Change in molecular weight distribution by elution of polymers from PVA cast gel

Poly(vinyl alcohol) (PVA) cast gels with high degrees of polymerization and hydrolysis were prepared at room temperature. After sufficient exchange of the outer solvent by fresh pure water, the gel was re-dissolved in pure water at a high temperature. Using this solution, the cast gel was prepared again. This re-dissolution process was repeated again, and a third cast gel was prepared. During each swelling process after each cast-drying process, the amount of elution from the gel was measured and the molecular weight distributions of PVA in the gel as well as in the outer solvent were quantitatively evaluated. The molecular weight distribution of the eluted PVA was found to depend on that of the PVA in the gel; the average molecular weight gradually increased as the re-dissolution process was repeated, while the polydispersity indices of both the PVA from the gel and the eluted PVA decreased.     

Properties of gels obtained by freezing/thawing of poly(vinyl alcohol)/water/dimethyl sulfoxide solutions

Melting points of poly(vinyl alcohol) (PVA) gels obtained by repeated freezing/thawing cycles of the solutions in mixed solvents of water and dimethyl sulfoxide (DMSO) were measured. PVA solutions in 1–30 and 90 vol % DMSO froze at ?40°C. The melting point of a gel increased with an increase in freezing/thawing cycles for all the solutions frozen and with an increase in freezing time in 10–30 vol % DMSO. The solutions in 1–5 vol % DMSO gelled even at very low polymer concentration of 1.0 g/dL by repeated freezing/thawing. The melting point of all the gels with a 5% polymer concentration reached ca. 73°C after 14 freezing/thawing cycles except for the gels in 90 vol % DMSO. However, the melting point of the gels with the same polymer concentration prepared at ?40°C using an unfrozen mixed solvent in 60 vol % DMSO was ca. 77°C and the largest. The maximum apparent enthalpy of fusion of the gels prepared by repeated freezing/thawing was ca. 40 kcal/mol. The gels in 90 vol % DMSO were transparent independent of cycles where as those in 1–80 vol % DMSO were opaque. The degree of opaqueness increased with decreasing DMSO content.     

Characteristics of zwitterionic sulfobetaine acrylamide polymer and the hydrogels prepared by free-radical polymerization and effects of physical and chemical crosslinks on the UCST

A zwitterionic sulfobetaine polymer, poly(N,N-dimethyl(acrylamidopropyl) ammonium propane sulfonate) (poly(DMAAPS)), and the hydrogels of this polymer were synthesized by free-radical polymerization in an aqueous redox system using a wide range of monomer concentrations (C_m). The resulting polymers were characterized in terms of polymer yield, intrinsic viscosity, molecular weight, gel fraction, and thermoresponsive phase-transition behavior. Parameters in the Mark–Houwink–Sakurada equation, including the molecular-weight exponent α, were determined for poly(DMAAPS) in 0.1 M NaCl aqueous solution. The physical state and transparency of the poly(DMAAPS) samples were strongly dependent on C_m and temperature. At higher values of C_m (i.e. above a critical molecular weight), poly(DMAAPS) became a gel comprising a physically crosslinked network consisting of entangled polymer chains and interchain associations of the zwitterionic groups. The poly(DMAAPS) solutions or gels exhibited a thermoresponsive phase transition with an upper critical solution temperature (UCST). The gels obtained were completely soluble in aqueous NaCl solution at ambient temperature as well as in water at temperatures above UCST. The effects of molecular weight, chemical crosslink density and copolymerization on the UCST were also elucidated.     

