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Suppression of phase coarsening in immiscible, cocontinuous polymer blends under high temperature quiescent annealing Xi-Qiang Liu, Ruo-Han Li, Rui-Ying Bao, Wen-Rou Jiang, Wei Yang,* Bang-Hu Xie and Ming-Bo Yang The properties of polymer blends greatly depend on the morphologies formed during processing, and the thermodynamic non-equilibrium nature of most polymer blends makes it important to maintain the morphology stability to ensure the performance stability of structural materials. Herein, the phase coarsening of co-continuous, immiscible polyamide 6 (PA6)–acrylonitrile-butadiene-styrene (ABS) blends in the melt state was studied and the effect of introduction of nano-silica particles on the stability of the phase morphology was examined. It was found that the PA6–ABS (50/50 w) blend maintained the co-continuous morphology but coarsened severely upon annealing at 230  C. The coarsening process could be divided into two stages: a fast coarsening process at the initial stage of annealing and a second coarsening process with a relatively slow coarsening rate later. The reduction of the coarsening rate can be explained from the reduction of the global curvature of the interface. With the introduction of nano-

Received 29th December 2013 Accepted 11th February 2014

silica, the composites also showed two stages of coarsening. However, the coarsening rate was significantly decreased and the phase morphology was stabilized. Rheological measurements indicated that a particle network structure was formed when the concentration of nano-silica particles was

DOI: 10.1039/c3sm53211a

beyond 2 wt%. The particle network inhibited the movement of molecular chains and thus suppressed

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the coarsening process.

I.

Introduction

Polymer blending is an easy and effective way to obtain new materials with desired properties and the properties of polymer blends constituted with given polymer components are to a large extent determined by the phase morphologies, such as the size, shape and distribution of the components.1–3 Factors governing the morphology of polymer blends include composition, rheological properties of components, interfacial interaction, processing conditions and introduction of llers with different surface characterisitics.4,5 Normally, two typical morphologies, i.e. sea–island morphology and co-continuous morphology can be observed in polymer blends. For polymer blends, one of the challenges is that the generated morphologies during processing are usually in a non-equilibrium state aer the melt mixing of the polymers.6 So, the morphologies and the resulting properties of polymer blends are generally unstable during further processing (such as compression and injection molding). In these cases, a reduction of interfacial areas and an increase of phase sizes oen occur, leading to a socalled coarsening of the phases.7–9 State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065 Sichuan, China. E-mail: weiyang@ scu.edu.cn; Fax: +86 28 8546 0130; Tel: +86 28 8546 0130

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The coarsening process in polymer blends has been regarded to be driven by interfacial tension and can occur by various patterns, viz. coalescence,10,11 and/or breakup, retraction and end-pinching.12–15 It is also found that the coarsening process is intensely dependent on the blend composition and blend morphology. Hashimoto et al.16–19 studied the coarsening behavior of the near-critical and off-critical mixtures, such as mixtures of poly(styrene-random-butadiene) copolymer with polyisoprene or polybutadiene, by using time-resolved light scattering. They found that the coarsening process continued with time for the near-critical mixture while the coarsening was effectively pinned at a certain time for the off-critical mixture. They believed that the spontaneous pinning originated from a “dynamical percolation-to-cluster (PC) transition” and a kinetically frozen diffusion–coalescence process. Their study denitely showed that the coarsening process was tightly associated with the blend composition. Willemse et al.8 summarized the coarsening of polymer blends with different initial morphologies in the molten quiescent state. If the initial morphology is a droplet/matrix structure, the coarsening takes place by coalescence of droplets provided that the volume fraction exceeds the percolation threshold of droplets. This is a comparatively slow process, leading to slow but continuous increase of the phase sizes. For initial structures with a brillar dispersed phase, coarsening takes place by the way of breaking up and/or

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retraction, leading to droplet/matrix morphology. This process is completed in a very short time and results in limited increase of the phase dimensions. Certainly, further coarsening may go on by the way of coalescence of droplets if the volume fraction exceeds the percolation threshold. For co-continuous morphology with lower concentration of the minor phase, signicant coarsening can occur either by breakup or retraction on a short time scale. Above a critical concentration, breakup does not take place, and only reaction can be observed, leading to a situation that the co-continuous structure is maintained but the phase sizes increase under annealing. Several classical models have been built to describe the coarsening process. Based on computer simulations, Binder– Stauffer et al.20 proposed a model with the assumption that for a range of times the relaxation proceeded by the diffusion and coalescence of large clusters. Lifshitz–Slyozov proposed evaporation–condensation theory for droplets growing from a slightly supersaturated broth.21 Siggia studied the inuence of hydrodynamic interactions on the coarsening rate and found that when the minority phase is continuous, as in a quench at the critical concentration, a surface-tension effect leads to a more rapid coarsening rate.22 To obtain blends with tailored properties, one of the prerequisites is to maintain the morphology stability of the blends. As a result, it is necessary to suppress the coarsening process of some special phase morphologies which are benecial to some particular properties of the blends. Traditional solution for this problem is incorporating organic compatibilizers, oen block copolymers, which can promote morphology renement by suppressing the coalescence of the dispersed droplets in polymer blends.23–25 Recently, nano-particle lled polymer blends have become attractive because generally the incorporation of nano-particles endows the resulted polymer blend based composites with enhanced properties or some functional performance. It should also be noted that the introduction of nano-particles can also signicantly inuence the morphology of polymer blends. The inuences involve morphology renement of the phases or the compatibilization of the components of the blends, which was rst reported for carbon black (CB) particle lled elastomers.26,27 Gubbels et al.28 reported that the co-continuous composition range of a polyethylene (PE)–polystyrene (PS) blend was enlarged owing to the introduction of CB particles and the macroscopic electrical resistivity of the lled blends strongly depended on the selective localization of CB in one phase or at the interface. Elias et al.29,30 observed a drastic reduction of the size of the dispersed PS phase in polypropylene (PP)–PS blends in the presence of nano-silica particles, especially the hydrophobic silica particles located at the interface of the two phases. Zhang et al.31 also found a drastic reduction of PS phase size and a very homogeneous size distribution of PS domains by introducing nano-silica into the PP–PS blend and the compatibilization effect was shown to be kinetics controlled. A similar effect of nano-silica was also found in PP–polyamide (PA) and PP–polycarbonate (PC) systems.32 Many papers reported the tailoring of the morphologies of immiscible polymer blends using layered silicates.33–35 Si et al.36 studied the morphology of

