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77-402: PEEK polymers are obtained by step-growth polymerization by the di alkylation of bis phenolate salts. Typical is the reaction of 4,4'-difluorobenzophenone with the disodium salt of hydroquinone , which is generated in situ by deprotonation with sodium carbonate . The reaction is conducted around 300 °C in polar aprotic solvents - such as diphenyl sulfone . [REDACTED] PEEK

154-460: A catalyst, and k is the rate constant. For a system with equivalent quantities of acid and glycol, the functional group concentration can be written simply as After integration and substitution from Carothers equation , the final form is the following For a self-catalyzed system, the number average degree of polymerization (X n ) grows proportionally with t {\displaystyle {\sqrt {t}}} . The uncatalyzed reaction

231-539: A catalytic complex of a cuprous salt and amine form aromatic polyethers , commercially referred to as poly(p-phenylene oxide) or PPO. Neat PPO has little commercial uses due to its high melt viscosity. Its available products are blends of PPO with high-impact polystyrene (HIPS). Polyethersulfone (PES) is also referred to as polyetherketone, polysulfone . It is synthesized by nucleophilic aromatic substitution between aromatic dihalides and bisphenolate salts. Polyethersulfones are partially crystalline, highly resistant to

308-735: A crosslinker such as glycerin , trimethylol propane . Introduction of covalent crosslinking improves in creep, increase in recovery temperature and recovery window. The PEO-PET block copolymers can be crosslinked by using maleic anhydride , glycerin or dimethyl 5-isophthalates as a crosslinking agent. The addition of 1.5 wt% maleic anhydride increased in shape recovery from 35% to 65% and tensile strength from 3 to 5 MPa. While shape-memory effects are traditionally limited to thermosetting plastics , some thermoplastic polymers, most notably PEEK , can be used as well. Light-activated shape-memory polymers (LASMP) use processes of photo-crosslinking and photo-cleaving to change T g . Photo-crosslinking

385-438: A dent in the fender, these materials "remember" their original shape. Heating them activates their "memory". In the example of the dent, the fender could be repaired with a heat source, such as a hair-dryer. The impact results in a temporary form, which changes back to the original form upon heating—in effect, the plastic repairs itself. SMPs may also be useful in the production of aircraft which would morph during flight. Currently,

462-576: A different wavelength (< 260 nm). Examples of photoresponsive switches include cinnamic acid and cinnamylidene acetic acid . The use of electricity to activate the shape-memory effect of polymers is desirable for applications where it would not be possible to use heat and is another active area of research. Some current efforts use conducting SMP composites with carbon nanotubes , short carbon fibers (SCFs), carbon black, or metallic Ni powder. These conducting SMPs are produced by chemically surface-modifying multi-walled carbon nanotubes (MWNTs) in

539-591: A hard to a soft phase which is responsible for the shape-memory effect. In shape-memory alloys martensitic / austenitic transitions are responsible for the shape-memory effect. There are numerous advantages that make SMPs more attractive than shape memory alloys . They have a high capacity for elastic deformation (up to 200% in most cases), much lower cost, lower density, a broad range of application temperatures which can be tailored, easy processing, potential biocompatibility and biodegradability, and probably exhibit superior mechanical properties to those of SMAs. One of

616-524: A mixed solvent of nitric acid and sulfuric acid , with the purpose of improving the interfacial bonding between the polymers and the conductive fillers. The shape-memory effect in these types of SMPs have been shown to be dependent on the filler content and the degree of surface modification of the MWNTs, with the surface modified versions exhibiting good energy conversion efficiency and improved mechanical properties. Another technique being investigated involves

693-477: A molecular dye to the material. By configuring both the amount of scatters and of the organic dye, a light amplification regime may be observed when the composites are optically pumped. Shape memory polymers have also been used in conjunction with nanocellulose to fabricate composites exhibiting both chiroptical properties and thermo-activated shape memory effect. Most medical applications of SMP have yet to be developed, but devices with SMP are now beginning to hit

770-492: A polyamide with two amine end groups incapable of further growth when the acid chloride was totally consumed. This can be expressed in an extension of the Carothers equation as, where r is the ratio of the number of molecules of the reactants. The equation above can also be used for a monofunctional additive which is the following, where N B is the number of monofunction molecules added. The coefficient of 2 in front of N B

