"Phase-Restricted Radiation Crosslinking in Semicrystalline
Polyolefins Investigated by Two-Dimensional Solid-State NMR"
Abstract
The utility of two-dimensional wideline separation (2D WISE) NMR in following
the phase dependence of the radiation-induced crosslinking of multiphase
materials has been demonstrated. Crosslink production in semicrystalline
polyolefins by high-energy electron irradiation in air was monitored via
the experimentally resolved 1H lineshapes of the respective amorphous and
crystalline regions. It was shown that radiation-induced crosslinks can
be primarily restricted to one phase or the other by proper functionalization,
or nonfunctionalization, of the host polymer. Poly(1,23-tetracosadiyne),
a diacetylene-functionalized polyethylene, was synthesized and shown to
predominantly undergo crystalline-phase crosslinking for dosages of 100
kGy and less. Thus, chain mobility within amorphous regions may be maintained
upon radiation-induced crosslinking. In comparison, linear polyethylene
exhibited amorphous-phase crosslinking for electron beam radiation dosages
of at least 400 kGy; application of 2D WISE NMR represents a direct method
for following this well-known process.
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"Molecular Nature of the Beta Relaxation in Poly(methyl
methacrylate) Investigated by Multidimensional NMR"
Abstract
The molecular motions underlying thc dielectric and dynamic-mechanical beta
relaxation in poly(methyl methacrylate) have been elucidated in detail by
means of two-dimensional (2D) and three dimensional (3D) 13C exchange NMR
of the carboxyl moiety, and 2D 2H exchange NMR of the methoxy group. The
identity of the motions observed by NMR and the beta relaxation dynamics
is proved by the agreement of the measured correlation times. The selective-excitation
"3D" NMR spectrum proves that for every mobile side-group, a relatively
well-defined motion between two potential-energy minima occurs. The 2D spectral
pattern shows that the OCO plane of the sidegroup undergoes 180 degrees(+/-20)
flips. Experiments with multiple exchange and selective saturation for analysis
of the growth of exchange signals (MESSAGE) prove that the molecular motions
responsible for the beta relaxation are associated with a distribution of
correlation times, which appears to be bimodal with both mobile and trapped
sidgroups. Consistently, analysis of the integral 2D exchange intensity
shows that around 330 K only about 50% of the sidegroups participate in
the large-amplitude dynamical process on the time-scale of the beta relaxation
correlation time. Thc 2D 2H NMR spectra, while exhibiting narrowing due
to methyl-group rotation around the C-OCH3 bond, exclude any significant
motion of the methoxy group around thc C-OCH3 bond. Both the 13C and the
2H 2D NMR spectra provide compelling evidence that the sidegroup flip is
accompanied by a main-chain rearrangement which can be characterized as
a random rotation around the local chain axis with a 20 degree root-mean-square
amplitude. This is ascribed to the fact that the asymmetric sidegroup, after
the flip does not fit into its original environment. These findings explain
the dielectric and the dynamic-mechanical beta relaxations of PMMA.
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"Mechanical Properties of Diacetylene-Functionalized
Polyamides"
Abstract
An aliphatic diacetylene-functionalized polyamide has been synthesized and
successfully processed into high quality fibres and films for mechanical
property measurements. The reactive diacetylene functional groups positioned
in the backbone of this polymer remained dormant during a wide variety of
processing schemes but were activated in a controlled manner by exposure
to fixed dosages of high energy electrons. Solid state 13C nuclear magnetic
resonance revealed that exposure to high energy electrons crosslinks the
polymer via the formation of conjugated polydiacetylene chains. In general,
crosslinking via the formation of polydiacetylene chains was found to increase
the ultimate tensile strength and modulus of the material and reduce its
critical failure strain. For specific fibres, the tensile strength of the
material was doubled, its modulus increased by 50%, and the failure strain
only reduced to 30%, thereby maintaining a high degree of toughness in this
system even after an 80 Mrad dose of electrons.
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"New Mechanism of Toughening Glassy Polymers. 1.
Experimental Procedures"
Abstract
Normally brittle, high molecular weight polystyrene can be very effectively
toughened by blending polybutadiene of 2.76 kg/mol at volume concentrations
of a few percent. An increase of toughness by about a factor of 10 above
that of homo-PS can be readily achieved with a 1.1% PB-2.76K blend if the
precipitated PB pools can be maintained at a size smaller than 0.2 µm.
It is proposed that this dramatic effect is the result of a controlled solvent
crazing action of the PB-2.76K on the homo-PS when the former becomes soluble
into the latter in the fringing layers of the craze borders subjected to
negative pressure by the local craze tuft drawing process.
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