Effects of Nanoporous Anodic Alumina Oxide on the Crystallization

Jan 8, 2016 - Poly(vinylidene fluoride) (PVDF) nanotubes were fabricated by melt-wetting into porous anodic aluminum oxide (AAO) templates with two di...
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Effects of Nanoporous Anodic Alumina Oxide on the Crystallization and Melting Behavior of Poly(vinylidene fluoride) Xiying Dai,† Jiali Niu,‡ Zhongjie Ren,† Xiaoli Sun,*,† and Shouke Yan*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering & The Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

‡

S Supporting Information *

ABSTRACT: Poly(vinylidene fluoride) (PVDF) nanotubes were fabricated by melt-wetting into porous anodic aluminum oxide (AAO) templates with two different interfacial properties: one is pristine AAO, and the other is modified by FOTS (AAO-F). Their crystallization and melting behaviors are compared with those of a bulk sample. For the PVDF in AAOF, the nonisothermal crystallization temperature is slightly lower than that of bulk, and the melting temperature is similar to that of bulk. For the PVDF in pristine AAO, when the pore diameter is 200 nm, the crystallization is induced by two kinds of nucleation: heterogeneous nucleation and interface-induced nucleation. On the contrary, in the AAO template with pore diameter smaller than 200 nm, only interface-induced nucleation occurs. The melting temperature of PVDF crystals in the pristine AAO is much higher than that of bulk which can be attributed to the presence of an interfacial layer of PVDF on the template inner surface. The interaction between PVDF and AAO template produces the interfacial layer. Such an interfacial layer plays an important role in enhancing the melting temperature of PVDF crystals. The higher melting peak is always observed when the PVDF is nonisothermally crystallized in the AAO template irrespective of the thermal erasing temperature suggesting the interfacial layer is very stable on the AAO template surface. If the PVDF nanostructures are released from AAO template, the higher melting peak disappears with the enhancement of thermal erasing temperature.

1. INTRODUCTION Materials in nanometer scale confinement always exhibit different features from the bulk. Recently, crystallization of polymer in the cylindrical templates has been widely investigated.1−13 Anodic aluminum oxide (AAO) membranes are frequently employed. A serious of studies has been focused on the nucleation mechanism of polymers in the nanopores.14 The nucleation mechanism of different polymers exhibits different pore diameter dependence. For the linear polyethylene (PE), in the pores with diameters larger than 62 nm, the homogeneous nucleation is predominant. In the smaller pores with diameters 15−48 nm, the heterogeneous nucleation prevails.10 By contrast, the isotactic polypropylene (iPP) infiltrated by self-ordered AAO with larger pores crystallizes mostly by heterogeneous nucleation. With the decrease of pore diameter, more and more iPP in the AAO pores crystallizes by homogeneous nucleation. There exists a critical pore diameter. Smaller than the critical value, only homogeneous nucleation occurs.15 The orientation of the polymer crystals affected by nanoporous confinement is also investigated extensively. The orientation of polymer crystals in the templates can be determined by the kinetics of nucleation and growth. With poly(vinylidene fluoride) (PVDF) as an example, if PVDF is © 2016 American Chemical Society

caped in separated nanopores, homogeneous nucleation dominates.8 Predominant PVDF crystals adopt the orientation with molecular chains aligned perpendicular to the long axis of nanopores. If PVDF nanotubes are connected by a bulk film of the same material on the surface of the template, nucleation predominantly happens in the bulk. The fastest growth direction of the PVDF spherulites (b-axis) points radically outward. When spherulites impinge on the surface of the template, only the lamellae with the growth direction parallel to the long axes of nanopores are able to grow. This leads to that macroscopic array with b-axis orientation; i.e., the b-axis of the lamellae inside the nanopores is along the long axis of the nanopores. The free surface can also affect the orientation of polymer crystals in the nanoporous template, as for syndiotactic polystyrene (sPS) crystals in the confined nanospaces with a connected layer on the template surface.16 On the side near the connected layer, crystals with the orientation of molecular chains perpendicular to the long axis of nanopores form. On the other side near the free surface, the molecular chains prefer to orient parallel to the long axes of the nanopores. This Received: November 15, 2015 Revised: January 7, 2016 Published: January 8, 2016 843

