Julio Guzmán

Surface Modification of Sepiolite in Aqueous Gels by Using Methoxysilanes and Its Impact on the Nanofiber Dispersion Ability

Surface modification reactions on needle-like sepiolite using alkyl and functional silanes have been carried out in the form of aqueous gels. In contrast with modifications in organic solvents, reactions in water make it possible to modify the surface of almost-individual sepiolite fibers and produce either a continuous coating or a nanotexturization of the sepiolite fiber surface, depending on the reaction conditions. This clean procedure substitutes advantageously organic solvent surface modifications and allows the tuning of surface properties such as specific surface area, wetting behavior, and chemical functionalization. A consequence of such tuning is, for example, the excellent dispersion of modified sepiolite nanofibers in a great variety of polymers by routine compounding and processing techniques.

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.

Synthesis and polymerization of acrylic monomers with hydrophilic long side groups. Oxygen transport through water swollen membranes prepared from these polymers

The synthesis and characterization of tetraethyleneglycol acrylate (TTEGA) and tetraethyleneglycol diacrylate are described. Radical polymerization reactions of TTEGA were carried out at different temperatures and the curves of conversion against time, obtained by using dilatometric techniques, allowed the determination of kp/kt12, where kp and kt are, respectively, the propagation and termination rate constants. The values found for this ratio were comparatively much higher than those reported in the literature for other acrylic monomers. The polymer, poly(tetraethyleneglycol acrylate), is soluble in water, exhibits low glass transition temperature (−45°C) and the percentage of syndiotactic dyads in the chains lies in the vicinity of 65 ± 5%, a value normally found in similar polymers. Both poly(tetraethyleneglycol acrylate-co-tetraethyleneglycol diacrylate) and poly(triethyleneglycol acrylate-co-triethyleneglycol diacrylate) membranes were prepared by radical polymerization of the corresponding monomers and a small quantity of diacrylic esters. Electrochemical techniques were used to evaluate oxygen transport through these membranes swollen in water. The apparent values of both the permeability and diffusion coefficients are unusually large as a consequence of a high swelling degree of these membranes. Although the solubility coefficient of oxygen in the swollen hydrogels is larger than in water, restrictions in the diffusion path caused by the polymer matrix decrease the diffusion coefficient of the gas to ca one-third of its value in water.

The development of electrical treeing in LDPE and its nanocomposites with spherical silica and fibrous and laminar silicates

Electrical treeing in LDPE and three LDPE nanocomposites, with spherical silica and fibrous and laminar phyllosilicates, has been studied. Electrical tests were performed at a 50 Hz frequency and voltages between 8 and 29 kV, and the time to inception of the first electrical partial discharges (TTI) of the electrical trees and the time to breakdown (TBD), related to the electrical stability of the insulator, were determined. Above 15 kV all the nanocomposites show longer inception times and shorter tree growth times than LDPE. It is proposed that both observations are caused by the modification of the polymer crystalline morphology induced by the presence of the fillers and by the development of a large number of interfacial structures, both organo–inorganic and amorphous–crystalline. Below 15 kV the TBD is increased in the nanocomposites with the laminar silicate because of tortuosity and the TTI is increased in the fibrous silicate containing a nanocomposite because of the LDPE crystalline morphology in the presence of the silicate. The nanosilica particles decrease the electrical stability in the whole voltage range by decreasing both TTI and TBD.

Crystallization of polyformals: 1. Crystallization kinetics of poly(1,3-dioxolane)

Abstract Poly(1,3-dioxolane) fractions ranging in molecular weight from 8800 to 120 000 have been isothemally crystallized in the temperature range 25–41°C. From the dilatometric isotherms, the Avrami exponent is an integral number, 3, and is independent of temperature and molecular weight. The level of crystallinity is dependent on molecular weight and there is a change from ∼55% for the highest molecular weight fraction to ∼80% for the lowest molecular weight fraction. The overall crystallization rate temperature coefficient was studied using two dimensional nucleation theory and it was found that the interfacial free energies do not change with molecular weight. However, the usual plots are nonlinear in the whole range of crystallization temperatures. For the high crystallization temperatures the slope is about twice the low crystallization temperature slope, this change being related to a morphological transition.

