Fibrillation process

Figure 22 Schematic representation of proposed models for the fibril formation in the cases of pH 3.3 and 7.5. (A) hCT monomers in solution (B) a homogeneous association to form the a-helical bundle (micelle) (C) a homogeneous nucleation process to form the P-sheet and heterogeneous association process (D) a heterogeneous fibrillation process to grow a large fibril, a-helix, antiparallel p-sheet, and parallel p-sheet forms are shown by a box, drawn by dark grey and grey, respectively. From Ref. 163 with permission.

The deformation of the polymer within a thin active zone was originally represented by a non-Newtonian fluid [31 ] from which a craze thickening rate is thought to be governed by the pressure gradient between the fibrils and the bulk [31,32], A preliminary finite element analysis of the fibrillation process, which uses a more realistic material constitutive law [36], is not fully consistent with this analysis. In particular, chain scission is more likely to occur at the top of the fibrils where the stress concentrates rather than at the top of the craze void as suggested in [32], A mechanism of local cavitation can also be invoked for cross-tie generation [37]. 

More work on a detailed description of the fibrillation process is needed to clarify the underlying mechanism and its relationship with molecular aspects, such as the entanglement density or the molecular mobility. Nevertheless, based on the observations reported by DOll [29,30] of time-dependent craze stress and Kramer s [31,32] description of fibrillation involving an active plastic zone, one can conclude that craze thickening is a viscoplastic process. 

In order to gain an insight into the deformation behavior of the fibrils in the craze and the time-dependence of the fibrillation process it is useful to examine the growth in the width and the length of the craze. 

In order to obtain information on the craze growth and on the fibrillation process the maximum craze widths are plotted against craze lengths s for high molecular... 

Continuing the discussion of the results in Fig. 3.22, for crack speeds above about 10 mm/s it may be observed that craze dimensions are nearly constant in PMMA of LMW and increase in HMW PMMA with crack speed up to a = 1(F mm/s. The reason for this constant or increasing craze size is to be seen in the following Due to the fibrillation process which is associated with internal friction, heat is produced in front of the propagating crack tip The local heat output due to the... 

Glover et al., 1997). Thus, an understanding of the fibrillization process in vitro will likely have direct implications for the molecular mechanism of Sup35 /W replication in vivo and perhaps the molecular basis for disease progression in the mammalian amyloidoses. 

PSF Blends with Fluoropolymers PSF/PPS/5-40 wt% PTEE fibrils processability, lubricity, anti-corrosive Bailleux etal., 1984... 

Table 7.6 provides a partial reference to studies on the effects of flow on the morphology of polymer blends [Lohfink, 1990 Walling, 1995]. Dispersed phase morphology development has been mainly studied in a capillary flow. To explain the fibrillation processes, not only the viscosity ratio, but also the elasticity effects and the interfacial properties had to be considered. In agreement with the microrheology of Newtonian systems, an upper bound for the viscosity ratio, X, has also been reported for polymer blends — above certain value of X (which could be significantly larger than the... 

Polypropylene fibers are produced by a larger variety of processes than other melt-spun fabrics. At one end of the range, the long air-quench process produces high-quality multifilament yarns, and, at the other end, fibrillating slit film produces coarser fibers. The success of the lower-cost polypropylene slit-film fiber is due to the lower price of the polypropylene resin and the unique adaptability of polypropylene to the less expensive slit-fibn fibrillation process. The water-quench process for monofilament has long been an established technique... 

Cellulose nanofibers from different sources have showed remarkable characteristics as reinforcement material for optically transparent composites [160, 161], Iwamoto et al. [160] prepared optically transparent composites of transparent acrylic resin reinforced with cellulose nanofibers extracted from wood pulp fibers by fibrillation process. They showed that cellulose nanofiber-reinforced composites are able to retain the transparency of the matrix resin even at high fiber content (up to70 % wt). The aggregation of cellulose nanofibers also contributes to a significant improvement in the thermal expansion properties of plastics. 

PSF/PPS/5 0 wt% PTFE fibrils Processability, lubricity, anticorrosive Bailleux et al. 1984... 

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