Polybutadiene crystallization

Suehiro, K. and Takayanagi, N. Structural studies of the high temperature form of trans-1,4-polybutadiene crystal. J. Macromol. Sci. Phys. B4, 39 (1970)... 

Figure 6. Schematic diagram of a trans- 1,4-polybutadiene crystal. The fold length is U and is approximately 3 butadiene units. The stem length Ls corresponds to 15 monomer units. The crystal thickness Lc is obtained from this value and the x-ray determined inclination angle of 114° (Schilling, F. C. Bovey, F. A. Tseng, S. Woodward, A. E. Macromolecules, 1983, 16, 808).

As indicated in Section 18.4.3, two types of polybutadiene are produced commercially, namely high ds- and cis-ltrans-, 4-polybutadiene. Although the two materials have broadly similar properties, they differ in one important respect. High ds-1,4-polybutadiene crystallizes on stretching (crystalline m.p. O C) whereas 1,4-polybutadienes with between 25 and 80% cw-content show no tendency to crystallize. Thus high ds-1,4-polybutadiene has higher tensile strength but poorer low temperature properties. 

High -cis polybutadiene has relatively high heat resistance, which is advantageous in the processing of HIPS. On the other hand, this type of polybutadiene crystallizes at about 0 °C, owing to its stereoregular structure, with the consequence that the low-temperature toughness of polystyrene, produced in this way, is reduced. 

Additional evidence for chain folding in solution-grown crystals comes from carbon-13 NMR studies of partially epoxidized 1,4-trans-polybutadiene crystals (49,50). This polymer was crystallized from dilute heptane solution and oxidized with m-chloroperbenzoic acid. This reaction is thought to epoxidize the amorphous portions present in the folds, while leaving the crystalline stem portions intact. 

Kayumova, M. A. (2007). Synthesis and Properties of the Oxygen-and Aryl-containing Derivatives of 1,2-Syndiotactic Polybutadiene Crystal, Ph.D, Chem. Science-Ufa, Bashkir State University, 115p. 

Several studies have concerned the microstnicture of lamellae in materials such as the block copolymers polystyrene-h/oc/r-poly-l-vinylpyridine [139] and polystyrene-h/oc/r-polybutadiene [140], as well as single crystals of poly-para-xylylene [139], and reveal features (such as intersecting lamellae (figure Bl.19.29)) that had not been previously observed. 

Processings and Properties. Polybutadiene is compounded similarly to SBR and vulcanised with sulfur. The high cis-1,4 type crystallizes poorly on stretching so it is not suitable as a "gum" stock but requires carbon black reinforcement. It is generally used for automotive tires in mixtures with SBR and natural mbber. Its low T (—OS " C) makes it an excellent choice for low temperature tire traction, and also leads to a high resilience (better than natural mbber) which ia turn results ia a lower heat build-up. Furthermore, the high i j -polybutadiene also has a high abrasion resistance, a plus for better tire tread wear. 

The polyols used are of three types polyether, polyester, and polybutadiene. The polyether diols range from 400 to about 10,000 g/mol. The most common polyethers are based on ethylene oxide, propylene oxide, and tetrahydrofuran or their copolymers. The ether link provides low temperature flexibility and low viscosity. Ethylene oxide is the most hydrophilic and thus can increase the rate of ingress of water and consequently the cure rate. However, it will crystallize slowly above about 600 g/mol. Propylene oxide is hydrophobic due to hindered access to the ether link, but still provides high permeability to small molecules like water. Tetrahydrofuran is between these two in hydrophobicity, but somewhat more expensive. Propylene oxide based diols are the most common. 

Polyester diols are often combined with polyether diols to provide green strength through crystallization or elevated r . Most prevalent and least expensive is hexamethylene diol adipate (HDA) with a Tm of about 60°C. A variety of polyesters are available with various levels of crystallinity — from wax-like to amorphous — and crystallization rate, and with values ranging well below 0°C to above room temperature. Polybutadiene diols are the most expensive and most hydrophobic. They provide low surface tension and thus good wet out of non-polar surfaces. 

