Rheology of Nanosilica-Compatibilized Immiscible Polymer Blends

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Rheology of Nanosilica-Compatibilized Immiscible Polymer Blends: Formation of a “Heterogeneous Network” Facilitated by Interfacially Anchored Hybrid Nanosilica Hengti Wang,†,‡,§ Xin Yang,† Zhiang Fu,† Xuewen Zhao,† Yongjin Li,*,† and Jingye Li‡ †

College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Rd., Hangzhou, 310036, P. R. China ‡ CAS Center for Excellent on TMRS Energy System, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, P. R. China § University of Chinese Academy of Sciences, Beijing, 100049, P. R. China S Supporting Information *

ABSTRACT: Exclusive localization of nanofillers at the interface of immiscible polymer blend has been confirmed to be effective in improving compatibility and facilitating the formation of nanofiller-network with very low percolation threshold, while the rheology of such nanofiller compatibilized blends has seldom been investigated. Herein, we present a systematic rheological study on nanosilicacompatibilized PVDF/PLLA (poly(vinylidene fluoride)/poly(L-lactide)) blends. The linear viscoelastic properties of the systems are evaluated using small amplitude oscillatory shear (SAOS). It is found that the interfacial jammed Janus grafted silica (JGS) located at the interface increases dynamic moduli at low frequency even with very low filler loadings. The nonterminal effects become more pronounced with increasing JGS loadings. Weighted relaxation spectra inferred from SAOS reveals that the shape relaxation of PVDF-droplets is strongly influenced by addition of JGS. The solid-like behavior of JGS-filled blends has been attributed to both the orderly arrangement of JGS at PVDF−PLLA interface and the molecular entanglement between the grafted long tails of JGS with the molecular chains of the component polymers. In other words, JGS at the interface not only promotes strong interfacial interactions between phases, but also stimulates the formation of unique nanoparticle−polymer hybrid network, termed as “heterogeneous network” with the silica as the junctions.

1. INTRODUCTION The nanoparticle- (NP-) compatibilized immiscible polymer blends, in which NP was localized exclusively at the polymer− polymer interface, are gaining attentions from both industries and academia.1,2 Owing to the fine dimension and rigidity, the blends compatibilized by NPs exhibit stronger interfacial stability than those compabilizied by graft/block copolymers during polymer blending, making them as a new type of efficient compatibilizers.1−3 Moreover, the NP-compatibilized blends usually have unique hierarchical morphologies, which leads to the enhanced performance such as electrical,3,4 mechanical,5−7 optical8 and magnetic properties,9−11 etc. The investigations so far have demonstrated the feasibility of this strategy by adjusting carbon nanotubes (CNTs),12,13 clay,14 silica (SiO2),15−17 carbon black (CB),18 graphene,19 polysilsesquioxane (POSS),20,21 and fullerene C6022 at the interface of specific immiscible polymer blends. The emphasis has mainly focused on the morphological observations and the characterization of the physical performances. The rheology behavior of blends, in contrast, has seldom been studied. Especially, the correlation between microstructure and viscoelasticity of the NP-compatibilized blends has been far from elucidated. © XXXX American Chemical Society

Linear rheological behavior of blend systems can provide a rich source of information regarding compatibilizing efficiency and the interfacial interactions, particularly in the terminal region.23−26 The results present high accuracy since morphological variation of blends during testing can be neglected against the applied flow fields with small amplitude oscillatory shear (SAOS).27−29 In principle, the improved compatibility can be detected sensitively by an additional contribution to the elastic modulus at low frequencies, corresponding to the longtime relaxation caused by interface.30−32 Macosko et al.33 reported that the addition of hydrophobic silica nanoparticles (C-SNP) into the polyethylene/poly(ethylene oxide) blends leads to a pronounced increase in the interfacial storage modulus (G′inf), with respect to the pristine blend. Such influence was more notable with increasing C-SNP loadings, owing to the larger interfacial area and finer characteristic pore size. Demarquette et al.34 have also found PMMA/PS (poly(methyl methacrylate)/polystyrene) (70/30) blends Received: October 5, 2017 Revised: November 19, 2017

A

DOI: 10.1021/acs.macromol.7b02143 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Location and Dispersion of Nanosilica with Different Chemical Surfaces: (a) Bare SiO2, (b) GS (PLLA Grafted Silica), and (c) JGS (Janus PLLA/PMMA Grafted Silica) and the Proposed Compatibilization Mechanism of JGS at the Interface of Immiscible Polymer Blends

Table 1. Molecular Characteristics of Different Types of Nanosilicas Mnb (g/mol) silica type

modificationa

SiO2 GS JGS SiO2-Lg GS-L JGS-L

bare silica PLLA grafts PLLA/PMMA grafts bare silica PLLA grafts PLLA/PMMA grafts

PLLA grafts − 8.9 8.9 − 8.9 8.9

× 104 × 104 × 104 × 104

PMMA grafts

fsilicac (wt %)

− − 4.6 × 103 − − 4.6 × 103

99.0 65.5 61.9 99.1 56.3 55.3

Dnd (nm)

σsilicae (nm−2)

location in PVDF/PLLA blendsf

± ± ± ± ± ±

− 0.26 0.29 − 2.9 3.3

PLLA phase PLLA phase PVDF−PLLA interface PLLA phase PLLA phase PVDF−PLLA interface

16.5 18.9 19.1 116 121 120

3.1 2.9 3.6 35 36 39

a

Detailed synthetic routes are described in our early work.41 bDetermined by GPC measurement using linear PS as calibration and THF as mobile phase (Mn: number-average molecular weight). cEstimated from TGA measurement ( fsilica: inorganic content). dDetermined by TEM images (Dn: number-average diameter of particles). ecalculated by TGA data (σsilica: grafting density of polymer chains). fAssumed from the interfacial energy (γ) and wetting coefficient (ω) values based on our previous work.41 gSiO2-L, GS-L and JGS-L refer to the nanosilica with larger dimensions.

PLLA grafted silica (GS) disperses well in its PLLA phase, which is resemble with bare silica (SiO2) (Scheme 1a,b). Oppositely, the self-assembled NPs with Janus polymeric hemisphere through reactive compounding, designated Janus grafted (PLLA/PMMA) silica (JGS), can be located exclusively at the polymer−polymer (PVDF−PLLA) interface. As shown in Scheme 1c, JGS behaves similarly to block copolymer-type compatibilizer to decrease interfacial tensions and strengthen adhesions by separately entanglement of grafts and suppress the coalescence maximally for the steric repulsion of NP core. The blends with the JGS exclusively located at the interface offer an interesting model system for the investigation on the rheology behaviors of NP compatibilized polymer blends, especially the correlations between microscopic morphologies and linear rheological properties of the NP compatibilized blends. In this work, we will specifically focus on the correlation between microscopic morphologies and linear rheological properties. We try to elucidate the underlying dynamics of polymeric morphology, NP dispersion and interfacial interaction of NP-polymer by means of linear rheology measurement. The linear viscoelastic properties are evaluated and discussed using SAOS data with four representations (i.e., Cole−Cole plot,43 van Gurp−Palmen plot,44 Han plot,45 and the plot of |η*(ω)| versus |G*(ω)|)46 to find detailed correlations between microstructure and the viscoelasticity. In addition, weighted relaxation spectrum inferred from SAOS is plotted and the interfacial tension of blends is quantitatively calculated by Gramespacher−Meissner model.30 It is revealed that the viscoelasticity of the blends are strongly influenced by the incorporation of JGS, evident from the prominently elevated elastic modulus, reduced interfacial tension and retarded form relaxation of PVDF droplets. Since each Janus graft is longer than the critical entanglement molecular weight, the ordered arrangement of JGS at PVDF−PLLA interface may entangle

