Standard Uniaxial Testing Deficiencies in Amorphous Polymer Characterization
Uniaxial tensile tests misrepresent multi-axial yield, physical aging, and strain-rate sensitivity in amorphous polymers, causing structural failure in real parts.
Free Volume Relaxation
Meaning
Post-solidification structural recovery describes the gradual densification of glassy amorphous polymers as hyper-equilibrium free volume escapes the polymer matrix over time. Experiencing free volume relaxation causes molded plastic parts to undergo subtle physical aging, embrittlement, and dimensional changes well below their glass transition temperature. The process governs long-term dimensional stability in precision optical components and tight-tolerance molded housings, but ceases once the polymer reaches thermodynamic equilibrium.
Physical Aging
Rapid cooling during injection molding traps excess unoccupied space between polymer chains. Glassy polymers trapped in non-equilibrium states continuously contract as chain segments slowly settle into tighter packings. Density increases while molecular mobility declines, causing an increase in yield stress and a reduction in impact strength.
Storage conditions alter aging velocity, as elevated ambient temperatures accelerate local chain motions.
Volumetric Shrinkage
Part dimensions contract beyond initial mold shrinkage allowances over months of storage. Amorphous resins like polycarbonate and polymethyl methacrylate show micro-scale dimensional drift as internal voids shrink. Annealing molded parts above their working temperature speeds up free volume loss, stabilizing part geometry before final machining or assembly.
High regrind levels increase free volume decay rates by introducing low molecular weight fragments that facilitate chain packing.
Property Evolution
Aging increases polymer yield strength while reducing impact toughness.