Article pubs.acs.org/Langmuir
Confinement Effects on the Crystallization of Poly(ethylene oxide) Nanotubes Jon Maiz, Jaime Martin, and Carmen Mijangos* Instituto de Ciencia y Tecnología de Polímeros (ICTP), Consejo Superior de Investigaciones Científicas (CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain
ABSTRACT: In this work, we show the effects of nanoconfinement on the crystallization of poly(ethylene oxide) (PEO) nanotubes embedded in anodized aluminum oxide (AAO) templates. The morphological characteristics of the hollow 1D PEO nanostructures were evaluated by scanning electron microscopy (SEM). The crystallization of the PEO nanostructures and bulk was studied with differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The crystallization of PEO nanotubes studied by DSC is strongly influenced by the confinement showing a strong reduction in the crystallization temperature of the polymer. X-ray diffraction (XRD) experiments confirmed the isothermal crystallization results obtained by DSC, and studies carried out at low temperatures showed the absence of crystallites oriented with the extended chains perpendicular to the pore wall within the PEO nanotubes, which has been shown to be the typical crystal orientation for onedimensional polymer nanostructures. In contrast, only planes oriented 33, 45, and 90° with respect to the plane (120) are arranged parallel to the pore's main axis, indicating preferential crystal growth in the direction of the radial component. Calculations based on classical nucleation theory suggest that heterogeneous nucleation prevails in the bulk PEO whereas for the PEO nanotubes a surface nucleation mechanism is more consistent with the obtained results.
1. INTRODUCTION The development of new synthesis routes and preparation methods for achieving polymeric nanostructures continues to be driven not only by interest of the potential applications but also by the interest of scientists in the new properties/behaviors that such nanoscale architectures show with respect to those of the bulk polymer. Among them, the striking consequences of the nanoscopic confinement over the crystallization of polymers are especially attractive.1−3 Currently, there is much scientific interest in and fruitful discussion of the existence of external effects involved in the crystallization and in proposing the exact mechanism by which the crystals nucleate. It is known that the nucleation step can affect both the total crystalline content in the semicrystalline material and the size of the spherulites.4−9 Some of the previous studies have focused mostly on the structure and crystallization behavior of polymers under 1D, 2D, or 3D confinement, revealing the details of the crystal structure in conjunction with the geometric features of confinement. In this case, the crystallization behavior of polymers could be strongly perturbed.10−16 There are two kinds of primary nucleation: © 2012 American Chemical Society
homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation involves the birth of small regions of the crystalline phase in the pure supercooled melt, and heterogeneous nucleation involves the formation of small crystalline regions on or near surfaces.17 It has been well known for a long time that heterogeneous nucleation is the dominant nucleation mechanism in bulk polymers and is initiated at defects or impurities extrinsic to the pure material.11,18 Homogeneous nucleation is less common and intrinsic to the polymer and requires a larger amount of supercooling so that the driving force for crystallization must overcome the intrinsic barrier to nucleation.4,19−22 In addition, it has been reported that for the crystallization of semiflexible homopolymers confined in isolated nanodomains nucleation occurs predominantly at the domain interface and the domain interface accelerates the process.23,24 Thus, our objective is to advance Received: April 26, 2012 Revised: July 24, 2012 Published: July 26, 2012 12296
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volume and the geometry of the nanostructure on the crystal nucleation and growth. PEO nanotubes are prepared by infiltrating the self-ordered porous alumina nanopores with the polymer melt. The morphology of the material was characterized by scanning electron microscopy (SEM). The isothermal crystallization kinetics of bulk PEO and PEO nanotubes were investigated by differential scanning calorimetry (DSC). Wide-angle X-ray diffraction (XRD) was employed to reveal the orientation of crystals.
