Hybrid liquid crystal polymers from the self-assembly of poly(vinylpyridine) and polyoxometalates via multiple non-covalent bonds

Abstract

A quaternary ammonium surfactant (S) carrying a phenol terminal was grafted on the surface of nano-sized polyoxometalates, H₄SiW₁₂O₄₀ (PM-1), K₆P₂W₁₈O₆₂ (PM-2) and Na₁₀[Co₄(H₂O)₂(VW₉O₃₄)₂] (PM-3), respectively, by ionic self-assembly. The obtained complexes were connected with the poly(vinylpyridine) (P4VP) backbone via hydrogen bonds, resulting in the formation of nano-hybrid supramolecular polymers. Differential scanning calorimetry, polarized light microscopy, temperature-dependent X-ray diffraction, and transmission electron microscopy confirmed that the hybrid polymers containing PM-1 exhibited a thermotropic smectic C phase, where the alkyl chains of the surfactant are equally distributed on either side of PM-1 in a compact manner with only interfacial hydrogen bonding interactions between the phenol groups and the pyridine units of P4VP. Detailed investigation revealed that the liquid crystal properties are dependent on the molar ratio between PM-1 and P4VP, the molecular weight of P4VP and the charges of PMs. An increase of charge from 4 to 6 results in a loss of liquid crystal behaviour. Although the liquid crystal structure of the hybrid polymers is independent of the molecular weight of the P4VP backbone, the transition temperatures are strongly related to the change of the molecular weight of P4VP. The UV-vis and XPS measurements reveal that these hybrid liquid crystal polymers show reversible photochromic properties because of the multi-electronic redox activity of PM-1. The present article opens up a door for developing nano-hybrid liquid crystal polymers via a multi-step supramolecular self-assembly strategy, and provide an insight into understanding the nature and mechanisms of the influence of nano-objects on the self-assembly behaviour of hybrid polymers.

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Introduction

Liquid crystal polymers (LCPs), consisting of repeating mesogenic monomer units in the main chain or side chain of the polymer backbone, are acknowledged to be fascinating objects in the fields of chemistry and materials science because they possess both the orientation characteristics of low-molecular mass liquid crystals and the physicomechanical properties of traditional polymers.^1,2 For example, the mesogenic units of LCPs can be orientated next to one another along the shear direction when they flow in the liquid crystal state, which create locally oriented domains, and in turn, macroscopic-oriented regions.³ The anisotropic fluid state of LCPs has been utilized for processing of fibers, films and three-dimensional structures with high strength, stiffness, chemical resistance, and good dimensional stability.^1,4–6 The rational design and synthesis of LCPs are undoubtedly important and the primary concern with current research focuses on engineering the LCPs with multifunctional properties. Addressing this theme, the construction of hybrid LCPs have been rapidly emerging as types of new LCPs with unexpected properties.^5,7–9 Supramolecular LCPs, which employs non-covalent interactions to hold different components together, provide an important contribution in the field of hybrid LCPs because the relatively low activation energy of the non-covalent interactions implement dynamic feature into LCPs.^10,11 More importantly, supramolecular polymerization opens up a wide variety of possibilities for integrating “rigid” inorganic components into LCP matrix, which endows this system with new properties, including color, luminescence, and magnetism.^9,12–14 There are various inorganic components available for the synthesis of hybrid LCPs. Among them, inorganic nano-objects are particularly attractive because the resultant nano-hybrid LCPs have significant potential for the creation of extraordinary properties in terms of functional applications in the fields of nanoscience and materials science. The introduction of nano-objects into the LCP matrix allows the development of new materials with unique traits of nano-objects.^14–16 Meanwhile, the intrinsic order character and high processability of LCPs may improve the macroscopic organization and orientation of inorganic nano-objects, which is crucial for use in devices.^17,18 In spite of the salient advantages, the number of supramolecular nano-hybrid LCPs is rather rare, and the investigations are restricted to Fe₃O₄,¹⁴ TiO₂,¹⁷ Ag,¹⁵ Au,¹⁶ and CdSe¹⁸ nanoparticles (NPs) covered with polymers. On the other hand, the structure–property relationships of the reported hybrid LCPs are virtually unknown but are highly desirable for the design of new nano-hybrid LCPs. However, pristine NPs possess a broad size distribution, which may negatively influence the ordered arrangement of mesogens. In addition, these NPs usually have different polyhedral structures, which may affect the bonding location and density of adsorbate molecules, leading to an inhomogeneous distribution of ligands at the surface of NPs. These unfavourable structural and morphologic nature of NPs hinders accessibility to LC properties of the resultant nano-hybrid LCPs, and hampers a better understanding of how the nature of NPs affects the liquid crystal property. To extend the molecular pool and advance the utility of nano-hybrid LCPs, the synthesis and characterization of new LCPs carrying mono-dispersed nano-objects present a high-priority task. Study on such kind of nano-hybrid LCPs not only allows one to develop unexpected functional materials, but also makes it possible to understand the nature and mechanisms of influence of nano-objects on the self-assembly behavior of the hybrid LCPs.

