Low molecular weight polybutadienes and styrene butadiene copolymers were anionically prepared with trialkyltin lithium initiator and end‐capped with either hydrogen or a trialkyltin group. These polymers were prepared with a variety of microstructures. Analysis by 119Sn‐NMR and comparison to model compounds showed no cis‐1,4‐initiation of the butadiene. The initiation sites found were trans‐1,4‐ and both 2,1‐ and 1,2‐additions of the tin‐lithium bound to a 1,3‐butadiene. At low levels of added polar modifier, the 2,1‐addition predominated. The 119Sn‐NMR spectra allowed the assignment of the sequence distribution associated with the nearest eight main chain carbon atoms (2‐4 monomer units) adjacent to the tin end groups. No initiation could be detected involving the styrene comonomer, but incorporation of styrene was detected as the first or second unit after initiation. The reaction of the allyl‐tin end groups of these polymers with 1,2‐napthoquinone was followed by NMR and was used to assign the peaks associated with 1,2‐addition of the trialkyltin lithium to 1,3‐butadiene. © 1995 John Wiley & Sons, Inc.
SYNOPSISTensile retraction measurements have been made on the same SBR filled with 50 phr of 10 different carbon blacks. The slopes and intercepts determined from the plots against A, , , were determined in three different elongation ranges. These carbon blacks were chosen to have a variety of different surface areas and structures such that the influence of these features on the slopes and intercepts could be measured. The tensile retraction parameters determined were also evaluated at elevated temperatures and Arrhenius plots were made. The activation energies calculated from these plots showed that the structure and surface area of the carbon blacks had no effect on the process being measured. A series of different molecular weight SBRs were also made by anionic techniques to have 0,1, and 2 tributyltin end groups. The changes in the constants determined from tensile retraction measurement on these cured filled stocks were explained in terms of the number of loose end groups still present after reaction with the carbon black.
SynopsisThe molecular weight between crosslinks was measured as a function of the maximum extension to which samples of solution cast films of ABA triblocks were tested. These measurements were done by applying the kinetic theory of rubber elasticity to the retraction portion of a tensile hysteresis curve. The molecular weight between chain restrictions, as determined by extrapolation to 0% elongation, was found to be identical to the entanglement molecular weight normally associated with the polybutadiene or polyisoprene that was used for the center block. The disentanglement of the small amount of AB diblock present could also be followed by fitting the data collected to what could be pictured kinetically as a simultaneous first-and second-order loss of crosslink density.
SYNOPSISTensile retraction measurements have been made on unfilled rubbers cured with either sulfur or dicumyl peroxide to different levels of crosslink density. The rubbers chosen were emulsion and solution polymerized anionic SBRs, cis-polybutadiene, natural rubber, and anionic 1,4-polybutadienes of different molecular weight. The measured molecular weight between crosslinks (M,) at each extension gave a smooth curve when plotted against the maximum extension (A,,,) used for the measurement. This curve was separated into three linear regions that characterize the cured rubber. The first region, above about 30% elongation, gave a Amax = 1 intercept that decreased with either increased polymer M,, or increased level of curatives. Subtracting this calculated crosslink density from the measured crosslink density below about 30% elongation gave another smooth curve when plotted against A, , , .A second linear relationship was described in the general range of 5-30%elongation. Below about 5% elongation, a third linear relationship was found. This line was determined from the differences between the calculated and the observed crosslink densities in the lowest strain region of the intermediate strain plot. Possible explanations of these three linear regions have been made. Supporting evidence was gained from temperature-dependent experiments as well as from correlation of experimental results with polymers of different values of Tg. I NTRO D UCTlO NThe retraction portion of the stress-strain tensile elongation curve of elastomers has been investigated over the years as a means to determine the effective crosslink density (v,) of the rubbers.'r2 The very early work of Flory et a1.l was done at equilibrium using a triple-beam balance to measure the stress. They reported that as the test specimen increased in thickness to about 7 mil a deviation from a constant response was noted. A particular interesting deviation from theory was also noted for those samples prepared at the lowest level of crosslink density. These samples appeared to have a much higher value of ve than what would be predicted from the level of curative used. This was partially attributed to the contribution of entanglements.* To whom correspondence should be addressed.Journal of Applied Polymer Science, Vol. 42, 1727-1738 (1991) Later, Fedors2 used an Instron stress-strain tester to determine the value of ve in a nonequilibrium method. This work was based on the retraction from X = 2.6, which was similar to the extension used in the work of Flory et al. ( A = 2.0). Fedors showed that the C2 of the Mooney-Rivlin equation had a value of zero under these conditions of testing, and as a result the data appeared to be equivalent to that collected under equilibrium conditions.In a more recent paper, Hergenrother and Doshak3 have demonstrated that tensile retraction ( T R ) can be used to determine the entanglement molecular weight ( M e ) of the rubbery center block of a thermoplastic elastomer. Here, the plot of the observed molecular weight between cros...
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