Mechanical properties, degradable networks

By adjusting postpolymerization temperature and cross-linking (curing) time, the mechanical properties, degradation profiles, and surface energies of the cross-linked polyester networks, POC can be tuned to fit a wide range of tissue engineering applications (see Table 16.2) [2,16-18,35,50-56]. An increase in postpolymerization temperature and cross-... 

R. Censi, T. Vermonden, H. Deschout, K. Braeckmans, P. di Martino, S.C. De Smedt, et al, Photopolymerized thermosensitive poly(HPMAlactate)-PEG-based hydrogels Effect of network design on mechanical properties, degradation, and release behavior. Biomacromolecules, 11, 2143-51,2010. 

R.J. Farris, "The Stress-Strain Behavior of Mechanically Degradable Polymers," In POLYMER NETWORKS STRUCTURAL AND MECHANICAL PROPERTIES, ed. A.J. Chompff and S. Newman, pp. 341-394, Plenum, New York, 1971. 

CEs are known to react with phenols to form iminocarbonates which eventually lead to polycyanurates with the liberation of more acidic phenol moiety. This can be a method to alter the gel point of the resin, Tg, and thermal stability of the network by co-curing diphenol with CE. Thus, copolymerization of dicyanate with diphenols resulted in polycyanurates with altered network structure and diminished crosslink density [237]. However, an earlier report claims poly(imi-nocarbonate) by reaction of these two in equimolar quantities. The thermoplastic so formed was reported to retain the mechanical properties like a polycarbonate. This approach can produce strong, non-toxic, biodegradable films and molded plastics that are degradable at temperatures above 140 °C [169,238]. Except for a few very early reports [239], the reaction of CE with anhydrides to form poly(iminocarbamates) has not been explored much. 

Water based poly(ester-imide) wire enamels were developed in the 1960s and 1970s. The resins were made water soluble in different ways. Resins with a defined acid number were neutralized with alkanolamines [144-148] or ammonia [149]. In another process the poly(ester-imide) resins were submitted to an ami-nolysis with alkanolamines [150-153] or ammonia [154-156], when the resin network is more or less degraded. Solvents for this poly(ester-imide) is water and usually a small amount of high boiling solvents like N-methylpyrrolidon or diethylene glycol monomethylether. Titanium catalysts stable to hydrolysis, like titanium-ammonium lactate and titanium lactate, were used [157]. To improve thermal and mechanical properties, phenol blocked isocyanates can be added to the water based poly(ester-imide)s. The blocked isocyanates are dispersed by means of an ethoxylated nonylphenol and are added to the water based wire enamel. Improvement of the property level is claimed [158]. 

The chemical stability against chain scission and oxygen attack also plays a role in the application of plastics at still higher temperatures. The statistical probability of chain scission is indeed practically the same for linear chains, cross-linked network polymers, and ladder polymers. But, whereas, a chain scission in linear polymers leads to lower degrees of polymerization because of chain degradation, with consequent diminished mechanical properties, chain scission with, for example, a ladder polymer, still leaves one chain of the ladder intact since it is improbable that both chains of the ladder should break at exactly the same distance from each end (see also Chapter 23). 

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