synopsisMelting and crystallization behavior of virgin polytetratluoroethylene have been studied using a differential scanning calorimeter. Following quantitative relationship was found between number average molecular weight of polytetrafiuoroethylen_e and the heat of crystallization in the molecular weight range of 5.2X 106 to 4.5XlO': M, = 2.1 X10" AH,-G.16, where a,, is number average molecular weight and AHc is the heat of crystallization in cal/g. The heat of crystallization is independent of cooling rate ranging from 4 to 32 'C/min. This relationship provides a simple rapid and reliable method for measuring the molecular weight of polytetrafluoroethylene.

Melting and crystallization behavior of polytetrafluoroethylene as polymerized in emulsion and suspension is shown to depend on molecular weight. DSC heating curves for virgin PTFE with low molecular weight below 3 × 105 have a single peak, whereas curves for higher molecular weight samples have double peaks. With increasing heating rate the areas of higher melting peaks become larger than the lower melting peaks. The morphology of polymer exhibiting double melting peaks is mainly folded ribbons or granular particles. The phenomenon of double melting is explained on the basis of two different crystalline states which correspond to the “fold regions” and the “linear segments” in a folded ribbon. The melting temperature of virgin PTFE is almost constant at ca. 330°C for molecular weights below 1 × 106, and rises as the molecular weight increases above 1 × 106. The heat of melting of virgin PTFE is nearly independent of molecular weight. On the basis of these results, we propose a model for melting and crystallization of low and high molecular weight PTFE and for the crystal structure.

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Worldwide, studies are being devoted to the development of adaptive drug release systems capable of delivering the right amount of drug at the right place and time. Many of these studies use responsive hydrogels capable of adapting reversibly under the influence of environmental conditions such as temperature, pH, concentration of chemical species, or electric charge.A new concept for a mechanically stable drug release system is to combine hydrogels with porous ion-track membranes, which have the advantage of high mechanical strength. Ion-track membranes can be produced with cylindrical pores of controlled pore size and narrow pore size distribution. The result is a responsive microcomposite that can be tailored for specific applications. The membrane consists of a mechanically strong polymer matrix onto which a soft hydrogel is grafted. Due to its symmetric sandwich structure the membrane remains flat and does not expand laterally during thermal cycling.The first component of the microcomposite is an etched ion-track membrane. The technique is based on energetic heavy ions from accelerators. On passing through a polymer each ion releases its energy within a cylinder of -10 nm diameter"] -the latent ion trackcorresponding to a high concentration of active chemical species,[21 which is associated with dramatic changes in molecular structure and properties. The preparation of an etched track membrane, as well as its grafting by a hydrogel, is illustrated in Figure 1. The etched ion-track membrane is characterized by adjustable, highly uniform, pore diameters (adjustable between 10 nm and 100 pm), identical pore length, and uniform pore orientation. The number of ion tracks per cm2 corresponds exactly to the number of impinging ions Intelligent gel Ion track etch techniqueFig. 1. Schematic diagram for the generation of responsive membranes by radiation-grafting of functional monomers onto the surface of ion-track membranes.from the ion accelerator and can be easily varied between lo3 and 10'' ions per cm2. Even individual ion tracks, i.e., one single pore per specimen, can be created.[31 The second component of such a microcomposite is the responsive hydrogel. Recently we found that the acryloylbased polymer with L-proline methyl ester (ProOMe) as a side chain confers an interesting property on the resulting polymer. It undergoes a coil-globule transition at a lower critical solution temperature (LCST) of 14 "C, in connection with the dissociation of structured water molecules surrounding hydrophobic ProOMe groups.[41 The crosslinked polymer is a temperature-responsive hydrogel undergoing a dramatic volume phase transition at 14°C. It swells below this temperature, absorbing water, and shrinks above this temperature, releasingThe volume of swollen hydrogel is approximately 400 times larger than that of the shrunken one. The swelling-shrinking process is reversible.['] Our next goal is thus to synthesize a thermoresponsive ion-track membrane by combining this hydrogel with an ion-track membrane.A 50 pm thick membrane...

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