Comparison of the effect of ethylene and hexene-1 co-monomers on the composition, microstructure, rheology, thermal and mechanical behaviour of randomized polypropylene hetero-phasic block co-polymers

Abstract

In this work, ethylene and hexene-1 co-monomers were applied in the matrix preparation stage (MPS) of isotactic polypropylene (iPP) reactor alloys to compare the effect of the co-monomer type used in MPS on their interfacial interaction and impact strength. For this purpose, before ethylene–propylene gas phase co-polymerization to produce dispersed phase, propylene was co-polymerized by 0.5, 1.0, 1.5 and 3.0% wt of ethylene and hexene-1 co-monomers in two separate slurry procedures to produce the partially randomized iPP (PR-iPP). Dispersed phase content of reactor alloys was strongly affected by the amount of the co-monomers used in MPS, while ethylene–propylene segmented co-polymers (EPs), as the compatibilizer between the matrix and dispersed phase, were little changed for all samples. SEM micrographs of the modified reactor alloys displayed that matrix modification technique (MMT) improves the morphology of the dispersed phase insofar as a single-phase morphology was seen when an appropriate amount of hexene-1 was used in the MPS. Pal and Palierne rheological models were applied to determine the interfacial tension of reactor alloys and the results illustrated that MMT affects the interfacial tension of matrix-dispersed droplets. Next rheological studies revealed that the interfacial interaction of EPR with the matrix modified by hexene-1 is higher than the matrix modified by ethylene. Improvement of interfacial interaction reduced the degree of crystallinity and overspread the melting thermograms of PR-iPP reactor alloys. Also, the presence of ethylene through the growth of β crystalline forms and hexene-1 via changing the α-form distribution help to improve the toughness of PR-iPP reactor alloys.

---

Introduction

Isotactic polypropylene (iPP) as a commodity polyolefin is used in a wide range of applications because of its acceptable mechanical performance, excellent processability, and high chemical resistance. The low impact strength of iPP does not meet certain engineering requirements, especially at low temperature. The impact strength of commercial grades of iPP can be improved by introducing structural irregularities in the backbone of the main chain. It is conducted through feeding a few other α-olefins along with propylene in the bulk polymerization to produce random PP co-polymers.^1–3 Although a series of process limitations, such as the solubility of random sequences in the liquid monomer and the adhesion of rubber-sticky particles, are obstacles to increasing the co-monomer content in random co-polymers of PP; PP random co-polymers that contain low amounts of co-monomer are produced in large quantities in the world.

An effective approach for improving the toughness of iPP is its combination with ethylene-based elastomers, such as ethylene–propylene rubber (EPR), as a dispersed phase through melt blending^4–6 or reactor alloying.^7–9 In these hetero-phasic block co-polymers the dispersed phase content and its interaction with the matrix affect significantly the impact strength of PP/EPR blends. The time-consuming process of impact modifier break-up (the balance between break-up and coalescence) and the high viscosity-difference of the components have restricted the increment of the dispersed phase content in PP mechanical blends.¹⁰ On the other hand, the coarse fractured surface of the pieces along with the spherical shape of the dispersed phase in PP/EPR blends is evidence for the low adhesion and weak interfacial interaction of the phases in these blends.^5,11 To enhance the toughness of PP-based mechanical compounds, comprehensive attempts have been made considering the molecular characteristics,^12–14 mode and dispersion state of the discrete rubbery phase.¹⁵

Regulating the chemical structure, composition, and distribution of the dispersed phase in reactor alloying is cheaper and easier than mechanical blending.^16–18 Also, co-generation of ethylene–propylene segmented co-polymers (EPs) as the compatibilizer, along with EPR in the co-polymerization stage, leads to an improvement of the phase compatibility and an increase in the impact strength of PP reactor alloys. To enhance the toughness of PP reactor alloys, some chemical^19,20 and physical^17,21 modifications have been made by regulating the polymerization conditions and adjusting the content and chemical composition of the ethylene–propylene co-polymers of these alloys. However, most of the chemical treatments are not affordable at an industrial level and are often confined to academic researches. On the other hand, the decay-type profile of polymerization, especially in the co-polymerization stage, is a considerable hurdle to enhance the toughener and compatibilizer content of industrial-grade PP reactor alloys.^17,22,23 This makes it difficult to control the microstructure of ethylene–propylene co-polymers which has a decisive role on the mechanical properties of these alloys.

In the case of physical reforms, the use of commercialized multi-zone circulating reactor technology^24–26 and the implementation of polymerization through adjusting switching frequencies between polymerization and co-polymerization stages are efforts to achieve to a fine morphology and higher impact strength.^27–29 Recently, adding a co-polymerization stage with low ethylene content between homo-polymerization and main co-polymerization in a two-step polymerization process,³⁰ and changing the monomer feed ratio³¹ in a periodic switching process, were examined to improve the toughness of PP/EPR reactor alloys. Also, matrix modification technique (MMT) is a new procedure that was used in our previous work to improve the mechanical properties of PP reactor alloys in which the matrix was modified with a small amount of ethylene.³²

In this study, ethylene and hexene-1 co-monomers were used in the matrix modification stage of PP reactor alloys. With the participation of a small amount of these co-monomers, the iPP matrix was converted to a partially randomized co-polymer with the reduced isotacticity. Subsequently, the effect of this change and the comparison of the type and amount of the participating co-monomers in MMT on the composition, microstructure, morphology, phase compatibility, and thermal and mechanical behaviour of PP-modified reactor alloys have been investigated.

Experimental

Chemicals

Gaseous propylene and ethylene with 99.99% purity (polymerization grade), an industrial 4^th generation heterogeneous Ziegler–Natta catalyst containing 2.7 to 3% wt Ti and industrial grade methanol and double-distilled hexane were supplied by Shazand Petrochemical Company, Iran. Methylcyclohexylmethyldimethoxy silane as the external electron donor (ED), triethyl aluminum (TEA, 1.0 M solution in heptane) as scavenger and co-catalyst, and hexene-1 as co-monomer were purchased from Sigma-Aldrich, Germany. Gaseous hydrogen and nitrogen with 99.99% purity were purchased from Roham Gas Company, Iran.

