Self-Assembly Behavior of Triphenylene-Based Side-Chain Discotic

Jul 10, 2017 - By checking conformation of molecules, the intracolumnar self-assembling patterns based on the discrete ... Brown, Seo, Sides, and Hall...
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Self-Assembly Behavior of Triphenylene-Based Side-Chain Discotic Liquid Crystalline Polymers Minqing Gong,† Qiuyan Yu,† Shiying Ma,†,‡ Fang Luo,† Rong Wang,*,† and Dongzhong Chen*,† †

Department of Polymer Science and Engineering, Key Laboratory of High Performance Polymer Materials and Technology of Ministry of Education, State Key Laboratory of Coordination Chemistry and Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210023, China ‡ College of Chemistry and Chemical Engineering, Taishan University, Taian 271021, China S Supporting Information *

ABSTRACT: We constructed a generic coarse-grained model of triphenylene-based side-chain discotic liquid crystalline polymers (SDLCPs). Then dissipative particle dynamics (DPD) simulation was employed to systematically study how composition and structural factors of SDLCPs such as molecular weight, main chain, spacer and aliphatic tails, and the incompatibility between mesogenic core and its substituents influence their mesophases and self-assembly behavior. Eight mesophases were obtained by changing the factors mentioned above. The eight phases are hexagonal columnar−amorphous (Colh-Am), nematic columnar−amorphous (Colne-Am), nematic columnar−clustered (Colne-Clu), nematic columnar−columnar (Colne-Col), random columns−amorphous (Colran-Am), random columnar−clustered (ColranClu), amorphous−amorphous (Am-Am), and sphere−amorphous (Sph-Am). The name of mesophase is denoted as “assembly of discogens-aggregation of backbone”. By checking conformation of molecules, the intracolumnar self-assembling patterns based on the discrete columnar stack (DCS) were observed in Colh-Am, Colne-Am, and Colne-Clu, while Colne-Col and Sph-Am adopted different packing modes. Moreover, molecular weight effect and positive coupling of spacer were discolosed, which matches well with experiments. Moderate or strong incompatibility between mesogenic cores and substituents and proper peripheral aliphatic tails are needed for SDLCPs to form ordered mesophases. The in-depth understranding of their superstructures may offer inspiring guidance for rational polymer design, preparation, and further exploration in optoelectronic field.

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diodes, and so forth.2,15,16 In the simplest case, the principal axes (normal to the plane of the disk) of individual discotic mesogens align to a certain degree along a common direction to form nematic phase (ND). More interestingly, discotic mesogens also tend to pile up into extended columns to form various columnar phases, which can be distinguished by the order of discogens within columns (disordered, ordered, tilted) and the two-dimensional (2D) symmetry of the arrangement of columns (hexagonal, rectangular, oblique).17−20 Therefore, to modulate structural order and realize alignment are of profound significance for achieving a high DLC-based device performance. Liquid crystalline polymers combine the attributes of liquid crystals with good processability and mechanical properties of polymers, while DLC polymers are underdeveloped in contrast to their intensively studied calamitic counterparts. The challenge in controlled preparation of DLC polymers and

INTRODUCTION The rodlike mesogenic molecules have become a generally accepted principle in liquid crystal research since Daniel Vorländer1 reported that “the crystalline liquid state results from a molecular structure which is as linear as possible”.2 However, Chandrasekhar et al. reported the work on hexaesters of benzene which is probably the first observation of thermotropic mesomorphism in pure, single-component systems of relatively simple disklike molecules.3 Their pioneer research opened up a whole new field of fascinating liquid crystal research and drew particular attention on discotic liquid crystal (DLC).4,5 The typical DLC is composed of a more or less rigid planar core such as benzene,3,6 triphenylene,7−9 thiophene,10 naphthalene,11 phthalocyanine,12 and hexabenzocoronene13 with usually six or more flexible chain substituents laterally attached to the core.2 Because of the π−π interaction of rigid aromatic cores and the secondary interactions of the peripheral chains, they can self-assemble into various ordered columnar phases.14 DLCs with ordered columnar mesophases are potential candidates for applications in molecular wires, photovoltaic solar cells, field effect transistor, light-emitting © XXXX American Chemical Society

Received: March 30, 2017 Revised: June 23, 2017

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Figure 1. Three representative triphenylene-based side-chain discotic liquid crystalline polymers (SDLCPs) denoted as Pl-n (l stands for the methylene number of the spacer and n for DP), with different polymer backbone of (a) rigid polythiophene,24 (b) rigid polyacetylene,25 and (c) flexible polyacrylate.17,21

