Polyacrylylglycinamide (PAG) and its thermally reversible aqueous gels have been investigated and comparisons made with gelatin. For unfractionated PAG homopolymers in 2M NaCNS at 25°C, the Mark-Houwink-Sakurada equation is [η] = 1.16 × 10?3 M?n0.52. The Huggins k′ value is found to be about 0.9 and the Flory-Huggins polymer-solvent interaction parameter, 0.49. A theoretically calculated value of Kθ is very close to the experimental one and it is for this reason and the observed μ and M?n exponent values that 2M NaCNS at 25°C approaches being a θ solvent for PAG. A thermodynamic approach based on equilibrium swelling and modulus measurements indicates that a single thermally reversible crosslink in a gelatin gel involves numerous peptide backbone units, whereas in PAG gels a crosslink perhaps involves only one residue from each of two chains. These results complement the very high exothermic heat of gelation crosslinking, δHc, for gelatin compared to PAG. ΔHc has been measured on PAG samples of different DP and found to be independent of chain length. Similar measurements on acrylylglycinamide copolymers and terpolymers containing basic and acidic groups produce no change in δHc so that coulombic forces do not appear to be significant. Data are also included relating to the density of PAG, its glass transition and thermal decomposition temperature, the tensile modulus of equilibrium-swelled PAG films in water and the chain-transfer constant of methanol with the acrylylglycinamide free radical. 相似文献
Polymethacrylylglycinamides (PMG), like polyacrylylglycinamides (PAG), form thermally reversible aqueous gels, but higher molecular weights and/or concentrations are required and the melting points of the gels are lower. The heats of crosslinking for aqueous PMG gels fall in the range of ?5 to ?10 kcal/mole of crosslinks, the same as for aqueous PAG gels, implying that the crosslinks are chemically similar. PMG and PAG are incompatible with each other but both are individually compatible with some types of gelatin. The solubilities of PMG and PAG are similar. Various reagents, however, affect PMG and PAG gels in quite different manners. Aqueous PMG solutions, just outside conditions required for gelation, are rheopectic. Intrinsic viscosities [η] of PMG in 2M NaCNS are about 2.5 times those in water. The Huggins' k′ value for PMG in 2M NaCNS has a value of 0.39–0.40, and both it and [η] are essentially temperature-independent over the range 25–45°C. In water at 25°C for PMG, k′ has an average value of about 1.4. With increasing temperature, for H2O, there is a considerable increase in [η] which is accompanied by a decrease in the value of k′. Osmotic molecular weight measurements on unfractionated PMG in H2O at 40°C yield π/c versus c plots having essentially zero slope, implying a value of close to zero for the second virial coefficient, a value of about 0.5 for the polymer–solvent interaction parameter, and a condition close to a θ condition. An approximate viscosity–Mn relationship for polydisperse PMG is [η]2M NaCNS, 25deg;C = 1.7 × 10?8Mn1.5. The low value of K and high value of the exponent do not result from large differences in polydispersity but rather from a stiff, rodlike configuration in solution. This steric hindrance to rotation also manifests itself in the extreme brittleness of PMG films and in a ΔHp for homopolymerization of only ?6 kcal/-mole. The infrared spectra of MG monomer and PMG are recorded as well as the density and refractive index for PMG. PMG has a glass transition at 226°C by DTA and by TGA, thermal decomposition sets in at about 300°C. From copolymerization with acrylic acid, values of 1.66 and +0.06, respectively, were obtained for the resonance factor Q and the electrical factor e for MG monomer. 相似文献
Network formation of gelatin gel is known to consist of three-dimensionally cross-linked triple helices among polypeptide
chains. The effects of added low molecular weight mono-ols, diols and polyols on the higher-order structure formation of gelatin
chains were investigated using the following measurements: melting temperature, viscoelasticity and spin-lattice relaxation
time (T1) of H172O of gels, and circular dichroism spectra of diluted gelatin solutions. Furthermore, hydration behaviors of these hydroxy
compounds were evaluated from the dynamic hydration numbers (nDHN) derived from T1 of H172O in the solutions. It was found that network structures of gelatin gels containing hydoxy compounds were influenced by the
number and position of hydroxyl groups as well as the number of carbon atoms of these coexisting compounds. The effect of
hydroxyl groups of hydroxy compounds was considered to stabilize the helices among gelatin chains. Especially, the addition
of polyols with large number of hydroxyl groups increased the number of cross-linking junctions in the gel networks, which
consist of the aggregation among the helices. On the contrary, the effect of carbon atoms of hydroxy compounds is to disturb
the formation of the helices and the aggregation among the helices.
