Effect of Dual Reactive Compatibilizers on the Formation of Co

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Effect of dual reactive compatibilizers on the formation of co-continuous morphology of low density polyethylene/ polyamide 6 blends with low polyamide 6 content Dawei Ren, Zhaokang Tu, Changjiang Yu, Hengchong Shi, Tao Jiang, Yingkui Yang, Dean Shi, Jinghua Yin, Yiu-Wing Mai, and Robert Kwok-Yiu Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00304 • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on April 1, 2016

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Industrial & Engineering Chemistry Research

Effect of dual reactive compatibilizers on the formation of co-continuous morphology of low density polyethylene/polyamide 6 blends with low polyamide 6 content

Dawei Rena, Zhaokang Tua, Changjiang Yua, Hengchong Shib,d*, Tao Jianga, Yingkui Yanga, Dean Shia*, Jinghua Yinb, Yiu-Wing Maic, Robert K.Y. Lid

a. Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Collaborative Innovation Center for Advanced Origanic Chemical Materials, Faculty of Materials Science and Engineering, Hubei University, Wuhan, 430062, P. R. China.

b. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences

c. Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering J07, The University of Sydney, Sydney, NSW 2006, Australia

d. Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, PR China

Abstract The coexistence of two kinds of reactive compatibilizers at the interface in immiscible LDPE/PA6 blends, namely, polyethylene graft maleic anhydride (PE-g-MAH) with a long backbone length and polybutadiene graft maleic anhydride (PB-g-MAH) with a short backbone length, promoted the formation of a flat interface and induced a cocontinuous morphology in blends containing 30 wt.% PA6. The relationships between the contents and content ratios of the dual reactive compatibilizers as well as the morphologies of the resulting LDPE/PA6 blends are examined quantitatively. The morphologies of the blends are characterized by SEM and TEM combined with a selective solvent extraction process. It is found that in these systems, when the total content of the dual compatibilizers is within 20~30 wt.% and the content ratio of PE-g-MAH and PB-g-MAH is within 2:1~3:1, the minor PA6 phase can also form a co-continuous morphology in the LDPE matrix. The balance between the stability (determined by in situ formed PE-g-PA6 grafted copolymer with long backbone length) and flexibility of the flat interface (determined by in situ formed PB-g-PA6 *

Corresponding authors: D. Shi ([email protected]); H. Shi ([email protected])

ACS Paragon Plus Environment


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grafted copolymer with short backbone length) in the compatibilized LDPE/PA6 blends is the key factor that controls the formation of the co-continuous morphology with low PA6 contents.

Keywords: co-continuous morphology, dual reactive compatibilizers, flat interface 1. Introduction

Co-continuous polymer blends are different to droplet/matrix blends, such as rubber toughened systems 1, since all components are continuous within a given volume. The properties of each component in co-continuous blends can be combined without any loss and may even possess synergistic effects 2. In recent studies, polymer blends with co-continuous phase morphologies have been used as polymer-based conducting materials

3, 4

, in biological tissue engineering

5, 6

, for controlled drug release

7, 8

, and

micro-porous films 9.

Unlike the spinodal decomposition mechanism for the formation of co-continuous morphology in miscible polymer blends, the formation mechanism of co-continuous morphology in blends with originally immiscible polymer pairs still remains unclear. Coalescence of preformed dispersed particles or fibers to networks 10, 11 and the sheet formation followed by sheet breakup to a network with extended structures 12 are two main widely accepted mechanisms now. In the past few decades, many studies were conducted on the effects of interfacial tension

13

, viscosity and viscosity ratio

14

,

shearing rate or force 15, feeding sequential 16, and mixing time17, 18 on the formation of co-continuous morphology in uncompatibilized immiscible polymer blends. Actually, these co-continuous morphologies are thermal dynamically unstable and can become coarsened or even degrade to the droplet/matrix morphology under high temperature annealing or secondary processing19-22. Hence, whether the co-continuous morphology can be obtained or not in uncompatibilized immiscible polymer blends is determined by the stability of the elongated dispersed fibrils23. In uncompatibilized


