Linear precursors of liquid crystalline thermosets Barbara Hirn," Cosimo Carfagna" and Rosa Lanzettab a Department of Materials and Production Engineering, University of Naples FEDERICO II, Piazzale Tecchio 80, 80125 Naples, Italy Department of Organic Chemistry and Biology, University of Naples FEDERICO II, Via Mezzocannone 16, 80134 Naples, Italy Polycondensation of glutaric acid with a diglycidyl terminated mesogenic molecule [p-(2,3-epoxypropoxy)-a-methylstilbene]was carried out with a basic catalyst. A variety of novel main chain thermotropic liquid crystal oligoesters were synthesized uia a short melt-copolycondensation process, by using different ratios of co-monomers. Preliminary studies by thermogravimetric analysis, differential scanning calorimetry, steric exclusion chromatography and Fourier transform infrared spectroscopy have been carried out on the reacting mixtures in order to optimize the reaction conditions (temperature, time).Carbon-13 nuclear magnetic resonance was used to identify the material structure. The mesophase stability ranges of the oligoesters were evaluated by means of DSC and optical polarizing microscopy. X-Ray diffraction analysis was applied to determine the nature of the mesophase. Crosslinking of glycidyl-terminated rigid rod monomers with various curing agents produces liquid crystalline thermosets with outstanding The chemistry of the curing reaction is not far from that described in literature and regarding conventional epoxies. Of course, the density and distribution of the crosslinks in the network, as well as the kinetics of the reaction, are affected by the level of order of the growing thermosets.It has been found that aromatic amines react with epoxy mesogens very quickly, producing nematic On the other hand, aliphatic diacids react much more slowly, giving smectic elastomers that can be subsequently oriented by means of a stress field.899 As in the case of conventional epoxies, it should be interes- ting to explore the possibility of synthesizing functionalized linear prepolymers from liquid crystalline epoxies that could be further cured. In this way, by balancing the length of the prepolymers with the amount and the type of curing agent, it should be easy to produce thermosets with the desired levels of order and transition temperature for different applications. One of the ways to synthesize these prepolymers is the polymerization reaction of a dicarboxylic acid with a diglycidyl endcapped mesogenic molecule. The reaction of the epoxy ring with a carboxylic acid presents several advantages.lO"' It can be carried out at moderate temperature (80-150°C) in the presence of basic catalysts [Scheme l(a)].It is an addition reaction from which no volatile compound is eliminated, so formation of bubbles in the material is avoided. Another advantage of this polymerization process is the formation of polymers containing hydroxy side groups, which can be very useful for further chemical modification of the polymer chains, such as grafting or crosslinking.However, there are three different undesirable side reactions [Scheme 1 (b)].In order to eliminate these reactions and obtain linear polymers containing hydroxy side groups, the polymeriz- ation must be base-catalysed. In a first step, a carboxy anion is formed (Scheme 2) and the reaction proceeds ionically through the epoxy group. The acid anion is then regenerated. For example, in the presence of a tertiary amine, if the carboxylic acid is introduced in excess in the reaction medium, the polymerization reaction stops when all of the epoxy is consumed; the resulting polymers are carboxy terminated, and these functions cannot react further with the pendant hydroxy groups of the main chain because of the presence of basic catalyst. In order to obtain semi-flexible linear liquid crystalline polyesters which contain hydroxy side groups and carboxy terminated, a bulk polycondensation of different mixtures of glutaric