Stepless shape morphing polymer

To change the situation of shape memory polymers (SMPs) that can only remember very few shapes and enable discretional morphing for practical application, the authors report a reversible stepless multiple SMP derived from ultrahigh molecular weight polyethylene (UHMWPE). As the crystals of semi‐crystalline polymers are assembled by those with slightly different melting temperatures, and each type of crystal can remember a single shape, the crystalline region of UHMWPE is allowed to remember plenty of temporary shapes after programming. Changing the temperature of the programmed polymer within the melting/crystallization temperature ranges would lead to releasing/recovery of the memorized temporary shapes. Accordingly, multiple shape memory effects can be easily realized without an elaborate design of material structure and training process in advance as before. The temperature‐dependent adjustability of the analog capacitor and soft lens with embedded programmed UHMWPE as actuators, characterized by the continuous/random/proportional responsivity, further reveals the utilization prospects of the controllable reversible stepless discretionary morphing effect. Moreover, the maximum work density of the programmed UHMWPE is found to be 210 kJ/m3, which is more than 10 times of piezoelectric ceramics, so that it can serve as a proof‐of‐concept mechanical driver for reversibly pumping of ethanediol‐droplet upon heating/cooling.


| INTRODUCTION
Shape memory polymer (SMP) is a smart material that can change its shape from a deformed state to a permanent shape. Recent achievements in this area include diversifying stimuli, [1][2][3][4][5][6][7] upgrading from one-way SMP to two-way SMP,  and implementation of multiple shape memory effect. [1][2][3][4][5][6][7] Compared with the colorful expectations at the beginning of the development of this type of material, however, their commercial application so far is very few. The representative industrialized products are mainly heat-shrinkable tubes and heat-shrinkable films. We believe that the following factors are responsible for the situation.
(1) Only a few shapes can be remembered by the existing SMPs, and shape change is discrete (rather than continuous transformation). The poor on-demand adaptively and adjustability are unfavorable for satisfying the diverse requirements of practical scenarios. (2) The work density of SMPs is low. Upon heating, the stiffness of SMPs used to be greatly reduced due to the disordering of the ordered regions. Sometimes, even self-support is a problem. It is hard for the softened materials to produce sufficient mechanical work output.
Here, in this study, we tackle the challenges with semicrystalline polymer represented by ultrahigh molecular weight polyethylene (UHMWPE) as the proof-of-concept base material (Figure 1). UHMWPE is known for its wide melting temperature range (~20°C) and heavy chain entanglements. When it is properly trained at a certain programming temperature that is over the upper limit of the melting temperature range (i.e., T m(end) , Figure 1B), all the crystals are converted into the switching phase, and the chain entanglements are reformed during the subsequent cooling, serving as the internal stress provider. This F I G U R E 1 (A) Mechanism of the reversible stepless discretionary multiple shape memory effect. (B) Programming for acquiring reversible stepless discretionary multiple shape memory effect. consideration has been evidenced in our previous work, 38 but what we did not realize is that the switching phase of the UHMWPE programmed in this way had factually remembered lots of temporary shapes. Because the crystalline phase of semi-crystalline polymers is composed of various crystals with different melting temperatures, each type of crystal with a specific single melting temperature can remember one temporary shape after programming. So long as the material is heated to a certain activation temperature, T activation , within the melting temperature range (T m(onset) < T activation < T m(end) , Figure 1A), the temporary shapes remembered by the crystals with melting temperatures below this temperature would be restored to the primary ones with the assistance of entropic elasticity, while the temporary shapes remembered by the crystals with melting temperatures above this temperature are still frozen. Supposing material temperature (i.e., T activation ) is gradually raised from T m(onset) to T m (end) with sufficiently small intervals, the successive restorations of temporary shapes would be rather smooth so that stepless morphing is enabled. Meantime, the abundant chain entanglements that survive at the programming temperature and activation temperature keep structural stability of the material, avoiding collapse in case the shape memory effect is brought into play.
