Carbon Quantum Dots Composites

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Luminescent poly(vinyl alcohol)/carbon quantum dots composites with tunable water-induced shape memory behavior in different pH and temperature environments Guanghui Yang, Xuejuan Wan, Yijin Liu, Rui Li, Yikun Su, Xierong Zeng, and Jiaoning Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11476 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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ACS Applied Materials & Interfaces

Luminescent poly(vinyl alcohol)/carbon quantum dots composites with tunable water-induced shape memory behavior in different pH and temperature environments Guanghui Yang,ab§ Xuejuan Wan,a§* Yijin Liu,a Rui Li,a Yikun Su,a Xierong Zenga and Jiaoning Tanga* a

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science

and Engineering, Shenzhen University, Shenzhen 518060, PR China. b

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and

Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, PR China.

KEYWORDS: Luminescent polymer composites; Carbon quantum dots; Shape memory; Tunable shape recovery behaviour; water environments

ABSTRACT: Luminescent water-induced shape memory polymer (SMP) composites with tunable shape recovery rate are developed by blending poly(vinyl alcohol) (PVA) and carbon quantum dots (CQDs). The oxygen and active hydrogen-rich CQDs can serve as extra physical crosslinking points in PVA via strong hydrogen bonding interaction, which largely improves the shape memory performances of PVA. At room temperature, water can successfully actuate the shape recovery of deformed PVA/CQDs composite. It is demonstrated that this water-induced shape recovery is mainly attributed to the plasticizing 1 ACS Paragon Plus Environment


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effect of water and its competitive hydrogen bonding. Furthermore, a quantitative bending test suggests that the shape recovery time of this water-induced SMP is tunable by altering the environmental pH value and temperature, and a relatively large shape recovery time window (from 20 s to 200 s) can be achieved. In addition, the introduction of CQDs endows the PVA/CQDs SMP composites with excellent luminescent property, which makes the shape change of SMP visible under UV-light. It should be noted that the mild stimulus condition and tunable shape recovery performances make the luminescent visible PVA/CQDs SMP feasible for diverse biological applications in smart medical devices, stimuli-responsive drug-release, and intelligent sensors in vivo and in vitro.

1. Introduction Shape memory polymers (SMPs), as one of the most extensively studied smart materials, are able to 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,1-5 In recent years, SMPs have gained great attention for their wide applications in biomedicine, aerospace, sensors, and textiles.6-9 Usually, there are two structural requirements for the shape memory effect, including a stable polymer network and reversible stimuli-sensitive switching transition domain.4 The stable network, which can be acted by chain entanglement, interpenetrated networks, physically or chemically cross-linked points, or crystalline phases, determines the original shape of the SMPs.10-13 The reversible transition domain is responsible to fix and unfix their temporary shape, and diverse transition processes

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have been utilized such as crystallization/melting, vitrification/glass transition, dimerization reaction, and hydrogen bonding.14-18 Thus, SMPs can accordingly respond to different external stimuli (electric/magnetic field, light, solvent, ultrasound, and heat),19-23 and the most extensively studied SMPs are thermal-induced SMPs, in which the shape recovery is triggered by heating.24-26 However, if the heating process is not accessible or appropriate, other athermal stimuli have to be considered. Organic solvent stimulus and water stimulus are two kinds of extensively investigated stimuli in recent years, due to their mild and easily accessible condition.27-29 However, from the perspective of biomedical applications, water is more expected to trigger the shape recovery of SMPs because it has no damage to interactive response to the blood and biological tissue.30-31 Since Huang and co-workers reported that moisture could trigger shape recovery in polyurethane (PU), water-induced shape recovery has received increasing research attention.29,

32-34

Generally, water-responsive SMPs are

reported to undergo a significant reduction in the transition temperature,35 because water can work as plasticizer and insert into the macromolecular chains, which reduce their entanglement and decrease the Tg of the SMPs. Most importantly, water can largely disrupt the hydrogen bonding in SMPs and increase the moving ability of the polymer chains.36-37 Considering water-induced shape recovery is harmless to living organisms, water-responsive SMPs possess great potential in biological applications, which deserves comprehensive and thorough research. However, this research area seems far from maturity, and the development of a universal SMP with tunable shape recovery speed is highly desirable for diverse application requirements.31, 38 3 ACS Paragon Plus Environment


