WHAT ARE POLYMERS
Polymers are large molecules made up of repeating subunits called monomers. They are also known as
macromolecules due to their size, which can range from thousands to millions of atoms. Polymers are
ubiquitous in modern society, and can be found in a wide range of applications including plastics, textiles,
coatings, adhesives, and many other materials.
Polymers have a wide range of physical and chemical properties that can be tailored to meet specific
applications. These properties include mechanical strength, thermal stability, electrical conductivity,
optical clarity, and chemical resistance. The properties of a polymer can be modified through the use of
additives, copolymers, and processing techniques.
HOMOPOLYMER
A homopolymer is a polymer that is composed of only one type of monomer unit. In other words, all of
the repeating units in a homopolymer are identical. Homopolymers can be formed through different types
of polymerization reactions, including addition polymerization and condensation polymerization.
For example, polyethylene (PE) is a homopolymer consists of a long chain of repeating units of -CH2CH2-. Another example of a homopolymer is polyvinyl chloride (PVC), which is formed through the
addition polymerization of vinyl chloride monomer units.
Homopolymers have several advantages over copolymers, which are polymers formed from two or more
types of monomer units. For instance, homopolymers have a simpler structure and fewer impurities than
copolymers. Homopolymers also tend to have more consistent properties because they are composed of
identical repeating units.
The properties of a homopolymer depend on the nature of the monomer unit and the degree of
polymerization. Homopolymers can have a range of physical and chemical properties, such as high or low
melting points, flexibility, rigidity, solubility, and resistance to chemicals, among others. These properties
can be modified by controlling the reaction conditions, such as temperature, pressure, and catalysts, during
the polymerization process.
Overall, homopolymers are an important class of polymers that have a wide range of applications in
various industries, including packaging, textiles, construction, and electronics.
THE UNIQUENESS AND IMPORTANCE OF POLYMERS
Polymers are unique and important because they exhibit a wide range of physical and chemical properties
that make them versatile materials for various applications. Some of the key attributes that make polymers
unique and important are:
o Molecular structure: Polymers have a complex and varied molecular structure that can be tailored
to provide specific properties such as strength, flexibility, rigidity, solubility, and durability. This
makes them useful for applications ranging from lightweight and flexible packaging materials to
high-strength structural components.
o Chemical and thermal stability: Polymers are resistant to chemical and thermal degradation,
making them suitable for use in harsh environments such as high-temperature applications,
exposure to chemicals, and weathering.
o Cost-effectiveness: Polymers are relatively low-cost compared to other materials, making them
ideal for mass production of products such as packaging materials, textiles, and consumer goods.
o Recycling: Polymers can be easily recycled and reused, making them a sustainable and ecofriendly material. This has led to the development of new technologies and processes for recycling
and repurposing waste polymers.
o Tailored properties: Polymers can be engineered to have specific properties such as conductivity,
biodegradability, and self-healing, making them useful for a wide range of applications in fields
such as electronics, medicine, and construction.
o Lightweight: Polymers are typically lightweight, making them ideal for applications where weight
reduction is important, such as in the aerospace and automotive industries.
Overall, the uniqueness and importance of polymers lie in their versatility, ability to be tailored to specific
applications, cost-effectiveness, and sustainability. The broad range of properties and applications of
polymers make them an indispensable material for modern society.
NATURAL POLYMERS
Natural polymers are polymers that occur naturally in living organisms, such as plants, animals, and
microorganisms. These polymers are synthesized by living organisms through biochemical pathways and
have a wide range of functions, such as providing structure and support, storing energy, and facilitating
cellular processes. Some common examples of natural polymers include:
•
Proteins: Proteins are natural polymers made up of long chains of amino acids. They play a vital
role in the structure and function of cells and tissues in living organisms. Examples of proteins
include collagen, which provides structural support to skin and bones, and enzymes, which
facilitate biochemical reactions in cells.
•
Nucleic acids: Nucleic acids, such as DNA and RNA, are natural polymers that play a critical role
in genetic information storage and transfer. These polymers are made up of nucleotides, which
contain a sugar molecule, a phosphate group, and a nitrogenous base.
