| Polymer properties can be manipulated through processing or chemical modification. Both methods are explored here, by (a) elucidating the origin of directional tear behavior in polyethylene (PE) films processed under different conditions, and (b) synthesizing new block copolymers, whose architectures permit precise control over crystal thickness and melting temperature.; Directional tear in films of PE and its copolymers was traced to the orientation imparted during film blowing, quantified through x-ray scattering. The blow-up ratio (BUR) was the most significant process parameter controlling crystal orientation. The Keller-Machin I structure was observed in low-density polyethylene (LDPE) films, which tore preferentially in the transverse direction (TD). Conversely, the Keller-Machin II structure was observed in ethylene-methacrylic acid copolymer films at low BUR, which also tore TD, but the orientation rotated 90° at high BUR, leading to preferred tear in the machine direction (MD). High-density and linear low-density PE films also exhibited the Keller-Machin I structure (as in LDPE) but tore either along MD (HDPE) or isotropically (LLDPE). These differences in tear behavior between chemically similar but architecturally distinct polymers, differing greatly in the type and level of branching, stem from intercrystallite tie molecules.; In the second area, crystalline-amorphous diblock copolymers were synthesized through ring-opening metathesis polymerization and subsequent hydrogenation, where the amorphous block was hydrogenated poly(ethylidene norbornene), hPEN, and the crystalline block was either hydrogenated polycyclopentene, hPCP (identical to HDPE) or hydrogenated polynorbornene, hPN. Acyclic metathesis discovered during the PCP synthesis focused the study on block copolymers containing hPN, which is atactic yet highly crystalline. The hPN crystal structure was solved as monoclinic-β (space group C2/c), with a = 6.936 Å, b = 9.596 Å, c = 12.420 Å, and β = 130.7°. hPN/hPEN diblocks of constant crystalline block length and different amorphous block lengths permitted a direct test of theoretical scaling laws based on the premise of an equilibrium degree of crystal chain folding. Stable chain folding induced by the amorphous block was confirmed, and the theoretical scaling laws were generally verified, though integral chain folding by the crystalline block superimposes a discreteness on the continuous domain scaling predictions. As the amorphous block length increased, producing thinner crystals, the melting point decreased commensurably. The melting point was also influenced by the monomeric and polymeric endgroups attached to the crystallizable block, through the crystal surface energy. |
