局部聚合物构象对等规，间规和无规聚（甲基丙烯酸甲酯）的不同玻璃化转变温度和动态脆性的作用,Macromolecules

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局部聚合物构象对等规，间规和无规聚（甲基丙烯酸甲酯）的不同玻璃化转变温度和动态脆性的作用
Macromolecules ( IF 5.1 ) Pub Date : 2019-05-14 00:00:00 , DOI: 10.1021/acs.macromol.9b00434
Jialong Shen ₁ , Erol Yildirim ₁ , Shanshan Li ₁ , Yavuz Caydamli ₁ , Melissa A. Pasquinelli ₁ , Alan E. Tonelli ₁

Affiliation

聚（甲基丙烯酸甲酯）（PMMA）的玻璃化转变温度（T _g）对立体序列的依赖性很大，从而有可能评估任何构象对其玻璃形成过程的构象贡献。对于给定的聚合物液体，接近T _g时，粘度和弛豫时间急剧增加，导致其动力学的温度依赖性偏离Arrhenius行为。偏差的程度称为动态脆性（m）。广泛报道的T _g超过70 K和m的变化90个样本需要使用相同的协议对每个样品进行新的测量，以保持比较的可靠性。使用差示扫描量热法（DSC）测量了虚构温度的极限值，作为冷却速率的函数，并用于计算两种立构规整的PMMA（等规（i）和间规（s））以及三种无规（a）的m值）具有不同分子量的PMMA。对于a-PMMA，m表现出对分子量的正相关性，而s-和i-PMMA的m值分别与先前的研究一致，分别为高和低。T _g处的热容量变化与m呈负相关，可能是由于其热流变简单性的破坏。在二聚体水平进行构象分析，以阐明局部聚合物构象的潜在作用。旋转异构状态的蒙特卡洛模拟和构象分析提供了对i-PMMA和s-PMMA的T _g和m差异的分子起源的见解。另一方面，我们证明了特征比不是表征PMMA柔韧性的合适参数，与m无关。提出了对玻璃形成聚合物的局部构象几何学的未来关注。

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Role of Local Polymer Conformations on the Diverging Glass Transition Temperatures and Dynamic Fragilities of Isotactic-, Syndiotactic-, and Atactic-Poly(methyl methacrylate)s

The profound stereosequence dependence of the glass transition temperature (T_g) of poly(methyl methacrylate)s (PMMAs) offers the possibility to evaluate any conformational contributions to their glass formation processes. Close to T_g for a given polymer liquid, the viscosity and relaxation time increase dramatically, leading to the temperature dependence of its dynamics to deviate from Arrhenius behavior. The degree of that deviation is called dynamic fragility (m). The broad reported variations for T_g of over 70 K and for m of 90 necessitated new measurements for each sample using the same protocols to keep comparisons reliable. The limiting values of fictive temperature as a function of cooling rates were measured using differential scanning calorimetry (DSC) and were used to calculate m values of two stereoregular PMMAs, isotactic (i) and syndiotactic (s), as well as three atactic (a) PMMAs with different molecular weights. For a-PMMAs, m exhibited a positive dependence on molecular weight, and the m values for s- and i-PMMAs were consistent with the previous studies to be high and low, respectively. Heat capacity changes at T_g correlate negatively with m, probably due to the breakdown of their thermorheological simplicity. Conformational analyses were conducted at the dimer level to elucidate the potential role of local polymer conformations. Rotational isomeric state Monte Carlo simulations and conformational analyses provide insights into the molecular origin of the T_g and m differences of i-PMMA and s-PMMA. On the other hand, we demonstrated that the characteristic ratio is not a suitable parameter for characterizing PMMA flexibility and is irrelevant to m. A future focus on the local conformational geometry of glass-forming polymers is proposed.

更新日期：2019-05-14

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