Phase Transitions in Solutions of Helical Macromolecules

由拉长的大分子组成的液晶在材料科学中具有重要意义。在这些聚合物的溶液中获得的高度取向有序性可以传递到固态。这种额外的取向顺序导致合成的材料具有非常高的强度和耐温性。这里概述的研究计划集中在由于其亚基的内部螺旋排列而被拉长的聚合物。我们将研究两类由于非常不同的原因而呈螺旋结构的聚合物：通过单体之间的内部氢键保持螺旋结构的多肽，以及由于侧链相互作用而保持螺旋构象的多异氰酸酯。我们将考察这种大分子溶液中的递进柔性对相变和液晶形成的影响，这种柔性通常是由溶剂性质的变化引起的。分子的取向顺序将与链的刚性相关，如通过聚合物的持续长度来衡量。我们将探讨链刚性对液晶弹性模数的影响。溶剂变化将考察磁场引起的液晶向各向同性转变温度的降低。最后，将进一步描绘其中一种多肽在稀溶液中的独特的双螺旋到螺旋的转变。

Liquid crystals composed of large, enlongated molecules have significance in materials science. The high degree of orientational order achieved in solutions of these polymers may be passed on to the solid state. This extra order of orientation results in synthetic materials of very high strength and temperature resistance. The research program outlined here focuses on polymers that are elongated due to an internal helical arrangement of their subunits. Two classes of polymers that are helical for very different reasons will be studied: polypeptides held in helical organization by internal hydrogen bonding among the monomers, and polyisocyanates that maintain the helical conformation due to side-chain interactions. We will examine the effects of progressive flexibility, usually brought on by variation of solvent character, in solutions of such macromolecules on phase transitions and liquid crystal formation. Molecular orientational order will be correlated with chain stiffness, as measured by the persistence length of the polymer. The influence of chain rigidity on a liquid crystal elastic modulus will be explored. Solvent variations will be examined on a magnetic filed induced depression of the liquid crystal to isotropic transition temperature. Finally, a unique double, helix-to-coil transition in dilute solution of one of the polypeptides will be further delineated.