Direct Evaluation of Local Dynamic Viscoelastic Properties of Isotactic

Letter Cite This: ACS Macro Lett. 2019, 8, 218−222

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Direct Evaluation of Local Dynamic Viscoelastic Properties of Isotactic Polypropylene Films Based on a Dynamic μ‑Beam X‑ray Diffraction Method Shuhei Nozaki,† Shiori Masuda,† Chao-Hung Cheng,† Chigusa Nagano,† Kazutoshi Yokomachi,‡ Kazutaka Kamitani,‡ Kohki Aoyama,∥ Hiroyasu Masunaga,∥ Ken Kojio,*,†,‡,§ and Atsushi Takahara*,†,‡,§ Graduate School of Engineering, ‡Institute for Materials Chemistry and Engineering, and §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan ∥ Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo 679-5198, Japan ACS Macro Lett. Downloaded from pubs.acs.org by TULANE UNIV on 02/07/19. For personal use only.

†

S Supporting Information *

ABSTRACT: The local mechanical properties of crystalline polymer were evaluated using synchrotron radiation X-ray diffraction with 10 μm lateral resolution. A nonoriented isotactic polypropylene (iPP) film with isolated spherulites in a crystallized matrix was used as a model sample. In situ wide-angle X-ray diffraction (WAXD) measurement was performed on the iPP film using a microbeam synchrotron radiation X-ray under sinusoidal strain. The lattice spacing of the crystal planes increased and decreased in response to the applied sinusoidal strain. Local dynamic viscoelastic functions (dynamic storage and loss moduli (E′ and E″)) were calculated at room temperature from the relationship between the calculated applied stress and the response strain obtained by dynamic μ-beam WAXD measurement inside and outside of the spherulites. The E′ values inside and outside of spherulite obtained from the change in spacing of the (110) plane were 1.8 and 1.1 GPa, respectively. Furthermore, the E′ value inside of spherulite obtained from the change in spacing of the (1̅13) plane was 6.0 GPa. These values can be explained by the deformation of crystallite, which depends on the direction of crystal planes. The results obtained here revealed that synchrotron radiation X-ray diffraction measurement gives not only structural information but also the local mechanical properties of the materials E′.

I

allowing time-resolved measurement in a local area. Hence, synchrotron radiation X-ray diffraction/scattering measurements have been used for structure analysis under external stimuli. Nozue et al. reported on the deformation behavior of isotactic polypropylene (iPP) spherulite during a uniaxial elongation process.10 They investigated local structural changes, e.g., in ordered crystal size and orientation, and the lamella stacking structure of parent and daughter lamellae. Hammond et al.11 and the authors12,13 reported on the microphase-separated structural changes and orientation behavior of soft segment chains of polyurethane elastomers during uniaxial elongation. Synchrotron radiation X-rays have provided important insights on various structural changes resulting from an external stimulus. If the time-resolved measurement and local area measurement are combined properly while taking rheo-optics into consideration, it is expected that a new measurement technique can be created. Therefore, we aimed to acquire information about the response strain from changes in the lattice

t is important to understand hierarchical structures of polymers, including the crystal lattice, amorphous chains surrounding crystallites, accumulated lamella structure, and so on, for the development of high-performance polymer materials. These structures need to be controlled to produce various properties. In situ structure analysis has been performed under various external stimuli, including mechanical deformation, temperature change, atmosphere, and pressure. Rheo-optics is one of the most powerful in situ structural analytical methods.1−9 In rheo-optical measurements, X-ray scattering/ diffraction measurements, infrared/Raman spectroscopic measurements, and polarized optical microscopic observation are simultaneously performed while rheological measurements are performed. As the result, rheo-optics connects molecular aggregation structure, molecular motion, and mechanical properties, which are important to investigate molecular aggregation structures. The operando X-ray scattering/spectroscopic technique has achieved remarkable progress based on the development of synchrotron radiation facilities, because synchrotron radiation X-ray has high flux, high brightness, and a highly collimated Xray beam with a small size (on the order of several micrometers), © XXXX American Chemical Society

Received: December 22, 2018 Accepted: January 30, 2019

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DOI: 10.1021/acsmacrolett.8b00994 ACS Macro Lett. 2019, 8, 218−222

