Tunable Temperature Memory Effect of Photo-Cross-Linked Star PCL

Feb 26, 2014 - Journal of Applied Polymer Science 2018 135 (8), 45909 ... Recent progress in shape memory polymer: New behavior, enabling materials, a...
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Tunable Temperature Memory Effect of Photo-Cross-Linked Star PCL−PEG Networks Lin Wang, Shubin Di, Wenxi Wang, Hongmei Chen, Xifeng Yang, Tao Gong, and Shaobing Zhou* Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, PR China S Supporting Information *

ABSTRACT: In this study, we synthesized one type of biocompatible and biodegradable cross-linked star poly(εcaprolactone)−poly(ethylene glycol) (c-4sPCL−PEG) with an excellent temperature memory effect (TME) by photo-crosslinking of cinnamon group terminated four arms poly(εcaprolactone) (4sPCL−CA) and cinnamon group terminated poly(ethylene glycol) (PEG−CA) through the irradiation of 365 nm ultraviolet (UV) light. The results of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) demonstrated that the c-4sPCL−PEG networks possessed a broad transition temperature region from 20 to 55 °C, which including a wide high elasticity transition region and a melting transition region. A remembered temperature in this temperature range, which is close to body temperature, could be gained by adjusting the deformed temperature (Td). Moreover, the TME could be tuned by simply changing the molecular weight or the content of PCL segment. The mechanism of the TME was investigated in detail for the first time with X-ray diffractometry (XRD) and two-dimensional infrared correlation spectroscopy (2D-FTIR) at different temperatures, and the results indicated that the TME was resulted from a change in partial crystallization of the star cross-linked polymer networks, which led to the wide transition temperature. The study provides a facile strategy toward the design and engineering of a promising smart material for applications in the field of smart biomedical devices. also the same as the deformed temperature.10,11 Compared to the traditional SMPs, TMPs can respond to various temperatures with the same polymer instead of different new polymers; moreover, the transition temperature can be adjusted to meet special requirements. It has been reported that copolymers consisting of crystallizable controlling units have a temperature memory effect (TME),10,11 and polymers with a broad thermal transition are also able to memorize a specific deformation temperature.12−14 For example, Poulin et al. were the first to report that the nanocomposite of polyvinyl alcohol and carbon nanotubes possess a temperature memory effect owing to a broadened glass transition.12 Leidelin et al. reported that the temperature memory of the copolyesterurethanes under stressfree and constant strain recovery could be adjusted systematically by variation of the deformed temperature in a temperature range from 32 to 65 °C.11 Xie reported the recovery stress based temperature memory effect of the perfluorosulfonic acid ionomer (PFSA).13 These insights establish a compelling rationale for synthesizing and studying the temperature memory polymer.

1. INTRODUCTION Thermal-induced shape memory polymers (SMPs) can deform and recover their initial shape when stimulated by heat.1−5 Nowadays, thermal-induced SMPs may be more suitable for applications in biomedical devices, such as stents6,7 and selfreparation.8 However, the application of conventional thermalinduced SMPs is limited in the face of ever-increasing complex demands on the using of intelligent medical devices due to the fact that they can remember only one temporary shape and thus exhibit a dual-shape memory effect. Currently, multifunctional SMPs gradually have become a hot topic owing to the ability of processing complex deformation on demand and the multifunctional applications, such as a minimally invasive surgical catheter9 and smart actuator.10 Among these multifunctional SMPs, temperature memory polymers (TMPs) have received more and more attentions in recent years. TMPs possess a capability to memorize a specified temperature instead of a temporary shape, which can be looked as their deformed temperature or recovered temperature.11−16 The recovered temperature of TMPs depends on the deformed temperature. In the recovery process on stress free mode, the remembered temperature at which there exists a maximum instantaneous strain recovery rate, it can be looked as the deformed temperature, and in the recovery process on constant strain mode, the temperature of the maximum recovery stress is © 2014 American Chemical Society

Received: November 9, 2013 Revised: February 18, 2014 Published: February 26, 2014 1828

