Thermally Induced Structural Transitions of Nylon 4 9 as a New Example of Even–Odd Polyamides

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.

Crystalline Structures and Structural Transitions of Copolyamides Derived from 1,4-Diaminobutane and Different Ratios of Glutaric and Azelaic Acids

“…Copolymers were able to crystallize from dilute glycerin solutions giving rise to lamellar crystals even for a high comonomer content (e.g., nylon 4,5+9_50). Crystallization of nylon 4,5+9_100 has been previously studied [19], with a morphology dependence with crystallization temperature being described. Figure 6a shows specifically the formation at a relatively high crystallization temperature (i.e., 130 °C) of lenticular basal crystals that could attain 2 μm in its longer dimension.…”

“…The electron diffraction pattern of this homopolymer was in agreement with a rectangular unit cell with a and b dimensions of 0.430 nm and 0.860 nm, respectively. The main relevant features of the diffraction patterns were the clear 2mm symmetry, which was postulated for the model with two hydrogen bonding directions [15,16,17,18,19], the presence of four and two prominent reflections at 0.385 and 0.430 nm, respectively, and a pattern resolution up to 0.160 nm (i.e., the 150 reflection could still be distinguished). The orientation of the diffraction pattern with respect to the crystal morphology indicates that the b * reciprocal axis was aligned with the long crystal dimension and that the lateral faces of rhombic crystals corresponded to 110 planes.…”

“…Structural transitions of nylon 4,5+9_100 have been previously studied by means of synchrotron radiation [19], being a complex behavior involving three crystalline phases determined. In brief, the sample recovered from synthesis and also that crystallized from dilute solutions correspond to the two dimensional hydrogen-bonded structure named form I and characterized by two strong reflections around 0.428–0.433 nm and 0.376–0.385 nm.…”

“…Rotation of amide groups in two opposite directions from the above indicated that the sheet gave rise to another structure characterized by two H-bonding directions (Figure 1). This has been postulated for different odd-even [7,8,9,10,11,12] and even-odd nylons [13,14,15,16,17,18,19], being their diffractions patterns highly similar to those described for even-even nylons and clearly different to those found for the γ-form. Pseudohexagonal arrangements with a random deviation of amide groups from the indicated plane constituted by methylene carbons were also postulated for the structure attained when nylons were heated at temperatures close to their melting point.…”

Change of thermophysical properties of polyamides under the influence of polyvinylpyrrolidone

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