Prevosto Daniele, Capaccioli Simone, Ngai K L
CNR-IPCF, Largo B. Pontecorvo 3, I-56127 Pisa, Italy.
Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127 Pisa, Italy.
J Chem Phys. 2014 Feb 21;140(7):074903. doi: 10.1063/1.4865752.
From ellipsometry measurements, Pye and Roth [Phys. Rev. Lett. 107, 235701 (2011)] presented evidence of the presence of two glass transitions originating from two distinctly different and simultaneous mechanisms to reduce the glass transition temperature within freestanding polystyrene films with thickness less than 70 nm. The upper transition temperature T(u)(g)(h) is higher than the lower transition temperature T(l)(g)(h) in the ultrathin films. After comparing their data with the findings of others, using the same or different techniques, they concluded that new theoretical interpretation is needed to explain the two transitions and the different dependences of T(u)(g)(h) and T(l)(g)(h) on film thickness and molecular weight. We address the problem based on advance in delineating the different viscoelastic mechanisms in the glass-rubber transition zone of polymers. Theoretical considerations as well as experiments have shown in time-scales immediately following the segmental α-relaxation are the sub-Rouse modes with longer length scale but shorter than that of the Rouse modes. The existence of the sub-Rouse modes in various polymers including polystyrene has been repeatedly confirmed by experiments. We show that the sub-Rouse modes can account for the upper transition and the properties observed. The segmental α-relaxation is responsible for the lower transition. This is supported by the fact that the segmental α-relaxation in ultrathin freestanding PS films had been observed by dielectric relaxation measurements and photon correlation spectroscopy. Utilizing the temperature dependence of the segmental relaxation times from these experiments, the glass transition temperature T(α)(g)associated with the segmental relaxation in the ultrathin film is determined. It turns out that T(α)(g) is nearly the same as T(l)(g)(h) of the lower transition, and hence definitely segmental α-relaxation is the mechanism for the lower transition. Since it is unlikely that the segmental α-relaxation can give rise to two very different transitions simultaneously, a new mechanism for the upper transition is needed, and the sub-Rouse modes provide the mechanism.
通过椭偏测量，派伊和罗斯[《物理评论快报》107, 235701 (2011)]给出了证据，表明在厚度小于70纳米的独立聚苯乙烯薄膜中存在两个玻璃化转变，这两个转变源于两种截然不同且同时起作用的机制，用于降低玻璃化转变温度。在超薄膜中，较高的转变温度T(u)(g)(h)高于较低的转变温度T(l)(g)(h)。在将他们的数据与其他人使用相同或不同技术的研究结果进行比较之后，他们得出结论，需要新的理论解释来阐释这两个转变以及T(u)(g)(h)和T(l)(g)(h)对薄膜厚度和分子量的不同依赖性。我们基于在描绘聚合物玻璃 - 橡胶转变区不同粘弹性机制方面的进展来解决这个问题。理论考量以及实验表明，在链段α弛豫之后紧接着的时间尺度内，存在长度尺度比Rouse模式更长但比其短的亚Rouse模式。包括聚苯乙烯在内的各种聚合物中亚Rouse模式的存在已被实验反复证实。我们表明，亚Rouse模式可以解释较高的转变以及所观察到的性质。链段α弛豫导致较低的转变。这得到了以下事实的支持：通过介电弛豫测量和光子相关光谱法已观察到独立超薄聚苯乙烯薄膜中的链段α弛豫。利用这些实验中链段弛豫时间的温度依赖性，确定了与超薄膜中链段弛豫相关的玻璃化转变温度T(α)(g)。结果表明，T(α)(g)与较低转变的T(l)(g)(h)几乎相同，因此链段α弛豫肯定是较低转变的机制。由于链段α弛豫不太可能同时引发两个非常不同的转变，所以需要一种新的较高转变机制，而亚Rouse模式提供了这种机制。
