2.1 Structural Design and Performance Characteristics of PU-ssPCM
2.1 PU-ss相变材料的结构设计及性能特点

To achieve simultaneously high thermal energy storage and mechanical properties, the chemical structure of the flexible PCM needs to be carefully chosen and designed. Specifically, a polyurethane-based solid-solid PCM polymer (denoted as PU-ssPCM) was prepared from hexamethylene diisocyanate (HDI), polyethylene oxide (PEO), and polycaprolactone (PCL) with D-sorbitol as a chain extender through a pre-polymerization method (Figure S1 and more details can be found in the Supporting Information). The advantages are that the introduction of ether groups can separate adjacent methylene groups, thereby weakening the mutual repulsion force between the hydrogen atoms of two methylene groups and contributing to the flexibility of PU-ssPCM. In addition, ester groups (COOR) with high cohesive energy can generate significant intermolecular forces, improving the stiffness of the PU-ssPCM. To confirm the formation of PU-ssPCM, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was performed. The disappearance of the infrared absorption peak of N═C═O at the wavenumber of 2283 cm⁻¹ indicated that the NCO group was completely converted and the polycondensation reaction of PU-ssPCM completed (Figure 1b). Moreover, a multi-hydrogen bonding interaction network structure was easily generated as follows: 1) The positive configuration hydrogen bonding between -NH and C═O bonds formed in the ethyl carbamate group; 2) Crossed-configuration hydrogen bonding between the -NH- bond and COC bonds. The PU-ssPCM with an ultra-long molecular chain structure consists of two segmented blocks (i.e., soft segments and hard segments). No difference was observed, and no liquid leakage was observed on the surface of the PU-ssPCM at 25 and 80 °C, indicating that the sample could maintain stable shaping (Figure 1a). The reason is that PU-ssPCM with a crystalline structure undergoes a solid-to-solid phase transition at high temperatures. Consequently, the synergistic construction of chemical crosslinking and physical crosslinking enabled the PU-ssPCM to achieve shape stability at different temperatures (Figure 1a (upper right)). The PU-ssPCM with the characteristic functional group (NHCOO) was essentially endowed with the expected performance of flexibility, stability, and self-supporting shaping through the combination of these factors (Figure 1a,b).
为了同时实现高热能存储和机械性能，需要仔细选择和设计柔性相变材料的化学结构。具体来说，以六亚甲基二异氰酸酯 （HDI）、聚环氧乙烷 （PEO） 和聚己内酯 （PCL） 为基聚氨酯基固-固相材料聚合物（表示为 PU-ssPCM）通过预聚合方法制备，其中 D-山梨醇作为扩链剂（图 S1 和更多详细信息可在支持信息中找到）。优点是醚基的引入可以分离相邻的亚甲基，从而减弱两个亚甲基的氢原子之间的相互排斥力，有助于 PU-ssPCM 的柔韧性。此外，具有高内聚能的酯基 （COOR） 可以产生显着的分子间作用力，从而提高 PU-ssPCM 的刚度。为了确认 PU-ssPCM 的形成，进行了衰减全反射傅里叶变换红外光谱 （ATR-FTIR）。N═C═O 在 2283 ^cm-1 波数处的红外吸收峰消失，表明 NCO 基团完全转化，PU-ssPCM 的缩聚反应完成（图 1b）。此外，多氢键相互作用网络结构很容易产生如下：1） 氨基甲酸乙酯基团中形成的 -NH 和 C═O 键之间的正构型氢键;2） -NH- 键和 COC 键之间的交叉构型氢键。具有超长分子链结构的 PU-ssPCM 由两个分段段（即软段和硬段）组成。 未观察到差异，在 25 °C 和 80 °C 时 PU-ssPCM 表面未观察到液体泄漏，表明样品可以保持稳定的形状（图 1a）。原因是具有晶体结构的 PU-ssPCM 在高温下发生固-固相转变。因此，化学交联和物理交联的协同结构使 PU-ssPCM 能够在不同温度下实现形状稳定性（图 1a（右上））。通过这些因素的组合，具有特征官能团 （NHCOO） 的 PU-ssPCM 基本上被赋予了柔韧性、稳定性和自支撑成型的预期性能（图 1a、b）。

Additionally, to elucidate the correlation between the structure and properties of the PU-ssPCM, we conducted supplementary analyses. The stacked fold morphology could be observed in the SEM images, which were produced by curling and stacking the long molecular chain (Figure S2, Supporting Information). It was also found that the peak value of the mechanical loss (tan δ) gradually increased as the input amount of PCL increased (Figure S3, Supporting Information), indicating that the activation energy required for molecular segment or functional group activation increased. This was related to the gradual increase in the degree of crosslinking of the chemical structure and the limitation level of the molecular chains. PCL and D-sorbitol, with their multi-hydrogen bonding characteristics, possess the advantage of providing more active sites. It was found that the absorption peak of the amino group (N-H) at a wavenumber of 3337 cm⁻¹ and the stretching vibration absorption peak of the C═O bond at a wavenumber of 1622 cm⁻¹ (hydrogen bonding) became wider and stronger with an increase in PCL dosage (Figure S4, Supporting Information). This further verifies the generation of additional hydrogen bond formation sites. Hydrogen bonds exist not only in hard segments but also within soft segments. This structural characteristic could be conducive to increasing the thermal stability of PU-ssPCM as shown in Figure S5 (Supporting Information). Furthermore, when the molar ratio of PEO to PCL was 1:400, the maximum elongation at break of PU-ssPCM was 1703%, and the tensile strength was 10.4 MPa under normal temperature and pressure (Figure 1a,d (bottom right); Figure S6, Supporting Information). To the best of our knowledge, this elongation at break is very significant and high-level for ssPCMs. When the dosage of PCL was low, the crosslinking degree or rigid segments of the structure were not sufficient to support high mechanical properties, especially the elongation at break. With an increase in PCL dosage, the hard segments of the polymer skeleton increased, accompanied by an increase in the degree of crosslinking. However, the asymmetry of the molecular chain segments affects the mechanical properties, and the elongation at the break of the flexible PCM decreases (Figure S6, Supporting Information). Therefore, when the molar ratio of PEO to PCL was 1:400, the similar length between two adjacent cross-linked molecular chains contributed to constructing a homogeneous network structure for PU-ssPCM, effectively avoiding stress concentration on shorter chains. This homogeneous network structure can manipulate the uniform distribution of load stress and avoid fracture nucleation on shorter molecular chains. This kind of structural symmetry not only prompted PU-ssPCM to maintain shape stability but also presented the advantages of few limitations and little resistance to deformation (Figure S6, Supporting Information). When the material is deformed by an external force, the net chains are oriented and elongated along the direction of the external force, similar to the stretching state of the muscle (Figure 1a (bottom right)). When the material was not subjected to external forces or exposed to external stimuli, the oriented alignment of net chains was restored, which was similar to the contraction of the muscle (Figure 1a (bottom right)). Therefore, inspired by the structure of skeletal muscles, the construction of numerous hydrogen-bonding interactions and homogeneous networks favors the PU-ssPCM with high toughness and exceptional flexibility (Figure 1a).
此外，为了阐明 PU-ssPCM 的结构和性能之间的相关性，我们进行了补充分析。在 SEM 图像中可以观察到堆叠折叠形态，这些图像是通过卷曲和堆叠长分子链产生的（图 S2，支持信息）。研究还发现，随着 PCL 输入量的增加，机械损失的峰值 （tan δ） 逐渐增加（图 S3，支持信息），表明分子段或官能团激活所需的活化能增加。这与化学结构交联程度的逐渐增加和分子链的限制水平有关。PCL 和 D-山梨醇具有多氢键特性，具有提供更多活性位点的优势。结果发现，随着 PCL 剂量的增加，波数为 3337 ^cm-1 的氨基吸收峰 （N-H） 和 C═O 键在 1622 ^cm-1 的波数处的拉伸振动吸收峰（氢键）变得更宽、更强（图 S4，支持信息）。这进一步验证了其他氢键形成位点的产生。氢键不仅存在于硬链段中，也存在于软链段中。这种结构特性可能有助于提高 PU-ssPCM 的热稳定性，如图 S5（支持信息）所示。 此外，当 PEO 与 PCL 的摩尔比为 1：400 时，PU-ssPCM 的最大断裂伸长率为 1703%，在常温常压下拉伸强度为 10.4 MPa（图 1a，d（右下）;图 S6，支持信息）。据我们所知，这种断裂伸长率对于 ssPCM 来说非常显着且水平很高。当 PCL 的用量较低时，结构的交联度或刚性段不足以支持高机械性能，尤其是断裂伸长率。随着 PCL 剂量的增加，聚合物骨架的硬段增加，伴随着交联程度的增加。然而，分子链段的不对称性会影响机械性能，并且柔性 PCM 的断裂伸长率降低（图 S6，支持信息）。因此，当 PEO 与 PCL 的摩尔比为 1：400 时，两条相邻交联分子链之间的相似长度有助于构建 PU-ssPCM 的均匀网络结构，有效避免应力集中在较短链上。这种均相网络结构可以纵载荷应力的均匀分布，并避免在较短的分子链上发生断裂成核。这种结构对称性不仅促使 PU-ssPCM 保持形状稳定性，而且还具有限制少、抗变形能力小等优点（图 S6，支持信息）。当材料因外力而变形时，网链沿外力的方向定向和拉长，类似于肌肉的拉伸状态（图 1a（右下））。 当材料不受外力或受到外部刺激时，网链的定向排列得以恢复，这类似于肌肉的收缩（图 1a（右下））。因此，受骨骼肌结构的启发，许多氢键相互作用和均相网络的构建有利于具有高韧性和卓越柔韧性的 PU-ssPCM（图 1a）。

