Title: Morphology of Low to Very Low Density Ethylene-Octene Metallocene Copolymers
Abstract: The melting phenomena of random, hexyl-branched ethylene copolymers have been examined for branch contents ranging from 4-53 SCB/1000C at near constant molecular weight with low polydispersity (ca. 2.0) corresponding to densities of ca. 0.94-0.866 g/cm3. We have found that, irrespective of morphology, melting after isothermal crystallization and bulk isothermal crystallization phenomena can be grouped into two types of behavior: Group 1 (Gl), nominally with up to 2% defects (as branches) and Group 2 (G2) with greater than 2% and up to 8.5% defects. The first group is characterized by single peak melting behavior above Tc and classical annealing via thickening as evidenced by Tm–log time relations, at least for short times. G1 follows the Avrami equation up to ca 90-98% of the developed crystallinity with growth as spherulites. Group 2 is generally characterized by two melting peaks above Tc (one narrow, one broad). These samples show distinctive two-stage Avrami crystallization curves with lower than expected initial Avrami index where spherulitic forms are growing (up to 0.88 g/cm3) and even lower indices where no spherulitic forms are observed (below 0.872 g/cm3). Three unexpected phenomena are observed for the melting of isothermally crystallized G2 samples: (a) the narrower peak melts at a lower temperature than an underlying, broad peak; (b) the early peak is absent at short times (less than 1 m) and develops with increasing time and shifts to higher melting temperatures with time and; (c) the early melting peak appears representative isothermally crystallized samples, at lower undercoolings, insofar as these melting points show parallel Tm-Tc behavior in a Hoffman-Weeks Plot whereas the broad melting peak (observed at short times) exhibits more conventional behavior. The use of WAXD, DSC, and to a lesser extent, density are used on slow cooled and quenched bulk crystallized samples to gain some insight into this process.
Publication Year: 1998
Publication Date: 1998-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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