Hierarchical Self-Organization and Uniaxial Alignment of Well

Article pubs.acs.org/Macromolecules

Hierarchical Self-Organization and Uniaxial Alignment of Well Synthesized Side-Chain Discotic Liquid Crystalline Polymers Bin Mu,† Bin Wu,† Shi Pan,† Jianglin Fang,‡ and Dongzhong Chen*,† †

Key Laboratory of High Performance Polymer Materials and Technology of Ministry of Education, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, and ‡Center for Materials Analysis, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Liquid crystalline polymers (LCPs) combine the attributes of liquid crystals and polymers, while discotic LCPs have been less developed in sharp contrast to their calamitic counterparts mainly due to lack of suitable discotic LCP materials. Here we successfully prepared a series of welldefined triphenylene (TP) based discotic LC polyacrylates via reversible addition−fragmentation chain-transfer (RAFT) polymerization for the first time, and through a combination of multiple analysis techniques and phase transition kinetics study, a remarkable molecular weight effect or polymer effect at a critical degree of polymerization (DP) around 20 has been disclosed. Moreover, the first proposed discrete columnar stacks (DCS) based hierarchical self-organization model accounts well for the formation and transformation of ordered hexagonal columnar lattice Colho dominated by side-chain TP stacking and oblique columnar superlattice Colob‑s induced by compaction and ordering of polymer backbones. The in-depth understanding of their superstructures and readily achieved uniaxial alignment pave the way for the rational design and preparation of such kind of solution processable cutting-edge polymeric semiconducting materials and may boost various fascinating optoelectronic applications.

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triphenylene (TP),5−7 hexabenzocoronene (HBC),2c,8 multialkynylbenzene,9 perylene,10 porphyrin, and phthalocyanine (Pc),11 whereas there is almost no systematic work pertinent to some fundamental questions of discotic LCPs such as molecular weight effect and the spacer length influence, despite such issues are closely correlated to their self-organized supramolecular structures thus crucial for organic electronic and optoelectronic applications.12 The underdeveloped research situation in discotic LCPs is mainly in respect that synthesis of discotic mesogens is relatively arduous compared with their calamitic counterparts, especially the preparation of discotic LCPs with well-controlled molecular weight (MW) and narrow polydispersity remains a real challenge. Although some synthetic examples of side-chain discotic liquid crystalline poly(meth)acrylates have been reported through conventional free radical polymerization6d,e,g or polymer analogous reaction,5,6a−c,j,9 the attempts to prepare well-controlled side chain DLC polymers using atom transfer radical polymerization (ATRP) only achieved very limited progress.6h Among DLC materials, TP derivatives are probably the most widely studied and particularly promising in developing new systems for various applications and have attracted continuing attention.13 Very recently, through an indirect

INTRODUCTION Discotic liquid crystal (DLC) research is relatively young with a successful industrial application example as widely used optical compensating films in liquid crystal display in the nematic discotic phase (ND),1 whereas more applications in organic electronic and optoelectronic such as in field-effect transistors, light-emitting diodes, photovoltaic solar cells have yet to be developed taking advantage of the unique quasi one-dimensional (1D) conductibility in variant columnar mesophases.2 To this end, the capability to modulate structural order and realize alignment is of profound importance to achieve a high device performance, while for low molar mass DLCs usually some complicated and costly techniques are required to prepare ordered thin film and achieve alignment.2b−d Combining the virtues of liquid crystals with good processability, film-formation and mechanical properties of polymers, liquid crystalline polymers (LCPs) constitute a kind of fascinating advanced organic materials. LCPs are usually composed of rodlike (calamitic) or disclike (discotic) mesogens either incorporated into the polymer main chains or attached as side chains through flexible spacers. In contrast with intensively studied calamitic mesogens based LCPs3 with extensive applications such as in high modulus fibers, optoelectronic devices, and sensors,4 thus far surprisingly limited progress has been made on discotic LCPs since first reported in 1983 by Ringsdorf.5 Although some examples of side-chain discotic LCPs have been explored adopting various discotic mesogens such as derivatives of © XXXX American Chemical Society

Received: February 26, 2015 Revised: April 1, 2015

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DOI: 10.1021/acs.macromol.5b00415 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Well-Controlled Synthesis of a Series of TP-Based Side-Chain Polyacrylates Pm-n with a Six-Methylene Spacer (m = 6) by RAFT Polymerization

room temperature around 20 °C, essentially equivalent to a very slow cooling process. And a rapid cooling process was implemented at a fast cooling rate of about 20−30 °C min−1 from the isotropic state to room temperature or predetermined temperatures. While normal cooling processes referred to cooling at a rate in between the above rapid and stepwise cooling rates, usually by experiencing a cooling procedure at a rate of 10 °C min−1, as carried out in DSC measurements. Preparation and Characterization of Uniaxial Oriented SideChain DLC Polymer Thin Film. A brief introduction to the involved basic optics of discotic liquid crystals is available in Supporting Information. The schematic illustration for sample preparation and Xray characterization was provided in Figure S35. The oriented DLC polymer thin films were prepared by sandwiching their concentrated solutions or melts cooled from the isotropic temperature range between two glasses and experienced a brief mechanical shearing manually then cooled to room temperature and the preparations were used for optical properties and X-ray scattering investigations.

