Biomedical Engineering Reference
In-Depth Information
properties of fatty acid-based polyanhydrides. Films of fatty acid polyanhydrides
were transparent and fl exible with a tensile strength of 4-19 MPa and elongation
at break in the range of 77-115%. The terpolymer of (fatty acid trimer [FAT]-
CPP -SA] in a 1:1:1 weight ratio formed the strongest fi lm. The polymer had a
tensile strength of 2.5-3.2 MPa and yield stress at break of around 20% in com-
parison to poly(EAD-SA) (1:1) and PSA, which had tensile strengths of 5.7 and
7.2 MPa and yield stress at break of 10% and 1.5%, respectively. Thus, introduc-
tion of nonlinear fatty acid structures in polyanhydrides provides hydrophobicity
and fl exibility to the polymers.
Stability .
The stability of polyanhydrides in solid state and dry chloroform solution
was studied [86]. Aromatic polymers such as poly(CPP) and poly(1,1-bis[
p
-
carboxyphenoxy] methane) maintained their original molecular weight for at
least 1 year in the solid state. In contrast, aliphatic polyanhydrides, such as PSA,
showed decreased molecular weight over time. The decrease in molecular
weight shows a fi rst-order kinetics, with activation energies of 7.5 kcal/(mol K).
The decrease in molecular weight was explained by an internal anhydride inter-
change mechanism, as revealed from elemental and spectral analyses. This
mechanism was supported by the fact that the decrease in molecular weight was
reversible and heating the depolymerized polymer at 180 ° C for 20 min yielded
the original high molecular weight polymers. However, under similar condi-
tions, the hydrolyzed polymer did not increase in molecular weight [86]. In many
cases, it was observed that the stability of polymers in the solid state or in organic
solution did not correlate with its hydrolytic stability [86]. A similar decrease in
molecular weight as function of time was also observed among the aliphatic-
aromatic copolyanhydrides and imide-containing polyanhydrides [6, 87]. Gamma-
irradiation technique is typically used to sterilize polyanhydrides [88]. Aliphatic
and aromatic homo- and copolymers were irradiated at 2.5 Mrad dose and the
change in properties was monitored before and after irradiation. Properties such
as molecular weight, melting temperature, and heat of fusion remained the
same, and
1
HNMRand FTIR spectra of the polymer were also similar before
and after irradiation [34, 89]. Using the same concept, these studies were
extended for saturated and unsaturated polyanhydrides [90]. RA-based copoly-
mers with SA and poly(CPP:SA) were irradiated under dry ice and at room
temperature, while poly(FA:SA) was irradiated only at room temperature. Satu-
rated polyanhydrides are stable enough during irradiation; however, the pres-
ence of double bonds conjugated to an anhydride bond creates an unstable
structure and leads to the formation of free radicals [90]. These free radical
polyanhydrides degrade into less conjugated polyanhydrides. The outcome of
this process is self-depolymerization via inter- and/or intramolecular anhydride
interchange to form polymers with lowered molecular weight. In general, poly-
mers with high melting points and crystallinity give the highest yield of room
temperature observable radicals. These endogenous free radicals were used to
study processes of water penetration and polymer degradation
in vivo
[88] . The
detection of gamma- sterilization - induced free radicals
in vivo
using EPR could
