Hydrogen Bonding-Mediated Phase Transition of Polystyrene and

May 30, 2019 - Acid hydrolysis of the tert-butyl groups in the precursor resulted in the AmBBCP with an ultrahigh molecular weight (∼2880 kDa) and r...
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Hydrogen Bonding-Mediated Phase Transition of Polystyrene and Polyhydroxystyrene Bottlebrush Block Copolymers with Polyethylene Glycol Yong-Guen Yu,† Chunhee Seo,‡ Chang-Geun Chae,† Ho-Bin Seo,† Myung-Jin Kim,† Youngjong Kang,‡ and Jae-Suk Lee*,† †

School of Materials Science and Engineering and Grubbs Center for Polymers and Catalysis, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea ‡ Department of Chemistry, Hanyang University, 222 Wangsimni-ro, Seongdong-Gu, Seoul 04763, Republic of Korea

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ABSTRACT: A simple strategy was explored to systematically control the phase transition of an amphiphilic bottlebrush block copolymer (AmBBCP), poly[(norbornene-graf t-styrene)-block-(norbornene-graf t-hydroxystyrene)], with polymeric additives, such as poly(ethylene glycol) methyl ether (mPEG), poly(2-vinylpyridine) (P2VP), and poly(methyl methacrylate) (PMMA). The precursor polymers, poly[(norbornene-graf t-styrene)-block-(norbornene-graf t-4-tert-butoxystyrene)], were synthesized by sequential ring-opening metathesis polymerization of ω-endnorbornyl polystyrene and poly(4-tert-butoxystyrene). Acid hydrolysis of the tert-butyl groups in the precursor resulted in the AmBBCP with an ultrahigh molecular weight (∼2880 kDa) and relatively low dispersity (∼1.21). The disordered structures of neat AmBBCP were transformed to ordered lamellae by solvothermal annealing. AmBBCP and mPEG blended well because of H-bonding, maintaining well-ordered lamellae up to 40 wt % mPEG. The phase transition from ordered to disordered state occurred when increasing more than 50 wt %. The AmBBCP blended with P2VP and PMMA was compared. The effect of mPEG on phase transition, domain size, and refractive index and the photonic properties were determined.

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INTRODUCTION The self-assembly of the block copolymer (BCP) has attracted considerable attention because of the control of their morphologies in nanodomains.1 In nanotechnology, many applications of the BCPs are derived from the ease of attaining a feature below 100 nm.1 The well-organized nanomaterials with large d-spacing having hundreds of nanometers suggest other uses, such as photonic crystals (PCs).2−6 However, the typical requirement is that the copolymer should have high molecular weights (MWs > 400 kDa), such that the domain spacing is above 130 nm.5,7 Chain entanglement of high MW BCPs impedes the generation of photonic characteristics.8 The bottlebrush BCPs (BBCPs) with ultrahigh MWs over 1000 kDa have been explored recently for BCPs with a large dspacing.9−15 The structural features of the BBCP make not only chain-entanglement reduced but also form elongated backbone because of repulsion among the sterically congested side chains. These characteristic properties facilitate selfassembly with a less kinetic barrier even for ultrahigh MW BBCP. The morphology of such BBCPs is usually transformed by changing side chain length and volume fraction.16 Alternatively, the self-assembly of ultrahigh MW BBCP © XXXX American Chemical Society

blended with an additive has been suggested for precisely controlling the size and shape of nanostructures.9−13 For example, Grubbs group mixed polystyrene (PS) and polylactic acid (PLA)-based BBCPs (gA-b-gB) with the chemically identical linear polymers, PS (A) and PLA (B).10 Addition of the BBCP (gA′-b-gB′) with different MWs into parent BBCPs (gA′-b-gB′) containing polyisocyanate derivatives has been also studied.9 Furthermore, Watkins and co-workers showed phase separation by blending polyethylene oxide (PEO)-based BBCPs (gA″-b-gB″) with chemically different nanoparticles and small compounds (C).11−13 The small molecule C was selectively incorporated in one domain of gA″b-gB″ utilizing hydrogen bonding. The latter approach of introducing functional additives to regulate the refractive index (RI) and domain spacing has attracted much attention. In the gA″-b-gB″/C blends, the miscibility between a specific domain of host polymer and an additive is an important factor.17−20 Hydrogen bonding is mainly used to improve Received: April 4, 2019 Revised: May 3, 2019

