Synthesis, Thermal Properties, and Morphologies of Amphiphilic

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

Synthesis, Thermal Properties, and Morphologies of Amphiphilic Brush Block Copolymers with Tacticity-Controlled Polyether Main Chain Takuya Isono,† Hoyeol Lee,§ Kana Miyachi,† Yusuke Satoh,† Toyoji Kakuchi,‡,∥ Moonhor Ree,*,§ and Toshifumi Satoh*,† †

Faculty of Engineering and Graduate School of Chemical Sciences and Engineering and ‡Frontier Chemistry Center, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan § Department of Chemistry, Division of Advanced Materials Science, and Polymer Research Institute, Pohang University of Science and Technology, Pohang 37673, Republic of Korea ∥ Research Center for Polymer Materials, School of Materials Science and Engineering, Changchun University of Science and Technology (CUST), Weixing Road 7989, Changchun, Jilin 130022, China S Supporting Information *

ABSTRACT: A series of brush block copolymers (BBCPs) consisting of poly(decyl glycidyl ether) (PDGE) and poly(10hydroxyldecyl glycidyl ether) (PHDGE) blocks, having four different types of chain tacticities, i.e., [at-PDGE]-b-[atPDEGE], [at-PDGE]-b-[it-PDEGE], [it-PDGE]-b-[atPDEGE], and [it-PDGE]-b-[it-PDEGE], where the it and at represent the isotactic and atactic chains, respectively, were prepared by t-Bu-P4-catalyzed sequential anionic ring-opening polymerization of glycidyl ethers followed by side-chain modification. The corresponding homopolymers, i.e., atPDGE, it-PDGE, at-PHDGE, and it-PHDGE, were also prepared for comparison with the BBCPs. The PDGE homopolymers were significantly promoted in the phase transitions and morphological structure formation by the isotacticity formation. In particular, it-PDGE was found to form only a horizontal multibilayer structure with a monoclinic lattice in thin films, which was driven by the bristles’ self-assembling ability and enhanced by the isotacticity. However, the PHDGE homopolymers were found to reveal somewhat different behaviors in the phase transitions and morphological structure formation by the tacticity control due to the additional presence of a hydroxyl group in the bristle end as an H-bonding interaction site. The H-bonding interaction could be enhanced by the isotacticity formation. The it-PHDGE homopolymer formed only the horizontal multibilayer structure, which was different from the formation of a mixture of horizontal and tilted multibilayer structures in at-PHDGE. The structural characteristics were further significantly influenced by the diblock formation and the tacticity of the counterpart block. Because of the strong self-assembling characteristics of the individual block components, all the BBCPs formed separate crystals rather than cocrystals. The isotacticity always promoted the formation of better quality morphological structures in terms of their lateral ordering and orientation.

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INTRODUCTION Brush polymers have attracted much attention for their unique chemical architecture featuring long chain bristles. The first report of the brush polymer dates back to the 1940s, in which polyacrylates with long alkyl side chains were synthesized by the free radical polymerization.1,2 With the progress in precision polymerization chemistry, brush polymers with various side chains, such as polystyrene,3 polyisoprene,4 and polyethylene,5 have been reported. Brush copolymers possessing two different kinds of bristles, such as the statistical and block copolymer type ones, are of particular interest because of their ability to self-assemble into nanoscopic hierarchical structures involving the side chain and main chain ordering as well as the microphase separation.6 Recently, structurally well-defined brush copolymers carrying various bristles have © XXXX American Chemical Society

been designed and synthesized aiming at the creation of functional materials, such as drug carriers, photonic crystals, surface modifiers for biological applications, and nanolithography.7−12 As a consequence of these significant research efforts, the side chain composition, side chain distribution, side chain length, and main chain length were found to be important factors governing the self-assembly of nanostructures that eventually affect the material properties. For further fine-tuning of the brush copolymer self-assembly, introducing an additional structural parameter, such as chain tacticity, is of significant interest. Received: February 1, 2018 Revised: March 12, 2018

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DOI: 10.1021/acs.macromol.8b00243 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Synthetic Pathway for a Series of PDGE-b-PHDGE with Four Different Combinations of the Main Chain Tacticities

nanostructures and phase behaviors, which can lead to a new design concept of high performance materials based on the brush copolymer system. To explore the effect of the chain tacticity on the brush copolymer self-assembly, a robust and concise route toward the tacticity-controlled brush copolymers is therefore highly desired. While many kinds of polymer backbones, such as poly(meth)acrylates,23−25 polynorbornenes,29−31 and polyacetylene,32−34 have been examined as the main chain of brush polymers, we have particularly focused on the poly(ethylene oxide) (PEO)-based brush copolymers because of its specific characteristics, such as highly flexible nature and biocompatibility. For example, we have synthesized a series of PEO-based brush random copolymers, carrying long alkyl bristles by

The tacticity of polymers is one of the important structural characteristics that determines the polymer properties and functions, such as the thermal and mechanical properties, crystallization behaviors, and so on.13−20 Furthermore, the tacticity effects on the block copolymer self-assemblies have also been studied, in which the strong impact of tacticity on the crystallization behaviors resulted in forming unique selfassembled structures.21−25 However, it is rather surprising that the role of the chain tacticity on the brush copolymer selfassemblies has been neglected. Unlike regular polymers, the chain tacticity of brush polymers would affect not only their overall crystallization behaviors but also their bristle’s packing and orientation. Thus, we envisioned that the chain tacticity of the brush copolymers could further enrich the hierarchical B


