Effect of the matrix modification technique (MMT) on the composition, microstructure, morphology, interfacial interaction and mechanical properties of polypropylene reactor alloys

Abstract

In this paper, a new approach was applied to improve phase compatibility of isotactic polypropylene (iPP) reactor alloys through a matrix modification technique (MMT). The matrix phase of a heterophasic block copolymer was prepared using a 4th generation Ziegler–Natta catalyst and was modified by 0.5, 1, 1.5 and 3 wt% ethylene as a co-monomer along with propylene. Evaluation of quantitative FTIR spectroscopy revealed that the ethylene content of the matrix is similar to the amount of injected ethylene in the reactor during the matrix preparation stage. Copolymerization and post homopolymerization conditions for all the samples were similar. It was found that the co-monomer content of the modified matrix strongly affects the cold soluble xylene fraction and also the microstructure of iPP reactor alloys. Morphological studies using scanning electron microscopy confirmed the significant improvement in the morphology of reactor alloys when 1 and 1.5 wt% ethylene were randomly copolymerized with propylene during the matrix preparation stage and almost single-phase morphology was observed when 3 wt% ethylene was added in the matrix. The interfacial interaction between ethylene propylene rubber (EPR) and the matrix was determined by Pal and Palierne emulsion models, and the results revealed an improvement of phase compatibility between the matrix and the dispersed phase when 1.5 wt% ethylene was used in its modified matrix. Thermal analysis showed that by increasing the co-monomer content in the modified matrix, the degree of crystallinity and the melting point (T_m) decrease. Mechanical tests indicated that the effect of the MMT on improving the phase compatibility and toughness of reactor alloys is higher than that of the dispersed phase content.

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Introduction

Polypropylene (PP) is a commodity polyolefin with good process ability, excellent mechanical properties and low cost. The low impact strength of isotactic polypropylene (iPP) has limited its use in some engineering applications. Mechanical blending^1,2 with tougheners such as ethylene-propylene rubber (EPR) and reactor alloying^3–5 is one of the main approaches to increase the impact strength of iPP. The particle size and particle size distribution of the dispersed phase in the blends,^6,7 the molecular weight and molecular weight distribution⁸ of the matrix and also the dispersed phase affect the impact strength of the blends. The impact strength of iPP/EPR blends is under the influence of the distribution and dispersion state of EPR in the matrix⁹ and also is directly related to interfacial interactions between the matrix and the dispersed phase.⁷ The strength of interfacial interactions in the mechanical blends is a significant obstacle to achieve the desired mechanical properties. Numerous attempts have been made to produce various grades of polypropylene reactor alloys due to their enhanced properties and the possibility of balancing between stiffness and toughness.^4,10,11 The mechanical properties of PP reactor alloys can be improved either chemically, by modifying the catalyst,¹² co-catalyst¹³ and external electron donors,¹⁴ or by changing physical parameters^15–17 such as temperature, pressure and the monomer feed ratio. The rubber phase content and its interfacial interactions with the iPP matrix are two main concerns of the impact properties of heterophasic block copolymers. It has been demonstrated that by changing the copolymerization time and the percentage of ethylene in the feed, the content of random and segmented copolymers in the heterophasic copolymer will slightly change and, consequently, affect the impact strength of reactor alloys.¹⁸ However, the active centers of Ziegler–Natta catalysts show a decay type profile during polymerization and more pronouncedly in the copolymerization steps.¹⁹ So, an increase in the copolymerization time rarely has a significant effect on enhancing the EPR content of iPP reactor alloys. The ethylene rich segmented copolymers, as a compatibilizer between the matrix and the dispersed phase, can increase the impact strength of iPP/EPR blends,²⁰ while a negligible amount of these copolymers has been observed in the industrial grades of reactor alloys.²¹ In such a situation, by increasing the EPR content, the agglomeration of dispersed droplets may occur for prepared reactor alloys by a two-stage polymerization process.²² It has been indicated that conducting copolymerization by more successive stages in the copolymerization step affects the size and morphology of the dispersed phase in the reactor alloys.²³ Using multi-zone circulating reactor technology with a 5th generation Ziegler–Natta catalyst is among the efforts made to improve the properties of iPP heterophasic block copolymers. A multi-stage sequential polymerization process was used to synthesize the copolymers with a fine dispersed phase morphology.²⁴ It is found that, by increasing the switching frequency in the multi-stage process, the particle size of the dispersed phase decreases and the mechanical properties of iPP/EPR reactor alloys improve.²³

