Search results for "Polymers"
showing 10 items of 3567 documents
Characterization of a fiber reinforced semirigid liquid crystalline polymer
1997
In this work, samples of a semirigid Liquid crystalline polymer reinforced with carbon and glass fibers has been characterized. Semirigid liquid crystalline polymers (LCPs) show some advantages with respect to rigid LCPs: in particular lower processing temperatures, and better compatibility with flexible thermoplastics, but also some disadvantages: lower mechanical properties and poor thermomechanical resistance. Both properties can be improved by adding inorganic fillers. Although elastic modulus and tensile strength of the glass fiber filled LCP improve remarkably with increasing the filler content, the same properties for the carbon fiber-filled samples do not increase with increasing ti…
Specialty and advanced polymers by carbocationic macromolecular engineering
1993
This study briefly surveys a variety of new advanced polymeric materials and controlled synthetic processes available by carbocationic macromolecular engineering techniques and having potential commercial interest. These recent developments, mainly by living carbocationic polymerization, have led to new opportunities in polymerization process control, and in designing microstructure, functionality, molecular weight and molecular weight distribution (MWD) and thus properties of a wide variety of unique polymer systems. The fundamentals of these new emerging technologies and the novel materials offered by them, such as macromonomers, telechelics, polymers with pendant functional groups (liqui…
Macro-oriented network of liquid-crystalline polyesters: crosslinking induced by g-irradiation and thermally activated reaction.
2000
Abstract The synthesis of a macro-oriented network from liquid-crystalline polymers, bearing pendant groups with unsaturations, is reported. Virgin polymers are processable to obtain oriented samples, e.g. fibres, which may be crosslinked through thermal activation and/or exposure to γ-radiation. The liquid-crystalline state is “frozen” and the macroscopic anisotropy is preserved even at high temperatures. Compared to virgin fibres, irradiated samples show an enhancement of tensile performances, thus indicating that the main effect of irradiation is crosslinking without significant degradation phenomena.
The glass transition behaviour of salted nylon 6
1981
Glass transition measurements of nylon-6/lithium halides mixtures have been carried out in wide range of frequency with the aid of different experimental techniques. The results show an increase of the glass transition temperature when the salt is present and prove the larger effectiveness of lithium chloride with respect of lithium bromide. This effect, in line with the large reduction of the specific volume caused by the salt, is due to the formation of a pseudo-cross-linking between lithium ions and the carbonyl-oxygen groups of the polyamide.
Composites of ethylene-vinyl acetate copolymers with modified magnesium hydroxide-thermo-oxidative ageing
2014
Flame retardant composites must perform their protective roles permanently, also under conditions of intermittent or long-term exposure to sunlight, moisture or increased temperature. The aim of our work was characterization of the thermo-oxidative ageing (90 °C) and the changes in flammability of ethylene-vinyl acetate copolymer composites with 40 and 60 wt.% of magnesium hydroxide. The neat polymer and its composites were investigated for their thermal (DSC) and structural (FTIR) characteristics as well as their density, melt flow index, tensile properties and combustibility during ageing test. As FTIR results identified the different carbonyl groups in copolymer were created, the highest…
Polymer Processing and Performance: 2018–2019
2020
The section “Polymer Processing and Performance” of Polymers has published, in the last two years, 175 high quality papers investigating the processing and properties of several polymer based systems [...]
Melt spinning and mechanical properties of semirigid liquid-crystal copolyesters
1993
Melt-spinning and mechanical properties of fibers of a new class of semirigid thermotropic liquid-crystal polymers are presented. These copolyesters are synthesized from 4-4′-dihydroxybiphenyl (B), 4-hydroxybenzoic acid (H), and flexible units provided by aliphatic diacids. The flexible units depress the melting temperature without strongly depressing the mechanical properties. These liquid-crystal polymers can be easily spun at high draw ratios. Indeed, unlike rigid liquid-crystal polymers, relatively high draw ratios are needed to attain high mechanical strength. Tensile moduli of about 28 GPa and tensile strengths of about 350 MPa are obtained. © 1993 John Wiley & Sons, Inc.
Phase identification of triphenylene-based discotic monomer and its main chain polymers
1998
Abstract A monomer, 2,3,6,7,10,11-hexakispentyloxy triphenylene (HPT) possesses a triphenylene core as a discotic mesogen. Polymers containing this discotic mesogen have been studied using wide-angle X-ray and electron diffraction. HPT is known to show a discotic liquid crystal phase, noted as Dho (h for hexagonal bidimensional lattice, o for ordered molecular spacing in each column). In this paper, however, HPT liquid crystalline phases, heated up from the crystalline state and cooled down from the isotropic state, were characterized in the diameter dimensions. In addition. the diameters of the columns are close to a parameter of two separate crystals. A core orientation was, therefore, pr…
Ferroelectric block copolymers
1997
A block copolymer consisting of polystyrene and a side chain ferroelectric liquid crystalline polymer was synthesized using polymer analogous chemistry on a monodisperse poly(styrene-b-isoprene). Composition was adjusted to give lamellar microstructure after addition of the mesogenic side groups. If placed in an LC cell without orientation of domains, no ferroelectric response was observed. After shearing the thin film, presumably due to alignment of lamellae, a bistable ferroelectric switching could be detected.
Molecular Mobility in Oriented and Unoriented Membranes Based on Poly[2-(Aziridin-1-yl)ethanol]
2021
Unoriented and oriented membranes based on dendronized polymers and copolymers obtained by chemical modification of poly[2-(aziridin-1-yl) ethanol] (PAZE) with the dendron 3,4,5-tris[4-(n-dodecan-1-yloxy)benzyloxy]benzoate were considered. DSC, XRD, CP-MAS NMR and DETA, contribute to characterize the tendency to crystallize, the molecular mobility of the benzyloxy substituent, the dendritic liquid crystalline group and the clearing transition. The orientation of the mesogenic chain somewhat hindered this molecular motion, especially in the full substituted PAZE. The fragility, free volume and thermal expansion coefficients of these membranes near the glass transition are related to the orie…