EFFECTS OF SUPERCRITICAL CARBON DIOXIDE PROCESSING ON THE CRYSTALLIZATION OF POLYVINYLIDENE FLUORIDE

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1 EFFECTS OF SUPERCRITICAL CARBON DIOXIDE PROCESSING ON THE CRYSTALLIZATION OF POLYVINYLIDENE FLUORIDE Ji Eun Lee 1, Yanting Guo 1, Mirek Macecek 2, and Siu N. Leung 1, 1 Lassonde School o Engineering, Department o Mechanical Engineering, York University, Toronto, ON, Canada 2 Techno Scientiic Inc., 259 Edgeley Blvd., Units #11 & 12, Concord, ON, Canada Abstract Polyvinylidene luoride (PVDF) is an environmentally riendly, durable and low-cost alternative to traditional piezoelectric materials in sensors and actuators. PVDF is a semi-crystalline polymer with dierent crystal phases. Among them, the polar β-phase is the crystalline structure that is responsible or its piezoelectric property. Conventional technology or promoting β-phase crystals in PVDF is mechanical stretching. In this paper, processing o PVDF with supercritical carbon dioxide (ScCO 2 ) was investigated to examine its eect on PVDF s crystallization behavior. In the long-run, elucidation o potential strategies to tailor PVDF s crystal structures would help to identiy easible route to tailor PVDF s crystalline structure or emerging applications including sensing and energy harvesting. The oam morphology o PVDF was analyzed by scanning electronic microscopy while its crystallization behavior was studied by dierential scanning calorimetry and Fourier transorm inrared spectroscopy. Experimental results reveal that PVDF samples oamed at 120 C and 160 C under 2000 psi showed the highest crystallinity (54%) and volume expansion ratio (15.4 times), respectively. The crystallinity increase in ScCO 2 processed PVDF represents a 16% increase over that o its compression-molded samples. Introduction Piezoelectric materials have been used extensively in sensors, actuators, and as possible energy harvesters. Polyvinylidene Fluoride (PVDF) is a thermoplastic luoropolymer rom the polymerization o vinylidene diluoride. It provides an inexpensive, environmentally riendly, and conormable alternative to the currently popular toxic piezoelectric ceramic Lead Zirconate Titanate (PZT) [1]. Unortunately, the main disadvantage o PVDF is its low piezoelectric properties. PVDF is a semi-crystalline polymer where its crystalline structure is mainly in the non-polar α-phase but can transer into other phases through some structuring techniques such as mechanical stretching. The polar β-phase o PVDF is the crystalline structure that contributes to its piezoelectric property. Thereore, a high degree o crystallinity and β- phase content are very desirable to convert PVDF to a commercially usable material [2]. In this context, there are many abrication methods to align the crystal dipoles o PVDF in one direction. The most commonly used abrication method is through unidirectional mechanical stretching. By applying a certain strain on the material at given temperature, the molecular chain groups in both the amorphous region and the non-polar α-phase o the crystalline region can achieve an completely trans-planar TTTT conormation, and consequently achieving very high crystallinity and β- phase contents, at low temperatures and high elongation percentage [3 ]. Physical oaming o polymeric materials is a process to generate a large number o bubbles within the polymer matrix, through injection o gas into the polymer melt. During the bubble growth o oaming process, biaxial stress will be generated within the polymer matrix around the expanding bubbles. It is possible that the biaxial stress rom bubble growth could be comparable to the strain by mechanical stretching, to align PVDF s crystal dipoles. Moreover, physical oaming has been reported as a green technique, being less environmentally harmul than other current processing methods that use organic solvents [3]. In light o this, the goal o this paper is to investigate the potential o physical oaming using supercritical carbon dioxide (scco 2 ) as the blowing agent to control the crystallization behavior and tailor the crystalline structure o PVDF. The eects o key processing parameters on the volume expansion ratio, oam morphology, degree o crystallinity, and β-phase content o PVDF oams were investigated. The indings in this study could provide some insight to uture research work on oaming to induce the alignment o crystal dipoles o PVDF, advancing its development in piezoelectric applications. Materials Experimental Commercially available PVDF (Arkema, Kynar 741) was used. The physical properties o PVDF are summarized in Tables 1, obtained rom the Arkema Data Sheet and TA Instruments. Carbon dioxide (CO 2 ) o 99.8% purity (Linde Gas Inc.) was used as the physical SPE ANTEC Anaheim 2017 / 886

