Abstract Hydrogels with various compositions of polyvinyl alcohol (PVA) and poly(1‐vinyl‐2‐ pyrrolidinone) (PVP) were prepared by irradiating mixtures of PVA and PVP in aqueous solutions with gamma‐rays from 60Co sources at room temperature. The states of water in the hydrogels were characterized using DSC and NMR T2 relaxation measurements and the kinetics of water diffusion in the hydrogels were studied by sorption experiments and NMR imaging. The DSC endothermic peaks in the temperature range ?10 to +10°C implied that there are at least two kinds of freezable water present in the matrix. The difference between the total water content and the freezable water content was referred to as bound water, which is not freezable. The weight fraction of water at which only nonfreezable water is present in a hydrogel with FVP=0.19 has been estimated to be gH2O/gPolymer=0.375. From water sorption experiments, it was demonstrated that the early stage of the diffusion of water into the hydrogels was Fickian. A curve‐fit of the early‐stage experimental data to the Fickian model allowed determination of the water diffusion coefficient, which was found to lie between 1.5×10?11 m2 s?1 and 4.5×10?11 m2 s?1, depending on the polymer composition, the cross‐link density, and the temperature. It was also found that the energy barrier for diffusion of water molecules into PVA/PVP hydrogels was ≈24 kJ mol?1. Additionally, the diffusion coefficients determined from NMR imaging of the volumetric swelling of the gels agreed well with the results obtained by the mass sorption method. 相似文献
Nylon 6 and 6,6 literature data are collected over a wide range of water concentrations and temperatures (0 ≤ [W]0 ≤ 40.8 wt%, 200 ≤ T ≤300 °C) and used to fit parameters in an updated batch reactor model. The resulting copolymerization model uses side reactions to account for the complex influence of water on kinetics and reaction equilibria. The proposed parameter estimates result in a significant improvement in the fit to the data, corresponding to a 73% reduction in the weighted‐least‐squares objective function compared to when the parameters of Arai et al. are used. Copolymerization simulations are conducted at industrially relevant conditions, shedding light on the complex influence of water and on the potential to include waste nylon 6 cyclic dimer in the feedstock. The model and parameter estimates will be helpful in future models of nylon 6/6,6 copolymerization in continuous reactor systems. 相似文献
We have measured 1H and 19F NMR relaxation times T1, T1ρ, and T2, and diffusion constants, in trace penetrants hexafluorobenzene and n-hexadecane dissolved in stretched cis-polyisoprene, as function of temperature, rubber elongation, and angle with respect to the stretch direction. Values of T1 and T2 in the rubber were also measured. At all temperatures (—40 ≤ T ≤ 85°C), T1 in rubber and penetrants is isotropic and independent of elongation; the differences between rubber and penetrants are related to penetrant diffusion. All T2 above—15°C are anisotropic and elongation dependent, and follow a motional narrowing model. For the penetrants, averaging the dipolar interactions implies averaging over a diffusion path; this correctly reproduces the observed much higher T2 anisotropy in the penetrants. Penetrant diffusion rates, however, are essentially isotropic and elongation independent. These effects depend only weakly on the shape of the penetrant molecules. 相似文献
The dynamic tensile mechanical properties (E′, E″, and tanδ) of nylon 6 have been studied over the frequency range 10?25?102 Hz and water content up to 12.6 wt % at a constant temperature of 23°C. From the dispersion maps in the coordinates of frequency and water content, the relaxation behavior can be classified into three regions of water content: (A) dry to 2 wt %, (B) 2-5 wt % and (C) 5 wt % to wet. For region B, it is found that the logarithmic frequency shift Δ logfα/Δx of the α dispersion per 1 wt % change of water content is 1.7. Taking into consideration that the change of glass transition temperature per 1 wt % change of water content Δ Tg/Δx is 3.7°C (according to Kettle), we find Δ logfα/ΔTg = 0.5. For regions A and C, such an evaluation cannot be made. The effect of absorbed water on the dynamic mechanical properties at 23°C is discussed in terms of two kinds of processes: (a) formation of water-amide hydrogen bonds with free amide groups and (b) scission of amide-amide hydrogen bonds. 相似文献
