Well-Organized Columnar Superlattices via Positive Coupling

Article pubs.acs.org/Macromolecules

Well-Organized Columnar Superlattices via Positive Coupling between Polymer Backbone and Discotic Side Groups Bin Mu,† Shi Pan,† Huafeng Bian,† Bin Wu,† Jianglin Fang,‡ and Dongzhong Chen*,† †

Key Laboratory of High Performance Polymer Materials and Technology of Ministry of Education, Collaborative Innovation Center of Chemistry for Life Sciences, 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: Very little is known about some fundamental issues such as the spacer length influence and molecular weight (MW) effect of discotic liquid crystalline polymers (DLCPs), despite the elucidation of such aspects are of crucial importance for their structure tuning and device performance. Here in this article, a systematic comparative study has been conducted to investigate the MW effect and especially gain a deeper insight into the spacer length influence of side-chain DLCPs based on a homologous series of well-defined discotic liquid crystalline polyacrylates with triphenylene (TP) side groups of variant spacer lengths. The series DLCPs of shorter spacers display various well-organized columnar superlattices based on multicolumn bundles organization with “coordination number” from two to six through individual discogens or discrete columnar stack (DCS)-based intracolumnar stacking modes. It is disclosed for the first time that the positive coupling effect (PCE) prevails in side-chain DLCPs, and proper coupling between discotic side groups and polymer backbone is desirable and required for achieving well-organized ordered columnar mesophases, in striking contrast with the renowned classical spacer decoupling principle directing the fruitful exploration of their side-chain calamitic counterparts for several decades. These findings are inspiring for in-depth understanding of self-assembly of aromatic interactions involved complex functional chemical and biological systems and especially opening an avenue for rational design and synthesis of well-controlled side-chain DLCPs for low-cost solution processable optoelectronic device applications.

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INTRODUCTION

Triphenylene (TP) derivatives are among the most extensively investigated and particularly promising discogens arousing continuing attention.8 The reversible addition− fragmentation chain-transfer (RAFT) polymerization exhibited very broad tolerability for monomer functionalities9 and demonstrated to work well for the controlled synthesis of discotic liquid crystalline polyacrylates.10 In our preceding report, a series of well-controlled TP-based side-chain DLC polyacrylates of medium six-methylene spacer with a wide range of degree of polymerization (DP) from 5 to 100 have been successfully synthesized by a RAFT polymerization protocol for the first time, and a remarkable MW effect with a sharp transition at a critical DP around 20 has been disclosed and well explained with the first proposed discrete columnar stack (DCS)-based hierarchical self-assembly model.10 Moreover, thus-obtained side-chain DLCPs manifested welldeveloped uniaxial alignment of columnar liquid crystalline mesophases via a brief mechanical shearing,10 which is highly desirable from the viewpoint of various optoelectronic applications.4

Aromatic interactions involved columnar supramolecular assemblies are crucial for understanding complex functional chemical and biological systems.1 Beyond rich well-organized columnar lattices and superlattices based on well-designed tapered or dendritic building blocks2 or from polyphilic block molecules,3 the particular quasi-one-dimensional (1D) conductible properties of columnar mesophases from various discogens endue discotic liquid crystals (DLCs) with promising electronic and optoelectronic applications.4 Combining the characteristics of DLCs with good processability as well as mechanical and thermal stability of polymers, discotic liquid crystalline polymers (DLCPs) constitute a class of promising and attractive advanced organic materials. Whereas compared with their counterparts of intensively studied and welldeveloped calamitic mesogen-based liquid crystalline polymers,5 much less progress has been made on the study of DLCPs. Although several DLCPs have been investigated,6,7 some fundamental questions pertinent to side-chain DLCPs such as molecular weight (MW) effect and the spacer length influence have not been systematically explored despite such issues being of profound importance to their supramolecular organizations and various promising applications. © XXXX American Chemical Society

Received: July 9, 2015 Revised: September 5, 2015

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DOI: 10.1021/acs.macromol.5b01510 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules The MW effect as one of the involved crucial fundamental questions of DLCPs has been largely answered from a selected sample with a medium six-methylene spacer in the previous paper;10 herein we focus on the spacer length influence and also the universality of the MW effect, which is pivotal for the design, synthesis, and structure control of DLCPs and also inspiring for hierarchical self-assembly of complex functional polymers11 and aromatic interactions involved chemical and biological systems.1 For calamitic LCPs, except for laterally attached mesogen-jacketed liquid crystalline polymers (MJLCPs),12 the spacer decoupling principle13 has been the unchallenged guideline effectively directing the design of sidechain calamitic LCPs for decades, where usually a flexible spacer was thought to be the required linkage to decouple the random coil conformation of main chain and the anisotropic organization propensity of side-chain mesogens. Longer and more flexible spacers demonstrated more effective decoupling and resulted in more ordered mesophases.5,14 Further strongly or even completely decoupled side-chain LCPs were achieved by Percec and co-workers with emphasizing the notion of microphase separation between immiscible polymer backbone and calamitic side groups.15 Nevertheless, as demonstrated from DLC polysiloxane with TP-based side-chain discogens the classical spacer principle well suitable for calamitic systems turned out to be invalid for side-chain discotic LCPs, where longer spacers generally did not result in higher ordered phases, but rather led to severe distortion or even breakdown of the columnar phase, which was ascribed to the increasing perturbation introduced by longer spacers exceeding their decoupling function.6c This early report was almost the only literature concerning the spacer influence of DLCPs and presented some profound suggestions and perspectives, which were particularly significant and instructive for our further exploring in this field, although a more pertinent and universal principle was not available largely due to the adoption of extremely flexible polysiloxane backbone and limited number of DLCP samples, especially lack of homologues with shorter spacers.6c Herein, we present the synthesis of a huge family of TPbased side-chain DLC polyacrylates of Pm-n with continuously increased spacer length m = 0−10, 14 (see Figure 1a) spanning from mesogen-jacketed polymers to those nearly completely decoupled ones, with DP n = 10 and 50, respectively, for representative oligomeric and polymeric samples of each spacer group based on the intensive investigation on the spacer m = 6 series of DLC polymers reported in our previous paper.10 The systematic comparative study of the well-prepared side-chain DLCPs with polyacrylate backbone of suitable flexibility provides an in-depth insight into the influence of spacer length and also MW effect; thereinto the cognition is revolutionary that the positive coupling effect (PCE) between discogens and polymer backbone prevails in properly designed side-chain DLCPs, in sharp contrast to the negative coupling effect (NCE) in their calamitic counterparts, which may guide the rational design and controlled preparation of DLCPs and enhance comprehending aromatic interactions involved self-assembled functional systems.

