PS Bilayer

Chain diffusion at the polyphenylene oxide/polystyrene (PPO/PS) interface of bilayer films were investigated using nanothermal analysis (nano-TA) and ...
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Direct Measurement of Chain Diffusion at Interfaces of PPO/PS Bilayer Films by Nano-Thermal Analysis and Time-of-Flight Secondary Ion Mass Spectrometry Noriyuki Tanji,† Hui Wu,‡,§ Motoyasu Kobayashi,‡,§ and Atsushi Takahara*,‡,§ †

Kao Corporation, 1334, Minato Wakayama-shi, Wakayama 640-8580, Japan Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § ERATO Takahara Soft Interfaces Project, Japan Science and Technology Agency, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡

S Supporting Information *

ABSTRACT: Chain diffusion at the polyphenylene oxide/polystyrene (PPO/PS) interface of bilayer films were investigated using nanothermal analysis (nano-TA) and time-of-flight secondary ion mass spectrometry (TOF−SIMS). An ultralow-angle slicing technique by the surface and interfacial cutting analysis system (SAICAS) was used to expand the size of the interfacial region effectively. The glass-transition temperature (Tg) at the interface was directly evaluated by nano-TA. The gradient of Tg at the PPO/PS interface became broad after annealing. An increase in annealing time caused an increase in the Tg of the initial PS layer, indicating that the PPO diffused across the interface. Since PPO/PS is a compatible blend system, the local chemical compositions were evaluated by Fox equation. The methodology presented here is useful for interfacial analyses of various polymer composite systems and is beneficial because the isotope labeling is not necessary.

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glassy and rubbery states.11,12 Positron annihilation lifetime spectroscopy (PALS) is also a useful tool for the measurement of Tg based on the properties of free-volume sites in polymer films.13 It is known that the lifetime of ortho-ositronium (o-Ps) is correlated with the size of the free volume hole. The free volume played a important role in the volume expansion in the glassy state. Tg is estimated from the free volume in temperature dependence and a bending point at the same temperature as Tg. Scanning viscoelasticity microscopy (SVM) and lateral force microscopic (LFM) measurements were applied to examine the surface Tg of the polymer films.14−16 SVM can be used to directly identify viscoelastic properties on solid surfaces by measuring an amplitude change in response force, and detecting a phase lag between a stimulus displacement and a response force. The glassy and rubbery states at the polymer film surface were observed by temperature dependence SVM measurement of the surface phase. An onset temperature at the change of the viscoelastic property is defined as Tg. LFM is gained the information on lateral force between

INTRODUCTION The interface of the composite polymer plays a critical role for practical application in material sciences, such as adhesiveness, stiffness, and elasticity. The performances of these applications are closely related to the interfacial structures that are formed via interchain penetration during the diffusion process. The physical properties, such as thickness, chemical composition, and morphology, corresponding to the polymer interface miscibility, have been studied using various techniques such as ellipsometry,1 neutron reflectivity (NR),2,3 nuclear magnetic resonance spectroscopy (NMR),4,5 infrared spectroscopy (IR),6 Raman spectroscopy,7 secondary ion mass spectrometry (SIMS),2,8 atomic force microscopy (AFM),9 and transmission electron microscopy (TEM).10 Thermal properties are also important for understanding the polymer interface miscibility. However, to obtain the local information on the interface of the composite polymer is difficult, especially in the interface with finite thickness. The thermal properties of the thin polymer film have been characterized by several different techniques. The variable temperature ellipsometry and X-ray reflectivity has been widely used for measuring the glass transition temperature (Tg) in thin polymer films by detecting the film thickness based on volumetric changes in temperature dependence between the © 2013 American Chemical Society

Received: August 14, 2013 Revised: November 10, 2013 Published: December 5, 2013 9722

