Morphology Evolution of Polymer Blends under Intense Shear During

Article pubs.acs.org/JPCB

Morphology Evolution of Polymer Blends under Intense Shear During High Speed Thin-Wall Injection Molding Yi Zhou, Feilong Yu, Hua Deng,* Yajiang Huang, Guangxian Li, and Qiang Fu College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Sichuan Sheng 610000, P.R. China S Supporting Information *

ABSTRACT: The morphology evolution under shear during different processing is indeed an important issue regarding the phase morphology control as well as final physical properties of immiscible polymer blends. High-speed thin wall injection molding (HSTWIM) has recently been demonstrated as an effective method to prepare alternating multilayered structure. To understand the formation mechanism better and explore possible phase morphology for different blends under HSTWIM, the relationship between the morphology evolution of polymer blends based on polypropylene (PP) under HSTWIM and some intrinsic properties of polymer blends, including viscosity ratio, interfacial tension, and melt elasticity, is systematically investigated in this study. Blends based on PP containing polyethylene (PE), ethylene vinyl alcohol copolymer (EVOH), and polylactic acid (PLA) are used as examples. Compatibilizer has also been added into respective blends to alter their interfacial interaction. It is demonstrated that dispersed phase can be deformed into a layered-like structure if interfacial tension, viscosity ratio, and melt elasticity are relatively small. While some of these values are relatively large, these dispersed droplets are not easily deformed under HSTWIM, forming ellipsoidal or fiber-like structure. The addition of a moderate amount of compatibilizer into these blends is shown to be able to reduce interfacial tension and the size of dispersed phase, thus, allowing more deformation on the dispersed phase. Such a study could provide some guidelines on phase morphology control of immiscible polymer blends under shear during various processing methods.

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INTRODUCTION

morphology evolution, especially under complex and intense shear fields during processing. Meanwhile, high speed thin wall injection molding (HSTWIM) has been considered as an effective tool to fabricate thin wall articles with balanced properties due to its short processing cycle and fast injection speed. More importantly, such method can provide strong shear field and fast cooling rate which could have important influence on the phase morphology of polymer blends. In our previous studies, the formation of multilayered structure consisting of polypropylene (PP) and high density polyethylene (HDPE) has been observed during HSTWIM.24 In general, such multilayered structure could lead to improvement in various properties, including: barrier property,25−28 dielectric property,29 mechanical property,30−33 anisotropic electrical property,34−36 sound absorbing property,37 etc. Fabrication methods reported in literature for such structure are either too complicated to be used in large scale38−40 or require complex machinery.41 Thus, HSTWIM can provide us with a simple and

Polymer blends provide the opportunity to prepare polymeric materials with balanced properties. Their property-structure relationship has been widely investigated, it is well established that the morphology of polymer blends plays vital role on the final performance of polymer blends.1−4 Thus, the control of phase morphology through various processing methods is considered as an effective tool to tune the final performance of various polymer blends. A number of typical phase morphologies often involve orientation under shear field have been reported.5−9 The extent of orientation can be obtained under shear in polymer blends is thought to be largely related with the viscosity ratio and interfacial tension between different phases, shear field as well as melt elasticity.10−12 In dilute droplet model systems consisting of Newtonian fluids, systematic studies have been carried out.13−15 It is demonstrated that the degree of confinement, viscosity ratio, and interfacial tension influence the breakup and coalesce of these dispersed droplets significantly.6,16−23 Nevertheless, immiscible blends for industrial application containing viscoelastic components and compatibilizer have received little attention regarding the relationship of these issues with phase © 2017 American Chemical Society

Received: April 10, 2017 Revised: June 2, 2017 Published: June 7, 2017 6257

DOI: 10.1021/acs.jpcb.7b03374 J. Phys. Chem. B 2017, 121, 6257−6270

Article

The Journal of Physical Chemistry B Table 1. Product Characteristics of the Polymers Studied materials PE

PLA EVOH PP EGMA PP-g-MAH

trademark

melt index

density (g/cm3)

company

2911 5000S 7000F 4032D DC3212HB T30S AX8900 E43

20 g/10 min (190 °C, 2.16 kg) 1 g/10 min (190 °C, 2.16 kg) 0.035 g/10 min (190 °C, 2.16 kg) 15 g/10 min (230 °C, 5 min) 12 g/10 min (210 °C, 2.16 kg) 3 g/10 min (230 °C, 2.16 kg) 6 g/10 min (190 °C, 2.16 kg)

0.96 0.949 0.956 1.24 1.19 0.91 0.94

Fu Shun Petrol. Chem., China Lan Zhou Petrol. Chem., China Yan Shan Petrol. Chem., China NatureWorks, USA Nippon Gohsei, Japan Lan Zhou Petrol. Chem., China Arkema, France Westlake Chemical corporation, USA

