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# FT-Raman investigation of biodegradable polymers: Poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) - ScienceDirect

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Vibrational Spectroscopy
Volume 54, Issue 2, 18 November 2010, Pages 127-132
FT-Raman investigation of biodegradable polymers: Poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)
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Celly M.S. Izumi
, Marcia L.A. Temperini
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https://doi.org/10.1016/j.vibspec.2010.07.011
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Abstract
This work presents a FT-Raman study (λ0 = 1064 nm) of naturally occurring polyester poly[(R)-3-hydroxybutyrate] (PHB) and its copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) with 5, 8 and 12 mol % of HV (hydroxyvalerate). The FT-Raman spectra of films indicate that full width at half height of the band centered at 1725 cm−1 and relative intensity of bands at 1443 and 1458 cm−1 can be use to estimate the crystalline degree in film samples. The similarity between Raman spectra of molten PHB and PHBV and theirs CDCl3 solutions suggested that molten polymers present similar conformation than polymers in solution. Raman data of these samples showed that bands at 1220, 1402, 1725, 2998 and 3009 cm−1 are due to crystalline helical structure and the bands at 1453, 1740, 2881, 2938 and 2990 cm−1 are originated from disordered domains. It is shown that composition of PHBV samples can be estimated by analyzing the ratio of the intensity of the bands at 2938 cm−1 (νC–H) and 1740 cm−1 (νCO) in the spectra of solutions and of bands at 1354 (wCH2) and 1740 cm−1 (νCO) in spectra of molten polymers.
Introduction
In the last decades, there is a growing interest for biodegradable polymers as a solution to problems concerning the solid waste management [1], [2]. Poly[(R)-3-hydroxybutyrate], PHB, and its copolymer poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate], PHBV (Fig. 1) are synthesized by a wide variety of bacteria from renewable carbon sources such as sugar and vegetal oil [3], [4]. One of the main reasons for the great interest on polyhydroxyalkanoates is their biodegradability in landfill, compost, and aquatic systems due to degrading enzymes excreted by a number of bacteria and fungi in the environment. Besides the environmental factors, the microstructure and properties of these polymers can significantly affect the degradation rates. This includes factors such as composition, crystallinity, additives, and surface area [5], [6]. PHB is a thermoplastic (Tm ∼ 175 °C) and highly crystalline polymer that is stiff, brittle and thermally unstable during processing [4].
In order to improve PHB mechanical and thermal properties other monomers such as hydroxyvalerate (HV) are copolymerized with PHB. PHBV has much improved mechanical properties; the polymer becomes more flexible and tougher as the HV fraction increases. Also, the melting temperature of PHBV decreases with increasing of HV content with no changing in the degradation temperature [1], [4].
Bacterial PHB and PHBV have the advantage of being optically pure because bacteria produce just one enantiomer. Due to this high purity, PHB has been used as a ideal model system for studies in morphology, crystallization, and nucleation of polymers [7]. The size of crystalline domains in PHB and PHBV polymers determined via 1H spin-diffusion NMR measurements was in the range of 76–11 nm [8].
Furthermore, PHB presents other interesting properties such as piezoelectric and nonlinear optical response [7], [9]. Consequently this polymer is promising for applications as materials for acoustooptical deflectors and modulators [10].
Raman spectroscopy allows characterizing polymer chemical structure, stereochemistry, morphology and crystallinity that are essential to understand and optimize the polymer properties. Fourier transform Raman (FT-Raman) spectroscopy is a powerful technique for characterizing polymers samples; although the majority of polymers are not intrinsically fluorescent, when irradiated with a visible laser, the spectra show an intense level of fluorescence overcoming the Raman signal. Raman excitation in the near-IR region commonly prevents electronic absorptions; as a consequence fluorescence is nearly eliminated [11].
Fourier transform infrared (FTIR) spectroscopy has been used for studying structure, crystallinity and composition of PHB and PHBV films or in solution [12], [13]. The melting behavior of PHB and PHBV was investigated using two-dimensional infrared correlation spectroscopy demonstrating that during phase transition spectral changes take place in three main spectral regions, the νC–H, νCO and νC–O–C band regions [14], [15], [16]. The surface crystallinity of PHB and PHBV samples prepared under different processing conditions was analyzed using attenuated total reflection infrared spectroscopy. The crystallinity degree measurement was performed using the intensity ratio between the bands at 1230 and 1453 cm−1 since the former band is characteristic of crystalline polymer and the latter one is independent of crystallinity [17]. Using FTIR and quantum chemistry calculations it was demonstrated that crystalline PHB present C–H⋯O hydrogen bonds between CO group in one helical structure and CH3 group in other helical structure [18], [19].
Although PHB and PHBV have been extensively studied using FTIR, there are few reports concerning the Raman characterization of PHB. Raman spectrum of PHB at low frequency region was reported investigating the longitudinal acoustic mode of this polymer [20]. Also, Raman spectroscopy was employed to probe the dispersibility and crystallinity of PHB/poly(l-lactic acid) blends [21] and to study the formation of a β-structure in PHB drawn films [22].
The present work aims to investigate the FT-Raman spectra of PHB and PHBV in solid, solution and molten states. The vibrational bands characteristic of amorphous and of helical crystalline structure was identified by analyzing FT-Raman spectra of solid and molten PHB and PHBV samples. The examination of these spectra also pointed vibrational bands sensitive to PHBV composition showing that FT-Raman has a potential for determining the crystallinity degree and composition of PHB and PHBV samples.
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Experimental
Poly[(R)-3-hydroxybutyrate] (PHB), poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) with different % mol of HV units were purchased from Aldrich and used as received. The composition of PHBV copolymers was determined using the 1H NMR spectra in CDCl3 solution as reported by Bloembergen et al. [12]. Briefly, the compositions were determined from the areas of the methyl resonances in the side groups of HB and HV repeating units at 1.26 and 0.88 ppm, respectively. PHBV samples
Results and discussion
FT-Raman spectra of poly[(R)-3-hydroxybutyrate] (PHB) and of copolymers poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) with 5, 8 and 12 mol% of HV units are shown in Fig. 1. The Raman spectra of PHBV copolymers are quite similar to the spectrum of PHB homopolymer. This can be explained due to the striking resemblance between structure of comonomers HB and HV. In addition, these polymers are isodimorphic and in this composition range PHB crystalline phase is accommodating the other
Conclusion
FT-Raman is a suitable technique to characterize bacterial biodegradable polymers. The FT-Raman spectra of PHB and PHBV with 5, 8 and 12 mol% of HV units in film, solution and molten state were investigated. The FT-Raman of films showed that relative intensity of bands at 1443 and 1458 cm−1 and the full width at half height of the band centered at 1725 cm−1 can be used to estimate the crystalline degree of film samples. The FT-Raman as a function of temperature showed that changing of
Acknowledgements
This work was supported by FAPESP and CNPq (Brazilian Agencies). Fellowships from FAPESP (CMS Izumi 06/55687-7) and CNPq (MLA Temperini) are also gratefully acknowledged.
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