Generally, soft and hard segments have random distributions and ordered arrangements in their respective phases. The hard-segment phase is composed of small particles with crystalline microdomains of varying sizes and densities. The average size was approximately dozens of nanometers, as shown in the AFM images (Figure S7, Supporting Information). In addition, the XRD patterns and POM images corroborated the crystalline molecular structure of the PU-ssPCM (Figure 1c; Figures S8 and S9, Supporting Information). Generally, polymerization can inhibit the movement of molecular chains. The increasing amounts of PCL caused more formation of carbamate, and thus, the number and area of crystalline micro-zones of the hard segment embedded in the crystalline regions of the soft segment gradually increased (Figure S10, Supporting Information). Restricted crystallization led to a decrease in crystallinity (Figure S10, Supporting Information). The increasing number of crystalline microdomains of the hard segment in the crystal regions of the soft segment might lead to incomplete two-phase separation. Therefore, the formation and existence of the microphase separation structure in the PU-ssPCM were related to the formation of a continuous hard segment (Figure S7, Supporting Information). Furthermore, the free movement of the molecular chain segments was limited by the isocyanate skeleton, resulting in an increase in the degree of hydrogen bonding and a decrease in enthalpy (Figure S11, Supporting Information). When the molar ratio of PEO to PCL was 1: 400, PU-ssPCM exhibited high chemical stereoregularity of molecular chain and possessed relatively high latent heat storage and release capacity (∆H_m = 110.3 J g⁻¹, ∆H_c = 109.5 J g⁻¹) (Figure 1c; Figure S11, Supporting Information).
通常，软段和硬段在各自的阶段具有随机分布和有序排列。硬段相由具有不同大小和密度的结晶微域的小颗粒组成。平均尺寸约为数十纳米，如 AFM 图像所示（图 S7，支持信息）。此外，XRD 图谱和 POM 图像证实了 PU-ssPCM 的晶体分子结构（图 1c;图 S8 和 S9，支持信息）。通常，聚合可以抑制分子链的运动。PCL 量的增加导致氨基甲酸酯的形成增加，因此，嵌入软链段结晶区域中的硬链段结晶微区的数量和面积逐渐增加（图 S10，支持信息）。结晶受限导致结晶度降低（图 S10，支持信息）。在软链段的晶体区域中，硬链段的晶体微域数量增加可能会导致两相分离不完全。因此，PU-ssPCM 中微相分离结构的形成和存在与连续硬链段的形成有关（图 S7，支持信息）。此外，分子链段的自由运动受到异氰酸酯骨架的限制，导致氢键程度增加而焓降低（图 S11，支持信息）。 当 PEO 与 PCL 的摩尔比为 1：400 时，PU-ssPCM 表现出较高的分子链化学立体规则性，并具有相对较高的潜热储存和释放能力 （∆H_m = 110.3 J ^g-1， ∆H_c = 109.5 J ^g-1） （图 1c;图 S11，支持信息）。

In brief, a typical sample (PU-ssPCM), prepared via a molar ratio of 1: 400 between PEO and PCL, simultaneously achieved excellent flexibility, high toughness (σ = 10.4 MPa, ε = 1703%), and latent heat storage/release capacity (∆H_m = 110.3 J g⁻¹, ∆H_c = 109.5 J g⁻¹). This can lay a solid foundation for achieving programmable deformations and exploring multi-responsive performance. In the entire structure, the long carbon chains with regular arrangements can be regarded as the crystalline skeleton of macromolecules, and the chain segments of each level remain relatively independent, which plays an important role in the physical crosslinking network. The reversible switching mechanism is associated with the temperature-dependent sequential transition of crystalline domains from an ordered arrangement to the amorphous state (Figure 1a; Figure S12, Supporting Information). Consequently, a multi-hydrogen bond network and continuous crystal phase with temperature dependency collaboratively constructed a 3D dynamic network structure of the PU-ssPCM.
简而言之，通过 PEO 和 PCL 之间 1：400 的摩尔比制备的典型样品 （PU-ssPCM） 同时获得了优异的柔韧性、高韧性（σ = 10.4 MPa，ε = 1703%） 和潜热储存/释放能力（∆H_m = 110.3 J ^g-1，∆H_c = 109.5 J ^g-1）。这可以为实现可编程变形和探索多响应性能奠定坚实的基础。在整个结构中，排列规则的长碳链可被视为大分子的结晶骨架，各能级的链段保持相对独立，在物理交联网络中起着重要作用。可逆开关机制与晶畴从有序排列到非晶态的温度依赖性顺序转变有关（图 1a;图 S12，支持信息）。因此，多氢键网络和具有温度依赖性的连续晶相协同构建了 PU-ssPCM 的三维动态网络结构。

2.2 Preparation and Properties of PU-ssPCM/GNP
2.2 PU-ssPCM/GNP的制备和性能

To further improve the thermal conductivity and enhance the photo-to-thermal energy conversion performance, graphene with a 2D hexagonal honeycomb-like single-layer lattice structure was selected for introduction into the self-supporting PU-ssPCM matrix. Nanosized graphene can effectively decrease the thermal resistance in polymers, efficiently improving the thermal interface conductivity and accelerating heat transfer. In this study, graphene was uniformly dispersed in the network structure of PU-ssPCM based on a suitable solvent system through ultrasonic cavitation. The microscopic surface morphology and microstructure of the PU-ssPCM/GNP membranes are shown in Figure S13 (Supporting Information). A diagrammatic sketch of the synthesis process and chemical structure of PU-ssPCM/GNP and the interaction mechanism between graphene and the molecular chains of PU-ssPCM are exhibited in Figure 2a. The ordered crystal structure of graphene can be used to replace partially disordered PU-ssPCM; thus, local thermal chains of PU-ssPCM/GNP were in contact with each other as the proportion of graphene increased, forming a thermally connected network. Consequently, although the effects of crystal defects, interface thermal resistance, and phonon scattering are inevitable at heterogeneous interfaces (Figure S14a, Supporting Information), the thermal conductivity of PU-ssPCM/GNP increased by 100.6% over that of PU-ssPCM (Figure S14b, Supporting Information). In addition, the hydrogen bonding interaction between graphene and net-chains of PU-ssPCM can be seen as a “heat transfer bridge” for constructing a 3D thermal conductivity network, which contributes to further reducing the interfacial thermal resistance. The regular crystalline arrangement of PU-ssPCM/GNP also increased the thermal conductivity for molecular chain interactions.^[13] Therefore, the physical cross-linked network construction between the continuous crystal phase and hydrogen bonding interactions can provide a fast channel for phonon thermal conduction. Thermal transfer can be rapidly achieved from the high-temperature region to the low-temperature region through phonon vibrations.
为了进一步提高热导率和增强光热能转换性能，选择具有 2D 六边形蜂窝状单层晶格结构的石墨烯引入自支撑 PU-ssPCM 基体中。纳米石墨烯可以有效降低聚合物中的热阻，有效提高热界面导电性并加速传热。在本研究中，石墨烯通过超声空化，基于合适的溶剂体系均匀分散在 PU-ssPCM 的网络结构中。PU-ssPCM/GNP 膜的微观表面形态和微观结构如图 S13 所示（支持信息）。图 2a 显示了 PU-ssPCM/GNP 的合成过程和化学结构以及石墨烯与 PU-ssPCM 分子链之间的相互作用机制的示意图。石墨烯的有序晶体结构可用于替代部分无序的 PU-ssPCM;因此，随着石墨烯比例的增加，PU-ssPCM/GNP 的局部热链相互接触，形成热连接网络。因此，尽管晶体缺陷、界面热阻和声子散射的影响在异质界面上是不可避免的（图 S14a，支持信息），但 PU-ssPCM/GNP 的热导率比 PU-ssPCM 的热导率增加了 100.6%（图 S14b，支持信息）。此外，石墨烯与 PU-ssPCM 网链之间的氢键相互作用可以看作是构建 3D 导热网络的“传热桥”，有助于进一步降低界面热阻。 PU-ssPCM/GNP 的规则晶体排列也增加了分子链相互作用的热导率。¹³ 因此，连续晶相和氢键相互作用之间的物理交联网络构建可以为声子热传导提供快速通道。通过声子振动，可以从高温区快速实现从高温区到低温区的热传递。

