Microgels 4: The preparation of novel microgels and their applications

Abstract

AbstractThis paper describes studies on the formation of microgels from high functionality mixtures of monomers which are able to polymerise by a step‐growth process to polyurethanes in the specially selected solvent butan‐2‐one. Using the reactants poly(ethylene glycol) 6000, 1,2,6‐hexanetriol and biscyclohexylmethane‐4,4′‐diisocyanate in formulations made to comprise exactly one isocyanate group per hydroxyl, it was found that microgels of number average molecular weight, as determined by Gel Permeation Chromatography, of less than 20,000 were obtained up to the Critical Gelation Concentration which ranged from w/w reactant to solvent concentrations of from ∼ 0.24 to 0.74 at PEG/HT ratios of 1:4 to 4:4 respectively. These polymerisations surprisingly, went to complete conversion without gelation and produced 95 – 99% yields of microgel product after precipitation into a non‐solvent. Intrinsic viscosity determinations on the microgels dissolved in chloroform were determined and used to obtain estimates of the swollen coil densities at infinite dilution from which the critical packing concentration of these swollen coils in the solvent used for the solvent could be estimated. The critical packing concentration was in all cases well below the Critical Gelation Concentration. It was thus clearly demonstrated that the formation of “soluble” microgels is progressing in spite of the system being critically packed. This is a surprising observation and especially so when it is combined with the very low measured weight average molecular weights and high conversions of the products in the critically packed region. The GPC molecular weights decreased at the higher concentrations which is contrary to expectations. The ability to form microgels above critical packing confirms the results obtained and reported previously by us, that a similar phenomenom occurs in the case of the polymerisation of polyfunctional free‐radically polymerised monomer systems. These observations will need to be taken into account when formulating mathematical analyses of the gelation process. This class of products are able to be crystallised from organic solvents and are soluble in water. They are potentially useful as components of pharmaceutical formulations for the controlled release of drugs, high molecular weight active materials and living microorganisms.