Glass transition temperatures of high-density poly(methyl methacrylate) brushes
Glass transition temperatures of high-density poly(methyl methacrylate) brushes
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DOI：
10.1021/ma0255128
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发表时间：
2002-07
期刊：
影响因子：
5.5
通讯作者：
S. Yamamoto;Y. Tsujii;T. Fukuda
中科院分区：
文献类型：
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作者：
S. Yamamoto;Y. Tsujii;T. Fukuda
导言。固体衬底上的超薄聚合物薄膜(支撑膜)无论在科学上还是在实践中都是非常有趣的对象。对其结构和性能的详细了解对于设计电子设备、润滑剂和光致抗蚀剂等先进材料是必不可少的。1-3在最近的一些文献中，已经研究了支撑聚合物膜的玻璃化转变温度(Tg)，其中Tg被证明强烈地依赖于膜的厚度和底物的性质。自由或排斥表面可以通过限制链构象并导致链端或短链与膜的整体分离来影响接触的聚合物。这些都是熵效应，通常会降低Tg。另一方面，一个有吸引力的表面对薄膜既有热效应又有熵效应，它可以使玻璃化转变温度降低或升高，这取决于相互作用的性质和强度。显然，对于较薄的薄膜，这些表面效应比较厚的薄膜更重要，从而导致Tg对薄膜厚度的依赖。这是首次报道在固体表面形成的一系列高密度聚合物刷子的Tg。通过溶剂溶胀的聚合物刷子在许多科学和技术领域都很重要，例如胶体稳定、粘合、润滑剂和流变学，17-20，它们已经得到了广泛的研究。21、22干聚合物刷子也是一种有趣的支撑膜。然而，内接对玻璃化转变温度的影响仅在有限的文献中进行了研究。5C、11、12C均用中低密度刷子处理，对TG的接枝效果较小或不清楚。我们观察到端接在硅片上的聚甲基丙烯酸甲酯(PMMA)超薄膜的玻璃化转变温度(Tg)显著增加，表面密度非常高，几乎是恒定的，但链长不同(膜厚不同)。接下来，我们将给出相关的实验结果，并表明Tg的增加不仅源于(化学结合链端的)表面效应，而且还来自于高密度刷子特有的整体结构/性质。实验区。如前所述，通过表面引发的原子转移自由基聚合(ATRP)23、24在硅片上制备了PMMA刷子。表1显示了样品的特征。数均分子量和重均分子量(mN和mw)是由pmma校准的凝胶渗透色谱(Gpc)测定的同时产生的游离聚合物的值。
Introduction. Ultrathin polymer films on a solid substrate (supported films) are extremely interesting objects both scientifically and practically. Detailed knowledge of their structure and properties is essential for the design of advanced materials such as electronic devices, lubricants, and photoresists. 1-3 The glass transition temperature (Tg) of a supported polymer film has been studied in a number recent papers, 4-16 in which Tg has been shown to strongly depend on the thickness of the film and the nature of the substrate. A free or repulsive surface can influence the polymer in contact by restricting the chain conformation and causing the segregation of chain ends or short chains from the bulk of the film. These are entropic effects and generally lower the Tg. An attractive surface, on the other hand, has enthalpic as well as entropic effects on the film, and it can make the Tg either lower or higher depending on the nature and strength of the interactions. Obviously, these surface effects are more important for thinner films than thicker ones, thus causing the dependence of Tg on film thickness.This communication is the first report on the Tg of a series of high-density polymer brushes formed on a solid surface. Polymer brushes swollen by a solvent are important in many areas of science and technology, eg, colloid stabilization, adhesion, lubricant, and rheology, 17-20 and they have been extensively studied. 21, 22 Dry polymer brushes are also interesting as a kind of supported films. However, the effect of endgrafting on Tg has been studied only in a limited number of papers. 5c, 11, 12c All of them dealt with low-to moderatedensity brushes, and the grafting effects on Tg were rather minor or unclear. We have observed a marked increase in Tg for the ultrathin films of poly (methyl methacrylate)(PMMA) end-grafted on a silicon wafer with an extremely high, nearly constant surface density but differing chain lengths (differing film thicknesses). In what follows, we will present the relevant experimental results and show that the increase in Tg originates not only from the surface effect (of the chemically bound chain ends) but also from the characteristic bulk structure/properties of the high-density brushes. Experimental Section. PMMA brushes on a silicon wafer were prepared by the surface-initiated atom transfer radical polymerization (ATRP), 23, 24 as described previously. 21, 25 Table 1 shows the characteristics of the samples. The number-and weight-average molecular weights (Mn and Mw) are the values, as determined by PMMA-calibrated gel permeation chromatography (GPC), for the free polymers that were simultaneously produced