Publication: Modificación de grafeno con cadenas de polisulfona e incorporación a matrices poliméricas. Evaluación de las propiedades y de la biocompatibilidad de los nanocomposites
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2017-03
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2017-03-27
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Abstract
La presente tesis trata del estudio y diseño de nanocomposites de grafeno empleando
como matrices poliméricas polisulfona y una mezcla de polímeros de polisulfona y resina
epoxi.
En el diseño de estos nanocomposites juega un papel muy importante la interfase del
nanomaterial; ya que una buena interacción entre la nanopartícula y la matriz en la región
interfacial podría mejorar la estabilidad y despersabilidad de los nanorrefuerzos. Para
aumentar la compatibilidad con la matriz, se optó por modificar la superficie de grafeno
con cadenas de polifulfona. La modificación superficial del nanorrefuerzo se realizó
mediante el método que se conoce como nitrene chemistry empleando dos estrategias
sintéticas diferentes; permitiendo obtener cadenas de polímero ancladas con diferente peso
molecular (diferente longitud).
La polisulfona es un polímero empleado generalmente en aplicaciones biomédicas y
medioambientales, por lo que además, se estudió la biocompatibilidad de los
nanomateriales de rGO con cadenas de polisulfona. Para que un material sea
biocompatible debe cumplir dos requisitos, por un lado que presente propiedades
antibacterianas, y además, que no presente citotoxicidad. El estudio realizado de la
biocompatibilidad de nuestro nanomaterial muestra que este puede ser potencialmente
empleado en aplicaciones que requieran contacto directo con el ser humano, ofreciendo
además propiedades antibacterianas.
La preparación de los nanocomposites de polisulfona tuvo lugar mediante la técnica
de extrusión-inyección. Se prepararon nanocomposites con diferente carga de
nanomaterial, 0.1, 0.5, 1.0 y 3 % en peso de rGO y grafeno modificado con polisulfona. La dispersión del nanorrefuerzo presenta una notable importancia en las propiedades del xnanocomposite, por lo que previo al estudio de sus propiedades se analizó mediante
reología, espectroscopía dieléctrica y técnicas microscópicas su dispersión en la matriz
polimérica. De los resultados obtenidos, todas las técnicas empleadas para el estudio de la dispersión concluyen que la modificación superficial con cadenas de polisulfona de mayor peso molecular ayuda a mejorar la dispersabilidad.
Se estudiaron las propiedades mecánicas, térmicas y eléctricas de los nanocompostites, y además, se realizaron estudios de biodegradabilidad. Los resultados
muestran que al aumentar la carga de nanorrefuerzo, mejoran significativamente sus
propiedades térmicas y mecánicas, obteniéndose mejoras en los nanocomposites
modificados con cadenas largas de polisulfona. El estudio conformacional de las cadenas,
concluye que las cadenas de mayor peso molecular se encuentran preferentemente en
conformación semidiluida y además, se encuentran a una distancia lo suficientemente
alejadas unas de otras para que las cadenas de polisulfona de la matriz interpenetren en las cadenas de polisulfona ancladas a la superficie del rGO. Esta interpenetración ayuda a
mejorar y fortalecer la región interfacial, observándose en la mejora de las propiedades.
Los estudios de biodegradabilidad muestran que este nanomaterial, a altos contenidos de
rGO, es altamente resistente a periodos largos de exposición en contacto con aguas
residuales industriales, debido a que sus propiedades iniciales se mantienen y además, es
capaz de inhibir la formación de biofilms.
Los nanorrefuerzos sintetizados fueron también embebidos en una mezcla de
polímeros epoxi polisulfona, con el fin de obtener una morfología de fases invertidas. Se
pretendía obtener esta morfología para introducir las láminas de grafeno preferentemente
en los canales que forma la fase co-continua, para dar lugar a lo que se conoce como
fenómeno de doble percolación. Los resultados del análisis de las propiedades estudiadas
de estos nanocomposites muestran cómo el nanorrefuerzo modificado con cadenas de
polisulfona ayuda a obtener la morfología deseada, obteniendo resultados satisfactorios en las propiedades mecánicas y térmicas estudiadas.
