Polyvinylidene fluoride membranes with engineered porosity: role of temperature and substrate morphology in phase inversion processes
Creators: name: Balzamo, Gianluca
Publisher: name: Loughborough University
Year: 2021
Subjects: subject: 91299 Materials Engineering not elsewhere classified
subject: FOS: Materials engineering

description: The first part of this project was inspired by the fascinating internal porosity of Polypore fungi, characterised by channel-like and micro-fibrous porosities, which create a highly efficient directional release system for their basidiospores.An initial analysis of their natural porous structures revealed the effect of their morphology on the sorption of liquids; sections of the channel-like regions of Ganoderma applanatum fungi were first dip-coated with octadecyltrichlorosilane to achieve superhydrophobicity (water contact angle higher than 150°) and selective sorption of oils and organic solvents. The treated fungi showed oil sorption capacity between 1.8 (rapeseed oil) and 3.1 g/g (chloroform), and the combination of channel-like porosity and hydrophobic properties suggested its use as naturally derived filtration element in micro fluidic networks, showing the possibility of efficiently separating water from chloroform.The second part of this research work replicated the Polypore fungi porosity through the fabrication of polymeric membranes by means of immersion precipitation technique, a specific variant of phase inversion methodologies, thus biomimicking the dual porosity of Polypore fungi.Poly(vinylidene fluoride) (PVDF) has been widely adopted to produce porous membranes due to its membrane forming capability and outstanding mechanical, chemical and thermal properties.N,N-dimethylformamide (DMF) (solvent) and water (coagulant) were then chosen as part of the PVDF membrane forming system to achieve the formation of a net separation between finger-like and sponge-like porosity regions within the membrane thickness.By precisely varying the coagulation bath temperature, the extension of the finger-like region was controlled; the use of 90 °C as temperature for the coagulant led to a predominance of sponge-like morphologies within the PVDF membranes, where only ≈2.5 % of the membrane thickness at the coagulation bath/casting solution interface was characterised by macrovoids; the macrovoids disappearance at high temperatures was addressed to the rapid polymer precipitation and skin layer formation at the early stage of the casting solution phase separation, which prevented the fast solvent/coagulant mass exchange within the inner casting solution layers.On the other hand, lowering the coagulation bath temperature led to a significant increase of the macrovoids length, which were present in more than 50% of the membrane thickness when the temperature was decreased below 20 °C.Also, the production of PVDF membranes at low coagulant temperatures affected the shape of the macrovoids, and finger-like porosity was obtained; this effect was addressed to the early gelation of the polymer-rich phase during the occurrence of phase separation, which generated a contraction force around the macrovoids which turned into narrow and long finger-like pores.Finally, the resulting membrane morphologies, which mimicked the Polypore fungi porosity, showed directional sorption and release of therapeutic essential oils to be applied as drug delivery systems.Novel research from the past years has shown the possibility of combining phase inversion techniques with more recent scaffolds production technologies, such as electrospinning, to achieve superior membrane properties.The last part of this work developed an innovative approach to improve the mechanical properties of polycaprolactone (PCL) electrospun membranes by first dip-coating them into PVDF casting solution and then inducing phase separation through water vapor. The slow phase separation of PVDF caused the solid-liquid demixing to occur and lead to the formation of PVDF spheres with diameters ranging from 4.8 to 10.3 m; the electrospun polycaprolactone nanofibers acted as nucleation sites, and the PVDF spherulites grew onto the porous surface of the fibers and physically bonded multiple fibers together. The fabricated composite scaffolds showed increased mechanical properties if compared to the polycaprolactone mats alone; the ultimate tensile strength, Young’s modulus and elongation at break increased from 1.2 MPa, 62.9 kPa, 634.1 % for PCL electrospun mats to 2.1 MPa, 764.5 % and 136.7 kPa for the fabricated PCL-PVDF composite scaffolds, respectively.Finally, cytotoxicity studies using neuroblastoma cells showed the biocompatibility of the fabricated composite scaffolds and suggested their potential to be applied in neural tissue engineering.