In addition, the introduction of graphene enhances the shape stability and mechanical robustness of the material.^[13] The excellent mechanical properties and durability of PU-ssPCM/GNP were underscored by the conspicuous load-bearing capacity and enduring multiple cycles of folding, bending, knotting, and stacking. Furthermore, the synergistic effect between chemical crosslinking and physical crosslinking, and graphene empowerment is beneficial for enhancing the rigidity of PU-ssPCM/GNP, effortlessly supporting a weight of up to 100 g, featuring 200 times that of a sample measuring 35 mm in length, 15 mm in width and 0.87 mm in thickness (Figure 2b). Moreover, mechanical reinforcement enables PU-ssPCM/GNP to exhibit exciting toughness, bearing loads up to 50 000 times its own weight (size of sample: length × width × thickness = 100 × 2 × 0.24 mm) (Figure 2b). In short, this flexible PCM with adjustable mechanical properties is capable of various configurations according to different needs, exhibiting a preeminent mechanical performance.
此外，石墨烯的引入增强了材料的形状稳定性和机械强度。¹³ PU-ssPCM/GNP 优异的机械性能和耐用性通过显著的承载能力和耐久的折叠、弯曲、打结和堆叠的多次循环得到强调。此外，化学交联和物理交联以及石墨烯赋能之间的协同效应有利于增强 PU-ssPCM/GNP 的刚度，轻松支撑高达 100 g 的重量，是长 35 毫米、宽 15 毫米、厚度为 0.87 毫米的样品的 200 倍（图 2b）。此外，机械加固使 PU-ssPCM/GNP 表现出令人兴奋的韧性，承受高达自身重量 50 000 倍的载荷（样品尺寸：长×宽 × 厚度 = 100 × 2 × 0.24 毫米）（图 2b）。简而言之，这种具有可调机械性能的柔性 PCM 能够根据不同的需求进行各种配置，表现出卓越的机械性能。

2.3 Thermophysical Performance of PU-ssPCM/GNP
2.3 PU-ssPCM/GNP 的热物理性能

The thermal performance and mechanical properties of flexible PCM are crucial for their practical application. The corresponding 2θ values of the principle characteristic peaks of PU-ssPCM/GNP were 19.0° and 23.5°, respectively (Figure 3b), implying that it possessed the same crystal structure as the original PU-ssPCM, and no new phase formed during the preparation process (Figure S15c,b; Supporting Information). PU-ssPCM remained the main active ingredient for latent heat storage and release in PU-ssPCM/GNP. Compared with PU-ssPCM, the phase transition temperature of PU-ssPCM/GNP (1 wt.%) slightly increased (Figure 3a; Figure S16a–c, Supporting Information) and the crystalline peak intensity was enhanced (Figure 3b). This was related to the introduction of graphene, which might lead to a tighter molecular arrangement and an increased wafer thickness of the substrate.^[26] However, the melting enthalpy value of PU-ssPCM/GNP lightly decreased from 110.3 to 104.0 J g⁻¹ with increasing amounts of graphene (Figure S16a–c, Supporting Information), which was related to the relative proportion of graphene in the crystalline phase. The latent heat storage/release capacity possessed excellent cyclic durability according to the indistinguishable difference in the 100 curves of the heating/cooling process (Figure 3c; Figure S16d, Supporting Information). In addition, as the PU-ssPCM was subjected to sustained heating up to its melting temperature (T_m), the crystalline peak intensities at 2θ of 19.0° and 23.1° gradually decreased until they vanished entirely (Figure 3d), indicating that the molecular chains underwent a transformation from a regular arrangement to an amorphous state in response to external thermal stimulation. This temperature-dependent transition was consistent with the changes from the anisotropic spherical crystallographic structure to the isotropic structure observed via POM (Figure 3e1–e6; Figure S12, Supporting Information), and this transition was reversible.
柔性 PCM 的热性能和机械性能对其实际应用至关重要。PU-ssPCM/GNP 主特征峰的相应 2θ 值分别为 19.0° 和 23.5°（图 3b），这意味着它具有与原始 PU-ssPCM 相同的晶体结构，并且在制备过程中没有形成新的相（图 S15c，b;支持信息）。PU-ssPCM 仍然是 PU-ssPCM/GNP 中潜热储存和释放的主要活性成分。与 PU-ssPCM 相比，PU-ssPCM/GNP 的相变温度 （1 wt.%） 略有升高（图 3a;图 S16a–c，支持信息），结晶峰强度增强（图 3b）。这与石墨烯的引入有关，石墨烯可能会导致更紧密的分子排列和衬底的晶圆厚度增加。²⁶ 然而，随着石墨烯含量的增加，PU-ssPCM/GNP 的熔融焓值从 110.3 略微下降到 104.0 J ^g-1（图 S16a-c，支持信息），这与石墨烯在结晶相中的相对比例有关。根据加热/冷却过程的 100 条曲线的不可区分的差异，潜热储存/释放能力具有优异的循环持久性（图 3c;图 S16d，支持信息）。 此外，当 PU-ssPCM 持续加热至其熔融温度 （T_m） 时，2θ 处的 19.0° 和 23.1° 的结晶峰强度逐渐降低，直到完全消失（图 3d），表明分子链在响应外部热刺激时经历了从规则排列转变为无定形状态。这种温度依赖性转变与通过 POM 观察到的从各向异性球形晶体结构到各向同性结构的变化一致（图 3e1-e6;图 S12，支持信息），并且这种转变是可逆的。

The binding energy of hydrogen bonds (RT≈2.5 kJ mol⁻¹) is higher than that of thermal energy and can survive for a long time under low-temperature conditions.^[27] The hydrogen bonding interactions gradually decreased with increasing temperature, as shown in Figure 3f–h. The characteristic peak intensities of the NH bonds and C═O bond weakened during the heating process, suggesting that hydrogen bonding in the remote structure of the hard segment began to dissociate when the molecular chain obtained sufficient energy to move. Strong and uniform hydrogen bonds existing in the ordered crystallization zones also caused the characteristic peak intensity of hydrogen bonds to decrease faster around the melting point. Thereby, there are inter- and intra-chain hydrogen bonds in the structure of PU-ssPCM, contributing to the formation of multi-hydrogen bond network chains, which display molecular time-bonding effects.^[27] The characteristic of hydrogen-bonding interactions is that they can break at high temperatures and re-form at room temperature.
氢键的结合能 （RT≈2.5 kJ ^mol-1） 高于热能，可以在低温条件下存活很长时间。²⁷ 氢键相互作用随着温度的升高而逐渐减少，如图 3f-h 所示。NH 键和 C═O 键的特征峰强度在加热过程中减弱，表明当分子链获得足够的能量移动时，硬链段远程结构中的氢键开始解离。存在于有序结晶区中的强而均匀的氢键也导致氢键的特征峰值强度在熔点附近下降得更快。因此，PU-ssPCM 的结构中存在链间和链内氢键，有助于形成多氢键网络链，从而表现出分子时间键效应。²⁷ 氢键相互作用的特点是它们可以在高温下断裂，在室温下重新形成。