of l = 1−14 revealed that various well-organized columnar superlattices were constructed based on multicolumn bundles organization with “coordination number” from 2 to 6 through individual discogens or DCS based intracolumnar selforganization. This organization behavior and phase structure evolution manifested that proper coupling between backbone and discogens was desirable or necessary for SDLCPs to form ordered clumnar phases, and the positive coupling effect was proposed for SDLCPs first, which is in sharp contrast with decoupling principle prevailed in their calamitic counterparts for several decades.21 Based on the cognition of molecular weight effect and positive coupling effect for SDLCPs, cyclic polymers with spacer length l = 2, 6 and variant DP were prepared via a one-step intrachain cyclization. The obtained polymers exhibited largely frustrated thermotropic liquid crystalline behaviors in the bulk state while remarkably enhanced fluorescence in solution due to cyclic topological constraint.26 This study gave a deep insight into the cyclic topological effect, which may act to supplement the aggregation-induced emission (AIE) phenomenon and provide as a guide in rational designing and preparing functional cyclic polymers bearing various side groups. Besides, a group of diblock copolymers were prepared through polymer reaction by introducing different length flexible block poly(ethylene glycol) (PEG) segments into triphenylene-based side chain liquid crystalline polyacrylate blocks to modulate mesophases and examine the interplay between liquid crystalline order and morphology.27 The evolution of microphase-separated superstructures and the close correlation between overall morphologies and discotic mesogenic orders as a function of the DLC weight fraction ( f w,DLC) were presented. The copolymers with lower f w,DLC formed lamellar structures of variant periods at different temperature ranges, while those with higher f w,DLC generated hexagonal columnar structure, nematic columnar, or nematic discotic phases.27 Compared with polythiophene (Figure 1a) or polyacetylene (Figure 1b), polyacrylate (Figure 1c) has suitable flexibility. It should be more general and representative for SDLCPs, and it exhibited much enhanced optoelectronic device performance for their significantly promoted side-chain discogens packing.28 In this work, we conducted dissipative particle dynamics (DPD) simulation29−31 on the generic coarse-grained SDLCP model by changing a number of molecular parameters (DP, length of main chain repeat unit, spacer, aliphatic tails, and the interaction between different components); eight morphologies and six phase diagrams were obtained. Phase transitions

especially lacking systematic work pertinent to some fundamental questions such as molecular weight effect, spacer length influence, and substituents influence may largely account for such a situation.17,21 For DLC polymers, the discogens are either incorporated into the polymer main chain or attached as side groups through flexible spacer.22 Our interest lies in sidechain discotic liquid crystalline polymers (SDLCPs). SDLCPs serve as an intriguing polymer model to explore the molecular composition and structural factors that affect their self-assembly behavior.14,23 They are usually composed of four structural components: polymer backbone, flexible spacer, discotic mesogenic core, and the peripheral substituents or alkyl tails (Figure 1). Over the past several decades, triphenylene and its derivatives are among the most extensively investigated and particularly promising discotic mesogens arousing continuing attention.5,21 Thahar-Djebbar et al.24 reported lamella− columnar mesophases of polythiophene with triphenylenebased side groups (Figure 1a), in which discotic mesogens selfassembled into a 2D oblique columnar lattice; then layers were developed by the alternation of segregated polythiophene together with aliphatic tails and spacers. Yu et al.25 studied triphenylene-based side-chain polyacetylene polymers (Figure 1b) with short spacer and found that each individual polymer possessed columnar shape and all of them packed into a hexagonal columnar (Colh) phase. The Colh phase was found to be of higher thermal and chemical stability, thanks to the socalled “jacket effect” of bulky side groups tethered through short spacers. Chen’s group have successfully prepared a large family of well-defined triphenylene-based liquid crystalline polyacrylate polymers Pl-n with spacer l = 0−14 and degree of polymerization (DP) n = 5−100 (Figure 1c) by reversible addition−fragmentation chain-transfer (RAFT) polymerization. Phase transition, thermodynamic properties, and kinetic behaviors were examined and analyzed in detail by applying multiple modern analysis techniques.17,21 For the series of P6-n hexyloxy substituents and with DP n = 5−100, a remarkable molecular weight effect exhibited with a sharp jump increase of phase transition temperatures from columnar phase to nematic phase with oblique columnar superlattice order at a critical DP around 20.17 The discrete columnar stack (DCS) based intracolumnar self-assembly model was proposed to account for the structure transformation, from the ordered hexagonal columnar lattice dominated by side-chain triphenylene stacking to the oblique columnar superlattice induced by compaction and ordering of the polymer backbone.17 Moreover, intensive investigation on the series SDLCPs with variant spacer length B

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fixes the other weight function, with the relation as shown in the following equation:

corroborated the molecular weight effect, positive coupling effect, and DCS based self-assembly pattern which were proposed in experiments.17,21 Besides, moderate or strong incompatibility between mesogenic cores and substituents and proper peripheral aliphatic tails are required for SDLCPs to form ordered mesophases. Such deep insights into SDLCPs are significant for rational polymer design, preparation, and application in optoelectronic field.

ωD(rij) = [ωR (rij)]2

As a simple choice, we take ω (rij) = ω(rij); i.e., ω (rij) is the same function as in the conservative force. There is also a relation between the two amplitudes and kBT:

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σ 2 = 2γkBT

SIMULATION METHOD AND MODEL The DPD method introduced by Hoogerbrugge and Koelman29 in 1992 and then improved by Español and Warren30 in 1995 is a widely used mesoscopic simulation approach.32 In DPD simulations, an individual particle represents a cluster of atoms or molecules. The use of soft potential and a reduced number of interaction sites makes it a valuable approach to describe the phase morphology and dynamics of numerous soft matter systems at mesoscale while retaining the correct hydrodynamics. The time evolution of the simulation system is governed by integrating Newton’s equation of motion:31 f⃗ dvi⃗ = i dt mi

d ri ⃗ = vi⃗ , dt

C

∑ (Fij⃗

D

Figure 2. Generic coarse-grained model of triphenylene-based SDLCPs (AkBlC4D5×m)n. k is the number of main chain beads A (colored light gray), l is the number of spacer beads B (colored cyan), m is the number of tail beads D (colored red), and n stands for DP.

