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The Effect of Spray-Freeze Drying of Montmorillonite on the Morphology, Dispersion, and Crystallization in Polypropylene Nanocomposites.

INTRODUCTION

Polypropylene is a thermoplastic linear hydrocarbon polymer that has excellent physical, mechanical, and thermal properties for room temperature applications. Inorganic fillers such as talc, mica, and layered silicates are commonly used to improve stiffness, reduce mold shrinkage, and enhance thermal stability, while elastomeric compatibilizers are added to compensate for the reduction in toughness resulting from filler incorporation [1]. Layered silicates and compatibilizers are important factors in determining blend morphology. Well-dispersed clay layers form a barrier and reduce agglomeration of the elastomeric domains, in addition to enhancing the mechanical properties, particularly toughness [2]. Moreover, the presence of an interfacial agent or a compatibilizer can stabilize blend morphology by reducing the interfacial tension, thus forming a protective layer around the clay as in the case of organoclays [3]. In polypropylene nanocomposites, the quality of dispersion of the filler and elastomer phases, as well as processing conditions, play an important role in determination of toughness and stiffness of the product [4, 5]. Polypropylene has no polar groups in its backbone, therefore direct intercalation or exfoliation of silicate galleries is quite difficult [6]. Thus, it is common to use organoclays, such as alkylammonium ion or alkylamine modified clays [7]. This renders the organophilic-hydrophobic clay more compatible with the polymer [8].

Maleic anhydride grafted polypropylene (PP-g-MA) is a commonly used compatibilizers in preparation of polypropylene nanocomposites. The maleic anhydride functionality of the PP-gMA forms hydrogen bonding with the clay while providing compatibility with the matrix through its polypropylene backbone. There are reports regarding the effect of PP-g-MA to organoclay ratio on the morphology and the performance of the PP-based nanocomposites [7, 9]. Other studies report on the effects of degree of functionality and maleic anhydride content of PP-g-MA on the nanocomposite structure [10]. PP-g-MA can intercalate between the silicate layers and expand the galleries, thus enhancing penetration by polypropylene macromolecules while providing compatibility between the phases [11-13]. In this study, maleic anhydride-grafted polypropylene was used as a compatibilizer.

Spray drying is a commonly used method to produce nanoparticle agglomerates from aqueous clay suspensions by atomization using a pressure multinozzle array feeding into a drying chamber [14]. This drying technique yields solid, hard, nonporous agglomerates where the nanoparticles adhere to each other due to strong, inter-particle hydrogen bonds. Such agglomerates are difficult to break down and disperse in polymer matrices by melt processing and thus present a serious challenge [15]. Another technique replaces spray drying by freeze drying. Here, shrinkage that occurs during spray drying is avoided due to the absence of capillary forces. Ice crystal growth is the dominant factor in the aggregation process [16]. The properties of freeze-dried products are strongly influenced by the freezing rate before the drying process. A slow freezing rate favors the formation of elongated ice crystals [17]. The spray-freeze drying (SFD) technique combines the advantages of atomization (spraying) and lyophilization (freeze drying) processes to prepare ultraporous nanoparticle agglomerates. The porosity and consequently, the density of granules are controlled by the solid loading of the suspensions, whereas the size distribution of the granules is a function of the viscosity and the solid content of the suspension, the flow rate employed for spraying and the pressure of the applied gas [14, 18]. In comparison to conventionally spray dried granulates spray freeze granulates are much weaker, because capillary forces can be excluded in this technique. The SFD process is described in the literature for several materials, for instance, alumina powders and other ceramic materials, calcium-phosphate powders, protein inhalation powders for pharmaceutical applications or proteins [18]. In very fast cooling, such as spraying under liquid nitrogen, an amorphous ice could is formed and agglomerate morphology is frozen and very highly porous structure is usually obtained [19]. The main challenge is to maintain the nanostructure of the materials during drying.

Recently, Khoshkava and Kamal [17, 20] used SFD in conjunction with cellulose nanocrystal (CNC) suspensions. The resulting SFD-CNC particles exhibited improved porous morphology compared to spray dried CNC particles. They also reported significant improvements in nanocomposite properties upon mixing with different polymers. In the case of polymer-clay nanocomposite, the dispersion quality depends highly on the clay morphology and the ability of the polymer to access readily the clay galleries or surfaces. Taking this into consideration, it was considered that spray-freeze dried clay agglomerates might yield enhanced dispersion of clay nanoparticles in polymer melts, compared to clay agglomerates produced by conventionally used spray drying or freeze drying methods.

The main objective of the present investigation is to evaluate the effects of SFD on the morphology of montmorillonite clays and on the subsequent dispersion quality when melt compounded to form polypropylene nanocomposites with and without maleated polypropylene as compatibilizer. Firstly, spray-freeze dried montmorillonite was prepared from originally spray dried montmorillonite. The effects of SFD-MMT on the dispersion quality were evaluated in PP-MMT nanocomposites obtained by melt compounding in a batch mixer with and without incorporation of PP-g-MA as compatibilizer. The morphology and thermal and mechanical properties of the various nanocomposites were characterized and compared with commonly produced PP/PP-g-MA/MMT nanocomposites. Morphology and clay dispersion were evaluated using X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Polarized light microscopy (PLM) was carried out in the temperature range 120-126[degrees] C to evaluate the effects of both MMT and SFD-MMT on the spherulite sizes and MMT clusters in the resulting PP-MMT nanocomposites. The thermal stability of the nanocomposites was evaluated using nonisothermal thermo-gravimetric analysis (TGA). The calorimetric thermal properties and isothermal crystallization behavior of PP samples were evaluated using differential scanning calorimetry (DSC). Mechanical properties of the various nanocomposites were compared.