Polyoxometalates (PMs) are mono-dispersed metal oxide clusters,^19–21 have precise chemical topology and compositions, and show multi-electronic redox activities, luminescence, photochromism, magnetism and catalysis.^22–27 This kind of nano-clusters have been considered as ideal building blocks for the fabrication of hierarchical ordered assemblies^28–31 and hybrid polymers.^32–36 We have demonstrated that anchoring mesogenic ligands onto the surface of PMs is a convenient and effective way to produce thermotropic liquid crystals.^37–39 We found that the nano-phase separation of incompatible molecular components, the aggregation of compatible units and the volume minimization effect of the ligands caused the molecular deformation, such as the redistribution or migration of the organic ligands on the surface of the PMs, which led to the formation of well-defined self-assembly structures.⁴⁰ Therefore, we anticipate that LCPs consisting of PM clusters and polymer backbones could be generated through rational molecular design. We herein developed a two-stage self-assembly process in which the PM clusters were first surface-functionalized via ionic self-assembly to create surfactant-encapsulated PMs (SEPs) with hydrogen donor moieties, and the resultant SEPs were subsequently connected with poly(4-vinylpyridine) (P4VP) via complementary hydrogen bonds, giving rise to the PM-containing LCPs. A schematic model depicting this procedure is illustrated in Fig. 1. The present article details the fabrication and characterization of side-chain LCPs based on PMs. We scrutinize the relationship between LC property and the nature of components. Interestingly, the photochromic property of the PMs is retained in the LCP matrix, which indicates significant potential in the development of multi-functional LCP materials.

Fig. 1 Schematic representation of the fabrication of PM-containing liquid crystal polymers via two-step supramolecular self-assembly strategy.

Experimental

Materials

H₄SiW₁₂O₄₀ (PM-1) was obtained from Aladdin GmbH, K₆P₂W₁₈O₆₂ (PM-2) and Na₁₀[Co₄(H₂O)₂(VW₉O₃₄)₂] (PM-3) were freshly synthesized according to reported procedures.^41,42 The synthesis and characterization of surfactant (S) carrying phenol terminal is described in the ESI.† The atactic P4VP_m (m is the number of repeated pyridine unit) with different molecular weight (Mv (P4VP₅₇₁) ≈ 60 000 g mol⁻¹, Mv (P4VP₃₂₅) ≈ 34 200 g mol⁻¹, Mv (P4VP₁₄₃) ≈ 15 000 g mol⁻¹, Mv (P4VP₆₇) ≈ 7500 g mol⁻¹) were acquired from Polymer Source Inc., and dried at 60 °C for 2 days under vacuum.

Measurements

Elemental analysis (EA) was operated in a Flash EA1112 from ThermoQuest Italia S.P.A. Electrospray ionization-mass spectra (ESI-MS) were obtained from Q-TOF SYNAPT G2 High Definition Mass Spectrometer (HDMS). The ESI source in negative ion mode was used. The source temperature was 120 °C, and desolvation gas temperature was 350 °C. The flow rates of cone and desolvation gas were set at 50 L h⁻¹ and 700 L h⁻¹, respectively. Capillary, cone and extraction cone voltages were set at 2.5 kV, 30 V and 5.0 V, respectively. Data was collected in full-scan mode in the mass range of 100−5000 Da. Leucine enkephalin was used as the lock mass. FT-IR spectra and temperature-dependent IR spectra were recorded on a Bruker Optics VERTEX 80v Fourier transform infrared spectrometer, equipped with a DTGS detector in pressed KBr pellets. A resolution of 4 cm⁻¹ was chosen, and 32 scans were signal-averaged. The thermogravimetry analysis (TGA) test was carried out on a Perkin-Elmer TG/DTA-7 instrument with a heating rate of 10 °C min⁻¹, and the temperature range is set from 30 to 900 °C. X-ray photoelectron spectroscopy (XPS) was performed on an ES-CALAB Mark (VG Company, UK) 250 spectrometer with a monochromic X-ray source (Al Kα line, 1486.6 eV) and the charging shift was corrected by the binding energy of C (1s) at 284.6 eV. The optical textures of the mesophases were studied with a Zeiss Axioskop 40 polarized light microscope equipped with a Linkam THMSE 600 hot stage, a central processor, and a DF1 cooling system. Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 204 with a scanning rate of 5 °C min⁻¹. All the samples were sealed in aluminum capsules in air, and the atmosphere in the holder was sustained under dry nitrogen. For temperature-dependent X-ray diffraction (TD-XRD) experiments, a Bruker AXS D8 ADVANCE X-ray diffractometer using Cu-Kα radiation of wavelength 1.54 Å and a mri Physikalische Geräte GmbH TC-basic temperature chamber was used. Transmission electron microscopy (TEM) experiments were carried out in JEOL-2010 electron microscope operating at 200 kV. The photochromic experiments are carried out with a 300 W xenon lamp as light source at room temperature. The LCP film samples were placed at a distance of 10 cm from the light source for a certain time.

