Control of microstructures to induce ductility in brittle amorphous polymers

I.A. Casteren, van

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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Abstract

In the last decades considerable effort is addressed to the toughening of brittle amorphous polymers like polystyrene (PS) and polymethylmethacrylate (PMMA). The brittleness of these polymers is the result of a catastrophic strain localization in the form of crazes. Considering the network density, the intrinsic toughness of PS is expected to be higher than polycarbonate (PC), which is known as a more ductile material and deforms via shear yielding. Previous studies indicated that macroscopic deformation behavior such as crazing or shear yielding is dominated by the post yield behavior. PS suffers from strong strain softening in combination with limited strain hardening, giving rise to an extreme localization of the stress during deformation within the crazes. Improving the ductility should hence focus on avoiding localization of the strain by eliminating the intrinsic strain softening and promoting the contribution of strain hardening such that the high intrinsic ductility can be transferred to the macroscopic level. Furthermore, a transition from crazing to shear yielding takes place if the matrix ligament thickness within a heterogenous system is reduced to below a critical value. Maximum toughness in PS and PMMA is, therefore, expected by the introduction of a nanosized core-shell rubber with minimal resistance against cavitation. Cavitation of the core causes a strong relieve of the hydrostatic stresses, whereas delocalization is enhanced by a stronger strain hardening behavior of the rubber shell. Control over morphology is the key issue for a successful toughening process, since the required morphology can not be achieved by conventional melt blending routes. Therefore, the self-assembly process of block copolymers is used. Block copolymers are known to self-assemble into well-de??ned ordered phases with nanoscopic length scales both in the undiluted state and in blends. The thermodynamic or equilibrium morphology of a given diblock copolymer is governed by the polymer molecular weight, block symmetry and interblock repulsion. In blends and solutions, the morphology is further dictated by solvent-block interaction parameters and blend composition. In the self-assembly process used in this study, the monomer styrene or methyl methacrylate (MMA) is polymerized in the presence of block copolymers, which should result in a thermoplastic matrix ??lled with small rubbery particles. The observations on the morphology development showed that for a large number of systems with block copolymers based on a hydrogenated polyethyleneco- butylene (PEB) (or polyethylene-co-propylene (PEP)) and an acrylate block, undesired macrophase separation occurs during the polymerization, resulting in a PS- or PMMA-rich phase, and a block copolymer-rich phase, having a structure similar to the characteristic microphase structure of the block-copolymer itself. Two possible routes were explored to avoid macrophase separation. For PMMA blends it is shown that by reducing the polymerization temperature to -40 ?? C, the order-disorder temperature (ODT) is passed, resulting in additional stabilization of the initial micellar morphology of the block copolymer/MMA solution upon polymerization. Furthermore, a decreased polymerization temperature results in an enhanced viscosity of the reacting MMA mixture, which prevents further coalescence of the block copolymer. The intrinsic behavior of the resulting nanosized PMMA blends showed a strong reduction in strain softening with block copolymer content. For blends based on PS, suppression of macrophase separation could be realized by introduction of intermolecular hydrogen bonding between the PS matrix material and the acrylate shell block. Dependent on the extent of interactions, a transition in morphology occurs from macrophase separated structures to nanosized microphase separated structures up to complete miscibility of the block copolymer shell and PS matrix. Although the tensile test results didn't reveal a strong synergistic toughening effect, the changes in intrinsic deformation behavior were evident. The strain hardening modulus remained unaffected, whereas strain softening was strongly reduced by the introduction of the sub-micron micellar morphology. With increased miscibility between the rubber shell and matrix, the yield stress and strain softening were enhanced resulting in a more localized deformation and thus brittle behavior. This indicates that a rubber shell is a prerequisite for optimal delocalization. Time-resolved small angle X-ray scattering using synchrotron radiation proved to be a powerful technique to determine the mode and development of the microscopic deformation. Depending on the rubber content and amount of