Molecular network structure

The origins of the present three-dimensional molecular-level branching concepts can be traced back to the initial introduction of infinite network theory by Flory [62-65] and Stockmayer [66, 67], In 1943, Flory introduced the term network cell, which he defined as the most fundamental unit in a molecular network structure [68]. To paraphrase the original definition, it is the recurring branch juncture in a network system as well as the excluded volume associated with this branch juncture. Graessley [69, 70] took the notion one step further by describing... 

The primary structure of macromolecules is defined as the sequential order of monomers connected via covalent chemical bonds. This structural level includes features such as chain length, order of monomer attachment in homopolymers (head-to-head, head-to-tail placement), order of monomer attachment in various copolymers (block copolymers, statistical and graft copolymers, chemical composition of co-monomers), stereoregularity, isomers, and molecular topology in different branched macromolecules and molecular networks. Structure at this primary level can be manipulated by polymer synthesis [4]. With AFM it is possible to visualize, under certain conditions, single macromolecules (Fig. 3.2) and it is even possible to manipulate these (i.e. push with AFM tips). Characteristics of chain-internal... 

The molecular network structure suggested for CTF-1 is isoelectronic with the structure of COF-1 [14], By contrast, COF-1 shows a staggered, P6Jmmc hexagonal structure prior to CTF-1 is proposed to have eclipsed P6lmmm symmetry [15]. No NMR data was given for these triazine networks, but elemental analysis implied partial decomposition or carbonization in some samples. For example, a network produced... 

Ishikawa, Y. Ohira. A. Sakata. M. Hirayama. C. Kunitake, M. A twc-dimensional molecular network structure of trimesic acid prepared by adsorption-induced self-organization. Chem. Commun. 2002. 2652-2653. 

Moderately vulcanised rubfaerlike substances represent essentially molecular network structures. The junction points consist of chemical cross links of considerable and equal strength. They may be regarded as being fixed and permanent and not affected by the solvent or by temperature within a reasonable range. Moreover, the mutual cohesion between the hydrocarbon chains is very weak and can be neglected to a first approximation. 

There remains hardly any doubt that the concept of a molecular network structure in cellulose gels is an essentially correct one. 

Polymer-based composites can be divided into thermoset and thermoplastic composites, which due to their different properties show diverse fracture mechanism. Due to tight three-dimensional molecular network structure of the most of thermoset matrixes such as epoxy resins, they exhibit inherent brittle fracture behavior and poor... 

All thermosets which form high molecular weight or molecular network structures after application fall into the classification of polymerizing types ... 

The monomers discussed thus far have an active bond that may react to form two covalent bonds with other monomers forming a two-dimensional chamUke molecular structure, as indicated earlier for ethylene. Such a monomer is termed bifimctional. In general, the functionality is the number of bonds that a given monomer can form. For example, monomers such as phenol-formaldehyde (Table 14.3) are trifiinctional they have three active bonds, from which a three-dimensional molecular network structure results. 

Thermoset moldable compounds can be mixed with a very wide variety of fillers to modify their properties to meet the requirements for a given application. Adding suitable fillers can produce coefficients of thermal expansion and elongation behavior virtually identical to those of copper. Once thermoset materials have cured, moreover, their three-dimensional molecular network structure gives them a very high level of dimensional stability. Consequently, in terms of temperature resistance to soldering many of the materials in this class are potentially suitable as MID substrates. Table 2.5 summarizes some of the important thermal properties of commercially available thermoset moldable compounds. Thermoset moldable compounds, moreover, have economic potential because in some cases the cost of the material Is low. Phenolic resin moldable compounds in particular are available at a price of less than about 7 (EUR 5) per kg and could therefore be considered an economical alternative to LDS high-temperature thermoplastics. 

