question about the constants in WLF function

[benutella]

Hello,

i have a question about WLF function. i want to calculate the two constants,C1 and C2, based on experiment data.
could the value of C2 be negative?
what does it imply when C2 is negative?

thanks a lot!
benutella

[Jorgen]

Typically, the shift factor is written:

log (aT) = C1 * (T-T0) / (C2 + T - T0)

If you use this definition then the constant C2 cannot be negative :!:

If you had a negative C2 value then the shift log(aT) would be negative for T slightly larger than T0, and positive when T is slightly smaller than T0 -- but this is backwards to what it should be.

[benutella]

Hello,Jorgen,

Thanks for your reply!

In some references, the WLF function is written as
loga=C1*(T-Tref)/(C2+T-Tref)
but in other books, WLF function has a form of
loga=-C1*(T-Tref)/(C2+T-Tref)
where there is a negative sign in front of the equation

which one is correct?or both of them are right?

may i calculate the shift factor by relaxation time?

Best regards!

[Jorgen]

Eithor of the two definitions of the WLF function is correct -- it all depends on how you define the direction of the shift. Just make sure you are consistent.

Yes, you should be able to determine the shift factor from relaxation times are different temperatures.