Kinetics and Applications of On-Surface Topochemical Polymerization of Diacetylene Striped Phases

Anni Shi, Purdue University

Abstract

Nature exhibits the power of chemical environment at nanoscale to efficiently regulate diverse processes on biological membranes, inspiring extensive applications of biotemplates on quantum computing to regenerative medicines. Approaches to design molecular-level chemical features often requires lattice structures of pristine inorganic surface to provide precise templates to routinely control placement of atoms, functional groups and molecules based on epitaxial assembly. However, emerging applications in biocompatible materials often involve in precise patterning on soft amorphous materials. The hetergeneties of such materials contribute significant challenges to express controllable nanoscopic features in a macroscopic scale. However, there is a great potential to leverage existed ordered chemical patterns to the nanoscopic functionalization of elastomers via crosslinking reactions. Interfacial self-assembly provides a means of creating nm-resolution chemical patterns below the limitation of lithography. Particularly, long-chain functional alkanes generate lamellar phases through epitaxial alignment with inorganic substrates, forming 1-nm resolution functional stripes separated by ~5 nm of exposed alkyl chains. Diacetylene groups embedded in striped phases can undergo photopolymerization, forming conjugated π-bonds. An extensive polymer network on sPDA monolayer enhances the robustness of monolayers and provides reactive sites for crosslinking reactions, enabling the transfer of nm-precision patterns to amorphous materials, such as PDMS and acrylamide hydrogels. Scanning probe microscopy (SPM) has been typically used to understand on-surface polymerization of certain diacetylenes at molecular level on inorganic layered materials. It reveals different conformational change during polymerization on surface from in bulk crystals, but it is incapable of acquiring polymerization status at scales to incorporate observations in bulk. However, we utilized the reactivity of sPDA to transfer striped phases from inorganic substrates to soft elastomers via crosslinking. The transfer recovers the fluorescence of conjugated PDA networks, allowing optical characterization of striped phase monolayers at micrometer scales. We verified the correlation between fluorescence signals in macroscopic scales and polymerization studies by SPM. The combination of nanoscopic and macroscopic views assist us to discover significant impacts of subtle molecular structures on sPDA reactivity. Polymerization of diacetylene, also known as topochemical reaction, was noticed by significant color change of bulk crystals. The reaction requires a delicate balance between angstrom-level movement and molecular arrangement, forming products with high molecular ordering. Studies of bulk PDA crystals have revealed the control of reaction efficiency by intermolecular interactions based on molecular structures. With the combination of SPM and fluorescence, we identified the importance roles of alkyl chain length and headgroups played in polymerization rate and efficiency. The interfacial polymerization exhibits similar reaction kinetics. However, due to the special “lifted” conformation of PDA evolved in interfacial polymerization, on-surface reaction is significantly sensitive to short-range interactions controlled by molecular structures. Maximizing polymerization of striped phases benefit for the transfer reaction to elastomers, which have exhibited great potentials in development of electronic and biocompatible devices. Amine functionalities are useful in functional interface design, since primary amines are good nucleophiles for a wide range of reactions. Striped phases on elastomers with primary amines provide ordered reactive sites, and can serve either as functional handles for post functionalization of the surface for advanced purposes; Or other functions, including localized assembly of inorganic nanoparticles.

Degree

Ph.D.

Advisors

Claridge, Purdue University.

Subject Area

Chemistry|Analytical chemistry|Medicine|Polymer chemistry

COinS