Molecular-Weight-Dependent Interplay of Brittle-to-Ductile Transition in High-Strain-Rate Cold Spray Deposition of Glassy Polymers

Abstract

Based on the cold spray technique, the solvent-free and solid-state deposition of glassy polymers is envisioned. Adiabatic inelastic deformation mechanisms in the cold spray technique are studied through high-velocity collisions (<1000 m/s) of polystyrene microparticles against stationary target substrates of polystyrene and silicon. During extreme collisions, a brittle-to-ductile transition occurs, leading to either fracture- or shear-dominant inelastic deformation of the colliding microparticles. Due to the nonlinear interplay between the adiabatic shearing and the thermal softening of polystyrene, the plastic shear flow becomes the dominant deformation channel over brittle fragmentation when increasing the rigidity of the target substrate. High molecular weights (>20 kDa) are essential to hinder the evolution of brittle fracture and promote shear-induced heating beyond the glass transition temperature of polystyrene. However, an excessively high molecular weight (∼100 kDa) reduces the adhesion of the microparticles to the substrate due to insufficient wetting of the softened polystyrene. Due to the two competing viscoelastic effects, proper selection of molecular weight becomes critical for the cold spray technique of glassy polymers.

Influence of Entanglements on Ultrahigh Strain Rate Deformation of Polystyrene Microprojectiles

“…Like the yield stress, the crazing stress increases with the strain rate but decreases with increased temperature. , The size of the PS projectile plays an important role in the degree of adiabatic heating. In the recent Lee et al’s work, a ∼40 μm diameter PS projectile was employed, thus for the same ν i , about a factor of about 1000 greater KE than for our 3.6 μm diameter projectile. They studied similar MW PS but found different deformation morphologies featuring nonaxisymmetric viscous flow and fracture-cracking without extensive shear banding and crazing even for 100 kDa PS.…”

“…These differences are likely due to the increased adiabatic heating and a more severe temperature rise as well as the presence of defects cited in the preparation of their microspheres. 9 ■ EXPERIMENTAL OBSERVATIONS OF PROJECTILE DEFORMATION By controlling the MW of the PS projectiles and their initial impact velocity, one can observe the interaction of sample temperature, MW, and strain rate on the deformation processes that ensue. Table S2 shows the percent of rebounded versus adhered projectiles as a function of MW and ν i .…”

“…They studied similar MW PS but found different deformation morphologies featuring nonaxisymmetric viscous flow and fracture-cracking without extensive shear banding and crazing even for 100 kDa PS. These differences are likely due to the increased adiabatic heating and a more severe temperature rise as well as the presence of defects cited in the preparation of their microspheres …”

“…The mechanical properties of a polymeric material depend strongly on molecular weight (MW), temperature, strain rate, and state of stress. Research into the MW dependence of deformation is quite extensive under low strain rate conditions, , but fewer investigations have been done in the ultrahigh strain rate regime. ,,− For a given MW, increasing the strain rate or decreasing the temperature makes the response time of the polymer much longer than the high rate loading time, thus inducing brittle behavior. By increasing the MW, the number of entanglements ( N e = MW/ M e , where M e is the entanglement MW) increases, which not only raises the yield strength but also strongly increases the response times of the polymer.…”

Extreme Plasticity, Adhesion, and Nanostructural Changes of Diblock Copolymer Microparticles in Cold Spray Additive Manufacturing

“…17 The molecularweight-dependent mechanical and rheological behavior of polystyrene μPs and substrates were also explored through LIPIT, and the results showed the collision-induced plastic shear flow and thermal softening resulting from viscoplastic work. 18 However, the UHR plastic response, the resultant interfacial rheological characteristics, and the irreversibly produced nanostructural changes of polymeric μPs have not been sufficiently understood, and the material diversity is limited to homopolymers. The rate-dependent mechanical and temperature-dependent rheological properties causing the collision-induced extreme plastic behavior and morphologies are determined by the composition of the monomers and the configurations of their domains, such as their molecular weight, entanglement density, and nanostructure.…”