Molecular weight/mass distribution

This results in values of du /d log M which are plotted against log M to yield the differential molecular weight/mass distribution curve (MWD). 

Molecular weightjmass detectors include light scattering detectors and viscosity detectors. When SEC is used in the characterisation of polymer systems, its main aim will be the production of a molecular mass/weight distribution and where possible absolute molecular weights. Mass calibration is a complicated matter (section 9.3.5.1) in that calibration curves differ for different polymer types, and for many commercial polymers, direct molar mass calibration is not possible because of the lack of suitable, known molecular weight standards. 

An alternative is to determine the polymers molecular weight/mass in the SEC eluent in situ, by use of on-line molecular mass sensitive detectors. Two such detectors are commercially available, the light scattering detector and the viscosity detector. These detectors are usually used in series with a mass concentration detector and require specialised data handling/software to compute the outputs from the twin detectors and to produce molecular weight/masses and distributions. 

FIGURE 12.8 On-tissue digestion aids the detection of high-molecular-weight species. Distribution of ion detected at in/z 1496.75 corresponding to a tryptic peptide of the 71 kDa protein, synapsin I. (Reproduced from Groseclose, M.R. et al., J. Mass Spectrom., 42, 254, 2007. With permission.)... 

The polymerization rate is also controlled by the particle geometry, mass-transfer coefficient at the polymer surfece, and the diSusivity of the monomer through the layer of polymer coating the primary catalyst particles. The catalyst chemistry influences such polymer attributes as average molecular weight and distribution, bulk density, degree of crystallinity, particle size and morphology. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Range of molecular weight distribution (mass/molar). 

In these experiments, simultaneous DRI (mass) and FALLS detection is applied to obtain absolute information about molecular weight (degree of polymerization) distribution and average values of degree of polymerization. 

---