EXPERIMENTAL

Materials

Commercially available Cloisite[R] [Na.sup.+] montmorillonite, initially spray dried (MMT) was purchased from Southern Clay Products (Gonzales, TX). Cloisite[R] [Na.sup.+] MMT had 92.6 meq/100 g clay and basal spacing of 1.16 nm. The off-white MMT had a density of 2.86 g/c.c. and d50 particle size <25 [micro]m and moisture content between 4 and 9%. This MMT clay was used to prepare spray-freeze dried MMT (SFD-MMT). Polypropylene (PP; Profax SR549M, LyondellBasell, Houston, TX) with 0.90 g/[cm.sup.3] density at room temperature and melt flow index (MFI) of 11 g/10 min (2.16 kg load, at 220[degrees]C) was used as the polymer matrix. Polybond 3,150 (PP-g-MA) PP modified with 0.5 wt % maleic anhydride with 0.91 g/[cm.sup.3] at room temperature (ASTM D-792) and 50 g/10 min MFI (ASTM D-1238) was purchased from Dupont (Wilmington, DE) and used as a compatibilizer. Maleic anhydride in the PP-g-MA structure can react with the hydroxyl and amine groups, and the polar character of the anhydride causes affinity for the silicate surface.

Preparation of Spray-Freeze Dried Montmorillonite

Two grams of the initially spray dried MMT were dispersed in 100 mL of reverse osmosis water using a shear mixer (IKA, ultraturrax T25; IKA, Staufen, Germany) and sonicated (Qsonica Q700; Newtown) for 5 min at room temperature. The suspension was sprayed using a spray gun (Campbell Hausfeld DH5300; pattern size 8 in.) into a liquid nitrogen medium, using a peristaltic pump. After the spray freezing step, the slurry containing the frozen droplets was transferred to the 2.5 L Labconco freeze dryer (Labconco Corp, Kansas City, MO), where the frozen droplets were lyophilized at -52[degrees]C for 2 days, yielding spray-freeze dried MMT (SFD-MMT). The MMT used in these experiments were dried at 80[degrees]C for 16 h in a vacuum oven.

Preparation of Polypropylene Nanocomposites

PP nanocomposites containing 2.0 wt % of MMT and SFD-MMT particles and 6.0 wt % PP-g-MA were prepared using a Rheocord System 40 batch mixer Haake Buchler Instruments Inc. (Saddle Brook, NJ) with counter-rotating roller blades (specimen chamber capacity ~60 [cm.sup.3]) driven by a Haake system torque 90. The system is equipped with nitrogen purge and consists of three independent temperature-controlled zones (Zones I-II-III), which were kept at the optimum processing temperature, as recommended by the resin producers, throughout the experiments. The maximum screw speed achievable was 100 rpm. After compounding, specimens were ready for grinding and compression molding. The specimen chamber has a capacity of about 40 g. Specimen pellets and powder are introduced to the mixer without any loss of sample. Under normal operating conditions, intensive mixing and shearing action can be applied to the specimens. Compounding was carried out at 190[degrees]C and 60 rpm rotor speed for 10 min under a nitrogen atmosphere. PP and PP-g-MA were dried at 80[degrees]C for 12-16 h. Pure clay (MMT) was dried at 80[degrees]C for 16 h in a vacuum oven and the spray-freeze dried clay (SFDMMT) was used after the freeze drying step.

Processed PP and the corresponding nanocomposite samples (Dumbbell and impact samples) were molded using a 12-ton Carver hot press (Model No. 4386, Wabash, IN). Samples were first sandwiched between two Mylar[R] sheets and then placed between two brass platens at 180[degrees]C. After, 5 min pre-heating, samples were pressed between 6" * 6" platens at 1 metric ton (tonne) for 1 min, 1 min at 2 metric tons, 1 min at 3 metric tons, 1 min at zero metric tons and finally for 1 min at 5 metric tons and then cooled to room temperature using a water cooling system. The final blended samples were compression molded to form several 1.4-1.6 mm bars and 0.08-0.13 mm thin films. The samples were then kept in a desiccator prior to testing.

Morphology and Clay Dispersion Measurements

X-ray diffraction. XRD analyses were performed at room temperature using a Philips X'Pert X-ray diffractometer that generates a voltage of 50 kV and current 40 mA. The X-ray source was a tungsten filament tube with a Cu-target ([K.sub.[alpha]] = 1.5418 [Angstrom]). The diffraction angle, 2[theta], of the PP compositions was scanned from 1[degrees] to 10[degrees]. In order to calculate the basal spacing distance between the silicate layers, Bragg's law was used.

Transmission electron microscopy. TEM photomicrographs of the clays were obtained by a FEI Tecnai G2 200 kV transmission electron microscope. To image the individual clay crystallites, less than 1 mgof clay powder was deposited on the TEM grid. Ultrathin sections of about 100 nm were cut from the PP compositions with a Reichert FCS microtome equipped with a diamond knife.

