Biomimetic, stimuli-responsive polymer nanocomposites based on a hydrophobic styrene-butadiene rubber (SBR) matrix and rigid, rod-like cellulose nanocrystals (CNCs) isolated from cotton were prepared by three different approaches, and their properties were studied and related to the composition, processing history, and exposure to water as a stimulus. The first processing approach involved mixing an aqueous SBR latex with aqueous CNC dispersions, and films were subsequently formed by solution-casting. The second method utilized the first protocol, but films were additionally compression-molded. The third method involved the formation of a CNC organogel via a solvent exchange with acetone, followed by infusing this gel, in which the CNCs form a percolating network with solutions of SBR in tetrahydrofuran. The thermomechanical properties of the materials were established by dynamic mechanical thermal analysis (DMTA). In the dry state, all nanocomposites show much higher tensile storage moduli, E', than the neat SBR or the SBR latex. E' increases with the CNC content and depends strongly on the processing method, which appears to influence the morphology of the SBR nanocomposites produced. The highest E' values were observed for the solution cast samples involving an SBR latex, where E' increased from 3 MPa for the neat SBR to ca. 740 MPa for the nanocomposite containing 20% v/v CNCs. Upon submersion in deionized water, a dramatic reduction of E' was observed, for example from 740 to 5 MPa for the solution-cast nanocomposite containing 20% v/v CNCs. This change is interpreted as a disengagement of the percolating CNC network, on account of modest aqueous swelling and competitive hydrogen bonding of water molecules with the CNCs. It is shown that the method of preparation also influenced the swelling behavior and kinetics of modulus switching, consistent with different arrangements of the CNCs, which serve as channels for water absorption and transport within the hydrophobic SBR matrix.
Although several anticancer drugs have been introduced as chemotherapeutic agents, the effective treatment of cancer remains a challenge. Major limitations in the application of anticancer drugs include their nonspecificity, wide biodistribution, short half-life, low concentration in tumor tissue and systemic toxicity. Drug delivery to the tumor site has become feasible in recent years, and recent advances in the development of new drug delivery systems for controlled drug release in tumor tissues with reduced side effects show great promise. In this field, the use of biodegradable polymers as drug carriers has attracted the most attention. However, drug release is still difficult to control even when a polymeric drug carrier is used. The design of pharmaceutical polymers that respond to external stimuli (known as stimuli-responsive polymers) such as temperature, pH, electric or magnetic field, enzymes, ultrasound waves, etc. appears to be a successful approach. In these systems, drug release is triggered by different stimuli. The purpose of this review is to summarize different types of polymeric drug carriers and stimuli, in addition to the combination use of stimuli in order to achieve a better controlled drug release, and it discusses their potential strengths and applications. A survey of the recent literature on various stimuli-responsive drug delivery systems is also provided and perspectives on possible future developments in controlled drug release at tumor site have been discussed.
A systematic study was done on morphological, electrical and rheological behavior of co-continuous or dispersed-type polycarbonate (PC)/acrylonitrile-styrene-butadiene (ABS) blends, containing different amounts of multiwalled carbon nanotubes (MWNT). The MWNTs gave substantial electrical conductivities to these nanocomposites at very low concentrations, owing to the effective melt processing method. Because of selective localization of MWNTs in the PC phase, along with double percolation phenomenon, the blend with co-continuous morphology showed a lower electrical and rheological percolation threshold, higher melt viscosity and elasticity, as compared to the system with dispersed morphology. The morphology of both the blend systems was refined as a result of MWNTs incorporation but the morphology type remained unchanged. A typical role of compatibilizer in refining blend morphology was observed in both the systems. The electrical conductivity of the system filled with MWNTs in presence of compatibilizer, was lower than the systems filled with MWNTs only, which was attributed to role of compatibilizer in directing a part of MWNTs from PC matrix toward ABS phase. With increasing compatibilizer/MWNTs ratio, the influence of compatibilizer on morphology refinement and conductivity reduction was intensified. By comparing TEM micrograph of PC/SAN/MWNTs with that of PC/ABS/MWNTs, it was revealed that small portion of MWNTs was also located on polybutadiene rubber fraction of ABS.
Model "supramolecular IPNs" were developed via the formation of a hydrogen-bonded, supramolecular network of 2-ureido-4-[1H]-pyrimidinone (UPy) telechelic poly(ethylene-co-1-butene) (SPEB) in the presence of photopolymerizable, hydroxyl-terminated polybutadiene (HTPB). The role of a supramolecular elastomeric phase in mechanical toughening of IPNs was explored through (1) dynamic dissociation and reassociation of the noncovalent, UPy supramolecular associations, and (2) interphase formation. While an ∼4× increase in tensile toughness of the HTPB matrix was observed through incorporation of 10 wt % ethylene−propylene rubber (EPR)as a conventional elastomeric toughening agentinto HTPB, it was shown that adding the same amount of supramolecular elastomer SPEB to HTPB led to ∼600× enhancement in tensile toughness. Strain rate-dependent mechanical response and fractography studies revealed that this dramatic toughness enhancement was due to dissociation/reassociation of the dynamic UPy linkages in the elastomeric phase that facilitated dilatational yielding of the IPN. This toughness enhancement was only observed in combination with the existence of strong interfacial coupling between the matrix and supramolecular phase as revealed by transmission electron microscopy and dynamic mechanical analysis. By exploiting noncovalent dynamics and interfacial control in interpenetrating networks, pathways are envisioned toward a new class of tough materials.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.