Rheologicaland Calorimetric Studies On PNIPAM Gel Systems

The thermal fluctuations of colloidal particles are similar to the motion of molecules,so they are often analogous to the structure or dynamics of molecules or atoms to simulate the structure and dynamics of glass and crystal.Compared with hard colloidal particles,soft colloid particles can achieve a wider range of dense packing styles.Among them,PNIPAM is a kind of temperature-sensitive microgel,the diameter of which is able to vary with small change in temperature.This sensitivity provides researchers with a knob for in situ control of the structural and dynamical behavior.And PNIPAM has been widely used as a model system.In order to simulate materials effectively,we can control the properties of colloidal particles by experimental methods: changing the composition of monomers.In this study,the effect of AA and MMA on the mechanical and dynamic properties of the PNIPAM concentrated suspensions was studied by rheological techniques.The solid-liquid transition in this system is studies by calorimetric method compared with glass transition.This study can improve the ability to make full use of colloid as model materials,to provide guidance for designing material structure and the optimization of material properties,to better understand the profound relationship between solid-liquid transition and glass transition.In this study,the volume fraction in the P(NIPAM-co-AA)system with mass fraction of 6 wt% increased first and then decreased with the addition of AA content.The solid-liquid transition temperature,yield stress and viscosity have the same trend.The hydrophilic effect strengthened when less AA was added,while the swelling property of the particles weakened when more AA involved due to the increase of the osmotic pressure.The volume fraction in the P(NIPAM-co-MMA)of 6 wt% increased first and then decreased with the addition of MMA content.The hydrophobic effect strengthened when less MMA was added,while MMA plays a role of “additive” when content is more.Suspensions with various concentrations of 0.2 wt%、0.5 wt%、1.0 wt%、2.0 wt%、3.0wt%、6.0 wt%、10.0 wt% and 15 wt% were studied by DSC.The result shows that the hydrophilic-hydrophobic transition and the solid-liquid transition are two independent processes,which can be distinguished by the direct comparison of rheology and calorimetry.The interaction between particles in dilute and concentrated suspensions isvery different and the solid-liquid transition occurs in concentrated suspensions does not involve any enthalpy,it is an entropy-driven process.