Silica

Article pubs.acs.org/crystal

Releasing and Freezing Phase Separation of Polyvinyl Alcohol/Silica To Control Polymorphs of Silica Jieyang Huang, Ke Li, Longbo Luo, Huina Wang, Xu Wang, Yan Feng, and Xiangyang Liu* State Key Laboratory of Polymer Material and Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, P. R. China

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 1, 2015 | http://pubs.acs.org Publication Date (Web): March 27, 2015 | doi: 10.1021/cg501511s

S Supporting Information *

ABSTRACT: Crystalline silica is prepared beyond 1500 °C in a traditional process. Here, we prepared both cristobalite-rich and quartz-rich silica by calcinating polyvinyl alcohol (PVA)/ silica films at 900 °C. Results of characterizations show that polymorphisms of silica were dependent on the phase separation of PVA and silica before calcinations. The phase separation is controlled by a coagulation bath. By soaking PVA/silica hybrid films in a coagulation bath before thermal treatment, phase separation of PVA and silica was frozen and prevented. When PVA/silica hybrid films were not soaked in a coagulation bath before thermal treatment, phase separation of PVA and silica was released. Further research reveals that different phase structures of PVA and silica generate distinct microscopical morphologies and molecular structures of silica, leading to variation of the final polymorphs. hybrid films which were obtained via a sol−gel process.11 Through a series of comparative experiments, we found that the crystallinity of silica was positively correlated with the interactions between polyimide and silica. However, as shown above, polymer-based crystallization controls of metal oxides and silica are both mainly focused on the crystallinity or the minimum temperature at which it can crystallize, while crystallization controls of oxides through pH value, temperature, or organic additives involve polymorph transition. Recently, Brázda et al. have found that polymorphs of ferric oxide after calcinations at 1200 °C were dependent on the additive amount of small molecules of sucrose before calcinations.12 In our view, the interactions between oxides and additive substances always play a key role in the polymorphs of oxides.11 It is worth noting that, in common preparation of polymer/oxides materials, phase separation of two substances is often accompanied, which differs from small molecular additives/oxides. For polymer science, phase separation is an important manifestation of intermolecular interactions, and it may influence the properties of composites profoundly. In this paper, considering the experience of wet-spinning, we used a coagulation bath to control phase separation of polymer and silica. Different polymorphs of silica were prepared by calcinating polyvinyl alcohol (PVA)/silica films. Although the silica source and polymer template are identical, with different phase separations of PVA and silica, the main polymorph of

1. INTRODUCTION Silica is the most abundant substance on earth, and human beings have utilized these most common materials for thousands of years. In fact, silica can be applied to different fields for its diverse phase structures that include an amorphous phase and crystalline polymorphs. For instance, amorphous silica is the main component of glass; quartz silica can be used to make communication optical fibers; and cristobalite silica can be used to prepare thermal-shock resistant materials. Through these structure-based applications of silica, phase transition of silica has been explored by human beings since silica materials were utilized, and the study of phase transition is a complex subject including both physical and chemical processes. Since the sol−gel process can be easily controlled by pH value, temperature, and solvent content, it has been widely adopted in preparation of silica.1 For pure silica, the most important factor influencing the phase structure is temperature. When silica sol is used as the silica source, silica is amorphous by heating below 1500 °C, and crystalline silica forms at a high temperature above 1500 °C.2−5 However, when silica-sol is mixed with other substances, the phase transition process is changed dramatically. Generally, silica incorporated with inorganic substance forms cristobalite at a lower temperature which is below 1000 °C.6,7 Polymers, which can be removed after a heating or calcinating process, are also used as additives to control crystallization of oxides. Holland et al. prepared various metal oxides by using polystyrene as a template.8 However, it is still seldom reported about how to control the crystallization process of silica by using polymer templates at a relatively low temperature.5,8−10 Recently, we have prepared cristobalite silica at 800 °C by calcinating polyimide/silica © 2015 American Chemical Society

