The collapse kinetics of microgels is determined experimentally and by mesoscale computer simulations.

Cross-linking density and distribution are decisive for the mechanical and other properties of stimuli-sensitive poly(N-isopropylacrylamide) microgels. Here we investigate the structure of ultra-low cross-linked microgels by static light scattering and scanning force microscopy, and show that they have an inverted cross-linking structure with respect to conventional microgels, contrary to what has been assumed previously. The conventional microgels have the largest polymer volume fraction in the core from where the particle density decays radially outwards, whereas ultra-low cross-linked particles have the highest polymer volume fraction close to the surface. On a solid substrate these particles form buckled shapes at high surface coverage, as shown by scanning force micrographs. The special structure of ultra-low cross-linked microgels is attributed to cross-linking of the particle surface, which is exposed to hydrogen abstraction by radicals generated from persulfate initiators during and after polymerization. The particle core, which is less accessible to the diffusion of radicals, has consequently a lower polymer volume fraction in the swollen state. By systematic variation of the cross-linker concentration it is shown that the cross-linking contribution from peroxide under typical synthesis conditions is weaker than that from the use of 1 mol% N,N'-methylenebisacrylamide. Soft deformable hydrogel particles are of interest because they emulate biological tissues, and understanding the underlying synthesis principle enables tailoring the microgel structure for biomimetic applications. Deformability of microgels is usually controlled by the amount of added cross-linker; here we however highlight an alternative approach through structural softness.

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Precise particle size customization is a key requirement for the applications of stimuli-sensitive poly( N -isopropylacrylamide) microgels. No empirically supported mechanism has been presented to explain the microgel particle size dependence on polymerization conditions. In this work, nonstirred precipitation polymerization is employed to address this question. It is argued that the reaction proceeds by radical solution polymerization mechanism rather than as emulsion polymerization. The number of particles in the batch, and the microgel particle volume, is shown to correlate with the charge density of the particles determined by the kinetic chain length. Cross-linking effi ciency is recognized to be an important factor in the particle nucleation. Properties of radical solution polymerization, such as differences in the rate of decomposition of thermally decomposing initiator and redox initiator, are used to tune the particle number density in the batch. Contemporary synthesis approaches, including temperature-programmed synthesis, are discussed in the context of these results. approaches clearly demonstrate that microgel particle synthesis can be advanced, but also the need for thorough understanding of the particle formation mechanism in order to devise new synthesis methods. In this work we investigate the fundamental aspects of microgel particle formation in precipitation polymerization using rapid, convenient, and highly reproducible nonstirred synthesis method and show that a correlation exists between the fi nal volume of the PNIPAM particles in the collapsed state and the average degree of polymerization predicted by the solution polymerization kinetics. As already recognized by Gao and Frisken, [ 14 ] connecting the radical polymerization mechanism with the fi nal particle volume would be an attractive explanation for the experimentally observed microgel size. However, the experimental evidence in support of this mechanism has been nonconclusive and even confl icting [ 14 ] until now.

Many applications of poly(N-isopropylacrylamide) microgels necessitate robust control over particle size.