Onset Reduction and Stabilization of Cocontinuous Morphology in

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Onset Reduction and Stabilization of Cocontinuous Morphology in Immiscible Polymer Blends by Snowman-like Janus Nanoparticles Wei You, and Wei Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02503 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Onset Reduction and Stabilization of Cocontinuous Morphology in Immiscible Polymer Blends by Snowman-like Janus Nanoparticles Wei You, Wei Yu* Advanced Rheology Institute, Department of Polymer Science and Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

Abstract The interfacial jamming of monolayer nanoparticles is often required to kinetically arrest the co-continuous morphology, which is not in favor of achieving high efficiency at low particle content. In this paper, we find that the shape asymmetry of the snowman-like Janus particles (JPs) has significant influence on the co-continuous morphology of polymer blends under the melt mixing process. The addition of 0.9 vol% snowman-like JPs can almost has the onset concentration of the cocontinuity in immiscible blends, which is much lower than the apparent interfacial jamming concentration. In addition, Janus particles show superior ability to stabilize the continuous morphology during annealing at high temperature. The mechanism behind the interfacial activity of asymmetric JPs is due to the decrease of the radius of the jamming curvature in the interfacial region as the shape asymmetry of the snowman-like JPs increases. This result implies a general strategy to prepare Janus nanoparticles for highly effective interfacial modification agent at low content, which can induce the dispersed phase continuity and suppress coarsening of co-continuous morphology simultaneously.

Keywords: Immiscible polymer blends; Co-continuous structure; Interface localization; Janus particle

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INTRODUCTION Polymer blends with co-continuous morphologies containing nanoparticles with selective location represent an important strategy for high-performance, functional and responsive materials.1-8 The reduction of the particle contents and the decrease in the onset of co-continuous morphology are beneficial to the improved processing performance and comprehensive properties, which have attracted a great deal of interest in the recent past.9-11 However, it is still a great challenge to achieve co-continuous blends with low dispersed-to-continuous transition (DCT) and superior stability against strong flow as well as thermal annealing at low particle loadings. It has been illustrated that nanoparticles have the ability to control the morphology of polymer blends especially when they are located at the interface between polymers. However, for spherical nanoparticles with homogenous surface property, the interfacial jamming is often a need in order to kinetically arrest co-continuous morphology.9,

12, 13

A possible solution is using

spherical Janus particles (JPs) with different physical properties on opposing hemispheres, which have better compatibilization effect than homogeneous particles as indicated by the size reduction of the dispersed phase at lower particle content.5, 14-17 The combination of interfacial affinity and the Pickering effect of Janus particles5 helps the selective location at the interface irrespective of the strong flow field under melt mixing, which is very important for large scale industrial application. Despite the greatly compatibilizing effect on sea-island morphology,5, 14-17 it is still unclear about whether the Janus particles can alter the onset of the co-continuous morphology in immiscible blends and stabilize it. It has been found that spherical JPs in blends with almost symmetric composition (60/40) can induce the co-continuous structure only under the condition of interfacial jamming, but the morphology becomes unstable under different melt mixing conditions.15 The requirement of interfacial jamming of both Janus particles15,

18, 19

and

homogeneous particles9, 20 is actually not in favor of the reduction in the particle content. Shape anisotropy of the modified agent in the polymer blends plays a crucial role in the mechanism of the dispersed phase continuity. As to asymmetric block copolymers, the more asymmetric of the block copolymers, the wider of the cocontinuous interval in the polymer blends despite the high contents of copolymers due to the form of micelles.21 In the case of high aspect ratio nanoparticles like clay localized at the interface, the reduction of the cocontinuity of polymer 2


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blends only needs a much low nanoparticle contents due to its easy to form percolating network at the interface.22 For the JPs, the shape anisotropy (Janus balance), which is another important factor affecting nanoparticle interfacial active23 but only its role in reducing the dispersed phase size has been recently verified.24 The shape anisotropic of JPs in the polymer blends on the formation and stability of co-continuous morphology is still less understood. In this work, we investigated the effect of the shape anisotropy of JPs on the DCT and the thermal stability of the poly(lactic acid)/polycarbonate (PLA/PC) immiscible blends. The selective location of JPs in the PLA/PC blend is achieved by adjusting the chemical asymmetry and the geometric asymmetry of the JPs. The DCT concentration and the thermal stability were evaluated by the rheological measurements and microscopic morphology analysis. By further quantifying the coverage fraction of nanoparticles at the interface, we reveal the relation between the compatibilizing efficiency of the shape anisotropy of JPs and the interfacial jamming state.

