ABSTRACT

A new homologous series of thermotropic liquid crystalline copoly(arylidene-ether)s based on 4-teriary butyl cyclohexanone moiety were synthesized by solution polycondensation of 4,4’-diformyl-α,ω-diphenoxyalkanes, I_a-d or 4,4’- diformyl-2,2’-dimethoxy-α,ω-diphenoxyalkanes II_a-d with the 4-teriary butyl-cyclohexanone III and cyclopentanone. A model compound IV was synthesized from the monomer III with benzaldehyde and characterized by elemental and spectral analyses. The inherent viscosities of the resulting polymers were in the range 0.22 - 0.92 dI/g. All the copoly(arylidene-ether)s were insoluble in common organic solvents but dissolved completely in concentrated H₂SO₄ and formic acid. The mesomorphic properties of these polymers were studied as a function of the diphenoxyalkane space length. Their thermotropic liquid crystalline properties were examined by DSC and optical polarizing microscopy and demonstrated that the resulting polymers form nematic mesophases over wide temperature ranges. The thermogravimetric analyses of those polymers were evaluated by TGA and DSC measurements and correlated to their structural units. X-ray analysis showed that copolymers having some degree of crystallinity in the region 2q = 5^° - 60^°. In addition, the morphological properties of selected examples were tested by Scanning electron microscopy.

1. Introduction

Thermotropic liquid crystalline behavior of polymeric materials is of considerable current interest, not only because of their potential as high-strength fibers, plastics, moldings etc. [1-3], but also because of their unique position in the theoretical scheme of structural order in liquid phases [4]. These thermotropic liquid crystalline (mesomorphic) polymers have two basic types [5]: one type has flexible aliphatic spacer groups, used to insulate the delicate intermolecular interactions in the liquid crystalline portions from small, but disruptive motions of the main chain. The other type of thermotropic mesogenic polymer is one in which the liquid crystalline moiety is incorporated in the main polymer chain. The introduction of long aliphatic segments “spacers” was avoided to retain the relative rigidity and high axial ratio of the macromolecules which seem to be necessary, not only to stabilize the mesophase, but also to obtain superior mechanical properties [6,7]. On the other hand, the anisotropy of the liquid crystalline mesophase offers the possibility of production of novel high performance materials, exhibiting excellent properties due to a proper arrangement of macromolecules in the mesophase during the processing. The spacer length can cause periodic changes in the thermodynamic properties of a material (in particular, the transition properties) based on whether there are an even or odd number of unit in the LC polymer spacers [8-13]. Our previous papers described how thertropic liquid crystal homoand copoly(arylidene-ether)s containing cycloalkanone moieties were synthesized [14-17] and the relationship between structure and liquid crystallinity was evaluated. It was found that the copolymers showed stable mesophases over the entire range of composition. In this paper, attention was addressed to 4-teriary-butyl-cyclohexanone moiety to prepare new thermotropic liquid crystal homoand copoly(arylidene-ther)s. A major purpose of this work was to study the effect of inclusion of the 4-teriary butyl cyclohexanone moiety in the polymer main chain on the LC properties of these polymers. In addition, other characterization of these polymers such as thermostability, solubility, morphology, and crystallinity were discussed.

2. Experimental

2.1. Measurements

Elemental analyses were carried out using an Elemental Analyses system GmbH, VARIOEL, V_2.3 July 1998 CHNS Mode. Infrared Spectra from 4000 - 600 cm⁻¹ of solid samples of the synthesized monomers and polymers were obtained by the KBr method using a Shimadzu 2110 PC Scanning Spectrophotometer. The ¹H-NMR spectra were recorded on a GNM-LA 400-MHz NMR spectrophotometer at room temperature in DMSO or CDCl₃ using TMS as the internal reference. Mass spectra were recorded on a Jeol JMS₆₀₀ mass spectrometer. The inherent viscosity was measured with an Ubbelhode Viscometer in DMSO at 25˚C (0.5 g/L). The solubility of polymers was examined using 0.02 g of polymer in 3 - 5 ml of solvent at room temperature. The X-ray diffractographs of the polymers were obtained with a Philips X-ray pw 1710 diffractometer, and Ni-filtered CuK_α radiations. Thermogravimetric analysis (TGA) and differential thermalgravimetric (DTG) were carried out in air with TA 2000 thermal analyzer at heating rate of 10˚C/min. in air. The maximum position of the melting endotherms was taken to be the m.p.s. The isotropization temperatures were determined by observing polymer melts with a polarizing microscope, GARL-ZEISS (JENA) equipped with a hot-stage Chaixmeca (Nancy, France). The temperature at which initial formation of isotropic phases occurred was taken as the isotropization temperature, T_i. At the same time, optical textures of the polymer melts were very closely followed to determine the nature of their mesophase. The morphologies of polymers were examined by scanning electron microscopy (SEM) using a Jeol JSM-5400 LV instrument.

