· Web viewValues of solubility parameters and cohesive energy of some polymers are given in the following tables: A quantitative measure of the magnitude of secondary bonding forces

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, variousapproaches 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 KBvalue. 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:


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.


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 :



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:


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:

Whereand M are the density and molecular weight, respectively, of the polymer repeating unit.

Table : Molar Attraction Constants, E (cal cm 3 )/mol


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.



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.


►We can relate the interaction parameter to the solubility parameter asfollows:

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 ᵪ

1 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


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 insolvents completely. The rigid chains cannot move gradually because separation of two rigid chains requires large energy.


♦ 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.


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· Web viewValues of solubility parameters and cohesive energy of some polymers are given in the following tables: A quantitative measure of the magnitude of secondary bonding forces