Polymer Characterization

Polymer characterization involves the determination of molecular weight distribution, structure, and physical properties (such as thermal and mechanical properties) of polymeric materials. These analyses are implemented to ensure the quality and performance of the newly synthesized polymers. A fully characterized product can be then released to serve its designated purpose.

Light scattering techniques have been used to measure the molecular weight and size of polymer molecules. In light scattering measurement, a monochromatic light source, usually a laser, is applied to a polymer solution where the light scatters. When contacted with small polymer coils, in all directions the scattering lights are then collected by one or more detectors from different angles. In static light scattering (elastic), the intensity of the scattered light at different angles are correlated to the molar mass and radius of polymer by Zimm equation: by measuring the intensity at any specified angle to obtain the molecular mass of a polymer chain; by measuring the angular dependence of scattered light to obtain the mean square radius of gyration of a polymer coil. In dynamic light scattering (inelastic), the time-dependent fluctuations in the intensity of scattered light are measured since the particles are undergoing Brownian motion. The velocity of this particle Brownian motion is measured and the translational diffusion coefficient D is determined. This diffusion coefficient can be converted into particle size by using the Stokes-Einstein equation.

Analysis of additives in polymers takes advantage of modern analytical instruments including gas chromatography (GC), liquid chromatography (LC), UV spectrometer, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), mass spectroscopy (MS). Typical variables in these methods include temperature, pressure, speed, wavelength, voltage, performance, sensitivity, selectivity, resolution, mass, fragmentation, dimensionality, orthogonality, equipment size, column technology, instrumental complexity, cost, timeliness. For example, gas chromatography is often used to separate the mixture of polymer additives and provide identification information for each component through a spectrometer. FTIR provides the chemical component information by measuring the absorption or emission infrared spectrum of polymer samples.

Knowing the properties and limitations of commercially available polymers is crucial for the development of next generation products. This, together with the demand of more affordable approaches, has inspired a rapid evolution in analytical methodology and instrumentation. A number of techniques have been developed for this purpose. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), rheometer and tensile testing are often employed to characterize the thermal and mechanical properties of polymer. DSC is a thermoanalytical technique which measures the temperature change when samples are under heat, the rate of temperature change for a given amount of thermal radiation is directly linked to the heat capacity as well as the phase change information of polymers. Rheology testing provides information about melt viscosity, flow behavior, viscoelastic properties of polymer, which are important for understanding and improving the flow behavior of polymer products when being processed. Tensile tests measure the tensile force required to stretch, yield, and fracture a polymer and the extent to which the polymer elongates to the yield or failure point.

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