Richard G. Crystal

Scanning and transmission electron microscopy studies on a model series of spherulitic segmented polyurethanes

Abstract Scanning (SEM) and transmission (TEM) electron microscopy studies were made on a model series of four segmented polyurethanes. The morphologies of both the homopolymers and of their mixtures were made by SEM while TEM was utilized to study only ultrathin films of the homopolymers. It was found that well-structured spherulitic morphologies could be induced in these systems and their mixtures, and that control over the textures could be maintained by solution casting conditions. The domains were observable via TEM, and the general orientation of this structure within the spherulites was noted. Deformation studies of the spherulitic textures were followed by both SEM and small-angle light scattering. It was concluded that, in general, the spherulites deform in a nonaffine manner–this is particularly true for the systems possessing a porous texture.

Synthesis and Thermal Transition Properties of Styrene — Ethylene Oxide Block Copolymers

Block copolymers are composed of two or more monomers which are segregated into blocks along the polymer chain. The copolymers described in this paper have one block composed of polystyrene (PS) and one or two blocks composed of poly (ethylene oxide) (PEO). These materials were first synthesized by Richards and Szwarc (1) and their investigation of the collodial properties of the copolymers in solution generated considerable interest in the PS-PEO system. Subsequent studies by Skoulios et.al. (2–5) showed that phase separation of the copolymer blocks occurred when concentrated solutions of the copolymers were prepared in a solvent preferential for one of the components. The mesomorphous phases were cylindrical, lamellar and spherical in structure and were interconvertible by varying the concentration of the polymer solution. Sadron (6,7) was able to preserve the various mesophases indefinitely by using polymerizable monomers as the preferential solvents and polymerizing the solvent. A recent study by Kovacs and Lotz (8,9) demonstrated that single crystals of the block copolymers can be grown from dilute solution when the solvent is preferential for PS and that they are composed of PEO folded chain lamellae sandwiched between surface layers of amorphous PS.

The Effects of Preferential Solvents on the Phase Separation Morphology of Styrene-Ethylene Oxide Block Copolymers

Phase separation in block copolymer systems exhibits rather striking effects on resultant physical properties (1,2). Skoulios et al (3–6) have studied the morphology of block copolymers of poly(ethylene oxide) (PEO) and polystyrene (PS) as well as poly (propylene oxide) and PS, in concentrated solutions using small angle x-ray techniques. By varying both concentration and solvent type, they were able to create several interesting gel structures. As each component in their block systems had markedly different solubilities they were able to explore the effect of dispersing these polymers in preferential solvents. As an example, the more polar PEO was soluble primarily in polar solvents (nitromethane, water, etc.) while the non polar PS segment was soluble primarily in non polar media (ethylbenzene, p-xylene, etc.). By using these preferential solvents, several structures, namely the sphere, the cylinder and the lamella, were identified from small angle x-ray data. More recently, Kovacs et al (7–9) have studied single crystals of PS-PEO AB type blocks and have shown that PEO crystallizes with little interference from the glass forming PS in dilute solution experiments. They also indicate that the resultant PEO crystal lattices are identical to that of the homopolymer.

Effect of Morphology on the Dielectric Properties of Polystyrene-Polyethylene Oxide Block Copolymers

To date, very little work has been done on the dielectric properties of block copolymer systems, particularly the study of dielectric properties as a function of polymer morphology. Ishida, Shimada, Matsuura and Takayanagi (1) studied polystyrene-polymethacrylate block copolymers and noted that loss mechanisms associated with the individual components were readily observable, i. e. αa of polystyrene (associated with Tg) and the αa and β mechanisms of polymethacrylate. No mention was made, however, of sample morphology, and accordingly the effects of phase separation morphology on dielectric response.

Surface Morphologies of Styrene-Ethylene Oxide Block Copolymers

Control of block copolymer bulk morphologies via preferential solvent techniques has been the subject of several investigations (1,2,3). Effects of preferential solvents on the bulk morphologies of styrene-ethylene oxide AB block copolymers having both varied compositions and molecular weights have been recently reported from this laboratory. (3)

Rheological Properties of Styrene-Ethylene Oxide Block Copolymers: Transition and Melt Flow Behavior

The block copolymers described herein are long linear sequences of styrene (non-polar, oleophilic, amorphous) units connected by primary chemical bonds to long linear sequences of ethylene-oxide (polar, hydrophilic, crystallizable) units. Because of the marked chemical dissimilarity of styrene and ethylene oxide, and the polymeric, long chain nature of the segmental units, the components are incompatible and phase separation in the melt is expected and observed. However, because of the primary chemical bonds tieing together the incompatible chain segments, phase separation is not complete and the rheological behavior of one phase is strongly affected by that of the other phase. Block copolymers are unique heterophase systems in this respect and form a distinct class of materials, the study of which should extend our knowledge of the manner of flow of mixed phases. The distinguishing feature of the ethylene oxide/styrene block copolymer system is the sensitivity of the ethylene oxide block to shear. This allows ethylene oxide blocks to be used as sensitive rheological probes.