Structure-property relationships--linear and star-branched macrostructures.

Commercial SSBRs

Microstructure and macrostructure analysis The continuously-prepared SSBR samples were also fully characterized. SSBR-A is a linear material, while SSBR-B is a silicon tetrachloride coupled product. Table 7 provides a comparison between the polymerization processes, microstructure and macrostructure of the two materials. The microstructures are somewhat different; however, the Tgs of the two materials are identical. The styrene content of SSBR-B is higher, but the vinyl content is lower such that the Tg is maintained. While the Mooney viscosities are also similar, differences in the macrostructure are seen when comparing SEC data. The SEC chromatagrams of the two materials are shown in figure 17. Weight average molecular weights are very similar, but the PD is higher for the coupled material. Since polar modification (TMEDA) was used to produce these materials, less than quantitative coupling is seen, resulting in the PD difference. Figure 18 shows the relative differences between the radii of gyration as a function of molecular weight for the continuous samples. As seen with the model PBDs, the SSBR sample with the star-branched macrostructure has a smaller radius at equivalent molecular weight.

[FIGURES 17-18 OMITTED]

Rheological characterization

The RPA was used to compare the theological characteristics of these samples. Figure 19 contains the frequency sweep plots. As seen in the model PBD samples, the linear SSBR-A material displays a crossover in the shear moduli showing terminal flow behavior. The coupled SSBR-B sample, in contrast, has an extended plateau zone and never reaches terminal behavior over the frequency range tested. Tan delta as a function of frequency is given in figure 20. This analysis indicates that despite the differences in polymerization processes between the batch-prepared model PBds and the continuous SSBRs, the relative theological differences between linear and branched (coupled) materials are maintained. For comparison, the [M.sub.e] of a generic SSBR (23.5% styrene) is 3,000 g/mol (ref. 52).

[FIGURES 19-20 OMITTED]

Compound analysis

The compound formulations are given in tables 8 and 9 for the carbon black and silica filled systems, respectively. The compounded (productive) stock was tested in the laboratory to determine the differences in the performance between the linear and branched base polymers. The effects of the macrostructural differences are highlighted by comparisons in two main performance areas: Processing and hysteresis. Table 10 contains the results of compound testing for both the carbon black and silica formulations.

Indications of processing differences are first evident when comparing minimum torque values from the ODR (Min. S'). Cure times (T90) are similar for both samples within each formulation. Regardless of the formulation, the Min. S' values for SSBR-B are significantly lower. The differences in shear modulus of the compounded and uncured stock are maintained as evident in the compounded Mooney viscosities and RPA uncured G' data. Garvey die extrusion data also support improved processing characteristics for the SSBR-B material. Although the mass per unit length values are similar within each formulation, the extrudate quality is remarkably improved with the coupled material. Figure 21 shows a picture of the extrudates. The surface of the SSBR-B containing compound is smoother and has shine. While the edges of the SSBR-A containing stocks are rough and saw-toothed, those for the SSBR-B stocks are much improved. Clearly, the processing characteristics have been improved with the silicon-coupled product.

[FIGURE 21 OMITTED]

However, as expected, the tests that measure hysteresis and correlate to the rolling resistance of the tire show advantages for the more linear macrostructure, especially in the carbon black formulation. RPA and RSA cured tan delta values are lower for SSBR-A in the carbon black formulation, while they are equal to lower in the silica formulation. Zwick rebound results confirm lower compound hysteresis for the SSBR-B containing compound in each formulation. The advantage in hysteresis for the linear macrostructure is best seen in the carbon black formulation; silica compounding requires silane coupling agents to promote dispersion that may also improve polymer-filler interaction, perhaps equalizing the performance of the two materials in the dynamic testing of compounds from that formulation.

