Properties of Polymers – Important Characteristics

The polymer comprises one of the biggest markets in production. We use various kinds of polymers both in high technology and daily customer products. Here, we will give you brief and introductory information about polymers, and the classification of polymers below. Here you can find information about the properties and characteristics of polymers.

What Is Polymer?

Polymers include a very long chain of molecules and atoms on regular basis. In general, these long chains of atoms are achieved with carbon atoms. Because of that, the term polymer takes its name from its molecular structure. The Greek word ‘poly’ means ‘many’, ‘meros’ means ‘part’ and ‘polymers’ means that many molecules and atoms constitute a part. There are a bunch of polymers available in nature, but the production of polymers on a serial basis is very late if we compare it with metals and ceramics. The use of natural polymers is not common as synthetic polymers’ properties. Production of synthetic polymers dated back around mids of the 1800s. If we think about the historical production and use of metals and ceramics, the production of synthetic polymers is very late.

Classification Of Polymers

The classification of polymers has three elements: Thermoplastic(TP) polymers, thermosetting(TS) polymers, and elastomers(E). We make the categorization characteristics of polymers according to their physical natures generally.

• Thermoplastic Polymers(TP): Thermoplastic polymers are a polymer class, that that is solid at room temperature. When they are heated up to several hundred degrees, they become liquid after some liquification phases. When we cool the thermoplastics to room temperature, they become the same solid structure again. This cycle is valid limitlessly for thermoplastics without significant degradation. This property of thermoplastics makes them very economical, that we can process them again and again. Thermo sets have the biggest market among other polymer types, which is around 70% of the polymer market. Polyethylene, polystyrene, polyvinylchloride, polypropylene, and nylon are the most known thermoplastic materials.

• Thermosetting Polymers(TS): The cycle explained above for thermoplastics, does not apply to thermosetting. Cooling after heating a thermosetting material changes the original molecular structure of the thermosetting polymer characteristics. Other chains occur after solidification which turns the thermosetting materials into ‘char’. Because of that, they use thermosetting materials only for one, such as molding. The most known thermosetting materials are phenolics and epoxies.

• Elastomers(E): Elastomers show a very specific physical property that they are very elastic materials. They can elongate up to 10x of their original shape by the effect of low mechanical loadings. After the withdrawal of this load, elastomers can return to their original shape. Because of this property of elastomers, they can find very specific applications in engineering and daily life. The most known natural rubber is vulcanized rubber. But synthetic rubber’s market is bigger than the natural one.

Properties of Thermosetting Polymers

Thermosetting polymers are one of three main groups of polymer materials. Specific properties of thermosetting find very important application areas in engineering and daily life.

The most important property of thermosetting characteristics polymers is the cross-linked molecular structure. Because of this cross-linked structure, we make the products from thermosetting materials that have one large macromolecule. They obtain the cross-linked structure in various ways for thermosetting;

We make the production process of the thermosetting polymer by mixing two or more chemicals. These chemicals create cross-linking between themselves that obtain a thermosetting polymer. They use heat generally to accelerate the chemical reaction. One of the most important examples of this group is epoxies.

We make the second form to produce some thermosetting polymer properties by the addition of catalysts to accelerate the cross-linked structure creation in liquid form. Without catalysts, these properties of thermosetting polymers stay stable for a long time.

In the shaping processes in thermosetting, they supply heat to melt the starting granular thermosetting material. Then we give the shape by molding it at high temperatures. In solidification, they create cross-linkings.

The physical and mechanical characteristics of thermosetting that because of cross-linkings;

Other Important Properties

• Thermosetting materials are less soluble in common solvents than thermoplastics, because of their cross-linked molecular structure.
• We can use thermosetting materials in much higher temperatures. But there is no chance of re-melt thermosetting materials because of cross-linked structures.
• Thermosets are generally in brittle structure, there are no apparent ductile characteristics as thermoplastics.
• Thermosets have rigidity and better mechanical characteristics than thermoplastics because of their cross-linked molecular structure. The modulus of elasticity of thermosetting materials is 2-3 times higher generally than thermoplastics.