8 wt%. The acid number and number-average molecular weight of PP-g-MAH (E43) are 45 mg KOH/g and 9100, respectively. Sample Preparation. Before compounding, polylactic acid (PLA) and EVOH are dried in a vacuum oven for 12 h at 60 and 80 °C, respectively. Compounding between PP and HDPE (2911, 5000S, 7000F), PLA, and EVOH is carried out in a TSSJ-25 twin-screw extruder with a weight ratio of 85/15. The extrusion was performed at 150 rpm with compounding temperature for different sections on the extruder between 160 and 190 °C. The compatibilizer including EGMA and PP-gMAH was added into corresponding blends in a second step extrusion after these blends were first compounded in a weight ratio of 85/15. Finally, these obtained pellets were dried and injection or compression molded into specimens for various characterizations. For high speed injection molding, injection temperature was set between 160 and 190 °C, the diameter of the screw is 25 mm, injection speed is 600 mm/s, and injection distance (short size) is 50 mm. Injection molded sample with thickness of 0.4 mm, width of 60 mm, and length of 80 mm was obtained, detailed sketch of the mold can be found elsewhere.25,34,43 During compression molding, these pellets were first under a pressure of 8 MPa and temperature of 200 °C for 8 min, then they were quickly moved to another cold mold with a pressure of 10 MPa until room temperature was reached. These compression molded specimens were used for dynamic rheological test and dispersed phase size analysis. Neat polymers were also compression molded under the same condition. It should be noted that specimens without compatibilizer were designated as PP/X, where X represents the dispersed phase, with PP/X ratio of 85/15. For instance, PP/PLA represents blends consisting of PP and PLA with a ratio of 85/ 15. The specimens containing compatibilizer were designated as PP/X/C-Y, where C represents the compatibilizer, Y represents the content of such compabilizer. For instance, PP/PLA/C-0.5 represents EGMA content of 0.5 Phr and PP/ PLA in a ratio of 85/15. Characterization. Capillary Rheology. The rheological measurements were carried out on a piston-mode Rosand RH70 (Malvern, Bohlin Instruments) capillary rheometer under 190 °C. The selected shear rate for measurement is in the range of 100−5000 s−1. The length to diameter ratio (L/D) of the mold is 16 with a diameter of 1 mm. To characterize the elasticity of these polymer melt under high shear rate, the entrance pressure drop (Δpent) was measured under 5000 s−1 with a mold length of 0.25 mm. Rheological property at shear rate of 5000 s−1 was chosen as decisive for the phase structure evolution during injection molding in following study, as the upper shear rate limit for our capillary rheometer: 5000 s−1, is close to the real shear rate during injection molding.34

efficient route for the fabrication of multilayered structure. Significant improvement in different properties has been demonstrated by us.25,34 However, the effect of abovementioned fundamental issues, viscosity ratio, interfacial tension, and melt elasticity, on the morphology is not clear; and the general applicability of such process for a range of thermoplastic polymers needs to be systematically studied. Meanwhile, other possible phase morphology under such intense shear during HSTWIM also needs to be explored. Interfacial tension characterizes the ability between two phases to prohibit the deformation of dispersed droplets under shear and restore the original shape after deformation, maintaining the dispersed phase as droplets. The addition of compatibilizer is known to be able to alter the interfacial tension between two phases, thus, influence the final phase morphology of polymer blends. It has been demonstrated that low interfacial tension, strong shear field, and relative high polymer matrix melt elasticity could enhance the deformation ability of dispersed phase.42 While the viscosity ratio is near 1, the deformation of dispersed phase is more likely to occur. It has been observed in our previous studies that the formation of multilayered structure is achieved by deformation of dispersed PE droplets into small platelets and subsequent coalesce between them in the runner and mold.25,34 Therefore, the formation of these small platelets is the key for such phenomenon. This is indeed largely influenced by the interfacial tension, viscosity ratio, and melt elasticity. Thus, it is needed to study the influence of these basic issues of materials on the deformation of dispersed phase during HSTWIM. Only then, the relationship between phase morphology of polymer blends and above three basic issues of materials can be determined, which could reveal the basic requirement for the formation of multilayered structure or other interesting structure during HSTWIM. Therefore, current work aims to study the HSTWIM process of polymer blends based on PP with fixed processing condition and composition ratio. Effort is made to tune the polymer blends composition: using polymer with different MFI and additional compatibilizer, in order to alter the viscosity ratio, interfacial tension and melt elasticity in the system. Then, the phase morphology of these blends can be correlated with above three issues in order to provide guidance for the formation of multilayered structure or other structure during HSTWIM.

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EXPERIMENTAL SECTION Materials. Commercial grades of different materials were used in the current study. The details of these materials are listed in Table 1. The vinyl content in ethylene vinyl alcohol copolymer (EVOH) is 32%, EGMA (AX8900) is an ethylenemethacrylate-glycidyl ester random copolymer, where methyl acrylate content is 24 wt% and glycidyl methacrylate content is 6258


Article

The Journal of Physical Chemistry B

viscosity η* is correlated with frequency ω in the following equation:44−46

Scanning Electron Microscope (SEM). SEM was used to study the phase morphology of compression and injection molded specimens. These specimens were immersed into liquid nitrogen for 3 h before facture. For injection molded specimens, fracture was done at both longitudinal and perpendicular to injection direction, respectively. These fractured specimens were etched and coated with gold before SEM observation under FEI Inspect F with an accelerating voltage of 20 kV. The detail for etching condition is listed in Table 2.

|η*(ω)| = η0[1 + (λω)a ]n − 1/ a

Where |η*(ω)| is the absolute value of the complex viscosity, η0 is the zero shear viscosity, λ is the time constant, ω is the frequency, a is a dimensionless constant describing transition of viscosity from Newtonian region to Non-Newtonian region, and n is the Non-Newtonian coefficient. Through this equation, the zero shear viscosity η0 can be obtained.

