Microphase Separation Temperature Research Articles

Study of the open porosity of carbon materials using thermoporometry

Thermoporometry is a calorimetric method for characterizing pore structure from the melting or freezing point depression of a liquid confined in a pore, by the reason of the added contribution of surface curvature to the phase-transition free energy. Porous materials are widely used in the production of noise and vibration-absorbing coatings, thermal insulation, filters, etc. We present the results of studying the porous structure using thermoporometry, a method based on lowering the melting point of the working substance (penetrant) which previously filled micro- and mesopores of the sample under study. The open porosity of carbon materials based on phenol-formaldehyde resins and a pore former obtained after pyrolysis under different conditions of microphase separation induced by polymerization is analyzed. The tests were carried out using a differential scanning calorimeter. Bidistilled water, which has a relatively high value of the enthalpy change upon melting of the crystalline phase is used as a penetrant to lower the error of measurements. Differential and integral curves of the size distribution of micro- and mesopores are presented. It is shown that an increase in the microphase separation temperature entails an increase in the total porosity. Moreover, an increase in the polymerization rate of phenol-formaldehyde resin due to resin modification with m-cresol also facilitated an increase in the cumulative volume of micro- and mesopores. It is shown that replacement of phenol with paracresol leads to an increase in the total porosity even under a significant decrease in resole resin polycondensation rate. The results obtained can be used in the development of carbon matrices with controlled parameters of the mass transfer.

Structural Behavior of Cylindrical Polystyrene‐block‐Poly(ethylene‐butylene)‐block‐Polystyrene (SEBS) Triblock Copolymer Containing MWCNTs: On the Influence of Nanoparticle Surface Modification

AbstractIn this work, the influence of carbon nanotubes (CNTs) on the self‐assembly of nanocomposite materials made of cylinder‐forming polystyrene‐block‐poly(ethylene‐butylene)‐block‐polystyrene (SEBS) is studied. CNTs are modified with polystyrene (PS) brushes by surface‐initiated atom transfer radical polymerization to facilitate both their dispersion and the orientation of neighboring PS domains of the block copolymer (BCP) along modified CNT‐PS. Dynamic rheology is utilized to probe the viscoelastic and thermal response of the nanoscopic structure of BCP nanocomposites. The results indicate that nonmodified CNTs increase the BCP microphase separation temperature because of BCP segmental confinement in the existing 3D network formed between CNTs, while the opposite holds for the samples filled with modified CNT‐PS. This is explained by severely retarded segmental motion of the matrix chains due to their preferential interactions with the PS chains of the CNT‐PS. Moreover, transient viscoelastic analysis reveals that modified CNT‐PS have a more pronounced effect on flow‐induced BCP structural orientation with much lower structural recovery rate. It is demonstrated that dynamic‐mechanical thermal analysis can provide valuable insights in understanding the role of CNT incorporation on the microstructure of BCP nanocomposite samples. Accordingly, the presence of CNT has a significant promoting effect on microstructural development, comparable to that of annealing.

Rheological characterization of nanostructured material based on Polystyrene-b-poly(ethylene-butylene)-b-polystyrene (SEBS) block copolymer: Effect of block copolymer composition and nanoparticle geometry

Block copolymer (BCP) nanocomposite systems are of broad interest; however, reports on the role of nanoparticles on microphase separation behavior are rare. The goal of present study is to investigate the preparation of composite nanostructured materials containing Multi-Walled Carbon Nanotubes (MWCNTs) or graphene nanoplates. BCP nanocomposites based on the linear triblock copolymer, Polystyrene-b-poly(ethylene-butylene)-b-polystyrene (SEBS), with different morphological structure were prepared by melt mixing. The results of temperature sweep experiments showed an enhancing effect of both MWCNT and graphene nanosheets on increasing the microphase separation temperature as well as accelerating its kinetic, resulting from the confinement of BCP segments, with graphene nanosheets providing a more severely confined geometry for polystyrene segments in contrast to MWCNTs. Additionally, DMTA results indicated a promotion of the BCP microphase separation by incorporation of nanoparticles. Transient flow measurements followed by time sweep test suggested the existence of a special 3D network microstructure caused by nanoparticles/domain interactions.

