You-Lo Hsieh

Crystalline structure of developing cotton fibers

The crystalline structure of dried cotton fibers at varying development stages has been investigated using wide angle x-ray diffraction (WAXS) techniques. The cellulose I crystalline structure has been confirmed on dried SJ-2 Acala cotton fibers collected at varying developmental stages and at maturity. The cellulose I crystalline structure is clearly evident at the early developmental stage of 21 days postanthesis (dpa). The crystal system remains unchanged during the cotton fiber biosynthesis and at maturity. The degree of crystallinity and crystallite dimensions in the cotton fibers increase with cell development. The most significant increments are observed between 21 and 34 dpa (i.e., during the first half of the secondary wall thickening process). The unit cell sizes slightly decrease and thus the crystal density increases with fiber development. The alignment of the glucosidic rings in respect to the 002 planes improves with fiber cell development.

Structural transformation of ultra-high modulus and molecular weight polyethylene fibers by high-temperature wide-angle X-ray diffraction

Wide-angle x-ray diffraction (WAXD) of the ultra-high modulus and molecular weight polyethylene (UHMWPE) fibers at room temperature shows a predominantly orthorhombic structure with trace amount of nonorthorhombic crystals and very low amorphous contents. The calculated unit cell dimensions a and b of the orthorhombic crystals are 7.36 (±0.04) A, and 4.89 (±0.04) A, respectively. The apparent crystallite sizes perpendicular to the orthorhombic 110 and 200 reflection planes are 169.8 and 143.4 A, respectively. The crystallite size perpendicular to the nonorthorhombic 010 reflection is 149.4 A. The crystal density is calculated to be 1.02 g/cc. With increasing temperature, the thermal expansion coefficient in the a direction is much higher than that in the b direction which explains the structural transformation from the orthorhombic crystals to a pseudohexagonal form. Tension along the fiber axis while being heated during the high-temperature x-ray diffraction (HTWAXD) scanning has shown enhanced structural transformation from the orthorhombic form to the monoclinic form. Structural transformation from the orthorhombic form to the pseudohexagonal phase is not observed on the UHMWPE fibers under axial tension or annealing conditions in HTWAXD.

Wetting characteristics of poly(p-phenylene terephthalamide) single fibers and their adhesion to epoxy

Abstract the circumferential dimension of poly( p -phenylene terephthalamide) (PPTA), or Kevlar 49, single fibers and their wetting behavior and adhesion properties with an epoxy resin were evaluated. A tensiometer employing the Wilhelmy principle was described for fiber size and wetting force measurements. For the fine PPTA fibers, the fiber size could be most consistently measured by the wetting force profiling along the fiber axis in a total wetting liquid. The wetting characteristics of Kevlar 49 fibers in stepwise steady-state wetting, dynamic wetting, and multiple immersions in water and in an epoxy resin were detailed. Argon glow discharge significantly improved water wettability of the Kevlar 49 fibers but lowered their wettability in the epoxy resin. The observation on wettability in water and resin and the effects of glow discharge on wettability suggested that the polar nature of the Kevlar fiber surface may be a dominant factor in determining its wettability in these liquids. Upon argon glow discharge, the resin wettability of the Kevlar 49 fiber and the interfacial shear strength between the fiber and the epoxy were both lowered.

Relationship of substratum wettability measurements and initial Staphylococcus aureau adhesion to films and fabrics

The adhesion of Staphylococcus aureus to six films and four fabrics of commercial sources was measured following 1-h contact with the bacteria in aqueous suspensions. Among the untreated and glow-discharged film, increasing S. aureus adhesion was correlated with decreasing surface water wettability as measured by higher water contact angle measurements and lower critical surface tensions. Explicit differences in water wettability (drop absorbency and wicking time) and hydrophobicity (moisture regain) among the untreated as well as among the glow-discharged fabrics were observed. However, little difference in S. aureus adhesion among these two groups of fabrics indicates that the water wettability and hydrophilicity measurements of the fabrics were not the domainant factors attributing to their S. aureus adhesion properties. Different correlations between surface wettability and S. aureus adhesion on films and fabrics of similar chemical compositions further suggest that S. aureus adhesion may be influenced by other surface properties due to their different fabrication and finishing processes. Glow discharge improved wetting characteristics of both types of substratum, but S. aureus adhesion on fabrics was lowered and that on films was increased by the treatment. The bacteria—water wettability correlation on glow—discharged discharge induced active species on the surface.

Argon glow discharge and vapor‐phase grafting of vinyl monomers on wettability of polyethylene

Argon glow discharge-induced vapor-phase grafting of vinyl monomers containing various polar pendant groups onto PE films was investigated. Relationships between the enhanced wetting properties and the level of grafting, the types of pendant groups, and the surface topographical features were established. Improved wettability of the grafted PE surfaces is attributed to both the increased surface polarity and topographical features. On AA-grafted PE surfaces with optimal wettability, microcracks, with depths of 130 ∼ 250 nm, lengths over 10 μm, and widths between 1.5 and 6.0 μm, are proved to be a topographical feature necessary for improved wettability. With sufficient microcracks, grafting with vinyl monomers containing carbonyl groups, i.e., carboxyl, aldehyde, and ketone groups, improved surface wetting more than grafting with those containing either hydroxyl and epoxy groups. The acquired wettability of vinyl monomer-grafted PE surface is attributed to the physicochemical synergism between the microcracks and the polar groups.

Bacterial Adherence on Fabrics by a Radioisotope Labeling Method

A radioisotope labeling technique was used to quantify the adherence of Staphy lococcus aureus, Staphylococcus epidermidis, and Escherichia coli on eight fabrics. The extent of bacterial adherence depends on the bacterial type, the fiber content, and the contact condition. Staphylococci adhered more than Escherichia coli, and Staph. epidermidis adhered more than Staph. aureus on all fabrics. There were apparently different degrees of bacterial adherence in the eight fabrics, but the ranking orders of adherence depended to a great extent on the contact condition. Bacterial adherence was increased by both agitation and saturated wetting due to increasing bacterial cell- fiber contact. There was an increasing trend in bacterial adherence on all fabrics as contact time increased up to 24 hours, with occasional zig-zag patterns. On most fabrics, cleaning and autoclaving did significantly influence the extent of bacterial adherence to the fabrics. The quantitative data along with the morphology of bacterial adherence by SEM suggested that factors other than hydrophilicity, water absorbency, and surface roughness may affect the extent of bacterial adherence to the fibers.

Evaluation of a Radioisotope Labelling Technique for Measuring Bacterial Adherence on Fabrics

A technique utilizing tritiated thymidine labelled bacteria to quantify bacteria on fabrics has been evaluated. Quenching or self-absorption of isotope solution and labelled bacteria suspension by some of the fabrics has been observed. The extents of self-absorption of both isotope and labelled bacteria solutions on various fabrics was found to be dependent upon the fiber contents, i.e. the chemical compositions, of the substrata. This observation confirms that reduction of scinitillation efficiency or self-absorption does occur when radio-labelled substances in suspensions were measured with the presence of some fabrics. Cautions should be taken when radio-labelling techniques are applied to detect isotope-labelled micro-organisms or other substances which are in contact with fabrics in the form of solutions. However, when there is no excess and nonattached labelled bacteria in the aqueous surrounding of the fabric, scintillation counting efficiency of the labelled bacteria on all fabrics studied remained constant over a period of 8 h. This indicates that the application of the described isotope labelling procedure is appropriate for quantifying adherent bacteria on fibrous substrate.