Morphology Relationships in SEBS-Compatibilized HDPE

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Property/Morphology Relationships in SEBS-Compatibilized HDPE/ Poly(phenylene ether) Blends Mohammed A. Bin Rusayyis, David A. Schiraldi,* and João Maia*

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Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio 44106, United States ABSTRACT: High density polyethylene suffers from poor mechanical properties at elevated temperatures (e.g., above 60 °C) due to its relatively low melting point and, particularly, low heat deflection temperature (HDT), which limits its usage in a wide range of applications. This work intends to improve the HDT of high density polyethylene (HDPE) by melt blending with two poly(2,6-dimethyl-1,4-phenylene ether) (PPE) resins of different molecular weights at different compositions and using two styrene−ethylene/butylene− styrene (SEBS) triblock copolymers of different viscosities to compatibilize the resulting blends. HDT values were found to increase linearly with PPE content but decreased as the concentration of the compatibilizer increased. Morphological analysis of HDPE/PPE blends with various amounts of SEBS as investigated by scanning electron microscope (SEM) confirmed the compatibilization effect of SEBS in HDPE/PPE blend systems. The mechanical properties suggest that HDPE/PPE blends are somewhat stable systems at low PPE content; uncompatibilized HDPE/PPE maintained good ductility up to 25% PPE content when the PPE used is of a high molecular weight.

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INTRODUCTION Mixing two or more different polymers is a cost-effective approach to achieve various property combinations in final materials;1 hence, academic and industrial activities in this area will likely continue to flourish despite new emerging areas in polymer science and technology.1 It is of special interest to modify commodity low-cost polymers to produce materials with tailor-made properties for advanced applications. Polyethylene (PE) is the most widely used polymer in the world, commercially attractive for its durability, low temperature impact, and excellent processability. Polyethylene materials do have some drawbacks, suffering from the limitation of poor heat resistance. The low upper service temperature of polyethylene makes it unsuitable for applications requiring high temperatures as it is susceptible to deterioration. Short-term exposure to high temperatures is also limited; the low heat deflection temperature (HDT) of PE makes pharmaceutical products made from polyethylene incompatible for heat sterilization whether it is by dry heat or steam autoclaves.3,4 Poly(2,6-dimethyl-1,4-phenylene ether) (PPE) is an amorphous polymer with high glass transition temperature, low water absorption, good dimensional stability, and high ductility. PPE is also categorized in the class of heat-resistant engineering plastics.5 PPE has a number of deficiencies such as poor solvent resistance and difficulty of processing due to its high softening temperature and high melt viscosity.5 Because of its processability issues and high cost, PPE is not generally used by itself on a commercial scale but is commonly used as an additive or a building block in many commercial blends. The important discovery of the thermodynamic miscibility of polystyrene (PS) and PPE paved the way for the commercialization of modified © XXXX American Chemical Society

PPE (mixture of PPE and PS) in 1966, making PPE/PS blends one of the most successful and best-known polymer blends on both an academic and industrial scale.5,6 PPE has also been blended with polyamides,7−10 poly(ethylene terephthalate) (PET),11 and acrylonitrile-co-butadiene-co-styrene (ABS) terpolymers.12 These blends are, however, immiscible, and different compatibilization technologies were used to improve the interfacial adhesion between the blends’ components. The newest commercial PPE blend combines PPE with polypropylene (PP), which was introduced to the market in 2001. PPE/PP blends are compatibilized using specific block copolymers.13 The compatibilized PPE/PP blends offer excellent chemical resistance, improved dimensional stability, good toughness/ stiffness balance, and higher heat resistance than neat PP.13 Application areas for PPE/PP blends include automotive, power-tool housings, and fluid engineering.13 Several studies on compatibilization and properties of PPE/PP blends can be found in the literature.14−18 The inherent properties of high density polyethylene (HDPE) and PPE suggest that a combination of HDPE and PPE should produce materials with balanced properties at a reduced cost. Because of the excellent low temperature impact of HDPE (due to its low Tg) and the high heat resistance of PPE, HDPE/PPE blend resins can be used in a wide range of applications where various extreme environments are encountered. Because HDPE and PPE are immiscible, an appropriate compatibilizer is needed, most commonly by the addition of a Received: May 3, 2018 Revised: August 5, 2018

