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.

[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.

[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.

[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.

[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)

Relations between morphology and crystallization behavior of poly (l-lactide) / poly (butylene succinate) bioblend nanocomposites with graphene oxide nano-sheets

S. Fenni1, D. Cavallo2, and N. Haddaoui1

1LPCHP, Faculty of Technology, University of Setif -1, Algeria.
2DCCI, University of Genova, Italy.

Bio-based blend nanocomposites of poly(L-lactic acid) (PLA) and poly(butylene succinate) (PBS) [1] with different concentration (from 0.1 wt% to 0.5 wt%) of Graphene Oxide nano-sheets (GOs) were prepared by melt blending. The resulting morphology is investigated with scanning and transmission electron microscopy (FE-SEM and TEM). FE-SEM of fracture surfaces revealed that the addition of GO to the bio-based PBS/PLA blend improves the adhesion between the two polymers, indicating that GOs nanosheets locate at the interface.[2] TEM analysis showed that the nanofillers are preferentially found in the PBS phase (minority component). The grapheme oxide nanosheets act as nucleting agents for both semicrystalline polymers. The nucleating effect of the added particle is compared to the one of own self-nuclei for each polymer, to define a convenient nucleating efficiency (NE) scale. A value of around 80% is determined for GO towards PBS, among the highest nucleating efficiencies ever reported for this polymer. On the other hand, the efficiency in nucleating PLA is equal to a modest 15%, due to the uneven distribution of the filler in the two polymers. A close relationship between the nanocompostie complex morphology and crystallization behavior of the two different polymers is thus established.

[1] Wang, R. et al. Polym. Eng & Scie. 49:1, 26–33. 2009. (link)
[2] Cao, Y. et al. ACS Nano. 5:7, 5920-5927. 2011. (link)

Tailoring Properties of Polypropylene through Crystallization in the Presence of Polymeric Nucleating Agents

C. De Rosa, F. Auriemma, O. Tarallo, C. Santillo, M. Scoti

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso di Monte S.Angelo, Via Cintia, I-80126 Napoli.

In this communication, we present a strategy for the addition of polymeric nucleating agents for the crystallization of isotactic polypropylene (iPP) that guarantees a perfect fine dispersion of nucleating particles within the entire mass of the polymer, with consequent their high efficiency even at very low concentrations. The Ziegler-Natta catalyst particles are coated by a thin skin of poly(trimethylallylsilane) (PTMAS) or poly(vinylcyclohexane) (PVCH) that will act as nucleating agents, by prepolymerization of the corresponding monomers. PVCH shows higher nucleation efficiency than PTMAS with greater increase of crystallization temperature by standard cooling from the melt. Both polymeric nucleating agents affect the crystal morphology greatly reducing the size of shperulites. This in turn affects the mechanical properties improving ductility and flexibility. The presence of the nucleating agent accelerates the crystallization of iPP and affords crystallization of the α form even upon fast crystallization by quenching the melt, condition that generally produces crystallization of the mesomorphic form of iPP (1). Crystals of α form so obtained show a nodular morphology and absence of spherulitic superstructure. This novel iPP material is characterized by outstanding and unexpected properties of high mechanical strength and modulus and contemporarily high ductility, flexibility and good transparency due to the nodular morphology of α form (2).

[1] C. De Rosa; F. Auriemma Ang. Chem. Int. Ed. 51, 1207 (2012). (link)
[2] C. De Rosa, F. Auriemma, O. Tarallo et al. Polym Chem. 8, 655 (2017). (link)

Dissection of elastomeric performances of ethylene based semicrystalline multi-block copolymers

F. Auriemma, C. De Rosa, M. Scoti, G. Talarico

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

The crystallization properties, morphology and elastomeric behavior of some ethylene/1-octene multi-block copolymers produced by chain shuttling technology [1] are analyzed. We evidence that the samples consist of a reactor blend of chains characterized by alternation of crystalline (hard) blocks with low octene content and amorphous (soft) blocks with high octene content, having different length and different number of blocks. The sample show similar degree of crystallinity and melting temperature, and good elastomeric properties at 25°C. Differences occur for the crystallization temperature, morphology and elastomeric properties at 60°C. These differences reflect differences in segregation strength. For samples containing a high fraction of chains with hard-blocks of short length, and long soft-blocks, the segregation strength is high, and the hard domains are well separated at high correlation distances. For samples containing a high fraction of chains characterized by long hard-blocks and short soft-blocks some kind of interpenetration of the hard segments in the soft domains occurs, with consequent decrease of segregation strength and interdomain distance. Since the long hard-segments can also connect different hard-domains, a well interpenetrated network is formed. Therefore, the samples forming an interpenetrating network crystallize at lower temperatures (more slowly), show high mechanical strength and ductility, and good elastomeric properties even at high temperatures. The samples with no inter-woven structure with short hard-blocks, instead, form a more heterogeneous morphology, show low mechanical strength and lose elastomeric properties already at 60°C.

[1] D.J. Arriola, E.M. Carnahan, P.D. Hustad, R.L. Kuhlman, T.T Wenzel, Science, 312, 714 (2006). (link)