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Layered Double Hydroxide Nanoplatelets with Ultra High Specific Surface Area for Significantly Enhanced Crystallization Rate and Thermal Stability of Polypropylene Baku Nagendra, Angel Mary Joseph, Balakondareddy Sana, Tushar Jana, and E. Bhoje Gowd ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00049 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Nano Materials

Layered Double Hydroxide Nanoplatelets with Ultra High Specific Surface Area for Significantly Enhanced Crystallization Rate and Thermal Stability of Polypropylene

Baku Nagendra,†,‡ Angel Mary Joseph,†,‡ Balakondareddy Sana,§ Tushar Jana,§ and E. Bhoje Gowd*,†,‡ †

Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695 019, Kerala, India. ‡

Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 001, India. §

School of Chemistry, University of Hyderabad, Hyderabad 500046, India.

--------------------------------------------------------------------------------------------------------* Corresponding author. E-mail: [email protected] Tel.: +91-471-2515474; Fax: +91-471-2491712. 1 ACS Paragon Plus Environment

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Abstract A facile method for the simultaneous delamination and the lateral size reduction of layered double hydroxides (LDH) is reported. This method directly resulted in the delaminated mesoporous LDH nanoplatelets (nanodot LDH) with the high specific surface area (lateral dimensions as low as 10-30 nm and featured a thickness of ∼1 nm). Such prepared LDH was used as fillers for isotactic polypropylene (iPP). For the purpose of comparison, LDH having different surface areas were also used as fillers for iPP. The incorporation of nanodot LDH showed a remarkable improvement in the polymer properties with only 1 wt % loading. The uniformly dispersed LDH particles have a significant effect on the nucleation ability, thermal stability and mechanical properties of iPP. The nucleation ability of iPP in the presence of nanodot LDH is the best compared to other iPP nanocomposites reported using LDH as fillers in the literature. Furthermore, the microstructure of the iPP nanocomposites was systematically investigated at multiple length scales in the presence of different-sized LDH, which is a key to understand the polymer properties. Keywords: nanocomposites, isotactic polypropylene, layered double hydroxides, solvent blending, thermal stability, crystallization, surface area 1.

Introduction

Engineering thermoplastics in general and polypropylene, in particular, received indispensable attention from the industry, since these are low cost and easily processable polymeric materials used for various applications like fibers, packaging, laboratory equipment, automotive components, electronics etc.1-2 The application of isotactic polypropylene (iPP) as an engineering plastic is often limited due to its low Young’s modulus and impact strength. It has been demonstrated that the addition of the nanofillers to iPP can result in the formation of

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nanocomposites with significantly improved thermomechanical and optical properties.3-6 Among the numerous nanofillers, two-dimensional sheet-like nanostructures such as layered silicates, graphene, and its analogs, and recently layered double hydroxides (LDH) have found interest by researchers as multifunctional nanofillers for polymers.3-4, 7-9 LDH expressed by the general formula [M1-x2+Mx3+(OH)2]x+Ax/mn-⋅nH2O (where M2+, M3+, and An- represent divalent metal cation, trivalent metal cation and interlayer anions) is a versatile class of synthetic clays, which has been extensively used as a nanofiller by the researchers in order to improve the thermomechanical and physical properties of polymers.8-12 The major advantage of these materials compared to other two-dimensional nanofillers is the flexibility in tuning the chemical composition of layers as well as balancing anions in the interlayer space.8, 12 In order to modify the performance of a polymer effectively, delaminated LDH layers must be highly dispersed within the polymer matrix to increase the interfacial area between the LDH and polymer matrix. There have been multiple strategies to disperse the LDH in a nonpolar polymer matrix.8, 11, 13-27 Among these, recently, direct delamination of as-synthesized LDH in polymer solutions attracted the interest of researchers as this method yield highly dispersed polymer nanocomposites without the aid of surfactants or organic modifiers.15, 18-24, 27 When we consider the effect of nanofillers, the particle size, shape, surface modification, the dispersion quality, the interaction with the polymer, etc. play the crucial role in determining the nucleation efficiency of the nanofillers.25-32 For example, it has been found in several cases that the high degree of dispersion of the nanofillers in polymer matrix increases the rate of crystallization. In some other cases, interactions between the filler surface and the polymer chains determine the nucleation efficiency. Among the commonly used nanofillers, it was showed that carbonaceous nanofillers (carbon nanotubes, graphene, and its analogs) exhibit



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stronger nucleating abilities for iPP.31, 33-34 Using carbon nanotubes, it was demonstrated that the aspect ratio is an important factor influencing the nucleating ability of the nanofillers and the low-aspect-ratio CNTs could provide more nucleating sites for polymer crystal growth compared with the large-aspect-ratio CNTs.29 Although some efforts have already been reported to understand the effect of surface modification, the dispersion quality, and shape of the nanofillers, a systematic study by tuning the surface area of the nanofillers is still missing. Specific surface area of the nanofiller plays an important role in reducing the nucleation barrier of the semicrystalline polymers, thereby influencing the crystallization rate of polymers. Compared to other two-dimensional materials, the surface areas of LDH can be easily adjusted during the synthesis or post-modification process.27, 35 Herein, we report a method for the simultaneous delamination and the lateral size reduction of LDH. This method involves the mixed solvents treatment of the as-prepared LDH having the particle sizes ∼200 nm and resulted in the delaminated LDH nanodots having single- or fewlayer nanosheets with reduced lateral sizes. To the best of our knowledge, this is the first study to obtain LDH nanodots with extremely high specific surface areas 620 m2/g. In the present study, we also clarify the effects of three different-sized LDH (having different surface areas) on the various physical properties of the host iPP such as thermal stability, dynamic mechanical behavior, and nucleation ability. The remarkable improvements in the physical properties of iPP reported here are with only 1 wt % LDH nanodots. Furthermore, the hierarchical structure of the iPP nanocomposites is investigated using POM, WAXS, and SAXS in the presence of differentsized LDH.


