[Show abstract][Hide abstract] ABSTRACT: 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.
Accounts of Chemical Research 08/2013; · 20.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
Physical Chemistry Chemical Physics 02/2011; 13(6):2021-6. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the present work, in situ polymerization method was used to prepare nearly monodispersed microcapsules with long chain normal alkanes as core and melamine–formaldehyde (M–F) resin as shell at reaction temperature both above and below the cloud point of nonionic surfactant. A previously neglected point has been clarified, i.e., changing the reaction temperature is proved to be an effective way to tune the microcapsule size, surface pore size and density. Nano-scaled pores (from 5 to 200nm) on the microcapsule surface were formed by the self-assembly template of nonionic surfactant micelles at different reaction temperatures. The dynamic morphological evolution in the encapsulation process was illustrated, for the first time, by scanning electron microscopy (SEM) at different reaction time. It is the alteration of the hydrophilic–lipophilic balance of crosslinked M–F preploymer in the polymerization process that leads the micelle droplets to migrate inside out, and consequently forms nano- or submicron-pores on the microcapsule surface. The prepared microcapsules have close inner space, providing a good 3-dimensional environment for the confined crystallization of alkanes within the polymeric shell. This methodology is versatile and effective for the synthesis of other porous microspheres, which can be applied potentially for encapsulating lipophilic functional materials.
Colloids and Surfaces A-physicochemical and Engineering Aspects - COLLOID SURFACE A. 01/2011; 384(1):219-227.
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry B 12/2009; 114(3):1388-92. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry B 03/2009; 113(11):3269-72. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The condensed structure of normal alkane (n-alkane) mixtures in confined geometry is an interesting topic concerning the difference in crystallization behavior of odd and even alkanes. In the present work, the crystallization of mixtures of normal octadecane (n-C18H38) and normal nonadecane (n-C19H40) in microcapsules with narrow size distribution was investigated using the combination of differential scanning calorimetry (DSC) and X-ray diffraction (XRD). A surface freezing monolayer for microencapsulated n-C18H38, n-C19H40, and their mixture was detected by DSC, which for the mixture is a mixed homogeneous crystalline phase with continuous change in the composition. A more stable rotator phase (RI) was observed for the microencapsulated n-C18H38/n-C19H40 = 95/5 (molar ratio) mixture, confirmed by an increased supercooling of the transition from RI to stable phase compared to that of the mixture in bulk. Two nucleation mechanisms were speculated as "liquid-to-solid" heterogeneous nucleation and "solid-to-solid" homogeneous nucleation, which occur at different crystallization stages in microcapsules and might be attributed to the surface effect and confinement effect, respectively, in the confined geometry.
The Journal of Physical Chemistry B 01/2009; 112(51):16485-9. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nearly monodisperse microcapsules with controllable porous surface morphologies were prepared by the in situ polymerization 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.
Journal of Materials Chemistry 01/2009; 19(36). · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Physical Chemistry B 10/2008; 112(42):13310-5. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microcapsules with controllable porous surface morphology and good monodispersity were prepared using the one-step synthetic strategy by employing the self-assembly template of nonionic surfactant micelles above its cloud point. Both the pore size (from 100 to 400 nm) and pore density are tunable by changing the amount of core materials or the ratio of core material to shell material. This methodology provides a versatile and effective route for preparation of porous microsphere materials, which can encapsulate lipophilic functional compounds.
Chemistry of Materials - CHEM MATER. 04/2008; 20(9).
[Show abstract][Hide abstract] ABSTRACT: A series of amphiphilic triblock copolymers poly(ethylene glycol)-block-poly(acrylic acid)-block-poly(n-butyl acrylate) (PEG-b-PAA-b-PnBA) differing only in the relative block lengths were synthesized by the acid-catalyzed elimination of the tert-butyl groups from poly(ethylene glycol)-block-poly(tert-butyl acrylate)-block-poly(n-butyl acrylate) (PEG-b-PtBA-b-PnBA), which was synthesized by atom-transfer radical polymerization (ATRP). The degree of polymerization, molecular weight and percentage of hydrolysis of the product PEG-b-PAA-b-PnBA were studied by gel permeation chromatography (GPC), NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF-MS). Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to study the aggregation states of copolymers in water solution. The radii of the copolymer micelles shrink as Ca2+ is introduced into the solutions. The crystallization behaviors of calcium carbonate controlled by copolymer 1 (PEG112-b-PAA86-b-PnBA60) and copolymer 2 (PEG112-b-PAA40-b-PnBA72) differing mainly in the length of PAA block were systematically studied. It was found that the crystallization products are composed of calcite and vaterite, and the ratio of vaterite to calcite increases with increasing the concentration of copolymer 1. For copolymer 2, however, only calcite is obtained at all the concentration range investigated in this work.
[Show abstract][Hide abstract] ABSTRACT: Crystallization and phase transition behaviors of n-nonadecane in microcapsules with a diameter of about 5 mum were studied with the combination of differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (XRD). As evident from the DSC measurement, a surface freezing monolayer, which is formed in the microcapsules before the bulk crystallization, induces a novel metastable rotator phase (R(II)), which has not been reported anywhere else. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R(II) phase and induces its appearance, while the lower free energy in the confined geometry turns the transient R(II) phase to a "long-lived" metastable phase.
The Journal of Physical Chemistry B 08/2006; 110(29):14279-82. · 3.61 Impact Factor