Development of ring-banded spherulitic morphologies and formation of radially oriented nano-pores in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) during crystallization in CO2

• Crystallization of PHVB films in CO2 up to 24 MPa is reported.
• Ring-banded spherulite formation is examined using polarized optical microscopy.
• Nano-porous structure and its directional nature is explored with scanning electron microscopy.

The melting temperature depression, crystallization, and morphological modifications of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) in CO2 have been investigated in a variable-volume view-cell that permits explorations with thin polymer films. Melting temperatures (Tm) were determined at pressures up to 35 MPa and were found to decrease at a rate of about 1.3 °C/MPa up to about 24 MPa and then leveling off with an overall melting temperature reduction of about 34 °C. Thin films were melt-crystallized in air at ambient pressure at different temperatures (60, 70, 80, 90, 100, 110, 120 and 130 °C) and also in CO2 at 90 °C at different pressures up to 24 MPa in the view-cell. They were analyzed using polarized optical microscopy (POM), and scanning electron microscopy (SEM) for morphological features. The films that were crystallized in air in the temperature range from 70 to 110 °C, and those crystallized at 90 °C in CO2 were all found to show distinct ring-banded spherulitic morphologies reflecting the development of flat-on, and edge-on lamellae domains during crystallization. The band-width of the edge-on lamella domains were found to increase with CO2 pressure, and further, observed to develop increasing level of mottling with pressure which were associated with the formation of radially oriented nano-pores. The formation of these nano-pores is interpreted as arising from the exclusion of CO2 from the crystal growth front and its accumulation in the inter-crystalline amorphous regions in between the bundles of the lamellar stacks.

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