Uniaxial Deformation and Crazing in Glassy Polymer-Grafted Nanoparticle Ultrathin Films

Abstract

The deformation behavior of neat, glassy polymer-grafted nanoparticle (PGN) monolayer films is studied using coarse-grained molecular dynamics simulations and experiments on polystyrene-grafted silica. In both the simulations and experiments, apparent crazing behavior is observed during deformation. The PGN systems show a relatively more uniform, perforated sheet craze structure and significantly higher strain at break than reference homopolymers of the same length. Short chain, unentangled PGN monolayers are also simulated for comparison; these are brittle and break apart without crazing. The entangled PGN simulations are analyzed in detail for systems at both high and moderate graft density. Stress−strain curves show three distinct regions: yielding and strain localization, craze widening, and strain hardening preceding catastrophic failure. The PGN stress−strain behavior appears more similar to that of longer chain, highly entangled homopolymer films than to the reference homopolymer films of the same length as the graft chains, suggesting that the particles effectively add additional entanglement points. The moderate graft density particles have higher strain-to-failure and maximum stress than the high graft density particles. We suggest this increased robustness for lower graft density systems is due to their increased interpenetration of graft chains between neighboring particles, which leads to increased interparticle entanglements per chain.

Projectile Impact Shock-Induced Deformation of One-Component Polymer Nanocomposite Thin Films

“…Compressive shock heating occurs for these films, as for the higher molecular weight films, but the absence of chain entanglements between canopies eliminates the extensive melt draw. 7 Therefore, energy absorption is highly localized at the edge of the circular fracture, consistent with previous quasi-static measurements of strongly decreasing fracture toughness of PGNs with decreased PS chain length, 12,34 and brittle failure of low MW (∼30 kg/mol) homopolystyrene films under quasistatic tests 12,34 as well as in LIPIT. 14,16 PGNs with PS grafts having high MW such that the canopies are well entangled along with highly grafted NP nodes are much tougher.…”

“…The mechanical behavior can be modified via choice of the nanoparticle core radius ( r 0 ), graft density (anchored chains per unit area of the nanoparticle, ∑), and graft chain length (N), which along with r 0 and ∑, determines the grafted chain conformation and hence the distribution of entanglement density and the number of entanglements per chain . Recent quasi-static uniaxial tensile studies have established the relationship between various PGN designs, intercanopy entanglement, and their quasi-static thin film mechanical behavior below T g . − Four types of PS-SiO 2 and PS-Fe 3 O 4 PGNs were synthesized by surface-initiated living radical polymerization methods, based on established protocols (see Supporting Information (SI)). The Fe 3 O 4 and SiO 2 spherical core nanoparticles have r 0 = 3.9 ± 0.6 and 8.0 ± 2.8 nm grafted with low (unentangled) and high molecular weight (well entangled) chains having graft densities from 0.04 to 0.57 chains/nm 2 , respectively (Table ).…”

“…Moreover, the strong dependence of polymer properties on strain-rate and temperature presents many challenges and opportunities. , At a given temperature, amorphous polymers transition from rubbery, to ductile-plastic, to brittle with increasing strain-rate. − The yield stress increases with both the hydrostatic pressure and strain rate, while it falls with increase in temperature . For glassy polymers, additives such as inorganic fillers or rubbery inclusions can both dramatically decrease or increase the fracture toughness, depending on the extent of shear banding and craze nucleation and stable craze growth. ,− Under low strain rate testing, polymer-grafted nanoparticles (PGNs), one component nanocomposites, enable mechanically robust films by avoiding classic nanoparticle agglomeration and property heterogeneities occurring in two component polymer-NP blends and provide load transfer between canopy-entangled chains as well as from chain grafting to the NPs. − …”

Static and Dynamic Behavior of Polymer/Graphite Oxide Nanocomposites before and after Thermal Reduction

“…Nanohybrids composed of polymers and different types of additives, which have at least one dimension in the nm range, have attracted scientific interest since they usually possess superior properties compared to neat polymers or to composites that contain large reinforcing moieties [1][2][3][4][5][6][7][8][9]. Examples of such nanoadditives are certain layered materials like clays, graphene-based and/or other 2-D materials, as well as nanoparticles like silicon oxide (SiO 2 ) and titanium oxide (TiO 2 ) among others, or tubular materials like carbon nanotubes.…”

Coexistence and Phase Behavior of Solvent–Polystyrene-Grafted Gold Nanoparticle Systems

“…Control of PGN dispersion, and assembly, in solvents, reactive resins, and polymers, is crucial to simultaneously optimize synthesis, processability, and performance in all these uses. The hierarchy of the structure, from the tethered polymer canopy to the local arrangement of nearest-neighbors and the morphology of the aggregates, all influence the property suite of the nanocomposite and single-component assembly. − …”