Received: October 10, 2014 Revised: March 2, 2015 Published: March 23, 2015 2072

DOI: 10.1021/cg501511s Cryst. Growth Des. 2015, 15, 2072−2078

Article

Crystal Growth & Design

Nicolet Magna 650 spectroscope. Scanning electron microscopy (SEM) measurement was carried out with a JEOL JSM-5900LV SEM. Thermal gravity analysis-differential scanning calorimetry (TGA-DSC) synchronous thermal analysis was carried out with a Mettler Toledo TGA/DSC 1 system under ambient air. Small angle X-ray scattering (SAXS) measurements were carried out using a NanoSTAR-U (Bruker AXS Inc.) with Cu Ka radiation (k = 0.154 nm). Phase separation of PVA and silica was measured with a FEI Tecnai G2 F20 S-TWIN transmission electron microscope (TEM), and samples were cut into 100 nm thick slices with a Leica EMUC6 ultramicrotome before measurement.

silica after calcinations can be quartz or cristobalite. Our work makes it possible to control polymorphs of silica with the identical polymer and enriches the crystallization behaviors of silica.


2. EXPERIMENTAL SECTION 2.1. Materials. N-Methyl-2-pyrrolidone (NMP) was obtained from Puyang MYJ Technology Co. Ltd. and was distilled under reduced pressure before use. PVA-1799 (degree of polymerization is 1700 and degree of alcoholysis is 99%), tetraethoxysilane (TEOS), oxalic acid, distilled water, and ethylene glycol (EG) were obtained from Chengdu Kelong Chemical Company. 2.2. Synthesis of PVA Hybrid Solution. PVA granules were dissolved in NMP at 120 °C under nitrogen atmosphere for 2 h, and the PVA solid content was kept at 10%.13 At the same time, oxalic acid was added into distilled water to keep the pH value at 2. The subacid water and TEOS were added into a one-neck round flask at a molar ratio of 4:1. Then NMP was added into the one-neck round flask as a cosolvent, making the mass fraction of TEOS 40%. The mixed TEOS solution was stirred at 40 °C for 2.5 h until it hydrolyzed completely and became homogeneous. Afterward, hydrolyzed TEOS solution was added into the PVA solution, and the mass ratio of PVA/silica was kept at 9/1. The mixture was stirred at 120 °C for 2 h to acquire excellent compatibility and form the PVA/silica hybrid solution. 2.3. Preparation of PVA/Silica Hybrid Film. The PVA/silica hybrid solution at 120 °C was poured onto glasses, and the solution films were molded on glasses by knife coating immediately. The next branching steps are described below. The films were then processed in different ways: (1) The films on glasses were placed under ambient air for 1 h, and NMP was retained in these films. The films that underwent this step were marked as “A”. (2) The films on glasses were placed into EG (coagulation bath) to remove NMP from the films for 1 h. The films that underwent this step were marked as “B”. The films (including A and B) were placed in a vacuum oven. The negative pressure of the vacuum oven was maintained by vacuum pumps, and the temperature in the vacuum oven was kept at 120 °C for 4 h. Thus, NMP and EG were removed in the vacuum oven, and we called this process thermal treatment. In principle, hybrid films which underwent step A or step B might result in great distinctions in behavior of phase separation after thermal treatment. For films that underwent step A, the dispersion effect of good solvent NMP on PVA and silica was weakened during thermal treatment with the gradual evaporation of NMP. Besides, the residual NMP worked as carriers of molecular movement which supported the aggregation of identical substances. Therefore, along with the disappearance of solvent effect on PVA and silica, phase separation of PVA and silica would occur inevitably. For films that underwent step B, good solvent NMP was extracted before thermal treatment. Therefore, the homogeneous phase structure was frozen and retained without containing good solvent NMP. In thermal treatment, no NMP could be used as carriers of molecular movement, so the phase separation of PVA and silica would be frozen and prevented. After thermal treatment, the films were taken out from the vacuum oven and named for PVAS-A and PVAS-B respectively. To demonstrate the expectations of phase structures stated above, some characterizations used for analysis can be seen below. 2.4. Calcination of PVA/Silica Hybrid Film. The obtained PVA/ silica films were placed into crucibles and heated in a furnace starting from room temperature to the predetermined temperature of 900 °C under ambient air. After keeping the temperature at 900 °C for 2 h to remove PVA completely, crucibles were cooled to room temperature in the furnace. White silica films formed and were named for Silica-A and Silica-B respectively. 2.5. Characterization. Wide angle X-ray diffraction (WAXD) patterns were measured with a Philips X’Pert PRO MPD. Fourier transform infrared spectroscopy (FTIR) spectra were obtained with a

