Polymer characterization
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Polymer science |
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Polymer characterization is the analytical branch of polymer science.
The discipline is concerned with the characterization of polymeric materials on a variety of levels. The characterization typically has as a goal to improve the performance of the material. As such, many characterization techniques should ideally be linked to the desirable properties of the material such as strength, impermeability, thermal stability, and optical properties.[1]
Characterization techniques are typically used to determine molecular mass, molecular structure, morphology, thermal properties, and mechanical properties.[2]
Molecular mass
The molecular mass of a polymer differs from typical molecules, in that polymerization reactions produce a distribution of molecular weights and shapes. The distribution of molecular masses can be summarized by the number average molecular weight, weight average molecular weight, and
Molar mass determination of
Molecular structure
Many of the analytical techniques used to determine the molecular structure of unknown organic compounds are also used in polymer characterization. Spectroscopic techniques such as
Morphology
Polymer morphology is a microscale property that is largely dictated by the amorphous or crystalline portions of the polymer chains and their influence on each other. Microscopy techniques are especially useful in determining these microscale properties, as the domains created by the polymer morphology are large enough to be viewed using modern microscopy instruments. Some of the most common microscopy techniques used are
Polymer morphology on a mesoscale (nanometers to micrometers) is particularly important for the mechanical properties of many materials.
Thermal properties
A true workhorse for polymer characterization is thermal analysis, particularly Differential scanning calorimetry. Changes in the compositional and structural parameters of the material usually affect its melting transitions or glass transitions and these in turn can be linked to many performance parameters. For semicrystalline polymers it is an important method to measure crystallinity. Thermogravimetric analysis can also give an indication of polymer thermal stability and the effects of additives such as flame retardants. Other thermal analysis techniques are typically combinations of the basic techniques and include differential thermal analysis, thermomechanical analysis, dynamic mechanical thermal analysis, and dielectric thermal analysis.
Mechanical properties
The characterization of mechanical properties in polymers typically refers to a measure of the strength, elasticity, viscoelasticity, and anisotropy of a polymeric material. The mechanical properties of a polymer are strongly dependent upon the Van der Waals interactions of the polymer chains, and the ability of the chains to elongate and align in the direction of the applied force. Other phenomena, such as the propensity of polymers to form crazes can impact the mechanical properties. Typically, polymeric materials are characterized as elastomers, plastics, or rigid polymers depending on their mechanical properties.[5]
The
The fracture properties of crystalline and semicrystalline polymers can be evaluated with Charpy impact testing. Charpy tests, which can also be used with alloy systems, are performed by creating a notch in the sample, and then using a pendulum to fracture the sample at the notch. The pendulum’s motion can be used to extrapolate the energy absorbed by the sample to fracture it. Charpy tests can also be used to evaluate the strain rate on the fracture, as measured with changes in the pendulum mass. Typically, only brittle and somewhat ductile polymers are evaluated with Charpy tests. In addition to the fracture energy, the type of break can be visually evaluated, as in whether the break was a total fracture of the sample or whether the sample experienced fracture in only part of the sample, and severely deformed section are still connected. Elastomers are typically not evaluated with Charpy tests due to their high yield strain inhibiting the Charpy test results.[10]
There are many properties of polymeric materials that influence their mechanical properties. As the degree of polymerization goes up, so does the polymer’s strength, as a longer chains have high Van der Waals interactions and chain entanglement. Long polymers can entangle, which leads to a subsequent increase in bulk modulus.[11] Crazes are small cracks that form in a polymer matrix, but which are stopped by small defects in the polymer matrix. These defects are typically made up of a second, low modulus polymer that is dispersed throughout the primary phase. The crazes can increase the strength and decrease the brittleness of a polymer by allowing the small cracks to absorb higher stress and strain without leading to fracture. If crazes are allowed to propagate or coalesce, they can lead to cavitation and fracture in the sample.[12][13] Crazes can be seen with transmission electron microscopy and scanning electron microscopy, and are typically engineered into a polymeric material during synthesis. Crosslinking, typically seen in thermoset polymers, can also increase the modulus, yield stress, and yield strength of a polymer.[14]
Dynamic mechanical analysis is the most common technique used to characterize viscoelastic behavior common in many polymeric systems.[15] DMA is also another important tool to understand the temperature dependence of polymers’ mechanical behavior. Dynamic mechanical analysis is a characterization technique used to measure storage modulus and glass transition temperature, confirm crosslinking, determine switching temperatures in shape-memory polymers, monitor cures in thermosets, and determine molecular weight. An oscillating force is applied to a polymer sample and the sample’s response is recorded. DMA documents the lag between force applied and deformation recovery in the sample. Viscoelastic samples exhibit a sinusoidal modulus called the dynamic modulus. Both energy recovered and lost are considered during each deformation and described quantitatively by the elastic modulus (E’) and the loss modulus (E’’) respectively. The applied stress and the strain on the sample exhibit a phase difference, ẟ, which is measured over time. A new modulus is calculated each time stress is applied to the material, so DMA is used to study changes in modulus at various temperatures or stress frequencies.[16]
Other techniques include
.Other techniques
- Field flow fractionation
- laser assisted mass analysis
- ACOMP[17][18]
- MFI
- Dual polarisation interferometry
- Matrix-assisted laser desorption/ionization
References
- ^ http://camcor.uoregon.edu/labs/polymer-character. Chartoff, Richard."Polymer Characterization Laboratory". University of Oregon CAMCOR. 2013.
- ^ Campbell, D.; Pethrick, R. A.; White, J. R. Polymer Characterization Physical Techniques. Chapman and Hall, 1989 p. 11-13.
- ^ S. Podzimek. The use of GPC coupled with a multiangle laser light scattering photometer for the characterization of polymers. On the determination of molecular weight, size, and branching. J. Appl. Polymer Sci. 1994 54 , 91-103.
- PMID 22591263.
- ^ "Mechanical Properties of Polymers".
- .
- S2CID 250838683.
- ^ Blaine, Roger L. "Determination of Polymer Crystallinity by DSC" (PDF).
- .
- .
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- ^ "Polymer Properties Database".
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- ^ Litozar, Blaz; Krajnc, Matjaz (2011). "Cross-Linking of Polymers: Kinetics and Transport Phenomena". Ind. Eng. Chem. Res. 50.
- ^ "Introduction to polymers: 5.4 Dynamic mechanical properties".
- ^ Mernard, Kevin (2008). Dynamic Mechanical Analysis: A Practical Introduction. CRC Press.
- ^ Alb, A.M.; Drenski M.F.; Reed, W.F. "Perspective automatic continuous online monitoring of polymerization reactions (ACOMP)" Polymer International,57,390-396.2008
- ^ US patent 6052184 and US Patent 6653150, other patents pending