Polymer characterization (II) - Semantic Scholar · characterization (Figure 1, Table 1) is a key...

Polymer characterization (II)

G. Marković1, M. Marinović-Cincović2, V. Jovanović3, S. Samaržija-Jovanović3 and J. Budinski-Simendić4 1 Tigar, Nikole Pasica 213, 18300 Pirot, Serbia 2 Institute of nuclear science Vinča, Mike Petrovica Alasa 11-13, 11000, Belgrade, Serbia 3 Faculty of Natural Science and Mathematics, University of Priština, Lole Ribara 29, Serbia 4 Faculty of technology, University of Novi Sad, Bulevar cara Lazara 1, 21000 Novi Sad, Serbia

Characterization describes those features of the composition and structure (including defects) of a material that are significant for a particular preparation, study of properties, or use, and suffice for reproduction of the material. The characterization of polymers may be said in a sense to have begun with the recognition and demonstration of the high molecular weight and long-chain nature of these substances. As a result of the development of many special characterization techniques for polymers and of the application to these materials of a large number of standard analytical methods, there is a wide selection of characterization methods from which to select those suitable for a particular system. The polymer characterization technique categories are: chemical, electrical, mechanical, molecular, physical, rheological, spectroscopic, thermal property, thermal transition and viscoelasticity. But unfortunately, many available techniques are not applicable to all polymer systems. The practical problem, however, is not so much the availability of characterization techniques but their application in an economically feasible, scientifically sound manner to the situation at hand.

Keywords: polymer; structure, methods, characterization

1. Introduction

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]. Characterization describes those features of composition and structure (including defects) of a material that are significant for a particular preparation, study of properties, or use and suffice for the reproduction of the materials. Polymer characterization is done with a variety of experimental approaches. Molecular characterization uses common methods from physical chemistry and often involves polymer solutions. Sometimes spectroscopic methods can be used. Some common spectroscopic techniques are UV-visible absorption spectroscopy, infrared spectroscopy (IR), Raman spectroscopy, nuclear magnetic resonance (NMR), electron spin resonance (ESR), and mass spectrometry (MS). These techniques are usually aimed at getting information about the chemical structure of polymer materials. Macroscopic property measurement is what might be referred to as conventional polymer characterization. It involves taking a macroscopic polymer specimen, often in the final solid form, and doing experiments that give information about properties of that polymer. Some of the more important properties include thermal properties, mechanical and failure properties, melt viscosity, viscoelasticity properties, friction, wear and electrical properties.

Fig. 1 Relation sheap between structure, procesing, properties and performance.

Table 1 The characterization techniques are development according to structure (length scale).

Structure (length scale)

Subatomic <0.2nm atomic 0.2-10nm microscopic 1-1000mm Macroscopic >1mm

processing

performance

properties

structure

characterization

Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.) _______________________________________________________________________________________________

397

The subjects are logically combined because understanding how structure affects properties, as measured in characterization (Figure 1, Table 1) is a key element of polymer materials science and engineering.

2. Polymer characterization techniques

The topic of polymer characterization covers techniques used to give information about structure or properties of polymers. Polymer characterization involves measuring any kind of property of a polymer material. It includes both molecular characterization, such as molecular weight, microstructural information, degree of crystallinity, etc., and macroscopic property measurement, such as thermal properties, mechanical properties, microstructural information, time dependence of properties, etc.

2.1 Polymer molecular (mass) weight characterization

Polymer molecular weight characterization is important because it determines many physical properties. The molecular weight of a polymer differs from typical molecules, in that polymerization reactions produce a distribution of molecular weights and shapes. The distribution of molecular weight can be summarized by the number average molecular weight, weight average molecular weight, and polydispersity. Some of the most common methods for determining these parameters are colligative property measurements, light scattering techniques, viscosimetry, and size exclusion chromatography. Gel permeation chromatography, a type of size exclusion chromatography, is an especially useful technique used to directly determine the molecular weight distribution parameters based on the polymer's hydrodynamic volume. Dynamic Light Scattering (DLS) is a good monitor of diffusive processes in soft matter (for example macromolecular solutions). This technique, also called quasielastic light scattering or photon correlation spectroscopy measures the correlation in time of a laser beam scattered from the sample at a fixed angle. Macromolecules undergo a diffusive Brownian motion when in solution.