approach by the combination of ATRP and polymer analogous reaction, a series of TP-based side-chain DLC diblock copolymers have been prepared with narrow polydispersity in our laboratory showing an intriguing microphase separated superstructure evolution and close correlation between overall morphologies and discotic mesogenic orders.14 Here we present the well-controlled synthesis of a series of TP-based DLC polyacrylates with gradually increased MWs and narrow polydispersity index (PDI) via reversible addition−fragmentation chain-transfer (RAFT) polymerization for the first time, thus obtained well-defined samples offer an unprecedented opportunity to explore and elucidate the hierarchical superstructure organization and further the MW effect as well as the spacer length influence of side-chain DLC polymers, and moreover, the easy achievement of macroscopic alignment of columnar LC phases by a brief mechanical shearing has been demonstrated, which may pave the way for their promising applications as solution processable low-cost organic optoelectronic materials.

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RESULTS AND DISCUSSION Well-Controlled Synthesis of TP-Based Side-Chain DLC Polymers by RAFT Polymerization. TP-based poly(meth)acrylates with larger PDIs were prepared through conventional free radical polymerization,6d,e,g but the attempt of controlled synthesis by ATRP only obtained oligomers of a limited DP around eight.6h Our group have also attempted to synthesize such kind of (co)polymers by ATRP but turned out to be unsuccessful, so we employed an indirect pathway for the preparation of a series of TP-based block copolymers.14 Fortunately, we found that RAFT method worked well for the controlled synthesis of TP-based discotic side-chain poly(meth)acrylates. Among the controlled radical polymerization (CRP) techniques, RAFT polymerization15 possesses closer similarity to traditional radical polymerization and bears some special characters as compared with other CRP methods such as metal-catalyzed ATRP.16 To the best of our knowledge, this is the first time adopting RAFT technique for the controlled synthesis of side-chain DLC polymers, which may serve as a versatile synthesis protocol and thus open the door for the design and precise preparation of well-defined (co)polymers with various discotic side groups. The synthesis route via RAFT polymerization is shown in Scheme 1. Reaction kinetics studies manifested that the polymerization of TP-based side-chain DLC polymers via RAFT method exhibited well controlled characteristics with a well-fitted linear relationship until very high conversion (Figure

EXPERIMENTAL SECTION

Controlled Synthesis of the Series DLC Polyacrylates. Most experimental details, including intensive characterization and measurements, and the synthesis procedures for monomers, chain transfer agent (CTA), and other intermediates are provided in Supporting Information. The typical polymerization was conducted with a 50% (w/v) monomer solution in redistilled dioxane (typically 0.3 g monomer in 0.6 mL solvent) under nitrogen at 60 °C for 45 h in a 10 mL pear-shaped Schlenk flask. The 2,2-azobis(isobutyronitrile) (AIBN) was used as the radical initiator and 2-(methyl propionate)-O-ethyl xanthate as CTA with AIBN/CTA in molar ratio 0.5:1. The polymerization solution was degassed with freeze−pump−thaw three cycles before initiating the reaction. The reaction mixture was purified by silica-gel column chromatography with petroleum ether/ethyl acetate (10:1, v/v) as the eluent first to remove the residual trace monomer and then the polymer was eluted with ethyl acetate, the concentrated crude product was redissolved in dichloromethane and precipitated in cold methanol. Finally, the polymer product was dried at 35 °C under vacuum overnight. Thermal Programs Description for Some Samples Preparation Used for DSC and X-ray Scattering Characterizations. The procedures for differential scanning calorimetry (DSC) measurements and simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) analyses are provided in the Supporting Information and also refer to our previous paper.14 The stepwise cooling process was carried out through isothermal annealing at a set of selected tempeartures about 10 °C interval step-down from isotropic state to B


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Figure 1. (a) Normalized GPC traces of thus obtained series polymers P6-n with DP ranging from n = 5 to 100. (b) Representative line-resolved MALDI-TOF-MS patterns of representative oligomer P6-10 and polymer P6-50 with dithranol as the matrix.

