Water Induced Shape Memory and Healing Effects by Introducing

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Water Induced Shape Memory and Healing Effects by Introducing Carboxymethyl Cellulose Sodium into Poly(vinyl alcohol) Jiyu Yang, Yanan Zheng, Linjuan Sheng, Hongmei Chen,* Lijuan Zhao, Wenhao Yu, Ke-Qing Zhao, and Ping Hu College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, People’s Republic of China

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S Supporting Information *

ABSTRACT: Water as a common, easily obtained, and environmentally friendly stimulus has been explored in stimuli-responsive materials. In this paper, a series of composites, named as PVA(n)-CMC(m)-GA, were prepared by filling the carboxymethyl cellulose sodium (CMC) into chemical crosslinked poly(vinyl alcohol) (PVA). These composites demonstrated both water induced shape memory and self-healing effect. CMC with water sensitivity help the composite absorbing water to decrease the Tg of composite to cause shape recovery. And CMC with water solubility is also a good healing agent, which can freely move across the cutting cross-section in wet condition and form hydrogen bonds with PVA chains in dry condition to achieve healing. Water can affect the hydrogen bonding interactions between CMC and PVA, and which play a key role in the water-stimuli responsive properties. This study provides a relatively simple and low cost way to obtain water-induced multifunction materials. segments to achieve shape fixing and recovery. And most of the electric or magnetic induced SMPs are called indirectly thermo-induced SMPs, because the mechanism of these polymers is that the electric/magnetic energy is transformed into Joule heating and inductive heating to cause temperature increasing to recover shape. In heat-assisted healing polymer, the high-molecular-weight species will diffuse to heal the damage part when temperature above its transition temperature. It should be noted that the transition temperature of thermo-induced responsive polymers is usually higher than room temperature or body temperature, and therefore limits their application fields, such as biomedicine.21−24 Water as a common, easily obtained, and environmentally friendly stimulus has been explored to overcome the shortage of heat. In the water-induced SMPs, water as a plasticizer enters the polymer chain to reduce the transition temperature and cause shape recovery.25,26 Water-induced SMPs can be applied as water induced self-tightening suture or self-retractable stent.27 Recently water-induced healing effects are reported in polyelectrolyte multilayer coatings,28 poly(vinyl alcohol)− nafion films,29 graphene oxide,30 and the conducting polymer polyethylenedioxythiophene doped with polystyrenesulfonate (PEDOT:PSS).31 Water can improve polymers chains to flow and interdiffuse at the fractured surface or form hydrogen bonds to achieve self-healing. In this paper, we wish to achieve multiresponsive effect by water. As shown in Figure 1,

1. INTRODUCTION The field of stimuli-responsive materials has become one of the important areas of research. Stimuli-responsive materials can ideally change their property in an intended, predictable, and purposeful way upon exposure to a specific stimulus. The stimuli-responses include changes in shape, mechanical properties, color, fluorescence and so on.1 Shape memory polymers (SMPs) and healing polymers as typical two types of the stimuli-responsive materials are reported mostly.1−4 SMPs can be deformed and fixed into a temporary shape under specifically external conditions and subsequently recover to their original shape upon exposing to an appropriate stimulus, such as heat,5,6 electric/magnetic,7,8 light9 and chemical.10,11 Healing polymer can remove a scratch or other defect or mend two broken pieces under the stimulus of heat or light, and with the goal of restoring their original mechanical properties, transport properties (electrical, ions, thermal, etc.) and optical characteristics.12−18 Polymers with multifunction in response to a single stimulus are the tendency of stimuli-responsive polymer research. Rodriguez et al. achieved a thermo-induced shape memory and healing responses by blending cross-linked poly(εcaprolactone) networks with linear poly(ε-caprolactone).19 A single heating step triggered a shape-memory effect that assisted in the closure of a damaged region of the sample, whereas the efficient diffusion of the (viscous) liquid linear component successfully healed the sample and restored its original properties. Heat as one type of the stimuli is generally used in both SMPs and healing polymers.3,20 Thermo-induced SMPs are needed heating above the transition temperature of switch © XXXX American Chemical Society