Figure 1. (a) Molecular structure of the synthesized series DLC polyacrylates Pm-n with m standing for the methylene number of the spacer and n for DP. (b) Schematic polymer repeat unit structure labeled with dimensions. (c) Spacer length L measured from ChemBio3D Ultra 11.0 for a range of continuously increasing spacers in an all-trans conformation. (d) Plot of the ratio of theoretical MW (Mtheor) to number-average MW determined by GPC (Mn,GPC) of oligomers Pm-10 and high-DP polymers Pm-50 versus the length of spacer (m) to reflect different contraction extent in solution closely in correlation to their corresponding bulk structural organizations dependent on spacer length.

are quite particular, showing a remarkable MW effect and hierarchical superstructure organization, have been intensively investigated in the preceding paper.10 The family of TP-based side-chain DLC polyacrylates of Pm-n with continuously increased spacer length m = 0−10, 14 and DP n = 10 and 50, respectively, as typical oligomeric and polymeric samples for each spacer group as shown in Figure 1a have been well synthesized via RAFT polymerization. The RAFT technique worked well for discotic TP-acrylate monomers and possessed living character showing a well-fitted linear relationship of conversion versus polymerization time.10 The polymerization of TP-acrylate monomers 3(m) (Scheme S6, Supporting Information) with spacer m ≥ 6 was conducted with the same protocol as for 3(6).10 Nevertheless, for monomers with m < 6, due to their relatively low reactivity with the acrylate reactive group attached onto TP discotic core via a shorter spacer resulting in a significant steric hindrance, high-DP polymers were implemented at elevated temperatures, despite low-DP (n < 20) oligomers could also be carried out under the same reaction conditions as employed for monomers with longer spacers. In all polymerizations, a sufficient reaction time was guaranteed to reach nearly complete conversion so that the DP of obtained products was very near the designed value determined by the ratio of feeding monomer to chain

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RESULTS AND DISCUSSION Controlled Synthesis of Triphenylene (TP)-Based Side-Chain DLCPs (Pm-n). Among the whole family of polymers, the P6-n series with a six-methylene spacer of the same length as other alkoxy side chains attached on the TP core B


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Figure 2. DSC traces of the family of DLCPs (Pm-n) with variant length spacers of DP 10 oligomers and DP 50 polymers upon first cooling, second heating runs, and heating after a stepwise cooling process at a heating or cooling rate of 10 °C min−1. The stepwise cooling process was performed through isothermal annealing at a set of selected temperatures about 10 °C interval step-down from isotropic melts to room temperature, essentially equivalent to a very slow cooling process.

last section. First, the thermal behaviors and hierarchically selforganized structures of the family DLCPs with variant length spacers in bulk were systematically examined by a combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small- and wide-angle X-ray scattering (SAXS/WAXS) analytic techniques. Thermal Analysis of the Family of Side-Chain DLCPs (Pm-n). All DLCP samples investigated displayed obvious birefringence, indicating their self-organized mesomorphic orders with some representative POM textures provided in Figure S3. Their DSC traces of first cooling, second heating, and also heating after a stepwise cooling process are shown in Figure 2, and the phase transition temperatures and associated enthalpy changes are summarized in Table 1. By a preliminary general comparison, the DLCPs showed a modest MW effect for polymer samples with shorter spacers from m = 1 to 5 while exhibited the most remarkable MW effect for that of spacer m = 6 and, further, the medium MW effect for polymers with spacer m larger than 7 until no perceptible MW effect was detected for those with m longer than 9 irrespective of thermal histories, implying almost complete decoupling with a sufficiently long

transfer agent (CTA). The MWs of thus-obtained polymers were measured by gel permeation chromatography (GPC) and assessed from the integral ratio of characteristic 1H NMR peaks (Table S1). Based on polystyrene standards, the numberaverage molecular weight (Mn,GPC) of oligomeric Pm-10 determined by GPC matched quite well with the theoretical molecular weight (Mtheor), which manifested their relatively normal conformation with negligible influence of aromatic interactions for low-DP DLCPs in solution. However, the MWs of high-DP polymers Pm-50 were significantly underestimated by GPC, as shown in Figure 1d that the ratio of theoretical MW to the number-average MW measured by GPC (Mtheor/Mn,GPC) versus spacer length for high-DP polymers exhibited an obvious deviation especially more striking difference in the medium spacer length region, which could not be simply explained by the difference of hydrodynamic behavior in solution between DLCPs and polystyrene standards considering low-DP samples not showing significant deviation. It is very interesting to notice that such remarkable deviation is in close correlation with the columnar order and intracolumnar stacking mode of high-DP DLCPs in bulk state as will be more detailedly discussed in the C


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Table 1. Summary of Transition Temperatures, Associated Enthalpy Changes, and Phase Assignments Based on a Combination of DSC, POM, and SAXS/WAXS Analyses of the Series Pm-n Samples with Spacer m = 0−10, 14 and DP n = 10 and 50 thermal transitions/°C (enthalpy changes/J g−1)a polymer code P0-10 P0-50 P1-10 P1-50 P2-10 P2-50 P3-10 P3-50 P4-10 P4-50 P5-10 P5-50 P6-10d P6-50d P7-10 P7-50 P8-10 P8-50 P9-10 P9-50 P10-10 P10-50 P14-10 P14-50

heating after a stepwise cooling

b

Colr‑sio 41(5.2) Colr‑s 98(0.2) I Colr‑sio 42(0.4) Colr‑s 145(0.3) I Colh‑sio 51(1.1) Colh‑s 147(0.9) I Colh‑sio 59(1.4) Colh‑s 159(1.7) I Colhoio/L 37(0.3) Colho/L 123(0.5) Colhoio/L 42(0.2) Colho/L 133(1.6) Colhoio/L 50(12.7) Colho/L 64(0.9) Colhoio/L 51(10.3) Colho/L 73(1.1) Colob‑sio 38(10.9) Ncob‑s 65(2.0) I Colob‑sio 41(4.5) Colob‑s 95(3.0) I Colob‑sio 42(15.3) Ncob‑s 56(0.1) I Colob‑sio 40(1.0) Ncob‑s 80(2.8) I Colr‑sio 45(4.3) Ncr‑s 64(1.4) I Colr‑sio 42(0.3) Ncr‑s 81(3.3) I Colrio 48(21.3) I Colh‑sio 45(9.4) Nch‑s 66(1.6) I Colrio 41(16.8) I Colrio 43(17.6) I Colrio 42(22.6) I Colrio 42(19.6) I

first coolingb

second heatingb ColrMJ ColrMJ

c

> 210 I > 230c I Colob‑s 170c I Colob‑s 220c I Colho 98(0.7) I Colho 145(1.3) I Colho 139(3.2) I Colho 154(4.5) I Colho 115(3.1) I Colho 127(4.0) I G 5 Colho 60(1.8) I G 13 Colho 70(2.1) I G −5 Colho 30(3.2) Ncob‑s 57(0.9) I G 3 Colob‑s 83(2.3d) Ncob‑s 89(0.4) I G −4 Colx 29(6.5) Nc 43(0.7) I G −1 Colx 32(2.4) Ncob‑s 77(1.9) I G −4 Colx 31(6.6) Nc 53(10.1) I G −2 Colx 33(4.6) Ncr‑s 79(2.0) I G −4 Colx 29(6.3) Colx2 47(9.6) I G −4 Colx 31(5.3) Colx2 49(2.0) I G −2 Colx 27(5.1) Colx2 46(11.3) I G −3 Colx 29(4.8) Colx2 47(3.1) I G −3 Colx 29(9.5) Colx2 41(5.9) I G −1 Colx 31(8.9) Colx2 42(0.7) I