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2.

Results and Discussion

2.1. Synthesis and Characterization of different sized LDH.

Figure 1. Characterization of µm-LDH. (a & b) SEM and TEM images of as-synthesized µmLDH, and (c & d) TEM and EDS spectrum of delaminated µm-LDH. Currently, the lateral size of LDH is tuned by varying the reaction environment used for the synthesis of LDH or by sonication of as-synthesized LDH for longer duration.27, 36 For example, Mg-Al LDH prepared using urea as the base resulted in the LDH with larger lateral sizes ∼3-4 µm as shown in Figure 1. On the other hand, Mg-Al LDH prepared under the similar experimental conditions using HMT as the base resulted in the LDH with lateral sizes in the range of 100 – 200 nm (Figure 2), which is several magnitudes lesser than the lateral size of LDH synthesized using urea. The energy-dispersive X-ray spectrum (EDS) confirmed the composition of the Mg-Al LDH (Figure 2b). Such difference in the lateral size of the LDH might be due to the increase in a number of nucleating sites in the presence of HMT. Compared to urea, HMT is a stronger base, and the ammonia release rate in HMT is faster than that of urea, and as a 5 ACS Paragon Plus Environment


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result, the rapid nucleation occurs in the presence of HMT that resulted in the smaller-sized particles.37 Another notable difference is the morphology of the platelets, which also occurs due to the difference in the nucleation and growth process in different reaction environments. It has been reported that several parameters such as metal ion concentration, reaction temperature, pH, the rate of addition, aging time and mixing efficiency dictate the nucleation and growth process of LDH and as a result the morphology and crystal size.8, 12, 37-39 The morphology and thickness of LDH were evaluated by the AFM. Figure 2c shows the typical AFM height image and corresponding height profile of as-synthesized Mg-Al LDH using HMT as a base. It is evident from Figure 2c that the as-synthesized sample shows the platelet like-morphology with the lateral size ∼200 nm and thickness ~16 nm. The aqueous miscible organic solvent treatment (AMOST) method was used to delaminate the LDH, and the TEM image of the delaminated LDH nanosheets (nm-LDH) is shown in Figure 2d. Compared to the as-synthesized LDH, the contrast of delaminated LDH (nm-LDH) sheets is weak that indicates the delamination of LDH to single- or few-layer nanosheets. The lateral sizes of LDH remained intact after the delamination in the AMOST method.18, 22 The corresponding EDS spectrum shown in Figure 2e is consistent with the EDS spectrum of bulk LDH (Figure 2b). However, after the delamination, i.e., the oxygen peak decreased in intensity drastically indicating the removal of water molecules upon delamination. AFM height image (Figure 2f) of the delaminated nm-LDH shows that the thickness of LDH layer is around 1.5 nm. This value is twice to the reported value of a single layer LDH indicating the presence of two LDH nanosheets.40 Recently, we have demonstrated that the sonication-assisted delamination of acetone washed LDH results in the fragmented LDH.20,

27

However, this is a time- and energy-consuming process. Herein, we developed a

simple method for the simultaneous delamination and size reduction of LDH.


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Figure 2. Characterization of nm-LDH. (a, b & c) TEM, EDS spectrum, and AFM image of assynthesized nm-LDH and (d,e & f) TEM, EDS spectrum, and AFM image of delaminated nmLDH. Right to AFM images, height profiles are shown to understand the lateral size and thickness of LDH layers.

Figure 3. Schematic illustration of simultaneous delamination and size reduction of LDH. As-synthesized larger-sized LDH nanoparticles having the lateral sizes ∼200 nm were dispersed in a mixture of dimethylformamide (DMF) and hydrochloric acid (HCl) (4:1 volume ratio) and stirred for 2 h. DMF is a known solvent that can effectively swell the LDH and acids can dissolve the LDH.40 LDH is completely soluble in concentrated HCl due to the surface protonation and that resulted in the detachment of metal species from the LDH surface into the



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solution. With the addition of large amount of DMF to HCl, the dissolution of LDH is suppressed due to the less probability of HCl interacting with the LDH. Hence instead of complete dissolution, random fragmentation of LDH occurs depending on the concentration of HCl. At the same time, wherever LDH surfaces are exposed to HCl, pits and cavities are formed on the nanosheets of LDH as shown schematically in Figure 3 (irregular white spots). The disappearance of characteristic bands of interlayer carbonate (CO32- ~1356 cm-1) and interlayer nitrate (NO3- ~1382 cm-1) confirmed the exchange of gallery anions with chloride ions (Figure S1). These two steps occurred simultaneously and resulted in the cleavage and delamination of the bulk LDH into single layered or few layered nanodots as shown in Figure 3, and these LDH were labeled as nd-LDH. Hence the judicious selection of these proper solvents in convenient proportions is important in controlling the lateral sizes of LDH. Similar phenomena have been reported in the exfoliation of graphene and molybdenum disulfide.41-42 Figures 4a & 4b show the TEM images of the nd-LDH. It is clear that the lateral size has been reduced drastically to ∼30 nm and the histogram given in the inset of Figure 4a shows the lateral size distribution, with the peak maxima at 30 nm. HRTEM image shows the pits and cavities within the 2D layers of nd-LDH as described in the schematic diagram shown in Figure 3. Figure 4d shows the AFM height image of the nd-LDH. EDS spectrum was documented after the dilute acid treatment, to probe whether there is any compositional variation and is given in Figure 4c. One can notice that the ratio of Mg-Al remains almost same compared to the nm-LDH, which confirms the compositional integrity of nanodots with nano-sized LDH. The height of the LDH has been reduced to ∼1 nm, typical for the single-layered sheets (Figure 4d), which reveals that these LDH were undergone the process of exfoliation rather than the dissolution of the material. To further understand the compositional integrity of LDH, powder X-ray diffraction patterns


