Propylene Random Copolymer Research Articles

Raman structural study of random copolymers of propylene with 1-pentene

Nascent form of random propylene/1-pentene copolymers was studied for the first time by Raman spectroscopy. For the purpose of detailed interpretation of the copolymer spectra, we investigated Raman spectra of neat PP with low and high crystallinity. It was found that the copolymer Raman spectra depend on the 1-pentene content. We registered the most significant spectral alterations in two lines at 809 and 841 cm−1. The first line corresponds to vibrations of polypropylene (PP) isotactic chains in the crystalline phase, while the second one is associated with vibrations of PP isotactic chains having isomeric defects. We found out that increase in the 1-pentene content results in reduction of the copolymer crystallinity and in the content of isotactic chains in the copolymer amorphous phase.

Polypropylene/polyethylene blends as models for high‐impact propylene–ethylene copolymers, part 1: Interaction between rheology and morphology

AbstractIn this work, composition effects on interfacial tension and morphology of binary polyolefin blends were studied using rheology and electron microscopy. The amount of dispersed phase (5–30 wt %) and its type [ethylene–octene copolymer, linear low‐density polyethylene (LLDPE), and high‐density polyethylene] was varied, and the influence of different matrix materials was also studied by using a polypropylene homopolymer and a ethylene–propylene (EP) random copolymer. The particle size distribution of the blends was determined using micrographs from transmission electron microscopy (TEM). A clear matrix effect on the flow behavior could be found from the viscosity curves of the blends. Analyzing the viscosity of the blends applying the logarithmic mixing rule indicated a partial miscibility of the EP random copolymer with low amounts of the LLDPE in the melt. Micrographs from TEM also showed a clear difference in morphology if the base polymer is changed, with PE lamellae growing out of the inclusions or being present directly embedded in the matrix. To verify these findings, the interfacial tension was determined. The applicability of Palierne's emulsion model was found to be limited for such complex systems, whereas Gramespacher–Meissner analysis led to interfacial tensions comparable with those already reported in the literature. The improved compatibility when changing the matrix polymer from the homopolymer to the random copolymer allows the development of multiphase materials with finer phase structure, which will also result in improved mechanical and optical performance. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Effect of phase separation on overall isothermal crystallization kinetics of PP/EPR in‐reactor alloys

AbstractIn the present work, the effect of phase separation on overall isothermal crystallization kinetics of two polypropylene/ethylene‐propylene random copolymer (PP/EPR) in‐reactor alloys was investigated. It is found that at lower crystallization temperatures (Tc), the overall crystallization rate decreases with increasing phase separation temperature (Ts). This is attributed to the lower linear spherulitic growth rate incurred by the lower PP content in the PP‐rich phase at higher Tss. In contrast, at higher Tcs, quenching from a higher Ts to Tc promotes nucleation as a result of more dramatic concentration fluctuation, leading to a faster overall crystallization rate. The overall crystallization rate of the PP/EPR in‐reactor alloy prepared by multi‐stage sequential polymerization process (MSSP) is retarded by increasing phase separation time (ts). However, prolonging phase separation time has little effect on the crystallization rate of the sample prepared by two‐stage polymerization process (TSP). This result can be attributed to the different phase separation rates of these two samples. The SAXS result confirms that at higher Tc, phase separation in the melt before crystallization can retard crystallization, when compared with the directly quenched samples. It is also found that the phase‐separated PP/EPR in‐reactor alloys exhibit a larger long period because of more amorphous phases included between the lamellar crystals. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Structure formation of random isotactic copolymers of propylene and 1-hexene or 1-octene at rapid cooling

The effect of variation the cooling rate in a wide range between 10−2 and 103Ks−1 on solidification the relaxed melt of random isotactic copolymers of propylene with low amount of 1-hexene or 1-octene has been studied. Emphasis has been placed on the structure formation at rapid cooling and an evaluation of the conditions required to permit crystallization, mesophase formation, or suppression of any ordering. The presence of low amount of either 1-hexene or 1-octene co-units in the propylene chain decreases drastically the critical cooling rate required for suppression of crystallization from about 150–200Ks−1 in the homopolymer to about only 10Ks−1 in the copolymers; increasing the cooling rate beyond these limits allowed mesophase formation or even generation of fully amorphous samples. The study of the kinetics of formation of specific structures is completed by a complementary analysis of the X-ray structure, morphology and superstructure of the ordered phase. The hindrance of non-isothermal crystallization and mesophase formation of random copolymers of propylene with 1-hexene or 1-octene is compared with that in propylene–1-butene copolymers; addition of only 2–3mol% 1-hexene or 1-octene into the propylene chain leads to even larger hindrance of the ordering process than the addition of more than 10mol% 1-butene.