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RESULTS AND DISCUSSION Theoretical Background. As well documented in literature, droplet deformation under bulk shear flow can be described with Maffettone−Minale (MM) model,47 where the aspect ratio rp and with b of a droplet can be given by

Table 2. Etching Solution and Condition for the Polymer Blends Studied blends PP/PE

PP/PLA PP/ EVOH

etching solution mixed acid (concentrated sulfuric acid: strong phosphoric acid in a volume ratio of 2:1, potassium permanganate content is 2 wt%) trichloromethane formic acid

etching condition 2.5 h, 25 °C

1.5 h, 25 °C 6.0 h, 25 °C

etched phase PP

⎡ (f 2 + Ca 2)1/2 + Caf ⎤1/2 1MM 2MM ⎥ ⎢ rp = ⎢⎣ (f 2 + Ca 2)1/2 − Caf ⎥ 1MM 2MM ⎦

PLA EVOH

b = 2R

Dispersed Phase Size Analysis. Image Pro Plus was used to analyze the dispersed phase size, at least 300 dispersed phases had been measured before the number-averaged diameter (Rn) and volume averaged diameter (Rv) were calculated using following equation: Rn =

Rv =

(1)

2 2 [f1MM + Ca 2(1 − f2MM )]1/6 2 (f1MM + Ca)1/6

(5)

f1MM =

40(p + 1) (2p + 3)(19p + 16)

(6)

f2MM =

5 3Ca 2 + 2p + 3 2 + 6Ca 2

(7)

In above equations, p represents the viscosity ratio between droplets and matrix. An important parameter, capillary number Ca, which plays important role on the deformation and breakup behavior of a droplet is defined as

∑i (Nv)i R i4 ∑i (Nv)i R i3

(4)

Where f1MM and f 2MM are defined as

∑i (Nv)i R i ∑i (Nv)i

(3)

(2)

Where (Nv)i is the number for dispersed phase with a size of Ri, the phase diameter distributed can be given by d = Rv/Rn. Dynamic Rheological Characterization. The rheological characterization was carried out on a TA advanced rheometric expansion system (ARES). Rheological test was performed in parallel plate mode with a plate diameter of 25 mm and distance of 1.7 mm. Tests were conducted at 190 °C with 1% strain in a frequency range between 0.01 rad/s to 100 rad/s under nitrogen atmosphere. Neat polymers were characterized between the frequency range of 0.01 rad/s to 100 rad/s. As all the polymer used in current study is linear polymer, based on the calculation for zero shear viscosity of neat polymer, complex

·

Ca = ηmγR /σ

(8)

·

where ηm, R, γ , and σ are matrix viscosity, droplet radius, shear rate, and interfacial tension, respectively. Larger capillary number indicates better deformability of these droplets. It should be noted that in MM model, the breakup and coalesce of droplets are not considered. Moreover, systems with numerous droplets are not considered either. Therefore, current system is too complex for such model only considering specific ideal scenario. Analytical model with the ability to describe systems with conditions more related to industrial

Figure 1. (a) Shear viscosity of the polymer as a function of shear rate. (b) Shear viscosity of the polymer at shear rate 5000 s−1. 6259


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Such model provides an effective method to measure the interfacial tension in blends system. With the dispersed phase morphology and the dynamic viscoelasticity of these blends components at the same temperature and frequency range, α/Ri can be used as a fitting parameter to fit the experimentally obtained dynamic modulus curve for blends system, then, the interfacial tension can be determined. Such method has already been used to determine the interfacial tension in a number of blends systems: PLA/PCL,54 PS/PA6,55 PP/EVA.56 When the dispersed phase and matrix are Newtonian liquid, and the distribution index of dispersed phase size (d = Rv/Rn) is below 2, the following equation can be used to calculate the interfacial tension in blends system:

practice is needed. Following study could provide some useful data for future theoretical study on the morphological evolution of polymer blends under intense shear and confinement. Shear Viscosity and Melt Elasticity of Various Polymers at High Shear Rate. Figure 1a shows the relationship between shear viscosity and shear rate for various polymers under 190 °C. It is noted that shear thinning effect is observed for all polymers. Figure 1b demonstrates the shear viscosity and viscosity ratio between different polymers and PP under a shear rate of 5000 s−1. It can be observed that EVOH illustrates the largest shear viscosity and maximum viscosity ratio (2.91) with PP. While PLA demonstrates the lowest shear viscosity and minimum viscosity ratio (0.41) with PP. Meanwhile, 5000S and 7000F shows viscosity ratio approaching 1. It should be noted that these two PE illustrate the most shear thinning effect compared with the rest, especially much stronger than 2911. The entrance pressure drop (Δpent) measured in capillary rheology can be used to indirectly characterize the elasticity of polymer melt.48 Figure 2 demonstrates the Δpent of various

τ1 =

R vηm (19p + 16)(2p + 3 − 2φ(p − 1)) 4γ 10(p + 1) − 2φ(5p + 2)

(9)

where Rv is the volume averaged radius of dispersed phase; γ is the interfacial tension between two phases; ϕ is the volume fraction of dispersed phase; K = ηd/ηm, ηd, and ηm are the zero shear viscosity of dispersed phase and matrix, respectively; τ1 is the relaxation time of blends interface. As well documented in the literature, polymer melt can be regarded as Newtonian liquids near zero shear, therefore, such equation can use the calculation of relaxation time as well as interfacial tension. Figure 3 shows SEM study on the fracture surface of PP/PE blends in a ratio of 85/15 consisting of PE with different MFI. PP is easier to be etched by mixed acid than PE, therefore, the darker area is PP, lighter area is PE.57,58 It is observed that 2911 and 5000s are dispersed as droplets in PP matrix, meanwhile, 7000F is dispersed as both spherical and elliptical droplets. The diameter of dispersed phase is 1.44, 1.73, and 3.55 μm (see Table 3), respectively. Besides, the distribution index of dispersed phase size is below 2 for all specimens. Therefore, the interfacial tension between two phases can be calculated.

