Effect of molecular weight

Viscoelastic properties of polymer melts can be characterized by zero-shear viscosity, r]o. This parameter can be obtained from dynamic experiments by determining the dynamic moduli at the limit of low frequency. Table 6.3 shows the zero-shear viscosity, rjo, and also the elastic coefficient, Aq (the ratio of the applied stress to the change in shape of an elastic body), for all samples at 200°C. 

The empirical equation relating viscosity at zero-shear rate (t]o) to molecular weight for linear amorphous polymers is used to compare PLLA melts with those of conventional polymers. The form of this equation has been applied to the elasticity coefficient, Aq, to further quantify the effect of molecular weight on the elasticity (Cooper-White and Mackay 1999)  

Witkze showed that the temperature effect on rjo for 15% D-lactide PLA can be described by (Witzke 1997)  

C2 = 110 11°C ai, 2, U3, and Ci do not have units and T C) is the testing temperature in °C. Equation (6.6) can be used to predict r]o of amorphous polylactides with L-monomer composition higher than 50% between Tg and Tg -I- 100°C. The equation predicts that rjo increases with increasing L-monomer and reduces as /weso-lactide content becomes higher. 

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T is the glass-transition temperature at infinite molecular weight and is the number average molecular weight. The value of k for poly(methyl methacrylate) is about 2 x 10 the value for acrylate polymers is approximately the same (9). A detailed discussion on the effect of molecular weight on the properties of a polymer may be found in Reference 17. 

Table 2. Effects of Molecular Weight and Branching on Mechanical Properties of Polyethylenes...

Fig. 1. Effect of molecular weight and hydrolysis on the properties of poly(vinyl alcohol) (8).

Figure 8.11. Effect of molecular weight on critical shear stress at onset of elastic turbulence in poly(methyl methacrylate). (After Howells and Benbow )...

Table 10.2 Effect of molecular weight and density (branching) on some mechanical and thermal properties of polyethylene...

Only a limited amount of information is available concerning the effects of molecular weight distribution. There is, however, evidence that the narrower the distribution the more Newtonian are the melt flow properties. It has been noted... 

All of the observations above relate well to our own with respect to the effects of molecular weight on the available cure speed in manufacturing, both in plywood and OSB. They also correlate with our experience on the effects of molecular weight and dry-out on bond quality. 

As with resoles, the central issue in design of novolacs is molecular weight. The effects of formaldehyde-to-phenol molar ratio and formaldehyde conversion on molecular weight of novolacs has been well studied and reported [192,193]. The effects of molecular weight on most of the important properties are also available [193]. These include Tg, melt viscosity, gel time, hot-plate flow, glass-plate flow. 

VI. EFFECT OF MOLECULAR WEIGHT AND MOLECULAR WEIGHT DISTRIBUTION... 

Vill. EFFECT OF MOLECULAR WEIGHT ON THE MELTING-CRYSTALLIZATION OF PE UNDER HIGH PRESSURE... 

According to the concepts, given in the paper [7], a significant difference between the values of yield stress of equiconcentrated dispersions of mono- and polydisperse polymers and the effect of molecular weight of monodisperse polymers on the value of yield stress is connected with the specific adsorption on the surface of filler particles of shorter molecules, so that for polydisperse polymers (irrespective of their average molecular weight) this is the layer of the same molecules. At the same time, upon a transition to a number of monodisperse polymers, properties of the adsorption layer become different. 

In comparing observed reactivity ratios between various polymerization systems, it is important to take into account the possible effect of molecular weight on copolymer composition.3475 19 In conventional radical eopolymeri/.ation, the specificity shown in the initiation and termination steps can have a significant effect on the composition of low molecular weight copolymers (usually <10 units). These effects are discussed in Section 7.4.5. In a living polymerization molecular weights are low at low conversion and increase with conversion. In these... 

Saxena A., Sachin K, Bohidar H.B., Verma A.K. 2005. Effect of molecular weight heterogeneity on drug encapsulation efficiency of gelatin nano-particles. Colloids and Surfaces B Biointerfaces 45,42-48. 

Wilson, A. D., Crisp, S. Abel, G. (1977). Characterization of glass-ionomer cements. 4. Effect of molecular weight on physical properties. Journal of... 

Recently, we explored the effect of molecular weight on the pattern and employed post-dewetting processes to alter the shape of the dewetted polymer droplets. Since the viscosity of a polymer solution is nonlinear with respect to concentration and also strongly dependent on polymer weight, we expected a drastic effect. Figure 11.4... 

Hong, B. K. and Jo, W. H. (2000) Effects of molecular weight of SEBS triblock copolymer on the morphology, impact strength, and rheological property of syndiotactic polystyrene/ ethylene-propylene rubber blends. Polymer, 41, 2069-2079. 

Stafford, C.M., Russell, T.P, and McCarthy T.J., Expansion of polystyrene using supercritical carbon dioxide, effects of molecular weight, polydispersity, and low molecular weight components, Macromolecules, 32, 7610, 1999. 

Figure 3. Effects of molecular weight on labelling efficiency. Three heparins were independently labelled at increasing degrees of substitution. Molecular weight ranges of 3,000 (open square), 4,000-6,000 (solid circle), and 12,000-15,000 (open diamond).

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