Carbon Dots as Fillers Inducing Healing/Self-Healing and Anticorrosion Properties in Polymers
碳点作为填料诱导聚合物中的愈合/自愈合和抗腐蚀性能
Self-healing is the way by which nature repairs damage and prolongs the life of bio entities. A variety of practical applications require self-healing materials in general and self-healing polymers in particular. Different (complex) methods provide the rebonding of broken bonds, suppressing crack, or local damage propagation. Here, a simple, versatile, and cost-effective methodology is reported for initiating healing in bulk polymers and self-healing and anticorrosion properties in polymer coatings: introduction of carbon dots (CDs), 5 nm sized carbon nanocrystallites, into the polymer matrix forming a composite. The CDs are blended into polymethacrylate, polyurethane, and other common polymers. The healing/self-healing process is initiated by interfacial bonding (covalent, hydrogen, and van der Waals bonding) between the CDs and the polymer matrix and can be optimized by modifying the functional groups which terminate the CDs. The healing properties of the bulk polymer–CD composites are evaluated by comparing the tensile strength of pristine (bulk and coatings) composites to those of fractured composites that are healed and by following the self-healing of scratches intentionally introduced to polymer–CD composite coatings. The composite coatings not only possess self-healing properties but also have superior anticorrosion properties compared to those of the pure polymer coatings.
自我修复是自然修复损害并延长生物实体寿命的方式。各种实际应用通常需要自修复材料，特别是需要自修复聚合物。不同（复杂）方法提供断裂键的重新键合，抑制裂缝或局部损伤传播。据报道，这种简单，通用且具有成本效益的方法用于启动本体聚合物的愈合以及聚合物涂层中的自我修复和抗腐蚀性能：将碳点（CDs），5纳米尺寸的碳纳米微晶引入聚合物基体中复合材料。 CDs被混合到聚甲基丙烯酸酯，聚氨酯和其他常见聚合物中。自愈过程是由CDs与聚合物基体之间的界面键(共价键、氢键和范德华键)启动的，可以通过修饰终止CDs的官能团来优化。通过比较原始(大块和涂层)复合材料和断裂复合材料的抗拉强度，以及在聚合物CD复合涂层中有意引入的划痕的自愈，来评价大块聚合物CD复合材料的愈合性能。 与纯聚合物涂层相比，复合涂层不仅具有自修复性能，而且具有优异的抗腐蚀性能。
The lifetime of the rich variety of modern products is limited by the degradation of the engineering (structural and functional) materials from which they are constructed leading to the product’s complete failure. Many future materials cannot be frequently replaced due to efficiency or cost considerations and require a long operational time. The ability of nature to self-repair local damage, sometimes to complete restoration, inspired material scientists/engineers to design and process “self-healing” materials in which the damage (e.g., breakage of bonds or crack formation) initiates a healing response. The self-healing process should take place spontaneously and without human intervention. A variety of (very often complex and expensive) concepts have been applied to achieve self-healing materials. The mending may be irreversible or reversible. White pioneered the idea of microencapsulation of a monomer and a catalyst to initiate restoration in the damaged region. This can be similarly achieved by nanoreservoirs and by hollow fibers containing a healing fluid as well as by microvascular polymeric composites. The healing process may be achieved by specially designed supermolecular systems and may utilize either physical or chemical bonding including hydrogen or Diels– Alder bonds. For a more detailed description of the current status of self-healing materials the reader is referred to several recent studies and reviews.
丰富多样的现代产品的寿命受限于工程（结构和功能）材料的退化，从而导致产品完全失效。由于效率或成本考虑，许多未来的材料不能经常更换，并且需要很长的操作时间。自然修复局部损伤的能力，有时是完成修复，激发了材料科学家/工程师设计和加工“自修复”材料，其中损伤（例如，粘合或裂缝形成的破裂）引发愈合反应。自我修复过程应该自发进行，无需人为干预。已经应用了各种（通常是复杂且昂贵的）概念来实现自修复材料。修补可能是不可逆转的或可逆的。 White开创了单体微胶囊化和催化剂在受损区域启动修复的想法。这可以通过纳米贮存器和含有愈合流体的中空纤维以及微血管聚合物复合物类似地实现。愈合过程可以通过专门设计的超分子系统来实现，并且可以利用包括氢或Diels-Alder键的物理或化学键合。有关自愈材料当前状态的更详细描述，读者可参考最近的几项研究和评论。
Here, we propose a novel, simple, versatile, and cheap methodology to initiate healing properties in a variety of bulk polymers (e.g., polymethacrylate (PMMA)) and self-healing properties in several thin polymer coatings (e.g., polyurethane (PU)): introduction of carbon dots (CDs), 5 nm sized carbon nanocrystallites, into the polymer matrix. The synthesized PMMA (or PU) polymer composites are blended with amino group-modified and oxidized CDs. The CD-bulk polymer blended composites present healing after fracture restoring ≈60% of the tensile strength of the pristine composite. Moreover, the introduction of CDs into the polymer composite coatings not only initiates self-healing characteristics but also induces improved anticorrosion properties.
