MS03.08.06 : High-Strain-Rate Plasticity of Block Copolymer Microspheres Correlating with Microphase Separation
4:15 PM–4:30 PM Dec 4, 2019
Ara Kim1 Jae-Hwang Lee11, UMASS Amherst, Amherst, Massachusetts, United States
In the use of advanced manufacturing, targeting certain mechanical performances by tailoring inherent material properties is a success factor, and material diversities are emphasized as the application area expands to non-metallic materials and mixed composites. Multi-phase polymers or phase-separated block copolymers (BCPs) consist of two or more mechanically distinctive nanoscale phases. The diverse nanostructures of BCPs, which can be tailored through thermal annealing process, cause obvious changes in mechanical properties. Moreover, the high-strain-rate deformation characteristics are generated and controlled by impact tests with different collision conditions. Polymers are emerging materials in additive manufacturing such as the cold spray technique, however, mechanical behaviors including extreme plastic deformations resulting from collisions have not been demonstrated thoroughly.
In order to obtain insight into the novel phenomena of BCP’s mechanical behaviors with anisotropic nanostructures, single-particle impact experiments of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) block copolymer (BCP) microparticles with different nanostructures were performed to demonstrate the effects of nanostructures of BCP microparticles before and after impact against rigid substrates. BCP particles with different volume fractions of PS and PDMS were annealed at diverse conditions to obtain different degrees of ordered nanostructures in two phases, such as cylinders and lamellae. Both annealed and non-annealed BCP particles were tested by using the laser-induced projectile impact test (LIPIT) method. A single BCP micro-particle was accelerated to a high velocity (70 – 600 m/s) and impacted onto a rigid substrate, and the collision was monitored with an ultrafast imaging system using femtosecond illumination pulses.
Coefficients of restitution, critical velocity, acceleration-force-induced inelastic deformation and collision-induced extreme plastic deformation features of the BCP particles were investigated for different nanostructures and impact conditions. Electron microscopy and focused ion beam milling were used to demonstrate ordered nanostructures and impact-induced morphological changes of the BCPs before and after the collisions. Acceleration-force-induced inelastic deformation was investigated by analyzing dimension changes during acceleration and until the moment of impact. As critical velocities and coefficients of restitution of BCPs were changed depending on the status of the nanostructures, the morphologies of nanostructures were important factors in deciding the mechanical characteristics of BCP before and after impact. One of the most critical findings of this study was that nanoscale structural changes of BCP microparticles cause microscale changes in mechanical behaviors, and it furthers research into how BCPs can be tailored to satisfy target performances in additive manufacturing.
* This material is based upon work supported by the National Science Foundation under Grant No. CMMI-1760924.