Crystallization and crystallinity of polymers

Polymer Chemistry

1. Crystallization and Crystallinity of Polymers Presented by Dr. Prasanta Baishya 1

2. Outline Crystallization of polymers Factors affecting the Crystallinity of polymers M ethods of evaluating the degree of crystallinity 2

3. 3 Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. On the basis of the nature of order present in the arrangement of molecular chain , the polymers are classified as Crystalline Polymers : Crystallization of polymers is a process associated with partial alignment of their molecular chains. Crystalline polymers have a certain degree of crystallinity in their structures owing to the long range order arrangement of some segments of the polymer chains. Examples : Polyethylene (PE ), Nylon - 6,6, Kevler , Isotactic polypropylene ( PP ), etc. Amorphous Polymers: Most of the polymers do not have orderness in their structure and hence these type of polymers do not have any degree of crystallinity and these are known as amorphous polymers. Examples : Butadiene rubber, atactic polypropylene, etc. Polymers are never completely crystalline. They contains crystalline regions with amorphous regions together.

4. 4 The crystals forms by folding of polymer chains. The chains are much longer than the dimension of crystal they belong to. A given chain may belongs to both crystalline and amorphous region A given crystal may consist of more than one chains. Crystallinity defines the degree of long-range order in a material, and strongly affects its properties. The more crystalline a polymer, the more regularly aligned its chains.

5. 5 Why it is important? It is one of the most important property of the polymers. Some physical properties of polymers depend on their crystallinity . For example, Crystalline polymers possess high density and high melting. Crystalline polymers are usually opaque because of light scattering C rystalline polymers are more difficult to stain than amorphous ones Crystallinity makes polymers strong but lowers their impact resistance . C rystallization of polymers are broadly classified under three groups : ( A) Crystallization during polymerization ( B) Crystallization induced by stress and ( C) Crystallization under quiescent condition.

6. 6 Factors affecting the crystallinity Length of polymer chain : Long chain (high degree of polymerisation) are less likely to be crystallise. Long chains are more likely to get entangled and formed amorphous region. Chain Branching : Branch chains are less likely to be crystallise Inter chain bonding ( Copolymers ) : On the arrangement of repeating unit Crystallinity Random –A-B-A-A-A-B-B-A-A-B- No Block -A-A-A-B-B-B-A-A-A- No Graft No Alternating -A-B-A-B-A-B-A-B-A YES

7. 7 Differential Scanning Calorimetry (DSC) : Additional energy is released upon melting a semi-crystalline 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. Degree of crystallinity = ((Delta H f - Delta H c )/Delta H f,100% ) x 100% where Delta H f is the enthalpy of melting, Delta H c is the enthalpy of crystallization, and Delta H f,100% is the enthalpy of melting for a fully crystalline polymer. Enthalpy is the area under each peak so to get enthalpy values for crystallization and melting M ethods of evaluating the degree of crystallinity :

8. 8 % Crystallinity= Total area of all crystalline peaks X 100% / Total Area of all peaks The calculation of crystallinity by XRD is based on the presumption that the broad peak comes from amorphous phase, the sharp peak comes from crystal phase . T he sharp peaks become slightly broad. This causes from the small size effect of crystalline X-ray diffraction (XRD) : 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 .

9. 9 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) : C rystalline 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 .

10. 10 The distribution of crystalline and amorphous regions can be visualized with microscopic techniques, such as Polarized Optical Microscopy (POM) and Transmission Electron Microscopy (TEM ) 30 sec 60 sec 120 sec 9 0 sec 150 sec 180 sec 420 sec 390 sec 330 sec 360 sec 210 sec 240 sec 270 sec 300 sec 450 sec 510 sec 540 sec 570 sec 600 sec 480 sec POM experiment is done on these parameter Heating cycle Rate ( o /min) = 50 Limit ( o ) = 190 Holding time (min) = 3 Cooling cycle Rate ( o /min) = 50 Holding temperature = 120c o time = 10 min Each images taken at 30 second interval upto 10 min

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