High Density Polyethylene blends with high amounts of reactive compatibilizer

European Polymer Journal 50 (2014) 177–189

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Morphologies in Polyamide 6/High Density Polyethylene blends with high amounts of reactive compatibilizer A. Argoud, L. Trouillet-Fonti, S. Ceccia, P. Sotta ⇑ Laboratoire des Polymères et Matériaux Avancés (LPMA), CNRS/Solvay, UMR 5268, R&I Centre de Lyon, 85 avenue des frères Perret, 69192 Saint Fons, France

a r t i c l e

i n f o

Article history: Received 16 April 2013 Received in revised form 6 August 2013 Accepted 28 October 2013 Available online 8 November 2013 Keywords: Non-miscible polymer blends Morphology Compatibilization PA-PE blends

a b s t r a c t The morphologies (dispersed, stretched dispersed, fibrillar and co-continuous) obtained in twin screw extruders in reactively compatibilized PA6/HDPE/Maleic Anhydride-graftedHDPE blends have been studied over a broad range of compositions, specifically with high amounts of compatibilizer, by Scanning Electron Microscopy (SEM) after selective phase dissolution. The conversion ratio of the compatibilization reaction is estimated to be very high (of order 80%) in co-continuous morphologies. The morphologies exhibit two characteristic sizes: subdispersions of a few tens of nm coexist with larger (micrometric) scale morphologies. Large scale morphology regions have been identified in ternary compositions diagrams. The location of the co-continuity region agree with theoretical models based on rheological properties. The observed large scale morphologies, however, are not compatible with the high fraction of copolymer formed in the system after reactive extrusion. This implies that most of the copolymers are under the form of nanometric subdispersions within large size domains. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Blending polymers allows developing new materials with tuned properties [1]. In immiscible polymer blends, depending on the composition and on process parameters, various multiphase morphologies, generally classified into dispersed, stretched dispersed/fibrillar and co-continuous, can be developed [1]. In order to obtain reproducible properties, it is key to control and stabilize the required morphologies according to the desired set of properties. Compatibilization is the most common way to achieve stabilization and reproducibility of the morphologies [1,2]. It consists in reducing the interfacial tension through an interfacial agent (or compatibilizer) which locates at the domain interfaces and allows to decrease the characteristic domain sizes by inhibiting coalescence [3,4]. The compatibilizer may be a copolymer added to the blend [1,2] or, in the case of reactive compatibilization, generated in situ ⇑ Corresponding author. Tel.: +33 472896466; fax: +33 472896963. E-mail address: [email protected] (P. Sotta). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.10.026

during the blending process by a chemical reaction leading to covalent bonding directly at the interface [1,2]. Reactive compatibilization may be achieved by ionic association [5]. Nanoparticles may also be used as compatibilizers [6,7]. Co-continuous morphologies, in which both immiscible components form a continuous domain, appear in the vicinity of phase inversion, which denotes the point at which a dispersed phase becomes continuous [8–10]. A first, heuristic model to predict the composition at phase inversion was proposed by Paul and Barlow [11] and then generalized by Miles and Zureck [12]. The volume fractions u1 and u2 ¼ 1  u1 at phase inversion are directly related to the viscosity ratio R ¼ g1 ðc_ Þ=g2 ðc_ Þ as u1 =u2 ¼ R. An alternative model with a more complicated function of the viscosity ratio was developed by Metelkin and Blekht 2 [13]: u1 =u2 ¼ R½1 þ 2:25 ln R þ 1:81ðln RÞ . Bourry et al. proposed to take into account the elasticity, relating the ratio u1 =u2 to the ratio of either the elastic moduli G0i or the loss factors tan di ¼ G00i =G0i [14]. Another approach to rationalize phase inversion is to consider that the system should choose the less viscous morphology for each com-

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position and/or set of material parameters [15]. Phase inversion should then correspond to the point at which the effective viscosity of an A-in-B emulsion equals that of B-in-A emulsion. These models do not involve the interfacial tension, assuming that the presence of an interfacial agent in compatibilized systems would not affect the topology, but only change the average domain size. Then, the nature and/or architecture of the interfacial agent is not considered. The range of existence of full co-continuity (defined as the situation in which both phases form single co-continuous domains, without isolated droplets. Note that phase inversion as discussed above does not necessarily correspond to full co-continuity defined in this way) was studied in various non-compatibilized systems and compared to a model in which co-continuity is interpreted as the coalescence of densely packed elongated droplets or threads [16,17]. Compatibilized Polyamide/Polyethylene (PA/PE) blends have been already studied in the literature [18–30]. In such blends, reactive compatibilization has been shown to give the highest efficiency [2,31,32]. However, the conditions to develop and stabilize co-continuous morphologies have not been fully described in PA/PE blends yet. The relationships between morphologies and material properties have been studied [33–36]. Dispersion of Polyethylene in Polyamide may enhance tribological properties and impact resistance of PA [26,34]. There has been also some interest in developing lamellar-like structures (formation of platelets of PA in a PE matrix during 2D stretching processing) in order to increase tortuosity and therefore enhance barrier properties [33,36,37]. In this work, Polyamide 6/High Density Polyethylene (PA6/HDPE) blends compatibilized by reactive extrusion are considered. Systems with a large amount of compatibilizer, in which a large amount of copolymer may be formed during reactive extrusion, are studied. The reaction completion has been measured to check that this is effectively the case. Large scale morphologies (sizes in the micrometer range) are observed and described over a broad range of compositions. In this paper, we emphasize the fact that the observed sizes are not compatible with the large fraction of copolymer formed in the material, which indicates that the standard processing conditions used here do not allow dispersion at the nanoscale. The region of occurence of co-continuous morphologies is discussed and seems to be described by quite simple rheological models as discussed above. Different extrusion processing tools (twin screw extrusion and batch mini extrusion) and materials with slightly different viscosity ratios have been used in order to check the robustness of the conclusions with respect to processing conditions. The paper is organized as follows. Raw polymers, processing and methods of observation are described in Section 2. The rheological behavior of the raw materials and the influence of the process on the materials, including the compatibilization reaction, are discussed in details in Section 3. Then the observed morphologies are described in terms of the blend compositions in Section 4. Specifically, the location of the co-continuity domain is discussed.

