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Liquid crystalline and photocrosslinkable Poly(4,4'-stilbeneoxy) Alkylarylphosphates.

INTRODUCTION

Considerable attention has been paid toward dual functional polymers such as liquid crystalline and photocrosslinkable polymers (LC-PCPs), ascribed to the presence of mesogen arid photoreactive groups in the polymer back bone [1]. The photocrosslinkability in the polymer is attributed to the greater [pi]-electron density of photoreactive group [2]. Photocrosslinkable polymer thin film find wider applications in photolithography, optical data storage, and information display. fabricating anisotropic networks, LC elastomers, and thermosets [3-8]. In general, thermal stability, solubility, and LC property are greatly influenced by number of aromatic and alkyl groups present in the polymer backbone [9-12]. It has been reported that less number of methylene groups containing polymers exhibited high thermal stability, poor solubility with non-LC properties [13] On the contrary longer aliphatic chain polymers exhibited enhanced solubility and LC property [14].

Stilbene (a mesogen) and its derivatives have been intensely studied for their nonlinear optical properties [15]. The stilbenoid derivatives

have been for a wide range of applications such as optical brightener, laser dye, photoresist, optical data storage device, and luminescent polymeric sensor [16, 17]. Young et al. [18] synthesized a series of planar and non-planar trans-stilbene derivatives. Non-planar transtilbene derivatives possess lower melting points than planar trans-stilbene derivatives. Rameshbabu et al. synthesized 4,4'-bis(m-hydroxydecyloxy) stilbene based liquid crystalline polymers which exhibited photocrosslinking [19] as well as fluorescence [20] properties.

Solubility, flame retardancy, flexibility, stability, adhesive, and shrinkage properties of LC polymers can be altered by incorporation of phosphorus moiety as a bridging unit to the mesogen [21, 22]. Kricheldorf et al. reported thermotropic liquid crystalline polyarylphosphatc esters These polymers find applications in high performance fibers and plastics. The glass transition temperatures ([T.sub.g]) in these polymers decreased with increase in phosphorus content [23]. Literature survey revealed that incorporation of phosphate group into polymer backbone improved their solubility, thetmo-oxidative stability, char yield, flame retardancy and also results in low [T.sub.g] [24-33]. In continuation of our research [34-36] in this domain, the present work describes the synthesis and characterization of liquid crystalline and photocrosslinkable polyarylphosphates containing 4,4'-bis(m-alkyloxy) stilbene moiety and detailed comparative study on the effect of pendant aryl substitution on their physical properties.

EXPERIMENTAL

MateriaIs

Bromoacetaldehyde diethylacetal, 2-chloroethat101, 4-chloro-1 -butanol, 1,6-hexane diol, 1,8-octane dial, 1,10-decane diol (Aldrich), 47% hydrogen bromide (HBr), anhydrous potassium carbonate, potassium iodide, potassium hydroxide, sodium hydroxide, and anhydrous sodium sulphate (Merck) were used as received. Phenol, 1-naphthol, and 4-phenylphenol (SRL, India), chloroform, methanol, tetrahydrofuran (THE), dimethylformamide, diethyl ether, acetone, benzene, toluene, triethylamine, phosphor usoxychloride (SD Fine, India) were purified and dried by the usual procedure before use (37, 38).

Synthesis

The precursors were synthesized from corresponding aliphatic diols with different number of methylene groups (m = 2, 4, 6, 8, and 10) and 47% HBr solution by the procedure used in the literature [38, 39]. 4,4'-Dihydroxystilbene (DHS) was synthesized from phenol and bromoacetaldehyde diethylacetal by the procedure reported elsewhere [40]. Phenylphosphorodichloridate (PPDC), biphenylphosphorodichlondate, and naphthylphosphorodichloridate were prepared from phenol, 1-naphthol, and 4-phenylphenol, respectively, with phosphorusoxychloride using procedure reported elsewhere [41]

