Handbook of Solvents - George Wypych - ChemTech - Ventech!

350 George Wypych
• evaporation-induced self association of the polymer with progressive entrapment of
adsorbed volatile molecules in the glassy microdomains (during transition from a
rubbery to a glassy state)
• elimination of residual molecules entrapped in the polymer.
The last two stages are discussed in Chapter 16 which deals with residual solvent. This
discussion concentrates on the effect of components on the drying process and the effect of
the drying process on the properties of the product.
A schematic of the drying process is
represented in Figure 7.1.18. The material
to be dried is placed on an impermeable
substrate. The material consists of solvent
and semicrystalline polymer which contains
a certain initial fraction of amorphous
and crystalline domains. The presence of
Figure 7.1.18. Schematic representation of drying a
polymer slab. [Adapted, by permission, from M O Ngui,
S K Mallapragada, J. Polym. Sci.: Polym. Phys. Ed., 36,
No.15, 2771-80 (1998).]
crystalline domains complicates the process
of drying because of the reduction in
diffusion rate of the solvent. Evaporation
of solvent causes an inward movement of
material at the surface and the drying pro-
cess may change the relative proportions of amorphous and crystalline domains. 29
Equations for the change in thickness of the material and kinetic equations which relate
composition of amorphous and crystalline domains to solvent concentration are needed to
quantify the rate of drying.
The thickness change of the material during drying is given by the equation:
v dL
D
dt
v
1 = ⎛ ∂
⎜
⎝ ∂
⎞
⎟
1
x ⎠x=
L
[7.1.13]
where:
v1 volume fraction of solvent
L thickness of slab as in Figure 7.1.18
t time
D diffusion coefficient
x coordinate of thickness
The rate of change of crystalline volume fraction is given by the equation:
∂v
2
∂t
c
= kv
[7.1.14]
1 1
where:
v2c volume fraction of crystalline phase
k1 rate change of crystalline phase proportional to folding rate
The rate of change of amorphous volume fraction is given by the equation:
∂v
2a∂⎛ ∂v1
⎞
= ⎜D
⎟− kv 1 1
[7.1.15]
∂t
∂x
⎝ ∂x
⎠