I I I I

I I I I I I I I I I I I I I I I I I I I

79(−0.7) Colho 123(−1.8) Colho 122(−3.4) Colho 137(−4.7) Colho 104(−3.4) Colho 113(−4.5) Colho 36(−0.8) Colho 49(−1.5) Colho 7(−3.2) Colho 61(−2.1) Colob‑s 7(−3.7) Colx 7(−1.4) Colx 11(−6.3) Colx 9(−2.3) Colx 13(−9.6) Colx 10(−5.5) Colx 13(−11.7) Colx 10 (−7.1) Colx 15(−13.0) Colx 14(−9.9) Colx

a

Abbreviations: G = glassy state; ColrMJ = discotic mesogen-jacketed LC in rectangular lattice organization; Colr‑s = rectangular columnar superlattice; Colh‑s = hexagonal columnar superlattice; Colho = ordered hexagonal columnar lattice; Colho/L = ordered hexagonal columnar lattice with some lamellar correlation; Colob‑s = oblique columnar superlattice; Colr = rectangular columnar lattice; Nc = proposed columnar nematic phase or columnar nematic phase with some superlattice order as indicated by a superscript; I = isotropic phase; Colx = unidentified columnar phase due to lack of sufficient assignable characteristic peaks in SAXS profiles. Some ordered columnar lattice or superlattice with a superscript “io” indicating enhanced intracolumnar order. 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. cThe transition temperatures were approximately determined by a combination of POM and variable temperature SAXS/WAXS for no thermal transition peak detected in the DSC heating or cooling runs. dFrom ref 10.

mesogen-jacketed P0-n and P1-n, an endothermic peak at about 40 °C or slightly higher always appeared during the heating run after a preceding stepwise cooling process, which was attributed to the melting of columnar phases with enhanced intracolumnar order within TP-stacked columns,10 while the followed peak of higher temperature, if any, corresponded to isotropization transition. Structural Evolution with Lengthening Spacer and Proposed Organization Models for Side-Chain DLCPs. All polymer samples of the whole family were investigated with variable temperature X-ray scattering analysis in their LC mesophases and other interested temperature region, and all the SAXS/WAXS patterns were recorded during heating process after cooling at 10 °C min−1 or through a stepwise cooling process from the isotropic state, in agreement with the thermal examining conditions employed in DSC heating cycles considering fully rich phase transitions exhibited in the heating runs. Detailed temperature-dependent SAXS/WAXS profiles and associated indexing and assignment data are available in the Supporting Information. The proposed organization models for polymers of variant length spacers in this family mainly based on SAXS and thermal analyses are deduced as follows. Linked by only an ester group of side-chain TP discotic mesogens onto the polymer backbone, P0-n without any flexible

spacer, which will be discussed in detail and rationalized from their various self-organized structures in a later section. Then in a close inspection of the DSC thermograms, except the discotic mesogen-jacketed LCP without a flexible spacer P0-n and the one with only one methylene spacer P1-n showed no detectable signals in both heating and cooling scans; all other DLCPs exhibited only an exothermic peak in the cooling runs, pertaining to phase transition from the isotropic state to columnar phases with transition temperatures showing a similar supercooling degree of about 20 °C compared with their corresponding columnar melting temperatures in the second heating runs. In the second heating cycles, the Pm-n with shorter spacers m = 2−5 presented only one endothermic peak corresponding to isotropization transition of columnar mesophases, while those with a medium length spacer of m = 6−8 displayed two endothermic peaks with the first one at about 30 °C corresponding to disorganization of long columnar structure into discrete columnar stacks (DCS)-based columnar nematic phase (Nc), except P6-50 displayed its uniqueness of a column disorganization temperature at very high 83 °C (for details see the previous paper10), and a second peak corresponding to isotropization transition. For those with longer spacers of m ≥ 9, double melting peaks appeared as will be discussed later in some detail. For all samples except the D


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Figure 3. SAXS patterns and proposed 2D columnar arrangement of Pm-n (with spacer m = 1−5, DP n = 10 and 50). (a) SAXS patterns with proposed indexing of P1-n at 50 °C after cooling from the isotropic state at 10 °C min−1 and (b) corresponding schematic illustration of oblique columnar superlattice Colob‑s (p2) based on “twin-column” bundles. (c, e, g) SAXS patterns with proposed indexing (the red indexing in brackets indicating the coexisting hexagonal columnar lattice superposed on the superlattic with accordingly enhanced reflections) at indicated temperatures after a stepwise cooling process from the isotropic state and (d, f, h) corresponding schematic illustration of P2-n of coexistence of a rectangular columnar superlattice Colr‑s (p2) based on two-column bundles with a hexagonal columnar lattice by side-chain TP stacking, P3-n of coexistence of a hexagonal columnar superlattice Colh‑s (p3m1) based on three-column bundles with a hexagonal columnar lattice by side-chain TP stacking, and P4-n and P5-n of side-chain TP-stacked hexagonal columnar lattice with some extent lamellar correlation Colho/L (p1) organized by incompact threecolumn bundles in an alternately inverse orientation.

