Author: Ramona Denise Barent

Publisher:

Published: 2023

Total Pages: 0

ISBN-13:

With the groundbreaking work of Hermann Staudinger in 1920, polymer science has evolved tremendously for more than 100 years into manifold directions, impacting countless parts of life. Naming only a few disciplines, polymers find an omnipresent application in the automotive and the construction sector. Their insulating or semiconducting properties are pivotal for electric and electronic devices, whereas membrane technologies rely on their separation capability, and both the medical and the agricultural sector benefit from advanced polymer structures for controlled drug release. Yet, macromolecules played a crucial role long before they were recognized, studied in depth and specialized for targeted applications. For instance, natural rubber has been commercialized for almost the past two centuries, with Charles Goodyear's and Thomas Hancock's vulcanization process providing the basis for stable elastomeric materials. The introduction of synthetic thermoplastic elastomers (TPEs) in the 20th century, possessing the processability of thermoplastics and the elasticity of vulcanized rubber, and progress in understanding the chemical nature and the resulting material properties allows for envisioning versatile characteristics. With the living anionic polymerization technique at hand, which is the method of choice for the synthesis of complex but well-defined polymer architectures, new challenges designing materials with tailored physical properties can be met. The first part of this thesis aspires to elucidate the influence of an (IS)nI multiblock architecture on the materials' properties in bulk, whereas the second part evaluates their solution characteristics. At the end of this work a fundamental framework for the living anionic polymerizability in non-polar media is outlined. Chapter 1 gives a general introduction to the versatile toolbox of living anionic polymerization against the background of their use as thermoplastic elastomers (TPEs). After a brief outline of the historical background of elastomers and current application fields of TPEs, this review focuses on synthetic approaches tailoring the polymers' and in turn the materials ́ properties. In particular, inherent mechanical properties are discussed with respect to the phase segregation strength and the resulting microdomain morphologies. The highlighted synthetic strategies focus on lithium initiated living anionic (co)polymerizations of different styrenic and 1,3-diene monomers in hydrocarbons, further emphasizing the influence of modifiers on the block profile in statistical copolymers. In this regard, not only the block profile, but also the influence of the block sequence, the block number, and the polymer chain architecture are elaborated. Furthermore, the benefit of in situ monitoring techniques to determine kinetic parameters is demonstrated and expanded on kinetic Monte Carlo simulations, which allow for the calculation of reaction times, greatly facilitating the workflow of multiblock syntheses. Chapter 2 reviews the use of plasticized poly(vinyl chloride) for medical devices. In the first part health concerns related to the commonly used plasticizer are discussed within the framework of human exposure and its metabolism. The second and third part evaluate potential solutions by the introduction of different plasticizers and by the replacement of PVC by alternative polymer materials, respectively. For the latter, a project funded by the European Commission addressing the implementation of polyolefins for medical bags and the use of styrenic thermoplastic elastomers for healthcare applications are considered in greater detail. Chapter 3 highlights the structural and mechanical investigation of polyisoprene-polystyrene multiblock copolymers with two polyisoprene end blocks. These structures are of interest due to the potential internal plasticizer effect of the flexible end blocks, probably rendering a soft but resilient material. Temperature-dependent small-angle x-ray scattering experiments were implemented to investigate the phase separated morphologies of both sequential and tapered (IS)nI multiblock copolymers covering a wide range of molar masses and block numbers. A strong decrease in the segregation strength and hence of the order-disorder temperature was proven, comparing sequential block copolymers with a defined transition between the adjacent blocks with tapered block-like copolymers with a sharp, but gradual comonomer transition in each polyisoprene-polystyrene diblock sequence. This is caused by the reduced enthalpic incompatibility in tapered structures. While the sequential multiblock copolymers exhibited well-ordered lamellar morphologies, the tapered counterparts showed weakly- ordered perforated layers. The