Dynamics in thermo-responsive nanogel crystals undergoing melting.
R G Joshi1, B V R Tata, J Brijitta
1Condensed Matter Physics Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India.
The Journal of Chemical Physics
|October 5, 2013
Summary
This study investigates the melting and freezing dynamics of poly(N-isopropylacrylamide) (PNIPAM) nanogel crystals. Researchers found deviations from established criteria, particularly in denser suspensions, due to polymer chain overlap.
Area of Science:
- Soft Matter Physics
- Colloidal Science
- Polymer Physics
Background:
- Thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) nanogels form colloidal crystals.
- Melting and freezing transitions in colloidal systems are characterized by diffusion coefficients.
- A dynamical criterion (D(L)/D(S) ≈ 0.1) exists for melting/freezing transitions in colloidal suspensions.
Purpose of the Study:
- To investigate the melting/freezing dynamics of PNIPAM nanogel colloidal crystals.
- To verify the applicability of the Löwen et al. dynamical criterion for these systems.
- To explore the influence of volume fraction on melting/freezing dynamics.
Main Methods:
- Utilized static and dynamic light scattering techniques.
- Measured long-time (D(L)) and short-time (D(S)) diffusion coefficients across the melting transition.
- Studied PNIPAM nanogel crystals at volume fractions φ = 0.49 and 0.79.
Main Results:
- Observed a ratio D(L)/D(S) < 0.1 in dense PNIPAM nanogel crystals during melting, deviating from the established criterion.
- The deviation from the criterion increased with higher volume fraction (φ).
- Subdiffusive behavior was observed in the mean square displacement near melting, attributed to polymer chain overlap.
Conclusions:
- The dynamical criterion for melting/freezing needs refinement for dense polymer nanogel systems.
- Polymer chain overlap significantly influences the dynamics near the phase transition.
- The findings provide new insights into the phase behavior of thermo-responsive colloidal systems.
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