Lamellar crystalline structure of hard elastic HDPE films and its influence on microporous membrane formation

The development of hard elastic high-density polyethylene (HDPE) precursor films and its influence on the microporous membrane formation have been investigated. As a first step, the HDPE precursor films with ‘row-nucleated lamellar crystalline’ structure were prepared by applying elongation stress to the HDPE melt during T-die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long-period lamellar spacing, crystallite size, and degree of crystallinity. The processing (melt extension and annealing temperature)-structure (lamellar crystalline structure)-property (hard elasticity) relationship of HDPE precursor films was also investigated. The uniaxial stretching of hard elastic HDPE precursor films induces the bending of crystalline lamellae, which leads to the formation of micropores between them. The observation of morphology and air permeability for the HDPE microporous membranes have revealed that the well-developed porous structures characterized by superior air permeability were established preferably from the precursor films prepared by the high stress levels and the high annealing temperatures. Finally, the relationship between the hard elasticity of HDPE precursor films and the air permeability of corresponding microporous membranes was discussed.     

Impact of different die draw ratio on crystalline and oriented properties of polypropylene cast films and annealed films

Four kinds of polypropylene (PP) cast films with different die draw ratios (DDR) were prepared. The impact of different DDR on the crystalline and oriented properties of PP cast films and annealed films was explored herein. Wide angle X-ray diffraction (WAXD) and fourier transform infrared (FTIR) methods were adopted to examine the orientation degree of crystalline and amorphous phases. Long period distance (L_p) of the crystalline structure was tested by small angle X-ray scattering (SAXS). Crystallization was determined by differential scanning calorimeter (DSC). The oriented and crystalline behaviors of the samples were carried out by the elastic recovery (ER) testing. Then, samples after being annealed were examined by the same methods. The influence of annealing process on the films’ structures and properties was explored. Besides, the final stretched microporous membranes manufactured via stretching the annealed films along machine direction were examined by scanning electronic microscope (SEM). No matter for cast films or for annealed films, it is found that the films’ orientation degree of crystalline and amorphous phases, as well as L_p and crystallinity are larger at higher DDR and relatively lower at lower DDR. When the DDR is overly high (DDR?=?170), both the oriented and crystalline properties will decline. Elastic recovery testing indicates that a film with better orientation of the crystalline and the amorphous phases as well as with higher crystallinity can be obtained at an appropriate DDR. SEM images show that stretched membranes with better microporous structure can be obtained when the precursor film is prepared at a proper DDR.     

Oriented structure of PP/LLDPE multilayer and blends films

Polypropylene/linear low-density polyethylene (PP/LLDPE) multilayer and blends blown films are investigated in terms of crystalline morphology and orientation. The crystalline structures were probed using microscopy, infrared trichroism and X-ray pole figures. Two different PPs were used, one was a homopolymer and the other was a copolymer. Similar orientation characteristics were observed for both PP components in the blends and multilayer films, the PE component, however, showed very different structure, especially for the crystalline a- and b-axes orientation. The row-nucleated structure, epitaxial crystallization and transcrystallization observed in the films are discussed.     

Structure–Property–Process Relationship for Blown Films of Bimodal HDPE and Its LLDPE Blend

Blown films of bimodal‐high‐density polyethylene (HDPE) (BPE) and its blend containing 40 wt% of linear low‐density polyethylene (LLDPE) are prepared in various neck‐heights (NHs). The crystal structures of both films are investigated in detail using small‐angle X‐ray scattering and wide‐angle x‐ray diffraction techniques. The results show that the blending of LLDPE notably modifies the crystal structure of BPE, including crystal density (ρ_c), crystallite size of the 110 plane (〈L₁₁₀〉), thickness of the lamellar crystal (L_c), and grain widths of the lamellae. The relationships between NH, crystal structure, and the resistance of dart‐drop impact (DDI) are investigated for both BPE and BPE/LLDPE films. The results indicate that the reorientation of lamellae might be a primary factor responsible for the DDI property. However, large values of ρ_c, L_c, and 〈L₁₁₀〉 are required for the film to achieve high DDI.     

