Reversible Lamellar Periodic Structures Induced by Sequential

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Reversible Lamellar Periodic Structures Induced by Sequential Crystallization/Melting in PBS-co-PCL Multiblock Copolymer Miaoming Huang,†,‡ Xia Dong,*,†,§ Lili Wang,†,§ Liuchun Zheng,† Guoming Liu,† Xia Gao,‡ Chuncheng Li,† Alejandro J. Müller,∥,⊥ and Dujin Wang†,§ †

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Engineering Plastics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Beijing Key Laboratory of Organic Materials Testing Technology and Quality Evaluation, Beijing Engineering Research Center of Food Safety Analysis, Beijing Center for Physical and Chemical Analysis, Beijing 100089, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ∥ POLYMAT and Polymer Science and Technology Department, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, Donostia-San Sebastián 20018, Spain ⊥ IKERBASQUE, Basque Foundation for Science, Bilbao, Spain S Supporting Information *

ABSTRACT: Reversible periodic structures in a symmetric poly(butylene succinate)-co-poly(ε-caprolactone) (PBS-co-PCL) multiblock copolymer have been detected for the first time. A phase-segregated structure can be observed under the phase contrast optical microscope at 150 °C, but it has no significant effect on the subsequent crystallization behavior of the PBS component, which can break out at lower temperatures (i.e., 82 °C) forming spherulites that contain the molten PCL component within them. During PBS chains crystallization at 82 °C, two peaks are detected by SAXS experiments. The high-q peak corresponds to a periodic structure formed within PBSrich domains consisting of PBS lamellae and amorphous regions containing PBS and molten PCL chains. The low-q peak arises from a periodic structure formed within PCL-rich domains consisting of PBS lamellae and thick amorphous layers needed to accommodate the large fraction of molten PCL chains at 82 °C. When the temperature is decreased to 36 °C, the PCL component crystallizes within the PBS spherulitic template, and an alternating double crystalline layer structure of PCL and PBS forms, which leads to a decreased intensity of the low-q peak and an increased intensity of the high-q peak. If the temperature is increased again, the PCL crystals remelt and the high-q peak can still be observed, while the low-q peak becomes stronger again, confirming the reversibility of the periodic structures. Based on the results obtained, a schematic morphological model of the reversible periodic structures in the crystallization/melting process is proposed.

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INTRODUCTION In the past few decades, the necessity of environmental protection has led to the development of biodegradable materials with low ecological impact, and their production and application have become a major scientific and technological challenge. The family of aliphatic polyesters in particular is very attractive in view of their biodegradability, biocompatibility, good comprehensive properties, and further potential applications in both biomedical materials and environmentally friendly materials.1−4 Poly(butylene succinate) (PBS) is one of the most promising biodegradable polyesters in view of its relative high melting temperature (∼110 °C), thermal stability, and good mechanical strength.5,6 Unfortunately, the slow biodegradation rate governed by the high crystallinity of PBS,7 together with the insufficient impact resistance, restricts its widespread applications. Therefore, copolymerization has been applied to control the degree of crystallization and to improve its physical properties. As a semicrystalline thermoplastic polyester with well-defined crystalline © XXXX American Chemical Society

structure and high impact strength, poly(ε-caprolactone) (PCL) is a fully biodegradable and biocompatible material.8,9 Unfortunately, its inherently low melting point and tensile strength of PCL hamper its application. Copolymers or blends of PBS and PCL may yield a variety of biodegradable materials with improved properties in comparison with those of the corresponding homopolymers.10−12 Recently, PBS-co-PCL multiblock copolymers have been successfully synthesized for the first time using chain-extension reaction by Zheng et al.13 The impact strength of multiblock copolymers was greatly enhanced with the incorporation of the PCL component. Taking into consideration that the morphology, physical properties, and biodegradation features are strongly governed by crystallization, the investigation on crystallization behaviors of Received: August 17, 2017 Revised: December 26, 2017

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DOI: 10.1021/acs.macromol.7b01779 Macromolecules XXXX, XXX, XXX−XXX

