writer：Yongjin Peng, Chenting Cai, Rongchun Zhang, et al.
keywords：glass transition, polystyrene, heterogeneous dynamics, solid-state NMR
source：期刊
specific source：Chinese Journal of Polymer Science 2016, 34, 446-456
Issue time：2016年
A two-stage transition upon crossing the glass transition of polystyrene with increasing temperature was precisely determined and interpreted by using solid-state nuclear magnetic resonance (SSNMR), ¹H-¹H dipolar couplings based Double Quantum-Filtered (DQF) and dipolar filter (DF) experiment and ¹³C Chemical Shift Anisotropy (CSA) based centerband-only detection of exchange (CODEX) experiment are used to fully characterize the time scale of molecular motions during the glass transition. While differential scanning calorimetry （DSC） and CODEX experiment predicted the first stage of glass transiton, DQF and DF experiment provided the evidence for second stage transition during which the time scale of molecular motions changed from very slow (t>ms) to very fast (t<μs). The first stage of glass transition begins with the occurrence of remarkable slow re-orientation motions of the polymer backbone segments and ends when the degree of slow motion reaches maximum. The onset and endpoint of the conventional calorimetric glass transition of polystyrene can be quantitatively determined at the molecular level by SSNMR. In the second stage, a subsequent dramatic transition associated with the melting of the glassy components was observed. In this stage liquid-like NMR signals appeared and rapidly increased in intensity after a characteristic temperature T_f (~1.1T_g). The signals associated with the glassy components completely disappeared at another characteristic temperature T_c (~1.2T_g).