peripheral alkoxy side chains can be further well rationalized as illustrated in the Supporting Information (see Figure S7 and the accompanying explanation for the structure evolution). A slight one methylene increase of spacer length from P1-n to P2-n gave rise to the structural transformation from the “twincolumn” bundles-based oblique columnar superlattice to an ordered hexagonal columnar phase Colho with lattice parameter a = 20.8 Å dominated by side-chain TP-stacking after cooling from the isotropic state at 10 °C min−1 (Figure S8), which might result from their enhanced freedom of the two opposite TP-stacked columns tethered on the same polymer backbone, considering that with increasing one more methylene spacer the horizontal distance between two opposite discogens of P2-n can be as large as (3.7 + 7.2) × 2 = 21.8 Å with an all-trans conformation of spacers, just larger than the intercolumnar distance 20.8 Å of TP-stacked columns. Furthermore, upon a stepwise cooling process a well-organized rectangular columnar superlattice Colr‑s (p2) with lattice parameter a = 36.0 Å and b = 61.6 Å based on two-column bundles organization was developed as indicated in Figure 3c with associated indexing assignments summarized in Table S5. Such kind of further ordering was in great accordance with the conclusion that compaction and ordering of polymer main chain could be readily induced by a stepwise cooling process as drawn from the intensive phase transformation kinetics study of TP-based polyacrylate DLCPs with a six-methylene spacer in our previous paper.10 Moreover, the hexagonal columnar lattice by the side-

spacer behaved as discotic mesogen-jacketed LCPs with thermal properties and phase behaviors (Figures S4 and S5 and the accompanying analysis) very similar to the discotic LCPs based on rigid polyacetylene backbone with shorter spacers6g or the counterpart conventional calamitic mesogenbased MJLCPs.12 Both oligomer P1-10 and polymer P1-50 with only one methylene spacer showed highly ordered 2D columnar mesophase with a large number of strong reflections in the small-angle region and an obvious WAXS peak around 3.56 Å corresponding to average TP π−π stacking distance (Figure S6). However, no thermal transition peak was detected in the DSC measurements similar to that of P0-n, and high clearing temperatures of about 170 °C for P1-10 and 220 °C for P1-50 were found by a combination of POM and temperaturedependent SAXS investigations, which implied that the P1-n series with the shortest only one methylene spacer might possess some characteristics of discotic mesogen-jacketed LCPs like P0-n and yet showing a distinct π−π stacking of TP discogens. All reflections of both P1-10 and P1-50 in the low angle region (Figure 3a) can be well assigned to an oblique columnar superlattice Colob‑s (p2) with lattice parameters of a = 33.1 Å, b = 21.6 Å, and γ = 83.2° from “twin-column” bundlesbased supramolecular organization, as shown in Figure 3b, and detailed reflection peak values and assignments are provided in Table S4. Their “twin-column” bundle-based superlattice organization with partly overlapping and adjustment of E


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Figure 4. SAXS patterns and proposed 2D columnar arrangement of Pm-n (with spacer m = 7−10, 14, DP n = 10 and 50). (a, c, e) SAXS and (g) SAXS/WAXS patterns with proposed indexing at room temperature 20 °C after a stepwise cooling process from the isotropic state and (b, d, f, h) corresponding schematic illustration of P7-n of four-column bundles-based oblique columnar superlattice Colob‑s (p2), P8-n of four-column bundlesbased rectangular columnar superlattice Colr‑s (p2gg), P9-50 of coexistence of a hexagonal columnar superlattice Colh‑s (p6mm) with the hexagonal columnar lattice by side-chain TP stacking, and P9-10, P10-n, and P14-n of rectangular columnar lattice Colr (C2/m) from nearly completely decoupled side-chain TP stacking, where φ represents the tilt angle of discotic cores with respect to the columnar axis, calculated as φ = arcsin(√3b/ a). And the dark green line in (g) is the X-ray scattering patterns of the crystalline monomer TP-acrylate 3(6) upon a rapid cooling from its isotropic state for comparison.

Colh‑s (p3m1) with lattice parameter a = 35.8 Å based on threecolumn bundles was formed with associated indexing assignments summarized in Table S6. The coexistence of the sidechain TP-stacked hexagonal columnar lattice with the hexagonal columnar superlattice was also unequivocally revealed by the notably intensified peaks at 18.0 Å (3.49 nm−1) and 10.4 Å (6.04 nm−1) in the reciprocal spacing ratio 1:1/√3, which belonged to the (11) and (30) reflections of hexagonal columnar superlattice, respectively (Figure 3e). The proposed 2D organization in top view is illustrated in Figure 3f, in which the regularly collocated triangular three-column bundles as a whole took the same orientation as indicated by the down triangles in light blue. The stacking organization mode transformation of side-chain TP discogens from twocolumn bundles of P1-n and P2-n to three-column bundles of P3-n also could be preliminarily rationalized from a simple geometric consideration of the spacer length increase; the maximum distance from the center of TP discogens to polymer backbone for P3-n was estimated to be 3.7 + 8.4 = 12.1 Å with an all-trans conformation (referring to Figure 1c), which agreed quite well with the distance 12.0 Å as calculated from the measured hexagonal lattice constant between the axis of sidechain TP-stacked column and the center of regular triangular three-column bundle where was proposed to accommodate the polymer backbone. Both P4-n and P5-n series exhibited ordered hexagonal columnar phase Colho (p6mm) with lattice parameter a = 21.1 Å after either a stepwise cooling process or cooling at 10 °C

chain TP-stacking coexisted with the rectangular columnar superlattice as manifested by the remarkably intensified peaks at 18.0 Å (3.49 nm−1) and 10.3 Å (6.10 nm−1) in the reciprocal spacing ratio 1:1/√3 (as assigned [10] and [11] indices in red for hexagonal lattice), which were assigned to the (13)/(20) and (33) reflections of rectangular columnar superlattice, respectively. The coexistence of TP-stacked hexagonal columnar lattice with the 2D rectangular columnar superlattic from both polymer backbone and side-chain TP-stacked columns packing as a whole is schematically presented in Figure 3d, of which the two-column bundles adopted different orientations with a certain angle thus exhibited simple rectangular superlattice. Besides, the absence of (01) reflection could be well explicable considering that the two-column bundles as highlighted by the light gray background in the a direction may generate a similar additional electron density modulation as indicated by a vertical dashed line, thus resulting in an extinction of (0k) reflections with k = 2i + 1 (i is an integer) along the b direction. Upon further elongation of the spacer length by one more methylene, the P3-n series, similar to that of P2-n, showed an ordered hexagonal columnar mesophase Colho with a = 20.8 Å after cooling from their isotropic melts at 10 °C min−1 (Figure S9). Similarly, simultaneously hierarchical ordering with a superimposed superlattice on the hexagonal columnar lattice was also produced with the informative SAXS patterns shown in Figure 3e for P3-n series polymers after a stepwise cooling process. While in this case, a hexagonal columnar superlattice F


