The Interplay of Polymer Bridging and Entanglement in Toughening Polymer-Infiltrated Nanoparticle Films

Abstract

Polymer-nanoparticle composite films (PNCFs) with high loadings of nanoparticles (NPs) (>50 vol %) have applications in multiple areas, and an understanding of their mechanical properties is essential for their broader use. The high-volume fraction and small size of the NPs lead to physical confinement of the polymers that can drastically change the properties of polymers relative to the bulk. We investigate the fracture behavior of a class of highly loaded PNCFs prepared by polymer infiltration into NP packings. These polymer-infiltrated nanoparticle films (PINFs) have applications as multifunctional coatings and membranes and provide a platform to understand the behavior of polymers that are highly confined. Here, the extent of confinement in PINFs is tuned from 0.1 to 44 and the fracture toughness of PINFs is increased by up to a factor of 12 by varying the molecular weight of the polymers over 3 orders of magnitude and using NPs with diameters ranging from 9 to 100 nm. The results show that brittle, low molecular weight (MW) polymers can significantly toughen NP packings, and this toughening effect becomes less pronounced with increasing NP size. In contrast, high MW polymers capable of forming interchain entanglements are more effective in toughening large NP packings. We propose that confinement has competing effects of polymer bridging increasing toughness and chain disentanglement decreasing toughness. These findings provide insight into the fracture behavior of confined polymers and will guide the development of mechanically robust PINFs as well as other highly loaded PNCFs.

Role of Polymer–Nanoparticle Interactions on the Fracture Toughness of Polymer-Infiltrated Nanoparticle Films

“…As shown in our previous work, 45 the fracture behavior of PINFs is strongly affected by the degree of confinement. To focus on the effect of polymer−NP interaction strength, we keep CR unchanged by selecting P2VP with a similar MW as PS and choosing NP packings with approximately the same pore size after silane passivation.…”

“…To estimate the degree of confinement, we define a CR as the ratio between the radius of gyration of the unperturbed polymer chain (R g ) to average pore radius (i.e., CR = R g /R pore ). 45 Our previous studies have shown that stronger interactions between polymer and NP as well as stronger confinement lead to a greater positive shift in T g (i.e., ΔT g = T g,confined − T g , bulk is positive) of the polymer in PINFs. 30,31,41,43,46 As shown in Figure 3, confined polymers exhibit higher-than-bulk T g for all the samples investigated.…”

“…PINFs are prepared as previously described. 45 Prior to film deposition, single-side-polished 4-inch Si wafers (100) with a 1 nm native oxide layer (University Wafer) are rinsed with isopropanol and deionized water, dried with nitrogen, and then treated by oxygen plasma for ∼4 min. To avoid crack formation during polymer infiltration, a polymer-top geometry is used, where the NP film is spin-coated first, followed by the polymer film.…”

“…30 The thicknesses and refractive indices of the fractured films are also measured and compared with the as-prepared films to determine the location of the crack path. 45 The surface morphologies of the as-prepared and fractured films are imaged using…”

Polymer solution structure and dynamics within pores of hexagonally close-packed nanoparticles

“…2,3 While past computational work and experiments have been focused on studying PNCs with nanofillers being the minority component, there is a push for engineering PNCs with larger nanofiller content (e.g., 450% by volume (%v) filler) as it has been found that the nanofiller-induced polymer confinement improves PNC toughness. 4,5 One approach to fabricate PNCs with higher filler content is through a solvent casting process whereby the nanofiller and polymer are mixed in the desired portions in a solvent, and upon solvent evaporation, one forms PNCs with the desired composition. [6][7][8] For such processes, there is a need to focus on polymer structure and dynamics around nanofillers in the presence of solvent and to study how the evaporation of the solvent affects the polymer structure and dynamics; the former is tackled in this paper.…”