在此，我们提出了一种新颖、简单、通用和廉价的方法，用于在多种大体积聚合物(如聚甲基丙烯酸酯(PMMA))中启动自愈性能，以及在几种薄聚合物涂层(如聚氨酯(PU))中启动自愈性能。：将碳点（CD），5nm大小的碳纳米微晶引入聚合物基质中。 合成的PMMA（或PU）聚合物复合材料与氨基改性和氧化的CDs共混。 CD-本体聚合物共混复合材料在断裂恢复后表现出愈合，约为原始复合材料拉伸强度的60％。 此外，将CD引入聚合物复合涂层中不仅启动自愈特性，而且还诱导改善的防腐性能。
CDs in the form of monodispersed graphite particles with diameters less than 10 nm have been recently intensively studied due to their unique photochemical (e.g., photocatalytic) and physical (e.g., photoluminescence) properties.The small sizes and the rich variety of optional functional groups they may be terminated with enable the simple and homogeneous dispersion of CDs in polymers. Particularly, oxygen- and aminecontaining functional moieties coupled to CDs facilitate a strong interaction (hydrogen bonding and other heterobonding) with polymers. The healing/self-healing features achieved by introduction of CDs into polymers are assigned to: (1) hydrogen bonding between the surface functional groups of the CDs (e.g., amino (-NH2), carboxyl (-COOH), and hydroxyl (-OH) groups), (2) covalent bonds of (C-O-NH) between CDs and the polymer chains, and (3) Van der Waals bonding induced between adjacent CDs. This new concept of introduction of terminated CDs to a variety of common polymers is simple, versatile, and cost effective. It can be implemented both in a host of bulk polymers (e.g., as structural materials) as well as in many polymer protective coatings where the combination of self-healing and anticorrosive features is advantageous.
由于其独特的光化学(如光催化)和物理(如光致发光)特性，单分散石墨颗粒直径小于10纳米的CDs近年来得到了广泛的研究。它们的小尺寸和丰富多样的可选官能团可以终止，使得CD在聚合物中的简单和均匀分散。特别地，与CDs偶联的含氧和胺的官能部分促进与聚合物的强相互作用（氢键和其他异质结合）。通过将CD引入聚合物中实现的愈合/自愈特征指定为：（1）CDs的表面官能团之间的氢键（例如，氨基（-NH 2），羧基（-COOH）和羟基（ - ） OH）基团），（2）CDs与聚合物链之间的（CO-NH）的共价键，和（3）相邻CDs之间诱导的范德华键合。这种将终止CDs引入各种常见聚合物的新概念简单，通用且具有成本效益。它可以在许多本体聚合物（例如，作为结构材料）以及许多聚合物保护涂层中实施，其中自愈合和防腐蚀特征的组合是有利的。
A series of CDs (CDs-m, m = 1–5) were prepared applying our electrochemical etching method followed by chemical treatments which terminate the CDs by different surface functional groups (see the Experimental Section). Transmission electron microscope (TEM), high-resolution TEM (HRTEM), and dynamic light scattering (DLS) of the CDs were used to study the structure and the size distribution of the CDs (Figures S1 and S2, Supporting Information). They are 3–5 nm sized graphitic nanoparticles with either the (100) interplanar lattice spacing of 0.21 nm or the (002) graphitic spacing of 0.34 nm as described elsewhere. X-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition of the CDs, while XPS and Fourier transform infrared spectroscopy (FTIR) were used to determine the bonding and the nature of the CDs termination with various surface functional groups comprising amino, amide, carboxyl, and hydroxyl moieties (Figures S3–S6, Supporting Information). The titration method was also applied to identify the content of the -OH/-COOH groups in the CDs (Figure S7, Supporting Information). The titration results and XPS derived elemental compositions are listed in Tables S1–S3 (Supporting Information). CDs-1–CDs-3 contain mainly -OH and -COOH groups while CDs-4 and CDs-5 also contain the -NH2 and -C-O-NH2 groups which are absent in CDs-1–CDs-3.
使用我们的电化学蚀刻方法制备一系列CDs（CDs-m，m = 1-5），然后进行化学处理，通过不同的表面官能团终止CDs（参见实验部分）。使用CD的透射电子显微镜（TEM），高分辨率TEM（HRTEM）和动态光散射（DLS）来研究CDs的结构和尺寸分布（图S1和S2，支持信息）。它们是3-5nm尺寸的石墨纳米颗粒，其具有0.21nm的（100）晶面间距或0.34nm的（002）石墨间距，如其他地方所述。 X射线光电子能谱（XPS）用于确定CDs的元素组成，而XPS和傅立叶变换红外光谱（FTIR）用于确定CDs的终止与各种包含氨基的表面官能团的键合和性质，酰胺，羧基和羟基部分（图S3-S6，支持信息）。滴定方法也用于识别CDs中-OH / -COOH基团的含量（图S7，支持信息）。滴定结果和XPS衍生的元素组成列于表S1-S3（支持信息）中。 CDs-1-CDs-3主要含有-OH和-COOH基团，而CDs-4和CDs-5也含有CDs-1-CDs-3中不存在的-NH2和-C-O-NH2基团。
CDs/PMMA composites were prepared by polymerization of a mixture of a CDs/ethyl alcohol (CDs-EtOH) solution and methyl-methacrylate (MMA) monomers as described in the Experimenal Section. The CDs/PMMA composites were made in the form of rods, 7 mm in diameter. The photoluminescence properties of CDs-5 are preserved after their incorporation into the polymeric materials, as demonstrated in Figure 1. The intense blue photoluminescence of the solid CDs/PMMA composites is visible by bare eyes when illuminated by UV light (365 nm). The uniform photoluminescence indicates even distribution of CDs in PMMA (Figure 1a). The healing test was carried out by fracturing the rods to two pieces (Figure 1b), soaking the two halves of the CDs/PMMA rods in a CDs aqueous solution for a few seconds, then bringing the two fractured pieces into contact along the axial directions with a compression force of ≈20 N (Figure 1c). The contact was kept for 30 min at 33 °C in dry air. After this treatment, the fractured rod halves were rejoined and healed (Figure 1d).