•
Polysaccharides: Polysaccharides are natural polymers made up of repeating units of sugar
molecules. They play a variety of roles in living organisms, such as providing structural support
to plant cell walls and acting as energy storage molecules in animals.
•
Carbohydrates: Carbohydrates are natural polymers that are made up of long chains of sugar
molecules. They are found in many natural sources, including fruits, vegetables, and grains.
Carbohydrates play a critical role in providing energy to living organisms, as well as serving as
structural components in many biological systems.
•
Natural rubber: Natural rubber is a polymer made up of repeating units of isoprene. It is harvested
from rubber trees and is used in a variety of applications, such as making tires, gloves, and medical
devices.
•
Cellulose: Cellulose is a natural polymer found in the cell walls of plants. It is composed of glucose
units linked together by beta-1,4 glycosidic bonds and forms a strong, rigid structure that provides
support to plant cells.
Natural polymers have several advantages over synthetic polymers, such as biodegradability,
biocompatibility, and sustainability. They are also often cheaper to produce and have a lower
environmental impact.
POLYMERIC BIOMATERIALS
Polymeric biomaterials are synthetic or natural polymers that are used in various medical applications.
These materials can be designed to mimic the properties of natural tissues or to provide unique functions
for medical treatments. Polymeric biomaterials have several advantages over other materials, such as
metals and ceramics, due to their ability to be tailored for specific biological and mechanical properties.
Polymeric biomaterials can be classified into several categories based on their composition and function.
Some examples include:
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Synthetic polymers: Synthetic polymers are polymeric biomaterials that are synthesized in the
laboratory. They are often used due to their ability to be designed for specific biological and
mechanical properties. Examples of synthetic polymers used in biomedical applications include
polyethylene glycol (PEG), poly(lactic-co-glycolic acid) (PLGA), and polyurethane. Poly(lactic
acid) (PLA) and poly(glycolic acid) (PGA): PLA and PGA are biodegradable and biocompatible
polymers that are commonly used in tissue engineering applications. They can be processed into
porous scaffolds that can support cell attachment, proliferation, and differentiation.
§
Natural polymers: Natural polymers are polymeric biomaterials that are derived from natural
sources. They are often preferred over synthetic polymers due to their biocompatibility and
biodegradability. Examples of natural polymers used in biomedical applications include collagen,
chitosan, and hyaluronic acid.
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Hydrogels: Hydrogels are polymeric biomaterials that are capable of absorbing water and swelling
to form a gel-like material. They are often used in tissue engineering and drug delivery due to their
ability to mimic the properties of natural tissues. Examples of hydrogels used in biomedical
applications include alginate, agarose, and poly(ethylene glycol) diacrylate (PEGDA).
Polymeric biomaterials have a wide range of applications in various medical fields, such as tissue
engineering, drug delivery, and medical devices. They can be designed to mimic the properties of natural
tissues or to provide unique functions for medical treatments. Polymeric biomaterials have the potential
to revolutionize the field of medicine by providing safer, more effective, and more personalized treatments
for a wide range of medical conditions.
MOLECULAR STRUCTIRE OF POLYMERS
The molecular structure of polymers refers to the way in which the individual monomer units are
arranged and linked together to form the polymer chain. Polymers can have different types of
molecular structures, which can significantly affect their physical and chemical properties.
1. Linear polymers: Linear polymers have a simple, straight chain structure in which the monomer
units are linked end-to-end to form a long, unbranched chain. Examples of linear polymers include
polyethylene and polypropylene.
2. Branched polymers: Branched polymers have a more complex structure than linear polymers, with
side chains branching out from the main polymer chain. The presence of these side chains can
affect the physical properties of the polymer, such as its melting point and viscosity. Examples of
branched polymers include low-density polyethylene and dendrimers.
