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. 25, 603-609 Ind. Eng. Chem. Prod. Res. D ~ v1980, Hamilton, E. J.; Korcek, S.; Mahoney, L. R.; Zinbo, M. Int. J . Chem. Kinet. 1980, 12, 577. Jensen, R. K.; Korcek, S.; Mahoney. L. R.; Zinbo, M. J . Am. Chem. SOC. 1979, 101, 7574. Jensen, R. C.; Korcek, S.; Mahoney, L. R.; Zinbo, M. J . Am. Chem. Sac. 4981, 103, 1742. Klaus, E. E.; Ugwuzor, D. 1.; Naidu, S. K.; Duda, J. L. Proceedlngs of the JSLE Internatlonal Tribologv Conference; Tokyo, Japan, 1985; p 859. Lahijani, J.; Lockwood, F. E.; Klaus. E. E. ASLE Trans. 1982, 2 5 , 25. Lockwood, F. E.; Klaus, E. E. A S E Trans. 1981, 2 4 , 276. Naidu, S. K. M.S. Thesis, The Pennsylvania State University, University Park, PA, 1981. N a b , S.K. Ph.D. Thesis, The Pennsylvania State Unhrersity, University Park, PA, 1985.
Naidu, S. K.; Klaus, E. E.; Duda, J. L. Ind. Eng. Chem. Prod. Res. D e v . 1984, 2 3 , 613. Reich, L.; Stivala, S. S. AutoxMstion of Hydrocarbons and Po&o/eflns; Marcel Dekker: New York, 1969. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properfies of Gases and Liquids, 3rd ed.;McGraw-Hili: New York, 1977. Ugwuzor, D. I . M.S. Thesis, The Pennsylvania State University, University Park, PA, 1982. Zuidema, H. H. The Performance of Lubricating Oils; Reinhold: New York, 1952.
Received for reuiew December 2 , 1985 Accepted April 8, 1986
Synthesis of Styrene/Maleimide Copolymers and Physical Properties Thereof Eugene R. Moore' and Dale M. Plckelman Styrene MOMing Polymers, Dow Chemical U.S.A., Midland, Mlchlgan 48667
A series of carefully prepared copolymers of styrene and maleic anhydride @/MA) have been reacted with ammonia and converted to their respective styrene/maieimide (S/MI) derivatives. This paper discusses the preparation of these copolymers and presents extensive data on T , heat distortion temperatures, thermal stability, and melt viscosity. Some physical property data are also presenled. Some new S/MA data concerning solubility parameter, density, coefficient of expansion, refractive index, and physical properHes are also included and used for comparison.
Introduction Polystyrene is well-known for its usefulness as a moldable and extrudable thermoplastic. Often applications for a clear thermoplastic, however, require higher heat resistance than that provided by polystyrene. Copolymers of styrene and maleic anhydride are known to have in-. creases in heat resistance proportional to the MA content for example, was found to increase (Moore, 1986). The Tg, linearly at 2.7 O F per added percent MA. Copolymers with styrene and maleimide have since been found to be even more heat resistant. While the copolymers for this study could have been produced from styrene and maleimide monomers, they were instead produced by first forming parent S/MA copolymers, then reacting the solid copolymer with gas-phase NH3, followed by heating under vacuum to complete conversion to the imide, as shown in Figure 1. This route has some potential advantage over production using the monomer formed from reaction of MA and NH,, since NH, can also react across the double bond of MA and thus produce a mixture of maleimide monomer and byproduds. This reaction mixture would likely require purification before polymerization. An additional problem develops with MI because, once purified, the MI monomer must be handled with care to prevent homopolymerization. MA monomer cannot homopolymerize and thus does not offer this problem. This is fortunate, since the melting point of maleic anhydride is 52 "C (Weast and Selby, 1966))well above the acceptable storage temperature for most monomers capable of freeradical polymerization. It has been very convenient to handle MA in the liquid state when polymerization is done on a larger scale. Maleimide monomer has a melting point of 93 "C (Weast and Selby, 1966), which may force it to be handled only in solution. The availability of a series 0196-4321l86l1225-0603$01.50l0
of well-characterized S/MA copolymers further influenced our decision to proceed with this study by reacting those available copolymers with NH3. The reaction of solid S/MA copolymers with ammonia has in the past been used to modify the nature of foamed S/MA (Moore and Nakamura, 1970). In this case the NH, was allowed to diffuse into the cooled foam.
