Polyisoprenes 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. 

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... 

Vulcanization converts the material from a high viscosity liquid to an amorphous solid, suppressing creep and flow, and enhancing long-range rubber elasticity. Because cis-polyisoprenes crystallize on extension, they form extremely tough, self-reinforcing materials. Before the advent of synthetic rubber such as SBR, natural rubber was the only material for automobile tires. Nowadays, it remains the material of preference for heavy-duty tires such as are used by airplanes and trucks. 

Figure 7.2 compares the DSC thermogram of the transA,4 polyisoprene crystal grown from hexane to that grown from amyl acetate. The appearance of a single peak in DSC together with the X-ray evidence indicates the crystallisation of r tn5-l,4-polyisoprene... 

Figure 7.2 DSC thermograms of a-trd s-polyisoprene crystals crystallised isothermally from hexane and amyl acetate at -20 °C. Scanning rate 10 °C/min.

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). 

Very frequent are the cases of stress-induced crystallizations. A typical case is that of slightly vulcanized natural rubber (1,4-m-polyisoprene) which, under tension producing a sufficient chain orientation, is able to crystallize, while it reverts to its original amorphous phase by relaxation [75],... 

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. 

How much of a crystallizable material X can I blend uniformly into a polymer until it starts to form crystals A series of blends with increasing amount of X is prepared. The samples are studied by WAXS (cf. Sect. 8.2) using laboratory equipment. Crystalline reflections of X are observed, as X starts to crystallize. Peak areas can be plotted vs. the known concentration in order to determine the saturation limit. Think of X being Ibuprofen and Y a polystyrene-(7 )-polyisoprene copolymer, and you have an anti-rheumatism plaster. 

The major results described could be partially anticipated from those previously reported for linear polyethylene (17) as well as those for cis polyisoprene. (] ) For the latter polymer, by taking advantage of its crystallization kinetic characteristics, it was possible to compare the relaxation parameters of the completely amorphous and partially crystalline polymer (31% crystallinity) at the same temperature, 0°C. This is a unique situation and allows for some unequivocal comparisons. It was definitively observed that for all the carbons of cis polyisoprene the T] s did not change with crystallization. 

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. 

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. 

Meanwhile, development of coordination catalyst was proceeding full scale. The polyisoprene prepared using this coordination catalyst (TiClj, AIR ) proved to be more suitable in physical properties than the one made by lithium metal or organolithium compounds in hydrocarbon media. The Ziegler polyisoprene, as it was called, has greater stereoregularity and stress-induced crystallization properties than polyisoprene made by the alkyl lithium catalyst. How-... 

The block copolymers shown in both Table V and VI were hydrogenated. The B-lU block produced polyethylene and the polyisoprene block produced ethylene propylene alternating copolymer. The physical properties of this copolymer, composed of crystalline polyethylene block and a soft elastomeric segment made of an EPR block, is tabulated in Table VII. The data in this table illustrate the fact that a diblock of hydrogenated polybutadi ene-polyisoprene gave excellent physical properties. This is a further illustration of the new concept of soft chain interpenetrating the crystalli zable polyethylene chain via chain folding. 

Andrews76 gave results of the work of Reed and Martin on cis-polyisoprene specimens crystallized from a strained cross linked melt and on solid state polymerized poly-oxymethylene respectively, explaining the results by simple two phase models. He also summarized the studies of Patel and Philips775 on spherulitic polyethylene which showed that the Young s modulus increased as a function of crystallite radius by a factor of 3 up to a radius of about 13 n and then decreased on further increasing spherulite size. 

Attempts to structurally characterize the generated Ln(III)/Al hetero-bimetallic complexes were not successful. Addition of coordinating solvents, such as THF or pyridine, afforded complex mixtures from which only lanthanide(III) chloride donor-adducts crystallized. The presence of reactive ethyl groups was confirmed by the reaction with D20 (generating CH3CH2D) [134], The catalytic relevance of such heterobimetallic complexes was confirmed by the quantitative conversion of a 7500-fold excess of iso-prene into polyisoprene within 5-10 minutes at ambient temperature, after the addition of one equivalent of i-Bu3Al cocatalyst. 

Significantly different seemed intiaUy the crystal morphology of polyethylene, polybutene-1, polypropylene, polystyrene, poly(4-methyl pen-tene-1), and polyisoprene polymerized with varying solvents and at varying temperatures (114, 123). Discrete hollow particles with a fibrous texture could be observed. The fibrils had an appearance similar to polyethylene crystallized from solution sheared by rapid stirring (118). A closer analysis of this similarity was carried out by Wikjord and Manley (124), Keller and Willmouth (117), and Ingram and Schindler (125) for polyethylene. 

Crystals of tris(o-phenylenedioxyde)cyclotriphosphazene (97) can act as hosts for the inclusion of a number of organic polymers, e.g. cis-1,4-poly butadiene, 1,4-polyisoprene, polyethylene (PE), poly(ethylene oxide) (PEO) and polytetrahydrofuran. X-ray studies of the PE and PEO inclusion compounds show that the polymer chains are extended along the tunnel-like voids of the host lattice. The formation of clathrates appears to be limited by the tunnel dimension of the host crystal lattice. The melting points of the inclusion adducts appear to be higher than those of either the pure host or the pure... 

Finite chain extensibility is the major reason for strain hardening at high elongations (Fig. 7.8). Another source of hardening in some networks is stress-induced crystallization. For example, vulcanized natural rubber (cw-polyisoprene) does not crystallize in the unstretched state at room temperature, but crystallizes rapidly when stretched by a factor of 3 or more. The extent of crystallization increases as the network is stretched more. The amorphous state is fully recovered when the stress is removed. Since the crystals invariably have larger modulus than the surrounding... 

---