Ordering Transition in Salt-Doped Diblock Copolymers

Lithium salt-doped block copolymers offer promise for applications as solid electrolytes in lithium ion batteries. Control of the conductivity and mechanical properties of these materials for membrane applications relies critically on the ability to predict and manipulate their microphase separation temperature. Past attempts to predict the so-called “order–disorder transition temperature” of copolymer electrolytes have relied on approximate treatments of electrostatic interactions. In this work, we introduce a coarse-grained simulation model that treats Coulomb interactions explicitly, and we use it to investigate the ordering transition of charged block copolymers. The order–disorder transition temperature is determined from the ordering free energy, which we calculate with a high level of precision using a density-of-states approach. Our calculations allow us to discern a delicate competition between two physical effects: ion association, which raises the transition temperature, and solvent dilution, w...

Self-Assembly of Coil/Liquid-Crystalline Diblock Copolymers in a Liquid Crystal Solvent

Diblock copolymers having a random-coil polymer block (polystyrene, PS) connected to a side-group liquid crystal polymer (SGLCP) self-assemble in a nematic liquid crystal (LC), 4-pentyl-4′-cyanobiphenyl, into micelles with PS-rich cores and SGLCP-rich coronas. The morphologies of block copolymers with varying PS content are characterized as a function of temperature and concentration using small-angle neutron scattering, rheometry, and transmission electron microscopy. Unlike conventional solvents, the nematic LC can undergo a first-order transition between distinct fluid phases, accessing the regimes of both strong and slight selectivity in a single polymer/solvent pair. Micelles dissolve away above a microphase separation temperature (MST) that is often equal to the solution’s isotropization point, TNI. However, increasing or decreasing the polymer’s PS content can shift the MST to be above or below TNI, respectively, and in the former case, micelles abruptly swell with solvent at TNI. Comparable effect...

Grazing incidence small-angle neutron scattering—an advanced scattering technique for the investigation of nanostructured polymer films

With grazing incidence small-angle neutron scattering (GISANS), the limitations of conventional small-angle neutron scattering with respect to extremely small sample volumes in the thin-film geometry are overcome. GISANS turned out to be a powerful advanced scattering technique for the investigation of nanostructured polymer films. Similar to atomic force microscopy the surface topography is probed. In addition, buried structures from inside the film are detectable. As an example of the actual limits, nanostructures resulting from destabilized diblock copolymer films of poly(styrene-block-paramethylstyrene) in the highly confined regime are investigated. The stability of the structure, introduced by toluene vapor treatment, against annealing above the micro-phase separation temperature is shown.

Dewetting of Thin Diblock Copolymer Films: Spinodal Dewetting Kinetics

The stability of thin diblock copolymer films with respect to the annealing above the microphase separation temperature is investigated on long time scales and at different film thicknesses. Poly(styrene-block-p-methylstyrene) diblock copolymer films on top of silicon substrates are examined with scanning force microscopy and off-specular X-ray scattering. At film thickness below the lamellar spacing of the bulk material a dewetting is observed. The kinetics of the film destabilization are explainable within a spinodal dewetting model. The observed dewetting structures belong to a wet dewetting. In the case of film thicknesses which enable the buildup of a few lamellae no sign of instability was detected.

Microphase separation in oxyethylene/oxybutylene copolymers with diblock and triblock architectures

EBE and BEB triblock copolymers were prepared and characterized. Microphase separation in the melt state was studied, and the results combined with those for EB and BEB copolymers reported previously. The microphase separation temperature (MST) was determined from the temperature dependence of SAXS. There was a large difference in MST between the diblock and triblock copolymers as expected from theory. The Flory-Huggins parameter (χ) was independent of block architecture for all three series provided that the E block lengths in the EBE copolymers exceeded 65.