Coexistence of micrometric morphologies with a high fraction of formed copolymers is finally discussed (Section 5). 2. Experimental 2.1. Materials One Polyamide 6 (PA6), three High Density Polyethylenes of different viscosities (denoted HDPE1, HDPE2 and HDPE3) and one standard compatibilizer (Maleic Anhydride grafted High Density Polyethylene (MA-g-HDPE), 1 wt% MA, denoted C) were used. The properties of neat HDPEs and compatibilizer are shown in Table 1. The PA6 has a molar mass of 27,600 g mol1 and an index of polydispersity of 2.2. Note that the polymers have relatively long chains, as compared to other studies on similar systems [18]. The condensation reaction between Maleic Anhydride (MA) moieties of the compatibilizer and amine end-groups NH2 of PA6, which leads to the formation of grafted copolymers at the interface during the process, is schematized in Fig. 1. It is known that this reaction effectively takes place in two steps. First, the reaction of a MA group with one amide forms a amic acid function, which then transforms into an imide cycle, generating one water molecule [38]. MA groups do not react with water at 290 °C. The stoichiometric ratios [MA]/[NH2] range from 0.15 to 13.7 in the compatibilized blends. At the scales relevant in processing, the compatibilizer C (MA-g-HDPE) is considered to be miscible with HDPE. Thus, the overall HDPE + C fraction corresponds to one PE phase, which will be denoted as E in what follows, with subscript 1 (resp. 2; resp. 3) for HDPE1 (high viscosity 1) (resp. HDPE2 (high viscosity 2); resp. HDPE3 (low viscosity)). However, there is some indication that MA groups may not be miscible in HDPE at the molecular scale, related to a high Flory interaction parameter v between PE monomers and MA groups (see Appendix). 2.2. Blend processing and characterization Raw materials were characterized at high shear rates by capillary rheometry (Göttfert Rheograph 2002), using values L=D ¼ 20=1. A Bagley correction was applied according to Goubert et al. [39]. Before rheology characterization and blending, the materials were dried 12 h in primary vacuum at 90 °C in order to reach a water content between 500 and 1000 ppm for PA. Blending of PA6/HDPE/MAg-HDPE was carried out by extrusion, using different processing tools:

Table 1 Properties of neat polymers according to supplier data and to Steric Exclusion Chromatography (SEC) measurements. IP: Index of Polydispersity. Materials

Mn (g mol1)

Mw (g mol1)

IP

HDPE1 HDPE2 HDPE3 MA-g-HDPE

29,300 21,300 21,200 26,000

145,300 107,600 81,600 75,700

5 5 3.9 2.9

A. Argoud et al. / European Polymer Journal 50 (2014) 177–189

Fig. 1. Compatibilization reaction between MA moieties of the MA-gHDPE and amine end-groups of PA6.

– A Leistritz co-rotating twin screw extruder (diameter D ¼ 34 mm, ratio L=D ¼ 35, screw speed 250 rpm, throughput 10 kg/h). The water produced by the compatibilization reaction was eliminated through a venting zone. Feeding and venting zones were used for sampling during extrusion. – A Coperion co-rotating twin screw extruder (D ¼ 40 mm, L=D ¼ 34, 180–300 rpm, 25–40 kg/h), with a screw profile giving a higher shear rate than in the Leistritz. In both continuous extruders, the temperature of the blends at the exit of the die is about 290 °C. – A batch mini-extruder (Microcompounder DSM Midi 2000, 10 g of material per batch), with various processing conditions: temperature from 270 °C to 315 °C, screw speed 10–200 rpm, residence time 1–11 min. Blends morphologies were observed using Scanning Electron Microscopy (SEM). Pellets were included in Epoxy resin (Araldite) and placed in an oven at 70 °C overnight. Then, the surface was cryotrimmed at 150 °C. Finally, depending on the minority phase (in terms of volume fraction u): – For uPA > uE : the PE phase was etched using Decahydronaphthalene (Decalin) (stirring at 115 °C for 1h30) or Toluene (stirring at 80 °C for 2 h), which is less efficient because it easily dissolves the PE amorphous phase only. – For uPA < uE : the PA6 phase was etched using formic acid at 90% (stirring at room temperature for 30 min). Pellets were observed parallel (denoted k) and perpendicularly (denoted ?) to the flow, as schematized in Fig. 2. Co-continuity is determined visually on SEM micrographs. The quantitative index of co-continuity (i.e. the fraction of each phase belonging to the co-continuous domain) was not quantified. Blends were also observed by Transmission Electron Microscopy (TEM) on 100 nm thick samples stained with phosphotungstic acid (H3PW12O40) to enhance the contrast between PA and PE domains, without any etching. The conversion ratio of MA moieties in the processed blends (see Fig. 1) was estimated by transmission Infrared (IR) spectroscopy (Bruker Vertex 70 spectrometer). In order to prepare samples without melting (which may lead to further completion of the reaction), samples were cryo-

