Volume Fraction Of Block Research Articles

Gyroid Structures and Morphological Control in Binary Blends of Polystyrene-block-polyisoprene Diblock Copolymers

We report the morphology and phase behaviors of binary blends of polystyrene-block-polyisoprene (SI) copolymers, which were studied by a small-angle X-ray scattering (SAXS) technique. The neat SI copolymers employed in this study have almost the same molecular weights but different volume fractions of polystyrene block (φPS = 0.26 and 0.65). Blends with various overall volume fractions of polystyrene block ( ), ranging from 0.41 to 0.60, were prepared from these two SI copolymers. It was found that the morphology can be controlled by adjusting . For the blends with 0.57 ≤ ≤ 0.60, the morphological transition from lamellae to gyroid phases was observed upon increasing the temperature. We present an experimentally determined phase diagram for the blend by plotting χZ vs , where χ is the interaction parameter and Z is the reduced value of the total degree of polymerization of the block copolymer.

Molecular Design, Synthesis, and Characterization of Liquid Crystal−Coil Diblock Copolymers with Azobenzene Side Groups

The synthesis and characterization of a family of well-defined liquid crystal−coil (LC−coil) diblock copolymers have been carried out. The block copolymers in this study have been designed to have nearly identical molecular weight azobenzene-containing LC blocks in order to eliminate possible variations in LC behavior caused by the differences in the LC block molecular weight. Quantitative hydroboration chemistry was used to convert the pendent double bonds of an isoprene block to hydroxyl groups to which the mesogenic groups were attached via acid chloride coupling. The LC homopolymer and the block copolymers (LC volume fraction from fLC = 0.82 to fLC = 0.20) all exhibited smectic mesophases with similar clearing transition temperatures. The clearing transition enthalpies strongly depend on the block composition ratio and decrease as the LC block volume fraction decreases. Solvent-casting of a lamellar LC−coil copolymer (SICN5-66/60) resulted in an oriented bulk film in which both the axes of the mesogenic groups and the lamellar interfaces lie parallel to the film surfaces. A LC cylinder morphology was observed with a fLC = 0.22 LC-containing block (SICN5-176/55) using TEM and confirmed by SAXS measurements. This is the first observation with the LC block in a cylinder microdomain. Other morphologies (bicontinuous, LC sphere) were observed by TEM while retaining the smectic order in the LC microphase.

Phase Behavior of Pure Diblocks and Binary Diblock Blends of Poly(ethylene)−Poly(ethylethylene)

We have determined the phase behavior of a series of model poly(ethylene)−poly(ethylethylene) (PE−PEE) diblock copolymers with poly(ethylene) block volume fractions (fPE) ranging from 0.25 to 0.46. Four ordered microstructures make contact with the ODT: spheres arranged on a body-centered cubic lattice (Im3m space group), cylinders packed on a hexagonal lattice, a bicontinuous structure of space group Ia3d, and lamellae. A fifth ordered phase, tentatively identified as hexagonally perforated layers (HPL), separates the cylindrical and lamellar morphologies at moderate or greater degrees of segregation. Binary blends of fPE = 0.37 and 0.46 diblocks were used to investigate the bicontinuous cubic phase region in greater detail; these experiments indicate that this phase extends as much as 100 °C below the ODT for 〈fPE〉 values in the blend from 0.385 to 0.420.

Synthesis and Characterization of Model Polyalkane−Poly(ethylene oxide) Block Copolymers

We describe the preparation of a new set of amphiphilic block copolymers with well-defined molecular weights and block volume fractions. The synthesis of a variety of polyalkane−poly(ethylene oxide) block copolymers was accomplished by a new polymerization−hydrogenation sequence. Initially, anionic polymerization of either butadiene or isoprene was performed followed by end capping with ethylene oxide. The resulting hydroxyl-terminated polydienes were catalytically hydrogenated to give the corresponding hydroxyl-terminated polyalkanes. These polymeric alcohols were then titrated with potassium naphthalenide to yield the analogous potassium alkoxides. This type of macroinitiator was employed in the polymerization of ethylene oxide. Seventeen polyalkane−poly(ethylene oxide) block copolymers were prepared in near quantitative yields with molecular weights ranging from (1.4 to 8.7) × 103 and poly(ethylene oxide) volume fractions ranging from 0.29 to 0.73. These polymers are model materials for block copolymer...

Effect of Volume Fraction on the Order−Disorder Transition in Low Molecular Weight Polystyrene-block-polyisoprene Copolymers. 2. Order−Disorder Transition Temperature Determined by Small-Angle X-ray Scattering

Small-angle X-ray scattering (SAXS) was employed to investigate the order−disorder transition of low molecular weight polystyrene-block-polyisoprene (SI diblock) copolymers with widely varying composition. The SI diblock copolymers investigated in the present study have molecular weight varying from 15 000 to 41 000 and the volume fraction of PS block varying from 0.186 to 0.811. Low-temperature resolution SAXS experiments, which involved a temperature increment of ≥10 °C, did not allow us to accurately determine the order−disorder transition temperatures (TODT) of the SI diblock copolymers, but high-temperature resolution SAXS experiments, which involved measurements of the scattering profiles across TODT with a small temperature increment of ≤1 °C, enabled us to determine the values of TODT with accuracy to within ±1 °C. TODT was determined to be the temperature at which the reciprocal of the maximum intensity and the square of the half-width at half-maximum of the first-order scattering maximum change discontinuously with the reciprocal of absolute temperature. We compared the values of TODT determined from SAXS experiments with those determined from rheological measurements reported in part 1 of this series.