Morphology Study of Polystyrene-Polybutadiene-polycaprolactone (PS-b-PB-b-PCL), Polybutadiene-Polycaprolactone (PB-b-PCL), and Polystyrene-Polycaprolactone (PS-b-PCL) Semicrystalline Block Copolymers

Abstract Semicrystalline block copolymers containing one crystalline block, such as PCL, show more complicated morphologies than amorphous block copolymers. This is because the microstructure of semicrystalline block copolymers is determined by the interplay between the microphase-separation temperature, crystallization temperature, and glass transition temperatures of the respective blocks. This paper describes initial results of thermodynamic and kinetic effects associated with microphase separation and crystallization in as-cast semicrystalline (PS)0.62(PCL)0.38, (PB)0.2(PCL)0.8 and (PS)0.35(PB)0.15(PCL)0.5 block copolymers, where the subscripts denote the mass fraction of each block. Semicrystalline block copolymers (PS-b-PCL with Mn=209,000 g/mol, PB-b-PCL with Mn=146,000 g/mol and PS-b-PB-b-PCL with Mn= 150,000 g/mol) were prepared by anionic polymerization. Films were cast from 5% (mass fraction) solutions in toluene into ∼lmm thick films. A relatively slow solvent evaporation over approximately a 10 d period was achieved by partially covering the dishes containing polymer solutions.

Ordering and structure formation in triblock copolymer solutions. Part I. Rheological observations

The linear viscoelastic properties of a poly(styrene)-poly(ethylene, butylene) triblock copolymer in the presence of a hydrocarbon oil have been investigated. The samples were followed rheologically, during annealing at different temperatures. In a relatively small temperature interval, tan δ-curves, recorded at different frequencies, evolved in a pattern which is typical for a physical gelation. It allowed determination of the gel time. The kinetics of gel formation were found to be affected by the annealing temperature, and the rate was maximum at an intermediate temperature that depended on the polymer content of the sample. Increasing the polymer content shifted this maximum to higher temperatures. At lower temperatures, a glass transition of the polystyrene end-blocks was detected. The microphase separation temperature however, could not be determined. In addition, the influence of strain and the applicability of time-temperature superposition was analysed.

Copolymers with controlled distribution of comonomers along the chain, 1. Structure, thermodynamics and dynamic properties of gradient copolymers. Computer simulation

AbstractProperties of copolymer systems with various composition distributions of comonomers A and B along the chain are analyzed by means of computer simulation using the Cooperative Motion Algorithm. Extreme cases of random and block copolymers, as well as the intermediate cases of gradient copolymers with various composition gradients along the chain are considered. Thermodynamic, static and dynamic properties of systems are predicted in a broad temperature range including both the homogeneous and the microphase separation regime. A possibility to control the microphase separation temperature and consequently a possibility to control a shift of the properties along the temperature scale by changing the monomer distribution within copolymer chains is demonstrated.

Symmetric Blends of Complementary Diblock Copolymers: Multiorder Parameter Approach and Monte Carlo Simulations

Symmetric diblock copolymer blends AfB1-f/A1-fBf (0 ≤ f ≤ 0.5) are theoretically discussed in terms of a multiorder parameter approach and numerically investigated by Monte Carlo simulations. Theoretically, our main result is that below f ≅ 0.3, but still in the microphase separation region given by 0.21 ≤ f ≤ 0.5, the concentration profiles of the long and short A-blocks as well as the long and short B-blocks are out of phase. Monte Carlo simulations were used to investigate the nature of the phase transition, micro versus macro, as a function of f. Using the canonical ensemble, the microphase separation temperature (MIST) was determined. The macrophase separation temperature (MAST) was studied with the semi-grand-canonical ensemble combined with the histogram extrapolation technique. The phase diagram differs considerably from the theoretical predictions due to the stretching/polarization of the molecules already far above the transition temperature, thus stabilizing the macroscopically homogeneous state. The out-of-phase behavior between the long and short blocks near the critical value f ≅ 0.21, separating the micro- and macrophase separation regimes, was confirmed by the simulations.

Density Discontinuity at the Microphase Separation Transition of a Symmetric Diblock Copolymer

The temperature dependence of the density near the microphase separation temperature for a symmetric polystyrene-polyisoprene diblock copolymer is investigated by dilatometry and reflection ellipsometry.

