Doctor of Philosophy, Case Western Reserve University, 2012, Macromolecular Science and Engineering

Crystallization of polypropylene (PP) droplets produced by thermal break up of multilayered films was investigated. Droplets produced from 12 nm PP layers resulted into exclusive homogeneous nucleation, which is usually viewed as the initial step during polymer crystallization. The structure, thermodynamics and crystallization kinetics of the droplets were analyzed to understand the homogeneous nucleation process. The stability of the homogeneously nucleated droplets was further probed by thermal treatment. Ability to produce homogeneous nucleation provided an opportunity to add various nucleating agents to the droplets and investigate the nature of heterogeneous nucleation. Effect of various nucleating agents on the crystallization of PP droplets is also discussed. In another approach, confined crystallization of a low crystallinity polymer, syndiotactic PP (sPP), was achieved by “forced assembly” coextrusion process. The crystal orientation was controlled by melt recrystallization of sPP layers under the hard confinement of polycarbonate. A transition from in-plane to on-edge lamellar orientation was observed with decreasing crystallization temperature. Controlled melt recrystallization of sPP layers was successfully used to manipulate the lamellar orientation of sPP and control the gas transport properties over one order of magnitude. Additionally, the effect of confinement on the physical aging and structural relaxation of a glassy polymer, polystyrene (PS), under the hard confinement of polycarbonate (PC) was also investigated in the thermally reset multilayered films.

Committee: Eric Baer D.Eng. (Advisor); Eric Baer D.Eng. (Committee Chair); James Anderson PhD (Committee Member); David Schiraldi PhD (Committee Member); LaShanda Korley PhD (Committee Member) Subjects: Polymers

Doctor of Philosophy, University of Akron, 2009, Polymer Science

One dimensionally (1D) confined crystallization based on semicrystalline diblock copolymers has been widely investigated for twenty years. Highly orientated lamellar samples, after large amplitude oscillation shear, provide the typical 1D confined environment to investigate crystallization behavior, such as crystal orientation and crystallization kinetics. However, inevitable defect generation, such as edge and screw dislocations during mechanical shear lead to “cross-talking” between grain boundaries and significantly affect the ideal 1D confinement crystallization kinetics by releasing the confinement. Meanwhile, the mechanism of the origin of specific crystal orientations (parallel or perpendicular) within the 1D confinement is still under debate. In this research, PS-b-PEO single crystals composed of one PEO nano-layer sandwiched by two PS glassy layers on the nanoscale were chosen as a template to investigate polymer crystallization in a 1D defect-free nanoscale confinement. Since the TgPS is higher than the TmPEO in such a “sandwich” lamellar structure, the PEO single crystal can be melted while keeping the PS layer in the vitrified state. The PEO blocks can be recrystallized between the two confining, glassy PS nano-layers at different re-crystallization temperatures, Trx, and monitored for different recrystallization behavior using electron diffraction (ED). Results indicate that the PEO block does not recrystallize until Trx = -5 °C, the limit of homogeneous nucleation. This observation confirms PEO recrystallization takes place in a defect-free confinement. Next, a puzzling ED pattern taken from PEO microcrystals grown at Trx > -5 °C via self-seeding was analyzed to be the result of the specific interaction between tethered PEO blocks having monoclinic symmetry and the glassy PS substrate. Two different inclined PEO microcrystals having an orthogonal relationship truly coexisting in the lamellar confinement during self-seeding were found. Furthermore, (open full item for complete abstract)

Committee: Stephen Z. D. Cheng Dr. (Advisor) Subjects: Materials Science; Polymers

Doctor of Philosophy, Case Western Reserve University, 2009, Macromolecular Science and Engineering

With the recent advances in processing and catalyst technology, novel morphologies have been created in crystalline polymers and they are expected to substantially impact the properties. To reveal the structure-property relationships of some of these novel polymeric systems becomes the primary focus of this work. In the first part, using an innovative layer-multiplying coextrusion process to obtain assemblies with thousands of polymer nanolayers, dominating “in-plane” lamellar crystals were created when the confined poly(ethylene oxide) (PEO) layers were made progressively thinner. When the thickness was confined to 25 nanometers, the PEO crystallized as single, high-aspect-ratio lamellae that resembled single crystals. This crystallization habit imparted more than two orders of magnitude reduction in the gas permeability. The dramatic decrease in gas permeability was attributed to the reduced diffusion coefficient, because of the increase in gas diffusion path length through the in-plane lamellae. The temperature dependence of lamellar orientation and the crystallization kinetics in the confined nanolayers were also investigated. The novel olefinic block copolymer (OBC) studied in the second part consisted of long crystallizable sequences with low comonomer content alternating with rubbery amorphous blocks with high comonomer content. The crystallizable blocks formed lamellae that organized into space-filling spherulites even when the fraction of crystallizable block was so low that the crystallinity was only 7%. These unusual spherulites were highly elastic and recovered from strains as high as 300%. These “elastic spherulites” imparted higher strain recovery and temperature resistance than the conventional random copolymers that depend on isolated, fringed micellar-like crystals to provide the junctions for the elastomeric network. In the third part, positron annihilation lifetime spectroscopy (PALS) was used to obtain the temperature dependence of the free volu (open full item for complete abstract)

