纸质出版日期:2019-8,
网络出版日期:2019-4-18,
收稿日期:2019-1-29,
修回日期:2019-3-2
扫 描 看 全 文
引用本文
阅读全文PDF
借助量热学方法研究了低分子量聚乳酸(PLLA)受限于阳极氧化铝(AAO)模板中的玻璃化转变、结晶和熔融行为. 实验结果表明,相比于本体状态,PLLA受限于AAO纳米孔道中的降温结晶行为受到明显的抑制,结晶焓随着孔道孔径的减小而逐渐降低. 大孔径AAO孔道中PLLA纳米棒与本体相近的结晶温度表明其成核过程仍为异相成核所主导. 与此同时,非等温结晶动力学实验结果显示纳米孔道中PLLA成核速率的温度依赖性弱于本体状态. 当AAO孔径小于28 nm时,PLLA纳米棒则观测不到降温结晶峰的存在. PLLA纳米棒的玻璃态呈现出双重玻璃化转变温度(Tg)的行为,较高的Tg对应于邻近孔壁界面吸附层内的高分子链,较低的Tg归属于孔道中心的高分子链,两者表现出截然相反的孔径依赖性. 在升温过程中,PLLA纳米棒存在显著的冷结晶行为,其结晶发生在较高的温度下且冷结晶峰较宽. 这一现象可归咎于PLLA在纳米孔道内成核速率的改变,成核活性和分布的不均匀性,以及孔道内分子链运动性的差异性. 由于界面吸附层的存在,PLLA从玻璃态升温过程中的表面诱导成核受到抑制. 结合非等温结晶动力学实验结果,发现界面吸附层和孔道中心的PLLA的冷结晶过程彼此相互独立,后者在较高的温度下进行. 此外,随着AAO纳米孔道孔径的减小,界面效应逐渐变得显著,生成的PLLA晶体稳定性较差,熔融重结晶现象较明显,同时其结晶度和熔点逐渐降低.
The glass transition, crystallization, and melting behaviors of oligomer poly(L-lactide) (PLLA) confined in anodic aluminum oxide (AAO) nanopores were invesitgated by calorimetry. Compared with the bulk counterpart, PLLA located inside AAO nanopores showed frustrated crystallization during the cooling process, and the crystallization enthalpy gradually decreased with the reduction of pore size. In large nanopores, the crystallization peaks of PLLA nanorods were very close to that of bulk sample, which indicated the predomination of heterogeneous nucleation. Meanwhile, the nonisothermal crystallization results displayed that temperature dependence of nucleation rate of PLLA in nanopores was weaker than that in bulk state. As the diameter of nanopore was smaller than 28 nm, the crystallization peak disappeared. The glass state of PLLA nanorods exhibited double glass transition temperatures (Tgs), the higher Tg attributed to chains in the interfacial adsorbed layer adjacent to pore walls, and the lower Tg belonged to chains in the pore center. The two Tgs showed opposite pore size dependences—the lower one decreased with the reduction of pore size, while the higher one increased. During the heating process, PLLA confined in nanopores showed the pronounced cold crystallization phenomenon, which took place at higher temperatures and the peak was much broader than that of bulk state, which could be ascribed to the supressed nucleation rate, the poor nucleation activity, and the broad nucleation dispersion in PLLA nanorods. Apart form the influence of nucleation, hetergeneous chain mobilities in nanopores also played an important role. Due to the existence of adsorbed layer, surface induced nucleation was hindered. Interestingly, by the nonisothermal crystallization experiments, PLLA chains in the interfacial layer and pore center displayed independent cold crystallization behaviors, and the latter happened at the higher temperatures. Finally, the interfacial effect gradually dominated as the pore size decreased. PLLA crystals formed in small nanopores became unstable, obvious melting-recrystallization phenomena occurred, and the crystallinity and melting temperature of PLLA crystals were lower in smaller nanopores.
The glass transition, crystallization, and melting behaviors of oligomer poly(L-lactide) (PLLA) confined in AAO nanopores were investigated by calorimetry. The PLLA nanorods revealed the frustrated melt-crystallization, heterogeneous chain segmental dynamics, and pronounced and broad cold-crystallization at higher temperatures. Our researches would benefit the control of physical properties and performances of polymer materials with nanostructures.
