Visible Light-Induced Plasticity of Shape Memory Polymers - ACS

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Visible light induced plasticity of shape memory polymers Shaobo Ji, Fuqiang Fan, Chenxing Sun, Ying Yu, and Huaping Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11188 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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ACS Applied Materials & Interfaces

Visible light-induced plasticity of shape memory polymers Shaobo Ji, Fuqiang Fan, Chenxing Sun, Ying Yu, and Huaping Xu* Key Lab of Organic Optoelectronics & Molecular Engineering Department of Chemistry, Tsinghua University, Beijing, 100084, China KEYWORDS. Photoinduce plasticity, visible light, shape memory polymers, diselenide bonds, dynamic covalent bonds.

ABSTRACT. Thermoset polymer plasticity has been realized by introducing exchangeable bonds, and the plasticity is mostly triggered via heat or UV light. Visible light is a relatively mild trigger that has not been used to induce plasticity in polymer materials. Herein, thermoset polyurethanes (PUs) containing diselenide bonds are fabricated and possess visible light-induced plasticity along with shape memory behavior. A series of PUs with different diselenide bond contents were tested, and their shape memory properties and plasticity varied. With a higher diselenide bond content, both shape memory and light-induced plasticity are achieved. By combining these two properties, reshaping the permanent shapes of the PUs is easier. Compared with heat or UV light, visible light has the advantage of spatial control. For instance, a pattern of visible light was introduced by a commercial projector to demonstrate facile reshaping of the

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materials. Because visible light can be introduced via various methods, PUs with visible lightinduced plasticity have great potential.

1. INTRODUCTION Shape memory polymers (SMPs) are materials that possess the ability to adopt to programmed temporary shapes under certain conditions and recover their permanent shapes upon external stimulation.1-5 The programmability and stimuli responsiveness of SMPs endow them with a “smart” characteristic. SMPs have attracted attention for applications in biomedical devices, functionally responsive devices, and others. Recent studies on multi-shape memory and reversible shape memory behaviors have further developed shape memory materials and extended their potential applications.6-8 To fabricate SMPs, two structural requirements are needed.2 The first requirement is switch segments that allow fixation/recovery of the material’s shape. The second requirement is chemical or physical cross-linking net points, which define the permanent shape. Physically cross-linked SMPs possess reprocessibility, but their shape fixity and recovery is hindered. On the other hand, SMPs with chemical cross-linking often possess good shape memory and recoverability, but their thermoset nature makes reprocessing their permanent shapes difficult. A number of studies have focused on combining reprocessable thermoset and SMPs to simultaneously achieve robust shape memory and plasticity in a single polymer. Recently, studies on the plasticity of thermosets have emerged, and the concept of a vitrimer was proposed. Vitrimers are derived from thermosets, but their covalent polymer networks can be reformed under specific conditions. The reformation of the polymer network occurs via bond exchange, e.g., transesterification reactions or dynamic covalent bonds.9-19 Among various


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polymer materials, polyurethane (PU) is one of the most common thermosets, and PUs have been widely used in industry and smart materials, such as self-healing polymers and SMPs, because of the advantages of their versatile designs and properties.20-25 However, the poor reprocessibility of thermoset PUs has restricted their applications, and only a few PUs with plasticity have been reported.26-31 Reprocessable PUs still have great potential for development, especially shape memory PUs. The reprocessing condition for most vitrimer or vitrimer-like materials is heating at a high temperature. Light-triggered reprocessing of vitrimer-like materials has also been studied and mainly focused on photochemical reactions or disulfide bond exchanges, which require ultraviolet (UV) light.32-34 Plasticity induced by visible light and not UV light has been rarely reported. Heating is difficult to spatially control and is not conducive to most shape memory conditions. UV light can damage the chains in polymer materials and hinder their mechanical properties and lifetimes.35,36 Additionally, UV light, e.g., light with wavelengths under 300 nm, cannot penetrate some PUs (Figure S1) even with a 1 mm thickness. The disadvantages of using heat or UV light have limited their applications. However, visible light has advantages, such as spatial and temporal control, low cost and no polymer chain damage. Thus, the realization of materials with visible light-induced plasticity is of great significance, and by combining various methods to introduce visible light-induced plasticity, a new area of applications might be opened. Diselenide bonds have been used in drug delivery and response systems due to their sensitive responsiveness to redox stimuli.37-43 Recently, we discovered diselenide bonds can act as dynamic covalent bonds, and their exchange reactions are triggered by visible light.44-46 This chemistry has been introduced into materials by fabricating PUs containing diselenide bonds that can self-heal under visible light.47 The self-healing processes were realized via polymer chain re-


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formation. If the chain re-formation occurs across the whole material, the loaded stress will relax, i.e., the material will be reprocessed (Scheme 1). Based on this assumption, we designed and fabricated diselenide bond-containing, cross-linked PUs with different phase transition temperatures. In addition, the PUs exhibited temperature-dependent shape memory behaviors and visible light-induced plasticity. Unlike heating enabled plasticity, the orthogonal conditions of visible light to heating made fixing a temporary shape and transforming the temporary shape into a permanent shape possible. The ability to control visible light enabled patterning of materials without a light mask utilizing a designed light source. These unique properties mark the superior advantages of visible light-induced plasticity.