Polarized light microscopy. PLM studies were conducted using a polarized light microscope (Olympus, BX50) in conjunction with a hot-stage (Linkam Scientific Instruments, THMS600, Surrey, UK) and equipped with CI93 temperature controller in order to compare particle sizes and clusters. A 20-pm specimen was microtomed from each of the compression-molded bars and the specimen was heated to 200[degrees]C and pressed for a minute to remove the wrinkles caused by cutting stresses and lower the thickness. Subsequently, it was rapidly cooled to room temperature. The specimens were re-heated to 200[degrees]C and held for 2 min before cooling to the isothermal crystallization temperature at 30[degrees]C/min. Photomicrographs were taken at 200[degrees]C using the polarized plates at parallel positions to capture the structure of the MMT and SFD-MMT in PP compositions and using crossed polarized light to capture the crystallization process. Each specimen was photographed after cooling back to room temperature to obtain the image of the spherulites.

Thermal Properties and Crystalline Growth Measurements

Thermo-gravimetric analysis. Thermal stability of the PP nanocomposites was evaluated using non-isothermal TGA. The experiments were carried out in a TGA 7 Perkin-Elmer apparatus (Norwalk, CT), controlled by Pyris 1 software (version 4.0). The microbalance was calibrated with a reference weight of 100 mg, and the furnace was calibrated using the Curie point of Alumel, Nickel and Perkalloy. Polymer samples (6-8 mg) were placed in an open platinum crucible and heated from 50 to 500[degrees]C at 10[degrees]C/ min under a nitrogen atmosphere (40 mL/min). Derivative mass/ temperature curves and the maximum decomposition temperature were obtained by using Pyris software.

Differential scanning calorimetry. The calorimetric thermal properties and isothermal crystallization behavior PP samples were obtained under nitrogen using differential scanning calorimetry (DSC) (Pyris 1, PerkinElmer) equipped with Intra-cooler 1P.

Calorimetric Thermal Measurements

The specimens were punched from thin film samples and then enclosed in aluminum DSC pans. Calorimetric thermal properties were measured by heating the pans from 20[degrees]C to 200[degrees]C at a scanning rate of 20[degrees]C/min heat-cool-heat procedure. They were kept for 2 min at 200[degrees]C to remove the thermal history.

Isothermal Crystallization

For the isothermal crystallization study, the specimens were heated from 20[degrees]C to 200[degrees]C at 20[degrees]C/min, then cooled to the isothermal temperature at 30[degrees]C/min and held for 10 min and subsequently cooled to 20[degrees]C.

Tensile Tests

Mechanical testing of the nanocomposites was performed on Dumbbell-shaped (gauge length: 25.4 mm, width: 1.9 mm, thickness: 3.2 mm) compression-molded samples. Tensile tests were conducted using an MTS Universal Tensile Testing machine (Eden Prairie, MN) according to ASTM D638. Tensile strength (MPa), Young's modulus (GPa), and percent elongation at break (%) values were determined from the stress-strain curves. The crosshead speed was 500 mm/min, and the averages of five test results were reported.

RESULTS AND DISCUSSION

Morphology and Clay Dispersion

The morphologies of spray dried MMT (MMT), freeze-dried MMT (FD-MMT) and spray-freeze dried MMT (SFD-MMT) were investigated in an earlier study [21] using SEM and XRD. Freeze drying and spray-freeze drying of MMT resulted in platy surface texture of the clays compared to the dense agglomerates of spray-dried MMT. SFD-MMT has a porous structure compared to spray-dried and freeze-dried clay particles and thus requires less hydrodynamic forces for good dispersion in polymer nanocomposites.

The XRD patterns of all PP compositions incorporating spray-dried MMT and spray-freeze dried MMT (SFD-MMT) are shown in Fig. 1.

The X-ray patterns of SFD-MMT clay revealed a shift in the position of (001) planes (29 changed from 7.63[degrees] to 6.98[degrees]), indicating a relatively slight but significant increase in the basal spacing of these planes from 1.16 nm to 1.27 nm (~9.5% increase). The spacings of FD-MMT remained the same as MMT. In spray drying, the presence of heat causes the elimination of some of the monomolecular water layer between the montmorillonite layers [22]. This monolayer elemination is not present in freeze drying or SFD. SFD presents another advantage over freeze drying, which is the uniform distribution of droplets over liquid nitrogen and thus more uniform structure of the agglomerates.

Two particular characteristics of layered silicates play an important role in the preparation of nanocomposites: the first is the ability of silicate sheets to disperse into individual layers, and the second one is the possibility to modify their surface chemistry through ion exchange reactions with organic and inorganic cations. The simple mixing of polymer and layered silicates does not always result in the generation of a nanocomposite, as this usually leads to dispersion of stacked sheets. This is due to the weak interactions between the polymer and the inorganic component. If these interactions become stronger, then the inorganic phase can be dispersed in the organic matrix at the nanometer scale. The basal spacing of the clay in the PP/PP-g-MA/MMT and PP/PP-g-MA/SFD-MMT ternary nanocomposites exhibited peaks corresponding to basal spacings of 1.01 nm and 2.70 nm, respectively (Fig. lb). This suggests that some of the weak spray-freeze dried clay is partially delaminated and dispersed in the polymer melt.

Transmission electron microscopy analysis supports and validates the XRD analysis results. TEM gives a clear distribution of the various phases through direct visualization at nanometeric scale. The TEM images of PP/MMT and PP/SFD-MMT are given in Fig. 2a,b), respectively. TEM micrographs of the PP/SFDMMT samples confirmed the presence of more open and flake-type structure, whereas large aggregates and agglomerates of MMT samples were present in PP/MMT samples. SFD of MMT clays shows clear dispersion improvement in the structure.