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RESULTS AND DISCUSSION Synthesis. According to Scheme 1, PDGE-b-PHDGEs possessing varied combinations of chain stereochemistries were synthesized in three steps. We aimed at the synthesis of a series of PDGE-b-PHDGE with a fixed molecular weight and monomer composition (the degree of polymerization (DP) for each block was fixed at around 50) in order to prove the pure effect of the chain tacticity on the self-assembly behaviors. In addition, to provide the reference samples for the tacticitycontrolled BBCPs, the atactic and isotactic PDGE and PHDGE homopolymers with the DP of ca. 50, i.e., at-PDGE, it-PDGE, at-PHDGE, and it-PHDGE, were also prepared in a similar manner. The synthetic details for the homopolymers can be found in section S3-2 of the Supporting Information. During the initial step for the BBCP synthesis, the t-Bu-P4catalyzed sequential block copolymerizations of (RS)-/(R)DGE and (RS)/(R)-DEGE were performed using 3-phenyl-1propanol (PPA) as the initiator to produce the PDGE-bPDEGEs with four different chain tacticities. It should be noted that (R)-DGE and (R)-DEGE were verified to be pure enough in terms of optical purity by chiral HPCL analysis (see section S3-1 in the Supporting Information for more details). The t-BuP4-catalyzed ROP of (R)-DGE was performed with the [(R)DGE]0/[PPA]0/[t-Bu-P4]0 ratio of 50/1/1 to produce itPDGE. After 20 h of reaction, an aliquot of the polymerization mixture was analyzed by size exclusion chromatography (SEC) and 1H NMR measurements, which revealed the complete monomer consumption as well as the successful formation of a narrowly dispersed it-PDGE with the Mn,NMR of 10 600 g mol−1 and Mw/Mn of 1.05 (Table S1). The chain extension from the living it-PDGE oxyanion was carried out by adding 50 equiv of (R)-DEGE with respect to PPA, giving the desired [it-PDGE]b-[it-PDEGE] in 86.5% yield. The complete monomer conversion of (R)-DEGE was confirmed by 1H NMR analysis of the polymerization mixture. The SEC analysis of the final product revealed definitive evidence of the successful formation of BBCP; the elution peak for the first product clearly shifted to the higher molecular weight region after the second polymerization (Figure S1). In addition, the 1H (Figure S2) and 13C NMR spectra (Figure 1) of the obtained final product showed signals assignable to both the it-PDGE and it-PDEGE backbones. Based on the 1H NMR analysis, the Mn,NMR and the mole fraction of the PDGE block (f DGE) were determined

varying the number of hydrophilic phosphorylcholine head groups as the bristle end, by the postpolymerization modification of poly(epichlorohydrin).35 On the other hand, we have developed a versatile synthetic platform for the welldefined and narrowly dispersed PEO-based brush block copolymers (BBCPs), which involves the t-Bu-P4-catalyzed sequential ring-opening block copolymerization of allyl glycidyl ether and ethoxyethyl glycidyl ether followed by the selective side chain transformations based on the selective deprotection/ functionalization and thiol−ene click chemistries.36 With the aid of the t-Bu-P4 catalyst system, simple amphiphilic BBCPs as well as the crystalline−amorphous BBCPs carrying electroactive functional groups were successfully obtained with a narrow dispersity value and desired monomer composition.37 Considering their potential applications in biomedical, nanofabrication, and organic electronics fields, the PEO-based BBCPs are highly promising as a model system for examining the tacticity effects on brush copolymer self-assembly. In this study, we first present a strategy to prepare PEObased BBCPs consisting of poly(decyl glycidyl ether) (PDGE) and poly(10-hydroxyldecyl glycidyl ether) (PHDGE) blocks, having four different types of chain tacticities, i.e., [at-PDGE]b-[at-PHDGE], [at-PDGE]-b-[it-P PHDGE], [it-PDGE]-b-[atPHDGE], and [it-PDGE]-b-[it-PHDGE], where the it and at represent the isotactic and atactic chains, respectively. Scheme 1 illustrates the synthetic route toward PDGE-b-PHDGEs, which include the t-Bu-P4-catalyzed sequential block copolymerization of decyl glycidyl ether (DGE) and dec-9-enyl glycidyl ether (DEGE) to give PDGE-b-poly(dec-9-enyl glycidyl ether) (PDGE-b-PDEGE) followed by a hydroboration−oxidation reaction of the olefinic bristle. The most important feature of this synthetic strategy is that the desired chain tacticity can be achieved by the choice of the monomer optical purity, in which the racemic and optically pure monomers produce atactic and isotactic stereosequences, respectively. According to our previous studies, the t-Bu-P4-catalyzed ROP of the optically pure epoxide monomers can produce the corresponding isotactic polyethers without stereoinversion.38,39 For example, when the optically pure (R)-DGE and (R)-DEGE are used as the monomers, the resultant BBCP should have an isotactic PDGE (it-PDGE) block and an isotactic PHDGE (it-PHDGE) block, i.e., [it-PDGE]-b-[it-PHDGE]. On the other hand, when the racemic (RS)-DGE and optically pure (R)-DEGE are used as the monomers, the resultant BBCP should have an atactic PDGE (at-PDGE) block and an it-PHDGE block, i.e., [atPDGE]-b-[it-PHDGE]. In this manner, the BBCPs with four different stereochemistries could be prepared using the possible combinations of (R)- or (RS)-DGE and (R)- or (RS)-DEGE. In this study, we employed the hydroboration−oxidation reaction for the transformation of the olefin into hydroxyl groups, which endowed the BBCPs with an amphiphilic nature. Specifically, the decyl and 10-hydroxydecyl side chains can be regarded as hydrophobic and hydrophilic, respectively, which would permit the BBCPs to phase separate. The PDGE-b-PHDGEs as well as the corresponding homopolymers were then subjected to differential scanning calorimetry (DSC) and grazing incidence X-ray scattering (GIXS) analyses to provide preliminary information about the tacticity effects on the thermal properties and morphological structures, respectively. Our results suggested that the isotactic stereochemistry in the BBCPs always promoted the formation of better quality morphological structure in terms of lateral ordering and orientation.