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DOI: 10.1021/acs.macromol.9b00678 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of the Self-Assembly of P(NB-g-St)-b-P(NB-g-HSt) and Polymeric Additives through Hydrogen Bonding (*the KA Values Are for Reference Only and Represent Relevant Each of H-Bond Formation);17−19 (a) Synthesis of Polymers, (b) Behavior of Mixtures in Solution, (c) Self-Assembly Process, and (d) Phase-Separated Nanostructures

Table 1. Synthesis Results of P(NB-g-St)-b-P(NB-g-HSt)a DPn

conv. (%)f

polymer code

[NPSt]0/[G3]0:[NPtBOSt]0/[G3]0

Mn,theo (kDa)b

Mn,obsd (kDa)c

Đc

Mn,deprot (kDa)d

f NPSt (%)e

NPSt/NPtBOSt

NPSt

NPtBOSt

SP1 SP2 SP3 SP4 SP5

270:270 320:320 370:370 460:460 560:560

1601 1898 2194 2728 3321

1444 1641 1920 2420 3415

1.12 1.09 1.18 1.18 1.21

1215 1400 1630 2051 2880

56 60 58 58 56

248:240 308:252 349:304 435:387 595:561

>99 >99 >99 >99 >99

>99 >99 >99 >99 >99

Polymerizations performed in THF at room temperature ([NPSt]0 = [NPtBOSt]0 = 0.2 M). bMn,theo = [NPSt]0/[G3]0 × conv. (%)/100 × MW of NPSt + [NPtBOSt]0/[G3]0 × conv. (%)/100 × MW of NPtBOSt. cDetermined from SEC−MALLS. dMW of P(NB-g-St)-b-P(NB-g-HSt) calculated by Mn of precursor. eVolume fraction of NPSt determined using the feeding ratios of MMs and density at room temperature (PSt = 1.05 g/cm3; PHSt = 1.16 g/cm3). fConversion from NPSt and NPtBOSt to BBCPs is determined by comparing the peak areas of bottlebrush polymers with residual NPSt and NPtBOSt from SEC−MALLS measurement of the crude product. a

hinder microphase separation due to poor chain diffusivity.29−31 From the synthetic point of view for AmBBCP that can form H-bonding, ring-opening metathesis polymerization (ROMP) of macromonomers (MMs) containing a norbornene-terminal group, as “grafting through” method, have been used to synthesize well-defined BBCPs.9−13,32−35 Recently, our group reported development of grafting through method by combining living anionic polymerization (LAP) with ROMP and subsequently confirmed PC characteristics of the BBCPs.36−38 The LAP is a powerful tool for preparing welldefined amphiphilic polymers.39 The ROMP is an effective method for the precise synthesis of amphiphilic polymers using highly active, functional-group tolerable, and fast-initiating Grubbs third generation catalyst (G3).40 As a result, the grafting through technique by LAP and ROMP can lead to expanding the library of amphiphilic bottlebrush polymers for creating functional nanomaterials with unique optical properties. Herein, we present the phase transition behavior of the newly designed AmBBCP, poly[(norbornene-graf t-styrene)block-(norbornene-graf t-hydroxystyrene)], P(NB-g-St)-bP(NB-g-HSt). The precursor block copolymer, poly[(norbornene-graf t-styrene)-block-(norbornene-graf t-4-tert-butoxystyrene)], P(NB-g-St)-b-P(NB-g-tBOSt), was synthesized

miscibility leading to various nanostructures. In particular, the additives forming stronger H-bonds are uniformly distributed in the host polymer enlarging its domain.17,21 Additives forming weak H-bonding led to the discovery of smart materials, such as thermally responsible nucleobase.22 The Hbonding is regulated by the acidity of H-bond donor and basicity of H-bond acceptor.20 Despite additives augmenting self-assembly, as seen for gA″-b-gB″/C blends,11−13 and for an amphiphilic polymers with ultrahigh MWs,23,24 only a few host BBCPs have been similarly studied. A defined relationship between H-bonding interaction and phase behavior of the BBCPs with large d-spacing remains mostly unexplored. For investigating this relationship, we chose poly(styreneblock-hydroxystyrene), P(St-b-HSt), as a model of amphiphilic BBCP (AmBBCP). The phenolic (−OH) group can take part in H−bonding with additives.17−20 The P(St-b-HSt) tends to induce order−disorder morphology transitions either inherently or by cross-linking with a suitable additive.17,25,26 However, controlling nanostructures of ultrahigh MW AmBBCPs containing PSt and PHSt is a challenge for the following reasons: (i) the phenolic proton of hydroxystyrene lacks compatibility with living polymerization27,28 and (ii) a high interaction parameter (χeff) of these polymers generally tends to increase kinetic barrier of self-assembly and thereby B