Figure 2. Δpent of polymer components at shear rate 5000 s−1.

polymers at 190 °C under a shear rate of 5000 s−1. It can be noted that 5000S shows the largest Δpent, indicating the largest melt elasticity. While 2911 shows the lowest Δpent, indicating the lowest melt elasticity. PP/PE Blends. The interfacial tension between immiscible polymer blends can be determined based on rheology data,49−51 the mechanism responsible is that while the “seaisland″ structure in immiscible polymer blends is under shear, enhancement in the elasticity of blends at low frequency region can be observed. Such enhancement is caused by the interfacial relaxation of deformed droplets. Palierne has derived the constitutive equations for the linear viscoelasticity of latex system.52,53 Such equation is a function between the linear viscoelasticity, the diameter of dispersed phase and its distribution (ϕi, Ri), and interfacial tension (α).

Table 3. Number-Average Radius, Volume-Average Radius and Polydispersity for PP/PE Blends with Different Melt Index of PE blend

Rn(μm)

Rv(μm)

Rv/Rn

PP/2911 PP/5000S PP/7000F

0.980(±0.391) 1.078(±0.483) 2.398(±0.946)

1.441(±0.601) 1.729(±0.810) 3.550(±1.491)

1.470 1.604 1.480

Figure 4 shows the storage modulus (G′) and loss modulus (G″) of PP, PE, and PP/PE under dynamic frequency sweep at 190 °C. It is demonstrated in Figure 4 that the storage and loss moduli of PP are higher than that of 2911 in all tested frequency for PP/2911 system. This indicates PP contributes

Figure 3. SEM micrographs of compression molded PP/PE (85/15) blends: (a) PP/2911; (b) PP/5000S; (c) PP/7000F. 6260


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Figure 4. Storage modulus (G′) and loss modulus (G″) of PP, PE, and PP/PE under dynamic frequency sweep at 190 °C: (a,b) PP, 2911, and PP/ 2911; (c,d) PP, 5000S, and PP/5000S; (e,f) PP, 7000F, and PP/7000F.

Figure 5. Weighted relaxation spectrum of the PP, PE, and PP/PE (85/15) blend.

more to the system viscoelasticity. Meanwhile, it is noted that the storage modulus of PP/2911 blend is higher than that of PP and 2911 at low frequency range. This is caused by the shape relaxation of dispersed droplets. Similar result is also obtained for PP/5000S system, where the storage modulus of blends is higher than that of PP and 5000S due to enhancement in the elasticity. For PP/7000F system, the storage and loss moduli of neat 7000F are higher than that of PP and PP/7000F blends in all tested frequency range. Meanwhile, the storage modulus of blends system is higher than that of neat PP. The storage modulus of blends system is not only related to the storage modulus of both compositions, the contribution of interface between two phases to elasticity should also be considered. As

the storage modulus of 7000F is much higher than that of neat PP, the addition of 7000F into PP led to an increase in the storage modulus of the blends system. Meanwhile, the shape relaxation of dispersed droplets will increase the storage modulus of the system. These mechanisms are thought responsible for the higher storage modulus of PP/7000F than neat PP in low frequency range. On the contrary, PP/7000F (85/15) can be considered as adding large amount of PP into 7000F, the elasticity contribution from the interface cannot make up the decrease in storage modulus caused by additional PP. Therefore, the storage modulus of PP/7000F blends is lower than that of neat 7000F. 6261


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The Journal of Physical Chemistry B According to eq 9, the interfacial relaxation time τ1 is needed to calculate the interfacial tension between two phases. τ1 can be obtained from the relaxation time spectrum. The relaxation spectrum of viscoelastic polymer system (H(τ)) has the following relationship with G′ and G″: ∞

G′(ω) =

2 2⎤

⎡

∫−∞ ⎢⎣ H1 (+τ)ωω2ττ2 ⎥⎦d(ln τ) ∞

G″(ω) =

the vertical injection direction, indicating the formation of highly oriented 5000S fibers in such system. Meanwhile, 7000F is dispersed as spherical or ellipsoidal particles with small aspect ratio in the core layer of PP/7000F. Therefore, from PP/2911 to PP/5000S, then to PP/7000F, the phase morphology of PE dispersed phase in PP matrix has changed from platelets to microfibers, then to spherical or ellipsoidal particles. At the same time, the phase size is also increasing. It is thought that the change in phase morphology is largely related to the viscosity ratio, melt elasticity, and interfacial tension in the system. Since these PE have very similar chemical structure, they should have little difference in interfacial tension. It is supported by data listed in Table 4. Therefore, viscosity ratio and melt elasticity are responsible for the phase morphology difference observed. From capillary rheological data shown in Figure 1 b, the viscosity ratio between 2911 and T30S is as small as 1.63 under high shear rate. Therefore, the dispersed 2911 phase in PP matrix can be deformed into smaller phases during processing and demonstrate platelets or fibers structure during HSTWIM. While 7000F also has relative small viscosity ratio with PP, however, the elasticity is quite large as shown in Figure 2. Hence, it is more difficult to be deformed into smaller phases and illustrate spherical or ellipsoidal phase morphology. The viscosity ratio in PP/5000S system is also approaching 1, with relative large elasticity (as shown in Figure 2), therefore, microfiber structure is observed. It is noted that the melt elasticity is an important issue influencing the deformation ability of dispersed droplets.59,60 Among these PE, 2911 has the lowest melt elasticity, 5000S and 7000F have much higher elasticity (see Figure 2). Such high elasticity is thought to be responsible for the difference in phase morphology. PP/EVOH Blends. Figure 7 shows SEM images for the fracture surface of compression molded PP/EVOH (85/15) containing different amount of PP-g-MAH. EVOH is removed with formic acid before observation, hence, illustrates as voids in the image. It is noted that EVOH is dispersed as droplets in PP matrix. The addition of compatibilizer (PP-g-MAH) can reduce the size of dispersed phase. This is caused by the enhanced interface and reduced interfacial tension effect from such compatibilizer. As shown in Table 5, the dispersed phase size is reduced from 4.276 to 2.87 μm with the addition of 1 phr compatibilizer, it is slightly increased to 3.596 μm with 2 phr compatibilizer content. The distribution index for dispersed phase size is below 2 for all specimens. Therefore, the interfacial tension between two phases can still be calculated. Figure 8 shows the storage modulus and loss modulus of PP/ EVOH blends and respective neat polymers at 190 °C. It is well-known that the storage modulus G′ reflects the energy stored during elastic deformation process of materials, it is often used to characterize the elasticity of a given polymer; while loss modulus G″ represents the energy loss in the form of heat due to viscous flow during deformation, it is often used to characterize the viscosity of a given polymer. It is noted that the storage modulus of PP/EVOH is obviously higher than that of neat PP and EVOH in low frequency range. The deformation relaxation of dispersed droplets is thought as responsible for such phenomenon. During the deformation relaxation process of dispersed droplets, the interface area and interfacial energy have been changed, then, the energy is released together with the elastic energy stored, leading to enhanced elasticity.61−63 Besides, similar behavior is also observed in PP/EVOH blends containing various amount of compatibilizer.