Properties of Thermoplastic Polymers

Thermoplastic materials have lower mechanical properties generally; lower stiffness, hardness, and greater ductility compared with ceramics and metals. These mechanical properties of thermoplastics depend on temperature.

As you know that thermoplastics can have crystalline or amorphous molecular forms. So mechanical properties of a thermoplastic material change according to its molecular form. If we consider amorphous thermoplastics, they have a very critical property called ‘glass-transition temperature(Tg)’. Below the glass transition temperature, amorphous thermoplastics show solid characteristics. If we heat the amorphous thermoplastics around Tg, they will start to show rubbery characteristics. We call this rubbery characteristic of thermoplastics viscoelasticity. At the melting temperature of these thermoplastic materials, they show completely liquid characteristics called a viscous state.

But for crystalline thermoplastics, do not have any Tg that show rubbery characteristics. Crystalline thermoplastics are completely solid up to their melting temperature. After melting temperature, crystalline thermoplastics become completely liquid abruptly.

But there is a degree of crystallinity for most thermoplastic materials. So they can have both crystalline molecular structures and amorphous structures. So, these kinds of thermoplastics show mechanical and physical characteristics between complete crystalline and complete amorphous thermoplastic materials.

Also, the degree of polymerization and molecular weight of thermoplastics are very important parameters for properties. If the degree of polymerization and molecular weight increases for thermoplastics, they become more strong and harder mechanically. But their flowability reduce. This makes it hard to process these high DP and MW of thermoplastics.

Characteristics of Elastomers

The most important characteristic of elastomers is that we can stretch them extensively if we compare them with thermoplastics and thermosetting. This stretching property of elastomers comes from the very long molecular chains and some degree of cross-linking in molecular structure. But this cross-linked structure is not high as thermosets, in elastomers. The kinked long molecular chains of elastomers give the ability of stretching. And they return to the original shape by cross-linking structure.

We can make the classification of elastomers as;

• Inorganic elastomers that we produce with conventional polymer production characteristics.
• Organic elastomers originated from natural sources.

Very low forces can stretch the elastomer materials extensively. But with the increasing force, stretching decreases, but in the meantime, mechanical properties increases in stretched form.

To show elastomeric characteristics as stated above, the elastomer material must be above the glass transition temperature(Tg). At below these temperatures, elastomer polymer will be much more rigid characteristics, which will not show any elastomeric characteristics.

Thermal Properties of These Polymers

There is a difference between elastomers and thermoplastics in that their temperatures are above the glass transition temperature. Elastomers can return their original shape after we take the force, but thermoplastics do not turn. This is because there is a low degree of cross-linked molecular structure which provides this ability for elastomers. But thermoplastics, there is no cross-linked structure. Because of that, thermoplastics show ‘viscoelastic’ behavior which is special for this situation.

We call elastomeric materials also ‘rubbers’ in the market. We adjust their level of cross-linking with some processes. Crude natural rubbers are very elastic that we can stretch with very low forces. With the process called ‘vulcanization’, we can increase the number of cross-linked structures. With that increased cross-linked structure of natural rubber, we obtain much more stiff ‘vulcanized rubber. Also if we increase the cross-linking, much harder rubbers. After a level, that we can not call rubber, they are properties of thermoset polymers.

The general cross-linking density for rubbers can range from 1 to 10 in 100 carbon atoms in their molecular structure.

Linear, Branched, And Cross-linked Structures In Polymers

Polymers have different structural properties and because of this, have different properties. These molecular structures are; Linear branched and cross-linked structures.