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according to the protocols reported elsewhere.26 The modified surfaces were uniform, as determined by contact angle measurements, as shown in Figure S1 in the Supporting Information. For simplicity, the pristine AAO and FOTS modified AAO template are referred to as AAO and AAO-F, separately. The AAO membranes were infiltrated with PVDF by means of melt infiltration method. A piece of PVDF film was placed on the surface of the AAO template at 200 °C, above the melting temperature of PVDF, in the vacuum oven. After 48 h infiltration of the PVDF, the samples were then cooled to room temperature. It was checked that the thermal degradation temperature of PVDF is around 400 °C. This kind of thermal treatment does not affect the nonisothermal crystallization and melting behavior of PVDF. Finally the template was first cleaned with the aid of a blade to avoid any remaining PVDF on the AAO template surface, and then cotton buds soaked with N,N-dimethylformamide (DMF) were used to further clean the template surface to make sure the residual PVDF was completely eliminated. Thus, prepared samples were subsequently melt-recrystallized. The nonisothermal crystallization from the melt is carried out at the cooling rate of 10 °C/min. The crystallization and melting behaviors of the PVDF in nanopores of AAO membrane were characterized by calorimetric measurements conducted in a TA Instruments Q2000 differential scanning calorimeter (DSC). For structure characterization of the PVDF formed in the nanopores, the AAO membranes were removed after crystallization of PVDF by placement into 40 wt % aqueous potassium hydroxide solution for 24 h, and thereafter, the suspension was centrifuged. Suspensions of PVDF formed in the nanopores in ethanol were then prepared. The suspension was dropped either on copper grids for TEM study or on silicon wafer for scanning electron microscopy observation. Scanning electron microscopy (SEM) micrographs were recorded with a JEOL JSM 6300 F. Prior to the SEM investigations, the template surfaces or PVDF nanotubes on silicon wafer were sputtered by platinum. X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance for Cu Kα radiation. The Θ/2Θ scans were carried out in the reflection mode with a 2Θ increment of 0.05° and an integration time of 20 s. The samples were placed in the diffractometer in such a way that the template surface with the pore openings was arranged perpendicular with respect to the plane defined by the incident and the scattered X-ray beams. The nanotubes were wellaligned within the templates during the measurements. In this geometry, only crystal lattice planes oriented parallel to the surface of the template contribute to the intensity of a Bragg reflection. The nonisothermal crystallization and melting behaviors of PVDF in the bulk and AAO templates were monitored by in situ X-ray. The samples were treated at the cooling and heating rate of 5 °C/min. Also, the collection time for each XRD pattern is 10 min.

variation in orientation was attributed to the different nucleus types. Recently, it is found that the molecular weight also plays an important role in the orientation of crystals. For example, the poly(ethylene oxide) (PEO) in the AAO template shows the absence of crystallites oriented with the extended chains perpendicular to the pore wall. Wang claimed that the orientation of PEO molecular chains in the confinement spaces is determined by the relationship between contour length of molecules and the pore diameter of AAO templates.17 Although the crystallization behavior of polymer in the nanoporous template has been extensively studied, the effect of interfacial properties of AAO template on the crystallization and melting behavior of polymer crystals in such confined spaces has been rarely studied.18 In general, the AAO template is used without further interface treatment of the inner wall. The melting temperature of polymer crystals in the confined spaces is lower than that of the bulk value due to the smaller crystal size caused by the confinement spaces. Both interface and size can affect the melting temperature of the crystals. Size effect can obviously reduce the melting temperature of crystals. Whether the interface effect can increase or decrease the melting temperature of crystals or not is not clear. Thus, more studies on the effect of nanoporous confinement on the melting and crystallization are needed in order to understand the influence of interface and size effect further. Poly(vinylidene fluoride) (PVDF) has remarkable chemical, electrical, and mechanical properties. There are many important usages of PVDF membranes such as gas separation, water treatment, medical applications, fuel cell membranes, etc.19−23 Taking the potential application of PVDF nanotubes in the field of ultrafiltration, the fabrication of PVDF nanotubes has received much attention. Studies on the crystallization and orientation of PVDF nanotubes fabricated by melt- or solutionwetting method have been well-explored.7,8,24,25 Here we reported the preparation of PVDF nanotubes into the pores of commercial AAO templates with different interfacial properties and pore diameters. Crystallization and melting behaviors of PVDF nanotubes were investigated by using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). From a nonisothermal crystallization experiment, it is found that nucleation mechanism of PVDF is determined by the diameter of AAO nanopores and interfacial properties. The interface-induced crystallization of PVDF in pristine AAO template dominates in the nanopores. In the FOTS modified AAO template, the crystallization and melting behavior is similar to that of the bulk. In addition, the crystals formed in the pristine AAO template melt at much higher temperature than the bulk crystals. The interfacial layer induced by the AAO surface contributes to the enhancement of melting temperature. Due to the counterbalance between interface effect and confinement effect, the melting temperature decreases slightly with the narrowing of nanopores.

