Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Making a Rapid Completion of Crystallization for Bisphenol A Polycarbonate by a Double-Layer Film Method Wentao Wang,† Miao Tang,† Xuehui Wang, Cui Xu, and Zhigang Wang* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ABSTRACT: Bisphenol A polycarbonate (PC) is difficult to crystallize. In this study, it was found that PC could rapidly accomplish melt crystallization only within 8 min by laying poly(ethylene glycol) (PEG) film on it. Polarized optical microscope (POM) was applied to trace the growth of spherulites of PC in PEG/PC double-layer films during isothermal crystallization in a wide range of temperatures, which were above the melting temperature for PEG. The dependence of spherulitic growth rate (G) on isothermal crystallization temperature (Tc) followed a “bell-shape” curve, just like some other semicrystalline polymers did. A PEG film was laid on a PC film at 250 °C for 5 min to obtain a PEG/PC double-layer film, which was subsequently quenched to different Tc’s for observation on its crystallization process by using POM. It was surprising to observe that the spherulitic growth rate of PC was significantly improved and the crystallization time was obviously shortened. The miscibility for PEG/PC blends was evaluated by applying differential scanning calorimetry (DSC), which played a crucial role in the crystallization accelerating effect. This method provides an operative way to surmount the particular disadvantages of PC, such as very slow crystallization rate and harsh conditions that PC needs to crystallize in the film processing.
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crystallization in PC/PCL blends.27 Saito and Tsuburaya studied the crystallization of PC induced by spinodal decomposition for PC/PEO blends.28 Supercritical CO2 was also used to facilitate the crystallization of the solid-state polymerized PC by DeSimone et al. The PC crystallization process with use of supercritical CO2 took at least 2 h on the optimum conditions.29 To summarize, the above methods for inducing crystallization of PC samples still have limitations, such as harsh experiment conditions, which restrict them to be applied to PC. In addition, the thermally induced crystallization rate for PC is considered extremely slow. Turska et al. demonstrated that obtaining the crystallinity of about 18−28% for PC samples at 190 °C even took about 170 h.30 Crystallization of the cast PC films and compression-molded PC samples induced by acetone vapor needs more than 24 h.20 Thin films of PC have been paid great attention because of its low production costs, excellent transparency for optical observation, and so on. Recently, our group reported that the crystallization kinetics of semicrystalline polymers such as polylactide (PLA) could be significantly accelerated through the double-layer film method.31−36 For PLA systems, the results showed that the miscibility of polymer pairs played an important role in the enhanced spherulitic growth rates for
INTRODUCTION Bisphenol A polycarbonate (PC), well-known with amazing properties, such as shock resistance, thermal stability, transparency, and optical properties, has aroused great industrial and academic interests. PC is one of the engineering polymers with the highest impact resistance and is known as the transparent material with the highest toughness.1−11 These characteristics endow PC a wide range of applications. However, the most undesired problem with PC is that PC is hard to crystallize. The presence of two aromatic rings bridged by methylene unit in PC leads to low chain flexibility.1,12 Although bulk PC is hard to crystallize, dense fibrillar lamellar crystals were observed by AFM in the 30 nm thick spin coated PC films after annealing at 200 °C for 24 h by Ata et al.13 Von Falkai et al. reported that the first crystallites of PC appeared after one full day and it took longer than a week to obtain a complete PC spherulite at 190 °C.14 Along these years, various methods have been tried to obtain high crystallinity for PC samples, such as by blending,15−18 adding plasticizers,3 surfactants,19 solvent vapor exposure,20−25 or ultrasound-assisted crystallization (sonocrystallization),26 and so on. In order to lower the temperature of crystallization, Sundararajan studied the crystallization of PC in the presence of a surfactant, which required a temperature of 80 °C for 24 h, far less than the others as reported to date,19 whereas the report of the crystallization of polycarbonate in the presence of a surfactant is limited. Blending is considered to be a convenient way to modify the polymer crystallization kinetics. For instance, Laredo et al. studied the miscibility and © XXXX American Chemical Society
