liquid crystalline block copolymers

Polymer 130 (2017) 1e9

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Interplay of microphase separation, crystallization and liquid crystalline ordering in crystalline/liquid crystalline block copolymers Zaizai Tong a, *, Junyi Zhou a, Rui-Yang Wang b, Jun-Ting Xu b, ** a

Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Department of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China b MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310827, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 August 2017 Received in revised form 28 September 2017 Accepted 30 September 2017 Available online 3 October 2017

A series of liquid crystalline/crystalline block copolymers (BCPs) containing poly(methacrylate) block with liquid crystalline (LC) azobenzene moieties in the side chains (PMMAzo) and crystalline block poly(L-lactide) (PLLA) were prepared. The interplay of microphase separation, crystallization and LC ordering in these BCPs was investigated. It is revealed that microphase separation between two blocks is favorable to the LC ordering, which is attributed to the enhanced local concentration of LC moieties in PMMAzo microdomains. For a similar reason, crystallization of PLLA can intensify microphase separation thus facilitate LC ordering of PMMAzo. PLLA crystallization may also stabilize the LC structure, leading to phase transition temperatures of the BCPs higher than that of PMMAzo homopolymer. On the other hand, the LC ordering can conversely affect crystallization of PLLA. The crystallizability of PLLA is weakened by the chemically linked PMMAzo block. The special PLLA ε-crystals, which are usually formed in the presence of organic solvents, are unexpectedly observed under suitable conditions. © 2017 Published by Elsevier Ltd.

Keywords: Crystallization Liquid crystalline Microphase separation

1. Introduction Block copolymers (BCPs) continue to attract increasing attention in the past few decades due to the richness of microphaseseparated morphologies, which are of great interest in the field of nanotechnology [1,2]. The introduction of a crystalline block into BCPs generally can further enrich the microphase-separated morphology and enhance the flexibility and controllability of morphology, which can be attributed to the complex interplay between microphase separation and crystallization [3e15]. For instance, the microphase structure in the melt of crystalline/ rubbery BCPs can either be retained or destroyed after crystallization, which are known as “confined crystallization” [16] and “breakout crystallization” [17], respectively, depending on the relative segregation strength of BCPs. Highly asymmetric lamellar (Lam) structure may be obtained when break-out crystallization occurs in BCPs with a body-centered cubic spheres (BCC) or hexagonally packed cylinders (HEX) structure in the melt [18]. Switch between

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Tong), [email protected] (J.-T. Xu). https://doi.org/10.1016/j.polymer.2017.09.071 0032-3861/© 2017 Published by Elsevier Ltd.

two different ordered structures can also be achieved via melting and crystallization of the crystalline block [18]. On the other hand, BCPs with a liquid crystalline (LC) block are also popular as a candidate for functional materials due to the functionality of the LC block [19e26]. After introduction of LC order into BCPs, structure-within-structure hierarchical structures can be formed with the LC ordering (1e10 nm) and the microphase structures (10e100 nm) [27e33]. As a consequence, the crystalline/ LC BCPs, in which the functionality of LC block and controllability in morphology of the crystalline block are integrated, are interesting and potentially useful in functional materials and nanotechnology. Nevertheless, so far crystalline/LC BCPs are rarely reported, most of which are limited to the PEO-based BCPs [34e43]. Both confined crystallization and break-out crystallization are observed in crystalline/LC BCPs, depending on the volume fraction of crystalline block [34e38]. For example, Kasi showed that the crystallization behavior of poly(ethylene oxide) (PEO) was controlled by the weight fraction of the LC segment in the block copolymer. A large undercooling was needed for PEO crystallization at a high LC content and thus the structure of “PEO cylinders in LC matrix” was formed [34]. Mourran also investigated the hierarchical structure of a PEO-based crystalline/LC BCP formed by complexation of a