如实验部分所述，通过CDs /乙醇（CDs-EtOH）溶液和甲基丙烯酸甲酯（MMA）单体的混合物的聚合制备CDs / PMMA复合材料。 CDs / PMMA复合材料以棒的形式制成，直径为7mm。 CDs-5的光致发光性质在掺入聚合物材料后得以保留，如图1所示。当用UV光（365nm）照射时，裸眼可见固体CDs / PMMA复合材料的强蓝光致发光。均匀的光致发光表明PMMA中CDs的均匀分布（图1a）。通过将棒压碎成两块（图1b）进行愈合试验，将CDs的两半CDs / PMMA棒浸泡在CDs水溶液中几秒钟，然后使两块断裂的块沿轴向接触压缩力约为20 N（图1c）。将接触在33℃下在干燥空气中保持30分钟。在该处理之后，将断裂的杆半部重新结合并愈合（图1d）。
To evaluate the self-healing properties of the composites, we compared the tensile strength of the pristine EtOH/PMMA and CDs/PMMA rods with those of the self-healed rods. Figure 2a shows, respectively, the strain–stress curves of EtOH/PMMA (blue line), pristine CDs-5/PMMA (black line), and healed CDs-5/PMMA samples (red line) with the same quantity of additives. Due to the healing process, the fractured and healed CDs-5/PMMA rod restored about 60% of its original tensile strength (3.6 compared to 6.0 MPa). We further checked the effect of the CDs-5 concentration on the healing properties of the CDs-5/PMMA composite (Figure 2b). It is obvious that the pure PMMA (free of CDs-5) cannot heal. The incorporation of CDs-5 however initiated healing properties which were improved with increasing CDs-5 concentration. The tensile strength of the healed samples increased with CDs-5 concentration in the MMA monomers up to a concentration of 0.006 gCDs-5/gMMA (strength 3.6 MPa), above which the strength decreased. Moreover, the strength of the pristine CDs-5/PMMA composites decreased with the CDs-5 concentration (blue bars in Figure 2b) due to the EtOH solution used to introduce the CDs-5 to the MMA monomers.
The percentage of restored strength (with respect to the strength of the pristine samples) of the healed samples (Figure 2b) thus increased more with CDs-5 concentration than the strength itself.
At high CDs-5 mass concentration, the polymerization reactions become more difficult due to the retention of the EtOH used to introduce the CDs-5 so that the tensile strength decreases.
Figure 2c shows the tensile strength of CDs-5/PMMA for repetitive cycles of fracture and healing in the same position. The tensile strength decreases with the increasing number of fracture-healing cycles.
The experiment nevertheless shows that the damaged and healed region still keeps some healing properties after local bonds are broken. As a matter of fact, it is more plausible that the strength loss is due to the repetitive fracturing of the same area which makes the fitting together of the two fractured surfaces less likely rather than the loss of bonding ability of the damaged region.
Next we measured the effect of the surface termination of the CDs on the healing properties of the CDs/ PMMA composite. Five different CDs denoted CDs-1–CDs-5 were blended to PMMA applying the same CDs/PMMA concentration of 0.006 gCDs-5/gMMA that was found optimal for the healing effect for CDs-5 (Figure 2b).
Figure 2d shows that the tensile strengths of the healed composites made of different CDs indeed depend on the CDs type. CDs-5/PMMA reaches a tensile strength of 3.6 MPa, the highest among all tested samples. The second highest value is obtained for CDs-4/ PMMA. Both CDs-4 and CDs-5 have N-containing surface groups (e.g., amidogen), which seem to be effective for the healing ability. CDs-5 has a better performance possibly due to its larger amount of N-containing groups. The strength of the other three healed CDs/PMMA composites increases with the amount of oxygen-containing surface groups.
In short, the healing efficiency depends on the type and the number of the surface groups terminating the CDs. The healing of the two fractured pieces can be achieved without any solution, but the strength of the rejoined and healed rod is small (Figure S8, Supporting Information). Reaching a higher strength requires immersion in an aqueous solution which modifies the surface groups of the exposed fractured rods surfaces. Immersion in pure water leads to better healing than immersion in acidic or basic aqueous solution (Figure S8, Supporting Information). Introduction of CDs-5 nanoparticles to this solution further improves the healing activity providing healing species to the surfaces in contact (Figure S9, Supporting Information). Increasing the pressure between the fractured rods improves the healing efficiency as well (Figure S10, Supporting Information). Finally, note that the CDs should be evenly distributed in the polymer matrix.
Inhomogeneity of CDs dispersion causes CDs agglomeration leading to low tensile strength (Figure S11, Supporting Information).
