碳纳米管改性聚乳酸及其共混物的研究
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摘要
聚乳酸(Poly(L-lactide), PLLA)是一种可生物降解的环境友好材料,但其结晶性能差、脆性大,严重限制了PLLA的应用。目前围绕PLLA改性的研究中,解决PLLA结晶困难的问题仍然是众多研究者所关注的重点之一。另一方面,在现有增韧PLLA的研究中,韧性提高的同时强度和模量有很大的降低,使得综合性能的改善不甚理想。本文从改善结晶性能和提高韧性两方面入手,采用改性过的多壁碳纳米管(Functionalized multi-walled carbon nanotubes,f-MWCNTs)来诱导PLLA结晶,力求改善PLLA结晶的同时,还能对PLLA起到增强增韧的作用。在结晶方面,利用碳纳米管极大的长径比和比表面积特点,在PLLA冷却或退火热处理过程中起到异相成核的作用改善PLLA的结晶行为；在此基础上进一步研究了低分子量聚合物聚乙二醇(Polyethylene glycol, PEG)为增塑剂促进PLLA链段运动,碳纳米管为成核剂增大PLLA成核密度,两者在诱导PLLA结晶方面的协同效应；围绕PLLA脆性较低的缺陷,采用极性弹性体聚乙烯醋酸乙烯酯(Ethylene-co-vinyl acetate, EVA)为增韧剂,研究了增韧剂中醋酸乙烯酯单体(Vinyl acetate, VA)含量的变化对共混物微观形态和基体结晶行为的影响,及其宏观力学行为相关性；在此基础上有选择性地研究了碳纳米管对不相容共混物PLLA/EVA的增强增韧作用。全文从微观结构入手,对改性PLLA的结构和性能做了深入的研究,得到的主要研究结果如下：
(1)通过强酸酸化再接枝马来酸酐的方法,成功地在碳纳米管表面接枝上大量极性团能团,增强了碳纳米管与PLLA之间的界面相互作用,成功制备了分散优良的f-MWCNs改性PLLA纳米复合材料。
(2)无论是在不同的降温速度条件下,还是在不同的升温速度条件下以及退火条件下,少量的f-MWCNTs都对PLLA具有很强的异相成核作用,能够明显促进其冷结晶。纳米复合材料相对纯PLLA,结晶度增大,玻璃化转变温度升高。
(3) f-MWCNTs与PEG对PLLA的结晶具有协同促进作用。三元共混体系比二元共混体系结晶更容易,在相同结晶条件下得到的结晶度更大,晶体结构更完善。并且,PLLA结晶行为与碳纳米管含量有关：当碳纳米管含量<2 wt%时,碳纳米管起到异相成核的作用,促进PLLA的结晶,提高结晶温度和结晶度；当碳纳米管含量≥2 wt%时,碳纳米管在基体中形成物理网络结构,阻碍PLLA链段的运动,从而阻碍PLLA球晶的生长。
(4)EVA中VA的含量影响PLLA与EVA的相容性和界面相互作用,进而影响PLLA/EVA的形态和结晶性能。当VA含量≤28 wt%时,共混物形成以PLLA为基体,以EVA为分散相的海-岛形态；当VA含量>28 wt%时,共混物形成双连续相形态。而VA含量对PLLA/EVA的结晶性能的影响表现为：VA含量较低时,EVA与PLLA不相容,EVA分子链中的极性段VA诱导PLLA分子链在界面上异相成核,促进PLLA结晶；VA含量较高时,PLLA与EVA相容性非常好,大量的极性段VA的存在使PLLA分子链很难从EVA相中分离出来,阻碍了PLLA的成核和晶体生长。
(5)EVA的加入能够明显提高PLLA的冲击强度和断裂伸长率。而模量和拉伸强度的降低,可以通过退火处理得到补偿。
(6)改性MWCNT对不相容共混物PLLA/40EVA的结晶行为有明显的促进作用。力学性能测试表明,PLLA/40EVA/f-MWCNTs拉伸强度和模量随碳管含量增加而增加,断裂伸长率在碳管含量为2 wt%时达到最大,表明碳管在共混物中同时起到增强增韧的作用。
Poly (L-lactide) (PLLA) is a kind of biodegradable eco-friendly material. While poor crystallization ability and fracture toughness greatly limit the application of PLLA. Recently, improving its crystallization is still a critical problem for many researchers who are researching the modification of PLLA. On the other hand, many recent researches on PLLA toughening show that, although the toughness of PLLA can be greatly improved by elastomers and plasticizer, the tensile strength and modulus usually deteriorate to a certain degree. As a consequence, the comprehensive properties of PLLA and its blends are not very good. In the present work, functionalized multi-walled carbon nanotubes (f-MWCNTs) have been introduced into PLLA to improve the crystallization behavior. It has been proved that f-MWCNTs have a great influence on cold crystallization of PLLA due to the large length-diameter ratio and specific surface area. However, the melt-crystallization behavior of PLLA is still poor. Thus, low molecular polymer, i.e. polyethylene glycol (PEG) which usually acts as a plasticizer for other polymers, has been introduced into PLLA. As expected, PEG improves the mobility of PLLA chain segments greatly and largely decreased glass transition temperature has been reported. Consequently, there is a synergestic effect of PEG and f-MWCNTs on crystallization behavior of PLLA and such effect is greatly dependent upon the content of f-MWCNTs. In the last of the work, we attempt to imprve the toughness of PLLA through combination of elastomer and f-MWCNTs. Polar elastomer, ethylene-co-vinyl acetate (EVA), has been introduced into PLLA. Our attention is focused on the effect of monomer vinyl acetate (VA) content in EVA on phase morphologies, crystallization and mechanical properties of PLLA/EVA blends. Furthermore, f-MWCNTs has been introduced into PLLA/EVA blends, too. According to the previous reaserch, some interesting results have been achieved as shown in the following.
