Carmen Mijangos

Tailored polymer-based nanofibers and nanotubes by means of different infiltration methods into alumina nanopores.

Template synthesis is one of the most effective methods for the preparation of one-dimensional polymer-based nanostructures (1DPNs). Both hollow nanotubes and solid nanorods or nanofibers with tailored dimensions can be obtained by simply templating a porous material with suitable pore size. The mechanism of polymer infiltration into the pores is also very important in order to obtain the desired one-dimensional nanostructures and to control their final morphology. In this study, several infiltration methods were explored with the aim to obtain different 1DPNs. It was shown that, with these infiltration methods, it is possible to obtain nanofibers and nanotubes of any diameter and length composed of polymers with a wide chemical nature (poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidene fluoride), etc.), or even composed of nanoparticulate composites. Finally, the selection of infiltration method for desired nanostructure is discussed.

In situ Synthesis of Magnetic Iron Oxide Nanoparticles in Thermally Responsive Alginate-Poly(N-isopropylacrylamide) Semi-Interpenetrating Polymer Networks

A novel semi-interpenetrating polymer network based on alginate and poly(N-isopropylacrylamide) (PNiPAAm) has been synthesized that shows response to temperature and magnetic fields. Highly homogeneous porous hydrogels are obtained by copolymerizing N-isopropylacrylamide and bis-acrylamide in the presence of an aqueous alginate solution. The synthesis of magnetic iron oxides by in-situ oxidation of iron cations coordinated to the alginate network results in a hydrogel with an enhanced deswelling rate with respect to pure PNiPAAm.

Structure of Poly(vinyl alcohol) Cryo-Hydrogels as Studied by Proton Low-Field NMR Spectroscopy.

The network structure of poly(vinyl alcohol) (PVA) hydrogels obtained by freezing−thawing cycles was investigated by solid-state 1H low-field NMR spectroscopy. By the application of multiple-quantum NMR experiments, we obtain information about the segmental order parameter, which is directly related to the restrictions on chain motion (cross-links) formed upon gelation. These measurements indicate that the network mesh size as well as the relative amount of nonelastic defects (i.e., non-cross-linked chains, dangling chains, loops) decrease with the number of freezing−thawing cycles but are independent of the polymer concentration. The formation of the PVA network is accompanied by an increasing fraction of polymer with fast magnetization decay (∼20 μs). The quantitative study of this rigid phase with a specific refocusing pulse sequence shows that it is composed of a primary crystalline polymer phase (∼5%), which constitutes the main support of the network structure and determines the mesh size, and a secondary population of more imperfect crystallites, which increase the number of elastic chain segments in the polymer gel but do not affect the average network mesh size appreciably. Correspondingly, progressive melting of the secondary crystallites with increasing temperature does not affect the network mesh size but only the amount of network defects, and melting of the main PVA crystallites at ∼80 °C leads to the destruction of the network gel and the formation of an isotropic PVA solution.

Confinement effects on the crystallization of poly(ethylene oxide) nanotubes.

In this work, we show the effects of nanoconfinement on the crystallization of poly(ethylene oxide) (PEO) nanotubes embedded in anodized aluminum oxide (AAO) templates. The morphological characteristics of the hollow 1D PEO nanostructures were evaluated by scanning electron microscopy (SEM). The crystallization of the PEO nanostructures and bulk was studied with differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The crystallization of PEO nanotubes studied by DSC is strongly influenced by the confinement showing a strong reduction in the crystallization temperature of the polymer. X-ray diffraction (XRD) experiments confirmed the isothermal crystallization results obtained by DSC, and studies carried out at low temperatures showed the absence of crystallites oriented with the extended chains perpendicular to the pore wall within the PEO nanotubes, which has been shown to be the typical crystal orientation for one-dimensional polymer nanostructures. In contrast, only planes oriented 33, 45, and 90° with respect to the plane (120) are arranged parallel to the pores main axis, indicating preferential crystal growth in the direction of the radial component. Calculations based on classical nucleation theory suggest that heterogeneous nucleation prevails in the bulk PEO whereas for the PEO nanotubes a surface nucleation mechanism is more consistent with the obtained results.

