Effect of γ-irradiation on slow crack growth in a copolymer of polyethylene

The effect of γ-irradiation on slow crack growth (SCG) in a medium density polyethylene (MDPE) was measured and compared with behavior of high density polyethylene (HDPE) and a recrystallized HDPE (RCHDPE). The three materials exhibited the same dependence on dose up to 3 Mrd. The HDPE became brittle above 50 Mrd. The resistance to SCG of MDPE and RCHDPE increased very rapidly with dose above 3 Mrd, until at 50–80 Mrd their resistance to SCG became extraordinarily high. This high resistance to SCG was accompanied by a transition from crazing to shear deformation at the root of a notch. It was found that for the same concentration, crosslinks are more effective than short chain branches for increasing resistance to SCG. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2349–2354, 1998     

Models for the Carbonyl–ene Cyclization Reaction: Open and Closed Transition States

Different cyclization products are formed with different Lewis acids in the ene cyclization of 5-hexenals (see scheme on the right; a: methylaluminum bisphenoxide, b: Me₂AlCl, c: Sc(OSO₂CF₃)₃). This is not surprising, but it is gratifying that these result can be rationalized with an internally consistent model.     

Dynamic light scattering of poly(n-laurylmethacrylate) in the melt and in concentrated ethyl acetate solutions

Dynamic light-scattering measurements have been made on the “soft” polymer: poly(n-laurylmethacrylate) (PLMA) in solution in ethyl acetate over the concentration range 0.1 < C < 0.9 g/mL as well as melt PLMA. With the concentrated solutions, the hydrodynamic screening length (ξ_h) scales with an exponent of −0.7 below C ≈ 0.3 g/mL. Importantly, ξ_h decreases above 0.3 g/mL, and the concentration dependence increases strongly above this concentration. The latter contrasts with the behavior in a variety of well-studied poly(styrene) solutions, which are inevitably investigated close to the glass point, and for which there is a pronounced increase of ξ_h over the same concentration range. In PLMA solutions, the decrease in ξ_h parallels the behavior of the static screening length (ξ_s) which has been observed in other flexible polymer systems. Dynamic “clusters” (also termed “long-range density fluctuations”) contribute a large part of the scattered intensity above the concentration 0.6 g/mL and also in melt PLMA which contrasts with poly(styrene) systems in which they are absent. In common with the clusters recently observed in another polymeric melt, poly(methyl-p-tolylsiloxane) (ref. 9) this component is diffusive. While, however, in the latter system the clusters “melt out” at 90°C, we find in the well-matured PLMA systems that the cluster amplitude is temperature-independent (within the temperature-time range studied) in the solutions up to at least 80°C and up to at least 150°C in the melt, which were the highest temperatures examined. The clusters in the PLMA systems have a correlation length of the magnitude 65 nm in the melt, whereas in the solutions this dimension varies between 73 nm at 0.9 g/mL to 126 nm at 0.7 g/mL. In PLMA the clusters are presumed to arise from local segregation owing to the large paraffinic side groups on the chain. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1013–1024, 1997     

The Influence of Powder Properties and Tabletting Conditions on the Surface Roughness of Tablets

Tablets of five different compression formulations were investigated for their surface roughness using scanning electron microscopy and non-contact laser profilometry. It was found that the composition of a formulation not only influenced the tabletting properties of the powder mixtures, but also the surface properties of the final product. The addition of larger quantities of very brittle materials such as dibasic calcium phosphate dihydrate increased the surface roughness of tablets. An increase in tabletting pressure reduced the tablet surface roughness. Tablets were found to have smoother edges than faces, presumably due to the comparatively higher shear stress at the die walls and a polishing effect during tablet ejection. The assessment of surface roughness in three dimensions appeared more powerful than a simple line profile measurement.