847-442: A polyesterification mechanism. The simple esterification is an acid-catalyzed process in which protonation of the acid is followed by interaction with the alcohol to produce an ester and water. However, there are a few assumptions needed with this kinetic model. The first assumption is water (or any other condensation product) is efficiently removed. Secondly, the functional group reactivities are independent of chain length. Finally, it

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924-558: A polymer and will ultimately form a cross-linked macrostructure or network even at low fractional conversion. The point at which a tree-like topology transits to a network is known as the gel point because it is signalled by an abrupt change in viscosity . One of the earliest so-called thermosets is known as bakelite . It is not always water that is released in step-growth polymerization: in acyclic diene metathesis or ADMET dienes polymerize with loss of ethene . The kinetics and rates of step-growth polymerization can be described using

1001-401: A polymerization is a mixture of polymer molecules of different molecular weights. For theoretical and practical reasons it is of interest to discuss the distribution of molecular weights in a polymerization. The molecular weight distribution (MWD) had been derived by Flory by a statistical approach based on the concept of equal reactivity of functional groups. Step-growth polymerization is

1078-508: A programmed shape-memory polymer first above the glass transition temperature and then above the melting transition temperature of the switching segment. Polymers exhibiting a shape-memory effect have both a visible, current (temporary) form and a stored (permanent) form. Once the latter has been manufactured by conventional methods, the material is changed into another, temporary form by processing through heating, deformation, and finally, cooling. The polymer maintains this temporary shape until

1155-419: A random process so we can use statistics to calculate the probability of finding a chain with x-structural units ("x-mer") as a function of time or conversion. Probability that an 'A' functional group has reacted Probability of finding an 'A' unreacted Combining the above two equations leads to. Where P x is the probability of finding a chain that is x-units long and has an unreacted 'A'. As x increases

1232-502: A safe way of information storage and release. Overt anti-counterfeiting labels have been constructed that display a visual symbol or code when exposed to specific chemicals. Multifunctional labels may even make counterfeiting increasingly difficult. Shape memory polymers have already been made into shape memory film by extruder machine, with covert and overt 3D embossed pattern internally, and 3D pattern will be released to be embossed or disappeared in just seconds irreversibly as soon as it

1309-477: A significant impact is photonics. Due to the shape changing capability, SMPs enable the production of functional and responsive photonic gratings. By using modern soft lithography techniques such as replica molding, it is possible to imprint periodic nanostructures, with sizes of the order of magnitude of visible light, onto the surface of shape memory polymeric blocks. As a result of the refractive index periodicity, these systems diffract light. By taking advantage of

1386-456: A small amount of disentangling. As a result, the majority of SMPs will form compact, random coils because this conformation is entropically favored over a stretched conformation. Polymers in this elastic state with number average molecular weight greater than 20,000 stretch in the direction of an applied external force. If the force is applied for a short time, the entanglement of polymer chains with their neighbors will prevent large movement of

1463-417: A small amount of monofunctional monomer, a monomer with only one functional group. The monofunctional monomer, often referred to as a chain stopper, controls and limits the polymerization of bifunctional monomers because the growing polymer yields chain ends devoid of functional groups and therefore incapable of further reaction. To properly control the polymer molecular weight, the stoichiometric imbalance of

1540-458: A traditional double-shape-memory polymer will change from a temporary shape back to a permanent shape at a particular temperature, triple-shape-memory polymers will switch from one temporary shape to another at the first transition temperature, and then back to the permanent shape at another, higher activation temperature. This is usually achieved by combining two double-shape-memory polymers with different glass transition temperatures or when heating

1617-456: A useful operating temperature of up to 250 °C (482 °F). The thermal conductivity increases nearly linearly with temperature between room temperature and solidus temperature. It is highly resistant to thermal degradation , as well as to attack by both organic and aqueous environments. It is attacked by halogens and strong Brønsted and Lewis acids , as well as some halogenated compounds and aliphatic hydrocarbons at high temperatures. It

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1694-600: A variety of potential uses, such as electrical and engine parts on automotive and aircraft components, coatings on cookware, coating and circuit boards for electronic and microelectronic devices, etc. Polymer chains based on aromatic rings are desirable due to high bond strengths and rigid polymer chains. High molecular weight and crosslinking are desirable for the same reason. Strong dipole-dipole, hydrogen bond interactions and crystallinity also improve heat resistance. To obtain desired mechanical strength, sufficiently high molecular weights are necessary, however, decreased solubility