Synthesis of 4,4'-bis(m-hydroxyalkyloxy)stilhene

Typical procedure for the synthesis of 4,4'-bis(2-hydroxyethyloxy) stilbene is as follows: 4,4'-dihydroxystilbene (0.05 mol) was dissolved in dry dimethylformamide (DMF, 20 ml). Anhydrous potassium carbonate (0.20 mol) and 5 wt% of potassium iodide were added to it. The mixture was heated to 90[degrees] C with constant stirring; 2-chloroethanol (0.10 mol) was added dropwise to the reaction mixture and stirring was continued for 2 days. The reaction mixture was cooled to room temperature and poured into ice-cold dilute hydrochloric acid. Product was filtered, washed with water, and dried in vacuum, yield: 65% (9.7 g). Other monomers (m = 4, 6, 8, and 10) were prepared in similar manner.

Polymerization

The polymers were synthesized by solution polymerization method using an acid scavenger. Typical procedure for the synthesis of polymer la is as follows: 4,4'-bis(2-hydroxyethyloxy) stilbene (0.05 mol) was dissolved in dry chloroform (25 ml) to which dry triethylamine (0.1 mol) was added with constant stirring at room temperature under nitrogen atmosphere. Phenylphosphorodichloridate (0.05 mol), dissolved in chloroform (20 ml), was added dropwise to the reaction mixture over a period of 30 min and the reaction was allowed to continue for 24 h. Then the reaction mixture was refiuxed for 2 h and concentrated, cooled, and pouted into excess methanol. The polymer was reprecipitated in chloroform/methanol solvent and dried in vacuo at 50[degrees]C. Similarly other polymers namely, poly(4,4'-stilbeneoxy)butylphenylphosphate (Ib), poly(4,4'-stilbeneoxy)- hexylphenylphosphate (Ic), poly(4,4'-stilbeneoxy)octylphenylphosphate (Id), poly(4,4'-stilbeneoxy)decylphenylphos phate (le), poly (4,4'-stilbeneoxy)ethylnaphthyl-phosphate(na), poly(4,4'-stilbeneoxy)butylnaphthyl phosphate (Jib), poly (4,4'-stilbeneoxy)hexyl naphthylphosphate(IIc), poly(4,4'-stilbeneoxy) octylnaphthylphosphate(Ild), poly(4,4'-stilbeneoxy) decylnaphthyl phosphate (He), poly (4,4'-stilbeneoxy) ethylbiphenylphosphate (IIIa), poly(4,4'-stilbeneoxy)butyl biphenyl phosphate (IIIb), poly(4,4'-stilbeneoxy)-hexylbiphenyl-phosphate (IIIc), poly(4,4'-stilbeneoxy) octylbiphe nylphosphate (Ind), and poly(4,4'-stilbeneoxy) decylbiphenylphosphate (Me) were synthesized.

la: IR (KBr pellets, [cm.sup.-1]): 2932 ( -- [CH.sub.2] -- stretching), 1607 (C=C stretching), 1175 (Ar -- O -- C stretching) 1250 (p=O stretching); (1) H-NMR ([DMSOd.sub.6], ppm) [delta]: 7.7 (S, -- HC=CH -- ),7.2 (m, -- Ph - O -- ), 4.1 (t, -- O -- [CH.sub.2] -- ), 3.6 (t, -- [CH.sub.2] - O -- ), 1.5 (m, -- (C112)8--), 7.1 (m, Ph -- P=0). (13) C-NMR ([DMSOd.sub.6], ppm) 6: 134 ( -- HC=CH -- ), 115-159 (and Ph -- P=0), 67.18 (-0 -- [CH.sub.2] -- ), 64.5 ( -- [CH.sub.2]-0 -- ), 25-30(--[([CH.sub.2]).sub.8] --).3'P-NMR ([DMSOd.sub.6] ppm) c5: -22.0 (main chain P), -14 (terminal P). Anal. calcd for la [([C.sub18][H.sub.18][PO.sub.6]).sub.n]: C, 59.83, H, 4.98. Found: C, 59.11, H, 4.42.