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were comparable, 5 × 103 and 5 × 103 g/mol for PBS and PCL oligomers, respectively, as determined by 1H nuclear magnetic resonance (NMR). Then, PBS-co-PCL multiblock copolymers with different composition were synthesized from PBS and PCL prepolymers by a chain extension reaction using hexamethylene diisocyanate as the chain extender. The details of the polymer synthesis can be found elsewhere.13 The molecular weight of the final polymer was determined by gel permeation chromatography (GPC, equipped with a refractive index detector), using chloroform as the solvent and monodisperse polystyrene as the calibration standard, as listed in Table 1. The copolymers

PBS-co-PCL multiblock copolymers is of great significance. However, the detailed study of the crystallization behavior and microstructure formation of PBS-co-PCL multiblock copolymers has not been reported, as far as we are aware. Generally speaking, the crystallization in double crystalline copolymers is in direct relationship with several factors such as composition, miscibility, and thermal history.14−24 If the crystallization temperature for one crystalline component is quite different from the other, the two-step isothermal crystallization process can be carried out to investigate the final morphology, which is usually directed by the crystallization morphology of the first component as a template. The second component may be confined within the foregoing lamellar stacks of the first component, or within the microdomain structure, or its crystallization could be nucleated.25 For instance, the crystallization behavior and morphology of poly(L-lactide)-b-poly(ε-caprolactone) (PLLA-b-PCL) diblock copolymers have been intensively investigated. The results showed that the crystallization of PCL block occurred within the previously formed PLLA spherulites, and the PLLA spherulitic structure, acting as a template, remained unaltered except for a change in the birefringence. Based on the crystallization degree of PLLA block, the PCL block could be nucleated by the already formed PLLA crystals.26−28 Han et al.29,30 have already studied the microstructure and morphology and effects of crystallization temperature on the crystallization behavior in poly(L-lactide)-b-poly(ethylene glycol) (PLLA-b-PEG) copolymer. It was found that PEG block should crystallize within interlamellar or interfibrillar spaces of PLLA crystals. The microstructure formation, crystal nucleation, and crystallization rate of PEG block were dependent on the crystallization temperature of PLLA block. To our knowledge, only few studies have dealt with the crystallization of biodegradable double crystalline multiblock copolymers.6,31−33 Yang et al.6,31 have investigated the crystallization kinetics, morphology, and crystal structure of poly(butylene succinate)-co-poly(ethylene glycol) (PBS-co-PEG) multiblock copolymers with different composition and chain length. They found that the overall crystallization rate of PBS increased with PBS fraction and chain length (LPBS), and the higher content or longer PEG had larger crystallization rate. Kim et al.33 have prepared PLLA-co-PCL multiblock copolymers by using the coupling reaction between the bischloroformates of carboxylated PLLA with PCL-diol in the presence of pyridine. They found that the multiblock copolymers exhibited only one melting peak, corresponding to the PLLA phase, even at a relatively high PCL content. However, the detailed investigation on the complicated properties such as crystallization, morphology, and microstructure formation of biodegradable double crystalline multiblock copolymers is still in a nascent stage. In this work, we carry out a systematic investigation on the reversible periodic lamellar structures induced by crystallization and melting behavior in double crystalline PBS-co-PCL multiblock copolymers, from micrometer to nanometer scale, by a combination of various techniques. Based on the experimental evidence, a schematic morphological model of the reversible periodic structures in the crystallization/melting process is proposed.

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Table 1. Molecular Weights and Composition for PBS, PCL, and Its Symmetric Multiblock Copolymer copolymer

Mn (×103) (g/mol)

Mw (×103) (g/mol)

PDIa

weight fraction of PBS (wt %)

PBS BS50CL50 PCL

85.6 109 89.4

295 363 334

3.44 3.33 3.73

100 50 0

a

PDI: polydispersity index; the ratio of weight-average molecular weight and number-averaged molecular weight: Mw/Mn.

