Segmental Dynamics and Density Fluctuations in Polymer Networks during Chemical Vitrification

Abstract

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.

Correlation between Structural Relaxation and Distribution of Particle Clusters in Glass-Forming Epoxy−Amine Mixtures Undergoing Step Polymerization

“…A value K ) 0.28 ( 0.02 was found, almost constant in the conversion range 0.32 < R < 0.56, in perfect agreement with PCS results. 36,56 It should be noted, however, that this procedure allows to describe the shape of the dielectric spectra by means of a single parameter in a narrow region around the maximum of ′′, but it remains largely …”

“…The extent of polymerization is quantified by the epoxy conversion R, i.e., the fraction of epoxy groups reacted, which can be measured by calorimetry [21][22][23][24][25][26][27][28][29][30] or infrared spectroscopy experiments. [35][36][37][38][39] At any value of R, the distribution of clusters is known in many cases from wellestablished statistical models. 54,55 From this information a connection between structural relaxation properties near the glass transition and properties of the growing clusters can be traced.…”

Effect of the segmental mobility restriction on the thermoset chemical kinetics

“…The chemical reaction of network‐forming polymers continuously changes the magnitude of intermolecular and intramolecular interactions, causing a continuous variation of the characteristic relaxation time during the macroscopic time. The material relaxation time increases during the chemical transformation from liquidlike values (10 −9 s) to glassy values (10 −2 s); consequently, the chain mobility decreases as the glass transition is approached, and the polymeric network grows, inducing a potential freezing 19. The evolution of the relaxation time has been modeled by both free volume theory (Doolittle equation20) and cooperative rearrangements (Adam–Gibbs theory21) and further used to analyze diffusion effects in thermoset curing 22…”

Physical and chemical vitrification: the role of configurational entropy

“…(2) conforms to experimental observations that relaxation time and viscosity behaviors are not coupled in these systems, and gelation occurs well before vitrification. Remarkably, no signature of gelation can be seen in DS and PCS spectra [14,15].…”