Bubble Seeding Nanocavities: Multiple Polymer Foam Cell Nucleation by Polydimethylsiloxane-Grafted Designer Silica Nanoparticles

Abstract

We describe a successful strategy to substantially enhance cell nucleation efficiency in polymer foams by using designer nanoparticles as nucleating agents. Bare and poly­(dimethylsilane) (PDMS)-grafted raspberry-like silica nanoparticles with diameters ranging from ∼80 nm to ∼200 nm were synthesized and utilized as highly efficient cell nucleators in CO2-blown nanocellular polymethyl methacrylate (PMMA) foams. The successful synthesis of core–shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer–Emmett–Teller measurements, and transmission electron microscopy. The cell size and cell density of the obtained PMMA micro- and nanocellular foams were determined by scanning electron microscopy. The results show that increased surface roughness enhances the nucleation efficiency of the designer silica particles. This effect is ascribed to a decreased nucleation free energy for foam cell nucleation in the nanocavities at the melt–nucleator interface. For PDMS grafted raspberry-like silica nanoparticles with diameters of 155 and 200 nm, multiple cell nucleation events were observed. These hybrid particles had nucleation efficiencies of 3.7 and 6.2, respectively. The surprising increase in nucleation efficiency to above unity is ascribed to the significant increase in CO2 absorption and capillary condensation in the corresponding PMMA during saturation. This increase results in the presence of large amounts of the physical blowing agent close to energetically favorable nucleation points. Additionally, it is shown that as a consequence of cell coalescence, the increased number of foam cells is rapidly reduced during the first seconds of foaming. Hence, the design of highly efficient nucleating particles, as well as careful selection of foam matrix materials, seems to be of pivotal importance for obtaining polymer cellular materials with cell dimensions at the nanoscale. These findings contribute to the fabrication of polymer foams with high thermal insulation capacity and have relevance in general to the area of cellular materials.

Designer Core–Shell Nanoparticles as Polymer Foam Cell Nucleating Agents: The Impact of Molecularly Engineered Interfaces

“…In principle, there are no physical restrictions that prevent the occurrence of more than one nucleation event per particle, that is, nucleation efficiencies exceeding unity are possible. 46 …”

Nanostructure of PMMA/MAM Blends Prepared by Out-of-Equilibrium (Extrusion) and Near-Equilibrium (Casting) Self-Assembly and Their Nanocellular or Microcellular Structure Obtained from CO2 Foaming

“…The presence of the micelle cores almost perfectly centered in the cells (Figures 7 and S2, see Supplementary Materials), and the proposed energetic preference of nucleation to happen directly inside the PBA phase (i.e., decreasing the energy barrier due to the lower Gibbs energy of the PBA phase and avoiding the contribution of the line tension) [23,28], suggest that the cell nucleation happens simultaneously at several points of the PBA shell (Figure 8a,b). Moreover, the remaining structures or filaments between the micelle cores and the cell walls could be considered evidence of multiple nucleation sites, as proposed in the work of Liu et al [44]. Another nucleation process, such as a single nucleation point or nucleation in the interfaces, would probably displace the micelle cores from the center.…”

Effect of the Molecular Structure of TPU on the Cellular Structure of Nanocellular Polymers Based on PMMA/TPU Blends

“…In the last years, several studies were conducted focusing on the effect of nucleating species in the fabrication of nanocellular polymers, such as nanoparticles [ 17 , 18 , 19 , 20 ] or an immiscible polymeric second phase [ 21 , 22 ]. For instance, Liu et al [ 23 ] synthesize silica nanoparticles with different surface roughness to enhance the nucleation of poly(methyl-methacrylate) PMMA. They employed the same quantity of silica nanoparticles on each compound (around 10 13 particles/cm 3 ) and observed that increasing the surface roughness promotes a higher nucleation density because the energy barrier which promotes the cell nucleation is lower for concave surfaces than for flat-convex surfaces [ 24 ].…”