In a recent effort to improve the toughness of reactor alloys, after homopolymerization of the matrix, copolymerization in the gas phase has been carried out in two successive stages with 17 and 40 mol% ethylene in the feed.²⁵

An approach to enhance the mechanical properties of heterophasic iPP block copolymers is to increase the compatibility between the matrix and the dispersed phase through modification of the matrix phase.²⁶

In this work, the matrix of heterophasic copolymers was modified using ethylene as the co-monomer via a slurry process in the matrix preparation stage. The matrix phase in the heterophasic iPP block copolymer was exchanged for an iPP random copolymer (R-iPP) using a small amount of ethylene monomer and the effect of this modification on the chemical composition, phase compatibility, interfacial interaction and mechanical properties was investigated.

Experimental

Materials

Propylene and ethylene with 99.99% purity (polymerization grade) were prepared from Bandar Imam petrochemical Company, Iran. A heterogeneous fourth-generation Ziegler–Natta catalyst (TiCl₄/MgCl₂/ID) with 2.7 to 3 wt% Ti was supplied by Shazand petrochemical Company, Iran. Triethylaluminum (TEA) as the co-catalyst and methyl cyclo hexyl methyl dimethoxy silane as an external electron donor (ED) were purchased from Sigma-Aldrich, Germany. Double-distilled hexane and industrial grade methanol were provided by Shazand petrochemical Company, Iran. Nitrogen gases with 99.99% purity and research grade H₂ were supplied by Roham Gas Company, Iran.

Synthesis of the reactor alloys

The polymerization was carried out in a 1 liter stainless steel Buchi reactor equipped with a helical mechanical stirrer. Polymerization was carried out in three steps: matrix preparation, copolymerization and post polymerization. The matrix preparation stage was performed in a slurry process at 60 °C and 6 bar pressure in hexane as media after a pre-polymerization step in a non-isothermal condition, from 40 to 60 °C. After the matrix preparation stage, reaction media including unreacted reactants were evacuated out of the reactor by vacuum.

Consequently, the main copolymerization step and post polymerization were accomplished in the gas phase at 60 °C and 6 bar. The optimized values of TEA/Ti and ED/Ti (480 and 0.05 mole ratio) were obtained according to the maximum yield of the reaction. Ethylene as the co-monomer was used in the matrix preparation stage for modification and partial randomization of the matrix, as well as during the copolymerization stage for synthesis of EPR in a gas phase process. Table 1 demonstrates the designed experiments for reactor alloys. Fig. 1 shows a proposed model for the growth of particles and final morphologies of reactor alloys of the present work.

Table 1 The process conditions used to prepare the reactor alloys^a

Sample	Matrix Preparation				MCPS		PHP
	Pre-Pol		Homo-Pol
	E/P	T	E/P	T	E/P	T	E/P	T
a Pre-Pol: pre-polymerization. Homo-Pol: homopolymerization. MCPS: main copolymerization stage. PHP: post homopolymerization. E/P: ethylene to propylene ratio in weight percentage. T: time in minutes.
PE0.0	0.0	30	0.0	60	27	45	0.0	40
PE0.5	0.5	30	0.5	60	27	45	0.0	40
PE1.0	1.0	30	1.0	60	27	45	0.0	40
PE1.5	1.5	30	1.5	60	27	45	0.0	40
PE3.0	3.0	30	3.0	60	27	45	0.0	40

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Fig. 1 Proposed model for the growth of the particles in the various steps.