Preparation of surfactant-encapsulated polyoxometalates (SEP-n)

SEP-1 was synthesized using reported procedures.^37–39 PM-1 was first dissolved in aqueous solution, and then the chloroform solution of quaternary ammonium (S) was added to the aqueous solution of PM-1 dropwise. The initial molar ratio of S to PM-1 was kept at 4 : 1. The mixture was stirred for 8 h at 50 °C under nitrogenous atmosphere. The resultant precipitate was washed three times with water and chloroform, respectively, to remove the free PM-1 and quaternary ammonium. Subsequently, the precipitate was dissolved in mixed solvent (methanol–chloroform, v/v = 1/1) and dried with anhydrous MgSO₄. Then, the solvent was removed using a rotary evaporator and the resultant solid sample was further dried in a vacuum until its weight remained constant. FT-IR (KBr, cm⁻¹): ν = 3376, 3037, 2924, 2852, 1611, 1595, 1490, 1468, 1394, 1289, 1148, 973, 920, 882, 794. Anal. calcd for SEP-1 (C₁₂₈H₂₄₀O₄₈N₄SiW₁₂, 4837.44): C, 31.78%; H, 5.00%; N, 1.16%. Found: C, 31.93%; H, 4.9%; N, 1.07%. TGA shows no mass loss from 30–160 °C, suggesting the absence of crystal water in SEP-1. Combining TGA and EA, SEP-1 should correspond to a tentative formula: (S)₄[SiW₁₂O₄₀].

SEP-2 was synthesized followed the similar procedure as for SEP-1, by using PM-2 instead of PM-1. The initial molar ratio of S to PM-2 was kept at 5.5 : 1. FT-IR (KBr, cm⁻¹): ν = 3327, 3037, 2923, 2853, 1612, 1594, 1490, 1466, 1393, 1288, 1147, 1091, 1021, 956, 914, 818, 786. Anal. calcd for SEP-2 (C₁₉₂H₃₆₆O₇₇N₆P₂W₁₈, 7362.02): C, 31.32; H, 5.01; N, 1.14. Found: C, 31.60; H, 4.88; N, 1.22. TGA suggests a mass loss of 0.2% from 30–160 °C, arising from crystal water in SEP-2. Combining TGA and EA, SEP-2 should correspond to a tentative formula: (S)₆[P₂W₁₈O₆₂]·H₂O.

SEP-3 was synthesized followed the similar procedure as for SEP-1, by using PM-3 instead of PM-1. The initial molar ratio of S to PM-3 was kept at 8 : 1. FT-IR (KBr, cm⁻¹): ν = 3223, 3039, 2922, 2852, 1613, 1594, 1490, 1467, 1392, 1332, 1288, 1148, 1074, 1044, 952, 875, 826. Anal. calcd for SEP-3 (C₂₈₈H₅₇₀O₁₀₁N₉NaCo₄V₂W₁₈, 9445.33): C, 36.62; H, 6.08; N, 1.33. Found: C, 36.18; H, 5.65; N, 1.33. TGA suggests a mass loss of 2.45% from 30–160 °C, arising from crystal water in SEP-3. Combining TGA and EA, SEP-3 should correspond to a tentative formula: Na(S)₉[Co₄(H₂O)₂(VW₉O₃₄)₂]·13H₂O.

Preparation of hydrogen-bonding induced hybrid polymers (P4VP_m/SEP-n)

P4VP_m/SEP-n hybrid polymers were prepared as follows: SEP-n was dissolved in dried dimethylfomamide (DMF) at 60 °C until a clear solution resulted. Subsequently, the DMF solution of P4VP_m was added dropwise, followed by mechanical stirring at 60 °C. The concentration of P4VP_m was kept low (less than 2.5 wt%) to ensure the homogeneous complex is formed between SEP-n and P4VP_m. The resultant mixture was stirred mechanically for approximately 5 h. After the reaction was completed, the solvent was first evaporated on a hot plate at 40 °C with a nitrogen flow. The final samples P4VP_m/(SEP-n)_x were dried in a vacuum at 60 °C for 2 days, and thereafter were stored in a desiccator. Here, x is the stoichiometric ratio of phenol groups of the SEP-n to pyridine units of P4VP_m. In this study, the values of x were taken to be 0.25, 0.35, 0.5, 0.75 and 1.0.

