Chemistry
Cross Linked Polymer
A cross-linked polymer is a three-dimensional network of polymer chains that are linked together by chemical bonds. This structure provides the polymer with increased strength, rigidity, and resistance to solvents and heat. Cross-linked polymers are commonly used in a variety of applications, including in the production of rubber, adhesives, and coatings.
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7 Key excerpts on "Cross Linked Polymer"
eBook - PDFAtomic Radiation and Polymers
International Series of Monographs on Radiation Effects in Materials, Vol. 1
- A. Charlesby(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
CHAPTER 9 THE PROPERTIES OF A CROSSLINKED NETWORK MANY polymers, after exposure to small doses of high energy radiation, show an increase in viscosity, average molecular weight and degree of branching. These changes are due to dimerization—the linking of mole-cules together—and the reaction has also been observed in low molecular weight compounds such as «-paraffins. As the reaction proceeds, closed loops are formed and produce a three-dimensional network polymer, the properties of which are very different from those of the original linear or branched material. This transition in properties from a one- to a three-dimensional structure is also observed in the vulcanization of rubber, in the chemical crosslinking of polymer molecules such as copolymers of styrene and divinyl benzene, and in the gelation of polyesters. Polymers can be considered as consisting of an assembly of units (not necessarily identical) which are mono-functional, di-functional or poly-functional. The mono-functional units (—A) are needed to end the chain and therefore act as terminal units. Di-functional units (—A—) extend the chain but do not provide any means of linking to other molecules to form a network. This function is assumed by the tri-functional (—A—) I or higher functional units which also occur in branched polymers. The possibility of network formation depends on the degree of functionality, i.e. on the ratio of poly-functional units to mono-functional units. A small discrepancy arises from the possibility of intramolecular crosslinks, i.e. links of one poly-functional unit in a molecule with another such poly-functional unit in the same molecule. In the theory as given here, the term crosslink is taken to refer to a tetrafunctional link or junction point, binding two long molecules together side by side: —A—A—A—A—A— .
eBook - PDFAtomic Radiation and Polymers
International Series of Monographs on Radiation Effects in Materials
- A. Charlesby(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
C H A P T E R 9 THE PROPERTIES OF A CROSSLINKED NETWORK M A N Y polymers, after exposure to small doses of high energy radiation, show an increase in viscosity, average molecular weight and degree of branching. These changes are due to dimerization—the linking of mole-cules together—and the reaction has also been observed in low molecular weight compounds such as «-parafBns. As the reaction proceeds, closed loops are formed and produce a three-dimensional network polymer, the properties of which are very different from those of the original linear or branched material. This transition in properties from a one- to a three-dimensional structure is also observed in the vulcanization of rubber, in the chemical crosslinking of polymer molecules such as copolymers of styrene and divinyl benzene, and in the gelation of polyesters. Polymers can be considered as consisting of an assembly of units (not necessarily identical) which are mono-functional, di-functional or poly-functional. The mono-functional units (—A) are needed to end the chain and therefore act as terminal units. Di-functional units (—A—) extend the chain but do not provide any means of linking to other molecules to form a network. This function is assumed by the tri-functional (—A—) i or higher functional units which also occur in branched polymers. The possibility of network formation depends on the degree of functionality, i.e. on the ratio of poly-functional units to mono-functional units. A small discrepancy arises from the possibility of intramolecular crosslinks, i.e. links of one poly-functional unit in a molecule with another such poly-functional unit in the same molecule. In the theory as given here, the term crosslink is taken to refer to a tetrafunctional link or junction point, binding two long molecules together side by side: —A—A—A—A—A— .
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Network polymers have covalent crosslinks between adjacent molecular chains. During heat treatments, these bonds anchor the chains together to resist the vibrational and rotational chain motions at high tempera- tures. Thus, the materials do not soften when heated. Crosslinking is usually extensive, in that 10% to 50% of the chain repeat units are crosslinked. Only heating to excessive temperatures will cause severance of these crosslink bonds and polymer degradation. Thermoset polymers are generally harder and stronger than thermoplastics and have bet- ter dimensional stability. Most of the crosslinked and network polymers, which include vulcanized rubbers, epoxies, phenolics, and some polyester resins, are thermosetting. Polymer chemists and scientists are continually searching for new materials that can be easily and economically synthesized and fabricated with improved properties or better property combinations than are offered by the homopolymers previously discussed. One group of these materials are the copolymers. Consider a copolymer that is composed of two repeat units as represented by and in Figure 4.9. Depending on the polymerization process and the relative fractions of these repeat unit types, different sequencing arrangements along the polymer chains are possible. For one, as depicted in Figure 4.9a, the two different units are randomly dispersed along the chain in what is termed a random copolymer. For an alternating copolymer, as the name suggests, the two repeat units alternate chain positions, as illustrated in Figure 4.9b. A block copolymer is one in which identical repeat units are clustered in blocks along the chain (Figure 4.9c). Finally, homopolymer side branches of one type may be grafted to homopolymer main chains that are composed of a different repeat unit; such a material is termed a graft copolymer (Figure 4.9d).
