Functional polymers with embedded nanoparticles

Abstract

The present thesis presents experimental work on functional polymers with embedded nanoparticles. The work was specifically aimed at synthesizing and characterizing anisotropic bulk polymer-clay nanocomposites (PCNs) with aligned clay nanoparticles, and is divided into two main parts: Nanocomposite hydrogels (NC-gels) The poly(N-isopropylacrylamide)/Laponte-clay nanocomposite hydrogel (NC-gel) is a type of hydrogel where the polymer network is cross-linked by Laponite clay nanoparticles instead of organic cross-links. In this work a preliminary study on the feasibility of using external electric fields to induce alignment of the Laponite nanoparticles, in such an NCgel, was conducted. We concluded that electric field alignment was highly inefficient on this system since it contained around 90 % water. Due to the high dielectric constant of water the alignment persisted only for a few microseconds. During the work on the NC-gels a highly interesting phenomenon was discovered when small fractions of oxygen was allowed to come in contact with the reaction mixture while it polymerized. Complex interactions among clay, oxygen and the polymer were found to induce phase segregation of the mixture, into one polymer-rich and one polymer-deficient water-clay phase. This work is presented in Paper I. Synthesis of a new type of ferro-NC-gels (FNC-gels) with embedded magnetic nanoparticles was performed. A detailed in situ small angle x-ray scattering (SAXS) study of both the NC-gels and FNC-gels was conducted to find evidence of structural anisotropy when the gels were synthesized inside magnetic fields. A study of a photopolymerization synthesis of the NC- and FNC-gels was also conducted, using riboflavin-5’-phosphate (R5P) as the initiator. With this we tried to find a simple setup for use during in situ SAXS studies that could replace the temperature induced free-radical polymerization technique we already employed, which used potassium peroxodisulphate (KPS) as initiator. The resulting photopolymerized gels showed inferior mechanical properties compared to the ones produced using KPS as initiator, and due to this we decided not to develop the synthesis further. Solid PCNs By incorporating clay nanoparticles in polymers large improvements in thermal and mechanical properties can be achieved, compared to the polymer alone, if the clay is welldispersed in the polymer matrix. In this work a range of surface modified clays and solid polymer-clay samples were synthesized and carefully analyzed using different techniques. Several interesting findings from this work are presented in Paper II, Paper III and Paper IV. First of all, an almost linear correlation between the amount of clay surface charge, and the quantity of surfactant that each clay could incorporate, was established. The surface charge of the clays further proved to indirectly determine the level of improvements in high temperature resilience, and the extent of dispersion of the clay. Lower surface charge was seen to lead to increased dispersion of clay in polystyrene-clay nanocomposites (PSNCs), which resulted in larger improvements in thermal properties. The exact opposite, however, was seen in nylon 6/clay nanocomposites (N6NCs). In these materials both thermal and mechanical properties showed the largest improvements when the surface charge was high. The mechanical properties of the PSNCs, on the other hand, were inferior to pure polystyrene, but interestingly a low surface charge induced the least reduction. In situ heating studies of dispersions of clay in liquid styrene (the monomer of polystyrene) indicated that almost full exfoliation prior to polymerization could be achieved when the surface charge was low. Electric field guided self-assembly was utilized to align the clay nanoparticles into long chain-like structures during synthesis, for production of anisotropic polystyrene-clay nanocomposites. Wide angle x-ray scattering, x-ray computed microtomography, and optical microscopy were used to determine the microstructure of both aligned and nonaligned samples. This self-assembly technique was, however, not compatible with the nylon 6/clay nanocomposites. The reason for this was investigated further and was concluded to be related to a large increase in the dielectric constant of ε-caprolactam (the monomer of nylon 6) as it was melted. When this occurred the dielectric constant became higher than for the clay particles, and polarization of the clay particles could thus not take place.