T.L. Boggs et al, AIAA J 8 (2), 370-72 (1970) CA 72, 113371 (1970) Scanning electron microscopy is used to study the surface structure of solid proplnts, prepd from AP (1) and polyurethane or caiboxylated polybutadiene. Polyurethane proplnts are self-extinguish-ing at high pressure due to the flow of molten binder over I crystals. I crystals formed a thin surface melt with gas liberation in the molten phase... 

Unlike polybutadiene, polyisoprene prepared at low temperatures shows little or no inclination to crystallize either on stretching or cooling. This may seem surprising in view of the even greater preponderance of trans-1 4 units in polyisoprene than in poly butadiene. The explanation for the contrasting behavior in this respect between low temperature synthetic polyisoprene, on the one hand, and guttapercha and low temperature polybutadiene, on the other, probably is to be found in the appreciable occurrence of head-to-head and tail-to-tail sequences of 1,4 units of the former. 

The properties of elastomers are much improved by strain-induced crystallization, which occurs only in polymers with high stereoregularity. The polymerization of butadiene using completely soluble catalysts composed of a) rare earth carboxylates, b) Lewis acids and c) aluminum alkyls leads to polymers with up to 99 % cis-1,4 configuration. These polymers show more strain-induced crystallization than the commercial polybutadienes and consequently their processability is much improved. 

The rate of crystallization of polybutadiene depends mainly on the cis content and therefore on the catalyst system. As far as commercial catalysts are concerned it increases in the order Li < Ti < Co < Ni. High-cis SE-BR, like U-BR, crystallizes more rapidly than all the other types (Table III). However, SE-BR with 93 % cis-1,4 content, and even one with only 90 % cis-1,4 content, crystallizes more rapidly than Ti-BR (93 % cis-1,4 content). In our opinion the reason for this anomaly is a structural disorder in molecules with different chain length. 

Preparation and Properties of Barium Salt. The catalyst used to prepare this new class of crystallizing polybutadienes consists of a barium t-butoxide-hydroxide salt in combination with an organolithium ( 8, 9, 10). Rather specific preparative techniques must be used in forming this barium salt, as shown in Figure 1. The use of an amine solvent provided quantitative conversion of the metal to barium salts. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

In observing the time dependent changes in birefringence and stress-optical coefficient, for elongated samples at 25 C, it was found that the rate of crystallization of high trans SBR s was very much faster, some 10 times more rapid, than that for NR (8). This is consistent with the reported rates of isothermal crystallization for NR (2.5 hours at -26°C) and for 807. trans-1,4 polybutadiene (0.3 hours at -3°C) in the relaxed state (12). 

The main conclusions of the strain induced crystallization behavior of high trans polybutadiene based rubber and natural rubber are (1) the rate of crystallization is extremely rapid compared to that of NR (2) the amount of strain induced crystallization is small compared to that of NR, especially at room temperature and (3) for the high trans SBR s relative to NR, crystallization is more sensitive to temperature at low extension ratios, and crystallization is less sensitive to strain. 

Morphology. Observations with the light microscope, under polarized light, showed that the end blocks in the case of both types of polymers crystallized in the form of the usual spheru-lites, but not as well as the analogous homopolymer, H2-l,4-polybutadiene. The formation of the spherulites was improved with increasing end-block content and/or higher molecular weight of the end blocks. 

Moisture embrittlement must be distinguished from other forms of embrittlement. Brittleness of propellants at low temperatures is normally caused by binders which stiffen excessively with decreasing temperatures, owing to partial crystallization of the binder matrix (e.g.9 copolymers of butadiene with acrylonitrile, some hydrogenated polybutadienes, etc.). Such propellants are brittle at low temperatures whether they have been exposed to moisture or not. 

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