Results and discussion

Characterization of SEP-n and P4VP_m/SEP-n

The synthesis and characterization of the ammonium surfactant (S) were shown in ESI (Scheme S1, Fig. S1 and S2†). The SEP-n were synthesized by using S to encapsulate [SiW₁₂O₄₀]⁴⁻ (PM-1), [P₂W₁₈O₆₂]⁶⁻ (PM-2) and [Co₄(H₂O)₂(VW₉O₃₄)₂]¹⁰⁻ (PM-3), respectively, through an ion-exchange reaction. All of the prepared SEP-n are not soluble in common organic solvent except DMF. We carried out FT-IR, EA, TGA and ESI-MS to characterize the SEP-n. Herein, we take the SEP-1 as a representative example to describe the detailed characterization. The characteristic vibrations attributable to surfactant (S) at 2924 (νas CH₂), 2852 (νs CH₂), 1611 (ν phenyl ring), 1595 (ν phenyl ring), 1490 (ν phenyl ring), 1468 (δ CH₂), and to PM-1 at 973 (νas W=O_d), 882 (νas W–O_b–W), 794 (νas W–O_c–W) were observed in the FT-IR spectrum of SEP-1 (Fig. 2), indicating the success of the encapsulation process. The detailed assignments were summarized in Table S1.† ESI-MS measurements were performed to confirm the molecular weight of SEP-1. It is expected that SEP-1, consisting of anionic cluster and cationic surfactants, may give several negative signals because the ammonium surfactants (S) have strong propensity to disassociate in a stepwise manner from the PM-1 cluster. As shown in Fig. S3,† three main peaks could be identified, which correspond to three different charge states of SEP-1. The peak at m/z = 718.78 corresponds to the SiW₁₂O₄₀⁴⁻ fragment (calcd 718.53) and peaks at m/z = 1121.87 and 1927.99 belong to the (S)[(SiW₁₂O₄₀)]³⁻ (calcd 1121.66) and (S)₂[(SiW₁₂O₄₀)]²⁻ (calcd 1927.89) fragments, respectively. The EA data revealed that all four negative charges of PM-1 were neutralized with cationic surfactant (S), hence the proposed formula of SEP-1 was (S)₄[SiW₁₂O₄₀]. Furthermore, the TGA data (Fig. S4†) also supported the formation of the proposed molecular weight. Supposing that the organic component decomposed completely at 900 °C, and the only residual species were inorganic oxides SiO₂ and WO₃, the measured weight residue of 59.27% coincided with the calculated value of 58.80% based on the postulated molecular formula. The above data indicates that the SEP-1 consists of one PM-1 and four ammonium surfactants. Similar methods were applied to characterize the SEP-2 and SEP-3 (Fig. S5†), and their final molecular formulas are proposed in “Experiment section”.

Fig. 2 FT-IR spectra of surfactant (S), PM-1 and SEP-1 at solid state.

As shown in Fig. 1, all of the SEP-n possess a nano-sized core and a hydrophobic organic shell composed of alkyl chains with phenol groups at the terminus, which allows the subsequent formation of P4VP_m/(SEP-n) polymers via hydrogen bond because of the presence of complementary functional groups (acidic phenol groups and basic pyridine units). In this scenario, we first synthesized a series of complexes P4VP₅₇₁/(SEP-1)_x (x = 0.25, 0.35, 0.5, 0.75, 1.0) to evaluate the influence of molar ratio between phenol groups and pyridine units on the liquid crystalline behaviour of the H-bond complexes. Then, we prepared the following complexes: P4VP₅₇₁/(SEP-1)_1.0, P4VP₃₂₅/(SEP-1)_1.0, P4VP₁₄₃/(SEP-1)_1.0, P4VP₆₇/(SEP-1)_1.0 to study the dependence of the LC property of the P4VP_m/(SEP-n) on the molecular weight of P4VP backbones. We also tried to clarify the influence of nature of PMs on the self-assembly behavior of the nano-hybrid LCPs by comparing the thermal properties of P4VP₅₇₁/(SEP-1)_1.0, P4VP₅₇₁/(SEP-2)_1.0 and P4VP₅₇₁/(SEP-3)_1.0. Finally, the photochromic property of the obtained LCPs was demonstrated by UV-vis and XPS spectra.

The characterization of hydrogen-bonding formation between P4VP-m and SEP-n

The hydrogen bonding formation can be confirmed by XPS measurements because the binding energy of N 1s in pyridine ring is very sensitive to the electronic distributions.^43,44 In general, the formation of hydrogen-bonded or protonated nitrogen will lead to an upshift of the binding energy of N 1s due to the decreased electron density in the nitrogen atom of pyridine ring.^43,45 Regarding the hydrogen-bonded nitrogen, the change in the binding energy of N 1s is less than 1.0 eV. For protonated nitrogen, however, its binding energy change is usually more than 2.0 eV.^43,46 The XPS spectra of SEP-1, P4VP₅₇₁, and P4VP₅₇₁/(SEP-1)_x complexes are given in Fig. 3, and the corresponding N 1s binding energies are summarized in Table 1. It is observed that the N 1s binding energy values of the free pyridine unit of P4VP and the ammonium group of SEP-1 were located at 398.5 eV and 402.45 eV, respectively. In the case of P4VP₅₇₁/(SEP-1)_x polymers, the ammonium N 1s peaks kept constantly at 402.4 eV, however, the pyridine N 1s peak clearly shifted to a higher binding energy region (399.05 eV). The change ranging from 0.5–0.75 eV in N 1s binding energy of pyridine suggests the formation of hydrogen bonding in P4VP₅₇₁/(SEP-1)_x. In addition, in relative to the intensity of the N 1s peak of pyridine, the intensity of the N 1s peak of ammonium gradually increases while increasing the content (x) of SEP-1. The changing trend in intensity of N 1s corresponds directly to the change of stoichiometric ratio between phenol groups and pyridine unites.

Fig. 3 XPS spectra of N 1s of SEP-1, P4VP₅₇₁, and P4VP₅₇₁/(SEP-1)_x complexes.