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In fact, in addition to water, changes of pH and temperature can also largely disrupt the hydrogen bonding interaction.39-40 Several reports have designed thermal- and pH-induced SMPs based on hydrogen bonding. Li et al. developed pH-induced SMPs by blending PU with functionalized cellulose nanocrystals.9 The hydrogen bonding interactions in the SMPs can be readily associated/disassociated by altering pH values, endowing the SMPs with pH-induced shape recovery behavior. In another work by Chen and co-workers, a thermal-induced triple-shape memory supermolecular composite composed of PU and mesogenic units was prepared via forming hydrogen bonding between the two components. The authors suggested that high temperature can decrease the hydrogen bonding density in the obtained SMPs and consequently lower their transition temperature.41 Inspired by these studies, we speculated that the water-induced shape memory effect in the hydrogen bonding-containing SMPs can be tunable via altering the pH value and temperature of water, leading to different response time. On the basis of the above-mentioned consideration, the aim of this study is to prepare a water-induced SMP composite containing plenty of hydrogen bonding, which probably presents tunable shape recovery time by altering the pH value and temperature of water. PVA is chosen as the polymer matrix in the composite due to its prominent advantages such as high hydrophilicity, nontoxic nature, good biological compatibility, high optical transparency and easy processability.42 Moreover, the -OH groups in the backbone of PVA make it easier to form hydrogen bonding.43 Carbon quantum dots (CQDs), a rising star in nanocarbon family, are chosen as fillers to enhance the hydrogen bonding because there are abundant oxygen/hydrogen-containing species such as -OH and -COOH on the surfaces of CQDs.44 4 ACS Paragon Plus Environment

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Moreover, due to the excellent luminescent property of CQDs, luminescence is also expected to be achieved in PVA-based SMPs by incorporating CQDs into PVA,45-46 which makes the shape recovery response traceable under UV-light. To our knowledge, this is the first report about luminescent SMP composite with tunable shape recovery response speed. The advantages of this composite, including luminescent performance from CQDs, water-induced tunable shape memory behavior, make it have great potential in various application fields. 2. Experimental 2.1 Materials Cellulose fibers were supplied by Daicel Chemical Industries, Ltd in Japan. (Product number KY100-S). PVA (alcoholysis degree is 99%) with an average degree of polymerization 1700 was purchased from Sigma-Aldrich and used as received. Ammonia solution (25%), hydrochloric acid and sodium hydroxide were provided by Sigma-Aldrich and used as received. Dialysis bags (1-0150-45) and 0.22-µm PTFE membranes (11806-47-N) were purchased from Membrane Filtration Products, Inc. and Sartorius, respectively. Ultrapure water was used throughout the experiments. 2.2 Synthesis of luminescent CQDs CQDs were synthesized via a hydrothermal method using cellulose fibers as carbon precursor, and typical synthesis procedure was shown in Figure S1. 0.75 g cellulose fibers were added into 30 mL ammonia solution and stirred at 70 oC for 1 h. Then, the mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 200 oC for 4 h in an oven. After it was naturally cooled to room temperature, the suspension was poured into a beaker, followed 5 ACS Paragon Plus Environment


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by adding HCl until the pH was 7. Then, a 0.22-µm PTFE membrane was used to filter the neutral suspension, followed by dialyzing the filtrate for 5 days. After purification, the CQDs-containing solution was freeze dried to obtain solid CQDs for further use. The yield of CQDs is calculated to be 9%. 2.3 Fabrication of PVA/CQDs composites Solvent casting method was used to prepare the PVA/CQDs composites. Taking the PVA/CQDs composite containing 1 wt% CQDs (PVA1) as an example, in brief, dissolving 1.98 g of PVA in 40 mL of water at 98 oC for 1 h; dispersing 20 mg of CQDs in 20 mL of water with ultrasonic treatment for 1 h. After that, gradually mixing the prepared CQDs dispersion and PVA solution, and then continuing to stir at 98 oC for 2 h. Subsequently, the obtained PVA/CQDs mixture was poured into a PTFE dish, followed by keeping it in an oven at 60 oC for 5 days to cast the composite films. In order to avoid the effects of water molecules in the air on the hydrophilic PVA, the as-prepared composite films were sealed in a desiccator before further analysis. 2.4 Water-induced shape-memory behaviors Shape-memory behaviours was measured by a bending test according to the literature.32 A straight strip of composite (40 mm × 3 mm × 0.2 mm) was first placed in an oven at 80 oC for 10 min and then bent to an angle (θo) close to 180o. After that, the deformation (θi) was fixed by cooling the strip down to room temperature with the aid of an external force. Finally, the shape memory effect was investigated by immersing the deformed strip into water with varied temperature and pH values and subsequently recording the change of bending angle (θf)