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PS–polymethyl methacrylate (PMMA), PC–styrene-acrylonitrile copolymer (SAN), and PMMA–ethylene vinyl-acetate copolymer blends and compared the morphologies with and without modied organoclay and found in each case a large reduction in domains' size and the localization of the clay platelets along the interfaces of the components. So, the effect of nano-particles on the morphology of polymer blends reported in the literature is obvious: the reduction of the size of the dispersed domains or droplets and the morphology renement. The reduction of the interfacial tension due to the distribution of the llers at the polymer–polymer interface is oen cited as a potential explanation for the morphology change.29,30,35 However, sometimes the steric-hindrance effect of the solid particles when they are located at the interface is more reasonable.29,35,37,38 Actually, a similar effect of particles has been extensively reported in low viscosity emulsions. The emulsion formation is a dynamic process including two competitive processes: fragmentation of the liquid phase into smaller droplets and coalescence of these droplets. Ramsden39 and Pickering40 reported that insoluble particles can effectively suppress the coalescence process. As a result, a stabilized emulsion with much smaller drop size can be obtained. The particle-stabilized emulsions are normally described as “Pickering emulsions”.40 Okubo41 and Vignati and Piazza42 demonstrated that the interfacial tension between the two liquids of the emulsion is unaffected in the presence of particles. Binks et al.43 proposed a steric stabilization mechanism that particles in fully covered droplets can suppress the coalescence if they protrude far outside the drops. Stancik et al.44 conrmed that particle monolayers bridging droplets were effective in suppressing the phase coalescence process. For the phase morphology stabilization of polymer blends, Gubbels et al.28 showed that the phase morphology of polymer blends could be stabilized under post-thermal treatment in the melt state in the presence of CB particles and the slowing down of the phase coalescence process was attributed to the viscosity increase of one phase. Vermant et al.45 also reported that in a model blend of polydimethylsiloxane–polyisobutylene, the lled nano-silica particles reduced the sensitivity of the dispersed phase/matrix microstructure to shear ow. The phase coalescence was suppressed or at least delayed within the time scales investigated in the presence of nano-silica, similar to what was observed in “Pickering” emulsions. In the PA–ethyleneethylene-methylacrylate copolymers blend, they found that both phase coalescence and break-up were affected by the presence of particles.46 Nagarkar et al.47 showed that the effect of particles on the droplet size was not monotonic and the particles could completely cover the interface and reduce the size of the dispersed phase only at high particles loadings. At present, the phase coarsening process of polymer blends has been studied widely. However, the phase coarsening process of polymer blends in the presence of nano-particles, especially at high temperature, is still scarce. Except for the work of Gubbels,28 no further reports concern the phase coarsening of particle lled polymer blends under quiescent annealing at high temperature and the morphological stabilization mechanism of particles is still not completely

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understood. In this paper, the phase coarsening of immiscible PA6–acrylonitrile-butadiene-styrene (ABS) blends with a cocontinuous morphology and the effect of nano-silica particles are extensively studied. We managed to effectively suppress the phase coarsening processes in polymer blends at high temperature and clarify the morphological stabilization mechanism of nano-particles, which will probably provide helpful guidance for the ne tuning of morphologies during processing and performance optimization of polymer blend based composites.

II.

