PMC:7498153 / 40832-42699 JSONTXT

{"target":"http://pubannotation.org/docs/sourcedb/PMC/sourceid/7498153","sourcedb":"PMC","sourceid":"7498153","source_url":"https://www.ncbi.nlm.nih.gov/pmc/7498153","text":"In contrast to these early reports, we have found the thickness and electron density profiles of the adsorbed layers for cyclic and linear PNDG films to be nearly identical. As a result, we postulate that the relative thermal stability of cyclic and linear PNDG films mainly arises from the structural disparity of their adsorbed layers and not from their thicknesses/total adsorbed amounts on the substrate. At 25 °C, the c-PNDG adsorbed monolayer shows higher surface roughness and better in-plane molecular ordering than the l-PNDG monolayer. Upon heating at T = 200 °C above the melting temperature (Tm), both cyclic and linear PNDG adsorbed layers become thicker and less dense as they lose orientational ordering. Interestingly, c-PNDG adsorbed chains at 200 °C adopt the same board-shape molecular geometry with the trans-amide backbone conformation as the free c-PNDG chains in the molten film. By contrast, the conformation of l-PNDG adsorbed chains at 200 °C are more disordered and notably different from the board-shape conformation of the free l-PNDG chains with the trans-amide backbone in the molten film. As a result, the disordered l-PNDG adsorbed chains “thermodynamically repel” the neighboring unadsorbed chains to minimize the unfavorable contacts and limit interpenetration of unadsorbed chains at the free polymer/adsorbed polymer interface, eventually resulting in autophobic dewetting of the l-PNDG film. We reason that the conformational disparity between the linear PNDG molecules in the adsorbed layer and the molten film provides the thermodynamic driving force for the autophobic dewetting for the linear PNDG films. In the case of c-PNDG film, the continuity in the chain conformation from the adsorbed monolayer to the neighboring polymer melt plays an active role in stabilizing the interface and suppressing the autophobic dewetting.","tracks":[]}