High Strength Silicone-Urethane Copolymers: Synthesis and Properties

Chapter 26

High Strength Silicone-Urethane Copolymers: Synthesis and Properties

Emel Yılgör, Ayşen Tulpar 1, Şebnem Kara1, and Iskender Y ı lgör

Koç University, Department of Chemistry, İstinye 80860, İstanbul, Turkey

High molecular weight silicone-urethane segmented copolymers were synthesized by the reaction of 4,4'-isocyanatocyclohexylmethane (HMDI), α,ω-hydroxyhexyi terminated polydimethylsiloxane oligomers (TPDMS) and 1,4-butanediol (BD). Reactions were conducted in two steps in tetrahydrofuran/dimethylformainide solvent mixture under the catalytic action of dibutyltindilaurate. PDMS oligomers with molecular weights of 900 and 2300 g/mole were used during the synthesis. Hard segment contents of the copolymers were varied between 23 and 50% by weight. Thermal (DSC) and thermomechanical (DMTA) characterization of the copolymers showed the formation of two-phase morphologies. Mechanical strengths of these silicone-urethanes were superior to those of conventional polyether based polyurethanes with similar structures and hard/soft segment compositions.

Block copolymers obtained by the combination of polydimethylsiloxane (PDMS) soft segments and organic hard segments display a unique combination of properties (1). Overall performance of these types of silicone (or siloxane) containing copolymers depends on the type and nature of the organic (hard) segments and the amount and average molecular weight of the PDMS soft segments in the system. As expected, elastomeric properties of these copolymers are determined by the PDMS in the system. On the other hand their mechanical strengths are generally dictated by the type and the nature of the hard segments. Unique properties provided to these copolymers by the presence of PDMS include extremely low glass transition temperatures (-120°C), very low surface energies, improved thermal and oxidative stability, high gas permeabilities, low water absorption, physiological inertness and blood and tissue compatibility (1,2). As a result of these properties, PDMS containing copolymers find applications as specialty elastomers, biomaterials and high performance coatings.

1Current address: Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, V A 24061.

© 2000 American Chemical Society 395

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In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Preparation and properties of a wide range of well-defined PDMS containing multiphase copolymers have been reported in the literature (1,3,4). Important organic blocks combined with PDMS in the preparation of these block copolymers include polycarbonate, polysulfone, polyurea, polyamide, polyimide, polycaprolactone, poly(methylmethacrylate) and polystyrene (1,3*4). Most of these systems display two phase morphologies due to substantial differences between the solubility parameters of very nonpolar PDMS and moderately to highly polar organic hard segments.

Preparation and characterization of conventional polyether or polyester based polyurethane elastomers with high molecular weights and good mechanical performance have been widely investigated and reported (5,6). Interestingly, there are only a limited number of reports available on the synthesis and characterization of homologous polyurethane systems containing PDMS as the soft segment (7-9). The data published clearly shows that although the formation of copolymers with two phase morphologies (determined by thermal analysis) are observed, the mechanical properties of silicone-urethanes are inferior to those of polyether or polyester based urethane elastomers. Considering the highly flexible nature of PDMS soft segments and achievement of better phase separation in silicone-urethanes leading to stronger hydrogen bonding in the urethane hard segments, one would normally expect better elastomeric properties in silicone-urethanes. We believe poor mechanical properties of silicone-urethanes are mainly due to low molecular weights of the copolymers produced. One of the main reasons for the failure in being able to synthesizing high molecular weight silicone-urethane copolymers is related to the end-group stabilities of commercially available hydroxyalkyl terminated PDMS oligomers, especially the a,co-hydroxypropyl and a,©-hydroxybutyl terminated systems. We have recently shown that end-groups on these oligomers are not stable due to the back-biting of silicon (linked to the alkyl bridge) by the hydroxyl oxygen during the vacuum distillation of these oligomers (10). This gives rise to the loss of hydroxyalkyl end-groups and to the formation of stable 5 and 6 membered heterocyclic rings for hydroxypropyl and hydroxybutyl terminated oligomers respectively. Another important reason is the sensitivity of the reaction system to the solvent used. Initially, during the prepolymer formation, the reaction system is fairly non-polar. At this stage tetrahydrofuran (THF) with a solubility parameter of 19.4 (J/em3)1/2 or 9.48 (cal/cm3)1/2 (11) is a fairly good solvent. However, as the prepolymer is chain extended with butanediol or with other low molecular weight glycols, the polarity and the solubility parameter of the copolymer starts increasing. In order to be able to compensate for this change and increase the polymer molecular weight, polarity of the reaction solvent must also be increased gradually during the chain extension reactions. This can be achieved by the addition of dimethylformamide (DMF) with a solubility parameter of 24.8 (J/cm3) ! / 2 or 12.12 (cal/cm3)1/2 (11) as a cosolvent into the system, a good solvent for urethane groups.