acid (GA) and of p-( 2,3-epoxypropoxy)-a-methyl-stilbene (DOMS) in the presence of N,N-dimethylbenzylamine (BDMA) were carried out.By varying the co-monomer ratios a new class of products was generated. Experimental Measurements Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and thermomechanical analysis (TMA) of all samples was performed by means of an integrated analytical system DuPont 2100 composed of the following modules: differential scanning calorimetry (DSC) 2910, thermogravi- metric analyser (TGA) 951, and thermomechanical analyser (TMA) 2940.The X-ray diffraction patterns were recorded by the photographic method with a Rigaku model III/D max generator by using Ni-filtered Cu-Ka radiation. The textures of the crystalline and liquid crystalline phases were observed in transmitted light with an optical microscope (Reichert-Jung Polyvar), equipped with crossed polarizers for polarised microscopy (POM). The temperature was controlled by means of a Linkam TH 600 hot stage apparatus. Fourier transform infra-red analysis (FTIR) was performed with a Nicolet 5PC spectrophotometer. The 13C NMR spectra of the oligomers were obtained on a Bruker 100MHz in FT mode in [2H,] DMSO at 30 "C. The signal integrations were obtained by an Inverse-Gated experiment performed with a delay of 150 s.Steric exclusion chromatography (SEC) was performed by means of an apparatus composed of a Refractive Index detector (Shimadzu Rid-6A), an HP+C pump (Varian 2510) and three columns ( lo3, 500 and 100 A). Synthesis of monomers The diglycidyl endcapped compound used in all polymerization reactions was p-( 2,3-epoxypropoxy)-a-methylstilbene(DOMS). The synthesis was previously described in the literature;8 mp 130"C, T,N115 "C (monotropic). Glutaric acid (GA) (99%, mp 95-98 "C) and N,N-dimethylbenzylamine (BDMA) (99%, bp 183-184 "C) were purchased from Aldrich and util- ized without any further purification. Their purity was checked by DSC. J. Mater. Chew., 1996, 6(9), 1473-1478 1473 HOC(CH2)3COH + \-CH20-@zCH-@XH2-/ -II II 0 -0 00 GA DOMSI BDMA t C(CH2)3COCH2CHCH20 ~~~CH~OCH2CHCH20 II II I I0 0 OH OH (b 1 estenfication via hydroxy groups of the main chain -+ HOC--roc-+ H20II oc-II0 II 0 hydration of epoxy groups + H20 vT HO HO ethenfication reaction of epoxy with hydroxy groups I OH Scheme 1 (a)General scheme of polyesterification reaction, (b)side reactions base --co-II0 - +T -COCH*CH-II I0 0- -COCH2CH-II I0 0- + -COH II 0 --COCH2CH-II I0 OH + -CO-II 0 Scheme 2 Initiation of polyestenfication reaction Synthesis of oligomers The reagents p-(2,3-epoxypropoxy)-a-methylstilbene(DOMS), glutaric acid (GA) and BDMA (10mol%) were blended and heated at 140°C After 20 min, the reaction mixture was pressed between two glass plates (previously treated with a surfactant, Surfasil@) and heated for another 20 min at 140 "C The ratio GA DOMS varies from 1 1 to 12 1 and the synthesized oligomers were indicated as DG1 A, DG105A, DG1 1A and DG1 2A The resulting films were studied by TMA, DSC, SEC, osmometry, X-ray diffraction, POM and I3C NMR spectroscopy Results Preliminary studies of the polycondensation reaction Preliminary studies of the reaction parameters of the polymer- ization process were carried out by TGA under air The weight 1474 J Muter Chem, 1996, 6(9), 1473-1478 loss percents determinated by TGA during the polymerization reaction at 140 and 180°C of the blends (DOMS-GA) with and without basic catalyst are presented in Table 1 DSC analysis (Fig 1)of the mixtures (DOMS-GA-BDMA) consists of a heating from 30 to 140"C, followed by an isotherm at 140 "C (Fig 1) During the heating step the melting endotherms characteristics of both GA (110 "C) and DOMS (130 "C) are well evidenced The exotherm present during the isotherm at 140°C is representative of the polymerization process of GA with DOMS In all cases, after 15 min, the reaction is completed Analysis of polymerization reaction of the blend DGlA The FTIR analysis of the