(1) f-MWCNTs have been prepared through the acidification of strong acid and graft modification with maleic anhydride (MAH). The interfcial interaction between PLLA and f-MWCNTs is greatly improved consequently. PLLA/f-MWCNTs nanocomposites with homogeneous distribution of f-MWCNTs have been prepared successfully.
(2) A few amount of f-MWCNTs exhibit great heterogeneous nucleation effect on crystallization of PLLA, improving its cold crystallization apparently in all conditions selected in this work. PLLA/f-MWCNTs nanocomposites exhibit higher glass transition temperature compared with pure PLLA due to their high degree of crystallinity.
(3) f-MWCNTs and PEG exhibit synergistic effect on PLLA crystallization. Compared with binary blends, ternary bends can crystallize more easily with higher degree of crystallinity and more perfect crystal structure. Furthermore, f-MWCNTs content can also affect the crystallize behavior of PLLA. When the content of f-MWCNTs is lower than 2 wt%, they exhibit heterogeneous nucleation effect for PLLA, improving the crystallization behavior of PLLA greatly. However, if the content of f-MWCNTs is over 2 wt%, percolated f-MWCNTs physical network structure forms in the material, which restricts the mobility of PLLA chain segment and prevents the growth of spherulites.
(4) VA content has influence on the miscibility and interfacial interaction between EVA and PLLA, which further affects the morphologies and crystallization behaviors of PLLA/EVA blends. When VA content is lower than 28 wt%, the blends exhibit typical sea-island morphology features; whereas when VA content is over 28 wt%, the blend exhibit cocontinuous morphology feature. Meanwhile, VA content influences the crystallization of PLLA/EVA. At lower VA content, EVA and PLLA are immiscible. Polar VA segments in EVA promote the mobility of PLLA chain segments and induce heterogeneous nucleation of PLLA at the interface, leading to the improvement of PLLA crystallization. At relatively high VA content, the miscibility of PLLA and EVA is very good. The presence of large amounts of VA segments prevents the phase separation of PLLA chain segments from EVA domains in the interface, preventing the nucleation and spherulites growth of PLLA consequently.
(5) With the addition of EVA, the ductility and fracture toughness are enhanced. The deteriorated tensile modulus and strength can be compensated by annealing.
(6) The crystallization behavior of PLLA in PLLA/40EVA is greatly improved by addition of functionalized MWCNTs. The tensile strength and tensile modulus of PLLA/40EVA/f-MWCNTs nanocomposites increase with the increasing of f-MWCNTs contents. The maximum elongation at break is achieved when 2 wt% f-MWCNTs are present in the nanocomposites, indicating the reinforcement and toughening effects of f-MWCNTs for immiscible PP/40EVA blend.
(1)通过强酸酸化再接枝马来酸酐的方法,成功地在碳纳米管表面接枝上大量极性团能团,增强了碳纳米管与PLLA之间的界面相互作用,成功制备了分散优良的f-MWCNs改性PLLA纳米复合材料。
(2)无论是在不同的降温速度条件下,还是在不同的升温速度条件下以及退火条件下,少量的f-MWCNTs都对PLLA具有很强的异相成核作用,能够明显促进其冷结晶。纳米复合材料相对纯PLLA,结晶度增大,玻璃化转变温度升高。
(3) f-MWCNTs与PEG对PLLA的结晶具有协同促进作用。三元共混体系比二元共混体系结晶更容易,在相同结晶条件下得到的结晶度更大,晶体结构更完善。并且,PLLA结晶行为与碳纳米管含量有关：当碳纳米管含量<2 wt%时,碳纳米管起到异相成核的作用,促进PLLA的结晶,提高结晶温度和结晶度；当碳纳米管含量≥2 wt%时,碳纳米管在基体中形成物理网络结构,阻碍PLLA链段的运动,从而阻碍PLLA球晶的生长。
(4)EVA中VA的含量影响PLLA与EVA的相容性和界面相互作用,进而影响PLLA/EVA的形态和结晶性能。当VA含量≤28 wt%时,共混物形成以PLLA为基体,以EVA为分散相的海-岛形态；当VA含量>28 wt%时,共混物形成双连续相形态。而VA含量对PLLA/EVA的结晶性能的影响表现为：VA含量较低时,EVA与PLLA不相容,EVA分子链中的极性段VA诱导PLLA分子链在界面上异相成核,促进PLLA结晶；VA含量较高时,PLLA与EVA相容性非常好,大量的极性段VA的存在使PLLA分子链很难从EVA相中分离出来,阻碍了PLLA的成核和晶体生长。
(5)EVA的加入能够明显提高PLLA的冲击强度和断裂伸长率。而模量和拉伸强度的降低,可以通过退火处理得到补偿。
(6)改性MWCNT对不相容共混物PLLA/40EVA的结晶行为有明显的促进作用。力学性能测试表明,PLLA/40EVA/f-MWCNTs拉伸强度和模量随碳管含量增加而增加,断裂伸长率在碳管含量为2 wt%时达到最大,表明碳管在共混物中同时起到增强增韧的作用。