A

DOI: 10.1021/acs.macromol.8b00894 Macromolecules XXXX, XXX, XXX−XXX

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samples. After drying the PPE resins, a series of HDPE/PPE and HDPE/PPE/SEBS blends with predetermined weight ratios were prepared using a KitchenAid Professional 600 mixer at room temperature. The blends were then extruded with a PRISEM EUROLAB 16 XL twin-screw extruder (Thermo Scientific) having a screw diameter of 16 mm and an L/D of 40. The extrusion temperature increased from 260 °C at the feeding zone to 280 °C at the die, and the screw speed was 160 rpm. The extruded blend pellets were dried at the same conditions described above and then injection-molded in a Boy 22S injection molding machine, equipped with a family mold containing an ASTM flexural bar and type IV tensile dog bone part, at 280 °C. The molded specimens were used for mechanical and heat deflection temperature characterization. While these processing temperatures are high for HDPE, we found no issues with degradation in the present blend studies. Differential Scanning Calorimetry (DSC). DSC measurements were performed using Q100 DSC model (TA Instruments, USA). All scans were conducted in a nitrogen environment with a purge rate of 50 mL/min and at a heating/cooling rate of 10 °C/min. All DSC experiments were completed in three cycles: heat−cool−heat. Unless otherwise stated, the melting temperature (Tm) and glass transition temperature (Tg) were determined from the second heating cycle while the crystallization temperature (Tc) was determined from cooling. The heat of fusion for 100% crystalline polyethylene and 100% crystalline PPE was taken as 293 J/g30 and 11.84 cal/g (or 50 J/g), respectively, for crystallinity calculations.31 Heat Deflection Temperature (HDT). The HDT was measured in accordance with ASTM D648 standard by a CEAST HV6 (Instron, USA) HDT apparatus. The measured HDT is the temperature at which the specimen (3.2 mm × 12.7 mm × 125 mm) deflects by 0.25 mm under a load of 0.45 MPa while heated in a silicon oil bath at a rate of 2 °C/min. Three specimens were tested for each sample, and the average values are reported herein. Rheological Analysis. Rheological measurements were performed using an ARES oscillatory rheometer (Rheometric Scientific) at 230 °C. Frequency sweeps were carried over a frequency range of 0.1 and 100 rad/s in the linear viscoelastic region (strain amplitude γ = 0.02). Samples used for rheological tests were compression molded into discs of 25 mm diameter and 1 mm thickness using a Carver model 4122 press at a predetermined temperature and 3000 psi for 20 min. The samples were then cooled for 10 min by circulating water inside the two plates of the press while keeping the pressure at 3000 psi after which the samples were removed from the press and left to cool to room temperature. Morphology Observations. The morphology of the prepared blends was observed by scanning electron microscopy (SEM) techniques. The molded bars were cryogenically fractured in liquid nitrogen. The fractured specimens were then attached to 12.5 mm diameter aluminum SEM pin stubs and Au−Pd sputter coated for 1−2 min at a deposition current of 25 mA under an Ar pressure of 0.1 mbar. SEM images of the fracture surfaces of the blends were taken by using a JEOL JSM-6510LV instrument at an accelerated voltage of 20 kV and different magnifications. Morphologies of the HDPE/PPE-LMW (90/ 10) blend before and after stretching were also examined by atomic force microscopy (AFM) to develop a better understanding of the mechanical properties of this blend composition. Mechanical Properties. The mechanical properties of the blends were evaluated using an MTS Insight (Electromechanical- 5 kN Standard Length) testing machine. Tensile tests were conducted at ambient temperature (23 ± 1 °C) on at least five specimens of each sample, using cross-head speeds of 20 mm/min. Flexural properties were measured in three-point bending mode on the same machine under ambient temperature (25 ± 1 °C) on at least five specimens at a speed of 1.3 mm/min. The flexural strength at 5% strain was recorded.

