Cross-linked polymer, defined

Fig. 25. Schematic representation of imprinting (a) cross-linking polymerization ia the presence of a template (T) to obtain cavities of specific shape and a defined spatial arrangement of functional groups (binding sites. A—C) (b) cross-linked polymer prepared from the template monomer and ethylene...

Performance Characteristics Polyester resins undergo a rapid transformation from a viscous Hquid to a soHd plastic state that comprises a three-dimensional cross-linked polymer stmcture. The level of polyester polymer unsaturation determines essential performance characteristics (Table 7), although polymer components can influence subtle features that affect thermal, electrical, and mechanical performance as defined by ASTM procedures. 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

The unsaturated side chain of the SoyOx repeating units could be used for cross-linking well-defined P(EtOx)-.yfaf-(SoyOx) copolymers. Thus, the effect of cross-linking on the thermal properties of the polymers was investigated. The thermal properties of the synthesized P(EtOx)-.yfaf-(SoyOx) copolymers before and after UV-curing are illustrated in Fig. 18. 

Macroporous and isoporous polystyrene supports have been used for onium ion catalysts in attempts to overcome intraparticle diffusional limitations on catalyst activity. A macroporous polymer may be defined as one which retains significant porosity in the dry state68-71 . The terms macroporous and macroreticular are synonomous in this review. Macroreticular is the term used by the Rohm and Haas Company to describe macroporous ion exchange resins and adsorbents 108). The terms microporous and gel have been used for cross-linked polymers which have no macropores. Both terms can be confusing. The micropores are the solvent-filled spaces between polymer chains in a swollen network. They have dimensions of one or a few molecular diameters. When swollen by solvent a macroporous polymer has both solvent-filled macropores and micropores created by the solvent within the network. A gel is defined as a solvent-swollen polymer network. It is a macroscopic solid, since it does not flow, and a microscopic liquid, since the solvent molecules and polymer chains are mobile within the network. Thus a solvent-swollen macroporous polymer is also microporous and is a gel. Non-macroporous is a better term for the polymers usually called microporous or gels. A sample of 200/400 mesh spherical non-macroporous polystyrene beads has a surface area of about 0.1 m2/g. Macroporous polystyrenes can have surface areas up to 1000 m2/g. 

The resistance to fluid flow is a measure of the physical structure of the foam. In order to control the flow through a foam, ceU size, degree of reticulation, density, and other physical factors must be controlled. The control of these physical factors, however, is achieved through the chemistry and the process by which the foam is made. The strength of the bulk polymer is measured by the tensile test described above, but it is clear that the tensile strengths of the individual bars and struts that form the boundaries of an individual cell determine, in part, the qualities of the cells that develop. A highly branched or cross-linked polymer molecule will possess certain tensile and elongation properties that define the cells. The process is also a critical part of the fluid flow formula, mostly due to kinetic factors. As discussed above, the addition of a polyol and/or water to a prepolymer initiates reactions that produce CO2 and cause a mass to polymerize. The juxtaposition of these two reactions defines the quality of the foam produced. Temperature is the primary factor that controls these reactions. Another factor is the emulsification of the prepolymer or isocyanate phase with the polyol or water. 

The number of possible conformations n for each mobile molecular segment is reciprocal to the density of cross-links AT (defined for 100 C-C groups in the polymer chain). 

Chemical gels are covalently cross-linked polymer networks, featuring very high viscosity and well-defined pore structure. Polyacrylamide is the most widely used chemical gel material, usually cross-linked with N,N-methylene-bisacryl-amide (BIS). The pore size of the gel is determined by the relative concentration of monomer and cross-linker used during polymerization (%T, total monomer concentration and %C, cross-linker concentration as a percent of the total monomer and cross-linker concentration [34]). Highly cross-linked ( 5%C) poly-... 

Albertsson and coworkers [240-244] carried out extensive research to develop polymers in which the polymer properties are altered for different applications. The predominant procedure is ring-opening polymerization which provides a way to achieve pure and well defined structures. They have utilized cyclic monomers such as lactones, anhydrides, carbonates, ether-lactones. The work involved the synthesis of monomers not commercially available, studies of polymerization to form homopolymers, random and block copolymers, development of cross-linked polymers and polymer blends, surface modification in some cases, and characterization of the materials formed. The characterization is carried out with respect to the chemical composition and both chemical and physical structures, the degradation behavior in vitro and in vivo, and in some cases the ability to release drug components from microspheres prepared from the polymers. 

Dendrimers are a unique class of hyperbranched polymers with well-defined size, shape, and chemical functionality and with properties not found in classical linear and cross-linked polymers. There have been significant developments in the last decade in many areas of dendrimer research, partly due to the commercial availability of dendrimers such as PAMAM, but also the result of the synthesis of several novel dendritic structures. Numerous pharmaceutical applications have been proposed for these hyperbranched polymers, and given the rate of current developments, it is envisaged that dendrimer-based formulations will appear in the next decade. 

Thermosetting foams can be defined as foams having no thermoplastic properties. Accordingly, thermosetting foams include not only cross-linked polymer foams, but also some linear polymeric foams having no thermoplastic properties, e.g., carbodiimide foams and polyimide foams. These foams do not melt and turn to char by heating. 

The polymer volume fraction 92 is defined as ratio of polymer volume to the sum of the volumes of polymer and solvent. To describe the swollen state of a cross-linked polymer, we introduce the volume degree of swelling as ratio of the... 

As it is known [61], in case of different chemical reactions proceeding, including cross-linked polymers curing, an essential role is played by the so-called steric factor p (p < 1), showing that not all reacting molecules collisions occur with proper for chemical bond formation these molecules orientation. This factor importance in such treatment is defined by its proportionality to reaction rate constant k - the smaller p the smaller k and reaction proceeds with less rate. 

If a paint is to be formulated for a particular purpose, the properties required from the dry film are defined first. These properties and the desired minimum solids of the paint, may well determine whether the dry film is to contain a linear or a cross-linked polymer. When the type of polymer film has been selected, the choice of drying mechanism will have been restricted to a narrow area. 

The term nanogel or microgel usually defines aqueous dispersions of hydrogel particles or colloidal networks formed by physically or chemically cross-linked polymer chains of nano-or microscale size. 

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