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Investigating the roles of hydrophobicity and electrostatics in the particle-scale dynamics and rheology of dense microgel suspensions

Sayantan Chanda, Chandeshwar Misra, Ranjini Bandyopadhyay

TL;DR

This work demonstrates that hydrophobic and electrostatic interactions critically shape the dynamics and rheology of dense PNIPAM microgel suspensions, with hydrophobicity dominating below the volume phase transition temperature ($VPTT$) and a cooperative effect of hydrophobicity and electrostatics governing behavior near and above $VPTT$. By introducing dissociating salts and non-dissociating sucrose, the authors reveal how inter-particle attractions and screening modify particle size, effective volume fraction, and network structure, as evidenced by DLS, FTIR, zeta potential, and cryo-FESEM, together with comprehensive rheology and SRFS measurements. The results show additive-induced hydrophobic shrinkage lowers rigidity below $VPTT$, while near and above $VPTT$ combined hydrophobic and electrostatic attractions promote stronger gels and complex yielding (including a two-step process), with microstructures corroborating the mechanical data. This tunability through simple, common additives offers pathways to novel PNIPAM-based metamaterials, self-assembly control, and temperature-responsive actuators or cargo-delivery platforms.

Abstract

Colloidal microgel particles such as poly(N-isopropylacrylamide) (PNIPAM) shrink reversibly in an aqueous medium due to the expulsion of water at a volume phase transition temperature, VPTT $\sim$33$^\circ$C. Romeo et al. [Adv. Mater. 2010, 22, 3441-3445] had previously shown that dense aqueous PNIPAM suspensions transformed from one viscoelastic solid-like phase to another when suspension temperature was increased, with an intermediate viscoelastic liquid-like phase near the VPTT. They attributed this observation to a change in the inter-particle interaction from hydrophilic to hydrophobic. Here, we show using a combination of experimental techniques that particle hydrophobicity can become significant even below the VPTT. We achieve this by incorporating dissociating additives such as sodium chloride and potassium chloride, or non-dissociating additives such as sucrose, into the aqueous medium. Above the VPTT, we observe that suspension rigidity is the highest in the presence of salts because of the combined effects of electrostatic and hydrophobic attractions. In the presence of non-dissociating sucrose, in contrast, the inter-microgel interaction remains hydrophobic across the VPTT. Such easy tunability of interactions by incorporating commonly available chemicals into the suspension medium opens up new avenues for the synthesis of novel metamaterials.

Investigating the roles of hydrophobicity and electrostatics in the particle-scale dynamics and rheology of dense microgel suspensions

TL;DR

This work demonstrates that hydrophobic and electrostatic interactions critically shape the dynamics and rheology of dense PNIPAM microgel suspensions, with hydrophobicity dominating below the volume phase transition temperature () and a cooperative effect of hydrophobicity and electrostatics governing behavior near and above . By introducing dissociating salts and non-dissociating sucrose, the authors reveal how inter-particle attractions and screening modify particle size, effective volume fraction, and network structure, as evidenced by DLS, FTIR, zeta potential, and cryo-FESEM, together with comprehensive rheology and SRFS measurements. The results show additive-induced hydrophobic shrinkage lowers rigidity below , while near and above combined hydrophobic and electrostatic attractions promote stronger gels and complex yielding (including a two-step process), with microstructures corroborating the mechanical data. This tunability through simple, common additives offers pathways to novel PNIPAM-based metamaterials, self-assembly control, and temperature-responsive actuators or cargo-delivery platforms.

Abstract

Colloidal microgel particles such as poly(N-isopropylacrylamide) (PNIPAM) shrink reversibly in an aqueous medium due to the expulsion of water at a volume phase transition temperature, VPTT 33C. Romeo et al. [Adv. Mater. 2010, 22, 3441-3445] had previously shown that dense aqueous PNIPAM suspensions transformed from one viscoelastic solid-like phase to another when suspension temperature was increased, with an intermediate viscoelastic liquid-like phase near the VPTT. They attributed this observation to a change in the inter-particle interaction from hydrophilic to hydrophobic. Here, we show using a combination of experimental techniques that particle hydrophobicity can become significant even below the VPTT. We achieve this by incorporating dissociating additives such as sodium chloride and potassium chloride, or non-dissociating additives such as sucrose, into the aqueous medium. Above the VPTT, we observe that suspension rigidity is the highest in the presence of salts because of the combined effects of electrostatic and hydrophobic attractions. In the presence of non-dissociating sucrose, in contrast, the inter-microgel interaction remains hydrophobic across the VPTT. Such easy tunability of interactions by incorporating commonly available chemicals into the suspension medium opens up new avenues for the synthesis of novel metamaterials.

