Polymer-grafted nanopartides

We have reported the scale-up synthesis of polymer-grafted silica nanopartide in a solvent-free dry-system. In solvent-free dry-system, the isolation and purification after the graft polymerization was easily achieved, because untreated monomer can be removed under high vacuum (Tsubokawa, 2007). 

Chen et al. [19] have reported very active, stable platinum nanopartide catalysts prepared by alcohol reduction of PtCls using poly(N-isopropylacrylamide) previously grafted on PS microspheres as stabilizing polymer. The observed catalytic activity in the hydrogenation of allyl alcohol was more than five times higher than with Pt/C. Moreover, it was possible to recycle the resin-based catalysts for at least six cycles, whereas Pt/C was not recyclable at all. When comparing the catalytic activity of free and heterogeneous colloidal platinum particles, only a small decrease in the reaction rate was observed. 

Research efforts on filled polymer blends have been more focused on nanopartide-filled systems [42, 43]. One usual observation is the same as those with microscopic fillers - polar nanofillers localize in more polar phases [44—53]. In cases where both phases are polar or nonpolar, the filler particles have been observed to be expelled from both phases in the blend [54—56]. Selective localization of nano-sized partides has been an interesting topic of research. We discuss some of the results here. Gahleitner et al. [57] observed a preferential localization of clay particles in PA6 droplets in PA6/PP blends. Recall that day, espedally montmorillonite, is highly polar in both its pristine and various organically modified forms [58-62]. Similarly, Wang et al. [63] reported selective localization of clay particles in maleic anhydride grafted ethylene-propylene-diene (EPDM-MA) rubber droplets in poly(trimethylene terephthalate)/EPDM-MA blends. Selective localization of fillers other than clay particles has also been reported. Eor instance, Ou and Li [64] observed that toluene diisocyanate modified titania particles selectively localized in PA6 droplets in PP/ PA6/titania blends. 

In addition grafting of pwlymers onto these surfaces interests us for designing new functional composite materials which have the excellent properties both of nanopartides as mentioned above and of grafted polymers, such as photosensitivity, biorepellent activity, antibacterial activity, and pharmacological activity (Tsubokawa, 2007). 

Nevertheless, in other cases, a plastidzing effect of the nanopartides has been reported, which leads to a decrease in both the values of Tg and the storage modulus. Artzi et al. [127] reported in EVOH-montmorillonite nanocomposites a decrease of Tg values from 5 wt% of day. The same authors predict two opposing effects on the transition the confined chain mobility owing to interaction buildup, and an increased mobility due to reduction in the crystallinity degree as a consequence of polymer-clay interaction [128]. The addition of a compatibilizer, either maleic anhydride-grafted ethylene-vinyl acetate (EVA-g-MA) or maleic anhydride-grafted linear low-density polyethylene (LLDPE-g-MA), led to a decrease in the Tg values [129]. This 5wt% day content maximum is also described by McAdam et cd. in PA6-day nanocomposites [130]. 

In order to avoid particle aggregation during the polymerization process, grafting techniques offer an excellent possibility for direct covalent linking of the particles and the polymeric matrix. Polymers can either be grafted to or from the nanoparticle surfaces. If polymers should be grafted to the nanopartide surface, polymers will be end-capped with functional groups (Scheme 1). 

Surface prop>erties can be modified by thin layers of grafted polymers on a surface (not only flat substrates, but also colloidal particles, fibers, etc). These layers can be fabricated by grafring-from (as radical polymerization at the interface) and grafring-to (as tethering of the polymer chains from solution) methods. Grafted surfaces using smart temperature-responsive polymers can modulate cell adhesion and detachment properties in dependence on the temprerature. Cells adhere and proliferate on hydrophobic surfaces rather than hydrophilic ones. They tend to adhere to the surface with appropriate hydrophobidty. Polymer brush systems can be used to control adsorption mechanism, for example, protein adsorption or adsorption of nanopartides. 

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