Rigid rod networks: Liquid crystalline epoxy resins

Composite Structures 27 (1994) 37-43

Rigid rod networks: Liquid crystalline epoxy resins C. Carfagna, E. Amendola & M. Giamberini Department of Materials and Production Engineering, University of Naples 'Federico II' Piazzale Tecchio 80, 80125 Napoli, Italy Liquid crystalline polymers are well known for their unique mechanical and rheological properties. In recent years, some interest has been devoted to the study of a new class of liquid crystalline thermoset based on epoxy resins. Liquid crystalline epoxy resins can be obtained either by curing glycidyl terminated prepolymers over a range of temperature in which the mesophase is stable, or by reacting epoxy functionalized rigid monomers with a suitable curing agent. In our work this second approach has been followed. An unusual behaviour has been found for the form of the exotherm during the isothermal curing. Fracture toughness, K., was found to decrease with increased cunng temperature. This experimental evidence has been correlated with the reduction of the extent of liquid crystalline character with temperature.

INTRODUCTION

High strength and stiffness, chemical resistance and good electrical properties characterize the unusual family of plastics with liquid crystalline properties. Since the molecular architecture is already ordered in the liquid phase, the transition to the solid phase occurs quickly and requires less molecular rearrangement than with traditional isotropic liquids. The result is an extremely low coefficient of thermal expansion and low mold shrinkage. Only 10 years ago the first papers were published on crosslinked liquid crystalline polymers. Chemical crosslinking of linear liquid crystalline polymers can yield elastomers, which combine the anisotropic physical properties of the mesophase with the specific properties of elastomers. The mechanical deformation of the elastomers by dilation or compression causes a macroscopic orientation of the liquid crystalline phase, which is normally achieved by external electric or magnetic fields for linear liquid crystalline polymers. In 1986 some patents were published on mesomorphic epoxy resins, and on the procedure to cure the monomers in an ordered crosslinked state. 2 This historical review indicates how recent is the interest of the scientific community on thermosets with liquid crystalline properties.

Over 100 years ago the Austrian botanist Professor Friedrich Reinitzer, in his publication Beitrage zur Kenntnis del Chloesterins, 1 described his observations resulting in the discovery of liquid crystals. Since then a lot of work has been dedicated by several scientists to the study of this fascinating class of materials, with the aim of understanding their peculiar properties.

Research milestones of anisotropic polymers 1888 1956 1965 1975

-- Liquid crystal recognition -- Theory o r e J. Flory -- Lyotropic aromatic polyamides -- Patent activity on thermotropic polymers 1981 -- Crosslinked liquid crystal polymers 1988 -- Patent activity on liquid crystal thermosets

In 1956, P. J. Flory developed his first theory on anisotropic solutions and about 10 years later Kevlar was produced. From 1972 many thermotropic polymers were synthesized by industry and academia. From that time many patents have been filed based on the unique mechanical and rheological properties of liquid crystalline polymers. 37

Composite Structures 0263-8223/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

38

C. Carfagna, E. Amendola, M. Giamberini

Even if the concept of a liquid seems to be unsuitable for application to crosslinked polymers, in which macro-Brownian motion is not allowed due to chemical interchain linkages, nevertheless some functionalized compounds can be cured in ordered textures, resembling those found for liquid crystalline polymers. Two different approaches can be pursued in the synthesis of liquid crystalline epoxy resins. Consider, for example, the synthesis of glycidyl terminated prepolymers, that upon curing maintain their ordered morphologies. According to this procedure rigid blocks can be connected by flexible spacers. The resulting prepolymer, whose molecular weight can range from 3500 to 900, is subsequently endcapped and cured in the mesophase. 3

Table

1.

4,4'-Glycidyloxy-a-methylstilbene (DOMS):

4,4'-Diglycidyloxydiphenyl(DIF):

4-Glycidyioxyphenyl-4'-glycidyloxybenzoate:

X

Another method is that of potentially mesogenic rigid rod molecules that are functionalized and cured in the liquid crystalline phase. In this case, the mesophase forms during the initial step of curing, when the epoxy monomers and the curing agent react and give rise to a linear prepolymer. The liquid crystalline phase is then made stable during the gelation of the thermoset.4

In this paper we describe our efforts to prepare liquid crystalline networks following the second synthetic approach. The physical properties will be discussed and related to the ordered structure exhibited by the crosslinked thermoset. EXPERIMENTAL Table 1 depicts the molecular formulas of the epoxy compounds selected for this work.