Unlike the batch-prepared PBds, the macrostructure of the coupled SSBR sample is less well defined. The fraction of uncoupled material present in the SSBR-B sample is higher than in the model PBds. Typically, quantitative modification with diamines limits coupling efficiencies (50-70%, depending on modifier) regardless of a batch or continuous process. The ratio of Si[Cl.sub.4] to lithium used to produce the commercial product is also less than stoichiometry. The SEC/MALLS characterization of the SSBR samples proves that SSBR-B had a lower [R.sub.g.sup.2] (at equivalent molar mass) when compared to the linear SSBR-A. and the rheological characterization of SSBR-B supports limited molecular motion indicative of a higher state of entanglement when compared to SSBR-A. These data confirmed the presence of extensive coupling in the SSBR-B sample. However, the uncoupled fraction remaining in the sample certainly contributes to the improvements in processing and increased hysteresis.

Conclusions

Anionic polymerization techniques have been used to produce a set of well-defined PBd samples with both linear and star-branched macrostructures. The analysis of these materials in both dilute solution and solid state reveals how the chain structure influences the bulk rheology. At equivalent molecular weights, the star-branched structures have smaller radii of gyration and lower solution viscosities. Detailed rheological analysis confirms that the linear and star-branched structures have very different responses when subjected to oscillatory shear over several frequencies. The star-branched structures, while maintaining similar plateau moduli at similar molecular weights, have extended the plateau zone to lower frequencies when compared to linear samples. These results are consistent with the molecular theory of chain entanglement. Consistent with the reptation model, if a branch has a molecular weight above the [M.sub.e], it further retards the translational mobility of the entire chain. The relaxation time for the branched chain essentially becomes the addition of the relaxation times of all the chains about a branch point. In the linear viscoelastic regime, higher low shear viscosities are seen for branched materials when compared to linear materials of equivalent molecular weight. However, the shear rate dependence of viscosity for branched materials is much higher, resulting in lower viscosities at higher shear rates. This inversion of relative viscosity was seen for the branched and linear PBd samples. The branched materials had lower viscosity at higher shear rates, suggesting improved processing characteristics.

An analysis of commercially available SSBRs with linear and star-branched macrostructures shows that the differences seen in both solution and bulk for the batch-prepared model PBds are also evident for the SSBR materials, despite the fact that the continuous polymerization process produces inherently broader polydispersities, and polar modification was used to increase the vinyl content: both changes that will lower the coupling efficiency. The radius of gyration for the coupled sample was smaller at equivalent molecular weights. In addition, the rheological differences between the SSBRs were consistent with those established using the model PBds.

The compound evaluation using the SSBR products completes the structure-property relationships. The branched SSBR displayed improved processing characteristics despite similar raw polymer Mooney viscosities. The processing advantages were best highlighted in the improvements in the Garvey die extrudates. However, branched structures also have a higher chain end concentration at a given molecular weight, often resulting in higher hysteresis measurements. This behavior was seen for the branched SSBR, especially in the carbon black filled formulation, as rebound values were lower and tan delta measurements were increased versus the linear product.

Table 8--carbon black-filled compound
formulation (amounts in parts per hundred rubber)

Non-productive
SSBR-A              137.5     0
SSBR-B                0     137.5
Carbon black N299   70.0    70.0
Zinc oxide           3.0     3.0
Stearic acid         2.0     2.0
Agerite resin D      1.5     1.5

Productive
Sulfur               1.4     1.4
Santocure CBTS       1.2     1.2

Table 9--silica-filled compound formation
(amounts in parts per hundred rubber)

Non-productive
SSBR-A                      137.5     0
SSBR-B                        0     137.5
Zeosil 1165 silica          70.0    70.0
Coupling agent (50 wt. %)   11.2    11.2
Microcrystalline wax         1.0     1.0
Paraffin wax                 0.5     0.5
Santoflex 13 antiozonant     2.5     2.5
Stearic acid                 3.0     3.0

Productive
Wingstay 100                 0.5     0.5
Zinc oxide                   2.5     2.5
Santocure CBTS               2.0     2.0
Diphenylguanidine            1.6     1.6
Sulfur                       1.7     1.7

Table 10--summary of compound testing in
selected areas of cure, processing and hysteresis