Besides, the shape memory effect acquired by the programmed UHMWPE should be reversible owing to the collaboration of the crystalline phase and internal stress provider. It means that as the heated material is cooled down to a certain recovery temperature, T recovery , within the crystallization temperature range (T c(end) < T recovery < T c (onset) , Figure 1A), the melted switching phases with crystallization temperature above the temperature would be turned from random coils to oriented crystals under the stress induced by the internal stress provider. As a result, the material would be transformed from its primary shape back to the remembered temporary shape, while the melted switching phases with crystallization temperatures below the temperature remain to be molten. Naturally, such a shape change between T c(end) and T c(onset) along with T recovery can also be rather gentle, if the temperature step is small enough.
On the other hand, the strong chain entanglements of UHMWPE can be regarded as physical crosslinking sites, which provide the material with high robustness even after melting the crystalline phase. [46][47][48][49][50][51] It would behave like elastomers when its shape memory effect operates, ensuring the capacity to work.
A broad melting range has been used for preparing reversible dual SMP, 30 but the lower melting temperature part and higher melting temperature part of the crystalline phase were assigned to be the switching phase and fixing phase (i.e., internal stress provider), respectively. An increasing number of temporary shapes was not the concern of the authors, and the entire crystalline region is not totally used as the switching phase. Xie 52 took advantage of broad glass transition as the switching phases, and the resultant one-way SMPs exhibited dual-, triple-, and quadruple-shape memory effects. The result supports our design from another aspect.
The above analysis demonstrates the feasibility of this study. Hereinafter, the idea is verified by examining the performance of the programmed UHMWPE and its potential applicability.

| Preparation of UHMWPE specimen
UHMWPE particles were compression molded into a 1-mm-thick sheet at 170°C under 20 MPa for 30 min. Then, a dumbbell-shaped specimen (total length = 50 mm, length of reduced section = 20 mm, width = 4 mm) was cut from the sheet for the subsequent tests.

| Programming
As shown in Figure 1B, the dumbbell-shaped specimen of UHMWPE was warmed up to 160°C (the programming temperature, T prog , which is obviously higher than the upper limit of the melting temperature range of UHMWPE [136.6°C, Supporting Information: Figure S1]) for 10 min and extended to 500% strain. Then, the stretched specimen was cooled to room temperature at the constant strain. Lastly, the applied stress was removed, and the specimen was warmed up to 138°C for 10 min, followed by cooling to room temperature once more.

| Characterization
A dynamic mechanical analyzer (DMA, Q800; TA Instruments) was employed to study the dynamic mechanical properties at 1 Hz. The variation in the specimen's length at zero force was measured under the controlled force mode.
For each cycle of measurement, the heating rate was 3°C/min and the cooling rate was −3°C/min.
The melting and crystallization behaviors of the polymers were characterized using a differential scanning calorimeter (DSC, Q10; TA Instruments) in nitrogen. The temperature was cyclically controlled. For each cycle of measurement, the heating rate was 3°C/min and the cooling rate was −3°C/min.
Tensile tests were conducted on a universal tester (CMT 6000, SANS) at a crosshead speed of 50 mm/min at 25°C according to ISO527-2.
Fourier transform infrared spectra were collected by a Nexus 670 spectrometer (Thermo Nicolet). The orientations of the crystalline and amorphous phases were determined in terms of dichroism of δ(-CH 2 -) absorption at about 731 and 723 cm −1 , respectively. 53 The extent of anisotropy of the oriented material was assessed by the corresponding dichroic ratio, R = A ∥ /A ⊥ , where A ∥ and A ⊥ are the absorption intensities measured with the radiation polarized parallel and perpendicular to the stretching direction, respectively. Accordingly, (R − 1)/(R + 2) gives the measure of the orientation degree. [54][55][56] Before the measurements, two background single-beam spectra were recorded with the polarizers parallel and perpendicular to the elongation direction with the same experimental parameters, respectively.