are denoted in an abbreviated form, e.g., BSxCLy, indicating the weight ratio of each component as subscripts (x and y). The chain extension reaction can be hypothesized as a random inclusion process of PBS and PCL component, which should result in a statistical distribution. Rheology Tests. A stress-controlled rheometer (DHR-2 from TA Instruments), equipped with parallel plate geometry (a gap of 0.5 mm, diameter 25 mm), was used. The disk samples with thickness of 1 mm were compressed at 150 °C under a pressure of 10 MPa. All rheological properties were obtained from small-amplitude oscillatory shear (SAOS) tests, and all experiments were conducted in the linear viscoelastic region determined by the strain sweeps. The dynamic frequency sweep tests at different temperatures were performed in the range of 0.01 and 500 rad/s with a strain amplitude of 4%. Before the collection of data, the samples were allowed to equilibrate at each temperature for 10 min. In order to prevent thermo-oxidative degradation, all measurements were performed under a nitrogen atmosphere. Optical Microscopy (OM). Polarized optical microscopy (POM) and phase contrast optical microscopy (PCOM) images of the copolymer were observed by an Olympus (BX51) optical microscope equipped with a Canon 40D camera system. A Linkam (THMS600E) hot stage was used to control the temperature. To enhance contrast and determine the sign of the spherulites, a λ wave plate was inserted between the polarizers. The two-step isothermal crystallization process was applied to study the crystallization morphology of the PBS and PCL components within the multiblock copolymer. The copolymer was preheated at 150 °C for 3 min, and first isothermally crystallized at the crystallization temperature of the PBS component (Tc,PBS = 82 °C) until saturation and then cooled to the chosen crystallization temperature of the PCL component (Tc,PCL = 36 °C). Finally, the copolymer was reheated to 150 °C at 10 °C/min. Differential Scanning Calorimeter (DSC). A PerkinElmer Diamond DSC was employed to study the isothermal crystallization behavior of the copolymers. The instrument was calibrated with indium. Twostep isothermal crystallization experiments were also applied to study the crystallization of PBS and PCL components within the multiblock copolymer. Wide- and Small-Angle X-ray Scattering (WAXS and SAXS). The two-step isothermal crystallization behavior of the copolymer was investigated by means of in situ WAXS and SAXS analysis, which were performed at BL14B1 and BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF), respectively. The wavelength λ of the radiation source is 1.24 Å. The WAXS patterns were collected by a MAR 225 detector with a resolution of 3072 × 3072 pixels.34 The SAXS patterns were collected by a Rayonix SX165 detector with a diameter of 165 mm and 2048 × 2048 pixels.35 The image acquisition time in WAXS and SAXS was 120 and 30 s, respectively. The sample-to-detector distance was 466.0 mm for WAXS and 2323.6 mm for SAXS. All the images were

EXPERIMENTAL SECTION

Materials. Double crystalline poly(butylene succinate)-co-poly(εcaprolactone) (PBS-co-PCL) multiblock copolymers were successfully synthesized by a two-step method. Dihydroxytelechelic-PCL and dihydroxytelechelic-PBS prepolymers (PBS and PCL oligomer) were first synthesized. The number-average molecular weights (Mn) of the prepolymers B


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Figure 1. Shifted (a) storage moduli (G′), (b) loss moduli (G″), (c) complex viscosity (η*), and (d) Cole−Cole plots from SAOS for BS50CL50. The vertical shift factor is taken to be unity, and the horizontal shift factor (aT) is relied on behavior at higher frequencies. The reference temperature is 150 °C. corrected for background scattering, air scattering, and beam fluctuations. The SAXS radially integrated intensities I(q) (q = 4π sin θ/λ) (2θ is the scattering angle) were obtained for integration in the azimuthal angular range of 0° ≤ Φ ≤ 360°. The long period (L) was calculated with the Bragg equation: L=

2π qmax

The storage moduli (G′), loss moduli (G″), and complex viscosity (η*) of BS50CL50 under different temperatures are shown in Figure 1. The reference temperature is 150 °C, and all the data have been shifted to it. The vertical shift factor almost has no change over the temperature range and is taken to be unity, while the horizontal shift factor (aT) is dependent on the behavior at higher frequencies. The failure of TTS is seen for BS50CL50 from G′, G″, and η* at various temperatures, implying that TTS of BS50CL50 fails within the experimental temperature range. As we know, the failure of TTS can be seen as a sign of microphase separation in monodisperse di- and triblock systems,39,40 indicating that the failure of TTS for BS50CL50 could be correlated with phase transition. In addition, it is shown that for the terminal behavior, G′ ∼ ω0.7 and G″ ∼ ω, which may be resulted from the effect of phase transition. The deviations from master curve are small, although the shifted G′, G″, and η* can be used to judge the failure of TTS. In order to be more precise, the shifted Cole−Cole plots (η″/aT vs η′/aT), where η″ = G′/ω and η′ = G″/ω, have been applied and shown in Figure 1d. The shifted dynamic viscosities (η″/aT vs η′/aT) curves eliminate the effect of temperature. Herein, if TTS succeeds, the shifted Cole−Cole plots under different temperatures should superpose well. However, it is evident that within the experimental temperature range the shifted Cole−Cole plots of BS50CL50 fail to superimpose, suggesting the inhomogeneity of BS50CL50 within the experimental temperature range. Besides, a tail develops on the right-hand side of the arc, indicating the formation of a second phase, namely, the existence of phase transition. Morphology Formation on Micrometer Scale. The superstructural morphology of BS50CL50 on the micrometer scale during two-step isothermal crystallization process was characterized by PCOM and POM in Figure 2.