Measurements

Fourier transform infra-red (FT-IR) spectra were recorded on a Brucker-IFS66V spectrometer using KBr pellet. Nuclear magnetic resonance (NMR) spectra such as (1) H-NMR. (400 MHz), (13) C-NMR (75.4 MHz) and (31) P-NMR (300 MHz) spectra were obtained on a Bruker spectrometer in dimethylsulphoxide-[d.sub.6] (DMSO-[d.sub.6]) using tetramethylsilane (TMS) and phosphoric acid (85%) as internal and external standards, respectively. Elemental analysis was performed using PerkinElmer 2400 elemental analyzer. Inherent viscosity of the polymers was measured in chloroform (0.5 g/dl) at 30[degrees]C using an Ostwald viscometer. Thermo-oxidative stability was investigated using Mettler TA 3000 thermogravimetric analyzer (TGA) at a heating rate of 20[degrees] C [min.sup.-1] under air with a sample weight of 3-5 mg. Differential scanning calorimetry (DSC) was performed on a Mettler Toledo [STAR.sup.e] system in a sealed aluminum pan with an empty aluminum pan as a reference under nitrogen atmosphere at a heating rate of 10[degrees] C [min.sup.-1]. Textures of the polymers were observed on a hot stage polarizing optical microscope (POM) using Einomex polarizing microscope equipped with a Linkem HFS91 model heating stage and a TP93 model temperature programmer. A small quantity of polymer sample was placed between two thin glass cover slips and processed with heating and cooling at a rate of 5[degrees]C [min.sup.-1]. The photographs were taken with a Nikon FM 10 model camera and exposed on a Konica film.

Photocrosslinking studies of polymers were investigated in thin film by observing absorption around 265 nm on a Shimadzu UV-160A UV-visible recording spectrophotometer. Typical procedure is as follows. A thin film of the polymer sample was coated on the outer surface of a 1-cm quartz cuvette using [10.sup.-2] M chloroform solution. Photocrosslinking was monitored by exposing the film to UV irradiation using a 125-W medium pressure meicury lamp kept at a distance of 10 cm from the sample at various intervals of time until reduction in absorbance ceased. Fluorescence spectra of polymers were recorded on a Hitachi (Tokyo, Japan) model F2000 spectrofluorimeter with excitation near absorption maxima of stilbene moiety.

RESULTS AND DISCUSSION

Synthesis of Polymers

Synthesis and the molecular structures of polymers are shown in Scheme I; 4,4'-Bis (m-hydroxy-alkyloxy) stilbenes, containing even number (m = 2. 4, 6, 8, 10) of methylene spacers, were synthesized by the reaction between dihydroxystilbene with m-bromo-l-alkanols in DMF. Polymers were synthesized by solution polycondensation method at ambient temperature. The obtained polymers are white in color and powdery. Intrinsic viscosity of the polymers was examined using suspended level Ostwald viscometer in chloroform at 30 [degrees] C and the results presented in Table I revealed that these polymers contain moderate molecular masses.

Thernw-Oxidative Stability

TGA provides direct information about thermo-oxidative stability in air and degradation mechanism by measuring weight loss of sample as a function of temperature. The representative TGA thermograms are shown in Fig. 1. The minimum decomposition temperature of the sample was considered at 5% weight loss and char yield measured at 700T. The TGA data on the polymers are summarized in Table 1. The data indicated that they were stable in between 253 and 347 [degrees] C. Weight loss data suggested that polymers underwent two-stage decomposition. First stage decomposition was in the range of 380-330t, attributed to cleavage of phosphate combined with aliphatic esters. Second-stage decomposition occurred around 370 400[degrees]C, ascribable to pyrolytic cleavage of ether linkage in the aromatic backbone 119, 381. The decomposition was completed at 600'C, and no further weight loss was observed. The TGA data suggested that thermal stability and char yield of the polymers decreases with increasing number of methylene units in the following order [42-44]:

TABLE 1, Yield, viscosity, and TGA data of la-c, Ila-e, and lila-e.

