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

P. Voudouris, D. Florea, P. P. A. M. van der Schoot, H.M. Wyss

Research output: Contribution to journalArticleAcademicpeer-review


Microgels of poly-N-isopropylacrylamide (pNIPAM) exhibit a remarkable sensitivity to environmental conditions, most strikingly a pronounced deswelling that occurs close to the lower critical solution temperature (LCST) of the polymer at ≈32 °C. This transition has been widely studied and exploited in a range of applications. Along with changes in size, significant changes are also expected for the mechanical response of the particles. However, the full elastic properties of these particles as a function of temperature, T, have not yet been assessed at the single-particle level. Here we present measurements of the elastic properties of pNIPAM particles as a function of both temperature and cross-linking density using capillary micromechanics, a technique based on the pressure-dependent deformation of particles trapped in a tapered glass capillary. The shear elastic modulus G increased monotonously upon increasing temperature. In contrast, but in qualitative agreement with previous experiments on macroscopic pNIPAM hydrogels, we found that the compressive elastic modulus K of our microgels exhibits a dip close to the LCST. Remarkably, this dip is less sharp and deep than that observed in macroscopic hydrogels. The Poisson ratio of the particles also exhibits a pronounced dip close to the LCST, reaching unusually low minimum values of σ ≈ 0.15. To rationalize this behavior, we compared our experimental data to Flory–Rehner theory; the theory is able to qualitatively predict the general mechanical behavior observed, thus indicating that the observed dip in the Poisson ratio can be accounted for by simple thermodynamic arguments.
Original languageEnglish
Pages (from-to)7158-7166
Number of pages9
JournalSoft Matter
Issue number29
Publication statusPublished - 2013


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