Article pubs.acs.org/JPCB
Confined Crystallization of n‑Hexadecane Located inside Microcapsules or outside Submicrometer Silica Nanospheres: A Comparison Study Dongsheng Fu, Yunlan Su,* Xia Gao, Yufeng Liu, and Dujin Wang* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Crystallization and phase transition behaviors of n-hexadecane (n-C16H34, abbreviated as C16) confined in microcapsules and n-alkane/SiO2 nanosphere composites have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As evident from the DSC measurement, the surface freezing phenomenon of C16 is enhanced in both the microcapsules and SiO2 nanosphere composites because the surface-to-volume ratio is dramatically enlarged in both kinds of confinement. It is revealed from the XRD results that the novel solid−solid phase transition is observed only in the microencapsulated C16, which crystallizes into a stable triclinic phase via a mestastable rotator phase (RI). For the C16/SiO2 composite, however, no novel rotator phase emerges during the cooling process, and C16 crystallizes into a stable triclinic phase directly from the liquid state. Heterogeneous nucleation induced by the surface freezing phase is dominant in the microencapsulated sample and contributes to the emergence of the novel rotator phase, whereas heterogeneous nucleation induced by foreign crystallization nuclei dominates the C16/ SiO2 composite, leading to phase transition behaviors similar to those of bulk C16.
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INTRODUCTION Polymer nanocomposites (PNC) are an emerging class of hybrid materials composed of hard nanoparticles or “fillers” dispersed in a soft polymer matrix. Owing to the improvement of properties including conductivity, toughness, and permeability, polymer nanocomposites are slated for applications ranging from membranes to fuel cells.1−6 Nevertheless, because of the complex crystallization process of high molecular weight polymers, a detailed molecular understanding of the microscopic mechanism is still a challenge. Because long chain normal alkanes are the simplest organic series and are fundamental building blocks of polymers such as polyethylene and poly(ethyl-α-olefin),7,8 the nanocomposites of n-alkanes with silica nanospheres can provide a well-defined model for studying the complex crystallization behavior of polymer nanocomposites. Because n-alkanes are such a fundamental and important system, their crystallization behaviors have been extensively studied,9−23 and it is generally recognized that bulk n-alkanes exhibit two unique features of phase transitions. The first is the surface freezing phenomenon occurring for chain lengths with n values ranging from 15 to 50, which has been widely investigated by X-ray reflectivity,9,10 grazing incidence X-ray diffraction,11 surface tension measurements,12 ellipsometry,13 nonlinear optics,14 and Gibbs adsorption isotherm studies.15 A surface crystalline monolayer is formed up to ∼3 °C above the bulk crystallization temperature and is stacked into a planar hexagonal phase, thus exerting a strong influence on the formation of the bulk solid phases.16 The second unique feature is the formation of so-called “rotator phase”, which has been © 2013 American Chemical Society
identified occurring between the isotropic liquid phase and fully crystallized states.17−20 The rotator phase exhibits long-range order in the molecular axis orientation and the center of mass position but lacks long-range order in the rotational degree of freedom of the molecules around their long axis. Odd- and even-numbered n-alkanes show different crystalline forms of rotator phases (from RI to RV phase).21−23 In recent years, many experiments have shown that nanoparticles in alkanes exhibit remarkable new physical phenomena and that nanoparticles can greatly enhance the physical properties of alkanes and rotator phases.24−27 Silica and silver nanoparticles, stabilized by long chain alkanes, have been prepared and characterized by Rensmo et al.24 They found that upon solvent evaporation, the alkane-stabilized silica and silver nanoparticles self-assemble into closely packed twoand three-dimensional structures. Zammit et al.25 studied the effect of disorder induced by the presence of silica nanoparticles on the RI−RV and RII−RI phase transitions in alkanes by analyzing hysteresis behavior, specific heat, and latent heat. They found that over the RI−RV transition in alkanes, where it has a first-order character in the pure sample, the disorder changes the profile of the specific heat and reduces the extent of the hysteresis region. Prasad et al.27 carried out DSC measurements on nanosilica aerosil particles with tetracosane, an alkane exhibiting different types of rotator phases. The presence of aerosil particles has the general effect of weakening Received: March 4, 2013 Revised: April 18, 2013 Published: April 25, 2013 6323
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Figure 1. SEM images of microcapsules prepared by in situ polymerization using melamine−formaldehyde as shell materials and C16 as core materials (a) and the nearly monodisperse SiO2 nanospheres (b).