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However, if these systems would be considered as biomaterials for medical applications, some deficiencies still remain. For example, the biocompatibility of the polymer would be primarily required, and the applied temperature should be close to body temperature. In addition, for a temperature memory polymer with a controllable crystalline unit, the process of crystallization is fast, and it can be affected by many factors. Although the temperature is an important factor, previous reports almost did not investigate its influence on the crystallization process of the temperature memory cycle. In this study, four arms poly(ε-caprolactone) (4sPCL) and poly(ethylene glycol) (PEG) components, cinnamon groups as photo-cross-linked agent were employed to synthesize the star c-4sPCL−PEG networks with temperature memory function for the first time. The c-4sPCL−PEG networks were achieved by simple photo cross-linking of cinnamon group terminated 4sPCL and cinnamon group terminated PEG with 365 nm UV light irradiation. PCL and PEG components can provide a controllable crystalline unit for the temperature memory effect, and the terminated cinnamon groups can connect PCL and PEG chains together and form the star networks by the photocross-linking, subsequently endowing a wide transition temperature to investigate the TME. More importantly, the TME can be tunable by altering the content and molecular weight of the 4sPCL segment. The mechanism of the TME influenced by the crystallization process of the network was also systemically investigated. Unlike other temperature memory polymers, in this system all of the employed materials and the resultant 4sPCL−PEG polymer network are biocompatible,17−20 and the reaction of the resultant network can be easily manipulated under mild conditiona by simple UV light irradiation.

Scheme 1. Synthetic routes of 4s-PCL−Diols and 4sPCL− CA (A), PEG−CA (B), and c-4sPCL−PEG networks (C)

types of white solid were obtained. Yield: 92% for 4sPCL−CA and 85% for PEG−CA. Synthesis of Photo-Cross-Linked 4sPCL−PEG (c-4sPCL−PEG) Networks. c-4sPCL−PEG networks with different weight ratios and molecular weights of PCL segment were synthesized through 365 nm UV light irradiation for different time period at a distance of 10 cm. To eliminate the photodegradation of the polymer under UV irradiation, 4sPCL−CA and PEG−CA were dissolved in dichloromethane (20%), and the reaction systems were kept under vacuum in ice water for 1 h before UV irradiation. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR of all samples were carried out using a Nicolet 5700 IR spectrometer. All samples were processed as KBr pressed plates at a weight ratio of 0.5− 1%. Samples were heated from 20 to 55 °C and kept at 3 min. Then the temperature decreased to 25 °C, and recorded the infrared spectra between 4000 and 400 cm−1 when reaching each desired temperature after isotherm for 1 min. The 2D correlation infrared spectra analysis was acquired using the software of 2Dshige. Proton Nuclear Magnetic Resonance. 1H NMR spectra of all the polymers were obtained on a Bruker AM-300 spectrometer. CDCl3 was used as solvent. Ultraviolet−Visible (UV−Vis) Spectrophotometer. The UV spectra of the each polymer were recorded on an Ultraviolet−visible (UV−vis) spectrophotometer (UV-2550, Shimadzu, Japan) in the wavelength range 200−400 nm. The Gel Content (Qg) and Swelling Ratio (Qs) of c-4sPCL− PEG Network. Both the gel content and the swelling ration in phosphate buffered saline (PBS) were measured by weighing the samples. Qg was obtained as follows. The samples (m0) were dried at 60 °C under vacuum before the test; then they were immersed in dichloromethane for 12 h and dried at 60 °C for 6 h and further dried under vacuum at room temperature for 20 h (m2). Qs could also be acquired as follows. The samples (m0) were dried at 60 °C under vacuum before the test; then they were immersed in phosphate buffered saline (PBS) at 37 °C for different time (m1). All tests have three parallel samples.

2. EXPERIMENTAL SECTION Materials. ε-Caprolactone (ε-CL) and pentaerythritol were purchased from Aldrich. Stannous chloride (SnCl2), cinnamic acid, N,N-dimethylformamide (DMF) were purchased from Kelong chemical reagent factory in Chengdu (China). Poly(ethylene glycol) (PEG2000) with an average molecular weight (Mn) of 2000 g/mol was purchased from Alfa. Before use, ε -CL and DMF were distilled under a reduced pressure before drying over freshly powdered calcium hydride. Cinnamic acid was recrystallized before use. All other solvents were used as received without further purification. Synthesis of Four Arms Shaped PCL (4sPCL−diols). 4sPCL− diols was synthesized by ring-opening polymerization of ε-CL using pentaerythritol as an initiator and SnCl2 as catalyst according to our previous report.21As shown in Scheme 1A, preweighed ε-CL, pentaerythritol, and SnCl2 were quickly added into a 50 mL flask with a stopcock. The reaction system was kept under vacuum for 3 h and after that was kept at 140 °C for 6 h. The obtained polymer was dissolved in dichloromethane and precipitated in ethanol. The white precipitate was obtained and dried under vacuum at room temperature for 24 h. The molecular weight of 4sPCL−diols was controlled by adjusting the feed molar ratio of ε-CL and pentaerythritol. Table S1 in the Supporting Information summarized the feed ratios of ε-CL, pentaerythritol, and SnCl2 for two kinds of 4sPCL−diols with different Mn. Synthesis of Cinnamon Group Terminated 4sPCL−Diols (4sPCL−CA) and PEG (PEG−CA). 4sPCL−CA and PEG−CA were synthesized through an esterification as displayed in Scheme 1B. In the two esterification reactions, cinnamoyl chloride was added in the system in excess. After the reaction, the solutions were filtered off and the filtrates were concentrated under reduced pressure to obtain a sticky solution. Then the 4sPCL−CA and PEG−CA solution were precipitated in ethanol and ether, respectively. The precipitates were dried under vacuum at room temperature for 24 h, and finally the two 1829

