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Microstructural analysis of polyethylenes and their blends and copolymers

Minick, Jill Suzanne
http://rave.ohiolink.edu/etdc/view?acc_num=case1058204252

Abstract Details

1995, Doctor of Philosophy, Case Western Reserve University, Macromolecular Science.
The morphology of polyethylenes, their blends and copolymers was examined as a function of thermal history and microstructural parameters. The blends were made with high and low-density polyethylene. The former is a linear homopolymer synthesized using low pressures. This leads to long chain branching but no short chain branching. Low-density polyethylene is synthesized under high pressures which results in both short and long branches along the main chain. Also, there is a distribution of short chain branches along the long chain branches. Blends of 75, 50, and 25 weight percent were melt-blended in a twin screw extruder. Compression-molding of the pellets was done and two different thermal histories were generated: slowly-cooled, approximately 18°C/hour, and rapidly-cooled, approximately 18°C/min. Both sets of blends showed two endothermic peaks which depended upon the blend concentration and thermal history. It is assumed that the homopolymers are phase-separated with an intermediate density phase composed of the long-chain branched HDPE and low branch content LDPE. The lamellae sizes decreased as the weight fraction of HDPE decreased. Furthermore, it appears as if a bimodal distribution of thicknesses is apparent, one due to the HDPE phase, the second to the LDPE phase. The te nsile properties of these blends showed that the systems do interact since the elongation at break exhibited negative deviations from the rule of mixtures. This is not due purely to the inhomogeneities in the blend but probably to the rougher interfaces between the crystalline regions. Both the HDPE and LDPE showed sharpening in the WAXS patterns over the undrawn blend indicative of crystallite perfection. A 50 weight percent HDPE blend had broader arcs than seen in the HDPE and no distinct higher order reflections. The fracture surface morphology showed a trend from fibrillar to ductile failure as the weight content of HDPE decreased. Linear low-density polyethylenes polymerized using single-site catalysts were also studied as a function of short chain branching content. The homogeneity of the distribution of branches removes the bimodality seen in DSC traces of conventional LLDPEs. As the number of short chain branches, in this case hexyl, increased, the morphology changed from lamellar, with thicknesses around 160A to fringed micelles for which the size is 70A. It is postulated that the increase in branching content decreases the ability of the chains to diffuse to the growing crystal front and instead, bundles of unbranched chain segments form the crystalline phase. By annealing these fringed micellar and mixed morphology copolymers, it is possible to observe the relative numbers of each population. In the density range below 0.88 g/cc, only one crystal fraction distribution is observed at low crystallite thicknesses. In the materials with densities above 0.88 up to 0.90, a bimodal distribution is seen. The fringed micelle crystals melt at the lower temperatures, hence smaller crystallite thicknesses and the lamellar crystals at higher temperatures. The 0.90 g/cc materials show a thermal history dependence on microstructure which enables manipulation of the relative populations of both fringed micelles and lamellar crystals.
Abdelsamie Moet (Advisor)
221 p.
Microstructural analysis; polyethylenes; blends; copolymers

Recommended Citations

Citations

  • Minick, J. S. (1995). Microstructural analysis of polyethylenes and their blends and copolymers [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1058204252

    APA Style (7th edition)

  • Minick, Jill. Microstructural analysis of polyethylenes and their blends and copolymers. 1995. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1058204252.

    MLA Style (8th edition)

  • Minick, Jill. "Microstructural analysis of polyethylenes and their blends and copolymers." Doctoral dissertation, Case Western Reserve University, 1995. http://rave.ohiolink.edu/etdc/view?acc_num=case1058204252

    Chicago Manual of Style (17th edition)

case1058204252
659
© 1995, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.
Release 3.2.12