A potentiostatic study of oxygen transmissibility and permeability through hydrogel membranes

The permeability characteristics of membranes prepared from hydrogels of poly(2-hydroxyethylmethacrylate) (PHEMA) and poly(2-hydroxyethylmethacrylate-co-N,N-dimethylacrylamide) (PHNDA) are described. True values of the permeability and transmissibility coefficients of oxygen in the membranes are determined by using electrochemical procedures involving the measurement of the steady state current either in membranes with different thickness or in a single membrane in which its thickness is varied with layers of moistened paper. Comparison of the results obtained for the transport properties in these hydrogels with others obtained in other hydrogels permit to conclude that the degree of swelling rather than the chemical nature of the hydrogels affects the permeation properties. The chemical structure presumably only affects in high degree the chemical stability and flexibility of the hydrogel membranes.

Microwave versus conventional heating in the grafting of alkyltrimethoxysilanes onto silica particles.

The scope of this work is the comparative analysis in terms of grafting rate, structure of the grafted layer, and wetting behavior of three series of silica nanoparticles modified with alkyltrimethoxysilanes by using conventional heating with and without acid catalysis, and microwave irradiation. A comprehensive characterization of the grafted layer by means of Fourier transform infrared (FTIR), microanalysis, and solid state NMR techniques has shown that microwave irradiation provokes a pronounced increase in the loading rate compared to conventional heating. This microwave effect is outstanding in the case of the reactions with methyltrimethoxysilane, because of the acceleration of the condensation rate. Moreover, solid state NMR spectra ((29)Si and (13)C) strongly suggest structural differences in the grafted layer obtained by the two heating sources. The wetting behavior of the modified nanoparticles was studied, concluding that these changes in the structure of the grafted layer induced by the synthetic procedure do not determine the values of the dynamic water contact angles.

Crystallization of polyformals: 2. Influence of molecular weight and temperature on the morphology and growth rate in poly(1,3-dioxolane)

Abstract The morphology and growth rates of crystallized molecular weight fractions of poly(1,3-dioxolane) covering the range M n = 8 800 to 120 000 have been studied by polarized light microscopy. Two different supermolecular structures, dependent on molecular weight and crystallization temperature have been found. Spherulites are formed after rapid crystallization and a more disordered morphology is formed at the lowest undercoolings but there is a temperature region where both forms are observed. The disordered form appears first and a consecutive spherulitic growth takes place. The crystallization kinetics were analysed over the temperature range 10°C to 36°C. At crystallization temperatures lower than 15°–18°C, the growth rate is linear and only spherulites are found. In the temperature range from 18°C to 36°C a well defined break is observed in the growth rate but the spherulitic growth rate is always higher than that of the irregular form. The growth rate temperature coefficient was studied and the usual plots are not linear in the whole range of crystallization temperatures. For the high crystallization temperature region, the slope is about twice as great as the low crystallization temperature slope. This is the region where regular spherulites are formed. The comparison between dilatometric and growth rate data has shown that the overall rate and growth rate temperature coefficients are the same.

Electrical treeing in LDPE nanocomposite materials

This work deals with the inception and the growth of electrical treeing inside different Low Density Polyethylene (LDPE) nanocomposites. Two nanoflllers were adopted: sepiolite and montmorillonite. Both nanoflllers belong to the family of silicate clay minerals but have different aspect ratio. The nanocomposites have been prepared by melt dispersion of sepiolite, montmorillonite and a mixture of both. The results evidence that the growth of the electrical treeing inside the original pure polymer can be altered significantly by the dispersion of inorganic nanoparticles.

Kinetic investigations on the photopolymerization of di‐ and tetrafunctional methacrylic monomers in polymeric matrices. ESR and calorimetric studies. II. Postpolymerization reactions

The reaction kinetics in the dark of photopolymerized mono- and dimethacrylates in a polymeric binder has been studied. Electron spin resonance spectroscopy (ESR) provided useful information regarding the nature of the radicals involved in postpolymerization reactions. Computer simulations were performed to study the decay of the propagating radicals by considering normal bimolecular termination and transfer reactions of the radicals to the binder. Differences were found in the termination reactions for mono- and difunctional monomers when they were photopolymerized in a solid medium. Absolute kinetic constants for H-transfer reaction with the binder, relative kinetic rate constants for radical-radical coupling, and average lifetimes for the radicals have been calculated.