(1)

where qmax corresponds to the peak position in the Lorentz-corrected Iq2 versus q curves scattering curves.36

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RESULTS AND DISCUSSION Rheological Properties and Melt Structure in PBS-coPCL Multiblock Copolymers. It is important to determine the segregation state of multiblock copolymer employed in this work. Multiblock copolymers of PBS and PCL have only been recently prepared,13 and data on their properties are scarce. Recently, the melt structure of polydisperse multiblock copolymers has been studied by rheological methods.37,38 Yu et al.38 have conducted three rheological methods with different sensitivities, including the scaling law of zero shear viscosity on molecular weight, terminal behavior and time−temperature superposition (TTS), and two-dimensional rheological correlation spectrum, to reveal the mesophase separation of polydisperse ethylene− octene block copolymers (OBCs). The results showed that the mesophase separation transitions (MST) could be observed in such low octene content and low molecular weight OBC systems, and the critical χN (Flory−Huggins interaction parameter χ times the overall degree of polymerization N) for mesophase separation of OBCs with equal volume in hard and soft blocks could approach 3.0 due to the polydispersity in molecular weight and multiblock chain structure. C


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Figure 2. Time-resolved PCOM and corresponding POM images of BS50CL50 obtained at (a) 150 °C, (b) Tc,PBS = 82 °C, and (c) Tc,PCL = 36 °C for different times; the copolymer was first annealed at 150 °C for 3 min, then crystallized at 82 °C until saturation, and finally cooled to 36 °C for the crystallization of the PCL component during two-step isothermal crystallization process. (d) Upon reheating to 60 °C, the PCL crystals melted again. In the PCOM images, for clarity, the centers of spherulites are marked by blue circles.

Figure 2a shows that after annealing at 150 °C for 3 min a phase-separated structure with several micrometers phase size is observed by phase contrast optical microscopy. However, this phase structure has no significant influence on the crystallization morphology of the PBS chain segments within the copolymer. At the isothermal crystallization temperature of 82 °C, the PBS chain segments in both phases within BS50CL50 crystallize, although with different crystallization rates evidenced by the following SAXS analysis. The crystallization of the PBS component can spread through the phase domains, and this phenomenon is usually termed breakout crystallization.14 In this way, the PBS component can form the typical banded spherulites that fill the whole microscope field of view (Figure 2b), forming a PBS semicrystalline template. The spherulites are clearly negative in view of the characteristic colors observed by using a λ wave plate. Quadrants 1 and 3 are close to yellow, while quadrants 2 and 4 are blue, a characteristic of negative spherulites, meaning that the highest refractive index is tangential and coincides with the chain direction. During the isothermal crystallization process of the PBS component, the molten PCL component can be rejected from the front of PBS crystal growth into the amorphous regions. The molten PCL component, covalently linked to the previously crystallized PBS component, cannot be rejected far from the PBS lamella. The resultant spherulites consist of PBS crystalline lamellae and amorphous regions composed of amorphous PBS and molten PCL. It is noteworthy that after the isothermal crystallization of PBS the crystallinity of the PBS component in BS50CL50 (X1c,PBS) is only about 15.8% (Table 2), which means that a large amount of the multiblock copolymer chains remains in the amorphous regions of the spherulites. After the PBS component finishes its crystallization process at 82 °C (and forms the loosely packed spherulites with only 15.8% of PBS forming the radially oriented lamellae) and the temperature is lowered to 36 °C, the PCL component begins to crystallize (Figure 2c). Figure 2 demonstrates that during the crystallization

Table 2. Thermal Properties Obtained from DSC Isothermal Crystallization Curve for PBS and PCL Components in BS50CL50 (Tc,PBS = 82 °C, Tc,PCL = 36 °C) BS50CL50

