Adsorbed network polymer chains

Figure 2.13 (a) Adsorption of terminal hydrophobic group giving no additional network links (b) adsorption of polymer chain giving an additional network link for each chain adsorbed... 

Macroporous and isoporous polystyrene supports have been used for onium ion catalysts in attempts to overcome intraparticle diffusional limitations on catalyst activity. A macroporous polymer may be defined as one which retains significant porosity in the dry state68-71 . The terms macroporous and macroreticular are synonomous in this review. Macroreticular is the term used by the Rohm and Haas Company to describe macroporous ion exchange resins and adsorbents 108). The terms microporous and gel have been used for cross-linked polymers which have no macropores. Both terms can be confusing. The micropores are the solvent-filled spaces between polymer chains in a swollen network. They have dimensions of one or a few molecular diameters. When swollen by solvent a macroporous polymer has both solvent-filled macropores and micropores created by the solvent within the network. A gel is defined as a solvent-swollen polymer network. It is a macroscopic solid, since it does not flow, and a microscopic liquid, since the solvent molecules and polymer chains are mobile within the network. Thus a solvent-swollen macroporous polymer is also microporous and is a gel. Non-macroporous is a better term for the polymers usually called microporous or gels. A sample of 200/400 mesh spherical non-macroporous polystyrene beads has a surface area of about 0.1 m2/g. Macroporous polystyrenes can have surface areas up to 1000 m2/g. 

In composite systems, 2H NMR is particularly suited to investigate interfacial properties. Indeed, isolated nuclei are observed, which potentially allows spatially selective information to be obtained. It has been used to investigate polymer chain mobility at the polymer-filler interface, mainly in filled silicon (in particular PDMS) networks. The chain mobility differs considerably at the polymer-filler interface, and this may be interpreted in terms of an adsorbed polymer layer at the filler surface. T1 relaxation measurements allowed to determine the fraction of chain units involved in the adsorption layer, or equivalently, the thickness of the layer [75, 76, 77]. The molecular mobility and the thickness of the adsorption layer are very sensitive to the type of filler surface [78]. 

The molecular model is confined to the case of weak adhesion (5 < 0) to ensure homogeneous contact [48, 65]. In this case, two sources contribute to the frictional stress of a gel elastic deformation of an adsorbing polymer chain Cei and the lubrication of the hydrated layer of the polymer network ffvis, which can be represented as follows (Fig. 12) ... 

Twiss and Carpenter (1938) had also proposed that the water soluble polymers induced creaming by a type of cross-linking mechanism. They pointed out that water soluble polymers of the type studied formed a loose network in solution, as witnessed by their exhibition of a high structural viscosity. Twiss and Carpenter argued that if the chains of the water soluble polymer were adsorbed onto the latex particles, then the particles might be interlinked by network formation between polymer chains adsorbed on different particles. 

The nanocoating of PPy on natural cellulose fiber was performed without disrupting the hierarchical network structure of individual fibers [245]. Since pyrrole was hardly adsorbed onto a cellulose surface, this approach was based on the adsorption of the growing polymer chain from the solution and on the subsequent immobilization to form thin film. The conformation of the PPy chain was parallel to the surface of the cellulose fiber, because the corresponding oligomer is adsorbed parallel to the surface and further polymerized in the lateral direction. 

This picture is useful but does not match all adsorbents. Gel-type ion-exchange resins have no permanent pores. Instead they consist of a tangled network of interconnected polymer chains into which the solvent dissolves. In effect, Cp = 0. Macroporous ion-exchange resins have permanent pores and Cp > 0, but often < 1.0 for large molecules. Many activated carbons have both macropores and micropores thus, there are two internal porosities. Molecular sieve zeolite adsorbents are used as pellets that are a omerates of zeolite crystals and a binder such as clay. In this case, there is an interpellet porosity (typically, 0.32), an intercrystal porosity ( pi 0.23) and an intracrystal porosity ( p2 0.19), which has < 1.0... 

