Dongsheng Fu

Crystallization Features of Normal Alkanes in Confined Geometry

How polymers crystallize can greatly affect their thermal and mechanical properties, which influence the practical applications of these materials. Polymeric materials, such as block copolymers, graft polymers, and polymer blends, have complex molecular structures. Due to the multiple hierarchical structures and different size domains in polymer systems, confined hard environments for polymer crystallization exist widely in these materials. The confined geometry is closely related to both the phase metastability and lifetime of polymer. This affects the phase miscibility, microphase separation, and crystallization behaviors and determines both the performance of polymer materials and how easily these materials can be processed. Furthermore, the size effect of metastable states needs to be clarified in polymers. However, scientists find it difficult to propose a quantitative formula to describe the transition dynamics of metastable states in these complex systems. Normal alkanes [CnH2n+2, n-alkanes], especially linear saturated hydrocarbons, can provide a well-defined model system for studying the complex crystallization behaviors of polymer materials, surfactants, and lipids. Therefore, a deeper investigation of normal alkane phase behavior in confinement will help scientists to understand the crystalline phase transition and ultimate properties of many polymeric materials, especially polyolefins. In this Account, we provide an in-depth look at the research concerning the confined crystallization behavior of n-alkanes and binary mixtures in microcapsules by our laboratory and others. Since 2006, our group has developed a technique for synthesizing nearly monodispersed n-alkane containing microcapsules with controllable size and surface porous morphology. We applied an in situ polymerization method, using melamine-formaldehyde resin as shell material and nonionic surfactants as emulsifiers. The solid shell of microcapsules can provide a stable three-dimensional (3-D) confining environment. We have studied multiple parameters of these microencapsulated n-alkanes, including surface freezing, metastability of the rotator phase, and the phase separation behaviors of n-alkane mixtures using differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD), and variable-temperature solid-state nuclear magnetic resonance (NMR). Our investigations revealed new direct evidence for the existence of surface freezing in microencapsulated n-alkanes. By examining the differences among chain packing and nucleation kinetics between bulk alkane solid solutions and their microencapsulated counterparts, we also discovered a mechanism responsible for the formation of a new metastable bulk phase. In addition, we found that confinement suppresses lamellar ordering and longitudinal diffusion, which play an important role in stabilizing the binary n-alkane solid solution in microcapsules. Our work also provided new insights into the phase separation of other mixed system, such as waxes, lipids, and polymer blends in confined geometry. These works provide a profound understanding of the relationship between molecular structure and material properties in the context of crystallization and therefore advance our ability to improve applications incorporating polymeric and molecular materials.

Crystallization Behavior of Binary Even-Even n-Alkane Mixtures in Microcapsules: Effect of Composition and Confined Geometry on Solid-Solid phase Separation

The crystallization behaviors of binary even-even normal alkane (n-alkane) mixtures (n-C(18)H(38)/n-C(20)H(42), abbreviated as C(18)/C(20)) with different compositions, both in the bulk state and in nearly monodisperse microcapsules, have been investigated by the combination of differential scanning calorimetry and temperature-dependent X-ray diffraction. The solid-solid phase separation, usually observed during the cooling process of bulk samples, is greatly suppressed and even eliminated after being microencapsulated, with the orthorhombic-ordered phase dominating in the low-temperature crystal. Such a crystallization transition is attributed to the special interaction between the two even n-alkanes and the confined environment in microcapsules. The triclinic ordered phase, solely formed by the single even n-alkanes (C(18) or C(20)), becomes less stable due to the weakening of the layered structure and the suppression of the terminal methyl-methyl interactions in the confined geometry, which favors the miscibility of the two components. Furthermore, besides the chain-length difference and the composition, the confined environment is proved to be another important factor to exert strong positive influence on suppressing the solid-solid phase separation of C(18)/C(20) binary system.

Phase change materials of n-alkane-containing microcapsules: observation of coexistence of ordered and rotator phases

In the present investigation, the crystallization and phase transition behaviours of normal alkane (n-docosane) in microcapsules with a mean diameter of 3.6 μm were studied by the combination of differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD) and variable-temperature solid-state nuclear magnetic resonance (VT solid-state (13)C NMR). The DSC and VT solid-state (13)C NMR results reveal that a surface freezing monolayer is formed prior to the bulk crystallization of the microencapsulated n-docosane. More interestingly, it is confirmed that after the bulk crystallization, the ordered triclinic phase coexists with the rotator phase I (RI) for the microencapsulated n-docosane. We argue that the reduction of the free energy difference between the two phases, resulting from the microencapsulation process, leads to the coexistence of the ordered triclinic and rotator phases of the normal alkanes.

Solid−Solid Phase Transition of n-Alkanes in Multiple Nanoscale Confinement

The crystallization behavior of n-C(19)H(40)/SiO(2) nanosphere composites was investigated by a combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). Three kinds of confined alkanes with different solid-solid phase transition supercoolings and a surface (or interface) freezing monolayer of n-C(19)H(40) at the bulk liquid/SiO(2) interface were found in the composites at high SiO(2) loading. The surface freezing monolayer induces the chain packing of bulk alkanes by forming a 2D close-packed arrangement without long-range positional ordering in 3D space. A homogeneous nucleation and growth mechanism is found for the solid-solid transition in confined geometry, in which the supercooling of the transition is sensitive to the confined size.

