聚乳酸生質高分子韌化改質研究

Study of toughening of biomass-derived polylactide

何長鴻(Chang-Hong Ho)

指導教授 ： 李育德

國立清華大學/工學院/化學工程學系/博士(2010年)

https://doi.org/10.6843/NTHU.2010.00650

摘要

聚乳酸（polylactide）是以玉米澱粉提煉的乳酸為單體，經過化學合成反應所得的新型生物可分解高分子材料，具有無毒、生物相容、高強度以及可生物分解及吸收等特點，是目前最具有發展前途的生物可分解高分子材料。聚乳酸具有與PET相同的優異機械物性，但是缺乏耐衝擊強度，所以其應用範圍著實受到限制。本論文著重於聚乳酸高分子的改質研究，以提升聚乳酸的耐衝擊強度與拉伸斷裂伸長量等特性。本研究主題可分為三個部分。
第一部份是建立一種新型製備熱塑性聚烯烴彈性體接枝聚乳酸共聚合物（TPO-g-PLA copolymer）的方法：首先在熱塑性聚烯烴彈性體結構中接枝酸酐官能基（TPO-g-MAH），並在4-二甲氨基吡啶（4-dimethylaminopyridine；DMAP）的催化下完成聚乳酸高分子的接枝反應。TPO-g-PLA的結構透過紅外線光譜儀（FT-IR）以及氫核磁共振光譜儀（1H-NMR）進行鑑定。在高溫以及高觸媒濃度的反應條件下，DMAP引發聚乳酸產生降解反應；立體障礙效應的減弱將使接枝反應的活性明顯提升。此接枝型共聚合物經過證實得以有效改善聚乳酸與熱塑性聚烯烴彈性體混摻材料（PLA/TPO blend）之相容性。聚乳酸均聚合物的抗拉韌性以及斷裂伸長量特性分別為2.3±0.5 (MJ/m3) 以及6.0±0.1 (%)。加入TPO後些微增加聚乳酸的抗拉韌性以及斷裂伸長量分別達4.6±1.0 (MJ/m3) 以及15.4±1.4 (%)。當加入TPO-g-PLA共聚合物，隨著改變不同化學構造的TPO-g-PLA與其組成比例，混摻材料的斷裂伸長量變化範圍由101.7±16.5 (%)到181.9±10.1 (%)，抗拉韌性則由24.7±3.8 (MJ/m3)到38.5±2.1 (MJ/m3)。關於馬來酸酐的接枝含量以及觸媒濃度對於TPO-g-PLA的相容效率以及PLA/TPO blend機械物性的影響，實驗結果顯示當觸媒濃度提升將導致混摻材料機械物性降低的現象，主要因為在低觸媒濃度條件下，TPO-g-PLA具有高分子量的PLA接枝鏈段，該側鏈聚乳酸鏈段得以與PLA主體產生深層的鏈糾纏，避免在受力的過程中自高分子的介面中剝離。最後，本研究所製備的TPO-g-PLA具有比TPO-MAH更加優異的相容效率，對於PLA/TPO blend機械物性的提升亦優於TPO-MAH。

第二部分是選擇poly(dimethyl siloxane)（PDMS）修飾聚乳酸高分子，以提升聚乳酸的拉伸斷裂伸長量。利用末端雙醇的PDMS巨起使劑透過開環聚合反應製備出以聚乳酸為外層鏈段的三嵌段共聚合物；隨後利用diisocyanate為鏈延伸劑提升整體分子量並形成多嵌段型共聚合物（multiblock copolymer）。嵌段共聚合物以FT-IR、1H-NMR以及凝膠滲透層析（GPC）進行結構鑑定與分析。引入PDMS鏈段的聚乳酸共聚合物斷裂伸長量大幅提升到310.4%，抗拉強度與抗拉模數分別降低至1.81（MPa）與0.002（GPa）。本研究並選擇FT-IR分析isocyanate官能基隨反應時間進行的變化，探討三嵌段共聚合物化學組成對於鏈延伸反應動力學的影響。結果顯示鏈延伸反應級數在triblock copolymer的分子量由7,000（g/mol.）增加到25,000（g/mol.），反應級數由二級改變成為三級，推論是因為在分子量的提升，將因為分子鏈的聚集與糾纏使得共聚合物末端的羥基官能基無法與觸媒金屬中心形成活化錯合物，因此原本反應控制（kinetic control）的反應過程轉變為擴散控制（diffusion control），導致二級反應無法描述整體反應的進行。關於縮合反應的活性，除了與末端官能基的本質特性相關之外，分子鏈的運動性同時影響官能基間的碰撞頻率與效率；在triblock copolymer中，聚乳酸鏈段的分子量是影響反應活性的重要因素，因為當PLLA鏈段分子量增加時，分子鏈的運動性的降低將導致反應速率減緩。

