Solid-State Blending of Polymers by Cryogenic Mechanical Alloying
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Solid-State Blending of Polymers by Cryogenic Mechanical Alloying Archie P. Smith,† Harald Ade,‡ Carl C. Koch† and Richard J. Spontak†§ Departments of †Materials Science & Engineering, ‡Physics and §Chemical Engineering North Carolina State University, Raleigh, NC ABSTRACT Cryogenic mechanical alloying has been employed to blend poly(methyl methacrylate) (PMMA) with up to 25 wt% polyisoprene (PI) and poly(ethylene-alt-propylene) (PEP). Mechanical milling of the individual polymers reveals that their molecular and bulk properties depend sensitively on milling time, post-annealing and, for PMMA, temperature. Characterization of the as-milled blends by scanning transmission x-ray microscopy and transmission electron microscopy has demonstrated intimate (nanoscale) dispersions within the blends, with the degree of mixing increasing with increasing milling time. Phase domains as small as 10 nm are observed after alloying for 10 h. Post-annealing of the blends above the Tg of PMMA (which depends on milling time) induces morphological changes, which differ for blends containing PI and PEP. In blends composed of PEP, the fine dispersions gained as a result of milling are largely compromised. Conversely, PI crosslinking hinders molecular mobility so that the milling-induced nanoscale dispersions of PI in PMMA are, for the most part, retained even after long-term annealing at elevated temperatures.
INTRODUCTION Blending two or more polymers to create a material with properties beyond those achievable with a single polymer has become a major emphasis of polymer science [1]. The largest hurdle to polymer blending is the inherent immiscibility between most polymer pairs. Consequently, various techniques have been developed to produce and retain a high degree of dispersion in multicomponent polymer systems [2-4]. Most of these methods rely on processing the materials in the melt or in solution, which enhance chain mobility. Degradation of the polymers at elevated temperatures, undesirable chemical reactions and disposal of volatile organic solvents can, however, significantly hinder the utility of these process strategies. While one way to overcome these problems is to process the polymers in the solid state, the large mechanical forces required, and the corresponding damage imparted, have traditionally limited the viability of solid-state processing [1]. Techniques such as solidstate extrusion [5] and mechanical ball milling [6-9] are only now becoming more prevalent. The present work addresses the efficacy of cryogenic mechanical alloying as a means by which to produce polymer blends without a compatibilizing agent. Mechanical alloying is defined here as the high-energy ball milling of two or more materials to produce intimate mixtures or alloys [10]. Originally developed for the production of oxide dispersion-strengthened alloys [11], it has subsequently been utilized to produce a wide variety of metastable inorganic alloys and nanocrystalline materials [12]. Application of this technique to polymers has lagged far behind, wi
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