Regulation of Crystallization Kinetics, Morphology, and Mechanical

Jan 2, 2014 - kinetics and morphology at various crystallization temperatures (Tcs) and corresponding .... Synchrotron Radiation Facility (SSRF) in Ch...
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Regulation of Crystallization Kinetics, Morphology, and Mechanical Properties of Olefinic Blocky Copolymers Zai-Zai Tong, Bing Zhou, Jie Huang, Jun-Ting Xu,* and Zhi-Qiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310827, China S Supporting Information *

ABSTRACT: Two olefinic blocky copolymers (OBCs) were quenched from different mixing states in the melt, and crystallization kinetics and morphology at various crystallization temperatures (Tcs) and corresponding mechanical properties were studied. It is observed that, at lower Tcs, premesophase separation in the melt accelerates crystallization of OBC-A with a weak segregation strength and a larger fraction of the crystalline hard blocks due to enrichment of the hard blocks in the hard-block-rich domains. By contrast, premesophase separation retards crystallization of OBC-B with a stronger segregation strength and lower fraction of the hard blocks because of the prevailing confinement effect at lower Tcs. Moreover, since the hard blocks dissolved in the soft-block-rich domains can crystallize at lower Tcs, which can bridge the crystals formed in different hard-blockrich domains, the crystal growth is not restricted. At higher Tcs, OBC-A crystallizes more slowly from the premesophaseseparated melt than that from the homogeneous melt, which is attributed to the weaker crystallizability of the hard blocks dissolved in the soft-block-rich domains and thus the restricted crystal growth. Nevertheless, mesophase separation always takes place prior to crystallization at higher Tcs for OBC-B because of the faster rate of mesophase separation. Therefore, the mixing state in the melt has little effect on crystallization and morphology of OBC-B at higher Tcs. It is found that the mechanical properties of OBCs can be regulated in a wide range by alteration of crystallization conditions. Better mechanical properties can be achieved when OBCs crystallize from the homogeneous melt and at a lower Tc.

1. INTRODUCTION Recently, a novel olefinic blocky copolymer (OBC) was synthesized via a so-called chain-shuttling technology in a continuous process with two catalysts and a chain shutting agent (CSA). 1−4 In polymerization, one catalyst shows poor copolymerization ability toward 1-octene, while the other promotes enchainment of comonomer. The CSA switches the growing chains from one catalyst to the other at random intervals. Therefore, the finally obtained polymer chains consist of crystallizable blocks (hard blocks) with a very low 1-octene content, altering with amorphous blocks (soft blocks) with a high 1-octene content.5 With the chain shuttling polymerization, the OBC chains have most-probable distributions in both the block length and the number of block per chain.6 The 1-octene contents in the hard and soft blocks, the lengths, and percentages of both blocks can be regulated by change of polymerization parameters.3 Consequently, OBCs with different chain architectures and thus different mechanical properties can be designed. Many researches have been conducted to study the structure and properties of OBCs, including crystallization behavior,5−12 phase behavior,13,14 mechanical properties,15−19 and blending.20−28 It is reported that the segregation strength of OBCs is dependent on the octene content difference between the soft and hard blocks (ΔC8). When ΔC8 is large enough, mesophase © 2014 American Chemical Society

separation may occur in the melt of OBCs, as revealed by rheology and small-angle X-ray scattering characterizations.13,14 The term “mesophase separation” is used for OBCs, instead of “microphase separation” for block copolymers, because the domain size of OBCs is usually larger than 100 nm. Such a domain size is much larger than that of monodispersed diblock copolymers with a similar molecular weight and is attributed to the polydispersity of OBCs.14,29 By contrast, OBCs with a smaller ΔC8 are weakly segregated in the melt, and mesophase separation is difficult to observe. Since mesophase separation may take place in the OBC melt, it can exert an effect on crystallization of OBC. Jin et al. observed that mesophase separation started prior to crystallization, and crystals could only go through interstitial spaces.7 For common crystalline/rubbery block copolymers, both breakout and confined crystallization can occur, depending on the volume fraction of the crystalline block and the relative segregation strength, which is defined as the Flory−Huggins interaction parameter at the crystallization temperature (χc) divided by the Flory−Huggins interaction parameter at the order−disorder transition temperature (χODT).30−32 Confined crystallization is Received: November 11, 2013 Revised: December 16, 2013 Published: January 2, 2014 333

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Table 1. Molecular Characteristic of OBCs sample

Mn (kg/mol)

Mw/Mn

C8 contenta (mol %)

C8 in hard blocka (mol %)

C8 in soft blocka (mol %)

ΔC8a (mol %)

f harda (wt %)

TcP b (°C)

TmP b (°C)

Xc c (%)

OBC-A OBC-B

29 40

3.1 2.3

13.2 15.9

1.4 1.0

23.7 21.9

22.3 20.9

35 20

103 90

119 120

15.7 7.0

a Determined from 13C NMR. bCrystallization peak temperature and melting peak temperature determined from DSC curves with a cooling and heating rate of 10 °C/min. cCrystallinity Determined by fusion enthalpy at a heating rate of 10 °C/min.

usually observed at a larger χc/χODT (>3.0) and a smaller volume fraction of the crystalline block. Similar results were reported for crystallization of OBCs. A larger ΔC8 results in a strong segregation strength and thus confined crystallization, whereas breakout crystallization occurs in weakly segregated OBCs.8 Fragmentary crystals are formed when crystallization of OBCs is confined. However, larger crystals across different domains sometime are observed even for strongly segregated OBCs. A “pass through” crystallization mode was proposed by Register to interpret this phenomenon.33 They believed that there were a large amount of hard blocks dissolved in the soft-block-rich domains due to the large polydispersity in OBCs, and these hard blocks could crystallize as well, which made the crystals formed in the hard-block-rich domains not stop at the interface of two phases and continue to grow. This new crystallization mode is rarely reported in monodispersed block copolymers. The “pass through” crystallization can also lead to formation of isotropic lamellar crystals when OBCs crystallizes from presheared melt.13 Although the effect of mesophase separation on crystallization of OBCs has been well studied, there are still some controversial results reported in the literature. For example, both confined crystallization and breakout crystallization were observed for strongly segregated OBCs.8,13 It is well-recognized that the hard blocks dissolved in the soft-block-rich domains play an important role in development of crystal morphology when OBCs crystallizes from the mesophase-separated melt. Moreover, even when OBCs are quenched from homogeneous melt to the crystallization temperature (Tc), there are two possibilities: occurrence of mesophase separation prior to crystallization or direct crystallization from the homogeneous melt, depending on the relative rate of mesophase separation and crystallization. A similar occurrence has been reported in blends containing a crystalline component.34−38 In the present work, crystallization of OBCs was carried out at different Tcs; thus, crystallizability of the hard blocks dissolved in the soft-block-rich domains and the relative rate of mesophase separation and crystallization could be regulated. On the other hand, the start mixing state in the melt is of great importance for well understanding the effect of mesophase separation on crystallization of OBCs. Herein the different start mixing states in the melt of OBCs were clearly characterized, and OBCs were quenched from homogeneous and mesophase-separated melt to crystallize. As a result, we combine several factors, including the chain structure, annealing temperature, and crystallization temperature, to change the segregation strength, the mixing state in the melt, the relative rate of mesophase separation versus crystallization, and crystallizability of the hard blocks dissolved in the soft-rich domains. The aim of the present work is to attempt to understand the crystallization pathways of OBCs under different conditions, so that the morphology and final mechanical properties of OBCs can be regulated.

