Properties and mechanism of two-way shape memory polyurethane composite under stress-free condition

Two-way shape memory polymer can exhibit reversible shape transformation, which shows great application potential as a smart material. It is necessary to develop new reversible shape memory composite systems to achieve accurate and wide-range control of two-way shape memory behavior, especially under the condition of no external force. A chemically cross-linked two-component crystalline block copolymer was prepared, and the mechanism of two-way shape memory behavior under stress-free condition of the material was revealed. The results show that both the crystallization and thermal property of PLA/PCL-PU can be significantly changed by adjusting the proportion between PLA and PCL, and the dosage of crosslinking agent, and thus the shape memory property of PLA/PCL-PU can be regulated accordingly. The two-way shape recovery rate of the material can reach to 41.11%. This work provides a facile strategy to fabricate a biodegradable thermally induced two-way shape memory material under the stress-free condition with tunable shape memory properties.

In order to realize the reversible shape memory effect, one strategy is to compound the preformed SMP with an elastomer or another SMP and take the stress stored in the preformed part as the internal force to drive the deformation of the composite.The mechanism of the 2W-SME is shown in Fig. 1a.Although the material prepared on this approach can achieve reversible shape transformation, the main problems of such material include that the deformation of the material cannot be controlled accurately and the reversible strain is too small.Furthermore, since the internal stress of the material has been determined in preparation, the deformability has also been determined and cannot be changed.Therefore, it is necessary to introduce stress into the network of material at the molecular level to form anisotropy, so as to achieve accurate and abundant regulation of deformation behavior.To achieve the purpose above, a double-block polyurethane cross-linking network (PLA/PCL-PU) was synthesized from poly (L-lactide) (PLA) and poly (ε-caprolactone) (PCL).The copolymer has two separate melting temperatures, corresponding to the PCL and PLA chain segments respectively, and the melting temperature of the PLA chain segment is higher than that of PCL.As is shown in Fig. 1b, as a crystalline phase, the PLA chain segment retains its basic shape, maintains anisotropy, and provides a template for reoriented crystallization of PCL.PCL chain segment is a reversible phase, and the change of chain segment in the process of orientation crystallization and melting can cause macroscopic deformation of the material.
Compared to other two-component crystalline polymers with two-way shape memory effect, this material has a higher shape recovery rate.

Synthesis of PLA/PCL-PU
PLA and PCL were dissolved in 1,2-dichloroethane according to certain proportion, butyl tin dilaurate was added as the catalyst, followed by chain extender, TDI, and cross-linking agent, glycerol.After the reactants were mixed evenly, the reaction was carried out at 80 ℃ for 5 h under the protection of nitrogen.Then the reaction solution was cast onto a Te on mold.The mold was placed in a vacuum drying oven at 25 ℃ and 0.1 MPa for 72 h.Finally, the product was peeled off from the mold and stored in a sealed bag.

Analyses
IR spectra of PLA / PCL-PU were recorded on an Avatar 360 spectrophotometer.The spectra were recorded in the transmittance mode over a spectral range of 4000 ~ 400 cm − 1 , with a resolution of 4 cm − 1 .
Thermogravimetric analysis was conducted on a TGA Q 50 thermogravimetric analyzer.Samples were heated from 30 to 600 ℃ at a heating rate of 10 ℃ / min, in a nitrogen atmosphere at a ow rate of 20 mL / min.A TAQ 2000 differential scanning calorimeter was used to analyze the thermal properties of PLA/PCL-PU.
The test temperature range was from 30 to 210 ℃ and the heating rate was 10 ℃ /min.The heating curve was recorded in a nitrogen atmosphere.
One-way shape memory performance of PLA/PCL-PU was characterized according to the following method.As was shown in Fig. 2, rstly, a sample strip was heated to 180 ℃ and held for 5 min to release the internal stress of the material.Then it was folded in half along the midline to make θ 1 be 180°.The open end of the spline was secured with a clamp and cooled naturally to room temperature and maintained for 30 seconds.Next, the clamp was removed and the angle of the spline in the temporary shape, θ 2 , was determined.The spline was reheated to 180 ℃, and θ 3 at which the spline returned to the original shape was measured.The shape xation rate (R f ) and shape recovery rate (R r ) were calculated according to the equations ( 1) and (2) respectively.Five samples were tested in each group, and the results were averaged.

