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This research project investigated the physical aging of glassy polymers in confined environments. Many recent studies of aging in glassy polymers have observed that aging behavior is often strongly affected by confinement. Understanding aging in confined environments (e.g., thin polymer films and nanocomposites) is vital for predicting long-term performance in applications that use confined glassy polymers, such as gas separation membranes and advanced nanocomposite materials. Aging in bulk and layered films produced via layer-multiplying co-extrusion was studied using gas permeability measurement and differential scanning calorimetry (DSC). The layered films consisted of polysulfone (PSF) and a rubbery co-layering material, with PSF layers ranging in thickness from ~185 nm to ~400 nm. Gas permeation aging studies at 35 °C revealed that the PSF layers in layered films aged in a manner that was similar to bulk PSF and independent of layer thickness. This finding differs from what was observed previously in freestanding PSF films, in which aging depended strongly on thickness and was accelerated relative to bulk. Isothermal aging studies at 170 °C and cooling rate studies were performed on both bulk and layered samples using DSC. The aging of the PSF layers was similar to aging in bulk PSF for films having PSF layer thicknesses of ~640 nm and ~260 nm, while the film with 185 nm PSF layers showed a slightly higher aging rate than that of bulk PSF. The results of the DSC studies generally support the conclusions of our gas permeation aging studies. The absence of strong thickness dependence in aging studies of layered films tends to support the idea that the effect of film thickness on physical aging stems from interfacial characteristics and not merely thickness per se. The physical aging of thin polystyrene (PS) films at 35 °C was also investigated using gas permeation techniques. PS films of 400 nm and 800 nm did not exhibit aging behavior that was highly accelerated relative to bulk or strongly dependent on film thickness. At the thicknesses and aging temperature considered, the aging of PS shows much weaker thickness dependence than that seen in polymers like PSF and Matrimid. / text

The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to Fickian, Case II, and anomalous absorption behavior. While previous models account for anomalous behavior, there is still a disconnect between theory and experiment, as data must be fit to the model with previously determined independent parameters. With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for a quantitative understanding of the manner in which a polymer’s network structure alters both the rate and the mode of penetrant transport. To this end, samples of glassy poly(methyl methacrylate), poly(2-hydroxyethyl methacrylate), and poly(vinyl alcohol) were synthesized primarily by an iniferter-mediated, thermally-initiated free radical polymerization procedure. The thermal and mechanical properties of these polymers, as well as the polymer network structure, were varied through crosslinking and confirmed by detailed characterization. The dynamics of small molecule penetrant transport were examined in each polymer, with an emphasis on the occurrence of non-Fickian and Case II transport. The degree of crosslinking and choice of crosslinking molecule were shown to be powerful tools in tuning the observed penetrant transport process. For instance, the transport dynamics were altered from Fickian to Case II by increasing the degree of crosslinking and from Case II to Fickian by increasing the crosslinking interchain bridge length. Within the purely Case II regime, the rate of penetrant transport, or the Case II front velocity, was shown to scale with the square root of the degree of crosslinking in all systems investigated. A novel procedure for the in situ examination of penetrant transport in glassy polymers was developed utilizing high-resolution X-ray computed tomography. This completely nondestructive technique was used to visualize features in the interior of opaque solid objects and obtain digital information on their 3-D structure and properties. In this manner, the time-dependent penetrant concentration profiles throughout a swelling polymer were determined and analyzed. / text