Padden and Keith [23] reported on the spherulitic crystallization in polypropylene and classified four distinct types of PP spherulites based on their birefringence characteristic. Varga [24] considered the supramolecular structure of isotactic PP. Most recently, Nakamura and co-workers [25] reported on differences among the [[alpha].sub.1], [[alpha].sub.2], and [beta] forms of isotactic PP. In this study, we used PLM experiments to observe the particle and agglomerate sizes and the overall particle size distributions in nanocomposites incorporating both the spray dried MMT and the spray-freeze dried MMT. Photographs were taken at 200[degrees]C using the polarized plates at parallel positions to view the MMT (Fig. 3a-d) and using crossed polarized light (Fig. 3e-f).The specimens were photographed after cooling back to room temperature to obtain the image of the sphemlites. Under cross-polarizers at 200[degrees]C (Fig. 3e,f), the polypropylene spherulites cannot be observed because they are melted. However, many bright clusters of MMT are observed and showing the interference patterns at 200[degrees]C (Fig. 3e-f) were due to the MMT clusters present in the PP matrix.

Figure 3a-d was analyzed for the particle-size distributions (PSD) using ImageJ software, as well as the cumulative count of these particles. The PSD for PP with MMT are shown in Fig. 4a. Moreover, the PLM images (Fig. 3a-d) show some clusters of various particles or agglomerates, which can be attributed to the imperfect mixing and non-uniform dispersion. The PSD for the PP/MMT and PP/SFD-MMT are quite similar, while that of the SFD-MMT in PP/PP-g-MA are almost double than that of PP/MMT. In an area of 70,000 [micro][m.sup.2], the total particle count for PP/MMT is 286 and 384 for PP/SFD-MMT (34% more). While that for PP/PP-g-MA/MMT and PP/PP-g-MA/ SFD-MMT are 436 and 913, respectively. The PP/PP-g-MA/ MMT showed a larger percentage (37%) of fine particles between 1.0 and 1.5 [micro]m, while only 24-29% for the PP/MMT. For PP/PP-g-MA/MMT, 90% of the particles were under 4.0 pm, while only 76-77% for the PP/MMT. The % cumulation as shown in Fig. 4b for the SFD-MMT is similar to MMT for both the PP and PP/PP-g-MA even though the PP/PP-g-MA/SFD-MMT produced twice as much total particles than that of PP/PP-g-MA/MMT.

Thermal Properties and Crystalline Growth Analysis

The heating curves of PP and PP/PP-g-MA with MMT and SFD-MMT and their derivatives are shown in Fig. 5. The thermal decomposition data of PP compositions taken under nitrogen atmosphere are given in Table 1.

Figure 5a,b shows the TGA mass loss of MMT and SFDMMT containing nanocomposites compared to PP and the PP/PP-g-MA blend. They show that the mixing of PP-g-MA elastomer with PP caused a slight delay in the decomposition of the PP. The TGAs of PP, PP/PP-g-MA/MMT, and PP/PP-g-MA/SFD-MMT show slight improvement in the thermal stability of the PP nanocomposite with SFD-MMT, whereas earlier onset decomposition of PP nanocomposite occurs with spray dried MMT. The quantity in parenthesis is the onset temperature at which 1% weight loss is detected. One would expect that the onset temperature should be shifted toward higher

value with the addition of MMT. The tortuous path in which MMT protects the PP, the higher MMT concentration shifted the heating curve to higher temperature. The derivatives of PP and PP/-g-MA/MMT are shown in Fig. 5c,d, respectively.

The barrier effect of the silicate layers is dominant, due to the formation of carbonaceous-silicate char on the surface of nanocomposites. Thus, the onset decomposition temperatures of PP/MMT and PP/SFD-MMT nanocomposites are 5[degrees]C and 4[degrees]C, respectively, higher than the onset temperature for PP. The addition of the elastomer to produce ternary nanocomposites raised the onset decomposition temperature slightly. MMT containing ternary nanocomposite exhibited a slightly enhanced onset decomposition temperature (265[degrees]C), whereas the onset decomposition temperature of SFD-MMT ternary nanocomposite was 293. PC corresponding to 30[degrees]C increase in onset thermal stability.

The melting peaks, crystallization onset temperatures, and crystallization peak temperatures were obtained from DSC analysis. They are summarized in Table 2. The value used for the heat of melting of 100% crystalline PP was 207 J/g [26].

In the case of PP, the MMT serves as a heterogenous nucleation site and the onset of crystallization was shifted from 120.16[degrees]C to higher temperatures. It appears that the SFD-MMT leads to higher onset of crystallization than that of the MMT. However, for the PP/PP-g-MA, addition of MMT shifted the [T.sub.C-Onset] to a lower temperature. These results are in agreement with the results obtained by Lai and co-workers [27] for Cloisite 20A in PP.

The crystallization peak temperatures for the melt-processed PP and PP/PP-g-MA and their MMT composites are quite similar (ranging from 116 to 118[degrees]C), in contrast with those study of Chan et al. [28] for PP with CaCo3, as well as the study of Khoshkava [29] on melt-mixing PP with 1% SFD-CNC, which shifted 10[degrees]C lower for their nanocomposites. The reason is that the earlier comparisons were based on original un-processed PP, which required a higher degree of supercooling for crystallization to take place.

