ionomer composites: Morphology and rheological behavior

ionomer composites: Morphology and rheological behavior

Accepted Manuscript Effect of ionomer interfacial compatibilization on highly filled HDPE/Al2O3/ionomer composites: Morphology and rheological behavio...

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Accepted Manuscript Effect of ionomer interfacial compatibilization on highly filled HDPE/Al2O3/ionomer composites: Morphology and rheological behavior Xin Chen, Jin Sha, Tao Chen, Haili Zhao, Huajian Ji, Linsheng Xie, Yulu Ma PII:

S0266-3538(18)31770-6

DOI:

https://doi.org/10.1016/j.compscitech.2018.11.007

Reference:

CSTE 7450

To appear in:

Composites Science and Technology

Received Date: 7 August 2018 Revised Date:

31 October 2018

Accepted Date: 3 November 2018

Please cite this article as: Chen X, Sha J, Chen T, Zhao H, Ji H, Xie L, Ma Y, Effect of ionomer interfacial compatibilization on highly filled HDPE/Al2O3/ionomer composites: Morphology and rheological behavior, Composites Science and Technology (2018), doi: https://doi.org/10.1016/ j.compscitech.2018.11.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Effect of Ionomer Interfacial Compatibilization on Highly Filled HDPE/Al2O3/Ionomer Composites: Morphology and Rheological Behavior Xin Chen, Jin Sha*, Tao Chen, Haili Zhao, Huajian Ji, Linsheng Xie, Yulu Ma* School of Mechanical and Power Engineering, East China University of Science and

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Technology; Shanghai, 200237, China

Corresponding author. Email: [email protected] (Y. Ma), [email protected] (J. SHA)

were

filled

fabricated

high-density by melt

poly(ethylene-co-methacrylic)-based

polyethylene/alumina

mixing

ionomer

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composites

Highly

with

direct

(EMAA-Na)

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ABSTRACT:

as

(HDPE/Al2O3)

incorporation an

of

interfacial

compatibilizer. SEM and EDX micrographs indicate EMAA-Na interfacial adhesion on the Al2O3 spheres. Under low EMAA-Na content conditions, the FT-IR characterization and an EMAA-Na neutralization degree analysis revealed the priority of the melt

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neutralization interaction between the acid groups of the ionomer and the Al2O3 spheres in the composite. Under high EMAA-Na content conditions, an AFM phase

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characterization revealed the formation of an EMAA-Na ionomer spherical domain (~300 nm) dispersed in an HDPE matrix due to the microphase separation of ionic

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chains. Capillary and dynamic rheology measurements were also conducted to investigate the phase morphology evolution. The polymeric adhesion on the Al2O3 sphere surfaces contributed to the increase of the melt viscosity and gradual elongation thickening behavior of the composite melts. The formed spherical domain structure of EMAA-Na of the composite melts contributed to the shear thickening, elongation thickening and yield behavior. Three rheology criteria plots indicate large complex formation in the composite melts. The EMAA-Na incorporation in the highly filled

ACCEPTED MANUSCRIPT HDPE/Al2O3 composite matrix not only improved the strength and toughness performance, with a 27% improvement in the elongation at break (EMAA-Na content 1 wt.%) and 21% improvement in the tensile strength (EMAA-Na content 10 wt.%), but also preserved the good thermal conductivity (~1.5 W/(m·K)). This study reveals the

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potential application of the EMAA-Na ionomer to resolve the challenge of the strength and toughness performance degradation in highly filled polymer composites.

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Keywords: ionomer; interfacial compatibilization; melt neutralization; morphology; rheology.

