Rheological technique for determining the order–disorder transition of block copolymers

A rheological technique is proposed for determining the thermally induced order–disorder transition of block copolymers. In the present investigation, a cone-and-plate rheometer was used to measure dynamic storage and loss moduli, G′(ω) and G″(ω), as a function of angular frequency ω of a commercial grade polystyrene-block-polyisoprene-block-polystyrene (SIS) tri-block copolymer (KRATON D-1107, Shell Development Company) in the temperature range from 140 to 240°C. For comparison purposes, dynamic viscoelastic properties of a commercial grade low-density polyethylene (LDPE) were also determined in the temperature range from 160 to 238°C. We have found that log G′ versus log G″ plots for the LDPE show no temperature dependence, whereas log G′ versus log G″ plots for the SIS block copolymer do show systematic temperature dependence in the temperature range 140–230°C. This observation leads us to conclude that the order–disorder transition of the SIS block copolymer takes place gradually as the temperature is raised from 140 to 230°C. This conclusion is in good agreement with that drawn from the study of Roe (Ref. 33), who employed the same block copolymer using small-angle x-ray scattering. It is not possible to reach such a conclusion using log G′(ω) versus log ω, log G″(ω) versus log ω, or log η′(ω) versus log ω plots in which η′ is the dynamic viscosity. We have demonstrated further that the use of frequency-temperature superposition is inappropriate for investigating the rheological behavior of block copolymer in the temperature range over which a thermally induced transition from an ordered structure to a disordered homogeneous phase occurs. We therefore suggest that when using information on dynamic viscoelastic properties, log G′ versus log G″ plots be used for determining the thermally induced order–disorder transition of block copolymers.     

High-temperature morphological transition in a styrene-butadiene-styrene block copolymer

Electron microscopy reveals a high-temperature morphological transition in a styrene-butadiene-styrene block copolymer of 7000 polystyrene block molecular weight and 43,000 polybutadiene block molecular weight (7S-43B-7S). Samples quenched in liquid nitrogen from temperatures above 150°C show no structure, whereas those quenched from temperatures below 140°C clearly show a multiphase structure. We previously reported that the 7S-43B-7S polymer exhibits a relatively sharp melt rheological transition in the temperature region between 140 and 150°C from highly viscoelastic and nonlinear viscous behavior to linear viscous behavior with insignificant elasticity. The dynamic viscoelastic properties are measured at different strain amplitudes in this study, and the results show that the melt rheological transition behavior is not influenced by the strain amplitude. This study clearly shows that the melt rheological transition in the 7S-43B-7S results from a morphological transition from a multiphase structure below about 140°C to a single-phase structure above about 150°C.     

Newtonian behavior of a styrene–butadiene–styrene block copolymer

The melt rheological behavior of an anionically polymerized styrene–butadiene–styrene (SBS) block copolymer sample (S: 7 × 10³ and B: 43 × 10³) was studied using a Weissenberg rheogoniometer. Highly non-Newtonian behavior, high viscosity and high elasticity, which are characteristics of ABA type block copolymers, were observed at 125°C, 140°C, and 150°C. The data at these temperatures superimposed well onto a master curve giving a constant flow activation energy. However, the data at 175°C indicated a marked change in the flow mechanism between 150°C and 175°C. At 175°C, the sample showed Newtonian behavior, negligible elasticity, and deviation from the master curve. These findings may be considered as an indication that the SBS block copolymer sample undergoes a structural change from a multiphase structure at low temperatures into a homogeneous structure at some temperature between 150°C and 175°C.     