EXPERIMENTAL SECTION Materials. Poly (lactic acid) (PLA, 2003D), was supplied by NatureWorks LLC. Polycarbonate (PC, 1220), was supplied by HONAM Petrochemical Corp. South Korea. Colloidal silica nanoparticles of radius (Rp) 38 nm was purchased from Dalian Snowchemical Co., China. γ-methacrylox-ypropyltrimethoxysilane (γ-MPS), styrene (St), potassium persulfate (KPS), sodium dodecyl sulphonate (SDS), buffer, sodium bicarbonate (NaHCO3), ethanol, were purchased from Sinopharm Chemical Reahent Co., Ltd China. tert-butyl acrylate (BA, 99%), Acetoacetoxyethyl methacrylate (AAEM), allyl methacry-late (AMA), were purchased from J&K Chemical Reagent Co., Ltd. St and BA were distilled under reduced pressure before use and other reagents were used without further purification. The purified St and BA monomers were stored in a refrigerator at 5 oC before use. Synthesis of MPS grafted silica. 1.0g MPS was added to a suspension of silica (10g) and SDS (0.05g) in water, and the suspension was stirred for 24h at room temperature. The suspension was repeatedly washed 3 times by centrifugation and re-dispersion in ethanol. Synthesis of homogenous polymer grafted silica particles (HPs). PBA grafted SiO2 were prepared by emulsion polymerization. The MPS grafted silica (2.5g) dispersed in 15ml of ethanol 3


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was added into a solution of SDS (0.15g), NaHCO3 (0.4g), water (180ml) under ultrasonication for 10min, and then were placed into a four-neck flask with a machine stirring under an argon atmosphere for 20min. Then, BA, AMA and KPS were added. The polymerization was carried out for 5h at 70 oC. HPs like PS-g-SiO2 particles are prepared by emulsion polymerization under similar conditions. Detail of the polymerization recipe can be found in Table 1. Core-shell morphologies of HPs can be seen clearly (Fig. S1), and the thickness of shell depends on the amount of monomer. Synthesis of snowman-like Janus particles (JPs). A schematic illustration of the formation of Janus particle is shown in Scheme 1. In a typical experiment, the P(BA-co-AAEM)-g-SiO2 particles seed was bubbled with nitrogen gas, while being stirred at a speed of 240 rpm for 20 min. Then, a certain amount of St was added. The stirred speed was increased to 300 rpm. After swelling for 30min at 25 oC, the system was heated to 70 oC. 1.0 g of KPS in an aqueous solution was added. The polymerization was carried out for another 4.0 h with the stirred speed 260 rpm. The suspension was purified by centrifuging/redispersing in water for 4 cycles. The JPs were dried in a vacuum oven at 30 oC for 48 h. The polymerization recipe used in the preparation of JPs is shown in Table 1. BA and St are chosen to cover two caps of Janus particles because their compatibilities with PLA and PC, respectively. More delicate control on the interactions between Janus particles and polymers are fulfilled by adding small amount of AAEM during polymerization.

Scheme 1. Schematic representation of the procedure for the synthesis of snowman-like JPs.

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Table 1. The polymerization recipe used in the preparation of Janus particles and homogenous particles.

Samples

MPS-SiO2/g

BA/g

AAEMa/wt%

AMAa/wt%

JP1

2.5

5.0

7

0

15.0

-

-

JP2

2.5

5.0

7

1

15.0

-

-

JP3

2.5

5.0

0

3

15.0

-

3

JP2:1

2.5

5.0

14

3

15.0

-

3

JP1:1

2.5

2.0

14

3

4.0

-

3

HP1

2.5

0

-

-

8.0

0

3

HP2

2.5

0

-

-

8.0

14

3

HP3

2.5

5.0

0

3

-

-

-

HP4

2.5

5.0

14

3

-

-

-

a

St/g AAEMb/wt%

AMAb/wt%

The composition percent is based on the BA monomer content. bThe composition percent is based on

the St monomer content.

