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Non-Linear Rheological Response as a Tool for Assessing Dispersion in Polypropylene/Polycaprolactone/Clay Nanocomposites and Blends Made with Sub-Critical Gas-Assisted Processing.

INTRODUCTION

The blending of polymers is an attractive way to obtain a specified range of properties that a single material might not possess. For example, in toughened blends, a glassy or brittle, but strong matrix, is blended with a small amount of rubbery or soft, but lower strength, minor phase resulting in increased impact properties without sacrificing too much tensile strength [1].

Getting good properties out of a blend depends on not only the physical properties of the components, but also on the morphology of the mixture (namely, the size and shape of the minor phase) and the chemistry of the polymers, which affects how they interact. Most polymers are immiscible as a result of a low entropy of mixing, which means that they tend to phase separate and have a low level of interaction with each other, thus necessitating the use of a compatibilizer [1-3]. Reducing the secondary phase size in blends increases the surface area of interaction between the two phases and can promote the desired properties [4].

While the use of organo-modified nanoclay for enhancing properties like stiffness, gas barrier, and flame resistance is well known, it has also been shown that this 2D nanofiller can be used as an effective solid-state compatibilizer in blends of immiscible polymers [5-11]. Furthermore, the use of a solid-state filler like nanoclay can help to reduce the droplet size of the minor component in blends by altering the viscosity ratio. In addition, the clay particles can reside at the interface between the droplets, surrounding the minor phase and preventing coalescence, thereby maintaining the desired morphology in post-extrusion steps like injection molding or film extrusion [3, 12, 13]. In this study, Cloisite 20A, a montmorillonite clay modified with dimethyl (dehydrogenated tallowalkyl) ammonium cations, was used as a reinforcing filler and compatibilizer.

Nanofillers are required to be mixed well into their host polymer to take advantage of their benefits; thus, obtaining a good distribution and dispersion is paramount. Methods for dispersing clay nanoparticles include solvent-based methods, in situ polymerization, and melt processing [5, 6, 14]. Melt processing is the most economically viable technique for scale-up and is the method used in this paper--specifically, twin-screw extrusion (TSE) with the addition of a sub-critical gas to foam the polymer during extrusion. This creates equibiaxial extensional stresses in the melt that help to break up agglomerates of nanofillers and disperse them well [15].

In previous works, while dispersing nanoclay using either supercritical fluids (SCF) or sub-critical gas-assisted processing (SGAP), a synergistic effect occurred. Foaming is nucleated by the presence of the filler and that foaming helps exfoliate the nanoclay into a finer dispersion, increasing the sites where bubble growth is initiated [16, 17]. Most foaming applications involve the use of SCFs, which are above their critical temperature and pressure, and have the viscosity of a gas but the density of a liquid [18]. This liquid-like density allows them to be pumped and metered accurately [19]. In the case of supercritical carbon dioxide (SC-C[O.sub.2]), typical uses include as a viscosity reducer, an organic solvent, or as a blowing agent in microcellular foams [20, 21]. While excellent for these applications, special and expensive devices are required such as high-pressure pumping, metering, and safety equipment. In this work, sub-critical nitrogen ([N.sub.2]) is used with simple, off-the-shelf parts (metal hosing, pressure regulator, valve, and gas cylinder) that make the process accessible to even small plastics processing operations.

Rheologically, it is possible to distinguish when a blend has a smaller secondary phase size and when a nanocomposite has a better degree of dispersion. In practice, many researchers use small amplitude oscillatory shear (SAOS) in a frequency sweep at a fixed strain, typically ~1%, to stay in the linear viscoelastic region (LVR) where stress and strain are proportional [22]. At low frequencies, a system with a larger degree of interaction between the polymers, or a better degree of dispersion, will exhibit non-terminal behaviors that present as a tail in the storage modulus (G) vs. the angular frequency [5, 6]. However, there is a more quantitative way to assess these measures of mixing.

