Poly(tetramethylene oxide)glycol Multiblock Copolymers - American

Article pubs.acs.org/JPCC

Unique Multifunctional Thermally-Induced Shape Memory Poly(p-dioxanone)−Poly(tetramethylene oxide)glycol Multiblock Copolymers Based on the Synergistic Effect of Two Segments Jingjing Zhang, Gang Wu, Caili Huang, Ying Niu, Chao Chen, Zhongtao Chen, Keke Yang,* and Yuzhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China ABSTRACT: Biodegradable and biocompatible poly(p-dioxanone)−poly(tetramethylene oxide)glycol (PPDO−PTMEG) multiblock copolymers with excellent shape memory effect and mechanical properties were developed by coupling PPDO-diol and PTMEG-diol with 1,6hexamethylene diisocyanate (HDI). The influences of chain structure, microphase separating morphology on shape memory effect, and biocompatibility were investigated systematically. The TEM observation of PPDO−PTMEG poly(ether-ester)-polyether exhibited the relationship between characteristic microphase separating morphology and shape memory effect. Interestingly, differential scanning calorimetry (DSC) analysis indicated that Tg,PPDO was close to Tc,PTMEG, which provided the possibility that the reversible phase combined PTMEG segments and the amorphous phase of PPDO segments. The synergistic effect ensured copolymers owning both high Rf and Rr (96.6% and 92.9% with PTMEG segments only 15% in weight fraction), as well as the mechanical properties when T > Ttrans. Moreover, biocompatible evaluation results showed that the cell adhesion and proliferation could be enhanced on copolymers with more PTMEG segments, indicating potential application in minimally invasive surgery.

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INTRODUCTION As a most promising intelligent material, shape memory polymers (SMPs) have the capacity of changing shape in response to various external stimuli.1 This characteristic offers SMPs a wide range of applications, especially in biomaterials,1−7 such as smart medical devices, implants for minimally invasive surgery,7−9 sensors and actuators.10,11 In order to fulfill complex requirements, multifunctional SMPs, that is polymers combing multi functions,12,13 are desired, for example, remarkable mechanical performance, biodegradability, drug release and biocompatibility as well as possessing shape memory. Up to now, the previous studies on multifunctional shape memory polymers used as biomaterials have mainly concentrated on thermotropic polymers, such as polyurethanes (PU), biobased polymers, and their copolymers.14−17 Those shapes are driven by surpassing a specific transition temperature, Ttrans.18−20 The Ttrans can either be a glass transition temperature (Tg)21,22 or a melting temperature (Tm)23 of soft segments. Whereas Tm is generally preferred because the melting transition is sharper than the glass transition, at temperature Tm the shape recovery that takes place can be better defined. Various kinds of multifunctional SMPs with crystallizable switching segments have been reported.5,24,25 All of those designs aim to achieve good shape memory effects and biomedical properties. However, as Ttrans = Tm style SMPs, they also show some downsides that significantly limit their application, for example, poor mechanical properties, especially when the temperature is above Tm. In the case of the SMPs of poly(glycerol− sebacate) tensile strength below 0.5 MPa was exhibited;26 © 2012 American Chemical Society

poly (p-dioxanone)−poly(ε-caprolactone) storage modulus determined by DMTA was in the range of 170 MPa at 25 °C, but that was lowered to 60 MPa when the temperature was up to 55 °C (higher than the Tm of the soft segments) even with a mass fraction of soft segments containing 33%,27 coupling with other SMPs.23,28 It is likely to be an unavoidable fact for SMPs that Ttrans is based on Tm. This pronounced drop of the storage modulus would make it hard to form various shapes for the SMPs with a high proportion of soft segments. However, if we depress the soft segments content, it would somewhat sacrifice the fixity of SMPs. This embarrassment would limit the application of this type of SMPs, so a new type of multifunctional SMPs containing excellent shape memory and mechanical properties will be required. Liu et al. presented enhancing mechanical properties by the synergistic effect of the star polymer and polymer network;29 however, it cannot be neglected that the more complex chain structure of those systems will affect the crystallization behavior of crystallizable segments significantly. In this work, we aimed to design a new type of thermally induced shape memory polymer with remarkable biodegradability, biocompatibility, and suitable mechanical properties in the whole programmed temperature range. For this purpose, a novel shape-memory PPDO−PTMEG multiblock copolymer with poly(tetramethylene oxide)glycol (PTMEG) as soft segments was synthesized, considering that not only PPDO30,31 Received: December 12, 2011 Revised: February 8, 2012 Published: February 10, 2012 5835

