Phase Behavior of Poly(methyl methacrylate)/Poly(vinylidene fluoride) Blends in the Presence of High‐Pressure Carbon Dioxide

Abstract

Previous efforts have demonstrated that high‐pressure CO2 can markedly influence the phase behavior of amorphous polymer blends. In this work, we examine the effect of high‐pressure CO2 on the miscibility of blends composed of glassy poly(methyl methacrylate) (PMMA) and semicrystalline poly(vinylidene fluoride) (PVDF). Blends of this type are known to exhibit lower critical solution temperature (LCST) behavior with partial miscibility up to ≈50–60 wt.‐% PVDF at ambient conditions. Two miscible PMMA/PVDF blends have been systematically exposed to high‐pressure CO2 at 35 °C and pressures below and above the critical pressure. Small‐angle X‐ray scattering reveals that the scattering intensity at high scattering angles shows little dependence on pressure at low CO2 pressures, but increases substantially at relatively high CO2 pressures. Transmission electron microscopy and differential scanning calorimetry analyses confirm that the blends are initially quasi‐homogeneous with diffuse PVDF‐rich dispersions and a single glass transition temperature. After exposure to relatively high CO2 pressures, however, the PVDF is found to crystallize within the PMMA‐rich matrix. Thermal recycling of these blends promotes homogenization, indicating that such CO2‐altered phase behavior is reversible.SAXS patterns acquired from the 69/31 w/w PMMA/PVDF blend.magnified imageSAXS patterns acquired from the 69/31 w/w PMMA/PVDF blend.

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

“…These endotherms indicate T m values (133°C) lower than that of neat PVDF (160°C) because of the imperfect formation of PVDF crystals under restricted chain motion at high-viscosity conditions and temperatures near the blend T g . The CO 2 -induced crystallization of PVDF in PVDF/PMMA blends has been established by Chiou et al, [118] but crystallization at low PVDF concentrations has been reported by Walker et al [107] The difference between the two studies can be attributed to the difference in CO 2 pressure ranges examined: 1-2.5 MPa versus 17-31 MPa, respectively. Higher CO 2 pressures apparently promote greater plasticization and thus enable higher chain mobility, which facilitates the formation of PVDF crystals even in blends predominantly composed of PMMA.…”

“…Crystallization has also been induced by CO 2 in polymer blends consisting of at least one crystallizable component. For instance, PVDF in a PVDF/PMMA blend has been reported [63,107,118] to crystallize upon exposure to CO 2 . However, the molecular process of crystallization in such blends is more complex than in neat semicrystalline PVDF since the presence of PMMA effectively serves to deplasticize the PVDF and hinders crystallization in blends with more than ca.…”

“…Similar scCO 2 -enhanced crystallization has been recently observed by Yu et al [119] in glassy-crystalline-glassy triblock copolymers. Walker et al [107] have also examined the time and pressure dependence of PVDF/PMMA blends in situ through the use of small-angle X-ray scattering (SAXS). The observed SAXS patterns are consistent with the formation of PVDF lamellae and confirm that changes in blend crystallinity are sensitive to CO 2 pressure and thus the extent of plasticization.…”

“…Interestingly, it has also been found that the CO 2 -grown crystals align along the same direction as the original PVDF crystals. Crystallization of PVDF in PVDF/PMMA blends after exposure to high-pressure CO 2 at 35°C and up to 31 MPa pressure has also been examined [107] by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Exposure to scCO 2 results in the formation of crystalline PVDF lamellae measuring 4-6 nm laterally, according to TEM analysis of blends with 12 and 31 wt% PVDF (Fig.…”

“…RamachandraRao et al [105] have reported similar behavior for blends composed of deuterated polybutadiene (PB) and PI. We have indirectly observed [106,107] the changes in phase behavior of another series of LCST blends composed of semicrystalline poly(vinylidene fluoride) (PVDF) and perdeuterated or hydrogenous poly(methyl methacrylate) (PMMA) upon exposure to scCO 2 . An investigation [108] of a similar system composed of PVDF and poly(ethyl methacrylate) (PEMA) probed the phase behavior directly with cloud point measurements.…”