Diseño y desarrollo de nanopartículas de alúmina con cadenas de polisulfona injertadas y su efecto compatibilizante en nanocomposites de polisulfona

Este trabajo de investigación presenta la preparación y caracterización de nanocomposites poliméricos mediante la incorporación de nanopartículas funcionalizadas superficialmente para conseguir propiedades específicas. El comportamiento de los nanocomposites en aplicaciones concretas requiere propiedades interfaciales específicas. Las investigaciones realizadas hasta ahora revelan que las interfases partícula/matriz son de carácter débil, siendo fundamental conocer mejor el papel que juegan las interfases para poder desarrollar materiales con un buen comportamiento mecánico y resistencia al desgaste. En este trabajo de tesis se han funcionalizado nanopartículas de alúmina con cadenas de polisulfona (PSU) mediante 1,3 cicloadición dipolar entre los grupos vinilo introducidos en la superficie de las nanopartículas y los grupos azida introducidos en los extremos de cadenas de PSU de dos pesos moleculares y han sido añadidas al 2, 5 y 10% en peso de nanopartículas en una matriz de PSU mediante los procesos de extrusión e inyección. La efectividad del injerto de cadenas de PSU en las nanopartículas en la dispersabilidad de las mismas en la matriz ha sido analizada en función de los diferentes parámetros que gobiernan la mojabilidad entre las cadenas injertadas y las cadenas de la matriz: Densidad de injerto (σ), Peso molecular de las cadenas injertadas (N) a un Peso molecular de la matriz (P) constante. Se han preparado además nanocomposites mediante una técnica de mezclado denominada Jet Milling que se presenta como una posible alternativa al proceso de mezclado en fundido. Esta técnica utiliza una alta energía para realizar la mezcla polímero/nanorrelleno. Tiene la ventaja que se realiza a temperatura ambiente lo que elimina cualquier problema de degradación. Además no requiere ninguna modificación química, haciéndolo adecuado para protocolos biocompatibles en aplicaciones biomédicas. Mediante esta técnica se prepararon nanocomposites nuevamente al 2, 5 y 10% en peso de nanopartículas sin funcionalizar. Los nanocomposites preparados por ambas técnicas de mezclado han sido caracterizados por diversas técnicas confirmando la efectividad que presenta la funcionalización de las nanopartículas. Presentamos por tanto, un trabajo novedoso en el que se pretende aumentar la adhesión interfacial entre polímero y nanopartícula, modificando la superficie de la alúmina con cadenas de la misma naturaleza que la matriz empleada, diseñando una estrategia viable para injertar covalentemente la cadena de polisulfona a la superficie de la nanopartícula. Además se analiza la viabilidad de una nueva técnica de mezclado en estado sólido como alternativa a la técnica de extrusión comúnmente empleada.Programa Oficial de Doctorado en Ciencia e Ingeniería de MaterialesPresidente: Juan Carlos Cabanelas Valcárcel.- Secretario: María Pilar Aranda Gallego.- Vocal: Amaia Soto Beobid

The effect of polymer grafting in the dispersibility of alumina/polysulfone nanocomposites

γ-Alumina nanoparticles have been modified with polysulfone (PSU) chains via 1,3-dipolar cycloaddition reaction between functionalized alumina with vinyl groups and terminal azide polysulfone chains of two different molecular weights. Homopolymer nanocomposites have been prepared for the first time by extrusion and microinjection. The effectiveness of the grafts on the dispersiblity has been analyzed in terms of the parameters that govern the wettability between grafted and matrix chains: graft density (σ), graft molecular weight (N) at constant matrix molecular weight (P). The dispersion state and interfacial adhesion of PSU grafted-nanoparticles have been evaluated from laser scanning confocal, FESEM and SEM microscopy. Results show that the incorporation of the modified g-alumina improves the dispersion state in comparison with bare alumina nanoparticles, reducing the average particle size from 5±9 to 1.3±1 microns. Although aggregates are still present the size of the aggregates are also substantially reduced even with low or moderate graft density used in this work, but further improves the interfacial adhesion between nanoparticle and matrix when long PSU chains are grafted even with low-moderate grafting density. These results can be explained by enthalpic compatibility between polysulfone grafted layer and host polysulfone matrix.Authors wish to acknowledge funding from Spanish Ministerio de Economía y Competitividad under grant MAT2014-57557-R. Authors also thank to I. García and A. Cervera from Euroortodoncia for their invaluable help in nanocomposite processing

