5.1 Functionalized Polyethylenes as Compatibilizers
for Polyethylene/Polyamide Blends
The functionalized ethylene copolymers described in Sect. 4.1 were investigated as
compatibilizers in blends of commercial low-density polyethylene (40 wt%) and
polyamide 6 (60 wt%) [164]. The compatibilizers differed in comonomer type
(10-undecen-1-ol, 10-undecenoic acid, N-methyl-10-undecenylamine) and amount
(0.2–1.2 mol%) as well as in molar mass (M w 11–136 kg/mol) and crystallinity
(ΔH m 106–190 J/g). Blends containing 5 or 10 wt% of the compatibilizer were
prepared by melt mixing of the polymers in a small-scale twin-screw extruder.
Blends containing 10 wt% of metallocene-based copolymer were found to be
significantly tougher and stiffer than the uncompatibilized blend. This is a clear
indication of the presence of the functionalized copolymer at the polyethylene/
polyamide interface, which lowers the interfacial tension. Higher values of elongation at break for the compatibilized blends provide support for enhanced adhesion
between the phases. The improved compatibility was also seen in the SEM
micrographs as much finer morphology and a more ragged interface between the
phases in the compatibilized blends. In the blends containing hydroxyl- or
carboxyl-functional polyethylenes, a higher molar mass of the compatibilizer was
found to be advantageous in terms of mechanical properties. This can be explained
by enhanced entanglement formation with the polyethylene phase at higher molar
mass [165, 166]. On the other hand, the N-methylamino-functional compatibilizer
also gave a substantial increase in stiffness and toughness, even though it had the
lowest molar mass of all compatibilizers.
Formation of chemical linkages between the functional groups of the
functionalized polyethylenes and the terminal groups of the polyamide was verified
by studying the IR spectra of the blends after dissolving the pure polyamide part
with formic acid. Ester groups were found in blends containing hydroxyl-functional
polyethylenes, indicating a grafting reaction between the hydroxyl groups
and carboxyl end groups of polyamide. Similarly, in blends containing
N-methylamino-functional polyethylene, the presence of amide groups arising
from the reaction of the secondary amino groups with the carboxyl groups of the
polyamide was evidence of graft copolymer formation. Carboxyl functionalities of
acid-functional polyethylenes are assumed to form amide linkages with the amine
end-groups of the polyamide, though it was not possible to obtain clear evidence by
IR measurement because of the overlapping peaks from polyamide. It is also known
that interfacial interactions can be substantially enhanced by the formation of
hydrogen bonds between the blend components, so that covalent bonding is not a
prerequisite for efficient compatibilization [167].
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
225
for Polyethylene/Polyamide Blends
The functionalized ethylene copolymers described in Sect. 4.1 were investigated as
compatibilizers in blends of commercial low-density polyethylene (40 wt%) and
polyamide 6 (60 wt%) [164]. The compatibilizers differed in comonomer type
(10-undecen-1-ol, 10-undecenoic acid, N-methyl-10-undecenylamine) and amount
(0.2–1.2 mol%) as well as in molar mass (M w 11–136 kg/mol) and crystallinity
(ΔH m 106–190 J/g). Blends containing 5 or 10 wt% of the compatibilizer were
prepared by melt mixing of the polymers in a small-scale twin-screw extruder.
Blends containing 10 wt% of metallocene-based copolymer were found to be
significantly tougher and stiffer than the uncompatibilized blend. This is a clear
indication of the presence of the functionalized copolymer at the polyethylene/
polyamide interface, which lowers the interfacial tension. Higher values of elongation at break for the compatibilized blends provide support for enhanced adhesion
between the phases. The improved compatibility was also seen in the SEM
micrographs as much finer morphology and a more ragged interface between the
phases in the compatibilized blends. In the blends containing hydroxyl- or
carboxyl-functional polyethylenes, a higher molar mass of the compatibilizer was
found to be advantageous in terms of mechanical properties. This can be explained
by enhanced entanglement formation with the polyethylene phase at higher molar
mass [165, 166]. On the other hand, the N-methylamino-functional compatibilizer
also gave a substantial increase in stiffness and toughness, even though it had the
lowest molar mass of all compatibilizers.
Formation of chemical linkages between the functional groups of the
functionalized polyethylenes and the terminal groups of the polyamide was verified
by studying the IR spectra of the blends after dissolving the pure polyamide part
with formic acid. Ester groups were found in blends containing hydroxyl-functional
polyethylenes, indicating a grafting reaction between the hydroxyl groups
and carboxyl end groups of polyamide. Similarly, in blends containing
N-methylamino-functional polyethylene, the presence of amide groups arising
from the reaction of the secondary amino groups with the carboxyl groups of the
polyamide was evidence of graft copolymer formation. Carboxyl functionalities of
acid-functional polyethylenes are assumed to form amide linkages with the amine
end-groups of the polyamide, though it was not possible to obtain clear evidence by
IR measurement because of the overlapping peaks from polyamide. It is also known
that interfacial interactions can be substantially enhanced by the formation of
hydrogen bonds between the blend components, so that covalent bonding is not a
prerequisite for efficient compatibilization [167].
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
225