Siloxane polymers prediction

The ring-opening polymerization of is controUed by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cycHzation equihbrium constants of dimethyl siloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cycHc is approximately equal to the equihbrium concentration of this cycHc, [(SiR O) Thus the total... 

The study of molecular dynamics in polymers is of crucial importance in the understanding of processes in which diffusion is involved. Examples of such processes are diffusion-controlled chemical reactions, sorption-extraction, dissolution, interactions across polymer-polymer interfaces, etc. This has been a very active area of research, especially during the past few years, due to new theoretical proaches and the development and refinement of experimental techniques. Different polymer systems have been used to test the theoretical predictions based on scaling methods. One of these systems is poly(dimethylsiloxane) (PDMS). Other factors have also contributed to the interest in studying the molecular dynamics of PDMS. " Siloxane polymers have widespread commercial applications,... 

R. Keshavaraj, R. W. Tock, R. S. Narayan, and R. A. Bartsch, Fluid Property Prediction of Siloxanes with the Aid of Artificial Neural Nets, Polymer-Plastics Technology and Engineering, i5(6) 971-982 ( 996). 

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

The molar cyclization equilibrium constants, Kx, of PDMS are measured. Using the Jacobson and Stockmayer equilibrium theory of macrocyclization, the dimensions of PDMS chains with 40-80 chemical bonds in the bulk polymer at 383 K are deduced. Dilution effects in the PDMS systems are contrasted with predictions of the Jacobson-Stockmayer theory, and the experimental molar cyclization equilibrium constants of the smallest siloxane rings are discussed in terms of the statistical properties of the corresponding oligomeric chains using tire RIS model of PDMS of Flory, Crescemi, and Mark [S 116]. 

Observed Tg s vary from -123°C for polyfdimelhyl siloxane) (1-43) to 273°C for polyhydantoin (11-2) polymers used as wire enamels and to even higher temperatures for other polymers in which the main chain consists largely of aromatic structures. This range of behavior can be rationalized, and the effects of polymer structure on Tg can be predicted qualitatively. Since the glass-to-rubber transition... 

By contrast, the corresponding values for the large cyclics decrease along the same series. The total weight fraction (experimental) of cyclics in these bulk equilibrates are listed in Table 4. The effect of dilution with cyclohexanone in the syntheses of these same polymers is illustrated in Figure 3. In all cases, a critical dilution point was reached as predicted but it was reached at a lower diluent volume than the theoretical value. Molar cyclization equilibrium constants for undiluted poly(methylphenyl)siloxane were later measured (19) and found to be similar to those previously obtained in the case of poly(ethylmethyl)siloxanes. 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

Non-polymerizable 1,3-dioxane non-polymerizable Non-polymerizable Non-polymerizable Formation of cyclic dimer and Siloxanes 5) tetramer was reported 31 . Hexamethyltrisiloxane 1,3,5-tnoxane formation 32 331 of Considerable cyclization. Distribu- cyclic oligo- and polymers report- tion Gf cycjjc oligomers as predicted ed no quantitative data. by the J-S theory... 

Figure 2.23. The reduced surface tension as a function of the reduced temperature for a number of polymers (A, poly(vinyl acetate) , polystyrene , polyisobutylene A, poly(dimethyl siloxane) , linear polyethylene o, branched polyethylene), compared with the prediction of a gradient theory using the Poser and Sanchez lattice fluid model and a square gradient term modified to account for loss of polymer configurational entropy near a surface (full line). After Sanchez (1992).

Venkatraman, S., Deformation-Behavior of Poly(dimethyl Siloxane) Networks 0.1. Applicability of Various Theories to Modulus Prediction. J.Appl. Polym. 

The strength of interactions is represented by the solubility parameter 5 in (cal/mL) / and p is the density in g/mL. This formula predicts surface tensions that are within 5-10% of measured values (3), based upon two experimental parameters that are readily available for a large number of polymers. In the case of poly(dimethyl siloxane), the group contribution prediction for the backbone, 20.9 mN/m, compares well with the value 20.4 0.07 mN/m estimated by extrapolation of experimental data to infinite molecular weight (7). 

Molecular Structure.—A review has appeared that compares the relationship between polymer structure and surface-active properties of the poly(dimethyl siloxane)s (PDMS) with that of hydrocarbon and fluorocarbon polymer systems." A mathematical study of the spreading of (PDMS) oil drops has been presented and experimental data shown to be in good agreement with predictions. Quantitative comparison of previously published n.m.r. spin-relaxation data for poly(diethyl iloxane)s with theoretical predictions for a variety of motional processes, have allowed both the nature and time scale of molecular motions to be identified."... 

The synthesis and characterization of poly(hexamethylene sebacate-tiimethyl-siloxane) block copolymers, prepared by coupling Cl- or McjNH-terminated dimethylsiloxanes with OH-terminated poly(hexamethylene sebacate), have been studied. The copolymers containing 19—90% siloxane were characterized by n.m.r., viscosity, DSC and CPC. All the polymers were found to be crystalline. The copolymers containing <69% siloxane, when cast from solution or melts, indicated a reduction in spherulite size as the siloxane concentration increased, although only a small m.pt. depression was observed. All the copolymers have critical surface tensions similar to dimethylsiloxane homopolymers. Polycarbonate-Siloxane Copolymers.—A model has been proposed to predict the micromorphology and mechanical properties of block copolymers of bisphenol-A polycarbonate and poly(dimethylsiloxane). N.m.r. data upon 65 35 (wt%) copolymer of poly(dimethylsiloxane) and bisphenol-A polycarbonate, with block lengths of 20—100 monomer units, were found to be in agreement with the predictions of a spin-diffusion model. ... 

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