Abstracts Category : Other

Critical size for bulk-to-discrete transition and segment-induced size effect melting of 2D aliphatic layers

by Zichao Ye

Size miniaturization of aliphatic layers is of interest in the fundamental understanding of their structure-property-relationship. This research studies the size-dependence of structure evolution and thermodynamic principles in a model system of silver alkanethiolate (AgSCn), which consists of uniform alkyl chains with spatially discrete interfaces and 2D Ag-S slabs. Two novel synthesis methods are complementarily combined to obtain AgSCn lamellar crystals with variable number of layers (m=1-4 and >10) and variable chain lengths (n=1-16). The vapor phase method is capable of controlling the number of layers for the synthesis of monodispersed AgSCn (n=7-18) with 1-layer to 4-layer structures. The solution reaction method is developed to synthesize well-defined single crystal lamellae of different chain lengths (n=1-16), with all-trans alkyl chains, well-registered interfaces and highly ordered intralayer lattice. This technique is highlighted by the growth of extremely short chain lamellae (n=1-3), which is previously considered almost disordered. Unit cell structures of the beam-sensitive AgSCn are common for species with n=2-16 and are determined for the first time by coupling synchrotron XRD and nano-beam electron diffraction. Nanocalorimetry and commercial DSC are employed to systematically measure the chain-length-dependence and layer-number-dependence of AgSCn chain melting (Tm). Three unique effects of size-dependent properties are discovered. Short-chain effect melting occurs when chain length is decreased to a critical size (ncr=7), below which Tm of AgSCn deviates from that predicted by the classical Gibbs-Thomson model by as large as 50 K. Such unique deviation is also observed in structural parameters related to the local environment and dynamics of carbon groups, which are utilized to divide the alkyl chains into three segments. The previous 2D Ag-S planes and interfaces are assigned 3D representations with quantified thicknesses that include the head segment and the tail segment of alkyl chains, respectively. The incremental melting enthalpy of AgSCn with nncr is contributed from the increasing length of the bulk-like mid-chain segment. At/below the critical size, none of the carbon groups are equivalent and the lamellae structure is dominated by the Ag-S region as well as the interface region. The short-chain effect is caused by the bulk-to-discrete transition of lamellae properties at/below the critical length scale. Stacking effect of lamellar melting demonstrates an increase of Tm as a function of number of layers. This effect only occurs when interlayer interfaces are well-registered such that different layers perform synergistic with each other and melt collectively. This effect is not observed in polyethylene in which interfaces are amorphous.Odd/even parity effect exists in stacked AgSCn over the entire range of chain length (n=2-16), despite the short-chain effect below n=ncr, but is absent in single layer crystals where interface is not formed. Any parity-dependence exhibited inAdvisors/Committee Members: Allen, Leslie H (advisor), Allen, Leslie H (Committee Chair), Braun, Paul V (committee member), Zuo, Jian-Min (committee member), Lyding, Joseph W (committee member).