Experimental Investigation of the Fiber Formation Process and Web Structures Using an Annular Meltblowing Spinneret

This research experimentally investigates the fiber and web formation process of an array of annular meltblown spinnerets. In this design, the molten polymer is extruded from an array of outlets, each of which is individually surrounded by a concentric high-velocity heated air stream. With its multirow capability, it potentially becomes a high-productivity microfiber fabrication process. We experimentally investigate the effects of critical processing parameters and material properties on the fiber and web formation process. First, the polymer thermal and rheological behavior is presented. Next, a detailed three-dimensional air temperature and velocity profile, measured in the absence of spinning fibers, is presented for an array of supplied temperatures and internal machine air pressures. Web analysis in relation to this air profile shows that smaller fibers in cooler air streams require shorter die-collector distances to form bonded fabrics. Calculations are then made that show polymer spinning temperature is largely determined by air temperature, a distinguishing feature of this meltblowing design. Finally, a full factorial variation of air temperature, air speed, and polymer throughput is shown that relates processing conditions to fiber diameter distribution. Median diameters are well described by an empirical model and ranged from less than 1 μm to almost 14 μm

Rheological and Tribological Behavior of Gels and Emulsions Containing Polymer and Phospholipid

Oil-in-water (O/W) emulsions are widely used in food, pharmaceutical, and personal care applications. These types of systems often contain hydrophobically modified polymers and phospholipids, where the polymer acts as a rheology modifier or emulsifier, while the phospholipid acts as a functional performance surfactant. However, the underlying mechanism through which the different components affect rheological and tribological characteristics is not well understood. To this end, we take a hierarchical approach, evaluating first the hydrophobically modified polymer alone, followed by polymer and phospholipid and then an O/W emulsion stabilized with the polymer or the phospholipid and polymer combined. We characterize the tribological behavior using a soft model contact consisting of polydimethyl­siloxane (PDMS), which has elastic modulus similar to human skin. Bulk rheology results show that the studied systems are shear thinning and have gel-like behavior with elastic modulus increasing substantially upon phospholipids addition. Friction coefficients increase in the elastohydrodynamic regime with increasing sample viscosity. However, systems containing the hydrophobically modified polymer and phospholipids show a lower friction coefficient at the boundary regime. Adsorption studies with a quartz crystal microbalance with dissipation (QCM-D) measurements show that phospholipids are being adsorbed onto the PDMS surface. Confocal laser scanning microscopy of the PDMS surfaces before and after immersion in a hydrophobically modified polymeric suspension containing rhodamine B shows the presence of the polymer on the PDMS surface even after DI water rinse, indicating polymer adsorption, thus resulting in lower friction coefficients at low speeds

Mesomorphic−α-Monoclinic Phase Transition in Isotactic Polypropylene: A Study of Processing Effects on Structure and Mechanical Properties

We report the enthalpy for the mesomorphic to α-monoclinic phase transition in polypropylene under varying thermal treatments. The mesomorphic phase is created by fiber spinning and rapid quenching methods and identified using wide-angle X-ray diffraction and differential scanning calorimetry. Fiber mesomorphs are found to have a 3-fold increase in enthalpy of transition per gram of mesophase compared with our measurements of quenched polypropylene and previous reports of quenched polypropylene. In addition, systematic tensile testing over a range of spin speeds and polymer morphologies reveals that the presence of mesomorphic regions does not correlate with reduced fiber strength as has been previously suggested. Fiber true stress−true strain curves obtained at varying take-up velocities are compared to determine the “tensile strain shift”, which should theoretically provide a measure of molecular orientation. We find that the tensile strain shift correlates with birefringence, thereby providing an alternative method to assess molecular orientation in fibers, an important factor for fiber strength. This approach can prove useful for fibers in which measuring the molecular orientation via birefringence is not an option

Surface-Constrained Foaming of Polymer Thin Films with Supercritical Carbon Dioxide

Microcellular polymer foams afford a wide variety of attributes relative to their dense analogues, and efforts remain underway to establish viable routes to generate foams with substantially reduced pore cell size and increased pore cell density. Barrier constraints are applied in the present work to achieve diffusion-controlled isothermal foaming of thin polymer films in the presence of supercritical carbon dioxide (scCO2). Poly(methyl methacrylate) (PMMA) films measuring ca. 95−100 μm in thickness are physically constrained between two impenetrable plates so that scCO2 exit diffusion is restricted to the film edges. Results obtained here demonstrate that the pore size can be systematically reduced to less than 100 nm in such systems by applying high saturation scCO2 pressures, relatively low foaming temperatures (near the glass transition temperature of the scCO2-plasticized polymer), and a rapid pressure quench. Classical nucleation theory (CNT) modified to account for the compressible nature of scCO2 is used to describe pore cell growth as a function of foaming temperature and scCO2 saturation pressure. Incorporation of a gradient model based on the Sanchez−Lacombe equation of state to account for PMMA−CO2 interfacial tension in conjunction with the CNT yields accurate predictions of foam cell densities as a function of relevant system parameters

