An investigation was conducted to determine how the advancement of
chemical reactions
and the prepolymer molecular weight affect the reorientational dynamics
and intermolecular cooperativity
in model epoxy−amine systems. Experimental results were obtained
by dielectric spectroscopy over a
wide range of frequency and temperature. A strong effect of the
progress of reaction on reorientational
dynamics was noted and an explanation was put forward within the
framework of the coupling theory,
marking the first time this concept was applied to reactive systems.
It was proposed that the molecular-level characteristics that govern the intermolecular cooperativity of
reactive systems can be classified
into two categories: (1) molecular architecture, determined by
molecular symmetry, rigidity, and steric
hindrance, and (2) dielectric architecture, determined by the type and
concentration of all dielectrically
active species. Both molecular and dielectric architecture vary in
the course of chemical reaction, and
the overall direction in which the cooperativity shifts is governed by
the interplay between these two
phenomena.
An examination was carried out of the reorientational
dynamics of dipoles during the
network formation in multifunctional epoxy−amine systems.
Experimental results were generated by
simultaneous dielectric and Fourier transform infrared (FTIR)
measurements. The observed changes in
reorientational dynamics during the advancement of reactions were
utilized to (1) describe the origin of
the α relaxation during the network formation, (2) propose a
methodology for the evaluation of the kinetics
of network formation, and (3) advance an interpretation of network
dynamics in terms of intermolecular
cooperativity based on the interplay between molecular and dielectric
architecture. Fragility or
cooperativity plots proved most informative in relating intermolecular
cooperativity to the molecular
characteristics of the growing network.
The segmental dynamics of PMPS chains are determined in the bulk liquid and cross-linked networks of varying cross-link density by broad-band dielectric relaxation spectroscopy. A large
fragility index [τ(T
g*/T) dependence], F
1/2 = 0.77, is found, independent of cross-link density, molecular
weight, and T
g. Also independent of the above variables is the relaxation shape (characterized by a KWW
β parameter of 0.45). Since these parameters quantifying the α process in PMPS networks are insensitive
to cross-linking, it is concluded that the length scale of cooperatively rearranging domains in PMPS
networks is smaller than the distance between cross-links, i.e., less than 5 nm, in agreement with the
current consensus. An argument based on the temperature dependence of the dielectric relaxation strength,
dipole moment, and ensemble average chain configuration was advanced, suggesting that the high fragility
of PMPS has an intramolecular origin.
Molecular dynamics of network-forming reactive polymers were examined as a function of
the advancement of chemical reaction as the materials structure undergoes a transition from liquid to
amorphous solid. The accompanying changes in the segmental relaxation time (α process) are a signature
of the materials state (within the liquid to amorphous solid spectrum of physical properties). Both broad-band dielectric relaxation spectroscopy (DRS), which probes the α process via dipolar reorientational
mobility, and dynamic light scattering (DLS), which probes the α process via density fluctuations, were
used to monitor the system under reaction conditions in the reaction bath (to our knowledge, the first
study of its kind). An excellent agreement was found between DRS and DLS for the α process characteristic
parameters: relaxation time and KWW, the stretched exponential parameter (characterizing the relaxation
breadth). That dipole dynamics exhibit the same α relaxation characteristics as time dependent density
fluctuations, suggests that the rotational motion observed in a DRS measurement is directly controlled
by the behavior of the density domains. It was concluded that the broadening of the α process is due to
a general phenomenon: the micro/nanoscale heterogeneous nature of glass formers. The results of the
study are discussed in terms of cooperative and local relaxation modes in the growing network.
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