Random Propylene Ethylene Copolymers Research Articles

Rheological properties and UV photo-oxidation of montmorillonite-filled random propylene–ethylene copolymers

Rheological properties and UV photo-oxidation of montmorillonite-filled random propylene–ethylene copolymers

Structural evolution in propylene-based elastomer with γ form during stress relaxation

The stress relaxation and the structural evolution of a random propylene-ethylene copolymer of about 12 mol% of ethylene co-units with pure γ form (iPPcoE12-γ), which was pre-stretched to various strains at 30 °C, were investigated by means of in situ wide angle X-ray diffraction techniques. The two-dimensional X-ray diffraction patterns were decomposed into oriented and unoriented parts. It turned out that the unoriented fraction of the iPPcoE12-γ sample became oriented in the process of the relaxation. Due to the unique “cross-β” arrangement in the γ form, the fraction of elastic stress of iPPcoE12-γ was higher than that of iPPcoE12-α with pure α form at a small pre-stretched strain (εH = 0.27) and the increased fraction of oriented parts was much less than that of iPPcoE12-α at medium strains (εH = 0.71 and 1.27). A comparative analysis of the relaxation behavior of iPPcoE12-γ and iPPcoE12-α revealed that the elasticity and stability of iPPcoE12-γ were more attractive at smaller strains (smaller than critical strain C—the strain of the starting of the fragmentation of crystalline blocks), while at larger strains (larger than 1.27) iPPcoE12-α had its advantages in elasticity. In addition, it was found that the γ to α transition occurred during the relaxation, and the unoriented crystallites with γ form were more difficult to transform than the oriented one.

Crystal structure: a way to control properties in cast films of polypropylene

The efficiency of the degree of cooling and shear-induced crystallization on the formation of different crystal polymorphs and morphology development of isotactic polypropylene (iPP) and random propylene–ethylene copolymer was analysed in cast films. Wide-angle X-ray scattering was applied to evaluate the crystal structure formed in films solidified at different chill roll temperatures (15, 40, and 70 °C) and throughputs of the line (60 and 120 kg/h). The crystal geometry and morphology were visualized by atomic force microscopy. For films generated at 15 °C or at large supercooling together with the nodular domain additional grained round-shaped structures composed of mesophase were detected on the film surface. Increase of the chill roll temperature or lower supercooling allowed formation of the monoclinic phase of iPP. Higher drawing rate triggered crystal formation preferably on the surface of films quenched at 15 °C. The crystal structure and morphology were correlated to selected optical and mechanical properties. At close-to-identical crystallinity, the crystal aspect ratio and film roughness play a major role in controlling the film transparency and stiffness.

Counits Content and Stretching Temperature-Dependent Critical Stress for Destruction of γ Crystals in Propylene-Ethylene Random Copolymers.

Tensile deformation behavior of three random propylene–ethylene copolymers with the same molecular weight and different contents of counit was investigated at different temperatures from room temperature to close to melting point via tensile tests, step-cycle tests, and in situ wide-angle X-ray diffraction techniques. Upon stretching, the original crystalline lamellae must be destroyed, generating new highly oriented ones. A critical stress has been suggested, under which the original crystallites can be destructed. The propylene–ethylene copolymer samples in pure γ-form transformed gradually into α-form during tensile stretching. This crystalline transition proceeded via a destruction (melting) of the original γ-form crystals followed by recrystallization of the freed polymeric chain segments into α-form along stretching direction. This result provides a marker for investigating the critical stress mentioned above. Such critical stresses triggering the destruction of γ-form crystals for the propylene–ethylene copolymers of different ethylene counit contents were successfully calculated. It turned out that this critical stress depended on the ethylene counit content and stretching temperature. Samples with less ethylene counits show higher critical stress because of a lower degree of insertion of the ethylene counits into the crystalline unit cell than samples with higher ethylene counit content. The critical stresses remained constant when the samples were stretched at low-temperature region, whereas they decreased significantly at high stretching temperatures close to the melting points due to strong thermal distortion of the crystalline lattices, making the crystallites less stable.

