In silico cooling rate dependent crystallization and glass transition in n-alkanes

Abstract

n-Alkanes (C_nH_2n+2) are linear chain compounds spanning length-scales from small molecules to polymers. Intermediate length alkanes (say, n = 10–20) have attracted much interest as organic phase change materials (PCM) for storing energy as latent heat. The cooling rate (γ) determines both the latent heat and temperature of crystallization. While slow cooling of the liquid leads to the crystalline state, rapid cooling leads to a glassy state (glass transition temperature T_g). Albeit scant theoretical investigations concerning the vitrification processes, the role of molecular conformations therein remains completely unexplored. Our work presents an all-atom molecular dynamics study of (a) cooling intermediate length alkanes (n = 12 and 16) at seven different rates, and (b) rapidly cooling 14 n-alkanes (4 ≤ n ≤ 50) for determining T_g(n). We find that for linear molecules the end-to-end distance (R_ee) is of special relevance: the crystal is composed solely of fully stretched molecules (maximal R_ee). Hence one may define the “degree of crystallization” as the area under the maximal R_ee peak in the R_ee distribution. Other peaks in the distribution represent conformations that existed in the supercooled liquid just before vitrification. A peak for the shortest, hairpin rotamer appears only for n ≥ n₀ = 18, and is also manifested in the minimum of R_g/R_ee(n) for liquid n-alkanes. The dependence of T_g on n is represented as two intersecting Ueberreiter and Kanig equations, intersecting near n₀ = 18. Extrapolation gives the asymptotic n → ∞ limit of T_g, T^∞_g = 250 K, which is probably its most accurate estimate obtained theoretically todate.