Characterization

Fractionation of the alloy components

Synthesized samples were fractionated by boiling xylene according to the temperature gradient extraction fractionation method (TGEF) using a modified Kumagawa extractor.²⁷ For each sample six fractions F₁, F₂, F₃, F₄, F₅ and F₆ were respectively collected at 30, 90, 110, 120, 130 and >130 °C.

Fourier transform infrared (FTIR) spectroscopy

A FTIR spectrometer, BRUKER EQUINOX 55, Germany, was used for quantitative evaluation of the ethylene content in the samples using a calibration curve. The calibration curve is built according to ASTM D 3900 by calculating the A (722 cm⁻¹)/A (1156 cm⁻¹) ratio for the obtained blends from mixing of various amounts of polyethylene (PE) and polypropylene in hot xylene. The A (722 cm⁻¹) and A (1156 cm⁻¹) belong to the methylene and methyl groups of PE and iPP respectively. The FTIR test was carried out on the film of samples with a thickness of about 80 microns.

Investigation of phase morphology

To study the morphology of reactor alloys and the EPR dispersion state, the cryo-fractured surface of samples was sputter coated by gold and analysed using Scanning Electron Microscopy (SEM) (VEGA\\TESCAN, Czech Republic).

Shear rheological behaviour

Rheological measurements were accomplished using a RHEOPLUS rheometer model MCR 300 Anton Paar-Germany, on the 25 mm diameter compression disc in oscillation mode with a 1 mm gap. Linear viscoelastic behaviour of the samples was investigated in a frequency range of 0.01–600 rad s⁻¹ with a strain amplitude of 5% which is determined by the strain sweep test at a frequency of 10 rad s⁻¹ at 200 °C. The interfacial interaction of the matrix and dispersed particles was also evaluated using the rheological data. Continuous nitrogen purging of the environment chamber was necessary to minimize oxidative degradation of the blends.

Thermal analysis

The melting and crystallization behaviours of the samples were studied using the Differential Scanning Calorimetry (DSC) method by the METTLER TOLEDO calorimeter model STARe 10, Germany. To remove thermal history and eliminate sample shape abnormalities, they were first heated to 220 °C at a heating rate of 40 °C min⁻¹. After 5 minutes of holding at this temperature, they cooled down to 25 °C at a rate of 10 °C min⁻¹ to detect crystallization behaviour such as crystallization temperature (T_c) and heat of crystallization (ΔH_c). In the second heating run they were heated up again to 220 °C with the same rate of 10 °C min⁻¹ to detect the melting characteristics, melting point (T_m) and enthalpy of fusion (ΔH_m). The degree of crystallinity χ_c of samples was calculated using eqn (1).where ΔH_m is the enthalpy of fusion, which is equal to 209 J g⁻¹ for iPP with 100% crystallinity. The degree of crystallinity was calculated considering the composition (w) of the insoluble part in cold xylene for each sample.

Wide-angle X-ray diffraction analysis

The Wide-Angle X-ray Diffraction (WAXD) experiments were carried out to study the crystalline structures of the samples. Measurements were done on a SIEMENS D5000 powder diffractometer at room temperature. Cu Kα radiation (λ = 1.54059Å) as a source was operated at 40 keV and 30 mA in a 2θ range of 5°–40°.

Mechanical properties

The notched Izod impact strength of the samples were measured on a ZWICK impact strength tester according to ASTM D 256. The flexural properties of the samples were determined on a SANTAM-STM 50 electronic tester according to ASTM D 790.