3. Crosslinked polymers: Crosslinked polymers have a network-like structure in which the polymer
chains are linked together through covalent bonds. This structure results in a material that is rigid,
insoluble, and has high mechanical strength. Examples of crosslinked polymers include silicone
rubber and epoxy resins.
4. Copolymers: Copolymers are polymers made up of two or more different monomer units. These
monomer units can be arranged in different ways, including block copolymers, alternating
copolymers, and random copolymers. The molecular structure of copolymers can significantly
affect their properties, such as their solubility and thermal stability.
The molecular structure of a polymer can be characterized using various analytical techniques, such
as nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and X-ray diffraction
(XRD). Understanding the molecular structure of polymers is crucial for designing and developing
new polymer materials with specific properties for various applications, such as in medicine,
electronics, and packaging.
In addition to the architecture, the degree of branching and crosslinking also affects the properties of
polymers. Higher degrees of branching and crosslinking typically lead to increased stiffness, hardness,
and resistance to deformation, while lower degrees of branching and crosslinking result in more
flexible and ductile materials.
Property
Linear Polymers
Branched Polymers
Crosslinked Polymers
Molecular
Structure
Straight, unbranched chains
Chains with side branches
Network structure
Flexibility
High flexibility due to the lack of
branching
Lower flexibility due to the
presence of side branches
Rigid, with
flexibility
Strength
Lower mechanical strength
compared
to
crosslinked
polymers
Higher strength
polymers
High mechanical strength due
to the network structure
Solubility
More soluble in solvents than
crosslinked polymers
May be less soluble than linear
polymers
Insoluble in most solvents
Processing
Easier to process and melt than
crosslinked polymers
May
require
specialized
processing techniques due to
branching
Difficult to process due to the
network structure
Thermal
Stability
May have lower thermal stability
than crosslinked polymers
May have higher thermal stability
than linear polymers
High thermal stability
Properties
Can be flexible or rigid,
depending on the specific
polymer
May have improved strength and
toughness
Rigid, stable, and strong
than
linear
little
to
no
It's important to note that these are generalizations and that the specific properties of a polymer depend on
a variety of factors, including the type of monomer units used, the degree of branching or crosslinking,
and the molecular weight of the polymer. Additionally, different applications may require polymers with
specific properties, and the choice of polymer architecture may be critical in achieving those properties.
COPOLYMERS
Copolymers are polymers made up of two or more different monomer units. They can be formed through
copolymerization, a chemical reaction in which two or more different monomers are polymerized together.
Copolymers can have different types of molecular structures, depending on the arrangement of the
monomer units in the polymer chain. Here are some common types of copolymers:
1. Random copolymers: Random copolymers have a random distribution of the monomer units along
the polymer chain. This results in a polymer with a random sequence of different monomer units.
Random copolymers can have a wide range of properties depending on the monomer units used.
2. Block copolymers: Block copolymers consist of two or more blocks of different monomer units
that are linked together. The blocks may be arranged in a linear or branched structure. Block
copolymers can have unique properties due to the presence of different blocks with distinct
properties.
3. Alternating copolymers: Alternating copolymers have a regular, alternating sequence of two
different monomer units along the polymer chain. This results in a polymer with a repeating unit
composed of two different monomers. Alternating copolymers can have a high degree of symmetry
and may exhibit unique properties due to the regular sequence of monomer units.
4. Graft copolymers: Graft copolymers consist of a main chain polymer with side chains of a different
monomer unit attached. The side chains can be either linear or branched, and the polymer
properties can be tuned by varying the length and density of the side chains.
Copolymers can exhibit a range of properties depending on the specific monomer units used and the type
of copolymer structure. For example, copolymers may have improved strength, flexibility, or thermal
stability compared to homopolymers made from a single monomer unit. Copolymers are used in a variety
of applications, including coatings, adhesives, and biomaterials.
POLYMER CRYSTALLINITY
Polymer crystallinity refers to the degree to which polymer chains are organized in a regular, ordered
manner. In a crystalline polymer, the polymer chains are arranged in a well-defined, repeating pattern,
whereas in an amorphous polymer, the chains are arranged randomly. The degree of crystallinity in a
polymer can have a significant impact on its properties, including its mechanical strength, thermal
stability, and optical properties.
