Segmental Mobility and Glass Transition Temperature of Freely Suspended Ultrathin Polymer Membranes
Segmental Mobility and Glass Transition Temperature of Freely Suspended Ultrathin Polymer Membranes
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自由悬浮超薄聚合物膜的片段迁移率和玻璃化转变温度
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发表时间：
2009
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通讯作者：
M. Wübbenhorst
中科院分区：
文献类型：
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作者：
C. Rotella;S. Napolitano;M. Wübbenhorst
对限制在纳米尺寸几何形状中的聚合物层的广泛研究表明，吸收基底或自由表面的存在会改变聚合物的性质，例如双轴蠕变行为、流动和分子间缠结、小分子在基质内的扩散、结晶动力学、物理老化、玻璃化转变温度（Tg），7以及局部链（链段）流动性。在散装，局部链的流动性可以通过几种实验方法进行研究。相反，由于明显的仪器困难，探测自由站立的聚合物膜的局部链动力学的技术是不可用的。在这篇文章中，我们介绍了一种新的实验方法，利用介电谱（DS）的灵敏度，能够在宽频率范围内（1 Hz-1 MHz）探测自由站立的聚合物层的链段动力学，而不改变或消除它们的两个自由表面。本通讯中描述的方法指定的玻璃化转变温度与文献中的数据非常一致。在我们的方法中，聚合物薄膜被悬挂在叉指梳状电极，IDE。通过将交流电压施加到由沉积在高度绝缘衬底上的两个叉指梳状结构组成的升高的金属电极上来测量电信号（参见图1a）。对于厚度D远小于两个双孔电极之间的间距（约10 μm）的薄膜，电场线以平行于表面的方向穿透层内。因此，根据CTOT *（ω，T，D））Σ iCi（ω，T，Di），构成聚合物膜的所有子层（厚度为Di）的各个复电容相加为总电容，该表达式适用于任何角频率ω和温度T。在这些条件下，对弛豫谱的不同贡献线性叠加。事实上，梳状电极几何形状避免了由通过与表面正交的电场探测的薄膜的光谱引起的模糊性，如在平行板几何形状中，其中电容进入串联模型，CTOT *（ω，T，D）-1）Σ iCi（ω，T，Di）。为了证明我们的技术的可行性，我们调查的结构松弛动力学的无规 * 到谁对应应解决。电子邮件：simone. fys.kuleuven.be（S.N.）或wubbenhorst@fys.kuleuven.be（M.W.）。图1. (a)IDE芯片的示意图;红指和蓝指处于相反的电位。结构的高度为0.8 μm，每个指的宽度为5 μm，两个相邻指之间的平均距离为8 μm。黑色箭头表示薄膜内部电场的方向。(b)在高于其Tg的70 °C下退火后，65 nm厚的PS膜的AFM图像20 μm × 20 μm（形貌）（见正文）。白色虚线再现空芯片的结构。在电极边缘的峰值金属堆积已经存在于空芯片中。第42卷第5期2009年3月10日
Extensive investigation of polymer layers confined in nanometer-sized geometries revealed that the presence of an absorbing substrate or a free surface alters properties of polymers such as biaxial creep behavior, flow and intermolecular entanglements, diffusion of small molecules inside the matrix, crystallization kinetics, physical aging, the glass transition temperature (Tg), 7 and thus local chain (segmental) mobility. In bulk, the local chain mobility can be investigated by means of several experimental approaches. On the contrary, due to obvious instrumental difficulties, a technique probing the local chain dynamics of freely standing ultrathin polymer films was not available so far. In this Communication, we introduce a novel experimental method taking advantage of the sensitivity of dielectric spectroscopy (DS) and being able to probe the segmental dynamics of freely standing ultrathin polymer layers over a broad frequency range (1 Hz-1 MHz) without altering or eliminating their two free surfaces. The glass transition temperatures assigned by the approach described in this Communication are in excellent agreement with data from the literature. In our approach, polymer films are suspended over interdigitated comb electrodes, IDE. The electric signal is measured by applying an ac voltage to elevated metallic electrodes made up by the two interdigitated comb structures deposited on a highly insulating substrate (see Figure 1a). For films of thickness D much smaller than the separation between two neighbored electrodes (∼10 μm), the electric field lines penetrate inside the layer with a direction parallel to the surface. Consequently, the individual complex electric capacitances of all sublayers (of thickness Di) constituting the polymer membrane add up to the total capacitance according to CTOT * (ω,T,D) ) ∑iCi(ω,T,Di), an expression that holds for any angular frequency ω and temperature T. Under these conditions, the different contributions to the relaxation spectra are linearly superimposed. The comb electrode geometry, in fact, avoids ambiguities arising from spectra of ultrathin films probed by an electric field orthogonal to the surface, as in the parallel plates geometry where capacitances enter in a series model, CTOT * (ω,T,D)-1 ) ∑iCi(ω,T,Di). To prove the feasibility of our technique, we investigated the structural relaxation dynamics of atactic * To whom correspondence should be addressed. E-mail: simone.napolitano@fys.kuleuven.be (S.N.) or wubbenhorst@fys.kuleuven.be (M.W.). Figure 1. (a) Schematic representation of the IDE chip; the red and blue fingers are at opposite potential. The height of the structures is 0.8 μm, the width of each finger is 5 μm, and the mean distance between two neighbored fingers is 8 μm. The black arrows represent the direction of the E-field inside the film. (b) AFM image 20 μm × 20 μm (topography) of a 65 nm thick film of PS after annealing at 70 °C above its Tg (see text). The white dashed lines reproduce the structure of the empty chip. The peaks at the edge of the electrodes metal buildups already present in the empty chip. Volume 42, Number 5 March 10, 2009