                                    Weight loss ([degrees]C)
Polymer  Yield  [[eta].sub.inh]    5%        50%       Char yield at
          (%)         (dL                                   (%)
                  [g.sup.-1])                          600[degrees]C

la          63             0.51     307           430             26
lb          68             0.54     285           421             25
Ic          65             0.58     272           410             24
Id          67             0.60     267           395             20
le          70             0.59     253           398             20
IIa         61             0.56     321           443             33
lIb         64             0.58     300           435             32
IIc         63             0.60     284           430             28
IId         62             0.62     272           412             26
IIe         71             0.64     263           405             23
IlIa        60             0.60     347           440             31
Illb        64             0.61     330           436             28
IIIc        6S             0.63     312           428             26
IIId        75             0.63     292           417             23
IIIe        60             0.65     271           399             22

[[eta].sub.inh]=inherent viscosity.


Ethylene < tetramethylene < hexamethylene < octamethylene < decamethylene.

The thermic-oxidative stability increased with increasing size and rigidity of the pendant aromatic groups, linked directly through the phosphorus atom in the polymeric chain in the following trend phenyloxy < 1-naphthyloxy < biphenyloxy. It is interesting to note that l-naphthyloxy and biphenyloxy containing polymers were more stable compared to phenyloxy containing polymers, attributed to increase in aromaticity, size, and number of aromatic rings.

1-Naphthyloxy containing polymers were less stable (IIa-e) than biphenyloxy (I11a-e) containing polymers ascribed to aromaticity and steric hindrance. According to Huckel's rule of aromaticity for bicyclic and fused ring systems, compounds containing more number of double bonds are highly stable than aromatic compounds containing less number of double bonds. Thus, biphenyloxy containing polymers exhibited better stability than 1-naphthyloxy polymers. Stem hindrance offered by peni hydrogen in 1-naphthyl group disrupts close packing of polymer chain and causes lowering in thereto-oxidative stability.

Char yield of la-e, IIa-e, and Illa-e was calculated by measuring the amount of residual substance present during the TGA analysis in air (Table 1). The order of the char yield was found to be ITa-e > IIIa-e > Ia-e. The polymers provided high char yield, indicating that they may behave as effective fire retardant polymeric materials adopting condensed phase mechanism [45, 46]. When the material starts burning, the phosphorus segment decomposes in the initial stage and then forms a phosphorus rich residue as charred glassy layer on the material that prevents further decomposition. Formation of high char yield during combustion of materials can usually limit the production of combustible carbon-containing gases and hinder the flow of oxygen, thereby decreasing the thermal conductivity of burning materials. Here flammability gets reduced. During decomposition, formation of phosphoric acid as a final product will act as barrier in the actual flame environment and retard the combustion processes [24].

Liquid Crystalline Property

POM observations (Table 2) revealed that Ia-b, Ila-b, and Illa-b were unable to exhibit LC property, whereas Ic-e, IIc-e, and Illc-e displayed grainy and nematic LC textures. Representative LC photographs of polymers are presented in Fig. 2. The POM observations revealed that longer alkyl spacer allows the mesogen to move freely with maximum possibility for alignment to form longer monodomains [47, 48] and leads to formation of LC phases. In addition, incorporation of phosphate group also renders additional flexibility to the polymer chain. On the other hand, in the case of Ia-b, IIa-b, and Illa-b, restricted movement of mesogen dominates due to shorter alkyl chain which disturbs textural alignment and ensuing smaller microdomains (49].

TABLE 2. DSC and POM data of Ia-e, Ila-e, and Illa-e.

                            DSC                       POM
                        ([degrees]C)             ([degrees]C)
Polymer   m  [T.sub.g]     [T.sub.m]  [T.sub.i]  [increment of
                                                            T]

la        2        128           184         --
lb        4        109           165         --             --
Ic        6         93           147        209             62
Id        8         --           133        192             59
Ie       10         --           111        178             67
IIa       2        135           210         --             --
lIb       4        123           192         --              _
IIc       6        102           175        238             63
IId       8         --           150        227             77
IIe      10         --           129        203             74
IlIa      2         --           267         --             --
IIIb      4         --           265         --             --
IIIc      6         92           196        257             61
IIId      8         84           157        236             79
IIIe     10         72           136        223             87

