Enhancement of Crystallizability and Control of Mechanical and

Oct 26, 2015 - The sc-SMPs exhibit thermally induced shape-memory behavior, and ... would be an effective way to improve the crystallizability and deg...
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Enhancement of Crystallizability and Control of Mechanical and Shape-Memory Properties for Amorphous Enantiopure Supramolecular Copolymers via Stereocomplexation Ruoxing Chang, Guorong Shan, Yongzhong Bao, and Pengju Pan* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Supramolecular stereo multiblock copolymers consisting of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) hard blocks, poly(ethylene-co-butylene) (PEB) soft blocks, and 2-ureido-4[1H]-pyrimidinone (UPy) selfcomplementary quadruple hydrogen-bonding units were prepared upon blending of UPy-end-functionalized PLLA− PEB−PLLA and PDLA−PEB−PDLA triblock oligomers. Supramolecular copolymers (SMPs) based on UPy-terminated PLLA−PEB−PLLAs or PDLA−PEB−PDLAs (denoted as LSMPs and D-SMPs, respectively) are amorphous and noncrystallizable. However, the enantiomeric blends of L-SMPs and D-SMPs become crystalline and show relatively fast crystallization as a result of stereocomplexation of the PLLA and PDLA blocks. Stereocomplexes, rather than homocrystallites, are exclusively formed in the stereocomplexed SMPs (sc-SMPs) with various L-SMP/D-SMP mixing ratios. Because of the preferential crystallization of PLLA and PDLA blocks, sc-SMPs show less ordered microphase-separated morphologies and different domain spacings than the L-SMPs and D-SMPs. Compared with those of the amorphous L-SMPs and D-SMPs, the tensile strength, moduli, and heat resistance of sc-SMPs are increased with the stereocomplex crystallization or an increase in stereocomplex content. The sc-SMPs exhibit thermally induced shape-memory behavior, and their shape deformation and recovery temperatures (Td and Tr, respectively) can be modulated over a wide temperature range by varying the crystallinity or stereocomplex content. The Td and Tr of sc-SMPs increase from 70 to 100 °C with increasing stereocomplex content.

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INTRODUCTION Stereocomplexation (or stereocomplex crystallization) of enantiopure polymers is a special crystallization manner of macromolecules in which the enantiomeric pair cocrystallize in 1:1 ratio in the crystal cell.1−3 Stereocomplexation can occur in blends or block copolymers of stereoisomeric polymers with different tacticities or chiralities, such as poly(lactic acid) (PLA),4 poly(methyl methacrylate),5,6 poly(propylene succinate),7 polypeptides,8 polyamides,9 and epoxide/CO2 polycarbonates.10−12 In the formation of stereocomplexes, two complementary polymers usually interact with each other through specific interactions (e.g., stereoselective van der Waals forces or intermolecular hydrogen bonds),11−16 causing denser chain packing in the stereocomplexes than in the homocrystallites of their parent enantiopure polymers. This unique structure often gives the stereocomplexed materials higher melting point (Tm), higher heat resistance,1 higher mechanical strength and modulus,17−19 better hydrolytic resistance,20,21 and also better physical performance in biomedical applications such as drug delivery and tissue engineering.3,22,23 One of the most studied examples of polymer stereocomplexation is PLA. Poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) are a representative enantiomeric pair that can crystallize as © XXXX American Chemical Society

stereocomplexes in their polymeric racemic blends and stereoblock copolymers.2,4 The stereocomplexed PLA has an enhanced Tm of 230 °C, which is about 50 °C higher than that of their homocrystalline parent polymers. Supramolecular polymers (SMPs) based on non-covalent interactions such as multiple hydrogen bonds, metal−ligand interactions, ionic interactions, or hydrophobic interactions have received great attention because of their diversified properties and functions.24,25 A well-studied group of SMPs are those bonded by 2-ureido-4[1H]-pyrimidinone (UPy), which is a self-complementary quadruple hydrogen-bonding unit having a high dimerization constant and an interaction strength close to that of conventional covalent bonds.26−28 However, the introduction of strong non-covalent interactions and supramolecular units (e.g., UPy groups) generally destroys the structural uniformity and symmetry of polymer chains and also interrupts the mobility and regular folding of polymer segments in crystallization. Accordingly, the SMPs are usually less crystallizable or amorphous than their polymer or oligomer Received: September 9, 2015 Revised: October 15, 2015

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DOI: 10.1021/acs.macromol.5b01986 Macromolecules XXXX, XXX, XXX−XXX

