PS

Article pubs.acs.org/Macromolecules

Characterization of Effects of Silica Nanoparticles on (80/20) PP/PS Blends via Nonlinear Rheological Properties from Fourier Transform Rheology Reza Salehiyan,† Hyeong Yong Song,† Woo Jin Choi,‡ and Kyu Hyun*,† †

School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Korea Chemical Materials Solutions Center, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea

‡

ABSTRACT: Effects of silica nanoparticles with different natures (hydrophilicity and hydrophobicity) on (80/20) PP/PS blends were investigated via linear and nonlinear rheological properties. The hydrophilic silica nanoparticle was fumed silica OX50 while the two hydrophobic ones were precipitated silica D17 and fumed silica R202. SEM images revealed that hydrophilic OX50 could not improve morphological properties of the blends. On the other hand, the two hydrophobic silica nanoparticles (R202 and D17) improved morphological properties. TEM examination showed that OX50 silica nanoparticles aggregated inside PS droplets, thereby making breakup of PS (dispersed) phase into smaller sizes more difficult. D17 and R202 improved morphological properties regardless of the different droplet size reduction mechanisms, and rheological properties improved as a result. Both linear rheological properties from SAOS (small-amplitude oscillatory shear) tests and nonlinear rheological properties from LAOS (large-amplitude oscillatory shear) tests were obtained. The nonlinear−linear viscoelastic ratio (NLR ≡ normalized nonlinear rheological properties/normalized linear rheological properties) was used to quantify the degree of droplet dispersion and distinguish the effects of silica particles on the morphology of PP/PS blends. Previous research has observed an inverse correlation between NLR and droplet size. PP/PS/OX50 blends with no alteration of droplet size showed constant NLR values (≅1) with increasing concentration of OX50 (hydrophilic silica). However, NLR values of PP/PS blends with hydrophobic silica nanoparticles (D17 and R202) were much larger than 1 (NLR > 1) and increased with silica concentration, which is consistent with morphological evolution, i.e., reducing droplet size. However, NLR values of PP/PS/R202 blends were relatively larger than those of PP/PS/D17 blends despite smaller droplet sizes. This can be attributed to a different morphology microstructure, i.e., R202 located in PP matrix phase and D17 at interface between PP and PS. Therefore, the NLR value of PP/PS/silica blend could be due to the combined effects of the interface between droplets (PP/PS blend) and particle−polymer interactions (PP/silica nanocomposites). Especially, R202 showed larger NLR values due to PP/R202 nanocomposites. Based on these findings, relative NLR (= NLRPP/PS/silica/NLRPP/silica) is proposed as an effective measurement of droplet size information in PP/PS blends by eliminating the effects of PP/silica nanocomposites. Relative NLR matched well with droplet size evolution from the SEM results.

1. INTRODUCTION

Recently, solid inorganic particles have been extensively used as an attractive alternative compatibilizer.1 Ray et al.2 studied the effects of C20A (Cloisite 20A, organomodified montmorillonite) on polypropylene/polystyrene (PP/PS) and maleic anhydride-grafted polypropylene/polystyrene (PP-g-MA/PS) blends. Reinforcement and compatibilization effects of C20A nanoparticles located at the interface areas between PS and PP phases were confirmed while both mechanical and interfacial properties of the blends improved. Huang et al.3 studied the effects of silica particles on phase separation temperature of poly(methyl methacrylate)/poly(styrene-stat-acrylonitrile) (PMMA/SAN) blends by time−

Blending polymers is an attractive way of producing new materials with interesting properties. However, most polymer blends are immiscible due to the low mixing entropy (ΔmixS). In the case of immiscible blends, various morphologies from lamella to droplet morphology can be obtained depending on various factors such as processing conditions, composition ratio, viscosity ratio, etc. Alternatively, each of these morphologies renders its own unique characteristics to the blends. Therefore, it is quite important to control and characterize the morphology of blends in order to obtain the desired final characteristics. More stabilized morphology with finer droplet dispersion can usually be obtained by suppressing coalescence, reducing interfacial tensions, and altering viscosity ratios when compatibilizers are added to an immiscible blend. © XXXX American Chemical Society

Received: April 1, 2015 Revised: June 15, 2015

A

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Information on Different Types of Silica Nanoparticles silica type grade

type

polarity

modification

tamped density [g/L]

SIPERNAT D17 AEROSIL R202 AEROSIL OX 50

precipitated fumed fumed

hydrophobic hydrophobic hydrophilic

dimethyldichlorosilane poly(dimethylsiloxane) methacryloyloxypropyltrimethoxysilane

