Page 120 - Polymer-based Nanocomposites for Energy and Environmental Applications

96 Polymer-based Nanocomposites for Energy and Environmental Applications

diffusion process in a high-pressure vessel; thus, the specimen is taken out from the
vessel and simultaneously the cell nucleation and growing in gas/polymer melt
processed promptly in the end of the die, in order to form polymer foams. In the semi-
continuous process, a larger sheet die and/or filament die was used in order to produce
the foam polymer with higher properties [102].

3.6.2 Structure of polymer nanocomposites foams

The polymer nanocomposite foams are classified in four categories specifying the
rigid, semirigid, semiflexible, or flexible cellular foams [90], depending upon their
overall characteristics that comprise their compositions, cellular morphology, and
other mechanical and thermal characteristics. As happens with polymer foams, the
nanocomposite foam characteristics are principally affected by factors depending
on the type of foaming processing, as well as the processing conditions and the type
of base polymer. In combination with the presence of the nanoparticles influenced
either by their nature and morphological characteristics, size, concentration, degree
of dispersion, and possibility of functionalization and [103] as stated earlier, the all
characteristics of nanocomposite polymeric foams are principally determined by
the succeeding cellular morphology to include the cell density, expansion ratio, cell
size distribution (unimodal and bimodal), cell content, cell integrity, and finally the
type of cellular structure (closed-cell, partially or fully open-celled, or interconnected)
[86]. This cellular morphology is strongly conditional on the intensity and kinetics for
nucleation and growth, furthermore on the degree of cell wall collapsing, or on cell
coalescence during expansion. Particularly, during foaming process, the cell walls
separating neighboring cells grow progressively, and then, the stretching of cell walls
collapses that takes place to generate the cell coalescence [87]; in the end, the neigh-
boring cells join to form one. The formation of cell coalescence during foaming pro-
cess can produce the open-cell foams that have a cellular network in which continuous
channels are available throughout the solid macromolecular phase for air to flow
through [104], while cell coalescence is undesirable in closed-cell foams that have
a cellular structure in which contiguous air bubbles are entrapped within a continuous
macromolecular phase.

3.7 Conclusion

This chapter is focused on the latent heat thermal energy storage technology using
phase-change material (PCMs) with different applications. Those technologies are
very advantageous for the energy conservation. This paper presents also the current
status and future directions in research and development of this technology, with a
detailed review of the literature concerning PCM classification based on their thermal
characteristics and their technical feasibility. Indeed, the measurement techniques of
thermal properties of PCMs, container requirement, and impregnation techniques of
PCM into the construction materials including encapsulation techniques, shape-
stabilized PCM, and form-stable composite PCM are discussed.