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1. Introduction

In recent decades, the incorporation of micro- or nanoparticles into a polymer matrix via melt mixing has attracted great interests due to its practical and significant industrial importance [1-4]. It is generally realized that a high concentration of solid

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particles must be added to a polymeric matrix to impart the filler properties to the composite matrix and produce desirable properties adapted to specific applications, e.g.,

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thermal and electrical conductivity [5, 6], flame retardancy [7], magnetic characteristics [8], and 3D printing [9]. However, the addition of particles beyond a critical value of the

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solid level make strength and toughness degradation inevitable due to a reduction in the material cohesion. Herein, tremendous efforts have been devoted to achieve a high-quality dispersion and distribution of the particles in the matrix to obtain a homogenous mixture with desirable properties. The capacity of mixing depends on several main factors, including the material properties of the neat polymer, particle properties, processing conditions and material formulation [10]. Novel processing techniques, including oscillation shear [11],

ACCEPTED MANUSCRIPT extensional flow [12] and reactor granule methods [13], have exhibited great potential for the preparation of high-performance polymer composites. Moreover, the dispersion of micro- or nanoparticles in a polymer matrix also depends on the basic interparticle and particle-polymer interfacial interactions. Polymeric surfactants are widely used as

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interfacial compatibilizers either to increase the polymer-particle interactions over the interparticle interactions or to increase the interparticle repulsive forces to avoid particle

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aggregation [14-16]. Recently, the emerging requirements of a low usage of volatile organic compounds (VOC) and ease of melt processing have made high molecular

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weight amphiphilic block copolymers attractive as surfactants for highly filled polymeric composite preparation. As copolymers that comprise electrically neutral repeating units and a fraction of ionized units covalently bonded to the polymer backbone, the ionomers possess not only high toughness and interfacial adhesion

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capacity for ceramics [17], clays [18], fibrils [19] and wood [20] via ionic interactions but also substantial phase compatibilization for polyolefin [21], polyamides [22],

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poly(lactic acid) [19], etc. These make ionomers potential high molecular weight polymeric surfactants to improve the performance of highly filled polymeric composites.

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It is also worth noting that the clustered polar ionic groups on the ionomer carbon chains work as reversible crosslinkers, leading to a microphase separation morphology of ionic-rich domains dispersed in the solid-state and shear-thickening behavior in the melt state [23]. Therefore, research on the phase morphology and rheological behavior of highly filled polymeric composites containing ionomers would benefit our understanding of the influence of ionic interactions on composite performance. The highly filled polyolefin/Al2O3 composite is a well-studied composite system

ACCEPTED MANUSCRIPT due to its high thermal conductivity [13] and good electronic insulation properties [24]. The spherical geometry and microsized particle distribution make alpha-phase Al2O3 spheres an ideal rigid particle filler with a minimum agglomeration tendency and a maximum packing fraction (up to 80 wt.%). However, as an amphoteric oxide, Al2O3

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spheres possess a hydrophilic surface that is not compatible with the nonpolar polyolefin. In the present study, a poly(ethylene-co-methacrylic acid)-based sodium

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ionomer (EMAA-Na) is used as a compatibilizer to improve the interfacial interaction between the Al2O3 spheres and HDPE matrix. SEM, AFM and EDX characterizations conducted

to

observe

the

morphology

and

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were

microstructure

of

the

HDPE/Al2O3/EMAA-Na composite. FT-IR and a subsequent curve-fitting analysis were applied to illustrate the reactive interfacial interaction between the EMAA-Na and Al2O3 spheres during the melt mixing. Rheological measurements were conducted to

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reveal the composite’s melt rheological behavior and microstructure. Finally, the thermal conductivity and tensile properties of the HDPE/Al2O3/EMAA-Na composite

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were measured to evaluate the compatibilization effect. The results obtained may contribute to the design of highly filled polymeric composites for engineering

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applications.

2. Experimental 2.1. Materials

High-density polyethylene (HDPE 2911, melt flow index=20.5 g/10 min at 190 °C) was provided by Lanzhou Petrol Co., Ltd. Alpha-phase spherical alumina (Al2O3, D50=4.13 µm) was obtained from Zhengzhou Sanhe New Materials Co., Ltd., China. A commercial poly(ethylene-co-methacrylic acid)-based sodium ionomer (EMAA-Na),

ACCEPTED MANUSCRIPT SURLYN® 8920 (density=0.95 g/cm3, melt flow index=0.9 g/10 min at 190 °C, melting temperature=88 °C, methacrylic acid concentration 5.4 mol% and neutralization level 60%), was used in this work and purchased from DuPont. 2.2 Composite preparation

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The HDPE/Al2O3/EMAA-Na composites were prepared by melt compounding in a two-rotor continuous mixer (diameter=30 mm, see supporting information Figure S1) at

