60th Annual Report on Research 2015

In order to suspend PS nanoparticles of varying diameters into glycerol and the ionic liquid, a portion of the surfactant-free emulsion polymerized PS nanoparticles originally suspended in water was mixed with either glycerol (³ 99.5%, Sigma-Aldrich) or [BMIM]-[CF3SO3] (³ 95%, Sigma-Aldrich). Subsequently, PS nanoparticles suspended in a water/glycerol or water/ionic liquid environment were dried in a vacuum oven for at least 3 days. Due to the low vapor pressures of glycerol and the ionic liquid, vacuum drying the PS nanoparticles suspended in these mixed liquids results in the removal of the water only. Thus, after 3 days of vacuum drying (i.e., water removal), the PS nanoparticles are suspended in either pure glycerol or [BMIM]-[CF3SO3]. The T_gs of varying diameters of PS nanoparticles were then measured using DSC with a heating rate of 10 K/min.

Figure 1a plots the glass transition temperature vs. diameter for PS nanoparticles suspended in glycerol (squares) and in the ionic liquid (diamonds). The inset shows the chemical structures of glycerol and [BMIM]-[CF3SO3]. Here, it is clear that for PS nanoparticles suspended in glycerol, as the diameter is reduced, the T_g decreases slightly in a systematic manner. For example, for a ~ 200 nm diameter PS nanoparticle sample, the T_g is ~ 7 K lower than the bulk T_g of PS. For PS nanoparticles suspended in an ionic liquid, the T_g does not deviate from the bulk T_g of PS, within experimental error, at all diameters examined in the current study. For example, the T_g of ~ 200 nm diameter PS nanoparticles in the ionic liquid remains at ~ 375 K, which is equivalent to T_g,bulk. Hence, a size-dependent T_g is not observed for PS nanoparticles suspended in [BMIM]-[CF3SO3].

In order to examine the impact of confinement on the T_g of PS nanoparticles measured in different environments, Figure 1b shows the T_g vs. diameter for PS nanoparticles suspended in water (circles), glycerol (squares), ionic liquid (diamonds), and in the dried state under a nitrogen atmosphere (triangles). For aqueous-suspended and dried PS nanoparticles, a large systematic reduction in the T_g is observed when the diameter was decreased. The significant decrease in T_g was attributed to an enhanced mobile layer at the polymer/water or polymer/nitrogen interface, which acts to locally reduce cooperativity requirements for the polymeric segments. Hence, the T_g is lowered at the interface and the effects are propagated into the interior of the nanoparticle, leading to a lower average Tg in the nanoparticles. Here, the T_g-confinement effect in PS nanoparticles measured in aqueous and nitrogen environments are identical, i.e., the value of the T_g measured is the same at the same extent of confinement. This may be a consequence of the relative high mobility of each medium compared to the polymer, i.e., each may be considered a soft (low-viscous) medium that does not impose significant constraints at the interface.

Clearly in Figure 1b, when PS nanoparticles are measured in a glycerol environment, the observed Tg-confinement effect is suppressed, when compared to either the aqueous or nitrogen environment. For example, at a nanoparticle diameter of ~ 200 nm, the Tg of PS in glycerol is ~ 368 K, whereas for PS in water or nitrogen, the measured Tg is ~ 345 K. The suppressed T_g-confinement effect for PS nanoparticles in glycerol may be due to a lower mobility at the polymer/glycerol interface (when compared to the polymer/water or polymer/nitrogen interface). That is, the viscosity of glycerol (= 1412 cP at 293 K) is much higher than the viscosity of water (= 1.002 cP at 293 K) or nitrogen (= 0.0178 cP at 298 K and ~ 7 atm). The higher viscosity of glycerol may impose constraints at the polymer/glycerol interface, causing the interfacial polymer chains to be more sluggish than in a low-viscous medium. However, the mobility of polymer chains at the polymer/glycerol interface is still enhanced when compared to that of the bulk or to that at an interface between the polymer and a rigid substrate, since the T_g does decrease slightly with decreasing nanoparticle diameter.