The effect of metallocene-catalyzed linear low-density polyethylene on the physicomechanical properties of its film blends with low-density polyethylene

Films comprising a metallocene-catalyzed linear low-density polyethylene (mLLDPE) blended with either of two different low-density polyethylene (LDPE) materials were prepared. The physicomechanical, optical and melt flow properties of the films were measured. A novel adaptation of conventional radar plots was used to process the acquired data to identify the level at which mLLDPE should be incorporated in either of the LDPEs to produce optimal overall properties. In general, the addition of mLLDPE to LDPE improved most of the properties considered and the LDPE material having the higher polydispersity produced blends having superior properties. A level of mLLDPE of between 20–30% (w/w) was required in order to achieve optimization.     

Compatibility and morphology of blends of isotactic and atactic polypropylene

The compatibility and morphology of blends of isotactic and atactic polypropylene have been studied by several means: X-ray scattering, differential scanning calorimetry, and electron microscopy. It was found that the atactic polymer was located mainly inside the spherulites of the isotactic polypropylene on a scale approximately equal to that of the crystalline lamellae. This means that these two polymers were more intimately mixed than are blends of polypropylene and ethylene-propylene copolymer, in which the location of the copolymer is unrelated to the spherulite structure. This difference can be explained by the fact that atactic and isotactic polypropylene are miscible in the melt, whereas polypropylene and ethylene-propylene copolymer are not.     

Lamellar orientation in the blends of linear low density polyethylene and isotactic polypropylene induced by dynamic packing injection molding

In this work, we have carried out 2 dimensional small and wide angle X-ray scattering experiments on the blends of linear low-density polyethylene (LLDPE) and isotactic polypropylene (iPP) obtained by dynamic packing injection molding in which the melt was firstly injected into the mold then forced to move repeatedly in a chamber by two pistons that moved reversibly with the same frequency as the solidification progressively occurred from the mold wall to the molding core part. iPP was found to form a shish-kebab structure with its lamellar stack oriented perpendicularly to the shear flow direction. Very interestingly, the lamellae of LLDPE were found tilted away from shear flow direction with molecular chain still along flow direction, and the tilted angle increases from the skin to the core part. This can be only understood if the intra-lamellar block slip in the chain direction is generally activated during shearing process achieved by dynamic packing injection molding. Our finding is important and seems to provide further support for the idea that the structure of the crystalline lamellae is not continuous but constructed of small building units with thin boundary in between.     

Crystallization of partially miscible linear low-density polyethylene/poly(ethylene-co-vinylacetate) blends

Miscibility and crystallization of linear low-density polyethylene (LLDPE)/poly(ethylene-co-vinylacetate) (EVA) blends were investigated by optical microscopy (OM) and differential scanning calorimetry (DSC). It was found that there existed liquid–liquid phase separation (LLPS) below 220 °C over the whole composition. However, the depression in the crystallization temperature, melting temperature and equilibrium melting temperature of LLDPE all indicated that this polymer pair was partially miscible. The crystallization and melting behavior of LLDPE were determined by the dilute effect of non-crystalline EVA and the probable co-crystallization of parts of EVA chains with LLDPE chains. The crystallization and melting behavior of EVA was determined by the competence between a nucleation effect of LLDPE crystals and partial miscibility between this polymer pair, which was different from that of LLDPE.     

On the morphology of blends of linear and branched polyethylene

A blend of linear and branched polyethylene was examined in view of establishing conditions of cocrystallization and/or segregation. Two members of the SCLAIR series were used in 11 proportions. Conditions of segregation on crystallization were first explored by DSC, which was followed by morphological examination of selected samples. This involved optical and electron microscopy, the latter using the permanganate etching technique of Bassett and Hodge. Dependent upon crystallization temperature and time, crystal segregation on different scales was observed revealing that the final product can be regarded as a morphological composite where the ratio and nature of the constituents are determined by the conditions of solidification. The particulars should be relevant to the presently evolving practice of blending different polyethylenes. Examples of the different morphologies will be demonstrated. The individuality of the lamellae of the more readily crystallizable species can become strikingly apparent which incidentally reveals details in the edge on view, such as have not been seen before, and may have relevance to the fundamentals of the structure of lamellar crystals in general.     

