Crystallinity in Polymers

Summary

This educational script explores the nature of polymers, highlighting their semi-crystalline state, where both amorphous and crystalline regions coexist. It explains how polymer chains fold to form crystals, which are significantly smaller than the chains themselves. Factors influencing crystallization, such as chain length, branching, copolymer structure, and tacticity, are discussed. The role of plasticizers in promoting amorphous structures and enhancing polymer flexibility is also covered, providing insights into polymer properties and their manipulation.

Takeaways

• 🧬 Polymers can exist in amorphous, crystalline, or semi-crystalline states, with the latter being a combination of both amorphous and crystalline regions.
• 🔍 Crystalline regions in polymers are formed by the folding of polymer chains, which are significantly longer than the dimensions of the crystals they form.
• 🔄 The concept of chain folding was crucial in explaining how long polymer chains could fit into smaller crystal unit cells, resolving early confusion in polymer science.
• 🌐 The degree of crystallinity is a key factor in polymers, defined as the mass of the crystalline region divided by the total mass of the polymer sample.
• 🔗 The structure of a polymer significantly influences its crystallization behavior, with factors such as chain length, branching, and copolymer structure affecting the likelihood of crystallization.
• 🌱 Longer chains are less likely to crystallize due to increased entanglement and the formation of amorphous regions.
• 🌿 Branched chains and copolymers with random sequences of monomers are less likely to form crystalline structures due to difficulties in achieving periodic packing.
• 🔄 Alternating copolymers have an inherent periodicity that allows for the possibility of crystallization, unlike random copolymers.
• 🔄 Tacticity, or the arrangement of side groups on the polymer backbone, affects crystallization, with isotactic and syndiotactic polymers being more likely to crystallize than atactic polymers.
• 💧 Plasticizers, which are low molecular weight additives, are used to promote amorphous structures in polymers, making them more pliable and less brittle by preventing crystalline packing.

Q & A

What are the two primary states of polymers?

Polymers can be either amorphous or crystalline.

Is it possible for a polymer to be completely amorphous or crystalline?

No, a given polymer is never fully amorphous or fully crystalline. A sample of polymer may have regions of both amorphous and crystalline phases, which is called semi-crystalline.

How do crystals form in a semi-crystalline polymer?

Crystals in a semi-crystalline polymer form by folding of the polymer chains.

Why are polymer chains much longer than the crystals they form?

Polymer chains are much longer than the crystals they form because historically, when crystal structures were determined using x-ray diffraction, the unit cell size and crystal size were found to be much smaller than the chain size, leading to the understanding of the folding mechanism.

What is the degree of crystallinity in a semi-crystalline polymer?

The degree of crystallinity is defined as the mass of the crystalline region divided by the total mass of the polymer sample, indicating the fraction of the polymer that is crystalline versus amorphous.

How does the length of a polymer chain affect its ability to crystallize?

Longer chains are less likely to crystallize because they are more likely to get entangled and form amorphous regions.

What impact does branching have on a polymer's ability to crystallize?

Branched chains are less likely to crystallize because the side branches interfere with the periodic packing necessary for crystalline structures.

How do different types of copolymers affect crystallization?

Random and block copolymers are less likely to form crystalline structures due to the random arrangement of monomers, while alternating copolymers have an inherent periodicity that may allow for crystallization.

What is the role of tacticity in polymer crystallization?

Tactic polymers, due to the randomness of side group placement, are less likely to crystallize. Syndiotactic and isotactic polymers, with their inherent periodicity, are more likely to form crystalline structures.

Why are plasticizers added to polymers and how do they affect crystallization?

Plasticizers are added to polymers to prevent crystallization and increase plasticity. They work by coming in between chains, preventing them from packing together in a crystalline manner, thus promoting amorphousness.

Give examples of polymers that use plasticizers and the plasticizers used.

Celluloid uses nitrocellulose as its polymer with camphor as a plasticizer, and cellophane uses cellulose with glycerol as a plasticizer.

Outlines

00:00

🔬 Structure of Semi-Crystalline Polymers

This paragraph explains the dual nature of polymers, which can be amorphous or crystalline, but are often semi-crystalline with regions of both. The crystalline regions form through the folding of polymer chains, which are significantly longer than the crystals themselves. This folding mechanism was crucial in resolving the historical puzzle of how long chains could fit into small unit cells, as observed in X-ray diffraction studies. The paragraph emphasizes the importance of understanding that a single polymer chain can span both crystalline and amorphous regions, contributing to the complex structure of semi-crystalline polymers.

05:02

🌐 Factors Influencing Polymer Crystallization

The second paragraph delves into the factors that affect the crystallization of polymers, focusing on the structural aspects of the polymers themselves. Key factors include the length of the polymer chains, which when long, tend to entangle and form amorphous regions, thus reducing the likelihood of crystallization. Branching in the polymer structure also hinders crystallization due to the difficulty in folding and packing branched chains into a crystalline structure. The type of copolymer (random, block, graft, or alternating) plays a significant role, with only alternating copolymers having the periodicity necessary for crystallization. The paragraph also touches on the concept of tacticity, where the arrangement of side groups on the polymer backbone influences crystallization, with isotactic and syndiotactic polymers being more likely to crystallize due to their inherent periodicity.

