Article pubs.acs.org/Macromolecules
Strain Hardening of Polyethylene/Polypropylene Blends via Interfacial Reinforcement with Poly(ethylene-cb-propylene) Comb Block Copolymers Carlos R. López-Barrón* and Andy H. Tsou ExxonMobil Chemical Company, Baytown, Texas 77520, United States S Supporting Information *
ABSTRACT: A poly(ethylene-cb-propylene) comb block copolymer (P(E-cb-P)), prepared by copolymerization of vinyl-terminated atactic polypropylene and ethylene, was used to compatibilize immiscible blends of high-density polyethylene (HDPE) and isotactic polypropylene (iPP). Addition of 5 wt % P(E-cb-P) resulted in 5-fold microdomain size reductions and the concomitant increase in the elastic modulus, as typically observed in immiscible blends compatibilized with linear block copolymers. We report an unexpected phenomenon, namely, the development of extensional flow hardening by the addition of P(E-cb-P) to the HDPE/iPP blends. This unprecedented effect is stronger in blends with cocontinuous morphology (50/50 HDPE/iPP) than in blends with matrix-droplet morphology (75/25 or 25/75 HDPE/iPP). We postulate that the melt strength enhancement and extensional strain hardening observed in the compatibilized blends may arise from the interfacial stiffening as a result of the interfacial stitching by the P(E-cb-P) comb block copolymer. This interfacial stitched network acts as an elastic membrane that resists interfacial deformations. Entanglements of the PP comb arms with iPP generates interfacial stitches which, in turn, could lead to the stretching of the PE backbone of the P(E-cb-P) comb block at large interfacial deformations and, hence, extensional flow hardening.
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separated domains reach a finite equilibrium size. Finer phase domains can be achieved during melt blending by reducing Γ,5,9,17 with corresponding improved physical properties, such as impact and tensile strength.18−20 Besides interfacial tension reduction and morphology stabilization, interfacial compatibilization modifies the viscoelastic properties of immiscible blends. In general, rheological behavior of noncompatibilized immiscible blends is characterized by an extra contribution to the elastic modulus at low frequencies, resulting from interfacial relaxation.21−26 This interfacial relaxation process is manifested in the plot of the elastic modulus (G′) versus frequency (ω) as a shoulder for blends with droplet-matrix morphologies and as a power law for blends with cocontinuous structures.25 The addition of a BC compatibilizer further raises the elasticity in the terminal relaxation region27−34 due to an increase in interfacial area (or a reduction in domain size). Furthermore, addition of a BC compatibilizer reduces the coarsening rate, characterized by the time evolutions of both the elastic modulus and the specific interfacial area.34 Rheological studies of compatibilized blends with both droplet-matrix28,29,31−35 and co-continuous morphologies27,30,34 concentrated mainly on the linear viscoelastic
INTRODUCTION Phase-separated microstructures in immiscible polymer blends are thermodynamically driven to minimize their interface, which results in the typical (spherical) droplet-matrix or cocontinuous morphologies for nonsymmetric or symmetric blends, respectively.1 The dispersion size and stability to coalescence or coarsening depend on the interfacial tension between the blend components. The interfacial tension is related to the Flory−Huggins parameter, χ,2 which, in turn, determines the interfacial adhesion between phases and the interface thickness. Physical properties (mechanical, transport, and optical) of these blends depend not only on the blend composition but also on the blend morphology (size, shape, and continuity) and on the interfacial adhesion.3 Therefore, control of the interfacial properties provides control of the blend properties. A common method to improve interfacial adhesion is to use a block or graft copolymer either by blending4−12 or by in situ synthesis (reactive compatibilization).13−15 In either case, the blocks of the copolymer are chemically compatible (or identical) to the blend components, which leads to thermodynamically driven segregation of the block copolymer (BC) to the blend interface. Besides enhancing the interfacial adhesion, the presence of BC at the interface lowers the effective interfacial tension, Γ.9,16 As Γ approaches zero, the driving force for coarsening is removed, and the microphase© XXXX American Chemical Society
Received: February 3, 2017 Revised: March 13, 2017