Preparation of polymer blends. Samples were dried at 40 oC for 24h under vacuum before mixing. Polymer blends were prepared by melting mixing different amounts of samples using a miniature conical twin-screw extruder (SJZS-10, Wuhan Ruiming Equipment Co. Ltd., China) for 8 min with the mixing speed is 45rpm and the mixing temperature at 170 oC. Particles contents are from 0.9 vol % to 3.4 vol%. For JPs, the effective volume of a particle was calculated from the sum of the two-part spherical cap (Fig. S2). Table 2 summarizes the chemical composition, particle shape, particle size and locations in polymer blends.

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Table 2. Compared of different particles and their located place in PLA/PC blends

a

Abb.

Chemical composition

Particle shape

Reffa/nm

Localization

HP1

PS-g-SiO2

Homogenous Particles

66

PC

HP2

(PS-co-AAEM)-g-SiO2


67

PC

HP3

PBA-g-SiO2


51

PLA

HP4

(PBA-co-AAEM)-g-SiO2


52

PLA and Interface

JP3

PBA-g-SiO2-g-PS

2:1 Snowman-like JPs

73

PLA and Interface

JP2:1

(PBA-co-AAEM)-g-SiO2-g-PS


78

Interface

JP1:1

(PBA-co-AAEM)-g-SiO2-g-PS


40

Interface

The radius of the bigger node of the snowman-like JPs and the radius of the HPs.

The composites were compression-molded into a disk-shaped specimen (25mm in diameter and about 1 mm in thickness) at temperature near or below the blending temperature under 10 MPa for 5 min. Characterization. Rheological analyses were performed by Small amplitude oscillatory shear (SAOS) measurements on a strain-controlled rheometer (ARES-G2, TA Instrument, USA) at 160 °C. Parallel-plates with 25 mm in diameter and 1mm in gap were used. Dynamic frequency sweeps were performed from 100 rad/s to 0.03 rad/s under sufficiently small strain amplitude ensuring the linear viscoelastic responses, which were verified by dynamic strain amplitude sweeps. Time sweep tests (Bohlin Gemini HRnano) were performed at 240 °C. Field Emission Scanning Electron Microscopy (FE-SEM) images were checked using SEM (Nova Nanosem 450, USA). Samples were prepared by cryofracture of the polymer blends bars under liquid nitrogen. For polymer blends, trimethyl phosphate was utilized to etch PLA. After drying, the etched surface was coated by a gold layer before they were observed through SEM. SEM images of Janus particles were taken by JEOL JSM-7800F Prime (SEM) & Thermo Scientific TM NORANTM System. Transmission electron microscope (TEM) images were obtained by a FEI Tecnai G2 Spirit 6


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Biotwin instrument operating at an accelerating voltage of 120 kV. The thickness of the samples, randomly cut from the nanocomposites using a diamond knife at -120 oC, was about 100 nm. Thermo gravimetric analysis was conducted by TGA instrument (Pyris 1, Perkinelmer, USA) with a heating rate of 20 oC /min under air atmosphere from 40 oC to 700 oC.

RESULTS AND DISCUSSION Shape asymmetry of snowman-like Janus particles Snowman-like JPs were synthesized by using modified two-step emulsion polymerizations procedures with silica particles used as the spherical core25, 26. In the first step, silica particles (diameter 76 nm) were coated with poly(tert-butyl acrylate-co-acetoacetoxyethyl methacrylate) (poly(BA-co-AAEM)) shell. Then St monomers were added to swell the PBA shell first due to the miscibility between monomers and PBA. When monomers were polymerized, oligomers formed inside the PBA shell and on the seed surface. Due to release crosslinking force created on PBA network, phase separation between PBA and PS happened in the follow polymerization process. Low molecular weight polymers expelled from the seed are growing bigger as they merged with the PS/St bulges on the seed surface, anisotropic particles are synthesized27, 28. The chemical and shape asymmetries are controlled by the amount of AAEM and styrene, respectively (Table 1). Two typical snowman-like JPs with different shape asymmetries are shown in Fig. 1. Since the radius ratio of the PS node versus the SiO2-poly(BA-co-AAEM) node near 1:1 and 2:1 in the two samples, they are denoted as JP1:1 and JP2:1, respectively (Fig. 1). The shape asymmetry is mainly controlled by the amount of BA and St (Table 1). Both the poly(BA-co-AAEM) shell and PS lobe are crosslinked during polymerization by addition of allyl methacrylate (AMA).