dx.doi.org/10.1021/jp211953q | J. Phys. Chem. C 2012, 116, 5835−5845

The Journal of Physical Chemistry C

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and PTMEG32,33 have outstanding biocompatibility, making them good candidates for biomaterials, but also PPDO exhibits good mechanical performance with high tensile strength and excellent flexibility. More important, the Tg of PPDO segments (Tg,PPDO) in this multiblock copolymer is close to the crystallization temperature of PTMEG segments (Tc,PTMEG), which allows the amorphous phase of the PPDO segments, contributing to shape fixing. We take advantage of the unique character of those multiblock copolymers to reduce the ratio of PTMEG segments to a low content to keep excellent mechanical properties, without sacrificing the shape fixity ratio (Rf). This is not consistent with the reported copolymers: one component determines the permanent shape, and the other responds to the fixation of temporary shape.20 The synergistic effect of two segments in this multiblock copolymer may get a fast response in SMPs with a significant enhancement in mechanical properties, especially Ttrans = Tm style SMPs.

another 2 h under vacuum. Purged three times with dry nitrogen and kept with a nitrogen stream, the reactor was immersed into a preheated oil bath (T = 150 °C) with persistent stirring, and a predetermined amount of HDI was injected to actuate the coupling reaction as soon as the reactants were molten completely. After 1.5 h, crude products were purified by dissolving in phenol/1,1,2,2-tetrachloroethane (1:1 V/V) and then precipitated in excessive methanol. For comparison, PPDO-H and PTMEG-H with similar molecular weight were prepared by following the same procedure from its own prepolymer diol with HDI, respectively.35 Characterization and the Shape Memory Effect Testing. 1H NMR spectra were obtained at 400 MHz using a Bruker 400 (Bruker, Switzerland), with deuterated chloroform as solvent. FTIR spectral analysis was carried out in a range of wavenumbers from 4000 to 700 cm−1 with a Fourier Transform Infrared Spectrometer (Nicolet 6700, USA). The intrinsic viscosity ([η]) of the resulting polymers was measured in phenol/1,1,2,2-tetrachloroethane (1:1 v/v) solution using an Ubbelohde viscometer maintained at 30 °C. The determination of the dynamic mechanical properties was conducted on DMA Q800 (TA Instruments, USA), with the sweep mode of 3 K/min from −110 to 80 °C, the amplitude of 0.2% at a frequency of 1 Hz, and a static force of 0.1 N. Differential Scanning Calorimetry (DSC) was performed with DSC-Q200 (TA Instrument, USA), over the temperature ranges −50 °C∼140 °C at a heating rate of 10 °C/min, and purged with nitrogen gas. TEM images were acquired with a Tecnai G2F20 S-TWIN electron microscope (FET, Holland) at an acceleration voltage of 200 kV. For these measurements, droplets of uranygl acetate were deposited onto the cast copper grids and then dried under air; excess solvent was wicked away by filter paper.36 The shape memory effect (SME) was conducted on a bending test according to Liu et al.37 This averaged results of three parallel specimens were tested under the same conditions. The program of the SME test was conducted as follows: first, the specimens (80 mm × 6 mm × 1 mm or 80 mm × 6 mm × 0.5 mm) were heated to Tm + 20 °C and kept for 10 min before they were bent to a given angle θmax; subsequently, they were quenched to a low temperature Tlow < Tm (Tlow = Tc or −20 °C) and kept for another 10 min under the bent condition; after that, the external force was released, and the samples turned to an angle θfixed; finally, each specimen was heated to Tm + 20 °C for 10 min. When the recovery ended, the angle θfinal was recorded. The Rf (shape fixity ratio) and Rr (shape recovery ratio) were determined according to the following equations

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EXPERIMENTAL SECTION Materials. Dihydroxyl-terminated poly(tetramethylene oxide) glycol (PTMEG-diol) (Mn = 2900 g/mol, Reagent grade) was purchased from Aldrich Co., and p-dioxanone (PDO) was provided by the pilot plant of the center for degradable and flame-retardant polymeric materials (Chengdu, China). 1,6Hexamethylene diisocyanate (HDI) (AR grade) from SigmaAldrich was used without further purification. 1,1,2,2-Tetrachloroethane and phenol with A. R. grade were supplied by Kelong Reagent Corp. (Chengdu, China). Preparation of PPDO−PTMEG Multiblock Copolymers. The poly(p-dioxanone)diol (PPDO-diol) prepolymer was synthesized by bulk polymerization of PDO according to our previous work.34 PPDO−PTMEG multiblock copolymers with different compositions were prepared by coupling the PTMEG-diol and PPDO-diol with HDI (Scheme 1). The Scheme 1. Synthetic Route to the PPDO−PTMEG Copolymers

