Nuzzo, Anna (2014) Manipulating the Morphology and Properties of Immiscible Polymer Blends Using Nanoparticles – A Viable Route to Enlarge the Fields of Application of Biopolymers. [Tesi di dottorato]

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Abstract

The overall aim of the research activity has been identifying bio-based and eco-sustainable polymeric formulations potentially suitable for applications of technological interest. Together with the typically high costs, the major technical challenge to widespread acceptance of bio-polymers is the difficulty in achieving physical and mechanical properties comparable with those of conventional petroleum-based polymers. Among bio-polymers, poly lactic acid (PLA) is one of the most promising candidate for the substitution, totally or partially, of many petroleum-based polymers. In fact, PLA exhibits the best compromise among eco-sustainability, physical and mechanical features and industrial development prospect. However, the low mechanical resistance at high temperature (Heat Deflection Temperature = 50÷60°C) has prevented its complete access to relevant industrial sectors. In the present work, the poor mechanical properties of the amorphous PLA above its glass transition are corrected by blending it with Polyamide 11 (PA11), a semicrystalline bio-based polymer, and promoting the continuity of the latter phase through the addition of nanoparticles. Specifically, three different kinds of nanoparticles, inclined to enrich PA11 phase, have been used: an organo-modified montmorillonite (OMMT), an organo-modified sepiolite (MS) and carbon nanotubes (CNTs). The preferential positioning of the three fillers inside the PA11 minor phase of PLA/PA11 blends has resulted effective in inducing its phase continuity. In particular, provided a critical nanoparticles loading is exceeded, the drops-matrix morphology of a blend at 70% wt of PLA converts in a co-continuous one. In such a way, remarkable improvements of the high temperature mechanical performances are achieved owing to the filled PA11 framework, which interpenetrates the PLA major phase and contributes to bear stress up to ~160°C, i.e. ~100°C above the PLA glass transition. Nanoparticles-induced co-continuity in immiscible polymer blends has been previously observed in many systems, but the underlying mechanism is still unclear. An additional goal of this work is to better clarify the mechanism behind the co-continuity development observed in our bio-based nanocomposite polymer blend. In particular, we focus on the roles played by (i) bulk rheology of filled phase, (ii) self-networking propensity of nanoparticles, and (iii) properties of the blend interface. The latter are differently affected by changing chemistry and geometrical features of the nanoparticles. Among others, two issues are examined more in depth: how effective it is to promote co-continuity by kinetically arresting the relaxation dynamics of the bulk polymer phases by means of nanoparticles? And, if so, is the self-networking ability of the particles the only relevant parameter? The cross-check of the experimental data obtained from morphological, dynamical-mechanical and rheological analyses reveals that simply slowing down the melt state relaxation dynamics of the minor phase through the aggregation of nanoparticles in a space-spanning elastic network may be not sufficient to promote co-continuity. The effect of the geometrical features of the particles is discussed, as well as their ability to affect interfacial tension. In particular, the lowering of the latter seems to play a crucial role, stabilizing irregularly-shaped domains whose merging eventually results into co-continuity. Concluding, the results obtained in the present thesis provide the experimental evidence that a judicial selection of the blend constituents, combined with a clever manipulation of the blend microstructure through the addition of nanoparticles, may effectively result in ‘‘engineered’’ materials with enhanced properties. Applying such an approach to bio-based polymers represents a viable route to expand and diversify the field of possible applications of such promising materials.

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