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a speed of 900 rpm with a feed rate of 4280 g/h, orifice setting of 40%, and a two-stage barrel temperature of 30 °C and 160 °C. HDPE, Al2O3 and EMAA-Na were vacuum

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dried at 80 °C overnight and stored in a desiccator prior to use. A series of composites were studied in which the EMAA-Na ionomer content was varied (1-10 wt.%) with a fixed spherical alumina concentration (50 wt.%). 3 Results and discussion

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3.1 Reactive interfacial compatibilization

The fracture surface morphologies of the HDPE/Al2O3/EMAA-Na composites

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were investigated by SEM, as shown in Figure 1. On the fracture surface of the HDPE/Al2O3 composite (Figure 1a and b), many Al2O3 spheres are partially embedded

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in the HDPE matrix and surrounded by some neat and well-shaped circular craters due to Al2O3 spheres being pulled out. The relatively clean Al2O3 sphere surface and apparent microvoids between the two components are evidence for a low interfacial adhesion that impedes the stretching force transmission from the HDPE matrix to the Al2O3 sphere. With 1 wt.% EMAA-Na incorporation (Figure 1c and d), most of the Al2O3 spheres are embedded in the HDPE matrix, and only seldom exposed Al2O3 spheres and light circular craters are observed. The polymeric adhesive on the Al2O3

ACCEPTED MANUSCRIPT sphere surface prevents HDPE/Al2O3 interfacial crack formation. As the ionomer content increases to 6 wt.% and 10 wt.% (Figure 1e-f), the Al2O3 spheres are fully embedded in the HDPE matrix, and very few have been pulled out. Dimples caused by polymeric matrix ductile breakage are also observed. The observed dimples and Al2O3

the EMAA-Na ionomer into the composite system.

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surface adhesion indicate improved interfacial compatibilization after introduction of

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As a random copolymer of ethylene and methacrylic acid, the partially neutralized EMAA-Na exhibits an amphipathic characteristic that enables it to interact with both the

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polyolefin HDPE and metal oxide Al2O3 spheres. The massive carboxylic groups tethered on the EMAA-Na ionomer chain skeleton offer the possibility of favorable interaction with Al2O3. Here, FT-IR characterization was performed to qualitatively probe the base-acid interaction during the melt mixing process. Previous studies [25, 26]

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have employed FT-IR to qualitatively probe the acid group interactions in pure EMAA-Na. As shown in Figure 2(a), there are four possible interactions for EMAA-Na,

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including lone free acid groups, unneutralized acid dimers, Na+ ion fully neutralized bicoordinated carboxylates and unneutralized acid associated with a neutralized

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complex in tetracoordinated carboxylates. In Figure 2(b), free acids (1750 cm-1) are not detected in any of the composite systems, in agreement with the previous study [25]. The pure EMAA-Na ionomer spectrum displays two main peaks at approximately 1695 cm-1 (corresponding to unneutralized acid dimers) and 1560 cm-1 (corresponding to overlapping peaks of neutralized or associated acid complexes). A relatively weak absorbance at 1735 cm-1 is also observed, which can be assigned to an ester impurity converted via carboxylic acid transesterification during the EMAA copolymer synthesis.

ACCEPTED MANUSCRIPT The incorporation of Al2O3 in the HDPE matrix incurs a weak absorption at 1640 cm-1, corresponding to surface hydroxyl groups on the Al2O3 sphere. In the spectra of the HDPE/Al2O3/EMAA-Na composite, two new absorption peaks at 1706 cm-1 and 1715 cm-1 are observed, which can be associated with the stretching vibration of the

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methacrylic ester bonds [21]. A hypothetical model of the interfacial interactions between the EMAA-Na ionomer and Al2O3 is proposed in Figure 2(c): the hydroxyl

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groups on the Al2O3 sphere interact with the free acid groups and unneutralized acid dimers of the ionomer molecules, which further form ester bonds (corresponding to

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absorption at 1715 cm-1) and associated complexes (corresponding to absorption at 1706 cm-1).