ΔHm (J/g)

X1c (%)a

X2c (%)b

PBS PCL

31.6 15.5

15.8 11.5

31.6 23.0

a Crystallinity X1c,PBS and X1c,PCL are calculated by dividing ΔHm by the enthalpy of fusion of 100% crystalline polymer, shown as follows: X1c,PBS = ΔHm,PBS/ΔH0m,PBS, X1c,PCL = ΔHm,PCL/ΔH0m,PCL, where ΔH0m,PBS and ΔH0m,PCL are the enthalpy of fusion of 100% crystalline PBS (200 J/g)41 and PCL (134.9 J/g),42 respectively. bCrystallinity X2c,PBS and X2c,PCL are the crystallinity of PBS and PCL, respectively, calculated by normalizing X1c,PBS and X1c,PCL by the weight percentage of PBS and PCL components in the copolymers.

of the PCL component the morphological features of PBS spherulites do not exhibit any significant morphological change at the micrometer scale, while only the magnitude of the birefringence slightly increases as indicated by color changes (quadrants 1 and 3 become more yellow and quadrants 2 and 4 lighter blue). Upon reheating to 60 °C (above the melting temperature of PCL), the PCL crystals melt, and the magnitude of the birefringence recovers to the original state before crystallization of the PCL component (Figure 2d). The above results further confirm that the crystallization of the PCL component occurs within the previously formed PBS spherulitic template in the amorphous regions. Summarizing, the PBS chain segments within BS50CL50 crystallize in banded spherulites, occupying the whole visible space. The foregoing superstructural morphology of PBS component templates the later crystallization of the PCL component, and the magnitude of the birefringence of PBS spherulites is slightly enhanced by the crystallization of PCL. The birefringence recovers to the original state after remelting of PCL crystals. Crystal Structure. The crystal structure of BS50CL50 during two-step isothermal crystallization and the subsequent melting D


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Figure 3. One-dimensional (1D) WAXS intensity profiles of BS50CL50 obtained during two-step isothermal crystallization process: (a) Tc,PBS = 82 °C; (b) Tc,PCL = 36 °C. (c) The relative content of the (020/1̅11) plane reflection of PBS (φ(020/1̅11)PBS) and (110) plane reflection of PCL (φ(110)PCL). (d) Calculated diffraction spacings (d) from (a, b) according to Bragg’s law, dhkl = λ/(2 sin θhkl). (e) 1D WAXS intensity profiles of BS50CL50 obtained at different conditions. The difference value of two adjacent data points in WAXS curves is 0.0024°.

when the copolymer is quenched from 82 to 36 °C. In other words, d spacings decrease upon quenching to 36 °C (Figure 3d). In the subsequent crystallization at 36 °C, two other obvious peaks appear at 17.00° and 18.77°, corresponding to the (110) and (200) planes of PCL orthorhombic crystal form.9,42−44 Besides, a small peak at 17.47° corresponding to the (111) plane of PCL orthorhombic crystal form is overlapped with that of the (021) plane of PBS monoclinic α crystal form. As a result, the d spacing of the (021) plane after the crystallization of PCL is not calculated. A similar phenomenon has been reported previously.26,27,29 After crystallization at 30 °C, the peaks of PLLA at 13.28° and

process was measured by in situ WAXS. Figures 3a and 3b show the one-dimensional (1D) WAXS profiles of BS50CL50 obtained at 82 and 36 °C for different crystallization times. At 82 °C, only the PBS component is able to crystallize, while the PCL component remains molten. At the beginning of the isothermal crystallization at 82 °C, the WAXS profile shows a broad peak, indicating an amorphous state. Three peaks, located at 15.54°, 17.43°, and 17.93°, gradually become obvious with the crystallization time increasing, which can be indexed as (020/11̅ 1), (021), and (110) planes from the monoclinic α crystal form of PBS.5 It is remarkable that the three PBS peaks at 15.58°, 17.41°, and 17.94° are shifted significantly to 15.65°, 17.48°, and 18.10° E