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was sufficiently active to polymerize NPtBOSt. The synthesis of the BBCPs was well controlled. All of the SEC curves exhibited the tailored BBCPs without any residual MW peaks of the two MMs (NPSt and NPtBOSt), which is an evidence for nearly 100% end-functionalization of norbornene groups into PSt or PtBOSt. Moreover, the observed total DPn of the resulting BBCP from SP1 to SP4 linearly correlated with the increasing total feeding ratios of NPSt and NPtBOSt ([MM]0/ [G3]0) except for SP5 in Figure 1b. In the case of SP5, the anomaly is presumably due to the effect of small impurity in the MM and chain transfer of the sterically hindered propagation.37,41 Acid Hydrolysis of P(NB-g-St)-b-P(NB-g-tBOSt). The deprotection of the tert-butyl group from poly(4-tertbutoxystyrene) block was performed by acidolysis with a 5fold excess of hydrochloric acid (HCl) in dioxane at 60 °C to afford amphiphilic bottlebrush block copolymer, AmBBCP (Scheme 1a). The chemical structure of the deprotected AmBBCP was confirmed by 1H NMR spectroscopy as shown in Figure 2a. After deprotection, the broad signal of the

by grafting through LAP and ROMP (Scheme 1a). Acid hydrolysis of P(NB-g-St)-b-P(NB-g-tBOSt) led to the AmBBCP, which was blended with poly(2-vinylpyridine) (P2VP), poly(ethylene glycol) methyl ether (mPEG), and poly(methyl methacrylate) (PMMA), to precisely determine the relationship between H-bonding and phase transition behavior (Scheme 1b,c). Order−disorder phase transition for all of the blends shows interesting self-assembly and photonic characteristics; the details of which are presented.

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RESULTS AND DISCUSSION Synthesis and Characterization of P(NB-g-St)-b-P(NBg-tBOSt) BBCP. Before preparation of poly[(norbornenegraf t-styrene)-block-(norbornene-graf t-hydroxystyrene)] (P(NB-g-St)-b-P(NB-g-HSt)), we did the sequential ROMP of ω-end-norbornyl polystyrene (NPSt) with ω-end-norbornyl poly(4-tert-butoxystyrene) (NPtBOSt) to synthesize BBCP, P(NB-g-St)-b-P(NB-g-tBOSt), as a precursor, as shown in Scheme 1a.36 For designing quality MMs, the MMs NPSt (Mn = 2.61 kDa, Đ = 1.11) and NPtBOSt (Mn = 3.32 kDa, Đ = 1.09) were prepared by multistep synthesis in LAP, by endcapping with protected amine moiety and then by amidation with the norbornene-containing pentafluorophenyl ester. The relevant data of the MMs are summarized in Table S1 and Figure S1. The observed BBCPs in Table 1 exhibited successful synthesis of the well-defined BBCPs with ultrahigh MWs (Mn = 1507−3415 kDa) and a low dispersities (Đ = 1.09−1.21). The MW of the obtained polymers increased with the changing molar ratios of NPSt and NPtBOSt to G3. The size exclusion chromatography (SEC) curves of P(NBg-St)-b-P(NB-g-tBOSt) BBCPs in higher MW region were shifted from that of the starting MM (NPSt) (Figure 1a). The results indicate that the propagating chain end of P(NB-g-St)

Figure 2. (a) 1H NMR spectra (400 MHz, CDCl3 and DMSO-d6 as mixed solvents) and (b) DSC analysis of deprotection of P(NB-gSt)248-b-P(NB-g-tBOSt)240.

hydroxyl group was observed at 8.21 ppm and that of the tertbutyl group at 1.55 ppm disappeared. Additional confirmation was obtained from the differential scanning calorimetry (DSC) analysis for deprotection as shown in Figure 2b. The glass transition temperatures (Tg) of P(NB-g-St)248-b-P(NB-gHSt)240 seen at 87 and 178 °C correspond to the PSt and PHSt, respectively. In contrast, P(NB-g-St)248-b-P(NB-gtBOSt)240 showed one Tg at 95 °C, suggesting that the Tg of each block overlapped.36,42 These results strongly indicate that

