Abstract
Stimuli-responsive shape memory polymer surfaces show switchable change in surface property in response to an external stimulus. Material’s surface properties like wettability, adhesion and surface elastic modulus are influenced by surface patterns. Shape memory surfaces reported in the literature thus far demonstrated only dual shape memory surfaces. This involves programming one permanent memory of a surface topography memory, which is thermo-mechanically deformed to a temporary shape. Subsequently, the permanent surface topography memory is recovered, resulting in a single switch in surface property in response to a stimulus. The existing dual shape memory surfaces have three major limitations: 1) there is no precise controllability of the surface property as there is no design flexibility over the choice of temporary shape morphology; 2) lack of multiple switches in surface properties as only one permanent surface topography memory has been programmed thus far; and 3) recovery of the surface topography when subjected to non-uniform deformations such as scratch remains challenging because it involves splitting of material along with deformed and displaced surface patterns.
To address the above-mentioned gaps, it is crucial to program two distinct surface patterns within the same material system, while maintaining structural integrity. The major limitation to program multiple surface topographies is that majority of materials used to fabricate shape memory polymer surfaces are homopolymers with a single melt transition temperature, which are unsuitable for the programming of multiple permanent surface topographies. Furthermore, commonly used surface patterning processes such as laser etching, photolithography, replica molding, spin coating and solution casting offer limited processing window, hence are limited to programming of one permanent surface topography memory. This thesis reports the investigation of heterogeneous shape memory polymer blends with two distinct melt transition temperatures to achieve tunable multiple shape memory effect. Polymer blends with preferential crosslinking have been designed with semi-interpenetrated polymer network (semi-IPN) and fully interpenetrated network (full-IPN) which enable the tuning of the thermo-mechanical memories. The fabrication utilizes thermal nanoimprint lithography (NIL) that offer multiple thermo-mechanical molding stages, enabling the programming of multiple surface patterns through sequential imprinting.
This thesis reports two ‘first of its kind’ results: 1) triple and quadruple shape memory effects on micro-patterned surfaces with two distinct surface topographies; and 2) recovery of uniform surface pattern from scratch induced damages such as material splitting, surface pattern deformation and displacement.
In the first part of this thesis, triple and quadruple shape memory surfaces were investigated using Polycaprolactone (PCL)/Polyethylene (PE) 7030 semi-IPN blend and PCL/PE 5050 full-IPN blend with two well separated melt transition temperatures. In the triple shape memory surface, the first permanent memory was the non-patterned planar surface of the 7030 PCL/PE blend, from which a second permanent memory was an array of micron-pillar, while the temporary shape was an array of micron-grating pattern. The difference in the morphology between the temporary shape’s surface topography and the recovered permanent memory’s topography resulted in 61% change in surface elastic modulus values when subjected to heat at 60 °C for 180 seconds.
In the case of quadruple shape memory surface, the first permanent memory was a non-patterned planar surface, the second permanent memory was achieved in an array of micro-pillar, and a third permanent memory was an array of micron-grating arranged hierarchically over the second permanent micron-pillar array. In all the above multiple memories of multiple surface topographies, near complete recovery of the permanent surface topographies have been achieved when presented with appropriate thermal stimuli. Furthermore, multiple switches in surface elastic modulus were also demonstrated.
In the second part of the thesis, scratch recovery in micron-patterned dual shape memory surfaces were investigated using Thermoplastic Polyurethane (TPU) / PCL 6040 semi-IPN blend. In this dual shape memory surface, with the first permanent memory being the non-patterned planar surface of the 6040 TPU/PCL film, the second permanent memory was formed in an array of micron-pillar. The programmed permanent memories subjected to scratch were recovered by the shape memory effect of crosslinked network and the chain diffusion enabled by the melting of the semicrystalline polycaprolactone at 60 °C. Thus, this thesis has contributed new knowledge that will be useful for advancing the field of shape memory surfaces.