our knowledge of the effects of confinement on the crystallization of polymers. Control of the process of crystallization and the crystal structure in confined polymers is crucial to the fabrication of nanomaterials with customized properties because many of their most important properties, such as mechanical, optical, or electrical, are directly dependent on the crystalline phase, degree of crystallinity, orientation, and perfection of the crystal.25,26 To date, various systems have been employed to study the crystallization and crystal structure of polymers confined in spaces with dimensions comparable to those of macromolecules. For example, Cheng and co-workers have studied the orientation of polymer crystallites within the microdomains of ultrathin films27 and block copolymers.11,18,28 They have reported that the crystal orientation of poly(ethylene oxide) (PEO) in lamellar domains of poly(ethylene oxide)-bpolystyrene diblock copolymer systems was altered with the crystallization temperature11,29−33 or the cavity dimension28 because of the effect of geometric confinement on the crystallite growth. In this kind of low-dimensional system, the nucleation process acquires a higher specific weight in the overall crystallization kinetics than the crystal growth process.21 In addition, the nucleation mechanism is often altered by the reduced volume of the nanostructure, favoring the homogeneous nucleation of crystals versus heterogeneous nucleation, which is commonly present in the crystallization of bulk polymers. In relation to the crystallization of 1D nanostructures, there are a few published papers, but among them we can mention work on cylinder-forming block copolymers,21,28,34,35 nanofibers obtained by electrospinning,3,36,37 and polymers infiltrating porous templates.10,15,38−43 Some previous studies on the crystallization of polyethylene in AAO templates introduced by Shin, Woo, and co-workers and another studies of syndiotactic polystyrene crystallized in cylindrical nanopores introduced by Wu and co-workers report the surface nucleation as an alternative mechanism to the homogeneous nucleation for polymer crystallization in confinement situations.23,38−40,42 Surface nucleation is proposed to take place when at least these two conditions are satisfied: (1) the crystallizing volume must be small so that common heterogeneous nucleation cannot develop; (2) the crystallizing polymer molecule must have a certain conformation that facilitates the formation of nuclei at temperatures above the homogeneous nucleation temperature for that polymer, for example, a flat-on conformation. This kind of conformation can typically be found near attractive surfaces, such as that of the alumina nanopores. Anodized aluminum oxide (AAO) templates were widely used as templates for the fabrication of polymer-based nanostructures, including nanotubes,44,45 nanorods,46−49 and nanospheres.48 This type of material was first introduced by Martin and co-workers and later used by Martin, Steinhart, Russell, Mijangos, and other groups to fabricate new polymer nanostructures50 and study polymer properties under confinement.44,45,51−54 Nevertheless, the crystallization within the nanopores has been studied only recently,10,41−43,55,56 and a preferential orientation of the crystals has been found.10,38,39,42,43,56−58 In this work, we report how the nanotubular geometry influences the crystallization of PEO. The observed differences in the crystallization temperature and in the orientation of crystals are discussed in terms of the effect of the reduced
2. EXPERIMENTAL SECTION Poly(ethylene oxide), supplied by Scientific Polymer Products, is a homopolymer with a high grade of crystallinity, approximately 80% in the bulk. The melting point (Tm) of PEO is about 65 °C, the glasstransition temperature of the amorphous blocks (Tg) is −55 °C,59 and the mass-average molecular weight, Mw, is 100 000 g/mol. Self-ordered anodic aluminum oxide (AAO) templates, having pores of 400 nm diameter and 100 μm length, were prepared by the two-step anodization method as reported elsewhere.60 For that, the AAO templates were first cleaned with solvents of different polarity (water, ethanol, and acetone) and were then electropolished. Subsequently, a first anodization is performed in an acidic electrolyte under constant voltage, the first alumina layer is dissolved, and then the second anodization is carried out.2 As in any molding process, the fabrication of the polymer nanotubes starts with the infiltration of a mold with a well-defined shape, AAO in this case, by a polymeric fluid (melt or solution), and then the polymer solidifies within the cavity. The polymer melt is a low-surface-energy liquid that spontaneously tends to wet the pore walls. If the experimental conditions allow the polymer to be in a high-mobility regime, that is, in a low-viscosity regime, then infiltration takes place by means of nanoscopic precursor films. Thus, if the AAO pore radius is larger than the thickness of the precursor film, then the films spread and the nanotubes are obtained. If necessary, the molded polymeric material can be removed from the cavity in a third step. PEO infiltrated the AAO nanopores by means of the precursor film infiltration method.2,44,61,62 The AAO templates were annealed at 200 °C in vacuum in