Core-crosslinked spheres

Figure 6.3 The chemical compositions for macromolecular objects that result from crosslinking within nanodomains of bulk phase separated block copolymers include (a) core-crosslinked spheres (b) core-crosslinked rods (c) mushroom -shaped objects.

Janus micelles are non-centrosymmetric, surface-compartmentalized nanoparticles, in which a cross-linked core is surrounded by two different corona hemispheres. Their intrinsic amphiphilicity leads to the collapse of one hemisphere in a selective solvent, followed by self-assembly into higher ordered superstructures. Recently, the synthesis of such structures was achieved by crosslinking of the center block of ABC triblock copolymers in the bulk state, using a morphology where the B block forms spheres between lamellae of the A and C blocks [95, 96]. In solution, Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) half-coronas around a crosslinked polybutadiene (PB) core aggregate to larger entities with a sharp size distribution, which can be considered as supermicelles (Fig. 20). They coexist with single Janus micelles (unimers) both in THF solution and on silicon and water surfaces [95, 97]. 

Recently, a versatile class of poly(ethylene propylene)/poly(ethylene oxide) block copolymer micelles were introduced they were stable due to a combination of high block incompatibility, kinetically frozen core, and high interfacial tension between core and solvent [53, 58]. Moreover, by using a co-solvent of varying composition, the aggregation number was controlled and soft spheres from star-like to micelle-like could be obtained. Another way is core stabilization via chemical crosslinking, say by UV radiation [59-64]. 

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

In another example, BSA, as a model protein, vras entrapped in biodegradable shell crosslinked nanocapsules via layer-by-layer assembly of tertiary amine and hydrazide grafted poly(aspartamide) and carboryl and aldehyde grafted poly(aspartamide) on amino-fiinetionalized silica spheres, by hydrazone crosslinking and with a silica core. Tbe BSA release rate increased significantiy as tbe ambient pH dropped from tbe physiological pH to acidic endosomal pH, and cell experiments showed that the obtained polymerie nanocapsules possessed good uptake properties and biocompatibility. ... 

Two types of poly(2-vinyl pyridine-b-styrene-b-2-vinyl pyridine) triblock copolymers were synthesised by anionic living polymerisation. These polymers formed monodispersed micelles in toluene or toluene/cyclohexane mixture. Poly(2-vinyl pyridine) sequences in the core part of the polymer micelles were crosslinked with 1,4-diiodobutane. After crosslinking, no macrogelation was observed. The morphology of the crosslinked products did not correspond with that of the original triblock copolymer. All products were polystyrene spheres and each of them had one poly(2-vinyl pyridine) core in its centre. It was therefore concluded that the poly(2-vinyl pyridine) core-PS shell type flower microgels were synthesised by crosslinking of the flower micelles in solution. 27 refs. 

Walther [111] was among the first to study and report the effect of the incorporation of Janus nanoparticles on the morphology of PS/PMMA blend. The organic Janus nanoparticles were composed of a crosslinked polybutadiene core grafted with two PS and PMMA half-sphere (Figure 11.8). 

Among other original studies, catanionic vesicles were also used as templates for the crosslinking reaction of tetramethylcyclotetrasiloxane or D inside their bilayers this led to the creation of nonporous, impermeable, highly crosslinked, water-filled hollow spheres of about lOOnm diameter [106,107]. Various reports on the preparation of inorganic/organic core-shell particles via a simultaneous or two-step radical and ionic emulsion polymerization are outlined later in the chapter. 

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