纸质出版日期:2020-2,
网络出版日期:2019-9-27,
收稿日期:2019-7-31,
修回日期:2019-8-23
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基于氢键作用和Diels-Alder (DA)交联的双可逆网络,制备了一种高性能、高修复效率的自修复弹性体. 首先使用异氰酸正丁酯和N-(2-羟乙基)丙烯酰胺合成氨基甲酸酯小分子(简称为HM),然后将HM、丙烯酸丁酯(BA)及甲基丙烯酸糠酯(FMA)共聚得到线形共聚物,最后用双马来酰亚胺(BMI)通过DA反应交联线形共聚物,得到既有氢键交联,又有DA键交联的双网络自修复弹性体. 当受到外力时,键能较低的氢键先断裂消耗能量,使得材料的韧性提高了6.2倍,断裂强度提高了12.3倍;而DA键则赋予了材料较高的弹性和形状回复能力. 除此之外,两重网络均为可逆网络,使材料的自修复效率高达98%.
Intrinsic self-healing elastomers, which can automatically heal themselves after damage without the addition of other reagents, have recently attracted increasing attention. However, a trade-off commonly exists between high mechanical properties and high self-healing efficiency, which is always the bottle-neck in advancing these high performance self-healing elastomers. To solve this problem, a high performance and high self-healing efficiency elastomer was developed in this work based on hydrogen bonds and Diles-Alder (DA) crosslinks. Firstly, a monomer (HM) functionalized with amido bond and carbamic acid ester for the generation of hydrogen bonds was synthesized byN-butyl isocyanate and N-(2-hydroxyethyl)acrylamide. Next, one-pot free-radical copolymerization of HM, butyl acrylate (BA), and furfuryl methacrylate (FMA) was carried out to afford a linear copolymer, which was only cross-linked with hydrogen bonds. Finally, bismaleimide (BMI) was used to crosslink the linear copolymer through DA reaction. A double network self-healing elastomer with two kinds of crosslinks, i.e. hydrogen bonds and DA bonds, was thus prepared. The heating-up and cooling down FTIR spectroscopy was used to characterize the hydrogen bonds, while the existence of DA bonds was proved by FTIR, DSC, and DMA techniques. When an external force was applied, the hydrogen bonds broke firstly to dissipate energy, which helped to increase the toughness by about 6.2 times, the tensile strength by about 12.3 times, and Young’s modulus of the elastomer by about 26 times. Meanwhile, DA crosslinks endowed the elastomer with certain elasticity and the capability of fast shape recovery. Moreover, thanks to the reversible ability of hydrogen bonds and DA crosslinks, the elastomer exhibited a high self-healing efficiency up to 98%.
A self-healing elastomer with high mechanical properties was fabricated by introducing two reversible networks of hydrogen bond and DA crosslinks.
弹性体由于具有较大的延展性和弹性,被广泛应用于航天航空、军事工业以及生活中的方方面面. 但迄今为止,大部分弹性体都是通过不可逆共价键进行交联的,导致材料不具有自修复能力,并且在失效以后不能被回收利用. 这不仅降低了弹性体的使用寿命和安全性,还造成了大量的资源浪费和严重的环境污染,所以亟需开发自修复弹性体. 本征型自修复弹性体是指在无需添加修复剂的情况下,材料能够利用分子网络中动态键实现自修复,并且自修复可以重复多次[
本文结合多重氢键和DA键合成了一种高修复效率的自修复弹性体,虽然单一氢键较弱,但当聚合物链中的多重氢键同时交联作用时则会产生较大的结合能,从而赋予材料较高的力学性能. 当受到外力作用时,氢键首先断裂耗散能量,提高材料的韧性和机械强度;撤去外力后,具有较高键能的DA键使得材料能快速回复到原始形状,保证材料的弹性和稳定性. 同时,在一定温度下,断裂的DA键和氢键分别重新结合成键,赋予材料较高的自修复性能. 因此这种含有双重可逆网络结构的弹性体的自修复效率高达98.6%,并且断裂强度达3.7 MPa.
