Polymers semicrystalline state

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

Hence polysaccharides have been viewed as a potential renewable source of nanosized reinforcement. Being naturally found in a semicrystalline state, aqueous acids can be employed to hydrolyze the amorphous sections of the polymer. As a result the crystalline sections of these polysaccharides are released, resulting in individual monocrystalline nanoparticles [13]. The concept of reinforced polymer materials with polysaccharide nanofillers has known rapid advances leading to development of a new class of materials called Bionanocomposites, which successfully integrates the two concepts of biocomposites and nanometer sized materials. The first part of the chapter deals with the synthesis of polysaccharide nanoparticles and their performance as reinforcing agents in bionanocomposites. 

The present discussion of physical structure and properties is intended to serve merely as a basis for appraising the characteristics of various polymers here surveyed. The nature of the semicrystalline state in polymers and its influence on their physical properties will be dealt with in greater detail in a later chapter. 

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

The second effect, the well-known odd-even dependence, may be explained in terms of different degrees of order for polymers having odd or even numbers of methylene units in the spacer. The overall order in the semicrystalline state and in the mesophase is larger in the even-numbered samples than in the odd-numbered. 

Another use of Raman spectroscopy for quantitative analysis is the determination of percent crystallinity in polymers. Both the frequency and intensity of peaks can shift on going from the amorphous to the semicrystalline state for polymers. The percent crystallinity can be calculated with the help of chemometrics software. 

PBT is one of the faster crystallizing polymers. The high crystallization rate is a consequence of the considerable mobility that is provided by the butylene imit in the chain. Pure PBT has a crystalhnity in the range of 30-40%. The enthalpy of fusion, AHf°, of the 100% crystalline PBT is 142 J/g. The melting temperature, T, is about 225°C. The crystallization temperatirre, T, is approximately 180°C. The glass transition temperature, T, of the semicrystalline state is about 40 C. ... 

In a semicrystalline material the chains are constrained because of the crystallization process. The constraints give the material different properties from those of a purely amorphous rubbery polymer. The states of stress and strain... 

Note The quantity a represents the volume coefficient of expansion. The linear coefficients of expansion are approximately 1/3 of the volume coefficients of expansion. The quantities Ug (determined at 20°C) and Or represent the glassy and rubbery states, respectively, the former being a typical value for the polymer (semicrystalline for polyethylene and polytetrafluoroethylene) while the latter is calculated for the 100% amorphous polymer. However, the quantities Ur - g and Or - (Xo)Tg are calculated on the basis of 100% amorphous polymer. 

Crist B (2013) Chapter 3 structure of polycrystalline aggregates. In Piorkowska E, Rutledge G (eds) Handbook of polymer crystallization. Wiley, Hoboken de Rosa C, Auriemma F (2013) Crystals and crystallinity in polymers diffraction analysis of ordered and disordered crystals. Wiley, Hoboken Dettenmaier M, Fischer EW, Stamm M (1980) Calculation of small-angle neutrrai scattering by macromolecules in the semicrystalline state. Colloid Polym Sci 258(3) 343-349 Donald AM, Windle AH, Hanna S (2006) Liquid crystal polymers, 2nd edn. Cambridge University Press, Cambridge... 

Although the rotational molding process is carried out with very little shear on the polymer melt, the cooling rate of the polyethylene after the fabrication step remains important in order to avoid excessive shrinkage and warpage. This is due to the crystallization process that takes place as the polymer melt cools from an amorphous, less dense state to a solid, semicrystalline state, higher density solid. 

The large strain response in the glassy or semicrystalline state is that of a nonlinear viscoelastic solid. However, both engineering and theoretical approaches to plasticity in polymers have largely developed as an independent discipline, in which (Ty plays a central role, in spite of its somewhat arbitrary definition (indeed it is not always possible to associate cty with a maximum in the force-deformation curve [5]). This is because in practice the yield point, rather than the ultimate strength, is usually considered to be the failure criterion for ductile materials. 

The parameter % in the Eq. (4.9) characterizes a polymer fraction, which does not participated in plastic deformation process, but subjects to elastic deformation. For semicrystalline pol5mier this fiaction consists of devitrificated amorphous phase and crystalline phase part, which was subjected to partial mechanical disordering [4]. In other words, the parameter % characterizes he deformed polymer structural state. For the considered in Refs. [2, 3] HDPE crystallinity degree K= 0.687 and, hence, amorphous phase fraction (p, makes up 1 - = 0.313. As estimations according to the Eq. (4.9),... 

Further cooling of the smectic phase may lead to crystallization or perhaps to the formation of a smectic glass. The semicrystalline state of the rigid-rod polymers is different from that of the flexible-chain polymers described in Chapter 7. The low segmental flexibility of the former prevents chain folding and the crystals should be of the fringed micelle type. 

Semicrystalline states of polymers mean polymers partially in both the amorphous and the crystalline states. Polymer... 

In the amorphous state of homogeneous polymer solutions or melt, polymer chains are fully disordered, as described by the random-coil model. The random-coil model was first proposed by Kuhn [1] as well as by Guth and Mark [2] to predict the entropic elasticity of polymer chains, and then was used to describe the amorphous state of polymers by Flory [3]. Polymer crystallization commonly chooses a pathway favoring its kinetics, which results in metastable semicrystalline states [4]. It takes a long time for people to figure out the structural... 

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