Effect of Random Ethylene Comonomer on Relaxation of Flow-Induced Precursors in Isotactic Polypropylene

The effect of comonomer on structure and relaxation of flow-induced precursors was investigated in a series of isotactic polypropylene and random propylene–ethylene copolymers. The polymers were subjected to flow by fiber pulling and allowed to relax above their nominal melting temperature for specific times. The type of morphology developed after cooling revealed whether flow-induced precursors were still present or the melt had fully re-equilibrated. Precursors were long-lived and, at fixed temperature, decayed significantly faster with higher ethylene content. The critical time for precursor relaxation followed an Arrhenius-type dependence with temperature. The apparent energy of activation for precursor dissolution decreased with increasing comonomer content, indicating that the rate-limiting step of the relaxation process becomes less difficult with higher ethylene fraction. This effect is attributed to ethylene counits acting as disruptors of precursor structure and is discussed in terms of quasi-cr...

Melt Temperature and Initial Polymorphs Dependencies of Polymorphs Selection during Subsequent Crystallization in Propylene-ethylene Random Copolymer

Subsequent crystallization behavior during cooling for two random propylene-ethylene copolymers with 6 mol % ethylene co-units but different molecular weights initially in a pure α-form or pure γ-form processed at different melt temperatures was investigated using differential scanning calorimetry and wide-angle X-ray diffraction techniques. The onset crystallization temperature (Tonset) and fraction of the two polymorphs after crystallization depended on the molecular weight, initial polymorph, and melt temperature (Tmelt). At high Tmelt (120–160 °C), Tonset and the fraction of the γ-form (fγ) increased with decreasing Tmelt. For the high molecular weight sample initially in α-form, a higher Tonset and fγ than the one in the γ-form was observed because a higher Tmelt was needed to fully homogenize the melt for samples initially in the α-form due to the different morphology between the samples of different polymorphs. For the low molecular weight sample, the initial polymorph did not affect this process.

Elasticity Reinforcement in Propylene–Ethylene Random Copolymer Stretched at Elevated Temperature in Large Deformation Regime

Tensile deformation behavior of a random propylene–ethylene copolymer with 12 mol % ethylene counits at room temperature and 63 °C was investigated using the in situ small and wide-angle X-ray scattering techniques. Under both conditions, the deformation mechanism changed from slip of crystalline lamellar blocks to stretching induced melting and recrystallization process at a critical strain of about 0.9. This critical strain in tensile deformation of semicrystalline polymers normally marks the starting of plateau value of elastic strain. Further stretching leads to increase of plastic deformation only due to the fibrillation. However, a peculiar elasticity reinforcement was observed at strain larger than 1.3 when the sample was stretched at 63 °C. Wide angle X-ray scattering results indicate that at this strain of 1.3 fibrillation of the originally unoriented crystals finished so that further stretching leads to a deformation of a rather stable entangled network embedded by fibrils that possesses high elasticity.

Propylene based systems for high voltage cable insulation applications

Crosslinked polyethylene (XLPE) remains the material of choice for extruded high voltage cables, possessing excellent thermo-mechanical and electrical properties. However, it is not easily recyclable posing questions as to its long term sustainability. Whilst both polyethylene and polypropylene are widely recycled and provide excellent dielectric properties, polypropylene has significantly better mechanical integrity at high temperatures than polyethylene. However, while isotactic polypropylene is too stiff at room temperature for incorporation into a cable system, previous studies by the authors have indicated that this limitation can be overcome by using a propylene-ethylene copolymer. Whilst these previous studies considered unrelated systems, the current study aims to quantify the usefulness of a series of related random propylene-ethylene co-polymers and assesses their potential for replacing XLPE.

