Chemistry Reference
In-Depth Information
of MTF was driven by a need for better
tools for characterizing long chain branch-
ing (LCB). Present state-of-the-art techni-
ques such as 13CNMR
[3]
and size-exclusion
chromatography (SEC) with molecular
weight sensitive detectors
[4]
provide informa-
tion about the average number of branches
per molecule. Neither technique provides
information about the molecular weights of
the branches nor the shapes of the branched
molecules. Additionally, for low levels of
branching, both techniques are operating at
relatively low signal to noise, making
detection of low levels of LCB a challenge.
Long chain branching is introduced into
polymers because it can have substantial
impact on the rheological behavior of the
system provided the branch lengths are
significantly larger than the entanglement
molecular weight.
[5]
Because their radii of
gyration are smaller, LCB polymers shear
thin to a greater extent than linear polymers
of the same molecular weight. On the other
end of the shear spectrum, LCB polymers
offer higher zero shear viscosity than their
linear counterparts. This is because LCB
polymers entangle much more effectively
than their linear counterparts. The enhanced
entanglement of LCB polymers was the
motivation for the present embodiment of
MTF. The idea was to create an entangling
environment within a chromatographic
column through which LCB molecules
and linear molecules would be forced to
flow. It should be noted here that the term
''entanglement'' in the MTF experiment is
interchangeable with the term ''pinning''.
The original idea behind MTF column
development was to create a series of posts
on which LCB chains or linear chains could
become pinned.
In the first successful demonstration of
MTF, poly(styrene-co-divinylbenzene) mono-
lithic columns, having macropores (channels)
of average diameter on the order of
100-200 nm, were used in the separation.
[1]
In that work, MTF was characterized by a
flow rate dependent reversal in elution
order of linear PS molecular weight stan-
dards. Additionally, chains possessing LCB
were shown to be retained longer than
linear chains of the same hydrodynamic
size.
It was recognized that a second mechan-
ism may also be operative in the MTF
separation. This second mechanism involves
chain restriction followed by relaxation/
reorientation. In this mechanism, dissolved
solutes may become restricted by a fraction
of the column macropores such that the
relaxation times for reorientation determine
the rate of transport. Either mechanism,
pinning or relaxation/reorientation, is exp-
ected to be sensitive to topology; both may
be operative in a real system.
In this work, new MTF columns are
introduced. The columns were prepared via
high pressure packing of sub-micron, non-
porous silica (surface functionalized with
PS) into stainless steel columns. The new
columns were used to study the elution
behavior of regular PS stars. In addition,
the columns were used to perform high
temperature MTF on relatively narrow
fractions of lightly cross-linked homoge-
neous ethylene octene copolymers.
Experimental Part
Materials and Samples
The silica used to pack MTF columns was
obtained from Admatechs Co. Ltd. (Aichi,
Japan). The product identification number
was SO-C2 lot BMI206. An SEM image of
these particles is shown in Figure 1. In
addition, particle size distribution data
provided by the vendor revealed that the
average particle diameter was 0.5 micro-
meters and the half-width of the distribu-
tion was
0.3 micrometers.
Narrow molecular weight polystyrene
(PS) standards were obtained from Poly-
mer Laboratories (Amherst, MA) and were
used as received. Polymer standard solu-
tions for MTF studies were prepared
individually with concentrations ranging
from 0.5 to 1 mg/ml depending on the
molecular weight, with lower molecular
weight standards prepared at higher con-
centration and vice-versa. The tetrahydro-
furan (THF) used for standard dissolution