Polymer-matrix nanocomposites formed by incorporation of graphene sheets in polymer matrices have attracted enormous attention in various fields of science and engineering, due to the excellent properties of graphene sheets. Specifically, graphene has been used to improve the mechanical, thermal, electrical, and barrier properties of polymers. Graphenebased polymer nanocomposites have been employed for diverse applications in electronics, aerospace, automotive manufacturing, and green energy. Polysulfones (PSUs) are high-temperature thermoplastic polymers that exhibit great chemical inertness, enhanced oxidative resistance, thermal and hydrolytic stability, as well as high mechanical strength. Additionally, PSUs might be easily processed as a film and thus, they are good candidates for different applications, such as gas separation, hemodialysis, nano/ultra-filtration, adhesives for metal to metal bonds, membranes for fuel cells, drug delivery, or matrices for fiber reinforced composites. Reduced graphene oxide (rGO) is of particular interest, since functional groups on its surface enhance its solubility in organic solvents and remarkably facilitate surface modification with organic molecules or polymers. The grafting of polymer brushes improves the dispersion state and the polymer/graphene interfacial adhesion with the subsequent increase in the nanocomposite properties. During the nanocomposite synthesis, both the rGO/polymer ratio and the molecular weight of the graft polymer play an important role regarding rGO dispersion. This is because the behavior of polymer brushes is strongly determined by the polymer chain conformation. The grafting density and the critical spacing between two neighboring chains determine the brush regime, i.e. mushroom, crossover, and brush-like regimes. Depending on the conformation of the vigrafted polymer chains, the interactions with the polymer matrix may be improved. Another factor to be considered in order to enhance the interphase region in nanocomposite materials is the preparation method of those. One of the objectives of this thesis is improving some of these parameters to obtain better properties in the final nanomaterial, studying the interphase of the system. The present dissertation reports, for the first time, the modification of reduced graphene oxide nanosheets with polysulfone chains through two different synthetic routes via nitrene chemistry. The PSU polymer was bonded to rGO at the end (rGO-PSU end) or randomly along the PSU chain (rGO-PSU mid). These strategies allowed to obtain polymer brushes with two different lengths on the surface. The resulting rGO-PSU synthetic products were carefully characterized by Raman, FTIR, XPS, TEM, and TGA, evidencing the successful grafting of PSU onto rGO surfaces. The long-term stability of these nanosheets was also determined in common solvents. In addition, the biocompatibility of the nanoparticles was tested. This study involved the antimicrobial properties and cytotoxicity in human cells. The nanoparticles of modified and non-modified graphene were tested at different concentrations to determine the most toxic concentrations to both, Gram-positive (B. subtilis) and Gram-negative (E. coli K12) microorganisms. The results showed a reduction of 97% in the growth of B. subtilis after three hours exposure, for the polymer modified nanosheets. The results also demonstrated that rGO-PSU mid exhibited better antimicrobial properties due to its shorter polymer chains, which improves the contact of the microorganisms with the graphene surface. Furthermore, cytotoxicity in human cells was evaluated, showing no toxic effect even at the highest concentration employed in antimicrobial properties. Unmodified and modified rGO with polysulfone brushes were included in polysulfone and epoxy resin matrices to evaluate their properties. PSU nanocomposites were prepared at four different percentages (up to 3 wt% rGO) by extrusion. The extruded material was further processed by injection molding to finally obtain specimens for evaluation of thermal, mechanical, electrical, and antimicrobial properties. The morphology and microstructure of the prepared samples were examined by scanning electron microscopy (SEM) and transmission electronic microscopy (TEM). Rheological and dielectric spectroscopy were employed to study the dispersion state of the nanocomposites, showing that nanocomposites with rGO-PSU end nanoparticles presented better dispersion than non-modified graphene ones. The results indicated that the extrusion-injection procedure was an efficient preparation method of the nanocomposites with good dispersion degrees of rGO in the matrix. Tensile test, dynamic mechanical thermal analysis (DMTA), and nanoindentation showed an important improvement in the Young modulus respect to the neat polymer. The enhancement of mechanical properties was interpreted in terms of the dispersion and interface modification of rGO. A theoretical study about the polymer brushes conformation at the interphase helped to understand the mechanical behavior. Thermal properties of the nanocomposites were also analyzed, showing a moderate increase in the thermal stability. The antimicrobial properties of the prepared nanocomposites, with unmodified graphene, were investigated in E. coli K12. Biofilm formation was studied by confocal microscopy. The antimicrobial properties of rGO were preserved in the nanocomposites and a decrease in the biofilm thickness was observed for the nanocomposites with 3 wt% rGO. A biodegradation study was also performed