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3. RESULTS AND DISCUSSION Characterization of c-4sPCL−PEG Networks. The c4sPCL−PEG networks were prepared by simply photo-crosslinking of 4sPCL−CA and PEG−CA, which can undergo an [2 + 2] photocycloaddition reaction when irradiated with 365 nm UV light. During this reaction, the double bonds in the cinnamon groups were opened to form a four-membered ring and in turn form a cross-linked network structure. Figure S1 in Supporting Information shows the FT-IR spectra of 4sPCL− CA, PEG−CA, and c-4sPCL−PEG. All those spectra exhibit the characteristic absorptions of CC bonds at 1630 cm−1 ascribed to the cinnamon groups. In the spectrum of the c4sPCL−PEG, the characteristic absorptions of PCL and PEG all remain. The peak at 1750 cm−1 is ascribed to the CO stretching vibration from PCL, the peak at 1100 cm−1 is due to the C−O−C stretching vibration from PEG; in particular, the absorption intensity of CC bond at 1630 cm−1 decreases compared to that of 4sPCL−CA and PEG−CA, indicating that the photocycloaddition reaction occurred after the system was irradiated by UV light. The structure of 4sPCL, 4sPCL−CA, and PEG−CA was further confirmed by 1H NMR as shown in Figure 1. In the 1H NMR spectrum of 4sPCL (Figure 1A) there exist chemical shifts of −CH2− at 4.12 (a), 4.05 (e), 3.62 (e′), 2.28 (b), 1.62 (c), and 1.36 ppm (d), suggesting a pure ester structure of PCL−diols. From the intensity of signals e and e′, the number of repeating units (n) was calculated. Consequently, Mn of 4sPCL could be calculated as shown in Table S1 in Supporting Information. In the 1H NMR spectrum of 4sPCL−CA (Figure 1B), the characteristic chemical shifts of 4sPCL all remain, except the chemical shift at 3.62 ppm, and the chemical shifts at 6.45−6.51 (f), 7.69 (g), and 7.50−7.35 ppm (h) are attributed to CCH−CO, Ph−CHC, and phenyl H atoms, respectively. In contrast to Figure 1A, the appearance of new chemical shift and the disappearance of the peak at 3.62 ppm indicated that 4sPCL−CA was synthesized successfully. In the 1 H NMR spectrum of PEG−CA (Figure 1C), the chemical shifts at 3.65 (g, h, i), 4.41 (f), 6.51 (a), and 7.35−7.69 ppm (b, c, d, e) are attributed to −CH2−CH2−O−CH2−, −CH2− OCO, −CCH−CO, and Ph−CHC, respectively. For the c-4sPCL−PEG network sample, the partial insolubility in CHCl3 solvent indicates that the photo-cross-linking of 4sPCL−CA and PEG−CA was successful. Gel Fraction and Swelling Ratio Characterization of c4sPCL−PEG Networks. After being photo-cross-linked, the star c-4sPCL−PEG networks formed. Table S2 in Supporting Information shows that the gel fraction of all species increases with increasing irradiation time, due to an increase of the degree of photodipolymerization. The marked ratio in parentheses is the weight ratio of 4sPCL and PEG. Additionally, the c-4sPCL(2K)−PEG (1:1) under 3 h irradiation has a lower gel fraction and consequently cannot been selected for the investigation of shape memory effect due to low mechanical property.24 While the c-4sPCL(4K)−PEG (1:1) under 3 h irradiation has the highest gel fraction among these samples. From UV−vis spectra of c-4sPCL(4K)−PEG (1:1) (Figure S2 in Supporting Information), we can find that the cinnamon groups exhibited a maximum absorption at 280 nm and the intensity of the absorption peak decreased with the increasing of irradiation time under the λ = 365 nm UV light, which also indicated that they underwent a [2 + 2] photocycloaddition reaction. The conversion rate of CC bond in the cinnamon

The Qs and Qg were calculated according to the following equations, eqs 1 and 2.

