Regenerated Silk and Carbon Nanotubes Dough as Masterbatch for High Content Filled Nanocomposites

Luca Valentini,Alan Dalton,Manoj Tripathi,Nicola M Pugno,Silvia Bittolo Bon

doi:10.3389/fmats.2019.00060

Abstract

Regenerated silk (RS) is a natural polymer that results from the aggregation of liquid silk fibroin proteins. In this work, we observed that RS dispersed in aqueous solution undergoes a reversible solid/liquid transition by programmed heating/cooling cycles. Fourier transform infrared, atomic force microscopy imaging and Raman measurements of the RS reveal that the transition fromrandomcoil to b-sheet structures is involved in this liquid–solid transition. The reversible solid-liquid transition of silk fibroin was then found to be helpful to prepare polymer-like carbon nanotube (CNT) dispersions. We demonstrate that the gelation of RS makes the CNTs with the consistency of a dough with polymeric behavior. Such RS can disperse carbon nanotubes at high concentrations of tens of weight percent. Finally, such carbon nanotube dough has been used for the realization of rubber composites. With this method, we pave the way for handling nanopowders (e.g. CNTs or graphene related materials) with safety and reducing the filler volatility that is critical in polymer-processing.

Highlights

Regenerated silk (RS) is a natural polymer made by the coagulation of silk fibroin that is a an aggregation of proteins with short and long chains; the combination of intermolecular interactions via hydrogen bonds between the chains leads to the formation of β-sheet structures that have a high crystalline local order
The liquid RS extracted from solution and a drop cast on silicon substrate at 80◦C shows a liquid–solid transition (Figure 1a) that recovers the liquid state under cooling at 25◦C within few minutes, which is lower than that of conventional silk-fibroin
The transition from solid to liquid state of RS was obtained by the addition of calcium chloride salts to water based silk fibroin solution

Summary

Introduction

Regenerated silk (RS) is a natural polymer made by the coagulation of silk fibroin that is a an aggregation of proteins with short and long chains; the combination of intermolecular interactions via hydrogen bonds between the chains leads to the formation of β-sheet structures that have a high crystalline local order. Increasing the fibroin concentration in a solution enhances the probability to generate chain interactions where the β-sheets form a stable gel transition. The dehydration of such crystalline structures results in an irreversible liquid-solid transition due to thermodynamic cross-linking of the β-structures that undergo gelation with time (Ayub et al, 1993; Hanawa et al, 1995; Kang et al, 2000; Wang et al, 2008). Liu et al (2014) demonstrated the thixotropy (i.e., a time dependent shear thinning property) of silk fibroin via dissolution nanofibrils gel in alcohol and sodium chloride while Bai et al (2014) showed a reversible sol—gel transition by self-assembly of nanofibers into supramolecular

Methods

Results

Conclusion