order to remove the possible adsorbed organic molecules from the pore walls. These molecules decrease the surface energy of the solid and therefore make the infiltration of the polymer, which is driven exclusively by interfacial forces, difficult. In the precursor film method, the polymer is in the molten state at a temperature well above the glass-transition temperature (Tg) or melting point (Tm) during the infiltration. A piece of PEO was placed on the surface of the AAO template at 110 °C, well above the melting temperature of the polymer, so that the precursor films can develop. After 60 min, the PEO is infiltrated, exclusively covering the pore walls, and then the samples are cooled to room temperature at 1 °C/min and finally cleaned with the aid of a blade to avoid any remaining PEO on the surface. Calorimetric measurements of bulk PEO and PEO nanotubes were conducted in a Perkin-Elmer 8500 differential scanning calorimeter to measure the nonisothermal crystallization kinetics in the cooling mode from the molten state at 20 °C/min. The PEO mass in PEO-infiltrated AAO was estimated from the mass difference between PEO-infiltrated self-ordered alumina and empty self-ordered alumina pieces having the same size and the same pore diameter. To prepare the samples according to DSC measurements, the aluminum substrate was removed with a mixture of HCl, CuCl2, and H2O. DSC traces of PEO-infiltrated self-ordered alumina were recorded using reference pans containing these empty alumina pieces. About 10 mg of each sample (in the case of PEO nanotubes, the mass of the polymer was 0.25 mg and the remaining mass was the alumina template) was heated to 110 °C and kept at this temperature for 10 min. The samples were cooled from 110 to −20 °C at a 20 °C/min cooling rate. Isothermal crystallization tests were performed at temperatures from 46 to 41 °C in the bulk and from 4 to −4 °C in the nanotube samples. The samples 12297
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were heated to 110 °C, held for 10 min, and cooled to the corresponding crystallization temperature (Tc). A Bruker D8 Advance diffractometer equipped with nickel-filtered Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 40 mA was used to characterize the crystalline structure of PEO nanotubes. X-ray diffraction (XRD) data between 5 and 35° 2θ were collected at a scan speed of 0.2 s. The profiles were collected using a Gobel mirror equipped with a position-sensitive detector (Vantec1). Measurements were performed at different temperatures. The samples were first cooled from 60 to −30 °C and then were heated to the melting point. For XRD measurements, the aluminum substrate was maintained; therefore, heat transfer between the equipment holder and our sample occurred through the aluminum contained in the bottom of our sample. The morphological characteristics of the samples were examined by SEM (Philips XL30). It is well known that typical protocols for template removal imply the use of aqueous solvents, in which PEO is soluble. These are not applicable to our system, so we fractured the template to see the morphological structure.
other, thus being able to crystallize independently. The thermograms of bulk PEO and PEO nanotubes are shown in Figure 2a,b, respectively. To compare the results, the
3. RESULTS AND DISCUSSION PEO nanotubes have been obtained for the first time using AAO templates as the mold and the precursor film as the infiltration method. The dimensions of the PEO nanostructures are 400 nm in outer diameter, 100 μm in length, and 35 nm thickness of the tube wall. Figure 1a shows an SEM micrograph
Figure 2. Heat flow vs temperature during nonisothermal sweeps of (a) bulk PEO and (b) PEO nanotubes at 20 °C/min for both.
thermograms have been normalized to the peaks. Bulk PEO displays a sharp crystallization exotherm and a sharp melting endotherm, as shown in Figure 2a, with a peak Tc value of 44 °C and a peak Tm value of 62 °C. This is the typical nonisothermal crystallization behavior of heterogeneously nucleated bulk PEO. However, as seen in Figure 2b, PEO nanotubes crystallize at a Tc value of −8 °C and melt at a Tm value of 60 °C. Similar results have been found for other infiltrated polymers.10,38−40,42,43 The Tm of PEO crystals developed in the nanopores decreases at 2 °C compared to neat PEO. We conjecture that this melting-temperature depression may be due to the decrease in crystallite size and the increase in its interfacial area.43 In our case, it is difficult to know what type of nucleation is being carried out in the polymer inside the nanopores. It is important to consider that the crystallization temperature after homogeneous bulk nucleation depends on the volume of the isolated crystallizable phase involved.4,63 A correlation between droplet volume and the crystallization temperature of homogeneously nucleated PEO was calculated by Müller et al. from a large number of literature data.35 Such a dependence is expected because the probability of nucleation and the nucleation rate depend on the sample volume. The correlation found by Müller et al.35 in terms of the volume of the crystallizing phase was
Figure 1. SEM micrographs of the surface of the AAO template (a) before and (b) after the infiltration of PEO. (c) A bundle of PEO nanotubes extracted from the nanopores.