Influence of precursor film microstructure on properties of HDPE microporous membranes prepared by stretching

Oriented HDPE films having a stacked lamellar morphology were prepared to develop microporous membranes through cast film extrusion followed by stretching. Applying higher draw ratios (DR) and annealing the cast films improved crystal alignments remarkably. It was shown the improvement in c‐axis orientation upon annealing was more significant for the films prepared with lower DR. Furthermore, applying Raman spectroscopy, the amorphous mass fraction of the films was obtained. Subsequently, considering a three‐phase structure (i.e., crystal, amorphous and interphase), the interphase content of the films was also determined. A distinct DR dependence of the interphase content was noticed. It was observed that in addition to crystal perfection upon annealing, the amorphous phase fraction was also decreased due to possibly increase in free volume, promoting lamellar separation and cavitation during the subsequent stretching step. SEM images of the membranes surfaces as well as their normalized water vapor transmission rate (WVTR) values indicated a more uniform pore formation and increased permeability with DR. Furthermore, it was found that lower interphase content in the films with higher DR favors lamellar separation. However, permeability in the stretched precursor films with DR higher than a certain level (DR = 93) was not improved significantly. Moreover, crystallinity and crystal orientation of produced membranes were investigated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44725.     

Preparation and characterization of isotactic polypropylene/high‐density polyethylene/carbon black conductive films with strain‐sensing behavior

Novel conductive films with a unique strain (ε)‐sensing behavior and based on a blend of isotactic polypropylene (iPP), high‐density polyethylene (HDPE), and carbon black (CB) were fabricated by an extrusion casting method. The morphology and ε‐sensing behavior of the films were investigated. Scanning electron microscope images showed that the oriented lamellae with a growing direction perpendicular to the extrusion direction were obtained in the HDPE phase and were accompanied by a cocontinuous structure of the iPP phase and HDPE/CB phase. The conductive percolation threshold (m_c) and resistivity–ε behavior of the thin films are affected by the drawing ratio during the process of film preparation. The m_c and electrical resistance of the iPP/HDPE/CB composite films increased with the drawing ratio. The gauge factor of the films within the elastic region decreased with increasing drawing ratio. Furthermore, the result of iPP/(HDPE/CB) 40/60 with a high drawing ratio shows that a reversible conductivity was obtained during the cyclic tensile testing (ε = 10%), but an irreversible conductivity makes the film fail during use at the applied ε values of up to 15%. This makes them good piezoresistive candidates for ε‐sensing materials. Moreover, a simple structural model was proposed to describe the reversible and irreversible phenomena in the electrical resistance behavior of the iPP/HDPE/CB films under tensile loading. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40686.     

Effect of molding temperature on crystalline and phase morphologies of HDPE composites containing PP nano-fibers

A high-density polyethylene (HDPE)/isotactic polypropylene (PP) (75/25) blend containing 25 wt% of PP was fibrillated by roller drawing at 138 °C. The fibrillated blend was processed again at temperatures ranging from 155 to 200 °C by compression molding or extrusion. The effects of molding temperature on the morphology and mechanical properties of the blend were investigated. Wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) were used to study the morphology of the samples. The roller-drawn blend exhibited a fibrous structure with the chain direction aligned parallel to the drawing direction. After molding at 155 °C, the HDPE formed parallel-stacked lamellae retaining the parallel orientation after the melting of the PE crystals. As the molding temperature increased the parallel orientation gradually vanished and some of the parallel-stacked lamellae changed into twisted lamellae. The PP phase existed as fibrils in the PE matrix and the crystals stayed with their molecular chain aligned parallel to the fibrillation direction even when the molding temperature was far above the melting temperature of PP. Nevertheless, the orientation of the crystals did not change as the molding temperature increased from 155 to 165 °C. The internal structure of the PP fibrils changed from a needle structure to a parallel-stacked one. The PP fibrils induced the crystallization of the PE melt, leading to the formation of a trans-crystalline layer at their surface. As the molding temperature increased, more PE lamellae protruded into the PP fibrils and the interface between the PP fibrils and the PE matrix became diffuse.     

