Local polymer optical fibers

Light is a versatile physical stimulus that can be localized with optical fibers whose penetration can be tuned using UV, near infrared (NIR) or two-photon sources. Photoresponsive polymers contain groups that aggregate or are cleaved after irradiation (for a recent review see [ 141 ]). Whereas the photoresponsive groups (photosensitizers) have minimal toxicity themselves, irradiation leads to generation of reactive oxygen species and cell death (photodynamic therapy, PDT). 

Light wave technologies provide a number of special challenges for polymeric materials. Polymer fibers offer the best potential for optical communications in local area network (LAN) applications, because their large core size makes it relatively cheap to attach connectors to them. There is a need for polymer fibers that have low losses and that can transmit the bandwidths needed for LAN applications the aciylate and methacrylate polymers now under study have poor loss and bandwidth performance. Research on monomer purification, polymerization to precise molecular-size distributions, and weU-controlled drawing processes is relevant here. There is also a need for precision plastic molding processes for mass prodnction of optical fiber connectors and splice hardware. A tenfold reduction in the cost of fiber and related devices is necessaiy to make the utilization of optical fiber and related devices economical for local area networks and tlie telecommunications loop. 

Banding has been observed in both lyotropic and thermotropic polymers examined by optical and electron imaging techniques [430, 431, 439-446]. Incomplete extinction has been observed for some of the aramids where 1 ands, normal to the fiber axis, are observed in polarized light [440] (Fig. 5.88). It is now weU known that the aramids exhibit axial banding having periodicities of about 500 nm, which is observed by dark field TEM [447]. Sinunens and Hearle [440] have proposed that the optical observations and the pleated sheet model of Dobb et ah [447] are compatible and that the optical bands are the bends or folds between the pleats which might well exhibit the local density differences observed by DF TEM. 

A major advantage of optical microscopy techniques is the ease of sample preparation. Thin fibers, films, or membranes can be placed directly in an appropriate immersion oil on a glass slide and information regarding the crystalline or dispersed phases, orientation, birefringence, and so forth, can be readily determined (see Section 4.1.1). Sectioning (see Section 4.3.2) of thicker materials is routinely accomplished in very short times, on the order of 30 min or so, with steel or glass knives. Observations and measurements of spherulite sizes, local orientation in molded parts, and fiber orientation are also conducted with the optical microscope. Phase contrast and Nomarski techniques provide contrast in multiphase polymers. Small differences in refractive index are enough to make the dispersed phases distinct, so the... 

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