by exposure to industrial wastewater for nine days. The mechanical properties of the nanocomposites with high rGO content were maintained and they exhibited antifouling properties. The results of this study showed that these materials may be employed in environmental field applications. Epoxy resins are widely used in many industry fields due to their inherent excellent thermal and mechanical properties. Nevertheless, in order to increase toughness and meet high performance applications, they are modified with elastomers, thermoplastics, and all sorts of nanoparticles embedded on the epoxy network. Modification with highperformance engineering thermoplastic modifiers, such as polysulfones, helps to improve epoxy resins toughness. Thermoplastics are usually partially miscible with epoxy resin precursors at several temperatures and compositions, but as curing progress, the decrease in the entropy of mixing leads to a two-phase structure by reaction-induced phase separation (RIPS). Morphology and performance of epoxy/PSU blends has been extensively studied. Several morphologies, such as sea-island, bicontinuous or doublephase, and nodular (phase-inverted) structures have been observed. In this work, rGO or rGO-PSU sheets, up to 1% by weight, have been incorporated to an epoxy/PSU blend containing 20 wt% PSU by weight. The resulting epoxy nanocomposites were characterized by differential scanning calorimetry (DSC), DMTA, and rheology. The morphology and microstructure of the prepared samples were examined by SEM. Phase-inverted morphologies (PSU as continuous phase) have been viiiobserved. Nanocomposites showed interesting morphology changes in the presence of rGO-PSU end, which may be due to rGO nanosheets were preferently embedded and dispersed in PSU channels. The interest in this morphology lies in the possibility of achieving the phenomena known as double threshold, introducing channels where graphene sheets are preferably located. Nanocomposites with PSU and rGO-PSU end nanoparticles showed also an improvement in the mechanical and thermal properties. In summary, nitrene chemistry was successfully applied to graft PSU onto rGO sheets. The synthetic strategy presented in this work demonstrates that modified graphene nanosheets can be easily obtained in high yields. The resulting nanomaterials have suitable dispersability and processability in organic solvents; and present improved antimicrobial behavior and lower cytotoxicity compared to non-modified rGO. PSU matrix nanocomposites show an improvement in mechanical, thermal, antimicrobial, and degradability properties, and epoxy/PSU polymer blends modified with rGO exhibit noticeable morphology and mechanical changes.
Polymer-matrix nanocomposites formed by incorporation of graphene sheets in polymer matrices have attracted enormous attention in various fields of science and engineering, due to the excellent properties of graphene sheets. Specifically, graphene has been used to improve the mechanical, thermal, electrical, and barrier properties of polymers. Graphenebased polymer nanocomposites have been employed for diverse applications in electronics, aerospace, automotive manufacturing, and green energy. Polysulfones (PSUs) are high-temperature thermoplastic polymers that exhibit great chemical inertness, enhanced oxidative resistance, thermal and hydrolytic stability, as well as high mechanical strength. Additionally, PSUs might be easily processed as a film and thus, they are good candidates for different applications, such as gas separation, hemodialysis, nano/ultra-filtration, adhesives for metal to metal bonds, membranes for fuel cells, drug delivery, or matrices for fiber reinforced composites. Reduced graphene oxide (rGO) is of particular interest, since functional groups on its surface enhance its solubility in organic solvents and remarkably facilitate surface modification with organic molecules or polymers. The grafting of polymer brushes improves the dispersion state and the polymer/graphene interfacial adhesion with the subsequent increase in the nanocomposite properties. During the nanocomposite synthesis, both the rGO/polymer ratio and the molecular weight of the graft polymer play an important role regarding rGO dispersion. This is because the behavior of polymer brushes is strongly determined by the polymer chain conformation. The grafting density and the critical spacing between two neighboring chains determine the brush regime, i.e. mushroom, crossover, and brush-like regimes. Depending on the conformation of the vigrafted polymer chains, the interactions with the polymer matrix may be improved. Another factor to be considered in order to enhance the interphase region in nanocomposite materials is the preparation method of those. One of the objectives of this thesis is improving some of these parameters to obtain better properties in the final nanomaterial, studying the interphase of the system. The present dissertation reports, for the first time, the modification of reduced graphene oxide nanosheets with polysulfone chains through two different synthetic routes via nitrene chemistry. The PSU polymer was bonded to rGO at the end (rGO-PSU end) or randomly along the PSU chain (rGO-PSU mid). These strategies allowed to obtain polymer brushes with two different lengths on the surface. The resulting rGO-PSU synthetic products were carefully characterized by Raman, FTIR, XPS, TEM, and TGA, evidencing the successful grafting of PSU onto rGO surfaces. The long-term stability of these nanosheets was also determined in common