The defect diffusion model, scaling, EV*/H* and monomer volume and correlation lengths for glass-formers

Four topics are treated within the framework of the defect diffusion model (DDM). First, it is shown how the relationship between EV*/H* (ratio of the apparent isochoric activation energy to the isobaric activation enthalpy) and monomer volume for polymers that has been pointed out by Floudas and co-workers [G. Floudas, K. Mpoukouvalas and P. Papadopoulos, J. Chem. Phys. 124 (2006) 074905] is predicted. Next, it is shown that in the DDM, scaling arises because the critical temperature can be represented approximately by a power law. Consequently, in the DDM scaling is always approximate and significant departures from scaling, as is observed in the case of hydrogen bonded materials for example, are matters of degree. It is also shown how the connection of scaling with EV*/H* is a natural consequence of the DDM. Finally, DDM calculations of the defect correlation length are carried out and compared with experimental dynamical correlation lengths measured using the 4D3CP solid state NMR method.