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ACS Macro Letters

could be accomplished, then the local mechanical properties at each phase in various phase-separated systems could be calculated based on rheological analysis. In other words, a local viscoelastic function with a size of several micrometers could be estimated. This information would be quite useful for material design. This is the usage of synchrotron X-ray diffraction/scattering measurements for the investigation of not only structural information but also local mechanical properties. In this study, μ-beam wide-angle X-ray diffraction (WAXD) measurement was performed for crystalline polymers while sinusoidal mechanical deformation was applied. The sample used in this study was an iPP film with an isolated spherulite with 100 μm in diameter surrounding unoriented crystallites. Then, dynamic storage and loss moduli (E′ and E″) were calculated at each region. Figure 1 (a) shows a polarized optical microscopic (POM) image of the iPP film with a sensitive tint plate. Isolated spherulites with a diameter of ca. 100 μm were observed. The region surrounding spherulites seemed to be in a crystalline state, because some organized structure with red and blue color can be observed due to birefringence of the crystallized molecules. Figure 1 (b−d) shows WAXD patterns outside and inside the spherulites, respectively, as indicated in Figure 1 (a). The brightness of the right side was enhanced to compensate for the weak (1̅13) diffraction. WAXD patterns outside and inside the spherulites exhibited a crystalline Debye ring and arcs, respectively. Figure 1 (e,f) shows WAXD profiles obtained from Figure 1 (b,c). Crystalline diffraction peaks were clearly observed at q = 10.1, 12.1, 13.3, 15.2, 15.7, and 32.7 nm−1. These peaks can be assigned to (110), (040), (130), (111), (131̅)/(041), and (1̅13) planes of the α-form of iPP, indicating that both the outside and inside of spherulite are in α-form crystals.14 Moreover, the fact that the crystalline Debye ring and arcs were observed outside and inside of spherulite implies that the sizes of crystallites in each region were smaller and larger, respectively, than the beam size (5 × 5 μm2) used in this study. To obtain Young’s modulus and the stress region in which a linear response behavior could be observed for the iPP film, uniaxial tensile testing was carried out. The stress−strain curve of the iPP film is shown in Figure S1. Young’s modulus of the iPP film was 643 MPa, and the linear response region was estimated to be a strain of 0.01. Therefore, the maximum imposed stress was set to be less than 0.01 during dynamic μ-beam WAXD measurements. Figure 2 shows WAXD profiles for (a) (110), (040), and (b) (1̅13) planes along the cyclic deformation direction for the inside of spherulites in the iPP film under sinusoidal mechanical deformation at 0.1 Hz. A video of the 2D pattern of the iPP film during dynamic μ-beam WAXD measurement is shown in the Supporting Information. The diffraction peak positions shifted to a larger q region when the strain was increased, and the peaks returned to their original positions when the strain was released. The magnitudes of peak shifts of various crystal planes were different: the magnitudes of shift of the (110) and (040) planes seemed to be larger than for the (1̅13) plane. Peak position was determined using curve fitting with Gaussian functions. Then, the response local strain (εr(t)) was calculated using eq 1

Figure 1. (a) Polarized optical microscopic (POM) image of the iPP film with a sensitive tint plate. WAXD patterns (b) outside and (c,d) inside of the iPP film spherulite. WAXD profiles of (e) outside and (f) inside of the iPP film spherulite obtained from Figure 1 (b,c). The brightness of the right side was enhanced for weak (1̅13) diffraction.

Figure 2. WAXD profiles inside of the spherulites in the iPP film at the elongation direction in the q range from (a) 9 to 13 and (b) 28 to 33.5 nm−1 at 25 °C.

εr(t ) = (dhkl(t ) − dhkl(0))/dhkl(0)

strain of crystalline polymers under the application of sinusoidal strain at different polymer phases, such as an isolated spherulite region and the surrounding matrix, using μ-beam X-rays. If this

(1)

where dhkl(t) and dhkl(0) are the spacings of the (hkl) plane at a certain time and the initial state, respectively. 219

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Figure 3. (a) Response strain for (110), (040), and (1̅13) planes of outside and inside of spherulites for the iPP film at 25 °C and (b) Lissajous curves obtained from (a).

Table 1. Local E′ and E″ Values for Inside and Outside of Spherulite and Macroscopic E′ and E″ Values of the iPP Film at 25 °C local E′ / GPa

local tan δ

local E″ / GPa

position

(110)

(040)

(1̅13)

(110)

(040)

(1̅13)

(110)

(040)

(1̅13)

macroscopic E′a/GPa

macroscopic E″a /GPa

macroscopic tan δa

outside inside-1 inside-2

1.1 1.8 1.6

1.2 1.8

6.0 6.1 6.0

0.34 0.23 0.32

0.37 0.29

0.18 0.13 0.13

0.31 0.13 0.20

0.32 0.16

0.031 0.022 0.022

1.0

0.049

0.047

a

Measured by RHEOVIBRON.

