Discover how positron annihilation spectroscopy reveals the nanoscopic free volume in Nylon 12/PVA films treated with supercritical carbon dioxide
Explore the ResearchImagine if every solid material around us was actually filled with invisible holes too small to see, yet large enough to determine how these materials behave.
This isn't science fiction; it's the fascinating reality of free volume in polymers, the empty spaces between molecular chains that determine everything from how gases flow through membranes to how strong our materials are 1 . Until recently, these nanoscopic spaces were largely theoretical, beyond the reach of direct measurement and analysis.
Today, scientists are using an extraordinary technique that employs antimatter—specifically positrons, the antimatter counterparts of electrons—to map these hidden landscapes. One of the most exciting applications of this technology involves studying how supercritical carbon dioxide (scCO₂) treatment can alter the properties of polymer films like Nylon 12 and poly(vinyl alcohol) (PVA) blends 4 .
Free volume holes in polymers are typically just 2-6 Å in size—about 10,000 times smaller than the width of a human hair!
The empty space between polymer chains that determines material properties like permeability and strength 1 .
A radioactive source emits positrons into the material being studied.
Some positrons form positronium atoms that become trapped in free volume holes.
Positronium annihilates with electrons, emitting gamma rays.
Detectors measure the time between emission and annihilation.
The research team followed these meticulous steps 4 :
| Blend Composition (Nylon 12/PVA) | Before Treatment Radius (Å) | After Treatment Radius (Å) | Percentage Change |
|---|---|---|---|
| 100/0 | 2.62 | 2.87 | +9.5% |
| 75/25 | 2.68 | 2.98 | +11.2% |
| 50/50 | 2.76 | 3.12 | +13.0% |
| 25/75 | 2.71 | 3.02 | +11.4% |
| 0/100 | 2.65 | 2.91 | +9.8% |
| Property | Before Treatment | After Treatment | Change |
|---|---|---|---|
| Oxygen Permeability (Barrer) | 12.5 | 18.7 | +49.6% |
| Tensile Strength (MPa) | 38.2 | 33.5 | -12.3% |
| Glass Transition Temperature (°C) | 45.3 | 41.7 | -7.9% |
| Material/Reagent | Function in Research | Notes |
|---|---|---|
| Nylon 12 | Polymer matrix component | Provides mechanical strength and thermal stability |
| Poly(vinyl alcohol) | Polymer matrix component | Enhances gas barrier properties; biodegradable |
| Supercritical CO₂ | Environmentally friendly processing medium | Plasticizes polymers without chemical residues |
| Sodium-22 source | Provides positrons for PALS measurements | Typically deposited on thin Kapton foil |
This research advances our fundamental understanding of structure-property relationships in polymers. By quantitatively linking processing conditions to nanoscopic structural changes and ultimately to macroscopic properties, this work provides a more complete picture of how materials behave 4 6 .
The non-linear relationship between blend composition and free volume changes suggests complex intermolecular interactions that are not yet fully understood. The observed time-dependent relaxation offers insights into polymer chain dynamics 4 .
Optimizing free volume creates membranes with precise selectivity and permeability 1 6 .
scCO₂ treatment adjusts barrier properties without chemical additives 4 .
scCO₂-based sterilization offers a low-temperature alternative .
Physical modification enhances environmental profile of biodegradable polymers 4 .
The combination of positron annihilation spectroscopy and supercritical carbon dioxide treatment represents a powerful toolkit for understanding and engineering the nanoscopic structure of polymers.
By using antimatter to probe voids too small to see with any other technique, and by employing environmentally friendly scCO₂ to modify these structures, scientists are developing unprecedented control over material properties.
Developing sophisticated models to predict free volume changes based on composition and processing
Exploring combinations of scCO₂ with other environmentally friendly techniques
Applying these approaches to bioplastics and smart responsive materials
As these techniques advance, we move closer to a future where materials can be precisely tailored for specific applications—membranes that separate gases with perfect selectivity, packaging that keeps food fresh for weeks, and medical devices that withstand sterilization without degradation.
The study of free volume in polymers reminds us that what matters isn't just the substance itself, but the spaces between—the void that gives form to function, and the emptiness that makes materials useful.