Synthesis, characterization and investigation of liquid crystalline properties for some cross-link polymers containing melamine

doi:10.21203/rs.3.rs-3952299/v1

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Synthesis, characterization and investigation of liquid crystalline properties for some cross-link polymers containing melamine

https://doi.org/10.21203/rs.3.rs-3952299/v1

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published 08 May, 2024

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In this study condensation polymerization was used to synthesize a number of novel liquid crystal polymers with 1,3,4-oxadiazole rings based on melamine. The new synthesized polymers were characterized by Fourier Transform Infrared (FTIR) and proton nuclear magnetic resonance (¹HNMR) spectroscopy. Differential scanning calorimetry (DSC) and optical polarization microscopy (OPM) were used to investigate their liquid crystalline properties. The results demonstrated that throughout a wide temperature range, most of the polymers exhibited columnar (Coh_X) and nematic (N) liquid crystalline phases.

4-oxadiazole

Melamine-core

Heterocyclic

Liquid crystal

Discotic liquid crystal

Copolymer

In recent years, heterocyclic polymers have gained attention for a variety of reasons, including their improved chemical and thermal stability and their broad pharmacological action, which includes analgesic and antipyretic effects, as well as their ease of synthesis. A variety of polymeric systems with different electrical, physical, and chemical properties are produced by the polymers.

Pyrrole, thiophene, and other heterocyclic monomers can be polymerized to synthesize heterocyclic polymers. Oxadiazoles are heterocyclic compounds with three heteroatoms and five members. There exist three distinct types of oxadiazole compounds: 1,2,4-, 1,2,5-, and 1,3,4-oxadiazole. These compounds are well-known for their intriguing biological and therapeutic properties, having been shown to have antibacterial properties. Additionally, a range of pharmaceutical drugs have been found to have antibacterial, antimicrobial, herbicidal, and anti-inflammatory properties [1–7].

Because of its unique features, the construction of innovative thermotropic liquid crystals has emerged as one of the most promising fields of study in recent years. Five-membered heterocyclic rings are a common component of liquid crystalline compounds, and these compounds have shown particular effectiveness with supramolecular liquid crystals, which combine the structural complexity and fluidity of supramolecular chemistry [8]. Choosing an appropriate mesogenic core unite, the appropriate length of one or more chains, and connecting groups are common steps in the design and synthesis of new thermotropic liquid crystals. Modifying the mesogenic core significantly alters its properties, such as molecular polarity [9] and can be further altered by adding heteroatoms [10, 11].

Melamine-based resins have found several uses because melamine may be used to synthesize polymers with advantageous dielectric properties and a reasonably high thermal stability [12–16]. Melamine resins are utilized in the production of coatings, dishes, floor coverings, and kitchenware. The primary disadvantage of melamine is its poor solubility in water and most organic solvents, making it challenging to use directly [17]. Actually, the synthetic method widely used in the polymer sector to synthesize melamine formaldehyde resins consists of the "activation" of the melamine ring by the production of a melamine-formaldehyde derivative. This usually acts as a precursor to form resins with different functional groups or to allow the reaction to proceed, forming a resin that is cross-linked [18]. Alternatively, some published studies discuss the production of polymers based on melamine by dissolving and reacting melamine with a "reactive solvent" (formaldehyde or cyclohexanone) [19, 20]. Other papers [21–23] address the analysis of catalytic circumstances that can aid in the polymerization reaction. Moreover, melamine can be acetylated by heating it in the presence of acetic anhydride [24]. Furthermore, in this case, the acetylated molecule can operate as a building block for further functionalization and crosslinking events. Lately, oxirane and melamine in excess have been employed to produce oligo- and polyetherols. This reaction can be carried out via one of two primary synthetic methods, both of which need costly and time-consuming steps. The first requires the presence of a quickly boiling solvent, usually dimethyl sulfoxide (DMSO), which needs to be extracted further from the final goods. Because of the breakdown products that occur during reactions, reduced pressure distillation is frequently used to achieve this removal. The second step needs to be carried out at 120–130 \(℃\) and requires rigorous temperature control because of a strong exothermic influence that can cause a spontaneous temperature increase of up to 200–230 \(℃\) [25].

