冷冻/解冻制备的聚乙烯醇水凝胶的结构和流变性研究

研究了冷冻/解冻法制备的不同浓度(5wt%～25wt%)聚乙烯醇(PVA)水凝胶的结构和流变行为之间的关系.由XRD确定了凝胶中PVA的结晶度和晶粒尺寸.用应力流变仪研究了凝胶的流变行为,包括动态模量和蠕变等.在频率为1Hz和低应力的条件下,测量了凝胶的储能模量和损耗模量.在该试验条件下,PVA水凝胶的形变是完全可以回复的.低频率区和低应变区的储能模量随浓度增加而变大,但当浓度超过20wt%时,储能模量增加速率明显降低.由PVA水凝胶在1Hz时的储能模量和结晶度的数据,理论分析得到了形成PVA水凝胶的最低PVA浓度和最小结晶度.当PVA浓度低于15wt%时,储能模量主要由PVA的微晶控制,分子链间的氢键影响很小.通过低应变区储能模量的数值计算出了凝胶网孔尺寸的结构参数.同时对不同温度下PVA水凝胶的储能模量数据进行了标度分析.PVA水凝胶的蠕变行为显示,随浓度提高,凝胶的蠕变黏弹性由线性向非线性转变.     

Structure and Properties of Poly(vinyl alcohol) Hydrogels Obtained by Freeze/Thaw Techniques

The relationships between the structure and the viscoelastic properties of freeze/thaw PVA hydrogels obtained by repeatedly freezing and thawing dilute solutions of PVA in D₂O(11% w/w PVA) in as-prepared and rehydrated states are investigated. Our results indicate that the PVA chains and solvent molecules are organized at different hierarchical length scales, which include the presence of micro- and macro-pores, into a network scaffolding. The porous network is ensured by the presence of crystallites, which act as knots interconnected by portions of PVA chains swollen by the solvent. X-ray diffraction and SANS techniques are used to obtain structural information at short (angstroms) and medium (nanometers) ranges of length scales, concerning the crystallinity, the size of small crystalline aggregates and the average distance between crystallites in PVA hydrogels. Indirect information concerning the structural organization on the large length scales (microns) are provided by viscoelastic measurements. The dynamic shear elastic moduli at low frequency and low strain amplitude, G′, are determined and related to the degree of crystallinity. These data indicate that a minimum crystallinity of 1% is required for these PVA samples to exhibit gel behaviour and have allowed obtaining the order of magnitude of the average mesh size in these gels. Finally, it is shown that the negative effect of aging, inducing worse physical and mechanical properties in these systems, may be prevented using a drying/re-hydration protocol able to keep the physical properties of the as-prepared PVA hydrogels.     

Characterization of Poly(vinyl alcohol)/Chitosan Hydrogels as Vascular Tissue Engineering Scaffolds

In this study, PVA/Chitosan hydrogels were fabricated by freeze/thaw cycles and further crosslinking with a KOH/Na₂SO₄ coagulation bath and the effect of freeze/thaw cycle number on cell behaviour was evaluated. The surface of the hydrogels were further modified with Collagen type I adsorption and seeded with bovine aortic vascular smooth muscle and endothelial cells. Increasing the number of freeze/thaw cycles resulted in a marked change in surface morphology, hydrophilicity and protein adsorption. Collagen coating caused an increase in initial attachment and proliferation. We concluded that hydrogels that have undergone 3 freeze/thaw cycles were optimum for cell attachment both in the presence and the absence of coating.     