The degree of crystallinity in a polymer is typically determined by its processing history and the specific
polymer structure. For example, some polymers may crystallize more readily than others due to the nature
of their molecular structure or the specific processing conditions used. Crystalline polymers typically have
higher melting points and tend to be more rigid and brittle than amorphous polymers.
One way to measure the degree of crystallinity in a polymer is through X-ray diffraction (XRD) analysis.
XRD can reveal the presence of a regular, repeating pattern in the polymer chain structure, which indicates
the presence of crystallinity. Another method is differential scanning calorimetry (DSC), which measures
the amount of heat absorbed or released during heating or cooling of a polymer sample. Crystalline
polymers typically exhibit a sharp melting point in DSC analysis, whereas amorphous polymers do not.
In some cases, polymers may exhibit both crystalline and amorphous regions, which is referred to as
semicrystalline. The degree of crystallinity in a semicrystalline polymer can be affected by factors such
as processing conditions, molecular weight, and polymer chain alignment. The degree of crystallinity can
be adjusted through various processing techniques, such as annealing or quenching, which can affect the
organization and alignment of the polymer chains.
CROSS-LINKING
Polymer cross-linking refers to the chemical bonding of polymer chains through covalent bonds to form
a three-dimensional network. Cross-linking can occur during polymerization, where monomers react to
form a network, or it can be induced after polymerization through chemical or physical methods. Cross-
linking can improve the mechanical, thermal, and chemical properties of polymers by increasing their
strength, stiffness, and resistance to deformation and degradation.
Crosslinked polymers have a number of useful properties that make them suitable for a wide range of
applications. They are often used in the manufacture of adhesives, coatings, and composites, as well as in
medical devices and other applications where biocompatibility is important. Crosslinked polymers can
also be used in applications that require high-temperature stability or resistance to harsh chemicals.
However, the process of crosslinking can also have some drawbacks. For example, it can make the
polymer more difficult to recycle or degrade, which can pose environmental challenges. It can also make
the material more brittle and difficult to process, which can limit its usefulness in certain applications.
THERMOPLASTICS VS. THERMOSETS
Thermoplastics and thermosets are two broad categories of polymers that differ in their physical and
chemical properties, as well as their processing methods and applications.
Thermoplastics are polymers that soften and become pliable when heated and can be molded into a variety
of shapes when heated above their melting temperature. When cooled, they become solid again and retain
their shape. This process can be repeated multiple times without causing significant degradation of the
material. Examples of thermoplastics include polyethylene, polypropylene, PVC, polystyrene, and nylon.
In contrast, thermosets are polymers that undergo a chemical reaction when heated to form a rigid and
durable material that cannot be reshaped or remolded. This process is called crosslinking, which creates
strong, permanent bonds between the polymer chains, making the material infusible and insoluble. Once
a thermoset is formed, it cannot be melted or reshaped again. Examples of thermosets include epoxy resins,
phenolic resins, and urea-formaldehyde.
Thermoplastics and thermosets differ in their mechanical, thermal, and chemical properties.
Thermoplastics typically have lower strength and stiffness than thermosets but are more ductile and have
higher impact resistance. They also have lower thermal stability, meaning they can soften and deform at
higher temperatures. Thermosets, on the other hand, have high strength, stiffness, and thermal stability
but are brittle and can fracture easily under impact.
In terms of processing, thermoplastics are typically easier to mold and process than thermosets. They can
be injection-molded, extruded, blow-molded, and thermoformed, among other methods. In contrast,
thermosets require a curing process to initiate the crosslinking reaction, which can involve heating,
pressure, or chemical initiators. Once cured, they cannot be remolded or reshaped.
Thermoplastics and thermosets are used in a wide range of applications, from consumer products to
automotive and aerospace components. The choice of material depends on the specific requirements of
the application, such as mechanical strength, temperature resistance, and durability.
MELTING & GLASS TRANSITION TEMPS.