Received: March 26, 2018 Revised: April 24, 2018 Accepted: April 30, 2018
A
DOI: 10.1021/acs.iecr.8b01295 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research semicrystalline polymers in the double-layer films.31−36 Therefore, the double-layer (multilayer) films method might be considered to replace the blending method from the economic aspect. In order to resolve the problems of the long induction time of crystallization for PC and the reduction of cost for PC materials through regular blending, in this study we applied the double-layer film method for PC material, that is, to lay a thin molten layer of another polymer on the PC layer. The molten polymer component that we chose was poly(ethylene glycol) (PEG) for its miscibility with PC component. It has been reported that the analogue of PEG, poly(ethylene oxide), with higher molecular mass is miscible with PC in the blends.37 We recorded the polarized optical micrographs for the PEG/PC films in the double-layer format when they experienced the crystallization process at isothermal crystallization temperatures chosen in a wide range. We found that the incorporation of molten PEG layer could significantly improve the growth rates of PC spherulites and surprisingly the time for a completion of PC crystallization could be shortened less than 5−10 min. We further evaluated the shift of glass transition temperature Tg for PC component by introduction of PEG component by taking the heat flow curves from differential scanning calorimetry, disclosing a great decrease in Tg due to PEG component. Scanning electron microscopy (SEM) was also applied to reveal the layer structures for the PEG/PC films. Interfacial diffusion between PEG and PC layers provided a key clue to catching the speeding-up effect on crystallization kinetics for PC layer. To the best of our knowledge, accelerating the spherulitic growth rate of PC in such a short time has not been reported ever before. Promising applications might be inferred from the double-layer film method, which could replace the general blending method in certain film application aspects.
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Afterward, the obtained blend samples were dried under vacuum at 35 °C until constant masses. Polarized Optical Microscopy (POM). The model of polarized optical microscope (POM) was Olympus BX51 made in Japan, which was connected to a CCD camera (model Tucsen TCC-3.3N). Isothermal crystallization processes for the PEG/PC double-layer films were operated at Tc in a wide range of temperature from 90 to 170 °C. The polarized optical micrographs were recorded at appropriate time intervals. When the PEG/PC double-layer films were obtained in situ at 250 °C, the interfacial diffusion was allowed to process for 5 min, and then the films were rapidly quenched to Tc’s, which were preset in another hot-stage. It is noted that at 250 °C the diffusion process between PEG and PC layers for 5 min was sufficient for the study. A longer time was not applied to avoid degradation of PEG. Differential Scanning Calorimetry (DSC). The model for DSC measurements was a TA Q2000 DSC from TA Instruments, USA. For an evaluation of miscibility, the glass transition temperature was the most desired parameter. To obtain the heat flow curves for PEG/PC blends, the following procedure was adopted. The temperature was raised up rapidly to 280 °C and held for 5 min and then was dropped down to −90 °C with a cooling rate of 50 °C/min, and finally the temperature was scanned up to 250 °C with a heating rate of 10 °C/min. Scanning Electron Microscopy (SEM). The model for field-emission scanning electron microscopy was SEM FEI Sirion200 made in USA. The samples were immersed in liquid nitrogen for 5 min and then fractured with a razor blade perpendicular to the glass for cross-section observation. Furthermore, the leftover of sandwiched films was separated to observe the surface morphology of the layers. A gold thin layer was coated on samples to allow SEM observation.