R

(2)

main chain.17,21,33 There are four different types of DPD particles: A (gray), B (cyan), C (blue), and D (red) for backbone, spacer, discotic mesogenic core, and aliphatic tails, respectively. All beads have the same size, except a bead of discotic core C represents a whole benzene ring with the size of 1.2rC, and all the beads have the same mass as m = 1. Within triphenylene-based SDLCPs, neighboring beads are bonded to each other by a harmonic spring force, F = −ksr, where ks is the spring constant. We set ks = 10.0 for main chain, spacer, and tail chains, while setting ks = 100.0 for discotic mesogenic core. To keep the discotic mesogen being rigid, a harmonic bending potential is also applied between beads C, which takes the form Uθ = 0.5kθ(θ − θ0)2, where θ and θ0 are the current and specified angles formed by the adjacent bonds and kθ is a stiffness constant. Besides, the Lennard-Jones potential is applied to reflect π−π intereaction between discogens. Referring to the real system, we set the bead in the center of discogen only has interplay with the bead in the center of other adjacent discogens, and the peripheral three beads only have interplay with other adjacent peripheral beads. The time scale is set to (mrc2/ε)1/2, and the energy scale is given by kBT = 1, where kB is the Boltzmann constant and T is the temperature. We use the modified velocity-Verlet algorithm with λ = 0.65 in integrating the equation of motion. What is more, we set time step Δt = 0.03 and the amplitude of random noise σ = 3.0 to avoid divergence of the simulation. An individual polymer molecule is represented by (AkBlC4D5×m)n, where k, l, and m are the numbers of backbone beads, spacer beads, and one tail chain beads, and n represents the DP of the polymer. We change k, l, m, and n and the interaction parameters in our simulation to investigate their

The sum of force acts over all particles within a certain cutoff radius rC, beyond which the force is neglected.31 Here rC is the only length scale in the system, which is considered as the unit of length, rC = 1. The conservation force of two particles is softrepulsive interaction acting along the line of the centers of two particles: C

Fij⃗ = aijω(rij)riĵ

(3)

where aij is the interaction parameter between particles i and j. The r-dependent weight function ω(rij) provides the range of interaction for DPD particles with a commonly used choice: ω(rij) = 1 − rij/rC for rij ≤ rC and ω(rij) = 0 for rij > rC. ri⃗ j = ri⃗ − rj⃗ , rij = |ri⃗ j|, and r̂ = ri⃗ j/rij. The dissipative force which is proportional to the relative velocity, vi⃗ j = vi⃗ − vj⃗ , is defined as D

Fij⃗ = −γωD(rij)(riĵ ·vij⃗ )riĵ

(4)

where γ is the friction coefficient controlling the magnitude of the dissipative force. The random force acting as a heat source to equilibrate the thermal motion of unresolved scales is given by R

Fij⃗ = σωR (rij)θijriĵ

(7)

(1)

+ Fij⃗ + Fij⃗ )

j≠i

R

The combined effect of the dissipative and random force amounts to that of a thermostat. The generic coarse-grained model (Figure 2) was constructed based on triphenylene-based SDLCPs with flexible

where ri⃗ , vi⃗ , mi, and fi⃗ denote the position, velocity, mass of the i particle, and the force acting on it, respectively. The total force fi⃗ acting on particle i is composed of conservative force FC, random force FR, and dissipative force FD; each of them is pairwise interactions. The total force fi⃗ is given by fi ⃗ =

(6) R

(5)

where σ is the noise amplitude governing the intensity of the random force and θij(t) is a randomly fluctuating variable with Gaussian statistics: ⟨θij(t)⟩ = 0 and ⟨θij(t)θkl(t′)⟩ = (δikδjl + δilδjk)δ(t − t′). Español and Warren29 showed that the two weight functions D ω (rij) and ωR(rij) can be chosen arbitrarily, and this choice C

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Figure 3. Representation of various phases self-assembled by SDLCPs under variant conditions: (a) Colh-Am; (b) Colne-Am; (c) Colne-Clu; (d) Colne-Col; (e) Colran-Am; (f) Colran-Clu; (g) Am-Am; (h) Sph-Am. The pictures of each phase are listed in order to show all the beads (left), only discotic mesogenic core beads (middle), and only polymer backbone beads (right).

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RESULTS AND DISCUSSION Morphology of SDLCPs. Eight phases were found for SDLCPs, by changing k from 1 to 6, l from 1 to 10, n from 4 to 30, aBC (aBC = aCD) from 30 to 80, and aAj = 30 or 60 (j = B, C, D). Figure 3 shows the eight phases: (a) hexagonal columnar− amorphous (Colh-Am), with the former accounting for the assembly of discotic mesogens, while the latter for the aggregation of main chain, hereinafter the same; (b) nematic columnar−amorphous (Colne-Am); (c) nematic columnar− clustered (Colne-Clu); (d) nematic columnar−columnar (ColneCol); (e) random columnar−amorphous (Colran-Am); (f) random columnar−clustered (Colran-Clu); (g) amorphous− amorphous (Am-Am); (h) sphere−amorphous (Sph-Am). The discotic mesogens in Colh-Am (Figure 3a) stack irregularly within columns, while the axes of columns orienting along a certain direction and arranging in hexagonal lattice. The top view and the structure factor (S(q)) for a Colh-Am are given in Figure S1. Those phases with columns adopting an irregular arrangement deviating from the hexagonal lattice or only with the axes of columns orienting along a common direction are called Colne (Figure 3b−d). Colran are referred to those phases with the axes of short or long discogen columns orienting randomly (Figure 3e,f). Amorphous of discogens (Figure 3g) which distribute uniformly in space occurs when discotic mesogenic core is compatible with its substituents. The conformation of Sph-Am (Figure 3h) will be discussed in detail in the next section. The conformation of main chain is sorted into amorphous (Am), clustered (Clu), and columnar (Col). Amorphous backbone (Figure 3a,b,e,g) means that

