Crystallinity and properties

Another type of geometric arrangement arises with polymers that have a double bond between carbon atoms. Double bonds restrict the rotation of the carbon atoms about the backbone axis. These polymers are sometimes referred to as geometric isomers. The X-groups may be on the same side (cis-) or on opposite sides (trans-) of the chain as schematically shown for polybutadiene in Fig. 1.12. The arrangement in a cis-1,4-polybutadiene results in a very elastic rubbery material, whereas the structure of the trans-1,4-polybutadiene results in a leathery and tough material. Branching of the polymer chains also influences the final structure, crystallinity and properties of the polymeric material. 

Polyvinylidene fluoride is a semicrystalline polymer (35-70% crystallinity) with an extended zigzag chain.f Head-to-tail addition of VDF dominates, but there are head-to-head or tail-to-tail defects that affect crystallinity and properties of PVDF. It has a number of transitions, and its density alters for each polymorph state. There are four known states, named as a, (3, y, and 8, and a proposed state. The most common phase is a-PVDF, which exhibits transitions at -70°C (y), -38°C (p), 50°C (ot"), and 100°C (a ). 

Bigg, D. M. (1996). Effect of copolymer ratio on the crystallinity and properties of polylactic acid copotymers, SPE ANTEC Technical Papers, pp. 2028-2039. 

The key step in applying Eq. 30 is to measure Xcr as a function of t. This can be obtained from the correlation between

suspended particles and of polymer networks [ 12,64]. According to this relation, the volume fraction of fibrous networks can be correlated to the specific viscosity rjgp as... 

A detailed investigation has been performed by Luo and Suib on redox reactions, various synthetic parameters and their effects on the crystallinity, and properties of resultant birnessite and buserite materials [24]. The use of redox methods to synthesize birnessite consists of three stages an induction period, a fast crystallization period, and a slow crystallization period. The lengths of each of these periods strongly depend on temperature. The increments of temperature can shorten the time of crystallization. However, high temperature can decrease the ion exchange capability of the resulting birnessite. Increments of basicity have effects that are similar to but less important than those of increased temperature. 

Richards, R. B. (1951). Polyethylene - structure, crystallinity and properties. Journal of Applied Chemistry, 7(8), 370-376. 

PVDF is a semicrystalhne polymer (35-70% crystallinity) with an extended zigzag chain [65,66,72]. Head-to-tail addition of VDF dominates, but there are head-to-head or tail-to-tail defects that affect the crystallinity and properties of PVDF. 

Figure 15.14 The influence of degree of crystallinity and molecular weight on the physical characteristics of polyethylene. (From R. B. Richards, Polyethylene—Structure, Crystallinity and Properties, /. Appl. Chem., 1, 370,1951.)...

Glass is the name given to any amorphous solid produced when a liquid solidifies. Glasses are non-crystalline and isotropic, i.e. their physical properties are independent of the direction in which they are measured. When a glass is heated, it does not melt at a fixed temperature but gradually softens until a liquid is obtained. 

This relationship is sketched in Fig. 4.7a, which emphasizes that P, must vary linearly with 6 and that P, ° must be available, at least by extrapolation. The heat of fusion is an example of a property of the crystalline phase that can be used this way. It could be difficult to show that the value of AH is constant per unit mass at all percentages of crystallinity and to obtain a value for AHj° for a crystal free from defects. Therefore, while conceptually simple, the actual utilization of Eq. (4.37) in precise work may not be easy. 

Figure 4.7 Various representations of the properties of a mixture of crystalline and amorphous polymer, (a) The monitored property is characteristic of the crystal and varies linearly with 0. (b) The monitored property is characteristic of the mixture and varies linearly with 0 between and P, . (c) X-ray intensity is measured with the sharp and broad peaks being P. and P., respectively.

Fiber stmcture is a dual or a balanced stmcture. Neither a completely amorphous stmcture nor a perfectly crystalline stmcture provides the balance of physical properties required in fibers. The formation and processing of fibers is designed to provide an optimal balance in terms of both stmcture and properties. Excellent discussions of the stmcture of fiber-forming polymers and general methods of the stmcture characterization are available (28—31). 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Commercial production of PVA fiber was thus started in Japan, at as early a period as that for nylon. However, compared with various other synthetic fibers which appeared after that period, the properties of which have continuously been improved, PVA fiber is not very well suited for clothing and interior uses because of its characteristic properties. The fiber, however, is widely used in the world because of unique features such as high affinity for water due to the —OH groups present in PVA, excellent mechanical properties because of high crystallinity, and high resistance to chemicals including alkah and natural conditions. 

To improve homogeneity, the preformed article is heated to 370—390°C. The time required for heating and sintering depends on the mold dimensions cooling, which affects the crystallinity and product properties, should be slow. 

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

The chemical and physical properties of limestone vary tremendously, owing to the nature and quantity of impurities present and the texture, ie, crystallinity and density. These same factors also exert a marked effect on the properties of the limes derived from the diverse stone types. In addition, calcination and hydration practices can profoundly influence the properties of lime. 

Mechanical Properties. The principal mechanical properties are Hsted in Table 1. The features of HDPE that have the strongest influence on its mechanical behavior are molecular weight, MWD, orientation, morphology, and the degree of branching, which determines resin crystallinity and density. 

Preparation and Properties of Organophosphines. AUphatic phosphines can be gases, volatile Hquids, or oils. Aromatic phosphines frequentiy are crystalline, although many are oils. Some physical properties are Hsted in Table 14. The most characteristic chemical properties of phosphines include their susceptabiUty to oxidation and their nucleophilicity. The most common derivatives of the phosphines include halophosphines, phosphine oxides, metal complexes of phosphines, and phosphonium salts. Phosphines are also raw materials in the preparation of derivatives, ie, derivatives of the isomers phosphinic acid, HP(OH)2, and phosphonous acid, H2P(=0)0H. 

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). 

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