Experimental section

Materials and sample preparation PA6 (Grade M2800), with a molecular weight of 36 000 and a polydispersity index of 1.6, was purchased from Guangdong XinHui MeiDa Nylon Company, China. The density was 1.14 g ml1 and the melt ow rate was 11 g per 10 min (according to ASTM D-1238). The ABS copolymer was purchased from Toray Engineering Corporation-TEK, Japan (Grade 100). The density was 1.03 g ml1 and the melt ow rate was 15 g per 10 min at 220  C (according to ASTM D-1238). The nano-silica particles used, hydrophobic Aerosil R974, were purchased from Evonik Degussa Corporation, Germany. The particles have an average diameter of 12 nm and a methyl density of 1.25 –CH3 nm2. The PA6–ABS (50/50 w) blend and two PA6–ABS (50/50 w) based composites containing 2 and 6 wt% nano-silica particles, were prepared on an SHJ-20 co-rotating twin-screw extruder (Nanjing Giant Electric Machine Corporation, China) with a screw diameter of 25 mm and a length/diameter ratio of 23 : 1. In all the samples the weight ratio of PA6 and ABS was maintained at 50/50 w. To avoid oxidation of ABS during processing, 0.3 wt% of an anti-oxidant (grade 1010) was used. Before mixing, all the materials were dried in a vacuum oven for 24 hours at 80  C. Then, the mixed materials were extruded with a screw rotation speed of 150 rpm and a temperature prole of 210, 225, 235 and 230  C from the feeding zone to the die, respectively. The extruded samples were quenched in water at room temperature and chopped into pellets, and then dried at 80  C for 24 hours. The samples obtained were compression molded into sheets with a thickness of 2 mm. The annealing process was conducted on the same compression molding machine without pressure. High temperature quiescent annealing was conducted at 230  C and the annealing time was set as 0, 1, 3, 10, 60 and 120 minutes. Morphology characterization The compression molded samples were cryo-fractured in liquid nitrogen and the extraction of the ABS phase was then performed using tetrahydrofuran (THF). The extraction time was 3 hours, which is sufficient to completely remove the soluble ABS phase on the fractured surfaces. The morphologies of the samples were observed with a JEOL JSM-5900LV scanning electron microscopy (SEM, JEOL, Japan) at an accelerating voltage of 5 kV. Before SEM observation, the fractured surfaces were cleaned repeatedly and sputtered with gold to avoid charge accumulation. Because it is hard to give a phase dimension of

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co-continuous polymer blends, here we take the diameter of the strands of one phase as the phase size and the average phase sizes were determined by analyzing the SEM images using Image-Pro Plus image analysis soware. The statistics was done on the SEM images and at least 150 diameter values at different locations in the images were counted to calculate the average phase size. We also did element analysis during SEM observation and for convenience, only carbon (C), oxygen (O) and silicon (Si) elements were shown on the energy spectrum. Transmission electron microscopy (TEM, JEOL JEM-2100F) was also used to reveal the distribution of nano-silica particles in the blends. Crosssections of the compression-molded composites were obtained by slicing the samples into thin lms of about 50 nm in thickness. Rheological measurements Rheological measurements were performed on an AR2000ex stress controlled dynamic rheometer (TA Corporation, USA) using a parallel plate geometry (25 mm in diameter). In order to prevent thermo-oxidative degradation, all the measurements were performed under a nitrogen atmosphere. Frequency sweep was carried out in the frequency range of 0.01–100 s1 at 230  C. The strain used was 2% which is within the linear viscoelastic range.

III.

Results and discussion

Rheological properties Fig. 1a shows the complex viscosity as a function of frequency for the raw materials used, the blend and the composites. The PA6 melt behaves like a Maxwell uid with a Newtonian plateau of viscosity at lower frequencies and shear-thinning behavior at higher frequencies. Pure ABS shows similar melt behavior to PA6 but with much higher viscosity. The PA6–ABS (50/50 w) blend shows much higher viscosity than that of the PA6 melt but lower viscosity than that of the ABS melt, and shows similar shear-thinning behavior to the ABS melt. From the curves, zeroshear viscosity h0 can be obtained by tting all the data in the curve using the Carreau–Yasuda-equation48,49 as follows: * h ðuÞ ¼ h0 ½1 þ ðluÞa n1=a (1) where h0 is the zero-shear-rate viscosity, l is a characteristic time, n is a power law index, “a” the width of the transition and u is the frequency. With the incorporation of nano-silica particles, the scope of shear-thinning behavior expands to the whole frequency range measured. This kind of shear thinning behavior is oen associated with a melt yield stress.50 In this case, zeroshear viscosity can be obtained using the modied Carreau– Yasuda-equation,51 which is applied in the pronounced shear thinning regime. * h ðuÞ ¼ h0 ðluÞn1 (2) All the zero-shear viscosity values are summarized in Table 1. Fig. 1b shows the storage modulus (G0 ) versus frequency for the pure components, the blends and the composites. Compared with the PA6–ABS blend, the nano-silica particle lled blends Soft Matter, 2014, 10, 3587–3596 | 3589

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Fig. 1

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(a) Complex viscosity and (b) storage modulus as functions of frequency for the pure components, the pure blend and the filled blends.

Zero-shear viscosity of the polymers, the blend and composites

Table 1

Sample

PA6 ABS

(PA6–ABS)/2 (PA6–ABS)/6 PA6–ABS wt% nano-silica wt% nano-silica

Zero-shear 710 10 200 3040 viscosity (Pa s)

17 400

phase sizes as a function of annealing time. Obviously, a signicant phase coarsening process occurred for the PA6–ABS blend during annealing. The phase size increased from about 10 mm to over 100 mm in only 10 min. Aer that, the coarsening