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immiscible polymer blends, the deformability of dispersed droplets is determined by the capillary number Ca which is given by Equation (1):

Ca = where

σ is interfacial tension,

η mγ& R0 σ

(1)

ηm matrix viscosity, γ& shear rate, and R0 starting

sphere radius. While the break-up time tb of the fibril structure can be calculated by Equation (2)24:

 ηmB tb =   Ω(l, λ )σ

  0.8B    ln    2α 0 

(2)

where B is the fiber diameter, α 0 is amplitude of the initial distortion and Ω (l , λ ) is the Tomotika function25. HighCa

and longtb

favor the formation of a stable

co-continuous morphology. High matrix viscosity and/or the low thread viscosity should result Ω in(llow , λ) of

tb and lead to long breakup times. So, to obtain high values

, from Eq. (2), the matrix viscosity should be high and/or the thread viscosity

low.

The situations in compatibilized polymer blends are more complex. Even though pre-made or in-situ formed compatibilizers can reduce the interfacial tension, increase interfacial strength and favor the formation of a stable co-continuous morphology, the composition ratio range that a co-continuous morphology can be generated is rarely increased, 26 but further shrunk, 27, 28 compared to those uncompatibilized blends since compatibilizers at the interface will prohibit the coalescence of the minor phase 29, 30.

Obviously, in compatibilized polymer blends, the role of the compatibilizer does not only need to reduce the interfacial tension and make fine dispersions31-40, but it also has to enhance the interface strength and induce the formation of a flat interface 41, 42 when co-continuous morphologies are obtained. In this circumstance, a compatibilizer with asymmetric structure should be used. Pu et al. utilized pre-made PMMA-g-PS block copolymer to compatibilize PS/PMMA blends42. When symmetric copolymer PMMA110-g-PS110 was used, co-continuous morphology could only be formed with



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45-55% PS in the blend; but when asymmetric copolymer PMMA110-g-PS433 was used as compatibilizer, a co-continuous morphology could be obtained with only 16% PS. However, preparing stable co-continuous morphologies in reactive compatibilized immiscible blends within a wide range of composition ratio or with low minor phase content remain a challenge. According to the self-assembly theory43, 44, if the in-situ formed graft copolymer is favored to form a flat interface, the grafted chain should be much longer than the trunk chain. In this case, such graft copolymer with asymmetric structure usually has a weak polymer interface or it is even dragged into one polymer phase during melt mixing, forming micelles and losing their compatibilizing effect 45. In 2002, Leibler et al. proposed a novel method

46

to prepare PE/PA6 blends with

stable co-continuous morphology in which the minor PA6 content was only 20 wt.% by using special maleic anhydride functionalized polyethylene (MAH-PE) as the compatibilizer. The broad polydispersity of the MAH-PE copolymer was believed to be the key factor for formation of co-continuous morphology in their PE/PA6 blends, which means a certain amount of MAH-PE with high molecular weight must exist. But there was no discussion on the effect of MHA-PE with different polydispersity on generation of the co-continuous morphology. Nonetheless, this method was limited by its special raw materials. MAH-PE copolymer was specifically synthesized and the molecular weight of PA6 was too low for a commercial product. In our previous work, a dual compatibilizer system, PE-g-MAH with a long trunk chain (i.e., Mw ≈ 2.8×105 g / mol ) and PB-g-MAH with a short trunk chain (i.e., M n = 3 ×103 g / mol ),

was used as compatibilizers with broad molecular weight polydispersity in LDPE/PA6 blends. Co-continuous morphology was obtained when the PA6 content is 25 wt.%. The co-continuous morphology of these compatibilized LDPE/PA6 blends could be maintained after annealing for 15 minutes above the melt temperature of PA6.