2. EXPERIMENTAL SECTION Materials. The ethylene−octene blocky copolymers (OBCs) were supplied as pellets by Dow Chemical Company. The information on the two OBCs on molecular weight, octene contents in the hard and soft blocks, the weight percentage of the hard block, and crystallinity are summarized in Table 1. DSC Experiments. The differential scanning calorimetry (DSC) experiments were carried out on a TA Q200 calorimeter. Specimens weighing 3−5 mg were cut from compression-molded films for thermal analysis. The samples were first heated to 180 °C at a rate of 10 °C/min and held for 5 min to erase thermal history. Subsequently, two different thermal treatments were applied to the samples. (a) The samples were rapidly cooled to preset crystallization temperature (Tc) at a rate of 40 °C/min for isothermal crystallization. (2) The samples were cooled to 130 °C, held for prescribed time, and then were cooled to preset Tc at a rate of 40 °C/min to complete crystallization. The heat flow curves upon isothermal crystallization were recorded. All the measurements were under a nitrogen atmosphere with a flow rate of 50 mL/min, and the temperature was calibrated with indium. POM Observations. Polarized optical microscopy (POM) observations at various thermal conditions were carried out on an Olympus microscope (BX51) equipped with a hot stage. Thin film specimens of about 0.1 mm thickness were used for observation. The thermal treatments and Tcs were the same as those in DSC experiments. TEM Characterizations. Specimens for transmission electron microscopy (TEM) observation were prepared by dropping 0.15% xylene solution of the polymers on the carbon film supported on a glass slide at 120 °C. After evaporation of the solvent, the samples were placed on a hot stage and underwent the same thermal treatments as those in DSC experiments. After thermal treatment, the glass slide was corroded by hydrofluoric acid (40% w/w), and the floating samples were captured by copper grids. Observations were carried out on a JEOL JEM-1200EX instrument at an acceleration voltage of 80 kV. Small-Angle X-ray Scattering. Small-angle X-ray scattering (SAXS) experiments were performed at BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF) in China. The wavelength of Xray was 1.24 Å, and the sample-to-detector distance was set as 5100 mm. Two-dimensional (2D) SAXS patterns at room temperatures were recorded. The average exposure time was 300 s for each scan. Bull tendon was used as standard material for calibrating the scattering vector. The 2D SAXS patterns were converted into one-dimensional (1D) SAXS profiles using Fit2D software. Mechanical Behavior. The stress−strain behavior of two OBCs after different thermal treatments under uniaxial tension was performed on a CMT 4204 instrument. The tensile specimens were cut from the compression-molded films with the thickness of about 0.5 mm. The distance of two grips was 20 mm, and the specimen width was 2.4 mm. A strain rate of 20 mm/min, i.e., 100% per min, was applied to uniaxial tension.

3. RESULTS AND DISCUSSION Mesophase Separation in the Melt. In our previous work, the chain architecture of two OBCs was characterized and compared in detail.12 The information on two OBCs on molecular weight, octene content both in the hard and soft blocks, and weight percentage of hard block as well as the crystallization temperature, melting temperature, and crystallinity are concluded in Table 1. The sample OBC-A has a slightly larger value of ΔC8 between the soft and hard blocks than OBC334

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Figure 1. TEM micrographs of (a) OBC-A and (b) OBC-B after annealing at 180 °C for 10 min followed by quenching with liquid nitrogen.

Figure 2. TEM micrographs of (a) OBC-A and (b) OBC-B after annealing at 130 °C for 24 h followed by quenching with liquid nitrogen.

the temperature is recovered to room temperature for TEM observation, and thus the hard blocks can still crystallize to some extent. Therefore, the morphology observed by TEM may be slightly different from the actual morphology of OBCs in the melt. The morphologies of two OBCs after annealing at 130 °C for 24 h followed by quenching with liquid nitrogen are shown in Figure 2. It is observed that OBC-A exhibits a rod-like morphology (similar to cylinder), while OBC-B shows an island-like texture. The dark rod- and island-like domains should be formed by the crystals of the hard blocks of OBCs due to the denser packing. Since the OBCs are quenched to very low temperature, the value of χc/χODT is very large, and crystallization should be confined in the hard-block-rich domains. Moreover, according to the phase diagram,39,40 the common weakly segregated diblock copolymers will also exhibit a cylindrical morphology and a spherical morphology at the compositions similar to those of OBC-A and OBC-B, respectively. Therefore, the observed textures in Figure 2 can basically reflect the mesophase-separated morphologies in these two samples. As compared with the morphologies of diblock copolymers, neither the cylindrical domains of OBC-A are hexagonally packed nor the spherical domains of OBC-B are body-centric cubic. This

B. However, the weight percentage of hard blocks in OBC-A is evidently larger than that in OBC-B, leading to higher crystallinity of OBC-A (Table 1). It is reported that OBCs exhibit an upper critical solubility temperature (UCST)-type phase diagram,24 which means that OBCs are homogeneous at a higher temperature (usually above 180 °C) but may become mesophase-separated at a lower temperature. As a result, both samples were first annealed at a higher temperature (Ts = 180 °C) to form homogeneous melt or at a lower temperature (Ts = 130 °C) to produce mesophase-separated melt, respectively, and then quenched with liquid nitrogen to preserve the morphology in the melt. The temperature of 130 °C is chosen for phase separation because it is low enough to get a strong driving force for mesophase separation but still higher than the melting temperature of OBCs. Figure 1 shows the TEM images of two OBCs quenched from 180 °C. It is observed that the dot-like domains are uniformly dispersed in the matrix of OBC-A, as shown in Figure 1a. For OBC-B, the mesophase-separated texture can hardly be observed, and only much fewer and smaller dot-like microdomains can be discerned (Figure 1b). We speculate that the dark dot-like microdomains are the crystals of hard blocks. Although the OBC samples are quenched with liquid nitrogen, 335