The percent crystallinity was almost identical for the PP and PP/PP-g-MA and their MMT nanocomposites (38.9-39.7%) during fast cooling and the percent crystallinity values were found to be in the range of 40.5^4-1.4% upon second heating.

Isothermal DSC crystallization experiments were carried out for PP and PP nanocomposites at temperatures ranging from 120[degrees]C to 126[degrees]C (Regime III). The isothermal baseline was subtracted from the exothermic curve and analyzed for the fraction of crystallinity. Typical crystallization isotherms are shown in Fig. 6 for the PP and nanocomposite systems of interest.

The Avrami model [30] shown in Eq. 1 was used to estimate the experimental half-time.

[mathematical expression not reproducible] (1)

where k is the rate of crystallization and time and t is in seconds. The experimental half-time for crystallization at each temperature was estimated from the area under the exothermic curve for PP and their MMT nanocomposites. The Avrami Index [n] reflects the type of dimensionality and nature of crystallization. It can be obtained by plotting the value of log[-ln(1-[X.sub.t])] versus log t. The value of [X.sub.t] is traditionally calculated based on the value of [X.sub.(t)]/ [X.sub.(t = [infinity])]. However, [X.sub.(t = [infinity])] is much smaller at high crystallization temperature because the crystallization is not always completed. The Avrami Index [n] was obtained at each temperature for PP as well as its MMT nanocomposites. PP exhibited the lowest crystallinity as expected and the process was nucleation controlled initially with a 2D growth process at higher crystallization temperatures. The crystallization half-time as obtained experimentally and the Avrami index are presented in Table 3 for PP and their nanocomposites. It shows that the MMT did not affect the half-time of PP, while the SFD-MMT increased the half-time of PP and markedly prolonged the half-time for PP/PP-g-MA.

The Avrami Indices for PP are about 2.0 for all temperatures. They suggest that PP crystallization is nucleation controlled and insensitive to the temperature gradient during cooling. Hambir and co-workers [31] found that the addition of 5 wt % Cloisite 6A clay shifted the crystallization half time of PP with 15 wt % Polybond at 125[degrees]C from 100 s to 60 s. They concluded that the presence of clay accelerates the crystallization process of PP. Our results showed a hindrance effect especially with the PP/PP-g-MA/SFD-MMT.

The crystallization half times obtained experimentally can be used to analyze the crystalline growth kinetics for each material by using the Lauritzen-Hoffman Theory for Regime III [32]:

[mathematical expression not reproducible] (3)

where U is the diffusion activation energy = 6,284 J/mol [33]. R is the universal gas constant = 8.314 J/mol/K, [K.sub.g] is the activation energy for crystal growth in regime III: 4[b.sub.o][sigma][sigma][T.sub.m]*/k[DELTA][H.sub.u]; [b.sub.o]: layer thickness = 0.549 nm [for polypropylene], [sigma] is the lateral surface energy of the crystal = 0.0115 (J/[m.sup.2]), [[sigma].sub.e]: edge surface energy; [DELTA]T: [T.sub.m]* - [T.sub.c]; [T.sub.m]*: Equilibrium melting point of 100% PP = 185[degrees]C [34]; k: Boltzmann's constant = 1.38[(10).sup.-23] (J/K), [DELTA][H.sub.u] is the heat of fusion for 100% PP crystal = 1.96 * [10.sup.8] (J/[m.sup.3]), f is the correction factor for [DELTA][H.sub.u]: 2 [T.sub.c]/([T.sub.m]* - [T.sub.c]) [32].

In order to evaluate Eq. 3 for the materials being investigated, it is necessary to obtain the so-called "effective degree of supercooling ([DELTA]T)", which required a knowledge of the equilibrium melting point of each material. There were several values used for the equilibrium melting point ([T.sub.m]*) of polypropylene which were summarized in Table 4.

For the current work, we used the lowest (185[degrees]C) and the highest value (212.1[degrees]C) of [T.sub.m]* for our analysis using the experimental half time obtained for each material.

The plot of Ln ([t.sub.1/2]) + U/RT versus 1/(fTc[DELTA]T)] for PP, using both [T.sub.m]* equals 458.16 K [33] and [T.sub.m]* equals 485.26 K [36] are shown in Fig. 7.

Higher edge surface energy of polymer crystals indicates a more strained or imperfect surface. The magnitudes of edge surface energy, [[sigma].sub.c], and the energy of work for chain folding, q, of PP and their nanocomposites are summarized in Table 5. The [[sigma].sub.c] value of 74.3 mJ/[m.sup.2] obtained for the processed PP agreed well with published data of 65-70 mJ/[m.sup.2] [33]. The energy of work for chain folding can be estimated using the relationship: q = 2[a.sub.o][b.sub.o][[sigma].sub.e] where [a.sub.o] = 5.49[(10).sup.-10] (m) and [b.sub.o] = 6.26[(10).sup.-10] (m) [33] for PP. The edge surface energy was re-calculated using [T.sub.m]* values of 485.26 K reported by Xu and co-workers [36]. The MMT did not affect the crystallization of PP and PP/PP-g-MA. However, SFD-MMT affected both PP and PP/PP-g-MA by showing higher edge surface energy of 122 and 161 mJ/[m.sup.2], respectively. The energy of folding for PP/SFD-MMT nanocomposites with and without PP-g-MA was found to be much higher than PP/MMT nanocomposites due to the hindrance effect of SFD-MMT on the edge surface. The hindrance effects of nanocomposites were reported by numerous researchers such as Ray and co-workers 2007 [38] and Khoshkava and coworkers 2015 [29]. Interestingly, PP/SFD-MMT and PP/PP-g-MA/SFD-MMT exhibited the lowest crystallinity due to rapid crystallization that took place on the surfaces of the MMT nanoparticles/clusters (heterogenous nucleation). These resulted in lowering the tensile strength of PP and PP/PP-g-MA. However, there appears to be some increase of Young's Modulus, which may occur due to chain entanglement of some loose ends in the amorphous region.

