Main-chain liquid crystalline ionomers with a nonplanar ionic segment

Abstract

A series of thermotropic main-chain liquid crystalline ionomers named poly(4,4′-bis(6-hydroxyhexyloxy)biphenyl bibenzoate-co-sodium-phenoxaphosphinate-2,8-dicarboxylate) (PBBPins) containing sodium salt of 10H-phenoxaphosphine-2,8-dicarboxylic acid, 10-hydroxy-, 2,8-dimethyl ester, 10-oxide (DMPPO-Na) as an ionic monomer, 4,4′-bis(6-hydroxyhexyloxy)biphenyl (BHHBP) and 4,4′-dicarboxymethylbiphenyl (DBB) as a mesogenic unit were prepared by transesterification. The thermal stability, the mesophase behaviour, and the relationship between the chain structure and thermal properties, as well as the mechanical properties of the main-chain phosphorus-containing liquid crystalline ionomers were investigated. The ionomers all exhibited mesomorphic (smectic A) properties even when the feed ratio of DMPPO-Na : DBB reached as high as 2 : 8 (PBBPi10). In the presence of the physical cross-linking of ionic groups, the viscoelastic behaviour of the ionomers was greatly changed, and mechanical properties were improved with the introduction of an ionic monomer. Thermal stability of the ionomers remained; moreover, residue of the ionomers at 700 °C increased, due to the positive contribution of the phosphorus-containing ionic structure.

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Introduction

Thermotropic liquid crystalline polymers (TLCPs) have received significant concern for many years due to their good thermal stability,^1–4 excellent chemical resistance,^5,6 high strength and high modulus,^7,8 as well as their low linear viscosity and a high degree of molecular orientation in the liquid crystalline state, which makes them attractive high performance engineering materials for numerous applications.^9–12 Among them, polybibenzoates are a kind of main-chain LCPs, synthesized by biphenyl units as a mesogenic group and aliphatic spacers, a type of smectic structure could be formed in this polybibenzoates, such as poly(hexanediol p,p′-bibenzoate) (BB-6), poly(pentanediol p,p′-bibenzoate) (BB-5),^13–16 etc. However, the major shortcoming that limits their application is the weak property transverse to the direction of orientation,¹⁷ so as to the mechanical properties of these LCPs. One way to improve the transverse properties of LCPs is to incorporate specific functional groups into the polymer chains that promote the interchain interactions.^18,19

Ionomers are those that have a small molar fraction (usually less than 15 mol%) of ionic groups, known to change considerably the properties of polymers, especially to modify rheological properties,^20,21 crystallization properties,^22,23 mechanical properties,^24–27 etc. Liquid crystalline ionomers (LCIs) have been of interest because they combine the characteristics of both liquid crystalline and ion-containing polymers, in other words, the formation of LC phase and the existence of ionic aggregates.^28–31 Introducing of a small amount of ionic groups into LCPs, which act as a cross-linking point, is an effective way to promote the interchain interactions and enhance the chain entanglement of the polymers, then improve the mechanical properties. Zhao,^29,32–35 Zental,^36–40 Barmatov,^41–44 et al. synthesized different comb-shape LC ionomers using radical copolymerization with mesogenic groups on the side chain, subsequent partial neutralization of carboxyl groups or partial alkaline hydrolysis of the side fragments to obtain comb-shaped thermotropic LC ionomers, with different ions. An increase in the content of ions not only led to an influence on the transition temperature, such as the glass transition temperature and the clearing point, but also the phase behaviour of the ionomers, the incorporation of metal ions in the nematic polymer matrix led to the appearance of SmA phase.^41–46 This comb-shape LC ionomers could produce a fast response to the external stimuli including electric, magnetic, mechanic, etc. Zental^38,47–49 et al. synthesized a ferrocene-containing liquid crystalline ionomers with mesogenic groups in the main chain which showed that incorporation of oxidized ferrocene segments into the polymer backbone had no pronounced changes in the phase state of the polymers due to the entropy and steric factors for hinder interaction of charged groups. But with ferrocene fragments onto the side chain, an increase in T_g and T_LC could be found, which were associated with micro-segregation of the ferrocene fragments, which favor packing of the mesogenic groups in the linear polymer. Two charged groups, sulfite and phosphate,^18,50 smaller than ferrocene ions were chosen to obtain more stable ionic aggregates. The LC ionomers with sulfite ions exhibit a monotonic decrease in the T_g which could be explained by the impossibility of formation of ionic clusters in the absence of a spacer connecting the charged groups to the polymer backbone. The basic conditions of forming ionic aggregates are low glass transition temperature and steric hindrance. The higher the glass transition temperature, the more difficult is to force the polymer chains to change their position at normal temperature, so as to the steric hindrance.⁵¹