1771-412: A wide range of aqueous and organic environment. They are rated for continuous service at temperatures of 240-280 °C. The polyketones are finding applications in areas like automotive, aerospace, electrical-electronic cable insulation. Poly(p-phenylene sulfide) (PPS) is synthesized by the reaction of sodium sulfide with p-dichlorobenzene in a polar solvent such as 1-methyl-2-pyrrolidinone (NMP). It

1848-538: Is a problem. One approach to solve this problem is to introduce of some flexibilizing linkages, such as isopropylidene, C=O, and SO 2 into the rigid polymer chain by using an appropriate monomer or comonomer. Another approach involves the synthesis of reactive telechelic oligomers containing functional end groups capable of reacting with each other, polymerization of the oligomer gives higher molecular weight, referred to as chain extension. The oxidative coupling polymerization of many 2,6-disubstituted phenols using

1925-476: Is a semicrystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained to high temperatures. The processing conditions used to mould PEEK can influence the crystallinity and hence the mechanical properties. Its Young's modulus is 3.6 GPa and its tensile strength is 90 to 100 MPa. PEEK has a glass transition temperature of around 143 °C (289 °F) and melts around 343 °C (662 °F). Some grades have

2002-558: Is achieved by using one wavelength of light, while a second wavelength of light reversibly cleaves the photo-crosslinked bonds. The effect achieved is that the material may be reversibly switched between an elastomer and a rigid polymer. Light does not change the temperature, only the cross-linking density within the material. For example, it has been reported that polymers containing cinnamic groups can be fixed into predetermined shapes by UV light illumination (> 260 nm) and then recover their original shape when exposed to UV light of

2079-436: Is also used in fused filament fabrication (FFF) printing to thermally separate the hot end from the cold end. PEEK melts at a relatively high temperature (343 °C / 649.4 °F) compared to most other thermoplastics. In the range of its melting temperature it can be processed using injection moulding or extrusion methods. It is technically feasible to process granular PEEK into filament form and 3D printing parts from

2156-445: Is also well known for his invention of Nylon . "Step growth polymerization" and condensation polymerization are two different concepts, not always identical. In fact polyurethane polymerizes with addition polymerization (because its polymerization produces no small molecules), but its reaction mechanism corresponds to a step-growth polymerization. The distinction between "addition polymerization" and "condensation polymerization"

2233-510: Is assumed that each step only involves one alcohol and one acid. This is a general rate law degree of polymerization for polyesterification where n= reaction order. If no acid catalyst is added, the reaction will still proceed because the acid can act as its own catalyst. The rate of condensation at any time t can then be derived from the rate of disappearance of -COOH groups and The second-order [ COOH ] {\displaystyle [{\ce {COOH}}]} term arises from its use as

2310-413: Is commonly used to produce high-quality plastic parts that are thermostable and both electrically and thermally insulating. Filled grades of PEEK can also be CNC machined, but special care must be taken to properly manage stresses in the material. PEEK is a high-performance polymer , but its high price, due to its complex production process, restricts its use to only the most demanding applications. PEEK

2387-607: Is defined for multi-chain polymerization, as the average number of functional groups present per monomer unit. For a system containing N 0 molecules initially and equivalent numbers of two function groups A and B, the total number of functional groups is N 0 f av . And the modified Carothers equation is The driving force in designing new polymers is the prospect of replacing other materials of construction, especially metals, by using lightweight and heat-resistant polymers. The advantages of lightweight polymers include: high strength, solvent and chemical resistance, contributing to

Polyether ether ketone - Misplaced Pages Continue

2464-710: Is heated; Shape memory film can be used as label substrates or face stock for anti-counterfeiting, brand protection , tamper-evident seals, anti-pilferage seals, etc. Using shape memory polymers as matrices, multifunctional composite materials can be produced. Such composites can have temperature dependant shape morphing (i.e. shape memory) characteristics. This phenomenon allows these composites to be potentially used to create deployable structures such as booms, hinges, wings etc. While using SMPs can help produce one-way shape morphing structures, it has been reported that using SMPs in combination with shape memory alloys allows creation of more complex shape memory composites that