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with time. The shape recovery ratio (Rr) was defined as (θi – θf)/θi. A digital camera was used to record the macroscopic shape change of the immersed strip with time. 2.5 Characterization Transmission electron microscopy (TEM) was conducted on a Tecnai F30 transmission electron microscope at 200 kV. Atomic force microscopy (AFM) was carried out on a Bruker Dimension Icon microscopy in the tapping mode. A Kratos AXIS Ultra DLD X-ray electron spectrometer was used to record the X-ray photoelectron spectroscopy (XPS) under focused monochromatized Al Kα radiation (15 kV). FTIR spectra were recorded on a Nicolet-6700 infrared microscope with an ATR mode. A Shimadzu UV-2450 Ultraviolet-Visible spectrophotometer was used to record UV-Vis spectra. The fluorescent excitation and emission spectra are recorded by a Jobin Yvon HORIBA NanoLog spectrofluorometer. A TA Instrument DMA (Q800) was used to study the thermomechanical properties with a heating rate of 5 oC min-1. The DMA operating frequency is 1 Hz and the tested temperature range is from -40 to 120 oC. Raman spectra were recorded on an Olympus microscope coupled Renishaw Raman spectrometer. A 512 nm wavelength laser was focused on a spot size of 0.35 µm using the microscope with a 50× objective lens and the exposure time was 100 s. 3. Results and Discussion 3.1 characterization of CQD CQDs are synthesized for the first time by using cellulose fibers as carbon precursor and ammonia solution as doping source (Figure S1). The morphology and size distribution of CQDs are analyzed by AFM and TEM observations. The AFM image in Figure 1a suggests 7 ACS Paragon Plus Environment


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that the as-obtained CQDs are approximately 5.5 nm in height with a homogeneous distribution. The TEM image and size distribution histogram shown in Figure 1b suggest that the as-obtained CQDs are quasi-spherical and the diameter is in the range of 3-7 nm.

Figure 1. (a) AFM topography image of the as-obtained CQDs, inset shows the height profiles along the lines. (b) Typical TEM image of the as-obtained CQDs, inset shows the diameter distribution histogram (counted about 100 CQDs). The surface composition of the CQDs is characterized by XPS and FTIR. Figure 2a clearly shows three prominent features of C1s, O1s and N1s, suggesting that C, O and N are the primary composition of CQDs. The C1s XPS spectrum in Figure 2b is deconvoluted into four peaks at 288.5, 287.8, 285.9 and 284.5, which represent the C1s states in C=O, C-O, C-N and C-C/C=C, respectively.47 The deconvoluted N1s spectrum in Figure 2c shows three peaks at 400.3, 399.6 and 398.9 eV, confirming that the N1s states in pyrrole-like N, pyridine-like N and graphite-like structure, respectively.48 Figure 2d shows the FTIR spectrum of as-obtained CQDs. The absorption peaks located at 3405 and 3244 cm-1 are ascribed to the stretching vibrations of -OH and –NH2, respectively. The peaks at 2951 and


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2872 cm-1 correspond to the C-H bond, while the peaks at around 1605 and 1405 cm-1 are assigned to the stretching vibration peak of -COO-.49-50 Both XPS and FTIR indicate that there are plenty of functional groups such as -COOH, -OH and -NH2 on the surface of CQDs, which will be beneficial to form hydrogen bonding.

