Abstract:

Crystalline block copolymers self-assemble on the nanoscale into a variety of morphologies dependent on block length and type. This thesis describes the synthesis and morphological characterization of several classes of crystalline block copolymers, all containing a linear polyethylene (LPE) block. Defect-free LPE forms thicker, higher-melting crystals with a higher degree of crystallinity compared to its branched counterparts.

Our synthesis method for LPE involves the ring-opening metathesis polymerization (ROMP) of cyclopentene followed by hydrogenation. We describe the implementation of a Ru-based initiator for ROMP of cyclopentene to produce well-defined polymers (polydispersity<1.2) over a wide range of molecular weight (5-60 kg/mol).

A new block copolymer synthesis technique was developed by combining Mo-initiated ROMP with anionic polymerization. The ROMP reaction is terminated to yield a styrene-like endgroup, which is subsequently lithiated to act as a macroinitiator from which the anionically-polymerizable block is grown. We employed this method to produce polycyclopentene-polystyrene (PCP-PS) diblocks which became LPE-polyvinylcyclohexane (PVCH) diblocks upon catalytic hydrogenation.

A series of LPE-PVCH diblocks was used to investigate the crystallization behavior of LPE in confined environments. Because the glass transition temperature of PVCH is higher than the crystallization temperature of LPE, the melt morphology is vitrified upon cooling, confining crystallization within or around the glassy microdomains. Confinement within spherical or cylindrical microdomains directly limits LPE's degree of crystallinity and crystal thickness. Lamellar morphologies impose lesser limitations, because the crystal stems lie parallel to the microdomain interface. Crystals confined within or between cylinders, or within lamellae, show pronounced orientation of their b-axes parallel to the long direction of the microdomain. Confinement can also exert a pronounced influence on the crystallization kinetics.

Crystalline-crystalline block copolymers of LPE and hydrogenated polynorbornene (hPN), another highly crystalline polymer, were synthesized by ROMP followed by hydrogenation. The block molecular weights determine which block crystallizes first and sets the spherulitic superstructure, within which the second block subsequently crystallizes.

Triblocks with LPE endblocks and a rubbery midblock were evaluated as thermoplastic elastomers. Although they showed low tensile strengths, they exhibited high extensibility and good structural recovery from deformation.