2.2 Spectroscopy methods characterization

The major techniques for the determination of chemical composition and molecular topology (table 2) involve the absorption of electro-magnetic radiation by polymers. The major techniques are IR, Raman, and NMR spectroscopy and the bulk of this course will involve these major analytic techniques. Absorption is a quantized inelastic phenomenon involving the transfer of energy from EM radiation to a material.

Table 2 Spectroscopic and scattering methods commonly used for studying polymers.

Vibrational Spin resonance Electronic Scattering

Infrared (IR) Nuclear magnetic resonance (NMR)(Proton and carbon -13)

Ultraviolet (UV)-visible X-ray

Raman Electron spin resonance (ESR) Electron

Fluorescence Neutron

2.2.1 Vibrational spectroscopy methods characterization

Infrared spectroscopy (IR spectroscopy or Vibrational Spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, which is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. For a given sample which may be solid, liquid, or gaseous, the method or technique of infrared spectroscopy uses an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum. Infrared spectroscopy based on change in dipole moment of polymer's molecule. The infrared portion of the electromagnetic spectrum (is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. Infrared absorption regions for characteristic bands are shown on figure 2.

Fig. 2 Infrared absorption regions for characteristic bands.


398

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling. Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system [3]. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified. Need change in polarizability i.e., symmetric vibration modes, especially sensitive to vibrations with little or no change of dipole moments i) e.g., C-C vibrations cis-trans isomerism sulfur crosslinks in rubber. As instruments are relatively expensive, Raman method is harder to apply to colored materials.

2.2.2 Spin spectroscopy resonance methods characterization

Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei. It determines the physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. The intramolecular magnetic field around an atom in a molecule changes the resonance frequency, thus giving access to details of the electronic structure of a molecule.

2.2.3 Electronic spectroscopy methods characterization

Ultraviolet visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near- infrared [NIR]) ranges. Principle of ultraviolet-visible absorption- Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals [3]. The more easily excited the electrons (i.e. lower energy gap between the HOMO and the LUMO), the longer the wavelength of light it can absorb. Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying materials with unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei. Electron Spin Resonance, ESR, spectroscopy requires free radical in sample excellent for mechanistic studies synthesis degradation usually plotted in derivative mode.

2.2.4 Scattering spectroscopy methods characterization

In addition to inelastic absorption phenomena, elastic interaction between EM radiation and a material is possible and this gives rise to diffraction and scattering phenomena. The small crystallite size and dominance of crystalline orientation in processed plastics lead to several unique analytic approaches in the analysis of x-ray diffraction data in these materials. The focus in this course will be on those tools used in diffraction which are specific to polymeric materials. X-ray crystallography [4] is a tool used for identifying the atomic and molecular structure of a crystal, in which the crystalline atoms cause a beam of incident X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystalographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their disorder and various other information. Small-Angle Neutron Scattering (SANS) probes length scales from the near atomic (nanometer) to the near optical (micrometer) sizes. SANS has been a major characterization method in research areas such as polymers, complex fluids, biology and materials science.

2.2.5 Mass spectroscopy method characterization

Mass spectroscopy is an analytical method used to identify the composition of a compound based on its mass-to-charge ratio of the charged particles. Chemical fragments of the sample are produced through bombardment from an ion source. They are then accelerated using an electric field and passed through a magnetic field that curves their trajectory; the heavier the fragment, the larger the trajectory radius. The abundance of the various fragments is determined.

2.3 Microscopy methods characterization

Optical microscopy is the conventional form of microscopy. It uses visible light and can observe sizes down to 0.2 micrometer. Modern improvements of this technique include confocal microscopy which consists in focusing on specific layers within the specimen in order to attain depth resolution. Electron microscopy can observe smaller size


399

scales down to 1 nanometer. Electron microscopy is used in the transmission mode (TEM) for thin samples or in the scanning mode (SEM) to image surfaces. Samples are stained in order to enhance the contrast.

2.3.1 Transmission electron microscopy method characterization

Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera.

2.3.2 Scanning electron microscopy method characterization

A scanning electron microscopy (SEM) (figure 3) is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition. SEM can achieve resolution better than 1 nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet conditions (in environmental SEM), and at a wide range of cryogenic or elevated temperatures.