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wedge-shaped ligand. The crystallization behavior of PEO block changed from “break-out crystallization” to “confined crystallization” as the molar ratio of ligand/pyridine increased [35]. We also studied the competition between the LC and crystalline moieties in a series of PEO cholesterol ethers. It was found that LC dominated the overall morphology when the PEO fraction was low, while the LC phase was absent and crystalline PEO block determined the morphology when the PEO block was long enough [39]. Above works mainly focus on the evolution of microphaseseparated structure and the correlation between overall morphology and LC ordering as a function of block composition. In crystalline/LC BCPs, there exist three ordering processes: microphase separation, LC ordering and crystallization. All these three processes exert their effects on the final structure of BCPs and the interplay among them is far from extensive understanding. For example, the question that microphase separation hinders or facilitates LC ordering has not been fully answered yet, since the effects of microphase separation and chemical linkage with crystalline block are not decoupled in literature. Moreover, in the PEO-based crystalline/LC BCPs, the effect of crystallization on the LC ordering cannot be explored due to the low melting temperature of PEO crystals. Compared with PEO, poly(L-lactide) (PLLA) has a wider range of crystallization temperature, higher melting temperature and slower crystallization rate, thus both crystalline and completely amorphous PLLAs can be readily prepared and the effect of solid structure of PLLA on the LC ordering can be revealed in various manners. Meanwhile, PLLA exhibits polymorphic behavior and can crystallize into many crystalline forms such as a (a0 ), b, g, d and ε, which are sensitive to crystallization conditions [44e49]. As a result, the possible effect of LC ordering on crystalline structure of PLLA can be explored. In the present work, a series of PMMAzo26-bPLLAx (x represents the polymerization degree of PLLA) LC/crystalline BCPs containing poly(methacrylate) moieties with liquid crystalline azobenzene moieties in the side chains (PMMAzo) and crystalline PLLA block were prepared (Scheme 1) [50]. The interplay of microphase separation, LC ordering and crystallization was investigated. It is observed that microphase separation can enhance the LC ordering, and crystallization of PLLA can not only induce the microphase separation, but also stabilize the LC structure. Moreover, the presence of LC ordering can conversely influence the crystalline modification of PLLA, leading to formation of PLLA ε-crystal under suitable conditions. 2. Experimental section

of Supporting Information and detailed synthetic procedure is reported elsewhere [50]. The obtained BCPs were characterized by GPC (Fig. S2) and 1H-NMR (Fig. S3), and the final molecular characteristics are summarized in Table 1. The result indicates welldefined BCPs with a fixed PMMAzo block length but different PLLA block lengths are successfully prepared. 2.2. Thermal treatment Thermal treatments of the samples were performed under a N2 atmosphere in an oil bath with 1  C of temperature fluctuation. In order to make sure that all the samples have a same thermal history, the as-prepared BCP samples were first heated to 180  C and held for 5 min to eliminate the thermal history, then they were quenched into the ice-water to obtain completely amorphous samples. Subsequently, the quenched samples were heated to a preset temperature and annealed for a 48 h. Finally, the annealed samples were quenched to ice-water to retain the pre-formed solid structure. The temperature profiles for thermal treatment are described in Fig. S4. The selected annealing temperatures are 180  C, 160  C, 130  C, 115  C and 85  C, respectively. At these annealing temperatures, the PMMAzo-b-PLLA BCPs are in different ordering states, as shown in Fig. S4. After thermal treatment, the samples were used for further characterizations. 2.3. Characterizations LC phase transition temperatures and melting temperature of PLLA were measured using differential scanning calorimetry (DSC, TA Q200) with a heating rate of 10  C min1. Polarized optical microscopy (POM) observations were carried out on an Olympus microscope (BX51) equipped with a hot stage. X-ray diffraction (XRD) was performed on a SIEMENS D5000 X-ray diffractometer with a Cu Ka radiation source at 35 kV. The scan step is 0.02 and the wavelength of X-ray is 0.154 nm. Wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS) experiments were performed at BL16B1 beamline at Shanghai Synchrotron Radiation Facility (SSRF) in China [52,53]. The wavelength of the X-ray source was 0.124 nm. The sample-to-detector distance was set as 405 mm for WAXS experiments, while 1900 mm for SAXS experiments. The 2D patterns were converted into one-dimensional (1D) profiles using Fit2D software. Tapping mode atomic force microscopy (AFM) was conducted on an XE-100E instrument (PSIA cooperation, Korea). Thin films of BCPs for AFM observations were prepared by dropping the BCP/THF solution (1 wt%) on a piece of silicon wafer and followed by spin-coating at 3000 r min1 for 30 s.