showed an excess values of storage modulus (G′) and complex viscosity (|η*|) in the terminal zone with the incorporation of the interfacial accommodated organoclay, compared to the noncompatibilized correspondent. Additionally, the extra elasticity can be elucidated by means of calculated interfacial tension and relaxation spectra, inferred from SAOS data using the Palierne model and the Honekamp−Weese method, respectively. However, several issues regarding rheology and microstructure of NPs compatibilized blends have yet to be answered. First, independent means of assessing the compatibilization efficiency is unsatisfactory for the blend with interfacial accommodated NPs. Owing to the high specific area, NPs (e.g., silica, carbon black and TiO2) exist more easily as agglomerates and are prone to form three-dimensional networks within blends,35−37 which exhibits the similar rheology response as compatibilizing effect.38,39 The accurate elastic contributions from polymer, NPs and their interfaces in binary polymer blends remain poorly to be understood. Second, convincing interpretations on interplay of internal structure and properties of blends have not been well comprehended. Moreover, the NPs that located at the interface in immiscible polymer blends are mainly controlled by the wetting coefficient with the NPs in the two components.40 Limited works so far have been investigated on dynamic rheology of blends emulsified by hybrid nanoparticle with long graft tails. We have very recently demonstrated a new reactive compatibilization strategy to emulsify the typical immiscible poly(vinylidene fluoride) PVDF/poly(L-lactide) (PLLA) blends by hybrid nanosilicas.41 The dispersion of nanoparticles in polymer blends can be regulated by varying the sorts and construction of polymer grafts on the exterior surface of NP.42Owing to reactive blending, the in situ formed single B


Article

Macromolecules Scheme 2. Chemical Formulas and TEM Images of (a) Bare SiO2, (b) GS, and (c) JGSa

a

For better observation, all nanosilicas were prepared by ultrasonic dispersion in ethanol. MPa) at 200 °C for 6 min and quenched in liquid nitrogen for 5 min. The specimens are formed into disk shape with a thickness of 1 mm and diameter of 25 mm for rheology measurements. 2.3. Rheological Characterization. A physica rheometer (MCR301, Anton Paar Instrument) with parallel configuration (diameter 25 mm, gap 1 mm) was utilized for rheology experiments. Measurements were carried out under dry nitrogen atmosphere to prevent thermos-oxidative degradation. First, strain sweep experiments were performed for blends with various compositions to determine the linear viscoelastic (LVE) region in SAOS tests. Detailed information was presented in Figure 1. It was observed that the LVE region of JGS-

with the components on both sides. On the one hand, JGS serve as entanglement attractor to generate molecular entanglements among phase via selective interaction of the grafts; on the other hand, it stimulates the formation of a unique percolating hybrid nanoparticle−polymer network (we called heterogeneous network) with interfacial JGS serving as junctions. The creep-recovery and stress relaxation experiments are also carried out for verification. To best of our knowledge, this is the first time to demonstrate the rheological behaviors of the polymer blends compatibilized by NPs with long chain grafts.12,47−49

2. EXPERIMENTAL SECTION 2.1. Materials. Poly vinylidene fluoride (PVDF, KF850, Mn = 1.0 × 105 g/mol, Mw/Mn = 2.0) used in this work was purchased from Kureha Chemicals (Japan). Poly L-lactide (PLLA, 3001D, Mn = 8.9 × 104, Mw/Mn = 1.8) was supplied from Nature Works (USA). All pristine SiO2 with average-diameter of 20 or 110 nm was provided from Carbot bluestar Co. (China). The modification of SiO2 with different polymeric coronas was fabricated through “attaching onto” method by reactive blending according to our previous study.41 Detailed characteristics of nanosilicas are shown in Table 1. The chemical formulas of various functionalizations for bare SiO2, GS and JGS are depicted in parts a−c of Scheme 2, respectively. Densely gray coronas were clearly observed for both GS (part b) and JGS (part c) in the TEM images as compared with bare silica (part a), confirming that long polymer chains have been attached onto the surface of nanosilica. 2.2. Blends Preparation. All materials were dried in a vacuum oven at 80 °C for at least 24 h prior to processing. The materials were compounded using a batch mixer (Haake Polylab, QC) at 190 °C and 50 rpm for 10 min. Blends of PVDF/PLLA were fabricated with three compositions, namely, 20/80, 50/50 and 65/35 (w/w). For the 50/50 composition, in which PVDF was the dispersed phase while PLLA was the matrix, several blends were prepared with various loading of nanosilica (SiO2, GS and JGS) ranging from 0 to 5 wt %. It should be mentioned that all the blend systems in this paper were prepared by simultaneous mixing except the blends with interfacially confined bare SiO2 (discussed in section 3.3.1). The blends were prepared by twostep mixing protocol (premixed with PVDF). Besides, excepting that the the larger JGS (110 nm) was used for comparison, nanosilicas with a diameter of 20 nm were used throughout the manuscript. The compounded blends were then compression molded in a hot press (10

Figure 1. (a) Storage modulus G′(ω) and (b) loss modulus G″(ω) as a function of strain amplitude from 0.01% to 1000% for neat PVDF, neat PLLA, PVDF/PLLA blends with different weight ratios (20/80, 50/50 and 65/35), and the PVDF/PLLA/nanocilia (50/50/1) blends with various NP structure at a temperature of 200 °C and a fixed frequency of 10 rad/s. filled blend was lower than the other blends, which was attributed to the complex microstructure of the blend system (discussed later). Herein the strain amplitude values within LVE were determined. Then, dynamic frequency sweep experiments were carried out for neat PVDF, PLLA and all blends at 200 °C. The strain amplitude was within linear region (1−5%) and these tests were performed from 0.01 to 500 rad/s. The weighted relaxation spectrum were calculated from C


Article

Macromolecules

Figure 2. (Part I) Linear rheology properties from SAOS experiments: (a) storage modulus G′(ω), (b) loss modulus G″(ω), (c) damping factor tan δ(ω), and (d) complex viscosity |η*(ω)| of the PVDF/PLLA (w/w) blends with various compositions (20/80, 50/50, 65/35) with an amplitude of 1% from 0.01 to 500 rad/s at the temperature of 200 °C. (Part II): SEM images of PVDF/PLLA blends with (e) 20/80, (f) 50/50, and (g) 65/35 weight ratios. dynamic moduli using an edge preserving regulation method and then, the interfacial tension which was necessary to evaluate compatibility between phases was determined based on the method of Gramespacher and Meissner (detailed in the Supporting Information, section S-1). Third, creep-recovery tests were performed at 200 °C with the optimized creep shear stress (5 Pa). The zero-shear viscosity (η0) and creep compliance (Jv, Je) of individual phase and the blends incorporated with nanosilica was calculated based on creep/recovery tests. Last, stress relaxation tests were carried out for all specimens with a strain of 1% at 200 °C. All of the rheology experiments were proven to be reproduced within 5%. 2.4. Morphology. Field-emission scanning electron microscopy (FE-SEM) experiments were performed for all blends using a Hitachi S-4800 with an accelerating voltage of 5.0 kV. The specimens were fractured in liquid nitrogen and then sputtered with gold.

and 50/50 (PVDF/PLLA, w/w) blends, G′(ω) and G″ (ω) remained the decreasing trend. A slight shoulder of G′(ω) in low ω region was observed for 20/80 blend, as shown in Figure 2a. This shoulder behavior became clearer for the 50/50 blend, as compared with pristine homopolymer that displayed the classical power-law dependence. Whereas for the 65/35 blend, G′(ω) values increased and even exceed the elastic moduli for pristine PVDF and 50/50 or 20/80 counterparts when ω → 0. This difference at low ω was clearer in the plots of tan δ (ω) and |η*(ω)|. As compared with neat components and blend of 20/80 or 50/50 weight ratios, tan δ (ω) curve of 65/35 blend obviously flattened and its peak shifted to lower ω (Figure 2c). Moreover, |η*(ω)| for 65/35 blend was apparently higher than the other blends in full ω range (Figure 2d). Such increments of |η*(ω)| became more pronounced in the low ω region, implying the improved elasticity27,35 which was consistent with the nonterminal behaviors of dynamic moduli. The enhancement of elastic and viscous responses of the 65/ 35 blend is undoubtedly related to the morphology evolution and interfacial tension between phases.26,37,40 It is particularly important to make correlation between the viscoelasticity and microstructure of all blends. As demonstrated in Figure 2e,f, PVDF/PLLA blends with different weight ratios displayed various morphologies: droplet/matrix (20/80), droplet/matrix (50/50), and cocontinuous (65/35), respectively. It can be observed from the enlarged views (upper right) that there are many clear gaps along the PVDF−PLLA interface, indicating thermodynamic immiscibility of PVDF/PLLA blends. The morphology observations showed a good agreement with the rheological results. Owing to strong immiscibility between PVDF and PLLA, the extra elasticity acquired in 65/35 blend can be ascribed to the formation of cocontinuous morphology which facilitates the development of network structures of phases.2,13,40