Figure 3 shows the time dependence of εr(t) for the (110), (040), and (1̅ 1 3) planes under sinusoidal mechanical deformation at 0.1 Hz and 25 °C. All response strains were clearly retarded for imposed stress because of the viscoelastic properties of the iPP film. Additionally, the response strains varied depending on crystal planes. Response strains obtained from the (110) and (040) planes were much larger than those for the (1̅13) plane. These trends can be easily seen in Lissajous curves shown in Figure 3 (b). Furthermore, the response strains of outside of the spherulites were obviously larger than for inside the spherulites. These results may be due to anisotropy of elasticity arising from the molecular orientation and ordering of the crystal structure in the iPP film. Dynamic viscoelastic functions (E′ and E″) were calculated using equations and the response strain data shown in Figure 3. A schematic illustration and equations for calculation are provided in Figure S3. Table 1 shows the local E′ and E″ values obtained from various crystal planes inside and outside spherulites of the iPP film at 25 °C obtained by the dynamic μ-beam WAXD method. The macroscopic E′ value obtained by RHEOVIBRON is listed as well. The abbreviations denote the position (outside or inside of spherulites) and crystal planes (E′out‑(110) denotes the E′ value obtained from the (110) plane outside a spherulite). First, all local E′ values obtained by the dynamic WAXD were greater than macroscopic E′ values. This is because the local E′ value was associated only with the crystalline region, whereas the macroscopic E′ value was based on both crystalline and amorphous regions. Second, local E′out values obtained from all crystal planes were lower than for local E′in values. As shown in Figure 1 (e) and (f), the peak width outside the spherulites was larger than for inside. Therefore, the ordering of the crystal structure outside the spherulites is lower than inside the spherulites, resulting in a lower local E′out value. This trend corresponds well with the opposite magnitude relationships of the E″ values. Third, the local E′(1̅13) values were much larger than the local E′(110) and E′(040) values for both outside and

inside of the spherulite. The deformation of (110) and (040) planes is governed by van der Waals interaction between molecular chains, while the (1̅13) plane is deformed with the change in the bond angle of the main chains. Because the energy required for the latter deformation is larger than the former, the local E′(1̅13) value was larger. Sakurada et al. and Nakamae et al. reported on the elastic modulus of highly oriented iPP filaments and fibers in directions both parallel and perpendicular to the molecular chain axis (Ep and Et).15−17 They obtained the moduli using WAXD measurements while applying a constant load, that is, the crystalline Young’s moduli. The Ep and Et values obtained were 41 and 2.8 GPa, respectively. Our data corresponding to Ep and Et were 6.1 and 1.8 GPa and were smaller than Ep and Et, respectively. Both Sakurada’s and our measurements focused only on the crystalline region of the film. The specimens used in Sakurada’s experiment were highly oriented, whereas our specimens were isothermally crystallized. Therefore, it is conceivable that the ordering of the crystal structure at the crystalline region in our iPP films was lower than in Sakurada’s sample. Therefore, the local E′in‑(1̅13), E′in‑(110), and E′in‑(040) values were smaller than Ep and Et. Furthermore, the Ep and Et values were ca. 6 and 1.4 times larger than the corresponding local E′in values. This difference may be because ordering of the crystal structure strongly affects molecular stiffness. The mechanical behavior of phase-separated polymer blends, blocks, and their interpenetrating network can be represented using the Takayanagi model.18 Frequently, one phase is elastomeric, and the other is plastic. Arrays of amorphous and crystal phases are indicated in this model (see Figure S3). The quantities, λ and φ, or their indicated multiplications, indicate the volume fractions of the materials. Because φ tends to be related to the degree of crystallinity (χc) of iPP,19 the χc values calculated from DSC measurement were applied for φ. λ was substituted by 0.7, which is obtained by RHEOBIVRON.19 All parameters and obtained E′ values are listed in Table S3. The E′ value of the iPP film obtained by a combination of dynamic μ220

