Influence of Magnetic Nanoparticle Size on the Particle Dispersion

Jan 30, 2014 - Yuan Yao , Ezzeldin Metwalli , Martin A. Niedermeier , Matthias Opel , Chen Lin , Jing Ning , Jan Perlich , Stephan V. Roth , and Peter...
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Influence of Magnetic Nanoparticle Size on the Particle Dispersion and Phase Separation in an ABA Triblock Copolymer Jinrong Wu, Hui Li, Siduo Wu, Guangsu Huang,* Wang Xing, Maozhu Tang, and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: Oleic acid modified iron oxide nanoparticles (IONs) with different sizes were synthesized and mixed with styrene-butadiene-styrene block copolymer (SBS) with a lamellar structure. The octadecene segments on the oleic acid molecules have chemical affinity with the polybutadiene (PB) blocks, which makes IONs tend to be selectively confined in the microphase-separated PB domains. However, the dispersion state strongly depends on the ratio of the particle diameter (d) to the lamellar thickness (l) of the PB domains, which further changes the phase separation of SBS. When d/l ∼0.5, most of IONs are concentrated in the middle of the PB layers at low particle loading. Upon increasing the particle loading, part of IONs contact each other to form long strings due to their strong magnetic interactions. Away from the strings, IONs are either selectively dispersed in the middle and at the interfaces of the PB domains, or randomly distributed at some regions in which the phase separation of SBS is suppressed. The phase separation of SBS transforms from the lamellar structure to a cylinder structure when the IONs loading is higher than 16.7 wt %. As d is comparable to l, IONs aggregate to form clusters of 100 to 300 nm in size, but within the clusters IONs are still selectively dispersed in the PB domains instead of forming macroscopic phase separation. It is interpreted in terms of the relatively small conformational entropy of the middle blocks of SBS; thus, incorporation of nanoparticles does not lead to much loss of conformational entropy. Although incorporation of IONs with d/l ∼1 significantly increases the interfacial curvature and roughness, it has less influence on the phase separation structure of SBS due to the inhomogeneous dispersion. When d is larger than l, IONs are macroscopically separated from the SBS matrix to form clusters of hundreds of nanometers to several micrometers. More interestingly, the phase separation of SBS transforms from the lamellar structure to a two-phase cocontinuous structure, probably due to the rearrangement of SBS molecules to cover the clusters with PB segments and the strong magnetic interaction exerting additional force on the SBS matrix during the evaporation of the solvent and the subsequent thermal annealing process.

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INTRODUCTION ABA triblock copolymer based thermoplastic elastomers possess advantages of both conventional rubbers and plastic polymers.1 They show excellent rubber elasticity, sealing, damping, and antifatigue properties, while in contrast to thermoset rubbers, ABA triblock copolymers are simpler to process and recyclable like plastics. These unique properties are strongly dependent on their phase morphologies. Thus, various approaches have been developed to tune their phase morphologies over the last 30 years. Usually chemical approaches were used to manipulate molecular parameters, such as chain architecture, nature of interface, chain topology, etc., which could result in plenty of phase morphologies.2−6 Physical approaches, including annealing under thermal and solvent conditions, processing with different methods and conditions, etc., also have strong influence on the phase morphologies.7−10 For practical application, block copolymers are frequently filled with various fillers. Actually, it is the rich diversity of phase morphologies that makes block copolymers attractive as scaffolds for the controlled dispersion of nanoparticles in the polymer matrices. By properly controlling the size and interfacial interaction, nanoparticles can be selectively confined © 2014 American Chemical Society

within a desired microphase-separated domain, which has particular applications in photonics, information storage, diagnostics, photovoltaics, sensors, etc.11−24 On the other hand, incorporation of nanoparticles can result in a change in the phase morphologies of block copolymers, because the domain in which the nanoparticles are confined is preferentially swelled, like changing the volume fraction of one block relative to that of the other.22−30 However, the above-mentioned phenomena are mostly observed in diblock copolymers, especially amphiphilic diblock copolymers.30−40 For diblock copolymers, each block has a free chain end, which makes the blocks easily undergo conformation rearrangement with the incorporation of nanoparticles. Compared with diblock copolymers, only a few works reported on using ABA triblock copolymers as templates to organize nanoparticles in an ordered fashion and create nanostructured arrays.16,41−46 Moreover, nanoparticles are generally dispersed in the domains of A blocks with free chain ends. Limited works disperse nanoparticles in the domains of B blocks,44−48 which have Received: October 27, 2013 Revised: January 7, 2014 Published: January 30, 2014 2186

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Figure 1. TEM images of ION nanoparticles synthesized within different solvents: (a) 1-hexadecene, (b) 1-octadecene, (c) trioctylamine.