The influence of styrene–butadiene diblock copolymer on styrene–butadiene–styrene triblock copolymer viscoelastic properties and product performance

AbstractThe effects of the styrene–butadiene (SB) diblock copolymer on the viscoelastic properties of styrene–butadiene–styrene (SBS) triblock copolymers were examined in both in the the neat state and within specific product applications. The addition of the SB diblock copolymer into a pure SBS triblock copolymer resulted in a significant decrease in the plateau storage modulus and a quantitative linear rise in tan delta. In a pure triblock, in which all endblocks are anchored in polystyrene domains, all entanglements are physically trapped. The SB diblock embodies untrapped polybutadiene endblocks that are able to relax stress by chain reptation through the rubbery polybutadiene matrix. The SB diblock copolymer quantitatively lowered the microphase separation temperature (MST) of the SBS triblock copolymer. These changes in linear viscoelastic behavior manifest themselves into a reduction in the efficiency and performance of the SBS triblock copolymer in asphalt pavement binders and hot‐melt adhesive blends. Specifically, the SB diblock diminished the complex shear modulus and elasticity of a polymer‐modified asphalt, which translated into lower predicted rutting specification values. The increase in diblock content altered the viscoelastic response of the hot‐melt adhesive blend, translating into a reduction in the shear holding power and shear adhesion failure temperature. The lack of network participation, coupled with the relaxation of the polybutadiene endblocks, accounts for the lower strength and greater temperature susceptibility of the diblock‐containing systems. © 1995 John Wiley & Sons, Inc.

Pattern and Segment Relaxation in a Block Copolymer Melt following Step Shear Flow

The relaxation dynamics of a homogeneous poly(styrene)-poly(butadiene) star diblock copolymer melt following step shear flow is studied as its temperature is lowered through the microphase separation temperature (MST). Transient Raman scattering, birefringence, and mechanical rheometry measurements are used to investigate segment and composition pattern relaxation dynamics. The birefringence was determined to be primarily form birefringence, and its relaxation rate was found to decrease dramatically in the vicinity of the MST. At temperatures close to the MST, the shear stress, first normal stress difference, and segment orientation anisotropy (from Raman scattering measurements) relaxation rates also decreased

Optical and Mechanical Properties of a Star Diblock Copolymer Melt in Oscillatory Shear Flow

Mechanical rheometry and birefringence measurements are used to study the low-frequency dynamics of a homogeneous, star diblock copolymer melt as it is cooled through its microphase separation temperature (MST). Anomalous low-frequency behavior in both mechanical and optical properties are observed at temperatures close to the MST. We conclude that a general condition must be satisfied before unusual low-frequency behavior is observed, namely that the time scale of flow must be less than the relaxation time of structural features perturbed by flow

Rheology of ordered and disordered symmetric poly(ethylenepropylene)-poly(ethylethylene) diblock copolymers

The linear dynamic mechanical properties of nearly symmetric 55:45 PEP-PEE diblock copolymers and the corresponding homopolymers are described. The results show the influence of composition fluctuations on the rheological properties around the microphase separation transition and confirms the first-order nature of this transition for hearly symmetric samples dynamic shear measurements obtained after a temperature quench from just above to just below the microphase separation temperature demonstrate the ability to supercool the disordered material and provide a measure of the subsequent ordering kinetics

Thermodynamics of tapered styrene-butadiene block copolymers

Abstract A modified version of the Leary-Henderson-Williams equilibrium thermodynamic theory is used to predict microstructural properties for tapered poly(styrene-butadiene) diblock copolymers containing a random copolymer between the blocks. The interphase thickness, interphase volume fraction, and repeat distance are all predicted to increase as the volume fraction of the added random copolymer increases, whereas the microphase-separation temperature is shown to decrease. Comparison of model results with experimental data for tapered poly(styrene-isoprene) diblock copolymers reveals good agreement for small amounts of the additional random copolymer (up to about 20 vol%). Trends indicate that, at large fractions of random copolymer, the tapered block copolymer may be in a homogeneous state when the nontapered block copolymer would be phase separated under the same conditions.