H NMR studies of poly(N-isopropylacrylamide) gels near the phase transition

The phase transition and critical phenomenon of equilibrium swollen poly(N-isopropylacrylamide) (NIPA) hydrogels were studied by ¹H NMR spectroscopy in liquid solution mode. The quantitative NMR observation shows that the peak height and line width of polymer proton and of the HOD proton, and relaxation times of HOD proton all transitionally change as the temperature approaches the transition temperature. The relaxation times of water protons are also measured quantitatively, which shows that the temperature dependence of relaxation times of HOD on temperature before the transition is not consistent with relaxation theory based on the assumption of dominated dipolar interaction between like-spin nuclei and isotropic rotational motion. To explain the surprising relaxation behavior of HOD, we suggest that the amount of bound water in gels increases gradually with temperature at the approach of the phase transition. The pulsed-gradient spin-echo NMR experiments of NIPA gel confirm this suggestion. We believe that these results have important implications concerning the mechanism of the phase transition of NIPA hydrogels.     

1H and 13C NMR spectra of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphorylcholine in CDCl3 solution and in sonicated dispersions in 2H2O

A mixed-acid monounsaturated lecithin, 1-palmitoyl-2-oleyl-sn-glycero-3-phosphorylcholine (POL), has been synthesized by phospholipase A₂ digestion of 1,2 dipalmitoyl-sn-glycero-3-phosphorylcholine followed by reacylation of the lysolecithin with oleic anhydride. ¹H (90 MHz) and ¹³C (25.2 MHz) NMR spectra of POL in CDCl₃ solution and in sonicated dispersions in ²H₂O have been obtained, and spin-lattice relaxation times measured. The relaxation times were characteristic of the type of structure formed and reflect molecular motion within the lecithin molecule in each structure. In both systems the spin-lattice relaxation times increase along the alkyl chains towards the terminal methyl group, showing a corresponding increase in the chain molecular motion, although there are significant differences in the gradation of the changes.     

The temperature dependences and methods for the functional representation of the rates of proton magnetic relaxation in aqueous solutions of electrolytes

The paper presents the results of temperature dependence measurements for the rate of proton relaxation (1/T ₁) in sea water with salinity 35‰ over the temperature range −22−+120°C at atmospheric pressure and in some salt solutions at two concentrations (0.5 and 1 mol/l). The possibility of approximating the temperature dependences of magnetic relaxation rates by various functions in pure water, sea water, and solutions of salts with various concentrations was studied. The parameters of these dependences and trends of their variations under the influence of salt components are reported. The most well grounded method for the functional representation of the temperature dependences of 1/T ₁ is the use of the sum of exponents with the number of terms depending on solution concentration. This representation takes into account structural changes in solutions as the concentration grows and corresponds to the Frenkel model of the thermal motion of molecules in aqueous solutions of electrolytes. The combined use of the parameters of the temperature dependences of the rate of relaxation represented by various functions can be a mutually augmenting method for studying the dynamic properties of aqueous solutions of electrolytes with low and moderate concentrations.     

Pulsed NMR study of molecular motion in the uncured diglycidyl ether of bisphenol-A

The diglycidyl ether of bisphenol-A, an uncured epoxy resin, has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1p, and T₂ have been measured over the temperature range from ?160 to 200°C. The resin was studied in its monomeric form and in two mixtures containing higher oligomers. The relaxation times are interpreted in terms of the molecular motion in the resins. The motion responsible for relaxation in the solid monomer form is thought to be methyl group reorientation at low temperatures and general molecular motion at high temperatures. The motions are characterized by activation energies of 5 kcal/mole and 33 kcal/mole, respectively. The solid mixtures exhibit similar effects to the monomer, but an additional relaxation mechanism is observed which is attributed to segmental motion. This motion is characterized by an activation energy of 12–15 kcal/mole. The self-diffusion coefficient was measured in the liquid monomer, and the activation energy for self-diffusion is found to be 11 kcal/mole.     