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reaches maximum values with crystallization time (Figure 4c,d), which agrees with the change of crystallinity with time. The density of PBS is 1.18 g/cm3 for the amorphous phase and 1.34 g/cm3 for the crystalline phase,45−47 and the density of PCL is 1.08 g/cm3 for the amorphous phase and 1.15 g/cm3 for the crystalline phase.48,49 In general, the density contrast between crystalline and amorphous phases in polymers is relatively small, on the order of 0.1 g/cm3. The PCL component starts to crystallize when the temperature drops to 36 °C (Figure 4b). Figures 4c and 4d show that with the crystallization of the PCL component the intensity of peak 1 decreases significantly to a value just above the experimental resolution. Meanwhile, the intensity of peak 2 gradually increases and reaches a plateau. The general observation can be described briefly as follows. Peak 1 appears with the crystallization of the PBS component and becomes much weaker after the crystallization of the PCL component, whereas peak 2 appears with the crystallization of the PBS component and is located at an almost constant q value during the crystallization of the PCL component, except for a small increase in intensity. A morphological model for explaining these observations is provided in the following sections. For the PBS homopolymer, only one scattering peak at q = 0.5122 nm−1 appears after crystallization at 82 °C (Figure S2a), which is ascribed to the long period of the PBS crystalline lamellar structure. It is interesting to note that the q value for the only SAXS peak in PBS homopolymer (0.5122 nm−1) is close to that of peak 2 in BS50CL50 (0.4439 nm−1). The similarity of peak 2 of BS50CL50 and the SAXS peak of PBS homopolymer indicates the existence of inhomogeneous, macroscopically phase-separated domains with different chain compositions, namely, PBS-rich domains and PCL-rich domains. This is likely to happen in a multiblock copolymer with polydisperse block length, where the two blocks exhibit poor miscibility. Furthermore, the PBS and/or PCL prepolymers are randomly included in the multiblock copolymer by a chain extension reaction, which may produce very long PBS or PCL blocks. We should point out that the possibility of having homopolymer chain impurities in BS50CL50 is extremely small because chain extension occurs 20 times on average for each copolymer chain and statistically the possibility of connecting 20 PBS or PCL prepolymer segments is negligible (roughly (0.5)20). The evidence for the existence of PBS-rich and PCL-rich chains is obtained by solvent extraction experiments and the analysis of the fractions shown in the Methods section of the Supporting Information. This result is in line with the rheology behavior and phase contrast images. Additionally, it is found that during the crystallization process of the PBS component the intensity of peak 2 increases more rapidly than that of peak 1 (Figure 4e), indicating a larger PBS crystallization rate for the PBS chains within PBS-rich domains. Consequently, the SAXS results presented above can be interpreted as follows: at 82 °C, the high-q peak (or peak 2) arises from a periodic structure formed within PBS-rich domains consisting of PBS lamellae and amorphous regions containing PBS and molten PCL chains. The low-q peak (or peak 1) corresponds to a periodic structure formed within PCL-rich domains consisting of PBS lamellae and thick amorphous layers needed to accommodate the large fraction of molten PCL chains at 82 °C. Upon cooling the copolymer to 36 °C, the PCL chains in PCL-rich domains start to crystallize, which probably leads to an alternating double crystalline layer structure. According to

15.20° in PLLA-b-PEG copolymers would shift to 13.44° and 15.38°, respectively. The authors considered that PLLA crystals might be stretched or sheared by the tethered PEG during the crystallization process of PEG block.29 However, it is not the case in this study. It is worth noting that the d spacings of three PBS peaks reduce just after quenching to 36 °C (before the crystallization of the PCL component), whereas those remain constant during the crystallization process of the PCL component, indicating that the crystallization of the PCL component has no influence on the d spacings of PBS peaks (Figure 3d). Furthermore, the three PBS peaks in PBS homopolymer shift to higher angles upon quenching from 82 to 36 °C and recover to lower angles in the followed reheating process as well, predominantly attributed to the thermal shrinking and expansion effects resulted from temperature variation, which respectively results in the decrease and increase of d spacings (Figure S1 and Table S1 in the Supporting Information). As a consequence, the reduction in temperature induces the thermal shrinking of PBS crystals, and this is the reason for the reduction of d spacings in BS50CL50. During the subsequent reheating process, it is the thermal expansion of PBS crystals induced by the elevated temperature rather than the melting effect of the PCL component that leads to the rise in d spacings (Table 3). It is concluded that the copolymerization of PBS with Table 3. Diffraction Spacings (d) of PBS Component at Different Conditions Calculated from Figure 3e According to Bragg’s Law BS50CL50 d(020/1̅11)PBS (Å) d(021)PBS (Å) d(110)PBS (Å)

