Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 6 – Fluid uptake as instructive factor
the polymer phase in composites considering the elevated temperature factor.
Polylactide(s) are known to hydrolytically degrade [349, 387] and various linear models
describing the relationship between molecular weight changes and temperature have
been proposed. They all are based on the fact that the number average molecular
weight (Mn) is correlated with the cleavage of the polymer chains. A generally
proposed linear model is [397]
(1 / Mnt) = (1 / Mn0) + k · t
where Mnt is the number average molecular weight at the time point t, Mn0 is the
number average molecular weight before hydrolytic degradation and k is the
hydrolytic degradation rate constant. However, the above described model does not
take into account the hydrolytic autocatalysis typical of polyesters, [349, 387] thus another
model has been proposed. [398] This one is based on the kinetics of ester hydrolysis
reaction and included the autocatalysis provoked by the carboxylic acid end residuals.
This model is described as
Mnt = Mn0 · e –(k · t) (1)
where k is the catalyzed hydrolytic degradation rate constant. If this relationship
holds true, a linear relationship between ln(Mnt) versus time should exist until the
complete mass loss. However, since in our studies it is not possible to use GPC to
estimate Mn, we can use the intrinsic viscosity (measured with Ubbelohde
viscometer) to estimate the catalyzed hydrolytic degradation rate constant. Based on
Mark–Houwink equation, intrinsic viscosity is related to the viscosity average
molecular weight Mv, which is close to the weight average molecular weight Mw. In
turn, Mw is usually proportional to Mn and thus, by assuming that Mv is also
proportional to Mn, equation (1) can be rewritten as
= · e –(k · t)
from which
ln( / ) = –k · t
where and are the intrinsic viscosities at the time point t and before
degradation respectively. Linear regression of the plot ln( / ) versus time t
will indicate the catalyzed hydrolytic degradation rate constant k. Large absolute
values of k indicate that (catalysed) hydrolysis of the polymer is faster.
6.2.5.2. Experimental settings
A phosphate buffered saline (PBS; Invitrogen, Darmstadt, Germany) solution having
initial pH of 7.5±0.05 was used. Sterile granules of each composite (75±0.5 mg) were
weighed before use (m0) and soaked in 30 mL PBS at 55±0.5°C for five weeks. The
temperature was set referring to the polymers Tg reported in Table 3. For each of the
three time points considered, i.e. 1, 3 and 5 weeks, three samples of each material
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