Interface-induced crystallization via prefreezing: A first order prewetting transition

Ann-Kristin Flieger, M. Schulz, and T. Thurn-Albrecht

Experimental Polymer Physics, Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany

Interface-induced crystallization of a liquid on a solid substrate can either occur via heterogeneous nucleation or via prefreezing. Whereas heterogeneous nucleation takes place at finite supercooling below the melting temperature Tm, in prefreezing a crystalline layer is formed at the surface of a solid substrate already above Tm. Wetting theory predicts a jump in thickness at the formation and a divergence upon approaching coexistence. However, the thickness of the prefreezing layer has not been experimentally measured so far.

We studied ultrathin films of polycaprolactone (PCL) during the crystallization on graphite. With in-situ AFM-measurements we observe prefreezing instead of heterogeneous nucleation. The corresponding crystalline layer is formed at a temperature above the bulk melting temperature. Similar observations were already made for polyethylene on graphite [1]. In that case however, a direct measurement of the thickness of the prefreezing layer was not possible. Here, we show directly the finite thickness of the prefreezing layer for PCL. It forms with a thickness of a few nanometers which further increases during cooling. This observation demonstrates the transition is of first order, as expected for a prewetting transition.

The results prove that prefreezing can be described by common wetting theory. The studied system PCL-graphite is of importance for applications since graphitic materials are widely used as fillers for PCL.

References
[1] A.-K. Löhmann, T. Henze, and T. Thurn-Albrecht, PNAS 49, 17368-17372 (2014). (link)

Deformation and nano-void formation of β-phase isotactic polypropylene during uniaxial stretching

T. Kawai and S. Kuroda

Graduate School of Science and Engineering, Gunma University, Ota, Gunma 373-0057, Japan

Pseudo-hexagonal β-form is known to transform into thermodynamically stable monoclinic α -form during elongation. It is also reported that the nano-sized void is formed during deformation. Since the crystal deformation/void formation mechanism of β-iPP is not fully understood, we aim in this study to clarify the deformation behavior of β-iPP in both terms of crystal transformation (angstrom scale) and the void formation (nanometer scale). The film of β-iPP was prepared by melt crystallization of PP with 0.2% DCNDCA as a nucleating agent (kβ = 0.94). The samples were drawn uniaxially at 100ºC with fixed strain rate of 0.66 min-1. Synchrotron radiation WAXD/SAXS measurements were performed at BL40B2 in SPring-8, Japan. Deformation of β -iPP proceeded as follows; (i) at the yielding point of ε = 0.1 β-form started to decrease followed by increase in amorphous fraction. (ii) at ε = 0.4, α-form crystal with the chain orientation parallel to the stretching direction was formed. Importantly, as soon as α -form crystallized, formation of nano-sized void was initiated. Above findings strongly suggest that the β -form transforms to amorphous and/or mesomorphic state before recrystallization into α-form crystal. A detailed analysis on void structure by means of SAXS streak scattering is also to be discussed based on lamellar deformation during elongation.

Multiplicity of Morphologies in Poly (L-lactide) Bioresorbable Vascular Scaffolds

Julia A. Kornfield

California Institute of Technology, Chemistry & Chemical Engineering,  Pasadena CA 91125

Poly(L-lactide), PLLA, is the structural material of the first clinically approved bioresorbable vascular scaffold (BVS), a promising alternative to permanent metal stents for treatment of coronary heart disease. BVSs are transient implants that support the occluded artery for 6 months, and are completely resorbed in 2 years. Clinical trials of BVSs report restoration of arterial vasomotion and elimination of serious complications such as Late Stent Thrombosis. It is remarkable that a scaffold made from PLLA, known as a brittle polymer, does not fracture when crimped onto a balloon catheter or during deployment in the artery. X-ray microdiffraction revealed how PLLA acquired ductile character and that the crimping process creates localized regions of extreme anisotropy; PLLA chains in the scaffold change orientation from the hoop direction to the radial direction over micron-scale distances. The multiplicity of morphologies in the crimped scaffold enable a low-stress response during deployment, which avoids fracture of the PLLA hoops and leaves them with the strength needed to support the artery. Thus, the transformations of the semicrystalline PLLA microstructure during crimping explain the unexpected strength and ductility of the current BVS and point the way to thinner resorbable scaffolds in the future.

