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, I(3), 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, E(act) = (1.52 +/- 0.05) eV, and a frequency factor, ln(k(0), s(-1)) = 25.3 +/- 0.4. In the decomposition region, the PALS data show the unexpected correlation (tau(3n))(3) = I(3n), linking the normalised values of tau(3), tau(3n), and of I(3), I(3n). This is explained by considering that the changes in I(3) with T arise from those in the surviving volume fraction of the lattice, p, whereas the changes in tau(3) 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(k(0), s(-1)) = 25.4 +/- 0.2, in excellent agreement with the values derived from TGA.

Characterization of a sucrose/starch matrix through positron annihilation lifetime spectroscopy: unravelling the decomposition and glass transition processes

“…[9][10][11] In a first step, we chose to examine one of the basic constituents of numerous natural polymers utilized at an industrial level, viz, starch. 12 However, it was soon discovered that decomposition of the starch matrices would hamper this quest, by inducing a pronounced decrease in both t 3 and I 3 before T g could be reached: t 3 was found to increase linearly with T up to the region of decomposition, without any sign of enhancement in the gradient and the data merely indicated that T g should lie above about 501 K, in qualitative agreement with previous data on starch of various origins with variable water content. 13 The useful information gathered was that the normalized values of I 3 and t 3 were strongly correlated.…”

“…This was due to the fact that I 3 varies in proportion to the surviving fraction, p, of the decomposing matrix, whereas R, the radius of the free volume nanoholes, and hence t 3 (which is closely linear with R in the range 1-1.6 ns), decrease in proportion to p 1/3 (this being the one-dimensional equivalent to a decrease in nanohole volume in proportion to p). 12 The corresponding equations were as follows:…”

Desenvolvimento de um Ingrediente Alimentar Funcional: Caracterização por Aniquilação do Positrão

“…Embora o volume aparente livre integre apenas os locais detetados pelo o-Ps [9], o modelo tem sido utilizado com sucesso em estudos por PALS de numerosas substâncias, incluindo polímeros, membranas [10,11] e agora as ciclodextrinas [5,12]. Embora a equação 1 tenha sido obtida para buracos esféricos, o que não é o caso nas CDs, nem tão pouco na maioria das outras substâncias referidas, o valor de R extraído de τ 3 pode ser considerado como um raio "médio" e, como tal, é correntemente associado ao "volume livre médio" dos sistemas estudados.…”

Microwave treatment regulates the free volume of rice starch

“…Currently, positron annihilation lifetime spectroscopy (PALS) is a highly suitable technique for characterization of FV holes in starch 9,16 , which involves the detection of the lifetimes of positron injected into a polymer. The first objective of this study was to evaluate the effect of moisture content and microwave parameters on the FV changes of rice starch during microwave heating.…”