Figure 2. Cross-sectional TEM images of (a) neat SBS, (b) SBS with addition of 9.1 wt % IONs-8.2, (c) and (d) SBS with addition of 25 wt % IONs-8.2.

lower conformation entropy and no free chain ends, and can only undergo stretching with incorporation of nanoparticles, so the relating physics is not well understood. On the other hand, triblock copolymers have smaller domain size compared with diblock copolymers of the same molecular weight. Thus, it is of interest to investigate the dispersion state and the phase morphologies of ABA triblock copolymers incorporated with nanoparticles with chemical affinity to the B blocks. Styrene-butadiene-styrene block copolymer (SBS) is the most widely used ABA triblock copolymer up to the present date. Nevertheless, few works used SBS as a template to assemble nanoparticles.44−46 Fewer works reported mixing magnetic nanoparticles with SBS,49 although magnetic nanopartilces have recently attracted increasing attention in the fields of ferrofluid, biomedicine, biosensing, magnetic storage media, magnetic resonance imaging, etc.50,51 Therefore, it is worthwhile to study the phase behaviors of nanocomposites based on SBS and magnetic nanoparticles. In the present work, SBS was used as the scaffold for the controlled dispersion of iron oxide nanoparticles (IONs) modified with oleic acid that

has chemical affinity with polybutadience (PB). The influence of size and content of IONs on the particle dispersion and phase separation in SBS was studied.

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EXPERIMENTAL SECTION Materials and Preparation. Styrene-butadiene-styrene block copolymer (SBS) kindly provided by Sinopec Beijing Yanshan Company was prepared by a coupling method. In brief, sec-butyllithium was used to initiate the polymerization of styrene, and subsequently butadiene, giving rise to living PS-PB macro-ions. The living PS-PB macro-ions then underwent coupling reaction with the addition of dimethyldichlorosilane to form SBS with well-defined structure. This specially prepared SBS has a PS weight fraction of 31%, a number average molecular weight of 1.55 × 105 g/mol, and a polydispersity of 1.157. Iron oxide nanoparticles (IONs) with different sizes were prepared according to the method of Park et al.52 In a typical synthetic procedure, 36 g of iron-oleate complex (prepared by us) and 5.7 g of oleic acid (Chengdu Kelong Chemical Co., Ltd., 90%) were dissolved in 200 g of 12187

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octadecene in a round flask equipped with a condenser. The flask was placed in a heating kettle and heated to 320 °C with a heating rate of about 3.3 °C/min, and then kept at that temperature for 30 min. The resulting solution containing IONs was then cooled to room temperature, and 500 mL of ethanol was added to the solution to precipitate the nanoparticles. The nanoparticles were separated by centrifugation and washed again with 500 mL of ethanol. The nearly monodispersed IONs prepared with different solvents have different diameters. As determined by TEM images shown in Figure 1, 8.2, 15.1, and 50.2 nm IONs were synthesized using 1-hexadecene (Acros Organics, 92%), 1-octadecene (Alfa Aesar, 90%), and trioctylamine (Chengdu Kelong Chemical Co., Ltd., 95%), respectively. IONs-n is used to represent IONs with a diameter of n. To incorporate IONs in SBS, SBS and IONs were both dissolved in tetrahydrofuran (THF) and then mixed together. The weight fraction of IONs in the nanocomposites ranged from 3.2% to 25%. The solution of the mixture was then placed in a Petri dish covered with cling film to let THF evaporate slowly. This process usually took one week at room temperature. The resulting sheet of about 1 mm in thickness was then placed in a vacuum oven and thermally annealed at 120 °C for 48 h. Characterization. The phase morphology of SBS and the dispersion state of IONs were observed by a transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN) at an accelerating voltage of 200 kV. Ultrathin sections were prepared using a Leica Ultracut UCT ultramicrotome with a diamond knife at −100 °C. Synchrotron small-angle X-ray scattering (SAXS) measurements were carried out at BL16B1 at the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai, China. The wavelength (λ) of the synchrotron radiation was 0.124 nm. Two-dimensional SAXS patterns were recorded every 60 s by a Mar345 CCD (MAR USA) detector system with a resolution of 2048 × 2048 pixels (pixel size: 79 × 79 μm2). The sample-to-detector distance was 5190 mm. Two dimensional SAXS patterns were background corrected and then 360° azimuthally integrated to obtain one-dimensional scattering intensity curves (I vs q).

