Rounding effects on doped sulfur’s living polymerization: The case of As and Se

Abstract

The role of ``impurities'' or doping on the living polymerization of sulfur has been put under focus by means of a temperature dependent Raman study of the ${\mathrm{As}}_{x}{\mathrm{S}}_{100\ensuremath{-}x}$ and ${\mathrm{Se}}_{y}{\mathrm{S}}_{100\ensuremath{-}y}$ ($x=0.5,2,5$ and $y=2,5$) systems. Reduced isotropic Raman spectra have been analyzed properly to yield the temperature dependence of the extent of polymerization, $\ensuremath{\phi}(T)$, in each case. The general trend is that the presence of either As or Se facilitates the $\text{monomer}\ensuremath{\rightarrow}\text{polymer}$ transition of sulfur giving rise to three major effects: (i) A reduction of the transition temperature ${T}_{\mathrm{tr}}$ from neat sulfur's ${T}_{\ensuremath{\lambda}}\ensuremath{\approx}159\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ to an extent which depends on the doping level and the type of the impurity, i.e., As or Se. (ii) The presence of As atoms engenders severe ``rounding'' on the sharpness of neat sulfur's $\ensuremath{\lambda}$-shaped transition as evidenced from both the less abrupt increase of $\ensuremath{\phi}(T)$ function and the wider and more symmetrical shape of the calculated heat capacity peak. In contrast, in Se-doped mixtures, the characteristics of a second order phase transition seem to be retained. (iii) The As-doped mixtures exhibit appreciable polymer content at temperatures below ${T}_{\mathrm{tr}}$, whereas in Se doping the low temperature behavior of $\ensuremath{\phi}(T)$ resembles that of neat S. Thermodynamic parameters concerning the propagation step of sulfur's polymerization reaction have been also calculated demonstrating the apparent reduction of the associated enthalpy with increasing the doping level of As and its approximate invariance with respect to Se doping level. Finally, an examination of the depolarization ratio of the scattered light for the mixtures studied in this paper demonstrated subtle but systematic changes, when crossing the transition temperature, originating from local structural changes associated with the polymerization process.