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the sample surface and a probe tip by detecting the torsion of the sliding cantilever. Lateral force is assumed to come from the energy dissipation of molecular movement. It becomes possible to examine the relationship the surface molecular motion and the scanning rate dependence of the lateral force. On the basis of detecting the lateral force in temperature dependence, the film surface of Tg was found to be much lower than the bulk Tg,. Recently, a local thermal analysis technique (nano-TA) utilizing scanning force microscopy was developed.17 This technique is an analogy of mesoscopic thermo-mechanical analysis of which penetration depth of loaded needle is measured as a function of temperature. This technique is performed by a specially designed probe to contact with the sample surface, heating the end of the cantilever, and measuring its deflection using the standard beam detection of AFM. At the Tg or melting temperature (Tm), the polymer sample surface is soften, allowing the cantilever with small normal load to indent the surface of the sample and enabling the probe to penetrate the sample and decrease the deflection of the cantilever. The change in slope of the deflection signal is an indication of a thermal transition. Nano-TA have been applied a variety of the polymeric materials17,18 to identify the domains formed in immiscible polymer blend film and blend distribution within just a few minutes. Nano-TA also allows in situ thermal property measurements on the cross section of multilayer films. This enables the identification of each layer, as well as the identification of individual defects within any layer. Additionally, the individual films can be mapped to detect the possible presence of transition temperature gradients or in homogeneities. Upon penetration of the probe, heat generated from the cantilever was conducted to a area surrounding the contact point of the probe.19 Therefore, direct analysis of polymer blend films, having an interface thickness on the nanometer scale, has not been conducted because of the difficulties in characterizing the nanometer range. In this study, we conducted nano thermal analysis by nanoTA combined with time-of-flight secondary ion mass spectrometry (TOF−SIMS) to measure the diffusion of the polymer bilayer interface. The surface and interfacial cutting analysis system (SAICAS) was used to prepare the sample for local thermal analysis of the bilayer polymer interface.20 SAICAS has great potential for use in the characterization of the cutting force of films. This technique is also capable of precise ultralow-angle slicing.21,22 A miscible pairs of PPO/PS laminates was used because PS was compatible with PPO on the segmental level through the interaction between the electrodeficient methyl groups of PPO and the π-orbitals of PS.23−27 The PS/PPO blend system is known that is miscible in all proportions and exhibits a single Tg when examined by a variety of techniques including DSC, dynamic mechanical relaxation, and the measurement of dielectric properties.11,28 The Tg of PS and PPO is 401 and 490 K, respectively, from the change in slope of the deflection signals of nano-TA. The difference in the Tg between these two polymers is large enough to be distinguished by nano-TA.29 With the highresolution of these two techniques for local property measurement, characteristics of the interface chain diffusion were investigated using polymer bilayer films annealed at 453 K, which is higher than the Tg of PS and lower than that of PPO.

Article

EXPERIMENTAL SECTION

Sample Preparation. Poly(2,6-dimethy1-1,4-phenylene oxide) (PPO, Mn = 1.57 × 104, Mw/Mn = 2.32, Aldrich Chemical Co.) and polystyrene (PS, Mn = 3.66 × 104, Mw/Mn = 1.08, Polymer Science Co.) were used as received. The PPO films were prepared by spincoating 5 wt % chloroform solution onto a silicon wafer, while the PS films were obtained by spin-coating onto a polyimide (PI) surface with 5 wt % chloroform solution. To prepare the bilayer of PPO/PS, the PPO films were peeled from the substrate of silicon wafer by immersing into water and floated on water surface, and then placed it on the top of PS film supported by polyimide (PI) film.1 To remove the water trapped within samples, bilayer films were dried at 333 K for 24 h under vacuum. For diffusion studies, bilayer films were annealed at 453 K under nitrogen for various times. The sliced samples with an ultralow angle were prepared using a surface and interfacial cutting analysis system (SAICAS, Daipla Wintes Co., Ltd.) (Figure 1a). A diamond knife with a width of 0.1 mm, angle

