The first part of this paper describes an interpretation of extrudate swell with an emphasis on the following aspects: (1) The material behavior in the entrance zone leading to capillary is in transient and not in steady state. (2) The material behavior is predominantly in elognation and not in shear. (3) The deformational memory introduced in this zone is carried into capillary. (4) The material behavior in capillary is the steady state flow with accompanying memory mentioned in (3). (5) The memory is disspitated (relaxation) during the flow. (6) The remaining memory results in extrudate swell after material exist from capillary. (7) Even with a long capillary, the memory does not dissipate completely, because the steady state flow itself maintains certain memory. (8) The extrudate swell is time-dependent and not instantaneous. (9) It is like a creep-recovery. (10) The extrudate swell is related to the normal stresses in the flow through capillary. The second part of the paper describes various subjects, which are required for the fundamental understanding of the memory behavior and extrudate swell in particular. (11) The deformation associated with extrudate swell is nonlinear. Therefore, a theoretical treatment of nonlinear viscoelasticity is needed. (12) Because the material behavior involves both shear and elongation, the relationship between them must be established. (13) A quantitative description of deformational memory is needed. (14) In the transient behavior a memory is being built and at the same time a part of memory is lost. A “balance sheet” of the memory at any instant of time must be available. (15) Stress relaxation accompanying steady state flow must be elucidated. A use of stress relaxation measurement is proposed as a quantitative means of describing the “memory balance”.
ABA type triblock copolymer, poly (oxyethylene)-poly (oxytetramethylene)-poly-(oxyethylene) (PEO-PTHF-PEO) was used as prepolymer for preparing segmented polyurethaneureas (SEUU). The introduction of hydrophilic PEO segments was very effective for improving both the blood-compatibility and the mechanical pro-perties of segmented polyurethaneureas. The best antithrombogenisity was attained by SEUU of 33 mol%-EO-content. In addition to a better antithrombogenicity, SEUU displayed excellent mechanical properties, lower modulus and higher elongation, as well as a high tensile strength as compared with polyurethaneurea (SPUU) without PEO segments. These mechanical characteristics are similar to those of a living soft tissue. The morphology of segmented polyurethaneureas was investigated by DSC, SAXS and dynamic mechanical measurements. The microphase separation was observed in all polymers. However, segmented polyurethaneureas having PEO segments showed phase separation more clearly than SPUU. The improvements in properties of SEUU might be attributed to better microphase separation than SPUU.
Research Center for Medical Polymers and Biomaterials, Kyoto UniversityDifferential constitutive equations for entangled polymer systems are evaluated experimentally. Applicability of three constitutive models proposed by Leonov, Giesekus and Larson, respectively, is investigated based on stress relaxation data for polystyrene melts. Predictions of these three models on strain-dependent relaxation modulus G (t,γ) are compared with experimental data obtained for samples with narrow and bimodal molecular weight distributions. It is found from the comparison that none of these three models can describe the following two experimental results: (1) In entangled polymer systems, there appears a nonlinear relaxation process at short times characterized by a relaxation time τeq, which is shorter than the maximum relaxation time τ1. (2) Even when linear relaxation spectra of two samples with narrow and bimodal molecular weight distributions have the same shape at long and intermediate times, shape of G (t,γ) curves of the two samples is different at short and intermediate times.These three models cannot give a quantitative description of G (t,γ) at short times, because they neglect the fast relaxation process.
Dynamic measurements were made on two thermotropic liquid crystalline cellulose derivatives abbreviated as HEC-C5 and TBC. HEC-C5 is a hydroxyethyl cellulose (HEC) fully acylated with pentanoic acid. Since the chemical structure of the HEC is disordered (DS=1.5 and MS=3.0), that of the tripentanoate also becomes disordered. TBC is a tri-O-β-butoxyethyl cellulose, fully etherified cellulose (DS=MS=3.0) having an ordered structure. In both samples, the temperature dependence as well as the frequency dependence of the dynamic viscoelastic functions clearly and unambiguously reflected the three states of the polymers, i.e., an anisotropic liquid, isotropic liquid, and anisotropic/isotropic co-existing state. TBC was found to undergo an anisotropic-to-isotropic transition in a very narrow temperature span (less than 10°C), the dynamic viscosity η′ increasing by a factor about 10 in this temperature span. Moreover, the temperature dependence of η′ of TBC measured at a constant frequency showed two characteristic temperatures, other than the anisotropic-to-isotropic transition temperature, which indicated certain structural changes in this polymer in the liquid crystalline state. No such detailed information was obtained from the η′-temperature diagram of HEC-C5, which showed only a very broad anisotropic/isotropic co-existing region extending from about 90°C to 145°C. These marked differences in the transition and other behavior between the two polymers are believed to originate from their difference in chemical structural order.
