A Fundamental Study of the Molecular Structure, Interactions and Self-Organization of 1,3:2,4-Dibenzylidene-D-Sorbitol

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Title: A Fundamental Study of the Molecular Structure, Interactions and Self-Organization of 1,3:2,4-Dibenzylidene-D-Sorbitol
Author: Wilder, Elizabeth A
Advisors: Keith E. Gubbins, Committee Member
Saad A. Khan, Committee Member
Richard J. Spontak, Committee Chair
Carol K. Hall, Committee Co-Chair
Abstract: 1,3:2,4-Dibenzylidene-D-sorbitol (DBS) is a relatively low-molecular-weight amphiphile that is capable of self-organizing into nanoscopic fibrils. At sufficiently high DBS concentrations, these fibrils assemble into a nanofibrillar network in a wide variety of organic solvents and polymer melts to produce "organogels." DBS has been shown to induce physical gelation at surprisingly low concentrations (< 1 wt%), making it ideal for applications requiring uncompromised physical or chemical properties of the matrix medium. Contemporary applications of DBS include personal cosmetics, biomedical materials, and (opto)electronic devices. Despite the many and diverse uses of DBS in existing, as well as emerging, technologies, a comprehensive study addressing the molecular structure, intermolecular interactions, nanofibrillar morphology and macroscopic properties of DBS-containing systems remains lacking. In this work, we seek to elucidate the molecular interactions governing DBS self-assembly, the impact of molecular structure on resultant nanofibrillar morphology, and the effect of this nanostructure on macroscopic mechanical properties. Molecular mechanics calculations performed with Cerius2 and InsightII software reveal two important features of the DBS molecule: (i) the pendant hydroxyl group tends to form intramolecular hydrogen bonds, and (ii) the phenyl rings prefer to lie in an equatorial position. The terminal hydroxyl group, however, possesses tremendous flexibility, indicating that it may be able to participate in intermolecular interactions. Molecular self-organization of DBS molecules, as discerned from both molecular mechanics calculations and molecular dynamics simulations of dimers, is sensitive to hydrogen bonding of the hydroxyl groups and pi interactions between phenyl rings, suggesting that the mechanism of network formation is complex, involving more than one type of local interaction. Transmission electron microscopy of organogels composed of poly(ethylene glycol) (PEG) and DBS reveals that DBS nanofibrils measure from about 10 to 70 nm in diameter, with a primary nanofibrillar diameter closer to 10 nm. Dynamic rheological measurements of DBS-containing PEG and PEG derivatives differing in endgroup substitution and, hence, polarity exhibit several interesting features. The rate of gelation, the gel dissolution/formation temperatures, and the magnitude of the dynamic elastic modulus are all sensitive to both DBS concentration and matrix polarity. Hydroxy-endcapped PEG/DBS systems require more time to gel and dissolve faster than their methoxy-endcapped analogs at constant DBS concentration. The elastic modulus, however, is less dependent on matrix polarity. Time-temperature superposition analyses provide evidence that the activation energy of gelation increases linearly with: (i) decreasing DBS concentration at constant matrix polarity and (ii) increasing matrix polarity at constant DBS concentration. Addition of DBS to a series of amphiphilic polypropylene glycol-b-polyethylene glycol-b-polypropylene glycol (PPG-b-PEG-b-PPG) triblock copolymers yields organogels with properties intermediate between those observed in PEG/DBS and PPG/DBS systems. Dynamic rheology reveals a maximum in the elastic modulus at temperatures near the gel dissolution and formation temperatures, both of which increase with increasing DBS concentration and PPG content. The magnitude of the elastic modulus is sensitive to copolymer composition and block length at low DBS concentration, but becomes matrix-independent as the DBS network saturates at a DBS concentration in excess of about 1 wt%. Transmission electron microscopy and microtomography of DBS networks in a nonpolar thermoplastic such as poly(ethyl methacrylate) reveal the existence of DBS nanofibrils measuring ca. 10 nm in diameter and ranging up to several hundred nanometers in length. At sufficiently high DBS concentration, these nanofibrils form a highly interconnected 3D network that can be altered through the further addition of a siliceous nanoparticle, such as colloidal silica. Dynamic mechanical property analysis reveals that, while DBS has little effect on glassy PEMA, it serves to increase, in systematic fashion, the elastic modulus of molten PEMA above the glass transition temperature.
Date: 2003-04-16
Degree: PhD
Discipline: Chemical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/3616


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