[article]
| Titre : |
Density and molecular weight of polymers |
| Type de document : |
texte imprimé |
| Auteurs : |
Tipanna Melkeri, Auteur |
| Année de publication : |
2020 |
| Article en page(s) : |
p. 69-74 |
| Note générale : |
Bibliogr. |
| Langues : |
Anglais (eng) |
| Catégories : |
Densité
Poids moléculaires
Polymères
|
| Index. décimale : |
668.9 Polymères |
| Résumé : |
Polymers are high molecular weight compound with low specific gravity (density) and high specific strength. Density of polymers are very low as compared to metal, however, the strength of these materials is relatively very high. Because of high strength to weight ratio polymers have replaced metals for many applications. The variation of density is because of the elements present in the polymer chain and structure of the polymer chain and arrangement of polymer chain. In Polyethylene, the arrangement of polymer chains can be vary, these are available in various densities. Polyethylene with high density have an ordered structure hence are more crystalline in nature with higher strength. Polymers contain polymer chains of unequal length, and so the molecular weight is not a single value - the polymer exists as a distribution of chain lengths and molecular weights. Different average values can be defined, depending on the statistical method applied. i) Number average molar mass or M, (also loosely referred to as Number Average Molecular Weight. ii) Mass average molar mass or M„, (also commonly referred to as Weight Average Molecular Weight iii) Z average molar mass or M, (z is for centrifugation; from German zentrifuge) and iv) Viscosity average molar mass or M. The number average molecular mass of a polymer can be determined by gel permeation chromatography, viscometry via the (Mark-Houwink equation), colligative methods such as vapor pressure osmometry, end-group determination or proton NMR. The mass average molecular mass can be determined by static light scattering, small angle neutron scattering, X-ray scattering, and sedimentation velocity. The graph of variation of number of polymer chains Ni vs molecular weight, is called molecular weight distribution curve. The polydispersity index is used as a measure of the broadness of a molecular weight distribution of a polymer. The ratio of the mass average to the number average is called the dispersity or the polydispersity index. MWD curve is narrow or wide depending upon the poly dispersity index. A narrow MWD is having better mechanical properties over wide MWD. Whereas the processability of the material is easy in wide MWD. Step polymerization reactions typically yield values of Mw/Mn of around 2.0, whereas chain reactions yield Mw/Mn values between 1.5 and 20. Molecular weight, Molecular weight distribution of the polymer is determined by various methods viz osmometry, end group analysis, light scattering method, sedimentation method, Gel permeable chromatography, Matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS) and Diffusion-ordered NMR spectroscopy (DOSY). |
| Note de contenu : |
- Density
- Determination of density of polymers
- Molecular weight distribution (MWD)
- Definitions of molar masses (molecular weight) : Number average molecular mass Mn - Weight average molecular weight : Mw - Z average molar mass, M" - Viscosity average molar mass, Mv - Molecular mass distribution curve
- Methods of molecular weight determination : Osmometry method - End group analysis method - Light scattering method - Sedimentation method
- Gel permeation chromatography (GPC)
- Matrix-assisted laser desorption/ionization mas spectroscopy (MALDI-MS)
- Diffusion-ordered NMR spectroscopy (DOSY)
- Table : Densities of some important polymers |
| En ligne : |
https://drive.google.com/file/d/1MUGifUS8EjGox1vy7THmlunq0778nrmZ/view?usp=drive [...] |
| Format de la ressource électronique : |
Pdf |
| Permalink : |
https://e-campus.itech.fr/pmb/opac_css/index.php?lvl=notice_display&id=34688 |
in PAINTINDIA > Vol. LXX, N° 2 (02/2020) . - p. 69-74
[article]
|
Aller sur edunet
Accueil
Density and molecular weight of polymers in PAINTINDIA, Vol. LXX, N° 2 (02/2020)