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DOI: 10.1021/acs.macromol.7b00264 Macromolecules XXXX, XXX, XXX−XXX
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Rheological Measurements. Dynamic frequency sweeps (DFS) measurements were performed at 190 °C for all the blends and their components using a strain-controlled ARES-G2 rheomether (TA Instruments) with parallel plate geometry and strain amplitude of 10%. This strain amplitude correspond to the linear viscoelastic regime, as shown in Figure S1. Additional measurements at temperatures ranging from 130 to 250 °C for PE and from 150 to 250 °C for PP were carried out and used to construct dynamic master curves (with reference temperature T0 = 190 °C) via the time−temperature superposition principle (tTs). DFS measurements of the blends were performed only at 190 °C using a frequency range from 0.0001 to 100 Hz. All measurements were carried out under nitrogen purge to minimize sample degradation. Extensional rheology was measured at 190 °C using a Sentmanat extensional rheometer (SER)44 attached to a stress-controlled DHR rheometer (TA Instruments). These measurements were conducted at four Hencky strain rates (ε̇H): 0.01, 0.1, 1.0, and 10 s−1. Microscopy. The polymer blends were cryo-faced at −120 °C using a cryo-microtome (Leica) for subsequent morphological examination by a bimodal atomic force microscope (AFM) using a Cypher microscope (Asylum Research) and by a scanning electron microscope (SEM) using a low-voltage SEM with field emission electron fluxes (Hitachi). SEM imaging was performed at 1 kV in secondary electron mode. For the 25/75 PE/PP blends, extraction of the minor (PE) phase was carried out by submerging the microtomed specimens in xylene at 115 °C. Extraction of the PE phase in the 50/ 50 blends was not successful except for in the blend with 5 wt % CB. Droplet size analysis was performed using ImageJ software.
response. To our knowledge, the effect of interfacial compatibilization on the extensional rheology of immiscible blends has not been systematically studied to date. In this study, we examine the effect of using a poly(ethylene-cb-propylene) comb block (CB) copolymer compatibilizer, synthesized by copolymerization of ethylene with vinyl-terminated polypropylene, on the linear and nonlinear rheology of immiscible polyethylene (PE)/polypropylene (PP) blends with matching viscosities. Matching viscosities is used to isolate the interfacial contributions to the rheological properties of the blend. The effectiveness of poly(A-cb-B) comb blocks (also known as PA-g-PB graft copolymers) in compatibilizing A/B immiscible blends has been demonstrated previously.5,36−41 Although syntheses of “all-polyolefin” comb blocks have been reported before,5,41−43 only one study reported their uses as blend compatibilizers.5 Here we found that the poly(ethylenecb-propylene) CB copolymer is an effective compatibilizer for immiscible PE/PP blends and provides the expected enhancement in melt elasticity. Additionally, we discovered the unprecedented effect of the CB on the extensional viscosity of the blends, namely, the appearance of strong strain hardening, which is absent in the noncompatibilized blends, and identified its origin.
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EXPERIMENTAL METHODS
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Materials. The synthesis of poly(ethylene-cb-propylene) comb block (CB) copolymer was recently reported.41 In short, the synthesis involves two solution reactor in series, where the first reactor prepares vinyl-terminated atactic polypropylene (aPP) macromers using an organometallic catalyst favoring β methyl elimination and the second reactor copolymerizes aPP macromers with ethylene using a different organometallic catalyst capable of incorporating macromers. The products are copolymers with bimodalities in molecular weight, composition, and long chain branching. Low molecular weight (MW) components are linear and of mixed compositions consisting of random copolymers of ethylene and propylene and residual aPP macromers. The high MW fraction accounts for ∼34% of the product and consists predominantly of CB copolymers with an average of 17 aPP branches of MW ∼ 10 000 g/mol separated by linear PE segments (with MW ∼ 42 000 g/mol) which contains less than 10 wt % propylene. The total PE backbone has a MW of ∼800 000 g/mol. Details of the characterization methods used to examine this CB can be found in ref 41. High-density polyethylene (PE) and isotactic polypropylene (PP) are commercial homopolymer grades from ExxonMobil Chemical Company, and their properties are tabulated in Table 1.