Figure 2. (a) XPS spectrum of the as-obtained CQDs. High-resolution of C1s (b) and N1s (c) peaks of the as-obtained CQDs. (d) FTIR spectrum of the as-obtained CQDs. Photoluminescence (PL) and UV-Vis spectra of the as-obtained CQDs are illustrated in Figure 3. As Figure 3 inset showing, the as-obtained CQDs solution is slightly yellow in daylight, while it displays a strong blue luminescence upon exposing to the UV light (365 nm). From Figure 3, two peaks located at 280 and 330 nm (a broad shoulder peak) are observed in the UV-Vis absorption spectrum, which can be ascribed to the π-π* and n-π*



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transition, respectively.51-52 The strongest PL excitation and emission wavelengths of the as-obtained CQDs are measured to be 360 nm and 435 nm, respectively. Moreover, the PL of CQDs shows excitation-wavelength dependent features (Figure S2), which is similar to that of CQDs reported in the previous investigation.48 Furthermore, the impact of pH on the PL intensity of CQDs is investigated. Figure S3 indicates that the PL intensity of CQDs changes slightly when the solution pH varies from 1 to 7, and decreases significantly as the pH value increases from 7 to 13.

Figure 3. The UV-Vis absorption (Abs) and PL (Ex, Em) spectra of the as-obtained CQDs solution. Insets: Digital photographs of the as-obtained CQDs solution under illumination of daylight (left) and 365 nm UV light (right). 3.2 Characterization of PVA/CQDs composites PVA/CQDs composites containing 1%, 2%, 3% CQDs are denoted as PVA1, PVA2 and PVA3 respectively. AFM is used to analyse the dispersion state of CQDs in PVA matrix. The topographic 3D images are depicted in Figure 4. It can be found that the composite surface becomes rougher with increasing CQDs contents. When the filler content is less than 2%, the 3D bumps caused by the hard CQDs are sparse and separated from each other (Figure 4a-b),


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demonstrating a homogeneous dispersion of CQDs in PVA matrix. Moreover, the homogeneous dispersion also implies sufficient interfacial interaction between nanofillers and polymer matrix, which may be attributed to hydrogen bonding formed between CQDs and PVA.53 However, as the content of CQDs increases to 3 wt%, the 3D bumps become dense and some bumps are integrated with each other (Figure 4c), which indicate the aggregation of CQDs.

Figure 4. AFM 3D images of PVA/CQDs composites with 1 wt% (a), 2 wt% (b) and 3 wt% (c) CQDs. The thermomechanical properties of PVA/CQDs composites with different CQDs contents are measured by DMA. As shown in Figure 5, the storage modulus of the prepared composites increases significantly with increasing CQDs content (from 4400 MPa of PVA to 6950 MPa of PVA3), and the Tg (determined as the peak position of maximum Tan δ) presents similar trends (increased from 56 oC of PVA to 65 oC of PVA3). The increases of storage modulus and Tg confirm that CQDs can serve as additional physical crosslinking points in PVA via interfacial interaction.54



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Figure 5. DMA of PVA and PVA/CQDs compsites. (a) Storage modulus as a function of temperature. (b) Tan δ as a function of temperature. The influence of CQDs content on crystallinty of PVA is measured by DSC and XRD (Figure S4 and Table S1). As can be seen, the melting temperature and crystallinity degree of the PVA/CQDs composites decrease with increasing CQDs content. This could be contributed to the strong interfacial interaction formed between CQDs and PVA. The formation of large crystalline domains could be restricted by this interaction, resulting in higher disorder and decrease of PVA nucleation. Similar results in PVA-based composites have been reported.32, 55 To confirm the interactions between PVA matrix and CQDs, FTIR experiment is carried out and the results are shown in Figure 6a. As for PVA, the broad absorption with a peak position at 3294 cm-1 is ascribed to the symmetrical stretching vibration of -OH groups. Generally, the -OH stretching peak is sensitive to the association/disassociation of hydrogen bonding, which will lower the stretching frequencies of the participating O-H bonds.32 As shown in Figure 6a, the -OH stretching peak shifts to lower wavenumbers in the PVA/CQDs composites along with the increasing amount of CQDs (from 3294 cm-1 of PVA to 3260 cm-1 of PVA3). Moreover, from the temperature-variable FTIR spectra in Figure 6b, it can be


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found that the -OH stretching peak shifts from 3262 cm-1 to 3292 cm-1 with increasing temperature from 30 oC to 130 oC, which suggests the broken of the hydrogen bonding.30 Therefore, both FTIR studies demonstrate the presence of additional hydrogen bonding between CQDs and PVA. Moreover, as shown in Figure S5, the mechanical strength of PVA is significantly improved by the addition of CQDs. This mechanical enhancement could be ascribed to the enhanced interfacial adhesion of the PVA/CQDs composites, which also implies the presence of additional hydrogen bonding within the composites. Similar results have been reported in PVA/GO composites.56

Figure 6. (a) FTIR spectra of PVA/CQDs composites. (b) Temperature-variable FTIR spectra of PVA2 at temperature of 30-130 oC.