viscoelastic responses measured by tensile tests evidenced superior resilience for the sequential structures, while the tapered structures revealed a higher softness and flexibility. The concomitant decrease of the block size by increasing the block number at a given overall molar mass is accompanied by an increased domain bridging, but at the same time a weakened microphase separation. Due to this trade-off, the higher molar mass tapered pentablocks and sequential heptablocks were found to best balance these opposite effects, resulting in a significant mechanical toughness. Interestingly, polymers with the same molar mass per block exhibited comparable domain spacings and hence softness, while the ultimate deformability was found to increase with the extension by a diblock sequence due to an enhanced domain bridging. Comparison with a literature-known analogous structure with two polystyrene end blocks revealed an easier deformability of the structure with two polyisoprene end blocks reasoned in a smaller domain size. Chapter 4 further elaborates the mechanical properties of polyisoprene-polystyrene multiblock copolymers with different architectures in terms of chain connectivity. This fundamental research is intriguing due to the potential to minimize the issue of permanent sets upon large deformation while preserving the advantage of easily processable and reprocessable materials. Uniaxial tensile tests as well as recovery measurements were performed comparing linear tapered SIS triblock copolymers with linear (SIS)2 pentablock and star-shaped (SIS)4 multiblock copolymers featuring vitrifying core and end blocks. The advanced architectures were synthesized by an “arm-first” approach coupling the (SIS) arms. For small molar masses per arm, the star-shaped multiblock architectures showed superior ultimate stress and strain at break reflected in a higher toughness, which can be ascribed to their higher bridging fraction. For higher molar masses, the ultimate mechanical properties of the linear pentablock copolymers and the star-shaped structures approached each other reaching a plateau value, showing that at this point the covalent linkage of the star architecture does not provide further resilience. Yet, they still outperformed the simple triblock structure, further emphasizing the importance of large fractions of bridged chain conformations. For polymers with the same overall molar mass, the star-shaped multiblock copolymers could not compete with the linear pentablock copolymers due to an inferior phase segregation and hence facilitated chain pull-out. For three counterparts of equal molar mass per arm, which showed decent phase segregation and comparable ultimate properties, the cyclic strain experiments evidenced striking differences in their recovery behavior. While the star-shaped (SIS)4 multiblock copolymer showed unprecedented final recovery after 5 minutes of rest, they exhibited poor initial recovery during receding. In contrast, the linear structures feature superior rapid recovery but smaller final recoveries after resting. These phenomena prove the higher chain flexibility of the linear architectures being responsible for fast restoring, while the even stress distribution in star-shaped architectures with a covalent core junction generates improved shape memory. However, at high stress levels all specimens experienced a permanent set due to substantial restructuring, resulting in converging restoring properties. Chapter 5 examines the three most decisive tribological parameters dictating the performance of viscosity modifiers in lubricating oils relating to not only commonly implemented, but also novel polymer classes. Moving metal parts have to be lubricated in order to prevent friction. However, lubricating fluids face a rapid viscosity decrease upon increasing temperature. Therefore, viscosity modifiers are added to attenuate the adverse effects of asperity contact. Yet, optimum performance in all of the three key metrics, i.e., a beneficial viscosity-temperature relationship, thickening efficiency, and shear stability has not been achieved so far. Since these tribological parameters are complexly intercorrelated, their balancing is challenging, which is the reason for the demand for novel lubricant additives. The influence of key polymer characteristics such as molar mass, dispersity, chain composition, and architecture on the hydrodynamic volume and by this on the tribological parameters are accentuated. Furthermore, the chemical nature of the commonly implemented polymer classes comprising poly(alkyl methacrylates), olefinic copolymers, and hydrogenated styrene- diene copolymers is reviewed with respect to their polymerization mechanism and the inherent thickening mechanisms for viscosity improvement. The latter include the coil expansion mechanism prominent