Elastomeric Block Copolymer Research Articles

A Supramolecular Approach to Photoresponsive Thermo/Solvoplastic Block Copolymer Elastomers

With the aim of preparing supramolecular photoresponsive block copolymer elastomers, a series of ABA triblock copolymers with a poly(n-butyl acrylate) (PnBA) middle block and poly(dimethylaminoethyl methacrylate) (PDMAEMA or PDM) outer blocks were synthesized by atom transfer radical polymerization (ATRP), followed by PDM quaternization (giving PDMQ-PnBA-PDMQ) and then by ionic complexation with methyl orange (MO), an azo-containing and sulfonate-functionalized commercially available compound (giving PDMQ/MO-PnBA-PDMQ/MO). The PnBA block, which has a subambient glass transition, and the quaternized and complexed blocks, which have high glass transitions, form phase-separated soft and hard blocks, respectively. Simple elasticity tests of solvent-cast films show that the PDMQ/MO-PnBA-PDMQ/MO with hard block content between 18 and 29 wt % (as well as PDMQ-PnBA-PDMQ with 18 wt % hard block content) have significant elastomeric character. AFM and TEM (atomic force and transmission electron microscopies) of spi...

Metallo-supramolecular complexes mediated thermoplastic elastomeric block copolymer

Orthogonal multi-phase strategy has been applied to generate a novel type of thermoplastic elastomeric block copolymer which was obtained via anionic polymerization. Functional terpyridine groups which can potentially form metallo-supramolecular complexes with a variety of metal ions has been introduced to the polystyrene-b-polyisoprene (PS-PI) living chain ends through the termination reaction in an optimized condition. Iron(II), cobalt(II) and zinc(II) metal ions have been used to form metal-ligand complexes with the terpyridine end groups, which can phase separate from the polymer matrix to form hybrid clusters. The formation of metallo-supramolecular hybrid clusters have dramatic effects on the micro-phase separated structures of the PS-PI diblock copolymer, which have been characterized by transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy. This type of hybrid material containing hard PS nanophase and metal-ligand clusters exhibit distinct mechanic properties such as increased modulus, higher yield strength and improved toughness, which is further discussed in light of nature of the metal-ligand bonds and the liability of the clusters. The utilization of metallo-supramolecular complex and its high tunability is potential to fabricate new types of supramolecular nanocomposite materials.

Sawtooth Tensile Response of Model Semiflexible and Block Copolymer Elastomers

We study via coarse-grained molecular modeling the elastic response of semiflexible elastomer networks with idealized diamond connectivity. Under strain-driven uniaxial deformation, these networks are found to exhibit first a perfect soft elastic behavior (including a buckling instability preceding hardening) up to moderate extension ratios (<2.9), followed by a sawtooth-shaped stress–strain curve for larger deformations at the nanoscale. This unique tensile response follows from a deformation mechanism that entails the successive creation and distortion of ordered (smectic CA) chain domains in concert with cross-link segregation and layering; such events arise from the proclivity of the chains to align (due to the chain stiffness) and to disintersperse (due to the network regularity and lack of trapped entanglements). Multiple segregation states hence occur for different strains, producing multiple minima in the stress. These regular networks also exhibit marked anisotropic elastic behavior and shape-mem...

Mechanical strength of sutured block copolymers films for load bearing medical applications

The mechanical behavior of three styrenic thermoplastic block copolymer elastomers with applied surgical sutures was studied by uniaxial tensile testing. The materials exhibited oriented cylindrical microstructure. Distinct macroscopic deformation mechanisms have been observed upon stretching of samples with vertical and horizontal orientation. Deformation progressed along the axis of the suture in samples with parallel orientation (P), while it in case of normal orientation (N) the whole sample responded to the applied force. Also the analysis of the stress-strain curves showed a significant difference between samples P and N. Greater stress at break was observed for samples P, while samples N showed the capability to tolerate higher strain. The influence of morphology on the tear-out shape has been also observed. The thread made a vertical tear out in samples P while for samples N ripping off the bottom was observed.

Thermo‐Active Elastomer Composite for Optical Heating in Microfluidic Systems

Single-walled carbon nanotubes are used as doping agents to form thermo-active composites with an elastomeric block-copolymer. Thermal imaging reveals that the temperature response upon irradiation with NIR laser light is dependent (among other things) on the mass fraction of the nanotubes in the polymer matrix.

Soft Conductive Elastomer Materials for Stretchable Electronics and Voltage Controlled Artificial Muscles

Block copolymer elastomer conductors (BEC) are mixtures of block copolymers grafted with conducting polymers, which are found to support very large strains, while retaining a high level of conductivity. These novel materials may find use in stretchable electronics. The use of BEC is demonstrated in a capacitive strain sensor and in an artificial muscle of the dielectric elastomer actuator type, supporting more than 100% actuation strain and capacity strain sensitivity up to 300%.