2 blowing agent or the oaming o PVDF. Table 1. Physical properties o PVDF 741 Powder Property Value Unit Melting temperature 168 C Density 1.78 g cm -3 Heat o Fusion J g -1 Degree o Crystallinity % Preparation o PVDF Thin Film Samples PVDF was received as powder. 10 g o PVDF powder was compression-molded into circular disc samples o 120 mm in diameter and 500 μm in thickness by the ollowing procedures: Step 1. PVDF powder was weighed and deposited into a circular disc mold and loaded into a compression molding machine (Craver Press, 4386 CH) preset at 185 C. Step 2. The sample and the mold were stabilized to 185 C Step 3. The sample and the mold were pressurized to and maintained at 5000 lbs-orce or 30 s, then lbs-orce or 30 s, and lbs-orce or 30 s, totaling a pressurized time o 90 s. Step 4. The heat was turned o and the compressionmolded sample was let in the machine until it reached room temperature. Supercritical CO 2 Foaming o PVDF The abrication o PVDF oams were perormed by supercritical carbon dioxide (ScCO 2 ) oaming. All samples were pre-cut into square-shaped samples o 0.5 inch by 0.5 inch beore being oamed. First, the samples were loaded into vessel and sealed. The temperature was increased to desired oaming temperature and saturated with CO 2 at an elevated pressure or 15 minutes. The saturation time had been selected to ensure suicient time to saturate the PVDF samples with CO 2. Ater 15 minutes, the pressure o the vessel was rapidly decreased by releasing the high pressure CO 2 rom the vessel, and the heat turned o. This would lead to thermodynamic instability in the saturated sample. Finally, the oamed PVDF sample with CO 2 was quenched in ice water to stabilize the oam morphology. The key conditions used in the oaming experiments are summarized in Table 2. Table 2. Conditions or CO 2 Foaming Experiments Variable Value Unit Saturation Pressure 1200, 2000, 2500 psi Saturation Time 15 min Foaming Temp. 100, 120, 140, 160, 180 C Sample Analysis Volume Expansion Ratio The volume expansion o PVDF oams were calculated using its density beore and ater the oaming experiment. Ater measuring its weight in air and in water, the apparent density (ρ ) and the volume expansion ratio (VER) can be determined using ASTM Standards by Equations (1) and (2), respectively. mair m air m water where ρ is the density o a sample ater oaming, m air and m water are the sample s weight measured in air and water, respectively, and ρ water@tw is the density o water at water temperature used to ind m water, T w. The weight o the samples were measured using a digital scale (New Classic MF MS54S). VER (1) (2) where ρ = 1.78 g cm -3 and is the density o the PVDF sample beore oaming. Scanning Electron Microscopy The phase and oam morphologies o PVDF oams were characterized by scanning electron microscopy (FEI Company Quanta 3D FEG). The cross-sections o all samples were exposed by cryo-racturing the samples under liquid nitrogen. The ractured suraces were sputtercoated with gold (Denton Vacuum, Desk V Sputter Coater). The cell size and cell population density were obtained by analyzing the SEM micrographs o the oams. The cell population density (N 0 ) with respect to the unoamed volume was determined by Equation (3). N 0 2 nm VER A where n is the number o cells in the SEM micrograph, M is the magniication actor, and A is the area o the micrograph. Dierential Scanning Calorimetry The degree o crystallinity was quantiied by dierential scanning calorimetry (TA Instruments, Q20), measuring heat low through in and out o the material as a unction o temperature. A heating rate o 10 o C/min between o C was used. The crystallinity, X c, can be calculated using Equation (4). 32 (3) SPE ANTEC Anaheim 2017 / 887

