Article pubs.acs.org/JPCC
On-Demand Shape Recovery Kinetics Modulation with a Wide Regulation Range and Spatially Heterogeneous Shape Recovery Rate Tingting Chen, Haijie Han, Fan Jia, Qiao Jin, and Jian Ji* MOE Key Laboratory of Macromolecule Synthesis and Functionalization of Ministry of Education, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China S Supporting Information *
ABSTRACT: A suitable deformation rate is crucial for shape memory polymers (SMPs) in real word applications. Yet on-demand modulation in shape recovery kinetics and its spatial heterogeneous control still need to be explored systematically. Herein, a near-infrared (NIR) light controlling strategy was demonstrated for in situ modulation in both shape recovery rate and its spatial heterogeneity. Polyvinyl alcohol and chitosan, two typical SMPs, were chosen to elaborate the strategy, due to their heat-responsive shape memory effect (SME) and chemoresponsive SME, respectively. Reduced graphene oxide was incorporated in the SMPs to endow them with NIR light controllability. Through light intensity adjustment, the shape recovery rate could be altered by nearly an order of magnitude without any modification of material composition. Similar shape transition kinetics at different ambient temperatures could be achieved. More impressively, spatially modulated recovery kinetics was successfully conducted to avoid undesired self-collisions or self-interferences in complex shape shifting processes and thus prevent possible shape transition failure. As the regulatory information was not encoded in the SMPs, deformation rate and its spatial differentiation could be adjusted flexibly after material preparation, allowing the adaptability of the shape shifting process under conditions with individual differences.
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INTRODUCTION Shape memory polymers (SMPs) that can programmably change their shapes upon appropriate external stimuli are of high scientific and technological significance,1−4 due to their broad applications in aircraft,5 electronics,6 biomedical devices,7,8 and other systems.9,10 A typical shape memory cycle (SMC) involves deformation of the SMP in a rubbery state, temporary shape fixation via reduction of chain mobility and stimuli induced shape recovery that resulted from regained chain mobility.11,12 Due to the viscoelastic nature of polymers, the shape transition of SMPs progresses along with time, which declares the significance of the shape recovery kinetics.13,14 However, the optimization of recovery rate is a complicated issue which remains to be studied systematically. Generally, it should be varied with the demands of different applications. In most cases, either an excessively fast or slow shape recovery rate is undesired, particularly with regards to applications in minimally invasive medical devices, aerospace components, automatic control systems, etc. For instance, in biomedical implants, moderate and tunable expansion time is preferred to prevent premature expansion or device migration and ensure suitable time to deliver the devices in different patients with different lesion positions.15 In addition to the overall deformation kinetics, its spatial controllability is also of great importance to avoid shape transition failure in complex shape shifting processes due to the possible self-collisions.16 Yet the overwhelming majority of SMPs developed up to now are spatially homogeneous in shape transition kinetics so that © XXXX American Chemical Society
different parts may collide with each other in the shape transition processes (as we will show in the present work). Thus, the regulation of shape recovery kinetics is critical to make SMPs more capable in many advanced applications. In the previous researches, methods to regulate the deformation rate can be divided into three categories: copolymerization or blending,17 material size modulation,18 and switching phase aggregation state adjustment.19 However, these commonly used strategies are either involving complex chemical synthesis or applicable only in certain cases and few of them achieve the spatial heterogeneity in shape recovery rate.16 Even more importantly, all of these modulation strategies mentioned above are dependent on material modifications, resulting in unfeasible in situ tunability after material preparation. This hinders the adaption of shape shifting rate and its spatially heterogeneous distribution under conditions with individual differences. Photothermal conversion agent filled SMPs have been widely investigated for light-responsive shape transition.20−23 Yet these pioneering works mainly put emphasis on remote turning on/ off the shape recovery of SMPs with light. Herein, we develop it into a simple, but generic, method for in situ modulation in shape recovery rate in a wide range through a remote and contactless way. Similar shape recovery kinetics at different Received: March 23, 2017 Revised: May 11, 2017 Published: May 11, 2017 A
DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 1. (a) Schematic presentation of the PVA/GO mixed solution and the solution casting method for fabricating the nanocomposite films. (b) UV−vis absorption spectra of nascent PVA film (black) and PVA/GO films before (red) and after (blue) heat treatment. (c) Plot of temperature change (ΔT) versus the light intensity applied on the PVA/GO film after heat treatment. (d) Plot of temperature change (ΔT) versus the light intensity applied on the PVA/GO film before heat treatment and photograph of the PVA/GO film upon local light irradiation at a light intensity of 2.28 W/cm2.