The effect of varying the polyethylene content and the co-polymer content on crazing in polystyrene—low-density polyethylene blends

The effect of varying the low-density polyethylene content and the polystyrenepolyethylene block co-polymer content on the rates of craze initiation and craze growth in polystyrene/low-density polyethylene blends has been studied. it was found that the parameters in the Eyring rate coefficients for craze initiation and craze growth are not dependent on the low-density polyethylene content. However, the rates of craze initiation increased with increasing low-density polyethylene content. This is explained tentatively by a new model for craze initiation. It is argued that effective crazes are only formed within clusters of low-density polyethylene particles that have overlapping stress-concentration fields. The dependence of the rate of craze initiation on the volume-fraction of dispersed phase that follows from this cluster model is qualitatively in agreement with experimental results. PS-PE co-polymer addition gives rise to changes in the Eyring parameters of the rate coefficients of craze initiation and craze growth. This may be a consequence of changes in morphology near the interface and of the different stress state at the interface.     

酸刻蚀玻璃纤维增强等规聚丙烯(iPP) 复合材料的界面结晶形态

将在稀酸溶液中刻蚀不同时间的玻璃纤维(GF)添加到等规聚丙烯(iPP)基体中制得GF/iPP复合材料。通过偏光显微镜研究了刻蚀不同时间的GF对界面结晶形态的影响。结果表明, 在刻蚀4 h的GF增强iPP复合材料中, GF表面在结晶初期诱导iPP基体生成独特的环带状晶核, 并在此晶核上可以进一步诱导生成β横晶。此外, 该复合材料在142 ℃等温结晶的结果进一步说明了刻蚀4 h的GF对基体的异相成核作用具有二元性, 即同时具有β成核和α成核的能力。      

Thermo-mechanical characteristics of thermally aged polyethylene/polypropylene blends

Polyethylene (PE), polypropylene (PP) and their blends have attracted a lot of attention due to their potential industrial applications. Therefore, the current work has been carried out with the main objective of investigating the impact of the thermal aging/treatment and blend ratio (composition range) on the mechanical (tensile and hardness) and thermal characteristics (using thermogravimetric analysis in a dynamic air atmosphere) of PE, PP and PE/PP binary blends. Samples of PE/PP blends containing 100/00, 75/25, 50/50, 25/75 and 0/100 wt.% were prepared via injection moulding technique and thermally treated/aged at 100 °C for 0, 2, 4, 7, 14 days. The tensile measurements indicated that the yield strength and the modulus decrease with increasing PE content. It was also observed that PE, PP and their blends deform in ductile modes. They undergo a uniform yielding over a wide range of deformation, which is followed by strain hardening and then failure. The strain to break for pure PE is found to be much higher than that for pure PP and for their blends, intermediate values have been observed. The hardness measurements have also revealed that increasing PE content in PE/PP blends reduced the hardness value of PP, however, thermal aging at 100 °C has not affected the polymers hardness which holds also true for the tensile properties, showing a good correlation between tested mechanical properties. The thermogravimetric analysis (TGA) in a dynamic air atmosphere and derivative thermogravimetric analysis (DTA) were conducted to study the thermal degradation and stability of thermally unaged and aged PE, PP and PE/PP blends in terms of the initial (T_d and T_d(1%)) and final (T_d(99%)) decomposition temperatures and maximum decomposition rate temperature (T_max). All polymers start to decompose at no less than 365 °C. As for mechanical properties, the blend ratio has affected the thermal properties however, aging time has not.     

Epitaxial crystallization of linear low-density polyethylene on high-density polyethylene

The oriented crystallization of linear low-density polyethylene (LLDPE) on high-density polyethylene (HDPE) has been investigated by transmission electron microscopy. From morphology and electron diffraction, it is confirmed that epitactic growth of LLDPE lamellae on the HDPE crystals takes place with an adoption of the HDPE crystal thickness at the interface and a continuous thinning of the LLDPE lamellae in the interface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Glass-fiber reinforcement of in situ compatibilized polypropylene/polyethylene blends