Fourier-transform (FT) rheology is a useful tool for assessing the behavior of complex materials outside of the LVR range, which is coincidentally where many polymer processing operations reside [22]. For many materials, the LVR ends somewhere around 10% strain at typical testing frequencies of 1 rad/s or higher. By probing higher strains at a constant strain-rate amplitude, more information about the material can be gleaned. These higher strains correspond to medium amplitude oscillatory shear (MAOS), from ~10% to ~100% strain, and large amplitude oscillatory shear (LAOS), with strains of 100% and higher [23]. The method takes advantage of the fact that the stresses in the material under sinusoidal strain can be decomposed into viscous and elastic portions, each containing harmonics of increasing order [24, 25]. The elastic stress response as a function of strain is the storage modulus G' [26]. In FT rheology, a parameter Q is introduced as the ratio of the intensity of the 3rd harmonic response of elastic stress to the 1st (denoted [G.sub.3'][G.sub.1'], [I.sub.3]/[I.sub.1] or [I.sub.3/1]) and divided by the strain amplitude squared ([[gamma].sub.0.sup.2]) (cf. Equation 1) [27].

Q ([omega], [[gamma].sub.0]) = [I.sub.3/1]/[[gamma].sub.0.sup.2]

Q is a measure of the non-linearity of a material as a function of strain amplitude and frequency ([omega]). Typically, Q is characterized with strain amplitudes from the MAOS and LAOS regions.

A slightly different measure, which represents the material's intrinsic non-linearity without a dependence on strain amplitude, is [Q.sub.0], known as the zero-strain non-linearity, as shown in Eq. 2.

[mathematical expression not reproducible] (2)

This parameter [Q.sub.0] is more sensitive to filler concentration, polymer-particle interaction in nanocomposites, and polymer interface interactions in blends, as compared to G' from SAOS measurements alone [23, 28, 29]. This makes it useful for quantifying the difference in how well a nanocomposite or blend is mixed. The benefit to using [Q.sub.0] vs. Q is that [Q.sub.0] is obtained within the SAOS region, thus avoiding effects like edge fracture, slip, and leaking during rheometer operation [23].

Here we combine polypropylene (PP), polycaprolactone (PCL), and nanoclay (NC) using SGAP extrusion and assess the PCL droplet size and dispersion of NC using several techniques, thus illustrating the usefulness of FT rheology for assessing dispersion and blend quality.

EXPERIMENTAL METHODS

Materials

Daploy WB130HMS (Borealis AG, Austria), a high melt strength polypropylene (PP) homopolymer in pellet form, was used as the matrix material in this study. Capa 6,500 (Perstorp AB, Sweden), a high molecular weight linear polycaprolactone (PCL) in pellet form, was used as the minor blend component. Cloisite 20B, an organo-modified NC, was donated by Southern Clay Products (now BYK Additives, Gonzales, Texas). Compressed N2 to be used as a sub-critical gas was obtained and used as-received from Airgas, Inc. (Madison, WI).

Equipment

Mixing was performed in a Leistritz ZSE-18 twin-screw extruder at 215[degrees]C and 150 rpm. In this study, blends contained 10 wt% PCL and nanocomposites contained 10 wt% PCL and 5 wt% Cloisite 20A (NC). Extrusion followed one of two routes: traditional twin-screw extrusion or a sub-critical gas-assisted process (SGAP) whereby sub-critical nitrogen (N2) gas was injected into the melt stream during extrusion. A reverse flow element prevented the gas from escaping from the hopper side. The polymer and gas formed a mixture that foamed and resolubilized several times during the material's residence in the extruder as the material passed through high- and low-pressure regions. This foaming added additional stresses to the melt via equibiaxial elongational flows on the surface and in-between growing bubbles, which helped to break up nanofiller agglomerates and reduce the minor or secondary phase size in blends. Implementation details have been discussed previously [15, 16, 30, 31]. The extrudate was pelletized, dried, and injection molded into ASTM Type I tensile bars on an Arburg Allrounder 270A at a final barrel temperature of 215[degrees]C. Samples were cut to 63.5 mm and notched for impact testing.

