Molecular weight and molecular weight distribution of polymers are one of the most basic data for studying the properties of polymers and polymer materials. It involves the mechanical properties of polymer materials and products, the rheological properties of polymers, the processing properties of polymers and the selection of processing conditions. It is also one of the basic data needed in the field of polymer chemistry and polymer physics for the study of specific polymeric reactions and the structure of specific polymers.
Characterization Method and Principle
1. Measuring Relative Molecular Weight Determination(M) by Viscosity Method
The intrinsic viscosity of diluted polymer solution was measured by Ukrainian viscometer. According to Mark-Houwink formula [_]=kMalpha, the values of K and alpha were found out from literature or related manuals, and the molecular weight of the polymer was calculated. Among them, the values of K and a vary with the solvent used and the experimental temperature.
2. Measurement of Weight Average
Molecular Weight (Mw)by Small Angle Laser Light Scattering _When incident photoelectric and electromagnetic waves pass through the medium, the electrons in small particles (such as macromolecule) in the medium will vibrate forcibly, thus producing scattered light waves with the same frequency as the oscillating electric field (incident photoelectric and electromagnetic wave) emitted by the secondary wave source in all directions. The intensity of the scattering wave is related to the number of dipoles in small particles (polymers), that is, to the mass or molar mass of the polymers. Based on the above principle, the scattering light intensity of dilute polymer solution is measured by laser light scattering instrument at a small angle (2–7 C) to calculate the absolute molecular weight of protein in dilute solution. The hydrodynamic radius distribution of particles (macromolecule) can be measured by dynamic light scattering, and the molecular weight distribution curve of macromolecule can be calculated.
3. volume exclusion chromatography (SES) (also known as gel permeation chromatography (GPC)).
When the polymer solution is filled with a column with special porous filler, the polymer in the solution shows different hydrodynamic volumes because of its different molecular weight. There are various large and small holes and channels on the surface and inside of the filler of the column. When the detected polymer solution is introduced into the column with the eluent, the macromolecule solute will permeate into the holes in the filler. The degree of permeation is related to the size of the polymer volume. The macromolecule larger than the filler hole diameter_can only travel between the filler particles, so the eluent will be taken out of the column first, while the other macromolecule smaller than the filler hole can be retained in the filler hole, the smaller the molecular volume, the more holes that can be retained in the filler_, so it will be leached out. The longer it takes. According to this principle, the molecular weight distribution curve in polymer can be obtained by using gel permeation chromatograph. The absolute molecular weights of polymers with different components can be obtained by mass spectrometry analysis. The relative molecular weight of each component can be obtained by molecular weight calibration of the above molecular weight distribution curve with known molecular weight. Because the dissolution temperatures of different polymers in solvents are different, it is sometimes necessary to prepare polymer solutions at higher temperatures, when GPC columns need to work at higher temperatures.
4. Mass Spectrometry
Mass spectrometry is a method for precise determination of molecular weight of substances. The molecular weight determined by mass spectrometry gives the ratio of molecular mass m to charge number Z, i.e. mass-charge ratio (m/Z). In the past, mass spectrometry was difficult to determine molecular weight of polymers. However, due to the development of ionization technology, mass spectrometry has been used to determine molecular weight for more than 20 years. Up to millions of macromolecule compounds. The instrument composed of laser desorption ionization technology and ionization time-of-flight mass spectrometry is called matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF_MS_laser mass spectrometry), which can measure the average molecular weight (Mw) of high molecules with narrow molecular weight distribution. The instrument consisting of electrospray ionization and ion trap mass spectrometry is called “electrospray ion trap mass spectrometry” (ESI- ITMS). The average molecular weight (Mw) of polymers can be measured.
Other methods for determining molecular weight of polymers include end-group method, boiling point increasing method, freezing point lowering method, membrane osmotic pressure method, vapor pressure osmosis method, small angle X-ray scattering method, small angle neutron scattering method, ultracentrifugal sedimentation method, etc.