Received: July 15, 2018 Revised: October 7, 2018 Accepted: October 12, 2018

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DOI: 10.1021/acs.iecr.8b03230 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

2.2. Preparation of PVA(n)-CMC(m)-GA. A typical preparation route of PVA(n)-CMC(m)-GA: 8g of PVA was dissolved in 240 mL boiling water with vigorous mechanical stirring. After 30 min, 1 g of CMC was added and stirring for 2 h to dissolve completely. After that time, a transparent colorless solution was obtained then cooling to room temperature. The pH of this solution was set to 4 by HCl. Then this solution stayed to remove bubbles for 10 min. Then 4 mL of GA as cross-linker was added, stirring for 5 min. The solution was poured into the mold and dried 72 h at 25 °C in the oven. Lastly, a transparent film can be obtained and named as PVA(8)-CMC(1)-GA for the weight ratio of PVA to CMC is 8:1 in this composite. A similar procedure was followed for fabricating composite films with different weight ratio of PVA to CMC, such as PVA(8)-CMC(0)-GA, PVA(8)-CMC(2)GA, PVA(8)-CMC(3)-GA, PVA(8)-CMC(4)-GA, and PVA(8)-CMC(5)-GA. In all the polymers, the ratio of GA to PVA is the same. 2.3. Swelling Ratio and Gel Content. First, the sample was dried at 50 °C for 20 min, and then quickly weighed for Mi. After that, the sample was immersed in the water for a period of time (10 s, 30 s, 1 min, 2 min, 5 min, 10 min, 20 min, 30 min separately), and wiped the surface water to weight for Mw. The swelling ratio was defined as (Mw − Mi)/Mi. To estimate the cross-link density of sample with varying CMC content, the gel content was tested. It was weighed for Md after the sample was soaked 24 h in water and dried at 50 °C for 30 min. And the gel content was defined as Md/ Mi. 2.4. Water-Induced Shape-Memory Behaviors. Shapememory behaviors were measured by a bending test according to the literature47 as shown in Supporting Information (SI) Figure S1. A straight strip sample (20 × 3 × 0.2 mm) was first placed in an oven at 80 °C for 10 min and then bent to an angle (θi) close to 180°. After that, the deformation was fixed by cooling the strip down to room temperature with the aid of an external force. After removing the external force, the sample stays for 3−5 min to release the elastic deformation, and the holden bending angle was θf. Finally, the shape memory effect was investigated by spraying water on the deformed strip, then keeping it into the 80% RH environment at room temperature in a humidity chamber and recorded the changed bending angle as θr. The shape fixed ratio (Rf) was defined as θf/θi.The shape recovery ratio (Rr) was defined as (θf − θr)/θf. 2.5. Self-Healing Behaviors. We made scratches with a knife on the surface of the sample, and measured the width of the scratches and tested mechanical properties with electron microscopy and universal testing machines. At this point, the width of the scratch was named L0, and the stress of untreated samples was named η0. Then they were sprayed with water and placed in the 80% RH environment at room temperature in the humidity chamber for 24 h. Finally, the sample was measured the width of the scratches and mechanical properties again after drying. At this time the width of the scratch reduced to Ls and the stress was ηs. The self-repairing ratio of width of the scratches was defined as (L0 − Ls)/L0 and the self-repairing ratio of stress was defined as ηs/η0. 2.6. Characterization. Scanning electron microscope (SEM) was used to investigate the microstructures of specimens on a Quanta 200 (FEI, USA) and the specimens were coated with gold. Fourier transform infrared (FTIR) spectroscopic analysis (Thermo Electron Nicolet 5700, U.S.) was carried out on films of sample by the ATR method. Differential scanning calorimetry (DSC) testing (TA Instru-

materials can achieve shape recovery and healing only by spraying water on it.

Figure 1. Illustration of water induced multiresponsive effects.

