The Decisive Role of Intra-Crystalline Chain Dynamics for the Morphology of Semicrystalline Polymers

M. Schulz, A. Seidlitz, R. Kurz, R. Bärenwald, K. Saalwächter and T. Thurn-Albrecht

Institute of Physics, Martin Luther University Halle-Wittenberg

A special feature of some semicrystalline polymers is the existence of the so-called αc-relaxation, caused by a translational motion of the chains in the crystal. Although it was recognized already early on that these crystal-mobile polymers generally have a higher crystallinity than crystal-fixed polymers [1], differences in the semicrystalline morphology have not been analyzed in detail and the relaxation process has not been taken into account in most crystallization models. Using an extended method for quantitative analysis of small angle x-ray scattering data [2] we here compare the structural characteristics for two model polymers, namely PEO (crystal-mobile) and PCL (crystal-fixed). PCL follows the expectations of classical crystallization theories. The crystal thickness is well defined and determined by the crystallization temperature Tc. In contrast, for PEO the amorphous thickness is better defined. We hypothesize that due to the αc-relaxation, the crystalline lamellae thicken directly behind the growth front until the amorphous regions reach a minimal thickness. This assumption is consistent with NMR experiments, which give the time scale of intra-crystalline dynamics [3] and which show that crystal reorganization takes place on the same time scale as crystal growth itself. In keeping with this model, crystal-mobile and crystal-fixed polymers exhibit a different melting behavior during heating. The crystallization of PCL leads to the formation of marginally stable crystallites which constantly reorganize during heating, while in PEO due to the presence of the αc-relaxation, much more stable (thickened) lamellar crystals form, which melt only at much higher temperatures.

References:
[1] Boyd, R.H.; Polymer 26, 323 (1985) (link)
[2] Seidlitz, A. et al. in Polymer Morphology, Wiley (2016) (link)
[3] Kurz, R. et al.; Macromolecules 50 (10), 3890 (2017) (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.

Chain Tilt in the Crystalline Lamellae of Poly(ethylene oxide) Investigated by Mid-Chain Defects

Martin Pulst, Yury Golitsyn, Christian Schneemann, Paweł Ruda, Detlef Reichert, and Jörg Kressler

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

While many traditional schematic models show the crystalline polymer chains aligned parallel to the lamella thickness Lc recent sophisticated studies on poly(ethylene) show that the crystalline polymer chains are tilted at an angle φ to the lamella thickness [1].
Here, we investigate the chain tilt of poly(ethylene oxide) (PEO) by the means of mid-chain defects using wide-angle X-ray scattering and solid state 13C MAS cross polarization NMR spectroscopy. At low temperatures, one polymer chain of PEO9-meta-PEO9 and PEO11-TR-PEO11 [2-4] containing a 1,3-disubstituted benzene and a 1,4-disubstituted 1,2,3-triazole defect in central position of the polymer chain, respectively, form crystals and the other PEO chain as well as the defect remain amorphous. The aromatic defects of these two polymers can be incorporated into the crystalline lamellae upon heating below Tm and the corresponding structure models confirm that the amorphous PEO chains exit the tilted lamellae at a preordained angle. Thus, the chain tilt angle φ can directly be calculated from the bent angle ξ between the amorphous and crystalline PEO chains, which is given by the substitution patterns of the aromatic mid-chain units and a tilt angle range between 36° ≤ φ ≤ 60° is determined. Furthermore, our studies on PEO9-para-PEO9 containing a 1,4-disubstituted benzene mid-chain defect confirm that also linear unbent PEO chains form tilted lamellae.

References
[1] K. J. Fritzsching, et al., Macromolecules 50, 1521-1540 (2017). (link)
[2] M. Pulst, et al., submitted (2017).
[3] M. Pulst, et al., Macromolecules 49, 6609-6620 (2016). (link)
[4] Y. Golitsyn, et al., J. Phys. Chem. B 121, 4620-4630 (2017). (link)

Amyloid Protein Aggregation in the Presence of Temperature-Sensitive Polymers

Zhanna Evgrafova1, Sonu Kumar1, Bruno Voigt1,  Juliane Adler2, Daniel Huster2, Jochen Balbach1 and Wolfgang H. Binder1

1Faculty of Natural Science II, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 4, D-06120 Halle (Saale), Germany
2Institute for Medical Physics and Biophysics, Leipzig University, Härtelstraße 16-18, D-04107 Leipzig, Germany

The formation of amyloid fibrils is considered to be one of the main causes for many neurodegenerative diseases, such as Alzheimer’s, Parkinson’s or Huntington’s disease [1, 2]. Current knowledge suggests that amyloid-aggregation represents a nucleation-dependent aggregation process in vitro, where a sigmoidal growth phase follows an induction period. Here, we studied the fibrillation of amyloid β 1-40 (Aβ40) and Parathyroid hormone (PTH) in the presence of thermoresponsive polymers, expected to alter their fibrillation kinetics due to their specific hydrophobic and hydrophilic interactions with proteins [3]. Mixtures in varying concentrations and the conjugates of PTH or Aβ40 with poly(ethylene glycol) methyl ether acrylate were studied via time-dependent measurements of the thioflavin T (ThT) fluorescence and transition electron microscopy (TEM). The studies revealed that amyloid fibrillation was altered, accompanied by either reduction or elongation of the lag phase of PTH or Aβ40 fibrillation in the presence of studied polymers [4].

References:
[1] Chiti, F.; Dobson, C.M., Annu. Rev. Biochem. 2006, 75, 333–366. (link)
[2] Hamley, I.W., Angew. Chem. Int. Ed. 2007, 46, 8128–8147. (link)
[3] Adler, J.; Huster, D., Phys. Chem. Chem. Phys. 2017, 19, 1839–1846. (link)
[4] Funtan, S.; Evgrafova, Z.; Adler, J.; Huster, D.; Binder, W.H., Polymers 2016, 8, 178. (link)

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)