PSLCs

After polymerization, polymer networks tend to stabilize the state in which they are formed. In a PSLC, the liquid crystal near the polymer network is ahgned along the polymer network. The strength of the interaction between the liquid crystal and the polymer network is proportional to the surface area of the polymer network. The surface area of the polymer network can be increased by using higher polymer concentrations or producing smaUer lateral size polymer networks. 

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FIGURE 2.2 The smectite clay structure. (From http //www.pslc.ws/macrog/mpm/composit/nano/stmct3 l. htm, access date 4.11.06.)... 

Everything you ever wanted to know about polymers, especially for kids. http //www.pslc.ws/macrog.htm... 

Polytetrafluoroethylene. Polymer Science Learning Center. Department of Polymer Science, The University of Southern Mississippi. Available online. URL http //www.pslc.ws/mactest/ ptfe.htm. Accessed Dec. 17, 2006. 

Figure 12 Beam coupling for PSLC composite and unpolymerized LC. Beam coupling in PSLC is observed down to 2.5 xm for applied voltage of 1.8 V. (Reproduced with permission from Ref. 79.)...

Figure 13 Kinetics of asymmetric beam coupling within PSLC for A = 4.8 xm. The inset shows the beam coupling kinetics for A = 2.5 xm. Beam 1 , Beam 2 O.

Figure 14 Rise, Xpp, and decay times xpp, of the photorefractive grating versus A for (a) PSLC and (b) unpolymerized LC. The unpolymerized LC decay times exhibit a quadratic dependence on fringe spacing, consistent with an ion diffusion, whereas the PSLCs show a steeper dependence versus A.

The time-resolved photoconductivity measurements shown in Fig. 15 give further support for a difference in the photoinduced charge transport in the polymerized samples versus the unpolymerized samples. For the incident laser of 100 mW/cm2 and a spot size of 2.5 mm, the decay time of the photoconductivity for the unpolymerized samples is 7.4 sec, whereas the photoconductivity of the polymerized samples does not significantly drop over a 30 sec period. Also, the photoconductivity of the polymerized sample is nearly twice that of the unpolymerized samples even at the peak of the unpolymerized photoconductive response. The unnormalized values for the dark conductivity in both samples is 1.7 x 10-10 S cm-1. The photoconductivity is 5.8 x 10-11 S cm-1 for the unpolymerized sample and 1.1 x 10-10 S cm-1 for the PSLC at an optical intensity of 2 W cm-2. 

The beam coupling, four-wave mixing, and photoconductivity data indicate that the mechanism for charge transport within the PSLCs is dramatically different than for the unpolymerized samples. The unpolymerized samples should exhibit a grating decay time that decreases quadratically with fringe spacing. This is supported by the data in Fig. 14b [47,80], However, the data for the... 

Figure 17 Difference in decay kinetics for one beam blocked versus two beams blocked in PSLC. The lifetime of the grating is enhanced when one beam is incident on the sample. The inset shows the decay kinetics for the unpolymerized LC when one beam is incident on the sample. No enhancement of the grating lifetime is observed. Both beams are incident on the sample at t = 0, and either one or both beams are blocked as specified above at t = 100 s.

PSLCs exhibit a weak dc signal when one beam is incident on the sample that decays over a much longer time scale (a few hours). No such lifetime increase is observed for photorefractive gratings in unpolymerized LCs, which is illustrated by the inset of Fig. 17. These experiments show that a photoinduced process occurs in the PSLCs that encourages charge separation, but also discourages charge recombination. 

This first section wiU study different regjoselective processes of several types of nucleosides depending on the lipase used. Application of biotransformations over these compounds has acquired great importance in order to prepare new derivatives with interesting pharmacological activities. Two lipases, namely, Candida antarctica type B (CALB) and Pseudomonas cepacea, free (PSL) or Pseudomonas cepacea, immobilized (PSLC), are selective towards one of the two hydroxyl groups of different 2 -deoxynucleosides. Thus, it is possible to prepare the acylated compounds in S -position with CALB [10], whereas PSL is selective towards the secondary hydroxyl group [11]. Vinyl or oxime esters can be used as acyl donors. 

In addition, regioselective enzymatic acylation of pyrimidine 3, 5 -diaminonu-deoside derivatives is possible depending on the biocatalyst employed in the process [28]. N-5 -Acylated products were obtained using CALB as a biocatalyst, whereas PSLC was selective towards the N-3 position. Molecular modeling studies... 

A practical example of separation of a mixture of a/P-anomers of the mother liquor collected from the commercial-scale synthesis of P-thymidine is shown in Scheme 10.14. PSLC is used as a biocatalyst because this lipase has shown the desired opposite regioselectivity toward the hydrolysis of a- and P-nucleosides. The hydrolysis reaction takes place with excellent selectivity toward the 5 -p-... 

Polymer Science Learning Centre, PTFE, http //pslc.ws/mactest/ptfe.htm... 

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