Semiflexible Polymers with Specific Interactions

具特定作用力之半軟性高分子研究

陳軍佑(Chun-Yu Chen)

指導教授 ： 陳信龍

國立清華大學/工學院/化學工程學系/博士(2010年)

摘要

This dissertation centers on the self-organization behavior of semiflexible polymers bearing specific interaction groups to form special aggregates or supramolecular assemblies with themselves or another macromolecule with complementary binding groups. The systems studied include (1) semiflexible conjugated polymers (bearing pi-pi interaction groups) for opto-electronic applications and (2) DNA (bearing anionic phosphate groups) capable of forming electrostatic complexes with cationic dendrimers for mimicking DNA-histone complexation and for applications as non-viral vectors for gene therapy.

For the study of conjugated polymers, we focused on the conformational and aggregate structures and phase separation-induced gelation of poly(9,9-dioctylfluorene) (PF8) and poly(3-hexylthiophene) (P3HT) in the solutions with relatively poor solvents. Using small angle neutron scattering (SANS), PF8 in toluene was found to display wormlike chain conformation in dilute toluene solution. The effective Kuhn length of the chain increased with increasing polymer concentration due to excluded volume interaction. In the semidilute regime, the polymer formed transient networks in which the sub-chains constituting the dynamic meshes were essentially rodlike. The originally viscous liquid solution transformed into a gel when the semidilute solution was aged at -20 oC. The gel composed of a PF8-enriched phase and an isotropic phase due to the occurrence of a macrophase separation at the subambient temperature. The PF8-enriched phase was mesomorphic, consisting of sheetlike aggregates or membranes with thickness of ca. 2.6 nm. A fraction of the PF8 chains or segments within these sheetlike aggregates formed the beta-phase which dominated the photoluminescence of the gel.

The mechanism of phase separation-induced gelation of PF8 was further investigated using the solution of PF8 with a poor solvent methylcyclohexane (MCH), as this system underwent gelation easily at room temperature and would hence allow the phase separation process to be conveniently monitored in situ. Light scattering and optical microscopy revealed that the gelation was driven by a macrophase separation occurred through a spinodal decomposition mechanism. Although the spinodal decomposition could proceed to the late stage, the interconnected morphology was arrested to give rise to the gel property. Like the toluene gel, the phase-separated MCH gel also composed of an isotropic phase and a PF8-enriched liquid crystalline phase, in which a fraction of PF8 chains formed the beta-phase embedded within the loose membranes. The MCH gel exhibited a better thermal stability than the toluene gel, and the thermally-induced gel-to-sol transition was accompanied with the disruptions of the loose membranes and the beta-phase that led to homogenization of the solution.

In contrast to PF8 systems, the gelation of the solution of P3HT with xylene occurred through a liquid-solid phase separation. Here we have demonstrated the use of small angle X-ray scattering (SAXS) to identify the formation of nanowhiskers in the P3HT gel. We have shown that the P3HT chains originally forming network aggregates in xylene solution underwent crystallization to yield the nanowhiskers. The networking of the nanowhikers may lead to gelation of the solution. In contrast to the highly crystalline nanowhiskers formed in dilute P3HT solutions, the whiskers in the gel showed relatively low crystallinity due to the strong inter-chain interaction of P3HT in the network aggregates that restricted the registry and organization of a fraction of chains into the crystallites. The integrity of the nanowhiskers was largely maintained by the crystallinity of P3HT; therefore, the nanowhisker morphology was disrupted upon heating to the melting point of P3HT crystallites.

For the study of DNA-dendrimer complexes (called “dendriplexes”), we focused on resolving the supramolecular structures of the complexes with the 4th generation (G4) polyamidoamine (PAMAM) dendrimer. Using synchrotron SAXS and scattering function calculation of model structures via Debye equation, we showed that, depending on the charge density of the dendrimer prescribed by its degree of protonation (dp), the dendriplexes exhibited three distinct nanostructures characterized by different degrees of DNA twist. At dp < 0.3 the dendriplex displayed a columnar mesophase in which the square-packed DNA bridged by dendrimer showed relatively extended conformation. Increasing dendrimer dp to 0.3 induced a moderate twist of DNA into superhelices organized in a hexagonal lattice. At dp ≥ 0.6 where the interior tertiary amine groups were protonated, DNA wrapped around the dendrimer by 1.4 turns and penetrated into the dendrimer interior as the electrostatic attraction well dominated its bending energy.

關鍵字

小角度X光散射 ； 高分子溶液 ； 高分子凝膠 ； 交互作用力 ； 小角度中子散射

並列摘要

無資料

並列關鍵字

SAXS ； Polymer solution ； Polymer gel ； Interactions ； SANS

參考文獻

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16. Chen, J. H.; Chang, C. S.; Chang, Y. X.; Chen, C. Y.; Chen, H. L.; Chen S. A. Macromolecules 2009, 42, 1306-1314.

17. Rahman, M. H.; Chen, C. Y.; Liao, S. C.; Chen, H. L.; Tsao, C. S.; Chen, J. H.; Liao, J. L.; Ivanov, V. A.; Chen, S. A. Macromolecules 2007, 40, 6572-6578.