The spectral pattern in the region 1800-1500 cm-1 was further resolved into eight band components (see supporting information Figure S2). In the curve-fitting procedure,

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a band at approximately 1640 cm-1 assigned to hydroxyl groups on the Al2O3 was taken into consideration in addition to the three bands in the neutralized region (1584, 1565,

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and 1540 cm-1) and four bands in the carboxyl acid region (1735, 1715, 1706 and 1695 cm-1) discussed above. It is observed that the hydroxyl component peak at 1640 cm-1

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and fully neutralized bicoordinated carboxylate component peak at 1584 cm-1 decrease as the EMAA-Na ionomer content increases, while the neutralized tetracoordinated carboxylate component peaks at 1540 cm-1 and 1560 cm-1 exhibit a continuously increasing tendency. Despite various carboxylic contributions in the neutralized region (1650-1500 cm-1) that are difficult to separate because of peak overlap, the curve-fitting spectra provide quantitative information about the influence of the Al2O3 spheres, as an alkali neutralization agent, on the neutralization degree of EMAA-Na. The previous

ACCEPTED MANUSCRIPT infrared analytical methods based on the intensity measurements of a single carboxylate band (or the total absorbance of a collection of bands in the 1500-1700 cm-1 region) are subject to large errors due to a number of different acid structures in the composite and a lack of well-established absorption coefficients. Instead, the degree of neutralization

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can be determined directly via a quantitative comparison of the absorbance of the carboxylic acid dimer (AD) and an internal standard such as the 1735-cm-1 ester

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carbonyl band (AE) [27]. From the spectrum of the pure EMAA-Na film sample, we can determine the corresponding absorbance ratio for the completely unionized case,

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AD0 / AE0 . The degree of neutralization is given by:

  A A0   neutralization% = 1 −  D E0   × 100   AE AD   The absorption coefficients and the influence of the HDPE component conveniently

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cancel out in this equation. The degree of neutralization for each sample was determined, and the results are shown in Figure 3. Compared to pure EMAA-Na, the neutralization degree of the EMAA-Na in the composite is much higher and shows a decreasing

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tendency as the ratio of the EMAA-Na and Al2O3 sphere concentrations increases.

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During the melt extrusion process, the neutralization degree increase can be ascribed to the instant base-acid interaction between the Al2O3 spheres and the massive carboxylic groups on the EMAA-Na backbone. However, there is still residual EMAA-Na distributed in the composite matrix that does not interact with the Al2O3 spheres, as indicated by the maximum EMAA-Na neutralization degree (approximately 97%) under low EMAA-Na content conditions (< 4 wt.%). 3.2 Composite phase morphology Figure 4 shows EDX mapping images of the aluminum and sodium elements for

ACCEPTED MANUSCRIPT the fracture surface of HDPE/Al2O3/EMAA-Na composite. Cyan dots are observed in every EDX image that tend to aggregate and form round or elliptical patterns with a diameter 5 µm, corresponding to the exposed Al2O3 spheres. Magenta dots, with a diameter of approximately 0.3 µm, are only observed in the HDPE/Al2O3/EMAA-Na

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composite. Here, the magenta dots exhibit a close distribution around the aggregated cyan dots under the 1 wt.% EMAA-Na content condition and a uniform distribution in

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the composite matrix with 6 wt.% and 10 wt.% EMAA-Na content. It can be speculated that these magenta dots are ionic-rich domains of EMAA-Na distributed in the HDPE

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matrix. A previous study indicated that microphase separation happens when the high content EMAA-Na is well dispersed in a continuous HDPE phase [28]. Here, the EDX mapping results qualitatively describe the distribution state of the EMAA-Na ionomer throughout the composite matrix.

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Figure 5 shows high-resolution SEM and AFM phase images of the polymeric matrix in the HDPE/Al2O3/EMAA-Na composite. The fracture surfaces of the

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HDPE/Al2O3/EMAA-Na composite polymeric matrix show a characteristic feature of peaks and fibrils due to the ductile fracture of the HDPE matrix, in clear contrast to the

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relatively flat surface of the HDPE/Al2O3 composite. Under the 1 wt.% EMAA-Na content condition, only HDPE continuous phase stretching is observed, and the fibrils are very fine (the fibril cross-section is generally less than 0.1 µm). Under the 6 wt.% and 10 wt.% EMAA-Na content conditions, the HDPE continuous phase is preferentially stretched, leaving the spherical EMAA-Na ionomer particles embedded and relatively unchanged. As a result, a microphase separation with a network-like structure of fibrils and cavities is generated. Additionally, AFM phase images are