The kind of ionic groups in LC ionomers mostly containing ferrocene, carboxylic acid,⁴⁴ sulfonic acid groups^52–54 etc. Among them, the mesophase behaviour, and the relationship between the chain structure and thermal properties, as well as the mechanical properties of the main-chain phosphorus-containing LC ionomers have been studied in much less detail. The glass transition, thermal stability, mechanical properties and inherent flame retardance of the LCPs might be influenced with the introduction of phosphorus-containing ionic groups, which might suit for the application of engineering materials. In this paper, 10H-phenoxaphosphine-2,8-dicarboxylic acid, 10-hydroxy-, 2,8-dimethyl ester, 10-oxide (DMPPO-OH) was used as phosphorus-containing ionic monomer to enhance the interchain interactions, while 4,4′-bisphenol (BP) and 4,4′-dicarboxymethylbiphenyl (DBB) were used as mesogenic monomers to synthesize a series of main-chain phosphorus-containing liquid crystalline ionomers. Chemical structure, liquid crystalline phase behavior, the viscoelasticity behavior, thermal and mechanical properties were comprehensively investigated.

Experimental

Materials

4,4′-Dicarboxymethylbiphenyl (DBB) and 4,4′-biphenol were provided by Bangcheng Chemical Co. Ltd. (Shanghai, China). 6-Chloro-1-hexanol was manufactured by Alfa Aesar Chemical Co. (Tianjin, China). 4,4′-Dimethyldiphenylether was purchased from Jiaxing Sicheng Chemicals Co. Ltd. (Jiaxing, China). Phosphorus trichloride, aluminum trichloride, aqueous hydrogen peroxide solution (H₂O₂, 30%), potassium permanganate (KMnO₄), methanol, sodium hydroxide (NaOH), and tetrabutyl titanate were all purchased from Kelong Chemical Industries Reagent Co. (Chengdu, China). All other reagents were commercial and used as received.

Synthesis of monomers

4,4′-Bis(6-hydroxyhexyloxy)biphenyl(BHHBP) was synthesized according to other reports.⁵⁵

¹H-NMR (DMSO-d₆, δ, ppm): 7.52–7.50 (d, 4H, Ar-H), 6.98–9.96 (d, 4H, Ar-H), 4.36 (s, 2H, –OH), 4.0–3.97 (t, 4H, Ar-O–CH₂), 3.42–3.38 (m, 4H, –CH₂–OH), 1.74–1.69 (m, 4H, Ar-O–CH₂–CH₂), 1.38–1.32 (m, 12H, Ar-O–CH₂–CH₂–(CH₂)₃–CH₂OH).

The ionic monomer precursor, 10H-phenoxaphosphine-2,8-dicarboxylic acid, 10-hydroxy-, 10-oxide (DCPPO-OH), and 10H-phenoxaphosphine-2,8-dicarboxylic acid, 10-hydroxy-, 2,8-dimethyl ester, 10-oxide (DMPPO-OH) was prepared according to the other reports.^22,56–58 The ionic monomer, sodium salt of 10H-phenoxaphosphine-2,8-dicarboxylic acid, 10-hydroxy-, 2,8-dimethyl ester, 10-oxide (DMPPO-Na), was prepared by neutralization with NaOH (Scheme 1): A 100 mL beaker with a magnetic stirring, 70 mL H₂O, 6.96 g (0.02 mol) DMPPO, 0.80 g (0.02 mol) NaOH was added into the beaker with stirring successively, then a transparent solution with the pH of about 8.0 was obtained, then filtered twice and poured into a 100 mL round bottom flask, distillation to remove the water. The final white product DMPPO-Na was obtained after cooling and drying.

Scheme 1 Synthesis of ionic monomer DMPPO-Na.