Figure 1. 13C NMR spectra of [it-PDGE]-b-[it-PDEGE] and [itPDGE]-b-[it-PHDGE] in CDCl3. C


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Macromolecules to be 21 100 g mol−1 and 0.50, respectively, which closely matched the calculated values of 21 500 g mol−1 and 0.50, respectively. When we closely examine the 13C NMR spectra of the methine carbon region (79.3−78.4 ppm), only one resonance line corresponding to the composite of the ii triad stereosequence for the PDGE and PDEGE blocks was observed, which confirmed that the obtained BBCP truly consisted of the it-PDGE and it-PDEGE block. On the other hand, [at-PDGE]-b-[at-PDEGE], [at-PDGE]-b-[it-PDEGE], and [it-PDGE]-b-[at-PDEGE] exhibited three resonance lines corresponding to the ii, is/si, and ss triad stereosequences. Importantly, the peak area ratios for the three peaks was found to be 20/49/30, 62/25/13, and 59/28/13 for [at-PDGE]-b-[atPDEGE], [at-PDGE]-b-[it-PDEGE], and [it-PDGE]-b-[atPDEGE], respectively, which were in good agreement with the expected values (Figure S3). Thus, we successfully obtained a series of PDGE-b-PDEGEs with the desired combination of main chain stereochemistry as well as fixed molecular weight and monomer composition. The obtained [it-PDGE]-b-[it-PDEGE] was then converted to the desired [it-PDGE]-b-[it-PHDGE] by the hydroboration−oxidation reaction of the olefinic bristle in the PDEGE block. The reaction of the CC double bond with 9-BBN followed by the addition of NaOH/H2O2 had been employed for the postpolymerization modification to introduce a hydroxyl group along the polymer backbone.40 Thus, we also employed this reaction condition to produce [it-PDGE]-b-[it-PHDGE] in 54.2% yield. The FT-IR analysis of the product indicated the disappearance of the absorption bands due to the olefinic side chain, while a broad absorption band due to the hydroxyl group appeared (Figure S4). The completion of the reaction to form the hydroxyl group was also verified by the complete disappearance of the 1H and 13C NMR signals due to the olefin group (Figures S2 and S1). Furthermore, the signal corresponding to the methylene adjacent to the hydroxyl group was clearly observed at 62.7 ppm in the 13C NMR spectrum. The 1H and 13C NMR signals were therefore reasonably assigned to the expected chemical structure of the [it-PDGE]-b[it-PHDGE]. The Mn,NMR value of the amphiphilic BBCP was calculated to be 22 000 g mol−1 by assuming the quantitative conversion from the PDEGE block into the PHDGE. In addition, the SEC trace showed a monomodal peak with the Mw/Mn value of 1.06 that implied the absence of the side reactions like main chain scission during the hydroboration− oxidation reaction (Figure S5). On the basis of the NMR and SEC analysis, we confirmed the successful synthesis of the targeted [it-PDGE]-b-[it-PHDGE] with the well-defined chemical structure. In a similar fashion, we also obtained the well-defined BBCPs with three different combinations of chain stereochemistries, i.e., [at-PDGE]-b-[at-PHDGE], [at-PDGE]b-[it-PHDGE], and [it-PDGE]-b-[at-PHDGE], by the aboveestablished synthetic procedure. Importantly, all the BBCPs have narrow Mw/Mn values and comparable Mn,NMR and f DGE values. The molecular characteristics of all the homopolymers and BBCPs are summarized in Table 1. Thermal Properties. A DSC analysis was conducted on the brush polymers in order to investigate their phase transition behaviors and tacticity effects. The results are shown in Figure 2 and Table 2. at-PDGE underwent crystallization at −11.2 °C (= Tc) in the cooling run and crystal melting at −3.6 °C (= Tm) in the heating run. The Tc and Tm were drastically raised to 38.2 and 57.4 °C, respectively, by the stereoisomerization to isotacticity. Furthermore, the heats of fusion of crystallization

Table 1. Molecular Characteristics of PDGE-b-PHDGEs in Various Tacticities and Their Homopolymers polymer at-PDGE it-PDGE at-PHDGE it-PHDGE [at-PDGE]-b[at-PHDGE] [it-PDGE]-b[at-PHDGE] [at-PDGE]-b[it-PHDGE] [it-PDGE]-b[it-PHDGE]

Mn,NMRa (g mol−1)

Mn,SEC (g mol−1)

Mw/Mn

f DGEa

9760 9850 12000 11700 21400

8560b 8670b 12300c 13200c 16800b

1.05b 1.05b 1.11c 1.14c 1.08b

0.52

19400

11700b

1.10b

0.51

23100

14600b

1.13b

0.50

22000

9000b

1.06b

0.50

a

Calculated based on 1H NMR analysis. bDetermined by SEC in CHCl3 using polystyrene standards. cDetermined by SEC in DMF (containing 0.01 M LiCl) using polystyrene standards.

and crystal melting transitions (ΔHf,c and ΔHf,m) were significantly increased by the isotacticity formation. These results collectively confirm that the crystallization of at-PDGE required a much higher degree of supercooling and produced relatively smaller and more defective crystals while that of itPDGE required a lower degree of supercooling and led larger and more perfect crystals. Overall, the tacticity effect was significant during the crystallization and crystal melting behaviors of PDGE. at-PHDGE also showed a single-step phase transition in the cooling run as well as in the heating run. Its Tc and Tm were much higher than those of at-PDGE; the ΔHf,c and ΔHf,m were much higher than those of at-PDGE. Considering the chemical structures, these differences in the crystallization and melting transitions might be caused by a mass effect and a possible hydrogen-bonding (H-bonding) interaction of the additional −OH group in the bristle end. During the heating run, no additional phase transition could be observed above the melting (Tm = 49.1 °C) of the primarily formed crystals. This result suggests that the H-bonding interaction in at-PHDGE is not strong enough to form liquid crystals above Tm = 49.1 °C. Nevertheless, the PHDGE polymer showed very interesting tacticity-dependent phase transition behaviors due to the presence of the −OH group in the bristle end, which were quite different from those observed for the PDGE polymer. By the isotacticity formation, both the Tc and Tm somewhat declined; however, the ΔHf,c and ΔHf,m increased. Moreover, in the cooling run, it-PHDGE exhibited a two-step crystallization rather than a single-step crystallization: 35.5 °C (= Tc,1) and 30.0 °C (= Tc,2). In the heating run, this isotactic polymer revealed a two-step crystal melting (Tm,1 = 35.5 °C and Tm,2 = 44.8 °C) and subsequent cold crystallization (48.5 °C (= Tcold‑xtal) and remelting (72.1 °C (= Tremelt)). These remarkable phase transition behaviors might be attributed to the tacticity-mediated H-bonding effects as follows. First, the observation of the lower Tc,1 and Tc,2 informs that the H-bonding interactions of the bristles in the melt state are relatively stronger in it-PHDGE compared to those in atPHDGE. The stronger H-bonding interactions could result from the closer positioning of the −OH groups due to the isotacticity. These H-bonding interactions may further cause certain levels of reduction in the mobility and packing ability of the polymer chains. The observation of the two crystallization D