• Linear structure in polymer molecules: In general, linear molecular structures occur in thermoplastics. In this molecular structure, molecules have occurred in linear alignments, and there is nearly no cross-linking or branched structure.
• Branched structure in polymer molecules: Branched structures in polymer molecules occur with the replacement of ‘H’ atoms with ‘C’ atoms in linear molecules, and because of this replacement with ‘C’, this ‘C’ makes additional and same chemical reactions with other ‘C’ atoms. Because of this phenomenon in polymer molecules, branched structures occur. Polyethylene is an example of this branched structure.
• Cross-linked structure in polymer molecules: This structure is the main type of structure that affects the polymer properties. Cross-linking occurs in polymer structures with the same logic as the branched structure explained above. When the branches created by ‘C’ atoms are linked with each other, as shown in the illustration, a cross-linked structure occurs.

Thermosets show different levels of cross-linking structures and this affects their properties of thermosets. With the application of heat and light, cross-linking structures increase in thermoset molecular structure. A cross-linked structure makes polymer harder and brittle. Also, when cross-linking has occurred in a polymer, there is no turn back for that material. Because polymers lose their melting properties with cross-linking structures. When you heat a cross-linked thermoset, it will not melt, just burn. Curing is the application of heat and light to create a much more cross-linked molecular structure in thermosets to be much harder and stronger.

Also, elastomers have cross-linked structures, but at a very low level compared with thermosets. Because of this low level of cross-linked structure, elastomers have very high ductility and resilience.

What is the Degree Of Polymerization?

Macromolecule chains that they produce with polymerization processes have ‘n’ repeating units. These units are the same as each other, and these same units constitute macromolecule chains of polymer properties. We call the of ‘n’ the ‘degree of polymerization(DP)’ of that polymer. Material properties of polymers change according to the degree of polymerization. The higher degree of polymerization provides more mechanical strength in the solid state of polymers, but in a liquid state, viscosity increases which make the processing of polymer more difficult. So the degree of polymerizations of different polymer characteristics are adjusted according to this basis.

What is the Molecular Weight(MW) Of Polymers?

As you know, molecules of polymers are the repeating structures of mers. We obtain these repeating structures with the de-bonding of carbon structures in monomers and attach these released bondings to other mers to obtain polymers. This most known structure of polymers is called ‘homopolymers’. But in copolymer structures, there are two types of mers to bond themselves. With the mixing and bonding of these two types of mers, we obtain different types of polymers. For example, they use ethylene and propylene to obtain a copolymer molecular structure that has elastomeric nature. There are types of copolymer structures according to their physical arrangements;

• Alternating copolymer structure: Constituent mers that form copolymer structures align in alternating structures.
• Random copolymer structure: Constituent mers align in the form of a random structure.
• Blocked structure of copolymers: The same group of constituent members bonds with themselves in the copolymer structure.
• Graft structure of copolymers: One type of mers attaches as branches to other mer that creates the main chain structure.

Like alloying metals in different percentages, they mix different polymers to obtain copolymer structures. Percentages are very important to obtain different properties just like in metal alloys. Also, ternary polymers are also available in the market that is synthesized with three different constituent mers. Plastic ABS(acrylonitrile–butadiene–styrene) is an important example of ternary polymers.

Polymer Production

In general, there are two most common processes in polymer production. These processes are;

• Step polymerization.
• Addition polymerization.

Step Polymerization

It has nearly the same logic as addition polymerization. But there are some kinds of differences in the process itself. In step polymerization, two monomers are brought together to obtain a new mer. All these new mers are brought together again and again at each step. They call all these steps n1, n2, n3… And with sufficient time and steps, we obtain long-chained polymers. Also, it depends on the number of steps, that number of steps also depends on time.

Unlike addition polymerization, byproducts can occur. In such polymerization processes, water(H2O) occurs as a byproduct. That water comes with condensation, which gives another name to this polymerization process ‘condensation polymerization’. In some step polymerization processes, ammonia(NH3) can be also produced as a byproduct.

Also, in step polymerization or condensation polymerization, we produce both thermoplastics and thermosetting. More specifically; nylon 6-6, and polycarbonate polymers are examples of thermoplastic polymers that they produce with step polymerization. Phenol formaldehyde and urea formaldehyde are examples of thermosetting polymers that are produced with step polymerization.