¹H NMR (DMSO-d₆, δ, ppm): 8.37–8.33 (d, 2H, Ar-H), 8.19–8.16 (d, 2H, Ar-H), 7.53–7.50 (t, 2H, Ar-H), 3.90 (s, 6H, O–CH₃).

Polymerization

Liquid crystalline ionomers named poly(4,4′-bis(6-hydroxyhexyloxy)biphenyl bibenzoate-co-sodium-phenoxaphosphinate-2,8-dicarboxylate) (PBBPin) were synthesized by melt-transesterification. PBBPi10 was given as a representative (Scheme 2), where the number 10 indicated the molar percentage of DMPPO-Na in the reactants. BHHBP (7.72 g, 0.020 mol), DMPPO-Na (1.48 g, 0.004 mol) and tetrabutyl titanate (0.3 wt% as catalyst) were added into a 100 mL three-neck flask that was equipped with a mechanical stirrer, a nitrogen inlet, and a distillation trap connected to a vacuum line. The flask was processed by nitrogen protection, to remove the oxygen before reaction. The flask was placed in a salt bath preheated to 190 °C with mechanical stirring, and kept at this temperature until the homogeneous melt solution was obtained. After that, the flask was cooled to room temperature under nitrogen atmosphere; then DBB (4.32 g, 0.016 mol) was added. The mixture was heated to 220 °C and the byproduct, methanol, was taken off by the nitrogen flow. After maintained at that temperature for about 3 h, a vacuum was applied, the reaction system was heated to 260 °C in 1 h, and kept at that temperature for 3 h with a high vacuum of 30 Pa. Afterwards, an obvious pole-climbing phenomenon could be observed, and the stirring speed was turned slowly. When the polymerization was completed, the flask was cooled to ambient temperature under high vacuum. Other samples with different DMPPO-Na contents were obtained according to the same process. The basic parameters of the liquid crystalline ionomers were given in Table 1. Intrinsic viscosity [η] tests indicated that [η] of the target ionomers ranged from 0.88 to 1.21 dL g⁻¹. The ionomers showed different colors from pale white to light yellow with increasing the content of DMPPO-Na. For comparison, PBBP (poly(4,4′-bis(6-hydroxyhexyloxy)biphenyl bibenzoate) derived from BHHBP and DBB was synthesized following the same procedure.

Scheme 2 Synthesis route of PBBPins with different molar percentage of DMPPO-Na.

Table 1 Characterization parameters of PBBPins

Sample	Molar ratio of DBB : DMPPO-Na		[η] (dL g^−1)
	Feed	Test^a
a Test values were calculated from ^1H NMR.
PBBP	—	—	0.94
PBBPi1	49 : 1	48.0 : 2.0	1.21
PBBPi3	47 : 3	46.3 : 3.7	1.01
PBBPi5	45 : 5	44.5 : 5.5	0.99
PBBPi7	43 : 7	42.5 : 7.5	1.14
PBBPi10	40 : 10	39.8 : 10.2	0.88

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Characterization

All the resulting ionomers for characterization were purified by dissolution in a phenol/1,1,2,2-tetrachloroethane mixed solution (6 : 4 w/w) and precipitation into excess ether. Then the filter residues were dried in a vacuum oven at 80 °C to a constant weight. Intrinsic viscosities [η] of the ionomers were measured in a mixed solution of phenol/1,1,2,2-tetrachloroethane (1 : 1 w/w) at 30 °C at a concentration of 2.0 mg mL⁻¹ with an Ubbelohde capillary viscometer.

NMR spectra (¹H, 400 MHz; ³¹P, 161.9 MHz) of samples were measured using a Bruker AVANCE AV II-400 NMR instrument, with DMSO-d₆ and CDCl₃/CF₃COOD (v/v, 9/1) as the solvent. Tetramethylsilane and phosphoric acid were used as the internal standard, respectively.

The element contents of carbon (C) and hydrogen (H) in PBBPi10 were measured by elemental analysis (EA) on CARLO ERBA1106 instrument (Carlo Erba, Italy).