self-assembly behaviors. The interaction parameters chosen are shown in a symmetric matrix i or matrix ii: ⎧ ⎪ ⎪A ⎪ aij = ⎨ B ⎪C ⎪ ⎪D ⎩

A B 25 30 30 25 30 aCB 30 25

C D ⎫ ⎪ 30 30 ⎪ ⎪ aBC 25 ⎬ (i); 25 aCD ⎪ ⎪ aDC 25 ⎪ ⎭

⎧ ⎪ ⎪A ⎪ aij = ⎨ B ⎪ ⎪C ⎪D ⎩

A B 25 60 60 25 60 aCB 60 25

C D ⎫ ⎪ 60 60 ⎪ ⎪ aBC 25 ⎬ (ii) ⎪ 25 aCD ⎪ aDC 25 ⎪ ⎭

When we fix the three-dimension simulation with a fixed system number density of 3.0, the relationship between the aij and Flory−Huggins interaction parameters χij is aij ≈ aii + 3.947χij

(8)

We set aii = 25 for the same type of DPD particles (i = A, B, C, D), guaranteeing the correct excluded volume of the molecules. aij rises from 25 with increasing the incompatibility between particles i and j. Considering the relative position and chemical components of all the beads, we always keep aBD = 25 and aAB = aAC = aAD = 30 or 60. Except for n = 30, we performed the dynamics of total 24 000 DPD beads in a cubic box (203) under the periodic boundary conditions. In these conditions, each simulation takes at least 1.5 × 106 steps for equilibrium, so we carried out 2.0 × 106 steps for each simulation. When n = 30, we set the length of box as 27.14, keeping the same number of molecules as (A1B2C4D5×2)12 in the box lengthed 20, ensuring that we can observe several whole polymer chains in the box without exceeding boundaries. And we run 3.0 × 106 steps for n = 30. D

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Figure 4. Schematic stacking models of molecular conformations and self-assembly modes. Only the backbones (white), discogen cores (blue), and tails (red) are shown. (a) The discrete columnar stack taken by columns from most Colh-Am, Colne-Am, and Colne-Clu. (b) Conformation from certain Colh-Am, in which a single polymer molecule (the inset) forms one subunit and then self-assembles into ordered hexacolumnar phases. (c) Conformation forming 2−4 subunits from Colh-Am, Colne-Am, and Colne-Clu. (d) Conformation forming 2−6 subunits from Colne-Col.

backbone takes random coil conformation. Clustered backbone (Figure 3c,f) is the conformation when main chain aggregates tightly and thus causes the axes of columns cannot parallel so well to form Colh. What is more, when the fraction of main chain is high enough with appropriate repulsive interactions, backbones get together to form columns with discogen columns around them (Figure 3d), generating a complex phase denoted as Colne-Col. The conformation of a single molecule or several molecules in box shows that a polymer in Colh-Am, Colne-Am, and ColneClu tends to appear in neighboring columns by stacking into 2−4 subunit columns (Figure 4a,c), which then self-organize into long-range columns. And each subunit contains discogens ranging from 1 to x (x < n). Our observation agrees well with the experimental results and theoretically validates the proposed DCS stacking model.17 But in a certain columnar phases we also find a single molecule can stack into short columns to form long-range columnar phase (Figure 4b). This differs from Figure 4a with the backbone surrounding the columns. Colne-Col takes DCS based self-assembly pattern as well, but it is special with several main chains piling in center surrounded by many discogen columns. In the next section, we will discuss the factors affecting phase transition. Molecular Weight Effect. In experiments, the molecular weight effect is shown by the dependence of phase transition temperatures vs DP.17 We studied molecular weight effect by the phase diagram versus n (DP) and aBC as shown in Figure 5.

Figure 5. Phase diagram for (A1B2C4D5×2)n by changing DP n from 4 to 30 and aBC from 30 to 80 with other repulsive parameters as shown in matrix i, and aBC = aCD.

