Semi-crystalline polymer blends: Thermodynamics, morphology, and toughness enhancement

This work demonstrates that isotactic polypropylene (iPP) can be modified for improved low temperature toughness and impact strength using a variety of techniques. Not only is chain architecture important for producing thermodynamically stable blends, but maintaining or enhancing interfacial anchors and tailoring the interaction parameter such that the interfacial width can accommodate entanglements are critical factors in designing semi-crystalline blends to have improved low temperature properties without compromising blend stiffness.;The nonideal mixing behavior of iPP was studied using a series of anionically polymerized polyolefins, poly(ethylene-random-ethylethylene) (PEExx), where the chain architecture was varied by changing the ethylethene content (xx). Random copolymers having chain architectures similar to iPP proved to form melt miscible blends with iPP. The iPP miscible molecules (PX) provided a mechanism for dispersing immiscible materials such as polyethylene (PE) and model ethylene-propylene rubber (PEP) in iPP. Tougher iPP based blends were created through four different routes: (1) blending iPP with metallocene PE, (2) blending iPP with PEExx, (3) blending iPP with PX based block copolymers, and (4) creating stabilized PE/iPP and PEP/iPP blends with the PX based block copolymers. The impact that the melt state thermodynamics and the degree of crystallinity have on the ultimate properties of these semi-crystalline polyolefin blends was studied using scattering, rheological, thermal, microscopy, and mechanical testing (tensile, peel, and impact) techniques.;The ability to manipulate the PE/iPP interface with a (PE-PX-PE) block copolymer lead to the fundamental discovery that interfacial entanglements control the ultimate mechanical properties of polyethylene/polypropylene blends. The contribution of interfacial structure that entangles and subsequently anchors itself into crystalline lamella was evaluated. A semi-crystalline interface decoupled by non-crystalline material resulted in interfacial failure under tensile strain. A phase boundary having entangled, crystallite anchored interfacial chains resulted in failure of the iPP matrix material. This change in failure mode was accompanied by a 40% increase in both tensile toughness and elongation at -10°C.