Multiarm Star Research Articles

Relaxation and viscoelastic properties of complex polymer systems

Dynamic behavior of various polymer melts is studied on the basis of a comparison of viscoelastic properties with the information obtained from dielectric spectroscopy. The experimental observations are compared with results of computer simulation of corresponding systems. The studies include simple melts of linear chains, block copolymer systems of miscible components, as well as the behavior of melts with molecular objects of complex topology-like stars or microgels. In the case of polyisoprene linear chain melts an equivalence of terminal relaxation times determined from mechanical and dielectric measurements is demonstrated. Using linear block copolymers of isoprene and butadiene, relaxation times of chain fragments (isoprene blocks) in relation to relaxation times of whole copolymer chains are determined and compared with theory and simulation. Both the experimentally determined block relaxation times and relaxation times of chain fragments in simulated linear chain melts show a disagreement with predictions of the reptation theory. In the case of multiarm star polymers and microgel melts, the slow relaxation modes observed in viscoelastic spectra are assigned to cooperative translational motions detected in corresponding simulated systems in which an ordering of such molecules is demonstrated. This suggests that the terminal relaxation in multiarm star or microgel melts is governed by another relaxation mechanism than in linear chain melts. High efficiency of the Cooperative Motion Algorithm in simulation of dense systems of complex molecules is demonstrated.

Textures in hexatic films of nonchiral liquid crystals: Symmetry breaking and modulated phases.

Novel modulated textures, such as stripes and multiarmed star defects, have been observed in freely suspended films of nonchiral liquid crystals just below the smectic-$C$ to hexatic phase transition. Detailed studies using depolarized reflection microscopy suggest that the stripes are locally chiral surface splay domains of the smectic-$L$ phase, a tilted hexatic not previously identified in thermotropic liquid crystals. Line defects which form additional domain walls in the hexatic lattice lead to characteristic modulations of the basic one-dimensional stripe pattern. Inside thick circular islands, for example, stripes form circumferentially and the lines form centered 12-armed stars, resulting in a regular arrangement of hexatic domains in the form of a brick wall pattern. The observation that line defects which are not pinned at the film boundaries always form closed loops supports a model of the stripe pattern based on local chiral symmetry breaking.

Molecular dynamics in linear and multi-armed star polymers of cis-polyisoprene as studied by broadband dielectric spectroscopy

Broadband dielectric spectroscopy (10 −1 −10 9 Hz) was employed to investigate bulk amorphous multi-armed stars of cis-polyisoprene and their linear counterparts, both having narrow molecular weight distributions. The star polymers resemble the linear polymer in showing two distinct regions of dielectric dispersion: a molecular weight independent segmental mode process and a normal mode process which depends strongly on the (arm) molecular weight. The segmental mode is nearly identical for linear and multi-armed star polymers. By contrast, the normal mode is shifted to longer relaxation times if one compares star polymers with linear systems having the same (arm) molecular weight. The relaxation processes are analyzed in terms of the β-parameter (‘streched exponential’) of the autocorrelation function. Below a critical molecular weight, M c (≈ 10 4), the dynamics of the normal mode can be described by the Rouse theory. Above M c, the relaxation time follows the 3.7 power of the (arm) molecular weight which is characteristic for entangled macromolecules. The relaxation for the star polymers can be quantitatively interpreted by conformational fluctuations of a tethered chain.

Characteristic Ratio and Plateau Modulus of 1,2-Polybutadiene. A Comparison with Other Rubbers

Abstract In the course of work on linear and ring polybutadienes with 62% 1,2 units, a number of discrepancies were noted with data on polybutadienes of various microstructure available in the literature. For example, GNο=870 kPa for our 62% 1,2-polybutadiene. This is larger than GNο=730 kPa for a 56% 1,2-polybutadiene and GNο=550 kPa for a 78% 1,2-polybutadiene sample. The cis : trans ratio of our 62% 1,2-polybutadiene, prepared with potassium counterion, is 1 : 4, On the other hand, the cis : trans ratio of 62% 1,2-polybutadiene prepared with a modified Li catalyst is estimated to be 1 : 2. It is conceivable that the different cis : trans ratio leads to different properties at constant 1,2 content. Nevertheless, the low levels of both the cis and the trans units are not expected to cause more than minor differences in the properties of the polybutadienes. Correct values for GNο of model polymers are important for the study of the influence of the chemical structure on the melt characteristics of a polymer. For this reason, it was thought useful to reinvestigate 1,2-polybutadiene itself in some detail. The synthesis of narrow molecular-weight distribution 1,2-polybutadiene by anionic polymerization techniques has been described recently. The dilute-solution properties of 1,2-polybutadiene has been investigated. The melt rheology of two 1,2-polybutadiene samples have been studied, but no systematic study of the molecular-weight dependence of the melt properties was made. 1,2-Polybutadiene has been used as a component in block copolymers with 1,4-polybutadiene. These studies have permitted an investigation of the phase behavior of two rubbery blocks at room temperature. Poly(l,4-butadiene-graft-l,2-butadiene)s with well-defined composition and architecture have also been prepared. Hydrosilylated 1,2-polybutadiene has found use as the coupling agent for multiarm star polymer, and this method can easily be extended to the preparation of poly( l,2-butadiene-graft-l,4-butadiene). Hydrogenated 1,2-polybutadienes are prepared as model polymers for poly(l-butene). The synthesis and characterization of a series of 1,2-polybutadienes are described here. Special attention is given to low-molecular-weight polymers. The linear viscoelastic properties of the melts are also described. In the discussion, the relation between the characteristic ratio, C∞, and the plateau modulus, GNο, of a number of model polymer systems is explored.

Further investigation on the molecular dynamics in linear and multiarmed star polymers of cis-polyisoprene studied by dielectric spectroscopy

Dielectric spectroscopy from 10−1 to 109Hz was used to investigate bulk amorphous multiarmed stars ofcis-polyisoprene and their linear counterparts, both having narrow molecular weight distributions. The star polymers resemble the linear polymers in showing two distinct regions of dielectric dispersion. A molecular weight independent segmental process occurs at nearly the same frequency as in the linear polymer. The relaxation time of the molecular weight dependent normal mode process for linear polymers can be described below a critical molecular weightMc (≈ 104) according to the Rouse theory; but aboveMc the relaxation time follows the 3.7 power ofMw, which is characteristic for entangled macromolecules. A similar dependence of the relaxation time is observed for the many arm star polymers. The arm relaxation is interpreted by means of conformational fluctuations of a tethered chain.

Molecular dynamics in linear and multiarmed star polymers of cis-polyisoprene as studied by dielectric spectroscopy

Molecular dynamics in linear and multiarmed star polymers of cis-polyisoprene as studied by dielectric spectroscopy

Synthesis and characterization of multiarm star polybutadienes

Preparation a partir d'oligobutadiene-1,2 hydrosilyle avec le methyldichlorosilane, utilise comme agent d'accrochage