Effects of mixed solvent on gelation of poly(vinyl alcohol) solutions

The relationship between the polymer–solvent interaction and gelation behavior of poly(vinyl alcohol) (PVA) solutions prepared from ethylene glycol/water (EG/water) mixed solvents was investigated using a viscometer, light scattering, FTIR, X‐ray, and pulsed NMR analyses. The viscometric result showed that the affinity to PVA for water is higher than that for EG. The light scattering result showed that the spinodal decomposition rate of the PVA solution decreases rapidly as the water content in the EG/water mixed solvent is increased. On the other hand, the FTIR and X‐ray results both indicated that the crystallinity of the PVA gel decreases with water content. These results imply that the water molecules must improve the affinity of the solvent to PVA to inhibit the aggregation or crystallization of PVA chains. The pulsed NMR measurement results showed that the spin–spin relaxation times related to the polymer‐rich and polymer‐poor phases of the PVA gel increase, and the fractional amount of the polymer‐poor phase increases while that of the polymer‐rich phase decreases with increasing water content. These facts indicated that the increase in the mobility of PVA chains must give rise to the difficulty in chain aggregation of PVA solutions with increasing water content. Two transition temperatures were found in the phase transition of the polymer‐rich phase. The lower transition temperature was attributed to the destruction of the denser chain entanglements in the polymer‐rich phase and the higher transition temperature was mainly concerned with the melting of the crystallites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1113–1120, 2001     

Solution properties and unperturbed dimensions of poly (vinylpyrrolidone)

The dilute solution properties of nine poly(vinylpyrrolidone) fractions in methanol covering the molecular weight range 6.76 × 10⁴ to 1.02 × 10⁷ were studied. Constants a and K_m of the Mark-Houwink-Sakurada (M.H.S.) equation were found to be 0.60 and 2.64 × 10^?4 respectively using light scattering and intrinsic viscosity data and were compared with the literature values. The second virial coefficient, A₂ decreases gradually as the molecular weight increases while the root-mean-square radius of gyration, ² increases. The dependence of A₂ on molecular weight is in agreement with other flexible polymers dissolved in moderate to good solvents. The unperturbed chain dimension, (r/M) was calculated using the Stockmayer-Fixman (S—F) equation and a value of 4.9 × 10^?17 cm was obtained. The S—F plot slightly bends in the region of high molecular weight which is according to expectation.     

N-methyl-2-pyrrolidone as a solvent for poly(vinyl alcohol)

NMP (N-methyl-2-pyrrolidone) and its mixtures with water have been investigated as solvents for PVA [poly(vinyl alcohol)]. Judging from the higher exponent in the Mark-Houwink equation, NMP appears to be a better solvent than water. The equation found is for solutions in NMP at 30°C. Although the viscosity of NMP–water mixtures goes through a maximum when the molar ratio of NMP to water is 1:2, the intrinsic viscosity for PVA exhibits neither a maximum nor a minimum in mixtures ranging from pure water to pure NMP. As in aqueous solutions, the intrinsic viscosity in NMP solutions decreases slightly with increasing temperatures from 20 to 50°C.     

Measurement of the unperturbed dimensions of stereoregular poly(methyl methacrylate)

The results of measurements of unperturbed dimensions for a series of stereoregular poly(methyl methacrylates) are reported. The measurements were made by a recently developed method involving a gel permeation chromatograph coupled with an on-line low angle laser light scattering photometer. Measurements were performed in a thermodynamically good solvent, tetrahydrofuran at 25°C. The unperturbed dimensions were obtained by means of viscosity plots. Comparison of the results obtained by this method with those currently available in the literature, as well as with values predicted by statistical calculations, show good agreement. It was determined that a measurable difference occurs in the Mark-Houwink relationship between isotactic and syndiotactic poly(methyl methacrylate); isotactic poly(methyl methacrylate) is 30% more extended than syndiotactic poly(methyl methacrylate) in its unperturbed state; and isotactic poly(methyl methacrylate) exhibits a smaller degree of polymer solvent interaction than the syndiotactic form.     

Gelation of poly(vinyl alcohol) in dimethyl sulfoxide/water solvent

Summary Gelation of poly(vinyl alcohol) (PVA) in dimethyl sulfoxide (DMSO)/water was observed at 23°C with viscometry, spectrophotometry, wide angle X-ray scattering and light scattering. Here transparent gel formation was found to take place prior to being turbid in some cases, whereas the solution became turbid prior to gelation in other cases. Whether transparent gel is formed at first or solution becomes turbid, depends on DMSO composition. PVA solution forms gel in the DMSO composition range from 20 to 80 wt.%. Below the boundary DMSO composition of 60–70 wt.%, gelation takes place at first (i.e. transparent gel is formed) and then becomes turbid eventually, while beyond this DMSO composition the solution becomes turbid and then opaque gel is formed.     