third component, usually a nonreactive block or graft copolymer.19 A very limited number of reports are available on the compatibilization and properties of HDPE/PPE blends. Schwarz and co-workers used the styrene−ethylene/butylene− styrene (SEBS) copolymer to compatibilize ternary blends of PEC (a polyether copolymer similar to PPE), PS, and HDPE where the latter was the minor component.20 Ternary HDPE/ PPE/PS blends were studied by Schellenberg where the styrene−ethylene/propylene−styrene (SEPS) copolymer was used as a compatibilizer.21 Blends of ultrahigh molecular weight polyethylene (UHMWPE) and PPE compatibilized with UHMWPE-g-PS were recently reported.22 Commercially available SEBS copolymers have long been known as effective compatibilizers for HDPE/PS blends. Several studies have shown that SEBS copolymers were able to reduce the interfacial tension, stabilize the morphology, and strengthen adhesion at the interface in HDPE/PS blends by polymer chain entanglements and bridging the interface.23−29 By considering the miscibility of PS and PPE and the compatibility of ethylene/ butylene rubber and HDPE, SEBS may be an excellent candidate for the compatibilization of HDPE/PPE blends. The present work aims to utilize SEBS as a coupling agent for HDPE-rich HDPE/PPE blends where HDPE is the continuous matrix. Herein, experimental investigations on the influence of viscosity and concentration of SEBS on the morphology and properties of HDPE/PPE blends are presented, and the influence of concentration and molecular weight of PPE on the properties of the blends is discussed.

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EXPERIMENTAL SECTION

Materials. High density polyethylene HDPE P6006N (MFI5 = 0.23, density = 949 kg/m3) was provided by SABIC (Saudi Arabia) in pellet form. Two PPE resins representing high and low molecular weights were used. High molecular weight PPE (PPO640 56 kDa Mw/ 20 kDa Mn; powder) and low molecular weight PPE oligomers (NorylSA120, pellets) were obtained from SABIC (USA). The properties of these materials are listed in Table 1. The two

Table 1. Properties of PPE Resins Used in This Studya type

code

intrinsic viscosity (dL/g)

Mw

Mn

PPO640 NorylSA120

PPE-HMW PPE-LMW

0.46 0.12

56200 6300

19900 2350

a

Provided by SABIC.

hydrogenated styrene−ethylene/butylene−styrene (SEBS) triblock copolymers, which were used as compatibilizers, were received from Asahi Kasei (Japan) in pellet form under the trade names Tuftec H1517 and Tuftec H1051 (Table 2). All materials used in this study are commercial products. Blend Preparation. Neat PPE-HMW powders were pelletized using a PROCESS 11 twin-screw extruder (Thermo Scientific) at 290 °C and a screw rotational speed of 180 rpm. The purpose of this step was to improve the mixing between PPE-HMW and polyethylene pellets. Prior to melt blending, all PPE pellets were dried in a vacuum oven at 100 °C overnight to remove any moisture present in the

Table 2. Properties of SEBS Resins Used in This Studya type

code

styrene/elastomer ratio

MFIa

Tuftec H1517 Tuftec H1051

SEBS1 SEBS2

43/57 55/45

3 0.8

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RESULTS AND DISCUSSION Differential Scanning Calorimetry (DSC). DSC thermograms for the first and second heating cycles of PPE-HMW powders as received are shown in Figure 1. The first heating

Provided by Asahi Kasei; MFI measured at 2.16 kg and 230 °C.

a

B

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Table 3. Summary of DSC Results for Prepared Blends

a

blend composition HDPE/PPE-HMW/ PPE-LMW/SEBS1/SEBS2

Tm (°C)

ΔHm (J/ g PE)

Xca (%)

Tc (°C)

100/0/0/0/0 90/10/0/0/0 75/25/0/0/0 60/40/0/0/0 75/25/0/5/0 75/25/0/10/0 75/25/0/15/0 75/25/0/0/10 60/40/0/10/0 90/0/10/0/0 75/0/25/0/0 75/0/25/10/0

135 135 133 134 131 133 132 132 134 133 134 133

170 176 168 164 171 177 181 170 179 169 162 178

58 60 57 56 58 60 62 58 61 58 55 61

112 111 113 111 114 113 113 114 112 113 113 114

Normalized crystallinity for HDPE phase; values ±2.