Synthesis of hydrosoluble polymers of oxazoline using Maghnite‐H as catalyst

2,4,4‐Triméthyl‐2‐oxazoline was polymerized, using an acid exchanged montmorillonite clay as catalyst, with the aim to study the influence of the methyl group size in the initiation and propagation processes. The effect of amount of catalyst temperature, time, and solvent on the polymerization yield and viscosity of the polymers were studied. The polymers obtained were characterized by spectrometric methods and their average molecular weights were determined by viscosimetrie and GPC data. The polymers presented similar spectrometric results and narrow molecular weight distributions. The kinetics indicated that the polymerization rate is first order with respect to monomer concentration. The polymerization results showed that the methyl groups present in the monomer affected the initiation process. Mechanism studies showed that polymerization of TMOX involves nucleophilic ring opening by attack of nitrogen to cleave the CH₂? O bond of the oxazoline ring. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1792–1800, 2007     

Thermoreversible hydrogels. II. Effect of some factors on the swelling behavior of N,N-dimethylacrylamide and n-butoxymethyl acrylamide copolymeric gels

A series of thermoreversible hydrogels are prepared from the various molar ratios of N,N-dimethylacrylamide (DMA), n-butoxymethyl acrylamide (nBMA), and N,N′-methylenebisacrylamide (NMBA). The influences of the amount of DMA in the copolymeric gels, temperature, and polymerization media on the swelling behaviors in water are investigated. Results indicate that the higher the DMA content in the hydrogel systems the higher the swelling ratio and the gel transition temperature. The effects of the gel thickness on the swelling ratio for DMA/nBMA copolymeric gels indicate that the equilibrium swelling time and diffusion coefficient for the thinner gel (1.5 mm) from the dried state to the completely swollen state are obviously faster than are those for the thicker gels (2.0 and 3.5 mm). The effects of the different polymerization media on the swelling ratio for DMA/nBMA copolymeric gels also show that the larger the solvent molecular size and the poor miscibility of the monomer and solvent the higher the swelling ratio and the diffusion coefficient. The drug release in these copolymeric gel systems are also investigated. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:909–916, 1997     

Effect of solvent exchange on the pore structure and dissolution behavior of cellulose

Effect of solvent exchange, i.e., the sequential immersion in water, acetone, and DMAc on the pore structure of cellulose and its dissolution behavior in lithium chloride/N,N‐dimethylacetamide (LiCl/DMAc) was investigated by using size exclusion liquid chromatography (SEC), dynamic light scattering (DLS), and small‐angle X‐ray scattering (SAXS). In the SEC experiment, poly(styrene)s, diethyl phthalate, and acetone were used as probe solutes and 2‐butanone was used as an eluent. Capacity factor of these solutes in the solvent‐exchanged cellulose were larger than those in the untreated one. This was remarkable when diethyl phthalate and acetone were used as solutes. Since the molecular radii of these solutes were estimated to be less than 1 nm, it was shown that the solvent exchange increases the amount of pores within cellulose with the radii of less than 1 nm. In the SAXS experiment, structural difference between the solvent exchanged and the untreated celluloses was observed when the celluloses were immersed in acetone. Values of specific inner surface and average chord length calculated from SAXS profile showed that the amount of small pores was increased in the solvent exchanged cellulose. Considering the results from SEC, DLS, and SAXS measurements, facilitated dissolution of the solvent exchanged cellulose in LiCl/DMAc was attributed to the increase in the pores with the radii of less than 1 nm. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3976–3984, 2007     

Structural and magnetic properties of CoFe2O4 nanopowders,prepared using a modified Pechini method

Cobalt ferrite nanopowders were synthesized by means of the sol-gel method, using citric acid as a chelating agent, and various alcohols as gelling agent: ethanol (ET), ethylene glycol (EG), polyvinyl alcohol (PVA), 1,3 propanediol (PD) and a mixture of PVA and EG. The simultaneous TG/DTA analysis revealed different thermal behaviours of the synthesized gels, depending on the gelling agent. The powders obtained at 500 °C and annealed at 700 °C and 1000 °C contain a single CoFe₂O₄ phase. Scanning electron microscopy (SEM) revealed the influence of the gelling agent on the morphology of cobalt ferrite particles. The coercivity and the saturation magnetization of the powders obtained at 500 °C showed a strong dependence on the crystallite size, determined by the nature of the gelling agent.     