2.2. Reagents and Solvents

4-Teriary butyl cyclohexanone (Merk) was used without purification. p-Hydroxy-benzaldehyde (Aldrich) was used without crystallization. 4-Hydroxy-3-methoxybenzaldehyde (vanillin) from EL-Nassr Chemical Company (Egypt) was used as it is. Dihaloalkanes (Aldrich) were used without purification. All solvents and other reagents were of high purity and were further purified by standard methods [18].

2.3. Monomers Syntheses

4,4’-Diformyl-α,ω-diphenoxyalkanes I_a-d and 4,4’-diformyl-2,2’-dimethoxy-α,ω-diphenoxyalkane II_a-d. These monomers were prepared as described in our previous papers [16, 19].

2.4. Polymerization

The polycondensation of the dialdehydes and 4-teriary butyl cyclohexanone were carried out using the hightemperature solution method under the following conditions: the concentration of the monomers was 0.2 mol/L in ethanol (95%) in the presence of KOH (few drops) as a catalyst. The reaction temperature was 75˚C - 80˚C. Polymers were precipitated during the reflux, filtered off, washed with hot ethanol, acetone, and then dried under reduced pressure (1 mm/Hg) at 60˚C for 24 hours. All the poly(arylidene-ether)s were synthesized by an analogous procedure. Their yields, inherent viscosities, and elemental analyses are listed in Table 1.

3. Results and Discussion

3.1. Synthesis of Dialdehyde Monomers I_a-d and II_a-d

Two series of dialdehydes, namely of 4,4-diformyl-α,ω-diphenoxyalkanes I_a-d and 4,4-diformyl-2,2-dimethoxy-α,ω- diphenoxyalkanes II_a-d were prepared by interaction of one mole of dihaloalkanes with two moles of the sodium salt of 4-hydroxybenzaldehyde or sodium salt of 4-hydroxy-3-methoxy benzaldehyde respectively, in DMF in the presence of anhydrous potassium carbonate, as mentioned previously in our previous papers [13-16] as shown in Scheme 1.

The structures of these monomers were confirmed by elemental and spectral analyses as shown in the experimental part.

3.2. Synthesis of Model Compound 2, 6-Dibenzylidene-4-Tert-ButylcycloHexanone IV

Before attempting the polymerization process, model compound IV for the desired poly(arylidene-ether)s was synthesized by the interaction of 1 mole of the monomer named 4-tert-butyl-cyclohexanone with 2 moles of benzaldehyde in absolute ethanol and in the presence of few drops of (30%) alcoholic KOH as basic catalyst according to Scheme 2.

Scheme 1. Synthesis of dialdehydic monomers I_a-d, II_a-d.

Table 1. Solubility characteristic and inherent viscosity of copoly(arylidene-ether)s containing 4-tertiary butyl cyclohexanone moiety in the main chain V_a-d and VI_a-d.

++: Soluble at room temperature (RT); +: Partially soluble at (RT); −: Insoluble; ^*Inherent viscosity was measured in CHCl₃ at 23˚C.

Scheme 2. Synthesis of model compound IV.