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Figure 7. (a) Typical shifts of the Raman band position located at 1095 cm-1 upon tensile deformation of pure PVA and PVA2 composites. (b) Comparison between the 1095 cm-1 band shifts as a function of tensile strain observed for pure PVA and PVA2 composites. To further confirm the hydrogen bonding interactions between CQDs and PVA, we record the Raman spectra of the composites during tensile deformation and the results are shown in Figure 7. The Raman spectra of 1050 to 1125 cm-1 are illustrated in this figure in order to include the 1095 cm-1 C-O stretching mode. No obvious shift in 1095 cm-1 peak can be found when the pure PVA is strained. This may be because this peak involved the bending of the hydroxyl side groups rather than the stretching of the primary bonds in the polymer backbone.57 However, as for PVA2, a significant shift of 1095 cm-1 peak is observed upon tensile deformation, which represents direct deformation along the C-O bond of the nanofillers.58 As indicated in literatures, this deformation can be ascribed to the stress-transfer occurred between PVA and fillers via hydrogen bonding interactions.57 Therefore, the Raman measurement also implies the presence of hydrogen bonding between PVA and CQDs. Optical properties of the as-obtained PVA/CQDs composite are studied by UV-Vis spectroscopy and the results are shown in Figure S6. The thickness of the analyzed film


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samples is ~20 µm and the dependence of the film’s optical transparency (light transmittance at 550 nm) against the CQDs content is plotted in the inset of Figure S6. Because of the excellent nanoscale dispersion of CQDs at low content (2 wt% and below) in PVA matrix, the obtained composite films still retain high optical transparency (88%) which is quite similar with pure PVA (91%). Further increase of the CQDs content (3 wt% and above) leads to a considerable drop in the optical transparency (to 70% and below), which may ascribe to the agglomeration of CQDs as evidenced by the AFM measurement (Figure 4c).45

Figure 8. (a) PL spectra of PVA and PVA/CQDs composite films illuminated at 350 nm, inset shows the photographs of these films under illumination of UV light (right) and daylight (left). (b) Excitation-wavelength dependent PL spectra of PVA2, with inset showing the normalized curves. PL measurement of the composite is conducted to explore the luminescence properties of the PVA/CQDs composite films, and the corresponding results are shown in Figure 8. The PL spectra of the obtained composite films are quite similar with that of CQDs excited at 350 nm, and the PL intensity varies obviously with different CQDs loading content (Figure 8a). The maximum PL emission of the films is observed when the CQDs content is 2 wt%. Higher 15 ACS Paragon Plus Environment


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CQDs content (3 wt%) may lead to a decrease of PL intensity due to a variety of interrelated factors such as inner filter effect, photoluminescence reabsorption and partial quenching due to particle agglomeration.45 All the PVA/CQDs composite films emit strong blue luminescence upon exposing to UV light (365 nm). The PL spectra of PVA2 excited at different wavelengths is recorded in Figure 8b, suggesting that PVA2 exhibits similar excitation-wavelength dependent PL emission performance as pure CQDs (Figure S2). Considering that 2 wt% CQDs exhibits an excellent dispersibility in PVA, and the obtained PVA2 composite possesses the best luminescence emission properties, PVA2 is used to evaluate the shape recovery behavior of this luminescent materials. 3.3 Water-induced shape recovery of PVA/CQDs composites PVA and PVA2 composite are first made into thin film strips, and their shape recovery behavior are evaluated via classical bending test.59 A straight strip (original shape) is first placed in an oven at 80 oC for 10 min and then bent into “U” like shape. After that, the deformation is fixed by cooling the strip down to room temperature with applying an external force (temporary shape). The shape recovery performance is finally investigated through immersing the deformed specimen into distilled water at different pH and temperature, and the momentary shape of the specimen at different times is recorded using a digital camera.