Micro-Raman analyses were conducted by a Renishaw inVia Qontor spectrometer coupled with a microscope (LEICA DM2700) and 532 nm visible laser. The laser power was set to 50 mW. The measurement was performed by placing a thin sample on the microscope motorized stage at room temperature, and the sample was scanned in mapping mode with steps of 1 μm integrating the signal for 0.3 s at each step. The selected area was 50 μm × 50 μm and the size of each spot was 1 μm. Chemical images were gained by integrating over 2880-2900 cm −1 Raman shift regions in the spectrum.
Wide-angle X-ray diffraction spectra were recorded by a Bruker AXS D8 Discover diffractometer with Cu Kα radiation under an accelerating voltage of 35 kV and a tube current of 35 mA. The scanning range of Bragg 2θ angle was from 10°to 30°with a scanning rate of 3°/min.
The internal stress was determined by the wellknown sin 2 ψ X-ray diffraction method. [57][58][59][60] The relationship between the diffraction peak position, 2θ, and the axial internal stress (σ) was estimated from 59 : (2 ) (sin ) , where E is Young's modulus; υ is Poisson's ratio; ψ is the tilting angle; θ 0 is the diffraction angle at a stress-free state.

| Programming and introduction of internal stress
In the beginning, the crystalline feature of the UHMWPE specimen is characterized by DSC (Supporting Information: Figure S1); a broad melting temperature range (116.5-136.6°C) and crystallization temperature range (102.9-122.7°C) are observed. Accordingly, the peak melting temperature, T m , and peak crystallization temperature, T c , are determined to be 132.9°C and 119.1°C, respectively.
To obtain the desired reversible stepless discretionary multiple shape memory effect, the UHMWPE specimen is programmed in accordance with the steps shown in Figure 1B. The structural variation of UHMWPE, especially the cultivation of the internal stress provider, along with the programming is interpreted in the following. In Step 1, the specimen is heated to T prog (i.e., 160°C, higher than T m of UHMWPE), allowing for partial disentanglements. Under the circumstances, the specimen can be easily stretched (Step 2) because of temporarily relieve of partial hindrances (i.e. entanglements). The succeeding cooling at the constant deformation (i.e., Step 3) results in the generation of oriented crystallites (with orientation degree of the crystalline phase of 0.63, see Supporting Information: Figure S2 and Supporting Information: Table S1) and reconstruction of entanglements of the disentangled parts. 61,62 When the external force is unloaded at ambient temperature (refer to Step 4), the temporary shape of the stretched specimen remains unaffected because of the poor mobility of the macromolecules. As the specimen is heated to the upper limit of the melting temperature range, T m(end) , the oriented crystals melt and transform into entropically favorable random coils. The length of the specimen is shortened due to entropy increase aroused shrinkage (Step 5). Meantime, the rebuilt entanglements are compressed, partially losing the preferred orientation, and tend to return to the expanded status. The competition between the opposite deformations leads to the great reduction of the ultimate specimen length, but it is still marginally longer than that of the primary version. In this context, the re-entangled disentanglements serve as the internal stress provider. Eventually, the specimen is cooled to room temperature again, offering the trained version (Step 6), and the oriented UHMWPE crystalline phase is regenerated (with orientation degree of the crystalline phase of 0.12, see Supporting Information: Figure S2 and Supporting Information: Table S1) under the tension exerted by the re-entangled oriented amorphous phase (Supporting Information: Figure S2 and Supporting Information: Table S1). Clearly, the internal tensile stress is reduced to a certain degree following the expansion preferentially resulting from recrystallization. Even so, the oriented structures still work in the programmed material. Its tensile strength is as high as 41.62 MPa, for example, 30.80% higher than that of the original UHMWPE specimen (31.82 MPa, Supporting Information: Figure S3).
