纸质出版日期:2019-5,
网络出版日期:2019-4-2,
收稿日期:2019-1-28,
修回日期:2019-3-1
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通过同时向聚合物的主链和交联点引入2种动态共价键—Diels-Alder (D-A)键和二硫(S―S)键,制备了具有自修复、可重加工性能的聚氨酯(PU-SSDA)弹性体. 一方面分布在主链和交联点的动态键使聚合物分子链通过可逆的断裂重组实现自修复,另一方面,交联的PU中同时存在线性的分子链,从而提高了断面间分子的运动性能,使被破坏的断面间迅速浸润,提高修复效率. PU-SSDA在室温下修复60 min的修复效率可达93%. 此外,这些聚氨酯材料可以利用无溶剂一步法制备,作为环境友好材料具有广泛的应用潜力.
We synthesized a self-healing polyurethane elastomer (PU-SSDA) which displayed excellent healing efficiency at room temperature due to the synergistic contribution of dynamic covalent D-A bond and S–S bond. Mechanical and self-healing properties of the polyurethane elastomers were characterized by tensile test and three-dimensional super depth of field microscope. The cross-linking structure of PU-SSDA is explored by extraction experiments, demonstrating that there are not only crosslinked structures but also linear molecular segments existed in the polyurethane elastomer. Mechanical properties of the PU-SSDA elastomer can be tuned by changing the cross-linking degree. As the crosslinking degree increases from 25% to 45%, the tensile strength increases from 2.9 MPa to 5.5 MPa while the elongation at break decreases from 795% to 304%. After optimization, when the crosslinking degree was 35%, the tensile strength was 3.7 MPa, the elongation at break was 606% and the repair efficiency could be restored to 93% after healing for 60 min at room temperature. Moreover, the healing efficiency still remains above 90% after 4 damage-healing cycles. In addition, the PU-SSDA elastomer can also be reprocessed by hot pressing at 120 °C. This excellent self-healing behavior and reprocessable property were attributed to the reversible fracture recombination reaction of dynamic D-A and S―S bonds and the quick infiltration of the linear polymer chains into damaged surface. The self-healing mechanism can be further confirmed by the dissolution experiments which showed that the PU-SSDA elastomer can be dissolved in DMF at 100 °C while the PU-control can only swell under the same conditions, demonstrating that the reversible cleavage and reformation of D-A and S―S bonds contribute a lot to the self-healing process. Due to the facile and friendly preparation method, fast self-healing behavior at room temperature and fully reprocessable properties, the as-prepared polyurethane elastomers displayed wide potential applications such as protection coatings and wearable electronic devices.
D-A and S ―S bonds were successfully introduced into a flexible polymer matrix to prepare a polyurethane elastomer with self-healing and reprocessable properties at room temperature, which displayed strong mechanical property and high healing efficiency.
在聚合物材料的加工和使用过程中,外部刺激如热、应力、辐照等对材料不可逆的损伤会造成材料力学性能下降,使用寿命降低,从而导致资源的浪费. 而修补、焊接等传统修复方式只能从宏观上修复而不能从微观上修复损伤. 因此具有主动自我修复能力的聚合物材料的合成也成为科研工作者们研究的热门课题. 目前实现材料自修复的方式主要有2种,第1种是外植型自修复,即通过将修复剂包覆在微胶囊[
Wudl等制备了第一个基于D-A键的可修复环氧树脂材料[
将D-A键[
聚(丙二醇)(PPG)(平均分子量大约2000),甲苯二异氰酸和其他试剂购自Aldrich,若非特殊说明,所有试剂无需进一步纯化,直接使用. 含有D-A键的二醇分子(化合物5)参考文献[
PU-DA、PU-SSDA和PU-control的合成过程如
Fig 1 Schematic diagram of the synthesis of PU elastomer
其中化合物2、4和5主要参考文献[
最后将化合物2(1.41 g,10 mmol)、苯(20 mL)及糠醇(0.99 g,10.1 mmol)倒入100 mL的圆底烧瓶中,回流24 h. 反应液冷却至室温后真空抽滤收集并用乙醚洗涤数次,真空干燥后得到化合物5,其产率为88.0%. 化合物4采用类似的步骤合成. 将化合物2(1.41 g,10 mmol)、苯(20 mL)及化合物3(2.61 g,10.1 mmol)倒入100 mL的圆底烧瓶中,回流24 h. 反应液冷却至室温后真空抽滤收集,用乙醚洗涤数次后,真空干燥后得到化合物4,其产率大约为87.0%.
1.3.1 力学性能测试
材料力学性能和修复性能的拉伸测试实验均在Instron 5567万能试验机上进行. 所用传感器为500 N,拉伸速率为20 mm/min. 拉伸测试的样品尺寸均为30 mm × 1.5 mm × 1.5 mm的矩形样条. 平行测试5个样品.
1.3.2 修复切口观察
自修复实验中,使用日本基恩士VHX-1000 3D超景深光学显微镜在侧光模式下观察修复前后材料表面切口的变化.
1.3.3 修复性能测试
将矩形长条样条从中间切断,然后迅速在室温下将断面贴合于相同尺寸的模具中密封保存,放置室温下分别修复5 min、10 min、30 min、1 h. 修复完成后在室温下迅速进行拉伸实验,测试修复前后的拉伸性能,而修复效率可定义为修复前后样品的断裂强度之比.
1.3.4 修复性能循环测试
按照修复性能测试方法,对同一样品进行修复性能测试,重复过程4次,得到样品的循环修复效率.
