Polymers are composed of long chain molecules The monomer unit is repeated many times to give average molecular weights ranging from 50,000 to 500,000 for most common polymers Of course, not all polymer chains are of the same length, so we have a MOLECULAR WEIGHT DISTRIBUTION (MWD) Different molecular weight averages are defined to express the breadth of the distribution The number average molecular weight , Mn, is the sum of the individual molecular weights divided by their number The weight average molecular weight, Mw, is the sum of the squares of the weights divided by the sum of the molecular weights The POLYDISPERSITY INDEX (PDI) Mw/Mn (weight average / number average) would be 1 0 if all chains had exactly the same length (only theoretically possible) Usual grades of polymers have PI values from 1 5 to 30 Broad distribution polymers usually have lower viscosity, but higher elasticity
The sum of the atomic weights of all the atoms in a molecule. Also called formula weight. Mass of a molecule of a substance, based on 12 as the atomic weight of carbon-12. It is calculated in practice by summing the atomic weights of the atoms making up the substance's molecular formula. The molecular weight of a hydrogen molecule (chemical formula H2) is 2 (after rounding off); for many complex organic molecules (e.g., proteins, polymers) it may be in the millions
Number average Molecular Weight is defined as the total weight of all the molecules in a polymer sample divided by the total number of moles present Weight average Molecular Weight is defined as the Molecular Weight Equation See Equation List
The sum of the atomic weights of the atoms in a molecule For example, the molecular weight of water (H2O) is 18, the sum of the atomic weights of two hydrogen atoms (1+1=2) and oxygen (16) ADVANCE \x 540
The molecular weight of a compound in grams is the sum of the atomic weights of the elements in the compound The molecular weight of sulfuric acid (H2SO4) in grams is 98
The molecular weight of a substance is the ratio of weight of one molecule of substance compared to weight of one atom of hydrogen ( under same conditions of temperature and pressure ) It is a dimensionless number It has no units It does not provide the actual weight of the molecule of a substance Weight of 1 molecule of a substance Molecular wt of a gas = ------------------------------------------------------ Weight of 1 atom of hydrogen
The sum of the atomic weights of all the constituent atoms in a molecule The atomic weight is the relative weight of the atom on the basis of carbon isotope C12