ambient temperatures and controlled spatial heterogeneity in shape recovery kinetics are further achieved with this strategy. In this work, polyvinyl alcohol (PVA) and cross-linked chitosan (CS) are chosen as the model SMPs with heat-responsive shape memory effect (SME)24,25 and chemoresponsive SME,26,27 respectively. Graphene is employed as the nanoheaters for its highly efficient photothermal conversion.28−31 Thus, light intensity and its spatial variation can be utilized to modulate the shape recovery kinetics of these SMPs filled with photothermal conversion agents. In this strategy, different deformation rate and its spatial differentiation could be realized from the same material through on-demand light modulation, allowing the adaptability of the shape deforming process to meet on-site individual requirements.
graphene oxide (PVA/RGO) nanocomposite. Chitosan (2% w/ v) was dissolved in DI water containing 1% (v/v) of acetic acid. 0.1 mg/mL GO (aq) was added into the chitosan solution dropwise. The blend was also cast onto a plastic plate and dried in air at room temperature. The obtained film was immersed in 5% (w/v) sodium hydroxide (NaOH) (aq) for deprotonation and GO reduction. Then, the CS/RGO film was immersed in DI water for 48 h with water exchanging for several times to remove excess NaOH and then dried in a vacuum oven. Characterizations. A UV−vis spectrophotometer (Shimadzu UV-2550 UV−vis spectrometer, Japan) was employed to determine the difference of light absorbance of PVA, PVA/GO, and PVA/RGO films. The temperature of PVA/GO and PVA/ RGO films under irradiation with an 808 nm continuous wave (CW) laser (LSR808H-7W, Lasever Inc., China) was recorded with an infrared thermal imaging camera (FLIR E60, Flir System, Inc., USA). Scanning calorimetry (DSC, Q100, TA Instruments, USA) was applied to measure the glass transition temperature and the melting temperature (temperature ramping rate: 10 °C/min) of PVA and PVA/RGO nanocomposite. Shape Memory Evaluation. The PVA/RGO nanocomposite was deformed at Thigh = 85 °C (Tg < Thigh < Tm) and cooled down to Tlow = 20 °C under stress. Shape recovery of the PVA/RGO nanocomposite under different NIR light intensities was performed. Light intensities to realize similar shape recovery rates at different ambient temperatures were predicted with a temperature rise−intensity curve and verified via shape recovery tests. Asymmetric geometry change kinetics upon irradiation by 808 nm light with an intensity gradient or spatially varied intensity was recorded. Shape recovery in an environment with a uniform temperature was conducted in an oven to illustrate the possible shaping failure of kinetically homogeneous SMPs.