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Figure 1. TEM images of the snowman-like JP with different shape asymmetries.

To verify the asymmetrical character of the snowman-like JPs, we synthesis snowman-like JPs without cross-linked of the shell. We first extract of PBA shell. If the PBA shell is not cross-linked (snowman-like JP1 in Table 1), almost all of the polymer shell departs from the silica core and only a small amount of polymer link in the silica surface (Fig. 2 a2). This means that the PS lobe in the second polymerization step is formed through swelling of the PBA shell in one side and only a few PS chains grafted onto the silica core. After the cross-linking of the PBA shell (JP2 in Table 1), the extraction process cannot separate the polymer shell from the silica core, but a clear trend of separation between silica and PS nodule is seen due to the extraction of non-crosslinked chains (Fig. 2 b1 and b2). Secondly, ruthenium tetroxide (RuO4) was used as the staining agent for PS.29 The Janus particles were stained for 60min before TEM observation. After stained, the PS nodules side become darker (Fig. 2 a3 and b3) while the PBA side exhibits nothing changes. The amphiphilic character of JPs can also be observed from the formation of an elastic interface between immiscible liquids water and toluene (Fig. S4).

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Figure 2. Morphologies of the snowman-like JPs with (a) AMA content 0wt% (JP1), and (b) with AMA content 1wt% (JP2).

Chemical asymmetry of snowman-like Janus particles The selective localization of particles in the polymer blends is one of the key parameters that determine the morphology of the dispersed phase. The location of nanoparticles at interfacial area is regulated by the surface chemistry of Janus particles. Due to the partial miscibility between PC and PS,

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and between PLA and PBA,31 silica particles with homogenously grafted PS or PBA

(HPs 1-HPs4 in Table 1) are selectively located in PC or PLA phase, respectively (Fig. 3 b1 and b2). However, when the PBA-g-SiO2-g-PS snowman-like JPs (JP3 in Table 1) are mixed with PLA/PC blends, only a small amount of JPs was dispersed at the interface and most of the Janus particles were still in the PLA matrix (Fig. 3 a2), which is attributed to the better compatibility between PLA (solubility parameter δ=17.6 (J/cm)1/2 based on the Van Krevelen group contribution method

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) and PBA (δ=18.0 (J/cm)1/2 32) than that between PC (δ=20.2 (J/cm)1/2 32) and PS

(δ=19.2 (J/cm)1/2 32).

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Figure 3. The selective location of particles in PLA/PC-85/15 polymer blends. (a1) without particles, (a2) with PBA-g-SiO2-g-PS (JP3), (b1) with PS-g-SiO2 (HP1), (b2) with PBA-g-SiO2(HP3).

In order to adjust the relative affinity of the two sides of the snowman-like JPs, a third component AAEM (δ=20.7 (J/cm)1/2

32

) is added to improve the affinity with PC domains.

Copolymerization of AAEM with St in poly(St-co-AAEM)-g-SiO2 HPs does not change the distribution of particles in PLA/PC blends, i.e., they are still in the PC phase (Fig. 4 a1). Copolymerization of AAEM with BA in poly(BA-co-AAEM)-g-SiO2 (HP4) makes a lot of the nanoparticles migrate to the interface (Fig. 4 a2). For the poly(BA-co-AAEM)-g-SiO2-g-PS snowman-like Janus particles (JP2:1), JPs are mainly dispersed at the interface (Fig. 4 b1 and c1). The location of snowman-like JP1:1 nanoparticles at the interface is also clear except with weak aggregation (Fig. 4 b2 and c2). Although AAEM has some regulatory capacity for HP affinity, the regulatory role in JPs is more pronounced. In summary, polymers (PBA and PS) that can be readily grafted on nanoparticles are chosen to cap two spheres of snowman-like Janus particles. The affinity between PBA and PLA, PS and PC makes selective location at interface possible. However, due to the stronger affinity between PBA and PLA (∆δPBA/PLA=0.4 (J/cm)1/2) than that between PS and PC (∆δPS/PC=1.0 (J/cm)1/2), less compatible component AAEM (∆δPAAEM/PLA=3.1 (J/cm)1/2) was chosen to copolymerize with BA to balance the affinity of the grafted polymers and polymers in blends, i.e., PBA/PLA and PS/PC, with the consideration on the role of shape 10


