MM – 538 Dr. Kausar Ali Syed Polymer Engineering Lecture No 3

July 14, 2010

POLYMER CRYSTALLINITY

If the atoms or ions of a solid are arranged in a pattern that repeats itself in three dimensions, they form a solid which is said to have a crystal structure and is referred to as a crystalline solid or crystalline material. Small molecules and ions form a three-dimensional lattice with an extended regular structure that makes large crystals possible. A small portion of the NaCl lattice is modeled in the diagram below.

Fig.1

We usually describe such lattices with a unit cell - the smallest repeating unit in the lattice. In the case of NaCl, the unit cell is said to be face-centered cubic.

Arrangement of Molecular Chains

A picture of an individual molecular chain has been built up as a long randomly twisted thread-like molecule with a carbon backbone. It must be realised, however, that each chain must co-exist with other chains in the bulk material and the arrangement and interaction of the chains has a considerable effect on the properties of the material. Probably the most significant factor is whether the material is crystalline or amorphous. The crystalline state may exist in polymeric materials also. At first glance it may seem difficult to imagine how the long randomly twisted chains could exist in any uniform pattern. In fact X-ray diffraction studies (Fig. 2) of many polymers show sharp features associated with regions of three dimensional order (crystallinity) and diffuse features characteristic of disordered (amorphous) regions. However, since it involves molecules instead of just atoms or ions, as with metals and ceramics, the atomic arrangements will be more complex for polymers. We think of polymer crystallinity as the packing of molecular chains to produce an ordered atomic array. Crystal structures may be specified in terms of unit cells, which are often quite complex. By considering the polyethylene molecule again it is possible to see how the long chains can physically co-exist in an ordered crystalline fashion. This is illustrated in Fig. 3 which shows the unit cell for polyethylene and its relationship to the molecular chain structure; this unit cell has orthorhombic geometry.

Fig. 2 Wide-angle X-ray scattering

(a) polystyrene, showing a diffusehalo from an amorphous sample

(b) highly crystalline polyethylene,showing sharp ‘powder’ rings

Of course, the chain molecules also extend beyond the unit cell shown in the figure. Molecular substances having small molecules (e.g., water and methane) are normally either totally crystalline (as solids) or totally amorphous (as liquids). As a consequence of their size and often complexity, polymer molecules are often only partially crystalline (or semicrystalline), having crystalline regions dispersed within the remaining amorphous material as shown for instance by the presence of broad halos in WAXS patterns from unoriented polymers in addition to any sharp rings due to crystalline material..

Fig. 3 Unit cell for polyethylene and its relationship to the molecular chain structure; this unit cell has orthorhombic geometry. The chain molecules also extend beyond the unit cell.

Any chain disorder or misalignment will result in an amorphous region, a condition that is fairly common, since twisting, kinking, and coiling of the chains prevent the strict ordering of every segment of every chain. Other structural effects are also influential in determining the extent of crystallinity which will be discussed later. The widths of the rings due to the crystallites indicate that, in some polymers, the crystallite dimensions are only of the order of tens of nanometres, which is very small compared with the lengths of polymer chains, which may be of order 3000 nm measured along the chain, i.e. about 100 times the dimensions of crystallites. The following questions therefore arise.

(i) How can long molecules give rise to small crystallites?(ii) What are the sizes and shapes of polymer crystallites?(iii) How are the crystallites disposed with respect to each other and to the non-crystalline material?(iv) What is the nature of the non-crystalline material?

These questions are the basis of polymer morphology, which may be defined as the study of the structure and relationships of polymer chains on a scale large compared with that of the individual repeat unit or the unit cell, i.e. on the scale at which the polymer chains are often represented simply by lines to indicate the path of the backbone through various structures. In addition to the four questions above, morphology is concerned with such matters as the directions of the chain axes with respect to the crystallite faces and with the relationship between the crystallites and the non-crystalline material, a particular aspect of which is the nature of the crystalline–‘amorphous’ interface. .

The morphology of a polymer plays an important role in determining its properties, but the molecular motions that take place within the polymer play an equally important role. We will study later the types of motion that can take place in solid polymers and the evidence for these motions.

Degree of Crystallinity

Before considering the details of how the chains are arranged in the crystalline and non-crystalline regions of a polymer, it is useful to consider how the amount of material contained within the two types of region can be determined.