Qg =

m2 × 100% m0

(1)

Qs =

m1 × 100% m0

(2)

The intensity changes of maximum absorption peak at 280 nm could reflect the degree of photo-cross-linking.22 The typical process is investigated as below: polymer/CH2Cl2 solution with a certain concentration, which had been kept under vacuum in ice water for 1 h, was irradiated under 365 nm UV light for 1, 2, and 3 h, respectively, and 2 mL of polymer solution was pipetted at each time point; then the changes of absorption intensity at 280 nm were determined by a UV−vis spectrophotometer. Static Tensile Tests. The static tensile tests were carried out using a universal testing machine Instron 5567 (Instron Co., Massachusetts) with a temperature control device kept a speed of 1 mm/min at room temperature and 37 °C, respectively. Differential Scanning Calorimetry (DSC). Thermal transition temperature were determined by DSC (TA DSC-Q100). The process of heating and cooling were repeated from −10 to +100 °C at a rate of 10 °C min−1, and the DSC curves of the second heating and cooling process were obtained. All data were used from the second heating process. Dynamic Mechanical Analysis (DMA). DMA measurements were used a specimen with size of 10 × 3 × 1 mm (length × width × thickness), performed on a DMA (TA DMA-Q 800) at a heating ratio of 3 °C/min from 0 to 70 °C and at a frequency of 1 Hz. The storage modulus E′ and tan δ were tested. X-ray Diffractometry. XRD (Phlips, X’Pert PRO, Netherlands) was used to determine the crystallization property of 4sPCL, PEG and c-4sPCL−PEG networks. The scanning range is 5−40°. First, samples were heated from 20 to 55 °C, and in the cooling process, the dates were recorded when each desired temperature was reached and was at the isotherm for 1 min. Polarized Microscope. A polarized microscope (XPN203, China) was used to determine the phase of crystallization at a temperature from 80 to 20 °C and to record the photographs when reaching the correct temperature in the cooling process. The Investigation of the Temperature Memory Properties. The temperature memory process of c-4sPCL−PEG networks were measured by using strip-shaped specimens on a DMA (TA, DMA-Q 800), using a controlled force mode according to our designed experiment. A typical investigation procedure was performed as previous report.23 First, the strip shaped polymer was heated to 55 °C for 1 min, and then cooled to the deformation temperature at a rate of 10 °C·min−1. Second, the specimen was strained at a constant stress rate to obtain the temporary shape, marked as ε1, and then cooled to −10 °C and the stress was released and kept there for 10 min, marked as ε2. Finally, the specimen was heated to 55 °C at a temperature ramping rate of 10 °C·min−1 and kept there for 5 min, with the strain marked as ε0. All the process of shape memory were repeated two times. The shape memory fixity ratio (Rf) and recovery ratio (Rr) were calculated according to eqs 3 and 4, respectively, and the instantaneous strain recovery speeds Vr were calculated according to eq 5. ε2 × 100% ε1

(3)

⎛ ε ⎞ R r = ⎜1 − 0 ⎟ × 100% ε1 ⎠ ⎝

(4)

Rf =

Vr = (10 °C·min−1) ×

∂ε × 100% ∂T

(5) 1830

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Static Tensile Properties of c-4sPCL−PEG Network. Good mechanical property is an important factor for polymer materials applied in biomedical devices. All of c-PCL(2K)− PEG samples and the c-PCL(4K)−PEG sample under 1 h irradiation have poor tensile property due to the very low degree of cross-linking (∼21%). Figure S4 in Supporting Information shows that the stress−strain curve of c-4sPCL(4K)−PEG (1:1) under 2 and 3 h irradiation at room temperature and 37 °C, respectively. The polymer exhibited a typical thermoplastic property under 3 h irradiation. At room temperature, the elongation at break increased from 322% to 378%, and the elasticity modulus increased from 2.54 to 6.05 MPa with the increasing of irradiation time from 2 to 3 h. While, at 37 °C, the elongation at break increased from 443% to 466%, and the elasticity modulus increased from 0.82 to 1.12 MPa with the increasing of irradiation time from 2 to 3 h. Moreover, compared with the properties at room temperature, the elongation at break increased but the elasticity modulus decreased due to the better flexibility of polymer chains at 37 °C. The increases of elasticity modulus and elongation at break at different times are mainly ascribed to the increase of the degree of cross-linking. The photo cross-linking degree can be presented by the volumetric cross-link density (n) which can be calculated by the eq 626

n=

E 3RT

(6)

