Polybutadiene Networks Research Articles

Threshold tear strength of elastomers

AbstractTear strengths have been measured for a wide variety of molecular networks under threshold conditions; i.e., at high temperatures, low rates of tearing, and with swollen samples. For all of the polymers examined, the threshold tear strength was found to be proportional to the square root of the average molecular weight Mc of network strands, in agreement with theory. However, for the same Mc, and hence for similar values of elastic modulus, different polymers showed major differences in threshold tear strength. The tear strength of polydimethylsiloxane networks was only about one‐third that for networks of polybutadiene and cis‐polyisoprene, and the values obtained for polyphosphazene networks were only about one‐fifth as large, at the same Mc. These striking differences are attributed to differences in network strand length and extensibility for the same molecular weight. The threshold tear strengths are shown to be in satisfactory quantitative agreement with theoretically predicted values on this basis.

Entanglements and elasticity in polybutadiene networks

AbstractPolybutadiene networks were prepared by peroxide crosslinking of monodisperse 1,4‐polybutadienes both in solution and in bulk. The effect of the entangled sol fraction on the elastic modulus of high‐molecular‐weight polybutadiene was observed in stress relaxation measurements. Sol fraction was shown to make a large contribution to the Mooney–Rivlin 2C2 term. This effect was also observed on the molecular level in NMR spin‐spin relaxation measurements. For networks crosslinked in bulk the stress relaxation measurements suggest the presence of trapped entanglements. The 2C2 term is insensitive to sol extraction in these networks. NMR spin‐lattice relaxation measurements in the rotating frame at 4.68 kHz verify the presence of additional effective crosslinks in these networks.

Stress relaxation and dynamic viscoelastic properties of end‐linked poly(dimethyl siloxane) networks containing unattached poly(dimethyl siloxane)

AbstractStress relaxation in uniaxial extension and dynamic shear moduli G′ and G″ have been studied in networks of vinyl‐terminated poly(dimethyl siloxane) (PDMS) of five different molecular weights (Mn from 1800 to 29,200) crosslinked with cis‐dichlorobis (diethyl sulfide) platinum (II) and containing 10 and 15 wt % of two samples of high‐molecular‐weight unattached linear hydroxyl‐terminated PDMS (Mw 700,000 and 950,000). The Mw/Mn ratio of both the network prepolymers and the unattached linear species was approximately 2. In stress relaxation the stretch ratio was 1.25 or less and the shear relaxation modulus was calculated from the neo‐Hookean stress‐strain relation. In the dynamic measurements, the strain amplitude was 15% or less; after conversion to the timedependent shear relaxation modulus G(t) the two sets of measurements were combined and the contribution of the unattached species G1(t) was calculated by difference. After multiplication by (1 − v)−1G/Ge, where v2 is the volume fraction of network, G is the plateau modulus of the uncrosslinked polymer, and Ge is the equilibrium modulus of the network containing unattached molecules, G1(t) was compared with G11(t), the relaxation modulus was essentially the same in both environments. The relaxation was slower in the networks than in the uncrosslinked polymer by 1 to 2 orders of magnitude, and it increased gradually with increasing Ge, which is a measure of total to pological obstacles represented by crosslinks plus trapped entanglements. A similar but less striking difference between relaxation in a network and in the homologous environment of a linear polymer was previously observed in end‐linked polybutadiene networks and the butadiene phase of a styrene‐butadiene‐styrene block copolymer. It appears that, in these systems where the topology of the obstacles is fixed, the reptation is severely restricted or else alternative modes of configurational rearrangement which contribute to relaxation in the uncrosslinked polymer are suppressed.

Rubber Elasticity of Well-Characterized Polybutadiene Networks

Rubber Elasticity of Well-Characterized Polybutadiene Networks

Radiation induced cis-trans isomerization of swollen polybutadiene networks

The kinetics of the radiation induced cis- trans- isomerization of cis- 1,4- polybutadiene gel swollen in toluene were studied by using diphenyl disulfide as sensitizer. The reaction follows first order kinetics. The values of the pseudo first- order rate constant of the isomerization vary with the crosslink density of the network. After correction for the degree of swelling, however, the isomerization reaction turns out to be independent of the crosslink density and proceeds with the same efficiency as in solution.

Thermoelastic studies on polybutadiene networks

An unequivocal correlation between the first coefficient of the Mooney-Rivlin equation and the degree of swelling is concluded from stress- strain measurements of cis- trans isomerized polybutadiene networks of varying cis- contents and from corresponding swelling measurements. The second coefficient turns out to be approximately constant, which means that the polymer network is not affected critically by the irradiation although the crosslink density diminishes. The temperature dependence of the unperturbed dimensions of the network was determined to be positive in the higher cis- region and to be negative at higher trans contents. A linear correlation was found with the amount of trans.

