Understanding Polymers: Properties, Classification, and Degradation Mechanisms | Summaries Chemistry

WHAT ARE POLYMERS

Polymers are long chains of smaller molecules joined together through a process

called polymerization, the molecular weight of which ranges from hundreds to

hundreds of thousands. Natural polymers found in the human body include proteins,

sugars (polysaccharides), and nucleic acids.

The molecular weight of a polymer determines the length of the chains and, therefore,

their physical characteristics. Polymers are divided into two classes: thermosets and

thermoplastics. Thermoplastic polymers are further divided into two groups:

amorphous ones, such as polycarbonate (PC) and polystyrene (PS), and semi-

crystalline ones (polypropylene and acetal are two examples).

Amorphous polymers have a random/messy chain structure. Below Tg, they are hard

and brittle. With the application of heat, they gradually begin to soften until they

become rough and rubbery. This transition is the glass transition. Continuing to apply

heat, the polymer gradually becomes molten (moldable), having passed the Tg to a

temperature where it begins to exhibit viscous flow. Common examples of amorphous

polymers are hard, rigid materials such as polystyrene (PS) and polymethyl

methacrylate (PMMA) which are used in the glassy state and well below their glass

transition temperature.

Semi-crystalline polymers have highly ordered crystalline regions and amorphous

regions. The amorphous regions exhibit the same behavior just described. However,

in semi-crystalline materials, once the amorphous regions have passed the Tg, the

crystalline regions remain highly ordered and provide structure to the loose material.

For this reason, many semi-crystalline materials can be used well beyond their Tg.

Semi-crystalline materials such as polypropylene (PP), which has a Tg of around -20°C,

are used above their Tg in applications such as patio furniture which exhibit strength

and flexibility in the hot summer months, but can become brittle in the cold Northern

winters.

Thermosetting polymers have crosslinks that link the chains together. These crosslinks

form between the chains, turning them into one large molecule. You should think

about that the next time you have a bowling ball in your hands. The crosslinks provide

a strong chain structure that allows elastomeric materials such as liquid silicone

rubber to be used well beyond their Tg. Other thermoset materials, such as phenolics,

are used below their Tg and are quite stiff. Crosslinks form links between molecular

chains so strong that the melting point of thermoset materials is higher than their

decomposition temperature.

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WHAT ARE POLYMERS

Polymers are long chains of smaller molecules joined together through a process called polymerization, the molecular weight of which ranges from hundreds to hundreds of thousands. Natural polymers found in the human body include proteins, sugars (polysaccharides), and nucleic acids. The molecular weight of a polymer determines the length of the chains and, therefore, their physical characteristics. Polymers are divided into two classes: thermosets and thermoplastics. Thermoplastic polymers are further divided into two groups: amorphous ones, such as polycarbonate (PC) and polystyrene (PS), and semi- crystalline ones (polypropylene and acetal are two examples). Amorphous polymers have a random/messy chain structure. Below Tg, they are hard and brittle. With the application of heat, they gradually begin to soften until they become rough and rubbery. This transition is the glass transition. Continuing to apply heat, the polymer gradually becomes molten (moldable), having passed the Tg to a temperature where it begins to exhibit viscous flow. Common examples of amorphous polymers are hard, rigid materials such as polystyrene (PS) and polymethyl methacrylate (PMMA) which are used in the glassy state and well below their glass transition temperature. Semi-crystalline polymers have highly ordered crystalline regions and amorphous regions. The amorphous regions exhibit the same behavior just described. However, in semi-crystalline materials, once the amorphous regions have passed the Tg, the crystalline regions remain highly ordered and provide structure to the loose material. For this reason, many semi-crystalline materials can be used well beyond their Tg. Semi-crystalline materials such as polypropylene (PP), which has a Tg of around - 20°C, are used above their Tg in applications such as patio furniture which exhibit strength and flexibility in the hot summer months, but can become brittle in the cold Northern winters. Thermosetting polymers have crosslinks that link the chains together. These crosslinks form between the chains, turning them into one large molecule. You should think about that the next time you have a bowling ball in your hands. The crosslinks provide a strong chain structure that allows elastomeric materials such as liquid silicone rubber to be used well beyond their Tg. Other thermoset materials, such as phenolics, are used below their Tg and are quite stiff. Crosslinks form links between molecular chains so strong that the melting point of thermoset materials is higher than their decomposition temperature.

