Effects of Crystallinity

If the polymer is capable of crystallization, the free volume will be obeyed only by the amorphous regions the crystalline regions will have very little free volume at temperatures below the melting point (7 ). 

Most photochemical reactions in the crystalline state cannot occur due to [874]  

the lack of free volume imposed by the rigidity of the crystal lattice. 

Unless the photon energy is sufficient to cause local melting of the lattice to provide the necessary free volume, it seems unlikely that photochemical processes, particularly of the type requiring large rearrangements of molecular structure, can occur efficiently in crystalline regions of a polymer. 

The glass transition temperature is difficult to detect when the crystallinity of the sample increases. The deflection temperature at the baseline cannot be defined because the mobility of the molecules in the amorphous region is restricted by the presence of the crystalline region. In order to determine T, other TA methods, such as thermomechanometry, should be used. 

When a crystalline polymeric sample is measured, the variation in depends on the higher order structure. In some cases, increases with increasing crystallinity, whereas in other cases it remains constant or decreases. 

A large number of polymers are semi-crystalline, i.e. they contain an amorphous and a crystalline fraction. The presence of crystallites may strongly influence membrane performance with regard to both the transport of gases and liquids. If the diffusion takes place primarily in the amorphous regions and if the crystallites are considered to be impermeable, the amount of crystallinity directly influences the diffusion rate and hence the flux. 

The diffusion coefficient can be described as a function of the crystallinity in the following manner [40]  

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Studies have considered the effect of crystallinity on the performance of CR adhesives (97), on segmental mobiUty as determined by nqr studies (101), on strain induced property changes (102), and on relaxation processes (103). 

It has proved difficult to decide which of these two theories of polymer crystallisation is correct, since both are consistent with the observed effects of crystallinity in polymers. These effects include increased density, increased stiffness, and higher softening point. However, the balance of opinion among those working with crystalline polymers favours the latter theory, based on lamellae formed by the folding of single molecules. 

The effect of Crystallinity on the time-scale plot is rather complex and not readily sketched. If the crystallites were thermally stable, the isothermal... 

Figure 7 Effect of crystallinity on the modulus- temperature curve. The numbers on the curves are rough approximations of the percent of crystallinity. Modulus is given in dyn/cnr.

VII. Effects of Crystallinity and Morphology Vin. Effects of Plasticizers and Copolymerization... 

When describing erosion of and drug release from surface erodible polymers, it is often implicitly assumed that the matrix erodes uniformly, thus resulting in a uniform release profile for a homogenously dispersed drug. While this may be a valid assumption for some homopolymer systems, neglecting the effects of crystallinity, some multicomponent... 

Effect of Crystallinity and Additives on the Thermal Degradation of Cellulose... 

The objective of this research was to examine the effect of crystallinity, additives and data analysis technique on isothermally pyrolyzed cellulose. The Ea, activation enthalpy (AH+) and activation entropy (AS+) were determined from the mass loss rates. This data was used to develop an understanding of how cellulose pyrolysis is affected by crystallinity and additives and how the results obtained are dependent on the data analysis technique. 

Figure 5.9 Effect of crystallinity on the solid-state polycondensation of PET, shown as the number-average molecular weight as a function of time. Conditions fluidized bed polymerization at 230°C particle size, 35-48 mesh superficial velocity of nitrogen, 43cm/s [6]. From Chang, T. M., Polym. Eng. Sci., 10, 364 (1970), and reproduced with permission of the Society of Plastics Engineers...

Figure 11.8 Effect of crystallinity on the PTT glass transition temperature...

Mizuno, A., Mitsuiki, M., and Motoki, M. (1998). Effect of crystallinity on the glass transition temperature of starch. /. Agric. Food Chem. 46, 98-103. 

Fig. 23. Energy level scheme of a single 3d electron showing the effect of crystalline fields (CF) of various symmetry. Electron occupation of levels is indicated by a circle in (d) and by arrows in (e) to denote spin polarization.

Regardless of the precise picture of order and disorder in polymers, the prime consideration that should be emphasized is that polymers have a tendency to crystallize. The extent of this crystallization tendency plays a most significant role in the practical ways in which polymers are used. This is a consequence of the large effect of crystallinity on the thermal, mechanical,... 

It is quite natural that the thermodynamic approach does not allow photocorrosion processes to be described comprehensively. In a number of cases, kinetic peculiarities of reactions play an important role (see, for example, Bard and Wrighton, 1977) these peculiarities are caused by the effect of crystalline structure, state of the semiconductor surface, etc. A detailed description of a complicated reaction with several particles in the solution and crystal lattice involved usually encounters considerable difficulties. Therefore, at this stage the kinetic approach is used to reveal purely qualitative regularities of corrosion processes. 

Other Properties. Delay et al (Ref 100) and Mohler (Ref 98) obtd infrared absorption spectra of LA and other expls in the range 3 to 19 p Kahovec Hohlrausch (Ref 109) detd the Raman Effect of crystalline LA. 

Effects of Crystallinity on Properties. In polymers that can crystallize, the ratio of crystalline to amorphous material has a profound effect on properties. Because the chains arc packed more tightly and efficiently in the crystalline areas Ilian in the amorphous, the cryslalline phase lias a higher density and greater mechanical strength. In fact, density is a common measure of degree of crystallinity,... 

The Effect of Crystallinity and Chain Orientation on the Glass Transition. 258... 

Sperati, C. A., and J. L. McPherson The effect of crystallinity and molecular weight on physical properties of polytetrafluoroethylene. Meeting of the American Chemical Society, Atlantiv City, September 1956. 

Effects of crystalline nicotinic acid-induced hepatic dysfunction on serum low-density lipoprotein cholesterol and lecithin cholesteryl acyl transferase. Am J Cardiol 1998 81(6) 805-7. 

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