Upcycling of thermoplastic polyurethane into covalent adaptable networks via chain breaking-crosslinking strategy
通过断链-交联策略将热塑性聚氨酯升级再造为共价适应性网络

2.1 Preparation of PU-CANs
2.1 制备 PU-CAN

In order to obtain PU-CANs, commercial TPU was dynamically crosslinked through a two-step method of chain breaking and crosslinking on a micro twin-screw extruder. In the chain breaking stage, polyols (sorbitol, dipentaerythritol) and TPU were blended and extruded through micro twin-screw extruder for 5 min. During extrusion, exchange reaction occurred between hydroxyl group and carbamate bond to produce branched polyurethane, polyurethane oligomer, and so forth (Figure 1A), which is characterized by rheology and Fourier infrared spectroscopy. As shown in the FTIR spectra (Figures 1B, S1, and S2), the peak at 3320 cm⁻¹ corresponds to hydroxyl groups. As the content of polyols increases, the peak of hydroxyl groups significantly increases, the increase of hydroxyl groups in the system is conducive to the formation of a more complete crosslinked network. Meanwhile, the shape of other peaks remained basically unchanged, indicating that no other side reactions occurred. The rheological test is an important method for characterizing polymer morphology. For linear PU, the variation of its complex viscosity with angular frequency in the low frequency region fully conforms to Newtonian fluid law. When the angular frequency increases to the critical value, the viscosity decreases more quickly, that is, shear thinning, which is the characteristic of a typical linear polymer melt. However, long-chain branched PU exhibits continuous shear thinning phenomenon throughout the entire experimental frequency range, which is triggered by the multiple entanglements of long-chain branched PU molecular chains.^{36, 37} Under the action of the shear field, the branch chain with shorter relaxation time takes priority over the main chain to untangle, leading to the shear thinning phenomenon at low angular frequencies. After the untangling of the branch chain is completed, the main chain begins to untangle, consequently shear thinning phenomenon can be displayed over a long frequency range. In other words, the more significant the shear thinning phenomenon, the higher the branching level of the long branch chain. It can be seen from Figure 1C,D that with the increase of polyol content, the branching level of chain-broken polyurethane (B-PU) gradually increases. Moreover, the branching degree of B-PU-Dx is higher, which might be due to the higher reactivity of the primary alcohol. In the high-frequency region, the complex viscosity of B-PU is lower than that of PU0, which is due to the plasticizing effect of low molecular weight PU chains generated during the chain-breaking stage.
为了获得聚氨酯-CAN，商用热塑性聚氨酯在微型双螺杆挤出机上通过断链和交联两步法进行动态交联。在断链阶段，将多元醇（山梨醇、季戊四醇）和热塑性聚氨酯混合，并通过微型双螺杆挤压机挤出 5 分钟。在挤出过程中，羟基和氨基甲酸酯键发生交换反应，生成支化聚氨酯、聚氨酯低聚物等（图1A），并通过流变学和傅立叶红外光谱对其进行表征。如傅立叶红外光谱（图1B、S1和S2）所示，3320 cm^-1处的峰值与羟基相对应。随着多元醇含量的增加，羟基的峰值明显增加，体系中羟基的增加有利于形成更完整的交联网络。同时，其他峰的形状基本保持不变，说明没有发生其他副反应。流变测试是表征聚合物形态的重要方法。对于线性聚氨酯来说，在低频区，其复合粘度随角频率的变化完全符合牛顿流体定律。当角频率增加到临界值时，粘度下降更快，即剪切变稀，这是典型线性聚合物熔体的特征。然而，长链支化聚氨酯在整个实验频率范围内都表现出持续的剪切变稀现象，这是由长链支化聚氨酯分子链的多重缠结引发的。^{36, 37} 在剪切场的作用下，弛豫时间较短的支链会优先于主链解开，从而导致低角频率下的剪切变薄现象。支链松弛完成后，主链开始松弛，因此剪切变薄现象会在较长的频率范围内出现。换句话说，剪切稀化现象越明显，长支链的分枝水平就越高。从图1C、D中可以看出，随着多元醇含量的增加，断链聚氨酯（B-PU）的支化程度逐渐增加。此外，B-PU-Dx 的支化程度更高，这可能是由于伯醇的反应活性更高。在高频区，B-PU 的复合粘度低于 PU0，这是因为在断链阶段产生的低分子量聚氨酯链具有塑化作用。