Figure 1. Schematic illustration of (a) cutting bilayer films with SAICAS and (b) nanothermal analyses at the interface of the polymer bilayer film. of 40°, and inclination angle 10° is precisely moved by piezoelectric actuators. The ultralow-angle slicing is achieved by controlling the velocities of vertical and the horizontal movements of the cutting knife. A constant-velocity shaving mode with vertical velocity of 50 nm/s and the horizontal velocity of 500 nm/s was used. The blade tilt of the cutting knife was keeping at 45° in the slicing direction. The bilayer film was cut into a size of 1 mm × 5 mm, and was embedded in visible light curing resin. For comparison, another sample with vertical cross section was prepared using a microtome (Leica Microsystems). Nano-TA. A nano-TA add-on (Anasys Instruments) combined with E-sweep AFM instruments (SII Co. Ltd.) was used to characterize the bilayer interface. An AN-2 silicon thermal probe was used (spring constant: 1.0 N/m, resonance frequency: 59 kHz). Local thermal analysis measurements were obtained using a temperature ramp of 1 K/s from 293 K up to the penetration temperature under vacuum (below 5.0 × 10−4 Pa). Temperature was calibrated using three semicrystalline polymers with known melting temperature at 333 (polycaprolactone), 403 (polyethylene), and 511 K (polyethylene terephthalate). AFM topographic and amplitude images with area of 30 μm2 were obtained in tapping mode before and after the local thermal analysis. TOF−SIMS. Time-of-flight secondary ion mass spectrometry (TOF−SIMS) measurements were performed using the TOF−SIMS IV instrument (ION-TOF GmbH) to visualize the distribution of PPO and PS on the interface. The pulse primary ion used was 209Bi32+ (pulse width time of 100 ns, pulse interval of 250 μs) with an accelerating voltage of 25 kV. The measurements were obtained by prioritizing high spatial resolution. The detection area was 30 μm × 30 μm with image pixels of 256 × 256. Negative ion results were acquired by scanning for 64 times. An electron flood gun was used for charge correction in the measurements of insulation samples.

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RESULTS AND DISCUSSION Comparison of Ultralow-Angle Slicing with Microtomed Cross Sections. Enhancement of spatial resolution was investigated by combining nano-TA and the ultralow-angle slicing system technique. To compare the sample prepared with 9723

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Figure 2. (a, b) Stereoscopic AFM topographic images, (c, d) temperature dependence curves of probe penetration depth measured by nano-TA and (e, f) Tg depth profiles by nano-TA of PPO/PS bilayer film cross sections (a, c, e) ultralow-angle slicing and (b, d, f) conventional microtomed specimen.

Local Thermal Analysis of PPO/PS Bilayer Film. To investigate the molecular diffusion at the interfaces, the PPO/ PS bilayer films annealed for various time were examined by nano-TA. Figure 3 shows AFM amplitude images obtained in

ultralow-angle slicing, a sample with vertical cross section was prepared using a microtome. All the samples were annealed at 433 K for 1 min. Figure 2a shows a stereoscopic AFM topographic image of an ultralow-angle cross section of the PPO/PS bilayer film. The right side is the PPO surface. A cross section of the polyimide substrate was also observed at the left side. The area along the arrow direction on the top is the ultralow-angle slicing cross section. An interface was clearly observed between the PPO and PS layers. Local thermal analysis in the depth direction on these cross sections was determined by nano-TA. Figure 2b shows vertical cross section was prepared by conventional microtoming. The boundary line between the surface and cross section of the PPO phase was set to the zero position for the depth analysis. Measurements were obtained with a nano-TA system at submicrometer intervals (0.5−1 μm) toward the depth direction of the PPO/PS bilayer film represented by the arrow. Parts c and d of Figure 2 show temperature dependence curves of probe penetration depth on these cross sections measured by nano-TA. Tg were evaluated from the change in slope of the deflection signals, which is shown in Figure 2e and 2f, respectively. Only a few points between the surface and back on vertical cross sections prepared with a microtome can be measured. However, more than 20 points can be measured on the ultralow-angle slicing cross section. Therefore, the ultralowangle slicing technique is capable of enlarge the apparent width of the cross section. The difference in Tg obtained with depth of the PPO/PS bilayer film was analyzed with higher resolution than that of the vertical cross section. These results revealed that the ultralow-angle slicing technique were more useful for obtaining high-resolution local information on the polymer interface.

