Brillouin light scattering from poly(vinyl chloride) gels

The hypersonic velocity and attenuation in PVC gels have been measured as a function of gel network volume fraction, using the technique of Brillouin light scattering. The experimental data have been analysed using the full theory for the elastodynamics of gels proposed recently by Johnson. It has been found that for two asymptotic approximations of the dynamic damping factor the tortuosity parameter is nearly always less than unity, contrary to the theoretical expectation.     

An investigation of aging aqueous poly(vinyl alcohol) solutions by the light scattering method

Aqueous poly(vinyl alcohol) solutions of various concentrations were investigated. The aged solutions were diluted and then analyzed by the light scattering method, size exclusion chromatography, and viscometry. It was found that a relatively small quantity of supermolecular formations arise during aging; they are dispersed in the molecular solution of the predominant part of the polymeric material present. The amount of these aggregated structures and their formation rate increase with concentration of the aging solution.     

Scattering structure factor of colloidal gels characterized by static light scattering, small-angle light scattering, and small-angle neutron scattering measurements

The scattering structure factor of a colloidal gel in a q range of 5 orders of magnitude has been determined by combining static light scattering, small-angle light scattering, and neutron scattering measurements. It exhibits simultaneously two types of structure information: a mass fractal scaling within the clusters that constitute the gel and a surface fractal scaling for length scales larger than that of the clusters. Such scattering behavior can be well interpreted by the pair-correlation function proposed in the literature to model an ideal structure constituted of mass fractal objects inside surface fractal objects.     

Comparison between neutron and quasielastic light scattering by polyacrylamide gels

The authors describe small-angle neutron scattering measurements of the screening length ζ in polyacrylamide-water gels. Although these are inhomogeneous systems, the screening length is clearly observable and is in good numerical agreement with the relation E = 3kT/4πζ³, where E is the longitudinal elastic modulus of the gel obtained from measurements of the intensity of qu-asielastically scattered light. Static light scattering observations reveal a larger-scale (ca. 30 nm) superstructure in the gel.     

Time-resolving analysis of cryotropic gelation of water/poly(vinyl alcohol) solutions via small-angle neutron scattering

The structural transformations occurring in initially homogeneous aqueous solutions of poly(vinyl alcohol) (PVA) through application of freezing (-13 degrees C) and thawing (20 degrees C) cycles is investigated by time resolving small-angle neutron scattering (SANS). These measurements indicate that formation of gels of complex hierarchical structure arises from occurrence of different elementary processes, involving different length and time scales. The fastest process that could be detected by our measurements during the first cryotropic treatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and an unfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallization of PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of a microgel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution at subzero temperatures, and although below 0 degrees C the PVA solutions used for preparation of cryogels should be below the spinodal curve, occurrence of liquid-liquid phase separation could not be detected in our experiments. Upon thawing, ice crystals melt, and transparent gels are obtained that become opaque in approximately 200 min, due to a slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprinted during the fast freezing by the crystallization of water. During the permanence of these gels at room temperature (for hours), the presence of a high content of water (higher than 85% by mass) prevents further crystallization of PVA. Crystallization of PVA, in turn, is resumed by freezing the gels at subzero temperatures, after water crystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVA crystallization during the permanence of these gels at subzero temperatures are the same shown by PVA during the first freezing step of the solutions.     

Morphology of poly(vinyl alcohol) gels assuring the greatest significant drawability

Atactic poly(vinyl alcohol) (at-PVA) and syndiotactic poly(vinyl alcohol) (st. PVA) prepared by gelation/crystallization using dimethyl sulphoxide/water mixtures were drawn in a hot oven at 160 °C under nitrogen. The degrees of polymerization of at- and st-PVA were 2000 and 1980, respectively. The drawability of at- and st-PVA films was affected by the composition of the solvent mixture as well as by quenching temperature. The drawability of at- and st-PVA films prepared by using the solvent mixture containing 60% of dimethyl sulphoxide and 40% of water became more pronounced as the temperature of gelation/crystallization decreased and the draw ratio reached maximum value at –80 °C. Namely, the greatest significant drawability was the same condition for at- and st-PVA films in spite of the different stereo-regularity. Even in this common best condition for significant drawability, however, the morphological properties of swollen gels and of the resultant dry gel films are different each other, dependent upon the tacticity. For at-PVA, small-angle light scattering under Hv polarization condition could not be observed in the swollen gels and in the dry films when the solutions were quenched at temperatures <–10 °C. In contrast, for st-PVA, the X-type scattering pattern from swollen gels became clearer as the temperature decreased but the pattern became indistinct under drying process at ambient condition. On the other hand, the fibrillar textures within the at- and st-PVA dry films became finer and the orresponding crystallinity became lower as the temperature of gelation/crystallization decreased. Thus it turned out that the morphological properties of the swollen gels and of the dried films play an important role to assure the greatest significant drawability.     