To obtain water stimuli-responsive effect, the choice of materials should be taken into account first. Materials for water-induced SMPs are composed of synthetic polymer and composites, such as commercial polyurethane (MM3520 and MM5520),25 polyurethane containing pyridine moieties,32 cross-linked poly(vinyl alcohol) (PVA),33 cross-linked poly(ethylene glycol) (PEG),34 PVA-graft-polyurethane,35 polyurethane/cellulose nanowhiskers, 36,37 poly( D , L -lactide) (PDLLA)/microcrystalline cellulose (MCC),38 poly(glycerol sebacate urethane) (PGSU) or polycaprolactone (PCL), and PEG/cellulose nanocrystals (CNCs).39,40 Some natural materials, such as bird feathers and animal hair, are also reported with water-induced shape memory effect.41−43 We noted that these materials, which respond to water, all have hydrophilic segments or hydrophilic fillers. In this paper, PVA was chosen as the substrate because it is hydrophilic, in which there are abundant hydroxyl groups that have a strong affinity toward water molecules.44−46 It is reported that PVA cross-linked by glutaraldehyde (GA) can achieve good shape memory.33 Cellulose and its derivatives are the most used fillers to prepare water-responsive composites for low cost, good biocompatibility, biodegradation, high modulus, adequate flexural tensile strength, and high affinity. To reference the cross-link method reported in ref 33, we chose carboxymethyl cellulose sodium (CMC) as the filler to incorporate in PVA to achieve both shape memory and healing effects. CMC with a lot of carboxyl is sensitive to water, which not only helps PVA absorb the water to decrease the Tg of composite, but also can be used as the healing agent for its solubility in water. In this study CMC was introduced into poly(vinyl alcohol) (PVA) and then cross-linked by GA to obtain the composites named as PVA(n)-CMC(m)-GA, in which n is mass fraction of PVA and m is mass fraction of CMC.

2. EXPERIMENTAL SECTION 2.1. Materials. Unless otherwise noted, all chemical reagents were obtained from commercial suppliers and utilized without further purification. Poly(vinyl alcohol) 1788 (PVA 1788, 1700 degree of polymerization and 88% alcoholysis, AR), carboxymethyl cellulose sodium (CMC, MW 6400 ± 1000), glutaraldehyde (GA, 25%), HCl (37%, analytical grade) purchased from Chengdu kelong chemical reagent factory. Distilled water was utilized throughout the experiment. B


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Figure 2. (a) Preparation process and structure of PVA(n)-CMC(m)-GA; (b) SEM images of cross-linked PVA and its composite with CMC.

Figure 3. DSC curves of (a) dry samples and (b) wet samples with water content between 20% and 25%; (c) swelling ratio and (d) gel content of PVA(n)-CMC(m)-GA.

between −50 and 150 °C at a heating/cooling rate of 10 °C/ min. For the wet samples, the water contents are between 20%

ments DSC Discovery, U.S.) was carried out on the CMC/ PVA composition sample (∼10 mg) for a thermal cycle C


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Figure 4. (a) stress−strain curves of CMC/PVA composites; (b) zoom of the yield part of stress−strain curves; (c) illustration of the CMC is orientated under external forces.

smooth, while after adding CMC the material becomes rough and there are some micro pores. 3.2. DSC. Transition temperature of material indicates the movement of molecular chain, and it is an essential parameter to judge whether the material is potential to show shape memory behavior under a certain temperature condition. The DSC curves of PVA(n)-CMC(m)-GA in Figure 3a and 3b, demonstrated that the water could indeed reduce the glass transition temperature of PVA(n)-CMC(m)-GA. As shown in Figure 3a, all dried samples only exhibit glass transition temperature (Tg) at 80−90 °C. But after absorbing water (water content around 25%) the Tg of wet samples decrease to 20−30 °C as shown in Figure 3b, which are corresponding to the reports that water as the plasticizer can dramatically decrease the Tg of composites.26,48−50 In Figure 3b, it also can be noted that a larger peak is observed after the glass transition, which are attributed to the evaporation of water in wet samples.51 For PVA(8)-CMC(1)-GA, the peak of water evaporation is over 100 °C, while for the other composites the peak of water evaporation is lower than 100 °C and gradually increased as the content of CMC increasing. Samples of PVA(8)-CMC(1)-GA are retested for many times, but all showed the same results. We deduced that in PVA(8)CMC(1)-GA the OH and COOH could reach a proper ratio and prefer to bond the water than absorbing water, so water evaporation is higher than 100 °C. 3.3. Swelling and Gel Content. Next the swelling ratio and gel content of PVA(n)-CMC(m)-GA are tested, and the

