Tack energy and switchable adhesion of liquid crystalline elastomers

压敏胶（psa）在包括电子产品和汽车组装在内的各种应用中都很重要。PSA的一种测量方法是粘性能，它量化了分离由PSA层连接的两个物体所需的能量。psa必须有几个特性才能有高的粘性能；它必须是软的（具有低剪切模量），它必须抵抗裂纹的扩展，并且它应该最初应变软化(即。当它被拉长时变得不那么僵硬，当它被拉长时变硬。如果我们能够调整这些材料的一些特性，那么PSA的粘性能就会改变，并且粘合剂的特性也会改变。液晶弹性体（LCEs）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为基础）。然而，他们将棒状分子（液晶）结合到聚合物上。棒状分子在高温下是无序的（各向同性），因此聚合物的链形平均为球形。在低温下，这些棒彼此排列在一起，导致聚合物骨架伸长。lcee的机械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，拉伸后的力学性能表现为一个较长的应力平台（应变软化），然后是应变硬化。这是具有高能量的PSA的最佳形状。通过改变液晶棒的状态，可以切换LCE的附加能量。例如，通过从粘接层平面上排列的棒的灌肠状态切换到各向同性状态（例如，通过光照射），可以切换粘合剂的粘性能量。这将是一个很大的变化，因为机械性能从具有长应力平台转变为传统橡胶。这个项目的目的是利用理论技术调查lce可分配psa的使用情况。计算不同液晶相和不同取向下的固相能，以确定固相能在开关时的变化。粗糙金属基材与胶粘剂层之间的接触特性也将被建模。

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.