supplied straightforwardly substantial evidence to the discrete columnar stacks (DCS) based hierarchical organization model as further discussed later in structural analysis section, wherein the side-chain TP discogens self-organized piecewise into DCS and fragmentation tended to occur at relatively weak junctions among DCS subunits during MALDI fracture process. Molecular Weight Effect/Polymer Effect. The successful preparation of a series of well-defined TP-based polyacrylates Pmn (m stands for the methylene number of the spacer and n for DP) with variant spacer lengths and increasing DP offers an ideal platform for systematically investigating the phase structures and molecular weight effect of side-chain discotic LCPs. We have synthesized a huge family of such kind of DLC polyacrylates Pm-n with spacer m = 0−10 and 14, among them the P6-n series with medium six methylene spacer of the same length as other peripheral hexyloxy side chains around the TP core exhibited the most representative superstructure transformation and typical molecular weight dependence and pregnant with significant implications for the whole family of side-chain DLC polymers. Herein in this leading report, we shall focus on the MW effect, hierarchical superstructure organization and transformation, and demonstrate the facile achievement of macroscopic LC alignment of P6-n series DLC polymers. The systematic analysis of spacer length influence and superstructure evolution of other series of analogues with variant spacers of the family of DLC polyacrylates will be soon reported separately. The DSC curves of the series DLC polymers P6-n exhibited two endothermic peaks and also unnoticeable glass transitions with extracted Tg temperatures from amplified traces at −8−5 °C, as listed in Table 1 in the second and subsequent heating runs, and one exothermic peak in the cooling runs (Figure 2). While in the first heating run of the pristine P6-n series samples, the lower temperature endothermic peaks were all around 40 °C without Tg detected (Figure 2a), which were attributed to the melting of ordered columnar phases with promoted intracolumnar order as clarified next in the kinetics study section. A pseudo focal-conic fan texture of small size (Figure 3a) corresponding to columnar phase was observed for P6-10 when slowly cooled to room temperature from the isotropic state, and after reheated to 45 °C and isothermally annealed for 3 h the

S2). The key feature of RAFT polymerization mechanism is the reversible addition−fragmentation sequential process. Rapid transfer equilibrium between the active propagating radicals and xanthate terminated polymer dormant species provided equal probability for all chains to grow and resulted in the produced polymers of narrow polydispersity. The MW and PDI of thus obtained series TP-based DLC polyacrylates were first examined by gel-permeation chromatography (GPC). As shown in Figure 1a, monomodal GPC curves shifting toward shorter retention time with increased DP indicated monotonous increase of the obtained polymer MWs with relatively narrow PDIs of 1.19− 1.42 (Table S3). While it should be noted that values of the calculated MWs from GPC especially the number-average molecular weight Mn,GPC increased very modestly, which was presumably due to the large difference in the hydrodynamic properties of the DLC polymers and the polystyrene standards employed for GPC calibration, moreover, the particular aggregation characteristics of side-chain TP mesogens in solution (Figure S4) probably affected significantly on their hydrodynamic behaviors thus the calculated relative MWs. Fortunately, the chemical structure of the carefully chosen CTA facilitated the MW estimation from integral ratio of high resolution 1H NMR measurements, and values of the numberaverage MW determined from NMR (Mn,NMR) agreed quite well with the designed MW (Table S3). Furthermore, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy (MS) measurements were conducted under different test conditions with various matrixes (Figure S5). With the assistance of dithranol matrix, which was reported to work well in MS measuring of triphenylene based side-chain polymers and dendrimers,17 well line-resolved MALDI-TOF-MS profiles were achieved (Figure 1b). Moreover, the intervals of corresponding peaks in adjacent groups were almost constant and equaled to the molar mass 899 g mol−1 of a single discotic TP acrylate repeat unit (Figure 1b and Table S4). It is quite striking that very similar MALDI-TOF-MS patterns exhibited for the whole series DLC polymer samples of variant DPs with signal peaks largely locating at MW around 3000−7000 Da (Figures 1b and S6). Such kinds of unexpected MS spectroscopic characteristics of line-resolved patterns almost independent of DPs C


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significant change, while after annealing at this temperature for 4 h, the (11) peak disappeared with the WAXS peak arising from π−π stacking still remained, and the broad peak at 1.6 nm−1 split into two separated peaks implying a more ordered arrangement of polymer backbones. All these features were characteristic of some oblique columnar superlattice Colob‑s organization, together with POM characteristic schlieren texture, indicating a nematic phase as clarified later due to the increased mobility of the DCS subunits while constrained by the polymer backbone to preserve largely the oblique columnar superlattice organization, thus, forming a special nematic phase with columnar order Ncob‑s. The detailed stacking model of pendant discotic units and the arrangement of polymer backbones will be discussed below in the structural analysis section. For P6-50, similar texture transformation was also observed when slowly cooled to room temperature (Figure 3c) and reheated to 83 °C (Figure 3d). The scattering patterns (Figure 4b) can be well assigned to oblique columnar superlattice structure (Colob‑s) with superlattice parameters a = 4.21 nm, b = 3.80 nm, and γ = 84.4° (see Figure 8c,d and Table S6). The calculated density 1.07 g cm−3 for a unit cell consisting of four discotic units is reasonable for the investigated organic polymers. Moreover, upon beyond the transition temperature around 83 °C disclosed by DSC analysis, despite significantly diminished SAXS patterns while the persisting (10), (01), (02) main peaks and other weak peaks superposed on the broad haloes at around 1.6 nm−1 and 3.4 nm−1 together with the wide angle area peak arising from π−π stacking (Figures 4b and S24) also indicated the nematic phase with oblique columnar superlattice order Ncob‑s. Such kind of transition was reminiscent of the results reported by Percec where the formation of Colh was kinetically controlled, while the corresponding nematic phase was thermodynamically stabilized based on disc-like supramolecular polymer self-assembled from the tapered side groups.18 The molecular weight effect expressed as dependence of phase transition temperatures on DP is shown in Figure 5. As DP increasing the isotropization transition temperature gradually increased and Tg increased quite modestly, which was similar to the change tendency and extent exhibited by rod-like side-chain LCPs.19 Moreover, Tgs of P6-n polymers with n greater than or equal to 20 were 5−13 °C higher than those of oligomers, which accounted for relative decrease of polymer main chains’ flexibility and resulted in backbone induced columnar superlattice arrangement of polymers compared with that of oligomers as discussed later. Very interestingly, in the immediately second and