Tc = −41.8 + 2.89log(vd)
corresponding to the AAO pore arrangements obtained in phosphoric acid solution under optimum conditions. Figure 1b,c show SEM micrographs corresponding to the AAO template surface where both PEO nanotubes within alumina and PEO nanotubes outside the pores exist. In Figure 1c, we can observe some PEO nanotubes that are outside the alumina nanocavity as a consequence of the fracture of the template. These micrographs show that the morphology has been completely transferred from the pores to the polymeric material. To study the crystallization and melting behavior of bulk PEO and PEO nanotubes, DSC experiments were carried out as described in the Experimental section. From Figure 1b, we can observe that all of the nanotubes are separated from each
(1)
where Tc is the peak crystallization temperature during a DSC cooling scan in °C and vd is the volume of the PEO phase in nanometers. The prepared templates for PEO infiltration had a length of 100 μm, hence the volume of one PEO tube is on the order of 2.2 × 109 nm3. Therefore, for homogeneous nucleation, eq 1 would predict an experimental peak crystallization temperature, during a cooling DSC run, of −15 °C. The experimental Tc value obtained from DSC in Figure 2 for the PEO nanotubes was −8 °C. Considering the crystallization temperature given by eq 1 (Tc = −15 °C) and the experimentally obtained (Tc = −8 °C) temperature, one can say that the most probably nucleation process in the PEO nanotubes originates on the surface of the AAO nanopores 12298
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diffraction, the most notable in the pattern of bulk PEO, is not present in that of the PEO nanotubes with the experimental geometry that is used. The (120) direction, which is parallel to the extended chain direction in the crystal, it is known to be the fastest growth direction of PEO crystals. Therefore, taking into account the results reported for PEO nanowires43 and many other similar systems,15,40−42,65 one could expect the (120) direction to be parallel to the pore axis. However, the observed diffraction maxima in the PEO nanotube pattern correspond to planes oriented 33, 45, and 90° with respect to the (120) plane. Thus, our results suggest that crystals grow preferentially in directions with a radial component rather than in an axial direction. As a consequence, we can speculate about a scenario in which nuclei are formed at the active interface for nucleation; in our case, the AAO−PEO interface and crystals grow mainly in the radial direction until they reach the inner nanotube wall, where they die. Figure 4 shows a schematic illustration of PEO crystals within a nanotube.
because the experimental temperature is higher than that obtained in eq 1 as proposed by Müller and co-workers. The crystal structure of PEO in the bulk and in the nanopores was characterized by XRD. Figure 3 shows the XRD
Figure 3. X-ray diffraction patterns of (a) bulk PEO at room temperature (25 °C), (b) PEO nanotubes at room temperature (25 °C), and (c) PEO nanotubes at different temperatures.
Figure 4. Schematic illustration showing the cross section of a PEO nanotube embedded in an AAO template.
pattern of (a) bulk PEO and (b) PEO nanotubes at room temperature. Strong Bragg reflection peaks appear at 2θ = 19.1 and 23.2° for the bulk sample. The peak centered at 19.1° is identified with (120) series planes and an interlayer spacing of 4.5 Å.6 The other strong peaks at 2θ = 23.2° are assigned to (032) series plane diffraction with an interlayer spacing from 3.8 to 4.0 Å. The pattern in Figure 3b is taken for the ensemble of PEO nanotubes located in the AAO matrix in such a way that the wave vector, Q, is parallel to the long axis of the nanotube. The pattern shows the absence of diffraction peaks due to the amorphous nature of the PEO nanotubes at room temperature. Considering that this sample is amorphous at room temperature, the XRD patterns were collected at lower temperatures. Figure 3c contains the XRD profiles of PEO nanotubes acquired during nonisothermal crystallization at different temperatures. Sample crystallization at low temperatures (∼ −30 °C) can be easily observed. The discrepancy between DSC and WAXD experiments is probably associated with poor heat transfer within the chamber of the diffractometer. The main Bragg reflection peak appears at 2θ = 22.8°, which is assigned to the (112) series plane diffraction. The other peaks appear near 24−26° and are also assigned as (024) and (131) series plane diffraction.64 The (120)
The presence of a strong exothermic peak due to the crystallization in bulk PEO and PEO nanotubes allows a comparison of the crystallization kinetics in both samples. Avrami analysis is the most popular and easiest methodology to use in reaching relevant parameters for characterizing the crystallization kinetics of these materials. The value of the Avrami exponent is assumed to range from 1 to 4 and is related to the geometric characteristics of nuclei: n = 1 is ascribed to the 1D structure, 2 is ascribed to the 2D structure, and 3 or 4 is ascribed to the 3D structure. The Avrami theory has been widely and successfully used for the interpretation of the isothermal crystallization process. In the Avrami equation 1 − X(t ) = exp( −kt n)
(2)
X(t) is the relative crystallinty at a specific time; K is the overall rate constant depicting the contribution of the growth rate, the nucleation rate, and the number of nuclei; and n is the Avrami exponent. The Avrami exponent, n, is considered to represent the time dependence of the crystallization in such a way that after considering the time dependence of the nucleation it reflects the dimensionality of the crystal growth when a constant growth rate is assumed. DSC thermograms and plots 12299
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Figure 5. DSC thermograms and Avrami plots for the isothermal crystallization of (a, c) bulk PEO and (b, d) PEO nanotubes. (e) Relationship between temperature and the Avrami exponent for both samples. (f) 1/n ln K vs 1/Tc for Avrami parameter K from isothermal crystallization.
