Chapter 9
Telechelic Aryl Cyanate Ester Oligosiloxanes: Impact Modifiers for Cyanate Ester Resins 1
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Steven K . Pollack and Zhidong Fu 1
Department o f Chemistry and the Polymer Science and Engineering Program, Howard University, Washington, D C 20059 Colgate-Palmolive, 909 River Road, P.O. Box 1343, Piscataway, N J 08855
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W e describe the synthesis o f monomers and characterization o f polymers and blends derived from telechelic siloxanes terminated with dimethyl(4-cyanatophenyl)silyl units. These are synthesized by base -catalyzed equilibration o f THP-protected 1,1,4,4-tetramethyl-1,4 -bis(4-hydroxyphenyl)disiloxane w i t h octamethylcyclotretrasiloxane ( D ) and octaphenylcyclotetrasiloxane (D ") followed by deprotection and reaction with cyanogen bromide. Homopolymerization o f the telechelic siloxane cyanate esters produce cross-linked materials whose glass transition temperatures range from 277°C(disiloxane) to -120°C(dimethyl-10-mer). Blending with commercial novolac based -cyanate ester resins (Primaset PT-30) produces a single phase material for the disiloxane and two-phased materials for the higher oligomers. The completely dispersed rubber phase has an average domain size o f 20 microns. 4
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Cyanate ester resins (CERs) have found extensive use i n applications where lowdielectric, high thermal stability materials are needed . Polymerization occurs by the cyclotrimerization of aryl cyanate ester groups 1
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© 2000 American Chemical Society
Clarson et al.; Silicones and Silicone-Modified Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2000.
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These applications include printed wire circuit boards ( P C W B s ) , and radomes. Recently, these resins have been proposed as materials for use as the matrix i n structural composites used in civilian air transport. In this application, the organic materials must exhibit low flammability as well as having the appropriate strength and toughness for this type of application. C E R s , by themselves possess appropriate flammability properties, but are too brittle for use alone as matrix materials for structure composites. However, their toughness can be greatly enhanced by the introduction of modifiers such as thermoplastics or elastomers. Siloxane based elastomers provide both the requisite improvement i n toughness as well as having excellent flame retardency. T o that end, there have been efforts to develop siloxane based impact modifiers for C E R s . Previous workers have described approaches utilizing either amine or alcohol terminated siloxanes as impact modifiers . There is also a patent issued for the development o f allyl(2-phenylcyanate) terminated siloxanes . In the former case, there is concern that the cross-linking reaction may introduce polar functionality into these materials causing excessive moisture absorbance. In the latter materials, the added hydrocarbon component of the allyl group is a potential problem in terms of combustibility of the siloxane. W e have focused our attention on developing a family of telechelic siloxanes terminated with p-cyanatophenyl groups. 2
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Experimental NMR The nuclear magnetic resonance spectra were obtained using a G E N M R Q E - 3 0 0 model spectrometer with chloroform-d (CDC1 ) as solvent and T M S or CDCI3 was used as internal reference. *H N M R operated at 300.6 M H z and C N M R operated at 75.2 M H z . IR The infrared spectra were recorded on Perkin-Elmer 1600 series F T - I R spectrophotometer. L i q u i d samples were placed on N a C l plates. Solid samples were ground and mixed with K B r power. Rubbery samples were ground i n liquid nitrogen and mixed with K B r powder. The mixtures were then pressed i n a hand K B r press to give a transparent thin film. S E M Scanning electron micrographs (SEMs) were obtained using an R . J . L e e Inc. Personnel S E M . Micrographs were obtained from uncoated samples using the low pressure mode o f operation. Elemental composition of the various phases i n the materials was analyzed using energy dispersive X - r a y analysis ( E D X A ) . Images were obtained utilizing a backscatter detector thus contrast is due primarily to difference in atomic number. Thermal Analysis Differential scanning calorimetry was performed using a Perkin-Elmer Pyrus 1 D S C . Scan rates were 20 °C/min and carried out under a helium atmosphere. Thermogravimetric analysis was carried out using a PerkinElmer T G A - 7 interfaced to a Pyrus Data Station. Analyses were run at 20 °C/min scan rates of 20 cc/min flow of either nitrogen or air (as indicated). Synthesis of b i s [ l , 3 - p - t e t r a h y d r o p y r a n y l o x y p h e n y l ] - l , 1,3,3tetramethyldi-siloxane (5): S o l i d tetramethylammonium hydroxide (0.30 g) was added to 34.0 g of 4 (synthesized v i a a modification of the method described by M c G r a t h and co-workers ). The mixture was placed in high vacuum (0.2 mmHg) while stirring for three hours. When the reaction was completed, the 3500-3000 c m broad peak disappeared. The resulting product is a white solid (29.9 g, 88 % yield) . IR (neat, c m ) : 2948 (strong, C H ) , 2875 (strong, C H ) , 1594 (strong, A r ) , 1564, 1500 (strong, S i A r ) , 1467, 1454, 1441, 1388, 1356, 1239 (strong, S i M e ) , 1202, 1180, 1117 (strong SiO), 1038 (broad, Si-O-Si), 966, 921, 872, 840, 791, 704, 642, 565 (SiO) cm" U N M R (CDCI3): 7.44 (d,4H), 7.03 (d, 4H), 5.44 (t, 2H), 3.6-3.9 (m, 4 H ) , 1.5-2.1 (m, 12H), 0.29 (s, 11H). C N M R (CDC1 : 158.0 (Ar), 134.4 ( A r ) , 132.0 ( A r ) , 115.7 (Ar), 96.9 (THP), 62.0 (THP), 30.3 (THP), 25.2 (THP), 18.7 (THP), 0.9 (SiMe). Synthesis of