Table 1 The binding energy values of N 1s obtained from XPS spectra

Sample	N+ (eV)	Pyridine-N (eV)
SEP-1	402.45	—
P4VP571/(SEP-1)1.0	402.3	399.01
P4VP571/(SEP-1)0.75	402.5	399.25
P4VP571/(SEP-1)0.5	402.4	399.08
P4VP571/(SEP-1)0.35	402.3	399.06
P4VP571/(SEP-1)0.25	402.4	399.05
P4VP571	—	398.50

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FT-IR spectroscopy is another useful tool to study the specific interactions between hydroxyl groups and pyridine.^47,48 Upon formation of hydrogen bond, the vibration bands of the pyridine ring usually shift to higher frequencies due to the change in local environment. Herein, we show the FT-IR spectra of the precursors alone and the resultant P4VP₅₇₁/(SEP-1)_x to identify the hydrogen bonding formation. As shown in Fig. 4a, the characteristic stretching modes of free pyridine ring of P4VP₅₇₁ alone are concerned at 1597, 1555, 993, and 627 cm⁻¹, and the absorption peaks of phenyl ring of SEP-1 alone are located at 1611, 1595, and 1490 cm⁻¹. In principle, P4VP has an absorption band at 1597 cm⁻¹, and this band will shift to 1603 cm⁻¹ on the formation of hydrogen bonds.^49,50 However, in the case of P4VP₅₇₁/(SEP-1)_x, it seems difficult to evaluate the binding degree between P4VP₅₇₁ and SEP-1 based on the band shift of 1597 cm⁻¹ due to the overlapping absorption peak at 1595 cm⁻¹ of SEP-1 alone. To clarify the formation of the hydrogen bonds in more detail, the IR data in the middle frequency region (1800–1400 cm⁻¹) was evaluated because there is no overlapping absorption at 1415 cm⁻¹. It is established that the absorption band of pyridine ring at 1415 cm⁻¹ is strong indicator of the local environment: upward band shift is indicative of the formation of hydrogen bonding.^47,51,52 It may be observed clearly from Fig. 4b that the absorption band of P4VP₅₇₁ at 1415 cm⁻¹ shift approximately +5 cm⁻¹ in the resultant P4VP₅₇₁/(SEP-1)_x, indicating the presence of hydrogen bonds. The peak located at 1554 cm⁻¹ also depicts upward shift. Additionally, the absorption intensity decreased with increasing the molar ratio x. In the low frequency region (1200–950 cm⁻¹, Fig. 4c), the peak situated at 993 cm⁻¹, which attributes to the vibration of free pyridine of P4VP₅₇₁ also shifts to higher wavenumber and diminishes with increasing the SEP-1 content. These band shifts related to the 1554, 1415 and 993 cm⁻¹ indicate the formation of hydrogen bonds in P4VP₅₇₁/(SEP-1)_x. In the case of P4VP₅₇₁/(SEP-1)_1.0, most pyridine units have constructed complexes with phenol groups via hydrogen bonds, because the absorption bands of the free pyridine rings at 1554 and 993 cm⁻¹ have almost disappeared.^53–55

Fig. 4 FT-IR spectra of P4VP₅₇₁, SEP-1 and P4VP₅₇₁/(SEP-1)_x (x = 0.25, 0.35, 0.5, 0.75, 1.0) at room temperature: (a) 3800–400 cm⁻¹; (b) 1800–1400 cm⁻¹; (c) 1200–950 cm⁻¹.

The temperature-dependent FT-IR spectra was also carried out to evaluate the thermal stability of hydrogen bonds in the P4VP₅₇₁/(SEP-1)_x. As a representative example, the FT-IR spectra of P4VP₅₇₁/(SEP-1)_1.0 is given in Fig. 5. It is clear that the stretching bands (1558, 1420, and 993 cm⁻¹) of the pyridine ring in P4VP₅₇₁/(SEP-1)_1.0 are almost constant in position and intensity even after the sample has been heated up to 175 °C (Fig. 5a), suggesting that the hydrogen bonding is stable throughout the temperature range of 30–175 °C. In addition, the framework structure of PM-1 is intact and not distorted during the heating process based on the constant band positions (972, 920, 798 cm⁻¹) in Fig. 5b. TGA measurements (Fig. S6†) further confirmed that P4VP₅₇₁/(SEP-1)_1.0 did not decompose until the temperature over 233 °C.

Fig. 5 Temperature-dependent Infrared spectra of P4VP₅₇₁/(SEP-1)_1.0: (a) middle frequency region (1750–1400 cm⁻¹); (b) low frequency region (1200–700 cm⁻¹).

