Molecular weight and molecular weight distribution are one of the most basic structural parameters of polymer materials. Many properties of polymer materials are related to molecular weight and molecular weight distribution: excellent performance (tensile resistance, impact, high elasticity) is brought by large molecular weight, but too large molecular weight affects processing performance (rheological performance, solution performance, processing performance, etc.) ).
Characteristics of polymer molecular weight:
(1) The molecular weight is between 103-107;
(2) The molecular weight is not uniform and has polydispersity.
Polymers have the same chemical composition and are composed of mixtures of homologues with different degrees of polymerization, so the molecular weight of polymers is only statistically significant.
The molecular weight determined by the experimental method is only the statistical average value. To accurately describe the molecular weight of the polymer, in addition to the statistical average value, the molecular weight distribution of the sample should be given.
The molecular weights of polymers include number average molecular weight Mn (statistical average by number of molecules), weight average molecular weight Mw (statistical average by weight), Z average molecular weight Mz (statistical average by Z amount), and viscosity average molecular weight Mƞ (with solution viscosity Method). The relationship between the statistical average values ​​of these molecular weights is: Mn <Mƞ <Mw <Mz. This is called the polydispersity coefficient and is used to characterize the degree of dispersion. The larger α indicates that the molecular weight is more dispersed, and α = 1 indicates that the molecular weight is monodisperse (as large).
The molecular weight test has the following methods:

Types of

method

Scope of application

Meaning of molecular weight

Types of

Chemical method

End-group analysis

3 × 104 or less

Number average Mn

absolute

Thermodynamic method

Freezing point reduction

5 × 103 or less

Number average Mn

relatively

Boiling point rise method

3 × 104 or less

Number average Mn

relatively

Gas-phase infiltration

3 × 104 or less

Number average Mn

relatively

Membrane permeation

2 × 104-1 × 106

Number average Mn

absolute

Optical method

Light scattering

1 × 104-1 × 107

Weight average Mw

relatively

Kinetic method

Ultracentrifugal sedimentation balance method

1 × 104-1 × 106

Weight average Mw, Z average Mz

relatively

Viscosity method

1 × 104-1 × 107

Sticky MÆž

relatively

Chromatography

Gel permeation chromatography (GPC)

1 × 103-1 × 107

Various averages

relatively


Gel chromatography has been widely used because of its short measurement period, easy operation, reliable data and good repeatability. The following figure is the molecular weight distribution of PC / ABS plastic measured by gel chromatography, using commonly used tetrahydrofuran solvent-soluble plastics:

The spectrum scanned by the test can be used to directly obtain the molecular weight results of the four middle schools: Mz, MÆž, Mw, and Mp. You can choose the results you need.

MTT is a third-party laboratory engaged in the quality inspection, identification, certification and failure analysis services of materials and components.

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