Processing-structure-property relationships in copolyester elastomer nanocomposites

Abstract

Thermoplastic copolyesters (TPE-Es) are high-performance thermoplastic alternatives to natural or synthetic cross-linked rubbers. TPE-Es are segmented block copolymers typically incorporating hard (polyester) and soft (polyether) blocks. This thesis investigates the “processing-structure-property” of nanocomposites of TPE-E reinforced with organically-modified nanoclays. Two methods of materials processing are investigated for the preparation of the nanocomposites: melt compounding and reactive extrusion. The main objectives of this research are the investigation of the utility of organically-modified nanoclays to enhance the properties of segmented TPE-E, and to understand in parallel how these nanofillers influence the underlying TPE-E microphase morphology, from which segmented TPE-E elastomers derive their properties. As additives for melt compounding, a fluoromica (aspect ratio ~ 650 nm/1 nm) and two organo-hectorites (aspect ratio ~ 80-120 nm/1 nm) were modified with single or dual surfactants with varying degrees of surface hydrophobicity, which were incorporated into a single soft TPE-E grade (Hytrel® 3078), at loadings of 2 and 4% (w/w). Octadecyltrimethylammonium bromide (ODTMA), cetylpryidinium bromide (CP), and choline chloride (CC) salts were utilised for modifying the clay surfaces via an aqueous ion exchange reaction. Ultimately, the high aspect ratio fluoromica modified with a single ODTMA surfactant was found to demonstrate the best overall mechanical performance, and was the most thermodynamically-compatible TPE-E nanocomposite system investigated. This system resulted in good filler dispersion, while fluoromica with a dual surfactant 75%/25% ODTMA/CC modification was less compatible. At 2% (w/w) loading, the resulting nanocomposite exhibited marginally-improved tensile strength and creep resistance, in addition to the best enhancement of exfoliation and barrier properties. The CP modified nanofillers demonstrated the worst overall performance, perhaps due to the slightly shorter surfactant length employed. Increased hard segment secondary interactions were observed, as indicated by enhanced hard segment nucleation and crystallinity. Low aspect ratio organo-hectorites with a single and dual surfactant were found to be more difficult to disperse and less thermodynamically-compatible with TPE-E. These nanocomposites displayed inferior mechanical and barrier performance, primarily due to unfavourable morphological influences, combined with inadequate levels of dispersion or sufficient permeant tortuosity levels (due, in-part, to the lower aspect ratio) to improve barrier performance. Further modified reactive nanofiller variants were explored through reactive extrusion. The hydroxyl-bearing ODTMA/CC organo-fluoromica was combined in toluene with a tetraglycidyl compound, TGDDM, as a reactive modifier to the dual surfactant organoclay. It was shown that the presence of alkyl ammonium cations in the clay promotes the intercalation of the TGDDM molecule into the interlayer space. The effects of this combined organoclay and further modification on the final properties of TPE-E, organoclay and TGDDM were investigated. In the first experimental stage, the optimum concentration of the reactive modifier was determined by melt compounding TPE-E, 2% (w/w) of organoclay and TGDDM in a twin-screw extruder. The 0.3% (w/w) concentration of TGDDM, which resulted in moderate tensile strength, was selected because of its balanced mechanical properties. Subsequently, by using 0.3% (w/w) TGDDM content, the effects of the modification route of the materials on the properties of nanocomposites were systematically investigated. Mechanical testing showed that different orders of addition of the components significantly influenced the mechanical properties. Of the investigated addition orders, the best sequence of component addition (HC1-T) was the one in which TPE-E was first compounded with organoclay early in the twin screw extruder barrel, followed by the side addition of the TGDDM further downstream. While this approach led to excellent enhancement in mechanical properties, the water vapour transmission rate was not improved. A masterbatch approach of combined organoclay (HC1-C2) was able to significantly enhance the resistance to water vapour through the host TPE-E.