2
The two-way shape memory performance of PLA/PCL-PU was analyzed and the method was shown in Fig. 3. First, the spline was given a temporary shape, labeled shape A, in the same way as the one-way shape memory test.Then, the spline was heated to 110 ℃, the temporary shape of the spline was marked as B, and θ 3 was measured.After cooling to room temperature, the temporary shape of the spline was marked as C, and θ 4 was measured.The two-way shape recovery rate R r,2W of the spline was calculated according to Eq. (3).Five samples were tested in each group, and the results were averaged.IR spectra of PLA3000/PCL2000-PU and PLA3000/PCL3000-PU are shown in Fig. 4. As can be seen from the gure, the characteristic peaks of the two samples are roughly the same in position.The wide peak at 3500 − 3200 cm − 1 was caused by N-H vibration in the polyurethane structure.The absorption peaks at 2950 cm − 1 and 2860 cm − 1 were attributed to methylene stretching vibration peaks.The absorption peak at 1725 cm − 1 was caused by C = O in the ester and polyurethane.The absorption peaks at 1245 cm − 1 , 1191 cm − 1 and 1093 cm − 1 were generated by the C-O vibration of fatty ester.The absorption peaks at 1597 cm − 1 , 1552 cm − 1 and 840 − 680 cm − 1 were the characteristic peaks of the benzene ring, which can be used to identify isocyanate components.The above characteristic absorption peaks indicate that PLA and PCL react with TDI to form polyester polyurethane block copolymers.

Thermal Property of PLA/PCL-PU
The DSC curves of PLA3000, PCL3000 and PCL2000 are shown in Fig. 5. From the curve of PLA3000, it can be found that the glass transition temperature (T g ) of PLA 3000 is 60 ℃.The melting temperature (T m ) and crystallization degree (X c ) of the prepolymers are shown in Table 1.

