Poly bimodal networks

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). 

Non-Gaussian EflFects and Intermolecular Correlations in Bimodal Networks of Poly(dimethylsiloxane)... 

C. Menduina, C. McBride, and C. Vega (2001) Correctly averaged Non-Gaussian theory of rubber-like elasticity - application to the description of the behavior of poly(dimethylsiloxane) bimodal networks. Phys. Chem,. Chem. Phys. 3, p. 1289... 

There is evidence of large-scale supramolecular structures in end-linked PDMS elastomers, particularly in the case of bimodal distribu-tions. ° Small-angle neutron scattering on bimodal networks of poly(tetrahydrofuran) suggests segregation of short and long chains. ... 

Menduina, C. Freire, J. J. Uorente, M. A. Vdgis, T., Correctly Averaged Non-Gaussian Theory of Ruhherlike Elasticity. Application to the Description of the Behavior of Poly(dimethylsiloxane) Bimodal Networks. Macromolecules 1986,19,1212-1217. 

Tang, M.-Y. Mark, J. E., Comparisons Between Poly(dimethylsiloxane) Networks Filled with Particles of Poly(dimethylsiloxane) Thermosets and Unfilled Bimodal Networks Prepared from the Same Components. Polym. Bull. 1984, 11, 573-578. 

Curro, J. G. Mark, J. E., A Non-Gaussian Theory of Rubberlike Elasticity Based on Rotational Isomeric State Simulations of Network Chain Configurations. II. Bimodal Poly(dimethylsiloxane) Networks. J. Chem. Phys. 1984, 80, 4521-4525. 

Wang, S. Mark, J. E., Unimodal and Bimodal Networks of Poly(dimeth)dsiloxane) in Shear. J. Polym. Sci., Polym. Phys. Ed. 1992,30,801-807. 

Galiatsatos, V Mark, J. E., Non-Gaussian Effects and Intermolecular Correlations in Bimodal Networks of Poly(dimeth siloxane). In Advances in Silicon-Based Polymer Science. A Comprehensive Resource, Zeigler, J. M. Fearon, F. W. 

Sharaf, M. A. Mark, J. E. Al-Ghazal, A. A.-R., Elastomeric Properties of Poly(dimethylsiloxane) Networks Having High-Functionality Crosslinks and Bimodal Chain-Length Distributions. J.Appl. Polym. Sci. Symp. 1994, 55, 139-152. 

K. Saalwachter, P. Ziegler, O. Spyckerelle, B. Haidar, A. Vidal, J.-U. Sommer, IH multiple-quantum nuclear magnetic resonance investigations of molecular order distributions in poly(dimethylsiloxane) networks evidence for a linear mixing law in bimodal systems, J. Chem. Phys. 119 (2003) 3468—3482. 

The freezing points of solvents absorbed into bimodal networks are also of interest since solvent molecules constrained to small volumes form only relatively small crystallites upon crystallization, and therefore exhibit lower crystallization temperatures. " Differential scanning calorimetry measurements on solvent molecules constrained in the pores of PDMS elastomers gave evidence for several crystallization temperatures, which could be indicative of an unusual distribution of pore sizes. Calorimetric measmements on bimodal poly(ethylene oxide) networks indicated that the short chains seemed to decrease the amount of crystallinity in the unstretched state. This is an intriguing result since they inaease the extent of crystallization in the stretched state. A similar study on poly (tetrahydrofuran) did not show any decrease, however. ... 

Fig. 7.2 The extrapolated equilibrium melting temperature of poly(tetrahydrofuran) networks as a function of molecular weight between cross-links. Filled and open circles represent unimodal and bimodal networks respectively. Solid curve calculated from Eq. 7.10. (From Roland and Buckley (21))...

Sharaf MA, Mark JE, Al-Ghazal AA-R. Elastomeric properties of poly(dimethylsiloxane) networks having high-functionality crosslinks and bimodal chain-length distributions. J Appl Polym Sci Symp 1994 55 139-52. 