Nanoparticle Enlarged Interfacial Effect on Phase Transition of 1-Octadecanol/Silica Composites

Motivated by the interest in an interfacial effect on crystallization behaviors and material properties of polymer nanocomposites, phase behaviors of a novel model system for polymer nanocomposite, 1-octadecanol/silica nanosphere composites (C18OH/SiO2), were studied by means of thermal analysis and wide-angle X-ray diffraction. Although a huge specific surface area of silica nanoparticles enlarges the surface-volume ratio of C18OH molecules, surface freezing phenomenon is not observed by DSC in the C18OH/SiO2 composites. While pure C18OH exhibits rotator RIV phase with molecules tilted with respect to the layer normal, the silica network favors and enhances untitled RII phase by disturbing the layering arrangement. Moreover, the confined C18OH shows a polycrystalline mixture of orthorhombic β form and monoclinic γ form. It is demonstrated that the interfacial interaction between the C18OH molecules and the silica surface contributes to the peculiar phase transition behaviors of C18OH/SiO2 composites. The investigation of the model system of long-chain alcohol/nano-SiO2 composites may help us to understand the complicated interfacial effect on phase behaviors and material properties of polymer nanocomposite systems.

Confined Crystallization of n-Hexadecane Located inside Microcapsules or outside Submicrometer Silica Nanospheres: A Comparison Study

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.

Binary n-alkane mixtures from total miscibility to phase separation in microcapsules: enrichment of shorter component in surface freezing and enhanced stability of rotator phases.

The crystallization behaviors of binary normal alkane (n-alkane) mixtures with a series of carbon number difference (denoted as Δn), both in the bulk state and in nearly monodisperse microcapsules, have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As revealed by the DSC data, the surface freezing temperature (denoted as T(s)) of spatially confined binary n-alkane mixtures with large Δn is lower than the calculated value due to the enrichment of shorter component in the surface freezing phase. More alkane molecules with shorter carbon chain are located on the interface between the inner shell of microcapsules and the bulk mixture, thus leading to the decrease of the average chain length of the surface freezing phase and corresponding lower T(s). Furthermore, XRD results have proved that the enhanced surface freezing phenomenon can contribute to the stabilization of the rotator phases in n-alkane mixtures and even induce the crossover of some certain rotator phase (RII) from transient to metastable. However, the decisive reason for such stabilization or crossover is attributed to the suppression of the orienting movement of alkane molecules toward their next-nearest neighbors within the layer of rotator phases.

Preparation of nearly monodisperse microcapsules with controlled morphology by in situpolymerization of a shell layer

Nearly monodisperse microcapsules with controllable porous surface morphologies were prepared by the in situpolymerization of melamine and formaldehyde with a template of nonionic surfactant micelles above the cloud point, inside which normal alkanes can be either encapsulated as phase change material or removed to obtain porous hollow spheres. The experimental results indicate that both the size and density of the pores on the microcapsule surface are tunable by changing the amount of core material (normal alkane) or the ratio of the polymer shell material to core material. The formation mechanism of the surface porosity was investigated by considering the polymerization temperature and the concentration of nonionic surfactants, which were used as the emulsifiers of core material droplets. The thermal gravimetry analysis proved that the microcapsules are thermally stable, and the heat treatment provided a new approach to preparing porous hollow microspheres.

Phase Transition Behavior of a Series of Even n-Alkane Cn/Cn+2 Mixtures Confined in Microcapsules: From Total Miscibility to Phase Separation Determined by Confinement Geometry and Repulsion Energy

The phase behaviors of binary consecutive even normal alkane (n-alkane) mixtures (n-C(n)H(2n+2)/n-C(n+2)H(2n+6), with mass ratios of 90/10 and 10/90) with different average carbon numbers n¯ both in the bulk state (abbreviated as C(n)/C(n+2)) and in nearly monodisperse microcapsules (abbreviated as m-C(n)/C(n+2)), have been investigated by the combination of differential scanning calorimetry and temperature-dependent X-ray diffraction. The phase behavior of n-alkane mixtures gradually shifts from complete phase separation, partial miscibility to total miscibility in both bulk and microcapsules with the increase of average carbon numbers n¯. There are critical points for average carbon numbers of C(n)/C(n+2), where the corresponding mixtures exhibit coexistence of a triclinic phase (formed by alkane with a longer chain) and an orthorhombic ordered phase (formed by the two components of mixtures). Due to the confinement from hard shells of microcapsules, the critical points of m-C(n)/C(n+2) are smaller than those of C(n)/C(n+2). Such a phase behavior originates from the delicate combined action of confinement and repulsion energy for the encapsulated n-alkane mixtures with different average carbon numbers n¯. When n¯ is less than the critical point, the repulsion energy between the two kinds of molecules exceeds the suppression effect of confinement, and phase separation occurs in microcapsules. It is believed that the average carbon number is another important factor that exerts strong negative influence on the phase separation of m-C(n)/C(n+2) systems.

Confined phase diagram of binary n-alkane mixtures within three-dimensional microcapsules.

The confined phase behaviors of microencapsulated normal hexadecane/octadecane mixtures (abbreviated as m-C16/C18) have been investigated by combination of differential scanning calorimetry and in situ wide-angle X-ray scattering. The binary alkane mixtures confined in three-dimensional geometrical space demonstrate two novel crystallization features. The surface freezing is significantly enhanced after C16/C18 mixtures being encapsulated, and the surface monolayer formed is proved to be an ideal solid solution composed by C16 and C18. Furthermore, m-C16/C18 mixtures are trapped into a stabilized rotator phase below the crystallization temperatures, whereas C16/C18 mixtures with certain compositions form the low-temperature crystalline structure directly. These confined crystallization features originate from the jointed effects of spatial confinement and chain mixing of the components. Moreover, the phase diagram of the confined binary alkane mixtures (m-C16/C18) is successfully established for the first time, which enlightens the crystallization features of other spatially confined soft-matter binary systems.