Plant biomass has been the most important source of renewable polysaccharides which includes starch, cellulose and hemicelluloses. The major hemicellulose mostly found in plants is xylan. This polysaccharide has a backbone consisting of β-(1-4) linked xylose units. Recently, xylan gains increasing importance for the basis of new biopolymeric materials. In this research, xylan was chemically modified by esterification (Fig 1). The products were characterized based on their structure, solubility, thermal and mechanical properties. The application of these xylan esters as bio-based nucleating agents for PLA was of primary interest. Film and nanofiber formation of the xylan esters were considered as well.

Xylan was first extracted from eucalyptus hardwood pulp by alkaline treatment with different concentrations of NaOH solutions. Higher yield was obtained with higher alkaline solution. Structure elucidation of the extracted xylan was done by NMR (Nuclear Magnetic Resonance). Results revealed that the extracted xylan did not contain acetyl groups and other sugars aside from xylose. This confirms that the extracted xylan was a homoxylan. Based on thermogravimetric analysis (TGA) the xylans extracted with higher alkaline concentrations had better thermal stability. This is possibly due to the extraction of higher molecular weight xylan. In the preparation of xylan esters, xylan extracted with 10% NaOH was used. This yielded 6-8% xylan. Although higher yield can be obtained at higher alkaline concentration, it required a significant amount of acid for neutralization. This may not be practical for industrial application considering that the yield was not significantly higher.

Acetylation of xylan was carried out by homogeneous reaction in DMAc/LiCl system. The reaction was done at different reaction times in order to monitor the DS. Fully acetylated xylan (DS=2.0) was achieved within 6 h. Results revealed almost the same DS values at C-2 and C-3 positions of xylan acetate (XylAc) indicating the non-selectivity of the reaction. In the 1H-NMR spectrum of XylAc (Fig 2), the signal at δ 2.0 ppm corresponds to the methyl protons. This indicates successful acetylation of xylan. Meanwhile, the signals between δ 3.3 to 5.0 ppm are assigned to the ring protons of XylAc. Changes in the structure of xylan brought about by acetylation lead to an improvement in its thermal stability. XylAc with higher DS had better thermal stability. Likewise, the solubility of xylan in CHCl3 also increased. Perpropionylation of partially substituted xylan further increased its thermal stability and solubility in CHCl3. DSC and WAXD results of xylan acetate propionate suggest pseudo-crystallization of the side chains. The mechanical properties of xylan acetate propionate films were dependent on the DS. The xylan acetate propionate film having a lower DSAc exhibited a higher tensile strength and elongation at break.

Synthesis of low molecular weight (LMW) and high molecular weight (HMW) xylan esters having varying alkyl chain lengths (C2-C12) were done by heterogeneous (TFFA/acid) and homogeneous (DMAc/LiCl) reactions, respectively. The structural features of the xylan esters (DS=2.0) were elucidated by NMR analysis. Based on TGA, the thermal stability of xylan increased after esterifcation. Xylan esters with longer alkyl chains had higher decomposition temperatures. The solubility of xylan in CHCl3 also increased after esterification.

The ability of the xylan esters to form films was dependent on their molecular weight and solubility in CHCl3. LMW xylan esters with alkyl chains containing n >= 6 carbons (XylHe, XylDe and XylLa) were able to form continuous films. In the case of the HMW xylan esters, films were formed when the alkyl chains contained n >= 3 (XylBu, XylVa, XylHe, XylDe and XylLa). Tensile test done on HMW xylan ester films showed that the tensile strength and Young's modulus of xylan esters decreased with increase in alkyl chain length while the elongation at break increases (Fig 3). From the contact angle measurements, the hydrophobicity of the xylan ester films increased with increase in alkyl chain length. The surface contact angle of xylan ester films with longer alkyl chains ranged from 95 to 99o. X-ray diffractograms of the xylan esters films show the presence peaks in the low angle region suggesting the presence of side chain crystals (Fig 4). However, the results from DSC measurement did not reveal any side chain melting (Fig 5). This indicates that only pseudo crystals were formed. Likewise, glass Tg was also not observed for all samples even when faster heating rate was employed during DSC analysis. This observation is in good agreement with the results obtained from DMA (Dynamic Mechanical Analysis) where a drop in storage modulus was not seen in the curve. Electrospinning of solutions of HMW xylan esters in HFIP produced beaded fibers. XylAc, XylPr, XylBu, XylVa and XylHe can be electrospun into nanofibers (Fig 6). However, XylDe and XylLa were not soluble in HFIP.