Figure 1. DSC thermograms for neat PPE-HMW powders.

Figure 3. HDT for uncompatibilized HDPE/PPE blends as a function of PPE content.

Table 4. Summary of HDT Results for All Prepared Blends blend composition HDPE/PPE-HMW/PPE-LMW/SEBS1/SEBS2

HDT (°C)

100/0/0/0/0 90/10/0/0/0 75/25/0/0/0 60/40/0/0/0 75/25/0/5/0 75/25/0/10/0 75/25/0/15/0 75/25/0/0/10 60/40/0/10/0 90/0/10/0/0 75/0/25/0/0 75/0/25/10/0

61.9 ± 1.6 79.2 ± 1.5 110 ± 2 127 ± 1 102 ± 8 80.2 ± 3.9 70.7 ± 1.8 71.5 ± 1.5 104 ± 8 76.4 ± 1.9 107 ± 6 90.9 ± 2.5

degree of crystallinity. The percent crystallinity of the sample was calculated to be 31%, which is in agreement with some values found in the literature.32 The powder crystallinity may be attributed to solvent-induced crystallization by toluene which is used in the polymerization of PPE.32,33 The melting peak completely disappeared in the second heating cycle after cooling from the melt, giving a completely amorphous material, probably due to the small difference in Tg and Tm as PPE has

Figure 2. Melting curves (a) and cooling curves (b) for SEBS compatibilizers used in this study.

cycle for the powders showed an endothermic melting peak in addition to the glass transition and the sample showed some C

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Figure 4. Frequency-sweep plots of (a) dynamic viscosity (η′) and storage modulus (G′) and (b) tan (δ) for neat HDPE and uncompatibilized/ compatibilized HDPE/PPE-HMW blends.

a uniquely high Tg/Tm ratio of >0.9 (in K), which does not allow enough time for the crystallization to occur on cooling from the melt.34 The thermal behavior for the two SEBS copolymers used in this study were also investigated by DSC. Like any other copolymer with immiscible segments, SEBS copolymers are expected to have two distinct Tg as a result of their microphase separated structure consisting of a rubbery phase of ethylene/ butylene (EB) blocks and hard glassy microdomains of polystyrene (PS) blocks.35,36 The DSC scans for the studied SEBS copolymers are presented in Figure 2a. As expected, the melting curves showed two different Tg, although one is less noticeable than the other. The first Tg is observed in the range of −55 to −32 °C for the EB rubber block, and the second Tg is located in the range of 90 to 100 °C for the PS blocks. A broad endothermic peak was also found around 29 °C in the DSC thermogram for SEBS2 which can be ascribed to the melting of very small crystallites formed by long ethylene segments.36,37

The broad range of melting and the small area of the endothermic peak support this conclusion. The lack of melting endotherm in the DSC scan for SEBS1 is believed to be a result of high level of butylene branches which interrupt the crystallization of ethylene segments.37 The interruption of crystallization is supported by the absence of an exothermic peak in the cooling scan for SEBS1 (Figure 2b). DSC was also used to study the melting and nonisothermal crystallization behaviors of the binary HDPE/PPE and ternary HDPE/PPE/SEBS blends. As shown in Table 3, the melting and crystallization temperatures of the polyethylene phase were almost insensitive to the concentration of PPE in the blends and were also independent of the presence or absence of the compatibilizers, typical for immiscible polymer blends with a crystallizable matrix.38 The high crystallization rate for polyethylene is another factor to which this finding may be attributed. D

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Figure 5. Frequency-sweep plots of (a) dynamic viscosity (η′) and storage modulus (G′) and (b) tan (δ) for neat HDPE and uncompatibilized/ compatibilized HDPE/PPE-LMW blends.