Effect of crosslinker on swelling and thermodynamic properties of polyacrylamide gels

PAA networks have been prepared by crosslinking copolymerization of acry- lamide (AA) and N,N'-methylen-bis-acrylamide (BA) in water. The degree of swelling was measured in the range v2 =0-0.16 as a function of crosslinker proportion, C (%w/w) and total concentration, Cr (g/ 100 ml). The swelling of two networks with BA contents of 3 ,;; and 10% were measured as a function of temperature in the range 10-30°C. The interpretation of elastic and swelling data in terms of the scaling law lfh I =Av 2m, shows a transition from a good solvent regime to a collapsed state for the polymer chains, when the crosslinker proportion is around 5 to 7%. Results of elastic and swelling measurements were used to determine the x parameter. A different dependence of x with v2 was found for gels with BA contents below and above 5-7%. The enthalpic contribution to the interaction parameter XH was calculated from swelling experiments as a function of temperature. The results show that when the BA proportion is increased, the enthalpicFinancial support from CAICYT (SPAIN) under grant nc PA86/0370 is gratefully acknoweldged

Stress-strain behavior of polyacrylamide networks

The mechanical properties of a number of polyacrylamide gels have been studied by means of elastic measurements in elongation. Different gels were prepared by varying the proportions of both comonomers, acrylamide and N, N-methylene-bisacrylamide which is used as crosslinker, and a very broad range of concentrations was covered. The most important results considered were the elastic phantom modulus, the degree of (equilibrium) swelling and the elongation at rupture. The data were interpreted in terms of the mechanism of the gelation process taking into account a number of parameters that affect the final structure of the gels: comonomer reactivities, sequence length distributions and cycles formation. Consideration of these factors can explain properly all the features of the elastic results as well as some other properties related to the gelation process such as gel point data.This work was supported by a grant from the CAICYT (0528/84). The authors wish to thank Dr. A. Horta and Dr. J. Freire for many helpful discussions

Nanoindentation and wear behavior of thermally stable biocompatible polysulfone-alumina nanocomposites

The authors would like to acknowledge funding from the Spanish Ministerio de Economía y Competitividad under grant MAT2014-57557-R. R. Prof. Ozisik would like to acknowledge financial support from the U.S. National Science Foundation (CMMI-1538730). The authors also would like to thank I. Garc´ıa and A. Cervera from Euroortodoncia for their invaluable help in nanocomposite processing

Network defects in polyacrylamide gels

The weight conversion at the gel point (αGᵂ) was determined by the gas bubble method in two sets of polyacrylamide (PAA) gels crosslinked with N,N′-methylene-bisacrylamide (BA). αGᵂ remained practically constant when the crosslinker ratio was C = 10% (w/w) and the total concentration of comonomers in the feed ranged from CT = 5 to 10 g/100 ml. When CT was kept constant at 5 g/100 ml and C changed from 0 to 25% (w/w), αGᵂ showed a minimum at C = 7%. By comparing the experimental values of αGᵂ with those calculated for an ideal network, it was concluded that the crosslinking efficiency of BA is very low. The dependence on C of αGᵂ and other properties like the elastic phantom modulus, |ƒₚₕ*|, and the polymer volume fraction at swelling equilibrium, v₂ₛo, was explained in terms of the formation of network defects: the formation of BA sequences longer than unity and that of intramolecular cycles both have an increasing contribution at C > 7% but can be neglected at lower values of C.This work has been supported by a grant from CAICYT No. GG85-0013. The authors thank Dr A. Horta and Dr J. J. Freire for many helpful discussions