Synthesis of the reactor alloys

In this study, a 1 liter stainless steel Buchi reactor equipped with a helical mechanical stirrer was used to synthesize the reactor alloys. The amount of AL/Ti and ED/AL ratios was respectively optimized at 480 and 0.05 according to the maximum yield of reaction for all experiments. PP reactor alloys were prepared in three stages to contain matrix preparation, co-polymerization, and post homo-polymerization stages. Slurry polymerization for 60 min was carried out at 7 bar pressure and 60 °C in hexane as media, after 30 min non-isothermal pre-polymerization from 40 to 60 °C at 4 bar. In the matrix preparation stage (MPS), ethylene and hexene-1 co-monomers were separately consumed to modify and induce the partial random structure in the matrix. After this stage the polymerization media, which contained unreacted chemicals, was evacuated. Then, the main co-polymerizations for producing EPR and EPs, for 45 min at an ethylene–propylene constant weight ratio of 27% and post propylene polymerization for 40 min, were conducted in the gas phase mode at 60 °C and 6 bar. In addition, a PP block co-polymer without any matrix modification was produced under the same polymerization conditions as the modified alloys to compare with the modified ones. The designed experiments for reactor alloys are illustrated in Table 1.

Table 1 Comparison of the co-monomer content used in feed and the amount entered into the matrix

Sample, PERC^a	Matrix modification stage		Sample, PHRC^b	Matrix modification stage
	E/P in feed (% wt)	E/P in the modified matrix^c (% wt)		H/P in feed (% wt)	H/P in the modified matrix^c (% wt)
a Propylene–ethylene random co-polymer (PR-iPP matrix).b Propylene–hexene-1 random co-polymer (PR-iPP matrix).c Co-monomer content of the modified matrix which was calculated by FTIR analysis.
PE0.5	0.50	0.50 ± 0.03	PH0.5	0.50	0.50 ± 0.02
PE1.0	1.00	1.00 ± 0.03	PH1.0	1.00	1.00 ± 0.02
PE1.5	1.50	1.50 ± 0.01	PH1.5	1.74	1.50 ± 0.03
PE3.0	3.00	3.00 ± 0.02	PH3.0	4.00	3.00 ± 0.01

---

Polymer characterization

Fractionation

To understand the chemical and co-polymer composition of the reactor alloys, the prepared samples were fractionated by the temperature gradient extraction fractionation technique (TGEF) with a modified Kumagawa extractor.³³ For each sample, six fractions F₁, F₂, F₃, F₄, F₅, and F₆ were, respectively, collected at 30, 90, 110, 120, 130 and >130 °C by using boiling xylene.

Fourier transform infrared (FTIR) spectroscopy

A FTIR spectrometer, BRUKER EQUINOX 55, Germany, was used to quantify the co-monomer content of PR-iPP matrix. To calculate the ethylene content of various fractions in the reactor alloys, by regarding the peaks 722 cm⁻¹ (methylene sequences of polyethylene) and 1156 cm⁻¹ (methyl groups of iPP), eqn (1) and (2) from our previous work³² were used.

Y = −0.1196X + 3.6567 (1)

Y = −0.0282X + 2.8523 (2)

In eqn (1) and (2), X and Y are, respectively, the weight percentage of ethylene in co-polymer and Ln (A₁₁₅₆/A₇₂₂).

Two other calibration curves were built to calculate the amount of hexene-1 in the modified matrix which was co-polymerized by this co-monomer. The first calibration curve was built based on A(725 cm⁻¹)/A(998 cm⁻¹ + 973 cm⁻¹) ratio which was obtained from various blends of polyhexene-1 and iPP in hot xylene.³⁴ Here, peak 725 cm⁻¹ belongs to the methylene sequences of polyhexene-1, and peaks 973 cm⁻¹ and 998 cm⁻¹ pertain to the crystallizable sequences of iPP.³⁴ A series of films of the samples, with about 80 microns thickness, were prepared through compression molding for FTIR tests in order to build this calibration curve. The eqn (1) and (2) which relates to ethylene, and the first calibration curve that belongs to hexene-1 were built according to ASTM D3900.

The second calibration curve, from mixing various amounts of hexene-1 co-monomer and n-hexane, was constructed by considering the ratio of A(1645 cm⁻¹)/A(1460 cm⁻¹) to calculate the amount of unreacted liquid co-monomer in the reaction medium.³⁵ The peaks 1645 cm⁻¹ and 1460 cm⁻¹, respectively, belonged to the C=C stretching bonds of hexene-1 and C–H scissoring bonds in hexene-1 + n-hexane. It should be noted that the density and the boiling point of n-hexane and hexene-1 are almost the same, so during sampling (after cooling down the reactor) the composition of the mixture remains almost constant. A Perkin Elmer FT-IR spectrometer RX I was applied to prepare the second calibration curve.

Scanning electron microscopy (SEM)

The phase morphology and dispersion state of the EPR domains were studied using a Scanning Electron Microscope (SEM) VEGA\\TESCAN, Czech Republic. The cryo-fractured surface of the molded specimens of the samples was coated with sputtered gold before analysis.

Shear rheological study

Dynamic oscillation rheological experiments were carried out by using a RHEOPLUS rheometer, model MCR 300 Anton Paar-Germany, with a 1 mm gap, on compression-molded disk-shaped samples with a 25 mm diameter, in the linear viscoelastic region. The linear viscoelastic region (strain amplitude of 5%) was obtained by running a strain sweep test at a fixed frequency of 10 rad s⁻¹. The frequency sweep tests, with a strain amplitude of 5%, in the frequency range of 0.01–600 rad s⁻¹ at 200 °C were carried out for all samples. The experiments were accomplished under a continuous flow of dry nitrogen to minimize oxidative degradation of the blends.

Thermal analysis

A Differential Scanning Calorimetry (DSC), model STARe 10 from METTLER TOLEDO, Germany, was used to investigate the thermal characteristics of samples. The thermal history and abnormal shapes of samples were omitted by the first heating run from room temperature up to 220 °C at a 40 °C min⁻¹ heating rate. After 5 min holding at 220 °C, the crystallization characteristics were recorded via cooling down to room temperature at the rate of 10 °C min⁻¹. The melting behaviour of samples was studied in the second heating run from room temperature to 220 °C at the same rate as the cooling rate, to detect the melting point (T_m) and the determination of the fusion enthalpy (ΔH_f) of the samples. The degree of crystallinity (χ_c) of the samples was calculated by considering the weight percentage of the crystalline phase, obtained from the insoluble fraction in cold xylene, and the heat of fusion of 209 J g⁻¹ for 100% crystalline iPP.

Wide-angle X-ray diffraction analysis

Wide-Angle X-ray Diffraction (WAXD) measurements were conducted in order to survey the changes in the crystalline structure of the samples. The experiments were performed on a ZIEMENS D5000 powder diffractometer at room temperature. Cu Kα monochromatic radiation (λ = 1.54 Å) as the source was applied at 40 keV and 30 mA in a 2θ range of 5–40°.

