How miscibility affects the crystallization rates of poly(lactide)-block-poly(2-isopropyl-2-oxazoline) block copolymers

Significance 

Often the properties of a single polymer cannot meet the standards for a given purpose whereas a mixture of two polymers would favorably improve the material’s performance. Therefore, miscibility of the components is a precondition to combine the properties of two polymers to a single phase material. This requirement unfortunately prohibits the majority of possible polymer-polymer combinations. Especially for emerging polymers the demand is high to find suitable partners to synergistically improve the properties and widen their applications. Presently, the emergence of poly(lactide) (PLA), a biodegradable linear polyester, as a promising alternative to the currently popular petroleum-based commodity plastics, has shifted the epicenter of research. Unfortunately, despite having many positive features, its poor mechanical properties as a pure homopolymer have to be overcome by blending with additives or other polymers. More advanced chemistry allows to marry PLA via a covalent bond to a second polymer in form of linear block copolymers (BCP). These form a permanent molecular unit to which the properties of the individual components are inscribed. Crystallization is a process during which a compound finds an energetically favored state with high degree of order from an initially unordered situation. In a BCP consisting of two crystallizable components in close spatial proximity the crystalline transformations of one component manipulate the transformations of the other component and vice versa. Therefore, comprehending and being able to control the structural evolution of crystallization in BCPs is of high fundamental and applied interest.

Recently, University of Helsinki researchers: Fabian Pooch (PhD student), Dr. Kirsi Svedström, Ms Marjolein Sliepen, Antti Korpi (PhD student), Professor Françoise M. Winnik and Professor Heikki Tenhu assessed the crystallization behavior of miscible A-B-type BCPs of poly(lactide) and poly(2-isopropyl-2-oxazoline). They purposed to unearth the implications of mixing on the crystallization behavior of the block copolymers and the structural evolution. Their work is currently published in the research journal, Polymer Chemistry.

The researchers commenced their study by synthesizing alkyne-terminated poly(L-lactide) (PLLA), alkyne-terminated poly(DL-lactide) (PDLLA) and azide-terminated poly(2-isopropyl-2-oxazoline) (PiPOx). Next, they prepared a series of 18 block copolymers comprised of PLLA/PDLLA and PiPOx by copper(I)-catalyzed azide–alkyne cycloaddition for the various subsequent uses. The authors found – in agreement with theoretical considerations – all 18 block copolymers to be miscible. The researchers then progressed to prepare films by dissolving the block copolymers in chloroform and drop casting onto a clean glass slide at room temperature. Lastly, they characterized the samples obtained using various techniques including polarized optical microscopy (POM, Figure below) and atomic force microscopy among others.

Upon isothermal crystallization at 130 °C the crystalline superstructures of pure PLLA and PiPOx were clearly distinguishable. The PLLA sample featured spherulites and the crystallization was completed after 5 min (Image A). In contrast, PiPOx crystallized in a small granular superstructure and the crystallization lasted 45 min (Image B). As result of mixing, the crystallization rates were inverted in a blend of the two polymers (Image C), granules appeared after 8.5 min and spherulites after 15 min. In the BCP a granular superstructure was observable after 6.5 min (Image D). Crystallization of PiPOx was the driving force for phase separation. In the blend PLLA was macroscopically expelled from the regions of crystalline PiPOx and subsequently crystallized. However, the fate of PLLA in the BCP was irreversibly linked to PiPOx. In the micro-phase separated state PLLA could not form spherulites.

The researchers concluded that the mixing had a plasticizing effect on PiPOx and a detrimental effect on the crystallization of poly(L-lactide). The crystallization rates of PLLA in the BCPs were dramatically reduced, or in most cases entirely prevented, while the crystallization rates of PiPOx increased considerably.

University of Helsinki researchers have disclosed the significance of miscibility as a vital influencing factor in the crystallization of block copolymers. It has been seen that the bulk behavior of unprecedented PLA-PiPOx block copolymers is dominated by the miscibility of the components. Altogether, a good understanding of the interplay of miscibility, stereochemistry and crystallization has potential to stimulate a variety of auspicious biomedical applications.

 

How miscibility affects the crystallization rates of poly(lactide)-block-poly(2-isopropyl-2-oxazoline) block copolymers. Advances in Engineering

 

How miscibility affects the crystallization rates of poly(lactide)-block-poly(2-isopropyl-2-oxazoline) block copolymers. Advances in Engineering
Figure 1 Polarized optical microscopy images after isothermal crystallization at 130 °C for the indicated time of PLLA (A), PiPOx (B), their blend (c), and a PLLA-PiPOx block copolymer (D) (scale bar: 1 mm). Spherulites in (A) start to grow immediately after reaching the crystallization temperature and the process is completed in 5 min, granules in (B) first appear after 12 min and crystallization lasts 45 min. In (C) granules appear after 8.5 min and spherulites after 15 min. The polymers mix in the melt and PLLA has a plasticizing effect on the crystallization of PiPOx. The crystallization of PiPOx drives the polymers to macro-phase separate and PLLA crystallizes eventually. In (D) granules appear after 6.5 min. PLLA cannot crystallize in this sample as it is covalently bound to PiPOx and its migration (macro-phase separation) is prevented.