Polymer  [T.sub.m]  [T.sub.i]  [increment  Types of
                                 of T]     mesophase

la             188         --          --         --
lb             166         --          --         --
Ic             149        213          64     Grainy
Id             134        195          61     Grainy
Ie             114        183          69    Nemalic
IIa            211         --          --         --
lIb            195         --          --         --
IIc            174        239          65     Grainy
IId            150        229          79     Grainy
IIe            131        208          77    Nemalic
IlIa           270         --          --         --
IIIb           266         --          --         --
IIIc           198        260          62     Grainy
IIId           160        240          80    Nemalic
IIIe           138        227          89    Nemalic

[T.sub.g] =glass transition temperature; [T.sub.m]= melting temperature;
[T.sub.i]=sotrophic temperature; [increment of T] = [T.sub.i]- [T.sub.m]
(LC phase duration).


DSC thermograms confirm the formation of liquid crystalline phases for Ic-e, IIc-e, and IIIc-e (Table 2). The representative DSC thermograms conducted during heating-cooling and heating cycles are shown in Figs 3 and 4 respectively. All the polymers showed two endothermic peaks in between 72 and 257'C (see Fig. 3) and one broad exothermic peak around 350cC (see Fig. 4). In all the polymers, temperature above 350T indicated thermal crosslinking nature followed by decomposition of stilbene moiety of the polymer backbone as reported in the Literature [40]. The two endothermic peaks correspond to crystalline-liquid crystalline ([T.sub.m]) and liquid crystalline-isotropic ([T.sub.i]) transition temperatures, respectively, and another peak at lower temperatures corresponds to glass transition temperature ([T.sub.g]. The observations noted in the thermogram for Ia-e, Ha-e, and Illa-e indicated that TO. [T.sub.m], and [T.sub.i] decrease with increasing methylene spacer length. Among the three pendant aryl phosphate contain mg polymers, Ila-e and Illa-e polymers showed higher transition temperatures than Ia-e polymers. While IIa-e and Illa-e polymers possess the same number of aromatic rings, IIa-e polymers exhibited lower 7 [T.sub.m] and 7 [T.sub.i] attributed to the presence of more number of bulky naphthyl groups leading to an increase in the disordered arrangement and hence exhibited lower phase transition temperatures. Effect of various methylene spacers on phase transition temperatures are shown in Fig. 5.

Photocrosslinking Properly

Photocrosslinking ability of the polymers was investigated in the form of thin film formed on the outer surface of a 1-cm quartz cuvette using [10.sup.-2] M chloroform solution, irradiated under UV light and monitored by UV spectrophotometer. Changes in UV spectral pattern during photolysis of representative polymers are shown in Fig. 6. Absorption bands around 255-285 rim corresponds to [pi] - [pi]* transition of oletinic double bond of mesogenic stilbene moiety in the polymer backbone. During successive irradiation, a decrease in the intensity of absorption was noted attributed to dimerization of the olefinic double bond in the polymer chain. The dimerization involves 2[pi] -- 2[pi] cycloaddition reaction leading to formation of cyclobutane ring [50-53]. The relative rate of photocrosslinking of I, II, and III series of polymers are shown in Fig. 7. The relative reactivity [A.sub.0] -- [A.sub.t] / [A.sub.0] is plotted against time t of irradiation (min), where A, is the absorption before irradiation and [A.sub.0] is after irradiation for time t. The rate of photocrosslinking of I series of polymers was slow as the complete disappearance of absorption around 255-285 nm occurred in 22 min. For II series of polymers absorption completely disappeared in 20 min On the other hand, the rate of photolysis of III series of polymers was much as the absorption of 255-285 nm disappeared totally within 16 min. Photolysis studies of various methylene spacer containing polymers revealed that the rate of photocrosslinking of exocyclic double bonds of the polymers increases in the following order: ethylene < tetramethylene < hexamethylene < octamethylene < decamethylene.

