Network structure defects

The degradation of the matrix in a moist environment strongly dominates the material response properties under temperature, humidity, and stress fatigue tests. The intrinsic moisture sensitivity of the epoxy matrices arises directly from the resin chemical structure, such as the presence of hydrophilic polar and hydrogen grouping, as well as from microscopic defects of the network structure, such as heterogeneous crosslinking densities. 

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

Here, v is Poisson s ratio which is equal to 0.5 for elastic materials such as hydrogels. Rubber elasticity theory describes the shear modulus in terms of structural parameters such as the molecular weight between crosslinks. In the rubber elasticity theory, the crosslink junctions are considered fixed in space [19]. Also, the network is considered ideal in that it contained no structural defects. Known as the affine network theory, it describes the shear modulus as... 

The tetrahedral network can be considered the idealized structure of vitreous silica. Disorder is present but the basic bonding scheme is still intact. An additional level of disorder occurs because the atomic arrangement can deviate from the fully bonded, stoichiometric form through the introduction of intrinsic (structural) defects and impurities. These perturbations in the structure have significant effects on many of the physical properties. A key concern is whether any of these defects breaks the Si—O bonds that hold the tetrahedral network together. Fracturing these links produces a less viscous structure which can respond more readily to thermal and mechanical changes. 

Diffusion of the molecular gases can be complicated by reactions with the glass network, especially at the sites of structural defects. The diffusion coefficient of water, for example, shows a distinct break around 550°C (110). Above 550°C, the activation energy is approximately 80 kj/mol (19 kcal/mol), but below 550°C, it is only 40 kj/mol (9.5 kcal/mol). Proposed explanations for the difference cite the fact that the reaction between water and the silica network to form hydroxyls is not in equilibrium at the lower temperatures. 

In contrast, on the surface of the amino-containing polymeric materials, protonated amino groups introduced in a small proportion under physiological conditions, destroy their surrounding hydrogen bonds to produce, here and there, gaps in the network [127, 128]. Thus, the network structures are considered to become more or less unstable. As a consequence, the residence time of protein molecules trapped by these defective networks will be shorter than in the case of polyHEMA or cellulose. On the surface of these amino-containing materials, reversible protein adsorption and desorption, and also replacement (Vroman effect) - or even protein rejection - will become possible. 

The structure of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aliphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the structure even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore structure. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and Applied Chemistry (IUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

Chapter II describes the various kinds of network structures which may exist before or arise during its formation. Of these, the various network defects resulting from the crosslinking statistics, have received far more attention in the literature than the effects of inhomogeneous network formation and syneresis (separation in a gel + diluent phase). This is reflected in this review (Chapter II, section 2) although a special emphasis is also laid on the latter aspect (Chapter II, section 3 and 4). 

The flexibility and extensibility of a crosslinked epoxy network are determined by the available glassy-state free volume. If the free volume is insufficient to allow network segmental extensibility via rotational isomeric changes then the brittle mechanical response of the epoxy glass is not controlled by the network structure but rather by macroscopic defects such as microvoids. For epoxies with sufficient free volume that allows plastic network deformation the mechanical response is controlled by the network structure. 

In the case under consideration different physical structures were realized due to the formation of the polymer network in the surface layers the filler surface, as usually happens in filled systems. As is known79, this induces considerable changes in the structure of the material. It is also possible that in these conditions a more defective network structure is formed. These results show that even the purely physical factors influencing the formation of the polymer network in the interface lead to such changes in the relaxation behavior and fractional free-volume that they cannot be described within the framework of the concept of the iso-free-volume state. It is of great importance that such a model has been devised for a polymer system that is heterogeneous yet chemically identical. 

Model networks, synthesized by endlinking processes, contain few structural defects and are close to ideality. Spring-suspended bead models seem to fit adequately with the structural data obtained on labelled model networks and with the swelling and uniaxial deformation behavior of these networks. (67 refs.)... 

In addition to these technical problems, the complexity inherent to physical properties of gels is, as exemplified above, that they depend very sensitively on the preparation condition. This is because, in a formal language, a gel is a frozen system and we need two sets of statistical information, the preparative ensemble and the final ensemble , to understand its equilibrium properties [29]. Hence, a gel is by nature more complex than the usual equilibrium systems. We should clarify the dependence of the properties of gels on preparation conditions, and also on structural defects of the network before going into precise investigations such as critical phenomena associated with the phase transition. 