PLA3000
PCL3000 PCL2000 The DSC curves of PLA 3000/PCL3000-PU with different mass ratios of PLA3000 to PCL3000 were shown in Fig. 6.The melting peak of PLA3000 can be observed on the curve of the sample with the mass ratio of 8:2 and the melting temperature is 161.4 ℃.Compared with the prepolymer of PLA3000, T m was 2.9 ℃ lower, and X c was 21.5% which was reduced by 50%.Furthermore, the glass transition can also be observed on the curve.The glass transition occurred at a wider temperature range, starting from 35 ℃ and ending at 90 ℃, which was 40 ℃ wider than that of the prepolymer of PLA3000.It can be attributed that for block copolymer T g was determined by the polymer segment with lower T g , while the softening point was determined by the polymer segment with higher T g .Therefore, the whole copolymer was in a high elastic state with a wide temperature range, and a variety of crystalline morphology and crystal structures formed.
When the mass ratio of hard segment (PLA) to soft segment (PCL) was 6:4, the T m of the hard segment (PLA segment) was 161.9 ℃, the crystallinity decreased to 8.35%.The melting peak of the soft segment still did not appear, and the glass transition temperature range of the copolymer was further widened.The initial temperature of the glass transition was 45 ℃, and the end temperature was 125 ℃.In the range, the curve roughly showed three baseline movements toward the endothermic direction.
As to the mass ratio of 5:5, the T m of PLA was 160.5 ℃, and the crystallinity was further reduced to 6.93%.On the other hand, the crystallization melting peak of the soft segment (PCL segment) could be observed on the curve.Since the glass transition of copolymer occurred in the same temperature range as the melting of PCL, the endothermic peak in the range of 40-95 ℃ was the superposition result of the two endothermic processes of melting and glass transition, so the peak area could not be used to calculate the crystallinity of soft segment.When the mass ratio was 4:6 and 2:8, the crystallinity of the hard segment (PLA) further decreased.The shape of the endothermic peak in the low-temperature zone was similar to that of 5:5.
When the ratio was 1.5:8.5, it can be found that the melting temperature of the hard segment (PLA) is 160.2 ℃, and the crystallinity is further reduced to 0.72%.Compared with other samples, the DSC curve of this sample was obviously different in the low-temperature region.A signi cant melting peak appeared at 44.4 ℃, which was 15.1 ℃ lower than that of the prepolymer of PCL3000.This might be attributed to the blocking effect of the cross-linking network on the directional arrangement of PCL3000 segments.It leaded to the reduction of grain size and the destruction of the integrity of the crystal form, the melting temperature reduced accordingly.However, due to the high content of PCL3000, the crystallinity of PCL3000 is 14.2%.On the curve, the endothermic process caused by the glass transition of PLA3000 was not observed, which may be due to the low content of PLA3000.
The effect of the crosslinker on the thermal property of PLA 3000/PCL3000-PU was studied.The results were shown in Fig. 7.The dosage of the crosslinker was controlled by changing the ratio of glycerol hydroxyl value to the sum of PLA3000 hydroxyl value and PCL3000 hydroxyl value (OH Gly :OH PLA3000+PCL3000 ), and the mass ratio of PLA3000 to PCL3000 was xed at 1.5:8.5.As can be seen from the Fig. 7, the crosslinker has a signi cant effect on the crystallization of PCL chain segments.
When the ratio is lower than 1:1, the melting peak of PCL3000 can be observed on the DSC curve, and its crystallinity is signi cantly higher than that of PLA3000.The melting temperature is 44 ℃.Furthermore, a weak melting peak can also be observed near 60 ℃, indicating that a small number of PCL3000 segments can achieve good crystallization in the copolymer, which makes the grain size and integrity similar to that of the prepolymer, and therefore the melting temperature is the same as that of the prepolymer.With the increase of the dosage of crosslinker, the crystallinity of PCL decreases continuously.When the ratio is 4:3 and 2:1, the glass transition process can be observed on the DSC curves.
The DSC curves of PLA 3000/PCL2000-PU with different mass ratios of PLA3000 to PCL2000 are shown in Fig. 8.As can be seen from the gure, when the mass ratios of PLA3000 to PCL2000 were 8:2, 6:4, 5:5, 4:6 and 2:8, only the melting peaks of PLA3000 were found on the curves.In addition, the endothermic process caused by the glass transition of different chain segments in copolymer molecules was observed in a wide temperature range (40-100 ℃) on these curves.When the mass ratios of PLA3000 to PCL2000 were 8:2 and 6:4, the shape of the melting peaks of PLA3000 was not symmetrical.The left part was wider, indicating that the crystal size and integrity of the crystals of PLA3000 segment were greatly in uenced by the cross-linking network.When the mass ratio of PLA3000 to PCL2000 was 5:5, the melting peak of PLA3000 showed left-right symmetry.With the increase of PCL2000 content, the crystallinity of PLA3000 segments decreased gradually.When the mass ratio of PLA3000 to PCL2000 was 1.5:8.5, the melting peak of PCL2000 appeared on the DSC curve.The melting temperature is 45.6 ℃ and the crystallinity of the hard segment (PLA3000) and soft segment (PCL2000) was similar.
The effect of the crosslinker on the thermal property of PLA3000/PCL2000-PU was investigated and the results were shown in Fig. 9.The mass ratio of PLA3000 to PCL2000 was xed at 1.5:8.5.From the gure we can nd that when OH Gly : OH PLA3000+PCL3000 was 1:1, a broad melting peak was observed on the curve at the range of 40 ℃ to 60 ℃, which were not present in the curves of other samples in this group.On the other hand, as in all of the samples, the crystallization of hard segments was not signi cantly affected by the amount of crosslinker.
Figure 10 shows the TG and DTG curves of PLA/PCL-PU.It can be seen from the gure that the temperature corresponding to the maximum weight loss rate is 248.8 ℃.According to the results of thermogravimetric analysis, the material has good thermal stability below 200 ℃.

