Polymer Physics

bK¼

<R^2 >

L

(2.13)

3. Stiffness parameter or steric hindrance

The stiffness parameter,orsteric hindrance, is defined by

s



<R^2 >

<R^2 f:j:>

 1 = 2

(2.14)

which reflects the degree of hindrance in internal rotations.

4. Unperturbed dimension

The unperturbed dimensionof the polymer chain is defined by

A



<R^2 >

M

 1 = 2

(2.15)

whereMis the molar mass of the polymer. The unperturbed dimension reflects the
expansion of the polymer coil relative to that predicted by the freely-jointed-chain
model.

5. Characteristic ratio

The characteristic ratio Cnis defined by

Cn

<R^2 >

nb^2

(2.16)

and when the number of chain unitsnapproaches infinity, one can obtain the
limiting characteristic ratioC 1.

2.3 Local Inter-Chain Interactions

Along a long polymer chain, each chain unit may carry specific chemical groups,
which bring various kinds of inter-chain interactions, either the common van der
Waals interactions, or some special supermolecular interactions, such as the hydrogen
bonding, Coulomb forces,p-pstacking, hydrophobic interactions, and coordination
interactions with metal atoms. Under proper conditions, one kind of inter-chain
interaction may play a dominant role in determining certain physical behaviors of
polymers. Therefore, we need to introduce multiple molecular energy parameters to
characterize the corresponding inter-chaininteractions, especially when we study the

2.3 Local Inter-Chain Interactions 21