Confinement of Elastomeric Block Copolymers via Forced Assembly Coextrusion

Forced assembly processing provides a unique opportunity to examine the effects of confinement on block copolymers (BCPs) via conventional melt processing techniques. The microlayering process was utilized to produce novel materials with enhanced mechanical properties through selective manipulation of layer thickness. Multilayer films consisting of an elastomeric, symmetric block copolymer confined between rigid polystyrene (PS) layers were produced with layer thicknesses ranging from 100 to 600 nm. Deformation studies of the confined BCP showed an increase in ductility as the layer thickness decreased to 190 nm due to a shift in the mode of deformation from crazing to shear yielding. Postextrusion annealing was performed on the multilayer films to investigate the impact of a highly ordered morphology on the mechanical properties. The annealed multilayer films exhibited increased toughness with decreasing layer thickness and resulted in homogeneous deformation compared to the as-extruded films. Multilayer coextrusion proved to be an advantageous method for producing continuous films with tunable mechanical response.

Degradable Elastomeric Block Copolymers Based on Polycaprolactone by Free‐Radical Chemistry

AbstractNew degradable, transparent, and elastomeric materials based on PCL are reported in this work. These are block copolymers of PCL with a copolymer of MMA and MDO, i.e. P(MMA‐co‐MDO)‐b‐PCL‐b‐P(MMA‐co‐MDO). A PCL‐based macro‐azoinitiator is used for preparing the block copolymers by free‐radical chemistry. The free‐radical copolymerization of MDO with MMA provides CL units distributed randomly on the PMMA backbone of the second block. This provides the advantage of bringing degradability to the whole system. The detailed structural characterization of the new polymers, their properties evaluation like thermal and mechanical are also reported. The materials are degradable, transparent, and elastomeric depending upon the copolymer composition and block length. magnified image

Processing dependency of percolation threshold of MWCNTs in a thermoplastic elastomeric block copolymer

We prepared carbon nanotube thermoplastic elastomeric block copolymer nanocomposites using a commercial styrene–ethylenebutylene–styrene (SEBS) block copolymer and multiwall carbon nanotubes (MWCNTs) at different filler concentrations. Hereto we applied two different processing strategies, namely direct melt mixing and solution-precipitation. We observe that the mechanical and electrical properties such as storage module and electrical percolation threshold are clearly affected by the processing approach. We studied these effects in detail by means of dielectric spectroscopy, which provided important information about the dispersion state of the MWCNT filler network. It revealed a fractal filler behaviour of the samples independent of the processing method. However, in samples prepared by melt mixing an additional dielectric response related to nanoscopic gap junctions of 1.5nm was identified. This response was not present in solution-mixed samples, which are characterized by improved polymer wetting. We discuss how the interactions between CNTs and the two phases of the block copolymer matrix are affected by the processing conditions resulting in the important differences in the filler network structure, which directly influence the final electrical and mechanical properties of the composite.

Elastic block copolymer nanocomposites with controlled interfacial interactions for artificial muscles with direct voltage control

Soft, physically crosslinking, block copolymer elastomers were filled with surface-treated nanoparticles, in order to evaluate the possibility for improvement of their properties when used as soft dielectric actuators. The nanoparticles led to improvements in dielectric properties, however they also reinforced the elastomer matrix. Comparing dielectric spectra of composites with untreated and surface-treated particles showed a measurable influence of the surface on the dielectric loss behaviour for high filler amounts, strongly indicating an improved host–guest interaction for the surface-treated particles. Breakdown strength was measured using a test bench and was found to be in good agreement with the results from the actuation measurements. Actuation responses predicted by a model for prestrained actuators agreed well with measurements up to a filler amount of 20%vol. Strong improvements in actuation behaviour were observed, with an optimum near 15%volnanoparticles, corresponding to a reduction in electrical field of 27% for identical actuation strains. The use of physically crosslinking elastomer ensured the mechanical properties of the matrix elastomer were unchanged by nanoparticles effecting the crosslinking reaction, contrary to similar experiments performed with chemically crosslinking elastomers. This allows for a firm conclusion about the positive effects of surface-treated nanoparticles on actuation behavior.

