Abstract
It is well known that block copolymers with cylindrical morphology show alignment with shear, resulting in anisotropic mechanical properties. Here we show that well-ordered bi-directional orientation can be achieved in such materials by slow injection moulding. This results in a microstructure, and anisotropic mechanical properties, similar to many natural tissues, making this method attractive for engineering prosthetic fibrous tissues. An application of particular interest to us is prosthetic polymeric heart valve leaflets, mimicking the shape, microstructure and hence performance of the native valve. Anisotropic layers have been observed for cylinder-forming block copolymers centrally injected into thin circular discs. The skin layers exhibit orientation parallel to the flow direction, whilst the core layer shows perpendicularly oriented domains; the balance of skin to core layers can be controlled by processing parameters such as temperature and injection rate. Heart valve leaflets with a similar layered structure have been prepared by injection moulding. Numerical modelling demonstrates that such complex orientation can be explained and predicted by the balance of shear and extensional flow.
Highlights
Block copolymers are of increasing interest because of their nano-scale morphologies, which can be utilized in a range of applications.[1,2,3] Their properties o en depend critically on molecular orientation induced during processing
Native biological tissues are more complex than the synthetic composite material examined here, in composition and in their patterns of molecular architecture, the ability of cylinder-forming block copolymers to mimic the anisotropic structural and mechanical properties of these native tissues makes this group of materials attractive for engineering of bio-inspired systems
The case of particular interest for us is the development of improved prosthetic heart valves; anisotropic block copolymer lea ets can mimic the natural structure and improve the mechanical performance of the valve.[29,30,31]
Summary
Block copolymers are of increasing interest because of their nano-scale morphologies, which can be utilized in a range of applications.[1,2,3] Their properties o en depend critically on molecular orientation induced during processing. One example is the properties of thermoplastic elastomers with a cylindrical morphology, where alignment of the cylindrical micro-domains results in orthotropic mechanical properties.[4,5]. Such a layered structure with bi-directional orientation has not previously been reported in a solid material, analogous bi-directional orientation zones have been observed in liquid crystalline systems subject to ow.[8,9,10,11,12,13] Composite materials with a bi-directional microstructure could nd a range of applications in the fabrication of functional devices. By linking the morphology to mechanical properties of the nal solid material we are able to propose practical applications, such as the fabrication of prosthetic heart valve lea ets
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