3. RESULTS 3.1. Unfilled PVDF/PLLA Blends. Linear rheology behaviors of neat PLLA, neat PVDF and the PVDF/PLLA blends with various compositions were performed using SAOS tests. Figure 2 illustrated the viscoelastic properties (part II) within LVE region and morphologies (part II) of all specimens. The frequency (ω) dependencies of storage modulus G′(ω), loss modulus G″(ω), damping factor Tan δ (ω) and complex viscosity |η*(ω)| for pure polymers and the uncompatibilized blends are plotted in Figure 2a−d. Neat PLLA and PVDF displayed a typical power-law behavior33 (G′ ∝ ω2, G″ ∝ ω) and thereby the G′(ω) or G″(ω) plots of the individual polymer was almost linear in double logarithmic coordinate (Figure 2a,b). It was further observed that the G′(ω) or G″(ω) values of PVDF/PLLA blends were slightly increased as a function of PVDF compositions within the whole range of frequency (ω) investigated. In higher ω region, the G′(ω) or G′(ω) of three blends were in basically between those of neat components. Meanwhile, the curve variation was different and complicated at lower ω for the blends with different compositions. For 20/80 D


Article

Macromolecules

Figure 3. (Part I) Linear rheology properties from SAOS experiments: (a) G′(ω), (b) G″(ω), (c) tan δ(ω), and (d) |η*(ω)|. Four criteria calculated from SAOS: (e) van Gurp−Palmen plots, (f) Cole−Cole plots, (g) Han plots, and (h) plots on |η*(ω)| versus |G*(ω)| of the PVDF/PLLA (50/50) blends with 1 wt % addition of nanosilica containing different exterior grafts with an amplitude of 5% at the temperature of 200 °C. (Part II) SEM images of PVDF/PLLA (50/50) blends incorporated with (i) 1 wt % bare SiO2, (j) 1 wt % GS, and (k) 1 wt % JGS.

SiO2 or GS filled system. As shown in Figure 3a,b, upon addition of JGS, G′(ω) values at low ω increased by 2 orders of magnitude. A frequency-independent plateau of G′(ω) or G″(ω) at low ω could be observed and the G′(ω) appeared to be more sensitive, indicating a pseudo solid-like responses at terminal region.18,23,26,30,33,35 Thereby the storage modulus values of G′(ω=0.01 rad/s) as a function of nanosilica species are plotted and inserted in bottom right of Figure 3a, suggesting that interfacial JGS led to a sudden increase in elastic response while the variation was less pronounced for bare SiO2 and GS. Tan δ (ω) and |η*(ω)| values showed the same behavior, as illustrated in Figure 3c,d. It can be inferred that JGS with distinct polymeric grafts contributed to the enhancement of rheological moduli while the bare SiO2 and GS (single graft species) became ineffective with the equal loadings (1 wt %), which was consistent with the morphological observations in which bare SiO2 or GS was well-dispersed in PLLA matrix (Figure 3i,j) while JGS located exclusively at the PVDF−PLLA interface (Figure 3k). Similar terminal behaviors can be obtained from the JGS filled blends with 20/80 or 65/35 weight ratios, as shown in Figures S-1 and S-2. In general, the elevated moduli and solid-like behaviors of JGS emulsified blends can strongly reflect the enhanced

The main purpose of this work is to investigate the functions of the NPs with long polymer chains at the interface on the rheological behaviors of the blends. To avoid the complex of the bicocontinuous structure, the 50/50 blend with typical sea− island morphology has been used as the model blend system for further investigation. 3.2. Nanosilica-Filled PVDF/PLLA (50/50) Blend Systems: Influence of Grafting Parameters. We have very recently41 elucidated that the nanosilicas with different surface grafts exhibited diverse dispersions in the multiphase PVDF/ PLLA blends. Bare SiO2 or GS containing single PLLA-brushes would be well dispersed in PLLA phase while JGS with two populations of polymer brushes (PLLA/PMMA) was selfassembled into hybrid particle with Janus grafting distributions and orderly arranged at the PVDF−PLLA interface via thermodynamic drives. Here, the three types of nanosilica were utilized for dynamic characterizations. Figure 3 illustrated rheological properties (part I, Figure 3a−h) and the corresponding morphology (part II, Figure 3i−k) of 50/50 blend incorporated with 1 wt % of indicated nanosilicas. Figure 3a−d demonstrated that the JGS compatibilized blends exhibited a completely different response of viscous and elastic in the SAOS tests, as compared with the pristine blend and bare E


Article

Macromolecules interfacial adhesions15,18,26,28,30,35 between PVDF and PLLA, which is dominated by the species and configuration of polymer brushes onto silica surface. However, the current PVDF/PLLA/ nanosilica blend system may turn out as a more complex issue since the effect of NP should be taken into account. In the case of filled polymer blend nanocomposites, the dispersion and interparticle interactions of NP itself highly affected the nonterminal behaviors.23,38,39 It is too difficult to clarify influence of each aspect separately only with the master curve such as G′(ω) or G″(ω) in SAOS measurements. For a better understanding on the viscoelastic response of PVDF/PLLA/ nanosilica systems, a further analysis based on dynamic modulus study is carried out. As plotted in Figure 3e−h, four advanced rheology criteria18,38 which were usually used to detect differences in the secondary structure of nanofilled polymer blend systems were acquired. The vGP (van Gurp−Palmen) plot, i.e., a curve of phase angle (δ) versus logarithmic complex modulus (|G*(ω)|), which has been found as useful characterization for polymer nanocomposites was chosen as the first criterion. For neat polymers or nanocomposites with low NP loadings, δ(ω) is close to 90°, corresponding to the completely viscous in terminal regime. As the NP content increased to a certain extent (i.e., percolation threshold), a marked decrease of δ (ω) at low |G*(ω)| is observed and this can be attributed to the aggregation of NP into extended construction (formation of jammed NP within polymer matrix, facilitating the hierarchical NP-buckling),50,51 formation of clustered NP network23,33,38,39 or even formation of percolating polymer-NP networks applying NP as connections.52 As shown in Figure 3e, the pristine blend and system with 1 wt % bare SiO2 or GS exhibited the curves expected for linear homopolymer: a plateau of δ(ω) with 90° at low |G*(ω)| values. Upon addition of 1 wt % JGS, δ(ω) of blend significantly reduced and shifted to the value less than 80°. This behavior suggested that the elasticity for PVDF/PLLA blend increased after introducing JGS, which could be ascribed to agglomeration of JGS at PVDF−PLLA interface as well as apparently improved interactions between JGS and homopolymer components. The veracity of this claim can be envisaged by the morphology observation (Figure 3k) and will be clarified in detail below. The second criterion is the Cole−Cole plot, which was confirmed to be sensitive in phase separation of two-phase systems such as polymer blends and polymer nanocomposites.38,53 Since the curve was formed by plotting imaginary viscosity (η′(ω)) versus real viscosity (η″(ω)), ω dependences would be eliminated.28,43 Detailed information on characteristic relaxation of polymer systems can illustrated via Cole−Cole plot accordingly. In our system, neat PVDF or PLLA displayed as one arc, whereas the 50/50 blends showed a slight deviation from typical hemicircular shape due to the immiscibility between phases (detailed data was plotted in Figure S-3). Similar trends were observed for blends containing 1 wt % bare SiO2 or GS (Figure 3f), implying the homogeneous dispersion of nanoslicas in polymer matrix. While for the case of 1 wt % JGS incorporated blend, both a significantly deviation and a long tail in the high η′(ω) region was observed, which reflected additional long-time relaxation processes in this blend nanocomposites.28 It seemed that due to enhanced interaction between nanosilica and polymer components by molecular entanglements, the interfacial JGS retarded the system’s relaxations significantly. This difference becomes clearer in the weighted relaxation spectra, as demonstrated later.