In the crosslinking stage, the B-PUs and IPDI (OH: NCO = 1:0.9) were extruded and crosslinked in a micro twin-screw extruder for 5 min, and finally PU-CANs were obtained. The crosslinking was characterized by Fourier infrared spectroscopy, gel content, and swelling test. As can be seen from Figures 1E and S3, it is evident that the peak at 3320 cm⁻¹ has shifted from wide and strong after chain-breaking to narrow and weak after crosslinking, possibly owing to the consumption of hydroxyl groups by isocyanates. Moreover, there is no characteristic absorption peak of NCO at 2270 cm⁻¹ and a new characteristic absorption peak of CH₂ appears at 2956 cm⁻¹, which proves the formation of new carbamate bond. Then, THF as a good solvent for PU was used to test the gel content of PU-CANs through the Soxhlet extraction experiment and recorded in Figure 1F and Table S1. As expected, the increase of polyol content can significantly increase the gel content (from 31.7% to 97%) of the sample, that is, more reaction sites (OH) are conducive to consummating the crosslinked network. In the swelling test, the PU-CANs also exhibited well crosslinked networks, they only swelled rather than dissolved in common organic solvents (Tables 1 and S2).
在交联阶段，B-PU 和 IPDI（OH: NCO = 1:0.9）在微型双螺杆挤出机中挤出并交联 5 分钟，最终得到 PU-CAN。傅里叶红外光谱、凝胶含量和膨胀试验对交联情况进行了表征。从图1E和S3中可以看出，3320 cm^-1处的峰值从断链后的宽而强转变为交联后的窄而弱，这可能是由于异氰酸酯消耗了羟基。此外，在 2270 cm^-1 处没有 NCO 的特征吸收峰，而在 2956 cm^-1 处出现了 CH₂ 的新特征吸收峰、这证明形成了新的氨基甲酸酯键。然后，使用四氢呋喃作为聚氨酯的良好溶剂，通过索氏提取实验检测聚氨酯-CAN 的凝胶含量，结果如图1F和表S1所示。不出所料，多元醇含量的增加能显著提高样品的凝胶含量（从 31.7% 提高到 97%），即更多的反应位点（OH）有利于交联网络的形成。在溶胀测试中，PU-CAN 也表现出良好的交联网络，它们在普通有机溶剂中只溶胀而不溶解（表 1 和 S2 ）。

2.2 Thermal and mechanical properties of PU-CANs
2.2 聚氨酯泡沫罐的热性能和机械性能

The increase in the proportion of polyols transformed the dynamic thermomechanical analysis (DMA) characterization of the material from typical thermoplastic curves to thermosetting material curves. The storage modulus of PU0 disappeared at 210 °C due to melting fracture, while the storage modulus of PU-Sx and PU-Dx kept at specific values (platform moduli), which can be used to calculate the crosslink density (Table 2). At this point, it can be concluded that the crosslink density gradually increases with the increase of polyol ratio, which can significantly increase the glass transition temperature (T_g) of polymer (Figure 2B,C and Table 2). The T_g of PU-Dx increased from 5.8 to 76.4 °C, and the T_g of PU-Sx achieved a maximum value of 80.2 °C. It is worth mentioning that the loss factor curve of PU0 (Figure 2B) gradually increases with the increase of temperature after 150 °C, contributing to the increase in motion ability caused by melting. While the curve of PU-D5, with the highest crosslinking degree, presents a straight line, which is also indicative of the complete crosslinking of the material. Compared with the tan δ curve of PU-Dx and PU-Sx, the PU-Dx exhibit obvious single peak below 150 °C, while the PU-Sx possess double peak, indicating that PU-Sx were composed of two phases. It might be attributed to the lower reaction activity of secondary alcohols, most of which react with isocyanate rather than participate in the exchange of carbamate. Therefore, it is speculated that the two phases are PU network and sorbitol network with large difference in crosslink density.
多元醇比例的增加使材料的动态热机械分析（DMA）特性从典型的热塑性曲线转变为热固性材料曲线。PU0 的储存模量在 210 °C 时因熔融断裂而消失，而 PU-Sx 和 PU-Dx 的储存模量保持在特定值（平台模量），可用来计算交联密度（表 2）。由此可以得出结论，交联密度随着多元醇比率的增加而逐渐增大，这可以显著提高聚合物的玻璃化转变温度（T_g ）（图2B、C和表2）。PU-Dx 的 T_g 从 5.8 ℃ 升至 76.4 ℃，PU-Sx 的 T_g 达到 80.2 ℃ 的最大值。值得一提的是，PU0 的损耗因子曲线（图2B）在 150 °C 之后随着温度的升高而逐渐增大，这也是熔化导致运动能力增强的原因。而交联度最高的 PU-D5 的曲线呈现一条直线，这也表明材料已完全交联。与 PU-Dx 和 PU-Sx 的 tan δ 曲线相比，PU-Dx 在 150 °C 以下表现出明显的单峰，而 PU-Sx 则具有双峰，表明 PU-Sx 由两相组成。这可能是因为仲醇的反应活性较低，它们大多与异氰酸酯发生反应，而不是参与氨基甲酸酯的交换。 因此，推测这两相为交联密度相差较大的聚氨酯网络和山梨醇网络。

