Polypropylene Block Copolymer Research Articles

Stress-whitening of polypropylene block copolymer in impact tests and its measurement

This paper describes two methods—one based on a simple measurement and another requiring a special instrument—for measuring the degree of stress-whitening in impacted propylene-ethylene block copolymer (PEBC) specimens. These methods have been applied to determine the response of PEBC specimens, containing various nucleating agents, to measured stress-whitening. Difficulties in measuring stress-whitening have been resolved. Accordingly, low dart weights (60 and 80 g) are preferable for producing whitening. The area method, requiring measurement of the whitened spot, gives better reproducibility when measurements are done on the reverse side. In the differential opacity method, any side may be used. This difference between the two methods arises because area is a surface property whereas opacity is a property averaged across the specimen thickness. However, it is shown that even the area method can be used to evaluate and compare stress-whitening (studied as a function of dart weight or nucleating agent type).

Dynamic mechanical properties and morphology of polypropylene block copolymers and polypropylene/elastomer blends

AbstractThe relation between the dynamic mechanical properties and the morphology of polypropylene (PP) block copolymers and polypropylene/elastomer blends was studied by dynamic mechanical analysis (DMA), light‐ and electron microscopy. The latter techniques contributed to an improvement in assignments of relaxation transitions in the DMA spectra. It was established that PP block copolymers had multiphase structure since the ethylene/propylene rubber phase (EPR) formed in the copolymerization contained polyethylene (PE) domains. An identical morphology was found in the case of PP/polyolefin thermoplastic rubber (TPO) blends. Impact modification of PP by styrene/butadiene block copolymers led to a multiphase structure, too, due to the polystyrene (PS) domains aggregated in the soft rubbery polybutadiene phase. In the semicrystalline polyolefinic and in the amorphous styrene/butadienebased thermoplastic rubbers, PE crystallites and PS do mains acted as nodes of the physical network structure, respectively. PP/EPDM/TPO ternary blends developed for replacing high‐density PE showed very high dispersion of the modifiers as compared to that of PP block copolymers. This fine dispersion of the impact modifier is a basic regulating factor of impact energy dissipation in the form of shear yielding and crazing.

Amorphous phase separation in polypropylene block copolymers as revealed by thermostimulated depolarization measurements. II. Thermal sampling analysis

We achieved a thermal sampling scanning of the β-relaxation region of polypropylene (PP) block copolymers together with the β process of isotactic polypropylene, high-density polyethylene, propylene-ethylene rubber, and part of the PP bloc copolymer soluble in xylene and insoluble in ether. The comparison of activation parameters determined in the above relaxations confirmed the hypotheses drawn from complex spectra studies concerning phase separation and the origin of dielectric relaxations in PP block copolymers. One of the dielectric relaxations of the part of PP block copolymer soluble in xylene and insoluble in ether could be attributed to polypropylene blocks. We related the preexponential factor of the relaxation time to chain environment. Then we discussed the compensation phenomenon in distributed relaxations and the relation between the compensation temperature and variation of thermal expansion coefficient through the compensating relaxation.

Amorphous phase separation in polypropylene block copolymers as revealed by thermostimulated depolarization measurements. I. Complex spectra study

The thermally stimulated depolarization (TSD) technique was used to observe the amorphous phase separation in polypropylene block copolymers and to attribute its various dielectric relaxations to precise mobile entities. The complex thermostimulated spectra of polypropylene block copolymers were compared to complex spectra of polymers of the same nature as those of its main components, i.e., isotactic polypropylene, high-density polyethylene, and propylene-ethylene rubber. With TSD we then studied a binary blend of polypropylene and high-density polyethylene and another blend of polypropylene and propylene-ethylene rubber and also the various parts of polypropylene block copolymer which were separated by a selective solvent extraction. From peak-position and intensity considerations we suggested the origin of the three relaxations appearing from 90 to 290 K in the TSD complex spectra of polypropylene block copolymer.