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asymmetry (i.e., the contact area). Therefore, the selectively locations of JP1:1 and JP2:1 at the interface between PC and PLA can be successfully fulfilled through the control of the chemical asymmetry.

Figure 4. TEM images (a,b) of the polymer blends and SEM images (c) after the extraction of the matrix in PLA/PC-85/15 polymer blends. (a1) with (PS-co-AAEM)-g-SiO2(HP2), (a2) with P(BA-co-AAEM)-g-SiO2(HP4), (b1, c1) with (BA-co-AAEM)-g-SiO2-g-PS (JP2:1) and (b2, c2) with (BA-co-AAEM)-g-SiO2-g-PS (JP1:1).

Compatibilizing effect Normally, particles at interfaces can act as compatibilizer to decrease the domain size. Such effect can be clearly seen in Fig. 3 and Fig. 4. As compared with pure polymer blends (Fig. 3 a1), both snowman-like Janus particles (JP1:1 and JP2:1) can reduce the domain size in polymer blends (Fig. 4) because of the role acting as compatibilizer in decreasing the interfacial tension, which is consistent with observations in the literatures.

5, 14, 15, 17, 24, 33, 34

Moreover, homogeneous particles

located in one component seem also have certain compatibilizing effect. This may be ascribed to the change of the effective viscosity ratio when the nanoparticles are selectively located in one component. However, for Janus particles, which are selectively located at the interphase, their influence on the effective viscosity ratio could be negligible. Therefore, the decrease in domain 11


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size in Janus particles filled blends should be attributed to the change of interfacial properties (interfacial tension as well as interfacial rheological properties). In addition to these observations, we found that the transition concentration from dispersed domains to co-continuous domains (DCT concentration) can also be affected by snowman-like Janus particles. The DCT concentrations of polymer blends with and without JPs are shown in Fig. 5 (others in Fig S5) and are summarized in Fig. 6 a. Here, the DCT concentration is determined by Winter-Chambon criterion using the linear viscoelastic data35-37 , which has the advantage of simplicity and high accuracy as compared with other methods like the solvent extraction,

38, 39

the image analysis

40

and the conductivity measurement.41 It is noted that such rheological character has also been observed in particle-filled polymer nanocomposites, where dispersed particles start to form percolated network.42 The DCT concentrations thus determined are well consistent with the direct observation on morphology. For example, if the PC content is higher than the DCT (e.g., 4 wt% for PLA/PC/JP2:1 with 2.4vol% particles), the PC phase in PLA/PC/JP2:1-95/05/2.4 can support itself after etching the major component PLA. On the other hand, if the PC content is low than the DCT (e.g., 6.7 wt% for PLA/PC/HP3 with 3.4 vol% particles), the droplet domains in PLA/PC/HP3-95/05/3.4 aggregate with each other and big aggregates are clearly seen (Fig. S6). In the case of Janus particle filled polymer blends, however, the transition is attributed to the continuity of minor polymer component because particle content is far below the percolation concentration of spherical particles.42

Figure 5. The determination of DCT concentration (φc) for polymer blends. (a) PLA/PC, (b) PLA/PC/JP1:1 with 3.4vol% JP, (c) PLA/PC/JP2:1 with 0.9vol% JP, (d) PLA/PC/JP2:1 with 2.4vol% JP. The concentration independent concentration at low frequency range is determined as the DCT concentration. 12