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Figure 3. Variation of the relative crystallinity with crystallization time for OBCs crystallized from different mixing states in the melt and at different Tcs: (a) OBC-A at Tc = 108 °C; (b) OBC-B at Tc = 108 °C; (c) OBC-A at Tc = 115 °C; (d) OBC-B at Tc = 115 °C. The annealing temperature and time as well as Tc are indicated in the figures.

shows that the arrangement of the mesophase-separated domains in OBCs is not so ordered as that in common diblock copolymers. Island-like texture was also reported for an OBC with a similar weight fraction of the hard blocks in OBC-B.7,19 One can see from Figure 2 that the size of the domains formed by the hard blocks in OBC-A is about 350 nm in length and 60 nm in width, and the average size of the dispersed spherical domains is about 150 nm in diameter for OBC-B. These values are evidently larger than the microdomain sizes of common polyethylene-containing di- or triblock copolymers with a similar molecular weight.13 The large domain size of OBCs was reported by other authors as well, and it was attributed to the large polydispersity of OBC.13 Since the domain sizes in OBC-A and OBC-B are much larger than the lengths of the hard blocks, extensive mixing of the hard and soft blocks occurs in the OBCs. The observed dispersed and continuous phases are indeed hardblock-rich and soft-block-rich phases. Comparing the morphologies of OBCs in Figures 1 and 2, one can see that annealing at 130 °C for a long time can undoubtedly form a well-developed mesophase-separated structure for both samples. By contrast, annealing at 180 °C may yield homogeneous melt, though the possibility of mesophase separation cannot be completely excluded. Overall Crystallization Rates. TEM result shows that mesophase separation in OBCs is null or weak at 180 °C, but it takes place at 130 °C. As a result, we can produce two start mixing states in the melt of OBCs: homogeneous state and mesophaseseparated state, by annealing the OBCs at different temperatures to study the effect of premesophase separation in the melt on crystallization kinetics of OBCs. Moreover, the OBCs are

annealed for different times so that the rate of mesophase separation can be evaluated. It should be noted that, when OBCs are quenched from the homogeneous melt to crystallization temperature (Tc), both mesophase separation and crystallization may take place. Crystallization of OBCs is conducted at two different Tcs; thus, the relative rate of phase separation and crystallization can be changed. Considering the different crystallization peak temperatures of two OBCs (103 °C for OBC-A and 90 °C for OBC-B in Table 1), rational Tcs for isothermal crystallization should be selected to avoid too fast or too slow crystallization rates. Therefore, we chose 108 °C as a low Tc and 115 °C as a high Tc to regulate the crystallization rate, at which crystallization will be finished within minutes and several hours, respectively. However, it is noted that crystallization at 108 °C for OBC-B is still much slower than that of OBC-A; thus, the data at a lower Tc (100 °C) were added for OBC-B for the purpose of comparison (see Supporting Information). Figure 3 shows the plots of the relative crystallinity versus crystallization time for OBCs at two Tcs after quenching from the melt with different mixing states, respectively. At a lower Tc (Tc = 108 °C), the samples were also annealed at 130 °C for different times. As shown in Figure 3a, at Tc = 108 °C, the relative crystallinity increases with crystallization time more rapidly for OBC-A quenched from 130 °C, indicating a faster crystallization rate, whereas OBC-A quenched from 180 °C exhibits a slower crystallization rate. This shows that premesophase separation in the melt can accelerate crystallization of OBC-A at this Tc. In addition, one can see that the crystallization rate of OBC-A after annealing at 130 °C for 24 h is faster than that with an annealing 336

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time of 6 h. This implies that mesophase separation of OBC-A is not completed yet after annealing at 130 °C for 6 h and more thorough mesophase separation can facilitate crystallization of OBC-A at Tc = 108 °C. By contrast, at Tc = 108 °C the crystallization rate of OBC-B quenched from the homogeneous melt (Ts = 180 °C) is faster than that of OBC-B quenched from the mesophase-separated melt (Ts = 130 °C) (Figure 3b). It is also found that the crystallization rate of OBC-B at Tc = 108 °C is faster after annealing at 130 °C for 3 h than that after annealing at 130 °C for 1 h, but almost the same as that after annealing at 130 °C for 6 h. This shows that mesophase separation is already finished after 3 h, and premesophase separation in the melt can retard crystallization of OBC-B at Tc = 108 °C. Comparing Figures 3a and 3b, one can see that the effect of premesophase separation in the melt on crystallization of OBC-B at Tc = 108 °C is not so pronounced as that on crystallization of OBC-A. This is because OBC-B has lower crystallization peak temperature than OBC-A because of the smaller content of the hard blocks (Table 1), though they have similar octene contents in the hard blocks. For OBC-B, Tc = 108 °C is not a low enough crystallization temperature. As a result, the effect of premesophase separation on crystallization of OBC-B at a lower Tc = 100 °C was studied as well (Figure S1 in the Supporting Information). It can be seen that premesophase separation in the melt hinders crystallization of OBC-B more severely at this Tc. Examining the effect of annealing time at 130 °C on crystallization rate of OBCs (Figures 3a and 3b), one can see that OBC-B has a faster rate of mesophase separation than OBCA at 130 °C. The different rates of mesophase separation of OBC-A and OBC-B may originate from their different chain structures. For diblock copolymers, the rate of microphase separation is proportional to the quench depth and chain radius of gyration.41,42 The quench depth is the difference between the order−disorder transition temperature (TODT) and the temperature for microphase separation, which is TODT −Ts for OBCs in this work. Ts is the same for OBC-A and OBC-B, and TODT is determined by the composition (the content of the hard blocks) and χN at TODT, i.e., (χN)ODT, where χ is the Flory−Huggins interaction parameter and N is the total polymerization degree of a hard block and a soft block. For OBCs, the value of χ is related to ΔC8 and inverse to temperature. The data in Table 1 show that the values of ΔC8 in OBC-A and OBC-B are similar, which means a similar χ of these two samples at the same temperature. Based on the phase diagram of diblock copolymers,39,40 the value of (χN)ODT is smaller for OBC-B, which has a lower content of the hard blocks. On the other hand, the chain radius of gyration is proportional to the square root of the total molecular weight of a hard block and a soft block, i.e., N1/2. As a result, the faster mesophase separation rate of OBC-B indicates that it possesses longer hard and soft blocks, leading to the larger chain radius of gyration and higher TODT and thus larger quench depth. In combination with the data of octene contents in the soft and hard blocks, the weight percentage of the hard blocks and the overall length of the soft and hard blocks, the chain structures of OBC-A and OBC-B can be schematically depicted in Scheme 1. The different effects of mesophase separation on crystallization rate of OBC-A and OBC-B at lower Tc can be interpreted in terms of the interplay of confinement and concentration of the hard blocks induced by mesophase separation. As mentioned above, mesophase separation in OBCs leads to formation of a hard-block-rich phase and a soft-block-rich phase, as shown in Scheme 2.24 This is more like the phase separation in a binary blend than the microphase separation in a monodispersed