Differential scanning calorimetry (DSC) was conducted on a TA Q200 with 5 ± 0.25 mg of samples under a dynamic nitrogen atmosphere (the flow rate = 50 mL min⁻¹). Indium was used as a reference for temperature calibration. The test procedures were given as follows: the samples were first heated to 250 °C, kept isothermal for 3 min, then cooled to 40 °C and finally heated back to 250 °C. The heating and cooling rates were both 10 °C min⁻¹.

Thermogravimetric analysis (TGA) was performed with a NETZSCH TG 209 F1 instrument at a heating rate of 10 °C min⁻¹ from 40 to 700 °C under a nitrogen atmosphere at a flowing rate of 50 mL min⁻¹ with 4–5 mg of samples.

Wild-angle X-ray diffraction (WAXD) was performed on a DX1000 diffractometer using Cu-Kα radiation at room temperature scanned from 2.0 to 35.0°. The rotated velocity of goniometry was 2° min⁻¹. Variable temperature WAXD powder experiments were performed on a Bruker D8 AVANCE diffractometer with a 3 kW ceramic tube as the X-ray source (Cu KR) and an X'celerator detector.

The liquid crystalline textures of the ionomers were observed through a Nikon ECLIPSE LV100PL polarizing optical microscopy (POM) equipped with a hot-stage operating from room temperature to 250 °C. The samples were prepared according to the literature:⁵⁹ firstly, the samples were dissolved in a mixed solvent of trifluoroacetic acid and chloroform (3 : 1 w/w) at a 10 wt% weight concentration; then, the solution was cast onto a glass slide and solvents were removed through soaking in deionized water followed by methanol in order to prepare films. Finally, the films were dried under vacuum at 80 °C to a constant weight.

Dynamic mechanical analysis (DMA) of the flake specimens (16.0 × 8.0 × 0.5 mm³) were tested via a dynamic mechanical analyzer (TA Q800) at a heating rate of 3 °C min⁻¹ from 0 to 180 °C.

Dynamic oscillatory rheological measurements of samples were performed with a parallel-plate fixture (25 mm diameter and 1 mm thickness) using a TA Discovery HR-2 rheometer in the oscillatory shear mode. The complex viscosities were measured as a function of frequency ranging from 0.01 to 100 Hz at the temperatures of 250 °C.

Tensile strength and elongation at break of the samples were measured on a Sansi Universal Testing Machine (CMT, Shenzhen, China) at a crosshead speed of 5 mm min⁻¹ at room temperature according to GB/T 1040-92. The samples were compression molded and shaped with a dumbbell-shaped cutter. The thickness and width of the specimens were 0.5 mm and 4 mm, respectively. The length of the sample between the two grips of the testing machine was 20 mm.

Results and discussion

Characterization of PBBPins

Chemical structures of ionomers were characterized by ¹H and ³¹P NMR (CDCl₃/CF₃COOD, v/v, 9/1). Fig. 1 showed the ¹H and ³¹P NMR spectra of ionomer PBBPi10, it could be found that the main peaks at 8.10 (aromatic H, peak a) and 7.67 (aromatic H, peak b) ppm were due to the presence of the aromatic protons of DBB, peaks at 7.45 (aromatic H, peak c) and 6.97 (aromatic H, peak d) ppm were ascribed to the presence of the aromatic protons of BHHBP, and the small peaks at 8.65 (aromatic H, peak i), 8.30 (aromatic H, peak j) and 7.53 (aromatic H, peak k) ppm should be attributed to the presence of the aromatic protons of DMPPO-Na. The other peaks at lower chemical shifts attributed to the methylene groups located in the α-position (C(O)–CH₂–CH₂–CH₂–, peaks e and f), β-position (C(O)–CH₂–CH₂–CH₂–, peak g), and the γ-position (C(O)–CH₂–CH₂–CH₂–, peak h) of the ether linkages, respectively. For ³¹P NMR, there was only one single peak appearing at 16.54 ppm, suggesting that only one type of phosphorus existed, which belonged to the DMPPO-Na segment. The experimental molar content of ionic monomer in the resulting ionomers was calculated from the corresponding integral peak areas in ¹H NMR, and listed in Table 1. It can be noticed that the experimental ionic monomer content was close to the feed molar content of ionomers.

Fig. 1 ¹H and ³¹P NMR spectra of PBBPi10.

