Investigating thermal properties of and melting-induced structural changes in cold-drawn P(3HB) films with α- and β-structures using real-time X-ray measurements and high-speed DSC

Abstract

In this study, we prepared two types of drawn films of poly[(R)-3hydroxybutyrate], a microbial polyester: those containing only α-crystals and those containing both αand β-crystals. Real-time wide-angle X-ray diffraction (WAXD) and real-time smallangle X-ray scattering (SAXS) measurements were performed during heating using synchrotron X-rays, along with high-speed differential scanning calorimetry (DSC), to measure the melting temperature of the β-crystals, which were present in a small amount. The changes induced in the highly ordered structure of the films were also observed. Based on the results of the real-time WAXD measurements, the melting temperature of the β-crystals was found to vary from 90 °C to 130 °C. The results of high-speed DSC supported these results. The real-time SAXS measurements allowed the changes in the highly order structure of the films to be observed. A model to explain the changes in the highly order structure is constructed based on the obtained results.

X-ray Crystal Structure Analysis of Poly(3-hydroxybutyrate) β-Form and the Proposition of a Mechanism of the Stress-Induced α-to-β Phase Transition

“…As well known, an increment of the molecular weight may improve the brittleness of the sample, as already reported in refs. [17][18][19][20][21][22][23][24][25][26][27]30 However, even when the PHB sample of an ultrahigh molecular weight of 10 6 g/ mol order is stretched, the transition from the αto βform is still not perfect. 17 The LP values increase in an aff ine mode with an increase of strain up to 240 Å (α-form) and 400 Å (β-form).…”

Tensile study of melt-spun poly(3-hydroxybutyrate) P3HB fibers: Reversible transformation of a highly oriented phase

“…A broad equatorial reflection is clearly visible for both fibers in the region between approximately 17.8° and 20.5° in Figure 1 as well as in Figure 2, and is labeled as P nc . This reflection has commonly been assigned to arise from a (para)crystalline form of P3HB referred to as 'β-form' [7,9,10,[12][13][14][15][16]. This β-form of P3HB has typically been considered to be generated from transformed non-crystalline regions between lamellar α-crystals, either from the tie chains between the lamellae or from completely free chains in the non-crystalline domain [7,10,14,15,[23][24][25].…”

“…The visibility of a periodicity along the drawing axis apparently seems to largely depend on the stretch factor (draw ratio) and on how the material in question is produced. In, e.g., stretched P3HB films [12], uniaxially cold-stretched PHBV films [14] and stretched and annealed P3HB films [9,34], weak streaks above the first layer line have been observed. However, in our melt-spun P3HB fibers [8], in melt-spun and ice water cooled P3HB [35] and P3HBV fibers [36], as well as in melt-spun P3HBV fibers [37], no such layer line streaks are visible.…”

“…However, this secondary crystallization can be prevented by establishing a lamellar morphology during processing. Therefore, drawn P3HB films and fibers do not undergo secondary crystallization, and the mechanical properties of drawn and annealed P3HB remain unchanged, at least for several months [7][8][9].…”

“…Yokouchi et al [12] were the first authors who have observed an additional intense equatorial reflection and streaks above the layer lines in a WAXD pattern of a highly stretched P3HB film, which cannot be assigned to the α-M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 form P3HB crystal structure. In the literature, these stress-induced additional reflections have been attributed to arise from a (para)crystalline β-form, which has been described to develop in the regions between α-form lamellar crystals by orientation of molecular chains [7,9,10,[12][13][14][15][16].…”