随着高分子材料在微纳米领域的广泛应用,聚合物受限于微纳尺度下的凝聚态结构、相转变行为、力学性能、抗老化性能等受到了广泛的关注. 人们研究发现聚合物在受限环境下的众多物理性质与本体状态存在着显著差别[
近十多年来,半结晶高分子受限于二维AAO纳米孔道中的结晶行为引起了众多研究者的关注,其中包括结晶的成核方式、结晶动力学、结晶度、晶体的熔点、结构取向和晶型转变等[
前人关于二维纳米孔道中聚合物的结晶主要集中在高分子量的半结晶型高分子,且通常聚合物的结晶速率较快,在降温过程中即可明显观察到聚合物成核机理和结晶速率的改变. 低分子量聚合物和较慢结晶速率的半结晶型聚合物受限于AAO纳米孔道中的结晶行为研究得较少. 早期,我们在低分子量无定型聚合物如聚苯乙烯(PS)、聚甲基丙烯酸甲酯(PMMA)等受限于AAO纳米孔道中的玻璃化转变的研究发现,孔道内聚合物的玻璃态会表现出多重玻璃化转变的行为[
聚L-乳酸(PLLA),Mn = 5 kg mol−1,PDI ≤ 1.2,购自Sigma-Aldrich公司. 3种不同孔径的AAO模板均购于合肥的普元纳米科技有限公司,其孔径大小经SEM表征确认,分别为380、95和28 nm,模板厚度为60 ~ 100 μm. 溶剂四氢呋喃、二氯甲烷和甲醇均为分析纯,购自国药集团化学试剂公司,未经纯化直接使用.
商用AAO模板使用前需依次在二氯甲烷、四氢呋喃和甲醇溶剂中超声清洗,然后再置于150 °C的真空烘箱中充分干燥. PLLA纳米棒受限于AAO模板的复合物样品的制备流程图如
Fig 1 (a) Schematic illustration of the sample preparation, (b − d) SEM images of PLLA (Mn = 5 kg mol–1) confined in AAO templates with different pore diameters: (b) 380 nm, (c) 95 nm and (d) 28 nm
利用Hitachi公司的电子扫描电镜(SEM) S-4800对所制备的PLLA-AAO复合物样品的表面形貌进行了测试,结果如上
本实验中,PLLA本体及其受限于AAO中的结晶熔融和玻璃化转变行为检测在Mettler Toledo公司的DSC 1 STARe上完成. 所有DSC测试前,先利用金属铟对仪器进行了温度校正. PLLA本体样品取样约4 mg,PLLA-AAO复合物样品称重10 mg左右. 所有样品的DSC测试在氮气氛下进行,测试的温度程序如下:首先,将制备的样品升温至200 °C并恒温2 min以消除热历史;然后,将熔融的PLLA样品以10 °C min− 1的速率降温至0 °C并恒温2 min;最后,将样品以10 °C min− 1的速率升温至200 °C. 此外,为获取PLLA-AAO复合物样品中高分子的含量,对其进行了热重(TGA)测试. TGA测试在Perkin Elmer公司的TGA 8000上进行,载气为氮气,温度范围为30 ~ 700 °C,升温速率10 °C min–1.
众所周知,基于纳米受限效应,高分子材料在纳米尺度下常表现出异于本体状态的物理现象. 本实验采用量热学的方法考察了低分子量的PLLA纳米棒受限于不同孔径的AAO模板中的玻璃化转变、结晶和熔融等相转变行为,探索了受限尺寸效应对这些相转变行为的影响.
Fig 2 (a) DSC cooling curves and (b) subseqent heating curves of bulk PLLA and PLLA confined in AAO nanopores with different pore diameters. Both of the cooling and heating rates are 10 °C min–1. The shaded areas mark the crystallization and cold-crystallization peaks. The inset in (b) shows the enlarged glass transition region.
