Network formation

Chemically cross-linked networks provide higher cross-linking densities to the polymer network, are more favorable for the sustained release of therapeutics, and allow for the fabrication of scaffolds with enhanced mechanical properties. However, the toxicity of the chemical cross-linking agents used may adversely affect cell behavior and the incorporated bioactive molecules. On the other hand, physical gelation of the network may avoid the use of cross-linking agents, but shows a limited performance in their physical properties. In the next sections, we will discuss the mechanisms involved in the solidification of injectable materials. 

If we exclude the case of pre-existing order, we have so far considered a network as a random, but completely homogeneous structure. It should now be mentioned that the crosslinking process itself may give rise to aggregation of network elements and therefore, in the swollen state, to significant fluctuations in segment density. 

In copolycondensation for example, the more reactive monomer is expected to become exhausted more rapidly than the less reactive one. If the functionalities of the polyfunctional crosslinker are more reactive, short chains are formed in the beginning of the reaction and long chains in the end. If we assume equilibrium conditions throughout the reaction, the unreacted functionalities of the crosslinker on different growing trees, with short links in the beginning, are expected to react more likely with each other and as a result a part of the final network may be more crosslinked than the other part. This may eventually lead to phase separation. If the reaction is diffusion-controlled (177), cores with higher crosslinking density may be formed. 

In crosslinking polymerization, the effect of unequal reactivity and dilution may be combined, because long primary chains possibly very rich in divinyl units with unreacted double bonds are predominantly present in the beginning of the copolymerization in a very dilute solution of the monomem and possibly other diluents. In addition, inhomogeneous crosslinking can induce local gel effects (acceleration of polymerization at the gel point) and thus contribute to the overall inhomogeneity of the system. 

From these considerations an important conclusion follows immediately. The structure of a network is insufficiently described if only the number of networks chains and their length distribution is specified. In addition, knowledge of the topography of the chain length distribution or the spatial distribution of the chain length distribution is required. That means that in networks of the same chain length distribution the chains may still be quite differently distributed in space (Fig. 12). 

A more complex behaviour is to be expected for crosslinking polymerization in solution. Phase separation can occur already at the gel point or in its close vicinity and the formation of porous structures is thus possible (157). 

FIGURE 2-4 Different types of branching. Network Formation 

Branching can also lead to the formation of densely connected networks. For example, if we start with a Y-shaped molecule, where each arm of the Y has a reactive group that is capable of reacting with and connecting to any other group on another Y, 

An example of forming a network from a trifunctional molecule is the reaction of phenol with formaldehyde. The hydrogens in the ortho and para positions to the OH 

FIGURE 2-6 Representation of the formation of a typical phenolic resin. 

Leo Baekeland (Courtesy Edgar Fahs Smith Memorial Collection, University of Pennsylvania Library), 

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S. netropsis Nettle extract Network analysis Network formation Network polymer Networks... 

Detailed discussions of network formation from amines and epoxy resins are provided in References 24 and 25. 

The low quantum yield of the photografting process (0 = 2 X 10 ) provides a good opportunity to control the network formation (curing time control), and accordingly, the desirable properties of the crosslinked or grafted copolymer might be obtained. 

Free energy of network formation before the first break is ... 

NETWORK FORMATION, STRUCTURE FACTORS AND ELECTRICAL CONDUCTIVITY OF NASN ALLOYS... 

The strong interactions between the water molecules also become obvious from NMR measurements by Tsujii et al..57) 13C-NMR experiments were used for determining the microviscosity of water in reversed micelles of dodecylammonium-propionate with 13C glycine cosolubilized. It was found that the apparent viscosity of the water-pool corresponds to the viscosity of a 78 % aqueous glycerol solution, obviously as a consequence of the extended network formation by strong hydrogen bonding. 

Three-dimensional polymerization appears to be the most universal and widely used technique for obtaining SAH. The network formation proceeds either with participation of a multifunctional branching agent or due to side reactions, e.g.,... 

Undoubtedly, the properties of superabsorbent hydrogels occupy the key position in the problem under consideration. Being directly connected with the network formation reaction, they provide all necessary information about the details of this process. Also, the SAH properties are found to be the most reliable basis for understanding and predicting their behavior in real systems, i.e. in the soil, in contact with plants, in physiological media, etc. 

DuSek, K. Network Formation in Curing of Epoxy Resins. Vol. 78, pp. 1—58. 

Kloosterboer, J. G. Network Formation by Chain Crosslinking Photopolymerization and its Applications in Electronics. Vol. 84, pp. 1 —62. 

With appropriate choice of reaction conditions, hyperbranched polymers can be formed by sclf-condcnsing vinyl polymerization of monomers that additionally contain the appropriate initiator (NMP, ATRP), when the compounds are called inimers, or RAFT agent functionality. Monomers used in this process include 340,716 341717 and 34204 (for NMP), 108714,714 and 344 and related monomers720 723 (for ATRP) and 343408 (for RAFT). Careful control of reaction conditions is required to avoid network formation. 

As mentioned previously, the use of multifunctional monomers results in branching. The introduction of branching and the formation of networks are typically accomplished using trifunctional monomers, and the average functionality of the polymerization process will exceed 2.0. As the average functionality increases, the extent of conversion for network formation decreases. In... 

This chapter emphasizes the recent mechanistic and kinetic findings on phenolic oligomer syntheses and network formation. The synthesis and characterization of both novolac- and resole-type phenolic resins and dieir resulting networks are described. Three types of networks, novolac-hexamethylenetetramine (HMTA),... 

Figure 7.37 Network formation of phenolic novolac and epoxy.

Negative neighboring group effect, 456 Network formation, 13 Networks. See also Epoxy-phenol networks Phenohc networks phenolic-based, 376 polyester-based, 58-60 Neutral hydrolysis, 564-565... 

J 8 Explain the role of chain length, crystallinity, network formation, cross-linking, and intermolecular forces in determining the physical properties of polymers (Section 19.12). 

Striking support of this contention is found in recent data of Castro (16) shown in Figure 14. In this experiment, the polymerization (60-156) has been carried out in a cone-and-plate viscometer (Rheometrics Mechanical Spectrometer) and viscosity of the reaction medium monitored continuously as a function of reaction time. As can be seen, the viscosity appears to become infinite at a reaction time corresponding to about 60% conversion. This suggests network formation, but the chemistry precludes non-linear polymerization. Also observed in the same conversion range is very striking transition of the reaction medium from clear to opaque. 

Citric acid is a nontoxic metabohc product of the body (Krebs or citric acid cycle), and it has been approved by FDA for its use in humans. It was found that the citric acid can be reacted with a variety of hydroxyl-containing monomers at relatively mUd conditions. " Citric acid can also participate in hydrogen bonding interactions within a polyester network. Citric acid was chosen as a multifunctional monomer to enable network formation. 

This result of resistivity or electron conductivity, which is the inverse of resistivity, is well known and is called percolation [40]. Dramatic increase of conductivity is ascribable to network formation of the electron-conductive path. 

Network Formation with Silica The Silanization Reaction.802... 

Stochastic Description of Copolymerization and Network Formation in a Six-Component, Three-Stage Process... 

In this study computational results are presented for a six-component, three-stage process of copolymerization and network formation, based on the stochastic theory of branching processes using probability generating functions and cascade substitutions (11,12). 

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