CHARACTERIZATION

Impact Strength

Izod impact tests were conducted following ASTM D256-10 on a Custom Scientific Instruments CSI-137 Pendulum impact tester. Samples were cut and notched from ASTM Type I tensile bars, five of each were tested. The impact strength test samples were cut from injection molded ASTM Type I tensile bars and were solid without a foam structure.

Rheology

Rheological measurements were made on a TA Instruments AR 2000ex rheometer. A 25 mm parallel plate fixture at 215[degrees]C was used to test the materials with a gap of 1,000 [micro]m. For small amplitude oscillatory shear (SAOS) tests, a strain of 1% and a frequency range of 0.1-100 rad/s were used. For medium amplitude oscillatory shear (MAOS) tests, a constant frequency of 1 rad/s and strains from 1% to 100% were employed. Stress and strain waveforms were analyzed using the MITLaos software package.

X-Ray Diffraction

X-ray diffraction (XRD) spectra were obtained on a Broker D8 Discovery using a [Cu.sub.k[alpha]] (1.54 [Angstrom]) emitter. Flat, injection molded samples were used.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) images were obtained on a LEO 1530 FESEM with a 3 kV accelerating voltage and a working distance of ~4 mm using gold-coated samples prepared on a Denton Vacuum Desk V sputter coater for 30 s at 45 mA. Image processing was performed with ImageJ software.

Droplet Size and Morphology Measurement

For each sample, four to five SEM images were used and the number of droplets measured in total ranged from approximately 85 to 165 droplets.

RESULTS AND DISCUSSION

Impact Strength

In Fig. 1, the Izod impact results are shown for the various samples. One of the main drawbacks for PP homopolymer is that it has a low impact resistance, especially at low temperatures near its glass transition temperature (~5[degrees]C). In practical use, many grades of PP are toughened using various ratios of ethylene-propylene co-polymer rubber that is softer and helps dissipate the energy of crack propagation [32]. In this study, we used PCL as the soft, rubbery phase to increase the impact strength of PP. In Fig. 1, the impact strength increased when 10 wt% of PCL was added. Likewise, when 5 wt% of NC was present, the impact strength increased. NC is known to be an effective reinforcing agent, but it also acts as a solid-state compatibilizer between PP and PCL. However, in the SGAP samples, this increase was larger, owing to a smaller secondary phase size and a better dispersion of the nanoclay.

Morphology

Droplet size distributions, as shown in Fig. 2, illustrate the effect of the SGAP process on reducing the size of PCL droplets in the blend. A sharper distribution means more uniformity while a peak shifted to the left (to a lower diameter) signifies a smaller average droplet size. In the PP + 10 wt% PCL samples, the standard sample had a droplet diameter average of 348 nm while the SGAP sample average was 258 nm. The samples with 5 wt% NC followed the same trend, with an average of 405 nm in the standard sample and 248 nm for SGAP. However, the standard sample had a much wider and shorter distribution, owing to the existence of large particles (in the micron size range) that were a combination of PCL droplets and agglomerated nanoclay.

Several SEM images were used to calculate the distributions in Fig. 2; representative SEM images are presented in Fig. 3. It should be noted that the samples with NC had irregular shapes due to the NC residing in and around the droplets, restricting their ability to minimize the surface area. This effect also contributed to the prevention of coalescence by acting as a barrier between two droplets [3], although this did not appear to be a factor in this study. It can be noted that the poorer dispersion of nanoclay in the non-SGAP samples resulted in larger droplet sizes than the samples without nanoclay.