Effect of Temperature on Dynamic Physical Behavior of Poly(vinyl chloride) Gel Structure with Ester Plasticizers

Dynamic viscoelastic properties of poly(vinyl chloride) (PVC)/bis(2-ethylhexyl) phthalate (DOP) and PVC/di-n-butyl sebacate (DBS) gels with molecular weight distribution (M_w/M_n), of 2.16 and various polymer concentrations c, have been studied as a function of temperature. These PVC gels exhibited an elastic solid at room temperature T, and gradually became liquid (sol) with increasing temperature. The sol-gel transition took place at a critical gel temperature at which the scaling law of G′(ω) ∼ G″(ω) ∝ ωⁿ held, allowing an accurate determination of the critical gel temperature by means of the frequency ω independence of the loss tangent. In this study the scaling exponent n, was 0.75–0.77. This is in good agreement with the previous results observed at different temperatures and suggests the formation of a similar fractal structure of the PVC gels. The gel strength S_g, at the gel point increased with increasing PVC concentration. These results suggest a unique character and structure for the gel points of PVC-plasticizers.     

Shear rheology of micro-fibrillar cellulose aqueous suspensions

Micro-fibrillar cellulose aqueous suspensions with different fiber lengths were prepared by mechanical refining of softwood pulp fiber suspensions at different specific refining energies. Effects of refining energy level, micro-fiber concentration and temperature on the rheological properties of these aqueous suspensions were studied. These microfibers form a three-dimensional network, which displays typical shear-thinning behavior with little thixotropic tendency, at concentrations as low as 0.5 wt%. A viscoelastic analysis showed that these micro-fibrillar cellulose suspensions at different concentrations (from 0.5 to 2 wt%) exhibit a viscoelastic gel-like behavior [G′ > G″ over an extended range of frequencies (ω) and a weak dependency of G′ on ω] at 25 °C. The storage modulus, G′, at 1 rad/s increased strongly upon increasing concentration from 0.5 to 2 wt% following a power law with an exponent of 3.2. However, increasing the temperature decreases the storage modulus, G′, due to weakening or disruption of intermolecular interactions at elevated temperatures. The viscoelastic behavior changes to liquid-like, with G″ > G′ at the investigated frequency range, for the suspensions at 85 °C.     

Creep behavior of amorphous ethylene–styrene interpolymers in the glass transition region

The viscoelastic behavior of amorphous ethylene–styrene interpolymers (ESIs) was studied in the glass transition region. The creep behavior at temperatures from 15°C below the glass transition temperature (T_g) to T_g was determined for three amorphous ESIs. These three copolymers with 62, 69, and 72 wt % styrene had glass transition temperatures of 11, 23, and 33°C, respectively, as determined by DMTA at 1 Hz. Time–temperature superposition master curves were constructed from creep curves for each polymer. The temperature dependence of the shift factors was well described by the WLF equation. Using the T_g determined by DMTA at 1 Hz as a reference temperature, C₁ and C₂ constants for the Williams, Landel, and Ferry (WLF) equation were calculated as approximately 7 and 40 K, respectively. The master curves were used to obtain the retardation time spectrum and the plateau compliance. The entanglement molecular weight obtained from the plateau compliance increased with increasing styrene content as 1,600, 1,870, and 2,040, respectively. The entanglement molecular weight of the ESIs was much closer to that of polyethylene (1,390) than to that of polystyrene (18,700); this was attributed to the unique chain microstructure of these ESIs with no styrene–styrene dyads. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2373–2382, 1999     

Effect of molecular weight on conformation of poly(β-benzyl L-aspartate) in films

In order to study the effect of the molecular weight on the crystallinity and conformational changes of poly(β-benzyl L aspartate) in films, a previous study on high molecular weight samples has been extended to included polymers of low molecular weight, about 3.3 × 10³. Films were prepared from chloroform solution by quick or slow evaporation at room temperature. The conformation and the thermal behavior were studied by means of infrared spectroscopy and differential scanning calorimetry. All films dried quickly are composed of polymer in the left-handed α-helical form. All samples studied which have molecular weights above 2.3 × 10⁴ are similar in crystallinity and the left-handed α-helices in them crystallize to ω-helices during slow evaporation. In the low molecular weight region, however, the left-handed α-helices reverse to right-handed α-helices during slow evaporation, and the right-handed α-helices, in turn, reverse and crystallize to highly ordered ω-helices upon heat treatment, although there is some simultaneous conversion to the β-form. The transition temperatures of the quick-dried films for conversion from the left-handed α-helix to the ω-helix and from the ω-helix to the β-form increase linearly with increasing molecular weight up to about 2 × 10⁴, but no large molecular weight dependence is observed beyond that region.     

