Hsu David D, Xia Wenjie, Song Jake, Keten Sinan
Department of Mechanical Engineering, ‡Department of Civil & Environmental Engineering, and §Department of Materials Science & Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3109, United States.
Department of Mechanical Engineering, Department of Civil & Environmental Engineering, and §Department of Materials Science & Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3109, United States.
ACS Macro Lett. 2016 Apr 19;5(4):481-486. doi: 10.1021/acsmacrolett.6b00037. Epub 2016 Mar 29.
Despite having very similar bulk properties such as glass-transition temperature (), density, and fragility, polystyrene (PS) and poly(methyl methacrylate) (PMMA) exhibit characteristically different depression in free-standing ultrathin films due to free surface effects. Here we explain this difference using our recently established chemistry-specific coarse-grained (CG) models for these two polymers. Models capture the dissimilar scaling of with free-standing film thickness as seen in experiments and enable us to quantify the size of the regions near free surfaces over which chain relaxation exhibits differences from bulk. Most interestingly, vibrational density of states (VDOS) analysis uncovers a relationship between the amplitude of side-chain fluctuations, associated with side-chain flexibility and -nanoconfinement. We discover that increasing backbone to side-chain mass ratio in CG models increases the amplitude of side-chain fluctuations and suppresses the free-surface effect on . We show that mass distribution and side-chain flexibility are central to explain dissimilar free surface effects on PS and PMMA. Our model predictions are further corroborated by experimental evidence showing the role of mass distribution in styrene thin films. Our study ascertains the significance of molecular characteristics on nanoconfinement and highlights the ability for chemistry-specific CG models to explore the thermomechanical properties of polymer thin films.
尽管聚苯乙烯（PS）和聚甲基丙烯酸甲酯（PMMA）具有非常相似的整体性质，如玻璃化转变温度（）、密度和脆性，但由于自由表面效应，它们在独立的超薄膜中表现出截然不同的自由体积分数降低。在这里，我们使用我们最近为这两种聚合物建立的特定化学粗粒化（CG）模型来解释这种差异。模型捕捉到了实验中观察到的自由体积分数随独立薄膜厚度的不同缩放关系，并使我们能够量化自由表面附近链松弛与本体表现出差异的区域大小。最有趣的是，振动态密度（VDOS）分析揭示了与侧链柔性和纳米限域相关的侧链波动幅度之间的关系。我们发现，在CG模型中增加主链与侧链的质量比会增加侧链波动的幅度，并抑制自由表面对自由体积分数的影响。我们表明，质量分布和侧链柔性是解释PS和PMMA不同自由表面效应的关键。我们的模型预测得到了实验证据的进一步证实，这些证据表明了质量分布在苯乙烯薄膜中的作用。我们的研究确定了分子特征对纳米限域的重要性，并突出了特定化学CG模型探索聚合物薄膜热机械性能的能力。