ACCEPTED MANUSCRIPT applied to reveal the distribution and dispersion state of EMAA-Na in the composite polymeric matrix. This phase image contrast is related to the variation of the local area stiffness, where a larger stiffness leads to a more positive phase shift and thus to a brighter contrast in the phase image [29]. Here, small, irregular bright domains

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dispersed in a continuous dark phase can be distinguished. The areas of bright contrast can be assigned to the EMAA-Na phase, and the continuous areas of dark contrast can

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be assigned to the HDPE phase. The size of the formed spherical EMAA-Na domains is up to several hundred nanometers (~300 nm) in diameter (see supporting information

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Figure S3). The EMAA-Na phase dispersion state in the composite matrix is illustrated in Figure 5(e). As the ionomer content increases, excess EMAA-Na that has not interacted with the Al2O3 spheres may disperse in the HDPE matrix and form a spherical domain structure, whose diameter is proportional to the ionomer content. This

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dispersion state evolution can also be reflected in the crystallization behavior change of the HDPE matrix (see supporting information Figure S4).

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3.3 Rheological behavior of composite

The rheological behavior of the HDPE/Al2O3/EMAA-Na composite melts gives us

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an overview of the composite’s internal structure and allows the determination of suitable industrial polymer processing conditions. In Figure 6(a), a typical Newtonian behavior region at a low shear rate and shear-thinning behavior region at shear rates above 10 s-1 are observed on the melt viscosity curves of the HDPE matrix and HDPE/Al2O3 composite. The incorporation of Al2O3 spheres raises the shear viscosity of the HDPE melt due to the increased flow resistance. For the melt viscosity curves of the HDPE/Al2O3/EMAA-Na composites, clear initial-region yield behaviors are

ACCEPTED MANUSCRIPT observed. The corresponding yield stress exhibits a proportional relationship to the ionomer content. This phenomenon consists to the rheological observation of composite melt

with

elastomer

component

incorporation

[30].

Additionally,

obvious

shear-thickening behavior is observed over a narrow shear rate range on the shear

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viscosity curves of the pure EMAA-Na and high ionomer content (above 4 wt.%) HDPE/Al2O3/EMAA-Na composites. It is worth noting that the viscosity curve patterns

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of the pure EMAA-Na melt and 8 wt.% and 10 wt.% ionomer content composite melts are almost the same, indicating that the highly viscous EMAA-Na melt plays a

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dominant role in the shear viscosity behaviors of the high ionomer content HDPE/Al2O3/EMAA-Na composite melts. The shear viscosity behavior of the HDPE/Al2O3/EMAA-Na composite melt depends on the dispersion state of the Al2O3 spheres, HDPE melt strength, and entanglements and interchain associations between

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the EMAA-Na melt and HDPE molecules. Here, the shear rate rise contributes to the entanglements and interchain associations between the EMAA-Na melt and HDPE

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molecules [23]. Meanwhile, the dispersed EMAA-Na melt domains start to trap the Al2O3 spheres via ionic interactions and form a large complex structure whose

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orientation behavior within a certain shear rate range consequentially contributes to the melt viscosity increase. For HDPE/Al2O3/EMAA-Na composites with a lower ionomer content (less than 4 wt.%), as the EDX and FT-IR section discussed, the EMAA-Na tended to adsorb on the Al2O3 spheres and form a core-shell structure during the melt mixing process, which hindered the formation of Al2O3/EMAA-Na complex structure found in the higher EMAA-Na contents composite melts. The competing and synergetic relationship of the three factors would be responsible for the irregular shear-thinning

ACCEPTED MANUSCRIPT behaviors of the composite melts. Figure 6(b) shows elongation viscosity curves of the HDPE/Al2O3/EMAA-Na composites. An elongation thinning behavior is observed for the pure HDPE and EMAA-Na melts, which is followed by a flat viscosity plateau due to the melt molecule

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orientation under the high elongation stress. The addition of Al2O3 spheres to the HDPE matrix does not change the slope pattern of the elongation viscosity curves but