Fractionation of poly(ethylene-co-vinyl acetate) in supercritical propylene: Towards a molecular understanding of a complex macromolecule

A commercial low-density polyethylene copolymer, poly(ethylene-co-vinyl acetate) (EVA), synthesized via the high-pressure free-radical polymerization process, was fractionated with supercritical propylene by isothermal increasing pressure profiling and critical, isobaric, temperature rising elution fractionation (CITREF™). Extensive characterization of the fractions by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) in combination with low-angle laser light scattering (LALLS), and differential scanning calorimetry (DSC) was used to map not only the molecular-weight and chemical composition distributions of the parent copolymer, but also its short-chain branch (SCB) and long-chain branch (LCB) distributions. Fractionation by increasing pressure profiling confirmed the broad molecular-weight distribution and the narrow acetate-branch distribution expected for this random copolymer but revealed the presence of a small amount (∼ 2 wt %) of low molecular-weight amorphous species containing a high level of alkyl SCBs (80 branches/1000 C). The LCB density estimated from the Zimm-Stockmayer relationship using the GPC data monotonically increases with increasing molecular weight above 60,000 g/mol, in agreement with the kinetics of free-radical polymerization. CITREF™ was found to fractionate this copolymer by crystallinity, which is influenced by both the alkyl SCBs and the acetate branches. Up to 18% difference in total branch density (<5% in crystallinity) between EVA molecules was identified using CITREF™. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2015–2030, 1997     

Synthesis and characterization of water soluble poly(N‐acetyl)iminoethylene and poly(ethyleneimine) by ion‐exchanged clay montmorillonite

The cationic polymerization of 2‐méthyl‐2‐oxazoline was carried out at 0°C in acetonitrile using an acid‐exchanged montmorillonite as acid solid ecocatalyst (Maghnite‐H⁺). The effect of the amount of catalyst, solvent, and times of polymerization on yield and viscosity of polymer was studied. A typical reaction product (PMOX) was analyzed by infrared and nuclear magnetic resonance spectroscopy as well as by gel‐permeation chromatography and MALDI‐TOF MS. The polymers presented similar spectrometric results and narrow molecular weight distribution. The poly(N‐acetyl)iminoethylene was hydrolyzed in acid medium obtaining a linear poly(ethyleneimine). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3741–3750, 2006     

VAc聚合过程工艺条件对链支化的影响

研究了以甲醇为溶剂、偶氮二异丁腈(AIBN)为引发剂的VAc聚合过程,通过比较醇解前后PVAc和PVA聚合度的变化研究过程参数如溶剂、引发剂量和分段加入方式以及反应时间对PVAc链支化程度的影响。结果表明,在溶剂总量为20%(wt)、引发剂总量为0.00013%~0.0013%范围内,分段加入引发剂使PVAc链支化程度增加,醇解后PVA聚合度与PVAc聚合度相比有所下降;过低的引发剂浓度虽然可以获得比较高的PVAc聚合度,但醇解后的PVA聚合度相比PVAc下降可达27.0%。在引发剂总量为0.000 63%情况下分批加入溶剂,在合适的反应时间内可控制PVAc支链化程度,缩小PVAc和PVA聚合度的差异并获得相对较高PVA聚合度。无论是一次性加入溶剂或分段加入溶剂,过长的反应时间都使PVAc的支链化程度增加,醇解后PVA聚合度与PVAc聚合度相比差异增大。     

Properties of hydroxypropyl methylcellulose–polyvinyl alcohol water systems,dispersants in vinyl chloride suspension polymerization