The mechanical properties were evaluated via tensile test, the results are summarized in Figure 2D–F and Table 3. Obviously, the tensile strength (9.65–21.9 MPa) and elongation at break (308%–928%) of PU-Sx cannot be compared with commercial PU0, only modulus (272–480 MPa) increases. On the contrary, the PU-Dx exhibit an obvious increase in strength (30.2–53.3 MPa) and modulus (324–805 MPa) as the crosslink density increases, but a slight decrease in elongation at break (719%–1150%). The high crosslink density of PU-D5 results in the lowest elongation at break and limited tensile strength. Chemically crosslinked network significantly improved the elastic recovery rate of materials. The poor mechanical performance of PU-Sx might be due to the non-uniformity of their networks, as demonstrated by the aforementioned DMA test. The weak parts of the networks (regions with low crosslink density) might make the sample more prone to fracture. As illustrated in Figures 2F and S5, the elastic recovery rate of PU-CANs was obtained after 10 cycles of cyclic stretching at an elongation of 500%. As the crosslink density increases, the elastic recovery rate of the samples increases (from 58.2% to 69.6%), much better than the commercial TPU.
机械性能通过拉伸试验进行评估，结果汇总于图2D-F和表3。显然，PU-Sx 的拉伸强度（9.65-21.9 兆帕）和断裂伸长率（308%-928%）无法与商用 PU0 相比，只有模量（272-480 兆帕）有所增加。相反，随着交联密度的增加，PU-Dx 的强度（30.2-53.3 兆帕）和模量（324-805 兆帕）明显增加，但断裂伸长率（719%-1150%）略有下降。PU-D5 的高交联密度导致其断裂伸长率最低，拉伸强度有限。化学交联网络大大提高了材料的弹性恢复率。如上述 DMA 测试所示，PU-Sx 机械性能不佳的原因可能是其网络不均匀。网络的薄弱部分（交联密度低的区域）可能会使样品更容易断裂。如图2F和S5所示，PU-CAN 的弹性恢复率是在伸长率为 500% 的条件下循环拉伸 10 次后得出的。随着交联密度的增加，样品的弹性恢复率也在增加（从 58.2% 增加到 69.6%），远远优于商用热塑性聚氨酯。

