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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, $\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.
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Submitted on : Tuesday, December 1, 2009 - 3:38:30 PM
Last modification on : Friday, May 8, 2020 - 1:10:01 AM

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Gaëlle Roudaut, G. Duplâtre. Positronium as a probe in natural polymers: decomposition in starch. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2009, 11, pp.9556-9561. ⟨10.1039/b911949c⟩. ⟨in2p3-00437863⟩

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