Chain-Folded Lamellar Crystals of Aliphatic Polyamides. Comparisons between Nylons 4 4, 6 4, 8 4, 10 4, and 12 4

AMB2018-04: Benchmark Physical Property Measurements for Powder Bed Fusion Additive Manufacturing of Polyamide 12

Laser sintering (LS) of polyamide 12 (PA12) is increasingly being adopted for industrial production of end-use parts, yet the complexity of this process coupled with the lack of organized, rigorous, publicly available process-structure-physical property datasets exposes manufacturers and customers to risks of unacceptably poor part quality and high costs. Although an extensive scientific literature has been developed to address some of these concerns, results are distributed among numerous reports based on different machines, materials, process parameters, and users. In this study, a single commercially important LS PA12 feedstock has been processed along four build dimensions of a modern production LS machine, characterized by a wide range of physical techniques, and compared to the same material formed by conventional melt processing. Results are discussed in the context of the literature, offering novel insights including distributions of particle size and shape, localization of semicrystalline phase changes due to LS processing, effect of chemical aging on melt viscosity, porosity orientation relative to LS build axes, and microstructural effects on tensile properties and failure mechanisms. The resulting datasets will be made publicly available to modelers and practitioners for the purpose of improving certifiability and repeatability of end-use parts manufactured by LS.

Biocatalysis for biobased chemicals

The design and development of greener processes that are safe and friendly is an irreversible trend that is driven by sustainable and economic issues. The use of Biocatalysis as part of a manufacturing process fits well in this trend as enzymes are themselves biodegradable, require mild conditions to work and are highly specific and well suited to carry out complex reactions in a simple way. The growth of computational capabilities in the last decades has allowed Biocatalysis to develop sophisticated tools to understand better enzymatic phenomena and to have the power to control not only process conditions but also the enzyme's own nature. Nowadays, Biocatalysis is behind some important products in the pharmaceutical, cosmetic, food and bulk chemicals industry. In this review we want to present some of the most representative examples of industrial chemicals produced in vitro through enzymatic catalysis.

Erasing conformational limitations in N,N'-1,4-butanediyl-bis(6-hydroxy-hexanamide) crystallization from the superheated state of water

Aliphatic polyamides consist of regularly distributed amide moieties located in an aliphatic chain, off which the segment length can be varied. The crystallization and hence the eventual performance of the material can be tailored by changing the aliphatic lengths, and thus the hydrogen bonding density and the directional chemical positioning of the amide motifs. In this paper, N,N'-1,4-butanediyl-bis(6-hydroxy-hexanamide) crystallized either from the melt or from the superheated state of water is investigated. A comparison with N,N'-1,2-ethanediyl-1,2-bis(6-hydroxy-hexanamide) reveals the role of the hydrogen bonding density on the accommodation of water molecules in amide based crystals grown from the superheated state of water. However, wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR), and solid state 1H and 13C NMR spectroscopy reveal hydrogen bonding between the amide planes, while aliphatic polyamides and N,N'-1,2-diethyl-bis(6-hydroxy-hexanamide) feature hydrogen bonds that reside within the amide plane only. As a consequence, the role of N,N'-1,4-butanediyl-bis(6-hydroxy-hexanamide) crystals as a model system for polyamide 4Y polymers is questionable. However, the thermodynamic and structural behavior as function of temperature is determined by a balance between thermally introduced gauche conformers and hydrogen bonding efficiencies. These crystals enable a thorough investigation in the effect of superheated water on the crystallization of these uniquely hydrogen bonded molecules. Crystallization from the superheated state of water results in denser molecular packing and enhanced hydrogen bonding efficiencies. The induced spatial confinement hinders molecular motion upon heating, and thermodynamically more stable crystals are observed. Although the amide-hydroxyl hydrogen bonded crystals do not favor the accommodation of physically bound water molecules in the lattice, saturation of the amide motifs during crystallization erases conformational restrictions of the planar amide moieties that facilitates maximum hydrogen bonding efficiencies in the eventual lattice.

Structures of Monodisperse Nylon 6 Oligoamides. Onset of Chain-folding

Crystals were grown of the 5-amide, 9-amide, and 17-amide nylon 6 monodisperse oligoamides and investigated using electron microscopy (real space and diffraction) and X-ray diffraction. Analyses of the data allowed us to determine the crystalline structures and relate them to the morphology. Under our crystallization conditions, the 5-amide chains are unfolded and crystallize in the usual nylon 6 alpha-structure, i.e., an apolar arrangement of chains, directed parallel to the layer normal, within hydrogen-bonded sheets which stack via van der Waals interactions. In chain-folded nylon polymers such as nylon 6, the length of the straight stems is approximately 5 to 7 nm; this is equivalent to about six to eight amide units. Therefore it is not too surprising that the 5-amide molecule, of length 4.6 nm, remains unfolded. If the 9-amide molecules (length 8 nm) fold, we would expect them to create hairpin-like structures with four, or close to four, amide units in adjacent straight stems. Our results show that, depending on the crystallization conditions, both unfolded and once-folded conformations can occur for this 9-amide oligomer. In the unfolded conformation, the straight-stem chains crystallize in the nylon 6 gamma-phase structure as opposed to the usual alpha-phase structure. The once-folded structure for the 9-amide chain represents the onset of folding in these nylon 6 monodisperse oligoamides and the lamellar stacking periodicity (LSP) is 4.60 nm. This once-folded conformation is similar to the usual nylon 6 alpha-phase structure. The 17-amide chains crystallize in a twice-folded conformation, also with the nylon 6 alpha-phase structure, reinforcing the notion that once the chains are long enough to start folding, the pattern of behavior approaches that of the nylon 6 polymer. In this case, the LSP value is 5.36 nm.