Polybutadiene- cross-linked polymer

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

Another class of hydrocarbon binders used in propints are the carboxy-terminated polybutadiene polymers which are cross-linked with either tris[l-(2-methyl)aziridinyl] phosphine oxide (MAPO) or combinations with phenyl bis [l -(2-methyl)aziridinyl] phosphine oxide (Phenyl MAPO). Phenyl MAPO is a difunctional counterpart of MAPO which makes possible chain extension of polymers with two carboxylic acid groups. A typical propint formulation with ballistic properties is in Table 11 (Ref 83) Another class of composites includes those using hydroxy-terminated polybutadienes cross-linked with toluene diisocyanate as binders. The following simplified equations illustrate typical reactions involved in binder formation... 

The temperature dependence of the relaxation activation energy of hydroxy-terminated polybutadiene-methyl methacrylate AB cross-linked polymer in the glass transition region is shown in Figure 4.8. The parameters C, = 8.77 °C, C2 = 85.07 °C and = 340 K were measured by means of the dynamic mechanical spectra 381 K was chosen as the reference temperature Tq. 

Nomenclature. The IPNs have a well-developed nomenclature (9), as do most multicomponent polymeric materials (10). Thus, a cross-linked polymer network is indicated by the prefix net-, and an IPN is indicated by the connective -ipn-. Thns, an IPN of polybutadiene and polystyrene is written as... 

Polyisoprene. As with polybutadiene, polyisoprene (PIP) and natural rubber are cross-linking polymers. The radical intermediates have been identified as allyl radicals (336,337,342). Radical srields at 77 K ranging from 0.28 to 0.73 have been reported. It is also clear that the mechanism of reaction of these radicals is similar to that discussed above for polybutadiene. A G-value of 6.7 for the decrease in imsaturation has been reported by Golub and Danon (347). Much later, using solid-state NMR, Whittaker (349) obtained a G(d.b.) value of 45.8 (0-1 MGy). Significantly lower yields of cross-links have been reported Boehm (350) has compiled a list of G(X) values ranging from 0.4 to 3.5 as determined by measurement of physical properties. 

Thermo-oxidative breakdown of the cross-linked polymer derived from toluene-2,4-di-isocyanate, methylene bis(4-amino-3-chloroph0iyl), and a hydroxy-terminated polybutadiene occurred, however, by a radical mechanism... 

FIG. 14-1. Logarithmic plots of G against radian frequency at 25 C for four cross-linked polymers showing the onset of the transition zone. (NR) natural rubber (PB) 1,4-polybutadiene, about 50% trans (SBR) styrene-butadiene rubber with 23.5% styrene (Butyl) butyl rubber. All cross-linked with sulfur except SBR by dicumyl peroxide. (A) Plotted directly (B) with scales shifted to make C, and to, coincide with the values for natural rubber (indicated by cross). Reproduced by permission, from Science and Technology of Rubber, edited by F. R. Eirich, Academic Press, New York, 1978. 

Interpenetrating networks of DMPPO and polymers such as polystyrene, polybutadiene, poly(urethane acrylate), and poly(methyl methacrylate) have been prepared by cross-linking solutions of DMPPO containing bromomethyl groups with ethylenediamine in the presence of the other polymer (68). 

FIGURE 1.12 Master curve of tear energy Gc versus rate R of tear propagation at Tg for three cross-linked elastomers polybutadiene (BR, Tg — —96°C) ethylene-propylene copolymer (EPR, Tg — —60°C) a high-styrene-styrene-butadiene rubber copolymer (HS-SBR, Tg — —30°C). (From Gent, A.N. and Lai, S.-M., J. Polymer Sci., Part B Polymer Phys., 32, 1543, 1994. With permission.)... 

The yield of cross-linking depends on the microstructure of polybutadiene and purity of the polymer as well as on whether it is irradiated in air or in vacuum. The cross-link yield, G(X), has been calculated to be lowest for trans and highest for vinyl isomer [339]. The introduction of styrene into the butadiene chain leads to a greater reduction in the yield of cross-linking, than the physical blends of polybutadiene and polystyrene [340]. This is due to the intra- and probably also intermolecular energy transfer from the butadiene to the styrene constituent and to the radiation stability of the latter unit. 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

A new stress-relaxation two-network method is used for a more direct measurement of the equilibrium elastic contribution of chain entangling in highly cross-linked 1,2-polybutadiene. The new method shows clearly, without the need of any theory, that the equilibrium contribution is equal to the non-equilibrium stress-relaxation modulus of the uncross-linked polymer immediately prior to cross-linking. The new method also directly confirms six of the eight assumptions required for the original two-network method. 

Butadiene and isoprene have two double bonds, and they polymerize to polymers with one double bond per monomeric unit. Hence, these polymers have a high degree of unsaturation. Natural rubber is a linear cis-polyisoprene from 1,4-addition. The corresponding trans structure is that of gutta-percha. Synthetic polybutadienes and polyisoprenes and their copolymers usually contain numerous short-chain side branches, resulting from 1,2-additions during the polymerization. Polymers and copolymers of butadiene and isoprene as well as copolymers of butadiene with styrene (GR-S or Buna-S) and copolymers of butadiene with acrylonitrile (GR-N, Buna-N or Perbunan) have been found to cross-link under irradiation. 

The other possibility is to coat the silica with a polymer of defined properties (molecular weight and distribntion) and olefin groups, e.g., polybutadiene, and cross-linked either by radiation or with a radical starter dissolved in the polymer [32]. This method is preferentially used when other carriers like titania and zirconia have to be surface modified. Polyethylenimine has been cross-linked at the snrface with pentaerythrolglycidether [41] to yield phases for protein and peptide chromatography. Polysiloxanes can be thermally bonded to the silica surface. Other technologies developed in coating fnsed silica capillaries in GC (polysiloxanes with SiH bonds) can also be applied to prepare RP for HPLC. 

The yield of cross-links depends on the microstructure and purity of the polymer as well as whether it was irradiated in air or in vacuo2 The rate of degradation was found to be essentially zero when polybutadiene or poly(butadiene-styrene) was irradiated in vacuo, but increased somewhat when irradiated in air. 

The information on physical properties of radiation cross-linking of polybutadiene rubber and butadiene copolymers was obtained in a fashion similar to that for NR, namely, by stress-strain measurements. From Table 5.6, it is evident that the dose required for a full cure of these elastomers is lower than that for natural rubber. The addition of prorads allows further reduction of the cure dose with the actual value depending on the microstructure and macrostructure of the polymer and also on the type and concentration of the compounding ingredients, such as oils, processing aids, and antioxidants in the compound. For example, solution-polymerized polybutadiene rubber usually requires lower doses than emulsion-polymerized rubber because it contains smaller amount of impurities than the latter. Since the yield of scission G(S) is relatively small, particularly when oxygen is excluded, tensile... 

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