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
Crosslinking tends to increase T g and increase strength and toughness. Among other applications, this process is used to strengthen rubbers in a process known as vulcanization, which is based on crosslinking by sulfur. Car tires, for example, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen their durability. Eraser rubber, on the other hand, is not crosslinked to allow flaking of the rubber and prevent damage to the paper. A cross-link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of crosslinking is referred to as a polymer network. Sufficiently high crosslink concentrations may lead to the formation of an infinite network, also known as a gel, in which networks of chains are of unlimited extent— essentially all chains have linked into one molecule. ________________________ WORLD TECHNOLOGIES ________________________ Chain length The physical properties of a polymer are strongly dependent on the size or length of the polymer chain.. For example, as chain length is increased, melting and boiling tempe-ratures increase quickly. Impact resistance also tends to increase with chain length, as does the viscosity, or resistance to flow, of the polymer in its melt state. Chain length is related to melt viscosity roughly as 1:10 3.2 , so that a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times. Increasing chain length furthermore tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (T g ). This is a result of the increase in chain interactions such as Van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
Crosslinking tends to increase T g and increase strength and toughness. Among other applications, this process is used to strengthen rubbers in a process known as vulcanization, which is based on crosslinking by sulfur. Car tires, for example, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen their durability. Eraser rubber, on the other hand, is not crosslinked to allow flaking of the rubber and prevent damage to the paper. A cross-link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of crosslinking is referred to as a polymer network. Sufficiently high crosslink concentrations may lead to the formation of an infinite network, also known as a gel, in which networks of chains are of unlimited extent— essentially all chains have linked into one molecule. ________________________ WORLD TECHNOLOGIES ________________________ Chain length The physical properties of a polymer are strongly dependent on the size or length of the polymer chain.. For example, as chain length is increased, melting and boiling temperatures increase quickly. Impact resistance also tends to increase with chain length, as does the viscosity, or resistance to flow, of the polymer in its melt state. Chain length is related to melt viscosity roughly as 1:10 3.2 , so that a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times. Increasing chain length furthermore tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (T g ). This is a result of the increase in chain interactions such as Van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures.
- Michael Bolgar, Jack Hubball, Joseph Groeger, Susan Meronek(Authors)
- 2015(Publication Date)
- CRC Press(Publisher)
27 CHAPTER 3 Crosslinking Compounds/Accelerators The chemical crosslinking process has its origins in the Goodyear process by which rubber was transformed from a viscous form into a shape-retaining rubbery material through reaction with sulfur and heat. This 'vulcanizing' process promoted bonding between unsaturated end groups in the natural rubber chains, thus forming 'crosslinks' between chains. With these crosslinks, the rubber molecules were no longer subject to mutual slip, thus a molded object would permanently retain its shape while retaining elastomeric behavior. Crosslinking is a major commercial process for elastomeric, thermoplastic, and liquid polymeric materials. The approaches for crosslinking reaction imitation vary with the type of polymer, the end application, costs, and technical approach. Many crosslinking processed are thermally initiated. Heat exposure increases oxidation risk, while considerably boosting energy requirements. The rate of crosslinking can be increased by careful technical development of a process and selection of organic compounds that function as cure accelerators. In some cases, the rate of cure may be moderated with chemical retarding agents to control exothermic reactions and physical stability. Crosslinking of many commercial thermoplastics is conducted with reactive peroxides, including dicumyl peroxide, benzoyl peroxide, and many others, all of which have different activation temperatures. At the crosslinking initiation temperature, the peroxide decomposes and forms free radicals that react with unsaturated end groups on the polymer chains, thus forming chemical crosslinks between chains. For thermoplastic addition, stability of the peroxides is increased for storage and handling by incorporation into master batches that may include waxes, clay, fatty acids, or resins.
eBook - PDFDynamics and Transport in Macromolecular Networks
Theory, Modelling, and Experiments
- Li-Tang Yan(Author)
- 2023(Publication Date)
- Wiley-VCH(Publisher)
53 3 Dynamic Bonds in Associating Polymer Networks Jiayao Chen 1 , Xiao Zhao 2 , and Peng-Fei Cao 1 1 State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beisanhuan Road East, Beijing 100029, China 2 GCP Applied Technologies Inc., Wilmington, Massachusetts 01887, USA 3.1 Introduction of Dynamic Bonds Polymeric materials with covalent cross-links have advantages of facile synthe- sis, high mechanical performance, and decent chemical resistance, despite the emerging problems regarding the reprocess ability and recyclability [1–3]. There are basically two ways to introduce dynamic bonds into polymers: (i) within the polymer backbone and (ii) as reversible cross-links between polymer chains. Polymers with dynamic bonds, often termed associating polymers, have been attracting great research interest in recent years because of their unique viscoelastic properties, self-healing ability, and recyclability. A variety of dynamic covalent and noncovalent chemistries have been explored with respect to their capabilities to form transient bonding in polymer networks. Recently, the micro phase-separated dynamic aggregates have also been developed due to their unique architectures and performances in comparison to associating networks that are cross-linked merely by binary associating groups. Moreover, various dynamic bonds that differ by the bond-dissociation energy were broadly investigated to reveal the mechanisms that control the characteristic time of the network topology rearrangements. In this section, two typical types of dynamic bonds, i.e. dynamic covalent bonds and dynamic noncovalent bonds, are presented with regard to their categories, rearrangement mechanisms, and characteristics. 3.1.1 Dynamic Covalent Bonds Dynamic covalent bonds typically cross-link polymer chains to form the dynamic covalent networks, which is also known as covalent adaptive networks (CANs) [3].
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