The dual reactive compatibilizers, PE-g-PA6 and PB-g-PA6, however, have opposite effects on the formation of the co-continuous morphology in LDPE/PA6 blends. The existence of PE-g-PA6 will reduce the curvature of the interface between LDPE and


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PA6, and form spherical domains of the minor PA6 phase

47

. Too much PB-g-PA6

with short PB backbone will greatly reduce the interfacial strength and interrupt the shear stress transfer from the LDPE matrix to the PA6 domains, which will not favor forming elongated PA6 particles. Herein, in this work, the quantitative relationship between the morphology of LDPE/PA6 blends and the contents of the dual reactive compatibilizers is studied systematically. The optimal range of the content and content ratio of dual compatibilizers to form co-continuous morphology in LDPE/PA6 blends is proposed.

2.Experimental Section 2.1 Materials Low density polyethylene (LDPE-7042, Mw = 2×105 g / mol ) was bought from Daqing Petroleum & Chemical Co. Low molecular weight functionalized rubber, polybutadiene grafted with maleic anhydride (PB-g-MAH, MAH: 10 wt.%, M n = 3×103 g / mol ) was provided by Sartomer Asia Ltd. Polyamide 6 (PA6-M2800,

Mn = 2×104 g / mol ) and PE-g-MAH (MAH: 1.0 wt.%, Mw ≈ 2.8×105 g / mol ) were

purchased from Guangdong Xinhuimeida Nylon Co. Ltd and Beijing Sunred Plastics Co., respectively.

2.2 Sample Preparation All the materials except PB-g-MAH were dried in an oven at 80 oC overnight before testing. First, low molecular weight PB-g-MAH was melt blended with LDPE and PE-g-MAH by using a HAAKE PolyLab internal mixer at 200 oC to prepare the polyolefin master batch. Second, the pre-polyolefin alloy was further melt mixed with PA6 in a Haake Polylab co-rotating twin screw extruder (screw diameter D* = 16 mm, L/D* ratio = 40, where L is screw length). The temperature profile along the extruder was 170-190-210-240-240-230-215 oC and the screw speed was set at 100 rpm.



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The contents of the dual compatibilizers were varied based on the following two rules: (a) the content of one component was fixed while the other component varied within a certain range; and (b) the content ratio between the two components was fixed while the total amount of the dual compatibilizers varied within a certain range. Further, the content of LDPE was altered according to the change of the dual compatibilizers to maintain a constant 30 wt.% of PA6 within all blending systems. The recipes of the blends are given in Table 1.

2.3 Sample Characterization 2.3.1 Selective Solvent Extraction Experiment The extruded strips after drying were immersed in formic acid, i.e., a good selective solvent for PA6, for a few days until the sample weight was unchanged. The initial and final weights of the sample were recorded. For simplicity, no matter whether fully co-continuous structure is formed or not. Following the method reported by Favis and Macosko48-50, the value D calculated by Eq. (3) is defined as the degree of continuity in this work:

D=

Winitial − W final Winitial × C

(3)

where W is weight, subscripts “initial” and “final” are self-explanatory, and C is wt.% of the minor phase (and fixed at 30 wt.%). All the samples for formic acid extraction experiments were over 3 g with a thickness of ~2 mm to eliminate errors brought by the intrinsic flaws of this method

51

. Five pieces of each sample are tested and the

average values are recorded.

Table 1. Recipes of LDPE/PE-g-MAH/PB-g-MAH/PA6 blends (PA6 content is fixed at 30 wt.%) Sample 1 2 3 4 5

LDPE (wt.%) 64 61 58 54 51

PE-g-MAH (wt.%)

PB-g-MAH (wt.%)

0 3 6 10 13

6 6 6 6 6


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6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

