Microgels: model polymers for the crosslinked state

Abstract

Polystyrene microgels, small network particles with an overall size in the region of 100 nm, are synthesized via radical copolymerization of styrene and m-diisopropenylbenzene in microemulsion. The resulting structures are characterized by viscosimetry, static and dynamic light scattering (LS), and dynamic mechanical experiments. These microgels represent strictly spherical particles with a moderate polydispersity where the size of the microemulsion droplets can only be adjusted in the restricted range 60 nm < d < 200 nm. This is possibly due to the polymeric surfactant used. The intrinsic viscosity data are related to molecular swelling where the internal density of the particles in dependence of the cross-linking density obeys the rules of rubber elasticity. From LS experiments it is concluded that the swelling occurs heterogeneously even on the size scale given by the microgel dimensions. The application of microgels as a model for macroscopic networks is extended to the examination of the influence of dilution during the crosslinking reaction. The efficiency of cross-linking decreases with increasing amount of solvent. Dynamic mechanical shear experiments of microgels result in a typical rubbery plateau at high frequencies and small strains known also from macroscopic networks. The absolute plateau value indicates a large amount of defects reducing the cycle rank of the network. Dilution during cross-linking causes a qualitative change of the complete spectrum, which exhibits a continuous loss of the moduli toward lower frequencies, a behavior known for systems close to gel transition.ABSTRACT: Conformational dynamics of short probes dispersed in a matrix is investigated. The matrix can be in two states, fast or slow, depending on the amount of free volume. Following the work of Anderson and Ullman,4 the amount of free volume is assumed to fluctuate in time, thus modifying the states of the matrix. The probe undergoes transitions between two states resembling trans-cis isomeric transitions. Correlation times of the probe-matrix system exhibit a clear transition as the free-volume fluctuations of the matrix become faster. In a fast matrix, the temperature dependence of correlation times for probes reflects both the intramolecular conformational barrier in the probe and the energy change of viscous origin in the matrix, while in a slow matrix only the activation energy of the matrix is observed. This is in agreement with the results from recent experiments with fluorescent probes dissolved in small-molecule solvents and in bulk polymer.

Light-scattering study of the coil-to-globule transition of linear poly(N-isopropylacrylamide) ionomers in water

“…At higher temperatures, the interchain aggregation stops because Table III are uniform spheres. 15,16 The average density »r… terchain aggregation (the increase of R vv (u)/KC) shown in Figure 1 and to the peak position of »R h … of the aggregates decreases as the aggregation number »N… agg decreases, which may be attributed shown in Figure 2. Logically, there is a minimum value of »S… ionic , at which the surface of the aggreto the imperfect packing of the polymer chains inside the aggregate when are only one or two gates is ''fully covered'' by the ionic groups so that further aggregation is impossible because of the chains inside each aggregate.…”

Architecture of Multi-Component Copolymer Brushes: Synthesis, Solution Properties and Application for Nanodevices

“…This parameter was calculated for various macromolecular architectures. The calculated values of R g =R h are 0.775 and >2 for hard sphere of uniform density 18 and for monodisperse rigid rod, 19 respectively. The obtained value of R g =R h (1.80) for prototype copoly- mer brush P4 is similar to that of the rigid rod model.…”

NMR Study of the Morphology of Polystyrene Linear/Microgel Blends

“…These particles are homogeneous, with a uniform segment density and lacking any surface "softness". 3 The respective mass densities of microgels and linear PS are essentially the same, by virtue of the initially spherical microgel particles assuming a polyhedral form when coagulated to a neat melt. Of course, the size of an individual microgel particle is much less than the volume of a linear PS chain having the same molecular weight.…”

“…Microgels have also been proposed as model networks, offering a means to investigate the elastic properties of rubbery materials. 3,8 Blends of linear chains with networks have been the focus of research into nematic coupling (i.e., orientational correlations) in deformed networks containing unattached probe chains [9][10][11][12][13][14] and on the role of constraint release in the terminal relaxation of entangled polymers. [15][16][17] In this paper we investigate PS microgels mixed with linear PS.…”