hydrogen bond interactions introduced, the deformation of PS/diblock copolymer blends in uniaxial tension can be altered from crazing to cavitation induced shear yielding. The formation of voids relieve the triaxial stress state, which subsequently enhances shear yielding and thus a more ductile behavior. The limited strain at break observed in these blends is mainly caused by the too low strain hardening modulus. The introduction of hydrogen bond interaction is necessary to gain the required compatibility of the constituents and, subsequently, to control the morphology (development). The consequent incomplete demixing between the PS matrix and the rubber acrylate shell may be a major drawback of using this type of interactions to prepare model blends to study the in- ??uence of core-shell-like morphologies on the mechanical performance of brittle amorphous polymers. Therefore, additional attention is paid on the microscopic deformation behavior of various types of microphase separated triblock copolymers consisting of a glassy PS or PMMA matrix, with special emphasis on the role of cavitation, crazing and shear yielding. The ??rst triblock copolymer is based on polyethylene-co-butylene-polybutylacrylate-polymethyl methacrylate (PEB-PBA-PMMA). Liquid-like PEB particles, surrounded by a PBA rubber shell, are embedded in a PMMA matrix. Upon deformation, the PEB cores cavitate resulting in delocalization of the strain caused by the subsequent yielding of the PMMA ligaments. The second triblock copolymer with sharp phase boundaries studied is polystyrenepolybutadiene- polycaprolactone (SBC) triblock copolymer, showing a cylindrical core-shell morphology. It is demonstrated that the con??ned crystalline PCL domains possess a strong tendency for cavitation under tension due to the internal stresses that develop during crystallization and contraction. Consequently, the triaxial stress state is relieved and shear yielding is promoted resulting in a drastically improved tensile toughness (??900%). The degree of long range order of the cylindrical microdomains determines the craze termination capability. For materials possessing a high degree of ordering, crazes propagate preferentially along the cylindrical axes and can not be terminated effectively. The ??nal triblock copolymer studied is a poly(styrene-butadiene-styrene) (PS-PB-PS) triblock copolymer with a polystyrene content of 75 wt%. The microscopic mode of deformation of this triblock copolymer is largely in??uenced by the microscopic morphology. Orientation of the lamellae either parallel or perpendicular to the applied load results in a drastic reduction in mechanical performance. For lamellae oriented parallel to the applied load, the deformation proceeds via the subsequent failure of PS domains and the rearrangement of these fragmented domains in the PB phase. The samples with the lamellae oriented perpendicular to the applied load predominantly deform via craze initiation and propagation in the PS lamellae. In contrast to samples with random oriented lamellae, crazes propagate in the preferential direction perpendicular to the load. Furthermore, it is demonstrated that the choice of the solvent in the solvent casting process causes a transition from a lamellar morphology to a random distribution of short wavy rods of PB surrounded by a matrix of PS. As a result, the deformation behavior as well as the microscopic mode of deformation changes. Whereas a ductile homogeneous deformation behavior was observed for the lamellar morphology proceeding via yielding and effective craze termination, a more brittle behavior was seen for the short wavy PB rods embedded in a PS matrix. The latter is caused by the subsequent cavitation of the PB domains and the absence of a morphology able to terminate propagating crazes. Diblock copolymers can thus be used to prepare more ductile heterogenous systems based on brittle amorphous polymers. It is demonstrated that strain softening diminishes and that the deformation predominantly proceeds via cavitation induced shear yielding. Both a liquid-like core as well as a semi-crystalline core may facilitate the occurrence of cavitation. However, in order to obtain high strains at break the deformation should be transferred more accurately to undeformed parts by increasing the strain hardening modulus. Additionally, it is shown that triblock copolymers consisting of either a PS or PMMA matrix can serve as perfect model systems to reveal the in??uence of cavitation, long range order and microscopic morphology on the intrinsic deformation behavior.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Mechanical Engineering
Supervisors/Advisors
  • Meijer, Han, Promotor
  • Lemstra, Piet, Promotor
  • Goossens, Han, Copromotor
Award date3 Sep 2003
Place of PublicationEindhoven
Publisher
Print ISBNs90-386-2615-0
DOIs
Publication statusPublished - 2003

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