Observation of Intermolecular Hydrogen Bonding Interactions in Biosynthesized and Biodegradable Poly [(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] in Chloroform and 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP)

Abstract

In this work, we describe polymer-solvent interactions in biosynthesized and biodegradable poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] (PHBHx) and the atactic homopolymer, poly(3-hydroxybutyrate) (a-PHB), which were studied both as neat polymers and in solutions of chloroform and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Specifically, infrared frequency shifts of the carbonyl band were observed in semi-crystalline PHBHx, but not in a-PHB, because it cannot form the helical conformation required for crystallization. The carbonyl band of PHBHx exhibited the high frequency associated with amorphous structure in chloroform and the lower frequency traditionally attributed to the helical crystalline structure in HFIP. The same results were obtained for a-PHB, demonstrating that the helical structure is not required for a lower frequency carbonyl-stretching mode. It is proposed that the band shift is due to hydrogen bonding between the carbonyl and hydroxyl hydrogen in HFIP. Therefore, the carbonyl frequency observed upon crystallization is most likely due to hydrogen bonding between the carbonyl and methyl hydrogen of the neighboring polymer chain in the crystal lattice as previously suggested.

Intermolecular Hydrogen Bonding between Poly[(R)-3-hydroxybutyrate] (PHB) and Pseudoboehmite and Its Effect on Crystallization of PHB

“…It is noted that the 1736 cm −1 frequency is lower than the typical "free" amorphous carbonyls of PHB in the bulk, which appear near 1740 cm −1 . 25,26 This 1736 cm −1 peak indicates that those unbonded carbonyls are located in a somewhat constrained environment. PHB has been also found to form H-bond inter with other components, and a variety of peak frequencies for the H-bond inter have been reported.…”

Two-Dimensional Correlation Spectroscopy (2D-COS) Studies of Solution Mixtures in the Low Frequency Raman Region

“…Interestingly, a shift of the carbonyl group was also found for the chemically synthesized atactic homopolymer, poly(3-hydroxybutyrate) (aPHB) in the HFIP solution, which cannot form crystals. 19 Thus, the shift of the carbonyl vibration of PHBHx in HFIP did not result from PHBHx crystal growth in the HFIP. We therefore proposed that this shift was due to the hydrogen bond formed between the hydroxyl group of HFIP interacting with the oxygen in the carbonyl.…”

Studies on Chemical IR Images of Poly(hydroxybutyrate–co–hydroxyhexanoate)/Poly(ethylene glycol) Blends and Two-Dimensional Correlation Spectroscopy

“…The crystalline C=O stretching band at 1722 (or 1725) cm −1 decreases with increasing temperature, while the amorphous C=O stretching band at 1735 cm −1 increases with temperature. The shift of carbonyl stretching vibration from 1722 cm −1 to a higher wavenumber around 1735 cm −1 for the amorphous component is a reflection of the reduced hydrogen bond interactions compared to the crystalline phase [41,42]. In the temperature-dependent FTIR spectra of parts A and B in PHBHx/PEG = 70/30 blend films (as shown in Figure 4A,B), the tendency of intensity changes of two bands at 1722 and 1735 cm −1 were similar, even though they had different absorbance values.…”