Structure and properties of crystalline polymer nanocomposites : filler dispersion and crystallization behavior

Abstract

The last part of this dissertation explores the applications of dye doped NPs with optimized compatibility between NPs and the silicone matrix as color converters. A unique design of concentric silica-dye-silica structures provide confinement against dye motions and π-π intermolecular aggregations of the conjugated dye molecules. The result was an improvement in the stability of the dye doped silica NPs. The dye-functionalized NPs and their nanocomposites open up new opportunities in the design of next-generation LEDs.

A second component of this work explores approaches to thermodynamically controlling NP dispersion in semicrystalline polyethylene matrices. By varying the filler/matrix compatibility by tailoring the graft chain length, graft density and ratio of brush/matrix chain length, NPs are organized into individual particles, strings, sheets or clusters. While lots of research has been done working on understanding the nucleation and growth of polymer materials under confinement in one dimensional space, exploration of crystallization kinetics of semicrystalline polymers confined within NP assemblies is rare. In this work, we investigated the crystallization kinetics of polyethylene in the presence of several NP assemblies. It is shown that NP agglomerations (strings/clusters) decrease the crystallinity. A surprising aspect here is that a well dispersed densely grafted NP with long grafted chains in the polyethylene matrix leads to a slower crystallization and a higher crystallinity than observed for the neat matrix. We find that the well dispersed NPs are too small to act as heterogeneous nucleation sites but slow down the crystallization; while strings/clusters increase the nucleation rate but don’t impact growth rate at the loading. Finally, we show that the permittivity can be optimized with low crystallinity and the addition of nanofillers. The elongated agglomerations can further improve the permittivity due to the electric field concentrated at the heterogenous filler/matrix interface.

To achieve the full promise of nanoparticle filled polymers, control over both filler organization and polymer morphology is required. In this dissertation, we are dedicated to developing the fundamental principles behind tailoring filler organization. Previous work has demonstrated the ability to order the nanoparticles (NPs) into structures in poly(ethylene oxide) (PEO) matrices by tuning the crystallization rate. In particular, NPs grafted with PMMA chains were assembled at different scales (interlamellar, interfibrillar and interspherulitic regions) with different crystal growth rates. Here, we established the generality of PEO based systems and suggest that crystallization can be used to control NP organization across different semicrystalline polymers. We first graft C18 onto spherical silica NPs to improve miscibility. In a low molecular weight polyethylene matrix, NPs are well dispersed in the polymer melt and can be selectively moved into interlamellar zones. However, the incompatibility between NPs and a matrix with a high molecular weight leads to a competition between NP agglomeration and crystallization-induced NP organization. The mechanical properties, such as Young’s modulus, are enhanced with the addition of NPs and the increased crystallinity, and are further improved by particle organization. Dielectric properties strongly depend on the scale of NP organization. Higher NP ordering with anisotropic string-like dispersion shows more significant increases in permittivity than a system with well dispersed NPs. We find that we can successfully tune NP assemblies within interlamellar regions using controlled crystallization kinetics, especially when the NP are well dispersed in the melt. The string-like NP assemblies with high aspect ratio associated with crystalline morphology can improve mechanical properties and optimize the permittivity.