The discussion is evidenced by the Raman spectroscopy study. Figure 2A-C gives the mapping of -CH 2peak positions of the UHMWPE. It is seen from Figure 2A that the -CH 2groups are uniformly distributed at the beginning. Having been stretched to five times its original length (Step 4, Figure 1B), the -CH 2peak is upshifted from 2884.2 to 2891.1 cm −1 ( Figure 2B,D), implying that the internal stress generated during programming is a compressive one 63 due to entropic elasticity. After training, the specimen significantly shrinks (Step 6 in Figure 1B), but it is still longer than the original version. Correspondingly, the compressive stress gradually decreases because of an increase in entropy, and the upshift of the -CH 2peak is also reduced to 2885.7 cm −1 ( Figure 2C,D). We believe that it is the introduction of the internal tensile stress provided by the re-entangled entanglements that prevent further increase of entropy. The magnitude of the internal stress provided by the re-entangled entanglements should be equal to the compressive stress aroused by entropy increase but in the opposite direction. The difference in -CH 2 -Raman shift before and after training ( Figure 2D) proves that internal stress has been successfully introduced into the material.
For quantifying the internal stress, the sin 2 ψ X-ray diffraction technique is applied, as described in Section 2.4. The {110} peak at~21.6°is used for the measurement as it is much stronger than the {200} peak at~24.0°( Figure 2E). Accordingly, the peak of the relevant specimen is measured at the inclination angles of ψ = 0°and 7.5°( Figure 2F,G), and the calculated internal stresses are listed in Supporting Information: Table S2. It is found that the internal stress of the as-prepared specimen is 0 MPa, which agrees with the general understanding. As for the programmed specimen, the residual stress equals −4.38 MPa. Considering that the reaction stress provided by the re-entangled chains and the compressive stress induced by entropy increase reach balance in the specimen, the internal tension stress provided by the re-entangled chains is known to be 4.38 MPa.

| Reversible multiple shape memory effects
After programming, the UHMWPE specimen exhibits a reversible shape memory effect in the absence of external force during heating and cooling between 90°C and 138°C as shown by the visual inspection ( Figure 3) and thermomechanical behaviors measured by DMA (Supporting Information: Figure S4). The two boundary temperatures are chosen to be close to the lower limit of the crystallization temperature range (102.9°C) and the upper limit of the melting temperature range (136.6°C) of UHMWPE. When the specimen is heated to the activation temperature of 138°C, for example, the oriented crystalline domains melt, and macroscopic contraction is detected ( Figure 3A and Supporting Information: Figure S4A). A drop in temperature of the specimen to 90°C (i.e., <T c(end) ) leads to the reproduction of the oriented crystals guided by the internal stress provider (re-entangled chains). The same cyclic deformation habit can be continuously repeated with temperature-induced melting/recrystallization of the crystalline phases and increase/relaxation of internal stress.
F I G U R E 3 Length changes of the reversible (A) dual shape memory ultrahigh molecular weight polyethylene (UHMWPE), (B) triple shape memory UHMWPE, and (C) quadruple shape memory UHMWPE during cyclic heating/cooling between 90°C and 138°C.
For the second group of tests, 129°C is used as the activation temperature, which is about in the middle of the melting temperature range of UHMWPE, and the specimen shrinks from Shape Ⅰ to Shape Ⅱ as it is heated from 90°C ( Figure 3B and Supporting Information: Figure S4B). When the specimen is further heated from 129°C to 138°C, additional shrinkage (from Shape Ⅱ to Shape Ⅲ) takes place ( Figure 3B) owing to the melting of the remaining oriented crystalline domains (with a melting temperature range of 129-136.6°C). In the reverse process, 116°C acts as the recovery temperature. When the specimen is cooled down from 138°C to 116°C, partial random coils of UHMWPE are restored to the oriented crystalline domains under the tension imposed by the internal stress provider (re-entangled chains) so that the specimen becomes longer ( Figure 3B). Further cooling from 116°C to 90°C leads to the recreation of oriented crystalline domains from the rest random coils of UHMWPE and the specimen length keeps on increasing. The shape changes are also reproducible following the temperature loop of 90°C, 129°C, 138°C, and 116°C.
Similarly, the reversible quadruple shape memory effect is implemented based on the same philosophy by arbitrarily choosing two activation temperatures (126°C and 134°C) in the course of heating and two recovery temperatures (132°C and 122°C) in the course of cooling, respectively ( Figure 3C and Supporting Information: Figure S4C).
