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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Crystallization of polymers | 2/3 | https://en.wikipedia.org/wiki/Crystallization_of_polymers | reference | science, encyclopedia | 2026-05-05T10:46:59.767344+00:00 | kb-cron |
=== 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.
Density measurements Crystalline areas are generally more densely packed than amorphous areas. This results in a higher density, up to 15% depending on the material. For example, polyamide 6 (nylon) has crystalline density ρc = 1.24 g/cm3 and amorphous density ρa = 1.08 g/cm3). However, moisture which is often present in the sample does affect this type of measurement. Calorimetry Additional energy is released upon melting a semicrystalline polymer. This energy can be measured with differential scanning calorimetry and compared with that released upon melting of the standard sample of the same material with known crystallization degree. X-ray diffraction Regular arrangement of atoms and molecules produce sharp diffraction peaks whereas amorphous regions result in broad halos. The diffraction pattern of polymers usually contains a combination of both. Degree of crystallinity can be estimated by integrating the relative intensities of the peaks and halos. Infrared spectroscopy (IR) Infrared absorption or reflection spectra from crystalline polymers contain additional peaks which are absent in amorphous materials with the same composition. These signals may originate from deformation vibrations of the regular arrangement of molecular chains. From the analysis of these bands, the degree of crystallinity can be estimated. Nuclear magnetic resonance (NMR) crystalline and amorphous areas differ by the mobility of protons. The latter can be monitored through the line shape of NMR signals and used to estimate the degree of crystallinity.
=== Kinetics of polymer crystallization === The methods used to determine the degree of crystallinity can be incorporated over time to measure the kinetics of crystallization. The most basic model for polymer crystallization kinetics comes from Hoffman nucleation theory. The crystallization process of polymers does not always obey simple chemical rate equations. Polymers can crystallize through a variety of different regimes and unlike simple molecules, the polymer crystal lamellae have two very different surfaces. The two most prominent theories in polymer crystallization kinetics are the Avrami equation and Lauritzen-Hoffman Growth Theory.
== Properties of semicrystalline polymers ==