Table 1. Summary of Transition Temperatures and Associated Enthalpy Changes of the Series P6-n and Monomers phase transitionsa (°C; enthalpy changes/J g−1) DP monomer 3(6) saturated 3(6) 5 10 15 20 30 50 100

first heatingb

second heatingb

first coolingb

K 50(49) I

K 47(41) I

I 24(−38) K

K 43(35) I

K 43(40) I

I 24(−37) K

Colrio 43, 47(19) I Colrio 49(14) Ncob‑s 58(1.2) I Colrio 43(14) Ncob‑s 63(1.1) I Colob‑sio 44(3.8) Colob‑s 75(2.0) I Colob‑sio 44(1.4) Colob‑s 82(2.8) I Colob‑sio 43(1.5) Colob‑s 87(2.5) I Colob‑sio 42(3.0) Colob‑s 90(2.4) I

G −8 Colho 29(7.2) Ncob‑s 42(0.2) I G −5 Colho 30(3.2) Ncob‑s 57(0.9) I G −4 Colho 31(2.7) Ncob‑s59(0.6) I G 1 Colob‑s 69(1.8) Ncob‑s 76(0.1) I G 5 Colob‑s 77(2.2) Ncob‑s 84(0.4) I G 3 Colob‑s 83(2.3c) Ncob‑s 89(0.4c) I G 4 Colob‑s 88(2.3c) Ncob‑s 93(0.1c) I

I 8(−5.8) Colho I 7(−3.2) Colho I 6(−2.8) Colho I 49(−1.3) Colob‑s I 56(−2.0) Colob‑s I 61(−2.1) Colob‑s I 64(−2.1) Colob‑s

a

Phase assignments based on a combination of DSC, POM and SAXS/WAXS analysis results. Abbreviations: K = crystalline phase; Colrio = rectangular columnar lattice with enhanced intracolumnar order; Colob‑sio = oblique columnar superlattice with enhanced intracolumnar order; G = glassy state; Colho = ordered hexagonal columnar lattice; Colob‑s = ordered oblique columnar superlattice; Ncob‑s = columnar nematic phase with oblique superlattice order; I = isotropic phase. bThe transition peak values or the median values for glass transitions obtained from DSC heating or cooling runs at rate of 10 °C min−1, with positive enthalpy changes standing for endothermic while negative values for exothermic transitions. cEnthalpy changes approximately calculated through multipeak convolution method for partially overlapped transition peaks.

brightness declined and gradually changed into schlieren texture (Figure 3b) indicating its nematic characteristic. The SAXS patterns of P6-10 during second heating process as shown in Figure 4a with a strong main peak at 3.36 nm−1 and a small peak at 5.82 nm−1 in the ratio of 1/√3, together with the WAXS profiles showing a diffuse halo corresponding to the liquidlike aliphatic chains at about 14.1 nm−1 (4.45 Å) and a small shoulder at 17.74 nm−1 (3.54 Å) due to π−π stacking of TP moieties (Figure S20), indicated an ordered hexagonal columnar (Colho) structure with lattice spacing a = 2.17 nm (see Figure 8b,d). Upon heating to 45 °C, the scattering profiles experienced no

Figure 2. DSC traces during first heating (a), first cooling (b), and second heating (c) of the P6-n series DLC polymers with DP ranging from n = 5 to 100 at a rate of 10 °C min−1. D


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Figure 3. Representative POM images under crossed polarizers of (a) P6-10, cooled to room temperature (rt) from the isotropic state showing pseudo focal conic texture characteristic of columnar phase and (b) reheated to 45 °C and annealing for 3 h displaying schlieren texture indicating nematic feature; (c) P6-50, cooled to rt from isotropic melt and (d) reheated to 83 °C showing columnar and nematic phase textures, respectively.

