Mesophasic and Thermal Properties

4.3.3.1. Side-Chain Polymers AmBn. In order to understand the influence of the molar ratio of covalent-bonded bent-core units on the mesomorphic, molecular stacking, and thermal properties, side-chain polymers AmBn were investigated by POM, DSC, and XRD measurements. The thermal properties and phase behaviors of side-chain polymers AmBn are illustrated in Figure 4.7a and Table 4.2. Polymers A1B0, A16B1, and A10B1 with higher m/n molar ratios (lower densities of covalent-bonded bent-core units) possessed the tilted smectic (SmC) phases, which

were verified by POM to show the enantiotropic schlieren texture and grainy domain.

For instance, the schlieren texture and grainy domain of polymers A1B0 and A10B1 are demonstrated in Figures 4.5a and 4.5b, respectively. However, polymer A4B1 revealed a nematic phase in both heating and cooling processes, and the POM texture of polymer A4B1 is shown in Figure 4.5c. Regarding the mesophases of polymers A1B2, A1B5, A1B13, and A0B1 with lower m/n molar ratios (higher densities of covalent-bonded bent-core units), the same enantiotropic smectic phase (SmC) was obtained, where one of the POM texture of A1B13 is shown in Figure 4.5d.

Table 4.2. Phase transition temperatures and tnthalpies of side-chain polymers polymer phase transition temperature/°C [enthalpy/kJ/g]

heating (top) / cooling (bottom) A1B0 K 71.2 [1.3] K’ 96.3 [1.7] SmC₁ 162.2 [22.4] I

I 155.1 [-27.5] SmC₁ 76.2 [-4.5] K A16B1 K 141.2 [1.3] SmC₁ 159.7 [10.4] I

I 153.6 [-12.9] SmC₁ 140.9 [-1.8] K A10B1 K 140.2 [6.4] SmC₁ 154.3 [1.7] I

I 153.3 [-7.6]^a SmC₁ 138.7^a K A4B1 K 112.3 [0.6] N 129.8 [4.5] I

I 129.7 [-3.6] N 105.3 [-1.0] K A1B2 K 71.3 [3.5] SmC₂ 96.0 [5.7] I

I 90.1 [-2.8] SmC₂ 74.3 [-0.7] K’ 59.2 [-1.4] K A1B5 K 75.9 [6.9] SmC₂ 98.8 [2.5] I

I 88.7 [-4.69] SmC₂ 74.8 [-4.5] K A1B13 K 120.4 [18.5] SmC₂ 140^b I

I 135^b SmC₂ 115.4 [-15.3] K A0B1 K 136.3 [23.5] SmC₂ 150^b I

I 148^b SmC₂ 130 [-19.4] K

The phase transitions were measured by DSC at the 2nd scan with a cooling rate of 5

°C/min. I = isoptropic state; N = nematic phase; SmC₁ and SmC₂ = tilted smectic phases; K = crystalline state. ^a means the enthalpy values of two cover transition peaks. ^b means the temperature data is observed in POM only. Phase transitions of monomer A was obtained as I 106.4 [20.6] SmC 55.4 [8.59] K. Phase transition of

Figure 4.5. POM textures at the cooling process: (a) the tilted smectic phase with schlieren texture of polymer A1B0 at 150 °C; (b) the tilted smectic phase with grainy domain of polymer A10B1 at 150 °C; (c) the nematic phase with schlieren texture of polymer A4B1 at 125 °C; (d) the tilted smectic phase with grainy domain of polymer A1B13 at 130 °C.

Comparing the phase transition temperatures of all side-chain polymers AmBn, homopolymers A1B0 and A0B1 revealed the highest isotropization temperatures for polymers AmBn with higher and lower m/n molar ratios (lower and higher densities of covalent-bonded bent-core units), respectively. Higher isotropization temperatures of homopolymers A1B0 and A0B1 indicated that the tighter molecular staking of intermolecular H-bonded linear-cores or bent-cores in homopolymeric systems, which also suggested that the looser molecular stackings were formed in copolymers duo to the disorder arrangements of both H-bonded linear-cores and covalent-bonded bent-cores. Especially, copolymer A4B1 reached the largest randomness to lose the lamellar packings for both H-bonded linear-cores and covalent-bonded bent-cores,