RESULTS AND DISCUSSION
Blend Compatibilization with CB. As shown by the upper panel of Figure 1, typical droplet-matrix morphologies can be found in the nonsymmetrical 25/75 and 75/25 PE/PP blends, with and without CB. A significant decrease in the droplet size
Table 1. Molecular Weight and Rheological Parameters of PE and PP Homopolymers WLF parameters Mwa,
Cross model parameters
materials
kg/mol
Mw/Mna
C1
C2
η0, kPa·s
A, s
n
PE PP
69 306
3.45 4.55
1.79 2.55
288 274
1.51 1.58
0.032 0.041
0.48 0.31
a
Determined by size exclusion chromatography using PE standards.
Solution Blending. PE/PP blends of 25/75, 50/50, and 75/25 w/ w ratios, and with 0, 1, or 5 wt % CB, were prepared by solution blending in reflux xylene (at 150 °C) followed by recovery in methanol and drying. Butylated hydroxytoluene (BHT) antioxidant (0.5 wt %) was added to each blend to prevent degradation. All blends and homopolymers (PE and PP) were compression molded and annealed for 30 min at 190 °C prior to their examinations by rheology and microscopy.
Figure 1. SEM images (upper panel) and droplet diameter distribution (lower panel) for PE/PP/CB blends with the indicated compositions. More than 200 droplets were measured in each blend. B
DOI: 10.1021/acs.macromol.7b00264 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules by the addition of 5 wt % CB is clearly demonstrated. The droplet diameter distributions are shown in the lower panel of Figure 1. A 4−5-fold decrease in the mean droplet size is achieved with the addition of CB, as indicated in Table 2, which Table 2. Droplet Size in PE/PP Blends with and without CB blend 25/75 25/75 75/25 75/25
PE/PP PE/PP PE/PP PE/PP
w/o CB + 5 wt % CB w/o CB + 5 wt % CB
Dna, μm
SDa, μm
Rvb, μm
2.37 0.47 4.24 1.21
0.67 0.36 1.94 0.71
1.45 0.62 3.71 1.50
Dn = ∑niDi/∑ni is the number-average droplet diameter, with standard deviation SD. bRv = ∑φiRi/∑φi is the volume average droplet radius, where φi is the volume fraction of the droplets with radius Ri. a
substantiates the interfacial compatibilization of the CB.19,45 As noted in the Experimental Methods section, this CB contains only 34% of the comb-block copolymer with linear homo- and copolymers balance the rest.41 Therefore, the actual combblock loading in the blend is 1.7 wt %. It should be noted that a considerable population of the droplets in the compatibilized 25/75 blend have diameters in the order of 100 nm (see Figure 1 and Figure S5), suggesting swollen micelles instead of dispersion droplets. For the compatibilized 75/25 blend, the size of the smaller droplets is ∼500 nm. As discussed below, this domain size difference has an effect on the rheological responses of these blends. Interfacial compatibilization is also effective in the 50/50 PE/PP blend, as illustrated in Figure 2. The cocontinuous domains in the symmetric blends decrease from ∼50 to ∼10 μm upon CB addition.
Figure 3. Dynamic master curves of the PE and PP homopolymers constructed via the tTs principle with the shift factors shown in the inset.
Palierne’s model predicts the linear viscoelasticity of immiscible blends having spherical viscoelastic inclusions in a viscoelastic matrix. Assuming zero interfacial viscosity and a constant interfacial tension, the complex modulus of the blend, G*b = G′b + iG″b, is given as 3
Gb*(ω) = Gm*(ω)
1 + 2 ϕH(ω) 1 − ϕH(ω)
(1)
with H(ω) = [2(G*d (ω) − G*m (ω))(19G*d (ω) Γ + 16G*m (ω)) + 8 (5G*d (ω) + 2G*m (ω))] Rv
Figure 2. SEM images of 50/50 PE/PP blends without CB and with 5 wt % CB.
