Crystallization of polymers
Encyclopedia
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 spherulite
s. Polymer
s can crystallize upon cooling from the melt, 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%, thus crystallized polymers are often called "semicrystalline". The properties of semicrystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains.
(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 very 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 silicone
s.
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.
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. 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.
The chains interact via various types of the van der Waals force
s. 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 case of strong gradient, the growth has a unidirectional, dendritic character. However, if temperature distribution is isotropic and static then lamellae grow radially and form larger quasi-spherical aggregated 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 aignment within the individual lamellae of a spherullite.
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. 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.
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
.
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.
deformation, i.e., the polymer begins to creep
. Heat resistance is thus given for amorphous polymers just below the glass transition temperature.
Relatively strong intermolecular forces in semicrystalline polymers prevent softening even above the glass transition temperature. Their elastic modulus changes significantly only at high (melting) temperature.It also depends on the degree of crystallinity: higher crystallinity results in a harder and more thermally stable, but also more brittle material, whereas the amorphous regions provide certain elasticity and impact resistance. Another characteristic feature of semicrystalline polymers is strong anisotropy of their mechanical properties along the direction of molecular alignment and perpendicular to it.
Plastics are viscoelastic
materials meaning that under applied stress, their deformation increases with time (creep). The elastic properties of plastics are therefore distinguished according to the time scale of the testing to short-time behavior (such as tensile test which lasts minutes), shock loading, the behavior under long-term and static loading, as well as the vibration-induced stress.
polypropylene, which has crystallinity ~50%, is opaque. Crystallinity also affects dyeing of polymers: crystalline polymers are more difficult to stain than amorphous ones because the dye molecules penetrate much easier through amorphous regions.
Spherulite (polymer physics)
In polymer physics, spherulites are spherical semicrystalline regions inside non-branched linear polymers. Their formation is associated with crystallization of polymers from the melt and is controlled by several parameters such as the number of nucleation sites, structure of the polymer...
s. Polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s can crystallize upon cooling from the melt, 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%, thus crystallized polymers are often called "semicrystalline". The properties of semicrystalline 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 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 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 polyethylenePolyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
(PE), polyethylene terephthalate
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
PET polytetrafluoroethylene
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
(PTFE) or isotactic
Tacticity
Tacticity is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects of tacticity on the physical properties of the polymer...
polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
(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 very much easier in isotactic
Tacticity
Tacticity is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects of tacticity on the physical properties of the polymer...
than in the atactic polypropylene form. Atactic polymers crystallize when the side groups are very small, as in polyvinyl
Polyvinyl
Polyvinyl is a group of polymers derived from vinyl monomers.Polyvinyl may also refer to:* Polyvinyl chloride* Polyvinyl acetate* Polyvinyl alcohol* Polyvinyl Record Co., an independent record label...
and don't crystallize in case of large substituents like in rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
or silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
s.
Nucleation
NucleationNucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by 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
Crystal growth is achieved by the further addition of folded polymer chain segments and only occurs for temperatures below the melting temperatureMelting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
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. 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.
The chains interact via various types of the van der Waals force
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
s. 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
Temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...
and is suppressed at the top and bottom of the lamellae by the amorphous folded parts at those surfaces. In case of strong gradient, the growth has a unidirectional, dendritic character. However, if temperature distribution is isotropic and static then lamellae grow radially and form larger quasi-spherical aggregated 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 aignment within the individual lamellae of a spherullite.
Crystallization by stretching
The above mechanism considered crystallization from the melt, which is important for injectionInjection
Injection or Injected may refer to:* Injection , insertion of liquid into the body with a syringe* Injective function in mathematics, a function mapping distinct arguments to distinct values...
molding of plastic components. Another type of crystallization occurs upon extrusion
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
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. Polymer strength is increased not only by extrusion, but also by blow molding, which is used in the production of plastic tanks and PET
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
bottles. Some polymers which do not crystallize from the melt, can be partially aligned by 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 results 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
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
.
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
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
measurement, differential scanning calorimetry
Differential scanning calorimetry
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature...
(DSC), X-ray diffraction (XRD), infrared spectroscopy
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
and nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
(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
Polarized light microscopy
Polarized light microscopy can mean any of a number of optical microscopy techniques involving polarized light. Simple techniques include illumination of the sample with polarized light. Directly transmitted light can, optionally, be blocked with a polariser orientated at 90 degrees to the...
and transmission electron microscopy
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
).
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
Differential scanning calorimetry
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature...
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.
Thermal and mechanical properties
Below their glass transition temperature, amorphous polymers are usually hard and brittle because of the low mobility of their molecules. Increasing the temperature induces molecular motion resulting in the typical rubber-elastic properties. A constant force applied to a polymer at temperatures above Tg results in a viscoelasticViscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
deformation, i.e., the polymer begins to creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
. Heat resistance is thus given for amorphous polymers just below the glass transition temperature.
Relatively strong intermolecular forces in semicrystalline polymers prevent softening even above the glass transition temperature. Their elastic modulus changes significantly only at high (melting) temperature.It also depends on the degree of crystallinity: higher crystallinity results in a harder and more thermally stable, but also more brittle material, whereas the amorphous regions provide certain elasticity and impact resistance. Another characteristic feature of semicrystalline polymers is strong anisotropy of their mechanical properties along the direction of molecular alignment and perpendicular to it.
Plastics are viscoelastic
Viscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
materials meaning that under applied stress, their deformation increases with time (creep). The elastic properties of plastics are therefore distinguished according to the time scale of the testing to short-time behavior (such as tensile test which lasts minutes), shock loading, the behavior under long-term and static loading, as well as the vibration-induced stress.
Optical properties
Semicrystalline polymers are usually opaque because of light scattering on the numerous boundaries between the crystalline and amorphous regions. The density of such boundaries is lower and thus the transparency is higher either for low (amorphous polymer) or high (crystalline) degree of crystallinity. For example, atactic polypropylene is usually amorphous and transparent while syndiotacticTacticity
Tacticity is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects of tacticity on the physical properties of the polymer...
polypropylene, which has crystallinity ~50%, is opaque. Crystallinity also affects dyeing of polymers: crystalline polymers are more difficult to stain than amorphous ones because the dye molecules penetrate much easier through amorphous regions.