Crystallization Behavior of Polymers
Under certain conditions, polymers cooled from the melt can arrange into regular crystalline structures. Such crystalline polymers have a less perfect structure than crystals formed from low molecular weight compounds. A common basic structure are lamellae that consist of layers of folded chains as illustrated below. The loops of the lamellae can be loose and irregular or thight and regular. The thickness of a typical crystallite is in the range of 10 - 20 nm. In the absence of a thermal gradient, the lamellae grow radially in all directions, resulting in spherical crystalline regions the so called spherulites. Usually, polymers can only produce partially crystalline structures, i.e. they are semicrystalline because polymers do not have a uniform molecular weight.
Crystallinity is usually induced by cooling a melt or a dilute solution below its melting point. The later can result in the growth of single crystals. Crystallization can also be induced by stretching a polymer. In this case, crystallization is caused by molecular orientation in the stretch direction. If the temperature is above the brittle point (preferentially above the Tg) and the polymer is stretched, the randomly coiled and entangled chains begin to disentangle, unfold, and straighten. This method is called strain-induced crystallization. It occurs when polymers are stretched beyond its yield point. One usually observes a noticeable increase in modulus due to the formation of crystals that act as a physical reinforcements similar to fillers. Thus when strain induced crystallization occurs, the stress increases as well.
The size and structure of the crystals and the degree of crystallinity depend on the type and structure of the polymer, and on the growth
conditions. Narrow molecular weight, linear polymer chains, and high molecular weight increase the crystallinity.
Crystallinity is also affected by extrinsic factors, like crystallization temperature, cooling rate, and in the case of strain-induced
crystallization, by the stretch ratio, strain rate, and by the forming process of the polymer film or fiber.
Nucleating agents such as organic salts, small filler particles and ionomers also affect the crystallization. They act as seeds and can increase the crystallization rate.
The degree of crystallinity also depends on the tacticity of the polymer. The greater the order in a macromolecule the greater the likelihood of the molecule to undergo crystallization. For example, isotactic polypropylene is usually more crystalline than syndiotactic polypropylene, and atactic polypropylene is considered uncrystallizable since the structure of the polymer chain lacks any regularity. In fact, most atactic polymers do not crystallize.
Strong intermolecular forces and a stiff chain backbone favor the formation of crystals because the molecules prefer an ordered arrangement with maximum packing density to maximize the number of secondary bonds. Thus the molecules tend to cooperatively organize and develop a crystalline structure. A good example is Kevlar which has a high degree of crystallinity. The polar amide groups in the backbone are strongly attracted to each other and form strong hydrogen bonds. This raises the glass transiton temperature and the melting point. The high crystallinity and strong intermolecular interactions also greatly increases the mechanical strength. In fact, Kevlar fibers are some of the strongest plastic fibers on the market.
Bulky side groups have the opposite effect on crystallinity. With increasing size of the side groups it becomes progressively more difficult for the polymer to fold and align itself along the crystal growth direction. Thus bulky side groups and branching reduce the ability and likelihood of a polymer to crystallize. For example branched polyethylene has a low dregree of crystallinity, even though polyethylene itself easily crystallizes. Similarly, most network polymers do not crystallize because the polymer subchains do not have the freedom to move.
Glass Transition Temperature versus Melting Point
Crystalline polymers are characterized by a melting point Tm and amorphous polymers are characterized by a glass transition temperature Tg. For crystalline polymers, the relationship between Tm and Tg has been described by Boyer as follows
Tg / Tm ≈ 1 / 2 → symmetrical polymers
Tg / Tm ≈ 2 / 3 → unsymmetrical polymers2
It was found that many polymers with a Tg / Tm ratio below 1/2 are highly symmetrical and consist of small repeating units of one or two main-chain atoms each carrying only single atom substituents. Examples are poly(methylene oxide), polyethylene, and polyacetal. These polymers are markedly crystalline. Polymers with Tg / Tm ratios above 2/3 are usually unsymmetrical. They can be highly crystalline if they have long sequences of methylene groups or are highly stereo-regular. The majority of the polymers, however, have Tg / Tm ratios between 0.5 and 0.75 with a maximum number around 2/3 (see figure above); both symmetrical and unsymmetrical polymers belong to this group.
- R.F. Boyer, Transitions and Relaxations in Polymers, Interscience, New York, 1967.
Unsymmetrical polymers are defined as polymers containing at least one main-chain atom in the repeat unit that does not have two identical substituents.