Third, Han plot was introduced because it usually applied as criteria for miscibility of polymer−polymer or polymer-NP by Han and Chuang.54 This plot, drawing from log G′ versus log G″, were expected to be composition independent for compatible systems while turned dependent on the component content for immiscible ones.53 It was worth mentioning that due to the incorporation of heterogeneous NP which greatly enhanced NP-NP and NP-polymer interactions, the filling effect was dominant in this system, as compared with the compatibilization efficiency on PVDF/PLLA blend. As depicted in Figure 3g, the plot of bare SiO2 or GS loading blends could overlap to that of the pristine blend, suggesting good dispersion of NP. However, addition of JGS with identical content caused obvious reduction of slope in terminal region. It can be speculated that the low-log G′ plateau was ascribed to the largescale heterogeneity in blend systems with quite small NP amounts (1 wt %), resulting from interfacial JGS with regular packing. Last, the plot of |η*(ω)| versus |G*(ω)|, being comparable to the curve of steady viscosity against stress under steady shear,38,55 was selected to explore the fluids’ flow behaviors.18 Traditional polymer systems show a |η*(ω)| plateau at low | G*(ω)| values, whereas percolated nanocomposites exhibit a divergence of |η*(ω)| in terminal region. Kotaet al.56 claimed that the appearance of |η*(ω)| divergence in PS/MWCNTs composites was attributed to the network formation of nanofillers. Beside, an asymptotic values of |G*(ω)|, termed as yielding point (|G*|η*→∞),57 was involved for NP’s dispersion quantifications. As shown in Figure 3h, |η*(ω)| plateau with the value of nearly 103 Pa.s was found for the bare SiO2 or GS filled PVDF/PLLA blends, demonstrating that they behaved as typical viscous fluids. A remarkable variation of flow behavior occurred upon addition of 1 wt % interfacial anchored JGS. The Newtonian plateau at low |G*(ω)| disappeared and the |η*(ω)| dependence turned very steep, implying the strong flow restrictions in the presence of JGS. Beside, |G*|η*→∞ could be achieved (calculated and inserted at the top right of Figure 3h), which may result in the formation of pseudo penetrated and percolated network facilitated by JGS (discussed later). It seemed that the viscous restriction was strong enough to suppress deformation of PVDF domains driven by reduced interfacial tensions14,15 and, hence, to restrain coalescence of phase structure under shear field. This assumption could be affirmed by morphology observations in which a refinement of PVDF-domains was obtained with incorporation of JGS (Figure 3k). This result was analogous with the recent study of Scherzer18 on the poly(methyl methacrylate)/polystyrene blend filled with carbon black. It could be assumed that a transition from insulating (bare SiO2/GS) to some associated networks (JGS) occurred in this blend system by varying grafting parameters of nanosilicas. Figure 2 demonstrated that the grafting parameters of NP strongly influenced the morphology and rheology property in the nanosilica filled PVDF/PLLA systems. JGS and bare SiO2/ GS brought out as the highest and lowest effective nanoparticles in compatibilizing the immiscible PVDF/PLLA (50/ 50) blend, respectively. It was found that JGS-incorporated blend with even 1 wt % particle contents showed distinct rheological response and behaved as viscoelastic solids in ω region investigated, as compared with SiO2 or GS counterparts. 3.3. JGS-Filled PVDF/PLLA (50/50) Blends. There are several factors that can influence the terminal behaviors in the JGS filled PVDF/PLLA systems: the chemical surface, particle F


Article

Macromolecules

Figure 4. (Part I) Linear rheology properties from SAOS experiments: (a) G′(ω), (b) G″(ω), (c) tan δ (ω), and (d) |η*(ω)|. Four criteria calculated from SAOS: (e) van Gurp−Palmen plots, (f) Cole−Cole plots, (g) Han plots, and (h) plots on |η*(ω)| versus |G*(ω)| of neat PVDF/PLLA (50/50) blend and the blend with 1 wt % addition of nanosilica through different mixing sequence with an amplitude of 5% at the temperature of 200 °C. (Part II): SEM images of (i)PVDF/PLLA (50/50) blend and the blend incorporated with (j) 3 wt % JGS by simultaneous mixing and (k) 3 wt % bare SiO2 premixed with PVDF and the compounded altogether with PLLA components.

were located in PVDF−PLLA interface (Figure 4, part II). Interestingly, the nonterminal behavior was more pronounced for JGS system: a higher plateau value of G′(ω) or G″(ω) in low ω region (Figure 4a,b), a more flattened tan δ (ω) curve with peak values at higher ω zone (Figure 4c), and more elevated |η*(ω)| in the lowest ω (Figure 4d). It should be noted that a slightly increase values of G′(ω) or |η*(ω)| were visible at higher ω for bare SiO2 filled blend, which may due to the presence of small amount of SiO2 in PLLA matrix. Advanced criteria were applied for further prediction. A gradual decrease plateau values of δ(ω) (Pristine blends: 87°, interfacial SiO2-filled, 74°; JGSfilled, 65°) was acquired, indicating progressive elasticity in PVDF/PLLA blends by introducing silica-core and longbrushes with interfacial confinement gradually, as shown in vGP plots (Figure 4e). Meanwhile, JGS filled system displayed more apparent deviations of slope on Cole−Cole plots and Han Plot as compared with interfacial SiO2-filled blends. Similar trends was observed in the |η*(ω)| versus |G*(ω)| plot, where a more steep |η*(ω)| dependence was observed referred to JGS system. These results suggested a more integrated gellike network with addition of Janus hemispheres onto surface of SiO2 was formed, owing to the improved interactions among JGS and phase components.

size, and JGS loadings. In order to differentiate these effects, rheology and morphology properties were performed as a function of structural variations. 3.3.1. Chemical Surface Dependence. As the sensitive elastic response have been observed in JGS compatibilized blend system, it was not entirely clear whether the elevated dynamic modulus was contributed to the presence of silica core itself, or originated from the release of considerable amount of polymer grafts which exhibited Janus configurations at the PVDF−PLLA interface. In this section, the influence of exterior chemistry of nanosilica which anchored at the interface will be assessed. A new system with bare SiO2 interfacially confined in PVDF/PLLA interface (shown in Figure 4k) was achieved by two-step mixing through premixing with the lower affinitive PVDF phase. Therefore, one can compare the rheological behaviors of the JGS compatiblized blends and interfacial SiO2 jammed blends. Figure 4 showed linear rheology and microstructures of pristine blend, JGS compatibilized blend and interfacial SiO2 jammed blend. Both filled blends exhibited enhanced elasticity in the terminal zone as compared with pristine reference (Figure 4a−d), which was consistence with the morphology observations where the nanosilicas with different surface nature G


Article

Macromolecules

Figure 5. (Part I) Linear rheology properties from SAOS experiments: (a) G′(ω), (b) G″(ω), (c) tan δ (ω), and (d) |η*(ω)|. Four criteria calculated from SAOS: (e) van Gurp−Palmen plots, (f) Cole−Cole plots, (g) Han plots, and (h) plots on |η*(ω)| versus |G*(ω)| of neat PVDF/PLLA (50/50) blend and the blend with 3 wt % addition of JGS with various particle diameters of an amplitude of 5% at the temperature of 200 °C. Part II: SEM images of (i) PVDF/PLLA (50/50) blend and the blend incorporated with (j) 3 wt % JGS with a diameter of 110 nm and (k) 3 wt % JGS with a diameter of 20 nm.