Mechanical properties

The notched Izod impact strength of the molded specimens was measured on a ZWICK impact strength tester according to ASTM D256. The flexural characteristics of the samples were calculated on a SANTAM-STM 50 electronic machine according to ASTM D790.

Results and discussion

Block co-polymer composition

The effect of matrix modification by ethylene and hexene-1 on the composition of reactor alloys was assessed through block co-polymer fractionation. Table 2 presents the composition of reactor alloys along with the co-monomer content of each fraction. As Table 2 shows, adding ethylene and hexene-1 co-monomers in MPS affects the yield of polymerization and distribution constituents in the modified reactor alloys. The different activity ratio of propylene/ethylene and propylene/hexene-1 affects the catalyst efficiency in MPS.^35,36 An increase of ethylene content from 0.0 up to 3.0% wt in MPS helps to enhance the yield of polymerization. The higher activity of ethylene compared to propylene increases the catalyst efficiency compared with the case where only propylene is used in the preparation stage of the matrix. In contrast, by the presence of hexene-1 in MPS, which has a lower activity compared to propylene, the yield of the reaction decreases due to the intensification of the penetration resistance, which is a result of the mobility reduction of chains containing this co-monomer. By enhancing the hexene-1 content in MPS and the increase of the penetration resistance, the yield of the reaction has gradually decreased according to Table 2. It is probable that MMT can change the composition of PR-iPP/EPR reactor alloys.

Table 2 Fractionation results of the samples and polymerization yield in the matrix preparation stage along with the co-monomer content of the fractionated constituents

Sample	Polymer yield^a	F1		F2		F3		F4		F5		F6
		WP^b	EC^c	WP^b	EC^c	WP^b	EC^c	WP^b	CC^d	WP^b	CC^d	WP^b	CC^d
a Yield of polymerization (kg polymer/g catalyst h) in the matrix modification stage for the modified matrix by 0.0 up to 3.0% wt co-monomer.b Average of weight percentage was obtained from the fractionated constituents of the reactor alloys which were obtained from three measurements.c Ethylene content of fractions F1, F2 and F3.d Co-monomer content of insoluble part in cold xylene.e Ethylene content of fractions F4, F5 and F6.f Hexene-1 content of fractions F4, F5 and F6.
PE0.0	3.24	15.89	55.1 ± 2.2	7.18	38.5 ± 2.7	5.84	9.3 ± 1.5	6.32	0.0	24.20	0.0	38.91	0.0
PE0.5	3.47	27.62	57.6 ± 3.4	5.08	40.2 ± 1.9	5.53	10.5 ± 1.9	5.75	0.13^e	17.85	0.15^e	37.10	0.20^e
PE1.0	3.55	19.60	54.6 ± 2.3	4.82	37.0 ± 2.1	5.41	8.8 ± 0.8	5.93	0.18^e	22.15	0.31^e	41.93	0.48^e
PE1.5	3.76	16.08	54.8 ± 1.8	4.74	37.3 ± 1.7	6.35	7.9 ± 1.2	6.26	0.28^e	23.50	0.44^e	42.62	0.79^e
PE3.0	3.82	12.54	55.9 ± 3.1	4.53	38.6 ± 3.1	7.01	7.7 ± 0.9	6.81	0.69^e	24.06	0.82^e	43.92	1.48^e
PH0.5	3.30	26.84	56.8 ± 2.9	5.83	39.4 ± 1.6	5.71	9.7 ± 1.1	5.76	0.11^f	21.15	0.17^f	33.39	0.21^f
PH1.0	3.11	18.26	55.7 ± 3.3	5.06	36.8 ± 1.7	5.68	8.1 ± 0.7	5.24	0.19^f	22.94	0.28^f	42.07	0.52^f
PH1.5	3.01	15.24	54.0 ± 2.4	5.62	37.5 ± 0.9	5.40	7.9 ± 0.7	6.17	0.30^f	24.88	0.38^f	41.35	0.80^f
PH3.0	2.86	11.40	56.1 ± 2.1	5.38	36.3 ± 2.2	6.35	7.2 ± 0.8	6.55	0.61^f	25.35	0.93^f	44.40	1.45^f

---

Fractions of F₁, F₂ and F₃ play an important role in the toughness of PP reactor alloys. Although the microstructure of these fractions has been discussed by researchers,^33,37 their chemical composition was estimated by eqn (2) and is listed in Table 2. The combination of feed (propylene) with 0.5% wt ethylene and hexene-1, as co-monomer in MPS, led to the enhancement of fraction F₁ from 15.9 in PE0.0 to 27.6 and 26.8% wt in the reactor alloys PE0.5 and PH0.5. In the presence of a very small amount of these co-monomers during MPS, a series of random sequences with reduced spatial regularity are produced instead of hard crystalline segments of iPP.^38,39 Therefore, the penetration of monomers toward the sub-layers of catalyst grains speeds-up, especially in the main co-polymerization stage. This enhances the cold xylene soluble (CXS) fraction of samples PE0.5 and PH0.5. According to Table 2, by increasing the co-monomer content in MPS, the fraction F₁ is reduced from about 27% wt in reactor alloys PE0.5 and PH0.5 to 12.5 and 11.4% wt in the samples PE3.0 and PH3.0. By adding the ethylene content from 0.5 to 3.0% wt in MPS, along with PR-iPP, some amorphous EPR may form from low isotactic sites which migrate out gradually from the catalyst particle into the reaction media. The low activity of active sites which produce the dispersed phase can help to interpret this phenomenon. The activity of low isotactic sites is very low, so they hardly re-activate in the main co-polymerization step after one-time activation in MPS. This leads to a reduced fraction of F₁ in samples PE1.0 to PE3.0.⁴⁰

By increasing the hexene-1 content in MPS, as a higher α-olefin with a lower activity compared to propylene, chain mobility,^34,41 which is an effective parameter of the productivity of active sites, decreased further, especially for low isotactic sites. Therefore by enhancing the hexene-1 content and reducing the chain mobility, the amount of fraction F₁ decreases. The interesting point is that amount of fraction F₁ is a function of the co-monomer content used in MPS and is independent of the co-monomer type which has been applied in MPS. It is remarkable that MMT has not significantly affected the amount of fractions F₂ and F₃ for all samples.

Surveying the chemical composition shows that MMT and changing the co-monomer type do not alter the ethylene content of fractions F₁, F₂, and F₃ because the monomer feed ratio in the main copolymerization stage is constant. Moreover, the co-monomer content of the insoluble fractions of reactor alloys in cold xylene (F₄, F₅, and F₆) was determined by eqn (1) (ethylene content) and Fig. 1a (hexene-1 content). The results revealed that the chemical composition of fractions F₄, F₅ and F₆ has altered, arising from matrix modification.

