Functionality of crosslinks

Parallel to the development of the new theoretical approaches considerable experimental work was done on model networks especially synthesized, to show the effects of pendent chains, loops, distribution of chain length, functionality of crosslinks, etc. on properties (5-21). In some instances, the properties turned out... 

The blends swelling ratio in pure water is shown as a decreasing function of crosslinking time [44],... 

As expected, addition of IV tended to increase the sol fraction in the cured specimens. The statistical theory of gelation (9) can be used to calculate the expected gel fraction as a function of crosslink functionality, extent of reaction, and proportion of difunctional units in the mixture. For these mixtures, an adequate fit to the extraction data was obtained by assuming that the primary reaction is isoindoline formation (Figure 5). Triazine or phthalocyanine formation alone cannot account for the data. 

Fig 15.8 The dependence of foam density on swell ratio as a function of crosslinking system formulation... 

Figure 5. Shear relaxation modulus of NR as a function of crosslink density at 25°C.

Table I shows a comparison of free film swell Index results as a function of crosslinker at a constant comonomer level (0.3 moles/kg of polymer). ABDA and AEP gave crosslinking performance Identical to a conventional crosslinker, BNMA. However, derivatives which cannot cycllze, either because the amide has an additional substituent, as In 13, or the chain connecting the amide to the blocked aldehyde Is too short, as In 14, did not exhibit efficient crosslinking. They also showed significant discoloration, presumably due to Increased aldol condensation relative to 1, 2, or BNMA.

Frechet, J. M. J. and M. J. Farrall, Functionalization of Crosslinked Polystyrene Resins by Chemical Modification A Review, pp. 59-83 in Chemistry and Properties of Crosslinked Polymers, S. S. Labana, ed., Academic Press, New York, 1977. 

Fig. 36 Comparison of hardness values of EPDM gum and rubber composites as a function of crosslinking irradiation dose...

Table 3. Comparison of PDMS networks with known functionalities of crosslinks...

Grillet et al. (1991) studied mechanical properties of epoxy networks with various aromatic hardeners. It is possible to compare experimental results obtained for networks exhibiting similar Tg values (this eliminates the influence of the factor Tg — T). For instance, epoxy networks based on flexible BAPP (2-2 - bis 4,4-aminophenoxy phenyl propane) show similar Tg values ( 170°C) to networks based on 3-3 DDS (diamino diphenyl sulfone). However, fracture energies are nine times larger for the former. These results constitute a clear indication that the network structure does affect the proportionality constant between ay and Tg — T. Although no general conclusions may be obtained, it may be expected that the constant is affected by crosslink density, average functionality of crosslinks and chain... 

Eqs. (8) and (16) allow calculation of the crosslinking density at the gel point and the weight fraction of gel after the gel point is reached as functions of crosslinking density for a polymer with known molecular weight distribution. Therefore, the next step is to find the relationship between crosslinking density and the time and intensity of the incident irradiation, i.e., exposure conditions. In order to solve this problem analytically, the simplified system was considered where the irradiation is monochromatic and the active... 

Chapter 7 of this text details the use of NMR imaging methods to determine structure and function of crosslinked rubbers. In this section we review the NMR imaging of rubbers swollen with small molecules. Two sets of experiments are described, namely the study of diffusion of small molecules into a previously unswollen rubber (macroscopic diffusion), and the measurement of images of the solvent and/or polymer after equilibrium has been achieved. 

However, if the refractive indices of the two components are identical Rayleigh scattering will not occur [110]. Due to the various values of refractive indices of epoxy and polyimides quoted in the literature [2] (this is speculated to be a function of crosslink density, monomers used and state of cure) phase separation of epoxy systems can only be inferred and not quantitatively determined visually. 

For mechanical stability against hydration and swelling, starch is chemically crosslinked (by adipic acid-acetic anhydride, phosphate, etc.) below Tgz, so that the physical integrity of the granules is not impaired by gelatinization. Weakly crosslinked starches (DS < 0.1) are acid-stable and thus find occasional application in recipes containing vinegar. The many industrial functions of crosslinked starch include use as an adhesive. 

Operating Conditions. Table 10 shows typical selectivities of a cation resin as a function of crosslinking. For the monovalent cations given, Li is the least strongly sorbed, and the other values are calculated relative to it. Divalent cations are more strongly held then monovalent ions for a typical resin, the order among cations is... 

