Melting T emperature D epression of n-Hexadecane C onfined in R ubber N etworks

1. Melting Temperature Depression of n-Hexadecane Confined in Rubber Networks Qian Qin and Gregory B. McKenna Department of Chemical Engineering, Texas Tech University, Lubbock, TX Introduction Results and Discussion Results and Discussion It is well accepted that small crystals of a pure material exhibit a downward shift in melting point as the size scale is reduced. This fact has been demonstrated in many systems such as polymer lamellae, metal particles with nanometer scale size, and small molecules confined in controlled pore glass. The melting point depression is often attributed to the finite size effect, which can be described theoretically by the Gibbs-Thomson equation. . Accordingly, it is expected that small molecules in soft confinement formed by a rubber network will melt at a reduced temperature from the bulk. . The Flory - Huggins theory Calibration of “pore” size of network Fig. 4. Melting point depression of n-hexadecane in Controlled Pore Glasses with different mean pore size. . The surface energy σsl = 14.3 erg/cm2 (estimated by Gibbs-Thomson equation) Tm and Tm0– melting temperatures of solvent crystals in the polymer mixture and in the bulk state respectively; ΔHf – bulk fusion enthalpy; v2 – volumefraction of polymer; χ– polymer-solvent interaction parameter. Table 1. The mean “pore” size of equilibrium swollen rubber network estimated by CPG calibration Objectives Fig. 1. Melting point depression of n-hexadecane in polyisoprene solution vs. the volume fraction of polyisoprene (v2).. • Measure experimentally the melting point depression of n-hexadecane in polyisoprene solutions and compare with the prediction from the Flory-Huggins theory. . • Investigate the finite size effect on the melting behavior of confined small molecules. . • Demonstrate that the melting point depression of organic solvents can be used to estimate the “pore” size and “pore” size distribution of a rubber network. . Fig. 5. Pore size distribution of phr10 obtained by DSC step scanning measurement and pore size calibration from Controlled Pore Glasses. . In concentrated solutions (larger polyisoprene volume fraction),n-hexadecane exhibits larger depression from the bulk melting temperature. However, this system doesn’t follow the Flory-Huggins theory. . Finite size or crosslink density effect Conclusions • Melting temperature of n-hexadecane in polyisoprene solution shows a deviation from the Flory – Huggins theory. • Nano-scale soft confinement from a rubber network has an effect on the melting behavior of n-hexadecane. The melting temperature exhibits a larger depression with greater crosslink density due to the smaller “pore” size of the network. . • Melting point depression of small molecules confined in CPG can be used to “calibrate” the network heterogeneity measurements. Experimental Materials • Polyisoprene (Mw = 305,000g/mol, PDI = 1.05) crosslinked by dicumyl peroxide. • Crosslink density is represented by parts of • dicumyl peroxide per hundred parts of • polyisoprene by weight (phr). . • n-hexadecane (GC grade, purity≥ 99.8%) • Controlled Pore Glass (CPG) of narrow distributed nanoscale pore size. . • CPG was pretreated as described in ref [1]. References Fig. 2. Melting point depression of confined n-hexadecane in rubber network with different crosslink densities. . [1] C. L. Jackson and G. B. McKenna, J. Chem. Phys. 93, 9002 (1990) [2] C. L. Jackson and G. B. McKenna, Rubber Chem. Technol. 64, 760 (1991) Method In a polyisoprene network, it is not only the polymer volume fraction, but also the crosslink density which has an impact on the melting temperature of constrained n-hexadecane. The effect of crosslink density tends to decrease with an increasing volume fraction of the rubber network (lower swelling). . • Melting temperatures of confined n-hexadecane were measured with a Perkin-Elmer Pyris 1 DSC at a heating rate of 5K/min after temperature and enthalpy calibration. . • The onset melting temperatures were used. Acknowledgements This work was supported by the National Science Foundation under the grant number of NSF02-I48.