Tensile Test

Table 6 summarizes the tensile strength and Young's modulus for the PP and PP/PP-g-MA series.

PP has tensile strength, Young's modulus (YM), % elongation at break values of 40.1 MPa, 1.17 GPa, and 301%, respectively. Addition of PP-g-MA in the PP/PP-g-MA blend decreased its Young's modulus due to its elastomeric, dilution character, since PP-g-MA has lower modulus compared to PP. While PP/PP-g-MA decreases YM, at the same time, it enhances the elongation at break of PP.

The additional of both types of MMT to PP and PP/PP-g-MA increased the Young's Modulus while lowering the tensile strength (10%) and elongation at break. These trends are similar to earlier studies of Lai and co-workers [27] on PP with 6% Cloisite 20A. However, the effect is more pronounce for the PP/SFDMMT, which increase Young's modulus as much as 25% than those of PP/MMT. For the PP/PP-g-MA/SFD-MMT, the Young's modulus improved of 30% than the PP/PP-g-MA/MMT (as much as 60% improvement over PP/PP-g-MA). This may be due to the large number of finer MMT particles in PP/PP-g-MA/SFD-MMT and led to a brittle failure during tensile test.

Elastomeric materials enhance the bonding between the polymer and the organoclay. Also, the elastomeric phase has a lower Young's modulus compared to the matrix and it acts as stress concentrator during elongation. Thus, yielding or crazing occurs around the elastomeric domains and the polymer would absorb a higher amount of energy. Under normal condition, when incorporating an organoclay with higher layer spacing in PP, it is expected that the organoclay would stiffen the matrix in the binary nanocomposite and thus result in an increase in the tensile strength and Young's modulus indicating an increase in the rigidity of the material. However, this is not the case, here since the clays did not show clear enhancement in the tensile strength of PP for both MMT and SFD-MMT. On the other hand, a clear increase in the Young's modulus was observed for PP binary and ternary nanocomposites incorporating spray-freeze dried clays. This may be attributed to the fact that spray-freeze dried clays, which are more porous, have a better tendency for intercalation compared to neat untreated clay. The higher aspect ratio of the clay is expected to contribute to the reinforcement effect because it creates a larger contact area with the polymer matrix. The interfacial adhesion between PP and the clay is also important in dispersing the clay homogeneously in the polymer matrix and increasing the strength of the material. This effect was observed with SFD-MMTs, which exhibited higher Young's modulus than untreated ones in the binary and ternary nanocomposites. The compatibility of the clay with PP matrix and the elastomer, in addition to the initial basal spacing of the clay are important factors influencing the mechanical properties of the final product. These results are in good agreement with TEM analyses, which showed that SFD-MMT agglomerates smaller, whereas MMT agglomerates were larger packed agglomerate structure.

CONCLUSIONS

Spray-freeze drying and sonication have an important influence on the morphology and nanostructure of montmorillonite agglomerates. SFD of clay powders produces an increase in basal spacing and porosity of spray dried MMT agglomerates. The porous structure of spray-freeze dried granulates renders them weaker, thus smaller and more easily dispersible during processing. This leads to larger interfacial contact area between the polymer and the MMT.

Spray-freeze dried MMT agglomerates have a better potential to produce enhanced dispersion of MMT nanoparticles in polypropylene melts compared to agglomerates produced by conventional spray drying or freeze drying methods. The main factors are the following characteristics of SFD-MMT compared to spray dried MMT: smaller agglomerates, larger d-spacing, and higher porosity. These factors allow more polymer-MMT interfacial area and more intimate polymer-MMT contact.

The spherulitic structure of processed PP and PP/PP-g-MA with MMT is quite similar when prepared under fast cooling condition (30-50[degrees]C/min) and showing spherulites of 6-8 [micro]m. The particle-size analysis showing similar PSD for both PP/MMT and PP/SFD-MMT. On the other hand, PP/PP-g-MA/SFD-MMT was found to have twice as many fine particles of MMT when compared to PP/PP-g-MA/MMT, which caused a hindrance effect of the crystallization of PP/PP-g-MA.

The presence of MMT in PP and PP/PP-g-MA retarded the degradation of the PP and PP/PP-g-MA. There were no significant differences between PP/MMT and PP/SFD-MMT. However, the PP/PP-g-MA/SFD-MMT system exhibited a significant increase (30[degrees]C) in decomposition temperature compared to PP/PP-g-MA/ MMT, which may be attributed to the large number of smaller MMT particles.

