Anomalous melting behavior of cyclohexane and cyclooctane in poly(dimethyl siloxane) precursors and model networks

Abstract

Building on previous observations of anomalous melting behavior of solvents in polyisoprene, we have expanded our insight into the melting behavior of organic solvents in polymers and polymer networks through a calorimetric investigation of cylcohexane and cyclooctane in poly(dimethyl siloxane) (PDMS) precursors and model networks. The results are contrary to general expectations. Besides deviations between the predictions of the Flory-Huggins model and observed melting point depression of the small molecule organics, it is found that the melting point depression of cyclohexane in model networks is lower than that in the uncrosslinked precursors and unaffected by the molecular weight between crosslinks, which is not consistent with the general observation that higher crosslinking density leads to greater melting point depression. We interpret the observed phenomenon in terms of phase separation. In the case of cyclooctane, it exhibits a double melting peak in the model networks with high molecular weight between crosslinks. Furthermore, the heats of fusion of both cyclohexane and cyclooctane decrease with increasing polymer volume fraction which violates the underlying assumption that the heat of fusion of solvent in the polymer is the same as that in the bulk for both the Flory-Huggins model and the Gibbs-Thomson equation.

Melting of Linear Alkanes between Swollen Elastomers and Solid Substrates

“…The broad thermal peaks observed in the DSC cooling and heating scans are associated with the size distribution of the crystal (Gibbs–Thomson effect shown in eq ). The broad thermal peaks have been reported for melting of small molecules in networks ,, and glasses with well-defined porosity. , The distribution of crystal sizes determined using the combination of Gibbs–Thomson and Flory–Rehner equation is provided in the Supporting Information. The DSC data during the cooling cycle are a function of nucleation rates, and we have not used the transition temperatures measured in the cooling cycle for any quantitative comparisons.…”

“…This was done to allow for the PDMS lens to swell in the alkanes. The equilibrium concentration was determined by swelling the lens in a vial of alkane and measuring the weight of the lens before and after swelling . A heating and cooling rate of 0.5 K/min was used.…”

“…Using this assumption, eq and the experimentally measured distribution in T m have been used to determine the distribution of pore sizes or porosity. This problem was revisited by McKenna and co-workers , recently to study the melting of benzene in natural rubber and other small molecules in PDMS and polyisoprene networks. , McKenna and co-workers concluded that these models were unable to predict the depression in T m , and the heat of fusion was used as a fitting function to model the depression in T m …”

“…This problem was revisited by McKenna and co-workers 19,20 recently to study the melting of benzene in natural rubber and other small molecules in PDMS and polyisoprene networks. 21,22 McKenna and co-workers concluded that these models were unable to predict the depression in T m , and the heat of fusion was used as a fitting function to model the depression in T m . 23 For soft swollen elastomers in contact with solid substrates, the situation is more complex because of the influence of the solid surface on the segregation of small molecules at the interface.…”

Characterizing Mesh Size Distributions (MSDs) in Thermosetting Materials Using a High-Pressure System

“…It is often found, , in both networks and pores, that the value of the heat of fusion of the crystals decreases with decreasing pore size, which can be problematic because the crystal size is deduced from an equation that assumes a constant heat of fusion. The second limitation, as described in previous reports, , is related to the fact that an organic solvent in a polymer should have a depressed melting point according to the Flory−Huggins theory. Therefore, the Gibbs−Thomson equation needs at least one term to include the depression of the melting point due to mixing thermodynamics.…”