A. James Clark School of Engineering
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Item STRUCTURE-PROPERTY RELATIONSHIPS IN CRYSTALLINE-AMORPHOUS BLOCK COPOLYMERS: SYNTHESIS, PHASE BEHAVIOR AND MECHANICAL PROPERTIES(2017) Hwang, Wonseok; Briber, Robert M; Sita, Lawrence R; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Block copolymers are composed of two or more polymer chains that are chemically distinctive and covalently bonded at the single point. Due to the incompatibility between blocks, the block copolymers undergo microphase separation generating long-range ordered microdomains. Considerable effort has been put on the understanding of the microdomain structure-mechanical properties relationships in block copolymers where both blocks are amorphous. Less research has been done on structure-property relationships in crystalline-amorphous block copolymers. In this thesis, the structure-property relationships in crystalline-amorphous block copolymers have been explored using two polymer systems, 1) well-defined isotactic-atactic-isotactic stereoblock (sbPP) and stereoirregular (irPP) polypropylene materials, 2) poly(1-hexene)-poly(methyl-1,3-cyclopentane) (PH-PMCP) block copolymers. The structure-property relationships in sbPP and irPP materials have been characterized using tensile testing, dynamic mechanical analysis (DMA) and rheology. The sbPP materials demonstrated higher tensile modulus exhibiting elongation ratios of more than 2,000 % while irPP materials showed lower tensile modulus with elongation of less than 100 %. Furthermore, sbPP materials exhibited more than 90 % of tensile recovery in contrast to 60-70 % tensile recovery for irPP materials. The discrete crystalline-amorphous-crystalline structure in sbPP materials provides both rigidity and flexibility. The morphologies of PH-PMCP materials have been investigated using in-situ small and wide angle X-ray scattering (SAXS/WAXS), rheology, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The melting of the crystalline PMCP followed by the microphase separation in PH-PMCP results in a modulus transition after the PMCP melting. In particular, sphere-forming PH-PMCP block copolymers exhibit a drop of 2 orders of magnitude in the storage modulus at the PMCP melting temperature. The recovery in storage modulus within 2 min results from the low molecular weight and the incompatibility between PH and PMCP blocks. By tuning PMCP content, this modulus transition has been demonstrated in sphere, cylinder, double gyroid, and lamellae forming block copolymers. The viscoelastic response in sphere-forming PH-PMCP block copolymers indicate that after the PMCP crystal melt the molten PMCP blocks participate the formation of block copolymer microdomains and improve the microdomain ordering.Item The Effect of Phase Constitution and Morphology on Room Temperature Deformation Behavior of Binary Titanium Alloys(2011) Wyatt, Zane W.; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Currently, titanium alloys are used in a variety of applications, including defense, aerospace, biomedicine, and even common consumer products such as bicycles and golf clubs. In many applications such as the landing gear of aircraft and geothermal energy production, titanium components may be subjected to stresses for extended periods of time. It has long been known that single-phase α (HCP), single-phase β (BCC), and two-phase α + β Ti alloys can creep at low temperatures (<0.25Tm). For this reason, creep is an important factor to consider when designing titanium alloys for various applications. The first part of this investigation is concerned with single-phase α-Ti alloys. It was found that the twin size (lamellar thickness) decreases with an increase in strain rate. This behavior is unexpected based on the classical understanding of instantaneous twinning. This investigation was able to for the first time demonstrate a time-dependent twinning phenomenon during high strain rate tensile deformation. The second part of this investigation is concerned with experimentally and theoretically studying low-temperature creep deformation behavior of two-phase α + β Ti alloys. Deformation mechanisms were seen in two-phase α + β Ti alloys that are not present during creep of the respective single-phase alloys with compositions equivalent to the individual phases. To investigate the possible interphase interaction stresses, 3D anisotropic Finite element modeling (FEM) was used. These simulations revealed that due to the Burgers orientation relationship between the two phases, deformation such as slip or twinning in the α phase can create very high additional shear stresses on different slip systems in the β phase. This work also revealed that the interfacial stresses that develop between the two phases during elastic deformation will often be much greater than the applied stress. These results were used to help explain the additional deformation mechanisms seen in two-phase alloys that are not seen in the respective single-phase alloys during creep. This work was supported by the National Science Foundation under Grant Number DMR-0906994.