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)

The non-equilibrium phase diagrams of flow-induced crystallization and melting of polymer

Zhen Wang, Youxin Ji, Jianzhu Ju, Xiaoliang Tang, Liangbin Li

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

With a combination of extensional rheology and in-situ synchrotron ultrafast x-ray scattering measurements, we have studied the flow-induced phase behaviors of polyethylene (PE) [1], isotactic polypropylene (iPP) [2] and Poly(1-butene) (PB-1) [3] over a wide temperature and flow strength range. Non-equilibrium phase diagrams are constructed in temperature-stress space for PE, and in temperature-strain rate space for iPP and PB-1, which reflect the non-equilibrium natures of flow-induced crystallization (FIC). Applying flow is recognized to favor the formation of structure with high entropy and low conformational order. The interplay of kinetic competitions and thermodynamic stabilities between different phases leads to rich kinetic pathways for FIC and diverse final structures. The non-equilibrium flow diagrams provide a detailed roadmap for precisely processing of polymers with designed structures and properties. It demonstrates that the non-equilibrium process stimulated by flow is fundamentally different from the equilibrium phase behaviors, where a rich source of physics is still waiting for us to dig out.

References
[1] Z. Wang, J. Ju, J. Yang, Z. Ma, D. Liu, K. Cui, H. Yang, J. Chang, N. Huang, and L. Li, Scientific Reports 6, 32968 (2016). (link)
[2] J. Ju, Z. Wang, F. Su, Y. Ji, H. Yang, J. Chang, S. Ali, X. Li, and L. Li, Macromolecular Rapid Communications 37, 1441 (2016). (link)
[3] Z. Wang, J. Ju, L. Meng, N. Tian, J. Chang, H. Yang, Y. Ji, F. Su, L. Li, and L. Li, Soft Matter 13, 3639 (2017). (link)

Multi-Shape Memory Effect of Columnar Side-Chain Liquid Crystalline Polymer

R. Y. Zhao, S. Yang, and E. Q. Chen

Department of Polymer Science and Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China

Recently, we are interested in side-chain liquid crystalline (LC) polymers bearing the hemiphasmid side-chain that contains a rod-like mesogen linked with a half-disk end group [1-3]. We found that they could self-organize into the hexagonal and/or rectangular columnar LC phase when the size of flexible tails on the half-disk was properly chosen and the dimension of columnar lattice could approach to 10 nm easily. It is identified that the supramolecular column in the columnar phase shall contain several chains (e.g., ~5 chains) laterally associated together rather than a single chain. This “multi-chain column” provides a new type of physical crosslinking. Namely, within the confined space of the column the backbones and pendant groups of the polymer can get entangled. Using hemiphasmid side-chain LC polynorbornene as the example [4], we demonstrate that such physical crosslinks can be rather robust, giving the polymer with the typical properties of thermal plastic elastomer. Furthermore, taking the physical crosslinks to define the permanent shape and the LC formation to fix the temporary shape, we realized the side-chain polynorbornene with excellent shape memory effect. For the dual shape memory, both the shape fixity (Rf) and shape recovery (Rr) are admirably high (approaching 100%), even when a large strain of 600% was applied. Benefited from a broad LC transition, the polymer can present the high-strain multi-shape memory effect, exampled by its triple- and quadruple-shape memory with high Rf and Rr at each step.

References
[1] J. F. Zheng, X. Liu, X. F. Chen, X. K. Ren, S. Yang, E. Q. Chen, ACS Macro. Lett. 1, 641 (2012). (link)
[2] X. Q. Liu, J. Wang, S. Yang, E. Q. Chen, ACS Macro Lett. 3, 834 (2014). (link)
[3] Y. S. Xu, D. Shi, J. Gu, Z. Lei, H. L. Xie, T. P. Zhao, S. Yang, E. Q. Chen, Polym. Chem. 7, 462 (2016). (link)
[4] R. Y. Zhao, T. P. Zhao, X. Q. Jiang, X. Liu, D. Shi, C. Y. Liu, S. Yang, E. Q. Chen, Adv. Mater. DOI 10.1002/adma.201605908 (2017) (link)

The role of entanglements for polymer crystallization

J.-U. Sommer

Leibniz-Insitut für Polymerforschung Dresden, Institut für Theorie der Polymere, Hohe Straße 6, 01069 Dresden
TU Dresden, Institut für Theoretische Physik, Zellescher Weg 17, 01069 Dresden