2.4 Mechanical Properties of PU-ssPCM/GNP
2.4 PU-ssPCM/GNP 的机械性能

The changes in the chemical structure and hydrogen bonding interactions of PU-ssPCM/GNP with increasing graphene content were explored through ATR-FTIR analysis (Figure 4a–c). No new chemical bonds were formed in the spectral analysis of PU-ssPCM/GNP, implying that no chemical reaction occurred between the graphene nanoparticles (GNPs) and the PU-ssPCM. As the proportion of graphene increased, the intensity of the vibration peak of the -N-H bond at wavenumbers 3337 and 1625 cm⁻¹ became stronger, and the vibration peak width of -N-H bond at wavenumbers of 3337 and 1625 cm⁻¹ increased, implying that hydrogen bonding interactions between graphene and molecular chain of PU-ssPCM generated and gradually increased. The hydrogen-bonding network collaborated with the crystalline skeleton to construct a 3D dynamic network structure in PU-ssPCM/GNP. This structural characteristic contributes to the strengthening of the thermal stability and storage modulus of PU-ssPCM/GNP (Figure 4d; Figure S15a, Supporting Information). In addition, owing to the introduction of graphene, interfacial force, and internal friction between polymeric molecular chains and graphene generated and gradually enhanced, resulting in higher damping behavior and tanδ values (Figure 4e). The storage modulus of PU-ssPCM/GNP demonstrated temperature-dependent characteristics, with an increased modulus at reduced temperatures (Figure 4d).
通过 ATR-FTIR 分析探讨了 PU-ssPCM/GNP 的化学结构和氢键相互作用随石墨烯含量增加的变化（图 4a-c）。在 PU-ssPCM/GNP 的光谱分析中没有形成新的化学键，这意味着石墨烯纳米颗粒 （GNP） 和 PU-ssPCM 之间没有发生化学反应。随着石墨烯比例的增加，-N-H 键在波数 3337 和 1625 cm⁻¹ 处的振动峰强度变得更强，在 3337 和 1625 cm⁻¹ 波长处-N-H 键的振动峰宽度增加，表明石墨烯与 PU-ssPCM 分子链之间的氢键相互作用产生并逐渐增加。氢键网络与晶体骨架合作，在 PU-ssPCM/GNP 中构建了 3D 动态网络结构。这种结构特性有助于加强 PU-ssPCM/GNP 的热稳定性和存储模量（图 4d;图 S15a，支持信息）。此外，由于石墨烯的引入，界面力以及聚合物分子链和石墨烯之间的内部摩擦产生并逐渐增强，导致更高的阻尼行为和 tanδ 值（图 4e）。PU-ssPCM/GNP 的储能模量表现出与温度相关的特性，在降低温度下模量增加（图 4d）。

In addition, as shown in Figure 4f,g, the mechanical properties of PU-ssPCM/GNP were strengthened as a result of the introduction of graphene. The PU-ssPCM/GNP (1 wt.%) presented remarkable elongation at break (ε = 1543%) and ultimate stress (σ = 19.2 MPa), and the maximum loading capacity was 87.4 N (Figure S17, Supporting Information). The enhanced mechanical performance of the PU-ssPCM/GNP (1 wt.%) might be caused by the participating stress produced by the surface crystallization of graphene and the interaction between graphene and molecular chains.^[26] Moreover, high elongation was conducive to a sharp increase in stress or modulus, and increased stress can accelerate the crystallization of the polymer. The generated microcrystals played a role in physical crosslinking. Thereby, the ordered arrangement of the molecular chains and hydrogen bonding interactions contributed to improving the mechanical strength.^[13] However, excess amounts of graphene were prone to agglomeration in the matrix, resulting in stress concentration and fracturing, thus damaging the mechanical properties of PU-ssPCM/GNP. Compared with other types of materials, PU-ssPCM and PU-ssPCM/GNP (1 wt.%) exhibited high tensile and damage resistance performance as well as mechanical stability (Figure 4h).
此外，如图 4f，g 所示，由于石墨烯的引入，PU-ssPCM/GNP 的机械性能得到了增强。PU-ssPCM/GNP （1 wt.%） 表现出显着的断裂伸长率 （ε = 1543%） 和极限应力 （σ = 19.2 MPa），最大负载能力为 87.4 N（图 S17，支持信息）。PU-ssPCM/GNP （1 wt.%） 的增强机械性能可能是由石墨烯表面结晶产生的参与应力以及石墨烯与分子链之间的相互作用引起的。²⁶ 此外，高伸长率有利于应力或模量的急剧增加，而增加的应力可以加速聚合物的结晶。生成的微晶在物理交联中发挥作用。因此，分子链的有序排列和氢键相互作用有助于提高机械强度。¹³ 然而，过量的石墨烯容易在基体中团聚，导致应力集中和断裂，从而破坏 PU-ssPCM/GNP 的机械性能。与其他类型的材料相比，PU-ssPCM 和 PU-ssPCM/GNP （1 wt.%） 表现出较高的拉伸和抗损伤性能以及机械稳定性（图 4h）。

Generally, the tensile force of stimulus-responsive soft actuators is determined by their characteristics and geometric shape. Work capacity, output energy density, and recovery power are common parameters for evaluating actuation efficiency. The work capacity and density are determined by the distance of movement, which is related to the temporary strain and elongation at the break of the materials. Therefore, to further evaluate the actuation efficiency level of our work, we conducted extensive literature reviews and compared the deformation ability and work density of different soft actuators, as shown in Figure 4i. Compared with LCEs, SMPs, and fiber materials, the flexible PCM developed in our study presented high deformation and prominent work density, which may function as soft actuators.
通常，刺激响应软致动器的拉力由其特性和几何形状决定。工作能力、输出能量密度和回收功率是评估驱动效率的常用参数。工作能力和密度由运动距离决定，这与材料断裂时的临时应变和伸长率有关。因此，为了进一步评估我们工作的驱动效率水平，我们进行了广泛的文献综述，并比较了不同软致动器的变形能力和工作密度，如图 4i 所示。与 LCEs、SMPs 和纤维材料相比，我们研究中开发的柔性 PCM 表现出高变形和突出的工作密度，可能起到软致动器的作用。