Rheology of the Melt of Acrylonitrile–Styrene Copolymer Modified with Montmorillonite

The rheological properties of the acrylonitrile–styrene copolymer modified with hydrophobized montmorillonite Cloisite 30B and composites are studied in a wide range of shear rates and shear frequencies at a temperature of 230°C. It is shown that the filler insignificantly affects the viscoelastic properties of the compositions, but the viscosity of composite melts considerably increases at low shear rates and monotonically grows with increasing concentration, whereas at high shear rates the effective viscosity declines appreciably in the concentration range of about 0.3 vol %.     

共聚物组成对线形苯乙烯-丁二烯-苯乙烯嵌段共聚物粘弹弛豫与相形态的影响

研究了不同组成的苯乙烯-丁二烯-苯乙烯嵌段共聚物(SBS)的相形态与粘弹弛豫.用透射电子显微镜(TEM)表征了SBS的形态,结果显示,几种SBS均呈层状结构,随着苯乙烯含量的降低,聚苯乙烯(PS)相的尺寸稍有减小,而聚丁二烯(PB)相尺寸明显增大.用动态流变学方法考察了不同温度下SBS嵌段大分子的弛豫行为,结果表明,苯乙烯含量减少,PS相玻璃化转变和有序-无序转变温度均降低;苯乙烯含量少的,在有序-无序转变过程中呈现出高且宽的损耗峰,表明有序-无序转变过程中能量的耗散主要由两相溶合时分子链间的内摩擦所决定,分子链越长,内摩擦越大,能量耗散越大.     

Radiation-induced grafting in the polyethylene–styrene system and differential thermal analysis of the grafted samples

The graft polymerization of styrene onto high-density polyethylene films was carried out by γ-irradiation in the vapor phase. Two methods were used for grafting in these experiments: a preirradiation method and a simultaneous irradiation method. The effects of these grafting methods on the reaction mechanism of grafting and on the properties of the grafted samples were investigated. The amounts of styrene homopolymer in the grafted samples is under 2% in the case of the preirradiation method and above 10% in the case of the simultaneous irradiation method. The activation energies were calculated to be 18 kcal/mole for grafting in the preirradiation method and 15 kcal/mole for weight increase of polyethylene films in styrene vapor. The difference in the dimensional expansion between in the direction of stretching and the direction prependicular to it is smaller with preirradiation grafting than with grafting by the simultaneous irradiation method. Differential thermal analysis of the grafted films shows an endothermic peak due thermal decomposition which decreases gradually from 450°C to 415°C with increase in degree of grafting from 30 to 60%. The lowering of this peak temperature appears at a lower degree of grafting when the preirradiation method is used. On the basis of these results, it is concluded that the reaction rate of radiation-induced grafting in the vapor phase depends closely upon the processes of adsorption, dissolution, and diffusion of styrene monomer in polyethylene films; in the case of simultaneous irradiation method, the reaction proceeds comparatively uniformly in the amorphous region, while in the case of the preirradiation method, the reaction proceeds mainly at the boundary of the crystalline and amorphous regions.     

Effect of the reference chain dimension on the viscoelastic and stress–strain behavior of poly(n-butyl methacrylate) networks

The linear viscoelastic and stress-strain behavior of poly(n-butyl methacrylate) networks at a content of crosslinking agent (ethyleneglycol dimethacrylate) of c? 0–1 × 10^?4 mole/cm³ was investigated in the main transition and rubberlike region in the temperature interval from 20 to 150°C. The dependence of the unperturbed chain dimensions on temperature was determined from thermoelastic measurements in the rubberlike region; this dependence was unaffected by the content of crosslinking agent. Application of time–temperature superposition to the linear viscoelastic behavior did not give a continuous superimposed curve in the proximity of the rubberlike region; superposition within the whole time region required introducing the change of the unperturbed chain dimensions with temperature. This correction was sufficient for a sample with a higher content of the crosslinking agent. However, for loose networks (c< 0.1 × 10^?4 mole/cm³) it was insufficient, because of another relaxation mechanism in the region of high temperatures. It was found that the intensity and temperature dependence of this relaxation mechanism, which is probably due to a change of the number of entanglements with temperature, are connected with the magnitude and the temperature dependence of the C₂ constant of the Mooney-Rivlin equation.     