Fig. 1 Calibration curves to determine hexene-1 content (a) in the matrix modified and (b) in the reaction media.

Microstructure characterization

FTIR spectroscopy is most widely applied to analyze the chemical composition of polyolefins. The quantity of reacted co-monomers in MPS can be determined by the calibration curve obtained from FTIR analyses. Eqn (1) and (2) were used to calculate the co-monomer content of the matrix modified by ethylene and EPs co-polymers.

To investigate the ethylene content of the PR-iPP matrix, the higher activity of ethylene to propylene assured us that all the fed ethylene to the reactor in MPS was consumed and the ethylene weight percentage of PR-iPP is the same as the ethylene content of the feed. However, for greater certainty, the ethylene content of the synthesized PR-iPP matrix was measured by eqn (1).

The hexene-1 content of the modified matrix was determined by the calibration curves of Fig. 1a and b. The weight percentage of hexene-1 in PR-iPP matrix was determined by the calibration curve Fig. 1a.³⁴ As Fig. 1a shows, the increase of A₇₂₅/A₉₇₃ + A₉₉₈ is proportional with the increase of hexene-1 content in the modified matrix. To confirm the accuracy of the calculations, the calibration curve of Fig. 1b was used to estimate the unreacted hexene-1 in the reaction media. The difference between the initial amount of co-monomer injected into the reactor and the remaining amount in the reaction media at the end of MPS is equal to the entered hexene-1 content of the modified matrix. The results obtained from calibration curves of Fig. 1a and b showed that the intended amount of hexene-1 has entered into PR-iPP with a good approximation. Table 1 shows that the intended amounts of co-monomers have successfully inserted into the modified matrix.

To understand whether the co-monomer used in MPS entered into the matrix or not, FTIR spectra of the fractionated constituents F₁ to F₆ for samples PE0.0, PE1.5, and PH1.5 (with the same composition) were investigated in Fig. 2a–c. The chemical composition of fractions F₁ to F₃ was discussed in previous work.³²

Fig. 2 FTIR spectra of the fractionated constituents of PP reactor alloys (a) PE0.0, (b) PE1.5 and (c) PH1.5.

Fractions F₄, F₅ and F₆ in the samples PE0.0, PE1.5 and PH1.5 illustrate the peaks at 840, 998 and 1156 cm⁻¹ due to the existence of high isotactic PP, while a broad, weak fluctuation at 720–750 cm⁻¹ is only visible in spectra of these fractions for the modified reactor alloys PE1.5 and PH1.5. This reveals the existence of numerous random sequences in the fractions F₄, F₅, and F₆ of the modified samples. This should reveal whether random sequences created in the modified matrix affect the phase compatibility of the R-iPP/EPR reactor alloys or not.

Morphological study/phase compatibility

A “spectroscopic–microscopic” analysis helps to understand the relationship between the microstructure and phase morphology of PP reactor alloys. The amount and molecular architecture of the components, especially EPs as the part compatible with EPR and propylene rich propylene–ethylene multi-block co-polymers (EPC) as the part compatible with PP matrix, affect the phase compatibility of PP reactor alloys.⁴² According to Table 2 and Fig. 2, the amount and microstructure of the fractions F₂ and F₃ are almost constant for all samples synthesized. Therefore, the main objective of the morphology study of the samples is limited to the investigation of the effect of MMT, and the amount and type of co-monomer used in MPS on the phase compatibility of PP reactor alloys, especially those that have the same composition.

Fig. 3 shows SEM micrographs of reactor alloys; dispersed EPR droplets are visible as dark zones on the fractured surface of the samples. Although an increase of dispersed phase content from 15.89 in PE0.0 up to about 27% wt in samples PE0.5 and PH0.5 caused coagulation of their EPR droplets, the soft cryo-fractured surface and non-globular shape of the dispersed particles in these samples are good signs to improve their phase compatibility (Fig. 3a–c). The EPR content of the reactor alloy PH0.5 is approximately equal to sample PE0.5, while the size of its dispersed droplets (Fig. 3c) is smaller than that of the sample PE0.5 (Fig. 3b). This is the result of more breaking-up of the EPR domains of sample PH0.5 than sample PE0.5, which can be evidence for the better adhesion of the modified matrix (by hexene-1) to EPR droplets. In addition, numerous white spots, which were discussed in previous work³² as evidence for the improved phase compatibility of PR-iPP reactor alloys, are seen in the cryo-fractured surface of the PH0.5 sample arising from strong adhesion between the matrix and toughener domains.

Fig. 3 SEM micrographs of PP reactor alloys (a) PE0.0, (b) PE0.5, (c) PH0.5, (d) PE1.0, (e) PH1.0, (f) PE1.5 and (g) PH1.5.

In the case of alloys modified with ethylene, according to Fig. 3d and f, the increment of ethylene content to 1.5% wt in the modified matrix and reduction of EPR content by up to 16.08% wt caused the appearance of a series of discrete non-spherical EPR domains in these samples. These feature the effect of the number of created random sequences in the PR-iPP matrix and the composition of reactor alloys is modified when their phase morphology is improved. The increase in the random sequence number of the modified matrix is proportional to the increase the number of connection areas (phase interaction) of the matrix and the dispersed phase which can improve the interfacial interactions of PR-iPP–EPR. As well as by increasing the hexene-1 content of matrix modified up to 1.5% wt, the non-spherical dispersed droplets are visible in the micrograph of Fig. 3e and a nearly single-phase morphology is visible in Fig. 3g, which proves the improved phase compatibility of the samples PH1.0 and PH1.5.

Samples PE0.0, PE1.5, and PH1.5 have the same CXS fraction. Investigation of the micrographs of Fig. 3 clarifies that MMT has softened the cryo-fractured surface of the modified samples and has improved the phase compatibility of the modified PR-iPP by appropriate co-monomer content and EPR. However, in the same composition, comparison of the stretched EPR domains in Fig. 3f and a nearly single-phase morphology in Fig. 3g illustrates the effect of the co-monomer type used in MPS on the improved phase compatibility of the PR-iPP reactor alloys. It can be concluded that hexene-1 is more successful than ethylene for the improvement of the phase compatibility of PR-iPP reactor alloys. In other words, fabricating the random structure irregularities by hexene-1 in MPS is a more effective approach to improve the phase compatibility of PR-iPP reactor alloys than creating a structural similarity between the matrix and EPR in these alloys.