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(Figure 4e) of nm-LDH and nd-LDH are compared. For that purpose, the nd-LDH obtained after the treatment of DMF/HCl was recollected by centrifugation and washed with acetone for the removal of DMF and HCl. Both the LDH powders show the characteristic 00n reflections and other reflections at higher 2θ corresponding to the unit cell of LDH. The LDH treated with DMF/HCl showed higher 2θ positions of (00n) planes compared to the as-synthesized LDH and it may be due to the exchange of anions as discussed above. The position of the (110) reflection remains same before and after the DMF/HCl treatment indicates the compositional integrity of LDH. Another notable difference observed between these LDH is the change in full-width at half maximum (FWHM) of (00n) planes indicating the different crystal sizes of the LDH after the DMF/HCl treatment.

Figure 4. Characterization of nd-LDH. (a & b) TEM and HRTEM images, (c) EDS spectrum, (d) AFM image, and (e) comparison of powder X-ray diffraction patterns of Mg-Al LDH before and after mixed solvents (DMF/HCl) treatment.



2.2. Dispersion of different sized Mg-Al LDH in Polypropylene. To investigate the effect of the lateral size of LDH on the polymer properties, iPP nanocomposites were prepared using three different sized LDH (µm-LDH, nm-LDH, and ndLDH). Figure 5 depicts the powder X-ray diffraction patterns of iPP nanocomposites containing three different-sized LDH (1 wt%) along with pristine iPP and LDH prepared using urea as the base. The reflections corresponding to the (00n) planes of LDH are not observed in the X-ray diffraction patterns of iPP nanocomposites containing different-sized LDH that indicates the fine dispersion of LDH within the iPP matrix without 2D layer stacking. Even with the higher % of loading (tested up to 5 wt%), the reflections corresponding to the (00n) planes of LDH are not observed in the X-ray diffraction patterns of iPP nanocomposites (Figure S2). The details about

111 (α ) 041(α )

117 (γ )

040 (α )

130 (α )

110 (α )

the crystal structure will be discussed in a later section.

1% nd-LDH 1% nm-LDH 1% µm-LDH iPP

10

006

003

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

LDH (Urea)

20 25 30 2 Theta (deg.)

Figure 5. Powder X-ray diffraction patterns of iPP and its nanocomposites containing differentsized LDH (for comparison, the powder XRD patterns of LDH prepared by two different bases are shown). Polymer and nanocomposites samples were prepared by cooling the melt at a rate of 10 °C/min in the DSC pan. The homogeneous dispersion of µm-LDH and nm-LDH in iPP is further visualized with the help of cryo-TEM using nanocomposites containing 5 wt% of LDH (Figure S2). In the case of 10 ACS Paragon Plus Environment

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iPP/µm-LDH nanocomposites, the sheet-like morphology of the LDH is preserved in nanocomposite structure. The partially or fully exfoliated LDH are dispersed in the polymer matrix homogeneously. The EDS spectrum of the dispersed LDH in iPP matrix is matching with the EDS spectrum of as-synthesized LDH other than the intensities of carbon and oxygen peaks. The SAED pattern (not shown here) indicated the similar crystallinity of LDH after the dispersion in iPP matrix. On the other hand, in iPP/nm-LDH nanocomposites, the dispersion is quite uniform, and the thin transparent sheets retain as such without any aggregation. Visualizing the nd-LDH in iPP matrix is difficult due to their small sizes and the TEM image of the iPP/ndLDH nanocomposites is not shown here. These results suggest that LDH platelets have been successfully dispersed into the polymer matrix without much agglomeration during solution blending. 2.3. Hierarchical Structure of iPP/LDH nanocomposites 2.3.1. Influence of Lateral Size of LDH on the Crystallization Kinetics and Microstructure of iPP. The influence of different-sized Mg-Al LDH on the crystallization rate of iPP was studied using DSC. Quite often, the crystallization temperature (Tmc) upon cooling from the melt is used to measure the crystallization rate of the polymer.43 Higher the Tmc, the higher is the crystallization rate of the polymer. Figure 6a shows the DSC thermograms of pristine iPP and iPP/LDH nanocomposites containing different-sized LDH (1 wt %) obtained during cooling at a rate of 10 °C/min. It can be seen that the size of the LDH has a significant role in determining the crystallization temperature and the nanocomposite samples showed the higher Tmc than pristine iPP, indicating that the addition of LDH accelerates the crystallization rate of iPP. The Tmc values measured for various samples are summarized in Table 1.



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Table 1. Summary of BET surface area of LDH, crystallization kinetics parameters of iPP and iPP/LDH nanocomposites with different-sized LDH. Mg-Al LDH samples

As-synthesized

iPP-Mg-Al LDH nanocomposites BET

crystallization

Avrami

melt-

surface

half-time (t1/2)

constant

crystallization

area

at 130 °C

(n)

temperature

(m2/g)

(min.)