Figure 4. SAXS patterns of representative oligomer P6-10 (a) and polymer P6-50 (b) at the indicated temperatures during the second heating process after a preceding cooling at 10 °C min−1 from the isotropic state.

subsequent heating runs, the transition temperatures of TCol‑NC for oligomers were all around 30 °C, and increased slightly from 69 to 88 °C for polymers as DP increasing from 20 to 100. Moreover, for all the oligomers and polymers consistent supercooling degree of about 20 °C exhibited in the cooling scans compared with their corresponding main endothermic peaks in the heating runs, which indicated the respective internal coherence of oligomers and polymers and manifested an abrupt jump increase of the columnar-nematic transition temperatures for polymers of DP 20 or larger in remarkable contrast with the rapid but continuous increase from smectic to nematic phase transition in calamitic side-chain LCPs,19 though the turning point of DP 20 to show marked molecular weight effect or polymer effect was comparable to the critical DP values reported for side-chain calamitic or dendritic LCPs.19,20 Furthermore, the corresponding discotic TP acrylate monomer and the saturated TP propionate serving as the lowest n = 1 oligomer showed crystalline phases with melting points locating between the TCol‑NC transition temperatures of oligomers and polymers (Figure 5), which was explicable for the backbone and terminal

groups acting as diluents disrupted the efficient packing into columns of TP moieties in oligomers thus lowered the disassociation temperatures, whereas for polymers the special stacking mode of the side-chain TP moieties as further discussed later and the induction effect of polymer backbone enhanced the orientational correlation of discogens to construct columnar superlattice structures manifesting marked polymer effect.21 Kinetics Comparison Study and Superstructure Transformation. To further elucidate the formation mechanism and transformation of the hierarchically organized structures, phase transition kinetics comparison study of LCP samples of variant DP has been performed mainly by a combination of DSC and temperature dependent SAXS analysis. The thermal behaviors of the series oligomers and polymers employing P6-10 and P6-50 as the typical example of each group have been investigated with DSC after a preceding cooling process at variant rates or experiencing different annealing processes (Figures 6 and S9− S11). DSC thermograms of P6-10 heating at 10 °C min−1 after cooling from the isotropic state at increased cooling rate of 2, 5, 10, and 30 °C min−1, respectively, revealed that the enthalpy E


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temperatures of first endothermic transition for oligomers were about 8 °C higher arising from enhanced intracolumnar organization of side-chain TP groups. The isotropization temperatures also increased because of compaction and ordering of polymer backbones, which thus induced the structure transformation from ordered hexagonal columnar phase Colho to oblique columnar superlattice structure Colob‑s, as evidently manifested by SAXS patterns (Figure 7a). Moreover, such kinds of structures were very stable and experienced no perceptible changes during subsequent heating for all polymers and oligomers such as P6-10 (Figure 7c), except for the shortest oligomer P6-5 showing some specialty (Figure S25), implying Colob‑s structure of thermodynamical stability. A set of experiments for examining the effects of variant annealing time at selected temperatures for polymer P6-50 were designed (Figure 6b). The enthalpy changes of the transition at around 40 °C characteristic of melting of well organized sidechain TP columns increased with prolonged annealing time or at lowered annealing temperature (Figure 6b). Such phase transitions of annealed samples were comparable to those observed in the first heating run of the pristine samples (Figure 2a), which underwent precipitation and drying procedures equivalent to an annealing process. Upon stepwise cooling, the melting enthalpy of primary transition increased notedly and isotropization temperature also upshifted, indicating more ordered phase development in virtue of gradually annealing during the stepwise cooling process (Figure S11). Besides, the temperatures of these transition peaks around 40 °C were almost independent of DP, which indicated that polymer backbones were not involved in these transitions, further confirming their originating from promoted intracolumnar organization of the side-chain TP moieties. Slow kinetics and a small tendency to crystallize was reported in triphenylene-based oligomers,22 while for the discotic LCPs investigated here no crystallization evidence of side-chain TP moieties displayed; thus, we attributed the obviously increased enthalpy changes of the melting transition at around 40 °C to the promoted intracolumnar order as reported in supramolecular assemblies from dendronized perylene bisimides by Percec et al.,23 herein the intracolumnar order enhancement was mainly ascribed to the increased correlation between adjacent DCS subunits of TP moieties as discussed further below. All the series TP-based DLC polyacrylates exhibited Colho phase with obvious (10) and (11) SAXS peaks of q ratio 1/√3

Figure 5. Dependence of phase transition temperatures of P6-n series vs DP (solid symbols with solid lines from the second heating cycle, while empty symbols with dashed lines from the first cooling run, all connecting lines just for guiding the eyes; red symbols indicating the melting point and crystallization temperatures of crystalline saturated monomer 3(6), representing the oligomer P6-1).

changes around 30 °C due to collapse of long columnar structures increased while those around 57 °C of isotropization transitions decreased obviously (Figure 6a), which indicated that a faster cooling rate favored the organization of side-chain TP columns, while a slower rate enhanced the compaction and ordering of polymer backbones as further confirmed by SAXS analysis next. It was noticed that there existed an exothermic process around 18 °C before the first principal endothermic transition, which was more significant after experiencing a faster rate cooling process, indicating thermal induced ordering and kinetically controlling feature under rapid cooling (Figure 6a). Moreover, the glass transition temperatures as guided by a dashed green line in Figure 6a slightly increased though with reduced thermal capacity changes, from −6.5 °C to −5.1, −2.8, and −1.3 °C after cooling at the rate of 30, 10, 5, and 2 °C min−1, respectively, in accordance with a slight rigidity increase of polymer backbones resulting from their ordering and enhanced correlation between polymer backbone and side-chain discogens upon cooling at slower rates. The two endothermic peaks upon reheating shifted to a little higher temperature after stepwise cooling process for all the oligomers (Figure S9), wherein the

