DiBenedetto Glass Transition Modeling in Diffusion Controlled Resins
DiBenedetto modeling governs vitrification prediction, preventing unreacted resin pockets and internal residual stress during structural composite cure cycles.
Free Volume Suppression
Meaning
Molecular packaging density in glass-forming polymers increases as the empty space between chains is reduced during densification. Free volume suppression describes this reduction in the unoccupied intermolecular spaces, which restricts chain mobility and raises the mechanical modulus. The phenomenon occurs during physical ageing or slow cooling through the glass transition region.
Tracking this behavior is essential for predicting changes in the barrier properties and stiffness of polymers.
Moisture Permeability
Barrier properties against water and gases depend directly on the available intermolecular spaces in the polymer matrix. As free volume suppression occurs, the diffusion path for small molecules becomes more tortuous, lowering the permeability of the material. This change is valuable for aerospace composites that must resist moisture absorption in humid environments.
A lower diffusion rate prevents internal moisture from causing blistering during high-temperature repair cycles.
Component Stability
Prototype testing can overestimate the long-term impact resistance of a polymer if the material is tested immediately after moulding. Over time, free volume suppression increases the yield strength but reduces the toughness of the polymer, making it more brittle. A composite part that passes a high-rate impact test when fresh might fail when subjected to the same load after months of storage.
This difference requires engineers to age test coupons before performing qualification runs.
Volume Measurement
Dilatometry monitors the change in specific volume of the polymer as a function of temperature and time. This technique provides the raw data needed to quantify the progress of free volume suppression. Measuring these changes helps manufacturers adjust their post-cure schedules to achieve a stable material state before shipping.