The degree of crystallinity may range from completely amorphous to almost entirely (up to about 95%) crystalline; in contrast, metal specimens are almost always entirely crystalline, whereas many ceramics are either totally crystalline or totally noncrystalline. Semicrystalline polymers are, in a sense, analogous to two-phase metal alloys. In principle, almost any property that is different in the crystalline and non-crystalline regions could be used as the basis for a method of determining the degree of crystallinity, χ, or, as it is usually more simply put, the crystallinity, of a polymer sample. In practice the most commonly used methods involve density measurements, DSC measurements and X-ray diffraction measurements. .The crystallization of a polymer from the melt is accompanied by a reduction in specimen volume due to an increase in density. The density of a crystalline polymer will be greater than an amorphous (molten or non crystalline) one of the same material and molecular weight, since the chains are more closely packed together for the crystalline structure. This effect provides the basis of the density method for the determination of the degree of crystallinity. The technique relies upon the observation that there is a large and measurable difference (upto 20 %) between the densities of the crystalline and amorphous regions of the polymer. This method can yield both the volume fraction φc and the mass fraction xc from measurement of sample density ρs.If Vc is the volume of crystals and Va the volume of amorphous material then the total specimen volume, Vs, is given by

Vs = Vc + Va

Similarly the mass of the specimen Ws is given by: Ws = Wc + Wa

Where Wc and Wa are the masses of crystalline and amorphous material in the sample respectively. Since density ρ is mass per volume then it follows that

ρsVs = ρcVc + ρaVa

Substituting for Va and rearranging leads to

Vc / Vs = [ (ρs – ρa) / (ρc – ρa) ] = φc

since φc is equal to the volume of crystals divided by the total specimen volume. The mass fraction χ c of crystals or degree of crystallinity is similarly defined as

χc = Wc / Ws = ρcVc / ρsVs

Substituting the value of Vc /Vs, χc = [(ρc / ρs) (ρs – ρa) / (ρc – ρa) ]

or % crystallinity = [ ρc (ρs – ρa) / ρs (ρc – ρa) ] x 100

where ρs is the density of a specimen for which the percent crystallinity is to be determined, ρa is the density of the totally amorphous polymer, and ρc is the density of the perfectly crystalline polymer. The values of ρa

and ρc must be measured by other experimental means.

The degree of crystallinity of a polymer depends on the rate of cooling during solidification as well as on the chain configuration. During crystallization upon cooling through the melting temperature, the chains, which are highly random and entangled in the viscous liquid, must assume an ordered configuration. For this to occur, sufficient time must be allowed for the chains to move and align themselves.

EXAMPLE PROBLEM

Computations of the Density and Percent Crystallinity of Polyethylene

(a) Compute the density of totally crystalline polyethylene. The orthorhombic unit cell for polyethylene is shown in Figure 1; also, the equivalent of two ethylene repeat units is contained within each unit cell.(b) Using the answer to part (a), calculate the percent crystallinity of a branched polyethylene that has a density of 0.925 g/cm3. The density for the totally amorphous material is 0.870 g/cm3.

Solution(a) Equation utilized to determine densities for metals, also applies to polymeric materials and is used to solve this problem. It takes the same form—viz.

ρ = nA / VC NA

where n represents the number of repeat units within the unit cell (for polyethylene ), and A is the repeat unit molecular weight, which for polyethylene is

A = 2(AC) + 4(AH)

= 2 x (12.01 g/mol) + (4x1.008 g/mol) = 28.05 g/mol

Also, VC is the unit cell volume, which is just the product of the three unit cell edge lengths in Figure 3;

or VC = (0.741 nm) (0.494 nm) (0.255 nm)= 9.33 x 10 -23 cm3/unit cell

Now, substitution into Equation 3.5, of this value, values for n and A cited above, as well as leads to

ρ = nA / VCNA

_ (2 repeat units/unit cell) (28.05 g/mol)___________ (9.33 x 10-23 cm3/unit cell)(6.023 x1023 repeat units/mol)

= 0.998 g / cm3

(b) We now utilize Equation of % crystallinity to calculate the percent crystallinity of the branched polyethylene with ρc = 0.998 g/cm3, ρa = 0.870 g/cm3, and ρs = 0.925 g/cm3. Thus,

% crystallinity = [ ρc (ρs – ρa) / ρs (ρc – ρa) ] x 100

= [0.998 (0.925 – 0.870) / (0.998 – 0.870 )] x 100 = 46.4%

POLYMER CRYSTALS

The most obvious question that needs to be answered about polymer crystallites is question (i) How can long molecules give rise to small crystallites? Two principal types of answer have been given; they lead to the fringed-micelle model and the chain-folded model for polymer crystallites. A further type of crystallite, the chain-extended crystal, can also occur when samples are prepared in special ways. These three types of crystallites are considered in the following sections.