and 25% by spraying water before DSC testing. Tensile testing (INSTRON 3367, U.S.) was used to investigate the mechanical properties of samples at a constant crosshead speed of 5 mm min−1 at room temperature (∼20 °C, ∼60% RH).

3. RESULTS AND DISCUSSION 3.1. Preparation of PVA(n)-CMC(m)-GA. To obtain PVA(n)-CMC(m)-GA, first PVA and CMC are mixed and dissolved in water, then the mixture is cross-linked by GA in pH 4, as shown in Figure 2a. In acidic conditions, −OH of the PVA and −CHO of the GA undergo intramolecular acetalization to form chemical cross-linked network structure,29 and the CMC chains penetrate into the networks. This reaction also can be confirmed by the FTIR spectrum in SI Figure S2, in which new two peaks at 1092 and 1643 cm−1 represented the newly formed CHO2 groups. The peak at 1092 cm−1 contributed to the stretching vibration of C−O in −CH(O−)2 group and the peak at 1643 cm−1 contributed to the symmetrical bending vibration of C−H in −CH(O−)2 group. In acidic condition, the −COO− of CMC can be converted into −COOH, which forms hydrogen bonds with −OH of the PVA. Therefore, characteristic peak of −COO− appeared around 1594 cm−1 in CMC cannot be found in the FTIR spectrum of cross-linked materials. From the images of scanning electron microscope (SEM) in Figure 2b, it clearly showed that CMC are uniformly dispersed in the PVA matrix. The surface of purely cross-linked PVA is D


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Figure 5. (a) Shape memory recovery of the material after spraying water in 80% RH; (b) Shape fixation and recovery of the composite under water mist for 30 min, the water content is basically maintained at between 20% and 25%; (c) Mechanism of water induced shape recovery.

results are shown in Figure 3c and d. Swelling test showed that swelling happened quickly in the first 5 min and gradually reach balance after 20 min. The swelling ratio increases with the increase of CMC content except PVA(8)-CMC(0)-GA. The composites with lower CMC content, such as PVA(8)CMC(1)-GA, PVA(8)-CMC(2)-GA, and PVA(8)-CMC(3)GA, the swelling ratio is below 100%. When the CMC content exceeded half of the PVA content, such as composite PVA(8)CMC(5)-GA, the swelling ratio sharply increases to 674%. While the gel content shows opposite tendency. As shown in Figure 3d, the gel content increased modestly for PVA(8)CMC(1)-GA and PVA(8)-CMC(2)-GA, and then decreased with the increase of CMC content, which of PVA(8)CMC(5)-GA even decreased to only 56%. The above variation trend of swelling ratio and gel content can be attributed to the content of CMC, which affect the hydrogen bonding interactions among carboxyl groups of CMC, hydroxyl groups of PVA and water. When the carboxyl group content is low, the carboxyl groups can sufficiently form hydrogen bonds with the hydroxyl groups of PVA. A large number of hydrogen bonds increase the stability of the crosslinked network. It makes the swelling ratio lower and the gel content higher than that of the pure cross-linked PVA. However, with the increase of CMC content, more and more carboxyl groups form hydrogen bonds with water and bring more water into the interior of the material. As a result, the swelling ratio increased as the CMC content increasing and even suddenly increased to 674% for PVA(8)-CMC(5)-GA. On another hand, more water was brought into the interior of materials to dissolve the soluble parts, such as CMC and/or uncross-linked PVA. As a result, the gel content is continually