Fig. 3 SEM micrograph of CSM/IIR (60/40) a) and CSM/CIIR (60/40) rubber blends b) The most common SEM mode is detection of secondary electrons emitted by atoms excited by the electron beam. The number of secondary electrons that can be detected depends, among other things, on the angle at which beam meets surface of specimen, i.e. on specimen topography. By scanning the sample and collecting the secondary electrons with a special detector, an image displaying the topography of the surface is created.

2.4 Thermo-analytical methods characterization

Calorimetry consists in measuring the heat emitted and/or absorbed during thermodynamic transitions or chemical reactions. Differential Scanning Calorimetry (DSC) measures the amount of heat absorbed or emitted by two samples undergoing heating and/or cooling cycles: the measured sample and a reference sample. The difference in heat flow constitutes the DSC signal. The measured and reference samples must remain at the same temperature in DSC. In the Differential Thermal Analysis (DTA) method, it is the heat flow to the two samples that remains constant.

2.4.1 Differential scanning calorimetry

Differential scanning calorimetry or DSC is a thermo-analytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.

2.4.2 Differential thermal analysis

Differential thermal analysis (or DTA) is a thermo-analytic technique, similar to differential scanning calorimetry. DTA is older technique now generally replaced by DSC. In DTA, the material under study and an inert reference are made to undergo identical thermal cycles, while recording any temperature difference between sample and reference [5]. This differential temperature is then plotted against time, or against temperature (DTA curve, or thermogram). Changes in


400

the sample, either exothermic or endothermic, can be detected relative to the inert reference. Thus, a DTA curve provides data on the transformations that have occurred, such as glass transitions, crystallization, melting and sublimation. The area under a DTA peak is the enthalpy change and is not affected by the heat capacity of the sample. Sample and reference heated by same source.

2.4.3 Thermo-mechanical analysis

Thermo-mechanical analysis (TMA) is a technique used in thermal analysis, a branch of materials science which studies the properties of materials as they change with temperature. Thermo-mechanical analysis is a subdiscipline of the thermo-mechanometry (TM) technique [5]. Dynamic mechanical spectroscopy and dielectric spectroscopy essentially extensions of thermal analysis that can reveal more phase transitions by change in modulus or volume with temperature as they affect the complex modulus or the dielectric function of the material.

2.4.4 Thermo-gravimetric analysis

Thermo-gravimetric analysis (TGA) is a method of thermal analysis in which changes in physical and chemical properties of materials are measured as a function of increasing temperature (with constant heating rate), or as a function of time (with constant temperature and/or constant mass loss) [6]. TGA can provide information about physical phenomena, such as second order phase transitions, including vaporization sublimation absorption adsorption and desorption. Likewise, TGA can provide information about chemical phenomena including chemisorptions, desolvation (especially dehydration), decomposition, and solid-gas reactions (e.g., oxidation or reduction) [7]. TGA is commonly used to determine selected characteristics of materials that exhibit either mass loss or gain due to decomposition, oxidation, or loss of volatiles (such as moisture). Common applications of TGA are (1) materials characterization through analysis of characteristic decomposition patterns, (2) studies of degradation mechanisms and reaction kinetics, (3) determination of organic content in a sample, and (4) determination of inorganic (e.g. ash) content in a sample, which may be useful for corroborating predicted material structures or simply used as a chemical analysis. It is an especially useful technique for the study of polymeric materials, including thermoplastic, thermoset, elastomers, composites, plastic films, fibers, coatings and paints.

2.5 Mechanical properties methods characterization

The characterization of mechanical properties in polymers typically refers to a measure of the strength of a polymer film. The tensile strength and Young's modulus of elasticity are of particular interest for describing the stress-strain properties of polymer films. Dynamic mechanical analysis is the most common technique used to characterize this viscoelastic behavior. Other techniques include viscometry, rheometry and hardness.

2.5.1 Tensile strength

Tensile strength (figure 4) is defined as a stress, which is measured as force per unit area. For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. In the International System of Units (SI), the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the SI prefix; or, equivalently to pascals, newtons per square metre (N/m²). A United States customary unit is pounds per square inch (lb/in² or psi), or kilo-pounds per square inch (ksi, or sometimes kpsi), which is equal to 1000 psi; kilo-pounds per square inch are commonly used when measuring tensile strengths.