Fringed Micelle Model

During the 1940’s it was proposed that partially crystalline polymers consisted of regions where the molecular chains were gathered in an ordered fashion whereas adjacent regions had a random distribution of chains. It has been proposed that a semicrystalline polymer consists of small crystalline regions (fibrillar crystallites), each having a precise alignment, which are interspersed with amorphous regions composed of randomly oriented molecules and which can grow both parallel and perpendicular to the chain axes. The crystalline regions were considered to be so small that an individual chain could contribute to both crystalline and amorphous areas as shown in Fig. 4. Thus, a single molecule might pass through several crystallites as well as the intervening amorphous regions and small crystallites can thus be reconciled with long chains. This was known as the Fringed Micelle Model and was generally accepted until the late 1950's.

Fig. 4 Fringed Micelle Model

It is now believed to be incorrect as the basic model for polymer crystallites, but it is worth describing for historical reasons and because it may be a good approximation to the true structure in special cases.

Chain-folded Model

This type of crystallisation was first suggested by Storks in 1938. He made films of gutta percha 27 nm thick by evaporation from solution. Electron diffraction showed that the films were composed of large crystallites with the chain axes normal to the plane of the film. The only possibility was that the chains folded back and forth upon themselves, so that adjacent segments were parallel and in crystal register. This idea lay dormant until the early 1950s, but then very small single crystals of polymers were produced from dilute solutions. These crystals are regularly shaped, thin platelets (or lamellae), approximately 10 to 20 nm thick, and on the order of 10 μm long. Frequently, these platelets will form a multilayered structure, like that shown in the electron micrograph of a single crystal of polyethylene, Figure 9.Shortly afterwards (in 1957) Fischer showed by electron microscopy that the crystallites in melt-grown spherulites of polyethylene and nylon were most likely to be lamellar rather than fibrillar, as would be expected from the fringed-micelle model.

Studies by Keller and his group using electron diffraction showed that the chain axes were parallel to the thickness direction of these lamellar crystals (i.e. perpendicular to the flat faces of the platelet). Since the length of an individual chain could be 1000 times greater than the thickness of the platelet the only conclusion was that the chains were folded. The molecular chains within each platelet fold back and forth on themselves, with folds occurring at the faces; and as shown in Figure 5 and Fig . 6 in which the parallel chains (shown in different colors in the simulated structure) are perpendicular to the face of the crystals. Appropriately, this was called the Folded Chain Theory. It is now accepted that chain-folded lamellar crystallites play an important part in the structure of most ordinary crystalline polymers.

Figure 5 The chain-folded structure for a plate-shaped polymer crystallite. ~ 10 nm (~100Å)

Fig. 6

Sometimes part of a chain is included in this crystal, and part of it isn't. When this happens we find the chains hanging out of lamella everywhere in a disorder randomly coiled state (Fig. 7).

Fig. 7

Of course, the polymer chains will often come back into the lamella after wandering around outside for awhile and re-enters in some distance away from where it left. When this happens, we get a picture like this:

Fig. 8

This is the switchboard model of a polymer crystalline lamella. (The name arises from the similarity in the appearance of the crystal surface in this model to the criss-crossing of the wires on an old-fashioned manual telephone switchboard.)

When a polymer chain doesn't wander around outside the crystal, but just folds right back in on itself, as shown in the first pictures, that is called the adjacent re-entry model.

Keller originally suggested that solution-grown crystals exhibited regular sharp folding with adjacent re-entry. A later model with irregular re-entry, the switchboard model, was proposed by Flory. For solution-grown crystals adjacent re-entry was found to predominate, whereas for melt-grown crystals adjacent reentry probably occurs quite frequently, but much less regularly.