Figure 2. DSC traces of n-C16H34 in bulk state (a) and different kinds of confinement (b) during the cooling process.
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MATERIALS AND METHODS n-C16H34 (purity >99%) was purchased from Acros Company and used as received. Nearly monodisperse and highly heatresistant microcapsules were prepared by in situ polymerization31 using melamine−formaldehyde (M−F) resin as the shell material, inside which n-C16 was confined to individual small microspaces surrounded by the M−F resin wall. The SiO2 nanospheres with a diameter of ca. 300 nm were synthesized following the Stöber synthesis method.32 The SiO2 nanospheres with untreated surfaces were weighed in the desired weight fractions and mixed with the alkane solution (alkane/ SiO2 = 10/90) using C16 as the solute and n-hexane as the solvent. The mixture was homogenized ultrasonically at 70 °C and then vaporized at room temperature in a vacuum oven until all the solvent was removed. The particle size and surface morphology of the prepared microcapsules and SiO2 nanospheres were examined by a JEOL-JSM-6700F scanning electron microscope (SEM) fitted with a field emission source and operated at an accelerating voltage of 5 kV. The differential scanning calorimetry (DSC) measurements were carried out on a TA Instruments DSC Q2000 calorimeter under a nitrogen atmosphere at a cooling/ heating rate of 2 °C/min. Specimens were heated from room temperature to 35 °C, cooled to −10 °C, and heated again to 35 °C. The first cooling and second heating thermograms were recorded. Temperature-dependent X-ray diffraction (XRD) experiments were performed on an X’Pert Pro MPD X-ray diffractometer over the temperature region from −10 to 40 °C, using Cu Kα radiation (1.54 Å), power of 40 mA/40 kV, and rotating angle 2θ = 5−40°. The samples with thickness of about 1 mm were enclosed in aluminum foil, first heated from room
all the transitions observed in the system. The nature of the corona of aerosol particles turns out to have an important influence on the phase transitions. In our previous report, we investigated the crystallization of n-nonadecane (C19)/SiO2 nanosphere composites using differential scanning calorimetry.28 We measured the solid−solid phase transition of n-alkanes for different compositions of nalkane/SiO2 nanosphere composites. We showed a depression of the solid−solid transition temperature as a function of the phase size, while the liquid−solid transition temperature is independent of the phase size. Furthermore, like n-alkanes in a microcapsule,29,30 a surface freezing monolayer was detected by DSC in n-alkane/SiO2 composites at high SiO2 levels (higher than 80 wt %). Compared with odd-numbered alkanes, evennumbered ones are distributed more widely in nature, and the study of phase behavior of even-numbered alkanes in confinement has more practical significance. In this work, we designed n-hexadecane (C 16 )/SiO 2 nanosphere composites and studied the crystallization behavior of n-alkanes in nanosized space formed by SiO2 nanospheres. At the same time, the crystallization behaviors of C16 confined by a hard shell microcapsule (abbreviated as m-C16) were also investigated. The main difference between the first and second cases of confinement is that the first case is outside the nanospherical shell and the second case is inside the microcapsule. The enhanced surface freezing phase has been observed by the normal DSC method in both the C16/SiO2 nanosphere composite and m-C16. We observed different liquid−solid and solid−solid phase transitions caused by different nucleation mechanisms in the two confined spaces. 6324
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temperature to 40 °C, kept for 5 min, and then cooled to −10 °C. The heating and cooling rates were all 2 °C/min, and at each temperature point, the samples were equilibrated for about 5 min before measurements.
