Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named spherulites. Polymers can crystallize upon cooling from melting, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer. The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10 and 80%, with crystallized polymers often called "semi-crystalline". The properties of semi-crystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains.
Crystallization mechanisms
Solidification from the melt 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 readily convert into amorphous solids. In other polymers, the chains rearrange upon freezing and form partly ordered regions with a typical size of the order 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).
Whether or not polymers can crystallize depends on their molecular structure – presence of straight chains with regularly spaced side groups facilitates crystallization. For example, crystallization occurs much easier in isotactic than in the atactic polypropylene form. Atactic polymers crystallize when the side groups are very small, as in polyvinyl and don't crystallize in case of large substituents like in rubber or silicones.
Nucleation
Nucleation starts with small, nanometer-sized areas, where a result of heat motions in 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.Georg Menges, Edmund Haberstroh, Walter Michaeli, Ernst Schmachtenberg: Plastics Materials Science Hanser Verlag, 2002,
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
Crystal growth is achieved by the further addition of folded polymer chain segments and only occurs for temperatures below the melting temperature Tm and above the glass transition temperature Tg. Higher temperatures destroy the molecular arrangement and below the glass transition temperature, the movement of molecular chains is frozen.GW Becker, Ludwig Bottenbruch, Rudolf Binsack, D. Braun: Engineering Thermoplastics. Polyamides. (in German) Hanser Verlag, 1998 Nevertheless, secondary crystallization can proceed even below Tg, 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.Wilbrand Woebcken, Klaus Stöckhert, HBP Gupta: Plastics Encyclopedia. (in German) Hanser Verlag, 1998,
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.Wolfgang Weissbach: Materials science and materials testing. Vieweg + Teubner Verlag, 2007,
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. in the IWF Knowledge and Media gGmbH (videos and articles on the dendritic crystallization of polypropylene) 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 (see, e.g. front images) 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 spherulite.
Cold crystallization (Recrystallization) For the intermediate temperature (above Tg but below the melting temperature Tm), the polymer chains gain enough mobility to reorganize into ordered, crystalline regions. This exothermic process is called cold crystallization. Computer simulations of short semi-crystalline polyamide 6,6 shows that at the later stage of cold crystallization, the chains in inter-lamellae amorphous regions gains enough mobility to exchange with the several folded chains in the lamellae. The free energy change of such exchange process is shown to be of order of thermal energy and it is important for the degrading process of semi-crystalline polymers.
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. 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.Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic articles Hanser Verlag, 2006,
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 coil 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.
Topochemical polymerization Polymers formed via topochemical polymerisation are generally crystalline. In many cases, the monomer to polymer transition occurs with the retention of crystallinity. Often one can determine the crystal structure of such polymers and the mechanism of polymerisation via single crystal X-ray diffraction. Since the polymerization happens in the crystalline lattice without the aid of solvents or reagents, it comes under the domain of green chemistry. Also, the topochemical polymerizations are mostly atom economical reactions. The product can be obtained without any further purifications. It can achieve unique products which cannot be synthesized through conventional methods.
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.
In addition to the above integral methods, the distribution of crystalline and amorphous regions can be visualized with microscopic techniques, such as polarized light microscopy and transmission electron microscopy.
+Degree of crystallinity (D, %) and densities of crystalline (ρc) and amorphous (ρa, g/cm3) polymers. Polymer D ρcρa Nylon (PA66 and PA6) 35–45 1.24 1.08 Polyoxymethylene (POM)70–80 1.54 1.28 Polyethylene terephthalate (PET) 30–40 1.50 1.33 Polybutylene terephthalate (PBT) 40–50 – – Polytetrafluoroethylene (PTFE) 60–80 2.35 2.00 isotactic polypropylene 70–80 0.95 0.85 atactic polypropylene ~0 – – High-density polyethylene 70–80 1.0 0.85 Low-density polyethylene 45–55 1.0 0.85