ALBERT

Notes: The small angle neutron scattering (SANS) technique has been used to study the concentration fluctuations of binary polymer mixtures under shear. Two different polymer systems, deuterated polystyrene/poly(vinylmethylether) and deuterated polystyrene/polybutadiene, have been studied as a function of temperature and shear rate. Due to the small wavelength of the incident neutron radiation compared with light, the shear dependence of concentration fluctuations in the one-phase region and in the strong shear limit has been obtained from the q dependence of the scattering structure factor for the first time. From a detailed analysis of the scattering structure factor S(q) a crossover value of the wave number qs has been obtained as a function of temperature and shear rate. This crossover wave number represents the inverse of the lowest fluctuation mode which is not affected by shear. The temperature, viscosity, and shear rate dependence of this experimentally determined qs agree well with a simple rotatory diffusion model and also the dynamic mode–mode coupling analysis of Kawasaki and Ferrell. The apparent spinodal temperature as a function of shear rate is shown to be consistent with the prediction of Onuki.

Notes: The small angle neutron scattering experiments were conducted on N-isopropyl acrylamide (NIPA) gels in D2O and on the corresponding NIPA solutions. The NIPA gels underwent a sharp, but a continuous volume phase transition at 34.6 °C from a swollen state to a shrunken state with increasing temperature. In the case of the gels, an excess scattering due to the presence of crosslinks was observed at low q region (q≤0.02 A(ring)−1), where q is the magnitude of the scattering vector. The scattered intensity function for the gel was well described with a combination of Gauss and Lorentz-type functions, i.e., I(q)=IG(0)exp[−Xi2q2]+[IL(0)/(1+ξ2q2)] as proposed by Geissler et al. IG(0) and IL(0) are the intensities at q=0 for the contributions of Gaussian and Lorentzian functions, respectively. The Gaussian part results from solidlike inhomogeneity, having a characteristic size of Xi, which is due to the introduction of crosslinks into the system. The Lorentzian part is originated from the liquid nature of the local concentration fluctuations of the gel characterized with a thermal blob of dimension ξ. Xi decreases systematically with polymer volume fraction, φ, indicating the nature of Xi being the solidlike inhomogeneity. On the other hand, the intensity function for solutions was well fitted with the so-called Ornstein–Zernike (OZ) equation (a Lorentzian function), i.e., I(q)=[IL(0)/(1+ξ2q2)].It was found that both ξ and IL(0) diverged at the spinodal temperature, Ts. The critical exponents, ν and γ, for the temperature dependence of ξ and IL(0), were estimated to be ∼0.60 and 1.2 for the gel, respectively, which were larger than the values for the solution of the same polymers (ν=0.45 and γ=0.8). These critical exponents for the gels support that the volume-phase transition of gels is classified to the three dimensional Ising model reported by Li and Tanaka. The concentration dependence of ξ and IL(0) was also well described with a power law relationship, i.e., ξ∼φνφ and IL(0)∼φγ;φ. The values of νφ and γφ at 23 °C are −3/4 and ∼−1/4, respectively, for the NIPA solutions, which are in good agreement with the theoretical prediction for polymer solutions in a good solvent. In the case of the NIPA gels, however, both νφ and γφ are ∼−1. These exponents were interpreted by taking account of the effects of crosslinking on the Flory's interaction parameter.

Notes: The static structure factor for poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) gels was investigated in terms of small-angle neutron scattering (SANS). The NIPA/AAc gels underwent a discrete volume phase transition at 50.8 °C from swollen to shrunken states as temperature increased. The static structure factors for NIPA/AAc gels were well described by a Lorentz-type scattered intensity function at temperatures below 34 °C which was near the so-called aitch-theta temperature of NIPA in D2O. At higher temperatures, the static structure factor had a distinct scattering maximum although the gel was still in the highly swollen state. The scattering maximum was located around q=0.02 A(ring) and was temperature and concentration dependent, where q is the magnitude of the scattering vector. The appearance of the peak indicates the strong concentration fluctuations that created polymer rich and poor regions in the system. The contrast in the concentrations appeared to result from the competing interactions in the network, i.e., the electrostatic repulsive interaction between AAc segments on the network and the hydrophobic attractive interaction among NIPA segments. The static structure factors were analyzed quantitatively with the theory of Borue and Erukhimovich for polyelectrolyte solutions in a poor solvent. It is found that the concentration fluctuations lead to a microphase separation between polymer rich and poor domains before the system undergoes the volume-phase transition.