On the whole, it can be concluded from the photos in Figure 3 that as the number of activation temperatures or recovery temperatures increases within the range of interests, the length change becomes more and more gentle. In addition, the number of recoverable shapes (or the specimen lengths) is allowed to discretionarily vary. These indicate that the desired reversible stepless morphing may come true.

| Application prototypes
To intuitively understand the features of the reversible stepless multiple SMP and the capability of driving something, the following three application prototypes are assembled with the programed UHMWPE specimens as the driving units. Figure 4A illustrates the set-up of an adjustable capacitor in which two strips of the programmed UHMWPE are sandwiched between two steel sheets and the working principle as well. The reversible shrinkage/extension of the former along the longitudinal direction would cause the corresponding expansion/ shrinkage in the vertical direction as a result of the Poisson effect. Consequently, the space between the steel sheets, that is, the capacitance, could be manipulated by temperature. Figure 4B shows that with a rise in temperature from 90°C to 138°C, the UHMWPE specimens gradually shrink in the axial direction and become thicker in the F I G U R E 4 (A) Setup and working principle of the adjustable capacitor with the reversible stepless discretionary multiple shape memory ultrahigh molecular weight polyethylene as the actuators. (B) Temperature dependences of capacitance and space between the steel sheets of the capacitor were measured during heating/cooling cycles between 90°C and 138°C. normal direction. Accordingly, the distance between the two steel sheets increases from 2.43 to 2.70 mm, and the average capacitance of the capacitor is reduced from 119.07 to 106.81 pF. In the cooling process, the UHMWPE specimens are elongated and become thinner at the same time, resulting in a gradual decrease in the distance between the two steel sheets. The average capacitance of the capacitor also recovers from 106.81 to 119.07 pF accordingly. In the subsequent three heating/ cooling cycles, the same variation trends of the average capacitance are detected, demonstrating the stable performance of the material.
More importantly, when the temperature dependence of capacitance in Figure 4B acts as the standard curve, either random adjustment (Supporting Information: Table S3) or proportional adjustment (Supporting Information: Table S4) of capacitance can be achieved with very low errors by simply changing temperature.
To highlight the critical role of the reversible stepless discretionary multiple shape memory effect of UHMWPE under thermal stimulation, in this case, the untrained UHMWPE specimens are used to assemble the control capacitor following the same design as Figure 4A. As shown in Supporting Information: Figure S5, the variation in capacitance of the control capacitor, caused by the conventional thermal expansion and contraction is rather insignificant during the same heating/cooling cycles. The range of relative difference in the measured capacitance is only 1.99%, which forms a sharp contrast to the value of 10.37% estimated in Figure 4B.
By taking advantage of a similar mechanism, the programmed UHMWPE can be used to change the focal length of the soft tunable lens. To demonstrate the effectiveness, an annular UHMWPE specimen is firstly programmed according to Figure 1B to make a ring-like actuator, which is then embedded in uncured SYLGARD TM 184 silicone elastomer with the excellent optical property. After curing in a simple mold (polypropylene Petri dish), a flexible silicone rubber lens with a built-in UHMWPE actuator is obtained ( Figure 5A). The shrinkage of the programmed F I G U R E 5 (A) The as-prepared deformable silicone rubber lens with reversible stepless discretionary multiple shape memory ultrahigh molecular weight polyethylene (UHMWPE) ring as the actuator. (B) Side views of the deformed lens accompanying temperature change, see also Supporting Information: Video S1. (C) Optical pathway diagram. (D) Temperature dependences of curvature radius and focal length of the lens were measured during three heating/cooling cycles between 90°C and 138°C. (E) Sunflower images were observed through the lens with different focal lengths tuned by changing temperature, see Supporting Information: Video S2. UHMWPE would squeeze the soft lens to change its surface curvature for zooming ( Figure 5B and Supporting Information: Video S1), which imitates the zooming mechanism of a human eye that the ciliary muscle squeezes the eye's lens to change its focal length. Moreover, the reversed deformation of the UHMWPE would recover the original focal length ( Figure 5B and Supporting Information: Video S1). Figure 5C shows the optical pathway diagram of the prepared lens.
