Abstract
Crystalline morphology and structure of nylon 4 9 have been studied by means of optical and transmission electron microscopies, and X-ray diffraction. Rhombic crystals were characteristic of crystallization from glycerin dilute solutions, although the final morphology was dependent on the crystallization temperature. In any case, a single electron diffraction pattern was always obtained, being characteristic a 2 mm symmetry and reflections at spacings that were indicative of a projected rectangular unit cell with hydrogen bonds established along two planar directions (i.e., the diagonals of the unit cell), as it was determined from related polyamides. Crystallization from the melt gave rise to negative birefringent spherulites with a morphology (axialitic, speckled or ringed) that was dependent on the crystallization temperature. Kinetic analysis indicated that melt crystallization took place according to two growth mechanisms (Regimes II and III), which reflect distinct secondary nucleation rates. A complex polymorphic behavior on heating and cooling processes was evidenced by real time synchrotron experiments, being determined an intermediate crystalline structure as well as the typical pseudohexagonal arrangement associated to the Brill transition. Polymorphic transitions were highly dependent on the initial crystalline structure, being enhanced the structural transition from the low temperature structure to the intermediate one when traces of the latter were initially present. Calorimetric and infrared studies supported also the detected thermal transitions of nylon 4 9.
Highlights
Aliphatic polyamides are the first synthetic and semicrystalline polymers that displayed outstanding mechanical and thermal properties to be used as fibers and engineering thermoplastics [1,2].Nylons are nowadays employed in a wide range of applications that mainly cover the textile, and automotive industries
FTIR and 1 H nuclear magnetic resonance (NMR) spectra of the synthesized nylon are given in Figure 1b,c, respectively
Nylon449 9crystallized crystallized from diluted glycerin solutions giving to lamellar
Summary
Aliphatic polyamides (nylons) are the first synthetic and semicrystalline polymers that displayed outstanding mechanical and thermal properties to be used as fibers and engineering thermoplastics [1,2]. Even nylons with many carbon atoms (e.g., nylon 12) crystallize according to a pseudohexagonal packing (defined by one or two close equatorial reflections in the 0.420–0.400 nm range), a deviation towards skew conformations for torsional angles of methylene-amide bonds, and a tilting of amide groups from the plane defined by the methylene carbon atoms [8] In this way, hydrogen bonds become established between parallel chains and along a single direction that becomes outside the sheet plane defined by the methylene groups. Thermal induced structural transitions are a common characteristic observed for conventional nylons displaying the indicated α/β forms In this way, equatorial reflections gradually merge into a single peak indicative of a pseudohexagonal arrangement at the named Brill temperature. (a) characterization of the low temperature predominant form from the study of structure and morphology of single crystals; (b) study of thermally induced phase transformations on heating and cooling processes by means of real time synchrotron experiments and having special consideration on the occurrence of intermediate crystalline structures; (c) evaluation of the influence of the sample preparation process on the crystalline structure and the derived thermal induced structural transitions; and (d) study of spherulites developed at different crystallization temperatures, considering morphology, properties and crystallization kinetics
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