Effect of contaminants and processing regime on the mechanical properties and moldability of postconsumer polyethylene terephthalate bottles.

In this research, post-consumer polyethylene terephthalate (PC-PET) bottles were processed and characterized. The aim was to investigate the influence of number and type of melt processing steps and the presence of contaminants on the mechanical properties and moldability of PC-PET. Results and observations of current research showed that direct processing of PC-PET by injection molding (i.e. skipping melt extrusion step) is preferable in order not to deteriorate mechanical properties especially the toughness of PC-PET. Contaminants found in PC-PET waste stream include labels and bottle cap rings made of polyethylene (PE) and paper labels. Unlike PE contaminants, paper contaminants can drastically reduce toughness of PC-PET and severely affect its moldability. Molded samples of PC-PET containing traces of papers appeared incoherent and fragmented pieces due to poor adhesion between PET and papers. The recommendation given by current research is to remove paper contaminants from PC-PET waste stream before melt processing and melt processing steps should be limited.

Recyclable, strong thermosets and organogels via paraformaldehyde condensation with diamines.

Nitrogen-based thermoset polymers have many industrial applications (for example, in composites), but are difficult to recycle or rework. We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4'-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young's moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.

Computational investigations on base-catalyzed diaryl ether formation.

We report investigations with the dispersion-corrected B3LYP density functional method on mechanisms and energetics for reactions of group I metal phenoxides with halobenzenes as models for polyether formation. Calculated barriers for ether formation from para-substituted fluorobenzenes are well correlated with the electron-donating or -withdrawing properties of the substituent at the para position. These trends have also been explained with the distortion/interaction energy theory model which show that the major component of the activation energy is the energy required to distort the arylfluoride reactant into the geometry that it adopts at the transition state. Resonance-stabilized aryl anion intermediates (Meisenheimer complexes) are predicted to be energetically disfavored in reactions involving fluorobenzenes with a single electron-withdrawing group at the para position of the arene, but are formed when the fluorobenzenes are very electron-deficient, or when chelating substituents at the ortho position of the aryl ring are capable of binding with the metal cation, or both. Our results suggest that the presence of the metal cation does not increase the rate of reaction, but plays an important role in these reactions by binding the fluoride or nitrite leaving group and facilitating displacement. We have found that the barrier to reaction decreases as the size of the metal cation increases among a series of group I metal phenoxides due to the fact that the phenoxide becomes less distorted in the transition state as the size of the metal increases.