The thesis describes the study of slow dynamics of confined polymers and soft colloids. We study the finite size effect on the dynamics of glassy polymers using newly developed interfacial microrheology technique. Systematic measurement have been performed to address the issue of reduction of glass transition under confinements. Slow and heterogeneous dynamics are the underlined observed behavior for dynamics in confined glassy polymers. The slow relaxation dynamics and dynamical heterogeneity in polymer grafted nanoparticles (PGNPs) systems were studied using advanced X - ray photon correlation spectroscopy (XPCS) techniques. Our studies presented in this thesis on dynamics of polymer grafted nanoparticle systems in melts and solution are the first attempt to study them experimentally. Thus our work shed the light about new technique to study confined system more accurately and explore new soft colloidal system to study fascinating dynamics and interesting phase behavior. In Chapter 1, we provide the theoretical background along with brief review of the literature for understanding the results presented in this thesis. The details of the experimental set up and their operating principle along with the details of the experimental conditions are provided in Chapter 2. In Chapter 3 we present our newly developed technique (interfacial microrhelogy) and its consequences to study the complex fluids at interface. Chapter 4 discusses the concentration and temperature dependent glassy dynamics in confined glassy polymers. In Chapter 5 we provide the structural and dynamical study of polymer grafted nanoparticles in melts and solutions. We provide the summary of our result and the future prospective of the work in Chapter 6. Chapter-1 provides the ground work and theoretical aspects for understanding the results presented in this thesis. It starts with the discussion about the slow dynamics of complex fluids and transit to dynamic behavior of polymer in confinement, glassy dynamics in confinements . This also discusses the basic aspects of studying viscoelastic properties using rheology, interface rheology, microrheology, interface microrheology techinques. In continuation it discusses structure and dynamics of different soft colloids investigated for last decade and then theoretical aspects of XPCS is discussed. Towards the end of this Chapter, we discuss the procedure to explain and understand systems dynamical heterogeneity near glass like phase transition. Chapter-2 contains the details of the experimental techniques which has been used for the study of confined polymers and soft colloids. Brief introduction to basic principles of the measurements followed by details of the material and methods have been provided. Chapter-3 we discuss the interafacial microrheology of different complex fluids and advantages of the techniques is discussed in Chapter 3. This includes discussion about the technique sensitivity at the surface using quantum dots (QDs) as a probe and about the configuration of the QDs at/on monolayer. Later on establishment of the technique has been demonstrated through easurements on arachidic acid, poly(methylmethacrylate) (PMMA), poly(vinylacetate) (PVAc), poly(methylacrylate) (PMA) monolayers. The extracted subdiffusive nature of QDs in on monolayers through mean square displacement has been explained using fractional Brownian motion model. Towards the end of the chapter we discuss about the extraction of real and imaginary elastic modulus from mean square displacement data using generalized Stokes-Einstein relation for the quasi two dimensional systems and explains about the possible viscoelastic transition in the different monolayers. The concentration and temperature dependent glassy dynamics of confined polymers (PMMA) are discussed in Chapter-4. We demonstrate the microscopic nature of spatio-temporal variation of dynamics of glassy polymers confined to a monolayer of 2 3 nm thickness as a function of surface density and temperature. It illustrates the systems dynamical heterogeneity and explain the observed large reduction of glass transition temperature in confined system through finite size effect. In Chapter 5 we discuss the result based on systematic studies of dynamics of PGNPs in melts and solutions. In addition it also illustrates the structural anisotropy and anomalous dynamical transitions in binary mixture of PGNPs and homopolymers in good solvent condition. It provides temperature and wave vector dependent XPCS measurements on polymer grafted nanoparticles with the variation of functionality. The functionality ( f ) dependent nonmonotonic relaxation in melts of PGNPs and solvent quality dependent non monotonic relaxation of PGNPs system have been elaborated in the continuation. We present possible phase behavior of PGNPs system in good solvent with addition of homopolymer of two different molecular weight. Chapter 6 contains the summary and the future perspective of the work presented.

Model glassy polymers, polymethyl methacrylate (PMMA) and polycarbonate (PC) are used to experimentally probe several aspects of polymer fracture. In Chapter 1, the method of pre-stress is employed as a means of improving the fracture properites of brittle PMMA. Samples are tested under equi-biaxial compression, simple shear and a combination of biaxial compression and shear. Equi-biaxial compression is shown to increase the threshold stress level for projectile penetration whereas shear pre-stress has a large effect on the overall energy absorbed during an impact. There is also an apparent interaction observed between compression and shear to dramatically increase the threshold stress. Pre-stressed laminates of PMMA and PC show an increase in damage area because of the unique formation of a secondary cone. In Chapter 2, the effect of stress state on stress relaxation in PMMA and PC is investigated. Direct comparisons are made between uniaxial and biaxial loading conditions. The experimental methods used highlight the effect of hydrostatic stress on the relaxation process. The data shows an increase in relaxation time and increase in the breadth of the relaxation spectrum with increases in hydrostatic stress. This suggests that the stress state can have a significant effect on the useful lifetime of pre-stressed articles. In Chapter 3, Mode I and II fracture studies are performed from quasi-static to low velocity impact rates on PMMA and PC. Mode II testing utilizes an angled double-edge notched specimen loaded in compression. The shear banding response of PMMA is shown to be highly sensitive to rate, with diffuse shear bands forming at low rates and sharp distinct shear bands forming at high rates. As the rate increases, shear deformation becomes more localized to the point where Mode II fracture occurs. PC is much less rate dependent and stable shear band propagation is observed over the range of rates studied with lesser amounts of localization. A new theory is formulated relating orientation in a shear band to intrinsic material properties obtained from true-stress true-strain tests. In a qualitative sense the theory predicts the high rate sensitivity of PMMA. A kinematic limit for orientation within a shear band is also derived based on entanglement network parameters. Mode II fracture in PMMA is shown to occur at this kinematic limit. For the case of PC, the maximum impact rates were not high enough to reach the kinematic limit. In Chapter 4, the deformation response, as observed in a shear band is interpreted through the characterization of the "intrinsic material properties" obtained from true stress - true strain 8compression tests. The relatively high rate sensitivity of PMMA deformed at room temperature is related to the proximity of the beta transition to the test temperature. This is also shown in corollary experiments on PC where deformation near the beta transition is accompanied by an increase in rate sensitivity. Physical aging results in a more narrow alpha transition and is shown to increase strain localization and decrease rate sensitivity at low strain rates.