Consequence of Excess Configurational Entropy on Fragility: The Case of a Polymer-Oligomer Blend

By taking advantage of the molecular weight dependence of the glass transition of polymers and their ability to form perfectly miscible blends, we propose a way to modify the fragility of a system, from fragile to strong, keeping the same glass properties, i.e., vibrational density of states, mean-square displacement, and local structure. Both slow and fast dynamics are investigated by calorimetry and neutron scattering in an athermal polystyrene-oligomer blend, and compared to those of a pure 17-mer polystyrene considered to be a reference, of the same $T_g$. Whereas the blend and the pure 17-mer have the same heat capacity in the glass and in the liquid, their fragilities differ strongly. Thus, the difference in fragility is related to an extra configurational entropy created by the mixing process and acting at a scale much larger than the interchain distance, without affecting the fast dynamics and the structure of the glass.

Figure 1

Absolute heat capacity versus temperature. Monodisperse PSh1790 (stars) and bidisperse blend PSh92k/h750 (crosses) of same $T_g$. First inset: Apparent $C_p$ versus $T$ measured by DSC on heating at $10\mathrm{K}·{\min }^{-1}$ after cooling at $10\mathrm{K}·{\min }^{-1}$. Second inset: $T_f^{\prime }$ versus logarithm of cooling rate. [View in color for better clarity].

Figure 2

Structure factors obtained from the coherent signal of the deuterated samples at 100 K. The pure components of the blend are also shown to illustrate the $M_n$ dependence of the structure.

Figure 3

MSD at the ns time scale of the pure components and the blend. Inset: rescaled data, $\frac{⟨u^2⟩}{⟨u^2{⟩}_g}$ versus $T/T_g$.

Figure 4

Dynamic structure factor of the blend and the 17-mer PS at 100 K. Inset: low temperature $C_p$ values for both systems measured by calorimetry.