where R is the gas constant, E is the elasticity modulus, and T is the temperature. Table S3 in Supporting Information shows that the n of c-4sPCL−PEG increases with increasing irradiation time, suggesting that more cinnamon groups formed the cross-linking structure. Thermal Properties of the Network. To determine the transition temperature of c-4sPCL−PEG, DSC was tested. Figure 2 shows that DSC curves of four species of c-4sPCL− PEG with different ratios of 4sPCL and PEG against different irradiation time, and the results were also summarized in Table S2. For c-4sPCL(2K)−PEG (1:1), with the increasing of the irradiation time, the temperature of crystallization (Tc) reduced from 20.48 to 18.47 °C, and the crystallized peak shifted from two peaks to a single one (Figure 2A). The crystallization behavior of c-4sPCL(2K)−PEG (2:1) is similar to c-4sPCL(2K)−PEG (1:1) (Figure 2B), and the melting temperature (Tm) decreased from 47.21 to 42.35 °C with the increasing of the irradiation time. There appeared two Tm peaks, one due to the Tm of PEG segments, the other due to the Tm of 4sPCL segments. For c-4sPCL(4k)−PEG (1:1), after 1 and 2 h irradiation, there appear two melting temperatures owning to 4sPCL and PEG segments, respectively (Figure 2C), while after 3 h irradiation, there is only one melting temperature, and a wider high elasticity transition region occurs than that of the species irradiated for 1 and 2 h. This result indicated that the network structure with higher degree of cross-linking could be obtained with the increase of the irradiation time. From Figure 2D, we can find that Tm and high elasticity regions have not obvious changes at various irradiation times, probably due to the fact that the movement of the macromolecule chains is restricted to some extent with the increase of the molecular weight of PCL segments, leading to the preferable photodipolymerization of PCL−CA. Therefore, c-4sPCL(4K)−PEG (1:1) under 3 h irradiation possesses a high elasticity transition region as well as

Figure 1. 1H NMR spectra of 4s-PCL−diols (A), 4sPCL−CA (B), and PEG−CA (C).

group can also be used as the degree of photo cross-linking.25 After 3 h irradiation, the degree (65.32%) of photo-crosslinking could be calculated by determining the changes of the intensity of the absorption peak at 280 nm, which is also matched with the gel fraction in Table S2. As a potential biomedical material, the swelling ratio of c4sPCL−PEG-1:1 in phosphate-buffered saline (PBS) were examined as shown in Figure S3 in Supporting Information. The swelling ratio can be calculated by the eq 2. From the results of Figure S3, we can find that the swelling ratio increased with the immersed time increasing until 7 days (115.18%). The swelling can be ascribed to the fact that the polymer network is an amphiphic structure due to the incorporation of hydrophilic PEG and hydrophobic PCL component. The high swelling in PBS could make the material be degraded easily via a hydrolysis of ester bonds of the polymer network. 1831

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Figure 2. DSC curves of c-4sPCL(2K)−PEG-1:1 (A), c-4sPCL(2K)−PEG-2:1 (B), c-4sPCL(4K)−PEG-1:1 (C) and c-4sPCL(4K)−PEG-2:1 (D) under different irradiation times.

temperature, due to the melting transition of 4sPCL and PEG segments. The continuous increase in tan δ until Tm of PEG and PCL segments, where a sharp increase is observed, is due to the melting transition temperature of 4sPCL and PEG segment, and the decrease of tan δ from ∼50 to 65 °C is due to the melting of the un-cross-linked molecular chains in the networks. Moreover, the SMPs with a higher storage modulus would possess a faster recovery ratio.27 Thus, the species of c4sPCL(4K)−PEG (1:1)-3h could be selected as a temperature memory polymer since it possesses a wide transition temperature. Moreover, after 3 h of irradiation, it had the highest gel fraction than any other species. On the basis of these guidelines, it can be concluded that the polymer possess a good temperature memory property. The Temperature Memory Effect of c-4sPCL(4K)−PEG (1:1)-3h. To study the temperature memory effect, we chose the c-4sPCL (4K)-PEG sample with PCL/PEG weight ratio of 1:1 under 3 h irradiation (c-4sPCL(4K)−PEG(1:1)-3h), whose transition temperature can be tailored over a wide temperature range up to 45 °C. The investigation of TME took place at 37 and 45 °C, respectively, spanning a temperature range near the temperature for biomedical devices application. The stress− strain-temperature curves are obtained from DMA testing. Parts A and B of Figure 4 show the quantitative demonstration of temperature memory properties with the deformation temperature set at 37 and 45 °C for two cycles, respectively. The results indicate that the material possesses a good shape fixity ratio (Rf) and shape recovery ratio (Rr) either Td = 37 or 45 °C. The Rf and Rr were calculated using eqs 3 and 4, and the results were summarized in Table S4 in the Supporting Information.

a broad melting transition region, which can meet the key requirements of temperature memory effect. Dynamic Mechanical Analysis. To further evaluate the transition temperature of these resultant polymers, dynamic mechanical analysis (DMA) was employed. Figure 3 shows the

Figure 3. DMA curves of c-4sPCL−PEG with different compositions.

