Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 569-571
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GENERAL ARTICLES
Cure Studies of Styrene-Terminated Polysulfone Oligomers Dana Garcia The BFGoodrch Company, R&D Center, Brecksville, Ohio 44 14 1
Oligomers end-capped by groups capable of thermally induced polymerization have found wide applications in advanced structural composites and adhesives. The styrene-terminated polysulfone reactive oligomers described in this paper represent an example of such thermosetting systems. The advantages of these compounds over high molecular weight polysulfone resin lie in their easier processing and improved solvent resistance. This paper presents our initial results regarding the cross-linking kinetics and properties of these materials.
Introduction In recent years (Chemorheology of Thermosetting Polymers, 1983; Eddy et al., 1980; Hergenrother, 1980) the curing mechanism and kinetics of a number of oligomers end-capped by groups capable of thermally induced polymerization have been investigated as a means to achieve polymers with improved high-temperature stability and solvent resistance. A class of compounds falling in this category is acetylene-terminated sulfone monomers and oligomers (Chemorheology of Thermosetting Polymers, 1983; Eddy et al., 1980; Hergenrother, 1980; Lee, 1984; Lery et al., 1983; h u n g , 1982; Pickard et al., 1979). The advantages of these compounds as compared to the high molecular weight, linear polysulfone resin lie in their easier processing (Hergenrother, 1980) and improved solvent resistance (Hergenrother, 1982). Additionally, such compounds can be used as plasticizers (Auman, 1984) for high molecular weight polysulfone. Nevertheless, a number of problems still appear to plague these systems. Network structures depend on cure pathways (Lee, 1984; Lery et al., 1983), which result in varying ultimate mechanical properties (Lee, 1984) as well as high temperature (Pickard et al., 1979),and long cure cycles (Lery et al., 1983) are necessary to achieve complete exhaustion of all reactive groups. To circumvent some of the problems mentioned above, a new class of reactive sulfone oligomers was recently synthesized for the first time by Percec and co-workers (Auman, 1984,Percec and Auman, 1984a,b). They contain styrene chain ends rather than acetylenic termination. These thermally reactive oligomers cross-link significantly faster at lower temperatures and result in better defined network structures. This paper presents our initial results regarding the cross-linking kinetics and properties of this new class of oligomers as a function of starting molecular weight and degree of cure. Experimental Section 1. Oligomer Preparation and Characterization. Styrene-terminated polysulfone oligomers were prepared according to the procedure described by Percec and coworkers (Auman, 1984;Percec and Auman, 1984a,b) as well 0196-4321/86/1225-0569$01.50/0
as a closely related alternative developed by Ramp (1984). Following the synthesis, the isolated oligomer samples were purified by dissolution in chloroform and precipitation in lightly acidified methanol. The purified oligomer was air-dried, followed by drying at temperatures below onset of polymerization in a vacuum oven with nitrogen bleed for 48-60 h. The structure of the prepared oligomer (Figure 1)was checked by 200-MHz ‘HNMR, and the molecular weight was determined by GPC. Only those samples that exhibited complete styrene chain end substitution were employed in subsequent testing. 2. Differential Scanning Calorimetry (DSC). DSC experiments were run on a Perkin-Elmer DSC-2 calibrated against indium at a number of heating rates. For each sample the initial glass transition temperature ( T f i )the , exothermic heat of reaction, and the final glass transition temperature (TBf)of the partially cross-linked polymer were measured. To obtain the glass transition temperature of the fully cured polymer (T@),oligomer samples were cured at a temperature above Tgobut below the depolymerization temperature for periods up to 16 h. In order to obtain the kinetics of curing, an isothermal reconstruction technique was employed, because “classical” isothermal runs were unsuccessful at yielding meaningful data as a result of the low exothermicity of the cure reaction. In such a procedure the sample is brought to the desired temperature, held there for a predetermined amount of time, quenched back to room temperature, and rescanned. The heat evolved during isothermal cure (AHH,,)is then the difference between the heat evolved during a normal dynamic scan (AHdp) and the heat evolved on rescan (AHres)after the preset cure time. To obtain the cured fraction ( x ) one has to know the heat evolved for 100% cure (AHo). In many cases investigators (Eddy et al., 1980; Pickard et al., 1979) have assumed that AHdyn= AHo. This, of course, is incorrect because complete cure is not achieved during a normal dynamic scan. Evidence for this phenomenon is exemplified by the glass transition temperature following such a dynamic cure cycle, which is lower than the final glass transition temperature (190 OC) of a fully cured material (see Figure 2). The absence of subsequent exotherms on rescanning is simply an indication of the significant degree of cure already 0 1986 American Chemical Society
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Figure 1. Molecular formula of the styrene-terminated polysulfone reactive oligomer.