48 38 32 55 53 51 50 48 63 59 52.8 48 43.6 37 26

16 26 32 13 13 13 13 13 5 8 12.4 16 19.2 24 32

6 6 6 2 4 6 7 9 2 3 4.8 6 7.2 9 12

2.3.2 Morphology Characterization The microstructures of the polymer blends were studied with a JEOL-6510LV SEM at an accelerating voltage between 15 and 30 kV. Transverse sections of cryo-fractured sample surfaces were obtained in liquid nitrogen, etched with formic acid for 3 h, and, after drying, were spluttered with a thin layer of gold. TEM photographs were taken at a TEM-Zeiss EM 900 transmission electron microscope (Zeiss, Germany) operating at an accelerating voltage of 80KV. The ultrathin specimens for the TEM-observations were cut by ultramicrotomy with a diamond knife at -120 oC. An aqueous solution of phosphotungstic acid (PTA) was used to selectively stain PA6 phase in the blend, where PA6 phase had a darker image. An in-house software was applied to analyze the SEM and TEM images to derive the characteristic size of the morphology. The detailed methods for characterizing droplet-in-matrix and co-continuous morphology are described in previous works

41,42

.For droplet-in-matrix morphology, dispersed

particle diameter is used and for co-continuous morphology, the width of elongated PA6 rods is used to represent the characterized size of PA6 phase.

2.3.3 Rheological Measurement



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The rheological behavior was investigated by using a Physica-301 rheometer. A 25 mm parallel plate arrangement was adopted. The measuring temperature was 240 °C. Specimens were prepared by molding in a hot press at 240 °C and 5 MPa. Prior to compression molding, pellets were dried in a vacuum oven overnight. The rheometer chamber was purged with dry nitrogen during measurement to avoid degradation. A frequency range of 0.1-100 rad/s and a strain of 3% were supplied during the measurement. A strain sweep was carried out to determine the strain limit for linear viscoelastic responses.

3. Results and Discussion Morphology Variation of LDPE/PA6 (70/30) Blends (I) One component in dual compatibilizers is fixed and another is varied Fig. 1 shows SEM images of samples 1-8 in Table 1 with increasing concentration of PE-g-MAH from 0 (sample 1) to 32 wt.% (sample 8) while fixing the content of PB-g-MAH at 6 wt.%. From Fig. 1a, when only one compatibilizer PB-g-MAH is used, even a small part of the PA6 phase exists as isolated fibrils, the blend displays the droplet-matrix morphology with a degree of continuity D of 20% (see Fig. 2). When the PE-g-MAH is increased to 3 wt.% (sample 2), more PA6 phase shows elongated rods (Fig.1b) but D is only 25%. As PE-g-MAH content becomes 6 wt.% (sample 3), most PA6 elongated rods coalesce and form long inter-crossed rods, but relatively large amount of randomly distributed PA6 droplets also exists (see arrows in Fig. 1c); and D rises to 80.8% approaching full continuity. When PE-g-MAH contents are further increased to 10 (sample 4 D = 94.3%), 13 (sample 5 D = 111.9%) and 16 wt.% (sample 6 D = 106.2%), these samples show perfect co-continuous morphology with no isolated PA6 particles (Figs. 1d to 1f). Here, D of samples 5 and 6 obtained by the solvent extraction method exceeds 100% caused by the extraction of the polyolefin phase encapsulated within the PA6 phase 23. Comparing the domain size in sample 5 and 6 with full co-continuous morphologies, the domain size in sample 6 is a little bit larger than those in sample 5. This phenomenon seems contradictory to our


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common sense for compatibilized polymer blends, where the domain size decrease with the increase of compatibilizer content. In our system, because the total compatibilizer contents are much higher than commonly used in commercial products, a lot of in-situ formed PE-g-PA6 copolymer might not locate at the interface but form micelles in PE matrix 52. The micelle number will increase along with further increase of PE-g-MAH content (see Figure 1g), which will lower the viscosity of PE matrix which results in relatively larger domain size in the according blends.

a

b

c

d

e

f



g

h

Figure 1. SEM images of LDPE/PA6 blends with different concentration of PE-g-MAH content but same PB-g-MAH content at 6 wt.% and PA6 content 30 wt.%: (a) PE-g-MAH 0 wt.%; (b) PE-g-MAH 3 wt.%; (c) PE-g-MAH 6 wt.%; (d) PE-g-MAH 10 wt.%; (e) PE-g-MAH 13 wt.%; (f) PE-g-MAH 16 wt.%; (g) PE-g-MAH 26 wt.%; and (h) PE-g-MAH 32 wt.%. The samples are etched by formic acid.

