|Ph.D Student||Alhazov Dmitry|
|Subject||Study of the Thermo-mechanical Properties of Electrospun|
Block Co-Polymer Nanofibers
|Department||Department of Mechanical Engineering||Supervisor||Professor Eyal Zussman|
|Full Thesis text|
While extensive studies exist for thermo-mechanical properties of bulk segmented co-polymers, the thermo-mechanical properties at the nano-scale are still in its earliest stages. It is hypothesized that structural features affecting the mechanical/thermo-mechanical properties of nanofibers are: 1) the supermolecular structure in the nanofibers, and 2) confinement of supermolecular structure.
The proposed research aims to use the electrospinning process to produce nanoscale objects made of segmented co-polymers, for example polyurethane nanofibers with diameters ranging from 300 nm to 1 microns. In this work, we pursue to evaluate the desired knowledge on the structure-property relationship and to study structural aspects of segmented co-polymers, while scaling up the nanostructure to macroscopic mechanical and thermomechanical properties. Suggesting the physical mechanisms describing the observed phenomenon are crucial in order to understand the peculiar behavior of nanofibers.
Main experimental methods used in the present work are: Wide and Small X-Ray scattering (WAXS/SAXS), differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA) and Fourier transform infrared spectroscopy (FTIR).
Upon heating, nanofibers began to massively contract, at ~ 70°C, whereas TPU films started to expand. WAXS profiles of the nanofibers and the films showed no diffraction peaks related to crystals, whereas their amorphous halo had an asymmetric shape, which is approximated by two components, associated with hard and soft segments.
Unexpectedly, we demonstrated that TPU films can also massively contract upon heating, but only after thermo-mechanical programming (shape memory effect, SME). This non-equilibrium stretched state is highly preserved, despite sample temperatures that significantly exceeded the glass transition temperature of the soft phase, and hard segments concentration in the TPU macromolecules is too low to form a percolated “solid” system.
Experimental evidences reveal the role of the supermolecular structure of the segmented polyurethane, combined from disordered and ordered hard segment clusters, in relaxation suppression above the Tg (~-50°C) of the polymer matrix. In addition, aging has a pronounced effect on the thermo-viscoelastic properties of the TPU, hence affecting the shape memory behavior of the polymer. In particular, the non-aged sample shows shift in the α-transition to higher frequencies and a shift in the melting type transition to lower frequencies compared to the aged TPU. These observations, accompanied by mechanical measurements, suggested gradual destruction of non-crystalline hard segment clusters (first disordered, then ordered) to be responsible for the observed phenomenon.
Lastly, taking advantage of the studied contraction phenomenon, a novel thermo-responsive fiber mat showing controlled release of liquids upon heating was demonstrated.
The possibility of the relaxation suppression in the TPU copolymer matrix above Tg both in the electrospun nanofibers and in cast films, provided support for the key arguments, where the phenomenon of relaxation suppression cannot be directly attributed to the confinement effect, rather to the hidden features of the segmented co-polymer matrix which can preserve/memorize the stretched state above the glass transition temperature, and consequently suppress relaxation of the polymer matrix. These features are strongly related to the supermolecular structure of the TPU polymer matrix. Finally, a new set of macroscopic thermo-mechanical properties were discovered, explained by microscopic observations.