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Figure 3. Variation of the glass transition temperature of partially cured oligomer samples vs. cure temperature (curing time 16 h for all samples).
Curing Exolherm
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Figure 2. DSC scans for a styrene-terminated polysulfone reactive oligomer. Heating rate 40 "C/mi?. (All figurgs refer to a styreneterminated reactive oligomer of M,, = 3200, M , = 4580, n = 6.2.)
achieved during the initial scan and the low exothermicity of the reaction. This problem is even more prevalent in the case of acetylene-terminated oligomers which exhibit substantially slower cross-linking. In our case we have determined AHo by correlating the heat of reaction (AHh) with the cured fractions obtained independently from molecular weight measurements and lH NMR experiments. 3. Cure Rheology. Isothermal cure rheology was obtained on a Rheometrics RMS7700 mechanical spectrometer in a parallel plate configuration at a number of temperatures. 4. Dynamic Mechanical Measurements. Dynamic mechanical measurements were run on a Plastech torsion pendulum. Appropriate samples of cured polysulfone oligomer were cooled slowly to -125 "C, and measurements were recorded during warmup. The temperature equilibration time was at least 10 min.
Results and Discussion Typical DSC results for the curing reaction of styreneterminated polysulfone oligomers are shown in Figure 2. The initial and final (fully cured) glass transition temperatures as well as the start of the cross-linking reaction are a function of the initial molecular weight of the oligomer: the lower the molecular weight, the lower the initial glass transition temperature and the start of the crosslinking reaction, and the higher the fully cross-linked glass transition temperature, in agreement with the initial results of Percec and co-wokers (Auman, 1984; Percec et al., 1984a,b). For molecular weight varying between 1600 and 5000 the inital Tgivaries between 61 and 126 O C , and the final Tgo,of the fully cross-linked polymer, varies between 200 and 180 "C as a result of decreasing cross-linked density. The heat evolved for complete cure is only dependent on the number of available double bonds per unit mass. When performing the kinetic analysis of the cure reactions, one generally has to be concerned with two competing effects. The fiist is premature quenching as a result of vitrification and depolymerization if the cure temperature is too high. It is not uncommon to be faced with a situation in which full cure is never achieved. This phenomenon is illustrated in Figure 3. The figure shows the attained glass transition temperature as a function of the cure temperature. As expected in the low-temperature region, we observe the manifestation of the self-quenching
(an)
Figure 4. First-order kinetic plot for the curing of styrene-terminated reactive oligomers ( x = cured fraction) at 180 "C. IE6
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Figure 5. Dynamic viscosity (poise) vs. time for styrene-terminated reactive oligomer at 180 O C . (The solid line is the fit of the data to eq 1.)
process. A small window (200-220"C)exists to obtain fuUy cured materials followed by depolymerization, which of course decreases the final Tgvalue. Thus the analysis that will be subsequently presented incorporates data in the time and temperature regime well above vitrification and below depolymerization. Figure 4 shows typical data fits to a first-order kinetic scheme that affords the best fit to the experimental data. The rate constants obtained from the slope of these lines at a number of temperatures yield the activation energy (Eact)and the A factor for the curing reaction of the oligomer. The activation energy obtained was 22.41 f 0.50 kcal/mol and is independent of the initial molecular weight of the oligomer. The A factors, on the other hand, increase slightly with increasing molecular weight as one would expect. An alternative approach to the kinetic analysis is to employ isothermal dynamic viscosity measurements. In such an analysis the dynamic viscosity can be successfully fit to a relaxation function (Chemorheology of Thermosetting Polymers, 1983) of the type shown in eq 1 where
d t ) = qo + (7- - v0)U - exp[-(t/~)@Il (1) v(t), vo, and vlmare the dynamic viscosity at time t, at t = 0, and at the plateau, T is the relaxation time, and @ is the width of the relaxation spectrum. The disadvantage of such an analysis is in the convolution of the pure curing kinetics with the flow characteristics of the curing system, all incorporated in the T value. An example of such an analysis is shown in Figure 5. Its advantage lies in obtaining a viscosity/time profile which becomes important for any applications involving processing.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 571
our spectrum (Figure 7) and may be due to the high 22 7 1 cross-linked density present in the cured polysulfone oli-
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Figure 6. Glass transition temperature vs. percent cross-linkingfor styrene-terminated polysulfone oligomer. (The solid line is an equation of the form Tg= TBoA / [ B C(1 - r2];where TBois the glass transition temperature of the fully cured oligomer, x is the cured fraction, and A, B, and C, are nonadjustable constants.)
