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Rheological investigation of polymers

Today, polymers are among the most important materials for technical use and in everyday life, as the properties of polymers can be adapted in countless ways to fit nearly every field of application. Besides natural polymers like proteins, starches, cellulose, etc., there are many different kinds of synthetically produced polymers like nylon, silicones, PVC, plexiglas, etc. Some polymers are hard and brittle, some are tough and shock-resistant, while others are soft and flexible. The production and characterization of polymers is therefore the focus of numerous industries and specialized research institutes.

Rheological behavior of polymers

Polymers are large molecules composed of many repeated subunits called monomers. The length of the molecular chains and the entanglements between them are crucial for the properties of the material. Many relevant polymer properties can be characterized using rheological tests. Describing these properties requires diverse testing procedures in order to obtain the desired information.

Polymers show complex rheological behavior which needs to be considered when using or producing these kinds of materials, like the viscosity of the melt, flow behavior, viscoelastic properties, temperature-dependent behavior, glass transition temperature, aging behavior, etc. Various tests and analysis methods are employed to optimize the polymer properties until they meet all requirements.  

Investigating polymers using rheological tests

Rheological tests are useful in:

  • Quality control of polymers, e.g. by determining viscosityviscoelastic parameters, and molar mass
  • Improvement of the processing behavior of polymers for e.g. injection molding, extrusion, fiber spinning, etc.
  • Optimization of the end product (e.g. plastic materials in automotive production)

Acrylic glass (PMMA)

Polymethyl methacrylate (PMMA) is also known as acrylic glass or as the well-known and established trademark Plexiglas™. In fact it looks similar to glass, but is lightweight and shatter-resistant. When modified accordingly, acrylic glass shows great scratch and impact resistance. The pure material is rarely sold as an end product; it is more often sold as a modified formulation with varying amounts of comonomers, additives, and fillers. Due to its composition, acrylic glass has some specific properties: It can transmit light better than normal glass; it is elastic, shock-resistant and easily shaped at temperatures above approx. 105 °C; furthermore, it can be bonded or welded. Some types are permeable to UV light and X-rays but not to infrared light, which makes them ideal for specific applications such as in greenhouses and X-ray lithography.

Acrylic glass can be found in many products in the medical, automotive, building, and optical industries, as indicator glasses, spectacle glasses, industrial floors, light covers for vehicles, optical fibers, lenses, furniture, etc.

Rheological tests on acrylic glass

One kind of rheological test frequently performed on acrylic glass is dynamic mechanical analysis (DMA) in torsion using an oscillatory rheometer. In this test, a solid bar specimen of acrylic glass is fixed between two clamps and deformed at a specific amplitude and frequency over a defined temperature range. At low temperatures, the polymer shows stiff and brittle behavior (-150 °C). As the polymer is heated to very high temperatures, it begins to melt, passing from the solid, glassy state to the softening range at the glass transition temperature, finally reaching the liquid, molten state. The precise measurement of a solid bar of acrylic glass over a wide temperature range provides a lot of information on the relationship between macromolecular structure and mechanical behavior.

This test requires a rheometer equipped with a convection heating system for solid torsion bar fixtures.

Polyethylene (PE)

Among other applications, polyethylene (PE) is used as packaging material in the form of bottles, bags, films, etc. It is classified into several different categories mostly based on its density and molecular branching. There is, for example, high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), etc. Depending on the extent and type of branching, semi-crystalline structure and molar mass, polyethylene might show various mechanical properties. Whereas LDPE is used for rigid containers, LLDPE has a higher tensile strength compared to LDPE and is used for packaging, particularly in the form of film for sheets, plastic bags, and wrapping films. HDPE has a high strength-to-density ratio and is used for products and packaging materials such as milk cartons, detergent bottles, butter tubs, garbage containers, and water pipes.

Rheological tests on polyethylene

Rheological measurements provide information about the chemical properties of PE. For example, the molar mass of PE can be determined by measuring its zero shear viscosity with the help of frequency sweepsFrequency sweeps are oscillatory tests carried out at a constant amplitude and variable frequencies. At low values of the angular frequency, the sample’s zero shear viscosity value can be determined. The zero shear viscosity is one of the most important properties of a polymer melt, as it is directly proportional to the average molar mass. A great advantage of characterizing a polymer melt with an oscillatory rheometer is the relatively short time taken for a typical measurement. 

