Network Molecules

A more complex arrangement results when two polymers form networks that overlap in space. One way of making such an interpenetrating network (IPN) is to swell a cross-linked polymer with a monomer. The IPN comes about when the monomer is polymerized into a second network, even though the two netwoiks may have no covalent bonds in common [9-11]. Despite the fact that two polymers are not compatible when mixed as linear molecules, the IPN may exhibit no macroscopic phase separation and thus can be much stronger than a mechanical mixture. 

Another kind of mixed network comes about when two linear polymers that are incompatible at room temperature are simultaneously cross-linked at a high temperature at which they are compatible. Phase separation on cooling may be inhibited by the covalent bonds, which limit the movement of polymer chains. The term semi-IPN can refer to a linear polymer trapped in a network of another polymer. 

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The value of a to be used in these expressions is given by Eq. (5.45) for the specific mixture under consideration. At the point of gelation - l/(f- 1) according to Eq. (5.46). Equation (5.55) shows that n becomes infinite at this point while n remains finite. This merely means that there are still many molecules present at the gel point in addition to the network molecule of essentially infinite molecular weight. The ratio h /n indicates an immense expansion of the degree of heterogeneity as a ->. Expressions are also avail-... 

Relationship 23 provides a method for evaluating the parameter "a" that is defined by Equation 2A. The cumulative molar concentration of polymeric species PT0T was numerically evaluated via integration of population density distributions. The contribution of network molecules to the zeroth moment of the distribution is negligible. Results are presented by Figure A and show that... 

Note added in proof. Earlier in the text it was mentioned that the model used to describe the structure function of low density H20(as) does not describe that of high density H20(as). However, Narten, Venkatesh and Rice 27) do show than an ice I-like network with a near neighbor distance of 2.76 A has the density and distance spectrum of high density H20(as) if one permits 45% of the cavities characteristic of this structure to be occupied by water molecules. These are not ordinary unbonded interstitials. If the cavity molecules are located on the c axis at a distance of 2.76 A from the nearest network molecule each cavity molecule would have second neighbor network molecules at a distance of 3.25 A. Moreover, since occupancy of 45% of the cavities implies that 81% of the water molecules are part of the tetrahedral network and 19% in cavity positions, the average coordination number of nearest neighbors in this model is 4.3, as is found for H20(as) 10 K/10 K. Structure functions calculated for this interstitial variant of a randomized ice I model (the randomization is effected as in the simple ice I... 

The molecular structure of plastics is of two general kinds long molecules, either linear or branched and space-network molecules. 

In the different stages of this reaction the formaldehyde adds to one or two or to all three functional positions of the phenol molecule, and a new carbon-carbon bond is established. Once the phenol alcohols are formed, polycondensation setsin, and by splitting out water and later formaldehyde, large chain and network molecules are built up. 

Polycondensations can be carried out in an aqueous or a solvent medium, or they can be performed while the reactions are in a liquid or in a molten state. In industry, reactions of polyfunctional monomers leading ultimately to the tridimenaonal-network molecules of thermosetting resins are usually interrupted at a stage where the polymers still are soluble and fusible. They then are shipped to the fabricators, who convert them by heat curing processes into the final thermosetting product. 

When the density of branch points is increased in a polymer, there is a progression, from a collection of branched molecules, through a single infinite tree molecule containing no closed rings (Fig. 2.9a), to a three-dimensional network molecule (Fig. 2.9b). When a single tree molecule forms, the gel point occurs if a solvent is added the majority of the polymer forms a swollen gel, rather than dissolving. Both thermosets and rubbers are... 

IPN s have two continuous networks. In the case of latex IPN s, each particle ideally is composed of two network molecules. In bulk-prepared materials, the two networks are often presumed to be continuous on a... 

The term silicon dioxide is a misnomer in the sense that no individual Si02 molecules are known. In its numerous crystalline modifications, Si02 exists in the form of giant three-dimensional network molecules in which each silicon atom is surrounded tetrahedrally by four oxygen atoms to form a hard, rigid lattice ... 

Network Molecule of about 400 nm long that forms Supporting structures for cells and... 

The point at which the first network molecule is formed is known as the gel-point because it is manifested by gelation, i.e. an abrupt change of the reacting mixture from a viscous liquid to a solid gel which shows no tendency to flow. 

Network molecules can form when n chains are expected to lead to more than n chains through branching of some of them. The maximum number of chains that can emanate from the end of a single chain, such as that analysed above, is (/- 1) and so the probable number of chains emanating from the chain end is a(f- 1). Network molecules can form if this probability is not less than one, i.e. a(/-I) l. Thus the critical branching coefficient, ac, for gelation is given by... 

Up to this point, it has been assumed that all of the repeat units that make up the body of the polymer (linear, branched, or completely cross-linked network molecules) are all the same. However, if two or more different repeat units make up this chainlike structure, it is known as a copolymer. If the various repeat units occur randomly along the chainlike structure, the polymer is called a random copolymer. When repeat units of each kind appear in blocks, it is called a block copolymer. For example, if linear chains are synthesized from repeat units A and B, a polymer in which A and B are arranged as... 

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