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The DCT concentration for the PLA/PC blends is around 11.0 %, much lower than the normal range 40-60 % due to the high viscoelastic asymmetry between PC and PLA at low mixing temperature (close to the glass transition temperature of PC).37 After adding JPs to the polymer blends, the DCT concentration dropped significantly but to different extents. As the snowman-like JPs migrate from the matrix (JP3) into the interface (JP2:1), the DCT concentration is greatly reduced. The snowman-like JP2:1 is more effective in decreasing the DCT concentration than the snowman-like JP1:1. The PLA/PC blends containing 3.4 vol% snowman-like JP1:1 particle had a DCT concentration of about 7.5%, while much less shape asymmetric Janus particle (0.9 vol% JP2:1 particles) showed more evident effect, where the DCT concentration is nearly half of that without particles. At 2.4 vol%, snowman-like JP2:1 particles can further reduce the DCT concentration to only about 4%. The reduction of the DCT concentration is more noticeable for polymer blends with snowman-like Janus particles as compared with homogenous particles (HP1 and HP3) in polymer blends. As a matter of fact, the mechanism behind the DCT reduction is complex in polymer blends especially in the presence of particles with different localizations. The DCT of two-phase polymer blends is well correlated with the deformation of the dispersed domains and the ability to maintain the aspect ratio of the dispersed domains33. The increase of the viscosity ratio and the increase of the elasticity ratio is helpful to slowdown the relaxation process and increase the stability of the deformed domains33, which is beneficial to maintain the aspect ratio of the dispersed domains. Homogeneous particles change the bulk viscoelastic properties when they are selectively located in one component. The DCT concentration is thus affected due to the change of viscoelastic asymmetry of blends. On the other hand, the effect of nanoparticles on the bulk properties of two components becomes negligible when they are located at the interface. Then, the decisive role becomes the interfacial rheological properties. Therefore, locations of nanoparticles both in polymer domains and the interface can slowdown the relaxation of deformed domains through the change of bulk viscoelastic asymmetry and interfacial properties, respectively. Our results clearly indicate that direct control of interfacial properties is much more effective because 0.9vol% JP2:1 nanoparticles can have the similar effect (about 7% of DCT concentration) as homogeneous particles with almost four times content (3.4vol%) of particles. 13


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Moreover, the change of interfacial properties depends on the shape asymmetry of Janus particles.

Figure 6. (a) The DCT concentration of PLA/PC blends without and with different contents of different JPs. (b) Normalized storage modulus of different polymer blends during annealing at 240oC at 1 rad/s.

To further illustrate the effect of snowman-like Janus particles on the stability of co-continuous morphology, the coarsening process during annealing in the molten state was evaluated by both rheological measurement (Fig. 6 b) and image analysis of SEM and TEM micrographs (Fig. 7). As shown in Fig. 6b, the storage modulus of pure PLA and pure PC decreased rapidly due to the scission of polymer chains at high annealing temperature for more than 300s. Degradation of PLA is more pronounced than that of PC. The decrease of normalized G' of PLA/PC blend is even more quickly at short time, which is related to the faster broken down of the continuous PC domains than the degradation of polymers. For PLA/PC blends containing snowman-like JP1:1 particles, the storage modulus decreased when the annealing time is shorter than 300s but started to increase after 300s. This means that the coarsening process of domains happened and nanoparticles may also rearrange along the interface.13, 43 In contrast, as to PLA/PC blends containing snowman-like JP2:1 particles, the normalized storage modulus is almost constant in the whole annealing process except a weak decrease at the start of annealing. The reduction of normalized storage modulus is within 10% for blends containing snowman-like JP2:1 particle, much smaller than that with snowman-like JP1:1 particles (about 30%), that with HP1 particles (about 40%) and that of pure blend (over 80%). The reason that apparent modulus does not exhibit great decrease during annealing is that the interfacial contribution to the elastic modulus is much 14