Elemental analysis was used to investigate the real carbon and hydrogen content of the resulting ionomers. For PBBPi10, the carbon and hydrogen content reached 73.77% and 6.48%, which was much close to the calculated value as 74.51% and 6.47%, respectively. Combing the NMR and elemental analysis results, it could be concluded that the liquid crystalline ionomers were successfully synthesized via melt-transesterification.

Thermal stability

Thermal stability of PBBPin was analysed using TGA under nitrogen atmosphere at a heating rate of 10 °C min⁻¹. The corresponding curves were given in Fig. 2, and the detailed data were summarized in Table 2. It was noticeable that all the ionomers showed one single degradation process, very similar to PBBP. The initial decomposition temperature, (T_5%, defined as the 5 wt% of original weight loss) of PBBPin were from 389 to 394 °C, and the temperature at maximum weight loss rate (T_max) of PBBPin were from 414 to 418 °C, indicating that the thermal stability of the ionomers was maintained with the introduction of the ionic monomer. Meanwhile, residue at 700 °C increased with increasing the DMPPO-Na content, suggesting that, due to the existence of phosphinate groups and high boiling point of ionic fragments, char formation was promoted during decomposition.

Fig. 2 TGA thermograms of PBBP and PBBPin ionomers at a heating rate of 10 °C min⁻¹ in nitrogen atmosphere.

Table 2 TGA data of PBBPins in nitrogen atmosphere

Sample	T5% (°C)	Tmax (°C)	Residue at 700 °C
PBBP	389	417	7.7
PBBPi1	391	418	8.5
PBBPi3	394	418	9.3
PBBPi5	390	417	9.9
PBBPi7	389	414	10.4
PBBPi10	389	416	13.0

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Phase transition and liquid crystalline behaviour

The phase transition temperature of the ionomers was determined by DSC experiments in nitrogen atmosphere with a heating and cooling rate of 10 °C min⁻¹. Fig. 3 showed the first cooling and the second heating DSC thermograms of the ionomers, and the experimental data were summarized in Table 3. It could be found that on the cooling process of PBBP, there were two exothermic peaks at 198 and 137 °C with the exothermic enthalpy of 24.9 and 14.0 J g⁻¹, which were attributed to the temperature from isotropic liquid state to liquid crystalline phase (T_i) and then to crystalline state (T_LC), respectively. Similar phenomena could be found on the first cooling scan of ionomers. During the first cooling, T_i of the ionomers gradually decreased from 198 to 169 °C with an increase of ionic monomer content, T_LC values of the ionomers first increased with an ionic monomer content of 1 mol%, from 137 to 141 °C, and then decreased from 141 to 129 °C, as the ionic monomer content reached 10 mol%. The exothermic values of the phase transition both from isotropic liquid state to liquid crystalline (I → LC, ΔH_i) and from liquid crystalline to crystalline (LC → K, ΔH_LC) decreased evidently. From the second heating curve of the PBBPi10, the first endothermic peak at around 139 °C was hard to be identified, suggesting that, due to the introduction of nonplanar ionic segments, crystallization capacity of the ionomer was impeded, which could be further confirmed by WAXD (Fig. 5). Very similar to the transition temperature from LC → K in the cooling scan, T_i of the ionomers in the heating scan first increased with 1 mol% of ionic monomer, from 210 to 215 °C, and then decreased with a further increase of ionic monomer, from 215 to 197 °C. However, the endothermic enthalpies of the phase transition (ΔH_i and ΔH_LC) both on heating and cooling scans decreased continuously. Generally, copolymerization of the third monomer could decrease the chain regularity, dull and broad peaks observed from DSC of the ionomers indicated that it would take more time for the polymer chains moving to an ordered alignment. However, the unusual increase of the transition temperature of PBBPi1 might be due to the ionic interaction, which could act as a physical crosslinking point and maintain the regular chain conformation at the crystalline state with low content of ionic fragments, thus the temperature of T_LC increased. However, the regular conformation of ionomer chains were affected with a further introduction of ionic fragments, thus both the transition temperature and the corresponding enthalpy decreased significantly. Consequently the layered arrangements of the liquid crystalline polymer chains were gradually destroyed. Further evidence would be obtained from the WAXD results.

Fig. 3 DSC curves of PBBPins on the first cooling (a) and second heating (b) scans in N₂.

