Alan I. Nakatani

Shear stabilization of critical fluctuations in bulk polymer blends studied by small angle neutron scattering

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 dif...

Modification of the phase stability of polymer blends by fillers

Abstract We investigate the influence of filler particles on the phase stability of a model blend of polystyrene (PS) and polybutadiene (PB). The upper critical solution temperature cloud point curve of a PS:PB blend is “destabilized” (upward shift of cloud point temperature) by the addition of untreated fumed silica filler particles. Preliminary cloud point measurements on surface-functionalized fumed silica particles were also performed. A small downward shift of the cloud point temperature was observed by functionalizing the filler particles with grafted polystyrene chains, while grafting with a silane coupling agent resulted in a large apparent “stabilization” effect or a decrease of the cloud point temperature. (Further measurements on surface-functionalized filler particles over a range of polymer compositions are required to determine the filler concentration dependence of the shift of the blend critical temperature.) Surface treatments were selected to demonstrate that modifying the filler particle surface chemistry can alter the blend phase boundary. These phase boundary shifts have significance in commercial blends containing dispersions of filler particles.

Deviation from mean‐field behavior in a low molecular weight critical polymer blend

A deviation from mean‐field behavior is observed in the static susceptibility and correlation length measured with small angle neutron scattering as a function of temperature near the phase boundary of a relatively low molecular weight critical polymer mixture. The possibility of a fluctuation influenced crossover from mean‐field to nonmean‐field behavior is considered.

The influence of shear on the ordering temperature of a triblock copolymer melt

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. A shear rate dependent ordering temperature, Tord(γ), 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 des...

A rheometer with two‐dimensional area detection for light scattering studies of polymer melts and solutions

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.

Apparatus for Simultaneous Small Angle Neutron Scattering and Steady Shear Viscosity Studies of Polymer Melts and Solutions

The design and construction of an apparatus for studying the simultaneous small angle neutron scattering (SANS) and steady shear viscosity behavior of polymer melts and concentrated solutions is discussed. Successful operation of the device is demonstrated on a blend of 20 weight percent deuterated polystyrene and 80 weight percent poly(vinylmethylether). The effects of shear on the critical behavior of the blend are observed in the SANS behavior as a function of temperature and shear rate and indicate shear induced mixing behavior for the range of shear rates examined. The steady shear viscosity results alone are insufficient for detecting the transition from one to two phases. The examination of shear effects in polymer blends is important for understanding the critical behavior of binary systems. Technologically, knowledge of the phase behavior of polymer blends under shear are important for the design and improvement of commercial blend processing.

Anisotropic phase separation kinetics in a polymer blend solution following cessation of shear studied by light scattering

Abstract A light scattering instrument capable of monitoring the scattering from samples under the influence of a simple shear field has been constructed. The apparatus consists of transparent cone-and-plate fixtures and a two-dimensional charge coupled device array detector. The detector unit is also capable of measuring the scattering patterns as a function of time. The phase separation kinetics of an 8% solution of a polystyrene/polybutadiene (50:50) blend in dioctyl phthalate following cessation of a steady shear is monitored with this instrument. The sample is two-phase in the quiescent state and the applied shear is sufficient to suppress all scattering observed from the quiescent sample (shear-induced mixing). The evolution of the scattering profiles following cessation of shear is quite different parallel and perpendicular to the original flow direction. In the normal direction, a spinodal growth and coarsening mechanism similar to that observed in temperature quench experiments is observed. In the parallel direction, a different mechanism is followed. We believe this is the first report of this type of anisotropic behaviour in the phase separation kinetics of a polymer blend.

Inelastic neutron scattering from filled elastomers

Inelastic neutron scattering experiments are powerful techniques for evaluating local molecular dynamics. These methods are especially sensitive to hydrogen atoms containing motions. An overview of these experimental techniques is presented. Neutron filter analyzer and time-of-flight spectroscopy methods are used to characterize the local dynamics of polymers in the presence and absence of fillers. Of particular interest is the comparison between bound rubber attached to the filler surface and the pure, unbound rubber. A commercial synthetic polyisoprene containing approximately 100% cis-1,4 isomers was compounded with three different carbon blacks: N299, G299 (graphitized N299), and N762. Soxhlet extraction on each of the samples was performed so that corresponding samples containing purely bound rubber with filler were obtained. The filter analyzer and time-of flight spectra show distinct differences between the bound and pure rubber as well as differences based on carbon black type. Correlation of the spectral differences to the type of carbon black and initial concentration of carbon black are discussed.

Flow‐induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Small-angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow-induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow-induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow-induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers.

Shear-induced changes in the order-disorder transition temperature and the morphology of a triblock copolymer

A summary of our work on a triblock copolymer under steady shear is presented. The experiments were conducted using small-angle neutron scattering (SANS), as a function of shear rate and temperature, and transmission electron microscopy (TEM) on quenched specimens. The triblock copolymer is composed of polystyrene-d 8 /polybutadiene/polystyrene-d 8 , and the ordered microstructure normally consists of hexagonally packed cylinders. Two temperature-dependent, characteristic shear rates, γ C1 and γ C2 , are identified from the scattering results. The first characteristic shear rate identifies the shear rate required to go from a disordered state to an ordered state, while the second characteristic shear rate is interpreted as the shear rate necessary to produce a different ordered morphology. This latter transition was previously identified as being analogous to a martensitic transition in metal alloys. The supporting SANS and TEM evidence for the new ordered phase is presented. Dynamical aspects of the structural transition between different ordered triblock morphologies are discussed using the model of a martensitic-like transformation.