Figure 1. Synthesis of BBCPs as precursor. (a) GPC curves of MM (NPSt) and BBCPs (P(NB-g-St)-b-P(NB-g-tBOSt)), (b) plots of DPn (=DPNPSt + DPNPtBOSt) as a function of [MM]0/[G3]0 (=[NPSt]0/ [G3]0 + [NPtBOSt]0/[G3]0). C

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Figure 3. Cross-sectional SEM images of the AmBBCP without mPEG treated by SVTA at (a) 25, (b) 60 °C and the images of the films obtained by blending of AmBBCP with (c) 15, (d) 30, (e) 40, and (f) 50 wt % mPEG treated by SVTA at 60 °C.

deprotection was quantitative, and P(NB-g-St)-b-P(NB-gtBOSt) was transformed into the AmBBCPs. Furthermore, volume fractions of the PS block were set in the range 56−60% to afford lamellar nanostructures, which is related to the onedimensional photonic application.2 Self-Assembly of P(NB-g-St)-b-P(NB-g-HSt). The P(NB-g-St)-b-P(NB-g-HSt) thin films without additives were prepared by solvothermal annealing (SVTA) and post-thermal treatment for 10 min at 180 °C as shown in Scheme 1c. SVTA is an effective method for self-assembly of low flexible polymers, such as metal−organic framework, and polymers with large χ.31,43 Briefly, films of pure AmBBCP were coated on the silicon or glass substrates by drop-casting the solution of the AmBBCP in dioxane, and then, solvent vapor annealing with heating was simultaneously performed in a chamber saturated with dioxane, a nonselective solvent.31,43,44 This induces self-assembly of the AmBBCP having a large χ parameter. When the AmBBCPs were annealed in solvent vapor, amorphous structures for SP1-0R to 5-0R were seen in the field-emission scanning electron microscopy (FE-SEM) of cross-sectional images (Figure 3a). To accelerate self-assembly, the films were subjected to SVTA at 60 °C and showed phase transition from disordered to well-ordered lamellar nanostructures, stacked parallel to the substrate, except for SP5-0, as shown in Figure 3b. The results implied that the SVTA is an effective method for self-assembly of the AmBBCPs with ultrahigh MWs and large χ. Domain spacing (L0) of SP1-0 to 4-0 without additives were increased from 67 to 114 nm with increasing MW (Mn) of AmBBCPs from 1215 to 2051 kDa. The observed L0 values were plotted as a function of the total number−average DP (DPn,total) of the norbornyl backbone (Figure 4). The L0 values correlated linearly with increasing DPn,total following the relationship: L0 ≈ l0DPα, where α and l0 are the scaling power and static segment length of P(NB-g-St)-b-P(NB-gHSt), respectively.37,45,46 In the case, L0 and DP are related by the expression: L0 = 0.138 nm × DP with α ≈ 1. The scaling coefficient (0.138 nm) is lower than the static segment length of P(NB-g-St)-b-P(NB-g-HSt). For polynorbornene, the scaling factor is smaller than the static segment length.37,45 Unlike SP1-0 to 4-0, the morphology of SP5-0 with an

Figure 4. Plot of domain spacing (L0) as a function of DPn,total (=DPn,PSt + DPn,HSt) for the AmBBCP lamellae.

ultrahigh MW (2880 kDa) displayed disordered structures. The AmBBCP with extreme high MW is not suitable for uniform ordering because of a higher kinetic barrier for efficient chain diffusivity.29 Self-Assembly of P(NB-g-St)-b-P(NB-g-HSt) and mPEG Blend. To precisely control the phase behavior in terms of domain size and shape of nanostructures, a suitable selection of the additives is important. The previous studies proved that hydrogen bonding of the homopolymer with linear BCPs improves miscibility, which influences the phase separation behavior.17−19 In all these case, hydrogen bonding played important role to determine miscibility, based on interassociation equilibrium constant (KA) values of linear PSt-bPHSt/P2VP, PEO, and PMMA corresponding to KA = 598,18 280,19 and 37.4,17 respectively. On the basis of these data, we decided to use homopolymers P2VP, mPEG, and PMMA as additives, where the phenolic OH group of the AmBBCPs interacted with the pyridyl (−N−), ether (−O−), and ester (−COO−) group via hydrogen bonding, the strength of which follow the order of their respective KA values. We prepared the blending formulation for P(NB-g-St)-bP(NB-g-HSt)/mPEG (Table S2, Mn = 2.00 kDa, KA = 280) to examine the H-bonding effect of the additive. Miscibility of the components was investigated by DSC analysis, as shown in D