(± 0.01)

(Tmc) °C

(± 5)

(± 0.2)

samples

(± 0.3)

115

iPP

16.0

2.6

110

200

1 wt% µm-LDH

9.2

2.7

115.2

5 wt% µm-LDH

8.7

2.5

116

1 wt% nm-LDH

3.8

3.1

120.6

5 wt% nm-LDH

2.7

3

122

1 wt% nd-LDH

0.7

3.5

129.5

5 wt% nd-LDH

1.2

3.3

127

Mg-Al LDH

µm-LDH

nm-LDH

nd-LDH

393

620

The change in Tmc values was found to depend upon the lateral size of the LDH used for the preparation of nanocomposites. The incorporation of µm-LDH has a little effect on the Tmc (∆Tmc = Tmc(nanocomposite)-Tmc(iPP)= 5.2 °C), whereas the incorporation of nm-LDH has a moderate effect on the Tmc (∆Tmc = 10.6 °C). On the other hand, the incorporation of nd-LDH dramatically shifted the Tmc of iPP to a higher temperature (∆Tmc = 19.5 °C). To the best of our knowledge, the Tmc value observed for the iPP containing 1 wt% of nd-LDH is the highest compared to the Tmc values reported for iPP/LDH nanocomposites so far. The Tmc values reported in the literature for iPP/LDH nanocomposites are summarized in Table S1. In order to understand the effect of filler content on the Tmc, DSC thermograms of iPP/LDH nanocomposites containing 5 wt % of 12 ACS Paragon Plus Environment

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different-sized LDH were obtained during cooling at a rate of 10 °C/min and the corresponding Tmc values are reported in Table 1. The Tmc values slightly increased in the case of nanocomposites containing µm-LDH and nm-LDH indicating that the crystallization rate of iPP is increased with the increase in filler loading from 1 wt% to 5 wt%. However, in the case of ndLDH, the Tmc value was slightly reduced indicating the slow crystallization compared to the nanocomposite containing 1 wt% of nd-LDH and it might be due to the agglomeration of ndLDH above the optimal loading. To further confirm the influence of different-sized LDH on the crystallization rate of iPP, isothermal crystallization was carried out at 130 °C. Figure 6b shows the DSC thermograms of pristine iPP and iPP/LDH nanocomposites containing different-sized LDH (1 wt %) during isothermal crystallization at 130 °C. The crystallization half-time (t1/2), which is the time required to complete 50% of the crystallization under isothermal conditions, measured for various samples are summarized in Table 1. Compared to the pristine iPP, both the induction time and the t1/2 decreased with the incorporation of different-sized LDH and the decrease is drastic in the presence of nd-LDH. The crystallization exotherm observed for the iPP/nd-LDH is very sharp and the t1/2 measured is ~0.7 min. It is seen that the lateral size of LDH plays an important role on the crystallization rate of iPP. The incorporation of 1 wt% of µm-LDH, nmLDH and nd-LDH reduced the t1/2 to 42.5%, 76.2%, and 95.6%, respectively, of that of the neat iPP at 130 °C. Figure 6c illustrates the relationship between the surface area of the LDH and ∆Tmc and ∆t1/2 of the nanocomposites. As evident from the graph, a strong correlation was observed between the surface area of the LDH and crystallization kinetics of iPP. By reducing the lateral size of the LDH, the heterogeneous nucleation has a lower free energy barrier compared to that of the nucleation on larger-sized flat LDH surfaces because of the large number



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of edges and defects in the smaller-sized LDH. It has been reported in the literature that nanofiller driven crystallization of polymers is mainly controlled by high specific surface area of the fillers.44-45 The t1/2 values were also measured for the iPP/LDH nanocomposites containing 5 wt% of different-sized LDH and the DSC thermograms are shown in Figure 6d. As discussed in the preceding section, the crystallization rate of iPP/LDH nanocomposites increased with the filler loading in the case of µm-LDH and nm-LDH. The nucleating agents are generally added to semicrystalline polymers in extremely low loadings. In order to verify the effect of lower loadings on the nucleation ability, in the case of nd-LDH, the t1/2 of the iPP/nd-LDH was measured using different (0.5, 1, 1.5 and 5 wt %) loadings. It was found that 1 wt% of nd-LDH is optimal loading for the effective crystallization of iPP (Figure S3).

Figure 6. DSC thermograms of pristine iPP and iPP/LDH nanocomposites obtained during (a) cooling from the melt at a rate of 10 °C/min, (b) crystallization isotherms obtained at 130 °C for the nanocomposites containing different-sized LDH (1 wt %), and (c) surface area of the LDH 14 ACS Paragon Plus Environment

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versus ∆Tmc and ∆t1/2 of the nanocomposites, (d) crystallization isotherms obtained at 130 °C for the nanocomposites containing different-sized LDH (5 wt %). Isothermal crystallization kinetics for pure iPP and its nanocomposites containing different-sized LDH were analyzed using Avrami equation as discussed in our previous paper.27, 46 Table 1 summarizes the Avrami constants estimated for iPP and various nanocomposites crystallized at 130 °C. The n values of pure iPP and iPP/µm-LDH were almost similar, and this n value might be attributed to a heterogeneous nucleation followed by diffusion controlled spherulite growth (mixed two-dimensional (2D) and three-dimensional (3D) crystal growth). On the other hand, n value for the nanocomposites containing nm-LDH and nd-LDH is slightly higher than 3, which is typical for a rapidly nucleating system mostly through heterogeneous nucleation undergoing three-dimensional crystal growth in the presence of the tiny LDH particles. Figure S4 shows the POM images of iPP and its nanocomposites prepared using different-sized LDH (1 wt%) after isothermal crystallization at 130 °C. The pristine iPP exhibits a characteristic spherulitic structure with well-defined boundaries due to the slow crystallization rate (Figure S4a). These spherulites showed negative birefringence (type II), and a Maltese cross extinction pattern is visible.47-49 In contrast, for the iPP/LDH (1 wt%) nanocomposites, the spherulite size decreases drastically, and the individual spherulites are not visible. No evidence of macroscopic aggregation of LDH particles was observed in the nanocomposites. The uniform size of the spherulites in iPP/LDH nanocomposites suggest that the well-dispersed LDH particles act as heterogeneous nucleation sites and barricade iPP spherulite growth. With the decrease in the lateral size of the LDH particles, the number of nucleation sites increased dramatically, and the higher number of spherulites (or axialites) was observed. By comparing the effect of the lateral size of LDH on the crystalline morphology of iPP, it is apparent that nd-LDH provides the



