Multi stimuli-responsive copolymers obtained from colloidal dispersions

Fang Liu


This dissertation describes the design, synthesis, and development of multi stimuli-responsive random copolymers that exhibit collective responsiveness to temperature, pH or electromagnetic radiation. New colloidal particles of poly(N-(DL)-(1-Hydroxyymethyl) propylmethacrylamide/ n -butyl acrylate) (p(DL-HMPMA/nBA) and poly(2-(N,N'-dimethylamino)ethyl methacrylate/ n -butyl acrylate) (p(DMAEMA/nBA)) were synthesized, which upon coalesce to form solid continuous films. The presence of lower glass transition (T g ) nBA components not only facilitate film formation, but also provide sufficient free volume for polymer chain rearrangements. These studies showed that coalesced films exhibit 3D dimensional changes upon external stimuli, which are attributed to the collapse and conformational changes of stimuli-responsive components as well as buckling of copolymer backbones. Incorporation of N,N'-(dimethylamino)azobenzene acrylamide (DMAAZOAm) in DMAEMA/nBA generates films with cilia-like features. While film morphological features allow the formation of wavy whiskers, variable chemical composition of the copolymer facilitates chemical, thermal, and electromagnetic responses manifested by simultaneous shape and color changes as well as excitation wavelength dependent fluorescence. These studies demonstrated for the first time that synthetically produced polymeric films can exhibit combined thermal, chemical, and electromagnetic sensing resulting in loco-motions and color responses which may find numerous applications in sensing devices and intelligent actuators. During the course of these studies, new thermal relaxations in solid stimuli-responsive copolymer films were also discovered by differential scanning calorimetry (DSC). In addition to the Tg , a new composition-sensitive endothermic stimuli-responsive transition (TSR ) was observed, which follows the empirical relationship: 1/TSR = w1 /T binary + w2 /T, where TSR is the temperature of the stimuli-responsive transition, Tbinary is the temperature of stimuli-responsive homopolymer in a binary polymer-water equilibrium, w 1 and w2 are weight fractions of each component, and T is the film formation temperature. This relationship allows predicting T SR transitions in stimuli-responsive solid copolymers. The TSR transitions in dual-responsive copolymers depend on synergistic effects of temperature and pH, where temperature changes activate molecular rearrangements, and pH induces inter/intra hydrogen-bonding. These studies also showed that regardless of the copolymer compositions, the primary requirement to achieve the T SR transitions in solid networks is TSR >T g .