Besbes S, Bokobza L, Monnerie L, Bahar I, Erman B. Molecular orientation in deformed bimodal networks. 2. Fourier transform infrared measurements on poly(dimethylsiloxane) networks and comparison with theory. Macromolecules 1995 28 231-5. 

Wang S, Mark JE. Unimodal and bimodal networks of poly(dimethylsiloxane) in shear. J Polym Sci Polym Phys Ed 1992 30 801-7. 

Stress-strain isotherms have also been calculated with this approach. Examples are unimodal networks of polyethylene and poly(dimethylsiloxane) (226), polymeric sulfur and selenium (227), short n-alkane chains (228), natural rubber (229), several polyoxides (230,231), and elastin (232), and bimodal networks of poly(dimethylsiloxane) (233). It is possible to include excluded volume effects (1), in such simulations (234). In the case of the partially helical polymer poly-oxymethylene, the simulations were used to resolve the overall distributions into contributions from unbroken rods, once-broken rods, twice-broken rods, etc. (231). It was also shown how applying stresses to the ends of chains of this type can be used to bias the distributions in the direction of increased helical content and increased average end-to-end distances (231). In this sense, imposition of a stress has the same effect on the helix-coil equilibrium as a decrease in the temperature (6). 

The advantage of the swelling method is that it is not limited by the crosslinking reactions of each phase so any interference from these will be limited. A good representative example is the synthesis developed by Hamurcu and Baysal [75]. They synthesized a bimodal PDMS (15 000 gmoD1)/ PDMS (75 000 g mol ) IPN with the same condensation curing system. First, the 75 000 g mol 1 PDMS network was formed from the corresponding a, tw-dihydroxypolydimethylsiloxane and tetraethylorthosilicate catalyzed by stannous 2-ethylhexanoate. It was then swollen in a 15 000 g mol 1 a,a>-dihydroxy-poly(dimethylsiloxane) monomer. The second monomer was then crosslinked via the same condensation cure. The sequential full IPN structure... 

The dynamic behavior of linear charged polyelectrolytes in aqueous solution is not yet understood. The interpretation of dynamic light scattering (DLS) of aqueous solutions of sodium poly(styrene sulfonate) (NaPSS) is particularly complicated. The intensity correlation function shows a bimodal shape with two characteristic decay rates, differing sometimes by two or three orders of magnitude, termed fast and slow modes. The hrst observations in low salt concentration or salt free solution were reported by Lin et al. [31] for aqueous solutions of poly(L-lysine). Their results are described in terms of an extraordinary-ordinary phase transition. An identical behavior was hrst observed by M. Drifford et al. in NaPSS [32], Extensive studies on this bimodal decay on NaPSS in salt-free solution, or solutions where the salt concentration is increased slowly, have been reported [33-36]. The fast mode has been attributed to different origins such as the coupled diffusion of polyions and counterions [34,37,38] or to cooperative fluctuations of polyelectrolyte network [33,39] in the semidilute solutions. 

This atom-abstraction route was also used for the preparation of polymeric networks from alkylated [3,3 ]bis-(trithia)ferrocenophanes. A bimodal molecular weight distribution, with maxima at 4/n 5,000 and 5 x 10, was obtained by GPC for the material from which a polymer fraction was isolated (4/ = 8.5 x 10 ). Low molecular weight poly(ferrocenylene perselenides) have also been prepared from the selenium analog of 114. These materials also undergo photodegradation upon exposure to UV light in air. ... 

Recently, particular interest has focused on bimodal poly(dimethylsiloxane) (PDMS) networks (9-17) containing large mol fractions of very short chains as well as chains of the... 

Madkour, T. Mark, J. E., Some Evidence on Pore Sizes in Poly(dimethylsiloxane) Elastomers Having Unimodal, Bimodal, or Trimodal Distributions of Network Chain Lengths. Polym. Bull. 1993, 31, 615-621. 

PDMS networks are unsuitable for characterizing the effects of bimodality on strain-induced crystallization, because of their very low crystallization temperatures. Poly(ethylene oxide), however, has a relatively high melting point ( 65 C) and thus readily undergoes strain-induced crystallization. Decreasing temperature increases the extent to which the ultimate strength of the... 

---