triclinic crystal in which all the alkane molecules stay in the alltrans conformation. More exothermic peaks are observed during the crystallization process of confined C16, characterized by a small one emerging at a higher temperature than the crystallization point both in microcapsules and in the C16/SiO2 composite. This small peak has been found in m-C1929 and C19/ SiO2 composite28 and is attributed to the enhanced surface freezing phenomenon in confinement. The surface freezing has also been observed for C24 in nanoporous glasses, in which nanopore confinement may allow one to establish the surface freezing state over much larger temperature ranges than is possible for bulk surfaces.34 Previously, surface freezing of bulk n-alkanes has been directly detected by the surface tension method12 or surface-grazing incidence diffraction (GID).11 Although it is assigned a thermodynamic first-order phase transition, the enthalpy of surface freezing is so tiny that it cannot be detected by the normal DSC method because of the small surface-to-volume ratio in bulk n-alkanes and thus few surface molecules standing on the vapor−liquid interface. However, the surface-to-volume ratio is dramatically enlarged in both kinds of confinement, and more C16 molecules stand on the interface between the liquid C16 and the inner wall of the microcapsules (1% of the total molecules)29 or the surface of SiO2 nanospheres (14% of the total molecules, Supporting Information). When the confined samples are cooled to the surface freezing temperature, the C16 molecules on the interface crystallize to form the surface freezing phase, releasing an amount of heat large enough to be detected by the normal DSC method. Novel Solid−Solid Phase Transition in Some Special Confinement. As shown in Figure 2b, in addition to the small surface freezing peak, the big exothermic peaks at 15.4 and 14 °C emerge during the cooling process of the microencapsulated
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RESULTS AND DISCUSSION Obviously, as shown in Figure 1, the nearly monodisperse microcapsules are three-dimensional (3D) hard confinement (d1 = 3−5 μm) because C16 was spherically microencapsulated and confined to individual small microdomains surrounded by the noncrystalline wall of M−F resin. For the C16/SiO2 composite, the n-alkane is homogeneously dispersed into the interspace between the SiO2 nanospheres (d2 = 300 nm), which adopt a face-centered cubic (fcc) closely packed structure at a high SiO2 volume fraction.33 Similar Enhanced Surface Freezing Phenomenon in Both Kinds of Confinement. All three samples (bulk C16, mC16, and C16/SiO2 composite) were first investigated by the DSC method, and the results are shown in Figure 2 and Table 1. A single exothermic peak emerges during the cooling process Table 1. Phase Transition Temperature and Temperature Range of Phases of n-C16H34 in Bulk State and Different Kinds of Confinement C16
Tsf (°C)
TL‑C/R (°C)
temperature range of surface freezing (°C)
TR‑C (°C)
temperature range of rotator phase (°C)
b-C16 m-C16 C16/SiO2
− 17.8 16.6
15.9 15.4 14.0
− 2.4 2.6
− 10.7 −
− 4.7 −
of the bulk C16 (Figure 2, black line). It is attributed to the liquid−crystal transition from the isotropic liquid state to the
Figure 3. Temperature-dependent XRD results of n-C16H34 in different kinds of confinement during the cooling process: (a) bulk state, (b) m-C16, (c) C16/SiO2. 6325
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Figure 4. Schematic illustration of the phase transition mechanism for n-C16H34 in the bulk state and different kinds of confinement.