Repulsion parameters were set as values shown in matrix i, in which aAj = 30 (j = B, C, D) (which represents low incompatibility between the main chain and the other parts) of (A1B2C4D5×2)12, and changing aBC (= aCD) (which represents incompatibility between discogen and its substituents) from 30 to 80 and number n for DP from 4 to 30. According to eq 8, χij is the major factor of the value of aij; however, χ ij is proportional to 1/T, so the larger a ij corressponds the lower temperature generally. E

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Macromolecules Figure 5 shows the phase diagram as a function of n and aBC; when aBC (= aCD) are fixed at 30 and 40, discogens cannot gather to form Clu not to mention Colne or Colh, except when aBC = aCD = 40 and n ≥ 20, discogens form Colh. As aBC > 40 and n > 4, the stable phase is Colne or Colh. Thus, it means that proper or strong incompatibility between mesogenic cores and its substituents is needed for the formation of Colne or Colh. When we fix aBC = aCD > 50, and n ≤ 8, polymers self-assemble into Colne, while n ≥ 12, they all form Colh. While n = 4, the phase is sort of distorted from Colne with several spheres appearing at the boundary domains. These spheres are made of several molecules with discogens inside and other parts covering them. As an individual bead represents a cluster of atoms, the molecules with n ≥ 8 can be considered as polymers, while n < 8 as oligomers. The phase transition occurs at aij = 50 for n = 8−16 and aij = 40 for n = 20, 24. Perhaps, we can say n = 20 is the critical DP for columnar phase transition. This confirms the molecular weight effect concluded from experimental investigations.17 We change n and l to explore whether molecular weight effect exists in polymers with variant spacer lengths (Figure 6).

value. Rectangular columnar formed when DP was larger than 34, while amorphous remained when DP was lower than 34. In contrast, all those polymethacrylates with a six-methylene spacer exhibited Colh phase of higher symmetry independent of DP. Influence of Spacer. Figure 7 presents polymer (A1BlC4D5×2)12 with changing the spacer length l from 1 to

Figure 7. Phase diagram for (A1BlC4D5×2)12 by changing the spacer length l from 1 to 10 and aBC from 30 to 80 (aBC = aCD) with other interaction parameters as shown in matrix i.

10 and aBC (= aCD) from 30 to 80. When aBC (= aCD) ≤ 40, the discogen core is compatible with its six substituents to some extent. So they all form Am-Am; except when aBC = aCD = 40 and l = 1, it forms Colh-Am. While aBC = aCD = 50, 60 and l ≤ 2, polymers form Colh-Am; when 2 < l ≤ 6, Colne-Am is formed, and only Colran-Am is obtained when l > 6. As aBC (= aCD) increases to 70 and 80, the phase transition from ColneAm to Colran-Am occurs at l = 8 and l = 10, respectively. On the one hand, these results indicate that the more incompatible the discogen core to its substituents is, the longer spacer it can endure to form ordered columnar phases. On the other hand, they tell that polymers with shorter spacers favor the 2D lattice arrangement of columns, which corroborates the positive coupling effect for SDLCPs as proposed based on experimental results.21 In fact, the concept of spacer was first proposed by Finkelmann et al. in 1978, and the function of spacer was proposed to decouple the motions of polymer main chain from that of mesogenic side groups.22 The positive coupling effect is in stark contrast to the renowned classical spacer decoupling principle taking effect in side-chain calamitic (rodlike) liquid crystalline polymers.7,22,34,35 The positive coupling effect has been well demonstrated by results of side-chain DLC polacrylate polymers.21,33 In 1993, Werth and Spiess reported some peculiar phase behaviors based on polysiloxane polymers with side-chain triphenylene mesogens attached through variant lengths of spacer. Their research manifested that the phase transform from Colh to Colne with the spacer length increasing.33 It was concluded that the appropriate length of spacer contributed to the formation of mesophases with high order, while too long spacers may breakdown the 2D-ordered columnar phase.33 The phase transition denoted as Colh-Am → Colne-Am (aBC = aCD > 40) in our work matches the above experimental observations well. In order to further illustrate the positive coupling effect between the polymer backbone and discotic mesogens, we

Figure 6. Phase diagram for (A1BlC4D5×2)n by changing the DP n from 4 to 30 and the spacer length l from 1 to 10 and fixing interaction parameters as shown in matrix i, and aBC = aCD = 60.

For polymers with shorter spacer l ≤ 2, the phase transition from Colne-Am to Colh-Am occurs at n = 12. While l = 3, 4, and 5, the phase transition from Colne-Am to Colh-Am occurs at n = 16, 20, and 24, respectively. And while l ≥ 6, polymers do not form Colh-Am. This result reveals that polymers with shorter spacers have modest or strong molecular weight effect, while those with longer spacer l ≥ 6 have no or weak molecular weight effect. Our simulation results match well with the conclusion obtained from experiments based on well-defined side-chain DLC polyacrylate polymers Pl-n by Mu et al.21 What is more, when the spacer is too short, like l ≤ 2, only the phase transition Colne-Am → Colh-Am can be observed, while for l = 3−5, the phase transforms through Colran-Am → Colne-Am → Colh-Am, and for l ≥ 6, the phase transformation is Colran-Am → Colne-Am. That shows SDLCPs with medium length spacer are likely to exhibit more mesophases. Ban et al.14 have synthesized two series of triphenylene-based side-chain DLC polymethacrylate polymers: one is a mesogen-jacketed polymer without spacer with DP from n = 11 to 128 and the other one with a six-methylene spacer and with DP from n = 11 to 80. They concluded that the phase behavior of the mesogenjacketed polymethacrylates was strongly dependent on the DP F

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form Colran. And when l is larger than 3, columns’ arrangement tends to be more disorder than those with l shorter than 3. While k ≥ 2, all polymers self-assemble into the Colne-Col phase. It gives a hint for researchers to explore further. The packing mode of Colne-Col is shown in Figure 4c. The striking differences of influence of spacer lengths in SDLCPs and side chain calamitic liquid crystalline polymers may come from the differences of assembly fashion and orientation of discotic and calamitic mesogens.21,33 Influence of Tails. Figure 9 shows the phase diagram of polymer (A1BlC4D5×m)12 with changing the spacer length l from

carried out simulations on polymer (AkBlC4D5×2)12 by changing the spacer length l from 1 to 10, k from 1 to 6 and fixing interaction parameters as shown in matrix i, and aBC = aCD = 60 (Figure 8a). When k = 1, 2, with l increasing from 1 to 6, the

Figure 9. Phase diagram of model SDLCP with composition of (A1BlC4D5×m)12 by changing the spacer length l from 1 to 10 and m from 1 to 6 and fixing interaction parameters as shown in matrix i, and aBC = aCD = 60.