Proton spin diffusion studies for characterizing minimum domain dimensions have been carried out on three blends: a 40/60 blend of nylon 6,6 with the aromatic rigid rod polymer poly(benzo[a,d]dithiazol-2,6-diyl-1,4-phenylene), PBZT, a 47/53 blend of an amorphous nylon with PBZT, and a 50/50 blend of a polyetherimide (PEI) and a polybenzimidazole (PBI). Polarization gradients necessary for these experiments were generated using both chemical shift differences (via multiple pulse techniques) and linewidth differences. Polarization readout techniques included proton lineshape deconvolution, multiple pulse proton lineshapes and 13C CPMAS spectra utilizing short CP times. The two nylon/PBZT blends are expected to phase separate from thermodynamic arguments; however, kinetic considerations, more than thermodynamics, determine domain size. In the 40/60 blend, the minimum domain dimensions of each of the nylon and the PBZT phases were about 4 nm with some scattered larger crystals of nylon. In the 47/53 blend, mixing in some regions indicated domain dimensions similar to the 40/60 blend. In contrast to the 40/60 blend, however, the 47/53 blend was still far from internal spin polarization equilibrium after spin diffusion times of 140 ms. The implication is that the sample-average composition is not found over dimensions like 40 nm; the problem is that the possible morphological explanations are manifold. By investigating the proton rotating frame relaxation, T1p, the possibility that some of the PBZT domains are isolated from the nylon, in a spin diffusion sense, was eliminated. It is more likely that about half of the nylon protons are isolated by spin diffusion from the PBZT protons on a 140 ms timescale. The PEI/PBI blend is a compatible blend of two aromatic polymers where mixing on a molecular scale is expected. We were interested in a measurement of the lower limit of domain size using proton spin diffusion. This lower limit turned out to be about 2.5 nm based on low temperature T1p measurements as opposed to room temperature multiple pulse methods. The latter measurements monitored the disappearance of a polarization gradient between the PEI methyl protons and all the remaining protons. The superiority of the T1p measurements over the multiple pulse method for establishing the smaller minimum domain dimension is not a general result and reasons are discussed. Finally, some general remarks about characterizing polymer blends by solid state NMR, particularly blends which have undergone spinodal phase separation, are included. 相似文献
The dehydrogenation of propanol-2 on sodium-zirconium phosphates (NZP) with the composition Na1 − 2xMxZr2(PO4)3 (x = 0.125 and 0.25) in which Na+ ions were replaced by M2+ = Co2+, Ni2+, and Cu2+ ions was studied. The experimental reaction activation energy Ea decreased while transition through the T* = 310−340°C temperature; above this temperature, the electrophysical and crystallographic properties of the material changed.
These changes were explained by the reversible transfer of Me2+ ions from position M1 to M2 in the NZP lattice. Me2+ centers with different alcohol adsorption forms at T < T* (one-point) and T > T* (two-point) participated in the dehydrogenation reaction. For the first form, Ea and the logarithm of the preexponential factor linearly correlated with the ionic radius of M2+. The activity of M-NZP catalysts altered in repeated experiments and in cases when the direction of temperature variations
changed. 相似文献
The synthesis of a new liquid crystalline block copolymer consisting of a polystyrene block and a side-chain liquid crystalline siloxane block is reported. The synthetic approach described is based on the anionic polymerization of styrene and cyclic trimethyltrivinyltrisiloxane monomers, followed by functionalization of the siloxane block with side chain mesogens. The siloxane block has a Tg well below 25°C and is designed to exhibit a chiral smectic C* phase at room temperature. These block copolymers are the first side-chain liquid crystalline block copolymers which contain both a high Tg glassy block and a low Tg liquid crystalline block. 相似文献
The 00 band maximum of the transition T3(π, π*) ← T1 (π, π*) of acridine occurs at ≈ 10200 ± 20 cm?1 in inert (n-hexane, benzene, CCl4), at 10220 ± 20 cm?1 in polar (acetonitrile) and at 10170 ± 50 cm?1 in hydrogen-bonding (methanol, 2-propanol and alkaline water) solvents. Based on the solvent-independent energy of T1 (π, π*), the T3(π, π*) state of acridine is estimated at 26050 ± 50 cm?1 in all the solvents. 相似文献