Carboxylic acid derivatives (e.g., 3,4,5-trialkoxybenzoic acid) have only recently been able to self-assemble into stable supramolecular discotic liquid crystalline structures in the presence of multiple heterocyclic molecules, because pure melamine (not functionalized melamine derivatives) has a strong self-complementary hydrogen-bonded matrix, which contributes to its extremely stable state. It was found that the thymine imido-dicarbonyl unit functioned best, and that melamine-core hydrogen-bonded supramolecular liquid crystalline complexes could be formed by mixing derivatives of melamine and thymine with other complimentary molecules [26]. But what a powerful addition, Yoshida et al. [27] mentioned the synthesis and flexibility-hardness of polymer coatings based on melamine with mesogenic groups.

The results of the investigation validated the potential advantages of liquid crystalline group-containing coatings for pre-painted steel, where a more advantageous balance between film hardness and flexibility was especially required. Our approach involves synthesizing and manufacturing novel polymers of long alkyl-tailed compounds, such as 1,3,4-oxadiazole and phenyl aromatic rings, by utilizing the melamine unit as a building block for discotic liquid crystals with stoichiometry of 2:1, 4:1, or 6:1. Also in this study the effects of the stiff and flexible units in the central core on the temperature stability and liquid crystalline phase behavior of the synthesized polymers was evaluated.

Materials and Techniques

All chemicals were purchased from Sigma-Aldrich company. The melting point are determined by Electro thermal melting Apparatus 9300 in open capillary tubes that are un corrected. Thin layer chromatography (TLC) was used for monitory the reaction and to check purity. Thin layer chromatography (TLC) was used for monitory the reaction and to check purity. The FTIR spectrum of samples were taken on a Shimadzu type (8400s) via using KBr disc, FTIR spectrophotometer at wave number range 4000–500 cm^− 1 with a resolution of 4.0 cm^− 1 at 25 \(℃\). ¹HNMR spectra were obtained on a Bruker, 300MHz origin: Switzerland, DMSO was used as solvents. An Instec hot-stage, a PM-30 exposure control device, and a Meiji MT9000 optical oolarizing oicroscope were used to measure the melting points, phase transitions, and mesomorphic textures. Thermal gravimetric analysis (TGA) and DSC were carried out on STAPT-1000 Linseis, German origin instrument with a heating rate of 10 \(℃\)/min under nitrogen atmosphere, with indium serving as the internal reference (156.6 \(℃\), 28.45 J/g).

Synthesis

All the new polymers P₁ and P₂ and their copolymers P₁C_n and P₂C_n (n = different mole ratio of melamine 2:1, 4:1, or 6:1) were synthesized according to the synthetic route outlined in Schemes 1. Diethyl adipate [I]_a, diethyl terephthalate [I]_b, 5-(hydrazineyloxy) hex-5-enehydrazide [II]_a and terephthalohydrazide [II]_b were prepared using conventional methodologies [28, 29].

The monomers 4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₁ and 4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₂

In phosphorus oxy chloride (5 mL), compound [II]_a,b (10 mmol) and 4- aminobenzoic acid (1.37 g, 10 mmol) were mixed. The reaction mixture was then gently refluxed for 9 h and then the mixture was treated with ice-water carefully and adding concentrated sodium bicarbonate solution. The resulting solid was filtered, washed by water and recrystallized from ethanol.

Analytical data

4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₁

Yellow solid; yield: 85%, m.p. 173 \(℃\). Rf = 0.65. FTIR (ν/cm^− 1): 3441 − 3328 (sym and unsym NH₂), 3068 − 3045 (C-H aromatic), 2916 − 2843 (C-H aliphatic),1656 (C = N), 1581 (C = C aromatic) and 1246 − 1050 (C-O). ¹HNMR (300 MHz, DMSO-d₆) δ (ppm): 7.4–7.9 (m, 8H, Ar-H), 6.52 (s, 4H, 2NH₂), 4.52 (m, 8H, -CH₂).

