Abstract." The analysis of dynamic mechanical data indicates that linear flexible polymer chains of uniform length follow a scaling relation during their relaxation, having a linear viscoelastic relaxation spectrum of the form H(2) = n I G ° × (2/~.max) nl for 2.---).max. Data are well represented with a scaling exponent of about 0.22 for polystyrene and 0.42 for polybutadiene. The plateau modulus G ° is a material-specific constant and the longest relaxation time depends on the molecular weight in the expected way. At high frequencies, the scaling behavior is masked by the transition to the glassy response. Surprisingly, this transition seems to follow a Chambon-Winter spectrum H(2) = Cit-% which was previously adopted for describing other liquid/solid transitions. The analysis shows that the Rouse spectrum is most suitable for low molecular-weight polymers M~-M C, and that the de Gennes-Doi-Edwards spectrum clearly predicts terminal relaxation, but deviates from the observed behavior in the plateau region.
P3HT is a semirigid and semicrystalline polymer with a strong tendency to postcrystallize in solid films upon thermal annealing. Recent investigations showed that macroscopic, crystalline P3HT fibers can already be precipitated from solution, depending on the combination of solvent systems used. In this paper we investigate the mechanism and the dynamics of gelation in P3HT solutions. Rheological and absorption measurements in solution reveal a distinct two-step mechanism. In a first step, aggregates are formed which, in a second step, link to each other into a thermoreversible gel. The correlation between the gelation dynamics and the molecular weight distribution is discussed in more detail for a nonhazardous, printing friendly solvent system.
This paper contains an extensive presentation of dynamic mechanical data (complex moduli), as obtained on the melts of a series of standard polystyrenes of narrow molar mass distributions. It also shows the way of obtaining structural parameters (plateau modulus and friction factor) which are needed for an interpretation of these data in terms of simple theoretical models (Maxwell elements, Doi-Edwards model). A linear mixing rule is used for taking into account the finite width of the molar mass distributions.
In this work, composition effects on interfacial tension and morphology of binary polyolefin blends were studied using rheology and electron microscopy. The amount of dispersed phase (5-30 wt %) and its type [ethylene-octene copolymer, linear lowdensity polyethylene (LLDPE), and high-density polyethylene] was varied, and the influence of different matrix materials was also studied by using a polypropylene homopolymer and a ethylene-propylene (EP) random copolymer. The particle size distribution of the blends was determined using micrographs from transmission electron microscopy (TEM). A clear matrix effect on the flow behavior could be found from the viscosity curves of the blends. Analyzing the viscosity of the blends applying the logarithmic mixing rule indicated a partial miscibility of the EP random copolymer with low amounts of the LLDPE in the melt. Micrographs from TEM also showed a clear difference in morphology if the base polymer is changed, with PE lamellae growing out of the inclusions or being present directly embedded in the matrix. To verify these findings, the interfacial tension was determined. The applicability of Palierne's emulsion model was found to be limited for such complex systems, whereas Gramespacher-Meissner analysis led to interfacial tensions comparable with those already reported in the literature. The improved compatibility when changing the matrix polymer from the homopolymer to the random copolymer allows the development of multiphase materials with finer phase structure, which will also result in improved mechanical and optical performance.
Plasticizing agents are known to shift the material functions of polymer melts like raised temperatures to shorter times and to higher frequencies. The influence of dioctyl sebacate, dioctyl phthalate and bibenzyl on the viscoelastic properties of polystyrene melts is sized by shift factors which can be evaluated very exactly from the dynamic moduli in the rubberlike liquid region. Besides, these shift possibilities enable us to enlarge the experimental window to the low frequency region for some decades. The typical diluent effect caused by the additives is quite similar to the one caused by not entangled polymer molecules. The lowering of the plateau modulus, i.e., the shift on the modul axis, is found to be equal to the square of the weight fraction of the polymer. The reduction of the relaxation times, i. e., the shift on the time or frequency axis, mainly is due to the increased fraction of free volume accounted for by A/&&,,, where A rates the fraction created by one mole additive and @, , is the number-average of the molar masses of polymer and additive. The influence of the free volume on the relaxation times is calculated with the aid of a Doolittle equation. The concentration dependence of the time-temperature shift factors is also accounted for by A/&?,,,. Therefore, besides the parameter A, only the three parameters used in (and known by) the analysis of the time-temperature superposition principle, i. e., the fraction of free volume of the pure polymer 0, the thermal expansion coefficient o f f (af) and the Doolittle parameter B, are needed for the calculation of the temperature-dependent timeconcentration shift factors and of the concentration-dependent time-temperature shift factors as well. 0 1995,
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