An investigation of a sol-gel/melt transition: The poly(ethylene oxide)/methanol/LiClO 4 system Two slow, long-ranged relaxations in melts of poly͑ethylene oxide͒ ͑PEO͒ were studied using the noninvasive dynamic light-scattering technique of photon correlation spectroscopy ͑PCS͒. Measurements were made for two PEO samples, one with a molecular weight distinctly above the entanglement value and a second sample with a molecular weight distinctly below the entanglement value. Based on several lines of evidence, it is concluded that the faster of these relaxations, which can be identified with the ''cluster'' or ''Fischer'' mode observed earlier in other homopolymer melts, results from the gel-like behavior of a physical network formed by the polymer in the melt, while larger-scale rearrangements in the polymer network structure are responsible for the slower relaxation.

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The ''wet gel'' model used earlier to describe the results of dynamic light scattering studies of neat poly͑ethylene oxide͒ ͑PEO͒ melts has now been successfully applied to PEO-melt/LiClO 4 solutions. The results of static light scattering and viscometry measurements along with measurements from dynamic light scattering revealed that with or without salt, the melt/gel was an overdamped system whose network diffusive relaxations were controlled by the damping liquid component of the gel. Important features of the melt/gel were significantly affected by the addition of LiClO 4 . Examples of important changes included increases in the network mesh size even as ''petrification'' of the network by the salt rendered it less elastic. Nevertheless, for 1 K PEO melts the essential nature of the melt/gel remained uneffected.

We demonstrate an experimental setup utilizing fiber-optic detection and coupling of scattered light into existing photon correlation spectroscopy (PCS) and Fabry-Perot (F-P) interferometry systems. The performance of the fiber coupled F-P as a high-resolution interferometer is considered in detail. Several practical issues and limitations are discussed, including the selection of optic and fiber-optic components, collimation, effects of the fiber mode structure and core diameter, and alignment issues. A series of test measurements on standard systems with well known properties shows that the proposed fiber-optic design meets the performance expectations for both PCS and F-P instruments and presents an attractive alternative to the classical pinhole design.

The crossover behavior of 50 000 molar mass poly(ethylene oxide)/methanol solutions from dilute solution to the melt/gel was examined. At first this behavior was investigated without LiClO(4) and then reexamined with LiClO(4). To better understand this behavior, the dependencies of dynamic light scattering (specifically, photon correlation spectroscopy) measurement results on polymer concentration, on the scattering wave vector and on temperature, and the dependence of static light scattering results on the scattering wave vector were studied. This study produced interesting and important results about network structure and behavior in poly(ethylene oxide) solutions and melts generally and about the effects of LiClO(4) on this structure and behavior more particularly.