Biomedical Engineering Reference
In-Depth Information
3.4
Properties
Thermal .
Because crystallinity is an important factor in controlling polymer
erosion, the effect of polymer composition on crystallinity was studied [69, 70].
Almost all polyanhydrides show some degree of crystallinity as manifested by
their crystalline melting points. Polymers based on SA, CPP, CPH, and FA were
particularly investigated. Some of the important physicochemical properties of
P(CPP:SA) and P(FAD:SA) are given in Table 3.2. Homopolyanhydrides of
aromatic and aliphatic diacids, for example, poly(CPP) and poly(FA), were crys-
talline (
50% crystallinity), whereas the copolymers possess a less degree of
crystallinity, which increases by enhancing the mole ratio of either aliphatic or
aromatic diacid monomers [15]. The heat of fusion (
>
H
) values for poly(CPP-
SA) demonstrated a sharp decrease from 36.6 to 2.0 cal/g as CPP is gradually
added up to 40%, while an increase in
Δ
H
value was observed up to 26.5 cal/g
on further addition of CPP [71]. The trend of decreasing crystallinity, as one
monomer is added, was noted using X-ray diffraction or DSC methods. The
decrease in crystallinity is a direct result of the random presence of other units
in the polymer chain. A detailed analysis of the copolymers of SA with the
aromatic and unsaturated monomers, CPP, CPH, FA, and trimellitic-amino acid
derivative, showed that copolymers with high ratios of SA and CPP, TMA-gly,
or CPH were crystalline while copolymers with equal ratios of SA and CPP or
CPH were amorphous [69]. In contrasts, the poly(FA-SA) series displayed high
crystallinity regardless of comonomer ratio [72]. Aliphatic polyanhydrides gener-
ally melt at lower temperatures than do aromatic polyanhydrides. The melting
point of aromatic-aliphatic copolymers is proportional to the aromatic content
in the copolymer. Introduction of fatty acids in copolymers also lowers the
melting point of the bulk polymer. Thermal properties along with the molecular
weight of representative fatty acid-based polyanhydrides are given in Table 3.3.
Δ
Inclusion of an aromatic amide linkage in the backbone is found to increase
the transition temperatures. The formation of intermolecular hydrogen bonds
is believed to cause this high crystallinity. Polyanhydrides-
co
- amide also have
high thermal stability [39] .
Mechanical.
Polyanhydrides show poor mechanical properties in comparison to
other polymers such as polyesters. Mechanical properties of various polyanhy-
drides and their copolyanhydrides were tested as transparent and fl exible fi lms
made by melt compression and solvent casting. It was observed that increasing
the CPP content in copolymer composition increases the tensile strength as well
as elongation of various polyanhydrides tested [7]. Despite the low molecular
weight (
M
n
6400) of poly(CPP -SA) (60:40), it has a higher tensile strength of
981 MPa (100 kgf/cm
2
) than it has in the 20:80 composition (
M
n
=
18,900),
441 MPa (45 kgf/cm
2
). Decreasing the
M
n
of fi lms of the same CPP content
(60%) from 12,100 to 6400 results in lower tensile strength. The elongation at
break of these fi lms ranges from 17% to 23%. Table 3.2 shows the mechanical
=
