Bulk modulus of poly(N-isopropylacrylamide) microgels through the swelling transition

Abstract

We report measurements of the bulk modulus of individual poly(N-isopropylacrylamide) microgels along their swelling transition. The modulus is determined by measuring the volume deformation of the microgel as a function of osmotic pressure using dextran solutions. We find that the modulus softens through the transition, displaying a nonmonotonous behavior with temperature. This feature is correctly reproduced by the theory of Flory for polymer gels, once the concentration dependence of the solvency parameter is properly incorporated.

Micromechanics of temperature sensitive microgels: dip in the Poisson ratio near the LCST

“…This could be a result of the nite size of the particles, leading to a qualitatively different behavior in the case of microgels than is observed for hydrogels of macroscopic dimensions. Such a trend is also in agreement with earlier experiments on microgel particles in which only the compressive modulus 27 or the Young's modulus E of single microgel particles 7 was measured as a function of temperature; in both cases, a much broader dip in K or E, respectively, is observed than for the case of macroscopic gels. The physical reason for this apparent nite-size effect is still unclear.…”

“…By introducing our expression for the c parameter in eqn (9), one obtains an explicit equation of state; for brevity we do not display it here but instead refer to eqn (3) of a recent paper by Sierra-Martin et al 27 This equation of state relates the temperature to the polymer volume fraction in the microgel. In the following, we compare the predictions that follow from this equation of state to our experimental data.…”

“…1(A), where at the same time the obtained tting parameters are in agreement with what is expected from the microgel synthesis and from previous experimental results. 7,27,31 …”

“…26 In another study, by applying an external osmotic pressure to pNIPAM particles, the compressive elastic modulus K of single particles was measured directly, revealing a non-monotonic behavior of K as a function of temperature in the vicinity of the LCST. 27 While these measurements ensure a pure compressive mode of deformation and should thus provide an accurate measure of the compressive modulus, no information on the shear elastic modulus or the Poisson ratio can be obtained.…”

Mechanical Spectroscopy: Some Applications On Structural Changes And Relaxation Dynamics In Soft Matter

“…Further investigation is needed in order to gain a deeper understanding of this concentration-induced glass-to-gel transition. Among most microgels, poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33°C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature [20,21]. By using mechanical spectroscopy, the liquid, glass and gel transition in suspensions of PNIPAM microgel with three cross-link densities have been studied [22].…”