The structure of model compound IV was confirmed by elemental and spectral analyses. The FT-IR spectrum of [2,6-dibenzylidene-4-tertbutylcyclohexanone] IV showed characteristic absorption bands at 2995 cm⁻¹ for (CH) stretching vibration of aromatic ring, at 1680 cm⁻¹ for (C =O) of cyclohexanone and at 1595 cm⁻¹ for (C=C) group of phenyl rings, beside other characteristics peaks were observed. 1H-NMR spectrum of IV in CDCl₃ showed singlet S at δ0.9 (9H of CH₃), multiple m at δ1.5 (1CH₂ cyclohexanone), multiple m at δ2.3 - 3.1 (2CH₂ cyclohexanone), multiple m at δ7.3 - 7.6 (2CH=C and 10 aromatic). The mass spectrum showed molecular ion peak at m/z = 330.45 (3.2%) which in agreement with its molecular formula (C₂₄H₂₆O), other peaks appeared at m/z = 273.52(M⁺-C₂₀H₁₇O, 2%), at m/z = 57.91(M⁺-C₄H₉ 2.1%), and at m/z = 90.48 (M⁺-C₇H₆ 5.4%).

3.3. Synthesis of Copoly(Arylidene-Ether)s V_a-d & VI_a-d

The present work had as its aim the synthesis of new thermotropic liquid crystal copoly(arylidene-ether)s V_a-d and VI_a-d by the solution of polycondensation of the two series of diformyl-α,ω-diphenoxyalkane (I_a-d) and diformyl-2,5-dimethoxy-α,ω-diphenoxyalkane (II_a-d) with 4- teriary butyl-cyclohexanone III and cyclopentanone and as shown in Scheme 3.

The structure of the resulting polymers was established from elemental and spectral analyses including (FT-IR and ¹H-NMR). The results of elemental analyses for the

Scheme 3. Synthesis of copoly (arylidene-ether)s V_a-f and VI_a-f.

copoly(arylidene-ether)s V_a-f and VI_a-f prepared agree with calculated values (see experimental part). It should be noted that the elemental analysis results from the polymer deviated by 2% - 5% for the proposed structures. These differences can be accounted for by: 1) poor solubility and hence poor reactivity in the initial stage of polymerization; 2) difference in the reactivity of monomers during the polymerization, which lead to polymers with different compositions, the so called “composition drift” [20-23].

The FT-IR spectra of the poly(arylidene-ether)s V_a-d and VI_a-d as examples, showed characteristic absorption bands at 3035 - 2926 cm⁻¹ for (CH) stretching vibration of aromatic ring, at 2869 - 2852 cm⁻¹ for (CH) stretching of (CH₂) groups, at 1691 - 1678 cm⁻¹ for (C=O) of cyclohexanone, at 1600 - 1594 cm⁻¹ for (C=C) group, at 1511 - 1465 cm⁻¹ for phenyl rings and at 1266 - 1240 cm⁻¹ for (C-O-C) bond (ether linkages). In addition, other characteristic bands, due to specific groups present in various polymers were also shown.

¹H-NMR of polymers in CDCl₃ such as V_a showed singlet S at δ0.94 (9H of CH₃), weak multiple m at δ1.4 - 1.7 (H of CH and 2H of 2CH₂ in cyclohexanone), multiple m at δ2.4 (H of CH₂ β to C=CH ph), weak multiple m at δ3 (2H of CH₂ near C=O bond), multiple m at δ4.3 (4H of CH₂ linked to oxygen in ether linkage) and multiple m at δ7 - 7.8 (8H of aromatic and H of CH=C).

¹H-NMR of polymers in CDCl₃ such as VI_b showed singlet S at δ0.94 (9H of CH₃), multiple m at δ1.4 - 1.7 (8H of middle methylene group, H of CH and 2H of 2 CH₂ in cyclohexanone), multiple m at δ2.3 (H of CH₂ β to C=CH ph), multiple m at δ3.1(2H of CH₂ near C=O bond), multiple m at δ3.9 (4H of CH₂ linked to oxygen in ether linkage) and multiple m at δ6.9 - 7.8 (8H of aromatic and H of CH=C).

4. Polymer Characterization

The various characteristics of the resulting polymers including solubility, viscometry, X-ray diffraction analysis, DSC, TGA, OPM and SEM were also determined and the data are discussed below.

