Crosslinked polymer particles

Crosslinked polymer particles with a rather complex structure, which have also been designated by the name microgels and recommended as components of metal effect paints, consist of carboxyl-terminated oligoesters of 12-hydroxy stearic acid which were reacted with glycidyl methacrylate, subsequently copolymerized with MMA and hydroxymethyl methacrylate and then crosslinked by hydroxy melamine [346]. 

Larger crosslinked polymer particles were prepared earlier for application as pigments [364]. 

For a long time crosslinking reaction steps in the polymerization of unsaturated monomers have been considered to lead inevitably to insoluble polymer materials, even with small amounts of the crosslinking component. Moreover, small crosslinked polymer particles were a nuisance in the production and characterization of polymers as unpredictable products of side reactions. 

Reactive microgels are crosslinked polymer particles which have a diameter of some 100 A and contain pendant reactive groups like vinyl groups. Such microgels may be copolymerized with bifunctional monomers to obtain crosslinked polymers in which the microgel particles act as multifunctional cross-linking sites (J ). In some respects such crosslinked polymers are comparable with other multiphase polymer systems like block or graft polymers. 

Grassi M, Colombo I, Lapasin R. Drug release from an ensemble of swellable crosslinked polymer particles. J. Control. Release 2000 68 97-113. 

The mechanism affecting the appearance and development of crosslinked polymer particles during suspension polymerization may be described usually by different schemes, depending on the monomer systems used. Starting from the suspension of water-insoluble monomers, the formation mechanism of the crosslinked polymers might be schematically depicted such as in Figure 3.1. 

Figure 10 illustrates the result of a swelling of crosslinked polymer particles. In this case 2 ym particles of styrene-divinyl benzene copolymer (5 % divinylbenzene) were first swollen with dioctyl adipate (20 % by volume) and then in the second step with chlorobenzene (125 times the volume of the polymer). It is seen that the cores all seem to lie at the side of the droplets. However, this position is an artefact in the way that it apparently is caused by rotational movements of the droplets under the microscope. 

Block and Michaels [45] formed crosslinked polymer particles from water soluble polymers and suitably reactive non-polymeric crosslinkers in an aqueous medium containing sufficient inorganic salt to prevent the water soluble polymer from dissolving. This novel suspension process avoided organic solvents as dispersing media. Polyacrylate absorbents were not given as specific examples but were included in the broadest claim of this application. 

Microgel particles are crosslinked polymer particles that change their volume (i.e. swell or deswell) according to (i) the solvency conditions of the medium in which they are dispersed (ii) the density of the crosslinking moieties within the particles. For comprehensive reviews of these types of particles, including their preparation and characterization, one is referred to previous articles [1-3]. 

In these cases, the polymer remains processible in the gelled state, because it is in the form of discrete PSA particles dispersed in the reaction medium. However, once the particles are dried, redispersion may be difficult if strong interactions develop between the particle surfaces. Polymerization of the acrylic PSA directly on the substrate, as in the case of UV polymerization, can also yield a covalently crosslinked polymer that does not require any further coating steps [71]. 

Research on the modelling, optimization and control of emulsion polymerization (latex) reactors and processes has been expanding rapidly as the chemistry and physics of these systems become better understood, and as the demand for new and improved latex products increases. The objectives are usually to optimize production rates and/or to control product quality variables such as polymer particle size distribution (PSD), particle morphology, copolymer composition, molecular weights (MW s), long chain branching (LCB), crosslinking frequency and gel content. 

Perhaps the most common particle type used for bioapplications is the polymeric microsphere or nanosphere, which consists basically of a spherical, nonporous, hard particle made up of long, entwined linear or crosslinked polymers. Creation of these particles typically involves an emulsion polymerization process that uses vinyl monomers, sometimes in the presence of... 

The question whether the intramolecularly crosslinked microparticles of non-aqueous polymer dispersions are really microgels is also justified, considering non-aqueous dispersions prepared from acrylic copolymers and melamine/formaldehyde crosslinker with particle sizes of about 300 nm. [45, 343]. In any case, these crosslinked polymeric microparticles are useful constituents of high-solids coatings, imparting a yield stress to those solutions which probably involves attractive forces between the microparticles. 

Experimental and analytical studies over the past 25-30 years revealed that microgels are intramolecularly crosslinked macromolecules, which represent a new class of polymers besides linear and branched macromolecules and crosslinked polymers of macroscopic dimensions. In some ways microgels may be considered as a transition from molecules to larger polymer particles or macroscopic polymer materials. 

By interactive in this context we mean that the filler can adsorb network polymer and thereby become involved in the crosslinking process, creating a larger number of network links. In some cases the polymer-particle interaction may be the main crosslinking mechanism. 

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