Towards Mechanical Reinforcement in Polymer Layered Silicates Nanocomposites of the Second Generation

This dissertation focuses on the optimization of nanofillers based on synthetic layered silicates for preparation of nanocomposites with improved mechanical reinforcement. The mechanical characterization was done in collaboration with the Polymer Engineering Department at Bayreuth University in the frame work of SFB 840 (Project B3). The synthetic Na-fluorohectorite clay applied in this thesis differs from conventionally used natural montmorillonites (MMT) in respect to three main features: the charge density of the clay and thus the interface chemistry is much more homogeneous and may be adjusted to any desired value. The nanoplatelet diameter is more than an order of magnitude larger, and it is of high phase purity with limited, if any accessory minerals present. In respect to reinforcement, these features allow to eliminate internal shear planes to realize stiffer platelets with higher surface area. Moreover, the larger diameter of the nanoplatelets is beyond the size of typical microcracks and the number of platelets acting as stress concentrators is two orders of magnitude lower at any given filler content. The synthetic Na-fluorohectorite was prepared by melt synthesis in a well optimized process at the Department of Inorganic Chemistry I. This material has huge lateral extension (>20 μm) and superb layer charge homogeneity. In order to maximize the aspect ratio (α) the nanoplatelets were exfoliated into thinner tactoids by mechanical shearing. A cation exchange of the interlayer Na+ cations with Mg2+ allowed a higher hydration state and produced a ‘shear-labile’ material with reduced Coulomb forces between the layers. Applying shear forces generated in a stirred media mill led to exfoliation of the tactoids with minimal breakage and consequently larger aspect ratio. This process was optimized by investigating the main parameters controlling the exfoliation efficiency such as: solid content, grinding media size, and number of passages in the milling chamber. A subsequent cation exchange with K+ cations yielded collapsed and shear-stiff, mica-like nanofiller with no interlamellar reactivity. The collapsed state, moreover, allowed a selective organophilization of external basal surfaces of the tactoids using a polycationic macro-initiator (MIn+) with no signs of intercalation. The multiple electrostatic anchoring groups not only provided reliable adhesion but at the same time allowed adjusting the degree of protonation to match the surface charge. Dispersing the large aspect ratio MIn+/nano-mica by melt compounding in poly(methyl methacrylate) (PMMA) showed significantly enhanced mechanical properties. These nanoplatelets promoted additional energy dissipating mechanisms in the nanocomposites such as crack deflection, crack pinning as well as debonding effects leading to improved fracture toughness, such improvements have never been reported for MMT-nanocomposites. Optimization of the interfacial interaction towards complete miscibility with the PMMA-matrix was attempted by attaching a PMMA brushes on the external basal surfaces. This was done in a grafting-from process via surface initiated atom transfer radical polymerization (SI-ATRP). Highly stable suspensions of the coated nanofiller in organic solvent showing birefringence of a nematic phase were observed. Although a significantly improved stiffness as compared to MIn+/nano-mica-filled composites was achieved, still the full potential, as predicted by Halpin-Tsai equations, could not be utilized. This was attributed to a non-wetting character of the densely packed PMMA on the surface preventing chain interdigitation needed for good wetting. Finally, two compounding techniques were compared to probe the influence of the dispersion quality. PMMA-nanocomposites filled with MMT and MIn+/nano-mica were made by melt compounding and solution blending, respectively. Gas barrier measurements proved to be an additional independent and very sensitive probe for the dispersion quality. The solution blended samples and melt compounded samples showed permeation reductions by 60% and 30%, respectively. Maximizing the interface area by optimum dispersion not only reduced the oxygen permeation but also improved the reinforcement and fracture toughness especially at low clay content. These results highlighted the prime importance of the dispersion quality to exploit the full potential of nanofillers. Mediocre compounding may easily counterpoise advantages of superior fillers.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Ziadeh, Mazen

Contributors dc:contributor

• Breu, Josef

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1690/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1690