PLA /PCL-PU one-way shape memory property
One-way shape memory performance of PLA3000/PCL3000-PU and PLA3000/PCL2000-PU was studied.
The results show that the R f values of all samples are 100%, which means the material has an ideal shape xing ability.According to the DSC curves, in the one-way shape memory cycle with T trans at 180 ℃ and T x at 25 ℃, the stress storage and release of the material are achieved by the melting of PCL and PLA segments and glass transition of PLA segments.
The in uence of the mass ratio of hard segment (PLA) to soft segment (PCL) on the one-way shape memory recovery rate is shown in Fig. 11a.As can be seen from the gure, when the content of the soft segment is low, that is, when the ratio of hard segment to soft segment is 8:2 and 6:4, the shape recovery rate of the samples is low and does not reach 20%.With the increase of PCL content, the shape recovery rates are signi cantly increased.For PLA3000/PCL3000-PU experimental group, when PLA3000:PCL3000 was 1.5:8.5, the shape recovery rate was the highest, which was 92.2%.For the PLA3000/PCL2000-PU experimental group, when PLA3000:PCL2000 are 2:8 and 1.5:8.5, the shape recovery rate reached 100%.Combined with the DSC results, it can be found that for the two groups of samples, when the ratios of PLA to PCL are 8:2 and 6:4, there is no melting peak of the PCL segment on the DSC curves.Therefore, the shape recovery force of the material mainly comes from the melting of PLA segments and the transformation of PLA segments from glass state to rubber state.Then, by the increase of the PCL content in the copolymer, it could be seen on the DSC curves that the crystallization of PCL segments became strong, and thus the ability to release stress from the melting process of PCL segments increased accordingly.In the meantime, due to the continuous decrease of PLA content, the melting process of PLA segments and its transition from glassy state to rubber state were less signi cant, but as is shown in Fig. 8a, the shape recovery rate of the material was signi cantly increased.Therefore, it can be speculated that the melting process of the PCL chain segment has a stronger stress release ability than that of the PLA chain segment.
Figure 11b shows the effect of the crosslinker on the one-way shape recovery rate of the material.As can be seen from the gure, with the increase of crosslinkers, the shape recovery rate of the two groups of samples showed the same decreasing trend.The DSC results showed that the crystallinity of both PCL and PLA decreased with the increase of the amount of crosslinker.Furthermore, the crystallinity of PCL decreased more signi cantly especially when the ratio of glycerol hydroxyl value to the sum of PLA hydroxyl value and PCL hydroxyl value is higher than 1:1.But at the same time, it can be found that when the ratio is higher than 1:1, the transition process of PLA segment from glass to rubber state appeared on the DSC curve.Under the comprehensive action of these two factors, the shape recovery rate of the material nally shows a gradual decrease.The amplitude was not as signi cant as that of changing the ratio of PLA to PCL, and the shape recovery rates of the materials were all higher than 80%.
By analyzing all the results in Fig. 11a and Fig. 11b, it can be found that there are six samples, whose one-way shape recovery rate is 100%, and their one-way shape xation rate is also 100%, showing ideal one-way shape memory performance.
3.3.2Two-way shape memory properties of PLA /PCL-PU The two-way shape memory behavior of PLA/PCL-PU was studied and the results show that the material has two-way shape memory performance under the stress-free condition.The original design idea of PLA/PCL-PU was inspired by Lendlein's work of multiphase copolyester urethane networks with two different crystallizable chain segments [42].Considering that PLA and PCL are two crystalline polymers with different melting temperatures, if these polyesters are used as raw materials to synthesize a polymer material with two independent melting temperatures by chemical crosslinking, the two-way shape memory effect may be realized in a real sense.By exploring the synthesis effect of PLA and PCL with different molecular weights, the molecular weights of the two prepolymers were nally determined.In the PLA/PCL-PU molecule, the PLA phase with a higher melting temperature was used to provide an anisotropic framework for the PCL phase with a lower melting temperature.The ratio of hard to soft segments and the amount of crosslinker that could make the material exhibit a two-way shape memory effect were explored.Based on the characteristics of thermal and crystallization of the material, combined with the actual use environment, an experimental method is designed to make materials display two-way shape memory behavior.The concrete operation process is described in 2.3.
Figure 12a shows the effect of the ratio of hard to soft segments on the two-way shape memory recovery rate.As can be seen from the gure, with the increase of PCL content, the shape recovery rate of both groups of samples gradually increased.A total of 6 samples in both groups showed two-way shape memory performance.For the group of PLA3000/PCL3000-PU, when the PLA3000:PCL3000 was 1.5:8.5, the two-way shape memory recovery rate was the highest, which was 25%.For the group of PLA3000/PCL2000-PU, when the PLA3000:PCL2000 was 1.5:8.5, the two-way shape memory recovery rate was also the highest, 41.11%.Overall, the two-way shape memory performance of the PLA3000/PCL2000-PU was better than that of the PLA3000/PCL3000-PU group.It can be seen that the higher the content of PCL in the soft segment, the more PCL chain segments oriented along the skeleton template provided by PLA during the cooling process, and the stronger the shape recovery ability is.Therefore, when the ratio of soft and hard segments in the two experimental groups exceeds their respective critical values, two-way shape memory behavior can be displayed macroscopically.
Figure 12b shows the effect of the crosslinker on the two-way shape memory recovery rate of the material.As can be seen from the gure, for PLA3000/PCL2000-PU, the two-way shape memory recovery rate increased rst and then decreased with the increasing amount of crosslinker.When the ratio of glycerol hydroxyl value to the sum of PLA3000 hydroxyl value and PCL2000 hydroxyl value was 1:1, the two-way shape memory recovery rate of the material was the highest.When the ratio increases to 2:1, the material no longer has two-way shape memory property.It can be observed from the DSC results that when the ratio is lower than 1:1, the crystallinity of the PCL phase and PLA phase is gradually enhanced with the increase of the amount of crosslinker, which makes the degree of anisotropy of the material at 110 ℃ become higher.When the ratio is higher than 1:1, the crystallinity of the PCL phase and PLA phase decreases signi cantly with the increase of the amount of crosslinker.The two-way shape memory performance of the material deteriorates.For PLA3000/ PCL3000-PU, the two-way shape memory recovery rate showed a downward trend with the increasing amount of crosslinkers.It can be observed from DSC curves that the crystallization of the PCL phase becomes weaker with the increase of the amount of crosslinker.When the ratio of glycerol hydroxyl value to the sum of PLA3000 hydroxyl value and PCL2000 hydroxyl value was 4:5, although the crystallization of the PCL phase was still high, the crystallization of the PLA phase was low, which affected the ability of the PLA phase as a skeleton template.Therefore, the two-way shape memory recovery rate was low, which was similar to the recovery rate when the amount of crosslinker was maximum.