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largest number of nuclei due to the increased surface area and better dispersion of LDH particles in iPP matrix. By combining the POM results with the aforementioned DSC results, it may be concluded here that nd-LDH particles have the outstanding efficiency on overall crystallization rate of iPP than the larger-sized LDH particles. 2.3.2. Influence of Lateral Size of LDH on the Crystal Structure and Lamellar morphology of iPP. In order to understand the effect of the lateral size of LDH on the crystal structure and lamellar morphology of iPP, the samples were prepared with the well-defined thermal history under controlled conditions. All the samples were melted at 200 °C in DSC and cooled to room temperature at a rate of 10 °C/min. The samples recovered from the DSC pans were used to acquire WAXS and SAXS data in transmission mode. The WAXS patterns (Figure 5) of iPP, iPP/µm-LDH, and iPP/nm-LDH samples showed strong X-ray reflections at 2θ = 14.2°, 17.0°, 18.7°, 20.2°, 21.3°, and 22.1° corresponding to the monoclinic α form.50 In iPP, the formation of the α form is a kinetically driven process. On the other hand, iPP/nd-LDH nanocomposite sample showed an additional reflection at 2θ = 20.1°, and it was assigned to the (117) plane of the γ form.51 It has been reported that the γ form can be obtained in several manners such as by introducing stereo and regio-irregularities using metallocene catalyst, in the case oligomers having molecular weights between 1000 and 3000 g/mol or at elevated temperature and pressure.51-54 It was also reported that the formation of the γ form is thermodynamically more favored compared to the α form. Thomann et al. reported that the formation of the γ form is favored under low supercoolings.51 As discussed in the preceding section, iPP/nd-LDH nanocomposites were crystallized rapidly at high Tmc ~129.5 °C. Presumably, the low degree of supercooling favors the formation of minor fractions of the γ form in iPP/nd-LDH 16 ACS Paragon Plus Environment

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nanocomposite sample. In order to confirm this, pristine iPP was crystallized at 130 °C and the corresponding WAXS pattern (not shown here) showed the presence of minor fractions of the γ form similar to that of the iPP/nd-LDH sample. The degree of crystallinity was estimated by deconvolution method and the results are summarized in Table S2. It is apparent that the degree of crystallinity is not affected much by the addition of different-sized LDH. To further understand the change in the lamellar thickness, SAXS measurements were carried out on the pristine iPP and its nanocomposites. Figure 7a shows the Lorentz-corrected SAXS patterns for both pristine iPP and the nanocomposites prepared using different-sized LDH. Pristine iPP sample shows a peak at ~0.45 nm-1 attributed to the alternating crystalline and amorphous microstructure of the lamellae. A broad scattering peak at higher q (~0.9 nm-1) in pristine iPP indicates the formation of ordered periodic lamellar structure. On the other hand, nanocomposite samples containing 1 wt% of different-sized LDH show similar SAXS pattern, albeit the broad scattering intensity at low q, which might be due to the presence of well-dispersed LDH particles within the iPP matrix. The slope of the scattering intensity at low q was found to correlate with the lateral size of the LDH particles. From the SAXS patterns showed in Figure 7a, it is obvious that the presence of different-sized LDH particles do not significantly modify the lamellar structure of iPP other than the morphological parameters such as long period (L), lamellar thickness (lc) and amorphous thickness (la). In order to estimate the morphological parameters, Fourier transformation of the scattering curves was performed based on the method illustrated by Strobl and Schneider as shown in Figure S5 to yield the normalized one-dimensional electron density correlation function curves (Figure 7b).55 The lamellar parameters obtained for iPP and its nanocomposites are listed in Table S2.



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As the crystallinity in these samples is around 60%, based on the earlier reports, we assume that the larger value is crystalline lamellar thickness, lc, and the smaller value is the amorphous thickness, la.56 These results indicate that the lamellar parameters are strongly influenced by the different-sized LDH. Both L and lc values were found to increase with the addition of differentsized LDH. It was also found that these values increased with the decrease in the lateral size of the LDH. The L and lc values of iPP/nd-LDH increased by 39 Å and 23 Å, respectively, with respect to the pristine iPP. In the preceding section, it was showed the lateral size of the LDH strongly influenced the Tmc of iPP (Figure 6a). In the case of iPP/nd-LDH nanocomposites, the overall crystallization rate of iPP increased significantly and as a result, the polymer crystallized at the higher temperature during cooling from the melt. The strong dependence of the crystallization temperature on the lamellar thickness has been reported for some semicrystalline polymers.57-58 On the basis of these results, we may say that the lateral size of the particles influences the Tmc of iPP under nonisothermal conditions, and that Tmc determines the lamellar parameters. The degree of crystallinity ((lc/L)×100) estimated from the SAXS is in good agreement with the degree of crystallinity calculated from WAXS and DSC. Another noteworthy point to highlight here is that larger LDH particles are not located in the inter-lamellar region of iPP. Nam et al. illustrated the dispersion of clay platelets in the PP matrix in semicrystalline PP nanocomposites.32 Similar to their case, LDH particles are expected to reside in the amorphous phase outside the stacked lamellar structure. As reported in our previous paper, the SAXS patterns of iPP/LDH nanocomposites containing higher loadings of LDH (5 wt% or more) are not that clear due to the overlap of strong scattering intensities at low q which is coming from the scattering of well-dispersed LDH particles within the iPP matrix.27 Hence lamellar parameters are not estimated for the iPP/LDH nanocomposites containing 5 wt% of LDH.