150 60 130

temperature superposition (TTS) measurement. Phase separation temperatures of the blends increased upon silica incorporation. Hong et al. 4,5 examined size reduction phenomena when clay was added to poly(butylene terephthalate)/polyethylene (PBT/PE) and poly(butylene terephthalate)/polystyrene (PBT/PS) blends. At lower concentrations, clay was located at the interface and suppressed coalescence. Moreover, clay particles were always located at the phase they are more compatible with regardless of the original mixing protocol. Furthermore, interfacial tensions for PBT/PE blends as predicted by extensional force measurements were reduced when clay was added to the blend. Elias et al.6,7 studied the effects of two different hydrophilic and hydrophobic fumed silica nanoparticles on the morphology and rheological properties of PP/PS and polypropylene/poly(ethylene-co-vinyl acetate) (PP/EVA) blends, respectively. Significant reduction of PS and EVA droplet sizes was observed in both blend systems regardless of silica location. The Palierne model was used to estimate interfacial tensions based on linear rheological properties. The results were consistent with the size reduction, as interfacial tensions significantly decreased as a consequence of silica addition. It was suggested that localization of hydrophilic silica nanoparticles inside the dispersed phases, PS and EVA, contributed to size reduction by decreasing interfacial tensions while hydrophobic silica nanoparticles located at the interface prevented coalescence of the dispersed phase droplets. Hiutric et al.8 investigated the effects of C30B (Cloisite 30B, organo-modified montmorillonite) nanoparticles on polyethylene/polyamide-12 (PE/PA-12) blends and concluded that PA size reduction in PE matrix can be attributed to suppressed coalescence when C30B clays were located at the interface. On the other hand, extra C30B clay nanoparticles were located in PA matrix due to their higher affinity toward PA phase and changed matrix viscosity, which promoted PE size reduction. Moreover, calculations based on the simple Palierne model showed remarkable reduction of interfacial tensions upon addition of C30B clay nanoparticles, which was consistent with droplet size reduction. Vandebril et al.9 suggested that interfacially trapped silica nanoparticles do not promote interfacial tension reduction in immiscible polyisobutylene/ poly(dimethylsiloxane) (PIB/PDMS) polymer blends like classical compatibilizers, and the change in droplet size is due to suppressed coalescence. Similarly, Tao et al.10 concluded that droplet size reduction in polyamide/acrylate−ethylene copolymer (PA/EA) blends is due to neither reduction of interfacial energy nor alteration of matrix viscosity when CNTs are trapped at the interface and matrix, respectively. Their rheological analyses based on the Palierne model revealed that the interfacial energy of the blends was unaffected despite CNT being located at the interface. Thus, droplet size reduction for systems in which CNTs are located at the interface could be ascribed to formation of a rigid shell around the dispersed phase, which hinders coalescence of the two approaching droplets. Therefore, the above-mentioned reports imply that the use of inorganic particles as an emulsifier or compatibilizer can be an interesting route to stabilize the

morphologies of immiscible polymer blends regardless of their final location. Although many researchers have taken advantage of linear rheological properties from small-amplitude oscillatory shear (SAOS) tests to characterize the microstructural morphology evolutions of immiscible blends, less attention has been paid to nonlinear rheological analyses. In our previous work, nonlinear rheological properties from large-amplitude oscillatory shear (LAOS) tests were used to investigate the effects of two types of nanoparticles with different shapes and surface treatment (spherical hydrophilic fumed silica and hydrophobic C20A nanoparticles) on morphological evolution of (80/20) PP/PS blends at various particle concentrations.11 For analyzing nonlinear rheological properties, Fourier transform rheology (FT-rheology) was used. Sensitivity of FT-rheology for detection of topology and morphology of various complex fluids has been frequently reported.11−21 Nonlinear−linear viscoelastic ratio (NLR ≡ normalized nonlinear viscoelasticity from FT-rheology/normalized linear viscoelasticity from SAOS tests) was used as the rheological indicator to quantify morphology of the PP/PS blends.21 NLR is defined as follows: NLR =

Q 0(φ)/Q 0(0) |G*(φ)| /|G*(0)|

(1)

where φ is the filler concentration, Q0 is the asymptotic limiting and constant value of Q at low strain amplitude from LAOS tests, and |G*| is the complex modulus from SAOS tests.11,21 Salehiyan et al.11 found that C20A nanoparticles would cover the PS dispersed phase at their interfaces and cause significant size reduction, and this phenomenon was reflected in increasing NLR values larger than 1 (NLR > 1). On the other hand, hydrophilic silica nanoparticles accommodate themselves inside PS phase in aggregated forms, leading to constant droplet sizes at all silica concentrations, in which case NLR values remained constant and below 1 (NLR < 1). These findings suggest that NLR values based on both linear and nonlinear rheological properties can be used as a promising tool to distinguish compatibilization effects of different nanoparticles, whereas linear rheological properties from SAOS tests showed only increments in moduli for both nanoparticles used. Since NLR is known to be able to interpret morphological evolutions of PP/PS blends,11 the aim of this study was to further investigate the application of NLR to probe the effects of three types of silica nanoparticles with different surface treatments (two hydrophobic silicas, D17 and R202, and one hydrophilic silica, OX50) on morphological and rheological properties of PP/PS blends. In this study, the effects of silica locations in PP/PS blends on the degree of compatibilization and correlation with NLR values were investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene (PP) (grade HP562T, MI = 60.0 g/ 10 min at 230 °C and load 2.16 kg, M̅ n = 56 000, M̅ w = 157 100, PDI = 2.81) was obtained from PolyMirae Company Ltd. (South Korea). Polystyrene (PS) (grade HF2680, MI = 17.6 g/10 min at 230 °C and load 3.8 kg, M̅ n = 59 200, M̅ w = 168 700, PDI = 2.85) was provided by B

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Samsung Cheil Industries Inc. (South Korea). All three different silica particles were provided from Evonik Industries (Germany). Information on silica nanoparticles is listed in Table 1. Chemical structures of different modifications for SIPERNAT D17, AEROSIL R202, and AEROSIL OX50 are shown in Figures 1a−c, respectively.

linear rheological responses under small-amplitude oscillatory shear (SAOS) flow. Strain amplitudes were within linear region (γ0 = 0.03− 0.07). Nonlinear rheological properties were obtained through strain sweep tests under large-amplitude oscillatory shear (LAOS) flow with increasing strain amplitude from γ0 = 0.01 to 5 at a fixed frequency of 1 rad/s. All measurements were carried out at 180 °C. 2.4. Morphology. Field emission scanning electron microscopy (FE-SEM) observations were carried out using a JSM 6700F microscope at 5 kV to evaluate morphology evolutions. Samples were fractured in nitrogen liquid and then covered with platinum. The number-average radius (Rn) was calculated using eq 2.

