Metathesis polycondensation chemistry has been employed to control the crystalline morphology of a series of 11 precision-branched polyethylene structures, the branch being placed on each 21st carbon and ranging in size from a methyl group to an adamantyl group. The crystalline unit cell is shifted from orthorhombic to triclinic, depending upon the nature of the precision branch. Further, the branch can be positioned either in the crystalline phase or in the amorphous phase of polyethylene, a morphology change dictated by the size of the precision branch. This level of morphology control is accomplished using step polymerization chemistry to produce polyethylene rather than conventional chain polymerization techniques. Doing so requires the synthesis of a series of unique symmetrical diene monomers incorporating the branch in question, followed by ADMET polymerization and hydrogenation to yield the precision-branched polyethylene under study. Exhaustive structure characterization of all reaction intermediates as well as the precision polymers themselves is presented. A clear change in morphology was observed for such polymers, where small branches (methyl and ethyl) are included in the unit cell, while branches equal to or greater in mass than propyl are excluded from the crystal. When the branch is excluded from the unit cell, all such polyethylene polymers possess essentially the same melting temperature, regardless of the size of the branch, even for the adamantyl branch.

The morphology of ADMET-synthesized polyethylene with n-butyl branches precisely spaced on every 39th carbon (EH39) was studied in comparison with an ethylene/1-hexene addition copolymer possessing the same branching probability, the goal being to elucidate the effect of the intramolecular sequence length heterogeneity on the lamella crystal thickness and its distribution. EH39 was found to have an orthorhombic crystalline polymorphism, which is normal for commercialized polyethylenes and different from that of the other ADMET polyethylenes with shorter CH 2 spacing (C15, C21). EH39 exhibits a narrow lamella thickness distribution; the average thickness (l c,av. ) corresponds exactly to the space length between two consecutive branches, suggesting the complete exclusion of n-butyl branches from the crystal stem. The average thickness, l c,av. mentioned above is also coincident with that obtained from WAXS and SAXS. On the other hand, the 1-hexene copolymer forms much thicker lamellae and a broader thickness distribution than ADMET polyethylene. Here, the average thickness l c,av. determined by TEM observation of the copolymer is 1.5 times larger than that calculated from the most probable ethylene sequence length obtained from 13 C NMR, or for a theoretical ethylene sequence length distribution, indicating that the lamellae are composed predominantly of the sparsely branched longer ethylene sequences that are statistically included. The intramolecular sequence distribution is considered significant to determine the lamella thickness and thickness distribution for short chain-branched polyethylenes with a narrow intermolecular chemical composition distribution.

Polymer 49 (2008) 2985-2995. doi:10.1016/j.polymer.2008.03.029Received by publisher: 2008-02-01Harvest Date: 2016-01-04 12:20:37DOI: 10.1016/j.polymer.2008.03.029Page Range: 2985-299

We report tensile testing and in situ X-ray scattering measurements of a homologous series of precise poly(ethylene-co-acrylic acid) copolymers (pxAA). The number of backbone carbons (x) between pendant acrylic acid groups along the polyethylene chain (x = 9, 15, 21) has a pronounced effect on both their tensile properties as well as their morphologies during deformation. The semicrystalline precise copolymer (p21AA) displays yielding behavior similar to polyethylene. Also, strain hardening in p21AA coincides with the originally isotropic acid-rich layered structures strongly aligning with acid layers perpendicular to the strain direction, demonstrating the facile nature of the H-bonding within the acid aggregates. When the alkyl spacer is only nine carbons (p9AA), the precise copolymer withstands strains of >1000% without failing, because the liquid-like assembly of acid aggregates permits the acid groups to exchange without developing substantial anisotropy in the structure. Both p21AA and p9AA maintain..

No abstract