New insights into crystalline transition in nylon 46

Jia-Ru Xu, Shuang Yang, and Er-Qiang Chen

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry,
Peking University, Beijing 100871, China

Nylon 46 (PA46) is an important engineering plastic. Two crystal structures, monoclinic and triclinic, have been proposed for the α form of PA46. To distinguish which one is more suitable for the melt crystallized PA46, we prepared double oriented samples of PA46 by mechanical rolling at 180 °C followed by annealing at 260 °C. The samples showed the typical α phase diffractions. Combining the two-dimensional (2D) wide-angle X-ray diffraction (WAXD) patterns obtained with the X-ray beam perpendicular and parallel to the chain axis, we concluded that PA46 adopted the monoclinic structure that was determined by Atkins based on the electron diffraction of single crystals grown from solution [1]. On heating, the room temperature α phase firstly transformed into high temperature α phase, and then changed into γ phase which is pseudo-hexagonal [2]. We observed that directly cooling down the sample after high temperature rolling could result in the γ phase retained at room temperature, which was evidenced by 2D WAXD. This room temperature γ phase was a new observation. It was relatively stable and would transform to high temperature γ upon heating. For comparison, we also studied the thermal transition of the melt quenched samples. The obtained amorphous PA46 crystallized in γ phase during heating. The non-oriented γ phase would change into α phase during cooling, showing the typical Brill transition.

References
[1] Atkins, E.D.T., Hill, M., Hong, S.K., Keller, A. Organ, S. Macromolecules 25, 917 (1992). (link)
[2] Ramesh, C. Macromolecules 32, 3721 (1999). (link)

Crystallization of Isotactic Poly(methacrylic acid) at the Air-Water Interface and in Thin Films

N. Hasan, T. M. H. Nguyen, A.-K. Flieger and J. Kressler

Faculty of Natural Sciences II, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 4, D-06120 Halle (Saale), Germany.

Perfect isotactic poly(methacrylic acid) (i-PMAA) was synthesized and characterized by NMR spectroscopy. Langmuir isotherms of i-PMAA were measured and compared with atactic PMAA. The crystallization of i-PMAA on the water surface strongly depends on the pH-value of the subphase. We found that i-PMAA solutions can immediately form crystalline nanoparticles upon spreading at the air-water interface at neutral pH value. They have diameters of approximately 14 nm to 40 nm and their size can be increased 60 nm to 100 nm without aggregation upon compression on the Langmuir trough. Helical structures are observed when i-PMAA is spread on water having pH value of 10. The morphology of i-PMAA was studied after transfer in LB films by AFM. i-PMAA can also be crystallized in thin films after treatment for several weeks with water. Wide-angle X-ray diffraction (WAXD) studies of these films show a new Bragg reflection at 2θ = 7° (d = 1.20 nm) indicating the crystallization of i-PMAA which might correspond to the helical pitch distance as known for isotactic poly(methyl methacrylate) (i-PMMA) [1-3]. The crystallization is also observed by polarized optical microscopy and AFM.

References
[1] E. van den Bosch, Q. Keil, G. Filipcsei, H. Berghmans and H. Reynaers, Macromolecules 37, 9673 (2004). (link)
[2] H. Ajiro, M. Akashi, Macromol. Rapid Commun 31, 714 (2010). (link)
[3] D. Kamei, H. Ajiro, C. Hongo and M. Akashi, Langmuir 25, 280 (2009) (link)

The Crystallization Transition and Microstructure Evolution of Long Chain Aliphatic Polyamide

Xia Dong*, Lili Wang, Ping Zhu, Yunyun Gao, and Dujin Wang

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry Chinese Academy of Sciences, Beijing, 100190, China

Deformation-induced microstructure evolution of a long chain aliphatic polyamide, with an emphasis on lamella development, polymorphism transition and molecular orientation, is investigated in this work. When the materials was deformed above Tg of polyamide1012, a series of complex SAXS patterns are continuously identified at intermediate strain including four points, a figure-eight and an X-shaped pattern, accompanied with two-bars pattern on the meridian, which corresponds to the transient microstructure. Such particular structure is resulted from the approach of tilted lamellae along the drawing direction, the insertion and the orientation of new lamellae. To investigate the temperature dependence, the microstructure developments at different temperature, which was below, above, or close to Tg, are compared. Based on the comprehensive results, the correlation between microstructure and mechanical response has been successfully established, which is featured by the synchronous occurrence of transient structure with slight strain hardening.

