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Comparative study of new thermotropicpolyazomethines y Luminita Marin*, Vasile Cozan and Maria Bruma Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda 41A, Iasi, Romania Received 29 November 2005; Revised 13 April 2006; Accepted 27 April 2006 Three series of novel modified polyazomethines were prepared with mesogenic units and varyingkinkinggroups.Theeffectoftheincorporationofthekinkinggroupsonthethermotropicpropertiesof the alternated and random polymers was studied. Structural elucidation was carried out byelemental analysis, infrared (IR), proton nuclear magnetic resonance ( 1 H-NMR) and UV-vis spec-troscopy. Inherent viscosity of the polymers measured in N , N -dimethyl formamide (DMF) at 20 8 Cwas in the range 0.15–0.44dl/g. Their molecular weight and molecular weight distribution weredetermined using gel permeation chromatography (GPC). Mesomorphic properties were studied bydifferential scanning calorimetry (DSC), hot stage polarizing optical microscopy and the thermalstability was determined by thermogravimetric analysis (TGA). Some polyazomethines exhibitedfine grain texture and good mesophase stability. It was observed that the random copolymers hadlower melting point and higher mesophase stability than alternated polymers. Copyright # 2006 John Wiley & Sons, Ltd. KEYWORDS: poly(azomethine-ether)s; poly(azomethine-ether-sulfone)s; NMR; gel permeation chromatography (GPC); differentialscanning calorimetry (DSC) INTRODUCTION Aromatic polyazomethines are an attractive class of highperformance polymers, due to their high thermal stability,excellent mechanical strength, chelate forming ability,semiconducting and good optoelectronic properties, andthermotropic liquid crystalline behavior. 1–4 However, poly-azomethines are generally infusible polymers and havepoor solubility, drawbacks which would minimize theirpractical applications. Various modified polyazomethines,such as poly(azomethine-ester)s, 5 poly(azomethine-ether)s, 6 poly(azomethine-carbonate)s, 7 poly(amide-azomethine-ester)s, 8 poly(acrylate-azomethine)s, 9 thermosetting polyazo-methines, 10 poly(azomethine-sulfone)s, 11–14 weresynthesizedwith the aim to reduce the melting temperature, to improvethe solubility and to promote specific properties such asmesomorphism. 15 However, aromatic polysulfones and poly(ether sulfone)sare a family of aromatic amorphous thermoplastics withunique high-performance properties as engineeringmaterials. They have high glass transition temperatures( T g ), very high melt thermal stability, long-term thermal-oxidative endurance, high distortion-temperature, chemicalinertness, good solubility, good electrical insulative proper-ties and flame retardancy. 16 Previous studies showed that poly(azomethine-ether-sulfone)s combine the valuable properties of bothpolyazomethines and polyethersulfones, due to the contri- bution of the anisotropic azomethine rigid cores andamorphous isotropic arylethersulfone moieties. 