4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₂

Dark yellow solid; yield: 78%, m.p. 194 \(℃\). Rf = 0.58. FTIR (ν/cm^− 1): 3456 − 3244 (sym and unsym NH₂), 3071 − 3052 (C-H aromatic), 1663 (C = N), 1587 (C = C aromatic) and 1319 − 1061 (C-O),. ¹HNMR (300 MHz, DMSO-d₆) δ (ppm): 7.5–8.1 (m, 12H, Ar-H), 6.52 (s, 4H, 2NH₂).

Polymer resins P₁ and P₂

Amine monomer [M₁] or [M₂] (20 mmol) was dissolved in 10% aqueous NaOH solution (20 mmol) in round bottom flask with three necks placed in oil bath at 50 \(℃\) for 1h. Then addition of epichlorohydrin (1.8 g, 20 mmol) with continues string and the temperature was raised to 90 \(℃\) for another 8 h. The solid was then separated from the liquid using the decantation process, washed with distillate water, and neutralized with 10% aqueous HCl. The product was air-dried at room temperature.

Analytical data

Polymer resin P₁

Light yellow solid; yield: 90%, m.p. > 300 \(℃\). FTIR (ν/cm^− 1): 3329 − 3323 (OH), 3068 − 3045 (C-H aromatic), 1645 (C = N), 1600 (C = C aromatic), 1409 (C-N), and 1261 − 1014 (C-O). ¹HNMR (300 MHz, DMSO-d₆) δ (ppm): 8.33 (s, 2H, 2NH), 7.98–6.73 (m, 8H, Ar-H), 6.10 (s, 2H, OH), 3.66 (m, 2H, -HO-CH), 1.24–1.91 (m, 16H, NH-CH₂, CH-CH₂ and -CH₂-).

Polymer resin P₂

Yellow solid; yield: 75%, m.p. > 300 \(℃\). FTIR (ν/cm^− 1): 3341 − 3325 (OH), 3072 − 3051 (C-H aromatic), 1648 (C = N), 1589 (C = C aromatic), 1405 (C-N), and 1258 − 1034 (C-O). ¹HNMR (300 MHz, DMSO-d₆) δ (ppm): 8.29 (s, 2H, 2NH), 8.07–6.65 (m, 12H, Ar-H), 5.99 (s, 2H, OH), 3.58 (m, 2H, -HO-CH), 1.63–2.11 (m, 8H, NH-CH₂ and CH-CH₂).

Cross-linked polymers P₁C_n and P₂C_n

Polymers P₁ or P₂ and melamine as curing agent were mixed with ratios 1:2, 1:4 and 1:6 which placed in suitable flask with stirring. The first step curing was performed at 60 \(℃\) for 30 min and post curing at 140 \(℃\) for 2 h. After the cooling, 30 mL cold water was added to reaction mixture; the yellow solid was filtered and dried by air.

Analytical data

Copolymer P₁C₆

Yellow solid; yield: 75%, m.p. > 300 \(℃\). FTIR (ν/cm^− 1): 3323 − 3315 (OH), 3095 − 3000 (C-H aromatic), 2910 − 2860 (C-H aliphatic), 1641 − 1630 (C = N), 1604 (C = C aromatic), 1415 − 1409 (C-N), and 1266 − 1014 (C-O).

Characterization

Polymers P₁ and P₂ were synthesized under heating of a mixture from amine monomers M₁ and M₂ with epichlorohydrin in 10% aqueous NaOH solution, respectively. The FTIR spectra showed disappearance of the absorption bands due to NH₂ and appearance the new stretching bands for O-H and C-N at 3329, 3323 cm^− 1 and 1409 cm^− 1, respectively. Besides another stretching bands for the following: C-H aromatic at 3061 − 3045 cm^− 1, C-H aliphatic at 2929 − 2856 cm^− 1, C = N at 1645 cm^− 1, C = C aromatic at 1600 cm^− 1, and C-O at 1261 − 1014 cm^− 1 [30].