Recently McGrath and coworkers reported the preparation and characterization of polyurethanes based on 4,4'-diphenyimethanediisocyanate (MDI) and mixed poly(tetramethylene oxide) (PTMO) and PDMS soft segments, where 1,4-butanediol was used as the chain extender (12). They used toluene/DMF (50/50) mixture as the reaction solvent. They reported number average molecular weights of 10,000 to 12,000 g/mole for the copolymers obtained.

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In this study siloxane-urethane copolymers were prepared by the reaction of α,ω-hydroxyhexyl terminated PDMS oligomers, 4,4'-isocyanatocyclohexylmethane and 1,4-butanediol, which was used as the chain extender. Influence of the reaction solvent, PDMS molecular weight and hard segment content on the overall molecular weight and thermal and mechanical properties of the products were investigated. Performance of PDMS based polyurethanes were compared with those of PTMO based systems, with similar structures and compositions.

E X P E R I M E N T A L

Materials

α,ω-Hydroxyhexyl terminated PDMS oligomers with number average molecular weights (Mn) of 900 and 2300 g/mole were kindly supplied by Th. Goldschmidt A G of Essen, Germany. Mn of these oligomers were determined by ^ - N M R spectroscopy by comparing the peaks of hexyl end-group protons to those of the methyl groups attached to silicon, as shown in Figure 1. PTMO oligomer with Mn of 2050 g/mole and 1,4-butanediol (BD) were products of DuPont. 4,4'-isocyanatohexylmethane (HMDI) was supplied by Bayer A G , Leverkusen, Germany. Purity of HMDI, as determined by standard dibutylamine back titration method was better than 99.5%. Reagent grade THF and DMF were purchased from Aldrieh. Dibutyltin dilaurate (DBTDL) catalyst was a product of Witco.

Polymer Syntheses

Both PDMS and PTMO based polyurethanes were prepared by following the two-step prepolymer method, which is given in the Scheme I outlined below. Reactions were carried out in 3-neck round bottom flasks equipped with an overhead stirrer, an addition funnel and a nitrogen inlet.

A typical procedure followed in the preparation of siloxane-urethane segmented copolymers is as follows: Calculated amounts of HMDI and PDMS oligomer are introduced into the reaction flask, stirred and heated up to about 60°C. This mixture is not miscible. However, when 0.5 mL of 1.0% DBTDL solution in toluene is added, the mixture turns clear in about one minute indicating a fairly fast reaction between PDMS and HMDI. There is also a dramatic increase in the reaction temperature from 60 to about 90°C, typical for very exothermic urethane formation reaction. Prepolymer formation is followed by FTIR, monitoring the disappearance of the broad hydroxyl peak centered around 3300 cm"1. Prepolymer obtained is then diluted with THF to about 50% solids and heated to reflux temperature of 64.5°C. Chain extender, BD, is dissolved in THF and added dropwise into the reaction mixture. As the system became viscous as a result of the chain extension reaction, it is diluted with THF and DMF to prevent the premature precipitation of the copolymer formed, which is indicated by the formation of a cloudy solution. Reaction was continued until the complete disappearance of sharp isocyanate peak around 2250 cm"1 in the FTIR