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decreases the value of the elongation viscosity. This can be ascribed to the limited elongation stress transition between the two components. In comparison, the

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HDPE/Al2O3/EMAA-Na composites show a typical elongation thickening behavior with an abrupt viscosity increase followed by a constant-viscosity plateau. The EMAA-Na content increase accounts for the reduced elongation rate at the onset of the elongation thickening behavior. For composite with low ionomer contents (1 wt.% and

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2 wt.%), the elongation thickening behavior is more gradual, and the overall elongation viscosity value remains in the viscosity range of the pure HDPE and HDPE/Al2O3

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composite. For high ionomer contents (above 6 wt.%), the elongation thickening behavior is much steeper, and the overall elongation viscosity value is obviously higher

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than that of the HDPE matrix. The results can be explained based on the alignment mechanism of the polymeric matrix and rigid Al2O3 spheres [31]. Under low ionomer content conditions, the gradual elongation thickening behavior mainly depends on the average orientation of the Al2O3 spheres. As the ionomer content increases above 4 wt.%, an Al2O3 sphere-HDPE network structure bridged by the EMAA-Na ionomer forms, which changes the elongation thickening behavior and increases the viscosity. With an ionomer content above 6 wt.%, the steep elongation thickening behavior mainly

ACCEPTED MANUSCRIPT depends on the orientation and stretching of the HDPE and EMAA-Na ionomer chains. Here, a strain sweep measurement was conducted to characterize the strain amplitude dependence of the storage and viscous moduli (G′ and G″) to clearify the linearity extent (see supporting information Figure S5). Afterward, the frequency (ω)

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dependencies of the storage modulus G′, loss modulus G″ and damping factor Tan δ for the HDPE/Al2O3/EMAA-Na composite melts are investigated via frequency sweep

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measurements, and the results are plotted in Figure 7 (a-c). The HDPE/Al2O3 and pure EMAA-Na ionomer melts display a typical power-law behavior (G'~ω2 and G"~ω1),

The

filler

topology

and

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and the individual G' and G″ plots are almost linear on double logarithmic coordinates. filler-polymer

interaction

strongly

influence

the

hydrodynamic-to-nonhydrodynamic transition behavior of the composite melts [32]. Here, with the ionomer incorporation, an apparent rubber plateau prior to the Rouse

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region (G'~G"~ω1/2) is observed on the G' and G″ plots of the HDPE/Al2O3/EMAA-Na composite. The plateau becomes more pronounced with the increasing ionomer content.

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Compared to the pure EMAA-Na, the composites show higher dynamic moduli in the Rouse region and lower moduli in the terminal region. It is further observed that

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increasing the ionomer content would delay the terminal relaxation and expand the Rouse region to lower frequencies. Though a higher neutralization degree of the ionomer would promote the transition of its melt viscoelastic behavior to a solid-like (elastic)

state,

the

elevated

modulus

and

solid-like

behaviors

of

the

HDPE/Al2O3/EMAA-Na composite still reflect the enhanced interfacial interactions between the EMAA-Na and Al2O3 spheres. The Tan δ values show the same sudden increase in the composite’s elasticity. It can be inferred that the incorporation of

ACCEPTED MANUSCRIPT EMAA-Na in the HDPE/Al2O3/EMAA-Na composite works more similar to an interfacial compatibilizer and ionic cross-linked elastomer components. Three rheology criteria plotted in Figure 7 (d−f) are used to detect the secondary structure differences in the HDPE/Al2O3/EMAA-Na composite. Figure 7(d) shows the

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vGP (van Gurp-Palmen) plot, i.e., curve of phase angle (δ) vs. logarithmic complex modulus (|G*|). The plot of the HDPE/Al2O3 composite exhibits a plateau of δ (nearly

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90°) at low |G*| values, indicating the completely viscous nature in the terminal region. The EMAA-Na addition makes the plateau value of δ decrease to approximately 40°,

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which indicates an elasticity increase due to the apparently improved interactions between the Al2O3 and HDPE as well as the aggregation of Al2O3 into an extended construction: the formation of a clustered Al2O3 network at low ionomer contents and the formation of percolating ionomer-Al2O3 networks at high ionomer contents. Han

