Short Diblock Research Articles

Ordered Structure in Blends of Block Copolymers. 4. Location of the Short Diblock

We used small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) to examine a blend of two polystyrene-block-polyisoprene polymers with different molecular weights (6.73 × 104 and 1.0 × 104) and volume fractions of polystyrene (0.81 and 0.52, respectively), i.e., a blend of the type (A−B)α/(A−B)β. The results show that the junctions of the short diblock copolymer are localized at the interface in the lamellar microdomains at low temperatures with a relatively strong segregation and that more of the short diblock copolymer is delocalized with their junctions moving away from the interface with increasing temperature. The domain spacing is seen to decrease with the addition of the short diblock to the long diblock copolymer at a given temperature. The domain spacing of blends with a large amount of the short diblock is seen to increase or stay constant with increasing temperature, which is radically different from the behavior of pure diblock copolymers or their blends with homopolymer....

Theory of lipid monolayers comprised of mixtures of flexible and stiff amphiphiles in athermal solvents: Fluid phase coexistence

Statistical thermodynamic theory developed recently to study self-assembled fluid monolayer films of short diblock copolymeric surfactants at the oil/water interface is applied to spread monolayers comprised of mixtures of amphiphiles that differ only in the flexibility of their chains. Although the oil and water solvents are assumed to be athermal for the two blocks of the amphiphiles, we predict large positive deviations from the ideal free energy of mixing that result in phase separation for sufficiently long chains. Pressure–composition phase diagrams and representative pressure–area isotherms are predicted for monolayer films over a range of chain lengths. By comparing predicted structural properties to the contributions to the mixing free energy, we explore the physical origins of the demixing, and consider its implications for experimental systems such as cholesterol/lipid mixtures in monolayers and bilayers.

Designing Compatibilizers To Reduce Interfacial Tension in Polymer Blends

We use self-consistent mean field methods and analytical theory to determine the behavior of AB copolymers at the interface between two incompatible homopolymers, A and B. We calculate the reduction in interfacial tension, γ, resulting from the copolymers localizing at the A/B interface. We examine the effects of chain length, composition, and molecular architecture on the efficiency of the copolymers. In particular, we compare the interfacial behavior of different linear copolymers (random, alternating, and diblock) and various branched copolymers (stars and combs). At fixed molecular weight, the diblock copolymers are the most efficient at reducing γ. However, when we compare random and comb copolymers with diblocks at different molecular weights, we observe that the longer random or comb copolymers are more efficient than short diblocks. These studies allow us to predict the reduction in interfacial tension produced by a wide variety of copolymers and, thereby, permit a rational design of cost-effective and efficient compatibilizers.

Effect of Short Diblock Copolymers at Internal Interfaces of Large Diblock Copolymer Mesophases

A model system of short polystyrene−polyisoprene diblock copolymers mixed with the corresponding strongly segregated long diblock copolymers is studied by small-angle X-ray scattering, electron microscopy, and self-consistent mean-field calculations of the ordered microphases. It is found that most of the small diblocks solubilize in the ordered domains, but a small amount segregates to the domain interfaces, in the manner of cosurfactants. Adding more short diblocks results in domain size shrinkage, consistent with a theoretical analysis of the changes in the interfacial energy and the chain stretching. The stability of the lamellar phase is discussed with respect to the structural fixed point in sections of the phase cube for diblock mixtures studied earlier. More complex systems, involving nonionic surfactants in solutions of water-soluble block copolymers, are expected to behave in the same way.

Competitive Adsorption at Homopolymer Interfaces from a Binary Mixture of Diblock Copolymers

Interfacial adsorption from a binary mixture of short and long polystyrene-polyisoprene diblock copolymers (PS-PI or its deuterated analogue dPS-PI) to interfaces of a PS(M = 3 x 10 6 ) homopolymer was studied using nuclear reaction analysis. Short symmetric diblocks dPS(M = 10 4 )-PI(M = 10 4 ) (designated dS), or a similar protonated analogue (hS), and a highly asymmetric, long diblock dPS(M = 10 6 )-PI(M = 10 4 ) (L) were used in the binary mixtures. We found that the shorter diblocks adsorb preferentially at the interfaces : the isotopic contrast between dS and hS in the mixtures with L enables us to extract detailed information on the surface segregation of the short and long species. Our data are analyzed in terms of a modified Flory-type mean field model due to de Gennes and Leibler. Interfacial segregation from the binary diblock mixture is well predicted by this model with no adjustable parameters, using the adsorption characteristics determined earlier for systems with a single diblock component.

Compatibilizing A/B blends with AB diblock copolymers: Effect of copolymer molecular weight

We use numerical self-consistent field (SCF) calculations to determine the interfacial behavior of AB diblocks in a blend of immiscible homopolymers, A and B. In particular, we compare the compatibilizing effect of relatively short and long AB copolymers. In the calculations, we explicitly take into account the formation of micellar or multilamellar phases. The results show that the interfacial tension can be reduced to zero only if the blocks in the diblock are longer than the corresponding homopolymer. Our two-dimensional SCF calculations reveal that short diblocks form multilamellar structures in the blend, whereas a microemulsion is formed when relatively long copolymers are added to the A/B mixture. These observations are compared with experiments on blends of polystyrene (PS), polymethyl methacrylate (PMMA), and PS-PMMA symmetric diblock copolymers. By measuring the contact angle of PS droplets on the PMMA layer, we can obtain a direct estimate for the interfacial tension at the PS/PMMA interface. The experiments also reveal that negligible interfacial tensions and emulsionlike structures are only obtained with relatively long diblocks.

Effects of Short Diblocks at Interfaces of Strongly Segregated Long Diblocks

Mixtures of small amounts of short diblocks (cosurfactants) and chemically identical large diblocks (surfactants) are considered, when the large diblocks are in the strongly segregated regime. Due to the enthalpy gain, the short diblocks segregate to the interfaces and change the interfacial properties. The competition between the interfacial free energy reduction and the packing free energy change determines the phase behavior of the system. It is shown that large changes can be expected in the global phase diagram when both short and long chain composition ratios are varied

The relation between the microphase separation transition and the glass transition in diblock copolymers

AbstractThe microphase separation transition (MST) has been studied for short chain diblock copolymers poly(styrene‐b‐isoprene) and poly(styrene‐b‐mma). A detailed analysis of small‐angle x‐ray scattering (SAXS) profiles in the homogeneous phase allows determination of the interaction parameter and the spinodal temperature Ts of the MST. Ts for the PS/PI diblocks is found to be lower than the glass transition temperature of their hard blocks. This results in a coupling of the MST and the glass transition. Using both structural (SAXS) and thermal differential scanning calorimetry (DSC) methods it is shown that an endothermal peak found in the DSC diagrams is related to the combined effect of the MST and the glass transition. © 1992 John Wiley & Sons, Inc.