Melting Temperature Depression of n-Hexadecane

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 ssl 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 ?Hf bulk fusion enthalpy
v2 volume fraction of polymer ?
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 doesnt 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.