A study of the microstructural and diffusion properties of poly(vinyl alcohol) cryogels containing surfactant supramolecular aggregates

Surfactant-containing poly(vinyl alcohol) (PVA) cryogels have been prepared by drying and reswelling hydrogel patches, previously obtained by the freeze/thaw procedure, in decyltrimethylammonium bromide (C10TAB) aqueous solutions. The microstructural and diffusive properties of the resulting material have been characterized by a combined experimental strategy. Gravimetric measurements show that the cryogel maximum swelling is not affected by the surfactant. The surfactant concentration within the cryogel, measured by ion chromatography, is the same as that in the rehydrating surfactant solution. Electron paramagnetic resonance (EPR) spin-probe and small-angle neutron scattering (SANS) measurements show that surfactant self-aggregation in the gel is similar to that in water, occurring at the same critical concentration and resulting in the formation of micellar aggregates whose structure is not affected by the cryogel polymeric scaffold. However, both the micelle intradiffusion coefficients, measured by PGSE-NMR, and the spin-probe correlation times, measured by EPR, indicate that dynamic processes in the hydrogel are much slower than in bulk water. A quantitative analysis of these results suggests that the cryogel polymer-poor domains, in which surfactant molecules are solubilized, have an average dimension of approximately 0.1 microm. Interestingly the experimental data also show that the polymer-poor phase contains more polymer than expected, suggesting that the spinodal decomposition, which occurs during the freezing step of cryogel preparation, is not complete or prevented by ice formation.     

Physico-chemical and structural properties of hydrogels formed by chitosan, in the presence and absence of poly(vinylpyrrolidone) and sodium decylsulfate

The structure of chemically-crosslinked chitosan and chitosan-poly(vinylpyrrolidone) (PVP) hydrogels is investigated by means of the combined use of small-angle neutron scattering (SANS), electron paramagnetic resonance spectroscopy (EPR), intradiffusion, and swelling degree measurements. These hydrogels may be described in terms of an inhomogeneous structure composed by polymer-rich and polymer-poor regions. The polymer-rich regions, whose correlation distance zeta is ranged between approximately 600 and approximately 850 A, are, in turn, characterized by the presence of a network formed by the chemical crosslinks, with a mean correlation distance xi approximately 90 A. The structures of chitosan and chitosan-PVP hydrogels have also been analyzed in the presence of sodium decylsulfate micelles that could provide a multidomain system useful, in principle, for drug delivery applications. Both SANS and EPR measurements show that sodium decylsulfate micelles do not significantly interact with both the gels. Finally, intradiffusion and swelling degree measurements show an improved hydrophilicity of chitosan-PVP gels, even further magnified by the presence of C10OS surfactant.     

Vesicle--biopolymer gels: networks of surfactant vesicles connected by associating biopolymers

The effect of adding an associating biopolymer to surfactant vesicles and micelles is studied using rheology and small-angle neutron scattering (SANS). The associating polymer is obtained by randomly tethering hydrophobic alkyl chains to the backbone of the polysaccharide, chitosan. Adding this polymer to surfactant vesicles results in a gel; that is, the sample transforms from a Newtonian liquid to an elastic solid having frequency-independent dynamic shear moduli. SANS shows that the vesicles remain intact within the gel. The results suggest a gel structure in which the vesicles are connected by polymer chains into a three-dimensional network. Vesicle-polymer binding is expected to occur via the insertion of polymer hydrophobes into the vesicle bilayer. Each vesicle thus acts as a multifunctional junction in the network structure. Significantly, gel formation does not occur with the native chitosan that has no hydrophobes. Moreover, adding the hydrophobically modified chitosan to a viscous sample containing wormlike micelles increases the viscosity further but does not give rise to a gel-like response. Thus, the formation of a robust gel network requires both the presence of hydrophobes on the polymer and vesicles in solution.     

Facile preparation of methylcellulose/poly(vinyl alcohol)physical complex hydrogels with tunable thermosensitivity

Novel complex hydrogels of methylcellulose(MC)and poly(vinyl alcohol)(PVA)with wide-spectrum thermoresponsivity were prepared via physical and mild process.Thermal phase transition of MC/PVA hydrogels exhibited two forms including sol/sol to gel/sol and sol/gel to gel/gel.The phase transition temperature of MC/PVA solution ranged from 38.7 to 60.6℃and was able to be adjusted by simply changing the feeding ratios of two components.The interior morphology of MC/PVA gels was examined with fluorescence analy...     

