Possible Causes of the Change of Dynamics in Glass-Forming Materials Subjected to Reduced Dimension
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ABSTRACT There is currently many ongoing investigations of the change in the glass transition temperature when a material is reduced in dimension from the normal bulk state. The reduction in dimension can be accomplished by casting the material as thin films with or without a substrate or putting it in nanometer size pores. In this work, we explore possible causes of the change in dynamics of the bulk material when the glass-former is subjected to such modifications. The existence of a growing cooperative length scale L(T) with decreasing temperature in bulk fragile glass-forming liquids reaching the size of approximately 1.5-2.0 nm at the glass transition temperature is the basis of our consideration. When the reduced dimension is comparable to L(Tg), cooperative dynamics within a lengthscale equal to L(Tg) can no longer be maintained in all three dimensions throughout the sample. The imposed reduction of the cooperative length scale speeds up the dynamics and causes a reduction of the glass transition temperature. For polymeric glass-formers particularly at higher molecular weights, reduction of one dimension in thin films engenders orientation of the polymer chains when their radius of gyration becomes comparable to the film thickness. The latter is known to cause also a reduction of the glass transition temeperature. INTRODUCTION The dynamics of glass transition is currently the subject of intensive study as evidenced by the papers published in this Volume. The current trend of research which extends the research to short microscopic times and temperatures much higher than Tg means a solution has to provide an understanding of the dynamics over immense time (frequency) and temperature ranges. Thus the problem of glass transition in bulk glass-formers at present days has become very difficult to solve due to the presence of many complicating factors. First and foremost is the many-body nature of the dynamics (sometimes loosely referred to as cooperativity) which makes it difficult to be accurately described, particularly over a large time range from microscopic times of the order of a picoseconds to macroscopic times of the order of 102 s. It is expected, at least from some models including the coupling model [1-4], that the many-body effects are responsible at least partly for the nonexponentiality [1,2], dynamically heterogeneous nature [5] and slowing down with decreasing temperature of the dynamics [6]. The many-body effects are thought of being responsible also for the relation between short-time and long-time dynamics observed experimentally. Second is the nontrivial effect caused by the significant change of a thermodynamic variable such as density and entropy which take place over the large temperature range. These large changes of thermodynamic variables are the basis of the free volume theory [7] and the configurational entropy theory [8,9]. It seems a complete description of glass transition requires a synthesis of all these relevant factors into a theory. 147 Mat. Res. Soc. Symp. Proc. Vol. 455 © 199
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