Solution properties and unperturbed dimensions of poly(vinylidene fluoride)

A commercial sample of poly(vinylidene fluoride) (PVF₂) was fractionated and characterized. The following relationships were found: (η) = 1.93 × 10^?4M^0.677 (in dimethylacetamide at 25°C); (η) = 2.13 × 10^?4M^0.62 (in acetophenone at 85°C); (η) = 6.86 × 10^?4M^0.50 (in benzophenone at 190°C), leading to a value of the steric factor: σ = (〈r²〉₀/〈r²〉_0f) = 1.66 ± 0.05, which is in agreement with the crystalline properties of the polymer. The concept of M(η) as a universal parameter for g.p.c. calibration was valid for PVF₂ in dimethylacetamide.     

On the viscoelastic properties of poly(vinyl alcohol) and chemically crosslinked poly(vinyl alcohol)

Poly(vinyl alcohol)(PVA) films chemically crosslinked with glutaraldehyde(GA) in the presence of HCl were prepared by casting from aqueous solutions. The PVA and PVA gels were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA); their swelling characteristics and tensile strength were also determined. The DSC results for the gels displayed depressions of the melting and crystallization temperatures, as well as a decrease of the heat of fusion, when compared to those of PVA free of crosslinker. The DMA analysis revealed that: (1) The glass transition temperature of the wet PVA was lower than that of the dry one, indicating that the water had a plasticizing effect. (2) The gels had a lower glass transition temperature than PVA. (3) The glass transition temperature of the wet gels increased with increasing crosslink density. Possible explanations are provided for these observations. Whereas the thermogravimetric curves of PVA exhibited a single degradation peak, two degradation peaks were detected for the crosslinked PVA. The wet PVA and PVA gels displayed lower tensile strengths and higher elongations than the dried ones. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1816–1823, 2001     

The plasticizing effect of calcium nitrate on poly(vinyl alcohol)

Calcium nitrate (Ca(NO₃)₂) was used as the plasticizer to improve the properties of Poly(vinyl alcohol) (PVA). The interaction between Ca(NO₃)₂ and PVA was investigated by Fourier transform infrared (FTIR) spectroscopy. The influence of Ca(NO₃)₂ on the crystalline, thermal, and mechanical properties of PVA was studied by X‐Ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and tensile testing, respectively. FTIR studies showed that Ca(NO₃)₂ could strongly interact with PVA molecule and reduce the intensity of the hydrogen bonding between PVA molecules. The crystallization of PVA was interrupted and the degree of crystallinity of PVA decreased due to the strong interaction between Ca(NO₃)₂ and PVA. DSC studies showed that the glass transition temperature of PVA decreased with the addition of Ca(NO₃)₂. However, the thermal stability of PVA decreased with the addition of Ca(NO₃)₂. The mechanical properties of PVA was improved with the addition of Ca(NO₃)₂, PVA film became soft and ductile, with lower tensile strength and higher elongation at break compared with original PVA. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Influence of the nanofiber dimensions on the properties of nanocellulose/poly(vinyl alcohol) aerogels

The investigation of aerogels made from cellulose nanofibers and poly(vinyl alcohol) (PVOH) as a polymeric binder is reported. Aerogels based on different nanocellulose types were studied to investigate the influence of the nanocellulose dimensions and their rigidity on the morphology and mechanical properties of the resulting aerogels. Thus, cellulose nanocrystals (CNCs) with low (10), medium (25), and high (80) aspect ratios, isolated from cotton, banana plants, and tunicates, respectively, microfibrillated cellulose (MFC) and microcrystalline cellulose (MCC) were dispersed in aqueous PVOH solutions and aerogels were prepared by freeze‐drying. In addition to the cellulose type, the PVOH‐ and the CNC‐concentration as well as the freeze‐drying conditions were varied, and the materials were optionally cross‐linked by an annealing step or the use of a chemical cross‐linker. The data reveal that at low PVOH content, rigid, high‐aspect ratio CNCs isolated from tunicates afford aerogels that show the least amount of shrinking upon freeze‐drying and display the best mechanical properties. However, with increasing concentration of PVOH or upon introduction of a chemical cross‐linker the differences between materials made from different nanocellulose types decrease. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41740.     