Tc(°C) | ΔHc(J g−1) | Tg(°C) | ΔCp(J g−1 K−1) | Tcc(°C) | ΔHcc(J g−1) | Tm(°C) | ΔHm(J g−1) | |
---|---|---|---|---|---|---|---|---|
Bulk PLLA | 98.9 | 36.9 | 52.4 | 0.253 | 91.9 | 16.7 | 146.6, 158.9 | 63.7 |
380 nm | 96.5 | 21.7 | 45.2, 66.1 | 0.351, 0.102 | 115.9 | 30.4 | 146.2, 157.9 | 52.0 |
95 nm | 90.2 | 7.93 | 38.4, – | 0.363, – | 117.6 | 15.2 | 142.6, 154.8 | 23.4 |
28 nm | – | – | 34.8, 82.0 | 0.330, 0.188 | 112.2 | 0.84 | 141.3, 153.4 | 2.82 |
与本体状态不同,受限于AAO模板中的PLLA纳米棒彼此独立,PLLA-AAO复合物的结晶过程应为其中各PLLA纳米棒结晶的独立事件的加和. Floudas等认为,通过对比聚合物本体中单位体积内异相成核位点数与单位体积内聚合物纳米棒的数目,可简单估算出受限于纳米孔道中聚合物纳米棒从熔体降温过程中的结晶行为. 当单位体积内聚合物纳米棒的数目与异相成核位点数相当时,聚合物纳米棒主要以异相成核的方式在较小的过冷度下发生结晶;而当单位体积内聚合物纳米棒的数目远大于异相成核位点数时,聚合物纳米棒将以均相成核的方式在较大的过冷度下结晶[
Fig 3 Pore diameter dependences of some physical properties associated with the (a) crystallization, (b) glass transition, (c) cold-crystallization and (d) melting processes of PLLA confined in AAO nanopores
首先,与我们之前在低分子量的无定形高分子材料(如聚苯乙烯PS[
Fig 4 Schematic illustrations of (a) chain mobility distribution of PLLA located inside AAO nanopores, and (b) the diversity of nuclei formation of PLLA nanodomains cooled from melt state
其次,PLLA受限于AAO孔道中的冷结晶行为表现出了显著的本体差异性. 如
最后,PLLA晶体的熔融表现为双熔融峰,受限于AAO孔道中PLLA纳米棒的双峰形态较为明显,且随着AAO孔径的减小,较低的熔融峰Tm,1呈逐渐增大的趋势. 基于熔融-重结晶的机理,较低的熔融峰应归属于降温结晶和升温冷结晶过程中生成的稳定性较差的晶体的熔融,其在升温过程中会发生熔融-重结晶行为而生成更稳定的晶体并在更高的温度Tm,2下发生熔融. 如
聚合物的结晶是由成核和片晶生长控制的动力学过程,总结晶速率由成核速率和片晶生长速率相互竞争而共同决定. 为更系统研究PLLA受限于AAO孔道中的结晶行为产生本体差异性的根源,我们改变了PLLA从熔体降温的降温速率Qc,考察了降温速率对降温过程中结晶峰以及再升温过程中玻璃化转变、冷结晶和熔融峰的影响. 如
Fig 5 Normalized DSC cooling curves of (a) bulk PLLA and PLLA located inside AAO nanopores with diameters of (b) 380 nm and (c) 95 nm cooled from melt state under different cooling rates from 50 °C min–1 to 1 °C min–1. The subseqent heating curves after the varied cooling rates of (d) bulk PLLA and PLLA loacted inside AAO nanopores with diameters of (e) 380 nm and (f) 95 nm. The heating rate is 10 °C min–1.
Fig 6 Cooling rate dependences of crystallization temperature (Tc) and crystallization enthalpy (ΔHc) of bulk PLLA and PLLA confined in 380 nm AAO nanopores
PLLA本体及其受限于大小孔径AAO纳米孔道中的样品在经过不同降温速率后的再升温曲线如
Fig 7 Cooling rate dependences of (a) cold crystallization temperature (Tcc), (b) melting enthalpy (ΔHm), and the ratio of cold crystallization and (c) melting enthalpies (ΔHcc/ΔHm) of bulk PLLA and PLLA confined in 380 and 95 nm AAO nanopores
本文以阳极氧化铝AAO模板为受限介质,利用量热学的方法研究了低分子量聚乳酸PLLA受限于二维纳米孔道中的结晶熔融行为. PLLA受限于AAO孔道中的降温结晶、玻璃化转变以及升温冷结晶和熔融行为均展现出明显的本体差异性. 相比于本体状态,PLLA受限于AAO纳米孔道中的结晶存在2个主要特征:一是AAO模板中PLLA的结晶为所有纳米棒独立结晶事件的总和,纳米棒数量的增加及尺寸的减小对样品的成核行为影响较大;二是受限于AAO纳米孔道中的界面效应较为显著,具体表现在孔道内PLLA分子链段运动性的改变,进而对结晶行为的影响.