As temperature increases, the silicone rubber lens becomes more arched ( Figure 5B,D and Supporting Information: Video S1), which causes changes in the curvature radius (from 75.10 to 44.49 mm, see Figure 5D) and the focal length (from 2739.33 to 894.33 mm, see Figure 5D). As a result, the sunflower observed through the lens looks smaller and smaller ( Figure 5E and Supporting Information: Video S2). When the temperature drops gradually from 138°C to 90°C, the embedded UHMWPE ring returns to its original shape. Accordingly, the lens gets back to be flat with decreasing the squeezing force, and the sunflower image becomes bigger and bigger ( Figure 5E and Supporting Information: Video S2) owing to the recovery of the curvature radius and focal length.
On the basis of the temperature dependence of focal length ( Figure 5D), additionally, proportional adjustment and random adjustment of the focal length of the lens are allowed by changing temperature (Supporting Information:  Tables S5 and S6). It means that we can adjust the lens to any desired focal length by changing the temperature.
For comparison, a control lens containing an unprogrammed UHMWPE ring is made. During the heating and cooling processes, the shape of the lens and the observed sunflower remains unchanged (Supporting Information: Figure S6). The result not only proves the vital function of the programmed UHMWPE but also the thermal deformability of the unprogrammed UHMWPE, which is not strong enough to overcome the constraint of the surrounding silicone rubber.
Lastly, the programmed UHMWPE is employed for liquid transportation. As shown in Figure 6, a horizontal silicone pipe is fixed on a wooden board by several isolated programmed UHMWPE strips and the board is perpendicular to the ground. When the UHMWPE strips were heated successively from left to right ( Figure 6A-F and Supporting Information: Video S3), the contraction of the strips would put pressure on the pipe, which partially squashes the pipe and forces the ethanediol F I G U R E 6 (A)-(U) Motion of the ethanediol droplet (with blue dye) in a silicone pipe driven by the thermal response of the programmed ultrahigh molecular weight polyethylene (UHMWPE) strips, refer to Supporting Information: Video S3. The red arrows indicate the moving direction of the heater. droplet to move forward. Meanwhile, the ethanediol droplet can be sent back to the original place by successively heating the UHMWPE strips from right to left ( Figure 6G-K and Supporting Information: Video S3). Such transportation of ethanediol droplets can be repeated by reheating the UHMWPE strips as mentioned above. Because the strips' temperature returns to the recovery temperature within~5 s after removal of the heating source, the transition of the moving direction of the ethanediol droplet can be rapidly completed. Evidently, the delivery system does not require specific driving devices and complex assembly processes before use.

| Capacity to do work
It is worth noting that the above discussion about the potential applications of the programmed UHMWPE has touched on a topic that reversible SMPs rarely talk about; that is, the ability of reversible SMPs to do work. The examples shown in Figures 4-6 can only be achieved by doing work against external forces, which are different from most cases reported in the area. To quantify the work output of the programmed UHMWPE, that is, the work done per unit volume of the material, DMA testing is employed to measure the reversible length change of the material subjected to different tensile forces (different loads) along the programming direction during the heating/cooling cycles between 90°C and 138°C (Supporting Information: Figure S7). Then, the work densities of the programmed UHMWPE are determined as a function of the applied tensile force.
