The Isothermal and Nonisothermal Crystallization Kinetics and Morphology of Solvent-Precipitated Nylon 66

Abstract

Solvent-precipitated nylon 66 (SP PA66) is a key material used to fabricate microfiltration membranes. The crystallization kinetics and behavior of SP PA66 were investigated through differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). The Avrami equation was used to describe the isothermal crystallization of SP PA66. Nonisothermal crystallization behaviors were analyzed using Avrami equations modified by Jeziorny, Ozawa and Mo. The Avrami analysis demonstrated that the k values of SP PA66 were higher than those of neat PA66. The was between 2 and 3 indicating the presence of two- and three-dimensional mode with thermal nucleation. With an increasing cooling rate, the Jeziorny crystallization rate constant increased for SP PA66; however, the Ozawa model was not satisfactory for all SP PA66 samples. The Mo method suggested that SP PA66 had a faster crystallization rate than neat PA66 during the nonisothermal crystallization process. The solvents dissolved nylon 66, rearranged it and formed a regular hydrogen-bonded region. These regions served as nucleation sites and increased the crystallization rate constant in the subsequent melting process. The crystal morphology of the SP PA66 under the POM investigation exhibited Maltese cross spherulites. The sizes of the spherulites of SP PA66 were significantly smaller than those of neat PA66. Wide-angle XRD revealed that SP PA66 had the same crystal structure and a higher crystal perfection than neat PA66.

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“…The heat generation or crystallization enthalpy was measured for the isothermal temperature designated in the range of 40–100°C. The isothermal crystallization rate and the half crystallization time were estimated by fitting Equations (1) and (2) [ 34 ] to the DSC data. where represents the relative crystallinity, k is the crystallization rate, and t is the time. n is the Avrami index, and is the half crystallization time.…”

“…For isothermal measurement, the temperature was first heated to 230 C, held for 1 s, reduced at a cooling rate of 4000 C/s, and held at a designated isothermal crystallization temperature. The heat generation or crystallization enthalpy was measured for the isothermal temperature designated in the range of 40-100 C. The isothermal crystallization rate and the half crystallization time were estimated by fitting Equations ( 1) and ( 2) [34] to the DSC data.…”

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“…Avrami parameters of n ¼ 1, 2, and 3 indicate crystal formation in one, two, and three dimensions, respectively. 47 However, the Avrami exponent n is not a simple integer when additional complicated elements, such as the uneven boundaries of the spherulites and the competition of diffusion-controlled development, are potentially involved.…”

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“…It is calculated as follows: The initial stage at each crystallization temperature is linear, and the Avrami equation is applicable for this part of the graph. At longer crystallization times, the curves deviate from the linearity due to sphaerocrystal collision, i.e., the collision of initially formed spherulites in the secondary stages of crystal growth [23,24]. The n and Z t values calculated from the linear part of the Avrami plots are listed in Table 1.…”