SBS with incorporation of 9.1 wt % of IONs-8.2. It can be seen that the nanoparticles are selectively confined in the PB domains. Moreover, most of the nanoparticles are located at the center of the PB domains, in good agreement with the theoretical study of a mean field approach, which predicts the concentration of nanoparticles at the center of the domains for d/l > 0.3.11,22,41 It indicates that organization of nanoparticles in the microphase of the block with no free chain end is similar to that with one free chain end. Due to the magnetic interaction, some of the confined nanoparticles exist in the form of aggregates composed of two or more particles, in addition to the form of a single particle. When the weight fraction of IONs-8.2 is as high as 25.0 wt %, the strong magnetic interaction leads to the formation of some dark strings in which many nanoparticles contact each other (these strings can also be found in SBS containing 9.1% of the nanoparticles, but fewer in number and shorter in length), as shown in Figure 2c. A similar phenomenon has also been observed in previous studies for magnetic nanoparticles.25,26,57 Away from the strings, two types of nanoparticle dispersion can be observed. One type is that IONs-8.2 are still selectively dispersed in the PB domains; however, due to the high loading, many nanoparticles are driven to the interfaces between PS and PB. Thus, it can be observed in Figure 2d that the interfacial curvature and roughness are evidently increased with the addition of 25 wt % IONs-8.2, compared with the rather smooth interfaces in Figure 2b. The other type of nanoparticle dispersion is that IONs-8.2 are randomly and uniformly dispersed in some regions, in which the microphase separation of SBS is suppressed, as highlighted in the enclosed area by the dashed lines in Figure 2c and d. Generally, it is reported that high nanoparticle loading would lead to a disorder structure of microphase separation,28,33,58−61 but the frustration of microphase separation to an amorphous structure is less reported.30,34 Xu et al.30,34 found that the as-cast film of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) filled with PMMA-grafted Fe3O4 nanoparticles showed amorphous structure with random distribution of the nanoparticles, but after thermal annealing both the block copolymer and the nanoparticles assemblied into ordered structure. In the present work, during the preparation process, the solvent is evaporated very slowly for a week and the sample is annealed at 120 °C for 48 h; nevertheless, the amorphous structure still remains. The reasons for this phenomenon are two-fold: (1) addition of IONs-8.2 significantly slows down the microphase separation of SBS, (2) the PB blocks with which IONs-8.2 selectively interact have no free chain ends, thus are slower to form ordered structure. Another important feature is that addition of IONs-8.2 with size smaller than the PB lamellar thickness can change the microphase separation of SBS. As clearly demonstrated in Figure 2c, addition of 25 wt % IONs-8.2 induces an evident morphological transformation from the lamellar structure to a hexagonal cylinder structure, which will be confirmed by SAXS analysis later. A similar phenomenon has been observed in diblock copolymers filled with a certain amount of nanoparticles.22−28,30 To further study the evolution of phase morphology of SBS with addition of IONs-8.2, two-dimensional SAXS measurement was performed. Figure 3a displays the representative twodimensional SAXS patterns of neat SBS and SBS filled with different amounts of IONs-8.2 at room temperature. The concentric, uniform-intensity rings indicate that the solvent-