at 82 °C, 26 min

at 36 °C, 0.5 min

at 36 °C, 30 min

reheating to 60 °C

reheating to 80 °C

4.577

4.555

4.555

4.570

4.575

4.100 3.977

4.081 3.943

3.943

4.093 3.963

4.099 3.973

PCL does not alter the crystal structure, where the PBS and PCL component in the multiblock copolymer crystallizes separately and form their own crystals. Reversible Periodic Structure Induced by Crystallization/ Melting on Nanometer Scale. SAXS patterns of BS50CL50 during two-step isothermal crystallization and subsequent melting process were measured to investigate the microstructure transition on the nanometer scale. Figures 4a and 4b show Lorentz-corrected 1D SAXS profiles for BS50CL50 obtained at selected crystallization temperatures and times during two-step isothermal crystallization process. The product between the intensity (I) and the square of the scattering vector q is plotted versus q. For the quantitative characterization of SAXS curves, the Peakfit software is used to simulate these curves, and the normalized intensity variations of the two peaks are shown in Figures 4c and 4d as a function of time. For the crystallization process of the PBS component, (Iq2)t indicates the intensity at crystallization time t, while (Iq2)end means the intensity at the end of PBS crystallization; for the crystallization process of the PCL component, (Iq2)t indicates the intensity at crystallization time t, while (Iq2)0 means the intensity at the beginning of PCL crystallization. As shown in Figure 4a, when the PBS component crystallizes at 82 °C, two scattering peaks are clearly observed at q1 = 0.2056 nm−1 and q2 = 0.4439 nm−1 (labeled as peak 1 and peak 2, respectively) after some time has elapsed (a few minutes), suggesting that a complex superstructure has formed. The normalized intensity ((Iq2)t/(Iq2)end) of both peaks (1 and 2) first increases and then F


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Figure 4. 1D SAXS intensity profiles of BS50CL50 obtained during two-step isothermal crystallization process: (a) Tc, PBS = 82 °C; (b) Tc, PCL = 36 °C; normalized intensity variations of (c) peak 1 and (d) peak 2, for the crystallization process of the PBS component, (Iq2)t indicates the intensity at crystallization time t, while (Iq2)end means the intensity at the end of PBS crystallization; for the crystallization process of the PCL component, (Iq2)t indicates the intensity at crystallization time t, while (Iq2)0 means the intensity at the beginning of PCL crystallization; (e) normalized intensity variations of peak 1 and peak 2 during the crystallization process of the PBS component; (f) long periods of peak 2 obtained from (a) and (b); (g) 1D SAXS intensity profiles obtained during the reheating process; (h) calculated long periods of peak 2 from (g).

our previous studies,50 this will lead to a decreased intensity of peak 1 and an enhanced intensity of the second-order peak. This explains the intensity increase of peak 2 when PCL crystallizes, as the second-order peak of peak 1 locates at a similar q value together with peak 2. In order to confirm the above conclusion, a simulation of the SAXS curves has been applied. The model with several assumptions makes the simulated curve fit the

experimental curve very well, as shown in the Simulation section of the Supporting Information. It is well-known that 1D electron density correlation function curves have usually been employed to estimate the crystalline layer thickness. However, in this study, it is difficult to directly analyze the 1D correlation functions of the SAXS curves based on the standard two-phase model, as the 1D correlation functions G


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Figure 5. Schematic illustration of the microstructure formation at the nanometer scale during two-step isothermal crystallization and subsequent reheating process.

