No abstract

The phase state of 25 poly(ethylene oxide-b-isoprene) (PEO-PI) diblock copolymers spanning the composition range 0.05 < f PEO < 0.8 has been studied using small-angle X-ray scattering and rheology. In addition, the thermal and thermodynamic properties have been obtained from differential scanning calorimetry and pressure-volume-temperature measurements. Twenty of the diblocks exhibit at least one order-to-order transition, and two show four ordered phases. The phase diagram consists of four equilibrium phases in the melt; lamellar (Lam), hexagonally packed cylinders (Hex), spheres packed in a body centered cubic lattice (bcc) and a bicontinuous cubic phase with the Ia3 hd space group symmetry known as the gyroid phase. The latter is formed for the range of compositions 0.4 < f PEO < 0.45 which are the highest ever reported for a stable gyroid phase. The high asymmetry in the present phase diagram is attributed to the high conformational asymmetry of the PEO and PI ( ) 2.72). At low temperatures, upon PEO crystallization, all phases revert to the crystalline lamellar structure (Lc). Within the composition range 0.66 < fPEO < 0.7 another intermediate phase is formed known as perforated layers (PL) which is clearly not an equilibrium phase. The thermal expansion coefficient was found to be a sensitive probe of the ordered microstructures.

The phase state and the kinetics of the order-to-order transitions have been studied in a series of poly(isoprene-b-ethylene oxide) (PI-PEO) diblock copolymers with a PI volume fraction in the range 0.25&lt;fPI&lt;0.92, using small angle x-ray scattering (SAXS), and rheology. The mean-field theory (MFT) structure factor is used to describe the SAXS profiles in the disordered phase and to extract the temperature dependence of the interaction parameter χ(T). In general, an agreement is found with the phase diagram proposed by an extended MFT, except at fPI=0.61 where the following sequence of phases was found: Lc→Hex→Gyroid→Dis (Lc is the crystalline lamellar phase, Hex signifies hexagonally packed cylinders, Gyroid is the bicontinuous cubic network with the Ia3̄d symmetry, and Dis is the disordered phase). We found that crystallization disrupts the amorphous ordered morphologies and imposes a layered structure (Lc). The study of the kinetics of the Hex to Lc and the Hex to Gyroid transitions is facilitated by the different viscoelastic contrast and the distinctly different scattering patterns of the three phases involved (Lc, Hex, Gyroid). Our studies show that it is possible to undercool and overheat ordered phases just as we can undercool the disordered phase. The transformation from the Hex to the Lc phase proceeds via a heterogeneous nucleation and growth process and results in the formation of a spherulitic superstructure composed from stacks of lamellar crystals. The transformation of the Hex to the Gyroid phase involves two steps. The first step—which is too fast to be picked up by rheology—involves fluctuations of the hexagonal phase. The second “slow” step involves a nucleation and growth process of elongated objects. The transformation proceeds nearly epitaxially and has an activation energy of 47 kcal/mol which is typical for a collective process.

We have investigated the influence of composition gradients on the microdomain structure and viscoelastic properties of tapered block copolymers by (i) varying the amount of interfacial material and (ii) the block sequence. Normal tapered and inverse tapered triblock copolymers of polystyrene and polyisoperne with a tapered midblock have been synthesized via anionic polymerization with a nearly symmetric composition and compared with the corresponding diblock copolymers. We found that increasing the amount of tapered material within the interface systematically increases the compatibility. Block sequencing is found to be an important factor controlling compatibility. Inverse tapered block copolymers are much more compatible than the corresponding normal tapered block copolymers. Results presented here could be used as a guideline for preparing copolymers with controlled compatibility at the synthesis level.

The living anionic copolymerization of isoprene and styrene in cyclohexane affords tapered block copolymers due to the highly disparate reactivity ratios of r I = 12.8 and r S = 0.051. Repeated addition of a mixture of these monomers was exploited to generate tapered multiblock copolymer architectures of the (AB) n type with up to 10 blocks (1 ≤ n ≤ 5), thereby subdividing the polymer chains in alternating flexible polyisoprene (PI) and rigid polystyrene (PS) segments. Three series of well-defined tapered multiblock copolymers with approximate molecular weights of 80, 240, and 400 kg/mol were prepared on the 100 g scale. Via this synthetic strategy polymer chains were divided in di-, tetra-, hexa-, octa-, and decablock tapered multiblock structures. Because of the living nature of the polymerization, low dispersities in the range 1.06–1.28 (decablock) were obtained. To ensure full monomer conversion prior to the addition of the isoprene/styrene mixture, kinetic Monte Carlo simulation was employed, permitting to simulate chain growth in silico by employing the known polymerization rates and rate constants k p. The synthesized tapered multiblock copolymers were characterized via SEC and selected samples via oxidative degradation of the polyisoprene block in solution, confirming the well-defined nature of the PS segments. Subsequently, the question was addressed, to which extent the tapered multiblock copolymers are capable of forming ordered nanosegregated morphologies. Detailed thermal, structural, and rheological investigations showed that the tapered multiblock copolymers with a molecular weight of 240 kg/mol formed ordered phases with the expected lamellar morphology. However, X-ray scattering data and transmission electron microscopy (TEM) images of the octablock and decablock copolymers reflect weakly ordered structures at ambient temperature. The domain spacing, d, was found to scale as d ∼ N 0.62, where N is the total degree of polymerization, suggesting stretching of chains and nonideal configurations. Following the structure factor, S(q), as a function of temperature revealed that the tapered multiblock copolymers undergo a fluctuation-induced first-order transition at the respective order-to-disorder transition temperature, T ODT. The viscoelastic response of the tapered copolymers was controlled by the nanodomain structure, the degree of segregation, nanodomain-bridging configurations of blocks, and also the proximity to the glass temperature of the vitrified PS domains. Tapered hexablock copolymers were found to best combine structural integrity and mechanical toughness, while maintaining a large strain at break (>900%).

