Browsing by Author "Dr. Alan E. Tonelli, Committee Chair"

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    Reorganization of Structure to Alter the Properties of PET
    (2005-07-13) Vedula, Jyotsna; Dr. Alan E. Tonelli, Committee Chair
    This research focuses on the study of the unique behavior and properties exhibited by as-received Polyethylene Terephthalate (PET) subsequently processed by a simple precipitation method, and mainly involves the comparison of the properties of as-received and precipitated PET's at the microscopic and macroscopic levels. As-received PET is dissolved in Trifluoro Acetic acid (TFA) at 50C and then precipitated by adding the solution drop wise to acetone stirred at a very high rate. The ar-PET is by nature a slowly crystallizing polymer. Bulk PET has been observed to reorganize both morphologically and conformationally by the precipitation method used. Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR), and X-ray studies of precipitated (ppt) PET show structures and morphologies that are different from those of the samples obtained by ordinary solution and melt processing techniques. Unlike as-received PET, the ppt PET is apparently repeatedly and rapidly crystallizable from the melt, with non-crystalline portions of the sample that do not evidence a glass transition nor a crystallization peak during the heating DSC scan. DSC results suggest that the precipitation method has transformed PET into a repeatedly, rapidly crystallizable polymer that is found to achieve very high levels of crystallinity even when crystallized from the melt at a very high cooling rate. The fact that the precipitated PET has high levels of crystallinity is also supported by FTIR analysis, which shows a higher amount of trans conformer. The reorganization induced in the ppt PET is different from ordinary solvent induced crystallization. Shrinkage of films made from both PET's is observed with an increase in temperature. Heating the as-received film, prepared by melt pressing and rapid quenching in water, results in the abrupt shrinking of the material at around the glass transition temperature of the polymer, whereas the ppt PET film continues to contract at a controlled rate with rising temperature. This suggests that the amorphous regions in the ppt PET film are organized differently than that of the as-received material. Density measurements also support the fact that the non-crystalline regions in ppt-PET are different from the amorphous portions of ar-PET. Atomic Force Microscopy suggests a molecular-level difference between the precipitated and the as-received PET's even in their melts. Melting of the ppt PET has not erased the structural organization. Preliminary observations of the stress-strain behavior of their films indicate that as-received PET exhibits tough plastic behavior characteristic of glassy amorphous polymers, whereas ppt PET shows more rigid behavior similar to semi-crystalline polymers like isotactic polypropylene. Characterization of macroscopic properties, such as rheology using the Minimelter, indicates that the melt viscosity of ppt PET is less than that of the as-received sample, which will make processing easier. Blending of the as-received and the precipitated PET's has also been done. Overall it is observed that ppt-PET is different from ar-PET in terms of both its microscopic (organization and conformation) and macroscopic behaviors.
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    Solid-State 13C-NMR Investigation of Di- and Tri-block Biodegradable Copolymers Isolated In Their Inclusion Compounds with Cyclodextrins
    (2003-04-16) Porbeni, Francis Ebikefe; Dr. Saad Khan, Committee Member; Dr. Alan E. Tonelli, Committee Chair; Dr. Jeffrey L. White, Committee Member; Dr. Mohan Srinivasarao, Committee Member; Dr. Hawthorne Davis, Committee Member
    We have synthesized and characterized two biodegradable copolymers: poly(e-caprolactone)-poly(L-lactide), (PCL-b-PLLA) diblock copolymer, and poly(e-caprolactone)-poly(propylene glycol), (PCL-PPG-PCL) triblock copolymer. The lengths of each block in the diblock were determined to be: PCL = 92 and PLLA = 84. While in the tri-block, each PCL block length was found to be 45, and the PPG was 60. Inclusion compounds (ICs) of these copolymers with alpha- and gamma- cyclodextrins (CDs) were formed and characterized. The solid crystals of the ICs were found to be in the channel packing mode, allowing the copolymers to be isolated as straight chains within the cylindrical channels of the CDs. We have investigated the conformations and dynamics of the isolated di- and triblock copolymer chains entrapped within the channels of alpha- and gamma-cyclodextrins (CD) using solid-state 13C-NMR techniques. The PCL block chains isolated within the cavities of alpha-CD, adopt a conformation similar to that of the bulk semi-crystalline di- and tri-block copolymers. While in gamma-CD/IC, an upfield shift by approximately 1 ppm was observed for the PCL block chains in the triblock. The spin-lattice relaxation time T1 (C) measurements confirmed the semi-crystalline morphology of both of these copolymers. A dramatic difference was observed in the mobility of the polymer chains in the bulk compared to the ICs. In the absence of intermolecular interactions, the isolated chains experienced much rapid mobility relative to their behavior in the bulk. This result reflects on the role of cooperative interactions between the polymer chains in both the bulk di- and tri-block copolymers. The length scale of proton spin diffusion was probed using T1(1H) and T1rho (1H). The T1(1H) in the bulk copolymers averaged to a single value, while the proton T1 indicated that the copolymers were phase-separated. In the ICs, exchange of proton magnetization through spin diffusion was observed between the polymer chains and the CDs, but it was not totally efficient. 2D heteronuclear correlation method was also employed to monitor the nature of proton communication in these samples. Intra-block exchange of proton magnetization was observed in the bulk copolymers at short mixing times. In the ICs, intra-block 1H-1H spin communication was observed for the isolated chains, in spite of the physical closeness between the isolated chains and CD molecules efficient proton spin diffusion was not observed.