Crystallinity and modulus

Iizuka, E. (1965). Degree of crystallinity and modulus relationships of silk thread from cocoons of Bombyx Mori and other moths. Biorheology 3, 1-8. 

Linear Low-Density Polyethylene (LLDPE). Ziegler and metallocene polyethylene can be modified by copolymerization with several percent (e.g., 10%) of butene, hexene, or octene, to reduce regularity/crystallinity and modulus, and thus produce... 

When plastic is dissolved in organic solvents, it can be viewed as individual molecules (chemical structure and molecular weight), instead of regarding it as a polymer aggregate. Thus, physical properties of the plastic such as crystallinity and modulus of elasticity would no longer affect its biodegradability [52, 53]. However, in practice, selection of suitable enzymes with solvent mixtures (i.e., solvents with different polarities) is necessary to avoid inactivation of enzyme. 

In general, the hydrolysis rate of solid plastic, PLLA, is affected by factors associated with higher order structure (crystallinity and modulus of elasticity) and those associated with surface conditions (surface area, hydrophilic, and hydrophobic properties). It is well known that the crystalline... 

The number of chain scissions increases when the processing is carried out in humid conditions and indeed the rise of crystallinity and modulus is lower for the samples processed in dry state. Similar results have been found for other semicrystalline polymers, like polypropylene [6], while this does not occur for amorphous polymers like polycarbonate [7]. 

Wide angle X-ray diffraction (WAXD) of multi walled carbon nanotube (MWCNTs). Reinforced polyethylene composites showed that the MWCNTs are very well distributed and dispersed in the PE matrix [61], There is a broadening and reduction in intensity of the 110 and 200 PE rejections with increasing MWCNT concentration, indicative of altered amorphous and crystalline phases. XRD results of calcium carbonate and PE composites were studied and the results showed that the adoption of calcium carbonate in polyethylene has two primary effects the reinforcement and the nucleating effect. The reinforcement effect increases the bulk crystallinity and modulus, while the nucleating effect decreases the spher-ulite size. 

Annealing of the neat PLA sample increases the crystallinity and modulus. 

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

Content of Ot-Olefin. An increase in the a-olefin content of a copolymer results in a decrease of both crystallinity and density, accompanied by a significant reduction of the polymer mechanical modulus (stiffness). Eor example, the modulus values of ethylene—1-butene copolymers with a nonuniform compositional distribution decrease as shown in Table 2 (6). A similar dependence exists for ethylene—1-octene copolymers with uniform branching distribution (7), even though all such materials are, in general, much more elastic (see Table 2). An increase in the a-olefin content in the copolymers also results in a decrease of their tensile strength but a small increase in the elongation at break (8). These two dependencies, however, are not as pronounced as that for the resin modulus. 

In cases where the copolymers have substantially lower glass-transition temperatures, the modulus decreases with increasing comonomer content. This results from a drop in crystallinity and in glass-transition temperature. The loss in modulus in these systems is therefore accompanied by an improvement in low temperature performance. However, at low acrylate levels (< 10 wt %), T increases with comonomer content. The brittle points in this range may therefore be higher than that of PVDC. 

Molecular movement above the Tg is restricted by crystallinity and, as with chemical cross-linking, the more the crystallinity, the more rigid the polymer. Some polymers tend to melt over a wide temperature range, in which case the modulus may fall over a range of temperatures leading up to the melting point T . The above effects are summarised in Figure 9.1. 

In the case of crystalline polymers such as types E and F the situation is somewhat more complicated. There is some change in modulus around the which decreases with increasing crystallinity and a catastrophic change around the. Furthermore there are many polymers that soften progressively between the Tg and the due to the wide melting range of the crystalline structures, and the value determined for the softening point can depend very considerably on the test method used. 

Important physical properties include the density, melt flow index, crystallinity, and average molecular weight. Mechanical properties of a polymer, such as modulus (the ratio of stress to strain), elasticity, and breaking strength, essentially follow from the physical properties. 

With DMA the effect of temperature on the modulus can be studied. By increasing the temperature from -150 to 300°C, one encounters several transitions in PA (Fig. 3.1). There is a transition at about —120°C, the y-transition, which is due to the mobilization of methylene units. There is also a transition at —30°C, which is present in wetted aliphatic PA this is due to non-H-bonded amide units and is termed the /J-transition. At about 50°C the glass Uansition (Tg) (a-transition) of the aliphatic polyamides PA-6 and PA-6,6 occurs. At this Uansition, the modulus is lowered considerably. For partially aromatic PA, the Tg occurs above 100°C. The last transition is the flow temperature, at which temperature the material melts the flow temperature and the melt temperature, as measured by DSC, correspond well. The modulus is a measure of dimensional stability and increases with crystallinity and filler content (Fig. 3.12). 

Describe the seven different classes of polyethylene and explain how the structural differences affect the tensile strength, crystallinity and flexural modulus of each of the materials. 

Annealing can reduce the creep of crystalline polymers in the same manner as for glassy polymers (89,94,102). For example, the properties of a quenched specimen of low-density polyethylene will still be changing a month after it is made. The creep decreases with time, while the density and modulus increase with time of aging at room temperature. However, for crystalline polymers such as polyethylene and polypropylene, both the annealing temperature and the test temperatures are generally between... 

In the above, the variable R is the radius between center to center fiber spacing, while r is the fiber radius. The shear modulus (Gm) can be approximated as Em/3. The matrix modulus is effected by the level of crystallinity and it is important that the samples are fully crystallized to ensure reproducibility. The value of (> for 30wt% glass-fiber-reinforced PET has been calculated as 3.15 x 104. Using the mathematical analysis shown above, the orientation function of the glass fiber... 

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