grinded into a fine powder which was incorporated in KBr plates of fixed thickness and sample content. Samples of unreacted blend formulations were prepared by physically mixing pellets of PA6, HDPE and MA-g-HDPE and cryogrinding. The amount of residual unreacted maleic anhydride (MA) moieties was determined by comparing the normalized intensities (integrated areas) of the anhydride carbonyl absorption band at 1791 cm1 [30,40–42] in unreacted and reacted blends at same compositions. 3. Characterization of raw materials and of processing effects 3.1. Viscosity ratios As extrusion temperature (290 °C) is very high for HDPE, the viscosities of the neat HDPEs, compatibilizer C (MA-gHDPE) and one representative sample of each PE phase (HDPE/C 60/40 vol%) were measured as a function of the shear rate, before and after extrusion at 290 °C. These materials indeed show some evolution during processing. It was checked by Size Exclusion Chromatography (SEC) measurements that the observed changes of viscosities are coherent with changes in M w . Thus, for HDPEs, we shall consider the viscosities measured after extrusion to be representative of their rheological properties during processing. For PA6, viscosities values are stable and SEC measurements do not show any significant effect of processing. As mentioned above, the compatibilizer C is considered to be miscible with HDPE. Thus, the overall HDPE + C amount corresponds to one PE phase (denoted as E, with subscript corresponding to each HDPE). However, compatibility at large scale (a few micrometers) between HDPE and MA-g-HDPE does not rule out a possible, at least partial, micellization of MA moieties in HDPE at the molecular level. Indeed, the presence of MA groups grafted along the chains have a quite strong effect on the rheology of the PE phase in the low frequency regime. This is discussed in Appendix. The viscosities of extruded HDPEs, representative PE phases (HDPEi/C 60/40 vol%) and PA6 as a function of shear rate are shown in Fig. 3. Note that the relative values of HDPE viscosities after processing are not described in a simple way by differences in the initial M w s. This may not be so surprising, since viscosities of HDPEs in general are quite sensitive to small differences in the degrees of branching, which indicates that M w alone may not be sufficient to determine the viscosity in HDPEs. Adding compatibilizer to HDPE1 and HDPE2 decreases the corresponding PE phase E1 and E2 viscosities and does not change the HDPE3 viscosity, as expected from the relative values of the pure component viscosities (see Table 1). The viscosity ratios at a given shear rate c_ are defined throughout the paper as:

Rðc_ Þ ¼ Fig. 2. Schematics of a pellet with the directions of observation in SEM indicated.

179

gE ðc_ Þ gPA ðc_ Þ

ð1Þ

The average shear rate during extrusion is of the order

c_  100 s1. The maximum shear rate in the extruder die

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HDPE 1 HDPE 2 HDPE 3

2

E1 E2 E3 C PA6

Viscosity [Pa.s]

1000 6 5 4 3 2

100 6 5 4 2

3

4 5 6

10

2

3

4 5 6

100

2

3

4 5

1000

Shear rate [s-1] Fig. 3. Viscosities of processed HDPEs, MA-g-HDPE (C), the corresponding PE phases Ei (HDPEi/C 60/40 vol%), and unprocessed PA6 as a function of the shear rate at 290 °C. This graph shows the viscosities of all raw materials which have been used to compute the viscosity ratios.

(diameter D) may be estimated as the shear rate on the wall of a capillary c_  16Q v =pD3 (for a Newtonian fluid), with Q v the volume throughput, which gives an order of magnitude 3500 s1 for a diameter D ¼ 2 mm. The viscosity ratios obtained with the three PE phases Ei at shear rates of 100 s1 and 3500 s1 are reported in Table 2. As shown in Fig. 3 and in Table 2, HDPE3 is a particular case in which the viscosity ratio is independent of the fraction of compatibilizer in E3 and of the shear rate. 3.2. Conversion rate of the compatibilization reaction The reaction conversion was quantitatively characterized in five blends a to e (with HDPE3) (shown in Figs. 4a and 8a) which all have the same PA6 fraction 60 vol% and compatibilizer fractions decreasing from 40 vol% to 0. Note that all blends a to e have the same viscosity ratio R ¼ 0:5 (because HDPE3 and C have the same viscosities, see Fig. 3) and correspond to co-continuous morphologies. Thus, the only changing parameter is the amount of MA moieties. The obtained IR spectra in the region of the absorption band of anhydride carbonyl at 1791 cm1 in the unreacted (resp. extruded) blends are shown in Fig. 4a (resp. Fig. 4b). In unreacted samples, it was checked that the areas, normalized by the mass of sample in the corresponding KBr test plate and by the area of a PA6 band at 1170 cm1, were proportional to the MA content. The normalized average areas Au (resp. Ar ) of the 1791 cm1 absorption band are Table 2 Viscosity ratios R ¼ gE ðc_ Þ=gPA ðc_ Þ calculated using the viscosities of the unprocessed PA6 and processed HDPE’s at c_ ¼ 100 and 3500 s1, at 290 °C. Materials

R (c_ ¼ 100 s1)

R (c_ ¼ 3500 s1)

HDPE1 HDPE2 HDPE3 E1 E2 E3 C

1.3 1.5 0.5 1.1 0.9 0.5 0.5

0.8 0.7 0.5 0.7 0.6 0.5 0.5

plotted as a function of the weight fraction of compatibilizer in unreacted (resp. extruded) samples in Fig. 5. The amount of residual MA groups in the blends is much lower after extrusion, which demonstrates the effectiveness of the reaction. The obtained conversion rates R, calculated as R ¼ ðAu  Ar Þ=Au , are plotted as a function of the initial molar ratio of MA groups on amine end-groups of PA6 [MA]/[NH2] in Fig. 6. The maximum theoretical reaction rate is also indicated in Fig. 6 (continuous line). Given the very small areas to be measured, particularly in reacted blends, IR spectra give only a good semi-quantitative estimate of the MA conversion rate. In any case, it may be reliably estimated that the compatibilization reaction rates in the co-continuous blends are higher than 80% for molar ratios [MA]/[NH2] from 0.5 to 1.2, which is in agreement with literature [30,43]. It follows that the formed copolymers represent a large fraction of the material after extrusion. This is discussed below.