We use a coarse-grained polymer model to study the crystallization and melting behavior of long polymer chains in the dense state under various conditions. Our primary goal was to investigate the impact of the entanglement length on the properties of the lamellar crystal, in particular on the thickness selection, using algorithms to calculate the primitive path of the chains. In situ analysis of the local entanglement length prior to crystallization and the stem length and crystallinity at the same location reveals a direct correlation between the entanglement length and crystallization properties in the nucleation-controlled regime [1]. We have investigated various scenarios to change the apparent entanglement length in the system such as rapid cooling and cold-crystallization protocols, dilution of the melt by short chains, and the influence of solid substrates [2-4]. All studies confirm the correlation between entanglement length and thickness selection in the dense state. First results on non-concatenated ring polymer melts display a strong increase of the lamellar thickness and the degree of crystallization as compared to otherwise identical linear counterparts subject to the same thermal history [5].

References
[1] C.-F. Luo and J.-U. Sommer; Physical Review Letters 112, 195701 (2014). (link)
[2] C.-F. Luo, M. Kröger and J.-U. Sommer; Polymer 109, 71 (2017). (link)
[3] C.-F. Luo, M. Kröger and J.-U. Sommer; Macromolecules 49, 9017 (2016). (link)
[4] C.-F. Luo and J.-U. Sommer; ACS Macro Letters 5, 30 (2016). (link)
[5] H.-Y. Xiao, C.-F. Luo, D. Yan and J.-U. Sommer, manuscript (2017).

Interplay between the Relaxation of the Glass of Random L/D Lactide Copolymers and Homogeneous Crystal Nucleation: Evidence for Segregation of Chain Defects

C. Schick1,2 and R. Androsch3

1Institute of Physics and Competece Center CALOR, University of Rostock, Albert-Einstein-Str. 23-24, 18051 Rostock, Germany
2Kazan Federal University, 18 Kremlyovskaya street, Kazan 420008, Russian Federation
3Center of Engineering Sciences, Martin Luther University Halle-Wittenberg, 06099 Halle/Saale, Germany

Random L isomer rich copolymers of poly(lactic acid) containing up to 4% D isomer co units have been cooled from the molten state to obtain glasses free of crystals and homogeneous crystal nuclei. The kinetics of enthalpy relaxation and the formation of homogeneous crystal nuclei have then been analyzed using fast scanning chip calorimetry [1]. It has been found that the relaxation of the glass toward the structure/enthalpy of the supercooled liquid state is independent of the presence of D isomer co units in the chain. Formation of homogeneous crystal nuclei in the glassy state requires the completion of the relaxation of the glass [1,2]. However, nucleation is increasingly delayed in the random copolymers with increasing D isomer chain defect concentration. The data show that the slower formation of homogeneous crystal nuclei in random L/D lactide copolymers, compared to the homopolymer, is not caused by different chain segment mobility in the glassy state but by the segregation of chain defects in this early stage of the crystallization process [3].

References
[1] E. Zhuravlev, et al., Polymer 52, 1983 (2011). (link)
[2] R. Androsch, et al., Europ. Polym. J. 53, 100 (2014). (link)
[3] R. Androsch, and C. Schick, J. Phys. Chem. B 120, 4522 (2016) (link)

Talk by Tuomas P. J. Knowles

Tuomas P. J. Knowles

University of Cambridge, Department of Chemistry, UK

Filamentous protein aggregation underlies a number of functional and pathological processes in nature. This talk focuses on the formation of amyloid fibrils, a class of beta-sheet rich protein filament. Such structures were initially discovered in the context of disease states where their uncontrolled formation impedes normal cellular function, but are now known to also possess numerous beneficial roles in organisms ranging from bacteria to humans. The formation of these structures commonly occurs through supra-molecular polymerisation following an initial primary nucleation step. In recent years it has become apparent that in addition to primary nucleation, secondary nucleation events which are catalysed in the presence of existing aggregates can play a significant role in the dynamics of such systems. This talk describes our efforts to understand the nature of the nucleation processes in protein aggregation as well as the dynamics of such systems and how these features connect to the biological roles that these structures can have in both health and disease.

