Polymer Crystallinity

Raghavendra R. Hegde, M. G. Kamath, Atul Dahiya

1. INTRODUCTION

Properties of textile fibers are determined by their chemical structure degree of polymerization, orientation of chain molecules, crystallinity, package density and cross linking between individual molecules. Polymer crystallinity is one of the important properties of all polymers. Polymer exists both in crystalline and amorphous form.

Fig 1. Shows the arrangement of polymer chain forming crystalline and amorphous regions. [1]. It can be seen that part of molecules are arranged in regular order, these regions are called crystalline regions. In between these ordered regions molecules are arranged in random disorganized state and these are called amorphous regions.

Crystallinity is indication of amount of crystalline region in polymer with respect to amorphous content.

Crystallinity influences many of the polymer properties some of there are

·         Hardness

·         Modulus

·         Tensile,

·         Stiffness

·         Crease

·         Melting Point

So while selecting polymer for required Application its crystallinity plays foremost role.

1.1  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

Fig 2 bellow shows the schematic diagram of development of PET fiber structure along the spin line. At the take up point, the stress reaches a level of dyn  by take up speed of 3000-4000 m min. As the polymer melt comes out from molten disoriented state, its diameter decreases. With further reduction in diameter oriented mesophase is formed. After mesophase neck like deformation is formed along spin line. After this neck like deformation has been completed, the diameter does not reduces [3,4].

 

Fig.2 Schematic of Development of fiber structure in spin line at high temperature. [3,4]

As the 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 in between glass transition and melting state. Crystallization is always exothermic.

2. Differential Scanning Calorimerty (DSC)

One of the methods to measure polymer crystallinity is Differential Scanning Calorimerty (DSC).DSC can be used to determine amount of crystallinity in a polymer. Instrument is designed 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. Fig 3 shows DSC curve of a PET bottle sample.

 

Fig 3 DSC curve of a PET bottle sample

 

 

2.1 Heat capacity

Heat flow  = = . …(1)

The heat rate is given by Change in temperature for given time,

Heating rate= . …(2)

Dividing Equation (1) by (2) we get,

 

Heat capacity =  =  = 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 comes the melting point where polymer completely loses its orderly arrangement. Lets 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.

2.2 Crystallinity

DSC evaluation can be used to measure amount of crystallinity in the sample.

Let heat of crystallization be , and total heat given off during melting be Ht,

H= -. …(3)

 

Where H is the heat given off by that part of polymer, which was already in crystalline state.

Now 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 = = Mc Grams…. (4)

 

This is total amount of polymer that was crystalline bellow , Crystallization temperature.

So Percentage of crystallinity in the polymer sample is

 

 ´ 100 = % of crystallinity in the sample.

 

 Where Mt is total mass of sample taken.

 

So by DSC we can determine Crystallinity, glass transition temperature and Melting Point [2].

3. X-Ray diffraction

X-Ray diffraction is also used to measure the nature of polymer and extent of crystallinity present in the Polymer sample. Fig 4 shows the schematic diagram of x-ray diffraction pattern. 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 [1].

Fig 4. Schematic diagram of X-ray diffraction pattern [1]

Crystalline structure is regular arrangement of atoms. As per our discussion 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. Fig. 5 shows schematic pattern of x-ray diffraction. 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 [3].

 

 

(a)                                               (b)

Fig 5. X-ray diffraction pattern of (a) amorphous sample and (b) Semi crystalline polymer sample [1]

 

3.1 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 () amorphous components ().

Percentage of crystallinity  % is measured as ratio of crystalline area to Total area.

 % = {+ Ac} 100  (%)…. (5)

Where

 = Area of crystalline phase

 = Area of amorphous phase

 = Percentage of crystallinity

Small Angle X-ray Scattering (SAXS), Infrared Spectroscopy, can also be used to measure crystallinity. [3]

References

1.                  V.R. Gowariker, N. V. Viswanathan, Jayadev Sreedhar “ Polymer Science” Published by New Age International (P) Ltd., p, 173, 1986.

2.                   www.psrc.usm.edu

3.                  V.B. Gupta and V.K. Kothari, “Manufactured Fiber Technology” Published by Chapman & Hall.p, 225,1997.

4.                  Shimizy, J., Okui, N. and Kikutani, T (1985) in High speed fiber spinning (eds A. Ziabicki and H.Kawai), Wiley-Interscience, New York, p.295.

 

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