Phase transition and paracrystalline order in solution‐crystallized solid n‐paraffins

Solution‐crystallized single crystals of various paraffins were investigated at room temperature by nuclear magnetic resonance and by small‐angle and wide‐angle x‐ray scattering. The samples were crystallized from various solvents and annealed 2, 5, 6, and 12°C below the melting point. The NMR spectra are separated into three components (α, β1 and γ) of different chain‐segment mobility. On combining these results with x‐ray measurements and with results on polyethylene, enough information is obtained to propose a model of the paracrystalline superlattice built up by at least 30 paraffin lamellae and to find where the chains of various conformation are located in the structure. The phase transition of C44H90 in the solid state gives further hints for this allocation. For instance, the β1 component of intermediate mobility increases anomalously from 3% before to 10% after the phase transition. The paracrystalline g value of the macrolattice of the orthorhombic phase is 1.5% and the gap between the lamellae is 2.2 Å. At the beginning of the phase transition, the chains incline stepwise to the lamella surface retaining the orthorhombic macrolattice distance P0 = 58 A and enlarging the gap to 8.8 Å without changing the g value of 1.5%. The growing monoclinic phase, on the other hand, has a constant g value of 3.5% from the beginning with Pm = 52 Å and has gaps of only 3 Å between adjacent lamellae. This explains the large β1 (ca. 10%) of these two new phases, because the mobile chain ends consist of approxiamately 1 CH3 and 1 CH2 group on the average. The γ component of highest micro‐Brownian mobility corresponds to the γ component in polyethylene with a line width of ca. 0.07 gauss. Unlike the case of polyethylene it is produced by only a small amount (0.02–0.13) of free CH3 groups per chain. Another fraction of the CH3 groups belongs to the rigid component α of the crystalline phase. They are located where adjacent lamellae touch each other in crystalline‐like order. Because of these contacts, stacks of lamellae about 1000 Å thick scatter coherently, and the long period Po of the orthorhombic phase remains undestroyed by annealing until the monoclinic domains collapse to the smaller period Pm of the monoclinic phase.