Page 19 of 37

(a)

iPP 1% µm-LDH 1% nm-LDH 1% nd-LDH

(b) (b)

K(Z )(a.u.) (a.u)


Iq2 (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3

0.6

0.9 q (nm-1)

1.2

1.5

0

100

200 o Z (A)

300

400

Figure 7. (a) Lorentz corrected SAXS patterns of iPP and its nanocomposites with differentsized LDH, (b) corresponding 1D electron density correlation function curves.

2.4. Performance of iPP/LDH nanocomposites 2.4.1. Thermal Stability of iPP/LDH nanocomposites The influence of different-sized LDH (1 wt%) on the thermal stability of iPP was examined by TGA under nitrogen atmosphere (Figure 8a). The single decomposition observed in all the nanocomposite samples indicates the random scission of polymeric chains is the major decomposition mechanism, quite similar to that of pristine iPP. The 50% weight loss temperature (T0.5) for pure iPP is ~ 410 °C and it increases to 436 °C, 448 °C, and 457 °C for 1 wt% loaded iPP/µm-LDH, iPP/nm-LDH, and iPP/nd-LDH nanocomposites, respectively. As the lateral size of LDH decreases, the thermal stability of iPP increases significantly, and highest thermal stability was observed in the case of iPP/nd-LDH nanocomposites. It has to be noted that though all the nanocomposite samples are reinforced with the same amount of LDH (1 wt%), a significant difference in the thermal stability of iPP was observed with the lateral size of the LDH particles. As reported in our previous paper, presumably, well dispersed LDH particles 19 ACS Paragon Plus Environment


capture the radicals that generated during degradation.27 It is expected that nd-LDH will have relatively large number of dispersed particles in the polymer matrix compared to that of the nmLDH and µm-LDH. Similar result was observed by O’Hare and co-workers using 1 wt% LDH loadings in surfactant-modified Mg-Al LDH/PP nanocomposites synthesized by microemulsion method.15 The influence of filler loading of different-sized LDH (5 wt%) on the thermal stability of iPP was also examined (Figure S6). The T0.5 for iPP/µm-LDH, iPP/nm-LDH, and iPP/nd-LDH nanocomposites containing 5 wt% of LDH are 440 °C, 436 °C, and 438 °C, respectively. The thermal stability of iPP/µm-LDH was increased slightly with the filler loading. However, the thermal stability of the iPP/LDH nanocomposites containing smaller-sized LDH particles reduced with the amount of filler loading, and it may be because of the agglomeration of LDH as discussed in our previous paper.27 To further understand the reason for the difference in the thermal stability of the nanocomposites, TGA thermograms of different LDH were taken (Figure 8b). All the LDH samples show similar thermal stability irrespective of their synthetic procedures. These results suggested the higher thermal stability of the nanocomposites come from the well dispersed of small-sized particles. 100

100

(a)

µm-LDH

nm-LDH nd-LDH

Weight (%)

80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

60 40 20 0 300


350

(b)

90

80

70 400

450

500

100

Temperature (°C)

200 300 400 500 Temperature (°C)

600

Figure 8. TGA thermograms of (a) iPP and its nanocomposites with different-sized LDH and (b) different-sized LDH. 20 ACS Paragon Plus Environment

Page 21 of 37

2.4.2. Dynamic Mechanical Analysis (DMA) The viscoelastic properties of the iPP and its nanocomposites were analyzed using DMA and the data are given in Figure 9. It has to be noted that for viscoelastic properties, nanocomposites were prepared with 5 wt% of different-sized LDH. It is observed that the storage modulus (Eꞌ) of the nanocomposites is higher than the pristine iPP in the entire temperature range and it decreases with increasing temperature with a plateau region observed near the glass transition temperature (Tg). Table S3 shows the summary of viscoelastic properties at 0 °C and room temperature. The sharp reduction of Eꞌ in the lower temperature region can be assigned to the amorphous phase relaxation of iPP.20, 59 It can be seen that the lateral size of the LDH has a significant role on the increase of storage modulus of iPP. Among the different-sized LDH, nanocomposites having nd-LDH showed higher Eꞌ due to the better reinforcing effect of smaller-sized LDH particles in the polymer matrix. These results indicated that higher the surface area of LDH, the stronger is the interfacial strength between iPP and LDH particles. 5k 4k

0.20

3k

0.15


2k 1k

(b)

0.10


0.05 0

Figure

0.25

(a)

tan δ

Storage modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-40

9.

0

40 80 120 Temperature (°C)

160

-40

0

40 80 120 Temperature (°C)

160

Temperature-dependent dynamic mechanical properties of iPP and its

nanocomposites containing different-sized LDH: (a) storage modulus (Eꞌ) and (b) tan δ.