140 PB-g-MAH 5.6wt%

120 100

D (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 60 40 20 0 0

5

10

15

20

25

30

35

PE-g-MAH content(wt%)

Figure 2. Degree of continuity for LDPE/PA6 blends as a function of PE-g-MAH content with fixed PB-g-MAH (6 wt.%) and PA6 (30 wt.%) content.

Obviously, a certain amount of PE-g-MAH has a positive effect on the formation of a co-continuous morphology for these compatibilized LDPE/PA6 blends. Excessive PE-g-MAH content, however, can destroy this morphology. The degree of continuity D of sample 7 with 26 wt.% PE-g-MAH is reduced to 71.8%. Some elongated PA6

particles are dispersed in LDPE matrix isolatedly. In sample 8, with 32 wt.% PE-g-MAH, a complete droplet-matrix morphology is obtained (Fig. 1h) without any


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trace of the stretched PA6 phase. Here, the droplet sizes of PA6 phase are similar to those found in in situ compatibilized PE/PA6 blends53, 54, which indicates that the effect due to the PB-g-MAH is fully suppressed. Thus, in summary, in the dual compatibilized LDPE/PA6 blends, when PB-g-MAH is fixed at 6 wt.%, the degree of co-continuity of the blends first increases with increasing PE-g-MAH content and then drops as it exceeds 16 wt.%. (See Fig. 2)

The effects of PB-g-MAH content on the formation of co-continuous morphology in LDPE/PA6 blends are studied by fixing PE-g-MAH at 13 wt.%, a value that is selected purposely mid-way between 10 wt.% (sample 4) and 16 wt.% (sample 6) which have excellent co-continuous morphologies when the PB-g-MAH content is 6 wt.%.

a

b

c

d



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e

Figure 3. SEM images of LDPE/PA6 blends with different PB-g-MAH content but same PE-g-MAH content of 13 wt.% and PA6 content of 30 wt.%. The images (a) PB-g-MAH 2 wt.%t; (b) PB-g-MAH 4 wt.%; (c) PB-g-MAH 6 wt.%; (d) PB-g-MAH 7 wt.%; and (e) PB-g-MAH 9 wt.%. The samples are etched by formic acid.

In sample 9, Fig.3a, when PB-g-MAH is fixed at 2 wt.%, a local continuous PA6 phase can be found in the middle although most of the areas are full of PA6 droplets. The degree of continuity obtained via the solvent extraction process is 49.6% (Fig. 4), which is much higher than that in commonly compatibilized PE/PA6 alloys53, 54. When PB-g-MAH content is further increased to 4 wt.% or more, i.e., samples 10 to 13, fully co-continuous morphology can be obtained (Figs. 3b to 3e). The degrees of continuity are 103.3, 111.9, 108.2 and 103.2%, respectively (Fig.4). The only difference among these samples is the size of the minor PA6 phase. Increasing the PB-g-MAH content increases the size of the PA6 phase. This result can be explained below: (a) short trunk chain of PB-g-PA6 graft copolymer will give weak polyolefin/PA6 interface and cannot effectively enhance the interfacial strength to promote the formation of the co-continuous PA6 phase 23; and (b) reaction of MAH in PB-g-MAH with amino groups in PA6 molecules will increase the shear viscosity of PA6 phase (see later part of this paper) and make it more difficult to deform under shear, which is believed to be more difficult to form co-continuous morphology

14

.