θ R f = fixed × 100% θmax

Rr =

(θfixed − θfinal) × 100% θfixed

Culture of L929 Fibroblasts on PPDO−PTMEG Films. PPPDO−PTMEG films were cut into squares (10 mm × 10 mm × 0.5 mm) to locate into 24-well culture plates. All the samples were sterilized in 75% ethanol for 24 h and then exchanged with phosphate buffered saline (PBS) for another 24 h, followed by removal of the PBS, sterilization by O3, and UV irradiation for 30 min, respectively. This was repeated three times. The L929 fibroblasts (State Key Laboratory Of Oral Diseases, China) were seeded in each well (with or without PPDO−PTMEG films) with a density of 5 × 104 cells per well in 1 mL of Dulbecco’s Modified Eagle’s Medium (DMEM, Invitrogen Corporation USA, supplemented with 10% calf bovine serum, 2.38 mg/mL of HEPES, 0.219 mg/mL of

reaction was performed in a glass reactor. First, PTMEG-diol was charged and dried under vacuum at 100 °C for 2 h, and then PPDO-diol was added after cooling to 60 °C and kept 5836



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Table 1. Composition and Mechanical Properties of Copolymersa samples

Mn(PTMEG‑diol) (g/mol)

Mν(PPDO‑diol) (g/mol)

PPDO-diol content (wt %)

[η] (dL/g)

E′1 (MPa)

E′2 (MPa)

E′3 (MPa)

PPDO-H PPDO9.9−PTMEG2.9-95/5 PPDO9.9−PTMEG2.9-85/15 PPDO9.9−PTMEG2.9-75/25 PPDO12.6−PTMEG2.9-75/25 PPDO12.6−PTMEG2.0-95/5 PPDO12.6−PTMEG2.0-85/15 PPDO12.6−PTMEG2.0-75/25 PPDO19.7−PTMEG2.9-85/15 PPDO19.7−PTMEG2.9-75/25 PPDO19.7−PTMEG2.0-85/15 PTMEG-H

 2900 2900 2900 2900 2000 2000 2000 2900 2900 2000 2900

9900 9900 9900 9900 12600 12600 12600 12600 19700 19700 19700 

100 95 85 75 75 95 85 75 85 75 85 0

1.51 1.76 1.62 1.75 1.98 1.7 1.5 1.63 1.42 2.31 1.68 1.18

5768.2 5336.6 5144.3 4789.8        

1835.1 1922.8 1667.9 1448.9        

419.4 377.5 307.9 287.2        

In the samples ID, PPDOa−PTMEGb-c/d represent the chain length of prepolymers and the feed ratio of PPDO-diol/PTMEG-diol (wt %/wt %), with a × 103 = the viscosity-average molecular weight of PPDO-diol, b × 103 = the number-average molecular weight of PTMEG-diol, and c/d = the feed ratio of PPDO-diol/PTMEG-diol (wt %/wt %). E′1, E′2, and E′3 are the storage modulus determined from DMA corresponding to −20 °C, 0 °C, and 46 °C, respectively. In view of the copolymers, there is poor solubility in the solvents commonly employed to perform gel permeation chromatography (GPC) at ambient temperatures, and GPC traces can not be obtained. a

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100 units/mL of penicillin, and 100 μg/mL of streptomycin) at 37 °C under 5% CO2 and 95% relative humidity atmosphere. After cell culturing for 2, 4, and 6 days, respectively, the viability and proliferation of the L929 fibroblasts were determined by an MTT assay. First, we removed the cell-culture medium by a pipet and then added 1 mL of fresh medium and 40 μL of MTT solution (5 mg/mL, Lvshengyuan Biotechnology) for each well, incubated at 37 °C in 5% CO2/ air for 3.5 h. After that, the culture medium was carefully removed, and dimethyl sulfoxide was added (DMSO, 440 μL/well). Then, shaking the plates for 30 min, allotted the solubilized dye into 96-well plates for 140 μL/well. The absorbance of the solubilized dye, which correlates with the number of living cells, was measured at 570 nm by a microplate reader (Thermo Varioskan Flash, Thermo, USA). For each sample, the final absorbance was the average of those measured from six wells in parallel. Data were represented as the mean ± SD. Cell morphology was investigated by SEM. After L929 fibroblast cell culturing in PPDO−PTMEG films for 24 h,38 the films were gently washed with PBS three times, located in glutaraldehyde solution (2.5−3%) at 4 °C for 2 h, and then sequentially dehydrated in 30%, 50%, 75%, 85%, 95%, and 100% ethanol each for 15 min, following added isoamyl acetate for 15 min. Those films were coated in vacuum with gold and observed by scanning electron microscopy (JSM-5900LV electron microscope JEOL Co. Japan). In Vitro Degradation of PPDO−PTMEG Multiblock Copolymers. The specimens were prepared by hot pressing and cut into 10 mm × 10 mm × 1 mm, and the samples were immersed in 30 mL of phosphate buffer solution (PBS) at 37 °C, pH 7.4. At preset intervals, the pH and the mass remaining were measured. The polymer mass remaining (%) was determined as follows