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higher than that from bulk polymers at the experimental frequency. Although the components’ moduli decrease greatly due to thermal degradation, the interfacial contribution does not change much since the morphology are greatly stabilized by Janus particles, which can be seen directly in Fig. 7. When the homogeneous silica particles (HP1) is dispersed in the minor phase (Fig. 7 a), the coarsening process cannot be effectively suppressed due to the poor ability to form particle network compared with fumed silica particles.44 Comparing PLA/PC blends containing snowman-like JP1:1 particle and snowman-like JP2:1 particle (Fig. 7b and 7c), it is obvious that snowman-like JP2:1 particle in polymer blends can effectively suppress coarsening of the dispersed phase and stabilize the co-continuous morphology. We also noticed that the cocontinuous morphology can only be clearly revealed in SEM images (Fig. 7a and 7b), while only dispersed domains can be seen in TEM images (Fig. 7c) because the volume fraction of the minor component is too low (10%) to show connected domains in a slice. However, TEM images can give a clear indication on the particle distribution in the blends. No aggregation and redistribution of snowman-like JP2:1 can be found, and all snowman-like JPs still locate at the interface (Fig. 7 c3). In contrast, coarsening of PC domains accompanied by the aggregation and redistribution of nanoparticles are seen for snowman-like JP1:1 particles during annealing (Fig. 7 b3).

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Figure 7. SEM images (1, 2) and TEM images (3) of the polymer blends before (1) and after (2, 3) annealing at 240 oC for 2000 s. (a) PLA/PC/HP1-90/10/3.4, (b) PLA/PC/JP1:1-90/10/3.4, (c) PLA/PC/JP2:1-90/10/2.4.

Interfacial jamming It has been reported that the interfacial jamming is a prerequisite for morphological stabilization.5, 9, 13, 15, 20, 33 However, it is evident from Fig. 4 that the visible silica particles are far away from jamming in the interface. Quantitatively, the interfacial packing density (IPD) of snowman-like JPs in the interfacial area of the continuous domains was evaluated by the ratio between particle cross area and the interfacial area. The interfacial area of co-continuous morphology can be estimated from the approximate morphology of the interconnected rods

45, 46

by Ssp = 3π a (1 − a ) 2lc where φ = 3a 2 − 2a3 with φ the volume fraction of PC and lc the characteristic length of PC domains. Diameter of silica particle is used to calculate the particle cross-sectional area with the assumption that all Janus particles are located at the interface and 16


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form a monolayer. For Janus particle with shape symmetry (JP1:1), the interfacial packing density is around 37% in PLA/PC 90/10 blend containing 3.4 vol% JPs (Table 3). For Janus particle with shape asymmetry (JP2:1), the IPD is only 5% in the PLA/PC (90/10) blends with 2.4 vol% JPs (Table 3). Increasing the content of PC in the blend and keep the volume fraction of Janus particles unchanged, the interfacial packing density will decrease. Apparently, IPD of snowman-like JP2:1 are smaller than those of snowman-like JP1:1 at comparable particle loading, indicating that shape asymmetry plays an important role in lowering the interfacial packing density but increasing ability to stabilize the morphology. In both cases, the apparent interfacial packing densities of silica particles are lower than that of the jamming state (IPD ~ 76% 47).

Table 3. The packing density (PD) of JPs at the interface of the dispersed phase.

Sample

Composition

PD /%

PLA/PC/JP1:1

95/05/3.4

47

PLA/PC/JP1:1

90/10/3.4

37

PLA/PC/JP1:1

85/15/3.4

29

PLA/PC/JP2:1

95/05/2.4

7.0

PLA/PC/JP2:1

90/10/2.4

5.0

PLA/PC/JP2:1

85/15/2.4

4.4

All these results manifest that the geometric asymmetry of snowman-like JP2:1 is more effective in controlling the co-continuous morphology. The extraordinary ability of snowman-like JPs with large shape asymmetry to reduce the domain size and resist the relaxation of the interface between polymer blends can be explained by the geometric confinement as illustrated in Fig. 8. For spherical particles (with homogeneous surface or with different surface chemistries), chemical asymmetry has little effect on the position of particle at the interface if the length scale of phase domain (R0) is much larger than the particle size (R1) (case I in Fig. 8). Interfacial jamming (or gelation) of particles is required to strongly alter the relaxation behavior of interfaces. However, the interfacial position of snowman-like JPs is strongly affected by both chemical asymmetry and shape asymmetry (case II-IV in Fig. 8). Affinity of both spheres of snowman-like particle with two 17