in poly(alkyl methacrylate) formulations and association phenomena featured by hydrogenated styrene-diene copolymers. Beyond this, advanced structures aiming at exceeding the current performance limits are discussed. Here, blending approaches and new polymer classes like poly(2-oxazolines) and poly(2-oxazines), but also sophisticated architectures like brush-like, comb-like or linear (tapered) multiblock copolymer structures are emphasized. This promising combination of several benefits gives food for further investigations. Chapter 6 addresses the self-organization of multiblock copolymers with both a defined and a gradual block profile in the polyisoprene-selective solvent heptane. Conclusions on the aggregate structures are drawn based on the diffusion behavior at varying concentrations. For this purpose, dilute and concentrated polymer solutions are examined by dynamic light scattering using a cross-correlation approach, which allows to study diffusion processes even in the presence of multiple scattering events. Diverging diffusion coefficients for the triblock and multiblock copolymers at a polymer concentration above 1 wt% proved the formation of non-ergodic systems, i.e., polymer networks, in case of the structures with several solvophobic polystyrene blocks. Complementary fluorescence correlation spectroscopy measurements permitted insights into the self-diffusion of unimers through these polymer networks. These processes were found to be most restricted in networks of sequential multiblock copolymers, which is the consequence of a denser network due to a higher bridging fraction in combination with larger unimer dimensions. The unimer dimensions themselves were studied in highly diluted polymer solutions, where no aggregation phenomena are present. Furthermore, micellar aggregates and their fraction increased going from dilute solutions to higher concentrations, finally adapting the discussed transient polymer networks. In order to enable fluorescence correlation spectroscopy investigations, a novel post-polymerization protocol for fluorescent dye attachment was established, which stands out by only a marginal alteration in the chemical nature of the labeled polymer. Chapter 7 emphasizes the concentration-dependent viscosity-temperature relationship of the (non)-hydrogenated tapered multiblock copolymers in the polyisoprene-selective solvents squalane and a highly isoparaffinic hydrocarbon lubricating oil against the background of the self-assembled structures discussed in Chapter 6. Combining temperature- and frequency- dependent dynamic viscosity measurements with temperature-dependent kinematic viscosity measurements at 40 °C and 100 °C, a clear correlation between the self-organized polymer aggregate structure and the performance as viscosity modifier could be established. At comparably low polymer concentrations, an enhanced viscosity-temperature relationship with increasing overall molar mass, block number, and isoprene content was identified. As an explanation, the formation of loose aggregates with several polystyrene cores and hence extended hydrodynamic volumes in case of multiblock architectures was deduced. Yet at high concentrations, the structures with the shortest individual solvophobic polystyrene blocks faced a deterioration in the investigated tribological key parameters, i.e., the viscosity- temperature relationship and the thickening efficiency. This is explained by the formation of large transient networks, which are more prone to partial disassembly upon shearing. This effect is further intensified by the migration of solvent molecules into the aggregate's cores, which is most pronounced for short polystyrene blocks. The resulting weakened van-der-Waals interactions promote chain pull-out and by this partial disassembly. Comparison of the investigated styrene-diene multiblock copolymers to a commercialized comb-like poly(alkyl methacrylate) displayed a superior performance of the presented structures at low polymer treat rates. This demonstrates the enormous potential of tapered multiblock architectures as advanced viscosity modifiers. Chapter 8 aims at understanding the structure-property relationships of multiblock copolymers with a defined or a gradual, albeit sharp block profile in dilute non-selective solution. For this purpose, series of sequential and tapered multiblock copolymers (IS)nI with molar masses ranging from 40-400 kg·mol-1 and block numbers of 3-13 were comprehensively characterized by complementary approaches. Subsequently, dilute solutions in toluene were systematically investigated, implementing classical scaling relationships between the hydrodynamic characteristics derived from analytical ultracentrifugation, intrinsic viscosity, and related experiments. Both rotational and translational diffusion experiments showed subtle