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Figure 9. Shape recovery of PVA2 (a) and PVA (b) immersed in distilled water (pH=7). (c) Comparative study of hot air and water on the shape recovery of PVA2. The PVA strip was coated with dark dyestuff which is easy to observe. The shape change of PVA and PVA2 strips upon immersing into distilled water (pH=7) at room temperature (25 oC) are shown in Figure 9a-b. Note that the whole recovery process of PVA2 strip is recorded under a 365 nm UV lamp. As we can see, the PVA2 strip fully recovers to its original shape after immersing in water for about 180 s, while the pure PVA strip can only regain about 20% of its original straight shape after 270 s. Moreover, as shown in Figure 9c, the deformed PVA2 strip could not fully regain its original shape even after it was kept in a dry oven at 80 oC for 30 min. However, this high temperature-processed strip can fully recover its original straight shape within 110 s when it is subsequently immersed in distilled water. These observations clearly indicate that the shape recovery of PVA is significantly improved by adding CQDs and is induced by water instead of heat. It may be



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ascribed to the enhanced hydrogen bonding interactions in the PVA/CQDs composite as observed in FTIR measurement (Figure 6). The interactions between the active hydrogen-containing functional groups (-COOH, -OH and -NH2) on CQDs and PVA can produce plenty of physical crosslinking point, which helps to decrease chain slippage and thus strengthen the hard domains in SMPs during deformation as literature reported.60-61 Additionally, the shape recovery of the stretched PVA/CQDs composite is also studied and the results are shown in Figure S7. It reveals that the shape memory effect measured by stretching test is similar to that measured by bending test. PVA2 shows a much faster shape recovery than pure PVA (Figure S7a-b), and the shape recovery of PVA2 can be further accelerated with increasing pH values and environmental temperature (Figure S7b-c).

Figure 10. DMA of PVA2 immersed in water for different times. (a) Storage modulus as a function of temperature. (b) Tan δ as a function of temperature. To investigate the mechanism of the shape recovery of PVA/CQDs composites, DMA measurement is used to study the thermomechanical properties of PVA2 composites with different immersion times (0-60 s). Figure 10a shows an evident decrease of storage modulus from 5140 to 3015 MPa when the immersion time increases from 0 to 60 s. In Figure 10b, the


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Tg (defined as the Tan δ peak position at higher temperature) of PVA2 decreases from 60 oC to 56 oC with increasing the immersion time from 0 s to 20 s. After immersing for 40 s, the Tg of the composite continuously decrease to ~40 oC, where the Tan δ peak is partially overlapped with the broad peak at ~7 oC (may be caused by the phase transition of water penetrated into the PVA2 samples) and becomes a shoulder. With increasing immersion time to 60 s, no apparent Tan δ peak is observed except the peak at ~7 oC. This may be attributed to the fact that the Tg decreases to a lower temperature, and thus the corresponding Tan δ peak is totally overlapped with the broad peak at ~7 oC. These results reveal that the absorbed water molecules play the role of plasticizers, leading to the decline of the switching transition temperature and the stiffness of PVA2. When the transition temperature of PVA chains drops below the environmental temperature, the shape recovery of PVA2 occurs as the active motion of tangled PVA chains starts.62 Therefore, water is identified as a stimulus to trigger the shape recovery of PVA2 SMPs.32-33

Figure 11. Volume swelling degree and weight swelling degree and of PVA2 as a function of immersion time. To further understand the mechanism of shape recovery of PVA/CQDs, the swelling 19 ACS Paragon Plus Environment


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degrees in volume and weight of PVA and PVA2 as a function of immersion time are comparatively studied. Figure 11 shows that the weight and volume of PVA2 increase obviously with increasing immersion time. The volume swelling degree and weight swelling degree increase from 1 to 1.61 and 1.25 respectively, with increasing immersion time from 0 s to 210 s. These experimental results are agreed well with the previous finding that the penetrated solvent molecules can increase the weight and volume of SMPs.27, 32 However, comparing with PVA2, no obvious swelling difference is found for pure PVA (Figure S8), indicating that the swelling effects alone cannot account for the shape memory effect of PVA2.

