Swelling of responsive-microgels: experiments versus models

Abstract

Interpenetrated Polymer Network (IPN) microgels of PNIPAM and PAAc have been investigated and the experimental data have been compared with theoretical models from the Flory–Rehner theory. We confirm that the swelling behavior of PNIPAM microgels is well described by this theory by considering the second order approximation for the volume fraction ϕ dependence of the Flory parameter χ(ϕ). Indeed the Volume-Phase Transition (VPT) of the PNIPAM-PAAc IPN microgel at neutral conditions and in D⁠2O solvents can be well described only\ud considering a third-order approximation. Interestingly we empirically find that sharper is the transition higher is the order of the χ(ϕ) relation which has to be considered. Moreover the VPT can be experimentally controlled by tuning the polymer/solvent interactions through pH and solvent allowing to directly modify the delicate balance between energetic and entropic contributions and to explore the swelling behavior in a wide range of environmental conditions. In particular we find that the most advantageous condition for swelling is in water at acidic pH

Study of network composition in interpenetrating polymer networks of poly(N isopropylacrylamide) microgels: The role of poly(acrylic acid)

“…Interestingly the A parameter increases with PAAc content indicating that mixing between polymer and solvent is not favored and swelling is inhibited by PAAc. These results strongly indicate that interpenetration of PAAc network affects the interaction between PNIPAM and water molecules, these polymer/solvent interactions can be therefore controlled through pH [46] or by varying PAAc content, as here highlighted for the first time. However it is worth noting that there is a discrepancy between the value of φ 0 obtained with the Flory Rehner theory and other methods as deeply discussed in Ref.…”

“…In order to understand the role played by the poly(acrylic acid) on the relaxation dynamics and on the aggregation process across the VPT, the temperature behaviour of the relaxation time, reported in Fig.4, has been obtained through DLS by fitting the g 2 (Q,t) with Eq.(1). In the low weight concentration range ( Fig.4(a)) the well known dynamical transition associated to the VPT is evidenced for both PNIPAM and IPN microgels [44,46]: as temperature increases the relaxation time τ(T ) slightly decreases up to the volume phase transition temperature above which it decreases to its lowest value, corresponding to the shrunken state indicating a fastening of the dynamics related to the reduced size of the particles and an increased diffusivity. Surprisingly as the weight concentration increases ( Fig.4(b)) relaxation time above the VPTT is strongly affected by PAAc content:…”

“…Finally in Ref. [46] the experimental radius of PNIPAM and IPN at fixed PAAc concentration has been compared with the one expected from the Flory-Rehner theory.…”

“…At variance with previous studies [38][39][40][41][43][44][45][46][47][48] the present research aims to understand the role played by poly(acrylic acid) on the relaxation dynamics and on the aggregation process across the VPT. Despite the work done up to now on IPN microgels, to the best of our knowledge a similar study has never been reported before.…”

Molecular mechanisms driving the microgels behaviour: A Raman spectroscopy and dynamic light scattering study

“…The relaxation time and the stretching parameter for both PNIPAM and IPN microgels are reported in Fig.2 as a function of temperature at four weight concentrations (C w =0.1 %, C w =0.3 %, C w =0.5 % and C w =0.8 %) as obtained by fitting the g 2 (Q,t) with Eq.(1). In the case of pure PNIPAM (Fig.2(a)) the well known dynamical transition associated to the VPT is evidenced [25,44]: as temperature increases, the relaxation time τ slightly decreases up to the volume phase transition temperature (VPTT), above which it decreases to its lowest value, corresponding to the shrunken state and indicating a fastening of the dynamics related to the reduced size of particles.…”