2. EXPERIMENTAL SECTION Poly(vinylidene fluoride) (PVDF) (Mw = 530 000) was purchased from Sigma-Aldrich Corp. The AAO membranes with pore diameters of 30, 60, 100, and 200 nm were bought from Puyuan Nano Ltd. To remove polar and nonpolar substances on the membranes, the AAO membranes were first cleaned with solvents of different polarity (deionized water, nhexane, ethanol, chloroform, and acetone). The surfacemodified hydrophobic AAO was fabricated by a vapor reaction using trichloro(1H,1H,2H,2H-perfluorooctyl)-silane (FOTS)

3. RESULTS AND DISCUSSION Figure 1a,b shows the surface SEM images of a used AAO template before and after PVDF infiltration. The original AAO template, see Figure 1a, exhibits nanopores with diameter of 60 nm. The diameter of the nanopores decreases after infiltration of PVDF, see Figure 1b, indicating the successful infiltration of PVDF into the AAO nanopores and the formation of PVDF hollow nanotubes. This has been confirmed by checking the PVDF nanostructures after removing the AAO template. As 844

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ature with the decrease of the AAO template pore diameter. For example, the exothermic crystallization peak of PVDF caped in AAO D100 appears at 75 °C, while the crystallization peak of the PVDF caped in D30 shifts to around 55 °C. From these results, it can be concluded that the crystallization of PVDF in nanopores is restrained due to the confined environment. For the PVDF caped in AAO D200, the crystallization peak is very broad and shifts to lower temperature compared with that of the bulk. Some areas of the broad peak are overlapped with the crystallization peak of bulk samples. Thus, one can infer that the crystallization of the PVDF in the nanoporous template of D200 is the combination of bulk crystallization and confinement induced crystallization. The crystallization behavior of PVDF in the AAO-F is very different from that in the AAO. For PVDF caped in AAO-F D200, there are two crystallization peaks. One is at around 133 °C, while the other tiny peak is observed at around 120 °C. When the diameter is decreased to 100 nm (AAO-F D100), two crystallization peaks are still observed. The higher crystallization temperature is 133 °C, and the lower crystallization temperature is 115 °C. With the further decrease of pore diameter, the crystallization peak at higher temperature disappears, and the crystallization peak at lower temperature is always observed. The crystallization peak at higher temperature for PVDF in AAO-F is overlapped with the crystallization peak of bulk samples suggesting that the crystallization of the PVDF in the AAO-F at relatively higher temperature is similar to that of bulk crystallization. Considering that the lower crystallization temperature of PVDF in AAO-F template at around 115 °C is only slightly lower than that of the bulk value, we prefer to attribute it to the crystallization similar to the bulk. With the decrease of diameter, the crystallization peak at higher temperature disappears, and only the crystallization peak at lower temperature is left. It should be noted that the crystallization peak at lower temperature for PVDF in AAO-F is much higher than that in the pristine AAO. The remarkably different crystallization peak implies that the crystallization type changes with the interfacial properties of AAO template. In the confined spaces (microdomains, nanopores), the nucleation typically changes from a heterogeneous nucleation to homogeneous or surface/interface-induced nucleation. For simplification, interface-induced crystallization will be referred. Homogeneous nucleation requires the maximum supercooling available to material, and generally it has a small difference between Tg and crystallization temperature (Tc). By comparison, interface-induced nucleation needs lower free energy. It is initiated at the surface of the microdomains or at the interface between the crystallizable polymer and the matrix surrounding