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Figure 2. DSC thermograms in cooling runs (a, c, e, and g) and subsequent heating runs (b, d, f, and h) of BBCPs and their homopolymers in various tacticities: (a, b) [at-PDGE]-b-[at-PHDGE] and its homopolymers; (c, d) [at-PDGE]-b-[it-PHDGE] and its homopolymers; (e, f) [itPDGE]-b-[at-PHDGE] and its homopolymers; (g, h) [it-PDGE]-b-[it-PHDGE] and its homopolymers. The black line curves represent the DSC traces of the BBCPs, whereas the red line curves represent those of the PDGE homopolymers in different tacticities and the blue line curves represent those of the PHDGE homopolymers in different tacticities.

drastically lowered compared to that of its homopolymer. The ΔHf,c was also decreased compared to that of its homopolymer. Such a declining fashion was observed in the Tm and ΔHf,m of the resulting at-PHDGE block crystals. Namely, the reductions in the Tc and ΔHf,c were reflected in the melting transition (Tc and ΔHf,c) of the resulting crystals. These results collectively inform that the crystallization and crystal melting transitions of the at-PHDGE block were significantly influenced by the presence of its at-PDGE block counterpart. The lower Tc and ΔHf,c could be caused by an entropy penalty due to the absence of one main chain end as well as by the restricted chain mobility due to the presence of the at-PDGE counterblock component. Similar effects are expected for the crystallization of the atPDGE counterpart. However, surprisingly, such reductions were not observed for the at-PDGE block. In contrast, the Tc and ΔHf,c of the at-PDGE block were both increased compared to those of its homopolymer. These increases could be attributed to the presence of the at-PHDGE block crystals grown before crystallization of the at-PDGE block started. Namely, such at-PHDGE block crystals might act as nuclei to crystallize the at-PDGE block counterpart and further promote the crystallization. In general, the crystallization of one block component in the presence of another block crystals formed beforehand is known to be highly retarded because of the geometrical confinement due to the crystals grown already. Considering this knowledge, the positive contribution of the atPHDGE block crystals to the crystallization of the at-PDGE block counterpart is quite remarkable. As a result, the resulting at-PDGE block crystals revealed much higher Tm than that of its homopolymer crystals. Despite the higher Tm, the ΔHf,m (i.e., overall crystallinity) was a much lower than that of the homopolymer. This significant reduction in the ΔHf,m might be a clue that the geometrical confinement effect due to the

temperatures indicates that the reductions in the chain mobility and packing ability are not uniform throughout the whole sample. Consequently, it-PHDGE requires a higher degree of supercooling for crystallization from the melt state, thus exhibiting lower Tc,1 and Tc,2 values. As a result of such restricted crystallizations, the resultant crystals showed relatively lower Tm,1 and Tm,2 values Second, the observations of Tcold‑xtal and Tremelt suggest that it-PHDGE favorably formed liquid crystals above Tm,2 via cold crystallization and the resulting liquid crystals underwent a phase transition to the isotropic, disordered state with a further increasing temperature, which were not detected for at-PHDGE. Such a liquid crystal formation could be ldue to the isotacticity-assisted Hbonding interactions of the bristles bearing the −OH end group. These results again support that the H-bonding interaction between the bristles is much stronger in itPHDGE than in at-PHDGE. Namely, the H-bonding interaction of the bristles in PHDGE could be significantly enhanced by the isotacticity formation. More interestingly, the phase transition behaviors of PDGE and PHDGE were further influenced by the diblock copolymer formation and the tacticity of their counterparts. All the BBCPs of PDGE and PHDGE were found to show crystallization exotherms and crystal melting endotherms of their block components (Figure 2 and Table 2). These results confirmed that for all BBCPs the block components favorably form separate crystals rather than cocrystals. During the cooling run of [at-PDGE]-b-[at-PHDGE], the crystallization of the atPHDGE block from the melt state first occurred, then followed by the crystallization of the at-PDGE block. As a result, during the heating run, the resulting at-PHDGE block crystals melted at a higher temperature compared to the at-PDGE block crystals. The Tc of the at-PHDGE block component was E


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Macromolecules Table 2. Phase Transition Behaviors of PDGE-b-PHDGEs in Various Tacticities and Their Homopolymers PDGE block atactic (at) polymer

Tca

(°C)

PHDGE block isotactic (it)

−1

ΔHf,c (J g ) b

Tc (°C)

atactic (at) −1

ΔHf,c (J g )

isotactic (it) −1

Tc (°C)

ΔHf,c (J g )

41.6

38.0

18.1

29.7

21.9

25.8

Tc (°C)

ΔHf,c (J g−1)

35.5 (30.0)

51.1

22.0

40.9

−1

cooling run (2.0 °C min ) at-PDGE it-PDGE at-PHDGE it-PHDGE [at-PDGE]-b-[at-PHDGE] [at-PDGE]-b-[it-PHDGE] [it-PDGE]-b-[at-PHDGE] [it-PDGE]-b-[it-PHDGE]

−11.2

28.1 38.2

5.1 −2.4

32.2 41.9 16.7 19.0 PDGE block atactic (at)

polymer

92.8

Tmd (°C)

ΔHf,me (J g−1)

73.1 103.1c isotactic (it)

Tm (°C)

103.1c

19.0 PHDGE block atactic (at)

ΔHf,m (J g−1)

Tm (°C)

ΔHf,m (J g−1)

49.1

43.2

isotactic (it) Tm (°C)

ΔHf,m (J g−1)

44.8 (35.5)f 48.5g 72.1 (80.0)

34.2 16.8h 29.3

35.1

40.6

30.8 57.4 79.2

7.4 32.5i 26.7

−1

heating run (10.0 °C min ) at-PDGE it-PDGE at-PHDGE it-PHDGE

−3.6

[at-PDGE]-b-[at-PHDGE] [at-PDGE]-b-[it-PHDGE] [it-PDGE]-b-[at-PHDGE] [it-PDGE]-b-[it-PHDGE]