Scheme 1. Possible Chain Architecture of Two OBCs

Scheme 2. Schematic Phase Diagram for OBCs

diblock copolymer, in which two blocks are usually completely segregated and a sharp interface is formed. At lower Tc, no further mesophase separation occurs after OBCs are quenched from the melt because crystallization proceeds faster than mesophase separation; thus, the hard blocks of OBCs crystallize at the concentration in the melt before quenching. As can be seen from Scheme 2, when OBCs are quenched from homogeneous melt, the concentration of the hard blocks at Tc is the average concentration of the hard blocks in OBCs, f. By contrast, when OBCs are quenched from the mesophase-separated melt, the concentration of the hard blocks in the hard-block-rich phase is f 2, which is larger than f. Because of the higher concentration of the hard blocks, the hard-block-rich phase may crystallize faster than the homogeneous melt. However, mesophase separation may also exert a confinement effect on crystallization of the hard blocks. In crystalline/amorphous diblock copolymers, confined crystallization usually occurs for a block copolymer with a larger segregation strength (χN) and a smaller fraction of the crystalline component.30,31 As revealed by the faster mesophase separation rate, OBC-B has a larger χN than OBC-A. Moreover, the weight percentage of the hard blocks in OBC-B is smaller than that in OBC-A (Table 1). As a result, the confinement effect is more severe in OBC-B. Confinement effect usually leads to a slower crystallization rate and a lower crystallization temperature.43−46 We also notice that the crystallization peak temperature of OBCB is much lower than that of OBC-A, though the octene content in the hard blocks of OBC-B is even slightly smaller than that in OBC-A (Table 1). This confirms the stronger confinement effect in OBC-B. In OBCs, both concentration of the hard blocks in the hard-block-rich phase and confinement of the mesophaseseparated structure influence crystallization of the hard blocks, and these two factors are competitive. For OBC-A, the confinement effect is weaker due to its smaller χN and higher weight percentage of the hard blocks, and the concentration effect is predominant; thus, OBC-A quenched from 130 °C crystallizes faster than OBC-A quenched from 180 °C at Tc = 108 337

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Figure 4. Avrami plots of OBC-A and OBC-B at Tc = 108 °C (a) and at Tc = 115 °C (b).

°C. On the contrary, in OBC-B the confinement overwhelms the concentration effect; thus, OBC-B crystallizes more slowly from the mesophase-separated melt than from the homogeneous melt at Tc = 108 °C. The crystallization rates of OBC-A and OBC-B at a higher Tc were investigated as well. The plots of the relative crystallinity versus crystallization time for OBCs at Tc = 115 °C after quenching from the melt with different mixing states are illustrated in Figures 3c and 3d, respectively. It can be seen from Figure 3c that at Tc = 115 °C OBC-A quenched from 180 °C crystallizes faster than the sample annealed at 130 °C for 24 h. Such a trend is reverse to that at Tc = 108 °C. The different effects of premesophase separation on crystallization of OBC-A at higher and lower Tcs are related to crystallizability of the hard blocks dissolved in the soft-block-rich phase. At higher Tcs, the hard blocks dissolved in the soft-block-rich domains are difficult to crystallize due to a small supercooling; thus, the growth of the crystals formed in the hard-block-rich domains is restricted, leading to a slower overall crystallization rate. This will be further confirmed by the smaller Avrami exponent and crystal morphology in the next section. It should be noted that the equilibrium melting temperature (Tm0 ) of the crystalline component in a miscible blend varies with its concentration, based on the Flory theory.47 The larger the concentration of the crystalline component, the higher the value of T0m is. For crystalline polymers, the crystallization rate is strongly dependent on the supercooling, which is defined by T0m − Tc.48 If Tc is not far from T0m, a smaller supercooling results in a slower crystallization rate. As a consequence, the supercoolings of the hard blocks in the soft-block-rich and hard-block-rich phases are different at the same Tc. The supercooling in the soft-block-rich domains is smaller due to lower concentration of the hard blocks, and thus crystallizability of the hard blocks dissolved in the softblock-rich domains is weaker. On the other hand, it is found that whether mesophase separation occurs in the melt has little effect on the crystallization rate of OBC-B at Tc = 115 °C, as revealed in Figure 3d. This result can be interpreted from the relative rate of mesophase separation versus crystallization. The data in Figure 3b have shown that OBC-B has a faster mesophase separation rate; thus, mesophase separation may take place quickly at Tc = 115 °C after OBC-B is quenched from homogeneous melt. Moreover, at a higher Tc, i.e., 115 °C, the crystallization rate of OBC-B is very

slow because of the smaller supercooling. This means that mesophase separation may take place prior to crystallization for OBC-B at Tc = 115 °C, irrespective of the previous mixing state in the melt. As a result, OBC-B always crystallizes from the mesophase-separated melt at Tc = 115 °C and the premesophase separation in the melt barely affects the crystallization rate. By contrast, at a lower Tc, crystallization of OBC-B proceeds faster than its mesophase separation; thus, the crystallization rate is strongly dependent on the mixing state in the melt. A more obvious difference in crystallization behavior is observed at Tc = 100 °C after quenching from different mixing states (Figure S1 in Supporting Information). Since complete crystallization at Tc = 108 °C consumes 30 min but only 5 min at Tc = 100 °C, mesophase separation should proceed to some extent prior to crystallization at 108 °C, which may weaken the effect of premesophase separation on crystallization. Avrami Exponents. The data of isothermal crystallization for OBCs quenched from melt with different mixing states and at different Tcs were analyzed with Avrami equation:49 1 − X t = exp( −kt n)