第三部分主要針對PLLA-PDMS-PLLA triblock copolymer的微相分離與結晶行為進行探討；首先利用熱示差掃瞄卡量計（DSC）以及動態機械分析儀（DMA）的分析結果確認聚乳酸-聚二甲基矽氧烷嵌段共聚合物具有微相分離的現象。PLLA-PDMS-PLLA triblock copolymer的熔融狀態結構以及經過等溫結晶後的微相分離結構透過穿透式電子顯微鏡（TEM）以及小角度X-ray光散射儀（SAXS）進行分析；改變嵌段共聚合物化學組成並提升嵌段共聚合物的排斥強度（χN）達239.5，則熔融狀態下的微相分離結構在經過等溫結晶熱處理之後得以保留。關於PDMS鏈段對於嵌段共聚合物中PLLA鏈段的結晶結構的影響，選擇FT-IR以及廣角度X-ray光散射儀（WAXS）進行分析：當聚乳酸為副組成（minor component）時，其FT-IR圖譜中可觀察1749 cm-1的吸收訊號，加上WAXS缺乏2θ = 24.5°的訊號得以證實即使在低結晶溫度條件下，PLLA的結晶結構僅以α-phase呈現，此現象主要是由於導入柔軟的PDMS鏈段將提升聚乳酸鏈段的運動性，因此即使在低結晶溫度的條件下（Tc< 90°C），高分子鏈段能夠迅速地運動與擴散，使PLLA結晶鏈段能夠排列出有序的結晶結構。嵌段共聚合物的結晶速率在導入PDMS後呈現降低的現象，而嵌段共聚合物的平衡熔點溫度隨著PLLA分子量增加而提升，因此在同樣的結晶溫度下，以低分子量PLLA組成的嵌段共聚合物將展現較低過冷程度（degree of undercooling），加上PDMS對於結晶速率的影響在PLLA鏈段提升的同時隨之降低，因此含有長聚乳酸鏈段的嵌段共聚合物具有高結晶速率。然而當聚乳酸成為副組成分散在PDMS主體時，將因為可結晶區域連續性遭到破壞，使得降低整體的結晶速率。