The addition of MMT to PP shifted the crystallization temperature higher is due to the heterogenous effect of MMT acted as nucleation sites for PP. The PP/SFD-MMT showing similar effect as the PP/MMT as the number of fine MMT particles are almost the same. Both the crystallization peak of PP/PP-g-MA/MMT and PP/PP-g-MA/SFD-MMT to lower temperatures was caused by the hindrance effect (higher edge surface energy) of the MMT on PP/PP-g-MA crystallization. However, under controlled cooling rate of 20[degrees]C/min, the percent crystallinity of the PP, PP/PP-g-MA was not affected by the presence of MMT and/or SFD-MMT.

The crystallization half time for the PP/MMT is quite similar to the processed PP in the temperature range studied (Regime III, 120-126[degrees]C), since the Avrami Index of ~2.0 signified that the crystallization was mostly controlled by nucleation. Moreover, the PP material used in this work was a commercial bottle grade random copolymer containing nucleation agent to control clarity of the product. The PP/SFD-MMT showed slightly lower growth rate even though they provide the nucleation sites for PP crystallization. PP/PP-g-MA/SFD-MMT showed a much bigger hindrance effect on the crystallization of PP/PP-g-MA as the calculated edge surface energy was twice that of PP/PP-g-MA and PP/PP-g-MA/MMT.

SFD-MMT exhibited higher Young's modulus and lower tensile strength than conventional MMT in the binary and ternary nanocomposites. As expected, the compatibilized SFD-MMT filled compatibilized PP/PP-g-MA/SFD-MMT composites yielded the greatest improvement in modulus (31%) over the PP/MMT nanocomposites. The results regarding elongation at break were not reliable. Thus, they are not reported.

ACKNOWLEDGMENT

The authors would like to acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), McGill University, Center for Applied Research on Polymers and Composites (CREPEC), and the Network for Innovative Plastic Materials and Manufacturing Processes (NIPMMP).

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Wissam Abdallah, Victor Tan, Musa R. Kamal

Department of Chemical Engineering, McGill University, 3610 University Street, Montreal, Quebec, H3A 2B2, Canada

Correspondence to: M. R. Kamal; e-mail: musa.kamal@mcgill.ca Contract grant sponsor: Faculty of Engineering, McGill University, contract grant sponsor: Network for Innovative Plastic Materials and Manufacturing Processes, contract grant sponsor: McGill University, contract grant sponsor: Natural Sciences and Engineering Research Council of Canada. DOI 10.1002/pen.25270

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

Caption: FIG. 1. X-ray diffractograms of PP, PP/MMT, PP/SFD-MMT, PP/PP-g-MA/ MMT, PP/PP-g-MA/sfd-MMT. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. TEM images of (a) PP/MMT nanocomposite and (b) PP/SFD-MMT nanocomposite at different magnifications.

Caption: FIG. 3. Photomicrographs under parallel polarizers at 200[degrees]C of (a) PP/MMT (b) PP/SFD-MMT (c) PP/PP-g -MA/MMT (d) PP/PP-g-MA/SFD-MMT (e) PP/SFD-MMT at 200[degrees]C under crossed-polarizers showing interference pattern of MMT and (f) PP/PP-g-MA/SFD-MMT at 200[degrees]C crossed-polarizers showing interference pattern of MMT. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. (a) PSD of MMT in PP and PP-g-MA analyzed under parallel polarizers and (b) comparison of the cumulative count of PP and PP/PP-g-MA with MMT and SFD-MMT. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Thermograms of (a) PP composites (b) PP/PP-g-MA composites and derivative TGA of (c) PP composites and (d) PP/PP-g-MA composites. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. (a) Crystallization isotherms of PP at 120-126[degrees]C (regime III) and its corresponding nanocomposites at different temperatures (b) crystallization isotherms of PP/PP-g-MA at 120[degrees]C to 126[degrees]C (regime III) and its corresponding nanocomposites at different temperatures. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Plot of In ([t.sub.1/2]) + U/RT versus 1/(fTc[DELTA]T)] for PP using both Tm* (regime III). [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Nonisothermal data of clays and all PP samples
obtained from TG/DTG thermograms.

                       Mass loss at    Tonset decomposition
                       190[degrees]C    temperature at 2%
                           (wt%)            mass loss
                                           ([degrees]C)

PP                         0.31               262.9
PP/MMT                     0.16               268.2
PP/SFD-MMT                 0.27               266.7
PP/PP-g-MA                 0.23               264.4
PP/PP-g-MA/MMT             0.30               265.0
PP/PP-g-MA/SFD-MMT         0.32               293.1

                       Decomposition      Temperature at
                       temperature at        maximum
                        5% mass loss    decomposition rate
                        ([degrees]C)       ([degrees]C)

PP                         276.7               352
PP/MMT                     285.1               369
PP/SFD-MMT                 284.7               373
PP/PP-g-MA                 281.0               357
PP/PP-g-MA/MMT             281.9               364
PP/PP-g-MA/SFD-MMT         311.5               391

TABLE 2. Thermal properties of PP with MMT and SFD-MMT

                           First heating            Cooling

                      [T.sub.m]     [X.sub.C]   [T.sub.C-Onset]
                     ([degrees]C)      (%)       ([degrees]C)

PP                      145.65        40.18         120.16
PP/MMT                  145.94        41.78         121.57
PP/SFD-MMT              146.28        41.71         122.75
PP/PP-g-MA              147.37        40.44         123.71
PP/PP-g-MA/MMT          149.19        43.47         121.03
PP/PP-g-MA/SFD-MMT      148.30        42.18         121.83