Conclusion
In this paper, a two-component crystalline block copolymer PLA/PCL-PU was prepared by PLA and PCL.
Based on the mutual independence of the two crystals, the material exhibited a reversible two-way shape memory effect under the condition of no external force.Through the study of the shape memory performance of the material, it was found that changing the ratio of PLA to PCL and the amount of crosslinker can signi cantly change the crystallization and thermal properties of the material, so as to realize the effective regulation of reversible two-way shape memory performance.The shape recovery rate can reach 41.11%.In addition, through the work in this paper, we found that, for PLA/PCL-PU to exhibit a two-way shape memory effect, the material needs to achieve a " ne" and "dynamic" balance in the following aspects, mainly including "crystallization induced elongation", that is, the ability of the soft segment to crystallize directionally under the "skeleton" provided by the hard segment to store stress, "melting induced contraction" refers to the ability of soft sections to release stress during melting, the strength of hard segments to maintain anisotropy, and even glass transition of molecular chains.

Declarations
Author Contribution H. Yang and R. Shi contributed to the study conception and design.Material preparation and data collection were performed by H. Yang and R. Shi.H. Yang, R. Shi, Q. Jiang and J.
Ren conducted data analysis and discussion.The manuscript was written by H. Yang, R. Shi, Q. Jiang and J. Ren.All authors read and approved the nal manuscript.
Funding information This work is supported by the Natural Science Foundation of Heilongjiang Province of China(E2018003) and the Fundamental Research Funds for the Central Universities (2572018BC31).
Con ict of interest The authors declare no competing interests.
Effect of soft and hard segment ratio on two-way shape memory recovery rate of PLA/PCL block copolymer (a), Effect of cross-linking agent dosage on the two-way shape memory recovery rate of PLA/PCL block copolymers (b)

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Results And Discussion 3.1 IR spectroscopy of PLA/PCL-PU