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precursors.29−33 For example, SMPs of telechelic UPyfunctionalized poly(ε-caprolactone) (PCL) oligomers have much lower crystallization rates and lower crystallinity than their PCL precursors. SMPs based on UPy-functionalized PLLA−poly(ethylene-co-butylene)−PLLA (PLLA−PEB− PLLA) triblock oligomers are amorphous even though the hydroxyl-terminated PLLA−PEB−PLLA triblock oligomers are semicrystalline.33 The low crystallizability of SMPs would impact their processing, thermal, and mechanical properties in practical applications. On the other hand, an advantage of stereocomplexation compared with conventional polymer crystallization is that it can significantly enhance the crystallizability or level of crystallinity.7,11 For example, enantiopure poly((S)-propylene succinate) and poly((R)-propylene succinate) crystallize extremely slowly, taking more than 2 weeks to crystallize. However, their 1:1 racemic blends show fast stereocomplex crystallization that can finish upon rapid cooling from the melt state.7 Enantiopure poly(limonene carbonate) is amorphous but becomes crystalline upon stereocomplexation with its complementary enantiomer.11 As inspired by these studies, we envisioned that stereocomplexation would be an effective way to improve the crystallizability and degree of crystallinity of amorphous or less-crystallizable polymers such as SMPs. The dimerized UPy stacks can serve as the physically crosslinked network or stationary phase in the SMP elastomers. In addition, the dimerization of UPy motifs is thermally responsive, and the UPy dimers can completely melt or dissociate at ∼80 °C.34,35 This affords the UPy-based SMPs specific functions such as shape-memory behavior.33,36−41 However, for thermally induced shape-memory behavior of SMPs with UPy stacks as the physical cross-linkers, the recovery temperature (Tr) must be lower than the dissociation temperature of UPy stacks (∼80 °C).33,40,41 Tr has been a key parameter determining the service conditions of thermally induced shape-memory polymers, and feasible control over Tr would be essential to widen their applications. On the other hand, stereocomplexation can cause the formation of physically cross-linked networks in blends of enantiomeric polymers.42−44 It has been demonstrated that the physically cross-linked networks induced by stereocomplexation are directly correlated to the shape-memory properties of the resulting materials.45 Therefore, stereocomplexation of SMPs could offer an alternative method to control and modify their shape-memory properties. In the present work, we synthesized enantiopure SMPs from PLLA−PEB−PLLA and PDLA−PEB−PDLA triblock oligomers by UPy end functionalization. These enantiopure SMPs are amorphous and have an alternating distribution of PLA and PEB blocks. The crystallizability of the amorphous enantiopure SMPs is enhanced upon stereocomplexation with their complementary enantiomeric SMPs. The crystallinity of the stereocomplexed SMPs (sc-SMPs) is controlled by varying the mixing ratio of the two amorphous SMPs containing the enantiopure PLLA and PDLA blocks. The effects of stereocomplexation on the microphase separation, thermal, mechanical, and shape-memory properties of the sc-SMPs were investigated. The shape deformation and recovery temperatures of SMPs were successfully tuned over a wide temperature range by varying the degree of stereocomplexation.

Article

EXPERIMENTAL SECTION

Materials. α,ω-Dihydroxy-terminated PEB oligomer (HLBH-P 3000, Mn = 3.6 kg/mol, Mw/Mn = 1.11) containing 38 wt % (55 mol %) ethylene units and 62 wt % (45 mol %) butylene units was purchased from Krasol Co. L- and D-lactide (>99.9%) were purchased from Purac Co. (Gorinchem, The Netherlands) and recrystallized from ethyl acetate before use. 2-(6-Isocyanatohexylaminocarbonylamino)-6-methyl-4[1H]-pyrimidinone (UPy−NCO) was prepared according to the published method.28 Telechelic hydroxyl-functionalized PLLA−PEB−PLLA and PDLA−PEB−PDLA triblock oligomers with different PLLA and PDLA lengths were synthesized by ringopening polymerizations of L-lactide and D-lactide, respectively, using the dihydroxy-terminated PEB as the macroinitiator.33 The Mn of the PEB midblock was kept as 3.6 kg/mol in all of these triblock oligomers. Telechelic UPy-functionalized PLLA−PEB−PLLAs and PDLA− PEB−PDLAs were synthesized through the reaction of PLLA−PEB− PLLA and PDLA−PEB−PDLA triblock oligomers, respectively, with excess UPy−NCO according to a published method.33 The chemical structures of UPy-functionalized PLLA−PEB−PLLAs and PDLA− PEB−PDLAs are illustrated in Scheme 1a. Gel-permeation chroma-

Scheme 1. (a) Chemical Structures of UPy-EndFunctionalized PLLA−PEB−PLLAs and PDLA−PEB− PDLAs; (b) Formation of Stereo Multiblock SMPs upon Blending of UPy-End-Functionalized PLLA−PEB−PLLAs and PDLA−PEB−PDLAs

tography (GPC) and 1H NMR spectroscopy confirmed the successful syntheses of telechelic hydroxyl- and UPy-terminated PLLA−PEB− PLLAs and PDLA−PEB−PDLAs (Figures S1 and S2). The molecular weights and copolymer compositions of the PLLA−PEB−PLLA and PDLA−PEB−PDLA triblock oligomers were characterized by GPC and NMR spectroscopy, as listed in Table 1. The prepared hydroxylterminated PLLA−PEB−PLLA and PDLA−PEB−PDLA triblock oligomers are denoted as L−EB−Lx (or Lx) and D−EB−Dx (or Dx), where L, D, and EB represent the PLLA, PDLA, and PEB blocks, respectively, and x denotes the Mn (in kg/mol) of each PLLA or PDLA block as derived from NMR spectroscopy. The SMPs formed from UPy-functionalized PLLA−PEB−PLLAs and PDLA−PEB− PDLAs by UPy dimerization are denoted as L-SMPs and D-SMPs, respectively. Preparation of sc-SMPs. The SMPs obtained from UPy-endfunctionalized PLLA−PEB−PLLAs and PDLA−PEB−PDLAs (i.e., LSMPs and D-SMPs, respectively) were blended in a common solvent to prepare the sc-SMPs. The L-SMP/D-SMP mass ratio was maintained B

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Macromolecules Table 1. Compositions and Molecular Weights of PLLA−PEB−PLLA and PDLA−PEB−PDLA Triblock Oligomers samplea

Mn of each block (kg/mol)b

Mn,NMR (kg/mol)c

Mn,GPC (kg/mol)d

PDId

mPLA (%)e

L−EB−L0.6 or L0.6 L−EB−L1.4 or L1.4 L−EB−L2.2 or L2.2 D−EB−D0.7 or D0.7 D−EB−D1.3 or D1.3 D−EB−D2.2 or D2.2