11–14 Thesepolymers exhibited good solubility and accessible meltingtemperatures; besides, thermotropic liquid crystalline behavior.Here the synthesis of three new series of modifiedpolyazomethines are reported: (1) poly(azomethine-ether)s, by the reaction of a dichloro-compound containing azo-methine groups with a bisphenol such as isopropylidene- bisphenol(bisphenol A)or fluorobisphenol (bisphenol F); (2)poly(azomethine-ether-sulfone)s, by the reaction of bis( p -chlorophenyl)sulfone with a bisphenol containing azo-methine group; (3) copoly(azomethine-ether-sulfone)s bythe reaction of bis( p -chlorophenyl)sulfone with a mixture of bisphenol A and bisphenol containing azomethine groups.Thesolubility,transitiontemperaturesandtheinfluenceof thestructureonthe thermalbehaviorofthesepolymershave been studied. In this paper it was decided that less attentionwould be paid to the mesophase characterization. EXPERIMENTALReagents p -Hydroxybenzaldehyde (98%, Fluka) was recrystallizedfrom water (melting point, mp ¼ 116–117 8 C). 4-Chloroben- POLYMERS FOR ADVANCED TECHNOLOGIES Polym. Adv. Technol. 2006; 17 : 664–672Published online 22 September 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.767 * Correspondence to : L. Marin, Institute of Macromolecular Chem-istry, Aleea Gr. Ghica Voda 41A, Iasi, Romania.E-mail:
[email protected] y 8th International Symposium on Polymers for AdvancedTechnologies 2005 (PAT 2005), Budapest, 13–16 September,2005, Part 1. Copyright # 2006 John Wiley & Sons, Ltd. zaldehyde (97%, Aldrich), therephthalaldehyde (98%,Aldrich), p -aminophenol (97.5%, Aldrich), 4,4 0 -oxy-bis(4-amino-phenylene) (98%, Aldrich), 4,4 0 -bis( p -aminophenoxy)biphenyl (mp ¼ 205 8 C, Kennedy & Klim, Inc., USA),2,2 0 -bis( p -hydroxyphenyl)propane (bisphenol A, mp ¼ 158–159 8 C, Merk), 2,2 0 -bis( p -hydroxyphenyl)hexafluoropropane(bisphenol F, Merk) were used as received. Bis( p -chlorophe-nyl)sulfone (Fluka) was recrystallized from toluene (mp ¼ 148–149 8 C). Dimethylsulfoxide (DMSO) was dried overcalcium hydride and distilled under vacuum before use.Anhydrous potassium carbonate was dried at 120 8 C in avacuum oven before use. All the solvents used for solubilitytesting were provided by Aldrich and used as received. Measurements Infrared (IR) spectra were recorded on a Specord M80CarlZeiss JenaSpectrophotometer, using KBr pellets.Protonnuclear magnetic resonance ( 1 H-NMR) spectra wererecorded by a Jeol 60MHz spectrometer using CDCl 3 assolventandtetramethylsilaneasinternalreference.Chemicalshifts are reported inparts per million (ppm). UV-visspectrawere obtained on a Carl Zeiss Jena SPECORD M42spectrophotometer in N , N -dimethylformamide (DMF)solutions using 10mm quartz cells fitted with poly(tetra-fluoroethylene) stoppers.Solubility tests of the polymers were performed in variousorganic solvents such as: acetone, methanol, carbon tetra-chloride, chloroform, DMSO, DMF, N -methylpyrolydine-2-one(NMP).Molecularweightdistributionsofpolymersweremeasured by gel permeation chromatography (GPC), inDMF solutions. Viscosities were measured with an Ubbe-lohde suspended level viscometer in DMF, 0.2wt%, at 20 8 C.Differential scanning calorimetry (DSC) was performedwith a Mettler TA Instrument DSC 12E, at heating ratesof 10 8 C/min or 15 8 C/min, under a nitrogen atmosphere.The transition temperatures were read at the top of theendothermalandexothermal peaks.The T g values werereadat the middle of the change in the heat capacity. The sampleswere first heated 10 8 C/min up to the isotropic state (asdetermined by polarized light microscopy), quenched toroom temperature, then reheated at 10 8 C/min. Mesophasesofthethermotropicpolymerswerestudiedbyobservationof the texture with an Olympus BH-2 polarized light micro-scope under cross polarizers with a THMS 600/HSF9I hotstage. The temperature at