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mixing P(NB-g-St)-b-P(NB-g-HSt) with PMMA (Table S2 and Figure S2b, Mn = 4.40 kDa, Đ = 1.08, KA = 37.4). The DSC analysis was used to confirm H-bonding between the hydroxyl group in the AmBBCP and a carbonyl group in PMMA. Figure 6a shows the second scans of the DSC

Figure 5. The glass transition temperature (Tg) of PHSt segment disappeared after blending with different mPEG

Figure 5. DSC analysis of SP1/mPEG blends with different mPEG weight percents.

weight percent ranging from 0 to 50 wt %, while the Tg of PSt segment remained unchanged at approximate 83 °C. The observed results mean that the miscibility between the PHSt block in AmBBCP and mPEG are a consequence of efficient H-bonding, unlike the immiscible PSt phase. PEG generally acts as a plasticizer in polymer blending due to flexibility arising from low Tg and low melting temperature (Tm). These characteristics allow suppressing the glass transition in PHSt segment.47 Furthermore, for the mPEG content of over 50%, the PHSt phase showed one Tm, which occurred at 54 and 58 °C for 70 and 100 wt % mPEG, respectively. The Tm for 70 wt % mPEG was slightly lower than that for 100 wt % mPEG because the tendency of mPEG phase to crystallize was impeded by the amorphous P(NB-g-St)-b-P(NB-g-HSt).19 Cross-sectional SEM measurements were also carried out to examine the structural changes in films of AmBBCP/mPEG blend. The films were annealed by SVTA and subsequent thermal treatment similar to the conditions for the pure AmBBCP (Scheme 1c). As shown in Figure 3b−e, wellordered lamellar structures within the range of mPEG contents of 0, 15, 30, and 40 wt % were observed, except for SP5. The results indicate that the mPEG is evenly distributed in the PHSt domain due to H-bonding interaction.48 Conversely, with increasing mPEG to 50 wt %, the ordered lamellae phase changed to the disordered state (Figure 3f). These results can be explained by the deterioration of ordered structures to exhibit wet-brush behaviors.19 When the amount of mPEG exceeded the accommodating volume of the AmBBCPs, the A/ B polymer interfaces gradually collapsed, destroying the lamellar structures.19,49 Furthermore, SP5, a high-MW AmBBCP, and mPEG blend, furnished disordered phase because high-MW AmBBCP has slow mobility that prevented phase separation of mixtures.29 As summarized in Table S3, the lamellar structure of the AmBBCP dominantly occurred in a broader range of volume fraction, when compared with linear BCP. This is presumably because the unique architecture with polymeric side chains of similar size makes the main backbone less curved and thereby diminish the effect of volume fraction on the interface curvature change.16,23,50,51 Self-Assembly of P(NB-g-St)-b-P(NB-g-HSt) and PMMA Blend. To investigate further the effect of the additive on morphology, another binary mixture was prepared by

Figure 6. Characterization of SP3/PMMA blends with different PMMA weight percents. (a) DSC analysis and (b) cross-sectional SEM images of self-assembled blend films.

thermogram of blended SP3/PMMA. While the Tg values of pure SP3 were observed at 87 and 178 °C, Tg of the PHSt segment in the AmBBCP gradually decreased from 178 to 120 °C with an increasing weight fraction of PMMA from 0 to 50%, without any change in the Tg of PSt. The DSC results imply that PHSt and PMMA are miscible due to effective Hbonding, but immiscible with the PSt block. A single Tg value for SP3 containing 70% PMMA was seen at 101 °C, which is 98 °C for pure PMMA. This tendency to alter the Tg of PHSt domain with a varying amount of the additives is because the Tg is affected by low MW additives.17 The change in the morphology of P(NB-g-St)-b-P(NB-gHSt)/PMMA microstructures was confirmed by using crosssectional FE-SEM images. As shown in Figure 6b, nearly wellordered lamellar structures for SP3/PMMA = 100/0 and 85/ 15 were maintained. Interestingly, an additional increase in PMMA concentration to 30% formed both the ordered and disordered structures. A high PMMA content tends to aggregate in the PHSt segment and hinders self-assembly into ordered structures.17,49 A completely disordered morphology was observed with 40 wt % PMMA. Therefore, weak Hbonding interaction between PMMA and PHSt restricted lamellar phase at low additive concentration. Self-Assembly of P(NB-g-St)-b-P(NB-g-HSt) and P2VP Blend. When the AmBBCP was blended with P2VP (Table S2, Figures S2a, and S3b, Mn = 3.79 kDa, Đ = 1.09, KA = 598), in which the blending ratio of SP4/P2VP is 7:3, a strong hydrogen bonding-induced cross-linking occurred. It was hard to generate a phase-separated structure as well as uniform films by solution process due to precipitation of the SP4/P2VP mixture. Their structure was predicted based on the previous report on macrogel formation by physically or chemically E

