Polymer Molecular weight and its Measurement methods.pptx

Polymer Molecular weight and its
Measurement methods
At the end of this lecture, you should be able to:
- Understand the distribution of polymer M.wt.
- Learn the ways to present the average M.wt
- Understand the measurement methods of polymer average M.wt.
- Calculate polymer average M.wt using different formula.
Feb. 2019
Mustansiriyah University/ Materials Eng. Dept.
Dr Ahmed AYASH

Molecular Weight:
Molecular weight of a chemical compound can be defined, simply, as the sum of the atomic weights of each of the
atoms in the molecule.
Examples:
Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18 g/mol
Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16 g/mole
Polyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000 g/mole
Average Molecular Weight
Polymers are made up of many molecular weights or a distribution of chain lengths. In other other words, if one
takes polyethylene as an example, this polymer may have chains of ethylene (C2H4) with different lengths; some
longer than others.
Example:
Polyethylene -(C2H4)-1000 has some chains with 1001 repeating ethylene units, some with 1010 ethylene units,
some with 999 repeating units, and some with 990 repeating units. The average number of repeating units or
chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28,000 g/mole .
Therefore, polymers must be represented by the value of average molecular weight. 2

There are four ways to represent the average molecular weight:
(1) Number-average molecular weight (Mn) for a discrete distribution of molecular weights is given as:
where Ni indicates the number of moles of molecules having a molecular weight of Mi and Wi, is the weight
of molecules with molecular weight Mi. Thus, Wi = Ni Mi.
The expression for the number-average molecular weight of a continuous distribution function is:
4

(3) Z-average molecular weight (Mz) for a discrete distribution of molecular weights is given as:
(2) Weight-average molecular weight (Mw) for a discrete distribution of molecular weights is given as:
The expression for the weight-average molecular weight of a continuous distribution function is:
5

(4) Viscosity-average molecular weight (Mv) can be obtained, experimentally, from dilute-solution
viscometer using Mark-Howink equation. The viscosity-average molecular weight falls between Mn and
Mw depending upon whether the solvent is a good or poor solvent for the polymer. In the case of a good
solvent, Mv = Mw .
- Mark-Howink-Sakurada equation:
NOTES:
1. A measure of the breadth of the molecular-weight distribution is given by the ratios of molecular-weight
averages. For this purpose, the most commonly used ratio is Mw/Mn ,called the polydispersity index or PDI.
PDI =
𝑀𝑤
𝑀𝑛
> 1 ; polymers having PDI = 1 are called monodiseperse polymers
2. Degree of Polymerization (DP): is the number of monomeric units in a macromolecule or polymer or
oligomer (consists of a few repeating units Or DP is the number of repeat units.
DPw =
𝑀𝑤
𝑀𝑜
, DPn =
𝑀𝑛
𝑀𝑜
; where M0 is the monomer molecular weight. 6

There are several reasons why we might want to measure polymer average molecular weight and its
distribution:
1. The molecular weight and its distribution determine the viscous and elastic properties of the molten polymer.
This affects the processibility of the melt and also the behavior of the resulting solid material. For example, a
resin suitable for extrusion must have a high viscosity at low shear rates so that the extrudate maintains its
integrity. To be suitable for injection molding, however, the same resin must have a low viscosity at high shear
rates so that the injection pressure not be excessive.
2. The molecular weight of a polymer can determine its applications. For example, the resin used for making
polycarbonate water bottles, for example, differs significantly in molecular weight from the polycarbonate that
goes into compact disks.
3. Differences in molecular weight distribution also influence the polymer properties. As a consequence, two
chemically similar polymers, processed identically, that have the same molecular weight but different
molecular-weight distributions may result in products that show significantly different shrinkages, tensile
properties, and failure properties. For this very important reason, it is advantageous to know the molecular
weight and molecular-weight distribution of the polymers used.
4. Other situations where the molecular weight and its distribution directly influence results include phase
equilibrium and crystallization kinetics. 9

Example 2.1:
A polydisperse sample of polystyrene is prepared by mixing three monodisperse samples in the following proportions:
1 g 10,000 molecular weight
Using this information, calculate the number-average molecular weight, weight- average molecular weight, and PDI of
the mixture.
Solution:
10