However, in our experiments, for 2 wt.% < PB-g-MAH < 9 wt.%, fully co-continuous morphologies can be obtained.


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130 120

PE-g-MAH 12.6wt%

110 100

D (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 80 70 60 50 40 2

3

4

5

6

7

8

9

10

PB-g-MAH content(wt%)

Figure 4. Degree of continuity of LDPE/PA6 blends as a function of PB-g-MAH content with fixed PE-g-MAH (13 wt.%) and PA6 (30 wt.%) content.

(II) The content ratio between PE-g-MAH and PB-g-MAH in the dual compatibilizers is fixed and their total amount is varied Here we start from a recipe in which most perfect co-continuous morphology can be obtained (sample 17). The contents of PE-g-MAH and PB-g-MAH are 16 wt.% and 6 wt.%, respectively, and the total amount of dual compatibilizer is 22 wt.%. Here, the total amount of the dual compatibilizers is proportionally reduced or increased from 0.3, 0.5, 0.8 to 1.2, 1.5 and 2-folds with the PE-g-MAH to PB-g-MAH ratio constant as in sample 17. However, to maintain the PA6 content at 30 wt.%, the LDPE content is varied accordingly. SEM images and degree of continuity for these samples are shown in Figs. 5 and 6.

a

b



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c

d

e

f

g

Figure 5. SEM images of LDPE/PA6 blends with different total content of the dual compatibilizers but the content ratio between PE-g-MAH and PB-g-MAH is fixed around 8:3 (a) PE-g-MAH 5 wt.%, PB-g-MAH 2 wt.%t; (b) PE-g-MAH 8 wt.%, PB-g-MAH 3 wt.%; (c) PE-g-MAH 12.4 wt.%, PB-g-MAH 4.8 wt.%; (d) PE-g-MAH 16 wt.%, PB-g-MAH 6 wt.%; (e) PE-g-MAH 19.2 wt.%, PB-g-MAH 7.2 wt.%; (f) PE-g-MAH 24 wt.%, PB-g-MAH 9 wt.%; and (g) PE-g-MAH 32 wt.%, PB-g-MAH 12 wt.%; The samples are etched by formic acid.

In Fig. 5a, sample 14 shows a typical droplet/matrix morphology with a degree of continuity of 24.4% (Fig. 6). PA6 particles exist in almost standard spherical shapes. But, in samples 15 and 16 (Fig. 5b and 5c), the total content of the dual compatibilizers are 50% and 80% of that in sample 17, respectively, spherical particles and elongated fibrils of PA6 can be found in both blends. The number of


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fibrils increases gradually with total content of dual compatibilizer. Degrees of continuity of samples 15 and 16 reach 64% and 84.5% (Fig. 6), respectively. Increasing the dual compatibilizer content changes all PA6 particles into inter-crossed fibrils (formed by the coalescence of elongated PA6 rods) in sample 17 (Fig. 5d). This co-continuous inter-crossed fibrils morphology remains in sample 18, wherein the total compatibilizer content is 1.2-fold of that in sample 17 (Fig. 5e). However, if the total compatibilizer content is too high, i.e., 1.5-fold and 2-fold of sample 17, the degrees of continuity in samples 19 and 20 are reduced to 67.5% and 63.8% (see Fig. 6), respectively. Some isolated PA6 particles can be found in the SEM images (Fig. 5f and 5g). This phenomenon shows that excessive compatibilizers do not favor the formation of a co-continuous morphology in immiscible polymer blends. This trend is very similar to increasing PE-g-MAH content with a fixed PB-g-MAH content.

120

PE-g-MAH/PB-g-MAH 8/3

100

D (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 5

10

15

20

25

30

35

40

45

Total compatibilizers content (wt.%)

Figure 6. Degree of continuity for LDPE/PA6 blends as a function of the total content of the dual compatibilizers at a fixed content ratio of PE-g-MAH/PB-g-MAH = ~8/3.

