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Thermochimica Acta 655 (2017) 313–318 Thermochimica Acta 655 (2017) 313–318 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Short communication The effect of supercooling of the melt on the semicrystalline morphology of PA 66 MARK ⁎ Anne M. Gohna,b, Alicyn M. Rhoadesa, , Nichole Wonderlingb, Tim Tigheb, René Androschc a School of Engineering, Penn State Behrend, 4701 College Drive, Erie, PA 16563, United States b Materials Characterization Lab, Penn State, N-050 Millennium Science Complex, Pollock Road, University Park, PA 16802, United States c Interdisciplinary Center for Transfer-oriented Research, Martin Luther University Halle-Wittenberg, 06009 Halle/Saale, Germany ARTICLE INFO ABSTRACT Keywords: Fast scanning chip calorimetry (FSC) was used to prepare isothermally crystallized samples of polyamide 66 (PA Polyamide 66 66) in a wide range of temperatures between 70 and 230 °C for subsequent analysis of the semicrystalline Crystallization morphology by X-ray diffraction, polarized-light optical microscopy and atomic force microscopy. At high Crystal nucleation crystallization temperatures stable triclinic α-crystals of lamellar shape, organized within a spherulitic super- Crystal morphology structure are formed. At low crystallization temperatures, in contrast, the less stable pseudohexagonal γ-crystal Crystal polymorphism structure/mesophase develops. The mesophase of PA 66, which forms at temperatures close to the glass tran- Fast scanning chip calorimetry sition, is of grainy, non-lamellar habit, and not organized within spherulites. The formation of such qualitatively different semicrystalline morphologies of PA 66 is suggested to be caused by different densities of crystal nu- cleation, supported by observation of a bimodal temperature-dependence of the crystallization rate. The ex- perimental findings reported in this work are important to allow tailoring of the microstructure of PA 66 by variation of the conditions of processing as well as contribute to the ongoing research about crystal nucleation in polymers. 1. Introduction rather high temperature, the molecules arrange in a triclinic unit cell. The methylene units adopt a planar zigzag conformation and the hy- Polyamide 66 (PA 66) is an important engineering thermoplastic drogen bonds, which connect the amide groups between neighbored material used in a variety of markets ranging from automotive, sporting chain segments, show a sheet-like planar arrangement [12]. On heating, goods, aerospace, and fiber industry [1–3]. It is an attractive polymer the α-form transforms at the Brill transition temperature into its high- for many applications because it is a tough, strong, durable material temperature α’-modification with a pseudohexagonal unit cell [19,20]. with excellent chemical and temperature resistance, and good wear The transition is reversible, that is, the α’-phase converts on cooling properties [4]. As in case of all polymeric materials, many of these into the α-phase, and cannot be observed at room temperature. In the properties depend on the semicrystalline structure [5–8], and therefore pseudohexagonal γ-mesophase, a non-planar arrangement of hydrogen much effort has been undertaken exploring the relation between the bonds is suggested [13]. It develops upon quenching of the melt [13], conditions of solidification, the resulting microstructure and the prop- and is only metastable at the temperature of formation and lower erties of this material [9–11]. Focus of the present study is the eva- temperatures, as on heating the γ-mesophase converts irreversibly into luation of microstructure/semicrystalline morphology of PA 66 forming α’/α-crystals [21]. at different condition of solidification the supercooled melt, with em- The transition of the melt into α/α‘-crystals on slow cooling/high phasis placed on the analysis of the crystal polymorphism, the forma- temperature is connected with the formation of lamellae and spher- tion of a higher-order superstructure, and the crystal morphology. ulites [22–25]. Details of the morphology and superstructure of the γ- PA 66 forms multiple/different crystal polymorphs whose devel- mesophase of PA 66, and their exact conditions of formation, however, opment is dependent on the conditions of crystallization [12–18]. With are completely unknown to date. In case of PA 6 and PA 11, which regard to crystallization of the quiescent melt, there has been reported exhibit a similar crystal/mesophase polymorphism controlled by the formation of either α-crystals or γ-mesophase. In the more stable α- cooling rate/supercooling as PA 66 [26–30], the mesophase is of non- crystals, which develop on slow cooling of the quiescent melt or at lamellar habit and not spatially organized in a higher-order ⁎ Corresponding author. E-mail address: [email protected] (A.M. Rhoades). http://dx.doi.org/10.1016/j.tca.2017.07.012 Received 18 May 2017; Received in revised form 10 July 2017; Accepted 18 July 2017 Available online 22 July 2017 0040-6031/ © 2017 Elsevier B.V. All rights reserved. A.M. Gohn et al. Thermochimica Acta 655 (2017) 