Results and discussion

Copolymer composition

The effects of adding an ethylene co-monomer in the matrix preparation stage on the polymer yield and fractionated constituents from reactor alloys at various temperatures along with their results are surveyed in Table 2. It is seen that adding a small amount of ethylene during the matrix preparation stage leads to an increase of the polymer yield. The fraction F₁ belongs to EPR + atactic PP, F₂ is moderate isotactic PP + ethylene rich segmented copolymers and F₃ is PE homopolymer + propylene rich multi-block copolymers respectively.^27,28

Table 2 Fractionation results of the reactor alloys PE0.0–PE3.0 and their polymerization yield in the matrix preparation stage

Sample	Polymer yield^a	F1 (wt%)	F2 (wt%)	F3 (wt%)	F4 (wt%)	F5 (wt%)	F6 (wt%)	Summation (%)
a kg of polymer per g of catalyst, yield of polymerization in the matrix modification stage using 0.0 to 3.0 wt% ethylene co-monomer.
PE0.0	2.41	14.95	6.04	5.74	6.07	23.78	43.29	99.87
PE0.5	2.47	24.17	4.34	5.18	5.50	19.05	41.65	99.89
PE1.0	2.59	17.26	4.26	5.29	5.62	23.61	43.93	99.97
PE1.5	2.66	14.60	4.46	5.91	6.10	24.19	43.88	99.14
PE3.0	2.73	10.38	4.25	6.72	6.54	25.67	45.95	99.51

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The presence of 0.5 wt% ethylene in the feed along with propylene in the matrix preparation stage leads to an increase of the F₁ fraction from 14.95 wt% in the PE0.0 reactor alloy to 24.17 wt% in the PE0.5 sample, improving the impact strength. In the PE0.0 sample, diffusion of the monomers to particle sub layers is hard due to the existence of the hard crystalline segments of iPP. By feeding a small amount of the ethylene co-monomer with propylene in the matrix preparation stage, a random copolymer with a low ethylene content and decreased crystallinity is obtained.^28,29 In this way, the high crystalline iPP matrix is exchanged into a random copolymer with lower crystallinity and consequently the monomers can diffuse easier to the particle sub layers in the copolymerization stage.

By increasing the percentage of ethylene in the copolymerization step 1, the EPR content in the alloy decreases continuously from 24.17 wt% in PE0.5 to 10.4 wt% in the PE3.0 sample. By increasing the ethylene co-monomer in the matrix preparation stage, in addition to an iPP random copolymer, some amorphous EPR may form and fill the pores of growing particles, while they migrate out from particles in the slurry process media.³⁰

Microstructure characterization

The effect of the MMT on the microstructure of reactor alloys was investigated using FTIR spectroscopy. As well, the fractionated parts at various temperatures were investigated and the microstructure of reactor alloys was quantitatively evaluated. The absorbance peaks positioning correlate to various ethylene-propylene random sequences are listed in Table 3.^31,32 Fig. 2 compares FTIR spectra of iPP and the R-iPP matrix in the presence of a 0.5, 1.0, 1.5 and 3.0 wt% ethylene co-monomer. The isotactic PP matrix does not show any significant trace from 720–750 cm⁻¹, while the existence of ethylene-propylene random sequences in the R-iPP matrix are detectable by a weak and broad peak without any doublet and shoulder in these wave numbers.³¹ The results revealed that matrix modification in the samples PE0.5 to PE3.0 has been accomplished successfully. The ethylene content of the samples was determined using the calibration curve shown in Fig. 3. It was found that all the ethylene injected into the reactor was copolymerised with propylene and the ethylene content of the samples was the same as the percentage of ethylene in the feed.

Table 3 Peak position of ethylene-propylene sequences in FTIR spectroscopy^a

Monomer sequence	P–P Sequence	Position (cm^−1)	n	Chemical structure
a HT: head-to-tail. HH: head-to-head. TT: tail-to-tail. a: broad peak. b: doublet peak. n: number of methylene groups.
P–P	HT	815	1	–CH2–CH(CH3)–(CH2)n–CH(CH3)–CH2–
P–P	TT	750^a	0	–CH2–CH(CH3)–CH(CH3)–CH2–
P–P	HH	752^a	2	–CH2–CH(CH3)–(CH2)n–CH(CH3)–CH2
P–E–P	TT	752^a	2	–CH2–CH(CH3)–(CH2)n–CH(CH3)–CH2–
P–E–P	HT	736^a	3	–CH2–CH(CH3)–(CH2)n–CH(CH3)–CH2–
P–E–P	HH	726^a	4	–CH2–CH(CH3)–(CH2)n–CH(CH3)–
P–E–E–P	HT	722^a	5	–CH2–CH(CH3)–(CH2)n–CH(CH3)–CH2–
P–(E)n–P	HH, HT	720–730^b	≥6	–CH2–CH(CH3)–(CH2)n–CH2–CH(CH3)–
				–CH2–CH(CH3)–(CH2)n–CH(CH3)–CH2–

