Crystallinity of Polymers PDF

Crystallinity of Polymers 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 fr...

Crystallinity of Polymers 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". When hardness and rigidity are required, a polymer with greater crystallinity may be preferred. Amorphous Polymers Amorphous polymers can be defined as polymers that do not exhibit any crystalline structures in X-ray or electron scattering experiments. They form a broad group of materials, including glassy, brittle and ductile polymers. Many applications of polymers and polymer coatings need flexibility at low to ambient temperatures. That’s where amorphous polymers are the right choice. Lamella: A lamella is a small plate or flake, and may also be used to refer to collections of fine sheets of material held adjacent to one another. The term lamella is often used as a way to describe crystal structure of some materials. Schematic model of a spherulite. Black arrows indicate direction of molecular alignment Spherulites: In polymer physics, spherulites (from Greek sphaira = ball and lithos = stone) are spherical semi-crystalline regions inside non-branched linear polymers. Spherulite diameter may vary in a wide range from a few micrometers to millimeters. Spherulites are composed of highly ordered lamellae, which result in higher density, hardness, but also brittleness when compared to disordered regions in a polymer. The lamellae are connected by amorphous regions which provide elasticity and impact resistance. Effects of Spherulites Formation of spherulites affects many properties of the polymer material; in particular, crystallinity, density, tensile strength and Young's modulus of polymers increase during spherulization. Stronger intermolecular interaction within the lamellae accounts for increased hardness, but also for higher brittleness. The amorphous regions between the lamellae within the spherulites give the material certain elasticity and impact resistance. Crystallization Mechanisms 1) 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. 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 semi crystalline. Examples of semi-crystalline polymers are linear polyethylene (PE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) or isotactic polypropylene (PP). 2) Nucleation Nucleation starts with small, nanometer-sized areas where as a result of heat motion some chains or their segments occur parallel. 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. Many of the good nucleating agents are metal salts of organic acids, which themselves are crystalline at the solidification temperature of the polymer solidification. 3) 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. This process affects mechanical properties of the polymers and decreases their volume because of a more compact packing of aligned polymer chains. 4) Crystallization by stretching 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. 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. 5) 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. 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. 6) 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. The unique crystal orientation of confined polymers imparts anisotropic properties (A material is anisotropic in the broad sense if its properties, when measured at the same location, change with direction).