Figure 3. AFM amplitude images of ultralow-angle slicing cross sections of PPO/PS bilayer films annealed at 453 K for (a) 0, (b) 1, (c) 10, (d) 20, and (d) 180 min.

tapping mode, in which the surface and cross section area of each phase can be clearly observed. Before annealing, the PPO and PS phases have the same thicknesses of 7 μm. After the samples were annealed at 453 K for 1 min, the thickness of the PS-rich phase increased, while that of PPO-rich layer decreased. The PPO/PS interface disappeared completely after 3 h. The growth of the interfacial layer with annealing time can be clearly observed. On these cross sections, the composition variations across the interfaces in the depth direction were determined by nano-TA. Figure 4 shows Tg depth profiles of the annealed ultralowangle slicing cross sections at various time as shown in Figure 3. The boundary line between the surface and cross section of the PPO phase was set to the zero position for the depth analysis. Measurements were taken at submicrometer intervals (500 nm to 1 μm) toward the depth direction of the PPO/PS bilayer film represented by an arrow in the AFM amplitude image. The axis of film depth in Figure 4 is the vertical depth of PPO/PS 9724

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Figure 5. PPO weight fraction dependency of Tgs of homogeneous PPO/PS blend films. Figure 4. Tg and depth profiles of PPO weight fraction on the ultralow-angle slicing cross-section of PPO/PS bilayer films before and after annealing at 453 K (●, before annealing; ⧫, after annealing for 1 min; ■, after annealing for 10 min; ◇, after annealing for 20 min; □, after annealing for 3 h). Three arrows with dot line indicate the bondary front at the PS/PPO interface after annealing process.

and pure PS, wPPO and wPS are the weight fractions of PPO and PS. Our results show that the measured Tgs of homogeneous PPO/PS blend films are in good agreement with the Fox equation. It is known that the Tg of thin film measured by DSC shows a similar value with the Tg calculated from the Fox equation.11 Figure 5 indicate that the Tg measured by nano-TA also is related to the Fox equation. The Fox equation is not applied to the case of film prepared the thickness under 100 nm, because the interaction between polymers and substrates is strongly reflected. In this study, the Fox equation can be applied to the estimation of the composition, because bilayer films had a sufficient thickness. Based on the Fox equation, the weight fraction of PPO calculated from Tg depth profiles on the cross section of PPO/PS interface was shown in the right vertical axis of Figure 4. The PPO and PS chain interdiffused at the interface to give gradient PPO content in the interface. In addition, the content of PPO apparently increased at the deep layer of the film with the annealing time. Calculations from amplified magnification of the slicing surface showed that PPO molecules diffused into the PS phase a few micrometers even after 1 min at 453 K. Distribution of the glass transition temperature was observed at the interface between PPO and PS in the range of 483 to 450 K. The interface movement pointed to a Fickian diffusion characteristics, although they do tend to show some non-Fickian (case II diffusion) characteristics.33 It is known that the displacement of the PPO/PS interface toward the PPO-rich phase is observed to scale at t1/2.30 The diffusion coefficient was calculated from the annealing time variation of the interfacial displacement1

bilayer films by calculation based on the amplified magnification of the ultralow-angle slicing and the nano-TA measurement distance on AFM images. The depth profile of the interface was symmetric while the Tg profile showed a very sharp transition from the PPO Tg value to the PS value before annealing. The profile gradient changed gradually as annealing proceeded. In addition, Tg values for the PS-rich phase increased with annealing time and temperature. However, Tg values for the PPO-rich phase did not vary with annealing, indicating that molecular diffusion across the interface of PPO molecules into highly mobile low Tg PS phase after annealing. This is because the annealing temperature in our study at 453 K is higher than the Tg of PS and lower than that of PPO. The PS is in the melt state while the PPO is still in the glassy state. The migration of PPO into PS blow the Tg has been observed previously.30 Because PS and PPO are miscible, low-Tg component (PS) can penetrates into the high-Tg component (PPO). This diffusion should not be Fickian, rather it should be similar to case II diffusion which is typical for a good solvent penetrating into a glassy polymer. Dissolution of the high-Tg component should occur rapidly after penetration. As a result, a front of the interface moved from the PS side into PPO side and eventually vanished 3 h later. Migration of component below the Tg was also observed in a PS/poly(vinylmethyl ether) blend using neutron reflectivity measurement.31 Nano-TA analysis combined with the ultralow-slicing technique was capable of determining the local thermal properties at the polymer−polymer interface. The Tg value at each position was affected by the PPO/PS composition ratio. To quantify the composition ratio in the interface area, the Tg values for the homogeneous PPO/PS blend films varying PPO weight ratios from 0 wt % to 100 wt % in steps of 10 wt % were measured by nano-TA. Figure 5 shows a relation between Tgs of PPO/PS blend films and the weight fractions of PPO. The relation between the Tg values and the composition ratio was evaluated using the Fox equation.11,32 This equation represents the dependence of the glass transition on the composition of copolymer or polymer blend. w w 1 = PPO + PS Tg Tg,PPO Tg,PS (1)