通过改变AAO孔道的尺寸,考察了尺寸效应对PLLA二维纳米受限结晶行为的影响. PLLA受限于AAO纳米孔道中的降温结晶受到抑制,结晶焓随着孔道孔径的减小而逐渐降低. 在大孔径AAO纳米孔道中,PLLA纳米棒的结晶为异相成核所引发,发生较低的过冷度下;在小孔径AAO纳米孔道中,PLLA纳米棒中异相成核位点较少,降温过程中基本观察不到结晶峰,再升温过程中,孔道内PLLA纳米棒的玻璃态呈现出双重Tg的行为,且两者表现出截然相反的孔径依赖性. 较高的Tg对应于临界孔壁的界面层内高分子链,而较低的Tg归属于孔道中心的高分子链. PLLA纳米棒再升温过程中的冷结晶较为明显,冷结晶峰较宽且冷结晶温度远高于本体. 非等温结晶动力学实验结果表明,这与降温过程中PLLA纳米棒内成核的数量与活性的多分散性以及孔道内分子链运动性分布的展宽相关. 从高温熔体逐渐降温过程中,受限于AAO纳米孔道中的PLLA成核的速率低于本体,晶核数量较少,生成晶核的活性亦较低. 此外,除了各PLLA纳米棒中成核分布不均匀外,AAO孔道中从界面至中心分子链运动性的差异也会导致孔道内部成核的差异性,孔道中心相对于孔壁界面层较难成核,其冷结晶发生在较高温度下. PLLA受限于AAO孔道中的结晶度和熔点随着孔径的减小而逐渐降低,小孔径AAO中生成的片晶稳定性较差,升温过程中易发生熔融重结晶现象. 相关研究揭示了聚合物在二维纳米孔道受限态下的结晶行为本体差异性的根源,有助于对纳米孔道内聚合物凝聚态结构和性能的调控.
Keddie J L, Jones R A L, Cory R A. Faraday Discuss , 1994 . 98 219 - 230 . DOI:10.1039/fd9949800219 . [百度学术]
Chai Y, Salez T, McGraw J D, Benzaquen M, Dalnoki-Veress K, Raphael E, Forrest J A. Science , 2014 . 343 ( 6174 ): 994 - 999 . DOI:10.1126/science.1244845 . [百度学术]
Fakhraai Z, Forrest J A. Science , 2008 . 319 ( 5863 ): 600 - 604 . DOI:10.1126/science.1151205 . [百度学术]
O'Connell P A, McKenna G B. Science , 2005 . 307 ( 5716 ): 1760 - 1763 . DOI:10.1126/science.1105658 . [百度学术]
Yang Z H, Fujii Y, Lee F K, Lam C H, Tsui O K C. Science , 2010 . 328 ( 5986 ): 1676 - 1679 . DOI:10.1126/science.1184394 . [百度学术]
Jones R L, Kumar S K, Ho D L, Briber R M, Russell T P. Nature , 1999 . 400 ( 6740 ): 146 - 149 . DOI:10.1038/22080 . [百度学术]
Rowland H D, King W P, Pethica J B, Cross G L W. Science , 2008 . 322 ( 5902 ): 720 - 724 . DOI:10.1126/science.1157945 . [百度学术]
Priestley R D, Ellison C J, Broadbelt L J, Torkelson J M. Science , 2005 . 309 ( 5733 ): 456 - 459 . DOI:10.1126/science.1112217 . [百度学术]
Yang Rong(杨榕), Li Hongmei(李红梅), Jiang Jing(姜菁), Zhou Dongshan(周东山). Acta Polymerica Sinica(高分子学报) , 2018 . ( 9 ): 1228 - 1235 . DOI:10.11777/j.issn1000-3304.2018.18024 . [百度学术]
Mijangos C, Hernández R, Martín J. Prog Polym Sci , 2016 . 54-55 148 - 182 . DOI:10.1016/j.progpolymsci.2015.10.003 . [百度学术]
Michell R M, Mueller A J. Prog Polym Sci , 2016 . 54-55 183 - 213 . DOI:10.1016/j.progpolymsci.2015.10.007 . [百度学术]
Mary Michell R, Blaszczyk-Lezak I, Mijangos C, Mueller A J. J Polym Sci, Part B: Polym Phys , 2014 . 52 ( 18 ): 1179 - 1194 . DOI:10.1002/polb.23553 . [百度学术]
Michell R M, Blaszczyk-Lezak I, Mijangos C, Mueller A J. Polymer , 2013 . 54 ( 16 ): 4059 - 4077 . DOI:10.1016/j.polymer.2013.05.029 . [百度学术]
Duran H, Steinhart M, Butt H J, Floudas G. Nano Lett , 2011 . 11 ( 4 ): 1671 - 1675 . DOI:10.1021/nl200153c . [百度学术]
Suzuki Y, Duran H, Steinhart M, Butt H J, Floudas G. Soft Matter , 2013 . 9 ( 9 ): 2621 - 2628 . DOI:10.1039/c2sm27618f . [百度学术]