Received: 18 April 1996/Accepted: 23 July 1996 相似文献
We investigated the relationship between the macroscopic viscosity lpar;ηN ), determined by creep measurement, and the microscopic viscosity, estimated by rotational correlation time (τc) of spin label determined by ESR method, for wheat and potato starch gels. Both ηNand τc. Of wheat starch gel increased linearly with an increase of starch concentration. Both ηN and τc of potato starch gel increased exponentially with an increase of starch concentration. The τc of wheat starch gel related linearly to the ηN of the gel at 30, 40 and 50°C. For potato starch gels, the τc.also related significantly to the ηN at 30, 40 and 50°C. The τc of free BrAcTEMPO was much smaller than that of spin-labeled AcTEMPO in wheat starch gel. It is concluded that the change in microscopic viscosity of starch gels agrees with their change in macroscopic viscosity. 相似文献
Summary: We developed a novel method of producing polymer gels in aqueous solution using UV irradiation. Persulfates were effective photosensitive initiators of polymerization and/or gelation of acryloyl‐type monomers/polymers. The gelation was confirmed by an abrupt increase in light scattering intensity, 〈I(q)〉T, at the gelation point. The gelation method entails significant advantages: it does not need any cross‐linkers, temperature control (heating), and additives except the persulfate.
The UV irradiation time dependence of light scattering intensity, 〈I(q)〉T, for pre‐gel solutions containing N‐isopropylacrylamide (NIPAm) and/or ammonium persulfate (APS). 相似文献
Summary : Temperature-sensitive hydrogels undergo a volume phase transition (VPT) when heated above a critical temperature Tc. For the poly(N-isopropyl acrylamide) (PNIPA)-water system, Tc. = 34 °C. Below Tc the gels are transparent and highly swollen. On warming above Tc they promptly turn white and start to deswell. The rate of deswelling, however, can be orders of magnitude slower than that of swelling below Tc. The unstable intermediate structure above Tc, can retain the solvent and conserve the sample volume for may days, even with millimetre-sized samples. Light scattering observations of the internal structure of these gels above Tc are precluded by their strong turbidity. Small angle X-ray scattering measurements (SAXS), on the other hand, are less subject to multiple scattering as X-rays penetrate more easily into the bulk material. Conventional (incoherent) SAXS observations reveal intense scattering from smooth internal water-polymer interfaces with an estimated surface area of about 7 m2/g in the swollen gel. The dynamics in the off-equilibrium high temperature state, investigated by X-ray photon correlation spectroscopy (XPCS), displays a relaxation rate that is linearly proportional to the wavevector q, rather than to q2 as in diffusion processes. The physical origin of this relaxation is consistent with jamming, a phenomenon that is common in other disordered systems. 相似文献
E. M. F. of the Cell, Cd-Hg (2-phase)/CdAc2(m), Hg2Ac2(s)/Hg was measured at 20°, 25°, 30° and 40°C. The standard e. m. f. of the cell, Cd/CdAc3(m), Hg2Ac2(c)/Hg was evaluated as E°=1.1500?11.09×10?4T+1.06×10?8T2 The thermodynamic data of the reaction, Cd(c) + Hg2Ac2(c)=2Hg(l)+Cd++(aq)+2Ac?(aq) at 25°C were estimated as ΔF°=?42,139, ΔH°=?48,698 cal mole?1 and ΔS°=?22.0 cal deg?1 mole?1 at 25°C. The thermodynamic data for the formation of Hg2Ac2(s) were evaluated as ΔFf°=?202.3, ΔHf°=?154.5 Kcal mole?1 and S°=72.9 cal deg?1 mole?1. From measurements of the heats of solution of CdAc2·2H2O in aqueous solution, the relative partial molal enthalpies of cadmium acetate in aqueous solution were estimated. 相似文献
Gel formation was observed at 25°C in a mono sodium N‐stearoylglutamate (C18GS)/water system by quick cooling (quenching, 15°C/minute), whereas coagel was formed by slow cooling (annealing, 1°C/minute). Two kinds of phase transition temperatures, Tgel (coagel‐gel) and Tc (gel‐liquid crystal or micelle), were detected in the annealing system using a differential scanning calorimeter (DSC). On the other hand, only Tc was observed in the quenching system. Since the phase transition entropies at Tc in both the quenching and annealing systems are similar, both gels are considered to be in the same structure, and the gel observed in the quenching system at low temperature is in the metastable, supercooled state. Judging from the 1H‐NMR data and microscopic observation, a homogenous gel is formed above 7 wt% of C18GS. With an increase in surfactant concentration, the thixotropic tendency of the gel increases due to the decrease in free‐water. Since it was difficult to show gel formation with the shorter chain homologs, C14GS and C12GS, the hydrocarbon chain length of the surfactant appears to be very important in the formation of a metastable, supercooled gel. 相似文献
Differential thermal analysis has been used to study the fusion of aqueous thermally reversible gels of gelatin and polyacrylylglycinamide (PAG). In the case of gelatin gels, endotherms close to the melting point are readily observed and these are sometimes preceeded by a small exothermic heat of gel reorganization. Calculations are presented to show that breaking of the gelatin gel network requires only a small fraction of the observed endothermic heat of fusion and that most of the heat is required for melting larger crystallites within gelatin aggregates and for perhaps a helix → coil transition. Failure to observe endotherms by DTA over the known temperature range of fusion of PAG gels is consistent with prior measurements and conclusions. The noncrystallinity of PAG gels and soluble aggregates together with a heat of crosslinking of only ?5 to ?10 kcal/mole of crosslinks places the heat of fusion of PAG gels outside the lower limits of DTA sensitivity. 相似文献
The formation of hydrogels from diacylphosphatidylcholine (PC) and water/glycerol mixtures and the properties of the gels
are reported. The gels are formed when Lα phases from the PC in the solvent mixtures are cooled from T >55 °C below the Krafft temperature of the PC (Tm ∼52 °C). The glycerol can also be replaced by other co-solvents like butylenglycol. Above Tm, the PC spontaneously forms Lα phases with multilamellar vesicles that show a strong stationary birefringence. On cooling below Tm, the Lα phases jellify to transparent gels. DSC measurements of the gels show that the PC molecules undergo a phase transition into
the crystalline state. This transition does not seem to be accompanied by a change of the morphological structure of the liquid
Lα phase. The hydrogels also have a stationary birefringence. The vesicles in the gels have been imaged by the CryoTEM method.
