Question

(Amorphous vs semi-crystalline)

Using words and visuals, describe the amorphous and semi-crystalline arrangements of polymer chains when forming a solid. Using examples of specific polymers, explain why some types of polymer chains do not readily align and tend to be amorphous while others readily form semi-crystalline structures.

Answer 1

Polymers are composed of long molecular chains which form irregular, entangled coils in the melt. Some polymers retain such a disordered structure upon freezing and thus convert into amorphous solids. In other polymers, the chains rearrange upon freezing and form partly ordered regions with a typical size of the order of 1 micrometer. Although it would be energetically favorable for the polymer chains to align parallel, such alignment is hindered by the entanglement. Therefore, within the ordered regions, the polymer chains are both aligned and folded. Those regions are therefore neither crystalline nor amorphous and are classified as semicrystalline.

Examples of semi-crystalline polymers are linear polyethylene (PE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) or isotactic polypropylene (PP).

Semi crystalline polymers form spherulite structures.It is for a relatively rapid growth.

When compared to semi-crystalline thermoplastics of a similar grade, amorphous plastics tend to have better dimensional stability and impact resistance. Drawbacks include amorphous plastics having poor fatigue resistance and are prone to stress cracking.

Advantages of semi-crystalline plastics are that they are excellent for bearing, wear and structural applications. When compared to amorphous thermoplastics, these semi-crystallines tend to have better chemical resistance, electrical properties and a lower coefficient of friction. Drawbacks include semi-crystalline plastics being difficult to thermoform, difficult to bond, have a sharp melting point and only average impact resistance.

Nucleation

Nucleation starts with small, nanometer-sized areas where as a result of heat motion some chains or their segments occur parallel. Those seeds can either dissociate, if thermal motion destroys the molecular order, or grow further, if the grain size exceeds a certain critical value.

Apart from the thermal mechanism, nucleation is strongly affected by impurities, dyes, plasticizers, fillers and other additives in the polymer. This is also referred to as heterogeneous nucleation. This effect is poorly understood and irregular, so that the same additive can promote nucleation in one polymer, but not in another. Many of the good nucleating agents are metal salts of organic acids, which themselves are crystalline at the solidification temperature of the polymer solidification.

Crystal growth from the melt

Schematic model of a spherulite. Black arrows indicate direction of molecular alignment

Crystal growth is achieved by the further addition of folded polymer chain segments and only occurs for temperatures below the melting temperature T_m and above the glass transition temperature T_g. Higher temperatures destroy the molecular arrangement and below the glass transition temperature, the movement of molecular chains is frozen.^[5] Nevertheless, secondary crystallization can proceed even below T_g, in the time scale of months and years. This process affects mechanical properties of the polymers and decreases their volume because of a more compact packing of aligned polymer chains.

The chains interact via various types of the van der Waals forces. The interaction strength depends on the distance between the parallel chain segments and it determines the mechanical and thermal properties of the polymer.

The growth of the crystalline regions preferably occurs in the direction of the largest temperature gradient and is suppressed at the top and bottom of the lamellae by the amorphous folded parts at those surfaces. In the case of a strong gradient, the growth has a unidirectional, dendritic character.^[8] However, if temperature distribution is isotropic and static then lamellae grow radially and form larger quasi-spherical aggregates called spherulites. Spherulites have a size between about 1 and 100 micrometers and form a large variety of colored patterns when observed between crossed polarizers in an optical microscope, which often include the "maltese cross" pattern and other polarization phenomena caused by molecular alignment within the individual lamellae of a spherullite.

The arrangement of the molecule chains upon crystallization by stretching.The above mechanism considered crystallization from the melt, which is important for injection molding of plastic components. Another type of crystallization occurs upon extrusion used in making fibers and films.

In this process, the polymer is forced through, e.g., a nozzle that creates tensile stress which partially aligns its molecules. Such alignment can be considered as crystallization and it affects the material properties. For example, the strength of the fiber is greatly increased in the longitudinal direction, and optical properties show large anisotropy along and perpendicular to the fiber axis. Such anisotropy is more enhanced in presence of rod-like fillers such as carbon nanotubes, compared to spherical fillers.^[9] Polymer strength is increased not only by extrusion, but also by blow molding, which is used in the production of plastic tanks and PET bottles. Some polymers which do not crystallize from the melt, can be partially aligned by stretching.

Some elastomers which are amorphous in the unstrained state undergo rapid crystallization upon stretching.

Crystallization from solution

Polymers can also be crystallized from a solution or upon evaporation of a solvent. This process depends on the degree of dilution: in dilute solutions, the molecular chains have no connection with each other and exist as a separate polymer coils in the solution. Increase in concentration which can occur via solvent evaporation, induces interaction between molecular chains and a possible crystallization as in the crystallization from the melt. Crystallization from solution may result in the highest degree of polymer crystallinity. For example, highly linear polyethylene can form platelet-like single crystals with a thickness on the order 10–20 nm when crystallized from a dilute solution. The crystal shape can be more complex for other polymers, including hollow pyramids, spirals and multilayer dendritic structures.

A very different process is precipitation; it uses a solvent which dissolves individual monomers but not the resulting polymer. When a certain degree of polymerization is reached, the polymerized and partially crystallized product precipitates out of the solution. The rate of crystallization can be monitored by a technique which selectively probes the dissolved fraction, such as nuclear magnetic resonance.

Confined crystallization

When polymers crystallize from an isotropic, bulk of melt or concentrated solution, the crystalline lamellae (10 to 20 nm in thickness) are typically organized into a spherulitic morphology as illustrated above. However, when polymer chains are confined in a space with dimensions of a few tens of nanometers, comparable to or smaller than the lamellar crystal thickness or the radius of gyration, nucleation and growth can be dramatically affected. As an example, when a polymer crystallizes in a confined ultrathin layer, the isotropic spherulitic organization of lamellar crystals is hampered and confinement can produce unique lamellar crystal orientations.Sometimes the chain alignment is parallel to the layer plane and the crystals are organized as ‘‘on-edge’’ lamellae. In other cases, "in-plane" lamellae with chain orientation perpendicular to the layers are observed.

The unique crystal orientation of confined polymers imparts anisotropic properties. In one example the large, in-plane polymer crystals reduce the gas permeability of nanolayered films by almost 2 orders of magnitude.

Degree of crystallinity

The fraction of the ordered molecules in polymer is characterized by the degree of crystallinity, which typically ranges between 10% and 80%. Higher values are only achieved in materials having small molecules, which are usually brittle, or in samples stored for long time at temperatures just under the melting point. The latter procedure is costly and is applied only in special cases.

Most methods of evaluating the degree of crystallinity assume a mixture of perfect crystalline and totally disordered areas; the transition areas are expected to amount to several percent. These methods include density measurement, differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared spectroscopy and nuclear magnetic resonance(NMR). The measured value depends on the method used, which is therefore quoted together with the degree of crystallinity.