Addition Polymerization

Addition polymerization is a special process to produce long-chained polymers. In addition to polymerization, there are special steps to obtain special long-chained polymer groups. Starting molecules that we call ‘monomers’, which we used to obtain polymer molecules from them. To do it, we must open bondings between carbon atoms in monomer molecules to react with other monomer molecules. Opening carbon bondings take place by a catalyst which we call an ‘initiator’. After the addition polymerization process starts, single monomers are tied up between themselves to become short-chained polymers. With the ‘addition’ of monomers to these short-chained polymers, we obtain long-chained polymers.

All the processes of addition polymerization take place in seconds. But in industrial production and large serial production of polymers, all the addition polymerization processes can take minutes, even hours.

In general, we produce thermoplastics with the addition polymerization process. Also, a polymer we call ‘polyisoprene’ is a kind of natural rubber, we can produce with an additional polymerization process. In such processes, the replacement of H atoms with other atoms is a special application. For example in the production of polytetrafluoroethylene, all the four H atoms bonding with Carbon are replaced with F atoms. They use polypropylene generally in this H atom replacement process to obtain other kinds of polymers such as polyvinylchloride and polystyrene.

Stereoregularity Properties of Polymers

Stereoregularity is a very important parameter in polymer chemistry that defines the properties of polymers. It is about the arrangement of atom groups in the unit of long-chain polymers. As you know that with the replacement of ‘H’ atoms from mers with another atom, different types of polymers are produced. Stereoregularity concerns the alignment of these different atom groups of individual types of polymers. For example, in polypropylene, ‘H’ atoms are replaced with ‘CH3’ molecules. And alignment of these ‘CH3’ molecules affects the properties of polypropylene material. Alignment types for stereoregularity are;

• Isotactic: All the substituted atoms are on the same side of the long-chain structure in the polymer.
• Syndiotactic: Alternating alignments of substitution atoms at opposite sides on the long-chain structure.
• Atactic: Random alignment of substitution atoms on the long-chain structure.

Propylene’s melting point is around 175 celsius degrees in isotactic form for example. The syndiotactic form of propylene is 131 celsius degrees and the atactic is 75 celsius degrees. This can be a very good example of the effects of tactics of stereoregularity in polymers’ properties.

Crystallinity Properties of Polymers

Polymers have a lower tendency to be a crystalline structure than other materials. Because of this reason, the degree of crystallinity of polymers is always less than 100%. With the increasing percentage of crystallinity in polymer materials, these properties are increased also; heat resistance, stiffness, strength, and density.

Polymers can be transparent if they are partially crystalline. This partially crystalline structure depends on a glassy(amorphous) structure.

Crystallinity occurs folding of linear long chains of molecules upon themselves on regular basis. For crystalline polymers, there is no situation of 100% crystallinity. There are always non-crystallized long chains that are dispersed around crystallized long-chained molecules in polymers. Lamellar structure occurs with that folding phenomenon.

Various polymers have various levels of crystallinity. This level of crystallinity depends on such parameters;

• Copolymers do not form crystalline structures because of their irregular molecular structure.
• Plasticizers that are promoted to soften polymers, reduce the crystallinity.
• The deformation of thermoplastics in a heated situation increases the crystallinity.
• Like in metals and ceramics, slower cooling rates promote crystalline structure in polymers.
• Linear characteristics of polymers can make crystal structures.
• The Stereoregularity of a polymer affects crystallinity; From atactic, syndiotactic to isotactic, the formation of crystalline structures in polymers increases. Atactic polymers generally never form crystalline structures.

Thermal Characteristics of Polymers

As we stated above, the crystalline structure or amorphous structure of polymer materials gives specific properties. For crystalline characteristics of polymers, the melting point is very high, and with the increasing temperature, it gets higher and higher. Also, the change of physical structure of the material from solid to liquid state is very abrupt.

