(217q) Long Chain Branching in Polyethylene By UV Irradiation

Rheological
and structural modification of polyethylene (PE) has been an area of great interest.
Most of the research done in this area has been focused on modifying the
rheology and consequently the structure of the PE either by creating selective
crosslinking or chain scission. These modifications are achieved through
numerous chemical reactions (mainly peroxides and silanes)
and high energy irradiation techniques including electron beams and gamma (γ)
rays. Although crosslinks are capable of enhancing many structural properties, they
do not improve other industrially important aspects such as processability.
Long chain branching (LCB), on the other hand, is highly desirable as it can
improve both the melt strength and the processability
of the polymer. Long chain branches in PE have been created by using
constrained geometry catalyst (CGC) systems. These catalysts produce long chain
branched polyethylene (LCBPE) with a relatively narrow molecular weight
distribution (MWD). Irradiation methodologies have also been employed to induce
LCB in polymers by using ionizing irradiation. In this study, a novel reactive
extrusion (REX) process was developed using ultraviolet (UV) photoinitiation to modify the rheological properties of a
high density polyethylene (HDPE) copolymer with a relatively narrow molecular
weight distribution (MWD) by inducing LCB. The UV radiation was selected due to
availability, and relatively safe and inexpensive operating conditions
employed. The modifications were performed to accommodate the following specifications:
(1) to minimally change the molecular weight (MW) or MWD of the selected PE,
and (2) to identify the effect of three important processing factors, namely,
photoinitiator concentration, polymer flow rate (throughput), and screw speed,
on the final rheological properties of the modified resin. A central composite
response surface design
was selected for the extrusion experiments. Three levels of photoinitiator (Benzophenone (BP)) concentration, flow rate (throughput),
and extruder screw speed were selected. Extruder temperature and the intensity
of the UV lamp were kept constant for all the experiments. Oscillatory
shear and shear creep experiments were conducted to identify the linear
viscoelastic (LVE) properties. Gel Permeation Chromatography (GPC) was further performed
to investigate the molecular architecture of the resins and to verify the
findings from the rheological studies.

Structural and Rheological Properties

A linear HDPE
with a negligibly low amount of short chain branches was selected for this
study. The Soxhlet extraction experiments showed an insignificant
amount of gel in the structure of the modified resins. Oscillatory shear
experiments were conducted to identify the changes in the LVE behaviour of the
modified resins due to LCB. The Cross model was fit to the data obtained from
oscillatory shear experiments to identify: (1) zero shear viscosity (η_o), (2) characteristic relaxation
time (λ), and (3)
power law index (n) of the modified and linear resins ^[1]. Estimates
of η_o indicated
an increase in zero shear viscosity of the resins. Zero shear viscosities
increased to a maximum of 11,600 Pa.s from a
starting value of 1,900 Pa.s (the increase was related
to a higher extent of entanglement due to LCB, resulting in more physical
obstacles for deformation, hence, higher viscosity). A quadratic model based on
BP, throughput and BP² was obtained for zero shear viscosity (see
Figure 1). η_oscaled
with both BP and BP², but decreased with throughput (a thicker PE
melt profile in the extruder lowered the effect/extent of irradiation). Characteristic
relaxation time was used to identify the width of the Newtonian plateau and
shear thinning region. ?λ? increased from a starting
value of 0.05 s to 4 s indicating a broader transition region (this
was again related to a higher extent of entanglement due to LCB, and
consequently a longer relaxation time). A model linear in BP (positive) and
throughput (negative) was obtained for lambda (see Figure 2). At lower
throughput, the residence time of the resins in the extruder is relatively
higher, which leads to a higher free radical concentration, and consequently a
higher degree of branching. A similar analysis was further applied to the power
law index. An enhancement in the shear thinning behaviour was observed as ?n?
decreased from a starting value of 0.64 to 0.55 (related to a decrease in the
hydrodynamic volume of the molecule with LCB). A model linear in BP content
(positive), throughput (negative) and screw speed (positive) was obtained for
the power law index (see Figure 3). Cole-Cole plots and van Gurp- Palmen plots ^[2]
were constructed to further validate the earlier observations. The formation of
LCB was clear in the Cole-Cole plots from a separation of the curves from those
of virgin PE at lower frequencies (related to the increase in storage modulus
with the extent of LCB, for PE with similar MWD). The van Gurp-
Palmen plots showed lower loss angle values for the
modified resins compared to the linear PE at the same complex modulus (|G*|).
Also a point of inflection appeared in the transition zone, which became more
enhanced as the extent of LCB increased (two inflection points for higher LCB
content, creating a plateau region in the loss angle). Creep experiments
conducted on selected samples further verified the zero shear viscosities
obtained earlier. This was facilitated by fitting the Maxwell model to the
creep data.

Molecular weight properties

Molecular architecture of the PE
resins was evaluated by GPC. It was found that the polydispersity index values of
the modified resins were similar to their linear parent PE. Both MWD and
average molecular weights slightly shifted to higher values as a result of LCB.
The differences in the molecular structure of the resins were apparent from
intrinsic viscosity determinations. The branched samples showed a lower
intrinsic viscosity as a result of a higher chain entanglement density, which
was more evident at the higher end of the MW spectrum (chains with MW higher
than 250, 000 g.mol^-1). Contraction factors (g) (the ratio of the
intrinsic viscosity of a branched sample to its linear counterpart at the same
molecular weight) were further evaluated ^[3]. These values were
employed to identify the average number of branched points (and, consequently,
the average number of branches per monomer unit of PE) for a tetra-functional
randomly branched LCB architecture. The data indicated that star-like LCB
structures with 0.055 branches per 1000 monomer units can be achieved in
the developed reactive extrusion processes (see Figure 4). Further analysis was carried out
from a theoretical point of view to find a correlation between zero shear
viscosity and the molecular weight of the LCB and the entanglement molecular
weight. From these studies it was concluded that modified resins with a higher
number of LCB (as a consequence of higher BP content and similarly higher
extent of available free radicals for the reaction) exhibited a decrease in the
length of branches.

References

[1] Sardashti,
P.; Tzoganakis, C.; M.; Polak,
M.A.; Penlidis, Macromol
React. Eng (2013, under review)

[1] S. Trinkle, C. Friedrich, Rheol. Acta. 2001,
40, 322-328.

[2] M. J. Scorah, C. Tzoganakis, R. Dhib, A. Penlidis, J Appl. Polym. Sci. 2007, 103, 1340-1355.

Figure 1: Zero shear viscosity (η_o) response surface

Figure 2: Relaxation
time (λ)
response surface

Figure 3: Power law index (n) response surface

Figure 4:
NLCB for a tetra-functional randomly branched structure