NMR and neutron-scattering studies on poly(ethylene) terminally attached at the polystyrene:water interface (original) (raw)
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Langmuir, 1990
A ternary phase diagram of CA-SLS-water was constructed in the dilute region. The structure of the phases was determined through DSC and related to the polymorphism of the alcohol. A birefringent gel phase was found in regions with molar ratios of CA/SLS at least greater than one. The structure of the gel is similar to the lamellar phase; however, the hydrocarbon chains are arranged in a solidified a crystal structure with hexagonal subcell packing. The a form of CA is supercooled to room temperature through a unique interaction between SLS and water with CA. In certain regions of the phase diagram, the a form of CA crystallizes into the 0 form, forming the coagel phase. This transformation is due to the decreased electrostatic repulsion between bilayers upon the desorption of SLS. The free SLS then acts to screen the charge on the bilayer, further reducing the interlayer distance. The expulsion of water from the bilayer removes a component necessary for the supercooling of the a crystal form of CA. Assuming that only a maximum amount of SLS can be incorporated in the a form, this mechanism also explains why the gel region is located in regions where the molar ratio of CA/SLS is greater than one. The structure of the gel and the emulsification mechanism are being examined through the measurement of the self-diffusion coefficients by Fourier transform spin-echo NMR.
The Journal of Physical Chemistry B, 2007
The structure of poly(vinyl alcohol) (PVA) hydrogels formed as a result of freeze/thaw treatments of aqueous solutions of the polymer (11 wt % PVA) in the freshly prepared state is analyzed through the combined use of small (SANS) and ultrasmall (USANS) angle neutron scattering techniques. The structure of these hydrogels may be described in terms of polymer rich regions, with dimensions of the order of 1-2 µm, dispersed in a water rich phase, forming two bicontinuous phases. The PVA chains in the polymer rich phase form a network where the cross-linking points are mainly crystalline aggregates of PVA having average dimensions of ∼45 Å. The structural organization of freeze/thaw PVA hydrogel membranes does not change either after rehydration of dried gels or in the presence of a tensile force. Finally, addition of surfactant micelles inside the gel provides a formulation with both hydrophobic and hydrophilic regions, which demonstrates the potential of the system for drug delivery. Both SANS and EPR measurements show that sodium decylsulfate (C10OS) micelles do not significantly interact with the PVA gel. Variation of the gel structure by the number of freeze/thaw cycles should modulate the rate of release of an active constituent, for example, in a dermal patch.
Macromolecules, 1986
Through the use of differential scanning calorimetry (DSC), the heat of fusion of the solvent in the gel is investigated as a function of polymer concentration. From this type of experiment, a parameter a, which stands for the number of solvent molecules adsorbed per monomer unit, can be determined. The knowledge of this parameter enables one to strongly support the existence of solvated structures responsible for the occurrence of physical cross-links. In addition, a allows one to coherently explain why the original gel structure is stable in some solvents but transforms into the 31 helical form prior to gel melting in some others. By neutron diffraction, three points are dealt with: it is shown (i) that the gel diffraction pattern resembles that of a liquid, (ii) that liquid decalin and the gel have virtually the same diffraction pattern with a pronounced reflection at 0.53 nm, and (iii) that there is no sign of any 0.53-nm reflection when only the polymer is deuterium labeled, which means that the 0.53-nm Bragg spacing does not characterize the chains. All these results are discussed and a molecular model for the gel is proposed. This model assumes that the chains are bridged together by the solvent molecules (ladder-like model). Although the chains certainly possess a rodlike conformation, they do not adopt a well-defined helical structure. It is then concluded that, while the 121 helix may exist in the gel, this form is far from being the main structure.
Microstructural investigation of monoglyceride-water coagel systems by NMR and CryoSEM
Journal of colloid and …, 2005
Monoglyceride coagels consist of a network of plate-like crystals and are formed from a swollen gel state (α-gel). In order to resolve the transition mechanism, coagels were prepared with monoglycerides that differ in fatty acid composition (monomyristate and palmitate/stearate, respectively). Rheology provided information on kinetics of coagel formation and the strength of the resulting crystal network. From NMR measurements, the surface-to-volume ratio, tortuosity, and dimensionality of the network were obtained. These findings were in line with qualitative and quantitative structural information obtained from CryoSEM. As a model for the behaviour of non-monoglyceride species, the dynamics of (perdeuterated) palmitic acid was monitored in both α-gels and coagels. The experimental data support a two-stage mechanism. In the first stage, two-dimensional separation of D-and L-isomers in the monoglyceride bilayers of the α-gel occurs. This process depends primarily on lateral diffusion rate of the monoglycerides. Palmitic acid can be accommodated in the α-gel bilayer, but in the coagels it is separated into relative mobile and mechanically weak junction zones between the crystal plates. In the second stage of coagel formation, the crystal plates also grow in the third dimension. Both monoglyceride type and concentration determine the kinetics of this process. 2004 Elsevier Inc. All rights reserved.