313–318 superstructure [31–33]. In the present study, we attempt proving that sample. When using the FSC for thermal preconditioning of samples, in PA 66 shows a similar dependence of the semicrystalline morphology case of XRD and POM analyses, all measurements were performed on a on the condition of melt-crystallization as observed before for PA 6 and single sample. In other words, the sample attached to the sensor was PA 11. It is, however, worthwhile noting that analysis of the crystal- removed from the FSC after a specific thermal treatment, analyzed re- lization behavior at high supercooling of the melt, at temperatures close garding its structure, and then re-inserted into the FSC to impose the to the glass transition, is complicated since crystallization often begins next thermal history. Only in case of the AFM analyses, sensors needed before the system reaches the target analysis temperature. Outpacing to be destroyed for optimum sample preparation. these kinetics requires application of high cooling rates, much higher than in conventional differential scanning calorimetry (DSC), which 2.2.2. X-ray diffraction (XRD) assures absence of crystallization at low supercooling of the melt. With XRD measurements of the preconditioned FSC samples were per- the use of fast scanning chip calorimetry (FSC) it is possible to transfer formed using a Rigaku DMAX-Rapid II diffractometer equipped with a the equilibrium melt to any temperature, allowing analysis of the iso- Cu X-ray tube and a graphite monochromator. Analyses were done in thermal crystallization kinetics starting with a supercooled melt free of transmission mode, and the scattered X-rays were recorded using a crystals [34,35]. In addition, samples subjected to well-defined crys- curved image plate coated with photo-stimulable phosphor (BaF tallization-histories in an FSC can then be used for structural analyses (Br,I):Eu2+)The beam diameter and exposure time were 300 μm and including atomic force microscopy (AFM) [31], polarized-light optical 600 s, respectively. The 2D XRD patterns were azimuthally integrated microscopy (POM) [36], or X-ray diffraction (XRD) [37,38]. In the to yield the intensity distribution as a function of the scattering angle present study, that approach is applied for PA 66 in order to provide for 2θ, followed by subtraction of an XRD curve obtained on an empty FSC this important polymer structure information after solidification of the chip. supercooled melt at temperatures close to the glass transition tem- perature. It was discovered that two maxima in crystallization rate were 2.2.3. Polarized-light optical microscopy (POM) observed around 160 and 115 °C. The high temperature crystallization POM images of FSC samples subjected to a specific crystallization was found to produce the stable triclinic α-crystal, whereas the low history were obtained using a Leica DMRX microscope operated in re- temperature crystallization produced the pseudohexagonal γ-meso- flection mode and with the sample placed between crossed polarizers. phase. Around 110–120 °C there was a transition from the spherulitic Images were captured using a Motic 2300 CCD camera and imaging structure to non-spherulitic structure at low and high supercooling, software. respectively. These information can then be used to intentionally pro- duce in an efficient and predictable way different semicrystalline 2.2.4. Atomic force microscopy (AFM) morphologies, by modification of processing routes, with the final aim AFM measurements were performed using a Bruker Dimension Icon. of tailoring properties. As prominent examples for demonstration of Samples were conditioned isothermally on the FSC chip before rapid structure-property relations, it was shown early for the specific case of cooling to room temperature. The sensor/membrane was then fractured PA 66, that mechanical properties are largely controlled by the size of away from the ceramic frame in order to transfer the sample to a spherulites [39] or, investigated on PA 6, that Young’s modulus and the stainless steel specimen disc using double-sided adhesive tape. hardness of the α-crystals are about twice than that of the γ-mesophase PeakForce tapping imaging mode was used with a ScanAsyst Air Probe [40]. (0.4 N/m spring constant). Image analysis was done using Nanoscope Analysis software. 2. Experimental 3. Results and discussion 2.1. Material Fig. 1 shows FSC crystallization curves, heat-flow rate as a function The material used in this study was Zytel 101 from DuPont (USA). It of time, recorded at temperatures between 220 °C (front curve) and fi is an un-lubricated and unmodi ed general-purpose molding-viscosity 75 °C (back curve), after the sample was cooled from 300 °C at a rate of PA 66 grade [41], selected in this study to ensure that no additives are 2000 K/s to the crystallization temperature. Inspection of the peak-time ff present which would a ect the crystallization process. The number- of crystallization reveals, as expected, a bimodal temperature-depen- – average molar mass of this polymer is around 17 18 kDa [42].
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