which isotropic phases occurredwas taken as the isotropization temperature ( T i ). Thermo-gravimetric analysis (TGA) was carried out using a MOM QDerivatograph (Hungary), at a heating rate of 9 8 C/min,in air.Wide-angle X-ray diffraction (WAXD) measurementswere performed at room temperature using a TUR-M62Diffractometer, and nickel-filtered CuK a radiation. Theinvestigation of electrical conductivity, s , was performedon thin film samples using a Keithley 6517 A electrometer.The geometrical features were observed by molecularmodeling, using the program HyperChem (TM). Synthesis of the monomers The preparation of the azomethine monomers (M1, M2, M3,M4 and M5) was performed by acid-catalyzed condensation,according to previous reported methods. 17–20 The structuresof these monomers and their characteristics are shown inTable 1. Synthesis of the polymers The synthetic pathway to alternated or random polymerswas the classical Williamson etherification method, inwhich an aromatic dichloride reacted with a bisphenol asnucleophile.Thepoly(azomethine-ether)sPAE1andPAE2areobtained by the reaction of the monomer M5 with bisphenol A or bisphenol F, respectively (Scheme 1).The poly(azomethine-ether-sulfone)s PAES1–PAES4 wereobtained by the reaction of bis( p -chlorophenyl)sulfone withthe coresponding monomers M1–M4 (Scheme 2).The copoly(azomethine-ether-sulfone)s CoPAES1–CoPAES3 were obtained by the reaction of bis( p -chlorophe-nyl)sulfone with a mixture of bisphenol A and a bisphenol Table 1. Structures of the monomers containing azomethine groups Code Structure Thermotropic behavior (PLM) a M14,4 0 -bis( p -hydroxybenzylidene-iminophenoxy)biphenylCr 299 LC 303 IM24,4 0 -bis( p -hydroxybenzylidene-imino)diphenyletherK 244 IM34,4 0 -bis( p -hydroxyaniline)terephthalaldehydeK 284 IM44-( p -hydroxybenzylideneimino)phenolK 218 IM54,4 0 -bis( p -chlorobenzylidene-iminophenoxy)biphenylK 243 LC 246 I a Thermotropic behavior determined by optical polarized microscopy (PLM); Cr ¼ crystalline; LC ¼ liquid crystalline; I ¼ isotropic. Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17 : 664–672DOI: 10.1002/pat New thermotropic polyazomethines 665 containing azomethine group (M1–M3) in 1:0.5:0.5 molarratio (Scheme 3).Atypicalpolymerizationprocedure(CoPAES1)isasfollows:0.45g (1.959mmol) of 2,2-bis( p -hydroxyphenylene)propane(bisphenol A), 1.12g (3.917mmol) of bis( p -chlorophenyl)sulfone, 0.8g (1.959mmol) of 4,4 0 -bis( p -hydroxybenzylidene-imino)diphenyl (M1), 0.31g anhydrous potassium carbonate(15% molar excess) and 8mL DMSO were charged into around bottom flask fitted with thermometer, condenser,nitrogen inlet and outlet and magnetic stirrer. The tempera-ture was increased gradually to 100 8 C during 1hr. Then, thetemperature was increased quickly to 160 8 C and the reactionmixture was maintained under stirring for 6hr. After coolingthe dark brown reaction mixture was poured into water toprecipitatethepolymer.Thepolymerwaswashedwithwater,refluxed in methanol for 2hr and hot filtered to extract thesolubleoligomersandthustoreducethepolydispersityindex(PDI). Finally, the polymers were vacuum dried at 60 8 C for18hr. RESULTS AND DISCUSSIONS Alternated and random polymers have been prepared usingthe classical Williamson condensation reaction of aromaticdichlorides with bisphenols having various structures.The structures of the polymers were confirmed by IR and 1 H-NMR spectra and by elemental analysis. For all thepolymers, the IR spectra showed the presence of theabsorption band corresponding to the azomethine linkage(–CH––N–) at 1620–1640cm À 1 . The sharp peaks occurring at1230–1250cm À 1 areduetotheasymmetricalvibrationsoftheether linkage. Other characteristic absorption bands are alsopresented in Table 2. 