Search results for "Crystallinity"
showing 10 items of 248 documents
Direct evaluation of the electron density correlation function of partially crystalline polymers
1980
A discussion of the general properties of the one-dimensional electron density correlation function K(z) of a partially crystalline polymer with lamellar structure shows that application of a graphical extrapolation procedure permits direct determination of the crystallinity, the specific inner surface, and the electron density difference ηc − ηa. The procedure is based upon the occurrence of a straight section in the “self-correlation” range of K(z). Curved and nonparallel lamellae do not invalidate the concept. In the case of heterogeneous samples composed of partially crystalline and totally amorphous regions, some of the parameters of the experimentally obtained correlation function, as…
Synthesis of low density poly(ethylene) using nickel iminophosphonamide complexes
2007
Ethylene polymerization using a catalyst derived from the reaction of the phosphorane (Me3Si)2NP(NSiMe3)2 (1) with either Ni(COD)2 or bis(π-allyl)Ni complexes affords branched poly(ethylene) (PE) of variable MW (103−106) depending on conditions. The branched PE of high MW is semicrystalline with Tm < 100 °C. High field 13C NMR spectra reveal the presence of methyl branches (ca. 10−15 per 1000 C atoms), branches longer than six C atoms (15−20 per 1000 C atoms) and trace levels of ethyl, propyl, n-butyl, and sec-butyl branches (total <2 per 1000 C atoms). The branching distribution changes modestly in response to changes in ethylene pressure in a manner consistent with a chain-walking mechani…
Compression-induced anti-nematic order in glassy and semicrystalline polymers
2020
We provide new insights into the molecular origin of the asymmetry between uniaxial tensile and compressive deformation of glassy and semicrystalline polymers using molecular dynamics simulations. The difference between the two responses strongly depends on the chain length and is the largest at intermediate chain lengths. Irrespective of chain length, the intra- and interchain organization of polymers under extension and compression are remarkably distinct. The chains align along the tensile axis leading to a global nematic order of the bonds and end-to-end vectors, whereas compression reorganizes polymers to lie in planes perpendicular to the compressive axis resulting in the emergence of…
The viscoelastic behavior of nylon 6/lithium halides mixtures
1980
Stress-relaxation experiments in the limit of small deformations have been carried out in a wide range of temperatures for pure nylon 6 and its mixtures with LiBr and/or LiCl. Master curves have been constructed, by allowing horizontal shifts only, showing the shape typical for semicrystalline polymers in the case of pure nylon and that of essentially amorphous polymers in the case of salted samples. The glass transition temperatures determined from the plot logaT− 1/T is increased when in the presence of salts. All such effects are considered to reflect the binding action of Li+ and Br− ions.
Determination of degree of crystallinity of drawn polymers by means of density measurements
1969
The well known procedure of determining the degree of crystallinity by means of measuring the density presupposes the knowledge of both the densities ρc and ρa of the crystalline and of the noncrystalline regions. By combination of small-angle and wide-angle x-ray scattering and of density measurements it can be shown that this method is not justified in the case of drawn polyethylene if the values of ρc and ρa known from isotropic material are used. Both ρc and ρa depend considerably on annealing and drawing conditions. In addition the effective density ρc* of the more densely packed phase in a two-phase structure is much lower than the value ρc calculated from the positions of the x-ray r…
Liquid crystalline phases from polymer functionalised semiconducting nanorods
2008
The orientation of semiconducting nanomaterials is a hot topic in optoelectronic applications. Liquid crystallinity offers the potential to orient inorganic anisotropic nanorods, if they can be solubilised sufficiently as realised by polymer functionalisation. In this work we functionalised TiO2, SnO2, ZnO and CdTe nanorods with PMMA, PS and PDEGMEMA (poly(diethylene glycol monomethyl ether) methacrylate) diblock copolymers containing anchor groupsvia grafting-to. The block copolymers were synthesised by RAFT polymerisation (PDI ≈ 1.2) via reactive ester diblock copolymers, which were functionalised later with anchor units polymer-analogously. The surface coverage of the nanorods (determine…
Role of the Intercrystalline Tie Chains Network in the Mechanical Response of Semicrystalline Polymers
2017
We examine the microscopic origin of the tensile response in semicrystalline polymers by performing large-scale molecular dynamics simulations of various chain lengths. We investigate the microscopic rearrangements of the polymers during tensile deformation and show that the intercrystalline chain connections known as tie chains contribute significantly to the elastic and plastic response. These results suggest that the mechanical behavior of semicrystalline polymers is controlled by two interpenetrated networks of entanglements and tie chains.
Ion bombardment of polyethylene—influence of polymer structure
2007
Abstract Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (Mw), molecular weight distribution (Mw/Mn) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate p…
Transparent thermoplastic polyurethanes based on aliphatic diisocyanates and polycarbonate diol
2016
Segmented thermoplastic polyurethanes (PUs) were synthetized using polycarbonate diol as soft segment with a molar mass of 500 and as a hard segment 1,5-pentanediol with a combination of isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HDI). Differential scanning calorimetry, differential mechanical analysis, Fourier transform infrared-attenuated total reflection spectroscopy, haze, transmittance, hardness, tensile properties and retention of tensile properties tests were employed to characterize the different PUs. The results of this study show that IPDI/HDI relation has a significant impact on the phase mixing, crystallinity and therefore on the PU’s properties. The variati…
Mesoporous silicon carbide via nanocasting of Ludox® xerogel
2016
Porous SiC with uniformly sized 12 nm and 22 nm spherical mesopores was synthesized from nanocomposites of polycarbosilane (PCS) preceramic polymer and xerogels of Ludox® SiO2 nanoparticles as templates. The influence of PCS type (Mw 800 and 2000 Da), PCS : SiO2 ratio, pyrolysis temperature 1200–1400 °C, and addition of Ni complex to the preceramic composite was studied with respect to the SiC porous morphology, crystalline structure and chemical properties. We found that the pore walls of Ni-free por-SiC are composed of relatively large (20 nm) crystallites embedded inside a poorly crystalline SiC/SiC1+x phase. Increasing the pyrolysis temperature resulted in an increase of the large cryst…