29.4 4.9

34.3 57.4

88.4

15.4 45.5 56.7 57.4

54.5 32.5i

46.9

28.6

31.1

17.3

a

Crystallization temperature. bHeat of fusion for crystallization. cSum of the heats of fusion for the crystallizations of it-PDGE and it-PHDGE blocks. Crystal melting temperature. eHeat of fusion for crystal melting. fTemperature of a shoulder peak for crystal melting. gCold crystallization temperature. hHeat of fusion for cold crystallization. iSum of the heats of fusion for the crystal meltings of it-PDGE and it-PHDGE blocks.

d

beforehand grown at-PHDGE block crystals was present and thus caused a certain level of negative contribution to the crystallization of the at-PDGE block counterpart. Similar phase transition behaviors were observed for [atPDGE]-b-[it-PHDGE]. However, the drop in the Tc of the itPHDGE block due to the presence of the at-PDGE block was relatively less than that in the Tc of at-PHDGE block caused by the at-PDGE block. In addition, the ΔHf,m of the it-PDGE block was enhanced, which was in contrast to the reduction in that of the at-PHDGE block for [at-PDGE]-b-[at-PHDGE]. Moreover, the ΔHf,m value was higher than that of the homopolymer. On the other hand, the increments in the Tc and Tm of the at-PDGE block by the it-PHDGE counterpart were relatively lower than those in [at-PDGE]-b-[at-PHDGE]. Nevertheless, the ΔHf,c and ΔHf,m of the at-PDGE block were significantly enhanced by the it-PHDGE counterpart, which were quite different from those of [at-PDGE]-b-[atPHDGE]. Furthermore, these values were higher than those of its homopolymer. A similar trend was observed in the phase transitions of atPHDGE and it-PHDGE in the BBCPs with the it-PDGE counterpart. However, the tacticity effects of the PHDGE block on the phase transitions of the it-PDGE counterpart were significantly different from those in the phase transitions of the at-PDGE counterpart. Namely, its Tc, ΔHf,c, and ΔHf,m were always reduced rather than enhanced. Its Tm was unchanged or slightly decreased.

Overall, the DSC results inform that the crystallization and crystal melting behaviors of the block components in [PDGE]b-[PHDGE] were significantly influenced by their tacticity as well as by the tacticity of their counterpart. Morphological Structures. With the thermal phase transition characteristics discussed above, the BBCPs and their hompolymers were further investigated to get information on any possible morphological structures in the thin film state and their tacticity dependencies by using synchrotron GIXS analyses. The measured scattering images are shown in Figures 3 and 4; the analysis results are listed in Tables 3 and 4. Figure 3a shows a representative picture of the grazing incidence wide-angle X-ray scattering (GIWAXS) images measured for the thin films of at-PDGE at room temperature. This scattering pattern showed only two amorphous halo peaks as hemispherical rings; the first halo peak at 3.14° (2.35 nm dspacing) corresponds to the mean interdistance between the polymer backbone chains, while the second halo peak at 16.29° (d = 0.46 nm) corresponds to the mean interdistance of the bristles and between the bristles and the polymer backbones. The GIWAXS results confirm that the at-PDGE film was amorphous at room temperature. This amorphous morphology was attributed to the low Tm (−3.6 °C) of at-PDGE. Different from the at-PDGE film, the it-PDGE film exhibited a featured GIWAXS image (Figure 3b). In particular, several scattering spots appeared at αf = 3.89° (d = 1.79 nm), 7.86° (0.88 nm), 11.69° (0.60 nm), 14.42° (0.48 nm), 15.48° (0.45 nm), 16.60° (0.42 nm), and 19.05° (0.37 nm) along the F


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Macromolecules

Figure 3. Synchrotron X-ray scattering images of the thin films of the PDGE and PHDGE homopolymers in various tacticities, which were measured at room temperature: (a) GIWAXS (λ = 0.1290 nm, αi = 0.155°, and SDD = 212 mm) and (e) GISAXS (λ = 0.1291 nm, αi = 0.126°, and SDD = 3930 mm) of at-PDGE; (b) GIWAXS (λ = 0.1213 nm, αi = 0.126°, and SDD = 222 mm) and (f) GISAXS (λ = 0.1135 nm, αi = 0.130°, and SDD = 2906 mm) of it-PDGE; (c) GIWAXS (λ = 0.1117 nm, αi = 0.110°, and SDD = 227 mm) and (g) GISAXS (λ = 0.1135 nm, αi = 0.126°, and SDD = 2906 mm) of at-PHDGE; (d) GIWAXS (λ = 0.1117 nm, αi = 0.130°, and SDD = 227 mm) and (h) GISAXS (λ = 0.1135 nm, αi = 0.132°, and SDD = 2906 mm) of it-PHDGE. Here, λ is the wavelength of the X-ray beam; αi is the grazing incidence angle of the X-ray beam; SDD is the sample-todetector distance. αf and 2θf are the out-of-plane and in-plane exit angles of the out-going X-ray beam, respectively.

meridian line at 2θf = 0°, where αf and 2θf are the out-of-plane and in-plane exit angles of the out-going X-ray beam, respectively. These observations suggest that the crystal lattice planes were stacked together in a regular manner along the outof-plane of the film. Another set of scattering spots appeared at αf = 2.72° (d = 0.72 nm), 5.21° (0.66 nm), 6.79° (0.62 nm), 9.12° (0.55 nm), 10.86° (0.50 nm), 12.70° (0.46 nm), 14.30° (0.43 nm), and 17.10° (0.38 nm) along the meridian line at 2θf = 9.89°. The appearances of these spots collectively suggested that crystals in the monoclinic lattice were formed in the film. Taking into consideration the monoclinic crystal lattice, some characteristic spots could be indexed, as shown in Figure 3b. From these assigned spots, the lattice parameters could be determined as a = 0.73 nm, b = 0.41 nm, c = 3.60 nm, α = γ = 90.0°, and β = 96.3°. These scattering results indicated that itPDGE formed well-defined monoclinic crystals in the film. The results further inform that the isotacticity could promote the lateral packing order of the alkyl bristles in a crystal lattice manner. It is additionally noted that the c value is slightly larger than twice the length of the bristle in a fully extended conformation. This fact informs that the bristles could have a fully extended conformation and align along the c-axis and then laterally packing together as a monoclinic lattice. There was no interdigitation between the stacks of such laterally ordered bristles along the c-axis. In conclusion, the it-PDGE homopolymer chains in the thin films favorably formed a

