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Posted: Dec 07, 2011

Spinning like the spider could cut energy cost of synthetic polymer fibers by 90%

(Nanowerk Spotlight) Researchers have, for the first time, compared the energetic cost of silk and synthetic polymer fiber formation and demonstrated that, if we can learn how to spin like the spider, we should be able to cut the energy costs for polymer fiber processing by 90%, leaving alone the heat treatment requirements. The two routes of polymer fiber-spinning – one developed by nature and the other developed by man – show striking similarities: both start with liquid feed-stocks sharing comparable flow properties; in both cases the 'melts' are extruded through convergent dye designs; and for both 'spinning' results in highly ordered semicrystalline fibrous structures. In other words, analogous to the industrial melt spinning of a synthetic polymer, in the natural spinning of a silk the molecules (proteins) align (refold), nucleate (denature) and crystallize (aggregate).
"We propose, that the key to this massive energy saving is through nature's use of a state of matter that is neither an isotropic polymer melt nor a true aqueous solution," Dr. Oleksandr O. Mykhaylyk, a researcher at the University of Sheffield's Polymer Scattering Group, tells Nanowerk. "Much like an individual polymer chain in a melt, silk proteins and their associated water molecules may be considered as a single processable entity, a nanocomposite state of biological matter we define as an 'aquamelt'. This newly defined state of biological matter is a nanocomposite consisting of water and protein bound by an energetic threshold which – if exceeded – causes an irreversible transformation from behaving like a polymer melt to a phase-separated fibrillar network and water as a by-product."
Working with members of his group and scientists from the Oxford Silk Group at University of Oxford, Dr. Mykhaylyk has published the team's findings in the November 23, 2011 online edition of Advanced Materials ("Silk and Synthetic Polymers: Reconciling 100 Degrees of Separation").
Key to the team's observations was a recently developed, relatively simple tool based on a shear-induced polarized light imaging (SIPLI) technique to access flow parameters responsible for the flow-induced crystallization of polymers ("Time-resolved polarized light imaging of sheared materials: application to polymer crystallization").
"SIPLI allows the moment of shear-induced nucleation as well as the moment of shear-induced fibrillation in polymers to be detected in situ and, therefore, the flow parameters associated with these phenomena to be measured," explains Dr. Mykhaylyk. "This technique enabled us to make an important step from a phenomenological description of polymer fibrillation towards a quantification of the process and, subsequently, a relation of the measured flow parameters – such as shear rate, time of shearing and strain – to the physical parameters of polymer molecules – such as relaxation time.
fiber front formation in both polyethylene and silk dope as observed in situ by shear-induced polarized light imaging (SIPLI)
The fiber front formation in both polyethylene and silk dope as observed in situ by shear-induced polarized light imaging (SIPLI) technique. (Image: Dr. Mykhaylyk, University of Sheffield and Oxford Silk Group, University of Oxford)
A recent review article ("Monodisperse macromolecules – A stepping stone to understanding industrial polymers"), first-authored by Mykhaylyk, which discusses the structural aspects of what is happening in synthetic polymers under flow conditions provides more details about this.
According to the team's findings, silks offer a potential roadmap for future energy efficient high-performance polymer design as they teach us how the 'blunt tool' of temperature processing can be replaced by the "fine instrument' of mechanical processing.
The results of this work can be applied in polymer science and polymer industry, in particular polymer processing industry. Specifically, this work shows that, energy-wise, there are much better polymer systems – what the team called 'aquamelts' – than the synthetic polymers currently used in polymer processing. As Mykhaylyk points out, the legitimate question arises if it is possible to modify existing synthetic polymers, or to develop new polymer systems, to mimic the properties of aquamelts in an industrial manufacturing environment?
Fibers forming in polymers and biopolymers as extreme case of morphology are only part of the team's work. In general, structural morphology of solidified polymers – which can be varied from an isotropic structure to well-oriented anisotropic fibers – determines physical and mechanical properties of the material and is strongly dependent on processing conditions.
"The SIPLI technique enables us to measure flow parameters and to classify polymers in terms of the energy barrier, required for the fiber morphology formation in polymers," says Mykhaylyk. "These flow parameters can be used to model and to predict behavior of polymers under complex flow conditions to optimize polymer articles produced by real industrial processes."
Since this technique is quite new, the scientists are currently collecting data on shear-induced nucleation and fibrillation in different polymers and polymer systems. This above work on silk is a part of these measurements. One of the advantages of the SIPLI technique is that it is a combinatorial technique that requires only a small amount of material (about 0.1 ml) to carry out the measurements. They are also testing the technique for other applications in soft matter.
"SIPLI gives a unique opportunity to study shear-induced phenomena such as stress, orientation and structural transitions taking place in soft matter – gels, polymers, copolymers, liquid crystals and colloids – in situ," notes Mykhaylyk.
He also raises the question if, with regard to fiber formation in polymers, one should only consider mechanical properties of the material as the main criterion for an assessment of the process; or if energy cost should be an important part of the equation.
"Our research suggests that the route chosen by nature in fiber formation outperforms existing synthetic routes in both cases – the energy required to produce natural silk fibers is at least ten times less than a classic synthetic polymer, yet still nature produces a fiber with superior mechanical properties. So our work is now trying to come up with answers as to what we have to do to implement the main principles of nature's approach in polymer production and polymer processing?"
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