Example 2.2:
A polymer is fractionated and is found to have the continuous molecular-weight distribution shown below as
a plot of the weight, W, of molecules having molecular weight, M, versus W. Given this molecular-weight
distribution, calculate Mn and Mw.
Solution:
11

M.wt Measurement Methods
Number average
M.wt
END-GROUP
ANALYSIS
COLLIGATIVE
PROPERTIES
Viscosity average M.wt
Viscometry
Weight average
M.wt
Light Scattering
Z-average M.wt
Ultracentrifugation
Gel Permeation Chromatography (GPC)
12
NOTE: The choice of method for polymer molecular weight determination is influenced by factors such as:
(i) information required, (ii) operative region, (iii) cost effectiveness, and (iv) experimental conditions and
requirements.

End-group Analysis
This method has presumed importance particularly in the determination of average molecular weight of
step-growth polymers (condensation). Consider the step-wise condensation polymerization of polyesters:
Under the assumption that each polymer chain contains one – OH and one – COOH groups, direct
measurement of the concentration of the groups can be done using chemical (titrimetric or pH
measurement) or spectroscopic (infrared or nuclear magnetic) techniques. This is also applicable to other
step-growth polymers.
LIMITATIONS:
1. The major setback of this technique is the decrease in sensitivity with increasing polymer chain length.
This method is restricted to polymer with molecular weight 20,000 amu. Aside this, it requires high
concentration of polymer.
2. This method requires that the polymer be free of impurities and other groups present in the chain
should not interfere in the determination of end group of interest.
13

colligative properties
The relations between the colligative properties and molecular weight for infinitely dilute solutions in a fact
that the activity of the solute in a solution becomes equal to its mole fraction as the solute concentration
becomes sufficiently small.
This method is based on:
•Vapour-pressure lowering,
•Boiling-point elevation (ebulliometry),
•Freezing-point depression (cryoscopy),
•Osmotic pressure (osmometry).
where
∆Tb, ∆Tf, and π are the boiling-point elevation, freezing-point depression, and osmotic pressure, respectively.
ρ is the density of the solvent,
∆Hv and ∆Hf are the enthalpies of vaporization and fusion, respectively, of the solvent per gram,
c is the solute concentration (gr/cm3),
Mn is the number-average molecular weight.
A2 is a constant.
Formula Used
14

An example of determining Mn from vapour pressure data
15

Light Scattering
This method is presumed most popular for the determination of weight average molecular weight (Mw). It
allows polymer molecular weight and structure to be assessed. Fundamentally, if light passes through a
medium, it scatters at different angles. This technique thus relies on the measurement of light scattered at an
angle to the incident ray as it passes through the target. The intensity of scattered light incident on polymer
sample is dependent of the polarity, chain size, and concentration. Consequent upon this, light technique
measures polymer molecular weight by quantifying the Raleigh scattering (elastic light scattering) from each
polymer molecule. The measurement approach is simple as illustrated in the Figure below.
16

where ∆R𝜃 is the change in Raleigh scattering at a specified angle, K is the optical constant, C is the polymer
concentration, and is the average particle scattering factor which accounts for the effects of measuring
scattering from large molecules with relatively small wavelength.
LIMITATIONS:
However, the light scattering technique is deemed time consuming because it requires that the sample
solution is completely free of dust and other impurities as these are capable of influencing the light
scattering pattern. Also, as could be deduced from the above equation, change in Raleigh scattering is
proportional to the sample concentration and molecular weight, hence this method requires high sample
concentration to produce a detectable signal.
17
The scattered light and the average molecular weight is related according to the following equation:
The famousness of this method stems from the fact that wide range of molecular weight (typically
104 to 6 X 105 g/mol) can be determined.