Figure 1. SEM images of the used AAO templates (a) before and (b) after PVDF infiltration. (c) SEM image showing the nanostructures of PVDF after removing the AAO template with an enlarged part inserted.

shown in Figure 1c, nanostructured PVDF with diameter of 60 nm, which is same as the pore diameter of the used AAO template, was observed. With careful inspection, hollow nanotubes of PVDF can be recognized, as indicated by the white arrows. This has been more clearly displayed in the enlarged part as inserted in the upper left corner of the SEM picture. The length of PVDF nanotubes can reach tens of micrometers, with the wall thickness about 20 nm. The PVDF nanotubes with diameters of 200, 100, 30 nm were prepared in the same way with the wall thickness of 25, 18, and 10 nm, respectively. The bright field micrographs of PVDF nanotubes with various diameters measured by TEM are displayed in Supporting Information (Figure S2). For simplification, the AAO templates with the pore diameter of 200, 100, 60, 30 nm are designated hereafter as D200, D100, D60, and D30, respectively. Figure 2a shows the DSC cooling curves of PVDF in the AAO nanopores at a cooling rate of 10 °C/min. For a direct comparison, the DSC cooling curve of bulk PVDF is also presented in Figure 2a. It is clear that the DSC cooling curve of bulk PVDF exhibits a main sharp exothermic crystallization peak at 135 °C. For PVDF caped in AAO D200, a broad crystallization peak appears with the peak position at around 131 °C, while another tiny peak is observed at around 70 °C. For the PVDF caped in the AAO with diameter less than 200 nm, only one low exothermic crystallization peak is observed. This crystallization peak shifts continuously to lower temper-

Figure 2. Cooling thermograms of bulk PVDF and PVDF located inside self-ordered (a) AAO, (b) AAO-F with pore diameters ranging from 200 to 30 nm (cooling rate, 10 °C/min). 845

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Figure 3. Heating thermograms of bulk PVDF and PVDF locating inside (a) AAO and (b) AAO-F (heating rate, 5 °C/min).

Figure 4. XRD patterns changes in the heating process for nonisothermal crystallized PVDF in (a) bulk and (b) AAO template with the pore diameter of 200 nm. (c) The changes of normalized diffraction peak areas by their maximum value of α(020) and α(110) at 142 °C in the melting process.

The above experimental results indicate the dependence of nucleation mechanism of PVDF on pore diameter and interfacial properties of AAO. For iPP in the AAO pores, even though homogeneous nucleation prefers to form with the decrease of pore diameter, heterogeneous nucleation exists until the pore diameter is reduced to 35 nm. For the PVDF in pristine AAO, the heterogeneous nucleation is already suppressed when the pore diameter reaches 60 nm and interfaceinduced nucleation is dominant. The difference may be caused by the interfacial properties of template. Clearly, the AAO-F surface favors the formation of heterogeneous nucleation, and pristine AAO favors the interface-induced nucleation. The melting behavior of PVDF crystals in nanopores is also investigated. Figure 3 shows the heating scans for the nonisothermal crystallized PVDF, either caped in the nanopores or not, with a heating rate of 5 °C/min. Two melting peaks are observed for the PVDF in the bulk. As for the origin of the double melting peaks, it is normally ascribed to (a) melting−recrystallization−remelting in the heating process, (b) presence of polymorphism, (c) variation of morphology (lamellar thickness, perfection of crystals).31 Since the multiple melting behavior of the PVDF bulk is not the focus of this work, it will not be discussed anymore. Here in this work the

