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Materials

Porous materials are typically categorized into three classes that have different pore sizes (1):

1. Macroporous with pore diameter larger than 50 nm,
2. Mesoporous (pore diameter between 2 nm and 50 nm), and
3. Microporous materials (pore diameter smaller than 2 nm).

While conventional polymer networks undergo pore collapse upon solvent removal as polymer strands can adopt many conformations in order to pack space efficiently, recent research efforts have popularized several classes of polymer networks that possess permanent porosity based on the use of rigid components.

1.1 Styropor

Otis Ray McIntire (1918-1996), a chemical engineer at Dow Chemical, rediscovered a process first patented by Swedish inventor Carl Munters (2).

According to the Science History Institute, "Dow bought the rights to the Munters method and began producing a lightweight, water-resistant, and buoyant material that seemed perfectly suited for building docks and watercraft and for insulating homes, offices, and chicken sheds (3). In 1944, Styrofoam was patented.

Before 1949, chemical engineer Fritz Stastny (1908-1985) developed pre-expanded poly(styrene) beads by incorporating aliphatic hydrocarbons such as pentane. These beads are the raw material for molding parts or extruding sheets. BASF and Stastny applied for a patent that was issued in 1949. The molding process was demonstrated at the Kunststoff Messe in Düsseldorf in 1952. These products were named Styropor (3).

The crystal structure of isotactic poly(styrene) was reported by Giulio Natta (4). In 1954, the Koppers company in Pittsburgh, Pennsylvania, developed expanded poly(styrene) foam under the trade name Dylite (5).

1.2 Porous Coordination Polymers

The design, analysis and applications of coordination polymers have been descried in a monograph (6).

A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally, a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions (7, 8)

Examples of coordination polymers are lanthanoid coordination polymers, organometallic networks, and organic-inorganic hybrids (6).

1.2.1 Multifunctional Pillared-Layer Material

A multifunctional pillared-layer porous coordination polymer, has been constructed based on a flexible viologen derivative, 1,1'-bis(4-carboxybenzyl)-4,4'-bipyridinium dichloride, and an oxalate co-ligand. 1,1'-Bis(4-carboxybenzyl)-4,4'-bipyridinium dichloride is shown in Figure 1.1.

Figure 1.1 1,1'-Bis(4-carboxybenzyl)-4,4'-bipyridinium dichloride.

Single-crystal X-ray analysis showed that the compound possesses multichannels with dimensions of about 6.1×6.6 Å along the [110] and [-110] directions and 4.2×7.6 Å along [100], and a void space of about 41.4%.

Hydrogen adsorption measurements at 77 K and 1 atm indicated that the compound exhibits a hydrogen uptake of 0.71%. Owing to the incorporation of bipyridinium acceptor units, the compound can selectively accommodate aromatic donors into its nano-sized pores based on charge-transfer interactions in an elastic way, and afford a specific color to different guests.

Furthermore, the effect of perturbation exerted by the guest molecules on its magnetic properties has been investigated. The results indicated that the donor inclusion has little effect on its antiferromagnetic behavior, whereas dehydration of the compound decreases the strength of the magnetic exchange couplings and results in a change of the antiferromagnetic transition temperature from 14.7 K to 9.8 K (9).

1.2.2 Porous Coordination Polymer-Ionic Liquid Composite

A porous coordination polymer-ionic liquid composite has been described that includes an insulating structure composed of a porous coordination polymer, and an ionic liquid retained inside pores of the porous coordination polymer. The porous coordination polymer preferably has a main chain containing a typical metal element (10).

It has been proposed to apply an ionic liquid owing to high ionic conductivity thereof to an electrochemical device as an electrolyte for a battery or an electrical double-layer capacitor. The ionic liquid has extremely high flame retardance, and hence when used as the electrolyte for the electrochemical device, there is no need for a combustible organic solvent, thus ensuring the electrochemical device with high safety (10).

A schematic diagram that shows that an ionic liquid is filled with particles of the porous coordination polymer to form particles after filling is shown in Figure 1.2.

Here a a plurality of particles 111 composed of a porous coordination polymer are filled with a ionic liquid 12. The composite 131 obtained by a molding process. The structure 11, which is used as an electrolyte for a battery or an electrical double-layer capacitor, has a dense structure, thus making it easier for ion conduction pathways between the particles to be connected to each other. Hence, the composite 131 is a satisfactory ion conductor. In the case of using the structure 11 obtained by subjecting a plurality of the particles 111 composed of the porous coordination polymer to compression molding, a plurality of voids are respectively formed between the particles 111 of the porous coordination polymer (10).

Figure 1.2 Synthesis of a porous coordination polymer-ionic liquid composite (10).

Examples of the porous coordination polymer are collected in Table 1.1.

Table 1.1 Porous coordination polymers (10).

The acids in Table 1.1 are shown in Figure 1.3.

Examples of hard acids, hard bases, soft acids, soft bases, intermediate acids, and intermediate bases are described in a monograph (11).

Figure 1.3 Acids in Table 1.1.

1.3 Networks

1.3.1 Microporous Polymer Networks

Microporous materials are defined as materials containing interconnected pores of less than 2 nm in diameter (12)

Due to their large surface area, many conventional microporous materials, such as zeolites and activated carbons, are widely used as catalysts, sorbents, and separation membranes. Recently, the field has evolved rapidly with the development of several novel types of microporous polymer networks. These materials not only benefit fundamental research by introducing modular approaches to accessing numerous sophisticated structures, but also provide new opportunities for various emerging applications (1).

The central design principle for introducing permanent microporosity into polymer networks involves the use of rigid building blocks. Such rigidity precludes the network strands from behaving effectively as entropic molecular springs and prevents the collapse of microporous structures upon solvent removal; consequently, the mechanical properties of these materials are stiff yet brittle.

Furthermore, the rigidity of the monomers prevents small loop formation and allows for establishing long-range order in the presence of self-error-correcting mechanisms, e.g., a reversible bond formation (1).

So, microporous polymer networks can be either amorphous or crystalline. Aside from the general use of very rigid components, the basic concepts of microporous polymer network synthesis are similar to those discussed above for either covalent or physical polymer networks.

1.3.2 Amorphous Microporous Polymer Networks

Amorphous microporous polymer networks of different types have been denoted by various names, such as:

1. Polymers with intrinsic microporosity (PIMs) (13),
2. Porous organic polymers (POPs) (14),
3. Conjugated microporous polymers (CMPs) (15), and
4. Hyper-crosslinked polymers (16).

It has been suggested to divide these materials into two categories, based on whether or not the strands are covalently crosslinked (1).

1.3.2.1 Conjugated Microporous Polymers

Conjugated microporous polymers...