DMA curves of c-4sPCL−PEG with different ratios of 4sPCL and PEG upon 3 h irradiation. Observed from the curves of storage modulus (E′) and tan δ vs temperature, all species have a wide transition temperature region from 20 to 60 °C, and below room temperature the E′ increases with the increasing of 4sPCL content and its molecular weight, owing to the crystallization of more 4sPCL segments and the higher molecular weight of 4sPCL. There is a decrease of more than 1 order of magnitude for the E′ with the increasing of 1832

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Figure 5. . Rr and Vr vs different temperatures from 10 to 55 °C (A). Shape recovery process with two deformation temperatures (B). Figure 4. Shape fixity and shape recovery process at different deformation temperatures tested with DMA two times. Td = 37 °C (A) and 45 °C (B).

drying oven, it could be observed that the right specimen, which was deformed at 37 °C, could recover its initial shape after 180 s, while the left sample, which was deformed at 45 °C, could not represent the temperature memory effect. Subsequently, the two species were placed in a 45 °C drying oven, and the left specimen was able to recover its initial shape. The result also demonstrated that c-4sPCL(4K)−PEG(1:1)-3h possesses an excellent temperature memory effect at 37 and 45 °C. As a potential biomedical material, the investigation of TME exposure to physiological conditions is necessary. Figure S5 in the Supporting Information shows a macroscopic temperature memory process in PBS at 37 °C for c-4sPCL(4K)−PEG(1:1)3h. A specimen was heated at 55 °C and kept for 2 min, and the specimen was cooled to 37 °C, then the specimen was bended to a spiral shape at 37 °C and moved into a 0 °C ice water for freezing stress and fixing spiral shape for 1 min. Lastly, when the spiral shape specimen was placed in PBS at 37 °C, it could be observed that the specimen recover its initial shape after 60 s. Compared with the shape recovery time at 37 °C in air, the recovery time was shortened in PBS at 37 °C, which is possibly due to the decrease of transition temperature of polymer resulted from the water absorption. Figure S6 in Supporting Information shows the DSC curves of c-4sPCL−PEG before and after immersed in PBS for 1 min. The result also confirmed that there is a pronounced decrease in transition temperature after immersed in PBS for 1 min. The Mechanism of TME for c-4sPCL−PEG Networks. Different from the previously reported temperature memory polymer, the star cross-linked network structure of c-4sPCL(4K)-PEG includes both a broad transition temperature region

Both the Rf and Rr are beyond 80%, indicating an excellent temperature memory function. DMA curves in Figure 5A show two curves of the recovery ratio (Rr) against temperature (T), which possesses two Tds at 37 and 45 °C. The Rr values have pronounced differences with the increasing of temperature. Apart from the outset and the end of the recovery process, the values increased with the temperature increasing from 10 °C to Td. The instantaneous recovery speeds (Vr) were calculated from the results of Rr using the eq 5. Vr, which represents the recovery speed at a certain temperature point, can pronouncedly reflect the strain changes in the process of the shape recovery. In the curves of Vr for Td = 37 °C and Td = 45 °C, two pronounced maximum peaks could be found corresponding to the two Tds (Figure 5A). The result also indicated that the c-PCL−PEG networks possess a good temperature memory effect. And the maximum Vr value increased from 12.1 to 13.7%·min−1 with the Td decreasing from 45 to 37 °C. The macroscopic temperature memory process for c4sPCL(4K)−PEG(1:1)-3h is demonstrated in Figure 5B and Movie S1 in the Supporting Information. Two copies of the sample were heated at 55 °C and kept for 2 min, and the species were cooled to 37 and 45 °C, respectively. Then the species were bent into a spiral shape at 37 and 45 °C, respectively. They were moved into a 0 °C ice water for freezing the stress and fixing the spiral shape for 1 min. Finally, when the two spiral shape specimens were placed in a 37 °C 1833

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Figure 6. XRD patterns of PEG−CA, 4sPCL(2K)−CA, 4sPCL(4K)−CA and c-4sPCL−PEG (A); XRD patterns of c-4sPCL(4K)−PEG-1:1-3h at different temperatures (B); DOC of c-4sPCL(4K)−PEG at determined temperatures (C); polarization microscope (POM) images at selected temperatures (D).