Scheme 1. Brief illustration of light-induced plasticity in diselenide bond-containing materials. 2. EXPERIMENTAL SECTION 2.1. Materials. Selenium powder, sodium borohydride, 4,4’-diphenylmethane diisocyanate (MDI) and polytetramethyleneglycol (PTMG, Mw = 2000 g/mol) were from Aladdin Reagent, Shanghai, China. 11-Bromoundecanol, glycerol (Gly) and super-dry N,N-dimethylformamide (DMF) were purchased from J&K Scientific, Beijing, China. Tetrahydrofuran (THF) and dichloromethane (DCM) were from Beijing Chemical Reagent Company, Beijing, China. Except for the super-dry DMF, all the materials for the polyurethane synthesis were dried under vacuum at 80 °C for 12 h before use and were directly used after drying.


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2.2. Instruments. The 1H NMR spectra were recorded on a Bruker Avance III HD 400 (400 MHz) spectrometer. The stress-strain tests were performed on an Instron Legend 2367 universal testing machine. The stress relaxation tests were conducted using a dynamic thermomechanical analysis (DMA) machine (TA-Q800 DMA apparatus). Differential scanning calorimetry (DSC) tests were conducted using a TA-Q2000 DSC apparatus to determine the phase transition temperature of each sample. The FT-IR spectra were obtained from Perkin Elmer Spectrum GX FT-IR System. 2.3. Synthesis of the diselenide monomer, DiSe. Di-(1-hydroxylundecyl) diselenide (DiSe) was synthesized as previously reported.37 For a typical procedure, 1.60 g of selenium powder (20.3 mmol) and 0.757 g of sodium borohydride (20.0 mmol) were added to 30 mL water in a 250 mL flask and stirred in an ice bath (Caution! H2 and toxic H2Se gas are generated!). After the gas generation slowed, a rubber plug was used to seal the flask. The reaction was heated to 50 °C and degassed with pure nitrogen for 20 min. Then, 5.10 g of 11-bromoundecanol (20.3 mmol) was dissolved in 120 mL of THF and injected into the reaction mixture. The reaction was stirred at 50 °C overnight. When the reaction was stopped, saturated brines and DCM were added into the mixture to extract the product. The organic layer was washed with saturated brines twice, dried and evaporated to produce 4.09 g of a yellow solid with a high purity and yield of 81.7 %. 1

H NMR (400 MHz, CDCl3) δ (ppm): 3.64 (4H, t, HOCH2), 2.91 (4H, t, SeSeCH2), 1.78-1.21

(36H, m, HOCH2 (CH2)9CH2SeSe). If the produce was not sufficiently pure, a recrystallization step with DCM was performed to purify the product. 2.4. Preparation of the diselenide bond-containing PUs. Before the reaction, all the reagents were dried and used immediately after drying. Using PU1 as an example, 1.80 g of PTMG, 0.20 g of DiSe and 60 mg of glycerol were dissolved in 2 mL of super-dry DMF at 60 °C in a glass


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bottle. After solids were no longer visible, 610 mg of MDI (Ratio of NCO to total OH group of 1.08) was added, and the mixture was stirred until all the solids dissolved. The mixture was poured into a mold and cured at 60 °C for 2 h. Then, the material was dried under vacuum at 80 °C overnight. In PU0 to PU10, the weight ratio of DiSe increased from 0 % to 100 % in increments of 10 %. The amount of glycerol was constant at 60 mg, and the amount of MDI increased with the total OH groups. 2.5. Characterization of the mechanical and thermal properties. The strain-stress tests were conducted on a universal testing machine at 21 °C with ramping forces at different rates (1 N/min for PU3 and 4, 2 N/min for PU 5 and 6, 6 N/min for PU 7 to 10) until the samples yielded. To determine the phase transition temperature of each sample, DSC tests were performed in the range of -60 °C to 120 °C using a heating rate of 20 °C/min and cooling rate of 20 °C/min. 2.6. Stress relaxation tests. Stress relaxation experiments were performed on a DMA at 25 °C, 80 °C or under white LED light source irradiation. The strain set for the samples was 50 % for PU 0 to 6 and 20 % for PU 7 to 10. Different fixed strains were used because the maximum loading force of the DMA is 18 N, and the stiffness of the materials increased with the diselenide content. For PU 7 to 10, a 50 % strain could not be achieved on the DMA. Samples were loaded at room temperature and equilibrated at the desired temperature for 10 min before loading the force. After equilibrium, the samples were stretched to a fixed strain, and the change in the stress was recoded. For stress relaxation under visible light, the temperature was not equilibrated because the furnace must remain open. The samples were irradiated using a white LED lamp, and the recording started immediately after the lamp was turned on. After 3 h, the loaded force was released, and another 30 min recovery segment was used to monitor the strain fixation.


