Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
Lec-7
Solubility
Polymer dissolution is a necessary step in many of the polymer
processing methods, such as blending, separation, coating, casting, etc.
SOLVENT POWER:
The usual problem of polymer engineering is a selection of proper
solvent(s) for a given polymer. This selection implies that the solvent
must form with polymer a thermodynamically stable mixture in the whole
range of concentrations and temperatures. Such choice is facilitated by
use of numerical criterion of a solvent power. Solvent power might be
taken from the thermodynamic treatment (for example, a change of
Gibbs. free energy or chemical potentials of mixing of polymer with
solvent), but these criteria depend not only on the solvent properties but
also on polymer structure and its concentration. For this reason, various
approaches were proposed to estimate solvent power.
♦ Kauri-butanol value, KB: is used for evaluation of dissolving ability
of hydrocarbon solvents. It is obtained by titration of a standard Kauri
resin solution (20 wt% in 1-butanol) with the solvent until a cloud point is
reached (for example, when it becomes impossible to read a text through
the solution). The amount of the solvent used for titration is taken as KB
value. The relationship between KB and solubility parameter, , fits the
following empirical dependence:
♦ Dilution ratio, DR: is the volume of a solvent added to a given solution
that causes precipitation of the dissolved resin. This ratio can characterize
the compatibility of a diluent with a resin solution in primary solvent.
When compatibility is high, more diluent can be added.
DR depends on the polymer concentration. With polymer concentration
increasing, DR increases as well.
♦Aniline point, AP: is the temperature of a phase separation of aniline in
a given solvent.
AP is a critical temperature of the aniline - solvent system. AP can be
related to KB value using the following equations:
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
AP depends on the number of carbon atoms in the hydrocarbon
molecule. AP is useful for describing complex aromatic solvents.
♦The solvent power can also be presented as a sum of factors that
promote solubility or decrease it:
where:
H: a factor characterizing the presence of active sites of opposite nature
in solvent and polymer that can lead to formation of hydrogen bond
between polymer and solvent
B: a factor related to the difference in sizes of solute and solvent
molecules
A: a factor characterizing solute melting.
C :a factor of the self-association between solvent molecules
D :a factor characterizing the change of nonspecific cohesion forces in
the course of transfer of the polymer molecule into solution.
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
Solubility Parameter():
The value of solubility parameter() can be calculated from the
evaporation enthalpy of liquid at given temperature:
The relation between the cohesive energy density (CED) and solubility
parameter () is:
As a first approximation and in the absence of strong interactions such
as hydrogen bonding, a polymer δ2 will dissolve in a solvent δ1 if :
δ1 – δ2 ≤ 1.7 – 2.0.
Values of solubility parameters and cohesive energy of some polymers
are given in the following tables:
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
A quantitative measure of the magnitude of secondary bonding forces is
the cohesive energy density (CED), which is the total energy per unit
volume needed to separate all intermolecular contacts and is given by:
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
In addition, the solubility parameter can be estimated from the molar
attraction constants, E, using the structural formula of the compound and
its density (Table bellow).
For a polymer:
Whereand M are the density and molecular weight, respectively, of
the polymer repeating unit.
Table : Molar Attraction Constants, E (cal cm 3 )/mol
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
Example 1:
Estimate the solubility parameters of the following polymers:
a. Low-density polyethylene (LDPE)
b. High-density polyethylene (HDPE)
c. Polypropylene (PP)
d. Polystyrene (PS)
Solution:
Example 2:
The table below shows the density and enthalpy of vaporization [ΔHvap]
of two solvents: methylethyl ketone and acetone.
Which is a better solvent for polystyrene at room temperature?
Notes: 1-The CED for polystyrene is 75 cal/cm3.
2-Assume room temperature is 27°C.
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
Since the CED of methylethyl ketone is closer to that of polystyrene
than that of acetone, methylethyl ketone should be a better solvent for
polystyrene than acetone.
Example 3:
Toluene and xylene have approximately the same cohesive energy
density (CED), but xylene is a more convenient solvent for polyethylene.
Solution:
Toluene has a boiling point of about 111°C, while xylene has a boiling
point of 138 to 144°C.
As a result of the highly crystalline nature of polyethylene it will
dissolve in solvents only at temperatures close to its melting point (about
135°C). When the boiling points of toluene and xylene are compared,
xylene is obviously a preferable solvent for polyethylene.
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
►We can relate the interaction parameter to the solubility parameter as
follows:
Effect of system variables on solubility:
Solubility in solvents depends on various internal and external factors.
Chemical structure, molecular mass of solute, and crosslinking of
polymer fall into the first group of factors, in addition to temperature and
pressure in the second group of factors involved
General Considerations:
♦The process of dissolution is determined by a combination of enthalpy
and entropy factors. The dissolution description can be based on the
Flory-Huggins equation.
Flory and Huggins calculated the entropy of mixing of long-chain
molecules under the assumption that polymer segments occupy sites of a
lattice and solvent molecules occupy single sites.
♦The critical value of ᵪ1 sufficient for solubility of polymer having large
molecular mass is 0.5. Good solvents have a low ᵪ1 value. ᵪ1 is a
popular practical solubility criterion.
♦Temperature is another factor. It defines the difference between polymer
and solvent. Solvent is more affected than polymer. This distinction in
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
free volumes is stipulated by different sizes of molecules of polymer and
solvent.
The solution of polymer in chemically identical solvent should have
unequal free volumes of components. It causes important thermodynamic
consequences. The most principal among them is the deterioration of
compatibility between polymer and solvent at high temperatures leading
to phase separation.
Chemical Structure:
Chemical structure and the polarity determine dissolution of polymers.
♦ If the bonds in polymer and solvent are similar, then the energy of
interaction between homogeneous and heterogeneous molecules is nearly
identical which facilitates solubility of polymer.
♦ If the chemical structure of polymer and solvent molecule differ greatly
in polarity, then swelling and dissolution does not happen.
It is reflected in an empirical rule that "like dissolves like"
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.
♦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.
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
(G < 0) . Therefore high-elasticity polymers (G < 0) are dissolved in
solvents completely.
The rigid chains cannot move gradually because separation of two rigid
chains requires large energy.
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C
♦ 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.
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.
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
temperature raise can cause improvement of compatibility.
♦The solubility of crystalline substances increases with temperature
increasing. Many crystalline polymers such as polyethylene or
polyvinylchloride are insoluble at the room temperature (only some
swelling occurs); however, at elevated temperature they dissolve in some
solvents.
♦The higher the melting temperature of the polymer, the worse its
solubility. Substances having higher melting heat are less soluble,
with other characteristics being equal.
♦The dependence of solubility on pressure can be described only by
modern theories taking into account the free volume of components.
Lec-7: Solubility ……………………………………………………………………………..Eng. Auda Jabbar Ms.C