Page 22 of 37

The tan δ plot obtained from DMA is given in Figure 9b, which gives information about the damping ability of the material. The subtle change in the molecular level motion and the chain relaxations of semicrystalline polymers can be probed from this analysis. In the present case, the peak near ~ 6 °C remains more or less at same temperature in iPP and its nanocomposites, indicating that Tg is not affected by the addition LDH.20, 32, 59-60 But it should be noted that the intensity of this peak is decreased with the decrease in lateral size of LDH and this might be due to the decreased damping of the stiffer nanocomposites formed with the highly dispersed ndLDH. In other words, these nd-LDH have more effective interfacial adhesion to the polymer matrix, thereby reducing the molecular chain motions which are indicated by the reduced height of the peak corresponding to Tg. The second broad peak in tan δ lying above 120 °C is arising from the relaxation and reorganization of the crystalline lamella and crystalline-amorphous interface of iPP.20, 56,

59

It is noticed that storage modulus is decreased and damping of the

polymer chains are increased in this temperature regime. It is to be underlined that damping is much lower in the case of nd-LDH composites and no peak is visible in the selected temperature region, possibly due to the fact that presence of smaller-sized LDH favored the lamellar thickening and restricts the free movement and molecular relaxations of amorphous iPP chains due to the increased number of LDH particles.20, 56, 59-60

3.

Conclusions

Different-sized Mg-Al LDH were prepared by the conventional co-precipitation method, and particularly, a new method was developed for the synthesis nanodot-LDH by treating the asprepared LDH with dilute acid. This approach provided directly delaminated LDH sheets with lateral dimensions as low as 10-30 nm and feature a thickness of ~1 nm with the same chemical


Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


composition. Highly dispersed iPP nanocomposites were prepared by the solvent mixing method using different-sized LDH. It was observed that there is a clear dependence of the physical properties of iPP on the lateral size of LDH. As the lateral size decreases, the properties such as thermal stability and mechanical properties are enhanced significantly. The uniformly dispersed nd-LDH particles have a significant effect on spherulite size, lamellar thickness, and crystal structure of iPP with very low LDH loadings (only 1 wt %). Isothermal and nonisothermal crystallization results revealed that the overall crystallization rate of iPP is faster in the presence of nd-LDH compared to that of the nm-LDH and µm-LDH and it might be due to the presence of the relatively large number of dispersed particles in the polymer matrix with high surface area.

4.

Experimental section

Materials: IPP pellets (Mw ~120000, Đ ~4.5) and metal salts like aluminium nitrate (Al(NO3)3·9H2O), and magnesium nitrate (Mg(NO3)2·6H2O)) were purchased from SigmaAldrich Co. Ltd. Other chemicals such as urea, hexamethylenetetramine (HMT), xylene, dimethylformamide (DMF), hydrochloric acid (HCl), ethanol, and acetone were received from Merck, India. Synthesis of LDH: The Mg-Al LDH with different lateral sizes was synthesized using the conventional co-precipitation method as follows. Micrometer-sized Mg-Al LDH (µm-LDH): Micrometer-sized LDH was synthesized using previously reported procedures.27 The lateral sizes of the LDH synthesized by this procedure are ~3-4 µm. Bulk LDH obtained by this method was further washed with 100 ml of acetone and vacuum dried at 70 °C overnight. Such obtained LDH was further dispersed in 100 ml of



Page 24 of 37

acetone, and then it was added to 100 ml of xylene. The resultant solution was stirred for 12 hours at room temperature to obtain the delaminated LDH nanosheets. Hereafter the delaminated micrometer-sized LDH will be labeled as µm-LDH. Nanosized Mg-Al LDH (nm-LDH): The nanosized LDH was synthesized using a strong base HMT.37 Magnesium nitrate (Mg(NO3)2·6H2O), aluminium nitrate (Al(NO3)3·9H2O), and HMT were dissolved in Millipore water in a molar ratio of 2:1:15 which results in a final concentration of 10, 5, 75 mM, respectively. The mixture was refluxed at 100 °C under continuous stirring for 24 hours in the inert atmosphere. The precipitate was washed with hot Millipore water repeatedly and vacuum dried at 60 °C for 24 hours. The lateral sizes of the LDH synthesized by this procedure are ~100-200 nm. Similar to the above, LDH obtained by this method was further washed with acetone and dispersed in xylene to obtain the delaminated LDH nanosheets. Hereafter delaminated nanosized LDH having lateral dimensions ~100-200 nm will be labeled as nm-LDH. Nanodot Mg-Al LDH (nd-LDH): Nanosized LDH (~100-200 nm) prepared using HMT as a base was agitated in a mixture (100 ml) of DMF and HCl (4:1 volume ratio) for 2 hours to obtain the nanodot Mg-Al LDH. In this procedure, the treatment of LDH with the acid leads to the simultaneous size reduction and the delamination of LDH with the yield of 60 – 70 %. The lateral sizes of the nanodot LDH ~10-30 nm and the specific surface area is very high due to its porous nature (∼ 620 m2/g). Finally, the LDH was transferred to 100 ml of xylene before adding to the polymer solution to maintain the same protocol for the preparation of nanocomposites. Hereafter these LDH were labeled as nd-LDH. Preparation of iPP/LDH nanocomposites: The nanocomposites of iPP/Mg-Al LDH containing three different sized LDH (µm, nm, and nd) were prepared using solution blending method. In


