Polybutadiene models

Ciolino, A. Villar, M. A. Valles, E. Hadjichristidis, N., Synthesis and Characterization of Model Polybutadiene-l,4-b-Polydimethylsiloxane-b-Polybutadiene-l,4 Copolymers. J. Polym. Sci., Part A Polym. Chem. 2007,45,1579-1590. 

We have recently extended the Flory model to deal with nonpolar, two-solvent, one polymer soltulons (13). We considered sorption of benzene and cyclohexane by polybutadiene. As mentioned earlier, a binary Interaction parameter Is required for each pair of components In the solution. In this Instance, we required Interaction parameters to represent the Interactions benzene/cyclohexane, benzene/polybutadlene, and cyclohexane/ polybutadiene. 

All values are with respect to that of reference polymer hydrogenated polybutadiene (HPB97), model ethylene-butene copolymer, x wt% butene. 

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques. By comparing the infrared absorption spectra of polybutadiene and of the olefins chosen as models whose ethylenic structures correspond to the respective structural units, it has been possible to show that the bands occurring at 910.5, 966.5, and 724 cm. are characteristic of the 1,2, the mns-1,4, and the m-1,4 units, respectively. Moreover, the proportion of each unit may be determined within 1 or 2 percent from measurements of the absorption intensity in each band. The extinction coefficients characteristic of each structure must, of course, be known these may be assigned from intensity measurements on model compounds. Since the proportions of the various units depend on the rates of competitive reactions, their percentages may be expected to vary with the polymerization temperature. The 1,2 unit occurs to the extent of 18 to 22 percent of the total, almost independent of the temperature, in free-radical-polymerized (emulsion or mass) poly butadiene. The ratio of trans-1,4 to cfs-1,4, however,... 

The data of Table II indicate that the etch rates for CB and its "homologues"—TP, CO (or TO), and EPM—tend to increase monotonically with a decrease in vinylene (-CH=CH-) unsaturation. The elastomeric EPM was chosen instead of crystalline polyethylene as a model for the fully saturated CB to avoid a morphology factor in etch rates, as was observed with crystalline TB. The difference in etch rates for the partially crystalline TO and the elastomeric CO (ratio of about 1.2 1.0) is attributable more to a morphology difference between these polyoctenamers than to the difference in their cis/trans content. Cis/trans content had likewise no perceptible effect on etch rates in the vinyl-containing polybutadienes (see Table I) if there was a small effect, it was certainly masked by the dominant effect of the vinyl groups. 

By analogy to simple olefins, we propose that 0(3P) initially adds to the 1,4 or 1,2 double bonds in polybutadienes at ambient temperature. Since the rate constants for 0(3P) addition to cis-2-butene and 1-butene (as models for 1,4 and 1,2 double bonds, respectively) are in the ratio 4.2 1 at 298 K ( 6), preferential addition to the 1,4 double bonds is assumed to persist to very high vinyl contents (-8011). The biradical adducts then rearrange to epoxides and carbonyl compounds or give rise to chain rupture and/or crosslinking as a consequence of PIF, according to the scheme ... 

This most simple model for the relaxation time spectrum of materials near the liquid-solid transition is good for relating critical exponents (see Eq. 1-9), but it cannot be considered quantitatively correct. A detailed study of the evolution of the relaxation time spectrum from liquid to solid state is in progress [70], Preliminary results on vulcanizing polybutadienes indicate that the relaxation spectrum near the gel point is more complex than the simple spectrum presented in Eq. 3-6. In particular, the relation exponent n is not independent of the extent of reaction but decreases with increasing p. 

Two types of networks were prepared (i) randomly crosslinked polybutadiene, and (ii) model urethane networks, (a) polybutadiene based, and (b) poly(e-caprolactone) based. The randomly crosslinked networks were prepared from polybutadiene (Duragen 1203 obtained from General Tire and Rubber Co.) crosslinked with di-cumyl peroxide. Specifications of the as obtained polybutadiene are given in Table I. Polybutadiene was purified by dissolving in benzene and precipitating in methanol. Precipitated polybutadiene was redissolved in benzene. Seven different weights of dicumyl... 