Amorphous polymers are often transparent (polycarbonate and acrylic are examples) rather than opaque like most semi-crystalline materials. They typically have better dimensional stability and are less likely to warp during the molding process. They are generally resistant to hot water and steam (think plumbing materials) and have good stiffness and impact resistance. They also tend to gradually soften when exposed to heat. Semi-crystalline thermoplastics, due to their internal structure, have very strong molecular bonds. This attribute makes them resistant to chemical attack. Like Teflon, many of these materials have a low coefficient of friction and are therefore a good choice for bearing and wear-prone surfaces or where structural loading is high. They are also much more resistant to fatigue than amorphous polymers. They soften when exposed to heat, but can be used above their Tg due to the crystalline regions retaining the structure up to the melting temperature of the polymer. Thermosetting materials, with their internal cross-linked structure, exhibit excellent chemical resistance, dimensional stability and heat resistance. Thermosets range from clear to opaque, elastomeric to rigid. They can be used below or above their Tg and do not have a melting point. POLYMERS DEGRADATION Polymer degradation is defined as an irreversible change of the chemical structure, physical properties, and visual appearance due to the chemical cleavage of the polymer’s constitutive macromolecules by one or more mechanism. More than one mechanism can simultaneously take place due to the action of external factors, and one mechanism can be more dominant than others at any time. External factors associated with the environment, such as heat, humidity, radiation, and acidic or alkaline conditions, could modify the degradation process and its rate. The degradation process can alter polymer properties such as mechanical, optical, electrical, discoloration, phase separation or delamination, erosion, cracking, and crazing. The main abiotic mechanisms associated with polymer degradation are mechanical, thermal (or thermo-oxidative), photo (photo-oxidative), and hydrolytic (chemical) degradation, some of which can be assisted by catalysis. In addition, ozone degradation (chemical) is considered a mechanism of degradation for polymers but is less common. The biotic degradation involves the action of microorganisms by enzymatic action. Microorganisms secrete enzymes that depolymerize the polymer chains into smaller molecules, which are then assimilated as a source of carbon and

There are various methods for determining the molecular weight of a polymer. One of them is the use of Gel Permeation Chromatography (GPC), a technique based on the separation of polymer chains according to their sizes. By measuring the retention time of polymer chains in a gel column, the relative molecular weight of the polymer can be calculated. In addition to molecular weight, the molecular structure of polymers is another important aspect to characterize. Techniques such as Infrared Spectroscopy (IR), Nuclear Magnetic Resonance (NMR), and Ultraviolet-Visible Spectroscopy (UV-Vis) can analyze the chemical structure and composition of polymers. These analyses provide information about the presence of functional groups, chain regularity, and molecular interactions within the polymer. The supramolecular structure of polymers can be studied through various thermal analysis techniques to evaluate the thermal behavior and phase transitions of the polymer, such as

Differential Scanning Calorimetry ( DSC ): Differential Scanning Calorimetry (DSC) is a thermal method used to measure the difference in heat flow between a sample and a reference material as they undergo a controlled temperature program. The DSC technique provides valuable information about the thermal properties and behavior of materials. There are two common types of DSC instruments:

Power compensation DSC: Measures the power required to maintain the temperature difference between the sample and reference at zero.
Heat flow DSC: Measures the temperature difference between the sample and reference using thermocouples. The differential heat flow is directly proportional to the difference in the output signals from the thermocouples. DSC allows the observation of various phenomena:

Chemical reactions (exothermic or endothermic): DSC can detect reactions such as melting (endothermic), crystallization (exothermic), evaporation (endothermic), condensation (exothermic), sublimation (endothermic), and solid-state transitions.
Glass transitions: DSC can identify transitions from a rigid amorphous state to a rubbery or liquid state, providing information about the thermal behavior of polymers and other materials.

In DSC, the measured parameters include heat flow rate (dQ/dt) as a function of temperature (T). The obtained data can be used to determine the heat capacity (Cp), enthalpy changes (ΔH), and other thermal properties of the sample.

Thermogravimetric Analysis ( TGA ): TGA (Thermogravimetric Analysis) is a technique used to measure the weight changes of a substance as it undergoes controlled temperature increases. It provides valuable information about the thermal stability, thermal-oxidative stability, composition of multicomponent systems, the effect of corrosive atmospheres on a material, and determination of moisture content. In TGA, the sample is heated, and as the temperature increases, chemical modifications occur, leading to the breaking of bonds and the formation of volatile products, particularly in the case of polymers. In addition to measuring the weight loss, the derivative of the weight loss (DTGA) can also be analyzed. DTGA offers several advantages, such as highlighting processes with small mass variations and providing better resolution for complex reactions.
Dynamic Mechanical Thermal Analysis (DMTA). Furthermore, microscopy techniques, including optical microscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Atomic Force Microscopy (AFM), can be used to observe the morphology and surface structure of polymers. These analyses allow visualization of the arrangement of polymer chains and identification of any defects or interesting features. The characterization of plastic materials also involves the evaluation of specific properties, such as melting range (measured by DSC), density (measured using a hydrostatic balance or other methods), optical characteristics (opacity, transparency, gloss), and mechanical behavior at room temperature (plasticity, toughness, flexibility, rigidity). Lastly, it is important to consider the heterogeneity of polymers, which can manifest in terms of composition, structure, and molecular weight. The presence of copolymers, chain regularity, branching, and molecular weight distribution all influence the properties and behavior of the polymer.

Understanding Polymers: Properties, Classification, and Degradation Mechanisms, Summaries of Chemistry

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WHAT ARE POLYMERS