34 200

show higher elasticity at low frequencies, which is proportional to the content of nano-silica particles. The slopes of G0 at low frequencies decrease from 0.81 for the blend to 0.43 for the 6% nano-silica particle lled blend. Compared with the 2 wt% nanosilica particle lled blend, the 6 wt% nano-silica particle lled blend shows stronger shear thinning and greater enhanced elasticity. The enhanced elasticity and the weakened frequency dependence of G0 indicate a transition from a liquid-like to a solid-like viscoelastic behavior, which can be attributed to the formation of a rheological percolation network in the blends. This behavior has been widely reported in the nano-silica particle29–31,52–56 and other inorganic particles lled polymer composites.57–59 That is to say, the lled nano-silica particles in the PA6–ABS blend form a network structure, and undoubtedly, the network structure is more prominent for the 6 wt% nanosilica particle lled blend. Phase coarsening of the unlled polymer blend Fig. 2 shows the morphologies of the pure PA6–ABS blend aer annealing at 230  C for different periods of time (0, 1, 3, 10, 60, 120 min). To give a clear view, the SEM micrographs showed the PA6 matrix that was le aer the ABS phase had been extracted by THF. It should be noted that Fig. 2(a, b and c) have a scale bar of 200 mm whereas other micrographs have a scale bar of 500 mm. It is clear that a co-continuous morphology was formed for the PA6–ABS blend directly aer extrusion. Aer annealing, the co-continuous morphology was maintained but a signicant increase of phase size was observed. Fig. 3 shows the average

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SEM micrographs of PA6–ABS (50/50 w) blends annealed at 230 C for (a) 0 min; (b) 1 min; (c) 3 min; (d) 10 min; (e) 60 min and (f) 120 min.

Fig. 2 

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Fig. 3 Average phase size of the PA6–ABS blend and 2 and 6 wt% silica particle filled PA6–ABS composites as a function of annealing time. (The inset graph shows the magnified curves of 2 and 6 wt% silica particle filled PA6–ABS composites.)

The coarsening rates of the pure blend and nano-silica particle filled blends

Table 2

Sample Coarsening rate before 10 minutes (m s1) Coarsening rate aer 10 minutes (m s1)

PA6–ABS (50/50)

(PA6–ABS)/2 wt% silica

(PA6–ABS)/6 wt% silica

1.49  107

7.33  109

2.78  109

1.35  108

1.99  1010

3.33  1012 Fig. 4 SEM micrographs of the PA6–ABS (50/50 w) blend filled with 2

process continued with a lower coarsening rate. The average size increased to about 200 mm aer annealing for 120 min, which was about 30 times larger than that of the initial size. The coarsening rates, dR/dt, obtained by a linear t of phase size vs. annealing time, are listed in Table 2. The values are 1.49  107 m s1 and 1.35  108 m s1 before and aer 10 minute annealing, respectively. It reveals that the coarsening process can be divided into two stages: a fast coarsening process at the initial time of high temperature annealing and a second coarsening process with a relatively slow coarsening rate later. Similar behavior was also observed in PS–PP (50/50 w),60 PS–HDPE (50/ 50 w),61 and PS–SAN (50/50 w) blends.62 Based on the fact that the driving force of coarsening is proportional to the surface curvature, Lopez–Barron and Macosko proposed a model for the coarsening process which considered both the characteristic size and the interfacial curvature50 and by analyzing both early and late stages of coarsening, they proved that the decrease in the coarsening rate in the second stage was due to a continuous reduction of the global curvature of the interface.63 Greatly suppressed phase coarsening of the nano-silica particle lled blends Fig. 4 shows the morphologies of 2 wt% nano-silica particle lled PA6–ABS composites aer annealing at 230  C for This journal is © The Royal Society of Chemistry 2014

wt% nano-silica particle annealed at 230  C for (a) 0 min; (b) 1 min; (c) 3 min; (d) 10 min; (e) 60 min and (f) 120 min.

different periods of time (0, 1, 3, 10, 60, 120 min). The SEM micrographs, with the same scale bar of 50 mm, also show the PA6 matrix that was le aer the ABS phase had been extracted by THF. Similar to the unlled PA6–ABS blend, the 2 wt% nanosilica particle lled composite also shows a co-continuous morphology directly aer extrusion but with smaller phase sizes. However, when the lled blend is annealed at high temperature, the co-continuous morphology and the phase size, are almost unchanged. According to the dependence of the average phase size on the annealing time also shown in Fig. 3, the phase sizes were almost constant during the high temperature annealing process. The inset in Fig. 3 shows that there are also two stages of coarsening the reason for which can also be explained by the reduction of the interface curvature. The coarsening rates of the lled blend are 7.33  109 m s1 and 1.99  1010 m s1 before and aer 10 minute annealing, which are almost two orders of magnitude less than that of the unlled blend, respectively. In fact, the enlargement of the average phase sizes for the 2 wt% nano-silica particle lled blend is less than two times, indicating that the phase morphology is signicantly stabilized.

