Viscoelastic properties of polybutadienes—linear and lightly crosslinked near the gel point

Abstract

SynopsisViscoelastic properties of four linear and three very lightly crosslinked polybutadienes (microstructure about 50% trans) were studied. Of the latter, two had not reached the gel point, and their molecular weight distributions were determined by sedimentation velocity analysis; the third was crosslinked just past the gel point, with only 32% gel fraction present.Complex shear compliances were measured over a frequency range from 0.1 to 1000 cps at temperatures from -70 to 30°C. with a Fitagerald transducer and a Plazek torsion pendulum; and torsional creep measurements were made over time periods up to about three days. The creep data were converted to the corresponding dynamic viscoelastic functions at very low frequencies by conventional approximation methods. All data were reduced to 25°C. by shift factors calculated from a previously adopted equation of the WLF form. In the transition zone, the viscoelastic properties of linear samples were almost independent of molecular weight. The entanglement spacing, derived from the minimum in the loss tangent and the inflection in the storage compliance, was 130 to 160 chain atom. The maximum in the retardation spectrum attributable to motions of individual network strands was closely similar to the corresponding maxima for more highly crosslinked vulcaniaates previously studied, showing that even in the latter it is associated with entanglement network strands rather than strands between chemical crosslinks. For a linear sample with molecular weight 180,000, the retardation processes disappear at times beyond about 10 sec. a t 25°C. With crosslinking short of the gel point (i.e., branching) the slow retardation processes are enormously increased and prolonged to longer times. With further crosslinking through the gel point and beyond, the slow retardation processes decrease progressively in magnitude. Qualitatively, this behavior resembles the sharp maximum in content of highly branched and aggregated molecular species which is predicted at the gel point by crosslinking statistics; but the slow processes (or low-frequency losses) persist farther past the gel point than would be expected on this basis. The steady-state compliances of the linear samples were smaller, but for a sample crosslinked short of the gel point were much larger, than the prediction of the Rouse theory modified for molecular weight distribution.

Transfusion independence in patients with myelodysplastic syndromes

“…The results demonstrate that the retardation spectra from the bulk response are independent of crosslink density, indicating the local nature of the underlying molecular motions. Also, shown in Figure 10 are the retardation spectra from the shear responses measured at atmospheric pressure, which show a narrowing with increasing crosslink density, consistent with the literature 17–20. To compare the bulk and shear retardation spectra, we first note that the bulk measurements were carried out at ∼67 MPa rather than at atmospheric pressure, leading to a suppression of the retardation spectrum (a decrease in the absolute magnitude) for the bulk response relative to the shear response at short times.…”

“…Moreover, the retardation spectra for the bulk response are independent of crosslink density. Previous studies17–21 indicated that crosslink density influences the viscoelastic properties only at long times, as can be observed in the shear retardation spectrum which narrows with increasing crosslink density. However, because the long‐time mechanisms are unavailable for the bulk response, this behavior cannot be observed for the bulk.…”

“…The effect of crosslink density on the viscoelastic shear response of crosslinked rubbers is well understood: with increasing crosslink density, the rubbery modulus increases and long‐time mechanisms are lost; in addition, the viscoelastic response can be reduced by time‐crosslink density superposition 17–21. Similar behavior is expected for thermosetting resins, and time‐crosslink density or time‐ T g superposition has been found to be valid both for describing the physical aging response and the evolution of the shear modulus during cure 22, 23…”

Temperature and Pressure Effects on Local Structure and Chain Packing in cis‐1,4‐Polybutadiene from Detailed Molecular Dynamics Simulations

“…These values are consistent with literature data reported, for example, by Paul and DiBenedetto [30,31] for a pure cis-1,4-PB system (a p ¼ 6.7 Â 10 À4 K À1 at T ¼ 298 K), by Smith et al [32] for a random PB copolymer sample through 13 C NMR experiments and MD simulations (a p ¼ 7.1 Â 10 À4 K À1 at T ¼ 353 K), by Barlow [28] (a p ¼ 7.35 Â 10 À4 K À1 in the temperature range 277-328 K) and by Valentine et al [33] (a p ¼ 7.5 Â 10 À4 K À1 ) for a random copolymer of PB (containing only 43% cis-1,4). Anderson et al [34] have also reported a p for a high cis-1,4-PB sample through NMR measurements (a p ¼ 5.7 Â 10 À4 K À1 at T ¼ 296 K).…”

Structure and Rheological Function of Side Branches of Carbohydrate Polymers

“…This trend can be reversed with significant coupling entanglements of the branches (Ferry 1980). Examples showing J,",B > J,",L in the entangle system can be found in star- (Masuda et al 1971(Masuda et al , 1981Graessley et al 1976;Graessely 1977;Graessley and Roovers 1979;Raju et al 1981;Masuda et al 1984), comb- (Fujimoto et al 1970;Roovers and Graessley 1981) and random-branched polymers (Valentine et al 1968;Jacovic et ul. 1979).…”