Looking for assistance? Contact Amco Polymers today. When we talk about molecular weight in terms of polymers, we are really talking about the length of the individual chains. Molecular weight is the average weight of the molecules that make up a polymer and gives an indication of the length of the polymer chains. The polymerization process is subject to variation so there is no single chain length, there is actually a wide range of lengths, so when we discuss molecular weight, we really mean the average molecular weight of the material.
This average is found by measuring samples of the material as it is produced. This table shows the effects of molecular weight on selected properties. In general, as molecular weight increase the strength, toughness and chemical stress crack resistance increase. Lower molecular weight will typically flow easier. This table shows the influence of melt flow on the impact properties of polycarbonate.
With increasing melt flow index, there is a significant drop off in notched Izod impact properties.
A liquid resin is then passed through the vascules and into the damage plane, allowing the cracks to be repaired. Vascular systems have a number of advantages over microcapsule based systems, such as the ability to continuously deliver large volumes of repair agents and the potential to be used for repeated healing. The hollow channels themselves can also be used for additional functionality, such as thermal management and structural health monitoring. Coatings allow the retention and improvement of bulk properties of a material. They can provide protection for a substrate from environmental exposure.
Thus, when damage occurs often in the form of microcracks , environmental elements like water and oxygen can diffuse through the coating and may cause material damage or failure. Microcracking in coatings can result in mechanical degradation or delamination of the coating, or in electrical failure in fibre-reinforced composites and microelectronics, respectively.
As the damage is on such a small scale, repair, if possible, is often difficult and costly. The capsule approach originally described by White et al. Liquid metal microdroplets have also been suspended within silicone elastomer to create stretchable electrical conductors that maintain electrical conductivity when damaged, mimicking the resilience of soft biological tissue.
Corrosion protection of metallic materials is of significant importance on an economical and ecological scale. To prove the effectiveness of microcapsules in polymer coatings for corrosion protection, researchers have encapsulated a number of materials. These materials include isocyanates   monomers such as DCPD   GMA  epoxy resin,  linseed oil   and tung oil.
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Cementitious materials have existed since the Roman era. These materials have a natural ability to self-heal, which was first reported by the French Academy of Science in Autogenous healing is the natural ability of cementitious materials to repair cracks. This ability is principally attributed to further hydration of unhydrated cement particles and carbonation of dissolved calcium hydroxide.
Self-healing of cementitious materials can be achieved through the reaction of certain chemical agents. Two main strategies exist for housing these agents, namely capsules and vascular tubes. These capsules and vascular tubes, once ruptured, release these agents and heal the crack damage.
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Studies have mainly focused on improving the quality of these housings and encapsulated materials in this field. According to a study by H. Erlich in Chemical Geology journal, the self-healing ability of concrete has been improved by the incorporation of bacteria, which can induce calcium carbonate precipitation through their metabolic activity. Jonkers and Erik Schlangen presented their research in which they had successfully used the "alkaliphilic spore-forming bacteria" as a "self-healing agent in concrete".
Later studies saw Jonkers use expanded clay particles  and Van Tittlelboom use glass tubes,  to protect the bacteria inside the concrete. Other strategies to protect the bacteria have also since been reported. Generally, ceramics are superior in strength to metals at high temperatures, however, they are brittle and sensitive to flaws, and this brings into question their integrity and reliability as structural materials. Micro cracks caused by wear or thermal stress are filled with oxides formed from the MAX phase constituents, commonly the A-element, during high temperature exposure to air.
Depending on the filling-oxide, improvement of the initial properties such as local strength can be achieved. Upon cracking, these particles are exposed to oxygen, and in the presence of heat, they react to form new materials which fill the crack gap under volume expansion. When exposed for long times to high temperatures and moderate stresses, metals exhibit premature and low-ductility creep fracture, arising from the formation and growth of cavities.
Those defects coalesce into cracks which ultimately cause macroscopic failure.