There are several proposals to account for the co-existence of crystalline and amorphous regions in the Folded Chain Theory . In one case, the structure is considered to be a totally crystalline phase with defects. These defects which include such features as dislocations, loose chain ends, imperfect folds, chain entanglements etc, are regarded as the diffuse (amorphous) regions viewed in X-ray diffraction studies. (In other words, in the complex structure of partially crystalline polymers, the crystalline regions consist of aligned and folded chains and the amorphous regions consist of crystal defects and randomly entangled chains.) As an alternative it has been suggested that crystalline (folded chains) and amorphous (random chains) regions can exist in a similar manner to that proposed in the fringed micelle theory.

Frequently, these platelets will form a multilayered structure, like that shown in the electron micrograph of a single crystal of polyethylene (Fig. 9).

Figure 9. Electron micrograph of a polyethylene single crystal.

Various pieces of evidence suggest that the fold surfaces are not perfectly regular, particularly for melt-crystallised materials. Buried folds exist up to 2.5 nm below the surface, but the number of sharp folds increases as the overall surface is reached and there are many folds near the mean lamellar surface. Outside

this there are loose chain ends and long loops (nonadjacent re-entry). In melt-crystallised material tie molecules pass from one lamella to adjacent ones in the multi-layer stacks that are generally found in such material (Fig. 10).There is thus an amorphous layer between the crystal lamellae. So a crystalline polymer really has two components: the crystalline portion and the amorphous portion. The crystalline portion is in the lamellae, and the amorphous potion is outside the lamellae. As the molar mass increases chain folding becomes less regular and the degree of crystallinity also falls, because the freedom of the chains to rearrange themselves on crystallisation decreases.

Figure 10

Spherulites

Many bulk polymers that are crystallized from a melt are semicrystalline and form a spherulite structure. As implied by the name, each spherulite may grow to be roughly spherical in shape; one of them, as found in natural rubber, is shown in the transmission electron micrograph (Fig. 11). The spherulite consists of an aggregate of ribbon-like chain-folded crystallites (lamellae) approximately 10 nm thick that radiate outward from a single nucleation site in the center.

Figure 11

In this electron micrograph, these lamellae appear as thin white lines. The detailed structure of a spherulite is illustrated schematically in Figure 12.

These lamellae also called “lamellar fibrils” grow out in three dimensions. The whole assembly is called a spherulite. In a sample of a crystalline polymer weighing only a few grams, there are many billions of spherulites.

In between the crystalline lamellae, there are regions where there is no order to the arrangement of the polymer chains. These disordered regions are the amorphous regions. As one can also see in the picture, a single polymer chain may be partly in a crystalline lamella, and partly in the amorphous state. Some chains even start in one lamella, cross the amorphous region, and then join another lamella. These chains are called tie molecules.

Figure 12. Schematic representation of the detailed structure of a spherulite.

As the crystallization of a spherulitic structure nears completion, the extremities of adjacent spherulites begin to impinge on one another, forming more or less planar boundaries; prior to this time, they maintain their spherical shape. These boundaries are evident in Figure 13, which is a photomicrograph of polyethylene using cross-polarized light. A characteristic Maltese cross pattern appears within each spherulite. The bands or rings in the spherulite image result from twisting of the lamellar crystals as they extend like ribbons from the center. In a sample of a crystalline polymer there are billions of spherulites.

Spherulites are considered to be the polymer analogue of grains in polycrystalline metals and ceramics. However, as discussed above, each spherulite is really composed of many different lamellar crystals and, in addition, some amorphous material. Polyethylene, polypropylene, poly(vinyl chloride), polytetrafluoroethylene, and nylon form a spherulitic structure when they crystallize from a melt.

Figure 13

Fig. 14

These spherulites may vary in size from fractions of a micron to several millimetres in diameter, depending on the cooling rate from the melt. Slow cooling tends to produce larger spherulites than fast cooling. As stated earlier, it is believed that the spherulites grow in all directions from a central nucleus, by the twisting of the folded chain platelets ( Fig. 15).

Fig. 15 Schematic representation of the detailed structure of a spherulite

The size of a spherulite will be limited by the growth of adjacent spherulites. If the polymer melt is cooled very quickly it may undercool, i.e. remain molten at a temperature below its melting point. This results in a shower of nucleation sites becoming available and a mass of spherulites will start to grow. The solid polymer will then consist of a large number of small spherulites.