References:
[1] Ailianou, A.; Ramachandran, K.; Kossuth, M.; Oberhauser, J.P.; Kornfield, J.A.*; “Multiplicity of Morphologies in Poly (L-lactide) Bioresorbable Vascular Scaffolds,” PNAS, 113, 11670-11675 (2016). (link)

Influence of Propylene-based Elastomer on Stress-whitening for Impact copolymer

Ying Lu2, Yingying Sun1, Lan Li1 and Yongfeng Men2

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

Two pure impact copolymer polypropylenes (ICP), one pure homo-polypropylene (HPP) and their compounds with different type and fraction of VistamaxxTM propylene-based elastomer (“Vistamaxx”) were used to investigate the stress whitening activated in the impact processes via ultra-small angle X-ray scattering technique. A characteristic macroscopic whitening of deformed materials was showed due to the formation of voids or cavities with a typical size up to the wavelength of visible light, that is, some hundreds of nanometers. The stress whitening presented in ICP is confirmed to be caused by the interfaces existed between ethylene-propylene (EP) rubber and polypropylene (PP) matrix, such kind stress whitening is apparently unlike the one initiated in the crystal phase of HPP. In this study, Vistamaxx with high crystallinity can adjust the compatibility of EP rubber and PP matrix which results in a reduction of interfaces, thus, a phenomenon of reduced stress whitening can be observed in blends of ICP with Vistamaxx. However, the enhancement of stress whitening can be found in blends of ICP with Vistamaxx which occupied low crystallinity. Such behaviors can be assigned to the poor compatibility between Vistamaxx and PP matrix.

Polythiophene Adsorption and Restructuring on Gold Surfaces

E. Schreck1, T. Simon1, S. Förster1, R. Hammer1, and W. Widdra1,2

1Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle, Germany
2Max Planck Institute of Microstructure Physics, Halle, Germany

Scanning tunneling microscopy is used to study the complex adsorption behavior of poly-3-hexyl-thiophene (P3HT) on Au(001) [1] and Au(011). Upon electrospray deposition under ultrahigh vacuum conditions, weakly adsorbed polymer chains are found on Au(001), which exhibit a truly 2D random-coil-like chain backbone structure. Their end-to-end distance and their radius of gyration are reported as function of the polymer length. Additionally, a fraction of the P3HT molecules is constraint into a fully stretched configuration along the high-symmetry [110] crystal direction, indicating a stronger molecule-substrate interaction. This adsorption is accompanied by local lifting of the Au(001) surface reconstruction [2] underneath the polymer chains [3].
For the more open Au(011) surface that exhibits a missing-row (2×1) reconstruction, we find a stronger interaction between P3HT and the gold surface. Polymer chains align predominantly along the missing rows by changing their conformation into an all-trans state. The presence of the polymer is inducing a local reorganization process changing the surface reconstruction from 2×1 to a 3×1 reconstruction. These results will be compared with similar findings for smaller thiophene oligomers (sexithiophene, 6T) on both substrates [4,5].

References:
[1] S. Förster, E. Kohl, M. Ivanov, J. Gross, W. Widdra, and W. Janke, J. Chem. Phys. 141, 164701 (2014). (link)
[2] R. Hammer, A. Sander, S. Förster, M. Kiel, K. Meinel, and W. Widdra, Physical Review B, 2014, 90, 035446. (link)
[3] S. Förster and W. Widdra, J. Chem. Phys. 141, 054713 (2014). (link)
[4] K. Duncker, M. Kiel, A. Höfer, and W. Widdra, Phys. Rev. B 77, 155423 (2008). (link)
[5] M. Kiel, K. Duncker, C. Hagendorf, and W. Widdra, Phys. Rev. B 75, 195439 (2007). (link)