1 to 10, the tail beads number m from 1 to 6 with fixing interaction parameters shown in matrix i, and aBC = aCD = 60. It can explicate the influence of tail length and symmery of discogens. The Colh-Am phase occurs when m ≥ l (l ≤ 2), except the polymer with m = 1 and l = 2 (m < l). When l = 1, 2, they both change from Colh-Am to Sph-Am, at m = 6 and m = 5, respectively. When l = 3−6 and m = 1, polymers display the Colran-Am phase while form Colne-Am as m = 2, 3 and exhibit Sph-Am at m > 3. As l is longer than 7, they all transform from Colran-Am to Sph-Am at m = 3. Polymers with m = 1 are more likely to form Colran-Am than those with longer tails. That is because m = 1 is too short to offset backbone’s interferences on packing of discogens.4 While for m > 3, too long tails from neighboring molecules are interdigitated with each other and thus frustrate polymers to form long columns. Therefore, we can conclude that polymers possessing medium length of flexible tails can exhibit more ordered mesophases. Figure 10 shows the Sph-Am phase and its related conformation. Spheres are distributed in high order (Figure 10a,b). By analyzing conformation, each polymer chain in SphAm form two small clusters (Figure 10c) and each sphere is the combination of two small clusters from two neighboring molecules (Figure 10d). These results are different from conclusions obtained by Xing et al. based on rigid backbone polymers.4 Their research on poly(1-alkyne)s carrying triphenylene discogens showed polymers with short and long alkyl substituents possessed a homogeneous hexagonal columnar structure, while those with intermediate ones formed mesophases with mixed structures.4 Or we can discern the SphAm phase from another point. The balance between mobility offered by substituents and π−π interaction of discogen cores needed by ordered columnar phases is breakdown by increased

Figure 8. Phase diagrams of model SDLCP with composition of (AkBlC4D5×2)12 by changing the spacer length l from 1 to 10 and k from 1 to 6 and fixing interaction parameters as shown (a) in matrix i, and aBC = aCD = 60, and (b) in matrix ii, and aBC = aCD = 70.

phase transition evolves as Colh-Am → Colne-Am → Colran-Am. When k = 3, 4, amorphous backbones transform into clustered at l = 5, 4, respectively. When k = 5, 6 and l ≥ 4, microphase separation takes place for backbones, and they aggregate into columns. Around with the backbone columns are nematic mesogenic columns, forming Colne-Col. Furthermore, as k = 6, the phases have two domains perpendicular to each other. This type of columnar phase is reminiscent of those complex perpendicularly oriented double hexagonal phases experimentally observed in polypeptide-based block copolymers.36 However, experimental examples have not yet been reported directly from SDLCPs. Therefore, it is concluded that long main chains disfavor the formation of Colh-Am, and short main chains contribute to the formation of Colh-Am. It is a direct evidence for positive coupling effect between the backbone and discogens. Well, as the coupling phenomena were shown in different types of polymers, we also studied the (AkBlC4D5×2)12 by changing l from 1 to 10 and k from 1 to 6, with interaction parameters setting as matrix ii, and aBC = aCD = 70 (Figure 8b). In this situation, backbones always tend to form clusters or columns. When k = 1, with l increasing, only Colran-Clu phase is formed. It is the compaction of main chain that “pull” Colne to G

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Figure 10. Sph-Am phase and its related conformation, formed by model SDLCP with composition of (A1B2C4D5×5)12 when interaction parameters are fixed as shown in matrix i and aBC = aCD = 60. (a) Sph-Am phase with showing all beads. (b) Ordered arrangement of discogen core spheres. (c) Conformation of a single polymer molecule. (d) Conformation of two neighboring molecules. The inserted conformation only shows the backbone and mesogenic core beads in (c, d).

mobility introduced by too long tails.37 When m = l, polymers do not have priority to form ordered columnar phases, saying the symmetry of substituents has little effect on the selforganization of polymers.

predict that SDLCPs with larger discogens, short backbone repeat unit, proper length of spacer, and tails are more preferable and possess higher propensity to form ordered columnar phases. The flexibility of main chain may be another significant factor that affects the self-assembly and phase orders, we will figure it out in our follow-up work.