Rn =

∑ niR i ∑ ni

(2)

where ni is the number of droplets with radius Ri. Radii of over 300 droplets were measured using image analyzer software (ImageJ). Transmission electron microscopy (TEM) FEI Tecnai G2 T-20s was used at an accelerating voltage of 200 kV to observe the location of silica particles. Samples were cut using a cryo-microtome device at −140 °C and put on a copper grid.

Figure 1. Chemical structures of (a) dimethyldichlorosilane of SIPERNAT D17, (b) poly(dimethylsiloxane) of AEROSIL R202, and (c) methacryloyloxypropyltrimethoxysilane of AEROSIL OX50.

3. RESULTS 3.1. Linear Rheological Properties from SmallAmplitude Oscillatory Shear (SAOS) Tests. Linear rheological properties of (80/20) PP/PS blends incorporated with different silica nanoparticles (two hydrophobic silicas, D17 and R202, and one hydrophilic silica, OX50) were measured by SAOS tests at a temperature of 180 °C. Figure 2 illustrates the storage moduli G′ (ω) (Figure 2a−c) and loss moduli G″ (ω) (Figure 2d−f) of the blends. Figure 2 shows that silica nanoparticle loadings led to increased elastic and viscous responses of (80/20) PP/PS

2.2. Blend Preparation. All materials were dried in a vacuum oven at 80 °C for 12 h prior to compounding. An internal Haake mixer (Thermo Fisher Scientific Inc.) was used to simultaneously mix the ingredients at 50 rpm and 200 °C for 8 min. The composition ratio was selected as (80/20), in which PP was the matrix and PS was the dispersed phase. Silica particles were added at concentrations of 1, 3, 5, 7, and 10 phr (parts per hundred). After melt blending, pellets were compressed and molded into disks with a diameter of 25 mm and thickness of 1 mm at 200 °C. 2.3. Rheological Measurements. An ARES-G2 (TA Instruments Inc., USA) with 25 mm parallel plate geometry was used to carry out rheological measurements. Frequency sweep tests were used to obtain

Figure 2. Storage moduli G′(ω) (a−c) and loss moduli G″(ω) (d−f) of the (80/20) PP/PS blends reinforced with various silica nanoparticles (two hydrophobic silicas, R202 and D17, and one hydrophilic, OX50) at different concentrations (1, 3, 5, 7, and 10 phr) and 180 °C. C

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

blends based solely on SAOS results. In a previous study, NLR (nonlinear−linear viscoelastic ratio) value was shown to be more reliable and precise for interpreting the morphological evolutions of (80/20) PP/PS blends based on nonlinear rheological analyses.11 Therefore, the nonlinear rheological properties from large-amplitude oscillatory shear (LAOS) tests should be determined to investigate morphological evolutions. 3.2. Morphology of PP/PS Blends with Various Silica Nanoparticles. Before determination of nonlinear rheological properties, morphological evolutions were investigated in order to examine the SAOS results. The impact of different silica nanoparticles on morphological evolutions of (80/20) PP/PS are addressed in Figure 4. Drastic size reduction of PP/PS/D17 blends was observed. Moreover, PS droplets were welldistributed within the PP matrix when D17 was used. Namely, coalescence was suppressed upon D17 loading. On the other hand, size reduction was not as pronounced in hydrophobic R202 silica as in PP/PS/D17. Finally, in PP/PS/OX50 blends, addition of hydrophilic OX50 silica nanoparticles did not improve morphological properties of PP/PS blends. This result is similar to a previous study in which OX50 silica nanoparticles aggregated inside PS phase apparently due to their very low surface area per unit volume.11 This led to formation of irregular droplet shapes as well as coexistence of large and small droplets, which is an indication of inefficient suppression of coalescence by OX50 nanoparticles. At 10 phr of OX50, the number of PS droplets dramatically decreased, resulting in a few large PS droplets as well as fibrils. Figure 5 shows specific SEM images of (80/20) PP/PS blends filled with 10 phr of OX50. Morphology seemed to change at higher OX50 concentrations (10 phr of OX50 in Figure 5). The morphology of PP/PS/10 phr OX50 droplets cannot be considered as perfect. Comparison of droplet sizes and distribution of droplet sizes is shown in Figure 6. Figure 6a shows the number-average radii (Rn) of the blends, and Figure 6b−e illustrates droplet size distributions of the neat and filled (80/20) PP/PS blends with 1, 5, and 10 phr of D17, R202, and OX50 silica nanoparticles. Morphological properties of the blends based on SEM analyses revealed that hydrophobic D17 silica nanoparticles were the best among all three types of silica nanoparticles used in this study. D17 nanoparticles not only caused dramatic size reduction of the dispersed phase (PS) but also narrowed down size distributions to very small sizes at higher silica concentrations (see Figure 6). This is an indication of suppressed coalescence (will be discussed later). However, the SEM results do not fully explain why PP/PS/R202 droplets showed the highest SAOS results (see Figures 2 and 3). Additionally, linear rheological properties of PP/PS/R202 were larger than those of PP/PS/D17, even though D17 showed the best compatibilizer effect. Namely, D17 resulted in the narrowest and smallest size droplets among all other silica particles. Furthermore, transmission electron microscopy (TEM) was used to investigate the mechanisms of size reduction in the blends by observation of silica locations. TEM images of (80/ 20) PP/PS blends reinforced with D17, R202, and OX50 at different concentrations are shown in Figure 7. TEM examination of the blends revealed that hydrophobic D17 nanoparticles were mostly located at the interface between PP and PS or in continuous phase PP at a concentration of 10 phr (Figure 7a−c). At high concentrations of D17 in PP/PS blends, PS droplets were fully covered by a rigid layer of D17 nanoparticles. As discussed before, many researchers have

