Nanophase Separation-Induced Anomalous Enthalpy Hysteresis in Poly(n-alkyl methacrylate)s

Enthalpy hysteresis (ΔHR), an easily accessible quantity determined from a well-defined cooling and subsequent heating cycle of heat capacity curves, was recently reported to bear a proportional relationship with the α-related excess entropy of polymers at glass transition temperature( Tg). The kinetic fragility m of polymers was thus verified to link with thermodynamic parameters of Tg·ΔCp/ΔHR, as predicted by the Adam–Gibbs equation. In this work, enthalpy hysteresis of poly(n-alkyl methacrylate)s was investigated systematically to clarify whether ΔHR is influenced by multifarious non-α-related contributions. The results showed that the ΔHR for higher poly(n-alkyl methacrylate)s is abnormally suppressed, as manifested by failure of the theoretic correlation between m and Tg·ΔCp/ΔHR. The anomalous suppression appears non-related to the βJG relaxation but stems from nanophase separation of the alkyl sidechains. Once the PE-like nanophase is completely destroyed, either by increasing the steric hindrance of side groups or by introducing small molecule-bridged hydrogen bonds, ΔHR recovers, and the theoretic correlation between m and Tg·ΔCp/ΔHR is re-established. Further experiments revealed that the abnormal ΔHR suppression occurs only when the periodicity of phase separation is comparable to the size of the cooperative rearrangement region of bulk α relaxation at Tg. Accordingly, the deviation of m and Tg·ΔCp/ΔHR correlations from the theoretic prediction must be related to the existence of two different but partly coupled glass transitions in higher poly(n-alkyl methacrylate)s. The extremely hindered and confined glass transition in the PE-like nanodomains is likely to cause the anomalous suppression of global enthalpy hysteresis.