n-hexadecane and C16/SiO2 composite, respectively. The big exothermic peak with smaller supercooling is assigned to the bulk crystallization, in which the peak is narrow in the former and broad in the latter because of the multiple nanoscale confinements induced by different confined sizes in the silica composite.28 Another exothermic peak at 10.7 °C with larger supercooling emerges in the low-temperature region during the cooling process of m-C16 but not during the cooling of bulk C16 or C16/SiO2 composite. Previously, some new exothermic peaks that do not emerge in the crystallization process of bulk nalkanes have been observed in the DSC traces after microencapsulation, and it was attributed to the novel rotator−rotator or rotator−crystal transitions of the confined n-alkanes. The hexagonally packed rotator phase II (RII) was found during the crystallization of m-C19,29 while the orthorhombic rotator phase I (RI) was detected in m-C18.30 Therefore, it is reasonable to speculate that the newly emerging peak of m-C16 in the low temperature is due to a new solid− solid phase transition, and a rotator phase which is originally transient in bulk C16 emerges as a metastable one in microcapsules. To further test the speculation, temperature-dependent X-ray diffraction (XRD) experiments were performed on all three samples. Figure 3 shows the variation of different diffraction
patterns with decreasing temperature. For bulk C16 (Figure 3a) above the melting temperature, the sample is in the isotropic liquid state characterized by a single halo. As the temperature decreases to 15 °C, the triclinic peaks of (010), (011), (100), and (111) emerge, indicating the completion of the liquid− crystal transition. Therefore, the bulk C16 crystallizes into the stable triclinic phase directly from the liquid state, and no rotator phase appears during the cooling process. Similar situations are observed for the C16/SiO2 composite (Figure 3c), and the liquid−crystal transition is dominant during the cooling process of the sample. As the m-C16 is cooled to 14 °C, however, two diffraction peaks (110) at 2θ = 21.1° and (200) at 2θ = 22.9° appear, which are the characteristic peaks of orthorhombic RI (Figure 3b). With further cooling to 10 °C, the characteristic peaks of (010), (011), (100), and (111) for the triclinic crystal emerge. At the same time, the (110) and (200) diffraction peaks still exist, showing the coexistence of RI and the triclinic crystal in this temperature range. With further decreasing temperature, the (110) and (200) peaks of RI disappear, indicating that the rotator−crystal phase transition has reached completion. Hence, the XRD results are in good agreement with DSC data that m-C16 crystallizes into the stable triclinic phase through a metastable rotator phase. The surface freezing was 6326
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heterogeneous nucleation induced by the enhanced surface freezing phase is dominant in the liquid-to-solid phase transition of the microencapsulated n-alkanes. According to Sirota and Ocko,10,23 the surface monolayer of n-alkanes tends to pack into a two-dimensional hexagonal crystal, which is similar to the in-planar structure of the RI phase and can be an ideal nucleation site of the RI phase, reducing the interfacial tension and energy barrier for its formation (Figure 4e).29 It was reported that the transient rotator phase of C16 crystallized from the liquid to the low-temperature triclinic phase in emulsified droplets (d = 30 μm) using time-resolved twodimensional small- and wide-angle X-ray scattering.43 The crystallization behavior of C16 solidified in mesoporous silicon (10 nm) showed a stable orthorhombic RI phase in both the cooling and melting processes.40,44 In addition, the layered structure of n-alkane is destroyed by the microencapsulated confinement, which is not beneficial to the transformation of RI to the triclinic phase, and induces the stabilization of the RI phase.45 With the decrease of temperature, the RI phase transforms to the triclinic phase (Figure 4f). Different from the microcapsule system, the fcc closely packed structures formed by the silica nanosphere are not closed systems. The interspace between the SiO2 nanospheres has some connections; therefore, the effect of the foreign crystallization nuclei on the whole alkane still exists. Although the large specific surface area provided by SiO2 nanospheres induces the enhancement of surface freezing, similar to the bulk n-C16 during the cooling process, heterogeneous nucleation induced by foreign crystallization nuclei is significantly dominant (Figure 4h). Therefore, the triclinic crystal phase emerges, and no rotator phase appears after the liquid−solid transition (Figure 4i).