2.3 Stress relaxation and reprocessing of PU-CANs
2.3 应力松弛和 PU-CAN 的后处理

Relaxation time is a crucial index for characterizing the reprocessing performance of CANs, which is related to the crosslink density of network, the activation energy of dynamic bonds, and so on. The relaxation time was obtained by selecting four temperatures for all samples through DMA relaxation mode testing (Figures 3A and S6–S13). Overall, the ratio of modulus after relaxation for certain time (G) and modulus before relaxation (G₀) of all samples can reach 1/e within 100 s at a specific temperature, and the increase of crosslink density significantly reduces their relaxation rate, which is due to the network rearrangement time gradually prolonging with the crosslinking degree of the network. As presented in Figure 3B,C, The PU-Sx required relatively longer relaxation times as a result of low reactivity of secondary alcohols. It is worth mentioning that PU-D5 and PU-S4 exhibit significantly longer relaxation times at 160 °C, which is attributed to their complete crosslinking networks, and the free polymer chains have a significant acceleration effect on the relaxation of CANs.³⁸ The activation energies (E_a) of the PU-Dx and PU-Sx were calculated using the Arrhenius formula (Figures 3D and S12). The E_a of PU-Dx decreases with the increase of crosslink density, while the E_a trend of PU-Sx is opposite and not significantly different. With the increase in crosslink density and hydroxyl content, the bond exchange ability of PU-Dx was enhanced. As previously demonstrated, the uneven distribution of PU-Sx network might result in the enrichment of hydroxyl groups in highly crosslinked regions, so the amount of hydroxyl groups has little effect on bond exchange ability.
松弛时间是表征 CAN 再加工性能的一个重要指标，它与网络的交联密度、动态键的活化能等有关。通过 DMA 松弛模式测试（图3A和S6-S13），为所有样品选择了四个温度，从而得到松弛时间。总的来说，在特定温度下，所有样品在一定时间内松弛后的模量（G）与松弛前的模量（G₀）之比都能在 100 s 内达到 1/e，而交联密度的增加会显著降低其松弛速率，这是由于网络重排时间随着网络交联度的增加而逐渐延长。如图3B、C所示，由于仲醇的反应活性较低，PU-Sx 需要的弛豫时间相对较长。值得一提的是，PU-D5 和 PU-S4 在 160 °C 时的松弛时间明显更长，这是因为它们具有完整的交联网络，游离聚合物链对 CAN 的松弛具有显著的加速作用。³⁸ 激活能（E_a ）（图 3D 和 S12 ）。 PU-Dx 的 E_a 随交联密度的增加而减小，而 PU-Sx 的 E_a 则相反，无显著差异。随着交联密度和羟基含量的增加，PU-Dx 的键交换能力增强。如前所述，PU-Sx 网络的不均匀分布可能会导致羟基在高交联区域富集，因此羟基的数量对键交换能力影响不大。

Thanks to the efficient stress relaxation of the material, all samples can be reprocessed using traditional PU processing methods such as extrusion and hot-press reprocessing. As shown in Figure 4A, PU-CANs can be extruded via twin-screw extruder and hot pressed into thin films at 190 °C for 5 min, which suggests that the recycling or reuse of PU-CANs can be easily achieved. The recycled PU-D4s (PU-D4-R1 and PU-D4-R2) have been subjected to tensile test, manifesting that their tensile strength recovery rates are 76.5% and 68.7%, their recovery rates of elongation at break are 93.4% and 91.9% (Figure 4B). Moreover, the FTIR spectra of PU-D4 before and after recycling manifest that the characteristic peaks and their height are basically consistent (Figure 4C), indicating that hot-press recycling would not affect the chemical structure of the material. After multiple reprocessing, the crosslink density (calculated by the platform modulus at 210 °C) and T_g of PU-D4 decreased slightly through DMA testing (Figure 4D,E). This might be due to the breakage of polyurethane chains during the remolding, which also explains the reduction of the mechanical properties.
由于材料的应力松弛效率高，所有样品都可以使用传统的聚氨酯加工方法进行再加工，如挤出和热压再加工。如图4A所示，PU-CAN 可通过双螺杆挤出机挤出，并在 190 °C 下热压 5 分钟制成薄膜，这表明 PU-CAN 可轻松实现回收或再利用。对回收的 PU-D4（PU-D4-R1 和 PU-D4-R2）进行拉伸试验，结果表明它们的拉伸强度回收率分别为 76.5%和 68.7%，断裂伸长率回收率分别为 93.4%和 91.9%（图 4B ）。此外，PU-D4 在回收前后的傅立叶变换红外光谱显示，特征峰及其高度基本一致（图4C），表明热压回收不会影响材料的化学结构。经过多次再加工后，通过 DMA 测试，PU-D4 的交联密度（按 210 °C 时的平台模量计算）和 T_g 略有下降（图 4D,E ）。这可能是由于聚氨酯链在重塑过程中断裂，这也是机械性能降低的原因。

