Bimodal networks properties

Strain-induced crystallization would presumably further improve the ultimate properties of a bimodal network. It would therefore obviously be of considerable importance to study the effect of chain length distribution on the ultimate properties of bimodal networks prepared from chains having melting points well above the very low value characteristic of PDMS. Studies of this type are being carried out on bimodal networks of polyethylene oxide) (55), poly(caprolactone) (55), and polyisobutylene (56). 

Some recent computational studies,287 however, indicate that it is possible to do simulations to identify those molecular weights and compositions that should maximize further improvements in mechanical properties. Results to date283 suggest that a trimodal network prepared by incorporating small numbers of very long chains into a bimodal network of long and short chains could have significantly improved ultimate properties. 

Bulky crosslinks or side-groups in the network chains, e.g., dendritic wedges [73], may also influence molecular mobility and viscoelastic properties of polymer networks. For example, UV curing of difunctional acrylates results in the formation of zip-like network junctions, which may be regarded as extreme cases of bimodal networks [52], Results obtained with the NMR T2 relaxation method agree well with those of mechanical tests... 

Isayev, A.I. Kim, S.H. Levin, V.Y. Superior mechanical properties of reclaimed SBR with bimodal network. Rubber Chem. Technol. 1997, 70, 194. 

But what happens in the case of bimodal networks having such overwhelming numbers of short chains that they cannot be ignored There is a synergistic effect leading to mechanical properties that are better than those obtainable from the usual unimodal distribution. The following sections describe these results in detail. 

Many of the stress-strain isotherms of bimodal networks were obtained on PDMS elastomers in the vicinity of 25°C, a temperature sufficiently high to suppress strain-induced crystallization. These elongational isotherms show greatly improved the ultimate properties (figure 7.17). ° ... 

The properties of several additional types of bimodal networks were also investigated. The first of these involves prereacting the short chains... 

Measurements of the mechanical and optical properties of bimodal networks as a function of temperature and degree of swelling were used to test further the conclusion cited in section 7.3.2.4 that crystallization was not the origin of the improved properties. - 244,246 p j, example,... 

Llorente, M. A. Andrady, A. L. Mark, J. E., Model Networks of End-Linked Polydimethylsiloxane Chains. Xlll. The Effects of Junction Functionality on the Elastic Properties of the Bimodal Networks. Coll. Polym. Sci. 1981,259, 1056-1061. 

Mark, J. E. Tang, M.-Y., Dependence of the Elastomeric Properties of Bimodal Networks on the Length and Amount of the Short Chains. J. Polym. Set,... 

Jiang, C.-Y Mark, J. E. Stebleton, L., Ultimate Properties of Filled Bimodal Networks Containing a Non-Volatile Diluent. J. Appl. Polym. Sci. 1984, 29 (12), 4411-4414. 

In recent years, with the development of model networks it has been possible to prepare networks of controlled and junction functionality These are prepared by endlinking functionalized prepolymers with cross-linking agents of known functionality. Therefore, by choosing the appropriate molecular weight distribution of the prepolymers it is possible to prepare unimodal and bimodal networks. Mark and coworkers (5-11) have performed extensive studies on model networks to test the various theories of rubber elasticity. In the case of unimodal networks they find that the macroscopic properties such as stress or swelling ratios can be described reasonably well by the Flory-Erman theory (12,13). 

In the case of such noncrystalUzable, unfilled elastomers, the mechanism for network rupture has been elucidated to a great extent by studies of model networks similar to those already described. For example, values of the modulus of bimodal networks formed by end-linking mixtures of very short and relatively long chains as illustrated in Fig. 6.4 were used to test the weakest-link theory [7] in which rupture was thought to be initiated by the shortest chains (because of their very limited extensibility). It was observed that increasing the number of very short chains did not significantly decrease the ultimate properties. The reason [85] is the very nonaffine... 

Llorente MA, Andrady AL, Mark JE. Model networks of end-linked polydimethylsiloxane chains. XIII. The effects of junction fimctional-ity on the elastic properties of the bimodal networks. Colloid Polym Sci 1981 259 1056-61. 

M.Y. Tang, L. Garrido and J.E. Mark, "The Effect of Crosslink Functionality on the Elastomeric Properties of Bimodal Networks," Polvm. Commun.. 25, 347-350 (1984). 

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