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Macromolecules min−1 (Figures S10 and S11), and moreover, a diffuse reflection peak in the low angle region of 36.7 Å (1.71 nm−1) appeared to be twice of the d value 18.3 Å (3.43 nm−1) of (10) reflection of Colho lattice (Figure 3g and Table S7). Such a SAXS signal presumably originated from polymer main chain and manifested three-column bundles structure as well, while rather than the same orientation as adopted in P3-n series, alternately inverse orientations were assumed here to further establish a lamellar correlation as shown in Figure 3h. Such proposed stacking mode for P4-n and P5-n series was also justified by their notable difference as compared with the four-column bundles organization based on DCS subunits of P6-n series with a slightly longer six-methylene spacer10 and their very similar behavior to P3-n series of phase transition temperature dependence upon molecular weight, which will be discussed later in particular. While both series did not develop 2D superlattices as in the case of P3-n series based on compact three-column bundles with polymer backbone significantly confined, which might result from the considerably increased conformational freedom of polymer main chain, profiting from one or two methylene elongation of the spacer. As a result, such relatively incompact three-column bundles took alternately inverse orientations to organize into hexagonal columnar lattice based on side-chain TP stacking; further, the allowed slip movement of three-column bundles within the horizontal direction led to lamellar electron density modulation upon TPstacked columns to further present some extent lamellar correlation with a period of √3 times larger than the primary hexagonal lattice parameter. Both P7-n and P8-n series exhibited two endothermic peaks in the second heating runs with the former of melting temperatures all around 30 °C, while the latter corresponding to isotropization transition showed an obvious MW effect with clearing temperatures of DP 50 polymers over 25 °C higher than those of their corresponding DP 10 oligomers (Figure 2). Only one exothermic peak around 10 °C displayed in the cooling scans showed a highly consistent supercooling degree of about 20 °C, which revealed that TP discogens were involved in both the first endothermic melting transitions in the heating cycles and the only exothermic peak during cooling in accordance with the slow kinetics of reorganization of bulky aromatic units, essentially similar to the kinetics performance of P6-n series.10 In contrast with the ordered columnar structures observed in P6-n polymers, all the P7-n and P8-n series samples exhibited a columnar mesophase of low order denoted as unidentified columnar phase (Colx) upon cooling from isotropic melts to room temperature at 10 °C min−1, with a weak peak of spacing about 3.5 Å in the wide-angle region characteristic of π−π stacking, while only a diffuse reflection peak around 18 Å (3.5 nm−1) showed in the SAXS patterns (Figures S12 and S13). Upon raising the temperature over the melting transition peak observed in the DSC second heating scans, the polymers P7-50 and P8-50 showed a columnar nematic phase with oblique or rectangular superlattice order denoted as Ncob‑s and Ncr‑s, respectively, while the oligomers P710 and P8-10 presented no significant change as discussed a little more later though with strong endothermic peaks around 30 °C as well. After a stepwise cooling process from their isotropic melts, as shown in Figure 4a,c, a series of strong SAXS peaks appeared, an oblique columnar superlattice Colob‑s (p2) with lattice parameters a = 88.5 Å, b = 42.5 Å, and γ = 89.0° for the P7-n series, and a rectangular columnar superlattice Colr‑s (p2gg) of a = 90.4 Å and b = 43.0 Å for the P8-n series could be

well assigned (see Tables S8 and S9 for detailed reflection data and assignments). Moreover, no significant changes in the SAXS/WAXS patterns occurred with temperature rising beyond the transition point around 40 °C (Figures S12 and S13) disclosed by DSC measurements with a strong endothermic peak corresponding to disorganization of long columns with enhanced intracolumnar order, which were ascribed to forming columnar nematic phases with columnar superlattice order through a DCS-based superstructure organization as compared to phase behavior of intensively studied P6-n series.10 Both Colob‑s and Colr‑s superlattices resulted from a little distortion of original hexagonal lattice disturbed by nonmesogenic moieties of main chain and slightly overlong spacers (Figure S14). Furthermore, based on a simplified straightforward geometric consideration of molecular dimensions especially spacer length (Figure S15) as successfully adopted for P6-n series,10 it was deduced that supercylinders constructed from DCS-based four-column bundles of single polymer chain with backbone plausibly in helical conformation and then further organized into oblique or rectangular superlattice for P7-50 and P8-50, respectively. While for P7-10 and P8-10 oligomers, DCS-based four-column bundles were constructed through nanosegregation of main chains and DCS subunits of several oligomers, owing to the insufficient main chain length for a complete helix to form a supercylinder based on a single oligomer. In addition, here the four-column bundlebased elliptical supercylinders as depicted with oval-shaped sections in the 2D top view cartoons adopted a herringbone orientation for both P7-n and P8-n, which was in significant contrast to the identical orientation adopted by P6-n series polymers in the same four-column bundle organization.10 Moreover, with one more methylene spacer elongation, P8-n gave rise to structure rectification from oblique columnar of P7n to more regular rectangular columnar superlattice. P9-n samples showed two endothermic peaks in the second heating scan and only one exothermic transition during cooling attributed to the formation of columnar mesophase (Figure 2). Both P9-10 and P9-50 exhibited unidentified columnar structures (Colx) of low order after cooling from isotropic melts at 10 °C min−1 (Figure S16), similar to that of P7-n and P8-n. However, upon a stepwise cooling process, P9-n series presented some extent MW effect with oligomer P9-10 showed very weak signals bearing some common features with that from polymers of longer spacers such as P10-n and P14-n as discussed further below, while high-DP polymer P9-50 exhibited a hexagonal columnar superlattice Colh‑s (p6mm) with lattice parameter a = 53.7 Å based on six-column bundles (Figure 4e and Table S10). As shown in Figure 4e, the SAXS pattern of P950 revealed that (12) peak was the strongest reflection, which implied a strong electron density modulation with a period √7 times shorter than the (10) spacing; thus, the coexistence of a hexagonal columnar lattice from side-chain TP stacking with their stretched backbone probably in helical conformation constituting superlattice points well accounted for the complex structure (Figure 4f). Similarly, the six-column bundles organization of P9-50 can also be straightforwardly understood based on simple geometric consideration referring to P6-n,10 for the maximum distance 19.5 Å between polymer backbone and the discogen center was smaller than the measured intercolumnar distance of 20.3 Å in the hexagonal superlattice (Figure S17); therefore, P9-50 might probably adopt DCSbased superstructure with six-column symmetrically surrounding the polymer backbone, which was also distinct from the G


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Figure 5. Schematic illustration of variant coordination number supercylindrical structure evolution versus spacer length based on different intracolumnar stacking modes from individual discogens stacking to discrete columnar stacks (DCS) and then to nearly complete decoupling with longer spacers more than nine-methylene of the Pm-n homologous series DLCPs showing in top view of (a) P1-n: Colob‑s (p2); (b) P2-n: Colr‑s (p2); (c) P3-n: Colh‑s (p3m1); (d) P4-n, P5-n: Colho/L(p1); (e) P6-n: Colob‑s (p2); (f) P7-n: Colob‑s (p2); (g) P8-n: Colr‑s (p2gg); (h) P9-50: Colh‑s (p6mm); (i) P9-10, P10-n, P14-n: Colr (C2/m).