Solute retention and the states of water in polyethylene glycol and poly(vinyl alcohol) gels

The states of water sorbed in a cross-linked polyethylene glycol (PEG) gel, TSKgel Ether-250, and cross-linked poly(vinyl alcohol) (PVA) gels of different pore sizes, TSKgel Toyopearl HW-40S, 50S, 55S and 75S, were investigated by means of differential scanning calorimetry (DSC). It was found that there were three types of water in these hydrogels, non-freezing water, freezable bound water and free water. The amount of water that functions as the stationary phase in the column packed with the each gel was also estimated by a liquid chromatographic method. The estimated amount of the stationary phase water is in good agreement with the sum of the amount of non-freezing water and that of freezable bound water for HW-40S, 50S and 55S, while it agrees with the amount of only non-freezing water for HW-75S and Ether-250. This means that the stationary phase water consists of non-freezing water and freezable bound water for HW-40S, 50S and 55S, while only non-freezing water functions as the stationary phase in HW-75S and Ether-250 gels. This result can be attributed to the difference in the structure of the gels; the PVA gels containing PVA at relatively high concentrations, HW-40S, 50S and 55S, have a homogeneous gel phase, whereas HW-75S and Ether-250 have a heterogeneous gel phase consisting hydrated polymer domains and macropores with relatively hydrophobic surface. The freezable bound water in Toyopearl HW-40S, 50S and 55S can be regarded as a component of a homogeneous PVA solution phase, while that in HW-75S and Ether-250 may be water isolated in small pores of the hydrophobic domains. The results obtained by the investigation on the retention selectivity of these hydrogels in aqueous solutions supported our postulated view on the structures of the hydrogels.     

聚乙烯醇/丝胶共混凝胶薄膜的制备与表征

采用反复冷冻-解冻的方法制备了聚乙烯醇/丝胶共混凝胶薄膜,考察了薄膜热性能、力学性能和在水中的失重率,并对其微观结构进行了分析。结果表明:当制备过程中循环冷冻-解冻次数增加,且共混薄膜中丝胶与聚乙烯醇的含量相等时,可提高薄膜的综合性能。     

Physically crosslinked poly(vinyl alcohol)-hydroxyethyl starch blend hydrogel membranes: Synthesis and characterization for biomedical applications

Poly(vinyl alcohol), PVA is a polymer of great importance because of its many appealing characteristics specifically for various pharmaceutical and biomedical applications. Physically crosslinked hydrogel membranes composed of different amounts of hydroxyethyl starch (HES) in (PVA) and ampicillin were prepared by applying freeze–thawing method. This freezing–thawing cycle was repeated for three consecutive cycles. Physicochemical properties of PVA–HES membrane gel such as gel fraction, swelling, morphology, elongation, tensile strength, and protein adsorption were investigated. Introducing HES into freeze–thawed PVA structure affected crystal size distribution of PVA; and hence physicochemical properties and morphological structure have been affected. Increased HES concentration decreased the gel fraction %, maximum strength and break elongation. Indeed it resulted into a significant incrementing of the swelling ability, amount of protein adsorption, broader pore size, and pore distribution of membrane morphological structure. Furthermore, an increase in HES concentration resulted in better and still lower thermal stability compared to virgin PVA and freeze–thawed PVA. The maximum weight loss of PVA–HES hydrogel membranes ranged between 18% and 60% according to HES content, after two days of degradation in phosphate buffer saline (PBS), which indicates they are biodegradable. Thus, PVA–HES hydrogel membranes containing ampicillin could be a novel approach for biomedical application e.g. wound dressing purposes.     

不同结晶度聚乙烯醇水凝胶中水氢键缺损的拉曼光谱学研究

聚乙烯醇 (ＰＶＡ)是一种水溶性高分子 ,它在一定条件下可以部分结晶形成水凝胶 ,其结晶度必然影响其机械性能及水在水凝胶中的状态 .部分结晶交联所得的ＰＶＡ水凝胶 ,由于无毒、机械性能好常用来作为生物医用材料 ,如接触眼镜、人工关节润滑软骨等[1,2 ] .水凝胶是轻度交联的高分子网络 ,其内含有大量的水 ,高分子交联网络与水之间的相互作用决定着水凝胶的物理性质和化学性质[3 ,4 ] .一般认为 ,水凝胶中的水以三种状态存在 ,即键合水 (Ｂｏｕｎｄｗａｔｅｒ)、自由水 (Ｆｒｅｅｗａｔｅｒ)和间隙水 (Ｉｎｔｅｒｓｔｉｔｉａｌｗａｔｅｒ…     