3. RESULTS AND DISCUSSION The WAXD patterns of Silica-A and Silica-B are shown in Figure 1, and the WAXD data are compared with data in the

Figure 1. WAXD patterns of silica after calcinating different PVA/ silica films.

cards (PDF#75-0923) and (PDF#85-0797). After calcinating of the PVA/silica films pretreated with different steps, silica exhibited different crystal forms. The main peaks of cristobalite and quartz are marked, and the relative content of cristobalite and quartz can be determined by the intensity of main peak in the WAXD patterns. The content of polymorphs are semiquantitatively calculated14 and summarized in Table 1. In Table 1. Percent Content of Cristobalite and Quartz in Different Samples silica sample

percent content of cristobalite (%)

percent content of quartz (%)

Silica-A Silica-B Silica-AB

100 24 92

0 76 8

Silica-A, the content of cristobalite is up to 100%. However, Silica-B contains only 24% cristobalite, while quartz is the major polymorph with content of 76%. Considering the same calcinating process, this significant distinction in silica polymorph between Silica-A and Silica-B can only be derived from the preparation process of PVA/silica hybrid films. PVASB exhibits a difference in that PVA was coagulated by a coagulation bath from PVAS-A film. In addition, a sample named Silica-AB was prepared to verify our speculation. The PVA/silica films were treated in a vacuum oven at 120 °C for 2073


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Crystal Growth & Design 1.5 h first, then were placed into EG for 1 h and treated in a vacuum oven at 120 °C again for 4 h. After the calcinations process of section 2.4, Silica-AB was obtained. The WAXD results of Silica-AB were also exhibited in Figure 1 and Table 1. Silica-AB contains 8% quartz and 92% cristobalite, and its content is in between that of Silica-A and Silica-B. The median may be derived from the weak change of phase structure between PVA and silica, which is caused by the thermal treatment of 1.5 h before treating in a coagulation bath. To eliminate other influences on silica polymorphs, such as the physical and chemical structure of NMP and EG, or contaminations in NMP and EG, two control samples (SilicaC and Silica-D) were prepared. The WAXD patterns of Silica-C and Silica-D are shown in Figure S1 in Supporting Information, eliminating the possibilities discussed above. To study the coagulated effect of PVA/silica films on final silica polymorphs, PVAS-A and PVAS-B were further characterized. The FTIR spectra of PVA/silica films are shown in Figure 2. The main distinction of two FTIR spectra is the peak at 1143

suggested that the strength of characteristic peak around 1443 cm−1 is sensitive to intramolecular hydroxy hydrogen bonds between two neighboring hydroxyls of PVA.16 Compared with PVAS-B, the PVAS-A film which was treated without soaking in coagulation bath possesses more intramolecular hydroxy hydrogen bonds between two neighboring hydroxyls of PVA macromolecules. Silica can destroy intramolecular and intermolecular hydrogen bonds in PVA and form new hydrogen bonds with hydroxyls of PVA by Si hydroxyls. In PVAS-B, the narrow spaces between two neighboring hydroxyls of PVA are occupied by silica more easily than PVAS-A, indicating PVAS-B tends to form more Si hydroxyls and smaller-size silica which can interact with PVA more strongly. PVA is a semicrystalline polymer in which molecular chains arrange orderly due to the strong hydrogen bonds between hydroxyl groups.17 In general, when a polymer contains a crystalline region, the WAXD peaks are sharp and exhibit high intensities, whereas for an amorphous polymer they are broad.18 To further characterize the hydrogen bonds between hydroxyl groups, the PVA/silica samples were measured with WAXD. In Figure 3a, the diffraction pattern reveals an important diffraction peak at around 20.0°,19,20 which is characteristic for the orthorhombic lattice.21 Compared with PVAS-A, PVAS-B presents a broad and low-intensity diffraction WAXD peak, indicating a low crystallinity of PVAS-B. Some reports also revealed that small molecules can interact with polymers and destroy the formation of crystalline regions.17,22−24 In most references, the decreasing crystallinity of polymer is dependent on the content of small molecules. With more small molecules, the interactions in polymers are destroyed severely, and polymer interacts with small molecules stronger, preventing macromolecules from arranging orderly to obtain high crystallinity. In our systems, the silica content of PVAS-A and PVAS-B is almost the same. Therefore, it is speculated that such huge distinctions in WAXD patterns may be derived from the interaction forms between silica and PVA. To exclude other likely impacts, PVA films treated without and with soaking in a coagulation bath (named PVA-A and PVA-B, respectively) were also prepared, and the WAXD patterns of PVA-A and PVA-B are shown in Figure 3b. As seen in Figure 3b, a coagulation bath has little impact on the crystallinity of PVA, which reconfirms that the coagulation bath influences the