blends. However, this increase was less pronounced for blends filled with hydrophilic silica nanoparticles (OX50). Further, high silica loadings resulted in nonterminal behavior during elastic responses of the blends, appearing as plateaus at lowfrequency regions. It is believed that this yield behavior in filled polymer systems is due to formation of interparticle networks resulting from strong particle−particle interactions.22 The storage modulus G′(ω) of a reinforced polymer system yields when network effects (particle−particle interactions) dominates the combined hydrodynamic (polymer−particle interactions) and viscoelastic effects of the matrix itself. However, the current (80/20) PP/PS/silica blend system is more complicated since the effects of the interface should be taken into account. A constitutive equation developed by Gramespacher and Meisner23 showed that linear moduli in immiscible polymer blends are a combination of the moduli of individual components as well as the interfaces. Therefore, a combination of hydrodynamic and interfacial effects can enhance rheological properties of filled immiscible blends. As shown in Figure 2, storage modulus G′(ω) is more sensitive to silica loadings. To compare the effects of different silica particles on linear rheological properties of (80/20) PP/PS blends, storage moduli G′ of the blends at a fixed frequency of 0.1 rad/s are plotted in Figure 3 as a function of silica concentration.

Figure 3. Storage moduli G′ of (80/20) PP/PS blends reinforced with various silica nanoparticles (two hydrophobic silicas, R202 and D17, and one hydrophilic silica, OX50) at a fixed frequency of 0.1 rad/s as a function of silica concentration.

Results show that the hydrophobic fumed silica R202 led to a sudden increase in rheological properties at high silica loadings while hydrophilic fumed silica OX50 was less effective compared to two hydrophobic silica nanoparticles (R202 and D17) at most concentrations. Based on these results, R202 and OX50 were the most and least effective particles in compatibilizing the (80/20) PP/PS blend, respectively. This is consistent with our previous results in which OX50 particles were jammed inside PS phase, resulting in large particle clusters that did not cause any morphological improvement.11 However, it is difficult to distinguish between the effects of D17 and OX50 on the PP/PS blend from the SAOS results. As shown in Figure 3, the storage modulus G′ of PP/PS/D17 was not much larger than that of PP/PS/OX50 at all concentrations. In addition, the two hydrophobic silica nanoparticles showed totally different effects in the SAOS results (G′ of PP/PS/R202 ≫ G′ of PP/PS/D17 at high concentrations, especially at 7 and 10 phr). Thus, we could make no conclusion regarding the effects of silica nanoparticles on morphological properties of the D

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. SEM images of (80/20) PP/PS blends (a) reinforced with D17 (b−d), R202 (e−g), and OX50 (h−j). Scale bar is 50 μm for all figures.

stuck to each other inside favorable phase PS and formed large silica clusters due to their relatively larger size (lower specific area per volume). Viscosity of PS was larger than that of continuous phase PP at 180 °C. Thus, when OX50 nanoparticles aggregated inside PS phase, the viscosity ratio ηd/ηm increased and breakup of PS domains became more difficult. This led to larger and irregular droplets at high OX50 concentrations (see Figure 5). This phenomenon in which particle additions promote coalescence in immiscible blends is known as “bridging−dewetting”.24−28 For this mechanism to occur, particles are required to be wetted by the dispersed phase. During collision of two particle-laden droplets, a particle can be adsorbed onto the surface of two droplets at the same time or two droplets collide through their particle-free surfaces, which can induce coalescence. These findings based on SEM and TEM reveal that linear rheological properties obtained from SAOS tests are not sufficient to elucidate the morphological effects of different types of silica nanoparticles on (80/20) PP/PS blends, as R202 was shown to have a larger impact on storage modulus G′ of the blends compared to other silica particles. On the other hand, SEM results showed that hydrophobic D17 was the most efficient nanoparticle in this study for reducing droplet size. Thus, further nonlinear rheological analyses from largeamplitude oscillatory shear (LAOS) tests should be carried out. From a previous study, NLR parameter (nonlinear−linear

Figure 5. SEM images of (80/20) PP/PS blends filled with 10 phr of OX50 silica nanoparticles. Scale bar is 20 μm for (a) and 50 μm for (b).

reported that size reduction in immiscible blends where nanoparticles are trapped at the interface may not be due to reduction of interfacial tensions but rather coalescence, which is strongly suppressed by formation of a layer around the dispersed phase that promotes steric hindrance.6−10 On the other hand, hydrophobic silica R202 nanoparticles were located mainly in the continuous phase PP (Figure 7d−f). In this case, size reduction could be attributed to altered viscosity ratios of the dispersed and matrix phases (ηd/ηm). Highly enhanced viscosity of the matrix phase PP due to strong particle−particle or polymer−particle interactions immobilized PS droplets and increased difficulty of film drainage between the two droplets. Hydrophilic silica OX50 nanoparticles were located inside the dispersed phase PS (Figure 7g−i). OX50 silica nanoparticles E

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 6. (a) Number-average radii Rn of the (80/20) PP/PS blends reinforced with R202, D17, and OX50 at different silica concentrations. (b) Size distribution of (80/20) PP/PS blend without silica nanoparticles. (c−e) Size distributions of (80/20) PP/PS blends incorporated with R202, D17, and OX50 at concentrations of 1, 5, and 10 phr, respectively. Inset plots (c−e) represent the zoomed-in plots.

