Reimund Stadler

Non-centrosymmetric superlattices in block copolymer blends

Materials with a macroscopic electric polarization display a variety of useful properties, such as piezo- and pyroelectricity and second-order nonlinear optical activity. Macroscopic polarization results when dipolar molecules are orientated in the same direction, or when ions are organized in a non-centrosymmetric crystal structure. Centrosymmetric molecules have no dipole moment and so cannot generate a macroscopic polarization. Non-centrosymmetry in amorphous materials can be engineered by depositing particular sequences of layers on top of each other, or by applying external fields (generally electric) to orientate the molecules. Here we report the formation of a non-centrosymmetric structure in an amorphous material through spontaneous self-assembly. Block copolymers are known to form ordered structures at the microscale owing to segregation of the different blocks, . We show that a mixture of a ternary triblock copolymer and a binary diblock copolymer will organize itself into a non-centrosymmetric layered structure in which the layers are occupied by different blocks. The structure is periodic with a length scale of around 60 nm.

Thermoplastic elastomers by hydrogen bonding 1. Rheological properties of modified polybutadiene

Polybutadiene of narrow molecular weight distribution was modified using 4-phenyl-1, 2,4-triazoline-3,5-dione. The degree of modification was 1% and 2% with respect to the repeating units. Hydrogen bonding between the highly polar urazole groups thus incorporated into the polymer gives rise to the formation of a thermoreversible elastomeric network. Dynamic mechanical measurements in the temperature range between 220 and 330 K support the picture of the thermoreversible hydrogen bond interaction. The rubber elastic plateau is shifted to higher temperatures and lower frequencies. The increase in the plateau modulus cannot be attributed solely to the contribution of the network structure but is mainly a consequence of the broadening of the relaxation time spectrum in the modified samples. From the temperature dependence of the shift factors log(aT) it is concluded that the general WLF approach fails. The strong temperature dependence of the apparent activation energy of flow is a consequence of the temperature dependence of the hydrogen bond interaction.

Influence of hydrogen bonding on the viscoelastic properties of thermoreversible networks: analysis of the local complex dynamics

Abstract The viscoelastic properties of thermoreversible polybutadiene networks in which junctions are formed by binary contacts between polar stickers (phenylurazole) are investigated by a dynamic mechanical spectroscopy within the frequency range 0.0079–79.5 Hz (0.05–500 rads−1). Time-temperature superposition is applicable in the terminal flow region and the glass transition regime, whereas thermorheologically complex behaviour is observed within the rubbery plateau region. For the terminal relaxation zone the polar stickers enhance the relaxation time and broaden the relaxation time spectrum. The thermorheologically complex behaviour within the rubbery plateau region results from the occurrence of an additional relaxation process which is attributed to the process of complexation/decomplexation of the reversible hydrogen bond contacts. The characteristic relaxation time of this process at 233 K is 0.044s. This process can also be monitored in dielectric relaxation. Discrepancies between dynamic mechanical and dielectric data are discussed.

Poly(styrene-b-methyl methacrylate) block copolymers as compatibilizing agents in blends of poly(styrene-co-acrylonitrile) and poly(2,6-dimethyl-1,4-phenylene ether): 1. Location of block copolymers in ternary blends — compatibilization versus micelle formation

Abstract The compatibilizing effect of the symmetric narrowly distributed block copolymer poly(styrene- b -methyl methacrylate) (P(S- b -MMA)) in blends of high-molecular-weight poly(styrene- co -acrylonitrile) containing 20 wt% (PSAN20) or 43 wt% acrylonitrile (PSAN43) with poly(2,6-dimethyl-1,4-phenylene ether) (PPE) was investigated by dynamic mechanical spectroscopy and transmission electron microscopy. In blends with the PSAN43, P(S- b -MMA) forms spherical micelles in the PPE phase with no dispersing efficiency. In contrast to this, for blends with PSAN20, the block copolymer is located at the phase boundary, causing an extremely fine dispersion of the components. Depending on the location of P(S- b -MMA), the PPE glass transition is altered by the PS blocks to different degrees. Even for block copolymer concentrations as high as 20 wt%, in ternary blends with PSAN20 and PPE no micelles could be detected. The results suggest that A/B blends compatibilized with a C-D block copolymer, in which the copolymer components show an exothermic heat of mixing with the bulk phases A/B, are superior to systems A/A-B/B with respect to emulsifying efficiency. Owing to the high tendency of the block copolymer to locate at the phase boundary, micelles do not compete at concentrations of practical interest.

Synthesis of block copolymers with poly(methyl methacrylate): P(B-b-MMA), P(EB-b-MMA), P(S-b-B-b-MMA) and P(S-b-EB-b-MMA)

SummaryWell-defined diblock copolymers poly(butadiene-b-methyl methacrylate) (=P(B-b-MMA)) and triblock copolymers poly(styrene-b-butadiene-b-methyl methacrylate) (=P(S-b-B-b-MMA)) have been prepared by sequential anionic polymerization in THF. The synthesis of P(B-b-MMA) and P(S-b-B-b-MMA) was most efficient in the presence of lithium alkoxides. By this method side reactions are suppressed and the polymerization can be performed at higher temperatures. The resulting triblock copolymers have narrow molecular weight distribution. The 1,2-PB midblock was quantitatively hydrogenated with tosylhydrazide to enhance thermal stability. Alternatively the hydrogenation can be performed at elevated pressure using hydrogen and Wilkinson-catalyst in butanone. Heterogeneous catalyst systems based on Palladium did not yield quantitative hydrogenation.

