Polybutadiene, diffusion

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Fig. 3.28 Melt self-diffusion data for hydrogenated (or deuterated) polybutadiene samples adjusted to 175 °C as a function of molecular weight [82] [83], S [84],H[85],S [86], [87]. (Reprinted with permission from [82]. Copyright 1999 The American Physical Society)...

The polymeric hydrocarbon also acts as a binder of the particles, holding them together so as to formulate a propellant grain. Ammonium perchlorate (AP) is a typical crystalline oxidizer and hydroxy-terminated polybutadiene (HTPB) is a typical polymeric fuel. When AP and HTPB are decomposed thermally on the propellant surface, oxidizer and fuel gases are produced, which diffuse into each other and react to produce high-temperature combustion gases. 

Fig. 7. Self-diffusion of linear (open symbols) and three-armed star (filled symbols) polystyrenes (squares) and polybutadienes (circles) in CC14 extrapolated to infinite dilution, as function of polymer molecular weight (Ref. 53>, with permission).

The first PGSE investigation of a rubber-based ternary solution was described by Ferguson and von Meerwall31), who measured diffusion of C6F6(19F NMR) and n-paraffin (n-dodecane or n-hexatriacontane 1H NMR) in a commercial polybutadiene as function of both concentrations. They showed that both concentration dependences in the ternary region can be derived from the measured diffusivity of each diluent i = 1, 2 in binary solution in the rubber. To do this it was necessary to extend the Fujita-Doolittle expression, as follows ... 

Fig. 10. Diffusion of 1,3 dimethyladamantane in polybutadiene as function of concentration, at four temperatures. Lines are single tit of Eq. (20) with Eq. (9) to data (Ref.M>, with permission).

The spin-lattice relaxation process is usually exponential. Theoretically, the effect of spin-diffusion, characterized by the coefficient D (order of 1(T12 cm2 s 1), has an influence on T, relaxation times when ix > L2/D, where Lis the diffusion path length. NMR studies of model systems f6r rubber networks, based on a styrene-butadiene-styrene block copolymer (SBSy, in which styrene blocks act as a crosslink for polybutadiene rubber segments of known and uniform length, indicate that spin diffusion operating between PS and PB phases causes a lowering of Tg for the PS component in SBS (as compared to the pure PS) and hindering of the motion of the PB component (as compared to the pure PB)51). 

Fig. 6. Proton Tj relaxation data for crosslinked polybutadiene samples with average number of repeat units per network chain 44 (A) and 14 (O), compared with the computed results from the modified BPP equation (assuming Gaussian distribution and the model based bn spin diffusion to locations of rapid spin-lattice relaxation) (reprinted from Ref.541 with permission)...

The effect of curing on the diffusion of polymer and the curing agent is studied for the system of hydroxyl-terminated polybutadiene (R-45-M)/isophorone disso-cyanate (IPDI). Both components contribute to the echo intensity and the plot of In P(x)/I(0)] vs (G5)2 (A — 5/3) consists of two exponentials (Eq. (22)) the fast component (the steep intial slope) is attributed to the IPDI, and the long component to the R-45-M. The dependence of both diffusion constants on the curing time is shown in Fig. 19. The accuracy for Dfast data is less pronounced than for the polymer D(Mn), because only the first few data points are relevant for its determination. Furthermore, the low tail of the R-45-M molecular weight distribution nearly coin-... 

Thermal Oxidative Stability. ABS undergoes autoxidahon with the polybutadiene component moie sensitive, to theimal oxidation than tire styrene-acrylonitrile component. Antioxidants substantially improve oxidative stability. Studies on the oven aging of molded parts have shown that oxidation is limited to the outer surface (<0.2 mm), i.e., the oxidation process is diffusion limited. 

Recently ICLwas used to follow the diffusion-limited oxidation of hydroxyl terminated polybutadiene at various temperatures and oxygen pressures. The CL intensity profiles were found to correlate well with theoretical oxygen concentration profiles [137]. 