(1)

where Xt is the relative crystallinity at crystallization time t; k and n are crystallization rate constant and Avrami exponent, respectively. The Avrami exponent n often provides useful qualitative information on the nature of crystallization process, including the crystal growth geometry and nucleation mechanism.50 The plots of log[−ln(1 − Xt)] versus log t for OBCs at different crystallization conditions are presented in Figure 4, and the derived Avrami exponents are listed in Table 2. One can see that both OBC-A and OBC-B exhibit an Avrami exponent close to 3.0 at lower Tc (108 °C), whether mesophase separation takes place in the melt or not. Such an Avrami exponent is typical for a crystallization process with a heterogeneous nucleation mechanism and three-dimensional crystal growth. Formation of threedimensional crystal, i.e., spherulite, will be verified by polarized optical micrographs in the next section. For crystalline diblock copolymers, spherulites can only be observed for a breakout crystallization process, while in confined crystallization the growth of crystals is restricted and even homogeneous nucleation occurs. Therefore, a smaller Avrami exponent is frequently observed for confined crystallization of block copolymers.31,51−56 Larger Avrami exponents close to 3.0 for OBC-A crystallized 338

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condition should be smaller because the confinement effect prevails over the concentration effect. Spherulites were also observed by Li for OBC with diblock and multiblock structures at a sufficiently high ΔC8.13,33 They proposed that the spherulites were formed via a “pass through” crystallization mode, in which the crystals formed in the hard-block-rich domain may continue to grow into an adjacent soft-block-rich domain rather than stopping abruptly at the interface, since the hard blocks dissolved in the soft-block-rich phase can still crystallize due to the large supercooling at lower Tcs.33 This can also explain the large Avrami exponent of OBC-B crystallized from the mesophaseseparated melt at Tc = 108 °C. At a higher Tc (Tc = 115 °C), OBC-A crystallized from the homogeneous melt exhibits an Avrami exponent close to 3.0, indicating formation of spherulites. By contrast, the value of Avrami exponent for OBC-A quenched from 130 °C, i.e., crystallized from the mesophase-separated melt, is around 2.0. OBC-B also has an Avrami exponent about 2.0 at Tc = 115 °C irrespective of the mixing state in the melt. The smaller Avrami

Table 2. Avrami Exponents of Two OBCs at Different Crystallization Conditions sample

Ts (°C)

Tc (°C)

n

OBC-A

180 130 180 130 180 130 180 130

108 108 115 115 108 108 115 115

2.87 2.80 2.84 2.16 2.90 2.95 2.03 2.12

OBC-B

from both the homogeneous and mesophase-separated melts are quite normal, since it has a weaker segregation strength and crystallization should be breakout.46,57 However, it is confusing that OBC-B crystallized from the mesophase-separated melt also has an Avrami exponent close 3.0 at Tc = 108 °C. It is expected that the Avrami exponent for OBC-B under such a crystallization

Figure 5. POM micrographs of OBCs after complete crystallization under various crystallization conditions (left a−h) and corresponding plots of spherulite radius versus crystallization time for OBCs (right i−l). (a) OBC-A quenched from 180 °C at Tc = 108 °C; (b) OBC-A quenched from 130 °C at Tc = 108 °C; (c) OBC-B quenched from 180 °C at Tc = 108 °C; (d) OBC-B quenched from 130 °C at Tc = 108 °C; (e) OBC-A quenched from 180 °C at Tc = 115 °C; (f) OBC-A quenched from 130 °C at Tc = 115 °C; (g) OBC-B quenched from 180 °C at Tc = 115 °C; (h) OBC-B quenched from 130 °C at Tc = 115 °C. (i) OBC-A at Tc = 108 °C; (j) OBC-B at Tc = 108 °C; (k) OBC-A Tc = 115 °C; (l) OBC-B at Tc = 115 °C. 339

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Figure 6. TEM micrographs of OBCs after crystallization under various conditions: (a) OBC-A quenched from 180 °C at Tc = 108 °C; (b) OBC-A quenched from 130 °C at Tc = 108 °C; (c) OBC-B quenched from 180 °C at Tc = 108 °C; (d) OBC-B quenched from 130 °C at Tc = 108 °C; (e) OBC-A quenched from 180 °C at Tc = 115 °C; (f) OBC-A quenched from 130 °C at Tc = 115 °C; (g) OBC-B quenched from 180 °C at Tc = 115 °C; (h) OBC-B quenched from 130 °C at Tc = 115 °C.

exponent implies that the growth dimension of the crystals is reduced. It should be noted that mesophase separation always takes place prior to crystallization at Tc = 115 °C, no matter the

melt of OBC-B is homogeneous or mesophase-separated. On the basis of this finding, we can draw the conclusion that at a higher Tc crystallization of OBCs from homogeneous melt is not 340

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which have a lower growth dimension than spherulites, the obtained Avrami exponent becomes smaller. Figure 6 shows the TEM micrographs of OBC-A and OBC-B crystallized at lower and higher Tcs after quenching from the homogeneous or mesophase-separated melt. It is revealed that well-developed spherulites with fibril lamellar stacks are observed for OBC-A quenched from the homogeneous melt at Tc = 108 °C, as shown in Figure 6a. The spherulites are closely packed with a clear boundary. Spherulite-like structure is also formed for OBC-A quenched from the mesophase-separated melt at Tc = 108 °C (Figure 6b). However, the spherulites in Figure 6b are not so well-developed. In addition, the spherulite size in Figure 6b is a little smaller than that in Figure 6a. These differences originate from the different mixing states of the melt before crystallization. When mesophase-separated domains are present in the melt prior to crystallization, nucleation is easier inside the hard-block-rich domains due to the higher concentration of the hard blocks (Scheme 2), leading to a higher nucleation density and a smaller size of spherulite. By contrast, when OBC-A crystallizes from the homogeneous melt, the nucleation density is lowered. The morphologies of OBC-A at Tc = 108 °C observed by TEM are basically in accordance with those observed by POM and the data of overall crystallization kinetics. However, we can see that the spherulites in Figures 6a and 6b are much smaller than those in Figures 5a and 5b. This is due to the different thicknesses of the films used for POM and TEM observations. Much thinner films with only nanometric thickness are used in TEM experiments, in which crystallization of OBCs is suppressed more severely due to confinement in the thickness direction. As for OBC-B at Tc = 108 °C, lamellar bundles instead of welldeveloped spherulites are observed (Figures 6c and 6d). Such morphology is also different from the spherulitic morphology observed by POM. This also results from the suppressed crystallization in the nanometric thin films used for TEM observation. The discrepancy in morphology between the POM and TEM observations is frequently reported in the literature for OBCs.7−10 POM revealed that spherulites could be formed for an OBC with only 7% crystallinity,6 but nanosized crystals were also observed by TEM.9 However, we notice that the crystal size in Figure 6d is still far larger than the size of the mesophaseseparated domains (Figure 2b). This shows that, even though OBC-B crystallizes from the mesophase-separated melt, the crystals growth at Tc = 108 °C is not restricted, agreeing with its larger value of Avrami exponent (Table 2). This can be ascribed to crystallization of the hard blocks dissolved in the soft-blockrich domains. On the other hand, the crystal size in OBC-B crystallized from the homogeneous melt is smaller, but the number of the crystal is larger (Figure 6c), as compared with that crystallized from the mesophase-separated melt (Figure 6d). This is an indication of retarded crystallization for OBC-B quenched from the mesophase-separated melt, which is consistent with the DSC and POM results. Morphologies are also compared after OBC-B crystallized from the melts with different mixing states at Tc = 100 °C, as shown in Figure S2 of the Supporting Information. Evident difference in morphology is also observed, which is similar to that at Tc = 108 °C. At Tc = 115 °C, branching fibrils inside the spherulites are observed for OBC-A quenched from the homogeneous melt (Figure 6e). Such crystal morphology agrees with the larger Avrami exponent (∼3.0). Nevertheless, OBC-A quenched from the mesophase-separated melt exhibits a quite different morphology from that quenched from the homogeneous melt