4. Results 4.1. Ternary phase diagrams for morphologies In this section, the observed morphologies are described in terms of the compositions of the blends and other parameters. We first describe and classify the various types of morphologies which are observed according to the ternary compositions of the samples. SEM micrographs in Fig. 7(1)–(5) show representative examples of the various types of morphologies observed in the compatibilized PA6/HDPE blends as composition varies. The observed morphologies were classified into five distinct categories: (1) PA6 droplets dispersed in a PE phase matrix; (2) PA6 stretched dispersion in a PE phase matrix; (3) Co-continuous; (4) PE phase stretched dispersion in a PA matrix; (5) PE phase droplets dispersed in a PA matrix. The observed morphologies agree with a well accepted general scheme according to composition [26]. Within the range of investigated processing conditions, the morphological observations are robust, except perhaps at boundaries between morphology regions which may be a little shifted. The types of morphologies do not depend on the process (note that the Coperion extruder gives smaller size morphologies, due to higher average shear rate). The investigated blend compositions can be located in ternary composition diagrams as shown in Fig. 8. These ternary diagrams may be analyzed along different pathways. Along a line parallel to the HDPE/C edge, the initial concentration (or chemical potential) of MA groups within PE phase changes at constant PA6 volume fraction. Along a line issuing from the pure PA6 vertex, only the PA (or PE phase) volume fraction changes, at constant initial concentrations of reactive species within each phase. Along a line issuing from the pure PE vertex, the initial molar ratio of reactive species [MA]/[NH2] is constant. The different types of morphologies illustrated in Fig. 7, obtained with HDPE2 and HDPE3, have been reported as regions on the ternary diagrams in Fig. 8. HDPE1 and HDPE2, which have very close rheological properties, exhibit nearly

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C

b

0,2

0,8

c 0.15

Neat C

0.085

a

0.080

b

0,9

d 0,8

l) (vo

0.20

0,7

0,3

E3

(vo l)

a

P HD

Absorbance Units

0,6

0,4

Absorbance Units

0.25

0,7

e 0,6

1

c

PA6 (vol)

0.10

d e

0.05 1900

1850

1800

(a)

0.075 a

0.070

b

0.065

c

0.060

(b)

0.055

1750

1810

Wavenumber [cm-1]

1800

1790

1780

1770

Wavenumber [cm-1]

1.0

100

0.8

80

Conversion ratio [%]

normalized absorbance at 1792 cm-1

Fig. 4. IR spectra of blends corresponding to formulations a to e in the region of the absorption band of anhydride carbonyl at 1791 cm1: (a) unreacted blends; (b) blends reacted during extrusion. Note that the scales are different on each graph. Compositions of formulations a to e are reported in the partial ternary diagram inserted in (a) and also in the full ternary diagram in Fig. 8a.

0.6

0.4

60

40

0.2

20

0.0

0 0

5

10

15

20

25

30

35

compatibilizer amount [wt%]

0.6

0.8

1.0

1.2

1.4

[MA] / [NH2] [mol]

Fig. 5. The average area of the absorption band at 1791 cm1 normalized by the absorbance at 1170 cm1 as a function of the weight fraction of compatibilizer C introduced in the blends for unreacted (black diamonds) and reacted samples (empty circles).

Fig. 6. Maximum theoretical (continuous line) and experimental (diamond symbols) conversion rates versus the initial molar ratio [MA]/ [NH2].

similar results. The reported points (with various symbols corresponding to different extrusion tools and process conditions) correspond to all investigated formulations. The overall extension of the different morphology regions in Fig. 8 is only indicative, morphologies being defined only at spots corresponding to the investigated formulations. In the regions corresponding to dispersed droplet phases, the Taylor size Dmin  2CCacrit =gm c_ (where Cacrit is the critical capillary number, c_ the estimated average shear rate, gm the matrix viscosity and C the interfacial tension) gives a generic estimate of roughly 50–100 nm (obtained by taking c_  100 s1, C  1 mN m1 [22], Cacrit  1 and the matrix viscosities from Fig. 3). For both PA6 or PE dispersed droplet phases (Figs. 7(1) and (5)), droplet sizes are indeed distributed between 50 and 500 nm in compatibilized blends. The droplet size distribution seems to be relatively broad. Larger droplets are significantly larger than the estimated Taylor size. On approaching phase inversion (the co-continuity region), large fibrils are present, reflecting the effect of the large applied deformation (Figs. 7(2) and (4)) [15]. This indicates that this type of morphology develops through formation of thread and break up of thread below a critical

radius. Two characteristic domain sizes coexist. This is particularly obvious at compositions close to phase inversion. Indeed, large fibrils and anisotropic, relatively large droplets (diameter of order 1–2 lm) coexist with spherical, small (50 nm or less) droplets (Figs. 7(2)–(4)). In the co-continuous region (Fig. 7(3)), a large co-continuous morphology (characteristic size 2–10 lm) coexists with a dispersion of small droplets (sizes 50 nm to 200 nm typically) embedded within domains of larger sizes, in both PA and PE phase domains, as illustrated in the TEM micrographs shown in Fig. 9. In this case, the two characteristic sizes differ by almost two orders of magnitude. While small size subdomains are always nearly spherical, large scale domains show the various types of morphologies described above, depending on the composition of the blends. 4.2. Influence of the compatibilizer on the kinetics of morphology development When comparing formulations a to e (see Figs. 8a or 4a), the PA6 volume fraction is constant (uPA ¼ 60 vol%), the viscosity ratio is constant and the morphologies are always

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PA6/HDPE 3/C: 31/42/27 vol%

PA6/HDPE 1/C: 38/38/24 vol%

PA6/HDPE 1/C: 50/30/20 vol%

PA6/HDPE 1/C: 60/24/16 vol%

PA6/HDPE 2/C: 63/15/22 vol%

Fig. 7. Representative examples of SEM micrographs (2500) of the different types of morphologies observed in twin screw extrusion diameter 34 mm: (1) and (2): PA6 phase etched with formic acid; (3)–(5): PE phase etched with Decalin.

co-continuous. Only the amount of MA moieties changes. Sampling in the melt at different stages during extrusion was performed on the two formulations c and e (with and without compatibilizer respectively), in order to determine the influence of the compatibilizer amount on the kinetics of morphology development. This is illustrated in Fig. 10. As expected, by increasing the compatibilizer amount in the PE phase, the domains become smaller (with sizes varying from roughly 10–20 lm in e down to roughly 1 lm in c). On the other hand, in formulation e (no compatibilizer), co-continuity develops progressively along the twin screw, whereas in compatibilized systems, the co-continuous morphology develops much earlier and remains practically unaltered along the extruder, including in the die, in spite of the high shear rate applied there. Thus, the compatibilizer both stabilizes the morphology and accelerates its development. In both cases, the extrusion residence times were long enough to have no influence on the obtained types of morphologies.