Quantification of organoclay dispersion and lamellar morphology in poly(propylene)–clay nanocomposites with small angle X-ray scattering

Intensity profiles of small angle X-ray scattering (SAXS) curves were analyzed to simultaneously gain quantitative information on nanoclay dispersion as well as lamellar ordering in polypropylene–clay nanocomposites. Different types of PP nanocomposites prepared with PP homopolymer (HPP), random propylene–ethylene copolymer (RCP) and a high impact polypropylene–ethylene propylene rubber (ICP) were analyzed. Various one-dimensional models for stacked structures were applied on Lorentz corrected SAXS spectra to derive long period, thicknesses of alternating high and low electron density layers and their distributions, and the number of stacks for both nanoclay and PP lamellae. We applied a mixed thickness distribution model comprising combined Gaussian and exponential for a simple stack of finite thickness, which was found to explain the experimental data better for both nanoclay tactoids and lamellar stacks, compared to simple Gaussian and exponential thickness distributions. Long period X and number of stacks N were derived as important parameters signifying changes in levels of nanoclay exfoliation in PP. Among the three types of polypropylenes studied, better nanoclay exfoliation was obtained for the high impact ICP grade compared to HPP and RCP. Complete exfoliation of nanoclay was achieved in ICP matrix, employing a masterbatch processing route. Moreover, role of nanoclay as a γ nucleating agent was evident from small and wide angle X-ray analyses, and was seen strongly in RCP. Changes in lamellar structure of PP as a result of nanoclay incorporation, double population consisting of both α and γ polytypes in the nanocomposites from that of a primarily α population in neat polymer matrices, were also analyzed in detail with the mixed thickness distribution model, thereby demonstrating its usefulness.

Maximum Rate of Crystallization and Morphology of Random Propylene Ethylene Copolymers as a Function of Comonomer Content up to 21 mol %

We report a study of the primary chain microstructure, crystallization kinetics, and crystalline morphologies of propylene ethylene copolymers (PE) synthesized with metallocene catalysts in a range of ethylene up to 21 mol %. A 13C NMR triad distribution analysis lends support for a random distribution of the comonomer in the complete series. Melting (Tm) and crystallization (Tc) temperatures decrease with increasing ethylene more rapidly than the decrease of the glass transition temperature (Tg). Consequently, for ethylene >10 mol % the crystallization is observed at Tcs relatively close to Tg allowing to observe experimentally the maximum of the crystallization rate. In the low Tc range, segmental dynamics play a prominent role in developing the semicrystalline structure, and the crystallization rate decreases dramatically with increasing molar mass. For a fixed molar mass and undercooling, the strong decrease of the crystallization rate with increasing ethylene, especially for copolymers with >10 mol %...

Linear growth rates of random propylene ethylene copolymers. The changeover from γ dominated growth to mixed (α+γ) polymorphic growth

The spherulitic linear growth rates of a homo-poly(propylene) and a series of propylene–ethylene copolymers, all synthesized with the same type of metallocene catalyst were analyzed. The inter-chain distribution of comonomer content is uniform in these copolymers and the intra-chain distribution adheres to the random behavior. Furthermore, the concentration of stereo and regio defects is constant for all copolymers. Thus, with these polymers it was possible to investigate the influence of ethylene content on the crystallization kinetics, as extracted from their linear growth rates. All iPPs investigated display mixed polymorphic behavior during isothermal crystallization and major emphasis was given to integrate the simultaneous development of the α and γ polymorphs, and their intimate structural relations during growth, in the analysis of the crystallization kinetics. A sharp break in spherulitic growth is found between times domains of mixed α+γ growth and growth of pure γ crystals reflecting a drastic change in growth mechanisms at the point where α development ceases. The rates corresponding to α growth are significantly higher than those characteristics of γ growth. In addition, growth data in the domain of mixed α+γ growth, which, following the structural models for α and γ branching, reflect the growth pattern of α crystals, display a discontinuity in the temperature gradient at the changeover from growth of mixed α+γ to γ dominated growth. This behavior correlates with that found in other systems that undergo a similar extended chain to folded change in crystallization mechanisms. The temperature coefficient of the linear growth rates is analyzed according to regime theory for both domains of growth. The results allow a quantitative framework for discussion of the interfacial free energies of α and γ crystals obtained from this analysis.

