The thermal cis f trans isomerization of azobenzene groups covalently attached to chains of undrawn and drawn polyethylene films (Azo-PE) has been examined in a temperature range that is well above the glass transition and well below the melting transition. Isomerizations in undrawn films at all temperatures and in a drawn film at 70°C, the highest temperature examined, follow singlecomponent first-order kinetics. At 20-65°C, two first-order components whose activation energies are both smaller than that of the undrawn film are needed to describe the isomerization kinetics in the drawn state. Doped (i.e., noncovalently attached) azobenzene groups in polyethylene films (Azo/PE) also exhibit a biexponential kinetics after drawing, but the fraction of the faster component is much smaller than that in the drawn Azo-PE films. Comparisons between the rate constants for isomerization of undrawn and drawn Azo-PE and Azo/PE films at the same temperature indicate that as a consequence of probe translocations the populations of sites occupied by the probe change significantly when the doped film is drawn. The faster and slower rate components in drawn Azo-PE are assigned to cis isomers residing in interfacial and amorphous sites, respectively, on the basis of differences in polymer chain ordering, rates of relaxation, and free volumes at occupied sites in interfacial and amorphous regions. These results are discussed in terms of how the micromorphological changes in PE when it is drawn affect the dynamics of the guest molecules and how the latter provides information about the local PE matrix.

A method is described to functionalize specifically the interior portions of polyolefinic films with chromophoric/lumophoric groups. In the first step, a film doped with dibenzothiophene 5-oxide (DBTO) is irradiated (λ > 300 nm) to produce O(3P) atoms that oxidize nearby C−H bonds of the polymer. Covalent attachment of acid chlorides (containing chromophoric/lumophoric groups) to the hydroxy bonds (via ester linkages) provides films that retain their absorption and luminescence in water or air for long periods. The esterified films have been characterized using UV/vis absorption spectroscopy and static and dynamic fluorescence techniques. Fluorescence from native or modified and then esterified films in 1.0 M methanolic 2-(dimethylamino)ethanol, a surface quencher, indicates that the vast majority of the chromophores/lumophores are located inside the films. Selective modification of the film surfaces by an analogous method was not efficient. However, a very low concentration of hydroxy groups is present throughout the native films. Their esterification reduces the fluorescence contrast between undoped/irradiated and DBTO-doped/irradiated parts of a film.

Linear polarizations have been measured for covalently attached and doped 9-anthryl and 1-pyrenyl groups residing in interior sites of stretched polyolefinic films. The influences of polymer crystallinity, the concentration of aromatic groups, and the length of the substituents attached to doped molecules or of the tethers to polymer chains of covalently attached species on the degree of polarization have been explored. The results demonstrate the utility of comparing orientational parameters from doped and covalently attached groups in analyzing the factors responsible for stretch-induced orientation. The anthryl and pyrenyl groups prefer to reside in interfacial regions more than amorphous regions even before film stretching, and the specificity of their orientations is determined by the nature of interactions with surrounding polymer chains. The magnitudes of orientation factors are dependent on polymer crystallinity and substituent or tether length, but are independent of aromatic group concentrations as long as they are low. There are significant differences between the orientations of doped and covalently attached groups of the same type due to the inability of the latter to translocate between site types during film stretching. The results, as interpreted in the context of current theories, demonstrate the necessity of crystallite surfaces (i.e., interfacial sites), but not stretching-induced translocation, for selective orientation of aromatic groups along the axis of stretching.

The weak bases tri-n-alkylphosphines (R3P, where R ) C14H29, C18H37) and phosphine oxides (R3PdO) have been reacted with the strong Lewis acid BF3 to form zwitterions (R3P + -OBF3or R3P + -BF3 -) and with Brønsted acids (pKa < 5) to form salts (R3P + -OH X -, where Xis several anion types, including chloride, p-dodecylbenzenesulfonate, and p-toluenesulfonate). The abilities of the zwitterions and some of the hydroxyphosphonium salts to gel a variety of organic liquids have been investigated and compared with those of the phosphine oxides (R3PdO, R ) C10H21, C14H29, C18H37). The gelating abilities of the zwitterions are similar to those of the phosphine oxides in that only liquids capable of donating protons or promoting ionic interactions could be gelled, and both are less efficient than the corresponding hydroxyphosphonium salts. Alcohols with short alkyl chains (<5 carbon atoms) were gelled preferentially by (C 18H37)3P + -OH X -(X -) chloride or p-toluenesulfonate), while those with longer alkyl chains were gelled better by the less polar p-dodecylbenzenesulfonate salt or (C18H37)3PdO. These observations suggest the participation of the solvent at the moment of network formation. Powder X-ray diffraction and temperature-dependent NMR measurements reveal that gel formation from the (C 18H37)3P + -OH Xsalts involves a competition among several concurrent processes, some of which are a consequence of salt dissociation during heating of sols. The one(s) that dominate determine the nature of the networks responsible for immobilization of the liquid components of the gels. Thus, some gel networks were comprised of (C 18H37)3PdO despite the fact that (C18H37)3P + -OH Clwas the added gelator and the same liquid was not gelled by the phosphine oxide alone! A hypothesis for the role of the acid in these gelation processes and models for molecular arrangements of the gelators in the neat solids and gel networks are presented.

The properties of several types of organogels that can undergo a chemical transition during gelation or while in the gel phase are described. The transitions can be physically induced by light or chemically triggered by the addition of an acid or a neutral molecule such as CO2 or CS2. In some cases, the gelation properties of the new species formed are markedly different from those of the precursors. The link between molecular structure and the nature of the gel networks as well as results obtained from the multidisciplinary tools used to study them are discussed.