(Figure 5f), more pronounced deviation at low G″(ω) from the Han plot (Figure 5g), and greater yielding point |G*|η*→∞ from the plot of |η*(ω)| versus |G*(ω)| (Figure 5h) was observed for 20 nm systems corresponding to 110 nm counterparts. Such results were in agreement with morphology observations in which 20 nm JGS exhibited more orderly arrangements at the interface (Figure 5i−k). It is clear that the incorporation of finer size JGS has a greater influence on rheology properties, since the very small NP with dimension comparable to Rg (∼15 nm) of polymers could impact exterior grafts’ molecular configuration58 which is necessary for polymer−NP interactions. This result is consistence with our previous study where the 20 nm JGS exhibited better compatibilization effect than the 110 nm counterpart.41 3.3.3. JGS Concentrations Dependence. Concentration dependence of NP always deemed as critical factor to ascertain percolation threshold (φ) values in polymer nanocomposites.15,18,30,38,39 As expected, the dynamic moduli of filled blend gradually elevated as the JGS loading increased (Figure 6a,b). The terminal G′(ω=0.01 rad/s) values was almost propor′ | ∼ Cn). At tional to the power of JGS concentrations (log|G(ω) JGS loadings less than 1 wt % (region I of the insertion in Figure 6a), G′(ω=0.01 rad/s) elevated with increasing JGS levels (n

It seemed that the extra elasticity in terminal region was closely connected to chemical surface of interfacial anchored NP in filled blend systems, which facilitated enhanced interactions mentioned above, orderly arrangements of nanosilica and even the formation of “heterogeneous networks” by virtue of JGS and polymer component (discussed later). 3.3.2. Particle Size Dependence. Ginzburget al.58 indicated that particle size (Rp) was a critical factor in changing the mobility and interactions among phases for the hybrid NP filled nanocomposites. If Rp was comparable with the mean square gyration radius (Rg) of polymers, enthalpy of the system was diminished due to strong polymer−polymer interactions and thereby the composite is stabilized. As Rg increased the influence of entropic surface tension turned stronger and NPrich phase would be segregated even at low loadings if Rp/Rg ≫ 1. In our system, two types of JGS with different diameters (20 nm, 110 nm) were introduced for comparison. Linear rheology properties (Figure 5a−d) demonstrated that the extra elasticity was greatly higher for the 20 nm JGS compatibilized blend than that for the 110 nm system. A better quantification of rheological differences could be achieved by the advanced criteria: lower δ(ω) at plateau from the vGP plot (Figure 5e), higher tail values at high η′ (ω) zone from the Cole−Cole plot H


Article

Macromolecules

Figure 6. (Part I) Linear rheology properties from SAOS experiments: (a) G′(ω), (b) G″(ω), (c) tan δ(ω), and (d) |η*(ω)|. Four criteria calculated from SAOS: (e) van Gurp-Palmen plots, (f) Cole−Cole plots, (g) Han plots, and (h) plots on |η*(ω)| versus |G*(ω)| of neat PVDF/PLLA (50/50) blend and the blend containing JGS with various concentrations of an amplitude of 5% at the temperature of 200 °C. (Part II) SEM images of PVDF/ PLLA (50/50) blend incorporated with (i) 0.5 wt % JGS, (j) 1 wt % JGS, (k) 3 wt % JGS, and (l) 5 wt % JGS.

= 2.53). Meanwhile, G′(ω=0.01 rad/s) values raised with a terminal slope of 0.31 at above 1 wt % of JGS. The slope values (n) of for G′(ω=0.01 rad/s) reduced and tended to be almost constant beyond 1 wt %, indicating some transition behaviors in JGS compatibilized blend as a function of JGS loadings. The plot of tan δ (ω) and |η*(ω) | at various JGS concentrations was shown in Figure 6c and Figure 6d respectively, which exhibited similar tendency as dynamic modulus. The four criteria demonstrated more sensitive to the variations. From vGP plots (Figure 6e), the peak value of δ(ω) monotonically decreased with increasing JGS loadings from 77° (0.5 wt %) to 58° (5 wt %). Gradual deviations were observed from Cole−Cole plots (Figure 6f) and Han plots (Figure 6g) as a function of JGS loadings, indicating enhanced interactions between JGS and homopolymers as well as improved ordering degree. It was worth noting that a similar variation of slope (|G*|η*→∞ versus JGS content) was obtained from the forth criteria (Figure 6h), as inserted in the left top of Figure 6h. More insights upon relating the rheological properties to microstructures were necessary, as shown in Figure 6i−l. It was obvious that PVDF domains reduced with increment of JGS concentrations from 0.5 to 5 wt %. On the basis of previous study,41 the thickness of interfacial layer elevated from 50 nm (0.5 wt %) to 58 nm (1 wt %) and then, kept constant with content of JGS from 3 to 5 wt %, implying that 1 wt % of JGS was enough to saturate the PVDF−PLLA interface. Therefore, higher JGS loading content

led to the accommodation of more interfacial area for refined morphologies. The observations were consistence with the variation of viscoelastic response. This result was agreed with the recent study from Hyun26 on poly(lactic acid)/natural rubber blend compatibilized by organoclay. Moreover, Mitchellet al.59 have demonstrated that in the single-walled carbon nanotubes (SWCNTs)/PCL composites, a percolation threshold could be attained by extrapolating |G*|η*→∞ since the yielding modulus displayed linearly dependent on SWCNT concentrations. We speculated that the “heterogeneous 3D network”, which was quite different with traditional gel of NP, was acquired with a critical JGS concentration of 1 wt% in the PVDF/PLLA blend.

4. DISCUSSION On the basis of rheology analysis, JGS brought out as the most effective hybrid nanosilica in compatibilizing the immiscible PVDF/PLLA (50/50) blend, as compared with bare SiO2 or GS. It was speculated that the nonterminal rheology behaviors of JGS system could be ascribed to the combined effects of the PVDF−PLLA interfacial interactions, JGS-JGS interactions (hydrodynamic factor) and arrangement of JGS (network’s factor). First, it has been confirmed that JGS functioned as traditional compatibilizers in stabilizing morphology, decreasing domain size and enhancing interactions between phases at the PVDF− I


Article

Macromolecules PLLA interface. According to Gramespacher and Meissner, the elastic moduli of typical immiscible polymer blends could be considered as the combination of both the contributions originated from the viscoelasticity of the components (domains and the matrix) and that from the polymer−polymer interface. Thus, the solid-like behavior in JGS compatibilized blend was partly down to emulsification effect as the consequence of reduced interfacial tension (calculated from GramespacherMeissner model and shown in the Supporting Infomration, section S-1 and Table S-2) and additional relaxation time (discussed later) owing to the presence of interfacial JGS. Second, the aggregation of particles driven by hydrodynamic forces should be taken into account. Traditional nanocomposites showed liquid-to-solid transition due to the formation of percolated particle network. Although Jancaret al.50 determined that the percolation concentration (φ) is even above 15 wt % for composites with well-dispersed NP in polymeric matrix. It was believed that nanocomposites with clusters or agglomerated particles may lead to an interconnected three-dimensional (3D) network with quite lower φ, according to Ahmad.51 As shown in morphology observations (Figure 6i−j), JGS arranged exclusively across the interface by thermodynamic forces, which facilitated the formation of extended aggregated38 in melt-state to influence system’s rheology performance. Furthermore, it is well-established that elevated dynamic moduli at low ω of the polymer blend nanocomposites strongly reflect formation of networks among components. It is conjectured a new type nanoparticle−polymer hybrid network, which contained both NP clusters and chemical gels was formed gradually during blending. Since the architecture of NP (Janus corona) and interactions between NP and polymers (including both chemical bonding and physical entanglement effects) in our system are quite unique with conventional hybrid networks, it is important to term it as “heterogeneous network” for better distinguish. The detailed assumption was illustrated schematically in Scheme 3. On the one hand, aggregation of JGS was developed due to inter and intraparticle interactions as mentioned above. On the other hand, a longrange “particle-polymer network” deriving from molecular interaction between vicinity polymers and the JGS could be envisaged, in which JGS applied as junctions. As expected, JGS seemed to be able to generate linkers around their surface with Janus-distributed brushes via the entanglement between the long tails on JGS and matrix polymer chain. The “JGS-JGS aggregates” and “JGS-polymer network” play a synergistic role in forming hierarchical clusters in blends. Therefore, a new type interpenetrating 3D network formed across phases although free chains in matrix or dispersed phases do not come in direct contact with each other, which was responsible for G′(ω) plateau at low frequency with rather low φ value. To confirm the existence of the “heterogeneous network” by virtue of JGS, the weighted relaxation spectra of the nanosilica filled PVDF/PLLA blend systems were evaluated by the edge preserving regulation method27,30,60 from SAOS tests. As illustrated in Figure 7, the spectra are divided into six groups according to their influential factors for clarify. Two peaks were observed for all specimens at the relaxation time (τ) in 0.004s and 0.33s, which was associated with PLLA and PVDF components, respectively. Besides, an additional peak at the τ in the range 3.8 s < ω < 81.3 s was induced by the form relaxation of PVDF-domains.26,30,34 For the 65/35 (w/w) blend (Figure 7a), the significantly rise of relaxation modulus at

