Pawan K. Agarwal

Orientation‐induced crystallization in isotactic polypropylene melt by shear deformation

Development of orientation-induced precursor structures (nuclei) prior to crystallization in isotactic polypropylene melt under shear flow was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and rheo-optical techniques. SAXS patterns at 165 °C immediately after shear (rate = 60 s -1 , t s = 5 s) showed emergence of equatorial streaks due to oriented structures (microfibrils or shish) parallel to the flow direction and of meridional maxima due to growth of the oriented layer-like structures (kebabs) perpendicular to the flow. SAXS patterns at later times (t = 60 min after shear) indicated that the induced oriented structures were stable above the nominal melting point of iPP. DSC thermograms of sheared iPP samples confirmed the presence of two populations of crystalline fractions; one at 164 °C (corresponding to the normal melting point) and the other at 179 °C (corresponding to melting of oriented crystalline structures). Time-resolved optical micrography of sheared iPP melt (rate = 10 s -1 , t s = 60 s, T = 148 °C) provided further information on orientation-induced morphology at the microscopic scale. The optical micrographs showed growth of highly elongated micron size fibril structures (threads) immediately after shear and additional spherulities nucleated on the fibrils at the later stages. Results from SAXS and rheo-optical studies suggest that a stable scaffold (network) of nuclei, consisting of shear-induced microfibrillar structures along the flow direction superimposed by layered structures perpendicular to the flow direction, form in polymer melt prior to the occurance of primary crystallization. The scaffold dictates the final morphological features in polymer.

Infra-red investigation of sulphonated EPDM polymers

Abstract Infra-red investigations have been conducted on a sulphonated ethylene propylene diene monomer system (sulpho EPDM) wherein the sulphonate is neutralized with various monovalent and divalent cations. Analysis of these systems was conducted on thin polymer films which were also analysed in spectra from which the unfunctionalized EPDM contribution was subtracted. These studies have revealed that the interaction of cation and sulphonated polymer differs for each cation, and some conclusions have been drawn regarding the symmetry of the cation-sulphonate complex. Surprisingly, the functionalization process induces ordering of the methylene groups in a trans fashion in the solid ionomer. The degree of ordering depends on the specific cation suggesting, for the first time with these polymers, that the ionic aggregation induces an ordered rearrangement of the polymer chains. The ordering in these systems disappears after prolonged heating, suggesting that thermal relaxation of the ionic aggregate destroys the order in the polymer chains.

Quiesent Crystallization Kinetics of Nucleated Metallocene and ZN Isotactic Polypropylenes

Crystallization kinetics and thermodynamic properties of nucleated isotactic polypropylene (PP) are analyzed using Hoffman—Lauritzen crystallization theory to determine the mechanistic effects of the nucleators. Calorimetric data provides quantitative comparisons between nucleating efficiences of the α (Millad) and β (NJSTAR) nucleator in Metallocene (M) and Ziegler—Natta (ZN) PP. The two types of PP without nucleators showed similar crystallization behavior though the T°m for ZN-iPP was about 10°C higher than M-iPP. Both nucleators show significant improvement in crystallization rate in both types of PP. In addition, Millad outperforms NJSTAR. The magnitude of the kinetic response is,however, different and both the nucleators appear to function better in ZN than in Metallocene PP. β nucleated PP shows predominantly the β form. The amount of the β form is thermal history dependent and changes with supercooling (ΔT=T°m−Tc). Similar equilibrium melting temperature (T°m) in the α nucleated and control PPs indicates the lack of any thermodynamic effect of the α nucleator. All nucleated PPs show a much lower secondary nucleation rate constant, Kg.

Influence of the functionality of the quaternizing agent and the polymer molecular weight on the viscoelastic behaviour of α,ω-(dimethylamino)polyisoprenes

Abstract Quaternization of the end-groups of α,ω-(dimethylamino)polyisoprenes with monofunctional, difunctional and trifunctional halides (iodides and bromides) has been undertaken on polymers of different molecular weights ( M n =6000 and 38000 g mol −1 ). The thermal stability of the ammonium end-groups is somewhat better when iodide counteranions are present. Quaternization increases the Tg of the material. This effect is more pronounced for low molecular weight chains, especially when a short alkyl group quaternizing agent is used. However, the size of the alkyl or aryl radical has a minor effect on polymer Tg when difunctional quaternizing agents are used. Nevertheless, the modulus of the rubber-like plateau of materials with difunctional agents is higher than with monofunctional quaternizing agents, owing to coupling reactions between the chain-ends. Rheological measurements indicate that the quaternized low molecular weight polyisoprenes exhibit an Arrhenius-type temperature dependence characterized by activation energies which vary between 121 and 146 kJ mol−1 and suggest that the relaxations attributed to dipole—dipole interactions dominate over long chain entanglements.