Experimental Section Copolymer Production. The base S/MA copolymers (Moore, 1986) were produced by free-radical polymerization in a well-mixed reactor with methyl ethyl ketone (MEK) as a solvent and with small amounts of either azobis(isobutyronitri1e) or benzoyl peroxide usually added as a free-radical initiator. A few copolymers were made by thermal initiation alone. Polymerization temperatures ranged from 76 to 172 "C. Higher molecular weights and higher MA contents both tended to require the lower temperatures. Devolatilization was carried out continuously to give strands of molten polymer, which were then cooled and chopped into granules. Three series of S/MA copolymers were produced, having 4-,8-, and 12-CPsolution viscosity (10% in MEK at 25 "C). Each viscosity series contained samples with aim compositions of 0,5,18, 25,33, and 48% MA. The granules were ground to about 60 mesh prior to reaction with gas-phase NH,. Conversion to Imide. About 200 lb of each copolymer (except the 0%) was converted to the maleimide form by reaction with gaseous NH3 at elevated temperature. Reaction was carried out in a large pressure-vacuum oven that held about 50 lb of the polymer in shallow trays with heated shelves where it could be heated while it was alternately pressurized with ammonia and then evacuated to remove the water of reaction. The reaction was generally carried out over a period of 2 or 3 days while the 0 1986 American Chemical Soclety
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Ind. Eng. Chern. Prod. Res. Dev., Vol. 25, No. 4, 1986
ANHYDRIDE
HALF AMIDE
IMIDE
Figure 1. Reaction of styrene/maleic anhydride copolymers with NH3 to produce styrene/maleimide copolymers.
oven jacket and shelves were heated with low-pressure steam. Temperatures were maintained just below the polymer Tg,with the maximum temperatures in the 150-160 "C range. After an initial evacuation, ammonia pressure (about 4.5 psig) would be maintained on the oven. As the reaction proceeded, water was produced (see Figure 1). Often, during the initial ammoniation cycle, water would be observed condensing on the oven window. After 12-24 h of exposure to ammonia, a vacuum was drawn on the oven for a period of about 2 h, and then the oven was recharged with ammonia gas. After each vacuum cycle a sample was withdrawn, and conversion to imide was estimated from neutron activation analysis for nitrogen. Each sample was subjected to a total of three ammonia pressurization cycles. The final cycle was followed by 72 h under vacuum with temperature maintained above 100 "C. A crude material balance was made on several batches, showing that about 25% excess ammonia was used. About 75% of the expected water of reaction was able to be recovered. Partially converted samples were often found to have significantly higher solution viscosities (10% MEK at 25 "C) than either the starting material or the imide product. The highest viscosity measured was on the 8-cP 18%MA copolymer (47 CPafter the first cycle, eventually decreasing to 8.4 CPfor the S/MI product). This temporary higher 10% solution viscosity may be due to intermolar polar association with the half-amide intermediate groups (see Figure 1). It could also be due to intermolecular imide formation. In this case, the half-amide structure could react with another anhydride group, forming an intermolecular link that would greatly increase the apparent molecular weight (or solution viscosity in this case). In either case, further reaction under reduced pressure removes these effects to the point of minimal influence on dilute solution viscosity. The heat distortion on these intermediate samples consistently showed a dramatic increase (over the anhydride form) after the first cycle and then very little additional change. Apparently the conversion was high enough, after one cycle, to have (within experimental error) the maximum effect. It also appears that the presence of some excess ammonia (perhaps as the ammonia salt) or of partially converted half-amide has little effect on the heat distortion. Molecular Weight. Molecular weight was determined in tetrahydrofuran (THF) by using gel permeation chromatography (GPC) for weight- and number-average molecular weights. A separate determination of number-average molecular weight was carried out by using high-speed