Study of the interfacial interaction of PP reactor alloys by rheological measurements

The interfacial tension (σ) of the polymer blends, which interprets relaxation of dispersed particle shape and their viscoelastic properties, can be studied by various approaches such as spinning drop, pendant drop, capillary break-up, and rheological models.^43–45 Dynamic shear rheology experiments have been used commonly to determine the interfacial tension of polymer blends based on a general expression of complex shear moduli (G*(ω)) that was proposed by Pal and Palierne for immiscible simple emulsions such as PP/EPR blends.^46–48 Details of these models were discussed in our previous work.³² An initial amount of interfacial tension is necessary to evaluate the interfacial interactions of the PP/EPR blend. A value of σ = 1.067 mN m⁻¹, as the initial amount of interfacial tension of PP/EPR blends, was obtained from Fowkes’ equation⁴⁹ to compare with the σ of the reactor alloys produced in this work.

To study the interfacial tension, the experimental G*(ω) of the produced samples was compared to the prediction of the Pal and Palierne models at low frequencies (0.03 rad s⁻¹) in Fig. 4 and 5. A negative deviation of G*(ω) from the Pal and Palierne models means that the interfacial interactions of reactor alloy have improved. Comparison of samples PE0.0 and PE0.5 in Fig. 4a and b infers that a modification of the matrix with 0.5% wt ethylene does not improve the interfacial interactions of EPR droplets and the modified matrix. Meanwhile, the use of 0.5% wt hexene-1 in MPS of the reactor alloy PH0.5 reduced the interfacial tension of the modified matrix and EPR according to Fig. 5a. It shows the important role of the co-monomer type used in MPS, even at low concentration, on the improvement of the interfacial interactions of PR-iPP reactor alloys. When the 1.5% wt co-monomer was used in MPS of samples PE1.5 and PH1.5, G*(ω) exhibits a negative deviation from the Pal and Palierne models, as is seen in Fig. 4c and 5b. This means that an increase in the number of random structures in the modified matrix can improve the interfacial interactions of PR-iPP–EPR. In the case of the alloys modified with ethylene, the created random structures in the modified matrix interact with EPR, which has a random structure (due to their structural similarity). This means that random sequences of the modified matrix act as connection areas between the matrix and EPR. The increase in the random sequence number of the modified matrix, which is proportional to the increase in the number of connection areas (phase interaction) of the matrix and dispersed phase, improves the interfacial interaction of PR-iPP–EPR.

Fig. 4 Comparison of the experimental complex shear modulus of the PP reactor alloys modified by ethylene with the Pal and Palierne models. (a) PE0.0, (b) PE0.5 and (c) PE1.5.

Fig. 5 Comparison of the experimental complex shear modulus of the PP reactor alloys modified by hexene-1 with the Pal and Palierne models. (a) PH0.5 and (b) PH1.5.

In the PP/EPR reactor alloys, EPR is known as an elastic component. On the other hand, the melt elasticity of the modified matrix increases via the copolymerization propylene with hexene-1. The similarity of the viscoelastic properties of the dispersed phase with the random sequences built into the modified matrix leads to the creation of the phase interaction between them. So, the increase of the number of random sequences in the modified matrix is proportional to the increase of the number of connection areas of the PR-iPP matrix with EPR, which improves the interfacial interaction of PR-iPP–EPR. Table 3 illustrates the predicted interfacial tension by the Pal and Palierne models along with the estimated error for the reactor alloys, modified and unmodified. An error function such as follows was used to estimate the error of the calculations.⁵⁰

Table 3 Prediction of the interfacial tension of PP reactor alloys by the Pal and Palierne models along with their relative error, estimation of system homogeneity by G′–G′′ diagram, and experimental and theoretical and VGF for PP reactor alloys and their components^a

Sample	EPR (% wt)	M^a	σ (mN m^−1)		Fi						VGF (%)
			Palierne	Pal	Palierne	Pal	PP	EPR	Alloy^b	Alloy^c
a m: slope of G′–G′′ diagram. Fi: error between the measured G′(ω) and G′′(ω) at 0.03 (rad s^−1) and the predicted value by Pal and Palierne models.b Experimental.c Theoretical, VGF: “viscosity growth factor’’.
PE0.0	15.89	1.48	1.1	1.2	1.48	1.53	3.80	4.63	4.04	3.93	+2.77
PE0.5	27.62	1.65	1.2	1.3	0.06	0.07	3.74	5.43	4.26	4.20	+1.43
PE1.5	16.08	1.82	0.6	0.6	0.18	0.19	3.78	4.71	4.27	3.93	+8.40
PE0.5	26.84	1.74	0.6	0.7	0.17	0.19	3.72	4.77	4.25	4.00	+6.25
PE1.5	15.24	1.92	0.6	0.6	0.51	0.26	3.65	4.68	4.14	3.80	+8.94

---

Eqn (3) is the relative error between the measured storage and loss moduli G′_i and G′′_i and the predicted amounts G′(ω_i) and G′′(ω_i) by the Pal and Palierne models at the specific frequency ω_i.

System homogeneity, which can be a good criterion to estimate the interfacial strength of polymer blends, is evaluated by a diagram of storage modulus (G′) versus loss modulus (G′′). The slope of the G′–G′′ log–log diagram which has been shown for reactor alloys PE0.0, PE0.5, PE1.5, PH0.5 and PH1.5 in Fig. 6a is usually used to investigate the homogeneity of multiphase polymer blends.⁵¹ The slope increment of the G′–G′′ diagram and its approach to 2 is proportional to the enhancement of system homogeneity and the augmentation of the interfacial region of polymer blends.^51,52

Fig. 6 (a) G′–G′′ diagram of the samples PE0.0, PE0.5, PE1.5, PH0.5 and PH1.5; (b) damping factor and EPR content versus the amount of co-monomer used in MPS for the unmodified reactor alloy PE0.0 (white blank), the modified reactor alloys by hexene-1 (red filled) and ethylene (dark filled).

The slope of the G′–G′′ diagram has increased from 1.48 in sample PE0.0 to 1.65 in reactor alloy PE0.5 according to Fig. 6a and Table 3. Meanwhile, the Pal and Palierne models did not show significant improvement in the interfacial strength of this sample. It is inferred that the enhancement of the EPR (elastic component) content of sample PE0.5 has increased the melt elasticity and slope of the G′–G′′ diagram of PE0.5. By increasing ethylene content up to 1.5% wt in MPS, the slope of the G′–G′′ diagram is raised from 1.65 in sample PE0.5 to 1.82 in the reactor alloy PE1.5 while its dispersed phase content is lower than that of PE0.5. This shows that number of random sequences created in the modified matrix is a key factor to increase the slope of the G′–G′′ diagram. By the increase of the random sequences in the modified matrix, the melt elasticity of the dispersed phase transfers more to the randomized matrix through the connection areas between PR-iPP–EPR. This shows that an increase of the number of random structures in the modified matrix enhances the strength of the interfacial region of the matrix and EPR droplets in PR-iPP reactor alloys.