RESULTS AND DISCUSSION Synthesis and Structural Analysis of PPDO−PTMEG Multiblock Copolymers. A series of PPDO−PTMEG multiblock copolymers, PPDO-H and PTMEG-H, were prepared via a two-step process as mentioned in the Experimental Section. The molecular weight of prepolymers, feed ratio, and viscosity-average molecular weight ([η]) of the resulting products are listed in Table 1. It shows that the multiblock copolymers have considerably high molecular weight compared with that of prepolymers, suggesting that HDI is a highly efficient chain extender to coupling these two diol prepolymers.35 1 H NMR was employed to characterize the chemical structure of PPDO-diol, PTMEG-diol, and PPDO−PTMEG multiblock copolymers (Figure 1). For PPDO-diol(a), the resonances

L-glutamine,

mass remaining (%) =

Figure 1. 1H NMR spectra of PPDO-diol (9900 g/mol), PTMEG-diol (2900 g/mol), and PPDO9.9−PTMEG2.9-85/15.

at 4.16 (δHa), 3.78 (δHb), and 4.33 (δHc) ppm are assigned to the three methylene protons in PPDO-diol repeat units, respectively. Signals that appear at 3.69 (δHb′) and 3.61 (δHc′) ppm should be attributed to the corresponding protons in the terminal group of PPDO-diol. For PTMEG-diol(b), the peak positions corresponding to the methylene protons of repeat units situate at 1.61 (δHi) and 3.41 (δHj), and the protons in the terminal group exhibit at 1.66 (δHi′) and 3.58 (δHj′) ppm.

Wr × 100 Wi

Wi and Wr are the weight of samples before and after the in vitro degradation with the constant weight at room temperature, respectively, and the pH of all was measured using Lei Ci pHS-3C (China). 5837



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As far as the multiblock copolymers, taking PPDO9.9− PTMEG2.9-85/15 for example, the characteristic shifts belonging to PTMEG-diol and PPDO-diol repeat units still exist, while the signals belonging to the end groups of the two prepolymers could not be found. The shifts of three kinds of methylene protons of HDI are observed at 1.50 (δHg), 3.16 (δHh), and 1.34 (δHf) ppm, indicating that the chain-extending reaction sufficiently proceeded. The FTIR spectra of the prepolymers and the multiblock copolymer are represented in Figure 2. The band at about 3480 cm−1

DSC was employed to record the heat flow of the samples with different compositions and PPDO-diol chain length, undergoing crystallization and melting with a controlled temperature program. Figure 3(a) shows the cooling run from melt with a rate of 10 °C/min after erasing the thermal history, and Figure 3(b) illustrates the following heating run with the same scanning rate. The relevant information is summarized in Table 2. PPDO-H conducts a glass transition at −12.1 °C, an exothermic cold crystallization peak (Tc,PPDO) about 43.6 °C, and the corresponding melting peak (Tm,PPDO) at 94.3 °C. Otherwise, PTMEG-H acts at a crystallization peak (Tc,PTMEG) of −11.4 °C and a melting peak (Tm,PTEMG) of 26.2 °C. As for multiblock copolymers, it is noteworthy for two interesting phenomena. First, in the cooling run, since the crystallization kinetics of PPDO segments is not so fast,39 there is only one exothermic crystallization peak of PTMEG segments ranging from −6.5 °C to −9.8 °C, which overlaps the Tg,PPDO. The Tg,PPDO is only detected in the subsequent heating trace at about −12 °C. Second, the Tc,PPDO of the multiblock copolymers shifts to lower temperature after introducing PTMEG segments into the system. Consequently, the endothermic melting peak of PTMEG segments is masked by the cold crystallization peak of the PPDO segments, owing to that the Tm,PTEMG is very close to the Tc,PPDO of the multiblock copolymers. On the basis of this fact, it is easy to explain why the values of ΔHm,PPDO coincide with the plus of ΔHc,PPDO and ΔHc,PTMEG. The separate crystalline and melting peaks as well as the unchanged Tg indicate the immiscibility of the two segments.39 Shown in Figure 4 is the crystallization and melt behavior of the PPDO−PTMEG containing different PPDO-diol chain lengths. With the increase of PPDO-diol chain length, the crystallinity of PTMEG segments decreases gradually (the cooling run in Figure 4(a)); meanwhile, the cold crystallization temperature of PPDO segments decreases, Tm,PPDO, and the value of ΔHm,PPDO increases (the subsequent heating run in Figure 4(b)). This phenomenon reveals that increasing the molecular weight of PPDO-diol will enhance the crystallization ability of PPDO segments but restrain the crystallization of PTMEG segments. This may be a consequence of morphological and regularity of chains:39 the longer the PPDO-diol is, the less physically constrained PPDO segments crystallization, but more for PTMEG segments. The influence of these