Highly oriented and crystalline semi-conducting and conducting polymer films prepared by high-temperature rubbing

Martin Brinkmann (1), Amer Hamidi-Sakr (1), Laure Biniek (1), Patrick Lévêque (2), Jean-Louis Bantignies (3),  David Maurin (3), Nicolas Leclerc (4)

(1) Université de Strasbourg, CNRS, ICS UPR22, F67000 Strasbourg, France
(2) Université de Strasbourg, CNRS, ENGEES, INSA, ICube UMR 7357, F-67000 Strasbourg, France
(3) Université de Montpellier, Laboratoire Charles Coulomb, F34095 Montpellier, France
(4) Université de Strasbourg, CNRS, ICPEES, UMR 7515, F67000 Strasbourg, France

This contribution focuses on recent advances in growth control and oriented crystallization of semi-conducting and conducting polymers. Particular emphasis will be given to the progress made in high-temperature rubbing of such polymers. This effective large scale alignment method can orient a large palette of polymer semiconductors (PSCs) with n- or p-type character without the use of an alignment substrate. The concurrent roles of the polymer molecular weight distribution and the rubbing temperature (TR) on the in-plane orientation have been rationalized for P3HT and PBTTT. Continue reading Highly oriented and crystalline semi-conducting and conducting polymer films prepared by high-temperature rubbing

Rheology of self-nucleated poly(ε-caprolactone) melts

D. Cavallo1, F. Barbieri1, L. Sangroniz2, A. Santamaria2, R.G. Alamo3 and A.J. Müller2

1Department of Chemistry and Industrial Chemistry, Genova (Italy)
2POLYMAT/Polymer Science and Technology Dept., San Sebastián, (Spain)
3Department of Chemical and Biomedical Engineering, Tallahassee, (USA)

Contrary to low molar mass molecules, semicrystalline polymers exhibit a strong influence of the melt-annealing conditions on the re-crystallization. An enhanced concentration of nuclei is found after a mild melting treatment, in comparison to annealing at high temperatures and long times. [1,2] The intimate nature of these “self-nuclei” is still under debate, since they are elusive to the most commonly adopted characterization techniques. [2,3]

In this work, the self-nucleation process is studied by probing the effect of self- nuclei on the rheological properties of the non-equilibrium melt. Three poly(ε- caprolactone)s with different molar mass were investigated by shear rheometry. The viscoelastic functions of the melt were determined at the same temperatures, reached either from the solid state or from the melt, i.e., with or without the presence of self-nuclei.
Continue reading Rheology of self-nucleated poly(ε-caprolactone) melts

Chain Folding of Semicrystalline Polymers

Toshikazu Miyoshi

Department of Polymer Science, The University of Akron, Akron, Ohio 44325, USA

There is a long-standing controversy about chain-folding structure and mechanism of long polymer chains during crystallization. This information is a key factor to understand the crystallization mechanisms at the molecular levels. Recently, solid-state (SS) NMR combined with selectively 13C isotopic labelling has been successfully utilized to determine local chain trajectory of folded polymer chains in single and bulk crystals.1-6 In this talk, we will discuss various physical effects on chain folding and crystallization mechanisms of several polymers such as Poly(L-Lactic, Acid),1, 2 Isotactic-poly(1-butene),1, 3, 4 and Isotactic-Polypropylene.1, 5, 6 Continue reading Chain Folding of Semicrystalline Polymers

Model Experiments for the Crystallization of Conjugated Polymers

G. Reiter1, F.M. Keheze1, D. Raithel2, R. Hildner2, D.Schiefer3, M. Sommer3

1Physikalisches Institut, Universität Freiburg, Freiburg, Germany
2Experimentalphysik IV, University of Bayreuth, Bayreuth, Germany.
3Institut für Makromolekulare Chemie, Universität Freiburg, Freiburg, Germany

Much insight into crystallization of long chain polymers can be gained by studying mono-lamellar single crystals. Because of the kinetically determined lamellar thickness and the corresponding variations in melting temperature, polymer crystals allow for self-seeding, i.e., crystals can be re-grown from a melt, which contains a few thermodynamically stable remnants of pre-existing crystals acting as seeds. Employing such a self-seeding approach, we demonstrated that large single crystals can be grown even from long poly(3-hexylthiophene) (P3HT) chains, with a control over the number density, size, and internal structure of these crystals exhibiting monoclinic form II with interdigitated hexyl side groups [1]. Continue reading Model Experiments for the Crystallization of Conjugated Polymers