Multiphase thermoplastic elastomers by combination of covalent and association chain structures: 2. Small-strain dynamic mechanical properties

Abstract Polybutadienes of narrow molecular-weight distribution, carrying statistically distributed polar phenylurazole (1), 4-ethoxycarbonylphenylurazole (2) and 4-carboxyphenylurazole (3) groups along the polymer chain, are analysed with respect to their dynamic mechanical properties in the linear viscoelastic region. In these systems thermoreversible networks are formed by hydrogen-bond complexes. In the case of 1 and 2, where only binary complexes are formed, the systems show thermorheologically simple behaviour, i.e. the construction of viscoelastic master curves is possible and the temperature dependence is described by an average apparent activation energy of flow for all relaxation processes. For polybutadiene carrying between 0.5 and 4 groups of 3 per 100 repeat units, thermorheologically complex behaviour is observed, which is related to the multiphase structure formed by phase separation between the covalent polybutadiene backbone and supramolecular ordered association polymers formed by the polar functional groups.

Thermoplastic elastomers by hydrogen bonding 4. Influence of hydrogen bonding on the temperature dependence of the viscoelastic properties

The temperature dependence of the viscoelastic properties of thermoreversible polybutadiene networks based on hydrogen bond linkages is analyzed from the logarithmic shift factors logaT. For binary hydrogen bond complexes thermorheologically simple behavior is observed. The temperature dependence of logaT is described by the Williams-Landel-Ferry (WLF) equation. The thermoreversible linkages cause an increase in the apparent activation enthalpy of flow which is related to the number of complexing sites in the polymer. Thermorheologically complex behavior is observed in a system with more complex association.

Ternary ABC block copolymers based on one glassy and two crystallizable blocks: polystyrene-block-polyethylene-block-poly(ε-caprolactone)

The hydrogenation of polystyrene-block-polybutadiene-block-poly(£-caprolactone) SBC triblock copolymers was performed in the presence of the Wilkinson catalyst RhCl(P(C 6 H 5 ) 3 ) 3 . Reaction conditions (hydrogen pressure, temperature and reaction time) were varied to ensure quantitative hydrogenation without detectable side reactions. Gel permeation chromatography showed no broadening of the molecular weight distribution during hydrogenation. The efficiency of the catalyst is markedly influenced by the molecular weight of the copolymer. Due to the presence of the polyethylene (PE) block, the resulting polymers exhibit a reduced solubility in comparison to the starting materials. Using differential scanning calorimetry (DSC), preliminary results about the crystallization and melting behavior of the PE-block were obtained. In the triblock copolymers, the PE-block showed a marked depression of the melting point and crystallinity when compared to pure hydrogenated polybutadiene of equivalent molecular weight and microstructure or to a comparable PE-block within a polyethylene-block-poly(e-caprolactone) diblock copolymer. A fractionated crystallization process of the PCL-block was observed when the PCL component in the hydrogenated triblock copolymers was present as a minor phase.

Influence of the phase separation on the linear viscoelastic properties of a polystyrene-poly(vinyl methyl ether) blend

The linear viscoelastic properties of a mixture of polystyrene (PS, Mw = 2.95 × 106 g mol−1, 10 wt%) and poly(vinyl methyl ether) (PVME, Mw = 75 000 g mol−1, MwMn = 1.6, 90 wt%) have been investigated in the temperature range 40–190°C. From small angle light scattering (SALS) and turbidity measurements the phase separation temperature has been determined to be at 106 ± 1°C. The composition was chosen because it was close to the critical composition. Thus, phase separation is expected to occur by spinodal decomposition. At temperatures T < Tc the relaxation behaviour is dominated by the relaxation of the lower molecular weight PVME. A second plateau zone which corresponds to a diluted PS is clearly resolved. Well within the phase-separated regime the frequency dependence of the storage and the loss modulus, G′ and G″, is proportional to ϵ0.6.

Cooperative structure formation by combination of covalent and association chain polymers: 4. Designing functional groups for supramolecular structure formation

Abstract A small number of polar functional groups are attached to polybutadiene by a polymer analogous reaction. Depending on their molecular structure, different degrees of supramolecular ordering result from the aggregation of these polar groups. In the case of phenylurazole units, which carry a single site for hydrogen-bond complexation, binary contacts are formed; whereas linear association chain structures are formed in the case of polybutadiene carrying phenylurazole units additionally substituted with a carboxy group in meta or para positions of the phenyl ring. In the latter case, association chains aggregate cooperatively to an ordered supramolecular structure. The different structural features are evident in the stress-strain properties of these thermoplastic elastomers.