Notes: Time-resolved small-angle neutron scattering (SANS) experiments have been performed on the self-assembling process of a binary mixture of deuterated polybutadiene and protonated polybutadiene at the critical composition. This mixture has an upper critical solution temperature type of phase diagram with the spinodal temperature at 99.2 °C. Specimens held in the single-phase state at an initial temperature (Ti) were quenched to a point inside the spinodal phase boundary at a final temperature (Tf) to induce phase separation via spinodal decomposition (SD). In order to examine the effect that thermal concentration fluctuations have on SD, three different initial temperatures, Ti=102.3 °C, 123.9 °C, and 171.6 °C, were chosen while Tf was fixed at −7.5 °C. The time-dependent SANS structure factor, S(q,t;Tf), showed clear scattering peaks corresponding to the early and intermediate stages of SD. The time changes in the wave number qm(t;Tf) and the intensity Sm(t;Tf) at the peak of S(q,t;Tf) followed different paths depending on the initial temperature. This fact evidences a definite effect of thermal concentration fluctuations on SD (i.e., a significant "memory'' effect). A critical test of the linearized Cahn–Hilliard–Cook theory led to the conclusion that this theory can describe satisfactorily the early stage SD in the deep-quench region.

Notes: The spinodal decomposition (SD) of a critical mixture of deuterated and protonated polybutadiene of nearly equal chain lengths was investigated. This mixture has an upper critical solution temperature type phase diagram and the spinodal temperature at the critical point is 99.2 °C. Phase separation was induced by quenching a single-phase specimen at an initial temperature, Ti (=102.3, 123.9, and 171.6 °C), to a final temperature, Tf (=−7.5, 1.1, and 10.5 °C). The subsequent SD was followed by time-resolved small-angle neutron scattering. The Onsager coefficient, Λ(q;Tf), as a function of wave number q and Tf, derived from experimental growth rates, R(q;Tf), of the Fourier mode of concentration fluctuations and estimation of ST(q;Tf), was compared to the reptation model theories of Pincus and Binder. Experimental Λ(q;Tf) was found to give a q-dependence greater than that given by the theories. Here, ST(q;Tf) denotes the virtual structure factor at Tf inside the spinodal region. The reduced wave number Qm(τ) and intensity S˜m(τ) at the peak of the scattering structure factor in the early and intermediate stages of SD were found to be scalable in terms of a reduced time τ when Ti was fixed and Tf was varied, but not when Tf was fixed and Ti was varied. The failure of the scaling law in the latter instance may be attributed to the fact that the concentration fluctuation at the onset of SD has a different memory of the thermal concentration fluctuation in the single-phase region depending on Ti, which affects the subsequent SD over an extended period of time.

Notes: A shear light scattering photometer with an optical microscope was constructed for the study of polymer blends in a simple shear field. This instrument utilizes a cone and plate or parallel plate geometry for the generation of shear field. The shear rates are controlled by a microstepping motor. The controllable range of shear rate is between 0.002 and 1000 s−1. The bottom plate of the shear cell has a special design to accommodate the microscope objective and a thin disk-type heater for temperature control. The accessible q range is from 0.7 to 4.6 μm−1 with a 632.8 nm He–Ne laser or from 0.9 to 6.0 μm−1 with a 488 nm Ar ion laser. The temperature can be controlled from ambient temperature to 250 °C with ±0.1 °C accuracy. A phase contrast microscope and a fluorescence microscope are built into this photometer for the in situ morphological study of materials of interest. The optics for light scattering and microscopy can be switched back and forth by a simple translational movement of a rail-mounted optical platform, without any realignment, for comparison of data from reciprocal space with that from real space. A bulk polystyrene/polybutadiene blend and a polystyrene/polybutadiene/dioctylphthalate blend were used to demonstrate the performance and versatility of this instrument.

Notes: A combined rheometer and light scattering photometer has been constructed to examine the light scattering behavior of polymer melts and solutions under the influence of a simple shear field. The device utilizes a special lens system and a two-dimensional charge-coupled device array detector which has not been used previously in an apparatus of this type to quantitatively measure the scattering intensity as a function of shear rate. The accessible q range of the instrument is from 3.75×10−4 to 3.0×10−3 nm−1 (2.2°–17.4° scattering angle, with λ=632.8 nm). The rheometer utilizes a cone and plate geometry to generate the shear gradient and is capable of measuring torque (1.8 N m maximum) and normal forces (50 N maximum). An 8% solution of a 50:50 polystyrene/polybutadiene blend in dioctyl phthalate was used to test the apparatus. This sample shows a shear-induced mixing behavior which is consistent with previous measurements by other investigators.

Notes: The effect of shear on the ordering temperature of a triblock copolymer melt of polystyrene-polybutadiene-polystyrene (SBS) is examined by in situ small angle neutron scattering (SANS). Results obtained by SANS are compared to the rheologically determined order–disorder transition temperature, TRODT=115±5 °C. The SANS measurements from a Couette geometry shear cell are then used to construct a "dynamical phase diagram'' based on characteristic changes in the scattering with temperature and shear rate, γ(overdot). A shear rate dependent ordering temperature, Tord(γ(overdot)), is identified as the system is sheared isothermally from the disordered state. The scattering behavior is shown to be highly strain dependent. We compare our findings on the shear rate dependence of the ordering transition in triblock materials with previous observations on diblock copolymer materials and theoretical expectations for the shear rate dependence of the order–disorder transition temperature. A simple scaling argument leads to a good description of the shear rate dependence of Tord(γ(overdot)) in both diblock and triblock copolymer measurements over the range of shear rates examined. © 1996 American Institute of Physics.