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EXPERIMENTAL SECTION Materials. An aqueous dispersion of graphene oxide (GO) was supplied by Chengdu Organic Chemicals Co., Ltd. (Chendu, China) and used as received. Polyvinyl alcohol (degree of hydrolysis: 98%), chitosan (viscosity < 200 mPa·s), and ethylene glycol diglycidyl ether were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). Other reagents such as acetic acid and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. Preparation of PVA/GO and CS/GO Nanocomposites. A solution-casting method was used to make both nanocomposite films. PVA powder was first dissolved in deionized (DI) water at 80 °C under stirring. 1 mg/mL GO (aq) was added in the PVA solution, and ultrasound was employed for GO dispersion. The blend was subsequently cast onto a plastic plate and dried in air at room temperature. The obtained PVA/ GO nanocomposite film was heated at 150 °C for 15 min for GO reduction and thus transformed into the PVA/reduced B
DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C For the CS/RGO nanocomposites, we first elucidated the effects of cross-linking time on the SME of the samples. The strips were wound onto mandrels and cross-linked with ethylene glycol diglycidyl ether at pH 10.5 for 0.5, 1, 2, 3, 5, and 8 h, respectively. After fixation, the strips kept on the mandrels were immersed in DI water for 24 h with water exchanging for several times to remove the excessive crosslinking reagents. Then, the spirals were removed from the mandrels. A temporary shape was obtained after straightening the spirals in water and fixed by dehydration. The obtained straight strips could recover (or partially recover) to their permanent shape in water. The diameters of the spirals in their permanent shape after cross-linking and in their recovered shape were measured to characterize the SME of the samples cross-linked for different times. Moreover, the swelling ratio of the samples was determined via the weight increase upon hydration to an equilibrium value. Shape recovery of the CS/ RGO nanocomposite upon NIR light irradiation with different intensities was performed to verify the controllability in shape recovery rate and spatially variable kinetics.
Figure 3. Plot of recovery ratio versus time of PVA/RGO sample exposed to 0.66 W/cm2 NIR light at 18 °C and that exposed to 0.30 W/cm2 NIR light at 50 °C.
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RESULTS AND DISCUSSION Fabrication of Nanocomposite Films. PVA was used as a model of heat-responsive SMPs.24,25 The nanocomposite films
Figure 2. Plot of recovery ratio versus time of PVA/RGO samples exposed to different laser powers at room temperature. Scatters: experiment results. Line: fitting results.
Table 1. Values of the Parameters in eq 1 When the Equation Was Used To Fit the Scatter Plot in Figure 2 2
0.57 W/cm 0.72 W/cm2 0.82 W/cm2
A1
τ1
A2
τ2
0.80 0.52 0.53
21.51 12.37 7.02
0.24 0.52 0.53
102.12 12.37 7.02
Figure 4. Shape recovery process of PVA/RGO samples. The permanent shape is a flat strip, and the temporary shape was programmed to a helix. (a) Schematic representation of the sample in temporary shape exposed to NIR light with intensity gradient. (b−h) The spatial differentiation in shape recovery process recorded with a digital camera. Scale bars: 200 μm.
reduced graphene oxide (PVA/RGO) nanocomposite. Actually, heat treatment of PVA/GO films not only endows the films with higher photothermal conversion efficiency as shown in the temperature rise curve (Figure 1c,d), but also improves the temperature predictability and the stability of the films when irradiated with light. Upon high-intensity NIR light irradiation, PVA/GO films without heat treatment would also generate a temperature rise, though less obvious compared with PVA/ RGO film (Figure 1d). When the temperature of the films was increased to a critical value, GO would be reduced to RGO and the photothermal conversion efficiency of the nanoheaters would be improved significantly to generate more heat. Thus, a
were fabricated via a PVA/RGO mixed solution casting method (Figure 1a). The content of GO in the prepared PVA/GO film was 0.1 wt % as designed. After heat treatment, the color of the PVA/GO film was significantly deepened (Figure S1), whereas the neat PVA specimen showed a barely visible color transition (Figure S2). UV−vis spectra indicated that the light absorbance of PVA/GO at 808 nm was much higher after heat treatment and the neat PVA film had scarcely any absorbance at 808 nm (Figure 1b). These results can be attributed to the thermal reduction of GO at high temperature, which implies that the PVA/GO nanocomposite was transformed into the PVA/ C
DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. (a) Schematic representation of the shape recovery process of the sample with a folded permanent shape and a flat temporary shape triggered with heat and the possible self-collision resulted incorrect folding. (b) Schematic representation of the same sample upon NIR irradiation with spatial intensity difference. (c) The shape recovery cycle of the sample recovered at 100 °C: permanent shape - temporary shape - intermediate shape in the recovery process - obtained shape. (d) The shape recovery cycle of the sample triggered with NIR light with spatial intensity difference: permanent shape - temporary shape - intermediate shape - obtained shape. Scale bars: 200 μm.