Figure 2. FTIR spectra of PPDO-diol (9900 g/mol), PTMEG-diol (2900 g/mol), PPDO-H and PPDO9.9−PTMEG2.9-85/15.

in the spectra of PPDO-diol and PTMEG-diol may ascribe to the −OH vibration. For PPDO-H and the multiblock copolymers (e.g., PPDO9.9−PTMEG2.9-85/15), however, the absorption band of the −OH is not found, and the absorption band at 2200 cm−1 of the −NCO− stretching disappeared, instead of the peak of the N−H vibration around 1540 cm−1. It confirmed again that isocyanate groups of HDI have been almost reacted with the −OH of the prepolymers. Both 1 H NMR and FTIR analysis demonstrated the successful synthesis of multiblock copolymers. Thermal Transition and Stress−Strain Behavior. Thermal properties of the PPDO-H, PTMEG-H, and multiblock copolymers were investigated by DSC and DMA. Here,

Figure 3. DSC thermograms of PPDO-H, PTMEG-H, and copolymers. (a) Cooling run after melt quenching at a rate of 10 °C/min. (b) Subsequent heating run at a rate of 10 °C/min. 5838



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Table 2. Results from DSC Cooling and Second Heating Scans of Copolymers, PPDO-H, and PTMEG-Ha cooling scan

a

second heating scan

sample

Tc1 (°C)

ΔHc1 (J/g)

Tg2 (°C)

Tm1 (°C)

ΔHm1 (J/g)

Tc2 (°C)

ΔHc2 (J/g)

Tm2 (°C)

ΔHm2 (J/g)

PPDO-H PPDO9.9−PTMEG2.9-95/5 PPDO9.9−PTMEG2.9-85/15 PPDO9.9−PTMEG2.9-75/25 PTMEG-H

 −7.2 −6.5 −9.8 −11.4

 6.1 11.7 16 56.0

−12.1 −12.3 −12.3 −12.1 

    26.2

    56.8

43.6 27.9 27.3 29.6 

50.36 36.1 26.3 16.9 

94.3 93.2 92.5 92 

50.3 44.2 38 32.2 

1 is the PTMEG segment, and 2 is the PPDO segment.

Figure 4. DSC thermograms of copolymers with the same soft segment molecular length and content but different PPDO-diol length. (a) Cooling run after melt quenching at rate of 10 °C/min. (b) Subsequent heating run at rate of 10 °C/min.

characteristics on the shape memory effect will be discussed later. The storage modulus (E′) determined from DMA at three typical temperature (−20, 0, and 46 °C) is recorded at Table 1. With increasing PTMEG segments content, in all temperature ranges, the E′ value of the samples decreases. At −20 °C, lower than the Tg,PPDO, PPDO-H shows an E′ value at 5768.2 MPa, while PPDO9.9−PTMEG2.9-75/25 is about 4789.8 MPa. When the temperature rises to 0 °C, higher than Tg,PPDO, the amorphous of PPDO segments turns to a rubber-elastic state, causing a sharp drop in E′, 1835.1 MPa for PPDO-H and 1448.9 MPa for PPDO9.9−PTMEG2.9-75/25, respectively. While the temperature increases to 46 °C, higher than Tm,PTMEG, PTMEG segments exhibit a viscous state, resulting in the E′ of PPDO9.9− PTMEG2.9-75/25 further declining to 287.2 MPa. However, for containing less soft segments, E′ still remains a quite high value compared with the reported SMPs.23,25−28 Traditionally, SMPs including less soft segments would lead to a poor shape memory effect, such as 5% or 15% in weight fraction.5,23,40 However, that phenomenon will not appear in the PPDO− PTMEG multiblock copolymers for its unique synergistic effect discussed below. Shape Memory Behavior. To explore the relationship among structure, microphase morphology, and shape memory behavior, systematical investigations were performed concerning the influence of different factors on the Rr and Rf, such as the composition of the multiblock copolymers, the chain length of soft and hard segments, and their micromorphology of samples. Figure 5 portrays the relationship of shape memory properties and the composition of the multiblock copolymers. Obviously, all the specimens (80 mm × 6 mm × 1 mm) show excellent shape memory properties (Rf is more than 93%, and the Rr

