Handbook of Solvents - George Wypych - ChemTech - Ventech!

128 Valery Yu. Senichev, Vasiliy V. Tereshatov
is accompanied by a decrease in ΔH and ΔS. The ΔS value changes faster than the ΔH value,
therefore the ΔG value becomes more negative, which means that the dissolution of polymeric
homologues of the higher molecular weight becomes less favorable.
Crystalline polymers dissolve usually less readily than amorphous polymers. Dissolution
of crystalline polymers requires large expenditures of energy for chain separation.
Polyethylene swells in hexane at the room temperature and dissolves at elevated temperature.
Isotactic polystyrene does not dissolve at the room temperature in solvents capable to
dissolve atactic polystyrene. To be dissolved, isotactic polystyrene must be brought to elevated
temperature.
4.2.4 CROSSLINKING
The presence of even a small amount of crosslinks hinders chain separation and polymer
diffusion into solution. Solvent can penetrate into polymer and cause swelling. The swelling
degree depends on crosslink density and compatibility of polymer and solvent.
The correlation between thermodynamic parameters and the value of an equilibrium
swelling is given by Flory-Rehner equation 23 used now in a modified form: 24
ln
2 ν2
⎛ 2ϕ
− ϕ2 + ϕ2 + χ1ϕ2 =− ⎜ϕ
−
V ⎝
Vs
f
( 1 )
13 / 2
2
⎞
⎟
⎠
[4.2.9]
where:
ϕ 2 polymer volume fraction in a swollen sample
ν2 /V volume concentration of elastically active chains
f the functionality of polymer network
The value of ν 2 /V is determined by the concentration of network knots. These knots
usually have a functionality of 3 or 4. This functionality depends on the type of curing agent.
Crosslinked polyurethanes cured by polyols with three OH-groups are examples of the
three-functional network. Rubbers cured through double bond addition are examples of
four-functional networks.
Eq. [4.2.9] has different forms depending on the form of elasticity potential but for
practical purposes (evaluation of crosslinking density of polymer networks) it is more convenient
to use the above form. The equation can be used in a modified form if the concentration
dependence of the parameter χ1 is known.
The value of equilibrium swelling can be a practical criterion of solubility. Good solubility
of linear polymers is expected if the value of equilibrium swelling is of the order of
300-400%. The high resistance of polymers to solvents indicates that the equilibrium swelling
does not exceed several percent.
In engineering data on swelling obtained at non-equilibrium conditions (for example,
for any given time), swelling is frequently linked to the diffusion parameters of a system
(see more on this subject in Subchapter 6.1). 25
An interesting effect of swelling decrease occurs when swollen polymer is placed in a
solution of linear polymer of the same structure as the crosslinked polymer. The decrease of
solvent activity causes this effect. The quantitative description of these processes can be
made by the scaling approach. 26
4.2.5 TEMPERATURE AND PRESSURE
The temperature effect on solubility may have different characters depending on the molecular
structure of solute. For systems of liquid-amorphous polymer or liquid-liquid, the tem-