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Environmentally friendly multifunctional fire retardant polymer hybrids and nanocomposites

Final Report Summary - NANOFIRE (Environmentally friendly multifunctional fire retardant polymer hybrids and nanocomposites)

In polymer nanocomposites (PN), a few wt.% of each filler is randomly and homogeneously dispersed on a molecular level in the polymer matrix. When molded, the mechanical, thermal and barrier properties of these materials are superior to those of pristine polymers and/or conventional composites.

The major use of polymers is in molded products. Polymers have been successfully reinforced using glass fibre, talc, calcium carbonate, carbon black and other inorganic fillers. The content of the filler is usually between 20 and 40 wt.% of a composite, and sometimes exceeds 50 wt.% in thermosetting resins. Polymers and fillers are not homogeneously mixed on a microscopic level, and are composed of different phases. The interface is not large, and interaction between the polymer (matrix) and the filler is limited.

Smectite clay minerals, especially montmorillonite (MMT), are potential candidates for a platelet-type filler for molecular composites, since they are composed of several layers of silicates.

These silicates are 1 nm thick and have a cross-sectional area of 100 nm2, which is very small compared to conventional fillers and also aramide fibers. MMT is the most common and ubiquitous clay mineral, and it is well known that it undergoes intercalation and swelling in the presence of water and organic cations.

Polymer clay nanocomposites are classified into the 'intercalated' type, where the structure of the clay is maintained to some extent, and the 'exfoliated' type, where silicate is randomly and homogeneously dispersed. If the silicates are in such a state, interactions between them must be avoided and the interface area is enormous.

Although the high aspect ratio of MMT is ideal for reinforcement, the nanolayers are not easily dispersed in most polymers due to their preferred face-to-face stacking in agglomerated tactoids.

Several strategies have been considered to prepare polymer-layered silicate nanocomposites. They include three main processes:
- Intercalation from a polymer solution: the layered silicate is exfoliated into single layers using a solvent in which the polymer is soluble. It is well known that such layered silicates, owing to the weak forces that stack the layers together can be easily dispersed in an adequate solvent. The polymer then adsorbs onto the delaminated sheets and when the solvent is evaporated (or the mixture precipitated), the sheets reassemble, sandwiching the polymer to form, in the best case, an ordered multilayer structure.
- In situ polymerisation: in this technique, the layered silicate is swollen within the liquid monomer (or a monomer solution) so as the polymer formation can occur in between the intercalated sheets. Polymerization can be initiated either by heat or radiation, by the diffusion of a suitable initiator or by an organic initiator or catalyst fixed through cationic exchange inside the interlayer before the swelling step by the monomer.
- Melt intercalation: the layered silicate is mixed with the polymer matrix in the molten state. Under these conditions and if the layer surfaces are sufficiently compatible with the chosen polymer, the polymer can crawl into the interlayer space and form either an intercalated or an exfoliated nanocomposite. In this technique, no solvent is required.

Carbon nanotubes (CNTs) are one of the fundamental research topics in the field of material science. The scientific and industrial interests involve both their synthesis and their application in many research areas. CNTs were discovered by Iijima as elongated fullerenes in 1991.

A single-wall carbon nanotube (SWNT) is best described as a rolled-up tubular shell of graphene sheet which is made of benzene type hexagonal rings of carbon atoms. The body of the tubular shell is thus mainly made of hexagonal rings of carbon atoms (in a sheet), whereas the ends are capped by half-dome shaped half-fullerene molecules. The natural curvature in the sidewalls is due to the rolling of the sheet into the tubular structure, whereas the curvature in the end caps is due to the presence of topological defects (pentagonal rings) in the otherwise hexagonal structure of the underlying lattice. The role of a pentagonal ring defect is to give a positive (convex) curvature to the surface, which helps in closing the tube at the two ends.

A multi-wall carbon nanotube (MWNT) is a rolled-up stack of graphene sheets into concentric SWNTs, with the ends again either capped by half-fullerenes or kept open.

The most common technique the production of CNTs uses an arc discharge through graphite electrodes under a continuum inert gas flow. In this process amorphous carbon and possibly fullerenes are also produced; the synthesized CNTs have lengths that do not exceed some micrometers.