5. Discussion The morphologies obtained in extrusion in compatibilized blends of long polymeric chains have been described. Reactive extrusion with high molarities of reactive species leads to the formation of a high fraction of graft copolymer, the copolymer being the majority component in some cases. From the observations described above, it may be concluded at this point that: – The observed large (micrometer) scale morphologies obey a general scheme which have been described in blends with a small fraction of compatibilizer, essentially driven by processing rheology. – Altogether, the amount of compatibilizer does not have a strong effect on the type of morphologies (discrete, co-continuous) which are formed at the micrometric (larger observed scale) scale.

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Co mp ati bil ize r( vo l)

(a)

0

1

0,1

0,9

0,2

0,8

0,3

0,7

0,4

0,6

0,5

0,5 0,4

0,6

a 0,7

b

0,2

0,8

0,9

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d 0 1

l) (vo

c

3 PE HD

0,3

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0,8

e

0,7

1

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Co mp ati bil ize r( vo l)

PA6 (vol)

(b)

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PA6 (vol) Fig. 8. Ternary diagrams of the morphologies obtained with HDPE3 (a) and HDPE2 (b). Blends processed with: : twin screw extruder diameter 34 mm; j: twin screw extruder diameter 40 mm; N: mini extruder, various conditions. The curves indicate the composition at phase inversion predicted by Eq. (3), using viscosity ratios at 100 s1 (order of magnitude of the shear rate within the extruder, full curve); at 3500 s1 (order of magnitude of the shear rate in the die, dashed curve). Regions are drawn to include all experimental points with a given morphology: white on dark dots: PE phase droplets dispersed in PA matrix; inclined stripes: PE phase stretched dispersion in PA matrix; light gray: co-continuous; horizontal stripes: PA stretched dispersion in PE phase matrix; black dots: dispersed PA droplets. Note that the extensions of morphology regions are speculative.

– The observed amount of interface in the blends is not compatible with the amount of graft copolymers formed during reactive extrusion, which has been checked by IR. Let us discuss these points in more details.

5.1. Location of co-continuity regions in the phase diagrams Let us first discuss the ternary morphology diagrams in Fig. 8, and more specifically the location of the co-continuity region, in terms of process rheology.

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Fig. 9. TEM micrograph of a blend in co-continuous morphology: PA6/ HDPE3/C 60/24/16 vol%. PA6 domains (in dark) were stained 15 min with phosphotungstic acid. This picture shows a representative example of subdispersions in both PA6 and E3 phase domains.

The morphologies obtained after extrusion in all studied formulations with different compositions and HDPE viscosities are reported as a function of the PA6 volume fractions uPA and the viscosity ratios in Fig. 11. This diagram has been established using the viscosity ratios measured at a shear rate value of either 100 s1 (order of magnitude of the average shear rate in extrusion, Fig. 11a) or 3500 s1 (order of magnitude of the shear rate in the extruder die, Fig. 11b) (see Fig. 3). Points with different viscosity ratios correspond to different HDPEs or different HDPE/C ratios in the case of HDPE1 or HDPE2. Even though the used viscosity ratios are quite close to each other, the morphology regions show a significant dependence or trend as a function of the viscosity ratio. Indeed, for example, co-continuous morphologies were obtained at various PA6 volume fractions (from 45 to 60 vol% of PA6) depending on the different HDPEs used.

Actually, in the case of compatibilized blends, it was observed that co-continuity develops inside the extruder and remains unaltered after exiting the die (Fig. 10). This would suggest that the relevant shear rate to consider is the one inside the extruder (of the order 100 s1 on average) rather that the shear rate at the die exit. As mentioned in Section 1, a way to rationalize phase inversion is to consider that the system should choose the less viscous morphology for each composition and/or set of material parameters [15]. Inversion point should then correspond to the point at which the effective viscosity of an A-in-B emulsion equals that of B-in-A emulsion. The effective viscosity geff of a emulsion of droplets of phase 2 (volume fraction u2 ) in a continuous matrix of phase 1 may be described (up to the concentrated regime) e.g. by the heuristic extension of the Krieger and Dougherty equation [44] proposed by Meijer et al. to account for fluid, viscous droplets [15]:



geff ¼ g1 1 

u2 um

2:5um Rþ0:4 Rþ1 ð2Þ

where um is some maximum packing volume fraction of droplets, to be considered as an effectively adjustable parameter, and R ¼ g2 =g1 is the viscosity ratio. This leads to the following relationship between the volume fractions at phase inversion (u1 ¼ 1  u2 ) and the viscosity ratio R: Rþ0:4 1þ0:4R     u 2:5um Rþ1 u 2:5um 1þR 1 2 ¼R 1 1

um

um

ð3Þ

The phase inversion compositions predicted by Eq. (3) (using um ¼ 1) have been reported on the graphs in Fig. 11a and b (full curves). The predictions of the model by Paul and Barlow [11] are shown also for comparison (dashed curves). Note that models based on elasticity ratios [14] would give a variation of the phase inversion composition as a function of the viscosity ratio opposite

Exit of the die Toluene dissolution

a

c

Sampling zone 1

Sampling Zone 2

Toluene dissolution

Toluene dissolution

2µm

2µm

2µm

2µm

10µm

10µm

e

10µm

Fig. 10. SEM micrographs of a (PA6/HDPE3/C 60/0/40 vol%), c (PA6/HDPE3/C 60/24/16 vol%) and e (PA6/HDPE3/C 60/40/0 vol%) formulations (see ternary diagram in Fig. 8a) along the screw and at the exit of the die (PE etched using Decalin unless otherwise specified).