丙烯酸丁酯(BA,> 99.0%,TCI,添加了对羟基苯甲醚(MEHQ)作为稳定剂),甲基丙烯酸糠酯(FMA,> 98%,Adams),双马来酰亚胺(BMI,95%,Sigma),异氰酸正丁酯,N-(2-羟乙基)丙烯酰胺,二月桂酸二丁基锡(DBT),乙酸乙酯,甲醇,石油醚,四氢呋喃(THF)购自Tansoole. com;偶氮二异丁腈(AIBN)购自Kermel. 除了BA用碱性氧化铝柱除去稳定剂以外,所使用的反应物未进行纯化.
利用羟基和异氰酸酯基发生反应制备氢键单体. 具体实验过程如下:将10 mL (0.0965 mol) N-(2-羟乙基)丙烯酰胺和15 mL (0.1144 mol)异氰酸正丁酯加入茄形瓶中,开启搅拌使2种反应物混合均匀. 然后将茄形瓶置于冰浴氛围中,再加入2 mL催化剂二月桂酸二丁基锡溶液(0.2 mL二月桂酸二丁基锡溶解在14.5 mL THF中),保持快速搅拌,反应5 h左右. 反应完成后,将产物用THF冲洗抽滤3次,然后在30 °C下,在真空烘箱中烘干,就得到聚合反应所需单体,即含氢键的小分子,我们称之为HM,其结构和核磁表征如
Fig 1 1H-NMR spectra of butyl isocyanate (a), N-(2-hydroxyethyl)acrylamide (b) and HM (c)
利用HM、BA、FMA上烯烃的自由基聚合反应,得到线形共聚物. 其具体实施过程如下:将22.30 g BA (0.087 mol),4.29 g HM (0.01 mol),1.00 g FMA (0.003 mol)以及少许AIBN(已提前溶解在40 mL的乙酸乙酯中)投入装有搅拌子的三颈瓶中,该三颈瓶与球形冷凝管连接,接冷凝水冷却. 然后将三颈瓶中的混合物在氩气氛围中鼓泡至少20 min以除去溶液中的氧气,再让单体在氩气氛围中,70 °C下聚合6 h左右. 聚合反应完成后,将产物在石油醚或正己烷中沉淀3次. 最后,将产物放在真空烘箱中,在60 °C下烘干至恒重. 其产物结构和核磁表征及归属如
Fig 2 1H-NMR spectrum of the linear copolymer product
利用呋喃基团和马来酰亚胺发生DA加成反应,合成交联聚合物. 其具体实施过程如下:将4 g (含1.985 mmol呋喃基团)上述所得的线型共聚物与0.36 g (0.9925 mmol) BMI分别在THF中溶解,然后将2种溶液混合,将混合均匀的溶液加在一个方形的四氟模具中,在60 °C下反应3天成膜,反应过程如
Fig 3 Synthesis of the double reversible networks
室温下,用Thermo Scientific Nicolet iS50 FTIR的衰减全反射模式记录了交联聚合物的FTIR光谱. 用Advance Ⅲ HD 400 MHz核磁波谱仪测量得到了氢核磁共振谱(1H-NMR). 在Q2000 (TA仪器)上测试获得了交联聚合物的热流曲线,在Q800 (TA仪器)上测试了其动态力学性能. 在INSTRON5967拉伸试验机上进行了拉伸实验获得了交联聚合物的应力-应变曲线;在应变速率为0.067 s−1的情况下,获得了循环拉伸曲线. 将每一个应变下的循环拉伸曲线对应的滞后圈面积比上该应变下的断裂韧性得到材料的能量耗散效率. 材料的自修复测试是将哑铃型材料垂直于拉伸方向从中间切断,然后将其断面再结合,在100 °C真空烘箱中放置1 h,使得DA键得以破裂,然后再在60 °C下放置24 h,完成修复. 将修复后的材料在同样拉伸速率下进行单轴拉伸,修复前后材料的断裂强度之比为材料的自修复效率.