Synthesis of the dihydroxy terminated molecules was carried out according to the procedure described in the literature. 5 Glycidyl termination was carried out with epichlorohydrin in a solution of KOH. A differential scanning calorimeter (DSC) DuPont Model 2910 was used to evaluate the curing kinetics of the resins. The fracture surface of the cured samples was analyzed by means of a scanning electron microscope. Transverse sections of the samples were obtained by fracturing them in liquid nitrogen. The textures of the resins were observed by an optical microscope (Reichert-Jung, Polyvar) equipped with a hot stage. The X-ray diffraction spectra were obtained by the photographic method, utilizing a flat-film camera; Cu K a radiation was used. The fracture toughness was evaluated according to the ASTM E-399 test procedure, using a 1 i n x l i n x l / 8 in (25.4 m m x 2 5 . 4 minx31.75 mm) compact tension specimens. RESULTS AND DISCUSSION Isothermal curing of liquid crystalline epoxy resins presents an unusual exothermal ~ . In a typical experiment, after an initial release of heat, the system reaches a pseudo plateau, followed by a new maximum (Fig. 1). The first peak is due t o reaction:~ the epoxy monomer ~ t h the curing agent. A linear pre-

39

Rigid rod networks: liquid crystalline epoxy resins

0 5

H2N- ~

CH3

4'

NH2

3J//,00°c ,le

LL

2 t400C

-r"

I.

120°C

.

t

5

3

Fig. 1.

7 9 Time (rain)

tl

13

15

DSC curves for isothermal cure of DOMS.

polymer is forming at this stage of the reaction, while the system is still isotropic: In fact, an optical microscopic analysis does not indicate any birefringence. Continuing the reaction, the increased length of the growing oligomer makes the liquid crystalline phase stable at the curing temperature. The microscopic observation of the sample under crossed polarizers indicates the immediate appearance of nematic textures. The second peak, due to heat release in the subsequent gelation, indicates the continuation of the curing reaction. Traditional kinked epoxy resins that do not develop ordered structures upon curing usually show a single exothermal peak during crosslinking. The unusual behaviour for nematic epoxy resins clearly indicates that, at a certain point of

the reaction, the curing mechanism changes as a consequence of the new state of the material. In general, the amine-epoxy resin curing reactions show complex kinetics typified by an initial acceleration due to the autocatalysis, while the later postgelation stages may exhibit retardation as the mechanism becomes diffusion controlled. The initial reaction of the epoxide with a primary amine produces a secondary amine and a secondary alcohol. The resulting secondary amine will react with another epoxide group, but more slowly, to form a tertiary amine with two secondary alcohol groups. At a certain temperature, higher than melting temperature of the two components, the mixture of mesogenic epoxy resin and curing agent is in its isotropic state. Upon reaction of primary reactive

40

C Carfagna, E. Amendola, M. Giamberini

groups a linear prepolymer forms, whose molecular weight increases with time. At this point the system is not yet crosslinked. It is well known that the range of stability of the liquid crystalline phase for a polymer is much broader than that of low molecular weight compounds. The liquid crystalline state broadens out and the transition temperatures increase with increasing molecular weight of the polymer. Continuing cure in the nematic state results in the development of permanent linkages due to the reaction of secondary reactive groups, which stabilize the nematic state further to the point that it cannot be destroyed by heating. When systems are cured at a temperature higher than a certain value Tni, the crosslinking takes place before the chain extension has been developed enough to stablize the nematic phase, and the system cures in its isotropic state. In fact, as revealed by Fig. 2, an increase of curing temperature causes a reduction of the birefringent regions and an increase of the isotropic parts. A nematic multidomain morphology, in which the ordered parts are surrounded by disordered

regions, can be supposed also at the lower curing temperature, with the difference that in this case the disordered isotropic portions are smaller. The physical properties of liquid crystalline polymers are a direct consequence of their ordered structure. The order parameter, which represents the statistical distribution of the orientation of mesogenic units, is one mean of characterizing the degree of order of mesomorphic materials. Other important parameters, affecting the resulting properties, are the disclinations or line defects. The orientation of the directors, the vectors indicating the average orientation of the mesogenic units in a region, changes systematically about the disclination lines so that the overall texture of the liquid crystalline phase is characterized by the spatial distribution of the disclination lines. The total volume of the polymer can be divided into subvolumes, within which the directors have an orientation correlation. These subvolume units are defined domains. Therefore, the size and the distribution of the domain in a liquid crystalline polymer are related to the density and the spatial distribution of the disclinations. As in

(a)

(b)

(c)

(d)

Fig° 2° Optical mierographs, between crosmd polarizers, of cured DOMS. Curing temperature: (a) 115"C; (b) 130"C; (c)

160"C;(d) 180"C.

Rigid rod networks: liquid crystalline epoxy resins

the case of thermotropic polymers, also in the case of crosslinked epoxy resin, exhibiting liquid crystalline order, it is possible to map the distributions of domains by means of optical microscopy. The dark brushes, in a typical optical micrograph, correspond to the extinction positions, indicating that the directors lie either parallel or normal to the electric vector of the polarizer or the analyzer. The points at which the dark brushes meet indicate singular points and correspond to the disclination lines normal to the film surface. The average spacing between neighboring disclination lines can be linearly related to the size of the liquid crystalline domain 6 (Fig. 3). An interesting correlation between fracture toughness, Kq, and extent of nematic regions in the cured sample can be made. An increased curing temperature causes a reduction of Kq. An increase of the curing temperature from 120 to 170°C causes a reduction of about 50% (Fig. 4). The lower fracture toughness at the higher temperature is due to the progressive reduction of the extent of nematic regions and a progressive increase of the isotropic part in the material, as revealed also by the trend of the density with the temperature for the cured thermoset (Fig. 5). The fracture toughness increase caused by the nematic structure can be explained as follows. The microstructure of isotropic material is homogeneous with properties equivalent in all directions, which results in straight, undeviating crack propogation. The liquid crystalline resin, having macroscopically isotropic properties, is nevertheless anisotropic on a microscopic level with pro-

