Intramolecular and Intermolecular Chiral Interactions in Chiral Polylactides

• Main Authors: Huang, Jian-Wei, 黃健維
• Other Authors: Ho, Rong-Ming
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/54553788832212182441

碩士 === 國立清華大學 === 化學工程學系 === 100 === Chirality is known as an important feature of molecules and macromolecules for the formation of helical architectures at different hierarchical levels. In nature, homochiral evolution is a key molecular process for the communication, replication and enzyme catalysis through the association of chiral molecules and macromolecules. Herein, we aim to examine the mechanisms of chiral information transfer for chiral polymers from configurational chirality to conformational chirality so as to give fundamental understanding of the self-assembly of helical polymer chains. Polylactide is a well-known biodegradable polymer, and chiral polylactides with specific optical activity are crystalline polymers due to their regular configuration. As a result, enantiomeric chiral polylactides were synthesized in this study so as to give a model system for the examination of chiral information transfer from molecular level. Opposite chiral entities (i.e., configurational chirality) with main-chain chirality as examined by circular dichroism (CD) will give rise to the PLLA and PDLA chains as helical conformations with preferential left- and right-handedness (i.e., conformational chirality), respectively, due to intramolecular interaction as evidenced by vibrational circular dichroism (VCD). Accordingly, the molecular chirality not only plays the key factor to give helical conformation but also controls its helicity. The dynamic helical polymers, such as chiral polylactides, possess a very low helical inversion barrier so as to give the reversal of handedness. This specific distance between helical reversal points named helical persistence length. Accordingly, the helical reversal occurs once the chain length is over the helical persistence length. To examine the behavior of the helical reversal, chiral polylactides with different molecular weights were synthesized so as to clarify the molecular weight dependence of optical activity by VCD. Our results indicate that the ellipticity of chiral polylactides is indeed strongly dependent upon molecular weight at which the ellipticity linearly increases with the molecular weight until the molecular weight reaches 1000 g/mol and then levels off at a higher molecular weight. Moreover, in comparison with the spectroscopic results, small angle neutron scattering (SANS) experiments are carried out to visualize the changes of scattering profiles for various chiral polylactides. According to the SANS results, the scattering results of chiral polylactides perfectly fit the worm-like chain model, indicating that the polymer chain of chiral polylactide is rod-like and possess several turning points. Namely, the behavior of rod-like chiral polylactide is in line with the suggested chain conformation with helical persistent length. Self-assembling plays an important role in the formation of many chiral structures and in the preparation of chiral functional materials. Therefore the control of chirality in synthetic or self-assembled systems is important either for the comprehension of recognition phenomena or to obtain materials with predictable and controllable properties. The intermolecular chiral interaction acts onto chiral polylactides during the aggregation process, so as to give the symmetric optical activity. This result is well observed from ellipticity contributed from ether group on chiral polylactides. Because the vibrational transition of ether group is almost parallel to helical conformational axis, and only supramolecular but no conformational chirality exist here. Whereas conformational chirality can be identified by the vibrational absorption of carbonyl group, and supramolecular chirality is recognized by the at ether group, a methodology is built to observe chiral information transfer from conformational chirality to supramolecular chirality by vibrational circular dichroism. Furthermore, the intensity of intermolecular chiral interaction will be enhanced by the degree of chain packing (that is, the crystallinity). The optical activities significantly amplify from amorphous states to crystalline phase.