McKenzie Iain, Fujimoto Derek, Karner Victoria L, Li Ruohong, MacFarlane W Andrew, McFadden Ryan M L, Morris Gerald D, Pearson Matthew R, Raegen Adam N, Stachura Monika, Ticknor John O, Forrest James A
TRIUMF, Vancouver, British Columbia V6T 2A3, Canada.
Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
J Chem Phys. 2022 Feb 28;156(8):084903. doi: 10.1063/5.0081185.
We investigated the depth, temperature, and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted Li and β-detected nuclear magnetic resonance. Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average Li spin-lattice relaxation time (T ) was measured near the free surface and in the bulk. Spin-lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence T . There is no clear correlation of the average entropy of activation (ΔS̄) and enthalpy of activation (ΔH̄) with MW, but there is a clear correlation between ΔS̄ and ΔH̄, i.e., entropy-enthalpy compensation. This results in the average Gibbs energy of activation, ΔḠ, being approximately independent of MW. Measurements of the temperature dependence of T as a function of depth below the free surface indicate the inherent entropic barrier, i.e., the entropy of activation corresponding to ΔH̄ = 0, has an exponential dependence on the distance from the free surface before reaching the bulk value. This results in ΔḠ near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a "double-exponential" depth dependence. This model can explain the depth dependence of 1/T in polystyrene films. The characteristic length of enhanced dynamics is ∼6 nm and approximately independent of MW near room temperature.
我们使用注入的锂和β检测核磁共振研究了聚苯乙烯玻璃中γ弛豫的深度、温度和分子量（MW）依赖性。对分子量范围为1.1至641 kg/mol的薄膜进行了测量。在自由表面附近和本体中测量了平均锂自旋晶格弛豫时间（T）的温度依赖性。自旋晶格弛豫是由苯环翻转引起的，苯环翻转涉及在具有焓和熵贡献的自由能垒上的局部最小值之间的转变。我们使用过渡态理论对γ弛豫以及因此对T的温度依赖性进行建模。平均活化熵（ΔS̄）和活化焓（ΔH̄）与MW之间没有明显的相关性，但ΔS̄和ΔH̄之间存在明显的相关性, 即熵 - 焓补偿。这导致平均活化吉布斯自由能ΔḠ大致与MW无关。作为自由表面以下深度的函数对T的温度依赖性的测量表明，固有熵垒，即对应于ΔH̄ = 0的活化熵，在达到本体值之前对距自由表面的距离具有指数依赖性。这导致自由表面附近的ΔḠ低于本体。综合这些观察结果得到一个模型，其中γ弛豫的平均涨落速率具有“双指数”深度依赖性。该模型可以解释聚苯乙烯薄膜中1/T的深度依赖性。增强动力学的特征长度约为6 nm，在室温附近大致与MW无关。
