High strain rate deformation of concentric layer diblock copolymer microspheres

An advanced laser-induced projectile impact test (LIPIT) apparatus was used to investigate high rate deformation of lamellar block copolymer (BCP) particles. By employing polyvinyl alcohol (PVA) as surfactant, chloroform as solvent and homogenization processing of a 42 kg/mol-32 kg/mol PS-PDMS BCP, (45 vol % PDMS) nearly defect-free, concentric microspheres were produced. These particles (diameters from 2.8 to 3.8 microns), were then individually launched at supersonic velocities (300~700 m/s) to impact a gold coated silicon substrate at normal incidence. Extreme microstructural deformations were observed after impact such as layer kinking to form axisymmetric chevron boundaries and layer compression as well as different types of overall particle shapes, depending on the magnitude of the KE at impact. Interestingly, spreading/wetting of the particle over the substrate occurred at higher speeds (480 m/s < v_i < 700 m/s), due to the shock compression adiabatic heating enabling rapid lateral diffusion of polymers along inter-material dividing surface (IMDS). The effects of different substrate surface energies and thermal conductivities on the deformation-spreading were also explored.