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

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 approximate to 32 degrees 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 sigma approximate to 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.