λ = 2 Dt

(2)

where λ is the interfacial displacement at the interface of PS phase side represented by black dot arrows in Figure 4. The change in slope was observed at this point in PPO weight fraction depth profile. D is the diffusion coefficient and t is the annealing time. D can estimate from the linear plot of λ versus t1/2 . Figure 6 shows the time variation of the interfacial displacement. D was evaluated to be 2.6 × 10−13 cm2/s. For PPO/PS blend annealed at 453 K, typical diffusion coefficient at the low PPO volume fraction area was evaluated to be the range from 5.0 × 10−14 cm2/s to 7.0 × 10−13 cm2/s.30,34 The evaluated value from eq 2 is in good agreement with the typical diffusion coefficient of PPO/PS blend, although eq 2 is for the case of the simplest polymer mixing condition.35 The diffusion coefficient of identical PS/PS at 447 K is in the range of 10−11 to 10−13 cm2/s,36 which is much higher than the PS/PPO case. In general, the interdiffusion between two miscible polymers, such as PS/PPO and poly(vinyl chloride)/ poly(ε-caprolactone) (PVC/PCL),37 proceed faster compared

Here Tg is the glass transition temperature of the PPO/PS homogeneous blends, Tg,PPO and Tg,PS are the Tg of pure PPO 9725

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analysis at the PVDF/PMMA mixing interface, however, it would be hard to determine the interfacial composition of SAN and PMMA, because their Tg are very close. TOF−SIMS Measurements of PPO/PS Bilayer Film. To further verify molecular diffusion in the PPO/PS bilayer film after annealing, TOF−SIMS measurements were carried out to determine the composition distribution along the thickness direction. Each film produced negative ion sputter on the film surface when the primary ion was irradiated. The mass spectrum of the PS film contained the characteristic mass/ charge (m/e) number 25, while the PPO film spectrum contained the characteristic mass/charge number 136, representing C2H and C8H8O2 ion, respectively (Figure S1). The CN ion (mass/charge number 26) was also detected from the polyimide films used as the indication of substrate of bilayer films. Therefore, these ions were chosen to analyze the composition distribution in the bilayer film. Parts a−c of Figure 7 show the secondary ion distribution images of PPO, PS, and polyimide at the bilayer interface shaved by SAICAS. The red-, green-, and blue- colored dots indicate the secondary ion distribution being specific to PPO, PS, and polyimide, respectively. Parts d−f of Figure 7 represent the mass spectra obtained at PS and PPO domains and PPO/PS interface area before and after annealing for 1 and 20 min at 453 K. The brightness of the dots represents ion intensity. These results indicate that the thickness of the PS phases increased with annealing time, which is in accordance with AFM changes as shown in Figure 3. Mass spectra show ion intensities of C8H8O2 ion in numbered points on the TOF−SIMS ion images. In addition, parts a−c of Figure 8 show linear profiles of secondary ion intensity obtained from the TOF−SIMS ion images. Ion intensities of C2H (m/z = 25), C8H8O2 (m/z = 136), and CN (m/z = 26) are mainly corresponding to PPO, PS, and polyimide substrates, respectively. PPO ion intensity in the PSrich phase (distance from 10 to 20 μm) increased with annealing time, which indicates that PPO molecules diffuse to the PS phase upon annealing. It seems that the PS molecules are also diffused into the PI and PPO phases from the ion profiles. C2H ion is the most characteristic ion to PS in

Figure 6. Relation of the interfacial displacement (λ) and the annealing time (t) at 453 K.