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2.7. Shape memory experiments. Shape memory cycles were tested on the DMA. Samples were loaded at room temperature, heated to 80 °C and equilibrated for 5 min. Then, the samples were stretched using a fixed stress of 1 MPa. After 10 min, the samples were cooled to 25 °C and equilibrated for 5 min before the loaded force was released. Afterwards, the samples were equilibrated at 25 °C for another 5 min to record the shape memory effect. Then, the temperature was increased to 80 °C again to test the recovery efficiency. For the visual illustration of the shape memory behavior, the samples were heated in an oven at 80 °C, reshaped and quickly cooled to room temperature to avoid diselenide metathesis at a high temperature. Then, the samples were placed into the oven for 5 min to recover their permanent shapes. Photos were taken in the beginning, after the heating and stretching, after cooling to room temperature, and after the recovery. In PU3 and PU4, the shapes were not retained, and the after recovery photos were not recorded. 2.8. Reshaping and patterning via visible light. To reshape the permanent shapes, the samples were first given temporary shapes and irradiated under a white LED lamp for 6 h. The fixing time was longer than that in the DMA test to ensure the shapes were well fixed. For the projector, 15 mm × 15 mm samples were stretched to a 100 % strain and irradiated using an EPSON commercial projector with the designed images. For PU 5, an image of “Se” was introduced, and for PU6, an image of a smiling face was introduced. After irradiation for 4 h, the force was released, and the patterns were fixed on the samples. 3. RESULTS AND DISCUSSION 3.1. Fabrication of diselenide-containing PUs. To realize visible light-induced plasticity, we incorporated diselenide bonds into synthesized shape memory PU. The formulas of the materials


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were designed to meet the requirements of SMPs, i.e., switch segments and cross-linking net points. Glycerol, a trifunctional alcohol, was selected to cross-link the polymer chains, and its content was optimized to achieve the desired mechanical properties. The phase transition temperature was selected as the switch for the shape memory behavior because it can be tuned by varying the hard chain/soft chain ratio in a polymer structure to tune the shape memory property. As shown in Figure 1, di-(1-hydroxylundecyl) diselenide (DiSe) was used to introduce diselenide bonds into PU, and polytetramethyleneglycol (PTMG, Mw = 2000 g/mol) was used as the soft chain. They were polymerized using 4,4’-diphenylmethane diisocyanate (MDI) with glycerol to fabricate the desired PUs. Different PUs were prepared with different ratios of DiSe and PTMG, e.g., PU0 contains no DiSe, PU1 contains 10 % DiSe and 90 % PTMG (weight ratio), and PU10 contains no PTMG. The variations in the DiSe/PTMG ratio changed the diselenide bond content and altered the soft/hard chain ratio because PTMG is much “softer” than DiSe. Based on this, the phase transition properties of the different PUs should be different, and we expect an optimized shape memory behavior can be achieved. The synthesized 11 PUs with different diselenide bond contents are shown in Figure 1, and the color of the PUs became darker with a higher diselenide bond content.


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Figure 1. The molecules used to synthesize the diselenide-containing PU and photos showing the PUs with different diselenide bond contents. The thermal properties of each PU were tested. The thermal analysis curves were recorded by a DSC, and the results are summarized in Table 1. The curves for each sample are in the Supporting Information (Figure S2 and S3). No obvious phase transition was observed within the analysis temperature range for PU0 to PU2, and only a low temperature phase transition was recorded for PU3, which should be a glass-transition. Based on these results, PU0 to PU3 should not exhibit shape memory behavior above room temperature. Therefore, the following studies focused on PU 4 to PU10, and PU3 was used as a comparison. PU4 to PU10 had phase transitions ranging from 20 to 60 °C, and these transitions could be suitable for heat-triggered SMPs. The transition temperature increased from PU4 to PU10 in accordance with the diselenide content due to the decrease in the soft PTMG chains. According to the curve shape of PU10, the higher temperature is a melting-transition, and as the DiSe content decreased from PU10 to PU4, the Tm declined and became less obvious (Figure S2 and S3). Thus, we concluded that the PUs with a higher diselenide bond content also possessed a better shape memory effect. The mechanical properties of the samples PU3 to PU10 were tested and are shown in Figure S4. Table 1. Phase transition temperatures of PU0 to PU10 calculated from the DSC curves. Sample

