THE SHEAR PROPERTIES OF LANGMUIR-BLODGETT LAYERS
THE SHEAR PROPERTIES OF LANGMUIR-BLODGETT LAYERS
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DOI：
10.1098/rspa.1982.0048
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发表时间：
1982-01-01
期刊：
影响因子：
--
通讯作者：
EVANS, DCB
中科院分区：
文献类型：
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作者：
BRISCOE, BJ;EVANS, DCB
采用分子光滑的云母衬底，以正交圆柱形接触的形式，研究了两层高取向单层膜之间的滑动摩擦。在Langmuir-Blodgett技术的帮助下，通过水基转移沉积了各种正常脂肪羧酸及其皂状的单层。正烷基的长度从14到22个亚甲基重复单位不等。还报道了羧酸部分皂化和烷基链氟化的影响。大多数的研究都局限于两个接触的单分子层，尽管关于多层相互滑动的数据有限。滑动运动的性质不仅取决于机器，而且取决于单层膜，特别是它们的化学性质。所研究的大多数单层材料提供连续的能量耗散速率。然而，一小部分，如某些肥皂，表现出不连续或粘滑运动。实验装置允许在滑动过程中同时测量滑动摩擦力、接触面积和薄膜厚度。在一些实验中，这种摩擦是单调滑动摩擦，但在其他实验中，它是粘滞阶段的平均最大值。薄膜厚度测量精确到0.2毫米，这使得在滑动过程中剪切平面的精确评估。在所有情况下，单层和多层材料都非常耐用，剪切总是发生在单层材料之间的原始界面上。滑动摩擦数据以动态比摩擦力或界面抗剪强度表示，并对一些接触变量进行了检查。这些包括单位接触面积或平均接触压力、温度和滑动速度所施加的正常载荷。在研究范围内，界面抗剪强度与平均接触压力呈近似线性关系，与温度呈线性关系。滑动速度的影响更为复杂。在检测到间歇运动的情况下，平均最大值随速度的对数线性降低。在光滑运动中，界面抗剪强度随滑动速度的对数呈线性增长。数据表明，界面剪切强度的大小及其随接触变量的变化对烷基链的长度相对不敏感，但该链的部分氟化会显著增加摩擦。钙离子的皂化作用使比摩擦功降低。这些数据的趋势可以通过简单的应力修正热激活Eyring模型进行分析。该模型假设活化能随平均接触压力线性增加，并对滑动过程中未知的接触压力延迟程度进行了任意修正。将数据拟合到该模型中得到的参数表明，滑动伴随着松弛过程，该过程涉及许多分子链或这些分子链的部分在剪切平面上的相对运动。这相当于一个在二维空间中运动的位错。
The sliding friction between two highly oriented monolayers has been studied by using molecularly smooth mica substrates in the form of contacting orthogonal cylinders. The monolayers in the form of various normal alipathic carboxylic acids and their soaps were deposited with the aid of the Langmuir-Blodgett technique by transfer from aqueous substrates. The normal alkyl group has been varied in length from 14 to 22 methylene repeat units. Data are reported also on the influence of partial saponification of the carboxylic acid and fluorination of the alkyl chain. Most of the investigation has been confined to two contacting single monolayers although a limited amount of data is presented for multilayers sliding over one another. The character of the sliding motion depends not only on the machine but also on the monolayers, particularly their chemistry. Most of the monolayers studied provide a continuous rate of energy dissipation. However, a small number, such as certain soaps, show discontinuous or stick-slip motion. The experimental arrangement allows simultaneous measurement of the sliding frictional force, contact area and film thickness to be madeduringsliding. In some experiments this friction is the monotonic sliding friction but in others it is the mean maximum value during the stick phase. The film thickness measurement is accurate to 0.2 mm which allows a precise assessment of the shear plane during sliding. In all cases the monolayers and multilayers were found to be extremely durable and shear invariably occurred at the original interface between the monolayers. The sliding friction data are presented as the dynamic specific friction force or interface shear strength, and a number of contact variables have been examined. These include the applied normal load per unit contact area or mean contact pressure, the temperature and the sliding velocity. The interface shear strength is found, to a good approximation, to increase linearly with mean contact pressure but to decrease linearly with temperature in the ranges studied. The influence of sliding velocity is more complex. In the case where intermittent motion is detected the mean maximum values decrease linearly with the logarithm of the velocity. During smooth motions there is a linear increase of interface shear strength with the logarithm of the sliding velocity. The data show that the magnitude of interface shear strength and its change with contact variables are relatively insensitive to the chain length of the alkyl group but partial fluorination of this chain produces a marked increase in friction. Saponification with calcium ions causes a reduction in the specific frictional work. The trends in these data have been found to be amenable to an analysis based upon a simple stress-modified thermally activated Eyring model. The model assumes a linear increase in activation energy with mean contact pressure, and an arbitrary correction is made for the unknown extent of contact-pressure retardation during sliding. The parameters obtained by fitting the data to this model suggest that the sliding is accompanied by relaxation processes that involve the relative movement of a number of molecular chains or parts of these chains in the shear plane. This is tantamount to a dislocation moving in two dimensions.