Dynamic viscoelastic behavior of polybutadiene networks

AbstractDynamic viscoelastic properties of polybutadiene networks, prepared by conventional sulfur vulcanization of polymers differing in primary molecular weight, molecular weight distribution, long‐chain branch content, and branching pattern, were measured in the region of the viscoelastic spectrum of principal interest in dynamic applications of rubbers. At lower frequencies, the storage compliance was found to correlate closely with the fraction of free (terminal) chains in the network, while the loss compliance could be correlated with either the free end fraction or with the fraction of chain entanglements not trapped into the network by primary valence crosslinks. At higher frequencies, approaching the plateau region of the viscoelastic spectrum, effects of network structural features diminished as contributions of untrapped entanglements became more pronounced. Networks prepared in presence of a diluent exhibited the same trends as pure gum vulcanizates. Carbon black produced a leveling effect, ascribed to a second entanglement entrapment mechanism by adsorption sites on the filler surface.

Polybutadiene/polystyrene interpenetrating polymer networks

AbstractInterpenetrating polymer networks (IPN's) of polybutadiene (PB) and polystyrene (PS) were prepared using both random (containing 36% cis, 55% trans, and 9% 1,2 vinyl) PB and high‐cis PB. For both series, a wide range of PB/PS compositions were synthesized. Using samples stained with osmium tetroxide, electron microscope studies revealed an irregular cellular structure of a few hundred Ångstrom diameter with the first component, PB, making up the cell walls. The size of the cells was found to depend on the PB crosslink density for the random materials. Modulus‐temperature data revealed two distinct glass transitions, confirming the microscopy finding of two phases. However, the transition temperature and transition slope varied with composition, and with the microstructure of the polybutadiene, giving evidence of significant molecular mixing. Stress‐strain data on the IPN's showed that materials rich in PB behave like self‐reinforced elastomers. Charpy impact resistance experiments on materials rich in PS indicated values of 5 ft‐lb/in. of notch, which compares well with graft‐type polyblends of similar PB/PS composition. The results were interpreted in the light of the recent theoretical work of Bragaw, who considered the importance of the distances between domain boundaries with respect to crack acceleration mechanics. Although the IPN's considered herein exhibited somewhat less than the predicted optimum phase dimensions, the arrangement of the domains is different from ordinary impact resistant plastics.

The CBS-Accelerated Sulfuration of Natural Rubber and Cis-1,4-Polybutadiene

Abstract The results of characterization of the natural rubber vulcanizates are consistent with the results of characterization of the sulfidic products from 2-methylpent-2-ene. In both the model olefin system and the rubber system the initially formed crosslinks are polysulfidic but these are subsequently reduced to di- and monosulfidic crosslinks as the cure time is increased. Similar amounts of zinc sulfide are formed during the sulfuration of 2-methylpent-2-ene and during the vulcanization of natural rubber. The efficiency of sulfur utilization for crosslinking in natural rubber is approximately half that in comparable sulfurations of 2-methylpent-2-ene, i.e. approximately twice as many sulfur atoms are needed to obtain a chemical crosslink in natural rubber as are needed to obtain a crosslink in 2-methylpent-2-ene. This is presumed to be a consequence of the intra-molecular sulfuration that occurs in natural rubber. There is no evidence to indicate the presence of vicinal crosslinks in the natural rubber vulcanizates. Hence in agreement with the views of other workers it is concluded that the crosslinks present in accelerated sulfur vulcanizates of natural rubber are tetrafunctional and dialkenyl. The results of the characterization of the polybutadiene vulcanizates are not fully supported by the results of the model olefin studies. In the vulcanization of polybutadiene the initially formed crosslinks are polysulfidic. As vulcanization proceeds, the chemical complexity of the network increases. After long reaction times, however, no significant amount of monosulfidic crosslinks are present in the network and very little of the reacted sulfur is present in the form of zinc sulfide. Nitrogen analyses of the polybutadiene vulcanizates showed that a substantial fraction of the accelerator, equivalent to 80–90% of the available 2-thiobenzothiazole groups, become combined in the network during vulcanization. It is proposed that the combination of accelerator with polybutadiene prevents the desulfuration of dialkenyl polysulfides to dialkenyl monosulfides (the normally observed pathway of accelerated sulfuration of natural rubber) and allows vicinal crosslinking to proceed. Some support for this proposal is that vicinal crosslinks and a substantial amount of nitrogenous product are formed during the accelerated sulfuration of cyclohexene. The findings of Gregg and Katrenick on the MBTS accelerated sulfuration of cis-cis-1,5-cyclooctadiene are also consistent with this proposal. The nitrogen analyses of the polybutadiene vulcanizates indicate that very little of the accelerator is permanently combined in the network during the initial stages of network formation. Hence by comparison with the observed pattern of sulfuration of hex-3-ene, where it was shown that negligible amounts of nitrogenous product are present, it is proposed that dialkenyl (tetrafunctional) polysulfidic crosslinks are initially introduced into the polybutadiene network. The polysulfidic crosslinks then presumably undergo desulfuration reactions leading to dialkenyl crosslinks of reduced sulfur chain length until the desulfurating agent is, in effect, removed from the system by the 2-thio-benzothiazole groups becoming combined in the network. Once most of these groups have combined, after ca. 60 min. at 140° C, the desulfuration reactions are probably less important than the reactions leading to vicinal crosslinking, and it is likely that a well cured-polybutadiene vulcanizate contains a substantial fraction of vicinal crosslinks.