reaction medium during the polymer- ization of the stoichiometric blend shows an increase in the absorbance at 3600-3200cm-' due to the formation of side hydroxy groups resulting from the addition reaction of epoxy and carboxy groups (Fig 2) During the reaction, the CO,H equivalent is determined by Table 1 Thermogravimetry analysis of blends (GA-DOMS) with and without basic catalyst composition DOMS-GA-BDM A/mol 110 1120 11 01 11201 T/OC weight loss (%) 140 67 180 88 140 91 180 92 140 17 180 49 140 28 180 71 0.5-7 I 0, P O-O-6= ri; -0.5-a,.c -1 .o-34100 32bo 3b 28100 wavenurnber/cm Fig.2 FTIR analysis of the blend DGlA during the polymenzation reaction titration using a solution of 0.1 mol dm-3 NaOH in the presence of phenolphthaleine as indicator (Fig. 3). The DSC analysis was carried out after 2, 5,10, 15 and 20 min of reaction (Fig. 4). For increasing reaction times, the endotherm moves towards lower temperature, and disappears after 20 min.This trend could be explained on the basis of the following consider- ations. During the early stages of the reaction, the endotherm is the result of the melting of the growing oligomers and unreacted monomers. With increasing time, the monomer is consumed and prepolymer is no longer crystalline, at least for the samples cooled from the reacted mixtures. 4.5-'' ' ' ' ' ' ' ' ' ' ' ' " " ' ' ' ' '. --4 ----0.5 i i iI I I I 1 I I I 1 I I I I I I 17 I 11 1 7 I:-Os4-= +-([I2 -0.8--1.2 I I 1 Molecular weight and functionality of the oligomers The weight of all prepolymers was determinated by osmometry and by SEC. The C02H equivalence per gram of product was evaluated by titration using a solution of 0.1 mol dme3 NaOH in the presence of phenolphthaleine as indicator. Both param- eters allow the evaluation of the functionality (Table 2). The osmometry was carried out at 90°C in DMF, and the SEC was performed at 20°C in THF.The signal integration of NMR spectra (Fig. 5) allows the determination of [OH]/[CO, R] and [OH]/[C02 H] ratios. The OH equivalence per gram of product was therefore calculated from the COzH equivalence per gram obtained previously by titration (Table 3). Thermal behaviour of the oligomers In order to determine all phase transition temperatures, all prepolymers were submitted to a DSC analysis consisting of a first heating made at a rate of 10"C min-l [Fig.6(u)] followed by a cooling at a rate of 15"Cmin-' in order to inhibit the crystallization of the oligomers [Fig. 6(b)],and by a second heating at a rate of 5 "C min-' [Fig. 6(c)] selected to enhance the sensitivity of the DSC technique (Table 4). Mesophase characterization X-Ray diffraction patterns of the crystalline oligomers were recorded at room temperature on samples quenched from the isotropic phase (Fig. 7) and allowed to crystallize at room corresponding to a spacing of 12.8 A. The weak reflections appearing at wider angles are probably due to the onset of crystallization of the quenched sample. In the pattern recorded for the crystallized sample, more crystalline reflections appear. that the mesophase developed during quenching is of the smectic type, the spacing corresponding to the distance between two adjacent smectic layers.The analysis of the texture as revealed by POM analysis confirms the X-ray diffraction analysis, even if the high viscosity of oligomeric compounds does not allow the observation of focal conics or a perfectly defined texture (Plate 1). Roviello and Sirigu reported the mesomorphic behaviour of some aliphatic esters of dunng the polymenzkon of DG1A-therefore a tilt within the layers could be supposed. J. Muter. Chem., 1996, 6(9), 1473-1478 1475 Table 2 Weights and functionality of the oligomers SEC functionality oligomers osmometryMJg mol-' Mn" MW" Ib CCO,Hl(equiv g- C d ~~~~~ DG12A 2280 2453 8044 33 122 x 10-3 28 3 DG11A 2873 2896 9275 32 728 x 10 21 21 DG105A 3448 2815 8198 29 5 56 x 10-4 19 16 DGlA 3363 3665 12209 35 5 98 x 10 2 22 "In equivalent PS bI = Mw/Mn=index