Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.2 Effect of system variables on solubility 127
Nonpolar polymers (polyisoprene, polybutadiene) mix infinitely with alkanes (hexane,
octane, etc.) but do not mix with such polar liquids as water and alcohols. Polar polymers
(cellulose, polyvinylalcohol, etc.) do not mix with alkanes and readily swell in water.
Polymers of the average polarity dissolve only in liquids of average polarity. For example,
polystyrene is not dissolved or swollen in water and alkanes but it is dissolved in aromatic
hydrocarbons (toluene, benzene, xylene), methyl ethyl ketone and some ethers.
Polymethylmethacrylate is not dissolved nor swollen in water nor in alkanes but it is dissolved
in dichloroethane. Polychloroprene does not dissolve in water, restrictedly swells in
gasoline and dissolves in 1,2-dichloroethane and benzene. Solubility of polyvinylchloride
was considered in terms of relationship between the size of a solvent molecule and the distance
between polar groups in polymer. 22
The above examples are related to the concept of the one-dimensional solubility parameter.
However the effects of specific interactions between some functional groups can
change compatibility of the system. Chloroalkanes compared with esters are known to be
better solvents for polymethylmethacrylate. Aromatic hydrocarbons although having solubility
parameters much higher than those of alkanes, dissolve some rubbers at least as well
as alkanes. Probably it is related to increase in entropy change of mixing that has a positive
effect on solubility.
The molecular mass of polymer significantly influences its solubility. With molecular
mass of polymer increasing, the energy of interaction between chains also increases. The
separation of long chains requires more energy than with short chains.
4.2.3 FLEXIBILITY OF A POLYMER CHAIN
The dissolution of polymer is determined by chain flexibility. The mechanism of dissolution
consists of separating chains from each other and their transfer into solution. If a chain
is flexible, its segments can be separated without a large expenditure of energy. Thus functional
groups in polymer chain may interact with solvent molecules.
Thermal movement facilitates swelling of polymers with flexible chains. The flexible
chain separated from an adjacent chain penetrates easily into solvent and the diffusion occurs
at the expense of sequential transition of links.
The spontaneous dissolution is accompanied by decrease in free energy (ΔG 0 then ΔG < 0. Therefore high-elasticity polymers are dissolved in
solvents completely.
The rigid chains cannot move gradually because separation of two rigid chains requires
large energy. At usual temperatures the value of interaction energy of links between
polymer chains and molecules of a solvent is insufficient for full separation of polymer
chains. Amorphous linear polymers with rigid chains having polar groups swell in polar liquids
but do not dissolve at normal temperatures. For dissolution of such polymers, the interaction
between polymer and solvent (polyacrylonitrile in N,N-dimethylformamide) must be
stronger.
Glassy polymers with a dense molecular structure swell in solvents with the heat absorption
ΔH > 0. The value of ΔS is very small. Therefore ΔG > 0 and spontaneous dissolution
is not observed and the limited swelling occurs. To a greater degree this concerns
crystalline polymers which are dissolved if ΔH < 0 and |ΔH|>|TΔS|.
When molecular mass of elastic polymers is increased, ΔH does not change but ΔS decreases.
The ΔG becomes less negative. In glassy polymers, the increase in molecular mass