Nitrogen-15 nuclear magnetic resonance spectroscopic investigation of the cure and degradation of imide polymers
Abstract
Solid state $\sp{15}$N NMR spectroscopy has been used to accurately and quantitatively measure details of the curing reactions and their kinetics of thermoset imide polymers used in high temperature composites. The para-isomers of the aromatic bismaleimide monomer 1,1$\sp\prime$-methylenedi-4,1-phenylene)-bismaleimide (MPBM) and the aromatic diamine 4,4$\sp\prime$-methylenedianiline (MDA) were synthesized with 100% $\sp{15}$N enrichment. The curing reactions of the neat resins were studied using both cross-polarization and single pulse experimental techniques. In all experiments magic angle spinning (5000 Hz) was used to improve spectral resolution. The curing reactions of the resin formulations were studied at MPBM:MDA stoichiometric ratios of 1:0 and 1:1, and at curing temperatures of 150$\sp\circ$C, 175$\sp\circ$C for the 1:1 system and 175$\sp\circ$C, 200$\sp\circ$C, and 225$\sp\circ$C for the 1:0 system. $\sp{15}$N NMR spectroscopy allows us to clearly identify and quantify reactions in the curing neat resin. Specifically, maleimide ring addition was found to be the only reaction in the homopolymerized bismaleimide resin. When co-reacted with the diamine, however, three reactions are observed: (1) Michael addition of the amine to the maleimide ring; (2) addition of the maleimide ring as observed in homopolymerization; and (3) a ring-opening reversible amidation reaction. The ring-opening amidation reaction has been conclusively shown for the first time in this research. In addition, we have performed studies of the curing and hydrolytic degradation of the polyimide resin AFR700B using $\sp{15}$N NMR spectroscopy and can clearly observe the reaction progression during imidization and cross-linking as well as degradation during hydrolysis. $\sp{15}$N solid state NMR spectroscopy yields well resolved spectra for this class of resins and offers tremendous promise for cure and degradation reaction studies of nitrogen-containing polymer systems. The cure kinetics of the 1:1 MPBM:MDA system were modelled using a novel approach based on the evolution of the viscoelastic timescale during cure. An expression for the reaction rate constant was developed that incorporated intrinsic reaction rate control initially and mobility control as the reaction proceeds towards vitrification. We found the GKAC based model accurately predicted mobility limitation on cure kinetics for the resin and that the volumetric changes during cure were predicted to be non-linear.
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering|Polymers
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