Influence of Entanglements on Ultrahigh Strain Rate Deformation of Polystyrene Microprojectiles

Abstract

Micron-sized polystyrene spherical projectiles made from three different molecular weight polymers spanning from below to well above the entanglement molecular weight were launched against a rigid substrate, causing ultrahigh strain rate deformation. Scanning electron microscopy, focused ion beam cross sections, and atomic force microscopy were used to elucidate the deformation mechanisms from the observed morphologies of the deformed specimens and to evaluate the important role of entanglements in the various deformation processes that dissipate the kinetic energy. Due to adiabatic heating from shock compression, the temperature at the bottom region of the polymer projectile is elevated above the glass transition temperature, enabling viscoplastic flow. The cooler portions of the unentangled sample undergo strain localization via micro-shear banding and brittle fracture, while the two entangled samples resist fracture and exhibit extensive viscoplastic flow and, in the upper, cooler regions, extensive shear banding followed by crazing. The interaction of molecular entanglements, temperature, shear rate, and total shear strain on melt viscosity influences the amount of additional plastic work and adiabatic heating occurring in the sample and hence the extent of lateral spreading of the projectile over the substrate.

Molecular Weight Controls Interactions between Plastic Deformation and Fracture in Cold Spray of Glassy Polymers

“…Results presented and discussed in previous sections demonstrate that the deposition mechanisms of glassy PS particles are complex, strongly dependent on molecular weight, and governed by the interplay between plastic deformation (and its consequent phenomena, e.g., local temperature increase, solid to liquid transformation, etc. 21 , 22 ), and particle fracture. Although the role of the former is established as one of the primary bond formation and deposition mechanisms in the cold spray process, our findings suggest that the latter is of no less significance, at least in the cold spray of relatively brittle glassy thermoplastics.…”

“…Despite the significant scientific understandings presented thus far, there still exist major gaps in the understanding of the behavior of cold-sprayed glassy polymers, mainly due to complexities arising from their brittle nature. In particular, there is still insufficient understanding of the interplay between plastic deformation (as the major mechanism for bonding in polymer cold spray) and the possible roles of fracture and state transformation as two secondary mechanisms attributing to the dissipation of kinetic energy of the particles. , The primary focus of the present work is to investigate the interplay between plastic deformation and fracture as the two primary mechanisms that govern the adhesion behavior of cold-sprayed glassy polymers. The study is designed such that the concurrent effects of the two aforementioned mechanisms, that is, plastic deformation versus failure, are investigated systematically via experimentally validated computer simulations.…”

“…Their results demonstrated that a successful deposition was only possible when small quantities of alumina nanoparticles were incorporated into the polymeric powders, improving interparticle adhesion by providing additional surface functionality. A recent report by Shan et al 21 showed that the deformation response of PS microparticles in ultra-high strain rate impact is strongly dependent upon molecular weight. It was concluded in this work that the interactions between molecular entanglements, temperature, shear strain, and the strain rate govern the degree of plastic deformation and the particle’s failure behavior, thereby affecting the energy dissipation mechanisms upon impact.…”

Elasto-plastic effects on shape-shifting electron-beam-patterned gel-based micro-helices

“…48,49 Modelling and experiments to understand such deformation remain an active research area. [50][51][52] Most relevant here is that shear bands tend to preferentially form under conditions of relatively high crosslinking 48,53,54 or spatial confinement. [55][56][57] Both conditions are present in region 2.…”