Positronium as a probe in natural polymers: decomposition in starch

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, Eact = (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 (τ3n)3 = I3n, linking the normalised values of τ3, τ3n, and of I3, I3n. 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 p1/3. Quantitative approaches on these bases lead to satisfactory fitting of all PALS data, yielding an activation energy, Eact = (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.