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Self-healing of early stage damage is thus a promising new approach to extend the lifetime of the metallic components. In metals, self-healing is intrinsically more difficult to achieve than in most other material classes, due to their high melting point and, as a result, low atom mobility.
Generally, defects in the metals are healed by the formation of precipitates at the defect sites that immobilize further crack growth. Improved creep and fatigue properties have been reported for underaged aluminium alloys compared to the peak hardening Al alloys, which is due to the heterogeneous precipitation at the crack tip and its plastic zone. A defect-induced mechanism is indicated for the Au precipitation, i. Healing agents selectively precipitate at the free surface of a creep cavity, resulting in pore filling. Work to translate the concept of creep damage healing in simple binary or ternary model systems to real multicomponent creep steels is ongoing.
Recently, a several classes of organic dyes are discovered that self-heal after photo-degradation when doped in PMMA and other polymer matrices. It was shown that, unlike common process like molecular diffusion,  the mechanism is caused by dye-polymer interaction.
Intrinsic Molecular Mobility and Toughness of Polymers II
Self-healing epoxies can be incorporated onto metals in order to prevent corrosion. A substrate metal showed major degradation and rust formation after 72 hours of exposure. But after being coated with the self-healing epoxy, there was no visible damage under SEM after 72 hours of the same exposure.
Numerous methodologies for the assessment of self-healing capabilities have been developed for each material class Table 1. Hence, when self-healing is assessed, different parameters need to be considered: type of stimulus if any , healing time, maximum amount of healing cycles the material can tolerate, and degree of recovery, all whilst considering the material's virgin properties.
The self-healing ability of a given material generally refers to the recovery of a specific property relative to the virgin material, designated as the self-healing efficiency. The self-healing efficiency can be quantified by comparing the respective experimental value obtained for the undamaged virgin sample f virgin with the healed sample f healed eq. In a variation of this definition that is relevant to extrinsic self-healing materials, the healing efficiency takes into consideration the modification of properties caused by introducing the healing agent. Accordingly, the healed sample property is compared to that of an undamaged control equipped with self-healing agent f non-healed equation 2.
For a certain property Pi of a specific material, an optimal self-healing mechanism and process is characterized by the full restoration of the respective material property after a suitable, normalized damaging process. The final average efficiency based on a number n of properties for a self-healing material is accordingly determined as the harmonic mean given by equation 3. The harmonic mean is more appropriate than the traditional arithmetic mean, as it is less sensitive to large outliers.
At least two companies are attempting to bring the newer applications of self-healing materials to the market. Arkema , a leading chemicals company, announced in the beginning of industrial production of self-healing elastomers. From Wikipedia, the free encyclopedia. Weinheim: Wiley - VCH. Concepts, realization and outlook: A review".
Express Polymer Letters. Retrieved 13 November Lawrence Berkeley National Laboratory. University of California. December 15, Retrieved 17 November Boston Globe. Proceedings of the National Academy of Sciences. Bibcode : PNAS.. Delft University of Technology. Retrieved 19 May Biomimetics in Materials Science: Self-healing, self-lubricating, and self-cleaning materials. Springer Series in Materials Science. Binder ed. Self-Healing Polymers. Fratzl; J. Dunlop; R. Weinkamer eds. RSC Smart Materials.
The Royal Chemical Society. Journal of Theoretical Biology. Bibcode : BiBi Cornerstone Research Group. Archived from the original on Retrieved Nature Materials.
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Bibcode : JMatS.. Bibcode : Sci ACS Appl. Bibcode : Natur. The laboratory which he had founded there, became well known for its innovative studies on the role of chain backbones in craze initiation, interdiffusion at interfaces and during high rates of loading. More than 20 PhD theses, some publications and several books testify to his creative activity, for which he was awarded the Dr. Through his research and his multiple engagements in international professional societies e.
Publications Other publications Kausch H. I and II, Kausch, H.