Water-Responsive and Mechanically Adaptive Natural Rubber Composites by in Situ Modification of Mineral Filler Structures.

A new biomimetic stimuli-responsive adaptive elastomeric material, whose mechanical properties are altered by a water treatment is reported in this paper. This material is a calcium sulphate (CaSO4) filled composite with an epoxidized natural rubber (ENR) matrix. By exploiting various phase transformation processes that arise when CaSO4 is hydrated, several different crystal structures of CaSO4· xH2O can be developed in the cross-linked ENR matrix. Significant improvements in the mechanical and thermal properties are then observed in the water-treated composites. When compared with the untreated sample, there is approximately 100% increase in the dynamic modulus. The thermal stability of the composites is also improved by increasing the maximum degradation rate temperature by about 20 °C. This change in behavior results from an in situ development of hydrated crystal structures of the nanosized CaSO4 particles in the ENR matrix, which has been verified using Raman spectroscopy, transmission electron microscopy, atomic force microscopy, and X-ray scattering. This work provides a promising and relatively simple pathway for the development of next generation of mechanically adaptive elastomeric materials by an eco-friendly route, which may eventually also be developed into an innovative biodegradable and biocompatible smart polymeric material.

In Situ Polymorphic Alteration of Filler Structures for Biomimetic Mechanically Adaptive Elastomer Nanocomposites.

A mechanically adaptable elastomer composite is prepared with reversible soft-stiff properties that can be easily controlled. By the exploitation of different morphological structures of calcium sulfate, which acts as the active filler in a soft elastomer matrix, the magnitude of filler reinforcement can be reversibly altered, which will be reflected in changes of the final stiffness of the material. The higher stiffness, in other words, the higher modulus of the composites, is realized by the in situ development of fine nanostructured calcium sulfate dihydrate crystals, which are formed during exposure to water and, further, these highly reinforcing crystals can be transformed to a nonreinforcing hemihydrate mesocrystalline structure by simply heating the system in a controlled way. The Young's modulus of the developed material can be reversibly altered from ∼6 to ∼17 MPa, and the dynamic stiffness (storage modulus at room temperature and 10 Hz frequency) alters its value in the order of 1000%. As the transformation is related to the presence of water molecules in the crystallites, a hydrophilic elastomer matrix was selected, which is a blend of two hydrophilic polymers, namely, epichlorohydrin-ethylene oxide-allyl glycidyl ether terpolymer and a terpolymer of ethylene oxide-propylene oxide-allyl glycidyl ether. For the first time, this method also provides a route to regulate the morphology and structure of calcium sulfate nanocrystals in a confined ambient of cross-linked polymer chains.

Strong Strain Sensing Performance of Natural Rubber Nanocomposites.

A detail study concerning the strain (tensile) dependent electrical conductivity of elastomeric composites is reported in this present paper. Multiwall carbon nanotubes (CNT), conducting carbon black (CB), and their combinations were considered as conducting filler in cross-linked natural rubber matrix. The loadings of the fillers were considered from 3 to 11 phr (filler concentration close to their percolation threshold). Without hindering the elastic nature of the composite (reversible stretchability up to several 100%), the change of relative resistance, ΔR/R0 (ΔR is the change in the resistance with respect to strain and R0 is the initial resistance of the sample) of the CB filled composites was found to be as much as ∼1300 at around 120% elongation. This value is much higher than any other reported values obtained from conducting polymeric composites. It was found that CNT offered a strong strain dependent character in the regime 100% to 150% elongation, whereas, the carbon black filled natural rubber showed strong strain dependencies at 50% to 100% elongation strain. The combination of two different fillers could be exploited to tailor and manipulate the sensing operating regime from 50% to 150% strain depending on the ratios of the two filler system. Additionally, after several loading-unloading cycles, the conductivity of the sample was very stable for CB filled system but for CNT filled system the conductivity of the sample was altered. This type of elastic materials could be used in structural health monitoring, sensors in different dynamic elastomeric parts like tires, valves, gaskets, engine mounts, etc.

Temperature-Dependent Reinforcement of Hydrophilic Rubber Using Ice Crystals.

This is the first study on the impact of ice crystals on glass transition and mechanical behavior of soft cross-linked elastomers. A hydrophilic elastomer such as epichlorohydrin-ethylene oxide-allyl glycidyl ether can absorb about ∼40 wt % of water. The water-swollen cross-linked network exhibits elastic properties with more than 1500% stretchability at room temperature. Coincidently, the phase transition of water into solid ice crystals inside of the composites allows the reinforcement of the soft elastomer mechanically at lower temperatures. Young's modulus of the composites measured at -20 °C remarkably increased from 1.45 to 3.14 MPa, whereas at +20 °C, the effect was opposite and the Young's modulus decreased from 0.6 to 0.03 MPa after 20 days of water treatment. It was found that a part of the absorbed water, ∼74% of the total absorbed water, is freezable and occupies nearly 26 vol % of the composites. Simultaneously, these solid ice crystals are found to be acting as a reinforcing filler at lower temperatures. The size of these ice crystals is distributed in a relatively narrow range of 400-600 nm. The storage modulus (E') of the ice crystal-filled composites increased from 3 to 13 MPa at -20 °C. The glass transition temperature (-37 °C) of the soft cross-linked elastomer was not altered by the absorption of water. However, a special transition (melting of ice) occurred at temperatures close to 0 °C as observed in the dynamic mechanical analysis of the water-swollen elastomers. The direct polymer/filler (ice crystals) interaction was demonstrated by strain sweep experiments and investigated using Fourier transform infrared spectroscopy. This type of cross-linked rubber could be integrated into a smart rubber application such as in adaptable mechanics, where the stiffness of the rubber can be altered as a function of temperature without affecting the mechanical stretchability either below or above 0 °C (above the glass temperature region) of the rubber.

Polymersomes as an effective drug delivery system for glioma--a review.

Glioma is one of the most commonly occurring malignant brain tumours which need proper treatment strategy. The current therapies for treating glioma like surgical resection, radiotherapy, and chemotherapy have failed in achieving satisfactory results and this forms a rationale for the development of novel drug delivery systems. Among them, polymersomes are superior novel carriers with diverse functions like enhanced stability, low permeability, tunable membrane properties, surface functionality, and long blood circulation time which make them suitable for cancer therapy. These are bilayered vesicles capable of encapsulating both hydrophilic and hydrophobic drugs used to target glioma effectively. In this review, we have discussed on general preparation, characterization, and targeting aspects of surface modified polymersomes for effective delivery of therapeutic agents to glioma.