Radiation Physics and Chemistry, 1999
Hydrogels that reversibly swell or shrink in water with decreasing or increasing temperature, respectively, were obtained by g ray-induced polymerization of acryloyl-L-proline methyl ester in the presence of dierent amounts of a crosslinking agent. The role of water in the hydrated polymers was investigated by DSC and 1 H-NMR relaxometry. From the curves of fusion of water determined by the former it was possible to ascertain that the amount of both freezing and non freezing water decreased with increasing the crosslinker percentage and/or swelling temperature. Moreover, at the temperatures higher than 378C, the water absorbed by the dierent hydrogels is mostly present as non freezing water. The 1 H-NMR relaxometry study enabled the spin±spin and spin±lattice relaxation curves to be analyzed. It was possible to distinguish three``populations'' of protons and identify two of them with protons of freezing and non freezing water determined by DCS. #
Biopolymer Gels: Nanostructure and Macroscopic Properties
Colloids for Nano- and Biotechnology
Small-angle neutron scattering (SANS) has been used to investigate the effect of salts (NaCl and CaCl 2) on the structure of DNA and polyacrylic acid (PAA) gels. In the absence of salt a distinct correlation peak is observed in the SANS spectra of both systems indicating that electrostatic interactions play an important role in the organization of the polymer chains. When the salt concentration is increased, the peak position shifts to smaller values of the scattering vector q, and progressively vanishes. Osmotic swelling pressure measurements show that Ca ions reduce the swelling pressure and lead to the collapse of these gels. The Ca/Na ion exchange process does not affect the shear modulus of PAA gels. However, the shear modulus of DNA gels decreases with increasing Ca ion concentration at high swelling degrees, and increases at low swelling degrees. The results indicate that changing the ionic composition provides a simple way to control the nanoscale structures and properties in polyelectrolyte gels.
Journal of Polymer Science Part B: Polymer Physics, 1993
The supermolecular structures of thermoreversible gels formed from either homogeneous or heterogeneous solutions were examined by scanning electron microscopy. The morphologies of gels of polyethylene and polystyrene of various tacticities were then related to the phase diagram of the polymer-solvent system. We confirmed the morphological findings of Aubert on isotactic polystyrene gels formed either above the binodal or inside the spinodal and extended his study to gels prepared within the metastable region of the phase diagram. For polystyrenes and polyethylene, the morphology of the gels formed inside the coexistence curve differs markedly from that of gels formed outside. Inside the binodal, gels of polyethylene and polystyrenes exhibit remarkable morphological similarities, indicating a common gelation mechanism, namely, liquid-liquid phase separation. Depending on the concentration, these gels exhibit either an open strut-like network structure or smooth spherical globules. The former is attributed to gelation inside the spinodal whereas the latter is believed to result from gelation in the metastable region. For crystalline polymers, gels formed inside the coexistence curve subsequently undergo crystallization within their polymer-rich phase. The morphology of isotactic polystyrene and polyethylene gels formed outside the binodal consists of overlapping lamellar structures, whereas that of atactic and epimerized polystyrene gels is characterized by a sheet-like structure, differentiating the crystallization-based mechanism from others.
Structures in polystyrene solutions: Their origins and their implications on physical gelation
Die makromolekulare Chemie, 1990
From studies on dilute and semi-dilute solutions it will be shown that gelation in atactic polystyrene proceeds via the creation of a nematic-like order (formation of a polymer-solvent intercalate with the highly syndiotactic sequences). A similar mechanism holds for isotactic polystyrene (intercalate wherein the chain adopts a near-31 helical form). It will be emphasized that the gelation phenomenon depends upon both the solvent size and the volume of the cavities created by the phenyl groups along the chain. Finally, diffraction experiments on well-organized structures (giant spherolites of 150-200pm diameter) grown from isotactic polystyrene solutions and possessing the same melting point as the gel show Therefore, gel formation is not due to the growth of this helical form. the absence of 121 helices.
Journal of Applied Polymer Science, 1995
The nature of water in copolymeric N-vinyl pyrrolidone/methyl methacrylate hydrogels has been investigated using dielectric thermal analysis (DETA) and differential scanning calorimetry (DSC). Dielectric tan 6 measurements are reported over a frequency range 500 Hz-20 kHz and a temperature range of-140 to +2OoC. The observed complex relaxations were attributed to the mobility of water and the relaxation processes of the copolymer matrix. In addition DSC was used to measure the melting endotherm of water in gels partially swollen to a pseudoequilibrium. The resultant data were fitted to a two-phase approximation model. 0 1995 John Wiley & Sons, Inc.