Chain extension of recycled PA6

YesRecycling of polymers is a necessity in our intensively consuming polymer world but the nature of polymers is such that they are prone to thermal degradation when re-extruded and this poses technical challenges to recycling. This article describes research that seeks to rebuild the structure of degraded PA6. We present data from controlled experiments with pristine pPA6 extruded to form a base recycle rPA6 to which we added two chain extenders, separately: one with anhydride multifunctionality (ANHY), highly reactive with amide groups and one with epoxy multifunctionality (EPOX), less reactive. We found from rheological data carried out in the linear viscoelastic region (so as to study structural changes) a striking difference in the ability of the chain extenders to rebuild structure: 306% increase in the complex viscosity of rPA6/ANHY compared to 25% in that of rPA6/EPOX of the base rPA6. Mechanical and thermal (DSC and TGA) tests confirmed the superior efficacy of the multifunctional anhydride chain extender. Beside the practical benefit that ensues from this research, it also provides a strategic platform to develop chain extenders for other degrading polymers on the basis of understanding the degradation chemical reaction and targeting the most reactive end group of the split chains

Polyamide-6,6-based blocky copolyamides obtained by solid-state modification

Copolyamides based on polyamide-6,6 (PA-6,6) were prepared by solid-state modification (SSM). Para- and meta-xylylenediamine (PXDA, MXDA resp.) were successfully incorporated into the aliphatic PA-6,6 backbone at 200 and 230 °C under an inert gas flow. In the initial stage of the SSM below the melting temperature of PA-6,6 a decrease of the molecular weight was observed due to chain scission, followed by a built up of the molecular weight and incorporation of the comonomer by post-condensation during the next stage. When the solid-state copolymerization was continued for a sufficiently long time, the starting PA-6,6 molecular weight was regained. The incorporation of the comonomer into the PA-6,6 main chain was confirmed by size exclusion chromatography (SEC) with UV detection, which showed the presence of aromatic moieties in the final high molecular weight SSM product. The occurrence of the transamidation reaction was also proven by 1H-NMR spectroscopy. Since the transamidation was limited to the amorphous phase, this SSM resulted in a non-random overall structure of the PA copolymer as shown by the degree of randomness determined by using 13C-NMR spectroscopy. The thermal properties of the SSM products were compared with melt-synthesized copolyamides of similar chemical composition. The higher melting and higher crystallization temperatures of the solid state-modified copolyamides confirmed their non-random, block-like chemical microstructure, whereas the melt-synthesized copolyamides were random

Electronic structure, photophysics, and relaxation dynamics of charge transfer excited states in boron-nitrogen-bridged ferrocene-donor organic-acceptor compounds

We present a study of the electronic, photophysical, and picosecond excited-state relaxation characteristics of a class of derivatives comprised of multiple bipyridylboronium acceptors covalently linked to a ferrocene donor. These compounds exhibit a broad visible absorption band, which we attribute to a metal-to-ligand charge transfer transition between the donor and the acceptor. A comparison of optical absorption, spectroelectrochemical, and theoretical results confirms the assignment of the band and provides information on the degree of electron delocalization between the donor and the acceptor. Picosecond transient absorption measurements reveal that the back-electron transfer relaxation is critically dependent on the structural flexibility of the bridging bonds between the donor and the acceptor. In the case where the acceptor substituents are free to rotate about the bridging bonds between the boron and the cyclopentadienyl rings of the ferrocene, a significant portion of the excited state decays directly back to the ground state on a time scale of ∼18 ps, whereas in the case where an additional ansa-bridge that connects acceptor substituents enforces a more rigid conformation, the ground-state recovery proceeds only on a ∼800-ps time scale. This demonstrates the importance of conformational degrees of freedom for the internal conversion and back-electron transfer in these systems