Thrombospondins 1 and 2 (TSP-1/2) belong to a family of extracellular glycoproteins with angiostatic and synaptogenic properties. Although TSP-1/2 have been postulated to drive the resolution of postischemic angiogenesis, their role in synaptic and functional recovery is unknown. We investigated whether TSP-1/2 are necessary for synaptic and motor recovery after stroke. Focal ischemia was induced in 8- to 12-week-old wild-type (WT) and TSP-1/2 knockout (KO) mice by unilateral occlusion of the distal middle cerebral artery and the common carotid artery (CCA). Thrombospondins 1 and 2 increased after stroke, with both TSP-1 and TSP-2 colocalizing mostly to astrocytes. Wild-type and TSP-1/2 KO mice were compared in angiogenesis, synaptic density, axonal sprouting, infarct size, and functional recovery at different time points after stroke. Using the tongue protrusion test of motor function, we observed that TSP-1/2 KO mice exhibited significant deficit in their ability to recover function (P<0.05) compared with WT mice. No differences were found in infarct size and blood vessel density between the two groups after stroke. However, TSP-1/2 KO mice exhibited significant synaptic density and axonal sprouting deficits. Deficiency of TSP-1/2 leads to impaired recovery after stroke mainly due to the role of these proteins in synapse formation and axonal outgrowth.

Colloidal dispersions of polymer-silica nanocomposite particles were synthesized in high yield by homopolymerizing 4-vinylpyridine (4VP) in the presence of an ultrafine silica sol using a free-radical initiator in aqueous media at 60°C. Copolymerization of 4VP with methyl methacrylate and styrene also produced colloidally stable nanocomposite particles, in some cases for comonomer feeds containing as little as 6 mol % 4VP. However, homopolymerization of styrene or methyl methacrylate in the presence of the silica sol did not produce nanocomposite particles in control experiments. Thus a strong acid-base interaction between the silica sol and the (co)polymer appears to be essential for nanocomposite formation. Transmission electron microscopy studies confirmed the presence of the ultrafine silica sols within the nanocomposite particles, which typically exhibited "currant-bun" particle morphologies. This is in contrast to the "raspberry" particle morphologies previously reported for conducting polymer-silica nanocomposite particles. The average silica contents and mean particle diameters of the vinyl (co)polymer-silica nanocomposites were surprisingly insensitive to the synthesis conditions, as judged by thermogravimetric analysis and disk centrifuge photosedimentometry studies, respectively. The latter technique also indicated that some of the copolymer-silica dispersions were appreciably flocculated, although the degree of dispersion could be improved by redispersion in alkaline media. 1 H NMR spectroscopy studies on the extracted nanocomposites confirmed incorporation of the 4VP comonomer, with reasonable agreement between copolymer compositions and comonomer feeds being obtained. Aqueous electrophoresis measurements confirmed that the surface of the 4VP-silica particles is polymer-rich, which is consistent with their currant-bun morphology. Timeresolved photon correlation spectroscopy studies during nanocomposite formation showed that particle growth occurred rapidly, with particles reaching their final size after approximately 1 h. Doubling the 4VP monomer concentration at a fixed 4VP/silica ratio led to an increase in particle size from 150 to 220 nm. IntroductionIn polymer nanocomposites the polymer chains are confined to nanoscale (1-10 nm) dimensions. Following pioneering work by Giannelis and co-workers, 1,2 it is now recognized that these materials can exhibit unusual, even unique, properties 3 which cannot be obtained simply by comixing the polymeric component with the inorganic phase. 4,5 In many literature reports polymer nanocomposites are synthesized by creating or modifying the inorganic phase in the presence of preformed polymer chains. For example, Messersmith and Stupp 6 prepared calcium aluminate in the presence of various water-soluble polymers and obtained "organoceramic" materials. In contrast, Mark and co-workers 7 prepared monolithic poly(methyl acrylate)/SiO 2 nanocomposites by dispersing surface-modified silica particles in methyl acrylate, followed by polymerization of the monomeric con...