solvents. In addition, the biocompatibility of the nanoparticles was tested. This study involved the antimicrobial properties and cytotoxicity in human cells. The nanoparticles of modified and non-modified graphene were tested at different concentrations to determine the most toxic concentrations to both, Gram-positive (B. subtilis) and Gram-negative (E. coli K12) microorganisms. The results showed a reduction of 97% in the growth of B. subtilis after three hours exposure, for the polymer modified nanosheets. The results also demonstrated that rGO-PSU mid exhibited better antimicrobial properties due to its shorter polymer chains, which improves the contact of the microorganisms with the graphene surface. Furthermore, cytotoxicity in human cells was evaluated, showing no toxic effect even at the highest concentration employed in antimicrobial properties. Unmodified and modified rGO with polysulfone brushes were included in polysulfone and epoxy resin matrices to evaluate their properties. PSU nanocomposites were prepared at four different percentages (up to 3 wt% rGO) by extrusion. The extruded material was further processed by injection molding to finally obtain specimens for evaluation of thermal, mechanical, electrical, and antimicrobial properties. The morphology and microstructure of the prepared samples were examined by scanning electron microscopy (SEM) and transmission electronic microscopy (TEM). Rheological and dielectric spectroscopy were employed to study the dispersion state of the nanocomposites, showing that nanocomposites with rGO-PSU end nanoparticles presented better dispersion than non-modified graphene ones. The results indicated that the extrusion-injection procedure was an efficient preparation method of the nanocomposites with good dispersion degrees of rGO in the matrix. Tensile test, dynamic mechanical thermal analysis (DMTA), and nanoindentation showed an important improvement in the Young modulus respect to the neat polymer. The enhancement of mechanical properties was interpreted in terms of the dispersion and interface modification of rGO. A theoretical study about the polymer brushes conformation at the interphase helped to understand the mechanical behavior. Thermal properties of the nanocomposites were also analyzed, showing a moderate increase in the thermal stability. The antimicrobial properties of the prepared nanocomposites, with unmodified graphene, were investigated in E. coli K12. Biofilm formation was studied by confocal microscopy. The antimicrobial properties of rGO were preserved in the nanocomposites and a decrease in the biofilm thickness was observed for the nanocomposites with 3 wt% rGO. A biodegradation study was also performed by exposure to industrial wastewater for nine days. The mechanical properties of the nanocomposites with high rGO content were maintained and they exhibited antifouling properties. The results of this study showed that these materials may be employed in environmental field applications. Epoxy resins are widely used in many industry fields due to their inherent excellent thermal and mechanical properties. Nevertheless, in order to increase toughness and meet high performance applications, they are modified with elastomers, thermoplastics, and all sorts of nanoparticles embedded on the epoxy network. Modification with highperformance engineering thermoplastic modifiers, such as polysulfones, helps to improve epoxy resins toughness. Thermoplastics are usually partially miscible with epoxy resin precursors at several temperatures and compositions, but as curing progress, the decrease in the entropy of mixing leads to a two-phase structure by reaction-induced phase separation (RIPS). Morphology and performance of epoxy/PSU blends has been extensively studied. Several morphologies, such as sea-island, bicontinuous or doublephase, and nodular (phase-inverted) structures have been observed. In this work, rGO or rGO-PSU sheets, up to 1% by weight, have been incorporated to an epoxy/PSU blend containing 20 wt% PSU by weight. The resulting epoxy nanocomposites were characterized by differential scanning calorimetry (DSC), DMTA, and rheology. The morphology and microstructure of the prepared samples were examined by SEM. Phase-inverted morphologies (PSU as continuous phase) have been viiiobserved. Nanocomposites showed interesting morphology changes in the presence of rGO-PSU end, which may be due to rGO nanosheets were preferently embedded and dispersed in PSU channels. The interest in this morphology lies in the possibility of achieving the phenomena known as double threshold, introducing channels where graphene sheets are preferably located. Nanocomposites with PSU and rGO-PSU end nanoparticles showed also an improvement in the mechanical and thermal properties. In summary, nitrene chemistry was successfully applied to graft PSU onto rGO sheets. The synthetic strategy presented in this work demonstrates that modified graphene nanosheets can be easily obtained in high yields. The resulting nanomaterials have suitable dispersability and processability in organic solvents; and present improved antimicrobial behavior and lower cytotoxicity compared to non-modified rGO. PSU matrix nanocomposites show an improvement in mechanical, thermal, antimicrobial, and degradability properties, and epoxy/PSU polymer blends modified with rGO exhibit noticeable morphology and mechanical changes.
Description
Mención Internacional en el título de doctor
Keywords
Nanocomposites, Grafeno, Matrices poliméricas, Polisulfona, Resinas epoxi, Biocompatibilidad