An investigation of optical clarity and crystalline orientation in polyethylene tubular film

A study of the crystalline orientation, light transmission, and surface roughness of polyethylene tubular film prepared in our laboratories is presented. The present studies were primarily carried out on low-density (LDPE) and linear-low-density (LLDPE) polyethylene films. The optical properties of a few films of high-density polyethylene (HDPE) prepared for a previous study of morphology were characterized for comparison to the LDPE and LLDPE films. Wide angle X-ray diffraction and birefringence were used to characterize orientation. Both the LDPE and LLDPE films exhibited crystalline texture in which the b-axes tended to be perpendicular to the film surface and the a-axes had some tendency to align with the machine direction. The c-axes tended to be concentrated in the plane of the film with nearly equal biaxial orientation with respect to the machine and transverse directions. Little variation in the crystalline orientation was found with changes of process conditions in the range studied. Birefringence results indicate that the amorphous regions developed an orientation in which the chains tend to be normal to the film surface. The majority of light scattering from these films and a series of HDPE films was from the surface and not from the film interior. The transmission coefficient for the surface contribution was found to be a monotonic decreasing function of the standard deviation of the surface height obtained from surface profiles measured by profilometer. The surface asperites were largest for the HDPE and smallest for the LDPE samples. The intensity of both the surface and interior contributions to the scattering increased with increasing frostline height, i.e., a slower cooling rate. As draw-down ratio and blow-up ratio increase the scattering contribution from the film interior decreases but the contribution from the surface increases somewhat. These effects are discussed in terms of the changes in crystalline morphology and surface roughness produced by flow defects generated during extrusion.     

Effects of melt‐extension and annealing on row‐nucleated lamellar crystalline structure of HDPE films

The row‐nucleated lamellar crystalline structure of high‐density polyethylene (HDPE) films was prepared by applying elongation stress to HDPE melt during T‐die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long‐period lamellar spacing, crystallite size, and degree of crystallinity. The contribution of melt‐extension represented by draw‐down‐ratio (DDR) to the overall orientation was found to be most noticeable than other processing variables. Meanwhile, the long‐period lamellar spacing, the crystallite size, and the degree of crystallinity were influenced predominantly by the annealing temperature. Finally, the processing (melt extension and annealing temperature) – structure (lamellar crystalline structure) – property (hard elasticity) relationship of HDPE films was investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3326–3333, 2007     

热处理条件对聚丙烯流延基膜取向片晶结构及拉伸成孔性的影响

摘要：为了优化挤出流延单向拉伸法制备聚丙烯微孔膜的工艺,以温度、时间和基膜受到的张应力做为热处理条件变量,通过DSC、FTIR、SEM、电子材料试验机和透气率测试仪等方法表征流延基膜的取向片晶结构参数和微孔膜的孔结构,考察热处理条件对聚丙烯流延基膜取向片晶结构和拉伸成孔性的影响。研究表明,随热处理温度从120℃提高到145℃,基膜的取向片晶结构得到改善,硬弹性增强,拉伸成孔性能变好;随热处理时间延长,基膜的结晶度、片晶厚度及取向程度不断提高,取向片晶结构趋于完善,当热处理时间达到30min后,取向片晶结构的完善程度接近稳定;在热处理过程中对基膜施加适当张应力可以提高基膜的片晶取向程度和硬弹性,改善其拉伸成孔性。     

Structural characterization of Celgard® microporous membrane precursors: Melt-extruded polyethylene films

“Row nucleated lamellar” structures are formed when highly crystalline polymers are melt-extruded and recrystallized under high stress. Polyethylene (PE) and polypropylene (PP) films with row lamellar structures have been utilized to produce microporous membranes. Birefringence measurements of melt-extruded PE films show that improved film orientation can be achieved by annealing, extruding at higher speed, and using higher molecular weight polymers. Images from scanning tunneling, atomic force, and field emission scanning electron microscopy (STM, AFM, and FESEM) clearly show the lamellar structures in the melt-extruded PE and PP films. Microscopy results also show that surface lamellar textures are more pronounced with thicker lamellae and are better aligned along the extrusion direction after annealing. X-ray diffraction results show that the increase in film orientation can be attributed to increased lamellar perfection and orientation during annealing and also to better crystallite alignment along the machine direction with higher extrusion speed or with higher molecular weight. High-resolution capabilities of STM, AFM, and FESEM prove to be very effective tools in elucidating lamellar structures in polymeric membrane precursors and can be used as an aid in establishing structure–process–property relationships in making microporous membranes. © 1994 John Wiley & Sons, Inc.     