The surfactant-free synthesis of colloidal dispersions of vinyl polymer-silica nanocomposite particles in aqueous media using a batch emulsion polymerization protocol has been previously described [Percy, M. J.; et al. Langmuir 2000, 16, 6913]. In the present work 2-hydroxypropyl methacrylate [HPMA] was copolymerized with 4-vinylpyridine [4VP] using ammonium persulfate in the presence of an ultrafine silica sol. 4VP is used as an auxiliary in these syntheses; the strong interaction of this basic monomer with the acidic surface of the silica particles is essential for successful nanocomposite particle formation. HPMA monomer was selected since it has appreciable water solubility (up to 13% at 20°C), but HPMA homopolymer is water-insoluble. This unusual solubility behavior ensured that these nanocomposite syntheses were conducted under true dispersion polymerization conditions. In view of the success of these syntheses, we conclude that emulsion monomer droplets and micelles are not a prerequisite for the formation of nanocomposite particles. Both thermogravimetric analysis and elemental microanalyses were used to determine the silica contents of the nanocomposite particles, which ranged from 5% to 42% by mass and depended on the proportion of 4VP in the comonomer feed. Under the conditions investigated the minimum amount of 4VP auxiliary required was around 15%. Depending on the synthesis conditions, the mean particle diameter of the HPMA-4VP/SiO2 particles varied from 205 to 330 nm, as judged by disk centrifuge photosedimentometry. Scanning electron microscopy studies of selected nanocomposites indicated that discrete particles were obtained on freeze-drying, partial particle coalescence occurred on drying at ambient temperature, and complete particle coalescence occurred on annealing at 50°C. The transmittance of the annealed films was greater than 80% over the entire visible wavelength range, indicating a high degree of dispersion for the ultrafine silica sol within the film. Aqueous electrophoresis measurements combined with X-ray photoelectron spectroscopy studies indicated that the surface compositions of these HPMA-4VP/SiO 2 particles are silica-rich.

The surfactant-free synthesis of vinyl polymer-silica nanocomposite particles has been achieved in aqueous alcoholic media at ambient temperature in the absence of auxiliary comonomers. Styrene, methyl methacrylate, methyl acrylate, n-butyl acrylate, and 2-hydroxypropyl methacrylate were homopolymerized in turn in the presence of three commercially available ultrafine alcoholic silica sols. Stable colloidal dispersions with reasonably narrow size distributions were obtained, with silica contents of up to 58% by mass indicated by thermogravimetric analysis. Particle size distributions were assessed using both dynamic light scattering and disk centrifuge photosedimentometry. The former technique indicated that the particle size increased for the first 1-2 h at 25 degrees C and thereafter remained constant. Particle morphologies were studied using electron microscopy. Most of the colloidal nanocomposites comprised approximately spherical particles with relatively narrow size distributions, but in some cases more polydisperse or nonspherical particles were obtained. Selected acrylate-based nanocomposites were examined in terms of their film formation behavior. Scanning electron microscopy studies indicated relatively smooth films were obtained on drying at 20 degrees C, with complete loss of the original particle morphology. The optical clarity of solution-cast 10 microm nanocomposite films was assessed using visible absorption spectrophotometry, with 93-98% transmission being obtained from 400 to 800 nm; the effect of long-term immersion of such films in aqueous solutions was also examined. X-ray photoelectron spectroscopy studies indicated that the surface compositions of these nanocomposite particles are invariably silica-rich, which is consistent with their long-term colloidal stability and also with aqueous electrophoresis measurements. FT-IR studies suggested that in the case of the poly(methyl methacrylate)-silica nanocomposite particles, the carbonyl ester groups in the polymer are hydrogen-bonded to the surface silanol groups. According to differential scanning calorimetry studies, the glass transition temperatures of several poly(methyl methacrylate)-silica and polystyrene-silica nanocomposites can be either higher or lower than those of the corresponding homopolymers, depending on the nature of the silica sol.

The surfactant-free synthesis of poly(methyl methacrylate)-silica nanocomposite particles has been achieved in aqueous alcoholic media at ambient temperature without the use of auxiliary comonomers. Stable colloidal dispersions with reasonably narrow size distributions can be obtained with silica contents of up to 58% by mass. X-ray photoelectron spectroscopy and aqueous electrophoresis studies indicate that these nanocomposite particles have silica-rich surface compositions.

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Colloidal poly(4-vinylpyridine)/silica and polystyrene/silica nanocomposite particles comprising 38% and 48% silica by mass, respectively, have been characterized by solid-state NMR spectroscopy in order to probe the nature of the molecular interactions between the polymer and silica phases. For P4VP/silica nanocomposite particles, our results indicate hydrogen bond formation between the pyridine nitrogen and a surface hydroxyl proton. In contrast, a π-interaction between the aromatic ring and the silica surface is the most likely model for the PS/silica nanocomposite particles. Nonspecific binding interactions do not appear to play an important role in nanocomposite particle formation in either case. The high-resolution proton-carbon-13 and protonsilicon-29 correlation experiments used to extract this information are a novel combination of multipulse proton decoupling and Lee-Goldberg cross-polarization.