Crystallinity of Polymers

CRYSTALLINITY OF POLYMERS

Polymer exists both in crystalline and amorphous form polymer chain forming crystalline and amorphous regions. Part of molecules arranged in regular order, called crystalline regions. In between ordered regions molecules arranged in random disorganized state called amorphous regions. Crystallinity is indication of amount of crystalline region in polymer with respect to amorphous content

Crystalline regions •A platelike structure (100 Angstrom thick) •Folded chain lamellae model

Crystallinity influences polymer properties, some of there are : · Hardness · Modulus · Tensile, · Stiffness · Crease · Melting Point ORIENTATION AND CRYSTALIZATION When polymer is extruded through the spinneret, the molecules orient themselves in the direction of the extruded melt. Polymer molecule orientation depends on many factors, some of them are, •Draw force •Screw speed •Melt temperature, •Stress force on melts •Number of spinneret holes

development of PET fiber structure along the spin line

At the take up point, the stress reaches a level of 107 dyne cm-2 by take up speed of 3000-4000 m/min . As polymer melt comes out from molten disoriented state,diameter decreases with reduction formation of oriented mesophase. After mesophase, neck like deformation formed along spin line. Diameter does not reduces after neck like deformation is completed. As fiber proceeds in spin line during the cooling molecules tend to curl and form ordered package. These orderly packed regions are called crystalline regions and are held together by less ordered regions called amorphous regions. This process of forming regularly ordered packing is called crystallization. The crystallization takes place between glass transition and melting state. Crystallization is always exothermic.

CRYSTALLINITY MEASUREMENT USING DSC DSC curve of a PET bottle sample

• used to determine amount of crystallinity in a polymer. • to measure amount of heat absorbed or evolved from sample under isothermal conditions. • DSC contains two pans, one reference pan that is empty and the other pan has polymer sample. In this method polymer sample is heated with reference to a reference pan. Both polymer and the reference pan are heated at same rate. The amount of extra heat absorbed by polymer sample is with reference to reference material.

HEAT CAPACITY Heat flow = heat / time = q / t …… (1) The heat rate is given by Change in temperature for given time, Heating rate = temperature / time = ΔT / t ….. (2) Dividing Equation (1) by (2) we get, Heat capacity = (q/t) / (ΔT/t) = q / ΔT = Cp = heat capacity of the sample. Big peak in the curve indicates crystallization temperature where polymer gives off huge heat to break hard crystalline arrangement. Next the melting point where take this temp as Tm. At this point polymer absorbs lot of heat; this is shown by huge dip in the curve. Heat of melting of the polymer is measured by area of this immerse in curve. Temperature at the tip of this immerse is Melting point Tm.

CRYSTALLINITY DSC evaluation can be used to measure amount of crystallinity in the sample. Let heat of crystallization be Hc and total heat given off during melting be Ht, H = HT – Hc Where H is the heat given off by that part of polymer, which was already in crystalline state. By dividing H by Hc (specific heat of melting) where Hc is amount of heat given off when 1gram of polymer is melted. H/Hc = joules/(joules/gram) = Mc Grams ..... (4) This is total amount of polymer that was crystalline below , Tc crystallization temperature. So Percentage of crystallinity in the polymer sample is Mc / Mt x 100 = % crystallinity in the sample Where Mt is total mass of sample taken

X-RAY DIFFRACTION used to measure the nature of polymer and extent of crystallinity . Crystalline regions in the polymer seated in well-defined manner acts as diffraction grating. So the Emerging diffracted pattern shows alternate dark and light bands on the screen. X-ray diffraction pattern of polymer contain both sharp as well as defused bands. Sharp bands correspond to crystalline orderly regions and defused bands correspond to amorphous regions