of polydispersity 'Functionality calculated from the osmometnc values dFunctionality calculated from SEC values Fig.5 13C NMR spectrum of DGlA Table 3 Determination of the OH equivalence per gram from the I3C NMR spectrum' oligomer ~~ [OH]/[CO,R] [OH]/[CO,H] [OH]/equiv g-' DG12A 07 21 2 52 x 10-3 DG11A 08 5 3 6 x loW3 DG105A 08 46 26 x 10-3 DGlA 07 54 3 2 x 10-3 Discussion The preliminary isothermal studies by TGA at temperatures of 140 and 180 "C (under air) of different blends of DOMS-GA without BDMA (Table 1)show that the weight loss percentage increases with the temperature and with the increasing excess of dicarboxylic acid present in the mixture The weight loss is representative of the extent of the esterification side reaction of carboxylic acid with the hydroxy groups of the main chain oligomers (Scheme 1) In the presence of basic catalyst, the weight loss is lower because the esterification reaction of the carboxylic acid and epoxy groups prevails, and after complete consumption of epoxy groups the end carboxylic groups of the synthesized oligomer do not react with the side hydroxy groups to any great extent, at least at 140 "C According to the literature," it can be noted that the presence of a basic catalyst and a decrease in the reaction temperature avoided the undesirable side reactions On the other hand, an increase of the co-monomer ratio [GA]/[DOMS] from 1 to 12 promoted the reaction of carboxylic acid with the hydroxy groups of the main chain oligomers In the case of the stoichiometric blend GA-DOMS, the analysis by FTIR, SEC and DSC and the determination of the C02H equivalence per gram of the reaction medium dunng the polymerization process allow verification of the simultaneous and total consumption of the co-monomers, the increase of the growing chain weight, the 1476 J Muter Chem, 1996, 6(9), 1473-1478 -0.7 I 2'0 do Id0 140 '9 08-Iul a OA-6 E c.04-c 0 2-0.d A0 2'0 &I Id0 140 -0.d -20 2b tib Id0 140 T/"C Fig.6 DSC analysis of the final oligomer, (a) first heating, (b)cooling, and (c) second heating formation of hydroxy groups and the reduction of the carboxylic group concentration The molecular weights of the synthesized oligomers deter- mined by osmometry and SEC (Table 2) are relatively low (2300-3400) and weakly affected by the variation of the co- monomer ratios The high values of the ratio M,/M,, (Table 2) indicate high polydispersity for the final products, probably resulting from the high values for the blend viscosity, but also possibly due to partial crosslinking via some of the numerous side reactions In the case of bulk polymerization, the increase of viscosity is one of the limiting parameters of the polymer Table 4 Phase transition temperatures and enthalpies determined by differential scanning calorimetry first heating second heating cooling oligomer TK ,LC I"C AHILLCIJ g-' TLc,1/3c AHLCSIJ g- T,LCI3C T,/T TLC,II0C AHLCJIJ g- DG1.2A 44.4 10.5 106.2 10.4 97.9 26.1 102.0 6.6 DG1.1A 41.3 9.3 126 10.8 112.4 21.9 123.3 11.8 DG1.05A 41.9 9.2 131.8 14.1 119.3 24.3 131.4 10.8 DGlA 41.7 8.7 123.3 6.4 118.1 26.4 127.4 8.8 Fig.7 X-Ray of quenched DGl.lA, room temperature Fig. 8 X-Ray of crystalline DGl.lA, room temperature chain growth and of the molecular weights of the resulting products. In our case it is not possible to carry out the polymerization at higher temperatures since an increase of temperature implies a growth of undesired side reactions. According to the literature, higher molecular weights could be obtained by polymerizing diglycidyl endcapped monomers with the proper dicarboxylic acid in an organic ~olvent.'