them. Distinguishing homogeneous nucleation from interfaceinduced nucleation is not easy. However, a comparison between the Tg and Tc difference provides an important clue for the most likely nucleation mechanism.27 In the present study, the Tc for the crystallization peak at lower temperature in pristine AAO template and Tg difference of PVDF is around 100 °C. An interface-induced nucleation mechanism is more plausible. The polymers like poly(ethylene oxide) (PEO) or polycaprolactone (PCL) have been reported to nucleate homogeneously at temperatures that are much closer to their corresponding Tg values.28−30 There are two crystallization peaks in the AAO D200. The nucleation of PVDF in AAO D200 can be attributed to a kind of combination of heterogeneous nucleation which is similar to that of bulk crystallization and surface or interface-induced nucleation. The lower crystallization temperature at 70 °C belongs to interface-induced nucleation. As far as the PVDF in AAO-F is concerned, both the crystallization peak at higher temperature and the crystallization peak at lower temperature are near the crystallization peak of the bulk. Thus, the crystallization type for PVDF in AAO-F is most probably the same as that of the bulk, and they are induced by the heterogeneous nucleation. 846

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PVDF crystals. To check the effect of the AAO template surface in the melting process, the melting behavior of the nonisothermal crystallized PVDF nanotubes released from AAO templates was measured. The DSC heating scan for the released nanotubes is essentially similar to the melting curve of the crystals within the AAO template (see Figure 5) except for

attention is paid to the melting behavior of the PVDF caped in the AAO template. From Figure 3a, we can see that there are also two melting peaks for PVDF crystals caped in the AAO D200. One melting peak locates at the same position as the lower melting peak of the bulk PVDF. This further implies that the crystallization peak at 131 °C for the PVDF caped in AAO D200 includes the crystallization of the PVDF molecular chains which is similar to bulk crystallization. The other melting peak locates at a higher temperature, even higher than the high melting temperature of the bulk PVDF. The melting temperature for PVDF crystals caped in the nanoporous templates with the pore diameter narrower than 200 nm is a single one and also locates at higher temperature than that of bulk. The single melting peak may suggest that no melt-recrystallization or reorganization occurs in the narrow template. The higher melting peak may relate to the interface-induced crystallization, which will be discussed in the following section. Figure 3b shows the melting curves for PVDF crystals caped in the AAO-F. It can be seen that the double melting peaks of PVDF in AAO-F D200 and D100 locate at the same position with that of bulk. It further suggests that the crystallization for the PVDF caped in AAO-F is similar to bulk crystallization. The melting peak of PVDF in AAO-F D60 and D30 is not observed due to the very low crystallization degree of PVDF. To further identify the higher melting temperature of PVDF crystals in the AAO template, the nonisothermal crystallization and melting behaviors of PVDF in the bulk and AAO templates are monitored by in situ X-ray. As for the nonisothermal crystallization process of PVDF in bulk, the α(020) diffraction peak at 2Θ = 18.24° starts to show up at 147 °C, as can be seen in Supporting Information (Figure S3a). And the peak intensity increases significantly when the temperature decreases to 142 °C. The crystallization of PVDF in AAO D200 occurs at slightly lower temperature than that in bulk, and the α(100), α(020), α(110) diffraction peaks appear simultaneously at 142 °C (Figure S3b). In the heating process, the crystals of PVDF in the bulk start to melt gradually from 142 °C, and they melt completely at 162 °C, as can be seen from Figure 4a. The melting temperature of PVDF in the AAO D200 is much higher than that in bulk (see Figure 4b). The α(100), α(020), α(110) diffraction peaks disappear completely at 170 °C. The changes of the α(020) and α(110) peak area as a function of temperature can be seen in Figure 4c. Obviously, the PVDF crystals are much more stable in AAO D200 nm than that in bulk. The results are consistent with the DSC results. It is worth noting here that the α(100) diffraction is not clear for the bulk PVDF, which may be caused by the different orientation of PVDF crystals in the thin film of tens of microns with those in the AAO nanopores. It is very interesting to observe that the crystals in nanoporous pristine AAO templates have much higher melting temperature than that in bulk, which may imply that the stability of interface-induced crystals is even higher than the bulk crystals although the crystallization occurs at relative lower temperature than that of bulk. By contrast, the melting temperature of PVDF in the AAO-F is similar to that in the bulk suggesting that the interfacial properties of the AAO template determine the melting temperature of PVDF crystals. There are two possibilities for the enhanced melting temperatures: One possibility is that the crystals formed in the nanopores are different from that of the bulk. The other possibility is that the crystals are similar the bulk, but the existence of the AAO template surface hinders the melting of