2541-596: Is inherently flame-resistant and stable toward organic and aqueous conditions; however, it is somewhat susceptible to oxidants. Applications of PPS include automotive, microwave oven component, coating for cookware when blend with fluorocarbon polymers and protective coatings for valves, pipes, electromotive cells, etc. Aromatic polyimides are synthesized by the reaction of dianhydrides with diamines, for example, pyromellitic anhydride with p -phenylenediamine . It can also be accomplished using diisocyanates in place of diamines. Solubility considerations sometimes suggest use of

2618-552: Is insoluble and infusible. Shape-memory polymer Shape-memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape when induced by an external stimulus (trigger), such as temperature change. Polymer that, after heating and being subjected to a plastic deformation, resumes its original shape when heated above its glass-transition or melting temperature Note: SMPs can retain two or sometimes three shapes, and

2695-729: Is not traditionally a shape-memory polymer ; however, recent advances in processing have allowed shape-memory behaviour in PEEK with mechanical activation. This technology has expanded to applications in orthopaedic surgery . Step-growth polymerization In polymer chemistry , step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers , then trimers , longer oligomers and eventually long chain polymers . Many naturally-occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters , polyamides , polyurethanes , etc. Due to

2772-401: Is often between 5/95 and 95/5, but ideally this ratio is between 20/80 and 80/20. The shape-memory polymers are effectively viscoelastic and many models and analysis methods exist. In the amorphous state, polymer chains assume a completely random distribution within the matrix. W represents the probability of a strongly coiled conformation, which is the conformation with maximum entropy, and is

2849-627: Is one of the few plastics compatible with ultra-high vacuum applications, which makes it suitable for aerospace, automotive, and chemical industries. PEEK is used in medical implants , for example in creating a partial replacement skull in neurosurgical applications. PEEK is used in spinal fusion devices and reinforcing rods. It is radiolucent , but it is hydrophobic causing it to not fully fuse with bone. PEEK seals and manifolds are commonly used in fluid applications. PEEK also performs well in high temperature applications (up to 260 °C/500 °F). Because of this and its low thermal conductivity, it

2926-445: Is rather slow, and a high X n is not readily attained. In the presence of a catalyst, there is an acceleration of the rate, and the kinetic expression is altered to which is kinetically first order in each functional group. Hence, and integration gives finally For an externally catalyzed system, the number average degree of polymerization grows proportionally with t {\displaystyle t\,} . The product of

3003-477: Is require since one B molecule has the same quantitative effect as one excess B-B molecule. A monomer with functionality 3 has 3 functional groups which participate in the polymerization. This will introduce branching in a polymer and may ultimately form a cross-linked macrostructure. The point at which this three-dimensional 3D network is formed is known as the gel point , signaled by an abrupt change in viscosity. A more general functionality factor f av

3080-486: Is soluble in concentrated sulfuric acid at room temperature, although dissolution can take a very long time unless the polymer is in a form with a high surface-area-to-volume ratio, such as a fine powder or thin film. It has high resistance to biodegradation. PEEK is used to fabricate items for demanding applications, including bearings , piston parts, pumps , high-performance liquid chromatography columns, compressor plate valves , and electrical cable insulation . It

3157-403: Is viscous flow strain and f α {\displaystyle f_{\alpha }} is strain for t >> t r {\displaystyle t>>t_{r}} . While most traditional shape-memory polymers can only hold a permanent and temporary shape, recent technological advances have allowed the introduction of triple-shape-memory materials. Much as

Polyether ether ketone - Misplaced Pages Continue

3234-499: The Carothers equation We can now obtain: The polydispersity index (PDI), is a measure of the distribution of molecular mass in a given polymer sample. However, for step-growth polymerization the Carothers equation can be used to substitute and rearrange this formula into the following. Therefore, in step-growth when p=1, then the PDI=2. There are two important aspects with regard to

3311-523: The Defense Advanced Research Projects Agency DARPA is testing wings which would change shape by 150%. The realization of a better control over the switching behavior of polymers is seen as key factor to implement new technical concepts. For instance, an accurate setting of the onset temperature of shape recovering can be exploited to tune the release temperature of information stored in a shape memory polymer. This may pave