SEBS2 since the SEBS1-compatibilized blend showed a higher crystallinity than the SEBS2-compatbilized blend at the same PPE and compatibilizer content. Heat Deflection Temperature (HDT). HDT is often used as a measure of a material’s ability to withstand short-term stresses at elevated temperatures. Figure 3 illustrates the dependence of the HDT of HDPE/PPE blends on PPE content. It was found that the HDT of the blends increases greatly (as much as 60 °C) with increasing the PPE concentration. A similar conclusion was reached by others in the case of low density polyethylene, medium density polyethylene, and polypropylene when blended with PPE.39 HDT results for all prepared blends are summarized in Table 4. The HDT of the neat HDPE is included for comparison. As expected, the HDT values of the HDPE/PPE/SEBS ternary blends decreased with increasing SEBS content, probably due to

The normalized heat of fusion and degree of crystallinity of HDPE in the blend were found to decrease slightly with increasing PPE content, though the changes are close to the limits of experimental error. If the decreasing values are to be believed, they suggest that the presence of a PPE phase, which is already in a solidified state at the crystallization temperature of HDPE, restricts the crystal growth and forces the HDPE crystallization to proceed in a confined space. The heat of fusion and degree of crystallinity of the HDPE phase increased with increasing SEBS1 content, possibly due to morphological changes. One might expect that the presence of a compatibilizer increases the interfacial area between the two polymer phases leading to smaller PPE domains, which consequently increases the crystallinity of the polyethylene phase as polyethylene crystals have more space to grow. With this reasoning in mind, it can be concluded that SEBS1 is a better compatibilizer than E

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blends as a function of angular frequency is shown in Figures 4 and 5. From Figure 4a, it can be seen that the storage modulus (G′) for the HDPE/PPE-HMW (75/25) blends is greater than that of neat polyethylene in the low frequency range, regardless of the presence or absence of a compatibilizer. This elasticity increase is typical for immiscible polymer blends and is believed to be driven by the relaxation of the dispersed PPE-HMW particles when it undergoes slight deformation in the low frequency range.40,41 G′ for HDPE/PPE-LMW (75/25), as shown in Figure 5, showed the opposite trend, decreasing slightly compared to neat polyethylene, perhaps due to partial miscibility of the PPE oligomers with polyethylene. The introduction of either of the SEBS compatibilizers to the HDPE/PPE-HMW (75/25) blend was found to decrease both dynamic viscosity (η′) and G′ and only showed slight difference with varying the compatibilizer concentration (5−15 phr). This phenomenon was observed by others for different blends when compatibilizers were added in a relatively high concentration.42−44 The unexpected drop in the dynamic viscosity and storage modulus may be associated with formation of micelles within the matrix, which have a plasticizing effect on the viscosity and the storage modulus of the blend.43−45 This effect is also supported by the increase in tan (δ) after the addition of SEBS (Figure 4b). Because both compatibilizers used in this study are of high molecular weight, they have a higher tendency to generate micelles instead of localizing at the interface when the interface is already saturated with the compatibilizer.43,44 One can see from Figure 5 that this is not the case for blends containing PPE oligomers. The SEBS1-compatbilized HDPE/ PPE-LMW (75/25) exhibited a similar viscosity and a higher elasticity as indicated by the higher storage modulus and lower tan (δ) in the low frequency ranges than the corresponding uncompatibilized blend, which is indicative of an increased interfacial interaction in the presence of compatibilizer.43−46 Hence, it can be concluded that the compatibilization effect of SEBS1 is dependent on the molecular weight of PPE where a lower molecular weight of PPE is preferred, probably because the viscosity ratio of the blend components as the viscosity of PPE-LMW is expected to be more similar to that of polyethylene than the viscosity of PPE-HMW. Morphology Observations. Figure 6 shows the SEM micrographs of uncompatibilized HDPE/PPE-HMW blends of different compositions. Poor adhesion between HDPE and PPEHMW is evident, as indicated by the smooth surface of the dispersed PPE-HMW particles and the presence of holes in the matrix, a sign of debonding of the PPE-HMW phase from the HDPE matrix. The HDPE/PPE-HMW (90/10) blend exhibited a continuous HDPE matrix with a dispersed PPE-HMW particles of an elongated pebble-like morphology. As the PPEHMW loading increases, PPE-HMW particles take a more irregular shape of a wider particle size distribution. The effect of SEBS compatibilization on the HDPE/PPEHMW (75/25) is shown in Figure 7. Compared with Figure 6, the blend exhibited a more homogeneous morphology, and the size of the dispersed PPE-HMW particles decreased when either SEBS1 or SEBS2 is added. This reduction is a result of the improvement in the interfacial adhesion which is believed to be caused by the localization of SEBS at the interface. The PPEHMW phase decreased in size as SEBS1 loading increased; beyond 10 phr loading, there was no significant reduction in the size of PPE-HMW particles. It is also worth to note that no obvious difference was observed between SEBS1 and SEBS2 as both compatibilizers showed similar effect on the morphology.