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plots, drawn from log G' against log G", are introduced to reflect the rheological compatibility and miscibility in the HDPE/Al2O3/EMAA-Na composite system, as

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shown in Figure 7(e). Here, the incorporation of EMAA-Na does not cause an obvious slope change in the terminal region. Instead, the plots of the HDPE/Al2O3/EMAA-Na

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composites shift to higher log G" values as the ionomer content increases. The dependence of the Han plots shift on the ionomer content indicates the large-scale heterogeneity in the composite system. To explain the consistency of the slope of the Han plots, it can be speculated that the EMAA-Na addition substantially improves the compatibility between the HDPE matrix and Al2O3 spheres. Last, the plot of the complex viscosity |η*| versus complex modulus |G*| is also used to characterize the melt flow behavior of the HDPE/Al2O3/EMAA-Na composite. In Figure 7(f), an |η*|

ACCEPTED MANUSCRIPT plateau with the value of nearly 103 Pa·s is found for the HDPE/Al2O3 composite, demonstrating

typical

viscous

melt

flow behavior.

For

the

plots

of

the

HDPE/Al2O3/EMAA-Na composites, the absence of a plateau at low |G*| and a steep dependence of |η*| are observed, indicating the strong melt flow restrictions. It is worth

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noting that the |η*| values increase along with the ionomer content rise, and a yield behavior (|G*|η*→∞) can be achieved with an ionomer content above 6 wt.%. Under high

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ionomer content conditions, the viscous restriction is strong enough to suppress the deformation of the percolating ionomer-Al2O3 domains and hence to restrain the

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coalescence of the phase structure under a shear field. 3.4 Mechanical properties and thermal conductivity

Stress-strain curves of the HDPE/Al2O3/EMAA-Na composites and the corresponding mechanical property dependencies on the ionomer content are presented

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in Figure 8(a) and (b), respectively. The HDPE/Al2O3 composite has a typical ductile stress-strain behavior with a characteristic yield point and extensive elongation before

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failure. Similar ductile behavior with a larger elongation is observed for the 1 wt.% ionomer addition HDPE/Al2O3/EMAA-Na composite. The additional ionomer results in

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a larger maximum stress but much shorter elongation. Figure 8(b) shows tensile strength, elongation at break and yield strength curves as functions of the ionomer content. It can be seen that the addition of ionomer substantially increases the tensile strength of the composite, to about an approximately 21% improvement under the 10 wt.% ionomer content condition. In contrast, the yield strength shows little dependence on the ionomer content, except for a slight increase under the 4 wt.% ionomer content condition. The influence of the ionomer content on the elongation at break is much more complex. A

ACCEPTED MANUSCRIPT 27% increase in the elongation at break is observed for the 1 wt.% ionomer addition HDPE/Al2O3/EMAA-Na composite. Further ionomer content increases result in a linear decrease in the elongation at break, followed by a small range of variation when the EMAA-Na content is higher than 6 wt.%. Here, with an ionomer content less than 2

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wt.%, most ionomers adsorb on the Al2O3 spheres surface, acting as a compatibilizer, which improves the interfacial interaction to the HDPE matrix and thereby enhances the

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ductile performance of the composites. As the ionomer content increases, additional EMAA-Na is dispersed in the HDPE matrix, forming discontinuous spherical domains,

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which leads to a tensile strength increase of the composite. The influence of the EMAA-Na content on the thermal conductivity κ of the HDPE/Al2O3/EMAA-Na composites is shown in Figure 8(c). Well-dispersed Al2O3 spheres in the HDPE matrix can form percolated pathways at a 50 wt.% content, which substantially improves the κ

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value to approximately 1.53 W/(m·K). On the one hand, the addition of EMAA-Na improves the interfacial interaction between the HDPE matrix and Al2O3 spheres via

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reactive compatibilization, which would substantially contribute to the overall composite thermal conductivity. On the other hand, the adequate ionomer preferentially