Melting temperature and glass transition temperature are two important thermal properties of polymers
that are used to describe their behavior when subjected to temperature changes.
The melting temperature, also known as the melting point, is the temperature at which a polymer changes
from a solid to a liquid. This temperature is specific to each polymer and depends on the strength of the
intermolecular forces holding the polymer chains together. Generally, polymers with stronger
intermolecular forces have higher melting temperatures. The melting temperature can also be influenced
by factors such as molecular weight, molecular structure, and degree of crystallinity.
The glass transition temperature, also known as the Tg, is the temperature at which an amorphous polymer
transitions from a glassy state to a rubbery state. In other words, it is the temperature at which the polymer
chains become mobile enough to move past each other and exhibit more flexible, rubbery behavior. The
glass transition temperature is influenced by factors such as the chemical composition of the polymer, the
degree of crosslinking, and the presence of plasticizers or other additives. Polymers with higher Tg values
generally have greater stiffness and higher resistance to deformation at room temperature.
It's important to note that not all polymers have a distinct melting temperature or glass transition
temperature. Some polymers, such as thermosetting resins, do not have a melting point because they
crosslink upon heating and become permanently rigid. Other polymers, such as semi-crystalline materials,
can exhibit a range of melting temperatures due to variations in their crystallinity.
MECHANICAL PROPERTIES OF POLYMERS – STRESS-STRAIN BEHAVIOR
The mechanical properties of polymers are determined by their response to applied forces or stresses. One
of the most common ways to describe the mechanical properties of polymers is by analyzing their stressstrain behavior, which is the relationship between the applied stress and the resulting strain or deformation.
When a polymer is subjected to an external force or stress, it initially responds with a linear elastic
behavior, where the deformation is directly proportional to the applied stress. This region of the stressstrain curve is known as the elastic region. Beyond a certain point, called the yield point, the polymer
undergoes plastic deformation, where the deformation continues to increase without a corresponding
increase in stress. This region is known as the plastic region. The stress required to cause permanent
deformation or fracture of the polymer is known as the ultimate tensile strength (UTS).
In addition to stress-strain behavior, other important mechanical properties of polymers include hardness,
toughness, impact resistance, and fatigue resistance. These properties can be evaluated using various
testing methods, such as hardness testing, impact testing, and fatigue testing.
CRAZING DURING FRACTURE OF THERMOPLASTIC POLYMERS
Crazing is a phenomenon that occurs during the fracture of thermoplastic polymers, where a network of
small cracks or voids known as crazes appears in the material. These crazes occur in the region near the
crack tip and can propagate ahead of the main crack. Crazing is a common mechanism of fracture in
thermoplastics and can significantly affect the mechanical properties of the material.
Crazing occurs when the tensile stress applied to the polymer exceeds its yield strength, causing plastic
deformation to occur. This deformation causes the polymer chains to align in the direction of the applied
stress, creating a region of high strain. The high strain region can cause microvoids to form within the
polymer, which coalesce to form the craze. The craze is a thin layer of polymer that is highly oriented in
the direction of the applied stress and is surrounded by a region of less oriented polymer.
Crazing can significantly affect the mechanical properties of thermoplastic polymers. It can reduce their
tensile strength, fracture toughness, and fatigue resistance. Crazing can also cause stress concentrations
that can lead to crack initiation and propagation, resulting in catastrophic failure of the material.
The severity and extent of crazing in a thermoplastic polymer depend on several factors, including its
molecular weight, molecular structure, degree of crystallinity, and the presence of additives or
reinforcements. For example, increasing the molecular weight of a polymer can increase its resistance to
crazing by increasing the number of entanglements between polymer chains. Adding fillers or reinforcing
fibers can also increase the strength and stiffness of a polymer, reducing the likelihood of crazing.
POLYMER ADDITIVES
Polymer additives are substances that are added to polymer formulations to improve their processing or
performance properties. Some common types of polymer additives and their examples include:
§
Plasticizers: These are additives that are added to increase the flexibility and toughness of
polymers. Examples include dioctyl phthalate (DOP) and diisononyl phthalate (DINP).