                                                       Carbon black

                   Cure                               SSBR A   SSBR-B

ODR                Min. S'                             10.9     6.5
                   Max. S'                             29.5     20.7
                   T'90                                17.5     16.4

                   Processing
Mooney viscosity   Compound Mooney                      93       85
RPA                Uncured G' (100[degrees]C,         466.6    400.4
                     15%, 8.33 Hz)
                   Cured G'(100[degrees]C,            3171.9   3424.2
                     1%, 11 Hz)
Garvey die         Extrudate (g/in.)                   1.62     1.54
Durometer A        Hardness                            66.2     68.8

                   Hysteresis
RPA                Cured tan [delta] (100[degrees]C,  0.238    0.252
                     10%,11 Hz)
RSA                Tan [delta] (60[degrees]C, 0.1%,   0.154    0.156
                     11 Hz)
Zwick              Rebound (100[degrees]C)             50.0     46.2

                                                          Silica

                   Cure                               SSBR-A   SSBR-B

ODR                Min. S'                             11.4     6.2
                   Max. S'                             34.3     20.9
                   T'90                                10.3     13.0

                   Processing
Mooney viscosity   Compound Mooney                      94       80
RPA                Uncured G' (100[degrees]C,         430.4    404.2
                     15%, 8.33 Hz)
                   Cured G'(100[degrees]C,            2678.8   3009.6
                     1%, 11 Hz)
Garvey die         Extrudate (g/in.)                   1.92     1.96
Durometer A        Hardness                            58.3     60.8

                   Hysteresis
RPA                Cured tan [delta] (100[degrees]C,  0.137    0.131
                     10%,11 Hz)
RSA                Tan [delta] (60[degrees]C, 0.1%,   0.097    0.097
                     11 Hz)
Zwick              Rebound (100[degrees]C)             61.7     61.0

References

(1.) F.C. Weissert and B.L. Johnson, Rubber Chem. Tech., 40, 590 (1967).

(2.) M.L. Kerns, S.K. Henning and M.J. Rachim, in Butadiene Polymers, Encyclopedia of Polymer Science and Technology; John Wiley & Sons, Inc., 2002.

(3.) H. Colvin and S. Henning, Tire Tech. Intl., Ann. Rev., 32 (2001).

(4.) J.E. Mark, B. Erman and F.R. Eirich, eds., Science and Technology of Rubber, Academic Press, New York, 1994.

(5.) P.J. Flory, Principals of Polymer Chemistry Cornell University Press, Ithaca, 1953.

(6.) S.L. Aggarwal, H.J. Fabris, I.G. Hargis and R.A. Livigni, Polym. Prepr.--Am. Chem. Soc. Div. Polym. Chem., 26, 3 (1985).

(7.) C. Lambert and P. von R. Schleyer, Angew. Chem. Int. Ed. Engl., 33, 1,129 (1994).

(8.) B. Wakefield, The Chemistry of Organolithium Compounds, Pergamon Press, Oxford, 1974.

(9.) F.C. Foster and J.L. Binder, Advances in Chemistry Series No. 19, Am. Chem. Soc., 1957.

(10.) H.L. Hsieh and R.P. Quirk, Anionic Polymerization: Principals and Practical Applications, Marcel Dekker, Inc., New York, 1996.

(11.) T.A. Antkowiak, A.E. Oberster, A.F. Halasa and D.P. Tate, J. Appl. Polymer Sci., 10, 1,319 (1972).

(12.) R. Milner, R. Young and A.R. Luxton, Polymer, 24, 543 (1983).

(13.) S. Bywater, in Recent Advances in Anionic Polymerization, T. Hogen-Esch and J. Smid, eds., Elsevier, New York, 1987.

(14.) J.C. Cho, K.H. Kim, K.U. Kim, S. Kwak, J. Kim, W.H. Jo, M.S. Chun, C.H. Lee, J.K. Yeo and R.P. Quirk, J. Polym. Sci., 36, 1,743 (1988).

