Chemistry Reference
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stable, the crystallinity degree is constant as well and even it may grow up
due to additional crystallization. On the second stage of hydrolysis, when
the MW of intermediates attain the “critical” value, which is equal about
30 kDa, these intermediates can dissolve and diffuse from the polymer
into buffer. Within this period the weight loss is clearly observed. The
intensity of hydrolysis characterized by the weight loss and the MW dec-
rement is enhanced in the series PHBV < PHB < PHB-PLA blend < PLA.
The growth of initial MW (a terminal group reducing) impacts on the
hydrolysis stability probably due to the increase of crystallite perfection
and crystallinity degree. The XRD data reflect this trend (see Fig. 5.3b).
Moreover, the surface state of PHB films explored by AFM technique de-
pends on the condition of film preparation. After cast processing, there is a
great difference in morphologies of PHB film surfaces exposed to air and
to glass plate. It is well known that the mechanism of hydrolysis could in-
clude two consecutive processes: (i) volume degradation; and (ii) surface
degradation. Under essential pore formation (in the surface layer exposed
to air) the volume mechanism prevails. The smooth surface of PHB film
contacted during preparation with the glass plate is degraded much in-
tensely than the opposite rough surface (Fig. 5.4).
In conclusion, we have revealed that the biopolymer MW determines
the form of a hydrolysis profile (see Fig. 5.1). For acceleration of this pro-
cess we have to use the small MW values of PHB. In this case we affect
both the degradation rate and the crystalline degree (Fig 5.3b). By con-
trast, for prolongation of service-time in a living system it is preferable to
use the high-MW PHB that is the most stable polymer against hydrolytic
degradation.
KEYWORDS
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Biopolymer
•
Crystalline degree
•
Degradation rate
•
Hydrolysis profile
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Hydrolytic degradation
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Polyblend
