Nanoparticles in Nanoporous Polymers

The progress in nanoporous polymer synthesis was strongly inspired by the need for polymers with low dielectric constant [26] although in this case closed pores are required. Commonly this is achieved by templating over block copoly-mers with a thermally labile block. Another use of porous polymer membranes with open pores is metal deposition which is well described in [27].

An interesting example of a porous polymeric system is polymer microspheres containing PAA-lined channels which are most likely continuous [28]. These micro-spheres were prepared by ultraviolet (UV) crosslink-ing of poly(f-butyl acrylate)-b/ock-poly(2-cinnamoyloxyethyl methacrylate) (PfBA-fo-PCEMA) block copolymer micelles followed by hydrolysis.

COOC(CH3):

ch3 I 3

(PtBA-b-PCEMA)

The microspheres were used for Pd nanoparticle formation that was performed by incorporation of the microspheres in the Pd(NO3)2 aqueous solution. Pd2+ ion uptake was 1 Pd ion per two carboxylic groups. To increase Pd loading, further Pd deposition during electroless plating has been used. This allowed an increase of Pd content to 27 and 63 wt% (from 13 to 14 wt%) which resulted in an increase of the Pd particle size. As the pores in the microspheres were comparatively large (~30 nm), they might provide restriction of the particle growth only at very high loading. In doing so, particles are getting so large that the advantages of the metal confinement are nearly lost (at least for catalytic applications).

Hyper-cross-linked polystrene (HPS) [29, 30] is another porous material of choice. This is a unique polymer network where pores form spontaneously during polymer synthesis. Due to its high cross-link density, which can exceed 100%, HPS consists of nanosized rigid cavities of comparable size in the 2-3 nm range. It is readily produced by chemically incorporating methylene groups between adjacent phenyl rings in dissolved polystyrene homopolymer or gelled poly(styrene-r-divinylbenzene) copolymer in the presence of ethylene dichloride. A unique feature of HPS is its ability to swell in a wide variety of different solvents, even thermodynamically poor ones (e.g., water). Such versatility greatly facilitates incorporation of various organometal-lic compounds into the nanostructured HPS matrix. Though HPS was known for absorption of heavy metal ions from solutions [31], the first use of HPS as a medium for metal nanoparticle formation was reported in 1999 [32]. Cobalt nanoparticles have been selected here due to the established [33] correlation between size and shape of Co particles and their magnetic properties, that is, their ferromagnetic resonance (FMR) spectrum characteristics. Particles measuring less than 1 nm in diameter are nonmagnetic, while those with diameters from 1 to 10 nm are superparamagnetic. Larger Co particles behave as ferromagnetic materials. Moreover, the width of the FMR signal (AH) for spherical particles depends on the size of the Co nanoparticles [33, 34], and the position corresponding to the zero signal (H0) contains information regarding the shape of the Co nanoparticles. This allows one to determine the general size and shape characteristics of Co nanoparticles solely from FMR spectra.

Impregnation of HPS by either Co2(CO)8 in 2-propanol or the [Co(DMF)6]2+[Co(CO)4]- complex in dimethylfor-mamide (DMF), followed by thermolysis at 200 °C, results in the formation of discrete Co nanoparticles. The concentration and characteristics of such nanoparticles were investigated by X-ray fluorescence spectroscopy, FMR spectroscopy, and transmission electron microscopy (TEM). The FMR data confirmed the formation of spherical nanoparticles. At Co concentrations of 2-8 wt% the magnitude of the FMR linewidth reveals that the mean Co nanoparticle diameter is about 2 nm, which agrees well with the mean particle diameter discerned by TEM. An increase in Co content higher than 8 wt% is accompanied by an increase in mean particle diameter due to an increase in the population of large Co nanoparticles up to 15 nm across. Regulated nanoparticle growth over a wide range of Co concentrations is attributed to nanoscale HPS cavities, which serve to physically restrict the size of growing particles. It is noteworthy to mention that the HPS employed does not contain functional groups that could specifically interact with growing metal nanoparticles and stabilize them. In fact, the nanoscale cavities in HPS are anticipated to be highly interconnected and, therefore, should not hinder the migration of Co atoms or even small Co clusters. A steric limitation arising from the predominance of phenyl rings represents the most probable reason for controlled Co nanoparticle growth in HPS; phenyl rings may interact with metal surfaces and nonspecif-ically stabilize metal nanoparticles. If the Co concentration in a HPS-Co composite exceeds a saturation limit (at Co loadings in excess of 10 wt%), a few large nanoparticles grow. Their size might (i) correlate with the statistics of the pore size distribution of the HPS matrix or (ii) identify a metal-induced rearrangement of the HPS network.

As was shown by the example of Co particle formation, HPS is a robust and convenient nanostructured polymer matrix which might be broadly used for growth of catalyt-ically active nanoparticles. Following this path, the authors [35] reported formation of Pt nanoparticles in HPS. Impregnation of HPS with tetrahydrofuran (THF) or methanol (ML) solutions containing platinic acid (H2PtCl6) results in the formation of Pt(II) complexes within the nanocavi-ties of HPS. Subsequent reduction of the complexes by H2 yields stable Pt nanoparticles with a mean diameter of 1.3 nm in THF and 1.4 nm in ML. As discussed above, for Co-containing HPS, the mean particle size was about 2 nm. This discrepancy in particle size can be explained by the different mechanisms of Co and Pt particle formation. As Co particles were formed at 200 °C, Co clusters could easily migrate between the HPS pores forming Co nanoparticles with the sizes matching the pore size. For Pt nanoparticles, H2 reduction occurs at room temperature and migration of Pt clusters or atoms or Pt precursor molecules from pore to pore hardly occurs. This results in the formation of Pt particles with a size matching the size of Pt precursor (salt or complex) filling the pores [35]. Thus, for Co nanoparti-cle formation, HPS controls the particle size due to physical limitations of their growth by the pore size (2 nm). In the case of Pt nanoparticles, pores limit the amount of precursor filling the single pore so during reduction the precursor particle shrinks (density increases from Pt complex to Pt metal) to Pt nanoparticle (1.3 nm in diameter). In both ways, HPS provides strong control over nanoparticle growth. The simplicity of this approach makes this polymer system very attractive for broader use (semiconductor and other catalytic particle formation). The limitation of this system lies in the cross-linked nature of HPS, which does not allow film and coating formation. This limitation is of no concern from the viewpoint of catalytic materials (moreover, it can be used without support in heterogeneous catalysis) but strongly limits prospects for HPS use for optical or magnetic materials. Another drawback of metallated HPS is an easy washing of the metal nanoparticles out of polymer in media where PS swells. So to prevent metal loss, HPS based catalysts should be used only in "poor" (for PS) solvents (water is the best).

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