TME.12,13 To demonstrate this, photo-cross-linked 4sPCL was prepared under 365 nm UV light using 4sPCL(4K)−CA sample, which has a DOC value of 40%. From the DSC curve of the c-4sPCL in Figure S7 in Supporting Information, we can find that there is only a melting transition temperature, which is closed to the melting temperature (50 °C) of 4sPCL, at which it displays a shape memory effect in spite of the different deformed temperatures (Figure S8 in Supporting Information). The deformed shape could recover its initial shape only at the temperature near the melting transition, instead of the deformed temperature. Previous studies have reported that polymers with a broadened transition temperature, which contain not a crystalline unit but particular molecular origins or wide relaxation time in those polymer system, can possess the temperature memory effect.10,11 So, from above discussion, we can find that the crystallizable controlling units and a broadened transition temperature are favorable to endow the temperature memory effect to the polymer. The broadened transition temperature provides a wide transition region, while the controlling crystallizable units offer a more precise deformation temperature. To further investigate the mechanism of TME, 2D-FTIR was used to determine the crystallization process of c-4sPCL−PEG networks (Figure 7B). 2D FTIR could provide additional useful information and pertinent characterization of spectral changes.28 Figure 7A shows that the FTIR spectra of the selected c-4sPCL−PEG at different temperatures. There are some small differences among the four spectra, the peak at

and suitable crystallizable units from 4sPCL and PEG components, most probably resulting in the temperature memory effect of c-4sPCL−PEG. To confirm this hypothesis, XRD patterns were first utilized to evaluate the crystallized region of the c-4sPCL−PEG networks. Figure 6A shows the XRD spectra of PEG−CA, 4sPCL(2K)−CA, 4sPCL(4K)−CA and c-4sPCL−PEG(1:1). The peaks at 2θ = 19.3° and 23.4° were attributed to the PEG block, while the peaks at 2θ = 21.6°, 22.1° and 23.8° were attributed to 4sPCL segments. Figure 6B shows the XRD spectra of c-4sPCL(4K)−PEG (1:1) under different temperatures. The degree of crystallinity (DOC) was calculated using Jade 5.0 software and summarized in Figure 6C. The DOC of 4sPCL increased with the increasing of the length of 4sPCL block. The low DOC for c-PCL(2K)-PEG is not enough as the controllable crystalline unit to influence the temperature memory effect of polymer.11 In the process of temperature memory, the DOC increased with the decreasing of temperature. The DOC values at Td depend on the deformed temperature, and they were 18.1% and 25.3% at Td = 45 and 37 °C, respectively. The DOC (36.8% and 37.3%) at 25 °C is slightly lower than that (40.4%) of the initial sample. Additionally, the polarization microscope (POM) images in Figure 6D show the crystalline state of the c-4sPCL(4K)−PEG (1:1) at different temperatures. The POM images also verify the change of the DOC valves in the process of the temperature memory. The polymer only with crystallizable units is failed in investigating the TME of polymer,10,11 and thus a broadened transition temperature is a necessary factor to investigate the 1834

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Macromolecules

Article

Figure 7B1 shows that the synchronous and asynchronous correlation spectra of CO stretching region of c-4sPCL− PEG. Generally, the obtained 2D FTIR results are always average results. The synchronous spectra shows two regions at 1735 cm−1, 1735 cm−1 and at 1725 cm−1, 1725 cm−1 assigned to amorphous phase and crystalline phase CO stretching vibration, respectively.29 There are two types of CO bands, which is agree with the results of FTIR spectra (Figure 7A). The asynchronous image shows a cross peak at 1735 cm−1, 1725 cm−1; this cross peak indicates that the intensity changes of the two dynamic bands at 1735 and 1725 cm−1 can provide the sequential order between the two peaks,30 and according to the previous reports by Noda,30 the band at 1735 cm−1 varies prior to the band at 1725 cm−1. 2D-FTIR spectra between different ranges could provide the relationship information for different chemical groups in the polymer. Figure 7B2 shows the synchronous and asynchronous 2D-correlation spectra of c-4sPCL−PEG in the range of 1380− 1240 and 1280−1170 cm−1. And from the analysis of the asynchronous correlation spectra in Figure 7 B2, some cross peaks are found. The positions at 1236 cm−1, 1365 cm−1 and at 1193 cm−1, 1295 cm−1 are negative, and the position at 1236 cm−1, 1295 cm−1 is positive. However, the cross peaks are positive at the synchronous image. The bands at 1295 and 1193 cm−1 are assigned to the CH2 vibration absorptions of the crystalline PCL segment, and the band at 1236 cm−1 belongs to the CH2 vibration of PEG segment. The sequence order of conformation transformation in those bands could be obtained: 1193 cm−1 > 1295 cm−1 > 1236 cm−1 > 1365 cm−1.31 The 2D-FTIR spectra of the CO stretching region is shown in Figure 7B3. Similar to Figure 7B3, Figure 7 B3 shows that the synchronous and asynchronous 2D-correlation spectra of c-PCL−PEG in the ranges 1400−1100 and 1800−1650 cm−1. The information on the relationship between different chemical groups could be gained from the selected various ranges. In the synchronous spectra, the position at 1725 cm−1 is negative, while the position at 1735 cm−1 is positive. And from the asynchronous spectra, based on prior reports,30,32 we inferred that a bond split occurs in the CO stretching. Moreover, the transition sequence at 1725 and 1735 cm−1 is ahead of the range at 1400−1100 cm−1. From the sequence of conformation transformation, we can draw a conclusion: first, the crystallizable 4sPCL segment forms a regular structure; later, the noncrystallizable PEG segment is separated from the crystallizable 4sPCL regions to form a tighter structure. The crystallization process and the different crystalline properties of c-PCL−PEG under different temperatures indicate that the polymer possessed a different crystalline state. 2D-FTIR results verify the crystallization process of cPCL−PEG theoretically. Therefore, from the results of XRD and 2D-FTIR, we can find that in this system the temperature memory is actually able to remember various crystalline states at different temperatures, which is similar to the previous reports.10,11 On the basis of the above results, the schematic model of how the crystallizable chains of the star networks under different temperatures were used to influence the temperature memory effect was constructed (as shown in Scheme 2). In this polymer network, there are two kinds of crystalline segments, and at the high temperature of above 55 °C, all of the crystallization chain and amorphous chain show freedom of movement. With the decreasing of the temperature to the deformation temperature of 37 or 45 °C, there is partial crystallization, and there is a