Figure 2. FTIR spectra of PVA/silica films.

cm−1, which is attributed to C−C symmetric stretching vibration.15 In the FTIR spectrum of PVAS-A, this peak is sharp and strong, while the peak of PVAS-B is weaker. Krimm

Figure 3. WAXD patterns of (a) PVA/silica films and (b) PVA films. 2074


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crystallnity of PVA/silica films through interacting forms between silica and PVA. SAXS measurement was used to characterize the interaction forms between silica and PVA. The Lorentz-corrected SAXS curves of samples are shown in Figure 4.25 The intensity of

which possess a higher electron cloud density than that of PVA. In preparation of PVA/silica films, no external force was introduced. Therefore, the average size of silica particles can be determined by assuming spherical objects to be spheres, with a diameter D. Diameter can be calculated by equation D = (3Φm/ 8π)1/3L, in which Φm is the volume fraction of silica phase.29 Thus, the diameter of silica particles is 1.87 nm by calculation. On the other hand, the shoulder peak locates at 0.531 nm−1 with peak-fitting, and it is attributed to the L of PVA lamellar. The lamellar thickness of PVA in PVAS-A is also displayed in Table 1, and it is pretty close to that of PVA in PVA-A. Meanwhile, in Lorentz-corrected SAXS curves of PVAS-B, no peak can be observed, supporting that phase separation is not obvious in this film. Moreover, as seen in Figure 5, SAXS curves with double logarithmic coordinates were analyzed to study the interactions

Figure 4. Lorentz-corrected SAXS curves of samples.

PVAS-A and PVAS-B are obviously higher than that of PVA-A, since PVA/silica films contain silica whose density is 2.2 g/cm3, higher than that of PVA (1.27−1.31 g/cm3). The Lorentzcorrected SAXS curve of PVAS-A exhibits a peak at 0.587 nm−1. A long period (L) of 10.7 nm was calculated from the equation L = 2π/qmax, where qmax is the peak value in the Lorentzcorrected SAXS plot. For that PVA-A is semicrystalline and contains no silica; the long period corresponds to the average interlamellar spacing of PVA. For semicrystalline polymer, lamellar thickness (Lm) can be calculated by using the equation Lm = XcL, where Xc is the crystallinity of polymer.26 To calculate lamellar thickness, peak-fitting of WAXD was carried out to determine the crystallinity of PVA.27 Crystallinity and lamellar thickness of PVA in different samples are summarized in Table 2.

Figure 5. SAXS curves of PVA/silica films with double logarithmic coordinates.

and fractal dimensions of PVA and silica. For the logarithmic curves of PVA-B, the slope is almost constant with the value of −1.659 ± 0.004. A mass-fractal dimension (Dm) of 1.659 ± 0.004 is obtained, indicating that silica is evenly dispersed in the PVA template with different observation scales.30,31 SAXS results sufficiently coincide with the WAXD and FTIR results, demonstrating that PVAS-B possesses little phase separation and strong interaction between PVA and silica compared with PVAS-A. While SAXS curves provide statistical data of phase separation, TEM images can exhibit visual phase separation structure. As seen in Figure 6, the TEM image of PVAS-A

Table 2. Crystallinity of PVA in Different Samples sample

crystallinity (%)

long period (nm)

lamellar thickness (nm)