4.1. Solubility

Room temperature solubility characterization of poly(arylidene-ether)s V_a-d and VI_a-d were tested using various solvents including: DMF, DMSO, CHCl₃, CHCl₃-acetone mixture, HCOOH, DMA, THF, DCM and conc. H₂SO4. A 0.5% (w/v) solution was taken as a criterion for solubility. It can be clarified from Table 1 that poly(arylidene-ether)s V_a-d and VI_a-d are soluble in protonic acids, e.g. conc. H₂SO₄ (concentrated H₂SO₄ gave reddishviolet color).In polar aprotic solvents, such as DMF and DMSO all polymers dissolved partially. In DMA all polymers dissolve partially except V_a,b, which are not soluble. In HCOOH all polymers are not soluble except V_e, which are dissolved partially. In CHCl₃ all polymers dissolved partially except V_d not soluble and V_b-d and VI_c,d are completely soluble. In CHCl₃-acetone mixture all polymers dissolved partially except VI_d not soluble. In THF all polymers are partially soluble except VI_d not soluble but V_b and VI_c are completely soluble. In DCM all polymers are partially soluble except V_b,e and VI_c,d are completely soluble. They have good resistance to most Solvents. The difference in solubility can be attributed to the crystallinity associated with various LCPs. The presence of methoxyl group as substitute in the phenyl ring with tertiary butyl group in cyclohexanone moiety caused some hindering between the repeating units so decrease chain packing distances and increasing inter chain interactions such as hydrogen bonding so that making salvation of polymers containing methoxyl group more difficult than those which contain hydrogen atom instead, while increase length of the chain increase this solubility even for those which contain methoxyl group.

4.2. Inherent Viscosity

The inherent viscosity (η_inh) of some poly (arylideneether)s V_b-d and VI_c,d were determined in CHCl₃ at 23˚C with Ubbelohde Suspended Level Viscometer. The inherent viscosity value is defined as:

The solution concentration C is 0.5 g/100 ml, η/η_° = relative viscosity (or viscosity ratio). The data are listed in Table 2. It can be clarified from this table that polymer VI_c,d have high viscosity value (0.92, 0.22 dL/g ) and this may be attributed to high molecular weight of these polymers. On the other hand, polymers V_b-d have low viscosity (0.049, 0.26, 0.08, 0.18 dL/g) respectively, and this may be attributed to low molecular weight of these polymers.

4.3. U.V. Visible Spectra

The electronic spectra of selected examples of poly(arylidene-ether)s V_b, V_d and V_f were obtained in chloroform CHCl₃ at a concentration 2 × 10⁻⁵ (w/v). The electronic spectra of V_b,d showed absorption band at 269 - 270 nm due to π-π^* transition within the benzenoid system for all polymers. For polymers V_b,d λ_max near 270 nm and at λ_max near 269 nm for polymer. All of these polymers showed absorption band at λ_max near 363 - 375 nm which was due to the π-π^* and n-π^* excitation of C=C and C=O groups. For polymer V_b λ_max near 364 nm, and at λ_max near 363 nm for polymer V_d.

4.4. X-Ray Analysis

The X-ray diffractograms of selected examples of the poly(arylidene-ether)s V_d, VI_b and are shown in Figures 1 and 2 as example. The polymers show few reflection peaks that are ranging between crystalline and amorphous lying in region 2θ = 5^° - 60^°. This indicates that there is a large class of structures that are intermediate in the ordered states between crystalline and amorphous phases (with pronounced long-range order) in the arrangement of their atoms and molecules. Moreover, the presence of C=O as polar group in addition to high content of C=C bonds provides some order between the two extents of crystallinity⁽¹²⁸⁾. More particularly, the diffractogram of polymer V_e which contains short spacer in Figure 1, Shows some reflection sharpness peaks, when the length of spacer increase as in polymer V_d which contains ten methylene groups, in Figure 2, the reflection sharpness increased and the polymer became semicrystalline. This is explained by the fact that increasing the number of methylene groups in the spacers results in increasing polymer chain flexibility, and hence

Table 2. Transition behavior of selected copoly(arylidene-ether)s containing 4-tertiary butyl cyclohexanone moiety in the main chain V_a-d and VI_a-d.

increases crystallinity. This can be explained by the trans molecular conformation of polymer when the number of methylene units is even, molecules can be fitted easily into the crystal lattice and the rate of crystalization is faster as it cools down from the liquid crystal states.

