Chemistry Reference
In-Depth Information
This critical size, expressed here as M
C
(linear PS), decreases with particle size (i. e.
macropore channel dimension). The func-
tional form of the relationship between M
C
and particle size may offer insight regarding
the separation mechanism, but further
work is needed to establish the relationship
between M
C
and particle size.
increased with increasing levels of branch-
ing. The fractionation provided by MTF
combined with the structural information
attainable with on line multiangle light
scattering detection will potentially enable
determination of a long chain branching
distribution for each hydrodynamic size
slice or fraction. Coupling of MTF and SEC
in an automated on line fashion would
facilitate determination of long chain
branching distributions across the molecu-
lar weight distribution of a sample. Present
state-of-the-art technologies such as SEC
with molecular weight sensitive detectors
or 13C NMR provide only an average
number of branches per chain. Thus, the
determination of branching distributions
across the molecular weight distribution
would provide structural information that is
not attainable at present.
In this paper, new columns for MTF
were introduced. The columns were packed
with submicron polydisperse silica that had
been surface treated with PS. MTF elution
order reversal was observed for PS stan-
dards and PE fractions injected onto these
new columns. Additionally, M
C
was found
to be dependent on packing particle size.
Thus, MTF elution order reversal was not
unique to the previously studied monolithic
columns,
[1]
but rather, appears to be a
general phenomenon attainable when the
interstitial channels (i. e. macropores) in a
column are of the appropriate dimensions,
and when the flow rate is reduced.
Although the mechanism of MTF is not
completely understood, two potential
mechanisms are pinning (or entanglement)
and polymer restriction followed by relaxa-
tion. Both may be operative in a MTF
experiment. Both are expected to be
sensitive to long chain branching topology.
Conclusions
In a detailed analysis of a 3-arm star PS
sample, it was shown that MTF can
separate LCB chains from linear chains,
and can separate LCB chains of differing
topologies. The example shown in this
paper included near baseline MTF resolu-
tion of at least 4 components present in a
nominal 3-arm star sample. Despite having
the same hydrodynamic volume, the 3-arm
star was separated from the 4-arm star by
MTF. Multidetector SEC results, knowl-
edge of the synthesis chemistry and on-line
low angle laser light scattering detection
was used to interpret the MTF fractogram
of the PS star sample. The clean MTF
separation of star components synthesized
via coupling of discreet arms will enable
experimental validation of the Zimm-
Stockmayer model for predicting the sizes
of branched polymer chains.
[13]
In a second application, polyolefin
samples having similar hydrodynamic size
distributions (i. e. similar breadth in SEC
chromatogram) were fractionated differ-
ently by MTF with the major differentiating
feature being the LCBf of the parent
polyolefin polymer. For fractions exceeding
M
C
, MTF was shown to produce increas-
ingly broader fractograms with increasing
branching frequency.
Further interpretation of the polyolefin
MTF fractograms with two angle light
scattering detection revealed little change
in R
G
across the fractogram, but significant
increases in M
W
across the fractogram,
especially for the highest branch frequency
samples. At roughly constant R
G
, branched
chains are higher in molecular weight than
linear chains. Thus,
[1] D. M. Meunier, P. B. Smith, S. A. Baker, Macromol-
ecules
, 5313-5320.
[2] P. B. Smith, D. M. Meunier, S. A. Baker, R. K.
Prud'homme, International Patent Application, Publi-
cation Number WO 03/098208 A1, November 27, 2003.
2005,
38
retention in MTF