Water effect on the gelation behavior of polyacrylonitrile/dimethyl sulfoxide solution

The gelation behavior of polyacrylonitrile (PAN)/dimethyl sulfoxide (DMSO) solution containing different amounts of water has been investigated using various methods. The ternary phase diagram of PAN/DMSO/water system indicated that water enhanced the temperature at which phase separation of PAN/DMSO solution occurred. Intrinsic viscosities [η] of dilute PAN/DMSO solution and PAN/DMSO/water solution at varied temperatures were measured to examine the influence of water on the phase behavior of PAN/DMSO solution. The presence of water in the solution gave rise to elevated critical temperature T_c. The gelation temperature T_g obtained by measuring the loss tangent tan δ at different oscillation frequencies in a cooling process was found to increase with increased water content in the solution. The critical relaxation exponent n value, however, changed little with varied concentration. During the aging process, the gelation rate of PAN/DMSO solution increases with the water level. The n values of the PAN/DMSO solutions with 2 wt% and 4 wt% water were a little larger than that of the solution without water, which may be explained by the turbid gel resulted from phase separation. The n values obtained in the aging process were larger than those obtained in the cooling process for the same three solutions, ascribed to the weaker gel with less cross-linking points formed in long time. Water led to the formation of denser gel structure. The coarser gel surface can also be attributed to the phase separation promoted by water.     

Proton spin-lattice relaxation rate studies. 1-Aryl-4,4-dimethyl-2-methylthio-2-imidazolin-5-ones

Proton spin-lattice relaxation rates (R₁ values) have been measured, at 270 MHz, for a series of 1-aryl-4,4-dimethyl-2-methylthio-2-imidazolin-5-ones, using both null-point and non-linear regression methods; the two procedures give the same rates. A normalization procedure has been used to enable comparisons to be made between the R₁ values of different compounds; this is necessary because the experimental values are affected by changes in motional correlation times associated with changes in molecular structure. A substantial (five-fold) dynamic range of R₁ values has been observed for the protons of individual compounds, and the values have been correlated with the molecular environments of the nuclei. Evidence for anisotropic molecular motion and inter-ring proton relaxation in these compounds has been considered.     

Interpretation of spin relaxation in polyisobutylene in terms of specific motions

Porton and carbon spin-lattice relaxation times T₁ and nuclear Overhauser enhancements are interpreted in terms of motions likely in linear polyisobutylene. Most of the interpretation is based on relaxation data in the literature, but some additional ¹H and ¹³C pulse Fourier transform experiments were conducted to resolve a disagreement in the literature concerning cross relaxation between the two types of protons present in polyisobutylene. Spin relaxation in solution and the bulk is accounted for by three specific motions considered as independent sources of motional modulation of the dipole–dipole interaction. The first motion is overall isotropic rotatory diffusion which has a known dependence on molecular weight, intrinsic viscosity, and solvent viscosity for polymers in solution, and a known dependence on molecular weight and viscosity for bulk polymers. The effects of overall tumbling account for a decrease of T₁ for the methylene and methyl carbons with increasing molecular weight in solution and increase of T₁ of methylene carbons with molecular weight in bulk. The second motion considered is backbone rearrangements caused by the three-bond jump. This motion dominates relaxation of the methylene carbons either in solution or in the bulk allowing for the determination of the associated correlation time. The correlation time characterizing the occurrence of the three-bond jump in a 5% (wt/vol) solution in CCI₄ at 45°C is 58 psec, and in the bulk at 45°C it is 11 nsec. The last motion included in the model is methyl-group rotation about the threefold symmetry axis. The methyl-group rotational correlation time is 0.20 nsec in a 5% (wt/vol) solution in CCI₄ at 45°C and 0.33 nsec in the bulk at 45°C. The concentration dependence of the backbone motion contrasts strongly with the corresponding dependence of methyl-group rotation.     

Solid‐state nuclear magnetic resonance relaxation times in crosslinked macroporous polymer particles of divinylbenzene homopolymers

Monodisperse porous particles of poly(divinylbenzene) prepared by the activated swelling method have been investigated by solid‐state ¹³C crosspolarization magic‐angle spinning (CPMAS) nuclear magnetic resonance (NMR) relaxation measurements. Homopolymeric combinations of two porogens (toluene and 2‐ethylhexanoic acid) and two monomers (meta‐ and para‐divinylbenzene) were studied. Residual vinyl groups were systematically reacted with increasing amounts of bromine, producing 20 different polymers samples for which we measured crosspolarization times, T_CH, proton rotating frame spin‐lattice relaxation, T, ¹³C spin‐lattice relaxation, T, and proton spin‐lattice relaxation, T. These parameters were chosen to reflect expected changes in a wide range of frequencies of motion as a function of structure. Relative differences in the molecular mobility of the major functional groups (aromatic, vinyl and aliphatic) is related to initial reactants used, vinyl concentration, relative reactivity of vinyl groups, distribution of vinyl groups, pore structure, and degree of crosslinking. Variable temperature ¹H combined rotation and multiple pulse NMR (CRAMPS) was used to derive activation energies for selected samples via measurement of the proton spin‐lattice relaxation time, T. Irreversible thermal effects were observed in ambient temperature relaxation after heating to temperatures in the range of 393–418 K. Simple univariate statistical analyses failed to reveal consistent correlations among the known variables. However, the application of more sophisticated multivariate and neural network analyses allowed excellent structure–property predictions to be made from the relaxation time data. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1307–1328, 1999     