differences in the polymer coil dimension and hence rigidity for polymer series with equal block number. For these homologous polymer series, a more rigid and hence expanded chain conformation for the sequential structures was deducible compared to their tapered analogues. Interestingly, the polymer series with different molar masses and block number, whose polymers featured equal degrees of polymerization per polystyrene block, did not comply with the classical scaling relationships. This demonstrates the influence of the block number on the solution properties even in non-selective solvents. Chapter 9 studies the polymerizability of rotationally constrained 1,3-dienes with a fixed cisoid or transoid geometry of the double bonds. Particularly, the reactivity in living anionic polymerization approaches in non-polar media is examined, whose mechanism is proposed to proceed via a coordinative mechanism. Theoretical simulation approaches of two new 1,3-diene monomers with a rigid, prescribed cisoid or transoid geometry in cyclohexane as a typical non-polar solvent are combined with synthetic and kinetic studies. Experimental observations correlated with the predicted reactivities and simulated reaction pathways, which showed that a cisoid geometry with in-plane double bonds is mandatory for propagation. Indeed, the cisoid diene was homo- as well as copolymerizable with isoprene, whereas the transoid diene lacked reactivity. The required ring distortion in case of the cyclic cisoid diene resulted in higher simulated activation barriers for the propagation step in comparison to the addition of the unrestricted common monomer isoprene. This was experimentally confirmed by real-time 1H NMR spectroscopic kinetic investigations, which evinced a gradient formation in statistical copolymerization experiments of the cisoid diene with isoprene, whose steepness became flattened upon temperature increase. Additionally, thermal characterization of the statistical copolymers revealed a weakened segregation strength for the tapered diblock copolymers with a smoother gradual distribution of the comonomers along the polymer chain. Chapter A1 comprises a complementary examination of the ultimate mechanical properties of the tapered star-shaped polyisoprene-polystyrene multiblock copolymers treated in Chapter 4. Two series of (SIS)4 multiblock copolymers varying in their comonomer ratio were compared to the linear SIS triblock copolymers representing the corresponding arm structures. While the star-shaped polymers with a lower isoprene content ordered into lamellae and therefore were stiffer and less elastic, the series with a higher isoprene content possessed morphologies with a continuous polyisoprene phase. Irrespective of the formed morphology, the star-shaped (SIS)4 architectures outperformed the corresponding linear SIS polymers with respect to their toughness and strain at break. Furthermore, selective catalytic hydrogenation of the PI blocks was exemplified for one star-shaped multiblock copolymer and its linear counterpart, this way increasing the segregation strength and decreasing the entanglement molecular weight of the polydiene segments. Interestingly, the influence of hydrogenation on the ultimate properties varied with the polymer architecture. While the linear tapered triblock copolymer experienced a substantial increase in its ultimate properties, the star-shaped multiblock copolymer faced a deteriorated performance. This is explained by differences in the magnitude of strain- hardening improvement in combination with higher amounts of entanglements. Chapter A2 focuses on the impact of solvents on the copolymerization kinetics of epoxides via an anionic ring-opening polymerization mechanism. For this purpose, the copolymerization kinetics of ethylene oxide (EO) with glycidyl ethers with varying coordination sites were monitored by in situ 1H NMR spectroscopy both in tetrahydrofuran (THF) and the highly polar solvent dimethyl sulfoxide (DMSO). The experiments revealed slightly higher reactivities of the glycidyl ethers compared to EO, which lacks in any side group, emphasizing a pronounced chelation effect of the potassium counterion by the side groups of the glycidyl ether monomers about to be added. With allyl glycidyl ether (AGE) featuring one and ethoxy vinyl glycidyl ether (EVGE) possessing two ether-type coordination sites per side group, EVGE showed slightly stronger incorporation preference than AGE. An increase in the disparity of the relative reactivities was found with decreasing solvent polarity, which relies on the degree of solvation of the propagating chain end and its counterion. Density functional theoretical simulation approaches were implemented to illustrate and further justify the pivotal role of the complexation capability of the ether-containing side groups.