Studies On The Structure Controlling And Behaviors Of Polymer Micelles

Polymer micelles have great potential for the applications in many fields such as preparation of advanced materials and controlled drug delivery systems, of which the properties and the functionalities are largely governed by the structure and behaviors of polymeric micelles. My dissertation focuses on the structure controlling of polymeric micelles and their behavior in solution and polymer matrix. The details are given in following three sections:1．The preparation of hybrid micelles with a relatively loose core and their inversion. The micellization of block copolymer polystyrene-b-poly (4-vinylpyridine) (PS-b-P4VP) was induced by the complexation between tetrachloroauric acid (HAuCl₄) and the pyridine groups of the P4VP block chains, producing hybrid polymeric micelle with HAuCl₄/P4VP complex as the core and PS as the shell, as is evidenced by our ¹H NMR measurements. The spheric core-shell structure of the composite micelles was observed by transmission electron microscopy (TEM). Dynamic light scattering (DLS) characterizations indicated that the micelle sizes increase with the molar ratio (MR) of HAuCl₄ to the pyridine units. HAuCl₄/P4VP complex aggregates to form the core and the PS block chains form the shell of the micelles when the MR is up to 0．3．The subsequent reduction of tetrachloroauric acid within the core leads to the formation of polymeric micelles encapsulating gold nanoparticles (PMGNs). DLS characterization data indicated the hydrodynamic diameters (D_h) of PMGNs are in the range of 82-113nm. X-ray diffraction (XRD) analysis showed five Bragg diffraction peaks at 38．36°, 44．45°, 64．62°, 77．84°and 81．86°, corresponding to the (111) , (200) , (220) , (311) , and (222) diffractions of the standard cubic phase of Au, respectively. The average size of the Au particles within the core of PMGNs was estimated to be 13．5nm according to the Debye-Scherrer equation, which is close to the size observed by TEM (15nm). The polymer shell surrounding the gold nanoparticles was visible in the TEM images with the size of the outline being close to the DLS result. Our further study demonstrate the importance of the protonation of pyridine units in the stabilization of the resultant PMGNs, i.e. the gold nanoparticles are encapsulated within the core of PMGNs when the P4VP chains are protonated, whereas when they are de-protonated, gold nanoparticles would release from PMGNs and precipitate.As the prepared PMGNs possess a low-density core in the medium, it is significant as PMGNs will be changed into the inversed PMGNs when the solvent was switched gradually from chloroform to the methanol/chloroform mixture (9/1, v/v), which is the solvent of P4VP chains or the protonated P4VP and the precipitator of PS chains. The inversed PMGNs are with PS as the core and protonated P4VP (P4VPH⁺)/gold nanoparticles as the shell, as is evidenced by our ¹H NMR characterization. The h> of the inversed PMGNs are 269 nm. The protonation of pyridine units is also indispensable to stabilize the gold nanoparticles in the inversed PMGNs, as the de-protonation of the P4VP block chains leads to the detachment of the gold nanoparticles and the shell. It is also found that the inversion behavior of PMGNs depends on the concentration of PMGNs and the structural parameters the PS-b-P4VP block copolymer. The inversion of the PMGNs from the copolymer with relative longer lengths of the two blocks has to be conducted at a concentration being lower than 0．1 mg/mL. However, when the lengths are relatively short, the inversion can occur at the concentration of the PMGNs of 1．0 mg/mL.2．Preparation of polymeric micelles composed of a small number of parent copolymer chains and the association/ dissociation behavior of the micelles.Chemically cross-linking P2VP block the triblock copolymer polystyrene-b-poly(2-vinylpyridine)-b-poly(ethyloxide) (PS-b-P2VP-b-PEO, SVE) in DMF, which is the common solvent of the copolymer, leads to the formation of micelles with the core being formed by the cross-linked P2VP block chains and the shell by the chains of both PS and PEO blocks. Due to the strong repulsion between PS and PEO block chains, the cross-linking reaction occurs among a small number (mainly 1～3 chains based on the DLS and TEM characterizations) of the copolymer chains, leading to polymeric micelles of oligo-chains (PMOs). It is found that the solubility of PS chains in the PMOs varied remarkably although they are well solubilized before the cross-linking reaction. The PS chains of PMOs tend to aggregate to form supermicelles with PMOs as the building block. The PS chains are within the core, the PEO chains construct the shell and the cross-linked P2VP chains (the core of PMOs) are located in the core-shell interface of the supermicelles. The critical micellization concentration (CMC) of the supermicelles is proved by DLS to be in the range of 5-8 mg/mL. However, the CMC for the supermicelles are about 2 or 3 order of magnitude higher than that of a block copolymer in its selective solvent and about 1 order of magnitude higher than that of Janus micelles. We think that the grafting of several PS chains on the small core formed by cross-linked P2VP chains leads to the crowding between PS chains, which will result in the decrease in the solubility when the entropy of the PS chains are taken into accounts.3．The preparation of core-crosslinked polymer micelles and the studies of dispersion of the micelles in polyacrylates matrix and the interface structure between the matrix and the micelles. Core-shell polymer micelles with a crosslinked core were synthesized by the approach similar to the "Arm-first" living polymerization method. The produced micelles were composed of linear polymer chains as the shell and crosslinked polymer network as the core. The size of the as prepared polymeric micelles is about 50nm, based on our DLS characterizations. The micelles prepared are with the compositions: 1) the micelles of PS-P(tBA-DVB) with PS as shell and crosslinked PtBA/PDVB as core and 2) the micelle PtBA-P(St-DVB) with PtBA as shell and crosslinked PS/PDVB as core. The composite films were prepared by casting tetrahydrofuran (THF) solutions of polyacrylates/PS-P(tBA-DVB) micelles mixtures. The dispersion of PS-P(tBA-DVB) micelles in the matrix and the structure of the interface between the matrix and the micelles are observed by SEM, TEM, DMA and solid state ¹³ C-NMR measured at 50℃. It is possibly due to the kinetic freezing, the micelles are uniformly dispersed in the matrix. Additionally, our solid ¹³ C-NMR measurements of the relaxation time of benzene rings at 50℃, which is above the glass transition temperature of the matrix but below that of PS chains, demonstrate that about 20% benzyl rings have a relaxation time being as low as 0．2 second. However, in the system formed by PS single chain spheres, only 15% benzene rings with a relaxation time of 3．9 second. This indicates that in the composite films formed by polyacrylates and PS-P(tBA-DVB) micelles, PS chains are partially immersed in matrix of rubber state, and the percentage 20% are much larger than the value of the PS chains existing in the interface, because the later, depending on the size of the particles, should be much smaller than the value (15%) detected in the system of single chain spheres. Our TEM observations viewed the immersion of PS chains within the matrix in the peripheral range of the micelles, supporting the speculation based on the solid state ¹³ C-NMR measurements.