Temperature dependence of the free volume holes in polyhydroxybutyrate biopolymer: A positron lifetime study

Abstract

The size of the free volume holes and their distributuion in polyhydroxybutyrate (PHB) biopolymer have been studied using positron annihilation lifetime spectroscopy (PALS). The measurements were performed as a function of the temperature from 243 to 363 K. The temperature dependence of the free volume hole shows a glass transition temperature, T g at 278 K. The free volume behaviour of PHB shows a small linear increase with temperature below T g and a steeper increase above T g . The thermal expansion coefficients of the free volume hole were determined to be 4.78 x 10 -3 and 12.5 x 10 -3 / K below and above T g , respectively. The free volume hole distribution shows a narrow distribution below T g . With increasing temperature, the maximum of the distributions of the free volume holes shifts to higher values. Meanwhile, widening of the distribution is smaller in lower temperature ranges and more pronounced at higher temperatures, especially above T g .1 Introduction Polymers contain local free volumes, these are cavities or holes of atomic and molecular dimensions that arise because of (1) irregular molecular packing in the amorphous phase (static and pre-existing holes) and (2) molecular relaxation of the polymer chains and terminal ends (dynamic and transient holes). Positron annihilation lifetime spectroscopy (PALS) has recently emerged as a unique nano-probe capable of measuring the free volume hole sizes in polymers [1][2][3][4]. When positrons are injected into materials, they lose their kinetic energy within a few picoseconds. After thermalization, a positron diffuses in the media, and annihilates with an electron from the surrounding emmiting two γ-rays. In polymers, a positron might annihilate from a positronium (Ps) state, a bound state of a positron and electron. A positronium has two spin states: a singlet state (para-Ps, p-Ps) and a triplet state (orthoPs, o-Ps).The most important component is the o-Ps lifetime, τ oPs which gives us information about the free volume holes in which annihilation of positrons takes place.Nowadays plastics and synthetic polymers are mainly produced using petrochemical materials that cannot be decomposed. Therefore they contribute to environmental pollution and are a danger to many animals. During the last decade, much attention has been focused on the production of bacterial polyesters. Different bacterial types [5] of microorganisms produce polyhydroxybutyrate (PHB) from renewable sources such as sugar and molasses as intracellular storage materials. The possible applications of PHB are in medicine, in pharmacology and in packaging for deep drawing articles in the food industry.Only one measurement was reported in the study of the properties of the free volume in PHB biopolymer [6]. The aim of this work is to invesigate the potential of PALS for studying the free volume properties of semicrystalline PHB bioolymer. This can be achieved via a systematic study of the temperature dependence of the mean size of local free volumes and their distributions.

Effect of thermal history on the free volume properties of semi‐crystalline poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) membranes by positron annihilation lifetime spectroscopy

“…3) was lower than that at a higher temperature range, but still higher than that in the glassy state. These behaviors have been observed in the literature 43, 44…”

Structural, topographical, and mechanical characteristics of purified polyhydroxyoctanoate polymer

“…The force-distance curves obtained in the FS experiments allowed us to determine the distributions of locally measured Young's moduli [ Figure 2(a-c)]. The half-width of the distribution is 28 61 measured for PHO strips (10 × 4 × 0.63-0.70 mm) obtained by extraction in a hypochlorite/chloroform dispersion followed by a purification procedure using acetone for dissolving and a methanol/ethanol mixture for polymer precipitation. On the other hand, PHO strips prepared by melting in 90 C polymer extracted earlier in chloroform and precipitated in methanol showed a Young's modulus of 9.3 MPa.…”

“…The technique allows for deep examination of a material structure without perturbing the arrangement or destroying the sample during a measurement . PAS is an appropriate method for studying the polymer's free volume regions . The presence of free volumes plays a crucial role in determining the macroscopic properties of polymers, namely density and thermal and mechanical properties …”

Evaluation of transport mechanism and nanostructure of nonperfluorinated PVA/sPTA proton exchange membrane for fuel cell application

“…Positron annihilation lifetime spectroscopy (PALS) has been used effectively to quantify the sizes of free volume holes in a variety of polymers. [36][37][38][39][40][41][42][43][44] The technique is based on the fact that positrons when they enter the material can interact with free electrons, which they annihilate by emitting two γ-lines with an energy of 511 keV each.…”