The existence of 0.5% wt hexene-1 in the modified matrix of reactor alloy PH0.5 has increased the slope of the G′–G′′ diagram to 1.74 for two reasons: increment of the EPR content; and invigoration of the interfacial region. The difference between the slope of the G′–G′′ diagram for samples PH0.5 and PE0.5, with the same CXS content, demonstrates the profound effect of the co-monomer type used in MPS on the interfacial interactions of PR-iPP reactor alloys.

Although the amount of dispersed phase, as an elastic component, in the sample PH0.5 is more than the EPR content of PH1.5, the slope of the G′–G′′ diagram of the reactor alloy PH1.5 is greater than that of PH0.5. It can be concluded that the increase of the molecular interactions between PR-iPP and EPR, through random sequences created in the modified matrix, acts much more effectively than EPR content to increase the melt elasticity of PR-iPP/EPR reactor alloys.

Damping factor (tan δ) and EPR content of reactor alloys vs. co-monomer content used in MPS have been shown in Fig. 6b. The damping factor decreased from 3.3 to about 2.0, when the EPR content increased from 15.9 in PE0.0 to about 27% wt in reactor alloys PE0.5 and PH0.5. The increase of EPR content in the sample PE0.5 and the enhancement of the interfacial strength along with the increment of dispersed phase content in the reactor alloy PH0.5 are effective parameters to decrease the damping factor of these samples. The decrease of EPR content to about 16% wt in samples PE1.5 and PH1.5 has decreased their damping factor. This highlights the role of the improved interfacial strength between PR-iPP–EPR on the enhancement of melt elasticity of PR-iPP reactor alloys in which their matrix has been modified with the appropriate co-monomer content.

Investigating the polymer viscosity is another method for evaluating the phase interaction of polymer blends. Polymer viscosity, at low frequencies, is changed by the change of molecular weight and temperature. The viscosity of multi-component polymer blends, without considering interfacial tension, is calculated by the logarithmic viscosity law according to eqn (4):⁵²

here, ϕ_i, and are, respectively, the volume fraction of constituents, the complex shear viscosity of components and the complex shear viscosity of the polymer blend. The experimental complex shear viscosity of sample PE0.0 and reactor alloys modified by 0.5 and 1.5% wt of ethylene and hexene-1, along with their fractionated components is, respectively, presented in Fig. 7 and 8. At a constant temperature, the complex shear viscosity at low angular frequencies, which is equivalent to , is a criterion of molecular weight. According to Fig. 7a, the increase of ethylene content in MPS up to 1.5% wt does not have a significant effect on the complex viscosity of the modified matrix. Meanwhile Fig. 7b shows that the of EPR increases when the matrix is modified by 0.5% wt ethylene and again drops when the matrix was modified by 1.5% wt ethylene in MPS. This is related to the ease of penetration of monomers into catalyst sub-layers and re-activation of low isotactic sites in the main co-polymerization stage (explained in the fractionation section) which affect directly the molecular weight and EPR content of PR-iPP reactor alloys. In comparison with the unmodified matrix, the matrix viscosity of the reactor alloys which have been modified by hexene-1 have decreased according to Fig. 8a. It should be noted that in Fig. 8a, PH0.0 is the same PE0.0. The lower activity ratio of hexene-1 to propylene decreases the chain mobility in MPS, which leads to a reduction in the polymerization rate, molecular weight and complex viscosity of the modified matrix by this co-monomer. As seen in Fig. 8b, the existence of 0.5 and 1.5% wt hexene-1 in the modified matrix does not change the of the EPR rubbery phase.

Fig. 7 Complex shear viscosity versus angular frequency of the samples modified by ethylene (a) matrix, (b) dispersed phase and (c) reactor alloys.

Fig. 8 Complex shear viscosity versus angular frequency of the samples modified by hexene-1 (a) matrix, (b) dispersed phase and (c) reactor alloys.

Complex viscosity of the reactor alloys modified vs. frequency is shown in Fig. 7c and 8c. The complex viscosity of reactor alloys depends on the complex viscosity of their components, according to eqn (4). However, a profound relationship exists between interfacial strength and of polymer blends. The difference between experimental and theoretical which is called the “viscosity growth factor” (VGF)³³ is proportional to the interfacial strength of the polymer blends.^32,53,54 A simple expression of VGF that is only dependent on the phase interaction between the matrix–dispersed phase is given in our previous work.³²

Table 3 presents the experimental and theoretical of PP reactor alloys along with their determined VGF. The existence of 0.5% wt ethylene in the modified matrix of the reactor alloy PE0.5 led to a reduction of its VGF compared to the sample PE0.0. This is due to the high EPR content of PE0.5, the low content of the random sequences of its modified matrix and the EPS shortage in the reactor alloy which weaken the interfacial region of the sample PE0.5. The reactor alloy PE1.5 shows significant improvement on VGF than samples PE0.0 and PE0.5, which has an EPS content almost equal to the other two. Two effective factors can be considered to enhance the VGF of sample PE1.5: an increase of the number of random sequences in the modified matrix, as compatibilizer segments of the modified matrix with EPR; and a reduction of the EPR content of the reactor alloy.

By introducing 0.5 and 1.5% wt hexene-1 in MPS of samples PH0.5 and PH1.5, their VGF is enhanced due to the improved strength of the interfacial region.

With equal amounts of the used co-monomers in MPS, the VGF of the reactor alloys modified by hexene-1 is higher than that of those modified by ethylene. This confirms that the interaction of propylene–hexene-1 sequences in the matrix of samples PH0.5 and PH1.5 with EPR domains is stronger than that of the propylene–ethylene random sequences of the modified-matrix-EPR of those modified with ethylene.

Morphology–rheology studies of samples PE0.0, PE1.5, and PH1.5, with the same CXS content, revealed the advantages of MMT, the type and amount of the used co-monomer in MPS on the improved phase compatibility and the increment of interfacial interactions of PR-iPP reactor alloys.