An experimental study of the unperturbed chain dimensions of poly(tetramethyl-p-silphenylene-siloxane)

Summary Experimental values of the characteristic ratio of the unperturbed dimensions of poly (tetramethyl-p-silphenylene-siloxane) were obtained from intrinsic viscosities and osmotic pressures. It was found that the characteristic ratio of this polymer is 1.55±0.30 when the phenylene group is treated as a virtual bond, and 4.51±0.40 when the individual bonds in the phenylene group are taken into account separately. This value is unexpectedly low, considering the usual stiffening effect of the p-phenylene group. Apparently this group can also have a buffering effect which decreases stiffness, by reducing interactions among the atoms or groups preceding it and succeeding it along the chain backbone.     

Biodegradability of poly(vinyl alcohol)

Plastic waste increases at a rate of several tons per year and the time required for their biodegradation is often unknown. The Phanerochaete chysosporium fungus oxidizes a wide variety of toxic organic compounds to nontoxic compounds or CO₂ and H₂O. The biodegradability of the poly(vinyl alcohol) (PVOH) caused by this fungus was analyzed by gel permeation chromatography (GPC) with differential refractive index (Δn) and ultraviolet (UV) detectors. During the GPC analyses, the PVOH is in contact with the components of the culture medium. These components can interact with the stationary phase and also with the polymer producing erroneous results in the molecular weight determination. Therefore, we compare results using differnt mobile phases. A phosphate-acetate buffer of pH 7.4 was found to be appropriate mobile phase to follow the degree of degradation of the PVOH by the change in the average molecular weight determined by GPC in this culture medium. UV detection enabled us to identify structural changes associated with transformations of the carbonyl groups, mainly to enolized β-diketones groups. Infrared absorption (FTIR) confirmed these changes and also showed important modifications in the C-O stretching region probably due-to apertures of cyclic ether groups or transformations of the OH groups by oxidation reactions.     

Characterization of poly(vinyl alcohol)

Molecular weight distributions, long chain branching frequency, and solution viscosities of samples of commercial poly(vinyl alcohol) (PVA) are reported. The PVA was fully reacetylated to poly(vinyl acetate) (PVAc) for characterizations by size exclusion chromatography using a low angle light scattering detector. The Mark–Houwink constants for PVAc in toluene were determined to be K = 0.106 cm³ g^?1 and α = 0.59, at 25°C. Long chain branching frequency in the commercial PVAs studied was small and was little affected by polymer molecular weight. Some 95% or more of the branches in these species were short. Aqueous solutions at 10% (w/v) of PVA were Newtonian. The polymers examined differed in chemical composition, molecular weight distributions, and mean block lengths of vinyl acetate residues. Variations in a single characteristic, like a solution or intrinsic viscosity, cannot be used to deduce structural differences between PVAs.     

The effect of solvent traces on the ultraviolet degradation of poly(vinyl chloride)

The ultraviolet degradation of poly(vinyl chloride) films was studied in a photoreactor which supplied energy near 3000 Å. It was noted that the rate of degradation and color development was increased when the level of residual solvent in the cast films was increased. Two solvents were employed in the study: tetrahydrofuran and dichloroethane. The amount of residual solvent was correlated with the height of a characteristic peak at 2800 Å for tetrahydrofuran and 1900 Å for dichloroethane. Films which had very small traces of solvent showed excellent resistance to ultraviolet degradation, even in the absence of ultraviolet stabilizers. The analysis of solubility data showed that chain scission was controlling in the early stages of exposure, while crosslinking was controlling at later stages. As a result, it was not possible to apply Charlesby's treatment of radiation-induced degradation. Comparison with the results obtained by other workers for degradation at 2537 Å showed that degradation at this wavelength was much faster than degradation at 3000 Å. Furthermore, degradation at 2537 Å appeared to be controlled by crosslinking at all stages of exposure.     

Characterization of polymeric biocomposite based on poly(vinyl alcohol) and poly(vinyl pyrrolidone)

Biocompatible and easily available materials from dairy production waste were used for modification of water‐soluble polymeric blends of Poly(vinyl alcohol) (PVA) and Poly(vinyl pyrrolidone) (PVP). The resulting biocomposites of PVA/PVP with various concentrations of lactose (L) or calcium lactate (CL) (0, 5, 15, 25, 35 wt%) were prepared by using a solvent cast technique and then characterized by optical microscopy, tensile test, water content determination, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy equipped by attenuated total reflectance device, and also tested for biodegradability. The films were transparent with a smooth surface. The results confirm that L and CL work as fillers in polymeric matrix. The tensile investigations showed enhanced Young's modulus (E) and tensile strength for low‐filled of composite materials (up to 5 wt% L and 15 wt% CL). The biodegradation test in aquatic conditions revealed improved biodegradability of modified blends. Both L and CL seem to be suitable for the modification of polymers, which can be convenient from economical and environmental point of view. POLYM. COMPOS., 27:147–152, 2006. © 2006 Society of Plastics Engineers.     

