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Thermally induced association/dissociation of polymers in dilute solutions. / CUHK electronic theses & dissertations collection

Chapter 1 briefly introduces the theoretical background of the association and dissociation of polymer chains or colloidal particles are briefly introduced, including thermodynamic consideration and viscoelastic effect on the formation of mesoglobular phase in dilute polymer solutions, as well as some basic theories and universal models of fractal aggregates. / Chapter 2 details the theories of static and dynamic laser light scattering (LLS) as well as the instrumental set-up. In addition, the invention and set-up of differential refractometer are briefly discussed. / Chapter 3 summarizes laser light-scattering (LLS) and stopped-flow studies of association of cyclic- and linear-poly( N-isopropylacrylamide) (c-PNIPAM and l-PNIPAM) chains in dilute aqueous solutions. Dynamic and static LLS results reveal that the heating leads to a microphase transition. Resultant structures of interchain aggregates depend on the heating rate and the chain structure. In comparison with l-PNIPAM chains, a slow heating of c-PNIPAM chains in the solution results in stable mesoglobules with a lower average aggregation number, a looser structure and a smaller average size (∼290 nm). The temperature-jump induced association of c -PNIPAM chains in the stopped-flow measurement reveals two kinetic stages; namely, the loose packing of contracted c-PNIPAM chains and further contraction-induced fragmentation of initially packed c-PNIPAM chains due to the lack of interchain entanglements. On the other hand, for l-PNIPAM chains, the intrachain contraction and interchain penetration/entanglement simultaneously occur as the temperatures increases, leading to larger and more compact aggregates whose size increases with the solution temperature. / Chapter 4 discusses the association of water-soluble PNIPAM-monolayer-protected gold particles in dilute dispersions induced by heating the dispersions to different final temperatures higher than the lower critical solution temperature (LCST) of PNIPAM chains via the slow and fast processes. LLS was used to trace and characterize the association process, supplemented by transmission electron microscopy (TEM) measurements. The slow heating-and-cooling cycle reveals that the association and dissociation of PNIPAM-protected gold nanoparticles can be easily induced by altering the solution temperatures and the association and dissociation are fully reversible. Fast heating the dispersion to three different temperatures reveals that both the aggregation rate and average aggregation number increase with the dispersion temperature. Furthermore, the fast heating leads to the formation of fractal aggregates. The fractal dimensions of such formed aggregates continuously increases as the time evolves, which can be ascribed to the simultaneous dissociation that leads to the restructuring and rearrangement of the aggregates, resulting in denser structures. It is interesting to note that the structure of aggregate always remains fractal during the whole process. / Chapter 5 shows how water-dispersible nanosized semiconductor CdS particles (quantum dots, QDs) can be synthesized with a protective layer of covalently grafting linear thermally sensitive PNIPAM chains as well as how these CdS particles can be induced into reversible association and dissociation via an alteration of the dispersion temperature. The formation and fragmentation of these QDs aggregates were systematically investigated by laser light scattering (LLS) and confirmed by transmission electron microscopy (TEM). There exists a hysteresis in one heating-and-cooling cycle. The CdS particles stabilized with shorter PNIPAM chains (Mn = 15,000 g/mol) can associate to form larger and denser spherical aggregates with a much higher aggregation number than those grafted with longer PNIPAM chains ( Mn = 31,000 g/mol) in the heating process. The dissociation (fragmentation) in the cooling process has two stages: initially, the aggregates dissociate as the temperature decreases, and then, the fragmentation stops over a wider temperature range before its final complete dissociation at a lower temperature. We attribute such a two-stage fragmentation to a balanced effect of inter- and intra-chain hydrogen bonding as well as to the hydrophobic interaction between PNIPAM chains and CdS particles. / In this Ph.D. thesis, temperature-induced association and dissociation of various polymeric systems were systematically investigated by a combination of static and dynamic laser light scattering (LLS), supplemented by other methods, such as stopped-flow temperature jump, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). / Ye, Jing. / Adviser: Wu Chi. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3533. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Identiferoai:union.ndltd.org:cuhk.edu.hk/oai:cuhk-dr:cuhk_344268
Date January 2008
ContributorsYe, Jing., Chinese University of Hong Kong Graduate School. Division of Chemistry.
Source SetsThe Chinese University of Hong Kong
LanguageEnglish, Chinese
Detected LanguageEnglish
TypeText, theses
Formatelectronic resource, microform, microfiche, 1 online resource (x, 101 leaves : ill.)
RightsUse of this resource is governed by the terms and conditions of the Creative Commons “Attribution-NonCommercial-NoDerivatives 4.0 International” License (http://creativecommons.org/licenses/by-nc-nd/4.0/)

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