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Fig. 2 FTIR spectra of the iPP and R-iPP matrix with 0.5, 1, 1.5 and 3 wt% ethylene as a co-monomer.

Fig. 3 Calibration curve to determine the ethylene content in the produced samples.

The FTIR spectra of the fractionated constituents F₁ to F₆ for reactor alloys PE0.0 and PE1.5 are compared in Fig. 4a and b respectively. The F₁ fraction is similar in both samples composed of EPR and atactic PP due to a weak-broad peak from 720–750 cm⁻¹ (ethylene–propylene random sequences) and two weak peaks at 973 and 1156 cm⁻¹ (methyl sequences of atactic PP).³¹ Two sharp peaks at 722 and 973 cm⁻¹ (by a moderate shoulder at 998 cm⁻¹) are respectively related to crystallisable methylene and propylene sequences in the F₂ fraction for samples PE0.0 and PE1.5. The peaks at 840 and 1156 cm⁻¹ prove the presence of isotactic PP in the F₂ fraction.

Fig. 4 FT-IR spectra of the fractionated constituents for reactor alloys (a) PE0.0 and (b) PE1.5.

It is believed that the F₃ fraction consists only of PE homopolymers,¹⁵ while peaks at 997 and 1156 cm⁻¹ in the spectrum of the F₃ fraction in Fig. 4a and b are evidence for the presence of multi block copolymers with large PP segments along with PE. High isotactic PP is identifiable in fractions F₄, F₅ and F₆ of samples PE0.0 and PE1.5 by absorbance peaks at 840, 998 and 1156 cm⁻¹. A weak-broad oscillation from 720–750 cm⁻¹ for fractions F₄, F₅ and F₆ of the PE1.5 sample shows the presence of ethylene–propylene sequences with a random structure in these fractions, while these fractions in the PE0.0 sample are composed only of high isotactic PP. The FTIR investigation of the fractionated parts confirmed that the composition and microstructure of the reactor alloys PE0.5 to PE3.0 are altered through the MMT and changing of the co-monomer content during the matrix preparation stage.

Investigation of phase compatibility by a morphological study

The copolymer composition of reactor alloys affects their microstructure and phase morphology. SEM micrographs of the fractured surface of synthesized reactor alloys are shown in Fig. 5. The extracted rubbery phase from reactor alloys is visible as discrete dispersed particles in the PE0.0 sample (Fig. 5a). By modifying the matrix with 0.5 wt% ethylene, a nearly smooth surface morphology is observed (Fig. 5b), which can be due to the improvement of phase compatibility in the PE0.5 sample.

Fig. 5 SEM micrographs of reactor alloys (a) PE0.0, (b) PE0.5, (c) PE1.0, (d) PE1.5 and (e) PE3.0.

White spots in the SEM images of samples PE1.0 and PE1.5 are visible, which are signs of deformation during fracturing of the sample in liquid nitrogen. On the one hand, this could be due to increased softness of the modified matrix with the ethylene co-monomer and on the other hand, this could be due to an increase of adhesion in the interface matrix and discrete dispersed droplets. During the freeze fracture of samples and pulling out of the EPR particles, the matrix shows more deformations in the interfacial region because of stronger interface adhesion and a softer matrix. The non-spherical particles in the samples PE1.0 and PE1.5 can be due to improved phase compatibility. By increasing the weight percentage of ethylene in the modified matrix, a nearly single-phase morphology is visible in the SEM image of the PE3.0 sample.