Aqueous solutions of partly hydrolyzed polyvinyl acetate (polyvinyl alcohol, PVA) and hydroxypropyl methylcellulose (HPMC) are used together as dispersants in vinyl chloride suspension polymerization. Surface tension, viscosity, and cloud point (CP) of diluted PVA and HPMC solutions at room temperature, viscosity of concentrated solutions as a function of temperature, and incipient gelation temperatures (IGT) were determined. Viscosity measurements show synergism of binary polymer mixtures. The synergetic effect diminishes with rise of temperature. IGT of PVA–HPMC solutions do not differ remarkably from those of solutions containing only HPMC and are much higher than the corresponding CP. At definite relative concentrations of HPMC and PVA, separation of the mixtures into two immiscible phases occurs. Each phase contains both polymers, but in the denser and more concentrated phase (lower layer), PVA is the prevailing component. The mutual influence of the two polymers causes changes in solute–solvent interaction, that is, dehydration of the polymers' molecules. It is assumed that in the processes occurring in the system, preferential molecules of definite structure take part. The merits of a system using both polymers as dispersants in the vinyl chloride polymerization are discussed in the light of ascertained data. © 1993 John Wiley & Sons, Inc.     

Aqueous tape casting and crystallization kinetics of Ce0.8La0.2O1.9 powder

An aqueous tape casting of Ce_0.8La_0.2O_1.9 (LDC) ceramics was developed using PAA as dispersant, PVA as binder, PEG as plasticizer, and deionized water as solvent. Surface properties of LDC powder with and without PAA dispersant were characterized by electrokinetic measurements. The rheology of the LDC slurries was evaluated with a rotary viscometer. The zeta potential measurement showed that the isoelectric point for LDC powders in the absence of dispersant corresponds to a pH value of 4.02. The experimental results showed the pH value greatly affects the rheology of the slurry. The optimum content to get a stable dispersed slurry is 1.5 wt% PAA at pH value of 9–10. In presence of 1.5 wt% PAA dispersant, 5 wt% PVA binder, 5 wt% PEG plasticizer, and 55 wt% LDC powders exhibited shear thinning behavior, indicating that LDC slurry was homogenous and well stabilized. With an appropriate formulation homogeneous, smooth, and defect-free green tapes were successfully obtained. Moreover, the crystallization kinetics of LDC powders prepared by coprecipitation process also has been investigated in this study. The activation energy of crystallization was calculated on the basis of differential scanning calorimetry (DSC) at different heating rates. From non-isothermal DSC data presented values in the range of 343.3–379.1 kJ/mol and 2.282–2.030 for the activation energy of crystallization and the Avrami exponent, respectively, at specific temperatures ranging from 280 to 285 °C.     

Carbothermic reduction synthesis of calcium hexaboride using PVA-calcium hexaborate mixed gels

In this study, PVA-CaB₆O₁₀·5H₂O precursor mixtures were prepared by coating the ceramic powders with PVA to synthesize CaB₆ via carbothermal reduction. Boron loss, the main problem in the synthesis of borides, was reduced by the use of metastable CaB₆O₁₀ as a transitional phase which is stable until the critical temperature ranges where the boron sub-oxides have higher volatilities. To minimize boron loss, due to the high hydrophilicity and ability to form cross-linked PVA-borate gels, PVA was used as a carbon source and carbon coating process was carried out via pyrolysis of the PVA - CaB₆O₁₀·5H₂O mixed gels. The effect of the molecular weight of PVA on the CaB₆ synthesis was also studied. Because of highly efficient interaction of CaB₆O₁₀·5H₂O with the PVA60-water solution, PVA60 was found to be the optimal carbon source. The CaB₆O₁₀·5H₂O-PVA60 composite powder was characterized by using Fourier transform infrared spectroscopy (FTIR) and the effect of molecular weight of the PVA’s on the thermal characteristics of mixed powders were analyzed by using simultaneous thermal analysis (STA). The effect of carbothermic reduction temperature and dwell time on the phase formation were examined via x-ray diffractometer (XRD) and scanning and transmission electron microscopy (SEM and TEM) techniques. The optimum synthesis conditions were determined for the formation of CaB₆ as 1450ºC for 12 h under an Argon flow by using the CaB₆O₁₀·5H₂O-PVA60 mixed precursor.     

