Computational study of hydrogen-bonding interactions in shape memory polymers

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Computational study of hydrogen-bonding interactions in shape memory polymers

 

Author: Zhang, Cuili
Title: Computational study of hydrogen-bonding interactions in shape memory polymers
Degree: Ph.D.
Year: 2014
Subject: Shape memory polymers.
Hydrogen bonding.
Polymers.
Polymeric composites.
Shape memory effect.
Hong Kong Polytechnic University -- Dissertations
Department: Institute of Textiles and Clothing
Pages: xxiii, 215 leaves : illustrations (some color) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2762956
URI: http://theses.lib.polyu.edu.hk/handle/200/7755
Abstract: Computational study plays an important role in understanding some important experimental processes and phenomena of shape memory polymers. In this project, the quantum-chemical calculations, molecular dynamics simulations (MD) and dissipative particle dynamics (DPD) method were chosen to investigate different properties of shape memory polymers. Firstly, the hydrogenbonding interactions of four widely used diisocyanate based hardsegment models in polyurethane, 2,4-toluene diisocyanate-methanol (2,4-TDI), 2,6-toluene diisocyanate-methanol (2,6-TDI), isophorone diisocyanate (IPDI) and 1,6-hexamethylene diisocyanate-methanol (HDI), were investigated theoretically by density functional theory (DFT). The equilibrium structures, Mulliken charges, hydrogenbonding energies, and infrared (IR) spectra were calculated using the Becke-three Lee-Yang-Parr (B3LYP) method, in good agreement with experimental data. Due to chairstructure of cyclohexane ring, the IPDI has a great variety to form hydrogen bonds. Different position of the methyl group on benzene ring for 2,4-TDI and 2,6-TDI results in different hydrogen bonds with various strengths. The formation of hydrogen bonding for HDI is more flexible due to simple straight-chain structure. With the forming of hydrogen bond, charge transfer for atoms nitrogen, hydrogen and oxygen related to hydrogen bonding appears. The hydrogen bonding forming in hardsegments with cyclohexane ring is much stronger than that with benzene ring or straight chain. This study can supply guidance for the selection of hard segment in polyurethane synthesis and in-depth understanding of the hydrogenbonding mechanism in polyurethane hard segments. Secondly, to investigate the mechanism for phase separation of a set of segmented polyurethanes, both in micro- and meso- scales, a combined theoretical study, density functional theory (DFT) and dissipative particle dynamics (DPD) simulations was performed. B3LYP method was used to obtain the geometric structures, bonding energies and infrared (IR) spectra for potential hydrogen bonding in polyurethanes; DPD method was performed to simulate the phase morphologies for polyurethanes with different hard segment contents. The hydrogen bonding between hard- and soft segments, and among hard segments are confirmed from the quantum-chemical calculations. Due to different hydrogen-bonding capacity, competitive hydrogen bonding is caused. The hydrogen bonding brought by hard segments governs the morphology evolution, from separated micelles, to connected micelles, and then reticulate cylinders. This study provides good understanding for the mechanism of phase separation in shape memory polyurethanes.
Thirdly, a combined theoretical study, density functional theory (DFT) and dissipative particle dynamics (DPD) method, was performed to investigate intrinsic mechanism for self-assembly of poly(styrene)-blockpoly(4-vinylpyridine) (PS-b-P4VP) and poly(4,4'-oxydiphenylenepyromellitamic acid) (POAA) blends in both microscopic and mesoscopic scales. The geometric structures, bonding energies and infrared (IR) spectra for the potential hydrogen bonding in the blends were obtained using the Becke-three Lee-Yang-Parr (B3LYP) method, and the morphologies for different blends were investigated by DPD simulations. From the quantum-chemical calculations, the hydrogen bonding between P4VP and POAA chains, and among POAA chains are confirmed. Due to different bonding strength, competitive hydrogen bonding is caused, the hydrogen bonding brought by POAA governs the morphology evolution, from lamellar to sphere. This study provides good understanding for the formation of morphology involved with competitive hydrogen-bonding interactions. Fourthly, a type of shape memory polyurethane with 60 w% hard segments (SMPU60) was prepared. Its material properties were tested by dynamic mechanical analysis (DMA) and Instron, and simulated using fully atomistic molecular dynamics (MD). The glass transition temperature (Tg) of SMPU60 determined by DMA is 316 K, which is slightly lower than that estimated through MD simulations (Tg=328 K), showing the calculated Tg is in good agreement with experimental data. A complex hydrogen bonding network was revealed with the calculation of radial distribution functions (RDFs). The CO···H bond is the predominant hydrogenbonding interaction. With increasing temperature, both the hydrogen bonding and the moduli decreased, and the dissociation of intermolecular hydrogen bonding induced the decrease of the moduli. The results in this project have offered a reference for the design and understanding of SMPs involved with hydrogen bonding, which makes up the limitations of experiments, and supplies a good guidance for future shape memory polymer research.

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