Committee: Anne Hiltner PhD (Committee Chair); Eric Baer PhD (Advisor); Adin Mann PhD (Committee Member); Lei Zhu PhD (Committee Member) Subjects: Chemical Engineering; Materials Science; Plastics; Polymers

Doctor of Philosophy, Case Western Reserve University, 2013, Macromolecular Science and Engineering

Multilayer co-extrusion, a highly flexible and unique process, has enabled the study of the permeation, mechanical, and optical properties of multilayer films. Gas separation membranes are strongly dependent upon the permeation of specific gases, which is controlled by the polymer structure and morphology. Poly(ether block amide) (PEBA) thermoplastic elastomers have an inherently high permeability and good selectivity for acid gases such as CO2. A series of PEBA copolymers containing poly(tetramethylene oxide) and polyamide-12 was studied to explore the influence of mechanically induced orientation and copolymer composition on gas permeability and morphology. Upon orientation, PEBA copolymers with high polyether content exhibited up to 3.5x reduction in permeation with increasing strain as a result of strain induced crystallization. To maintain high flux for membrane applications, elastic recovery and thermal treatment proved beneficial in reversing the effects of uniaxial orientation on PEBA copolymers. Gas separation membranes were produced through co-extrusion and subsequent orientation of films containing PEBA as the selective material and PP composites as the support, which are made porous through two methods: 1) inorganic fillers or 2) crystal phase transformation. Two membrane systems, PEBA/(PP + CaCO3) and PEBA/¿-PP, maintained a high CO2/O2 selectivity while exhibiting reduced permeability. Incorporation of an annealing step either before or after orientation improves the membrane gas flux by 50 to 100 %. The improvement in gas flux was a result of either elimination of strain induced crystallinity, which increases the selective layer permeability, or improvement of the PP crystal structure, which may increase pore size in the porous support layer. Forced assembly multilayer co-extrusion of commercially available polyurethane (PU) and polycaprolactone (PCL) polymers was used to create a continuous periodic alternating layer architecture that exh (open full item for complete abstract)

Committee: Eric Baer (Advisor); LaShanda Korley (Committee Member); Lei Zhu (Committee Member); Kenneth Singer (Committee Member) Subjects: Engineering; Plastics; Polymers

Doctor of Philosophy, Case Western Reserve University, 2013, Macromolecular Science and Engineering

Layer multiplying “forced assembly” micro and nanolayer coextrusion is used to produce films with thousands of layers containing two or more alternating polymers. This dissertation focuses on the structure-property relationships in layered polymeric systems for applications including confined crystallization for high gas barrier applications and multilayer dielectric films for high energy density capacitors. CHAPTER 1: A review of recent progress of confined polymer crystallization using forced assembly nanolayer coextrusion is described. Confinement of crystalline polymer materials in layer thicknesses ranging from hundreds to tens of nanometers thick resulted in multilayer films possessing enhanced gas barrier properties. The enhanced gas barrier has been attributed to nanolayer confinement of the crystalline polymer resulting in a highly ordered intralayer lamellae orientation extending over micron or larger scale areas. Research into the confined crystallization mechanism of the multilayered polymer films has resulted in several material case studies as well as an understanding of the chemical and thermodynamic parameters that control the degree and rate of the confinement in multilayer polymer systems. This review highlights our recentstudies on the confinement of poly(ethylene oxide), poly(caprolactone), polypropylene, and poly(vinylidene fluoride) polymers in multilayered films. CHAPTER 2: The structure and morphology of polyethylene terephthalate (PET) and poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDFTFE)] were investigated under nanolayer confinement. In addition, both biaxial stretching and isothermal melt recrystallization were used as an additional approach to manipulate the morphologies of these confined polymers. The synergistic combination of nanolayering, biaxial stretching, and isothermal recrystallization facilitated the formation of unique morphologies in the P(VDFTFE) layers. These structures, specifically the high aspect ratio in-pl (open full item for complete abstract)

Committee: Eric Baer (Advisor); Anne Hiltner (Advisor); LaShanda Korley (Committee Member); Lei Zhu (Committee Member); Donald Schuele (Committee Member) Subjects: Polymers