You are missing something in your question - what is the thing diffusing, and what is it diffusing into?
Do you want the diffusivity of water vapor into HDPE, or air, or some other gas/fluid? If so (seems most likely) - WHAT fluid or gas?
Or, do you want the diffusivity of HDPE into some solvent?
Or do you mean the heat diffusivity? Realize that the heat diffusivity, alpha = k/rho*Cp, most people cite the term k (thermal resistance) of materials, not its reciprocal, alpha. You should be able to find data for k, rho, and Cp in consistent units to allow you to calculate alpha.
OR do you mean magnetic diffusivity?