Fleisher G, Appel M (1995) Chain length and temperature dependence of the self-diffusion of polyisoprene and polybutadiene in the melt. Macromolecules 28(21) 7281-7283 Flory PJ (1953) Principles of polymer chemistry. Cornell Univ. Press, New York Flory PJ (1969) Statistics of chain molecules. Interscience, New York... 

FIG. 18.3 Activation energy of diffusion as a function of Tg for 21 different polymers from low to high temperatures, ( ) odd numbers (O) even numbers 1. Silicone rubber 2. Butadiene rubber 3. Hydropol (hydrogenated polybutadiene = amorphous polyethylene) 4. Styrene/butadiene rubber 5. Natural rubber 6. Butadiene/acrylonitrile rubber (80/20) 7. Butyl rubber 8. Ethylene/propylene rubber 9. Chloro-prene rubber (neoprene) 10. Poly(oxy methylene) 11. Butadiene/acrylonitrile rubber (60/40) 12. Polypropylene 13. Methyl rubber 14. Poly(viny[ acetate) 15. Nylon-11 16. Poly(ethyl methacrylate) 17. Polyethylene terephthalate) 18. Poly(vinyl chloride) 19. Polystyrene 20. Poly (bisphenol A carbonate) 21. Poly(2,6 dimethyl-p.phenylene oxide). 

Th-FFF can be applied to almost all kinds of synthetic polymers, like polystyrene, polyolefins, polybutadiene, poly(methyl methacrylate), polyisoprene, polysulfone, polycarbonate, nitrocelluloses and even block copolymers [114,194,220]. For some polymers like polyolefins, with a small thermal diffusion coefficient, high temperature Th-FFF has to be applied [221]. Similarly, hydrophilic polymers in water are rarely characterized by Th-FFF, due to the lack of a significant thermal diffusion (exceptions so far poly(ethylene oxide), poly(vi-nyl pyrrolidone) and poly(styrene sulfonate)) [222]. Thus Th-FFF has evolved as a technique for separating synthetic polymers in organic solvents [194]. More recently, both aqueous and non-aqueous particle suspensions, along with mixtures of polymers and particles, have been shown to be separable [215]. 

Colloid characterization is not the classical application of Th-FFF. Nevertheless, Th-FFF was first applied to silica particles suspended in toluene testing a correlation between thermal diffusion and thermal conductivity [397]. Although a weak retention was achieved, no further studies were carried out until the work of Liu and Giddings [398] who fractionated polystyrene latex beads ranging from 90 to 430 nm in acetonitrile applying a low AT of only 17 K. More recently, polystyrene and polybutadiene latexes with particle sizes between 50 pm and 10 pm were also fractionated in aqueous suspensions despite the weak thermal diffusion [215] (see Fig. 30). Th-FFF is also sensitive to the surface composition of colloids (see the work on block copolymer micelles), recent effort in this area has been devoted to analyzing surfaces of colloidal particles [399,400]. 

Since the pioneering work of Stejskal, the pulse field gradient method is currently used to characterize the diffusion process of small molecules or of macromolecules in dilute or semi-dilute solutions [18-20]. In this Chapter, the NMR approach is illustrated from the self-diffiision of ( dohexane molecules through polybutadiene. Variations of the Ds self-diffusion coefficient of cyclohexane in polybutadiene have been reported as a temperature function considering several concentrations [21]. 

There is an interesting feature about the limiting value of the trace diffusivity of solvent determined from experimental curves when the solvent concentration is set equal to zero. The diffusion coefficient Ds(0,T) is found to obey the free volume law of the pure polymer when the above numerical values are assigned to the parameters. Furthermore, the Cs(0/T) translational friction coefficient, derived from the ratio kT/Dg(0,T), is equal to (1.64 0,06) x lO- N.s.m-i, at 298 K this value is in good agreement with the result of viscoelastic measurements performed on polybutadiene (1.8 x 10 N.s.m ). 

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