Recent Developments in Shape Memory Elastomers for Biotechnology Applications.

Shape memory elastomers have revolutionised the world since their introduction in the 20th century. The ability to tailor chemical structures to produce a family of materials in wide-ranging forms with versatile properties has propelled them to be ubiquitous. Recent challenges in the end-of-life management of polymeric materials should prompt us to ask, 'what innovations in polymeric materials can make a strong case for their use as efficient materials?' The development of smart elastomers that can acquire, convey, or process a stimulus (such as temperature, pressure, electromagnetic field, moisture, and chemical signals) and reply by creating a useful effect, specifically a reversible change in shape, is one such innovation. Here, we present a brief overview of shape memory elastomers (SMEs) and thereafter a review of recent advances in their development. We discuss the complex processing of structure-property relations and how they differ for a range of stimuli-responsive SMEs, self-healing SMEs, thermoplastic SMEs, and antibacterial and antifouling SMEs. Following innovations in SEMs, the SMEs are forecast to have significant potential in biotechnology based on their tailorable physical properties that are suited to a range of different external stimuli.

Non-Covalent Interaction on the Self-Healing of Mechanical Properties in Supramolecular Polymers.

Supramolecular polymers are widely utilized and applied in self-assembly or self-healing materials, which can be repaired when damaged. Normally, the healing process is classified into two types, including extrinsic and intrinsic self-healable materials. Therefore, the aim of this work is to review the intrinsic self-healing strategy based on supramolecular interaction or non-covalent interaction and molecular recognition to obtain the improvement of mechanical properties. In this review, we introduce the main background of non-covalent interaction, which consists of the metal-ligand coordination, hydrogen bonding, π-π interaction, electrostatic interaction, dipole-dipole interaction, and host-guest interactions, respectively. From the perspective of mechanical properties, these interactions act as transient crosslinking points to both prevent and repair the broken polymer chains. For material utilization in terms of self-healing products, this knowledge can be applied and developed to increase the lifetime of the products, causing rapid healing and reducing accidents and maintenance costs. Therefore, the self-healing materials using supramolecular polymers or non-covalent interaction provides a novel strategy to enhance the mechanical properties of materials causing the extended cycling lifetime of products before replacement with a new one.

The Relationship between the Morphology and Elasticity of Natural Rubber Foam Based on the Concentration of the Chemical Blowing Agent.

Concentrated natural latex was used to produce a rubber foam that is porous, elastic and well ventilated. The mechanical properties can be either soft or firm, depending on the formulation of the latex used. Briefly, concentrated natural latex was mixed with chemical agents to make the rubber foam on a laboratory scale using the Dunlop process. In this work, we changed the concentration of the chemical blowing agent in the latex. The morphological properties of the rubber foam were characterised using scanning electron microscopy, and the mechanical properties, or elasticity, were studied using compression experiments and the Mooney-Rivlin calculation. The results show that the concentration of the chemical blowing agent affects the morphological properties of the rubber foam but not the mechanical properties, indicating the heterogeneous structure of the rubber foam. The thermodynamic parameters (∆G and ∆S) and the internal energy force per compression force (Fu/F) of the rubber foam with various amounts of chemical blowing agent were also investigated. This study could be applied in the foam industry, particularly for pillow, mattress and insulation materials, as the present work shows the possible novel control of the morphological structure of the rubber foam without changing its mechanical properties. The difference in cell sizes could affect the airflow in rubber foam.

Current challenges in thermodynamic aspects of rubber foam.