Figure 5. Effect of hard segments content in the shape fixity and recovery.

ranges between 87.3% and 96.4%). The Rf increases with the addition of PTMEG segments. However, in the case of the Rr, it reduces. For a Ttrans = Tm SMP, commonly, crystallization of the soft segments dominates the shape fixing, while the crystallization of hard segments manages the shape recovery according to the previous investigations.5,17,41 From DSC analysis, the crystallization ability of each segment could be enhanced when its contents increase. The stationary phase can prevent the deforming forces from causing viscoelastic deformation; therefore, in these multiblock copolymers, the Rr value increases as PPDO segment content increases.42 Here, an aspect that cannot be ignored is that the PPDO-H shows shape memory properties itself, which is similar with the PLLA.43,44 The crystalline phase and the amorphous 5839



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Figure 6. Recovery process of PPDO9.9−PTMEG2.9-85/15 from the different cooling temperature. A, −20 °C; B, −6.5 °C.

the original shape was recovered. However, when the cooling temperature was at −6.5 °C, it ws above the Tg of PPDO segments. The amorphous phase of PPDO segments always stayed in the elastomeric state; therefore, it would not contribute to the shape fixity (Figure 8). In this situation, the shape fixity only depended on the crystallization of PTMEG segments. Spontaneously, the Rf is lower than the former case, particularly the copolymer with lower mass fraction of soft segments. Effects of chain length as well as the micromorphology on the SME are also considered. Two different series of multiblock copolymers were designed and well prepared: (1) multiblock copolymers with the same PTMEG-diol length (Mn,PTMEG‑diol = 2900 g/mol) but different PPDO-diol chain length (Mν, PPDO‑diol = 9900, 12 600, 19 700 g/mol) corresponding to PPDO9.9− TMEG2.9-75/25, PPDO12.6−PTMEG2.9-75/25, and PPDO19.7− PTMEG2.9-75/25, respectively; (2) multiblock copolymers with same PPDO-diol chain length (Mν,PPDO‑diol = 19 700 g/mol) but various PTMEG-diol chain lengths (Mn,PTMEG‑diol = 2000, 2900 g/mol) coded as PPDO19.7−PTMEG2.0-85/15 and PPDO19.7−PTMEG2.9-85/15, respectively. Figure 9 compares the SME of samples in series one. It can be found that the Rf decreases but Rr increases with elongating PPDO-diol chain length. According to the results of DSC analysis above (Figure 4), the crystallization ability of PTMEG segments is weakened by increasing the PPDO-diol chain length, which leads to a decrease in Rf. Otherwise, the crystallization ability of PPDO segments is enhanced, which benefits Rr. A similar phenomenon is observed at the SME test in series two. With the same PPDO-diol chain length, the crystallization ability of PTMEG segments is improved by increasing its chain length.45 Accordingly, the deformation fixation ratio is enhanced (Figure 10).5,23 In addition, the deformation temperature is Tm + 20 °C for each sample, namely, 40 °C for PPDO19.7− PTMEG2.0-75/25 and 46 °C for PPDO19.7−PTMEG2.9-75/25. As previously discussed, the increasing chain mobility of PPDO segments makes the deformation easily and leads to an increase in Rf. However, the Rr is in the opposite direction, which should be attributed to the generation of more irreversible deformation. Shape memory behavior is a result of the appropriate structure/ morphology combination; even more important, it would depend on the special microstructure of the SMPs,22,45 which possesses incompatible hard and soft segments distributed at a molecular level to form a microphase separation.42 The characteristic micromorphology of the multiblock copolymers could be elucidated with the help of the TEM (Figure 11). The PTMEG domains act as the dispersed phase (not stained) spread into the continuous phase (stained) of PPDO domains. All samples display the microphase separation morphology, but the phase dimension and microstructure vary with different