2.1. Materials 3. Result and discussion The block copolymers, PMMAzo-b-PLLA, with various compositions were prepared via atom transfer radical polymerization and ring opening polymerization using bifunctional initiator according to previous paper [50,51]. The synthetic route is described in Fig. S1

3.1. Thermal behavior To identify the thermal transitions of crystalline/LC BCPs, DSC measurements are first conducted. The PMMAzo homopolymer Table 1 Molecular characteristics of PMMAzo-b-PLLA BCPs.

Scheme 1. Structure of PMMAzo-b-PLLA diblock copolymer.

Sample code

MGPC n

PDI

Structure

WPLLAa

LA19 LA38 LA45 LA49 LA54 LA56

13300 15900 16700 18000 23800 25600

1.10 1.11 1.14 1.12 1.14 1.13

PMMAzo26-b-PLLA17 PMMAzo26-b-PLLA44 PMMAzo26-b-PLLA58 PMMAzo26-b-PLLA68 PMMAzo26-b-PLLA83 PMMAzo26-b-PLLA92

0.19 0.38 0.45 0.49 0.54 0.56

a

WPLLA is the weight fraction of PLLA block.

Z. Tong et al. / Polymer 130 (2017) 1e9

3.2. Effect of microphase separation on the LC ordering In PMMAzo26-b-PLLAx BCPs, the microphase separation behavior may vary with the fraction of PLLA block, which is also an important factor affecting the LC ordering of PMMAzo block. As a result, the microphase separation behavior of PMMAzo26-b-PLLAx BCPs needs to be examined. Fig. 2 shows the room-temperature

LA19 LA38 LA45 LA54

Log I

exhibits two phase transitions upon heating, which correspond to smectic-to-nematic and nematic-to-isotropic transitions, respectively, as revealed by the DSC curve of PMMAzo26 (Fig. S5) [33]. Fig. 1 shows the DSC cooling and second-run heating scans of five BCPs with various compositions and the first-run heating scans are supplied in Fig. S6. One can see that glass transition of PLLA, LC phase transitions of PMMAzo and melting of PLLA crystals can be detected in the first-run heating scans (Fig. S6). After eliminating the thermal history by annealing at 180  C for 5 min, the LC phase transitions and glass transition can still be observed in the cooling scans (Fig. 1a). However, the crystallization peak of PLLA cannot be observed because of the slow crystallization of PLLA block. Finally, in the second-run heating scans, two interesting phenomena are observed: (1) Cold crystallization and melting of PLLA can only be detected in LA45 (Fig. 1b). For rapidly cooled PLLA homopolymer, cold crystallization and melting can usually be observed upon heating. (2) Both LC phase transition temperatures of PMMAzo block, which correspond to smectic-to-nematic (TS/N) and nematic-to-isotropic transitions (TN/I), respectively, increase as the fraction of LC block decreases (Table 2 for the samples with amorphous PLLA), although the enthalpies of phase transition of PMMAzo block is decreased as PLLA fraction increases. This phenomenon is quite abnormal, since usually the ordering of two blocks in BCPs is mutually inhibited [54]. Therefore, the temperature of phase transition (including crystallization and melting temperatures, LC phase transition temperature) of one block with a fixed block length should be depressed as the fraction of the other block increases [33,35,55]. The abnormal change of TS/N and TN/I with PLLA fraction observed here will be discussed in the following section based on SAXS result.

3

0.2

0.4

0.6 -1 0.8

q (nm )

1.0

Fig. 2. Room-temperature SAXS profiles of four PMMAzo-b-PLLA BCPs after quenching from 180  C.

Fig. 1. DSC curves of PMMAzo26-b-PLLAx BCPs. (a) cooling scans and (b) second-run heating scans. The TI/N and TN/S in the cooling scans indicate the temperatures of isotropic-tonematic and nematic-to-smectic LC phase transitions, respectively. The TS/N and TN/I in the heating scans refer to the temperatures of smectic-to-nematic and nematic-toisotropic LC phase transitions, respectively.