These results can be explained by the increase of viscosity ratio when the total content of dual compatibilizers increases in LDPE/PA6 blends55,

56

. We have LDPE,

PE-g-MAH, PB-g-MAH and PA6 four components altogether in our LDPE/PA6



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system. Even in the LDPE/PE-g-MAH/PB-g-MAH polyolefin master batch phase separation may happen. (See Supporting Information). If PE-g-MAH and/or PB-g-MAH react with PA6, it will be located at the interface and increase the viscosity of the minor phase57-59; however, that part of the low molecular weight PB-g-MAH does not react with PA6 and exists in LDPE phase and decreases the viscosity of the polyolefin matrix. Clearly, in this case, the viscosity ratio cannot be simply defined as the ratio of viscosities of PA6 and LDPE, respectively. Although the accurate distribution of the compatibilizers cannot be detected and the real viscosity ratio is difficult to calculate, we can consider two extreme situations to evaluate the trend of the viscosity ratio of the modified dispersed phase to modified matrix after adding dual compatibilizers: (a) all compatibilizers and PA6 are considered as modified PA6 phase; and (b) all compatibilizers as well as LDPE are considered as modified PE phase. Modified PA6 phase A is the master batch of PA6 with the highest compatibilizer contents (PE-g-MAH32/PB-g-MAH12/PA630). Modified PA6 phase B is

the

master

batch

of

PA6

with

the

lowest

compatibilizer

contents

(PE-g-MAH5/PB-g-MAH2/PA630). Modified PE phase A is the master batch of LDPE with the lowest compatibilizer content (LDPE63/PE-g-MAH5/PB-g-MAH2). And modified PE phase B is the master batch of LDPE with the highest compatibilizer content (LDPE26/PE-g-MAH32/PB-g-MAH12). The number behind each

component

represents

PE-g-MAH32/PB-g-MAH12/PA630

their means

weight the

ratio, weight

for

example, ratio

of

PE-g-MAH:PB-g-MAH:PA6 is 32:12:30. The shear viscosities of the above four master batches as well as pure LDPE and PA6 are shown in Figure 7 and summarized in Table 2.


Page 17 of 29

Complex viscosity Pas

modified PA6 phase A modified PA6 phase B modified PE phase A modified PE phase B

10000

1000

1

10

100

-1

Angular Frequency s

(a)

Complex viscosity Pas

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PA6 LDPE

1000

100 1

10

100 -1

Angular Frequency s

(b) Figure 7. (a) Complex viscosities of modified PE phase and modified PA6 phasewith highest and lowest content of dual compatibilizer (PE-g-MAH/PB-g-MAH). (b) Complex viscosities of pure LDPE and PA6.

Figure 7 shows the complex viscosities of modified matrix and modified dispersed phase with the highest (44 wt.%) or lowest (7 wt.%) dual compatibilizer content in this study, respectively. Modified PA6 phase A (dual compatibilizer is 44 wt.%) and B (dual compatibilizer is 7 wt.%) represent all compatibilizers that exist with PA6 phase. Modified PE phase A (dual compatibilizer is 7 wt.%) and B (dual compatibilizer is 44



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Page 18 of 29

wt.%) represent all compatibilizers within LDPE matrix. The viscosities of each sample under processing conditionare summarized in Table 2. Here, the complex viscosity at 100s-1 are approximately used to present the shear viscosity under processing condition (screw speed 100rpm) according to Cox-Merz rule Heidemeyer’s method

23, 60, 61

and

. It can be seen that the highest viscosity ratio ( ρmax ) in all

62

the blends studied in this work should be that of the modified dispersed phase A and LDPE ( ρmax = 841/ 465 ≈ 1.8 ); and the lowest ratio ( ρ min ) is that of PA6 and modified matrix A ( ρmin = 436 / 539 ≈ 0.8). For practical compatibilized polymer blends, their viscosity ratios should lie in between ρmax

and

ρmin .