Fractionation of Polypropylene Impact Copolymers Using Temperature Rising Elution Fractionation (TREF)

AbstractImpact polypropylene copolymers (ICPs) have complex microstructures compared to homo‐ or random copolymers of propylene. Standard analytical techniques are unable to unpack the complex structure of impact copolymers, hence the need to fractionate the material. The investigation of poly(propylene‐ethylene) block copolymers was conducted in order to understand the copolymer composition and the role of particular components by fractionation. Temperature rising elution fractionation (TREF) was used for fractionation of ICPs which involves breaking up the polymer into small fractions that can be analysed further to give more understanding on the complex microstructures. Two ICP samples (ICP1 and ICP2) with different physical properties were fractionated. The bulk samples as well as the resulting fractions were further analysed using DSC, FTIR and DMA. Below 0 °C, the DMA results of the bulk sample with high ethylene content (ICP1) showed both low storage modulus and stiffness which is in line with expectations. However, it was interesting to note that above 0 °C the trend was reversed, thus ICP2 with low ethylene content had lower storage modulus and stiffness. The DSC and FTIR results of the fractions of the two samples revealed different or inhomogeneous distribution of ethylene content between the samples which explained differences in mechanical properties. The findings emphasize the contributions of phase separation and compatibility between the rubber and the matrix towards physical and mechanical properties.

Effect of co-unit type in random propylene copolymers on the kinetics of mesophase formation and crystallization

Fast scanning chip calorimetry has been employed to study the effect of the type and concentration of co-units on the rate of mesophase formation and crystallization in random isotactic copolymers of propylene and 1-alkenes, including ethylene, 1-butene, 1-hexene, and 1-octene. The dependence of the rate of ordering on temperature of the propylene homopolymer shows two distinct maxima around 300 and 340–350 K which are related to mesophase formation and crystallization, respectively. Addition of 1-alkene co-units leads to a decrease of the maximum rate of both crystallization and mesophase formation. At comparable temperature and molar percentage of co-units in the propylene chain, ethylene, and 1-butene co-units cause less reduction of the maximum rate of ordering than 1-hexene or 1-octene co-units. The experimental observations are discussed in the context of possible incorporation of these chain defects into the ordered structures.

Evaluating the foamability of polypropylene with nitrogen as the blowing agent

The foamability of polypropylene (PP) was carefully evaluated with nitrogen (N2) as the blowing agent. Four PP samples with different molecular architectures, namely isotactic polypropylene (iPP), ethylene propylene block copolymer (E-b-P), ethylene propylene random copolymer (E-r-P) and branched PP (high melt strength PP, HMSPP), were selected to observe the relationship between the molecular structure and polymer foamability. Two measuring parameters, the foaming temperature window and the correlation factor of foam density with foaming temperature, were used to characterize the foamability. The results showed that the foaming temperature window of HMSPP was more than 34 °C, whereas that of linear PPs was as narrow as 2–4 °C. In addition, unlike linear PPs, the foam density of HMSPP was insensitive to foaming temperature. HMSPP was indeed the best of all PP resins at present available for PP industrial foam production.

Development of high temperature comprehensive two-dimensional liquid chromatography hyphenated with infrared and light scattering detectors for characterization of chemical composition and molecular weight heterogeneities in polyolefin copolymers

The application of high temperature comprehensive two-dimensional (2D) liquid chromatography for quantitative characterization of chemical composition and molecular weight (MW) heterogeneities in polyolefins is demonstrated in this study by separating a physical blend of isotactic-polypropylene, ethylene- random-propylene copolymer, and high density polyethylene. The first dimension separation is based on adsorption liquid chromatography that fractionates the blend from low to high ethylene content. The second dimension is size-exclusion chromatography connected with light scattering (LS) and infrared (IR) detectors. The IR detector shows desired sensitivity and linearity for monitoring analyte concentrations in the eluent after 2D separations. In addition, the compositions of the analytes are also determined from the ratio of two IR absorbances at the specified wavelength regions, an absorbance for measuring the level of methyl groups in polyolefins and another absorbance for measuring concentration. The LS detector is used to determine absolute molecular weight of the analytes from the ratio of the light scattering signal to the IR concentration signal. The ability to obtain concentration, chemical composition, and MW of polyolefins after 2D separation provides new opportunities to discover structure–property relationships for polyolefins with complex structures/architectures.