1 H-NMR spectra of the polymers showed three types of signals which were assigned correspondingly: the singlet at8.4–8.7ppm, –CH––N–; the multiplet at 6.63–7.72ppm,aromatic protons; the singlet at 1.43–2ppm for aliphaticprotons from the bisphenol A segment (CH 3 ) 2 C < . The ratio Scheme 1. Synthesis of poly(azomethine-ether)s PAE1 and PAE2. Scheme 2. Synthesis of poly(azomethine-ether-sulfone)sPAES1–PAES4. Scheme 3. Synthesis of copoly(azomethine-ether-sulfone)s CoPAES1–CoPAES3. Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17 : 664–672DOI: 10.1002/pat 666 L. Marin, V. Cozan and M. Bruma values of the integrals corresponding to aliphatic versusaromatic protons agree with the proposed structure.Elementalanalysisdatafornitrogenandsulphurshowedagood agreement between the calculated and experimentalvalues (Table 2).UV-vis electronic spectra were recorded for all thepolymers in DMF solutions. The data are collected inTable2.Allthesamplesexhibitedanintenseabsorptionbandranging between 328 and 342nm, due to the p – p * electronictransition of the Ph–CH––N–Ph systems. Similar absorptionmaxima were reported for polyazomethines containing benzylidene ketone moieties. 21 The solubility of these polymers (PAE, PAES, CoPAES)was tested in various organic solvents. All the polymersshowed good solubility in dipolar aprotic solvents, such asDMF,DMSO,NMP,tetrahydrofuran(THF),and1,4-dioxane.In other solvents, such as chloroform, methanol, acetone,carbon tetrachloride, the polymers showed poor solubility,except for the polymer PAES4, which contains the smallestmesogen unit. The monomers M1, M2, M3 and M5 used forthe synthesis of these polymers were only soluble at hightemperature in DMSO, DMF, and NMP. The enhancedsolubility of polymers compared with that of the monomerscan be explained by the presence of the bis( p -chlorophe-nyl)sulfone or/and bisphenol A units in the macromolecularchains. It was also observed that the polymers containingether linkages inside the mesogenic units (PAE1, PAE2,PAES1, PAES2, CoPAES1 and CoPAES2) have bettersolubility than the others and the copolymers have bettersolubility than the corresponding homopolymers.Inherent viscosity ( h inh ) measurements on DMF solutionsof polymer samples gave values ranging between 0.15 and0.44dl/g, respectively (Table 3). GPC measurements of thesame DMF samples provided numerical average molecularweight ( M n ) values comprised between 5.313 (PAES4) and11.695 (PAES1), which indicate fairly good molecular weight Table 2. Spectral and elemental analysis data CodeMolecularformula/FormulaweightElementalanalysisIR absorption bands due to the various stretchingvibrations (cm À 1 ) l amax (nm)N (%)S (% Calcd.