Self-assembling Morphologies Obtained from Helical Polycarbodiimide Copolymers and Their Triazole Derivatives

Summary

Here, we present a protocol to prepare and visualize secondary structures (e.g., fibers, toroidal architectures, and nano-spheres) derived from helical polycarbodiimides. The morphology characterized by both atomic force microscopy (AFM) and scanning electron microscopy (SEM) was shown to depend on molecular structure, concentration, and the solvent of choice.

Abstract

A facile method for the preparation of polycarbodiimide-based secondary structures (e.g., nano-rings, "craters," fibers, looped fibers, fibrous networks, ribbons, worm-like aggregates, toroidal structures, and spherical particles) is described. These aggregates are morphologically influenced by extensive hydrophobic side chain-side chain interactions of the singular polycarbodiimide strands, as inferred by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. Polycarbodiimide-g-polystyrene copolymers (PS-PCDs) were prepared by a combination of synthetic methods, including coordination-insertion polymerization, copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) "click" chemistry, and atom transfer radical polymerization (ATRP). PS-PCDs were found to form specific toroidal architectures at low concentrations in CHCl₃. To determine the influence of a more polar solvent medium (i.e., THF and THF/EtOH) on polymer aggregation behavior, a number of representative PS-PCD composites have been tested to show discrete concentration-dependent spherical particles. These fundamental studies are of practical interest to the development of experimental procedures for desirable architectures by directed self-assembly in thin film. These architectures may be exploited as drug carriers, whereas other morphological findings represent certain interest in the area of novel functional materials.

Introduction

The helix is a ubiquitous chiral motif observed in nature. Complex biological systems and their components, such as proteins, polypeptides, and DNA, all utilize the helical structure as a means of performing complex tasks for applications like information storage, tissue molecular transportation support, and localized chemical transformations.

Helical polymeric macromolecules¹ have been a target for the design of functional materials and composites possessing interesting properties, which enabled their practical use in many areas^2,3,

Protocol

NOTE: All reactions were performed in a glove box (or fume hood, when noted) using standard scintillation vials.

1. Synthesis of the (R)- and (S)-series of Ethynylpolycarbodiimides

1. Place 1.0 g (0.00442 mol) of N-(3-ethynylphenyl)-N'-hexylcarbodiimide monomer (ET) and 0.894 g (0.00442 mol) of N-phenyl-N'-hexylcarbodiimide monomer (Ph) as transparent, viscous liquids in a clean scintillation vial (20 mL) with a magnetic stirring bar (glove box) to obtain a representative R-50-ET-50-Ph composition.
   NOTE: Use only one monomer ....

Results

Figure 1 (upper panel) illustrates BINOL (R)- or (S)-titanium (IV) catalyst-mediated coordination-insertion polymerization leading to the (R)- and (S)-series of ethynylpolycarbodiimides with an altering ratio of the repeat units (i.e., aryl- and alkyne aryl). Monomers and catalysts were obtained as described elsewhere¹⁸. Both (R)- and (S)-family alkyne random copolymers were selected fo.......

Discussion

In summary, the spin-coating deposition method represents a convenient way to reproducibly generate multiple-type morphologies, including fiber-like aggregates, ribbons, worm-like structures, fibrillar networks, looped fibers, toroids, and superhelices, from either alkyne polycarbodiimides or from their respective PS-derivatives (i.e., polycarbodiimide-g-polystyrenes). Thus, coordination-insertion polymerization, along with further functionalization using a "click" reaction followed by ATRP, pro.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
styrene	Sigma-Aldrich	S4972-1L	reagent
N,N,N′,N′′,N′′- Pentamethyldiethylenetriamine (PMDETA)	Sigma-Aldrich	369497-250ML	reagent
Copper(I) iodide	Sigma-Aldrich	215554-5G	reagent
Copper(I) chloride	Alfa-Aesar	14644, 5G	reagent
1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)	Sigma-Aldrich	139009-100G	reagent
N,N-dimethylformamide (DMF)	Sigma-Aldrich	227056-100mL	solvent
Tetrahydrofuran (THF)	Acros-Organics	B0320346	solvent
Chloroform	Sigma-Aldrich	372978-100mL	solvent
Methanol	Fisher-Chemical	A411-20	solvent
20 mL glass scintillation vials	Cole-Palmer	UX-08918-03	glassware
1-Dram vials (15 x 45 mm)	Kimble-Chase	KIM-60965D-1	glassware
13 mm syringe filter with 0.45 µm PTFE membrane	VWR International	28145-493	membrane filter
Silicon wafer disks (25.4 ± 0.5 mm)	Wafer World, Inc	S076453	AFM substrate
Corning Stirrer/Hot Plate	Hot Plate	PC-420	heating device
single stage Unilab mBraun glove box	Unilab	12-109	glove box
Nanoscope IV-Multimode Veeco AFM-machine	Veeco	3100 Dimension V Atomic Probe Microscope	AFM-instrument