Crosslinked network, swelling

If an inert good solvent is used in solution polymerization, the gel thus obtained will have a supercoiled (expanded) structure (Gel B). Gel B swells in good solvents much more than Gel A which is synthesized in bulk. If the amount of the crosslinking divinyl monomer in the reaction mixture is increased while the amount of solvent remains constant, highly crosslinked networks are formed that cannot absorb all solvent molecules present in the reaction mixture and a heterogeneous structure results (Gel C). A part of the solvent separates from the gel phase during polymerization and the formed Gel C consists of two continuous phases, a gel and a solvent phase. If the amount of solvent is further increased, a... 

The relationship between swell ratio and foam properties was initially established for LDPE crosslinked by DCP alone. Figure 15.7 shows that an increase in swell ratio is accompanied by a decrease in foam density and an increase in mean cell size. The higher swell ratios are associated with a more loosely crosslinked network (i.e. lower crosslink density) that has a greater ability to expand and hence lowers the foam density. 

Kuhn et al. [22] extended their studies to crosslinked networks of poly-(acrylic acid) (PAA) and found that these gels swelled to a high degree in alkaline environment and contracted rapidly when acid was added to the surrounding solution. Dilations and contractions of the order of 300% were observed. They were reversible and could be repeated at will. A contracting and expanding gel... 

Many studies of vulcanized elastomer blends have revealed discontinuities in physical property trends attributable to poor interfacial bonding. Recently Rehner and Wei (5) have observed discontinuities in the swelling of blended crosslinked networks swollen in a common solvent. This departure from an averaged swelling behavior, based upon compositional ratios and the swelling behavior of the two homophases, re-... 

Table I. Swelling of Various Elastomer Crosslinked Networks in Differential Solvents at — 25°C...

The system Cl-butyl-cis-polybutadiene has been studied in some detail because it was suitable for the developed differential swelling technique and because this system of blends vulcanized with zinc oxide, sulfur, and thiuram disulfide first revealed the presence of interfacial bonds. This curative system has the feature of a flat cure —i.e.y the two homophases are vulcanized rapidly, and the crosslinked density does not increase radically as vulcanization time is prolonged. This is observed in Table IV by swelling and extractable levels of a series of crosslinked networks cured at increasing times and swollen in a common solvent, cyclohexane. 

Macromolecular 10-100 nm Network chains/strands, crosslinks network defects (dangling chains) Macro- molecular science Rubber elasticity, solvent swelling... 

There are two principal approaches to forming sequential IPNs (1) form the first network, swell it with the second monomer, crosslinker and catalyst and then form the second network (2) blend the two monomers, crosslinkers and catalysts together and then crosslink them. Two different initiation processes (e.g. different temperatures) can be used in what is called in-situ sequential synthesis. Finally, an alternative consists in blending the monomers and then adding the catalysts and/or the crosslinkers sequentially. 

To evaluate the swelling behavior and the crosslinked parameters of coal networks, swelling data were obtained for seven American coals supplied in fine particles by the Pennsylvania State University coal bank. To eliminate experimental artifacts, several separations and purifications were performed prior to swelling, according to the general experimental protocol reported in earlier communications (1,12). 

Although the kinetics of liquid uptake to attain gel-saturation is history-dependent, the composition at the true end-state (i.e. thermodynamic equilibrium in excess liquid) is not therefore the observed end-state is usually reproducible [19]. Gel-saturation is attained when the restraining force (per unit area) of the polymeric crosslinked network becomes equal and opposite to the osmotic pressure that causes the system to swell [20], In other words saturation is achieved when the chemical potential of swelling liquid, p1 in the swollen network is equal to the chemical potential of the excess pure liquid, p , outside the network. It was logical to anticipate that the volume of liquid sorbed per gram of polymer, at this state of thermodynamic equilibrium with excess liquid, would correlate with the molecular structure of the liquid. In fact two parameters already exist which relate the sorption affinity to the molecular structure, namely the solubility parameter, 8, first proposed by Hildebrand [21], and the interaction parameter, %, introduced by Flory [22] and Huggins [23-26],... 

The process of dissolving a polymer is usually a slow one. Frequently—and particularly for highly crosslinked network polymers—the addition of a solvent results only in swelling as the solvent permeates the polymer matrix. For other polymers solution takes place over a prolonged period of time after the first swelling occurs. In general, the portions with lower molar mass are more soluble this property can be used to separate the polymer into fractions of different average molar mass. 

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