Doctor of Philosophy
School of Chemistry
Gestos, Adrian, The development of electrospun polymer nanofibres and measurement of their individual mechanical properties with the atomic force microscope, Doctor of Philosophy thesis, School of Chemistry, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3640
Electrospun nanofibres are being developed at an ever increasing rate for a range of applications. The intended applications vary widely, from tissue engineering inspired by natural tissue scaffolds, to energy applications as high surface area electrodes in batteries and fuel cell devices. However, the ability to routinely characterise the individual fibres themselves is still catching up with the rate and sophistication of their development.
The characterisation of individual electrospun fibres in regards to their mechanical properties is the main focus of this thesis. Understanding individual nanofibre mechanical properties is important for predicting their suitability in an intended application and can also provide unique fundamental insights into the mechanical behaviour of polymers at any scale. Current techniques for characterising individual electrospun nanofibres can be prohibitively expensive, overly time consuming or not widely available in most research labs. Although the use of the atomic force microscope (AFM) is not new in the field of characterising the mechanical properties of individual nanofibres, this thesis presents a more streamlined and inexpensive approach, which is achievable by any research lab with access to an AFM.
The lateral twist of AFM cantilevers was employed as the force sensor for the mechanical tests and the crucial step of lateral force calibration was investigated in Chapter 3. A recently published method using a glass fibre of known dimensions was utilised for the lateral force calibration but slightly modified and expanded upon. The significant effect on the lateral conversion factor of the fibre snagging point along the tips height and the non-linearity of the AFM photosensitive detector were investigated and able to be accounted for.
The mechanical properties of individual electrospun nanofibres made from a diverse range of polymers were then measured in Chapter 4. The polymers studied were glassy (poly(styrene) (PS), poly(lactic-co-glycolic acid) (PLGA) and poly(acrylic acid) (PAA)) and rubbery(poly(styrene-b-isobutylene-b-styrene) (SIBS) and synthetic elastin (SE)). While no significant change in the Young’s modulus compared to bulk values was observed for the glassy polymers, the rubbery polymers showed more than an order of magnitude increase in Young’s modulus compared to bulk values. All the glassy polymers showed a brittle to ductile transition at the nanoscale which was attributed to the lack of significant defects and possibly to the alignment of polymer chains in the fibres axial direction caused by the electrospinning process.
Hydrogel nanofibres were produced by crosslinking neat electrospun PAA with short wave, 100 nm to 280 nm UV (UVC) radiation as presented in Chapter 5. While the crosslinking of neat PAA with UVC radiation had been described previously, it is practically utilised here for the first time. The actuation force generated by the individual PAA nanofibres when held at increasing constant pre-strains was measured successfully for the first time and found to follow models developed for bulk scale diffusion driven actuators. The degradation of individual PLGA nanofibres in PBS solution at 37oC was then monitored in Chapter 6. However, the large shrinkage and resulting breakage of the PLGA nanofibres themselves prevented monitoring of their mechanical properties over the full degradation life time.