Stimuli-Responsive Materials

Covalent attachment of multilayers (CAM)

These studies show covalent attachment of multilayers (CAM) to chemically alter surfaces to achieve pH switchable antimicrobial and anticoagulant properties. Polyethylene (PE), poly(tetrafluoroethylene) (PTFE), and silicon (Si) surfaces were functionalized by tethering pH-responsive “switching” polyelectrolytes consisting of poly(2-vinyl pyridine) (P2VP) and poly(acrylic acid) (PAA) terminated with NH2 and COOH groups, respectively. At pH < 2.3, the P2VP segments are protonated and expended, but at pH > 5.5, they collapse while the PAA segments are expanded. The presence of terminal NH2 or COOH moieties on P2VP and PAA, respectively, facilitated the opportunity for covalently bonding ampicillin (AMP) and heparin (HEP) to both polyelectrolyte chains. Such surfaces, when exposed to S. aureus, inhibit the growth of microbial films (AMP) as well as anticoagulant properties (HEP).

Stimuli-Responsive Polymeric Surfaces: Mimicking Cilia

Our research interests are fairly diversified. One of the current efforts focuses on stimuli-responsive polymeric materials, molecular level events which govern their physico-chemical behaviors, and the influence of heterogeneity of networks on film formation. Mobility of individual components near surfaces and interfaces as well as responses of cilia-like morphologies are selected examples.

Cilia-like Responses (Temperature, UV, pH)

Stimuli-Responsive Photochromic Polymer Composites

We have developed a new family of azobenzene crosslinked brominated vinyl ester polymer networks that exhibit reversible photochromic and fluorescence properties in which azobenzene serves as a crosslinker as well as a molecular sensor of structural and conformational changes of the surrounding molecular segments. The “built-in” crosslinker functions as a highly sensitive light emitting group capable of sensing network stresses or damages. Such networks are also capable of sensing electromagnetic radiation that results in isomerization, thereby modifying the UV-Vis absorption and fluorescence emission properties. These reversible processes induced by structural network rearrangements may offer numerous application possibilities ranging from molecular stress sensors to nano-crack detectors in materials.

Expandable Temperature-Responsive Polymeric Nanotubes

Materials with the ability of dimensional changes on demand exhibit many potential applications ranging from adaptive composites that mimic biological functions under extreme conditions to microfluidics or neural implants to stimulate components of the nervous systems. These studies show the synthesis of temperature-induced reversibly expandable nanotubes that were prepared by polymerization of N-isopropylacrylamide (NIPAAM) in the presence of biologically active 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC8,9PC) diacetylenic phospholipids (PL). As a result, thermally responsive poly-NIPAM-phospholipid nanotubes (PNNTs) were prepared. Polymerization reactions occur within hydrophilic regions of PL bilayers, whereas PL hydrophobic zones facilitate transport and supply of the monomer for polymerization. The unique feature of PNNTs is that, above 37 °C, the outer diameter (OD) as well as the wall thickness (WT) shrink by 20 and 55%, respectively, whereas the inner diameter (ID) increases by 16%. This behavior is attributed to the PNIPAM backbone buckling induced by local rearrangements within PL bilayered morphologies. The presence of acetylenic moieties along the PL bilayers in PNNTs provides an opportunity for irreversible “locking” of designable dimensions, which is facilitated by the formation of cross-linked PNNTs (CL-PNNTs).

Stimuli-Responsive Polymer Films

These studies show that the relationship between the newly discovered TSR and known Tg relaxations can be predicted by the following formula: 1/TSR = [Tg1 × Tg2 × (Tbinary − T)]/[Tbinary × T × (Tg1 − Tg2) × Tg] + (Tg1 × TTbinary × Tg2)/[Tbinary × T × (Tg1 − Tg2)], where TSR is the stimuli-responsive transition temperature, Tg is the glass transition temperature of the copolymer; Tbinary is the temperature of stimuli-responsive homopolymer in a binary polymer–water equilibrium, Tg1 and Tg2 are the glass transition temperatures of stimuli-responsive and non-stimuli-responsive homopolymers, respectively, and T is the film formation temperature.

These studies showed that molecular rearrangements responsible for the TSR transitions are attributed to the backbone buckling and collapse of stimuli-responsive components. Based on empirical data, the relationship between Tg and TSR was established: log(V1/V2) = (P1(TSR − Tg))/(P2 + (TSR − Tg)), where the V1 and V2 are the copolymer total volumes below and above the TSR, respectively, Tg is the glass transition temperature of the copolymer, and P1 and P2 are the fraction of the free volume (ffree) at Tg (P1) and (Tg,midpoint − TSR)50/50) for each random copolymer (P2), respectively.
These studies report for the first time new thermal relaxations in stimuli-responsive solid-phase copolymers detected by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). When 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and n-butyl acrylate (nBA) monomers were copolymerized into colloidal dispersions and allowed to coalesce to form solid continuous films, in addition to the glass-transition temperature (Tg), which follows the Fox equation for random copolymers, a new composition-sensitive endothermic stimuli-responsive transition (TSR) was observed. The TSR transition changes with the composition of the stimuliresponsive component of the copolymer, the temperature, and the rate of temperature change. On the basis of the experimental data, the following relationship was established: 1/TSR ) w1/Tbinary + w2/T or 1/TSR ) w1(1/Tbinary -
1/T) + 1/T, where TSR is the temperature of the stimuli-responsive transition, Tbinary is the temperature of the stimuli-responsive homopolymer in a binary polymer-water equilibrium, w1 and w2 (w2 ) 1 - w1) are weight fractions of each component of the copolymer, and T is the film-formation temperature.