2.4 Improvement in creep resistance
2.4 提高抗蠕变性

Generally, PU fixes the molecular chain through a large number of intermolecular hydrogen bonds, but as a physical crosslinking point, its binding energy is significantly lower than the covalent bond. Besides the large number of hydrogen bonds, the dynamic covalently crosslinked networks of the PU-CANs limit the movement of molecular chains and endow the materials with better dimensional stability. As can be seen from Figure 5A, after 4 min in a 250 °C oven, the original TPU slice (PU0) melted and shrank (Video S1), while PU-D4 slice showed a complete and stable size. This can be explained by the melting of PU0 in a 250 °C oven, while the highly covalently crosslinked network maintained the shape of the PU-D4. Creep experiments were performed on all samples at 50 °C via DMA, and the results are shown in Figure 5B,C. Obviously, the strain recovery rates of highly crosslinked samples such as PU-D5 (87%) and PU-S4 (82%) are better than 66% of PU0, and the recovery rate increased with the increase of crosslink density. However, there are several exceptions. The creep recovery rate of PU-D1 and PU-D2 was not significantly improved and even worse, which might be due to the inability to construct a complete crosslinked network with such low polyol loading, and the negative effect from the chain breaking. It is worth mentioning that PU-Sx have relatively good creep resistance at low polyol loading, and the recovery rate increases with the addition content. Their more complete crosslinked networks (higher gel content at the same loading) should be the main reason.
一般来说，聚氨酯通过大量分子间氢键固定分子链，但作为物理交联点，其结合能明显低于共价键。除了大量氢键外，聚氨酯-CAN 的动态共价交联网络还限制了分子链的移动，使材料具有更好的尺寸稳定性。从图5A中可以看出，在 250 °C 的烘箱中放置 4 分钟后，原始热塑性聚氨酯切片（PU0）熔化并收缩（视频S1) ，而 PU-D4 切片则显示出完整稳定的尺寸。这可以解释为 PU0 在 250 °C 的烘箱中熔化，而高度共价交联的网络保持了 PU-D4 的形状。通过 DMA 在 50 °C 下对所有样品进行了蠕变实验，结果如图5B、C所示。显然，PU-D5（87%）和 PU-S4（82%）等高交联样品的应变恢复率优于 PU0 的 66%，而且恢复率随着交联密度的增加而增加。但也有几个例外。PU-D1 和 PU-D2 的蠕变恢复率没有明显改善，甚至更差，这可能是由于在如此低的多元醇负载量下无法构建完整的交联网络，以及断链带来的负面影响。值得一提的是，PU-Sx 在较低的多元醇添加量下具有相对较好的抗蠕变性，且恢复率随添加量的增加而增加。主要原因应该是它们的交联网络更完整（相同添加量下的凝胶含量更高）。

4.1 Materials 4.1 材料

Polyurethane (PU, trade name 1190A) and the antioxidant (Irganox 1010) were purchased from BASF. Polypropylene (PP, trade name B4808) was provided by Sinopec. Dipentaerythritol (Di-PE, 90%), sorbitol (99%), isophorone diisocyanate (IPDI, 99%), N, N-dimethylformamide(DMF), THF, toluene, chloroform, methanol, ethanol, and acetone were bought from Aladdin. PU pellets, Di-PE, Irganox 1010, and sorbitol were dried in vacuum oven at 70 °C for 12 h before use, and IPDI was used as received.
聚氨酯（PU，商品名 1190A）和抗氧化剂（Irganox 1010）购自 BASF。聚丙烯（PP，商品名 B4808）由中国石化提供。二季戊四醇（Di-PE，90%）、山梨醇（99%）、异佛尔酮二异氰酸酯（IPDI，99%）、N，N-二甲基甲酰胺（DMF）、四氢呋喃、甲苯、氯仿、甲醇、乙醇和丙酮购自阿拉丁公司。聚氨酯颗粒、Di-PE、Irganox 1010 和山梨醇在使用前在 70 °C 的真空烘箱中干燥 12 小时，IPDI 按收到的原样使用。

