New Trends in Polymers - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 186

phosphate buffer (pH = 7.4) over a period of 58 days. The M n value decreased from
768 to 245 kDa for 48 days. The ¿ lm thickness increased from 65 to 75 —m for 48
days, suggesting that water permeated the polymer matrix during the hydrolytic
degradation. Examination of the surface and cross section of PHB ¿ lms before and
after hydrolysis showed that surface after 48 days of hydrolysis was apparently
unchanged, while the cross section of the ¿ lm exhibited a more porous structure
(pore size<0.5 —m). It was shown also that the rate of hydrolytic degradation is
not dependent upon the crystallinity of PHB ¿ lm. The observed data indicates that
the non-enzymatic hydrolysis of PHB in the aqueous media proceeds via a random
bulk hydrolysis of ester bonds in the polymer chain ¿ lms and occurs throughout
the whole ¿ lm, since water permeates the polymer matrix [20, 21]. Moreover, as
the molecular weight distribution was unimodal over the whole degradation time
which, together with the observed ¿ rst order kinetics, indicates a random chain
scission both in the crystalline and the amorphous regions of PHB [11, 29]. For
synthetic amorphous atactic PHB it was shown that the hydrolysis of PHB is the
two step process. First, the random chain scission proceeds. The scission accompa-
nies by a molecular weight decrease. Then, at a molecular weight of about 10,000,
mass loss begins [23].
The analysis of literature data shows a great spread in values of rate of PHB hy-
drolytic degradation in vitro . It can be explained by different thickness of PHB ¿ lms
or geometry of PHB devices used for experiment as well as by different sources, pu-
rity degree and molecular weight of PHB (Table 1). At 37°C and pH = 7.4 the weight
loss of PHB (unknown M w ) ¿ lms (500 —m thick) was 3% after 40 days incubation
[32], 0% after 52 weeks (364 days) and after 2 years (730 days) incubation (640
kDa PHB, 100 —m ¿ lms) [11, 12], 0% after 150 days incubation (650 kDa PHB, 50
—m ¿ lm) [20], 7.5% after 50 days incubation (279 kDa PHB, unknown thickness of
¿ lms) [31],, 0% after 3 months (84 days) incubation (450 kDa PHB, 40 —m ¿ lms),
12% after 3 months (84 days) incubation (150 kDa PHB, 40 —m ¿ lms) [24, 25], 0%
after 180 days incubation of mono¿ lament threads (30 —m in diameter) from PHB
(470 kDa) [17, 18]. The molecular weight of PHB dropped to 36% of the initial
values after two years (730 days) of storage in buffer solution [12], to 87% of the
initial values after 98 days [32], and 58% of the initial values after 84 days [24, 25]
(Table 1).
At acidic or alkaline aqueous media PHB degrades more rapidly: 0% after 20
weeks (140 days) incubation in 0.01 NaOH (pH = 11) (200 kDa PHB, 100 —m ¿ lms)
with surface changing [33], 0% after 180 days incubation of PHB threads in phosphate
buffer (pH = 5.2 and 5.9) [18], complete PHB ¿ lms biodegradation after 19 days (pH =
13) and 28 days (pH = 10) [31]. It was demonstrated that after 20 weeks of exposure to
NaOH solution, the surfaces of PHB samples became rougher, along with an increased
density of whole formation on their surfaces. From these results, it can be surmised
that the non-enzymatic degradation of PHAs progresses on their surfaces before no-
ticeable weight loss occurs (Figure 1) [33].