Cross-linker poly

Figure 64 Hydrosilation reactions between l,7-bis(vinyltetramethyldisiloxyl)-m-carborane monomer and the polymeric cross-linker, poly(methylhydrosiloxane), producing hard, colorless networked plastics. (Adapted from ref. 133.)...

It is noteworthy that the load capacity and the resolving capability increased when the tri-functional cross-linker PETRA was used instead of EDM A [20]. The same features have been seen with polymers prepared from TRIM, another trifunc-tional cross-linker. Poly(methacrylic acid-co-TRIM) imprinted with a dipeptide was able to resolve 1 mg of the racemate with almost base-line separation (analytical column 4.6 x 250 mm) (Fig. 17.7). 

Poly(vinyl alcohol) (different cross-linkers) Poly(aayclic acid) (different cross-linkers) Various poly(amide)s (different substituents) Various poly(imide)s (different substituents) Nafion... 

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Because of the many choices of hydrophilic monomers, cross-linkers, and hydrophobic monomers, a large number of formulations have been developed and manufactured into hydrogel lenses. The water content of these hydrogel lenses ranges from about 38%, for HEMA-based lenses, to 80%, for poly(vinyl alcohol) and partially hydrolysed acrylonitrile lenses. Table 2 gives a representative Hst of FDA approved hydrogel materials available to the consumer in the early 1990s. 

Figure 1. Sketch of the primary structure (vide infra) of cross-linked polystyrene (a) and of a typical cross-linked poly-vinyil co-monomer-functional co-monomer-cross-linker (b) [14].

In a series of papers, Kennedy and co-workers studied the amphiphilic networks and their membrane characteristics. The use of poly(pentamethyl)cyclopentasiloxane)32 as an oxyphilic cross-linker in the novel tricontinuous membranes, consisting of hydrophilic and lipophilic polymers, was reported.466 The presence of polysiloxane brought about high oxygen permeability, which was studied in the water-swollen membranes.467 The use of polysiloxanes as sorbents,468 and as carriers for the receptor molecules in the ion-exchange membranes,469 470 was reviewed. 

This was conhrmed in 1967 by Kubin et al., who were the first to accomplish polymerization directly in a glass colnmn [13], The resulting swollen poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) gel, which was prepared in the presence of 1% cross-linker, was strongly compressed after pressnre application and consequently exhibited exceedingly low permeability (4.5mL/h for a 25 X 220 mm colnmn) and poor efficiency. 

Figure 3.11 Poly(4-vinylpyridine) brushes subsequently exposed to metallic cross-linkers. The addition of 4a decreases the lateral friction, but the addition of 4b increases the lateral friction.

Yount et al. (2005) reported the formation of organogels using poly(vinylpyridine) (PVP) and ditopic metallopincer cross-linkers. This study provided particularly pertinent information on the dynamic elements of MSPs and elegantly demonstrated how they control the material s properties. PVP dissolved in dimethylsulfoxide (DMSO) is cross-linked with either bis-Pd - (18a) or Pt -pincer compounds (18b, Fig. 7.13 Yount et al. 2005). Addition of 5% 18a -Pd to a PVP solution results in a viscous material (77 = 6.7 Pa s), whereas the corresponding PVP 18b Pd is a gel (77 = 550 Pa s). Changing R from methyl to ethyl does not affect the thermodynamics of the pyridine/Pd interaction however, the rate of exchange decreases by approximately 2 orders of magnitude. Further studies on these materials and their... 

The dependence of the friction coefficient of the poly(acrylamide) gel on the total concentration of polymers of the gel, including both the main constituent and cross-linker, has been studied under the experimental conditions of a constant molar concentration of the cross-linker at 1 mol%. The total concentration was changed from 400 mM to 2.8 M which corresponds to a concentration from about 3 to 20 wt.%. The temperature was fixed at 20 0.1 °C. 

The results of the friction measurements are shown in Fig. 8. The friction coefficient of the poly(acrylamide) gel decreases with an increase of the crosslinker concentration in the major part of the concentration range studied, except that it sharply decreases at a mole fraction of 0.2% of the cross-linker concentration. The measurements cannot be performed below this concentration because the gel becomes too soft to be used in the present experimental setup. It suggests that the system is dose to the gelation threshold. The sharp decrease of the friction observed in the gel with a mole fraction of 0.2% may be due to the effect of the sol-gel transition. It is also found from visual inspection that the gels becomes opaque above a cross-linker concentration of 3 mole-% fraction. 

Fig. 8. The cross-linker concentration dependence of the friction coefficient of the poly(acrylamide) gel. The total concentration of the gel is constant at 700 mM...

The sample used to study the relationship between the volume phase transition and the frictional property is poly( /V-isopropylacrylamide) gel which shows a small discontinuous volume phase transition at 33.6 °C. The sample gel is prepared by free radical polymerization 7.8 g of re-crystallized N-iso-propylacrylamide (main constituent, Kodak), 0.133 g lV,iV -methylenebis-acrylamide (cross-linker, Bio-Rad), 240 ml tetramethylenediamine (accelerator, Bio-Rad), and 40 mg ammonium persulfate (initiator, Mallinckrodt) are dissolved in distilled water (100 ml) at 0°C. The gel mold is immersed in the pre-gel solution and then degassed for 40 min at 0°C. The temperature is raised to 20.0 °C after this treatment to initiate the gelation reaction. The sample gel thus obtained is homogeneous and transparent, at least by visual inspection. 

Poly(vinyl acetal)s can be formulated with other thermoplastic resins and with a variety of multifunctional cross-linkers. When cross-linking takes place the resin becomes thermoset. Thermosetting generally increases thermal stability, rigidity, and abrasion resistance, and improves resistance to solvents ancl to acids and bases. It also severely limits proccssibility by making the resin insoluble and impossible to extrude. 

Poly(acrylic acid) and Poly(rnethacrylic acid). Poly(acrylic acid) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (widi cross-linker for superadsorber applications) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78°C provides a syndiotactic form that can be hydrolyzed to syndiotactic PAA. 

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