Positronium as a probe in natural polymers: decomposition in starch

Area of Science:

• Materials Science
• Physical Chemistry
• Polymer Science

Background:

• Ortho-positronium (o-Ps) is a valuable probe for characterizing free volumes in materials using positron annihilation lifetime spectroscopy (PALS).
• Understanding the behavior of free volumes in natural starch as a function of temperature is crucial for material characterization and processing.

Purpose of the Study:

• To investigate the free volume dynamics in natural starch samples subjected to varying temperatures using PALS.
• To correlate PALS data with thermal gravimetry analysis (TGA) to understand starch decomposition mechanisms.

Main Methods:

• Positron annihilation lifetime spectroscopy (PALS) was employed to measure ortho-positronium (o-Ps) lifetime (tau(3)) and intensity (I(3)).
• Thermal gravimetry analysis (TGA) was used to study the thermal decomposition of starch samples.
• Analysis involved correlating PALS parameters (tau(3), I(3)) with temperature and TGA data.

Main Results:

• Up to approximately 540 K, o-Ps intensity (I(3)) remained constant, while lifetime (tau(3)) increased linearly, indicating stable free volumes.
• Above 540 K, both I(3) and tau(3) decreased due to sample decomposition and shrinking.
• An unexpected correlation, (tau(3n))^3 = I(3n), was observed during decomposition, linking normalized lifetime and intensity.

Conclusions:

• The study suggests a glass transition temperature for starch above 501 K.
• PALS data and TGA measurements yielded consistent activation energies and frequency factors for starch decomposition.
• The observed correlation during decomposition is explained by the interplay between surviving lattice volume fraction and free volume radius.