New Aspect of Poly(3-hydroxybutyrate) Pyrolysis
Mechanisms（ 九 工 大 院 ・ 生 命 体 、 University Putra Malaysia）（ 学 ） ○ Hidayah Ariffin（ 九 工 大 ETC）（ 正 ） 西 田 治 男 ＊ 、（ 九 工 大 院 ・ 生 命 体 ） 白 井 義 人（ University Putra Malaysia） Mohd Ali Hassan
Abstract
The thermal degradation behavior of poly (3-hydroxybutyrate), PHB was studied
kinetically and by characterization of the PHB pyrolyzates using 1 H/ 1 3 C NMR, FTIR, and FAB-MS.
From the kinetics analysis of weight loss behavior, it is proposed that there are some kineticallyfavored scissions occurring at the chain ends, which were assumed to be unzipping reactions
occurring at ester groups neighboring the crotonate end groups. Newly proposed thermal degradation
pathways of PHB bring new aspect to the PHB pyrolysis mechanisms.
Introduction
Random chain scission by β-elimination has been considered as the dominantmechanism in PHB thermal degradation based on typical structures of pyrolysis products, i.e. crotonic
acid and oligomers with a crotonate end-group. Recently, Lehrle et al. 1, 2 reported that the analyticalresults of partial pyrolysis of PHB showed some deviations from the theoretical predictions of randomscission statistics due to reactions induced by the end groups. However, a clear confirmation has not
yet been reported. In this paper, we propose an expected thermal degradation pathway of PHB basedon the kinetics analysis of weight loss behavior.
Materials and Methods
Dynamic pyrolysis of PHB film samples was conducted using TG andPy-GC/MS. In the TG analysis, the sample was heated at prescribed multiple heating rates of 1, 3, 5, 7
and 9 ºC min -1 in the range of 60–400 ºC under a steady N 2 flow of 100 mL min -1 . For pyrolysis inPy-GC/MS, the sample was heated from 40 ºC to a prescribed temperature at a heating rate of 9 ºC
min - 1 . Isothermal pyrolysis was took place in a glass tube oven at 260ºC for 30 min. Pyrolyzatesobtained were analyzed using 1 H and 1 3 C NMR, FTIR and FAB-MS.
Results and Discussion
PHB pyrolysis at multiple heating rates.
TGcurves of PHB film degradation, which werenormalized in accordance with sample weight =1, are illustrated in Figure 1. All the TG curves
showed a smooth decomposition from thebeginning until completion, with TG traces
shifted up the temperature scale by an increasein the heating rate. Calculation of activationenergy E a value showed that thermal degradationof PHB has a constant E a value in the range of
117 ± 3 kJ mol - 1 . This suggests that the weightloss over the whole period resulted mainly fromone main reaction.

Figure 1. TG curves of PHB degradation.

Composition of pyrolyzates.
Isothermal degradation of PHB in a glass tube oven was conducted at260 ºC in vacuo to determine the components of distilled pyrolyzates. Pyrolysis products werecharacterized and categorized as major and minor products. Major pyrolysis product was seen ascrystals, whereas the minor product was an oily liquid. From 1 H NMR analysis, it was found that themajor component was trans-crotonic acid, which
contributed to 67.7 % of the total PHB pyrolyzates. Minor
components were cis-crotonic acid and its derivatives(3.1 %) and oligomers (29.2 %) having crotonyl end groups.
13C NMR spectrum of the minor component showed at least
four kinds of carbonyl carbons detected at 175.5, 170, 166,
161 ppm (Figure 2), which were assigned as carbonyl
carbons of 3HB units (a) 3, 4 , chain-end carboxyl (a’) andcrotonyl (a”, a”’) groups. As one minor component having
the crotonyl end group, crotonic anhydride was assigned tothe signal at around 161 ppm. This was evident by the 13 C
NMR and FTIR analyses.
Kinetics analysis of PHB pyrolysis.
Plots in Fig. 3
show the integral simulation analysis of the observed
TG data (9 ºC min - 1 ) and model reaction plots withparameter values: E a : 118 kJ mol -1 and A: 1.18 x 10 9s - 1 . Interestingly, the observed plot gradually
decreased in weight similar to the random
degradation process in the initial period, andthereafter the weight loss curve showed a linear
relationship between w and Aθ in parallel to the0th-order simulation plot 5 .

Figure 2. 125MHz of 1 3 C NMR
spectrum of PHB pyrolyzates.

Figure 3. Plots of w vs Aθ for TG data ofPHB at heating rate 9ºC min - 1PHBPHBOOOO
Expected thermal degradation pathways of PHB.
OHOO
HOHOO nOCHOO-H O
Based on the above results, it was considered thatOO
β-elimination
β-elimination
α-deprotonation
there are some kinetically favored scissions occurringauto-acceleratedOO
O
trans-esterificationnear the ends of the molecules, which proceeded by aHOOHOCHO
n-1-H Oβ-elimination
OCH
0th-order weight loss process in the middle stage.
unzippingOOOOO
β-elimination
This reaction is identified as unzipping reaction
OOHOO lmoligomeric anhydrides (l=0-1)
oligomers
occurring at ester groups neighboring the crotonateOβ-elimination
unzippingOH
β-elimination
end groups.
crotonic acid
Apart of the initial random degradation and
Scheme 1. Expected thermal degradation
following unzipping reaction, crotonic anhydride andpathways of PHB
its oligomers were detected as minor products fromcondensation reactions between carboxyl groups. The anhydrides production must be one out ofconforming to the random degradation statistics and contribute to the complexity of the PHB pyrolysis.
By considering all these, expected thermal degradation pathways of PHB are proposed in Scheme 1.
-22References
1) Lehrle, R. S.; Williams, R. J. Macromolecules, 1994, 27, 3782.
2) Lehrle, R.; Williams, R.; French, C.; Hammond, T. Macromolecules, 1995, 28, 4408.
3) Abe, H.; Doi, Y.; Hori, Y.; Hagiwara, T. Polymer, 1997, 39, 59.
4) Barnard, G. N.; Sandars, J. K. M. J. Biol. Chem., 1989, 264, 3286.
5) Ariffin, H., Nishida, H., Shirai, Y., Hassan, M. A. Polym. Degrad. Stab., 2008,doi:10.1016/j.polymdegradstab.2008.05.020.
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