(15.) B.J. Bauer and L.J. Fetters, Rubber Chem. Tech., 51, 406 (1978).

(16.) V. Ruiz Santa Quiteria, C.A. Sierra, J.G. Fatou, C. Galan and L. Fraga, paper #78 presented at the 148th ACS Rubber Division Meeting, 1995.

(17.) M.L. Kerns, Z. Xu and S. Christian, paper #158 presented at the 156th ACS Rubber Division Meeting, 1999.

(18.) N. Sommer, Kautsch. Gummi, 28, 131 (1975).

(19.) JP 47-2688 assigned to Asahi Chemical Industry.

(20.) H. Eschwey and W. Burchard, J. Polym. Sci., Polym. Syrup., 53, 2 (1975).

(21.) W. Burchard and H. Eschwey, Polymer, 16, 180 (1975).

(22.) P. Lutz and P. Rempp, Makromol. Chem., 189, 1,051 (1988).

(23.) C. Strazielle and J. Herz, Europ. Polym. J., 13, 223 (1977).

(24.) M. Hadjichristidis, A. Guyot and L.J. Fetters, Macromolecules, 11, 889 (1978).

(25.) D.H. Rein, P Rempp and P.J. Lutz, Macromol. Chem. Phys., 199, 569 (1998).

(26.) M.L. Kerns and S.K. Henning, Rubber Chem. and Tech., 75, 299 (2001).

(27.) D.P. Tate, A.F. Halasa, F.J. Webb, R.W. Koch and A.E. Oberster, J. Polym. Sci., 9, 139 (1971).

(28.) M. Farral and J. Frechet, J. Org. Chem., 41, 3,877 (1976).

(29.) H.L. Hseih and C.F. Wofford, J. Polym. Sci., 7, 449 (1969).

(30.) T. Kremer, S. Harder, M. Junge and P. Schleyer, Organometallics, 585 (1996).

(31.) M. Sclosset, Pure & Appl. Chem., 60, 1,627 (1988).

(32.) L. Lochmann, Eur. J. Inorg. Chem., 1,115 (2000).

(33.) A.F. Halasa, G.B. Mitchell, M. Stayer, D.P. Tate, A.E. Oberster and R.W. Koch, J. Polym. Sci., 14, 497 (1976).

(34.) H.A.J. Battaerd and G.W. Tregear, Graft Copolymers, Interscience, New York, 1967.

(35.) R.J. Ceresa, ed., Block and Graft Copolymerizations, Vol. 1 and 2, Wiley-Interscience, New York, 1973.

(36.) K. Hatada, T. Kitayama and O. Vogl, Macromolecular Design of Polymeric Materials, Marcel Dekker, New York, 1997.

(37.) P. Rempp, E. Franta and J.-E. Herz, Adv. Polym. Sci., 86, 145 (1988).

(38.) R.W. Pennisi and L.J. Fetters, Macromolecules, 21, 1,094 (1988).

(39.) R.P. Quirk and T. Roo, Polym. Bull., 29 (1993).

(40.) Y. Tselikas, N. Hadjichristidis, R.L. Lescanec, C.C. Honeker, M. Wohlgemuth and E.L. Thomas, Macromolecules, 29, 3,390 (1996).

(41.) H. Iatru, E. Siakali-Kioulafa, N. Hadjichristidis, J. Roovers and J.W. Mays, J. Polym. Sci., Polym. Phys., 33, 1,925 (1995).

(42.) R.P. Zelinski and C.F. Wofford, J. Polym. Sci., 3, 93 (1965).

(43.) C.A. Uraneck and J.N. Short, J. Appl. Polym. Sci., 14, 1,421 (1970).

(44.) J.E.L. Roovers and S. Bywater, Macromolecules, 5, 385 (1972).

(45.) N. Hadjichristidis and L.J. Fetters, Macromolecules, 13, 191 (1980).

(46.) F. Wang, J. Roovers and P.M. Toporowski, Macromol. Syrup., 95, 205 (1995).

