Slow Dynamics and the Glass Transition in Confining Systems
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Slow Dynamics and the Glass Transition in Confining Systems Li-Min Wang, Fang He, and Ranko Richert Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, U.S.A. ABSTRACT The slow dynamics associated with the structural relaxation of glass forming materials near the glass transition is very sensitive to the effects of small confining geometries. Based upon the experimental results of triplet state solvation dynamics, we explore the extent to which confinement effects can be rationalized solely in terms of interfacial dynamics which are modified relative to the bulk situation. The importance of the interfacial conditions is emphasized by observing the changes due to the surface chemistry, by comparing relaxation times at and further away from the surface, and by studying the effects of 'soft' versus 'hard' confining materials. While 'hard' confinement by porous solids is observed to result in slower dynamics and an increased glass transition temperature Tg for propylene glycol, our 4.6 nm nanodroplets suspended in a more fluid environment display faster structural relaxation, equivalent to a reduction of Tg as observed in free standing polymer films. INTRODUCTION A tremendous amout of experimental as well as theoretical studies is devoted to the question of how the properties of materials change when the dimensions of the sample are reduced from macroscopic sizes to the scale of a few nanometers. There is a large variety regarding how the geometrical restriction is realized in experiments, models, and simulations. Confining films of a liquid as thin as a few molecular layers between atomically smooth surfaces is a case of extreme confinement to a well defined geometry [1,2,3]. Free standing or substrate supported polymer films in a thickness range between approximately 10 nm and 500 nm are also well defined spatial restrictions in one dimension [4,5,6]. Confinement in more dimensions can be realized by zeolites or similar highly regular and open structures [7], by the more irregular pores and channels of porous silica glasses [8,9], or by polymer solutions [10]. At spatial dimensions as small as several nanometers, the majority of liquids, supercooled liquids and polymers display changes in their relaxation behavior if compared with the bulk counterpart. This is particularly obvious in the case of glass-forming liquids near their glass transition temperature Tg, i.e. in the regime of high viscosities. For these slowly relaxing systems, confinement to sizes of order 10 nm can alter the relaxation time scale by orders of magnitude. It is common to quantify the confinement effect in terms of a glass transition shift ∆Tg, for which both positive and negative values have been reported [11]. Liquids are called supercooled in the case of a gradual increase of their viscosity instead of crystallizing if the temperature is taken below their melting point. The most characteristic properties of supercooled liquids are the complex behavior regarding the time and temperature dependence o
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