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A significant reduction in the glass transition temperature (Tg) is found for poly(l-(−) lactide) (PLLA), crystallized under partially constrained conditions, where the polymer is prevented from shrinking. It is proposed that the constrained crystallization process results in increasing the net free volume in the amorphous phase. This is a result of the fixed sample volume in which there is an increasing crystal volume fraction at a higher density than the amorphous fraction, depleting polymer mass from the amorphous fraction, which is not able to contract to maintain its nominal density. The Tg depression increases with increasing degree of crystallization or crystallization temperature, Tc. On the other hand, unconstrained PLLA samples (free to shrink during crystallization) exhibit the conventional trend of increasing Tg with crystallinity or Tc. Although the difference in Tg between samples prepared by the two methods could be large, as much as 30 °C, the melting point, heat of fusion, and overall degree of crystallinity in PLLA are not influenced. Finally, shear-induced crystallization does not affect any of the physical properties studied here but only modifies the crystallization rates, increasing them by enhancing nucleation.

We have investigated a set of small molecules, which, in terms of chemical groups and overall molecular architecture, are representative of several of the more complex epoxide/amine network-forming systems. The static and dynamic dielectric relaxation properties of selected model compounds are obtained and discussed. The static properties are of interest for their relation to the fundamental material property, the dipole moment. From this relation, the potential for the determination of the chemical composition of reactive mixtures via an additivity of group dipole moments is explored. Dipole dynamics of the model compounds are determined and are related to cooperative molecular and submolecular motions in these simple molecules and it is shown how these findings may be extended to complex polymer networks composed of chemically similar moieties.

Molecular dynamics via broad-band dielectric relaxation spectroscopy (DRS) are obtained on a class 3 network-forming system:  a generic cyanate ester resin. The potential for in-situ monitoring of the network-forming reaction is explored. The segmental dynamics of the growing network are investigated, and a hitherto unknown secondary relaxation process, β*, is found. This relaxation is distinctly different from the Johari−Goldstein slow β process. Model compounds and conformational strain analysis are used to elucidate the β* relaxation molecular mechanism. The α process in selected cyanate networks was analyzed with the Adam−Gibbs thermodynamics-based relations (AG model). The AG model provides a link between the relaxation time of a cooperatively rearranging domain (CRD) and configurational entropy. The model is explored via DRS and calorimetrically for various network structures. A good agreement is found between independent quantities derived from both techniques when examined in the AG framework. These results lend support to the CRD concept used in the AG model and, more generally, toward the establishment of the heterogeneous view of relaxations in glass-formers.

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.

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(Tg*/T) dependence], F1/2 = 0.77, is found, independent of cross-link density, molecular weight, and Tg. 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.

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.