Handbook of Solvents - George Wypych - ChemTech - Ventech!

5.4 Mixed solvents - polymer solubility 273
5.4.3 NEW COSOLVENTS EFFECTS. SOLUBILITY BEHAVIOR
Cosolvent effect of alkyl acrylates on the phase behaviour of poly(alkyl acrylate)supercritical
CO2 mixtures has been reported. 89 Cloud-point data to 220 and 2000 bar are presented
for ternary mixtures of poly(butyl acrylate)-CO2-butyl acrylate (BA) and poly(ethylhexyl
acrylate)-CO2-ethylhexyl acrylate) (EHA). The addition of either BA or EHA to the respective
polymer-solvent mixtures decreases the cloud-point pressures by as much as 1000 bar
and changes the pressure-temperature slope of the cloud-point curves from negative to positive,
which significantly increases the single-phase region.
The literature presents many studies on coil dimensions of synthetic polymers in
mixed solvents. Most investigations involve liquid mixtures composed of a good and a poor
solvents for the polymer. The action of mixed solvents has been reported to change coil dimensions,
not only because of excluded volume effect or due to the interactions existing between
the two liquids 90,91 but also due to the preferential adsorption of one of the solvent by
the polymer.
Recently, the behavior of polysiloxanes with amino end-groups in toluene/nitromethane
mixtures has been reported. 92 This mixture is solvent/non-solvent for the
polymer.
The transition concentrations separating the concentration domain 93 chain flexibility
aspects, excluded volume effects 94 and total and preferential adsorption coefficients 95 of the
same system have been discussed.
The solubility curves, the cloud point curves and vitrification boundaries for several
poly(lactide)-solvent-nonsolvent systems have been reported. 96 The liquid-liquid miscibility
gap for the systems with the semicrystalline poly(L-lactide) (PLLA) were located in a
similar composition range as the corresponding systems with the amorphous
poly(DL-lactide) (PDLLA). The solvent-nonsolvent mixtures used for the experiments
were: dioxane/water, N-methyl pyrrolidone (NMP)/water, chloroform/methanol and
dioxane/methanol. For all PLLA solvent-nonsolvent systems studied solid-liquid demixing
was preferred thermodynamically over liquid-liquid demixing. Attempts were made to correlate
the experimental finding with predictions on the basis of the Flory-Huggins theory for
ternary polymer solutions using interaction parameters derived from independent experiments.
Qualitative agreement was found for the relative locations of the liquid-liquid miscibility
gaps. The Flory-Huggins description of the solubility curves was less satisfactory.
The phase separation processes occurring in poly(L-lactide) (PLLA)-chloroform-methanol
mixtures and poly(DL-lactide) (PDLLA)-chloroform-methanol mixtures
have been also studied using differential scanning calorimetry, cloud point measurements
and optical microscopy. 97 It was demonstrated that liquid-liquid demixing occurs in ternary
solutions of PDLLA at sufficient high methanol concentrations. For PLLA-containing-solutions,
both liquid-liquid demixing processes and soli-liquid demixing processes occur.
Only a low cooling rates and high polymer concentration does solid-liquid demixing take
place without the interference of liquid-liquid demixing. 97
Another interesting effects are the changes of a polyelectrolyte in binary solvents. The
complex inter and intramolecular interactions that take place due to the presence of hydrophilic
and hydrophobic structural units in the macroion can modify the balance of the interactions
and for this reason can change the solubility. 98