The data in Supporting Information: Figure S7F reveal that the reversible length of the material first increases and then decreases, and reaches the maximum at the tensile force of 1.8N. In fact, the deformation of the specimen comes from the contribution of two parts. Besides the reversible length change during the heating and cooling cycles, the specimen would also deform under the tension. The greater the tensile force, the greater the deformation of the specimen. When the tensile force is much lower than the recovery force caused by entropy increase at a certain activation temperature, the deformation produced by the tensile force can be easily recovered. As the tension force increases from 1.5 to 1.8 N, the strain generated by the tension force gradually increases, resulting in the corresponding increase of the reversible length change of the programmed UHMWPE specimen. As shown in Supporting Information: Figure S7, the reversible length change reaches the maximum value of 3.46 × 10 −3 m, when the tension force is 1.8 N. But it decreases significantly to 1.81 × 10 −3 m when the tension force reaches 1.9 N, which indicates that the tension force is very close to the recovery force caused by entropy increase. In case the applied tension force and the recovery force reach the balance, the specimen would not be able to retract through the reversible shape memory effect at a certain activation temperature and it cannot do work anymore. Therefore, the specimen has to fail when the tensile force reaches 2.0 N, which exceeds the recovery force of the specimen.
With respect to the tensile force of 1.8 N under which the maximal reversible length change is detected, it means that the programmed UHMWPE specimen (0.0284 g weight) can lift almost 6500 times heavier than its own weight (1.8 N/9.8 N/kg/0.0284 × 10 −3 kg) with a reversible length change (reversible strain) of 32.86%. Considering that the density of UHMWPE is 938 kg/m 3 , the maximum work density is thus known to be 1.8 N × 3.46 × 10 −3 m/ (0.0284 × 10 −3 kg/938 kg/m 3 ) = 210 kJ/m 3 . The value is over 26 times higher than that of mammalian skeletal muscle (8 kJ/m 3 ) 64 and more than 10 times higher than that of piezoelectric ceramics (20 kJ/m 3 ). 65 When the tensile force is further increased to 1.9 N (equivalent to 0.194 kg gravitational force), the specimen (0.0211 g weight) can still perform normally, and a 17.12% reversible length change is detected. Under the circumstances, the lifted weight is 9188 times heavier than the specimen's own weight, which is higher than those of all reversible semicrystalline SMPs to the best of our knowledge. In the meantime, the untrained UHMWPE specimen experiences the same test (Supporting Information: Figure S8). Only creep can be seen under the tensile force of 1.8 N during the heating/cooling cycles. Such a high work density is attributable to the strong chain entanglements of UHMWPE, which provide the material with high robustness even after melting the crystalline regions.

| CONCLUSION
Reversible stepless discretionary multiple shape memory UHMWPE is successfully prepared in this study. The crystalline phase and the uniformly dispersed chain entanglements serve as the switching phases and the internal stress provider, respectively. After being simply trained by thermal treatment associated with stretching, the crystals of UHMWPE memorize many temporary shapes as independent switching phases. As a result, the programmed UHMWPE specimen is able to show multiple shape memory effects through activation at different temperatures within the melting range during heating. Moreover, the thermally induced multiple transformations can be reversed by cooling the specimen to different temperatures within the crystallization temperature range. The number of temporary shapes can be changed at will, eventually leading to controlled stepless transformation, as demonstrated by the example applications of the adjustable capacitor and soft lens. In these cases, the capacitance of the capacitor and focal length of the lens is allowed to be proportionally or randomly tuned by changing temperature. Such a continuous morphing manner completely differs from the step-type transformation of current SMPs and has wide application prospects. Besides, the work density of the programmed UHMWPE is 210 kJ/m 3 , which is higher than those of the reported semi-crystalline reversible SMPs as far as we know. Therefore, the material is competent for mechanical driving systems like the ethanediol droplet transport shown by the example application.
The stepless shape morphing polymer developed by the authors realizes what traditional SMPs want to realize but fail to realize. Soft robots, flexible actuators, biomedical devices, smart textiles, switchable micropatterned surfaces, and so on, which have always been the favorite target applications of SMPs, may thus be endowed with practicability.
UHMWPE is a member of the family of commercial polyolefin, and no chemical reactions are required during the aforesaid training processes. Hence the prepared reversible stepless discretionary multiple SMP is suitable for industrial mass production. In principle, moreover, the method for preparing reversible stepless discretionary multiple SMPs proposed in this paper can be further generalized to other reversible SMPs even with not that wide melting temperature range by precise temperature control and maintaining mechanical robustness of the materials during operation. People will benefit from the technological advances in this area accordingly.