Orientational enhancement mechanism

A number of other characteristics are required in order to ensure a viable polymeric conductor. Chain orientation is needed to enhance the conducting properties of a polymeric material, especially the intermolecular conduction (i.e., conduction of current from one polymer molecule to another). This is a problem with many of the polymers that are amorphous and show poor orientation. For moderately crystalline or oriented polymers, there is the possibility of achieving the required orientation by mechanical stretching. Liquid crystal polymers would be especially advantageous for electrical conduction because of the high degree of chain orientation that can be achieved. A problem encountered with some doped polymers is a lack of stability. These materials are either oxidants or reductants relative to other compounds, especially water and oxygen. 

Results will be split into various sections the first of which will be fundamentals of hot spots. This will include a summary of the most important developments in the theory of SERS hot spots for both the EM and CT enhancement mechanisms. The second section will cover developments in tip-enhanced Raman spectroscopy (TERS) which represents the idealized hot spot. Then some issues regarding hot spots and the single molecule will be tackled such as the magnitude of enhancement required for single-molecule detection, the effects of molecular orientation with respect to the hot spot as well as the possible influence of optical forces. Sections 4.4 and 4.5 will cover developments in the imaging and fabrication of SERS hot spots, respectively, which have important implications for theoretical modeling as well control of SERS hot spots. The chapter will conclude by summarizing some of the applications of SERS hot spots that have been recently reported. 

The deformation of long chain polymer molecules has always been of great industrial interest as more value can be placed on a material that has improved properties. Molecular extension, or alternatively molecular orientation, is of particular interest as it can enhance mechanical properties of an otherwise weak polymer. Oriented materials are inherently anisotropic. These anisotropic regions can be found directly in semicrystalline polymers where chains organize themselves into crystalline domains. 

As described in Section II.B.l above, doping causes a drastic change in the electrical properties of polyacetylene. The initial values of electrical conductivity were of the order of 10 S cm" for unoriented materials d24-i30 when doped by iodine and AsFs, were enhanced to the order of 10 S cm, which was obtained in the parallel direction of the doped films oriented by mechanical stretching 31 Improvements in polymerization methods and in the catalyst systems also enhanced the electrical conductivity. Highly oriented films prepared in liquid crystal solvents (Section II.A.l.d.iii) exhibited a conductivity higher than 10 S cm, as did also a well stretch-oriented film prepared by Ti(OBu)4-EtsAl dissolved in silicon oil and aged at 120°C. In further studies Naarmann and Theophilou and Tsukamoto and coworkers attained a conductivity of ca 10 S cm k... 

Orientation A process of drawing or stretching of as-spun synthetic fibers or hot thermoplastic films to orient polymer molecules in the direction of stretching. The fibers are drawn uniaxially and the films are stretched either uniaxially or biaxially (usually longitudinally or longitudinally and transversely, respectively). Oriented fibers and films have enhanced mechanical properties. The films will shrink in the direction of stretching, when reheated to the temperature of stretching. 

During the last few years, most attention has been paid to the blending of PLCs with less expensive thermoplastic engineering polymers (EPS). Addition of PLCs to such polymers not only enhances mechanical properties (strength and stiffness) of the resulting composites obtained due to the orientation of the PLC phase, but also improves their processing properties. Even relatively small amounts of a PLC may induce a reduction in the melt viscosity and thus improve the processability. In most cases, under appropriate processing conditions the dispersed PLC phase can be deformed into a fibrillar one. The... 

A fibrillar polymer-polymer composite consists of an isotropic matrix polymer with fibrils of a second polymer dispersed within it. The idea was developed by Fakirov et al. [40] with the knowledge that drawing of polymers with good molecular orientation enhances their mechanical properties. Depending on the fibril diameters, such composites are referred to as microfibrillar composites (MFCs) or nanofihrillar composites (MFCs). For simplicity, MFCs are discussed because the manufacturing process is essentially the same. One method of creating MFCs is to produce a blend of the two selected polymers in the form of a continuous wire. 

Comonomers do have a significant effect on the stabilization process, enhancing the segmental mobility of the polymer chains [22,23] resulting in better orientation and mechanical properties of the precursor and resulting carbon fibers. Comonomers can also reduce the temperature of initiation of cyclization [24,25]. 

The use of cellulosic nanofibers in the productiOTi of all-cellulosic based composites can greatly improve the mechanical performance of these composites. With these nanofibers and a cellulosic anisotropic matrix a synergy between the percolation of the nanofibers and its matrix-induced orientation can lead to composites with enhanced mechanical properties. 

M. Hwang, S.S. Kim, K.U. Polym Eng, Sci in press). The morphological illustration for the enhanced mechanical properties by incorporating PSF is given in Figure 18. The flexible amorphous PSF reduces free volume at the interface of the crystalline PPS and LCP polymers and seems to facilitate the fibril formation and the molecular orientation of LCP under shearing. Hence, the spherical or ellipsoidal LCP domains observed in the PPS/LCP blend are deformed into rod-like or thread-like fibrils in the PPS/LCP/PSF blend. 

With increasing sensitivity of detectors, the detection of unenhanced surface Raman scattering becomes a possibility, and if this is achieved a wide range of effects can be investigated. In particular the determination of adsorbate orientation from depolarisation ratios and the angular dependence of the Raman signal will be possible. Currently, such measurements only serve as a means of investigating the enhancement mechanism. 

The most common orientation process is used to produce fibres and is described in research disclosures from ICI, academic literature and manufacturer processing guidelines. Melt spun fibres of PEKEKK have exhibited moduli as high as 13 GPa [33]. There are also a number of academic references to oriented material which, as expected, shows enhanced mechanical performance. Techniques such as solid-state extrusion [34] and die drawing have been applied to PEEK and a modulus of 11 GPa has been obtained [35]. Orientation processes are similar to those used for polyethylene terephthalate but there are some key differences in terms of orientation-induced crystallisation [36, 37]. 

Preferred orientation in polymers (see Sect. 1.6.1) can deliver enhanced mechanical properties, especially when the level of preferred orientation is high. Most familiar to many is the case of nylon rope. Polyethylene is another material where very high strength fibres have been used to make ropes and bullet-proof vests. Two kinds of these are commercially available, namely Dyneema from DSM and Spectra from Honeywell. 

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