PU0

PU1

PU2

PU3

PU4

PU5

PU6

PU7

PU8

PU9

PU10

Tg [°C]

- a)

-

-

-

28.4

35.3

45.3

44.4

55.1

56.9

56.6

Tm [°C]

-

-

-

-29.9

-26.1

-25.1

-9.2

-13.3

1.7

7.4

9.2

a)

No obvious phase transition observed. Same for other samples marked as ‘-’.


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3.2. Shape memory behavior. To evaluate the shape memory properties of these materials, shape memory cycles between 25 °C and 80 °C were tested on a DMA instrument. The curves for PU3, PU4, PU9 and PU10 are shown in Figure 2, and the curves for PU5 to PU8 are shown in Figure S5. As expected, the materials exhibited shape memory behaviors. The shape memory efficiency and recovery efficiency increased from PU3 to PU10 along with the more obvious phase transition and suitable temperature. PU3 had almost no shape memory effect, but when the diselenide bond content was 100 %, the shape memory efficiency reached 90 % based on the DMA measurements. The PU3 shape memory effect might be caused by an inconspicuous phase transition that was not observed in the DSC curve. The shape memory cycle of PU2 (Figure S6) showed no memory behavior.

Figure 2. Shape memory cycles of A) PU3, B) PU4, C) PU9 and D) PU10. Curves for PU5 to PU8 are in the Supporting Information.


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Figure 3. Stress relaxation of PU3 to PU10 at A) 80 °C and B) 25 °C. Stresst: stress measured at time t. Stress0: the stress measured at the start. C) Visual illustration of the shape memory behavior for PU3 to PU10. For PU3 and PU4, the samples after recovery at 80 °C are not shown because no obvious shape memory effect was observed after they were cooled to room temperature. The recovery was problematic, and the strains did not totally recover during the measurement. However, the diselenide metathesis would also proceed at 80 °C and more slowly under visible


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light in solution using our LED light source, which could explain why the final strains did not fully recover.44 To confirm this hypothesis, the stress relaxation of the samples was measured at 80 °C. As shown in Figure 3A, the stress relaxed more quickly at a higher temperature. As the diselenide content increased, the relaxation procedure became faster and more stress was relaxed (Figure 3B), which confirmed the diselenide metathesis at high temperatures. The relaxed stress indicated that the PUs were plastic at high temperatures. And to confirm the relaxation was not caused by post-polymerization, FT-IR measurements were conducted. Samples of PU3 to PU10 were submitted to FT-IR test before and after heated at 80 °C for 3 h. For each sample, the spectra were almost the same before and after heating (Figure S7 and S8), which indicated that the stress-relaxation test at 80 °C was not through post-polymerization. For the shape memory cycle tests, the materials were stretched and maintained at 80 °C, and the permanent shapes partially changed. Thus, as the diselenide bond content increased from PU3 to PU10, the stress relaxation was faster, and the permanent shape change was more obvious during the DMA test. To avoid these problems, we visually demonstrated the shape memory behavior (Figure 3C). The samples were heated in an 80 °C oven, stretched and quickly cooled to room temperature. As illustrated, all 8 samples could fully recover their original shape with different shape memory abilities (Table 2). The shape memory efficiency trend agreed with the shape memory cycle tests. These results confirmed that the prepared PUs were SMPs with regulatable shape memory behavior.


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Table 2. Shape fixity and recovery ratio calculated from Figure 3C. Sample

PU3

PU4

PU5

PU6

PU7

PU8

PU9

PU10

0

0

30

32

62

64

81

91

-

-

100

100

97

100

95

97

Shape-fixity ratioa) [%] Shape-recovery ratiob) [%] a)

∆L (after shape memory) / ∆L (stretched);

b)

-∆L (after recovery) / ∆L (after shape memory)

3.3. Light-induced plasticity. Subsequently, the visible light-induced plasticity of the materials was tested. A white LED was selected as the light source, and the total power of the LED was 6 W, which is similar to daily use LED lamps. First, the light-induced plasticity was characterized by stress relaxation under LED irradiation. As shown in Figure 4A, the stress in PU3 to PU10 relaxed under the LED light, and as the diselenide bond content increased, the relaxation rate also increased. These results indicated that the prepared materials possessed lightinduced plasticity. The mechanism was determined to be a light-induced diselenide bond exchange between the polymer chains based on the fact that PU0 had almost no stress relaxation after 3 hours. The strain changes in the test were tracked as well (Figure 4B). In agreement with the stress relaxation trend, the strain fixation also increased from PU3 to PU10, which indicated different plasticities under light. PU7 to PU10 had nearly total strain fixations. Along with the shape memory tests, we can regard PU7 to PU10 as temperature-triggered SMPs with good visible light-induced plasticity.