(47.) M. Morton, T.E. Helminiak, S.D. Gadkery and F. Bueche, J. Polym. Sci., 57, 471 (1962).

(48.) L.J. Fetters and M. Morton, Macromolecules, 7, 552 (1974).

(49.) D. Wittenburg and H. Gilman, Quart. Rev., 13, 116 (1959).

(50.) R.G. Jones and H. Gilman, Org. Reactions, 6, 339 (1951).

(51.) T. Altares, Jr., D.P. Wyman, V.R. Allen and K. Meversen, J. Polym. Sci., A, 3, 4,131 (1965).

(52.) J.D. Ferry, Viscoelastic Properties of Polymers, 2nd Ed., Wiley, New York, 1970.

(53.) J.M. Dealy and K.F. Wissburn, Melt Rheology and Its Role in Plastics Processing, Van Nostrand Rheinhold, New York, 1990.

(54.) W.W. Graessley, Adv. Polym. Sci., 16, 1 (1974).

(55.) G. Kraus, in The Stereo Rubbers, W.M. Saltman, ed., Wiley, New York, 1977.

(56.) P.E. Rouse, Jr., J. Chem. Phys., 21, 1,272 (1953).

(57.) B.H. Zimm, J. Chem. Phys., 24, 269 (1956).

(58.) F. Bueche, J. Chem. Phys., 20, 1,959 (1952).

(59.) J.T. Gruver and G. Krause, J. Polym. Sci., 2, 797 (1964).

(60.) H.H. Meyer and W Ring, Kaut. Gummi Kunstst., 24, 526 (1971).

(61.) G. Kraus and J.T. Gruver, J. Polym. Sci., 3, 105 (1965).

(62.) G. Berry and T. Fox, Adv. Polym. Sci., 5, 261 (1968).

(63.) F. Bueche, W.M. Cashin and P. Debye, J. Chem. Phys., 20, 1,956 (1952).

(64.) F. Bueche, J. Chem. Phys., 25, 599 (1956).

(65.) P.G. DeGennes, J. Chem. Phys., 55, 572 (1971).

(66.) M. Doi and S.F. Edwards, The Theory of Polymer Dynamics, Oxford University Press, Oxford, 1986.

(67.) M. Doi, W.W. Graessley, E. Helfand and D.S. Pearson, Macromolecules, 20, 1,900 (1987).

(68.) S.F. Edwards and J.W.V. Grant, J. Phys. A: Math. Nucl. Gen., 6, 1,169 (1973).

(69.) R.G. Mancke, R.A. Dickie and J.D. Ferry J. Polym. Sci., 6, 1,783 (1968).

(70.) I.M. Ward, Mechanical Properties of Solid Polymers, Wiley, New York, 1971.

(71.) D.S. Pearson, Rubber Chem. Tech., 60, 439 (1987).

(72.) W.W. Graessley, J. Chem. Phys., 47, 1,942 (1967).

(73.) R.A. Stratton, J. Colloid Sci.. 22, 517 (1966).

(74.) E.V. Menezes and W.W. Graessley, J. Polym. Sci: Polym. Phys., 20, 1,817 (1982).

(75.) A. Ghijels and H.J.M.A. Mieras, J. Polym. Sci: Polym. Phys., 11, 1,849 (1973).

(76.) W.L. Hsu, et al., U.S. 5,422,403 (1995).

(77.) D.J. Zanzig, et al., U.S. 5,798,448 (1998).

(78.) F. Tsutumi, M. Sakakibara and N. Oshima, Rubber Chem. Tech., 63, 8 (1990).

(79.) J.L. White, Rubber Chem. Tech., 42, 257 (1969).

(80.) N. Nakajima, Polymer International, 36, 105 (1995).

(81.) B.M.E. Van der Hoff, Rubber Plast. Age, 46, 821 (1965).

(82.) N. Nakajima, Journal of Elastomers and Plastics, 32, 46 (2000).

(83.) L.J. Fetters and M. Morton, Macromolecules, 7, 552 (1974).

