Atomic scale investigation of the volume phase transition in concentrated PNIPAM microgels

Abstract

Combining elastic incoherent neutron scattering and differential scanning calorimetry, we investigate the occurrence of the volume phase transition (VPT) in very concentrated PNIPAM microgel suspensions, from a polymer weight fraction of 30 wt% up to dry conditions. Although samples are arrested at the macroscopic scale, atomic degrees of freedom are equilibrated and can be probed in a reproducible way. A clear signature of the VPT is present as a sharp drop of the mean square displacement of PNIPAM hydrogen atoms obtained by neutron scattering. As a function of concentration, the VPT gets smoother as dry conditions are approached whereas the VPT temperature shows a minimum at about 43 wt%. This behavior is qualitatively confirmed by calorimetry measurements. Molecular dynamics simulations are employed to complement experimental results and gain further insights into the nature of the VPT, confirming that it involves the formation of an attractive gel state between the microgels. Overall, these results provide evidence that the VPT in PNIPAM-based systems can be detected at different time-and length-scales as well as in overcrowded conditions.

Molecular insights on poly(N-isopropylacrylamide) coil-to-globule transition induced by pressure

“…9 Moreover, at low temperatures, PNIPAM microgels were found to efficiently retain and confine water, 10 the polymer-water interactions being a key ingredient for the PNIPAM solution behavior. [11][12][13][14] Several studies using different experimental techniques have addressed the changes induced by temperature across the VPT, from both the structural [15][16][17] and the dynamical 18,19 points of view, and more recently the aid of numerical simulations turned out to be crucial in deeply understanding microgels' phase behavior. 20 Although studying equilibria of polymers in solution is computationally challenging, [21][22][23][24] atomistic simulations have provided useful insights on the effects of the stereochemistry and the nature of the functional groups on the lower critical solution temperature of the polymers.…”

Gellan Gum Microgels as Effective Agents for a Rapid Cleaning of Paper

“… 7 , 8 These states are arrested on the macroscopic scale but not completely on the molecular one. 9 , 10 Hence, the individual particles may still retain some degree of motion and be useful for purposes where solid-like and liquid-like properties are simultaneously needed. For the above reasons, microgels have become a favorite model system for fundamental physics, replacing hard spheres as the new paradigm of soft repulsive particles.…”

Thermal Behaviour of Microgels Composed of Interpenetrating Polymer Networks of Poly(N-isopropylacrylamide) and Poly(acrylic acid): A Calorimetric Study

“…Therefore, to investigate the basic mechanisms of thermoresponsive smart microgels, Differential Scanning Calorimetry (DSC) turns out to be one of the most suitable techniques [ 45 ], generally employed to study the thermal transitions of polymeric materials, food and pharmaceutical products, glasses and ceramics, biological macromolecules such as proteins, and in general of soft materials with significant transitions with temperature [ 46 , 47 , 48 , 49 , 50 , 51 , 52 ]. Several works have been conducted over the past 20 years, with the aim to investigate, through DSC, the thermal behaviour of hydrogels [ 53 , 54 , 55 ] and microgels [ 56 , 57 , 58 , 59 ]. For instance, the thermal behaviour of PNIPAM-PNIPMAM core shell microgels in D O solution has been studied by varying the core-shell mass ratio in order to control the mutual influences of the two polymers [ 60 ].…”

“…In a further study, the VPT and the fluid-to-glass transition of PNIPAM microgels have been studied through rheological and calorimetric measurements at various concentrations [ 58 ]. Always on PNIPAM microgel, the investigation of the volume phase transition has been carried out combining elastic incoherent neutron scattering and differential scanning calorimetry starting from polymer weight concentration of 30% up to dry conditions [ 59 ]. They outlined that the VPT emerges from the complex amphiphilic nature of the polymer that is composed by a hydrophobic backbone decorated with side chain groups containing both hydrophilic amide moieties and hydrophobic isopropyl ones.…”