A. Argoud et al. / European Polymer Journal 50 (2014) 177–189

1.0

(a) φPA6 [vol%]

0.8

0.6

0.4 PE dispersion PE stretched dispersion Cocontinuous PA stretched dispersion PA dispersion Paul-Barlow Krieger-Dougherty

0.2

0.0

2

3

4

5

6

7 8 9

0.1

2

1

R = ηPE phase/ηPA6 1.0

φPA6 [vol%]

0.8

the type of morphologies which are formed. The dependence of the morphology regions on the compatibilizer amount may be entirely explained by the effect of changing viscosity ratios. The above model does not involve the interfacial tension, which does not appear in Eq. (3). It is assumed that the presence of an interfacial agent in compatibilized systems does not affect the topology, but only changes domain sizes. Then, the nature and/or architecture of the interfacial agent is not considered. Indeed, chain lengths have been chosen such that the spontaneous curvature induced by the graft copolymer formed at the interface should be close to zero. This may be estimated in the following way. It may be assumed that the graft copolymer behaves in the same way as a block PA with two HDPE arms, with total average length (of the two arms) equal to the average length between graft points. Then the condition for zero mean curvature is [45,46]:

V 3PA

(b)

R2PA

0.6

0.4

0.2

0.0 2

3

4

5

6

7 8 9

0.1

2

1

R = ηPE phase/ηPA6 Fig. 11. The various morphology regions reported as a function of PA6 volume fraction (in ordinate) and viscosity ratios (in abscissa), taken at 100 s1 (in a) or at 3500 s1 (in b). All symbols are experimental points. The continuous curves corresponds to the phase inversion model of Paul and Barlow (dashed) and based on the Krieger-Dougherty description of the viscosities (Eq. (3)) (plain curves).

to experimental data. Thus, the results reported in Fig. 11 are consistent with Eq. (3). The phase inversion predicted by Eq. (3) has been also reported on the ternary diagrams in Fig. 8. For HDPE3 (Fig. 8a), the model prediction only depends on the PA6 volume fraction and thus follows a line parallel to the C/HDPE edge, since the viscosity ratios are similar at 100 and 3500 s1 and since the MA-g-HDPE content does not affect the PE phase viscosity (Fig. 3). Conversely, for HDPE2, increasing the compatibilizer amount decreases the viscosity ratio (Fig. 3), which explains the curvature of the phase inversion curve predicted by the model in this case. The predicted curvature is less pronounced using the viscosity ratios at 3500 s1. In fact, at this higher shear rate, MA-g-HDPE and HDPE2 have the same viscosities (Fig. 3). 5.2. Curvature at interfaces From the above discussion, it may be concluded that the amount of compatibilizer does not have a strong effect on

185

¼

16V 3PE R2PE

ð4Þ

in which V PA (resp. V PE ) is the average molar volume of PA chains (resp. of each PE arm) and R2PA and R2PE are the corresponding squared radii of gyration. With V PA ¼ M nPA =qPA where q is the density) (and the same for each PE arm) and using the values for the ratios R2 =M n tabulated by Fetters et al. [47], it is estimated that V 3PA =R2PA ffi 1:17M 2PA and V 3PE =R2PE ffi 2:0M 2PE (with lengths in Å and M in g mol1), which gives for the zero mean curvature condition MPA  5:3M PE . Experimentally, we have M PA  27; 000 g mol1 and M PE  4900 g mol1 (number average mass of a chain portion between graft points), which gives MPA  5:5M PE , close to the theoretical ratio estimated above. Thus, the grafted copolymer structure resulting from the compatibilization reaction does not induce a strong curvature at interfaces. 5.3. Compatibilization and domain sizes On increasing the compatibilizer amount, the characteristic domain sizes decrease (from typically 10 lm in blend e down to 1 lm in blend c, for example), as expected from the associated decrease of the surface tension. However, in compatibilized systems, the observed sizes are not coherent with the estimated graft copolymer content. Considering the blend shown in Fig. 7(3) (composition PA6/HDPE1/C: 50/30/20 vol%), which has a ratio [MA]/ [NH2]  1, for an estimated reaction conversion ratio of about 80% (see Fig. 6), 80% of PA6 chains are grafted to MA-g-HDPE, while 32 vol% of the PE phase (i.e. 80% of the 40 vol% of C within the PE phase) is constituted of the PE blocks of the graft copolymer. It results that the graft copolymer is the majority component in the material. Thus, at thermodynamic equilibrium, with all copolymers located at interfaces, the whole system should exhibit a morphology at the nanometric scale (the radius of gyration of constitutive blocks being of the order 10–15 nm). Considering the particular case shown in Fig. 7(3), the fraction of graft copolymer located at the micrometer scale,

186

A. Argoud et al. / European Polymer Journal 50 (2014) 177–189

2 μm

t ≈ 20 nm

a ≈ 4 μm

a/2 ≈

Fig. 12. Schematics of a ‘unit cell’ in a co-continuous morphology. a=2 is the characteristic domain size. t roughly represents the thickness of the layer occupied by copolymers at interfaces.

