0428同步年報-2021-全

Soft Matter 033 the non-destructive method, microbeam X-ray analysis is the most powerful tool to determine the micro- architectures of a lamellar distribution in a specific location of micrometres. So we specifically chose a poly(β- hydroxy butyrate) (PHB) spherulite with band spacing > 20 μm (in PHB/poly(ethylene oxide) (PEO)) crystallized at T c = 50 o C, band spacing ~25 µm). A mechanism of 3D-grating structure assembly of RBS is positively proven with synchrotron microbeam small-angle X-ray scattering (SAXS) and various microscopic techniques. This grating structure in the banded PHB resembles many of nature’s iridescent crystals and is further proved by photonic reflection results as a critical novel finding. The powerful synchrotron microbeam of small-angle X-ray scattering is ~15 μm available at TPS 25A of the NSRRC, where the microbeam facility was recently made possible by a group of experts led by Wei-Tsung Chuang, and his collaborator Yi-Wei Tsai (NSRRC).. The morphological microstructures of a PHB ring-band spherulite were examined with microscopic techniques and powerful synchrotron microbeam SAXS by a research team led by Eamor M. Woo (National Cheng Kung University), assisted and supported by Chuang and Tsai of the NSRRC. The microbeam analysis was recorded through two sets of alternate valleys and ridges. In the periodic ring, the statistical-average lamellae orientation across the film thickness ata specific location of alternate valley/ridge periodicity was investigated withsynchrotron microbeam SAXS. Figure 2 shows (a) a banded pattern of PHB across which a microbeam moved, and (b) expected SAXS 2D patterns as the beam moved in a radial direction on five separate spots (A−E). The POM-banded pattern of a typical PHB-banded spherulite is exemplified; a yellow arrow is marked from its nucleus center to a periphery to guide the movement of the microbeam. The interior lamellar construction of periodic rings with respect to alternate ridge and valley is given in Fig. 2(b) . When the microbeam was passed through a ridge, the feasible 2D-SAXS signals are showcased in Fig. 2(b) for conceptual understanding. As the lamellar plates are vertically aligned in a ridge, the contrast of electron density between the amorphous and crystalline phases of a lamellae plate can be deduced in the X-ray pattern. In the valley, lamellar plates are arranged in a horizontal direction, so the microbeam X-rays cannot reduce the phase contrast of electron density, resulting in no signal in a 2D-SAXS pattern. Note that one should not be misled by the top surface morphology, as the X-ray beam would actually penetrate the interior lamellae in much greater path length ( ca . ~20 μm) than the thin top surface (< 1 μm). This effect is to hint that the resulting X-ray signal is built not by the thin top surface layer, but mostly by the interior bulk lamellae and their assembly patterns (crystal orientation relative to the X-ray beams etc. ). The scheme in Fig. 2(b) shows that, as the X-ray microbeam is stepwisely moved along the radial direction of a PHB banded spherulite from its central nucleus to the outer periphery, the interior crystal orientation traverses from tangential to radial orientations in a repetitive periodic cycle. The general universality of such periodicity with cross- hatch grating architectures can be demonstrated, which tends to be ubiquitous in periodically banded crystals or crystal aggregates. In earlier work on another polymer system, Nagarajan and Woo 2 analysed 2D- Wide Angle X-ray Diffraction (WAXD)/SAXS microbeam data of poly(ethylene adipate) (PEA)-banded spherulites to provide evidence for a corrugate-grating lamellae assembly, using similar synchrotron microbeam SAXS and WAXD analyses, with a 1-µm microbeam at the SPring-8 facility, Japan, in collaboration with Kohji Tashiro. The analysis of the PEA banded spherulite has proved that SAXS/WAXD signals indicate that alternate strut-to-rib crystal plates in PEA are interfaced with a discontinuity; abruptly changed orientations from the ridge to valley bands account for the periodic optical birefringence (band spacing = 6.5 µm). Using NSRRC microbeam SAXS, an analysis of the interior assembly of the PHB-banded spherulite was performed. Although the available beam size in this work is a bit large, the band spacing in PHB is much larger (15−30 µm) than that in PEA. Considering the constraint of the large beam size, we specifically chose a PHB spherulite with band spacing > 20 µm (in PHB/PEO crystallized at T c = 50 o C) as a model for such analysis. The X-ray beam was directed to travel stepwise along the radial direction originating from the nucleus of the banded PHB spherulites; signals were collected in each intermittent step-move. The results are shown in Fig. 3(a) (see next page). The beam size covers a fixed area ca. ~177 μm 2 ; the signal is thus a statistical average of crystals distributed in this region and crystal plates across the entire film thickness. Figure 3(b) shows 2D signals for each spot marked on Fig. 2(a) , Fig. 2 : (a) POM graph showing alternate colored rings, and (b) expected 2D-SAXS pattern variation with respect to a microbeam moving position along the radial direction at five spots: tangential (edge- on) lamellae 1, radial (flat-on) lamellae 2, back to tangential (edge-on) lamellae in next cycle 3, radial (flat-on) lamellae 4, back to tangential (edge-on) lamellae in next cycle 5. [Reproduced from Ref. 1]