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CONCLUSIONS The reversible periodic structures in a symmetric PBS-co-PCL multiblock copolymer are induced by sequential crystallization and subsequent remelting process. A phase-segregated structure with several micrometers phase size is detected by phase contrast optical microscopy at 150 °C. However, this phase structure has no significant effect on the crystallization morphology of the PBS component that can form spherulitic superstructural templates, filling the whole microscope field of view. During the crystallization process of the PBS component at 82 °C, two scattering peaks appear. The high-q peak (or peak 2) originates from a periodic structure consisting of PBS lamellae and amorphous regions containing PBS and molten PCL chains, while the low-q peak (or peak 1) corresponds to a periodic structure consisting of PBS lamellae and thick amorphous layers needed to accommodate the large fraction of molten PCL chains at 82 °C. After crystallization of the PCL component, an alternating double crystalline layer structure of PCL and PBS forms, resulting in a decreased intensity of peak 1 and an increased intensity of peak 2. Upon reheating, after the melting of PCL crystals, peak 1 becomes larger and stronger once again, while peak 2 can still be observed, indicating the reversibility of the periodic structures induced by crystallization/melting. The PBS and PCL components within the multiblock copolymer crystallize separately and form their own crystals without modifying the crystal structure of each other. Upon cooling from the melt, PBS forms well-developed spherulites that template the morphology of the copolymer. The crystallization of the PCL component within the amorphous regions of the PBS spherulites does not alter the spherulitic morphology, only the magnitude of the birefringence. The enhanced birefringence recovers to the original state upon the remelting of PCL crystals. These results will add significantly to the understanding of periodic structures and crystallization/melting behaviors in biodegradable double crystalline multiblock copolymers.

estimated after the crystallization of PBS and PCL components respectively display a complex shape due to the overlap of periodic structures (Figure S3). Even though only PBS crystals remain at 82 °C, the equation lc = L × χv (lc is the crystalline layer thickness, L is the long period, and vc is the volumetric crystalline fraction approximated to the mass crystalline fraction (χm)) cannot be applied to measure the crystalline layer thickness of PBS as well. The reason is that the χm values obtained from the DSC isothermal crystallization data correspond to the entire mass crystalline fraction of the PBS component. It is impossible to calculate the separate contributions to χm that correspond to peak 1 and peak 2. After crystallization of the PBS component at 82 °C, the long periods of peak 1 (L1) and peak 2 (L2) are respectively about 30.5 and 14.1 nm. Just upon quenching from 82 to 36 °C (before crystallization of the PCL component), L1 decreases to about 30.1 nm (L1′) and L2 drops to about 13.1 nm (L2′). We assume that this phenomenon is mainly ascribed to the thermal shrinking effect of amorphous layer thickness. This assumption is relied on the fact that for PBS homopolymer, upon quenching from 82 to 36 °C, the crystalline layer thickness (lc) of PBS crystals varies little, whereas the amorphous layer thickness (la) reduces much more (Table S2). Moreover, the rise of long period (L) in PBS homopolymer during the reheating process (before the melting of PBS crystals) also comes from the increase of la. It is the thermal shrinking and expansion of PBS crystals due to the temperature change that brings about the variation of la and L (Figure S2 and Table S2). This phenomenon is in good accordance with the above WAXS results. During the reheating process, after the melting of PCL crystals, peak 2 can still be observed, while peak 1 becomes larger and stronger once again (Figure 4g), indicating the reversibility of the periodic structures induced by crystallization/melting. It is apparent that after the melting of PCL crystals and before the melting of PBS crystals the long period of peak 2 shows a nearly linear increase with temperature (Figure 4h), as has been discussed above in detail, primarily ascribed to thermal expansion. By the combination of the above results, a model is suggested to schematically illustrate the microstructural morphologies at the nanometer scale formed during two-step isothermal crystallization and subsequent reheating process (Figure 5).

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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.7b01779. H


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Macromolecules

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1D WAXS intensity profiles and d spacings of PBS homopolymer obtained during crystallization and reheating process; 1D SAXS intensity profiles and density correlation functions, long period (L), crystalline layer thickness (lc), and amorphous layer thickness (la) of PBS homopolymer obtained during crystallization and reheating process; 1D density correlation functions of BS50CL50; solvent extraction experiments and analysis of the fractions obtained; simulation of SAXS curves (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel and Fax +86-10-82618533; e-mail [email protected] (X.D.). ORCID

Xia Dong: 0000-0002-6409-7011 Guoming Liu: 0000-0003-2808-2661 Alejandro J. Müller: 0000-0001-7009-7715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the generous financial support of the following grants: National Natural Sciences Foundation of China (No. 21574140), China Postdoctoral Science Foundation (No. 2016M591111), and CAS President’s International Fellowship Initiative (PIFI) (No. 2016VMA010). We also thank the Shanghai Synchrotron Radiation Facility (Beamline BL14B1 and BL16B1) for beam time and help during the experiments.

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Reversible Lamellar Periodic Structures Induced by Sequential

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