Physical Hydrogels of Poly(vinyl alcohol) with Different Syndiotacticity Prepared in the Presence of Lactosilated Chitosan Derivatives

Poly(vinyl alcohol) (PVA) physical hydrogels were prepared by repeated freeze–thawing cycles using aqueous solutions of two PVA samples having different degrees of syndiotacticity, a‐PVA and s‐PVA with 55% and 61% of syndiotactic diads, respectively. The hydrogels were prepared in the presence of different amounts of lactosilated chitosan derivatives (LC) of different molecular weight. The PVA stereoregularity was found to have a dramatic effect on the amount of PVA incorporated into the hydrogels, leading to remarkable differences in the swelling degree and porosity of a‐PVA and s‐PVA hydrogels. A significant amount of LC was retained in the hydrogels after equilibrium swelling. The swelling of the a‐PVA hydrogels was found to increase significantly by increasing the amount of LC while it was only slightly increased in the case of s‐PVA hydrogels. The amount of LC released after equilibrium swelling was lower when chitosan derivatives with higher molecular weights were used. Increased initial concentrations of LC resulted in much higher porosity of the hydrogels. TGA and DSC studies showed that LC is stabilized by the incorporation in the PVA hydrogels. The melting temperature of the crystalline regions of PVA was not significantly influenced by LC. Conversely, the extension of the crystalline domains increased in the presence of LC. The retention of a chitosan derivative bearing β‐D ‐galactose side chain residues makes these hydrogels potentially useful as scaffolds for hepatocytes culture.

Scanning electron micrographs of PVA‐LC hydrogels: (a) a‐PVA; (b) a‐PVA/LC₁₅₀ 80:20; (c) a‐PVA/LC₁₅₀ 50:50.     

PVA-PAMPS-PAA三元互穿网络型水凝胶的合成及其性能研究

以2-丙烯酰胺基-2-甲基丙磺酸(AMPS)、丙烯酸(AA)以及聚乙烯醇(PVA)为原料,制备了PVA-PAMPS-PAA三元互穿网络型(T-IPN)水凝胶.红外分析表明,PVA与PAA以及PAMPS之间形成了较强的氢键,使得PVA分子上的C—O伸缩震动吸收峰移向了低波数处.X射线衍射以及电镜分析表明,当PVA用量较低时,PVA能均匀的穿插于凝胶网络中,形成完善的互穿网络结构,当PVA用量过高时,部分的PVA结晶而使得凝胶出现相分离.研究了该三元互穿网络型水凝胶的溶胀性能,结果表明,该水凝胶的平衡溶胀比在200至340之间,并且随着AA以及AMPS用量的增加,凝胶的溶胀速率以及平衡溶胀比均升高.该三元互穿网络型水凝胶在酸性溶液中和在碱性溶液中表现出截然不同的消溶胀性能;并且随着溶液pH的升高,凝胶在pH=9.0附近出现体积突变,表现出pH敏感性.通过研究T-IPN水凝胶的抗压缩性能发现,利用线型高分子、柔性高分子网络以及刚性高分子网络制备的三元互穿网络型水凝胶能在高溶胀比下保持较高的强度.溶胀比为180的T-IPN水凝胶,其最大抗压缩强度可达12.1 MPa.进一步研究发现,凝胶的组成以及溶胀比均对凝胶的抗压缩强度和压缩应变均存在较大的影响.     