The 355 nm emission is sharp and intense at the start of irradiation, and the intensity decreases with prolonged irradiation time. The 440 nm emission is weak and broad, and the intensity does not change with the irradiation time. Emission spectra of PMPrS obtained at ion fluences of 0.15,0.76, and 1.53 p,C/cm2 shows emission bands at 350 nm and 440 nm. The decrease in the intensity of the main peak indicates that main chain scission (photolysis) occurs under ion beam irradiation. Intense and sharp emission at 340 nm and weak broad emission at 440 nm for PDHS at 354 K are observed at the beginning of the irradiation and decrease on further irradiation. At 313 K and 270 K, sharp intense main emissions at 385 nm are seen. The 340 nm and 385 nm emission bands are assigned to a - a fluorescence. Experimental results have shown the presence of a phase transition at 313 K for PDHS.102,103 Below 313 K, the backbone conformation of PDHS is trans-planar, and above the solid-solid phase change temperature, a disordered conformation is seen. Fluorescent a -a transitions occur at 355 nm for PMPS, 350 nm for PMPrS, and 385 nm and 340 nm for PDHS. Emissions around 440 nm are observed at all temperatures examined and are assigned to defect and network structures induced by ion beams. 

Mechanical properties of crosslinked elastomers are influenced not only by the volume-average crosslink density but also by network heterogeneity. The influence of structural defects (such as residual sol, dangling chains, chain loops and the heterogeneity of the junction distribution) on the viscoelastic properties and the equilibrium swelling data is still under discussion. Local methods which probe molecular properties are very suitable for the determination of the degree of network heterogeneity [11]. 

Network structure analysis is discussed in Chapters 7, 8,10 and 13. These chapters deal with the characterisation of the structure of chemical and physical networks, rubber-filler physical network, network defects and its heterogeneity using NMR relaxation techniques and NMR imaging. 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

The description of a network structure is based on such parameters as chemical crosslink density and functionality, average chain length between crosslinks and length distribution of these chains, concentration of elastically active chains and structural defects like unreacted ends and elastically inactive cycles. However, many properties of a network depend not only on the above-mentioned characteristics but also on the order of the chemical crosslink connection — the network topology. So, the complete description of a network structure should include all these parameters. It is difficult to measure many of these characteristics experimentally and we must have an appropriate theory which could describe all these structural parameters on the basis of a physical model of network formation. At present, there are only two types of theoretical approaches which can describe the growth of network structures up to late post-gel stages of cure. One is based on tree-like models as developed by Dusek7 I0-26,1 The other uses computer-simulation of network structure on a lattice this model was developed by Topolkaraev, Berlin, Oshmyan 9,3l) (a review of the theoretical models may be found in Ref.7) and in this volume by Dusek). Both approaches are statistical and correlate well with experiments 6,7 9 10 13,26,31). They differ mainly mathematically. However, each of them emphasizes some different details of a network structure. 

Computer simulations were performed taking into account the component ratio P, the relative reactivity of primary and secondary amines, and different probabilities of monocycle formation. The simulations showed the structural features of networks (both topology and defects) at all stages of the cure process. Two examples of network structure received from computer simulation are shown in Fig. 2. Here... 

Besides PTBs, A-site defective perovskite oxides are known to be formed when B = Ti. Nb.Ta and soon "13. Such compounds exhibit metallic properties and perovskite structures when the B atom occurs in a low oxidation stale. Compositions such as A0 5Nb03 (A = Ba. Pb etc.) where niobium is in the highest oxidation state adopt non-perovskite network structures. An interesting example20-21 of a A-site defective perovskite is Cu 5Ta03 which crystallizes in a pseudocubic perovskite structure. The unit cell is orthorhombic with a = 7.523, />= 7.525 and c = 7.520 A and eight formula units per cell. Tantalum atoms form... 

Ferric ferrocyanide, commonly known as Prussian blue (PB), was first synthesized > 300 years ago (39) and is still used in the manufacture of blueprints. Prussian blue is a prototypical mixed-valence compound with formula Fe 4[Fe (CN)6]3 I4H2O. In its canonical form, the pigment consists of ferrocyanide anions linked by Fe cations (Fig. 1) to form an extended pcu network. A defect structure that arises from the necessity of charge balancing in the cubic framework results in vacancies at 25% of the [Fe(CN)6]" sites (40). Analogous compounds are formed when one or both iron atoms are replaced by a variety of other metals. This substitution affords compounds of the formula M [My(CN)6]j where M and M can be Cr, Mn, Fe, Co, and many others, and where x andy depend on the identity and oxidation states of the metals. Because of their structural similarities we will refer to the entire class of compounds as blues PBs. 

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