Crystallinity and crystalline phase orientation of poly(1,4-cis-isoprene) from Hevea brasiliensis and Taraxacum kok-saghyz

Crystallization is studied for poly(isoprene-1,4-cis) from Hevea brasiliensis (natural rubber [NR]) and from taraxacum kok-saghyz, mainly by collecting wide-angle X-ray diffraction patterns after processing and stretching. Although rubber samples before stretching are generally fully amorphous, crystallization can be induced in NR samples by processing at room temperature under moderate pressure. This phenomenon is possibly associated with nucleation by saturated fatty acid components. For rubber samples being fully amorphous in the undeformed state, strain-induced crystallization occurs only at high strain ratios (α>4), leading to high degrees of crystalline phase orientation (fc>0.9 for α=5). Rubber samples presenting some crystallinity already in the unstretched state, on the contrary, reach much lower degrees of axial orientation, even for high strain ratios (fc<0.7 for α=5). These differences in crystallinity and in crystalline phase orientations produce large differences in stress-strain behavior of the rubber. By room temperature processing, the considered NR samples can also develop an unreported disordered crystalline modification, with low intensity of 120 and 121 reflections. This disordered crystalline modification, which is also maintained after axial stretching procedures, can rationalized by a structural disorder along the b axis, possibly associated with statistical sequences of A+TA- or A-T A+ conformations for poly(isoprene-1,4-cis) chains.