1.3.5 溶解/抽提实验
将制备得到的PU-SSDA和PU-control样条剪取一小段,完全浸没在DMF中,放置于100 °C下,间隔10 min、1 h和12 h分别观察聚氨酯的溶胀情况. 在室温下进行一组同样的测试作为参照.
1.3.6 重加工实验
将聚氨酯弹性体剪成小颗粒后,放入不锈钢模具中,在R-32022015热压机中成型. 热压条件为:温度120 °C,压力5 kPa.
1.3.7 弹性性能测试
弹性性能测试在Instron 5567万能试验机上进行. 所用传感器为500 N,拉伸速率为10 mm/min. 拉伸测试的样品为40 mm × 5 mm × 1 mm的矩形样条. 在200%、300%及450%应变下循环拉伸各1次.
为了探究聚氨酯材料的力学性能,在相同条件下合成了不同交联度的2种聚氨酯材料,分别命名为PU-DA和PU-SSDA. 其中DA代表只含有D-A键,而SSDA代表既含有D-A键又含有S―S键. 并且我们对其进行了拉伸测试,
Fig 2 Stress-strain curves for several polyurethane elastomers at different crosslinking degrees: (a) PU-DA and (b) PU-SSDA
由于含DA键的二醇小分子是一种相对刚性的分子,而与PU-DA相比较,PU-SSDA中D-A键的含量为PU-DA中D-A键含量的2倍,刚性链段增多,因此力学强度进一步提高. 在25%的交联度时,PU-SSDA的断裂强度为2.9 MPa,是同等交联度下的PU-DA的1.5倍,其杨氏模量0.7 MPa,是同等交联度下的PU-DA的2.25倍,其断裂伸长率达795%,是同等交联度下的PU-DA的1.8倍. 在45%的交联度下,PU-SSDA的断裂强度为5.5 MPa,是同等交联度下PU-DA的1.57倍.
本文也对聚氨酯的修复性能进行了表征.
Fig 3 Digital photos of PU-SSDA with a crosslinking degree of 35%: the cut sample (b) before (a) and after (c) self-healing at room temperature
Fig 4 (a) Cyclic tensile curves of PU-SSDA under different strains; (b) Stress-strain curves of PU-SSDA before and after healing; (c) Healing efficiency of 35% crosslinked PU at room temperature for different healing time; (d) Healing efficiency of 35% crosslinked PU after multiple “damage/healing” cycles
为了进一步探究聚氨酯材料的修复性能,本文对聚氨酯材料的修复效率进行了表征.
将制备得到的PU-SSDA和PU-control样条剪取一小段,并将样条完全浸没在DMF中,放置于100 °C下溶解,间隔10 min、1 h和12 h后分别观察聚氨酯的溶胀情况.
Fig 5 (A) Dissolution process of PU-SSDA in DMF at 100 °C for different time; (B) Swelling process of PU-contorl in DMF at 100 °C for different time; (C) Swelling process of PU-SSDA (a) and PU-DA (b) in DMF at room temperature; (D) Optical microscope photos of fracture surface of PU-SSDA (a) and PU-DA (b) (5 min after the sample was cut.)
从
在后续的实验中,我们将室温溶解实验中的溶解部分抽提出来分别计算,得到聚合物的交联度. 其中ρ1为根据投料比按照公式(1)计算的交联度,其含义为三官能度的异氰酸酯聚氨酯占总异氰酸酯聚氨酯的比例. ρ2为根据溶解实验按照公式(2)计算的交联度,表示实际交联的聚氨酯占总样品的比重. 由
Fig 1 Dynamic crosslinked polyurethane
Sample | Feed ratio[tri-NCO]:[bis-NCO]:[bis-OH] | Crosslinking degree ρ1 | Crosslinking degree ρ2 |
---|---|---|---|
PU-DA-40 | 16:30:60 | 40% | (58 ± 3.5)% |
PU-SSDA-40 | 16:30:60 | 40% | (53 ± 3.5)% |
PU-control-40 | 16:30:60 | 40% | (96 ± 2.7)% |
通过向高分子链分别引入动态可逆的D-A键[
Crosslinking degree-ρ1:
ρ1=3×[tri-NCO]/(3×[tri-NCO]+2×[bis-NCO]) 1
Crosslinking degree-ρ2:
ρ2=W2/W1 2
其中W1为样品原始重量,W2为抽提回流后剩余样品的质量.
Fig 6 Reprocessing of (A) PU-SSDA and (B) PU-control: (a) original materials, (b) materials cut into pieces, (c) materials after reprocessing
通过同时向聚合物引入2种动态共价键—Diels-Alder (D-A)键和二硫(S―S)键的方法,制备了具有可室温自修复、可重加工性能的聚氨酯(PU-SSDA)材料. 修复实验表明PU-SSDA在室温下修复60 min的修复效率可达到93%,并且循环4次,修复效率仍然高达90%以上,重复性好. 在40%的交联度下,PU-SSDA的断裂强度可以达到4.5 MPa. S―S键和D-A键的可逆断裂重组赋予了材料良好的重加工性能. 而本材料特有的双重结构,即在交联网络中同时存在运动性较好的线性分子链的结构使得材料在受损时,运动性较好的线性分子链迅速浸润切口断面,通过动态键的断裂和重组实现材料的自修复. 此外,无溶剂的制备条件也使这种聚氨酯材料可以作为环境友好材料从而具有广泛的应用潜力.
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