A synthetic hydrogel with thermoreversible gelation,III: an NMR study of the sol–gel transition

The sol–gel transition mechanism of a thermoreversible hydrogel composed of a copolymer comprising poly(N-isopropylacrylamide) and poly(ethylene glycol) (PNIPAAm–PEG) was studied by NMR. The ¹H– and ¹³C–NMR spectra measured on a PNIPAAm–PEG solution in 99.9% D₂O showed a remarkable line width broadening of the PNIPAAm block of more than that of the PEG block, during thermally induced hydrogel formation. This result suggested that the mobility of the PNIPAAm block is more restricted than that of the PEG block during gelation. A crosslinked polymer network formation was ascertained by a sudden reduction in the spin-lattice relaxation time (T₁) of the residual HDO proton during gelation. The temperature dependency of the T₁ values for the PNIPAAm and PEG blocks revealed that the microscopic condition of the PNIPAAm block in water was drastically changed during gelation, while that of the PEG block was unchanged. The experimental results from NMR supported the following gelation mechanism; that an aggregation of PNIPAAm blocks in the separate copolymers caused by hydrophobic interaction forms crosslinking points to give an infinite three-dimensional network structure. The hydrated PEG chains in the copolymers provide the network with a swelling property in water, and prevent the aggregation from causing a macroscopic phase separation.     

Pulsed NMR study of molecular motion in the diglycidyl ether of bisphenol-A cured with 4,4′-methylenedianiline

The monomeric diglycidyl ether of bisphenol-A cured with methylenedianiline has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1p, and T₂ have been measured over the temperature range ?160 to 200°C. The system was studied after being fully postcured at 180°C and after being cured at 100°C and at 54°C. The relaxation times are interpreted in terms of molecular motion in the cured resins, i.e., methyl group reorientation, segmental motions, and general molecular motion. The results are compared with those obtained previously by us for the uncured resin. Correlation frequencies for the segmental motions are compared with those obtained from dielectric relaxation and mechanical loss studies. There are at least two principal segmental motions present in the cured system, and the nature of these motions is found to depend on the cure temperature. These effects are discussed in terms of crosslinking and annealing of the system.     

Associative thickeners. Part I: Synthesis,rheology and aggregation behavior

Nonionic associative thickeners with systematic changes in chemical composition have been synthesized. Rheological measurements of thickened latexes are presented as well as measurements of relaxation times, intrinsic viscosity and osmotic pressure of polymers in pure water solution. We find that the general hydrophobicity of the polymers' end groups control both rheology and efficiency. Hydrophobic parts in the interior of the polymer do not seem to effect rheology in latex systems. Viscosity increases with molecular weight in the low molecular region (Mw<10000), and passes through a maximum in the high molecular region. The thickeners seem to form micelle-like aggregates even at very low concentrations, while at higher concentrations the viscoelastic properties may be modeled by means of one Maxwell element. Only the network relaxation times but not the network structure seems to be dependent on the polymers' end group.     

Structure and dynamics of MG rubber studied by dynamic mechanical analysis and solid‐state NMR

Methyl‐methacrylate‐grafted natural rubber was prepared by free radical polymerization of methyl methacrylate in natural rubber latex, and their structure and dynamics were investigated by dynamic mechanical analysis and solid‐state nuclear magnetic resonance (NMR). Samples were prepared by chemical initiation and high‐energy radiation. The changes of glass transition temperature and tan δ max with different total poly(methyl methacrylate (PMMA) content are reported. The effect of the change in composition in copolymers on tan δ peak width, tan δ max, and area under the tan δ curve are used to understand the miscibility and damping properties. Solid‐state ¹³C‐NMR measurements were carried out to determine several relaxation time parameters, such as rotating frame and laboratory frame proton and carbon relaxation times. Cross polarization times and carbon relaxation times were interpreted based on the changes in the molecular motion. Proton relaxation times were interpreted based on the heterogeneity of the matrix. Results confirmed phase separation and a presence of an interfacial region. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1141–1153, 1999     

