1,4-Polybutadiene strain-induced

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 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. 

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. 

Within a single chain cis- and trans configuration can also both occur in a random sequence the chain is than irregular (some kinds of polybutadiene). This has consequences for the occurrence of strain-induced crystallization (see Chapter 4). 

With barium-containing anionic initiators [88] polybutadiene with a high trans content [89] and less then 5 % of vinyl double bonds can be synthesized. This rubber does not crystallize at room temperature but can undergo crystallization upon stretching. The properties of rubber, e. g., green strength, tackiness of the compound, as well as tensile strength of the vulcanizate, are much improved by strain-induced crystallization. 

The effects of HAF black on the stress relaxation of natural rubber vulcanizates was studied by Gent (178). In unfilled networks the relaxation rate was independent of strain up to 200% extension and then increased with the development of strain induced crystallinity. In the filled rubber the relaxation rate was greatly increased, corresponding to rates attained in the gum at much higher extensions. The results can be explained qualitatively in terms of the strain amplification effect In SBR, which does not crystallize under strain and in cis-polybutadiene, vulcanizates of which crystallize only at very high strains, the large increase in relaxation rate due to carbon black is not found (150). 

Although elastomers are usually amorphous, strain-induced crystallization occurs in rubbers such as cA-l,4-polybutadiene, butyl rubber, and NR. Crystallization under stress, discovered 200 years ago [239], increases the modulus and most failure properties of rubber and is essential to performance in many... 

Part of the effect is usually strain-induced crystallinity especially for natural rubber and cis-polybutadiene. 

Strain-induced crystallization also has a pronounced reinforcing effect within the network, and this increases the ultimate strength and maximum extensibility (26). The capability of crystallizing with extension while remaining elastomeric at low elongations provides an important self-reinforcement mode for natural rubber. The only other elastomer with these properties is cis-polybutadiene. These materials are the elastomers of choice for many heavy-duty applications. 

Fig. 13. Onset of strain-induced crystallization for polybutadienes of different cis contents.

In summary, the anomalous upturn in modulus observed for crystallizable polymers such as natural rubber and cw-1,4-polybutadiene is largely, if not entirely, due to strain-induced crystallization. In the case of the noncrystaUizable PDMS model networks it is clearly due to the limited chain extensibility, and thus the results on this system will be extremely useM for reliable evaluation of the various non-Gaussian theories of rubber-like elasticity. 

Some relevant results on the effects of strain-induced crystallization on ultimate properties have been obtained for 1,4-polybutadiene networks [112]. As has already been mentioned, the higher the temperature, the lower the extent of crystallization and, correspondingly, the lower the ultimate properties. The effects of increasing swelling parallel those for increasing temperature, since diluent also suppresses crystallization of the network. For noncrystallizable networks such as those of PDMS, however, neither change is found to be very important [118]. 

Comparisons of stress-strain measurements on such bimodal PDMS networks with those in crys-tallizable polymer networks, such as natural rubber and c 5-1,4-polybutadiene were carried out, particularly as a function of temperature and presence of a plasticizing diluent [85,91]. The results showed that the anomalous upturn in modulus observed for crystallizable polymers, such as natural rubber is largely if not entirely due to strain-induced crystallization. 

Su T-K, Mark JE. The effect of strain-induced crystallization on the elastomeric properties of c 5-1,4-polybutadiene networks. Macromolecules 1977 10 120-5. 

A strain-crystallizing material like NR shows much better autohesion. It can be processed to a relatively low viscosity for easy wetting on contact, and still exhibit green strength due to strain-induced crystallization. Several other strain-crystallizable elastomers have been synthesized and shown to exhibit autohesion and green strength comparable or superior to that of NR. These include rran.s-polypen-tenamer, fran -butadiene-piperylene elastomers, and uranium-catalyzed polybutadiene. 

The preparation of high trflns-l,4-polybutadiene ( 80%) is of current interest because it has been reported that these polymers can exhibit strain-induced crystallization analogous to natural mbber. One class of initiators that form high trans-1,4-polybutadiene contain barium salts. For example, a polybutadiene with 79% tram-1,4- and 7% vinyl microstructure (Tg = -91 °C) was prepared from an initiator from n-butyllithium with 0.5 equivalents of a barium (t-butoxide-hydroxide) salt with 9mol% hydroxide ion in toluene at 30 The trans-l,4-content decreased at higher temperamres. A more recent modification utilizes a catalyst formed from a barium(t-alkoxide-hydroxide) or a barium (t-alkoxide)2 salt with the complex of a dialkylmagnesium and a trialkylaluminum. Polybutadienes with 90% 1,4-content were prepared when the ratio of [Ba]/[Mg] was approximately 0.20 and the [Mg]/[A1] ratio was 6 for polymerizations in cyclohexane at 50 The trans-... 

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