confined, whereas the crystal growth is restricted when OBCs crystallize from mesophase-separated melt. This can also be attributed to the incapability of crystallization for the hard blocks dissolved in the soft-block-rich phase at a smaller supercooling (a higher Tc). As a result, crystals formed by the hard blocks dissolved in the hard-block-rich domains are difficult to penetrate into the soft-block-rich domains to form crystals of larger size. Morphology after Crystallization. In order to verify the effect of premesophase separation on crystallization kinetics of OBCs, the morphologies of OBCs after crystallization under various conditions were characterized with POM and TEM. The POM micrographs of OBCs after complete crystallization under various crystallization conditions, and the corresponding variations of spherulite radius with crystallization time are shown in Figure 5. It is observed that at Tc = 108 °C spherulites with a smaller size and an enhanced nuclei density are formed for OBC-A crystallized from the mesophase-separated melt, as compared with those in OBC-A crystallized from the homogeneous melt (Figures 5a and 5b). The linear spherulitic growth rate of OBC-A crystallized from the mesophaseseparated melt at Tc = 108 °C is also larger than that of OBCA crystallized from the homogeneous melt (Figure 5i). This can be attributed to the higher concentration of the hard blocks in the hard-block-rich domains than that in the homogeneous melt. The POM result shows that the faster overall crystallization rate of OBC-A crystallized from the mesophase-separated melt at Tc = 108 °C is due to both the enhanced nucleation density and larger linear spherulitic growth rate. Spherulites are also formed in OBC-B crystallized from both the homogeneous and mesophase-separated melt at Tc = 108 °C (Figures 5c and 5d). A slight difference in nucleation density is observed. In addition, the linear spherulitic growth rate of OBCB crystallized from the mesophase-separated melt at Tc = 108 °C is smaller than that crystallized from the homogeneous melt (Figure 5j), which may be the main factor responsible for the slower overall crystallization rate of the former sample. One can see from Figures 5e and 5f that spherulites of OBC-A become more open and coexist with crystallites of smaller size at a higher Tc, i.e., Tc = 115 °C. The linear spherulitic growth rate of OBC-A crystallized from the mesophase-separated melt at Tc = 115 °C also becomes smaller (Figure 5k), as compared with that of OBC-A crystallized from the homogeneous melt. This result agrees with the slower overall crystallization rate of premesophase-separated OBC-A at Tc = 115 °C. Both spherulites and small crystallites are observed for OBC-B at Tc = 115 °C (Figures 5g and 5h). This is possibly due to the large polydispersity of OBCs as well. Crystal growth in some domains is unrestricted, and spherulites are formed, whereas crystal growth in other domains is restricted and thus small crystallites are formed. There is no big difference in morphology and linear spherulitic growth rate for OBC-B crystallized from the homogeneous and mesophase-separated melt (Figure 5l). As pointed out above, this is due to occurrence of mesophase separation prior to crystallization at Tc = 115 °C; thus, OBC-B crystallizes from the similar melt state. The observation of spherulite in OBC-A crystallized from the mesophase-separated melt and OBC-A at Tc = 115 °C is out of our expectation, since an Avrami exponent close to 2.0 is yielded from the overall crystallization kinetics, which indicates a two-dimensional growth of the crystals. The possible explanation is that the Avrami exponent is an apparent value reflecting the growth dimension of different crystals in the sample. Since there exist lots of smaller crystallites at Tc = 115 °C, 341

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Scheme 3. Illustration for the Effects of Ts, Tc, and Segregation Strength (χN) on Crystallization Behavior of OBCsa

a

MS melt and homo-melt denote mesophase-separated melt and homogeneous melt, respectively.

Scheme 4. Schematic Illustration for Morphology Evolution of OBCs Crystallized from the Mesophase-Separated Melt at Different Tcsa

a

At lower Tcs crystals with a branching structure are formed. The crystals can span different hard-block-rich domains, and the crystal growth is unrestricted. At higher Tcs crystal growth is restricted, and crystals with few branches are formed. These differences mainly arise from the different crystallizability of the hard blocks dissolved in the soft-block-rich domains at various Tcs.

at Tc = 115 °C (Figure 6f). Rod-like crystals scatter in the sample and the size of the crystals (700 nm in length and 150 nm in width) is about twice that of the mesophase-separated domains (350 nm in length and 60 nm in width) in the melt (Figure 2a). Such a crystal size is far smaller than that of OBC-A crystallized from homogeneous melt at Tc = 115 °C or crystallized at Tc = 108 °C. This shows that the crystal growth is severely restricted when OBC-A crystallizes from the mesophase-separated melt at a higher Tc, which agrees well with its smaller Avrami exponent. As pointed out above, the restricted growth of the crystals in OBC-A quenched from mesophase-separated melt at Tc = 115 °C is due to the weak crystallizability of the hard blocks dissolved in the soft-block-rich domains. It should also be noted that

crystallization of the hard blocks dissolved in the soft-block-rich domains just becomes slower at higher Tcs. When mesophaseseparated OBC melts are slowly cooled or the crystallized OBCs are annealed near the melting temperature, the hard blocks dissolved in the soft-block-rich domains can still crystallize, leading to unrestricted growth of the crystals.8 Because of this, the term “restricted crystallization” instead of “confined crystallization” is used in the present work. Moreover, comparing the crystal morphologies of OBC-A at Tc = 108 °C and Tc = 115 °C crystallized from the mesophase-separated melt (Figures 6b and 6f), the possibility that the restricted crystal growth at Tc = 115 °C is due to confinement effect can be excluded. For the same polymer, the relative segregation strength (χc/χODT) is 342

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Figure 7. Lorenz-corrected SAXS profiles of OBC-A (a) and OBC-B (b) after crystallization under various conditions.