phase of PPDO-H may be responsible for the hard and soft segments, respectively. Although its performance in shape memory effect is not prominent, this feature will help us to understand why the PPDO−PTMEG multiblock copolymers show outstanding shape memory performance. Just as we mentioned above, the Tg,PPDO is very close to the Tc,PTMEG from the DSC measurement, so the amorphous phase turns to a glassy state; meanwhile, PTMEG segments crystallize at the fixing temperature. In conclusion, both PPDO amorphous domains and PTMEG domains act as the reversible phase in this SMP and dominate the shape fixity. To verify this view, the multiblock copolymer with PPDO segment content of 85% (wt %) and PPDO-H were chosen for the SME test, programming with different fixing temperatures at −20 °C and −6.5 °C (Figure 6). Since −20 °C is lower than Tg,PPDO and Tc,PTMEG, −6.5 °C is the Tc,PTMEG of PPDO9.9− PTMEG2.9-85/15, but higher than Tg,PPDO. Figure 7 represents

Figure 7. Shape fixity in −20 °C and −6.5 °C for the PPDO segment content of 100% and 85%.

that the Rf drops by a big margin from 91.9% in −20 °C to 69.3% in −6.5 °C for PPDO9.9−PTMEG2.9-85/15 and from 86.8% to 58.2% for the PPDO-H. When the samples were bathed in 46 °C, the amorphous phase of PPDO segments exhibited elastomeric state and PTMEG segments in viscous state, so they were easy to deform. When the temperature decreased to −20 °C, the amorphous phase of PPDO segments would be frozen, while PTMEG segments crystallized. Then the deformed samples would keep the material relatively rigid with a higher modulus than in the previous state, so as to fix the deformed shape.42 While the temperature located 46 °C again, the amorphous phase of PPDO segments was in the elastomeric state and PTMEG segments in the viscous state; the chains were flexible; the temporary shape disappeared; and 5840



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Figure 8. Schematic representation of the molecular mechanism of SME.

Figure 10. Effect of the number-average molecular weight of PTMEGdiol in Rf and Rr.

Figure 9. Effect of the viscosity-average molecular weight of PPDOdiol in Rf and Rr.

microstructure becomes more complex and detailed, the PTMEG domains contain several isolated PPDO domains (Figure 11(B)), and PPDO promotes the connection network

PPDO chain length. With increasing PPDO-diol chain length, the dimension of the dispersed phase decreases; otherwise, the 5841



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Figure 11. Transmission electron microscopy pictures of PPDO9.9−PTMEG2.9-75/25 (A), PPDO12.6−PTMEG2.9-75/25 (B), and PPDO19.7− PTMEG2.9-75/25 (C) with the same magnification.

Figure 12. Scanning electron micrograph of L929 fibroblasts seeded on PPDO12.6−PTMEG2.0 copolymers. PPDO12.6−PTMEG2.0-95/5 (A,a), PPDO12.6−PTMEG2.0-85/15 (B,b), and PPDO12.6−PTMEG2.0-75/25 (C,c) films after 24 h. Original magnification: 100× (A, B, C); 2000× (a,b,c).

for PPDO19.7−PTMEG2.9-75/25 (Figure 11(C)). This phase separation structure plays a crucial role in determining the shape memory properties since the shape memory effect is greatly affected by the incompatibility.46 In view of the preceding DSC and TEM facts, both PPDO and PTMEG segments in PPDO−PTMEG exhibit independent crystallization and melting behavior, and one can conclude apparently that PPDO segments are incompatible with PTMEG segments. Combining the preceding shape memory effect and unique microstructure of the PPDO−PTMEG, it can be concluded that a smaller dispersed phase would inhibit Rf but arise Rr. This increase of shape recovery might result from the PPDO segment phase changing alternately to interconnected states, and the smaller domain of the PTMEG segment phase should answer for lower Rf. This theoretically analyzed micromechanism of SME may be a sign for preparing SMPs with excellent shape memory effect. Biocompatibility and Biodegradability of PPDO− PTMEG Multiblock Copolymers. Cells in contact with a

surface will attach, adhere, and spread. So, their morpholog, capacity of proliferation and differentiation will be influenced by the qualities of the adhesion. Fibroblasts are slow-moving cells, and the cytoskeleton plays a very important role in cell motility and shape.47 To assess the modification of cell morphology, SEM was used to analyze the L929 fibroblast cultures on PPDO12.6−PTMEG2.0 films of various compositions (Figure 12). The fibroblasts could attach all the films, and even many pseudopodial protrusions can be observed connecting to the materials’ surfaces, such as filopodia and lamellipodium. It shows that the PPDO12.6−PTMEG2.0 presents a good surface for cells to adhere and spread. Nevertheless, the films are characterized by a lack of cell adhesion and spreading with the gradual decrease in mass fraction of PTMEG segments (Figure 12, from a to c). The cells display more rounded shape in spite of many pseudopodial protrusions catching the material's surface for PPDO12.6− PTMEG2.0-95/5, and then the shape of L929 fibroblasts changes to spindle-shaped for better spreading in PPDO12.6− 5842