This test requires a rheometer equipped with a Peltier temperature control system.

Polypropylene (PP)

Polypropylene (PP) is a tough and flexible polymer with diverse applications, including flexible packaging, textiles, polymer banknotes, and engineering materials. PP has properties similar to polyethylene, but PP has lower density, a higher melting point (TM > 160 °C), and excellent chemical resistance. Products made from PP can be manufactured in a variety of ways, including film extrusion (for packaging), blow molding (for stronger containers such as bottles, tubs, fuel tanks), and injection molding for heavier-duty applications such as safety helmets, electrical tools, and TV casing. Such versatility in manufacturing means that various additives (e.g. dyes and pigments) and reinforcing agents can be used to modify the polymer’s properties. For example, glass-fiber reinforcement gives PP even better tensile strength at higher temperatures.

Rheological tests on polypropylene

To see how PP (or glass-fiber-reinforced PP) reacts to mechanical stress at various temperatures, dynamic-mechanical analysis (DMA) is used. The main aims of such a test are to see at what point the polymer starts to soften (its glass transition temperature, Tg), and up to which temperature the polymer can still resist a certain mechanical load. There are other thermal analysis methods for performing these tests (differential scanning calorimetry, DSC, or thermo-mechanical analysis, TMA) but DMA is usually much more accurate for finding the Tg.

In the DMA torsional test, a solid sample (e.g. with a rectangular or circular cross section) of the polymer is fixed between two clamps and deformed at a specific sinusoidal amplitude and frequency by using an oscillatory rheometer. The sample is strained or stressed over a defined temperature range, and the polymer’s response to the preset mechanical loading is measured over increasing temperature.

This test requires a rheometer equipped with a convection heating system for solid torsion bar fixtures.

Polystyrene (PS)

Comparatively inexpensive to produce, polystyrene is among the most widely used plastics. In the form of styrofoam, it is commonly used for protective packaging. In its rigid form it is used for building materials, yogurt containers, CD/DVD cases, bottles etc. Polystyrene has a relatively low melting point (about 100 °C) and is transparent. In industrial uses it is usually colored.  

Rheological tests on polystyrene

In order to investigate the short-term and long-term behavior of a polystyrene melt, a frequency sweep can be carried out using an oscillatory rheometer. Frequency sweeps are oscillatory tests performed at a constant amplitude and variable frequencies. The polymer sample placed into the measuring cell can be in the form of granules, a powder, or a pre-formed plate. Analysis of the cross-over point between the storage modulus and loss modulus curves makes it possible to obtain a qualitative picture of the polymer sample’s average molar mass. With further methods of analysis the molar mass distribution (MMD) can be determined. In contrast to other methods such as GPC analysis (gel permeation chromatography) this test method does not require any solvents and there are no limits to the MMD determination. 

This test requires a rheometer equipped with a Peltier temperature control system.

Polyurethane (PU)

Polyurethanes (PU) are the polymers most often used to make foams. PUs are formed by a two-component reaction between an isocyanate and a polyol. When water is present the reaction produces CO2 which leaves bubbles in the polymer to create a foam. PUs are highly versatile polymers, their applications ranging from foam seating and insulation to synthetic fibers such as Spandex, as well as diverse automotive products, such as seals and gaskets, suspension bushings, coatings, and sealants. 

The great versatility of PU arises because the two components used to make them can be so diverse. Long chains and low crosslinking give a polymer that is soft and stretchy, while short chains with lots of crosslinks produce a hard polymer. By carefully selecting the components, and closely monitoring the rheological properties of the reaction, the end product can be finely engineered.

Rheological tests on polyurethane

PUs are manufactured by mixing the two liquid components, the isocyanate and the polyol, and dispensing the mixture into a mold. The curing reaction then proceeds and the viscosity increases until the reaction is complete, and the final solid product can be demolded. The sample’s viscosity and other properties can be analyzed over the entire curing process by using a rheometer with a parallel-plate measuring system. Here, for example, an oscillation test with a constant low amplitude, such as 0.05 %, can be preset. Important points can be measured, such as the pot life (the point up to which the sample can still be processed, e.g. injected into a mold), the sol-gel point (where the sample turns from liquid into a gel-like solid), and the curing time, as shown in the graph below.

This test requires a rheometer equipped with a Peltier temperature control system and a disposable plate-plate measuring system.