Biomedical Engineering Reference
In-Depth Information
them, polyglycolide or polyglycolic acid (PGA) is the most important one because most other biode-
gradable polymers are derived from PGA either through copolymerization, for example, poly(glycolide-
l-lactide) copolymer or through modified glycolide monomer, for example, PDS.
5.2.1 Glycolide-Based Biodegradable Homopolymer Polyesters
PGA can be polymerized either directly or indirectly from glycolic acid. The direct polycondensation
produces a polymer of
M
n
less than 10,000 because of the requirement of a very high degree of dehydra-
tion (99.28% up) and the absence of monofunctional impurities. For PGA of molecular weight higher
than 10,000, it is necessary to proceed through the ring-opening polymerization of the cyclic dimers of
glycolic acid. Numerous catalysts are available for this ring-opening polymerization.
They include organometallic compounds and Lewis acids (Chujo et al., 1967a; Wise et al., 1979).
For biomedical applications, stannous chloride dihydrate or trialkyl aluminum are preferred. PGA
was found to exhibit an orthorhombic unit cell with dimensions
a
= 5.22 Å,
b
= 6.19 Å, and
c
(fiber
a x is) = 7.02 Å (Chujo et al., 1967b). The planar zigzag-chain molecules form a sheet structure parallel to
the ac-plane and do not have the polyethylene-type arrangement (Chatani et al., 1968). The molecules
between two adjacent sheets orient in opposite directions. The tight molecular packing and the close
approach of the ester groups might stabilize the crystal lattice and contribute to the high melting point,
T
m
, of PGA (224−230°C). The glass transition temperature,
T
g
, ranges from 36 to 40°C. The specific
gravities of PGA are 1.707 for a perfect crystal and 1.50 in a completely amorphous state (Chujo et al.,
1967a). The heat of fusion of 100% crystallized PGA is reported to be 12 kJ/mol (45.7 cal/g) (Brandrup
and Immergut, 1975). A study of injection-molded PGA disks reveals their IR spectroscopic character-
istics (Chu et al., 1995). As shown in Figure 5.1, the four bands at 850, 753, 713, and 560 cm
−1
are associ-
ated with the amorphous regions of the PGA disks and could be used to assess the extends of hydrolysis.
Peaks associated with the crystalline phase included those at 972, 901, 806, 627, and 590 cm
−1
. Two
broad, intense peaks at 1142 and 1077 cm
−1
can be assigned to C−O stretching modes in the ester and
oxymethylene groups, respectively. These two peaks are associated mainly with ester and oxymeth-
ylene groups originating in the amorphous domains. Hydrolysis could cause both these C−O stretching
modes to substantially decrease in intensity.
(d)
(c)
(b)
(a)
1900
1750
1600
1450
1300
Wavenumber
1150
1000
850
700
550
FIGURE 5.1
FTIR spectra of PGA disks as a function of
in vitro
hydrolysis time in phosphate buffer of pH 7.44 at
37°C: (a) 0 day; (b) 55 h; (c) 7 days; (d) 21 days.