Crystallization Tendency (Factors Determining Crystallinity of Polymers)

No polymer is completely crystalline. Crystallinity makes a material strong, but it also makes it brittle. A completely crystalline polymer would be too brittle to be used as plastic. The amorphous regions give a polymer toughness, that is, the ability to bend without breaking.

But for making fibers, we like our polymers to be as crystalline as possible. This is because a fiber is really a long crystal. Many polymers are a mix of amorphous and crystalline regions, but some are highly crystalline and some are highly amorphous. Here are some of the polymers that tend toward the extremes:

Some Highly Crystalline Polymers Some Highly Amorphous Polymers:

Polypropylene Poly(methyl methacrylate)Syndiotactic polystyrene Atactic polystyreneNylon PolycarbonateKevlar and Nomex PolyisoprenePolyketones Polybutadiene

 Thermosetting polymers do not crystallize. The ability of a thermoplastic polymer to partially crystallize is determined by the ease with which the molecules can move and be efficiently packed together to create long-range order. The ease with which a polymer will form into crystalline regions depends on the chemical composition and the structural details of particular polymers.

Several factors favor a polymer with a high percent crystallinity, including:

1. The size of side groups.2. The extent of chain branching.

3. Tacticity.

4. The complexity of the repeat unit.

5. Strong intermolecular forces (i.e., the degree of secondary bonding between parallel chain segments)

6. A low degree of polymerization.

7. A slow rate of cooling, and oriented molecules.

1. Size of Side Groups or Pendant Groups

Regular polymers with small pendant groups crystallize more readily than do polymers with large, bulky pendant groups. Thus, one can predict that polypropylene, [C2 H3 (CH3)]n, is less likely to crystallize than polyethylene, (C2H4)n, but more likely to crystallize than polystyrene, [C2H3(C6H5)]n, since the CH3 group is larger than a single H atom but smaller than a C6H5 group. Poly(vinyl alcohol) (PVA) is made by the hydrolysis of poly(vinyl acetate) (PVAc).

   

  PVAc   PVA  

PVA crystallizes more readily than PVAc because of the bulky acetate groups in PVAc. The -OH groups in PVA also form strong hydrogen bonds. Hence, the bulkier or larger the side-bonded groups of atoms, the less tendency there is for crystallization.

2. Chain Branching

For linear polymers, crystallization is easily accomplished because there are few restrictions to prevent chain alignment. Any side branches interfere with crystallization (although limited crystallization can take place if a small number of branches are present), such that branched polymers never are highly crystalline; in fact, excessive branching may prevent any crystallization whatsoever. Crystallization is favored by a regular arrangement along the polymer chain giving the structure a high degree of symmetry.

Polyethylene is a good example. It can be crystalline or amorphous. Linear polyethylene for example can form a solid with over 90% crystallinity in some cases. This is made possible by the planar zig-zag structure easily assumed by the molecule. But the branched stuff just can't pack the way the linear stuff can, so it's highly amorphous.

The density, strength, and modulus of PE all increase as the degree of crystallization increases. For example a sample of PE with a moderate concentration of chain branches may be approximately 40 – 60 % crystalline (LDPE) with a density of ~0.94 g/cm3, a strength of 4 – 16 MPa, and a modulus of 100 – 260 MPa. In contrast, a sample of PE with a low concentration of chain branches may be ˃95% crystalline (HDPE) with a density of ~1.0 g/cm3, a strength of 20 – 40 MPa, and a modulus of 400 to 1200 MPa.

For copolymers, as a general rule, the more irregular and random the repeat unit arrangements, the greater is the tendency for the development of noncrystallinity. For alternating and block copolymers there is some likelihood of crystallization. On the other hand, random and graft copolymers are normally amorphous.

 

3. Tacticity

Normal polystyrene is atactic with no regular order in the position of the benzene rings along the chain. The irregularity prevents the chains from packing closely to each other.

Atactic polystyrene, is amorphous. It is comparatively soft, low melting, and becomes swollen in solvents.

 

 

Isotactic and syndiotactic polymers crystallize much more easily because the regularity of the geometry of the side groups facilitates the process of fitting together adjacent chains.

In syndiotactic polystyrene the benzene rings are on alternate sides of the chain. This allows the chains to

 

pack into crystals.

