Research ArticlePHYSICS

Evidence of a low-temperature dynamical transition in concentrated microgels

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Science Advances  28 Sep 2018:
Vol. 4, no. 9, eaat5895
DOI: 10.1126/sciadv.aat5895
  • Fig. 1 EINS data.

    (A) Incoherent elastic intensities I(Q,0) measured on PNIPAM microgels in D2O with mass fraction concentrations of 43 and 60% as a function of temperature T. Top (bottom) panels report data recorded under cooling (heating). (B) Integral over Q of I(Q,0) as a function of T. The integrated EINS intensities are normalized to 1 for T → 0. (C) Temperature evolution of MSD as obtained using the double-well model (see the Supplementary Materials). In (B) and (C), measurements under cooling are presented with filled symbols, whereas those under heating are presented with open symbols.

  • Fig. 2 MD simulation results for 40% mass fraction.

    (A) Snapshot of the simulated PNIPAM microgel. Chain atoms are displayed in green to highlight the network structure. (B) SISF of PNIPAM hydrogen atoms calculated at Q = 2.25 Å−1 as a function of temperature. (C) Time evolution of the MSDs of water molecules. The inset shows the water MSD values at 150 ps as a function of T.

  • Fig. 3 Water-PNIPAM coupling.

    (A) Temperature dependence of water diffusion coefficient (squares); SISF relaxation times for PNIPAM hydrogen atoms (circles) and water oxygen atoms (diamonds); RMSF of backbone hydrogen atoms (triangles) calculated from MD simulations of PNIPAM 40 wt %. Dashed lines are guides to the eye. Note that data sets are represented on different scales on the y axis to improve visualization. (B) Temperature evolution of RMSF of the PNIPAM hydrogen atoms for 40 wt % (light blue) and for 60 wt % (dark green) averaged over 150 ps: methyl groups (circles) and backbone atoms (triangles).

  • Fig. 4 Quantitative comparison between experiments and simulations.

    (A) Neutron spectra as a function of temperature, as calculated from the MD simulations of the PNIPAM microgel at 40%. The total intermediate scattering functions are displayed at a value of 150 ps for comparison with the experimental data. (B) EINS spectra measured on the sample with a PNIPAM concentration of 43% as a function of temperature. Data shown at 252 K compare the spectra measured under cooling (dark green) and heating (green). (C) Temperature dependence of experimental (open symbols) MSD for PNIPAM mass fractions of 43% (circles), 60% (triangles), and 95% (squares) and numerical (filled symbols) MSD, calculated at 150 ps for PNIPAM hydrogen atoms only, for PNIPAM mass fractions of 40% (circles) and 60% (squares). The dotted line is a guide to the eye, suggesting a linear behavior for the dry sample, for which the dynamical transition is suppressed.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/9/eaat5895/DC1

    Section S1. EINS measurement and data analysis

    Section S2. PNIPAM model development

    Section S3. MD simulations procedure

    Section S4. Reproducibility of numerical results

    Section S5. Calculation of I(Q) from numerical simulations

    Fig. S1. EINS data for PNIPAM 43 wt % sample.

    Fig. S2. EINS data for PNIPAM 60 wt % sample.

    Fig. S3. D2O contribution to EINS spectra.

    Fig. S4. Schematic representation of the microgel network model.

    Fig. S5. Aging effect on water dynamics.

    Fig. S6. Aging effect on PNIPAM dynamics.

    Fig. S7. I(Q,0) from numerical simulations.

    Table S1. Thermal protocol followed during the EINS measurements.

    References (3845)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. EINS measurement and data analysis
    • Section S2. PNIPAM model development
    • Section S3. MD simulations procedure
    • Section S4. Reproducibility of numerical results
    • Section S5. Calculation of I(Q) from numerical simulations
    • Fig. S1. EINS data for PNIPAM 43 wt % sample.
    • Fig. S2. EINS data for PNIPAM 60 wt % sample.
    • Fig. S3. D2O contribution to EINS spectra.
    • Fig. S4. Schematic representation of the microgel network model.
    • Fig. S5. Aging effect on water dynamics.
    • Fig. S6. Aging effect on PNIPAM dynamics.
    • Fig. S7. I(Q,0) from numerical simulations.
    • Table S1. Thermal protocol followed during the EINS measurements.
    • References (3845)

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