Table 3 Phase transition parameters of PBBPins obtained from DSC

Sample	The first cooling				The second heating
	TLC (°C)	ΔHLC (J g^−1)	Ti (°C)	ΔHi (J g^−1)	TLC (°C)	ΔHLC (J g^−1)	Ti (°C)	ΔHi (J g^−1)
PBBP	137	14.0	198	24.9	168	3.4	210	22.4
PBBPi1	141	10.4	198	23.9	159	1.9	215	22.0
PBBPi3	139	9.0	194	22.2	159	1.2	213	20.4
PBBPi5	137	7.4	183	17.0	155	0.6	205	15.8
PBBPi7	131	4.4	174	14.1	148	0.4	200	12.9
PBBPi10	129	2.4	169	10.4	139	0.32	197	9.8

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Liquid crystalline textures of the ionomers were observed by POM equipped with a hot-stage heated from room temperature to 250 °C. Sample films were casted from CHCl₃/CF₃COOH (v/v 9 : 1) mixed solution and slowly dried at ambient temperature. Fig. 4 showed the liquid crystalline textures of PBBP, PBBPi1, PBBPi3, PBBPi5, PBBPi7 and PBBPi10 taken at different temperatures, respectively. It was noticed that the textures of all testing polymers during cooling and heating processes were almost the same; therefore, only the photographs obtained during heating were chosen as representatives illustrated in the manuscript (Fig. 4). All the samples could found the apparent birefringence during both heating and cooling process. A typical Schlieren texture developed at the temperature of phase transition under cooling, which indicated that the introduction of DMPPO-Na did not change the morphology of liquid crystalline texture. However, the birefringence of the ionomers became weaker gradually with the introduction of DMPPO-Na content. With the content of DMPPO-Na reached 10 mol%, the ionomer PBBPi10 still exhibited liquid crystalline behavior, which was in accord with the DSC results.

Fig. 4 Polarized optical microscope photographs of sample at different temperature: (a) PBBP (190 °C); (b) PBBPi1 (190 °C); (c) PBBPi3 (192 °C); (d) PBBPi5 (180 °C); (e) PBBPi7 (176 °C); (f) PBBPi10 (176 °C).

The phase transition behaviors of the polyesters were further investigated via WAXD instrument. The WAXD patterns of PBBP and PBBPin ionomers at room temperature were given in Fig. 5. From all polymers, a typical diffraction peak appeared at 2θ = 5.65 (15.6 Å), corresponding to the spacing of the smectic layers. This spacing was significantly less than the calculated length of the fully extended repeat unit of about 18.2 Å, which was attributed to the presence of the gauche or eclipsed conformations of the spacer. The q (q = 4π sin θ/λ, where λ is the X-ray wavelength and 2θ is the scattering angle) ratio of the diffraction peaks appeared at 2θ of 5.65 and 11.20° was 1 : 2, meaning that PBBP and ionomers with different content of ionic monomer exhibited the positional order of the molecules' centers of mass, leading to the layered structure of the smectic phases. With the introduction of DMPPO-Na, the intensity of the layer-spacing peak was decreased gradually, which indicated that the regular smectic layer packing of PBBPin was deteriorated. Although the crystalline reflections could still be found for PBBPi5 one by one, the diffractions became dull and broad, indicating the regular arrangement of the ionomer chains was destroyed. Firstly, as illustrated above, copolymerization of the third monomer could decrease the chain regularity. On the second, the introduction of nonplanar structure (DMPPO-Na) into PBBP, the regular spacing arrangement of the mesogenic moieties was affected or even destroyed. However, a new diffraction peak came out at 2θ = 31.9 (2.80 Å) with the incorporation of DMPPO-Na and got stronger with the content of DMPPO-Na increased, which as corresponding to the ion-π stacking between the sodium ion and the phosphate structure in DMPPO-Na. It should be mentioned that, the diffraction peak at 2θ of 9.4° was attributed to the instrument (Fig. 5 and 6).

Fig. 5 WAXD patterns of PBBP and PBBPin ionomers at room temperature.

Fig. 6 WAXD patterns of PBBP and PBBPin ionomers at different temperature on the cooling scan (a) PBBP; (b) PBBPi5; (c) PBBPi10.

