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.
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.Go To Polymer Chemistry