Positronium as a probe in natural polymers: decomposition in starch

Abstract

Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I₃, remains constant at 26.2% while its lifetime, τ₃, is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival probability (p) of the starch lattice, with an activation energy, E_act = (1.52 ± 0.05) eV, and a frequency factor, ln(k₀, s⁻¹) = 25.3 ± 0.4. In the decomposition region, the PALS data show the unexpected correlation (τ_3n)³ = I_3n, linking the normalised values of τ₃, τ_3n, and of I₃, I_3n. This is explained by considering that the changes in I₃ with T arise from those in the surviving volume fraction of the lattice, p, whereas the changes in τ₃ reflect the shrinking of the radius of the free volumes, the latter decreasing in proportion to p^1/3. Quantitative approaches on these bases lead to satisfactory fitting of all PALS data, yielding an activation energy, E_act = (1.53 ± 0.03) eV, and frequency factor, ln(k₀, s⁻¹) = 25.4 ± 0.2, in excellent agreement with the values derived from TGA.