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長期以來，人們知道壁面受限湍流中的聚合物添加劑將顯著降低摩擦阻力，但是對聚合物在熱對流熱傳輸的影響還沒有太多研究。作為第一步，一項最近的工作[1]研究了在穩態邊界層流中熱量傳輸是怎樣被聚合物添加劑所影響的。在這項工作中[1]，聚合物是用Oldroyd-B模型來描述，這個模型允許聚合物無限伸展而沒有限制。 / 在這篇論文中，我們用一個更加真實的聚合物模型來研究聚合物在穩態邊界層流中對熱量傳輸的影響。我們採用FENE-P（有限擴展非線性彈性Peterlin）模型，在這個模型中，聚合物僅可以被伸展到一個最大的長度。聚合物的有限伸展性由參數L來衡量，它是聚合物最大長度與平衡長度的比例。基於該模型，我們發現，相對於與聚合物溶劑在底板處粘度相同的牛頓流體，熱量傳輸可以被提高或者被降低，這取決於聚合物不同的L值。而在不同的L值下，流場中底板的阻力始終加強。在早期的工作中，可以用一個隨位置改變的有效粘度來理解聚合物的效果。我們探討了聚合物的有效粘度和流速場是怎樣被聚合物改變的，以理解這個問題。我們也對熱量傳輸與不同參數的依賴關係進行了研究，這些參數包括威森博格數，普朗特數和聚合物對零剪切下溶劑粘度作出的貢獻的比例。 / It has long been known that friction drag will be reduced signicantly due to polymer additives in turbulent wall-bounded flows, but the effect of polymers on heat transport in thermal convection has not been studied much. As a rst step, a recent work [1] has studied how heat transport in a steady-state boundary layer flow might be influenced by the addition of polymers. In this work [1], polymers are modeled by the Oldroyd-B model, in which they can be extended innitely without a limit. / In this thesis, we study the effect of polymers on the heat transport in steady-state boundary layer flow using a more realistic model of polymers. We apply the FENE-P (nite extensible nonlinear elastic-Peterlin) model, in which the polymers can only be extended up to a maximum length. The nite extensibility of the polymers is measured by the parameter L, which is the ratio of the maximum length to the equilibrium one. Based on the model, we nd that compared to a Newtonian flow with the same viscosity as that of the polymer solution at the plate, heat transport can be enhanced or reduced depending on L. The fraction drag is always enhanced by the polymers for all different L. In the earlier work, the effect of the polymers has been understood to produce an effective viscosity that is position-dependent. We have explored the effective viscosity of the polymers and how the velocity eld is modied by the polymers to understand our results. We have also studied how the results depend on the different parameters, including Weissenberg number, Prandtl number and the ratio of polymer contribution to the total zero-shear viscosity. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Wang, Yiqu = 有限伸展的聚合物對邊界層流中熱量傳輸的改變 / 王異曲. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 68-69). / Abstracts also in Chinese. / Wang, Yiqu = You xian shen zhan de ju he wu dui bian jie ceng liu zhong re liang chuan shu de gai bian / Wang Yiqu. / Chapter 1 --- Introduction --- p.1 / Chapter 2 --- Prandtl-Blasius boundary layer flow --- p.7 / Chapter 3 --- Earlier work with Oldroyd-B polymers --- p.13 / Chapter 4 --- Theoretical formulation of the problem with polymers of finite extensibility --- p.20 / Chapter 4.1 --- Equations of motion --- p.20 / Chapter 4.2 --- Quantities of interest --- p.30 / Chapter 5 --- Checking validity of fixed angle approximation --- p.34 / Chapter 6 --- Results and Discussion --- p.42 / Chapter 6.1 --- Calculations --- p.42 / Chapter 6.2 --- The effect on heat transport --- p.45 / Chapter 6.3 --- The effect on drag --- p.48 / Chapter 6.4 --- The velocity field due to polymers --- p.49 / Chapter 6.5 --- Effective viscosity --- p.55 / Chapter 6.6 --- Dependence on Weissenberg number --- p.58 / Chapter 6.7 --- Dependence on Prandtl number --- p.61 / Chapter 6.8 --- Dependence on the ratio of polymer contribution to the total zero-shear viscosity --- p.64 / Chapter 7 --- Conclusion --- p.66

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.

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Polymers, by virtue of their chemical composition and molecular architecture, exhibit a diverse range of microstructural features and properties. As thin films, due primarily to effects associated with confinement and interfacial interactions, their properties may be film-thickness dependent. The significance of their thicknessdependent behavior is underscored by the fact that polymer films are of technological interest in areas that include, sensors, catalysts and organic electronics. One challenge associated with the use of thin film polymers is to understand the role of confinement and interfacial interactions on thermally induced transitions, such as vitrification and various morphological transitions. To this end, the work presented in this dissertation focuses on the behavior of thermally induced transitions in two thin film polymer-based systems: (1) an A-b-B diblock copolymer which can undergo a disorder-to-order transitions (ODT), wherein the ordered state exhibits varying geometrical symmetries, depending on the relative volume fractions of the A and B components; (2) an amorphous polymer filled with particles of nanoscale dimensions. The first of three problems examined is the influence of supercritical carbon dioxide (scCO₂) on the order-disorder transition of thin film symmetric A-b-B diblock copolymer systems. We show that the transition (xN)ODT, where x is the energetic A-B Flory-Huggins interaction parameter and N is the total degree of polymerization of the copolymer, of the thin film decreased ~ 20% compared to the bulk; the decrease was more significant in scCO₂ environments. The decrease of (xN)ODT in scCO₂ is contrary to observations in bulk copolymer-scCO₂ systems where the effective A-B interactions are weaker, hence the condition for the transition increases to higher (xN)ODT values. With regard to the second problem, we show for the first time experimentally that nanoparticles induced order into thin films of a symmetric A-b-B diblock copolymer at temperatures below the bulk ODT. Finally, we examine the influence of polystyrene (PS) grafted nanoparticles on the glass transition of PS films of varying molecular weight and thickness. We demonstrate that by controlling spatial distribution of nanoparticles, through driving forces of entropic origin, the glass transition temperature of the film can be changed drastically, as much as tens of degrees.

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