One of the primary interests of this research is the application of xylan esters as bio-based nucleating agents for PLA (PLLA and PDLA). The effect low molecular weight (LMW) and high molecular weight (HMW) xylan esters (C2-C12) on the crystallization behavior of PLLA were investigated. Non- isothermal crystallization study on PLLA and PLLA blends containing LMW and HMW xylan esters were studied. In both cases, only XylPr and XylBu were effective in lowering the crystallization temperature (Tc) of PLLA. Other xylan esters with longer alkyl chains showed an inhibitory effect on the crystallization of PLLA. Figure 7 shows the endotherms of PLLA blends containing 1% LMW xylan esters. Blending of LMW XylPr and LMW XylBu decreased the Tc of PLLA from 126 to 95 and 97 oC, respectively. Similar results were obtained when HMW xylan esters were used. Different concentrations of LMW XylPr and LMW XylBu in the PLLA blends produced comparable results. Melt crystallization study revealed that these xylan esters can enhance the crystallization of PLLA even at faster cooling rate. In addition, LMW XylPr can act as better nucleator than LMW XylBu for PLLA. It is speculated that the aggregation of these xylan esters acts as nucleating centers and thus enhancing the crystallization of PLLA.

Isothermal crystallization study show that the half time crystallization (t1/2) of PLLA decreased when blended with LMW XylPr and LMW XylBu (Fig 8). The t1/2 of PLLA, 1% LMW XylPr/PLLA and 1% LMW XylBu/PLLA isothermally crystallized at 130 oC were 13.6, 3.1 and 5.7 min, respectively. POM images reveal that the spherulites of PLLA blends containing the xylan esters were smaller and denser compared to PLLA (Fig 9). These results confirm the increase in the number of nucleating centers in the presence of the xylan esters. WAXD results showed that the crystallinity (Xc) of the PLLA blend films was higher compared to neat PLLA. When PLLA was annealed for 10 min at 100 oC, the Xc was only 32%. However, in the presence of the xylan esters, the Xc almost reached 50%. Although the PLLA blends had higher Xc, the films were more optically clear than neat PLLA based on the haze measurements (Fig 10). This is attributed to the presence of smaller spherulites in PLLA blends. In addition, the PLLA blend films had lower thermal expansion compared to PLLA based on TMA; hence, more resistant to heat deformation. Similar results were also obtained with PDLA blends.

The effectiveness of XylPr and XylBu as nucleators for PLA could be a result of the aggregation of these xylan esters. These aggregates act as nucleating centers which initiates the crystallization of PLA. It is hypothesized that the sum of the d-spacings of these aggregates is approximately the same with one of the lattice parameters of PLA.

Fig 1. Esterification of xylan.

Fig 2. 1H-NMR spectrum of xylan acetate (XylAc).

Fig 3. Stress-strain curves of HMW xylan ester films.

Fig 4. X-ray diffractograms of HMW xylan ester films.

Fig 5. DSC traces of xylan esters in the 2nd heating scan.

Fig 6. HMW XylAc nanofibers.

Fig 7. Endotherms of PLLA and its blends containing 1% LMW xylan esters.

Fig 8. Plots of t1/2 vs Tc of PLLA and its blends containing LMW esters.