为了评估复合材料的自愈性能，我们比较了原始EtOH / PMMA和CDs / PMMA棒的拉伸强度与自愈棒的拉伸强度。图2a分别显示了具有相同量添加剂的EtOH / PMMA（蓝线），原始CDs-5 / PMMA（黑线）和愈合的CDs-5 / PMMA样品（红线）的应变 - 应力曲线。由于愈合过程，骨折和愈合的CDs-5 / PMMA棒恢复了其原始拉伸强度的约60％（3.6与6.0MPa相比）。我们进一步检查了CDs-5浓度对CDs-5 / PMMA复合物的愈合性质的影响（图2b）。很明显，纯PMMA（不含CDs-5）无法愈合。然而，CDs-5的掺入引发了愈合特性，其随着CDs-5浓度的增加而得到改善。愈合样品的拉伸强度随着MMA单体中CDs-5浓度的增加而增加，直至浓度为0.006gCDs-5 / gMMA（强度3.6MPa），高于该浓度，强度降低。此外，由于用于将CDs-5引入MMA单体的EtOH溶液，原始CDs-5 / PMMA复合材料的强度随着CDs-5浓度（图2b中的蓝色条）而降低。
因此，随着CDs-5浓度的增加，恢复强度(相对于原始样品的强度)的百分比(图2b)比强度本身增加的更多。
在较高的CDs-5质量浓度下，由于引入CDs-5的EtOH的保留使得抗拉强度降低，聚合反应变得更加困难。
图2c显示了CDs-5 / PMMA在相同位置的重复断裂和愈合循环的拉伸强度。 随着断裂愈合循环次数的增加，拉伸强度降低。
然而，该实验表明，在局部粘连破坏后，受损和愈合的区域仍保持一些治愈特性。 事实上，更强有力的是，强度损失是由于相同区域的重复断裂造成的，这使得两个断裂表面的配合不太可能而不是损坏区域的粘合能力的损失。
接下来我们测量了CDs表面终止对CDs/ PMMA复合材料愈合性能的影响。使用0.006 gCDs-5/gMMA的相同cd /PMMA浓度，将CDs-1 - CDs-5与5种不同的CDs-1 - CDs-5混合到PMMA中，发现CDs-5的愈合效果最佳(图2b)。
图2d显示了不同CDs材料的愈合复合材料的拉伸强度确实取决于CDs的类型。CDs-5/PMMA的抗拉强度为3.6 MPa，是所有测试样品中最高的。CDs-4/ PMMA的值第二高。CDs-4和CDs-5都有含有n的表面基团(如酰胺原)，这似乎对治疗能力有效。CDs-5有更好的性能，可能是因为它含有更多的n基团。另外三种愈合的CDs/PMMA复合材料的强度随含氧量的增加而增加。
简而言之，愈合效率取决于终止CDs的表面基团的类型和数量。两个断裂的碎片可以在不需要任何溶液的情况下愈合，但是重新连接和愈合的棒的强度很小(图S8，支持信息)。达到更高的强度需要浸入水溶液中，这样可以改变暴露的断裂杆表面的表面基团。浸泡在纯水中比浸泡在酸性或碱性水溶液中愈合更好(图S8，支持信息)。在这种溶液中引入CDs-5纳米颗粒进一步提高了愈合活性，为接触的表面提供了愈合物种(图S9，支持信息)。增加断裂棒之间的压力也可以提高愈合效率(图S10，支持信息)。最后，注意CDs应均匀分布在聚合物基体中。
CDs分散度的不均匀性导致CDs团聚导致低抗拉强度(图S11，支持信息)。
To evaluate the nature of the bonding which might be associated with the healing of the CDs-5/PMMA composites, we performed detailed XPS and FTIR analyses of: (1) CDs-5 powders at room temperature, CDs-5 annealed to 33 °C, and CDs-5 annealed to 180 °C, (2) pristine CDs-5/PMMA composites at room temperature and annealed to 33 °C, and (3) healed CDs-5/PMMA composites at room temperature and annealed to 33 °C. The rationale of checking the bonding of CDs-5 powders annealed to different temperatures was to follow possible bond formation between CDs-5. Note that annealing of the CDs-5 powder to 180 °C initiated agglomeration of the powder nanoparticles indicating that particle–particle bonding indeed occurred. The XPS and FTIR data of the CDs annealed to 180 °C allowed determination of the bonding of agglomerated CDs. The comparison of the pristine and the healed composites enabled the evaluation of bond formation during the healing process.
为了评估可能与CDs-5 / PMMA复合材料愈合相关的粘合性质，我们对以下材料进行了详细的XPS和FTIR分析：（1）室温下CDs-5粉末，CDs-5退火至33° C和CDs-5退火至180°C，（2）原始CDs-5 / PMMA复合材料在室温下退火至33°C，（3）在室温下愈合CDs-5 / PMMA复合材料并退火至33 C。
检测在不同温度下退火的CDs-5粉体的结合的基本原理是跟踪CDs-5之间可能形成的结合。注意，将CDs-5粉末退火至180℃引发粉末纳米颗粒的附聚，表明确实发生了颗粒 - 颗粒结合。 退火至180℃的CDs的XPS和FTIR数据允许确定附聚的CD的结合。 原始和愈合复合材料的比较使得能够评估愈合过程中的粘合形成。
The XPS data of the CDs-5 powders annealed at different temperatures (Figures S4 and S12–S14, Supporting Information) yielded five deconvoluted peaks for C1s ((i) C-C and/ or C-C, (ii) C-N, (iii) C-O, (iv) C-O and/or C-O-NH, and (v) COOH) and two for N1s (C-N and N-H). The annealing to 180 °C was associated with an increase of the C-N and C-O-NH bonds of C1s and the C-N bond of N1s. Both the C1s and the N1s data thus indicate a significant increase of the C-O-NH bond in the 180 °C annealed CDs-5 powder. The FTIR spectra (Figure 3c) of the 180 °C annealed powder show a strong 1739.1 cm−1 peak absent in the room temperature and 33 °C powders which is attributed to C-O-NH. It is thus validated by both XPS and FTIR that a large amount of C-O-NH is generated upon 180 °C annealing of CDs-5 nanoparticles and their agglomeration (Figure 3a–c).