Thermal behavior of P4VP₅₇₁/(SEP-1)_x hybrid polymers

As shown in the DSC traces of Fig. 6a, upon heating, P4VP₅₇₁ alone exhibits a glassy transition at 122.5 °C, SEP-1 alone undergoes a single phase transition at 84.2 °C and then quickly develops into an isotropic liquid. These phase transitions were also confirmed by polarized light microscopy (PLM). In contrast, the heating thermograms of the hybrid polymers at 5 °C min⁻¹ exhibit different phase behavior. P4VP₅₇₁/(SEP-1)_1.0 undergoes two endothermic transitions at 99.2 and 144.9 °C, respectively. Furthermore, it is observed from PLM measurement that birefringent textures together with homeotropic regions grow slowly upon heating the P4VP₅₇₁/(SEP-1)_1.0 sample (Fig. 7a and S7†). It should be noted that the hybrid polymers exhibit high viscosity, but fluid textures could be observed with the help of extra mechanical stress. We come to a conclusion that the high viscosity resulted from the multiple hydrogen bonding interactions between P4VP and SEP-1 together with the cross-linking network structures. The stated results above strongly identify that the nano-hybrid polymer exists in a liquid crystalline state in the temperature range of 100–145 °C. The broad peak at the clearing point corresponds to a slow phase transition from liquid crystalline state to isotropic phase. Interestingly, the LC properties of the hydrogen-bonding complexes are strongly dependent on the phenol/pyridine molar ratios (x). First, the PLM measurements show that the fluid birefringent textures are maintained when decreasing the molar ratio until 0.5, where the fluid birefringence disappears (Fig. 7) and a single phase transition from solid to isotropic liquid is observed. Secondly, the molar ratio difference is regularly reflected in the clearing point temperatures, which decrease markedly from 144.9, 127.2 to 110.9 °C while decreasing phenol/pyridine molar ratio from 1.0, 0.75 to 0.5 (Fig. 6b). When the molar ratio is below 0.5, the transitions are significantly suppressed. The above results suggest that the hydrogen-binding polymer can form liquid crystalline state when the phenol/pyridine molar ratio reaches a critical value, and the ability to stabilize the LC state corresponds to the following trend: the higher the molar ratio, the more stable the mesophase.

Fig. 6 (a) DSC curves of P4VP₅₇₁/(SEP-1)_1.0, P4VP₅₇₁, SEP-1; (b) DSC curves of P4VP₅₇₁/(SEP-1)_x (x = 1.0, 0.75, 0.5, 0.35, 0.25). (All the traces were obtained upon heating run with a scanning rate of 5 °C min⁻¹).

Fig. 7 PLM images of P4VP₅₇₁/(SEP-1)_x: (a) P4VP₅₇₁/(SEP-1)_1.0 at 130 °C; (b) P4VP₅₇₁/(SEP-1)_0.75 at 115 °C; (c) P4VP₅₇₁/(SEP-1)_0.5 at 97 °C; (d) P4VP₅₇₁/(SEP-1)_0.35 at 94 °C; (e) P4VP₅₇₁/(SEP-1)_0.25 at 94 °C.

X-ray diffractograms of various blends at room temperature are shown in Fig. 8a. For the hybrid polymers with phenol/pyridine molar ratios of 0.5 and more, all of the diffraction peaks are present at similar positions. In the small-angle region, equidistant diffractions are found, which are indexed with the 00l Miller indices of a 1D lamellar ordering with a lattic parameter d = 4.8 nm (d was calculated according to the formula [d₀₀₁ + 2d₀₀₂ + 3d₀₀₃ + 4d₀₀₄]/4. It is also noted that the intensity of the 001 reflection is weak relative to that of 002, implying that the lamellar order is short-ranged, and the position of the diffracting centers is not correlated from layer to layer. An additional reflection centered at d′ ≈ 1.05 nm is commonly found in the ordered state for in-plane inter-cluster distance, i.e. an intra-layer order associated with a specific arrangement of the strongly diffraction inorganic cores.^56,57 Below 0.5, however, the diffraction patterns are significantly different, the relative broadness of the diffractions and the disappeared 001 reflection suggest that SEP-1 is dispersed throughout the P4VP₅₇₁ polymer matrix and is not able to condense and self-assemble into successively ordered domains any more.

Fig. 8 (a) XRD pattern of SEP-1, P4VP₅₇₁, and P4VP₅₇₁/(SEP-1)_x (x = 1.0, 0.75, 0.5, 0.35, 0.25); (b) temperature-dependent XRD pattern of P4VP₅₇₁/(SEP-1)_1.0 upon heating process.

X-ray diffraction scans as a function of temperature allowed the identification of the liquid crystals. As shown in Fig. 8b, the lamellar structures and the corresponding periodicity d (4.8 nm) of P4VP₅₇₁/(SEP-1)_1.0 are almost invariant upon heating in the temperature region corresponding to the DSC transition (about 105–135 °C). The diffractions at small angles gradually become broad and/or disappear when the temperature rises further, indicative of the gradual growth of the isotropic state. The layer periodicity of the hybrid polymers with phenol/pyridine molar ratios above 0.5 are very similar to that of SEP-1 alone, suggesting that the SEP-1 complexes within the polymer matrix form lamellar sandwich structures, in which the peripheral alkyl chains are equally distributed on either side of the PM-1 cluster in a compact manner (cylindrical molecular conformation) with only interfacial hydrogen bonding interactions between the phenol groups of SEP-1 and the pyridine units of P4VP₅₇₁. It is clear that the SEP-1 act as a cross-linker to stabilize the lamellar mesophase via multiple hydrogen bonds. The broad halo centered at wide angle (2θ ≈ 20°) reveals that the liquid crystalline polymers possess less ordered smectic phase. Considering the diameter (1.0 nm) of PM-1,⁵⁸ the ideal molecular length (2.01 nm) of S with zig–zag conformation, and the thickness (0.41 nm) of P4VP backbone, the layer distance of P4VP₅₇₁/(SEP-1)_1.0 should be around 5.82 nm. The calculated layer distance is much larger than that of the experimental periodicity (4.8 nm), indicating that the sublayers containing the alkyl chains and hydrogen-binding units adopt tilted conformations (smectic C phase, Fig. 1), which will lead to the shortened layer spacing.⁵⁹

