Polyphosphazene membranes. II. Solid-state photocrosslinking of poly[(alkylphenoxy)(phenoxy)phosphazene] films

✍ Scribed by Richard Graves; Peter N. Pintauro

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 284 KB
• Volume: 68
• Category: Article
• ISSN: 0021-8995
• DOI: 10.1002/(sici)1097-4628(19980502)68:5<827::aid-app14>3.0.co;2-n

✦ Synopsis

The solid-state UV photocrosslinking mechanism and the properties of dense crosslinked films composed of poly[(methylphenoxy)(phenoxy)phosphazene], poly[(ethylphenoxy)(phenoxy) phosphazene], and poly[( isopropylphenoxy)(phenoxy)phosphazene] were investigated, where the alkyl substituent was in either the metaor para-position. Solution-cast films containing dissolved benzophenone photoinitiator (at a concentration of 1-25 mol %) were crosslinked at either 25 or 70ЊC. The ordering of benzophenone disappearance during polymer irradiation was methylphenoxy ú ethylphenoxy ú isopropylphenoxy, indicating that the rate controlling step for photoinitiator disappearance was the consumption of benzophenone, either by benzopinacole formation (with the creation of a polymer crosslink) or by reaction of a benzophenone-derived ketyl radical with a polymer macro-radical. The presence of such a ketyl adduct in crosslinked ethylphenoxy/phenoxy and isopropylphenoxy/phenoxy phosphazene films was verified by solid-state NMR. The ordering of crosslinked polymer swelling (for a given initial benzophenone concentration) when films were equilibrated in dimethylacetamide (DMAc) was isopropylphenoxy/phenoxy ú ethylphenoxy/phenoxy ú methylphenoxy/phenoxy, indicating that steric effects of the alkyl group were playing a role during crosslink formation. The methylphenoxy/phenoxy phosphazenes were the best materials for crosslinking; the glass transition temperature increased by approximately 25ЊC (from 015 to 10ЊC) and the film swelling (in DMAc) decreased from infinity (complete solubilization) to 35% as the benzophenone concentration was increased from 0 to 25 mol %.