Elastic properties of highly crosslinked polyacrylamide gels

Mechanical properties of polyacrylamide gels covering a wide range of polymer concentrations have been studied. Gels were synthesized by using N,N'-methylenebis (acrylamide) as cross-linking agent whose weight percentage, with respect to the total weight of comonomers, ranged from 0.663% to 14.50%. The resulting gels were analyzed by means of their stress-strain isotherms in elongation at 30 °C. Mooney-Rivlin type plots of the data show a large increase of the modulus or upturn, particularly on gels with high percentage of cross-linking monomer and at high polymer concentrations, due to non-Gaussian effects arising from the very heterogeneous molecular network structure. The elastic modulus was found to increase exponentially with total comonomer concentration, keeping constant the percentage of bisacrylamide comonomer. On the other hand, the modulus passes through a maximum as the amount of cross-linking agent is increased. The ultimate properties found and the comparison of the cross-linking densities obtained from the elastic results with the theoretical ones, determined from the initial comonomer compositions, confirm the very high heterogeneity of polyacrylamide gels.We thank Dr. P. Deporcellinis for his kind cooperation. This work was supported in part by Grant 0528/84 from the Comision Asesora de Investigación Científica y Técnica. Registry No. (Acrylamide) (N,N'-methylenebisacrylamide) (copolymer), 25034-58-6

Polyacrylamide gels. Process of network formation

This paper refers to the crosslinking copolymerization of acrylamide (AA or monomer-1) and N,N′-methylene-bisacrylamide (BA or monomer-2) in aqueous solution at 22°, covering a broad range of comonomer concentrations. The extent of reaction, pT, the composition of the remaining comonomer mixture, f₂, and the copolymer composition, F₂, have been determined by high resolution 1H-NMR with polymerization in situ. The non-linear regression fit of f₂ as a function of pT yields almost the same values for the reactivity ratios for any of the studied comonomers feeds. The instant copolymer composition changes with the copolymerization time due to a shift in the residual comonomer composition and, to a smaller extent, to changes in the comonomer relative reactivities. This effect contributes to the heterogeneity of the final network.Financial support from CAICYT (Spain) under Grants No. GG85/0013 and PA86/0370 is gratefully acknowledged. We are also indebted to Dr R. Gonzalez Rubio for his computer assistance, and to Dr J. Guzman for sorne interesting comments

Jet milling as an alternative processing technique for preparing polysulfone hard nanocomposites

This work describes how a solid-state blending method such as jet milling can be used to successfully prepare polysulfone (PSU)/-alumina nanocomposites. For comparison purposes, conventional melt extrusion was used as well. Morphological analysis revealed how jet mill blending allows obtaining well-dispersed -alumina nanoparticles within a polysulfone matrix without any surface treatment, with an important decrease of particle size promoted by the breakup of agglomerates and aggregates due to the particle-particle impacts during processing, which was not observed in the extruded nanocomposites. DSC analysis demonstrated that jet-milling processing promoted T-g enhancements with alumina addition, while TGA experiments confirmed the increment of thermal stability of the nanocomposites prepared by jet milling when compared with the composites prepared by extrusion. The tensile tests showed that ductility remains at a high value for milled nanocomposites, which agreed with the fracture surface images revealing large plastic deformation as a function of the alumina content. This comparative study indicates that the dispersion of nanoparticles in PSU was more homogeneous, with smaller nanoparticles when preparing nanocomposites using jet milling, showing a strong correlation with the enhanced final properties of the nanocomposites.The authors acknowledge funding from Spanish Ministerio de Economía y Competitividad under grant MAT2014-57557-R. R. Ozisik would like to acknowledge financial support from the US National Science Foundation (CMMI–1538730)

Polyacrylamide networks. Kinetic and structural studies by high field H-NMR with polymerization in situ

The crosslinking polymerization of acrylamide (AA) and N,N′-methylene-bis-acrylamide (BA) in aqueous solution at 22°C has been studied by H.R. H-NMR spectroscopy. The initial comonomer mixture was placed inside the NMR spectrometer probe head and the polymerization was followed for 10 hr. This procedure allows measurement of the instant composition of the residual comonomer mixture even during and after gelation and therefore to calculate the composition of the formed copolymer or network. When the reaction was finished, the structure of the most mobile part of the network was also analyzed