Surface Chemical Functionalization to Achieve Extreme Levels of Molecular Confinement in Hybrid Nanocomposites

Abstract

Polymer confinement is realized in hybrid nanocomposites where individual polymer molecules are confined by a nanoporous matrix to dimensions less than the molecular size of the polymer. Here it is shown that by functionalizing the interior pore surfaces of a nanoporous organosilicate matrix, the pores can be filled with polystyrene molecules to achieve extreme levels of molecular confinement not previously possible. This provides opportunities for unique thermal and mechanical properties. It is shown that pore surface functionalization markedly impacts the polymer mobility during polymer infiltration by affecting the polymer-pore surface interaction, addressing the challenge of filling high-molecular-weight polymer molecules into nanoscaleconfined spaces. This allows for achieving extreme levels of molecular confinement with the loss of interchain entanglement and extensive polymer elongation along the pore axis. The glass transition temperature of the polymer is suppressed compared to bulk polymer melt, and is significantly affected by the polymer-surface interaction, which changes the polymer segmental mobility. The polymer-surface interaction also affects the interfacial polymer-pore sliding shear stress during polymer pullout from the nanopores, markedly affecting the fracture resistance of the nanocomposite.of the polymer, which has been shown to reduce entanglements, [4] form ordered morphology, [5] influence T g , [6] improve polymer diffusivity, [7] and enhance fracture resistance of the nanocomposites. [8] A fundamental limit of these studies, however, has been to achieve extreme levels of molecular confinement and the ability to control the interfacial interaction of the confined polymer with the matrix pores.Here we demonstrate that pore surface chemical functionalization allows us to achieve extreme levels of molecular confinement and manipulate the nanocomposite thermal and mechanical properties by controlling the interaction of the polymer with the pore surface (Figure 1). We show that the polymer filling rate can be significantly improved through selection of appropriate surface chemistry, which enables filling polymers with unprecedentedly high molecular weight (M w ) into the porous matrix to create extreme levels of molecular confinement not previously possible. This induces polymer hyperconfinement [[8a]] in which polymer chains entirely lose interchain entanglements and elongate extensively along the pore axis. [9] We also show that T g of the polymer is decreased by hyperconfinement, and can be significantly affected by tuning the polymer-surface interaction to impact the polymer segmental mobility.In addition, we show that the polymer-surface interaction affects the fracture of the nanocomposites where molecular bridging that involves the pullout and stretching of individual confined polymer chains from the nanopores leads to a marked increase in fracture resistance. The surface chemical functionalization significantly changes the sliding shear stress at the polymer/pore surface interfac...

Solvent-Driven Infiltration of Polymer (SIP) into Nanoparticle Packings

“…It has been postulated that interactions between PS and the untreated SiO2 surface could be attributed to SiOH-π hydrogen bonds forming between the phenyl ring of the PS and the silanol groups on the SiO2 surface. 42 Such an interaction has been observed previously in gas adsorption studies of benzene and toluene onto SiO2. 43 Unlike the dynamics, the partitioning behavior of polymer into the interstitial voids of the nanoparticle packings in the SIP system does not strongly depend on the confinement ratio (Figure 4).…”

“…A previous report suggests that some interactions exist between PS and SiO2. 42 To test whether PS in toluene can adsorb on the surface of SiO2, we perform quartz crystal microbalance with dissipation (QCM-D) experiments. In this experiment, a SiO2coated quartz crystal is exposed to a continuous flow of dilute PS dissolved in toluene (M n = 173,000 g/mol, c = 0.05wt%) and the resulting changes in the frequency (Δf) and dissipation (ΔD) are monitored in situ.…”

Effect of Confinement on Solvent-Driven Infiltration of the Polymer into Nanoparticle Packings

“…Modeling of this final adsorbed layer using an extended viscoelastic model indicates that the solvated adsorbed layer is roughly 12 nm thick, which is comparable to the R g of the PS in solution (Figure S2). It has been postulated that interactions between PS and the untreated SiO 2 surface could be attributed to SiOH-π hydrogen bonds forming between the phenyl ring of the PS and the silanol groups on the SiO 2 surface . Such an interaction has been observed previously in gas adsorption studies of benzene and toluene onto SiO 2 …”

“…Previous simulation studies indicate that the polymer can partition more strongly into confined spaces ( K > 1) when there are strong interactions between the polymer and pore surface. , Although PS has been observed to interact with and adsorb onto unfunctionalized SiO 2 surfaces under theta conditions, − there are a significantly smaller number of studies that investigate the adsorption behavior of PS onto SiO 2 surfaces from a very good solvent (toluene). A previous report suggests that some interactions exist between PS and SiO 2 …”

“…A previous report suggests that some interactions exist between PS and SiO 2 . 42 To test whether PS in toluene can adsorb on the surface of SiO 2 , we perform QCM-D experiments. In this experiment, a SiO 2 -coated quartz crystal is exposed to a continuous flow of PS dissolved in toluene (M n = 173,000 g/mol, c = 0.05 wt %) and the resulting changes in the frequency (Δf) and dissipation (ΔD) are monitored in situ.…”