Table 2 LC phase transition temperatures obtained from DSC heating scans with initial amorphous and crystalline states of PLLA. Sample

PMMAzo LA19 LA38 LA45 LA54 LA56 a b c d

WPMMAzo

1.00 0.81 0.62 0.55 0.46 0.44

Crystalline (130  C)

Amorphous TS/Nb ( C)/DHf (J/g)d

TN/Ic ( C)/DHf (J/g)

TS/N ( C)/DHf (J/g)

TN/I ( C)/DHf (J/g)

89.4/4.80 87.1/2.76 87.6/1.88 89.1/-a 89.5/0.65 89.9/0.58

129.0/3.19 118.0/2.52 120.3/1.65 -a 121.1/0.58 121.4/0.52

87.1/2.74 89.4/1.82 91.8/1.12 92.3/0.43 92.7/0.46

118.1/2. 46 122.0/1.54 123.0/1.05 127.5/0.60 128.2/0.47

Difficul to detect due to cold crystallization of LA45 from an amorphous state. The temperature of smectic-to-nematic transition of PMMAzo block. The temperature of nematic-to-isotropic transition of PMMAzo block. The enthalpy of phase transition of PMMAzo block.

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SAXS profiles of four selected PMMAzo-b-PLLA BCPs after quenching from 180  C (the thermal treatment procedure is depicted in Fig. S4). One can see that the profiles of LA19 and LA38 are smooth without obvious scattering peaks, indicating the PLLA and PMMAzo blocks are miscible and possibly form a homogeneous structure in the melt (180  C) of these two samples. As the PLLA becomes longer, a weak scattering peak appears in the SAXS profiles of LA45 and LA54, implying that the PLLA and PMMAzo blocks are weakly segregated in the melt. It is noted that, due to the small segregation strength between PLLA and PMMAzo blocks, no ordered microphase structure is formed (as revealed by the absence of high-order SAXS peaks), though microphase separation is observed. In BCPs, the microphase separation behavior is dependent on the composition and segregation strength, which is the product of the Flory-Huggins interaction parameter c and the polymerization degree N [56]. As a consequence, the tendency of microphase separation in the melt of PMMAzo26-b-PLLAx BCPs becomes stronger with increasing the PLLA block when the fraction of PLLA block is less than 0.5. As a result, the abnormal change of TS/N and TN/I with PLLA fraction may be related to the enhanced microphase separation. Fig. 2 shows that the microphase separation of PMMAzo-b-PLLA BCPs at 180  C is rather weak because of the small segregation strength cN. Since c is inversely proportional to temperature, we anneal the BCPs at a lower temperature (160  C) for 48 h to intensify microphase separation, so that the effect of microphase separation on LC ordering can be revealed more clearly. Fig. 3a shows the room-temperature SAXS profiles of LA54 after quenching from 180  C followed by annealing at 160  C for 48 h. The corresponding XRD patterns and DSC heating traces are illustrated in Fig. 3b and c. One can see that long time annealing at 160  C indeed leads to a stronger scattering peak, confirming the intensified microphase separation. Accordingly, the diffraction peak at 2q ¼ 3.5 , which reflects the d-spacing of the lamellar smectic LC structure, becomes stronger (Fig. 3b, detailed calculation see Fig. S7). By contrast, the broad halo around 2q ¼ 20 which corresponds to the lateral distance among azobenzene LC moieties, moves to a slightly larger angle. This shows that the azobenzene LC moieties stack more closely after long time annealing at 160  C. In addition, the TN/I shifts to higher temperature (Fig. 3c). It should be emphasized that, the PLLA block is completely amorphous under both thermal treatments. Therefore, these differences are solely caused by the difference in microphase separation in the melt. The XRD and DSC results reveal that the LC ordering is enhanced by microphase separation with a strong diffraction peak of LC ordering and higher phase transition temperatures.