Elastic properties of crosslinked poly(vinyl alcohol) gels: Network topology

Current network theory exhibits inconsistencies which show up particularly clearly in deformation of networks prepared by crosslinking a polymer in solution. A check of theory can be obtained if one knows precisely the number of crosslinks in the network and if a range of deformations is applied to the network. In an effort to explore this problem we have examined the relation of shear modulus to crosslink density, primary molecular weight, and polymer concentration for a series of poly(vinyl alcohol) gels at low to intermediate concentrations. Aqueous poly(vinyl alcohol) solutions were crosslinked to form infinite networks using terephthalaldehyde. We find a large discrepancy with these poly(vinyl alcohol) gels between measured shear modulus and that calculated from classical elasticity theory assuming quantitative reaction of crosslinking. The ratio of measured to calculated modulus is independent of crosslink density for a given primary molecular weight and concentration. It shows linear dependence on polymer concentration prior to crosslinking and extrapolates to a critical concentration which is consistent with the effective sizes of the polymer molecules.     

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.     

Solid‐state 13C and 129Xe NMR study of poly(vinyl alcohol) and poly(vinyl alcohol)/lactosilated chitosan gels

Dry and hydrated poly(vinyl alcohol) (PVA) gels with 55% (a‐PVA) and 61% (s‐PVA) syndiotacticity and related PVA/lactyl chitosan (LC) blends have been investigated with ¹²⁹Xe and cross‐polarization/magic‐angle‐spinning ¹³C NMR techniques. Although the dry gels exhibit two broad ¹²⁹Xe resonances in the slow‐to‐intermediate exchange limit, both hydrated gels show three resonances. The corresponding dry blends exhibit two signals, the chemical shifts and line widths of which change with respect to those of pure PVA, whereas one (a‐PVA/LC) or two (s‐PVA/LC) signals appear in the spectra of the hydrated blends. A comparative analysis of the data demonstrates that LC rearranges the domains of the polymeric matrix in both the dry and hydrated blends according to the syndiotacticity of the PVA chains. Information on the molecular motions of the amorphous and swollen polymeric domains in the kilohertz range has been obtained from an analysis of the spin‐lattice relaxation times. These data indicate that the dynamics and arrangement of the PVA chains in the gels are strongly affected by their tacticity and the addition of the copolymer LC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3123–3131, 2003     

The microstructure of poly(vinyl chloride) as revealed by x-ray and light scattering

Small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) as well as small-angle light-scattering (SALS) techniques have been applied to investigate the microstructure of a number of commercial poly(vinyl chloride) (PVC) samples. From the wide-angle x-ray scattering, crystallinity and crystal size parameters have been determined. The crystallinity of the samples investigated range from 5% to 10%. Superstructure parameters such as crystallite thickness, distribution functions of crystallite and amorphous thicknesses, and size of ordered regions have been obtained by an analysis of the SAXS curves using the cluster model. The crystallinity agrees well with the WAXS crystallinities indicating that most of the crystals are lamellar shaped, though some rodlike entities are present in the sample as is shown by the small-angle light scattering. From the SAXS analysis, the microstructure is described as clusters of lamella stacks which are identical with the subprimary particles. Their size is determined to be 220–240 Å. Emulsion type PVC also contains lamellar-shaped crystals. The superstructure, however, of this type of PVC is different from that of mass or suspension-polymerized material. The SAXS curve does not reveal any correlation between the crystals.     

Structural and dynamical studies of poly(vinyl alcohol) gels by high-resolution solid-state C NMR spectroscopy

The ¹³C CP/MAS NMR spectra of isotactic, syndiotactic and atactic poly(vinyl alcohol) (PVA) gels were measured in order to clarify the structure of the immobile component of PVA gel. In the ¹³C CP/MAS NMR spectra, the three CH carbon peaks I, II and III (at about 77, 71 and 65 ppm) were clearly observed, which originate from the formation of strong intermolecular or intramolecular hydrogen bonds between hydroxyl groups like solid PVA. It has been assigned that these peaks originate from the crosslinked region in the gel state. On the basis of the experimental results, intermolecular hydrogen bonds play an important role in the formation of the crosslinked-region in the gel state. Further, the effect of PVA's tacticity on the amount of the crosslinked regions by intermolecular interactions was discussed. In addition, molecular motion in the immobile and mobile region of PVA gel was discussed through the observation of ¹³C spin-lattice relaxation time T₁.     

