Pooja Bajaj

Applications of Low Voltage Field Emission Scanning Electron Microscopy (FE-SEM) for Characterization of Polyethersulfone/ Polyvinylpyrillidone (PES/PVP) Based Materials for Membrane Separations

Polymer blending offer an interesting route to modify the material properties with the goal to combine the unique key attributes that the individual constituents have to offer for improved performance. Polyethersulfone/polyvinylpyrrolidone (PES/PVP) presents unique properties when combined owing to their mechanical and chemical stabilities for preparing ultrafiltration membranes with a low molecular weight cut-off for biotechnology applications [1]. The morphology and structure often assumes a key role in governing the polymer properties. These membranes were cast using PES (BASF E6020P) at 14 weight% and polyvinylpyrrolidone (PVP) K90 at 2 weight %, PVP K30 at 5 weight %, using phase inversion process from n-methylpyrrolidone (NMP) into water [1]. The blending of low and high K-value PVP is shown to be important to controlling the surface pore structure and tendency towards macro void formation [1]. For a functional membrane, surface provides insight into the roughness and selective porous structure, while the cross sectional morphology reveal information about the mechanical stability of the membrane. In this work, low voltage field emission scanning electron microscopy characterization using Carl Zeiss Supra 40 VP FE-SEM of PES/PVP based flat membranes will be discussed.

Understanding the Structure-Process-Property Balance in PC/PEI blends by Morphology (using Scanning Transmission Electron Microscopy in Field Emission -Scanning Electron Microscopy) and Correlative Deformation Mechanics

Nanoparticle dispersion has been reported to have a critical impact on the properties of polymer composite materials [1, 2]. Here in, we present the application of STEM in FESEM to study the phase morphology and correlative deformation phenomenon in Polycarbonate (PC)/ Polyetherimide (PEI) blends to bridge the understanding between functional and failing material performance at different processing conditions. PEI is an amorphous, transparent engineering thermoplastic with a unique chemical structure. The aromatic backbone offers superior heat resistance with high Tg ~216°C, low smoke, hydrolytic and improved solvent resistance and modulus. However, high Tg poses a challenge for processability, brittleness and limit its broader use. The colorability of PEI is also limited due to its natural amber color with yellowness index (YI) > 50. Blending PEI with PC, offers gainful advantage for PEI processability, imparts toughness and excellent colorability to the blend. PC/PEI exhibits a distinctly phase separated morphology with PEI phase forming a discrete phase residing in the continuous matrix of PC with pigment dispersion (Figure 1a). For an immiscible blend, the properties are mainly dominated by the continuous phase, but other variables such as the composition, viscosity of the blend, and processing conditions can influence the nanoparticle pigment dispersion and distribution, discrete phase size and shape, and interfacial free volume (compatibilization) yielding different tensile stress vs strain behaviors (Figure 1b,c).

From Flat Membranes to Fabrication of Hollow Spun Fibers of Polyethersulfone/ Polyvinylpyrillidone (PES/PVP) - Correlative Scanning Electron Microscopy (FE-SEM), 3-D X ray Microscopy and Ultrafiltration Properties

In a previous report, we presented the application of low voltage field emission scanning electron microscopy to characterize the structure and morphology of lab scale flat membranes cast using Polyethersulfone/polyvinylpyrrolidone (PES/PVP) (Figure 1a, b) [1 and all references within]. Morphology was insightful in tuning the compositional variables and optimizing the design space for translation into hollow spun fibers. In this work, morphological comparison of flat membranes with the hollow spun fibers and their performance properties will be presented. For (PES/PVP) based hollow fiber spinning, the dope solution containing PES (BASF E6020P) at 14 weight% and polyvinylpyrrolidone (PVP) K90 at 2 weight %, PVP K30 at 5 weight %, were blended into a homogenous solution. The blending of low and high K-value PVP is shown to be important in controlling the surface pore structure and tendency towards macro void formation using phase inversion process from n-methylpyrrolidone (NMP) into water [2]. Fiber spinning was performed by dry-wet immersion precipitation using a bore solution of 70 wt% deionized water and 30 wt% NMP [2]. Dope solution along with the bore liquid were simultaneously pumped through a double orifice spinneret and after passing the air gap, immersed into the water coagulation bath. Figure 1c visualizes the fiber spinning process. The fibers were post treated by washing in 70°C pure water for 3h and air dried. Some fibers were immersed for 24 into a mixture of water/glycerol (80wt%/ 20wt %) prior to the drying step. Lab scale hollow fiber membrane modules were prepared from these as spun and dried, and glycerol post treated and dried fibers and tested for the clean water flux and molecular weight cut off measurements. Non-destructive 3-D pore distribution was visualized using Carl Zeiss Xradia 510 Versa and Xradia 810 Ultra X ray microscopy using Zernike phase contrast mechanism to benefit for the otherwise low attenuating membrane material [3].