Enthalpy relaxation of rigid poly(vinyl chloride)

The changes which take place on annealing rigid PVC in the vicinity of the glass transition have been followed by differential scanning calorimetry. The changes appear as an increase in the glass-transition temperature and a decrease in the enthalpy with time of annealing. For annealing at 75°C, the enthalpy after 50–100 hr approaches the value characteristic of the equilibrium liquid state. The results obtained for annealing at 65°C and 75°C are in accord with those expected for the relaxation of an amorphous material, and are at variance with those expected on the basis of crystallization taking place on annealing. The enthalpy relaxation process is characterized by a distribution of activation energies centered about 18.8 kcal mole^?1, and seems to reflect a multiplicity of molecular processes.     

Crystallization of polyethylene at high pressure: A kinetic view

A model for the crystallization kinetics of polymers is outlined and is used to interpret observations of the crystallization of polyethylene at high pressures. This model introduces a distinction between σ_e the lamellar surface energy which controls the lamellar thickness, and σ_e′, the surface nucleus surface energy which controls the growth rate. Differential scanning calorimetry and electron microscopy data for several polyethylenes crystallized at pressures of up to 8 kb are presented. From the dependence of lamellar thickness on the crystallization undercooling at 5 kb, it is found that σ_e increases markedly with pressure leading to the formation of very thick crystals at high pressures. The magnitude of the increase in σ_e is in agreement with σ_e values calculated from the dependence of melting temperatures on pressure. The nucleus surface energy σ_e′ is not expected to vary significantly with pressure, and estimates of growth rates of 5 kb which indicate that the growth rate does not vary significantly with pressure at constant under-cooling confirm this. Fractionation effects and the differences in behavior between different polyethylenes are also discussed.     

Gene Synthesis [New Synthetic Methods (77)]

Oligonucleotide synthesis, until a few years ago the rather exotic preserve of a few experts, has become an integral part of the arsenal of molecular-biological techniques. The last decade, in particular, has seen unbelievably rapid development in this area. DNA synthesis has been automated and can now produce genes greater than 1000 base pairs in length. Tailor-made synthetic genes also permit the synthesis of altered or even novel proteins (de novo protein design) by gene-technological methods. Together with modern methods of gene isolation, sequencing, and expression, gene synthesis has played a major part in the enormous advances achieved in gene technology.     

Multiple-step martensitic transformations in Ni-rich NiTi alloys--an in-situ transmission electron microscopy investigation

Multiple-step martensitic transformations in Ni-rich NiTi shape memory alloys have so far been rationalized on the basis of dislocation stress fields, coherency stress fields around Ni 4 Ti 3 precipitates and evolving Ni concentrations between precipitates during ageing. The primary objective of the present paper is to show that such transformations can also occur owing to heterogeneous microstructures that form during ageing of solution annealed defect-free materials. These microstructures are characterized by Ni 4 Ti 3 grain-boundary precipitation and by precipitate-free grain interiors. Two microstructures which give rise to two and three distinct differential scanning calorimetry (DSC) peaks on cooling from the B2 regime are subjected to in-situ cooling and heating cycles in the transmission electron microscope. Martensitic transformations are directly studied and the observations provide a new explanation for multiple-step martensitic transformations in Ni-rich NiTi alloys. Most importantly the results of the present study allow us to understand why DSC chart features on cooling from the B2 regime change during ageing, where they lose their one-step character (after solution annealing at 1123 K for 900 s) and evolve from two-step (after ageing at 773 K for 3.6 ks) to three-step (after ageing at 773 for 36 ks K) transformations.     

On ϵ-Collocation for Pseudodifferential Equations on a Closed Curve

This paper is devoted to the approximate solution of one-dimensional pseudodifferential equations on a closed curve via spline collocation methods with variable collocation points and represents a continuation of [11]. We give necessary and sufficient conditions ensuring the L²-convergence for operators with smooth and piecewise continuous coefficients.