⎡

(10)

⎤

d(ln τ ) ∫−∞ ⎢⎣ 1H+(τω)ωτ 2 2⎥ τ ⎦

(11)

The weighted relaxation spectrum (τ H(τ)) of blends system from above equations, the relaxation time spectrum presented in this article is obtained from ARES (detailed calculation method can be found in the helpfile of TA Orchestrator V7.2.0.2). Figure 5 presents the relaxation time spectrum of PP/PE blends, neat PP and different PE at 190 °C. The relaxation time of neat PP and 2911 are 1.6 and 0.003 s, respectively. While for PP/2911 blends system, a relaxation time shoulder peak near 1 s is observed, which is associated with the relaxation of PP phase; a secondary relaxation time peak is observed at 22.07 s corresponding to the shape relaxation of dispersed phase. For PP/5000S system, the relaxation time for neat 5000s is 0.287 s, while the relaxation time corresponding to the deformation relaxation of dispersed droplets in PP/5000s system is 20.851 s. For PP/7000F system, no relaxation peak is observed in tested frequency range for neat 7000F, this might be due to the relative long relaxation time of 7000F molecules. The relaxation time for the interface of PP/7000F blends is 11.635 s. It is shown in Table 4 that the interfacial tension in PP/2911 system Table 4. Calculated Interfacial Tension and Parameters Used

sample PP/ 2911 PP/ 5000S PP/ 7000F

volume fraction of dispersed phase (ϕ)

volume average radius of dispersed phase (Rv, μm)

visocsity of matrix of PP (ηm, Pa.s)

zero shear visocsity ratio (K)

τ1 (s)

γ1 (mN/ m)

0.143

1.441

16261

0.031

22.067

1.52

0.145

1.729

16261

20.851

3.550

16261

11.635

is 1.52 mN/m. However, the interfacial tension of PP/5000S and PP/7000F system could not be calculated as the zero shear viscosity can not be obtained from the viscosity data in tested frequency range. Figure 6 shows the phase morphology of the core layer in PP/PE blends processed by HSTWIM, where the upper row is observed from parallel injection direction and lower row is observed from vertical injection direction. It is observed that 2911 is dispersed in PP phase as relative thin and short, highly oriented strip structure in the parallel injection direction, while relative short and highly oriented strip structure is observed in the vertical injection direction. This indicates the formation of dis-continuous 2911 platelets or fibers. This is caused by the intense shear during HSTWIM, which can deform dispersed 2911 phase into platelets. For PP/5000S system, relative thin and short, highly oriented strip structure is observed in the parallel injection direction, while spot structure is obtained in 6262


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Figure 6. SEM micrographs of the core layer in PP/PE (85/15) thin-wall parts viewed parallel (up column) and perpendicular (below column) to the flow direction, respectively. (a),(d) PP/2911; (b),(e) PP/5000S; (c),(f) PP/7000F.

compatibilizer due to enhanced interfacial interaction between PP and EVOH.64,65 Figure 10a shows the relaxation time spectrum of PP/EVOH, neat PP, and EVOH at 190 °C. The relaxation time of neat PP (1.6 s) is higher than that of EVOH (0.03 s). There are two relaxation peaks for PP/EVOH blends, where the shoulder peak near 0.33 s corresponding to the superposition of molecular relaxation process of both polymers; while relaxation peak at 6.39 s originates from the long relaxation of the interface between PP and EVOH. Figure 10b shows the relaxation time spectrum of PP/EVOH containing different amount of compatibilizer. It is noted that the relaxation time is increased from 6.39 to 10.66 s with the addition of 1 phr compatibilizer. This is caused by the relaxation of interface which is influenced by two issues: the viscous force elongates droplets ηmγ, and interfacial stress restore droplets to sphere σ/ R. With the addition of compatibilizer, the interfacial interaction can be enhanced, interfacial tension between two phases can be reduced. Hence, the force which prohibits droplets restoring their original shape has been enhanced, leading to increased relaxation time for these droplets. However, with further increasing compatibilizer content to 2 phr, the secondary relaxation time is reduced from 10.66 to 4.68 s. The calculated interfacial tension and parameters used are listed in Table 6. The interfacial tension for PP/EVOH without compatibilizer is near 16.25 mN/m, while the value decreases to 6.53 mN/m with 1 phr PP-g-MAH. However, the interfacial tension increases to 18.64 mN/m when compatibilizer content increases to 2 phr. It is indeed strange that the interfacial tension and dispersed phase size first decrease with the addition of compatibilizer, then increases rapidly, with some even exceeding the original value without compatibilizer. Normally, good compatibilizer or nanofiller with compatibilzing effect could reduce the dispersed phase size with quite small content, then the dispersed phase size or interfacial tension will keep steady with further increasing compatibilizer or filler content.11,66 Herein, we do not have a clear answer to this phenomenon. In literature, only few studies with similar trend