solution osmometry (HSO), also in THF. Solution viscosities were measured at 25 OC with 10 wt % solids in MEK. Results are reported in centipoise (cP). High-Shear Melt Viscosity. Measurements were made on a capillary rheometer, which has been described elsewhere (Karam et al., 1955). The shear stress of lo7 dyn/cm2was selected because it was believed to approximate the shear in modern high-speed injection-molding equipment. Measurements were taken at three temperatures where possible. Thermogravimetric Analysis. Measurements were carried out in air at a heating rate of 10 "C/min. The weight of a 15-17-mg powdered sample was continuously recorded. Heat Distortion and Glass Transition. Measurements were carried out by using standard ASTM techniques. ASTM D1525-65 at 264 psi was used for Vicat, and ASTM D648 at 264 psi was used for deflection temperature under load (DTUL). Samples for DTUL were annealed for 2 h at a temperature 25 O C below the Vicat softening point. Glass transition temperature (T,)was determined by using differential thermal analysis (DTA). Powdered samples were heated in air at 25 OC/min. Expansion. The linear coefficient of thermal expansion was determined by using ASTM D696. Conversion. The completeness of conversion to the MI was calculated from nitrogen analysis. While initially both infrared and neutron activation analyses were used, it was noted that neutron activation analysis was more reliable and faster. Consequently, neutron activation was used for all results reported here. It was assumed that all nitrogen remaining was in the imide form. This assumption was supported by the fact that nitrogen levels along with both solution viscosity and heat distortion temperatures (HDT) did not change even with further prolonged heating. Density. The measurement of density was carried out by the standard water displacement technique and is given in grams per cubic centimeter. Refractive Index. These measurements were carried out by using a standard temperature-controlled refractometer. Solubility Parameter. Solubility parameter values reported here were determined by using a cloud-point titration technique at room temperature. Samples were first dissolved in MEK. Two turbidity titrations were then carried out using methanol to titrate to the high-solubility parameter cloud point and n-pentane for the low cloud point. The solubility parameter of the mixed solvents was calculated at both cloud points. The average of these two was then taken as the solubility parameter of the polymer. Similar results were obtained when methanol and n-heptane were used as the nonsolvent pair. Before the cloud-point technique was selected, the more traditional intrinsic viscosity technique was explored. Intrinsic viscosity of a styrene/ 15.2% maleic anhydride copolymer (3.74 cP) was measured in a series of solvents: solvent chlorobenzene 1,1,2-trichloroethane 1,1,2,2-tetrachloroethane ethylene dichloride o-dichlorobenzene nitropropane
solubility parameter
intrinsic viscosity, dL/g
9.50 9.60 9.70 9.80 10.0 10.7
0.580 0.668 0.743 0.633 0.550 0.468
A maximum in intrinsic viscosity indicates the closest match in solubility parameter between the solvent and the polymer, where the maximum extension of the polymer chain in solution occurs. With this copolymer, the maximum occurs very close to 9.70, which agrees very well with
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 605 looooo
Table I. Conversion of P a r e n t Maleic Anhydride Copolymer t o Maleimide" conversion, % 8 CP 12 CP init 4 CP % MA init visc init visc init visc 0 5 91.3 >99 94.7 18 98.4 92.7 92.9 25 98.4 95.5 99.2 33 97.5 94.9 97.2 48 >99
" Weight percent
r
SOLN VlSC = 4CPS
VlSCOSlTY IN PplSE
1000
V 0% MI X
5% MI
based on neutron activation analysis of nitro-
gen.
r
I
100
520°F
SOLUBILITY
I
I
I
480°F
440'F
400°F
1ITEMP ( " R )
//
Figure 3. High-shear melt viscosity (at lo7 dyn/cm2) of low molecular weight (4 cP) styrene/maleimide copolymers as a function of temperature and maleimide content.
STYRENE/MALEIMIDE
10.00
100%/70%~30%,
lOWW VIS