41

perties such as strength, ranging with molecular orientation, which results in the deviation of crack propagation from a straight line. This suggests that the inhomogeneity and localized anisotropy of nematic texture is the main reason for fracture toughness increase for the liquid crystalline material. The unique microstructure of liquid crystalline polymers is demonstrated in the rheological, optical and electrical properties of these materials. A principal feature of the nematic phase of polymers is the polydomain morphology. This dynamic structure can orient in electric, magnetic or shear fields. 7

1600

\ 1400 C

1200

x

Z

ooo

800

~ 100

80

' 120

' 140

Curing Temperature

Fig. 4.

J 160

180

(C)

Fracture toughness versus curing temperature for DIE

1.29

L~

1.26 v >,

C @

1.2,.3

J Fig. 3.

1.20

8O

Optical micrographs of cured DOMS, between crossed polarizers.

I

I

I

I

100

120

140

160

Curing Temperature

Fig. 5.

180

(C)

Density versus curingtemperature for DIF.

42

C. Carfagna, E. Amendola, M. Giamberini

the mesogenic groups to lie in subvolumes with their major axes parallel.

Fig. 6.

X-ray diffraction pattern of DOMS.

Thermotropic polymers can often be processed with ordinary thermoplastic processing equipment. Because of low viscosity and low extrudate swells, this is even easier than processing of ordinary thermoplastics. Hence, the name self-reinforcing plastics is sometimes used for thermotropic polymers. Domains that, under shear, slide over each other are responsible for the easier processability. In a similar way, it is expected that, if the liquid crystalline prepolymer is oriented during the initial step of the curing reaction, a highly anisotropic material could result. The subsequent gelation will freeze the resin in this oriented mesophase, so maintaining the macroscopic anisotropy during the postcuring (Fig. 6). The curing agent is crucial to the formation of a liquid crystalline epoxy resin. In fact, as previously stated, the mesophase is forming during the initial step of curing during the reaction of the curing agent and the epoxy monomers.

If the chemical constitution of the curing agent does not lead to the growth of a linear liquid crystalline prepolymer, the resulting crosslinked resin will form an isotropic material. This implies that, in the case of a kinked curing agent, the microBrownian motion of the network does not allow

Moreover, if the curing agent, even though showing the geometrical requirements to link with the epoxy monomers in a mesophase, reacts in a range of temperature above the Tni of the prepolymer, the resulting crosslinked material will be isotropic. CONCLUSIONS New epoxy resins can be cured to a liquid crystalline phase. By epoxy endcapping nematogenic dihydroxy terminated monomers it is possible to obtain a reactive compound that can be subsequently cured in a nematic state. The liquid crystalline phase forms during the initial step of reaction and is subsequently maintained during the gelation and postcuring. A large difference in fracture toughness has been observed between isotropic and liquid crystalline resins. Nematic thermosets are largely tougher than the isotropic ones. The nature of the curing agent is also important for the resulting state of order of the network. A kinked curing agent cannot allow the rigid blocks to arrange themselves in a nematic state. Flexible or rigid curing agents can crosslink the epoxy monomers in a liquid crystalline phase, provided that their reactivity is carried on in the thermal range of stability of the nematic phase of the prepolymer. From the practical point of view, the major difference in proce~ing conditions, which results in nematic or isotropic structures of the cured product, is a ~hold' at a certain temperature below the isotropization temperature of the p r e ~ r . The ~ hold time d q ~ a d s on the selected temperature. If the curing process contaim such a 'hold', the system develops a liquid crystalline structure which does not ~ at ldg5 temperatures.

ACgNO~'qbllm61mL~tr The financial assistance of Shell Co. is gratefully acknowledged.

Rigid rod networks: liquid crystalline epoxy resins REFERENCES 1. Reinitzer, E, Monatsh. Chem., 9 (1888) 421. 2. Muller, H. P., Gipp, R. & Heine, H., Bayer AG, DOS 36 22 610 (1986). 3. Barclay, G., Ober, C. K., Papathomas, K. & Wang, D., Proc. ACS Div., Polymeric Materials: Sci. and Eng., 63 (1991)356.

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4. Kirchmeyer, S., Karbach, A., Muller, H. P., Meier, H. M. & Dhein, R., Proc. Int. Conf. on Crosslinked Polymers, Luzern, 1990, pp. 167-76. 5. Zaheer, S. H., Singh, B., Bhushan, B., Bhargava, P. M., Kacker, I. K., Ramachandran Sastri, V. D. N. & Rao Shanmukha, N., J. Chem. Soc., Part lll, (1954) 3360. 6. Shiwaku, T., Nakai, A., Hasegawa, H. & Hashimoto, T., Polymer Commun., 28 (1987) 174. 7. Wissburn, K. F., Br. PolymerJ., 12 (1980) 163.