Figure 7. TEM images of (80/20) PP/PS blends reinforced with (a−c) D17, (d−f) R202, and (g−i) OX50 silica nanoparticles. Scale bar is 1 μm for PP/PS blends filled with D17 and R202 and 0.5 μm for the PP/PS blends filled with OX50.

viscoelastic ratio) can be used to detect differences in morphological evolutions of (80/20) PP/PS blends.11 3.3. Nonlinear Rheological Properties from LargeAmplitude Oscillatory Shear (LAOS) Tests. Nonlinear rheological properties of (80/20) PP/PS blends reinforced with different silica nanoparticles were studied under LAOS flow. Figure 8 demonstrates storage moduli G′(γ0) of the blends reinforced with R202, D17, and OX50 at a fixed frequency of 1 rad/s and temperature of 180 °C. Similar trends as the SAOS

results were observed for LAOS responses. Higher silica fractions increased moduli of the blends as well as their strain thinning (or softening) behavior. Similar to the SAOS results, R202 and OX50 fumed silica showed the most and least effects on moduli of the blends, respectively. Moreover, LAOS results revealed that the limit of linearity was reduced to lower strain amplitudes when silica nanoparticles were added. This nonlinearity effect, strain amplitude dependency of dynamic rheological properties of filled polymers, is often referred as the F

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 8. Storage moduli G′(γ0) of the (80/20) PP/PS blends reinforced with various silica nanoparticles (two hydrophobic silicas, R202 and D17, and one hydrophilic, OX50) at different concentrations at a frequency of 1 rad/s and temperature of 180 °C.

Figure 9. Normalized third relative (I3/1) intensities of the (80/20) PP/PS blends reinforced with various silica nanoparticles at different concentrations acquired at a frequency of 1 rad/s and temperature of 180 °C.

one is usually the largest among higher harmonics.33−35 Sensitivity of the FT-rheology method in analyzing various complex fluids has been frequently discussed. Normalized third relative intensities (I3/1) of the blends reinforced with R202, D17, and OX50 are shown in Figure 9a−c, respectively. Once again, the significant difference between D17 and R202-filled blends can be observed from Figure 9a,b. High D17 concentrations induced plateau behavior in the I3/1 results. In the case of OX50-filled blends, a significant boost in intensities occurred from 7 to 10 phr. At this point, a transient (cocontinuous-like) morphology with both droplets and fibrils was shown to exist at the same time. This change in morphology appeared as a sudden increase in rheological properties of the PP/PS blend filled with 10 phr of OX50. Thus, nonlinear analyses methods could be beneficial to characterize internal structures of the blends to some extent.

Payne effect. It has been suggested that the Payne effect originates from interparticle interactions in filled polymer systems. Deagglomeration of silica clusters or breakdown of the clay network is believed to be the reason behind this phenomena.29 Furthermore, Ziegler and Wolf30 reported that as deformability of the droplets is lessened, shear thinning behavior is weakened in immiscible polymer blends. This explains why OX50-filled blends exhibited a weaker Payne effect and strain thinning compared to the other two cases. Jammed silica particles localized inside PS droplets changed the viscosity of the PS phase, which made it more difficult to deform. As a result, OX50-filled blends showed longer linear limit regions at all silica concentrations. On the other hand, the two hydrophobic silica nanoparticles dispersed mostly in more compatible phase PP and reduced viscosity ratios of the corresponding blends, which resulted in stronger strain softening behavior. The linearity limit shifted to lower strain amplitudes in 10 phr D17 compared to that of 10 phr R202 filled blends (see Figure 8b). This can be attributed to deformation and possible breakup of droplets, which led to smaller droplets compared to that of R202-filled blends. This is in agreement with the SEM images and morphological analyses discussed in the previous section. Generally, the definitions of the phenomenological concepts of nonlinear viscoelasticity, Payne effect, and strain softening from LAOS tests provide information to interpret morphological evolutions of complex fluids. Additionally, in order to quantify the nonlinear rheological properties of the blends for better understanding of microstructural changes, we utilized the FT-rheology method.17,31,32 In a nutshell, nonlinear effect manifests as odd higher harmonic intensities in which the third

4. DISCUSSION 4.1. Nonlinear−Linear Rheological Properties Ratio (NLR). Lim et al.21 established a new parameter referred to as nonlinear−linear viscoelastic ratio (NLR, see eq 1). They observed a direct relationship between NLR value and degree of dispersion of nanoparticles in a polycaprolactone (PCL) matrix, where the highest NLR value corresponds to the highest dispersion quality of nanoparticles. In addition, in previous study,11 we found that this NLR value could be used to detect the compatibilizing effects of nanoparticles with a different shape and surface treatments, C20A (clay) and OX50 (fumed silica) on (80/20) PP/PS blends. A good compatibilizer C20A (clay) led to NLR values increasingly larger than 1 (NLR > 1) and OX50 (fumed silica), which failed to show morphological improvements with NLR values constantly lower than 1 (NLR G

