The effect of molecular structure on the properties of biomedical polymers - Durability and Reliability of Medical Polymers - page 44

Rubber-fluid + Crystal.

Melt

Glass + crystal.

10 9

Stiff

Semi-stiff

Fluid

Crystalline

T g T m T

Temperature

2.29 Schematic representation of E - T curve for a generic crystalline
polymer.

in the proximity of T m , there is a mechanical collapse owing to gradual
crystallite fusion (Braun, 2005).

2.5.1 Thermoplastic and thermosetting polymers
the behaviour described in the previous section occurs in thermoplastic
polymers, where the single molecules are bonded together by thermosensitive
intermolecular attractive forces (van der Waals, dipole-dipole, hydrogen
bonding). crosslinked polymers (thermosetting polymers) show a different
behaviour owing to primary bonds between main chains. these types of bonds
are not thermosensitive and they limit viscous flows. The segments of chain,
included between transversal bonds, are obstructed in their movement.
In general, thermosetting polymers are in amorphous state (Fig. 2.30). The
presence of a high density of transversal bonds can produce the disappearance of
the glass transition. thermosetting polymers thus maintain shape stability until
high temperature and then thermal decomposition occurs. typical examples
of thermosetting polymers are unsaturated polyesters, epoxy resins, phenolic
and urea-formaldehyde resins (Pascault, 2002). Urea-formaldehyde resins
are not widely used in biomedical applications owing to their potentially
toxic, allergenic and carcinogenic characteristics. Unsaturated polyesters
have been considered as potential bone cement and scaffolds in bioresorbable
composite materials for tissue engineering (Kharas et al. , 1997). epoxy resins
have been used in combination with other biomaterials for plates in bone
fixation devices (Ramakrishna et al. , 2001). Phenolic resins have been used
to develop activated carbon for many biomedical applications (cai et al. ,
2004).

￿ ￿ ￿ ￿ ￿ ￿