Browsing by Author "Gong, Kai"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Density Functional Theory Study of Microstructure and Phase Behavior of Stimuli-Responsive Polymer Brushes
    (2013-12-04) Gong, Kai; Chapman, Walter G.; Verduzco, Rafael; Riviere, Beatrice M.
    Stimuli-responsive polymer materials can change their structure and physical properties drastically on external signals like a change in temperature, solvent properties (pH, ionic strength), the magnetic or electrical field etc. Such "smart" polymer materials play an important role in various fields such as biology, medicine, and soft materials. However, it is a great challenge to investigate such "smart" polymer materials due to highly inhomogeneous structure at the molecular scale and the complex interactions. In this thesis, we have systematically studied three common types of stimuli-responsive polymer brushes such as temperature responsive polymer brushes, copolymer brushes, and mixed polymer brushes by using classical density functional theory. We find a surface outer layer switch for both copolymer brushes and mixed polymer brushes with a selective solvent. Without using any temperature-dependent parameter, our theory successfully captures the lower critical solution temperature behavior of the associating polymer brushes. Related parameters such as molecular weight, grafting density, and solvent properties that affect the phase behavior of these stimuli-responsive polymer brushes have been also investigated. Qualitatively consistent with experimental observations, our results provide physical insight and helpful guidance for the experimental design of such stimuli-responsive polymer materials.
  • Thumbnail Image
    Item
    Modeling lower critical solution temperature behavior of associating polymer brushes with classical density functional theory
    (AIP Publishing LLC, 2013) Gong, Kai; Marshall, Bennett D.; Chapman, Walter G.
    We study the lower critical solution temperature (LCST) behavior of associating polymer brushes (i.e., poly(N-isopropylacrylamide)) using classical density functional theory. Without using any empirical or temperature-dependent parameters, we find the phase transition of polymer brushes from extended to collapsed structure with increasing temperature, indicating the LCST behavior of polymer brushes. The LCST behavior of associating polymer brushes is attributed to the interplay of hydrogen bonding interactions and Lennard-Jones attractions in the system. The effect of grafting density and molecular weight on the phase behavior of associating polymer brushes has been also investigated. We find no LCST behavior at low grafting density or molecular weight. Moreover, increasing grafting density decreases the LCST and swelling ratio of polymer brushes. Similarly, increasing molecular weight decreases the LCST but increases the swelling ratio. At very high grafting density, a partial collapsed structure appears near the LCST. Qualitatively consistent with experiments, our results provide insight into the molecular mechanism of LCST behavior of associating polymer brushes.
  • Thumbnail Image
    Item
    Response behavior of diblock copolymer brushes in explicit solvent
    (American Institute of Physics, 2012) Gong, Kai; Marshall, Bennett D.; Chapman, Walter G.
    The understanding of phase behavior of copolymer brushes is of fundamental importance for the design of smart materials. In this paper, we have performed classical density functional theory calculations to study diblock copolymer brushes (A-B) in an explicit solvent which prefers the A block to B block. With increasing B-block length (NB), we find a structural transition of the copolymer brush from mixed to collapsed, partial-exposed, and exposed structure, which is qualitatively consistent with experiments. The phase transitions are attributed to the interplay between entropic cost of folding copolymer brushes and enthalpic effect of contact between unlike components. In addition, we examine the effect of different parameters, such as grafting density (ρg), the bottom block length (NA), and the chain length of solvent (NS) on the solvent response of copolymer brushes. The transition chain length (NB) increases with decreasing ρg and NA, and a smaller solvent molecule makes the collapsed structure less stable due to its lower penetration cost. Our results provide the insight to phase behavior of copolymer brushes in selective solvents from a molecular view.