(w/w), no visible GO aggregates could be observed in the dispersion. There were not any CS/GO precipitates when the solution was concentrated in the film-forming process. This phenomenon is relevant to the interaction between CS and GO. When a highly concentrated GO dispersion was added into the CS solution, multiple GO nanosheets would interact with the same chitosan chain via electrostatic interaction due to the high regional concentration of GO, resulting in the aggregation of GO nanosheets (Figure S3b). On the contrary, a piece of GO nanosheet would be coated with several CS chains, when a dilute GO dispersion was added instead. The hydrophilicity of the chitosan and the electrostatic repulsion between different chitosan molecules contributed to the stability of GO in the dispersion.32 Second, the shape recovery of cross-linked CS/RGO occurs in the presence of water and the specific heat capacity of water is relatively high. Therefore, the content of the nanofiller in the CS/RGO film is designed to be as high as 1 wt % to provide enough temperature rise of the film in water. The as-prepared CS/GO nanocomposite can be slowly dissolved in water. To suppress the solubility of the film, 5% (w/v) NaOH (aq) was employed for deprotonation of the amino groups. The GO in the film would be reduced to RGO in such an alkaline condition.33 As a result, the film presents an obvious change in color (Figure S3c). Shape Memory Effect and Remote Adjustable Shape Recovery Kinetics of the PVA/RGO Nanocomposite. PVA is a commercially available heat-responsive SMP with a crystalline phase to act as the physical cross-linking.20,21 The neat semicrystalline PVA has a glass transition temperature (Tg) of 80 °C and a crystallization temperature (Tm) of 185 °C (Figure S4). The addition of 0.1 wt % RGO in the
Figure 6. Recovery time of the CS/RGO samples exposed to different laser powers.
positive feedback loop was formed and an abrupt temperature increase or even PVA carbonization would occur at a critical light intensity. In our experiment, the critical light intensity was 2.28 W/cm2, and the irradiated part of the film was partially carbonized at this light intensity (Figure 1d). CS/RGO nanocomposites were also fabricated in order to explore the feasibility of this strategy in chemoresponsive SMPs. The fabrication process was roughly the same with that of PVA/RGO films. Yet, there are a couple of details worth noting. First, the concentration of the GO aqueous dispersion was the key point in preparing uniform CS/GO aqueous dispersions. As shown in Figure S3a, GO aggregates could be observed, when 10 or 1 mg/mL GO dispersion was added in the CS solution (CS/GO = 99/1 (w/w)). However, when the concentration of the GO dispersion was decreased to 0.1 mg/ mL and the proportion of CS and GO was maintained as 99/1 D
DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C semicrystalline PVA had no significant effect on its Tg, while Tm decreased to 167 °C. Thus, the deforming temperature was set as 85 °C, which was above Tg but below Tm. Under this condition, the shape recovery would take place when the sample was heated above the deforming temperature. Shape recovery under NIR light irradiation with different light intensities was conducted, illustrating that the shape recovery rate could be regulated in a wide range via the light intensity adjustment. As shown in Figure 2, the time consumed for shape recovery was 285, 78, and 34 s, when the light intensity was set as 0.57, 0.72, and 0.82 W/cm2, respectively. That means the shape recovery time would decrease by 88% when the light intensity was increased from 0.57 to 0.82 W/cm2. To establish the quantitative relationship between the recovery ratio and the recovery time upon light irradiation with different light intensities, the scatter plot was fitted, and we found that the strain versus time curves could be approximated using a twoexponential decay function derived from previous work:13,14,34,35 t