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Differences in the types of surface treatments of silica nanoparticles (hydrophobicity and hydrophilicity) can be assessed from this plot. Size reduction in the blends filled with two hydrophobic types of silica nanoparticles (D17 and R202) was observed. A previous study showed that NLR is inversely related to size reduction, which suggests that NLR values will be increasingly larger than 1 (NLR > 1) for the efficient silica nanoparticles (R202 and D17). On the contrary, NLR was constant at 1 (NLR ≈ 1) for the inefficient silica nanoparticle used (OX50). Moreover, the reason for the sudden increase in NLR values at 10 phr of OX50 is that morphology is in a transient state where droplet morphology changes to a cocontinuous state, as discussed in a previous section (see Figure 5). The results are more interesting regarding NLR values of the blends filled with two different hydrophobic silica nanoparticles (precipitated D17 and fumed R202). We already showed in previous section that size reduction of PP/PS/D17 blends was greater than that of PP/ PS/R202 blends. An inverse relationship between droplet size and NLR was reported previously.11 Therefore, larger NLR values were expected for PP/PS/D17 than for PP/PS/R202. However, Figure 10 shows different behaviors from the expected results. In the case of PP/PS/silica nanoparticle systems, NLR could be the combination of two contributions: morphology or interfacial effect of the dispersed and matrix phase, which determines droplet sizes, and dispersion effect of nanoparticles within PP matrix phase. This implies that the effect of particle dispersion itself within the polymer matrix PP should be taken into account. Both D17 and R202 are more compatible with PP phase, and TEM images show localization of hydrophobic silica nanoparticles inside the matrix PP or at the interface. In other words, for the blends filled with silica nanoparticles having the same chemical affinity (hydrophobic), interparticle or polymer−particle interactions in PP silica nanocomposites can play a significant role in determining NLR values. Therefore, in order to evaluate the effects of hydrophobic silica nanoparticles (D17 and R202) on the matrix PP itself, PP/silica nanocomposites were prepared at different concentrations with the same thermal and shear history as the blends. 4.2. Effect of PP/Silica Nanocomposites. Linear and nonlinear rheological properties of PP/silica nanocomposites were analyzed using SAOS and LAOS tests, similar to those of the blends at 180 °C and frequency of 1 rad/s. Figure 11 shows the storage moduli G′(ω) from the SAOS tests. Linear rheological properties in Figure 11 show that incorporation of R202 silica nanoparticles had a more pronounced impact on rheological properties of PP than D17. At higher silica concentrations, nonterminal behavior was stronger in the case of PP/R202, in which the width of the storage modulus G′(ω) at the low-frequency region was much larger than that of PP/ D17 nanocomposites. This nonterminal behavior in filledpolymer systems is due to formation of three-dimensional network structures caused by strong particle−particle interactions.22,29 Figure 12 shows the storage moduli G′(γ0) from LAOS tests of the PP/silica nanocomposites. PP/R202 nanocomposites exhibit stronger strain softening (Payne effect) compared to PP/D17 nanocomposites due to deagglomeration of silica nanoparticle networks within the matrix PP. From TEM picture, it is found that PP/R202 composite is better dispersed than PP/D17. Figure 13 shows dispersion state of PP/R202 and PP/D17 nanocomposite, respectively.

< 1). Further, NLR was shown to be inversely proportional to droplet size changes.11 In this study, therefore we applied NLR parameter to investigate the effects of silica nanoparticles with different surface treatments on morphological improvements of (80/20) PP/PS blends. Tables 2−4 show the acquired Q0 from FTTable 2. Required Parameters for Calculation of NLR Values for PP/PS/R202 Blends composition

Q0

|G*| [Pa]

NLR

PP/PS PP/PS/1 phr R202 PP/PS/3 phr R202 PP/PS/5 phr R202 PP/PS/7 phr R202 PP/PS/10 phr R202

6.1990 × 10−3 0.0107 0.3345 0.3465 3.7390 22.9335

467.6220 606.0880 527.4790 646.0270 854.7510 1413.2600

1.0000 1.3347 47.8378 40.4622 329.9804 1224.1191

Table 3. Required Parameters for Calculation of NLR Values for PP/PS/D17 Blends composition

Q0

|G*| [Pa]

NLR

PP/PS PP/PS/1 phr D17 PP/PS/3 phr D17 PP/PS/5 phr D17 PP/PS/7 phr D17 PP/PS/10 phr D17

6.1990 × 10−3 9.7873 × 10−3 0.0332 0.1246 0.6581 27.9511

467.6220 545.9390 764.0090 765.7767 789.7330 750.1465

1.0000 1.3524 3.2796 12.2772 62.8601 2810.7910

Table 4. Required Parameters for Calculation of NLR Values for PP/PS/OX50 Blends composition

Q0

|G*| [Pa]

NLR

PP/PS PP/PS/1 phr OX50 PP/PS/3 phr OX50 PP/PS/5 phr OX50 PP/PS/7 phr OX50 PP/PS/10 phr OX50

6.1990 × 10−3 7.3203 × 10−3 0.0100 8.9156e-3 0.0117 0.2916

467.6220 617.9330 596.0350 650.6720 746.4450 882.1200

1.0000 0.8936 1.2604 1.0336 1.1832 24.9343

rheology (LAOS test), |G*| from SAOS test, and corresponding NLR values of the blends filled with R202, D17, and OX50, respectively. |G*| values were acquired from SAOS tests at a fixed frequency of 1 rad/s for all blends. Figure 10 illustrates the NLR values of the blends filled with different silica nanoparticles.

Figure 10. Comparison of NLR values of PP/PS blends filled with R202, D17, and OX50 at different silica concentrations. H

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 11. Storage moduli G′(ω) of the (a) PP/R202 and (b) PP/D17 nanocomposites at strain amplitudes within linear regions at a temperature of 180 °C.

Figure 12. Storage moduli G′(γ0) of the (a) PP/R202 and (b) PP/D17 nanocomposites at a frequency of 1 rad/s and temperature of 180 °C.

With FT-rheology, normalized third relative intensities I3/1 of the PP silica nanocomposites along with their corresponding NLR values are plotted in Figures 14a and 14b, respectively. As better dispersion of silica nanoparticles yields higher NLR values in filled-polymer systems,21 R202 silica nanoparticles formed a stronger network, which led to better dispersion than D17 silica nanoparticles (see Figure 13). Thus, stronger interparticle interactions between R202 silica nanoparticles inside PP matrix compared to D17 silica nanoparticles was the dominant factor causing NLR values to be larger at medium silica concentrations (3−5 phr) in the case of PP/PS/R202 blends. At the highest silica concentration (10 phr),

Figure 13. TEM images of PP/silica 10 phr nanocomposites: (a) PP/ R202 and (b) PP/D17. Scale bar is 200 nm.