PVA-A PVAS-A PVAS-B

54 50 10

10.7 11.8

5.8 5.9

In Lorentz-corrected SAXS curves of PVAS-A, there is a strong peak at 0.760 nm−1, which corresponds to an L of 8.2 nm. Some reports revealed that mixing a second substance into semicrystalline polymer increases the long period which relates to the average interlamellar spacing,28 and it can be simply explained by additives embedding in the amorphous phase broaden the interlamellar spacing. However, in our results, the long period seems to decrease with addition of silica. In fact, in Lorentz-corrected SAXS curves of PVAS-A, the peak corresponding to L of 8.2 nm is asymmetric, and an obscured shoulder peak exhibits at low q value. Besides, the peak at 0.760 nm−1 possesses a different peak shape from that of PVA-A. All these analyses above indicate that the L of 8.2 nm is not an average interlamellar spacing value of PVA, and it can be L derived from an average spacing between two silica particles

Figure 6. TEM images of PVA/silica films. 2075


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According to TGA curves, PVA are almost removed by calcinating to 600 °C. To observe phase morphologies of silica intuitive, SEM images of hybrid films after calcinating at 600 °C are tested and shown in Figure 8. The residual silica of PVAS-A


shows a grainy structure, and the TEM image of PVAS-B has a cloudy appearance. Thus, TEM images are consistent with the phase structures that are deduced according to SAXS curves. Even though the size of grains in PVAS-A TEM image is larger than the diameter of particles calculated by SAXS results, these difference may be derived from the local aggregation of silica particles. Then the graphic abstract can be supported by both statistical data (SAXS) and visual images (TEM). Would the difference in PVA/silica films cause the final distinction of the silica polymorph? The results of TGA/DSC measurement under ambient air are analyzed below. TGA/DSC synchronous thermal analysis curves of PVA and PVA/silica films under air atmosphere are shown in Figure 7. In the

Figure 8. SEM images of PVA/silica films (a for PVAS-A and b for PVAS-B) after calcinating to 600 °C.

are separated and discontinuous, while the residual silica of PVAS-B looks compact under 80000-times magnification. The cavities in residual silica of PVAS-A are derived from the calcinated PVA. Meanwhile, The SEM results of microscopical morphology are in consistent with the low density of cristobalite and the phase separation analysis of SAXS results. FTIR spectra of Silica-A and Silica-B are displayed in Figure 9 to provide the molecular structure of silica. The absorbance

Figure 7. TGA/DSC synchronous thermal analysis curves of PVA and PVA/silica films under air atmosphere.

temperature ranging from 270 to 400 °C, PVA-A exhibits a higher weight loss rate than PVA/silica films. Moreover, in this temperature range, PVAS-B possesses a lower weight loss rate than PVAS-A. The distinction discussed above indicates that silica of PVAS-B increases the thermo-oxidative stability more than that of PVAS-A, since PVAS-B obtains stronger interaction with PVA. A strong endothermic peak at 425 °C is observed in DSC curves of PVAS-B, while no endothermic peak can be obviously found in the other two DSC curves. The endothermic peak at 425 °C is derived from condensation of Si hydroxyls.32−34 Because of strong interaction with the PVA template, the complete condensation temperature is increased significantly. In the temperature ranging from 450 to 600 °C, strong exothermic peaks are observed in all DSC curves. These exothermic peaks can be attributed to the degradation and oxidation of polymer to form carbonaceous gases.35−37 Compared with PVA/silica films, PVA-A exhibits a sharper exothermic peak, indicating a more intense oxidation process.38 The broad exothermic peak in DSC curves of PVA/silica films reflects better thermo-oxidative stability that is derived from interactions between PVA and silica. Interestingly, the broad exothermic peak in DSC curves of PVAS-B is a single peak, while the one in DSC curves of PVAS-A is an overlap of two peaks. The overlapping exothermic peak in the DSC curves of PVAS-A reconfirms its heterogeneity of phase structure due to phase separation of PVA and silica. On the contrary, the single exothermic peak in DSC curves of PVAS-B indicates its homogeneous phase structure.

