Crystal, tensile properties

In the late 1980s, new fully aromatic polyester fibers were iatroduced for use ia composites and stmctural materials (18,19). In general, these materials are thermotropic Hquid crystal polymers that are melt-processible to give fibers with tensile properties and temperature resistance considerably higher than conventional polyester textile fibers. Vectran (Hoechst-Celanese and Kuraray) is a thermotropic Hquid crystal aromatic copolyester fiber composed of -hydroxyben2oic acid [99-96-7] and 6-hydroxy-2-naphthoic acid. Other fully aromatic polyester fiber composites have been iatroduced under various tradenames (19). 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

More definitive evidence of enzymatic attack was obtained with 1 1 copolymers of e-caprolactone and 6-valerolactone crosslinked with varying amounts of a dilactone (98,99). The use of a 1 1 mixture of comonomers suppressed crystallization and, together with the crosslinks, resulted in a low-modulus elastomer. Under in vitro conditions, random hydrolytic chain cleavage, measured by the change in tensile properties, occurred throughout the bulk of the samples at a rate comparable to that experienced by the other polyesters no weight loss was observed. However, when these elastomers were implanted in rabbits, the bulk hydrolytic process was accompanied by very rapid surface erosion. Weight loss was continuous, confined to the... 

For both EPDM-LDH and XNBR-LDH nanocomposites, the various tensile properties are summarized in Table 13 and their typical stress-strain plots are shown in Fig. 58 [104]. In Fig. 58a, the gum vulcanizates of both rubber systems showed typical NR-like stress-strain behavior with a sharp upturn in the stress-strain plot after an apparent plateau region, indicating strain-induced crystallization. With the addition of LDH-C10 in the XNBR matrix, the stress value at all strains increased significantly, indicating that the matrix undergoes further curing (Fig. 58b). 

There have been many related works by other research groups that are concerned with the phase behavior, isothermal crystallization kinetics, and tensile properties of cellulose propionate (DS = 2.75)/PHB blends [129], the miscibility and crystallization behavior of CAB/PHB blends [ 130], or the melt... 

The prevention of further deterioration requires arresting the chain scission reaction caused by acid or enzymatic hydrolysis of the glycosidic linkages. The reduction in DP through chain scission has a dual character to its negative effect upon properties. In addition to the inherent reduction in the tensile properties of the fibers, the lower DP enhances the opportunity for crystallization with resulting embrittlement. 

Commerically available samples of biaxially oriented polystyrene and SMA copolymer sheet material, having a thickness of 0.0381cm, were used in this investigation. It is generally recognized that crystallization under stress can enhance the tensile properties of a semi-crystalline polymer through a special arrangement of the crystalline portion ( 23). Therefore, the physical properties of the styrene-maleic anhydride copolymers chosen were compared to those of polystyrene produced in the same manner and are shown in Table I (24). 

In terms of nanocomposite reinforcement of thermoplastic starch polymers there has been many exciting new developments. Dufresne [62] and Angles [63] highlight work on the use of microcrystalline whiskers of starch and cellulose as reinforcement in thermoplastic starch polymer and synthetic polymer nanocomposites. They find excellent enhancement of properties, probably due to transcrystallisation processes at the matrix/fibre interface. McGlashan [64] examine the use of nanoscale montmorillonite into thermoplastic starch/polyester blends and find excellent improvements in film blowability and tensile properties. Perhaps surprisingly McGlashan [64] also found an improvement in the clarity of the thermoplastic starch based blown films with nanocomposite addition which was attributed to disruption of large crystals. 

A novel technique of directional devitrification from a molten zone has been developed to produce silicate O-CaCSiOs]) rods, crystallized in a truly aligned and fibrous habit, which have far superior tensile properties to those prepared by conventional methods. The technique is probably applicable to any silicate (or other system) which crystallizes in a sheet or chain structure. 

Several authors have analyzed the miscibility of iPP and PB-1, by means of different analytical approaches. Piloz et al. (16) found a single, composition-dependent, glass transition behavior for these blends, and concluded that they are compatible in the amorphous state. Sjegmann (17,18) reported that the composition dependence of tensile properties evidences a high degree of compatibility of iPP and PB-1 and observed a marked effect of the composition on the morphology of melt-crystallized samples. Conversely, the analysis of the crystallized blends indicates the presence of separated crystal phases of the two polymers, even if a mutual influence during the crystallization cannot be excluded. 

The miscibility of olefin copolymers such as ethylene-a-olefin copolymers was found to be controlled by the structural composition and the primary strucmre of the copolymers. Using these copolymers, binary blends with various compatibilities were prepared and the effects of compatibihty on mechanical properties in the binary blends were investigated. The tensile properties in binary blends of iPP with rubbery olefin copolymers are considerably influenced by the miscibility between iPP and the copolymers. The miscibility of iPP with other polyolefins is described in detail based on the dynamic mechanical properties, morphology observation, and solidification process. It is found that EBR, EHR, and EOR having more than 50 mol% of a-olefin are miscible with iPP in the molten state. In the solid state, the miscible copolymers are dissolved in the amorphous region of iPP, although the copolymers are excluded from crystalhne lattice of iPP. The isotactic propylene sequence in the EP copolymers with a propylene-unit content of more than 84 mol% participates in the crystallization process of iPP, resulting that a part of the EP copolymers is included in the crystalline lattice of iPP. 

We present these data on PE crystals as being qualitatively representative of the tensile properties of such crystals but recognize that the quantitative information must be regarded as preliminary. Not only must more accurate knowledge of the specimen cross-sectional area be obtained, but we must make certain the crystal surface is not contaminated with residue suflScient to affect the mechanical properties. 

These observations are illustrated in Figure 3c, showing the average tensile properties for four crystals of arbitrary orientation across an original 2-/i.m gage length. 

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

The effects of SCB and side groups are similar. They disrupt the ability of the polymer to crystallize. If the disruption is not complete, the added bulkiness will make the rate of crystallization slow down. SCB has little effect on the flow properties of a polymer, but LCB has a profound effect. We will discuss this more when we look at the differences in behavior between HDPE, LDPE, and LLDPE in Chapter 4. For now, we can illustrate the effects of branching by comparing LDPE with HDPE. The densities differ, the tensile properties differ, and the elastic character of the polymers differs greatly, even though both are made from the same monomer. 

G. P. Karayannidis, N. Papachristos, D. N. Bikiaris, and G. Z. Papageor-giou. Synthesis, crystallization and tensile properties of poly(ethylene tere-phthalate-co-2,6-naphthalate)s with low naphthalate units content. Polymer, 44(26) 7801-7808, December 2003. 

Samples were compression molded and slow cooled in air tensile properties were determined at a draw rate of 25.4mmmin percent crystallinity was determined by DSC at a heating rate of 10°Cmin crystal phase structures were determined by Raman internal mode technique (LAM) a, aj,/ nd Oj refer to fraction of chain units in the perfect crystals, interfacial region, and amorphous region, respectively of a lamella. 

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