decreasing. In all, content of CMC can effectively affect the water absorption of PVA network. CMC can stabilize the network structure for it prefers to form hydrogen bonds with PVA at lower level. But when the CMC content reaches a higher level, it prefers to form hydrogen bonds with water and bring more water into the composites, so the stability of the network decreased. Regulation content of CMC can control the water absorbing of materials. 3.4. Tensile Testing. Further tensile testing of the PVA(n)-CMC(m)-GA is measured to find how the CMC affect the mechanical properties of PVA composites. These composites can be divided into two groups, one is the CMC content at lower levels for PVA(8)-CMC(1)-GA and PVA(8)CMC(2)-GA, and the another group is the CMC content at higher levels for PVA(8)-CMC(3)-GA, PVA(8)-CMC(4)-GA and PVA(8)-CMC(5)-GA. From the gel content of Figure 3(d), we know that gel content of the group with lower levels CMC is higher, while that of the group with higher levels CMC is lower. Composites in these two group, both showed that the stress gradually decreased as CMC content increasing, as shown in Figure 4a. It can be attributed that as the CMC increasing, the hydrogen bonding formed by OH in PVA was destroyed and the mobility of PVA chains increased, so the stress decreased. It is also noteworthy that all samples appear a plateau when the strain reaches about 1%, shown in Figure 4b. The existence of the plateau means that the material has an atypical yield. We deduced that unbinding polymer chains in composites and their orientation during tensing induce this yield, as shown in Figure 4c. There are several types of hydrogen bonding interactions in composites. Hydrogen bonds can be formed E


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Figure 6. Self-healing effect of the PVA(8)-CMC(5)-GA at 80% RH:(a)surface scratch self-healing effect; (b)Mechanical properties after selfhealing; (c) Mechanism of water induced self-healing.

OH of PVA acts as a stationary phase to maintain the original shape. Glassy PVA segments cooperated with hydrogen bonds formed by PVA and CMC are used as a reversible phase to fix a temporary shape, as showm in Figure 5c. From the DSC curves in Figure 3b, it clearly showed that the glass transition temperature of composites decreased after absorbing water. When water molecules come into the networks of material, the volume of material will increase, which reduces the obstruction of movement of PVA and CMC chains. As a result, the glass transition temperature of materials decrease. When the transition temperature is lower than room temperature, strain energy fixed by PVA and CMC will release which causes the shape recovery. It also should be noted that PVA(n)-CMC(m)-GA composite not only demonstrate water induced shape memory effect, but also showed thermo-induced shape memory effect, as shown in SI Figure S3, the samples can recover from the bending temporary shape to the straight initial shape by heating to 80 °C. The thermo-induced shape memory property of PVA(n)-CMC(m)-GA are corresponding to the report that chemical cross-linked PVA showed thermo-induced shape memory effect.29,42 This paper is focused on the water-induced shape memory property, so there is no more discussion about thermo-induced shape memory properties. 3.6. Self-Healing Effect. We tested two types of self-heal behaviors of the composite materials, which were the recovery of scratches width and mechanical properties under humidity condition. PVA(8)-CMC(0)-GA without CMC cannot heal, whereas the self-healing effect of other composites gradually become obvious as the increase of CMC content. PVA(8)CMC(5)-GA showed the best self-healing behavior, as shown in Figure 6. As shown in Figure 6a, we carve out scratches with a knife on the surface of the PVA(8)-CMC(5)-GA sample (in order to conveniently observe the same location, scratches are depicted as cross-grain pattern). After 24 h at 80% RH, the scratches on