In BCPs microphase separation usually has two opposite effects on the ordering process of the block. On the one hand, the local concentration in the microdomains becomes higher, as compared with that in the homogeneous melt, which is advantageous to the ordering. On the other hand, nano-spatial confinement will be produced by microphase separation, leading to an unfavorable effect on the ordering. Moreover, the chemical link with the other block has a structural confinement effect, which may also retard the ordering [57e59]. In the PMMAzo-b-PLLA LC/crystalline BCPs, the increase of TS/N and TN/I with increasing PLLA fraction and degree of microphase separation indicates that the concentration effect overwhelms the spatial and structural confinement effects. This is possible because the LC moieties are located at the side chains of the PMMAzo block, thus are less affected by confinements. The effect of microphase separation on LC ordering is similar to that on the crystallization to some extent, which is related to the competition between the concentration effect and confinement effect. In our previous work, we found that crystallization of the crystalline block in an olefin block copolymer (OBC) with a relatively low value of cN could be accelerated after microphase separation [60]. However, the local concentration of PMMAzo will not exceed the concentration of PMMAzo, the thermal transition temperatures (including nematic-isotropic temperature and smectic-nematic temperature) in the BCPs are lower than that of PMMAzo homopolymer (Table 2). Nevertheless, when the crystallizability of the crystalline block is too weak, the confinement effect might prevail, leading to retardation of crystallization by microphase separation [60]. In the present work, the effect of microphase separation on PLLA crystallization cannot be conducted, because microphase separation always takes place prior to the crystallization due to the slow crystallization of PLLA. 3.3. Effect of crystallization on the LC ordering Because the crystallization of PLLA is slow and the melting temperature (Tm) of PLLA is higher than the TN/I of the PMMAzo, both amorphous and crystalline PLLA can be obtained and the effect of solid state structure of PLLA block on the LC ordering of PMMAzo block can be investigated. The PMMAzo-b-PLLA BCPs were first held at 180  C for 5 min, quenched into ice-water, followed by annealing at 130  C to obtain crystalline PLLA blocks. Note that the annealing temperature of 130  C is higher than the TN/I of the PMMAzo26, so that the PMMAzo block is isotropic upon annealing. Fig. S8 shows the WAXS profiles of LA54 after quenching from 180  C and after annealing at 130  C for 48 h, respectively, which confirm the crystalline and amorphous states of the PLLA block

Fig. 3. Room-temperature SAXS profiles (a), XRD patterns (b) and DSC heating scans (c) of LA54 after quenching from 180  C or annealing at 160  C for 48 h. The wavelength of X-ray for XRD experiments is 0.154 nm.

Z. Tong et al. / Polymer 130 (2017) 1e9

under these two thermal treatments. Fig. 4 shows the DSC heating scans of PMMAzo-b-PLLA BCPs after annealing at 130  C for 48 h. One can see that all the samples except for LA19 exhibit melting peaks of PLLA around the temperature of 160e170  C (Fig. 4a). With increasing PLLA block length, the Tm of PLLA increases gradually and the melting peak of PLLA becomes stronger. The LC phase transitions of PMMAzo after PLLA crystallization are shown in Fig. 4b, which is the enlargement of the framed area in Fig. 4a. The data of TS/N and TN/I for the PMMAzo block are summarized in Table 2. It is observed that both the TS/N and TN/I of the BCPs with crystalline PLLA block are evidently larger than those of the samples with amorphous PLLA block, except for LA19, in which the PLLA block cannot crystallize (Fig. 4a). This result implies that crystallization of PLLA can promote the LC ordering, instead of disrupting the LC arrangement. Moreover, when the PLLA block is crystalline, the LC phase transition temperatures also increase as the PLLA block becomes longer (Table 2). This is similar to that for the BCPs with amorphous PLLA block. On the other hand, compared the enthalpy detected from amorphous phase with that from crystalline phase (Table 2), the crystallization of PLLA has negligible effect on the enthalpy of phase transition of PMMAzo, indicating the enthalpy of phase transition of PMMAzo block mainly related to the fraction of PMMAzo block. The effect of PLLA crystallinity on the LC ordering of PMMAzo block is further studied. Fig. 5 shows the DSC heating scans of LA54 after annealing at 130  C for different times. One can see that, with prolongation of annealing time at 130  C, the melting peak of PLLA block becomes stronger and stronger, indicating the increase of PLLA crystallinity. Moreover, the TS/N and TN/I also increase as the annealing time increases. This result further confirms that the PLLA crystallization is favorable to the LC ordering, thus the LC phase transitions occur at higher temperatures. When a polymer crystallizes from the homogeneous melt of its mixture with another polymer, it is usually accompanied by enrichment of the crystalline component and segregation of the different components. Fig. 6 shows the room-temperature Lorentzcorrected SAXS profiles of BCPs after annealing at 130  C for 48 h. It can be seen that the scattering peak is strong, implying obvious microphase separation. As pointed out in the previous section, microphase separation can increase the local concentration of LC moieties. As a result, the enhancement of LC ordering by PLLA crystallization may partially originate from crystallization-induced microphase separation. However, we notice that, the temperatures of smectic-to-nematic LC phase transition of LA45, LA54 and LA56 after annealing at 130  C for 48 h are 91.8  C, 92.3  C and 92.7  C, respectively, which are evidently higher than the TS/N of 89.4  C for the PMMAzo26 homopolymer (Table 2). This cannot be

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Fig. 5. DSC heating scans of LA54 after annealing at 130  C for different times.

