Effect of supercritical carbon dioxide on morphology development during polymer blending

Abstract

Supercritical carbon dioxide (scCO2) was added during compounding of polystyrene and poly(methyl methacrylate) (PMMA) and the resulting morphology development was observed. The compounding took place in a twin screw extruder and a high‐pressure batch mixer. Viscosity reduction of PMMA and polystyrene were measured using a slit die rheometer attached to the twin screw extruder. Carbon dioxide was added at 0.5, 1.0, 2.0 and 3.0 wt% based on polymer melt flow rates. A viscosity reduction of up to 80% was seen with PMMA and up to 70% with polystyrene. A sharp decrease in the size of the minor (dispersed) phase was observed near the injection point of CO2 in the twin screw extruder for blends with a viscosity ratio, ηPMMA/ηpolystyrene, of 7.3, at a shear rate of 100 s−1. However, further compounding led to coalescence of the dispersed phase. Adding scCO2 did not change the path of morphology development; however, the final domain size was smaller. In both batch and continuous blending, de‐mixing occurred upon CO2 venting. The reduction in size of the PMMA phase was lost after CO2 venting. The resulting morphology was similar to that without the addition of CO2. Adding small amounts of fillers (e.g. carbon black, calcium carbonate, or nano‐clay particles) tended to prevent the de‐mixing of the polymer blend system when the CO2 was released. For blends with a viscosity ratio of 1.3, at a shear rate of 100 s−1, the addition of scCO2 only slightly reduced the domain size of the minor phase.

Preparation of interconnected poly(ε-caprolactone) porous scaffolds by a combination of polymer and salt particulate leaching

“…Similar changes in coarsening dynamics were observed in different polymer/ polymer/particulate blends. For example, addition of a small amount of fillers (typically, 3 wt%) such as nano-clays, carbon black and calcium carbonate prevented the de-mixing or coalescence of the PS/PMMA polymer blend in presence of carbon dioxide [41]. In another investigation, the addition of black carbon in PE/PS blends-selectively localized into the PE phase-increases the stability of the co-continuous morphology during thermal treatment [42].…”

Thermodynamics and Kinetic Processes of Polymer Blends and Block Copolymers in the Presence of Pressurized Carbon Dioxide

“…[173] In a comparable blend, an initial viscosity ratio of 7.3 is lowered to 3.4 upon addition of scCO 2 , and the dispersion size correspondingly decreases in both batch and continuous mixing processes. [176] Indeed, the dispersion size in a 25:75 (w/w) PMMA/PS blend systematically decreases with increasing CO 2 concentration, as illustrated by the sequence of TEM images displayed in Figure 10. The level of dispersion achieved with 2.75 wt% CO 2 is superior to that generated even after extended mixing (for 4 h) without scCO 2 .…”

“…[178] While scCO 2 tends to reduce the size of a dispersed phase in a polymer blend, subsequent phase coalescence can proceed if the blends are further mixed in the absence of CO 2 at high temperatures in both batch and continuous processes. [174,176] Twin-screw extrusion of a 25/75 PMMA/PS blend with CO 2 vented near the end of the barrel increases the diameter of the PMMA dispersion from 1.1 to 2.1 lm, despite the short mixing time after venting. [176] Mixing the same blend at 13.8 MPa and 200°C for 4 h, followed by CO 2 -free mixing for 2 h, results in the progressive growth of the diameter of the PMMA domains from 2.6 to 5.1 lm.…”

“…[174,176] Twin-screw extrusion of a 25/75 PMMA/PS blend with CO 2 vented near the end of the barrel increases the diameter of the PMMA dispersion from 1.1 to 2.1 lm, despite the short mixing time after venting. [176] Mixing the same blend at 13.8 MPa and 200°C for 4 h, followed by CO 2 -free mixing for 2 h, results in the progressive growth of the diameter of the PMMA domains from 2.6 to 5.1 lm. Coarsening also occurs in LDPE/PS blends after CO 2 venting, again despite a relatively short post-ventilation mixing time.…”

“…Another route by which stabilized highly dispersed morphologies are achieved in CO 2 -regulated polymer processing is through the inclusion of inorganic species, such as CaCO 3 , carbon black, and organically-modified clay. [176] Surface-functionalized nanoparticles with CO 2 -philic species may further enhance the tunability of physical properties under supercritical conditions. [181] Lastly, the use of CO 2 has been extended to other processes, [182] for example, high-energy ball milling.…”