Enhancing the Electrospinnability of Low Molecular Weight Polymers Using Small Effective Cross-Linkers

The electrospinning of continuous fibers normally requires the presence of a network of topological entanglements in solution, limiting the spinnability of low molecular weight polymers. Here, we show that a supramolecular approach can improve, and even render possible, the electrospinning of low molecular weight polymers, illustrated here with poly(4-vinylpyridine) (P4VP), by creating effective (i.e., physical) cross-links via hydrogen bonding or coordination interactions in solution using small molecules. The addition to P4VP solutions in dimethylformamide of 4,4′-biphenol (BiOH), which hydrogen bonds to P4VP, and nitromethane, a poor solvent for P4VP that increases BiOH hydrogen bonding to P4VP, decreases the concentration needed to prepare fibers of 50 kg/mol P4VP by a factor of 2 and enables the formation of unbeaded fibers. Hydrogen bonding in solution is quantified by infrared spectroscopy, and the impact of the supramolecular interactions on the P4VP concentration needed to form a physical network is shown by rheological studies. BiOH can also be removed by sublimation without damaging the fibers. Replacing BiOH by 4-hydroxy-4′-biphenylcarboxylic acid (HBCA), whose acid group hydrogen bonds more strongly than OH to P4VP, or by just 1% ZnCl 2 (relative to pyridine), which promotes metal coordination interactions with P4VP, improves the cross-linking efficiency still further. Most spectacularly, HBCA enables the electrospinning of unbeaded fibers of P4VP with the very low molecular weight of 5.2 kg/mol, which is well below the entanglement molecular weight. These results establish the supramolecular cross-linking approach as a powerful strategy for preparing nanofibers of pure polymers having limited electrospinnability.