0.6−3.6−0.6 1.4−3.6−1.4 2.2−3.6−2.2 0.7−3.6−0.7 1.3−3.6−1.3 2.2−3.6−2.2

4.8 6.4 8.0 5.0 6.2 8.0

7.87 10.5 10.3 8.12 9.72 10.1

1.07 1.07 1.15 1.09 1.10 1.12

25 44 55 28 42 55

a For brevity, the L−EB−Lx and D−EB−Dx triblock oligomers are also denoted by the abbreviated labels Lx and Dx, respectively, where x represents the Mn (in kg/mol) of each PLLA or PDLA block. bThe numerals denote Mn (in kg/mol) of corresponding PLA and PEB blocks, as derived from NMR data. cMn measured by 1H NMR spectroscopy. dMn and polydispersity index (PDI) measured by GPC. eMass fractions of PLA in triblock oligomers derived from 1H NMR spectroscopy.

For dynamic mechanical analysis (DMA), films were cut into rectangular specimens with dimensions of 30 mm × 6.0 mm × 0.6 mm and loaded onto the Q800 DMA instrument. The storage and loss moduli were collected as functions of temperature from −100 to 170 °C at a heating rate of 3 °C/min and a frequency of 5 Hz.

at 5/5, 6/4, 7/3, 8/2, or 9/1, in which L-SMP was the rich component. Preweighed L-SMP and D-SMP with similar molecular weights and PLA block lengths were separately dissolved in chloroform (50 g/L) and then mixed under stirring. The mixed solution was then cast onto a polytetrafluoroethylene (PTFE) dish, followed by evaporation of the solvent at 25 °C for 24 h. Residual solvent was removed by further drying at 70 °C in vacuo for 12 h. For comparison, PLLA−PEB− PLLA/PDLA−PEB−PDLA oligomeric blends with various blend ratios were prepared by a similar method. The sc-SMPs and oligomeric blends with different mixing ratios are denoted as Lx/Dy-SMP m/n and Lx/Dy-OLG m/n, respectively, in which x, y, and m/n represent the Mn (in kg/mol) of the PLLA block, the Mn of the PDLA block, and the mixing mass ratio of the two components, respectively. Measurements. 1H NMR spectra were acquired on a 400 MHz Bruker AVANCE II NMR spectrometer (Bruker BioSpin Co., Switzerland) using CDCl3 as the solvent. GPC analysis was carried out on a Waters GPC instrument (Waters Co., Milford, MA, USA) consisting of a Waters degasser, a Waters 1515 isocratic HPLC pump, a Waters 2414 RI detector, and two PLgel mix C columns. The temperature of the column was 30 °C, and the eluent was THF at a flow rate of 1.0 mL/min. Polystyrene was used as the standard for calibration. The viscosities of the PLA−PEB−PLA triblock oligomers and UPyend-functionalized PLA−PEB−PLAs at various concentrations were measured on a Ubbelohde viscometer at 25 °C using dichloromethane as the solvent. Differential scanning calorimetry (DSC) analysis was performed on a NETZSCH 214 Polyma DSC (NETZSCH, Selb, Germany) equipped with an IC70 intracooler. In the first scan, the sample (8− 10 mg), sealed in an aluminum pan, was heated from −70 to 190 or 220 °C at 10 °C/min. After this temperature was held for 2 min to erase the thermal history, the sample was cooled to −70 °C at 10 °C/ min and then reheated to 190 or 220 °C at 10 °C/min to observe the melting behavior. Wide-angle X-ray diffraction (WAXD) analysis was carried out on a Rigaku RU-200 diffractometer (Rigaku Co., Japan) using Ni-filtered Cu Kα radiation (λ = 0.154 nm) at 25 °C. The instrument was worked at 40 kV and 200 mA. The sample was step-scanned from 7 to 40° at a 2θ scanning rate of 2°/min. Small-angle X-ray scattering (SAXS) measurements were performed on beamline BL16B1 at the Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the radiation source was 0.124 nm, and the sample-to-detector distance was 2.0 m. The scattering pattern was recorded using a Rayonix SX-165 CCD detector (Rayonix, Evanston, IL, USA) with a resolution of 2048 × 2048 pixels and a pixel size of 80 μm × 80 μm. All of the data were corrected for background and air scattering. The two-dimensional data were averaged to obtain the onedimensional SAXS profile by circular averaging with Fit2D software. Mechanical tests were performed on a Zwick/Roell Z020 testing machine at 23 °C. Specimens were cut into a dumbbell shape with a length of 50 mm, a cross-sectional width of 4.0 mm, and a thinness of ∼0.6 mm from the solvent-cast film. The crosshead speed of the tensile test was 20 mm/min. The reported results represent averages taken from at least five measurements for each sample performed under the same conditions.

