc.     

Effect of additives on the initial stage of emulsion polymerization of styrene (St) using poly(vinyl alcohol) as a protective colloid

To realize the mechanism of particle formation followed by the grafting of styrene (St) onto poly(vinyl alcohol) (PVA) more precisely, we investigated the influence of additives on model experiments of emulsion polymerizations in 1 ml styrene (St) per 100 ml water at 70 °C using PVA as a protective colloid. In the case of a standard system without additive, experimental results indicate new particle formation, and that 30% of St feed and 30% of the PVA used were grafted. The sulfate radical seldom enters into particles but reacts with PVA, forming the PVA radical that yields the grafting of St onto PVA. The experimental results are thought to arise from a hydrogen abstraction from PVA with a sulfate radical. The addition of low-molecular-weight alcohol, such as isopropyl alcohol (i-PrOH) and n-propyl alcohol (n-PrOH), strongly affected the mechanism of particle formation in terms of a decrease in grafting, due to competition between hydrogen abstraction from PVA and the alcohols with a sulfate radical. Surprisingly, the addition of a low-reactive alcohol, such as t-butyl alcohol (t-BuOH), also resulted in the particle formation with a decrease in grafting, influenced the polymerization locus to be a monomer droplets dispersed by t-BuOH, and confirmed homogeneous nucleation with the increase in the solubility of St in the aqueous phase.     

Preparation of novel thermoplastic poly(vinyl alcohol) with improved processability for fused deposition modeling

In this work, thermoplastic poly (vinyl alcohol) (PVA) with improved processability for fused deposition modeling (FDM) was successfully prepared via intermolecular complexation and plasticization. The glycerol and water, which were non‐toxic and have a complementary structure with PVA, were adapted to realize FDM processing of PVA, thus providing a novel biomaterial with FDM processability. The result showed that the water and glycerol could interrupt hydrogen bonding in PVA and reduce the melting point of PVA to 127.4°C. Moreover, the water fraction of the plasticizer had a significant effect on the FDM processability and usability of the final parts. When the water fraction was greater than 75%, the PVA/plasticizer was unsuitable for FDM processing. However, when the water fraction was lower than 25%, the glycerol precipitated from the modified PVA. Thus, a mixture of 50% water and 50% glycerol was most suitable for FDM processing. A 0.3 mm layer thickness with a nozzle temperature of 175°C was chosen as the optimal processing condition for FDM using thermoplastic PVA. Finally, complex PVA parts with high dimensional accuracy, good mechanical properties, and designated structures were fabricated by FDM machine.     

Pervaporation separation of water-acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde

Poly(vinyl alcohol) (PVA) membranes crosslinked with glutaraldehyde (GA) were prepared by a solution method for the pervaporation separation of acetic acid-water mixtures. In the solution method, dry PVA films were crosslinked by immersion for 2 days at 40°C in reaction solutions which contained different contents of GA, acetone and a catalyst, HCl. In order to fabricate the crosslinked PVA membranes which were stable in aqueous solutions, acetone was used as reaction medium in stead of aqueous inorganic salt solutions which have been commonly used in reaction solution for PVA crosslinking reaction. The crosslinking reaction between the hydroxyl group of PVA and the aldehyde group of GA was characterized by IR spectroscopy. Swelling measurements were carried out in both water and acetic acid to investigate the swelling behavior of the membranes. The swelling behaviour of a membrane fabricated at different GA content in a reaction solution was dependent on crosslinking density and chemical functional groups created as a result of the reaction between PVA and GA, such as the acetal group, ether linkage and unreacted pendent aldehydes in PVA. The pervaporation separation of acetic acid-water mixtures was performed over a range of 70–90 wt% acetic acid in the feed at temperatures varying from 35 to 50°C to examine the separation performances of the PVA membranes. Permeation behaviour through the membranes was analyzed by using pervaporation activation energies which had been calculated from the Arrhenius plots of permeation rates.     

Multistimuli responsive grafted poly(ether tert‐amine) (gPEA): Synthesis,characterization and controlled morphology in aqueous solution

Multistimuli responsive grafted poly(ether tert‐amine) (gPEAs), which were comprised of poly(propylene oxide) (PPO) in backbone and poly(ethylene oxide) (PEO) as grafted chain, were successfully synthesized through nucleophilic addition/ring‐opening reaction of commercial poly(propylene glycol) diglycidyl ether and Jeffamine L100. These gPEAs exhibit very sharp response to temperature, pH and ionic strength with tunable cloud point (CP). The CP of gPEA aqueous solution increases with increasing the PEO content or decreasing pH value, varying from 27 to 77 °C. Compared with linear PEA101, gPEA110 of completely grafted structure in aqueous solution exhibits sharper response to temperature with ΔT around 1 °C. The results obtained from TEM and dynamic light scattering reveal that gPEAs are dispersed as uniform sized nano‐micelles in aqueous at room temperature, which can further aggregate into mesoglobules of complex structure at high temperature (>CP). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6353–6361, 2009     

The kinetics of poly(N-isopropylacrylamide) microgel latex formation

Conversion versus time curves were measured for poly(N-isopropylacrylamide) microgel latexes prepared by polymerization in water with sodium dodecyl sulfate, SDS. Polymerization rates increased with temperature with methylenebisacrylamide crosslinking monomer consumed faster thanN-isopropylacrylamide. The particle diameter decreased with increasing concentrations of SDS in the polymerization recipe and there was evidence that the rate of polymerization increased somewhat with SDS concentration. Particle formation occurred by homogeneous nucleation as micelles were absent.Comparison of particle size distributions from dynamic light scattering to those from a centrifugal sizer led to the conclusion that larger particles within a specific latex were less swollen with acetonitrile than were the smaller ones. This was interpreted as evidence for the polymer in larger particles having a higher crosslink density. Particle swelling was estimated from swelling ratios defined as the particle volume at 25 °C divided by the volume at 50 °C. In the absence of crosslinking poly(N-isopropylacrylamide) linear chains would disolve at 25 °C. The swelling results indicated that the average crosslink density in the particles decreased with conversion. This was explained by the observation that the methylenebisacrylamide was consumed more quickly and is typical of crosslinking in emulsion polymerization where polymer particles have high polymer concentrations at their birth.