Raman spectroscopy reveals differences in collagen secondary structure which relate to the levels of mineralisation in bones that have evolved for different functions

Bone is a composite material comprising a collagen fibril scaffold surrounded by crystals of carbonated‐hydroxyapatite mineral. It is well established that the relative proportions of mineral and collagen in mature bone are not definite and are adapted in order to ‘tune’ its mechanical properties. It is not known, however, how the mineral to collagen ratio is controlled. This paper uses Raman spectroscopy (which permits the probing of both the mineral and the collagen phases of bone) to explore the hypothesis that the control mechanism is related to the nature of the collagen and that bones with different levels of mineralisation have qualitatively different collagen. Raman spectra of functionally adapted bones with varying levels of mineralisation are presented and features that indicate the differences in the collagen's secondary structure (amide I band profiles) and post‐translational modification (hydroxyproline/proline ratios) are highlighted. The study demonstrates that Raman spectroscopy can provide a means to investigate the mechanisms that control the mineral to collagen ratio of bone. Understanding these mechanisms could pave the way towards the therapeutic alteration of the mineral to collagen ratio and, thus, the control of the mechanical properties of bone. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel methods for synthesizing halide-free alane without the formation of adducts

Many of the current synthesis methods for aluminum hydride (alane—AlH₃) involve reacting AlCl₃ and LiAlH₄ in solvents. The reaction requires the formation of an alane adduct such as AlH₃⋅[(C₂H₅)₂O] prior to obtaining crystallized stable α-AlH₃. This process requires several hours of pumping in a vacuum system to remove the ether and convert the alane etherate into stable α-alane. This crystallization process is both costly and hazardous because a large amount of highly flammable material (e.g. ether) is removed by vacuum pumps over several hours. Conversely, the work presented herein describes novel methods to synthesize adduct-free alane. It is demonstrated here that AlH₃ can form by mixing AlCl₃ and LiAlH₄ in the solid state and heating to 75^∘C; only α-AlH₃ was obtained. The α-AlH₃ product can be washed with minimal solvents leading to zero formation of alane adducts. In addition, the unwanted LiCl by-product is also removed during the solvent wash, resulting in halide-free α-alane. Although simply mixing and heating the reactants led to a 40% yield of alane, having the reactants compacted and mechanically pressed while heating increases the yield to 60% crystalline α-AlH₃.     

Neck propagation in polyethylene

Intrinsic true stress–true strain response was evaluated at room temperature for three linear polyethylene samples deformed in conventional tensile tests. It was observed that high crystallinity is associated with a low rate of strain hardening that results in a sharp neck and a large drop in nominal stress. The maximum and minimum deformation loads are accounted for by Considère's analysis of neck initiation and stabilization, respectively. Following stabilization, neck propagation occurs at a load or nominal stress that is lower than the yield stress. The jump analysis of Ericksen and Hutchinson/Neale predicts steady state neck propagation stresses that are in very good (ca. 10%) agreement with experiment. Although the jump analysis is done in terms of uniaxial stress, the actual value of the propagation stress is established by the triaxial stress state in the neck shoulders. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2081–2091, 2004     

Plastic deformation of glassy polystyrene: A unified model of yield and the role of chain length

A study was made of yield and plastic flow in glassy polystyrene. A range of 12 linear atactic polystyrenes was studied: monodisperse, bimodal blends, and a polydisperse commercial sample. M_n varied between 66,000 and 490,000 g/mol. These were given standardized thermal treatments and then subjected to uniaxial compression tests in the glassy state over the temperature range 40 to 95 °C and nominal strain-rates 10⁻⁴ to 10⁻³ s⁻¹. Their constitutive responses were interpreted in terms of the physically based three-dimensional constitutive model for small or large deformations in amorphous polymers proposed earlier (Polymer 1995, 36, 3301–3312), including plastic strain-induced structural rejuvenation. In multimode form, the model captured closely both linear viscoelastic response and yield and plastic flow. When the reduction of Vogel temperature caused by chain ends was incorporated in the model, it predicted a fall in yield stress with reducing molecular length. This was also observed in experimental data, with the rate of fall approximately in agreement. The results provide further support for the model as a unifying framework for describing the physical properties of polymer glasses. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2027–2040, 2004