140

typical size [μm]

120 100

non-compatibilized

80 60 40 20

compatibilized 0 0

5

10

15

20

annealing time [min] Fig. 13. Typical domain size as a function of the annealing time in static conditions, in blends 5 (compatibilized) and 6 (non-compatibilized).

co-continuous morphology, can be roughly estimated as follows (see Fig. 12). For a typical domain size of the order a=2  2 lm, the average size of a ‘unit cell’ is a  4 lm. From the estimated radius of gyration of PA chains [47], the interfacial thickness (on PA side) may be estimated to be of the order n  10 nm, which is coherent with an average interfacial area per chain of order 4–5 nm2 [48,49]. Then, within a ‘unit cell’, the interface area is of the order 2:5a2 and the interface volume on the PA side, i.e. the volume occupied by PA blocks of the graft copolymer is of order 2:5a2 n. The volume fraction of PA grafted to interface (refered to the PA fraction) is thus of the order 2:5n=ðauPA Þ, that is of the order 1%, to be compared to the estimated 80% of reacted species. Therefore, assuming a high reaction rate, of the same order as those estimated in Section 3.2, only a small fraction (of the order a few % at most) of the formed copolymer is effectively located at the interfaces corresponding to the large scale morphologies as observed in Fig. 7. Similarly, blend a (Fig. 8a) is very near the stoechiometric point [MA]/[NH2] = 1 and does not contain non-reactive HDPE. Thus, the observed size of the order 1 lm is not compatible with the estimated amount of 80% overall volume fraction of the material being in the form of graft copolymer. Thus, even though the compatibilization reaction has a high conversion degree, only a relatively small fraction of

the formed copolymer is efficient to form the well-defined (flat at the molecular scale) interfaces, observed at the micrometer scale, within the process conditions which have been used. This rises issues as regards the morphologies observed at the micrometric scale (or even larger scale in some cases), since copolymer molecules must be located at interfaces, which imposes some structuration at the scale of the polymer chains themselves, i.e. at the scale of 10–20 nm. Therefore, the observed morphologies are strongly non-equilibrium morphologies, with characteristic sizes governed by the process rheology, similar to those observed in non-compatibilized blends, with smaller sizes however, due to a reduced interfacial tension. A relatively small fraction of the graft copolymer locate at interfaces of the large scale morphologies, in coexistence with a reservoir of copolymer, probably in the form of small (nanometric) size droplets (or micelles). This small fraction may evolve during annealing/melting and/or shearing associated to subsequent processing of the materials. Thus, in order to check the influence of this reservoir of interfacial agent, the evolution of the observed large scale morphologies has been studied under quiescent annealing and shearing. Annealing under static conditions was performed in DSC at 290 °C during 5 and 15 min under Helium flow. When applied, controlled shearing was performed subsequently in a capillary rheometer at 200 s1 and 2000 s1 at 290 °C. In non-compatibilized systems, the characteristic size of the morphologies increases drastically by coalescence upon annealing, as it is well known [50]. An example of the evolution of the typical domain volume as a function of annealing time is plotted in Fig. 13 for the system PA6/HDPE2 60/40 vol%. Also, in the case of fibrillar morphologies, a relaxation of the domain anisotropy is observed. The evolution of the characteristic domain size is reported in Fig. 13 for the compatibilized system PA6/HDPE2/ C 60/32/8 vol%. In compatibilized systems, the morphologies do not evolve towards a nanometer scale morphology, as could be expected from the large reservoir of copolymers present in the system. As expected, the stretched morphologies, being non-steady state morphologies, show a stretching release after static annealing. However, the domain size remains stable whatever the annealing time. This indicates that, even though thermal equilibrium would correspond to a morphology with domain sizes comparable to chain dimensions (of order a few 10 nm), the free energy barrier to overcome in order to reach this hypothetic final, equilibrium state, is extremely high in the used conditions. When a subsequent shear is applied, morphologies initially close to the co-continuous regions systematically evolve toward Co-continuity, leading to a broadening of this region. This evolution corresponds to coalescence of large size fibrils and elongated droplets.

6. Conclusion PA6/HDPE blends compatibilized in reactive extrusion have been studied over a broad range of compositions. Spe-

A. Argoud et al. / European Polymer Journal 50 (2014) 177–189

cifically, blends with large amounts of compatibilizer were studied. It was checked (in co-continuous morphologies with HDPE3) that the compatibilization reaction has a large conversion ratio. Morphologies with two characteristic sizes have been observed. At the larger scale, the characteristic domain sizes vary from 10 lm down to 1 lm, specifically in the case of co-continuous morphologies, depending to the compatibilizer/HDPE ratio. The types and sizes of the morphologies are determined by process rheology. Composition (the volume ratio PE phase/PA6) is the predominant system parameter which determines the type of morphology. The type of morphology does not depend strongly on the amount of compatibilizer (for a compatibilizer structure which does not induce a strong curvature at interface). As expected, by increasing the compatibilizer amount, the characteristic size becomes smaller. Compatibilizer also suppresses coalescence and stabilizes the micrometer scale morphologies. The major observation emphasized here is that in systems which contain a large amount of compatibilizer, only a very small fraction of the formed copolymer is located at the well defined interfaces of the larger scale (micrometer) morphologies. Thus, even in the presence of a large amount of compatibilizer, the morphologies are still driven by process rheology. Introducing a large fraction of compatibilizing agent is not sufficient to obtain morphologies at the nanometric scale (which would be expected at equilibrium), in the standard processing conditions which have been used. Processing rheology by itself does not allow organizing the whole amount of formed block copolymers in the form of well-defined interfaces. Thus, the non-equilibrium large scale morphologies coexist with some nanodispersions of the remaining copolymers within the large scale domains. Acknowledgements This work has been funded by the Duramat project of the Axelera pole of competitiveness of the Region RhôneAlpes and funded by FUI. We thank Solvay Research and Development teams at Lyon Research Center (V. Curtil, C. Basire, L. Odoni, N. Bulgarelli, F. Le Guyader and L. Tribolet). We thank the LCPP lab in CPE Lyon (O. Boyron) for high temperature SEC experiments and N. Peduto (from Solvay). Appendix A. Estimation of the solubility parameters for PE and Maleic Anhydride A.1. Miscibility of MA-g-HDPE in HDPE Compatibility at large scale (a few micrometers) between HDPE and MA-g-HDPE does not rule out a possible, at least partial, micellar organization of MA moieties in HDPE at the molecular level. An effective Flory interaction parameter v12 between Polyethylene monomers (entity 1) and MA entities (2) may be estimated from Hildebrand’s solubility parameters d1 and d2 [51]:

v12 ¼

pffiffiffiffiffiffiffiffiffiffiffi

v1v 2 RT

ðd1  d2 Þ2

ð5Þ

187

v 1 and v 2 , the molar volumes of molecular entities 1 (ethylene monomers) and 2 (MA groups) respectively, were calculated using the values qPE ffi 0:72 g cm3 for PE chain density at 290 °C and qMA ffi 1:5 g cm3 for MA. For MA, which is a polar entity, the Hansen’s method, in which solubility parameters d are splitted into three components (dispersive, polar and hydrogen bonding interactions), was used [51]: – Dispersive (non-polar) interactions: dd ¼ RvF2di . pffiffiffiffiffiffi2ffi RF pi – Polar interactions: dp ¼ v 2 . qffiffiffiffiffiffiffi – Hydrogen bonding interactions: dh ¼ RvE2hi with v 2 ¼ 65 cm3 mol1. The group contribution method was applied using Van Krevelen values at 25 °C, which gives the following factors [51]: – Two ACOA groups: F d ¼ 290 J1/2 cm3/2 mol1; F p ¼ 700 J1/2 cm3/2 mol1; Eh ¼ 2000 J mol1; – One = CHA groups: F d ¼ 200 J1/2 cm3/2 mol1; F p ¼ 0 J1/2 cm3/2 mol1; Eh ¼ 0 J mol1; – One AOA group: F d ¼ 100 J1/2 cm3/2 mol1; F p ¼ 400 J1/2 cm3/2 mol1; Eh ¼ 3000 J mol1. According to these factors, the three terms of Hansen are dd ¼ 13:5 J1/2 cm3/2; dp ¼ 17:8 J1/2 cm3/2; dh ¼ 10:4 J1/ 2 cm3/2. Then, the solubility parameter is calculated as:

d2 ¼ d2d þ d2p þ d2h

ð6Þ

which gives an estimated solubility parameter d ¼ 24:6 MPa1/2 for MA groups. For Polyethylene, several values for the solubility parameter ranging from 16 to 18.4 MPa1/2 (obtained from calculation or from experiments) have been tabulated [51]. The group contribution method can also be applied. The tabulated molar attraction constant for ACH2A groups is F ¼ 280 MPa1/2 cm3 mol1. Thus, for an ethylene monomer (i.e. two ACH2A groups), with a density of 0.91 g cm3, the solubility parameter estimated in this way is 18.2 MPa1/2, which is within the range of tabulated values. According to these values, the effective interaction parameter is estimated from Eq. (5) to range between v12 ffi 0:6 (for dPE ffi 18:4 MPa1/2) and v12 ffi 1:2 (for dPE ffi 16 MPa1/2). Thus, MA-g-HDPE should be considered as a copolymer containing entities not miscible with PE. Results of dynamical rheometry measurements at 290 °C in the linear viscoelasticity domain for HDPE3, neat MA-g-HDPE (C) and phase E3 (HDPE3/C, 60/40 vol%) are shown in Fig. 14. It is clear that the presence of MA groups grafted along the chains have a quite strong effect on the rheology of the material as compared to unmodified HDPE, in the low frequency regime. An upward inflexion (or possible onset of a plateau) is observed in the storage modulus G0 at low frequency in both neat C and E3, in contrast to the neat reference HDPE3 in which no inflexion is observed. This inflexion may be attributed to structuration at the molecular scale, i.e. it could possibly indicate the presence of micelles of MA moieties bridged by PE chains [52,53]. An average volume d3 surrounding each micelle may be roughly estimated from the relationship d3  kT=G0 , where G0 is the value at onset of the shoulder. With G0  103 Pa at onset of shoulder, the average volume surrounding each

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A. Argoud et al. / European Polymer Journal 50 (2014) 177–189

10

4 6 4

G' [Pa]

2

10

3 6 4 2

10

2

PE Phase 3 HDPE-g-AM HDPE 3

6 4 4 5 6

2

1

3

4 5 6

2

10

3

4 5 6

100

ω [rad.s-1] Fig. 14. Elastic modulus G0 versus frequency at 290 °C for HDPE3 (plain curve), C (black diamonds) and phase E3 (HDPE3/C 60/40 vol%) (open triangles). Data obtained in cone plate dynamical rheometry measurements at 290 °C in the linear viscoelasticity domain (shear amplitude 4%).

micelle is estimated to be of the order d3  4  1024 m3, which gives an average distance between micelles d  16 nm. Not that the slope of G0 for HDPE (in log–log plot) is close to one, in contrast to the value 2 expected in the low frequency limit (beyond the terminal relaxation time). This is most certainly due to the relatively high polydispersity of this material (see Table 1). There is an average of about 350 monomers (ACH2ACH2A), or molar mass 9800 g mol1, between each grafted MA group, which corresponds to a gyration radius hR2 i1=2 ¼ ð1:25M n Þ1=2 of the order 11 nm [47]. This radius can possibly reach 20 nm when PE segments are stretched to some extent. Thus, the distance between micelles is roughly compatible with the average size of PE segments between grafted MA groups. The neat compatibilizer contains 1 wt% of MA moieties, that is a volume fraction of MA moieties uMA  0:63%. The average volume of the micelles can then be estimated as V micelle  uMA d3  25 nm3 and the aggregation number as N  V micelle qMA N a =M, with M ¼ 98 g mol1 the MA molar mass and N a Avogadro’s number. N is estimated to be of the order of 250. As expected, the plateau of the modulus is less pronounced, with a seemingly lower value, in E3 (HDPE3/C, 60/40 vol%) than in neat MA-g-HDPE, which would correspond to a larger volume surrounding MA micelles, due to dilution of MA moieties in the PE phase. Indeed, the variation in the plateau value corresponds very roughly to a factor 2– 3, which is coherent with the dilution ratio of MA groups by a factor 2.5 in E3 and which indicates that the micelle size would not change significantly. On the other hand, note that the storage modulus of the E3 mixture is a little higher than those of its components. This may also be related to some small-scale structure of this phase. References [1] Utracki LA, editor. Polymer Blends Handbook. Dordrecht (Boston, London): Kluwer Academic Publishers; 2003.

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