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EXPERIMENTAL SECTION
RESULTS AND DISCUSSION Temperature Dependence of Spherulitic Growth Rate. The PC spherulitic growth rates at different isothermal crystallization temperatures in a wide range of 90−170 °C were measured for PEG/PC double-layer films. Nitrogen was purged for sample protection. The selected typical POM micrographs during the growth of spherulites for PEG/PC double-layer films are shown in Figure 1. No crystals can be seen at 180 °C or above. When the temperature decreased, small spherulites with radius of about 5 μm formed and crystallization was completed within only 10 min, which was 60 times shorter than the most recent result reported by Lan and co-workers.38 In addition, along with temperature decreasing, the area ratio occupied by spherulites representing crystallinity increased significantly. And the spherulites could form even at a temperature as low as 100 °C, which was 50 °C lower than Tg of PC,9,39,40 illustrating that the chain segmental mobility of PC was greatly enhanced with applying the double-layer method. In the POM micrographs, the spherulites-occupied area ratio roughly represents the crystallization degree, and the crystallization rate could be evaluated against the crystallization temperature.38,41 The crystallization kinetics is correlated to the spherulitic growth rate. Figure 2 presents the increases of spherulite radius with crystallization time during isothermal crystallization for PEG/PC double-layer films. The linearly fitted line provides a slope, representing the value of spherulitic growth rate. The spherulitic growth rates for PC at 115, 135, and 155 °C are 1.1, 1.7, and 1.0 μm/min, respectively. We can
Materials. Bisphenol A polycarbonate (PC, weight-average molecular mass, Mw = 44 kg/mol, polydispersity index, PDI = 2.7) and poly(ethylene glycol) (PEG, Mw = 3.5 kg/mol, PDI = 1.5) used in this study were purchased from Sigma-Aldrich Company. The Mw and number-average molecular mass Mn values for these two materials were obtained by using the sizeexclusion chromatography (SEC).31 Both PC and PEG materials were dried under vacuum at 35 °C for 24 h prior to use. Preparation of PEG/PC Double-Layer Films and PC/ PEG Blends. PC was dissolved in methylene dichloride to obtain 2 wt % solution, and PEG was dissolved in chloroform to obtain 5 wt % solution. The solutions were stirred at room temperature for 12 h, and then drops of the solutions were cast on cover glasses. PC and PEG films were obtained when methylene dichloride and chloroform evaporated, and these films were further dried under vacuum at 35 °C until constant masses. The thicknesses of PC and PEG films were about 20 and 15 μm, respectively. By turning over the cover glass with PEG film and laying it on the top of PC film, the sandwiched films between the two cover glasses were obtained, which were termed as the PEG/PC double-layer films. These films were melted on the hot-stage at 250 °C prior to observation by applying the polarized optical microscope (POM). For the characterization of miscibility between PC and PEG, PC/PEG blends were prepared by dissolving PC and PEG in methylene dichloride. The solutions were stirred at room temperature for 12 h, and then methylene dichloride was evaporated in air. B
DOI: 10.1021/acs.iecr.8b01295 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. Dependence of spherulitic growth rate on isothermal crystallization temperature for PEG/PC double-layer films. The solid line is a guide to the eye.
films. The result is consistent with the general “bell-shaped” dependence of the crystallization rate on crystallization temperature for semicrystalline polymers. The spherulitic growth behavior in this study is in accordance with other research.3,7,31,32,42 POM Observation of PEG Spherulites Formed at the Two-Step Quench for PEG/PC Double-Layer Films. For the PEG/PC double-layer film, a sequential crystallization of PC and PEG components might be observed, depending on the applied crystallization temperatures. So we applied a sequential temperature quench procedure, which was also used by Woo et al. to observe the birefringence enhancement for poly(L-lactic acid)/poly(1,4-butylene adipate) (PLLA/PBA) blend with 50/ 50 mass ratio.43 The first temperature quench step reached Tc of 135 °C, at which the nucleation and crystal growth in PC layer occurred, and the second temperature quench step reached Tc of 40 °C, at which the crystallization in the top covering PEG layer occurred. Figure 4 demonstrates the observation, from which we can see that underneath PC crystallized at 135 °C and formed spherulites (Figure 4a), while the top PEG layer remained in the clear molten state (as a
Figure 1. Selected POM micrographs taken at different times during crystallization for PEG/PC double layer films at (A) 120 °C, (B) 125 °C, (C) 135 °C, (D) 145 °C, and (E) 155 °C. PEG film had thickness of about 15 μm, and PC film had thickness of about 20 μm. The scale bar in panel A of 100 μm is applicable to all micrographs.
Figure 2. Plots of spherulite radius with crystallization time during isothermal crystallization for PEG/PC double-layer films at Tc of 115, 135, and 155 °C, respectively.