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RESULTS AND DISCUSSION Nanoparticles with Size Smaller than the Lamellar Thickness of PB Domains. The chemical affinity of the nanoparticles with each of the blocks of SBS can be evaluated with interfacial tension, which is related to the surface tension (γ, including dispersion component, γd, and polar component, γp). γPS = 40.7 dyn/cm2(γdPS = 32.5 dyn/cm2,γpPS = 8.2 dyn/ cm2),γPB = 37.0 dyn/cm2, and γoctadecene = 28.1 dyn/cm2 at 20 °C.25,53 According to the literature,54,55 the polar component contribution to the surface tension of PB and octadecene is negligible. Thus, the values of interfacial tension calculated by harmonic mean equation are γPS/NP = 8.5 dyn/cm2 and γPB/NP = 0.3 dyn/cm2.56 It suggests that the nanoparticles are more miscible with the PB blocks than the PS blocks. Figure 2a shows the cross-sectional TEM image of SBS used in the present work. As can be clearly distinguished from the TEM image, neat SBS has a lamellar structure consisting of alternating dark and bright microphase-separated domains, corresponding to the PB and PS phases, respectively. The lamellar thickness of the PB domains is about 15.7 nm. Thus, the ratio of the diameter of IONs-8.2 to the lamellar thickness of PB (d/l) is about 0.52. Figure 2b shows the TEM image of 2188

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incorporating highly selective solvent swelling one block of a copolymer, which eventually leads to a change in the phase morphology.62,63 Furthermore, as observed in the TEM image of Figure 2d, the addition of 25 wt % IONs-8.2 remarkably increases the interfacial curvature and roughness, which could also induces order−order transitions.27,28 In addition to the change in microphase separation, three other features which are of great interest should be noted in Figure 3b: the relative intensity of the higher order peaks which is determined by long-range order of microphase separation, the full width at half-maximum (fwhm) of the primary peak which is related to the relative local order and defects of the morphology, and the primary peak position (q*) which is defined by the d spacing.45 The intensity of the higher order peaks decreases with the weight fraction of IONs-8.2, which has been observed in the two-dimensional SAXS patterns in Figure 3a, indicating the gradual loss of long-range order of microphase separation. In order to determine fwhm and q*, a nonlinear regression function including a Gauss peak and a background of the following form are used to fit the primary peak, 2 2 A I = y0 + e−2(q − q*) / w + Bq + Cq2 w(π /2) fwhm = w(2 ln 2)1/2. As shown in Figure 4, fwhm monotonously increases with increasing IONs-8.2 weight

Figure 3. (a) Representative two-dimensional SAXS patterns of neat SBS and SBS filled with different amounts of IONs-8.2 at room temperature; (b) SAXS intensity vs scattering vector based on 360° azimuthally integration of 2D images.

casting process results in a random microdomain orientation. On the other hand, the higher-order intensity rings gradually weaken as the weight fraction of IONs-8.2 increases, suggesting the gradual loss of long-range order in SBS. Based on 360° azimuthally integration of 2D images, SAXS profiles of integrated intensity (I) as a function of scattering vector (q) are shown in Figure 3b. It can be seen that neat SBS displays a q/q* ratio of 1:2:3:4, suggesting the lamellar structure of microphase separation. Upon addition of 9.1 wt % IONs-8.2, the lamellar structure of microphase separation is still maintained in SBS, while it transforms into the coexistence of lamellar structure and cylinder structure when the weight fraction of the nanoparticles is 16.7 wt %, because the q/q* ratio shows both 1:2:3 and 1:√3:√7. Nevertheless, only the hexagonal cylinder structure is attained when the weight fraction of IONs-8.2 is up to 25.0 wt %, which is in good agreement with the TEM observation. The transformation in SBS morphology can be attributed to the change in the relative weight fraction of PS. By adding 25.0 wt % of IONs-8.2, the weight fraction of PS is reduced from 31 wt % to 25 wt %. Meanwhile, IONs-8.2 have chemical affinity to the PB phase, thus are selectively confined in the PB microdomains, which should not affect the degree of segregation strength between PB and PS. Thus, the reduction in the weight fraction of PS with addition of the nanoparticles is similar to the case of

Figure 4. Fwhm of the primary peak and d-spacing of SBS as a function of IONs-8.2 weight fraction.