3 H X c 100% (4) H ( Crys ) preserve the cellular structure. where ΔH is the heat o usion o the sample and ΔH (Crys) is the heat o usion o the 100% crystalline PVDF which was given as J g -1. Fourier Transormed Inrared Spectroscopy The content o β-phase present in the sample was observed through an inrared spectroscopy (Bruke Alpha P FTIR ATR) rom cm -1 wave numbers. This range was determined to include the α and β-phase peaks o interest. The bands at 766 cm -1 and 840 cm -1 were used or α and β band characterization, respectively. The β-phase content, F(β), was calculated using Equation (5). X A F( ) X X 1.26 A A where X β and X α correspond to the degree o crystallinity o β and α-phases respectively. This equation can be rearranged to use A β and A α, the absorption bands o β and α-phases. The absorption bands o β and α-phases can be obtained using the Beer Lambert Law in Equation (6). (5) A 2 log(t%) (6) where T% corresponds to the transmittance percentage obtained rom the inrared spectroscopy at the β and α- phase peaks at 840 and 766 cm -1 wave numbers, respectively. Results and Discussion Eects o Foaming Conditions on the Foam Morphology o PVDF Figure 1 shows the eects o oaming pressure and temperature on the volume expansion ratio o the PVDF oams. It can be seen that 160 C resulted in highest volume expansion ratio, regardless o the oaming pressures. For the other oaming temperatures being investigated, the PVDF oams were almost unoamed. When the oaming temperature was ixed at 160 C, as the oaming pressure increased rom 1200 psi to 2000 psi, the volume expansion ratio o PVDF oams increased rom 3.8 to 15.4, due to a higher degree o supersaturation rom higher pressure. However, as the pressure urther increased to 2500 psi, the volume expansion ratio dropped to This indicates that the plasticization eect o the CO 2 has shited the optimal temperature or achieving highest volume expansion ratio to a lower temperature region, and oaming temperature o 160 C was too high to Figure 1. Comparison o the eect o CO 2 on volume expansion ratio o PVDF at various temperatures and pressures In addition, the volume expansion ratio as a unction o oaming temperature showed a mound shape. When the temperature was too low (i.e., 140 C), the PVDF matrix would still be too sti to generate cellular structure; however, when the temperature increased too much (i.e., 180 C), the gas loss would become more signiicant and hence the existing cells collapsed. The oam morphologies o PVDF oams abricated at 2000 psi illustrated in Figure 2 urther conirmed these. As the temperature varied rom 140 C to 180 C at 2000 psi, the morphology o PVDF oams changed rom ew tiny voids to uniorm cellular structure, ollowed by some big voids and gas pockets (i.e., deterioration o oam structure). This is evident that the transition o oaming temperature rom too low to an optimum, and then too high occurred. The PVDF oams with oaming temperature o 160 C and oaming pressure o 2000 psi possessed a cell density o cells/cm 3 and an average cell size o µm. Eect o ScCO 2 Processing on the Degree o Crystallinity in PVDF The degree o crystallinity o PVDF oams at various oaming conditions was plotted in Figure 3. As a reerence case, the degree o crystallinity o compressionmolded sample was measured, which was 38%. For all the oaming pressures, there existed an optimal temperature or achieving highest degree o crystallinity. The optimal temperature shited rom 140 C to 120 C as the pressure increased rom 1200 psi to 2000 psi and 2500 psi. This suggests that there is an optimal level o ScCO2 plasticizing eects to obtain the highest inal crystallinity. As the PVDF matrix has some pre-existing crystals used as the seeds, an optimal level o plasticizing eect would SPE ANTEC Anaheim 2017 / 888

promote the chain mobility around the crystal nuclei and thereby promote the crystal perection.

However, when the oaming temperature increased to 160 C, the suppression o melting point by ScCO 2 plasticizing eect led to the disruption o the pre-existing crystal structure.

This did not allow suicient amount o time or the crystal to grow. As a result, the degree o crystallinity decreased despite the high volume expansion ratio.