The confined crystallization of polymer in anodized aluminum oxide template

Xiaoli Sun, Xiying Dai, Shouke Yan

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

The one dimensional (1D) polymer nanostructures with controlled morphologies and properties can be easily achieved from Anodized aluminum oxide (AAO) template. In AAO template, confinement effect and interface effect determine the crystallization and dynamical behavior.[1-4] Some aspects of questions are worthwhile for deeper understanding: (1) discrimination between the interfacial and confinement effect on the dynamics of multiscale chain-motions; (2) discrimination between segmental dynamics and chain dynamics on the crystallization of polymer. Moreover, the studies on the comparison of confinement effect between 1D polymer nanostructures and two dimensional (2D) polymer thin film by employing the same polymer is also less explored. In the present study, poly(3-hydroxybutyrate) (PHB) was selected as a model to explore the above aspects. Interfacial effect slows down the dynamics of PHB segmental mobility and shows strong dependence on pore size. Spatial confinement accelerates the dynamics of segmental mobility. Both effects slow down the chain mobility which consequently slow down the crystallization kinetics of PHB. The inhibited crystallization of PHB in AAO nanopores can be attributed to both segmental and chain mobility. By contrast, only chain mobility plays the role on the crystallization of thin film. Segmental mobility does not change with film thickness.

References
[1] L. Li, D. Zhou, D. Huang, G. Xue, Macromolecules 47, 297 (2014). (link)
[2] M. Steinhart, P. Göring, H. Dernaika, M. Prabhukaran, U. Gösele,  E. Hempel, and T. Thurn-Albrecht, Phys. Rev. Lett. 97, 027801 (2006). (link)
[3] X. Sun, Q. Fang, H. Li, Z. Ren, S. Yan, Langmuir 32, 3269 (2016). (link)
[4] X. Dai, J. Niu, Z. Ren, X. Sun, S. Yan, J. Phys. Chem B 120, 843 (2016). (link)

Effects Of Phase Separation and Crystallization on Morphology of Poly(propylene carbonate)/Poly(3-Hydroxybutyrate) Blend Thin Films

Huihui Li, Shujing Zhang and Shouke Yan

Beijing University of Chemical Technology, Beijing, China

For partially miscible crystalline/amorphous polymer blends, the combined effects of phase separation and crystallization can result in multiple morphology. In this study, the effects of composition and melting time on the morphology and structures of poly(propylene carbonate)/poly(3-hydroxybutyrate) blend thin films was investigated. A low PPC content in the film resulted in compact PHB spherulites, filling the whole space, whereas the amorphous PPC spherical microdomains scatter in the PHB region. With increasing PPC component and melting time, a large amount of PPC aggregates to the surface to form a network uplayer, whereas the PHB thick domains connected by its thin layer form a continuous PHB region, leading to a superimposed bilayer structure. If PPC content reached 70 wt%, a PPC-top and microporous PHB-bottom bilayer structure can be developed. We suggested that phase separation can take place mainly along the normal direction of the film surface at 190 ºC, attributed to the different surface energies of the two components. After cooling, the crystallization occurred, leading to further segregation and solidification along film planes. This superimposed bilayer by interplay between phase separation and crystallization may provide availability to tailor the final structure and properties of crystalline/amorphous polymer films.

References
[1] S. Zhang, X. Sun, Z. Ren, H. Li and S. Yan Phys. Chem. Chem. Phys. 17, 32225 (2015). (link)
[2] S. Zhang, Z. Ren, X. Sun, H. Li and S. Yan Langmuir 33, 1202 (2017). (link)

Epitaxial Crystallization of Polyethylene via Prefreezing: Effect of Strength of Substrate Interaction

M. Tariq, A. -K. Flieger, and T. Thurn-Albrecht

Institute of Physics, Martin Luther University, Halle-Wittenberg, Germany

The process of crystallization is most often initiated at an interface to a solid surface due to a decreased nucleation barrier. A solid surface can induce crystallization either by heterogeneous nucleation [1] or by prefreezing [2], where these two processes are very different from each other from a thermodynamics point of view. Previously we observed prefreezing occurring in polyethylene on the surface of graphite [2]. In this system a thin crystalline layer is first formed at the solid interface at a temperature 16 K higher than bulk melting temperature and the thickness of this prefreezing layer increases upon approaching melt-solid coexistence. Here we present another case where polyethylene crystallizes via prefreezing on a molybdenum disulfide substrate which has a stronger effect on the crystallization of polyethylene. Using in-situ high temperature AFM measurements we show that the prefreezing layer is stable in a temperature range up to about 47 K above the bulk melting temperature.