to the self-diffusion of identical polymers because of attractive interactions between different segments according to the Flory−Huggins theory for the free energy of mixing of a polymer blend.35 However, such a thermodynamic acceleration due to negative Flory−Huggins parameters (χ) are usually exhibited by polymer diffusion above Tg. In this paper, interdiffusion experiment of PS/PPO was performed at 453 K, which is higher temperature than Tg of PS but lower than Tg of PPO. Therefore, the obtained diffusion coefficient of PS/ PPO was lower than that of identical PS/PS system. The diffusion coefficient at the interface between other miscible polymer systems have been reported. For instance, Wu et al. estimated the diffusion coefficient of poly(methyl methacrylate) (PMMA) at the interface of poly(vinylidene fluoride) (PVDF)/PMMA to be 3.42 × 10−10 cm2/s at 463 K,38 by transmission electron microscopic (TEM) observation at the interface assuming the case II diffusion. In the case of poly(styrene-co-acrylonitrile) (SAN)/ PMMA, diffusion coefficient of PMMA was 1.3 × 10−13 cm2/s at 433 K by the energyfiltering TEM.39 These diffusion experiment were carried out much higher temperature than Tg of SAN and PMMA at 371 and 378 K, respectively, and the crystal melting temperature of PVDF at 445 K. Therefore, these diffusion coefficient values are higher than that of PS/PPO at 453 K. In addition, interdiffusion of SAN/PMMA were accelerated due to a large negative χ parameter (−0.38). Nano-TA technique would be useful for the

Figure 7. TOF−SIMS images of PPO/PS bilayer films annealed at 453 K for (a) 0 min, (b) 1 min, and (c) 20 min. (green dot, m/z = 25 (C2H; PS); red dot, m/z = 136 (C8H8O2; PPO); blue dot, m/z = 26 (CN; PI)). (d−f) TOF−SIMS mass spectra of PS and PPO phase and PPO/PS interface area at (1−9) in the images a−c. 9726

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Figure 8. Ion intensity profiles of TOF−SIMS at PPO/PS bilayer interfaces annealed at 453 K for (a) 0, (b) 1, and (c) 20 min. (black soild line, m/z = 136 (C8H8O2; PPO); gray solid line, m/z = 25 (C2H; PS); black dot line, m/z = 26 (CN; PI)). The profiles were obtained along the orange horizontal arrows in Figure 7a−c.

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comparison with mass spectra of PPO, PS and PI phases. However, C2H ions are generated from PPO and PI molecules as well as PS. Therefore, depth profile of C2H ion does not represent the accurate PS distribution directly. The C2H ion profile are consisting of the both elements from PS and PPO. In the strict sense, PS distribution at the interface cannot described simply by the depth profile of C2H fragment. As shown in Figure 7, the increase of interface thickness and change in Tg of the annealed sample were consistent with the predicted diffusion of PPO and PS molecules by nano-TA. The PPO/PS blend is miscible when the annealing temperature is higher than the individual Tg values. The annealing of polymer bilayer films was conducted at 453 K which is are higher temperature than the Tg of PS and lower than that of PPO. Therefore, PPO molecules are in a glassy state and PS molecules exist in a rubbery condition under these annealing conditions. PPO molecules at the interface of the bilayer films were plasticized by the PS molecules under these conditions, followed by diffusion of PPO chains into the PS phase across the PPO/PS interface. In contrast, PS chains were not able to diffuse into the PPO across the interface because the PPO phase was in a glass form. Therefore, the PPO phase remained undiluted with rubbery PS molecules and stayed at a constant PPO concentration. Mixing of PPO and proceeded with an increase in annealing temperature or time. At 3-h annealing time, each phase was completely miscible. This observation is in good agreement with results reported previously on element concentration profiles using dynamic SIMS.8

Corresponding Author

*E-mail: (A.T.) [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS Chain diffusion at the interface of PPO/PS bilayer films was studied by nanothermal analysis and time-of-flight secondary ion mass spectrometry. The ultralow-angle slicing technique was useful for the sample preparation of polymer interfaces. The depth distribution of the Tg broadened and the Tg value for the PS-rich phase increased after annealing. An increase in annealing time increased the Tg value in the PS layer, indicating that PPO molecules diffused into the PS-rich phase across the interface between the glassy PPO phase and rubbery PS phase. The difference between the Tg value and PPO/PS blend ratio was described by the Fox equation. We suppose that the thermal probe technique is useful for determining the local chemical compositions as well as the thermal properties of the miscible bilayer interface.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

TOF−SIMS mass spectra of PPO and PS monolayer films. This material is available free of charge via the Internet at http:// pubs.acs.org. 9727

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