Nanoscale lamellar assembly and segregation mechanism of poly(3-hydroxybutyrate)/poly(ethylene glycol) blends

Characterization of polymeric structures is an important step in the identification of different polymeric materials. The currently used characterization techniques exhibit low spatial resolution that prevents full exploitation of the polymeric materials. To this end, researchers have been looking for more advanced alternative characterization methods. This includes incorporation of a microscope with a high-resolution capability to explore the phase separation and miscibility of the polymer blends.

Among the newly developed techniques, atomic force microscopy coupled with infrared spectroscopy has been effectively used in polymers, polymer blends, multilayer films, and composite materials. However, it produces infrared spectra similar to the initially used technique in terms of band shapes and peak positions, meaning the database can as well be used in chemical analysis and materials identification.

Recently published research has shown that understanding the structure and growth mechanism of the spherulites and polymer diffusion blends is of great significance in controlling the properties of crystallizable materials, especially for miscible blends. Alternatively, the morphology and crystallization mechanism of the blends is largely affected by the bend ratios and polymer melting points. For instance, the low melting temperature can lead to simultaneous crystallization of the polymers where the spherulites of one component can develop inside the spherulites of the other component or between the lamellae of the other. Consequently, at large melting temperature, crystallization of the polymer with the high melting point is given high priority. The crystallization process is generally a two-step process and, therefore, the polymer with the low melting point is trapped between the lamellae of that with a high melting point. However, despite the advances in the microscopy and X-ray techniques, characterization of polymers in miscible blends especially at a sub-micrometer level is a great challenge.

To this note, Professor Phuong Nguyen Tri at Université du Québec à Trois-Rivieres and Professor Robert Prud’homme at University of Montreal cross-examined the feasibility of using atomic force microscopy coupled with infrared spectroscopy combined with scanned electron microscopy for investigating the crystallization behavior and the segregation mechanism of the crystalline blends of poly(3-hydroxybutyrate) and poly(ethylene glycol) at nanoscale levels. The polymer blends were isothermally crystallized at different temperatures followed by a quenching-etching procedure. The main objective was to explore the three-dimensional interior spherulitic structure and lamellar assembly of the blend. The work is published in the journal, Macromolecules.

The authors observed concentrically banded spherulites at high melting points while both con-like and banded spherulites were observed at low melting temperature due to the perpendicular lamellae orientation of the banded spherulites. Additionally, lamellae twisting occurring clockwise and exhibiting helical twisting mechanism were mainly observed at high melting temperatures. However, no twisting of the lamellae spherulites was reported at low melting temperature due to the presence of nanoscale needle-like crystals.

In summary, Canadian researchers are the first to observed polymer distribution at the nanoscale level during the crystallization of the miscible poly(3-hydroxybutyrate) and poly(ethylene glycol) blend. Specifically, a two-step crystallization process was demonstrated whereby, crystallization of the component with high melting temperature took place first at the ridge banded spherulites regions while crystallization of the component with the low melting temperature occurred latter in the valley regions. Furthermore, identification of the chemical nature of the individual components in the blends was possible at the nanoscale level. In general, the authors provided new insight regarding the polymer distribution and segregation mechanism in miscible crystalline blends. Further improvement of the atomic force microscopy coupled infrared spectroscopy technique will advance nanoscale polymer crystallization.