STUDY OF SOME PROPERTIES OF TRANS-1,4-POLYISOPRENE CRYSTAL

Stirrer crystallization of a trans-1,4-polyisoprene fraction ((')M(,n) = 3 x 10('5)) was carried out from n-butyl acetate and from n-heptane solutions (2% w/v). Fibrous crystals in the (beta)-form were obtained at temperatures of 46-48(DEGREES)C and 36(DEGREES) to 40(DEGREES)C in the two solvents respectively. At 36(DEGREES) to 46(DEGREES)C from n-butyl acetate and 25(DEGREES)-35(DEGREES)C from n-heptane lamellar crystallization took place leading predominantly to the (alpha) form. Melting endotherms and densities for various samples were obtained. The maximum T(,ENDO) for (alpha) was 74(DEGREES)C and for (beta) 79(DEGREES)C and the maximum weight fraction crystallinity for (beta) was 0.78. The stability of preformed (alpha) nuclei in n-butyl acetate and n-heptane using a fraction with (')M(,n) = 2.5 x 10('5) was monitored as a function of temperature. The dissolution temperature of fibrous (beta)-TPI with the maximum T(,ENDO) was measured in thirteen liquids and the results analyzed in terms of the Flory-Huggins parameter. The heat of fusion for the (alpha) and (beta) form obtained by extrapolation of heat of fusion vs the change in specific volume, were found to be 8.0 and 10 kJ mole('-1), respectively. A (beta) to (alpha) transformation was found to occur in the presence of n-butyl acetate, heptane and amyl acetate as diluents. From experimental results 74(DEGREES)C is chosen as the temperature at which the (alpha) and (beta) forms coexist in the bulk. The pressure coefficients of the melting temperature were calculated to be 38 and 43(DEGREES)K kilobar('-1) and the fold surface free energies recalculated to be 42 (+OR-) 1 and 53 (+OR-) 1 ergs cm('-2) for the (alpha) and (beta) forms, respectively. The number of monomer units per average non-crystalline chain traverse for the most crystalline fibrous (beta)-TPI was estimated at 36. The entropy of fusion of (alpha) and (beta)-TPI were calculated to be 30 and 22 JK('-1) mole('-1), respectively. The contribution to the entropy of fusion made by the volume change on fusion were calculated to be 21 and 18 J/K mole('-1) for (alpha) and (beta)-TPI, respectively.