(Found))PAE1 (C 54 H 43 N 2 O 4 ) n (783.96) n 3.57(4.28)— 3060 ( n C–H aromatic), 2960, 2885 ( n CH 3 ) 1625 ( n CH––N),1590, 1490 ( n C––C, aromatic), 1230, 1250 ( n C–O–C), 1085 (C–Cl para).336PAE2 (C 55 H 40 F 6 N 2 O 4 ) n (906.93) n 3.09(3.82)— 3075 ( n C–H aromatic), 1620 ( n CH––N), 1590, 1495( n C––C, aromatic), 1250 ( n C–O–C), 1090 (C–Cl para).328PAES1 (C 50 H 34 N 2 O 6 S) n (790.90) n 3.54(3.51)4.05(3.22)3075 ( n C–H aromatic), 1630 ( n CH––N), 1595, 1510 and 1500( n C–C, aromatic), 1330 ( n SO 2 asymmetry) and 1160( n SO 2 symmetry) 1245 ( n C–O–C), 1095 (C–Cl para).342PAES2 (C 38 H 26 N 2 SO 3 ) n (622.7) n 4.5(4.67)5.15(5.42)3090 ( n C–H aromatic), 1625 ( n CH––N), 1595, 1510 and 1500( n C–C, aromatic), 1330 ( n SO 2 asymmetry) and 1160( n SO 2 symmetry) 1250 ( n C–O–C), 1100 (C–Cl para).337PAES3 (C 32 H 22 N2O 4 S) n (530.61) n 5.28(3.7)6.04(6.49)3080 ( n C–H aromatic), 1640 ( n CH––N), 1595, 1510 and1500 ( n C––C, aromatic), 1300 ( n SO 2 asymmetry) and 1160( n SO 2 symmetry) 1250 ( n C–O–C), 1085 (C–Cl para).342PAES4 (C 25 H 17 NO 4 S) n (427.48) n 3.28(3.6)7.5(6.66)3080 ( n C–H aromatic), 1625 ( n CH––N), 1585, 1505 and1490 ( n C––C, aromatic), 1320, 1295 ( n SO 2 asymmetry) and 1150( n SO 2 symmetry) 1245 ( n C–O–C), 1100 (C–Cl para).332CoPAES1 (C 79 H 62 N 2 O 10 S 2 ) n (1065) n 2.22(2.87)5.08(6.84)3050 ( n C–H aromatic), 2980, 2930, ( n CH 3 ) 1630( n CH––N), 1595, 1510 and 1500 ( n C––C, aromatic), 1330( n SO 2 asymmetry) and 1160 ( n SO 2 symmetry) 1250( n C–O–C), 1090 (C–Cl para).332CoPAES2 (C 65 H 48 N 2 O 9 S 2 ) n (1263.51) n 2.63(3.28)6.02(5.84)3080 ( n C–H aromatic), 2990, 2940, ( n CH 3 ) 1635( n CH––N), 1590, 1510 and 1495 ( n C––C, aromatic), 1330( n SO 2 asymmetry) and 1160 ( n SO 2 symmetry)1250 ( n C–O–C), 1070 (C–Cl para).337CoPAES3 (C 59 H 44 N 2 O 8 S 2 )(973.15) n 2.88(3.17)6.59(5.47)3060 ( n C–H aromatic), 2990, 2920, ( n CH 3 ) 1635( n CH––N), 1595, 1515 and 1500 ( n C––C, aromatic), 1305( n SO 2 asymmetry) and 1160 ( n SO 2 symmetry) 1250( n C–O–C), 1090 (C-Cl para).332 a Measured for polymer solutions in DMF. Table 3. Molecular weight measurements of the polymers Code M na M w / M n b h inhc (dl/g À 1 )PAE1 5 403 1.13 0.32PAE2 5 947 1.08 0.33PAES1 11 695 2.66 0.33PAES2 8 362 1.19 0.15PAES3 8 812 1.21 0.44PAES4 5 313 1.15 0.33CoPAES1 6 543 1.24 0.29CoPAES2 7 100 1.3 0.31CoPAES3 8 126 1.124 0.24 a From GPC measurements on DMF solutions. b Polydispersity index. c Inherent viscosity measured for polymer solutions in DMF, at aconcentration of 0.2wt% at 20 8 C. Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17 : 664–672DOI: 10.1002/pat New thermotropic polyazomethines 667 forthesepolymerswitharigidbackbone.Thevaluesofindexof polydispersity are low and comprised between 1.08(PAE2) and 1.3 (CoPAES2) (except for PAES1), so, thesepolymers have a narrow molar mass distribution (Table 3).Thermal behavior of the polymers was observed by TGA,DSC and polarized light microscopy (PLM). The data areshown in Table 4.Figure 1 shows representative plots of weight residue(in wt%) versus temperature, up to 600 8 C. The polymersexhibit high thermal stability: slight decomposition startedabove 200 8 C but with insignificant weight loss up to 370 8 C;a10%weightlosswasseenattemperaturesabove400 8 C.Thehighest thermal stability was recorded for the polymersPAE1 and PAE2, which contain bisphenol A or bisphenol F,respectively, as a non-mesogenic unit.A birefringent fluid is the confirmation of liquid crystal-linity 22 and PLM data revealed that some of the polymersexhibited thermotropic liquid crystalline (LC) behavior. Thepolymers PAE1 and PAE2 showed a strong birefringence on both heating and cooling, while the polymers PAES1,CoPAES1, CoPAES2 only at first heating, probably due tothe partial decomposition of the polymer near the clearingtemperature. Another explanation for this behavior could bethe high rigidity of the backbone which hinders the orderingof macromolecular architectures from a melted state. Thethermotropic polymers (PAE1, PAE2, PAES1, CoPAES1,CoPAES2) when observed by PLM showed a fine texture(Fig. 2), similar to those reported for thermotropic polymershaving stiff chains without flexible spacers. 