Figure 9. FTIR spectra of silica after calcinations.

peaks at 622 and 695 cm−1 are characteristic for cristobalite and quartz respectively,39,40 which is consistent with WAXD patterns. The peaks in region 1000−1100 cm−1 and 400−500 cm−1 are attributed to Si−O−Si asymmetric stretching vibration and Si−O−Si bending vibration, respectively. Both of the characteristic peaks of Silica-B locate at a lower wavenumber than that of Silica-A, indicating that Silica-B tends to form a relatively linear structure while Silica-A tends to form a relatively cyclic structure.41 The distinction of the silica molecular structure is consistent with the endothermic peak in the DSC curves. Thus, with coagulation bath treatment of PVA/silica films, the microscopical morphology and molecular structure of silica after calcinating PVA/silica films are influenced deeply. Besides, the peak at around 3500 cm−1 is seriously influenced by water. At first, silica exhibits a high water absorbing capacity. Second, after our calcinating process, silica was obtained at room temperature, and at this temperature absorption of water 2076


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The distinction of crystallization mechanism between SilicaA and Silica-B is a complicated issue. For Silica-A, phase separation of PVA and silica leads to condensation of Si hydroxyls to a great extent before removing PVA. After PVA is removed, the remaining Si hydroxyls continue to condense. For Silica-B, condensation of Si hydroxyls is delayed by interactions between PVA and silica. When PVA is almost removed at about 425 °C, Si hydroxyls of monomeric silica or silica with low molecular weight condense suddenly. It is noted that six-membered oxygen loop is planar in cristobalite, while it is quite irregular in quartz, resulting in a more compact structure in quartz.44 Jones and Segnit demonstrate that silica with a high degree of condensation trends to form cristobalite, and it cannot form quartz without breaking Si−O bonds.45 At a temperature below 900 °C, although quartz is the most stable polymorph at this temperature, silica with a high degree of condensation (SilicaA) cannot form quartz, due to the low mobility of silica source with large volume. On the contrary, monomeric silica or silica with a low degree of condensation (Silica-B) can form quartz, for the high mobility of the silica source with a tiny volume.45 In other words, the freezing phase separation of PVA and silica protects monomeric silica from condensing. To some extent, the freezing method is similar to the theory that break Si−O bonds in silica with a high degree of condensation to rearrange bonds to form quartz.44 By analyzing above, controlling the polymorph of silica can been achieved by releasing and freezing phase separation of PVA and silica.


was difficult to avoid. Therefore, we think the peak at 3500 cm−1 is not enough evidence of a Si−OH group. Many classical references discuss Si−OH groups at wavenumbers below 1400 cm−1.41−43 The peak at around 3500 cm−1 exhibits more information about absorption of water. In Silica-A, the peak at around 3500 cm−1 exhibits a larger area than that in silica-B, indicating stronger absorption of water in Silica-A. The peak at around 3500 cm−1 in Silica-A is consistent with the loose structure shown in Figure 8a, resulting in higher specific surface area and stronger absorption of water. With all the data collected from our study, we can explain how a coagulation bath influences the final polymorphs of silica in Figure 10. The theory of three primary colors is utilized to

4. CONCLUSIONS Cristobalite-rich and quartz-rich silica were prepared by calcinating PVA/silica films at 900 °C. Without soaking PVA/silica hybrid films in a coagulation bath, the polymorph of silica is mainly cristobalite. With soaking PVA/silica hybrid films in coagulation, the polymorph silica is mainly quartz. By characterization and analysis of hybrid films, it is found that PVAS-A, which has not been soaked in a coagulation bath, possesses a phase separation structure and weaker interactions between PVA and silica. PVAS-B, which has been soaked in a coagulation bath, possesses a uniform phase structure and stronger interactions between PVA and silica with more Si hydroxyls containing. At the level of the mechanism, PVA protects monomeric silica from condensing, leading to a change of polymorph. Our method aims to preserve monomeric silica to obtain quartz, while a theory45 suggest that breaking Si−O bonds leads to rearrangement of bonds to form quartz. As a result, by releasing and freezing molecular motions to influence phase separation, silica obtained after calcinating hybrid films presents different microscopical morphology and molecular structure, resulting in a final difference in polymorphs of silica. Our work will make it possible to control the content of silica polymorphs by controlling a different degree of phase separation in PVA/silica films through a coagulation process. The work will enrich the crystallography, and this effective method (polymorphs dependent on phase separation) may be adopted in controlling crystallization behaviors of the other inorganic substances.