關鍵字

聚乳酸 ； 熱塑性聚烯烴彈性體 ； 聚二甲基矽氧烷 ； 微相分離 ； 韌化 ； 結晶動力學 ； 反應動力學

並列摘要

Polylactide (PLA) is an environmentally friendly polymer derived from biomasses, and has been emerged as an alternative to conventional petroleum-based polymeric materials. Although PLA is a high-strength and high-modulus polymer analogous to PET, its inherent brittleness and low toughness restrict the range of applications. The main purpose of this research is to modify the mechanical properties of PLA and enhance the toughness of PLA. This thesis comprises three sections.
In the first section, a thermoplastic polyolefin elastomer-graft- polylactide (TPO-g-PLA) was prepared by grafting polylactide onto maleic anhydride-functionalized TPO (TPO-g-MAH) in the presence of 4-dimethylaminopyridine (4-DMAP). The structures of the TPO-g-PLA copolymers were conducted by FT-IR and 1H-NMR. The effects of reaction temperature and concentration of 4-DMAP on the reactivity of graft polymerization were investigated by FT-IR, which revealed that a high reaction temperature and a high DMAP concentration are associated with dramatic depolymerization of PLA and reduction of steric hindrance effect in the graft reaction. Upon addition of the graft-type copolymers, acting as a premade compatibilizer, the compatibility of the PLA/TPO blend system was significantly improved. As the concentration of TPO-g-PLA copolymer increased, the tensile toughness and elongation at break increased with compatibilizer concentration up to 2.5 wt%, beyond which it declined. The effect of the chemical compositions of the TPO-g-PLA copolymers on the efficiency of compatibilization and mechanical properties of the PLA/TPO blends was examined by altering the number of grafting sites and concentration of 4-DMAP, suggesting that DMAP concentration dominated the properties of the ternary blend materials. Two compatibilizers, TPO-g-MAH and TPO-g-PLA, were used to compatibilize the PLA/TPO blends; the results suggested that TPO-g-PLA was more efficient in reducing the interfacial tension between the two immiscible polymers and in improving the mechanical properties of PLA/TPO blending specimens.
In the second section, the (AB)n-type multiblock copolymers containing poly(L-lactide) (PLLA) and poly(dimethyl siloxane) (PDMS) segments were synthesized by chain extension of the hydroxyl-telechelic PLLA-PDMS-PLLA triblock copolymers, which were prepared by the ring-opening polymerization of L-lactide initiated by α,ω-functionalized hydroxyl poly(dimethyl siloxane), using 1,6-hexamethylene diisocyanate as a chain extender. The triblock and the multiblock copolymers were characterized by FT-IR, 1H-NMR and GPC. The effect of the chemical composition of the triblock copolymers, including the molecular weight and the constitutive segment chain length of the macrodiol, on the development of the Mw of the multiblock was discussed based on diffusion effect. Furthermore, the consumption of the isocyanate groups was determined by FT-IR to investigate the dependence of the reaction kinetics of the urethane formation on the chemical composition of the triblock copolymer. The results reveal that the order of the chain extension reaction depended on the Mw of the triblock copolymer: a second order reaction was transformed into a third reaction as the Mw of the triblock copolymer increased from 7000 to 25,000 (g/mol) perhaps because of the inhibition of the formation of an active complex involved in the catalyzed-urethane reaction by the polymer chain aggregation. Finally, the mechanical properties of the multiblock copolymers demonstrated that the introduction of the extremely flexible PDMS segment substantially improved the elongation at breakage, and the tensile strength and the tensile modulus declined due to the intrinsic elasticity of such segments.
In the third section, the behaviors of microphase separation and the crystallization kinetics for triblock copolymers composed of PLLA and PDMS were characterized. From the results of thermal analysis, two glass transition temperatures which were measured by DSC showed the occurrence of phase separation phenomena in the PLLA-PDMS-PLLA triblock copolymers. And their the molten morphologies following isothermal crystallization of were characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The break-out and preservation of the nanostructure of the triblock copolymer depended on the segregation strength, which was manipulated by varying the degree of polymerization. The crystallization kinetics of these semicrystalline copolymers and the effect of isothermal crystallization on their melting behaviors were also studied using DSC, FT-IR and WAXS. The exclusive presence of α-phase PLLA crystallite was verified by identifying the absence of the WAXS diffraction signal at 2θ = 24.5° and the presence of IR absorption at 1749 cm-1 when the PLLA segment of the block copolymers was present as a minor component. The dependence of the crystallization rate (Rc) on the chemical composition of the triblock copolymers reveals that the Rc of the triblock copolymers was lower than that of PLLA homopolymer and the Rc were substantially reduced when the minor component of the crystallizable PLLA domains was dispersed in the PDMS matrix.

並列關鍵字

polylactide ； thermoplastic polyolefin elastomer ； poly(dimethyl siloxane) ； microphase separation ； toughening ； crystallization kinetics ； reaction kinetics

參考文獻

19. Shiomi T, Takeshita H, Kawaguchi H, Nagai M, Takenaka K, Miya M. Macromolecules 2002:35(21):8056-8065.

1. Mark JE, Allcock HR, Robert W. Inorganic Polymers. New York: Oxford University, 2005.

39. Yang JH, Zhang Y, Zhang YX. Polymer 2003:44(17):5047-5052.

7. Martin O, Averous L. Polymer 2001:42(14):6209-6219.

26. Ueda M, Sakurai K, Okamoto S, Lohse DJ, MacKnight WJ, Shinkai S, Sakurai S, Nomura S. Polymer 2003:44(22):6995-7005.