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CONCLUSIONS In summary, we constructed a generic model of triphenylenebased SDLCPs with composition of (AkBlC4D5×m)n and variable values for k, l, m, n and interaction parameters aBC (= aCD). Eight different phases and six phase diagrams were obtained. The DCS-based intercolumnar self-assembly mode has been demonstrated in distinct columnar phases Colh and Colne. Besides, according to the phase diagram versus DP n and aBC (aCD), strong incompatibility between mesogenic core and its substituents is needed, and molecular weight effect is remarkable at around n = 20. The phase diagram versus DP n and the spacer length l further reveals that modest or strong molecular weight effect takes effect for polymers with shorter spacers, while weak or no molecular weight effect displays for those containing too long spacers. The phase evolutions versus l−aBC (aBC = aCD) and k−l (with aAj = 30, j = B, C, D) also manifest that positive coupling effect prevails in triphenylenebased SDLCPs, which is in sharp contrast to the classical decoupling principle taking effect for side-chain calamitic liquid crystalline polymers. These results match well with conclusions drawn from experiments. Moreover, we have got two special phases: one is Colne-Col from k−l phase diagram (with aAj = 60, j = B, C, D), and the other is Sph-Am from that versus m−l. The phase Colne-Col is special with dual columnar phases of discogens and backbones, and the relative positions of columns and backbones are different from other phases. The molecular conformations from phase diagram of m−l (in which k = 1, n = 12, aAB = aAC = aAD = 30, and aBC = aCD = 60) validate that tails offer the mobility for discogens. And too short tails cannot offset backbone’s interferences on packing of discogens. We can

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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.7b00655. Characteristic morphological snapshots of Colh-Am and corresponding discogen columns arrangement; calculated structure factor for Colh-Am compared with that for theoretical hexagonal lattice (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(R.W.) E-mail [email protected]; Tel +86-25-89686802. *(D.C.) E-mail [email protected]; Tel +86-25-89686621. ORCID

Rong Wang: 0000-0001-7525-1400 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21674047, 21474051, 21574062, and 21074053) and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). The numerical calculations in this paper have been done on the IBM Blade cluster system in the High H

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(20) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Discotic Liquid Crystals: A New Generation of Organic Semiconductors. Chem. Soc. Rev. 2007, 36, 1902−1929. (21) Mu, B.; Pan, S.; Bian, H.; Wu, B.; Fang, J.; Chen, D. WellOrganized Columnar Superlattices via Positive Coupling between Polymer Backbone and Discotic Side Groups. Macromolecules 2015, 48, 6768−6780. (22) Finkelmann, H.; Ringsdorf, H.; Wendorff, J. H. Model Considerations and Examples of Enantiotropic Liquid-crystalline Polymers - Polymereactions in Ordered Systems 0.14. Makromol. Chem. 1978, 179, 273−276. (23) Stewart, D.; McHattie, G. S.; Imrie, C. T. Convenient Synthesis of a Discotic Side Group Liquid Crystal Polymer. J. Mater. Chem. 1998, 8, 47−51. (24) Tahar-Djebbar, I.; Nekelson, F.; Heinrich, B.; Donnio, B.; Guillon, D.; Kreher, D.; Mathevet, F.; Attias, A. Lamello-Columnar Mesophase Formation in a Side-Chain Liquid Crystal pi-Conjugated Polymer Architecture. Chem. Mater. 2011, 23, 4653−4656. (25) Yu, Z.; Lam, J. W. Y.; Zhao, K.; Zhu, C.; Yang, S.; Lin, J.; Li, B. S.; Liu, J.; Chen, E.; Tang, B. Z. Mesogen Jacketed Liquid Crystalline Polyacetylene Containing Triphenylene Discogen: Synthesis and Phase Structure. Polym. Chem. 2013, 4, 996−1005. (26) Mu, B.; Li, Q.; Li, X.; Pan, S.; Zhou, Y.; Fang, J.; Chen, D. Cyclic Polymers with Pendant Triphenylene Discogens: Convenient Synthesis and Topological Effect on Thermotropic Liquid Crystal Behavior and Fluorescence Enhancement. Polym. Chem. 2016, 7, 6034−6038. (27) Wu, B.; Mu, B.; Wang, S.; Duan, J.; Fang, J.; Cheng, R.; Chen, D. Triphenylene-based Side Chain Liquid Crystalline Block Copolymers Containing a PEG block: Controlled Synthesis, Microphase Structures Evolution and Their Interplay with Discotic Mesogenic Orders. Macromolecules 2013, 46, 2916−2929. (28) Mu, B.; Hao, X.; Chen, J.; Li, Q.; Zhang, C.; Chen, D. Discotic Columnar Liquid Crystalline Polymer Semiconducting Materials with High Charge-Carrier Mobility via Rational Macromolecular Engineering. Polym. Chem. 2017, 8, 3286−3293. (29) Hoogerbrugge, P. J.; Koelman, J. Simulating Microscopic Hydrodynamic Phenomena with Disspative Particle Dynamics. Europhys. Lett. 1992, 19, 155−160. (30) Espanol, P.; Warren, P. Statistial-Mechanics of Disspative Particle Dynamics. Europhys. Lett. 1995, 30, 191−196. (31) Groot, R. D.; Warren, P. B. Dissipative Particle Dynamics: Bridging the Gap Between Atomistic and Mesoscopic Simulation. J. Chem. Phys. 1997, 107, 4423−4435. (32) Liu, X.; Yang, K.; Guo, H. Dissipative Particle Dynamics Simulation of the Phase Behavior of T-Shaped Ternary Amphiphiles Possessing Rodlike Mesogens. J. Phys. Chem. B 2013, 117, 9106−9120. (33) Werth, M.; Spiess, H. W. The Role of the Spacer in Discotic Polymers. Makromol. Chem., Rapid Commun. 1993, 14, 329−338. (34) Finkelmann, H.; Rehage, G. Liquid-crystal Side-Chain Polymers. Adv. Polym. Sci. 1984, 60−1, 99−172. (35) Shibaev, V. P.; Plate, N. A.; Freidzon, Y. S. Thermotropic Liquid Crystalline Polymers. I. Cholesterol-Containing Polymers and Copolymers. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1655−1670. (36) Klok, H. A.; Lecommandoux, S. Supramolecular Materials via Block Copolymer Self-assembly. Adv. Mater. 2001, 13, 1217−1229. (37) Allen, M. T.; Diele, S.; Harris, K.; Hegmann, T.; Kariuki, B. M.; Lose, D.; Preece, J. A.; Tschierske, C. Intermolecular Organisation of Triphenylene-based Discotic Mesogens by Interdigitation of Alkyl Chains. J. Mater. Chem. 2001, 11, 302−311.

