Biomedical Engineering Reference
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stage is characterized by the preferential attack of the ester linkages in the more
accessible amorphous regions, while the second stage is characterized by the
attack of the less accessible crystalline regions (Li
1999
; Weir et al.
2004
). The
degradation mechanism of PLGA polymers was extensively studied by Li (
2006
).
It has been reported that the cleavage of an ester bond yields a carboxyl end
group and a hydroxyl one and thus formed carboxyl end groups are able to cat-
alyze the hydrolysis of other ester bonds (Li
2006
). This phenomenon is called
autocatalysis.
R
−
COO
−
R
′
+
H
2
O
−→ [ −
COOH
]
R
−
COOH + HO
−
R
′
The kinetics of the autocatalyzed hydrolytic degradation was given as follows:
D
[
COOH
]
DT
D
[
E
]
DT
(2.1)
=−
=
K
[
E
]
•
[
H
2
O
]
•
[
COOH
]
where [COOH], [H
2
O] and [E] represent carboxyl end group, water and ester
concentrations in the polymer matrix and k is the rate constant. The following
relationship can be obtained after a series of integrations and simplifications:
M
N
M
NO
=−
K
′
T
(2.2)
ln
In this equation,
M
N
the number average molecular weight at time t
M
NO
the initial number average molecular weight
K
′
rate constant
t
degradation time
According to this equation, semilog plots of
M
N
versus degradation time should
be linear prior to the onset of weight loss. Different researchers had observed the
uncatalysed and autocatalysed degradation of semicrystalline PLLA The number
average molecular weight
M
N
was used for the modeling of these two mecha-
nisms. The unanalyzed model was proposed by Chu (
1995
) and Anderson (
1995
).
The kinetic relationship which does not account for the possibility of autocatalysis
accelerating the polymer degradation rate is as follows:
1
M
NT
1
M
NO
+
K
′
T
=
(2.3)
The second model, the autocatalyzed model was proposed by Pitt and Zhong-
Wei (
1987
). A relationship based on the kinetics of the ester-hydrolysis reaction
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