temperatures. The black spots in Figure 3 show the shape recovery ratio versus time of the PVA/RGO sample exposed to 0.66 W/cm2 NIR light at 18 °C. According to the results in Figure 1c, the temperature rise of the sample under this condition was about 75 °C, so the temperature of the irradiated sample was approximately 93 °C. When the ambient temperature was increased to 50 °C, the temperature rise should be 43 °C to reach 93 °C and thus the theoretical intensity of the light should be 0.37 W/cm2 according to the plot of temperature rise. We conducted the experiment and found that, when the intensity was set as 0.30 W/cm2 at 50 °C, the shape recovery rate of the sample was almost consistent with that exposed to 0.66 W/cm2 NIR light at 18 °C, demonstrating another aspect of shape recovery rate regulation. This characteristic makes the SMPs more capable in the real world with varied temperatures. In addition to the powerful modulation in the shape recovery rate, another impressive feature of this strategy is the spatial controllability in shape recovery rate. This advantage is mainly derived from the spatially patterned potential of light.36,37 We programmed a flat strip to obtain a temporary helix state at an elevated temperature and then exposed the helix to an NIR light with an intensity gradient along the helix for shape recovery. The recovery process was recorded by a digital camera. We found that the recovery rate varied in different parts of the helix and its gradient was in consistence with the light intensity gradient (Figure 4), which verifies the feasibility to realize different shape transition kinetics in different parts of a uniform SMP. Actually, the spatially variable shape recovery rate is quite desired when the shape transition process is complex in which the self-collision and self-interference should be considered. To demonstrate the importance of spatially controlled shape recovery kinetics and the benefit of the light regulated shape recovery path, we designed a shape memory PVA/RGO sample with a folded permanent shape which was unfolded into a flat sheet as the temporary shape (Figure 5a,b). The shape recovery occurred upon heating to 100 °C or irradiation with NIR light. It could be observed that the structure failed in correct folding because of the self-collision within different parts of the structure when uniform heating was employed as the trigger (Figure 5a,c). However, although the sample itself was homogeneous, spatially patterned light could be utilized to generate spatial heterogeneity. As shown in Figure 5b,d, the target permanent shape could be reached accurately when NIR light with spatially variable intensity was employed to induce the shape recovery. This experiment fully embodies the spatially modulated recovery kinetics and its significance in complex shape transition processes. As the spatial heterogeneity is defined by the modulated light, the material preparation process is greatly simplified and the heterogeneity could be tuned flexibly. Remote Adjustable Shape Recovery Kinetics of the CS/RGO Nanocomposite. The light controllable shape recovery kinetics strategy is also valid in chemoresponsive SMPs. The CS/RGO nanocomposite was chosen as the model in this research. The permanent shape was obtained by winding the strips onto mandrels and cross-linking with ethylene glycol diglycidyl ether at pH 10.5.38,39 The obtained helices were flattened in water and dried to generate the temporary shape. The flat temporary shape can recover to the helix by immersing in water. To elucidate the effects of cross-linking time on the shape memory effect of CS/RGO film, the diameters of the cross-linked helices were measured after being removed from the mandrels (Figure S5a,b). Meanwhile, the diameters of these
t
R r(t ) = 1 − A1e− τ1 − A 2 e− τ2
(1)
In this equation, Rr is the recovery ratio; A1 and A2 are defined as the pre-exponential factors; τ1 and τ2 denote the relaxation times of the soft segments and hard segments, respectively; and t is the light irradiating time. The fitting results are shown in Figure 2 and Table 1. The standard deviations R in all three cases are greater than 0.99, confirming the rationality of the fitting model. We can see that, when the light intensity was set as high as 0.72 and 0.82 W/cm2, τ1 ≈ τ2, which means that only one relaxation mode existed. However, when the light intensity was 0.57 W/cm2, τ1 and τ2 were of distinct values, indicating the presence of both soft segments and hard segments. This phenomenon can be attributed to the forced high-elastic deformation of partial chain segments. Actually, the glass transition of PVA/RGO occurred in a broad temperature range and there was still part of chain segments whose movement was frozen at 85 °C, which was slightly above the measured Tg. However, these chain segments would undergo forced highelastic deformation under static external force. The τ2 mode was associated with the slower relaxation process of these chain segments at low light intensity. Upon higher power light irradiation, the temperature of the SMP samples was much higher than 85 °C, and the difference in chain mobility was eliminated, resulting in a single relaxation