Deformation of elastomeric polyolefin spherulites

AbstractThe ethylene‐octene block copolymers in this study consist of long crystallizable sequences with low comonomer content alternating with rubbery amorphous blocks with high comonomer content. The crystallizable blocks form lamellae that organize into space‐filling spherulites even when the fraction of crystallizable block is so low that the crystallinity is only 7%. These unusual spherulites are highly elastic and recover from strains as high as 300%. This new class of thermoplastic elastomers is fundamentally different from conventional elastomeric olefin copolymers that depend on isolated, fringed micellar‐like crystals to provide the junctions for the elastomeric network. The elastomeric block copolymers are shown to be unique in that a hierarchical organization of space‐filling lamellar spherulites provides the junctions for the elastomeric network. The deformation of the elastic spherulites is readily studied with small angle light scattering, wide angle X‐ray diffractograms, and atomic force microscopy. At strains in excess of 300%, the spherulites break up into a fibrillar structure following lamellar deformation processes that are similar to those established for high density ethylenic polymers. The crystalline transformation produces a stiffer elastomer that exhibits complete recovery on subsequent loadings. Similar experiments on elastomeric random ethylene‐octene copolymers where fringed micellar crystals provide the physical crosslinks that connect the rubbery, amorphous chain segments reveal significant differences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1313–1330, 2009

Functionalization of carbon nanofibers with elastomeric block copolymer using carbodiimide chemistry

Surface functionalization of carbon nanofibers (CNFs) with aminopropyl terminated polydimethylsiloxane [(PDMS-NH 2)] and other organic diamines was achieved using carbodiimide chemistry. The carbodiimide chemistry provides faster reaction rate so that the reaction occurs at lower temperature compared to amidation and acylation–amidation chemistry. CNF functionalized with PDMS-NH 2 fibers were further functionalized with oligomer of polyimide (6FDA-BisP) using imidization reaction. The formation of block copolymer on the surface of CNF is proposed as an effective method to engineer the interphase between the fiber and the polymer, which is essential to modulate and enhance the properties of the nanocomposite. The efficiency of the carbodiimide chemistry to functionalize amine terminated groups on CNF and the functionalization of block copolymer was characterized using thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and UV–vis spectroscopy.

Crystallization behavior of elastomeric block copolymers: Thermoplastic polyurethane and polyether‐block‐amide

AbstractThe isothermal crystallization kinetics of melt‐blown webs produced from a series of elastomeric block copolymers was studied through differential scanning calorimetry (DSC). Three hardness grades were selected for a polyester and a polyether Elastollan® thermoplastic polyurethane and Pebax® polyether‐block‐amide copolymers. The Avrami crystallization kinetics parameters, k and n, were derived from two different methods: (1) traditional Avrami model and (2) derivative of the Avrami model proposed by Kurajica et al. (Croat Chem Acta 2002, 75, 693). The kinetic parameters from both models were consistent and showed good correlation. For all polymer types and hardness grades, crystallization kinetics were interpreted with the derivative model (Kurijica et al.) since it could be directly fitted to untransformed DSC isothermal crystallization data, and thus reduces the errors involved in Avrami analysis. The values of the Avrami exponent, n ranged between 2.59 and 3.41, indicating similar nucleation and growth mechanisms. These n values and morphological observations indicate that crystallization occurs in these copolymers in three dimensions from pre‐existing nuclei and the crystals grow under isothermal conditions. This suggests that, in these elastomeric copolymers, crystallization of the hard segments drives microphase separation into crystalline and amorphous regions rather than formation of hard and soft domains. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Supramolecular Triblock Copolymers Containing Complementary Nucleobase Molecular Recognition

A novel difunctional alkoxyamine initiator, DEPN2, was synthesized and utilized as an efficient initiator in nitroxide-mediated controlled radical polymerization of triblock copolymers. Complementary hydrogen-bonding triblock copolymers containing adenine (A) and thymine (T) nucleobase-functionalized outer blocks were synthesized. These thermoplastic elastomeric block copolymers contained short nucleobase-functionalized outer blocks (Mn ∼ 1K−4K) and n-butyl acrylate rubber blocks of variable length (Mn ∼ 14K−70K). Hydrogen-bonding interactions were observed for blends of the complementary nucleobase-functionalized block copolymers in terms of increased specific viscosity as well as higher scaling exponents for specific viscosity as a function of solution concentration. In the solid state, the blends exhibited evidence of a complementary A−T hard phase, which formed upon annealing, and dynamic mechanical analysis (DMA) revealed higher softening temperatures. Morphological development of the block copolymer...

Buckling in elastomer/plastic/elastomer 3‐layer films

AbstractBuckling during recovery of stretched elastomer/plastic/elastomer 3‐layer films was studied using a block copolymer elastomer and polyolefin plastics. An elastic model was developed to describe the critical wavelength, critical compressive strain, and critical compressive stress for buckling without delamination. The predicted linear dependence of critical wavelength on the plastic layer thickness was confirmed, and the modulus ratio between the plastic and the elastomeric layers was extracted. Determination of the critical compressive strain provided an independent calculation of the modulus ratio. Similar modulus ratios were obtained by the two methods. The critical compressive stress in the plastic layer led to reasonable values for the compressive modulus of the stretched plastic layer and the bulk modulus of the elastomeric layers. To test the model further, the modulus ratio was altered by changing the stretch ratio and by varying the plastic layer material. No buckling occurred if the modulus ratio was too small. Polym. Compos. 25:653–661, 2004. © 2004 Society of Plastics Engineers.