Figure 12. FTIR spectra of PVA2 composites immersed in water for different times. Considering that the hydrophilic PVA and CQDs all exhibit strong hydrogen bonding affinity towards water, a large number of physical crosslinking points between PVA and CQDs with hydrogen bonding will be destroyed during the water immersing process, which may also give rise to improve the shape recovery of PVA2.36-37 FTIR spectroscopy is utilized to detect this possible variation of hydrogen bonding in PVA2 composites immersed in water


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for different times at room temperature (Figure 12). It is clear that the -OH stretching peak shifts to higher wavenumbers with increasing immersion time, indicating that the water absorbed in PVA2 composite can indeed form strong hydrogen bonding with PVA and CQDs. As demonstrated in the previous literatures,32, 63 the newly formed hydrogen bonding from water may significantly disrupt the primary hydrogen bonding in PVA/CQDs, leading to a declined transition temperature and improved shape recovery performance of PVA/CQDs. 3.4 Tunable shape memory behavior of PVA/CQDs composites Despite the fact that SMPs possess a great potential for the application in biomedical field, the conversion of SMP technology to actual medical equipments are still severely limited nowadays, and SMPs with tunable shape memory recovery rate are urgently needed to cope with the different application demands.31 For example, in order to prevent the possible impact on physiological environment which would be caused by sudden shape change of SMPs, slower shape recovery is more suitable in applications such as medical implants.6 On the contrary, as for the SMPs used in actuating applications such as artificial muscles and stents, a key performance indicator is speed, and thus faster shape recovery is usually more desired.64 Therefore, the development of tunable shape recovery properties would make SMPs have greater potential in biomedical field. It is known that changes of pH and temperature can largely disrupt the hydrogen bonding interaction.39-41 Since hydrogen bonding contributes to the shape recovery of PVA/CQDs SMPs, the effects of pH and temperature on the shape recovery of these SMPs are further studied.



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Figure 13. Shape recovery of PVA2 immersed in water with different temperatures and pH. (a) 25 oC, pH=2; (b) 25 oC, pH=12; (c) 37 oC, pH=7; (d) 50 oC, pH=7; (b) 50 oC, pH=12. Figure 13 shows the shape change of PVA2 upon immersing into water with different pH values and temperatures. As shown in Figure 13a-b and Supplementary movie 1-2, PVA2 strip fully regains its original straight shape after 200 s when immersed in pH 2 water at room temperature (Figure 13a, Supplementary movie 1), while it can recover to original shape after being immersed in pH 12 water within 140 s (Figure 13b, Supplementary movie 2). This phenomenon indicates that alkali environment shows a faster shape recovery effect than acidic environment. This can be understood as due to the effect of pH values on the hydrogen bonding between the -COOH groups on CQDs and -OH groups in PVA.9,

65

In alkali 22

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environment, the vast majority of -COOH groups are deprotonated and negatively charged, which may apparently lower their hydrogen bonding formation ability. Therefore, combined influence of water and the deprotonation effect is supposed to weaken hydrogen bonding between CQDs and PVA, leading to a quicker shape recovery speed. Conversely, as the environmental pH is adjusted to 2, the hydrogen bonding between -COOH and -OH groups will be spontaneously strengthened, resulting in a relatively slower recovery response of PVA2 composite. Note that the full spectrum XPS analysis of CQDs provides the atom ratio of O:N:C as of 8.6:1:12, which means the content of nitrogen-containing groups on CQDs is much less than that of oxygen-containing groups. Therefore, the effect of -NH2 groups on the hydrogen bonding is relatively slight. The influence of environmental temperature on the shape recoery of PVA2 is also investigated, and the shape changes of PVA2 strips immersed in pH 7 water at different temperatures are shown in Figure 13c-d. As we can see, the shape recovery time of PVA2 decreases from 180 s (Figure 9a, Supplementary movie 3) to 80 s (Figure 13c, Supplementary movie 4) with increasing water temperature from 25 oC to 37 oC. When the environmental temperature is further increased to 50 oC, the deformed PVA2 strip recovers its original straight shape even more quickly, which is within 50 s (Figure 13d, Supplementary movie 5). As hydrogen bonding interaction could be quickly broken in the composite by increasing temperature,41 the shape recovery rate of PVA2 in pH 7 water at high temperature will be significantly improved due to the enhanced mobility of the PVA chains. Furthermore, as Figure 13e and Supplementary movie 6 showing, a much faster shape recovery response of PVA2 immersed in pH 12 water at 50 oC is observed, where the shape recovery time 23 ACS Paragon Plus Environment


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decreases dramatically to 20 s.

