Elsevier

Polymer

Volume 189, 17 February 2020, 122165
Polymer

Effects of hydrogen-bonding density on polyamide crystallization kinetics

https://doi.org/10.1016/j.polymer.2020.122165Get rights and content

Highlights

  • Crystallization kinetics of six polyamide samples were compared via Flash DSC measurement.

  • High and low hydrogen-bonding densities favor fast crystallization with different mechanisms.

  • Avrami indexes approve the cross-over between homogeneous and heterogeneous nucleation.

Abstract

We employed Flash DSC1 apparatus for fast-scan chip-calorimeter measurement in a broad temperature range to compare the isothermal crystallization rates among six polyamide/nylon (PA) samples, i.e. PA 46, PA 66, PA 610, PA 612, PA 1012 and PA 12. The double parabolic curves of crystallization half-time versus crystallization temperature revealed that PA 46 crystallizes relatively fast due to its high hydrogen-bonding density favoring thermodynamic driving forces for crystallization in the high temperature region, while PA 1012 and PA 12 crystallize relatively fast due to their low hydrogen-bonding densities favoring short-range diffusion for crystallization in the low temperature region. Furthermore, the cross-over temperatures between two parabolic curvatures of six samples follow their sequences in hydrogen-bonding density, and the Avrami indexes reveal a potential switching in the modes between heterogeneous and homogeneous crystal nucleation.

Introduction

Polyamide (PA), as its bulk materials commonly known as nylon, is a linear polymer holding repetitive structures that form Cdouble bondO⋅⋅⋅H–N hydrogen-bonding interactions between polymer chains [1]. The hydrogen-bonding offers enhanced mechanical properties, good chemical resistance and high thermal stability for nylon products in broad daily applications [2]. For nylon crystallization, hydrogen-bonding dominates the modes of chain packing in the crystalline phases [3], and hence the equilibrium melting points. In practice, the crystals are composed of hydrogen-bonding sheets with chain stems holding adjacent chain-folding, which are the fundamental for the beta-sheet as one of the most basic secondary structures in proteins [4]. There are two hydrogen-bonding schemes of stem packing in the crystalline sheets of nylons: terraced or staggered [[5], [6], [7]]. Brill transition can be observed below the melting point of nylon [8,9]. The harvest of crystallinity in nylon industrial processing is mainly dominated by crystallization kinetics. It is well-known that the hydrogen-bonding has been commonly existing in the melt, but not so much as in the crystalline state. Therefore, hydrogen-bonding densities of nylons as determined by the lengths of methylene sequences in diamine alkane and diacid alkane strongly influence the crystallization kinetics. However so far, our knowledge about this influence is still limited.

It is well-known that the overall crystallization kinetics in bulk polymers is dominated by primary crystal nucleation. According to the classical nucleation theory, the free energy barrier for crystal nucleation dominates the nucleation rate in the high temperature region, while the activation barrier for short-range diffusion dominates the nucleation rate in the low temperature region [10], as shown in Eqn. (1) for the nucleation rate I,I=I0exp(ΔEa/kTc)exp(ΔEc/kTc)where the activation barrier for diffusion ΔEa/kTc=A/(TcTV) with the Vogel temperature TV about 50 °C below the glass transition temperature Tg, and A is a diffusion constant; ΔEc=8πσ2σe[Tm0/ΔHc/(Tm0Tc)]2 is the free energy barrier for primary crystal nucleation supposed in a cylindrical bundle, k is the Boltzmann's constant, Tc is the crystallization temperature, σ and σe are separately the surface free energy densities on the lateral and folding-end surfaces of lamellar polymer crystals, Tm0 is the equilibrium melting point and ΔHc is the heat of fusion. Therefore, the curvature of crystallization rate (or crystallization half-time) versus crystallization temperature appears as parabolic, with a maximum (or minimum in case of half-time) locating near the middle between Tg and Tm0.

Recently, fast-scanning chip-calorimeter technique has significantly expanded our DSC measurement of polymer crystallization behaviors into the low temperature region [11,12]. The isothermal crystallization kinetics of many polymers in the broad temperature range have thus been characterized [13]. In particular, double parabolic curvature was observed [14], which has usually been assigned to heterogeneous nucleation and homogeneous nucleation dominating in the high and low temperature regions, respectively [15]. The crystallization kinetics of PA 6 was compared to that of Polyketone holding similar melting points and heats of fusion but without hydrogen-bonding [16]. The results revealed that the hydrogen-bonding in the melt of PA 6 brings a low mobility to suppress crystallization in the low temperature region, while the hydrogen-bonding in the crystal sheets of PA 6 leads to a low surface free energy to accelerate crystallization in the high temperature region [16]. In this study, we focus our attention on the hydrogen-bonding density and compare the isothermal crystallization kinetics of PA 46, PA 66, PA 610, PA 612, PA 1012 and PA 12 via Flash DSC measurement. The chemical structures of six aliphatic nylon samples with a series of hydrogen-bonding densities are demonstrated in Fig. 1. Our results reveal some important effects of hydrogen-bonding density on crystallization kinetics among six nylon samples.

Section snippets

Samples and instruments

PA 46 granules used in this work were kindly supplied by DSM Company. The viscosity was 100 Pa s under the temperature 300 °C and the shear rate 100 s−1. The sample was dried in a vacuum oven at 80 °C for 12 h before use. The equilibrium melting point of PA 46 was 307.1 °C, obtained with conventional DSC measurement [17].

PA 66 (molecular weight 262.35 kg mol−1, density 1.14 g mL−1 at 25 °C) and PA 12 (density 1.01 g mL−1 at 25 °C) were obtained from SIGMA-ALDRICH. The equilibrium melting points

Erasing thermal history

In order to avoid any memory effect of crystallization in this study, it is necessary for us to erase completely the thermal history of the nascent samples at the very beginning. On the one hand, the sample should be heated to a high enough temperature for staying long enough time to erase its thermal history. On the other hand, the temperature should be lower than the decomposition temperature to make sure the sample is intact. We used PA 610 as an example for demonstration. Firstly, PA 610

Conclusion

We employed Flash DSC measurement to characterize the isothermal crystallization kinetics of PA 46, PA 66, PA 610, PA 612, PA 1012 and PA 12 in a broad temperature range. The double parabolic curves of crystallization half-time versus crystallization temperature reveal that PA 46 crystallizes fast because of its high hydrogen-bonding density favored by strong thermodynamic driving force for crystallization in the high temperature region, while PA 1012 and PA 12 crystallize also fast because of

Declaration of competing interest

There is no declare of conflict interests!

Acknowledgements

We thanks Shandong Guangyin New Materials Company offering PA 610, PA 612 and PA 1012, and DSM China Campus offering PA 46 for this study. The financial support of National Natural Science Foundation of China (Grant Nos. 21474050 and 21973042), Key Lab of National Defense Science and Technology of China (Grant No. 61426030107), Program for Changjiang Scholars and Innovative Research Teams (IRT1252) and the CAS Interdisciplinary Innovation Team are appreciated.

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