Eugenio Rodríguez-Núñez

Rheological behaviour of an amide pectin

Forced-oscillation measurements of aqueous dispersions of an amide pectin were carried out at pH 3.83 in the presence of 50% sucrose. The results show a predominant viscous behaviour corresponding to the so-called terminal region of the general mechanical spectrum of biopolymer solutions. The polymer chains have been described by the bead-spring model with dominant hydrodynamic interactions among beads. At pH 3.31, samples are more elastic than viscous and the mechanical spectra reveal an entanglement network (weak gel), although not dense enough to store the stress during a long period of time after having been deformed. At low pectin concentration, the plot of loss moduli versus frequency shows a minimum that suggest monodisperse junction zones and the existence of two regions with different relaxation times. The short relaxation process corresponds to movements of galacturonic units inside the junction zones (local configuration adjustment), and the longer one to movements of the junction zone as a whole (rearrangements of couplings through entanglement and cross-linking). The comparison of the results obtained here at different sucrose concentrations and temperatures with those for methoxy pectins, suggests that the hydrogen bonds formed by amide groups have an important role in the formation of the gel. The role of the hydrophobic interactions in the stability of the junction zones has also been discussed.

Physical aging for an epoxy network diglycidyl ether of bisphenol A/m-xylylenediamine

Abstract The physical aging of the epoxy network consisting of a diglycidyl ether of bisphenol A (BADGE n=0) and m-xylylenediamine (m-XDA) were studied by differential scanning calorimetry. The following aging temperatures have been used in this work: 60, 70, 80, 90, 100 and 110 °C. The glass transition temperature and the variation of the specific heat capacities have been calculated using the method based on the intersection of both enthalpy–temperature lines for glassy and liquid states. The endothermic aging peak, relaxation enthalpy and fictive temperature were also calculated for each aging temperature and aging time.

Aggregation behaviour of polygalacturonic acid in aqueous solution

Static and dynamic light scattering techniques have been used to study the aggregation behaviour of polygalacturonic acid (PG) in aqueous solution at different pH values. The dependence of apparent molecular weight, radius of gyration and hydrodynamic radius with time or concentration have been determined. Other structural parameters as contour length, Kuhn segment numbers, mass per unit length, persistence length, and average number of chains per cross section have been deduced from Casassa –Holtzer plots. The weight average molecular weight increases linearly with time suggesting that the aggregation kinetics follows a simple Smoluchowski equation in which all the kernels are identical, the aggregation rate being approximately two chains per day at pH 3.83. As a consequence of this growth the polydispersity of the aggregates increases with time. The growth of the hydrodynamic radius with time suggests that no water molecules remain trapped in the aggregate, explaining why the PG does not gel. q 2003 Elsevier Science Ltd. All rights reserved.

Calorimetric study of nanocomposites of epoxy network DGEBA n = 0/m-XDA with gold nanoparticles

The optimisation of cure conditions for epoxy system diglicidil ether of bisphenol A (DGEBA n = 0)/meta-xylylenediamine (m-XDA) has been made using experimental design. A quantitative analysis by Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) as well as making of nanocomposites of epoxy system with gold nanoparticles has been carried out. The knowledge of the optimum values for the variables of experimental design obtained with DSC in the system BADGE/m-XDA allows to study the adequate conditions of the nanocomposites curing process. DSC was used to measure the glass transition temperature (Tg) and the degree of conversion (a). The FTIR method should be a more precise indicator of degree of conversion than DSC and it was used to corroborate the degree of conversion.

CHARACTERIZATION OF NANOCOMPOSITES USING MICROWAVES FOR CURING PROCESS

The main objetive of this work is the synthesis of epoxy nanocomposites with gold nanoparticles. The curing process was optimized by microwave taking into account variables such as time and temperature.