Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 31

was derived which accounted for the autocatalysis by the generated carboxylic
acid end groups, was given as follows:

(2.4)

ln M NT = K ′ T + ln M NO

In both models, the assumption was that the extent of degradation is small.

2.4.4 Degradation Characteristics

It has been reported that the hydrolytic degradation of aliphatic polyesters such
as PGA, PLLA, PCL, PHB and PHBV is a complex process that involves several
phenomena (Li 2006 ). These phenomena involve water absorption, ester bond
cleavage, neutralization of carboxyl end groups at the surface, autocatalysis inside
and the diffusion and solubilization of soluble oligomers. Several factors influence
these phenomena such as matrix morphology, chemical composition and configu-
rational structure, molecular weight, size, distribution of chemically reactive com-
pounds within the matrix and the nature of the degradation media (Li 1999 ).

2.4.4.1 Polymer Morphology

In the degradation process, the morphology of a polymeric material, i.e., amor-
phousness or semicrystallinity plays an important role. It was described in the
earlier section that degradation of semicrystalline polyesters in aqueous media
proceeds in two stages. Several researchers reported preferential degradation of
amorphous areas in the cases of semicrystalline polymers (Fischer et al. 1973 ;
Reed and Gilding 1981 ; Leenslag et al. 1987 ; Li 1999 ). They reported an increase
of crystallinity. It was also reported that the presence of imperfections and defec-
tive crystalline regions influences the degradation as when the spherulitic crystal-
lization develops within a matrix containing impurities, monomers or oligomers,
these noncrystallizable species are generally concentrated at the interspherulitic
boundaries (Li 2006 ). With the amorphous region, these defects are preferentially
degraded. Li et al. observed from 45 to nearly 80 % increase in crystallinity of
PCL films after 120 weeks of implantation (Li et al. 1997 ). This was attributed to
the crystallization of tie segments after chain cleavage in the amorphous phase and
low glass transition temperature of PCL (T g = − 60 °C) facilitated the recrystal-
lization. Due to the high crystallinity and hydrophobicity of PCL, water absorp-
tion was less than 2 % during the first 63 weeks. Due to decrease in molecular
weight and the formation of carboxyl end groups, water absorption reached 8 %
at 133 weeks and 12 % at 200 weeks. The weight loss reached 3 % at 63 weeks,
11 % at 133 weeks and 14 % at 200 weeks and molecular weight decreased stead-
ily from an initial 58,700 to 7000 after 200 weeks. Li et al. also reported that in the
case of a PLA 75 CL 25 copolymer, crystallinity increased from an initial 14 to 52 %
at 63 weeks (Li et al. 1997 ).