The static and kinetic aspects of the order-disorder transition (ODT) in newly synthesized model 3-miktoarm star copolymer (simple graft) of SI2 type, (polystyrenexpolyisoprenek, and a 3-miktoarm star terpolymer of SIB type, (polystyrene)fpolyisoprene)(polybutadiene), have been studied using smallangle X-ray scattering (SAXS) and rheology. The morphology and the order-disorder transition temperature (TODT) have been identified from the two-dimensional SAXS patterns with shear-oriented samples. Hexagonally ordered cylindrical microdomains aligned along the direction of shear and with TODT = 379 K have been formed for both samples studied. The SAXS profiles at temperatures well above the Tom have been fitted to the mean-field theory (MFT) for graft copolymers. Near the ODT deviations from the theory have been observed and the SAXS data provide unambiguous evidence for the existence of fluctuations. The Tom obtained from rheology is in excellent agreement with the one from SAXS.Discontinuities in the SAXS peak intensity and m the storage modulus near the Tom are more pronounced in these systems as compared to linear diblwks. The ordering kinetics have been studied with rheology and complementary with SAXS. The width of the kinetically accessible metastable region is enlarged as compared to linear diblocks. For shallow quenthes the ordering proceeds by heterogeneous nucleation and growth of three-dimensional grains with cylindrical microstructure. Our kinetic studies probe the metastable states near but below the Tom.

Small angle X-ray scattering (SAXS) and rheology are employed to study the microphase separation transition in a symmetric poly(styrene-b-isoprene) diblock copolymer. The order-disorder transition temperature Toor has been identified with a good precision ( ToDT =359 K) and the interaction parameter is extracted using mean field theory at temperatures well above TODT. The kinetics of microphase separation have been studied by means of rheology in a small temperature range below the order-disorder transition, following a quench from the disordered state.The time evolution of the storage and loss moduli exhibited shapes which are reminiscent of the crystallization isotherms of semicrystalline materials (of the Avrami type). For shallow quenches, we find a temperature range of 3 K below To,, where the ordering proceeds by heterogeneous nucleation and growth of three-dimensional objects with lamellar microstructure. At higher undercooling, the kinetic curves change in form, indicating a different mechanism of structure formation. Our results are compared with theoretical predictions for the ordering kinetics of symmetric diblocks.

A series of homologous dendronized polymers (DPs) with generations (g) 1−3 and backbone nominal degrees of polymerization (P n ) in the range 50−3000 have been synthesized and characterized in order to investigate the g-and P n -dependent viscoelastic properties and packing of this class of densely grafted, associating and effectively "thick" macromolecules in their molten state. Rheological measurements reveal an unusually long thermal equilibration time, attributed to (i) the tendency of DPs to minimize local density gradients, as realized via their mutual weak interpenetration, and (ii) the intermolecular DP−DP correlations and inter-and intramolecular hydrogen bonding and π−π stacking interactions. With the help of simulations and X-ray scattering measurements, a scenario emerges, according to which DPs interact via local cooperative rearrangements of the dendrons, akin to a Velcro fastening process. In this picture, neighboring bonds accelerate the local interpenetration process. Results from X-ray scattering show increased lateral backbone−backbone correlations with a columnar arrangement of backbones and a liquid crystalline underlying order. Linear viscoelasticity is characterized by plateau moduli which originate from intermolecular bonding and whose extent in frequency and absolute value depends on P n and g and can be lower than or comparable to that of the backbone. Very long relaxation times can be probed (sometimes via creep measurements) and attributed to the lifetime of the bonds. The nonlinear shear rheology data suggest a resemblance in behavior to unentangled linear chains with finite extensibility and point to reduced deformability of the DPs in flow. These findings indicate that DPs constitute a promising class of functional macromolecules with tunable properties.