Viscosity studies of poly(DL‐lactic acid) in supercritical CO₂

Abstract

The effect of supercritical CO 2 on the viscosity and activation energy to viscous flow of P DL LA is investigated, using a high pressure parallel plate rheometer, over a range of temperatures (50-140 C) and pressures (5-12 MPa). The Cross model is fitted to the data to enable calculation of the zero shear viscosity and critical shear rate. A significant decrease in the viscosity is observed on increasing both variables; however, at high temperatures, the pressure effect becomes negligible. An increase in the critical shear rate is also observed on raising the pressure, indicative of a reduction in the relaxation time of the polymer. Manipulation of the Arrhenius equation shows a reduction in the activation energy to viscous flow as the pressure is increased. Together, these results show that the melt processing temperature of P DL LA can be reduced in the presence of supercritical and high pressure CO 2 .

High-pressure rheological analysis of CO2-induced melting point depression and viscosity reduction of poly(ε-caprolactone)

“…The zero shear viscosity is dramatically decreased under CO 2 , in agreement with results obtained on other polymers such as amorphous poly(lactic acid) [31], poly(ethylene glycol) [19], and poly(dimethylsiloxane) [59]. At pressures above 120 bar the shear thinning region was less detectable in this shear rate range.…”

“…Newtonian plateau [31]. The critical shear rate at the onset of shear thinning ( g cr ) is, in fact, inversely proportional to the relaxation time [60][61][62][63] The ability of scCO 2 to plasticise higher MW PCL was confirmed by viscosity studies carried out on PCL80 (Figure 9).…”

“…Kelly et al [31] who observed that at sufficiently high temperatures (well above T g ) no changes in relaxation time could be observed upon the addition of CO 2 . A final set of experiments were conducted at ambient pressure and under CO 2 (120 bar) over a temperature range of 80 to 180 °C in order to correlate the CO 2 -induced viscosity depression to the temperature-induced reduction, and in order to obtain activation energies with and without pressurised CO 2 (E a ) ( Table 2).…”

“…These interactions often results in high sorption levels and decrease the intermolecular bonding between the chains [31]. This causes an increase in the free volume fraction (v f ) and therefore a higher mobility of the polymer chains, hence leading to a decrease in viscosity with the CO 2 molecules acting as molecular lubricants [16,31].…”

Controlled polymerisation and purification of branched poly(lactic acid) surfactants in supercritical carbon dioxide

“…Thus we can reasonably infer why the reaction continues even at this low temperature despite the fact that the reactants visually remain solid. As the reaction progresses, the scCO 2 is thought to lower the viscosity and reduce the T g of the S-PLA product, 58 aiding both further polymerisation (lowering viscosity), solubilisation and mass transport of the monomers, allowing the reaction to continue. 1 H-NMR measurements allow us to follow the levels of conversion and in doing so the apparent rate of polymerisation (k app ) could be monitored, allowing us to visualise the influence of temperature.…”