Effect of co-monomer types on crystallization behaviour

Study of DSC and WAXD analyses helps to understand the relationship between phase interaction of the modified PR-iPP/EPR reactor alloys and their thermal behaviour. Table 4 shows thermal characteristics of the reactor alloys produced in this work. By increasing the co-monomer content in the modified matrix, X_c, T_m and T_c are decreased. DSC thermograms of Fig. 9a and 10a reveal that modification of the matrix with a small amount of ethylene or hexene-1 co-monomers shifts the melting and crystallization peaks of PR-iPP reactor alloys to lower temperatures. This can be due to a change in the crystalline structure of the matrix, which displays a polymorphic behaviour,^55,56 and formation of shorter stereo-regular blocks of crystals⁵⁷ in the modified reactor alloys.

Table 4 The degree of crystallinity and thermal properties of PP reactor alloys along with their mechanical properties^a

Sample	Xc (%)	Tm	Tc	Impact strength (kJ m^−2)	Flexural modulus (MPa)	Flexural strength (MPa)
a NB: non-break.
PE0.0	40	164	107	29.63	800	25.82
PE0.5	38	161	106	NB^a	680	22.13
PE1.0	34	160	105	NB	702	22.00
PE1.5	35	153	102	NB	610	20.44
PE3.0	27	153	97	NB	600	18.56
PH0.5	39	163	106	NB	727	25.31
PH1.0	36	161	105	NB	672	22.30
PH1.5	32	159	103	NB	647	20.25
PH3.0	29	158	103	NB	520	19.26

---

Fig. 9 (a) DSC thermograms and (b) WAXD patterns of iPP and reactor alloys modified by ethylene in the matrix preparation stage.

Fig. 10 (a) DSC thermograms and (b) WAXD patterns of reactor alloys modified by hexene-1 in the matrix preparation stage.

To study the crystalline structure distribution of the reactor alloys, their WAXD patterns were surveyed in Fig. 9b and 10b. It is reported that in an ethylene–propylene random co-polymer, ethylene sequences reduce the crystallization rate of the α, β, and γ forms while the β form has the highest crystallization rate among them.⁵⁷ So, it can be concluded that the presence of low amounts of ethylene–propylene sequences, which are prone to beta nucleation, lead to the formation of the β crystal form in reactor alloys PE0.5 and PE1.5, according to Fig. 9b. The β crystal form melts at 154 °C, so the increase of ethylene content in the modified matrix leads to a reduction of the T_c of the samples PE0.5 and PE1.5. The presence of β modification in the reactor alloys modified by ethylene helps to improve their toughness.

Although in the case of reactor alloys modified by hexene-1, beta modification is not observed in the modified samples (Fig. 10b), it seems that matrix modification by this co-monomer changes the distribution of α₁, α₂ and α₃ forms in the reactor alloys PH0.5 and PH1.5. According to the WAXD patterns of Fig. 10b, by increasing the hexene-1 content in the modified matrix, the α₁ form is reduced and the α₂ form, which is melted at lower temperatures, is increased. Therefore, the decrease of the melting temperature of samples PH0.5 and PH1.5 is related to a change in the distribution of the crystalline structures.

Mechanical properties

The flexural properties and impact strength of polymer blends, which are listed in Table 4, are influenced by X_c, EPR content and the interfacial interactions of reactor alloy ingredients. The reactor alloys modified with ethylene and hexene-1 co-monomers, with different composition, show nearly similar mechanical properties. This shows that the impact strength of the modified reactor alloys has improved in two ways: the increase of EPR content; and augmentation of the interfacial region.

The comparison of reactor alloys PE0.0, PE1.5, and PH1.5, with the same CXS content, describes the role of MMT, the used appropriate co-monomer content in the modified matrix and improvement of interfacial strength of PR-iPP–EPR in the mechanical properties of modified samples.

Conclusion

To investigate the effect of MMT on some of the properties of PP/EPR reactor alloys, their matrices were randomized by ethylene and hexene-1 co-monomers in two separate processes. In a series of experiments, ethylene, and in other series hexene-1, in small amounts, along with propylene were used in the matrix preparation stage. The dispersed phase was produced with a constant ratio of ethylene and propylene in the gas phase co-polymerization.

The following results were obtained:

(1) The microstructure of the matrix and the composition of EPR in the modified reactor alloys changed through MMT while that of EPS remained nearly constant.

(2) The type and the co-monomer content used in MMT affected strongly the dispersion state of EPR and an almost single-phase morphology was observed in micrographs of the reactor alloys PH1.5.

(3) A negative deviation of experimental G*(ω) from the Pal and Palierne models, the increment of VGF and reduction of the damping factor confirmed the improvement of interfacial interactions of PR-iPP–EPR, especially for reactor alloys PE1.5, PH0.5 and PH1.5.

(4) Randomization of the matrix formed a small amount of β-form crystals in the reactor alloys modified by ethylene and changed the distribution of the α forms in the reactor alloys modified by hexene-1; and both of the co-monomers increased the width of DSC melting thermograms.

(5) Investigation of mechanical properties revealed that MMT improves the toughness of the reactor alloys via an increase of EPR content and augmentation of the interfacial region between the matrix and the EPR dispersed phase.

(6) Comparison of reactor alloys modified by ethylene with those modified by hexene-1 demonstrated that creating a viscoelastic similarity between the matrix and EPR is a more effective approach than creating a structural similarity between them for improving the toughness of the PR-iPP reactor alloys.

(7) Comparing PE0.0 with PE1.5 and PH1.5 with the same CXS clarified the significant effect of MMT on the mechanical properties of PP hetero-phasic block co-polymers.