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Discussion For the pure PA6–ABS blend with co-continuous morphology, the phase coarsens via retraction rather than breakup. In fact, Stone et al.13,14 reported that break-up of bres (via sinusoidal distortions) only occurs in the case of highly extended bres. For moderately extended bres, end pinching or retraction occurs. The critical extension ratio of the bers necessary for breakup is greatly determined by the viscosity ratio of the components. According to rheological measurements, the zeroshear viscosities of PA6 and ABS, obtained by tting all the data using the Carreau–Yasuda-equation, are 710 and 10 200 Pa s, respectively. As a consequence, the viscosity ratio is 14.4. According to Stone, the critical extension ratio of the bers is larger than 10 at a viscosity ratio of 14.4. Obviously, the aspect ratio of both phases of the PA6–ABS blend between two adjacent joints in the phase morphologies is less than 10 from the SEM image in Fig. 6. So the co-continuous structure does not break up during annealing. The coarsening process only happened via retraction, leading to a co-continuous structure with continuously increasing phase sizes. Since the co-continuous structure can be considered as a three-dimensional (3D) structure formed by elongated domains that are interconnected, the coarsening process of a co-continuous morphology is also similar to breakup and retraction of isolated polymer bres in a polymer matrix. The growth rate for retraction or breakup is oen thought to be related to interfacial tension, viscosity and phase size. For example, Tjahjadi64 expressed the retraction growth rate (qr) for a polymer bre as: qr ¼ SEM micrographs of 6 wt% nano-silica particle filled PA6–ABS (50/50 w) composites annealed at 230  C for (a) 0 min; (b) 1 min; (c) 3 min; (d) 10 min; (e) 60 min and (f) 120 min. Fig. 5

Fig. 5 shows the morphologies of 6 wt% nano-silica particle lled PA6–ABS composites aer annealing at 230  C for different periods of time. Compared with the composites containing 2 wt% nano-silica particles, the phase sizes are even smaller and more constant under annealing. According to Fig. 3, the change of the average phase sizes is not more than 2 mm. However, the coarsening is still a two-stage process. The coarsening rates of the 6 wt% nano-silica particle lled blend are 2.78  109 m s1 and 3.33  1012 m s1 before and aer 10 minute annealing, which are almost two and four orders of magnitude less than that of the unlled blend, respectively. Now, it can be concluded that the nano-silica particle lled blends also show two-stage coarsening, similar to the pure blend. However, in both stages, the coarsening rate is signicantly reduced. The effectivity to suppress the coarsening process depends on the content of the lled particles. With introduction of more nano-silica particles, the coarsening rate is further decreased to a level that can be neglected. Our results assuredly show that the phase coarsening process of the cocontinuous morphology can be signicantly suppressed and the introduced nano-silica particles can effectively stabilize the cocontinuous morphology of the blends.

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s D0 h m

(3)

According to this equation, Veenstra et al.65 thought that the rate of the coarsening process is therefore proportional to the interfacial tension and inversely proportional to the viscosity of the system. They proposed an equation to express the coarsening rate as follows: dD s ¼c dt he

(4)

High magnification SEM image of the PA6–ABS (50/50 w) blend without experiencing high temperature annealing.

Fig. 6

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where dD/dt is the average diameter of the network strands changing with time, he is the effective viscosity depending on the composition of the blends and c is a dimensionless factor. By comparing the experimental coarsening rate for the pure PA6–ABS blend, a value of c ¼ 0.07 was chosen for the calculation according to eqn (3) to get the best agreement with the experimental coarsening rate.53 For the pure PA6–ABS blend, an interfacial tension value of 3.5 mN m1 at 190  C is obtained from eqn (4), which is close to the interfacial tension values of most polymer blends. When nano-silica particles were added, some of the particles were located in the ABS phase, which would change the surface energy and thus the interfacial interaction with the PA6 phase. But most importantly, some of the nano-silica particles were located at the interface as also shown in Fig. 6. So the initial PA6–ABS interface has been partially replaced by the PA6–silica interface. According to our previous thermodynamic analysis, the interface tension of PA6–ABS and PA6–silica are 0.136 and 4.05 mN m1 at room temperature, respectively.31 That is to say, the interface tension is increased, which is not benecial for the stabilization of the co-continuous morphology. However, the co-continuous morphology of the lled blend was proved to be more stable. The results demonstrate that the change in thermodynamic properties is not the main factor responsible for the morphology stabilization. The increase of viscosity may play a signicant role. For 2% and 6% nano-silica lled composites, the viscosity increased by 6 and 10 times compared with the pure PA6–ABS blend, respectively. According to eqn (4), the increased viscosity would lead to a linear decrease of the coarsening rate. However, the coarsening rates of 2% and 6% composites were found to be decreased by two orders of magnitude compared with that of the pure PA6–ABS blend. That is, the great viscosity difference between 2% and 6% nano-silica lled composites did not lead to a corresponding change in the coarsening rate. In fact, the change of the phase size is so small that it can be ignored for both 2% and 6% nano-silica lled composites. So the increase of viscosity cannot arouse a very signicant suppression effect towards coarsening, either. That