well-defined horizontal multibilayer structure with a long period dL of 3.58 nm in which the bristles in a fully extended conformation were laterally well packed like a monoclinic crystal lattice without interdigitation. The at-PHDGE film also revealed a featured GIWAXS image (Figure 3c). However, the scattering image was quite different from that of the it-PDGE film. Scattering peaks appeared at αf = 1.78° (T; d = 3.42 nm), 2.03° (R; d = 3.42 nm), 3.66° (T; d = 1.71 nm), 3.92° (R; d = 1.71 nm), and 5.67° (R) along the meridian line at 2θf = 0°, where it is noted that the spots marked with “T” were the scattering peaks generated by the transmitted X-ray beam, while those marked with “R” were the scattering peaks generated by the reflected X-ray beam. Similar scattering spots were also weakly observed along the direction defined with the azimuthal angle μ of 60° with respect to the out-of-plane of the film (see the spots along the red dotted line in Figure 3c). The observation of these spots collectively indicated that at-PHDGE formed a lamellar structure with dL = 3.42 nm in which the lamellae were stacked along two different directions, namely, the directions defined by μ = 0 and 60°. The horizontally oriented lamellar structures were formed in a major portion, while the tilted lamellar structures were only a minor portion. An additional scattering peak was discernible at 2θf = 14.82° (d = 0.43 nm) as an arc shape. Considering such lamellar structures and the chemical structure of the brush polymer, this scattering arc might be originated from the lateral G


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Macromolecules

Figure 4. Synchrotron X-ray scattering images of thin films of [PDGE]-b-[PHDGE]s in various tacticities, which were measured at room temperature: (a) GIWAXS (λ = 0.1213 nm, αi = 0.117°, and SDD = 221 mm) and (e) GISAXS (λ = 0.1122 nm, αi = 0.151°, and SDD = 2501 mm) of [at-PDGE]-b-[at-PHDGE]; (b) GIWAXS (λ = 0.1213 nm, αi = 0.117°, and SDD = 221 mm) and (f) GISAXS (λ = 0.1122 nm, αi = 0.116°, and SDD = 2501 mm) of [at-PDGE]-b-[it-PHDGE]; (c) GIWAXS (λ = 0.1213 nm, αi = 0.117°, and SDD = 221 mm) and (g) GISAXS (λ = 0.1122 nm, αi = 0.125°, and SDD = 2501 mm) of [it-PDGE]-b-[at-PHDGE]; (d) GIWAXS (λ = 0.1122 nm, αi = 0.117°, and SDD = 222 mm) and (h) GISAXS (λ = 0.1122 nm, αi = 0.122°, and SDD = 2501 mm) of [it-PDGE]-b-[it-PHDGE]. Here, λ is the wavelength of the X-ray beam; αi is the grazing incidence angle of the X-ray beam; SDD is the sample-to-detector distance.

A similar scattering feature was measured for the it-PHDGE film (Figure 3d). However, no scattering spots were discernible along the direction defined by μ = 60°. Furthermore, all the scattering peaks were relatively stronger in intensity than those of the at-PHDGE film. The scattering results collectively confirmed that in the film it-PHDGE formed only horizontal multibilayer structures (dL = 3.42 nm) with no interdigitation in the laterally ordered bristles (0.43 nm d-spacing). The overall crystallinity was relatively higher than that of the at-PHDGE film. These enhanced structural features could be driven by the isotacticity. All the homopolymer films were found to reveal featureless grazing incidence small-angle X-ray scattering (GISAXS) images (Figures 3e−h). These results inform that the homopolymers did not form any discernible microstructures in the films, regardless of the tacticities. With the homopolymers’ morphologies above as well as the DSC analysis results, the BBCPs in the thin films were tried to be analyzed. Figure 4a shows a representative picture of the GIWAXS images measured for the thin films of [at-PDGE]-b[at-PHDGE] at room temperature. This BBCP film revealed several scattering arcs. In the large angle region, two scattering arcs appeared at (αf = 0° and 2θf = 16.69°) and (αf = 13.35° and 2θf = 10.06°) (see the peaks remarked by the red arrows “1” and “2”); their d-spacings were estimated to be 0.42 nm. In particular, the scattering arc at (αf = 0° and 2θf = 16.69°)

Table 3. Thin Film Morphologies of PDGEs and PHDGEs in Various Tacticities phase structure structure

dLa (nm) bristle orientation bristle packingb (nm)

at-PDGE

it-PDGE

at-PHDGE

it-PHDGE

amorphous

Hor multibilayer

Hor multibilayer

random

3.58 Ver with 6.3° tiltc 0.41, 0.73

Hor and Hor multibilayer with 60° tiltd 3.42 Ver and Ver with 60° tilt 0.43

0.46

3.42 Ver 0.43

a

Long period of multibilayer structure. bMean interdistance of bristles. Tilt angle with respect to the out-of-plane of film. dTilt angle with respect to the in-plane of film. Hor and Ver in the table mean to horizontal and vertical, respectively. c

packing of the bristles aligned vertically with respect to the film plane. The dL value is close to twice the length of the bristle in a fully extended conformation. Overall these results inform that the bristles could have a fully extended conformation and laterally packed together without any well-defined ordering, forming a horizontal multibilayer structure in a major portion and the tilted multibilayer structure in a minor portion. There was no interdigitation between the neighboring layer stacks. H


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Macromolecules Table 4. Thin Film Morphologies of PDGE-b-PHDGEs in Various Tacticities thin film morphology microstructure type La (nm) phase structure at-PDGE structure dLb (nm) bristle orientation bristle packingc (nm) it-PDGE structure dL (nm) bristle orientation bristle packing (nm) at-PHDGE structure dL (nm) bristle orientation bristle packing (nm) it-PHDGE structure dL (nm) bristle orientation bristle packing (nm)

[at-PDGE]-b-[at-PHDGE]

[at-PDGE]-b-[it-PHDGE]