Fig 9. Spherulite images of PLLA and its blends containing LMW esters isothermally crystallized at 130 ℃

Fig 10. Haze as a function of time of PLLA and its blend containing LMW esters annealed at 100℃

木材はセルロース、リグニン、ヘミセルロースから構成され、セルロースは繊維やフィルムなど古くから化学工業製品として広く利用されています。また、リグニンについては、固形燃料を中心としたエネルギー源としての利用が検討されています。しかし、ヘミセルロースに関しては、これまでほとんど利用されてきませんでした。本研究は、ヘミセルロースの中で最も多く存在するキシランに着目し、化学合成法によるプラスチック材料化を目的としています。さらに、現在最も研究開発が進んでいるバイオマスプラスチックの一つであるポリ乳酸に少量添加し、その結晶化速度を大幅に改善するための結晶核剤としての応用について検討した論文です。

第1章の序論に引き続き、第2章では、2種類の化学合成法(均一反応法と不均一反応法)を用いて、最も置換基の長さが短いアセチル基を用いて、反応条件の検討を行いました。合成したキシランアセチルの化学構造、分子量、置換基分布、熱的性質、機械的強度、溶解性を、核磁気共鳴スペクトル(NMR)、ゲルパーミエーションクロマトグラフィー(GPC)、示差走査熱量計(DSC)、引張試験、溶解試験により解析しました。その結果、均一反応の方が不均一反応より、分子量が高いことがわかりました。さらに、反応条件により分子量、置換度、置換基分布、溶媒への溶解度を制御可能であることがわかりました。

第3章では、様々な長さを有するキシランエステル誘導体を合成し、それらの構造と物性を詳細に解析しました。均一反応法により、アセチル基(C2)からラウリル基(C12)までのエステル誘導体の化学合成に成功し、NMRにより置換度は100%であることを確認しました。キシランエステル誘導体の熱分解温度を解析したところ、キシランに対し、熱分解温度は70度以上高くなることがわかりました。ソルベントキャスト法によりフィルムを作製したところ、非常に透明性の高いフィルムが得られることがわかりました。引張試験の結果、置換基の長さにより、硬くて強いフィルムから、軟らかくて弾性に富むフィルムまで、様々な物性を有するフィルムを作製することに成功しました。これらの物性は、ポリエチレンやポリプロピレンに匹敵することがわかり、キシランから有用なプラスチックを合成できることがわかりました。X線により構造解析を行ったところ、キシランエステル誘導体は結晶性を有しておらず、ポリスチレンのような、非晶質のプラスチックであることがわかりました。さらに、キシランエステル誘導体から、直径がナノオーダーの極細繊維を加工することに成功し、マスクや不織布などの高機能性繊維材料に応用可能であることが示唆されました。

第4と第5章では、第3章で合成した一連のキシランエステル誘導体をバイオマスプラスチックの一つであるポリ乳酸に少量添加し、結晶核剤としての有用性について検討を行いました。ポリ乳酸は、トウモロコシやサトウキビから乳酸発酵により生合成され、現在、バイオマスプラスチックの中で最も利用が検討されています。しかし、ポリプロピレンなどの汎用樹脂に比べ、成形加工速度(結晶化速度)が遅いこと、熱的安定性が低いことなどが大きな課題で、今のところその利用は限られています。今回、ポリ乳酸に、キシランエステル誘導体を1%添加したところ、ポリ乳酸単体では結晶化に約2分が必要であるのに対し、キシランエステル誘導体を添加すると、ポリプロピレン並みの30秒以下へと劇的に結晶化速度が増加することがわかりました。この結果は、添加量を0.1%にまで減らしても有効であることもわかりました。また、成形したフィルムは非常に高い透明性を有しているだけでなく、熱安定性にも非常に優れていることを見出しました。この原因を偏光顕微鏡で解明を試みたところ、フィルム中に発生する球晶と呼ばれる高分子結晶の大きさが、キシランエステル誘導体を添加すると劇的に小さくなっていることがわかりました。

以上のように、本研究は木材成分の中の未利用成分であるキシランをエステル誘導体化によりプラスチック材料へと変換することに成功すると共に、バイオマスプラスチックであるポリ乳酸への結晶核剤としての有用性を見出したもので、審査委員一同は、本論文が博士(農学)の学位論文に値するとの結論に達しました。