Scheme 3. Localization and Dispersion of Nanosilica (SiO2, GS and JGS) within PVDF/PLLA (50/50) Blend and the Possible Formation Mechanism of “Heterogeneous Network” Facilitated by Interfacial JGS

longer time indicates enhanced interfacial relaxation due to cocontinuous phase structures as confirmed before. For better understanding of the molecular processes, we mainly focus on the nanosilica filled 50/50 blend systems in the following discussion. As shown in Figure 7b, the amplified spectra for pristine 50/ 50 blend revealed that the shape relaxation of PVDF was quite small, suggesting poor compatibility between phases.26 Interestingly, the form relaxation peak of JGS-filled system became more amplitude while that for SiO2 or GS containing system was unchanged, indicating the strengthened relaxation effects at PVDF−PLLA interface by virtue of interfacial anchored nanosilicas (Figure 7c). More pronounced variations were observed for blends containing nanosilicas with Janusdistributed brushes (Figure 7d), finer particle sizes (Figure 7e). To put this results in context, the form relaxation of PVDFdomains (i.e., interfacial relaxation) was closely related to the hierarchical networks across phases. As shown in Scheme 4, a rigid “shell” formed by interfacial JGS inhibited the retraction of PVDF-domains into spherical shapes, which was familiar to the “Pickering emulsification effect”;1−3 at the same time, the interfacial interactions (PVDF−PLLA) were dramatically improved by the long-range “JGS-Polymer network”, further restraining molecular motions adjacent to PVDF−PLLA interface. The reinforcement of form relaxation turned greater as a function of JGS concentrations (Figure 7f) since a more penetrated “heterogeneous network” was formed by increasing JGS loadings (Scheme 3). More quantitative characterizations, such as calculations of interfacial tensions (Tab. S-1, S-2), stress relaxation tests (Figure S-4) and creep-recovery tests (Figure S5), were carried out for verification. It is worth mention that the stress relaxation is quite an appropriate means of characterizing relaxation behaviors in concentrated immiscible polymer blends J


Article

Macromolecules

Figure 7. Weighted relaxation spectrum of pristine and nanosilica filled PVDF/PLLA blends at 200 °C: (a) PVDF/PLLA blends with various compositions (w/w: 20/80, 50/50 and 65/35), (b) high resolution spectra of the 50/50 blend, (c) PVDF/PLLA (50/50) blend with 1 wt % of interfacial nanosilica with different chemical surface (bare SiO2 and JGS), (d) PVDF/PLLA (50/50) blend with 1 wt % of nanosilica possessing different surface configurations (bare SiO2, GS and JGS), (e) PVDF/PLLA (50/50) blend with 1 wt % addition of JGS with various particle diameter (110 and 20 nm), and (f) PVDF/PLLA (50/50) blends incorporated of JGS with different loadings (0, 0.5, 1, 3, and 5 wt %).

Scheme 4. Mechanism of “Heterogeneous Network” by Interfacial (PVDF−PLLA) Anchored JGS and the Particle-Percolation Assumption by JGS Functions as Entanglement Attractors, Which Causes an Increased Entanglement Density

with high concentration from macro perspective.61 Main factors influencing stress relaxation process of binary blend systems include the molecular relaxation and interfacial relaxation, in which the latter are much slower due to the dependences of the microstructures, domains’ interactions and initial-strain values.62,63 Since the morphology evolution resulted from flow field can be neglected in test, the relaxation modulus (G(t)) are fully determined by interfacial tensions as well as the interactions between droplets. The remarked increase of G(t) upon addition of JGS (Figure S-4a, b) and further improvement of JGS systems with finer size (Figure S-4c) and higher concentrations (Figure S-4d) were exhibited as a direct evidence for the formation of a “heterogeneous network” facilitated by JGS.

5. CONCLUSION The rheology of nanosilica filled immiscible PVDF/PLLA blends has been investigated. It was revealed that JGS-filled immiscible blend systems exhibited a solid-like flow behavior in terminal region while the blends containing bare SiO2 or GS behaved as viscous fluids which were similar to linear homopolymers, implying strong emulsifying effect of JGS in blends. Moreover, the interfacial Janus-hemisphere nanoparticles with smaller particle size led to more pronounced plateau in dynamic moduli. It was further found that the solidlike behaviors were strong dependent on JGS concentrations and thus percolation threshold (φ) of JGS in PVDF/PLLA blends was calculated to be 1 wt % via frequency sweeping tests. On the basis of these results, we concluded that the nonterminal behavior for the JGS filled blend system was attributed to the emergence of interfacial JGS in PVDF/PLLA K


