Primary Thickening in Nascent Single Crystals of High-Molecular-Weight trans-1,4-Polybutadiene

Fine-tuning the polymerization conditions, in particular, solvent medium, catalytic system, and thermodynamic parameters, it is feasible to crystallize the growing polymer chain directly into single crystals. Using these basic concepts, the resultant polymer, having a trans-isomer of ∼99% and a molecular weight of ∼500 kg/mol, during polymerization forms platelet-like single crystals having crystal thickness of 7.8 nm. The crystal thickness corresponds to the attachment of approximately 63 covalently linked carbon atoms along the c-axis of the unit cell. Considering the crystal thickness and chain length of approximately 37 000 covalently linked carbon atoms, electron diffraction of the nascent crystals suggests chain folding along the ab-plane of the single crystal. In view of the polymerization kinetics, followed by crystallization leading to chain packing, these platelet-like single crystals are expected to have the low entangled state. DSC thermogram of the crystals suggests solid-solid transition from the low entropy (monoclinic) to high entropy (hexagonal) phase, prior to melting. The phase transition temperature, from the monoclinic to hexagonal phase, shifts to higher temperatures on annealing crystals in the high entropy phase. Cause for the shift in the phase transition temperature, on annealing, is attributed to increase in crystal thickness. To follow chain mobility during thickening, ¹³C solid-state NMR techniques are employed. Similar to the trans-1,4-polybutadiene, prepared from solution crystallization, the reversal phase transition from the hexagonal to monoclinic phase is followed in the single crystals by ¹³C solid-state NMR. The SEM images and ¹³C CP/MAS spectra of the nascent and the annealed single crystals in the hexagonal phase have proven the lamellae thickening of the isolated single crystals, accompanied by the reversal in the phase transition to the monoclinic phase with crystal thickening. We attribute crystal thickening in the isolated single crystals as the primary thickening that requires high-chain mobility facilitated by the high entropy hexagonal phase and driven by the thermodynamics of reducing the surface energy. Amazing aspect is that the primary thickening that requires desired cooperative motion of the covalently linked carbon atoms reduces the crystal planar surface area to accommodate crystal thickening that starts from several nanometers and reaches several tens of nanometers with the phase transformation from the monoclinic to hexagonal phase. The ease in crystal thickening, in the high entropy phase, is suggestive of adjacent re-entry perceived with deposition of the growing chain after suppression of the nucleation barrier.