DOI: 10.1021/acsmacrolett.8b00994 ACS Macro Lett. 2019, 8, 218−222

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(molten phase), and then quenched to 20 °C. As a result, the iPP film with isolated spherulites and a surrounding crystallized matrix without the organized structure, was obtained. The crystallinity and melting point of the iPP film were 43.6% and 165.2 °C by differential scanning calorimetry. The thickness of the iPP films was ca. 70 μm, which is comparable with the size of spherulites. To observe the morphology of the prepared iPP film, polarized optical microscopic observation (POM) was performed using a POM (Nikon, Ltd.) with a sensitive tint plate and two polarizers. The dynamic viscoelastic properties of the bulk iPP film were evaluated using RHEOVIBRON (ORIENTEC Ltd., DDV-01FP) under an N2 atmosphere. The heating rate was 2 °C/min, and the temperature range was from 25 to 200 °C. The sample size was 20 mm (length) × 5 mm (width) × 0.07 mm (thickness). The strain amplitude and frequency were 0.2% and 0.1 Hz, respectively. Dynamic μ-beamWAXD measurements were performed at 25 °C for the inside and outside of spherulites of the iPP film during an imposed dynamic strain amplitude (εd) of 0.005 at 0.1 Hz by a homemade stretcher, as shown in Figure 4. WAXD data were collected every 500 ms with an exposure time of 150 ms using a CCD detector, ORCAFlash4.0 (42 × 42 μm2 pixel size, Hamamatsu Photonics, K. K.) at BL40XU in SPring-8, Hyogo, Japan. A camera was used to determine the beam position in the sample. Therefore, the data number per cycle was 20. The beam size and wavelength at the sample were 5 × 5 μm2 and 0.1 nm, respectively. FIT2D was used to reduce the data from the 2D to 1D format. To confirm the reliability of the measurements, the same experiment was done at BL05XU in the SPring-8. WAXD data were collected every 500 ms with an exposure time of 150 ms using PILATUS 1 M (172 × 172 μm2 pixel size, DECTRIS).

beam WAXD measurements and calculation with the Takayanagi model was 0.81 GPa, which corresponded well with the E′ value, 1.0 GPa, measured by RHEOVIBRON. A notable feature of the dynamic μ-beam WAXD method is that the modulus value along the direction of sinusoidal mechanical deformation can be obtained at different polymer phases at a single micrometer scale and for various crystal planes in only one measurement. This is a quite useful method, particularly for material design, not only for polymers but also for any material, including metallic and inorganic materials. In conclusions, local E′ and E″ values inside and outside spherulites in the iPP film were successfully obtained based on a dynamic μ-beam WAXD method. The local E′in‑(110) and E′out‑(110) values inside and outside spherulites obtained from the change in spacing of the (110) plane were 1.8 and 1.1 GPa, respectively and were larger than the macroscopic E′ value obtained by RHEOVIBRON. Furthermore, the local E′in‑(1̅13) value inside spherulites, obtained by the (1̅13) plane, was 6.0 GPa. These values can be explained well by the ordering of crystal structure and stiffness of crystals depending on crystal planes. The E′ values of the iPP film obtained by combination of dynamic μ-beam WAXD measurement and of calculation with the Takayanagi model were quite consistent.

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EXPERIMENT

The specimen films were prepared from a 5 wt % solution of iPP (Sigma-Aldrich, Mn = 67 000, Mw = 250 000) in toluene (Wako Pure Chemical Industries, Ltd., 99.5%) using a solution casting method onto a glass plate. The film was dried at 90 °C for 5 h in vacuo to remove residual solvent, isothermally crystallized at 140 °C for 3 h from 200 °C

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00994. Experimental details; Figures S1: Stress−strain curves of the iPP film measured at 25 °C; Figure S2: Schematic illustration for calculation of the dynamic viscoelastic properties; Figure S3: Mechanical model and equation for calculating the modulus of the equivalent model; Table S1: Parameters and calculated E′ values by the Takayanagi model (PDF) 2D pattern of the iPP film during dynamic μ-beam WAXD measurement (AVI)

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AUTHOR INFORMATION

Corresponding Authors

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

Ken Kojio: 0000-0002-6917-7029 Atsushi Takahara: 0000-0002-0584-1525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Impulsing Paradigm Change through Disruptive Technology (ImPACT) Program and the Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Wide-angle X-ray difracton measurements were done at BL03XU, BL05XU, BL40XU, the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal No. 2012B1506,

Figure 4. Experimental setup for dynamic μ-beam WAXD measurement at BL40XU beamline, SPring-8, and a schematic illustration. 221

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crystalline regions of isotactic polypropylene. Kobunshi Ronbunshu 1985, 42, 241−247. (18) Takayanagi, M.; Minami, S.; Uemura, S. Application of equivalent model method to dynamic rheo-optical properties of crystalline polymer. J. Polym. Sci., Part C: Polym. Symp. 1964, 5, 113− 122. (19) Takayanagi, M.; Harima, H.; Iwata, Y. Viscoelastic behavior of polymer blends and its comparison with model experiments. Mem. Fac. Eng. Kyushu Univ. 1963, 23, 1−96.

2013B1186, 2014B1198, 2015A1514, 2015B1325, 2015B1459, 2016A1012, 2016A1406, 2016A1414, 2016B1032, 2016B1436). We gratefully acknowledge Dr. Taizo Kabe (JASRI), Dr. Taiki Hoshino (RIKEN), Dr. So Fujinami (RIKEN), and Dr. Tomotaka Naktani (RIKEN) for their assistance on the WAXD measurement.

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REFERENCES

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DOI: 10.1021/acsmacrolett.8b00994 ACS Macro Lett. 2019, 8, 218−222