Polymerization of 2-(substituted methyl)acrylate bearing ω-methoxyoligoethyleneoxy groups as side chains to new low Tg polymer

The polymerization of methyl 2-(ω-methoxyoligoethleneoxymethyl)acrylates, which were synthesized from reactions of methyl 2-bromomethylacrylate with ω-methoxyoligoethylene glycol, was investigated. All these monomers polymerized readily to high molecular weight despite expectation of the considerable steric hindrance against propagation. No influence of ceiling temperature was observed. The 2-(2-methoxyethyleneoxymethyl)acrylate exhibited similar reactivities to methyl 2-alkoxymethylacrylate reported previously in copolymerization with styrene and methyl methacrylate. The homopolymers synthesized, except for those from methyl 2-methoxymethyl- and 2-(2-methoxyethyleneoxymethyl)acrylates, were soluble in water, and all were soluble in benzene. These polymers were thermally less stable than poly(methyl methacrylate) as confirmed by thermogravimetric analysis, and all showed glass transition temperatures below 0°C. © 1993 John Wiley & Sons, Inc.     

高聚物两相体系相行为的流变学研究──Ⅱ．嵌段高聚物微相溶合的研究

本文从流变学角度研究了三嵌段高聚物苯乙烯－丁二烯－苯乙烯（ＳＢＳ）中的微相转变，从相转变温度前后流变行为的变化，确定了相转变温度，其结果与小角Ｘ光散射的结果一致，在此基础上，研究了嵌段高聚物的微相溶合过程，发现相溶合过程可分为四个阶段，在第二，第四阶段，Ｇ’～ｅαｔ，’～ｅβｔ，矿是相溶合时间，α，β随相溶合温度及阶段不同而改变．以Ｄｉｃｋｉｅ模型为基础，考虑到分子运动及剪切条件下的流体力学相互作用，分析了各阶段的流变行为，初步说明了微相溶合过程中，流变行为时间依赖性的本质．     

Temperature dependence of viscosity–shear rate behavior in undiluted polystyrene

The time—temperature superposition principle is well-established for linear viscoelastic properties of polymer systems. It is generally supposed that the same principle carries over into nonlinear phenomena, such as the relationship between viscosity η and shear rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document}. Guided by this principle and the forms of various molecular theories, one would expect that η—\documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} data on the same polymer at different temperatures would superimpose when plotted as η/η0 versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document}η0/ρT, η0 being the limiting viscosity at low shear rates, ρ the polymer density, and T the absolute temperature. Data on polystyrene melts, obtained in a plate-cone viscometer, appear systematically to violate this principle in the range 140–190°. Such anomalies are absent in concentrated solutions of polystyrene. The trends are similar to those reported by Plazek in the steady-state compliance of polystyrene melts near T_g, but they appear to persist to higher temperatures than the compliance anomaly.     

Structure of styrene–butadiene–styrene block copolymers by diffusion analysis

In this study, solvent sorption was used to investigate the morphology of a styrene–butadiene–styrene (SBS) triblock copolymer. The sorption process was found to deviate from the normal Fickian character, usually found in conventional elastomer–solvent systems, because of the presence of an interfacial region for both polybutadiene and polystyrene. This interphase absorbed solvent at a temperature below its glass transition and contributed to the resulting non-Fickian time-dependent diffusion process. The equilibrium diffusion coefficient was estimated to be 3.2 × 10^?7 cm²/sec regardless of the casting surface. Nevertheless, according to the sorption measurements, the casting surface did have an effect on the approach to equilibrium. The results indicated a denser packing of the molecules and hence a decreased diffusion coefficient for Teflon and glass cast films, because of internal stresses left within the films during casting.     

