Carmen S. Renamayor

Porous structures controlled by segregation of ordered mesophases in poly(N,N-dimethylacrylamide) hydrogels polymerized from an isotropic AOT/water medium

N,N-dimethylacrylamide with cross-linker (N,N′-methylenebis(acrylamide)) is polymerized in the presence of various amounts (4.5–51% v/v) of the anionic surfactant AOT. All initial mixtures are isotropic homogeneous solutions, but with polymerization they become microheterogeneous due to development of an anisotropic mesophase whose structure varies from lamellar to cubic to hexagonal (on increasing the AOT concentration). The polymerization is started in two ways: with initiator (AIBN), and induced by X-rays (without initiator). The hydrogels obtained at the end of polymerization with AIBN are analysed regarding swelling capacity and porous structure. Swelling increases with AOT concentration, and the rate of this increase is larger when the mesophase formed is hexagonal than when it is lamellar. These ordered domains induce the following porogen effects: (i) more macropores with a better defined geometry are formed when the mesophase is hexagonal. (ii) The mean diameter of the pores is controlled by the type of mesophase: 25 μm (lamellar); 55 μm (hexagonal). The evolution of polymerization induced by X-rays is followed during a period of time to study the development of the mesophase that occurs by a segregation process, and a model in four steps is proposed to explain the results.

Salting-in effect of ionic liquids on poly(N-vinylimidazole) hydrogels

This work attempts to study the interaction of two ionic liquids (1-ethyl-3-methylimidazolium tosylate and 1-hexyl-3-methylimidazolium chloride) with chemically crosslinked poly(N-vinylimidazole) in aqueous media, and to compare it with that of NaCl, a typical salting-out electrolyte. The three salts show a salting-in effect whose intensity was measured on the basis of the decrease of the polymer–solvent interaction parameter and the increase of swelling with increasing ionic strength. It was thus found that the salting-in effect is the same for the two salts with the same anion (chloride), while the intensity of the salting-in effect exerted by the tosylate ionic liquid is larger. The coefficient of selective sorption, which expresses the salt excess inside the swollen gel with respect to the external solution was determined by comparing the initial and equilibrium compositions of the immersion bath. These results are discussed in terms of the hydrophobic character of the ions involved.

Polymerization of N,N-dimethylacrylamide in Aerosol OT–water mixtures: from lamellae to segregation

The polymerizable N,N-dimethylacrylamide (DMAA) is added to the lamellar liquid crystalline system formed by the anionic surfactant Aerosol OT and water. Small angle X-ray scattering, gives the same bilayer thickness and hydrophilic group cross-sectional area as without DMAA, and an environment sensitive fluorescent probe indicates that DMAA is likely to be in the water layers between the AOT bilayers. This shows that the structure of the AOT bilayer in the lamellar phase is not affected by DMAA. However, DMAA induces instability of the lamellar phase, and a second phase, an isotropic solution, appears for larger DMAA contents The effect of DMAA in stabilizing this phase with respect to lamellae is much larger than that expected from a simple dilution with water. Furthermore, there is a combined influence of temperature and DMAA concentration on the coexistence of the lamellar and the isotropic phases. When an initiator is added and the monomer polymerizes the tendency to phase separate increases strongly. The lamellar spacing after polymerization is below the value predicted by the global AOT/H2O ratio. This indicates that the polymer segregates from the AOT phase and that the lamellar phase is osmotically compressed by the polymer rich phase

Copolymer-surfactant complexes obtained in a lamellar lyotropic medium.

Polymer-surfactant complexes formed between charged copolymers and oppositely charged surfactants are analyzed as a function of the charge density in the macromolecule. Copolymers of ionizable diallyldimethylammonium chloride (DADMAC) and neutral acrylamide are obtained at different comonomer ratios. When mixed with the lamellar medium formed by the anionic surfactant 1,4-bis(2-ethylhexyl)sodium sulfosuccinate (AOT) in water, they give rise to highly condensed lamellar phases in equilibrium with another lyotropic phase. The structure of these phases is studied by SAXS and optical microscopy revealing the formation of copolymer-surfactant complexes which present a lamellar structure. The composition of the phases is inaccessible to direct determination, because they do not separate macroscopically (in most of the samples). Thus, the stoichiometry is determined using a model which considers the charge density of the copolymers. This model allows, from the experimental data provided by SAXS, to calculate the composition and volume ratio of the phases. The results indicate that these complexes are nonstoichiometric, containing a lesser amount of DADMAC than surfactant units. The neutral sequences of acrylamide can be considered as bridges along the water domains remaining anchored to the AOT bilayers by the cationic DADMAC units. When the charge density diminishes, the bridges become longer, rendering structures with higher water content.

