Author: Hoe H. ChuahPublisher:
ISBN:
Category : Nylon
Languages : en
Pages : 450
Book Description
Structure-property Studies in the Deformation of Semi-crystalline Polymers
Author: Hoe H. ChuahThe Physics of Deformation and Fracture of Polymers
Author: A. S. ArgonPublisher: Cambridge University Press
ISBN: 0521821843
Category : Science
Languages : en
Pages : 535
Book Description
A physical, mechanism-based presentation of the plasticity and fracture of polymers, covering industrial scale applications through to nanoscale biofluidic devices.
Multiscale Modeling of the Deformation of Semi-crystalline Polymers
Author: James Ellison ShepherdPublisher:
ISBN:
Category : Crystalline polymers
Languages : en
Pages :
Book Description
The mechanical and physical properties of polymers are determined primarily by the underlying nano-scale structures and characteristics such as entanglements, crystallites, and molecular orientation. These structures evolve in complex manners during the processing of polymers into useful articles. Limitations of available and foreseeable computational capabilities prevent the direct determination of macroscopic properties directly from atomistic computations. As a result, computational tools and methods to bridge the length and time scale gaps between atomistic and continuum models are required. In this research, an internal state variable continuum model has been developed whose internal state variables (ISVs) and evolution equations are related to the nano-scale structures. Specifically, the ISVs represent entanglement number density, crystal number density, percent crystallinity, and crystalline and amorphous orientation distributions. Atomistic models and methods have been developed to investigate these structures, particularly the evolution of entanglements during thermo-mechanical deformations. A new method has been created to generate atomistic initial conformations of the polymer systems to be studied. The use of the hyperdynamics method to accelerate molecular dynamics simulations was found to not be able to investigate processes orders of magnitude slower that are typically measurable with traditional molecular dynamics simulations of polymer systems. Molecular dynamics simulations were performed on these polymer systems to determine the evolution of entanglements during uniaxial deformation at various strain rates, temperatures, and molecular weights. Two methods were evaluated. In the first method, the forces between bonded atoms along the backbone are used to qualitatively determine entanglement density. The second method utilizes rubber elasticity theory to quantitatively determine entanglement evolution. The results of the second method are used to gain a clearer understanding of the mechanisms involved to enhance the physical basis of the evolution equations in the continuum model and to derive the models material parameters. The end result is a continuum model that incorporates the atomistic structure and behavior of the polymer and accurately represents experimental evidence of mechanical behavior and the evolution of crystallinity and orientation.
Uniaxial Plastic Deformation of Isotactic Polypropylene Studied by Solid-state Nmr
Author: Jia KangPublisher:
ISBN:
Category : Crystalline polymers
Languages : en
Pages : 163
Book Description
At alleviated temperatures, some semicrystralline polymers can be stretched to very large deformation ratios. Such deformations of semicrystalline polymers have been extensively studied since 1960s. Based on experimental observations and theoretical investigations, solid-state transformation (three stage model) proposed in 1971 and local melting and recrystallization in 1978 have been considered two major mechanisms to explain the deformations of polymer crystals. With the elucidation of molecular dynamics in the last two decades, it was proposed in 1999 that helical jump motion plays an important role in crystal deformation. On the other hand, the new structures induced by deformation also influence the molecular motions and resultant properties of deformed polymers. Such processing-structure-property relationship is very important to understand the polymer behaviors as well as to inform the polymer industry. However, to conclude the deformation mechanism of semicrystalline polymer is still challenging, because there is no appropriate tools to trace structural evolution of polymer chains inside the polymer bulk. And detailed understanding of the relationships between hierarchical structures and specific motions and properties need to be achieved. In this dissertation, using the advanced tool of solid-state NMR (ss-NMR), we achieve three goals: Firstly, we investigate the hierarchical crystalline structural changes of isotactic polypropylene (iPP) upon high temperature stretching to understand the deformation process. Secondly, we evaluate the roles of local packing structure and crystal thickness in determining the stem motions and thermal properties of deformed a-form iPP. Thirdly, we utilize 13C-labeled isotactic polypropylene (iPP) to trace the change of chain folding number as a function of e to conclude molecular-level deformation mechanism. To realize the first and second goals, the chain packing, crystal thickness, molecular dynamics, and melting temperature (Tm) of a-form iPP drawn uniaxially at high temperatures of 100 - 150 °C were investigated using solid-state (SS) NMR and DSC. Two types of iPP samples with disordered (a1) and relatively ordered (a2-rich) packing structures were prepared via different thermal treatments and drawn up to an engineering strain (e) of approximately 20. High-resolution 13C NMR detected continuous a2-to-a1 transformations in the original a2-rich samples over the entire deformation range at all drawing temperatures (Tds). A sudden a1-to-a2 transformation was found to occur in the original a1 sample in the small e range of approximately 3 - 8 at Td = 140 °C. Then, in the late stage, the newly grown a2 structure reversely transformed into a1 structure with further increase in e, as observed in the original a2-rich sample. These results indicate that at least two different processes are involved in large deformations. On the basis of crystallographic constraints, the continuous a2-to-a1 transformation over the entire deformation range is attributed to molecular-level melting and recrystallization facilitated by chain diffusion. The steep a1-to-a2 transformation in the smaller e range is assigned to isotropic melting and recrystallization induced by stress. After the large deformations (e ̃ 20) of the original a2-rich and a1 samples at Td = 150 and 140 °C, respectively, 1H spin diffusion verified increases in the crystal thickness in both the former (14.1 nm at e = 0 and 20.1 nm at e = 20) and the latter (9.2 to 17.0 nm). Centerband-Only Detection of EXchange (CODEX) NMR at 120 °C demonstrated that the correlation time (tc) of the helical jump for the former drastically decreased from tc = 52.4 ± 5.2 at e = 0 to 9.3 ± 1.8 ms at e = 20 but slightly increased from 4.2 ± 1.3 to 7.1 ± 0.9 ms for the latter. Additionally, DSC indicated that the melting temperature (Tm) for the former decreased considerably from 173 °C at e = 0 to 165 °C at e = 20, whereas the melting temperature (Tm) remained nearly invariant at 163 °C for the latter. Based on these findings, we conclude that the local packing structure plays a crucial role in determining the molecular dynamics of the stems and Tm of largely deformed iPP materials. The established relations among the structures, the dynamics, and the thermal properties provide a useful guide to achieving improved properties of iPP materials under processing. To realize the third goal, 13C-13C Double Quantum (DQ) NMR was applied to trace the structure evolution of 13C-labeled iPP chains inside the crystallites under stretching at 100 oC. DQ NMR based on spatial proximity of 13C labeled nuclei proved conformational changes from the folded chains to the extended ones of the iPP chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays critical role to achieve the large deformability of iPP.