not detected by XRD perhaps because of the spherical structure of microcapsules.29 Heterogeneous Nucleation Induced by Different Agents in Different Kinds of Confinement. It has been reported that the crystallization process is composed of two parts: nucleation and crystal growth.35,36 The small amount of foreign crystallization nuclei plays such an essential role during the nucleation process that they can be regarded as the crystal nuclei of the bulk materials, and as a result, the interfacial tension is significantly reduced. Therefore, the heterogeneous nucleation induced by foreign crystallization nuclei is the most common mechanism during crystallization. However, once the effect of foreign crystallization nuclei is suppressed, the crystal nuclei are formed only by the bulk material itself; thus, homogeneous nucleation emerges, characterized by a large supercooling.37 For the n-alkanes, homogeneous nucleation is usually observed in the confinement because the foreign crystallization nuclei are isolated and fail to contribute to the nucleation. Ueno et al.38 used synchrotron radiation (SR) X-ray diffraction of small- and wide-angle areas combined with differential scanning calorimetry to measure the crystallization behavior of C16 in oil-in-water emulsion droplet (0.9 μm) and observed the orthorhombic rotator phase and 15 °C supercooling after the addition of a high-melting-point surfactant. The crystallization behavior of C16 in miniemulsion (d = 218 nm) showed 16 °C supercooling in the cooling process and 0.7 °C supercooling in the melting process compared to the bulk.39 This is explained by the fact that in miniemulsions, each droplet must be nucleated separately, and the nucleation mechanism is shifted from heterogeneous to homogeneous nucleation. For the bulk even-numbered n-alkanes, phase transitions and rotator phases are not so abundant as for the odd-numbered nalkanes. In the crystallization process, RI is first observed in nC20, whereas during the heating process, rotator phases emerge from C22 to C40.40 For the even-numbered n-alkanes with short chain length (n < 20), no rotator phase is detected, and the nalkanes will be trapped directly into the triclinic crystal phase after the liquid−solid phase transition. However, previous investigations have shown that the confinement can contribute to the emergence of rotator phases, and the RI phase has been found in confined C14, C16,41 and C18.30 In the present work, nC16 was trapped in two kinds of confinement, the enhanced surface freezing phenomenon was observed in both, but the rotator phase (RI) was found only in m-C16. We believe that different kinds of nucleation play a decisive role in the liquid− solid phase transition. For the bulk n-C16, heterogeneous nucleation induced by foreign crystallization nuclei is significantly dominant during the cooling process (Figure 4a); therefore, the triclinic crystal phase emerges after the liquid−solid transition (Figure 4b). The situations in the crystallization of m-C16 are totally different. After microencapsulation, the foreign crystallization nuclei are distributed among a large number of isolated microcapsules. As seen in the emulsion systems, there are no links among the alkanes in these microcapsules (Figure 4d).38 The probability of any individual microcapsule containing a crystallization nucleus is, however, practically zero.42 Therefore, the influence of foreign crystallization nuclei on the whole alkane is significantly suppressed. The enhanced surface freezing phase due to the larger specific surface area in microcapsules emerged before the main liquid−solid phase transition and was regarded as the perfect template and crystal nuclei for the bulk n-alkane molecules.29 Therefore, the
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CONCLUSION In summary, differences exist between the low-temperature solid−solid phase transition of C16 in microcapsule and in SiO2 nanosphere composite. The microcapsule provides completely separate confined spaces in which the effect of the foreign crystallization nuclei on the crystallization of alkanes can be ignored. The heterogeneous nucleation induced by surface freezing dominates the liquid−solid phase transition, contributing to the formation of the RI phase. The confined spaces in silica nanospheres have connections, and the heterogeneous nucleation induced by the foreign crystallization nuclei dominates the liquid−solid phase transition. Compared with the confined C16, bulk C16 shows a simple phase transition: it crystallizes into a stable triclinic phase directly from the liquid state.
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ASSOCIATED CONTENT
S Supporting Information *
Calculation of the weight of C16 molecules on the surface of SiO2 nanospheres. This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Phone and fax: +86-10-82618533. E-mail: Y.S.,
[email protected]; D.W.,
[email protected]. 6327
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Author Contributions
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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51103166) and China National Funds for Distinguished Young Scientists (Grant 50925313) for financial support.
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