Figure 14. (a) Normalized third relative intensities I3/1 of the PP/R202 and PP/D17 nanocomposites. (b) Nonlinear−linear viscoelasticity ratio (NLR) value of the PP silica nanocomposites. I

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

interface or PP matrix phase and formed a rigid layer around PS phase, which suppressed coalescence. On the other hand, other hydrophobic silica nanoparticle R202 dispersed within PP phase and altered the viscosity ratio, which led to reduced droplet size. Finally, hydrophilic OX50 silica nanoparticles stuck inside PS phase with increasing OX50 concentrations did not have any significant effect on droplet size, whereas 10 phr of OX50 produced an unstable morphology consisting of both droplets and fibrils. The “bridging−dewetting” mechanism could explain the coalescence of OX50 systems. The contradiction between morphological and linear rheological properties of PP/PS/R202 and PP/PS/D17 was convincing enough to perform nonlinear rheological analyses with FT-rheology. Despite greater enhancement of the moduli of PP/PS/R202 blends, the nonlinear effect was stronger in PP/PS/D17 blends, as the limit of linearity was reduced compared to that of PP/ PS/R202 blends. Strong strain softening behavior in immiscible polymer blends could be ascribed to droplet deformation, which led to remarkable size reduction. Eventually, in order to correlate rheological and morphological properties, NLR parameter was utilized. The following notes could be implied from the NLR results: • NLR parameter captured the differences caused by silica nanoparticles having different chemical affinities. For the two hydrophobic silica (R202 and D17)-filled blends in which size reduction was observed, NLR value gradually increased. For hydrophilic silica (OX50)-filled blends, NLR value remained constant up to 7 phr similar to its corresponding droplet sizes. • NLR value for 10 phr OX50-filled PP/PS blend dramatically increased. This could be due to changes in morphology from droplet morphology to a transient unstable morphology, where droplets and fibrils coexist at the same time. • Despite better efficiency of D17 nanoparticles in reducing droplet sizes, NLR values corresponding to D17-filled blends were relatively lower than those of R202-filled blends. Therefore, NLR is a combination of the dispersion effect of nanoparticles in PP matrix and the morphology effect caused by interfacial tension between PS and PP. In cases where nanoparticles of similar shape and chemical affinity are used, the effects of particle−particle interactions (dispersion quality) of the silica particles on matrix phase (PP) should be taken into account as they have similar affinities toward the corresponding phase. • According to NLR values of PP/R202 and PP/D17 (PP silica nanocomposites), well-dispersed nanoparticles in PP/ R202 nanocomposites led to larger NLR values than those of PP/D17 nanocomposites. Thus, dispersed nanoparticles in PP matrix is the reason behind the larger values of PP/PS/R202 with respect to PP/PS/D17 blends. • In order to eliminate the effects of PP/silica nanocomposites, use of relative NLR (= NLRPP/PS/silica/NLRPP/silica) was proposed. This can cancel out the particle−particle effects and roughly reflect morphological contributions. Relative NLR values of D17-filled systems were larger than those of R202filled systems. This is consistent with higher efficiency of D17 silica nanoparticles in enhancing morphological properties of (80/20) PP/PS blends. These findings further reveal that the NLR parameter is quite sensitive to small changes in the internal structures of polymer blends.

morphology effect (droplet size) overwhelmed the particle− particle effects, which led to the largest NLR value for the lowest droplet size (PP/PS/10 phr D17). Relative values of NLR are useful in cases in which the effects of nanoparticles show similar chemical affinity (hydrophobic in this study). Therefore, in order to normalize the effect of silica dispersion in PP matrix, the ratio of NLR values of PP/PS/silica blends to NLR values of PP/silica nanocomposites as a function of silica concentration is defined as follows: relative NLR = NLRPP/PS/silica/NLRPP/silica. That is the effects of silica particles on the matrix PP are canceled out and due to interfacial modification, leaving only the effects of droplets. The relative NLR values for the PP/PS/R202 and PP/PS/D17 blends are depicted in Figure 15.

Figure 15. Relative NLR values NLRb/NLRc for hydrophobic silica nanoparticle-filled systems at different concentrations.

Interestingly, relative NLR values for D17 silica nanoparticlefilled systems were relatively larger than those of R202 silica nanoparticle-filled systems. This result matches with the inverse of droplet size of PP/PS blends in Figure 6a. This is also consistent with the significant impact of D17 on morphological evolutions of PP/PS blends. Thus, when investigating the effects of similar nanoparticles (shape and chemical affinity) on morphological evolutions of polymer blends via rheological analyses, the effects of nanoparticles on the matrix phase must be taken into account. However, the capability of NLR parameter to differentiate between effective and ineffective types of fillers for (80/20) PP/PS blend systems still holds. Namely, NLR value could distinguish the effects of nanoparticles with different shapes and surface treatments based on the previous and current study results.