Figure 8. One-dimensional correlation functions for OBC-A (a) and OBC-B (b) after crystallization under various conditions. The curves are vertically shifted for better clarity.

(segregation strength, χN), the mixing state in the melt (Ts) and crystallization temperature (Tc), as depicted in Scheme 3. We can also see that crystallizability of the hard blocks dissolved in the soft-block-rich domains, which is mainly determined by Tc, has a great influence on the overall crystallization rate, crystal morphology, and Avrami exponent of OBCs. At a lower Tc, the hard blocks dissolved in the softblock-rich domains can crystallize and then can bridge the crystals formed in different hard-block-rich domains, leading to a larger crystal size and a branching crystal structure. By contrast, when OBCs crystallize from the mesophase-separated melt and at a higher Tc, the hard blocks dissolved in the soft-block-rich domains are difficult to crystallize; thus, the growth of the crystals formed in the hard-block-rich domains is restricted, and branching of the crystals is suppressed as well. These two situations can be schematically depicted in Scheme 4. The variable crystallization with Tc of the hard blocks dissolved in the soft-block-rich domains results in some unique crystallization characteristics of OBCs differing from common crystalline/ amorphous diblock copolymers. SAXS Results. Although TEM can provide the information on crystal morphology, the lamellar thickness of the crystals is

smaller at a higher Tc;30−32 thus, breakout crystallization should take place more easily at Tc = 115 °C instead of at Tc = 108 °C. On the other hand, scattered crystal bundles of small size are observed for OBC-B quenched from both 180 and 130 °C (Figures 6g and 6h), since mesophase separation always occurs prior to crystallization at Tc = 115 °C. This shows that the crystal growth of OBC-B is restricted under both crystallization conditions; thus, smaller Avrami exponents are observed accordingly. The crystals in OBC-B quenched from 180 °C are slightly larger than those quenched from 130 °C. This is probably due to the slightly different concentrations of the hard blocks in the hard-block-rich domains because the mesophase separation in OBC-B quenched from 180 °C takes place at Tc = 115 °C, while it occurs at 130 °C for OBC-B quenched from the mesophase-separated melt. It should be noted that the morphologies observed TEM (Figures 6g and 6h) are quite different from that observed by POM (Figures 5g and 5h) for OBC-B at Tc = 115 °C. This is due to that the confinement effect of the film thickness is much stronger for a polymer with weaker crystallizability. Above results show that the crystallization behavior of OBCs is simultaneously affected by three parameters: the chain structure 343

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Figure 9. Stress−strain curves of OBC-A (a) and OBC-B (b) at room temperature after different thermal treatments. The annealing and crystallization temperatures are indicated in the figures. The crystallization time at Tc = 108 and 115 °C is 1 and 6 h, respectively. After crystallization, the samples were quenched with liquid nitrogen.

crystallization kinetics and morphology but also results in thinner lamellar crystals. By contrast, if crystallization of OBC is not restricted, the lamellar crystal thickness increases with Tc, which is frequently observed for homopolymers. As can be seen from Figure 8a, the value of lc for OBC-A crystallized from the homogeneous melt at Tc = 115 °C is 8.2 nm, larger than the value of 7.3 nm at Tc = 108 °C. Tensile Behavior. The uniaxial stress−strain curves at room temperature for two OBCs after various thermal treatments are shown in Figure 9. The mechanical data as well as the crystallinity (Xciso) are summarized in Table 3. All the samples exhibit a

difficult to obtain from TEM. SAXS is a good method to characterize the structure of polymer crystals at the nanometer level. Figure 7 shows the Lorenz-corrected SAXS profiles of two OBCs after various thermal treatments. The long period L can be calculated by L = 2π/qmax, where qmax is the scattering vector at the peak position of the SAXS profile. It is found that OBCs crystallized at higher Tc (115 °C) always exhibit a larger long period than those crystallized at lower Tc (108 °C). Moreover, premesophase separation in the melt has more evident effect on the long period at higher Tc than at lower Tc. Therefore, both higher Tc and restricted crystallization will lead to a larger long period. This is because parts of segments with weak crystallizability (such as the hard blocks dissolved in the softblock-rich phase), which originally form crystals inserted between thicker crystals, cannot crystallize at higher Tc or under restricted crystallization conditions, leading to increase of L. Figure 8 shows the normalized one-dimensional correlation functions of SAXS profiles for two OBCs based on the method proposed by Strobl.58 The normalized correlation function is given as follows:59

Table 3. Tensile Properties of Two OBCs after Various Thermal Treatments sample OBC-A

OBC-B

∞

γ (r ) =

∫0 I(q)q2 cos(qr ) dq ∞

∫0 I(q)q2 dq

(2)

Ts (°C)

Tc (°C)

Xciso a (%)

secant modulus (MPa)

stress at break (MPa)

strain at break

180 130 180 130 180 130 180 130

108 108 115 115 108 108 115 115

10.5 10.3 9.8 8.9 5.9 5.9 5.0 4.9

6.3 ± 0.2 5.9 ± 0.2 5.0 ± 0.2 4.3 ± 0.1 3.5 ± 0.1 3.2 ± 0.1 2.3 ± 0.1 2.3 ± 0.1

8.1 ± 0.6 6.7 ± 0.4 6.9 ± 0.4 6.5 ± 0.3 5.1 ± 0.4 4.4 ± 0.3 2.7 ± 0.1 2.6 ± 0.1

13.9 ± 0.5 12.3 ± 0.4 12.8 ± 0.5 14.8 ± 0.6 18.1 ± 0.6 17.9 ± 0.8 15.5 ± 0.5 16.7 ± 0.3

a

Crystallinity is determined from the fusion enthalpy by DSC after isothermal crystallization.