氢键小分子HM是通过异氰酸丁酯和N-(2-羟乙基)丙烯酰胺在常温下发生加成反应制备而成. 利用核磁共振氢谱(1H-NMR)对HM结构进行表征,其结果如
之后,通过两步法合成出双网络弹性体. 首先通过一锅共聚法制备出线形共聚物,然后通过将线形共聚物与双马来酰亚胺混合反应制备出弹性体. 其中,HM侧链上酰胺键和氨基甲酸酯之间形成氢键交联点,而FMA和BMI之间形成DA键交联点. 通过控制HM的含量调控交联聚合物中氢键的相对含量,合成了一系列力学性能可调的自修复弹性体. 利用1H-NMR对材料结构进行表征,其结果如
Sample | Content of HMin DNs (mol%) | Content of FMA in DNs (mol%) | Tg (°C) |
---|---|---|---|
DN-0-3 | 0 | 2.73 | −41.2 |
DN-6-3 | 5.64 | 2.96 | −32.6 |
DN-20-3 | 20.14 | 3.16 | −27.7 |
DN-31-3 | 31.04 | 3.07 | −12.2 |
为了检测材料中是否形成氢键,对交联后的样品进行了升温傅里叶变换红外光谱(FTIR)测试,如
Fig 4 FTIR spectra of DNs: (a) heating-up and (b) cooling down
为了检测材料中是否形成DA键,本文对BMI交联前后的样品进行FTIR测试,如
Fig 5 (a) FTIR spectra of DN-20-3 before and after the DA reaction with BMI; (b) DSC curve of DN-20-3, which shows an endothermic peak at about 90 °C; (c) Storage modulus of DN-20-3 obtained from DMA
为了研究自修复弹性体DN的力学性能,以0.067 s−1的应变速率,对哑铃状试样进行了单轴拉伸试验. 研究发现,只含氢键交联作用的材料表现出很大的黏性和流动性,不易成型,如
Fig 6 Mechanical properties: (a) stress-strain curves of DNs (inset: DN-15-0 sample which contains no DA crosslinks), (b) Young’s modulus of DNs, (c) toughness of DNs calculated by the integral area of stress-strain curves
如
为了探究为什么氢键的引入会提高材料的断裂强度和断裂韧性,本工作对材料进行了循环拉伸测试,并计算材料的能量耗散效率,如
Fig 7 Energy dissipation of DNs: (a) cyclic tension of DN-20-3 and (b) energy dissipation efficiency of the DNs; (c) Schematic illustration of the proposed mechanism for energy dissipation within DNs under deformation
基于以上结果,我们提出如下的增韧机理,如
为了研究材料的自修复性能,我们用刀片将哑铃型试样的中部切断,然后迅速将断切面对齐贴合,再将样品放入真空烘箱中在60 °C下修复24 h. 然后将该样条在相同的条件下进行单轴拉伸测试,得到修复后的应力-应变曲线,如
Fig 8 Self-healing properties of DNs: representative stress-strain curves of (a) DN-6-3 and (b) DN-20-3 in the original state and after healing for 24 h; (c) Healing efficiency of DN-6-3 and DN-20-3
通过将氢键和DA键的双重动态网络结合在一起,合成了一种力学性能优良且自修复效率高的弹性体. 其中,一重网络由氢键作用形成,在外力作用下,氢键首先断裂以耗散能量,赋予材料良好的力学性能. 另一重网络是由DA键形成的,在室温下作为永久交联以保持材料的形状,但在一定温度下表现出动态行为. 所以在一定温度下,这2种机制对材料的自修复性能都起着重要的作用. 结果表明,材料具有高强度(3.7 MPa)、高的自修复效率(98%)以及良好的弹性.
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