Effect of Crosslink Density

Crosslink density may be defined as the number of effective crosslinks per unit volume. The crosslink density is a key parameter in determining the properties of an epoxy resin after cure. It is dependent on the number of reactive sites (functionality), the molecular distance and chain mobility between functional sites, and the percentage of these sites that enter into reaction. Crosslink density is inversely related to the molecular weight between crosslinks Mc. 

TABLE 3.8 Effect of Various Formulation Parameters on the Flexibility of an Epoxy Adhesive 

The crosslink density ultimately defines the rheological and mechanical properties of the polymer. Polymers that have a high crosslink density are thermosets and are infusible, insoluble, and dimensionally stable under load. These properties make epoxy resin systems useful as structural adhesives as well as important materials in other applications. Polymers that have a low crosslink density are more flexible and show greater resistance to stress concentration, impact, and cold. 

In the DGEBA product (n = 1) shown in Fig. 3.13, the epoxy groups are separated by seven units (the aromatic ring is counted as one unit). In other resins the groups may be 

FIGURE 3.12 Effect of molecular weight between crosslinks on the physical state of epoxy resins. 

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Effect of Crosslink Density on Elastic and Viscoelastic Properties... 

To appreciate an eventual effect of crosslink density on mass density, we have to compare networks having the same hydrogen bond concentration. This is possible, for instance, in the series B of Table 10.4 (Morel et al., 1989). 

To summarize, the bulk modulus of thermosets is proportional to the cohesive energy density and does not depend practically on temperature in the 200 K - (Tg — 30 K) temperature range. There is no significant effect of crosslink density on K, which can be predicted (in the temperature interval under consideration) using K=ll (CED), with an incertitude of about... 

Results presented in Fig. 13.8 could have been interpreted as an effect of crosslink density on toughening. But this is an incorrect concept, because crosslink density can be increased by the use of a low-molar-mass aliphatic diepoxide. This would decrease the matrix Tg and increase its toughenabil-ity, in spite of the increase in crosslink density. But also, it may be stated that at the same Tg — T, other factors related to the chemical structure, such as sub-Tg relaxations, will play a role on toughening mechanisms. 

Misra, S. C., et al., Effect of Crosslink Density Distribution on the Engineering Behavior of Epoxies, Epoxy Resin Chemistry, ACS Symposium Series 114, R. S. Bauer, ed., American Chemical Society, 1979, pp. 137-156. 

The glass transition temperature and resulting free volume are calculated from the DiBenedetto equation 23.) or by an approach which incorporates the effect of crosslink density on the glass transition temperature (22). This part of the model is iterative in order to account for diffusion-limitations on the reaction rate in the later stages of thermoset cure 23). ... 

Pierre et al [173] considered the effects of crosslink density on dynamic mechanical properties and plastic deformation of epoxy-amine networks, varying chain stiffness by using... 

Oh KS, Oh JS, Choi HS, Bae YC (1998) Effect of crosslinking density on the swelling behavior of NIPA gel particles. Macromolecules 31 7328-7335 Okay O (2000) Macroporous copolymer networks. Prog Polym Sci 25 711-779 Okay O, Gundogan N (2002) Volume phase transition of polymer networks in polymeric solvents. Macromol Theory Simul 11 287-292... 

Lu Lu, F., Kausch, H.-H., Cantwell, W. J., Fischer, M. The effect of crosslink density on the fracture toughness of core-shell modified epoxy resins. J. Mater. Sci. Lett. 15 (1996) 1018-1021. 

Liu Liu, J., Sue, H.-J., Thompson, Z. J., Bates, F. S., Dettloff, M., Jacob, G., Verghese, N., Pham, H. Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles. Polymer 50 (2009) 4683-4689. 

Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles. Polymer 50 (2009) 4683 689. 

The DMTA data of Figs 3.17 and 3.18 enable information to be derived concerning the effect of crosslink density on these properties. In brief, the overall effects of the large crosslink density changes are small, with the... 

A. A. Donatelli, L. H. Sperling, and D. A. Thomas, A Semiempirical Derivation of Phase Domain Size in Interpenetrating Polymer Networks, /. Appl Polym. Sci. 21(5), 1189 (1977). Equations for phase domain size in IPNs and semi-I IPNs. Effect of crosslink density, composition, interfacial tension. 

B. N. Kolarz, Ion Exchangers XIX. Some Properties of the Carboxylic Cation Exchangers Obtained by Intermesh Polymerization of Methacrylic Acid into Styrene and Divinylbenzene Porous Copolymers, /. Polym. Sci. 47C, 197 (1974). PS/PMA IPNs. Both polymers crosslinked with DVB. Effect of porosity of network I. Effect of crosslink density of network I. Ion exchange properties. 

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