Short Diblock Copolymers Research Articles

Controlled node growth on the surface of polymersomes.

Incorporating nucleobases into synthetic polymers has proven to be a versatile method for controlling self-assembly. The formation of strong directional hydrogen bonds between complementary nucleobases provides a driving force that permits access to complex particle morphologies. Here, nucleobase pairing was used to direct the formation and lengthening of nodes on the outer surface of vesicles formed from polymers (polymersomes) functionalised with adenine in their membrane-forming domains. Insertion of a self-assembling short diblock copolymer containing thymine into the polymersome membranes caused an increase in steric crowding at the hydrophilic/hydrophobic interface, which was relieved by initial node formation and subsequent growth. Nano-objects were imaged by (cryo-)TEM, which permitted quantification of node coverage and length. The ability to control node growth on the surface of polymersomes provides a new platform to develop higher-order nanomaterials with tailorable properties.

Thermodynamic Model of Solvent Effects in Semiflexible Diblock and Random Copolymer Assembly

We present a field-theoretic model to predict the equilibrium thermodynamic behavior of semiflexible diblock copolymers and random copolymers in the presence of solvent. We find that in both systems polymer–solvent contacts dramatically influence the thermodynamic behavior with decreasing the copolymer segment length (i.e., molecular weight). When a copolymer has unequal monomer composition, both polymer length and solvent concentration have a strong influence on the phase transition spinodal and magnitude of the critical wave modes. Diblock copolymers exhibit an expanded region of the lamellar phase in the phase diagram with decreasing chain length and polymer concentration. Such effects suggest a breakdown of the dilute approximation for solutions of short diblock copolymers. Random copolymer solutions also exhibit changes in the phase-transition spinodal and critical wave mode at asymmetric chemical compositions. This effect is highly relevant to most random copolymer materials, since a monomer is typi...

Computer simulation study of polydispersity effect on the phase behavior of short diblock copolymers

Recent progress in producing short chain length diblock copolymers facilitates the fabrication of block copolymer nanostructures with extremely small feature sizes. In this work, phase diagrams for monodisperse, one-sided and two-sided polydisperse short diblock copolymer melts are constructed using dissipative particle dynamics (DPD) simulations. Detailed comparisons are carried out between these systems to elucidate the influence of chain length polydispersity on the phase behavior of short diblock copolymers. In particular, we find an unexpected stability of a bicontinuous structure over a wide composition range between 0.7 < f ≤ 0.8 in one-sided polydisperse system, where f is the volume fraction of the polydisperse component. In the case of the two-sided polydisperse system, due to the presence of highly asymmetrical block copolymers and the pulling-away of short blocks of these molecules into opposite domain, an up-shift is found in the order–disorder transition boundary to larger χN values over the composition range we investigated.

Determination of the Lamellae-to-Disorder Heat of Transition in a Short Diblock Copolymer by Relaxation Calorimetry

Relaxation calorimetry experiments were used to measure the temperature dependence of the heat capacity near the order–disorder phase transition (ODT) in a low molecular weight volumetrically symmetric, lamellae-forming, poly(1,4-isoprene-b-dl-lactide) diblock copolymer with Mn = 2750 g/mol (dispersity Đ = 1.10) and composition fPLA = 0.51. This enabled accurate determination of the latent heat of the ODT, yielding a value ΔHODT = 0.26 ± 0.02 J/g, consistent with previous measurements using differential scanning calorimetry. The relatively small magnitude of the latent heat supports the results of recent molecular simulations and reinforces the importance of composition fluctuations on the thermodynamics of block polymers near the ODT. These thermal measurements reveal no signature of the fluctuation effects in the disordered state as T → TODT, consistent with previous experimental and theoretical work, which supports the notion that composition fluctuations develop over a relatively wide range of tempera...