Segmental motion in polyoxymethylene

The proton spin-lattice relaxation times (T₁) of melt-crystallized, solution-crystallized, and solid-state-polymerized polyoxymethylene (POM) were measured between ?60 and +150°C. The three types of samples each have a pronounced T₁ minimum near room temperature which is a high-frequency manifestation of the γ process. From the quantitative dependence of the relaxation intensity on crystallinity as well as from the absolute magnitude of the relaxation times, it is concluded that the γ process in POM arises from hindered rotation of noncrystalline chain segments. The relation between the relaxation times and the long period indicates that these noncrystalline segments constitute disordered lamellar surface layers, the thickness of which depends on thermal history of the material. The temperature dependence of the motion of the relatively thin surface layers of solution crystallized POM is quite straightforward. The γ process in the bulk-crystallized material involves cooperative motion, however, leading to temperature-dependent kinetic parameters.     

Thermoreversible gelation of isotactic-rich poly(N-isopropylacrylamide) in water

We report the experimentally determined phase diagram for an aqueous solution of isotactic-rich poly(N-isopropylacrylamide) (PNiPAM) composed of the sol-gel transition curve and the cloud-point curve. The meso diad content of isotactic-rich PNiPAM is 64%, and it is soluble in water at low temperatures, but undergoes a sol-to-gel transition with increasing temperature in the investigated concentration range of 1.8 wt. %-6.0 wt. %. With a further increase in temperature, the system becomes turbid. The gel formation and clouding behavior are thermally reversible. This is the first observation of thermoreversible gelation under the cloud-point temperature for an aqueous solution of PNiPAM. On the basis of the determined phase diagram, we carried out light scattering experiments to characterize the sol-gel transition behavior as a function of temperature.     

Polymer electrolyte membranes based on p‐toluenesulfonic acid doped poly(1‐vinyl‐1,2,4‐triazole): Synthesis,thermal and proton conductivity properties

Throughout this work, the synthesis, thermal as well as proton conducting properties of acid doped heterocyclic polymer were studied under anhydrous conditions. In this context, poly(1‐vinyl‐1,2,4‐triazole), PVTri was produced by free radical polymerization of 1‐vinyl‐1,2,4‐triazole with a high yield. The structure of the homopolymer was proved by FTIR and solid state ¹³C CP‐MAS NMR spectroscopy. The polymer was doped with p‐toluenesulfonic acid at various molar ratios, x = 0.5, 1, 1.5, 2, with respect to polymer repeating unit. The proton transfer from p‐toluenesulfonic acid to the triazole rings was proved with FTIR spectroscopy. Thermogravimetry analysis showed that the samples are thermally stable up to ～250 °C. Differential scanning calorimetry results illustrated that the materials are homogeneous and the dopant strongly affects the glass transition temperature of the host polymer. Cyclic voltammetry results showed that the electrochemical stability domain extends over 3 V. The proton conductivity of these materials increased with dopant concentration and the temperature. Charge transport relaxation times were derived via complex electrical modulus formalism (M*). The temperature dependence of conductivity relaxation times showed that the proton conductivity occurs via structure diffusion. In the anhydrous state, the proton conductivity of PVTri1PTSA and PVTri2PTSA was measured as 8 × 10^?4 S/cm at 150 °C and 0.012 S/cm at 110 °C, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1016–1021, 2010     

Effect of temperature and molecular weight on enthalpy relaxation in polystyrene

Isothermal enthalpy relaxation in polystyrene was measured as a function of temperature and molecular weight on a differential scanning calorimeter. Relaxation spectra were derived from the data and expressed as a distribution of relaxation times. For a given molecular weight the relaxation spectra at different temperatures could not be superimposed by a shift in time. The relaxation curves of samples of different molecular weights could be superimposed only when the difference between the temperature at which the relaxation was monitored (T_a) and their respective T_g was the same. The relaxation spectrum at any temperature for a given molecular weight was also expressed as a distribution of energies. The average energy represented by this distribution was associated with an activation energy required for the motion of a chemical repeat unit. The activation energy extracted from the temperature shift in the relaxation spectra corresponded to the motion of a statistical unit (Kuhn's segment) in polystyrene.     