deduce from the straight lines that the spherulites grow up linearly. Another significant character is that the spherulites at 135 °C grow faster than at 115 and 155 °C, so there must be an appropriate temperature for PC spherulites to grow fastest. It is noticed here that some of the spherulites in Figure 1 are out of focus. There are two reasons for the result. One is the crystals of PC grow so fast that the dispersion of spherulites becomes inhomogeneous in space. The other reason is that the sizes of PC spherulites are small with their radius values below 12 μm as shown in Figure 2. This infers that PC spherulites look like they are floating in the supercooled PC melt with film thickness larger than the sizes of PC spherulites. Both reasons cause the loss of focus for some spherulites. However, the sizes of PC spherulites do become larger and larger with increasing crystallization time as shown in Figure 1, which allows us to pick up the focused spherulites to measure the sizes with certain precision. Figure 3 displays the temperature dependence of spherulitic growth rate on isothermal crystallization temperature for the
Figure 4. POM (a−d) for PEG/PC double-layer films during isothermal crystallization at Tc = 135 and 40 °C, respectively: (a) Tc = 135 for 10 min; (b) Tc = 40 °C for 5 min; (c) Tc = 40 °C for 10 min; (d) Tc = 40 °C for 15 min. Before the first step quench to Tc = 135 °C the film samples were melted at 250 °C for 5 min. The scale bar of 50 μm in (a) is applicable to all micrographs. The solid blue lines in (b) and (c) point at the growth front of PEG spherulites. C
DOI: 10.1021/acs.iecr.8b01295 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research liquid-like layer). Only if the top covering PEG layer was quenched from Tc = 135 to 40 °C, it started to form PEG spherulites. Solid blue lines are drawn to clearly remark PEG spherulitic growth front (Figure 4b,c). PEG crystallization can apparently enhance the birefringence of the films, and the earlier-grown PC spherulites seem to serve as the template for the latter-grown PEG spherulites. Overall, the sequential crystallization of PC and PEG was apparent due to their much different melting points and undercoolings. SEM Observation on Layer Structures for PEG/PC Double-Layer Film. To further evaluate the possible interfacial diffusion between PEG and PC layers, SEM was used to observe the fractured surface and the transversal surface of the films. Figure 5A shows several large PEG spherulites (pointed at by blue arrows) with average radius of 490 μm formed, indicating that although the interfacial diffusion between PEG and PC appeared at interface, neat PEG could still crystallize after the double-layer film was cooling down. Figure 5B displays a large number of PC spherulites well corresponding to those shown in the POM micrograph in Figure 4. To take a close view of them, Figure 5C shows the interior structure of PC spherulites (pointed at by blue arrows) with an average radius of 6.4 μm, which coincides with the data analyzed from POM measurements. Transversal surface of the film is shown by Figure 5D, in which PC spherulites can also be observed and the interior structure looks similar to that in Figure 5C. In summary, we reach two major conclusions: (i) the interfacial diffusion between PC and PEG exists; however, it is different from polymer blends because the much large spherulites of neat PEG are still observed; (ii) the influence of a molten PEG layer on the underneath PC layer is significant; therefore, PC spherulites are full of the vision in SEM. Characterization of Tg in PEG/PC Blends. If the two polymer films contact intimately, an interfacial diffusion might occur, especially when the two polymers stay in amorphous state above Tg or in molten state above Tm.44 The result in the previous section indicates that for PEG/PC double-layer films, the nucleation of PC spherulites occurs at much lower temperatures and the PC spherulites grow in much less time than neat PC film, inferring possible occurrence of an effective interfacial diffusion between PEG and PC layers. It is predicted that in the PEG/PC double-layer films the Tg of PC in the interfacial layer is to decrease when it is mixed with PEG with much lower Tg. As a conventional method, differential scanning calorimetry (DSC) can be used to determine the Tg of PC. However, because the interfacial layer might be too thin to provide sufficient heat flow signal intensity from glass transition, we used PEG/PC blends for DSC measurements. Figure 6 shows the DSC thermograms for PC/PEG blends with different PEG compositions. In Figure 6A, the signals of glass transition in the Cp curves at the temperatures below 150 °C are significant. The endothermic peaks at the temperatures above 220 °C for the blends with PEG compositions of 15 and 20 wt % represent the melting of PC crystals formed in the blends. The Tg values for PC component can be obtained from temperature derivative curves of Cp. Tg = 147 °C for neat PC film can be clearly determined. As the PEG composition increases, the Tg of PC goes down sharply, as indicated by the arrows, pointing at the glass transition positions. Figure 7 shows the change of Tg for PC component in PEG/PC blends with increasing PEG composition. In the plot, the Tg value for PEG was −74 °C, which was taken from our previous publication.32 The data points in Figure 7 cannot be fitted by using the Fox
Figure 5. SEM micrographs taken from fractured PEG/PC doublelayer films. The film sample was melted at 250 °C for 5 min and then crystallized at Tc = 135 °C for 5 min. (A) ×100 showing the spherulitic structure in PEG layer; (B) ×500 mainly showing the whole PC layer with PC spherulites; (C) ×5000 mainly showing the interior structure of PC spherulites; (D) ×5000 showing the transversal surface of the film (pointed at by blue arrows).
