Baoquan Xie

Crystallization behaviors of n-octadecane in confined space: crossover of rotator phase from transient to metastable induced by surface freezing.

In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 microm were studied with the combination of differential scanning calorimetry (DSC), temperature dependent Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The main discovery is that the microencapsulated n-octadecane crystallizes into a stable triclinic phase via a mestastable rotator phase (R I), which emerges as a transient state for the bulk n-octadecane and is difficult to be detected by the commonly used characterization methods. As evident from the DSC measurement, a surface freezing monolayer, which is formed at the interface between the microcapsule inner wall and n-octadecane, induces the crossover of the R I from transient to metastable. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R I phase, and consequently the lower relative nucleation barrier in the confined geometry turns the transient R I phase into a metastable one.

A tough hydrogel–hydroxyapatite bone-like composite fabricated in situ by the electrophoresis approach

Mechanically strong hydrogel-HAp composites have been successfully fabricated through in situ formation of hydroxyapatite (HAp) in a tough polyacrylamide (PAAm) hydrogel with a modified electrophoretic mineralization method. The pre-swelling of the PAAm hydrogels in CaCl2 buffer solutions makes the electrophoresis method able to produce large area (10 × 8 cm2) hydrogel-HAp composites. At the same time the CaCl2 solution with different concentrations could control the HAp contents. The obtained hydrogel-HAp composites exhibit enhanced mechanical properties, namely higher extensibility (>2000%), tensile strength (0.1-1.0 MPa) and compressive strength (up to 35 MPa), in comparison to the as-synthesized PAAm hydrogels. FTIR and Raman characterizations indicate the formation of strong interactions between PAAm chains and HAp particles, which are thought to be the main reason for the enhanced mechanical properties. The hydrogel-HAp composite also shows excellent osteoblast cell adhesion properties. These composite materials may find more applications in biomedical areas, e.g. as a matrix for tissue repair especially for orthopedic applications and bone tissue engineering.

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.

A novel biocompatible double network hydrogel consisting of konjac glucomannan with high mechanical strength and ability to be freely shaped

A novel physically linked double-network (DN) hydrogel based on natural polymer konjac glucomannan (KGM) and synthetic polymer polyacrylamide (PAAm) has been successfully developed. Polyvinyl alcohol (PVA) was used as a macro-crosslinker to prepare the PVA-KGM first network hydrogel by a cycle freezing and thawing method for the first time. Subsequent introduction of a secondary PAAm network resulted in super-tough DN hydrogels. The resulting PVA-KGM/PAAm DN hydrogels exhibited unique ability to be freely shaped, cell adhesion properties and excellent mechanical properties, which do not fracture upon loading up to 65 MPa and a strain above 0.98. The mechanical strength and microstructure of the DN hydrogels were investigated as functions of acrylamide (AAm) content and freezing and thawing times. A unique embedded micro-network structure was observed in the PVA-KGM/PAAm DN gels and accounted for the significant improvement in toughness. The fracture mechanism is discussed based on the yielding behaviour of these physically linked hydrogels.

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.

Suppression of the Phase Separation in Binary n-Alkane Solid Solutions by Geometrical Confinement

The crystallization of binary n-alkane solid solution n-C(18)H(38)/n-C(19)H(40) = 90/10 (molar ratio) (abbreviated as C(18)/C(19) = 90/10) and the microencapsulated counterpart (abbreviated as m-C(18)/C(19) = 90/10) has been investigated by a combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). The solid-solid phase separation was obviously detected in C(18)/C(19) = 90/10 by XRD, which is absent in m-C(18)/C(19) = 90/10. The XRD data also show that the chain packing of m-C(18)/C(19) = 90/10 is different from that of bulk C(18)/C(19) = 90/10. The packing mode of m-C(18)/C(19) = 90/10 molecular chains is unique; i.e., the n-alkane chains pack along the longitudinal direction and the neighboring layers interdigitate with each other, subsequently resulting in the deconstruction of lamellar ordering. The extinction of phase separation in m-C(18)/C(19) = 90/10 can be understood in terms of the suppression of longitudinal chain diffusion caused by the special three-dimensional confinement effect provided by microcapsules.

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.

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.

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.