Article

Macromolecules

on Poly (Propylene)-Graphene Nanocomposites. RSC Adv. 2016, 6, 87828−87835. (5) Wang, Y. H.; Chen, K. L.; Xu, C. H.; Chen, Y. K. Supertoughened Biobased Poly(lactic acid)-Epoxidized Natural Rubber Thermoplastic Vulcanizates: Fabrication, Co-continuous Phase Structure, Interfacial in Situ Compatibilization, and Toughening Mechanism. J. Phys. Chem. B 2015, 119, 12138−12146. (6) Ehlert, S.; Taheri, S. M.; Pirner, D.; Drechsler, M.; Schmidt, H. W.; Förster, S. Polymer Ligand Exchange to Control Stabilization and Compatibilization of Nanocrystals. ACS Nano 2014, 8, 6114−6122. (7) Lin, N.; Dufresne, A. Physical and/or Chemical Compatibilization of Extruded Cellulose Nanocrystal Reinforced Polystyrene Nanocomposites. Macromolecules 2013, 46, 5570−5583. (8) Lundén, H.; Liotta, A.; Chateau, D.; Lerouge, F.; Chaput, F.; Parola, S.; Brännlund, C.; Ghadyani, Z.; Kildemo, M.; Lindgren, M.; Lopes, C. Dispersion and Self-Orientation of Gold Nanoparticles in Sol-Gel Hybrid Silica-Optical Transmission Properties. J. Mater. Chem. C 2015, 3, 1026−1034. (9) Horechyy, A.; Zafeiropoulos, N. E.; Nandan, B.; Formanek, P.; Simon, F.; Kiriy, A.; Stamm, M. Highly Ordered Arrays of Magnetic Nanoparticles Prepared via Block Copolymer Assembly. J. Mater. Chem. 2010, 20, 7734−7741. (10) Gorrasi, G.; Sorrentino, A. Mechanical Milling as a Technology to Produce Structural and Functional Bio-Nanocomposites. Green Chem. 2015, 17, 2610−2625. (11) Cohen, E.; Zonder, L.; Ophir, A.; Kenig, S.; McCarthy, S.; Barry, C.; Mead, J. Hierarchical Structures Composed of Confined Carbon Nanotubes in Cocontinuous Ternary Polymer Blends. Macromolecules 2013, 46, 1851−1859. (12) Gegenhuber, T.; Krekhova, M.; Schöbel, J.; Gröschel, A. H.; Schmalz, H. Patchy” Carbon Nanotubes as Efficient Compatibilizers for Polymer Blends. ACS Macro Lett. 2016, 5, 306−310. (13) Bharati, A.; Wübbenhorst, M.; Moldenaers, P.; Cardinaels, R. Dielectric Properties of Phase-Separated Blends Containing a Microcapacitor Network of Carbon Nanotubes: Compatibilization by a Random or Block Copolymer. Macromolecules 2017, 50, 3855− 3867. (14) Si, M. Y.; Araki, T.; Ade, H.; Kilcoyne, A. L. D.; Fisher, R.; Sokolov, J. C.; Rafailovich, M. H. Compatibilizing Bulk Polymer Blends by Using Organoclays. Macromolecules 2006, 39, 4793−4801. (15) Salehiyan, R.; Song, H. Y.; Choi, W. J.; Hyun, K. Characterization of Effects of Silica Nanoparticles on (80/20) PP/PS Blends via Nonlinear Rheological Properties from Fourier Transform Rheology. Macromolecules 2015, 48, 4669−4679. (16) Mofokeng, J. P.; Kelnar, I.; Luyt, A. S. Effect of Layered Silicates on the Thermal Stability of PCL/PLA Microfillar Composites. Polym. Test. 2016, 50, 9−14. (17) Jalali Dil, E.; Favis, B. D. Location of Micro and Nano-Silica Particles in a High Interfacial Tension Poly (lactic acid)/Low Density Polyethylene System. Polymer 2015, 77, 156−166. (18) Scherzer, S. L.; Pavlova, E.; Esper, J.; Starý, Z. Phase Structure, Rheology and Electrical Conductivity of Co-continuous Polystyrene/ Polymethylmethacrylate Blends Filled with Carbon Black. Compos. Sci. Technol. 2015, 119, 138−147. (19) Imperiali, L.; Clasen, C.; Fransaer, J.; Macosko, C. W.; Vermant, A. Simple Route Towards Graphene Oxide Frameworks. Mater. Horiz. 2014, 1, 139−145. (20) Zhang, W. H.; Fu, B. X.; Seo, Y.; Schrag, E.; Hsiao, B.; Mather, P. T.; Yang, N. L.; Xu, D. Y.; Ade, H.; Rafailovich, M.; Sokolov, J. Effect of Methyl Methacrylate/Polyhedral Oligomeric Silsesquioxane Random Copolymers in Compatibilization of Polystryene and Poly (methyl methacrylate) Blends. Macromolecules 2002, 35, 8029−8038. (21) Fu, Z. A.; Wang, H. T.; Dong, W. Y.; Li, Y. J. Synthesis of Reactive Nanoparticles and Their Compatibilization Effects on Immiscible Polymer Blends. Acta Polym. Sin. 2017, 2, 334−341. (22) Han, D.; Zhang, Q.; Chen, F.; Fu, Q. Using POSS-C60 Giant Molecules as a Novel Compatibilizer for PS/PMMA Polymer Blends. RSC Adv. 2016, 6, 18924−18928.

blend, which is assumed to be the consequence of both emulsification effects in immiscible blends and the formation of 3D ordered nanosilica network in polymer nanocomposites. To be specific, the new type network, termed as “heterogeneous network”, could be envisaged: on the one hand, aggregation of JGS at the PVDF−PLLA interface was developed due to inter and intraparticle interactions; on the other hand, a long-range “particle−polymer network” deriving from molecular interaction between vicinity polymers and the JGS was gradually formed, in which JGS was applied as junctions. This assumption was further verified from the molecular relaxation process using weighted relaxation spectra and stress relaxation measurements.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02143. Additional data including the Gramespacher−Meissner model, rheology parameters of PVDF/PLLA (50/ 50)blend (Table S-1), the calculated form relaxation time and interfacial tension values of PVDF/PLLA (50/ 50) blends (Table S-2), creep parameters (Table S-3), linear rheology properties from SAOS experiments for the PVDF/PLLA (80/20, 65/35) blends with or without JGS (Figures S-1 and S-2), van Gurp−Palmen plots (Figure S-3), stress relaxation tests (Figure S-4), and creep-recovery experiments (Figure S-5) of PVDF, PLLA and related PVDF/PLLA blends (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail (Y.L.): [email protected]. Fax: +86 57128867899. Telephone: +86 7128867026. ORCID

Yongjin Li: 0000-0001-6666-1336 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21674033, 21374027) and National Key R&D Program of China (2017YFB0307704). The authors thank Prof. Quan Chen (Changchun Institute of Applied Chemistry, Chinese Science Academy) for a kind suggestion.

■

REFERENCES

(1) Zolali, A. M.; Favis, B. D. Compatibilization and Toughening of Co-continuous Ternary Blends via Partially Wet Droplets at the Interface. Polymer 2017, 114, 277−288. (2) Virgilio, N.; Marc-Aurèle, C.; Favis, B. D. Novel Self-Assembly Close-Packed Droplet Array at the Interface in Ternary Polymer Blends. Macromolecules 2009, 42, 3405−3416. (3) Tan, Y.; Fang, L.; Xiao, J.; Song, Y.; Zheng, Q. Grafting of Copolymers onto Graphene by Miniemulsion Polymerization for Conductive Polymer Composites: Improved Electrical Conductivity and Compatibility Induced by Interfacial Distribution of Graphene. Polym. Chem. 2013, 4, 2939−2944. (4) Kim, S. H.; Kim, K.; Park, O. O. Poly (Propylene)-Grafted Thermally Reduced Graphene Oxide and Its Compatibilization Effect L