§
Stabilizers: These are additives that protect polymers from degradation due to heat, light, or
chemical exposure. Examples include hindered phenols, such as Irganox 1010 and Irganox 1076.
§
Flame Retardants: These are additives that improve the fire resistance of polymers.
§
Antioxidants: These are additives that protect polymers from oxidation and degradation due to
exposure to air or heat. Examples include butylated hydroxytoluene (BHT)
§
Fillers: These are additives that are added to improve the mechanical and thermal properties of
polymers. Examples include calcium carbonate, glass fibers, and carbon black.
§
Lubricants: These are additives that improve the processability of polymers by reducing friction
between polymer chains. Examples include stearic acid and calcium stearate.
§
Processing aids: These are additives that improve the processability of polymers during
manufacturing. Examples include slip agents
§
Colorants - These are compounds that are added to polymers to impart color to them. Examples of
colorants include pigments, dyes, and color masterbatches.
§
Pigments: These are additives that are added to impart color to polymers. Examples include
titanium dioxide, iron oxide, and phthalocyanine blue.
§
Fillers and Reinforcements - These are compounds that are added to polymers to improve their
mechanical properties, such as strength, stiffness, and toughness. Examples of fillers and
reinforcements include glass fibers, carbon fibers, and silica.
PROCESSING OF POLYMERS
The processing of plastics refers to the various methods used to convert raw plastic materials into finished
products with desired shapes, sizes, and properties. There are several methods for processing plastics, and
the choice of method depends on the type of plastic, the product requirements, and the production volume.
Some of the common methods for processing plastics are:
§
Injection Molding - This is the most commonly used method for producing plastic products in
large quantities. In this method, molten plastic is injected into a mold cavity, cooled, and then
ejected as a finished product.
§
Extrusion - This is a continuous process used to produce products with a constant cross-section,
such as pipes, tubing, and sheeting. In this method, molten plastic is forced through a die to form
a continuous shape, which is then cooled and cut to length.
§
Blow Molding - This method is used to produce hollow products, such as bottles, containers, and
tanks. In this method, a tube of molten plastic is extruded, and then air is blown into it to expand
and shape it to the desired form.
§
Thermoforming - This is a process used to produce products with a shallow depth, such as trays,
lids, and packaging materials. In this method, a sheet of plastic is heated and then pressed into a
mold using vacuum or pressure to form the desired shape.
§
Rotational Molding - This is a method used to produce large, hollow products, such as tanks,
playground equipment, and garbage cans. In this method, a mold is filled with a powdered plastic
resin, which is then rotated in an oven until the plastic melts and coats the inside of the mold to
form the desired shape.
§
Compression Molding - This is a method used to produce products with complex shapes and high
strength, such as automotive parts and electrical switches. In this method, plastic resin is placed in
a mold and then compressed under heat and pressure to form the desired shape.
the choice of plastic processing method depends on various factors such as the type of plastic, the required
properties of the final product, and the production volume. The selection of an appropriate processing
method is crucial in determining the quality and cost-effectiveness of the final product.
Processing Method
Extrusion
Advantages
Disadvantages
High production rates
Limited complexity of shapes
Suitable for continuous production
High tooling costs
Wide range of materials
Precise control over wall thickness
Blow Molding
Suitable for hollow parts with complex shapes
Limited material options
Low tooling costs
Limited control over wall thickness
High production rates
Wide range of materials
Thermoforming
Low tooling costs
Limited complexity of shapes
High production rates
Limited material options
Suitable for large parts
Limited control over wall thickness
Precise control over wall thickness
Rotational Molding
Ability to produce large, complex shapes
Limited material options
High strength and durability
Slow production times
Low tooling costs
Lack of precise control over wall thickness
Flexibility in design
Compression Molding
Suitable for high-strength parts
Limited complexity of shapes
Low tooling costs
Limited control over wall thickness
Wide range of materials
Slow production times
Precise control over dimensions