Effect of Volume Fraction on the Order-Disorder Transition in Low Molecular Weight Polystyrene-block-Polyisoprene Copolymers. 1. Order-Disorder Transition Temperature Determined by Rheological Measurements

Low molecular weight polystyrene-block-polyisoprene (SI diblock) copolymers having a wide range of compositions were synthesized via anionic polymerization, and the order-disorder transition temperatures (T ODT ) of the block copolymers were determined using dynamic viscoelastic measurements. We have shown that logarithmic plots of dynamic storage modulus (G') versus dynamic loss modulus (G) are very effective to determine the T ODT S of the block copolymers. The experimentally determined T ODT S are compared with the predictions made from currently held theories. We point out that the reliability of the predicted T ODT of a block copolymer depends, among many factors, on the accuracy of the expression for the Flory-Huggins interaction parameter X. We observed that the SI diblock copolymers having a low volume fraction of polystyrene or polyisoprene block (e.g., less than about 0.2) exhibited liquidlike rheological behavior at temperatures below their T ODT S, for instance at temperatures as low as about 30°C below the T ODT . We attribute this observation to the existence of defects in the long-range spatial order of microdomains (grain-boundary defects) which are continuous in three-dimensional space. Using the Onuki analysis that the extent of composition fluctuations of a block copolymer near the critical temperature, at a given temperature difference AT = T - T c from the critical temperature T c , is determined very much by the temperature dependence of the Flory-Huggins interaction parameter x, we have shown that the extent of composition fluctuations for SI diblock copolymers in the disordered state near the critical temperature is very small. Specifically, we have shown that for the same extent of composition fluctuations e, defined by e = 2 (1 - X/X c ), where X c is the value of X at the critical point, a temperature difference ΔT of 1°C for an SI diblock copolymer corresponds to ca. 20°C for a 1,2-polybutadiene-block-1,4-polybutadiene copolymer. This difference in AT between the two block copolymer systems is attributed to the difference in the temperature coefficient B appearing in the expression, X = A + B/T. Using the Onuki analysis, we have shown further that the extent of composition fluctuations in the disordered state near the critical temperature varies little with block copolymer composition.

Order and Disorder in Symmetric Diblock Copolymer Melts

The thermodynamic and dynamic properties of several partially deuterated symmetric (equal block volume fraction) polyethylene-poly(ethylethylene) (PE-PEE), poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE), and polyethylene-poly(ethylene-propylene) (PE-PEP) diblock copolymers were characterized above and below the order-disorder transition (ODT). A lamellar morphology was established for all temperatures below the ODT using small-angle neutron scattering (SANS) measurements with shear-aligned specimens. As T ODT is approached, the lamellar order weakens as evidenced by the loss of higher order SANS reflections and azimuthal smearing of the principal equitorial scattering peaks. SANS and rheology experiments also provided unambiguous evidence of composition fluctuations above the order-disorder transition temperature in each system. However, the temperature range of the fluctuations in the disordered state depends on the degree of polymerization, in accordance with fluctuation theory. A crossover in the disordered state between slightly and moderately stretched coils was also documented by SANS, coincident with the temperature where fluctuations become apparent rheologically.

Monte Carlo simulation of phase separations of block copolymers and of corresponding blends

AbstractThe Monte Carlo method has been used to simulate the phase separations of block copolymers and of corresponding blends with very high concentration (sum of volume fractions of blocks A and B: ϕA + ϕB = 0,9545). Our main findings are as follows: (1) The mixing is nonrandom even in the athermal limit. (2) The nonselective good solvent molecules (ϕV = 0,0455) are mostly located at the interface between A‐ and B‐rich phases, thus, it is not true that solvent and monomeric units will remain mixed at all temperatures. (3) Even for the same microscopic A‐B interaction energy, ε, and at the same temperature, the Flory‐Huggins parameter χ of block copolymers is always higher than that of corresponding blends, and the χ values of block copolymers and corresponding blends have different ε‐dependencies. (4) The critical values of χ both for block copolymer and corresponding blend are obtained and compared with the meanfield theoretical predictions. It is found that the ratio of χc (block)/χc (blend) is qualitatively compatible with the prediction of the Flory‐Leibler theory.

Influence of the interphase on mechanical properties in block copolymers

AbstractPolystyrene‐block‐poly(styrene‐ran‐isoprene)‐block‐polyisoprene characterized by a morphology of spherical rubbery microdomains in a continuous polystyrene matrix is considered as a model for rubber‐toughened thermoplastics in which the volume fraction of the interfacial layer can be varied by varying the volume fraction of the interblock. Stress‐strain measurements of these systems are presented in relation to their morphology, interblock volume fraction and molecular weight of the polystyrene block. The results indicate that the energy of fracture is increased with increasing width of the interfacial layer without change of Young's modulus.