Comparing the micrograph of samples PE0.0 and PE1.5 (by the same cold xylene soluble content) shows the effect of the MMT on the morphology and phase compatibility of the reactor alloys as well. The results show that the MMT and ethylene content in the matrix preparation stage play an important role in the improvement of morphology and phase compatibility of reactor alloys.

Study of the interfacial interaction using rheological measurements

The interaction between dispersed particles and the matrix is an important aspect in polymer alloys. The strength of the interface can be measured using the pendant drop, spinning drop, and capillary breakup methods as well as rheological models. The pendant drop, spinning drop and capillary breakup methods are time consuming and coincide with a risk of thermal degradation of the polymer melt. The dynamic shear rheology test in the linear viscoelastic region is commonly used to investigate the interfacial interactions of polymer blends. Pal³³ and Palierne³⁴ models were formulated to assess the interfacial interaction of polymeric immiscible suspensions and emulsions according to Kerner³⁵ and Oldroyd³⁶ models. PP/EPCs reactor alloys are known as emulsions of the EPR dispersed droplets in the PP matrix.² To determine the interfacial interaction in these emulsions, experimental results were compared to Palierne and Pal models (eqn (2) and (7)).Here, is defined in eqn (3)., and are the complex shear modulus of the matrix, dispersed phase and alloy, respectively. σ, R and ϕ₂ stands for the interfacial tension, average particle diameter and volume fraction of the dispersed phase, respectively. The Palierne model is valid for small deformations of the spherical dispersed droplets at low concentration. In the Palierne model, the volume fraction of the dispersed phase can be taken into account from zero to 100%, when it is virtually impossible. The maximum packing volume fraction of the dispersed phase (ϕ_m) is generally less than a unit and is proposed to be 0.64 (ref. 37) or 0.56 (ref. 33).

Pal considered the actual volume fraction of the dispersed phase (ϕ₂) and agglomeration of the droplets to measure the effective volume fraction of the dispersed phase in emulsions with appropriate boundary conditions according to eqn (4)–(6).³⁷

ψϕ₂ = 0 at ϕ₂ = 0 (4)

ψϕ₂ = 1 at ϕ₂ = ϕ_m (6)

In this way, the general Palierne model extends to the Pal model by eqn (7):

where ψϕ₂ is the effective volume fraction of the dispersed phase and has the same expression as the Palierne model. The effective volume fraction of the droplets is represented by eqn (8) by considering boundary conditions (4)–(6):³⁷

The initial value for interfacial tension in the Pal and Palierne models was calculated using Fowkes equation:³⁸

where σ, α₁, α₂ and χ stands for interfacial tension, surface tension of the components and the interaction parameter of the phases, respectively. χ is close to one for most of the polymer blends. The initial estimation for interfacial tension was determined to be 1.067 mN m⁻¹. Fig. 6 compares the experimental storage modulus of reactor alloys PE0.0, PE0.5 and PE1.5 with the Pal and Palierne model predictions. The results show that the Pal model, for predicting interfacial interaction of the samples PE0.0 and PE0.5, is more successful than the Palierne model.

Fig. 6 Comparison between the experimental storage modulus and the Pal and Palierne predictions. (a) PE0.0, (b) PE0.5 and (c) PE1.5.

By increasing the ethylene content to 1.5 wt% in the modified matrix of the PE1.5 sample, experimental G′(ω) shows extremely positive deviations from the Pal and Palierne models, which can be attributed to an improvement in phase compatibility. The results reveal that copolymerization of the matrix with 1.5 wt% ethylene will be able to improve phase compatibility in reactor alloys.