23 The meltedpolymersPAES2andPAES4didnotshowanybirefringence.ThepolymersPAES3andCoPAES3didnotmeltupto350 8 C,so that the thermotropic behavior could not be studied; theirhigher values of melting temperature ( T m ) are probably dueto the increase of the rigidity of the azomethinic mesogenicunits, which allow a more efficient packing of themacromolecular chains.ThetransitiontemperaturesofthepolymersobtainedfromPLMmeasurementswerecomparedwiththosemeasuredbyDSC. The observed small differences could be explained bythe variations of the heating/cooling rate, the amounts of sample used, and the presence of an inert gas (nitrogen)during the DSC measurement, versus air during PLM.TherepresentativethermogramsobtainedbyDSCanalysisof thermotropic polymers are given in Fig. 3. All the samplesexhibited at least two endothermal peaks in DSC analysis,duringthefirstheatingscan(1H).Thefirstendothermalpeakwas assigned to a solid–liquid crystalline transition ( T m )and the last one was assigned to a liquid crystalline–isotropic transition (T i ). During the second heating (2H) onlyPAE1 and PAE2 samples showed endothermal peaks(Fig. 3a), while the other polymers showed only a glasstransition ( T g ). The polymers PAES1, CoPAES1 andCoPAES2 showed two peaks in the first heating scan(Fig. 3b). Integrating the DSC thermogram peaks over arange of temperatures enabled the enthalpy of the samplesforthattransitiontobeobtained.The D H valuesareshowninTable 4. For PAE2, PAES1, CoPAES1, CoPAES2 polymers, D H m and D H i are low and as expected, the solid—liquidcrystal transition involved more energy than the liquidcrystal—liquid transition ( D H m > D H i ). For PAE1 a less thanusual aspect of the thermogram was observed. In the firstheatingscan, D H m islowerthan D H i ,whileinthesecondscanthe D H m ishigherthan D H i .Theexplanationforthisbehavior Table 4. Thermal behavior of the polymers CodePLM DSC T o / T 10a T m ; T i T g T m ; T i Corresponding enthalpies b (J/g) D T PAE1 Cr 262 LC 312 I (2H) 120 Cr 263 LC 295 I (1H) D H m ¼ 6.8; D H i ¼ 57.6 32 248/411Cr 265 LC 285 I (2H) D H m ¼ 52.8; D H i ¼ 1.11 20PAE2 Cr 278 LC 293 I 128 Cr 260 LC 303 I (1H) D H m ¼ 6.8; D H i ¼ 4.6 43 282/438Cr 262 LC 280 I (2H) D H m ¼ 7.3; D H i ¼ 3.8 18PAES1 Cr 287 LC 305 I 127 Cr 284 LC 303 I D H m ¼ 3.38; D H i ¼ 2.04 19 210/470PAES2 Cr 180 I 162 Cr 190 I 205/475PAES3 Cr, T m > 350 8 C — — 207/378PAES4 Cr 188 I 110 — 203/420CoPAES1 Cr 225 LC 258 I 125 Cr 214 LC 261 I D H m ¼ 12.5; D H i ¼ 4.6 47 200/450CoPAES2 Cr 159 LC 191 I 143 Cr 153 LC 188 I D H m ¼ 9.06; D H i ¼ 3.2 35 195/445CoPAES3 Cr T m > 350 8 C — — 204/394 a From TGA measurements in air: T 0 ¼ initial decomposition temperature; T 10 ¼ temperature at which 10% weight loss occurred. b Enthalpies subscript: m ¼ melting; lc–i ¼ liquid crystalline to isotropic transition. Figure 1. The thermal stability of some polymers evaluatedby TGA in air. Copyright # 2006 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2006; 17 : 664–672DOI: 10.1002/pat 668 L. Marin, V. Cozan and M. Bruma
