Core-Crosslinked Systems

Micelle formation with occlusion of homopolymer in the core 

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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],... 

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)...

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]. 

Permeability Reduction Screening. ITie intent of the preliminary gel screening in core is to determine if candidate polymer/crosslinker systems produce a lower permeability (higher RRF values) than the base polymer. Gels that meet the permeability reduction criteria for a specific field application are then evaluated fully in the final core test phase. 

Selected systems have been studied to compare the trends observed in beaker tests to core performance. If correlations can be developed between beaker test measurements and core test results, the screening of polymer/crosslinker systems can be expedited. These tests would ideally... 

Abdullah Al, N., Lee, H., Lee, Y. S., Lee, K. D. and Park, S. Y. (2011) Development of disulfide core-crosslinked pluronic nanoparticles as an effective anticancer-drug-delivery system. Macromol Biosci, 11,1264-1271. 

The ATRP of 4-vinylpyridine initiated by a PEO-based macroinitiator was conducted in an ethanol-water mixture in the absence or in the presence of a tfivinyUc comonomer to crosslink the pattide core. The system resulted in block copolymer micelles stabilized by the PEO-soluble blocks. To improve the PSD in dispersion ATRP, a two-stage method was applied, starting from a standard free radical polymerization, which allowed the nudeation step to proceed in fast conditions. The controlled character of the second polymerization step was demonstrated. 

Similar organic-inorganic systems, which were ultimately crosslinked by sol-gel chemistry, were prepared with cores composed of high- Tg dendrimers [69, 70]. Tough materials with high heat resistance were obtained. Also, coreshell structures were prepared via silylation, or hyrosilylation. The resulting structures were further crosslinked to give supramolecular assemblies [71]. 

The AB2 + AB system is equivalent to AB2 except that AB2 units are separated from each other by AB units. The AB2 + B3 system modifies the AB2 system by using B3 as a central core from which polymerization radiates and offers greater control of molecular shape. The A2 + B3 system is one of the standard systems used to produce crosslinked polymers (Sec. 2-10). It is useful for synthesizing hyperbranched polymers only when crosslinking is minimized by limiting conversion and/or diluting the reactants with solvent. 

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... 

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