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Figure 4. A) Stress relaxation of the PUs under visible light. Stresst: stress measured at time t. Stress0: the stress measured at the start. B) The strain fixation of PU3 to PU10 after visible light irradiation. C). Process of the shape memory behavior and visible light-induced reshaping of the permanent shape. Scale bar: 2 cm. D) Patterning of the synthesized materials utilizing a commercial projector as the light source. The light was introduced as the pattern below. Left, PU5. Right, PU6. Scale bar: 1 cm. After the characterization of the shape memory and plasticity, we attempted a combination of these two properties to examine their potential applications. First, as shown in Figure 4C (first row), two strips of PU10 were curved into the shape of “Se” at 80 °C , and the temporary shape was memorized by cooling to room temperature. Upon heating the materials back to 80 °C, the


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straight strips were recovered. This process illustrated their shape memory behavior combined with light-induced plasticity, and the permanent shape could be easily altered. To demonstrate changing the shape, two strips of PU9 were given the temporary shape of “Se”. Afterwards, the samples were irradiated with white LED for 6 h, and the permanent shapes were successfully changed. The “Se” could be processed as a temporary shape, and upon heating to 80 °C again, the “Se” shape was well recovered (Figure 4C, second row. Also see the video in the Supporting Information). Furthermore, we reshaped the sample again from a permanent shape “S” to a “T” to show the materials were capable of being reconfigured at least twice (Figure 4C, third row). Using visible light to reshape materials could avoid the potential damage to polymer chains caused by heating or UV irradiation. In addition, the orthogonality of light and heating make it possible to utilize a temporary shape for reprocessing. A mold or manual fix of the samples is not required during the reshaping process. To further show the unique advantages of visible lightinduced plasticity, a projector was used as a shaped light source. As shown in Figure 4D, a certain image was projected on the stretched materials for 4 h, and the stress of the irradiated parts was relaxed. When the sample recovered its original state, the stress relaxed part bulged to show the patterning of the material. Since the visible light was introduced by a commercial projector, reshaping our materials was inexpensive, and the pattern was easily designed using a computer. Based on these experiments, the advantages of SMPs with visible light-induced plasticity were exhibited. The orthogonality of light and heating made altering the permanent shapes easier. The mild conditions of visible light can avoid possible damage caused by heat or UV light. In addition, the various sources of visible light provide various means to pattern or reshape the materials. 4. CONCLUSIONS


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We fabricated diselenide bond-containing polyurethanes with visible light-induced plasticity. In addition, these materials possessed shape memory properties. For the PUs containing different amounts of diselenide bonds, their shape memory behavior could be adjusted by altering their thermal properties, and their light-induced plasticity was also related to the diselenide bond content. The mild conditions of visible light avoid the possible damage caused by heating or UV light during reshaping. In addition, the orthogonality of light and heat make altering the permanent shape easier because a mold or manual fix are not needed for the shape during this process. Furthermore, using different light sources, the materials can be easily reshaped or patterned inexpensively; e.g., a commercial projector directed patterning was realized. For the first time in these PUs, diselenide bonds were introduced into shape memory materials and shape memory behavior was successfully integrated with visible light-induced plasticity. With further development, these materials have potential applications in braille printing and erasable visiblelight printing materials. In addition, if well controlled lasers are used for micro-patterning, they also have potential for microdevice fabrication.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. DSC curves of PU0 to PU10, stress-strain curves of PU3 to PU10, shape memory cycles of PU2 and PU5 to PU8 (PDF) Shape memory process, recovery from strips to shape “Se” (AVI)


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AUTHOR INFORMATION Corresponding Author *Huaping Xu E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Science Foundation for Distinguished Young Scholars (Grant 21425416) and the National Natural Science Foundation of China (Grant 91427301). Thanks to Zizheng Fang and Prof. Tao Xie from Zhejiang University for their help with the PU synthesis and discussion. REFERENCES 1.

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Table of Contents


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Visible Light-Induced Plasticity of Shape Memory Polymers - ACS

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