Scientific and Technical Aerospace Reports
Author: Damage Mechanisms Under Tensile Stress of Amorphous and Low Semi-crystalline Polymers
Author: Stéphanie DjukicPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
In the last few years, a new class of polyamides has been developed for specific applications requiring better mechanical and thermal properties, such as electronics or automotive industry. Polyphthalamides (PPA) are semi-aromatic polyamides containing aromatic rings in their main chain. Recently, the damage mechanisms have been studied in the case of semi-crystalline (PA66) and amorphous (cellulose acetate) polymers. The aim of the PhD thesis was to study the damage mechanisms of amorphous and semi-crystalline polymers. In addition, few studies have been conducted to characterize the properties of pure PPA. More detailed data regarding their mechanical properties are needed. The study of the properties of this class of polymers is important, especially since their applications are different from aliphatic polyamides. We first characterized these polymers and then studied their mechanical behavior in traction, compression and Charpy impact strength. We have thus been able to highlight the appearance of a necking phenomenon and a strain hardening regime from 20% deformation for three PPA studied. Strain hardening has also been observed in other amorphous ductile polymers such as polycarbonate (PC) or poly(methyl methacrylate) (PMMA). The strain hardening stabilizes the deformation by avoiding the localization of the damage. In order to describe the microscopic mechanisms associated with the initiation and propagation of damage under tensile deformation of our polymers, we have carried out observations by scanning transmission electron microscopy (STEM) and optical microscopy as well as by Ultra-small angle X-ray scattering (USAXS). The analysis of these different polymers (amorphous and semi-crystalline PPA, PC, PMMA) by USAXS highlights different modes of damage. The simultaneous nucleation of nanometric crazes around the pre-existing defects (defects related to the injection process), then the limited growth of these crazes are observed for the amorphous PPA, the PC and the PMMA studied, in a mechanism similar to that studied in the case of cellulose acetate. The damage is blocked in the first place by the strain hardening. However, in the case of polycarbonate and PMMA, when the stress applied becomes sufficiently high, a small fraction of these crazes grows faster to cause the rupture of the sample, which allows to observe the evolution of a second family of larger crazes. We also observe the growth of a second family of crazes for one of the amorphous PPA, but their growth stops at the appearance of necking. No failure of the sample is observed. The second amorphous PPA is damaged by cavitation up to about 5% deformation. The cavities stop to grow at the appearance of shear bands. When the shear bands propagated on either side of the sample, the failure is observed. The two semi-crystalline PPA deform by necking without failure. No cavitation was observed by USAXS. The damage of the polymers studied can be classified into three categories. The first category concerns polymers that behave similarly to cellulose acetate. Crazing nucleation is observed, which growth is initially blocked by strain hardening. When the stress applied becomes sufficiently high, a fraction of these crazes increases more rapidly until the sample is broken, which makes it possible to observe the evolution of a second family of larger crazes. This category concerns polycarbonate and PMMA. The second category concerns one of the amorphous PPA with crazes whose growth is initially blocked also by strain hardening. The growth of a second family of crazes stops at the appearance of necking, and no failure of the specimen is observed. The last category concerns polymers that do not show any damage. These are the two semi-crystalline PPA. We propose in this thesis an interpretation of these different mechanisms of damage.
Polymer Microscopy
Author: Linda SawyerPublisher: Springer Science & Business Media
ISBN: 0387726284
Category : Science
Languages : en
Pages : 568
Book Description
This extensively updated and revised Third Edition is a comprehensive and practical guide to the study of the microstructure of polymers. It is the result of the authors' many years of academic and industrial experience. Introductory chapters deal with the basic concepts of both polymer morphology and processing and microscopy and imaging theory. The core of the book is more applied, with many examples of specimen preparation and image interpretation leading to materials characterization. Emerging techniques such as compositional mapping in which microscopy is combined with spectroscopy are considered. The book closes with a problem solving guide.
Polymer Blends Handbook
Author: L. A. UtrackiPublisher:
ISBN: 9789400760653
Category : Polymer engineering
Languages : en
Pages : 1800
Book Description
Written by an international group of highly respected contributors, this fundamental reference work covers all aspects of polymer blends: science, engineering, technology and applications.
Two Studies on Mechanical Properties of Semicrystalline Polymers
Author: Wen-Bin LiauPublisher:
ISBN:
Category : Polymers
Languages : en
Pages : 156
Book Description
Deformation and Fracture of High Polymers
Author: H. KauschPublisher: Springer Science & Business Media
ISBN: 1475712634
Category : Technology & Engineering
Languages : en
Pages : 648
Book Description