LC 에폭시 수지의 경화반응에서의 상전이

여만경, 심미자*, 김상욱

서울시립대학교 화학공학과,

*서울시립대학교 생명과학과

The Phase Transition in Curing Reaction of A Liquid Crystalline Epoxy Resin

Mangeng Lu, Mija Shim． Sangwook Kim

Department of Chem. Eng., The University of Seoul, Seoul 130-743, Korea

．Department of Life Sci., The University of Seoul, Seoul 130-743, Korea

Introduction

It is common knowledge that liquid crystalline (LC) main-chain polymers show a high modulus and a very low thermal expansion in the direction of the macroscopic orientation. In the direction transverse to the orientation of the chains, however, the properties are rather poor. These differences can be reduced by the introduction of cross-links between the chains, which improves the dimensional stability of these ordered systems.

It has been reported that the overall curing reaction kinetics is strongly affected by the presence of a LC phase. On the other hand, the reaction mechanism has not been investigated directly. If the conventional reaction scheme proposed for the addition reaction of organic amines to epoxy groups holds true, then the overall reaction can be modeled, and the contribution of the phase transition occurring during the hardening reaction can be evaluated and modeled.

In this article, the curing kinetics of a system containing 4,4□-diglycidyloxy-biphenyl and different functionality amines, phenylene diamine (PDA), methylene dianiline (MDA), and sulphanilamide (SAA), are studied by differential scanning calorimeter (DSC) under isothermal and dynamic curing conditions. The phase transformations during curing of the above systems have been monitored by a polarized optical microscopy equipped with a hot-stage and photo detector.

Experimental

The liquid crystalline epoxy monomer, 4,4□-diglycidyloxy-biphenyl, was synthesized according to the method reported in literature. Phenylene diamine, methylene dianiline, and sulphanilamide were purchased from Aldrich. Their chemical structures are shown in Scheme 1.

□                                    □

□□                                  □□

□□□□□□□□□□□□□□□□□□□□□□□□□□;  □□□□□□□□□□

4,4□-diglycidyloxy-biphenyl                    Phenylene diamine

□□□□□□□□□□□□□□□□□;              □□□□□□□□□□□□□

methylene dianiline                             sulphanilamide

The epoxy monomer and hardeners were mixed in Tetrahydrofuran (THF), then the solvent was removed and dried in vaccum oven at room temperature. A loose white powder was obtained and stored in the freezer.

Isothermal and dynamic curing were performed in a Solomat DSC 4000 under nitrogen purge (20 ml/min). For the isothermal curing, the DSC cell was preheated to the reaction temperature before the sample was put in. The texture of the liquid crystalline phase was observed through an Nikon polarizing microscopy. The temperature was controlled by a Mettler FP 82HT hot-stage.

Results and Discussion

The first two diamines, Phenylene diamine, methylene dianiline, are symmetrical compounds in which amine functions have approximately equal reactivity. The latter diamine (SAA) has, however, two different amine functions of unequal reactivity and seems to favor formation of a smectic phase as described below. PDA reacts with epoxy monomer at the lowest temperature of the four diamines. However, exothermic heat flow above 200℃ indicates that additional reaction also occurs when heated to this higher temperature. Epoxy monomer reacts with MDA more slowly than with PDA, while the reaction between epoxy and SAA instead occurred quite gradually. The lower reactivity of SAA comes from the low electron density on the nitrogen atom of the sulfonamide group. A summary of the properties of thermoset mixtures of epoxy monomer as cured with different diamines as given in Table 1. Reaction temperature, initial phase, and properties of the cured networks are listed and should be referred to during the following discussion of LC monomer.

Table 1. Curing conditions and properties of networks.

Curing agent	Curing condition			Cured polymer
	Temperature, ℃	Time, h	Initial state	Phase	Tg, ℃
MDA	140	5	isotropic	S	108.6
	175	4	isotropic	S	114.4
SAA	175	8.5	isotropic	S	101.2
	200	4	isotropic	S	100.5
PDA	140	4	isotropic	N	82.9
	175	3	isotropic	N	90.7

The initial melt mixture of epoxy monomer and MDA did not show liquid crystallinity at any temperature since it forms an isotropic liquid above the melting point. As the reaction proceeded, the reactant mixture first developed a fluid texture which then became locked-in as the network formed. The cured polymer network retained mesophase birefringence until decomposition. The mixture cured at 140 and 175℃ all had the same occurrence of dense disclination lines (threaded texture).

The uncured mixture with SAA was not initially birefringent. Three minutes after the start of curing, two transitions appeared at temperature 127 and 174℃. The transition at 127℃ was identified as a crystalline-smectic transition. The endothermic peak at 174℃ was a smectic-isotropic transition as confirmed by POM. The smectic-isotropic transition temperature increased gradually as the reaction advanced. Twelve minutes after the start of curing, another endothermic peak appeared at 140℃. No apparent change was detected around 140℃ using POM.

There are two exothermic peaks in the isothermal scans (Fig. 1). The early stage of the reaciton follows the tendency of this parallel model. However, an enhancement of the reaction rate appears in the latter stage of the reaction. This phenomenon is quite abnormal because a rate retardation is usually observed in the later stage of reaction due to the diffusion effect in non-LC epoxy systems. Also, the discrepancy of the experimental data from the autocatalytical kinetic model starts at a conversion of around 35-45%  and the maximum of reaction rate enhancement happpens at a conversion of about 50%, which is lower than the critical conversion (over 65%) in non-LC systems.

The presence of this second maximum can be interpreted by the occurrence of a phase transition of the reacting medium during the crosslinking reaction. This hypothesis is supported by the optical observation as already described. The contribution of the second exotherm decreases continuously with increasing curing temperature up to the limit of 190℃, where the last trace of the shoulder on the exothermic peak is lost. If the reaction is carried out at a temperature higher than 190℃, then the nematic phase cannot be observed by optical analysis and the DSC exotherm exhibits the usual single peak feature associated with the reaction exotherm.

If the curing temperature is increased up to 190℃, the isotropic phase obtained after the melting of the reacting mixture is maintained throughout the complete cure, and the crosslinked resin does not exhibit any trace of birefringence between crossed polarizers.

Fig. 1  DSC curves in isothermal mode at 130, 150 and 175℃ for SAA system.

The increase of temperature plays a different role in these systems. It is observed that the autocatalytical reaction becomes more dominant at higher temperatures for PDA, while the catalytical effect from preexisting impurities containing active hydrogens becomes more obvious at higher temperatures for SAA system. The reaction constants of curing under isothermal conditions from 120-180℃ for PDA and SAA systems were listed in Table 2.

Table 2. Reaction constants under isothermal conditions*.

Curing agent	102k1, min-1				103k2, min-1
	120	140	150	175	120	140	150	175
PDA	4.9	9.6	28.4	56.7	8.2	7.8	9.5	10.8
SAA	1.2	2.4	2.9	3.4	3.7	6.5	11.6	27.6

*data were obtained by the kinetic model as:

[eqn: { dH} over {dt } =( { k}_{ 1}+ { k}_{2 } alpha )(1- alpha )^2]

Conclusions

A epoxy monomer was cured in liquid crystalline phase from 120 to 175℃ with three different amines to form LCT. The thermal transitions of the LCT was studied by DSC and POM. The phase transitions of LCT's were shown to be highly dependent on the state of cure. The curing reaction of various systems follows a autocatalytical model, but exhibits different temperature dependence for each system.

References

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(1993)

3. Q. H. Lin, et al. J. Polym. Sci., Part  B, 35, 2363 (1997)