Fluctuations, Phase Transitions, and Latent Heat in Short Diblock Copolymers: Comparison of Experiment, Simulation, and Theory

We present a detailed quantitative comparison of experimental results and theoretical predictions for the structure and thermodynamics of low molecular weight symmetric (fL ≈ 1/2) poly(1,4-isoprene-b-dl-lactide) (IL) diblock copolymers near the order–disorder transition (ODT). Small-angle neutron and X-ray scattering (SANS and SAXS) measurements obtained in the disordered phase with IL degree of polymerization N = 39 were fit to the renormalized one-loop (ROL) theory in order to estimate the effective interaction parameter χe(T). Calorimetric measurements of the latent heat of the ODT for the same copolymer compare well with that obtained from corresponding coarse-grained simulations, when the comparison is based on this estimate of χe(T). The corresponding estimate of (χeN)ODT at the experimental ODT of this polymer is much closer to the value obtained from simulations than to any theoretical prediction but differs from the simulation result by somewhat more than the bounds implied by experimental uncertainties. A larger discrepancy between simulation and experimental results for (χeN)ODT is obtained for longer chains, with N ≥ 50, when also calculated using χe(T, N = 39). We discuss possible reasons for this discrepancy, including the possibility of a significant end-group effect for these polymers. These results confirm the overwhelming importance of fluctuation effects in short diblock copolymers, and the usefulness of coarse-grained simulations as a starting point for quantitative modeling, but also indicate the need for attention to nonuniversal features of specific polymers that can also become more important with decreasing chain length.

Fluctuations, Order, and Disorder in Short Diblock Copolymers

The phase behavior for a series of poly(1,4‐isoprene‐b‐DL‐lactide) diblock copolymers characterized by a relatively large Flory–Huggins segment–segment interaction parameter (χ) and low degrees of polymerization (N) over a range of compositions was reported. Ordered‐state morphologies were deduced from small‐angle x‐ray scattering (SAXS) measurements, and χ(T) was determined from order‐disorder transition temperatures (TODT's) associated with the compositionally symmetric specimens and assuming the mean‐field theory, that is, (χN)ODT=10.5. The ODT was determined by differential scanning calorimetry, SAXS, and dynamic mechanical spectroscopy, and shown to be weakly first‐order, with latent heats of transition that vary strongly with composition. We interpret this behavior in terms of the mean and Gauss interfacial curvature of the ordered‐state morphologies and with respect to composition fluctuations in the disordered state. These results offer a fresh strategy for investigating weakly first‐order phase transitions within the Brazovskii universality class. © 2013 American Institute of Chemical EngineersAIChE J, 59: 3502–3513, 2013

Phase Behavior of Binary Blends of Diblock Copolymers

The phase behavior of binary blends of a long symmetric AB diblock copolymer and a short asymmetric AB diblock copolymer is studied using the self-consistent mean-field theory. The investigation focuses on blends with different short diblocks by constructing phase diagrams over the whole blending compositions and a large segregation regime. The influences of the chain length ratio (R) of the long and short diblock copolymers on their miscibility and on the stability of various ordered structures are explored. The theoretical results reveal that the blends have a much more complex phase behavior than each constituent copolymer. With the increase of the volume fraction of the short diblocks in the blends, multiple transitions from a long-period lamellar phase to phases with nonzero interfacial curvatures including cylindrical and spherical phases, and finally to a short-period lamellar phase or disordered phase, are predicted. In particular, consistent with experiments, the theory predicts that the cylindrical phase is stabilized over a wide blending compositions region in the strong segregation region, even though the two constituent diblock copolymers are both lamella-forming. When the ratio R is large enough, macrophase separation occurs over a wide range of blending compositions in a relatively strong segregation regime. Various coexisting phases, including those of lamellar and disorder, lamellar and cylindrical, cylindrical and cylindrical, cylindrical and disorder, spherical and disorder, and cylindrical and spherical, are predicted. In addition, the density profiles of the typical ordered structures are presented in order to understand the self-organization of the different copolymer chains.

STUDY OF THE PHASE BEHAVIOR OF BINARY BLENDS OF SYMMETRIC LONG AND NEARLY SYMMETRIC SHORT DIBLOCK COPOLYMERS

The phase behavior of binary blends of symmetric long diblock copolymers and nearly short symmetric diblock copolymers with different ratio of chain lengths is studied using the self-consistent field theory.The occurrence of order-order transition,disorder-order transition and regions of order-disorder coexistence has been examined.Comparisons are also made with the theoretical study of the binary blend of symmetric long and short diblock copolymers and experiments on the similar systems.The effects of the short nearly symmetric diblock copolymer on the symmetric long diblock copolymer in their blends were analyzed.

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....

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.