Melt spinnabilities of thermoplastic paramylon mixed esters.

The low thermoplasticities of polysaccharide esters make them unsuitable for melt spinning. In this study, we aimed to overcome this problem by mixed esterification of paramylon, a euglenoid ß-1,3-glucan with short- and medium-chain acyl groups, as melt-spinnable materials. Thermal analyses revealed that all the synthesized paramylon mixed esters exhibited glass transition temperatures greater than 100 °C; some of them showed large differences between the melting and 5%-weight-loss temperatures (Td5s) and are extrudable through a spinneret at a temperature ~100 °C below Td5, rendering them potential candidates for the production of melt-spun filaments. Among the various compounds investigated, paramylon acetate propionates, in which the degrees of acetyl- and propionyl-group substitution were 0.5-0.7 and 2.2-2.5, respectively, could be melt-spun to yield mechanically tough crystalline monofilaments. In contrast, the melt spinning of cellulose acetate propionate, analogous to the paramylon acetate propionates in terms of acyl substituents, their substitution degrees, and molecular weights, but differs from it in terms of the glucose linkage mode (i.e., ß-1,3 vs ß-1,4), yielded brittle, charred, and short filaments. Curdlan acetate propionate, another analogue with a degree of polymerization five times larger than that of paramylon mixed esters, was not extrudable due to the lack of thermoplasticity. Therefore, we herein confirmed the superiority of paramylon as a primary raw material for melt-spun filaments.

Thermal, crystalline, and pressure-sensitive adhesive properties of paramylon monoesters derived from an euglenoid polysaccharide.

The thermal, crystalline, and pressure-sensitive adhesive properties of thermoplastic monoesters made from paramylon, a storage polysaccharide of Euglena gracilis, and a long-chain acyl chloride, were examined. Differential scanning calorimetry revealed that the thermal properties of these paramylon monoesters were dependent on the chain length and the average degree of substitution of the long-chain acyl group (av. DSlca). X-ray diffractometry revealed that the product solids with a myristoyl or palmitoyl group had a less ordered lateral acyl chain structure than those with a stearoyl group. Tackiness testing showed that the introduction of a myristoyl group into paramylon with an av. DSlca of ∼2.6 to ∼2.9 yielded palpable pressure-sensitive adhesion. A slight deviation of the chain length and/or av. DSlca from those of tacky paramylon myristate solids weakened or dispersed the tackiness. These results demonstrate the feasibility of using paramylon myristate solids with the av. DSlca in a specific range as a practical pressure-sensitive adhesive.

Synthesis and thermal properties of paramylon mixed esters and optical, mechanical, and crystal properties of their hot-pressed films.

Acylation of paramylon, a storage polysaccharide of Euglena gracilis, using multiple acid anhydrides yielded thermoplastic paramylon mixed esters without significant depolymerization. DSC examination showed that the shorter the acyl chain, the higher both the melting and glass transition temperature of the ester. TG analyses revealed their higher thermostability with the 5% weight loss temperature of ∼330°C. Melt volume flow rate examination revealed that the longer the acyl chain, the higher the thermoplasticity of the ester and that the esters exhibited higher thermoplasticity than structurally analogous esters made from cellulose and curdlan. A notable feature of the thermoplastic paramylon mixed esters is the availability of hot-pressing as a means of molding them into a film. Light transmittance and XRD measurements revealed that these films were transparent and in the amorphous state. Tensile tests indicated that the films had adequate mechanical strength comparable to those of the cellulose and curdlan analogues.

Synthesis of nanofiber-formable carboxymethylated Euglena-derived ß-1,3-glucan.

Carboxymethylation of paramylon, a storage polysaccharide of Euglena, was explored using homogeneous and heterogeneous reaction systems. Homogeneous reactions yielded carboxymethyl paramylons with a low degree of substitution of carboxymethyl group (DScm) of ∼0.13. Heterogeneous reactions of paramylon with chloroacetic acid in a mixture of 2-propanol and a NaOH aqueous solution yielded carboxymethyl paramylons with a high DScm of up to ∼0.80. An essential step for successful carboxymethylation using the heterogeneous reaction systems devised in this study was the pretreatment of a paramylon particle to break its inherent highly crystalline state into an amorphous one. A high degree of substitution provided carboxymethyl paramylons with a nanofiber-forming ability in the aqueous phase while a low degree provided them with a fiber-forming ability in both the aqueous and solid phases.

Preparation of transparent self-standing thin films made from acetylated euglenoid ß-1,3-glucans.

The feasibility of using acetylparamylon to prepare self-standing optical films was investigated. Its thermal stability, represented by the glass transition temperature, and 5% weight loss temperature are equivalent to those of acetylcellulose. The mechanical properties (maximum stress, elongation at break, and elastic modulus) of acetylparamylon films prepared by solution casting were comparable to those of acetylcellulose films. The degrees of their transparency and birefringence were the same as those of the acetylcellulose film. These results, together with the finding that acetylparamylon with a higher degree of acetylation formed a thin film with fewer micrometer-sized defects and that acetylparamylon with a lower degree of acetylation formed a thin film with a nanometer-level flat surface, suggest that acetylparamylon is a feasible material for self-standing optical film.

One-pot synthesis of thermoplastic mixed paramylon esters using trifluoroacetic anhydride.

Mixed paramylon esters prepared from paramylon (a storage polysaccharide of Euglena), acetic acid, and a long-chain fatty acid by one-pot synthesis using trifluoroacetic anhydride as a promoter and solvent were shown to have thermoplasticity. Size exclusion chromatography indicated that the mixed paramylon esters had a weight average molecular weight of approximately 4.9-6.7×10(5). Thermal analysis showed that these esters were stable in terms of the glass transition temperature (>90°C) and 5% weight loss temperature (>320°C). The degree of substitution of the long alkyl chain group, a dominant factor determining thermoplasticity, was controlled by tuning the feed molar ratio of acetic acid and long-chain fatty acid to paramylon. These results implied that the one-pot synthesis is useful for preparing structurally-well defined thermoplastic mixed paramylon esters with high molecular weight.

Fabrication of doughnut-shaped particles from spheroidal paramylon granules of Euglena gracilis using acetylation reaction.

An anhydrous type of paramylon, the micro-sized granular storage carbohydrate (ß-1,3-glucan) of Euglena, was transformed from a spheroidal to a doughnut-like shape by acetylation. Fourier transform infrared spectroscopic measurements suggested that the doughnut formation is due to removal of accessible regions of paramylon particles by acetylation of glucans. A time-course observation of the paramylon granules during acetylation by using field emission scanning electron microscopy revealed that the doughnut-making process begins with the removal of an outer membrane of the granule and that the central region of the granules is preferentially removed with the survival of a thick rim part to give the doughnut-like particles.