Effect of phase separation on spherulitic growth rate of PP/EPR in‐reactor alloys

AbstractIn this work, the effect of phase separation on the spherulitic growth rate of a polypropylene/ethylene–propylene random (PP/EPR) copolymer in‐reactor alloy was investigated. The PP/EPR in‐reactor alloy was either directly quenched from homogeneous melt to crystallization temperature or held at various temperatures for phase separation prior to crystallization. It is found that at lower crystallization temperatures previous phase separation in the melt retards the crystallization rate. The higher the phase separation rate, the smaller the spherulitic growth rate. This can be attributed to faster crystallization rate than the rate of secondary phase separation. The composition of the PP‐rich phase and corresponding depression of the equilibrium melting temperature of PP vary with phase separation temperature. On the other hand, at higher crystallization temperature, previous phase separation in the melt has little effect on the spherulitic growth rate because secondary phase separation can take place prior to crystallization. The transition temperature from regime II to regime III also shifts to lower temperature as the phase separation temperature increases. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Characterization of the microstructure of impact polypropylene alloys by preparative temperature rising elution fractionation

Two polypropylene alloys (Samples A and B), as impact polypropylene (PP) with similar ethylene contents and melt indices but different impact properties at low temperatures, are fractionated into eight fractions using preparative temperature rising elution fractionation. The microstructure of the original samples and their fractions are studied using high-temperature gel permeation chromatography, Fourier transform infrared spectroscopy, 13C nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. The results indicate that the two alloys are mainly composed of four portions: ethylene–propylene random copolymer (EPR), ethylene–propylene segmented copolymer, ethylene–propylene block copolymer, and propylene homopolymer. Sample A contains more EPR and more fractions with higher isotacticity eluted at 120 and 140 °C than Sample B. The difference in the microstructure distributions of both PP alloys results in observable differences in their mechanical properties: Sample A has better impact toughness and possesses higher rigidity than Sample B. Sample A also exhibits better balance between toughness and stiffness.

Random copolymers of propylene with 1,5-hexadiene containing only cyclopentane units in main chain and tailoring structure and mechanical properties of the copolymers

In the presence of [Ph3C][B(C6F5)4] and Triisobutylaluminium, dimethyl pyridylamidohafnium complex exhibited remarkable catalytic activity in the copolymerization of propylene with 1,5-hexadiene, affording high molecular weight copolymers with unimodal molecular weight distributions. More noticeably, the resultant copolymers contain absolutely cyclo-group in the polymer chains without pendent vinyl groups and crosslinking. The solid state structures and properties of the homogeneous copolymers were studied by examining melting behavior, wide-angle X-ray scattering, and tensile deformation. Chain microstructures were analyzed by 13C NMR spectra, and further confirmed by the measurement of reactivity ratios of the two monomers. These copolymers display a random distribution of the comonomer, and crystallinity decreases with increasing comonomer content. The copolymers contain variable amounts of comonomeric units, showing desired physical properties ranging from thermoplastic to elastoplastomeric to elastomeric.

In situ X-ray analysis of mesophase formation in random copolymers of propylene and 1-butene

The solidification of random isotactic copolymers of propylene and 1-butene has been followed in real time by wide-angle X-ray scattering as a function of the rate of cooling the quiescent liquid. The experimental setup allowed simultaneous recording of cooling curves—sample temperature as a function of time—and X-ray patterns at high sampling rate of 20 Hz. This approach allowed establishing a correlation between cooling rate, temperature of crystallization/mesophase formation, and X-ray structure, which formerly has only been observed ex situ, after completion of structure formation during cooling and subsequent aging. It is quantitatively confirmed that addition of 1-butene co-units into the propylene chain allows mesophase formation on cooling the melt at distinctly lower rate than in case of the homopolymer. The experimental results are compiled into a continuous cooling transformation (CCT) diagram and compared with data obtained earlier on random copolymers of propylene with ethylene.