The system homogeneity of blends can be interpreted by the slope of the G′–G′′ diagram in the Han plot. The Han plot of reactor alloys PE0.0, PE0.5 and PE1.5 is shown in Fig. 7. When the slope of the G′–G′′ diagram is close to two, it means that the alloy components are compatible.^39,40 The slope of G′–G′′ for PE0.0 and PE0.5 is 1.42 and 1.60, respectively. An increase of the Han plot slope arises from an increase of the EPR content as an elastic component in PE0.5. The dispersed phase content of PE1.5 is lower than that of PE0.5 (according to Table 2), while the interfacial interaction and the slope of the G′–G′′ plot for PE1.5 are greater than that of PE0.5. These results reveal an improvement of phase compatibility in the reactor alloy of PE1.5 more than that of PE0.5 due to the appropriate co-monomer content of its modified matrix.

Fig. 7 Modified Cole–Cole plot of samples PE0.0, PE0.5 and PE1.5.

In Table 4, the slope of the G′–G′′ plots and the predicted interfacial strength of the reactor alloys PE0.0, PE0.5 and PE1.5 are compared.

Table 4 Comparison of experimental interfacial tension of the reactor alloys using Pal and Palierne predictions^a

Sample	ϕ2 (wt%)	m	σ (mN m^−1)
			Palierne model	Pal model
a m: slope of the Han plot.
PE0.0	14.95	1.42	1.654	1.572
PE0.5	24.17	1.60	2.293	2.151
PE1.5	14.60	1.64	2.406	2.328

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The viscosity plot versus angular frequency can be a good measure for the interfacial adhesion between the alloy components. The viscosity of polymers changes by changing the molecular weight and temperature and is more pronounced at low frequencies. In the polymer alloys, the interfacial adhesion is also an effective parameter to enhance viscosity.

The logarithmic rule of viscosity is expressed by eqn (10) as an estimation for two-component blend viscosity.⁴⁰

The complex viscosity and storage modulus versus angular frequency of the matrix, EPR dispersed phase and reactor alloys PE0.0, PE0.5 and PE1.5 are shown in Fig. 8a–c. The complex viscosity at low angular frequencies/shear rates-η₀ is a measure of the molecular weight. The of the matrix for PE0.0, PE0.5 and PE1.5 is increased by increasing the ethylene content of the matrix and is more pronounced at 1.5 wt% according to Fig. 8a.

Fig. 8 Complex shear viscosity and storage modulus versus angular frequency of the (a) matrix, (b) dispersed phase and (c) reactor alloy.

The rheological study reveals that MMT modification leads to an increase molecular weight of the EPR dispersed phase. Almost the same increase in the molecular weight of EPR is observed by increasing the ethylene content of the matrix to 0.5 and 1.5 wt% ethylene according to Fig. 8b. The comprehensive increase in the molecular weight of the matrix and EPR dispersed particles leads to an enhanced complex viscosity of the reactor alloys and improved mechanical properties. The predicted values in eqn (10) were compared with the experimental viscosity data in Table 5. The difference between the experimental findings and theoretical predictions (viscosity growth factor) is an appropriate criterion for the phase interaction of the blends.^41,42 The modification of the matrix by 0.5 wt% ethylene showed a peak in the EPR content from 14.95 in PE0.0 to 24.2 in PE0.5, which declined to 14.6% in the PE1.5 sample. It does not lead to a considerable change in the viscosity growth factor of the PE0.5 sample compared to PE0.0. The viscosity growth factor of the PE1.5 sample, with a matrix modified by 1.5 wt% ethylene, enhanced more than that of PE0.0 and PE0.5 due to improved interfacial interactions (Fig. 8c and Table 5).

Table 5 Comparison of the viscosity growth factor of the reactor alloys PE0.0, PE0.5 and PE1.5

Sample	log				VGF^c (%)
	EPR	PP	Alloy^a	Alloy^b
a Experimental.b Theoretical.c Viscosity growth factor.
PE0.0	4.49	3.55	3.82	3.69	+3.52
PE0.5	4.78	3.66	4.04	3.93	+2.79
PE1.5	4.71	3.75	4.17	3.89	+7.19

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The improved interfacial interaction affects the damping factor (tan δ). By increasing melt elasticity, the damping factor decreases.⁴³ Fig. 9 shows the damping factor versus angular frequency for the reactor alloys PE0.0, PE0.5 and PE1.5. The damping factor of PE1.5 is lower than that of PE0.5 and PE0.0 due to improved interfacial strength and phase compatibility.