Figure 10. Diagrammatic drawing of PVA-induced silica crystallization.

color a diagrammatic drawing visually. The coagulating process in wet-spinning was adopted with good solvent NMP and coagulation bath EG. First, PVA (yellow), silica-hydrolysate (blue), and solvent NMP (red) are mixed. Depending on the binding effect of solvent, PVA and silica-hydrolysate are dispersed evenly. By soaking mixed films in an EG coagulation bath , the solvent NMP is extracted quickly without destroying the uniform phase of PVA and silica. Then thermal treatment is used to solidify the PVA/silica films. For PVAS-A, the binding effect of NMP is lost in a slow evaporation process of NMP, leading to phase separation of PVA and silica. Silica aggregates into nanoscale particles, dispersing in amorphous PVA domains that are divided by PVA lamellar. For PVAS-B, since NMP is extracted quickly by a coagulation bath, the molecular motions of PVA and silica are frozen and prevented in thermal treatment. After solidification via thermal treatment, phase dispersion occurs in PVAS-A, while PVAS-B maintains a uniform phase. In the end, silica is obtained after calcinations. Because of phase separation in PVAS-A, Silica-A is separated and discontinuous, and forms a more cyclic molecular structure. On the contrary, due to the large amount of Si hydroxyls and uniform phase in PVAS-B, Silica-B is continuous and forms a more linear molecular structure.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed preparation process of control samples (Silica-C and Silica-D) and WAXD patterns of control samples (Silica-C and 2077


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(28) Kubo, J. i.; Rahman, N.; Takahashi, N.; Kawai, T.; Matsuba, G.; Nishida, K.; Kanaya, T.; Yamamoto, M. J. Appl. Polym. Sci. 2009, 112, 1647−1652. (29) Dahmouche, K.; Santilli, C. V.; Pulcinelli, S. H.; Craievich, A. F. J. Phys. Chem. B 1999, 103, 4937−4942. (30) Martin, J. E.; Hurd, A. J. Appl. Crystallogr. 1987, 20, 61−78. (31) Koga, T.; Takenaka, M.; Aizawa, K.; Nakamura, M.; Hashimoto, T. Langmuir 2005, 21, 11409−11413. (32) Barrado, E.; Rodríguez, J.; Prieto, F.; Medina, J. J. Non-Cryst. Solids 2005, 351, 906−914. (33) Pathak, S.; Sharma, A.; Khanna, A. Prog. Org. Coat. 2009, 65, 206−216. (34) Rubio, F.; Rubio, J.; Oteo, J. Thermochim. Acta 1997, 307, 51− 56. (35) Fernandes, D.; Hechenleitner, A.; Pineda, E. Thermochim. Acta 2006, 441, 101−109. (36) Sarangi, P. P.; Naik, B.; Ghosh, N. N. Powder Technol. 2009, 192, 245−249. (37) Iyer, A.; Garofano, J. K.; Reutenaur, J.; Suib, S. L.; Aindow, M.; Gell, M.; Jordan, E. H. J. Am. Ceram. Soc. 2013, 96, 346−350. (38) Chen, Y.; Qian, Q.; Liu, X.; Xiao, L.; Chen, Q. Mater. Lett. 2010, 64, 6−8. (39) Graetsch, H.; Gies, H.; Topalović, I. Phys. Chem. Miner. 1994, 21, 166−175. (40) Saikia, B. J.; Parthasarathy, G.; Sarmah, N. Bull. Mater. Sci. 2008, 31, 775−779. (41) Mauritz, K.; Warren, R. Macromolecules 1989, 22, 1730−1734. (42) Šimon, I.; McMahon, H. J. Chem. Phys. 1953, 21, 23−30. (43) Ocana, M.; Fornes, V.; García-Ramos, J. V.; Serna, C. Phys. Chem. Miner. 1987, 14, 527−532. (44) Williams, L. A.; Crerar, D. A. J. Sediment. Res. 1985, 55. (45) Jones, J.; Segnit, E. J. Geol. Soc. Aust. 1972, 18, 419−422.

Silica-D). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 28 85403948. Fax: +86 28 85405138. E-mail: [email protected]. Notes

The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 50973073). We acknowledge Analytical & Testing Centre Sichuan University, P. R. China for characterization. This work was financially supported by the National Natural Science Foundation of China (Grant No. 50973073) and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme 2014-2-04). The authors acknowledge Analytical & Testing Centre of Sichuan University, People’s Republic of China for characterization.

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