Figure 6. SEM micrographs for HDPE/PPE-HMW blends of (a) 90/ 10 and (b) 75/25 weight ratios.

that the addition of more SEBS decreases the concentration of PPE in the overall composition and further softens the material compared to the unmodified/uncompatibilized HDPE/PPE binary blends. The SEBS-compatibilized/modified blends nonetheless exhibited higher HDT values than that of the neat polyethylene. A similar trend for SEBS-compatibilized PP/PPE blends was reported in the literature.14 It can be further concluded from Table 4 that SEBS1 is a more efficient compatibilizer than SEBS2, which will be further discussed below. When HDPE/PPE (75/25) is compatibilized with 10 phr of SEBS2, the reduction in HDT was more significant than in the case of SEBS1 at the same SEBS concentration. Although blends with high molecular weight PPE and blends with low molecular weight PPE oligomers showed similar HDT at the same PPE concentration, it is interesting to see that when 10 phr of SEBS1 is added to the HDPE/PPE (75/25) blends, HDT for the blend containing PPE oligomers was about 10 °C higher than the blend containing high molecular weight PPE. These results suggest that SEBS1 induces a better improvement in interfacial adhesion between the low molecular weight PPE oligomers and polyethylene than in the high molecular weight PPE and polyethylene blends. This suggestion is supported by the mechanical properties and morphological observations, as discussed below. Rheological Analysis. The isothermal dynamic rheological response of the neat polyethylene and uncompatibilized/ compatibilized HDPE/PPE-HMW and HDPE/PPE-LMW F

DOI: 10.1021/acs.macromol.8b00894 Macromolecules XXXX, XXX, XXX−XXX

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Figure 7. SEM images for compatibilized HDPE/PPE-HMW (75/25) blends containing (a) 5 phr SEBS1, (b) 10 phr SEBS1, (c) 15 phr SEBS1, and (d) 10 phr SEBS2.

Figure 8. SEM images for (a) HDPE/PPE-LMW (90/10), (b) HDPE/PPE-LMW (75/25), and (c) HDPE/PPE-LMW/SEBS1 (75/25/10).

PPE of a conventional molecular weight (PPE-HMW) (Figure 7b), confirming the heat deflection temperature and rheological data. AFM results (Figure 9) confirmed the compatibility of PPELMW with HDPE at low concentration (i.e., 10 wt %) as manifested by the good adhesion between the dispersed oligomers particles and the HDPE matrix as well as the absence of any sign of interfacial debonding. As shown in Figure 9b, when the tensile specimen was subjected to tension, the

The morphologies of the uncompatibilized and compatibilized HDPE/PPE-LMW blends are illustrated in Figure 8. The PPE oligomers showed a high degree of compatibility with polyethylene without a compatibilizer, particularly for the blend containing 10% oligomers, evidenced from the very small size of the PPE oligomers that are barely detected. No voids or detached oligomer particles were observed. The SEM analysis (Figure 8c) also suggests that SEBS1 is a more effective compatibilizer in the case of PPE oligomers (PPE-LMW) than G

