Block copolymers spherulites

It should be re-emphasized that although our block copolymers do not display spherulitic morphology when they are compression molded, they are nevertheless crystalline. Hence, this indicates that under this mode of film preparation, aggregation into well developed superstructure is apparently kinetically limited. 

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

Light microscopy has been used in a number of contexts to characterize block copolymer morphology. For crystalline block copolymers, spherulitic structures that result from organization of crystalline lamellae can be examined using microscopy. In solutions, polarized light microscopy can reveal the presence of lamellar and hexagonal-packed cylindrical micellar phases. Cubic micellar phases are optically isotropic, and consequently cannot be distinguished from sols only on the basis of microscopy. 

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

Finally, a thermoplastic elastomer polybutyleneterephthalate/polytetramethyleneoxide (PBT/PTMO) block copolymer was also investigated, because it is a partly crystalline elastomer with large spherulites. Details of the composition can be found in [12]. 

Closer inspection reveals that besides fibrils, spherical domains are also observed in STS-1 and to a lesser extent in ST-1 and ST-2. Thus, fibrils and spherical domains co-exist. Increase in PS content in the block copolymers results in progressively smaller spherulites, and a very rough surface for the STS-5 film is observed in the optical micrograph. 

Figure 1. Cross polarized photomicrographs of films of EO-Is-EO block copolymers with relatively large fractions of EO segments revealing less perfect development of spherulitic crystalline texture as the Is content increases...

The core of the book is devoted to subjects starting with anelastic behavior of polymers and rubber elasticity, but proceeds with greater emphasis in following chapters to mechanisms of plastic relaxations in glassy polymers and semicrystalline polymers with initial spherulitic morphology. Other chapters concentrate on craze plasticity in homo-polymers and block copolymers, culminating with a chapter on toughening mechanisms in brittle polymers. To make the... 

By melting a PS-PEO block copolymer and then subsequently cooling (Figures 6.3 and 6.4), Kovacs (1967) showed that spherulites could be obtained with copolymers containing at least up to about 50 % PS. Even though such block polymers contain a relatively large fraction of an uncrystal-... 

Figure 6.3. Spherulitic texture of a thin film of a styrene-ethylene oxide block copolymer (w = 0.40) obtained on quenching to 20°C (Kovacs, 1967). Photomicrograph taken with film between crossed Nicols ( 100 x).

PLLA generally form non-banded spherulites from the melt that exhibit negative birefringence. Banding can appear after special annealing conditions or in blends and block copolymers. 

Furthermore, the crazes in PP show other similar characteristics to those of amorphous polymers. They grow apparently normal to the direction of major tensile stress which somewhat deviates from the tensile direction because of spherulitic structure. There are similar environmental effects on craze initiation (see also Environmental stress cracking of polypropylene in this book). Crazing is also an important source of toughness in toughened PP alloy systems such as propylene-ethylene block copolymers. 

Nojima et al. [160] examined the nature of spherulites in blends of PCL with 8-caprolactone-butadiene diblock copolymers. Blends were prepared by solventcasting mixtures of the components, using a series of block copolymers and PCL samples. Optical microscopy showed that all blends investigated, and cast at... 

It is now firmly established that confinement of crystalline stems has a profound influence on crystallization in block copolymers. Confinement can resnlt from the presence of glassy domains or simply strong segregation between domains. In contrast crystallization can overwhelm microphase separation when a sample is cooled from a weakly segregated or homogeneons melt (152-154). The lamellar crystallites can then nncleate and grow heterogeneonsly to produce spherulites (152,155), whereas these are not observed when crystallization is confined to spheres or cylinders. 

The synthesis and characterization of poly(hexamethylene sebacate-tiimethyl-siloxane) block copolymers, prepared by coupling Cl- or McjNH-terminated dimethylsiloxanes with OH-terminated poly(hexamethylene sebacate), have been studied. The copolymers containing 19—90% siloxane were characterized by n.m.r., viscosity, DSC and CPC. All the polymers were found to be crystalline. The copolymers containing <69% siloxane, when cast from solution or melts, indicated a reduction in spherulite size as the siloxane concentration increased, although only a small m.pt. depression was observed. All the copolymers have critical surface tensions similar to dimethylsiloxane homopolymers. Polycarbonate-Siloxane Copolymers.—A model has been proposed to predict the micromorphology and mechanical properties of block copolymers of bisphenol-A polycarbonate and poly(dimethylsiloxane). N.m.r. data upon 65 35 (wt%) copolymer of poly(dimethylsiloxane) and bisphenol-A polycarbonate, with block lengths of 20—100 monomer units, were found to be in agreement with the predictions of a spin-diffusion model. ... 

Cast films exhibit a range of morphologies due to the effect of solvents, substrates and orientation. In the case of block copolymers the choice of solvent is quite important to the final structure. Spherulites, a common textural structure observed in crystalline pol5maers, are formed in many industrial processes where the polymer is melted prior to forming the article of interest. Films produced by these industrial processes differ from films used in model studies as the former are usually not thin in the microscopic sense. In true thin films, the spherulitic texture is two dimensional, whereas in these thicker film materials the spherulites are three dimensional. 

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