Figure 6. DSC thermograms of (a) P6-10 reheating at 10 °C min−1 after a preceding cooling process at variant rate of 2−30 °C min−1 from the isotropic state, and (b) P6-50 reheating at 10 °C min−1 after cooling from the isotropic state at 10 °C min−1 to the indicated temperatures and then annealing for different times. F


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Figure 7. SAXS patterns of P6-n with DP n = 5−100 at room temperature after (a) stepwise cooling from isotropic melts, and (b) rapid cooling at a rate of 20−30 °C min−1 from isotropic melts. Variable temperature SAXS profiles upon subsequent heating of (c) P6-10 after stepwise cooling from the isotropic state, and (d) P6-50 after rapid cooling from the isotropic state at a rate of 20−30 °C min−1 showing transformation from lower temperature ordered hexagonal columnar phase to oblique columnar superlattice structure of higher temperature beyond 40 °C.

Figure 8. Schematic illustration of superstructure transformation between Colho lattice structure and Colob‑s superlattice organization. (a) Chemical composition and structure of the series DLC polyacrylates P6-n with a six methylene spacer of DP n = 5−100. (b) Cartoon showing ordered hexagonal columnar phase Colho (with the main chain omitted). (c) Ordered oblique columnar superlattice structure Colob‑s. (d) Representation of structure transformation between Colho lattice dominated by side-chain TP stacking and polymer backbone induced Colob‑s superlattice organization (top view).

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Figure 9. (a) Geometric size and arrangement of the polymer main chain and pendant TP discogens in polyacrylates P6-n. (b) Schematic illustration of columnar lattice Colho, superlattice organization Colob‑s, and columnar nematic phase with partially preserving oblique columnar superlattice Ncob‑s, and thermally induced phase transitions. The white disc space indicating the gaps between the two neighboring DCS subunits just for guide the eye to differentiate them from the intervals among TP discs within DCS though all the interplanar spacings showing the same average period of 3.5 Å, as revealed by X-ray scattering analysis.

largely in parallel with the axial direction of polymer backbones.5,6a,b Discrete Columnar Stacks (DCS) Based Hierarchically Organized Structures and Molecular Weight Dependent Phase Transitions. Elucidating the detailed superstructure organization is crucial for better understanding the molecular weight (MW) effect, superstructure transformation, and phase transitions. The next question is how the side-chain TP discogens self-assemble into columns, which is closely related to another question; Why the MW effect occurred at a critical MW of around DP 20? For answering these questions and better understanding the pendant TP discogens stacking surrounding the polymer backbone, three relatively independent moieties of TP rigid core, spacer and polymer backbone are brought into consideration. The Colob‑s superstructure was a slight distortion from Colho phase induced by the compaction of polymer backbones with enhanced correlation with side chain TP discogens and these two structures were readily transformable with comparable lattice parameters (Figure 8d and also Figure S32 and the analysis followed). For simplification, the welldefined Colho structure is employed for illustrating the stacking of pendant TP discs and accommodation of spacer and backbone, as shown in Figure 9a. Considering the well interconvertibility between the side-chain dominated lattice Colho and main-chain induced superlattice Colob‑s, that each polymer chain surrounded by four lateral columns forms a supercylinder individually without intercalation of the pendant TP discs into neighboring polymer chains is the most probable packing arrangement. Such

after rapid cooling from their isotropic melts to room temperature around 20 °C as shown in Figure 7b, of which the splitted two peaks around 1.6 nm−1 in Colob‑s superstructure became broader and almost merged implying lowered ordering of the polymer backbones. The SAXS/WAXS patterns of oligomer P6-10 during subsequent heating after a rapid cooling process were almost unchanged with SAXS profiles the same as those shown in Figure 4a upon second heating. However, obvious structure transformation occurred for polymer P6-50, upon heating beyond 40 °C, the (11) peak of Colho phase diminished, while characteristic peaks of Colob‑s superstructure gradually emerged with the broader peak around 1.6 nm−1 prominently splitting into two peaks again accounting for the (10) and (01) reflections of Colob‑s superlattice organization (Figure 7d). Therefore, the involved kinetics and transformation between Colho phase and Colob‑s superlattice organization can be well examined through cooling at different rates and annealing processes by a combination of DSC and temperature dependent SAXS/WAXS analyses. The self-organized superstructures and their transformation are schematically illustrated in Figure 8. The strong π−π stacking interactions of side-chain TP discotic moieties dominated during rapid cooling process to form Colho lattice with a single exothermic peak in DSC traces upon faster cooling. On the other hand, more dense polymer backbones with enhanced correlation with side chain TP moieties during slow cooling processes resulted in main-chain induced Colob‑s superlattice, wherein arrangement of TP-stacking columns was H


