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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