quantitative analysis of molecular weights Flashcards
(16 cards)
What are the two methods for measuring molecular weight
Absolute: measured quantity is theoretically related to average molecular weight
and
Relative: measured quantity is in some way related to molecular weight but the exact relation must be established by calibration—indirect but preferred
Two methods under absolute and relative method
Absolute:
1. End group analysis
2. Collimating property measurement
Relative:
1. Intrinsic-viscosity measurement
2. Gel-permeation chromatography
Which analysis is based on first principles
End group analysis
What is an end group?
-functionalities or constitutional units that are at the extremity of a macromolecule or Oligomer
Linear polymer has how many ends? Condition?
Two, if the repeating unit is symmetrical
For a polymer with two ends, how would you use end group analysis to calculate
- how many polymer chains are there in the sample
- Number molecular weight
-use titration to obtain molar number of polymer-COOH
- (check the functionality) calculate the number of polymer chain in moles by dividing the moles of -COOH by the number of -COOH per polymer chain.
To find actual number, use avagadro constant
- Mn = mass of sample / molar number of polymer chains
Limitations of the end group analysis
- chemical nature of the end group must allow it to be quantified(most polymers have end groups that cannot be determined quantitatively
- only accurate to polymers with Mn < 10000
- accuracy deteriorates at higher Mn because the number of ends decreases
- it doesn’t give Mw, only Mn. A single GPC will give all the information of the polymer
How does colligative property measurement work?
colligative properties:
freeze point depression, boiling point elevation and osmotic pressure (polymers don’t generally don’t boil, hence the elevation is not valid)
- when a solute(polymer) is dissolved in a solvent, it causes a change in the chemical potential of the solvent
- magnitude of the change is directly related to the solute concentration c and Mn
write down the three formulae that theoritically relates colligative properties and Mn and say how will you measure the values
- boiling point elevation and freeze point depression
- measure change in temperature at different concentrations
- plot change in temp/c vs c
-then extrapolate to c= 0 - draw the diagram on page 12, write the formula for osmotic pressure
membrane osmometer measures the pressure difference across a semipermeable membrane.
Setup:
A chamber is divided into two compartments by a semipermeable membrane.
One compartment holds the pure solvent, and the other holds the polymer solution.
Process:
Solvent naturally flows into the solution compartment due to osmosis.
Pressure is applied to stop this flow.
The applied pressure at equilibrium is the osmotic pressure (π) of the solution.
why is osmotic pressure more feasible and commonly used but not BP/FP method
- change in temperature is minute so need expensive equipment
- wide applicability of osmotic pressure measurement. Suitable for 50000< Mn< 1 000 000
define osmotic pressure
additional pressure that must be imposed to keep the solvent and solution sections at the same level
osmotic pressure is affected by
concentration and molecular weight of the polymer
Gel permeation Chromotography
1. application
2. mobile and stationary phase
3. specifications for stationary phase
4. how are molecules sorted?
5. what do we obtain at the end of the experiment?
- determining molecular weight
molecular weight distribution - stationary phase: highly porous silicon gel
mobile phase: solvent flushing through the tap - porous beads, pore size ranging from 1nm to 1mm in diameter => equivalent to size of polymers in solution
- Small amount of polymer solution passed
through the column packed with porous beads - molecules are sorted based on size exclusion principle
- Large sized polymer molecules will elute first, as
they are too big to penetrate into the pores. - Smaller molecules have easier access to the
beads and diffuse in and out of the pores,
following a more circuitous and longer route
- Small amount of polymer solution passed
- each fraction collected represent the different molecular weight and to the very left and right, there is only solvent coming out because initially the solvent needs to run through and finally everything is sorted so only the solvent is running out again
GPC is a _____method and _______ is needed
relative, calibration
explain the flow path and parts of a GPC set-up
Flow Path Summary:
Solvent Reservoir → Pump → Injector → Column → Detector → Data System
🧪 1. Solvent Reservoir
Function: Holds the solvent used to dissolve the polymer sample and carry it through the system.
The solvent must be chemically compatible with both the polymer and the column material.
💧 2. Pump
Function: Delivers the solvent at a constant flow rate and pressure to ensure consistent elution.
A steady flow rate is crucial for accurate molecular weight determination.
💉 3. Injector
Function: Introduces a small, measured volume of the polymer solution into the solvent stream.
Typically, a loop injector is used to ensure reproducibility.
🧱 4. Chromatographic Column
Function: The core of GPC, packed with porous gel beads that separate polymer chains by size.
📏 5. Detector
Function: Measures the concentration of polymer as it elutes from the column.
Common detectors include:
Refractive Index (RI) Detector: Measures changes in refractive index (most common in GPC).
UV Detector: Detects UV-absorbing polymers.
Light Scattering Detector: Provides absolute molecular weight.
💻 6. Chart Recorder
Function: Records and analyzes the detector signal to create a chromatogram.
The chromatogram plots detector response/signal vs. elution time, allowing determination of molecular weight distribution and number-average molecular weight
Analysing GPC chart
- axes
- height
- beginning and end of peak
- y axis is time, x axis is detector signal
- height corresponds to the quantity of polymer molecule
- at the beginning of the peak corresponds to when large molecules exit and the end of the peak signifies the exit of small molecules