It is the most intricate of the methods for determining the molecular-weights of high polymers. This
method is useful for biological materials, such as protein molecules.
An ultracentrifuge consists of an AL rotor (ø ∼1-2 inch) that is rotated at high speed in an evacuated
chamber. The solution being centrifuged is held in a small cell within the rotor near its periphery. The rotor
is driven electrically or by oil or air turbine.
The concentration of polymer is determined by optical methods based on measurements of refractive
index or absorption. The solvents must have difference both density and refractive index from the polymer.
The density differences allow the sedimentation and the refractive index differences allow the
measurement.
Ultracentrifugation (Sedimentation) Method
In the sedimentation equilibrium experiment, the ultracentrifuge is operated at a low speed of rotation for
times up 1 or 2 weeks under constant conditions. A thermodynamic equilibrium is reached in which the
polymer is distributed in the cell according to its molecular weight and molecular-weight distribution.
18

https://www.britannica.com/technology/ultracentrifugation 19

The force on a particle (F):
F = ω2r(1− υρ)m
where:
ω is the angular velocity of rotation,
r is the distance of the particle from the axis of rotation,
υ is the partial specific volume of the polymer,
ρ is the density of the solution,
m is the mass of the particle.
For an ideal solution in the equilibrium condition:
where: c1 and c2 are the concentrations at 2 points r1 and r2 in the cell.
LIMITATIONS: The disadvantage of sedimentation equilibrium experiment is taking quite long time to reach
equilibrium.
20

Ultracentrifugation Method
21
Example: At rotation rate 400 rpm (41.8 m/s) and 20 oC, the concentration ratio (C2/C1) is 1000. If the r2
and r1 are 5 cm and 10 cm , respectively, the density is 1 gm/cm3 and the partial specific volume of the
polymer is 0.0077 cm3/gm, what is the molecular weight.
SOLUTION:
Mw =
(2)(8.314)(293)ln(1000)
1 − 0.0077 ∗ 1 (41.8)2 ((0.1)2−(0.05)2)
Mw = 2587.8 𝑔𝑚/𝑚𝑜𝑙𝑒

Viscometry
A method that is widely use for routine molecular-weight determination is based on the
determination of intrinsic viscosity, η, of a polymer in solution through measurements of solution
viscosity.
The fundamental relationship between η and molecular-weight is given in Mark-Howink equation (mentioned
in Page 6):
22
It is clear in the above equation that the value of intrinsic viscosity of the diluted polymer is needed to
determine the molecular weight. To find the intrinsic viscosity, a series of experimental measurements and
calculations have to be made. These steps are as follows:
1. The relative viscosity can be measured experimentally using a suitable viscometer:

23
2. The intrinsic viscosity can be calculated as shown below:

24
Fig. 1 Reduced viscosity and inherent viscosity of nylon 66 in 90% formic acid (Adapted from Ph.D thesis of
R Walia, P. S., Chemical Engineering, West Virginia University, Morgantown, 1998).
Intrinsic Viscosity

Viscosity-Molecular Weight Constants

One of the most widely used methods for routine determination of molecular weight and molecular-weight
distribution is GPC. This method based on the principle of size-exclusion chromatography to separate samples
of polydisperse polymers into fractions of narrower molecular-weight distribution.
The equipment:
Several small-diameter columns (L = 30 – 50 cm) are packed with small highly porous beads (∅ = 10 – 107 Â).
Pure pre-filtered solvent is continuously pumped through the columns at a constant flow rate
(1 – 2 mL/min). Then, a small amount (1 – 5 mL) of a dilute polymer solution is injected by syringe into the
solvent stream and carried through the columns. The smallest polymer molecules are able to penetrate deeply
into the bead pores but the largest may be completely excluded.
Gel Permeation Chromatography (GPC)
The process is repeated until all polymer molecules have been eluted out of the column in descending
order of molecular weight. The concentration of polymer molecules in each eluting fraction can be
monitored by means of a polymer-sensitive detector, such as IR or UV device. The detector is usually a
differential refractometer (differ the refractive index between the pure solvent and polymer solution).
For a given polymer, solvent, temperature, pumping rate, and column packing size, elution volume (Vr) is
related to molecular weight.

29
Summary of the Measurements Method of M.wt
A = Absolute
E = Equivalent
R = Relative

Polymer Molecular weight and its Measurement methods.pptx

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