Influence of the nature of components on the LC behavior of hybrid polymers

The thermal properties of P4VP₅₇₁/(SEP-1)_1.0, P4VP₃₂₅/(SEP-1)_1.0, P4VP₁₄₃/(SEP-1)_1.0, and P4VP₆₇/(SEP-1)_1.0 are compared to detect the LC behavior dependence of the hybrid polymer on the molecular weight of P4VP_m. The temperature-dependent XRD patterns of all the hybrid polymers (Fig. 9) reveal the presence of layer structures with similar layer periodicity, indicating that the molecular weight of the P4VP_m backbones has no apparent influence on the self-assembly structures.

Fig. 9 Temperature-dependent XRD pattern of LCPs: (a) P4VP₃₂₅/(SEP-1)_1.0; (b) P4VP₁₄₃/(SEP-1)_1.0; and (c) P4VP₆₇/(SEP-1)_1.0.

We further investigated the frozen samples by TEM to confirm the structure of the LC state. The sample of hybrid polymers were heated until they changed to LC state, holding the state isothermally for 2 min and then quenching to preset annealing temperature for 1 h. Then, the thin films were removed from the glass slide, floated on top of the water surface, and recovered using copper grids for TEM observations. As shown in Fig. 10, well-defined layer structures of the hybrid polymers were obtained. The dark region corresponds to the inorganic PM clusters whereas the light region corresponds to the organic part. The lamellar structures give ca. 4.8 ± 0.5 nm of layer-to-layer spacing, which is consistent with the layer periodicity estimated from XRD. It is further concluded from the TEM results that the self-assembly behavior of the liquid crystal polymers improve the ordered organization of inorganic nano-objects. The PLM measurements showed that the hybrid LCPs exhibited birefringent textures (Fig. S8†) in the LC temperature range. On further heating, the LC textures gradually disappear and change into isotropic states. Although the liquid crystal structure is independent of the molecular weight of P4VP backbone, the transition temperatures of the hybrid polymers are related to the change of the molecular weight. It was observed from the PLM measurements that the clearing point temperatures of the hybrid polymer decrease with decreasing the molecular weight of P4VP_m (see Table S2†). The heating thermograms of the hybrid polymers further support the dependence of transition temperature on the molecular weight.

Fig. 10 TEM images of hybrid polymers quenched by liquid nitrogen under liquid crystalline temperatures: (a) 130 °C for P4VP₅₇₁/(SEP-1)_1.0; (b) 125 °C for P4VP₃₂₅/(SEP-1)_1.0; (c) 120 °C for P4VP₁₄₃/(SEP-1)_1.0; (d) 115 °C for P4VP₆₇/(SEP-1)_1.0.

The mono-dispersed size of PMs allows one to study the LC property dependence of the hybrid LCPs on the nature of PMs. To do this, SEP-2 and SEP-3 were used respectively to connect with P4VP₅₇₁ to form hybrid polymers. However, the resultant hybrid polymers P4VP₅₇₁/SEP-2 exhibit different self-assembly behavior. No typical LC characteristics were observed for P4VP₅₇₁/SEP-2 based on the PLM measurements (Fig. S9†). XRD patterns (Fig. S10†) showed that the hybrid polymers indicating the formation of less ordered structures. We have summarized in our recent review article how the charges of PMs affect the surfactant density and the LC structures of PM-surfactant hybrids (SEPs).⁴⁰ In fact, when the organic surfactants are bounded on the surface of rigid nano-sized cores through electrostatic interaction, the ammonium groups are located in a highly restricted environment. We found that the SEPs are deformable due to the redistribution or migration of the alkyl chains of surfactant on the surface of PMs. The overall architecture of the SEPs is dependent on the charge of PMs or the surfactant density on the surface of PMs. The charge of PMs is proportional to the surfactant density of the resultant SEPs. The increased negative charge of PMs will lead to the high surfactant density. The higher surfactant density, the higher interfacial curvature of the PM-surfactant hybrids, and in turn the less ordered structures. In the present article, we have concluded that the surface charges of PM-2 are higher than that of PM-1. The high surface charges of PM-2 result in the number or density of surfactant (S) in SEP-2 is larger than that of SEP-1. We proposed that the higher density of surfactant in SEP-2 cause an increase of in surface curvature, which leads to a less ordered self-assembly structure. This speculation was supported by P4VP₅₇₁/SEP-3, which depicted the carrying of more surface charges.