fraction. This could be attributed to the addition of the nanoparticles resulting in variance in domain size, defects at domain boundaries, and interfacial curvature and roughness.45 The domain periodicity, i.e., d spacing, can be calculated from q* by equation q* = 2π/d. It can be seen in Figure 4 that the d spacing increases at first and then decreases with addition of IONs-8.2. This is due to the fact that when the IONs-8.2 weight fraction is lower than 9.1 wt %, SBS retains the lamellar structure, and most of the nanoparticles are selectively confined within the PB domains, which leads to an increase in the thickness of the PB domains. However, when the IONs-8.2 weight fraction is up to 16.7 wt %, the lamellar structure transforms into the cylinder structure; meanwhile, some regions with suppressed microphase separation appears, thus the d spacing decreases. Nanoparticles with Size Comparable to the Lamellar Thickness of PB Domains. IONs-15.1 synthesized with 1octadecene as the solvent have an average diameter of 15.1 nm, which is comparable to the lamellar thickness of PB. It can be 2189

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Figure 5. Cross-sectional TEM images of SBS with addition of (a) 9.1 wt % IONs-15.1, (b) and (c) 25 wt % IONs-15.1 nanoparticles.

seen in Figure 5a and b that IONs-15.1 are not uniformly dispersed in the SBS matrix, but rather aggregate to form clusters of 100 to 300 nm in size at all loadings. A closer observation in Figure 5 reveals that IONs-15.1 are still selectively confined in the PB domains and thus form oriented strings along the PB phase. This phenomenon is inconsistent with previous simulation and experimental results, which reported that nanoparticles were not capable to locate within the domains but rather macroscopically separated from the polymer matrix when the size of the nanoparticles was comparable to the domain size of block copolymers.22,34,60 The simulation of Huh et al.22 found that when the size of nanoparticles became comparable to the radius of gyration of the minority block, the nanoparticles self-assembled inside the copolymer micelles, which was caused by the interplay between the particle−particle excluded volume interactions, preferential particle/block interactions, and the enthalpic and stretching interactions within the diblock copolymers. Lo et al.60 proposed that when the size of nanoparticles was comparable to the domain size, the huge loss of conformational entropy caused by the presence of nanoparticles prevented particles from assembly in the preferred domains. Thus, the system underwent macrophase separation of particles from the polymer phase even with the small addition of particles. However, our result suggests that even IONs-15.1 aggregate to form clusters when their size is comparable to the domain size of the middle blocks of SBS, the chemical affinity still makes the nanoparticles selectively dispersed in the PB domains. This probably can be interpreted in terms of the relatively small conformational entropy of the middle blocks of triblock copolymers compared with that of end blocks, thus incorporation of nanoparticles does not lead to a huge loss of the conformational entropy. Although IONs-15.1 are selectively dispersed in the PB domains, they are randomly distributed both at the interface and in the middle of the PB domains. Therefore, the interfacial curvature and roughness of SBS are significantly increased within the clusters of IONs-15.1. Nevertheless, the microphase separation of SBS retains the lamellar structure. Thus, the nanoparticles with size comparable to the lamellar thickness of PB domains have less influence on the microphase separation of SBS. Two-dimensional SAXS patterns of SBS filled with different amounts of IONs-15.1 are shown in Figure 6a. Evidently, the higher order intensity rings gradually disappear, while the first order intensity ring gradually becomes diffusion-like with addition of the nanoparticles. There are two factors that may lead to this phenomenon. One is that the addition of the nanoparticles disrupts the long-range order of SBS; the other is that the clusters of IONs-15.1 strongly scatter the incident Xray. Figure 6b displays the corresponding integrated intensity as

Figure 6. (a) Representative two-dimensional SAXS patterns of neat SBS and SBS filled with different amounts of IONs-15.1 at room temperature; (b) SAXS intensity vs scattering vector based on 360° azimuthally integration of 2D images.

a function of scattering vector. It can be seen that q/q* ratio is 1:2:3 when the weight fraction of IONs-15.1 is lower than 9.1 wt %. Higher weight fraction of the nanoparticles leads to a broad diffuse-like peak that obscures the higher order intensity peaks, from which we cannot know that what structure is formed. However, the TEM images demonstrate that SBS still has the lamellar structure when the IONs-15.1 weight fraction is up to 25 wt %. Figure 7 presents the variation of fwhm of SBS as a function of IONs-15.1 weight fraction. It can be seen that fwhm increases with addition of the nanoparticles, indicating the increase of local disorder and defects. In the clusters of IONs15.1, PB domains are selectively swelled, leading to an increase 2190