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oligomer backbones and side-chain TP stacks, wherein the main chain bundle of 2−4 short chains encircled by at least four DCS subunits constructed a supercylinder as the building block of the Colob‑s superlattice, which was reminiscent of the four columnbundle host−guest supramolecular dendrimers.25a Moreover, we believe that the intracolumnar order promotion through longterm annealing, as shown in Figure 6b, or as in the pristine samples self-organizing into intracolumnar order enhanced rectangular columnar lattice Colrio for side-chain dominated oligomers or oblique columnar superlattice Colob‑sio for polymers was presumably attributed to the significant correlation enhancement or short-range registration with each other among adjacent DCS subunits. Such intracolumnar order enhancement was a self-repairing process, as manifested by the notable thermal effects, whereas imperceptible SAXS/WAXS signal changes, which might constitute a common feature of self-assembly system, as also demonstrated in the case of perylene bisimides based complex supramolecular helical columns.23 Such kinds of intracolumnar ordering through an error correction self-healing process is of particular significance for organic optoelectronic applications. Additionally, both oligomers and polymers underwent a transition from ordered columnar phase to columnar nematic phase Ncob‑s upon reaching their main transition temperatures, wherein the Ncob‑s phase demonstrated the coexistence of side-chain columnar nematic character with main chain induced oblique columnar superlattice ordering, as characterized by their specific thermal properties, characteristic POM schlieren textures, and SAXS/WAXS patterns. The generation of such special nematic phase due to partial disorganization of the long columns might probably occur at the junctions among DCS subunits (Figure 9b, bottom left). Such a special nematic mesophase with concurrently existing backbone columnar ordering was reminiscent of the unusual nematic hexagonal columnar structure proposed for thermotropic polypeptides bearing side-on mesogens,28 while the flexible polyacrylate backbone adopted herein compared to the rigid helical polypeptide was significantly compensated by the large size bulky DCS subunits in contrast to laterally attached simple calamitic mesogens. The employed polyacrylate backbone of suitable flexibility with Tg around 0 °C in this work compared with either relatively rigid polymethacrylate6c−e or too flexible polysiloxane5,6a−c backbone systems, together with the narrow MW distribution and well-defined structure in contrast with the previously reported polydisperse DLC polymers6c−e,g may constitute the key issues accounting for their well developed hierarchical structures and rich phase transitions. Facile Achievement of Uniaxial Alignment of SideChain DLC Polymer via Brief Mechanical Shearing. The alignment of discotic semiconducting materials is of pivotal importance to realize high device performance in optoelectronic applications, for the properties of the oriented materials can exceed those of their unoriented counterparts by orders of magnitude.29 Since the DLC polymers possessing excellent film formation property, some more convenient and practical alignment methods can be employed besides those complex or expensive equipment dependent techniques required for aligning low molar mass DLCs. Thanks to their particular side-chain DCS jacketed structure, this series of DLC polymers exhibited some characteristics similar to main chain LCPs4b and can be easily oriented in larger dimensions by a brief mechanical shearing of sandwiched sample between two glasses of their melts cooled from the isotropic temperature range or concentrated solutions (also see Figures S36 and S37). The resulting thin film displayed

kind of structures are reminiscent of three-column organization demonstrated by Watanabe et al. from cellobiose heptadecanoate-based side-chain poly(viny1 ether)24 and four-column superlattice organization from polymethacrylate with twintapered dendritic benzamide side groups by Percec and coworkers.25 Furthermore, to form four-column supercylinder both an infinite column structure for an individual polymer chain and the most straightforward arrangement of four TP discs sequentially rotating along the polymer backbone axis in a helix period can be precluded considering the largest backbone identity period of 2.5 Å in an all-trans conformation, and limitation of spacer length compared with the 2D lattice dimensions and the intracolumn average interplanar spacing 3.5 Å by X-ray scattering analysis. Therefore, that a certain number of neighboring TP discs of the same polymer chain self-assemble into discrete columnar stacks (DCS) subunits is the most plausible structure, then the DCS subunits further surround the polymer backbone to form a supercylinder, which further self-organizes into extended columns with 2D intercolumnar order to construct variant columnar phases (Figure 9b). The TP disc number involved in a DCS subunit for P6-n series polymer was estimated mainly based on the spacer and bond linkage geometric consideration, an approximate range of less than eight was obtained with a rough estimation method (see Figure S32 and accompanied detailed derivation process). Moreover, the disc number of different DCS subunits in the same supercylinder should be roughly the same to facilitate the extension along the column axis and comply with the minimum energy requirement. Taking the critical DP value of 20 from oligomer to polymer showing remarkable MW effect into consideration, the average disc number of each short column stack is 20/4 = 5, agreeing essentially with the fragment molecular weights of principally around 3,000 to 7,000 detected by MALDI-TOF-MS measurements (Figure 1b, also see Figures S5 and S6 and Table S4), which implied that about five TP discs for each DCS subunits, and only when at least four discrete columns formed satisfying a complete helix period around the polymer backbone, the bulky side-chain pendants could impose significant steric constraints on the main chain and drive the backbone into an extended shape probably with a helical conformation,26 then the whole polymer chain was rigid enough to self-organize into a columnar superlattice structure. Despite discrete aromatic stacks have been well explored in organometallic compounds and organic conjugated systems based on multiple template motifs through various covalent and noncovalent interactions,27 such self-organized mode based on DCS was herein first suggested for DLC polymer system. In our opinion similar DCS subunits might also account for the imperfect four-column-bundle supramolecular cylindrical dendrimers with some specific empty spaces proposed by Percec and co-workers in their polymethacrylates with side-chain dendritic benzamide groups.25a Upon heating, for polymers, the Colho lattice transformed into Colob‑s superlattice with main chain compaction so as to reduce the larger tension arising from the constrained main chain conformation, while for oligomers with less than the critical four DCS subunits to significantly constrain the main chain to form a supercylinder, the Colho lattice was preferred dominated by the side-chain TP stacking. Moreover, it was found that oligomers with 1−3 DCS subunits could also further organize into columnar superlattice structure under suitable conditions favoring bundling of main chains. Undergoing stepwise cooling or a very slow rate cooling process, Colob‑s superstructure was also formed due to nanosegregation of I