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polymers can make two spheres distribute symmetrically (case II) or asymmetrically (case III) at the interface. Because of the stronger geometric confining effect inside the domain, the inner part of snowman-like particle may jam at lower particle concentration, i.e., the curvature radius of the jamming regime RJ is R0-R1 (case II) or R0-2R1 (case III). Then, for outer particle at the interface with radius R0, the number density per area is proportional to (RJ/R0R1)2 and the IPD for jamming is about 0.76(RJ/R0R1)2R12. They are 65% and 55% for case II and case III, respectively, using the domain size R0 about 500 nm and silica particle size R1 about 38 nm. If there is shape asymmetry (case IV), the IPD for jamming becomes 0.76(RJ/R0R2)2R12 with RJ=R0-R1-R2. The IPD for jamming for snowman-like JP2:1 is then only 11% when R2= 2R1. Such value can be 7% if the domain size R0 decreases to 300 nm. It should be mentioned that the above estimation on the interfacial jamming concentration assumes that all particles are uniformly distributed at the interface. If aggregated, the actual jamming concentration at interface should be higher. Some morphology observations can partially support the schematics in Fig. 8. For example, as shown in Fig. 7(c3), black silica particles are located at the interface. The larger semisphere of Janus particles (JP2:1), which is not seen in the TEM images due to weak contrast between PS and PC, should be located inside the PC domains. This situation is exactly similar to the case IV in Fig. 8. Actually, the hypothesis in Fig. 8 is an idealized case, where particles are hard spheres. In this work, the PS semisphere is a soft colloid although PS are crosslinked. But the softness of particles will not change the jamming character at the interface. Therefore, large shape asymmetry JPs is more efficient in reducing the dispersed phase size and in increasing the relaxation of the interface (Fig. 8). Therefore, it is implying that increase of the shape asymmetry of JPs is an efficient approach to form and stabilize the co-continuous morphology in immiscible polymer blends. Such mechanism is also helpful to understand the stabilizing effect more quantitatively. The higher interfacial coverage of particles, the slower the coarsening of the domains. When the interfaces are fully covered by particles, coarsening will stop due to the interfacial jamming. The effective interfacial coverage (ψ), defined as the ratio of the interfacial packing density and the interfacial jamming concentration, can be used to evaluate the stabilizing effect. For example, ψ in PLA/PC/HP1-90/10/3.4 blend should be much smaller than 4.5% if all HP1 particles are located at interface (actually HP1 are mostly in PC domains), while ψ in PLA/PC/JP1:1-90/10/3.4 blend (case 18


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III in Fig. 8) and PLA/PC/JP2:1-90/10/2.4 (case IV in Fig. 8) are 67.3% and 71.4%, respectively. Such trend of effective interfacial coverage (ψ) is consistent with the evolution of characteristic length during annealing (Fig. S7).

Figure 8. Schematic representation of the interfacial state of different nanoparticles and the dispersed phase changed for polymer blends at different process.

CONCLUSION In conclusion, we propose a novel co-continuous phase modified strategy that can reduce the interfacial coverage fraction of nanoparticles by adjusting the anisotropy degree of snowman-like JPs in immiscible polymer blends. The interfacial location of snowman-like JPs is successfully achieved by controlling the chemical asymmetry and the geometric asymmetry of the JPs. The increase in the degree of anisotropy of the JPs favors the formation and stability of the co-continuous structure at low particles loading according to the mechanism of geometric confinement. Such understanding in the cocontinuity mechanism of Janus particles can be used to guide the development of highly efficient Janus particles phase acting as morphology controlling agent towards functional cocontinuity multicomponent polymer blends.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author: [email protected]

ACKNOWLEDGMENTS The authors thank the support from the National Natural Science Foundation of China (No. 51625303, No. 21790344 and No. 21474063). This work was also partially supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

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Onset Reduction and Stabilization of Cocontinuous Morphology in Immiscible Polymer Blends by Snowman-like Janus Nanoparticles Wei You, Wei Yu* Advanced Rheology Institute, Department of Polymer Science and Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, P. R. China

TABLE OF CONTENTS GRAPHIC

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Onset Reduction and Stabilization of Cocontinuous Morphology in

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