LA CONVIVENCIA EN EL AULA (Primaria)

1.3.1 Structure and morphology

To tune the polymers for the right properties, the structures of the networks should be understood. Nevertheless, characterisation of networks morphology is considered to be difficult. As most of them are insoluble, solution-phase techniques, such as solution nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) and mass spectroscopy (MS), cannot be used. Also, amorphous structures lead to the difficulty in characterisation by X-ray crystallographic techniques. Infrared spectroscopy (IR) is a useful technique to identify the functional groups in the polymers. If monomers or networks contain detectable functionalities, the reaction could be tracked by observing the reduction of starting material functional groups and presence of product functional groups. However, the limitation is IR spectra usually provide only qualitative data, quantitative information often cannot be obtained.

To measure the chemical compositions in the polymeric material, elemental analysis is a tool that can be used. Nevertheless, it still give an error due to porous properties of materials which can easily adsorb gases and water vapour as well as trap catalyst in their porous structures resulting in inaccurate information.72, 81-83 Energy-dispersive X-ray spectroscopy (EDX or EDS)53, 79, 83, 84 and X-ray photoelectron spectroscopy (XPS)85-88 are another analytical methods which provide the elemental contents information used in confirming the amount of end groups and residual catalyst in the networks.

The most powerful method up to date may be the solid state NMR spectroscopy (ssNMR) to elucidate the quantitative information by different carbon environments. However, ssNMR normally takes a long experimental time, which can be hours or days. Recently, ssNMR was developed by using high-field dynamic nuclear polarization (DNP) to enhance the sensitivity of signal.89 The time used to get a good signal-to-noise ratio was reduced to minutes compared to hours without the enhancement by DNP. The shortened time allows the high-throughput characterisation can be obtained although unfortunately there are very few suitable NMR spectrometers available for use.

27 | P a g e Macroscopic morphology can be investigated by the images of the polymers using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The size and morphology of materials can be observed by SEM, while TEM can provide the porous texture information.

Physical stability of polymers is generally confirmed by thermogravimetric analysis (TGA). TGA is also used in some gas sorption experiment. The chemical stability of the network is normally studied by immersing materials in water, acids and bases.

1.3.2 Porosity

Porosity is the most important property of porous networks. In general, the porosity is measured by using nitrogen gas as a sorbent at 77 K providing nitrogen isotherms. Nitrogen is widely used because it has low cost and is highly abundant gas. However, other gases can be used in different purposes. For example, argon, hydrogen and carbon dioxide, which are smaller than nitrogen molecules, were used to probe the smaller pores in the network where nitrogen cannot access. While nitrogen and hydrogen adsorption measurements are normally performed at 77 K, carbon dioxide uptakes usually operate around room temperature. Ar isotherms are normally measured at 87 K.

The nitrogen isotherm is the plot between the amounts of nitrogen gas adsorbed against the equilibrium relative pressure (p/p0) measured at the constant temperature, normally at 77 K. Much porous information can be obtained from nitrogen isotherms. The shape of nitrogen isotherms can be grouped into six types as shown in Figure 1.5 (left). Type I isotherms (or Langmuir isotherms) demonstrated the microporous structure. Gas can be largely adsorbed at relative low pressure then saturated at the higher relative pressure where the micropores are filled. Type II isotherms are normally given by the non-porous or macroporous materials where unrestricted monolayer-multilayer adsorption occurred. Point B in Figure 1.5

indicates the point where the monolayer coverage is complete and the multilayer adsorption begins. Type III isotherms are not generally observed. Such isotherms happen as the adsorbate-adsorbate interaction is greater than the adsorbent-adsorbate

28 | P a g e interaction. Type IV isotherms are similar to Type II isotherms but possess the hysteresis loop attributed to capillary condensation in mesopores. Type IV isotherms are obtained by many mesoporous adsorbents. Type V isotherms are related to Type III isotherms but with hysteresis and also uncommonly found. Type VI isotherms represent the stepwise multilayer adsorption which each steps is responsible to each monolayer adsorption capacity.90

Hysteresis is observed when the adsorption and desorption curves do not coincide which is resulted from the capillary condensation. Hysteresis divided into four types as shown in Figure 1.5 (right) according to its shape. Type H1 is the hysteresis which the adsorption curve is vertically parallel to the desorption one. For Type H4, the adsorption and desorption curves are horizontally parallel. Type H2 and H3 are the intermediate shape between Type H1 and H4. Type H1 hysteresis is often found in materials that have uniform spheres and narrow pore size distribution (PSD). Meanwhile, Type H2 hysteresis is observed in materials which the shape and size is not uniform. Type H3 and H4 are often obtained by slit-shaped pores with the narrow pores in Type H4.90

29 | P a g e Gas sorption data can also be used to provide other useful porosity information such as surface areas, pore volumes and PSD. Surface areas can be calculated from either Langmuir91 or Brunauer-Emmett-Teller (BET)92 theories. Langmuir theory is based on the monolayer adsorption while BET theory is calculated from the multilayer adsorption assumption. Unfortunately, the accurate surface area is difficult to obtain as both theories are based on the assumption of rigid networks while MOPs could swell upon guest adsorption.33, 93 Nevertheless, the calculated surface area provides the common way to compare between networks. PSD can also be calculated from gas sorption data. The use of different models results in different PSD. However, there is no universal model to date. For MOPs, density functional theory (DFT) and non-local DFT (NL-DFT) are generally used.