equation.45 However, the data points can be satisfactorily fitted by using the Couchman−Karasz equation, indicating that PEG and PC components have much different heat capacity changes at the glass transition.46 Nevertheless, the result in Figures 6 and 7 demonstrates that PEG and PC components in the experimentally measured PEG composition range are miscible in the blends. Mechanism for Enhancement of Crystallization in PEG/PC Double-Layer Films. PEO, the closest analogue of D
DOI: 10.1021/acs.iecr.8b01295 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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applicable. By covering a miscible amorphous polymer layer (or molten semicrystalline polymer layer) on the PC layer, the crystallization kinetics can be improved obviously. We are currently running some tests on the double-layer films made through the coextrusion technique for the PC materials.
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CONCLUSIONS The method of double-layer is advantageous in enhancing crystallization and improving the spherulitic growth rate of PC. We prepared PEG/PC double-layer films and measured the spherulitic growth rates of PC layer by using polarized optical microscope. The temperature dependence of spherilitic growth rates of PC layer clearly demonstrates that the amorphous molten PEG layer greatly lowers the crystallization temperature by nearly 150 °C, which does significantly accelerate the crystallization kinetics. The glass transition temperatures in PEG/PC blends as examined by DSC measurement show a great decrease for PC component, inferring a reduced glass transition in the interfacial area (or layer) of PEG/PC doublelayer films. The interfacial areas (or layer) are considered to be crucial for the observed result. Obviously enhanced crystallization of PC becomes achievable, and surprisingly the crystallization temperature with the highest spherulitic growth rate eventually approaches below the glass transition temperature of neat PC. Potential applications can be implied from the double-layer film method, employing a coextrusion technique to produce PEG/PC multilayer films, providing a prospective way to overcome the disadvantages of PC, such as slow crystallization rate and harsh conditions that PC needs to crystallize in the film processing. In addition, the results in this study indicate that the simple double-layer method for accelerating PC crystallization delivers potential extensive application prospect and theory values, since it overcomes the shortcomings of traditional molten blending method.
Figure 6. DSC thermograms for PC/PEG blends with PEG composition of (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, (d) 15 wt %, and (e) 20 wt %. (A) Cp curves. (B) Temperature derivative Cp curves. Vertical shifts of the curves have been applied for clarity.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86 0551-63607703. Fax: +86 0551-63607703. E-mail:
[email protected]. ORCID
Figure 7. Change of glass transition temperature Tg of PC with PEG composition for PEG/PC blends. The blue solid line represents the curve of the Fox equation. The olive solid line is the fitted curve on the basis of the Couchman−Karasz equation.
Zhigang Wang: 0000-0002-6090-3274 Author Contributions †
W.W. and M.T. contributed equally to this work.
Notes
PEG, was reported to be miscible with PC.28,37,47 Compared with PEO, PEG has much lower molecular mass. Therefore, PEG is considered to be a much effective plasticizer to PC because of its miscibility with PC. PC chain segmental mobility can be improved if an interfacial diffusion occurs when PEG layer is thermally contacted with PC layer. As is reported, entanglements have a great influence on glass transition of polymers including PC. 11,48 Thus, the nucleation and spherulitic growth rate for underneath PC layer can be obviously enhanced by the top covering PEG layer, and the double-layer film method can greatly reduce the crystallization temperature and time of neat PC from the academic point of view. More important, the layer-by-layer film method implies potential applications from industrial perspective. To produce multilayer polymer films, a coextrusion technique has been conventionally applied.49,50 Regarding very low crystallization rate in PC processing, the layer-by-layer film method might be
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
Z.W. acknowledges the financial support from the National Science Foundation of China (Grant 51673183). The work was also financially supported by the opening project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant Sklpme2017-4-06).
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DOI: 10.1021/acs.iecr.8b01295 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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