Effect of flow history on the rheological properties of a well-known thermotropic liquid crystalline copolyester which was synthesized from poly (ethylene terephthalate) and 60 mol % p-hydroxybenzoic acid was investigated. The development behavior and steady values of shear stress, σ, and first normal stress difference, N1, and frequency dependence of dynamic viscoelastic functions were measured by means of a cone-plate type rheometer. Flow histories were controlled by steady and interrupted flows. The development of σ and N1 on a virgin sample shows a two-step overshoot. For sheared samples, the maxima of the stresses at the first overshoot drastically decreased, as compared with those for virgin samples. The initial behavior can not be recovered by resting the sheared samples for as long as 20 min. The values of dynamic viscoelastic functions measured after steady-state in shearing flow are significantly lower than those for virgin state. On the other hand, the steady state values of a and N1 are independent of the flow histories. It is concluded that the structure of the thermotropic liquid crystalline copolyester at rest changes to other one irreversibly after being subjected to steady shear flow, but the structure under steady-shear flow is unique and not affected by the shear histories.
Birefringence and linear viscoelasticity were investigated for solutions of the mixture of polystyrene (PS) and poly (vinyl methylether)(PVME) in dibutyl phthalate.Molecular weight,MW, of PVME was 6.0×104 and those of PS samples were 9.5×105 and 2.89×106. The storage modulus, G′, and the loss modulus, G″, of mixed solutions were measured at PVME concentrations varying from 0 to 0.30 g cm-3, where MW of PS was 2.89×106 and PS concentration was 0.10 g cm-3. The plateau modulus was independent of the PVME concentration. Thus the stress at long times is supported by the entanglement network composed of long PS chains. The birefringence, Δn, and the shear stress, σ, were measured after an instantaneous shear deformation. Under assumption that the Δn and σ are sums of independent contributions from PS and PVME chains, and that the stress-optical law holds good for the contribution from each chain, the stresses, σPS and σPVME, attributable to the PS and PVME chains, respectively, were separately evaluated. It was found that at long times σ was originated solely by PS-PS entanglements. This is consistent with the result of the dynamic measurement.
We investigated sol-gel transition of isotactic polypropylene (i-PP) mixed with a small amount of small-molecule gelling agents, PDTS (1, 3:2, 4-p, p′-ditolyliden sorbitol) and DBS (1, 3:2, 4-dibenzylidene sorbitol). The i-PP has viscosity average molecular weight 1.61×105 and MW/Mn =4.4. The sol-gel transitions of PDTS/i-PP and DBS/i-PP systems were studied as a function of concentration of the gelling agents (0, 0.3, 0.5 and 1% by weight) using linear dynamic mechanical analyses. The results indicated that (i) the two systems underwent thermoreversible sol-gel transition and that (ii) G′ and G″ obtained at a given ω as a function of temperature during the cooling and heating cycles exhibited remarkable hysteresis and two sol-gel transition temperatures Tfg,H and Tfg,C (Tfg,H>Tfg,C). Tfg,H designates the sol-gel transition obtained during the heating cycle and corresponds to the critical temperature for dissolution of the percolation network formed by physical association of PDTS or DBS in the matrix of PP melts. Tfg,C designates the sol-gel transition obtained during the cooling cycle and corresponds to the critical temperature for formation of the PDTS or DBS percolation network. Thus, in this sense, the sol-gel transition is analogous to the first-order phase transition. The master curves of G′ and G″ were obtained by applying the time-temperature superposition separately for the sol and gel states for 0.5% PDTS/i-PP system. The master curve for the sol state was found to be identical with that of the pure i-PP melt. However, the master curve for the gel state was found to be distinctly different from those of the sol state and of the pure i-PP melt. At low reduced frequencies corresponding to the terminal flow region of the sol and pure i-PP, the gel exhibited a plateau in G′, and a significantly high G″, which implies that the real part of the dynamic viscosity goes to infinity at zero frequency limit. These results imply that the PDTS forms a macroscopic percolation network. The loss tangent for the gel was found to be nearly equal to unity overall ωαT covered in this experiment, implying that the association and dissociation of the PDTS molecules are in dynamic equilibrium, contributing to the large loss tangent.
Stabilization of a coal-water mixture (CMW) is important for an establishment of the total coal utilization system including the preparation, the pipe line transportation, the tank storage and the ship transportation. The stability means that a sedimentation of coal particles is prevented during the processing of CWM. Both the particle-size distribution and the stabilizing additive affect on the fluidity and stability of CWM. In order to study the mechanism of the internal structure formation in the CWM, the effect of pH on the stability of the CWM with and without the stabilizing additive was examined. The experimental results have shown that the viscosity of the CMW with the stabilizing additive was increased with increasing pH value, if pH>7.0. On the other hand, the viscosity of the CWM without the stabilizing additive showed the reverse tendency. The estimation of both thickness of electric double layer and the adsorbed layer of the stabilizer gave a possible explanation on the mechanism of an internal structure formation of the CWM.