Figure 7. SEM micrographs of compression molded PP/EVOH (85/ 15) blends with different PP-g-MAH content: (a) 0 phr; (b) 0.5 phr; (c) 1 phr; (d) 2 phr.

Table 5. Number-Average Radius, Volume-Average Radius, and Polydispersity for PP/EVOH Blends with Different PPg-MAH Contents blend

Rn(μm)

Rv(μm)

Rv/Rn

PP/EVOH PP/EVOH/C-0.5 PP/EVOH/C-1 PP/EVOH/C-2

2.604(±1.177) 1.359(±0.794) 1.264(±0.728) 1.898(±0.985)

4.276(±2.044) 2.393(±1.251) 2.183(±1.127) 3.596(±1.961)

1.642 1.761 1.727 1.890

Figure 9 shows the complex viscosity of PP/EVOH blends (85/15) containing different amount of compatibilizer. It is noted that all specimens demonstrate shear thinning effect with increasing frequency, illustrating non-Newtonian liquid behavior. The viscosity is slightly increased with the addition of 6263


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Figure 8. Dynamic modulus of PP/EVOH (85/15) blend and of pure phases at 190 °C: (a) storage modulus; (b) loss modulus.

1b) between PP and EVOH are quite large, which are unfavorable for the deformation of EVOH phase. Therefore, the deformation observed could be attributed to the relative low melt elasticity of EVOH phase (as shown in Figure 2) and intense shear during HSTWIM, which makes these EVOH droplets relatively easily deformable. It is worth it to notice that the phase size in skin layer is obviously smaller than that in the core layer. This might due to the more intense shear in skin layer which leads to better compatibility, easier break up, and dispersion of dispersed phase. Then, the fast cooling rate in the thin wall structure can freeze the structure. While core layer possesses weaker shear and longer cooling time, this is beneficial to the cohesion of deformed droplets, hence, leads to larger dispersed phase size. Figure 12 shows the phase morphology of PP/EVOH blends containing different amount of compatibilizer in the core layer, where EVOH phase has been removed with formic acid before observation. It is noted that relative small length-to-width ratio platelets or low aspect ratio fiber structure is obtained in blends without compatibilizer. With the addition of 0.5 phr compatibilizer, the aspect ratio of EVOH fiber is significantly enhanced. Such behavior is caused by the increased interfacial interaction between PP and EVOH, which leads to enhanced dispersion of EVOH in PP phase and reduced interfacial tension. Thus, the ability to resist deformation and restore original shape is reduced, and finally leads to enhanced deformation of dispersed phase. With further increasing compatibilizer content to 1 phr, slight further decrease in interfacial tension is obtained, which leads to slightly smaller phase size. For compatibilizer content of 2 phr, ellipsoidal structure with much larger phase size is observed instead of larger aspect ratio fiber structure. This might be caused by the significantly enhanced interfacial tension at 2 phr, thus, leads to

Figure 9. Complex viscosity of the PP/EVOH (85/15) blends with different PP-g-MAH contents.

have been reported,65,67 but no clear explanation is given. It might be caused by the rather moderate compatibilzing effect from the compatibilizer used. So, phase separate might occur at higher content. Another possible mechanism might be the dissolution of low molecular weight PP-g-MAH in PP matrix, which leads to a decrease in its viscosity and, therefore, calculation of incorrect interfacial tension from eq 9. Further study is needed to understand this. Figure 11 shows the phase morphology of PP/EVOH (85/ 15) blends after HSTWIM, where EVOH phase has been removed with formic acid before observation. EVOH fibers with relative large aspect ratio are observed in the skin layer of the specimen. This is caused by the strong shear and fast cooling rate in the skin layer of HSTWIM specimen. While the dispersed phase is in the form of small platelets or fibers with low aspect ratio in the core layer, this indicates that the dispersed EVOH phase is still relatively largely deformed. The interfacial tension and viscosity ratio (2.91 as shown in Figure

Figure 10. (a) Weighted relaxation spectrum of the PP, EVOH, and PP/EVOH (85/15) blend. (b) Weighted relaxation spectrum of the PP/EVOH (85/15) blends with different PP-g-MAH contents. 6264


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The Journal of Physical Chemistry B Table 6. Parameters for Interfacial Tension Calculation and Calculated Interfacial Tension sample PP/EVOH PP/EVOH/ C-0.5 PP/EVOH/ C-1 PP/EVOH/ C-2


volume average radius of dispersed phase (Rv, μm)

viscosity of matrix of PP (ηm, Pa.s)

zero shear viscosity ratio (K)

τ1 (s)

γ1 (mN/ m)

0.119 0.119

4.276 2.393

16261 16261

0.123 0.123

6.39 10.23

16.25 5.68

0.119

2.183

16261

0.123

10.662

4.97

0.119

3.596

16261

0.123

4.68

18.64

Figure 11. SEM Micrographs of PP/EVOH (85/15) thin-wall parts viewed parallel (up column) and perpendicular (below column) to the flow direction, respectively. (a),(d) Whole cross-section; (b),(e) skin layer; (c),(f) core layer.