Functionalization of crosslinked polymers by Mannich reaction" includes mainly polystyrenes and polyacrylics such as styrene/divinyl benzene copolymers 537-539114-117 gpjj acrylic ester/divinyl benzene copolymers 540, respectively." These materials are involved in the reaction as substrate (539)" or, more frequently, as amine reagent (sec also Fig. 163, Chap. Ill) when the crosslinked product, containing amino groups, is allowed to react with phosphorous acids (537,538, and 540). Thus, chelating properties are assumed by the resins. 

Poly(S -benzoyl O-ethylmethacrylate xanthate) resin [poly(BEMX)], obtained by functionalization of crosslinked 2-hydroxyethyl methacrylate/ethyleneglycol dimethacrylate beads, results [135], upon irradiation at 400 nm, in a heterogeneous photoinitiator for the polymerization of MMA and styrene more effective than the low-molecular-weight analogue BEX. 

What would be the size of a phantom network, made from strands with N monomers of size h and with functionality / of crosslinks, if it were not... 

However, the efficacy of immunotoxins targeted to these proteins indicates that at least a portion of the cell-surface envelope protein is reinternalized. This is not a function of crosslinking, since even monomeric CD4-based immunotoxins are effective. Our studies with anti-HIV immunotoxins not only demonstrate that there is a recirculating pool of envelope protein, but that the rates of internalization are subject to regulatory influences. 

Figure 1.6 Classification of polyurethanes as function of crosslink density and stiffness...

A model network should, at least, satisfy the following conditions [5] (i) the linear chain element of a model network should exhibit known length and, if possible, a narrow molar mass distribution ( >m)- ch elastic chain should be connected by its two ends to two different crosslink points (ii) a model network should be homogeneous crosslinking density should be constant throughout the gel and (iii) a model network should exhibit a known and constant functionality of crosslink points. 

It is also desirable to treat network topology in greater detail that is, to incorporate the functionality of crosslinks, their distribution in space, and loop formation. The effect of mutual interaction between chains in the condensed state appears to be accounted for satisfactorily by the tube model for... 

Rubber networks will imbibe solvent liquids until the elastic retractive force of the network crosslinks counterbalances the swelling force exerted by the liquid. If no crosslinks are present, the rubber dissolves completely on immersion in an excess of solvent. The degree of swelling is thus a function of crosslink density. As crosslink density increases, the degree of swelling decreases and vice versa. The average crosslink between junction points can be related to swelling measurements from potential considerations, as shown below. 

Where, is volume fraction of the polymer, is molar volume of solvent, X is the polymer solvent interaction parameter. Me is the crosslink density of polymer, pi is solvent density, P2 is polymer density, / is functionality of crosslink, Fequ is equilibrium volume of the hydrogel. 

Latex with hydroxyl functionalised cores of a methyl methacrylate/butyl acrylate/2-hydroxyethyl methacrylate copolymer, and carboxyl functionalised shells of a methyl methacrylate/butyl acrylate/methacrylic acid copolymer was prepared by free radical polymerisation. The latex was crosslinked using a cycloaliphatic diepoxide added by three alternative modes with the monomers during synthesis dissolved in the solvent and added after latex preparation and emulsified separately, then added. The latex film properties, including viscoelasticity, hardness, tensile properties, and water adsorption were evaluated as functions of crosslinker addition mode. Latex morphology was studied by transmission electron and atomic force microscopy. Optimum results were achieved by introducing half the epoxide by two-step emulsion polymerisation, the balance being added to the latex either in solution or as an emulsion. 8 refs. 

J. L. Thiel and R. E. Cohen, Synthesis, Characterization, and Viscoelastic Behavior of Single-Phase Interpenetrating Styrene Networks, Polym. Eng. Sci. 19, 284 (1979). Polystyrene/polystyrene homo-IPNs. Swelling equation for single-phase IPNs. Equilibrium swelling studies as a function of crosslink level. 

FIG. 8 Variation of In (Vq/N,) as a function of crosslinking time for emulsions stabilized by 0.05% P-casein polymers crossUnked by transglutaminase. N, is the concentration of droplets at time t. No is the initial concentration of droplets at time 0. The ratio of NqIN, was calculated from the turhidity measurement. The smaller the ratio change, the more stable the emulsion is. A typical crosslinking reaction system contained 1% P-casein in 0.1 M tris-HCl buffer, pH 7.5, at the ratio of enzyme to substrate of 4.25 units/g protein. The reaction mixtures were incubated at 37°C for the period shown in the figure, and the reaction was terminated at various time intervals by heating up to 85°C for 5 min. (From Ref. 41.)... 

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