                               Cooling

                      [T.sub.C]      [X.sub.C]
                     ([degrees]C)   ([degrees]C)

PP                      116.14         39.07
PP/MMT                  117.57         38.92
PP/SFD-MMT              116.88         38.89
PP/PP-g-MA              118.84         39.18
PP/PP-g-MA/MMT          115.50         39.66
PP/PP-g-MA/SFD-MMT      117.89         39.62

                            Second heat

                      [T.sub.m]     [X.sub.C]
                     ([degrees]C)      (%)

PP                      146.97        40.50
PP/MMT                  148.12        40.71
PP/SFD-MMT              147.47        40.60
PP/PP-g-MA              149.57        40.94
PP/PP-g-MA/MMT          149.74        41.39
PP/PP-g-MA/SFD-MMT      147.39        40.87

[T.sub.m], melting peak temperature; [X.sub.C], crystallinity;
[T.sub.C-Onset] cystallization onset temperature; [T.sub.C],
crystallization peak temperature.

TABLE 3. Half time of crystallization and the Avrami index
for PP and its MMT nanocomposites.

                                     Avrami index (n)

Sample                120[degrees]C   122[degrees]C   124[degrees]C

PP                        24.02           31.06           47.15
PP/MMT                    21.01           32.45           48.98
PP/SFD-MMT                23.59           46.22           84.97
PP/PP-g-MA                13.87           23.50           34.18
PP/PP-g-MA/MMT            24.38           38.81           59.49
PP/PP-g-MA/SFD-MMT        43.97           94.08          220.65

                         Avrami        Experimental half time (s)
                        index (n)

Sample                126[degrees]C   120[degrees]C   122[degrees]C

PP                        74.07           2.339           1.930
PP/MMT                    76.58           1.921           1.916
PP/SFD-MMT               158.90           1.935           1.939
PP/PP-g-MA                53.51           1.935           1.939
PP/PP-g-MA/MMT            92.72           1.964           1.957
PP/PP-g-MA/SFD-MMT                        1.840           1.701

                       Experimental half time (s)

Sample                124[degrees]C   126[degrees]C

PP                        2.011           1.990
PP/MMT                    1.900           1.923
PP/SFD-MMT                1.868           1.584
PP/PP-g-MA                1.838           1.584
PP/PP-g-MA/MMT            2.003           1.736
PP/PP-g-MA/SFD-MMT        1.750

TABLE 4. Values used in the literature for
equilibrium melting point ([T.sub.m.sup.*]) of
polypropylene.

Authors               Material         [T.sub.m.sup.*]
                                         ([degrees]C)

Krigbaum and             PP           186.0 [+ or -] 2.0
Uematsu [35]

Clark and                PP                 185.0
Hoffman [33]

Xu et al. [36]           PP                 212.1

Yamada et         99.6% isotatic PP         186.0
al. [37]

Khoshkava,               PP                 205.4
Ghasemi et al.
2015 [29]

Khoshkava          PP + 1% SFD-CNC          213.8
et al. [29]

Authors           [sigma] (mJ/   [[sigma].sub.c]
                   [m.sup.2])    (mJ/[m.sup.2])

Krigbaum and           --              --
Uematsu [35]

Clark and             11.5            65-70
Hoffman [33]

Xu et al. [36]         --              146

Yamada et              --              --
al. [37]

Khoshkava,            12.6             139
Ghasemi et al.
2015 [29]

Khoshkava             12.6             173
et al. [29]

TABLE 5. Edge surface energy and energy of work for chain folding
calculated from experimental half time for PP and its MMT
nanocomposites.

                      [T.sub.m.sup.*] (458.16 K)

Sample                [[sigma].sub.e]   q (kJ/Mol)
                      (mJ/[m.sup.2])

PP                         74.26          30.74
PP/MMT                     83.46          34.54
PP/SFD-MMT                122.16          50.57
PP/PP-g-MA                 85.89          35.55
PP/PP-g-MA/MMT             86.11          35.64
PP/PP-g-MA/SFD-MMT        160.99          66.64

                      [T.sub.m.sup.*] (485.26 K)

Sample                [[sigma].sub.e]   q (kJ/Mol)
                      (mJ/[m.sup.2])

PP                        173.49          71.81
PP/MMT                    184.23          76.25
PP/SFD-MMT                269.71          111.64
PP/PP-g-MA                189.66          78.50
PP/PP-g-MA/MMT            190.12          78.69
PP/PP-g-MA/SFD-MMT        352.16          145.77

TABLE 6. Tensile strength values (MPa) and Young's modulus
values (GPa) of PP compositions.

                       Tensile strength    Young's modulus (GPa)
                             (MPa)

PP                     40.1 [+ or -] 3.8    1.17 [+ or -] 0.10
PP/MMT                 36.3 [+ or -] 0.9    1.18 [+ or -] 0.22
PP/SFD-MMT             35.9 [+ or -] 0.9    1.43 [+ or -] 0.12
PP/PP-g-MA             37.7 [+ or -] 1.0    0.96 [+ or -] 0.19
PP/PP-g-MA/MMT         34.1 [+ or -] 1.5    1.20 [+ or -] 0.10
PP/PP-g-MA/SFD-MMT     32.5 [+ or -] 1.6    1.55 [+ or -] 0.03
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Author:Abdallah, Wissam; Tan, Victor; Kamal, Musa R.
Publication:Polymer Engineering and Science
Date:Jan 1, 2020
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