Abstract
Despite the growing application of nanostructured polymeric materials, there still remains a large gap in our understanding of polymer mechanics and thermal stability under confinement and near polymer–polymer interfaces. In particular, the knowledge of polymer nanoparticle thermal stability and mechanics is of great importance for their application in drug delivery, phononics, and photonics. Here, we quantified the effects of a polymer shell layer on the modulus and glass-transition temperature (Tg) of polymer core–shell nanoparticles via Brillouin light spectroscopy and modulated differential scanning calorimetry, respectively. Nanoparticles consisting of a polystyrene (PS) core and shell layers of poly(n-butyl methacrylate) (PBMA) were characterized as model systems. We found that the high Tg of the PS core was largely unaffected by the presence of an outer polymer shell, whereas the lower Tg of the PBMA shell layer decreased with increasing PBMA thickness. The surface mobility was revealed at a temperature about 15 K lower than the Tg of the PBMA shell layer. Overall, the modulus of the core–shell nanoparticles decreased with increasing PBMA shell layer thickness. These results suggest that the nanoparticle modulus and Tg can be tuned independently through the control of nanoparticle composition and architecture.
Highlights
Nanoscaled polymeric materials are increasingly used in advanced technologies, for example, as separation membranes,[1,2] drug-delivery vehicles,[3] reinforcing modifiers,[4] and photonic[5] and phononic crystals.[6]
We found that the high Tg of the PS core was largely unaffected by the presence of an outer polymer shell, whereas the lower Tg of the poly(n-butyl methacrylate) (PBMA) shell layer decreased with increasing PBMA thickness
We found a significant change in the elastic modulus and thermal behavior of NPs by the introduction of ultrathin shell layers with different chemical compositions via Brillouin light-scattering spectroscopy (BLS).[38]
Summary
Nanoscaled polymeric materials are increasingly used in advanced technologies, for example, as separation membranes,[1,2] drug-delivery vehicles,[3] reinforcing modifiers,[4] and photonic[5] and phononic crystals.[6]. The determining factor for either reduction or enhancement of the modulus for film−substrate systems is still unclear Another system of polymer nanofibers such as PS and Nylon-11 shows a dramatic elastic modulus increase as the diameter drops below 200 nm.[61,62] Very recently, the enhanced elasticity in films of polymer-tethered nanoparticles at a low grafting density was attributed to strong polymer−polymer interactions compared to densely tethered NPs with short chains.[63] For the spherical confinement of Figure 2, the NP shear modulus can be rationalized assuming hardening for the PMMA and softening for the PBMA shell atop the same PS core. It does relate to Tg,l, and the difference Tg,l − Ts ∼ 15 K seems to be virtually independent of the PBMA thickness as shown in Figure 4a, suggesting a constant free surface contribution to Ts
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