Non-diffusion-controlled excimer formation with indane and acenaphthene.: Kinetics and thermodynamics from picosecond-time-resolved fluorescence

Abstract Excimer formation with acenaphthene (AN) and indane (IN) in THF was studied using steady-state and picosecond-time-resolved fluorescence data as a function of temperature (173–298 K). Several methods of decay analysis (single decay deconvolution, global analysis and excimer deconvolution with monomer) were employed to extract all kinetic and thermodynamic parameters. In both cases, the transition temperature between the high and low temperature limits is low (182±5 K) due to efficient excimer dissociation. Excimer formation with these compounds is non-diffusion-controlled (the rate constants of excimer formation are eight- (AN) or nine- (IN) fold lower than the diffusional limit. This conclusion is opposite to the generally accepted concept that excimer formation is diffusion-controlled.

Lyotropic Lamellar Structures of a Long-Chain Imidazolium and Their Application as Nanoreactors for X-ray-Initiated Polymerization

The lyotropic behavior of the ternary system formed by 1-tetradecyl-3-methylimidazolium chloride, 1-decanol, and water is investigated. A lamellar mesophase is formed for a wide range of compositions and is characterized by polarized optical microscopy, low-temperature scanning electron microscopy, small- and wide-angle X-ray scattering with synchrotron radiation, and differential scanning calorimetry. This phase presents onionlike structures. Two lamellar structures are formed: an Lα mesophase between 25 and 50 °C, with an isobaric thermal expansivity of the bilayer thickness of -3.2 × 10-3 K-1, and a lamellar gel phase, when the temperature decreases below 25 °C. This new medium is employed to perform in situ X-ray-initiated polymerization of N-isopropylacrylamide. When the monomer is incorporated in the lamellar structure, it is distributed between the water layer and bilayer interface and its polymerization can be followed by variations in the diffractograms with time.

Ionic liquid–water mixtures as solvents for poly(N-vinylimidazole)

Ionic liquids extend the Hofmeister series and create a wide range of new possibilities for processes involving salt effects on both soluble and crosslinked systems. This work reports on some mixtures of water with NaCl or an ionic liquid, either 1-ethyl-3-methylimidazolium tosylate or 1-hexyl-3-methylimidazolium chloride, which are better solvents for linear poly(N-vinylimidazole) (L-PVI) than water, i.e., that exhibit a salting-in effect. The intensity of the salt effects was measured on the basis of the polymer solubility, the decrease in polymer–solvent interaction parameter (measured by light scattering), and the increase of coil size (measured through the intrinsic viscosity). It was thus found that the intensity of the salting-in effect of either NaCl or 1-hexyl-3-methylimidazolium chloride on L-PVI is different (larger for the ionic liquid), which denotes that salt effects are not under anion control, and the mechanisms operating in the linear and crosslinked polymers are different. These results are discussed after accounting for the role of ion–polymer interactions.

Intramolecular excimer formation in copolymers of methylmethacrylate and indene

A set of fifteen samples of indene (IN) with methylmethacrylate (MMA) copolymers was synthesized by free radical polymerization in bulk. Reactivity ratios, mean sequence lengths and distribution-functions of monomeric units indicate the formation of quasirandom copolymers, with a larger tendency to blockiness for MMA units. UV and NMR results are consistent with these observations. No typical signals of strongly interacting aromatic moieties can be identified either in the UV short-wavelength spectra region, or in the 1H NMR aromatic proton region. Photophysical behavior is analysed. © 2000 Society of Chemical Industry