Fig. 6. Room-temperature Lorentz-corrected SAXS profiles of PMMAzo-b-PLLA BCPs after annealing at 130  C for 48 h.

interpreted in terms of the enhancement of local concentration related to microphase separation, since the concentration of the LC moieties in the phase-separated microdomains cannot exceed that in the homopolymer. Here we attribute the higher TS/N of the

Fig. 4. (a) DSC heating scans of PMMAzo-b-PLLA BCPs after annealing at 130  C for 48 h (b) enlargement of the framed area in (a).

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PMMAzo-b-PLLA BCPs to the stabilization effect of the crystalline PPLA block. Since the melting temperature of PLLA block is higher than the TS/N and TN/I of the LC moieties, the crystalline PLLA block is still in solid state when LC phase transitions occur. Due to the chemical linkage between the PMMAzo and PLLA blocks, the solid PLLA block may stabilize the ordered LC structure to some extent, leading to higher LC phase transition temperatures of the BCPs, which are even higher than that of PMMAzo homopolymer. On the other hand, after crystallization of PLLA, the scattering peak of LA38, LA45 and LA54 appeared in a similar q value, which indicates a similar long period of these BCPs. This should mainly due to the crystallization of PLLA. Since after annealing at 130  C for 48 h, the PLLA block is crystalline and has a similar lamellar thickness. 3.4. Effect of LC ordering on crystallization Above results show that crystallization of PLLA block can affect the LC ordering of PMMAzo block in the PMMAzo-b-PLLA BCPs. On the other hand, the LC ordering of PMMAzo block may also exert its influence on PLLA crystallization. To reveal such an influence, the PMMAzo-b-PLLA BCPs are annealed at 130  C, 115  C and 85  C,

respectively. At these temperatures the PMMAzo blocks are isotropic, nematic and smectic accordingly. Fig. 7 shows the POM micrographs of three BCPs after annealing at different temperatures. One can see that the LC textural structures of the PMMAzo block are predominant in all the samples at room temperature. However, the dot-like crystals of PLLA can be observed after the samples are heated to 140  C, at which the LC ordering disappears while the PLLA crystals remain. With decreasing annealing temperature, the size of the PLLA crystals decreases but the number increases. Moreover, as the PLLA block becomes longer, more PLLA crystals are formed (Fig. 7). The PLLA crystals can be identified more clearly by AFM. The AFM phase images of LA45 after annealing at 85, 115 and 130  C are shown in Fig. 8. The granular-like PLLA crystals can be clearly observed. As the annealing temperature increases, the size of PLLA crystals also increase, which is consistent with POM observation. The change of the number and size of PLLA crystals with annealing temperature and BCP composition can be explained from two aspects. Firstly, fewer nuclei are formed at a higher annealing temperature, thus crystals of a larger size are formed. Secondly, the crystallizability of PLLA block is weakened by the PMMAzo block. When the LC moieties are more ordered (i.e. at lower temperature), such an unfavorable effect is more evident

Fig. 7. POM micrographs of LA38 (top), LA45 (middle) and LA54 (bottom) after crystallization at (A) 130  C, (B) 115  C and (C) 85  C. Left column is captured at room temperature with the presence of LC order and PLLA crystals, and right column is captured at 140  C, where the LC order is lost and PLLA crystals are still present. All images are all captured by 20  50 magnification. The scale bar in the figure is 100 mm.

Fig. 8. AFM phase images of LA45 after crystallization at (a) 85  C, (b) 115  C and (c) 130  C for 48 h. The scale bar is 1000 nm.