The ¹HNMR spectrum of P₂ (in DMSO as a solvent) exhibited the following: a one signal at δ 8.29 ppm for NH proton, many signals in the region δ 8.07–6.65 ppm that related to aromatic protons, a one signal at δ 5.99 ppm for proton of OH and a signal at δ 3.65 ppm for aliphatic protons of HO-CH groups in polymer. As well appeared many signals in the range δ (1.24–1.91) ppm for aliphatic protons of CH₂ termination, cyclic ends and NCH₂ groups [31].

The copolymers were synthesized from the reaction of P₁ or P₂ and melamine as curing agent with ratios 2:1, 4:1, or 6:1 and designated as P₁C_n and P₂C_n (n = different mole ratio of melamine). The IR spectra showed disappearance of the absorption bands due to NH₂ and appearance the stretching bands for O-H at 3323 − 3315 cm^− 1, C-N at 1415 − 1409 cm^− 1, C-H aromatic at 3095 − 3000 cm^− 1, C-H aliphatic at 2910 − 2860 cm^− 1, C = N at 1641 − 1630 cm^− 1, C = C aromatic at 1606 − 1602 cm^− 1 and C-O at 1266 − 1014 cm^− 1 [32–34].

Thermogravimetric analysis TGA

TGA measurements provide valuable information that can be used to select materials for certain end-use applications, predict product performance and improve product quality. The thermograms of synthesized polymers show the percent weight loss as a function of samples temperature for the polymers. The TGA thermograms of samples were obtained at the heating rate of 10\(℃\)/min under argon, and measured in temperature range (0-600\(℃\)). Typical TGA thermograms of representative polymers are shown in Fig. 1. The data of thermogravimetric analysis indicated that the synthesized polymers generally have relatively good thermal stability. It was found that the polymers containing central aromatic rings in their backbone i.e., P₂C_n, have higher thermal stability than those containing central polymethylene flexible chains, P₁C_n. This can be explained by the stiffening effect introduced by phenylene ring to the polymer backbone.

Liquid Crystalline Behavior

The transition temperatures and mesophase type for the synthesized polymer and copolymers in this work were investigated by DSC and OPM. The mesomorphic properties, phase transition temperature and enthalpy change of the synthesized polymers were listed in Table 1. Figure 2 shows the DSC thermogram of the representative polymers. On heating, the first endothermic peak is accompanied with high enthalpy indicates a transition from solid crystalline state to a liquid crystalline mesophase. The second endothermic transition at higher temperature presented lower associated enthalpy, which indicates transitions between distinct mesophases and to the isotropic liquid state. The optical textures were used as the basis for the phase identification, with each mesophase type of texture being classified according to the approaches described by Dierking [35] and Gray with Goodby [36]. The first type of polymer P₁C_n, i.e., the polymer P₁, P₁C₂ and P₁C₄ did not show any liquid crystalline properties only simply transition from crystalline solid to isotropic liquid. That may be related to the presence of flexible methylene group in the center of the repeating unit. At the same time, the polymer P₁C₆ showed dimorphism nematic (N) and columnar (Col_X) phases (Fig. 3). Upon further cooling of the nematic phase of this polymer, the emergence of the Col_X phase was observed in its mosaic texture. These results may be because of increasing in the relative of melamine leads to an increase in the width and rigidity of the molecule of this polymer. The second type of polymer P₂C_n, the polymers P₂C₂ and P₂C₄ displayed a discotic columnar phase (Fig. 4) [37]. This behavior may be related to the presence of a melamine unit and phenyl ring as a rigid core in the polymer molecule leading to the formation discotic phase. On the other hand, the polymer P₂C₆ showed dimorphism behavior; nematic and columnar liquid crystal phases [38]. Finally, the P₂ do not reveal any liquid crystalline behavior, but simply changes from the solid crystalline state to the isotropic liquid at 426 \(℃\).