Effect of pH on Protein Distribution in Electrospun PVA/BSA Composite Nanofibers

We examine the protein distribution within an electrospun polymer nanofiber using polyvinyl alcohol and bovine serum albumin as a model system. We hypothesize that the location of the protein within the nanofiber can be controlled by carefully selecting the pH and the applied polarity of the electric field as the pH affects the net charge on the proteins. Using fluorescently labeled BSA and surface analysis, we observe that the distribution of BSA is affected by the pH of the electrospinning solution. Therefore, we further probe the relevant forces on the protein in solution during electrospinning. The role of hydrodynamic friction was assessed using glutaraldehyde and HCl as a tool to modify the viscosity of the solution during electrospinning. By varying the pH and the polarity of the applied electric field, we evaluated the effects of electrostatic forces and dielectrophoresis on the protein during fiber formation. We surmise that electrostatic forces and hydrodynamic friction are insignificant relative to dielectrophoretic forces; therefore, it is possible to separate species in a blend using polarizability contrast. A coaxial distribution of protein in the core can be obtained by electrospinning at the isoelectric point of the protein, whereas surface enrichment can be obtained when the protein carries a net charge

Morphological and Thermochemical Changes upon Autohydrolysis and Microemulsion Treatments of Coir and Empty Fruit Bunch Residual Biomass to Isolate Lignin-Rich Micro- and Nanofibrillar Cellulose

Autohydrolysis and microemulsion treatments followed by microfluidization are employed to isolate micro- and nanofibrillar cellulose (MNFC) from coir fibers and palm tree empty fruit bunches (EFB) with residual lignin content of ∼24 and ∼31 wt %, respectively. The fibers and associated MNFC are characterized in each treatment for their chemical, structural, and thermal properties. The most significant findings include the fact that two MNFC populations are produced, with distinctive structural differences and characteristic lateral dimensions of 20–70 nm and 1–3 μm. The lignin distribution after possible recondensation occurred in the form of nanodroplets. Finally, a correlation between thermal degradation of MNFC with spatial arrangement of lignin is hypothesized and a defibrillation mechanism is proposed. The detailed structural and thermochemical analyses presented here are expected to facilitate further interest in the development of new materials from MNFC isolated from coir and EFB, two abundant bioresources that are most suitable for their valorization

Multiscale Constitutive Modeling of the Mechanical Properties of Polypropylene Fibers from Molecular Simulation Data

We present a multiscale approach to create a constitutive model that predicts the mechanical properties of polypropylene fibers based on chemical and physical characteristics. The development of this method relies on validation with experimental stress–strain curves from nine different isotactic polypropylene (iPP) fibers with their varying molecular weight characteristics, Hermans orientation factors, and crystallinity. Complementary molecular models were built by using molecular dynamics (MD) simulations with united atom models. Tensile deformation simulations adapting a quasi-static procedure resulted in stress–strain curves that aligned well with the experimentally measured ones. A neural network model was trained on the MD simulation data to create correlations that predict parameters for a chosen constitutive model that describes the mechanical properties of the polypropylene fibers. This computational approach is amenable to be applied to polymer fiber systems and aims to aid in the design of polymeric materials to achieve targeted mechanical properties

Kinetics of Enzymatic Depolymerization of Guar Galactomannan

A new mathematical model based on Michaelis Menten (MM) kinetics is developed to predict the changes in molecular weight distribution (MWD) during the enzymatic depolymerization of guar galactomannan. The model accounts for the effect of branching by considering the guar molecule as a substrate having three types of bonds with different MM kinetic parameters. The overall kinetics of the enzymatic reactions then can be represented in terms of composite kinetic parameters that are functions of the MM parameters for the individual bonds. The depolymerization is assumed to follow a random scission mechanism, in which an enzyme randomly attacks the substrate molecule at any one of the three types of bonds, and leaves the substrate on cleavage of the bond. Expressions for the variation in molecular weights during depolymerization are developed by applying moment generating techniques to the kinetic model. The model is evaluated against the complete MWD obtained using gel permeation chromatography. During the initial stages of depolymerization, the enzymatic reaction is in the zero-order regime of MM kinetics and the polydispersity index (PDI) increases with time. Subsequently, the PDI decreases as the depolymerization tends to follow first order kinetics. We also show that for a zero-order, random or nonrandom scission, the variation of PDI with time can exhibit a maximum. These analyses confirm that an increase in PDI during the depolymerization is not necessarily due to nonrandom scission, as previously concluded

<i>In Situ</i> Cross-Linking of Electrospun Poly(vinyl alcohol) Nanofibers

We examine single step reactive electrospinning of poly(vinyl alcohol) (PVA) and a chemical cross-linking agent, glutaraldehyde (GA), with hydrochloric acid (HCl) as a catalyst to generate water insoluble PVA nanofibers. Such an approach using a conventional setup with no modification enables the fibers to cross-link during the electrospinning process, thereby eliminating the need for post-treatment. Significant changes in the rheological properties occur during in situ cross-linking, which we correlate with electrospinnability. In particular, we associate changes in dynamic rheological properties to changes in fiber morphology for two regions: (1) below the critical concentration to electrospin PVA only and (2) above the critical concentration to electrospin PVA only. In region 1 fiber morphology changes from beaded fibers to uniform fibers to flat fibers, and in region 2 fiber morphology changes from uniform fibers to flat fibers. Electrospinning windows to generate uniform fibers for both regions are determined and can be manipulated by changing the molar ratio of GA to PVA and the volume ratio of HCl to GA. The electrospun fibrous material generated can be rendered insoluble in water, and the uniform fiber morphology can be maintained after soaking in water overnight. The reactive electrospinning process also lowers the critical PVA concentration required for successful electrospinning of the system