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Raman scattering from binary Ge x Se 1−x glasses under hydrostatic pressure shows onset of a steady increase in the frequency of modes of corner-sharing GeSe 4 tetrahedral units when the external pressure P exceeds a threshold value P c . The threshold pressure P c ͑x͒ decreases with x in the 0.15Ͻ x Ͻ 0.20 range, nearly vanishes in the 0.20Ͻ x Ͻ 0.25 range, and then increases in the 0.25Ͻ x Ͻ 1 / 3 range. These P c ͑x͒ trends closely track those in the nonreversing enthalpy, ⌬H nr ͑x͒, near glass transitions ͑T g s͒, and in particular, both ⌬H nr ͑x͒ and P c͑x͒ vanish in the reversibility window ͑0.20Ͻ x Ͻ 0.25͒. It is suggested that P c provides a measure of stress at the Raman-active units, and its vanishing in the reversibility window suggests that these units are part of an isostatically rigid backbone. Isostaticity also accounts for the nonaging behavior of glasses observed in the reversibility window.

Advanced fibers revolutionized structural materials in the second half of the 20 th Century.However, all high-strength fibers developed to date are brittle. Recently, pioneering simultaneous ultrahigh strength and toughness were discovered in fine (<250 nm) individual electrospun polymer nanofibers (NFs). This highly desirable combination of properties was attributed to high macromolecular chain alignment coupled with low crystallinity. Quantitative analysis of the degree of preferred chain orientation will be crucial for control of NF mechanical properties. However, quantification of supramolecular nanoarchitecture in NFs with low crystallinity in the ultrafine diameter range is highly challenging. Here, we discuss applicability of traditional as well as emerging methods for quantification of polymer chain orientation in nanoscale one-dimensional samples. Advantages and limitations of different techniques are critically evaluated on experimental examples. It is shown that straightforward application of some of the techniques to subwavelength-diameter NFs can lead to severe quantitative and even qualitative artifacts. Sources of such size-related artifacts, stemming from instrumental, materials, and geometric phenomena at the nanoscale, are analyzed on the example of polarized Raman method, but are relevant to other spectroscopic techniques. A proposed modified, artifact-free method is demonstrated. Outstanding issues and their proposed solutions are discussed. The results provide guidance for accurate nanofiber characterization to improve fundamental understanding and accelerate development of nanofibers and related nanostructured materials produced by electrospinning or other methods. We expect that the discussion in this review will also be useful to studies of many biological systems that exhibit nanofilamentary architectures and combinations of high strength and toughness.

Ternary (Ge 2 X 3) x (As 2 X 3) 1−x glasses with X = S or Se are of interest because they span a mean coordination number r in the 2.40 < r < 2.8 range that is characteristic of stressed-rigid glasses. We have examined X = S glasses in Raman scattering and T-modulated differential scanning calorimetry measurements over the 0 < x < 1.0 range. Glass transition temperatures, T g (x), increase monotonically in the 0 < x < 0.8 range and decrease thereafter (0.8 < x < 1) to display a global maximum near x = 0.8. Raman scattering provides evidence of sharp modes of As 4 S 4 and As 4 S 3 monomers, with scattering strength of these modes showing a global maximum near x = 0.3 and 0.5 respectively. The results suggest that at low x (0 < x < 1/2), addition of Ge 2 S 3 to the As 2 S 3 base glass results in insertion of Ge(S 1/2) 4 tetrahedra in the As(S 1/2) 3-based backbone as compensating As-rich monomers segregate from the backbone to deliver the requisite S. At higher x (0.4 < x < 0.8), the Ge 2 S 3 additive continues to enter the glass in a majority (As 2 S 3)(GeS 2) backbone and several minority nanophases including an ethane-like Ge 2 (S 1/2) 6 and a distorted rock-salt-like GeS. In the 0.8 < x < 1 range, the nanophases grow qualitatively at the expense of the backbone as T g values decrease and the end-member composition (x = 1) is realized. Heterogeneity of glasses near x = 1/2 or mean coordination, r = 2.60 derives intrinsically from the presence of several minority nanophases and a majority backbone showing that stressed-rigid networks usually phase separate on a nanoscale.

Spherical aberration is probably the most important factor limiting the practical performance of a confocal Raman microscope. This paper suggests some simple samples that can be readily fabricated in any laboratory to test the performance of a confocal Raman microscope under realistic operating conditions (i.e., a deeply buried interface, rather than the often-selected alternative of a bare silicon wafer or a thin film in air). The samples chosen were silicon wafers buried beneath transparent polymeric or glass overlayers, and a polymer laminate buried beneath a cover glass. These samples were used to compare the performance of three types of objectives (metallurgical, oil immersion, and dry corrected) in terms of depth resolution and signal throughput. The oil immersion objective gave the best depth resolution and intensity, followed by a dry corrected (60x, 0.9 numerical aperture) objective. The 100x metallurgical objective was the worst choice, with degradations of approximately 5x and 8x in the depth resolution and signal from a silicon wafer, comparing a bare wafer with one buried under a 150 microm cover glass. In particular, the high signal level obtained makes the immersion objective an attractive choice. Results from the buried laminate were even more impressive; a 30x improvement in spectral contrast was obtained using the oil immersion objective to analyze a thin (19 microm) coating on a PET substrate, buried beneath a 150 microm cover glass, compared with the metallurgical objective.