High-density amorphous , generally

There is great interest in the development of methods that allow the identification of a reasonably good structure with which to start the simulation of dense atomistically detailed polymer systems. The problem of generating dense polymer systems is formidable due to the high density and the connectivity of polymer systems. For crystal structures this can be systematically achieved [33,34] for amorphous structures, however, there is no generally satisfactory method available. Two recent developments in methods for generating amorphous packing (Santos, Suter) are reviewed in Section 3. 

The modulus of PE resins increases with increasing solid density. Thus, a HDPE resin has a higher modulus than an LDPE resin, as shown by the data in Table 2.3. In general, resins with low solid densities feel soft to the touch while resins with high densities feel hard. The and Tg for selected semicrystalline and amorphous materials are given in Table 2.3. 

If the film contains a high density of impurity states that act as electron donors or acceptors, the space-charge region may become thinner than the film itself. A more general expression for the photocurrent coversion efficiency that takes this possibility into account can be derived if the simplifying assumption is made that the diffusion length of minority carriers is likely to be small in polycrystalline or amorphous films that contain high densities of recombination centres... 

It is often challenging to achieve immediate release with amorphous solid formulations due to the physical nature of HME particles and the polymers generally used for this purpose. Many of the commonly used excipients tend to be enteric polymers (e.g HPMCAS) or binders (PVP, PVP/ PVAc). Furthermore, the high density and low porosity natnre of the dispersion results in lower specific surface area that drives slower release rates. Coupled with the lower disintegration rates for the monolithic HME products, release rates will tend to remain slower after introduction to the aqueous environment. However, for CR applications these attribntes present unique advantages that can be exploited in the development process. 

Miller et al. have recently reported infra-red dichroic data obtained for high density polyethylene crystallised under the orientation and pressure effects of a pressure capillary viscometer. Their data for a number of crystalline bands (including the 1894 cm" absorption) showed that the crystal c-axes were almost perfectly oriented (f 1) in the initial extrusion direction. The amorphous orientation functions were generally lower, but corresponded to an extension ratio between 2 and 7 when compared with the above results of Read and Stein and of Glenz and Peterlin. Further evidence was also obtained for the relatively high orientation of the amorphous component of the 2016 cm" band (U = 0-66-0-72). 

In the case of crystalline polymers such as high density polyethylene (FiDPE), the effect of orientation on the morphology has been extensively studied [32,48]. The isotropic sample consists of crystalline lamellae (thickness 100-400 A) embedded in an amorphous matrix. Each lamella is composed of a mosaic of crystalline blocks of lateral dimension 100-200 A with boundaries defined by dislocations. The lamellae are randomly oriented and generally arrange themselves end to end in ribbon-like structures which grow out from nucleating centers to form... 

Figure 16.28 Thermal energy required to heat thermoformable amorphous and crystalline polymers to their respective forming temperatures. HOPE, High density polyethylene at 960 kg/m MDPE, Medium-density polyethylene at 945 kg/m LDPE, Low-density polyethylene at 920 kg/m POM,- Polyoxymethylene PA-6, Polycaprolactam or polyamide PP, Homopolymer polypropylene, PS, General purpose polystyrene MIPS, Medium-impact or rubber-modified polystyrene ABS, Polyacrylonitrile-polybutadiene-polystyrene terpolymer PMMA, Polymethylmethacrylate FPVC, Flexible polyvinyl chloride RPVC, Rigid polyvinyl chloride.

The thermosetting polymers noted above are generally high modulus, amorphous materials with a high crosslink density. As such, they are brittle and thus many blends have been inves-... 

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