Fig. 6 gave the variations of WAXD intensity with scattering angle 2θ for an as-cast PBBP, PBBPi5 and PBBPi10 specimens recorded at various temperatures during the first cooling cycle, as indicated on the curves. At the very beginning of cooling process of PBBP, at the temperature of 220 °C, which was higher than its T_i, there was only a broad diffusion peak at 19.30°, confirming the presence of an isotropic liquid state. When the temperature was cooled to 170 °C, a diffraction peak at 5.70° (15.5 Å) corresponding to the smectic layer spacing was appeared, indicating the formation of a SmA phase, where the director n and the optic axis were perpendicular to the smectic layer plane. The peak at 19.30° became sharp and divided into two peaks until the temperature was cooled from 170 to 140 °C, corresponding to the crystal form between the well-defined smectic layers of PBBP. When the temperature was further cooled to 40 °C, it was clear that six diffraction peaks appeared at 16.90, 20.00, 20.80, 22.00, 23.50 and 26.60°, respectively, indicating that the polyester was fully crystallized. The WAXD patterns of PBBP and ionomers at 40 °C in the angle region of 16–27° in Fig. 6 were little different from that of the WAXD patterns at room temperature in Fig. 5, particularly the shifts of angles. The former signals were obtained after a programming cooling process; however, the latter ones were obtained after a long time of thermal relaxation history, which allowed the polymer fully crystallized. This result indicated that during the cooling process, PBBP underwent first from isotropic state to smectic A (I → SmA), and then to crystal state (SmA → K). But for PBBPi5 and PBBPi10, the reflection about 20° could not be splitted both in LC and K state, but the broad reflection transformed into stronger and sharper, and the spacing regular arrangement still remained.

By combining the experimental results of various tests for phase transition and liquid crystalline behaviour of the polymers, we demonstrated that PBBP and the corresponding PBBPin ionomers exhibited the phase transition sequence of I → SmA → K under cooling. Because of the copolymerization and nonplanar structure of DMPPO-Na, when ionic monomer was introduced into the ionomers, the regular molecular packing and crystal ability of PBBP were destroyed, but the mesophase of the ionomers stayed the same. Otherwise, when the feed ratio of DMPPO-Na : DBB reached 2 : 8, the regular layer spacing arrangement still remained.

Dynamic mechanical analysis

Fig. 7 showed the storage modules (E′) (a) and tan δ (b) plots of PBBP and ionomers obtained from a dynamic mechanical analyzer (DMA). When the temperature was below T_g, all the samples exhibited no significant changes in E′. When the temperature turned close to T_g, the E′ began to decline, and the peaks of tan δ associated with T_g could be observed. It was noticeable that the E′ of ionomers was higher than that of PBBP before the glass transition, due to that the ionic groups played a positive role on increasing the interaction between the polymer chains, which enhanced the elasticity of the liquid crystalline ionomers. T_g values of ionomers decreased with an increase of DMPPO-Na content. On the one hand, the nonplanar structure of DMPPO-Na would decrease the chain rigidity of ionomers; on the other hand, the existence of ionic groups could enhance the intermolecular attraction near the ionic groups (acted as the physical crosslinking sites), which could decrease the chain mobility and increase the T_g of the ionomer. Considering the two factors, it could be concluded that the decreasing of chain rigidity was dominant, thus the glass transition of the ionomers decreased with the introduction of DMPPO-Na.

Fig. 7 Storage modulus (E′) (a) and tan δ (b) of PBBP and ionomers.

In summary, the high orientation of the polymer chains due to the formation of the layer spacing arrangement, the high steric hindrance of the ionic monomer due to the absence of a spacer connecting the ionic groups to the polymer backbone, the high T_g (higher than 110 °C), all those factors resulted in that the ionic groups in the ionomers just played a role of physical cross-linking, could not form ionic aggregates.