Effect of molecular weight on molecular orientation and morphology of polypropylene sheets containing a β-nucleating agent

In this study, polypropylene (PP) films containing the β nucleating agent, N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide, were prepared using PP with three different molecular weights (low, medium, and high) by extrusion process with T-shaped die. The structure and morphology of the films were studied after stretching. It was found that a unique molecular orientation, in which both the c-axis and crystalline lamellae were oriented perpendicular to the flow direction, was formed in all undrawn film samples, irrespective of the molecular weights of the PP. In the PP sheets stretched in the machine direction, the low-molecular-weight sample containing the nucleating agent exhibited brittle properties owing to a lack of tie chains in the stretching direction. In contrast, cavitation was prominent in the medium (M-PP)- and high (H-PP)-molecular-weight samples. Notably, M-PP containing the nucleating agent, with a high degree of molecular orientation, promoted the formation of a large number of voids. In H-PP containing the nucleating agent, the presence of numerous tie chains inhibited cavitation, resulting in fewer voids. The experimental results demonstrated the influence of the molecular weight on the void structure, which will be useful in the field of microporous membranes.     

Structure and properties of MDO stretched polypropylene

Two polypropylene cast films of different crystalline structures (one with coexisting small rows of lamellae and spherulites and the other with only a spherulitic structure) were prepared by extrusion. The produced cast films were uniaxially hot drawn at T = 120 °C using a machine direction orientation (MDO) unit and the changes in structure and morphology were examined and related to barrier as well as tear and puncture properties. Structural changes in terms of the degree of crystallinity and crystal size distribution, orientation of the amorphous and crystalline phases, and the deformation behavior at the crystal lattice and lamellae scales were investigated using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), respectively. A significant effect of the original crystal morphology on the alignment of the amorphous and crystalline phases was observed from FTIR and WAXD. The results also revealed that the deformation behavior of the crystal structure was dependent on the draw ratio (DR). Our findings showed that by increasing DR the crystal lamellae first broke up and oriented along the drawing direction and then, at large DR, they were deformed and created a fibrillar structure. Morphological pictograms illustrating the effects of original morphology and draw ratio on the stretched film microstructure are proposed. The tear resistance along the machine direction (MD) decreased significantly with increasing DR whereas the puncture resistance increased drastically. Finally, the oxygen transmission rate (OTR) of the MDO stretched films could be correlated with the orientation parameters as well as the β-relaxation peak magnitude of the amorphous tie chains.     

Effect of confinement on glass dynamics and free volume in immiscible polystyrene/high‐density polyethylene blends

The effect of confinement on glass dynamics combined with the corresponding free volume changes of amorphous polystyrene (PS) in blends with semi‐crystalline high‐density polyethylene (HDPE) have been investigated using thermal analyses and positron annihilation lifetime spectroscopy (PALS). Two different glass transition temperatures (T_g) were observed in a PS/HDPE blend due to the dissimilarity in the chemical structure, consistent with an immiscible blend. However, T_g of PS in the incompatible PS/HDPE blend showed an upward trend with increasing PS content resulting from the confinement effect, while T_g of the semi‐crystalline HDPE component became lower than that of neat HDPE. Moreover, the elevation of T_g of PS was enhanced with a decrease of free volume radius by comparing annealed and unannealed PS/HDPE blends. Positron results showed that the free volume radius clearly decreased with annealing for all compositions, although the free volume hole size agreed well with linear additivity, indicating that there was only a weak interaction between the two components. Combining PALS with thermal analysis results, the confinement effect on the glass dynamics and free volume of PS phase in PS/HDPE blends could be attributed to the shrinkage of HDPE during crystallization when HDPE acted as the continuous phase. © 2015 Society of Chemical Industry     

Study on the Crystallization Properties of Polypropylene/Montmorillonite Composites

The crystalline size of polypropylene (PP) filled with montmorillonite (MMT) was studied by X-ray diffraction (XRD). The isothermal crystallization behavior of polypropylene was studied by means of differential scanning calorimetry (DSC). The Avrami equation was used to describe the isothermal crystalline kinetics of PP/MMT composites. The result showed that the addition of MMT decreased the crystalline size L _hkl of the polymer. MMT was used as nucleating agent during isothermal crystallization process of polypropylene. The addition of montmorillonite decreased the crystallization time of the polypropylene and the melt point was raised. The value of Avrami exponent n was related with the crystallization temperature. The value of Avrami pre-index factor k of PP/MMT composite was decreased with increasing crystallization temperature. The value of half crystallization time t _1/2 of PP/MMT composite was less than that of PP at a given crystallization temperature, signifying that montmorillonite acted as nucleating agent, accelerated the overall crystallization process.     