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distributes around the Al2O3 spheres and forms ionic-rich domains, which might increase the thermal interfacial resistance due to phonon scattering. In all, the dependence of the thermal conductivity on the ionomer content is not obvious. The κ values of the HDPE/Al2O3/EMAA-Na composites remain within a small range close to that of the HDPE/Al2O3 composite when the ionomer content is less than 6 wt.%. 4. Conclusion The incorporation of EMAA-Na ionomer in the highly filled HDPE/Al2O3 composite

ACCEPTED MANUSCRIPT system improves the interfacial interaction between the Al2O3 spheres and HDPE matrix by polymeric adhesion on the Al2O3 sphere surface. The neutralization degree characterization of the EMAA-Na ionomer in the composites indicates the preferential melt neutralization interaction between the acid groups of the ionomer and Al2O3

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spheres. Additional EMAA-Na ionomer dispersed in the HDPE matrix is observed to form spherical phases, corresponding to the formation of ionic-rich domains via

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microphase separation. This phase morphology also contributes as ionic cross-linkers forming large complexes in the composite melt, leading to the rheological observations

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of shear thickening, elongation thickening and yield behavior for composite melts. Finally, the obtained HDPE/Al2O3/EMAA-Na composite shows improved strength and toughness performance with an unchanged thermal conductivity. This study reveals the potential application of an ionomer to resolve the challenge of the strength and

Acknowledgements

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toughness degradation for highly filled polymer composites.

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The authors sincerely acknowledge the support of National Natural Science Foundation of China (5150306, 51273065) and National Center for International

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Research of Micro-nano Molding Technology (MMT2016-05). Author contributions

J.S. and Y.L.M. conceived, designed, and directed the project. X.C. and J.S.

performed

the experiments. T.C., H.L.Z,

H.J.J

and

L.S.X.

supported the

characterizations. X.C., J.S. and Y. L. M. wrote the paper. All authors analyzed the data, discussed the results, and commented on the manuscript. References

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Figure 1. Fracture surface SEM micrographs of (a, b) HDPE/Al2O3, (c, d)

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HDPE/Al2O3/1 wt.% EMAA-Na, (e, f) HDPE/Al2O3/6 wt.% EMAA-Na and (g, h)

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HDPE/Al2O3/10 wt.% EMAA-Na composites.

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Figure 2. (a) Possible acid group interactions in EMAA-Na ionomer, (b) FT-IR spectra for HDPE/Al2O3/EMAA-Na composites from 1800 to 1500 cm-1 and (c) hypothetical

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melt neutralization model for interactions between EMAA-Na and Al2O3 spheres.

Figure 3. Degree of neutralization (%) and absorbance ratio (AD/AE) for HDPE/Al2O3/EMAA-Na composites with different ionomer contents.

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Figure 4. Energy-dispersive X-ray spectra (EDX) mapping images of the elements Al (cyan) and Na (magenta) for the cryo-fracture surfaces of (a) HDPE/Al2O3, (b)

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HDPE/Al2O3/1 wt.% EMAA-Na, (c) HDPE/Al2O3/6 wt.% EMAA-Na and (d)

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HDPE/Al2O3/10 wt.% EMAA-Na.

ACCEPTED MANUSCRIPT Figure 5. High-resolution SEM images (suffix -1) and AFM phase images (suffix -2) of polymeric matrixes of (a) HDPE/Al2O3, (b) HDPE/Al2O3/1 wt.% EMAA-Na, (c) HDPE/Al2O3/6 wt.% EMAA-Na and (d) HDPE/Al2O3/10 wt.% EMAA-Na, (e) scheme

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of EMAA-Na morphology evolution in HDPE/Al2O3/EMAA-Na composite.

Figure 6. Viscosity of the HDPE/Al2O3/EMAA-Na composite as a function of the

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ionomer content: (a) shear viscosity and (b) elongation viscosity.

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Figure 7. Dynamic rheological properties of the HDPE/Al2O3/EMAA-Na composite as a function of the ionomer content: (a) G', (b) G", (c) tan δ, (d) van Gurp-Palmen plots,

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(e) Han plots, and (f) plots on complex viscosity versus complex modulus.

Figure 8. Mechanical properties of HDPE/Al2O3/EMAA-Na composites: (a) tensile stress-strain curve, (b) tensile strength, elongation at break and yield strength curves and (c) thermal conductivity as a function of the ionomer content.