Polypropylene and low-density polyethylene (LDPE) were melt-blended at proportions 75/25, 50/50, and 25/75 w/w, respectively. These blends were reinforced with two types of glass fibers added at an amount of 20 wt %: the E-type fibers without any surface treatment and the M-type fibers, which were treated with y-methacryloxy propyltrimethoxy silane coupling agent. Poly(propylene-g-maleic anhydride) with 0.8 mol % maleic anhydride content and poly(ethylene-co-vinyl alcohol) with 7.5 mol % vinyl alcohol content were added at a 50/50 w/w proportion as in situ reactive compatibilizers at an amount of 10 wt %. The thermoplastic composite materials have higher tensile strength as well as impact strength compared to the unreinforced blends. The simultaneous process of the in situ blend compatibilization, along with the incorporation of glass fibers in the thermoplastic matrix, leads to a significant improvement of the mechanical properties as compared to the properties of the composite materials with the uncompatibilized matrix. Scanning electron microscopy and micro-Raman spectroscopy have been used to study the adhesion of the thermoplastic matrix onto the glass fibers. Significantly better adhesion characteristics were observed in the composites containing M-type glass fibers, with LDPE adhering the most on the fibers. This better adhesion was reflected in the improved mechanical properties of the composites.     

Vibration-dependent morphology and crystal structure of isotactic polypropylene

A small homemade device was used to study the influence of mechanical vibration on the crystal structure and morphology of isotactic polypropylene (iPP) under different melting temperatures, vibration times, vibration frequencies, and cooling rates. The crystallite size, crystal structure, and crystallinity of iPP under or without vibration treatment were investigated by means of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), and polarized microscopy observation (PLM). The crystallization of iPP varied with the length of vibration time, vibration frequency, cooling rate, and melt temperature. Compared with the data of conventional samples measured by DSC, vibration could increase the crystallinity of iPP, make melting peak of α-crystal move toward higher temperature and make that of β-crystal shift to lower temperature. Meanwhile, WAXD measurements showed that the vibration could reduce the content of β-crystal evidently, particularly at the lower vibration frequency, lower cooling rate, and higher melting temperature. Furthermore, PLM measurements showed that the vibration made the spherulite size smaller.     

On morphology and the competition between crystallization and phase separation in polypropylene–polyethylene blends

Blends of polypropylene (PP) and low density polyethylene (LDPE) have been examined for a series of compositions using differential scanning calorimetry and permanganic etching followed by transmission electron microscopy. Thermal analysis of their melting and recrystallization behaviour suggests two possibilities, either that below 15 wt% PP the blends are fully miscible and that PP only crystallizes after LDPE because of compositional changes in the remaining melt, or else that the PP is separated, but in the form of droplets too small to crystallize at normal temperatures. Microscopic examination of the morphology shows that the latter is the case, but that a fraction of the PP is nevertheless dissolved in the LDPE. © 1998 Kluwer Academic Publishers     

The effect of annealing temperature on the fracture performance of isotactic polypropylene

The influence of annealing temperature on the fracture behaviour of a commercial extrusion-grade isotactic polypropylene was studied. Fracture mechanics analysis was carried out at room temperature and at a low crosshead speed underJ-controlled conditions. Parameters characterizing fracture initiation,J _IC, and crack propagation,T _M, were determined. Some thermal treatments induced “ductile instability” after a certain amount of crack extension while others strongly enhanced the fracture toughness parameters and promoted completely stable behaviour. Aiming to correlate mechanical properties with the supermolecular structure, the different morphologies induced by thermal treatments were studied by differential thermal analysis. In addition, a qualitative fracture surface analysis was carried out by SEM. Craze formation appeared to be the principal plastic deformation mechanism present. The degree of crystallinity and the degree of interconnection related to the thermal treatment the sample had undergone, are the main structural factors controlling fracture performance.     

The origin of heteroepitaxy in the system of uniaxially oriented isotactic polypropylene and polyethylene

The epitaxy of polyethylene (PE) on oriented and -form of isotactic polypropylene (PP) at different crystallization temperatures and rates have been investigated. The result shows that the epitaxy of PE on -form of PP does not differ from that on the -form of PP. The epitactic alignment of the PE lamellae on the PP films becomes more perfect with higher crystallization rates of PE. The epitaxy of PE on - or -form of PP does not occur at crystallization temperatures higher than 123 °C. A nucleation controlled process for the alignment of the PE is proposed.     