of log {ln[1 − X(t)]} versus log t are shown in Figure 5a−d. It is clearly observed that PEO in the nanopores and in the bulk exhibited different crystallization behavior, especially in the initial stage of isothermal crystallization. From the slope and the intersection of the plots in Figure 5c,d, the values of n and k were calculated, respectively, and the results are summarized in Table 1. For the bulk, n values were found to range between 1.6 to 2.1 whereas for the PEO nanotubes, the values ranged from 1.1 to 1.2. The results are compared in Figure 5e. These lower n
values in nanoconfined PEO are comparable to those observed in other confined semicrystalline polymer systems such as within discrete microdomains of block copolymers,14,21,66 and it has been claimed that they are a consequence of the small crystallizable volume, which hinders crystal growth. The crystallization process for bulk PEO is assumed to be thermally activated, thus Avrami parameter K can be used to determine the energy of crystallization. Several authors67 have tried to describe the variation in terms of the expression of the kinetics 12300
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ACKNOWLEDGMENTS We acknowledge financial support from grants MAT200801073 and MAT2011-24797 from MICINN, Spain. J.M. acknowledges support from grant FPI BES-2009-026632, and J.M. acknowledges support from grant FPU AP2005-1063. We are also grateful to D. Gómez for the SEM experiments and E. Verde for the XRD measurements.
Table 1. Avrami Exponent for Different Crystallization Temperatures for Different Samples of PEO bulk PEO temperature (°C)
n, Avrami exponent
PEO nanotube temperature (°C)
n, Avrami exponent
46 45 44 43 41
2.0 1.9 2.1 1.9 1.6
4 2 0 −2 −4
1.2 1.2 1.2 1.2 1.1
■ (3)
where K0 is the constant of proportionality; R is the perfect gas constant, 8.314 J/K mol; and Ea is the apparent activation energy. Both samples can be compared by plotting 1/n ln K against the reciprocal of the crystallization temperature (Tc). The corresponding plots for PEO bulk and PEO nanotubes can be seen in Figure 5f, where the slope is related to the activation energy of the process. The total activation energy consists of the free energy of formation of nuclei of critical size at their respective Tc values and the activation energy required to transporting molecular segments across the phase boundary to the crystallization surface. According to this definition, the activation energy for nanotubes is much lower than the activation energy for the bulk. If we assume that in surface nucleation the nucleation rate is almost independent of the temperature at sufficiently low Tc and that nucleation governs the overall kinetics because of the geometrical limitation for crystal growth, then we can speculate that in PEO nanotubes, where the surface-to-volume ratio increases, the polymer chain is likely to have more chances to form a nucleus from the surfaces of nanopores.
4. CONCLUSIONS PEO nanotubes have been successfully obtained by templating AAO with 400 nm pore diameter. The crystallization temperature of PEO nanotubes in the nanoporous alumina is significantly reduced compared to that of neat PEO (ΔTbulk→tubes ≈ 50 °C) as a consequence of the transition from a heterogeneous nucleation process in bulk PEO to a surface nucleation process in PEO nanotubes. XRD experiments confirmed the isothermal crystallization results obtained by DSC. XRD analysis shows that at room temperature PEO in the confined system is amorphous and crystallization occurs at low temperatures, which give different diffractions. The PEO crystals in the nanopores influenced by the wall are oriented at different planes compared to the most intensive (120) reflection at 19.1° for the isotropic bulk sample, as evidenced by XRD measurements. We found from the calculations based on classical nucleation theory that heterogeneous nucleation prevails in bulk PEO whereas for the PEO nanotubes a surface nucleation mechanism is more consistent with the obtained results.
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constant, and it was found that the experimental Arrhenius law was the most adequate,
E 1 ln K = ln K 0 − a n RT
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest. 12301
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