Figure 14. The shape recovery behavior of PVA2 immersed in water with different pH and temperatures. Bending test is carried out to quantitatively assess the shape memory effect according to the literature.59 The shape recovery ratio (Rr) of PVA2 immersed in water with different pH and temperatures are shown in Figure 14. Although all the deformed PVA2 strips possess a high shape recovery ratio (Rr>90%), there is a large difference in shape recovery time. As shown in Figure 14a-c, alkali environment is beneficial to reduce the shape recovery time. In addition, water temperature has a more significant effect on shape recovery time. From Figure 14b and Figure 14e, it is clear that the shape recovery time of PVA2 dramatically drops from 180 s to 50 s as the water temperature increases from 25 oC to 50 oC. These results indicate that the shape recovery time of PVA2 can be tailored by altering the pH value and temperature of water.


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Figure 15. Sketch diagram of the shape recovery of PVA/CQDs composites and illustration of the evolution of hydrogen bonding interactions within the composites upon immersing in water. To better understand the mechanism of the tunable shape recovery behaviour, a sketch diagram of the shape recovery of PVA/CQDs composite is drawn according to the above discussions (Figure 15). After the composite is fabricated, the hydrogen bonding interactions between the active hydrogen-containing functional groups (-COOH, -OH and -NH2) on CQDs and the -OH groups on PVA can produce plenty of physical crosslinking points, which helps to decrease chain slippage and strengthen the hard domains in the composite during deformation, resulting in the improvement of shape recovery performance. When water molecules penetrate into the deformed composite, the primary hydrogen bonding in the 25 ACS Paragon Plus Environment


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composite would be disrupted by competitive hydrogen bonding from water on the one hand and on the other hand, Tg of the composite would be decreased by the plasticizing effect of water. When the transition temperature of PVA/CQDs composite is reduced below environmental temperature, the active motion of polymer chains starts, which will lead to the shape recovery of the deformed composite. Furthermore, as the pH and/or temperature of water increases, the primary hydrogen bonding in the composite will be weakened and broken more rapidly due to the deprotonation of -COOH groups on CQDs in alkali environment and the enhanced mobility of the PVA polymer chains in high temperature environment. As a result, a quicker shape recovery speed of PVA/CQDs composite is achieved. 4. Conclusions In summary, we have successfully prepared a novel luminescent SMP composite by incorporating CQDs in PVA. The fluorescent CQDs can act as additional physical crosslinking points in PVA via strong hydrogen bonding interaction, and the obtained PVA/CQDs composite exhibits an excellent water-induced shape memory performance at room temperature. Experimental results suggest that the competitive hydrogen bonding from water and its plasticizing effect are the two main factors for triggering the shape recovery. Moreover, the bending test shows that the shape recovery time can be adjusted by altering the pH value and temperature of water at a range of 20 s to 200 s. As the shape recovery can be induced and modulated with mild condition in water, which is also visible under UV-light, the obtained PVA/CQDs SMPs possess tremendous research potential in diverse biological field such as smart medical devices, stimuli-responsive drug-release, and intelligent sensors in 26 ACS Paragon Plus Environment

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vivo and in vitro.

ASSOCIATED CONTENT Supporting Information The following Supporting Informations are available free of charge via the Internet at http://pubs.acs.org.. Synthesis procedure of CQDs. Excitation-wavelength dependent PL spectra of CQDs. PL spectra of CQDs solutions with different pH values. Stress-strain behavior for PVA/CQDs composites. DSC and XRD results of PVA/CQDs composites. Swelling result of PVA. Optical transparency of PVA/CQDs composites. Movies of the shape recovery process for PVA2 immersed in water different pH values and temperatures. AUTHOR INFORMATION Corresponding Author *

[email protected]

*

[email protected]

Author Contributions §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest. 27 ACS Paragon Plus Environment


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ACKNOWLEDGMENT The financial support from National Natural Science Foundation of China (51472165 and 21204042), and Fundamental Research Project of Shenzhen (JCYJ20160422102541990, JCYJ20160520160830116 and JCYJ20140509172609160) are gratefully acknowledged. REFERENCES 1.

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Table of Contents Graphic


Carbon Quantum Dots Composites

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