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Knowledge of the glass transition temperatures (T g s) as function of composition reflects miscibility (or lack of it) and is decisive for virtually all properties of polymer-based materials. In this article, we analyze single blend-average and effective T g s of miscible polymer blends in full concentration ranges. Shortcomings of the extant equations are discussed to support the need for an alternative. Focusing on the deviation from a linear relationship, defined as DT g ¼ T g À u 1 T g,1 À u 2 T g,2 (where u i and T g,i are, respectively, the weight fraction and the T g of the i-th component), a recently proposed equation for the blend T g as a function of composition is tested extensively. This equation is simple; a quadratic polynomial centered around 2u 1 À 1 ¼ 0 is defined to represent deviations from linearity, and up to three parameters are used. The number of parameters needed to describe the experimental data, along with their magnitude and sign, provide a measure of the system complexity. For most binary polymer systems tested, the results obtained with the new equation are better than those attained from existing T g equations. The key parameter of the equation a 0 is related to parameters commonly used to represent intersegmental interactions and miscibility in binary polymer blends. V V C 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 80-95, 2008 SCOPEThe glass transition is an important phenomenon that characterizes a range of amorphous systems, including homopolymers, polymer blends, copolymers, and polymer networks. The related experimental signals and the underlying process on a molecular scale involve both kinetic and thermo-dynamic features. 1,2 For example, the influence of the direction of the change (freezing a liquid, melting a solid) and of variations in the change rate (heating or cooling) on the location of the glass transition region are illustrative of its kinetic character, whereas the experimental observation of features attributed to a second order transition is in line with the thermodynamic character. 2 Although the change from the glassy state into either a liquid or a rubbery state is a gradual one, and thus the glass transition phenomenon spans a wide temperature window, experimentalists

The thermally stimulated current (TSC) signatures of the primary (alpha) transition and its precursor, the Johary-Goldstein (beta) relaxation, are used to probe effects of nanoconfinement on the dielectric relaxation dynamics of poly(methyl methacrylate) (PMMA) radically polymerised in situ 50 angstroms mean pore size silica-gel. Nanoconfinement leads to a broadened and low-temperature-shifted beta band (peaking at Tbeta, with deltaTbeta = T(conf.)beta - T(bulk)beta = -15 degrees C for a heating rate of 5 deg/min), signifying the occurrence of faster relaxing moieties compared to the bulk-like PMMA film. Furthermore, both TSCs and differential scanning calorimetry (DSC) estimate a rise of the glass transition temperature for the confined phase ([Formula: see text]= +13 degrees C) and an increased width for the corresponding transition signals, relative to the signals in the bulk. Simple free-volume and entropy models seem inadequate to provide a collective description of the above perturbations. The observation of a spatial heterogeneity regarding the relaxation dynamics is discussed in terms of the presence of a motional gradient, with less mobile segments near the interface and more mobile segments in the core, and the interplay of adsorption ( e.g., strong physical interactions that slow down molecular mobilities) and confinement effects ( e.g., lower entanglements concentration and local density fluctuations that provide regions of increased free space). The results suggest that in the case of high-molecular-weight polymers confined in small-pore systems, adsorption effects have considerable bearing on the glass transition phenomenon whereas confinement primarily influences side-chains' rotational mobilities. The confinement effect is expected to dominate over adsorption for PMMA phases occluded in higher pore sizes and silanised walls.

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