Thu, 09 October, 2025
A new research paper has been published in the Elsevier journal 'Polymer,' detailing an investigation into the free volume of polymers, an important concept in the use of these materials.
The paper, ‘Fractional Free Volume of Polymers Above and Below the Glass Transition Temperature,’ was co-written by TWI Technology Fellow, Dr Bernadette Craster and TWI Consultant, Dr Tim Jones.
The free volume of polymers is a central concept for the physical science, ageing, and mechanical properties of polymers. It also impacts polymer coatings and the transport properties of polymers. Because the permeability and viscosity of polymers is dependent on free volume, large changes in transport properties can be achieved with small changes in free volume.
However, despite the importance of polymer free volume, it has been shown to be a complex concept with no universally-agreed definition and no current means of direct measurement. As a result of this, there is a significant range in the reported values of the relative free volume of polymers – known as the fractional free volume (FFV).
There are different approaches to determine the value of FFV, including the polymer physics-based Williams-Landel-Ferry (WLF) theory, which is typically applied to amorphous polymers above their glass transition temperatures, and the Bondi-Lee approach, which is commonly applied to polymers below their glass transition temperatures.
Since these different approaches create different FFV values, this newly-published research paper proposes an approximate expression for FFV, derived from WLF theory, as well as examining the conditions under which the values of FFV calculated by the Bondi-Lee method are equal to those derived from WLF theory.
You can see the paper, in full, here:
https://www.sciencedirect.com/science/article/abs/pii/S0032386125011103