Crystalline structure is regular arrangement of atoms. Polymer contains both crystalline and amorphous phase within arranged randomly. When beam of X-ray passed through the polymer sample, some of the regularly arranged atoms reflect the x-ray beam constructively and produce enhanced intense pattern. Amorphous samples gives sharp arcs since the intensity of emerging rays are more, where as for crystalline samples, the incident rays get scattered. Arc length of diffraction pattern depends on orientation. If the sample is highly crystalline, smaller will be the arc length

X-ray diffraction pattern of (a) amorphous sample and (b) Semi crystalline polymer sample

CALCULATION OF CRYSTALLINITY The crystallinity is calculated by separating intensities due to amorphous and crystalline phase on diffraction phase.Computer aided curve resolving technique is used to separate crystalline and amorphous phases of diffracted graph. After separation, total area of the diffracted pattern is divided crystalline (Ac ) amorphous components (Aa ). Percentage of crystallinity Xc % is measured as ratio of crystalline area to Total area. Xc % = {(Ac / Aa ) + Ac } x 100 (%) ......... (5) Where Ac = Area of crystalline phase Aa = Area of amorphous phase Xc = Percentage of crystallinity Small Angle X-ray Scattering (SAXS), Infrared Spectroscopy, can also be used to measure crystallinity.

CRYSTALLISABILITY • The maximum crystallinity that a polymer can achieve at particular temperature • Depends on chemical nature of the polymer chain, geometrical structure, molecular weight, monomer weight disribution. STRUCTURAL REGULARITY AND CRYSTALLISABILITY •Most important factor in crystallisability of polymer is geometrical regularity •Stereo polymers i,e. isotactic and syndiotactic crytallises while atactic doesn’t •Linear polyehtlene having high regular configuration is highly crystalline but drops sharply when there is branching, branching polymers are hard to crystallise •Linear polyethylene is highly crystalline but random copolymers ethylene and propylene is non crystalline . •Random copolymers do not crystallize but alternating copolymers with repeating units in regular alteration shows tendency to crystallize.

Gutta percha

Natural rubber

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OTHER FACTORS FFECTING CRYSTALLISABILITY Polarity affects crystallisability. Example – nylon66 is highly crystalline due to polar group in the molecule which leads to formation of Hydrogen bonds. The carbonyl oxygen atom of one polymer chain and NH groups of another polymer chain forming hydrogen bonds, increasing the interaction forces of attraction and facilitating tighter packing and perfect bonding of chain elements with each other. Polymers with bulky side groups like polyvinyl carbazole finds it hard to crystallise because their bulky groups comes in the way of closer molecular packing But in case of smaller side groups like polyvinyl alcohol or polyvinyl fluoride the polymer can crystallise

EFFECTS OF CRYSTALLINITY ON PROPERTIES OF POLYMERS • Crystallinity affects properties of polymers like density, modulus ,hardnes, permeability ad heat capacity. • Partial polymer with crystalline and amorphous regions exhibit different properties even though the region are chemically same. • Density of crystalline region is higher than that of amorphous region • Thus properties of polymer depend on percentage of crystalline material present in bulk. • Example – plot a graph of Young’s modulus values against crystallinity of natural rubber . • Graph shows initial low value , characteristic of amorphous polymer and steeply increases along with amount of crystalline component in sample. • Crystallinity affects permeability as it depends on extent and rate of penetration of liquid or vapour molecules through matrix which depends on physical structure of polymer. crystalline regions put stiffer resistance to penetrating molecule and are less permeable • Amorphous regions are rapidly attacked by oxygen and acid hydrolysis in case of cellulose is done at the amorphous regions

Tg mostly depends on structural rigidity and secondary intermolecular forces -Tm mostly depends on molecular symmetry

4. Pendant Groups Bulky pendant groups – benzene ring restricts rotational freedom , increases Tg Flexible pendant groups – aliphatic chains limit how close chain can pack ,increases rotaional motion and lowers Tg

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Plasticizers low molecular weight compounds added to plastics to increase their flexibility and workability weaken intermolecular forces between polymer chain and decrease Tg