Dynamics of Ethylene−Propylene Random Copolymers by 1H and 13C Solid-State NMR

The dynamic behavior of three completely amorphous ethylene-propylene random copolymers with slightly different structural properties was investigated by means of a multifrequency global fitting approach, presented in a previous paper. To this end, the trends of NMR (1)H and (13)C spin-lattice relaxation times with temperature, obtained at different frequencies, were analyzed by means of unified motional models for the segmental main-chain motion and rotation of methyl groups about their ternary symmetry axis. To evaluate the reliability of both the models and the method used with respect to the dimensionality of the data set, as well as to identify the minimal data set to be used for getting reliable dynamic information on similar systems, the fitting results obtained from the analysis of three different experimental data sets collected on the same polymer were compared. The data collected on the three polymers using the minimal data set were analyzed and the results discussed to evaluate the degree of sensitivity of the method to differences in the dynamic behavior of structurally similar systems. The results demonstrated that the method shows good sensitivity to the use of different dynamic models depending on the number and/or type of relaxation curves included in the analysis and that variations in the dynamic features of the same motions on different systems can be revealed even in the presence of polymeric systems characterized by very similar structural properties.

Formation and reorganization of the mesophase of random copolymers of propylene and 1-butene

Fast scanning chip calorimetry (FSC) has been employed to study the kinetics of formation of the mesophase of random copolymers of propylene and 1-butene from the glassy amorphous state and its reorganization on heating. The experiments performed consistently prove a distinct decrease of the rate of mesophase formation with increasing concentration of 1-butene chain defects. The time required for isothermal mesophase formation at 300 K is of the order of 0.1 s in case of the homopolymer, while it is prolonged by one order of magnitude to 1 s in the copolymer with 11 mol-% 1-butene. Similar, cold-ordering of amorphous structure on continuous heating at 1000 K s−1 is only completed in the homopolymer while it is almost completely suppressed in the random copolymer containing about 11 mol-% 1-butene. The perfection of the mesophase and/or its reorganization into crystals is faster in the homopolymer than in copolymers containing 1-butene. The critical heating rate for complete inhibition of perfection and reorganization is reduced from about 40,000 K s−1 in the homopolymer to about 10,000 K s−1 in copolymers. The reduced rate of mesophase formation in random copolymers of propylene and 1-butene is attributed to the decrease of the thermodynamic driving force for the phase transformation.

Raman Structural Study of Copolymers of Propylene with Ethylene and High Olefins

AbstractNascent form of random copolymers of propylene with ethylene, 1‐butene, 1‐hexene, 1‐octene, and 4‐methyl‐1‐pentene was studied by Raman spectroscopy. The most significant spectral alterations with a change in propylene content were observed in two lines at 809 and 841 cm−1. The first line corresponds to vibrations of polypropylene helical chains in the crystalline phase, while the second one is associated with vibrations of polypropylene helical chains having isomeric defects. Raman data confirm that conformational composition and phase state of copolymer macromolecules strongly depend on the comonomer content as well as on the size of the comonomer units.

X-ray study of crystallization of random copolymers of propylene and 1-butene via a mesophase

Wide-angle X-ray scattering has been employed for evaluation of the crystal structure of cold-crystallized random copolymers of propylene with 1-butene. The crystallization procedure included the formation of a metastable mesophase at ambient temperature by quenching the quiescent melt, and the reorganization of the semimesomorphic structure into a semicrystalline structure on subsequent heating/annealing. Research was performed to gain information about incorporation/exclusion of 1-butene chain defects into the crystalline phase when formed via the intermediate step of mesophase formation. The X-ray data suggest that 1-butene co-units up to a concentration of at least 11mol-% are incorporated into both the mesophase formed on quenching and crystals formed on subsequent heating/annealing according to their concentration in the chain. This observation is in agreement with results obtained on samples which were crystallized directly from the supercooled liquid state, indicating that incorporation of 1-butene co-units into the crystalline phase is not kinetically controlled. Furthermore, information about the α/γ polymorphism and the kinetics of the transition from the semimesomorphic into semicrystalline structure are provided.

