Abstract:Creep and recovery rheooptical measurements were conducted on linear polyisobutylenes, with viscosity-average molecular weights in the range from 4.2 × 10 5 to 4.7 × 10 6 g/mol. To isolate the contribution of chain ends to the birefringence, the temperature of each experiment was adjusted to yield equivalent creep strains for each sample. Higher molecular weight PIB exhibited less change in the birefringence during creep, but the birefringence during recovery was independent of molecular weight. The sign of th… Show more
“…20 Polyisobutylene (PIB) is one of the more interesting rubbers. The properties of the linear polymer have been studied by many techniques, including mechanical spectroscopy, [21][22][23][24] light scattering, 25 optical birefringence, 26,27 neutron scattering, 28,29 dielectric spectroscopy 29 and computer simulation. 30,31 A few studies of star-branched PIB have appeared, 32,33 while our recent interest has been the viscoelastic response of hyperbranched polyisobutylene.…”
The microdispersion of carbon black filler in linear and hyperbranched polyisobutylene (PIB) was assessed from dynamic mechanical and volume resistivity measurements. While no significant differences were observed in the carbon black concentration necessary for formation of a filler network, in comparison to the linear polymer, the highly branched PIB was found to have substantially more carbon black agglomeration. This occurs despite its higher viscosity, due to the relative inaccessibility of large portions of the molecule, as a result of the profuse treelike branching. The consequence is more extensive interaggregate interaction, and thus a larger Payne effect and greater mechanical hysteresis.
“…20 Polyisobutylene (PIB) is one of the more interesting rubbers. The properties of the linear polymer have been studied by many techniques, including mechanical spectroscopy, [21][22][23][24] light scattering, 25 optical birefringence, 26,27 neutron scattering, 28,29 dielectric spectroscopy 29 and computer simulation. 30,31 A few studies of star-branched PIB have appeared, 32,33 while our recent interest has been the viscoelastic response of hyperbranched polyisobutylene.…”
The microdispersion of carbon black filler in linear and hyperbranched polyisobutylene (PIB) was assessed from dynamic mechanical and volume resistivity measurements. While no significant differences were observed in the carbon black concentration necessary for formation of a filler network, in comparison to the linear polymer, the highly branched PIB was found to have substantially more carbon black agglomeration. This occurs despite its higher viscosity, due to the relative inaccessibility of large portions of the molecule, as a result of the profuse treelike branching. The consequence is more extensive interaggregate interaction, and thus a larger Payne effect and greater mechanical hysteresis.
“…(This observation remains true also after 200 s of relaxation, i.e., near the end of the relaxation period shown in Figure .) This contrasts with what was observed in polystyrene-based statistical ionomers, where the polystyrene orientation increases with ion content, and with what would be expected if the ionic aggregates act as effective cross-links. , 5 Effect of temperature on the orientation function calculated from the band at 1368 cm -1 at the end of the deformation for samples of Z67, Z52, and Z20 stretched at a rate of 2 cm/min to a draw ratio of 3.2.…”
Section: Resultsmentioning
confidence: 82%
“…This contrasts with what was observed in polystyrene-based statistical ionomers, where the polystyrene orientation increases with ion content, 17 and with what would be expected if the ionic aggregates act as effective crosslinks. 13,14 Part of the explanation for these observations may lie with the difference in morphology of the aggregates (lamellar for Z20 and cylindrical for Z52 and Z67) or with differences in their sizes or coherence domains, which are not known. The hard-phase glass transition temperature, which is broad and which occurs at similar temperatures for all three zwitterionomers (see Table 1), cannot be responsible for the differences in orientation behavior.…”
Section: Resultsmentioning
confidence: 99%
“…We have recently investigated the phenomenon of strain-induced crystallization in one of these segmented zwitterionomers by infrared linear dichroism . Molecular orientation and relaxation, in general, have been studied in single-phase polymers, − in ionomers, − and in other multiphase polymer systems. − In this paper, we have investigated the orientation and relaxation behavior of three zwitterionomers of different PTMO segment lengths (designated Z20, Z52, and Z67 for segmental PTMO molar masses of 2000, 5200, and 6700 g/mol, respectively), mainly under conditions where no strain-induced crystallization occurs. This has allowed us to focus on correlations between the noncrystalline morphology and the orientation−relaxation behavior.…”
The orientation and relaxation behavior of segmented poly(tetramethylene oxide) (PTMO) zwitterionomers was studied under conditions where no strain-induced crystallization occurs, using polarization modulation infrared linear dichroism. Samples of three different PTMO segment lengths, designated Z67, Z52, and Z20 for PTMO blocks with molar masses of 6700, 5200, and 2000 g/mol, respectively, were used. During deformation to a draw ratio of 3.2, the PTMO chains progressively orient along the stretch direction, the maximum induced orientation decreasing with increase in temperature. In contrast, the orientation function of the sulfonate units increases only at the beginning of the deformation process, reaching a maximum of about 0.015 at a draw ratio of about 1.5, and then decreases to become negative before the end of the stretch period. During the subsequent relaxation period, the PTMO orientation decreases as a function of time and the negative orientation function of the ionic groups further decreases slightly with time. By analogy with segmented copolymers characterized by a crystalline hard phase and an elastomeric soft phase, the following model is proposed. Before deformation, the ionic aggregates, which have previously been shown to have lamellar (Z20) or hexagonal (Z52, Z67) morphology, are like rigid anisotropic entities randomly oriented throughout the sample. Upon deformation to low draw ratios, the flexible PTMO chains orient along the stretch direction and the ionic rigid entities orient perpendicular to the stretch direction, the latter entailing orientation of the ionic side chains parallel to the stretch direction. At higher draw ratios, the applied stress causes progressive disruption of the ionic aggregates, resulting in a reorientation of the "freed" ionic side chains perpendicular to the deformation direction.
“…Importantly, such softening was attributed by Shi and McKenna1, 2 to a simple nonlinear viscoelasticity that is well captured by such constitutive equations as the K‐BKZ theory of Kaye, Bernstein, Kearsley and Zapas 18–20. It is worth mentioning that although past work on branched polymers21 suggests that reversing flows may not deviate as strongly from, for example, the KBKZ theory constitutive behavior, there is still evidence in the literature22–26 that reversing flows can be the cause of significant nonlinearities in some polymeric fluids. The mechanical hole burning spectroscopy may be very sensitive to this and future work needs to be performed to establish how important is this.…”
Mechanical Spectral Hole Burning (MSHB) was previously developed to investigate dynamic heterogeneity for polymeric materials, which exhibit relatively weak dielectric responses. In our previous work, MSHB was applied to a densely entangled block copolymer and successfully distinguishes the heterogeneous from the homogeneous states. Here, a series of polystyrene (PS) solutions was chosen to further investigate the effect of different types of heterogeneity on mechanical spectral hole burning. The three types of heterogeneity of interest include the entanglement spacing, the entanglement density (or number of entanglements per chain), and chain end density. The heterogeneity was varied by changing either solution concentration or molecular weight of the PS. Different types of dynamics from close to the Rouse regime into the terminal region were also examined. Our results are consistent with a heterogeneous dynamics over the time scales from close to Rouse regime into the rubbery plateau regime and the rubbery plateau-to-terminal flow transition regime. Terminal relaxation dynamics, on the other hand, were found to be homogeneous for the PS/diethyl phthalate solutions investigated. The results also indicate the hole properties are dominated by the type of dynamics rather than the length scale of heterogeneity.
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