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Physical Chemistry Chemical Physics 11 (2009) 9556-9561
Positronium as a probe in natural polymers: decomposition in starch
G. Roudaut1, G. Duplâtre2

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, $\tau$3, is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival probability (p) of the starch lattice, with an activation energy, Eact = (1.52 ± 0.05) eV, and a frequency factor, ln(k0, s-1) = 25.3 ± 0.4. In the decomposition region, the PALS data show the unexpected correlation ($\tau$3n)3 = I3n, linking the normalised values of $\tau$3, $\tau$3n, 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 3 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.
1 :  ENSBANA - Lab Eau Mol Act Macromol Act,
2 :  DRS-IPHC - Département Recherches Subatomiques
Chimie/Chimie théorique et/ou physique