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PTMEG2.0-85/15. On PPDO12.6−PTMEG2.0-75/25 films, the fibroblasts tend to develop a confluent monolayer. Cell adherence and spread may influence cell proliferation. Therefore, the MTT assay, as a suitable way for detection of biomaterials cytotoxicity, was performed to evaluate the proliferation of cells on the film's surface (Figure 13 and Figure 14).48−50

condition, pH shift release of toxic materials (catalyst, low molecular weight chains) are accompanied. Faster degradation rates would result in increased pH shift and toxicity.52−55 In addition, the monomer from biodegradation could reduce the metabolic activity of culture fibroblasts.55 These would inhibit the proliferation of the L929 fibroblasts and also maybe the reasons for the various cell morphologies. Degradation studies of PPDO−PTMEG (Figure 15 and

Figure 13. Mitochondrial activity of L929 fibroblasts on PPDO12.6− PTMEG2.0 films on the MTT assay.

The results obtained by the MTT assay state that the multiblock copolymers do not inhibit the viability and proliferation of L929 fibroblasts compared with the control. Furthermore, the mitochondrial activity of cells that cultured on the PPDO−PTMEG films is significantly higher than that of the control, confirming the good biocompatibility of these materials. The stimulation of mitochondrial activity produced by PPDO−PTMEG is in agreement with the results of Serrano and Ignatius et al.47,51 Regarding the effect of the composition, there is a slight increase of mitochondrial activity with the addition of PTMEG segments, which may be caused by the following factors: as a biodegradable polymer, the degradation is inevitable under the culture

Figure 15. In vitro degradation of PPDO12.6−PTMEG2.0, and the change in mass remaining performed as a function of degradation time.

Figure 16) show that the mass remaining and pH shift are related with the compositions of PPDO−PTMEG; that is, the mass remaining increases continually, and the pH decreases more slowly by increasing the ratio of PTMEG segments. The slower biodegradation rate would result in better viability and proliferation of L929 fibroblasts. Thus, the MTT assay is reasonable for the PPDO−PTMEG with different composition.

Figure 14. MTT stained L929 fibroblasts after different time periods. A, B, and C were the control, while a, b, and c were the samples of PPDO12.6− PTMEG2.0-75/25. Aa, 2 days of contact; Bb, 4 days of contact; Cc, 6 days of contact. 5843



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Figure 16. In vitro degradation of PPDO12.6−PTMEG2.0, and the change in pH performed as a function of degradation time.

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CONCLUSIONS Coupling PPDO-diol and PTMEG-diol prepolymers with HDI is a successful route to synthesize PPDO−PTMEG multiblock copolymers. The DSC and TEM tests give solid evidence that the multiblock copolymers exhibit the characteristic morphology of microphase separation. The DSC analysis also indicates that the Tg,PPDO is close to the Tc,PTMEG, and then the reversible phase combines by both the PTMEG segments and the amorphous phase of the PPDO segments. The test of the shape memory effect conducting at different fixing temperature demonstrates that the frozen amorphous phase of the PPDO segments plays an important role in improving the fixity of the sample, cooperating with the crystallized PTMEG segments. The synergistic effect of two segments makes this novel multiblock copolymers exhibit excellent shape memory performance, besides the mechanical properties. A typical sample PPDO9.9− PTMEG2.9-85/15 shows Rf and Rr at 96.6% and 92.9%, respectively. Furthermore, the better crystallization ability of PTMEG segments would increase Rf, and the better crystallization ability of PPDO segments would result in increased Rr. The microphase separating morphology of the multiblock copolymers also plays an important role in the SME: the larger dispersed phase leads to lower Rr but higher Rf. Finally, the cell culture experiments indicate that samples possess good biocompatibility, especially the samples with more PTMEG segments.

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AUTHOR INFORMATION

Corresponding Author

*Ke-Ke Yang. Fax: +86-28-85410284. Tel.: +86-28-85410755. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Science Foundation of China (50873064), the Funds for Young Scientists of Sichuan Province (2010JQ0015), Program of International S & T Cooperation (2011DFA51420), and Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026).

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