~,'~ The functionality of the oligomers (Table 2) was determined Plate 1 POM of DGl.lA, T= 1OOT from both molecular weight and CO,H equivalence per gram.The values of this parameter increase with the excess of glutaric acid introduced into the reaction medium. This result confirms the preliminary TGA analysis showing the crucial influence of co-monomer ratios on the esterification side reaction of the carboxylic groups with the side hydroxy of the growing chains, forming branched or crosslinked polymers.The DG1.2A has a functionality of three but is soluble in organic solvents (THF, DMF, DMSO etc.), so for this reason it is reasonable to hypothesize that it is not a cured product but a branched one. 13C NMR spectra show that the resulting prepolymers were not completely linear because the integration of the signal characteristic of the ester groups (6 172.5) is higher than the corresponding value of the carbon bonded to the hydroxy group (6 66.8) (Table 3). This result also confirms that some part of the hydroxy side groups of the prepolymer main chain are involved in secondary reactions. 13C NMR spectra allow the determination of the OH equivalence per gram of prepolymers, which can be very important in the case of a further curing reaction.The synthesized oligomers crystallize at room temperature but the crystallization can be inhibited by a quick cooling to give glassy products (T, of around 25 "C) which slowly crys- tallize at room temperature (Table 4). The TKqLCwas found to be in the range of 40-45"C, and the corresponding TLc,,was found to be in the range 100-130 "C. The high values of the J. Mater. Chem., 1996, 6(9), 1473-1478 1477 isotropization enthalpies (6-14 J g-') strengthen the pre-viously discussed hypothesis that the mesophase exhibited by the prepolymer is of the smectic type, even if the AH values cannot be considered conclusive The relatively wide range of the liquid crystalline phase denotes the high stability of the smectic structure, which might be a consequence of interchain hydrogen bonds Conclusion A variety of novel thermoplastic liquid crystal carboxy- terminated oligoesters containing hydroxy side groups was synthesized vzu a short melt-polycondensation of a dicarboxylic acid and a diglycidyl mesogenic molecule, in presence of a basic catalyst These products were crystalline at room tem- perature The crystallinity can be inhibited by quenching the melt to get glassy products with a 5 of 25°C The liquid crystalline phase of these products was smectic and proved stable over a relatively wide temperature range (80-100 "C) Their molecular weights decrease from 3400 to 2300 with the increase of the acid content in the blend The functionality of the oligomers was equal to or higher than two, and the OH equivalence per gram of product was lower (2 5 x to 3 6 x than the theoretical value (4 x because of OH participation in secondary reactions The presence of these hydroxy side groups made these oligoesters potential candi- dates as precursors for liquid crystalline thermosets References 1 G G Barclay, C K Ober, K I Papathomas and D W Wang, J Polym Sci ,Polym Chem ,1992,30,1831 2 G G Barclay,S G McNamee,C K Ober, K I Papathomasand D W Wang, J Polym Sci, Polym Chem, 1992,30,1845 3 C Carfagna, E Amendola, M Giambenni, A Filippov, and R S Bauer, Liq Cryst, 1993,13,571 4 C Carfagna, E Amendola and M Giambenni, Compos Struct , 1994,27,37 5 C Carfagna, E Amendola, M Giamberini and A Filippov, Macromol Chem Phys , 1994,195,279 6 C Carfagna, E Amendola and M Giambenni, Macromol Chem Phys , 1994,195,2307 7 E Amendola, C Carfagna, M Giambenni and G Pisaniello, Macromol Chem Phys , 1995,196,1577 8 M Giambenni, E Amendola and C Carfagna, Mol Cryst Liq Cryst, 1995,266,9 9 C Carfagna, E Amendola and M Giamberini, Makromol Chem , Rapid Commun , 1995,16,97 10 H Lee and K Neville, in Epoxy resins, their applications and tech- nology, McGraw-Hill, NY, Toronto, London, 1957 11 P J Madec and E Marechal, in Aduances in polymer science, Springer-Verlag, Berlin, 1985, vol 71 12 A Roviello and A Sirigu, Gazz Chim Ital, 1977,107, 333 13 E Haertel, M Fedtke, D Pospiech and H Gulbe, Plaste Kautsch , 1984,31,405 14 F B Jones, USP 3,639,655, 1972 (Chem Abstr ,1972,76, 141 764) Paper 6/02795D, Received 22nd April 1996 1478 J Muter Chem, 1996, 6(9), 1473-1478