Effect of addition of saccharose on gelation of aqueous poly(vinyl alcohol) solutions

The properties of poly(vinyl alcohol) (PVA) hydrogels containing saccharose were examined. The effect of the addition of saccharose to atactic PVA (α-PVA) gels on their melting temperatures was larger than that for syndiotacticity-rich PVA (s-PVA) gels and the melting temperature was above 100°C for α-PVA gels with saccharose contents of 60 wt %. However, the fusion enthalpy (ΔH) of the α-PVA gels was at most 100 kJ/mol. The release of solvent (water/saccharose) from gels in air decreased with an increase in the saccharose content and the equilibrium was achieved after standing for 20 days for the α-PVA and s-PVA gels with saccharose contents above 40 and 20 wt %, respectively. © 1995 John Wiley & Sons, Inc.     

X-ray investigation of the crystallization behaviour of swollen polyvinyl alcohol

X-ray measurements as function of temperature were made from pure polyvinyl alcohol (PVA) and concentrated PVA-solutions in water, glycerine, and glycol. The solutions form gels at adequate low temperatures. As the formation of a chemically bound network can be excluded gel formation must be related to physical crosslinking. From the results of wide angle X-ray measurements it is concluded that the gels are partially crystalline systems. In this case the polymer network is a physical one, consisting of crystalline regions connected by macromolecules in the amorphous state. At room temperature the used PVA is about 20% crystalline. By quenching from temperatures above the melting point down to temperatures far below the glass temperature we succeeded in producing approximately amorphous PVA. Water is the best and glycol is the poorest of the agents used for solving PVA. Crystallization and gel formation following from it take place at the lower temperatures the better the solvent is. The differences in the degree of crystallinity of PVA in these solvents is not caused, however, by the solvent itself but mainly by the different gel formation temperatures. Crystallisation and gel formation in bad solvents occur at higher temperatures than in good solvents. The degree of crystallinity of the gels is the same as that of pure PVA samples crystallized at the same temperatures. In the case of glycol the highest degree of crystallinity was found similar to the case of pure PVA, of about 42%. In the wide angle X-ray diagrams of the gels investigated the positions of the crystal interferences are unchanged. Therefore it can be concluded that the solvents do not enter into crystals to an appreciable amount. The continuous scattering curves of the amorphous part, however, are changed during gel formation. In comparison with the pure polymer the medium distance of the amorphous part of the sample is enlarged and the distance distribution around the medium distance is broadened.     

Aqueous processing of Ce0.8Sm0.2O1.9 green tapes

An aqueous tape casting of Ce_0.8Sm_0.2O_1.9 (SDC) ceramics was developed using poly(acrylic acid) (PAA) as dispersant, poly(vinyl alcohol) (PVA) as binder, poly(ethylene glycol) (PEG) as plasticizer, and deionized water as solvent. Surface properties of SDC powder with and without PAA dispersant were characterized by electrokinetic measurements. The rheology of the SDC slurries was evaluated with a rotary viscometer. The zeta potential measurement showed that the isoelectric point (IEP) for SDC powders in the absence of dispersant corresponds to a pH value of 3.66. The experimental results showed that the pH value greatly affects the rheology of the slurry. The optimum content to get a stable dispersed slurry is 2 wt% PAA in pH value range of 9–10. In presence of 2 wt% PAA dispersant, 55 wt% SDC powders exhibited shear thinning behavior, indicating that SDC slurry was homogenous and well stabilized. Homogeneous, smooth, and defect-free green tapes were successfully obtained by an appropriate slurry formula.     