Effect of zeolite 5A on the crystalline behavior of polypropylene (PP) in PP/β‐nucleating agent system

The effect of zeolite 5A on the crystalline behavior of polypropylene (PP) in PP/β‐nucleating agent system was investigated with X‐ray diffractometer (XRD), differential scanning calorimeter (DSC), and polarized light microscope (PLM) in this study. Zeolite 5A has less effect on crystalline phase of PP/β‐nucleating agent than does n‐CaCO₃, which is ascribed to the stronger surficial polarity and better dispersibility of zeolite 5A. The data of DSC indicated that the crystallization peak temperature of PP increases by 8.3°C in the presence of zeolite 5A, and n‐CaCO₃ increases 5.7°C in PP/TMB‐5 system, relative to pure PP. And the initial crystallization temperature (T_c0) and the relative crystallinity (X_c) of PP is much more dramatically raised in the presence of zeolite 5A than CaCO₃. The efficiency of zeolite 5A in reducing the spherulites size of PP was seen clearly from the PLM photographs. The mechanical testing results showed that the flexural strength and impact strength of PP/β‐nucleating agent system increased in the presence of zeolite 5A. POLYM. COMPOS., 2008. © Society of Plastics Engineers     

Study of the mechanical,thermal, and thermodegradative properties of virgin PP with recycled and non-recycled HDPE

In order to show the significance of plastic recycling, a study of the properties of blends of PP with non-recycled and recycled high density polyethylene (HDPE) is made. The results of the Young's modulus for polypropylene (PP)/non-recycled and recycled HDPE blends present a slight synergism, although no significant dependence of this property on the compound was observed. The values of elongation at break and impact strength reflect the incompatibility of the blends. In thermal studies of the blends, the values of fusion enthalpy are below the values of the components. The results of the thermodegradative studies show that activation energies (E_a) obtained are lower, in the case of the blends, than the E_a corresponding to pure polymers. In PP/recycled HDPE blends, activation energy, at 5% to 20% concentration, is maintained and falls abruptly with an increase in the concentration of the recycled material. Based upon the facts previously exposed, it is possible to recycle the recycled HDPE up to 20% concentration, in PP blends. The addition of the compatibilizer at 5% represents the optimal concentration for improving the final properties of the finished product.     

Hydrophilic modification of polypropylene microporous membranes by grafting TiO2 nanoparticles with acrylic acid groups on the surface

Hydrophilic microporous membranes were prepared based on polypropylene (PP) cast films blended with a commercial acrylic acid grafted polypropylene (PP-g-AA) via melt extrusion followed by grafting titanium dioxide (TiO₂) nanoparticles on its surface, annealing and stretching. ATR-FTIR, XPS and EDS analyses showed that the hydrophilic segments of an amphiphilic modifier (PP-g-AA) acted as surface functional groups on the film surface. The results indicated that the presence of the modifier was very important for grafting TiO₂ nanoparticles on the film surface. Compared to PP and PP/PP-g-AA blend films, the water contact angle decreased by a factor of 2.5 after grafting TiO₂ on the surface of the films, meanwhile the water vapor permeability of the microporous membranes prepared from those films increased by a factor of 1.5. All these results indicated that the hydrophilicity of the modified PP membranes was improved.     

Oriented structure and anisotropy properties of polymer blown films: HDPE, LLDPE and LDPE

Different types of polyethylene blown films (HDPE, LDPE, LLDPE) differ significantly in the ratio between machine and transverse direction tear resistance. In this paper, low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) blown films at different draw-down ratios are studied, and the relation between crystalline structure and anisotropy of blown film properties is investigated. The crystalline morphology and orientation of HDPE, LDPE, LLDPE blown films were probed using microscopy and infrared trichroism. Significant differences in crystalline morphology were found: at medium DDR HDPE developed a row-nucleated type morphology without lamellar twisting, LDPE showed rod-like crystalline morphology and turned out to the row-nucleated structure with twisted lamellae at high draw-down ratio (DDR), while a spherulite-like superstructure was observed for LLDPEs at all processing conditions. They also showed quite different orientation characteristics corresponding to different morphologies. The morphologies and orientation structure for LDPE, LLDPE and HDPE are related to the stress applied (DDR) and their relaxations in the flow-induced crystallization process, which determine the amount of fibrillar nuclei available at the time of crystallization and therefore, the final crystalline morphology. These structure differences are shown to translate into different ratios of machine and transverse direction tear and tensile strengths.