Figure 5. Heating thermograms of bulk PVDF, PVDF located inside AAO template with pore diameter of 200 nm, and PVDF nanotubes released from the AAO template (heating rate, 5 °C/min).

the observation that the melting enthalpy at relatively lower temperature increases slightly for the nanotubes released from the AAO template. This implies that the molecular chains in released nanotubes are cold crystallized in the heating process, and the melting temperature of such crystals is the same as that in bulk. On the other hand, it can also be realized that the melting temperature of interface-induced crystals is not affected by the removal of the AAO template, as can be evidenced from the unchanged higher melting peak position and enthalpy. Thus, the AAO template surface does not play a very important role in the melting of incorporated PVDF crystals induced by template surface. The high melting temperature of the PVDF nanotubes originates from the intrinsic structure changes of the crystals generated in the nanopores of AAO in the cooling process. Clearly, the crystals formed in the nanopores of AAO have higher stability. Two factors can affect the intrinsic structure of PVDF crystals in nanopores. One is the size of the confinement space effect, and the other is the interface effect exerted by the porous wall. The size effect generally lowers the nucleation temperature of polymer and consequently lowers the melting temperature of polymer crystals. Hence the interface effect exerted by the porous wall should play the key role in enhancing the thermal stability of the PVDF crystals in the nanopores. From the comparative study on the crystallization and melting temperature of PVDF in AAO and AAO-F template, there are indications that the interface effect is a key factor to determine the stability of PVDF crystals. The AAO-F is a hydrophobic interface with significantly lower surface energy. After PVDF is confined in AAO-F, only weak interactions exist between the PVDF and the interface of AAO-F. Hence it is expected that the interface effect of the AAO-F wall on the crystallization of PVDF is different from that of the AAO wall which consequently influences the intrinsic structure of PVDF crystals. Although the intrinsic structure changes of PVDF crystals cannot be observed directly, the crystallization and melting temperature reflects the change of PVDF crystals. Upon comparison of part a of Figure 3 with part b, it is clear that the melting temperature of PVDF confined in AAO-F is much lower than that of AAO. This result implies that the 847

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Figure 6. DSC heating thermograms of PVDF initially prepared in AAO D200, then heat-treated at different temperature for 10 min to erase thermal history (a) before and (b) after removing the AAO template, and finally nonisothermally crystallized during cooling at a rate of 10 °C/min. The heating rate is 5 °C/min.

isotropic chain conformation at elevated temperature. Consequently, its effect on the crystallization and melting behavior of PVDF disappears. Another thing worth discussing is the pore diameter dependence of melting temperature. It can be seen from Figure 3 that the double melting peaks for PVDF in AAO change to a single melting peak for PVDF with the narrowing of pores. With the consideration that the lower melting peak at 157 °C in AAO D200 relates to the crystals which are similar to the bulk, and the higher melting peak at 164 °C correlates to the crystals induced by the porous wall, the absence of lower melting peak in the narrower pores implies that only the crystallization which is induced by porous wall occurs. The melting temperature decreases slightly with the narrowing of pore diameters. Actually, in the nanopores, both confinement effect and interface effect influence the crystallization and melting behavior of PVDF. The confinement effect caused by the size of nanopores generally alters the nucleation mechanism and consequently changes the melting temperature of polymer crystals. In the nanopores with smaller size, stronger confinement is expected, and the crystallization prefers to occur at lower temperature, leading to a lower melting temperature of the crystals. On the other hand the interface effect exerted by the AAO porous wall favors enhancing the melting temperature of PVDF crystals. Under the influence of the dual-function, it can be easily understood that the melting temperature of PVDF in AAO nanopores decreases with the narrowing of pores.