Neutron scattering study of the dynamics of a polymer melt under nanoscopic confinement

Poly(ethylene oxide) confined in an anodic aluminum oxide solid matrix has been studied by different neutron scattering techniques in the momentum transfer (Q) range 0.2<or=Q=/Q/<or=1.9 A(-1). The cylindrical pores of the matrix present a diameter (40 nm) much smaller than their length (150 microm) and are parallel and hexagonally ordered. In particular, we investigated the neutron intensity scattered for two orientations of the sample with respect to the incident beam, for which the Q direction was either parallel or perpendicular to the pores for a scattering angle of 90 degrees . Diffuse neutron scattering at room temperature has shown that the aluminum oxide has amorphous structure and the polymer in the nanoporous matrix is partially crystallized. Concerning the dynamical behavior, for Q<1 A(-1), the spectra show Rouse-like motions indistinguishable from those in the bulk within the uncertainties. In the high-Q limit we observe a slowing down of the dynamics with respect to the bulk behavior that evidences an effect of confinement. This effect is more pronounced for molecular displacements perpendicular to the pore axis than for parallel displacements. Our results clearly rule out the strong corset effect proposed for this polymer from nuclear magnetic resonance (NMR) studies and can be rationalized by assuming that the interactions with the pore walls affect one to two adjacent monomer monolayers.

Surface modification of PVC films in solvent-non-solvent mixtures

Appropriate reaction conditions for the selective surface modification of PVC films with sodium azide and amino thiophenol were investigated. Whilst the use of a phase-transfer agent makes it possible for the nucleophilic substitution reaction to occur, the morphology of the film is changed by the reaction, leading to a loss in both surface smoothness and transparency of the film. On the other hand, performing the reaction in mixtures of a good and a non-solvent for PVC modification takes place in a controlled manner and the film remains both smooth and transparent. The degree of modification can be varied as a function of time, of temperature and the proportion solvent to non-solvent used in the reaction. The determination of the overall degree of modification of films of different thickness allows one to draw conclusions about the surface selectivity of the reaction. According to this study, the reaction with sodium azide occurs homogeneously through the film, whilst the use of amino thiophenol leads to a polymer film which is preferentially modified at the surface.

Effect of physical aging on the gas transport properties of PVC and PVC modified with pyridine groups

Abstract The gas transport coefficients of polyvinyl chloride (PVC) and PVC modified with pyridine groups have been studied. It has been observed that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. For PVC there is a two-fold reduction of the diffusion coefficients during the first two days, and about one order of magnitude, a month after the membranes are prepared, and no stabilisation of the trend is seen after a month. Membranes prepared from modified PVC show a short-term diffusion rate reduction which is similar to that found in PVC, while at longer times the diffusion rate decrease levels off quickly, attaining constant values after about ten days. The time dependence of the transport coefficients is attributed to the samples’ physical aging and an attempt is made to fit the experimental data by considering a stretched exponential time dependence of the volume contraction on aging.

Magnetic Hydrogels Derived from Polysaccharides with Improved Specific Power Absorption: Potential Devices for Remotely Triggered Drug Delivery

We report on novel ferrogels derived from polysaccharides (sodium alginate and chitosan) with embedded iron oxide nanoparticles synthesized in situ and their combination with thermally responsive poly(N-isopropylacrylamide) for externally driven drug release using AC magnetic fields. Samples were characterized by Raman spectroscopy, transmission electron microscopy, and magnetic measurements. The obtained nanoparticles were found to be of ∼10 nm average size, showing magnetic properties very close to those of the bulk material. The thermal response was measured by power absorption experiments, finding specific power absorption values between 100 and 300 W/g, which was enough for attaining the lower critical solution temperature of the polymeric matrix within few minutes. This fast response makes these materials good candidates for externally controlled drug release.

Influence of iron oxide nanoparticles on the rheological properties of hybrid chitosan ferrogels

Magnetite nanoparticles have been successfully synthesized in the presence of chitosan using an in situ coprecipitation method in alkali media. This method allows obtaining chitosan ferrogels due to the simultaneous gelation of chitosan. The chitosan concentration has been varied and its effects on the particle synthesis investigated. It has been demonstrated that high chitosan concentrations prevents the formation of magnetite due to the slow diffusion of the alkali species through the viscous medium provided by chitosan, instead iron hydroxides are formed. The presence of magnetite nanoparticles increases the elastic modulus which results in a reinforcement of the chitosan ferrogels. This effect is counterbalanced by the disruption of hydrogen bonding responsible for the formation of chitosan hydrogels in alkali media.

PVC modification with new functional groups. influence of hydrogen bonds on reactivity, stiffness and specific volume

Abstract The chemical modification of PVC with new bifunctional thiol compounds is reported. Aliphatic as well as aromatic reactives were tested and the influence of protic and non-protic functionalities on reactivity was studied. The chemical structure of the polymers was analyzed using 1 H NMR spectroscopy. The structural changes in the modified samples were monitored by means of density and T g determinations, which are a measure of interchain spacing and chain stiffness. While protic functionalities lead to polymers with strongly enhanced T g values indicating a considerable stiffening of the system due to physical interaction by hydrogen bonds, the softening-point temperature of PVC modified with non-protic substituents does not change very much. The interchain spacing of the polymer is not significantly altered by the presence of mobile hydrogen in the polymer.