How Positron Annihilation Reveals Hidden Spaces in Materials
Imagine trying to understand why one plastic bottle allows carbonated drinks to go flat faster than another, or why some advanced membranes can miraculously turn seawater into drinking water. The secrets to these phenomena don't lie in what we can see, but in what we cannot—the invisible nanoworld of "free volume" within materials. These are the empty spaces between molecules, the molecular-scale gaps and holes that determine how gases and liquids move through solids.
For decades, this free volume was merely a theoretical concept—scientists knew it must exist but lacked tools to measure it directly. That changed with the development of an extraordinary technique called Positron Annihilation Lifetime Spectroscopy (PALS), which uses antimatter to map these invisible landscapes.
When combined with innovative processing methods like supercritical CO₂ treatment, researchers can literally engineer these nanospaces to create advanced materials with remarkable properties. This article explores how scientists are using these cutting-edge technologies to probe and manipulate the hidden architecture of materials, opening new frontiers in everything from water purification to pharmaceutical development.
Empty spaces between polymer chains that allow for movement and transport
Using antimatter to probe nanoscale voids in materials
A molecular sculptor that modifies polymer structure
In the molecular structure of any material, particularly polymers, not every space is occupied by atoms. The free volume refers to the empty spaces between polymer chains that allow for movement and transport. As one research group explains, "Free volume = total volume - occupied volume" 1 . Think of it as the difference between a completely packed suitcase and one with gaps between items—those gaps allow you to fit more in or rearrange contents easily.
These nanoscale voids typically range from 0.1 to several nanometers in size 7 and exist only for incredibly short timescales (from microseconds down to femtoseconds) 1 . Despite their transient nature, they play a crucial role in determining key material properties:
Positron Annihilation Lifetime Spectroscopy (PALS) represents one of the most powerful techniques for studying these free volume holes. The method employs positrons—the antimatter counterpart of electrons—as molecular-scale probes.
Positrons are introduced into the material from a radioactive source
Positrons form bound states with electrons called positronium (Ps)
Ortho-positronium (o-Ps) gets trapped in nanoscale voids
o-Ps annihilates, emitting gamma rays that are detected and analyzed
When a positron enters a material, it can form an atom-like bound state with an electron called positronium (Ps). There are two types of positronium that play different roles in PALS 7 :
Where the positron and electron spins are antiparallel. Shorter lifetime, less useful for free volume studies.
Where the spins are parallel. Longer lifetime, preferentially trapped in free volume holes - ideal for measurement.
The o-Ps state is particularly valuable for studying free volume because it's preferentially trapped in the empty nanospaces within materials. Once trapped, the o-Ps eventually annihilates with another electron, emitting gamma rays that can be detected. The longer the o-Ps survives before annihilation, the larger the free volume hole in which it was trapped 7 .
Researchers have developed mathematical models, most notably the Tao-Eldrup model 7 , that establishes a quantitative relationship between the measured o-Ps lifetime and the actual size of the free volume holes. This transforms PALS from a mere observational technique into a precise measuring tool for the nanoscale world.
Supercritical carbon dioxide (scCO₂) is carbon dioxide held at specific temperature and pressure conditions where it displays unique properties between a gas and a liquid. This state gives it remarkable abilities to penetrate and modify polymers.
When polymers are exposed to scCO₂, the carbon dioxide molecules infiltrate the free volume spaces, effectively plasticizing the polymer chains and increasing their mobility 6 . As the chains gain freedom to move, they can rearrange themselves, often creating larger and more interconnected free volume elements. Even after the CO₂ is removed, these structural changes can remain, permanently altering the material's transport properties.
While specific PALS studies on Nylon 12/PVA films treated with scCO₂ aren't detailed in the available literature, extensive research has been conducted on similar systems that illustrates the principles and methodologies. One particularly revealing study examined poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes treated with scCO₂ for enhanced separation capabilities 6 .
Researchers prepared both unfilled PTMSP membranes and composite membranes containing hydrophobic silica particles through solution casting techniques.
The membranes were subjected to supercritical CO₂ under varying conditions of pressure (12-24 MPa) and temperature (40-150°C) to explore how different treatment parameters affect free volume.
The researchers used positron annihilation lifetime spectroscopy to measure changes in free volume characteristics before and after scCO₂ treatment. The o-Ps lifetime (τ₃ and τ₄) and intensity (I₃) were particularly monitored as indicators of free volume size and concentration.
The treated membranes were evaluated in pervaporation experiments separating dilute ethanol/water mixtures to correlate free volume changes with separation performance.
The findings demonstrated that scCO₂ treatment significantly increased the free volume cavity sizes in PTMSP membranes. The researchers observed a clear correlation between larger free volume elements and enhanced permeation rates in separation applications 6 .
| Treatment Condition | Free Volume Size Increase | Permeation Rate Change | Selectivity |
|---|---|---|---|
| 70°C, 24 MPa | Significant | +76% | Maintained |
| With silica fillers | Moderate | +72% | Slight decrease |
For unfilled PTMSP membranes treated at 70°C and 24 MPa, the specific permeation rate increased by 76%—an enhancement even greater than what could be achieved by adding silica fillers alone 6 . Crucially, unlike traditional modification methods that often trade selectivity for permeability, the scCO₂ treatment maintained or even improved the membrane's separation capabilities.
| Free Volume Cavity Size (nm) | Specific Permeation Rate (kg μm m⁻² h⁻¹) |
|---|---|
| 0.35 | ~4.5 |
| 0.39 | ~6.0 |
| 0.42 | ~7.5 |
This research demonstrated that scCO₂ treatment provides a powerful method for enhancing membrane performance without the typical trade-offs, offering potential applications in biofuel purification, water treatment, and gas separation processes.
| Tool/Material | Function | Example/Specification |
|---|---|---|
| Positron Source | Emits positrons for probing materials | Typically ²²Na radioisotope source 3 |
| Gamma Detectors | Detect annihilation radiation | NaI scintillation detectors 3 |
| Coincidence Circuitry | Identifies simultaneous gamma rays | Timing resolution of ~250 ps 3 |
| Supercritical CO₂ System | Processes materials at molecular level | Pressure: 12-24 MPa, Temperature: 40-150°C 6 |
| Polymer Membranes | Materials under investigation | PTMSP, Nylon 12, PVA, or other polymers |
| Data Analysis Software | Interprets positron lifetime data | LT ver.9.0 fitting program 5 |
The implications of understanding and controlling free volume extend far beyond basic research:
By tailoring free volume characteristics, engineers can create more efficient membranes for water desalination, gas separation, and biofuel purification 6 .
The same principles that govern transport through membranes also apply to transdermal drug delivery. Research has even used PALS to study free volume in human skin, revealing nanospaces with radii around 0.269 nm in the stratum corneum 5 .
Understanding free volume helps packaging engineers design better containers to protect foods, pharmaceuticals, and electronic components from moisture and gases.
Controlled free volume enables the development of sensitive detection systems that rely on selective transport of target molecules.
The invisible nanoworld of free volume, once merely theoretical, has been brought to light through the remarkable combination of positron annihilation spectroscopy and supercritical CO₂ processing. This powerful partnership allows scientists not just to observe but to actively engineer the hidden architecture of materials at the molecular level.
As research continues, particularly with promising polymer systems like Nylon 12 and PVA films, we move closer to a future where materials can be custom-designed with precisely controlled transport properties—offering solutions to some of our most pressing challenges in clean water, sustainable energy, and advanced medicine.
The antimatter probe has opened a window into spaces we cannot see, yet which hold the key to creating the materials of tomorrow.