SANS from Pluronic P85 in d-water

Small-angle neutron scattering (SANS) has been used to investigate Pluronic P85 (EO₂₆PO₄₀EO₂₆) copolymer in deuterated water. A range of P85 fractions were measured for a wide sample temperature window. A rich phase behavior is reported. Unimers were observed below the critical micelle formation condition. At fixed P85 fraction, a number of micellar phases were observed upon increasing temperature; first spherical micelles, then cylindrical micelles, then lamellar micelles. At the highest temperature, a demixed lamellae phase was observed. Analysis of the SANS data consisted in fits to an empirical Guinier-Porod model that was appropriate for data fitting in the various phases at low P85 fractions. When the P85 fraction increased, an inter-particle structure factor was included to analyze SANS data from concentrated spherical micelles. At high P85 fractions, paracrystalline structures were observed as evidenced by an enhanced inter-particle interaction peak. A phase diagram for P85/d-water was obtained showing the various phases. Focusing on the spherical micelles phase for one sample composition, a core-shell model was used to fit SANS data and obtain sizes and scattering length densities. Using material balance equations, information such as the aggregation number (i.e., number of Pluronic macromolecules per micelle) and the number of hydration water molecules in the shell region are determined.     

Small-angle neutron scattering and theoretical investigation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) stabilized oil-in-water microemulsions

The aim of this study is to determine the effects of oil solutes and alcohol cosolvents on the structure of oil-in-water microemulsions stabilized by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The systems investigated involved the solubilization of 1,3,5-trimethylbenzene or 1,2-dichlorobenzene by P123 (EO(20)-PO(70)-EO(20)) pluronic surfactant micelles in water and water + ethanol solvents. The structures of these swollen micelles were determined by small-angle neutron scattering (SANS). A thermodynamic model was employed to interpret the characterization data. The results of the thermodynamic model for micellization agreed well with the SANS data from samples of micelles swollen by both oils. The model predicted the size of the micelles within 5% accuracy using only one fitting parameter, the micelle polydispersity. Ethanol had significantly different effects on the polymer micelles that contained solubilized oil compared to pure polymer micelles. For pure polymer micelles, the addition of ethanol increased the solubility of the polymer and, therefore, decreased the total volume fraction of micelles, while for polymer-oil aggregates, ethanol tended to have a positive effect on the volume fraction of micelles. SANS results showed that the greatest divergence from pure aqueous solvent results occurred at oil concentrations above the microemulsion stability limit.     

Thermally exfoliated graphene oxide reinforced stress responsive conductive nanocomposite hydrogel

Lightweight and flexible biosensors that can sustain mechanical deformation and can be adhered to human skin is an interesting field of study. In the current article, a systematic study on development of thermally exfoliated graphene oxide (TEGO)–reinforced poly(vinyl alcohol) (PVA)–based conductive hydrogel nanocomposites has been reported. The free‐standing hydrogels were synthesized using controlled and repetitive freeze‐thaw cycles. The samples were then studied for their mechanical as well as electrical properties. The hydrogels were characterized for their microstructural, chemical, and rheological properties to understand the observed macroscopic properties. Additionally, a study on the behavior of hydrogels immersed in phosphate‐buffered saline (PBS) was carried out to investigate their hydrolytic stability within simulated biological environment. Overall, the nanocomposite hydrogels demonstrated excellent static and dynamic mechanical performance, stability in PBS, considerable electrical conductivity, and significant electrical response to applied external stress, establishing their potential for use as flexible biosensors.     

Visualization of an adsorption model for surfactant transport from micelle solutions to a clean air/water interface using fluorescence microscopy

The micro- and mesoscopic structure of reverse Pluronic 25R4 in aqueous mixtures has been studied by SANS, SAXS and shear rheology. These techniques have been able to give a deep insight into the complex structure of the system phase diagram, that includes an isotropic water-rich liquid phase L(1), and liquid crystalline phases with hexagonal, E, or lamellar order, D. Particular attention has been paid to the isotropic water-rich phase L(1), which has a large stability region in the temperature-composition phase diagram. This region is crossed by a large "cloudy zone". Below it, namely at low temperature and composition, SANS data show the presence of polymer unimers in a gaussian coil conformation. Above the "cloudy zone", at higher temperature and composition, the L(1) phase is structured as a network of interconnected multimeric micelles. Rheology adds information about the structuring of the L(1) phase showing its incipient hexagonal pre-structuring. This technique is also able to highlight the defective structure of the E phase itself. In the temperature and concentration ranges in which a lamellar phase D is present, SANS and SAXS results are in complete agreement, showing how interlamellar distance is influenced by both polymer composition and temperature according to an "ideal deswelling" or a "not ideal swelling" mechanism. In addition, in the D phase rheology suggests the presence of densely packed vesicles.     

