Transparent semicrystalline polymer

The SFA, originally developed by Tabor and Winterton [56], and later modified by Israelachvili and coworkers [57,58], is ideally suited for measuring molecular level adhesion and deformations. The SFA, shown schematically in Fig. 8i,ii, has been used extensively to measure forces between a variety of surfaces. The SFA combines a Hookian mechanism for measuring force with an interferometer to measure the distance between surfaces. The experimental surfaces are in the form of thin transparent films, and are mounted on cylindrical glass lenses in the SFA using an appropriate adhesive. SFA has been traditionally employed to measure forces between modified mica surfaces. (For a summary of these measurements, see refs. [59,60].) In recent years, several researchers have developed techniques to measure forces between glassy and semicrystalline polymer films, [61-63] silica [64], and silver surfaees [65,66]. The details on the SFA experimental procedure, and the summary of the SFA measurements may be obtained elsewhere (see refs. [57,58], for example.). 

Crystallinity is important in determining optical properties because the refiaetive index of the crystalline region is always higher than that of the amorphous component irrespeetive of whether the amorphous component is in the glassy or rubbery state. This difference in refractive indices of the component phases leads to high scattering and consequently, the translucency or haziness of semicrystalline polymers. For a purely amorphous polymer, this does not occur, and hence amorphous polymers are usually transparent. Therefore the state of polyethylene terephthalate can be explained as follows ... 

In a thermoplastic, the macromolecules are not cross-linked so that the material can melt, i.e., above the glass transition temperature, the material begins to soften. Thermoplastics can be amorphous or semicrystalline. In microfluidics, amorphous polymers are often preferred because of their optical transparency. Amorphous polymers include polymethylmethacrylate (PMMA), polycarbonate (PC), polystyrene (PS), and cyclic olefin polymers (COP) and copolymers (COC). COP and COC, unlike most other polymers, are also transparent for UV Ught... 

The presence of crystalline regions tends to rednce the level of light transmission, and pnre semicrystalline polymers in moderate thickness are generally translucent. They include the polyolehns, polyamides, and thermoplastic polyesters. However, several crystallizing polymers can be made into highly transparent, relatively thick prodncts. They are polymethylpentene and polyethyleneterepthalate. Films of many crystallizing polymers, particnlarly oriented hhns, can also be transparent See also optical properties. 

Amorphous polymers (transparent in the solid state to be precise, it is not a solid but rather a supercooled liquid) are usually easy to dissolve in the good solvent. In contrast, crystalline and semicrystalline polymers (opaque in the solid state) are sometimes not easy to dissolve. Within a crystallite, polymer chains are folded into a regular, thermodynamically stable arrangement. It is not easy to unfold the chain from the self-locked state into a disordered state in solution even if the latter state is thermodynamically more stable. Heating may help the dissolution because it facilitates the unfolding. Once dissolved, polymer chains take a random-coil conformation unless the chain is rigid. 

Despite the importance of the problem, the effect of recycling on optical properties has been marginally analysed in the literature [36]. The optical transparency in semicrystalline polymers is mainly related to the crystallinity and surface properties... 

Due to their noncrystalline structure, amorphous materials have better optical transparency than semicrystalline materials, and a decrease in the crystallinity of a semicrystalline polymer enhances the clarity [40]. However, an excessive reduction of crystallinity can result in unacceptable reductions in strength, stiffness and resistance to softening, so a compromise must be reached that is appropriate for the application. 

Amorphous plastics, such as acrylics, are transparent to infrared rays. Some semicrystalline polymers, such as PFA, have surprisingly high levels of infrared transmission. PFA can be welded by infrared under certain circumstances. For example, a transparent tube of PFA was welded to a black sheet of PFA that absorbed the infrared light generated by a Nd YAG laser (1,064 nm).[ ] A tube composed of natural PFA (6.4 mm outer diameter and 3.2 mm inner diameter) was pressed into a 6.4 mm diameter hole in an aluminum sheet. The aluminum sheet was used to shield the black PFA sheet from stray radiation. The IR light was defocused to a diameter of 6.4 mm and aimed at the end of the tube in the aluminum. A laser power of 30 W with a tube length of 50 mm produced a strong weld in a few seconds. 

The semicrystalline, ethylene-based ionomers of commerce are flexible, transparent polymers notable for high strength and elasticity in both soUd and molten states. The ionic bonding is completely reversible (8) and has a strong influence on properties, even at temperatures well above the melting point. 

Oriented polypropylene is used for clear films used as food wrap. What advantageous properties do the oriented films have over standard films How can a polymer that is, typically semicrystalline be transparent ... 

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