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the case of µm-LDH and nm-LDH, 10 g of iPP was dissolved in 100 ml of xylene in a round bottom flask at 150 °C with continuous stirring at ambient conditions. However, in the case of nd-LDH, the LDH dispersed in the mixture of DMF and HCl was added to 100 ml of xylene and stirred at 100 °C to remove the HCl. Then the required amount of LDH solution prepared above was added to the flask under continuous stirring. In all the cases, the amount of LDH was adjusted to 1 wt % and 5 wt% to prepare nanocomposites. The resultant solution was refluxed at 150 °C for 24 h, and the solution was poured into 100 ml of ethanol. The precipitate was filtered, washed with ethanol several times and dried in a vacuum oven at 100 °C for 24 h. Characterization XEUSS 2D SAXS/WAXS system with a Genixmicro source from Xenocs was used for the WAXD and SAXS measurements. The Cu Kα radiation (λ = 1.54 Å) was collimated with a FOX2D mirror and two pairs of scatterless slits. The X-ray system was operated at 50 kV and 0.6 mA and the measurements were carried in the transmission geometry. The fiber diagrams were recorded on a image plate (Mar345) and processed using the Fit2D software. Samples crystallized under controlled conditions using a differential scanning calorimeter were used for the X-ray measurements. The two-dimensional SAXS images were azimuthally integrated to obtain 1D scattering intensity profiles as a function of q, where q is the magnitude of scattering vector, q = 4π sin θ/λ, with 2θ being the scattering angle and λ is the X-ray wavelength. The resulting scattering intensity SAXS profiles were corrected for background scattering. To resolve the structure parameters of the lamellar structure such as long period (L), lamellar thickness (lc) and amorphous thickness (la), normalized one-dimensional electron density correlation function was derived from the small-angle X-ray scattering curves.



Page 26 of 37

With the assumption of stacked lamellar structure (crystalline and amorphous layers), the normalized one-dimensional electron density correlation function (K(z)) is defined as follows.55

= cos

(1)

Where z is normal to the layer faces in the lamellar stack, and I(q) is the scattered intensity. The various lamellar parameters were calculated from the K(z) curve as shown in Figure S5. The morphology of various LDH prepared were analyzed with TEM (JEOL 2010 transmission electron microscope operating at 300 kV), SEM (Zeiss EVO 18 cryo SEM with an accelerating voltage of 20 kV) and AFM (Digital Instruments, Inc., Santa Barbara with imaging performed using a Dimension 3100 and a CP microscope Park Scientific Instrument, Inc in the tapping mode) techniques. The elemental constitution of the materials was determined on an energy dispersive spectrometer (Technai G2 30LaB6, ST with EDS). The specific surface area was measured using the Brunauer, Emmet, and Teller (BET) surface area analyzer (Micrometrics TriStar 3000 V6.05A). A Perkin Elmer Series FT-IR spectrum-2 was used to measure the infrared spectrum over the wavenumber range of 4000 – 400 cm-1. The spectra were collected with 32 scans and a resolution of 1 cm-1. The dried LDH powder was mixed with KBr and pressed in the form of pellets for FTIR measurements. In order to view the dispersion of LDH in polymer nanocomposites, ultrathin sections of the samples (thickness ~ 70 nm) were sliced with a diamond knife (35° knife angle; DIATOME, Switzerland) using ultramicrotome EM UC/FC 6, Leica (Austria) at -140°C. The sections were flooded with a DMSO/water mixture on carbon filmed TEM copper grid. These thin slices of the samples polymer nanocomposites were analyzed using a TEM LIBRA. A polarized optical microscope (Universal polarizing microscope ZPU01, Carl Zeiss Inc.) equipped with a Linkam


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hot stage was used to monitor the spherulites. The thin-film specimens were prepared by melting the samples at 200 °C for 1 min and then rapidly cooled to the crystallization temperature. The samples were imaged at the isothermal crystallization temperature of 130 °C. DSC measurements were performed on a TA Instruments DSC Q2000 model equipped with a refrigerated cooling system under nitrogen gas flow. The crystallization half-time (t1/2) was measured to evaluate the crystallization rate of iPP and its nanocomposites. Molten samples were rapidly cooled to the desired crystallization temperature (Tc) (130 °C) at a rate of 100 °C/min, and then the samples were allowed to crystallize at that temperature. The non-isothermal crystallization data were collected while cooling the samples from the melt to room temperature at 10 °C/min. These samples were again reheated to 200 °C to analyze the melting behavior. DMA (TA Instruments Model Q800) was used to study the temperature-dependent dynamic mechanical properties of the samples. The samples with dimensions 25 × 6 × 0.4 mm3 were prepared using the hot press and all samples were annealed at 130 °C for 5 h before the measurements. The prepared samples were used to measure the temperature-dependent storage modulus (E′) and tan δ at a constant frequency (ω) of 6.28 rad/s with the strain amplitude of 0.05%. The measurements were carried out in a temperature range of -50 to 170 °C, with a heating rate of 2 °C/min. Thermal stability of the prepared nanocomposites was analyzed with thermo gravimetric analysis TA Q50 instrument in the heating process at a heating rate of 10 °C/min under the nitrogen atmosphere (nitrogen gas flow rate of 60 mL/min for the furnace and 40 mL/min for balance). Acknowledgements Authors thank Dr.T. Narasimhaswamy, CSIR-CLRI, Chennai for the POM, Mr. Kiran Mohan for TEM, Dr.Yoosaf Karuvath and Mr. Aswin for AFM measurements. Authors also thank Dr. Petr



Page 28 of 37

Formanek and Mrs. Uta Reuter, IPF, Dresden, Germany for TEM analysis of polymer nanocomposites. EBG thanks the Department of Science and Technology (Government of India) for the research project vide No.: SB/S3/CE/070/2014 and Council of Scientific and Industrial Research, India, for financial support in the form of 12th FYP project (CSC-0135). BN thanks the Council of Scientific and Industrial Research for the award of a research fellowship.

Associated Content: Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website. FTIR spectra, powder X-ray diffraction patterns, TEM and POM images, DSC and TGA thermograms, electron density distribution η(z) and the one-dimensional electron density correlation function K(z) are provided. Summary of thermal data, lamellar parameters and viscoelastic properties are provided in the tabular form.

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