Shell-Crosslinked Systems

re-hydrates the core causing a small increase in the size from 28 to 30 nm. 

That crosslinking has indeed occurred is confirmed by the very existence of aggregates at 25 °C, as in its absence the diblock copolymers are completely soluble at this temperature. Tunability of the solubility of the PMEMA block in water arises from the fact that its lower critical solution temperature (LCST) lies between 25 and 60 °C. This reversible hydration of the core could be a very useful feature to trigger release of occluded guest molecules from the core interior. More recently, utilizing a similar methodology, zwitterionic shell-crosslinked systems have also been prepared wherein the core and shell domains contain amine and carboxylic acid groups, respectively, or vice versa. Such systems exhibit an isoelectric point, at a pH wherein the crosslinked micelles ( 40 nm) become electrically neutral and precipitate out in water addition of acid or base causes complete redissolution of these nanospheres [58]. 

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Temperature- and pH-sensitive core-shell microgels consisting of a PNIPAAm core crosslinked with BIS and a polyvinylamine (PVAm) shell were synthesized by graft copolymerization in the absence of surfactant and stabilizer [106] The core-shell morphology of the microgels was confirmed by TEM and zeta-potential measurements. Other examples of core-shell microgel systems are PNIPAAm-g-P(NIPAM-co-styrene) colloids [107] or PS(core)-g-PNIPAAm (shell) particles [108],... 

Wooley L. Karen. Shell crosslinked polymer assemblies Nanoscale constructs inspired from biological systems. J. Polym. Sci. Part A Polym. Chem. 38 no. 9 (2000) 1397-1407. 

Wooley, K.L. (2000) Shell crosslinked polymer assemblies nanoscale constructs inspired from biological systems. 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

Recently, core-shell type microgels, which contain a hydrophobic core and a hydrophilic thermosensitive shell, have become attractive for scientists because such systems can combine the properties characteristic of both the core and the shell [53], We have prepared core-shell microgel particles consisting of a poly(styrene) core onto which a shell of polyCA-isopropylacrylamide) (PS-PNIPA) has been affixed in a seeded emulsion polymerization [54-56], In this case, the ends of the crosslinked PNIPA chains are fixed to a solid core, which defines a solid boundary of the network. In this respect, these core-shell latex particles present crosslinked polymer brushes on defined spherical surfaces. The solvent quality can be changed from good solvent conditions at room temperature to poor solvent conditions at a temperature... 

Fig. 3 Formation of metal nanoparticles in the PS-PNIPA core-shell system. The crosslinked PNIPA chains absorb metal ions step 1) which are reduced to produce corresponding metal nanoparticles immobilized in the thermosensitive network step 2)...

As expected for two-phase polymer systems, two glass transitions are detected for thermoplast grafted elastic siloxane particles by means of thermal analysis (DSC, DMTA) [10]. In Table 1, glass transition temperatures of elastic siloxane graft copolymer particles with different crosslink densities (1-20 mol% T units) and distinct thermoplastic shells (PMMA, PS) are listed. 

Shell has developed a catalyst system for the RIM polymerization of DCPD which is the reaction product of 2 mol of 2,6-diisopropylphenol and 1 mol of WClg the co-catalyst is a trialkyltin hydride. Both components are soluble in DCPD and inherently storage-stable. In addition, this catalyst system has the advantage of being able to polymerize DCPD of technical quality. In a very fast exothermic reaction a complete conversion of the DCPD monomer takes place into the crosslinked polymer [78]. 

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