Crystallization and Tensile Deformation Mechanism of Propylene/Ethylene Copolymers in α and γ Polymorphs

Jiayi Zhao1, Yingying Sun2, and Yongfeng Men1

1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P.R.China
2ExxonMobil Asia Pacific Research & Development Co., Ltd, 1099 Zixing Road, Minhang District, 200241 Shanghai, P.R.China

Isotactic polypropylene (iPP) is a widely used commercial polymer with various polymorphs. γ-form, which can only crystallize at high pressure for isotactic polypropylene, is a usual polymorph for propylene copolymers. The subsequent crystallization behavior during cooling for copolymer initially in pure α-form and γ-form processed at different melt temperature has been studied [1]. Specifically, sample initially in α-form led to higher fraction of γ-form (fγ) and onset crystallization temperature (Tonset) than that of sample in γ-form under low melted temperature, which was caused by the difference between morphology of samples previously in α-form and γ-form.
Tensile deformation properties of the propylene/ethylene copolymers were also studied [2, 3]. The influence of stretching temperature and content of co-unit in transition behavior from γ-form to α-form during stretching was investigated. The critical stress for the polymorph transition was obtained which depends strongly on the stretching temperature and content of co-unit. The critical stress was higher for sample with lower co-unit content with less partitioning of ethylene co-unit in propylene crystalline lattices.
This work is supported by NSFC (21134006, 51525305) and ExxonMobil.

References
[1] Zhao, J.; Sun, Y.; Men, Y. Ind. Eng. Chem. Res. 56, 198 (2017). (link)
[2] Zhao, J.; Sun, Y.; Men, Y. Macromolecules 49, 609 (2016). (link)
[3] Zhao, J.; Sun, Y.; Men, Y. Submitted.

Effects of Melt Structure on Shear-induced Crystallization of Isotactic Polypropylene

B. Zhang and J. B. Chen

School of Materials Science & Engineering, Zhengzhou University, Zhengzhou 450002, People’s Republic of China

Based on a control of the melt structure at temperatures near but below the equilibrium melting point we investigated the role of shear stress imposed by the wall of the capillary die on crystal morphology of isotactic polypropylene (iPP). Bundles of partially ordered nanoscale chain segments within the quiescent melt at temperatures between the nominal melting temperature and the equilibrium melting point allowed for the possibility of shear-induced or shear-assisted formation of crystalline cylindrites which were investigated by means of polarized optical microscopy and small/wide-angle X-ray scattering.1-4 The SAXS patterns of near melting point structured melt monitored at 180 °C can be fitted by using a form factor for polydisperse cylinders. It was found that the average radius and height of the bundles of partially ordered chain segments were about 17 nm and 40 nm, respectively. For a given structured melt, the number of cylindrites increased with shear stress. Concomitantly, the nucleation density of α-iPP within a single cylindrite structure increased with shear stress at the expense of β-iPP nucleation density.

References
[1] Zhang, B.; Chen, J.; Zhang, X.; Shen, C. Polymer 52, 2075 (2011). (link)
[2] Zhang, B.; Chen, J.; Ji, F.; Zhang, X.; Zheng, G.; Shen, C. Polymer 53, 1791 (2012).  (link)
[3] Zhang, B.; Chen, J.; Cui, J.; Zhang, H.; Ji, F.; Zheng, G.; Heck, B.; Reiter, G.; Shen, C. Macromolecules 45, 8933 (2012). (link)
[4] Zhang, B.; Wang, B.; Chen, J.; Shen, C.; Reiter, R.; Chen, J.; Reiter, G. Macromolecules 49, 5145 (2016). (link)

A new microscopic kinetics model for nucleation of polymer crystallization

Jun Xu

Institute of Polymer Science & Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