between PVA chains themselves, PVA and CMC chains, and CMC chains. Under external force, the hydrogen bonds will be destroyed, and then PVA and CMC segment will be reoriented. As shown in Figure 4b, the stress is consumed as the strain increases, which is the yield phenomenon. 3.5. Shape Memory Effect. Shape memory effect was tested as fellow: as shown in Figure 5a, a straight strip with the initial shape, was heating to 80 °C to bent under force, and cooling to room temperature to fix the bending state as temporary shape. Then bending samples were sprayed by water and kept in 80% relative humidity (RH) environment at the room temperature to observe the shape recovery. As shown in Figure 5a, bending PVA(8)-CMC(2)-GA can gradually recover to its straight initial shape in 5 min by spaying water. Shape fixing and recovery ratio are plotted in Figure 5b. For all the PVA(n)-CMC(m)-GA samples, the shape fixed ratio are around 95%, while the water induced shape recovery ratio showed a wave tendency as the CMC content increasing. It can be seem that the composites with lower CMC content (m = 0, 1, and 2) showed better shape recovery effect, and the shape recovery ratio reaches 95%. Then the shape recovery ratio decreased even to 26% as the CMC content increasing (m = 3, 4). But when the CMC content exceeded half of the PVA content for PVA(8)-CMC(5)-GA, the shape recovery ratio of which increase to 63%. From the swelling and gel testing, we realized that CMC can stabilize the network structure at lower levels and promote the interaction between PVA chains and water at higher levels. But here for water induced shape recovery ratio, it does not obey this law. So the hydrogen bonds formed between PVA and PVA, PVA and CMC, and CMC themselves should be considered as we have discussed in tensile testing part. For the shape memory mechanism, it can be attributed to the decrease of glass transition temperature by water absorption according to the literature.25 In these composites, the chemical cross-linked points formed by GA reaction with F



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ACKNOWLEDGMENTS This work was supported by the Key Subject Project of Education Department of Sichuan Province (18ZA0398), the Open Project Funds of Key Laboratory of Advanced Functional Materials in Universities of Sichuan Province (KFKT2014-1), and the Project of Experimental Technology and Management in Sichuan Normal University(SYJS2017009)

the sample surface become invisible and the width of scratches dropped from 40.8 to 5.3 μm. From the scratch width point of view, the repair ratio is 87%. In terms of mechanical properties, the material repair ratio is higher, even more than 100%. As shown in Figure 6b, the stress of original material can reach 52 MPa, and that of healed material can increase to 55.6 MPa after healing from damage. The proposed mechanism of self-healing is shown in Figure 6c. In dry materials, PVA and CMC formed hydrogen bonds. In moist conditions, the movement of the material segment becomes easier, which is a prerequisite for the material to be self-repairing. In a wet environment, the absorbed water damaged the hydrogen bonds and swelling caused the cut cross-sections to expand contact with each other. CMC chains on the cutting edge can move to another cutting cross-section and tangle with each other. During the drying process, PVA and CMC formed additional hydrogen bonds again. Therefore, we can observe the reduction of the scratch width and the restoration of the mechanical properties. CMC play an important role in the self-healing property, which can move to each side and formed a new hydrogen bond with PVA to achieve self-healing. That is why PVA(8)-CMC(5)-GA with the most CMC content showed the best self-healing effect.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information (file type PDF) The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03230.

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REFERENCES

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4. CONCLUSIONS In summary, we successfully obtained a type of material with water induced shape memory and self-healing property, which are prepared by filling CMC into cross-linked PVA. The study showed that the formation of hydrogen bonds between CMC and PVA affected the shape memory effect of materials. And CMC as the healing agent, the content of which play an important part in the self-healing effect. This study provides a relatively simple and low cost way to obtain multifunction material. These composites with water-induced shape memory and self-healing properties have potential application in many fields, such as the protective layer of devices. When damaged or scratched, the surface will recover only by spraying some water.

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(The bending method for testing water induced shape memory effect (Figure S1), FTIR spectrum of polymers and composites (Figure S2) and Thermo-induced shape memory effect (Figure S3) PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongmei Chen: 0000-0002-4674-3278 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. G


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Water Induced Shape Memory and Healing Effects by Introducing

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