The effects of annealing (solid and melt) on the time evolution of the polymorphic structure of polyamide 6

Abstract

To understand whether and how the thermal history, especially the melting annealing, affects the polymorphism and thermal property of polyamide 6 (PA6), the temperature-modulated differential scanning calorimetry technology was used to investigate the effects of thermal histories, including annealing temperature, annealing time, and cooling rate, on the polymorphic behavior and thermal property of PA6. It was found that longer annealing time and faster cooling rate favored the formation of a crystal when PA6 samples were annealed in the solid-state at 175 C. As the annealing temperature was elevated to 195 C, faster cooling rate also favored the formation of a crystal, whereas longer annealing time was more favorable for the formation of c-form crystals. When PA6 samples were annealed in the melt-state (245 C), however, although the a crystal was dominating crystalline phase, the formation of c crystal was greatly enhanced with increasing annealing time and cooling rate. Moreover, a small endothermic peak was observed in the low-temperature region in PA6 samples annealed at 175 C and 195 C, which might be related to the melting of microcrystals formed in the amorphous regions during annealing.

Rheological, crystal structure, barrier, and mechanical properties of PA6 and MXD6 nanocomposite films

“…These additional peaks can be attributed to the formation of small crystals during annealing . They were also related by some authors to an enthalpy relaxation process of the interphase between the crystalline and amorphous phases, which can be induced by isothermal crystallization or annealing process. A remarkable feature in Fig.…”

Preparation of Polyesteramides in a Reactive Extrusion Process

“…In the PCLA homopolymer, the polymorphic behavior is reported to depend strongly on the thermal history. When annealed in the melt and subsequently crystallized non‐isothermally, a transition from the thermodynamically stable α‐phase to the kinetically favored γ‐phase is reported when the cooling rate increases . Another factor favoring the formation of the γ‐phase is found to be the presence of interfaces being capable of forming hydrogen bonds with the amide groups of the PCLA segments, as could be the case in the presence of PCLO polymer chains.…”

Morphology and local chain structure of polyamide 6 modified in the solid state with a semi-aromatic nylon salt

“…These studies showed the presence of three components: two crystalline phases (␣-and ␥-forms) and an amorphous fraction, where the 13 C chemical shifts of both carbons adjacent to the amide group (C1 and C5, see inset of Table 2) are particularly sensitive to the crystal form of PA6; i.e., in the ␣-form, C1 and C5 are shifted ∼3 ppm downfield from their 13 C chemical shift values in the ␥-form [28]. These differences in 13 C chemical shift between the ␣-and ␥-forms further show that the PA6 chains predominantly exist in the trans configuration (fully extended) in the ␣-form, whereas they are twisted and bended with more gauche-like character in the ␥-form.…”

“…There are also other crystal forms of PA6 reported in literature, which are mainly variations of these two crystal forms, however, most of them are not stable [4,[6][7][8][9]. The molecular packing of PA6 is highly dependent on processing conditions, thermal treatment, applied stress and presence of moisture and additives [5,[10][11][12][13]. It is well known that the ␣-form can be transformed into the ␥-form by treatment with an aqueous iodine/potassium iodide solution [14,15].…”