Extension and compression behavior of polybutadiene networks

Extension and compression behavior of polybutadiene networks

Stress softening in styrene‐butadiene block copolymers: Rheo‐optical studies

AbstractThe stress‐strain and the optical properties of styrene/butadiene block polymers indicate that at strains below those at which finite extensibility of the polybutadiene chains become Important, the mechanical properties of the polymers are determined by the combined effects of the elastic deformation of the polybutadiene phase and the degree of interaction between the polystyrene regions. The process of stress‐softening in styrene/butadiene block polymers is due to the break‐up of polystyrene regions, rather than to flow and rupture of weak crosslinks. Some additional softening occurs in the polybutadiene phase due to loosening‐up of the polybutadiene network structure. Specimens which were softened to the same degree of internal strain of the polybutadiene phase had similar stress‐strain properties when account was taken of the filler effect of the polystyrene regions. Block polymers which were prestretched to a strain below the finite extensibility of the polybutadiene chains exhibited a stable network structure. At large strains non‐Gaussian behaviour of the polybutadiene chains, probably combined with a recoverable plastic deformation of the polystyrene regions, determined the stress‐strain properties of the block polymers. The stress‐strain and the optical studies of the original and the softened block polymers indicate that under the experimental conditions described here the physical crosslinks formed by the polystyrene regions were stable.

Transient Changes in Topology and Energy on Extension of Polybutadiene Networks

Abstract The theory of elasticity of polymer networks has been developed along two lines. The phenomenological approach leads to the Mooney-Rivlin relation between stress and extension ratio for uniaxial extension. The statistical theory of elasticity, based on a model for polymer molecules, predicts a similar relation with one of the constants zero. Actual elastic properties of rubbers do not agree fully with either theory. Experimental results are reported obtained with quantitatively cured polybutadiene and polyisoprene vulcanizates. These data are near-equilibrium results through the use of a cyclic stress sequence which largely eliminates the influence of long-time creep. The dependence of the initial modulus and the parameters of the Mooney-Rivlin relation on the chemical nature and the degree of branching of the polymer, the type of cross-links, and temperature has been investigated. A possible relation between the energy component of the elastic force and one of the parameters is discussed. These ...

Viscoelastic Relaxation of Rubber Vulcanizates Between the Glass Transition and Equilibrium

Abstract In agreement with the results of dynamic experiments of Stratton and Ferry, this study of relaxation of rubber vulcanizates entirely confirms the existence of peculiar, slow, viscoelastic processes in high polymer networks. Characteristic differences with the rheological behavior of unvulcanized polymers are best reflected by the shape of the end of the distribution functions of relaxation times. The box distribution found for free chains is replaced, for crosslinked polymers, by a long incline extending during several decades of time. The slope of this linear part of the spectrum is only slightly dependent on nature of the polymer and type of vulcanizate. On the other hand, the position of the incline along the time scale is very sensitive to the mean molecular weight Mc of the vulcanizates, by far the most important factor controlling the phenomenon. The downward deviations observed at the end of the incline also occur later for larger values of Mc. A useful step towards theoretical understanding of this behavior should be a quantitative knowledge of the effect of molecular weight in a broader range of Mc than studied here. If the chain entanglements are of primary importance, as considered probable by Ferry it seems that some singularity should occur for a critical molecular weight fitting the corresponding value for the viscosity of free chains. The role of crosslink mobility might be tested by comparing the relaxation of ordinary random vulcanizates with that of eventually more regular polybutadiene networks prepared by end group crosslinking of carboxy-terminated and mono-disperse chains. In fact, the displacement of a crosslink away from its affine position requires, apart from the Brownian fluctuations, an unbalance between the forces exerted by the four radiating chains. This implies that the lengths of the strands present large differences and that the shortest chains are approaching their limit of extensibility. As the latter condition can hardly be fullfilled at small deformations, it seems doubtful that this mechanism may be predominant either for dynamic properties or the relaxation experiments reported here. Another cause sometimes invoked is the presence of free chains attached to the networks and we are presently studying their effect on viscoelastic relaxation. At this stage, it is already apparent that they do not have a large effect, as might be expected on theoretical grounds. In our opinion, special attention should be paid to the reason why the experimentally found relaxation times are so large, in spite of the relatively short average length of the network strands. If the usual notion of entanglements developed for free chains, as an extension of the Rouse theory, should fail in this respect, it would be necessary to reconsider the non-equilibrium statistics of single chains with fixed ends, taking into account the proper inter- and intramolecular forces hindering their motion. This more direct approach to the problem, already outlined by Kirkwood, ought to express mathematically the fact that the presence of crosslinks tends to prevent longitudinal slippage of large parts of the chains. The slow changes of configuration should occur therefore rather through lateral motions to which the neighboring medium opposes a much greater resistance.

