Trans-polyisoprene crystallization

Cis-polyisoprene crytallizes partially between 0° and 35 °C and the glass transition temperature Tf of the amorphous fraction corresponds to - 35 °C. On the other hand, trans-polyisoprene crystallizes in two forms, the a-form which melts at 65 °C and the metastable /J-form, obtained by quick cooling down, which melts at 56 °C. 

In contrast to the spin-lattice relaxation parameters, which remain invariant, a sijbstantial broadening of the resonant lines occurs upon crystallization. The effect is relatively modest for cis polyisoprene at 0°C and 57.9 MHz, where comparison can be made at the same temperature. Here there is about a 50% increase in the linewidths upon the development of 30% crystallinity. Schaefer (13) reports approximately 3- to 5-fold broader lines (but they are still relatively narrow) for the crystalline trans polyisoprene relative to the completely amorphous cis polyisoprene at 40°C and 22.5 MHz. It is interesting to note in this connection that for carbon black filled cis polyisoprene the line-widths are greater by factors of 5-10 relative to the unfilled polymer. 

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). 

A typical optical micrograph for trans-1,4-polyisoprene crystallized from the melt is given in Fig. 10 where a spherulite morphology is observed.Spherulite structures can also be obtained by solvent evaporation from a thin layer of a dilute trans 1,4 polyisoprene solution, as seen in Fig. 11. 

The bromination of trans-l,4-polyisoprene crystals in CCl suspension at 0 C has received some study.In that work the disappearance of bromine was monitored with a correction made for polymer solubility. Under the conditions used the total amount of bromine consumed becomes essentially constant after 1-3 hours. from surface bromination exceeds that from epoxidation ([MCPBA] =. 011 M) for all preparations used and therefore it appears that substitution as well as addition might be occurring during the... 

It was known from early X-ray diffraction work that polymers never crystallize to 100%. The prevailing view of polymer crystals was that they were fringed micelles (Fig. 7.10). The modern view of chain folding was first introduced by Storks (1938). Storks concluded that the chains of semicrystalline trans-(polyisoprene) had to fold back and forth. This proposal went by largely unnoticed by the scientific community. Three papers were independently published by Keller (1957), Till (1957) and Fischer (1957) reporting that single crystals were 10 nm thick... 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

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. 

Tacticity and geometric isomerism affect the tendency toward crystallization the tendency increases as the tacticity (stereoregularity) is increased and when the geometric isomers are predominantly trans. Thus isotactic PS is crystalline, whereas atactic PS is largely amorphous and c/s-polyisoprene is amorphous, whereas the more easily packed trans isomer is crystalline. 

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. 

Gutta percha consists primarily of the polymer of trans-l,4-isoprene (Fig. 10.2). It is a structural isomer of natural rubber, which is CM-l,4-polyisoprene, and the structure of gutta percha means that it has greater linearity and crystallizes more readily than natural rubber. These features mean that gutta percha is harder, less tough and more brittle than its structural isomer [11],... 

Figure 10 - Trans-1,4-polyisoprene (purified, unfractionated) crystallized from the melt at 55 C for two weeks.

Figure 11 - Trans-1,4-polyisoprene (Mq = 2.5x10 ) crystallized from a thin film cast from amyl acetate at 32 C. ...

Pure tra i-l,4-polyisoprenes as well as 1,4-polybutadienes can be synthesized by polymerization in inclusion compounds [266-269]. As typical hosts for this dienes, the inclusion compounds or clathrates of urea, thiourea, or perhydrotriphenylene [PHTP Eq. (36)] are used [270,271]. The host forms the frame of the crystal and the guest is placed in the cavities existing in the lattice. Polymerization is generally started by subjecting the inclusion compound to irradiation with a-, y-, or x-rays and proceeds by a radical mechanism [272,273]. Also, free radical initiators such as di-/cr/-butylperoxide could be used [274]. Inclusion in urea yields crystalline trans-, A polymers, whereas trans-lA-polyisoprene obtained in PHTP is amorphous. There is no trace of, A-cis units or of 1,2, 3,4, and cyclic units. The reason for the amorphous product is the presence of a substantial number of head-to-head and tail-to-tail junctions in addition to head-to-jail junctions [275, 276]. 

Polymers of some of the higher 2-alky 1-1,3-butadienes give vulcanizates with tensile strength and elasticity comparable to that of natural rubber. Poly(2-ethylbutadiene) and poly(2-phenylbutadiene) are most important. 2-Ethyl-1,3-butadiene can be polymerized in the same way as isoprene [333,334]. The polymer has a glass transition temperature of —76°C [335]. A polymer rich in trans-, A structures is obtained by catalysis with vanadium(III) chloride/triisobuthylaluminum. In contrast to tmw -l,4-polyisoprene, the product can be used as rubber, due to its reduced tendency to crystallization [336, 337]. 

Boochathum and co-workers [29] applied ozonisation-GPC to a study of the structure of solution-grown trans 1,4-polyisoprene (TPI) crystals. 

The cis-1,4-polyisoprene has very low crystallinity, low Tm ( 28°C) and Tg (-73 C) values, and is an excellent elastomer over a temperature range including room temperature. More than two billion pounds per year of cis-1,4-polyisoprene are used in the U.S. for tires, coated fabrics, molded articles, adhesives, rubber bands, and other elastomeric applications. Trans-1,4-polyisoprene known in commerce as gutta percha is harder than natural rubber since it can crystallize to a greater degree due to symmetry and has relatively high T (74 C) and Tg (-58°C) values. 

The commercial polydienes are elastomers. Q s-1,4 polybutadiene has a Tg of -100 °C and has a crystalline melting point of less than 0 °C. Q s-1,4 polyisoprene has a Tg of -70 °C and has a crystalline melting point of 35 C. Both polymers crystallize rather slowly. Trans-1,4 polybutadiene and polyisoprenes are crystalline thermoplastics at room temperature. They are not, however, used commercially because of their poor aging characteristics relative to polyolefins. This is associated with the double bonds in their backbones. Polybutadienes with high atactic 1,2-contents have been widely used in the tire industry. Their Tg is about -15 °C. Isotactic and syndiotactic 1,2-polybutadienes are high melting crystalline thermoplastics, but age poorly compared to polyolefins. The 1,2-polybutadienes have been used as packaging for additives in the rubber industry. 

They are also natural polyisoprenes and are isomers of natural rubber. They are extracted from sheets of plants in the family of sapotaceae (Southeast Asia and Equatorial America). The polymer chains are constructed from repeating units of 1,4-trans isoprene structure. Contrary to natural rubber, they partially crystallize spontaneously according to two conformations one is 2i helical (a-form), the other one is totally transplanar and is equivalent to a li helix ( -form). The mechanical properties of these materials are determined by the values of the transition temperatures. Amorphous zones undergo a glass transition at —60 C and the two crystalline forms (a and P) melt at 64°C. The corresponding polymers are thus thermoplastics and acquire (after vulcanization) a highly elastic behavior beyond 70°C. 

TEM (Fig. 5.1a). The width can be several micrometers and the thickness only a few hundred angstroms. The electron diffraction pattern shows that only the hkO reflections are detected in the case of trigonal POM, which indicates that the chain stems stand up in the direction normal to the crystal surface. Stokes is the first to indicate the possibility of chain folding by measuring the electron diffraction patterns of unoriented trans-l,A-polyisoprene, he found that the chain axis is normal to the film plane [9]. Later, Fischer, Till, and Keller made direct observations of polymer single crystals with a chain-folded structure. The concept of chain folding originates from this experimental evidence [10]. 

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