Architecture-Induced Microphase Separation in Nonfrustrated A–B–C Triblock Copolymers

The extent of block microphase separation in nonfrustrated A–B–C triblock copolymers forming a “three-domain, four-layer” lamellar morphology is examined. Specifically, the extent of separation between the B and C blocks is probed, for the case where the B and C blocks are sufficiently compatible that they would not be microphase-separated if they were connected as a diblock. However, attachment of the A block, and consequent localization of the A–B block junction to the A–B lamellar interface, induces extensive separation between the B and C blocks. This separation is revealed both through the small-angle X-ray scattering pattern in the melt, and by distinct glass transitions observed in the solid state for the B block at low B–C segregation strengths, and for both the B and C blocks at higher segregation strengths. The particular polymers studied here have polyethylene as the A block; except for the most weakly segregated triblock, upon cooling from the melt, crystallization of the polyethylene block is confined within the lamellar structure established in the melt, with the polyethylene crystals stacking orthogonally to the microdomain lamellae

Imaging Block Copolymer Crystallization in Real Time with the Atomic Force Microscope

The crystallization of two diblock copolymers, one forming a cylindrical mesophase and the other a spherical mesophase in which the crystallizable unit, polyethylene, is the minority component, has been followed in situ using high-temperature atomic force microscopy. Contrast is obtained between the crystallizable and noncrystallizable microdomains in the melt as well as between the melt and the crystalline material, allowing the influence of the melt structure on crystallization to be observed in real time. In both cases the mesophase structure is destroyed by crystallization, but its influence over the growth dynamics is followed. In the cylinder former, transport of material to the crystal growth front is found to occur primarily by diffusion, rather than by flow, leaving the melt structure unperturbed. Crystal growth rates vary considerably between individual crystallites, with more rapid growth along existing ethylene-rich domains. Melting occurs randomly along the crystallites, leading to their breakup into small blocks. In the sphere former crystallization leads to a densely branched seaweed structure, the morphology controlled by the directionally varying diffusion rate between the ethylene-rich spherical domains. On increasing supercooling the extent to which the spherical melt structure controls the crystal morphology increases, with crystal growth directions dominated by sphere−sphere nearest-neighbor angles

Thermoplastic Elastomers with Composite Crystalline−Glassy Hard Domains and Single-Phase Melts

We report the synthesis and characterization of thermoplastic elastomers (TPEs) containing both crystalline and glassy hard segments, with the aim of capturing the mechanical properties of conventional all-amorphous triblock TPEs, while forming the solid-state structure by crystallization from a single-phase melt. To accomplish this, we used living ring-opening metathesis polymerization (ROMP) and subsequent hydrogenation to synthesize symmetric pentablock copolymers with the architecture crystalline−glassy−rubbery−glassy−crystalline. Analogous crystalline−rubbery−crystalline triblocks show a high initial modulus, yielding, and poor recovery, resulting from platelike crystalline hard blocks. By contrast, with the pentablock architecture and appropriate selection of block lengths, crystallization from a single-phase melt causes a layer rich in the glassy block to form around the crystallites, limiting their lateral growth and generating composite hard domains with both crystalline and glassy components. The pentablocks show the low initial modulus, strain-hardening behavior, and small permanent set desired for TPEs, while retaining an easily processed single-phase melt

Block Copolymers Synthesized by ROMP-to-Anionic Polymerization Transformation

A two-step site transformation technique is presented to convert from living ring-opening metathesis polymerization (ROMP) to living anionic polymerization of unsaturated monomers. This method permits the synthesis of well-defined diblock copolymers of controllable molecular weight and narrow molecular weight distribution, combining ROMP and anionically polymerizable monomers. The technique employs a functional terminating agent to add a styryl group to the end of the ROMP chain. The end group is then metalated to yield an active site for anionic polymerization, so that the ROMP chain acts as a macroinitiator from which the anionic block is grown. Both polycyclopentene/polystyrene and polynorbornene/polystyrene diblocks were successfully synthesized, and the method should be applicable to a broad range of monomers

Imaging Block Copolymer Crystallization in Real Time with the Atomic Force Microscope

The crystallization of two diblock copolymers, one forming a cylindrical mesophase and the other a spherical mesophase in which the crystallizable unit, polyethylene, is the minority component, has been followed in situ using high-temperature atomic force microscopy. Contrast is obtained between the crystallizable and noncrystallizable microdomains in the melt as well as between the melt and the crystalline material, allowing the influence of the melt structure on crystallization to be observed in real time. In both cases the mesophase structure is destroyed by crystallization, but its influence over the growth dynamics is followed. In the cylinder former, transport of material to the crystal growth front is found to occur primarily by diffusion, rather than by flow, leaving the melt structure unperturbed. Crystal growth rates vary considerably between individual crystallites, with more rapid growth along existing ethylene-rich domains. Melting occurs randomly along the crystallites, leading to their breakup into small blocks. In the sphere former crystallization leads to a densely branched seaweed structure, the morphology controlled by the directionally varying diffusion rate between the ethylene-rich spherical domains. On increasing supercooling the extent to which the spherical melt structure controls the crystal morphology increases, with crystal growth directions dominated by sphere−sphere nearest-neighbor angles

Regular Mixing Thermodynamics of Hydrogenated Styrene–Isoprene Block–Random Copolymers