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RESULTS AND DISCUSSION

Preparation of sc-SMPs. Three series of PLA−PEB−PLA triblock oligomers and UPy-end-functionalized PLA−PEB− PLAs with different stereostructures and molecular weights of the PLA blocks were prepared. Table 1 summarizes the compositions and molecular weights of the PLLA−PEB−PLLA and PDLA−PEB−PDLA triblock oligomers. As shown in Figure S3, L-SMP has higher viscosity than its triblock precursor in solution at the same concentration, which is much more significant at high concentration. This suggests formation of the SMP and chain extension induced by the UPy units. The viscosities of the triblock oligomers show nearly linear increases with concentration. However, the viscosities of the L-SMPs exhibit two different linear regions at low and high concentrations, reflecting the fact that the SMP dissociates in solution at low concentration because of dissociation of the hydrogen bonds between UPy motifs. Upon blending of complementary L-SMPs and D-SMPs, stereo multiblock SMPs containing both PLLA and PDLA blocks with an alternating distribution of PLA and PEB blocks are prepared, as the UPy units can exchange between different dimers. Since the dimerization of UPy groups is random, PLLA blocks can be linked with either PLLA or PDLA blocks and vice versa through the self-complementary quadruple hydrogen bonds in the stereo multiblock SMPs, as illustrated in Scheme 1b. UPy end functionalization and stereocomplexation between PLLA and PDLA blocks influence the physical states of scSMPs and oligomeric blends remarkably. L−EB−L/D−EB−D oligomeric blends are viscous semifluids or opaque, brittle solids at room temperature (Figure S4). As shown in Figure 1, all of the sc-SMPs composed of the L-SMP and D-SMP with the shortest PLA blocks (i.e., the L0.6/D0.7-SMP series) are transparent, elastic solids under various mixing ratios. However, the physical states of sc-SMPs with longer PLA blocks (i.e., the L1.4/D1.3-SMP and L2.2/D2.2-SMP series) are affected by the mixing ratio. L1.4/D1.3-SMPs and L2.2/D2.2-SMPs with low D-SMP mass fractions (f D) (e.g., L-SMP/D-SMP = 9/1 and 8/ 2) are transparent, elastic solids, while those having high f D (e.g., L-SMP/D-SMP = 6/4 and 5/5) are opaque, brittle solids. In conclusion, the sc-SMPs with shorter PLA blocks and lower f D values easily form flexible and ductile films. The different physical states of sc-SMPs are ascribed to the effect of C

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in the cooling and subsequent heating processes (Figure 3). This is consistent with the WAXD data (Figure 4), in which the

Figure 1. Representative images of solvent-cast sc-SMP films with different L-SMP/D-SMP mixing ratios.

crystallinity and stereocomplex content, as described in the following part. Thermal and Crystallization Behavior. The thermal properties and crystallization behavior of sc-SMPs with different PLA block lengths and L-SMP/D-SMP mixing ratios were studied and also compared with those of the corresponding triblock oligomeric blends. Figure 2 shows the

Figure 4. WAXD patterns of solvent-cast (a) L0.6/D0.7-SMPs and (b) L2.2/D2.2-SMPs with different L-SMP/D-SMP mixing ratios. Diffraction peaks of stereocomplexes are indicated by “sc”. The profiles have been arbitrarily shifted in the vertical direction for clarity.

solvent-cast L-SMP and D-SMP do not show an obvious diffraction peak except for the broad amorphous halo. These results indicate that the enantiopure L-SMPs and D-SMPs are amorphous and noncrystallizable, even though the L−EB−L and D−EB−D triblock oligomers are semicrystalline.33 The restrained crystallization of the PLA blocks in the L-SMPs and D-SMPs is ascribed to the enhanced intermolecular interactions and suppressed chain mobility induced by the UPy dimerization and chain extension.32,33 Interestingly, the crystallizability of the L-SMPs and D-SMPs increases significantly upon blending with the complementary pair. Obvious melting peaks are seen in the DSC curves of solvent-cast sc-SMPs with various L-SMP/D-SMP mixing ratios (Figure 2). The Tm values for the solvent-cast L2.2/D2.2-SMPs are ∼200 °C, which is much higher than that of homocrystalline PLA (∼170 °C). In the WAXD patterns of solvent-cast scSMPs with various L-SMP/D-SMP mixing ratios (Figure 4), the characteristic diffractions of the stereocomplex (e.g., 110, 300/ 030, and 220 diffractions at 2θ = 12.0°, 20.9°, and 24.1°, respectively46) are exclusively observed. This demonstrates that PLLA/PDLA stereocomplex crystallization, rather than homocrystallization of the individual enantiomers, takes place upon blending of the complementary enantiopure L-SMPs and DSMPs. As shown in Figure 3, a new crystallization peak is observed for the L2.2/D2.2-SMPs in the cooling process from the melt, in contrast to the amorphous L-SMP and D-SMP. The crystallization enthalpy of the sc-SMP increases and its crystallization peak shifts to higher temperature as f D is increased from 0.1 to 0.5, suggesting an enhancement of the crystallinity and crystallization rate. Similar variation trends of the crystallization enthalpy and temperature with changing f D are seen for the L1.4/D1.3-SMP series (Figure S5). Because of the short PLLA and PDLA blocks, the crystallization and melting peaks of the L0.6/D0.7-SMPs are less obvious in the DSC cooling and heating curves (Figure S6), but the melting peaks and characteristic diffractions of the stereocomplexes can be clearly seen in the DSC heating curves (Figure 2a) and WAXD patterns (Figure 4a) of solvent-cast L0.6/D0.7-SMPs, which are attributable to the facilitated crystallization in the solvent-casting process. To elucidate the role of the UPy units in crystallization, we also compared the crystallization behavior of the sc-SMPs with that of the corresponding L−EB−L/D−EB−D oligomeric

Figure 2. DSC curves of solvent-cast (a) L0.6/D0.7-SMPs and (b) L2.2/D2.2-SMPs with different L-SMP/D-SMP mixing ratios collected in the first heating scan at 10 °C/min. The curves have been arbitrarily shifted in the vertical direction for clarity.