Fig. 9 Damping factor versus angular frequency for reactor alloys PE0.0, PE0.5 and PE1.5.

Effect of the MMT on crystallization behavior

The microstructure along with degree of crystallinity (χ_c) are effective parameters for the mechanical properties of the PP/EPR blends. The melting and crystallization temperature, degree of crystallinity and isotacticity index of samples are shown in Table 6. By increasing the co-monomer content in the modified matrix, T_m, T_c and χ_c are decreased. The iPP crystalline structure can be altered by adding various amounts of different co-monomers.^44,45 Introducing ethylene into propylene sequences promotes formation of short stereoregular blocks⁴⁶ that leads to a decreased melting temperature and crystallinity.^47,48

Table 6 Melting and crystallization properties of the reactor alloys

Sample	Isotacticity (%)	Tm (°C)	Tc (°C)	χc (%)
PE0.0	97.52	165	107	51.1
PE0.5	97.12	163	107	48.8
PE1.0	96.26	162	106	44.7
PE1.5	95.49	160	104	39.1
PE3.0	94.65	154	97	38.2

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Polypropylene has α, β and γ crystalline forms.^49–51 The DSC thermograms along with WAXD patterns of the samples are shown in Fig. 10. The β form of crystals for the reactor alloys PE0.5 and PE1.5 can be seen. This shows that random sequences of the R-iPP matrix act as a β-nucleating agent in the R-iPP/EPCs reactor alloys that affects the degree of crystallinity and width of the melting thermogram.

Fig. 10 Thermal behaviour along with crystalline structure of the samples: (a) DSC thermograms and (b) WAXD patterns.

Mechanical properties

Impact strength and flexural modulus of the reactor alloys are surveyed in Table 7. Increasing the EPR content as well as the strength of the interfacial interaction in the produced reactor alloys by the MMT causes an improvement in the toughness of samples PE0.5-PE3.0 compared to that of PE0.0. Comparing the mechanical properties of PE0.0 and PE1.5 with the same cold soluble xylene shows well that the presence of 1.5 wt% ethylene in the matrix preparation stage has a profound effect on the fracture mechanism of the reactor alloys.

Table 7 Mechanical properties of the reactor alloys at room temperature^a

Samples	Flexural modulus (MPa)	Impact resistance (kJ m^−2)
a NB: no breakage.
PE0.0	865	26.7
PE0.5	700	NB
PE1.0	680	NB
PE1.5	640	NB
PE3.0	620	NB

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Conclusion

Regulating the co-monomer content in the matrix preparation stage by the MMT could increase the disperse phase content of the R-iPP/EPCs/PP reactor alloys. However, the amount of segmented copolymers as compatibilizers in the produced reactor alloys is almost constant. Creating random structures in iPP could improve the phase morphology of R-iPP reactor alloys, as in the case of the PE3.0 single phase morphology, which was roughly observed due to the use of the proper co-monomer content during its matrix preparation stage. Positive deviation of the experimental storage modulus from the Pal and Palierne predictions in the PE1.5 sample was proven to improve its phase compatibility. Increasing the melt elasticity, viscosity growth factor, slope of Han plot and toughness are signs of improvement of the interfacial interaction between the matrix and the dispersed phase detected in the produced reactor alloys with the MMT. The matrix modification increased χ_c and the β crystal content, while melting thermograms of the R-iPP reactor alloys became wider. Toughness of the samples increased in two ways: (1) increasing ethylene-propylene random copolymers as the dispersed phase and (2) improving interfacial interaction between the matrix and the dispersed phase. The results show that the role of creating structural similarity between the matrix and the dispersed phase for the improvement of phase compatibility and toughness of the reactor alloys is more complicated than just modifying the EPR content.

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