XRD Spectra

Figure 4 contains XRD spectra of the samples containing 5 wt% NC as well as the neat NC powder. The peak located at 3.84[degrees] 2[theta] is associated with the interlayer spacing of NC, which has a distance of ~2.3 nm [33, 34]. After melt processing, this distance increased due to the polymer chains entering the gallery spacing of the clay, a process known as intercalation. As the distance between the platelets grows, 2[theta] is shifted to a lower value. For the standard extrusion and SGAP samples, this value became 2.51[degrees] and 2.61[degrees], respectively. These values translated to interlayer distances of 3.5 nm and 3.4 nm, respectively. Because the amount of intercalation did not change appreciably between standard and SGAP extrusion, it can be concluded that the gas had little effect on increasing the diffusion of PP/PCL into the clay layers of Cloisite 20B. This effect has been shown previously [16]. However, a striking difference between the two blends ' spectra can be seen in the reduction of the intensity of the peak. The peak intensity value diminished by 40% in the SGAP sample. This can be interpreted as there being less stacks or agglomerations of clay in the SGAP sample [5, 6, 16, 35]. Because the loading percent was the same, the nanoclay in the SGAP sample was better exfoliated and dispersed. Note that this was not the case for the pure Cloisite 20A spectrum, which has been scaled for clarity.

Rheological Properties

Figure 5 contains rheological data from SAOS and LAOS experiments. The solid line represents a typical SAOS frequency sweep experiment at fixed strain (1%) while the dashed line is from a LAOS strain sweep at a fixed frequency (1 rad/s). In frequency sweeps (solid lines), an increase in storage modulus (G') over the range of testing frequencies is due to the presence of a filler. A deviation from linear behavior (a change in slope) in G' at low frequency can be due to increased polymer-particle interaction (the so-called pseudo-solid network effect) in filled polymers and due to additional relaxation mechanisms in blends [5, 6, 14, 36-38]. Both effects are enhanced by increasing the contact surface area between the filler or minor component and the matrix polymer.

In Fig. 5, the increase in G' across the entire frequency range was present for the samples with 5 wt% NC, but the change in slope at low frequencies was small. This was likely due to the clay preferentially residing at the surface of the PCL droplets (see Fig. 3 C and D), which is a known effect in ternary blends of immiscible polymers with a nanofiller [3, 12]. In this case, it is difficult to assess the dispersion of clay via G'. There was little to no difference in G' between the neat PP and the blends containing 10 wt% PCL, thus indicating that the presence and droplet size of PCL was difficult to discern with this method.

In strain sweeps (dashed lines), the storage modulus (G') for the samples with NC also increased at low frequencies, but experienced an earlier onset and sharper decrease at lower strains than the neat PP and blends thereof. This change in the non-linear transition is referred to as the Payne or Fletcher-Gent effect, where the storage modulus of filled polymers decreases at lower strain amplitudes than their unfilled counterparts. This effect is filler-concentration dependent and is thought to have roots in particle-particle interaction and structure breakup [3, 29, 39, 40]. Strain amplitude sweeps can better tell the difference between the unfilled samples and those with NC, but not between the standard extrusion samples and SGAP samples. Likewise, the frequency sweeps cannot differentiate the standard and SGAP extruded samples. Thus, a more sensitive measure of non-linearity is required.

Fourier-Transform Rheology

As described earlier, the rheological property [Q.sub.0] is a measure of the intrinsic non-linearity of a material. Table 1 lists the value of [Q.sub.0] for various samples. From these values, the addition of PCL is detectable, as is the addition of NC to the blends. More importantly, there is a distinct difference in the samples processed with and without SGAP. For PP + 10 wt% PCL, [Q.sub.0] increased by 62% with SGAP. For the PP+10 wt% PCL + 5 wt% NC, [Q.sub.0] increased by 40% with SGAP. These changes in [Q.sub.0] follow the same trends as the impact strength, particle average diameter, and XRD intensity--all of which were affected by the dispersion of the nanofiller or the droplet size of the PCL phase. While there is not a 1:1 correspondence, having a measure to assess how these properties are expected to behave is convenient.

There are other measures of non-linearity that contain more information, such as the Chebyshev coefficients [e.sub.3] and [v.sub.3] that arise from the use of Chebyshev polynomials instead of Fourier transforms to extract the higher harmonic responses of G'. The Chebyshev coefficients are more useful because they contain a physical interpretation that can inform about the strain stiffening or softening and the shear thickening or thinning behavior. Also useful are the dimensionless indices S and T, which better capture the non-linear response over the range of deformation [25]. These measures are readily available from the MITLaos software used in this study.