The hydrogels are already formed with as little as 1% of PC in the mixed solvent. The rheological properties of the gels were
determined from oscillating rheological measurements. Samples with 10% of PC have a storage modulus of >10,000 Pa. 相似文献
Solutions of isotactic polystyrene in either trans-decalin or 1-chlorodecane were transformed into gels by quenching from a high temperature (ca. 180°C) to ?20°C. The relaxation modulus in compression of these gels was measured over a range of concentrations of from 0.04 g/g to 0.40 g/g. At 22°C, the gels show a double logarithmic stress relaxation rate, m, which is higher than for PVC and gelatin gel systems. 120 s isochronal modulus concentration diagrams exhibit non-power law behavior, i.e., not only is the general trend such that the double logarithmic slope decreases with increasing concentration, but there are also regions in which abrupt changes in modulus occur over narrow ranges in concentrations. These features in the concentration dependence of the modulus are less pronounced than those found previously1 in isotactic polystyrene/cis-decalin gels. The behavior is interpreted to be inconsistent with a fringed micelle picture of the gel structure. Preliminary results are reported indicating that polymer fraction and temperature of gel formation can significantly affect the modulus of the gels. 相似文献
The density, equilibrium heat of fusion and equilibrium melting temperature of Nylon 1010 were determined by means of infrared spectrum, differential scanning calorimetry, wide angle X-ray diffraction and density measurement techniques. According to Starkweatber' s method crystalline density ρ_c and amorphous density ρ_a were estimated to be 1.098 and 1.003 g/cm~3 respectively by extrapolating the straight lines of the IR absorbanee against density to zero intensity. Owing to the less intense in absorbance and less sensitive to the change in crystallinity of the amorphors band the thus obtained ρ_c was too low in value. Thereby the value of the ratio ρ_c /ρ_a is far less than generally accepted mean value for most crystalline polymers. Accordingly, traditional X-ray diffraction method was used through determining thc crystalline dimension(a=4.9, b=5.4, c=27.8, α=49°β=77.0°, γ=63.5°), and a rather correct value of ρ_c or the crystal density 1.13 g/cm~3 was obtained. The equilibrium heat of fusion △H_m~0 was estimated to be 244.0 J/g piotting △H_m 's of specimens with different crystallinity against their corre sponding specific volumes _(sp), and extrapolating to completely crystalline condition (_(sp)~c= 1/ρ_c) As to the equilibrium melting temperature T_m~0, because of the easiness of recrystallization of melt crystallized Nylon 1010 specimen, the well-known Hoffman's T_m-T_c method failed in determining this value and an usually rarely used Kamide double extrapolation method was adopted. The so obtained value of T_m~0 487 seems to be fairly reasonable. 相似文献
Self‐metathesis of erucic acid by [(PCy3)(η‐C‐C3H4N2Mes2)Cl2Ru = CHPh] (Grubbs second‐ generation catalyst) followed by catalytic hydrogenation and purification via the ester yields 1,26‐hexacosanedioate (>99% purity). Polyesterification with 1,26‐hexacosanediol, generated from the diester, affords polyester‐26,26, which features a Tm of 114 °C (Tc = 92 °C, ΔHm = 160 J g−1). Ultralong‐chain model polyesters‐38,23 (Tm = 109 °C) and −44,23 (Tm = 111 °C), generated via multistep procedures including acyclic diene metathesis polymerization, underline that melting points of such aliphatic polyesters do not gradually increase with methylene sequence chain length. Available data suggest that to mimic linear polyethylenes thermal properties, even longer sequences, amounting to at least four times a fatty acid chain, fully incorporated in a linear fashion are required. 相似文献