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is, the increase of the viscosity is not the main stabilization mechanism either, at least not the most important one. Except for interfacial tension and melt viscosity, the stabilization mechanism must be closely related to the microstructure of the blend aer the incorporation of nano-silica particles. Fig. 7(a) gives a clear view of the morphology of 6% nano-silica particle lled composite. It is seen that the lled composite exhibits a co-continuous morphology and the two phases are easy to be distinguished since the ABS phase shows a rough appearance due to the existence of the rubber component (the area circled by white lines). It is observed that the nano-silica particles are almost dispersed completely in the rough ABS phase. The corresponding elemental analysis for PA6 and ABS areas in Fig. 7(b) shows that silicon was only detected in the ABS phase, demonstrating that the lled silica particles were selectively dispersed in the ABS phase, which is in agreement with our previous study.38 TEM images in Fig. 8 further reveal the distribution of the lled nano-silica particles. In Fig. 8(a) and (b), it is easy to observe the interface between the two phases since it is occupied by a layer of the lled nano-silica particles. This particle layer separates the two phases and gives a clear view of the distribution of the nano-particles in the two phases. In one phase, a lot of aggregations of nano-silica particles are seen while in another phase, few aggregations can be found. Combining these results with SEM images, we can conclude that the lled nano-silica particles were selectively distributed in the ABS phase. With the increase in concentration of nanosilica particles, as shown in Fig. 8(c) and (d), more aggregations of nano-silica particles can be seen, and a network structure formed by the aggregations of nano-silica particles can be observed. In Fig. 9(a), a plateau in the storage modulus is also observed in the low frequency range when the particle concentration is beyond 2 wt%, which normally indicates the formation of a particle network of the nano-silica particles in the ABS phase. The complex viscosity versus frequency curve in Fig. 9(b) shows that the lled composites also show extreme shear thinning behavior when the particle concentration is beyond 2 wt%, indicating the existence of a yield stress. The

Fig. 7 (a) SEM image of 6% nano-silica filled composite and (b) corresponding energy spectra of PA6 and ABS areas.

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TEM images of (a and b) 2% and (c and d) 6% nano-silica filled PA6–ABS (50/50 w) blends.

Fig. 8

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development of a nite yield stress, which is oen associated with the formation a percolated particle network structure, can also be manifested by a diverging h* vs. G* plot.53,54 As shown in Fig. 9(c), for the 2 wt% nano-silica particle lled ABS composite, a divergence in the value of h*is observed, which is consistent with inference of a percolated particle network structure drawn from the frequency dependence of G0 shown in Fig. 9(a). In the case of nano-silica lled PA6–ABS (50/50) blends, a similar rheological behavior is also observed in Fig. 1 (of course, in that case the contribution of the phase interface must be taken into account). Obviously, due to the selective distribution of nanosilica particles, the rheological characteristics of a particle network structure in the blends come mainly from the percolation of the nano-silica particles in the ABS phase.66–68 Veenstra et al.65,69–71 studied the coarsening process of cocontinuous morphology in the PS–poly(ether–ester) blend and found that the coarsening of blends was closely related to the melt structure of the components. They found that at 220  C, the poly(ether–ester) melt behaved like a Maxwell uid with a Newtonian plateau in viscosity at low frequencies and shear thinning behavior at higher frequencies. However, the melt showed shear thinning behavior over the complete range of frequencies measured at 210  C or below. They demonstrated that the

Fig. 9 (a) Storage modulus and (b) complex viscosity as functions of frequency and (c) complex viscosity versus complex modulus of nano-silica particle filled ABS composites.

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extreme shear-thinning behavior was associated with a melt yield stress. This change is shown to be caused by a gradual decrease in crystallinity resulting in a transition which takes place, upon heating, from below 207 to 224  C. From their points of view, the residual crystals in the melt formed physical crosslinks or a network structure. Their study showed that the coarsening process of co-continuous morphology in the PS– poly(ether–ester) blend could be severely slowed down or even completely stopped when physical cross-links were present in the thermoplastic elastomers. The idea that physical crosslinks72 or a yield stress73 can prevent coarsening of co-continuous morphologies has also been suggested by other researchers. According to the analysis above, we can infer that the particle network plays a similar role to the “cross-links” proposed by Veenstra in suppressing the coarsening behavior of the blends with co-continuous morphology. However, the particle network does differ from Veenstra's “cross-links”. In this case, the network is formed by the incorporated nano-particles. Certainly, the morphology stabilization mechanism owing to the existence of the particle network is different. We think that the morphology stabilization mechanism can be attributed to the inhibited movement of molecular chains and the coarsening process, or phase changing, is the result of the movement of molecular chains. At a higher temperature, the movement of molecular chains is increased and the coarsening behavior becomes more serious. When the incorporated particles form a network, the movement of molecular chains is greatly inhibited. So the phase changing, for example, the retraction of the elongated phase, becomes very difficult. With the driving force for coarsening that is too small to overcome the stabilizing force due to the particle network, the coarsening is suppressed. So the particle network structure in the ABS phase suppresses the retraction of the phases and thus suppresses the coarsening of the co-continuous morphology. As a result, the co-continuous morphology of PA6– ABS is greatly stabilized. The effectivity to suppress the coarsening process depends on the content of the lled particles, or the perfection of the network structure. That is, with more nano-silica particles, the network structure becomes perfect and the coarsening behavior is more signicantly suppressed.

IV. Conclusions Coarsening behavior and stabilization of PA6–ABS blends with co-continuous morphology were studied. The raw blend maintained co-continuous morphology upon annealing in the melt state but showed a great increase in phase sizes. Detailed analysis showed that the coarsening process could be divided into two stages: a fast coarsening process at initial high temperature annealing and a second coarsening process with a relatively slow coarsening rate at later time, which resulted from the reduction of the global curvature of the interface. The coarsening rate of the blend was signicantly decreased with the incorporation of nano-silica particles, indicating that the coarsening process was suppressed and the morphology was stabilized. It was found that viscosity increase alone could not explain the morphology stabilization. The particle network in the ABS phase formed by the lled nano-silica particles was This journal is © The Royal Society of Chemistry 2014

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believed to decrease the movement of molecular chains and thus suppressed the coarsening process.

