Dilute solution properties of asymmetric six-arm star polystyrenes

Six-arm star polystyrenes having varying numbers of short and long arms attached to the same molecule have been synthesized by anionic polymerization. The molecules have been characterized by high resolution size exclusion chromatography using multiangle light scattering and viscosity detectors. This technique has allowed the radii of gyration and intrinsic viscosities to be measured for stars with each possible combination of arms. The branching parameters g and g′ are computed and compared with theoretical expectations. It is found that short arms add preferentially to the stars, because of reduced steric effects. The molecule with one long and five short arms exhibits behavior closest to that of a linear chain (largest branching ratios). The effect of arm polydispersity on solution properties of stars is discussed. © 1995 John Wiley & Sons, Inc.     

Synthesis and dilute solution properties of divinylbenzene-linked polystyrene stars with mixed arm lengths: Evidence for coupled stars

Sequential anionic polymerization of styrene and divinylbenzene (DVB) is known to lead to the formation of star-shaped polymers. This ‘arms-first’ method has been widely used and studied. It is known that this polymerization forms stars with anionically active cores. This article is concerned with the attempt to make asymmetric-star polymers utilizing these living carbanionic sites present in the core to form a second set of shorter arms growing out from the star core. The presence of remaining unreacted DVB within the core was found to cause the stars to couple to form linked double stars and other larger structures. Results from detailed dilute solution studies of the resulting polymers are reported. It was found that the results obtained from size exclusion chromatography for the double stars were flow rate dependent; only at low flow rates was a true size separation obtained. © 1997 John Wiley & Sons, Inc.     

Dilute solution properties,chain stiffness,and liquid crystalline properties of cellulose propionate

The solution properties of cellulose derivatives are of interest from both technological and purely scientific aspects. At high concentrations these solutions form liquid crystalline structures. In dilute solution cellulosic chains can be described as semiflexible or wormlike with properties intermediate between random coils and rigid rods. A series of fractions of cellulose propionate have been examined by dilute solution viscometry, static and dynamic light scattering, and polarizing microscopy. Power law exponents are considerably larger than those observed for flexible chains and analysis of the intrinsic viscosity and hydrodynamic radii has yielded chain diameters and Kuhn statistical segment lengths. Corresponding aspect ratios from the hydrodynamic measurements are in good agreement with those obtained from polarizing microscopy, as analyzed in light of Flory's theory. Some aggregation and specific solvent effects have been observed, however separation of these effects has proven to be difficult. Results of these studies are compared to previous work for other cellulose derivatives. ©1995 John Wiley & Sons, Inc.     

Theory of Multiple‐Detection Size‐Exclusion Chromatography of Complex Branched Polymers

SEC separates complex branched polymers by hydrodynamic volume, rather than by molecular weight or branching characteristics. Equations relating the response of different types of detectors are derived including band broadening, by defining a distribution function N′(M,V_h), the number of chains with molecular weight M and hydrodynamic volume V_h. While the true molecular weight distribution of complex polymers cannot be determined by SEC, irrespective of the detector used, the formalism enables multiple detection SEC data to be processed to both analyze the polymer sample and reveal mechanistic information about polymer synthesis. The formalism also shows how the true weight‐ and number‐average molecular weight, and , can be obtained from correct processing of the hydrodynamic volume distributions.

     

Aggregation and disaggregation of Aeromonas gum in an aqueous solution under different conditions

The aggregation and disaggregation of Aeromonas (A) gum, an acidic heteropolysaccharide, were investigated by viscometry, a fluorescent probe, and gel permeation chromatography combined with laser light scattering techniques in aqueous solutions containing desired NaCl at different temperatures. The A gum had a strong tendency of aggregation and high viscosity in the aqueous solutions. The weight‐average molecular weight, z‐average radius of gyration, weight‐average molar number (w_ag), and apparent aggregation number (N_ap) of the aggregates were investigated and discussed. The results indicated that there were three regions that corresponded to three kinds of aggregates and two transition temperatures at about 35 and 75 °C in the disaggregation course. When the temperature was higher than 75 °C, the w_ag hardly changed, and there was still a certain amount of aggregates even at 100 °C, indicating that the aggregates were difficult to disrupt completely. Moreover, the aggregation was thermally irreversible. Decreasing polysaccharide concentration reduced the content of the aggregate. However, N_ap remained constant around 20, independent of the polysaccharide concentration in a 0.5 M NaCl aqueous solution at 25 °C. At a salt concentration greater than or equal to 0.05 M, the aggregation was almost independent of the salt concentration used here. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2644–2651, 2000     

Solution characterization of the novel organometallic polymer poly(ferrocenyldimethylsilane)