But for amorphous structure characteristics of polymers, there is no abrupt change of physical structure like crystalline one. When the temperature decreases from liquid to solid state, amorphous characteristics of polymers show a gradually changing physical appearance from liquid to solid. Thermal expansion of amorphous polymer lowers and lowers with the decreasing temperature. There is a specific point called ‘glass-transition temperature(Tg)’. At this point, the thermal expansion of amorphous polymer lowers abruptly upon lowering the temperature.

At the level of crystallinity between these two extremes, characteristics of polymers show thermal characteristics between these extremes again. Above melting point(Tm), polymers show viscous characteristics and, between Tg and Tm, they show viscoelastic properties.

As you know from the formation of crystalline structures in materials, upon slow cooling, crystalline structure formation is induced. It is valid for polymers also. With the increasing crystalline structure, Tm also increases.

If we take a look at the thermal characteristics of thermosets and elastomers, they show amorphous characteristics by decreasing temperature from the liquid state. But with the decreasing temperature, cross-linkings also occurred. These cross-linkings inhibit the formation of crystalline structures. Also, they can not be melted and liquified again because of these cross-linked structures.

Recycling Properties of Polymers is Tough

The first reason of it, general polymer materials include additives, dyes, and fillers to obtain specific properties. And decomposition of these additives is very hard and requires various kinds of processes in the recycling phase characteristics of polymers.

The second reason, if we consider the number of metals that are recycled, they are much higher in tonnage compared with plastics. Metals that are used in big structures such as bridges, ships, etc. recycling of them is much more profitable because of their higher tonnage. But recycling plastics is not profitable compared with metals, because gathering the same types of plastics to recycle is much harder. Products produced from plastics are smaller than metals to be recycled generally.

Another important reason for this, the plastics that they came for recycling can have various kinds of chemical structures, which makes them very hard to mix, and fuse to obtain a new product. For example, recycling glass waste is much easier compared with plastics, because all glass products depend on silicon dioxide.

We stated that there are different kinds of thermoplastic characteristics of polymers, and because of that, the decomposition of different kinds of characteristics of polymers is hard. To overcome this situation, the Society of Plastics Industry developed the Plastic Identification Code(PIC) which refers to the type of thermoplastic of produced plastic products to separate them into same groups to recycle them. You can see this code on plastic products in the shape of a triangle the corners are arrows. And inside these arrows, there is a number states the type of that thermoplastic.

Also…

With this code, people can separate the plastic parts into the same types. Here we give the meaning of these numbers to separate the thermoplastics;

• 1: Polyethylene terephthalate that is used in beverage containers.
• 2: High-density polyethylene that is generally used in shopping bags and milk containers.
• 3: Polyvinyl chloride that is used in juice beverage bottles and PVC pipes.
• 4: Low-density polyethylene which has the acronym LDPE used in compressible bottles and container lids.,
• 5: Polypropylene is generally used in yogurt containers and margarine containers.
• 6: Polystyrene is used in disposable plates, egg cartons, and cups, and as foamed packing materials.
• 7: Other: such as ABS or polycarbonate.

This system makes the decomposition of plastics much easier. Despite this system to decompose thermoplastics, the use of recycled thermoplastics is around 6% in the United States.

This separation system is valid for only thermoplastics. Recycling thermosetting and elastomers is almost impossible. Because of the cross-linked molecular structure in elastomers and thermosetting, melting of these materials to recycling them is not easy. But they can be recycled as filler material for other applications. Also, recycled elastomers can be used as granules, chunks, and nuggets in various applications.

Conclusion on Properties of Polymers

These are the main and the most important points about the characteristics of polymers. So, you can learn lots of things here about the properties of polymer materials.

In general, there are three classes of polymer materials. There are various properties of these polymer materials in general.

Polymer production methods are also very important to know about this material.

Finally, do not forget to leave your comments and questions below about the properties of glass.

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