Preparation and characterization of novel physically cross-linked hydrogels composed of poly(vinyl alcohol) and amine-terminated polyamidoamine dendrimer

Poly(vinyl alcohol) (PVA) and polyamidoamine (PAMAM) dendrimers are water-soluble, biocompatible and biodegradable polymers, which have been widely applied in biomedical fields. In this paper, novel physically cross-linked hydrogels composed of PVA and amine-terminated PAMAM dendrimer G6-NH(2) were prepared by cyclic freezing/thawing treatment of aqueous solutions containing PVA and G6-NH(2). The FT-IR analysis and elemental analysis indicated that PAMAM dendrimer G6-NH(2) was successfully introduced into the formed hydrogels, possibly via hydrogen bonds among hydroxyl groups, amide groups and amino groups in PVA and PAMAM dendrimer in the process of freezing-thawing cycle. Compared with physically cross-linked PVA hydrogel, PVA/G6-NH(2) hydrogels show higher swelling ratios and faster re-swelling rate due to the higher hydrophilicity of PAMAM dendrimer G6-NH(2). Higher contents of G6-NH(2) in PVA/G6-NH(2) hydrogels resulted in higher swelling ratios and faster re-swelling rates. With increasing freezing/thawing cyclic times, the swelling ratios and re-swelling rates of PVA/G6-NH(2) hydrogels decreased, which is similar to that of physically cross-linked PVA hydrogel. Combining the special host property of polyamidoamine dendrimer, these novel physically cross-linked hydrogels are expected to have potential use in drug delivery, including improving drug-loading amounts in hydrogels and prolonging drug release time. Swelling ratios of physically cross-linked PVA/G6-NH(2)-50 hydrogels prepared by three, six, nine freezing/thawing cycles. The swelling equilibrium experiments were carried out in distilled water at 25 degrees C.     

Determination of the rheological properties of aqueous solutions of branched PEO‐PPO‐PEO copolymers. Confrontation with small‐angle neutron scattering studies.

The physico‐chemical properties of Tetronic^® 908 in aqueous solution have been explored on a wide range of temperature and concentration. This system presents interesting rheological properties depending on the polymer concentration in solution, and temperature. For weight percentage p comprised between 5% and 20%, the viscosity of the solution passes through a maximum. Small angle neutron scattering experiments show that the increase in the viscosity is due to the progressive aggregation of the chains. For higher weight percentages, the viscosity of the system diverges and the mixture becomes “gel‐like”. SANS studies of the same solution indicate that the micelles are organized in a cubic structure. We have developed a model which suitably describes the SANS curves of the system in the region where micelles are formed. It allows the calculation of several parameters (volume fraction of the micelles, size, composition). The evolution of these parameters with temperature and polymer concentration is detailed. Comparisons with models of literature are discussed.     

SANS and DSC study of water distribution in epoxy-based hydrogels

Distribution of water in stoichiometric hydrophilic epoxy network swollen in heavy water to different degrees (epoxy-based hydrogels) at 25 °C has been investigated by small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC). Nanophase separated structure of the hydrogels consisting of water-rich and water-poor domains was revealed by SANS. Two regimes for hydrogel structure were found: (a) at low water content hydrogel consists of isolated water-rich domains dispersed in continuous water-poor phase and (b) at high water content the water-rich domains form another continuous phase. Isosbestic point of scattering curves was found by SANS in the latter region and attributed to conservation of Porod’s length of the nanophase separated structure. Thermal properties of the system are qualitatively different in the two regions: in the former one the glass transition temperature decreases with growing water content while in the latter one it remains constant. Percolation threshold separating both regimes is reflected in a jump of glass transition temperature and inversion of the dependence of the specific heat difference at glass transition.