A series of four well‐defined poly(ferrocenyldimethylsilane) (PFS) samples spanning a molecular weight range of approximately 10,000–100,000 g mol⁻¹ was synthesized by the living anionic polymerization of dimethyl[1]silaferrocenophane initiated with n‐BuLi. The polymers possessed narrow polydispersities and were used to characterize the solution behavior of PFS in tetrahydrofuran (THF). The weight‐average molecular weights (M_w ) of the polymers were determined by low‐angle laser light scattering (LALLS), conventional gel permeation chromatography (GPC), and GPC equipped with a triple detector (refractive index, light scattering, and viscosity). The molecular weight calculated by conventional GPC, with polystyrene standards, underestimated the true value in comparison with LALLS and GPC with the triple detection system. The Mark–Houwink parameter a for PFS in THF was 0.62 (k = 2.5 × 10⁻⁴), which is indicative of fairly marginal polymer–solvent interactions. The scaling exponent between the radius of gyration and M_w was 0.54, also consistent with marginal polymer–solvent interactions for PFS in THF. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3032–3041, 2000     

Comparison of the molecular dimensions of di-alkyl and aryl-alkyl-substituted polysilanes in solution using SEC/LS

Polysilanes provide an opportunity for exceptional control of the chain hindrances to rotation through the choice of substituents on each backbone silicon. Two alkyl substituents on each silicon result in a large characteristic ratio of at least 19 for poly (di-n-hexylsilane), determined by extrapolation of intrinsic viscosities. Bulky aromatic substituents provide even more hindrance to backbone rotations, and can be expected to result in a more extended polymer chain. Direct measurement of the dimensions of these polymers by scattering techniques has been limited by the small quantities available, and by the polydispersity of samples. The recent introduction of light-scattering detectors for size exclusion chromatography enables the simultaneous measurement of light scattered at as many as 15 scattering angles as the fractionated polymer elutes from the column. Determination of both M and the root-mean-square radius of gyration R_g of narrow fractions eluting from a column allows determination of the R_g M relation over as much as a decade in M with less than a milligram of sample. Values of R_g and M across the distribution have been determined for alkyl and aryl substituted polysilanes with this technique. Estimation of R_g,0/M unperturbed by long-range interactions is made by an extrapolation procedure. The dependence of R_g,0 on M across the distribution is compared among the different substituents and with other measurements reported for these polymers. © 1994 John Wiley & Sons, Inc.     

Retracted: Effects of nanoparticles to the dynamical behavior of polymer chains in semidilute polymer solution: A dynamic light scattering study

The following article from the Journal of Polymer Science Part B: Polymer Physics: “Effects of nanoparticles to the dynamical behavior of polymer chains in semidilute polymer solution: A dynamic light scattering study,” by Mojammel H. Mondal, published online on 18 January 2011 in Wiley Online Library (onlinelibrary.wiley.com), has been retracted by agreement between the author, the journal Managing Editor, Dr. Victoria Cleave, and Wiley‐Blackwell. The retraction has been made as the research article was submitted without the author's PhD supervisor and principal investigator's knowledge or consent. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Comparison of methods for the determination of diffusion coefficients of polymers in dilute solutions: The influence of polydispersity

A comparison between various methods to determine diffusion coefficients of polymers in dilute solutions has been made. It is shown that Taylor dispersion analysis (TDA), dynamic light scattering (DLS), hydrodynamic chromatography (HDC), and size exclusion chromatography (SEC) can all be used to accurately determine diffusion coefficients when the polymer samples have low polydispersities. By the analysis of a series of practically representative styrene acrylonitrile copolymer (SAN) samples, it is shown that polydispersity of the samples and the presence of low‐molecular‐mass material cause considerable differences between the methods. It was found that TDA is mostly disturbed by the presence of low‐molecular‐mass material, whereas DLS is more sensitive to the polydispersity of the polymer. With broad samples, DLS gives the Z‐average diffusion coefficient. SEC can be used to obtain a diffusion coefficient distribution as well as an average diffusion coefficient of a polydisperse sample. Although, the same was expected for HDC, it was found that this method could only be successfully used for polymer samples having low polydispersities. Deviations between SEC, HDC, and TDA found for narrow samples were not related to the chemical composition of the samples. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 593–603, 1999     

Effects of the thermal history and concentration on the aggregation of Erwinia gum in an aqueous solution

The aggregation of Erwinia (E) gum in a 0.2 M NaCl aqueous solution was investigated by multi‐angle laser light scattering and gel permeation chromatography (GPC) combined with light scattering. The GPC chromatograms of five fractions contained two peaks; the fractions had the same elution volume but different peak areas, suggesting that aggregates and single chains coexisted in the solution at 25 °C. The apparent weight‐average molecular weights (M_w) of the aggregates and single chains for each fraction were all about 2.1 × 10⁶ and 7.8 × 10⁴, respectively. This indicates that the aggregates were composed of about 27 molecules of E gum in the concentration range used (1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ g/mL). The weight fraction of the aggregates (w_ag) increased with increasing concentration, but the aggregates still existed even in an extremely dilute solution. The fractionation process and polymer concentration hardly affected the apparent aggregation number but significantly changed w_ag. The E‐gum M_w decreased sharply with an increase in temperature. When the E‐gum solution was kept at 100 °C, w_ag decreased sharply for 20 h and leveled off after 100 h. Once the aggregates were decomposed at a higher temperature, no aggregation was observed in the solution at 25 °C, indicating that the aggregation was irreversible. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1352–1358, 2000