泥鳅体表黏液的流变行为

研究了无鳞鱼———泥鳅的体表黏液流变行为,发现黏液的稳态流动存在着3个不同区域:第一区域内,黏度随剪切速率(γ)变化不显著,呈现牛顿流动行为;第二区域内,随γ增大,黏度下降,呈现非牛顿行为;第三区域内,随γ继续增大,黏度又基本保持不变.黏液表观黏度(ηa)与γ的关系可用Carreau模型很好地拟合,其增比黏度(ηsp)与浓度(c)的关系为ηsp∝c1.5,表明黏液处于亚浓缠结区域.在测试频率(ω)范围内,黏液的动态储能模量(G′)大于动态损耗模量(G″),表明与黏性相比较,弹性占优,且G′及G″随ω变化不显著.存在一临界温度(35℃),当低于35℃时,黏液黏度随温度变化不显著,当高于35℃时,黏液变性,表现出不同的流变行为.     

Dependence of the frequency dispersion of the bulk viscosity coefficient of solutions of electrolytes on the nature of the decay of relaxing fluxes

The region of the frequency dispersion of the bulk viscosity coefficient η_V(ω) of solutions of electrolytes is studied as a function of the nature of the decay of the stress tensor in the momentum and configuration space, the analytical expressions of which are derived by means of kinetic equations. Numerical calculations of η_V(ω) for a water solution of NaCl are performed over a wide range of frequencies, temperatures, and densities using a selection of the potentials of intermolecular interaction Φ{in{itab}}(|\(\vec r\)|) and radial distribution function {itg}{in{itab}}(|\(\vec r\)|). It is shown that the region of frequency dispersion η_V(ω) based on the power law of the decay of the stress tensor is wide (~10⁵ Hz), while the region based on the exponential law is narrow (~10² Hz).     

Influences of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile solutions around the sol-gel threshold

Viscoelastic experiments were performed to study the influence of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile (PAN) solutions around the sol-gel threshold. The dynamic critical parameters around the sol-gel threshold were determined using dynamic rheometer. The sol-gel transition takes place at a critical gel temperature at which the scaling law of G′(ω) ∼ G″(ω) ∝ ωⁿ holds, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. Although the gel points of PAN solutions increase with increasing H₂O content, the results show that the scaling exponent n at the gel point is found to be universal for all ternary PAN solutions, which is independent of temperature and H₂O content, indicating the similarity of the fractal structure in the critical PAN gels. The gelation of ternary PAN solutions induced by adding a nonsolvent and by decreasing the temperature is demonstrated to be a thermoreversible process, which implies that the PAN gels are physical gels. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2637–2643, 2008     

Nonlinear stress relaxation of a styrene–butadiene random copolymer

The isothermal uniaxial stress relaxation response in the vicinity of the glass-to-rubber transition has been measured for a lightly crosslinked poly(styrene–butadiene) random copolymer, 85% styrene by weight. The volume change during stress relaxation was determined by measuring the time-dependent lateral contraction of the specimen with a Hall-effect proximity detector. The specimen exhibited an instantaneous dilation upon application of the strain and a subsequent time-dependent volume decrease. The stress relaxation behavior and the associated volume relaxation were determined for a variety of strains and temperatures in both the linear and nonlinear viscoelastic regime. As the applied strain was increased the isothermal tensile modulus decreased and the shape of the log(modulus) vs. log(time) curve was altered. At equal levels of strain the tensile modulus exhibited increasing deviations from the linear viscoelastic response as the temperature was decreased. The maximum difference between the nonlinear tensile modulus and the linear viscoelastic response was observed at short times. Subsequently, the nonlinear tensile modulus began to approach the linear viscoelastic modulus with increasing time. Both the instantaneous dilation and the magnitude of the time-dependent part of the volume change increased as the level of applied strain was increased and/or as the temperature was decreased. The observed nonlinearity in the tensile stress relaxation response has been quantitively related to the experimentally measured volume relaxation with a free-volume model.