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Figure 7. Dynamic mechanical results for cross-linked styreneterminated polysulfone reactive oligomer. (G'and G" are in units of Pa.)
Once the system has attained a certain degree of cure, its properties can be evaluated from its glass transition temperature and its mechanical response. Employing the theoretical equation of Kaelble and Cirlin (1971), we can model the glass transition temperature as a function of percent cure as shown in Figure 6. The equation does not contain any adjustable parameters, and ita excellent fit to the experin ental data indicates a linear polymerization path for our system. The dynamic mechanical data (Figure 7) of the cured polysulfone sample exhibits in addition to the glass transition another transition, which maximizes at approximately -85 "C. This transition has been attributed (Sasuga et al., 1984) to a relaxation involving the phenylsulfonyl group. Another subambient relaxation peak has been observed around -20 "C in the linear polymer analogue and has been attributed14 (Sasuga et al., 1984) to the relaxation of the Bisphenol A units. Such a transition is absent in
gomer. Additionally, a small transition appears at 0 "C, which so far remains unidentified. The styrene-terminated polysulfone oligomers do not exhibit network structures dependent on the cure pathway employed as has been suggested for the acetyleneterminated sulfone (Lee, 1984; Lery et al. (1983). The consequence of different network structures is reflected in cure-dependent mechanical properties (Lee, 1984) for the latter oligomers, while our system would not exhibit this drawback. Furthermore, the cure of styrene-terminated polysulfone oligomers does not result in a structure still containii maturation in the chain, which can reduce the thermooxidative stability, and does not require long post-cure times at elevated temperature as needed for the acetylene-terminated sulfones (Leung, 1982). While substantial work still remains to be done before a complete evaluation of the applicability of these styrene-terminated reactive polysulfone oligomers is achieved, our preliminary results indicate that these oligomers exhibit attractive properties for applications in the area of advanced structural composites.
Acknowledgment I would like to thank Drs. L. Traynor and R. Ramp (BFGoodrich Co.) for providing the samples of styreneterminated polysulfone reactive oligomers. The technical assistance of W. Johnson is also gratefully acknowledged. Registry No. (bisphenol A potassium salt) (4,4'-dichlorodiphenylsulfone)(copolymer)(SRU), 25135-51-7.
Literature Cited Auman, 8. C., M.S. Thesis, Case Western Reserve University, Cleveland, OH, 1984. Chemorheology of Thermosetting Polymers; May, C. A., Ed.; ACS Symposium Serles 227; American Chemical Society: Washlngton, DC. 1983. Eddy, S.; Lycarelll, M.; Helminiak, T; Picklesimer, L. Org Coat. Plast. Chem. 1980, 42, 502. Hergenrother, P. M. J . Mecromol. Sci., Rev. Macromol. Chem. 1980, C19,
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Hergenrother, P. M. J . Polym. Sci., Po/ym. Phys. Ed. 1982, 20. 3131. Kaelble, D. H.; Cirlln, E. H. J . Polym. Scl., Part C , 1971, 35, 79. Lee, C. Y.C. Polym. Mater. Scl. Eng. 1984, 51, 406. Lery, R. L.; Liud, A. C.; Saudreczki, T. C. Net/. SAMPE Tech. Conf. 1983, 15, 27. Leung, C. Rockwell Science Center, Thousand Oaks, CA, Air Force Contract F33615-604-5142, Monthly Report, 1982. Percec, V.; Auman, 8. C. Polym. Prepr. (Am. Chem. SOC.,Dlv. Polym. Chem.) 1984, 25(1), 122. Percec, V.; Auman, B. C. Makroml. Chem. 1984, 185, 1867. Pickard, J. M.; Jones, E. 0.;GoMfarb, I. J. Polym. h e r . (Am. Chem. Soc., Div. Polym. Chem.) 1979, 20(2), 370. Ramp, F. The BFGoodrlch Co., private communication, 1984. Sasuga, T.; Hayakawa, N.; Yoshida, K. J . Polym. Scl., Polym. Phys. E d . 1984, 22, 529.
Received for review March 20, 1986 Accepted July 21, 1986
