Melt Spun Bi/Tri-Component Fibers Exhibiting Shape Memory: A Mesoscale to Macroscale Experimental and Theoretical Study

Formations of shape memory polymers (SMP) and soft smart structures that respond to the external stimuli have been in the forefront of polymeric materials development for the past three decades. Physical combinations of network forming and stimuli-sensitive macromolecular species ensure a highly versatile, scalable and cost effective means of creating SMPs materials. The incorporation of physically cross linked network formers not only allows excellent processing methods like melt spinning and injection molding, but also permits the structures thus formed to be reused and recycled. A sustainable route to achieving large scale production of SMP materials is explored by coprocessing linear low density polyethylene and poly-(styrene-co-(ethylene butadien)-co-styrene (SEBS) into bicomponent filaments. By controlling the composition, interfacial area and cross-sectional geometries of the bicomponent filaments, tunable SMP filaments were produced that respond to thermal activation. Bicomponent filaments composed of LLDPE/SEBS exhibit shape fixity at ambient temperatures due to the increased orientation in the homopolymer component resulting from plastic deformation. While the SEBS core component retains rapidly recoverable elastic strain. The recovery is observed to be spontaneous following the supply of heat that elevates the temperature of the LLDPE component and thereby making it mobile. Conventional shape memory behavior induced by heated programming is also possible in the bicomponent filaments and is investigated by means of thermal (differential scanning calorimetry), mechanical (tensile) and morphological(scanning electron microscopy) studies. The storage of macroscopic strain in the ambient temperature programming of SEBS/LLDPE filaments is supported by studies on increase of crystalline and amorphous orientation using optical birefringence. The release of the shape after heating the filaments is rapid and mechanical and microscopic evidence points out the permanent loss of properties due to SMP cycling. The SMP behavior is quantified in terms of strain recovery and strain fixity ratios and is dependent upon the composition, interfacial area and intended maximum strain of the SEBS/LLDPE filaments. The stability of the interface even at high strains is identified as the key to maintaining shape memory performance of these bicomponent filaments. Mesoscale simulations using dissipative particle dynamics (DPD) were employed to study the microphase segregation and network formation in the triblock copolymers interfaces with homopolymers. Firstly, the adaptation of the simulations to the present coarse graining and implementation of a spatial clustering approach to quantify the fraction of chains existing as bridges loops and dangling ends is discussed. Interfacial thickness, microphase segregation and network strength is studied with respect to varying incompatibilities among endblock, midblock and homopolymer species. In the simulation of experimental equivalent system, the fraction of bridges along the SEBS/LLDPE interface is estimated to linearly decrease with increasing PE volume content. Whereas the fraction of chains with both endblocks unsegregated increases rapidly from an insignificant number (0.05) at 50% to a large fraction (0.84) at 90% PE content by volume. Lastly, the investigation of molecular network formation in asymmetric triblock copolymers with increasing second endblock is conducted. A linear increase in the bridging and looping fraction with the asymmetry parameter is reported with a corresponding decrease in dangling ends.