2.5 Mechanism of Shape Memory Effect
2.5 形状记忆效应的机制

PU-ssPCM/GNP (1 wt.%) displayed notable shape programmability, wherein the polymer exhibited substantial stretching up to 600% of its original length. Subsequently, when subjected to temporary conditions, the polymer can revert to its original dimensions upon heating by a hot air fan (Movie S1, Supplementary Movie) or full- spectrum sunlight in a short time, as shown in Figure 5d. This coupling effect between excitation and contraction ensured that the PU-ssPCM/GNP (1 wt.%) actualized coordinated and synchronous contraction under external environmental stimuli (such as thermal stimulation and photic stimulation). Subsequently, we explored the shape behavior of the polymer using DMA (Figure 5b,c). The experimental results revealed that the PU-ssPCM/GNP (1 wt.%) exhibited a maximum recoverable strain (ɛ_r,max) of 300% under an applied maximum recovery stress (σ_r,max) of 3 MPa. The combination of high extensibility and prominent recovery stress confers polymers with high work density.^[28] Furthermore, the PU-ssPCM/GNP (1 wt.%) presented a shape fixity (R_f) of 76.0% and shape recovery (R_r) of 90.3%, respectively (Equations (S1) and (S2), Supporting Information, Figure 5b). Furthermore, following a series of four cyclic tests, the R_f of the sample exhibited a progressive trend with values of 52.3%, 56.5%, 60.4%, and 60.3%, respectively. Because partial prestored stress was employed to balance the loading gravity, a localized shape recovery behavior emerged after the removal of the external force. Concurrently, the R_r of the sample demonstrated an interesting pattern, with intestinal values of 47.8%, 51.0%, and 56.3%, culminating in a remarkably high value of 99.0% after the fourth cycle. The observed increase in R_f and the significant increase in R_r may be attributed to the continuous soft segment phase, which is characterized by a high degree of elongation orientation. This phase facilitates plastic deformation under mechanical stress.^[30] However, it is hypothesized that the initial plastic deformation may undergo a transformation through a process of mechanical conditioning, manifesting as a substantial increase in elastic deformation. This phenomenon may indicate that the composite undergoes structural reorganization that enhances its ability to revert to its original state following deformation. We also discovered that the elastic behavior of PU-ssPCM/GNP (1 wt.%) can be enhanced by exerting a controlled degree of stress (Figure S18, Supporting Information). During continuous stretching and release, the disordered dispersion state of graphene within the polymer matrix contributes to the entropy increment process. The implications of these findings are significant as they highlight the potential of PU-ssPCM/GNP to exhibit improved resilience and adaptability in applications where materials are subjected to repetitive mechanical stress.
PU-ssPCM/GNP （1 wt.%） 显示出显着的形状可编程性，其中聚合物表现出高达其原始长度 600% 的实质性拉伸。随后，当受到临时条件时，聚合物可以在热风风扇（电影 S1、补充电影）或全光谱阳光下在短时间内恢复到其原始尺寸，如图 5d 所示。这种激发和收缩之间的耦合效应确保了 PU-ssPCM/GNP （1 wt.%） 在外部环境刺激（如热刺激和光刺激）下实现协调和同步收缩。随后，我们使用 DMA 探索了聚合物的形状行为（图 5b，c）。实验结果表明，在施加的最大恢复应力 （σ_r，max） 为 3 MPa 的情况下，PU-ssPCM/GNP （1 wt.%） 表现出 300% 的最大可恢复应变 （ɛ_r，max）。高延展性和突出的恢复应力相结合，赋予聚合物高工作密度。²⁸ 此外，PU-ssPCM/GNP （1 wt.%） 的形状固定性 （R_f） 为 76.0%，形状恢复率 （R_r） 分别为 90.3%（方程 （S1） 和 （S2），支持信息，图 5b）。此外，经过一系列的 4 次循环测试，样品的 R_f 呈渐进趋势，值分别为 52.3%、56.5%、60.4% 和 60.3%。由于采用了部分预存储应力来平衡加载重力，因此在去除外力后出现了局部形状恢复行为。 同时，样本的 R_r 表现出一种有趣的模式，肠道值为 47.8%、51.0% 和 56.3%，在第四个周期后达到 99.0% 的非常高的值。观察到的 R_f 增加和 R_r 的显着增加可能归因于连续的软段阶段，其特征是高度伸长取向。此相有助于在机械应力下发生塑性变形。³⁰ 然而，据推测，最初的塑性变形可能会通过机械调节过程发生转变，表现为弹性变形的大幅增加。这种现象可能表明复合材料发生了结构重组，从而增强了其在变形后恢复到原始状态的能力。我们还发现，PU-ssPCM/GNP （1 wt.%） 的弹性行为可以通过施加受控程度的应力来增强（图 S18，支持信息）。在连续拉伸和释放过程中，石墨烯在聚合物基体中的无序分散态有助于熵增量过程。这些发现的意义重大，因为它们强调了 PU-ssPCM/GNP 在材料承受重复机械应力的应用中表现出更好的弹性和适应性的潜力。

We speculated that the structural transition of the polymer is related to the crystal structural transformation and hydrogen bonding interactions under different external conditions. Figure 5a depicts the process of structural transformation that the sample undergoes in different settings: 1) the transition of molecular chains from a regular arrangement to the amorphous state generated when external thermal stimulation was applied (corresponding to Figure 3d,e1,e2); 2) the hydrogen bonding interaction between and within molecular chains underwent dissociation once the molecular chains obtained sufficient energy (corresponding to Figure 3f–h). In addition, the molecular chains of the flexible PCM gradually underwent directional arrangement and elongation along the direction of the force under an external force at room temperature. The ordering degree of the crystals increased significantly as the stretching state changed from 60% to 200%, as displayed in XRD pattern (Figure 5e). As the stretching state increased from 200% to 400%, local lamellar fragmentation or strain-induced melting of the crystal structure resulted in a decreased crystalline peak intensity. As the stretching deformation further increased to 800%, the generation of melt recrystallization resulted in a significantly increased crystalline peak intensity. Flory and Yoon^[31] first proposed the strain-induced melting-recrystallization model, which described the molecular micromechanisms of polymer crystals during the multi-stage deformation process.^[30] The crystal structure of the flexible PCM transformed from the disordered intermediate state of the original crystal into new crystallites with a preferable net-chain orientation along the stress direction, forming a fibrillation structure, which implies orientation-induced crystallization enhancement. Therefore, the degree of orderliness of the net chains of PU-ssPCM gradually improved after stretching by an external force, causing the formation of new crystals (Figure 5e). Additionally, an increase in the degree of orderliness and crystallinity of the network chain contributed to an enthalpy increase, as shown in Figure 5g. Deformation and relaxation are related to the bond rotation. The energy and recovery stress in a rigid thermosetting network can be stored through bond rotation and bond length variation during the programming process,^[32] mainly through an increase in enthalpy. Thus, the stretching process was capable of inducing orientational crystallization, which was accompanied by the accumulation and storage of energy, as shown in Figure 5f,g. The stress recovery and energy output depend on the energy input during programming and energy storage of the temporary shape after programming. The stored energy is composed of entropy and enthalpy. Stress and energy storage in polymer networks are achieved by entropy reduction during deformation. Based on the increase in enthalpy through stretching bonds, we speculated that storing enthalpy during programming might also be a method to enhance stress recovery and energy output.^[33
我们推测聚合物的结构转变与不同外部条件下的晶体结构转变和氢键相互作用有关。图 5a 描述了样品在不同设置下经历的结构转变过程：1） 分子链从规则排列转变为施加外部热刺激时产生的无定形状态（对应于图 3d，e1，e2）;2） 一旦分子链获得足够的能量，分子链之间和分子链内的氢键相互作用就会发生解离（对应于图 3f-h）。此外，在室温下，柔性相变材料的分子链在外力作用下沿力的方向逐渐发生定向排列和伸长。当拉伸状态从 60% 变为 200% 时，晶体的有序度显著增加，如 XRD 图所示（图 5e）。当拉伸状态从 200% 增加到 400% 时，局部层状碎裂或应变诱导的晶体结构熔化导致结晶峰强度降低。随着拉伸变形进一步增加到 800%，熔融再结晶的产生导致结晶峰强度显着增加。 Flory 和 Yoon³¹ 首先提出了应变诱导熔融再结晶模型，该模型描述了聚合物晶体在多阶段变形过程中的分子微观机制。³⁰ 柔性相变材料的晶体结构从原始晶体的无序中间状态转变为新的微晶，沿应力方向具有优选的净链取向，形成纤化结构，这意味着取向诱导的结晶增强。因此，PU-ssPCM 的净链在受到外力拉伸后逐渐改善，导致新晶体的形成（图 5e）。此外，网络链的有序性和结晶度的增加也导致了焓的增加，如图 5g 所示。变形和松弛与键旋转有关。刚性热固性网络中的能量和恢复应力可以通过编程过程中的键旋转和键长变化来储存，³² 主要是通过焓的增加。因此，拉伸过程能够诱导取向结晶，并伴随着能量的积累和储存，如图 5f，g 所示。应力恢复和能量输出取决于编程过程中的能量输入和编程后临时形状的能量存储。储存的能量由熵和焓组成。聚合物网络中的应力和能量存储是通过变形过程中的熵减少来实现的。 基于通过拉伸键增加焓，我们推测在编程过程中存储焓也可能是增强应力恢复和能量输出的一种方法。^33]

