論文の公開元へPhysical property and structure analyses for bio-based films and fibers of β-1,3-glucan esters
概要
Chapter 1. Introduction
In recent years, eco-friendly bio-based plastics have attracted a lot of attentions because the manufacturing and waste disposal process of plastics derived from petroleum will accelerate global warming. Polysaccharides consisting of glucose with - or -glucoside linkages are one of the attractive raw materials for bio-based plastics.
Paramylon, which photosynthesized from Euglena of microalgae, is a storage polysaccharide with a structure of -(1,3)-glucan. Since considerable amounts of paramylon can be photoproduced by using readily cultured Euglenoid alga from CO2 and water, it is a prospective biomass. However, unmodified polysaccharides do not show plasticity because of their strong inter- and intra-molecular hydrogen bonding. Accordingly, esterification of chemical modification is usually adopted for inhibiting the formation of hydrogen bonding, eventually obtain thermoplastic polymeric materials. For the plastic materials, physical property is strongly correlated with polymer structure. Therefore, investigation of the relationship between the structure and the property is important for determining the utilization of the materials.
In this study, paramylon triesters with different side-chain lengths were taken as research target, their fundamental properties were tested. Besides, further study about correlation between structure and properties were conducted by measurement of crystalline elastic modulus through time-resolved X-ray diffraction method. At last, melt spinning ability of crystalline paramylon esters were tested.
Chapter 2. Preparation of paramylon esters and physical properties of their films
In this chapter, various paramylon triesters with different alkyl chain lengths (carbon numbers 2−12) were successfully prepared through one-pot synthesis. All the paramylon triesters have higher thermal degradation temperatures than that of neat paramylon. Moreover, it was found that the paramylon triesters with C2−C6 alkyl chains are crystalline polymers with melting temperatures from 281 °C to 114 °C, and those with C8−C12 alkyl chains are amorphous polymers, confirmed by both DSC and Xray diffraction analysis. Paramylon triesters with C3−C12 alkyl chains could shape self-sustaining films by both solvent-casting and melt-quench methods with high optical transmittance. Thermal and mechanical properties of paramylon triesters can be controlled freely by substituted acyl length. In the cases of the crystalline paramylon triesters, highly oriented and crystallized films could be fabricated by the thermally stretched method, and their tensile strengths have been obviously improved.
Chapter 3. Crystal structures and crystalline elastic modulus of PaPr, PaBu, and PaVa
In this chapter, three paramylon ester derivatives paramylon propionate (PaPr), paramylon butyrate (PaBu), paramylon valerate (PaVa) were prepared and their crystal structures were determined by wideangle X-ray diffraction. All the reflections were indexed to a hexagonal crystal system and all the molecular chains have five-fold screw symmetry along the molecular axes, despite of the different sidechain lengths of the ester groups. The crystal lattice parameters increase with increasing of side-chain length of the ester groups (Tab. 2).
The crystalline elastic modulus of PaPr, PaBu, and PaVa parallel to the fiber axis (El) were determined by time-resolved X-ray diffraction at SPring-8, which is a large synchrotron radiation facility (Fig. 3). The crystalline elastic modulus of PaPr, PaBu, and PaVa are 2.5, 1.9, and 1.0 GPa, respectively (Fig. 4). The decreasing El values are mainly caused by increasing of cross-sectional area of the molecular chains, which is affected by the side-chain length. The El values of the paramylon esters are one-tenth those of previously reported cellulose esters. This seems to be because of the different helical structure of the backbone chain.
Chapter 4. Melt spinning of PaPr, PaBu, PaVa and tensile properties of the fibers
In this chapter, thermal formability of paramylon esters was discussed by producing melt-spun fibers, following with characterization of the fibers’ crystallinities and tensile properties. As results, melt-spun fibers of paramylon propionate (PaPr, Fig. 5), paramylon butyrate (PaBu), and paramylon valerate (PaVa) were successfully manufactured at varied take-up rate, without using any additives. To investigate relations between crystallinities and tensile properties, Polarized microscope, Wide-angle Xray diffractions and tensile test were adopted for the PaPr fibers. All the PaPr fibers spinning at varied take-up rate show good orientation, however, those at higher take-up rate of 300 and 500 rpm were crystallized, while the other at lower take-up rate of 50 and 100 rpm were amorphous. With further annealing of the PaPr fibers, crystallization was promoted with high degree of orientation of crystallites. On the other hand, the tensile strengths and Young’ modulus of the PaPr fibers increased with increasing of take-up rate (Fig. 6) while nearly almost stayed unchanged after annealing. It was considered that the status of amorphous part in the fibers is one of major factors that affect the tensile properties.
Chapter 5. Discussion about crystal structures of PaPr, PaBu, and PaVa
In this chapter, molecular models of PaPr, PaBu, and PaVa were firstly established through computer simulation. After that, the models were packed in crystal lattices combining with the results of wideangle X-ray diffractions obtained in the chapter 3. Calculated X-ray diffractions and profiles were obtained though the simulated structure models. The calculated profiles fitted well with experimental data from the X-ray diffractions. However, some diffractions were difficult to confirm so that further study need to be conducted for investigating the crystal structures.
Chapter 6. Conclusions
In this study, a series of paramylon triesters with different alkyl side-chain lengths have been prepared, and their thermal properties, mechanical properties, and molecular structures have been investigated. By adjusting the side-chain length, the paramylon esters with C2−C6 alkyl chains have been prepared as crystalline polymers with melting temperatures of 281−114 °C. The crystal structures of PaPr, PaBu, and PaVa have been determined by indexing the reflections extracted from X-ray fiber diagrams obtained from stretched and annealed films. All the three paramylon esters have hexagonal crystal systems. The crystalline elastic modulus of PaPr, PaBu, and PaVa were determined to be 2.5, 1.9, and 1.0 GPa, respectively, by time-resolved X-ray diffraction measurements. The decrease of the El value with increasing side-chain length seems to be caused by increasing cross-sectional area of the molecular chain and loose molecular chain packing in the crystal. Besides, PaPr, PaBu, and PaVa were used for manufacturing fibers by melt spinning method. All the paramylon esters showed good thermal fluidity at the melt processing temperatures. For the melt spinning, successive melt spinning was achieved for all the paramylon esters at varied take-up rate of 50, 100, 300, and 500 rpm, without using any additives and the molecular weight almost had no change.
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