Acknowledgements The authors are grateful to the National Natural Science Foundation of China (grant nos 51073110, 51121001), Fok Ying Tung Education Foundation (grant no.: 122022), the Fundamental Research Funds for the Central Universities (grant no. 2011SCU04A03) and Program for Sichuan Provincial Science Fund for Distinguished Young Scholars (grant no. 2010JQ0014) for the nancial support.

Notes and references 1 J. W. Barlow and D. R. Paul, Polym. Eng. Sci., 1984, 24, 525– 534. 2 C. W. Macosko, P. Gu´ egan and A. K. Khandpur, Macromolecules, 1996, 29, 5590–5598. 3 R. B. Grubbs, J. M. Dean, M. E. Broz and F. S. Bates, Macromolecules, 2000, 33, 9522–9534. 4 B. Majumdar, D. R. Paul and A. J. Oshinski, Polymer, 1997, 38, 1787–1808. 5 N. Kitayama, H. Keskkula and D. R. Paul, Polymer, 2000, 41, 8041–8052. 6 A. G. C. Machiels, K. F. J. Denys, J. Van Dam and A. Posthuma de Boer, Polym. Eng. Sci., 1996, 36, 2451–2466. 7 R. C. Willemse, E. J. J. Ramaker, J. van Dam and A. Posthuma de Boer, Polymer, 1999, 40, 6651–6659. 8 R. C. Willemse, E. J. J. Ramaker, J. van Dam and A. Posthuma de Boer, Polym. Eng. Sci., 1999, 39, 1717–1725. 9 J. J. Elmendorp and A. Van Der Vegt, Polym. Eng. Sci., 1986, 26, 1332–1338. 10 U. Sundararaj and C. W. Macosko, Macromolecules, 1995, 28, 2647–2657. 11 I. Fortelny and A. Zivny, Polymer, 1995, 36, 4113–4118. 12 S. Tomotika, Proc. R. Soc. London, Ser. A, 1935, 150, 322–337. 13 H. A. Stone, B. J. Bentley and L. G. Leal, J. Fluid Mech., 1986, 173, 131–158. 14 H. A. Stone and L. G. Leal, J. Fluid Mech., 1989, 198, 399–427. 15 M. Kozlowski, Polym. Networks Blends, 1993, 3, 213–220. 16 T. Hashimoto, M. Takenaka and T. Izumitani, J. Chem. Phys., 1992, 97, 679–689. 17 M. Takenaka, T. Izumitani and T. Hashimoto, J. Chem. Phys., 1993, 98, 3528–3539. 18 H. Takeno and T. Hashimoto, J. Chem. Phys., 1997, 107, 1634–1644. 19 H. Takeno, M. Iwata, M. Takenaka and T. Hashimoto, Macromolecules, 2000, 33, 9657–9665. 20 K. Binder and D. Stauffer, Phys. Rev. Lett., 1974, 33, 1006– 1009. 21 I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids, 1961, 19, 35–50. 22 E. D. Siggia, Phys. Rev. A: At., Mol., Opt. Phys., 1979, 20, 595– 605. 23 A. O. Gozen, J. Zhou, K. E. Roskov, A. C. Shi, J. Genzer and R. J. Spontak, So Matter, 2011, 7, 3268–3272.

Soft Matter, 2014, 10, 3587–3596 | 3595

View Article Online

Published on 11 February 2014. Downloaded by Brigham Young University on 17/08/2014 10:53:42.

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24 M. A. Huneault and H. Li, Polymer, 2007, 48, 270–280. 25 P. Van Puyvelde, S. Velankar and P. Moldenaers, Curr. Opin. Colloid Interface Sci., 2001, 6, 457–463. 26 M. J. Hore and M. Laradji, J. Chem. Phys., 2007, 126, 244903– 244911. 27 J. E. Callan, W. M. Hess and C. E. Scott, Rubber Chem. Technol., 1971, 44, 814–837. 28 F. Gubbels, S. Blacher, E. Vanlathem, R. J´ erˆ ome, J. R. Deltour, O. F. Brouers and P. Teyssib´ e, Macromolecules, 1996, 28, 1559–1566. 29 L. Elias, F. Fenouillot, J. C. Majeste and P. Cassagnau, Polymer, 2007, 48, 6029–6040. 30 L. Elias, F. Fenouillot, J. C. Majeste, P. Alcouffe and P. Cassagnau, Polymer, 2008, 49, 4378–4385. 31 Q. Zhang, H. Yang and Q. Fu, Polymer, 2004, 45, 1913–1922. 32 F. Laoutid, E. Estrada, R. M. Michell, L. Bonnaud, A. J. M¨ uller and P. Dubois, Polymer, 2013, 54, 3982–3993. 33 D. Voulgaris and D. Petridis, Polymer, 2002, 43, 2213–2218. 34 S. S. Ray, S. Pouliot, M. Bousmina and L. A. Utracki, Polymer, 2004, 45, 8403–8413. 35 J. S. Hong, H. Namkung, K. H. Ahn, S. J. Lee and C. Kim, Polymer, 2006, 47, 3967–3975. 36 M. Si, T. Araki, H. Ade, A. L. D. Kilcoyne, R. Fisher, J. C. Sokolov and M. H. Rafailovich, Macromolecules, 2006, 39, 4793–4801. 37 M. Feng, F. Gong, C. Zhao, G. Chen, S. Zhang and M. Yang, Polym. Int., 2004, 53, 1529–1537. 38 X. Q. Liu, Y. Wang, W. Yang, Z. Y. Liu, Y. Luo, B. H. Xie and M. B. Yang, J. Mater. Sci., 2012, 47, 4620–4631. 39 W. Ramsden, Proc. R. Soc. London, Ser. A, 1903, 72, 156–164. 40 S. U. Pickering, J. Chem. Soc., Abstr., 1908, 91–92, 2001–2021. 41 T. Okubo, J. Colloid Interface Sci., 1995, 171, 55–62. 42 E. Vignati and R. Piazza, Langmuir, 2003, 19, 6650–6656. 43 B. P. Binks, Curr. Opin. Colloid Interface Sci., 2002, 7, 21–41. 44 E. J. Stancik, M. Kouhkan and G. G. Fuller, Langmuir, 2004, 20, 90–94. 45 J. Vermant, G. Cioccolo, K. G. Nai and P. Moldenaers, Rheol. Acta, 2004, 43, 529–538. 46 J. Vermant, S. Vandebril, C. Dewitte and P. Moldenaers, Rheol. Acta, 2008, 47, 835–839. 47 S. P. Nagarkar and S. S. Velanka, So Matter, 2012, 8, 8464– 8477. 48 P. J. Carreau, Trans. Soc. Rheol., 1972, 16, 99–127. 49 K. Yasuda, R. C. Armstrong and R. E. Cohen, Rheol. Acta, 1981, 20, 163–178.

