PPG/Water matrix dynamics was studied using Differential Scanning Calorimetry, DSC, and photophysical techniques.  Sample mixing was a slow process, taking about two weeks of subsequent shaking, stirring and sonicating to fully homogenize the system.  Supercooling of 60% PPG and 40% water was studied using DSC at cooling/heating rates ranging from 0.75 to 6.5 Kelvin/minute.  No regular crystallization was observed at cooling rates above 1.5 K/min; the enthalpies of cold crystallization and melting were equal when no regular crystallization took place.  The enthalpies of cold crystallization were found to be dependent on heating rate.  The enthalpy of melting increased and leveled off at 28 J/g as the heating rate was lowered.  The Kissinger plot for cold crystallization was found to be non-linear. The best fit obtained is: ln(�/Tc2) = 300 - 1.3*105 *(1/Tc) + 1.5*107 *(1/Tc)^2. A luminescent probe, ruthenium tris-bipyridine, was used to comment on the matrix dynamics.  The rate constant for unimolecular inherent decay versus temperature increased when the sample underwent cold crystallization indicating that the probe was more constrained in the glassy state than the crystalline state.  The experiment was conducted in a way that turbidity in the sample could be simultaneously monitored. Upon heating, the glassy (transparent) sample crystallized and became opaque and returned to a transparent liquid state.  The temperatures of these transitions were found to be in agreement with DSC data.  Related studies as a function of molar fraction of water are in progress.

DSC and photophysical study of cold crystallization of poly(propylene glycol)/water mixtures

247th National Meeting (2014)

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