Effect of the molecular structure of the polymer and nucleation on the optical properties of polypropylene homo- and copolymers.

Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt%. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation

A hierarchical algorithm for predicting the linear viscoelastic properties of polymer melts with long-chain branching

The “hierarchical model” proposed earlier [Larson in Macromolecules 34:4556–4571, 2001] is herein modified by inclusion of early time fluctuations and other refinements drawn from the theories of Milner and McLeish for more quantitative prediction. The hierarchical model predictions are then compared with experimental linear viscoelastic data of well-defined long chain branched 1,4-polybutadienes and 1,4-polyisoprenes using a single set of parameter values for each polymer, which are obtained from experimental data for monodisperse linear and star polymers. For a wide range of monodisperse branched polymer melts, the predictions of the hierarchical model for monodisperse melts are very similar to those of the Milner–McLeish theories, and agree well with experimental data for many, but not all, of the branched polymer samples. Since the modified hierarchical model accounts for arbitrary polydispersity in molecular weight and branching distributions, which is not accounted for in the Milner–McLeish theories, the hierarchical algorithm is a promising one for predicting the relaxation of general mixtures of branched polymers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47217/1/397_2004_Article_415.pd

What do research ethics committees say about applications to conduct research involving children?

Objective: To identify issues raised by research ethics committees (RECs) in letters about applications to conduct research involving children. Methods: Analysis of 80 provisional and unfavourable opinion decision letters written by RECs in response to applications to conduct research involving child participants. Results: RECs were most likely to be concerned about issues relating to consent, recruitment, care and protection of participants, scientific design and confidentiality. RECs focused on children's status as “vulnerable”. They sought to ensure that children would be protected, that appropriate written language would be used to communicate with children and that an appropriate person would give consent for children to participate. Implications: Researchers should be attentive to issues of potential vulnerability when preparing applications. REC letters may be improved by giving clear and explicit reasons for their opinions