Performance Computing Center (HPCC) of Nanjing University.

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REFERENCES

(1) Vorlander, D. The Influence of the Molecular Figure on the Crystallised-fluid State. Ber. Dtsch. Chem. Ges. 1907, 40, 1970−1972. (2) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Haegele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Discotic Liquid Crystals: From Tailor-made Synthesis to Plastic Electronics. Angew. Chem., Int. Ed. 2007, 46, 4832−4887. (3) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Liquid-crystals of Disc-like Molecules. Pramana 1977, 9, 471−480. (4) Xing, C.; Lam, J. W. Y.; Zhao, K.; Tang, B. Z. Synthesis and Liquid Crystalline Properties of Poly(1-alkyne)s Carrying Triphenylene Discogens. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2960− 2974. (5) Pal, S. K.; Setia, S.; Avinash, B. S.; Kumar, S. Triphenylene-based Discotic Liquid Crystals: Recent Advances. Liq. Cryst. 2013, 40, 1769− 1816. (6) Balagurusamy, V.; Prasad, S. K.; Chandrasekhar, S.; Kumar, S.; Manickam, M.; Yelamaggad, C. V. Quasi-one Dimensional Electrical Conductivity and Thermoelectric Power Studies on a Discotic Liquid Crystal. Pramana 1999, 53, 3−11. (7) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H. W.; Vill, V. Handbook of Liquid Crystals: High Molecular Weight Liquid Crystals; Wiley-VCH: Weinheim, Germany, 1998; Vol. 3. (8) Kumar, S. Recent Developments in the Chemistry of Triphenylene-based Discotic Liquid Crystals. Liq. Cryst. 2004, 31, 1037−1059. (9) Kumar, S. Triphenylene-based Discotic Liquid Crystal Dimers, Oligomers and Polymers. Liq. Cryst. 2005, 32, 1089−1113. (10) Cai, R.; Preston, J.; Samulski, E. Liquid-crystalline Aromatic Polyesters Derived from 2,5-Thiophene. Macromolecules 1992, 25, 563−568. (11) Praefcke, K.; Kohne, B.; Gutbier, K.; Johnen, N.; Singer, D. Syntheses and Unusual Mesophase Sequences of Hydrocarbons of a Novel Type of Non-calamitic Liquid-crystal. Liq. Cryst. 1989, 5, 233− 249. (12) Piechocki, C.; Simon, J.; Skoulios, A.; Guillon, D.; Weber, P. Discotic Mesophases Obtained from Substituted Metallophthalocyanines-toward Liquid-crystalline One-demesional Conductors. J. Am. Chem. Soc. 1982, 104, 5245−5247. (13) Herwig, P.; Kayser, C. W.; Mullen, K.; Spiess, H. W. Columnar Mesophases of Alkylated Hexa-peri-hexabenzocoronenes with Remarkably Large Phase Widths. Adv. Mater. 1996, 8, 510−513. (14) Ban, J.; Chen, S.; Li, C.; Wang, X.; Zhang, H. Influence of the Spacer and Molecular Weight on the Phase Behavior of Side-Chain Liquid Crystalline Polymers Containing Triphenylene Discotic Mesogen Units as Side Groups. Polym. Chem. 2014, 5, 6558−6568. (15) Xiao, S. X.; Myers, M.; Miao, Q.; Sanaur, S.; Pang, K. L.; Steigerwald, M. L.; Nuckolls, C. Molecular Wires from Contorted Aromatic Compounds. Angew. Chem., Int. Ed. 2005, 44, 7390−7394. (16) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-organized Discotic Liquid Crystals for High-efficiency Organic Photovoltaics. Science 2001, 293, 1119− 1122. (17) Mu, B.; Wu, B.; Pan, S.; Fang, J.; Chen, D. Hierarchical Selforganization and Uniaxial Alignment of Well Synthesized Side-Chain Discotic Liquid Crystalline Polymers. Macromolecules 2015, 48, 2388− 2398. (18) Gupta, M.; Pal, S. K. Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A New Class of Blue-Light-Emitting Materials with Long-Range Columnar Self-Assembly. Langmuir 2016, 32, 1120−1126. (19) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbachi, K. H.; Ringsdorf, H.; Haarer, D. Fast Photoconduction in the Highly Ordered Columnar Phase of a Discotic Liquid-Crystal. Nature 1994, 371, 141−143. I

DOI: 10.1021/acs.macromol.7b00655 Macromolecules XXXX, XXX, XXX−XXX