在不同温度下退火的CDs-5粉体的XPS数据(图S4和S12-S14，支持信息)为C1s ((i) C-C和/或C-C， (ii) C-N， (iii) C-O， (iv) C-O和/或C-O- nh， (v) COOH)和N1s (C-N和N-H)产生了5个解旋峰。
退火至180℃与C1s的C-N和C-O-NH键和N1s的C-N键的增加有关。
因此，C1s和N1s数据都表明180℃退火的CDs-5粉末中C-O-NH键的显着增加。 180℃退火粉末的FTIR光谱（图3c）显示在室温和33℃粉末中不存在强的1739.1cm -1峰，这归因于C-O-NH。 因此，通过XPS和FTIR验证，在CDs-5纳米颗粒的180℃退火及其附聚时产生大量的C-O-NH（图3a-c）。
Now we analyze the XPS and FTIR data of the pristine and healed CDs-5/PMMA composites (Figures S15 and S16, Supporting Information). The C1s data show stronger C-N and C-O-NH peaks for the healed composites with respect to the pristine composites. The N1s peak is too noisy for comparison. The FTIR spectrum of the healed composite shows a 1732 cm−1 peak attributed to C-O-NH, which is absent in the FTIR spectrum of the pristine sample. In conclusion, both healing of CDs-5/PMMA at low temperatures and 180 °C annealing of CDs-5 powder are associated with the formation of C-O-NH bonding.
现在我们分析原始和愈合的CDs-5 / PMMA复合材料的XPS和FTIR数据（图S15和S16，支持信息）。 C1s数据显示愈合的复合材料相对于原始复合材料具有更强的C-N和C-O-NH峰。 对于比较，N1s峰值太嘈杂。 愈合的复合材料的FTIR光谱显示1732cm -1峰归因于C-O-NH，其在原始样品的FTIR光谱中不存在。 总之，CDs-5 / PMMA在低温下的愈合和CDs-5粉末的180℃退火都与C-O-NH键的形成有关。
Fracturing or cracking exposes CDs and polymer chains on the fractured surface. We may now speculate that the healing of CDs-5/PMMA fractured composites proceeds either by bonding of two CDs to each other or by CD-PMMA bonding. Bonding of two CDs-5 to each other is possible by the formation of a covalent bond of C-O-NH. Hydrogen bonding may be formed either between two CDs-5 or between a CDs-5 and the polymer chain. The hydrogen bonding may involve the different functional surface groups of CDs forming O-H-N, O-H-O, N-H-N and N-H-O. Finally, Vvn der Waals forces between CDs may also contribute to the healing of the composite though their strength is expected to be smaller than that of the covalent or hydrogen bonding (Figure 3d and Figure S17 (Supporting Information)).
压裂或裂缝暴露出破裂表面上的CDs和聚合物链。 我们现在可以推测，CDs-5 / PMMA断裂复合材料的愈合可以通过将两个CD彼此粘合或通过CD-PMMA粘合来进行。 通过形成C-O-NH的共价键，可以将两个CD-5彼此键合。 氢键可以在两个CD-5之间或在CDs-5和聚合物链之间形成。氢键可能涉及到CDs不同的表面官能团形成O-H-N, O-H-O, N-H-N和N-H-O。 最后，CDs之间的Vvn der Waals力也可能有助于复合材料的愈合，尽管它们的强度预计小于共价或氢键的强度（图3d和图S17（支持信息））。
The healing of CDs-5/PMMA fractured rods can be performed with no immersion in any solution but requires some intervention (application of a compression force between the fractured pieces in contact). The strength of the healed rod could be increased by a stronger intervention (immersion in an aqueous solution for a few seconds (which modifies the surface functional groups) and/or an increased force (Figures S8–S11, Supporting Information)). This means that the above experiments have shown healing between fractured surfaces but not self-healing. The ability to heal fractured surfaces and the formation of an O-C-NH bond in healed CDs-5/PMMA (Figures S15 and S16, Supporting Information) and in agglomerated CDs-5 (Figure S11, Supporting Information) however indicates that self-healing and suppression of crack propagation in the CDs5/ PMMA composite is plausible and cannot be ruled out.
CDs-5 / PMMA骨折棒的愈合可以在不浸入任何溶液的情况下进行，但需要一些干预（在接触的骨折片之间施加压缩力）。 通过更强的干预（浸入水溶液几秒钟（其改变表面官能团）和/或增加的力（图S8-S11，支持信息））可以增加愈合棒的强度。 这意味着上述实验已显示断裂表面之间的愈合但不能自愈。 在愈合的CDs-5 / PMMA中愈合断裂表面和形成OC-NH键的能力（图S15和S16，支持信息）和聚集的CD-5（图S11，支持信息）然而表明自愈 抑制CDs5 / PMMA复合材料中的裂纹扩展似乎是合理的，不能排除。
Beside CDs-5/PMMA composites, we performed further work to check the generality of the approach of using CDs to achieve healing properties in other polymers blended with CDs. The other polymers included polyethyl methacrylate (PEMA), and mixtures of PMMA with PEMA, PMMA with polybutyl methacrylate (PBMA), and PMMA with polystyrene (PS). The tensile strengths of all CDs-5/polymer composite rods, 7 mm in diameter (pristine and healed) were measured (Figure S18 and Table S4, Supporting Information). All the tested composites exhibited healing properties leading to restoration of 22–36% of the initial tensile strength.