Hydrogen bonding is a direct attraction between electron-deficient and high-electron-cloud density atoms and is a widely existing weak force at the molecular level.^[34] The energy of hydrogen bonds is usually between 5 and 20 kJ mol⁻¹, which is much weaker than the covalent and ionic bonds, but slightly stronger than the π–π stacking forces and van der Waals forces.^[35] We discovered that the stability of the stretching state was closely related to hydrogen bonding interactions, according to ATR-FTIR analysis (Figure 5h–j). Although the dissociation energy of a single hydrogen bond is low, aggregation can form strong bond energies with covalent bonds in the microdomains,^[36] contributing to more rigidity in the crystal. Therefore, the abundant hydrogen bonding between the inter- and intra-chains may serve as an internal stress provider and contribute to maintaining the elongation state of the molecular chain without external force, as well as maintaining stable deformation with external force. The weakened hydrogen bonding (at ɛ = 800%) was related to melt recrystallization of crystal structure (Figure 5h–j). Moreover, hydrogen bonds with unique directionality and adjustability can act as molecular switches to achieve a dynamic equilibrium. The strength and degree of hydrogen bonding in the hard segments of PU-ssPCM/GNP decreased with increasing temperature, exhibiting dynamic reversibility and unique responsiveness to external environmental stimuli (Figure 3f–h).^[37] In short, chemical bonding forces and hydrogen bonding interactions synergistically facilitated the mechanical performance and programmable features of PU-ssPCM and PU-ssPCM/GNP (Figure 1d and Figure 4f). We summarized that the stretching process of dumbbell shapes at room temperature can be regarded as the accumulation and temporary storage of energy through crystallization regions, simultaneously achieving effective “locking” and shape fixation via hydrogen bonding interactions.
氢键是缺电子原子和高电子云密度原子之间的直接吸引，是分子水平上广泛存在的弱力。³⁴ 氢键的能量通常在 5 到 20 kJ ^mol-1 之间，这比共价键和离子键弱得多，但比 π-π 堆叠力和范德华力略强。³⁵ 根据 ATR-FTIR 分析，我们发现拉伸状态的稳定性与氢键相互作用密切相关（图 5h-j）。尽管单个氢键的解离能很低，但聚集可以在微域中与共价键形成强键^能，36 从而有助于提高晶体的刚度。因此，链间和链内之间丰富的氢键可以作为内应力提供者，有助于在没有外力的情况下维持分子链的伸长状态，以及在外力作用下保持稳定的变形。减弱的氢键（ɛ = 800%）与晶体结构的熔融再结晶有关（图 5h-j）。此外，具有独特方向性和可调节性的氢键可以充当分子开关以实现动态平衡。 PU-ssPCM/GNP 硬段中的氢键强度和程度随着温度的升高而降低，表现出动态可逆性和对外部环境刺激的独特响应性（图 3f-h）。³⁷ 简而言之，化学键和氢键相互作用协同促进了 PU-ssPCM 和 PU-ssPCM/GNP 的机械性能和可编程特性（图 1d 和图 4f）。我们总结说，哑铃形状在室温下的拉伸过程可以看作是通过结晶区积累和临时储存能量，同时通过氢键相互作用实现有效的“锁定”和形状固定。

When thermal stimulation was applied, the transition of the polymeric molecular chains from the stretching state to a randomly coiled conformation occurred, and the deformation of the material was restored to its original length (Figure 5d and 1a). At this time, molecular chains moved toward the “thawing” direction after hydrogen rupture. This means that the stored energy underwent “unlocking” and was controllably released upon applying external thermal stimulation. The amorphous state possesses the highest entropy in the Boltzmann equation (S = κ lnW, where S is entropy, κ is Boltzmann's constant, and W is the possibility of a polymer chain conformation).^{[10, 38]} Entropy energy can be stored through the phase transition of crystal domains, and the entropy increment induced by length contraction can overcome the tension effect provided by the hydrogen-bonding network. Therefore, entropy energy release can drive deformation recovery. The shape recovery caused by energy release may generate a significant force for implementing actuation tasks. In summary, we believe that hydrogen bonding interactions (i.e., internal stress provider) and the sequential change of orientation crystallization domains serve as reversible switches, which play an important role in the shape memory behavior and programmable deformation of the flexible PCM.
当施加热刺激时，聚合物分子链从拉伸状态转变为随机卷曲构象，并且材料的变形恢复到其原始长度（图 5d 和 1a）。此时，氢破裂后分子链向“解冻”方向移动。这意味着储存的能量经历了“解锁”，并在施加外部热刺激时被可控地释放。非晶态在玻尔兹曼方程中具有最高的熵（S = κ lnW，其中 S 是熵，κ 是玻尔兹曼常数，W 是聚合物链构象的可能性）。^10、38熵能可以通过晶畴的相变来储存，长度收缩引起的熵增量可以克服氢键网络提供的张力效应。因此，熵能释放可以驱动变形恢复。由能量释放引起的形状恢复可能会产生用于执行驱动任务的重要力。综上所述，我们认为氢键相互作用（即内应力提供者）和取向结晶域的顺序变化起到了可逆开关的作用，它们在柔性 PCM 的形状记忆行为和可编程变形中起着重要作用。

In brief, our work is mainly based on the regulation of the cross-linked structure of molecular chains to construct a flexible PCM. It not only solves the problems of leakage in traditional solid-liquid PCMs but also possesses programmable deformation and shape stability. Due to the limitations of hard segments on the free movement of molecular chains of solid-liquid PCM, the crystallinity is reduced to some extent, but it exhibits better high-temperature stability and mechanical performance. The symmetry and regularity of the network structure can significantly improve rigidity and anti-damage tensile performance. In addition, the chemical bonds and physical interactions (such as crystalline domains, hydrogen bonds, and entanglement of polymer chains) in the network chains can effectively store entropy energy and regulate molecular mobility through crystallization/melting phase-transition processes to achieve fixed/released temporary shapes, thus achieving a shape-memory effect.
简而言之，我们的工作主要基于调节分子链的交联结构来构建柔性 PCM。它不仅解决了传统固液相变材料中的泄漏问题，而且具有可编程的变形和形状稳定性。由于硬链段对固液相变材料分子链自由运动的限制，结晶度在一定程度上降低，但表现出较好的高温稳定性和力学性能。网络结构的对称性和规则性可以显著提高刚度和抗损伤拉伸性能。此外，网络链中的化学键和物理相互作用（如晶域、氢键和聚合物链的纠缠）可以有效地储存熵能，并通过结晶/熔融相变过程调节分子迁移率，以实现固定/释放的临时形状，从而达到形状记忆效应。

2.6 Photo-to-Thermal Conversion and Actuation Performance
2.6 光热转换和驱动性能

To further explore the development potential of flexible PCM in the application of photothermal actuation, the photo-to-thermal conversion performance of PU-ssPCM/GNP was investigated. The absorptivity of the PU-ssPCM/GNP (1 wt.%) (≈96%) is obviously higher than that of PU-ssPCM in the wavelength range of 200–2500 nm via UV-vis-NIR (Figure S19, Supporting Information). The surface temperature variations of the samples with illumination time were collected using a thermocouple thermometer under simulated solar sunlight irradiation (100 mW cm⁻², AM 1.5). Due to its capacity to capture incident photons, graphene can quickly absorb solar energy to induce structural changes in PU-ssPCM/GNP upon exposure to solar radiation, as shown in Figure 6a. The corresponding temperature-time evolution curves of PU-ssPCM/GNP and PU-ssPCM were used to explore the light-to-thermal energy conversion performance, as shown in Figure 6b. The formation of turning points, attributed to the absorption or release of heat during phase transitions, indicates the phenomenon of absorbed solar energy conversion into latent thermal energy inside the PU-ssPCM/GNP composite. The light-to-thermal conversion and storage efficiency (η) of PU-ssPCM and PU-ssPCM/GNP were 39.5% and 97.8%, respectively, as determined by the formula provided in the supplementary material (Equation (S8), Supporting Information). Additionally, the differences in the thermal distribution in the infrared thermal images of PU-ssPCM and PU-ssPCM/GNP indicated that the intense vibration and mutual interaction of phonons within the graphene layer resulted in a rapid accumulation of thermal energy within the polymer molecular chains following exposure to light irradiation, as shown in Figure 6c (additional data can be found in Figures S20 and S21, Supporting Information). These findings corroborated the pivotal role of graphene, acting as both an efficient photon-absorbing agent and a molecular heat generator, effectively enhancing the thermal conductivity and overall photothermal conversion efficiency of the PU-ssPCM (1 wt.%). Additionally, PU-ssPCM/GNP (1 wt.%) demonstrated a consistent and enduring capacity for converting light to thermal energy. This was evidenced by the comparable cyclic thermal profiles during the heat storage and release cycles at a simulated solar sunlight irradiation of 100 mW cm⁻² (AM 1.5), as shown in Figure 6d. In addition, subsequent experiments revealed that the η values increased and the heating time diminished with an increase in solar intensity, indicating that photothermal conversion can be accomplished within a shorter timeframe under strong light irradiation (Figure S22, Supporting Information). In short, the findings from the photothermal conversion performance offered compelling evidence and a solid theoretical foundation for further research on the photothermal-driven performance.
为进一步探索柔性相变材料在光热驱动应用中的发展潜力，研究了 PU-ssPCM/GNP 的光热转换性能。通过 UV-vis-NIR 在 200–2500 nm 波长范围内，PU-ssPCM/GNP （1 wt.%） （≈96%） 的吸收率明显高于 PU-ssPCM（图 S19，支持信息）。使用热电偶温度计在模拟太阳光照射 （100 mW ^cm-2， AM 1.5） 下收集样品表面温度随光照时间的变化。由于其捕获入射光子的能力，石墨烯可以在暴露于太阳辐射时快速吸收太阳能以诱导 PU-ssPCM/GNP 的结构变化，如图 6a 所示。使用 PU-ssPCM/GNP 和 PU-ssPCM 的相应温度-时间演变曲线来探索光能转换性能，如图 6b 所示。由于相变过程中热量的吸收或释放而形成转折点，表明 PU-ssPCM/GNP 复合材料内部吸收的太阳能转化为潜热能的现象。PU-ssPCM 和 PU-ssPCM/GNP 的光热转化率和存储效率 （η） 分别为 39.5% 和 97.8%，由补充材料中提供的公式确定（方程 （S8），支持信息）。 此外，PU-ssPCM 和 PU-ssPCM/GNP 红外热图像中热分布的差异表明，石墨烯层内声子的强烈振动和相互作用导致热能在暴露于光照射后在聚合物分子链内迅速积累，如图 6c 所示（其他数据见图 S20 和 S21、支持信息）。这些发现证实了石墨烯的关键作用，石墨烯既是高效的光子吸收剂，又是分子热发生器，有效提高了 PU-ssPCM 的导热性和整体光热转换效率 （1 wt.%）。此外，PU-ssPCM/GNP （1 wt.%） 表现出将光转化为热能的一致和持久的能力。如图 6d 所示，在模拟 100 mW ^cm-2 （AM 1.5） 的太阳光照射下，储热和释放循环期间的可比循环热曲线证明了这一点。此外，随后的实验表明，随着太阳强度的增加，η值增加，加热时间减少，这表明在强光照射下可以在更短的时间内完成光热转换（图 S22，支持信息）。简而言之，光热转换性能的研究结果为进一步研究光热驱动性能提供了令人信服的证据和坚实的理论基础。