Z. Tong et al. / Polymer 130 (2017) 1e9

[61]. This can also be seen from the disappearance of cold crystallization when heating LA19, LA54 and LA56 with amorphous PLLA (Fig. 1b). Moreover, one can see from Fig. 8 that, the size of PLLA crystals is much larger than microdomain size calculated from SAXS irrespective of annealing temperature, indicating that break-out crystallization occurs for the PLLA block and the microphase-separated structure in the melt is destroyed by PLLA crystallization [62,63]. This agrees well with the weak segregation strength in the PMMAzo-b-PLLA BCPs. The crystal structure of PLLA is further characterized with WAXS. The obtained WAXS patterns of different BCPs after annealing at 130  C and 115  C for 48 h are shown in Fig. S9. It is observed that all the BCPs except for LA19 exhibit two major diffraction peaks of a(110)/(200) and a(203) reflections of PLLA crystals (Fig. 9). Interestingly, in the WAXS pattern of LA45 after annealing at both 130  C and 115  C, the unexpected (220) reflection of PLLA ε-crystal is observed at 2q ¼ 14.8 besides the major a crystal reflections of (110)/(200) and (203). In order to verify the formation of ε-crystal, we also prepared another BCP, PMMAzo26-b-PLLA68 (named as LA49), to reveal the condition for formation of ε-crystal. Fig. 9 shows WAXS patterns of LA45 and LA49 after annealing at 130  C, 115  C and 85  C for 48 h, respectively. It is found that, the relative intensity of ε-crystal with respect to that of a-crystal is the strongest in both LA45 and LA49 when the annealing temperature is 115  C, at which the PMMAzo block is nematic. By contrast, the content of ε-crystal is reduced or even absent when the samples are annealed at 85  C and 130  C, which is particularly evident for LA49. Surprisingly, ε-crystal cannot be formed in another sample with a similar PLLA fraction but a shorter PLLA block, PMMAzo14-b-PLLA35 (46 wt% of PLLA fraction) at any annealing temperature (Fig. S10). Moreover, no ε-crystals are observed either when the polymerization degree of the PLLA block in PMMAzo26-b-PLLAx BCPs is smaller than 38 or larger than 54, irrespective of annealing temperature (Fig. S9 and Fig. S11). This implies the formation condition of PLLA ε-crystal in PMMAzo-b-PLLA BCPs is very strict. All the requirements about suitable block length, composition and annealing temperature must be met. In the literature, the ε-crystal of PLLA was first observed by Marubayashi in the presence of the specific organic solvents such as tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) below room temperature [64]. It is reported that the solvent molecules can be incorporated into the crystal lattice of PLLA, leading to expansion of crystal lattice and thus ε-crystals [49,64,65]. When the temperature rises, the incorporated organic molecules will escape from the ε crystal lattice, and a transition from ε to a crystal will occur [65]. We also examined the thermal stability of ε-crystal using temperature-variable WAXS. The

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WAXS patterns of LA45 at various temperatures upon heating are shown in Fig. S12. It is found that the ε-crystal is still preserved at 160  C. This is different from the result reported in literature that the ε-crystal usually transforms into a-crystal at the boiling temperature of the incorporated molecules, which is due to the solvent desorption from the ε-crystal lattice [65]. This is the first example for formation of PLLA ε-crystals in the absence of small organic molecules. Here we proposed a possible explanation for the formation of PLLA ε-crystals in the PMMAzo-b-PLLA BCPs. The ordering of two blocks in the PMMAzo-b-PLLA BCPs is mutually affected. When the PLLA block is too long or its fraction is too high, the effect of the PMMAzo block on the crystallization of PLLA is very weak. By contrast, in the PMMAzo-b-PLLA BCPs with very a long PMMAzo block or with a high fraction of PMMAzo, crystallization of PLLA is severely retarded. Only under suitable conditions including proper block lengths, composition and annealing temperature, the effect of the PMMAzo block on PLLA crystallization is subtle, i.e. neither too strong nor too weak. For example, the pre-formed nematic LC phase of PMMAzo may prevent the PLLA blocks from densely stacking into a-crystal, but will not completely inhibit crystallization of PLLA, thus the PLLA blocks stack loosely in the crystal, i.e. form ε-crystal. The loose stacking of PLLA chains in the ε-crystals may lead to a fast transformation rate of this kind of crystal, thus cold crystallization and melting can be observed upon heating LA45 with amorphous PLLA even in the presence of unfavorable effect of the PMMAzo block (Fig. 1b). As a result, the finding in the present work provides a new way to prepare PLLA ε-crystal in bulk. Moreover, the chemically linked PMMAzo block may also stabilize the formed PLLA ε-crystal, thus the transition from ε to acrystals cannot be observed upon heating the PMMAzo-b-PLLA BCPs, as shown in Fig. S12. Above results show that the final structure of the PMMAzo-bPLLA BCPs are determined by the interplay among microphase separation, crystallization of PLLA block and LC ordering of the PMMAzo block, as depicted in Scheme 2. Microphase separation may enhance the LC ordering of PMMAzo block due to the local concentration effect, and crystallization of PLLA can intensify the microphase separation and further enhance LC ordering. Sometimes PLLA crystallization may stabilize the ordered LC structure. On the other hand, LC ordering of the PMMAzo block can weaken the crystallizability and even alter the crystal modification of the PLLA block. 4. Conclusion In summary, the interplay of microphase separation, crystallization, and LC ordering in the PMMAzo-b-PLLA BCPs was investigated