除CDs-5 / PMMA复合材料外，我们还进行了进一步的工作，以检查使用CD实现其他与CD混合的聚合物的愈合性能的方法的一般性。 其他聚合物包括聚甲基丙烯酸乙酯（PEMA），PMMA与PEMA的混合物，PMMA与聚甲基丙烯酸丁酯（PBMA）和PMMA与聚苯乙烯（PS）的混合物。 测量所有CDs-5 /聚合物复合材料棒的拉伸强度，直径7mm（原始和愈合）（图S18和表S4，支持信息）。 所有测试的复合材料表现出愈合性能，导致恢复22-36％的初始拉伸强度。
The work by now was focused on the healing properties of bulk CDs/polymer composites. We have established the ability of CDs (introduced as fillers to polymers) to initiate healing (and possibly self-healing) in bulk composites which may serve as structural materials. Following this success, we further explored the possibility of applying the same concept to polymer coatings which require both healing capabilities and anticorrosion properties. The first model system investigated was 600 µm thick composite coatings of CDs-5 introduced to PU in three concentrations denoted CDs-5/PU-1, CDs-5/PU-2, and CDs-5/PU-3 (see the Supporting Information for details). The healing properties were evaluated in two ways: (1) studying the time evolution of the size of a scratch introduced by a sharp surgical blade to the 600 µm thick coating and (2) comparing the tensile strengths of the pristine and healed 600 µm thick selfsupported ribbons. In the first healing experiment, the scratch depth profiles (depth and width) of blade-scratched CDs-5/ PU-3 and PU ribbons held at room temperature were probed by an alpha step profilometer (Figure 4a–c). The initial 8.3 µm deep and ≈100 µm wide scratch became gradually smaller and almost completely disappeared after 48 h at room temperature. In contrast, the PU scratch did not heal and remained almost the same (within the experimental error). Strikingly, at 60 °C (Figure S19, Supporting Information), a similar scratch (7.3 µm deep and 78 µm wide) in a CDs-5/PU ribbon almost completely vanished (0.4 µm deep and 26.8 µm wide) after 10 min while a PU scratched ribbon at 60 °C remained intact. These experiments clearly and unambiguously demonstrate the self-healing characteristics of CDs-5/PU coatings. The second way of evaluation of the healing properties of CDs/PU composites included fabrication of CDs-5/PU ribbons first. The introduction of CDs-5 to the PU coating increased the tensile strength of the composite coating (Figure S20, Supporting Information). Each ribbon was cut to two pieces that were brought into contact without any compression force for 30 min at 33 °C in dry air. The two pieces were joined to each other restoring 14% of the initial tensile strength of a CDs-5/ PU ribbon (Figure 4d). In contrast, the PU ribbons showed no healing (joining) following the same process. The above experiments clearly show that the CDs/PU composite ribbons possess self-healing properties, i.e., self-mending without any intervention. Furthermore, the tensile strength of the healed CDs/PU composite ribbons could be increased to 70% of that of the pristine CDs/PU ribbons (Figure 4d) by immersion for a few seconds in a CDs-5 aqueous solution, exhibiting the bonding capabilities of the CDs. Notably, the same immersion treatment was capable of rejoining the pure PU cut pieces again demonstrating the bonding characteristics of the CDs in a PU matrix (Figure S21, Supporting Information).
目前的工作重点是散装CD /聚合物复合材料的愈合性能。 我们已经建立了CD（作为聚合物填料引入）以在可用作结构材料的大块复合材料中引发愈合（和可能的自愈合）的能力。 在此成功之后，我们进一步探索了将相同概念应用于需要愈合能力和抗腐蚀性能的聚合物涂层的可能性。 研究的第一个模型系统是600微米厚的CDs-5复合涂层，以三种浓度引入PU，表示为CDs-5 / PU-1，CDs-5 / PU-2和CDs-5 / PU-3（见支持） 详情信息）。
通过两种方式评估愈合特性：（1）研究由锋利的手术刀片引入到600微米厚涂层的划痕尺寸的时间演变和（2）比较原始和愈合的600微米厚的拉伸强度 自支撑的丝带。 在第一次愈合实验中，通过α步骤轮廓仪探测在室温下保持的刮刀刮擦CD-5 / PU-3和PU带的划痕深度分布（深度和宽度）（图4a-c）。 在室温下48小时后，最初的8.3μm深和约100μm宽的划痕逐渐变小并且几乎完全消失。 相比之下，PU划痕没有愈合并且保持几乎相同（在实验误差内）。 引人注目的是，在60°C（图S19，支持信息）下，CDs-5 / PU色带中的类似划痕（7.3μm深和78μm宽）在10分钟后几乎完全消失（0.4μm深和26.8μm宽）。 在60°C的PU划痕色带保持完好无损。
这些实验清楚且明确地证明了CDs-5 / PU涂层的自愈特性。评估CD / PU复合材料的愈合性能的第二种方法包括首先制造CD-5 / PU带。将CDs-5引入PU涂层增加了复合涂层的拉伸强度（图S20，支持信息）。将每个条带切割成两片，在33℃下在干燥空气中在没有任何压缩力的情况下接触30分钟。两片彼此连接，恢复CDs-5 / PU带的初始拉伸强度的14％（图4d）。相反，PU带在相同的过程后没有显示出愈合（连接）。上述实验清楚地表明CD / PU复合带具有自修复特性，即无需任何干预的自修复。此外，通过在CDs-5水溶液中浸泡几秒钟，愈合的CD / PU复合带的拉伸强度可以增加到原始CD / PU带（图4d）的拉伸强度的70％，表现出粘合能力。的CD。值得注意的是，相同的浸渍处理能够再次重新连接纯PU切片，证明了PU基质中CD的粘合特性（图S21，支持信息）。
The corrosion properties of PU and CDs-5/PU 600 µm thick coatings on stainless steel were evaluated through electrochemical measurements (see the Supporting Information). Tafel polarization curves are a standard method for studying the corrosion properties. In such curves a more negative corrosion potential (Ecorr) corresponds to a higher corrosion probability, while the corrosion current Icorr is a measure of the corrosion rate.The Tafel polarization curves of pristine PU and CDs-5/ PU-1 in 3.5 wt% NaCl solution are displayed in Figure 4e. The Ecorr of the pristine PU coating (black curve) is about −0.56 V, while the Icorr is about 3.07 × 10−6 A. The corrosion of the CDs-5/PU is significantly lower than that of the PU. Ecorr becomes more positive (−0.139 V) and Icorr reduces to 1.45 × 10−8 A. This means that the composite coating reduces both the corrosion probability and the corrosion rate. Figure S22 (Supporting Information) presents the corrosion potential and the corrosion current of PU and CDs-5/PU electrodes at different stages: (1) initial stage, (2) cut by a surgical blade, (3) healed in dry air at 33 °C for 3 h, and (4) corroded in 3.5 wt% NaCl for different periods of time. The conclusions of this set of experiments are: (1) pristine CDs-5/PU-1 shows less corrosion than PU. (2) The healing of the CDs-5/PU-1 reduces the corrosion while the same healing treatment does not affect the corrosion of PU. (3) The corrosion of both PU and CDs-5/PU-1 increases with time, but that of CDs-5/PU-1 is significantly lower than that of PU. Electrochemical impedance spectroscopy (EIS) was also applied to study the CDs-5/PU-1 and PU coating behaviors in corrosive environments (3.5 wt% NaCl). The typical Nyquist plot (Figure S23, Supporting Information) shows two capacitive loops for both coatings, indicating that the corrosion process consists of two relaxations or two time constants. A detailed analysis of this plot suggests that the accumulation of electric charges on the CDs-5/PU-1 coated substrate is impeded. Hence the corrosion activity of the substrate is minimized due to the increased polarization resistance. The evolution of the Tafel plots with time, displayed in Figure S24 (Supporting Information), shows that the CDs-5/PU is stable after 7 d immersion in 3.5 wt% NaCl solution while a steel substrate coated by PU corrodes within 2 d.