Natural rubber (NR) foam can be prepared by the Dunlop method using concentrated natural latex with chemical agents. Most previous studies have focused on the thermodynamic parameters of solid rubber in extension. The main objective of this study is to investigate the effect of the NR matrix concentration on the static and dynamic properties of NR foams, especially the new approach of considering the thermodynamic aspects of NR foam in compression. We found that the density and compression strength of NR foams increased with increasing NR matrix concentration. The mechanical properties of NR foam were in agreement with computational modelling. Moreover, thermodynamic aspects showed that the ratio of internal energy force to the compression force, Fu/F, and the entropy, S, increased with increasing matrix concentration. The activation enthalpy, ∆Ha, also increased with increasing matrix concentration in the NR foam, indicating the greater relaxation time of the backbone of the rubber molecules. New scientific concepts of thermodynamic parameters of the crosslinked NR foam in compression mode are proposed and discussed. Our results will improve both the knowledge and the development of rubber foams based on the structure-properties relationship, especially the new scientific concept of the thermodynamical parameters under compression.

Effect of Fillers on the Recovery of Rubber Foam: From Theory to Applications.

Natural rubber foam (NRF) can be prepared from concentrated natural latex, providing specific characteristics such as density, compression strength, compression set, and so on, suitable for making shape-memory products. However, many customers require NRF products with a low compression set. This study aims to develop and prepare NRF to investigate its recoverability and other related characteristics by the addition of charcoal and silica fillers. The results showed that increasing filler loading increases physical and mechanical properties. The recoverability of NRF improves as silica increases, contrary to charcoal loading, due to the higher specific surface area of silica. Thermodynamic aspects showed that increasing filler loading increases the compression force (F) as well as the proportion of internal energy to the compression force (Fu/F). The entropy (S) also increases with increasing filler loading, which is favorable for thermodynamic systems. The activation enthalpy (∆Ha) of the NRF with silica is higher than the control NRF, which is due to rubber-filler interactions created within the NRF. A thermodynamic concept of crosslinked rubber foam with filler is proposed. From theory to application, in this study, the NRF has better recoverability with silica loading.

Performance of Nano- and Microcalcium Carbonate in Uncrosslinked Natural Rubber Composites: New Results of Structure-Properties Relationship.

Calcium carbonate (CaCO3) is one of the most important inorganic powders and is widely used as filler in order to reduce costs in the rubber industry. Nanocalcium carbonate reduces costs and acts as a semireinforcing filler that improves the mechanical properties of rubber composites. The objective of this study was to investigate the effect of nano-CaCO3 (NCC) and micro-CaCO3 (MCC) on the properties of natural rubber composites, in particular, new results of structure-properties relationship. The effects of NCC/MCC on the properties of rubber composites, such as Mooney viscosity, bound rubber, Mullins effect, and Payne effect, were investigated. The result of the Mullins effect of rubber composites filled with NCC was in good agreement with the results of Mooney viscosity and bound rubber, with higher Mooney viscosity and bound rubber leading to higher stress to pull the rubber composites. The Payne effect showed that the value of different storage moduli (ΔG') of rubber composites filled with 25 parts per hundred rubber (phr) NCC was the lowest due to weaker filler network, while the rubber supplemented with 100 phr NCC had more significant ΔG' values with increase in strain. The results of rubber composites filled with MCC showed the same tendency as those of rubber composites filled with NCC. However, the effect of specific surface area of NCC on the properties of rubber composites was more pronounced than those of rubber composites filled with MCC. Finite element analysis of the mechanical property of rubber composites was in good agreement with the result from the experiment. The master curves of time-temperature superposition presented lower free volume in the composites for higher loading of filler, which would require more relaxation time of rubber molecules. This type of nanocalcium carbonate material can be applied to tailor the properties and processability of rubber products.

Recent Progress in Processing Functionally Graded Polymer Foams.

