Influence of polyethylene glycol (PEG) chain length on the thermal behavior of spin-coated thin films of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)/PEG blends

• 2D correlation analysis of the temperature-dependent IRRAS spectra of thin films of PHBHx and PHBHx/PEG blends.
• Effect of PEG chain length on the miscibility and thermal properties of PHBHx/PEG blends.
• Possible existence of two different amorphous states in the 70/30 PHBHx/PEG 400 blend.

The influence of the polyethylene glycol (PEG, Mn = 400, 1500, and 3400) chain length on the miscibility and thermal properties of spin-coated films of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx, HHx = 6.9 mol %)/PEG blends was elucidated by using differential scanning calorimetry (DSC) and temperature-dependent infrared (IR) spectroscopy. To extract more detailed information about the spectral variations induced by the temperature, 2D correlation spectroscopy was applied to the temperature-dependent IR spectra of PHBHx/PEG blends. It was found that PEG 400 was completely miscible with PHBHx while PEG 1500 and 3400 were only partially miscible, reflecting that PHBHx/PEG miscibility decreased with the increasing molecular weight of PEG. The amorphous band of carbonyl group of 70/30 PHBHx/PEG 400 blend is resolved into two bands at 1744 and 1754 cm−1 in the asynchronous spectrum, which is not observed in the corresponding asynchronous 2D correlation spectra of PHBHx and its blend with PEG 1500 and 3400. This observation suggests that we captured the possible existence of two different types of amorphous state in 70/30 PHBHx/PEG 400 blend and the band at 1744 cm−1 is related to the amorphous mixture of PHBHx and PEG 400. Furthermore, 2D correlation analysis and the normalized peak height trends demonstrate that PEG 400 disrupts the crystalline structure of PHBHx, indicating low molecular weight PEG 400 has a clear effect on the thermal properties of PHBHx as well as depressing its melting temperature.

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