Nucleation is a fundamental step of polymer crystallization and the mechanism is not fully understood yet [1]. Classical nucleation theory based on the capillary approximation has achieved success in the field of polymer crystallization; however, there are still some open questions remained: (1) Which pathway is chosen for nuclei formation of polymer lamellar crystals, stem by stem or cluster by cluster? (2) How to describe the many intermediate states during nucleation? In this work, we propose a microscopic kinetics model without the prerequisite thermodynamics parameters. In our new model, crystal nucleation is considered as a series of elementary processes: attaching and detaching of units. Correlation factors were introduced to describe the variation of the rate constants to attach and detach a unit with the cluster size. Via the microscopic kinetics, we can determine the equivalent thermodynamics parameters and simulate the time evolution of cluster size distribution [2]. Application of the new model to some polymers will be given. The critical size of nuclei in poly(butylene succinate) during crystallization and melting will be estimated. The model describes nucleation of small molecules and polymer chains in a unified view, which we believe can be applied to other kinetic processes far from equilibrium.

References
[1] M. C. Zhang, Y. Gao, B. H. Guo, J. Xu, Crystals 7(1), 4 (2017). (link)
[2] K. L. Xu, B. H. Guo, R. Reiter, G. Reiter, J. Xu, Chinese Chemical Letters
26, 1105 (2015). (link)

Fiber surface-induced nucleation of polylactide

B. Wang,1 T. Wen,2 X. Zhang,3 D. Wang,4 D. Cavallo1

1 Department of Chemistry and Industrial Chemistry, Genova (Italy)
2 Department of Chemical Engineering, Hsinchu (Taiwan)
3 Beijing Institute of Fashion Technology, Beijing (China)
4 Institute of Chemistry Chinese Academy of Sciences, Beijing (China)

Fiber-reinforced semicrystalline polymer composites are largely employed for their improved strength with respect to the pure polymer matrix. The adhesion between the polymer and the fiber is known to play a key role in determining the overall mechanical behavior. When semicrystalline polymers are employed, the heterogeneous nucleation on the surface of the solid fiber is an efficient way to improve the adhesion and speeding up the composite production rate. However, fiber induced nucleation studies are still scarce and mainly limited to polyolefins, despite the increasing importance of bio-based polymers and composites.
In the work, the nucleation process of polylactide (PLA) on several fibers was studied in-situ by means of hot-stage polarized optical microscope. Several commercially available fibers (i.e., PET, Kevlar and glass fibers) are employed and compared to stereocomplex enantiomeric PLA blend and annealed homochiral PLA fibers. The nucleating efficiency of the various heterogeneous substrates is quantitatively compared on the basis of the derived free energy barrier for critical nucleus formation, ΔG*.
It results clear that the PLA stereocomplex fibers has higher nucleating ability, due to the identical surface chemistry between the substrate and PLA homocrystal, although the crystalline structure of stereocomplex and homochiral crystals is not the same. On the other hand, the nucleation kinetics of PLA homocrystal on fibers of the very same crystal is even more efficient, and simply follows the secondary nucleation process: the induction times for the birth of the nucleus display the same temperature dependence of crystal growth.

Bond-orientational Order Assisted Crystal Nucleation in Polyethylene

Xiaoliang Tang, Junsheng Yang, Tingyu Xu, Fucheng Tian, Chun Xie, Liangbin Li

National Synchrotron Radiation Lab and CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, China

Flexibility and connectivity are the most prominent characteristics of polymer, how does flexible chain transform into rigid conformational ordered segments may be the key step for crystallization [1]. We investigate the nucleation process of polyethylene with full-atom molecular dynamic simulation, during which the structural evolution is analyzed with three order parameters, including conformational order, modified bond-orientational order [2] and density. Coupling between conformational and bond-orientational orderings results in the formation of hexagonal clusters first, which is dynamic in nature and absence of density order. Whilst nucleation of orthorhombic clusters occurs inside hexagonal clusters later, which involves all three order parameters and proceeds via the coalescence of neighboring hexagonal clusters rather than standard stepwise growth process. This demonstrates that nucleation of PE crystal is a two-step process with the assistance of bond-orientational order, which is different from early models for polymer crystallization but in line with that proposed for spherical “atoms” like colloid and metal.

