CommunicationsFinally, it is worth noting that the oriented films were found to be extremely stable: the films could be stored under ambient conditions (exposure to air and light) for months without any noticeable change to their properties. We attribute the latter, most relevant, phenomenon to the encapsulation of the conjugated polymer in the highly crystalline PE matrix.In conclusion, we have shown that gel processing and subsequent tensile deformation of blends of different PPEs and ullra-high molecular weight polyethylene leads to an outstanding orientation of the conjugated polymer guest, resulting in state of the art polarized photoluminescence and absorption of the prepared films. Maximum orientation and polarization is obtained when the conjugated guest is of high molecular weight and derivatized with sterically hindered, rather than only linear, side chains. The orientation process used appears to induce a transformation of an initially phase-separated system into an apparent molecular dispersion of the conjugated polymer guest in the PE host.Experimental 0-OPPE, EHO-OPPE, and HMW EHO-OPPE used in this work were prepared according to previously published procedures [15,18]. UHMW-PE (Hostalen Gur 412) was obtained from Hoechst AG.Thin films were prepared by casting a solution of the PPE (5-125 mg) and UHMW-PE (0.5 g) in xylene (50 mL) (dissolution at 130 "C after degassing the mixture in vacuum at 25 "C for 15 min) into a petri dish 11 cm in diameter. The gels were dried under ambient conditions for 24 h. All resulting blend films had a homogeneous thickness of about 70 pm. The films were drawn at temperatures of 90-120 "C on a thermostatically controlled hot shoe. Draw ratios were calculated from the displacement of distance marks printed on the films prior to drawing.Polarized UV-vis spectra were recorded with a Perkin Elmer Lambda 900 instrument, fitted with motor driven Glan-Thomson polarizers. PL spectra were recorded on a SPEX Fluorolog 2 (Model F212f), using unpolarized light (350 nm) for excitation and a Glan-Thomson polarizer on the detector side. For the photophysical experiments, the polymer films were sandwiched between two quartz slides, applying a silicon oil fluid in order to minimize light scaltering at the film surfaces. The remaining scattering effects were compensated in the absorption measurements by subtracting the spectra of pure UHMW-PE films of comparable draw ratio and thickness. Received
An interpretation of the effect of resin molecular weight on the dissolution of Novolak is offered. It is based on Eyring's transition state theory and on the percolation model of Novolak dissolution. The rate-determining step of Novolak dissolution is the deprotonation of phenol by base at the front edge of the penetration zone. In order for this reaction to occur at a particular site, an ion pair of base must appear there, and to make this possible, all base ions of the corresponding percolation channel have to move forward in synchronism. That requires the simultaneous thermal activation of all the sites of the channel. At this point the mechanism of energy transport intervenes: In a system of polymer chains, thermal (vibrational) energy propagates much faster along the chains than between them, and the critical energy fluctuations needed for the activation of a site will reach this site almost exclusively via the chain to which the site belongs. It can be shown that the probability that a particular site will receive an activating quantum is inversely proportional to chain length. The probability that all sites of a percolation channel will be activated simultaneously is inversely proportional to chain length to a power that is the number of sites involved in the move. The probability of this event decreases steeply with chain length, as is observed. These principles lead to a quantitative description of the dissolution of Novolak films as a function of the molecular weight of the resin.
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