Hor lamellae 17.50

Hor lamellae 17.50

Hor and Ver multibilayer 3.52, 3.44 Ver, Hor, and Hor with 150° tilte 0.45

amorphous

[it-PDGE]-b-[at-PHDGE]

[it-PDGE]-b-[it-PHDGE]

Hor lamellae (suspected)

Hor lamellae (suspected)

Hor and Ver multibilayer 3.37 Hor and Hor with 150° tilte 0.44

Ver multibilayer 3.65 Hor and Hor with 167° tilte 0.43

random 0.46

Hor multibilayer 3.52, 3.44 Hor and Hor with 143° tilte 0.42

Hor and Ver multibilayer 3.37 Hor and Hor with 149° tilte 0.42 Ver multibilayer 3.45 Hor 0.42

Ver multibilayer 3.65 Hor and Hor with 167° tilte 0.40

a Long period of lamellar structure. bLong period of multibilayer structure. cMean interdistance of bristles. dTilt angle with respect to the out-ofplane of film. eTilt angle with respect to the in-plane of film. Hor and Ver in the table mean to horizontal and vertical, respectively.

resembles that of the at-PHDGE homopolymer film in shape and d-spacing value. Taking this fact into account, the arc “1” could originate from the lateral packing of the at-PHDGE block’s bristles aligned along the out-of-plane of film, while the arc “2” might be attributed to the lateral packing of the atPHDGE block’s bristles aligned along a direction normal to the line defined with an azimuthal angle μ of 37°. The arc “1” was much stronger in intensity than that of the arc “2”, indicating that the population of the vertically aligned bristles was relatively much higher than that of the bristles aligned along a direction normal to the line defined with μ = 37°. Interesting scattering arcs were additionally discernible at (αf = 7.66° and 2θf = 13.81°), (αf = 16.02° and 2θf = 0°), and (αf = 0° and 2θf = 15.87°) (see the peaks denoted by the red arrows “3”, “4”, and “5”); they all had a d-spacing of 0.45 nm. These arc peaks could be attributed to the lateral packings of the at-PDGE block’s bristles aligned along (i) a direction normal to the line defined with μ = 30°, (ii) the in-plane of the film, and (iii) the out-of-plane of the film, respectively. The arc peak intensities were in a decreasing order: peak “3” > peak “4” ≫ peak “5”. These inform that the population was relatively the highest for the bristles aligned along a direction normal to the line defined with μ = 30°, intermediate for the horizontally aligned bristles, and the lowest for the vertically aligned bristles. On the other hand, relatively strong arc peaks were observed in the low angle region: (αf = 0° and 2θf = 2.05°), (αf = 0° and 2θf = 4.05°), and (αf = 3.95° and 2θf = 0°). Taking into consideration the GIWAXS features of its hompolymers, the arc peaks in the low angle region could be generated from the presence of multibilayer structures in the film. For the scattering arc at (αf = 3.95° and 2θf = 0°), which was the second-order peak, dL was estimated to be 3.52, which might correspond to the long periods of the horizontal multibilayer structures formed with

the at-PHDGE and at-PDGE block chains, respectively. For the other arcs along the equatorial line, dL was estimated to be 3.44 nm, which was slightly larger than that (3.42 nm) of the multibilayer structure formed in the at-PHDGE homopolymer film but slightly lower than that (3.52 nm) of the multibilayer structure formed with the at-PDGE block chains. Considering this fact and the bristles’ alignments, the arc peaks along the equatorial line could result from both the vertical multibilayer structures formed by the at-PDGE and at-PHDGE block chains, respectively. These scattering results collectively inform that in the film the at-PDGE and at-PHDGE block chains formed their own block domains via phase separation. The microstructure formation of such phase-separated block domains was detected in the GISAXS image, as shown in Figure 4e. Two weak, broad scattering peaks were discernible at αf = 0.29° and 0.44° along the meridian line (see the two red arrows “T” and “R”); they were generated by the transmitted (T) and reflected (R) X-ray beams, respectively. These results indicated that a horizontal lamellar structure was formed in the film where the lamellar domains of the at-PDGE and the atPHDGE blocks were alternatively stacked along the out-ofplane of film. Its long period L was estimated to be 17.50 nm. The [at-PDGE]-b-[it-PHDGE] film revealed a featured GIWAXS pattern (Figure 4b). In the large angle region, an isotropic halo scattering appeared at 15.20° (d = 0.46 nm) could be originated from the mean interdistance of the bristles in the at-PDGE block chains in the melt state due to its low Tm. A scattering arc was observed at αf = 16.54° (d = 0.42 nm) along the meridian line (see the peak denoted by the red arrow “1”), which might be attributed to the lateral packing of the horizontally aligned bristles in the it-PHDGE block chains at crystalline state. In the low angle region, two arc peaks were observed at (αf = 0° and 2θf = 2.01°) and (αf = 0° and 2θf = I


