Search results for "Tacticity"

New Thermoshrinkable Materials of Radiation Modified Polypropylene-Elastomer Composites with Cross-Linking Agents

2014

In this work, electron beam modified heterogeneous composites of isotactic polypropylene (PP) with chlorinated polyethylene (CPE) have been investigated. The PP/CPE blend composites with an excess of elastomer (30/70 wt%) have been modified with trimethylolpropane triacrylate (TMPTA) and bisphenol-A-dimethacrylate (BPDMA) cross-linking agents and have been irradiated with accelerated electrons up to ionizing radiation doses from 25 to 150 kGy. The internal stresses characterizing the thermoshrinkage properties (TMP) thermorelaxation stresses formed in thermal heating and the residual stresses resulting in the process of full setting and cooling of materials have been investigated for radiat…

Influence of morphology and chemical structure on the inverse response of polypropylene to gamma radiation under vacuum

1999

Abstract In this work the influence of the chemical structure and of the morphology on the gamma-radiation effects on polypropylene based polymers is studied on the basis of a previously discussed kinetic model [1] . For this aim an isotactic polypropylene and a random ethylene–propylene copolymer were irradiated under vacuum at one dose rate and several absorbed doses after well defined solidification conditions. We show that the model is reliable varying both the chemical structure and the morphology of the polypropylene based polymer. An inversion of the response of the material to gamma radiation under vacuum is always observed, and the inversion conditions depend on the irradiation par…

Novel polyolefin materials via catalysis and reactive processing

1998

Recent advances in transition metal catalyzed olefin polymerization and melt processing stimulate the production of new polymers derived from old monomers Modern polyolefin processes do not require polymer purification and give excellent control of molecular and supermolecular polyolefin architectures. Progress in catalyst design and preparation of tailor-made homo-and copolymers is highlighted for isotactic, syndiotactic, atactic and stereoblock polypropylene (PP), novel 1-olefin copolymers, and ethylene copolymers with polar monomers, e.g., CO and acrylics Today polyethylene short-and long-chain-branching is controlled either by uniform ethylene copolymerization with 1-olefins using singl…

Small angle scattering study of the structure of isotactic polypropylene-hydrogenated oligo(cyclopentadiene) blends

1996

Abstract Blends of isotactic polypropylene ( i PP) and hydrogenated oligo cyclopentadiene (HOCP) have been studied by means of small angle X-ray scattering in the temperature range 70–160°C. The structure of blends containing less than 25% HOCP is very similar to the one of plain i PP, i.e. lamellae whose thickness increases by increasing the temperature. Blends containing more than 25% HOCP are characterized by two kinds of lamellae formed by layers of i PP and amorphous material rich in i PP and in HOCP, respectively. The crystallizable i PP present in both phases crystallizes from the melt, in analogy to what happens in HDPE/HOCP blends and in agreement with the values of the crystallini…

The use of the indentation test for studying the solidification behaviour of different semicrystalline polymers during injection moulding

2005

Summary: An in-line method for monitoring the solid-ificationprocess during injection molding of semicrystallinepolymers (demonstrated previously in J. Appl. Polym. Sci.2003, 89, 3713) is based on a simple device, where anadditional ejector pin is pushed on the injection molded partatdifferenttimesduringthesolidificationphase.The‘inden-tation depth profile’, i.e., residual deformation as a functionoftime,wasobtainedandallowedtodeterminetheevolutionof the solidification front in the mold as a function of thecooling time. The present work shows the reliability andthe powerfulness of the aforementioned method for a largevariety of different semicrystalline polymers (PET, PBT,polyamide-6 PA6, isota…

The use of master curves to describe the simultaneous effect of cooling rate and pressure on polymer crystallization

2003

In a previous work a master-curve approach was applied to experimental density data to explain isotactic polypropylene (iPP) behaviour under pressure and high cooling rates. Suitable samples were prepared by solidification from the melt under various cooling rate and pressure conditions with the help of a special apparatus based on a modified injection moulding machine. The approach here reported is more general than the case study previously shown, and is suitable to be applied to several materials and for different measures related to crystalline content. The proposed simple model is able to predict successfully the final polymer properties (density, micro-hardness, crystallinity) by supe…

Ylide als polymerisationsinitiatoren. I. Salzhaltige und salzfreie phosphoniumylide

1972

Phosphorylide, R3PCHX und R3PCH2·LiBr, konnen eine anionische Polymerisation von Vinylmonomeren starten. Dabei werden im Falle des Methacrylnitrils und des Methylmethacrylats die Ylide als Endgruppen in die Polymeren eingebaut. Bei den salzfreien Yliden hangt die Startwirksamkeit von der Basizitat des Ylidcarbanions ab, die Struktur dieser Ylide hat aber keinen mesbaren Einflus auf das Molekulargewicht und die Taktizitat der entstehenden Polymeren. Im Gegensatz dazu wird bei der Initiierung mit den salzhaltigen Yliden auch die Taktizitat der Polymeren von den Substituenten der Phosphorylide beeinflust. Phosphorous ylids, R3PCHX and R3PCH2·LiBr, are able to start an anionic polymerization of…

Modification of Chiral Properties Due to Interaction of Polymers and Small Molecules or Ions

1979

The first part of this report deals with changes in the optical rotatory dispersion and circular dichroism resulting from the chemical modification of optically active polymers. Amylose derivatives have very different chiroptical properties compared with cellulose derivatives. Variations in glucosidic bonds and differing secondary structures are described and suggested as causes for this. The N-chlorination of synthetic optically active polyamides is also described, which in all probability proceeds without changing the configuration at the asymmetric C atoms. Reactions of inactive polymers with low molecular weight chiral reagents usually give optical rotation values proportional to the de…

Rotationsdispersion bei monomerem und polymerem L-(–)-Lactid

1965

Bei der kationischen Polymerisation von L-Lactid entsteht ein hochkristallines, isotaktisches Polymeres. Physikalische Eigenschaften des Monomeren und Polymeren werden mitgeteilt. Fur die Rotationsdispersion von L-Lactid in verschiedenen Losungsmitteln gilt die eintermige DRUDE-Gleichung. Das Polymere hat eine Dispersions-Kurve mit einem ersten Extremum bei 275 mμ. Die Dispersionskurve des Poly-L-lactids last sich durch die MOFFITT-YANG-Gleichung beschreiben. Hieraus werden Schlusse auf die Sekundarstruktur des gelosten Polymeren gezogen. In the cationic polymerization of L-lactide a high cristalline isotactic polymer is obtained. Physical properties of monomer and polymer are reported. The…

1978

A newly designed automatically controlled stirred reactor suitable for kinetic measurements of reactions with half lives ≥2s has been applied to follow the anionic polymerization of methyl methacrylate in THF with Na+ as a counter ion in the presence of an excess of NaB(C6H5)4. As initiators were used: benzylsodium reacted with α-methylstyrene (I), fluorenylsodium (II), and 9-methylfluorenylsodium (III). With I the initiation is fast as compared with the polymerization reaction which is first order in monomer concentration. Within the range of −50°C to −100°C an almost unperturbed “living” polymerization is observed. The Arrhenius plot of the rate constants results in a straight line with a…