Spin-lattice relaxation times (T1), spin-spin relaxation times (T2), and nuclear Overhauser enhancements (NOE), at 75.5 MHz are reported for α-13C nuclei of poly (γ-benzyl-L -glutamate) in deuterated dimethylformamide at 60°C and of poly(γ-hexyl-L -glutamate) in cyclohexanone at 48 and 79°C. It is shown that for molecular weights above 105, the polypeptides cannot be considered as essentially rigid helices with internal librational motions; additional backbone flexing motions contribute to the relaxation behavior. 相似文献
The deuterium NMR lineshape and spin—lattice relaxation time, T1, have been measured in deuterium molybdenum bronze, D1.6MoO3, over the temperature range 166–400 K. The 2D quadrupole coupling constant is 21 kHz at room temperature. The temperature dependence of the 2D T1 has been interpreted in terms of two independent motional processes for deuterium. The data suggest that one of the processes corresponds to diffusion of the 2D nuclei whereas the other may arise from a 180° flipping of the OD2 moieties. This specific interpretation agrees with the results obtained for proton T1 and proton lineshape data reported earlier. 相似文献
Single-pulse 13C NMR spectra and spin-lattice relaxation times T1(1H), detected indirectly via 13C carbons, and T1(13C) were measured at 31°C for virgin pelletized and annealed polylactic acid (PLA) samples using the magic-angle spinning technique. The structural relaxation resulting in more regular crystals with narrower conformation distribution and increase in the lamellae thickness and crystallinity brought about by annealing at 100°C was deduced from the narrowing of the 13C NMR lines and proton spin-lattice relaxation times T1(1H). The spin-lattice relaxation times T1(13C) related to the respective carbons of the α-polymorph of PLA are also discussed in the study. 相似文献
The polymerization behavior of N-(p-aminobenzoyl)caprolactam was studied. It was found that polymerization could proceed by either elimination of caprolactam or by ring opening. Polymers prepared at temperatures above 200°C showed a greater tendency for ring opening to produce alternating aromatic/aliphatic copolymers than did polymers prepared at lower temperatures. Block copolymers of poly(p-benzamide) and nylon 6 were prepared by a two-stage hydrolytic polymerization process or by anionic polymerization at temperatures > 200°C. Polymer microstructures were determined using 13C-NMR spectroscopy by comparison with homopolymers and model alternating copolymers. The alternating copolymer prepared by condensation of N-(p-aminobenzoyl)-6-caproic acid showed a melting transition at 300–305°C in the DSC and a Tg in subsequent heating cycles of 116–119°C. Copolymers made with the two-stage process were rich in p-benzamide sequences and showed no Tg or Tm below 400°C. Copolymer made with NaH was rich in nylon 6 units, showed a Tm of 175–180°C and a Tg of 80–81°C, and was homogeneous in both the melt and solid. 相似文献
Summary: Nylon‐6/Na+‐montmorillonite (MMT) nanocomposites (NNNs) are synthesized by a hydrolyzed ring‐opening polymerization. At a loading of only 2 wt.‐% MMT, the tensile modulus, the flexural modulus, and the heat distortion temperature of the NNNs exhibit increases of nearly 20%, 60%, and 63 °C, respectively. Compared with that of neat nylon‐6, the temperature of the main α‐relaxation (Tα) of the NNNs is shifted 3.6 °C toward higher temperatures and two β‐relaxation peaks are observed. Another interesting phenomenon is that there is a new melting peak (at about 206 °C) for the NNNs.
DSC second heating curves of neat nylon‐6 (N6), nylon‐6/Na+‐MMT nanocomposites with highly swollen Na+‐MMT (NHM), and nylon‐6/Na+‐MMT nanocomposites with slightly swollen Na+‐MMT (NSM) with various amounts of Na+‐MMT. 相似文献
Summary: The sol‐gel transition of one thermoreversible gelling mixture made of xanthan gum and locust bean gum has been studied by using in situ time‐resolved dynamic light scattering (DLS) and measuring the spin‐lattice relaxation time T1 of several protons. A critical dynamical behavior was observed near the sol‐gel transition, which is characterized by the presence of power‐law spectra over four decades of the delay time in the time‐intensity correlation function g2(t)−1 ∼ t−μ at 48 °C. The increase in T1 with increasing temperature becomes steeper at 50 °C indicating a significant change in the local mobility of one anomeric proton of the xanthan side chain and the anomeric protons of the locust bean gum mannose backbone.
Temperature dependence of the spin‐lattice relaxation time T1 for the equatorial anomeric proton of the mannopyranosic unit located next to the main chain of the xanthan. 相似文献