Poly (L-lactide) (PLLA) is a kind of biodegradable eco-friendly material. While poor crystallization ability and fracture toughness greatly limit the application of PLLA. Recently, improving its crystallization is still a critical problem for many researchers who are researching the modification of PLLA. On the other hand, many recent researches on PLLA toughening show that, although the toughness of PLLA can be greatly improved by elastomers and plasticizer, the tensile strength and modulus usually deteriorate to a certain degree. As a consequence, the comprehensive properties of PLLA and its blends are not very good. In the present work, functionalized multi-walled carbon nanotubes (f-MWCNTs) have been introduced into PLLA to improve the crystallization behavior. It has been proved that f-MWCNTs have a great influence on cold crystallization of PLLA due to the large length-diameter ratio and specific surface area. However, the melt-crystallization behavior of PLLA is still poor. Thus, low molecular polymer, i.e. polyethylene glycol (PEG) which usually acts as a plasticizer for other polymers, has been introduced into PLLA. As expected, PEG improves the mobility of PLLA chain segments greatly and largely decreased glass transition temperature has been reported. Consequently, there is a synergestic effect of PEG and f-MWCNTs on crystallization behavior of PLLA and such effect is greatly dependent upon the content of f-MWCNTs. In the last of the work, we attempt to imprve the toughness of PLLA through combination of elastomer and f-MWCNTs. Polar elastomer, ethylene-co-vinyl acetate (EVA), has been introduced into PLLA. Our attention is focused on the effect of monomer vinyl acetate (VA) content in EVA on phase morphologies, crystallization and mechanical properties of PLLA/EVA blends. Furthermore, f-MWCNTs has been introduced into PLLA/EVA blends, too. According to the previous reaserch, some interesting results have been achieved as shown in the following.
(1) f-MWCNTs have been prepared through the acidification of strong acid and graft modification with maleic anhydride (MAH). The interfcial interaction between PLLA and f-MWCNTs is greatly improved consequently. PLLA/f-MWCNTs nanocomposites with homogeneous distribution of f-MWCNTs have been prepared successfully.
(2) A few amount of f-MWCNTs exhibit great heterogeneous nucleation effect on crystallization of PLLA, improving its cold crystallization apparently in all conditions selected in this work. PLLA/f-MWCNTs nanocomposites exhibit higher glass transition temperature compared with pure PLLA due to their high degree of crystallinity.
(3) f-MWCNTs and PEG exhibit synergistic effect on PLLA crystallization. Compared with binary blends, ternary bends can crystallize more easily with higher degree of crystallinity and more perfect crystal structure. Furthermore, f-MWCNTs content can also affect the crystallize behavior of PLLA. When the content of f-MWCNTs is lower than 2 wt%, they exhibit heterogeneous nucleation effect for PLLA, improving the crystallization behavior of PLLA greatly. However, if the content of f-MWCNTs is over 2 wt%, percolated f-MWCNTs physical network structure forms in the material, which restricts the mobility of PLLA chain segment and prevents the growth of spherulites.
(4) VA content has influence on the miscibility and interfacial interaction between EVA and PLLA, which further affects the morphologies and crystallization behaviors of PLLA/EVA blends. When VA content is lower than 28 wt%, the blends exhibit typical sea-island morphology features; whereas when VA content is over 28 wt%, the blend exhibit cocontinuous morphology feature. Meanwhile, VA content influences the crystallization of PLLA/EVA. At lower VA content, EVA and PLLA are immiscible. Polar VA segments in EVA promote the mobility of PLLA chain segments and induce heterogeneous nucleation of PLLA at the interface, leading to the improvement of PLLA crystallization. At relatively high VA content, the miscibility of PLLA and EVA is very good. The presence of large amounts of VA segments prevents the phase separation of PLLA chain segments from EVA domains in the interface, preventing the nucleation and spherulites growth of PLLA consequently.
(5) With the addition of EVA, the ductility and fracture toughness are enhanced. The deteriorated tensile modulus and strength can be compensated by annealing.
(6) The crystallization behavior of PLLA in PLLA/40EVA is greatly improved by addition of functionalized MWCNTs. The tensile strength and tensile modulus of PLLA/40EVA/f-MWCNTs nanocomposites increase with the increasing of f-MWCNTs contents. The maximum elongation at break is achieved when 2 wt% f-MWCNTs are present in the nanocomposites, indicating the reinforcement and toughening effects of f-MWCNTs for immiscible PP/40EVA blend.
引文
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