Transcript Slide 1
Advanced GPC Part 1 – GPC and
Viscometry
Introduction
The
GPC experiment with a single concentration detector is called
conventional GPC
This is by far the most common form of GPC
However there are some limitations to this technique
Recently, developments in detector technology have made viscometers more
widely available
These detectors avoid some
of the problems associated with conventional
GPC
This presentation outlines GPC viscometry as an analysis methodology
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Re-cap - Gel Permeation
Chromatography (GPC)
Gel
permeation chromatography separates
polymers on the basis of size in solution
Separation occurs through the partitioning of
polymer molecules into the pore structure of beads
packed in a column
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Conventional GPC
Calibrate
the column by chromatographing a number of narrow standard
polymers of known molecular weight, correlating MW with molecular size
For
unknown samples slice the peak into components of weight Mi and
height/area Ni, sum to determine molecular weight averages
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Limitations with Conventional GPC
Column
separates on basis of molecular
size NOT molecular weight
two
different polymers
differently with solvent
will
interact
At any molecular weight, the two polymers
will have different sizes in solution
Molecular weights from conventional GPC
are dependent on a comparison in size
between the standards and the sample
The
result – practically speaking the
majority of conventional GPC experiments
give the wrong results!
Viscometers get round this problem…
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Viscosity of Polymers
All polymers increase the viscosity of solutions by increasing the resistance to
flow
Different
types of polymers have differing viscosities depending on the
interactions with the solvent
Viscometers are used to determine intrinsic viscosity, IV or [ŋ]
Intrinsic viscosity can
be though of as the
inverse of the molar
density
At
any given MW, a
high IV means the
sample is a large diffuse
molecule, a small IV
means a compact, dense
molecule
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Intrinsic Viscosity
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Effect of Solvent and Temperature
on Intrinsic Viscosity
Polystyrene
Solvent affects the intrinsic viscosity of
polymers by altering how well solvated
they are
Large
changes occur in solvents of
different polarities
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Temperature has less of an effect
So Why do Viscometry? –
The Universal Calibration
If
a calibration of size versus
retention time could be generated
then one true calibration would hold
for all sample types
Hydrodynamic volume = [] M
A
Universal Calibration plot of
log[]M versus RT holds true for all
polymer types
Can
use measured intrinsic
viscosity and retention time to get
accurate molecular weights
Ref : Grubisic, Rempp, Benoit, J. Polym. Sci., Part B,
Polym. Lett., 5:753 (1967)
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Accurate Molecular Weights
As a result of using the viscometer, a universal calibration can be set up
that gives the same calibration line regardless of the type of standards
employed
The chemistry of the sample is also unimportant – the column is separating
on size and that is the parameter we have calibrated
Therefore
the GPC/viscometer experiment will give accurate molecular
weights for any samples regardless of their or the standard’s chemistry
assuming that pure SEC takes place
We are still doing chromatography – the column must be calibrated
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Comparisons of Conventional
and Universal Calibrations
Conventional calibrations are offset due to differences in the molecular size of
polystyrene and polyethylene
Universal calibrations account for the offset to the calibrations overlay
Discrepancy at low molecular weight is due to a conformation change
polyethylene
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in
The Mark-Houwink Plot
IV
A Mark-Houwink plot of log IV versus log M should give a straight line as
long as the Universal Calibration is obeyed (i.e no interactions occur)
K and alpha vary between different solvents and polymers
Alpha is an indication of the shape of the polymer in solution
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The PL-BV 400 Series
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Viscometer Operation
T
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Measuring Intrinsic Viscosity What do we need?…
A viscometer that measures specific viscosity
A concentration detector that tells us how much
material is eluting from the
column
Can be any type that gives a response proportional to concentration
Typically a differential refractive index detector is used
DRI detector response proportional to concentration
Operation identical to conventional GPC, determines the concentration
material eluting from a GPC column
RIsignal = KRI (dn/dc) C
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of
GPC/Viscometry Experimentation
Calibration with a series of narrow standards of known Mp and concentration
Calculate detector constant (Kvisc) using one standard for which IV is known
For the remainder of the standards, calculate [] from the viscometer response
Plot log M[] versus retention time to generate the Universal Calibration
For unknown sample, for each slice across the distribution determine [] from
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the viscometer, and then convert to molecular weight via the Universal
Calibration curve
Typical Chromatograms
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Analysis
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Analysis of Poly(styrene-co-butadiene)
Columns: 2 x PLgel 5µm MIXED-C Eluent: Tetrahydrofuran
Flow rate: 1.0 ml/min
Temperature: 40˚C
Detector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer
Example chromatograms of one sample
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Only small differences in the MWD of the two samples
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The Mark-Houwink plots indicate the materials are structurally similar
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Analysis of Polylactide
and Poly(lactide-coglycolide)
Columns: 2 x PLgel 5µm MIXED-D Eluent: Tetrahydrofuran
Flow rate: 1.0 ml/min
Temperature: 40˚C
Detector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer
Example chromatograms of one sample
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The
copolymer (red) has a considerably lower molecular weight than the
homopolymer (blue)
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Structurally
the co-polymer is very different to the homopolymer across the
molecular weight range
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Analysis of Cornflour
Columns: 3 x PLgel 10µm MIXED-B Eluent: Dimethyl sulphoxide + 0.1% lithium bromide
Flow rate: 1.0 ml/min
Temperature: 50˚C
Detector: PL-GPC 50 Plus differential refractive index, PL-BV 400RT viscometer
Example chromatograms of one sample
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Large differences in the MWD of the two samples
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Large
differences in the Mark-Houwink plot indicate the samples are
structurally dissimilar
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Summary
Conventional
GPC has limitations in that the results obtained are purely
comparative
The situation can be remedied by adding a viscometer to the system
The viscometer allows calibrations of retention time as a function of molecular
size to be generate
This
give accurate molecular weight information regardless of the type of
standards used in the analysis
The
Mark-Houwink plot allows the change in density of the polymers as a
function of molecular weight to be analysed
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