DSC curves of solvent-cast sc-SMPs collected in the first heating scan. Figure 3 shows the DSC curves of L2.2/D2.2SMPs with different L-SMP/D-SMP mixing ratios collected upon non-isothermal melt crystallization and subsequent melting. Solvent-cast L-SMP and D-SMP do not show discernible melting endotherms in the first heating scan (Figure 2) and also do not exhibit any crystallization and melting peaks

Figure 3. DSC curves of L2.2/D2.2-SMPs with different L-SMP/DSMP mixing ratios recorded upon (a) cooling and (b) subsequent heating. Both the cooling and heating rates are 10 °C/min. The curves have been arbitrarily shifted in the vertical direction for clarity. D

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Macromolecules blends that did not bear the UPy functionalities. As shown in Figure S7, the L-EB-L2.2 and D-EB-D2.2 triblock oligomers undergo cold crystallization in the heating process. Their crystallization rates are further improved upon blending with the complementary oligomers. In the oligomeric blends, a crystallization peak is present upon cooling from the melt state and the cold crystallization peak is decreased upon subsequent heating, implying an enhanced crystallization rate. This agrees with the results for the sc-SMPs and indicates that the crystallization rate of oligomeric blends is also enhanced upon stereocomplexation. Because PLLA and PDLA are 1:1 in the stereocomplex, the asymmetric L−EB−L/D−EB−D oligomeric blends with much different L−EB−L and D−EB−D fractions crystallize as both stereocomplexes and homocrystallites. For the oligomeric blends with comparable L−EB−L and D−EB−D contents (i.e., f D = 0.4−0.5), the melting endotherm and characteristic diffractions of stereocomplexes are mainly observed in the DSC heating curves (Figure S7b) and WAXD patterns (Figure S8). However, for the asymmetric oligomeric blends with f D = 0.1− 0.3, two melting endotherms are observed in the temperature regions of 120−150 and 180−210 °C in the DSC heating curves, corresponding to the melts of homocrystallites and stereocomplexes, respectively (Figure S7b). This is consistent with the WAXD results (Figure S8), in which both the characteristic diffractions of stereocomplexes and homocrystallites (e.g., 110/200 and 203 diffractions at 2θ = 16.8° and 19.1°, respectively47) are observed for the asymmetric L−EB− L/D−EB−D oligomeric blends with f D = 0.1−0.3. However, this is different from those observed for the corresponding scSMPs, in which the stereocomplexes are exclusively formed regardless of f D. Therefore, it is concluded that the UPy groups significantly depress the homocrystallization but not the stereocomplex crystallization of PLA blocks. Previous studies have demonstrated that the interactions between PLLA and PDLA blocks are essential for their stereocomplexation and that the stereocomplexation can be facilitated by either covalent (i.e., stereoblock copolymerization)19,48 or non-covalent interactions, in compared with homocrystallization.32,49 Therefore, the exclusive stereocomplex formation in sc-SMPs can be ascribed to the depressed homocrystallization caused by the UPy groups and the enhanced interactions between enantiomeric blocks induced by the multiple hydrogen bonds between the UPy units. On the basis of these DSC data, the Tm values and melting enthalpies of the stereocomplexes (T m,sc and ΔH m,sc , respectively) were calculated, and the results are plotted as functions of mixing ratio in Figure 5. As f D is increased from 0.1 to 0.5, Tm of the sc-SMPs increases slightly or keeps nearly unchanged, while ΔHm,sc and the diffraction intensities of the stereocomplexes gradually increase (Figures 3−5). This demonstrates that the crystallinity and stereocomplex content are enhanced when the mass fractions of the L-SMP and D-SMP in the sc-SMP become similar. Both Tm,sc and ΔHm,sc for the scSMPs and oligomeric blends increase with increasing PLLA and PDLA block lengths as a result of the increased perfection and content of stereocomplexes. Because of the shorter block length and the steric effect of the PEB block on crystallization, the Tm,sc values of the sc-SMPs and oligomeric blends (100−200 °C) are lower than those of PLLA/PDLA racemic blends (220−240 °C).50 In addition, the sc-SMPs exhibit lower Tm,sc and ΔHm,sc than the corresponding oligomeric blends at the same mixing ratio, suggesting that the strong interactions

Figure 5. Plots of (a) melting temperature and (b) enthalpy of stereocomplexes as functions of D−EB−D or D-SMP mass fraction in the sc-SMPs and oligomeric blends.

between UPy units diminish the crystalline perfection of the formed stereocomplexes. Microphase Separation of sc-SMPs. The microphaseseparated morphologies of sc-SMPs with various L-SMP/DSMP mixing ratios were investigated via SAXS. As shown in Figure 6, all of the sc-SMPs show broader and less resolved scatterings than the amorphous enantiopure L-SMPs and DSMPs, indicating that the order of the microphase-separated structures is decreased by stereocomplexation of the PLLA and PDLA blocks. For block copolymers with crystallizable segments, crystallization of the crystallizable blocks and microphase separation of the different blocks occur simultaneously. The breakout crystallization of crystallizable segments can span different microdomains and decrease the order of the microphase-separated structure when crystallization has a stronger driving force than microphase separation.51 Because of the hydrogen bonds between the enantiomeric segments,13−15 the driving force for stereocomplexation in scSMPs would overwhelm that of microphase separation. Previous studies have demonstrated that stereocomplexation results in aggregation of the PLLA and PDLA chains from the solution.52 Therefore, stereocomplexation would precede microphase separation upon solvent-casting the sc-SMPs and thus would diminish the order of the microphase-separated morphology. On the other hand, it has been verified that stereocomplexation of PLLA and PDLA segments can induce the formation of physically cross-linked networks and molecular bridges,42−44 which may also destroy the order of the microphase-separated structure. As shown in Figure 6a, L-SMP and D-SMP having low PLA contents exhibit sharp principal scattering peaks at q* = 0.72 nm−1 followed by well-resolved oscillations at √3q* and √7q*. This indicates that these amorphous SMPs separate into the hexagonally packed cylindrical morphology53 with an average spacing (D = 2π/q*) of 8.7 nm. However, the principal scattering peak shifts to a smaller angle at q* ≈ 0.56 nm (D ≈ 11.2 nm) and an additional scattering peak at √3q* is observed for the L0.6/D0.7-SMPs with f D ≤ 0.3. Therefore, such scSMPs with low PLA content (i.e., L0.6/D0.7-SMPs) separate into the cylindrical morphology but have larger domain sizes than the amorphous enantiopure L-SMP and D-SMP. The strong intermolecular interactions and physical cross-linking between PLLA and PDLA blocks upon stereocomplexation as well as the increased polymer chain length resulting from the hydrogen bonds between UPy moieties may cause more polymer segments to be included in the PLA cylindrical domains, thereby enlarging the domain size. E