mode. In addition, the fitting results show that τ1 (0.82 W/cm2) ≈ τ2 (0.82 W/ cm2) < τ1 (0.72 W/cm2) ≈ τ2 (0.72 W/cm2) < τ1 (0.57 W/ cm2) < τ2 (0.57 W/cm2). It suggests that the intrinsic nature of kinetic regulation in the shape recovery process relies on the light controllable polymer chain relaxation. Theoretically, the recovery rate can be adjusted continuously in a quite predictable mode by this method, which can be hardly realized by chemical modification. The shape recovery rate is adjusted via light modulation after material preparation, suggesting the feasibility to adapt the recovery rate on-demand to the specific operating conditions. Moreover, similar shape recovery kinetics is achievable at different ambient temperatures. It is easy to understand that, by changing the light intensity, the temperature of the SMP samples can reach the same value at different ambient temperatures. The relationship between the light intensity and the temperature rise (Figure 1c) provides the prediction basis for the determination of the light intensity at different E
DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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helices after undergoing a shape memory cycle were also measured (Figure S5a,c). The results indicate that the diameters of the helices in both states decreased over crosslinking time, and the difference between the permanent shape and the recovered shape after an SMC also decreased. The relationship between the shape memory effect and cross-linking time is attributed to the increased cross-linking degree along with time as shown in the swelling ratio test (Figure S5d). According to the results, 5 h was chosen as the cross-linking time in the following experiments. The recovery time of the CS/RGO composite exposed to different light intensities was measured and is shown in Figure 6. The recovery process of the sample immersed in water with spatially controlled light irradiation was recorded (Figure S6). The results suggest that the shape recovery rate and the spatial heterogeneity in shape recovery kinetics can be modulated with light.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Qiao Jin: 0000-0002-6584-4111 Jian Ji: 0000-0001-9870-4038 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51333005) is gratefully acknowledged.
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CONCLUSIONS
Remote and contactless controllability of shape recovery kinetics was demonstrated in this research. By adjusting the intensity of the light, the shape recovery rate could be tuned on-demand in a wide range without any modification in polymer structure or material composition. The feasibility of similar shape transition kinetics at different environment temperatures was also verified in this research. Meanwhile, spatially variable shape recovery, which is quite desired in kinematically complex shape shifting processes, was realized to avoid self-collisions or self-interferences. This kind of spatial controllability provides an easy, but effective, solution to prevent shape transition failures in the construction of complicated target shapes. These two levels of controllability fully reflected the powerful modulation in shape recovery kinetics. More impressively, as the regulatory information was decoupled from the composition and the structure of the SMPs in this strategy, the deformation rate and its spatial differentiation could be adjusted flexibly after material preparation. This allows the adaptability of the shape shifting process to meet the requirements under conditions with individual differences.
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Article
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02759. Photographs of PVA/GO nanocomposite films before and after heat treatment, photographs of PVA films before and after heat treatment, photographs of CS/GO aqueous dispersions prepared under different conditions, schematic illustration of the GO dispersity when GO dispersions were added in the CS solution, photographs of CS/GO nanocomposite films before and after alkali treatment, DSC heating curves of neat PVA and PVA/ RGO, the diameters of the CS/RGO helices after being cross-linked and after a shape memory cycle, photographs of the helices which were cross-linked, photographs of the helices after a shape memory cycle, the swelling ratio of the samples which were cross-linked, CS/RGO shape memory polymer immersed in water with localized NIR irradiation (PDF) F
DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.7b02759 J. Phys. Chem. C XXXX, XXX, XXX−XXX
