Melting of Linear Alkanes between Swollen Elastomers and Solid Substrates

Abstract

We have measured the melting and freezing behavior of linear alkanes confined between cross-linked poly(dimethylsiloxane) (PDMS) elastomers and solid sapphire substrates. Small molecules are often used as lubricants to reduce friction or as plasticizers, but very little is directly known about the migration or changes in physical properties of these small molecules at interfaces, particularly the changes in transition temperatures upon confinement. Our previous studies highlighted striking differences between the crystal structure of confined and unconfined pentadecane crystals in contact with sapphire substrates. Here, we have used surface-sensitive infrared-visible sum-frequency-generation spectroscopy (SFG) to study the melting temperatures (Tm) of alkanes in nanometer thick interfacial regions between swollen PDMS elastomers in contact with sapphire substrate. We find that confined alkanes show depression in Tm compared to the melting temperature of unconfined bulk alkanes. The depression in Tm is a function of chain length, and these differences were smallest for shorter alkanes and largest for 19 unit long alkanes. In comparison, the DSC results for swollen PDMS elastomer show a broad distribution of melting points corresponding to different sizes of crystals formed within the network. The Tm for confined alkanes has been modeled using the combination of Flory-Rehner and Gibbs-Thomson models, and the depression in Tm is related to the thickness of the confined alkanes. These findings have important implications in understanding friction and adhesion of soft elastomeric materials and also the effects of confinement between two solid materials.

Heterogeneous ice nucleation correlates with bulk-like interfacial water

“…As the annealing time increases, the size of the formed ice crystals of the bound water may become smaller due to the effects of confinement of the PVA network. The depression of T f,a and T m,a likely stems from the size reduction of ice crystals ( 42 – 44 ). In contrast, T f,b increases from −26.8° to −19.8°C and then remains constant as the annealing time exceeds 60 min (see Fig.…”

Morphology and Thermal Properties of Precision Polymers: The Crystallization of Butyl Branched Polyethylene and Polyphosphoesters

“…As calculated from the electron diffraction pattern shown in the inset of Figure 4a Additionally, n-alkanes can serve as a simple model system in order to allow us to come to conclusions on the fold length. The melting point of PE21-butyl (Figure 2) lies between that of n-pentadecane (10 °C) 37 and n-hexadecane (18 °C). 38 Accordingly, the above estimation of a fold length of 16 CH 2 units for solution crystallized lamellae is in unexpectedly good agreement with the experimental data.…”

The Effect of a Solid Surface on the Segregation and Melting of Salt Hydrates

“…The reflectivity of the helium–neon (He–Ne) beam (632.8 nm wavelength) was measured to determine the phase transition temperatures of the bulk salt solutions . These measurements were performed in a total internal reflection geometry using the SFG sample cell.…”

“…The reflectivity of the helium−neon (He−Ne) beam (632.8 nm wavelength) was measured to determine the phase transition temperatures of the bulk salt solutions. 34 These measurements were performed in a total internal reflection geometry using the SFG sample cell. Similar heating rates and equilibration time used in the SFG experiments are also used for reflectivity experiments.…”