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Figure 6. SAXS patterns of solvent-cast sc-SMPs with various compositions and L-SMP/D-SMP mixing ratios: (a) L0.6/D0.7-SMPs; (b) L1.4/D1.3SMPs; (c) L2.2/D2.2-SMPs. The profiles have been arbitrarily shifted in the vertical direction for clarity.

As shown in Figure 6b,c, the amorphous L-SMPs and D-SMPs with the Mn values for the PLA blocks of ∼1.4 kg/mol (i.e., the L1.4 and D1.3 SMPs) and 2.2 kg/mol (i.e., the L2.2 and D2.2 SMPs) exhibit sharp principal scattering peaks at q* = 0.55 and 0.49 nm−1, demonstrating the formation of lamellar microphase-separated structures53 with D = 11.4 and 12.8 nm, respectively. These principal scattering peaks are all followed by the well-resolved higher oscillations at 2q* and 3q*. However, an additional principal scattering peak at q* ≈ 0.65 (D ≈ 9.7 nm) followed by the higher oscillations at 2q* and 3q* is observed for the L2.2/D2.2-SMPs with f D = 0.1−0.4. This principal scattering peak shifts toward lower q as f D increases from 0.4 to 0.5. A similar trend is also observed for the L1.4/ D1.3-SMPs with varying f D. Therefore, the sc-SMPs with longer PLLA and PDLA blocks (e.g., L1.4/D1.3-SMPs and L2.2/D2.2-SMPs) form lamellar morphologies having thinner domains than the corresponding amorphous L-SMPs and DSMPs. On the other hand, the stereocomplexed L2.2/D2.2 oligomeric blends also exhibit a smaller domain spacing (D ≈ 11.4 nm, q* ≈ 0.55 nm−1) than the L-EB-L2.2 and D-EB-D2.2 triblock oligomers (D ≈ 14.3 nm, q* ≈ 0.44 nm−1) with the homocrystalline PLA domains (Figure S9). Previous studies have demonstrated that PLA stereocomplexes have smaller long periods than the homocrystallites.50,54 Therefore, the smaller domain spacing of sc-SMPs and oligomeric blends having longer PLLA and PDLA blocks may be attributed to the smaller long periods and thinner crystalline layers of the stereocomplexes. Mechanical Properties. The mechanical properties of the sc-SMPs with various L-SMP/D-SMP mixing ratios were measured using uniaxial tensile tests, as shown in Figures 7a and S10. The results for the tensile strength, Young’s modulus, and elongation-at-break of the sc-SMPs are plotted as functions of D-SMP mass fraction in Figure 7b−d, respectively. Because the L1.4/D1.3-SMPs and L2.2/D2.2-SMPs with larger f D (≥0.3) cannot form uniform films in solvent-casting, their mechanical properties were not determined. As shown in Figures 7a and S10, the sc-SMPs show relatively lower strength and modulus but larger elongation-at-break compared with neat PLA, characteristic of the thermoplastic elastomers. The strengths and moduli of the sc-SMPs increase and their elongation-at-break decreases with increasing f D or crystallinity. The L0.6/D0.7-SMP 5/5 sample has a tensile strength of 5.5 MPa, Young’s modulus of 23 MPa, and elongation-at-break of 45%, compared with 3.6 MPa, 9.6 MPa, and 162%, respectively, for the amorphous enantiopure L0.6-SMP. Stereocomplex crystallization of the PLLA and PDLA blocks in the sc-SMPs leads to an increase in tensile strength of about 53% and an

Figure 7. (a) Representative stress−strain curves and (b−d) plots of (b) tensile strength, (c) Young’s modulus, and (d) elongation-at-break for sc-SMPs with different compositions and L-SMP/D-SMP mixing ratios.

enhancement of the Young’s modulus of 139% due to the higher strength and modulus of stereocomplexed PLA compared with its amorphous analogue.17 For the sc-SMPs with the same f D, the tensile strength and modulus increase while the elongation-at-break decreases with increasing PLLA and PDLA block lengths (Figure 7b−d). The storage modulus and loss tangent (tan δ) curves of scSMPs with various L-SMP/D-SMP mixing ratios are shown in Figures 8 and S11. It can be seen from these figures that the storage moduli of sc-SMPs with f D ≤ 0.2 exhibit abrupt decreases in the temperature ranges of −50 to −20 °C, 20 to 60 °C, and >80 °C, which are attributable to the glass transitions of PEB and PLA and softening of PLA (or dissociation of UPy dimers), respectively. Two relaxation peaks are observed at −50 to 0 °C and 50 °C in the tan δ curves for the sc-SMPs with f D ≤ 0.2, which are attributable to the glass transitions of the PEB and PLA domains, respectively (Figures 8b and S11b). The Tg of the PEB-rich domain in the sc-SMP was relatively invariant with respect to the L-SMP/D-SMP mixing ratio. The relaxation peak corresponding to PLA domains becomes less obvious for the sc-SMPs with f D ≥ 0.3 because of the increased crystallinity F