In addition, according to basic thermodynamics (ΔG = ΔH−TΔS, where ΔG, ΔH, and ΔS are the changes in Gibbs free energy, enthalpy, and entropy, respectively, and T is the absolute temperature), the stored energy is composed of entropy and enthalpy. In general, entropy elasticity is a recognized driving force for shape and stress memory in SMP. Fan et al suggested that storing enthalpy through bond rotation and bond length changes during the programming process may be another way to increase recovery pressure and energy output.^[32] Therefore, we considered that storing enthalpy can act as a way to further increase stress recovery and energy output. The chain conformation and interchain stacking of the original lattice may undergo a phase transition by applying tensile stress,^[30] and the flexible chains undergo tensile orientation-induced crystallization, resulting in an enthalpy increase and a decrease in entropy (Figure 5f,g). The stored energy in the network and the restored stress are increased through bond rotation and bond length changes.^[32] When the external force is eliminated, solidification of the switch segment of PU-ssPCM/GNP prevents the molecular migration rate from being triggered. Applying only a heating-stimulus triggers the molecular migration rate, and the molecular chains transform from an ordered arrangement to a random coiled conformation with the highest entropy, thus restoring their original shape. The high-energy conversion occurring inside the material puts it in a metastable state during this process. Therefore, the material tends to release this energy, which can be considered the driving force for actuation. Therefore, the latent capacity (ordered crystal arrangement) of PU-ssPCM/GNP (1 wt.%) contributes to driving more energy storage and reducing relaxation during the recovery process, achieving high recovery stress and energy output. In addition, the introduction of GNPs can significantly increase the steric hindrance and interaction between molecular segments, which is conducive to improving the recovery stress.
此外，根据基本热力学（ΔG = ΔH-T ΔS，其中 Δ G、ΔH 和 ΔS 分别是吉布斯自由能、焓和熵的变化，T 是绝对温度），存储的能量由熵和焓组成。一般来说，熵弹性是 SMP 中形状和应力记忆的公认驱动力。Fan 等人认为，在编程过程中通过键旋转和键长变化来存储焓可能是增加回收压力和能量输出的另一种方法。³² 因此，我们认为储存焓可以作为进一步增加应力恢复和能量输出的一种方式。原始晶格的链构象和链间堆叠可以通过施加拉伸应力发生相变，³⁰ 并且柔性链发生拉伸取向诱导的结晶，导致焓增加和熵减少（图 5f，g）。网络中存储的能量和恢复的应力通过键旋转和键长变化而增加。³² 当外力消除时，PU-ssPCM/GNP 开关段的凝固会阻止分子迁移速率的触发。仅施加加热刺激会触发分子迁移速率，分子链从有序排列转变为具有最高熵的随机卷曲构象，从而恢复其原始形状。在此过程中，材料内部发生的高能转换使其处于亚稳态。 因此，材料倾向于释放这种能量，这可以被认为是驱动的驱动力。因此，PU-ssPCM/GNP 的潜在容量（有序晶体排列）（1 wt.%） 有助于驱动更多的能量存储并减少恢复过程中的松弛，实现高恢复应力和能量输出。此外，GNPs的引入可以显著增加分子片段之间的空间位阻和相互作用，有利于改善恢复应力。

We demonstrated that PU-ssPCM/GNP (1 wt.%) possessed excellent extensibility (ɛ > 1500%), toughness, and programmable deformation. However, further experimental verification is required to determine whether the shape recovery of this flexible PCM can generate a tensile force. The flexible PCM exhibited thermal-driven actuation (Figure S23, Movies S1–S5, Supplementary Movie) and photothermal-driven actuation (Figure 6e; Figure S24, Movies S6–S10, Supplementary Movie). The pre-strained thin spline of the PU-ssPCM exhibited hydration actuation and displayed a work density of 405 kJ m⁻³ (Figure S23 and Movies S2–S5, Supplementary Movie). In addition, a pre-strained thin spline of the PU-ssPCM/GNP (1 wt.%) was able to lift a 55 g object (2620 times its own weight) accompanied by shrinkage deformation rate of 53.3% under light stimulation, presenting a work density of 1330 kJ m⁻³ (Figure 6e; Movie S7, Supplementary Movie). Similarly, another pre-strain thin spline (45 mg) was capable of hoisting a 105 g weight upon exposure to a full spectrum sunlamp (Figure S23 and Movie S8, Supplementary Movie). The lifting ratio (m₁/m_a) exceeded 2333, translating to an energy output of 460 J kg⁻¹. We conducted a comprehensive analysis of the work density of PU-ssPCM/GNP (1 wt.%) by lifting objects of varying weights when exposed to light stimulation as shown in Figure 6f. The working capacity is related to the distance of movement, which is related to the temporary strain and elongation at the break of the material.^[39] PU-ssPCM/GNP (1 wt.%) has been experimentally proven to possess high toughness (ε = 1543%, σ = 19.2 MPa) and rigidity. Therefore, actuators with a higher elongation at break or temporary strain contribute to the generation of larger movement distances and exhibit a greater working capacity. In addition, to maximize the measurement of the work capacity caused by photothermal actuation, lightweight samples with a small volume are preferred for testing. However, intermolecular interactions of molecular chains (such as binding and entanglement) can introduce higher energy barriers during the phase transitions. Thereby, when being a photothermal stimulus, a large molecule requires a longer time to adapt to and reach a new equilibrium, especially during the process of lifting heavy objects. Furthermore, one of the advantages of developing flexible PCMs with high toughness is that the crystalline skeleton is conducive to energy storage during the stretching orientation process. The tensile force of an optical actuator is determined by its own characteristics and geometric shape, and high mechanical performance and large cross-sectional area contribute to generating significant tensile force.^[39] In brief, the flexible ssPCM integrated with high thermal energy storage, toughness, and multi-stimuli responsive performance, is extremely attractive for artificial muscles and soft robots with the requirement for low consumption and energy conservation in complex scenes to expand developments.
我们证明 PU-ssPCM/GNP （1 wt.%） 具有出色的可扩展性 （ɛ > 1500%）、韧性和可编程变形。然而，需要进一步的实验验证来确定这种柔性 PCM 的形状恢复是否可以产生拉力。柔性 PCM 表现出热驱动驱动（图 S23，电影 S1-S5，补充电影）和光热驱动驱动（图 6e;图 S24，电影 S6-S10，补充电影）。PU-ssPCM 的预应变薄花键表现出水化驱动，并显示出 405 kJ ^m-3 的工作密度（图 S23 和电影 S2-S5，补充电影）。此外，PU-ssPCM/GNP （1 wt.%） 的预应变薄花键能够在光刺激下举起 55 g 的物体（其自身重量的 2620 倍），收缩变形率为 53.3%，呈现 1330 kJ ^m-3 的工作密度（图 6e;电影 S7、补充电影）。同样，另一种预应变薄花键 （45 mg） 在暴露于全光谱太阳灯时能够举起 105 g 的重量（图 S23 和电影 S8，补充电影）。提升比 （m₁/m_a） 超过 2333，转化为 460 J ^kg-1 的能量输出。如图 6f 所示，当暴露于光刺激时，我们通过举起不同重量的物体对 PU-ssPCM/GNP （1 wt.%） 的工作密度进行了全面分析。 工作能力与运动距离有关，而运动距离与材料断裂时的临时应变和伸长率有关。³⁹ PU-ssPCM/GNP （1 wt.%） 已被实验证明具有高韧性 （ε = 1543%， σ = 19.2 MPa） 和刚度。因此，具有较高断裂伸长率或临时应变的致动器有助于产生更大的运动距离，并表现出更大的工作能力。此外，为了最大限度地测量光热驱动引起的工作能力，首选小体积的轻质样品进行测试。然而，分子链的分子间相互作用（如结合和纠缠）可以在相变过程中引入更高的能垒。因此，当成为光热刺激时，大分子需要更长的时间来适应并达到新的平衡，尤其是在举起重物的过程中。此外，开发具有高韧性的柔性 PCM 的优势之一是结晶骨架有利于在拉伸取向过程中储存能量。光致动器的拉力由其自身的特性和几何形状决定，高机械性能和大横截面积有助于产生显着的拉力。³⁹ 简而言之，柔性 ssPCM 集成了高热能存储、韧性和多刺激响应性能，对复杂场景下需要低功耗和节能的人工肌肉和软体机器人极具吸引力，以拓展发展。