Linear Viscoelasticity. The linear viscoelastic master curves of the homopolymers are shown in Figure 3. These are constructed via the tTs principle using the shift factors, aT, shown in the inset, and a reference temperature of 190 °C. The solid lines in the inset are best fits to the Williams−Landel− Ferry (WLF) equation (log(aT) = −C1(T − Tr)/(C2 + (T − Tr))), using the parameters C1 and C2 listed in Table 1. Although slight differences in the moduli curves, viscosities of the two polymers are nearly matched. The lines shown in the lower panel of Figure 3 are Cross model (η = η0/(1 + (Aω)1−n)) fits, using the zero-shear viscosity, η0, and the parameters A and n, given in Table 1. This matching viscosity of the two homopolymers simplifies the evaluation of the interfacial effects on the rheological properties of the blends. The linear viscoelastic response of the nonsymmetrical blends is shown in Figure 4. The solid and dashed lines are calculated using the Palierne’s emulsion model21 and the dynamic moduli data of each component (given in Figure 3).
/[(2G*d (ω) − 3G*m (ω))(19G*d (ω) + 16G*m (ω)) Γ + 40 (G*d (ω) + G*m (ω))] Rv (2)
where G*m and G*d are the complex moduli of the matrix and the disperse phases, respectively, ϕ is the volume fraction of the disperse phase, and Rv is the volume average radius of the disperse phase (given in Table 2). Average moduli of the components (corresponding to the dashed lines in Figure 4) are computed with eq 1 by setting Γ = 0, i.e., assuming that there is no interfacial contribution to the stress in the blend. In Figure S2, the computed average moduli and complex viscosity for the three blend compositions are plotted. Comparing those averages with the measured moduli of the blends, extra contribution from the interface is evident for all the blends in the low-frequency region. In the case of the 75/25 blend w/o CB, the interfacial contribution can be fitted C
DOI: 10.1021/acs.macromol.7b00264 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 4. Dynamic frequency sweeps of the 25/75 and 75/25 PE/PP blends with and without CB. Dashed lines are average moduli values computed with eq 1 by setting Γ = 0. Solid lines are best fits to the Palierne’s model, using Γ = 1.86 mN/m and the Rv values given in Table 2. Lower panel: schematic illustration of the 2D CB network at small patch of the blend interface.
Figure 5. Dynamic frequency sweeps of the 50/50 PE/PP blend with and without CB. Dashed lines are average moduli values computed with eq 1 by setting Γ = 0.
measurements, as shown in Figure S4. The origin of the fitting deviation, with respect to the measured data, at frequencies below 0.02 rad/s is not clear. Graebling et al. determined that
with Palierne’s model using the Rv value in Table 2 (3.72 μm) and Γ = 1.86 mN/m (solid line in Figure 4b). Notice that the droplet diameter of the blends did not increase after the DFS D
DOI: 10.1021/acs.macromol.7b00264 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
in η0 is achieved by adding 5 wt % of CB. Similar effect can be found in the nonsymmetric blends (Figure S3). Extensional Rheology. The effect of CD addition on the nonlinear rheological response of the blends is studied by transient extensional flow measurements. The transient viscosities of the neat PE and PP homopolymers are shown in Figure 6. Because of their low viscosities, these measure-
eq 1 is valid for blends where the droplet size polydispersity Rv/ Rn < 2.46 The polydispersity for the 75/25 blend w/o CB is 1.75; therefore, polydispersity is not likely to be the origin of the fitting deviation. Elucidation of the fitting deviation at very low frequencies is beyond the scope of this work and requires further research. The fitting of the dynamic moduli for the 25/ 75 blend w/o CB using eq 1 was not possible, as both G′ and G″ deviate from the component average values at much higher frequency values. The solid line shown in Figure 4a does not represent a fitting result to Palierne’s model. Rather, it shows calculated values using eq 1 with the Rv value in Table 2 (1.45 μm) and Γ = 1.86 mN/m. Attempts to improve the fitting were not possible, which could be the consequence of the nonnegligible droplet population with sizes