From a practicality standpoint, [Q.sub.0] is attractive for several reasons. It is convenient compared to other measures of non-linearity because it can be performed at low strain amplitudes in the SAOS region where edge fracture and leaking are unlikely to occur in parallel plate rheometry. Measures like [e.sub.3], [v.sub.3], S, and T require LAOS, or at least MAOS, to fully describe the material of interest. [Q.sub.0] can be thought of as a "single" measure of a material's non-linearity, akin to using the melt flow index (MFI) in lieu of a viscosity curve. MFIs are commonly reported on material data sheets to give the process engineer an idea of how viscous a material is. [Q.sub.0] could also become a common parameter for the process engineer as a quality control measure in blending and formulating nanocomposites. Of course, more investigation is needed to fully understand the relationships therein.

CONCLUSIONS

Polypropylene homopolymer (PP) was blended with polycaprolactone (PCL) and nanoclay (NC) using a traditional extrusion process and a sub-critical gas-assisted process (SGAP) that was previously shown to increase the dispersion of nanofillers and reduce the minor phase droplet size in blends. In this case, the SGAP process showed improvements in impact strength. The effects of SGAP were evident in scanning electron microscopy (SEM) images as smaller droplets and particles, as well as in Xray diffraction (XRD) spectra as a lower intensity peak for better-dispersed NC. The common rheological measure of storage modulus (G') was not able to capture the difference between the traditionally extruded samples and those made with SGAP. However, the parameter [Q.sub.0] based on Fourier-Transform (FT) rheology, the intrinsic non-linearity, was useful in differentiating the degrees of mixing and dispersion in samples prepared by these two processes.

ACKNOWLEDGMENTS

The authors would like to acknowledge the support of the Kuo K. And Cindy F. Wang Professorship, the Vilas Distinguished Achievement Professorship, Wisconsin Distinguished Graduate Fellowship, 3M Fellowship, Nanocor, the Wisconsin Institute for Discovery, the Society of Plastics Engineers Extrusion Division, and the Society of Plastics Engineers Milwaukee Education Foundation for their support. The MITLaos package was provided courtesy of Prof. Randy Ewoldt and Prof. Gareth McKinley.

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Thomas Ellingham, (1,2) Galip Yilmaz, (1,2) Lih-Sheng Turng (iD) (1,2)

(1) Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, Wisconsin

(2) Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, Wisconsin

Correspondence to: L.-S. Turng; e-mail: turng@engr.wisc.edu DOI 10.1002/pen.25258

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Izod impact strength of various sample types. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 2. PCL droplet size distributions. Samples with 5% NC had irregular shapes. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. SEM images, (a) PP + 10 wt% PCL, (b) PP + 10 wt% PCL SGAP, (c) PP + 10 wt% PCL + 5 wt% NC, and (d) PP + 10 wt% PCL + 5 wt% NC SGAP. Scale bar is 2 [micro]m.

Caption: FIG. 4. XRD spectra of samples with nanoclay. The peaks near 2.5-4[degrees] correspond to the interlayer spacing of the nanoclay. The Cloisite 20A spectra were scaled down for clarity. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Frequency sweeps (solid lines) and strain sweeps (dotted lines). [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. The rheological property [Q.sub.0],
a dimensionless measure of the intrinsic non-linearity
of a material

Sample       PP    PP+10 wt%   PP+10 wt%    PP+10 wt%     PP+10 wt%
                      PCL      PCL SGAP    PCL + 5 wt%   PCL + 5 wt%
                                               NC          NC SGAP

[Q.sub.0]    2.9     18.34       29.73        45.34         63.59
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Author:Ellingham, Thomas; Yilmaz, Galip; Turng, Lih-Sheng
Publication:Polymer Engineering and Science
Date:Jan 1, 2020
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