The effect of molecular structure on the properties of biomedical polymers - Durability and Reliability of Medical Polymers - page 41

transition points of poly(vinyl chloride) and polypropylene (low polarity),
a difference of 85 °C is found.
Plasticised PVc is the most widely utilised material for the production
of blood and blood-component contacting devices, including blood storage
bags, catheters and tubing for extracorporeal circuits (Fig. 2.26) (Manfredini
et al. , 2003), whereas polypropylene has a wide use in medical disposables
and as surgical implantable meshes (Alariqi et al. , 2009). Poly(vinyl chloride)
and polypropylene have the same main chemical structure and also a similar
steric burden given by the methyl group (-CH 3 ) and chlorine atom (-Cl), but,
in this case, the polarity plays an important role (Fig. 2.27). the presence of
large molecular groups limiting chain movements can promote a transition
point rise. An example is the presence of aromatic rings.
the glass transition temperature is therefore a fundamental material
parameter for manufacturing and future applications. Polymers such as PVc
are usually processed by compression/injection moulding or extrusion and
an adequate manufacturing temperature is required (normally 50-100 °C
over the T g point). The application temperature must be significantly higher
than T g (typically 50-60 °C) in order to avoid mechanical problems such as
creep.
table 2.1 records glass transition temperatures for some common biomedical
polymers. it allows comparisons between polymers such as polypropylene,
poly(vinyl chloride) and polystyrene.
Linear amorphous polymers in the solid state usually show a behaviour
very close to inorganic glasses: very stiff, devoid of plasticity, with high
Young's modulus, high tensile strength and low elongation at break. there
are a number of exceptions. Some linear amorphous polymers in a glass-like
state display certain flexibility and a moderate plasticity. The reason is the
presence inside the macromolecule of very flexible chain segments (methylene
sequence) or short lateral chains. this allows some local molecular movement
below T g . in these cases, as temperature decreases, there is a secondary
transition temperature which results in freezing the macromolecule structure
(Stuart, 2002; Dattelbaum and Rae, 2004).

￿ ￿ ￿ ￿ ￿ ￿

Besides the Young's modulus, mechanical damping is a good criterion

· · · · · ·

CH 2 CH CH 2 CH CH 2 CH CH 2 CH ( T g = 75 °C)

Cl Cl Cl Cl

2.26 Polyvinyl chloride (PVC).

· · · CH 2 CH CH 2 CH CH 2 CH CH 2 CH · · · ( T g = -10 °C)

CH 3 CH 3 CH 3 CH 3

2.27 Polypropylene (PP).