Polyethylene mixed crystal

The mixed crystal formation in ethylene/vinyl alcohol copolymers was previously quoted 3). The examination of the X-ray data shows a different structure from both that of polyethylene and of polyvinyl-alcohol. The repeat along the chain axis is 2.5 A, indicating a planar... 

In addition to the general steric requirements reported in the introductory section for macromolecular isomorphism, if chains differ in chemical structure, they must also show some degree of compatibility to intimate mixing and not too much different crystallization kinetics. The first condition is strictly similar to the one that applies to liquid mixtures. As a well known example, liquids without reciprocal affinity in general cannot form a unique phase. Attempts to obtain mixed crystals from polyethylene and polyvinyl or polyvinylidene fluoride has been unsuccessful hitherto, in spite of the similarity in shape and size of their chains. In view of the above somewhat strict requirements, it is not surprising that relatively few examples of this type of isomorphism have been reported. 

The infrared-absorption spectra of mixed crystals of ordinary polyethylene, -(CH2, and fully deuterated polyethylene, yielded infor-... 

Beginning in 1968,Tasumi and Krimm (103) undertook a series of experiments using a mixed crystal infrared spectroscopy technique. Mixed single crystals of protonated and deuterated polymer were made by precipitation from dilute solution. The characteristic crystal field splitting in the infrared spectrum was measured and analyzed to determine the relative locations of the chain stems of one molecule, usually the deuterated portion, in the crystal lattice. The main experiments involved blending protonated and deuterated polyethylenes (104-106). 

In addition to simple binding there are many examples where a low molecular weight species enters either the crystal interior or the interlamellar space with compound formation. These situations, although not uncommon, must obviously be very specific in nature and are termed inclusion compounds or clathrates. An example is given by the phase diagram of Fig. 3.25 for polyethylene-perhydrotriphenylene mixtures.(112) A compound is formed that melts congruently at 178.2 °C. This inclusion compound does not exist in the liquid phase and does not form mixed crystals with the pure species. 

Blends of HDPE-l-low density polyethylene were immiscible even in the molten state [43], and formed separate crystal structures with generally poorer properties [27,41,42,46-48,50,53-55]. On the other hand, one study observed that they formed mixed crystals with intermediate melting point [185]. Some studies reported that practical properties were proportional to blend ratio [56-60] and a few even showed synergistic improvement of properties such as film drawdown and stiffness [56, 57,61,62]. 

Solution blending Polar as well as nonpolar solvents can be used in this method. The polymer is solubilized in a proper solvent and then mixed with the filler dispersion. In solution, the chains are well separated and easily enter the galleries or the layers of the fillers. After the clay gets dispersed and exfoliated, the solvent is evaporated usually under vacuum. High-density polyethylene [24], polyimide (PI) [25], and nematic hquid crystal [26] polymers have been synthesized by this method. The schematic presentation is given in Scheme 2.2. 

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

To uniquely identify the intrinsic feature of the material, one method of sample preparation is to pelletize the explosive powders or crystals [14], It is standard practice in far-infrared (THz) spectroscopy to press samples into pellet form to measure the THz transmission spectra. When the sample is a powder with a grain size comparable to the THz wavelength (about 300 microns), the powder strongly scatters the THz radiation. Another method of sample preparation is to mix the material (e.g., RDX) with an inert matrix or filler material to create a pellet. The filler is typically a material that is transparent in the THz such as polyethylene. This allows dilute concentrations of a highly absorbing agent to be measured. 

The assumption that one of the monomer units is excluded from the polymer crystal can entail several consequences. In this case, the, the size of the crystal is limited by the sequence length of the crystallizing monomer units, so the effect of comonomer concentration becomes mixed up with the effect of crystal size. In addition, if one of the units is excluded, it will probably reside at the crystal surface and affect the melting point. In many cases, comonomer units get included in the crystal, for example polypropylene in polyethylene. Thus, the situation is more complicated than what is described by the mathematics in this section. 

---