Lamellar semicrystalline structure

Polymers containing unsatmated rubber and semicrystalline polymers are often effectively stained using OSO4. What about materials that do not show such differential staining Two examples will be described where reactive (unsaturated) materials are included into the polymer to provide reaction sites. Inclusion of a stainable unsaturated polymer was shown for cellulosics [179] and synthetic fibers [202]. The initial work focused on improvement of the properties of cellulosics by inclusion of an elastomer between the microfibrils. OSO4 staining revealed that a lamellar sheet structure was present. Marfels and Kassenbeck [202] used a similar method with polyester and nylon fibers. 

Modeling the Interface Distribution Function for a ID Lamellar Stack As demonstrated in the last section, the nonideality of a real semicrystalline polymer can lead to a broadening and overlapping of the peaks in K (z), which makes it difficult to extract the correct structure parameters simply from the peak positions. The one-dimensional paracrystalline stack has been suggested as an analytical model for the semicrystalline structure [2,13,16], We here present a procedure that allows simulating and modeling the measured IDF based on this model. A simulated IDF... 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

The major feature of polymers that have been bulk crystallized under quiescent conditions are polycrystalline structures called sphemlites. These are roughly spherical supercrystalline structures which exhibit Maltese cross-extinction patterns when examined under polarized light in an optical microscope. Spheruliies are characteristic of semicrystalline polymers and are also observed in low-molecular-weight materials that have been crystallized from viscous media. Sphemlites are aggregates of lamellar crystallites. They are not single crystals and include some... 

Figure 25. Schematic drawing of different lamellar structures observed in asymmetric methyl-branched alkane Ci9iH383CH(CH3)C9gHi99 (a) (b) semicrystalline form obtained at high Tc (c) double layer crystalline form obtained from panel b on cooling (d) semicrystalline form obtained at low Tc (e) triple layer crystalline form obtained from panel d on cooling. Chain tilt is neglected in this and Figure 26, for simplicity (from ref 151).

Figure 26. Schematic drawing of lamellar structures in Y-shaped alkane Ci2oH24iCH(C6iHi23)Cii9H239 (a) (b) high-temperature semicrystalline form (c) low temperature double layer crystalline form (d) the hypothetical energy favored but kinetically unattainable structure (from ref 151).

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

DiMarzio et al. developed a scaling model for semicrystalline lamellar structures with alternating layers of amorphous and crystalline blocks, with crystalline chains folded perpendicular to the interface [24]. The model system in-... 

In the lamellar crystals of semicrystalline materials and the extended chain structure of oriented polymers, chain packing is usually much more efficient than in the amorphous, isotropic state. The efficiency of chain packing in the crystalline phase reduces the free volume available for transport to such an extent that, as a first approximation, the crystalline phase may be regarded as impermeable relative to the amorphous phase. 

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