Providing a thermodynamic basis to kinetic fragility has been pursued for decades. This objective is particularly challenging for glass-forming polymers because of the difficulty in gauging configurational excess entropy ΔS ex . Herein, we report that enthalpy hysteresis ΔH R determined from a well-defined cooling and subsequent heating cycle of heat capacity curves bears a proportional relationship with αrelated excess enthalpy at the glass transition temperature (T g ). Correlations of fragility (m e ) with ΔH R and heat capacity jump ΔC P at T g are explored in a broad range of polymers, including monodispersed polystyrene with a molecular weight of 580−2 400 000, homopolymers with a fragility ranging from 50 to 200 and T g ranging from 200 to >400 K, and copolymers and polymer/small-molecule mixtures with different compositions and varied intermolecular strengths. Surprisingly, all of the presented data follow m e = 0.75ΔC P •T g /ΔH R + 15, a formula that can be theoretically derived from the Adam−Gibbs equation if the prefactor τ 0 is given at 10 −13 s. These experimental results not only provide a possible routine for estimating the α-related excess entropy of noncrystallizable polymers but also validate the thermodynamic link to the kinetic fragility of glass-forming polymers.

This report studied changes in dynamic fragility (m) of poly(butyl methacrylate) (PBMA) by introducing guest hindered phenols capable of forming double or triple intermolecular hydrogen bonds (inter-HBs) per molecule with...

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Establishing the relationship between interfacial layer chain packing and dynamics remains a continuing challenge in polymer nanocomposites (PNCs). This issue is expected to be significant in our understanding of the mechanism of the dynamic response of such materials and the manner in which these parameters affect the macroscopic properties of PNCs. In this study, we report the dynamics of free polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrix chains, as well as those of polymer chains surrounding the spherical silica nanoparticles (NPs) where silica NPs are either bare or PS grafted, to discriminate the role of grafted chains and interfacial interactions between grafted NPs and the matrix. The α-relaxation dynamics of the PS matrix is unaffected by silica NP loadings, it slows down in PMMA nanocomposites because of polymer-NP interfacial interactions and steric hindrance. More interestingly, we probe the enhanced mobility of the interfacial layer (α'-relaxation) in PNCs filled with grafted NPs, and this phenomenon is further corroborated by the accelerated Maxwell-Wagner-Sillars polarization process in the presence of grafted silica NPs. Moreover, the α'-relaxation time in the vicinity of glass transition temperature of the polymer matrix unexpectedly increases with increasing temperature. Such an anomalous temperature-dependent behavior can be attributed to the influence exerted by slow α-relaxation dynamics. Considering these phenomena and the mechanical properties, we propose a three-layer model to explain the observed behavior of grafted silica NP-filled nanocomposites. These findings provide new insight into the mechanisms responsible for mechanical reinforcement and therefore provide guidance in designing PNCs with tunable macroscopic properties.