Sticky poly(vinyl alcohol) hydrogels

Low-temperature gelation of poly(vinyl alcohol) (PVA) solution was performed in the presence of CaCl₂. The resulting PVA hydrogels showed high stickiness and excellent water-holding ability. These properties became more remarkable with increasing CaCl₂ concentration in solvent. Furthermore, both the peel strength on stainless steel and the degree of swelling of the PVA hydrogel drastically increased as CaCl₂ concentration was increased from 20 to 30 wt %. It is concluded that the sticky and water-holding properties of PVA hydrogels are intimately related to the state of hydration of Ca ions in aqueous environment. © 1993 John Wiley & Sons, Inc.     

Study on ketalization reaction of poly(vinyl alcohol) by ketones. VII. Reaction between poly(vinyl alcohol) and aromatic ketones and behavior of poly(vinyl ketal) in water

The ketalization reaction of poly(vinyl alcohol) (PVA) by aromatic ketones, with dimeth-ylsufoxide (DMSO) as solvent, under the presence of an acidic catalyst in homogeneous system, was carried out. The synthesis of poly(vinyl ketal) (PVKL) for the case of phenylacetone and benzylacetone was thus successfully performed, but for the case of acetophenone was performed only with a ketalization degree of a few molecular percent, and for the case of benzophenone it could not be performed at all. It seems that these different behaviors in synthesis are due to steric hindrance of the bulky side chain of ketones. The equilibrium constant at 40°C was ca. 0.12. in the case of phenylacetone and benzylacetone, and the value is somewhat higher compared with that of aliphatic ketones, but somewhat lower compared with the case of cyclic ketones. Because the heat of reaction is 7.5 kcal/mol in these two aromatic ketones, all ketalization reactions are considered to proceed by the same reaction mechanism. The rate of hydrolysis, contact angle, surface free energy, moisture regain, and water vapor permeability of PVKL films were measured. All results show the PVKL obtained from phenylacetone is nearly equal to PVKL obtained from methyl n-butyl ketone. However PVKL obtained from benzylacetone shows different behavior compared with other ketones, because the side chain of benzylacetone is flexible and bulky. The hydrophobicity of PVKL seems to depend upon the kind of the original ketones and the flexibility of the side chain. © 1995 John Wiley & Sons, Inc.     

Solvent effect on structural change of poly(vinyl alcohol) physical gels

In this study, the relationship between the polymer–solvent interaction and the network structure of poly(vinyl alcohol) (PVA) gels prepared with organic solvents such as N-methylpyrrolidone (NMP) and ethylene glycol (EG) are investigated. The values of the intrinsic viscosity [η] and Huggins constant k′ of dilute PVA solutions indicate that the attractive interaction between PVA and NMP is higher than that between PVA and EG. The X-ray result shows that PVA–EG gels have a (101) diffraction peak of PVA crystal that appeared at about 2θ = 19°, while PVA–NMP gels only show a broad amorphous scattering peak. On the other hand, Fourier transform infrared results of PVA/EG gels also clearly show an intense peak at 1141 cm⁻¹ due to the crystalline absorption. The results of H¹ pulsed nuclear magnetic resonance show that the spin–spin relaxation time, and respectively, related to the polymer-rich and polymer-poor components decrease, and the fractional amount of the polymer-rich component, f^s, increases, while that of the polymer-poor component, f^l, decreases with an increase in the concentration of polymer. At a given concentration, the value of f^s in the PVA–EG gel is larger than that in the PVA–NMP one. These facts indicate that the crystallinity in the PVA–EG gel is higher than that in the PVA–NMP gel, implying that the aggregation of PVA chains is much easier in the poor solvent, EG, than in the good solvent, NMP. The structural change with aging time in the PVA–EG gel is very remarkable because of the significant syneresis, indicating that the opaque PVA–EG gel with higher crystallinity has a comparatively heterogeneous and unstable network structure than the PVA–NMP gel does. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2477–2486, 1998