The rate of diminution on exocyclic double bond, during photolysis, is faster for longer aliphatic chain containing polymers than shorter aliphatic chain containing polymers [25] attributed to folding tendency of methylene units present in the polymer chain. The folding tendency normally increases with increase in the number of methylene units, which brings stilbene units closer, thereby photocrosslinking reaction, occurred at a much faster rate. Thus, the rate of photocrosslinking was slower for Ia, containing ethylene unit, and the absorption band at 285 nm, completely disappeared in 22 min, while the rate of crosslinking was faster for Ie, containing decamethylene unit, and absorption at 255 nm completely disappeared within 12 min. A similar trend was observed in IIa-e and 11.1a-e series of polymers. The photocrosslinking studies of various aryl phosphate esters containing polymers revealed that the rate of photocrosslinking increased in the following order: biphenyloxy > 1-naphthyloxy > phenyloxy.

It is interesting to note that the rate of photocrosslinking is much faster for IIIa-e polymers than the IIa-e and la-e polymers attributed to the close packing of the biphenyl rings through improved [pi] - [pi] interactions. However, [pi] - [pi] interaction is more effective in Ila-e polymers than Ia-e polymers and hence lIa-e polymers showed faster crosslinking than Ia-e polymers 15.31. After completion of photocrosslinking study, the films were found to be insoluble in chloroform solvent. The irradiated samples were further subjected to NOM and DSC analysis and found that they lost their LC property with increase in transition temperatures. The DSC thermogram of irradiated Ind is depicted in Fig. 4. It was noticed that [T.sub.g] of the polymer shifted to high temperature. In addition to that [T.sub.m] observed for the sample before irradiation shifted to higher temperature associating with decomposition substantiating the photocrosslinked product.

Fluorescence Property

The fluorescence emission of I, II, and III series of polymers were investigated in the form of thin films with constant excitation at 255-285 nm for various time intervals. The fluorescence emission was recorded at 350-550 nm. Representative fluorescence spectra of Ia, IIa, IIIa are shown in Fig. 8. The decrease in intensity of emission, on successive irradiation, revealed the effect of dimerization of olefinic double bond. It has been indicated that the degree of overlapping between two mesogenic units depends on their spacer lengths. The polymer with longer alkyl chain shows larger redshift in fluorescence than the shorter alkyl chain attributed to the packing nature of more flexible alkyl chain. The degree of overlapping between two stilbene units for Ia-e, IIa-e, and IIIa-e was found to be decreased with increase of methylene groups [541.

CONCLUSIONS

Three series of liquid crystalline and photocrosslinkabl spectroscopic techniques. Inherent viscosity revealed that they have moderate molecular masses. The data of TGA suggested that the polymers were stable between 325 and 367 [degrees] C. These polymers exhibited good thermo-oxidative stability and char yield as the requisites to act as fire retardants adopting condensed phase mechanism. The incorporation of methylene spacer and phosphate group in the ene spacers that leads to the formation of liquid crystalline phases. All the polymers demonstrated photocrosslinking behavior under UV irradiation. Polymers containing longer alkyl chain with pendant biphenyloxy group showed faster photocrosslinking than that of lower alkyl chain with pendant naphthyloxy and phenyloxy group containing polymers. All the polymers exhibited fluorescence properties. The emission intensity was found to decrease at various intervals of time attributed to dimerization of olefinic double bond. The synthesized poly(4,4'-stilbeneoxy) alkylarylphosphates demonstrated liquid crystallinity, good thereto-oxidative stability, flame retardancy, fluorescence, UV-curable properties suitably utilized in multifunctional organic coatings, sensors, and photolithography.

Correspondence to: P. Hannan; pakannan[congruent to]annauniv.edu

Contract grant sponsor: Department of Science and Technology, New

Delhi, India; contract grant number: SR/SI/PC-14/2003.

DOI 10.1002/pen.22122

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2011 Society of Plastics Engineers

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Angayan Ravikrishnan, Posa Sudhakara, Palaninathan Kannan Department of Chemistry, Anna University, Chennal 600 025, India
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Author:Ravikrishnan, Angayan; Sudhakara, Posa; Kannan, Palaninathan
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9INDI
Date:Mar 1, 2012
Words:4272
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