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Figure 9. AFM images for HDPE/PPE-LMW (90/10) before stretching (a) and after stretching (b) with a draw ratio of 2.5.

stages or plateaus are observed in the curve for the blend containing 10% PPE-LMW. These stages are not shown in the case of pure polyethylene or in the case of blends containing 10% PPE-HMW. The stress−strain curves together with the AFM results presented earlier suggest that the dispersed PPE oligomers act as organic reinforcements and that there is a high efficiency of load transfer from the HDPE matrix to the reinforcement oligomers particles. During the tensile test at room temperature, the oligomers are well below their glass transition temperature (Tg = 165 °C), and hence, they are in a glassy solid state. When the specimen is stretched, at some point, the applied load is transferred to the dispersed oligomers particles. Because there are no chain entanglements in the oligomers and the oligomers are in the glassy state, the applied load forces the oligomers to break into smaller pieces as confirmed by AFM analysis. We postulate that the stress transfer between the matrix and the oligomers combined with the fracture mechanism of the oligomers delays the fracture of the polyethylene matrix, leading to improvement in elongation at break and toughness. When the oligomers content was increased to 25%, the elongation at break and toughness deteriorated significantly, possibly attributable to the weakness of the oligomers’ chains taking into consideration that the molecular weight of these oligomers is lower than the entanglement molecular weight (Me) of PPE (Me = 3620).47 Similar to blends containing PPE-HMW, elongation at break and toughness increased greatly when SEBS1 was added. This improvement was more significant for blends containing oligomeric PPE (PPE-LMW) than in the blends with conventional PPE (PPE-HMW). It can also be seen from Table 5 that the HDPE/PPE-HMW (75/25) blend compatibilized with SEBS1 exhibits higher elongation at break with less fluctuations than the blend compatibilized with SEBS2 while maintaining good stiffness and strength. The mechanical results and the higher melt flow index of SEBS1 suggest that SEBS1 is more efficient in locating itself at the HDPE/PPE-HMW interface and, hence, provide better compatibilization than SEBS2. The effect of the compatibilizer amount on the flexural properties and toughness of HDPE/PPE-HMW (75/25) is depicted in Figure 12. Both flexural strength and modulus decreased continuously with increasing SEBS1 content. This behavior is very common when polymers are modified by elastomers. The reduction in flexural properties may, thus, be due to the excess concentration of SEBS1, which was more than required for compatibilization. Therefore, a considerable amount of SEBS1 is likely located in the bulk of the polyethylene

Table 5. Summary of Tensile Properties for the Prepared Blends blend composition HDPE/PPEHMW/PPE-LMW/SEBS1/ SEBS2

tensile strength (MPa)

Young’s modulus (MPa)

elongation at break (%)

100/0/0/0/0 90/10/0/0/0 75/25/0/0/0 60/40/0/0/0 75/25/0/5/0 75/25/0/10/0 75/25/0/15/0 75/25/0/0/10 60/40/0/10/0 90/0/10/0/0 75/0/25/0/0 75/0/25/10/0

27.6 ± 0.8 28.2 ± 0.5 30.4 ± 0.7 33.8 ± 0.9 27.3 ± 1.0 28.0 ± 0.4 28.7 ± 2.3 27.1 ± 0.3 28.6 ± 0.3 25.1 ± 0.6 25.8 ± 1.0 27.1 ± 0.3

462 ± 30 507 ± 10 503 ± 21 603 ± 18 479 ± 24 416 ± 10 382 ± 4 447 ± 7 524 ± 22 431 ± 17 581 ± 18 473 ± 19