(a) (b)

(c) (d)

and the nematic phase was preferred instead. Hence, three cartoon diagrams were drawn in Figure 4.6 to explain possible intermolecular arrangements in polymers AmBn. Based on the molar ratios of bent-core units, polymers A1B0, A16B1, and A10B1 with the high density of benzoic acidic groups displayed the smectic stacking by the intermolecular acidic H-bonds (with H-bonded cross-linking structures) as shown in Figure 4.6a, and the stacking order was reduced by decreasing m/n molar ratio. As m/n ratio reached 4/1 (polymer A4B1), the acidic H-bonded linear-cores (H-bonded cross-links) would be separated into a more random stacking by the introduction of covalent-bonded bent-core unit B as shown in Figure 4.6b. Afterwards, to the other extreme of more covalent-bonded bent-core units (B), polymers A1B2, A1B5, A1B13, and A0B1 demonstrated another smectic arrangement due to the major intermolecular stackings of bent-core units (see Figure 4.6c).

(a)

(b)

(c) (a)

(b)

(c)

Figure 4.6. Cartoon diagrams of possible intermolecular arrangements of (a) polymers A1B0, A16B1, and A10B1 with larger m/n molar ratios mainly contributed from the self H-bonded acidic dimmers, (b) polymer A4B1 with a medium m/n molar ratio, and (c) polymers A1B2, A1B5, A1B13, and A0B1 with smaller m/n molar

A1B0 A16B1 A10B1 A4B1 A1B2 A1B5 A1B13 A0B1 60

80 100 120 140 160

Temp. (o C)

SmC1 N SmC2 K

(a)

A1B0N A16B1NA10B1N A4B1N A1B2N A1B5N A1B13N 60

80 100 120 140 160

(b)

Temp. (o C)

SmC1 SmCP SmC2 K

Figure 4.7. Phase diagrams (upon 2nd cooling) of (a) side-chain polymers AmBn and (b) bent-core side-chain polymer complex AmBn-N.

4.3.3.2. Bent-Core Side-Chain Polymer Complexes AmBn-N. The influence of molar ratios of bent-core covalent- and H-bonded units on the mesomorphic, molecular stacking, and thermal properties of bent-core side-chain polymer complexes AmBn-N were also investigated by POM, DSC, and XRD measurements.

The thermal properties and phase behaviors of bent-core side-chain polymer

complexes AmBn-N are illustrated in Figure 4.7b and Table 4.3. According to Figure S1 of the supporting information, compound S12 and supramolecular analogue H12 (H-bonded complex) both exhibited the SmCP phase, so bent-core side-chain polymer complexes AmBn-N via the copolymerization of these two units (S12 and H12) were prepared and surveyed for the generation of the SmCP phase. However, due to the addition of acrylate termini in their similar structures, B and A-N units did not possess any SmCP phase, where the phase transition temperatures of monomers A and B along with complex A-N were obtained as A: I 106.4 °C SmC 55.4°C K, B: I 90.3 °C K, and A-N: I 76.7 °C K, respectively.

Figure 4.8. POM textures at the cooling process: (a) the polar smectic phase with fan-like texture of polymer complex A10B1-N at 130 °C; (b) the tilted smectic phase

(b)

(a)

Table 4.3. Phase transition temperatures and enthalpies of bent-core side-chain polymer complexes

polymer complex

phase transition temperature/°C [enthalpy/kJ/g]

heating (top) / cooling (bottom) A1B0-N K 125.6 [43.3] SmC1 158.0 [2.5] I

I 138.7 [1.9] SmC1 97.3 [36.8] K

A16B1-N K 80.0 [2.7] K’112.7 [13.3] SmCP 145.5 [15.0] I I 140.1 [-12.2] SmCP 85.6 [-1.8] K’ 59.4 [-9.0] K A10B1-N K 85.4 [2.8] K’116.1 [18.7] SmCP 143.0 [3.2] I