Macromolecules

Article

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CONCLUSIONS Side-chain discotic LCPs have been less developed for lack of well-defined polymer material samples to gain a deep insight into their self-organized structures and exploit potential applications. In this work, a series of well controlled triphenylene (TP) based side-chain DLC polyacrylates of suitable polymer backbone flexibility have been successfully synthesized by a RAFT polymerization protocol for the first time, and a remarkable MW effect or polymer effect exhibited with a sharp jump increase of phase transition temperatures from columnar phases to a special nematic phase with columnar superlattice order Ncob‑s at a critical DP around 20, which was well explicable with the first proposed DCS based hierarchical organization model. The formation of ordered hexagonal columnar phase Col ho dominated by the side-chain TP stacking and oblique columnar superlattice structure Colob‑s induced by compaction and ordering of polymer backbone accounted for the distinct molecular weight effect and phase transformation. Furthermore, well developed uniaxial alignment of thus obtained side-chain DLC polymers has been demonstrated via a brief mechanical shearing. The controlled synthesis of desirable side-chain discotic LCPs and in depth understanding of their hierarchical structures and some crucially fundamental issues such as MW effect and spacer length influence will pave the way for the design and preparation of such kind of cutting-edge solution processable low-cost polymeric semiconducting materials, and their easy achievement of uniaxial alignment by a brief mechanical shearing may arouse various fascinating optoelectronic applications.

maximum birefringence when the shearing direction was oriented 45° to the direction of the polarizers and almost a dark field showed as the shearing direction in parallel with that of the polarizer or analyzer (Figure 10a,b), as well as the change in

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Figure 10. Uniaxial alignment of side-chain DLC polymer P6-100 via mechanical shearing. POM images of a typical DLC polyacrylate P6-100 film sandwiched between two glasses at ambient temperature after expericing brief mechanical shearing of their melts cooled from the isotropic temperature range with the shearing direction at (a) 45° to the directors of the crossed polarizers and (b) in parallel with the director of the polarizer. And POM images of the same sheared sample as in (a) after insertion of a λ plate retarder with the slow axis (c) in parallel with and (d) perpendicular to the shearing direction. (e) A cartoon showing the alignment of side-chain columns and polymer backbone. (f) Schematic illustration of the sample shearing geometry. (g) The anisotropic SAXS/WAXS patterns of thus obtained oriented film after isothermal annealing at 60 °C for 12 h at directions perpendicular to (X axis) or in parallel with the shearing direction (Y axis). Red arrows indicate the shearing direction, white arrows for the directions of polarizer and analyzer, and green arrows for the slow axis of the λ plate retarder.

ASSOCIATED CONTENT

S Supporting Information *

Detailed information regarding the synthesis and characterization of CTA, the monomers and corresponding polymers P6n, representative POM images, DSC thermograms, variable temperature SAXS/WAXS profiles and detailed diffraction data for resulting polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-25-83686621. Fax: +8625-83317761. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 20874044) and also partially by Program for Changjiang Scholars and Innovative Research Team in University and the National Science Fund for Talent Training in Basic Science (No. J1103310). We thank Ms. Yuhua Mei and Dr. Ying Ding for assistance in MALDI-TOF-MS measurements.

interference colors exhibited after a λ plate retarder inserted with the slow axis in parallel with or perpendicular to the shearing direction (Figure 10c,d). All these characteristic optical behaviors revealed that the TP-stacked columns were in unidirectional edge-on alignment with the column axis parallel to the shearing direction. Furthermore, transmission-mode SAXS/WAXS patterns of thus obtained as-prepared film (Figure S38) or after further isothermal annealing at 60 °C for 12 h at directions perpendicular to or in parallel with the shearing direction (quasi 2D X-ray scattering analysis, Figure 10g) unequivocally demonstrated well developed uniaxial orientation of the side chain DLC polymer, wherein strong SAXS reflections corresponding to the oblique columnar ordering appeared in the X axis direction, while the WAXS signal of 3.5 Å characteristic of the TP π−π stacking distance was only observed in the Y axis in parallel with the shearing direction together with almost disappeared SAXS signals.

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REFERENCES

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