3388-465: The SMP. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials. SMPs are known to be able to store up to three different shapes in memory. SMPs have demonstrated recoverable strains of above 800%. Two important quantities that are used to describe shape-memory effects are the strain recovery rate ( R r ) and strain fixity rate ( R f ). The strain recovery rate describes

3465-604: The ability of the material to memorize its permanent shape, while the strain fixity rate describes the ability of switching segments to fix the mechanical deformation. where N {\displaystyle N} is the cycle number, ε m {\displaystyle \varepsilon _{m}} is the maximum strain imposed on the material, and ε p ( N ) {\displaystyle \varepsilon _{p}(N)} and ε p ( N − 1 ) {\displaystyle \varepsilon _{p}(N-1)} are

3542-416: The bifunctional monomer or the monofunctional monomer must be precisely adjusted. If the nonstoichiometric imbalance is too large, the polymer molecular weight will be too low. It is important to understand the quantitative effect of the stoichiometric imbalance of reactants on the molecular weight. Also, this is necessary in order to know the quantitative effect of any reactive impurities that may be present in

3619-614: The chain and the sample recovers its original conformation upon removal of the force. If the force is applied for a longer period of time, however, a relaxation process takes place whereby a plastic, irreversible deformation of the sample takes place due to the slipping and disentangling of the polymer chains. To prevent the slipping and flow of polymer chains, cross-linking can be used, both chemical and physical. Representative shape-memory polymers in this category are polyurethanes , polyurethanes with ionic or mesogenic components made by prepolymer method. Other block copolymers also show

3696-404: The control of molecular weight in polymerization. In the synthesis of polymers, one is usually interested in obtaining a product of very specific molecular weight, since the properties of the polymer will usually be highly dependent on molecular weight. Molecular weights higher or lower than the desired weight are equally undesirable. Since the degree of polymerization is a function of reaction time,

3773-439: The desired molecular weight can be obtained by quenching the reaction at the appropriate time. However, the polymer obtained in this manner is unstable in that it leads to changes in molecular weight because the ends of the polymer molecule contain functional groups that can react further with each other. This situation is avoided by adjusting the concentrations of the two monomers so that they are slightly nonstoichiometric. One of

3850-451: The filament material using fused deposition modeling – FDM (or fused filament fabrication – FFF) technology. PEEK filaments have been demonstrated for producing medical devices up to class IIa . With this new filament, it is possible to use the FFF method for different medical applications like dentures . In its solid state PEEK is readily machinable, for example, by CNC milling machines and

3927-636: The first conceived industrial applications was in robotics where shape-memory (SM) foams were used to provide initial soft pretension in gripping. These SM foams could be subsequently hardened by cooling, making a shape adaptive grip. Since this time, the materials have seen widespread usage in, for example, the building industry (foam which expands with warmth to seal window frames), sports wear (helmets, judo and karate suits) and in some cases with thermochromic additives for ease of thermal profile observation. Polyurethane SMPs are also applied as an autochoke element for engines. One field in which SMPs are having

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4004-491: The first polymerization reaction whose results had been predicted by scientific theory. Carothers developed a series of mathematic equations to describe the behavior of step-growth polymerization systems which are still known as the Carothers equations today. Collaborating with Paul Flory , a physical chemist, they developed theories that describe more mathematical aspects of step-growth polymerization including kinetics, stoichiometry, and molecular weight distribution etc. Carothers

4081-468: The half acid-half ester of the dianhydride, instead of the dianhydride itself. Polymerization is accomplished by a two-stage process due to the insolubility of polyimides. The first stage forms a soluble and fusible high-molecular-weight poly(amic acid) in a polar aprotic solvent such as NMP or N,N-dimethylacetamide . The poly(amic aicd) can then be processed into the desired physical form of the final polymer product (e.g., film, fiber, laminate, coating) which

4158-524: The literature is a copolymer consisting of polycyclooctene (PCOE) and poly(5-norbornene-exo,exo-2,3-dicarboxylic anhydride) (PNBEDCA), which was synthesized through ring-opening metathesis polymerization (ROMP). Then the obtained copolymer P(COE-co-NBEDCA) was readily modified by grafting reaction of NBEDCA units with polyhedral oligomeric silsesquioxanes (POSS) to afford a functionalized copolymer P(COE-co-NBEDCA-g-POSS). It exhibits shape-memory effect. The main limitation of physically crosslinked polymers for

