DriveAFM for polymer chemistry

Normal alkanes C_nH_2n+n are linear chain molecules, which are typically extended, and they form ordered lamellar structures and crystals. When the size of the chains increases to n > 120, such ultra-long alkanes manifest chain folding – the main driver behind polymer crystallization. The longest alkane addressed in this application note, C₃₉₀H₇₈₂ , is a perfect model of linear polyethylene, and crystallization of these species have many common features. The lamellar 2D organization has been detected in thin and ultra-thin layers of alkanes on highly oriented pyrolytic graphite (HOPG) and other layered crystals such as MoS₂. On these substrates the molecular orientation exhibits three-fold symmetry that guides the related orientation of the alkane lamellae. As the lamellae are formed of fully elongated chains, their width equals to the alkane length when the molecules are aligned perpendicular to the lamellar edges. The length of different alkanes from C₁₈H₃₈ to the ultra-long one - C₃₉₀H₇₈₂ changes as listed in Figure 1a. The lamellar width can be smaller when molecules are tilted at some angle to the lamellar edges.