Designer nanoparticles as efficient nucleation agents in polymer foaming
Low density nanocellular polymer nanocomposite foams are considered as a promising new class of materials with many potential applications, for instance, in high performance thermal insulation. Nanoparticles in these foams act as energetically favorable heterogeneous nucleation sites. However, the currently reported cell nucleation efficiency in polymer nanocomposite foaming is very low, i.e. below 0.01. This results in foams with relatively low cell densities. In this Thesis the design and synthesis of surface functionalized nanoparticles and their exploitation as highly efficient cell nucleation agents in CO2 assisted polymer nanocellular foaming is presented and discussed. Poly(methyl methacrylate) (PMMA) was selected as the foam matrix polymer due to its reported potential for the fabrication of nanocellular foams with a high porosity. As a comparison, polystyrene (PS) was used, as well. The influence of nanoparticles size, surface chemistry and surface texture on foam cell nucleation are discussed in detail. The investigation of interfacial interactions between individual nanoparticle and CO2 swollen polymer matrix is presented as well. Decoration of silica nanoparticles with a low surface energy and high CO2-philic thin polymer shell, e.g. PDMS, is found to be an efficient strategy to increase the nucleation efficiency of silica nanoparticles. The low surface energy and relatively high CO2 sorption in the PDMS shell is favorable for heterogeneous nucleation and as a consequence these particles outperform every currently known nanoparticles as heterogeneous nucleation sites.
In Chapter 1, a short introduction to the topics, the motivation and an overview of the work described in this Thesis are presented. A literature review concerning the general topics discussed in this thesis is described in Chapter 2. Emphasis is on CO2 based foaming strategies, limitations and challenges of heterogeneous nucleation in polymer foaming and thermal insulation performance of nanocellular polymer foams. In addition, the synthesis and subsequent surface derivatization strategies for silica nanoparticles are elucidated, as well.
Chapter 3 describes the synthesis of hybrid silica polymer core-shell nanoparticles and their exploration as highly efficient nucleation agents in CO2 batch foaming of PS and PMMA. Silica nanoparticles with a diameter of 80 nm were synthesized and surface grafted with PS, Fluorolink E10 and poly(dimethylsiloxane) (PDMS). Following melt blending of the prepared nanoparticles with PS and PMMA, batch foaming was used to produce nanocomposite foams. The results emphases that proper selection of the polymer grafts, e.g. PDMS, results in achieving nucleation efficiencies of up to approximately 0.5 (i.e. 1 foam cell per 2 particles on average).
In Chapter 4, the influence of nanoparticle curvature on cell nucleation is described. Bare and PDMS grafted nanoparticles with (core) diameter from 12 nm to 120 nm were synthesized and used as heterogeneous nucleation agents in CO2 batch foaming of PMMA. The results show that nanoparticles with a diameter below 40 nm were less efficient as nucleation agents compared to particles with a diameter of 60 nm or larger. This is ascribed to the presence of a line tension at the three phase contact line of a nucleated bubble with the nanoparticle and viscoelastic polymer. The line tension contributes to a higher nucleation energy and thus renders these particles less efficient compared to their larger counter parts. This extra energy penalty is not considered in the classical nucleation theory and its adaptations. Experimentally the presence of a positive line tension was confirmed by the absence of the smallest nanoparticles (diameter < 40 nm) at the polymer cell wall gas phase interface, i.e. the particles were engulfed by the polymer. Particularly interesting is the fact that when the CO2 saturation pressure was increased from 55 bar to 300 bar, which resulted in an increase CO2 concentration in the PMMA matrix, 60 and 80 nanometer particles were also nearly completed engulfed. This is ascribed to an expected increase in the line tension length. As a consequence of this effect, in combination with an increased homogenous nucleation rate the heterogeneous nucleation efficiency was low. The results emphasize the need for the development of new particle designs that are expected to further enhance the nucleation efficiency of nanoparticles in polymer nanocellular foaming.
In Chapter 5, the quasi 2D foaming mimicking experiments confirms the results as described in Chapter 4. More importantly it is demonstrated that the position of particles existing at the gas viscoelastic polymer interface is strongly size-dependent. The embedding of nanoparticles in CO2 swollen PMMA films exhibits a double transition upon reducing particle size, from adhesion to wetting and eventually to engulfment. Complete particle engulfment is observed for nanoparticles with a diameter of approximately 12 nm or less. These findings are explained quantitatively by a thermodynamic analysis, combining elasticity, capillary adhesion and line tension.
To further enhance the cell nucleation efficiency of nanoparticles, Chapter 6, PDMS decorated core-shell raspberry-like nanoparticles were synthesized and exploited as nucleation agents in PMMA nanocellular foaming. With these particles we report for the first time that a cell nucleation efficiency of above 1 was achieved. The highest nucleation efficiency obtained was ~ 6.2 for PDMS decorated raspberry-like nanoparticles with a silica core of ~ 200 nm, which is approximately 40 times higher compared to that of a pristine nearly spherical silica nanoparticle with a similar diameter. The unusual high nucleation efficiency of these nanoparticles is ascribed to CO2 capillary condensation in the surface cavities combined with the known favorable nanocavity nucleation energy. Hence the results show that PDMS decorated raspberry-like nanoparticles are very promising to be used as a new class of highly efficient nucleation agents.
Following the experimental work as described in the previous Chapters, Chapter 7 provides an outlook concerning new particle designs (e.g. PDMS decorated mesoporous particles), optimization of polymer foam matrix and foaming strategies, as well as for the measurement of the thermal insulation properties of nanocellular polymer foams. Overall, in this Thesis, nanoparticles with specifically designed surfaces were synthesized and exploited as highly efficient nucleation agents in CO2 assisted nanocellular polymer foaming. The obtained fundamental insights into heterogeneous cell nucleation at the nanometer length scale offers a framework for the design of highly efficient nucleation agents in the relevant foam processing windows. Future work on these particles is expected to result in foams with a desired cell density and cell size, especially when these nanoparticles are combined with different polymer matrixes that allow i) a good foam cell nucleation and ii) vitrification of the foam matrix at a time scale fast enough so that cell coalescence is prevented.