通过电化学测量评估PU和CDs-5 / PU600μm厚涂层在不锈钢上的腐蚀性质（参见支持信息）。塔菲尔极化曲线是研究腐蚀性能的标准方法。在这样的曲线中，更负的腐蚀电位（Ecorr）对应于更高的腐蚀概率，而腐蚀电流Icorr是腐蚀速率的量度。原始PU和CDs-5 / PU-1的Tafel极化曲线为3.5wt％ NaCl溶液显示在图4e中。原始PU涂层的Ecorr（黑色曲线）约为-0.56 V，而Icorr约为3.07×10-6A。CDs-5 / PU的腐蚀明显低于PU的腐蚀。 Ecorr变得更正（-0.139 V），Icorr降低到1.45×10-8 A.这意味着复合涂层降低了腐蚀概率和腐蚀速率。图S22（支持信息）显示了PU和CDs-5 / PU电极在不同阶段的腐蚀电位和腐蚀电流：（1）初始阶段，（2）手术刀片切割，（3）在干燥空气中愈合33℃下3小时，和（4）在3.5wt％NaCl中腐蚀不同的时间。这组实验的结论是：（1）原始CDs-5 / PU-1显示出比PU更少的腐蚀。 （2）CDs-5 / PU-1的愈合减少了腐蚀，而相同的愈合处理不会影响PU的腐蚀。 （3）PU和CDs-5 / PU-1的腐蚀随时间增加，但CDs-5 / PU-1的腐蚀明显低于PU。电化学阻抗谱（EIS）也用于研究腐蚀环境（3.5wt％NaCl）中的CDs-5 / PU-1和PU涂层行为。典型的奈奎斯特图（图S23，支持信息）显示了两个涂层的两个电容回路，表明腐蚀过程包括两个弛豫或两个时间常数。对该图的详细分析表明，CDs-5 / PU-1涂覆的基底上的电荷累积受到阻碍。因此，由于增加的极化电阻，衬底的腐蚀活性被最小化。图S24（支持信息）中显示的Tafel图随时间的演变表明CDs-5 / PU在3.5wt％NaCl溶液中浸泡7天后是稳定的，而PU涂覆的钢基材在2d内腐蚀。
Rotating disk electrode (RDE, see the Experimental Section of the Supporting Information) and linear sweep voltammetry (LSV) measurements indicate that the oxygen reduction current density increases with the disc revolution as illustrated in Figure 4f. The inset of Figure 4f gives the Koutecky–Levich plots (the inverse current density j−1 versus the inversed square root of angular velocity, 1 2 ω − at different corrosion potentials). The electron transfer number (n) of CDs-5 calculated from the slope of Koutecky–Levich plots (Figure 4f inset) is 3.8, indicating the more efficient four-electron pathways to reduce oxygen to H2O directly during the oxygen reduction reaction (ORR) process. Note that we have found CDs-5 to be more efficient in improving the anticorrosion properties than carbon nanotubes (CNTs) or graphene (Figures S25 and S26, Supporting Information). We attribute this to the small electron transfer number (n) for CNTs and graphene (2.3 and 2.0, respectively), which corresponds to two-electron pathways to reduce oxygen to H2O2 (a more active oxidant than O2) (Figure S27, Supporting Information).