About the author

Fabian Pooch started his scientific training in Polymer and Colloid Chemistry (Bachelor’s degree) and Polymer Science (Master’s degree) at the University of Bayreuth (Germany). In 2012 he received an Erasmus grant to complete his Master program in the Laboratory of Polymer Chemistry at the University of Helsinki (Finland). Since 2013 he is a PhD student in the same group under the supervision of Prof. Heikki Tenhu and Prof. Francoise Winnik. In this time he also worked as a visiting scientist at the University of Montreal.

His research covers ring-opening polymerization of lactide and 2-alkyl-2-oxazolines, block copolymer synthesis and characterization in solid and dispersed states. The materials are responsive to external triggers and especially designed for use in nanomedical applications. He has mastered a wide selection of soft-matter characterization methods ranging from scattering, microscopy and spectroscopic techniques to thermal analysis of polymers. He presented his results at 9 international conferences and published 4 peer-reviewed articles.

About the author

Kirsi Svedström is currently working as a university researcher at the X-ray Laboratory, Department of Physics, University of Helsinki (UH), Finland. She got her Master’s Degree and PhD in Physics, specialized in Materials Physics, at the UH in 2007 and 2012, respectively. She has worked as a lecturer and researcher at the UH since 2014. Her research focuses on the studies of the structure of natural polymers using x-ray techniques, especially wide- and small- angle x-ray scattering and microtomoghraphy techniques.

She conducts experiments both using advanced in-house x-ray equipment and international synchrotron light sources. She has been involved in materials science studies regarding characterization of e.g. various metal nanoparticles, archaeological wood, lignin, nanocellulose and other novel cellulose based materials, and effects of ionic liquids on multilamellar liposomes. She is currently a PI of a research project focusing on the hierarchical structure of plant cell wall, especially reaction wood cell wall, where the goal is to characterize the structural changes of tree saplings in various environmental conditions.

About the author

Marjolein Sliepen studied chemistry at Maastricht University and obtained her B.Sc in 2014. In 2016, she graduated in polymer chemistry from the University of Helsinki. Currently she is working as a researcher at Akzonobel Paints and Coatings in the Netherlands. Her research interests include functional coatings, stimuli-responsive polymers, and self-assembly.

About the author

Antti Korpi is a Ph.D. student in Biohybrid Materials research group at the Department of Bioproproducts and Biosystems at Aalto University, Finland. He acquired his master’s degree from University of Helsinki, Finland, in 2016 and began his Ph.D. studies the same year. In the master’s studies he specialized in polymer chemistry, especially polymer synthesis and analysis. His other expertise is structural material analysis, especially small angle X-ray scattering (SAXS).

He combines these skills in his Ph.D. studies, as he produces and characterizes self-assembling biohybrid systems based on synthetic polymers and natural proteins or viruses. The focus of the research is mainly in electrostatic self-assembly of crystalline materials for functional systems or scaffolds.

About the author

Françoise M. Winnik was born and educated in France. She obtained her PhD in chemistry from the University of Toronto (Canada). She worked as a research scientist in the Xerox Research Center of Canada, before joining McMaster University in Hamilton (Canada) in 1993 as an Associate Professor and, from 2000 to 2018 the Université de Montréal, Montreal, Canada as a Professor.

She is currently a Professor and Principal investigator in the University of Helsinki, Finland. She is the editor in chief of Langmuir, a journal of the American Chemical Society. Her research interests include self-assembly of amphiphilic polymers, nanoparticles and biointerfaces.

About the author

Heikki Tenhu is a Professor of polymer chemistry at the University of Helsinki, since 1992. His research interests include synthesis and characterization of amphiphilic polymers, responsive polymers, polyelectrolytes and polyelectrolyte complexes, as well as various hybrid nanomaterials. An important part of the research is the investigation on self-assembling processes in aqueous solutions by scattering, spectroscopic and rheological methods.

 

Reference

Fabian Pooch, Marjolein Sliepen, Kirsi J. Svedström, Antti Korpi, Françoise M. Winnik and Heikki Tenhu. Inversion of crystallization rates in miscible block copolymers of poly(lactide)-block-poly(2- isopropyl-2-oxazoline). Polymer Chemistry, 2018, volume 9, page 1848.

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