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C H 3 C H 3

I I

HO-(CH 2 )6-[Si-0]n-Si-(CH2 )6-OH

C H 3 C H 3

+

Excess 0=C=N-R-N=C=0

4

H O C H 3 C H 3 0 H

I II I I II I 0=C=N-R-N-C-0-(CH 2 )6-tSi-0]n-Si-{CH2)«-0-C-N-R-N=C=0

C H 3 C H 3

+

Equivalent HO-(CH2 )4 -OH

C H 3 C H 3 O H H O O H H O

I I II I I I! II I I II

-KCH 2 )6-[Si-0]n-Si-{CH2 )6-0-[C-N-R-N-C-0-(CH2 )4-0]m-C-N-R-N-C-0}x

C H 3 C H 3

• O O SCHEME I

Reaction scheme followed in the preparation of PDMS-urethane segmented copolymers

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spectrum. Polymer produced was coagulated in methanol/water (95/5) mixture, washed several times with methanol, filtered and dried to a constant weight in a vacuum oven at 50°C. Yields were determined gravimetrically.

A similar two-step procedure was used for the preparation of PTMO based polyurethanes, where the solvent used was DMF. Reactions were conducted at 80°C.

A model polyurethane based on HMDI and BD was also prepared in DMF at 80°C by reacting equimolar amounts of each reactant. DBTDL was used as catalyst during the reaction. Product obtained was coagulated in methanol/water (80/20) mixture, washed and dried to a constant weight in a vacuum oven. This sample was used to determine the thermal transitions of the hard segment in DSC.

Table I gives the compositions of silicone-urethane and polyether-urethane segmented copolymers prepared in this study.

Table I. Compositions of the Silicone-Urethane and Polyether-Urethane Reactions

Sample Soft Segment Composition of the reaction mixture (wt.%) m Mn (g/mole) Soft Segment HMDI BD PSU-l PDMS 900 77.4 22.6 -PSU-2 PDMS 900 61.1 33.6 5.3 PSU-3 PDMS 900 59.0 34.8 6.2 PSU-4 PDMS 2300 78.7 17.8 3.5 PSU-5 PDMS 2300 70.1 24.0 5.9 PSU-6 PDMS 2300 63.3 28.8 7.9 PSU-7 PDMS 2300 56.7 33.3 10.0 PEU-1 PTMO 2050 70.1 24.6 5.3 PEU-2 PTMO 2050 63.2 29.4 7.4 PEU-3 PTMO 2050 56.8 34.1 9.1

Product Characterization

FTIR spectra of starting materials, intermediates and products were recorded on a Nicolet Impact 400D spectrometer. GPC chromatograms were obtained on a Polymer Laboratories, PL 110 instrument equipped with PLgel columns of 500, 103 and 104

angstroms. THF was used as the solvent and the flow rate was 1 mL/min. Intrinsic viscosities were determined in Ubbelohde viscometers, at 25°C, in THF. Thermal analyses of the products were obtained on a Rheometrics PL-DSC Plus instrument, under nitrogen atmosphere with a heating rate of 10°C per minute. Dynamic mechanical thermal analyses (DMTA) were determined using a Polymer Laboratories D M T A Mk III system, operating in bending mode. Measurements were made simultaneously at frequencies of 1 and 100 Hz, with a heating rate of 5°C per minute. Stress-strain tests were carried out on an Instron Model 4411 Universal Tester, at room temperature, with a crosshead speed of 20 rnm/min. For each test a minimum of 5 specimens were used. Films for D M T A and stress-strain tests were prepared by compression molding at 200°C using a Carver, hydraulic press. Transmission

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micrographs of the solvent (THF) cast polymer films were obtained by using a Leica, Model D M L M , light microscope.

