Polymer molecular mass, changes

It is difficult, however, to derive accurate values of ccr and Mcr from experimental data, because the change of slope in a log rj - log M curve or a log rj - log c curve is very gradual. An additional difficulty is that the concentration is expressed in different units in different articles. Moreover, the values mentioned for the polymer molecular mass are not completely comparable. 

Decrease of polymer molecular mass takes place during the process of destruction. In the work [195] there has been shown linear behaviour of the dependence of lg(da/dt) on t (time), which speaks about the fact that the quantity of broken bonds a at any moment of time is defined by the equation a=l-e hence, decrease of molecular mass at PETP destruction happens as a results of bonds decay according to the law of randomness. This conclusion is proved by PETP destruction in nitrogen flow at 28O C which does not lead to the change of the character of molecular - mass distribution (MMD) [196]. 

This book, with chapters by the editors and other experts in the field of polymer science, covers a broad selection of important research advances in the field, inclnding an update on photoelectric characteristics, a study on the changes in the polymer molecular mass during Itydrolysis, an update on enzymatic destraction, a study on a new type of bioadditive for motor fuel, an exploration of the interrelation of viscoelastic and electromagnetic properties of densely cross-linked polymers, and much more. 

Changes in the Polymer Molecular Mass During Hydrolysis... 

Additionally, online monitoring methods have been developed to adapt off-line characterization methods into in situ (i.e., in-reactor) probes for determination of kinetics and monomer conversion with optical methods such as mass spectroscopy (MS), ESR, FTIR, near IR, and Raman spectroscopy. However, frequently, due to high turbidity and viscosity of the polymer reaction milieu, the optical surfaces are easily fouled, leading to frequent sensor failure. Furthermore, data acquired with these probes are model dependent the empirical and inferential calibration schemes used can be expensive and time consuming to develop and can drift and become unreliable as reactor conditions change and as sensors become fouled. Another limiting feature of these methods is that they usually measure only one characteristic of the reaction, such as monomer conversion and are not directly sensitive to polymer molecular mass and intrinsic viscosity. More detailed discussion of these techniques can be found in Chapters 6-10 of this book. 

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

E. F. Vainstein, A. A. Sokolovskii, and A. S. Kuzminskii, Kinetics of the Changing Products Molecular-Mass Distribution in Thermodegradation of Associated Polymers, Polymer Yearbook, (R. A. Pethrick and G. E. Zaikov, eds.) Gordon and Breach, London, vol. 9, pp. 79-101 (1993). 

As suggested by Barrett (2), it is assumed that following the particle nucleation stage, the polymerization proceeds in the particle (monomer/polymer) phase with no mass transfer limitation. Therefore, the dispersion polymerization is similar to a mass or suspension polymerization, and kj can not be assumed to be constant even at isothermal conditions, since kp and even kp are dependent on the degree of polymerization because of a gel effect. (2., ,D However, since the application of the model is for a finishing step, with polymer molecular weight and viscosity fairly well established, further changes in kp and kp should be minimal. 

The use of polymeric coatings in catalysis is mainly restricted to the physical and sometimes chemical immobilization of molecular catalysts into the bulk polymer [166, 167]. The catalytic efficiency is often impaired by the local reorganization of polymer attached catalytic sites or the swelling/shrinking of the entire polymer matrix. This results in problems of restricted mass transport and consequently low efficiency of the polymer-supported catalysts. An alternative could be a defined polymer coating on a solid substrate with equally accessible catalytic sites attached to the polymer (side chain) and uniform behavior of the polymer layer upon changes in the environment, such as polymer brushes. 

Changes in the specific properties of the polymer can also be taken as a hint of some form of degradation. Properties examined include different aspects of physical behaviour of the polymer, different microscopic images, or changes in simple parameters such as the total weight of the polymer in the test or an altered molecular mass of the polymer under evaluation. 

The model simulates an experimentally observed trend (25) that the solubility of chains in a SCF shows a strong inverse dependence on the molecular mass of the polymer. Figure 5 shows that changing the molecular weight of the chain molecule from 100 to 700 causes a reduction in solubility of nearly 6 orders of magnitude. The model also shows that all the solubility plots tend to flatten out around 300 bar, as observed in experiments (25). Classical EOS like a modified cubic EOS (2JD > when applied to such systems, produce solubility curves which tend to show a sharp maximum around 200 bar. For polymer-SCF systems, therefore, the lattice EOS is believed to be superior to modified cubic EOS. 

Like polymer melts, concentrated polymer solutions show the phenomenon of a critical molecular mass. This means that in a plot of log tj against log M the slope of the curve changes drastically if M exceeds a critical value Mcr. For polymer solutions, Mcr increases with decreasing polymer concentration. A similar phenomenon is observed if log tj is plotted against log c for a constant value of M the slope of the curve changes at c = ccr. 

---