Morphology, microstructure and compatibility of impact polypropylene copolymer

The morphology of impact polypropylene copolymer (IPC) was studied through scanning electron microscope (SEM) and transmission electron microscope (TEM) observation, and a modified dispersed phase model of IPC with core-shell structure was proposed. Through fractionation of IPC, the glass transitions of the ethylene–propylene random copolymer (EPR) fraction, ethylene–propylene block copolymer (EbP) fraction and propylene homopolymer (iPP) fraction were detected, respectively. Moreover, the glass transitions and crystallization behaviors of EbP/iPP and EPR/EbP fraction blends were systemically investigated and several reasonable chain structures of EbP component were confirmed. The results reveal that the EbP component presents three glass transition peaks, and the glass transition temperature of ethylene–propylene random copolymer in IPC sample is remarkably lower than that of pure EPR fraction due to the existence of special structure of EbP component in IPC. In addition, co-crystallization occurring between the polypropylene chains in EbP fraction and in iPP fraction was found for solution-mixed EbP/iPP blends, and it is believed that there exists a dilute effect of EPR on the crystallization of EbP fraction for the solution-mixed EPR/EbP blends. Accordingly, it can be inferred that EbP fraction has good compatibility with both EPR and iPP fraction, and indeed it confirms that the compatibilization effect of EbP fraction in IPC was good.

Polarized Raman study of random copolymers of propylene with olefins

The polarized Raman spectroscopy is employed in the study of structural modifications in the films of isotactic polypropylene (PP) whose chain contains ethylene, 1-butene, 1-hexene, 1-octene, and 4-metyl-pentene-1, which represents an isomer of 1-hexene. It is demonstrated that the phase and conformational compositions of copolymer molecules depend on the comonomer content and the side-chain length of the second monomer. The content of the PP molecules in the helical conformation in the crystalline and amorphous phases of the copolymers monotonically decreases with increasing content of the second monomer. The decrease in the content of helical macromolecules in the crystalline phase is faster than the decrease in the amorphous phase. At a certain content of comonomers, the total content of the helical fragments decreases with increasing length of the side chain of the second monomer. The structures and Raman spectra of the copolymers of propylene with 1-hexene and 4-methyl-1-pentene are similar.

Tensile properties of random copolymers of propylene with ethylene and 1-butene: effect of crystallinity and crystal habit

The tensile modulus of elasticity and yield strength of semicrystalline random copolymers of propylene with different amount on ethylene or 1-butene co-units were analyzed as a function of the crystallinity and the crystal habit/shape. Samples were prepared by cooling the melt to ambient temperature, and subsequent annealing at elevated temperature. Variation of the cooling rate between 10−1 and 103 K s−1 and of the temperature of annealing allowed preparation of semicrystalline specimens with either lamellar or non-lamellar crystals of different size, and with different crystallinity between about 30 and 70%. Young’s modulus and yield strength increase with increasing crystallinity and consistently are lower for samples containing nodular, that is, almost isometric, non-lamellar crystals of low aspect ratio. For samples of identical crystallinity and crystal habit, an only minor effect of presence of co-units in the crystalline and amorphous phases is observed.

The morphology and mechanical properties of recyclated polyethylene/polyamide laminate

The influence of the compatibilizers on the morphology and mechanical properties of the blend of polyethylene (PE) and polyamide 6 (PA6) was studied. The waste of PE/PA6 laminate obtained after processing of barrier packaging foils was studied. The waste of PE/PA6 laminate containing 80 wt. % of PE and 20 wt. % of PA6 was processed with two compatibilizers: maleic anhydride functionalized ethylene propylene random copolymer (EPM-g-MA) and carboxyl functionalized polyethylene (PE-COOH). The compatibilizers were added to the waste in the amount of 5 wt. % or 10 wt. %. The compatibilization was performed in a Brabender plasticizer at 220°C and at rotor speed of 50rpm in a two-step mixing procedure. In the first step the blend was melted for 2min. That was followed by the addition of compatibilizer in the second step and the mixing was continued for the next 3min. The mixed melted samples formed thin films by compressive molding. Morphologies were examined using transmission electron microscope TESLA BS 500. The morphology was altered by an addition of compatibilizers and the system became more homogeneous. The dimensions of PA6 domains dispersed in PE matrix decreased dramatically, compared to that of uncompatibilized system. After addition of 5 wt. % or 10 wt. % of a compatibilizer to PE/PA6 blend its mechanical properties improved (twice higher Young's modulus).