Syndiotactic polystyrene is crystalline. It is rigid, high melting, and not penetrated readily by solvents.

As was the case with chain branches, the influence of tacticity on degree of crystallization ultimately influences the macroscopic properties of the polymer. For example, at room temperature isotactic polypropylene (~70% crystalline) is hard and rigid while atactic polypropylene (~0% crystallinity) is a useless gummy substance.

4. Complexity of the Repeat Unit

Crystallization is not favored in polymers that are composed of chemically complex repeat units (e.g., polyisoprene). On the other hand, crystallization is not easily prevented in chemically simple polymers such as polyethylene and polytetrafluoroethylene, even for very rapid cooling rates.

Polymers with long repeat units such as PET and many nylons, require more extensive chain segment motion to establish LRO. As such they typically crystallize slowly, if at all, and can be easily formed into glasses using modest quench rates.

 

   

5. Intermolecular Forces (Secondary Bonds between Chains)

Intermolecular forces can be a big help for a polymer if it wants to form crystals. A good example is nylon. You can see from the picture that the polar amide groups in the backbone chain of nylon(6,6) are strongly attracted to each other. They form strong hydrogen bonds. This strong binding holds crystals together.

Polyesters are another example. Let's look at the polyester we call poly(ethylene terephthalate).

The presence of polar and hydrogen bonding groups favors crystallinity because they make possible dipole-dipole and hydrogen bonding intermolecular forces. A polyester, such as poly(ethylene terephalate), contains polar ester groups. Dipole-dipole forces between the polar groups hold the PET molecules in strong crystals.

Crystallinity in poly(ethylene terephalate) is also favored by the structural regularity of the benzene rings in the chain. The benzene rings stack together in an orderly fashion.

6. Degree of Polymerization

Relatively short polymer chains form crystals more readily than long chains, because the long chains tend to be more tangled. High crystallinity generally means a stronger material, but low molecular weight polymers usually are weaker in strength even if they are highly crystalline. Low molecular weight polymers have a low degree of chain entanglement, so the polymer chains can slide by each other and cause a break in the material.

7. Processing

A major difference between small molecules and polymers is that the morphology of a polymer is dependent on its thermal history. The crystallinity of a polymer can be changed by cooling the polymer melt slowly or quickly, and by "pulling" the bulk material either during its synthesis or during its processing

A. Cooling Rate

When they are processed industrially, polymers often are cooled rapidly from the melt. In this situation, crystallization is controlled by kinetics rather than thermodynamics. There may not be time for the chains, which are entangled in the melt, to separate enough to form crystals, so the amorphous nature of the melt is "frozen into" the solid. A polymer is more likely to have a higher percent crystallinity if it is cooled slowly from the melt.

B. Orientation

Another important feature of long chain molecules is the ease with which they can be rearranged by the application of stress. If a plastic is stretched the molecules will tend to align themselves in the direction of the stress and this is referred to as orientation. Molecular orientation leads to anisotropy of mechanical properties. This can be used to advantage in the production of fibres and film or may be the undesirable result of a moulding process. However, it is important that orientation should not be confused with crystallinity. It is possible to have an orientated polymer which shows no evidence of crystalline regions when X-ray diffraction studies are carried out. Equally, a polymer may be crystalline but optical measurements will show no signs of orientation.

Crystallinity can be enhanced by pulling the bulk material either when it is synthesized or during its processing. This is common for both films and fibers.

When a film is formed the small crystallites tend to be randomly oriented relative to each other. Drawing (stretching) the film pulls the individual chains into a roughly parallel organization as is shown in the schematic diagram at the right. Films can either be uniaxially oriented (oriented in only one direction) or biaxially oriented (oriented in two directions).

Fibers normally are drawn so that they are oriented in one direction. Unstretched nylon fibers are brittle, for example. When the fibers are stretched the oriented fibers are strong and tough.

Orientation can be introduced into plastics such as polyethylene and polypropylene (both semi-crystalline) by cold drawing at room temperature. Other brittle plastics such as polymethyl methacrylate and polystyrene (both amorphous) cannot be cold drawn but can be drawn at elevated temperatures.

Polyethylene can be unentangled by forming a gel with a low molecular weight solvent. When the gel is drawn, the resulting fibers are highly oriented. Ultra-oriented PE formed in this way is used in bullet-proof vests.

