The microstructure and mechanical properties of β-nucleated iPP before and after being annealed at different temperatures (90-160 °C) have been analyzed. Annealing induced different degrees of variation in fracture toughness of β-nucleated iPP samples, namely, slight enhancement at relatively low annealing temperatures (<110 °C) and great improvement at moderate temperatures (120-130 °C), whereas dramatic deterioration at relatively high temperatures (>140 °C) has been observed. The variation of fracture toughness of β-nucleated iPP is observed to be dependent on the content of β-NA. Experiments, including scanning electronic microscope (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA), are performed to study the variations of microstructures as well as the toughening mechanism of the β-nucleated iPP after being annealed. The results indicate that the decreased number of chain segments in the amorphous region and the formation of microvoids, which is easily triggered by the secondary crystallization at 120-130 °C, are mainly responsible for the great improvement of toughness through promoting the lamellae to slip or elongate along the impact direction and inducing the intense plastic deformation during the fracture process.

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The field of tissue engineering and regenerative medicine will tremendously benefit from the development of three dimensional scaffolds with defined micro- and macro-architecture that replicate the geometry and chemical composition of native tissues. The current report describes a freeform fabrication technique that permits the development of nerve regeneration scaffolds with precisely engineered architecture that mimics that of native nerve, using the native extracellular matrix component hyaluronic acid (HA). To demonstrate the flexibility of the fabrication system, scaffolds exhibiting different geometries with varying pore shapes, sizes and controlled degradability were fabricated in a layer-by-layer fashion. To promote cell adhesion, scaffolds were covalently functionalized with laminin. This approach offers tremendous spatio-temporal flexibility to create architecturally complex structures such as scaffolds with branched tubes to mimic branched nerves at a plexus. We further demonstrate the ability to create bidirectional gradients within the microfabricated nerve conduits. We believe that combining the biological properties of HA with precise three dimensional micro-architecture could offer a useful platform for the development of a wide range of bioartificial organs.

Hydrogel‐based scaffolds are widely used for culturing cells in three dimensions due to their tissue‐like water content and tunable biochemical and physical properties. Most conventional hydrogels lack the macroporosity desirable for efficient cell proliferation and migration and have limited flexibility when subject to mechanical load. Here microribbon‐like elastomers that, when photocrosslinked, can form macroporous and highly flexible scaffolds that support cell proliferation in 3D are developed. These microribbons are produced by wet‐spinning gelatin solution into microfibers, followed by drying in acetone, which causes asymmetrical collapse of microfibers to form microribbon‐like structures. Gelatin microribbons are then modified using methacrylate anhydride to allow further photocrosslinking into 3D scaffolds. The macroporosity and mechanical properties of the microribbon‐based scaffold may be tuned by varying wet‐spinning rate, drying temperature, choice of drying agent, level of glutaraldehyde crosslinking, and microribbon density. When encapsulated in the microribbon‐based scaffold, human adipose‐derived stromal cells proliferated up to 30‐fold within 3 weeks. Furthermore, microribbons‐based scaffold demonstrate great flexibility and can sustain up to 90% strain and 3 MPa stress without failing. The unique mechanical properties of microribbon‐based scaffolds make them promising tools for engineering shock‐absorbing tissues such as cartilage and intervertebral discs.

In the present work, the crystalline structures and the melting behaviors of poly(L‐lactide) (PLLA) obtained after being annealed at different conditions have been investigated through differential scanning calorimetry and wide‐angle X‐ray diffraction, respectively. To improve the crystallization of PLLA, functionalized multiwalled carbon nanotubes (f‐MWCNTs) are introduced into PLLA. Our results show that by prolonging the annealing duration or enhancing the annealing temperature, the degree of crystallinity of PLLA gradually increases. Very important, the addition of f‐MWCNTs promotes the cold‐crystallization of PLLA dramatically even at relatively lower annealing temperature or in shorter annealing duration. Further results show that, whether in neat PLLA or in PLLA/f‐MWCNTs nanocomposite, only α form crystal forms during the annealing process. The glass transition temperature shifts to high temperatures because of the increase of crystallinity. F‐MWCNTs exhibit great heterogeneous nucleation effect for PLLA crystallization through enhancing the nucleation density, leading to homogeneous and tiny spherulites formation in a very short time. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 326–339, 2009

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ABSTRACT:The synergistic toughening effect of nucleating agent (NA) and ethylene-octene copolymer (POE) on polypropylene was studied in the present work. Two different nucleating agents, such as a-form nucleating agent 1,3 : 2,4-bis (3,4-dimethylbenzylidene) sorbitol (DMDBS, Millad 3988) and b-form nucleating agent aryl amides compounds (TMB-5), were selected to blend with PP or PP/POE blends, respectively. The results show that PP containing 0.5-0.25 wt % DMDBS or 0.5-0.25 wt % TMB-5 has relatively low impact strength. For PP/POE blends, although the impact strength increases gradually with the increasing of POE content, high content of POE is needed to obtain the available PP toughness. However, once nucleating agent and POE are simultaneously added into PP, PP/POE/NA blends show great improvement of toughness even at low POE content. Furthermore, the synergistic toughening effect of POE/TMB-5 is more apparent than that of POE/DMDBS. SEM results show that whether DMDBS or TMB-5 has no apparent effect on the morphologies of POE in the PP/POE/NA blends. Further investigations using DSC and POM indicate that both DMDBS and TMB-5 induce the apparent enhancement of the crystallization temperature of PP and the sharp decrease of spherulites size of PP in the PP/POE/NA blends. The possible synergistic toughening mechanism is discussed in the work.

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