Urethane networks

Urethane network polymers are also formed by trimerization of part of the isocyanate groups. This approach is used in the formation of rigid polyurethane-modified isocyanurate (PUIR) foams (3). 

For imperfect epoxy-amine or polyoxypropylene-urethane networks (Mc=103-10 ), the front factor, A, in the rubber elasticity theories was always higher than the phantom value which may be due to a contribution by trapped entanglements. The crosslinking density of the networks was controlled by excess amine or hydroxyl groups, respectively, or by addition of monoepoxide. The reduced equilibrium moduli (equal to the concentration of elastically active network chains) of epoxy networks were the same in dry and swollen states and fitted equally well the theory with chemical contribution and A 1 or the phantom network value of A and a trapped entanglement contribution due to the similar shape of both contributions. For polyurethane networks from polyoxypro-pylene triol (M=2700), A 2 if only the chemical contribution was considered which could be explained by a trapped entanglement contribution. 

Two types of networks were prepared (i) randomly crosslinked polybutadiene, and (ii) model urethane networks, (a) polybutadiene based, and (b) poly(e-caprolactone) based. The randomly crosslinked networks were prepared from polybutadiene (Duragen 1203 obtained from General Tire and Rubber Co.) crosslinked with di-cumyl peroxide. Specifications of the as obtained polybutadiene are given in Table I. Polybutadiene was purified by dissolving in benzene and precipitating in methanol. Precipitated polybutadiene was redissolved in benzene. Seven different weights of dicumyl... 

The term S represents the strength of the network. The power law exponent m was found to depend on the stochiometric ratio r of crosslinker to sites. When they were in balance, i.e. r = 1, then m - 1/2. From Equations (5.140) and (5.141) this is the only condition where G (co) = G (cd) over all frequencies where the power law equation applies. If the stochiometry was varied the gel point was frequency dependent. This was also found to be the case for poly(urethane) networks. A microstructural origin has been suggested by both Cates and Muthumkumar38 in terms of a fractal cluster with dimension D (Section 6.3.5). The complex viscosity was found to depend as ... 

Bruin, P., Smedinga, J., Pennings, A.J., and Jonkman, M.E, Biodegradable lysine diisocyanate-based poly(glycolide-co-e-caprolactone)-urethane network in artificial skin. Biomaterials 11 191-295, 1990. 

Model urethane networks prepared from polyoxypropylene tetrols and hexamethylene diisocyanate are studied with the aid of the computer, and good agreement of theory with experiment is found for gel paints when the crosslinkers are treated as sticks. 

Polyoxypropylene triol-based urethane networks are simulated. Simulations show that cyclic molecules are present in substantial amounts in the sol fraction when the vhnmotr-. tin rss1.0. Simulations also show that the fractions of loops are much higher than those obtained from modified cascade theory. 01 lc r... 

Problem 5.38 Calculate the sol fraction and the degree of cross-linking in the urethane, networks formed by stepwise polymerization of 2-hydroxymethyl-2-ethyl-l,3-propanediol and 1,6-hexamethylene diisocyanate to 90% conversion of the hydroxyl groups. Compare these properties for two urethane systems with (a) r = 1 and (b) r = 0.75, where r is the mole ratio of hydroxyl to isocyanate groups. 

Finally, as the foam achieves full rise, urethane network formation becomes the predominant chemical reaction and the foam develops a true, longer-range bulk modulus. 

When the process involves two competitive reactions, some people prrfer to call those modified polymers interpenetrated polymer networks (IPNs) [5]. The formation of a polyether-urethane network in a loosely crosslinked poly(methyl methacrylate) matrix to increase its toughness can serve as one of the examples. From a general point of view, the analysis of the reaction-induced phase separation is the same (perhaps more complex) for IPNs than for rubber-modified epoxies or for high-impact polystyrene. 

Storey RF, Wiggins JS, Puckett AD. Hydrolyzable poly(ester-urethane) networks from L-lysine diisocyanate and D,L-lactide/e-caprolactone homo- and copolyester triols. JPolym Sci, Part A Polym Chem 1994 32(12) 2345-2363. 

Supramolecular networks based on linkages containing adjacent urea groups have recently been studied. Ni and coworkers examined polyurea-urethane networks connected by H-bonding between triuret and tetrauret blocks [162]. 

G. LUgadas, J. C. Ronda, M. GaUa and V. Cadiz, Poly(ether urethane) networks from renewable resources as candidate biomaterials Synthesis and characterization , Biomacromolecules, 2007,8, 686-92. 

Wiggins,J.S. and Storey, R. (1992) Synthesis and characterization of l.-lysine based poly(ester-urethane) networks. Polym. Preprints., 32, 516-517. 

The materials used and their descriptions are listed in Table 5.9. Three urethane networks, two polyether based (PU-1,2) and the one polyester based (PU-3), were synthesized. 

Wilson, T.S., Bearinger, J.P., Herherg, J.L., Marion, J.E., Wright, W.J., Evans, C.L., Maitland, D.J., 2007. Shape memory polymers based on uniform aliphatic urethane networks. Journal of Applied Polymer Science 106,540-551. 

Richter, E.B. and Macosko, C.W. (1980) Viscosity changes during isothermal and adiabatic urethane network polymerization. Polym. Eng. Sci., 20, 921-924. 

Alteheld, A., Feng, Y., Kelch, S., Lendlein, A. (2005). Biodegradable, amorphous copolyester-urethane networks having shape-memory properties. Angewandte Chemie International Edition, 44(S), 1188-1192. 

Lendlein, A., J. Zotzmann, Y. Feng, A. Altheld and S. Kelch (2009), Controlling the switching temperature of biodegradable, amorphous, shape-memory poly(rac-lactide)urethane networks by incorporation of different comonomers. Biomacromolecules, 10(4) pp. 975-982. 

Ahir, S. V., Tajbakhsh, A. R.,Terentjev, E. M. (2006), Self-assembled shape-memory fibers of tri-block liquid-crystal polymers,. Advanced Functional Materials, 16,556-60. Alteheld, A., Feng, Y. K., Kelch, S., Lendlein, A. (2005), Biodegradable, amorphous copolyester-urethane networks having shape-memory properties, Angewandte Chemie - International Edition, 44,1188-92. 

Siloxane-based electrolytes appear to be an excellent choice due to the low glass transition temperature of many siloxane polymers. The si loxane-based electrolyte described here consists of a methyls loxane backbone with pendent polyether sidechains and crosslinks. Other siloxane electrolytes which have been studied Include a copolymer liquid electrolyte comprised of dimethylsiloxane and ethylene oxide repeat units, and a urethane network of poly(dlmethyIsiloxane grafted ethylene oxide). Initial reports have also appeared on siloxane-... 

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