The dynamics of crystalline and amorphous samples of poly(l-lactide) and three l-lactide/meso-lactide random copolymers were investigated in the frequency domain using broadband dielectric
spectroscopy (DRS). Two relaxations, the segmental (α) and subglass (β) processes, were observed in the
temperature range between −100 and 105 °C. Modification of the characteristics of the α process, i.e.,
the relaxation strength (Δε), mean relaxation time (τm), and shape parameters, as a function of crystal
content was investigated on “fully crystallized” samples as well as in real-time crystallization experiments.
As expected, the strength of the α relaxation was smaller and its distribution broader for materials with
higher degrees of crystallinity. The relaxation strength of the α process of the amorphous samples increased
with temperature, while that of the crystalline materials changed very little or in the opposite direction
with temperature. This behavior can be explained by the existence of a rigid amorphous component.
Changes in τm and T
0 (the Vogel temperature) for the copolymers crystallized at various T
c are influenced
by crystalline microstructure as well as by crystalline content. A relaxation is also observed in higher
temperature (125−155 °C) experiments on amorphous and crystalline materials. The evidence at hand
suggests that these processes are associated with the normal mode and αc process, respectively.
Tapping mode atomic force microscopy was used to investigate the lamellar morphology of poly(L-lactide) and two poly(L-lactide-co-meso-lactide) random copolymers containing 3% and 6% meso-lactide. Samples were isothermally crystallized at selected temperatures, and qualitative and quantitative analyses of lamellar structure were performed using height and phase images. This is the first study of the morphology of polylactide stereocopolymers using a real-space probe, and the important effects of scanning parameters on the acquired images are described. More open spherulites with an abundance of screw dislocations between edge-on lamellar stacks were observed in samples crystallized at higher temperatures. Mean lamellar thicknesses are lower for the random copolymers compared to PLLA, particularly at lower ∆T, in agreement with the results of our previous small-angle X-ray scattering (SAXS) experiments. Mean lamellar thicknesses derived from the current real-space examination are in good agreement with those determined previously from SAXS. Internal surfaces-from microtomed specimens-were also studied to investigate the bulk crystal morphology. Although quantitative analysis was not feasible (for reasons discussed in the text), lamellar organization similar to that seen in the surface experiments is observed at high magnifications.
The volumetric expansion of bisphenol-A and aniline-based benzoxazine (BA-a) has been studied by comparing the room temperature density of the cured BA-a under different polymerization conditions with the amorphous BA-a monomer. The glass transition temperatures (Tg) of BA-a at various degrees of polymerization have been investigated by differential scanning calorimetry (DSC). The degrees of conversion at different polymerization temperatures were compared with the respective glass transition temperatures. It was found that substantial development of Tg occurs at low degrees of conversion. A fluorinated polybenzoxazine was synthesized from the ring-opening polymerization of hexafluoroisopropylidene-containing benzoxazine monomer. The properties of the fluorine-containing polymer were compared to those of nonfluorinated polybenzoxazine. Fluorine incorporation had a profound effect on the glass transition temperature of polybenzoxazine. The thermal stability also improved upon fluorination.
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