crystals of PVDF formed on the pristine AAO wall are very different from those formed on the AAO-F wall. The former has a relatively higher melting temperature. The molecular chain mobility of PVDF on the pristine AAO wall is different from that on the AAO-F wall, which consequently alters the crystallization behavior of the PVDF. Many researchers have announced the tremendous change in chain conformation and diffusion rate of polymers in contact with a foreign surface.32−37 Nogales et al. reported that there is a strong interaction between the PVDF molecular chains and AAO template surface as indicated by the highly constrained relaxation behavior of PVDF.38 Both van der Waals force and hydrogen bonds contribute to the attractive interaction between alumina walls and PVDF chains. The strong interaction leads to the formation of a layer with reduced mobility. The molecular chains in such a kind of layer are anisotropically coiled which is parallel to the surface. In combination with the present results, it can be inferred that such an interfacial layer leads to the formation of PVDF crystals with higher melting temperature than the bulk crystals. The intrinsic structure of PVDF crystals, which is altered by the interfacial layer of PVDF on pristine AAO wall, needs to be further studied. To further explore the formation of interfacial layer and its influence on the stability of PVDF crystals in nanopores, the effect of thermal history on the melting temperature of produced PVDF crystals is studied. Figure 6a shows the melting curves of PVDF crystals located inside the D200. Before crystallization, the samples are heat-treated at different temperatures. It can be seen that the melting curves do not show any change with heat-treatment temperature. This suggests that the interfacial layer of PVDF formed on the AAO wall is very stable, and the nanoporous wall induced crystals with higher stability always form irrespective of thermal history. However, the effect of thermal history is completely different if the nanotube of PVDF is released from the AAO templates (see Figure 6b). When the heat-treatment temperature is lower than 210 °C, the melting curve is the same as that at lower temperature. However, once the temperature is set at 220 °C, the melting enthalpy at higher temperature decreases significantly, and its position shifts to lower temperature. This implies that the AAO wall induced crystals in the released nanotubes decrease with the increase of thermal treatment temperature. Such a phenomenon can be understood from the viewpoint that the interfacial layer released from AAO templates disappears gradually with the enhancement of thermal treatment temperature. The anisotropic molecular chain conformation in the interfacial layer changes gradually to

4. CONCLUSION The PVDF nanotubes have been prepared by using the AAO templates with two different interfacial properties. One is pristine AAO, and the other is modified by FOTS with lower surface energy. For nonisothermal crystallization, the nucleation mechanism of PVDF in the AAO templates is determined by the pore diameter and interfacial property. In the AAO-F, the nonisothermal crystallization temperature is slightly lower than that of bulk, and the melting temperature is similar to that of bulk. In the pristine AAO, when the pore diameter is 200 nm, the crystallization is induced by two kinds of nucleation: heterogeneous nucleation and interface-induced nucleation. By contrast, in the AAO template with the pore diameter smaller than 200 nm, only interface-induced nucleation occurs. The wall interface-induced crystals have higher melting temperature than the bulk crystals. The interaction between the PVDF and AAO wall produces the interfacial layer. Such an interfacial layer plays an important role in changing the intrinsic structures and enhancing the stability of PVDF crystals. The higher 848

DOI: 10.1021/acs.jpcb.5b11178 J. Phys. Chem. B 2016, 120, 843−850

Article

The Journal of Physical Chemistry B

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melting peak is always observed when the PVDF is nonisothermally crystallized in the AAO template irrespective of the thermal history, suggesting the interfacial layer is very stable on the porous wall. If the PVDF nanotubes are released from AAO template, a higher melting peak disappears when the heattreatment temperature is set at 220 °C, implying that the interfacial layer is destroyed and loses its effect on the crystallization of PVDF.

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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/acs.jpcb.5b11178. Contact angle of AAO and AAO-F templates; the TEM images of PVDF nanotubes with diameters of 200, 100, 60, and 30 nm; and the XRD patterns for bulk PVDF and 200 nm PVDF annealed at different temperature (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundations of China (Nos. 51221002, 21004003, 21574010)

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REFERENCES

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DOI: 10.1021/acs.jpcb.5b11178 J. Phys. Chem. B 2016, 120, 843−850

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DOI: 10.1021/acs.jpcb.5b11178 J. Phys. Chem. B 2016, 120, 843−850