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Syntheses of Organic Polymer/inorganic Composite Nanoparticles and Polymer Brushes by Surface-initiated Radical Polymerization Reactions

Author: NiGang
Tutor: YangWu
School: Northwest Normal University
Course: Polymer Chemistry and Physics
Keywords: surface-initiated polymerization organic/inorganic hybrid composite nanoparticles TiO2 nanoparticles macro-initiator polymer brush "living" / controlled polymerization grafted modification
CLC: TQ316.322
Type: PhD thesis
Year: 2006
Downloads: 753
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Abstract

As nanoscopic science, technology, and engineering have been rapidly developed in recent years, nanomaterials have attracted widespread attention for their specific capability in the optics, electrics, calorifics, magnetics, catalysis and mechanics because of their high activity, good selectivity, tremendous specific surface and small size. Polymer films with thicknesses in molecular dimensions attached to solid substrates are expected to have a wide application. Recently, as an important model surface grafting technique, surface-initiated polymerization （SIP） was successfully applied in modifying inorganic surfaces with polymers. In this "grafting from" approach, a reactive unit on the surface initiates the polymerization, and consequently the polymer chains grow from the substrate. Comparing with other methods, because of smaller steric barriers, the SIP is more appropriate to prepare higher grafting density polymers because the monomers more easily diffuse to the reactive sites than the macromolecules do during the grafting process. Because the polymerization in SIP occurs from an initiator covalently immobilized on a surface, it should be possible to control both the thickness and graft density of the polymer. And as a result the prepared polymer films have higher stability. Furthermore, because the outer polymer prepared by SIP still have some free initiators, the hybrid nanocomposites can be used as macro-initiators to initiate the polymerization of a second block polymer. So SIP can be applied not only in the homopolymerizations but also in the block and/or copolymerizations of a variety of monomers. Therefore the SIP technique opens up a new and wide route for "precision" surface modification.In this thesis, the recent development of surface-initiated polymerization （SIP） is reviewed. The theory, the experimental approaches and the research prospects of the surface- initiated polymerization are introduced extensively. The applications of this technique such as the patterned nanostructures of polymer films and polymer brushes are also discussed in detail. Some new methods of surface-initiated polymerization have been studied and some structurally well-defined polymer brushes have been prepared. With those functional thin films we could modify the inorganic particles and improve effectively the surface properties. The main contents are lists in the following: （1） A novel method of the surface initiated nitroxide-mediated radical polymerization was developed. With the common industrialized reagent, thionyl chloride （SOCl₂） and tertiary butyl hydroperoxide （TBHP） replacing the specific silane coupling agent, the peroxide initiation groups were immobilized on the inorganic material surfaces, and with 2,2,6,6-tetramethylpiperidinooxy （TEMPO）, polystyrene （PS） were grafted onto inorganic material surfaces by nitroxide-mediated living free radical polymerization. The experiment results indicated this surface-initiated nitroxide-mediated radical polymerization is a "living" / controllable free radical polymerization.（2） A new method of the surface-initiated atom transfer radical polymerization （ATRP） was investigated. After reacted with 2-bromoisobutyrate bromide, the tethered 2-bromoisobutyrate was used as the surface immobilized initiator for ATRP and homopolymer brushes of polystyrene （PS） was prepared by surface-initiated atom transfer radical polymerization. The polymerization displayed the diagnostic criteria for a living/controlled radical polymerization. Through a GPC analysis, it is found that the molecular weight of the grafted polymer increased linearly with the increase of the conversion. The linear relationship between ln（[M₀]/[M]） and time can be observed.（3） The surface-initiated reverse atom transfer radical polymerization was also studied. The peroxide group initiator was covalently immobilized on inorganic material surfaces and the polystyrene （PS） thin film was prepared by surface-initiated reverse atom transfer radical polymerization. Because this SIP is a living/controlled radical polymerization, both the thickness and graft density of the polymer can be well controled.（4） The structurally well-defined, polymer-grafted hybrid nanocomposite such as PS/SiO₂ and PS/TiO₂ composite nanoparticles were prepared by some surface-initiated radical polymerization. The molecular weight （MW）, the molecular weight distribution （MWD）, the thickness and graft density of the grafted polymer can be controlled. Grafting of the polymers onto inorganic particles can enhance effectively dispersedness of those inorganic nanoparticles and increase the affinity of the surface for the solvent and polymer matrix.（5） The smooth and uniform, structurally well-controlled, polymer brush was prepared by modifying the surface of silicon substrate with surface-initiated polymerization. Because the process of surface-initiated polymerization was a living/controlled process, we could effectively control both the thickness and graft density of the polymer by changing reaction condition and monomer concentration. The homopolymer covalently tethered to the silicon atoms at the surfaces could impart well-structured functionalities directly onto the inorganic single-crystal surfaces and improved effectively the surface properties.（6） The polymer-grafted hybrid nanocomposite prepared by above surface-initiated radical polymerization could be further used as macro-initiators to initiate the polymerization of a second block polymer, because their outer polymer still have some free initiators （such as Cl or Br）. So using the same general procedure as described previously, the PS/SiO₂ and PS/TiO₂ hybrid nanoparticles can be used as the functional macro-initiator to proceed block copolymerization reaction with MMA, so that the PS-b-PMMA copolymer/inorganic hybrid nanocomposite was prepared by surface-initiated atom transfer radical polymerization.

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