Energy storage and release of flexible PCM can be achieved through intermolecular interactions (such as van der Waals forces and hydrogen bonds) during phase transitions. The advantage of a high energy storage density is that it improves energy utilization and addresses the issue of energy distribution, which plays an important role in energy conservation. Owing to its high latent storage/release capability and photothermal-driven performance, when subjected to thermal or photothermal stimulation, the flexible PCM undergoes deformation recovery due to absorbed heat, resulting in actuation. This stimulation and driving process takes a very short time, and energy conversion can be completed within a few seconds. After the heat source is removed, the flexible PCM remains in an exothermic state and can maintain a contracted state or deformation. Thus, the phase transition characteristics are beneficial for thermal energy utilization and conversion.
柔性 PCM 的能量储存和释放可以通过相变过程中的分子间相互作用（例如范德华力和氢键）来实现。高储能密度的优点是提高了能源利用率，解决了能源分配问题，这在节能方面起着重要作用。由于其高潜伏储存/释放能力和光热驱动性能，当受到热或光热刺激时，柔性相变材料会因吸收热量而发生变形恢复，从而导致驱动。这个刺激和驱动过程需要很短的时间，能量转换可以在几秒钟内完成。去除热源后，柔性相变材料保持放热状态，可以保持收缩状态或变形。因此，相变特性有利于热能的利用和转换。

To analyze the obtained results and evaluate the photothermal absorption efficiency and energy output, numerical simulations were conducted to obtain the energy intensity distribution of light inside PU-ssPCM/GNP (1 wt.%) via using COMSOL software according to Beer-Lambert law and Gaussian distribution (Figures S25 and S26, Equations S9–S11, Supporting Information). The changes in the absorbed photothermal energy and output energy of the PU-ssPCM/GNP (1 wt.%) when lifting objects of various weights can be observed in Figure 6g. It is simple to lift a relatively heavy object for a pre-stretched sample strip of small size. The photothermal energy absorbed inside the material is closely related to the irradiation distance (Figures S25 and S26, Supporting Information). Therefore, after setting the illumination distance, the output energy can increase with the mass of the heavy object, and the internal thermal energy gradually tends to stabilize from a decreasing trend. The output capacity was determined by the movement distance, which was related to the temporary strain and elongation at the break of the flexible PCM. The PU-ssPCM/GNP (1 wt.%) can respond to light stimulation and achieve energy output by controlling the appropriate illumination distance, which contributes to remote control and thermal management in the application of artificial muscle and soft robots. Owing to its high latent heat storage/release capacity, the PU-ssPCM/GNP (1 wt.%) can maintain the state of lifting heavy objects without the requirement for additional energy input, which effectively reduces energy consumption compared to other stimulus-responsive materials. Photothermal stimuli can trigger the reorganization of the molecular structure within PU-ssPCM/GNP (1 wt.%) during deformation, leading to the formation of a stable configuration to achieve a stable catch state. This characteristic is highly beneficial for energy conservation and simplification of control systems. To further clarify the application value of this flexible PCM in the field of artificial muscles, we designed and printed an arm prosthetic inspired by the human arm (Figure 6g). After fixing the fibrous sample (46 mg) onto the prosthetic limb, the arm model could undergo a bending motion similar to that of the biceps brachii, achieving a bending close to 90° under photic stimulation. The fibrous sample exhibited a work capacity of 622 J kg⁻¹ (Figure 6h,i; Movie S11, Supplementary Movie). In summary, PU-ssPCM/GNP (1 wt.%) with high latent heat storage capacity is capable of responding to thermal/photothermal stimuli, generating actuation distance and driving force simultaneously (Figure 6e–i; Movies S1,S6–S11, Supplementary Movie). The flexible PCM with high toughness and programmable deformation possesses photothermal actuation features due to its high latent storage capacity, which demonstrates that it has enormous potential for the application of artificial muscles and soft actuators with requirements for energy-saving and thermal-management.
为了分析获得的结果并评估光热吸收效率和能量输出，使用 COMSOL 软件根据比尔-朗伯定律和高斯分布（图 S25 和 S26，方程 S9-S11 ）进行了数值模拟，以获得 PU-ssPCM/GNP （1 wt.%） 内光的能量强度分布（图 S25 和 S26，方程 S9-S11、支持信息）。在图 6g 中可以观察到 PU-ssPCM/GNP （1 wt.%） 在举起各种重量的物体时吸收的光热能和输出能量的变化。对于小尺寸的预拉伸样品条，抬起相对较重的物体很简单。材料内部吸收的光热能与照射距离密切相关（图 S25 和 S26，支持信息）。因此，在设定照明距离后，输出能量可以随着重物质量的增加而增加，内部热能从减小的趋势逐渐趋于稳定。输出容量由移动距离决定，这与柔性 PCM 断裂时的临时应变和伸长率有关。PU-ssPCM/GNP （1 wt.%） 可以响应光刺激并通过控制适当的照明距离来实现能量输出，这有助于在人工肌肉和软体机器人应用中进行远程控制和热管理。由于其高潜热储存/释放能力，PU-ssPCM/GNP （1 wt.%） 可以保持提升重物的状态，而无需额外的能量输入，与其他刺激响应材料相比，有效降低了能耗。 光热刺激可以在变形过程中触发 PU-ssPCM/GNP （1 wt.%） 内的分子结构重组，从而形成稳定的构型以实现稳定的捕获状态。这一特性对于节能和简化控制系统非常有益。为了进一步阐明这种柔性 PCM 在人工肌肉领域的应用价值，我们设计并打印了一种受人类手臂启发的手臂假肢（图 6g）。将纤维样本 （46 mg） 固定到假肢上后，手臂模型可以进行类似于肱二头肌的弯曲运动，在光刺激下实现接近 90° 的弯曲。纤维样品的功容量为 622 J ^kg-1（图 6h，i;电影 S11、补充电影）。综上所述，具有高潜热储存能力的 PU-ssPCM/GNP （1 wt.%） 能够响应热/光热刺激，同时产生驱动距离和驱动力（图 6e-i;电影 S1、S6–S11、辅助电影）。这种具有高韧性和可编程变形的柔性相变材料由于其高潜在存储容量而具有光热驱动特性，这表明它在应用具有节能和热管理要求的人造肌肉和软致动器方面具有巨大的潜力。