Polymer foams are an important class of engineering material that are finding diverse applications, including as structural parts in automotive industry, insulation in construction, core materials for sandwich composites, and cushioning in mattresses. The vast majority of these manufactured foams are homogeneous with respect to porosity and structural properties. In contrast, while cellular materials are also ubiquitous in nature, nature mostly fabricates heterogeneous foams, e.g., cellulosic plant stems like bamboo, or a human femur bone. Foams with such engineered porosity distribution (graded density structure) have useful property gradients and are referred to as functionally graded foams. Functionally graded polymer foams are one of the key emerging innovations in polymer foam technology. They allow enhancement in properties such as energy absorption, more efficient use of material, and better design for specific applications, such as helmets and tissue restorative scaffolds. Here, following an overview of key processing parameters for polymer foams, we explore recent developments in processing functionally graded polymer foams and their emerging structures and properties. Processes can be as simple as utilizing different surface materials from which the foam forms, to as complex as using microfluidics. We also highlight principal challenges that need addressing in future research, the key one being development of viable generic processes that allow (complete) control and tailoring of porosity distribution on an application-by-application basis.

Effect of Stabilizer States (Solid Vs Liquid) on Properties of Stabilized Natural Rubbers.

The main objective of this work is to study the effect of hydroxylamine sulfate or stabilizer states (solid vs liquid) on the storage hardening of natural rubber (NR). Several types of natural rubber samples were prepared: unstabilized NR samples and stabilized NR samples: (i) dry NR with 0.2 and 2.0 parts per hundred rubber (phr) of dry hydroxylamine sulfate, and (ii) natural latex with 0.2 and 2.0 phr of liquid hydroxylamine sulfate. The samples were characterized immediately (time 0) and after 12 weeks of storage at room temperature, respectively. We found that the Mooney viscosity, gel content, and Wallace plasticity of NR without a stabilizer increases with storage hardening for 12 weeks. However, two types of stabilized NR samples represent constant values of those three parameters, because hydroxylamine sulfate inhibits network and gel formation in NR. Interestingly, the mixing states (solid vs liquid) between natural rubber and the stabilizer affect the properties of stabilized NR. This could be explained by the better dispersion and homogeneous nature of liquid stabilizers in natural latex (liquid state), and thus the higher loading of the stabilizer in the liquid state. This is important, as the stabilization of NR properties as a function of time is required by rubber industry. This study is a utilization model from theory to application.

Noncovalent Sericin-Chitosan Scaffold: Physical Properties and Low Cytotoxicity Effect.

This research aims to utilize sericin, which is the waste from boiling silk cocoon, for the supramolecular scaffold preparation with chitosan. A suitable method for the self-assembled scaffold formation of sericin and chitosan at 1:1 stoichiometry is presented and the morphological and physical properties of the scaffold are studied. The effect of an alcohol/NaOH solution on the secondary structure of sericin protein within the sericin-chitosan scaffold, with adjusted pH, was investigated. Additionally, the scaffold was tested in a native phosphate buffer solution (PBS). The results show that sericin increases the porosity of scaffold while chitosan increases the rigidity. The self-assembled sericin and chitosan material is nontoxic to human cells and which can adhere and spread well on such support. For the effect of the molecular weight of chitosan (15,000 and 100,000 g/mol), the scaffold made from lower molecular weight (MW) chitosan provides a somewhat smaller porosity, but a similar swelling ratio and water uptake. On the basis of this research, sericin, which is a silk waste from the textile industry, can be utilized to produce a self-assembled scaffold with chitosan in order to increase the porosity of the scaffold. This type of scaffold is not toxic and can be used for the adhesion of fibroblast cells.

Noncovalent self-assembling nucleic acid-lipid based materials.

We detail a method originally described by Okahata et al. (Macromol. Rapid Commun. 2002, 23, 252-255) to prepare noncovalent self-assembling films by exchanging the counter-ions of the nucleic acid phosphate moieties with those of cationic lipid amphiphiles. We are able to control the strength and surface properties of these films by varying the composition between blends of DNA of high molecular weight and RNA of low molecular weight. X-ray and AFM results indicate that these films have a lamellar multilayered structure with layers of nucleic acid separated by layers of cationic amphiphile. The tensile strength of the blended films between DNA and RNA increases elastically with DNA content. The length as well as the molecular structure of nucleic acids can affect the topology and mechanical properties of these films. We suspect that the permeability properties of these films make them good candidates for further biological applications in vivo.