(84.) Gas chromotography was performed on a Hewlett Packard 5890 Series H machine using a DB-5 60 m x 0.25 mm column with helium as the carrier gas. A Hewlett Packard 3396 Series II integrator was used for analysis.

(85.) F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd Ed., John Wiley and Sons, New York, 1984.

(86.) J.C. Moore, J. Polym. Sci., 2, 832 (1964).

(87.) P.J. Wyatt, Analytica Chimica Acta, 272, 1 (1993).

(88.) Determined in THF at 23[degrees]C for high molecular weight poly(butadiene). American Polymer Standards Corporation, 2003.

(89.) B.L. Johnson, H.E. Adams, F.C. Weissert and K. Farat, The Proceedings of the International Rubber Conference, Maclaren and Sons, London, 1967.

(90.) J.C. Moore, J. Polym. Sci., 2, 832 (1964).

(91.) W.H. Stockmayer and M. Fixman, Annals. N. Y. Acad. Sci., 68, 323 (1953).

(92.) W. W. Graessley, "Characterization of macromolecular structure," in NAS Pub. 1573, D. McIntyre, ed., National Academy of Sciences, Washington, D.C., 1968.

(93.) P.J. Flory and T.G. Fox, J. Polym. Sci., 5, 745 (1950).

(94.) P. Debye and A.M. Bueche, J. Chem. Phys., 16, 573 (1948).

(95.) M.L. Huggins, J. Am. Chem. Soc., 64, 2,716 (1942).

(96.) E.O. Kraemer, Ind. Eng. Chem., 30, 1,200 (1938).

(97.) M. Morton, T.E. Helminiak, S.D. Gadkary and F. Bueche, J. Polym. Sci., 57, 471 (1962).

(98.) R. Endo, J. Rubber Ind. Japan, 35, 658 (1962).

(99.) J. Shaefgen and P.J. Flory, J. Am. Chem. Soc., 70, 2, 709 (1948).

(100.) A.J. Charlesby, J. Polym. Sci., 17, 379 (1955).

(101.) W.W. Graessley, T. Masuda, J.E.L. Roovers and N. Hadjichristidis, Macromolecules, 9, 127 (1976).

(102.) T. Masuda, Y. Ohta and S. Onogi, Macromolecules, 4, 763 (1971).

(103.) M. Doi and N.Y. Kuzuu, J. of Polym. Sci., Polym. Letters., 18, 775 (1980).

(104.) D.S. Pearson and E. Helfand, Macromolecules, 17, 888 (1984).

(105.) B.J. Bauer, N. Hadjichristidis, G. Quack, J. Vitus and L.J. Fetters, Polym. Prepr., 20, 126 (1979).

(106.) L.J. Fetters, Polym. Prepr., 23, vii (1982).

(107.) L.J. Fetters, A.D. Kiss, D.S. Pearson, G.F. Quack and F.J. Vitus, Macromolecules, 26, 647 (1993).

(108.) E. Maekawa, R.G. Mancke and J.D. Ferry, J. Phys. Chem., 69, 2,811 (1965).

(109.) J. Roovers, Polymer, 26, 1,091 (1985).

(110.) T. Masuda, K. Kitagawa, T. Inoue and S. Onogi, Macromolecules, 3, 116 (1970).

(111.) J. Meier, E. Giebeler, M. Klippel and R.H. Schuster, Paper #XX presented at the 159th ACS Rubber Division Meeting, 2001.

(112.) L.J. Kasehagen, W. Macosko, D. Trowbridge and F. Magnus, J. Rheol., 40, 689 (1996).

(113.) J. Sezna, Rubber and Plastics News, 9, 12 (1998).

(114.) W.P. Cox and E.H. Merz, J. Polym. Sci., 28, 619 (1958).

(115) R.I. Tanner and R.R. Huilgol, Rheol. Acta, 14, 959 (1975).

(116.) R.G. Larson, Non-Newt. F1. Mech., 23, 249 (1987).

Steven K. Henning, Goodyear (This is the conclusion of a four-part series)