Ravi K. Verma

Novel flow apparatus for investigating shear-enhanced crystallization and structure development in semicrystalline polymers

An instrument to study the effects of shearing on the crystallization process in semicrystalline polymers is described. It can impose transient stresses similar to those encountered in polymer processing and provides in situ monitoring of microstructure development during and after cessation of flow. Box-like wall shear stress profiles (rise and fall times under 50 ms with maximum wall shear stress on the order of 0.1 MPa) can be applied for controlled durations. A unique feature of our device is that it accommodates a wide variety of real-time probes of structure such as visible and infrared polarimetry and light and x-ray scattering measurements. The design also allows us to retrieve the sample for ex situ optical and electron microscopy. Data are acquired with millisecond resolution enabling us to record the extent of shear deformation of the polymer melt during the pressure pulse. Our device works with small sample quantities (as little as 5 g; each experiment takes ~ 500 mg) as opposed to the kilogram quantities required by previous instruments capable of imposing comparable deformations. This orders-of-magnitude reduction in the sample size allows us to study model polymers and new developmental resins, both of which are typically available only in gram-scale quantities. The compact design of the shear cell makes it possible to transport it to synchrotron light sources for in situ x-ray scattering studies of the evolution of the crystalline structure. Thus, our device is a valuable new tool that can be used to evaluate the crystallization characteristics of resins with experimental compositions or molecular architectures when subjected to processing-like flow conditions. We demonstrate some of the features of this device by presenting selected results on isotactic polypropylenes.

A.C. dielectric and TSC studies of constrained amorphous motions in flexible polymers including poly(oxymethylene) and miscible blends

Poly(oxymethylene) ( POM ) and its miscible blends were studied by multifrequency A.C. dielectric and thermally stimulated currents (TSC). The blends contained small amounts of either poly(vinyl phenol), which is a high glass transition (T g diluent, or a styrene-co-hydroxy styrene oligomeric low T g diluent. The variation of the 10°C β transition with blend composition proves that it is the glass transition, and that the -70°C y transition is a local motion. Dielectrically the P transition is very weak in pure POM even in fast-quenched samples. The TSC thermal sampling method also detected two cooperative transitions, and β, in POM and its blends, and was used to directly resolve the transition into low and high activation energy components. If one considers the contribution of exclusion of the diluents from the crystal lamellae, it is shown that the blends behave like typical amorphous blends as a function of concentration. The effect of crystals on amorphous motions is examined in light of comparison with van Krevelens 37 predictions of an amorphous T g , and the transitions in POM are contrasted with those for other semicrystalline polymers.

Crystallization study of poly(ether ether ketone)/poly(ether imide) blends by real-time small-angle x-ray scattering

Abstract Morphological changes during isothermal melt crystallization in poly(aryl ether ether ketone) (PEEK)/poly(ether imide) (PEI, Ultem® 1000) blends were monitored via real-time small-angle x-ray scattering (SAXS) using synchrotron radiation. SAXS data were analyzed using a novel combination of correlation and interface distribution functions to determine the lamellar crystal thicknesses l c and interlamellar noncrystalline or “amorphous” layer thicknesses l a. The higher glass transition noncrystalline PEI component slows the PEEK crystallization substantially, but l a (about 40 A) and l c (about 85 A) are independent of crystallization time and blend composition. This is consistent with the known independence of melting temperature with blend composition. These results indicate that PEEK crystallizes in densely crystalline lamellar stacks through all stages of primary crystallization, and that the noncrystalline PEI is almost entirely excluded from the stacks at all times during spherulitic growth....

Note: Development of a cryocooler based high efficiency cryosorption pump

In this communication, we report the development of a cryosorption pump, whose pumping speeds are higher at least by a factor of 1.5 compared to the commercial pumps in specific pressure ranges. This uses a material of high adsorbing capacity along with an adhesive of high thermal conductivity. The measured pumping speeds of these pumps for different gases such as nitrogen, argon, hydrogen, and helium are higher than the commercial cryopumps in the pressure range from 5 × 10-6 mbar to 5 × 10-5 mbar. The developed pump will be quite useful for fusion applications.