Multigraft copolymers with polyisoprene backbones and polystyrene branches, having multiple regularly spaced branch points, were synthesized by anionic polymerization high vacuum techniques and controlled chlorosilane linking chemistry. The functionality of the branch points (1, 2 and 4) can be controlled, through the choice of chlorosilane linking agent. The morphologies of the various graft copolymers were investigated by transmission electron microscopy and X‐ray scattering. It was concluded that the morphology of these complex architectures is governed by the behavior of the corresponding miktoarm star copolymer associated with each branch point (constituting block copolymer), which follows Milner's theoretical treatment for miktoarm stars. By comparing samples having the same molecular weight backbone and branches but different number of branches it was found that the extent of long range order decreases with increasing number of branch points. The stress‐strain properties in tension were investigated for some of these multigraft copolymers. For certain compositions thermoplastic elastomer (TPE) behavior was observed, and in many instances the elongation at break was much higher (2‐3X) than that of conventional triblock TPEs.

Summary: The morphology and tensile deformation behaviour of a highly asymmetric styrene/butadiene star block copolymer (polystyrene (PS) content = 74%) containing random PS‐co‐PB (polybutadiene) copolymer as a rubbery phase were investigated. The existence of double yielding, similar to that observed in some semicrystalline polymers, was detected in this nanostructured amorphous polymer. The observed phenomenon may be correlated with two different micromechanical processes taking place at the initial stage of deformation.The stress‐strain curve of the star block copolymer prepared here (each curve represents a different method). The two yield points are clearly visible (labelled I and II).imageThe stress‐strain curve of the star block copolymer prepared here (each curve represents a different method). The two yield points are clearly visible (labelled I and II).

No abstract

The influence of the catalytic system and synthesis conditions on the reactor powder morphology and the molecular packing in the nascent UHMWPE is studied with the help of various electron microscopic methods. The potentiality of different morphologies for producing strong consolidated material by sintering at temperature lower than the melting temperature is considered. It is shown that the small catalytic particles of colloidal sizes (reactor powders of M-series) produce homogeneous broccoli type morphology consisting of small nodules with the sizes less than 0.2-0.5 lm, which in turn, comprise crystalline domains and disordered regions. Comparison analysis of transmission electron microscopic data with the DSC and NMR results enabled to conclude that the disordered regions are predominantly comprised of tie molecules with low degree of coiling, taut tie molecules, and a number of tight folds. This type of morphology best fits for compaction and sintering. Reactor powder morphology arising upon synthesis lab-scale and commercial UHMWPE on supported catalysts is not so homogeneous, and consists of miscellaneous morphological units, such as spirals, flakes, secondary fibrils, interconnecting the subparticles, large and small lamellae in depending of the catalyst system. The density of disordered regions in these reactor powders is less than that in the particles of M-series. The tensile strength of the samples obtained by sintering of the M-powders is higher than the strength of the other ones by a factor 2.5, which makes them good precursors for orientation drawing.

The morphology and deformation behavior of blends comprising an asymmetric styrene/butadiene star block copolymer and polystyrene homopolymer (hPS) were investigated by means of transmission electron microscopy (TEM) and uniaxial tensile testing. The hPS samples used had a wide range of molecular weights. In spite of the high polystyrene (PS) content, the star block copolymer exhibited a morphology consisting apparently of a rubbery matrix, attributable to its asymmetric architecture and presence of polystyrene/polybutadiene random copolymer (PS‐co‐PB) as soft rubbery blocks. In the blends, depending on the molecular weight and modality of the added hPS and on the composition of the mixtures, different morphologies (ranging from microphase‐separated and macrophase‐separated ones to a combination of both of them) were observed. Consequently, it was possible to tailor the tensile mechanical properties based on the nature of the nanoscopic morphology of the star block copolymer/PS blends. Copyright © 2005 John Wiley & Sons, Ltd.