Ethylene-propylene Diblock Copolymers Research Articles

Dynamic birefringence and non-linear rheology of diblock copolymer micellar solutions

Dynamic viscoelasticity, dynamic birefringence, and non-linear rheology of styrene–ethylenepropylene diblock copolymer, SEP, in isotridecyl isononanoate solution were examined in order to clarify the rheology–structure relationship for block copolymer micellar solutions. SAXS experiments at room temperature indicated formation of the BCC super-lattice of the micelles, composed of S chain core and EP chain corona. The complex strain-optical coefficient, being the complex ratio of birefringence to strain in dynamic measurements, changed its sign with angular frequency, ω, and exhibited a very complicated ω dependence, indicating that the stress relaxed through several relaxation mechanisms. Assuming the stress-optical rule for each mechanism, the complex shear modulus was separated into three components. The three components could be assigned to the reorientation of corona chains, reorientation of core chains, and deformation of the lattice. Steady shear viscosity, η, showed strong thinning behavior. Shear rate dependence of η and the first normal stress difference were described well with the BKZ constitutive equation composed of the above three mechanisms having different damping parameters.

Molecular and microdeformation characteristics of ethylene-propylene copolymers during tensile loading

The microstructural modification during uniaxial tensile deformation of ethylene-propylene diblock copolymers (EP copolymers) at varying displacement rates was studied using atomic force microscopy (AFM) and Raman spectroscopy. Characterisation by AFM of the undeformed surface of the two copolymers (designated EP-M and EP-TC) indicated differences in arrangement (regular or irregular) of fibrils depending on their melt flow conditions. Type EP-M is a long chain copolymer manufactured with restricted flow, whereas EP-TC is a short chain copolymer obtained by controlled rheology. The microfibrils in both copolymers exhibited small kinks/nodules of ethylene-propylene. At the molecular level, long chain EP-M copolymer exhibited greater non-uniformity in the alignment of chains of molecules in comparison with short chain EP-TC copolymer. Atomic force micrographs taken at different fields of view for long chain EP-M copolymer tensile deformed at a low displacement rate indicated excessive stretching and merging of fibrils/microfibrils in the direction of the tensile axis, whereas short chain EP-TC copolymer exhibited reduced stretching of fibrils and microfibrils. With a subsequent increase in displacement rate of the tensile test, long chain EP-M copolymer continued to stretch along the tensile direction, but with reduced ability for stretching and merging of fibrils/microfibrils. On the other hand, tensile deformation of short chain EP-TC copolymer at a high displacement rate was irregular, and the fibrils/microfibrils were not preferentially stretched in the direction of tensile stress. The irregular deformation behaviour was a consequence of reorganisation of the microstructure. At the molecular level, stretching of individual molecular units occurred in both cases with an increase in displacement rate. However, the differences between the two copolymers were insignificant. Raman spectroscopy indicated that the rate of shift of frequency of Raman band per percentage strain was only marginally greater for long chain EP-M copolymer in comparison with short chain EP-TC copolymer, confirming AFM observations and suggesting only small differences in molecular deformation between the two copolymers.

Ductile behaviour of ethylene-propylene block copolymers

Flow behaviour during the tensile loading of ethylene-propylene diblock copolymers (EP copolymers) has been investigated at various strain rates. The strain rate sensitivity index indicated that the short chain (EP-TC) copolymer is more sensitive to strain rate than is the long chain (EP-M) copolymer. Scanning electron microscopy of ethylene-propylene diblock copolymers deformed at various strain rates revealed two types of fracture morphology, fibrillation and ductile ploughing. The area fraction of fibrillation decreased with an increase in strain rate, with a consequent increase in ductile ploughing. The surface of tensile deformed specimens of both block copolymers was characterised by wedge formation that increased with strain and strain rate of the tensile test. The density of wedges was greater in the centre of the longitudinal direction of the tensile specimen and less away from the centre. The higher plastic flow associated with wedges led to fibrillation type failure in the centre of the fracture, surrounded by a less ductile mode of failure, referred to as ductile ploughing. Regarding the difference in ductile behaviour of the two copolymers, long chain EP-M diblock copolymer exhibited greater fibrillation compared with short chain EP-TC diblock copolymer at a constant strain rate.

Atomic force microscopy assessment of mechanically induced scratch damage in polypropylenes and ethylene–propylene di-block copolymers

Atomic force microscopy (AFM) in tapping mode has been used to determine the extent of surface damage induced during a scratch test in different grades of polypropylenes (PPs) and ethylene–propylene (EP) di-block copolymers. The surface damage is examined for long chain polymeric materials and their respective short chains. The extent of surface damage is evaluated in terms of average surface height of the plastically deformed region, depth of the scratch tracks, and thickness and density of the scratch tracks. The ability of the polymeric materials to resist scratch deformation under identical conditions of scratch test follows the sequence (from maximum resistance to minimum resistance): high crystallinity PPs>low crystallinity PPs>EP di-block copolymers. Also, short chain polymeric materials have greater resistance to scratch deformation than their respective long chain polymers. The scratch tracks are zigzag in PPs and parabolic in EP di-block copolymers with localized plastic flow involving voids. It is interpreted that modulus and yield strength are the determining factors that dictate the scratch resistance of polymeric materials.

Microstructural evolution during tensile deformation of polypropylenes

Tensile deformation processes occurring at varying strain rates in high and low crystallinity polypropylenes and ethylene–propylene di-block copolymers have been investigated by scanning electron microscopy. This is examined for both long and short chain polymeric materials. The deformation processes in different polymeric materials show striking dissimilarities in spite of the common propylene matrix. Additionally, the deformation behavior of long and their respective short chain polymers was different. Deformation mechanisms include crazing/tearing, wedging, ductile ploughing, fibrillation, and brittle fracture. The different modes of deformation are depicted in the form of strain rate-strain diagrams. At a constant strain rate, the strain to fracture follows the sequence: high crystallinity polypropylenes<low crystallinity polypropylenes<ethylene–propylene di-block copolymers, indicative of the trend in resistance to plastic deformation.