10:02

💧 Role of Plasticizers in Polymer Crystallization

The final paragraph discusses the role of plasticizers in polymer systems. Plasticizers are low molecular weight additives that prevent crystallization by interfering with the close packing of polymer chains, which is necessary for crystalline structures. By promoting amorphous regions, plasticizers increase the plasticity of the polymer, making it more flexible and less brittle. Examples of plasticizers include camphor in celluloid and glycerol in cellophane, both of which are used to enhance the polymer's flexibility and prevent crystallization, thus maintaining the amorphous nature of the material.

Keywords

💡Amorphous

Amorphous refers to a state of a polymer that lacks a well-defined crystalline structure. In the context of the video, it is mentioned that no polymer is fully amorphous, and a sample may contain regions of amorphous and crystalline phases. The amorphous regions are characterized by randomly oriented polymer chains, which contribute to the polymer's flexibility and are represented by 'randomly wiggling chains' in the script.

💡Crystalline

Crystalline is a term used to describe the orderly and repeating arrangement of atoms or molecules in a solid. In the video, crystalline regions in polymers are formed by the folding of polymer chains, resulting in an organized structure. These regions are depicted as 'parallel chains' in the script, and they contribute to the polymer's strength and rigidity.

💡Semi-crystalline

Semi-crystalline is a term used to describe polymers that contain both amorphous and crystalline regions. The video script explains that a given polymer sample may have regions of amorphous phase and regions of crystalline phase, which together form a semi-crystalline state. This dual nature influences the polymer's overall properties, such as its mechanical strength and flexibility.

💡Chain folding

Chain folding is a mechanism by which polymer chains fold upon themselves to form crystalline regions within a polymer. The video script emphasizes that the length of a given chain is much longer than the length of the crystal, and this folding is crucial for fitting long chains into smaller crystalline structures. This concept is historically significant as it resolved the confusion regarding how long chains could fit into small unit cells as observed in early polymer research.

💡Degree of crystallinity

The degree of crystallinity is a measure of the proportion of a polymer that is in a crystalline state. It is defined as the mass of the crystalline region divided by the total mass of the polymer sample. The video script explains that this factor indicates what fraction of a semi-crystalline polymer is crystalline and what fraction is amorphous, which directly affects the material's properties.

💡Chain length

Chain length refers to the number of repeating units in a polymer chain. The video script states that longer chains are less likely to crystallize due to increased entanglement, which favors the formation of amorphous regions. This is an important factor in determining the crystallization behavior of polymers.

💡Branching

Branching in polymers refers to the presence of side chains or branches extending from the main polymer backbone. The video script explains that branching makes it difficult for chains to fold and pack periodically, thus branched chains are less likely to crystallize. This factor influences the overall crystallinity and the physical properties of the polymer.

💡Copolymers

Copolymers are polymers made from more than one type of monomer unit. The video script discusses different types of copolymers, including random, block, graft, and alternating, and how their structures affect crystallization. For example, random copolymers with a random arrangement of monomers are less likely to form crystalline structures due to the lack of periodicity in their chain structure.

💡Tacticity

Tacticity refers to the spatial arrangement of substituent groups on a polymer chain. The video script explains that if the side groups are arranged randomly (atactic), the chain will not be able to form a crystalline structure. However, if the groups alternate (syndiotactic) or are all on the same side (isotactic), the chain has an inherent periodicity that allows for crystallization.

💡Plasticizers

Plasticizers are low molecular weight additives that are added to polymers to increase their flexibility and prevent crystallization. The video script mentions that plasticizers like camphor and glycerol are used in polymers such as celluloid and cellophane, respectively. They work by coming between the polymer chains, preventing them from packing together in a crystalline arrangement and thus promoting amorphousness.

Highlights

Polymers can be amorphous, crystalline, or semi-crystalline, with some regions being amorphous and others crystalline.

Crystals in polymers form by the folding of chains, which is a key mechanism for understanding polymer structure.

The length of a polymer chain is much longer than the length of the crystal it forms, which was historically a point of confusion in polymer science.

The folding mechanism of chains was crucial in explaining how long chains fit into smaller unit cells or crystals.

A single polymer chain can be part of both crystalline and amorphous regions within the same sample.

A single crystal in a polymer can be formed from more than one chain, indicating the complexity of semi-crystalline structures.

The degree of crystallinity is defined as the mass of the crystalline region divided by the total mass of the polymer sample.

The degree of polymerization affects crystallization; longer chains are less likely to crystallize due to entanglement.

Branching in polymers reduces the likelihood of crystallization due to the difficulty in folding and packing.

Copolymers, especially random and block types, are less likely to crystallize because of the random arrangement of monomers.

Alternating copolymers have an inherent periodicity that allows for the possibility of crystallization.

Tacticity influences crystallization; atactic polymers are less likely to crystallize due to randomness, while isotactic and syndiotactic polymers have more potential.

Plasticizers are low molecular weight additives that prevent crystallization, making polymers more plastic and less brittle.