Figure 7. The FTIR spectra of c-4sPCL(4K)-PEG-1:1−3h at different temperatures (A); the synchronous (left) and asynchronous (right) 2D infrared correlation spectroscopy of the sample (B) at 1800−1700 cm−1 (1), 1380−1240 and 1280−1170 cm−1 (2), and 1400−1100 and 1800−1650 cm−1 (3).

1725 cm−1 assigned to the CO stretching vibration of high crystalline state, while the peak at 1735 cm−1 assigned to the CO stretching vibration of amorphous state. 28 The absorbance peak at 1365 cm−1 is attributed to −CH2−, the peak at 1295 cm−1 is assigned to the crystalline PCL phase.29 and the peak at 1168 cm−1 is assigned to C−O−C stretching of the amorphous 4sPCL phase. The FTIR spectra indicate that the polymer possessed different crystallization states at different temperatures. 1835

dx.doi.org/10.1021/ma4023229 | Macromolecules 2014, 47, 1828−1836

Macromolecules

Article

Natural Science Foundation of China (Nos. 51173150, 51373138), Research Fund for the Doctoral Program of Higher Education of China (20120184110029), and Fundamental Research Funds for The Central Universities (SWJTU11ZT10).

Scheme 2. Schematic of the Variation of the Crystalline Unit in the Star Polymer Networks at Different Temperatures in the Process of Temperature Memory

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different DOC at different temperatures. Subsequently, with the decreasing of temperature to the low temperature of below 20 °C, the polymer possesses a good crystalline characterization. The tunable crystallite segment acts as the reversible phase, and the photo-cross-linked region acts as the fixed phase.

4. CONCLUSIONS In summary, we have designed a new strategy for achieving highly responding temperature memory polymer networks through simple photo-cross-linking with cinnamon groups. In this system, the star polymer network was composed of a widely high elasticity transition region owing to the photo cross-linking and a partly overlap melting transition temperature due to the melting transition of PCL and PEG. The results of Rr and Vr against temperature indicated that the polymer possess a good temperature memory effect, and there is maximum recovery speed at the deformed temperature. The temperature memory of the star polymer networks is actually to remember various crystalline states at different temperatures.

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ASSOCIATED CONTENT

S Supporting Information *

Data for the two kinds of 4sPCL−diols (Table S1)m FTIR of the 4sPCL−CA, PEG−CAm, and c-4sPCL−PEG (Figure S1), UV−visible spectra of c-4sPCL(4K)−PEG (Figure S2), mechanical properties of c-PCL(4K)−PEG at room temperature and 37 °C (Table S3), shape fixity ratio and shape recovery ratio for two cycles with different Tds (Table S4), swelling ratio of c-4sPCL−PEG-1:1 in PBS at 37 °C (Figure S3), stress−strain curves of c-4sPCL(4K)−PEG-1:1 (Figure S4), TME in PBS at 37 °C (Figure S5), DSC curves of c4sPCL(4K)−PEG-1:1 before and after immersed in PBS for 1 min (Figure S6), DSC curves of 4s PCL−diols and c-4sPCL (Figure S7), and shape memory process of photo-cross-linked 4sPCL (Figure S8) and a movie (Movie S1) showing the process of temperature memory effect (×3 speed). This material is available free of charge via the Internet at http:// pubs.acs.org/.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(S.Z.) E-mail: [email protected]; shaobingzhou@ swjtu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by National Basic Research Program of China (973 Program, 2012CB933600), National 1836

dx.doi.org/10.1021/ma4023229 | Macromolecules 2014, 47, 1828−1836