Tack energy and switchable adhesion of liquid crystalline elastomers

压敏粘合剂（PSA）在包括电子产品和汽车的组装的各种应用中是重要的。PSA的一个例子是粘性能量，它量化了分离由PSA层连接的两个物体所需的能量。PSA必须具有几种特性，才能具有高粘性能;它必须是软的（具有低剪切模量），它必须能抵抗裂纹的扩展，并且它应该在最初应变软化（即随着它的伸长而变得不那么刚性），并且应变硬化以获得更大的伸长率。如果我们可以调整这些材料的一些属性，那么PSA的粘性能量就可以改变，并且粘合剂属性也可以改变。液晶弹性体（LCE）具有与普通PSA相似的聚合物化学（丙烯酸酯或聚二甲基硅氧烷基）。然而，它们将棒状分子（液晶）结合到聚合物链上。棒状分子在高温下呈无序（各向同性），因此聚合物的链形状为平均球形。在低温下，这些杆彼此对齐，导致聚合物主链伸长。LCE的力学性能主要取决于棒状分子的取向。当延伸方向垂直于取向方向时，力学性能表现出一个长的应力平台（应变软化），随后是应变硬化.这是具有高粘性能的PSA的最佳形状。通过改变液晶棒的状态，可以切换LCE的粘性能量。例如，通过从杆在粘合剂层的平面中对齐的热机械状态切换到各向同性状态（例如，通过用光照射），可以切换粘合剂的粘性能量。这将是一个很大的变化，机械性能切换从具有长的应力平台，一个经典的rubber.The项目的目的是研究使用LCE asswitchable PSA使用理论技术。计算了不同液晶相和不同取向的粘着能，以确定在切换时粘着能的变化。粗糙的金属基板和粘合剂层之间的接触的属性也将被建模。

Pressure sensitive adhesives (PSAs) are important in a wide variety ofapplications including assembly of electronics products and cars. Onemeasure of a PSA is the tack energy, which quantifies the amount ofenergy required to separate two objects joined by the PSA layer. ThePSA must have several properties for it to have a high tack energy; itmust be soft (have a low shear modulus), it must be resistant to thepropagation of cracks, and it should strain soften initially(i.e. become less stiff as it is elongated) and the strain harden forlarger elongations. If we could tune some of these material propertiesthen the tack energy of the PSA could be changed, and the adhesiveproperties switched.Liquid crystal elastomers (LCEs) have similar polymer chemistry(Acrylate or Polydimethylsiloxane based) to common PSAs. However theyincorporate rodlike molecules (liquid crystals) onto the polymerchains. The rodlike molecules are disordered (isotropic) at hightemperature so the chain shape of the polymers is on averagespherical. At low temperature the rods align with each other and causethe polymer backbone to elongate. The mechanical properties of the LCEare crucially dependent on the orientation of the rodlikemolecules. When the elongation direction is perpendicular to thealignment direction the mechanical properties show a long stressplateau (strain softening) on extension followed by strainhardening. This is the optimal shape for a PSA with high tackenergy. By modifying the state of the liquid crystal rods the tackenergy of the LCE can be switched. For example by switching from thenematic state with the rods aligned in the plane of the adhesivelayer, to the isotropic state (e.g. by illuminating with light) thetack energy of the adhesive could be switched. This will be a largechange as the mechanical properties switch from having a long stressplateau, to a classical rubber.The aim of this project is to investigate the use of LCEs asswitchable PSAs using theoretical techniques. The tack energy indifferent liquid crystalline phases, and with various orientationswill be calculated to ascertain the tack energy change onswitching. The properties of the contact between a rough metalsubstrate and the adhesive layer will also be modeled.