Article

Macromolecules (23) Ock, H. G.; Ahn, K. H.; Lee, S. J.; Hyun, K. Characterization of Compatibilizing Effect of Organoclay in Poly (lactic acid) and Natural Rubber Blends by FT-Rheology. Macromolecules 2016, 49, 2832− 2842. (24) Van Puyvelde, P.; Velankar, S.; Moldenaers, P. Rheology and Morphology of Compatibilized Polymer Blends. Curr. Opin. Colloid Interface Sci. 2001, 6, 457−463. (25) Huang, C. W.; Yu, W. Role of Block Copolymer on the Coarsening of Morphology in Polymer Blend: Effect of Micelles. AIChE J. 2015, 61, 285−295. (26) Sailer, C.; Handge, U. A. Reactive Blending of Polyamide 6 and Styrene-Acrylonitrile Copolymer: Influence of Blend Composition and Compatibilizer Concentration on Morphology and Rheology. Macromolecules 2008, 41, 4258−4267. (27) Hong, J. S.; Kim, Y. K.; Ahn, K. H.; Lee, S. J.; Kim, C. Y. Interfacial Tension Reduction in PBT/PE/Clay Nanocomposite. Rheol. Acta 2007, 46, 469−478. (28) Ezzati, P.; Ghasemi, I.; Karrabi, M.; Azizi, H. Rheological Behavior of PP/EPDM Blends: The Effect of Compatibilization. Iran. Polym. J. 2008, 17, 669−679. (29) Shah, D.; Maiti, P.; Jiang, D. D.; Batt, C. A.; Giannelis, E. P. Effect of Nanoparticle Mobility on Toughness of Polymer Nanocomposites. Adv. Mater. 2005, 17, 525−528. (30) Salehiyan, R.; Yoo, Y.; Choi, W. J.; Hyun, K. Characterization of Morphologies of Compatibilized Polypropylene/Polystyrene Blends with Nanoparticles via Nonlinear Rheological Properties from FTRheology. Macromolecules 2014, 47, 4066−4076. (31) Vandebril, S.; Vermant, J.; Moldenaers, P. Efficiently Suppressing Coalescence in Polymer Blneds Using Nanoparticles: Role of Interfacial Rheology. Soft Matter 2010, 6, 3353−3362. (32) Yu, W.; Zhou, W.; Zhou, C. X. Linear Viscoelasticity of Polymer Blends with Co-continuous Morphology. Polymer 2010, 51, 2091− 2098. (33) Huang, S. J.; Bai, L.; Trifkovic, M.; Cheng, X.; Macosko, C. W. Controlling the Morphology of Immiscible Cocontinuous Polymer Blends via Silica Nanoparticles Jammed at the Interface. Macromolecules 2016, 49, 3911−3918. (34) Genoyer, J.; Yee, M.; Soulestin, J.; Demarquette, N. Compatibilization Mechanism Induced by Organoclay in PMMA/PS Blends. J. Rheol. 2017, 61, 613−626. (35) Lee, S. H.; Kontopoulou, M.; Park, C. B. Effect of Nanosilica on the Co-continuous Morphology of Polypropyrene/Polyolefin Elastomer Blends. Polymer 2010, 51, 1147−1155. (36) Cassagnau, P. Melt Rheology of Organoclay and Fumed Silica Nanocomposites. Polymer 2008, 49, 2183−2196. (37) Bai, L.; Fruehwirth, J. W.; Cheng, X.; Macosko, C. W. Dynamics and Rheology of Nonpolar Bijels. Soft Matter 2015, 11, 5282−5293. (38) Song, Y. H.; Zheng, Q. Linear Rheology of Nanofilled Polymers. J. Rheol. 2015, 59, 155−191. (39) Wu, G.; Asai, S.; Sumita, M.; Hattori, T.; Higuchi, R.; Washiyama, J. Estimation of Flocculation Structure in Filled Polymer Composites by Dynamic Rheological Measurements. Colloid Polym. Sci. 2000, 278, 220−228. (40) Trifkovic, M.; Hedegaard, A. T.; Sheikhzadeh, M.; Huang, S.; Macosko, C. W. Stabilization of PE/PEO Cocontinuous Blends by Interfacial Nanoclays. Macromolecules 2015, 48, 4631−4644. (41) Wang, H. T.; Fu, Z. A.; Zhao, X. W.; Li, Y. J.; Li, J. Y. Reactive Nanoparticles Compatibilized Immiscible Polymer Blends: Synthesis of Reactive SiO2 with Long Poly(methyl methacrylate) Chains and the in Situ Formation of Janus SiO2 Nanoparticles Anchored Exclusively at the Interface. ACS Appl. Mater. Interfaces 2017, 9, 14358−14370. (42) Parpaite, T.; Otazaghine, B.; Caro, A. S.; Taguet, A.; Sonnier, R.; Lopez-Cuesta, J. M. Janus Hybrid Silica/Polymer Nanoparticles as Effective Compatibilizing Agents for Polystryene/Polyamide-6 Melted Blends. Polymer 2016, 90, 34−44. (43) He, P.; Shen, W.; Yu, W.; Zhou, C. X. Mesophase Separation and Rheology of Olefin Multiblock Copolymers. Macromolecules 2014, 47, 807−820.

(44) White, K. L.; Li, P.; Sumi, Y.; Sue, H. J. Rheology of Disentangled Multiwalled Carbon Nanotubes Dispersed in Uncured Epoxy Fluid. J. Phys. Chem. B 2014, 118, 362−371. (45) Kim, J. K.; Lee, H. H.; Son, H. W.; et al. Phase Behavior and Rheology of Polystyrene/Poly (α-methylstryene) and Polystyrene/ Poly (vinyl methyl ether) Blends Systems. Macromolecules 1998, 31, 8566−8578. (46) Mitchell, C. A.; Krishnamoorti, R. Dispersion of Single-Walled Carbon Nanotubes in Poly (ε-Caprolactone). Macromolecules 2007, 40, 1538−1545. (47) Caro, A. S.; Parpaite, T.; Otazaghine, B.; Taguet, A.; LopezCuesta, J. M. Viscoelastic Properties of Polystyrene/Polyamide-6 Blend Compatibilized with Silica/Polystyrene Janus Hybrid Nanoparticles. J. Rheol. 2017, 61, 305−310. (48) Kar, G. P.; Bharati, A.; Xavier, P.; Madras, G.; Bose, S. The Key Role of Polymer Grafted Nanoparticles in the Phase Miscibility of an LCST Mixture. Phys. Chem. Chem. Phys. 2015, 17, 868−877. (49) Xavier, P.; Bose, S. Nanomechanical Mapping, Hierarchical Polymer Dynamics, and Miscibility in the Presence of Chain-End Grafted Nanoparticles. Macromolecules 2016, 49, 1036−1048. (50) Jancar, J.; Douglas, J. F.; Starr, F. W.; Kumar, S. K.; Cassagnau, P.; Lesser, A. J.; Sternstein, S. S.; Buehler, M. J. Current Issues in Research on Structure-Property Relationships in Polymer Nanocomposites. Polymer 2010, 51, 3321−3343. (51) Zohrevand, A.; Ajji, A.; Mighri, F. Relationship between Rheological and Electrical Percolation in a Polymer Nanocomposites with Semiconductor Inclusions. Rheol. Acta 2014, 53, 235−254. (52) Moll, J. F.; Akcora, P.; Rungta, A.; Gong, S.; Colby, R. H.; Benicewicz, B. C.; Kumar, S. K. Mechanical Reinforcement in Polymer Melts Filled with Polymer Grafted Nanoparticles. Macromolecules 2011, 44, 7473−7477. (53) Bazli, L.; Khavandi, A.; Boutorabi, M. A.; Karrabi, M. Correlation between Viscoelastic Behavior and Morphology of Nanocomposites Based on SR/EPDM Blends Compatibilized by Maleic Anhydride. Polymer 2017, 113, 156−166. (54) Han, C. D.; Chuang, H. K. Criteria for Rheological Compatibility of Polymer Blends. J. Appl. Polym. Sci. 1985, 30, 4431−4454. (55) Vermant, J.; Ceccia, S.; Dolgovskij, M. K.; Maffettone, P. L.; Macosko, C. W. Quantifying Dispersion of Layered Nanocomposites via Melt Rheology. J. Rheol. 2007, 51, 429−450. (56) Kota, A. K.; Cipriano, B. H.; Duesterberg, M. K.; Gershon, A. L.; Powell, D.; Raghavan, S. R.; Bruck, H. A. Electrical and Rheological Percolation in Polystyrene/MWCNT Nanocomposites. Macromolecules 2007, 40, 7400−7406. (57) Abbasi, S.; Carreau, P.; Derdouri, A.; Moan, M. Rheological Properties and Percolation in Suspension of Multiwalled Carbon Nanotubes in Polycarbonate. Rheol. Acta 2009, 48, 943−959. (58) Ginzburg, V. V. Influence of Nanoparticles on Miscibility of Polymer Blends. A Simple Theory. Macromolecules 2005, 38, 2362− 2367. (59) Mitchell, C. A.; Krishnamoorti, R. Dispersion of Single-Walled Carbon Nanotubes in Poly (ε-Caprolactone). Macromolecules 2007, 40, 1538−1545. (60) Roths, T.; Maier, D.; Friedrich, C.; Marth, M.; Honerkamp, J. Determination of The Relaxation Time Spectrum from Dynamic Moduli Using an Edge Preserving Regulation Method. Rheol. Acta 2000, 39, 163−173. (61) He, Q.; Yu, W.; Zhou, C. X. Phase Morphology and Rheological Properties of Concentrated Immiscible Polymer Blends. Gaofenzi Xuebao 2010, 010, 114−119. (62) Yu, W.; Bousmina, M.; et al. Quantitative Relationship between Rheology and Morphology in Emulsions. J. Rheol. 2002, 46, 1381− 1399. (63) Wu, Y. J.; Yu, W.; Zhou, C. X. A Study on Interfacial Tension between Flexible Polymer and Liquid Crystal. J. Colloid Interface Sci. 2006, 298, 889−898.

M


Rheology of Nanosilica-Compatibilized Immiscible Polymer Blends

Recommend Documents