higher than 40 °C. The single sharp peaks displayed around 42 °C in the heating runs after a stepwise cooling equivalent to very slow cooling process manifested remarkable enhancement of intracolumnar order. Such particular thermal behaviors and kinetics characteristics were also confirmed by SAXS/WAXS analyses showing columnar phase of low order upon cooling at 10 °C min−1 (Figures S18 and S19). Whereas after a stepwise cooling process, SAXS/WAXS patterns of both oligomers and high-DP polymers of P10-n and P14-n series also of oligomer P910 provided very inspiring results as compared to that of TPacrylate monomer 3(6) (Figure 4g). All these oligomers or polymers of longer spacers presented rectangular columnar lattice Colr (C2/m) with lattice parameter a = 37.4 Å and b = 15.9 Å showing varying degrees of SAXS/WAXS features of crystalline phase of monomer 3(6), with enhanced characteristic diffraction peaks for polymers of lengthened spacer length such as P14-n series (Figure 4g and Table S11). Such thermal behaviors and structural characteristics were reminiscent of some extent crystalline phase features exhibited in the SAXS/ WAXS patterns of the pristine low-DP samples of P6-n series10 and revealed that almost completely decoupling achieved between the side-chain discotic mesogens and polymer backbone with a sufficient spacer length such as longer than

hexagonal superlattice of P3-n based on three-column bundles. Such complex hexagonal columnar superlattices with the contracted backbone serving as superlattice points were also observed in hexabenzocoronene (HBC)-based side-chain supramolecular block copolymers16 and TP-based discotic liquid crystalline poly(propylenimine) dendrimers.17 As shown in Figure 2, all samples of P10-n and P14-n series exhibited two quite close endothermic peaks with peak temperature difference less than 20 °C in the second heating cycles, whereas only one sharp endothermic peak at 41−43 °C displayed upon undergoing a preceding stepwise cooling process, and the oligomer P9-10 also revealed similar thermal behaviors. Such double melting phenomena were in sharp contrast with thermal behaviors of other homologues with shorter spacers and reminiscent of multiple transition behaviors observed in other TP-based DLCPs with flexible backbone or longer spacers.6a,i Upon cooling to 20 °C at a relatively fast rate of 10 °C min−1, a kinetically controlled unidentified columnar phase (Colx) of low order formed, which melted at around 30 °C upon heating and simultaneously transformed into another columnar phase (Colx2) with slightly higher order thanks to thermal induced structural adjustment and intracolumnar order promotion to exhibit a second melting temperature at a little H


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The four-column bundles-based columnar cross section was assumed to be elliptical and showed spacer length-dependent orientations. Similarly, the most confined four-column bundles of P6-n adopted an identical orientation (Figure 5e), while with slightly reduced confinement close-packed herringbone orientations of minimized energy were adopted for P7-n (Figure 5f) and P8-n (Figure 5g). With further elongation of spacer length, P9-50 presented a six-column bundles-based superlattice (Figure 5h), while P9-10, P10-n, and P14-n displayed ill-defined rectangular columnar lattice comparable to some extent the crystalline behaviors of the TP-acrylate monomers (Figure 5i), revealing nearly complete decoupling of side-chain discogens from main chains. Therefore, higher “coordination number” was adopted with increasing the length of the spacer to alleviate the interference of redundant part of spacers on the stacking of TP discogens and strengthen the orientational correlation between side-chain TP columns and the confined polymer main chain to form a supercylinder then further organize into various 2D superlattices with the supercylindrical building blocks. Such a 2D periodic arrangement based on multicolumn bundles with coordination number from two to six was reminiscent of the complex compartmentalization tiling patterns from polygonal tiles of polyphilic block molecules intensively explored by Tschierske’s group.3 Two distinct intracolumnar stacking modes of individual discogens stacking and discrete columnar stacks (DCS)-based stacking well accounted for the superstructure transformation from lower coordination number two or three for DLCPs of shorter spacers m = 1−5 to four or more for those with longer spacers m ≥ 6. Such stacking mode transition of side-chain TP discogens within the column was manifested by their significantly different thermal behavior in the DSC second heating runs that two endothermic peaks for the series Pm-n with m ≥ 6 showed in sharp contrast with only a single peak for m = 1−5 (Figure 2), implying their kinetics difference as further clarified below. Further, based on geometric dimension consideration, polymers of spacer length m = 1−5 exhibited highly consistent of the estimated molecular dimensions with the experimentally measured lattice parameters based on individual discogens surrounding the polymer backbone to stack into columns; otherwise, the formation of DCS-based column was either not allowed by the very short spacer or would result in much higher conformational tension. While those polymers with m ≥ 6 could not adopt individual discogens stacking mode from geometric and energy consideration, DCS-based stacking was preferred, as comprehensively demonstrated for Pm-n with m = 6 in the preceding report10 and illustrated for m = 7−9 in the previous section that DCS-based self-organizations took effect and developed some special DCS-based columnar nematic phases above the melting temperature upon disorganization probably at junctions among DCS subunits. Additionally, Pm-n with m = 10, 14 might follow DCS-based stacking as well, while they could also assume other stacking manner such as just in a random way thanks to the more freedom offered by the much longer spacers in an almost completely decoupled state. Despite discrete aromatic stacks have been well explored in organometallic compound and organic conjugated system based on multiple template motifs through various covalent and noncovalent interactions,20 such self-organized mode based on DCS subunit was first introduced for the DLC polymer system by our group,10 which accounted well for the MW effect and spacer length-dependent hierarchical self-organization and superstructure transforma-

