Amounts Of Free Chains Research Articles

Rheological properties of silicon polymer networks: The influence of the crosslink density

AbstractThis study was focused on the influence of the microstructural properties of a silicon polymer network on its rheological properties. Two commercial silicon oils were mixed in different ratios to form, by hydrosilylation, networks with different crosslink densities. The chemical compositions of the oils were determined by NMR, whereas their molecular weights and viscosities were studied with gel permeation chromatography and viscosimetry, respectively. The different networks were characterized through their crosslink densities. Afterward, the rheological properties were studied. The formulation notably influenced some characteristic values of the rheological behavior: the α‐transition temperature and the onset temperature of the caoutchoutic plateau shifted toward higher temperatures as the crosslink density increased, the storage modulus at the onset temperature of the caoutchoutic plateau increased with the crosslink density, and the amplitude of the peak associated with the α‐transition temperature decreased. These behaviors were explained as follows: as the crosslink density increased, a drastic decrease in the amount of free chains in the network occurred, and both phenomena induced a large decrease in the chain mobility, which might explain the aforementioned behavior. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1152–1160, 2003

Osmotic and scattering properties of polymer gels containing free chains

Measurements are made of the osmotic swelling pressure and small angle X-ray scattering from lightly crosslinked poly(vinyl acetate) (PVAc) gels swollen in toluene and containing known amounts of free PVAc chains. Gels with different amounts of free chains, but with identical total polymer concentration, exhibit similar behaviour. The differences observed between the thermodynamic properties of the gels and the solutions are consistent with a reduction in the fraction of polymer participating in the dynamic concentration fluctuations.

Block copolymer micelle solutions: 2. An intrinsic excimer fluorescence study

We have utilized photostationary and transient excimer fluorescence to extend our study of the concentration dependence of PSPEP in heptane, given in the previous paper. The concentration dependence of photostationary observables, namely the excimer-to-monomer intensity ratio I D I M , the excimer bandwidth Δv D, 1 2 and the excimer band maximum position v D, can be explained in terms of the variation of solvent content in the core. Using a three-dimensional energy migration lattice model developed previously by Gelles and Frank in 1983 for studying miscible blends of polystyrene (PS) and poly(vinyl methyl ether), we have estimated the volume fraction of PS in the core as a function of concentration. In addition, both photostationary and transient results indicate that the free chain-micelle equilibrium is changed in favour of more free chains above 0.5% for the heptane solution and above 0.25% for the dodecane solution. The amount of free chains in the dispersed phase could also be estimated from the lattice model by a lever-rule approach. The critical micelle temperature (c.m.t.) can be measured by monitoring the changes of the excimer bandwidth at elevated temperatures. At temperatures below the c.m.t., two different transitions, in the form of small increases in I D I M , were observed, and have been attributed to the solvation of the micelle core and the disordering of the core-corona interface.

Chondroitin sulphate proteoglycan in the substratum adhesion sites of Balb/c 3T3 cells. Fractionation on various ion-exchange and affinity columns.

Proteoglycans on the cell surface play critical roles in the adhesion of fibroblasts to a fibronectin-containing extracellular matrix, including the model mouse cell line Balb/c 3T3. In order to evaluate the biochemistry of these processes, long-term [35S]sulphate-labelled proteoglycans were extracted quantitatively from the adhesion sites of 3T3 cells, after their EGTA-mediated detachment from the substratum, by using an extractant containing 1% octyl glucoside, 1 M-NaCl and 0.5 M-guanidinium chloride (GdnHCl) in buffer with many proteinase inhibitors. Greater than 90% of the material was identified as a large chondroitin sulphate proteoglycan (Kav. = 0.4 on a Sepharose CL2B column), and the remainder was identified as a smaller heparan sulphate proteoglycan; only small amounts of free chains of glycosaminoglycan were observed in these sites. These extracts were fractionated on DEAE-Sepharose columns under two different sets of elution conditions: with acetate buffer (termed DEAE-I) or with acetate buffer supplemented with 8 M-urea (termed DEAE-II). Under DEAE-I conditions about one-half of the material was eluted as a single peak and the remainder required 4 M-GdnHCl in order to recover it from the column; in contrast, greater than 90% of the material was eluted as a single peak from DEAE-II columns. Comparison of the elution of [35S]sulphate-labelled proteoglycan with that of 3H-labelled proteins from these two columns, as well as mixing experiments, indicated that the GdnHCl-sensitive proteoglycans were trapped at the top of columns, partially as a consequence of their association with proteins in these adhesion-site extracts. Affinity chromatography of these proteoglycans on columns of either immobilized platelet factor 4 or immobilized plasma fibronectin revealed that most of the chondroitin sulphate proteoglycan and the heparan sulphate proteoglycan bound to platelet factor 4 but that only the heparan sulphate proteoglycan bound to fibronectin, providing a ready means of separating the two proteoglycan classes. Affinity chromatography on octyl-Sepharose columns to test for hydrophobic domains in their core proteins demonstrated that a high proportion of the heparan sulphate proteoglycan but none of the chondroitin sulphate proteoglycan bound to the hydrophobic matrix. These results are discussed in light of the possible functional importance of the chondroitin sulphate proteoglycan in the detachment of cells from extracellular matrix and in light of previous affinity fractionations of proteoglycans from the substratum-adhesion sites of simian-virus-40-transformed 3T3 cells.