Paper Structure

This paper contains 17 sections, 6 figures.

Table of Contents

  1. Introduction:
  2. Materials and methods
  3. Synthesis and sample preparation:
  4. Differential scanning calorimetry (DSC):
  5. Fourier transform infrared (FTIR) spectroscopy:
  6. Zeta potential:
  7. Dynamic light scattering (DLS):
  8. Rheology:
  9. Cryogenic field emission scanning electron microscopy (cryo-FESEM):
  10. Results and Discussion
  11. Modifying hydrophobicity and electrostatic interactions in aqueous PNIPAM suspensions by incorporating additives:
  12. Influence of additives on the temperature-dependent viscoelastic properties of aqueous PNIPAM suspensions:
  13. Influence of additives on particle dynamics and suspension rheology below the VPTT:
  14. Influence of additives on particle dynamics and suspension rheology near the VPTT:
  15. Influence of additives on particle dynamics and suspension rheology above the VPTT:
  16. ...and 2 more sections

Figures (6)

  • Figure 1: (a) The relative strength of the amide-amide bonding, $f_A$, used here to parametrize particle hydrophobicity, is plotted as a function of additive concentration for NaCl (red), KCl (green) and sucrose (blue) both below (18$^{\circ}$C, bold symbols) and above (45$^{\circ}$C, hollow symbols) the volume phase transition temperature (VPTT). (b) Temperature-dependent zeta potential of 1% w/v PNIPAM particles suspended in water and in different additive solutions.
  • Figure 2: Temperature-dependent (a) mean hydrodynamic diameters, $<d_{H}>$, and (b) storage moduli, G$^{\prime}$, of PNIPAM particles suspended in water and in various additive solutions.
  • Figure 3: (a) Frequency-dependent elastic moduli G$^{\prime}$ (solid symbols) and viscous moduli G$^{\prime \prime}$ (hollow symbols), measured at 18$^{\circ}$C (below the VPTT), of dense suspensions of PNIPAM particles prepared in different additive solutions. (b) G$^{\prime}$ (solid symbols) and G$^{\prime \prime}$ (hollow symbols) of PNIPAM particles suspended in pure water as a function of $\omega$ for five different $\dot{\gamma_{0}}$ at 18$^{\circ}$C. (c) Data shown in (b) is scaled and collapsed on a single master curve. The inset shows the scaling parameters. (d) Structural relaxation timescales of dense suspensions of PNIPAM particles as a function of additive concentration at 18$^{\circ}$C. (e) Strain amplitude dependent elastic moduli, G$^{\prime}$, (solid symbols) and viscous moduli, $G^{\prime\prime}$, (hollow symbols) of dense suspensions of PNIPAM particles at 18$^{\circ}$C. (f) Yield strains of dense suspensions of PNIPAM particles as a function of additive concentration estimated from (e).
  • Figure 4: Frequency-dependent elastic moduli G$^{\prime}$ (solid symbols) and viscous moduli G$^{\prime \prime}$ (hollow symbols), of dense suspensions of PNIPAM particles, prepared in pure water and in additive solutions, at (a) 27$^{\circ}$C and (b) 34$^{\circ}$C. Strain amplitude-dependent elastic moduli G$^{\prime}$ (solid symbols) and viscous moduli G$^{\prime \prime}$ (hollow symbols) of the same suspensions at (c) 27$^{\circ}$C and (d) 34$^{\circ}$C.
  • Figure 5: (a) Frequency-dependent elastic moduli, G$^{\prime}$ (solid symbols) and viscous moduli, G$^{\prime \prime}$ (hollow symbols), of dense suspensions of PNIPAM particles prepared with and without additives at 45$^{\circ}$C (above the VPTT). (b) Crossover timescales of these suspensions as a function of additive concentration. (c) Strain amplitude-dependent elastic moduli, G$^{\prime}$ (solid symbols), and viscous moduli, G$^{\prime \prime}$ (hollow symbols), of the same samples at 45$^{\circ}$C. The inset shows a zoomed-in view of the two-step yielding of a PNIPAM suspension containing 0.6M NaCl. (d) Yield strains, $\gamma_{y1}$ and $\gamma_{y2}$, of the samples at 45$^{\circ}$C as a function of additive concentration.
  • ...and 1 more figures