nine methylenes, which demonstrated relatively independent discogens stacking comparable to crystalline features of the corresponding monomer, while of low order due to significantly perturbed by the nonmesogenic overlong spacers and polymer main chains. Additionally, the rectangular columnar structure can be considered to evolve from the regular hexagonal columnar lattice by tilting of discotic mesogens with respect to the column axis at an angle of φ = 47.4° (Figure S20), which was also manifested to some extent by the closer intracolumnar spacing of about 3.43 Å. Such kind of structure adjustment and evolution is also rationalized by the cognition that for discotic mesogens there mainly exist two types of stable columnar structures of hexagonal columnar mesophase with the aromatic core plane perpendicular to the columnar axis and rectangular columnar crystalline phase of tilted aromatic cores to optimize their π-orbital interactions.18 Although following distinctly different mechanism and bearing meaningful scientific implications as will be further elucidated below, the turning point of about ten-methylene spacer of nearly complete decoupling in side-chain DLCPs is comparable to the observation that in calamitic mesogen-based LC polyacrylates about 11-methylene spacer sufficiently decoupled the mesogens from the coil backbone,19 while further elongated spacers led to better organized smectic mesophases in contrast with nematic phases of low order generally for side-chain calamitic LC polymers of shorter spacers.12,14b Evolution of Supercylindrical Organization of Variant Coordination Number with Individual Discogens to DCS-Based Intracolumnar Stacking Modes Transition upon Increasing the Spacer Length. Above discussed superstructure examination and analyses of the homologous series DLCPs with spacer length increasing are summarized in Figure 5. For P0-n without a flexible spacer linked only via an ester group exhibited no discotic columnar WAXS signal and just behaved as discotic mesogen-jacketed polymers showing the highest clearing temperature over 200 °C with relative rigid chain as a whole. The P1-n and P2-n series adopted two-column bundles-based superlattices, where for P1-n series two columns in the same polymer chain were so jam-packed that their peripheral alkoxy tails partially overlapped (Figure 5a). Therefore, P1-n series exhibited much higher clearing temperatures compared with that of P2-n, with thermal behaviors much like discotic mesogen-jacketed polymers P0-n. Nevertheless, relatively independent two columns of P2-n organized into Colr‑s (p2) superlattice based on the close-packed hexagonal columnar lattice of side-chain TP-stacking (Figure 5b), where two-column bundles with reduced confinement took different orientations, while an identical orientation was adopted in more confined P1-n series. Similarly, the P3-n series as the most confined three-column bundles organization took the same orientation for the whole regular triangular three-column bundles to shape a hexagonal columnar superlattice (Figure 5c). Further, with reduced confinement of three-column bundles, P4-n and P5-n adopted alternately inverse orientations, so that they exhibited hexagonal TP-stacked columnar lattice with some lamellar correlation (Figure 5d), which was comparable to the nematic arrangement of three-column bundles produced by polymethacrylate with twin-tapered dendritic side groups.2a Whereas all the four-column bundles-based superlattices in P6-n, P7-n, and P8-n series were readily evolved from the TPstacked hexagonal columnar lattice with slight distortion perturbed by nonmesogenic main chains and partial spacers. I


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Figure 6. Plots of phase transition temperatures versus the length of the spacer m of Pm-n with DP n = 10 and 50 determined from DSC with heating rate of 10 °C min−1 during (a) heating after a stepwise cooling process and (b) second heating after a normal cooling at a rate of 10 °C min−1, where the red symbols and lines stand for Pm-50 while blue for Pm-10. The transition temperatures for P1-n determined from POM and temperaturedependent SAXS investigations.

of various columnar or columnar nematic mesophases showed medium to strong MW effect for polymers of spacer length m = 1−8 with 10−50 °C difference of clearing temperatures between high-DP polymers Pm-50 and oligomers Pm-10, while no MW effect exhibited for polymers of spacer length m > 9 owing to their nearly completely decoupling. Surprisingly, overall the clearing temperatures decreased from P1-n to P8-n and then to Pm-n with longer spacer m ≥ 9 of nearly completely decoupling polymers showing clearing points very close to or even merged with their melting temperatures. Especially the clearing temperatures declined monotonically in subdivisional groups of the same stacking mode for all the coupling systems with spacer length m = 1−8. For two-column bundles organization subdivision, the clearing temperatures decreased from 220 °C of P1-50 to 145 °C of P2-50 and showed about 50 °C higher than that of their corresponding oligomers. For three-column bundles subdivision, during second heating the clearing temperatures showed 10−15 °C temperature difference between polymers and their corresponding oligomers and decreased from 154 °C of P3-50 to 127 °C of P4-50, and then to 70 °C of P5-50 with extended spacer length (Figure 6b and Table 1). For four-column bundles subdivision of Pm-n with m = 6−8, the clearing temperatures displayed modestly monotonic decrease with the spacer length increasing and showed a decline of about 30 °C for corresponding oligomers (Figure 6a,b). Such monotonic decreasing tendency of clearing temperatures in a subdivisional group of polymers of the same coordination number multicolumn bundles organization was presumably ascribed to the gradually reduced constraint with extension of spacer as discussed in the previous sections. It is worth to note that as another turning point besides the P6-n series of the whole family DLCPs with regard to the spacer length influence, the phase behaviors of P9-n samples behaved some uniqueness. During cooling and second heating runs, the P9-n series exhibited no MW dependence and showed double melting transition peaks the same as those of P10-n and P14-n series with longer spacers of almost completely decoupling systems. However, after a stepwise cooling process, high-DP polymer P9-50 showed six-column bundles-based superlattice and presented a columnar nematic mesophase with superlattice

tion. The disclosed DCS-based stacking mode might be universal for other discogens containing discotic LCPs and thus greatly enriched their supramolecular structures for various potential applications. Furthermore, such individual discotic units or discrete aromatic stacks-based assembly mimicked in some way the hierarchical self-organization found in nature, such as the arrangement of preassembled biological macromolecular subunits into complex and functional superstructures.11 The π−π stacking interactions of aromatic rings have been proved to be of crucial importance in both chemical and biological recognition and supramolecular assemblies.1a Aromatic interactions were commonly employed in engineering and modulating the self-assembled structures in liquid crystalline phases and organic crystals21 and also accounted for the protein structure stabilization1b and charge transfer in DNA.1c Influence of Spacer Length on Thermotropic Phase Behaviors and Positive Coupling Effect (PCE) in SideChain DLCPs. Figure 6 shows the phase transition temperatures and their change trends as a function of the spacer length. It is very interesting to notice that the melting temperatures during heating after a stepwise cooling process were all around 40 °C except P1-n of the shortest spacer without thermal transitions in DSC measurements similar to that of the discotic mesogen-jacketed polymers P0-n (Figure 6a). Such melting transition temperatures were quite near the melting temperatures of their corresponding crystalline monomers (Table S2), which were attributed to partial disorganization (π−π stacking modulation of TP discogens or column breakup due to nonpersistent extending conformation of polymer backbone with a longer spacer) of various columnar phases with enhanced intracolumnar order some similar to crystalline columnar phases. While the melting temperatures during second heating immediately after the first cooling process at 10 °C min−1 were all around 30 °C except that Pm-n with shorter spacers m = 1−5 did not show a melting transition temperature due to their stronger coupling interactions between discogens and polymer backbone and slow kinetics of column formation, and P6-50 displayed a very high value of 83 °C due to its unique structure and marked MW effect (Figure 6b). More interestingly, the isotropization temperatures J