Most network and crosslinked polymers are almost totally amorphous because the crosslinks prevent the polymer chains from rearranging and aligning into a crystalline structure. A few crosslinked polymers are partially crystalline.

In sum, the factors that hinder crystal formation in polymers are an atactic configuration, bulky side groups, and chain branches. It can be seen, for example, that if the polyethylene molecule has a high degree of branching then it makes it difficult to form into the ordered fashion. Polar side groups tend to aid in the formation of crystalline regions in polymers. Polymers with large repeat units are generally slow to crystallize. Polymer with simple symmetric structures are generally semi crystalline. If the side groups are large, it is not easy for a polymer with an atactic structure to form ordered regions. On the other hand isotactic and syndiotactic structures do have sufficient symmetry to be capable of crystallisation. Although the rules given in this section are incomplete, they lead to correct predictions in most common engineering situation. We pause to point out that the inability to form extensive crystalline regions is not necessarily a disadvantages; however, it means that the useful temperature range of the material may be determine by Tg

rather Tm. In fact, the reciprocal property, known as glass-forming ability, is often a highly desired materials property.

Even polymers for which all factors are favorable (small polar side groups in a stereo regular configuration ) are never fully crystalline. As such, they are referred to as semi crystalline, or partially crystalline, polymers. Because the macromolecules are highly entangled I the melt and diffusion rates are low, the chains do not have sufficient time to completely disentangle during solidification.

Amorphous Polymers Semi - Crystalline Polymers

* Random structure - The molecular structure of an amorphous polymer is completely random i.e., the molecular chains are randomly oriented and entangled with no recognized pattern.

* Ordered structure - In semi-crystalline polymers, regions of highly ordered molecular arrays exist in the molecular structure. The crystalline region are typically platte like. (folded chains stacked called lamella)

* Atactic configuration * Isotactic & Syndiotactic configuration

* Weak secondary bonding forces * Closer molecular spacing hence high secondary bonding forces

* Broad soflening range – thermal agitation of the molecules breaks down the weak secondary bonds. The rate at which this occurs throughout the formless structure varies producing broad temperature range for softening.

* Sharp melting point – the regular close- packed structure results in most of the secondary bonds being broken down at th same time.

* Usually transparent - the looser structure transmits light so the material appears transparent. E.g., PS, Acrylic, PVC, PC.

* Usually opaque – the difference in refractive indices between the two phases (amorphous and crystalline) causes interference so the material appears translucent or opaque. E.g., PE, Polyamides (Nylon).

* Low Strength - Low Strength than comparable semi- crystalline polymer

* Higher strength - Higher strength, Stiffer and high density than comparable amorphous polymer

* Tough – Amorphous region gives toughness to the materials

* Rigid – Crystalline region gives rigidity to the materials

* Fully amorphous polymers have only Tg and no Tm. Amorphous depend upon Tg.

* Semi-crystalline have both Tm and Tg. Semi-crystalline depends upon Tm.

* Low shrinkage – all thermoplastics are processed in the amorphous state. On solidification, the random arrangement of molecules produces little volume change and hence low shrinkage.

* High shrinkage - as the material solidifies from the amorphous state the polymers take up a closely packed, highly aligned structure. This produces a significant volume change manifested as high shrinkage.

Predict which member of each pair of polymers has the better glass-forming ability.

A. Isotactic [C2H3(CH3)], or Syndiotactic [C2H3F],,?B. Atactic [C2H3CH3],, or isotactic [C2H3CI]

SOLUTIONA. Both configurations are symmetric and can form semi crystalline structures. The relatively

bulky CH3 groups hinders crystal formation in [C2H3(CH3)], the secondary bonds associated with the polar F side group in [C2H3F] favor crystal formation. Thus, we predict that isotactic [C2H3(CH3)] is a better former, and syndiotactic [C2H3F] is more likely to form crystalline regions.

B. Atactic [C2H3Cl] has two factors working against the establishment of extensive crystalline regions: the atactic configuration and a bulky CH3side group. Isotactic [C2H3Cl] however, has a regular structure, no bulky side groups, and relatively strong secondary bonds associated with its Cl atom. Therefore, we predict that atactic [C2H3CH3],, is a better glass former and isotatctic [C2H3Cl] is more likely to form crystalline region.