The temperature‐dependent behavior of individual components within metallocene‐catalyzed semisyndiotactic polypropylenes (semi‐sPP) with a wide range of stereoregular content (26 to 96% rr) is studied using Fourier transform infrared (FTIR) spectroscopy and temperature‐modulated differential scanning calorimetry (DSC). Changes in sensitive, high‐resolution absorbance spectra are observed as melt‐slow‐cooled thin films are subjected to stepwise temperature increases. In general, spectral bands previously identified as being sensitive to ordered structures (e.g., conformed chains, crystal morphs) appear to follow overall trends of shifting to lower wavenumbers (energies), broadening, and decreasing in peak area intensity as temperature increases. Peaks that appear due to “splitting” (observed in more stereoregular materials) show a trend toward coalescence as temperature increases; this corresponds to a gradual loss of chain conformational order. Gauche‐gauche‐trans‐trans (ggtt)n helical and all‐trans (tttt)n planar zigzag‐conformed chains that participate in the crystalline‐amorphous interfacial region (“mesophase”) appear to be more stable (i.e., they do not lose their conformational order as easily) with increasing temperature in materials with a greater degree of syndiotacticity. Moreover, IR data correspond well with modulated DSC endotherms located near 50 °C and 70 °C. At each transition temperature—thought to represent, respectively, a thermally driven chain conformation from planar zigzags to helices, and a dynamic disorder of helices marked by rapid gauche ↔ trans isomerization—the IR absorbance ratio, A978/A963, which represents the relative population of helical chains, undergoes an accelerated decrease. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 439–461, 2005

The tensile properties of poly (paraphenylene benzobisoxazole) or PBO fiber strands were studied using two variables: gage length and the number of twists per inch. The gage length was varied from 1 to 10 inches with 2 twists of the fiber per inch. The effect of the number of twists per inch was studied by varying the number of twists from zero to 10 along a 5-inch gage length. The trends of tensile strength and modulus due to these variables were established and appropriate explanations of these behaviors are provided.The coefficient of thermal expansion (CTE) was studied on bare strands of PBO and Kevlar 49 (a product of Du Pont de Nemours & Co.) fibers, using a Du Pont 943 Thermomechanical Analyzer (TMA) equipped with a film and fiber tension assembly. The axial CTE of both fibers exhibited a dependence on the small dead load employed to keep the fibers straight. Kevlar 49 fiber, when wet, attained a much less negative value of CTE than when dry. In contrast, PBO fiber absorbed very little moisture, and the CTE remained unchanged.