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Macromolecules cross-linked polymer network as depicted in Scheme 1b.25,52 Structural analysis by using dynamic light scattering and transmission electron microscopy (TEM) supported the formation of strong hydrogen bonding-induced macrogel, as shown in Figure 7. The AmBBCP was soluble in dioxane to

Figure 8. Plot of (a) domain spacing (L0), and (b) RI contrast as a function of mPEG concentration with 0, 15, 30, and 40 wt % mPEG at λ = 632.8 nm.

arising from similar refractive indices of PSt (∼1.60) and PHSt (∼1.61) is circumvented by adding mPEG. Figure 8b shows the Δn values of PHSt/mPEG blends increasing from 0 to 0.1 corresponding to mPEG contents from 0 to 40 wt %. Although the observed Δn values with 0 and 15 wt % mPEG were a good agreement with theoretical values, those for 30 and 40 wt % showed a departure from the calculated data. A high free volume of PHSt/mPEG blend allows mPEG to act as the plasticizer, increasing segmental flexibility47 and reduces the effective RI.55 Consequently, the Δn values disproportionately increased and hence deviated from the calculated values. The photonic films of mixture P(NB-g-St)-b-P(NB-g-HSt)/ mPEG and pure polymers were fabricated by using SVTA and subsequent thermal treatment as shown in Figure 9a. The disordered structures for SP1-0R to 4-0R, which were prepared by solvent vapor annealing (SVA) without heating, displayed no structural color. Photonic films without an additive for SP10 to SP4-0 were likewise transparent because of their small dspacing and similar refractive indices, even though these formed well-organized structures. Upon adding mPEG, the colors of photonic films appeared in the range from blue to red, which were distinctly visible to the naked eye. Interestingly, for the mPEG content, more than 50 wt % of the samples again became transparent. The results suggest that well-ordered lamellar structures displaying the desired color are modulated by mPEG. The reflectance of photonic films was measured by a UV/ vis/NIR spectrophotometer as shown in Figure 9b−d. The reflectivity from SP2, SP3, and SP4 with 40 wt % additive was stronger than that with 15 and 30 wt %. Thus, the mPEG efficiently enhances the Δn values with a simultaneous decrease in the RI of PHSt segment in the AmBBCP (Figures

Figure 7. (a) Particle size distribution plot, and (b) TEM image of SP4/P2VP blend (7:3) at a solution concentration of 1.0 and 0.2 g L−1 in dioxane.

form a clear solution (Figure S3a). Upon adding P2VP, interchain cross-linking due to strong H-bonding interaction led to the macrogel structure (Figure S3b).25,53 Effect of mPEG on Optical Properties AmBBCP. The impact of mPEG on phase behavior, domain size, and the RI on PC property of AmBBCP was studied.4 To generate intense structural color, lamellar phase must be well-ordered with defined domain spacing (L0) and RI contrast (Δn) of each block. The d-spacing of the AmBBCP increased on introducing mPEG in interdomain space of the PHSt segment.20,49 A plot of L0 and Δn in various mPEG blending ratio is given in Figure 8. The L0 values for SP1, SP2, SP3, and SP4 with the mPEG content varying from 0 to 40 wt % were seen to follow a linear trend (Figure 8a). The additive significantly enhanced domain spacing from 67 to 133 nm for SP1, 77 to 183 nm for SP2, 91 to 207 nm for SP3, and 114 to 230 nm for SP4. The added mPEG led to only stretching of the AmBBCP domain reminiscent of the wet-brush behavior.17,21 Such a modulated control of the domain spacing by an additive caused by Hbonded self-assembly is rare and is a significant step forward in tuning the color expression of the PCs. Furthermore, the RI of materials is tuned by varying mixture composition, based on Lorentz−Lorenz equation.54 The RI of the domains in AmBBCP and mPEG were measured by prism coupler at λ = 632.8 nm, as shown in Figure S5. The limitation F

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Figure 9. Optical characteristics of self-assembled mixtures (P(NB-g-St)-b-P(NB-g-HSt)/mPEG) for (a) images of the photonic films, scale bar: 0.5 cm, (b−d) reflectance spectra.