References
[1] R. P. Sear, J. Phys.: Condens Matter, 19, 033101 (2007). (link)
[2] A. –K. Löhman, T. Henze, T. Thurn-Albrecht, Proc Natl Acad Sci, 111, 17368 (2014). (link)

Novel three-dimensional nano-oriented crystals of polyesters

K. Okada1, Y. Tanaka2, H. Masunaga3, and M. Hikosaka1

1Graduate school of integrated arts and sciences, Hiroshima University, Japan.
2Teijin Ltd., Japan.
3Japan synchrotron radiation research institute (JASRI), SPring-8, Japan.

We crystallized the supercooled melt of polyesters by the melt-elongation. We used three kinds of polyesters, such as poly(ethylene terephthalate); PET, poly(ethylene-2,6-naphthalene dicarboxylate); PEN and poly(butylene terephthalate); PBT. We found that the novel three-dimensional (3D) morphology of “nano-oriented crystals (NOCs)” was formed, while in the case of isotactic polypropylene, one-dimensional morphology of NOCs was formed [1,2]. We observed the structure and morphology of NOCs by means of polarizing optical microscope, small/wide angle X-ray scattering. The nano crystals of 10 nm in order showed single crystal like monoclinic arrangement. The molecular chains were mainly oriented along the elongational direction. We clarified the mechanism of formation of 3D-NOCs of polyesters and an important role of the Benzene plane. In the elongational flow, the Benzene planes will be arranged by the effect of hydrodynamics [3] and packed locally in parallel. The parallel-packed Benzene planes should become a precursor of a nucleus, which would result in homogeneous nucleation [4] and formation of 3D-NOCs.

References
[1] K. Okada et al., Polymer J. 42, 464 (2010). (link)
[2] K. Okada et al., Polymer J. 45, 70 (2013). (link)
[3] T. Tatsumi, Ryutairikigaku, p.171 (Baifukan Co. Ltd., Tokyo, Japan, 1982).
[4] F. P. Price, in Nucleation (ed. A. C. Zettlemoyer) Ch.8 (Marcel Dekker, Inc., New York, USA, 1969).

Bi-axial nano oriented crystals (NOCs) of Polyamide 66

M. Hikosaka1, K. Okada1, K. Yasui2, M. Ishikawa3 and H. Masunaga4

1Graduate school of integrated arts and sciences, Hiroshima University, Japan.
2Bridgestone corp., Japan, 3ASAHI KASEI corp. , Japan.
4Japan synchrotron radiation research institute (JASRI), SPring-8, Japan.

We found that polyamide 66 (PA66) crystallizes into novel “bi-axial nano-oriented crystals (bi-NOCs)”, when the supercooled melt was elongated above a critical elongational strain rate.
We used PA66 (Mw=87×103, Mw/Mn=2.31). We used roll system to generate strain rate. We observed the structure and morphology of NOCs by means of polarizing optical microscope and small/wide angle X-ray scattering from three directions, through, edge and end. Polarizing optical micrographs suggested the formation of NOCs. SAXS patterns showed typical two-point pattern, which indicates the formation of NOCs. The two-point pattern showed orientation along machine direction (MD) for through and edge view, while along normal direction (ND) for end view. Size of a nano crystal (NC) was 11nm along MD and ND. WAXS patterns showed chain orientation along MD for through and edge views, while along ND for end view. From these observed facts, we concluded that the arrangement of NCs and chains showed bi-orientation along MD and ND.
The bi-orientation of NCs and chains suggests the important role of hydrogen bond planes (HBPs) in formation of NOCs. As the crystals of PA66 includes “rigid” HBPs[1], the HBPs should change into hydrogen bond clusters (HBCs) after melting. The HBCs should become nuclei. Under large strain rate, the HBCs would be parallelly oriented to the roll surface due to hydrodynamic effect, which should be the reason of the formation of bi-NOCs.

References
[1] Bunn, C. W. & Garner, E. V. Proc. Royal Soc. London, A(189), 39 (1947).  (link)