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the minority block self-assemble into a “core”, surrounded by a “shell”, that have chemical affinity with the nanoparticles.22 Another possible reason is that strong magnetic interaction exists between the clusters, which exerts additional force on the SBS matrix during the evaporation of the solvent and the subsequent thermal annealing process, thus leading to the change of the phase morphology. SAXS measurement is also performed on SBS filled with IONs-50.2 nanoparticles. However, due to the strong scattering influence of the clusters, no intensity peaks of SBS can be observed, as shown in Figure 9. Figure 7. Fwhm of the primary peak and d-spacing of SBS as a function of IONs-15.1 weight fraction.

in the lamellar thickness. While out of the clusters, the lamellar of PB still remains its original thickness. Thus, the inhomogeneous dispersion of IONs-15.1 leads to a variation in the lamellar thickness of the PB domains. On the other hand, the domain interfacial curvature and roughness are also significantly increased within the clusters of IONs-15.1, as clearly confirmed by the TEM images in Figure 5. These two phenomena are the major reasons for the increase of local disorder and defects. Figure 7 further illustrates the dependence of d spacing on the IONs-15.1 weight fraction. Although there is no variation trend, all the samples with nanoparticles have larger d spacing than pure SBS due to the selective swelling of the PB domains. Nanoparticles with Size Larger than the Lamellar Thickness of PB Domains. IONs-50.2 synthesized with trioctylamine as the solvent have an average size of 50.2 nm, which is much larger than the lamellar thickness of the PB domains. TEM images in Figure 8 show that IONs-50.2 strongly aggregate to form clusters of hundreds of nanometers to several micrometers. Selective dispersion is not observed due to the fact that the PB domains cannot accommodate nanoparticles with size larger than their lamellar thickness. Nevertheless, it is interesting to note that incorporation 16.7% of IONs-50.2 has significantly changed the microphase separation of SBS from the lamellar structure to a two-phase co-continuous structure. The exact cause leading to this new structure needs to be further explored. We propose that although IONs-50.2 are not confined in the PB domains, the outer interfaces of the clusters have chemical affinity to the PB phase, and thus the SBS molecules need to be rearranged to cover the clusters with PB segments, resulting in the deviation of microphase separation from the equilibrium morphology of SBS. This is similar to the Monte Carlo simulation that nanoparticles with size comparable to the radius of gyration of

Figure 9. Two-dimensional SAXS patterns and integrated intensity of neat SBS and SBS filled with different amounts of IONs-50.2.

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CONCLUSION Oleic acid modified ION nanoparticles with size smaller than, comparable to, and larger than the lamellar thickness of the PB domains of SBS were synthesized and solution mixed with SBS. Figure 10 summarizes the evolution of phase behavior of SBS filled with IONs with different sizes. When d/l ≈ 0.5, IONs can be selectively confined in the middle of the PB domains (Figure 10a), in addition to the formation of strings. Increasing particle loading leads to the dispersion of IONs both in the middle and at the interfaces of the PB domains, which increases interfacial curvature and roughness, and the transformation from lamellar structure to cylinder structure of SBS due to the selective swelling of the PB domains (Figure 10b). Moreover, some other phase behaviors are observed, including ION strings and IONs randomly distributed at some regions where phase separation of SBS is suppressed. When d/l ≈ 1, IONs form clusters even at low particle loading; however, they are still selectively dispersed in the PB domains. The phase separation remains the lamellar structure with increasing IONs loading

Figure 8. Cross-sectional TEM images of SBS with addition of 16.7 wt % IONs-50.2 nanoparticles. 2191

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Figure 10. Schematic presentation of the evolution of phase behavior of SBS filled with IONs with different sizes; the upper insets show the phase separation of SBS.

(Figure 10c). When d/l > 1, IONs cannot be confined in the PB domains, but rather form clusters of several hundreds of nanometers to several micrometers, which intriguingly changes the phase separation of SBS from lamellar structure to twophase co-continuous structure (Figure 10d).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Key Basic Research Program of China (grant no. 2011CB606000). REFERENCES

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