Crystallization and melting behaviors of poly(vinylidene fluoride) examined by fast-scan calorimetry: Hoffman-Weeks, Gibbs-Thomson and thermal Gibbs-Thomson plots
Crystallization and melting behaviors of poly(vinylidene fluoride) examined by fast-scan calorimetry: Hoffman-Weeks, Gibbs-Thomson and thermal Gibbs-Thomson plots
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DOI：
10.1016/j.polymer.2019.02.017
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发表时间：
2019-04
期刊：
影响因子：
4.6
通讯作者：
A. Toda;K. Taguchi;Gaku Kono;K. Nozaki
中科院分区：
文献类型：
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作者：
A. Toda;K. Taguchi;Gaku Kono;K. Nozaki
用快速扫描量热法(FSC)结合小角X射线散射(SAXS)和广角X射线衍射(WAXD)研究了聚偏氟乙烯的结晶和熔融行为。用FSC测量了在Tc等温形成的片状晶体的熔点随升温速率的变化关系，并根据熔化动力学模型进行了标定，以确定零升温速率下的熔点。Tm对Tc的Hoffman-Week(H-W)图在大于30 K的T_c范围内是一条直线，并提出了扩链无限大晶体的平衡熔点Tm_0≅ 200 °C的α形式。另一方面，利用SAXS测定的晶片厚度，TMandTc对(DC)−1的Gibbs-Thomson(G-T)图中的熔融线和结晶线都严重偏离直线。通过考察长时间等温退火时晶化的第二阶段，结合熔融比热Δ和熔融比热的变化，提出了一种新的TM对(ΔHFS)−1的热G-T图，证实了由H-W图和T相关的折叠表面自由能E所确定的α形式的TM的可靠性，在G-T图中产生弯曲的熔化和结晶线。此外，在长时间等温退火过程中，SAXS和广角X射线衍射仪观察到的微晶尺寸的变化证实了在第二阶段，随着亚稳态链状折叠聚合物晶体的增厚和完善，Tm和Δ的增大是由分子机制造成的。
Crystallization and melting behaviors of chain-folded polymer crystals have been examined by fast-scan calorimetry (FSC) combined with small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) for poly(vinylidene fluoride). The melting point of lamellar crystals formed isothermally atTcwas measured by FSC in terms of the heating rate dependence and calibrated on the basis of the modeling of melting kinetics for the determination of the melting point at zero heating rateTM. The Hoffman-Weeks (H-W) plot ofTMagainstTcwas on a linear straight line overTcrange broader than 30 K, and suggested the equilibrium melting pointTM0≅ 200 °C of chain-extended infinite-size crystals of the α form. On the other hand, utilizing crystalline lamellar thicknessdcdetermined by SAXS, both of the Melting and Crystallization lines in the Gibbs-Thomson (G-T) plots ofTMandTcagainst (dc)−1, respectively, were found to seriously deviate from linear straight lines. By examining the secondary stage of crystallization on long time isothermal annealing atTcin terms of the changes inTMand the specific heat of fusion Δhfsby FSC, a newly proposed thermal G-T plot ofTMagainst (Δhfs)−1has confirmed the reliability ofTM0of the α form determined by the H-Wplot and theTcdependent folding surface free energyσe, which brings the curved Melting and Crystallization lines in the G-T plots. In addition, on long time isothermal annealing, the changes indcby SAXS and in crystallite size by WAXD have confirmed the molecular origin of the increase inTMand Δhfsin the secondary stage with thickening and perfecting of metastable chain-folded polymer crystals.