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Species distribution during the cure of dicyanate ester.


Bisphenol A-based cyanate ester resin systems are gaining widespread interest in the electronic and the composite industries because of their excellent dielectric properties, good dimensional stability, glass transition temperatures in the range of 250 to 300[degrees]C, solubility in ketone solvents, and low moisture absorption (1-7). The resins cure by addition polymerization without volatile products. The major reaction pathway is cyclotrimerization of the cyanate groups to form a highly crosslinked polytriazine network (1-11). The trimerization reaction for catalyzed system has been reported to follow second kinetics while the uncatalyzed system shows autocatalytic behavior (4-9).

The basic factors that affect the network structure include the resin functionality, and the system composition. Since variations in the processing conditions and the system composition may change the reaction mechanism, and consequently, the polymer structure, the choice of the optimum curing conditions and the composition requires a knowledge of the network formation process and the network structure.

The objective of this study was to examine the network formation process during the cure of uncatalyzed bisphenol A-based cyanate resin using size exclusion chromatography (SEC) and differential scanning calorimetry (DSC).


The bisphenol A dicyanate (BADCy) monomer with the trade name Arocy B-10 and 99.9% purity was supplied by Rhone-Poulenc, Inc.

SEC studies were performed using a Waters Associates 150C ALC/GPC equipped with a RI detector. Tetrahydrofuran (THF) was used as the solvent. The column was operated at 30[degrees]C and a solvent flow rate of 1.0 ml/min was maintained. Polystyrene standards were used for the column calibration. In the low molecular weight range, cumylphenyl cyanate and its trimer were also used for column calibration.

Samples for the SEC studies were cured in an oven under various conditions in a nitrogen atmosphere. The extent of reaction of the partially cured samples was determined from dynamic DSC experiments. The soluble portion for samples cured beyond the gel point was obtained by extraction in THF for 48 h.

Dynamic DSC studies were performed under a constant nitrogen purge using a Mettler DSC-30 equipped with a low temperature cell. The samples were scanned after the initial cure to 370[degrees]C at 10[degrees]C/min to determine the residual heat of reaction and the glass transition temperature. The temperature corresponding to the only onset of endothermic deflection of the baseline was taken as the [T.sub.g]; the residual heat was calculated from the exotherm area. Conversion, [alpha], from the DSC experiments was calculated as:


where [delta]H.sub.tot is the total heat of reaction determined to be 730 J/g, and [delta]H.sub.res is the residual heat of reaction for a partially cured sample.


The Network Formation Process

The structural build-up of the polycyanurate network in both the pregel and postgel regions was examined using SEC and uncatalyzed BADCy samples cured to various extents of reaction. The SEC chromagrams for BADCy cured to various conversions below the gel point are shown in Fig. 1. The first step in the build-up of the network is the formation of trimer and species identifiable in increments of two monomers up to n = 7, where n is the number of monomer units. As the cure proceeds, interactions between the growing chains and with the monomer result in a wide range of oligomeric species. The overall species distribution becomes broader as the system approaches gelation, as evidenced by the chromatogram at 51.6% conversion; gelation in these systems occurs at 60 to 65% conversion (2, 4, 5, 7, 12). Figure 2 shows the normalized peak area as a function of sample conversion for the monomer, trimer, 5-mer, and for species of n [greater than or equal] 7 in the pregel region.

The maximum amount of the trimer and the 5-mer is achieved at about 25% conversion. As a result of interactions with the monomer and with the growing chains, the peak intensities of the trimer and the 5-mer decrease at conversions greater than 25%. At the gel point, an insoluble network is formed and the species undergo retroversion.

A scheme representing the interactions in the reaction medium is given by the following equations:


where M is the monomer and [P.sub.x] is x-mers.

A kinetic model based on the reaction scheme was derived to predict the species distribution in the reaction medium in the pregel region. The assumptions used in the derivation are:

(i) the only reaction pathway is the trimerization reaction; (ii) each trimerization step is second order; (iii) the principle of equal reactivity is valid. The mole balance equations for the chemical species present in the reaction mixture are:


where [ M] is the concentration of the monomer at time t, [P.sub.n] is the concentration of n-mer; [P] is the total concentration of species with n [greater than or equal to] 7; and k is the rate constant. The set of differential equations was solved using an integration algorithm and a conjugate search algorithm (13) to determine the species distribution. Sensitivity analysis was performed on the values of the powers of the reacting species in Eqs. 6 to 9. The specified convergence criteria were satisfied when the powers of each reacting species was 0.67. As shown in Fig. 2, good agreement between the predicted and the experimental values was obtained. The scheme thus provides the means of monitoring the changes that occur during the network formation process prior to gelation.

The SEC chromatograms for the sol portions of BADCy cured to 56, 61, 67, and 76% conversions are shown in Fig. 3. At 56% conversion, a wide range of oligomeric species is obtained. After gelation, the peak intensities of the high molecular weight species decrease as the insoluble network begins to form as shown by the chromatogram at 61 % conversion. This is because interactions between the high molecular weight species become dominant in the vicinity of the gel point due to the higher number of reactive units attached to the chains; consequently, the high molecular weight species are attached to the growing network. At 76% conversion, the high molecular weight species are absent in the sol as they have become part of the insoluble network structure. The soluble portion of the sample thus becomes richer in the low molecular weight fractions. This behavior is consistent with the observation made by Flory that at conversions greater than the gel point, the molecular species of the sol undergo retroversion over the course followed up to the gel point (14,15).

To further analyze the network formation process, the sol and the gel portions of the reacting system were separated and analyzed at various stages in the curing proess. The glass transition temperatures for the sol, the network, and the sample as a function of sample conversion are shown in Fig. 4. At conversions less than 60%, the [T.sub.g's] of the sol and the sample are the same, since no insoluble material exists in the sample, and increase with conversion. After gelation, the [T.sub.g] of the sample and the network increase while the sol [T.sub.g] remains relatively constant with conversion, as the large soluble growing chains in the sol have become part of the network. After 80% conversion, the network [T.sub.g] coincides with the sample [T.sub.g] and both increase rapidly with conversion as the network "tightens."

Cure Path Independence

The effect of cure conditions on the network structure was studied from SEC experiments on BADCy samples cured at 195[degrees]C to a conversion of 51.2% and at 235[degrees]C to a conversion of 50.5%. As shown in Fig. 5, the molecular species distributions for the two samples are the same, indicating that the system is cure-path independent.


The network formation process of cyanate ester resin systems was examined using SEC and DSC experiments. In the vicinity of the gel point, interactions between the high molecular weight species become dominant and lead to the formation of an insoluble network. After gelation, the species undergo retroversion over the course followed up to gelation. A scheme based on kinetic relationships was used to predict the molecular species distribution in the pregel region.


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[4.] A. O. Owusu, G. C. Martin, and J. T. Gotro, SPE ANTEC Tech. Papers, 37, 727 (1991).

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[6.] A. 0. Owusu, G. C. Martin, and J. T. Gotro, Polym Eng. Sci., 32, 535 (1992).

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[12.] A. O. Owusu and G. C. Martin, in preparation.

[13.] L. T. Biegler and J. E. Cuthrell, Comp. Chem Engr., 9, 257 (1985).

[14.] P. J. Flory, J. Amer. Chem Soc., 63, 3091 (1941).

[15.] D. S. Argyropoules, R M. Berry and H. I. Bolker, Macro-molecules. 20, 357 (1987).
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Author:Owusu, A. Osei; Martin, G.C.
Publication:Polymer Engineering and Science
Date:Aug 1, 1995
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