Crystallization of the ? form in random propylene-ethylene copolymers

Wide-angle X-ray scattering and differential scanning calorimetry measurements have been conducted on seven random copolymers of propylene with ethylene in order to study the γ phase formation as a function of the comonomer content. The lamellar morphology of the samples was also investigated by small-angle X-ray scattering. The content of the γ phase was found to go through a maximum with crystallization temperature and to increase with comonomer concentration, up to a point (ethylene ≥6.5 wt%) where the latter parameter became less influential. The multiple melting endotherms behaviour of the samples was studied by DSC and temperature-controlled diffractometric techniques. The attribution of the DSC peaks to the different isotactic polypropylene polymorphs that form in these conditions was confirmed. The results obtained permitted us to ascertain that, in the experimental conditions chosen, some further formation of crystallites takes place during the quenching to room temperature after the crystallization isotherm. In this phase, the chains organize themselves in stacks with thin lamellae, forming a distinct population with respect to those formed on isothermal crystallization. The melting of the thinner lamellae determines a convergence of the two populations into just one, still retaining an organization in stacks, that gradually disappears until complete melting of the material. Copyright © 2004 Society of Chemical Industry

Crystallinity and amorphous segmental properties in random propylene-ethylene copolymers: NMR correlation

We present a characterization of the state of semi-crystallization of random propylene–ethylene copolymers, observing the transverse relaxation of protons attached to the chains. The degree of crystallinity was determined step by step, from NMR, during the progressive annealing of the copolymers, quenched at room temperature from the melt. The ethylene content varied over the range 0–4.7 (w/w). It is shown that the empirical temperature dependence of the degree of crystallinity, as detected by NMR, obeys a single curve; this curve is translated along the temperature axis in accordance with the melting point depression when the ethylene content varies. The degree of crystallinity is primarily a function of the undercooling, whatever the ethylene content. Considering any state of semi-crystallization, a strong correlation between the degree of crystallinity and the relaxation rate of the protons attached to amorphous segments is established; the NMR sensitivity to crystallinity is considerably enhanced when detected by amorphous properties. © 1999 Society of Chemical Industry

Crystalline structure of propylene-ethylene copolymer fractions

Crystalline structure were studied on both block and random propylene–ethylene copolymer fractions, which were obtained by temperature rising elution fractionation. The peak characteristic of α-polypropylene (PP) was observed for all fractions, except the fraction eluted at room temperature. A characteristic peak of polyethylene crystals [i.e., (200) planes] was observed in some fractions, indicating the existence of long ethylene sequence in these fractions. This is in accordance with the analysis from Fourier transform infrared spectroscopy and 13C-NMR. The γ-form crystal of PP was observed in these copolymer fractions at atmospheric pressure. It is suggested that the insertion of comonomer into the isotactic PP chain makes the crystallizable sequences sufficiently short and produces the γ-form crystal. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:381–386, 1998

Evaluation of Multiple Melting Peaks of Propylene-Ethylene Copolymers

Multiple melting peaks phenomena have been investigated in terms of compositional heterogeneity, crystallization temperature, cooling rate, heating rate, annealing and crystal structure for low ethylene content block and random propylene-ethylene copolymers. The experimental results suggested that the compositional heterogeneity and crystal morphology play an important role in the cause of multiple melting behavior.

Structural analysis of random propylene-ethylene copolymers

Abstract Structural properties of random propylene ethylene copolymers with low ethylene contents between 0 % and 11 mol % were investigated using mainly X-ray techniques. One of the essential issues of this paper is that a cocrystallization of the comonomer units does not take place. This is based on the following observations: The long period and the thickness of the crystalline lamellae decrease with increasing ethylene content. The electron density difference between crystalline and amorphous phase, however, as determined with the invariant, does not show any changes. The heat of fusion, normalized to the degree of crystallinity, does not decrease. This leads to the conclusion that the observed changes are not due to an incorporation of the ethylene units as defects, but to a reduced extension of the crystalline regions, now confined by the ethylene units which interrupt the helical chain conformation. The γ-modification, detectable by means of wide-angle X-ray diffractometry from an ethylene content of 5 mol % on, partially converted into the α-modification on annealing.