Nonconventional lecithin gels in hydrocarbon oils

Nonconventional gels formed by lecithin in hydrocarbon oils containing a small amount of water have been studied with the help of temperature- and water-content-dependent phase transitions,³¹P NMR and FT-IR techniques. Gels are obtained in cyclohexane,n-octane, andn-dodecane. Addition of cholesterol in lecithin/water/oil ternary system either eliminates the gelation or shifts the transition temperature to higher values. The gels are thermoreversible, optically clear and are not birefringent. The composition of these gels indicates that the gelation takes place even at 15 weight % of lecithin and water. The lateral motion of the phosphate part of lecithin head group along surfactant monolayer, as well as the tumbling motion of the entangled assemblies of the rodshaped aggregates are fast enough to average out the chemical shift anisotropy of³¹P NMR spectra. From the FT-IR studies, it has been concluded that the gel formation involves the conformational changes in the polar head part of lecithin molecules in such a way that further hydration of phosphate group is drastically hindered.This paper is respectfully dedicated to the sixtieth birthday of Professor Hans F. Eicke, University of Basel, Switzerland     

Diglycidyl ether of bisphenol-A with 4,4′-methylenedianiline: A pulsed NMR study of the curing process

The curing process, during which the monomeric diglycidyl ether of bisphenol-A is cured with 4,4′-methylenedianiline, has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1ρ, and T₂ have been measured as a function of time as the resin system cures at constant temperature. The relaxation times are interpreted in terms of the decrease in general molecular motion which results from the cure. Plots of correlation frequency versus time for the constant-temperature cure were constructed for three temperatures. It is shown that these three plots can be represented by a reduced curve. With certain simplifying assumptions, the shape of this reduced curve. With certain simplifying assumptions, the shape of this reduced curve can be accounted for in terms of the chemical rate constant and an exponent relating molecular weight to viscosity. The activation energy for the cure is estimated to be 11.7 kcal/mole.     

Studies of solution dynamics of poly(N-vinyl pyrrolidone) and its iodine adduct

Carbon-13 NMR spin-lattice relaxation times T₁ of poly(N-vinyl pyrrolidone) (PVP) and PVP-iodine have been studied in several solvents and at different temperatures. Three kinds of motion can be identified from the T₁ data: segmental motion, ring rotation, and ring puckering. The effective correlation time for segmental motion is calculated to be 1 × 10^?9s, in good agreement with published proton NMR data. Another solvent, 1,1,2,2-tetrachloroethane, behaves like D₂O, the segmental correlation time being 3 × 10^?9s. In benzene, however, the linewidths are very broad and tend to narrow with increasing temperature, but the T₁s are not very different from those of PVP in D₂O. The results suggest association of pyrrolidone rings in benzene that reduces chain dimensions and also restricts chain mobility. As for PVP-iodine in water, again broad resonances are observed which sharpen considerably at higher temperatures. The result agrees with previous suggestions of specific interactions between the pyrrolidone group and iodine.     

Water penetration in nylon 6,6: Absorption,desorption, and exchange studied by NMR microscopy

The uptake of water by nylon 6,6 [42DB Adipure (trade name of Dupont Canada Inc.)] at 100°C has been monitored by a combination of one-dimensional proton NMR spectroscopy, relaxation time (T₁ and T₂) measurements and proton microscopic NMR imaging techniques. The relaxation times of the water absorbed into the nylon matrix are very short at room temperature, (T₂ < 1 ms and T₁ ≈ 1 s) indicating that the water is located in a highly restricted environment and suggesting that strong interactions exist between the absorbed water and the polymer. The diffusion profiles measured at room temperature indicate that the diffusion of water into nylon 6,6 at 100°C is Case I Fickian diffusion. The spatial dependence of the T₂ relaxation time constant and its variation with the water content was also examined. The results reveal that both T₂ and T₂^* decrease toward the center of the sample in samples that have a concentration gradient of sorbed water. In fully saturated samples, no spatial dependence was observed. The overall values of T₂ and T₂^* are also observed to increase as a function of exposure time. An evaluation of the desorption process at room temperature and at 100°C was performed. A continuous, exponentially decreasing solvent profile was observed for the desorption process which again indicates Case I Fickian kinetics. The exchange process of external bulk and atmospheric water with deuterium oxide (D₂O) saturated nylon rods has also been studied using the microscopic imaging technique. © 1993 John Wiley & Sons, Inc.