Porous structure, permeability, and mechanical properties of polyolefin microporous films

Physics of the Solid State, 2012

The surface structure of polypropylene and polyethylene microporous films prepared by the extrusion of the polymer melt with the subsequent stages of annealing, uniaxial extension, and thermal fixa tion of the samples has been analyzed using scanning electron microscopy. It has been shown that percolation through pores corresponds to the axial texture of the surface with the channel structure described by the frac tal cluster model. The transition from open pores (through flow channels) to closed pores leads to the forma tion of surface regions with a biaxial texture. An increase in the density of the solid phase cluster is accompa nied by the formation of a homogeneous biaxial texture with a period of alternation of the density in two mutually perpendicular directions, one of which coincides with the direction of orientation of the films.

Polymer, 2009

Cast films of a high molecular weight linear polypropylene (L-PP) were prepared by extrusion followed by stretching using a chill roll. An air knife was employed to supply air to the film surface right at the exit of the die. The effects of air cooling conditions, chill roll temperature, and draw ratio on the crystalline orientation, morphology, mechanical and tear properties of the PP cast films were investigated. The crystallinity and crystal size distribution of the films were studied using differential scanning calorimetry (DSC). It was found that air blowing on the films contributed significantly to the uniformity of the lamellar structure. The orientation of crystalline and amorphous phases was measured using wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The amount of lamellae formation and long period spacing were obtained via small angle X-ray scattering (SAXS). The results showed that air cooling and the cast roll temperature have a crucial role on the orientation and amount of lamellae formation of the cast films, which was also confirmed from scanning electron microscopy (SEM) images of the films. Tensile properties and tear resistance of the cast films in machine and transverse directions (MD and TD, respectively) were evaluated. Significant increases of the Young modulus, yield stress, tensile strength, and tensile toughness along MD and drastic decreases of elongation at break along TD were observed for films subjected to air blowing. Morphological pictograms are proposed to represent the molecular structure of the films obtained without and upon applying air cooling for different chill roll temperatures. Finally, microporous membranes were prepared from annealed and stretched films to illustrate the effect of the PP cast film microstructure on the morphology and permeability of membranes. The observations of SEM surface images and water vapor transmission rate of the membranes showed higher pore density, uniform pore size, and superior permeability for the ones obtained from the precursor films prepared under controlled air cooling.

Journal of Membrane Science, 2008

Journal of Plastic Film and Sheeting, 2000

Microporous films from immiscible blends were produced via melt processing and post-step treatments. Polystyrene (PS)/polypropylene (PP) and poly(ethylene terephthalate) (PET)/polypropylene blend systems with different viscosity ratios were studied. The blends were first compounded in a co-rotating twin-screw extruder and subsequently extruded through a sheet die to obtain the precursor films. These were uniaxially or biaxially drawn (100-400%) with respect to the original dimension to induce microporous structure and post treated at elevated temperature to maintain the porous structure which consisted of uniform microcracks in the order of a few hundred nanometers. The fabrication process here is shown to be a promising technique for producing microporous films that can be used for liquid and gas separations.

Polymer Science Series A, 2010

Effects of the degree, velocity, and temperature of repeated extension of porous PE films on their structure and transport and mechanical properties are investigated. The sizes of through flow channels and the permeability of porous films are determined via filtration porosimetry. A rise in the temperature and the degree of extension in the course of orientation extension noticeably increases the sizes of pores, the overall porosity, the specific surface area, and the permeability of the films and improves their mechanical character istics.

Journal of Plastic Film and Sheeting, 2005

Four different polypropylene (PP) resins are extruded using the tubular blown and cast film processes. The resin morphology is observed by SEM and the effect of extrusion processing variables on the morphology is investigated. Melt rheological experiments are also carried out to characterize the polymer melts. It has been found that the molecular weight distribution and the chain structure as well as the processing conditions have important effects on the morphology. Efforts have been focused on developing a row-nucleated morphology from PPs through the control of processing conditions. The possibility of generating a porous membrane from the initial row-nucleated morphology using a stretching technique is evaluated. It has been found that the initial lamellae arrangement of the precursor films and the stretching conditions play a significant role in obtaining a porous structure.

Desalination, 2004

An alternative method using established polymer processing and membrane formation processes to prepare hydrophilic membranes based on acid modified polyolefins is introduced. This process includes blend formation by reactive extrusion of functionalised polyolefins (polypropylene PP) and a hydrophilic polyamide (PA). In a first step the pore formation is realised by extracting non-covalently bound polyamide. The resulting membranes are tested by pure water as well as protein filtration. The properties are discussed in terms of permeate flux, retention capability and fouling behaviour.

Journal of Polymer Research, 2010

This paper presents a study on the synthesis of porous polymers with an application to microneedles. A range of Poly (ethylene glycol-co-methacrylic acid) polymers are synthesised using bulk polymerisation techniques to produce porous polymers with various strength and fluid transport characteristics. The synthesised materials are morphologically and mechanically characterised. Using different porogens in the polymer synthesis results in different mechanical strength and fluid flow characteristics. The results indicate that the fluid flow characteristics of the polymers can be sacrificed to improve strength. Optimum polymer strength can be attained by synthesising polymers with macropores that are interconnected via nanopores using the minimum amount of porogenic solvents.

Journal of Membrane Science, 2009

Polypropylene/high density polyethylene (PP/HDPE) multilayer as well as monolayer films were prepared to develop microporous membranes using cast film extrusion followed by stretching. The effects of draw ratio (DR), cooling air flow rate (AFR), and annealing on the crystalline structure and orientation of the monolayer and components in the multilayer films were investigated using wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). Annealing and high DR and AFR enhanced the crystal alignments significantly. However, the orientation of the PP and HDPE in the multilayer film was slightly lower compared to the monolayer films. The amount of lamellae formation, long period spacing (L p ), and lamellae thickness (l c ) were obtained via small angle X-ray scattering (SAXS). A significant influence of annealing on L p and l c was observed for the PP. The row-nucleated lamellar crystallization and transcrystallization observed in the surface and cross-section of the films, respectively, are discussed. Scanning electron microscopy (SEM) images of the membrane surface and cross-section obtained for the cold and hot stretch ratios of 55% and 75%, respectively, showed larger pores and higher porosity for the HDPE layer compared to the PP. As the level of the applied extension during cold stretching increased, the water vapor transmission rate (WVTR) of the HDPE monolayer improved while the effect was inversed for the PP single layer. In addition, compared to the monolayer membranes, the multilayer ones showed smaller WVTR. Tensile properties of the precursor films and microporous membranes in the machine and transverse directions (MD and TD, respectively) as well as puncture resistance in the normal direction (ND) were evaluated.

Journal of Membrane and Separation Technology, 2012

Microporous high density polyethylene flat membranes were fabricated via thermally induced phase separation (TIPS) method. Effects of polymer concentration and coagulation bath temperature on the membrane morphology and porosity were investigated. To the best of our knowledge, there is no work about the order of magnitude and degree of importance of influential parameters and their interactions on the microstructure of fabricated membranes. The results showed that the porosity of membranes decreased as the polymer concentration increased. It was also shown that, due to the short contact time and rapid phase inversion between coagulation bath and membrane's outer surfaces, bath temperature mainly affects on the outer surface porosity. The results obtained from analysis of variance (ANOVA) using 95% confidence interval on the membrane porosity revealed that the effect of polymer concentration is more important than coagulation bath temperature.