where I(q) is the scattering intensity, q is the scattering vector defined as q = 4π sin θ/λ, and λ is the X-ray wavelength. The details have been reported in our previous works.11,12 The values of the lamellar crystal thickness (lc) analyzed from the correlation function are indicated in Figure 8. One can see that at low Tc the mixing state in the melt has no effect on lc for both OBCs, which is 7.3 nm for OBC-A and 4.5 nm for OBC-B. The lamellar crystal thicknesses of OBCs at higher Tc are different from those at lower Tc and also vary with the mixing state in the melt. In combination with the results of crystallization kinetics and morphology, we can see from Figure 8 that restricted growth of the crystals leads to a smaller lc, even though the Tc is higher. For example, OBC-A crystallized from the mesophaseseparated melt at Tc = 115 °C has a smaller lc than OBC-A crystallized at Tc = 108 °C. Moreover, the values of lc for OBC-B crystallized from both the mesophase-separated and homogeneous melts at Tc = 115 °C are smaller than those at Tc = 108 °C. This shows that restricted crystallization not only alters

stress−strain curve of typical thermoplastic elastomer, i.e., diffused yielding point, strain-hardening at the late stage and large strain at break. It is observed that OBC-A always shows larger elastic modulus and stress at break but smaller strain at break than OBC-B (Table 3). This is due to the higher content of the hard blocks and thus higher crystallinity of OBC-A (Tables 1 and 3), leading to stronger physically cross-linking and lower elasticity. We also notice that the mechanical properties of OBCs vary with the thermal treatment. Tensile modulus and stress at break of OBCs crystallized at lower Tcs are always larger than those of the same OBCs crystallized at higher Tcs (Table 3). This is possibly due to more complete crystallization of the hard blocks at lower Tcs, especially the hard blocks dissolved in the soft-block-rich domains, resulting in larger Xciso (Table 3). The strain at break is usually larger at lower Tcs as well. At lower Tcs, 344

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soft-rich domains at a smaller supercooling, leading to thinner lamellar crystals. On the other hand, no matter the melt of OBCB is homogeneous or mesophase-separated, further mesophase separation will take place at a higher Tc due to the faster rate of mesophase separation than crystallization; thus, the mixing state in the melt has little effect on crystallization of OBC-B at a higher Tc. Moreover, since the dissolved hard blocks in the soft-rich domains are difficult to crystallize at a higher Tc, restricted growth of the crystals is observed for both OBCs crystallized from the mesophase-separated melt. Both the mixing state in the melt and crystallization condition have a great influence on morphology, crystallinity, and thus the mechanical properties of OBCs. Better mechanical properties can be yielded when OBCs crystallize from the homogeneous melt and at a lower Tc.

both OBC-A and OBC-B crystallized from the mesophaseseparated melt show smaller modulus, stress at break, and strain at break than the same OBCs crystallized from the homogeneous melt, although they have similar crystallinity, Xciso (Table 3). This shows that crystallinity is not the sole factor affecting the tensile behavior. At Tc = 115 °C, Xciso of OBC-A crystallized from the mesophase-separated melt is evidently smaller than that from the homogeneous melt, leading to a smaller tensile stress. By contrast, the tensile modulus and stress at break of OBC-B crystallized from the homogeneous melt are comparable to those of OBC-B crystallized from the mesophase-separated melt. The above result reveals that the mechanical properties of OBCs can be regulated by the starting state in the melt and Tc. Typically, larger tensile modulus, stress, and strain at break can be achieved for OBCs crystallized from the homogeneous melt and at a lower Tc. The effect of thermal treatment on tensile behavior of OBCs can be interpreted in terms of their morphology and crystallinity. As can be seen from Figures 5 and 6, the crystal growth at lower Tcs is not restricted, and interconnected crystals with a branching structure and higher crystallinity are formed. This may lead to fewer defects in the materials, and thus higher tensile stress and elasticity can be yielded simultaneously. By contrast, due to the incapability of crystallization or reduced crystallizability of the hard blocks dissolved in the soft-block-rich domains at higher Tc or under restricted crystallization conditions, nonuniform morphology with lower crystallinity is formed (Figure 5e−h); thus, the materials are easily broken at a smaller strain and a smaller stress. Moreover, the SAXS data (Figures 7 and 8) show that the ratio of the lamellar crystals thickness over the thickness of the amorphous layer is larger for OBCs crystallized from the homogeneous melt at a lower Tc, which may lead to a high modulus and larger stress at break. POM reveals that the structure of the spherulites is more open at a higher Tc (Figure 5), indicating that more amorphous components are included into spherulites. Fewer amorphous components among the spherulites, i.e., in the continuous phase, will result in a smaller strain at break for OBCs crystallized at a higher Tc.

■

ASSOCIATED CONTENT

S Supporting Information *

Plots of relative crystallinity versus crystallization for OBC-B at Tc = 100 °C and TEM images of OBC-B after crystallization from the mesophase-separated and homogeneous melt at Tc = 100 °C. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel +86-571-87952400 (J.-T.X.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (2011CB606005) and the National Natural Science Foundation of China (51073138). The authors also thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. The OBC samples were kindly supplied by Dow Chemical Company.

■

4. CONCLUSIONS The results show that the rate and morphology of mesophase separation are strongly dependent on the chain structure of OBCs. OBC-A with a weak segregation strength and a larger fraction of the hard blocks exhibits a rod-like morphology and a slower rate of mesophase separation, while OBC-B with a strong segregation strength and a small fraction of the hard blocks exhibits an island-like morphology and a faster rate of mesophase separation. Crystallization temperature strongly affects the relative rate of crystallization and mesophase separation and crystallizability of the hard blocks dissolved in the soft-block-rich phase, thus the crystallization kinetics and morphology. At a lower Tc, OBC-A crystallizes faster from the mesophaseseparated melt than that from the homogeneous melt due to enrichment of the hard blocks in the hard-block-rich domains, whereas the confinement effect prevails in OBC-B, leading to slower crystallization rate of OBC-B crystallized from the mesophase-separated melt. The hard blocks dissolved in the soft-block rich domains can also crystallize at a lower Tc due to a larger supercooling; thus, the crystal growth in the hard-blockrich domains is not restricted when OBCs crystallize from mesophase-separated melt. At a higher Tc, premesophase separation in the melt retards crystallization of OBC-A due to the weaker crystallizability of the dissolved hard blocks in the

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dx.doi.org/10.1021/ma4023263 | Macromolecules 2014, 47, 333−346