References

1. R. G. Alamo, Polimeros, 2003, 13, 270–275.
2. C. De Rosa, F. Auriemma, O. R. De Ballesteros, L. Resconi and I. Camurati, Macromolecules, 2007, 40, 6600–6616.
3. J. Kang, F. Yang, T. Wu, H. Li, D. Liu, Y. Cao and M. Xiang, J. Appl. Polym. Sci., 2012, 125, 3076–3083.
4. M. Louizi, V. Massardier and P. Cassagnau, Macromol. Mater. Eng., 2014, 299, 674–688.
5. C. Lotti, C. A. Correa and S. V. Canevarolo, Mater. Res., 2000, 3, 37–44.
6. J. Karger-Kocsis and V. Kuleznev, Polymer, 1982, 23, 699–705.
7. P. Galli and J. Haylock, Prog. Polym. Sci., 1991, 16, 443–462.
8. P. Galli, Prog. Polym. Sci., 1994, 19, 959–974.
9. R. Li, X. Zhang, Y. Zhao, X. Hu, X. Zhao and D. Wang, Polymer, 2009, 50, 5124–5133.
10. C. Grein, K. Bernreitner, A. Hauer, M. Gahleitner and W. Neißl, J. Appl. Polym. Sci., 2003, 87, 1702–1712.
11. C. Grein, M. Gahleitner and K. Bernreitner, eXPRESS Polym. Lett., 2012, 6, 688–696.
12. P. S. Lima, J. M. Oliveira and V. A. F. Costa, J. Appl. Polym. Sci., 2015, 132, 42011–42033.
13. F. Chen, B. Qiu, B. Wang, Y. Shangguan and Q. Zheng, RSC Adv., 2015, 5, 20831–20837.
14. R. Babu, N. Singha and K. Naskar, eXPRESS Polym. Lett., 2010, 4, 197–209.
15. E. Martuscelli, in Polypropylene Structure, Blends and Composites, Springer, 1995, pp. 95–140.
16. P. Galli and J. Haylock, Makromol. Chem., Macromol. Symp., 1992, 63, 19–54.
17. Y. Fan, C. Zhang, Y. Xue, W. Nie, X. Zhang, X. Ji and S. Bo, Polym. J., 2009, 41, 1098–1104.
18. P. Doshev, G. Lohse, S. Henning, M. Krumova, A. Heuvelsland, G. Michler and H. J. Radusch, J. Appl. Polym. Sci., 2006, 101, 2825–2837.
19. P. Li, S. Tu, T. Xu, Z. Fu and Z. Fan, J. Appl. Polym. Sci., 2015, 132, 5069–5108.
20. Z. Fu, S. Tu and Z. Fan, Ind. Eng. Chem. Res., 2013, 52, 5887–5894.
21. Y. Q. Zhang, Z. Q. Fan and L. X. Feng, J. Appl. Polym. Sci., 2002, 84, 445–453.
22. Y. V. Kissin and L. Rishina, Polym. Sci., 2008, 50, 1101–1121.
23. H. Mahdavi and M. E. Nook, Polym. Int., 2010, 59, 1701–1708.
24. J. C. Chadwick, Macromol. React. Eng., 2009, 3, 428–432.
25. M. Covezzi and G. Mei, Chem. Eng. Sci., 2001, 56, 4059–4067.
26. G. Mei, P. Herben, C. Cagnani and A. Mazzucco, Macromol.Symp., 2006, 245–246, 677–680.
27. R. Mehtarani, Z. Fu, Z. Fan, S. Tu and L.-F. Feng, Ind. Eng. Chem. Res., 2013, 52, 13556–13563.
28. Y. Li, J.-T. Xu, Q. Dong, Z.-S. Fu and Z.-Q. Fan, Polymer, 2009, 50, 5134–5141.
29. Q. Dong, X. Wang, Z. Fu, J. Xu and Z. Fan, Polymer, 2007, 48, 5905–5916.
30. L. Moballegh, S. Hakim, J. Morshedian and M. Nekoomanesh, J. Polym. Res., 2015, 22, 1–11.
31. R. Mehtarani, Z.-S. Fu, S.-T. Tu, Z.-Q. Fan, Z. Tian and L.-F. Feng, Ind. Eng. Chem. Res., 2013, 52, 9775–9782.
32. S. O. Mousavi, Y. Jahani and H. Arabi, RSC Adv., 2015, 5, 107445–107454.
33. Q. Dong, Z. Q. Fan, Z. S. Fu and J. T. Xu, J. Appl. Polym. Sci., 2008, 107, 1301–1309.
34. I. Kim, Macromol. Rapid Commun., 1998, 19, 299–303.
35. A. Piloz, Q. Pham, J. Decroix and J. Guillot, J. Macromol. Sci., Chem., 1975, 9, 517–537.
36. G. Natta, G. Mazzanti, A. Valvassori, G. Sartori and A. Barbagallo, J. Polym. Sci., 1961, 51, 429–454.
37. Z.-S. Fu, Z.-Q. Fan, Y.-Q. Zhang and L.-X. Feng, Eur. Polym. J., 2003, 39, 795–804.
38. B. Poon, M. Rogunova, A. Hiltner, E. Baer, S. Chum, A. Galeski and E. Piorkowska, Macromolecules, 2005, 38, 1232–1243.
39. M. Gahleitner, P. Jääskeläinen, E. Ratajski, C. Paulik, J. Reussner, J. Wolfschwenger and W. Neißl, J. Appl. Polym. Sci., 2005, 95, 1073–1081.
40. E. P. Moore, Polypropylene Handbook, Hanser/Gardner Publications, 1996.
41. R. E. Hoff and R. T. Mathers, Handbook of Transition Metal Polymerization Catalysts, Wiley Online Library, 2010.
42. Z.-Q. Fan, Y.-Q. Zhang, J.-T. Xu, H.-T. Wang and L.-X. Feng, Polymer, 2001, 42, 5559–5566.
43. O. Del Rıo and A. Neumann, J. Colloid Interface Sci., 1997, 196, 136–147.
44. M. Wagner and B. Wolf, Macromolecules, 1993, 26, 6498–6502.
45. J. Maldonado-Valderrama, A. Torcello-Gómez, T. del Castillo-Santaella, J. A. Holgado-Terriza and M. A. Cabrerizo-Vílchez, Adv. Colloid Interface Sci., 2015, 222, 488–501.
46. J. Palierne, Rheol. Acta, 1990, 29, 204–214.
47. R. Pal, Polym. Eng. Sci., 2008, 48, 1250–1253.
48. F. Mighri, M. Huneault, A. Ajji, G. Ko and F. Watanabe, J. Appl. Polym. Sci., 2001, 82, 2113–2127.
49. F. M. Fowkes, Ind. Eng. Chem., 1964, 56, 40–52.
50. J. Stokes, B. Wolf and W. Frith, J. Rheol., 2001, 45, 1173–1191.
51. H. K. Chuang and C. D. Han, J. Appl. Polym. Sci., 1984, 29, 2205–2229.
52. H. Kwak, D. Rana and S. Choe, J. Ind. Eng. Chem., 2000, 6, 107–114.
53. K. Li, J. Peng, L. S. Turng and H. X. Huang, Adv. Polym. Technol., 2011, 30, 150–157.
54. S. Danesi and R. S. Porter, Polymer, 1978, 19, 448–457.
55. C. De Rosa, S. Dello Iacono, F. Auriemma, E. Ciaccia and L. Resconi, Macromolecules, 2006, 39, 6098–6109.
56. J. Chen, Y. Cao, J. Kang and H. Li, J. Macromol. Sci., Part B: Phys., 2011, 50, 248–265.
57. C. Silvestre, S. Cimmino and R. Triolo, J. Polym. Sci., Part B: Polym. Phys., 2003, 41, 493–500.

---