Trans-1,4-Polybutadiene crystal

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).

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. 

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. 

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 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). 

Natta, Porri, Carbonaro and Lugli (25) have prepared copolymers of 1,3-butadiene with 1,3-pentadiene in the whole range of compositions. The properties of the copolymers, in which all butadiene and pentadiene comonomer units are in the trans-1,4 configuration, clearly show the isomorphous replacement between the two types of units. The melting point/composition data show that the copolymer melting temperatures are a regular function of composition and are always comprised between those of trans-1,4-polybutadiene modification II and trans-1,4-polypentadiene. Also the X-ray diffraction spectra of the copolymers show that the trans-1,4-pentadiene units are isomorphous with the trans-1,4-butadiene units crystallized in the crystalline modification of the latter stable at high temperatures (form II). 

Fig. 22. Scanning differential calorimeter trace of heating (top) and cooling (bottom) of a melt crystallized trans-1,4-polybutadiene (Drawn after Ref.14S), Perkin-Elmer DSC, unspecified heating and cooling rates)...

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). 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

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. 

If, however, the phenylene rings are para-oriented, the chains retain their axial symmetry and can crystallize more readily. Similarly, double bonds in trans configuration maintain the chain symmetry thus allowing for crystallite formation. This is highlighted by a comparison of the amorphous elastomeric ciy-polyisoprene (Tm = 28°C) with highly crystalline trans-polyisoprene (Tm — 74°C) which is a non-elastomeric rigid polymer, or c -l,4-polybutadiene (Tm = - 11°C) with ram-l,4-polybutadiene (Tm = 148°C). 

Fig. 4.10. Thermal analysis and proton NMR second moment curves of trans-l,4-polybutadiene. The heat capacity for the solid (glass and low temperature crystal form I) have been computed by fitting at low temperature to an approximate skeletal vibration spectrum. The gradual decrease of the second moment of the proton resonance spectrum below T comes mainly from increasing mobility in the amorphous phase, Ref. The melting curve is of an approximately 90% crystalline sample. Ref.. Compare also to Fig. 1.3...

However, the excellent cold properties of the lithium polymer can be explained on the basis of microstructure in Table II. It seems reasonable to assume that of the three possible microstructures the 1,2 structure is the least desirable for low temperature flexibility followed by the frans-1,4 structure, with the cis-1,4 structure the most desirable. A comparison of the low temperature flexibility of balata (or gutta-percha) vs. Hevea rubber would indicate a preference for the cis-1,4 structure over the trans-1,4 structure, although these natural products are polyisoprenes rather than polybutadienes. In the case of the 1,2 structure, it is generally assumed that the prevalence of this structure in sodium-catalyzed polybutadiene, or butadiene copolymers, accounts for its poor cold properties however, the occurrence of a natural or synthetic product with an entirely 1,2 structure would help to confirm this more definitely. The relative predominance of any single structure is another important consideration in the performance of a rubber at low temperatures because a polymer with a large percentage of one structure would be more likely to crystallize at a low temperature. 

The series of five polymers with particularly low entropies of melting in Fig. 5.124 show crystals that are disordered, most likely to condis crystals (see Sect. 2.5), as shown aljove for PTFE. Of special interest is the difference between cis- and trans-1,4-polybutadiene. Their entropies are shown in Fig. 2.113. 

In Sect. 2.5 a similar two-step melting was discussed for the condis state of trans-1,4-polybutadiene. The c/ -isomer shows in Fig. 2.113 complete gain of the entropy of fusion at a single melting temperature, while the trans isomer loses about 2/3 of its entropy of transition at the disordering transition. The structure of the trans isomer is close to linear, so that conformational motion about its backbone bonds can support a condis crystal sttucture with little increase in volume of the unit cell. 

The polymer was amorphous at room temperature like cis-1, 4-polybutadiene. However, at —30°C the polymer was crystalline like the all cis polymer under the same conditions. The evidence indicated, therefore, that the mixed polymer contained long blocks of cis-1,4 units separated by shorter sequences of trans-l,A units, the latter being incapable of crystallization. 

The lamellar nature of flexible polymers crystallized from dilute solution is clearly evident in the electron micrographs given in Figs. 1-5 for trans—1,4-polybutadiene,TPBD, and trans—... 

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