References
[1] Su F, Ji Y, Meng L, et al. Coupling of Multiscale Orderings during Flow- Induced Crystallization of Isotactic Polypropylene. Macromolecules, 2017, 50(5): 1991-1997. (link)
[2] Yang J, Tang X, et al. Coupling between intra-and inter-chain orderings in flow-induced crystallization of polyethylene: A non-equilibrium molecular dynamics simulation study. The Journal of Chemical Physics, 2017, 146(1): 014901. (link)

Crystallinity and Stress Induced Crystallization in Ethylene-Propylene Rubbers

M. Scoti, F. Auriemma, and C. De Rosa

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso di Monte Sant’Angelo via Cintia, 80126 Napoli, Italy.

In this communication we report a study of the influence of the presence of crystallinity in the undeformed state and of the occurrence of stress-induced crystallization (SIC) on the mechanical properties and elastic behavior of ethylene-propylene-diene (EPD) random terpolymers, which are the basic material of many commercial rubbers. EPD terpolymers with ethylene concentration in the range 44-78 wt% have been analyzed. EPD samples with high ethylene content of 78 wt% are crystalline in the undeformed state and crystallize from the melt in the pseudohexagonal form of polyethylene. Samples with 71-64 wt% of ethylene show broad diffraction profiles typical of amorphous copolymers but show endothermic peaks in the DSC heating curves and well-defined correlation peaks in the small angle X-ray diffraction (SAXS) patterns. This indicates that presence of a certain amount of crystals that are not visible by wide angle diffraction. Further crystallization of the pseudohexagonal form of PE occurs during deformation (SIC). These results indicates that in EPD terpolymers, even when the concentrations of propene units is high in the range 30-40 wt%, ethylene sequences are long enough to crystallize, due to partial inclusion of propene units in the crystals (1). The crystallinity present in the undeformed state and its improvement during deformation are keys for understanding the remarkable mechanical performances of these rubbers.

References
[1] O. Ruiz De Ballesteros, F. Auriemma, G. Guerra, P. Corradini, Macromolecules 29, 7141 (1996). (link)

Mechano-optical rheology of semi-crystalline polymers

O. Mykhaylyk

Soft Matter Analytical Laboratory, Department of Chemistry, The University of Sheffield, Sheffield, S3 7HF, UK

Since polymeric liquids subjected to an external field (flow) often respond with a related anisotropy of their refractive index and stress, flow birefringence is commonly used for structural characterization of these materials. In this respect, rotational geometries are well suited for visual observations of the flow and, consequently, mechano-optical rheology of polymeric liquids. New applications of a rheo-optical method based on a combination of rotational rheology and a recently developed optical technique – shear-induced polarized light imaging (SIPLI) are presented [1]. Simultaneous rheo-optical studies using rheo-SIPLI have already been successfully used for characterization of self-assembled copolymers and liquid crystals [1, 2]. The proposed rheo-optical method is also effective for studying flow-induced crystallization of semi-crystalline polymers (FIC) [3]. Simultaneous optical measurements and mechanical rheology are performed during FIC. These experiments enable a relationship between the shish formation, detected by SIPLI, and the viscosity upturn, measured by the rheometer at the same time, to be established. The results are compared with small-angle x-ray scattering. It is also shown that SIPLI setup can be used for birefringence measurements. The normal stress difference calculated from the birefringence of a sheared polymer melt (polyethylene) correlate well with both the total normal force measured by the rheometer transducer during the same experiment and the first normal stress differences measured independently on the same polymer using cone-and-plate geometry.

References
[1] O. O. Mykhaylyk, N. J. Warren, A. J. Parnell, G. Pfeifer, and J. Laeuger, J. Polym. Sci., Part B: Polym. Phys. 54, 2151 (2016). (link)
[2] O. O. Mykhaylyk et al., Macromolecules 45, 5260 (2012). (link)
[3] O. O. Mykhaylyk, Soft Matter 6, 4430 (2010). (link)