The effect of molecular structure on the properties of biomedical polymers - Durability and Reliability of Medical Polymers - page 31

2.12 Block copolymers.

2.13 Graft copolymers.

this is a special kind of branched copolymer in which the side chains are
structurally distinct from the main chain (Fig. 2.13) (Hadjichristidis et al. ,
2003).

2.2 Molecular weight and polymer properties
In general, polymers are defi ned as 'polydisperse' because they show a
non-uniform molecular size. this parameter is important in determining
many physical properties such as transition temperature, strength, viscosity,
toughness and stiffness. For this reason it is necessary to identify an average
value of molecular weight (Misra, 1993). In polymer science the most practical
w a y to identify polymer molecular weights is the a verage molecular weight
( M n ) and the weight average molecular weight ( M w ). Let n i be the number
of molecules with molecular weight M i , then the number average molecular
weight is:

= ∑

nM nM

i

nM

i i

[2.1]

M

n

∑ n i

if the polymer property is dependent on the weight of each polymer
molecule, the weight average molecular weight is:

￿ ￿ ￿ ￿ ￿ ￿

•

2

 nM

nM nM

i

•

i=1

[2.2] Â

M wM =

i i
w i i wM

w

•

i=1

 nM

nM nM i

i=1

where w i represents the weight fraction of polymer with molecular
weight i .
these parameters are very important for the mechanical properties of the
polymer. A high value of average molecular weight corresponds to high
mechanical resistance (Gowariker et al. , 1986). the polydispersity index (PDi)
is a measure of the distribution of molecular mass in a given sample:

PDi = M w / M n [2.3]