Table 1

Phase transition temperatures (\(℃\)) and (transition enthalpies \(\varDelta H\)(kJ/mol)) of polymers P₁, P₂, P₁C_n and P₂C_n determined by DSC (10\(℃\)/min) during first heating
Compounds	Phase transition (\(℃\)) and (enthalpy kJ/mol)
P₁	Cr 429 I
P₂	Cr 426 I
P₁C₂	Cr 318 I
P₁C₄	Cr 327 I
P₁C₆	Cr 306 (25.68) Col_X 386 (4.62) N 419 (0.84) I
P₂C₂	Cr 302 (22.04) Col_X 426 (dec) I
P₂C₄	Cr 325 (17.95) Col_X 432 (dec) I
P₂C₆	Cr 298 (10.08) Col_X 375 (3.99) N 416 (dec) I
Cr = Crystal; N = Nematic; Col_X = Columnar; I = Isotropic transition; dec = Decomposition

We successfully carried out the polycondensation of heterocyclic aromatic monomers with the epichlorohydrin to get a new polymer which was used with melamine as the copolymerization process to get new cross-link copolymers. The chemical structures of these polymers were examined by FTIR and ¹HNMR spectroscopy. The results are in agreement with the considered molecular structure. The mesomorphic properties and optical textures of the resultant compounds were characterized by OPM and DSC. The resultant copolymers show columnar discotic with nematic liquid crystalline phases accompanied by the formation of new mesogens.

Author contribution All authors contributed to the study conception and design. MTS and JHT were responsible for the writing original draft preparation. BJA sample preparation and analysis; AHA supervision, conceptualization, review and editing last version.

Conflict of interest The authors declare that they have no confect of interest.

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No competing interests reported.

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You are reading this latest preprint version

Synthesis, characterization and investigation of liquid crystalline properties for some cross-link polymers containing melamine

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Experimental

Materials and Techniques

Synthesis

The monomers 4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₁ and 4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₂

Analytical data

4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₁

4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₂

Polymer resins P₁ and P₂

Analytical data

Polymer resin P₁

Polymer resin P₂

Cross-linked polymers P₁C_n and P₂C_n

Analytical data

Copolymer P₁C₆

Results and discussion

Characterization

Thermogravimetric analysis TGA

Liquid Crystalline Behavior

Conclusions

Declarations

References

9. Niori T, Sekine T, Watanabe J, Furukawa T, Takezoe H (1996) Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules. J Mater Chem 6(7):1231–1233

Additional Declarations

Status:

Journal Publication

Version 1

Synthesis, characterization and investigation of liquid crystalline properties for some cross-link polymers containing melamine

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Experimental

Materials and Techniques

Synthesis

The monomers 4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]1 and 4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]2

Analytical data

4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]1

4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]2

Polymer resins P1 and P2

Analytical data

Polymer resin P1

Polymer resin P2

Cross-linked polymers P1Cn and P2Cn

Analytical data

Copolymer P1C6

Results and discussion

Characterization

Thermogravimetric analysis TGA

Liquid Crystalline Behavior

Conclusions

Declarations

References

9. Niori T, Sekine T, Watanabe J, Furukawa T, Takezoe H (1996) Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules. J Mater Chem 6(7):1231–1233

Additional Declarations

Status:

Journal Publication

Version 1

The monomers 4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₁ and 4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₂

4,4'-(methylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₁

4,4'-(1,4-phenylenebis(1,3,4-oxadiazole-5,2-diyl))dianiline [M]₂

Polymer resins P₁ and P₂

Polymer resin P₁

Polymer resin P₂

Cross-linked polymers P₁C_n and P₂C_n

Copolymer P₁C₆