Swelling and scattering investigations on chemically cross-linked poly(vinyl alcohol) hydrogels

Swelling and scattering measurements are reported from chemically cross-linked polyvinyl alcohol) hydrogels and the corresponding semi-dilute polymer solutions. The mixing free energy in the swollen network is found to be significantly smaller than that of the corresponding polymer solution at identical concentration. Static light scattering and small-angle neutron scattering measurements indicate the presence of large-scale static structures in the solution. Reasonable agreement is found between the osmotic moduli obtained from light scattering measurements and macroscopic osmotic observations.     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.     

Factors influencing the swelling and elution properties of poly(vinyl alcohol) cast gels

Swelling and elution properties of physically crosslinked poly(vinyl alcohol) (PVA) cast gels depend on the network structure of the PVA and crosslink, which is characterized by the size, number, and distribution of microcrystallites. Therefore, the swelling and elution ratios can be manipulated by adept control of the conditions adopted for the preparation of gels. Among the various factors that influence the formation of microcrystallites, the temperature and relative humidity at gelation play an important role. In addition, the size of gel is also a key factor that determines the network structure of gels. To this end, this study quantitatively evaluates the macroscopic properties of swelling and elution, and the microscopic properties of the network structures in disk‐shaped PVA cast gels of the same diameter prepared by casting different weights of PVA solution in the same dish. Although the drying speed can be controlled by adjusting the three processing parameters, namely, drying temperature, humidity, and cast weight, the changes in swelling and elution ratios, microcrystallite size, and crystallinity independently depended on each parameter. Regardless of the three factors, the swelling ratio was found to correlate strongly with the elution ratio. Optimum factors to minimize the elution ratio are discussed on the basis of the change in the network structures obtained by varying the preparatory conditions. Based on the results of the systematic analyses, this study proposes a method to control the elution ratio while retaining high water‐absorbance ability. Copyright © 2015 John Wiley & Sons, Ltd.     

Sorption of water by poly(vinyl alcohol)

The isotherms of water sorption by poly(vinyl alcohol) have been obtained by static sorption methods in a wide range of vapor activities. The properties of poly(vinyl alcohol) at various values of relative humidity have been studied by DSC, X-ray diffraction analysis, and mechanical testing. It has been shown that the correct thermodynamic analysis of sorption isotherms for sorbents with complex organization requires knowledge of their structural features. A method of allowing for the effect of osmotic pressure on the polymer sorption capacity is proposed. The pair interaction parameters estimated in this study are compared with the published data.     

Structural properties of partially charged poly (acrylic acid) gels as probed by light scattering

We report results of static light scattering for partially charged gels at different swelling degrees and different ionization degrees. We measured both the ensemble-average and the rms fluctuations of the scattered intensity by scanning through various positions in the gel. It is shown that the dynamic concentration fluctuations are, to the first order, the same as in semi-dilute solutions at the same concentration. The excess of scattering of the gel with respect to the solutions arises mainly from frozen-in scattering domains with spatial extent less than ∼ 300 Å.     

Study of the effect of poly(vinyl alcohol) concentration on the gelation point of poly(vinyl alcohol) poly(acrylic acid) semi‐IPN systems as determined by viscoelastic measurements

We report on the determination of the gelation point of semi‐interpenetrating polymer networks (semi‐IPNs) composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAAc) formed by a sequential method. The evolution of the viscoelasticity during the gelation reaction of acrylic acid (AAc) in solutions of PVA has been monitored through the sol‐gel transition with dynamic mechanical experiments. The gelation time of the system increased with PVA concentration; however, the molecular structure of the gel, composed of swollen clusters, is rather independent of the presence of PVA. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1944–1949, 2005     

Effects of microcrystallites on swelling behavior in chemically crosslinked poly(vinyl alcohol) gels

We report the swelling ratio and network structure of a poly(vinyl alcohol) (PVA) gel chemically crosslinked by glutaraldehyde with different degrees of crosslinks. Microcrystallites were formed in a chemical PVA gel during a drying process and were confirmed by X‐Ray diffraction (XRD) measurements and Fourier transform infrared (FTIR) spectroscopy. The formation of microcrystallites in the dried gels was suppressed by increasing the degrees of chemical crosslinks. When the dried samples were immersed in pure water at 25 °C, the swelling ratio depended on the degree of chemical crosslinks resulting from the destruction of physical crosslinks by microcrystallites. On the other hand, when the dried samples were immersed in a poor solvent of a mixture of dimethyl sulfoxide and water at 8 °C, the gels did not swell and stayed in the collapsed state. Starting from the collapsed state, the equilibrium swelling ratios were measured while the temperature was increased to 90 °C and then decreased to 8 °C. As a result, irreversible swelling behaviors were observed for all gels with different degrees of crosslinks, which were attributed to the destruction of microcrystallites. The swelling behavior is discussed in terms of the formation and destruction of additional physical crosslinks in the chemical PVA gels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011