4.2 Preparation of PU-CANs
4.2 制备 PU-CAN

To avoid the impact of water, all materials (PU pellets, Di-PE, Irganox 1010, and sorbitol) were dried in a 70 °C vacuum oven for 12 h until the vacuum level did not change. As a representative example, PU pellets (15 g), Irganox 1010 (0.06 g), and Di-PE (0.45 g) were mixed evenly in a plastic bag. Later, the mixture was added into micro twin-screw extruder (SJZS-7A) bought from Wuhan Ruiming Co., Ltd through a hopper. Under nitrogen protection, the screw speed was set to 40 rpm and the four temperature ranges were set to 30, 150, 180, and 190 °C. After the mixture was completely added, it was subjected to cyclic extrusion for 5 min to get B-PU for testing. Then, IPDI (0.99 g) was added to the feeding port and cyclicly extruded for 5 min to obtain PU-CANs. Before each use of micro twin-screw extruder, it was cleaned with PP pellets for 3–4 times. The feed compositions of other different samples are listed in Table 4.
为了避免水分的影响，所有材料（聚氨酯颗粒、Di-PE、Irganox 1010 和山梨醇）都在 70 °C 的真空烘箱中干燥 12 小时，直到真空度没有变化为止。举例来说，将聚氨酯颗粒（15 克）、Irganox 1010（0.06 克）和 Di-PE （0.45 克）在塑料袋中混合均匀。然后，通过料斗将混合物加入从武汉瑞明有限公司购买的微型双螺杆挤出机（SJZS-7A）中。在氮气保护下，螺杆转速设定为 40 rpm，四个温度范围设定为 30、150、180 和 190 °C。混合物完全加入后，进行 5 分钟的循环挤压，得到供测试用的 B-PU。然后，在喂料口加入 IPDI（0.99 克）并循环挤压 5 分钟，得到 PU-CAN。每次使用微型双螺杆挤出机前，都要用 PP 粒子清洗 3-4 次。表 4 列出了其他不同样品的喂料成分。

4.3 Swelling tests 4.3 膨胀试验

The gel content of PU-CANs was characterized by the Soxhlet extraction method. The sample (around 0.1 g) was wrapped with filter paper. The THF was used as the extraction solution and reflux for 24 h. Later, the samples were dried at 60 °C for 24 h in a vacuum oven. The gel content was calculated by m₁/m₀, where m₀ is the initial mass and m₁ is the final mass after drying.
采用索氏提取法对 PU-CANs 的凝胶含量进行表征。样品（约 0.1 克）用滤纸包裹，以四氢呋喃为萃取液，回流 24 小时。然后在真空烘箱中于 60 °C 下干燥 24 小时。凝胶含量的计算公式为：m₁/m₀ 、其中，m₀ 为初始质量，m₁ 为干燥后的最终质量。

After soaking in ordinary solvents (tetrahydrofuran, toluene, chloroform, methanol, ethanol, DMF, acetone, and water) at 25 °C for 12 h, the solvent resistance was verified by observing the integrity of the sample and calculating the swelling ratio. The swelling ratio was calculated by m₁/m₀, where m₀ is the initial mass and m₁ is the final mass after swelling.
在 25 °C的普通溶剂（四氢呋喃、甲苯、氯仿、甲醇、乙醇、DMF、丙酮和水）中浸泡 12 小时后，通过观察样品的完整性和计算膨胀率来验证耐溶剂性。膨胀率的计算公式为：m₁/m₀ 、其中，m₀ 为初始质量，m₁ 为膨胀后的最终质量。

4.4 Dynamic thermomechanical analysis measurement
4.4 动态热机械分析测量

The stress relaxation test was conducted via a Q800 DMA (TA instruments, America) using a stress relaxation mode. The samples were examined between 150 °C and 180 °C by applying a constant strain of 10%. Before the tests, all the samples were held at the test temperatures for 5 min to achieve thermal equilibrium and preloaded a 1 × 10⁻³ N force to prevent the samples from bending.
应力松弛测试通过 Q800 DMA（美国 TA 仪器公司）使用应力松弛模式进行。通过施加 10% 的恒定应变，在 150 °C 和 180 °C 之间对样品进行了测试。测试前，所有样品在测试温度下保持 5 分钟以达到热平衡，并预先加载 1 × 10^-3 N 的力以防止样品弯曲。

