# Melting Points as Function of P

1. Apr 16, 2013

### robousy

Hey folks,

I'm trying to obtain the melting points of several materials in a vacuum.

It's pretty straightforward to look up the melting point in 1 atm, but I'm having difficulty doing this in a vacuum.

One material in particular I'm looking at is Teflon.

If anyone can provide any resources, formula or hints on how to do this I'd be grateful.

Thanks!

2. Apr 17, 2013

### DrDu

The question bears some problems. First, at least in an ideal vacuum, any material will vapourize sooner or later, so both solid and liquid states are thermodynamically unstable. Hence melting becomes a non-equilibrium phase transition. However, from a practical point of view, I think that the changes in melting point with pressure are negligible between 0 and 1 atmosphere. The slope of the melting curve $d\ln T/dP =\Delta V/\Delta H$, where $\Delta V$ is the volume change in melting and $\Delta H$ the heat of melting. The volume change being very small leads to the slope also being very small.

Secondly, teflon is a polymer and polymers don't have a sharp melting point. Rather ( I don't remember the details) one defines the melting point as the temperature where the viscosity becomes lower than a predefined (and quite arbitrary) level.