Figure 12. SEM micrographs of the core layer in PP/EVOH (85/15) blends with different PP-g-MAH contents viewed parallel (up row) and perpendicular (down row) to the flow direction, respectively. (a),(e) PP/EVOH; (b),(f) PP/EVOH/C-0.5; (c),(g) PP/EVOH/C-1; (d),(h) PP/ EVOH/C-2.

enhanced ability to resist deformation and restores original spherical shape. Therefore, EVOH phase is hardly deformed under intense shear, dispersed as ellipsoidal structure. PP/PLA Blends. Figure 13 shows SEM images of fracture surface from PP/PLA (85/15) containing different amount of

EGMA content. PLA has been etched with chloroform, showing as voids in the image. Similar with PP/EVOH system, PLA phase is dispersed as spherical droplets in PP matrix. The addition of compatibilizer decreases the PLA phase size as indicated in Table 7. The phase size is decreased from 1.691 to 6265


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as shown in Figure 13, their difference in storage and loss moduli is as insignificant as shown in Figure S1 and S2. Figure 15 shows the complex viscosity of PP/PLA (85/15) blends containing different amounts of EGMA over a range of

Figure 15. Complex viscosity of the PP/PLA (85/15) blends with different EGMA contents. Figure 13. SEM micrographs of compression molded PP/PLA (85/ 15) blends with different PP-g -MAH content: (a) 0 phr; (b) 0.5 phr; (c) 1 phr; (d) 2 phr.

frequency. It is noted that the complex viscosity of PP/PLA containing compatibilizer is higher than that of PP/PLA without compatibilizer. With increasing compatibilizer content, complex viscosity of these blends increases until a maximum is reached at 1 wt%. This indicates that the interfacial interaction between PP and PLA is enhanced by EGMA. With further increase in EGMA content to above 1 phr, the viscosity is decreased. This might be attributed to the phase separation caused by additional EGMA, leading to significantly reduced EGMA content at the interface.64,68 Figure 16a shows the relaxation time spectrum of PP/PLA blends, neat PP and PLA under 190 °C. It is noted that the relaxation time of neat PP and PLA are 1.6 and 0.01 s, respectively. For PP/PLA blends, there are two relaxation peaks located at 0.5 and 16.70 s, respectively. Former shoulder peak at 0.5 s corresponds to the superposition of molecular relaxation process between two polymer phases, while later relaxation peak at 16.70 s is caused by the interface long time relaxation between PP and PLA. Figure 16b shows the relaxation time spectrum of PP/PLA blends containing different amount of compatibilizer. It is noted that the secondary relaxation time increases from 16.70 to 17.74 s with the addition of 0.5 phr compatibilizer. With further increase in compatibilizer content, the secondary relaxation is reduced to 9.149 s at 2 phr. In Table 8, PP/PLA without compatibilizer is shown to have an interfacial tension of 2.42 mN/m. Such value is reduced to a

Table 7. Number-Average Radius, Volume-Average Radius, and Polydispersity for PP/PLA Blends with Different EGMA Contents blend

Rn(μm)

Rv(μm)

Rv/Rn

PP/PLA PP/PLA/C-0.5 PP/PLA/C-1 PP/PLA/C-2

0.896(±0.452) 0.874(±0.386) 1.417(±0.685) 1.581(±0.789)

1.691(±0.915) 1.422(±0.671) 2.442(±1.233) 2.844(±1.489)

1.887 1.627 1.724 1.799

1.422 μm for 0.5 phr compatibilizer content; with further increase in compatibilizer content to 1 phr and 2 phr, the dispersed phase size is increased to 2.442 and 2.844 μm, respectively. Besides, the distribution index for dispersed phase size is below 2 for all specimens. Therefore, the interfacial tension between two phases can be calculated. Figure 14 shows the storage modulus and loss modulus of PP, PLA, and PP/PLA blends at 190 °C. In low frequency range, PP/PLA blends demonstrate higher storage modulus than that of PP and PLA. This is caused by the increase in melt elasticity due to deformation relaxation of dispersed droplets. Such behavior should be influenced by the dispersed phase size. Nevertheless, as the difference in dispersed size is not as much

Figure 14. Dynamic modulus of PP/PLA (85/15) blend and of pure phases at 190 °C: (a) storage modulus; (b) loss modulus. 6266


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Figure 16. (a) Weighted relaxation spectrum of the PP, PLA, and PP/PLA (85/15) blend. (b) Weighted relaxation spectrum of the PP/PLA (85/ 15) blends with different EGMA content.

Table 8. Parameters for Interfacial Tension Calculation and Calculated Interfacial Tension sample


volume averge radius of dispersed phase (Rv, μm)

visocsity of matrix of PP (ηm, Pa.s)

zero shear visocsity ratio (K)

τ1 (s)

γ1 (mN/m)

PP/PLA PP/PLA/C-0.5 PP/PLA/C-1 PP/PLA/C-2

0.115 0.115 0.115 0.115

1.691 1.422 2.442 2.884

16261 16261 16261 16261

0.109 0.109 0.109 0.109

16.7003 17.741 15.113 9.15

2.42 1.92 3.87 7.44

Figure 17. SEM micrographs of PP/PLA (85/15) thin-wall parts viewed parallel (up column) and perpendicular (below column) to the flow direction, respectively. (a),(d) Whole cross-section; (b),(e) skin layer; (e),(f) core layer.