Reactions between Peroxide and Polydiolefins

Abstract Reactions of cumyl peroxide with polyisoprenes and polybutadienes are compared. Hydrogen abstraction from polyisoprenes and recombination of polymer radicals yield a network with crosslink concentration equal to that of decomposed peroxide. The concentration of crosslinks in polybutadiene networks is much greater than that of decomposed peroxide. Values of the ratio of these two concentrations as high as 50 have been calculated from swelling measurements of the vulcanizates. The polybutadienyl radical attacks double bonds in the polymer; thus crosslinks are formed by propagation. Infrared analysis shows that propagation proceeds via unsaturation in the main chain as well as via double bonds of vinyl side groups. The experimental relation between crosslink concentration and antioxidant concentration is consistent with a reaction scheme based on termination of polymer radicals by antioxidant only.

Chain entanglements and elastic behavior of polybutadiene networks

AbstractThe role of chain entanglements in determining the stress‐strain properties of polybutadiene networks has been investigated. The number of entanglements was varied by changing the primary molecular weight and chemical crosslink concentration. Networks essentially free of entanglements were prepared by endgroup coupling of carboxy terminated polybutadiene of 5500 molecular weight. Conventional sulfur or peroxide vulcanizates obeyed the Mooney‐Rivlin stress‐strain relation, the constant C2 which represents the deviation from simple kinetic theory diminishing with the time allowed for approaching elastic equilibrium. The constant C2 was found to increase with both the total physical crosslinking and the entanglement contribution to this quantity. The endgroup vulcanizates obeyed the simple kinetic theory of rubber elasticity and approached elastic equilibrium much more rapidly than sulfur or peroxide vulcanizates. The evidence presented suggests that the apparent deviation of elastomeric vulcanizates from the kinetic theory of rubber elasticity resulting in the appearance of the Mooney‐Rivlin C2 term in the stress‐strain relation arises from a slow relaxation process involving the entanglement crosslinks.

Reactions between Peroxide and Polydiolefins

Abstract Reactions of cumyl peroxide with polyisoprenes and polybutadienes are compared. Hydrogen abstraction from polyisoprenes and recombination of polymer radicals yield a network with crosslink concentration equal to that of decomposed peroxide. The concentration of crosslinks in polybutadiene networks is much greater than that of decomposed peroxide. Values of the ratio of these two concentrations as high as 50 have been calculated from swelling measurements of the vulcanizates. The polybutadienyl radical attacks double bonds in the polymer; thus crosslinks are formed by propagation. Infrared analysis shows that propagation proceeds via unsaturation in the main chain as well as via double bonds of vinyl side groups. The experimental relation between crosslink concentration and antioxidant concentration is consistent with a reaction scheme based on termination of polymer radicals by antioxidant only.

Quantitative characterization of polybutadiene networks

AbstractUsing a relationship found to be applicable for natural rubber networks in establishing contributions of chemical crosslinking, entanglements, and networks defects to physical crosslinking, a physical versus chemical crosslinking “calibration” is developed for polybutadiene prepared by organolithium initiation. This calibration is then used to determine chemical crosslink yields for sulfur‐sulfenamide, thiuram, and peroxide vulcanization. Crosslink yields for polybutadiene in the sulfur‐sulfenamide and thiuram curing systems are shown to be similar to those of natural rubber. In peroxide vulcanization the crosslink yield is in excess of unity for polybutadiene. Evidence is presented which indicates that this is due to a polymerization type reaction involving vinyl groups resulting from 1,2‐addition during polymerization.