401

a) b)

Fig. 4 Mechanical tester a) and typical specimens for measuring tensile strength b).

2.5.2 Dynamic mechanical analysis

Dynamic mechanical analysis (DMA, also known as dynamic mechanical spectroscopy) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus. The temperature of the sample or the frequency of the stress are often varied, leading to variations in the complex modulus; this approach can be used to locate the glass transition temperature of the material, as well as to identify transitions corresponding to other molecular motions.

2.5.3 Young's modulus

Young's modulus, which is also known as the elastic modulus, is a mechanical property of linear elastic solid materials. It defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material.

2.6 Other characterization methods

2.6.1 Viscosity method characterization

A viscometer (also called viscosimeter) is an instrument used to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. Viscometers only measure under one flow condition. In general, either the fluid remains stationary and an object moves through it, or the object is stationary and the fluid moves past it. The drag caused by relative motion of the fluid and a surface is a measure of the viscosity. The flow conditions must have a sufficiently small value of Reynolds number for there to be laminar flow. At 20.00 degrees Celsius the dynamic viscosity (kinematic viscosity x density) of water is 1.0038 mPa s and its kinematic viscosity (product of flow time x Factor) is 1.0022 mm2/s. These values are used for calibrating certain types of viscometers.

2.6.2 Rheology method characterization

Rheometry generically refers to the experimental techniques used to determine the rheological properties. One of the goals of rheology is to establish relationships between deformation or flow and applied stress. Rheometers are operated either in a constant stress or constant shear mode. The choice of the adequate experimental technique depends on the rheological property which has to be determined. This can be the steady shear viscosity, the linear viscoelastic properties (complex viscosity respectively elastic modulus), the elongational properties, etc. For all real materials, the measured property will be a function of the flow conditions during which it is being measured (shear rate, frequency, etc.) even if for some materials this dependence is vanishingly low under given conditions (see Newtonian fluids).


402

Rheometry is a specific concern for smart fluids such as magnet or rheological fluids and electro rheological fluids, as it is the primary method to quantify the useful properties of these materials.

2.6.3 Hardness

Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a compressive force is applied. Some materials, such as metal, are harder than others. Macroscopic hardness is generally characterized by strong intermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness: scratch hardness, indentation hardness, and rebound hardness. Hardness is dependent on ductility, elastic stiffness plasticity strain strength toughness viscoelasticity and viscosity. Common examples of hard matter are ceramics concrete, certain metal, and super hard materials, which can be contrasted with soft matter.

3. Conclusion

The best solution calls for meaningful characterization, based on the selection, from all possible molecular and physical parameters, of those whose determination will insure the desired performance within the limits of current knowledge. The hope is that by appropriate compromise, adequate information can be obtained with an economically feasible amount of testing. Infinitely more needed are skilled persons capable of choosing the appropriate techniques, knowing both their limitations and applicability.

Acknowledgements Financial support for this study was granted by the Ministry of Science and Technological Development of the Republic of Serbia (Projects Numbers 45022 and 45020).

References

[1] Mukherjee S, Gowen A A Review a recent trends in polymer characterization using non-destructive vibrational spectroscopic. modalities and chemical imaging. Analytica Chimica Acta. 2015; 895: 12-34.

[2] Pethrick RA,Dawkins JW Modern techique for polymer characterization , Wiley 1999. [3] Murthy S, Recent developments in polymer characterization using X-ray diffraction, The rigaku Journal. 2004; 21 (1):15-24. [4] Mulla S M., Phale P S, Saraf M R. Use of X-Ray Diffraction Technique for Polymer Characterization and Studying the Effect

of Optical Accessories, presented at the AdMet 2012; 2012. [5] Manfred S Polymer Surface and Interface Characterization Techniques; Polymer surfaces and interfaces Characterization,

modification and applications, Springer 2008; 1-6. [6] Nicholas P C Polymer Characterization, Noyes Publications 1996; 17-24. [7] Arza S Characterization and Analysis of Polymers, John Wiley & Sons, 2008, 237-247.


403

Polymer characterization (II) - Semantic Scholar · characterization (Figure 1, Table 1) is a key...

Documents

Transcript of Polymer characterization (II) - Semantic Scholar · characterization (Figure 1, Table 1) is a key...