Self-assembled and flow induced nano-structures in polymer nanocomposites

• Main Author: Ratnaweera, Dilru R
• Format: Dissertation
• Language: English
• Subjects: chemistry, general; chemistry, polymer
• ISBN: 1267409886; 9781267409881

Distribution of nanoparticles (NPs) in polymers and role of NPs in modifying the structure of the polymeric matrices were studied using X-ray diffraction, neutron scattering, neutron reflectivity and atomic force microscopy techniques. Dispersion of NPs in polymers is challenging due to the aggregation tendency of NPs and inherent mixing challenges of polymer. Several strategies were used to control the NP distribution in polymers, including "smart blending" and tailoring the interactions of the polymers.
In smart blending chaotic flow fields were used to disperse and orient montmorillonite NPs in homopolymer melt of polyamide 6 (PA6). The roles of duration of chaotic advection on the melt (N) and NP concentration on polymer chain orientation and crystalline morphology were investigated. The major crystalline form of PA6 changed from a stable form to a meta-stable, which is defined as a state with local stability to small perturbations in the system, gamma form upon addition of NPs. This crystalline transformation is enhanced with increasing N for lower NP concentrations. At higher NP concentrations, polymer chain packing is mainly controlled by the surface forces of NPs. In these nanocomposites the polymer chains orient perpendicular to the NP surface as well as to the extrusion direction. Upon annealing and stretching, the fraction of the stable crystalline form increases and the polymer chains orientate parallel to the NP surface. Using small angle neutron scattering we were able to show that in blended films amorphous and crystalline domains form stacks where water uptake of amorphous domains is significantly higher than the crystalline ones.
A second approach included tailoring interaction of polymer and NPs. For this purpose we used block copolymers. Block copolymer can be used as a template to disperse NPs into specific domains of the matrix polymer. Using fluorinated segments affects many of the polymer properties. The first stage of this study investigates the effects of fluorine on a model diblock copolymer, polytrifluoro propyl methylsiloxane-b-polystyrene, in solution. We found that this polymer forms assemblies with different shapes ranging from spherical to elliptical micelles in solutions. These micelles exhibit unique temperature stability and associated into micelles at small volume fractions of the fluorinated block compared to the diblock copolymer micelles in lower segregation limit. As the temperature increases the micelles dissociate into free chains to form unimolecular micelles.
In the second phase of the study, copolymer templates were used to control the dispersion of NPs in thin polymer films. A semi-fluorinated random copolymer, biphenyl perfluorocyclobutyl, was used as a matrix polymer. Fluorinated blocks segregated toward the lower surface energy air/polymer interface while the hydrogen rich blocks moved to the substrate/polymer interface due to differences in surface energies of the fluorinated and protonated blocks. This segregation results in multi layer thin films with alternative fluorine rich and hydrogen rich layers. The dimensions of the NPs and the combine size of the fluorinated and protonated blocks were about the same. NPs migrate to internal surface induced interfaces in contrast to block copolymer in the lower segregation limit where NPs were segregated to the external interfaces due to translational and conformational entropic contributions. Modifying the NPs with a single matrix polymer chain further reduced the tendency of the NPs to migrate to the external interfaces and induced layering at the center of the film. The different distributions of NPs in the polymer affected the distribution of water molecules, which are absorbed from saturated vapor, in polymer films. We found that the amount of solvent penetration in thin films is governed by the density of NPs at the air/polymer interface.