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in these experiments, and both were varied over a wide temperature range of 70−140 °C. The selected temperature range is in view of two factors: the temperatures must be higher than Tg but lower than Tm of the PLA domains in the sc-SMPs. As shown in Figure 9, the shape-memory properties of the sc-SMPs are strongly influenced by Td, Tr, and the stereocomplex content. At Td = 70−90 °C, the L0.6/D0.7-SMPs with high stereocomplex content ( f D ≥ 0.2−0.3) are too stiff and cannot be fixed to the helix shape. Only C-shaped specimens can be obtained for these stiffer sc-SMPs. After reheating to Tr = 70 °C, only the sc-SMPs with low stereocomplex content (f D = 0.1−0.2) can recover their permanent shape in 10 s, similar to the L-SMP (Figure S14). However, the sc-SMPs with high stereocomplex content (f D = 0.3−0.5) cannot fully recover their permanent shapes at Td = Tr = 70 °C. However, when Td and Tr are increased to 80−90 °C, the sc-SMPs with high stereocomplex content ( f D = 0.3−0.5) return to their permanent shape while those with low stereocomplex content ( f D = 0.1−0.2) merely recover to the C-shaped specimens. Similar temperature-dependent shape-memory behavior is also observed for the L1.4/D1.3-SMPs and L2.2/D2.2-SMPs with low stereocomplex contents (f D = 0.1−0.2). They show good shape-memory ability at a low Td and Tr of 70−80 °C that becomes worse when Td and Tr are increased to ≥90 °C. Interestingly, as Td and Tr are further increased to 100 °C, the L0.6/D0.7-SMPs with high stereocomplex content (f D = 0.4− 0.5) exhibit a good balance of shape deformation and recovery abilities. They can not only be deformed to the helix shape but also fully resume the permanent shape after reheating. The higher Td is, the better the helix shape is fixed. However, the shape-memory ability of sc-SMPs diminishes drastically when Td and Tr are increased from 100 to 140 °C, even though all of the specimens have good shape deformation ability in this temperature range. In order to quantitatively evaluate the shape-memory property, rectangular specimens (30 mm × 0.5 mm × 0.5 mm) of sc-SMPs and L-SMPs were folded to a right angle (90°)

Figure 8. Representative DMA curves of (a) storage modulus and (b) tan δ as functions of temperature for the L0.6/D0.7-SMPs with different L-SMP/D-SMP mixing ratios.

and decreased amorphous content. Stereocomplex crystallization of PLLA and PDLA blocks significantly improves the thermal resistance and storage modulus. As shown in Figure 8a, at temperatures above 0 °C, the storage moduli of sc-SMPs gradually increase with increasing f D from 0 to 0.5. The scSMPs with f D ≤ 0.2 soften completely upon heating to ∼90 °C; this temperature is enhanced to ∼130 °C when f D increases to ≥0.3. All of these results demonstrate that the mechanical properties of sc-SMPs can be feasibly tuned by the crystallization of the PLA blocks or by varying the stereocomplex content. Shape-Memory Properties. The thermally induced shapememory behavior of the sc-SMPs and its dependence on the LSMP/D-SMP mixing ratio and PLA block length were investigated, as illustrated in Figures 9, S12, and S13. A solvent-cast sc-SMP film (thickness ≈ 0.5 mm) with a rectangular shape was curled into a temporary helix shape at different deformation temperatures (Td), and the deformed shape was fixed by cooling to room temperature for 30 min. The deformed specimen was then reheated to a certain Tr to observe the shape-memory behavior. Td and Tr were the same

Figure 9. Representative images showing the shape-memory behavior of L0.6/D0.7-SMPs with different L-SMP/D-SMP mixing ratios: (a) 5/5; (b) 6/4; (c) 7/3; (d) 8/2; (e) 9/1. Td and Tr are the deformation and recovery temperatures, respectively. G

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Macromolecules at different Td (70−120 °C) and then held at room temperature for 30 min to fix the shape (Figures S13 and S15). The folded specimens were then reheated to Tr to evaluate the shape-memory ability. The folded angles in the deformed and resumed specimens were measured, as shown in

