Yunlan Su

Mechanical properties of PNIPAM based hydrogels: A review.

Materials which adjust their properties in response to environmental factors such as temperature, pH and ionic strength are rapidly evolving and known as smart materials. Hydrogels formed by smart polymers have various applications. Among the smart polymers, thermoresponsive polymer poly(N-isopropylacrylamide)(PNIPAM) is very important because of its well defined structure and property specially its temperature response is closed to human body and can be finetuned as well. Mechanical properties are critical for the performance of stimuli responsive hydrogels in diverse applications. However, native PNIPAM hydrogels are very fragile and hardly useful for any practical purpose. Intense researches have been done in recent decade to enhance the mechanical features of PNIPAM hydrogel. In this review, several strategies including interpenetrating polymer network (IPN), double network (DN), nanocomposite (NC) and slide ring (SR) hydrogels are discussed in the context of PNIPAM hydrogel.

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.

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.

Hollow hydroxyapatite spheres fabrication with three-dimensional hydrogel template

Hydroxyapatite [HAp, Ca10 (PO4)6 (OH)2] crystals were successfully prepared by the electrophoresis approach and an ion diffusion method using a template of polyacrylamide (PAAm) hydrogel. Flower-like porous hollow HAp spheres were both obtained in the PAAm hydrogel by the two methods. The diameters of the HAp spheres could be controlled in the range of 500 nm to 28 μm. The formation of flower-like porous hollow HAp crystals is believed to be the combined result of the three-dimensional hydrogel template and electrostatic interaction.

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.

Effects of Biological Soil Crust on Desert Insect Diversity: Evidence from the Tengger Desert of Northern China

Biological soil crusts are widespread in arid and semiarid regions. They are one of the major components of desert ecosystems, and their importance has been demonstrated by numerous researchers. However, little research has been carried out on the relationship between biological soil crusts and the diversity of desert insects. In this study, psammophilous Caragana korshinskii-Artemisia ordosica communities occurring on semifixed dunes and fixed dunes in the Shapotou region at the southeastern fringe of the Tengger desert are investigated. Observation sample plots and the survey quadrats were arranged in the vegetation areas with different types and coverage of biological soil crusts. The insects were surveyed in 10 m × 10 m quadrats and caught by a sampling frame and net-trap methods. The insect number was recorded and samples were collected for identification. The results show that as compared to the un-crusted vegetation area, the presence of biological soil crusts in the Tengger desert significantly increases the diversity and abundance of insect species. Diversity and abundance in the quadrats mainly covered by moss and lichen crusts are significantly higher than those in the quadrats mainly covered by cyanobacteria and algal crusts. The contribution of biological soil crusts to the diversity of insect species may be attributed to the stabilized soil surface providing suitable habitats and some food sources for the insects.

Nano-hydroxyapatite/polyacrylamide composite hydrogels with high mechanical strengths and cell adhesion properties.

Nano-hydroxyapatite/polyacrylamide composite hydrogels were successfully fabricated by physically mixing nano-hydroxyapatite (nHAp) particles into a peroxidized micelles initiated and cross-linked (pMIC) polyacrylamide (PAAm) hydrogel. The nanocomposite hydrogels exhibited excellent mechanical properties. The fracture tensile stresses of the gels were in the range of 0.21-0.86 MPa and the fracture tensile strains were up to 30 mm/mm, and the compressive strengths were up to 35.8 MPa. Meanwhile the introduction of nHAp endowed the composite hydrogels with good cell adhesion properties. This nHAp/PAAm nanocomposite hydrogel is expected to find potential applications in tissue engineering.

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.