Page 338 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOPOLYMERS 315

(random, alternating, block, graft) and must be accompanied by the structure name to specify copoly-
mer type.

O H
O CH 3
O C CH O C CH
n m
Poly(lactide-co-glycolide)

Block copolymers often phase segregate into an A-rich phase and a B-rich phase. If one repeat
unit (or phase) is a soft phase and the other a hard glassy or crystalline phase, the result can be a ther-
moplastic elastomer. The crystalline or hard glassy phase acts as a physical cross-link. The advan-
tage of thermoplastic elastomers, unlike chemically cross-linked elastomers, is that they can be melt
or solution processed. Many polyurethanes are thermoplastic elastomers. They consist of soft seg-
ments, either a polyester or polyether, bonded to hard segments. The hard segments are ordinarily
synthesized by polymerizing diisocyanates with glycols.
Similarly thermoplastic hydrogels can be synthesized by using a hydrophilic A block and a
hydrophobic B block poly(ethylene oxide-b-lactide) (PEO-b-PLA) are biodegradable hydrogel poly-
mers which are being developed for drug delivery applications. 2–6 PEO is a water-soluble polymer that
promotes swelling in water, and PLA is a hard degradable polymer that acts as a physical cross-linker.

13.2.2 Polymer Mechanical Properties

Solid polymer mechanical properties can be classified into three categories: brittle, ductile, and elas-
tomeric (see Fig. 13.7). Brittle polymers such as PMMA are polymers with a T much higher than
g

Brittle

Stress Ductile Ductile

(necking)

Elastomeric

Strain
FIGURE 13.7 Mechanical behavior of polymers. [Reproduced from Ency-
clopedia of Materials Science and Engineering, M. B. Bever (ed.). Cambridge,
MA: MIT Press, 1986, p. 2917.]