Random copolymerization of A and B monomers represents a versatile method to tune interaction strengths between polymers, as A<i>r</i>B random copolymers will exhibit a smaller effective Flory interaction parameter χ (or interaction energy density <i>X</i>) upon mixing with A or B homopolymers than upon mixing A and B homopolymers with each other, and the A<i>r</i>B composition can be tuned continuously. This approach can also be used to tune the segregation strength in A–A<i>r</i>B “block–random” copolymers. Simple models of polymer mixing thermodynamics suggest that the effective interaction energy density in such block–random copolymers should follow <i>X</i>_{A–A<i>r</i>B} = <i>f</i>_B²<i>X</i>_A–B, but this prediction has not been tested quantitatively. The present work systematically assesses the validity of this rule for thermally stable hydrogenated derivatives of styrene–isoprene block copolymers, through measurements of the order–disorder transition (ODT) temperature on near-symmetric diblock and diblock–random copolymers of varying composition and suitable molecular weight (M). Both hydrogenated derivatives wherein the styrene aromaticity is retained, and derivatives wherein the styrene units are saturated to vinylcyclohexane, are examined, and both are found to closely obey the <i>X</i>_{A–A<i>r</i>B} = <i>f</i>_B²<i>X</i>_A–B prediction, thereby confirming the utility of this simple relationship in designing block copolymers with targeted interaction strengths using only these two common monomers. The reduction in <i>X</i>_{A–A<i>r</i>B} over <i>X</i>_A–B permits the synthesis of polymers having much larger <i>M</i> and domain spacing <i>d</i> while maintaining a thermally accessible ODT; measured domain spacings are found to closely follow the expected scaling, <i>d</i> ∼ <i>X</i>^1/6<i>M</i>^2/3

Imaging Block Copolymer Crystallization in Real Time with the Atomic Force Microscope

The crystallization of two diblock copolymers, one forming a cylindrical mesophase and the other a spherical mesophase in which the crystallizable unit, polyethylene, is the minority component, has been followed in situ using high-temperature atomic force microscopy. Contrast is obtained between the crystallizable and noncrystallizable microdomains in the melt as well as between the melt and the crystalline material, allowing the influence of the melt structure on crystallization to be observed in real time. In both cases the mesophase structure is destroyed by crystallization, but its influence over the growth dynamics is followed. In the cylinder former, transport of material to the crystal growth front is found to occur primarily by diffusion, rather than by flow, leaving the melt structure unperturbed. Crystal growth rates vary considerably between individual crystallites, with more rapid growth along existing ethylene-rich domains. Melting occurs randomly along the crystallites, leading to their breakup into small blocks. In the sphere former crystallization leads to a densely branched seaweed structure, the morphology controlled by the directionally varying diffusion rate between the ethylene-rich spherical domains. On increasing supercooling the extent to which the spherical melt structure controls the crystal morphology increases, with crystal growth directions dominated by sphere−sphere nearest-neighbor angles

Minimum Molecular Weight and Tie Molecule Content for Ductility in Polyethylenes of Varying Crystallinity

Semicrystalline polymers of low glass transition temperature, such as polyethylene (PE), can be either brittle or ductile depending on their content of intercrystallite stress transmitterssuch as tie molecules (TMs), chains that directly bridge the intercrystalline amorphous layer. TM content will increase with increasing molecular weight (M) or with the fraction of high-M chains in a disperse polymer and with decreasing intercrystallite repeat spacing d, which can be manipulated through thermal history and the incorporation of comonomer. The present work examines the failure mode of model narrow-distribution linear PEs (LPEs) of high crystallinity, where d is varied through crystallization history (either quenching or slowly cooling), and ethylene-butene copolymers (hydrogenated polybutadienes (hPBs)) of moderate crystallinity, where d is limited by the short-branch content. For each series (LPEs with different thermal histories and quenched hPBs), a rather sharp brittle-to-ductile transition (BDT) is observed with increasing M, at a value MBDT. However, across the three series, the value of MBDT does not depend solely on the value of d; indeed, a higher M is required to achieve ductility in quenched samples of hPB than in LPE, despite the much lower values of d for hPB. Consequently, the calculated value of TM fraction at the BDT increases strongly as crystallinity decreases, by a factor of ∼50 from slow-cooled LPE to quenched hPB. This strong dependence is explained by considering the influence of TMs on the brittle fracture stress (σb), with the BDT occurring when there are sufficient TMs for σb to exceed the yield stress (σy), which is strongly dependent on crystallinity but independent of TM content

Synthesis and Properties of Well-Defined Elastomeric Poly(alkylnorbornene)s and Their Hydrogenated Derivatives

Synthesis and Properties of Well-Defined Elastomeric Poly(alkylnorbornene)s and Their Hydrogenated Derivative

Synthesis and Phase Behavior of Block-Random Copolymers of Styrene and Hydrogenated Isoprene

“Block-random” copolymerswherein one or more blocks is itself a random copolymerpresent a useful and convenient variation on the typical block copolymer architecture, as the interblock interactions and physical properties can be tuned continuously through the random block’s composition. However, typical living or controlled polymerizations produce compositional gradients along the “random” block, which can in turn influence the phase behavior. Organolithium initiation in a cyclohexane/triethylamine mixture is shown herein to yield narrow-distribution copolymers of styrene and isoprene of any desired composition, with no measurable down-chain gradient. These random copolymers (SrI) are also successfully incorporated into well-defined symmetric block copolymers (I-SrI diblocks). Isoprene-selective hydrogenation yields thermally stable hI-SrhI diblocks, which self-assemble into well-defined lamellar morphologies with sharply defined order–disorder transitions, whose temperatures TODT scale predictably with diblock molecular weight. The use of SrhI in lieu of a styrene homopolymer block allows the diblock molecular weight and domain period to be substantially increased for a given value of TODT. The measured interaction energy density between hI and SrhI is consistent with the mean-field “copolymer equation”, providing a first step toward the design of styrene–isoprene block-random copolymers of desired molecular weight and TODT