Figure 17 shows the phase morphology of PP/PLA (85/15) after HSTWIM, where PLA is etched with chloroform before observation. It is observed that PP/PLA demonstrate similar morphology as PP/EVOH specimen, namely highly oriented platelets with small length-to-width ratio and fibers with relative small aspect ratio are observed in the skin layer due to intense shear as fast cooling rate. While in the core layer, the dispersed phase is in the form of fiber with relative large aspect ratio, demonstrating PLA phase has been relatively largely deformed. The viscosity ratio between PP and PLA under high shear rate is quite small (with the viscosity of PLA is much smaller than that of PP), which is not beneficial for the deformation of dispersed phase as the difference in viscosity is quite large. However, PLA has quite low melt elasticity, and relative low interfacial tension between PP and PLA is observed, therefore,

minimum of 1.92 mN/m, indicating the addition of EGMA has reduced interfacial tension significantly. It is then increasing with increasing compatibilizer content, reaching 7.44 mN/m at 2 phr. Indeed, the droplet size shown in Table 7 is only slightly reduced by adding compatibilizer, with further increasing content, the droplet size is actually increased. The trend in interfacial tension agrees with the trend of droplet size. Comparing with result in literature,65 the interfacial tension obtained here is rather similar. The trend observed for this system is similar to PP/EVOH system discussed above and some results observed in literature.65,67 Yet, no clear explanation is given. As discussed above, it might be caused by the rather moderate compatibilzing effect from the compatibilizer or the reduced viscosity for PP matrix due to the dissolution of compatibilizer in the matrix. 6267


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Figure 18. SEM micrographs of the core layer in PP/PLA (85/15) blends with different PP-g-MAH contents viewed parallel (up row) and perpendicular (down row) to the flow direction, respectively. (a),(e) PP/PLA; (b),(f) PP/PLA/C-0.5; (c),(g) PP/PLA/C-1; (d),(h) PP/PLA/C-2.

dispersed PLA phase is largely deformed into fiber structure under intense shear. Figure 18 shows the phase morphology of PP/PLA blends containing different amount of compatibilizer in the core layer, where PLA has been etched with chloroform before observation. It is noted that the blends without compatibilizer show fiber structure with relative small aspect ratio in the core layer. With the addition of 0.5 phr compatibilizer, the PLA phase size is obviously decreased, illustrating small platelets structure. This is caused by the improved dispersion and reduced phase size of PLA in PP matrix; meanwhile, the interfacial tension between PP and PLA is reduced, leading to reduced ability of dispersed phase to resist deformation or restore its original morphology. Above issues finally led to significant deformation in dispersed phase, forming platelets structure under shear. With further increase in compatibilizer content, the size of dispersed phase is obviously increasing. This might be caused by the increased interfacial tension, which leads to enhanced ability to resist deformation and restore original spherical shape. Therefore, the dispersed phase is in the form of relative large platelets. From above analysis, different blends system illustrates various morphology in injection molded article due to difference in interfacial tension, melt elasticity and viscosity ratio. Table 9 demonstrates the relationship between morphology in core layer and interfacial tension, viscosity ratio, melt elasticity. It is noted that the dispersed phase can be deformed into layered-like structure if interfacial tension, viscosity ratio, and melt elasticity are relatively small. While some of these values are relatively large, these dispersed droplets are not easily deformed under HSTWIM, forming ellipsoidal or fiber-like structure.

Table 9. Relationship between the Morphology and Interfacial Tension, Viscosity Ratio, Melt Elasticity as well as Melt Viscosity of the Polymer Components viscosity ratio

melt elasticity ratio

morphology in core layer

blends system

1.50 1.90

1.63 0.41

0.48 0.60

large platelets small platelets

5.68

2.91

0.52

long fiber

0.41 1.08 2.91 0.93 2.91

0.60 1.12 0.52 1.60 0.52

long fiber short fiber short fiber ellipsoid ellipsoid

PP/2911 PP/PLA/C0.5 PP/EVOH/ C-0.5 PP/EVOH/ C-1 PP/PLA PP/5000S PP/EVOH PP/7000F PP/EVOH/ C-2

interfacial tension (mN/m)

4.97 2.40 assumed 1.50 16.30 assumed 1.50 18.60

blends based on PP under HSTWIM. The morphology of PPbased blends under HSTWIM is shown to have close relationship with the melt elasticity, interfacial tension, and viscosity ratio between different components. The dispersed phase is more easily deformed into platelets structure with small viscosity ratio, small melt elasticity, and small interfacial tension. While some of these three values are relatively large, the dispersed phase will only be deformed into fiber structure or ellipsoidal structure. Moreover, the addition of moderate amount of compatibilizer into these blends is shown to be able to reduce interfacial tension and the size of dispersed phase, thus, allowing more deformation on the dispersed phase. Through controlling above three issues by selecting different materials and using different compatibilizers, different phase morphology can be obtained.

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CONCLUSION Current study investigates the relationship between some intrinsic properties of different polymers, such as viscosity ratio, interfacial tension, melt elasticity, and the deformation of dispersed phase as well as phase morphology of their respective 6268


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b03374. The storage modulus and loss modulus of PP/PLA and their compatibilized blends (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Yajiang Huang: 0000-0002-1803-1580 Qiang Fu: 0000-0002-5191-3315 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We express our sincere thanks to the National Natural Science Foundation of China for financial support (51421061). H. Deng would like to thank the Ministry of Education (Program for New Century Excellent Talents in University, NCET-130383).

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Morphology Evolution of Polymer Blends under Intense Shear During

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