5. CONCLUSIONS Effects of three types of silica nanoparticles with different surface treatments (two hydrophobic silicas, fumed R202 and precipitated D17, and one hydrophilic silica, fumed OX50) on the rheological and morphological properties of (80/20) PP/ PS blends were investigated. SAOS (small-amplitude oscillatory shear) tests revealed that OX50 was the least while R202 was the most effective nanoparticle for enhancing linear rheological properties. However, SEM examination showed that D17 was the best candidate for improving morphology of the blends, as size reduction of PP/PS/D17 blends was sharper compared to the other hydrophobic silica nanoparticle R202. Moreover, hydrophilic OX50 was not able to induce any morphological enhancements. TEM examination revealed mechanisms of morphological evolutions. D17 was located mostly at the J

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

(31) Ewoldt, R. H.; Hosoi, A. E.; McKinley, G. H. J. Rheol. 2008, 52 (6), 1427−1458. (32) Ewoldt, R. H. J. Rheol. 2013, 57 (1), 177−195. (33) Wilhelm, M.; Maring, D.; Spiess, H. W. Rheol. Acta 1998, 37 (4), 399−405. (34) Wilhelm, M.; Reinheimer, P.; Ortseifer, M. Rheol. Acta 1999, 38 (4), 349−356. (35) Wilhelm, M.; Reinheimer, P.; Ortseifer, M.; Neidhöfer, T.; Spiess, H. W. Rheol. Acta 2000, 39 (3), 241−246.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program (No. 2010-0024466) and Global Ph.D. Fellowship Program (2014H1A2A1015767) through the National Research Foundation of Korea (NRF) and BK21 PLUS Centre for Advanced Chemical Technology (21A20131800002).

■

REFERENCES

(1) Fenouillot, F.; Cassagnau, P.; Majesté, J. C. Polymer 2009, 50, 1333−1350. (2) Ray, S. S.; Pouliot, S.; Bousmina, M.; Utracki, L. A. Polymer 2004, 45 (25), 8403−8413. (3) Huang, Y.; Jiang, S.; Li, G.; Chen, D. Acta Mater. 2005, 53, 5117−5124. (4) Hong, J. S.; Kim, Y. K.; Ahn, K. H.; Lee, S. J.; Kim, C. Rheol. Acta 2007, 46 (4), 469−478. (5) Hong, J. S.; Kim, Y. K.; Ahn, K. H.; Lee, S. J. J. Appl. Polym. Sci. 2008, 108 (1), 565−575. (6) Elias, L.; Fenouillot, F.; Majesté, J. C.; Cassagnau, P. Polymer 2007, 48, 6029−6040. (7) Elias, L.; Fenouillot, F.; Majesté, J. C.; Alcouffe, P.; Cassagnau, P. Polymer 2008, 49, 4378−4385. (8) Huitric, J.; Ville, J.; Médéric, P.; Moan, M.; Aubry, T. J. Rheol. 2009, 53, 1101−1119. (9) Vandebril, S.; Vermant, J.; Moldenaers, P. Soft Matter 2010, 6, 3353−3362. (10) Tao, F.; Auhl, D.; Baudouin, A. C.; Stadler, F. J.; Bailly, C. Macromol. Chem. Phys. 2013, 214, 350−360. (11) Salehiyan, R.; Yoo, Y.; Choi, W. J.; Hyun, K. Macromolecules 2014, 47 (12), 4066−4076. (12) Filipe, S.; Cidade, M. T.; Wilhelm, M.; Maia, J. M. Polymer 2004, 45 (7), 2367−2380. (13) Grosso, M.; Maffettone, P. L. J. Non-Newtonian Fluid Mech. 2007, 143, 48−58. (14) Hyun, K.; Wilhelm, M. Macromolecules 2009, 42 (1), 411−422. (15) Wagner, M. H.; Rolón-Garrido, V. H.; Hyun, K.; Wilhelm, M. J. Rheol. 2011, 55 (3), 495−516. (16) Reinheimer, K.; Grosso, M.; Wilhelm, M. J. Colloid Interface Sci. 2011, 360, 818−825. (17) Hyun, K.; Wilhelm, M.; O. Klein, C.; Cho, K. S.; Nam, J. G.; Ahn, K. H.; Lee, S. J.; Ewoldt, R. H.; McKinley, G. H. Prog. Polym. Sci. 2011, 36, 1697−1753. (18) Hyun, K.; Lim, H. T.; Ahn, K. H. Korea-Aust. Rheol. J. 2012, 24 (2), 113−120. (19) Salehiyan, R.; Hyun, K. Korean J. Chem. Eng. 2013, 30 (5), 1013−1022. (20) Lee, S. H.; Song, H. Y.; Hyun, K.; Lee, J. H. J. Rheol. 2015, 59 (1), 1−19. (21) Lim, H. T.; Ahn, K. H.; Hong, J. S.; Hyun, K. J. Rheol. 2013, 57, 767−789. (22) Wu, G.; Asai, S.; Sumita, M.; Hattori, T.; Higuchi, R.; Washiyama, J. Colloid Polym. Sci. 2000, 278, 220−228. (23) Gramespacher, H.; Meissner, J. J. Rheol. 1992, 36, 1127−1141. (24) Thareja, P.; Velankar, S. S. Rheol. Acta 2008, 47, 189−200. (25) Thareja, P.; Moritz, K.; Velankar, S. S. Rheol. Acta 2010, 49, 285−298. (26) Nagarkar, S. P.; Velankar, S. S. Soft Matter 2012, 8, 8464−8477. (27) Nagarkar, S. P.; Velankar, S. S. J. Rheol. 2013, 57 (3), 901−926. (28) Zou, Z. M.; Sun, Z. Y.; An, L. J. Rheol. Acta 2014, 53, 43−53. (29) Cassagnau, P. Polymer 2008, 49 (9), 2183−2196. (30) Ziegler, V.; Wolf, B. A. J. Rheol. 1999, 43, 1033−1045. K

DOI: 10.1021/acs.macromol.5b00679 Macromolecules XXXX, XXX, XXX−XXX