Polymer Surfaces INDEX

Fig. 8. R/Platelet in individual platelets adhering to polymer surfaces. HSB data were statistically confirmed to be different from PSt (P < 0.5), HSR (P < 0.5) and PHEMA (P < 0.5) after 40 s R/Platelet (an index of cytoplasmic free calcium concentration) is the ratio of fluorescence emission intensitie of a Ca2 + indicator dye (Fura 2) loaded in platelets when they are excited at 340 nm and 380 nm. (Reproduced from J Biomed Mater Res [Ref 84 Prevention of changes in platelet cytoplasmic free calcium levels by interaction with 2-hydroxyethyl methacrylate/styrene block copolymer surfaces] through the courtesy of John Wiley Sons, Inc.)...

The observed enhancement in oxygen index could be attributed to phase segregation in these block copolymers, which leads to domination of siloxane on the polymer surface. Siloxanes have solid-phase activity rather than vapor-phase activity and reduce flammability through increased formation of pyrolytic char. 

FIGURE 49 The influence of the mesoporosity of the catalyst on its activity and on the polymer melt index and MW. Cr/silica-titania hydrogel (450 m2g ) was dried by first replacing the pore water with a solvent of varying surface tension to produce catalysts having a variety of pore volumes that were then activated at 800 °C and tested at 105 °C. 

Another interesting use of empirical AN and DN indexes obtained by the method of refs.30,35 is exemplified in Table 3. This reports the temperature dependence of acid/base interactions in two commodity polymers, PVC and PC. The relevant AN and DN parameters for these polymers were evaluated by IGC at temperatures ranging from 30 - 130 C. As expected, at room temperature PVC again classifies as acid, and the relative acidity of the polymer surface increases steadily from 30 to about 70 C. Above 80 -90 C, however, the trend is reversed and acid/base interaction forces diminish. PC behaves as an amphoteric solid, its AN and DN finite and roughly equal at room temperatures. Above about 60 C however, the relative behavior of the polymer is that of a Lewis base. The results may be rationalized as follows ... 

Previous results were used for computer simulation of optical properties of the fabricated structures. By substitution to the RSoft software of measured polymer refractive index and the structure surface profile the inside and output optical field distribution were calculated using beam propagation method. The calculated output optical field is given in Fig 20C. In addition, calculation proved that the effective refractive index for fundamental mode is approximately 1.6276 at 632.8 ran. The prepared multimode structure supported 30 modes at wavelength 632.8 ran. 

The manipulation of surface forces by suitable choice of medium is central to the experiments in the present study. In the case of polymers, the choice of a suitable medium is greatly restricted by potential interactions, as a solvent, for example, with the polymer surface. In view of these constraints, we have chosen perfluorinated decalin (CioFis) as a measurement medium Employment of this nonpolar liquid, which displays both a low dielectric constant and a low refractive index and is inert toward most polymers, greatly facilitates the measurement of the dispersion component of the Van der Waals forces between the probe tip and the polymer surface. 

To explore the use of PFD as a contrast-enhancing medium in force distance measurements on polymer surfaces, two series of polymers were chosen for the experiments a first series, consisting of apolar polymers with different refractive indexes, and a second series, in which polymers of different hydrophobicities/hydrophi-licities were selected. 

First Polymer Series. The first series of polymers was investigated, in order to limit the interaction between an AFM probe and polymer surfaces to only the dispersion (London) component of the Van der Waals force. Adhesional forces were measured between a S10,t probe and a set of nonpolar polymers that provided a range of refractive indexes (as measured) polystyrene (1.582), isotactic polypropylene (1.501), poly(vinylidene fluoride) (1.407), and poly(tetrafluoroethylene-co-hexafluoropropylene) (1.348). The histograms of the pull-off forces, measured with a SiOx probe, are shown in Figure 2 and tabulated with the calculated values for adhesion energy in Table 1. 

Attenuated Total Reflectance (ATR) is another form of reflectance requiring contact of the polymer surface with an IR-transparent medium of high refractive index. 

The possibility of extinguishing a polymer flame depends on the mechanism of thermal decomposition of the polymer. Whereas ignition of a polymer correlates primarily with the initial temperatnre of decomposition, steady combustion is related to the tendency of the polymer to yield a char, which is produced at the expense of combnstible volatile fragments. Therefore, the dependence of steady combnstion on the amonnt of char seems to be simple, and in an early study it was established that the oxygen index shows a very good correlation with the char yield. In reality, char also serves as a physical barrier for heat flux from the flame to the polymer surface, as weU as a diffusion barrier for gas transport to the flame. Therefore, the contribution of the char can be more significant than is expected from a simple reduction in combustible gases. 

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