Probing the two-stage transition upon crossing the glass transition of polystyrene by solid-state NMR |
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Authors: | Yong-jin Peng Chen-ting Cai Rong-chun Zhang Tie-hong Chen Ping-chuan Sun Bao-hui Li Xiao-liang Wang Gi Xue An-Chang Shi |
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Institution: | 1. School of Physics, Nankai University, Tianjin 300071, China;2. Key Laboratory of Functional Polymer Materials of Ministry of Education and College of Chemistry, Nankai University, and Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China;3. Department of Polymer Science and Engineering, The School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry ,Nanjing University, Nanjing 210093, China |
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Abstract: | 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) experiments 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 experiments provided the evidence for the 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). |
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Keywords: | Glass transition Polystyrene Heterogeneous dynamics Solid-state NMR |
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