Two of the widely used programs for the generation of 3D structures are CONCORD and CORINA. CONCORD was developed by Pearlman and co-workers (17, 18] and is distributed by TRIPOS (19). The 3D-structure generator CORINA originates from Gasteiger s research group [20-23] and is available from Molecular Networks [24],... 

Traditional rubbers are shaped in a manner akin to that of common thermoplastics. Subsequent to the shaping operations chemical reactions are brought about that lead to the formation of a polymeric network structure. Whilst the polymer molecular segments between the network junction points are mobile and can thus deform considerably, on application of a stress irreversible flow is prevented by the network structure and on release of the stress the molecules return to a random coiled configuration with no net change in the mean position of the Junction points. The polymer is thus rubbery. With all the major rubbers the... 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

Homopolymerization of macroazoinimers and co-polymerization of macroinimers with a vinyl monomer yield crosslinked polyethyleneglycol or polyethyleneglycol-vinyl polymer-crosslinked block copolymer, respectively. The homopolymers and block copolymers having PEG units with molecular weights of 1000 and 1500 still showed crystallinity of the PEG units in the network structure [48] and the second heating thermograms of polymers having PEG-1000 and PEG-1500 units showed that the recrystallization rates were very fast (Fig. 3). 

To determine the crosslinking density from the equilibrium elastic modulus, Eq. (3.5) or some of its modifications are used. For example, this analysis has been performed for the PA Am-based hydrogels, both neutral [18] and polyelectrolyte [19,22,42,120,121]. For gels obtained by free-radical copolymerization, the network densities determined experimentally have been correlated with values calculated from the initial concentration of crosslinker. Figure 1 shows that the experimental molecular weight between crosslinks considerably exceeds the expected value in a wide range of monomer and crosslinker concentrations. These results as well as other data [19, 22, 42] point to various imperfections of the PAAm network structure. 

Analysis of data pertaining to the modulus of PEO gels obtained by the polyaddition reaction [90] shows that even in this simplified case the network structure substantially deviates from the ideal one. For all samples studied, the molecular weight between crosslinks (M p) exceeds the molecular weight of the precursor (MJ. With decreasing precursor concentration the M xp/Mn ratio increases. Thus, at Mn = 5650 a decrease in precursor concentration from 50 to 20% increases the ratio from 2.3 to 12 most probably due to intramolecular cycle formation. 

On a molecular scale, the difference between the two classes of materials is rather small. Thermoplastics consist of individual long chain molecules not connected with each other. Addition of a few crosslinks results in an infinite network structure that is the characteristic of thermosets. 

From the foregoing it is clear that indentation anisotropy is a consequence of high molecular orientation within highly oriented fibrils and microfibrils coupled with a preferential local elastic recovery of these rigid structures. We wish to show next that the influence of crystal thickness on AMH is negligible. The latter quantity is independent on 1 and is only correlated to the number of tie molecules and inter-crystalline bridges of the oriented molecular network. 

The intrinsic moisture sensitivity of the epoxy resins is traceable directly to the molecular structure. The presence of polar and hydrogen bonding groups, such as hydroxyls, amines, sulfones and tertiary nitrogen provides the chemical basis for moisture sensitivity, while the available free volume and nodular network structure represent its physical aspect. 

The large deformability as shown in Figure 21.2, one of the main features of rubber, can be discussed in the category of continuum mechanics, which itself is complete theoretical framework. However, in the textbooks on rubber, we have to explain this feature with molecular theory. This would be the statistical mechanics of network structure where we encounter another serious pitfall and this is what we are concerned with in this chapter the assumption of affine deformation. The assumption is the core idea that appeared both in Gaussian network that treats infinitesimal deformation and in Mooney-Rivlin equation that treats large deformation. The microscopic deformation of a single polymer chain must be proportional to the macroscopic rubber deformation. However, the assumption is merely hypothesis and there is no experimental support. In summary, the theory of rubbery materials is built like a two-storied house of cards, without any experimental evidence on a single polymer chain entropic elasticity and affine deformation. 

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