R E S U L T S A N D DISCUSSION

It is well documented that excellent elastomeric properties of thermoplastic, segmented or multiblock polyether or polyester based polyurethanes are a direct result of their backbone composition, the extent of phase separation between their hard and soft segments and the strength of hydrogen bonding in the hard domains (13-15). Extensive phase mixing between polyether or polyester soft segments and urethane hard segments usually lead to poorer mechanical properties, since it effectively reduces the strength of the hydrogen bonding in the system. In that respect, high molecular weight siHcone-urethane copolymers can be interesting model polymer systems which may provide valuable information towards a better understanding of the influence of phase separation and hydrogen bonding on the microphase morphology and elastomeric properties of segmented polyurethanes. Extremely nonpolar PDMS soft segments, with very low solubility parameters of 15.6 (J/cm3)1/2 or 7.6 (cal/cm3)1/2 (11) are expected to show almost complete phase separation from the urethane hard segments, which are fairly polar, as indicated by their high solubility parameters around 37.8 (J/cm3)1/2 or 18.5 (cal/cm3)172 (16). This will lead to stronger hydrogen bonding in the urethane hard segments in silicone-urethanes, when compared with polyether-urethanes since in the latter system ether oxygen in PTMO soft segments will form weak hydrogen bonding with the urethane groups in hard segments, leading to phase mixing and also effectively reducing the strength of these copolymers. In fact, the dramatic influence of extensive phase separation in silicone-urea systems on the formation of extremely strong thermoplastic elastomers has already been demonstrated (17,18).

Preparation of high molecular weight silicone-urethane copolymers were achieved by using athe conventional two-step, prepolymer technique. During prepolymer formation PDMS oligomers were reacted with excess HMDI in bulk, at 80°C under the catalytic action of DBTDL. Prepolymer mixture was diluted with THF and cooled down to THF reflux temperature of 64-65°C and held constant in this range throughout the chain extension process. For chain extension stoichiometric amount of BD was dissolved in THF (20/100, v/v, BD/THF), introduced into an addition funnel and added dropwise into the reaction mixture. As the urethane hard segment molecular weights grew, polarity of the product increased. In order to prevent premature precipitation of the product, polarity of the solvent was also increased by adding DMF into the reaction flask throughout the chain extension step. This seems to be critical in obtaining high molecular weight segmented siloxane-urethane copolymers. Table II gives various characteristics of the polymers produced. As can be seen from this Table, silicone-urethane copolymers with hard segment contents of up to 50% by weight were prepared in high yields and high molecular weights as indicated by their intrinsic viscosities and GPC chromatograms.

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Table II. Characteristics of Siloxane-Urethane and Polyether-Urethane Segmented Copolymers

Sample Soft Segment Hard Segment Yield l u T 0

No Mn(g/mole) Content fwt.%") (wt.%) ( W PSU-1 PDMS 900 22.6 92 0.90 PSU-2 PDMS 900 38.9 93 0.72 PSU-3 PDMS 900 41.0 93 0.80 PSU-4 PDMS 900 49.9 90 0.68 PSU-5 PDMS 2300 21.3 89 0.67 PSU-6 PDMS 2300 29.9 90 0.73 PSU-7 PDMS 2300 36.7 95 0.79 PSU-8 PDMS 2300 43.3 92 0.81 PEU-1 PTMO 2000 29.9 95 —

PEU-2 PTMO 2000 36.8 96 —

PEU-3 PTMO 2000 43.2 95 —

DSC thermograms of these copolymers all showed two glass transition temperatures, one at around -120°C, due to PDMS soft segments and the other around +70°C due to the urethane hard segments made of BD and HMDI. Silicone-urethane copolymers with high hard segment contents (e. g. PSU-2, PSU-3, PSU-4, PSU-7 and PSU-8) also showed a broad endotherm between 120-150°C due the melting of HMDI-BD hard segments. High temperature DSC thermograms of model polyurethane composed of HMDI-BD and PSU-4 are reproduced in Figure 2. HMDI-BD model polymer shows a Tg at 77°C and a broad melting endotherm between 150 and 170°C whereas Tg for PSU-4 is at 73°C, very close to that of the model system. As can be seen from Figure 2, there is a slight difference in the temperature range of the melting endotherm for the model hard segments and that of PSU-4, which is most probably due to the effect of hard segment molecular weight. However, these results clearly show the formation of two-phase copolymers with good phase separation.