recovery ratio with temperature and L-SMP/D-SMP mixing ratio are shown in Figure 10c. As shown in Figures 10 and S13, at Td = Tr = 70 °C, only the L0.6/D0.7-SMPs with low stereocomplex content (f D ≤ 0.2) can be well-deformed and resume their shapes, showing recovery ratios of ∼100%. In contrast, the recovery ratios of sc-SMPs with high stereocomplex content ( f D = 0.4−0.5) are just around 50−60%. The recovery ratio of sc-SMPs with low stereocomplex content (f D ≤ 0.2) significantly decreases with increasing Td and Tr at ≥80 °C. At Td = Tr = 80−100 °C, the sc-SMPs with high stereocomplex content ( f D = 0.4−0.5) show high recovery ratios of ∼100%, compared with ratios of 0−40% for the scSMPs with low stereocomplex content (f D ≤ 0.2). The recovery ratios for all of the sc-SMPs decrease significantly with increasing Td and Tr at >100 °C. All of these results indicate that the stereocomplex crystallization of PLLA and PDLA plays a key role in the shape deformation and recovery ability of scSMPs. The shape-memory properties of sc-SMPs can be feasibly tuned by varying the degree of stereocomplexation, allowing for the preparation of SMPs with good shape-memory properties over a wide range of service temperatures (70−100 °C). The prepared SMPs containing stereocomplexes and supramolecular UPy motifs meet the structural requirements of dualshape memory effects.55,56 The stereocomplexed physical networks45 and dimerized UPy stacks33,40,41 in sc-SMPs can act as the stationary phase or physical cross-linkers to set the permanent shape. The glass transition of the PLA amorphous phase57,58 and the reversible dimerization/dissociation of UPy motifs36 can provide the reversible thermal transitions for the temporary shape fixing and recovery. The temperaturedependent shape-memory behavior of sc-SMPs with different L-SMP/D-SMP mixing ratios can be explained from the balance of stationary and mobile phases, as illustrated in Figure 11. For sc-SMPs with low crystallinity or stereocomplex content ( f D ≤ 0.2), the dimerized UPy stacks mainly serve as physical crosslinkers at a lower temperature of 70 °C (Figure 11a). The dimerization of UPy groups is strongly temperature-dependent, and UPy dimers can completely melt or disassociate upon heating to ∼80 °C.34,35 Therefore, the shape-memory ability of sc-SMPs with low stereocomplex content ( f D ≤ 0.2) is good at the lower Td and Tr (∼70 °C) but is significantly depressed at

Figure 10. Effects of the deformation and recovery temperatures on the shape-memory behavior of L-shaped L0.6/D0.7-SMP specimens with different L-SMP/D-SMP mixing ratios: (a) angles of the initially deformed specimens; (b) angles of the recovered specimens; (c) calculated shape recovery ratios.

Figure 10a,b. The shape-memory ability of sc-SMPs is quantified by the shape recovery ratio (R), defined as

R=

θr − θd × 100% θ0 − θd

where θ0, θd, and θr are the folded angles of the specimen with permanent shape (180°), the deformed specimen, and the resumed specimen, respectively. The changes in the sc-SMP

Figure 11. Schematic illustration of structural changes for L0.6/D0.7-SMPs at different Td and Tr values in shape-memory experiments. H

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the higher Td and Tr (>80 °C) because of the loss of the stationary phase (Figure 11a,b). For the sc-SMPs with high crystallinity or stereocomplex content (f D ≥ 0.3), the stereocomplexes mainly act as the stationary phase and physical cross-linkers (Figure 11c). Parts of the stereocomplexes start to melt in L0.6/D0.7-SMPs upon heating to ∼100 °C (Figures 2a and S6b). Because most of the PLA segments are stereocomplexed (or crystallized) and the reversible phase (e.g., amorphous PLA) is not enough at a low temperature of ≤90 °C (Figure 11c), the sc-SMPs with high stereocomplex content (f D ≥ 0.3) are difficult to deform into complicated shapes. At Td = Tr ≈ 100 °C, the content of the reversible phase increases with the melting of partial stereocomplexes. In addition, the thermally triggered dimerization and dissociation of UPy groups can also serve as the reversible transition for the temporary shape fixing and recovery.36 Therefore, the sc-SMPs with high stereocomplex content have balanced contents of reversible and physically cross-linked stationary phases under these conditions, leading to the good deformation and shape-recovery abilities of these sc-SMPs (Figure 11d). However, the shape-recovery ability of the scSMPs decreases remarkably when Td and Tr are further increased to ≥110 °C because of melting of the stereocomplexes (Figure 11e).

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87951334. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge the National Natural Science Foundation of China (21274128 and 21422406) and the Fundamental Research Funds for the Central Universities (2015XZZX00408) for financial support and beamline BL16B1 of the SSRF (China) for SAXS measurements.

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CONCLUSIONS A series of sc-SMPs with the controllable crystallizability and thermal, mechanical, and shape-memory properties were prepared upon blending of the complementary enantiopure LSMP and D-SMP precursors. Even though the PLLA−PEB− PLLA and PDLA−PEB−PDLA triblock oligomers are semicrystalline, the UPy-bonded enantiopure L-SMPs and D-SMPs are amorphous and noncrystallizable. Interestingly, the amorphous L-SMPs and D-SMPs become crystalline upon blending with their complementary analogues as a result of stereocomplex crystallization between the PLLA and PDLA blocks. Stereocomplexes are exclusively formed in the sc-SMPs regardless of the L-SMP/D-SMP mixing ratio. The preferential stereocomplex crystallization of PLA blocks prevents the formation of the ordered microphase-separated morphology in the sc-SMPs. The sc-SMPs exhibit higher tensile strength and moduli and better heat resistance than the corresponding amorphous enantiopure L-SMPs and D-SMPs. The as-prepared sc-SMPs have excellent thermally induced shape-memory properties, which can be tuned by varying the crystallinity or stereocomplex content. The Td and Tr of sc-SMPs increase from 70 to 100 °C with increasing stereocomplex content. This study has found an example that an amorphous, enantiopure SMP becomes crystalline upon stereocomplexation with its complementary analogue. We anticipate that this will provide potential methods to control the thermal, mechanical, and shape-memory properties of supramolecular materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01986. Partial results on GPC, NMR, DSC, WAXD, SAXS, tensile test, DMA, and shape-memory properties of PLA−PEB−PLA triblock oligomers, oligomeric blends, and sc-SMPs (PDF) I

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