4 promote the chain mobility around the crystal nuclei and thereby promote the crystal perection. This would be beneicial to the crystal growth and thereby increased the degree o crystallinity o the PVDF matrix. However, when the oaming temperature increased to 160 C, the suppression o melting point by ScCO 2 plasticizing eect led to the disruption o the pre-existing crystal structure. Furthermore, the ast CO2 consumption during the oaming process would increase the melting temperature back to the original level rapidly. This did not allow suicient amount o time or the crystal to grow. As a result, the degree o crystallinity decreased despite the high volume expansion ratio. increased rom 160 C to 180 C, indicating a loss in crystallinity in many phases including the piezoelectric β- phase. This can be explained by the melting temperature o PVDF 741 being 168 C. As the temperature is increased, it creates more mobility in the chain groups o the PVDF to rotate rom the random amorphous region to the orderly crystalline structure. When the temperature is urther increased, the mobility o the chain groups is too high that instead o aligning themselves to be orderly, the chain groups orm into the random amorphous region, reducing all crystalline phases [6]. When the temperature went past the polymer s melting point, the mobility o the chain groups were deinitely very high with a low viscosity, allowing or even the previous crystalline structure to become unorderly, creating more amorphous region. (a) (b) (c) Figure 2. SEM micrographs o PVDF oams abricated at 2000 psi and with oaming temperature: (a) 140 C; (b) 160 C; and (c) 180 C Figure 3. Comparison o the eect o CO 2 on the degree o crystallinity o PVDF at various temperatures and pressures Nie et al. [5] ound that at higher temperatures, larger strains were required to initiate crystallization. Thereore, the highest crystallinity was obtained rom the samples with lower oaming temperature and negligible volume expansion ratio. This also implies that in order to achieve a higher crystallinity o samples with oaming temperature o 160 C, we would need a higher volume expansion ratio. However, when the oaming temperature urther decreased, the molecular chains did not have enough mobility to align or old and then crystallize. As a result, the degree o crystallinity dropped. Fourier Transorm Inrared Spectroscopy on Foamed PVDF Fourier transorm inrared spectroscopy was conducted at various oaming conditions and the raw data or 1200 psi can be seen in Figure 4. It can be seen that peaks are deinitely a lot smaller as the temperature Figure 4. FTIR spectra o CO 2 oamed PVDF at 1200 psi and various temperatures SPE ANTEC Anaheim 2017 / 889

5 Conclusions This paper reports the abrication o Polyvinylidene Fluoride (PVDF) oamed with supercritical CO 2 at various conditions with promoted degree o crystallinity. The highest volume expansion ratio o 15.4 was achieved at 2000 psi and 160 C, and decreased as pressure increased due to the plasticization eect shiting the optimal temperature. It was observed rom SEM micrographs that to create oam cells, the operation temperature needs to heat the polymer matrix enough to generate cellular structure but not excessive heat where the existing cells would collapse and lose gas. The highest crystallinity o 54.12% was achieved at 2000 psi and 120 C. It was observed that the optimal temperature or high crystallinity decreased as pressure increased, due to the amount o strain needed or crystallization. The reduction o the peaks in the FTIR analysis justiied that the high mobility o the chain groups o PVDF caused by high temperature will convert a considerable amount o the crystalline structure to the random amorphous region. This paper has shown an initial assessment o the potential o CO 2 oaming through comparison o various pressures and temperatures. Reerences 1. J. Rocha, L. Goncalves, P. Rocha, M. Silva and S. Lanceros-Mendez, IEEE Trans. on Ind. Elec., 57, 3, V. Sencadas, V. Moreira, S. Lanceros-Mendez, A. Pouzada and R. Gregorio Jr., Mat. Sci. For., , Y.T Shieh, T.T. Hsiao, and S.K. Chang, Polymer, 47, , Y. Xiang, F. Liu, L. Xue, J. Shen, and H. Lin, Chinese Journal O Polymer Science., 32, 12, Y. Nie, H. Gao, M. Yu, Z. Hu, G. Reiter, and W. Hu, Polymer, 54, , A. Salimi and A. Yousei, Polymer Testing, 22, , SPE ANTEC Anaheim 2017 / 890

DIELECTRIC PERMITTIVITY OF THERMOPLASTIC POLYURETHANE/PZT COMPOSITE FOAMS

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