Formation of two-phase morphologies, with good phase separation in these silicone-urethane segmented copolymers can also clearly be seen from the results of D M T A studies. Figure 3 gives the D M T A curves for PSU-4 where Tan 8 and Storage Modulus are plotted against temperature. The peaks for PDMS glass transition (-100°C), PDMS melting (-30°C) and hard segment glass transition (60°C) are all clearly visible. After about 80°C, since the sample became too soft it was not possible to observe the transition due to melting of the hard segment in D M T A , which was clearly visible in DSC. Slight shifts in the transition temperatures between DSC and D M T A are typical and are due to differences in the heating rates employed and also due to differences in the nature (static versus dynamic) of these techniques.

Investigation of the tensile behavior of silicone-urethane copolymers and their comparison with those of silicone-urea copolymers (18) or THF based polyether-urethane homologs yielded interesting and somewhat unexpected results. The data on

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^ I I I 1 1 1 1 50 100 150 200

Temperature ( ° C ) Figure 2. DSC thermograms for (a) model HMDI-BD based polyurethane and(b)PSU-4.

-140 -80 -20 40 100 Temperature ( °C)

Figure 3. D M T A curves for PSU-4 ( ) storage modulus (••••) bending tan 8.

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stress-strain tests is given in Table III. Silicone-urethanes with low hard segment contents, such as samples PSU-1 and PSU-5 with hard segment contents of 22.6 and 21.3% by weight, respectively, gave nice films when compression molded or cast from THF solutions, however, they were very weak with almost no tensile strength or mechanical integrity. This is quite different than the tensile test results obtained on silicone-urea copolymers of similar compositions, which show the formation of extremely strong elastomers with very high tensile strengths (17,18). This simple comparison between silicone-urethane and silicone-urea copolymers of similar structure and composition clearly indicate the importance of the strength of hydrogen bonding on the mechanical performance of well phase separated silicone containing thermoplastic elastomers.

Table III. Comparison of the Tensile Properties of Siloxane-Urethane and Polyether-Urethane Segmented Copolymers

Hard Segm. Ultimate Elong. Sample Soft Segment Content Modulus Strength at Break No Type Mn f e/moD (%wt.) (MPa) (MPa) £%) PSU-1 PDMS 900 22.6 — — — PSU-3 PDMS 900 41.0 6.75 3.45 400 PSU-4 PDMS 2300 21.3 — — ~ PSU-5 PDMS 2300 29.9 14.3 11.0 350 PSU-6 PDMS 2300 36.7 35.1 16.4 310 PSU-7 PDMS 2300 43.3 63.9 17.7 250 PEU-1 PTMO 2050 29.9 10.7 3.56 475 PEU-2 PTMO 2050 36.8 23.9 7.68 450 PEU-3 PTMO 2050 43.2 50.9 17.2 425

As expected, the moduli and the ultimate strengths of silicone-urethane copolymers increase with increasing hard segment content in the copolymer. On the other hand there is a small drop in the elongation at break as the amount of the hard segment is increased.

Stress-strain results given in Table III clearly show that for copolymers of similar structures and compositions, silicone-urethanes always show higher tensile moduli and tensile strength when compared with PTMO based polyether-urethanes. This is also very interesting, since at room temperature PDMS is about 145°C above its Tg (-120°C) and approximately 75°C above its melting point (-50°C) (1). On the other hand PTMO is about 90°C above its Tg (-65°C) but 25°C below its melting point (50°C) at room temperature (19). This would intuitively suggest higher tensile strengths for polyether-urethanes due to the contribution of crystalline PTMO soft segments to mechanical performance. However, the experimental results indicate just the opposite. We believe this is due to much better phase separation in silicone-urethanes, which results in more effective intermolecular hydrogen bonding between the urethane hard segments, which effectively determines the modulus and tensile strength of the material. As for the polyether-urethanes, there is usually fairly high

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ELONGATION (%) Figure 4. Comparison of the tensile behavior of polydimethylsiloxane and po ly(tetramethylene oxide) based segmented polyurethanes with similar compositions and structure. (A) PSU-8, (A) PEU-3, (o) PSU-7 and (•) PEU-2.

amount of phase mixing between urethane and polyether soft segments due to hydrogen bonding between ether linkages in polyethers and -N-H groups of methanes (6). This effectively reduces the strength of the hydrogen bonding between urethane hard segments and as a result the modulus and overall tensile strength of the polyurethane. Figure 4 gives the stress-strain curves for PSU-7, PSU-8 and PEU-2 and PEU-3 for a better comparison.