Model networks were prepared using hydroxyl terminated polymer and isocyanates, (a) Bifunctional hydroxyl terminated polybutadiene (Butarez, from Phillips Petroleum) was crosslinkined with tris (p-isocyanatophenyl)-thiophosphate (Desmodur RF, from Mobay Chemical Co.). This crosslinked... 

Based on these results and the reaction mode of phenylazide with unsaturated olefine monomers as the model compound of the polymer, mechanisms [i] - [ill] are proposed for the crosslinking of 1,2-polybutadiene by bisazide. 

A detailed scrutiny of the Gaussian approximation (Eq. 2.7) reveals that for KTr deviations occur. This was studied later in more detail for the case of polybutadiene (PB) [55]. These simulations demonstrated that the deviations from the Gaussian approximation relate to intermolecular correlations that are not included in any of the analytical models at hand. 

For molecules at a degree of polymerization n or larger, the mathematical model incorporates branch formation reactions which include a free radical of size j and a polymeric specie of degree of polymerization m n. The consequence is the formation of a free radical of molecular size j + m. Furthermore, due to the relatively high concentration initially of the 1,2-polybutadiene constituent at j = n, the derivation assumes that all polymeric species of size j n are unsaturated and are capable of branch and/or crosslink formation. Polymeric species are denoted by Pj free radical intermediates are described by Aj. Therefore, the first activated intermediate capable of formation by branching reactions is Ajj via Aq + Pj, -> Ajj. Conservation laws yield... 

Figure 5. Modulus-composition curves for crass-polybutadiene-inier-cross-polystyrene semi-I and full IPNs (16). (a) Kerner equation (upper bound) (b) Budiansky model (c) Davies equation and (d) Kerner equation (lower bound). (Reproduced from ref. 23. Copyright 1981 American Chemical Society.)...

Curing Agents for Carboxyl-Terminated Polybutadiene Prepolymers. The types of curing agents used to prepare binders for CTPB propellants are the same as those for PBAN or PBAA. The bifunctionality of CTPB, however, requires that part of the curing agents be polyfunctional to provide for the formation of the tridimensional network. Almost without exception, the polyfunctional aziridines and epoxides used with CTPB undergo side reactions in the presence of ammonium perchlorate, which affects the binder network formation. Kinetic studies conducted with model compounds have established the nature and extent of the cure interference by these side reactions. The types and properties of some of the crosslinkers and chain extenders used to prepare solid propellants are summarized in Table IV. 

Charge distribution on butadiene one unit model and % 1,2 structure in polybutadiene. 19... 

Monte-Carlo simulations are applied to estimate the characteristic ratios and p parameters from the RIS models for PE, POM, polybutadiene, and polyisoprene. Here the p parameter is defined as the ratio of the radius of gyration to the hydrodynamic radius. The p parameters of these real chains in the unperturbed state show only a slight dependence on the microconformation in the limit of large molecular weights and are found close to 1.504, which is the value for an idealized Gaussian chain. The estimated p parameters of the real chains appear to be correlated to the chain stiffness and increase with the characteristic ratios. 

The characteristic ratios of stereoirregular 1,4-polybutadiene and 1,4-polyisoprene chains are theoretically Investigated by the Monte Carlo procedure in accordance with the model proposed by Mark (V 002 and V 004). It Is pointed out that the presence of discrete cis units in trans-rich chains significantly reduces the characteristic ratio while that of discrete tram units in cis-rich chains has little effect on the characteristic ratio. The characteristic ratio and its dependence on both the trans and cis contents and their sequence distribution is calculated for stereoirregular polymers in accordance with the interdependent RIS model proposed by Mark (V 002 and V 004), and tsbikawa and Nagai (V 006 and V 008). 

The energy difference AF between gauche and trans conformers, resulting from rotation about single bonds of the chain backbone in trans-1,4-polybutadiene, c/s-1,4-polybutadiene and 1,5-hexadiene, is evaluated from IR measurements on the bands characteristic of bending vibration of CH2 groups. Consideration of the experimental results, on the basis of the RIS model, leads to the conclusion that a value of 0.4 to 0.8 kJ mol-1 is the best estimate of AF. 

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