47 ± 6 47 ± 7 36 ± 6 15 ± 2 199 ± 48 279 ± 32 360 ± 104 190 ± 74 103 ± 10 185 ± 19 8±1 392 ± 66

oligomers particles were fractured and broken into particles of smaller sizes. The breakage of the PPE oligomers prior to the fracture of the specimen during stretching indicates stress transfer from the HDPE matrix to the dispersed oligomers phase as will be discussed in the next section. Mechanical Properties. Tensile properties for the prepared binary HDPE/PPE and ternary HDPE/PPE/SEBS blends are summarized in Table 5. Flexural properties and toughness for uncompatibilized HDPE/PPE blends as a function of PPE content are presented in Figure 10. Tensile strengths and Young’s moduli generally increased with PPE-HMW content but decreased with increasing compatibilizer concentrations. Flexural properties were also improved significantly as PPEHMW or PPE-LMW content increased, especially at 25% and higher. Notably, the blends containing 25% or less of PPEHMW exhibited no or slight reduction in elongation at break or toughness as evaluated from the stress−strain curves, strongly suggesting that the HDPE/PPE-HMW blend system is partially stable at low PPE-HMW content even without any further modification. However, elongation at break and toughness were decreased by nearly 70% when PPE-HMW loading increased to 40%. This reduction in ductility is expected to be a result of the poor adhesion between the PPE-HMW phase and the polyethylene matrix. The addition of 10% of PPE oligomers (PPE-LMW) to polyethylene surprisingly increased the elongation at break 3fold. By examining the stress−strain curves (Figure 11), multiple H

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Figure 11. Stress−strain curves of pure HDPE, HDPE/PPE-HMW (90/10) blend, and HDPE/PPE-LMW (90/10) blend.

Figure 10. Effect of PPE content on the mechanical properties of uncompatibilized HDPE/PPE blends: (a) flexural strength, (b) flexural modulus, and (c) toughness.

Figure 12. Effect of SEBS1 content on the mechanical properties of HDPE/PPE-HMW (75/25 wt/wt) blends: (a) flexural properties and (b) toughness.

matrix acting as a toughening agent; this is confirmed by the substantial increase in toughness as SEBS1 loading increased. With only 5 phr of SEBS1, the toughness was increased to 45 MJ/m3, which is 4 times higher than that of the uncompatbilized

blend and neat polyethylene. This toughening effect may be attributed to the good flexibility of the rubbery EB block in SEBS; further increases in the compatibilizer loading beyond 10 phr only led to small improvements in toughness. I

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CONCLUSIONS HDPE/PPE blends with different compositions were prepared, and the properties of the blends were studied. The heat deflection temperature of polyethylene was significantly increased when blended with PPE. Blends containing high molecular weight PPE without SEBS exhibited good mechanical properties up to 25% PPE content. The stiffness of the blends increased linearly with increasing PPE amount while elongation at break and toughness were maintained. When PPE content was further increased, the ductility of HDPE/PPE blends were severely reduced owing to weak interfacial adhesion. Reductions in the PPE particles’ size upon the addition of SEBS copolymers indicate improved interaction between the dispersed PPE phase and the HDPE matrix, also reflected inasmuch as 3× increase in toughness of the compatibilized blends. The present work showed that viscosity and architecture of the compatibilizer play a role in the compatibilization effectiveness of HDPE/PPE blends. SEBS copolymers with higher melt flow rates, and hence lower viscosity, are more efficient compatibilizers than their lower melt flow rate counterparts, even if they contain more butylene branches. The present study showed that crystallinity of the HDPE phase in the prepared blends was influenced by both PPE and SEBS content. A higher PPE content resulted in lower crystallinity while higher SEBS concentration increased the crystallinity compared to the uncompatibilized blends, a sign of morphological changes. It can be further concluded from this study, as supported by heat deflection temperature, rheological, morphological, and mechanical results, that the efficiency of SEBS copolymers as compatibilizers is dependent on the molecular weight of PPE, where a lower molecular weight is favored. Overall, the objective to improve the heat resistance of high density polyethylene was successfully achieved by blending with poly(phenylene ether)s while maintaining excellent balance in stiffness/toughness properties.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.A.S.). *E-mail [email protected] (J.M.). ORCID

David A. Schiraldi: 0000-0001-5111-0558 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.macromol.8b00894 Macromolecules XXXX, XXX, XXX−XXX