Examples of polymers with plasticizers include celluloid, which uses camphor, and cellophane, which uses glycerol.

Plasticizers promote amorphous structures in polymers, enhancing their plasticity and deformability.

Transcripts

polymers are can be either amorphous or crystalline in fact a given polymer is never fully amorphous

or fully crystalline a given sample of polymer may have regions of amorphous field and regions

of crystalline phase such a state is called semi crystalline

semi crystalline so here we schematically a two dimensional schematic of a semi crystalline

polymer is shown what you should know here is that the crystals form by folding of the

chain you can see this blue chain or the red chain are folding so the first point is that

there are both amorphous region you can see the amorphous plus crystalline region

so where we have i am showing this parallel chains i am trying to represent crystalline

region and where i have shown these randomly wiggling chains that is the amorphous region

in the same polymer sample then the crystalline region the crystals form by chain folding

by folding of chains which really means and it's a very very important point that length

of a given chain it's much much longer than length of the crystal

so for example the green chain if you see is much much longer than the length of the

green crystal which is much smaller so the chains are much longer than the dimensions

of the crystal chains are much longer than the dimensions of the crystal they belong

to this is a very very important point historically as well because this was a big issue when

schrodinger and proposed the macromolecule or long chains as basic structure of polymers

because when crystal structure was determined using x ray diffraction the unit cell size

and the crystal size was found to be much smaller than the chain size

this led to the confusion that how come in a in a small unit cell or in a small crystal

long chains can fit so then this folding mechanism was thought of and gave an explanation for

long chains fitting into smaller unit cells or smaller crystals so chains are much longer

than crystals they belong to and a given chain if you see for example the green chain is

part of both crystalline and amorphous region so a given chain may belong to both crystalline

and amorphous regions and at the same time if you look at a given crystal so for example

this crystal look at this crystal this is a single crystal but then part of it is coming

from the blue chain and the part of it is coming from the red chain

so a given crystal can be formed from more than one chain so a given crystal consists

of more than one chains so these are basic aspects of a semi crystalline polymer a factor

called degree of crystallinity is defined which is simply mass of crystalline region

by total mass of the polymer sample this tells us that in a semi crystalline polymer what

fraction is crystalline and what fraction is amorphous instead of mass fraction one

can use volume fraction as well now what are the factors there are certain

aspects of polymer hm certain aspects of crystallization of polymers which depend upon the structure

of polymer a structural aspects of polymer so these factors are now listed so one of

the factors is length of the chain if you have long chain that is if you have high degree

of polymerization then these chains are less likely to crystallize this is simply be called

long chains are more likely to get entangled more likely to get entangled and form amorphous

amorphous regions so longer chains are less likely to crystallize

another factor which affects is branching if you recall there is a polymer backbone

and on that backbone sometimes you can have side branches we have discussed this so if

you have now branched chains like this it will be more difficult to fold them and pack

them periodically so these branches will come in the way of packing crystalline packing

of the polymer chains so branched chains are less likely to crystallize

now if you have copolymers then we discussed variety of copolymers random block graph and

alternating you can see that in random copolymer the monomers the black and the blue circles

are now representing monomers so the monomers come in some sort of random sequence so a

given chain itself has a random arrangement of monomers so crystallinity is much less

likely so it will not be forming a crystalline structure the block copolymers also has monomer

blocks of monomers coming in a random sequence so this is also unlikely to form crystalline

polymer graft polymer is like a branched polymer you can see and these branches will come in

the way of packing like the crystalline structure so the only alternating copolymer in which

the monomers are alternating has an inherent hm periodicity within the chain itself and

then one can think of folding and periodically arranging them so this may crystallize

we have discussed tacticity or stereoisomerism and if there is a special fide group those

specified group can come on either side of the main backbone if they are coming randomly

on either side then you have a tactic if they are coming alternately on each side you have

syndiotactic and if they are coming all on the same side it is isotactic you can see

that if in a a tactic chain or an a tactic polymer since because of the randomness of

the side group the main backbone or the main chain itself is random and will not be possible

to arrange them in a crystalline form so a tactic and will not be crystallizing where

a syndiotactic because it is coming alternately so has an inherent periodicity within the

chain and one can think of packing them in a crystalline way and same is true for the

isotactic the final factor which affects crystallization

of polymers is plasticizers sometimes low molecular weight additives are added to ah

polymers to prevent crystallization crystalline regions the crystalline polymers are usefully

brittle crystalline implies brittleness where as amorphous leads to lends plasticity to

the polymer plasticity so they can be deformed easily so these low molecular weight additives

are called plasticizers because they prevent the chains they come in between the chains

and prevent them from coming together so that therefore a crystalline periodic packing

of these chains are not possible and by promoting amorphousness they make the plastic or make

the polymer more plastic so that's why they are called plasticizers so some ah well known

bayer plasticizers and polymers made out of those plasticizers are celluloid which has

nitrocellulose as its polymer but camphor is added as plasticizer another polymer cellophane

has cellulose as its main chain but glycerol is added as plasticizers so both camphor camphor

and glycerol these are plasticizers in these polymers and they promote they promote amorphous

structure