Effect of liquid additives on morphology and properties of thermoplastic elastomers prepared from phase-modified EPDM elastomer and isotactic polypropylene blends

Thermoplastic elastomers (TPEs) were prepared from ternary blends of ethylene propylene diene poly methylene rubber (EPDM), isotactic polypropylene (PP), and low loadings (5–10 phr) of different types of interfacial phase modifiers (like maleated EPDM, styrene-ethylene-co-butylene-styrene block copolymer, and maleated PP). These showed much improved physico-mechanical properties compared to the binary blend of EPDM-PP. The effects of non-polar paraffin oil and polar di-octyl phthalate liquid additives (5–20 phr) were investigated in these phase-modified ternary and binary EPDM-PP blends. Only 5 phr of liquid additives provided synergistic improvement in physical properties (maximum stress, modulus, and elongation at break) and generated improved finer morphology of the ternary blends as revealed from scanning electron and atomic force microscopy studies. Enhanced physical properties and dynamic mechanical properties of these blends were explained with the help of better phase morphology and enhanced crystallinity of the blends.     

The effect of annealing on the elastoplastic and viscoelastic responses of isotactic polypropylene

Observations are reported on isotactic polypropylene (i) in a series of tensile tests with a constant strain rate on specimens annealed for 24 h at various temperatures in the range from 110 to 150 °C, (ii) in two series of creep tests in the subyield region of deformations on samples not subjected to thermal treatment and on specimens annealed at 140 °C, and (iii) in a series of tensile relaxation tests on non-annealed specimens. Constitutive equations are derived for the elastoplastic and non-linear viscoelastic responses of semicrystalline polymers. A polymer is treated as an equivalent transient network of macro-molecules bridged by junctions (physical cross-links, entanglements and lamellar blocks). The network is assumed to be highly heterogeneous, and it is thought of as an ensemble of meso-regions with different activation energies for separation of strands from temporary nodes. The elastoplastic behavior is modelled as sliding of junctions in meso-domains with respect to their reference positions driven by macro-deformation. The viscoelastic response is attributed to detachment of active strands from temporary junctions and attachment of dangling chains to the network. Constitutive equations for isothermal deformations with small strains are derived by using the laws of thermodynamics. Adjustable parameters in the stress–strain relations are found by fitting the experimental data.     

Melt processing of polypropylene/clay nanocomposites modified with maleated polypropylene compatibilizers

Maleic anhydride-grafted polypropylene (PPgMA) and organically modified clay composites were prepared in a plasticorder. PPgMAs, including PB3150, PB3200, PB3000, and E43, with a wide range of MA content and molecular weight were used. The structure was investigated with X-ray diffraction (XRD) and transmission electron microscopy (TEM). PPgMA compatibilizers gave rise to similar degree of dispersion beyond the weight ratio of 3 to 1 with the exception of E43, which had the highest MA content and the lowest molecular weight. It was found that thermal instability and high melt index were responsible for ineffective modification by E43. Furthermore, PPgMA with low melting point and high melt index was compounded at low equilibrium temperature in order to maintain a certain level of torque. We then modified polypropylene/organoclay nanocomposites with different levels of PPgMA compatibilizers on a twin-screw extruder. The PP/E43/clay system, as shown through XRD patterns and TEM observation, yielded the poorest clay dispersion among the compatibilizers under investigation. The relative complex viscosity curves also revealed a systematic trend with the extent of exfoliation and showed promise for quantifying the hybrid structure of the nanocomposites. Mechanical properties and thermal stability were determined by dynamical mechanical analysis and thermogravimetric analysis, respectively. Though PPgMA with lower molecular weight and higher MA content could lead to good clay dispersion in PP/clay composites, it caused the deterioration in both mechanical and thermal properties of PP/PPgMA/clay composites.     

Structure–property relationships in polyethylene blends: the effect of morphology on electrical breakdown strength

Blends of linear and branched polyethylene were prepared covering the composition range 1–20% linear polyethylene, and three thermal treatments were subsequently chosen to produce a range of different morphologies. Isothermal crystallization at 124 °C gives rise to compact linear inclusions within a matrix of branched polyethylene, isothermal crystallization at 115 °C produces an open, banded spherulitic morphology and, finally, quenching leads to a continuous spherulitic texture. Ramp testing was then employed to investigate the effect of morphology on electrical strength. It was found that the electrical strength of the blend depends primarily on the morphology and that, by optimizing thermal treatment and linear polyethylene content, substantial improvements in properties can be obtained. This revised version was published online in November 2006 with corrections to the Cover Date.