Suzuki Y, Duran H, Akram W, Steinhart M, Floudas G, Butt H J. Soft Matter , 2013 . 9 ( 38 ): 9189 - 9198 . DOI:10.1039/c3sm50907a . [百度学术]
Shi G Y, Liu G M, Su C, Chen H M, Chen Y, Su Y L, Muller A J, Wang D J. Macromolecules , 2017 . 50 ( 22 ): 9015 - 9023 . DOI:10.1021/acs.macromol.7b02284 . [百度学术]
Li L L, Liu J W, Qin L L, Zhang C, Sha Y, Jiang J, Wang X L, Chen W, Xue G, Zhou D S. Polymer , 2017 . 110 273 - 283 . DOI:10.1016/j.polymer.2016.12.081 . [百度学术]
Guan Y, Liu G M, Ding G Q, Yang T Y, Mueller A J, Wang D J. Macromolecules , 2015 . 48 ( 8 ): 2526 - 2533 . DOI:10.1021/acs.macromol.5b00108 . [百度学术]
Steinhart M, Goring P, Dernaika H, Prabhukaran M, Gosele U, Hempel E, Thurn-Albrecht T. Phys Rev Lett , 2006 . 97 ( 2 ): 027801 DOI:10.1103/PhysRevLett.97.027801 . [百度学术]
Guan Y, Liu G M, Gao P Y, Li L, Ding G Q, Wang D J. ACS Macro Lett , 2013 . 2 ( 3 ): 181 - 184 . DOI:10.1021/mz300592v . [百度学术]
Wu H, Wang W, Huang Y, Su Z H. Macromol Rapid Commun , 2009 . 30 ( 3 ): 194 - 198 . DOI:10.1002/marc.v30:3 . [百度学术]
Wu Hui(吴慧), Wang Wei(王巍), Su Zhaohui(苏朝晖) . 2009 . ( 5 ): 425 - 429. [百度学术]
Su Cui(苏萃), Shi Guangyu(施光宇), Wang Dujin(王笃金), Liu Guoming(刘国明) . 2019 . 50 ( 3 ): 281 - 290. [百度学术]
Su C, Shi G Y, Li X L, Zhang X Q, Mueller A J, Wang D J, Liu G M. Macromolecules , 2018 . 51 ( 23 ): 9484 - 9493 . DOI:10.1021/acs.macromol.8b01801 . [百度学术]
Yao Y, Sakai T, Steinhart M, Butt H J, Floudas G. Macromolecules , 2016 . 49 ( 16 ): 5945 - 5954 . DOI:10.1021/acs.macromol.6b01406 . [百度学术]
Shin K, Woo E, Jeong Y G, Kim C, Huh J, Kim K W. Macromolecules , 2007 . 40 ( 18 ): 6617 - 6623 . DOI:10.1021/ma070994e . [百度学术]
Sun X L, Fang Q Q, Li H H, Ren Z J, Yang S K. Langmuir , 2016 . 32 ( 13 ): 3269 - 3275 . DOI:10.1021/acs.langmuir.6b00251 . [百度学术]
Mi C, Zhou J D, Ren Z J, Li H H, Sun X L, Yan S K. Polym Chem , 2016 . 7 ( 2 ): 410 - 417 . DOI:10.1039/C5PY01532D . [百度学术]
Li L L, Zhou D S, Huang D H, Xue G. Macromolecules , 2014 . 47 ( 1 ): 297 - 303 . DOI:10.1021/ma4020017 . [百度学术]
Li L L, Chen J, Deng W J, Zhang C, Sha Y, Cheng Z, Xue G, Zhou D S. J Phys Chem B , 2015 . 119 ( 15 ): 5047 - 5054 . DOI:10.1021/jp511248q . [百度学术]
Teng C, Li L L, Wang Y, Wang R, Chen W, Wang X L, Xue G. J Chem Phys , 2017 . 146 ( 20 ): 203319 DOI:10.1063/1.4978230 . [百度学术]
Zhang C, Li L L, Wang X L, Xue G. Macromolecules , 2017 . 50 ( 4 ): 1599 - 1609 . DOI:10.1021/acs.macromol.6b02469 . [百度学术]
Vanroy B, Wubbenhorst M, Napolitano S. ACS Macro Lett , 2013 . 2 ( 2 ): 168 - 172 . DOI:10.1021/mz300641x . [百度学术]
74
浏览量
32
下载量
0
CSCD
相关文章
相关作者
相关机构