Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Mechanically Tough Syndiotactic Polypropylene (sPP) Gels Realized by Fast Quenching Using Liquid Nitrogen Fuyuaki Endo,† Ryusuke Okoshi,† Keita Takaesu,† Naruki Kurokawa,† Hiroki Iwase,‡ Tomoki Maeda,† and Atsushi Hotta*,† †
Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Research Center for Neutron Science and Technology, Comprehensive Research Organization for Science and Society (CROSS), 162-1 Shirakata, Tokai, Ibaraki 319-1106, Japan
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S Supporting Information *
ABSTRACT: Physical gels have attracted considerable attention due to the remoldability deriving from the reversible cross-linking points made of noncovalent bonds. In this study, we report a facile method to realize a mechanically tough syndiotactic polypropylene (sPP) gel, one of the physical gels made of semicrystalline polymers. The sPP/ decahydronaphthalene gel quenched using liquid nitrogen (GEL LN) presented the fracture stress of over 2 MPa with the fracture strain as high as ∼0.7, while the sPP gel prepared at room temperature (GEL 25) presented the fracture stress of only 315 kPa and the fracture strain of ∼0.3. The fracture stress was found to increase with the increase in the sPP concentration. Furthermore, the structural analyses of the sPP gels revealed that the quenching of the sPP gel suppressed the phase separation, eventually forming homogeneous nanocrystalline cross-linking network structures to produce a mechanically tough sPP gel.
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INTRODUCTION Physical gels are polymer gels with noncovalent cross-linking points forming a three-dimensional polymer network containing a large amount of liquid in the network.1,2 As the noncovalent cross-linking points are reversible in response to the environmental conditions such as temperature, ionic strength, and pH, physical gels can exhibit reversible sol−gel transition and thus are readily remoldable.1,3−5 However, despite the interesting remoldability of the physical gels, the studies on mechanically tough gels have mainly focused on chemical gels that possessed much stronger covalent bonds as cross-linking points, and few researches have been reported as for the toughening of physical gels.6−10 Semicrystalline polymers form physical gels when dissolved in proper solvent.3,4,11−22 Quite a few studies on the physical gels made of semicrystalline polymers have been conducted, and some important factors affecting the phase behavior, the structures, and the physical properties of semicrystalline polymer gels have been revealed. The solvent selection for the gel preparation greatly influenced the gelation behavior of the resultant gel, such as the gelation speed, the gel morphology, and the gel crystalline structures.3,4,12−14 The solvent selection also determined whether the system possessed the sol−gel transition or not. The polymer concentration also affected the sol/gel behavior of the solution and the gelation/ melting temperatures of the gels.15,23 Additionally, it was reported that the gel preparation temperature could influence © XXXX American Chemical Society
the optical characteristics of the poly(vinyl alcohol) (PVA) gel,17 where the PVA gels cooled at near room temperature (40 or 23 °C) became opaque, while those cooled at lower temperatures (−2 or −40 °C) were transparent. However, when gels were formed in organic solvents, the obtained physical gels with semicrystalline polymers usually possessed brittle mechanical characteristics.16 In this work, we found that an easy preparation processthe quench of syndiotactic polypropylene (sPP) solution by liquid nitrogencould realize the formation of an unusually elastic and mechanically tough sPP gel. We also report the effects of the gel preparation temperature on the mechanical properties and the structures of the sPP gel. In detail, sPP was first dissolved in decahydronaphthalene (decalin) at 150 °C, and then the sPP solution was cooled at different temperatures (38, 25, or 0 °C) or quenched in a liquid nitrogen bath. The compression tests were performed to investigate the mechanical properties of the sPP gels. The structural analyses were also conducted using polarized optical microscopy (POM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and small-angle neutron scattering (SANS) measurements to Received: November 16, 2017 Revised: February 21, 2018
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DOI: 10.1021/acs.macromol.7b02426 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. sPP gels cooled at (a) 38 °C, (b) 25 °C, (c) 0 °C, and (d) quenched with liquid nitrogen. temperature. The distance between the X-ray source and the detector was set at 500 mm. The scattering intensity of decalin was subtracted from the gel profile. Small-Angle Neutron Scattering (SANS) Analysis. The smallangle neutron scattering (SANS) measurements were performed at BL-15 (Taikan) in the Materials and Life Science Experimental Faculty (MLF), Japan Proton Accelerator Research Complex (J-PARC), Tokai, Japan.24 The sPP/deuterated decalin gels with the thickness of 1 mm, prepared at each temperature, were put into a quartz cell, and the white neutron beam was radiated. The data of the scattered beam were detected with 3He-sensitive detectors (for Q > 0.006 Å−1) and an ultrasmall-angle detector consisting of a position-sensitive photomultiplier tube and a ZnS/6LiF scintillator (for Q < 0.006 Å−1). The necessary corrections, such as the empty cell scattering, the inhomogeneity of detector efficiency, and the wavelength-transmittance dependence, were made on the scattering data, and the corrected data were merged with respect to the scattering vector Q (Q = 4π sin θ/λ, where 2θ is the scattering angle). The obtained scattering profiles were then normalized to the absolute intensity by the glassy carbon standard. Afterward, in order to analyze the scattering from the gel structures, the scattering from deuterated decalin and the incoherent scattering were subtracted from the measured profiles of the sPP gels.
discuss the relationship between the structures and the physical properties of the sPP gels.