Photochromic properties of the hybrid polymers

PM clusters are known for their remarkable photochromic properties due to the obvious visible color changes in their reduced state (as heteropoly blues) and their ability to accept and release electrons without structural changes.⁶⁰ To verify that the photochromic properties of the PM core were well retained in the LCP matrix, the film of P4VP₅₇₁/(SEP-1)_1.0 was irradiated with UV light under methanol atmosphere because the alcohol molecules acting as sacrificial reagents can improve the occurrence of photochromism.^61,62 The film was set at a distance of 10 cm from the light source. After UV irradiation with a 300 W xenon lamp, the colorless film of P4VP₅₇₁/(SEP-1)_1.0 turns blue (Fig. 11a). The UV-vis spectra (Fig. 11b) of the hybrid film before and after photo irradiation showed that the characteristic O → W ligand-to-metal charge-transfer (LMCT) band of PM-1 at ca. 264 nm can be clearly observed in both of the spectra, suggesting that the structure of PM-1 is well retained during the photochromic process. The new broad absorption band appearing in the visible region at ca. 748 nm (Fig. 11b, inset) after UV irradiation attributes to the W⁶⁺ → W⁵⁺ inter-valence charge transfer (IVCT), indicative of the formation of reduced PM-1.^62,63

Fig. 11 Photochromic process of P4VP₅₇₁/(SEP-1)_1.0 film: (a) photographs of the hybrid film (1) before and (2) after UV irradiation for 15 min; (b) UV-vis absorption spectra of the hybrid film before and after irradiation for 15 min.

We further employed X-ray photoelectron spectroscopy (XPS) to characterize the valence changes of W atoms. As shown in Fig. 12a, the binding energy at 38 and 35.9 eV is attributed to the W⁶⁺ 4f_7/2 and 4f_5/2, respectively, indicating that the valence of all W atoms is +6 in the initial state. After 15 min of light irradiation, the peaks at 38 and 35.9 eV maintained meanwhile two new bands at 36.9 and 34.8 eV were observed, indicating that some of W⁶⁺ ions had been reduced to W⁵⁺ accompanied by the coloration process. The amount of W⁵⁺ can be estimated as ca. 32% by analysis of peak separation and integral area ratio of W⁶⁺ and W⁵⁺. When the irradiation stopped, the blue film of reduced P4VP₅₇₁/(SEP-1)_1.0 faded gradually, and the decoloration process was completed in 2 h at room temperature in the dark. The absorption and XPS spectra of the faded film are identical with its original state, and no W⁵⁺ signal is detected, indicating that all W⁵⁺ are oxidized to W⁶⁺ again during the bleaching process. In addition, it should be noted that the blue color could last for a quite long time if the hybrid film is stored in a vacuum. This phenomenon is similar to reported results in the literature,⁶⁴ which suggested that O₂ acted as a catalyst for the oxidation of the reduced PM-1 in the hybrid film during the bleaching process. The color change of the hybrid film is reversible, and the coloration–decoloration cycle can be repeated several times (Fig. 12b), implying highly photochromic stability and potential applications in the areas of photo switches or memory devices.

Fig. 12 XPS spectra of W 4f level of P4VP₅₇₁/(SEP-1)_1.0 film (a) before and (b) after UV irradiation for 15 min.

Conclusions

We designed and synthesized a type of PM-containing LCPs based on two-step supramolecular self-assembly process. These hybrid polymers exhibited smectic phase upon heating. The SEP-1 carrying phenol terminals acted as cross linkers to hold the P4VP backbone together and form a sandwich layer structure. In this case, the stability of the mesophases could be enhanced by increasing the molar ratio (above 0.5) between phenol groups and pyridine units, or by increasing the molecular weight of the P4VP backbone in order to increase cross-linking strength. It was determined that the charge of PMs plays an important role in the formation of liquid crystal phase. An increase of charges from 4 to 6 causes a loss of liquid crystal behavior. These results suggest that the molar ratio, molecular weight of P4VP and charges of PMs should be carefully selected during the fabrication of PM-containing hybrid LCPs. TEM measurements reveal that the self-assembly of the LCPs improve the ordered arrangement of PM nano-cluster. More importantly, the obtained LCPs show reversible photochromic property. The present investigation not only provides an insight into understanding the nature of nano-objects on the self-assembly behavior of the hybrid LCPs, but also provides insight in order to improve the ability of developing functional liquid crystal polymers. We believe that multi-step supramolecular self-assembly strategy is available for the creation of hybrid LCPs based on other nano-objects. And the structure–property relationship observed in this article will provide helpful contribution in the design of hybrid LCPs carrying different nano-objects.

Acknowledgements

This work was supported by National Basic Research Program (2013CB834503), National Natural Science Foundation of China (91227110, 21221063, 20703019), Sci-Tech development project of Jilin Province (20130522133JH), 111 Project (B06009), and Open Project of State Key Laboratory of Polymer Physics and Chemistry of CAS.

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis and characterization of surfactant (S), FT-IR and TGA data of SEP-n, PLM and TEM images of the hybrid LCPs. See DOI: 10.1039/c4ra12174k

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