Flow-induced crystallisation of polymers from aqueous solution

Abstract

Synthetic polymers are thoroughly embedded in the modern society and their consumption grows annually. Efficient routes to their production and processing have never been more important. In this respect, silk protein fibrillation is superior to conventional polymer processing, not only by achieving outstanding physical properties of materials, such as high tensile strength and toughness, but also improved process energy efficiency. Natural silk solidifies in response to flow of the liquid using conformation-dependent intermolecular interactions to desolvate (denature) protein chains. This mechanism is reproduced here by an aqueous poly(ethylene oxide) (PEO) solution, which solidifies at ambient conditions when subjected to flow. The transition requires that an energy threshold is exceeded by the flow conditions, which disrupts a protective hydration shell around polymer molecules, releasing them from a metastable state into the thermodynamically favoured crystalline state. This mechanism requires vastly lower energy inputs and demonstrates an alternative route for polymer processing.

Emerging Silk Material Trends: Repurposing, Phase Separation and Solution-Based Designs

“…The seminal work published in 2020, by Oleksandr Mykhaylyk and co-workers [ 22 ], has now made a significant step into uncharted territory by applying flow-induced crystallization of poly(ethylene) oxide. This work borrows key principles from silk spinning, particularly the phase transition from liquid to solid.…”

“…Unlike thermoplastics spinning, natural silk spinning is, by orders of magnitude, a more energetically efficient process and only needs flow at ambient conditions to induce crystal nuclei [ 35 ]. Mykhaylyk and co-workers have exploited this process to demonstrate, for the first time, how a metastable poly(ethylene) oxide solution can be converted into a crystalline solid with flow under ambient conditions in the absence of a chemical reaction, removal of heat or evaporation of solvent [ 22 ]. This phase transition requires the flow to exceed the required energy threshold that disrupts the protective hydration shell around poly(ethylene) oxide.…”

“… Summary of emerging silk material trends. Repurposing [ 23 , 25 , 27 , 28 ], solution-based designs [ 24 ], phase separation [ 30 , 31 ] and flow [ 22 , 26 , 29 ] impacting manufacturing and creating living materials. …”

“…In this review, I have selected key primary research papers published in 2020 (references [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]) that I believe will change the silk material space ( Figure 1 ). Typically, technological innovations have catalyzed the fields that ultimately delivered the most impact.…”

Stretching of Bombyx mori Silk Protein in Flow

“…Protein-based natural materials persistently inspire the development of novel human-made materials, owing to their biocompatibility, unique combinations of strength and toughness [ 1 , 2 , 3 , 4 ], low-energy processing [ 5 ] and efficient solvent recycling [ 6 ]. While the industrial production of polymer-based fibres is challenged by a highly non-trivial interdependence between the molecular level of bond-orientation-dependent nucleation, and the macroscopic level, where the temperature-dependent rheology generates stretch of entire chain segments [ 7 , 8 , 9 , 10 , 11 ], silk is processed in semi-dilute aqueous conditions [ 5 ], where nucleation can be induced through the stretch-induced disruption of the solvation layer [ 12 ]. In order to generate sufficient stretch at modest flow rates, the silk protein has evolved to contain ‘sticky’ patches (which are assumed to be consisting of ionic calcium bridges between the carboxylated side groups of aspartic and glutamic acids) that significantly slow down stretch relaxation in flow [ 6 , 13 ].…”