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EXPERIMENTAL SECTION
Materials. Syndiotactic polypropylene (sPP) with the molecular weight of 174 000 was purchased from Sigma-Aldrich Corporation. The melting point of sPP was 134 °C, which was determined by the DSC measurements. Decalin (the mixture of cis- and transdecahydronaphthalene) with the boiling point of 190 °C was purchased from Wako Pure Chemical Industries and used without any purification. For the SANS measurements, deuterated decalin was purchased from Armar Chemicals and used without any purification. Gel Preparation. sPP was dissolved into decalin at 150 °C to obtain a homogeneous sPP/decalin solution with the sPP concentration ranging from 6 to 20 wt %. The sPP/decalin solution was cooled at 25 and 38 °C in temperature-controlled baths (GEL 25 and GEL 38), cooled at 0 °C in a water bath (GEL 0), or quenched in a liquid nitrogen bath (GEL LN). These solutions were confirmed to be in the gel state by the tube inversion experiments, where specimens are recognized as gel when the inversed specimens show no-flow behavior. The gels were further aged at room temperature for 72 h to complete the gelation. Light Transmittance Measurement. The transparency of the sPP gels was measured using a UV−vis spectrophotometer (U-2810, Hitachi High-Technologies Corporation). The sPP gels with the thickness of 10 mm were prepared in a quartz cell in the same way as mentioned in the gel-preparation section. The transmittance at the wavelengths ranging from 400 to 750 nm was scanned at 25 °C with the scan speed of 100 nm min−1. Mechanical Property Analysis. The compression tests of the gels were performed by using a dynamic mechanical analyzer (RSA3, TA Instruments Japan Inc.) to determine the mechanical properties of the sPP gels. The cylindrical gels, with a diameter of 8 mm and a length of 8 mm, were sandwiched and compressed with two parallel metal disks at room temperature. The compression was performed with the strain rate of 3.3 × 10−3 s−1. Microscopic Observations. The microstructures of the sPP gels were observed with polarized optical microscopy (POM) (BX51P, OLYMPUS Corporation) at room temperature. The gels were set between two glass slides, which were then compressed and circularly spread to the specimen size of ∼10 mm in diameter. The thickness of the compressed gels was set at ∼60 μm. The microstructures of the sPP gels were also observed by field emission scanning electron microscopy (SEM) (JSM-7500F, JEOL Ltd.) at a voltage of 3 kV. The gel specimens were dried for longer than 24 h, and then the fracture surfaces of the dried gels were coated by osmium before the SEM observation. Differential Scanning Calorimetry (DSC) Analysis. Thermal analyses were conducted to analyze the fusion enthalpy of the sPP gels using the differential scanning calorimetry (DSC) (DSC822, MettlerToledo International Inc.). The heating rate was set at 10 °C min−1, and the temperature range was set between 0 and 130 °C. Approximately 10 mg of the gel sample was put and sealed in an aluminum pan with a volume of 40 μL. Wide-Angle X-ray Scattering (WAXS) Analysis. The wide-angle X-ray scattering (WAXS) measurement was performed at BL40B2 in the synchrotron facility of SPring-8, Harima, Japan. The X-ray beam with the wavelength of 0.154 nm was radiated to the gel specimens prepared in a capillary tube with the diameter of 2.0 mm at room
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RESULTS AND DISCUSSION Optical Properties of sPP Gels. Figure 1 shows the appearance of the prepared sPP gels cooled at 38, 25, and 0 °C, and the sPP gel quenched with liquid nitrogen. The sPP concentration was fixed at 10 wt %. GEL 25, GEL 0, and GEL LN could maintain their cylindrical shapes, whereas GEL 38 collapsed. As for the transparency of the gels, GEL LN was highly transparent, whereas GEL 38, GEL 25, and GEL 0 seemed opaque. In fact, the light transmittance of GEL LN was above 85% at the wavelengths ranging from 400 to 750 nm, which was quite higher than that of other gels (GEL 38 (30− 45%), GEL 25 (9−14%), and GEL 0 (