8b and S4). The RI ratio (nL/nH, nL < nH) is related to reflectivity, whereby the reflectivity theoretically increases with increasing the Δn values.2,4 The observed reflectance shift to longer wavelength with high mPEG content (Figure 9b−d) indicates that mPEG expands the PHSt domain in the AmBBCP and can increase Δn. The peak positions were rationalized using the equation, λmax = 2(d1n1 + d2n2), where d1 and d2 are domain spacing, n1 and n2 are the corresponding refractive indices, and λmax is the wavelength of maximum reflectance.2,4 The self-assembled AmBBCP/mPEG photonic films demonstrated full color-display covering the entire visible range from blue (SP2-15, λmax = 429 nm) to red (SP4-40, λmax = 680 nm).

effective additive was mPEG that led to selectively expanding d-spacing of the PHSt domain as well as generated the desired RI contrast in the AmBBCP. The embedding of mPEG opens up a simple approach to modulation of phase behavior, domain size, and RI to control the full-color photonic display. In future studies, we will further regulate the self-assembly of the AmBBCPs in solution by using a diverse range of H-bondforming small and large functional molecules.

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EXPERIMENTAL SECTION

Materials. (H2IMes)(pyr)2(Cl)2RuCHPh (G3), two MMs of NPSt and ω-end-norbornyl poly(4-tert-butoxystyrene) (NPtBOSt), and polymeric additives, such as P2VP and PMMA were prepared according to the literature procedures.32,36,56 1,4-Dioxane (Thermo Fisher Scientific, >99%), mPEG (Merck, Mn ≈ 2000 Da), and secBuLi (Merck, 1.4 M in cyclohexane) were purchased and used without further purification. Styrene (Merck, 99%), 4-tert-butoxystyrene (Merck, 99%), 2-vinylpyridine, and methylmethacrylate were passed through alumina columns, dried for 24 h over CaH2, and distilled under reduced pressure. These compounds were then further distilled from dibutyl-magnesium (Merck, 1 M in heptane), CaH2, and trioctyl aluminum (Merck, 25 wt % in hexanes) under high vacuum system (∼10−6 mm Hg), respectively. Dried tetrahydrofuran (THF) was prepared by distillation using the ketyl solution of sodium-benzophenone under vacuum. All initiators and monomers were stored at −30 °C in ampoules equipped with break-seals. Characterization. 1H and 13C NMR spectra were measured using a JEOL JNM-ECX 400 instrument in the CDCl3 solvent or CDCl3 and DMSO-d6 mixed solvent. The MW and polydispersity of the polymers were determined in THF containing 2% trimethylamine at 1.0 mL/min flow rate using SEC−multiangle laser light scattering (SEC−MALLS), which is equipped with four columns (HR 0.5, HR 1, HR 3, and HR 4; and pore sizes 50, 100, 500, and 1000 Å, respectively), an Optilab DSP interferometric refractometer (Wyatt Technology), and a DAWN EOS laser photometer (Wyatt Technology). The dn/dc values for each injection were determined by assuming 100% mass elution. The RI detector was utilized for MW

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CONCLUSIONS We demonstrated an effective strategy for phase transition through self-assembly of an AmBBCP, with mPEG, PMMA, and P2VP as additives. The poly(hydroxystyrene) domain favorably interacted with the additives of the varying interassociation equilibrium constant (KA) as a measure of effective H-bonding. The film annealed by solvent vapor without heating resulted in disordered structures, which became ordered upon SVTA. The highly ordered lamellar phase, fabricated by self-assembly of AmBBCP/mPEG blends, was maintained without phase transition up to 40 wt % mPEG content. H-bonding interaction led to the selective introduction of mPEG in the PHSt block. Increasing the mPEG content to more than 50 wt % destroyed lamellae from order to disorder because of disruption at the interface of each block. On blending with PMMA, weak H-bonding interaction led to the accumulation of PMMA out of the PSt/PHSt interface, leading to disordered phase behavior above 15 wt %. The interaction of AmBBCP with P2VP led to the formation of macrogel due to the strong H-bond interaction. Thus, the most G