Block Copolymer Thermoplastic Elastomers for Microcontact Printing

Two commercial thermoplastic block copolymer elastomers with a high stiffness were investigated as stamp materials for microcontact printing and compared to conventional poly(dimethylsiloxane) (PDMS). Stamps with a relief structure were produced by hot embossing techniques, utilizing the thermoplastic properties of these rubberlike block copolymers. It is shown that the stamps based on these copolymers are able to transfer a thiol ink to a gold substrate. After printing, the thiol ink acts as an etch resist, which indicates that a coherent self-assembled monolayer is formed. Like PDMS stamps, specific copolymer stamps can be used for repeated printing without re-inking. Moreover, the higher stiffness of the thermoplastic stamps increases the load above which structural collapse occurs by a factor of 10−15 in comparison to that of identical PDMS stamps, which is potentially useful in the reproduction of structures which are sensitive to sagging, buckling, or pairing. An example is presented of relief struc...

The effect of hard and soft segment composition and molecular architecture on the morphology and mechanical properties of polystyrene–polyisobutylene thermoplastic elastomeric block copolymers

This paper reports the effects of hard (polystyrene, PS) and soft (polyisobutylene, PIB) segment composition and the molecular architecture (linear versus star, PS and PIB block length) on the morphology and mechanical properties of polystyrene/polyisobutylene (SIBS) block copolymers synthesized by living carbocationic polymerization. Atomic force microscopy, dynamic mechanical thermal analysis and tensile testing verified the phase-separated nature of the block copolymers, which behaved as thermoplastic elastomers (TPEs). The morphology of these TPEs is similar to polydiene-based TPEs, and is defined by the soft/hard segment composition. Interestingly, topology (linear vs star) did not have a major influence on morphology. Tensile testing showed that for both linear and three-arm star block copolymers, the modulus and tensile strength increased while elongation at break decreased with higher PS content. However, three-arm star block copolymers showed larger moduli than their linear homologues with similar PS content and PIB arm length, indicating the influence of molecular architecture on mechanical properties. These results might serve as a foundation for macromolecular engineering design for optimizing properties.

Collapse of the Ia3d cubic symmetry by uniaxial stretching of a double-gyroid block copolymer.

We report an experimental observation of collapse of the Ia3d symmetry by stretching of an elastomeric block copolymer that forms a double-gyroid (DG) microdomain structure. Some new diffraction spots were observed for a deformed DG structure. These spots were assigned to [110] and [200] reflections. For the Ia3d symmetry, these reflections are prohibited by the extinction rule and therefore none of them appeared for the sample before deformation. Appearance of these reflections indicates breakdown of the extinction rule and can be explained as a result of collapse of the symmetry of glide when three-dimensional DG networks are partially ruptured upon stretching.

Processing and Properties of Polyolefin Elastomers and Fully Hydrogenated Styrenic Block Copolymer Elastomers

Improving the processability while maintaining excellent stretch and elasticity is very important for elastomeric materials. Here is discussed the processing and properties of several recently developed elastomers, and it is shown that optimum elastic recovery can be achieved by blending polyolefin elastomers with fully hydrogenated styrenic block copolymers, the Figure showing a co-continuous morphology.

Microphase Separation and Rheological Properties of Polyurethane Melts. 3. Effect of Block Incompatibility on the Viscoelastic Properties

The accompanying paper described the structural characterization of polyurethane multi- block copolymers that varied block incompatibility at fixed block length and composition. Their linear viscoelastic properties are presented in this paper. Dynamic mechanical experiments in temperature sweep mode confirmed that the materials ranged from almost homogeneous to highly microphase- separated. Dynamic mechanical frequency sweep experiments showed a Rouse-like frequency response in all materials at high temperatures, including those polyurethanes that were highly microphase- separated. This is in stark contrast to the numerous reports on microphase-separated block copolymers that show nonliquidlike terminal behavior at low frequencies. We attribute this homopolymer-like response of microphase-separated polyurethanes to a lack of long-range order in their microphase-separated structure and regard it to be a crucial feature of most commercial multiblock copolymers. The apparent activation energy for terminal flow was found to be insensitive to the extent of microphase separation, indicating that the thermodynamic penalty N expected for chain motion in block copolymers plays an insignificant role in the dynamics of the present materials. We demonstrate that bare effects (primarily proximity to the Tg) and dynamic asymmetry (friction in one block much larger than in the other) play a major role in the dynamics of polyurethanes. The latter is expected to be especially important in the dynamics of elastomeric block copolymers whose blocks usually have widely separated Tg's.