I 135.8 [-17.0] SmCP 87.7 [-2.3] K’ 66.7 [-10.6] K A4B1-N K 78.5 [5.4] SmC1 117.3 [11.4] I

I 112.1 [-8.4] SmC1 66.8 [-2.7] K A1B2-N K 77.4 [6.0] SmC1 110.8 [14.3] I I 107.6 [-7.0] SmC1 75.5 [-14.7] K A1B5-N K 79.9 [9.9] SmC2 95.7 [17.1] I

I 90.0 [-19.7] SmC2 60.1 [-9.0] K A1B13-N K 75.9 [1.6] SmC2 115.2 [12.9] I

I 108.9[-13.9] SmC2 61.7 [-1.2] K

The phase transitions were measured by DSC at the 2nd scan with a cooling rate of 5

°C/min. I = isoptropic state; SmCP = polar smectic phase; SmC1 and SmC2 = tilted smectic phase2; K = crystalline state. Phase transitions of complex A-N was obtained as I 76.7 [36.4] K.

In comparison with side-chain polymers AmBn, bent-core side-chain polymer complexes AmBn-N have lower isotropization temperatures due to their H-bonded pendants assembled by pyridyl and acidic groups, which have less intermolecular acidic H-bonds (with less H-bonded cross-links). Therefore, the novel enantiotropic polar smectic (SmCP) phase was surprisingly generated in some compositions of bent-core side-chain polymer complexes AmBn-N. Regarding the mesophasic types, the enantiotropic tilted smectic phase was observed in polymer complexes A1B0-N, A4B1-N, A1B2-N, A1B5-N, and A1B13-N, and the enantiotropic polar smectic (SmCP) phase was achieved in polymer complexes A16B1-N and A10B1-N. The mesophasic textures were observed by POM experiments, for instance, polymer

complex A10B1-N revealed the polar smectic phase with a fan-like texture in Figure 4.8a, and polymer complex A1B13-N exhibited the tilted smectic (SmC) phase with a grainy domain in Figure 4.8b, which were the characteristics of the tilted smectic phases.

With regard to the variation of mesophasic transition temperatures of polymer complexes AmBn-N, the isotropization temperatures and mesophasic ranges were reduced as the m/n molar ratio decreased (except A1B13-N). Side-chain copolymers AmBn with higher m/n ratios possessed more H-donor groups exhibited more extensive mesophasic ranges and higher isotropization temperatures, which indicated that the acidic H-bonded linear-cores (H-bonded cross-links) would extend and stabilize the mesophase. However, due to the intermolecular acidic H-bonds (with H-bonded cross-linking structures) of side-chain copolymers AmBn being replaced with side-chain H-bonded pendants of the analogous polymer complexes AmBn-N, bent-core side-chain polymer complexes AmBn-N with higher m/n molar ratios did not exhibit more extensive mesophasic ranges but still possessed higher isotropization temperatures. Compared with side-chain copolymers AmBn, the corresponding polymer complexes AmBn-N generally exhibited more extensive mesophasic ranges and lower transition temperatures, except polymer complexes A1B2-N and A1B5-N.

In addition, the nematic phase in copolymer A4B1 was replaced by a tilted smectic phase in polymer complex A4B1-N. More excitingly, the polar smectic phase (the switching current behaviors will be demonstrated later) was achieved in polymer complexes A16B1-N and A10B1-N, though the individual components of H-donor side-chain copolymers A16B1 and A10B1 as well as H-acceptor N did not possess the SmCP phase (see Tables 4.2 and 4.3). Hence, it suggested that the mesomorphic and thermal properties of polymer complexes AmBn-N were strongly dependent on the

respectively), where the bent-core H-bonded units were formed by the acidic H-donor groups (A groups from side-chain polymers AmBn) incorporated with H-acceptor N.

Therefore, we have discovered a special technique that the construction (or stabilization) of the SmCP phase can be acquired by copolymerization of bent-core covalent- and H-bonded units in side-chain polymer complexes (with proper m/n molar ratios) from both bent-core covalent- and H-bonded monomers (i.e., B and A-N units, respectively) without the SmCP phase (see Figure 4.9).

Figure 4.9. The SmCP phase was introduced by copolymerized frameworks bearing both bent-core covalent- and H-bonded monomers (B and A-N units with proper m/n molar ratios) without the SmCP phase.