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Macromolecules 4.03°). From these peaks, a d-spacing value was estimated to be 3.45 nm, which corresponded to the long period L of the vertically oriented multibilayer structure formed in the itPHDGE block domains. These GIWAXS results inform that the block components underwent phase separation, forming amorphous at-PDGE block domains and crystalline it-PHDGE block domains. The microstructure of the phase-separated block domains was further identified in the GISAXS pattern (Figure 4f). A scattering peak was detected at αf = 0.37° along the meridian line, confirming the presence of a horizontal lamellar structure in the film; its L value was 17.50 nm. In the [it-PDGE]-b-[at-PHDGE] film, the block components were also found to form individual crystalline structures via phase separation (Figure 4c). The at-PHDGE blocks’ bristles were mainly aligned along a direction normal to the line defined by μ = 31° and in minor along the in-plane of the film (see the peaks denoted by the red arrows “1” and “2”); their dspacing in the scattering pattern was 0.42 nm. Similar alignments and lateral packings were observed for the itPDGE blocks’ bristles. They were mainly aligned along a direction normal to the line defined with μ = 30° and in minor along the in-plane of the film (see the peaks denoted by the red arrows “3” and “4”); their d-spacing in the scattering pattern was 0.44 nm. With such bristle alignments and packings, the block components formed their own multibilayer structures. For each block component, vertical multibilayer structure was formed in a major portion while a horizontal multibilayer structure was produced in a minor portion; their dL value was estimated to be 3.37 nm. Nevertheless, the presence of their microstructure could not be clearly identified from the measured GISAXS image (Figure 4g). Such an appearance might be caused by the small electron density contrast between the crystalline it-PDGE and crystalline at-PHDGE domains due to their crystalline states. However, a horizontal lamellar structure formation would be expected for the [it-PDGE]-b-[atPHDGE] film in regard to that observed for the [at-PDGE]-b[at-PHDGE] film. Similar structural features were observed for the [it-PDGE]b-[it-PHDGE] film (Figure 4d,h). Both blocks’ bristles were mainly aligned along a direction normal to the line defined with μ = 23° and in minor along the in-plane of the film (see the peaks denoted by the red arrows “1”, “2”, “3”, and “4”); the dspacing value was 0.40 nm for the it-PHDGE blocks’ bristles and 0.43 nm for the it-PDGE blocks’ bristles. Their multibilayer structures had dL = 3.65 nm. A horizontal lamellar structure of the block domains would be expected. However, such a microstructure could not be clearly confirmed because of the small electron density contrast between the block domains due to their crystalline states. Overall, all the brush polymers exhibited a strong tendency to form a multibilayer structure. The formation of such a structure could be driven by the self-assembling ability of welldefined bristles and further enhanced by controlling the polymer backbone chain’s tacticity. In particular, the overall crystallinity and orientation of such a multibilayer structure were significantly enhanced by the isotaticity formation; remarkably, a monoclinic lattice structure was achieved with it-PDGE. For their BBCP systems, all the block components built up their own multibilayered crystalline domains, producing horizontal lamellar structures as a microstructure. These results confirmed that the individual blocks have very exclusive self-organization characteristics regardless of the tacticities, leading to the formation of separate crystals rather

than cocrystals in their BBCP systems. In the domains of each block component, the orientation of the crystals (i.e., multibilayer structured crystals) was severely influenced by its tacticity as well as the counterpart’s tacticity. Moreover, the orientation of the block chain’s bristles was significantly varied by its tacticity and the counterpart’s tacticity.

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CONCLUSIONS We have successfully prepared a series of BBCPs consisting of PDGE and PHDGE blocks, having four different types of chain tacticities, i.e., [at-PDGE]-b-[at-PHDGE], [at-PDGE]-b-[it-P PHDGE], [it-PDGE]-b-[at-PHDGE], and [it-PDGE]-b-[itPHDGE], as well as their corresponding homopolymers, i.e., at-PDGE, it-PDGE, at-PHDGE, and it-PHDGE, by the aid of the t-Bu-P4-catalyzed ROP. The well-controlled nature of this polymerization system gives rise to BBCPs with a narrow dispersity as well as a fixed molecular weight and monomer composition. at-PDGE was amorphous at room temperature because of its low Tm; however, it is suspected to form a crystalline structure from the DSC and its BBCP data. Very interestingly, it-PDGE formed a horizontal multibilayer structure with a remarkable monoclinic lattice in the thin films, which was driven by the bristles’ self-assembling ability and enhanced by the isotacticity. The crystal characteristics (Tc, ΔHf,c, Tm, ΔHf,m, crystallinity, and crystal orientation) were significantly influenced by the diblock formation and the tacticity and initial crystallization of the counterpart block. at-PHDGE was found to inherently form multibilayer structure, which was driven by the bristles’ self-assembing ability. A mixture of a horizontal (in the majority) and tilted (in the minority) multibilayer structures was developed in the thin films, which could be attributed to the presence of weak Hbonding interactions between the hydroxyl end groups in the bristles. Interestingly, the isotacticity formation in PHDGE was found to create advantageous contributions and, on the other hand, cause disadvantageous contributions to the morphological structure formation and phase transition due to the Hbonding interaction of the hydroxyl groups in the bristles. The isotacticity formation could significantly enhance the Hbonding interaction between the hydroxyl groups in the bristles. Because of such a tacticity-mediated H-bonding interaction, surprisingly, the Tc and ΔHf,c of it-PHDGE severely decreased, consequently causing reductions in Tm and ΔHf,m. A cold crystallization behavior was additionally observed in the heating run. Moreover, the formation of a liquid crystal phase was found to form above the Tm. Nevertheless, the orientation of the polymer chains (backbone and bristles) and their multibilayer structure were significantly improved. The structural characteristics (Tc, ΔHf,c, Tm, ΔHf,m, crystallinity, and crystal orientation) were further significantly influenced by the diblock formation and the tacticity of the counterpart block. Overall, all the BBCPs in this study made very complicated morphologies due to the strong self-assembling characteristics of both the PDGE and PHDGE blocks. The isotacticity formation always promoted the formation of a higher quality morphological structure in terms of lateral ordering and orientation. Such improvement in the morphology through tacticity control should be of fundamental interest as a new means to fine-tuning the properties and functions of various brush copolymer-based thin film materials, such as organic memory, photovoltaic, and light-emitting diode devices as well as biocompatible surfaces for medical devices and biosensors. J


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As the CC double bond can be used as a key intermediate for the incorporation of various functional groups via a number of organic transformation reactions, including thiol−ene, epoxidation, hydrosilylation, and cross-metathesis reactions, our tacticity-controlled BBCP system is highly promising as a versatile platform for the creation of such high-performance thin film materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00243. Experimental details, additional tables and figures, and additional discussions for monomer preparation and homopolymer synthesis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(M.R.) E-mail: [email protected]. *(T.S.) E-mail: [email protected]. ORCID

Takuya Isono: 0000-0003-3746-2084 Moonhor Ree: 0000-0001-5562-2913 Toshifumi Satoh: 0000-0001-5449-9642 Author Contributions

T.I. and H.L. contributed equally. Funding

This study was financially supported by the MEXT Grant-inAid for Scientific Research (B) (16H04152) and the MEXT Grant-in-Aid for Scientific Research on Innovative Areas “Advanced Molecular Transformation by Organocatalysis” (24105503 and 26105703). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron X-ray scattering measurements at the Pohang Accelerator Laboratory were supported by MSIT, POSTECH Foundation, and POSCO Company. We thank SANYO FINE Co., Ltd., for providing (S)-epichlorohydrin.

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REFERENCES

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