Depletion forces can be of use in biomedical applications. Non-adsorbing polymer chains promote the adhesion of cells to surfaces [256] and enhance adsorption of lung surfactants at the air/water interface in lungs so as to help patients suffering from acute respiratory syndrome [257]. The physical properties of actin networks are affected by non-adsorbing polymers [258], which also modify phase transitions in vims dispersions [228]. 

Fig. 4.24 Sketch of a gelation by arrested spinodal phase separation. A space spanning network of colloidal spheres is aggregated through depletion of non-adsorbing polymer chains...

Neutron scattering is being used in a wide variety of applications in the study cf polymer structure. These include studies of dimensions of polymer chains in solution, conformation of chains in networks to test theories cf rubber elasticity, miscibility of blends, structure of block copolymws and semicrystalline polymers, and size and shape of bio-macromolecules. Adsorbed polymer layers can be studied by neutron reflectivity. 

A-network model is based on that the adsorbed polymer chain may be decomposed into (f-1) and f-body constituent chains by adsorption on filled particles as shown in Fig. lb. The constituent chain by adsorption on filled particles with different sizes is shown in Fig. 2b. Similar to the E-network model, they may be divided into the terminal or tail and loop forms which are located at the end and middle of polymer chains, respectively. The adsorbed segments with different lengths have the tail and loop segments which are located at the end and middle of polymer chains, respectively. 

The most important unsolved problem in this area is the nature of the bonding between the filler particles and the polymer chains [137]. The network chains may adsorb strongly onto the particle surfaces, which would increase the effective degree of crosslinking. 

There are probably numerous other ways in which a hller changes the mechanical properties of an elastomer—some of admittedly minor consequence [5,137]. For example, another factor involves changes in the distribution of end-to-end vectors of the chains due to the volume taken up by the hller [136,137,175-178]. This effect is obviously closely related to the adsorption of polymer chains onto hller surfaces, but the surface also effechvely segregates the molecules in its vicinity and reduces entanglements. Another important aspect of hller reinforcement arises from the fact that the particles inhuence not only an elastomer s stahc properties (such as the distribution of its end-to-end vectors), but also its dynamic properhes (such as network chain mobihty). More specihcally, the presence of hllers reduces the segmental mobility of the adsorbed polymer chains to the extent that layers of elastomer close to the hller particles are frequently referred to as bound rubber. [179-182]... 

As discussed in the previous section, polymer chain flexibility is important for cooperative adsorption. Both a single polymer chain and 3D networks in a salt solution exhibit flexibility. In the case of the adsorption of N-pyridinium chloride (CnPyCl, = 4, 8, 10, 12, 16, and 18) onto such flexible polymers, the cooperativity increases in proportion to the hydro-phobicity (the length of the aliphatic chain) [40, 41]. In this section, the character of the adsorbed surfactant will be discussed. 

To illustrate how the effect of the adsorption on the modulus of the filled gel may be modelled we consider the interaction of the same HEUR polymer as described above but in this case filled with poly(ethylmetha-crylate) latex particles. In this case the particle surface is not so hydrophobic but adsorption of the poly (ethylene oxide) backbone is possible. Note that if a terminal hydrophobe of a chain is detached from a micellar cluster and is adsorbed onto the surface, there is no net change in the number of network links and hence the only change in modulus would be due to the volume fraction of the filler. It is only if the backbone is adsorbed that an increase in the number density of network links is produced. As the particles are relatively large compared to the chain dimensions, each adsorption site leads to one additional link. The situation is shown schematically in Figure 2.13. If the number density of additional network links is JVL, we may now write the relative modulus Gr — G/Gf as... 

Polysaccharides that have been modified chemically, or altered physically, have been used as adsorbents for affinity chromatography. The modification of the structure of polysaccharides has been achieved by introducing cross-linkages between the chains of the polymer and bifunctional reagents. The alteration of the properties of polysaccharides by physical means can be effected by embedding the polysaccharide in a network of the support material. The molecular in-... 

---