Thesis (M.S.)--Rochester Institute of Technology, 1994. / Typescript. Includes bibliographical references (leaves 118-124).

Zugl.: Berlin, Techn. Univ., Diss., 2008

Thesis (Ph. D.)--Auburn University, 2009. / Abstract. Vita. Includes bibliographical references.

Zugl.: Ulm, Universiẗat, Diss., 2009.

Composites based on binary polymer blends of polystyrene (PS)/poly(ethylene-co-vinyl alcohol) (EVOH) (70/30 wt%) containing natural Montmorillonite, Na-MMTs (Nanomer PGW or Cloisite Na+) and organically modified Montmorillonite clays, OMMTs (Nanomer I.30T, Cloisite 30B or Cloisite 10A) were prepared via melt compounding. The interactions between the polymers and clays were studied using flow micro-calorimetry (FMC). Data obtained from FMC indicated that the probe molecule mimicking EVOH (butan-2-ol) interacted with the MMTs and OMMTs much more strongly than PS. Scanning electron microscopy (SEM) revealed that composites based on binary blends had dispersed/continuous morphologies, in which EVOH was dispersed in a PS matrix. The size of the EVOH droplets in the PS matrix increased with increasing clay loading. Transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD) were used to determine the extent of dispersion and location of clay in the PS/EVOH/clay composites. These techniques confirmed the formation of intercalated clay structures. As predicted by FMC, the clay platelets were selectively located in the EVOH phase, independent of the blending sequence and the type of organic modifier in the OMMT. Composites containing OMMTs showed better dispersion of platelets within the EVOH phase than those containing Na-MMTs. Differential scanning calorimetry (DSC); showed the crystallisation behaviour of EVOH to depend on the clay loading and the nature of the organic modifier in the OMMT. Nanomer PGW, Cloisite Na+ and Cloisite 30B acted as weak nucleating agents. In contrast, Nanomer I.30T and Cloisite 10A significantly hindered the crystallisation of EVOH in the blends due to the restriction of chain segment mobility. Dynamic mechanical thermal analysis (DMTA) confirmed that the presence of clay increases the storage modulus of the composites compared to an unfilled blend. In addition, the improvement in storage modulus reflected the dispersion state of the different clays and their interaction with the polymers of the blend. Ternary-blend based composites were formed by adding poly(styrene-co-acrylonitrile) (SAN) to the composites based on binary PS/EVOH blends. This resulted in a finer dispersion of the EVOH phase and the development of a core-shell morphology, in which SAN encapsulated and formed shells around EVOH droplets. In contrast to binary blend composites, the clay platelets were found at the interface between SAN and EVOH in the ternary blends.

Cellulose-based materials draw their good mechanical properties from the cellu-lose crystal. Improved understanding of crystal properties could lead to a wider range of applications for cellulose-based materials, Cellulose crystals show high axial Youngs modulus. Cellulose can attain several allomorphic forms which show unique structural arrangements in terms of both intra-molecular and inter-molecular bonding, as well as unit cell parameters and chain packing. Although several studies have confirmed that mechanical tensile properties of cellulose differ between different allomorphic forms, few reports have investigated the deformation mechanisms explaining the differences.In the first part of this thesis, the tensile elastic Youngs modulus of cellulose allo-morphs Iβ, II and III I were calculated under uniform conditions using Molecular Dynamics simulation techniques. As expected, a difference in modulus valuesc ould be observed, and the cooperative nature of energy contributions to crys-tal modulus is apparent. The allomorphs also show large differences in terms of how contributions to elastic energy are distributed between covalent bonds,angles, dihedrals, electrostatic forces, dispersion and steric forces.In the second part of this thesis, the cellulose Iβ and II allomorphs were sub-jected to a more detailed structural study. The purpose was to clarify how the deformation of the central glucosidic linkage between the monomer units depends on the hydrogen-bonding structures. This was carried out by studying simulated vibrational spectra and local deformations in the crystals.The results presented in this thesis confirm the differences in the tensile elastic properties of these cellulose allomorphs. These differences can in part be explained by the different intra-molecular hydrogen bonding patterns between allomorphs. Deformation mechanisms are discussed. The results are in supportof the so called ”leverage effect” proposed in the literature. The present analysis shows significant differences in details of deformation mechanisms compared with previous simpler analyses. / <p>QC 20150518</p>

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Fibers in biological scaffolds like fibronectin stiffen when they experience forces between cells. It will expose binding sites under contractile forces and then form disulfide bonds. This on-demand strain-stiffening is a desirable property in synthetic materials. Tran et.al. (2017) mimicked the “cryptic” design of fibronectin by copolymerizing thiol crosslinking sites with monomers containing poly (ethylene glycol) chains. When the PEG chain increased from 350 to 950 g/mol, the strains-stiffening became on-demand while the curing process extended from 3 hours to 15 hours. Extra steric hindrance brought by longer PEG chains caused decreasing mass transfer rates of cryptic sites while the same level of strain rate was introduced. I proposed to use stronger ultrasound mechanical perturbation so that higher strain rate can be induced, and the shielding effect brought by the PEG chain can be overcome more easily. Utilizing ultrasound as a stimuli has the potential to improve the gelation speed or achieve high mechanical performance while retain the long shelf life of the “cryptic” materials. To test this hypothesis, I synthesized “cryptic” polymer with aceto-acetoxy and primary amine as crosslinking sites such that, the only time limiting step is brought by the long PEG chain. This is because the bond formation reaction between these two reactive groups is rapid and spontaneous. When switching from weak to strong mechanical perturbation, the change in gelation speed owing to accelerated mass transfer between crosslinking sites can be easily compared. When the PEG chain is 300 Mw and 30 mol % crosslinking sites density, this “cryptic” polymer only showed strain-stiffening under ultrasound while strain under a rheometer was not able to overcome steric hindrance. Signs of chain scission appeared when the ultrasound amplitude was set at 75 %, but was counteracted by reducing amplitude mode over the time. The crosslinking was optimized by varying the ultrasound amplitude and intensity and a final mode of 1 hour 75 % amplitude, 0.5 hour 50 % amplitude and 3.5 hours 25% amplitude provided greatly improved on demand crosslinking. The estimated kinetic constant using this mode was two times higher than that of under simple shear strain. Through this study, I found that ultrasound can improve the curing time of this “cryptic” polymer system since it induces higher strain rate and expedite the mass transfer rate between crosslinking sites and optimizing the ultrasonic amplitude profile to limit chain scission provides improved crosslinking performance.

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