DOI: 10.1021/acs.macromol.9b00678 Macromolecules XXXX, XXX, XXX−XXX

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estimation of MMs and polymeric additives based on calibration of PS and PMMA standard. DSC (TA Instruments Q-20) was used to characterize thermal properties under a nitrogen atmosphere at a heating rate of 10 °C/min. The RI was measured by employing a prism coupler (SPA-4000, Sairon Tech. Inc.) equipped with a He−Ne laser (λ = 632.8 nm) in the angle range from −15° to 15°. The cross section of the films was imaged using FE-SEM (Hitachi S-4700 instrument). All films were fractured to obtain a perpendicularly cut sample to the surface. The samples were then coated with platinum before measuring the SEM. The statistical analysis of the domain size of the samples was performed using the SEM images. A LAMBDA 750 UV/vis/NIR spectrophotometer (PerkinElmer) was employed for measuring the reflection spectra of the photonic films. Bottlebrush Block Copolymerization of NPSt and NPtBOSt (P(NB-g-St)-b-P(NB-g-tBOSt)) ([NPSt]0/[G3]0 = [NPtBOSt]0/[G3]0 = 270). The polymer was synthesized according to a previously reported procedure.36 NPSt (0.14 g, 52.0 μmol), NPtBOSt (0.17 g, 52.0 μmol), THF (520 μL), and a G3 stock solution (20 μL, 0.19 μmol, 9.63 mM in THF) were used for the synthesis of the BBCP, which was obtained as a white solid (0.30 g, 97%). The conversion was quantitative based on gel permeation chromatography (GPC) data of dn/dc = 0.145 mL/g. 1H NMR (400 MHz, CDCl3, δ): 7.23− 6.22 (br), 5.86−5.19 (br), 3.77−0.78 (br), 0.77−0.42 (br). Preparation of P(NB-g-St)-b-P(NB-g-HSt) by Acid Hydrolysis of P(NB-g-St)-b-P(NB-g-tBOSt). P(NB-g-St)-b-P(NB-g-HSt) was prepared by using a previously reported procedure44 that was suitably modified. The BBCP P(NB-g-St)248-b-P(NB-g-tBOSt)240 (0.20 g) and a fivefold excess of hydrochloric acid (0.42 mL) in dioxane (5 wt % solution, 3.9 mL) were added to a round-bottom flask equipped with a magnetic stirrer. The mixture was reacted under a nitrogen atmosphere at 60 °C for 24 h. The residue was purified by precipitation from a large excess of water. After neutralization with 5 wt % NaOH aqueous solution, the desired P(NB-g-St)-b-P(NB-gHSt) was filtered and dried in vacuum oven at 60 °C. The obtained polymer was further purified by precipitation in excess cold methanol, filtered, dried in a vacuum oven at 40 °C, and collected (0.18 g, 90%). 1 H NMR (400 MHz, CDCl3 and DMSO-d6 as mixed solvents (f CDCl3 (%) = 75), δ): 8.20 (s), 6.84−5.89 (br), 2.20−0.68 (br), 0.51−0.32 (br). Preparation of PC Films of AmBBCP/Additive Blends. The AmBBCP/additive blends were dissolved in dioxane to make a 2 or 4% (w/v) stock solutions. The solution was stirred overnight and then purified through a 0.2 μm syringe filter. The solution was drop-cast onto the substrate treated with piranha solution. This was immediately subjected to dioxane vapor annealing at 60 °C for 24 h. After evaporating solvent, the dried film was thermally treated at 180 °C for 10 min under the reduced pressure.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2015R1A2A1A01002493 and 2018R1A2B6003616) and “Novel Research Project” grant of the Grubbs Center for Polymers and Catalysis funded by the GIST in 2019.

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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.9b00678. Molecular characteristics of MMs and polymeric additives, photograph of pure polymers and blending mixture with P2VP in solution, and the RI of PHSt/ mPEG blends (PDF)

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Corresponding Author

*E-mail: [email protected]. Phone: (+82)-62-715-2306. ORCID

Youngjong Kang: 0000-0001-5298-9189 Jae-Suk Lee: 0000-0002-6611-2801 Notes

The authors declare no competing financial interest. H

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