Table 2 Complex viscosity of PA6, LDPE and related modified dispersed phase and modified dispersed matrix with the highest and lowest dual compatibilizers

a

Sample

Viscosity at 100 s-1 (Pa.s)

PA6

436

LDPE

465

Modified PA6 phase A

841

Modified PA6 phase B

537

Modified PE phase A

539

Modified PE phase B

472

b


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c

d

Figure 8 TEM images of LDPE/PA6 blends with different total content of dual compatibilizers but the content ratio between PE-g-MAH/PB-g-MAH is fixed at ~8/3. (a) PE-g-MAH 5 wt.%, PB-g-MAH 2 wt.%t; (b) PE-g-MAH 12.4 wt.%, PB-g-MAH 4.8 wt.%; (c) PE-g-MAH 19.2 wt.%, PB-g-MAH 7.2 wt.%; (d) PE-g-MAH 32 wt.%, PB-g-MAH 12 wt.%; The samples are stained by PTA where PA6 phase is darker.

Figure 8 displays TEM images of LDPE/PA6 blends with different total content of dual compatibilizers but the content ratio of PE-g-MAH/PB-g-MAH is fixed at ~8/3. Fig. 8a shows that when the content of dual compatibilizer is relatively low, PA6 exist in typical droplet form. With increasing dual compatibilizer content, elongated PA6 particles (Fig. 8b) and co-continuous morphology (Fig. 8c) can be found. If the dual compatibilizer content is too high (44 wt.%), however, fully co-continuous morphology is broken into many elongated irregular structures. This trend agrees with the morphologies exhibited in the SEM images in Fig. 5. The characteristic sizes of PA6 phases in TEM (Fig. 8) and their corresponding data in SEM images (Fig.5) are summarized in Table S1 (See supporting information). It can be found that the PA6 particle sizes are almost equal in SEM and TEM when droplet-in-matrix morphology is formed; however, the characteristic sizes of elongated PA6 phase in these SEM images are relatively larger than those in TEM images (see Table S1). This can be explained by the fact that in the blends, LDPE particles are encapsulated in PA6 phase which is etched away by formic acid61, 63 (see arrows indicated in Figure 8b and 8d).

The cooperative and competitive effects of compatibilizers PE-g-MAH (with a high molecular weight backbone) and PB-g-MAH (with a low molecular weight backbone)



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Page 20 of 29

on forming a co-continuous morphology in immiscible LDPE/PA6 blends with low PA6 content is schematically summarized in Figure 9. Taking in to account experimental variations, the range of conditions for the formation of fully co-continuous

morphology

in

LDPE/PA6

blends

compatibilized

by

dual

compatibilizer with PA6 content at 30 wt.% are: (a) content ratio of PE-g-MAH/PB-g-MAH is between 3/1 and 2/1; and (b) total amount of dual compatibilizers is between 20 wt.% and 30 wt.%. If the total amount of dual compatibilizers is fixed but further increase the PE-g-MAH content, (so the content ratio of PE-g-MAH/PB-g-MAH exceeds 3/1), the interface curvature will be increased by the high content of the PE-g-PA6 copolymer with long trunk chain at the interface, yielding a droplet-matrix morphology. However, if the PB-g-MAH content is increased with fixed total compatibilizer amount, (so the content ratio of PE-g-MAH/PB-g-MAH is less than 2/1), slightly elongated PA6 rods are formed due to insufficient PE-g-MAH with long trunk chain, not favoring shear stress transfer.

By contrast, if the content ratio of PE-g-MAH/PB-g-MAH is fixed but varying the total amount of the dual compatibilizers, fully co-continuous morphology cannot be obtained when the total amount is lower than 15 wt.% or exceeds 30 wt.%. In the latter case, (>30 wt.%), the viscosity of the PA6 phase increases, which makes it more difficult to deform so that only irregular elongated droplet morphology is found. However, in the former case (

Effect of Dual Reactive Compatibilizers on the Formation of Co

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