As in the case of polymer silicate layered nancomposites, there are three main routes to produce nanocomposites: the dispersion of CNTs in a polymer solution and then the evaporation of the solvent; the dispersion of CNTs in a monomer and then its polymerization; the melt compounding.

Silsesquioxanes are a class of organosilicon compounds with general formula (RSiO1.5)n where n is an even number and R is hydrogen or an organic group, such as alkyl, aryl or any of their derivatives. Each Si atom is therefore bound on average to 1.5 oxygen atoms (giving the prefix sesqui-) and to an hydrocarbon group (giving the -ane termination). Very different silsesquioxane structures have been prepared and studied, with different degrees of symmetry, including random structure, ladder structures, polyhedral structures, as well as mixed structure and incompletely condensed polyhedra.

Polyhedral structures are the most widely studied silsesquioxanes, being also known as POSS from the acronym for Polyhedral Oligomeric SilSesquioxanes (POSS is a trademark from Hybrid Plastics).

POSS are particularly interesting when aiming to tailor the properties of a polymer based materials by the modification of the macromolecular structure; in this sense POSS are the nanoscale building blocks for new materials. POSS are based on an Si-O-Si thermally and chemically robust inorganic core which is the smallest possible silica particle, being also known as molecular silica. At the same time, the possibility to choose the group on POSS surface permits the solubility within the organic matrixes, while the possible presence of reactive groups allows chemical bonding onto the organic macromolecules. On the whole, POSS are highly tailorable and controlled structures: the ideal nanoparticle for a molecular level reinforcement of organic polymers.

There are basically three main synthetic ways to prepare and modify POSS:

- Direct complete condensation: complete condensation of RSiY3 precursors leads to the formation of a polyhedron. In general, complete condensation is favoured with small R groups, whereas difficulties are found with large and rigid organic groups, due to their steric hindrance.

- Incomplete condensation-corner capping reaction: incompletely condensed POSS structures can be obtained depending on the synthetic conditions; in particular, a trisilanol 'open cage' with one missing corner is easily obtained with high steric hindrance substituents, such as cyclopentyl and cyclohexyl. This incomplete cage can be corner capped with a functional R'SiY3 to obtain a monofunctional complete POSS cage, allowing a precise control on the final POSS structure. This approach is particularly interesting for the synthesis of metal functionalised POSS, which have been used as homogeneous and heterogeneous catalysts for alkene polymerisation, alkene methathesis and alkene epoxidation.

- Hydrosilylation: is a way to modify completely condensed POSS, through reaction on a double bond; this is possible both on hydro-POSS and on POSS functionalised with unsaturated-end chains. This reaction requires a catalyst, usually a Pt based complex, such as the Karstedt catalyst.

The preparation of POSS-polyolefines hybrids by copolymerisation has been widely studied; two main preparation routes have been reported:
- Ring opening metathesis copolymerisation (ROMP): involving the ring opening of a cyclic polyolefin and subsequent reaction with a norbornene-functionalised POSS, in presence of Grubbs catalyst.
- Metallocene catalysed copolymerisation: of ethene (or propene) with double bond-functionalised POSS or norbornene POSS, activated by methylalumoxane (MAO).

An extensively used approach for preparing nanocomposites is the sol-gel-technique which allows the formation of metal oxide frameworks. The sol-gel process based upon hydrolysis-condensation reactions of the metal alkoxide leads to an inorganic network that causes the precipitation and dispersion of inorganic fillers throughout the polymer matrix. From the literature, two types of organic-inorganic materials produced by sol-gel process are considered. In the first type, there is no covalent bonding between organic and inorganic phases but mainly hydrogen bonding between both organic and inorganic phases. Generally, these organic-inorganic materials are elaborated by mixing an organic polymer with a metal alkoxide such as tetraalkoxysilane in the presence of a solvent. The word of nanocomposites seems to be more appropriate for such materials because they behave as thermoplastics filled with inorganic domains. The second class of materials is organic-inorganic hybrids materials with the existence of covalent bonds between polymer matrix and inorganic phase. Hybrids systems prepared by sol-gel process can be obtained using different synthetic approaches, the most common one being the hydrolysis-condensation reactions of alkoxysilanes in the presence of alkoxysilane prefunctionalised polymers.

From a processing point of view, these materials behave as thermoset polymers. However, a major problem associated with the sol-gel process is the use of organic solvent which limits the application to coating of glass, metal and polymer substrata.