Effects of resonant phonon scattering from internal molecular modes on the thermal conductivity of molecular glasses

The thermal conductivity $\kappa \left(T\right)$ of the crystalline and glassy phases of the two isomers of propyl alcohol has been measured. The two isomers differ by a minor chemical detail involving the position of the hydroxyl group with respect to the carbon backbone. Such a difference in molecular structure leads, however, to disparate behaviors for the temperature dependence of $\kappa \left(T\right)$, for both glass and crystal states. The $\kappa \left(T\right)$ for the glass shows for 1-propanol an anomalously large plateau region comprising temperatures within $6–90\mathrm{K}$, while data for isomeric 2-propanol show only a small plateau up to $10\mathrm{K}$ which is comparable to data on lower alcohols. The results emphasize the role played by internal molecular degrees of freedom as sources of strong resonant phonon scattering.

Figure 1

(Color online) The thermal conductivity of both isomeric alcohols as compared to that measured for glassy ethanol (Ref. 2). Experimental data are depicted by symbols: open triangles for glassy ethanol, open squares for 2-propanol, and filled cyan circles for glassy 1-propanol. Filled purple circles depict data for the orientationally ordered crystals of 1-propanol and filled green squares show data for crystalline 2-propanol. Solid black lines are fitted curves using the soft potential model and yield parameter values shown in Table . The solid red line corresponds to the calculation including the resonant-scattering term given by Eq. (9).

Figure 2

Estimated phonon mean free paths for the three glasses here considered.

Figure 3

(Color online) A comparison of the temperature dependence of the conductivites for both glasses. Circles depict data for 1-Pr and open squares data for 2-Pr. Lines drawn through data represent power laws which yield an approximate description of the temperature dependence of $\kappa \left(T\right)$ over restricted ranges of temperatures. The solid and dotted lines display the predictions grounded on the minimal thermal conductivity model (see text), which are calculated assuming that the only relevant dependence with temperature comes through the specific heat term.

Figure 4

(Color online) A comparison of the spectral frequency distributions for both glasses as derived from inelastic neutron scattering experiments reported in Ref. 13. Data for 1-Pr are depicted by open circles with a dot. Data for 2-Pr are shown by inverted triangles. The inset depicts the angle-averaged spectra $S\left(\omega \right)=Z\left(\omega \right)∕{\omega }^2$.