3596 | Soft Matter, 2014, 10, 3587–3596

Paper

50 D. C.-H. Cheng, Rheol. Acta, 1986, 25, 542–554. 51 F. J. Stadler and H. M¨ unstedt, J. Non-Newtonian Fluid Mech., 2008, 151, 129–135. 52 R. V. Krishnamoorti, R. A. Vaia and E. P. Giannelis, Chem. Mater., 1996, 8, 1728–1734. 53 C. A. Mitchell, J. L. Bahr, S. Arepalli, J. M. Tour and R. Krishnamoorti, Macromolecules, 2002, 35, 8825–8830. 54 C. A. Mitchell and R. Krishnamoorti, Macromolecules, 2007, 40, 1538–1545. 55 N. Jouault, P. Vallat, F. Dalmas, S. Said, J. Jestin and F. Bou´ e, Macromolecules, 2009, 42, 2031–2040. 56 V. Geiser, Y. Leterrier and J. A. E. M˚ anson, Macromolecules, 2010, 43, 7705–7712. 57 C. A. Mitchell, J. L. Bahr, S. Arepalli, J. M. Tour and R. Krishnamoorti, Macromolecules, 2002, 35, 8825–8830. 58 F. Du, R. C. Scogna, W. Zhou, S. Brand, J. E. Fischer and K. I. Winey, Macromolecules, 2004, 37, 9048–9055. 59 K. Ke, Y. Wang, K. Zhang, Y. Luo, W. Yang, B. H. Xie and M. B. Yang, J. Appl. Polym. Sci., 2012, 125, 49–57. 60 T. S. Omonov, C. Harrats, G. Groeninckx and P. Moldenaers, Polymer, 2007, 48, 5289–5302. 61 A. Pyun, J. R. Bell, K. H. Won, B. M. Weon, S. K. Seol, J. H. Je and C. W. Macosko, Macromolecules, 2007, 40, 2029–2035. 62 C. R. L´ opez-Barr´ on and C. W. Macosko, Langmuir, 2009, 25, 9392–9404. 63 C. R. L´ opez-Barr´ on and C. W. Macosko, So Matter, 2010, 6, 2637–2647. 64 M. Tjahjadi, J. M. Ottino and H. A. Stone, AIChE J., 1994, 40, 385–394. 65 H. Veenstra, J. Van Dam and A. Posthuma de Boer, Polymer, 2000, 41, 3037–3045. 66 M. Sumita, K. Sakata, S. Asai, K. Miyasaka and H. Nakagawa, Polym. Bull., 1991, 25, 265–271. 67 M. Sumita, K. Sakata, Y. Hayakawa, S. Asai, K. Miyasaka and M. Tanemura, Colloid Polym. Sci., 1992, 270, 134–139. 68 Y. J. Li and H. Shimizu, Macromolecules, 2008, 41, 5339–5344. 69 H. Veenstra, J. Van Dam and A. Posthuma de Boer, J. Polym. Sci., Part B: Polym. Phys., 1998, 36, 1795–1804. 70 H. Veenstra, J. Van Dam and A. Posthuma de Boer, Polymer, 1999, 40, 1119–1130. 71 H. Veenstra, J. J. Barbara, J. Van Dam and A. Posthuma de Boer, Polymer, 1999, 40, 6661–6672. 72 W. P. Gergen, Kautsch. Gummi Kunstst., 1984, 37, 284–290. 73 J. J. Elmendorp and A. K. van der Vegt, Polym. Eng. Sci., 1986, 26, 1332–1338.

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Suppression of phase coarsening in immiscible, co-continuous polymer blends under high temperature quiescent annealing.

The properties of polymer blends greatly depend on the morphologies formed during processing, and the thermodynamic non-equilibrium nature of most pol...
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