Melt rheological behaviours

Fig. 8(a) showed the complex melt viscosity of PBBP and PBBPin ionomers with different oscillation frequencies at 250 °C. It could be noticed that the introduction of ionic groups considerable affect the complex viscosities of the liquid crystalline ionomers. A Newton platform could be obtained in low shear region (oscillation frequency) for PBBP, and an obviously shear thinning behavior could be found for PBBPi5 and PBBPi10 due to the interaction between ionic groups was deteriorated with an increase of shear rate. Because of the interaction between the ionic groups, the intermolecular attraction near the ionic groups was enhanced, which acted as the physical crosslinking; therefore the mobility and slippage of the polymer chains were hindered. Consequently, the complex viscosities of PBBPin ionomers in isotropic state were increased with the increase of DMPPO-Na content. The storage modulus (G′) and loss modulus (G′′) were both increased with the introduction of DMPPO-Na (Fig. 8(b) and (c)). Fig. 8(d) showed the plots of G′ against G′′, it could be found that for PBBP, G′′ was much higher than G′ under shearing, meaning that the irreversible energy loss was in the dominant position.

Fig. 8 Complex viscosity (a), loss (G′′) (b), storage (G′) (c) modulus and plots of G′ against G′′ for PBBP and PBBPin ionomers obtained in dynamic frequency sweep at 250 °C.

However, with the introduction of ionic groups, the G′′ of the ionomers was still higher than G′ under low shear rate, and the G′ was higher than G′′ under high shear frequency, as a result, there was an intersection point between G′ and G′′ of the ionomers. This result revealed that the reversible deformation could be in the dominant position under a suitable condition, and the viscoelastic behavior of the ionomers was greatly affected by introduction of ionic groups, the mechanical properties of the ionomers should also be changed.

Combining the dynamic mechanical analysis and rheological characterization, it could be revealed that the introduction of ionic groups enhanced the intermolecular attraction of the chain segments near the ionic groups by physical crosslinking, therefore greatly changed the viscoelastic behavior of the ionomers compared with PBBP. Thus a positive role in mechanical properties of the ionomers could be expected.

Tensile properties

In order to further investigate the development of mechanical performances, a tensile test was applied. The typical tensile stress-strain responses of PBBP, PBBPi5 and PBBPi10 were illustrated in Fig. 9. Generally, for a molded sample, a major shortcoming of main-chain liquid crystalline polymers (MCLCPs) is the anisotropy of their mechanical properties. Oriented MCLCPs have excellent properties at the direction parallel to the orientation; however, mechanical properties at perpendicular direction tend to be very poor. In this manuscript, it was noticeable that PBBP was very easy to break with extremely low strain and stress, which was due to its anisotropic nature. Addition of ionic groups, however, significantly enhanced both the stress and strain at break of PBBPi10: the tensile strength of PBBPi10 reached 29.5 MPa, and the elongation at break was 36.4%. Compared with PBBP, the tensile performance of liquid crystalline ionomers improved. Combining with the results of DMA tests, the storage modulus (E′) increased with the incorporation of ionic groups, suggesting an increase of the elasticity of the ionomers. Therefore, the incorporation of ionic groups resulted in improving the mechanical properties of the liquid crystalline ionomer.

Fig. 9 Representative tensile stress-strain curves of PBBP, PBBPi5 and PBBPi10.

Conclusions

In summary, a series of thermotropic main-chain liquid crystalline ionomers PBBPin containing DMPPO-Na as the ionic monomer were synthesized successfully. The chemical structures were characterized by ¹H NMR and ³¹P NMR. It was found that all the polymers exhibited a kind of smectic phase liquid crystalline behavior, and the introduction of the ionic groups decreased the layer spacing arrangement and the rigidity of molecular chain due to the nonplanar structure of the ionic segments. The ionic groups in the main chain of the ionomers could not form the ionic aggregates because of the high glass transition temperature, great steric hindrance, and the orientation tendency of the thermotropic polymer chains. The ionic groups acted as the physical crosslinking points, which could enhance the intermolecular attraction of the chain segments near the ionic groups; therefore the mobility and slippage of the polymer chains were hindered. As a consequence, compared with PBBP, the viscoelastic behavior of the ionomers was greatly changed, and the mechanical properties of the ionomers were improved with the introduction of ionic groups. In addition, thermal stability of the ionomers still remained, and the residue of the ionomers at high temperature increased due to incorporation of the phosphorus-containing ionic monomer.

Acknowledgements

Financial support by the National Natural Science Foundation of China (grant nos 51273115 and 51421061) are sincerely acknowledged. The authors would also like to thank the Analysis and Testing Center of Sichuan University for the NMR measurements.

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