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Macromolecules ordering similar to that of the adjacent homologous Pm-n samples with shorter spacers m = 6−8. Such general decreasing tendency of isotropization temperatures for the whole homologous DLCPs especially a monotonic decline for the subdivisional group samples of the same organization mode was in sharp contrast to the trend exhibited in their counterparts of side-chain calamitic LCPs,5 where overall virtually unchanged clearing temperatures for nematic phase were usually observed or even displayed a modest increase for smectic mesophases often showing odd− even effect with the elongation of flexible alkyl spacer.14a,22 More important and inspiring, as discussed in previous context all side-chain DLCPs of shorter spacers with the spacer length m ≤ 9 displayed various well-organized 2D columnar superlattices showing quite strong characteristic π−π stacking peaks in the wide-angle area of X-ray analysis only with the exception of discotic mesogen-jacketed P0-n without any flexible spacer, while those with longer spacers m > 9 exhibited ill-defined columnar mesophases. Therefore, the effective coupling between side-chain discogens and polymer backbone for DLCPs of shorter spacers favored the π−π stacking of sidechain TP discogens and resulted in well-organized columnar mesophases, while obvious even almost complete decoupling in those with longer spacers m > 9 led to weakened π−π stacking and, if any, columnar mesophases of low order. It indicated that suitable coupling with a short spacer equivalent to or shorter than the side chains of TP discogens was strongly preferred except the discotic mesogen-jacketed P0-n with too strong coupling to form π−π stacking of TP side groups. Thus, in sharp contrast with the negative coupling effect (NCE) presented in calamitic side-chain LCPs where decoupling was the primary goal and constituted the core idea of renowned classical spacer concept,5,13 here for side-chain DLCPs positive coupling effect (PCE) at work and suitable coupling between discogens and polymer backbone was definitely required and preferred for well-organized columnar mesophases creation. The distinct differences exhibited by side-chain DLCPs compared with their calamitic counterparts might mainly result from the strong contrast of 1D columnar stacking of discotic mesogens and thus caused polymer backbones’ sterically constrained within the small channels of the intercolumnar regions, with the 2D lamellar arrangements in smectic phases of calamitic mesogens with affiliated main chains in random coil retaining considerable conformational freedom.6c Although with some loss of high mobilities along the column axis compared with their unlimited counterpart low molar mass discotic molecules, the tethering to a backbone will restrict the rotation angle to some extent and lead to greater electronic coupling,23 and moreover, TP discogen columns can be readily aligned along the main chain in an almost perfect way.6b,10 Considering that the identical orientation arrangement inside the superlattice with less perturbation during shearing alignment may offer an intrinsic advantage for the columnar phase alignment and device performance, thus presumably that the first polymer of each organization mode subdivision such as P1n for two-column, P3-n for three-column, and P6-n for fourcolumn bundles-based columnar superlattice may be the best choice as candidate for high performance polymer semiconducting materials for various optoelectronic applications, especially for organic field effect transistors (OFETs) for their facilely achieved uniform planar alignment.10 The device fabrication and properties study based on some promising

DLCP materials is underway in our laboratory in close collaboration with colleagues of optoelectronic expertise. Close Correlation between Contraction Behavior in Solution and Bulk Columnar Stacking Structure of SideChain DLCPs of Variant Length Spacers. As aforementioned, the MWs of high-DP polymers Pm-50 showed a remarkable underestimation by GPC (Figure 1d), with all the ratio Mtheor/Mn,GPC larger than 2 even over 5 for the polymers of medium length spacers showing the most notable deviation. Considering that the high-DP polymers Pm-50 of variant length spacers self-organized into hierarchical structures based on single-chain supercylinders in most cases in the bulk state as discussed above, thus some close relationship with their solution behavior of usually involving individual polymer chains could be envisioned. In general, as shown in Figure 1d the ratio of Mtheor/Mn,GPC reflecting the degree of contraction behavior of high-DP DLCPs in solution correlated closely with their various self-organized structures in bulk with elongated spacer length (Figure 5). Such as the discotic mesogen-jacketed polymers P0-50 and P1-50 showed modest deviation in accordance with their almost rigid structure of the polymer chain as a whole. Whereas the Pm-50 of medium length spacers with m = 2−5 exhibited extraordinary deviation, manifesting the strongest contraction behavior in solution in relation to the highest ordered columnar structures revealed by SAXS analyses and their individual discogens-based intracolumnar stacking mode in bulk. Then similar contraction performance revealing relatively mild conformational adjustment in solution for Pm-50 with m = 6−9 was ascribed to the same stacking mode based on preorganized DCS showing modest deviation. Further, P10-50 and P14-50 exhibited the weakest contraction behavior due to the nearly complete decoupling of discotic side groups from the backbone enduing them increased freedom and alleviative deviation. Thus, the hydrodynamic behavior in solution of highDP side-chain DLCPs of variant length spacers manifested close correlation with their bulk structure evolution as summarized in Figure 5 and also indirectly confirmed their distinct columnar superstructure organizations in each subdivision based on variant intracolumnar stacking modes.

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CONCLUSIONS In summary, based on the well-controlled synthesis of a homologous series of side-chain DLCPs Pm-n with continuously increased spacer length m = 0−10, 14 and DP n = 10 and 50 for each spacer group of suitably flexible polyacrylate backbone, a systematic comparative study has been conducted to provide an in-depth insight into the influence of spacer length and also MW effect. The series DLCPs displayed modest to strong MW effect for those with shorter spacers, while no MW effect with longer spacers of m > 9. All polymers of shorter spacers with m ≤ 9 displayed various well-organized columnar superlattices based on multicolumn bundles organization with “coordination number” from two to six through individual discogens or DCSbased intracolumnar stacking modes, only except the discotic mesogen-jacketed P0-n without a flexible spacer, while those with longer spacers m > 9 of almost completely decoupling displayed ill-defined columnar mesophases. Therefore, the positive coupling effect (PCE) prevailed, and proper coupling between side-chain discogens and polymer backbone was desirable and required for achieving well-organized columnar mesophases for side-chain DLCPs, in striking contrast with the renowned classical spacer decoupling concept for side-chain calamitic LCPs. These findings enhance a better understanding K


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Macromolecules

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of self-assembly especially for aromatic interaction involved complex systems and may boost rational design and synthesis of well-defined side-chain DLCPs and further advance their low-cost solution processable device fabrication and various promising optoelectronic applications.

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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.5b01510. Full experimental details and characterization data for monomers and corresponding polymers, representative POM textures, and variable temperature SAXS/WAXS profiles and indexing assignments (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +86-25-83686621; Fax +86-2583317761 (D.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21574062 and 20874044) and also partially by the Program for Changjiang Scholars and Innovative Research Team in University and the National Science Fund for Talent Training in Basic Science (No. J1103310).

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REFERENCES

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