Another interesting observation was made in sample PSU-4, which is based on PDMS-900 and has a urethane hard segment content of 49.9% by weight. When THF cast films of PSU-4 were stretched slightly they showed strain induced crystallization, indicated by the formation of regular patterns in the films, which were crystal clear and transparent originally. Figure 5 gives the micrograph of PSU-4 obtained under a light microscope operating in the transmission mode. Figure 5 clearly shows the formation of a very well organized lamellar structure with dimensions of each layer being around 2.5 um.

CONCLUSIONS

High molecular weight silicone-urethane copolymers with urethane hard segment contents of up to 50% by weight were synthesized in high yields. Thermal, thermomechanical and microscopic characterization of these copolymers clearly indicated the formation of two-phase systems with very good phase separation.

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Fig

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Tensile strengths of silicone-urethanes were higher than those of PTMO based polyether-urethanes of similar structure and compositions. We believe, this is due to better phase separation in silicone-urethanes leading to more efficient and stronger hydrogen bonding between urethane hard segments, the main force, which provides the excellent elastomeric properties to polyurethane elastomers.

R E F E R E N C E S

1. Yilgor, I.; McGrath, J. E. Adv. Polym. Sci. 1988, 86, 1-87. 2. Noll, W. Chemistry and Technology of Silicones, Academic Press: New York,

N Y , 1968. 3. Noshay, A.; McGrath, J. E . Block Copolymers: Overview and Critical Survey,

Academic Press: New York, NY, 1978. 4. Burger, C.; Kreuzer, F. -H. , in, Silicone in Polymer Synthesis, Ed., Kricheldorf, H .

R., Springer-Verlag: Berlin, 1996, Chapter 3, pp 113-222. 5. Woods, G.,The ICI Polyurethanes Book, John Wiley: New York, 1990. 6. Lelah, M . D.; Cooper, S. L . Polyurethanes in Medicine, CRC Press: Boca Raton,

Florida, 1986. 7. Kossmehl, G.; Neumann, W.; Schafer, H. Makromol. Chem. 1986,187, 1381. 8. Sha'aban, A. K., et al. Polym. Prepr. 1983, 24(2), 130. 9 Pascault, J. P.; Chamberlin, Y. Polym. Commun. 1986,27, 230. 10. Yilgor, E.; Yilgor, I. Polym. Bul l . 1998, 40, 525. 11. Grulke, E . A. in Polymer Handbook, Eds. Brandrup, J.; Immergut. H . John Wiley:

New York, NY, 1989, Chapter VII, pp VII/519-559. 12. McGrath, J. E . ; Wang, L. F.; Mecharn, J. B.; Ji. Q. Polym. Prepr. 1998, 39(1),

455. 13. Cooper, S. L . ; Tobolsky, A. V. J. Appl. Polym. Sci. 1966, 10, 1837. 14. Marcos-Fernandez, A.; Lozano, A. E.; Gonzales, L. ; Rodriguez A.

Macromolecules, 1997, 30, 3584. 15. Abouzar, S.; Wilkes, G. L. J. Appl. Polym. Sci. 1984, 29, 2695. 16. Van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1992, Chapter

7, pp 185-225. 17. Yilgor, I.; Sha'aban, A. K.; Steckle, W. P., Jr.; Tyagi. D.; Wilkes, G. L.; McGrath,

J. E . Polymer, 1984, 25, 1800. 18. Tyagi, D.; Yilgor, I.; McGrath, J. E., Wilkes, G. L . Polymer, 1984, 25, 1807. 19. Dreyfuss, P. Polyftetrahydrofuran), Polymer Monographs 8; Gordon and Breach:

New York, NY, 1982, Chapter 6, pp-157-190.

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High Strength Silicone-Urethane Copolymers: Synthesis and Properties

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Transcript of High Strength Silicone-Urethane Copolymers: Synthesis and Properties