旋转圆盘电极（RDE，参见支持信息的实验部分）和线性扫描伏安法（LSV）测量表明，氧气还原电流密度随着圆盘旋转而增加，如图4f所示。图4f的插图给出了Koutecky-Levich图（逆电流密度j-1对角速度的倒置平方根，在不同的腐蚀电位下为12ω）。从Koutecky-Levich图的斜率（图4f插图）计算的CDs-5的电子转移数（n）为3.8，表明在氧还原反应（ORR）期间直接将氧还原为H2O的四电子通路更有效。处理。注意，我们发现CDs-5在改善抗腐蚀性能方面比碳纳米管（CNT）或石墨烯更有效（图S25和S26，支持信息）。我们将其归因于CNT和石墨烯（分别为2.3和2.0）的小电子转移数（n），其对应于将氧还原为H 2 O 2的双电子途径（比O 2更活跃的氧化剂）（图S27，支持信息） ）。
We have checked the generality of the approach of introducing CDs-5 to polymeric coatings to improve their healing and anticorrosion properties by evaluating four additional commercial polymers (PU1, poly acrylic acid, epoxy, and a fluorocarbon paint). All composites showed improved anticorrosion properties of steel surfaces (Figure S28 and Table S5, Supporting Information). Finally, we propose a plausible mechanism by which CDs improve the healing and the anticorrosion properties of polymer coatings (Figure 4g). CDs-5 have a relatively high specific area due to their nanometric size and are terminated with several surface functional groups (-NH2, -COOH, -OH, -CONH2). The connection and interaction between adjacent CDs-5 or between CDs-5 and polymer chains (see the above self-healing discussion and Figure S17, Supporting Information) lead to self-healing of local pores/cracks, which reduces the formation and prevents the expansion of micropores/capillaries. Thus CDs-5 effectively obstruct crossing oxygen and water through defects paths. Further, the hydrophilic functional groups on the surface of CDs-5 adsorb H2O as well as trap oxygen. The O2 molecules adsorbed on CDs-5 surface are directly reduced to H2O through four-electron pathways (Figure 4g) and thus suppress the corrosion process.
我们已经检查了将CDs-5引入聚合物涂层的方法的一般性，通过评估另外四种商业聚合物（PU1，聚丙烯酸，环氧树脂和氟碳涂料）来改善其愈合和抗腐蚀性能。所有复合材料都显示出改善的钢表面防腐性能（图S28和表S5，支持信息）。最后，我们提出了一种合理的机制，通过该机制，CD可以改善聚合物涂层的愈合和抗腐蚀性能（图4g）。 CDs-5由于其纳米尺寸而具有相对高的比表面积并且以若干表面官能团（-NH 2，-COOH，-OH，-CONH 2）封端。相邻CD-5之间或CDs-5与聚合物链之间的连接和相互作用（参见上述自我修复讨论和图S17，支持信息）导致局部孔隙/裂缝的自我修复，从而减少形成并防止微孔/毛细管扩张。因此，CDs-5有效地阻碍了通过缺陷路径的氧气和水的交叉。此外，CDs-5表面上的亲水性官能团吸附H 2 O以及捕获氧气。吸附在CDs-5表面的O2分子通过四电子通路直接还原为H2O（图4g），从而抑制腐蚀过程。
In summary, we present a simple, versatile, and cost-effective concept for the synthesis of healing (and possibly selfhealing) bulk polymer materials and for self-healing polymer coatings with improved corrosion resistance properties: introduction of CDs, nanometric carbon crystallites terminated by different surface functional groups. The method was successfully tested on four different common bulk polymers (the best of which was PMMA) and other four polymer coatings. The healing of the bulk polymer composites was measured by the tensile strength of fractured rods healed together to restore up to 6.5% of the original tensile strength under dry air that could be increased to 30% by a few second immersion in pure water and to 60% of its original tensile strength by a short dipping in a CDs aqueous solution. The most effective CDs fillers found were CDs-5 which possesses hydroxyl, carboxyl, and amidogen functional groups. Fracturing exposes CDs and polymer chains on the fractured surfaces which interact and bond to each other. Healing/self-healing between the fractured surfaces of the polymers is initiated via bonding between CDs (covalent bonding by formation of C-O-NH or hydrogen bonding between functional groups) or by CDs-polymer chain bonding (between functional groups of CDs and the polymer chain). Van der Waals bonding is optional as well. CDs-5/PU coatings clearly exhibited self-healing properties, as evident from the complete healing with no intervention of a scratch introduced by a surgical blade and by the joining of two cut ribbon pieces put in contact in dry air without applying any pressure. Moreover, CDs-5 as fillers contribute to the improvement of the anticorrosion properties of a variety of polymer coatings due to (1) self-healing and suppression of pores and (2) the four-electron path ORR capability which reduces oxygen to water. The selfhealing and corrosion resistance properties initiated by the concept of introducing CDs to polymers calls for a further and deeper research on one hand and opens the door for discovering potential future applications in industry and engineering on the other hand.
总之，我们提出了一种简单，通用且具有成本效益的概念，用于合成愈合（和可能自愈合）的本体聚合物材料以及具有改善的耐腐蚀性能的自愈合聚合物涂层：引入CD，纳米碳微晶终止于不同的表面官能团。该方法在四种不同的常见本体聚合物（其中最好的是PMMA）和其他四种聚合物涂层上成功测试。通过一起愈合的断裂棒的拉伸强度来测量本体聚合物复合材料的愈合，以在干燥空气下恢复高达原始拉伸强度的6.5％，通过在纯水中浸泡几秒钟可以将其增加至30％并且达到60％。通过在CDs水溶液中短暂浸渍，其原始拉伸强度的％。发现的最有效的CD填料是CDs-5，其具有羟基，羧基和氨基官能团。压裂过程中，裂缝表面的CDs和聚合物链相互作用并结合在一起。聚合体断裂表面的愈合/自愈合是通过CDs(通过C-O-NH或官能团之间的氢键形成共价键)或CDs-聚合物链(CDs官能团与聚合物链之间的官能团)之间的结合开始的。范德瓦尔斯粘合也是可选的。 CDs-5 / PU涂层清楚地表现出自愈合性能，从完全愈合可以看出，没有介入手术刀片引起的划痕和通过在干燥空气中接触而没有施加任何压力的两个切割的带状件的连接。此外，CDs-5作为填料有助于改善各种聚合物涂层的抗腐蚀性能，这是由于（1）自愈合和抑制孔隙和（2）减少氧气到水的四电子路径ORR能力。将CD引入聚合物的概念引发的自愈合和耐腐蚀性能一方面需要进一步深入研究，另一方面为发现工业和工程领域未来的潜在应用打开了大门。