4.5. Scanning Electron Microscope (SEM) Measurement

The morphology of selected examples of copoly(arylidene-ether)s V_d, VI_d were examined as example by SEM using a low dose technique. Figure 3(a) (X = 1000) shows that polymer V_d has aggregates of layer structure. The higher magnification (X = 3500, 1500) in Figure 3(b) show that the aggregates show cavity shape. After dissolving of V_d in (DCM) and evaporation of solvent. Figures 4(a) and (b) (X = 500, 1000) show that polymer V_d has sponge structure.

4.6. Thermotropic Liquid Crystalline Properties of Poly(Arylidene-Ether)s V_a-d and VI_a-d

The thermal properties of the selected poly(arylideneether)s V_b,d and VI_b,d were characterized by TGA and optical polarized microscope (OPM) with heating stage. The copoly(arylidene-ether)s V_b,d and VI_b,d did not exhibit liquid crystalline properties while observed under optical polarized microscope with heating rate (6˚/min). The mesophase could be observed when the microscopic observation was made by placing the sample on a preheated hot stage. The liquid crystalline states of polymers V_b,d, and, VI_b,d measured on a preheated hot stage of polarizing microscope at temperature 109˚C, 89˚C, 102˚C, 170˚C, 197˚C, 80˚C, 160˚C and 110˚C for the eight respective polymers; upon continued heating or heating at higher temperatures the liquid crystalline properties appeared at 112˚C, 92˚C, 114˚C, 182˚C, 210˚C, 87˚C, 162.7˚C and 119˚C for these polymers (Figures 5 and 6).

4.6.1. TGA Studies

The thermal behavior of the selected copoly(arylideneether)s V_b-f and VI_b-f was evaluated by TGA and DTG under nitrogen atmosphere at heating rate of 10˚C·min⁻¹. The thermographs of these polymers samples are given in Figures 5 and 6 while Table 3 gives the temperature for various% weight loss. TGA curves show initial decomposition of these polymers (10% loss) is considered to be the polymer decomposition temperature (PDT), which occurred in the range 503˚C to 694˚C for the samples.

In Figure 5, for polymer V_b the mass loss is seen to be rapid loss between ~282.02˚C - 557.46˚C (−81.95%). For polymer V_d in Figure 6, the mass loss was rapid between ~310.22˚C - 574.38˚C (−85.38%). From these data, it appears that the polymers containing methoxyl groups instead of hydrogen atoms with tertiary butyl group tend to exist in separated rather than more packed chains, appeared to be readily susceptible to easier thermal decomposition [24,25].

4.6.2. Texture Observations

The phase behavior of the selected copoly(arylideneether)s V_a as example according to optical polarizing microscope is summarized in Table 3. The examples ofpoly(arylidene-ether)s V_a is shown in Figure 7. Observation of polymer IV_a under polarizing microscope revealed that this polymer exhibited a good spheroid structure Figure 7(a) (before melting), in polarized light show that polymer has yellow color Figure 7(b). The meso-

Table 3. Thermal properties of selected copoly(arylideneether)s containing 4-tertiary butyl cyclohexanone moiety in the main chain V_b,d and VI_b,d.

^*Heating rate: 10˚C·min⁻¹.

phase extended up to the isotropic temperature at > 81˚C (T_i). After cooling to room temperature a highly spheroid structure with coalescence appeared as shown in Figure 7(b) in polarized light. Observation of polymer V_a under polarizing microscope revealed that this polymer exhibited good spheroid structure with thick dark rim Figure 7(a) (before melting), and the mesophase to the isotropic temperature at 87˚C (T_i). After cooling to room temperature a spheroid structure appeared as shown in Figure 7(b).

5. Conclusion

A novel series of liquid crystalline copoly(arylideneether)s containing 4-teriary butyl cyclohexanone moiety have been synthesized. A solution polycondensation technique at ~80˚C was used. All the copoly(arylidene-ether)s were insoluble in common organic solvents but dissolved completely in concentrated H₂SO₄ and formic acid. The majority of the polymers are insoluble in common organic solvents and halogenated hydrocarbons. Most of them exhibited melt birefringence and stirred opalscence during polarized microscope observation. Both (T_m) and (T_i) values increased as the length of the flexible aliphatic spacers increased and decreased with introduction of the methoxy group as a substituent in the polymers main chain.

REFERENCES