Key Insights into the Differences between Bimodal Crystallization Kinetics of Polyamide 66 and Polyamide 6

Abstract

Polyamide 66 (PA66) and polyamide 6 (PA6) share many comparable properties due to their similar chemical structures. However, their crystallization kinetics and morphological differences are not as well understood as other properties. This work establishes the crystallization kinetics and morphology of additive-free PA66 and PA6 at high undercooling conditions using a modified fast scanning calorimetry technique. Two polyamides show similar kinetics profile and morphology, but the transitions associated with polymorphs occur at different temperatures. Regarding kinetics, PA66 always crystallizes faster than PA6 regardless of the polymorphs formed, supported by the temperature-dependent Avrami kinetics coefficients k. Both PA66 and PA6 show a bimodal kinetics profile with a local crystallization rate minimum at 135 and 110 °C, respectively. Apart from the crystallization rate, a sudden broadening of the exothermic crystallization peak is found near the rate minimum. The broadening is described by a drastic change of the Avrami index n from 3 to 2. The morphology at the micro- and nanoscales of polyamides was followed by a polarized optical microscope (POM) and atomic force microscopy (AFM). The POM reveals that both polyamides turn translucent from transparent near the rate minimum. The temperature-dependent AFM micrographs show multistep transitions from amorphous-like morphology, cauliflower-like crystal, crystal aggregates, and lamellar structure after T c changes from near T g to above the kinetics break temperature. Although two polyamides have similar molecular weight and the same content of amide groups, the morphological transition in PA66 is found to always be 20 °C higher than in PA6, suggesting a difference in their thermodynamic drive to nucleate. The conclusions drawn from the Avrami analysis in the final part of this study provide a universal explanation of the drastic peak broadening observed in many previously studied thermoplastics.

Temperature-Dependent Triple Crystal Polymorphism of Poly(hexamethylene terephthalate): Long-Range Ordered Mesophase Formation and Crystal Structure–Property Relationships

“…The similar bimodal dependence of t 0.5 on T c has been reported in other T c -dependent polymorphic polymers such as iPP, PA11, PET, and PA6. 28,41,42,44,53 Two mechanisms have been mainly proposed to explain the bimodal dependence of the crystallization rate on T c : (i) the change of the nucleation mechanism from heterogeneous to homogeneous nucleation with the decrease in T c 41,43,54 and (ii) the temperature-dependent formation of crystal polymorphs with different growth rates. 44,53 We consider that both of these scenarios can interpret the bimodal dependence of crystallization rate on T c in PHT.…”

Effect of addition of fillers (MoS2 and PFA) on the tribological and mechanical properties of biobased semi‐aromatic polyamide (PA10T)

“…Furthermore, SEM images of the worn surfaces of samples, after being subjected to a wear test, demonstrated a significant enhancement in the wear resistance of PA10T‐FM composites due to the incorporation of fillers (MoS 2 and PFA) into PA10T. DSC analysis indicated that the melting points of PA10T‐FM composites ranged between 275.4°C and 278.0°C, implying that the addition of fillers helped to appropriately elevate the melting temperature of PA10T 50 . XRD results showed the presence of diffraction peaks around 2θ = 21° in PA10T‐FM composites containing MoS 2 and PFA, suggesting that the crystal structure of PA10T remained unaffected by the addition of fillers.…”

“…DSC analysis indicated that the melting points of PA10T-FM composites ranged between 275.4 C and 278.0 C, implying that the addition of fillers helped to appropriately elevate the melting temperature of PA10T. 50 XRD results showed the presence of diffraction peaks around 2θ = 21 in PA10T-FM composites containing MoS 2 and PFA, suggesting that the crystal structure of PA10T remained unaffected by the addition of fillers. Overall, the TGA and DTG results demonstrated the excellent thermal stability of PA10T-FM composites, indicating the potential of MoS 2 and PFA fillers for developing high-temperature-resistant polyamides.…”

Kinetics Comparison of Solid-State Polycondensation and Melt Postpolycondensation for Polyamide 6 with Controllable Extractive Content

“…Polyamide 6 stands out in the engineering world for its remarkable mechanical and thermal stability, alongside impressive optical and dimensional properties. − It is widely used in electronic and electrical, automotive, construction, optics, pharmaceutical, and packaging industries. − The predominant industrial method for synthesizing polyamide 6 involves the hydrolysis of caprolactam (CPL). This reversible reaction requires rigorous control to achieve high yield and superior product quality. − Affected by the reaction balance, polyamide 6 melt, formed through ring-opening hydrolysis, contains roughly 8–10% extractable small molecules (75% caprolactam monomer and 25% other oligomers). − The caprolactam monomer’s volatility is a known issue, often causing bubble formation during fiber spinning, leading to breakage .…”