INVESTIGATING THE EXISTENCE OF LONG-CHAIN
BRANCHING IN POLYMERS THROUGH THE EVALUATION OF
FLOW ACTIVATION ENERGIES.
Emilio Lucchelli, Lino Locatelli and Giorgio Giannotta
Centro Ricerche Novara “Istituto Guido Donegani”, Polimeri Europa,
Via Giacomo Fauser 4, I-28100 Novara, Italy.
An approach proposed by Mavridis and Shroff [1] was followed to ascertain the
existence of long-chain branching (LCB) in two commercial polyolefines (one
LDPE and one LLDPE) and one commercial polyethyleneterephthalate (PET.)
All of them were tested as received from the production plants; LLDPE and
PET were further investigated after being modified by additives in extrusion.
The method basically consists in measuring a dynamic function (e.g., the elastic
modulus G′) in response to a varying shear frequency ω and at different
temperatures T; all the curves are then overlapped to that one obtained for a
reference temperature T0, by shifting them either horizontally (i.e., along the ω
axis) or vertically. The two sets of shift factors so calculated can be expressed
in terms of a “horizontal” flow activation energy EH and a “vertical” flow
energy EV, respectively. If EV turns out to have a significant value, this can be
taken as evidence for the existence of LCB. The present investigation led to the
following findings:
 The values of EH and EV obtained for LDPE are in good agreement with
data for the same material published in [1].
 LLDPE modified by 0.0625 % terbutylperoxide (TBP) and 0.125 % TBP
revealed values of EV amounting to about one fourth of EH in both cases; on
the opposite, EV equals zero in absence of additives. This points out that
LCB shows up in linear low-density polyethylene upon addition of TBP.
 The non-modified PET showed a negligible value of EV (confirming its
linearity), while the addition of 1.2 and 1.5 % pyromellitic dianhydride
(PMDA) resulted in significant values of EV (increasing with the content of
PMDA and demonstrating the presence of induced LCB.)
Therefore it can be concluded that the method suggested by Mavridis and
Shroff is a sensitive and useful tool to establish the existence of LCB both in
polyethylenes and in polyethyleneterephthalate.
[1] H Mavridis, RN Shroff Polym Eng Sci 1992; 32 (23): 1778-1791