Two series of well-defined poly(urethane urea) multiblock copolymers were synthesized to investigate the phase-separated morphology of this family of materials. Series I copolymers were synthesized from 2000 g/mol poly(tetramethylene oxide) [PTMO], 4,4‘-methylene di(p-phenyl isocyanate) [MDI], and ethylenediamine (EDA), with hard segment contents ranging from 14 to 47 wt %. Series II copolymers (all with hard segment concentrations of 22 wt %) were prepared from the same PTMO and MDI and a diamine mixture of EDA and 1,4-diaminocyclohexane. The microdomain morphology was characterized using small-angle X-ray scattering, and the scattering data were analyzed using the approach of Bonart and Müller. The series I and II copolymers were found to have relatively low overall degrees of phase separation [ranging from ∼20% at the lowest hard segment contents to greater than 40%], contrary to the common notion that these copolymers are well phase separated materials. The introduction of the second diamine results in reduced phase separation, presumably as a consequence of disruption of hard segment hydrogen bonding.

As part of our continuing study of the solid-state morphology of poly(urethane urea) segmented block copolymers, we focus in the current paper on the use of atomic force microscopy to visualize the structure of phase-separated microdomains. Free surface and bulk images were obtained from two series of PUU films, for copolymers varying in hard segment content from 14 to 47 wt %. Using a progression of AFM tapping forces, the morphology of the hard domains at the free surfaces was found to be in the form of randomly oriented cylinders with additional spherical domains, both having lateral dimensions on the order of 5−10 nm. At higher tapping forces, phase images of microtomed surfaces of relatively high hard segment content PUU copolymers appear to exhibit hard segment-rich domains at two size scales. However, images of surfaces of specimens freeze-fractured at liquid N2 temperatures (and acquired at relatively low tapping forces) reveal the larger structures to be aggregates of smaller hard microdomains.

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Fibers of poly(lactic acid) produced by twostep melt spinning have been studied. The morphology is elucidated with respect to the thermal and mechanical properties of fibers produced at cold-draw ratios of 1-8. With atomic force microscopy and small-angle X-ray scattering, a fibrillar morphology is found, with microfibril diameters ranging from 30 to 60 nm. Shrinkage properties indicate that, with increasing draw ratio, the fibers undergo a transition from class 2 to class 1 within the classification proposed by Keller. A supramolecular model for the morphology of the fibers is presented that entails a highly oriented skin with a core consisting of microfibrils. The orientation of the crystalline blocks within the microfibrils is similar to what has been reported for nylon fibers.

Selected poly(urethane urea) block copolymers were prepared under different conditions and their microphase-separated morphologies analyzed primarily with small-angle X-ray scattering (SAXS). Preparation conditions were varied by adjusting the temperature and vacuum pressure during solution casting. Copolymers with relatively high hard segment contents prepared under “low” vacuum conditions, where solvent is removed comparatively slowly and the copolymers spend a longer period of time in the presence of a plasticizer (i.e., solvent), produced films with higher degrees of phase separation. The longer casting times associated with the low vacuum pressure conditions also resulted in larger mean interdomain spacings. We speculate that this may be due to secondary hard domain coalescence. Attenuated total reflectance FT-IR and wide-angle X-ray diffraction experiments were performed, but these were not sensitive to the microdomain organization of these copolymers.

Fibers of poly(lactic acid) (PLA) produced by two-step melt spinning have been studied. The PLA resins used contain a 96:04 ratio of L:D stereochemical centers; however, one of the materials is branched by a peroxide treatment. The thermal, mechanical, and morphological properties of the fibers are compared for the two different molecular architectures. In the branched material, at least some of the branches exceed the entanglement molecular weight. The branched material is accordingly characterized by greater shear and extensional viscosity than the linear material. Fiber properties are highly influenced by the draw ratio; both branched and linear materials reach a plateau of about 35% crystallinity. The branched polymer reaches the plateau at a lower draw ratio, and this is indicative of faster crystallization kinetics. Both materials shrink in boiling water, and the amount of shrinkage decreases with increasing draw ratio. At an intermediate draw ratio of 6, the branched material is characterized by significantly larger shrinkage. With small-angle X-ray scattering and atomic force microscopy, the morphology is found to be fibrillar. Microfibril diameters range from approximately 20 to 30 nm and are almost identical for the two molecular architectures studied.

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The mechanism underlying the large electric-field-induced strains in terpolymers of vinylidene fluoride, trifluoroethylene, and chlorotrifluoroethylene was investigated. The electrostrictive strain increased by an order of magnitude with increasing temperature, up to the Curie transition, and was essentially invariant to temperature thereafter. Infrared absorption spectra, obtained as a function of both temperature and electric field strength, revealed no change in the crystal phase structure for electric fields sufficient to induce longitudinal strains of ∼1%. Thus, the electrostriction observed herein is not due to crystal phase conversion. The Maxwell strain was also negligible under all conditions, because of the terpolymer’s high mechanical modulus (10 to 100 MPa). The mechanical properties exhibit an anomalous change in behavior near the Curie transition, whose origin is unclear.