Preparation of ketoprofen‐loaded high‐molecular‐weight poly(vinyl alcohol) gels

High‐molecular‐weight atactic poly(vinyl alcohol) (a‐PVA) gels loaded with (R,S)‐2‐(3‐benzoylphenyl)propionic acid (ketoprofen) were prepared from 5, 6, 7, and 8 g/dL solutions of a‐PVA with a number‐average degree of polymerization of 4000 in an ethylene glycol/water mixture with an aging method to identify the effect of the initial polymer concentration on the swelling behavior, morphology, and thermal properties of a‐PVA gels. Then, the release behavior of ketoprofen from a‐PVA gels was investigated. As the polymer concentration decreased, the ability for network formation decreased, and the degree of swelling of the a‐PVA gels increased. In addition, the enthalpy increased with an increase in the a‐PVA concentration, but the melting temperatures of the gels prepared at different initial polymer concentrations were the same; this indicated that tighter gel networks would be formed by a higher polymer chain density. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Thermoreversible hydrogels. IV. Effect of some factors on the swelling behavior of N-tetrahydrofurfurylacrylamide

A thermoreversible hydrogel, poly(N-tetrahydrofurfurylacrylamide) [poly-(NTHFAAm) gel], was prepared from N-tetrahydrofurfurylacrylamide, which was synthesized from N-tetrahydrofurfurylamine and acryloyl chloride (through acylation), with N,N′-methylenebisacrylamide, a crosslinker, in various aqueous solutions. The influences of temperature, gel thickness, and polymerization media on the swelling behaviors in water were investigated. The effect of the gel thickness on the swelling ratio for NTHFAAm gel indicated that the equilibrium swelling time and diffusion coefficient for the thinner gel were faster than those for the thicker gels. The effects of different polymerization media on the gel swelling ratio showed that the larger the solvent molecular size and the poorer the miscibility of the monomer and solvent, the higher the swelling ratio and diffusion coefficient. The drug release profiles in the various gels were also investigated. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1597–1603, 1998     

Understanding the swelling of poly (N-isopropyl acrylamide) gels through the study of free volume hole size distributions using positron annihilation spectroscopy

Poly (N-isopropyl acrylamide) gels were prepared by UV polymerization with different degree of cross-linking in different solvents. Ethylene glycol dimethacrylate and penta-erythritol tetra-acrylate were used as cross-linkers, and methanol and dimethyl formamide (DMF) were used as solvents for gel preparation. The free volume fraction and hole size distribution in the dry gels were measured using positron annihilation lifetime spectroscopy. The equilibrium swelling of the samples in water was measured at room temperature. Both swelling properties of gels and the free volume distributions were seen to be sensitive to the amount, type, and functionality of cross-linkers as well as solvent medium used for synthesis. The gels prepared in DMF showed poor swelling properties than those prepared in methanol. The mean free volume hole size was higher while the variance of hole size distribution was smaller in the gels prepared in DMF medium compared to those prepared in methanol. The free volume fractions in the gels were found to be inversely correlated to the extent of equilibrium swelling for similar chemical compositions. The possible reasons are discussed.     

Deacetylation behavior of binary blend films of cellulose acetate and various polymers

For binary blend films of cellulose acetate (CA) and various polymers, the elution behavior of the polymers from the CA films in different environments (i.e., soil, water) was examined. For the CA film containing poly(ethylene glycol) (PEG), the PEG eluted to the periphery of the film completely. On the other hand, polyvinylpyrrolidone blended with CA remained in the CA film. A CA film containing acrylic acid was prepared, and this film was heated. The elution of acrylic acid was inhibited by its polymerization. These results suggested that the internal polymers were capable of remaining in the CA film by polymer entanglement. Second, we examined the deacetylation and biodegradation behavior of CA films containing polymers with a phosphoric acid moiety in the side chain, such as poly(2‐hydroxyethyl methacrylate phosphoric acid ester) [poly(HEMA‐P)]. Poly(HEMA‐P) had the ability to deacetylate the CA, and the biodegradation rate of the CA films containing poly(HEMA‐P) increased in comparison with that of the nonadditive CA films. The elution of internal 2‐hydroxyethyl methacrylate phosphoric acid ester was inhibited by the copolymerization with 2‐hydroxyethyl methacrylate or crosslinking. In the case of both 2‐hydroxyethyl methacrylate phenyl phosphoric acid ester and 10‐methacryloyloxydecyl dihydrogen phosphate, the acetone‐soluble polymers were obtained by radical polymerization in a mixture of acetone and water. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1816–1823, 2006