Creep experiments were performed on a Q800 DMA (TA instruments, America). The samples were exposed to a constant stress of 1 MPa for 30 min at the test temperature, after which the tension was removed and a 30 min recovery period was conducted. The strain of the sample was recorded throughout this process.
蠕变实验在 Q800 DMA（美国 TA 仪器）上进行。样品在试验温度下承受 1 兆帕的恒定应力 30 分钟，然后去除拉力，再进行 30 分钟的恢复期。在整个过程中记录样品的应变。

4.5 Mechanical property 4.5 机械性能

The samples were compression-molded at 10 MPa and 190 °C for 5 min. The tensile properties of the PU-CANs were measured using an Instron 5567 Electric Universal Testing Machine (Instron, America) at room temperature. A gauge length of 20 cm, a preload force of 0.1 N, and a cross-head speed of 100 mm min⁻¹ was used. Each sample with dimensions of around 400 mm (length) × 5 mm (width) × 0.2 mm (thickness) was measured at least three times for accuracy.
样品在 10 兆帕和 190 摄氏度的条件下压缩成型 5 分钟。使用 Instron 5567 电动万能试验机（Instron，美国）在室温下测量了 PU-CAN 的拉伸性能。仪器长度为 20 厘米，预紧力为 0.1 牛顿，十字头速度为 100 毫米/分钟^-1。每个样品的尺寸约为 400 毫米（长）×5 毫米（宽）×0.2 毫米（厚），至少测量三次，以确保准确性。

4.6 Rheological behavior 4.6 流变行为

Extrusion-obtained B-PU was hot pressed at 190 °C for 1 min using press vulcanizer XLB-D produced by Huzhou Shunli Machinery Co., Ltd. to prepare circular plates for rheological testing. The rheological behavior of B-PU was observed using a Discovery HR-3 Rheometer (TA Instrument, America) operated with a 25 mm diameter parallel plate geometry. Frequency sweep experiments were performed at 190 °C with a strain of 1%. The variations of the complex viscosity, storage modulus, and loss modulus were monitored as a function of frequency.
使用湖州顺力机械有限公司生产的 XLB-D 加压硫化机在 190 ℃ 下热压 1 分钟，制备用于流变测试的圆板。使用 Discovery HR-3 流变仪（美国 TA 仪器公司）观察 B-PU 的流变行为，平行板的几何形状直径为 25 毫米。频率扫描实验在 190 °C 和 1%应变条件下进行。监测了复合粘度、存储模量和损耗模量随频率的变化。

4.7 Fourier transform infrared characterization
4.7 傅立叶变换红外特性分析

The B-PU samples were first dissolved in THF and then precipitated in water to wash away free polyols, and dried in a vacuum oven at 80 °C for 24 h. Infrared spectroscopy (FTIR-ATR) was recorded on an Agilent Micro-FTIR Cary660 with a scanning range of 4000–400 cm⁻¹ and 32 scans.
B-PU 样品首先溶解在 THF 中，然后在水中沉淀以洗掉游离的多元醇，并在 80 °C 的真空烘箱中干燥 24 小时。红外光谱（FTIR-ATR）在 Agilent Micro-FTIR Cary660 上记录，扫描范围为 4000-400 cm^-1 ，扫描次数为 32 次。

4.8 Processability 4.8 可加工性

The processability of the PU-CANs was verified by extrusion twice in a micro twin-screw extruder. Under nitrogen protection, the screw speed was set to 40 rpm and the four temperature ranges were set to 30, 170, 180, and 190 °C. Compression molding was performed on a flat-panel vulcanizer XLB-D produced by Huzhou Shunli Machinery Co., Ltd. The extruded sample was compression-molded at 10 MPa and 190 °C for 5 min to prepare a 10 mm (length) × 10 cm (width) × 0.2 mm (thickness) film.
通过在微型双螺杆挤压机中挤压两次，验证了聚氨酯-CAN 的加工性能。在氮气保护下，螺杆转速设定为 40 rpm，四个温度范围设定为 30、170、180 和 190 °C。在湖州顺力机械有限公司生产的平板硫化机 XLB-D 上进行压缩成型。挤压样品在 10 兆帕和 190 °C的条件下压缩成型 5 分钟，制备出 10 毫米（长）×10 厘米（宽）×0.2 毫米（厚）的薄膜。