Fig. 9. WAXS patterns of LA45 (a) and LA49 (b) after annealing at different temperatures. The wavelength of X-ray is 0.124 nm.

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Z. Tong et al. / Polymer 130 (2017) 1e9

Scheme 2. Schematic description of the interplay of microphase separation, crystallization and LC ordering in PMMAzo-b-PLLA BCPs.

in detail. The thorough microphase separation will lead to a higher local concentration of the LC moieties in the PMMAzo microdomains, resulting in an enhanced LC ordering. Meanwhile, crystallization cannot only intensify microphase separation, but also stabilize the LC structure. On the other hand, the LC ordering can also conversely affect PLLA crystallization, including crystallizability and crystal modification. Special PLLA ε-crystal, which is usually produced in the presence of solvent molecules, can be formed in the bulk of PMMAzo-b-PLLA BCPs with suitable block composition and block length under proper crystallization conditions. Notes The authors declare no competing financial interest. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21274130, 21604073), MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Zhejiang University (2016MSF002). The authors also would like to thank the Shanghai Synchrotron Radiation Facility (SSRF) for providing time on beamline BL16B1. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.polymer.2017.09.071. References [1] J.K. Kim, S.Y. Yang, Y.M. Lee, Y.S. Kim, Functional nanomaterials based on block copolymer self-assembly, Prog. Polym. Sci. 35 (2010) 1325e1349. [2] C.M. Bates, F.S. Bates, 50th anniversary perspective: block polymers-pure potential, Macromolecules 50 (2017) 3e22. [3] L. Zhu, S.Z.D. Cheng, B.H. Calhoun, Q. Ge, R.P. Quirk, E.L. Thomas, B.S. Hsiao, F.J. Yeh, B. Lotz, Crystallization temperature-dependent crystal orientations within nanoscale confined lamellae of a self-assembled crystalline-amorphous diblock copolymer, J. Am. Chem. Soc. 122 (2000) 5957e5967. [4] Y.L. Loo, R.A. Register, A.J. Ryan, Modes of crystallization in block copolymer microdomains: breakout, templated, and confined, Macromolecules 35 (2002) 2365e2374. [5] J.T. Xu, J.J. Yuan, S.Y. Cheng, SAXS/WAXD/DSC studies on crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer with lamellar morphology and low glass transition temperature, Eur. Polym. J. 39 (2003) 2091e2098. [6] C.Y. Chu, H.L. Chen, M.S. Hsiao, J.H. Chen, B. Nandan, Crystallization in the binary blends of crystallineamorphous diblock copolymers bearing chemically different crystalline block, Macromolecules 43 (2010) 3376e3382. [7] Y.Y. Kim, B. Ahn, S. Sa, M. Jeon, S.V. Roth, S.Y. Kim, M. Ree, Self-assembly characteristics of a crystallineeamorphous diblock copolymer in nanoscale thin films, Macromolecules 46 (2013) 8235e8244. [8] J. Ge, M. He, N. Xie, X.B. Yang, Z. Ye, F. Qiu, Microphase separation and crystallization in all-conjugated poly(3-alkylthiophene) diblock copolymers,

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