4235-524: The market. Recently, this technology has expanded to applications in orthopedic surgery . Additionally, SMPs are now being used in various ophthalmic devices including punctal plugs, glaucoma shunts and intraocular lenses. SMPs are smart materials with potential applications as, e.g., intravenous cannula, self-adjusting orthodontic wires and selectively pliable tools for small scale surgical procedures where currently metal-based shape-memory alloys such as Nitinol are widely used. Another application of SMP in

4312-468: The material to resume its original (permanent) form. Below T trans , flexibility of the segments is at least partly limited. If T m is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above T m and subsequently cooled below T m . These crystallites form covalent netpoints which prevent the polymer from reforming its usual coiled structure. The hard to soft segment ratio

4389-575: The mechanical properties of SMPs to enable traditional plastics processing ( extrusion , blow molding , injection molding , resin transfer molding , etc.) and allows thermoset SMPs in complex geometries. The customizable mechanical properties of traditional SMPs are achievable with high throughput plastics processing techniques to enable mass producible plastic products with thermosetting shape-memory properties: low residual strains, tunable recoverable force and adjustable glass transition temperatures. Shape memory polymers may serve as technology platform for

4466-584: The medical field could be its use in implants: for example minimally invasive, through small incisions or natural orifices, implantation of a device in its small temporary shape. Shape-memory technologies have shown great promise for cardiovascular stents, since they allow a small stent to be inserted along a vein or artery and then expanded to prop it open. After activating the shape memory by temperature increase or mechanical stress, it would assume its permanent shape. Certain classes of shape-memory polymers possess an additional property: biodegradability . This offers

4543-540: The most likely state for an amorphous linear polymer chain. This relationship is represented mathematically by Boltzmann's entropy formula S  =  k  ln  W , where S is the entropy and k is the Boltzmann constant . In the transition from the glassy state to a rubber-elastic state by thermal activation, the rotations around segment bonds become increasingly unimpeded. This allows chains to assume other possibly, energetically equivalent conformations with

4620-474: The nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight . The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain—each person has two hands (= reactive sites). There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers production take place. IUPAC has deprecated

4697-467: The option to develop temporary implants. In the case of biodegradable polymers, after the implant has fulfilled its intended use, e.g. healing/tissue regeneration has occurred, the material degrades into substances which can be eliminated by the body. Thus full functionality would be restored without the necessity for a second surgery to remove the implant. Examples of this development are vascular stents and surgical sutures . When used in surgical sutures,

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4774-414: The other hand, are the segments with the ability to soften past a certain transition temperature ( T trans ) and are responsible for the temporary shape. In some cases this is the glass transition temperature ( T g ) and others the melting temperature ( T m ). Exceeding T trans (while remaining below T perm ) activates the switching by softening these switching segments and thereby allowing

4851-415: The polymer's shape memory effect, it is possible to reprogram the lattice parameter of the structure and consequently tune its diffractive behavior. Another application of SMPs in photonics is shape changing random lasers. By doping SMPs with highly scattering particles such as titania it is possible to tune the light transport properties of the composite. Additionally, optical gain may be introduced by adding

4928-402: The probability decreases. The number fraction distribution is the fraction of x-mers in any system and equals the probability of finding it in solution. Where N is the total number of polymer molecules present in the reaction. The weight fraction distribution is the fraction of x-mers in a system and the probability of finding them in terms of mass fraction. Notes: Substituting from

5005-402: The reactants is present in slight excess. The polymerization then proceeds to a point at which one reactant is completely used up and all the chain ends possess the same functional group of the group that is in excess. Further polymerization is not possible, and the polymer is stable to subsequent molecular weight changes. Another method of achieving the desired molecular weight is by addition of

5082-450: The reaction mixture either initially or that are formed by undesirable side reactions. Impurities with A or B functional groups may drastically lower the polymer molecular weight unless their presence is quantitatively taken into account. More usefully, a precisely controlled stoichiometric imbalance of the reactants in the mixture can provide the desired result. For example, an excess of diamine over an acid chloride would eventually produce

5159-418: The shape change into the permanent form is activated by a predetermined external stimulus. The secret behind these materials lies in their molecular network structure, which contains at least two separate phases. The phase showing the highest thermal transition, T perm , is the temperature that must be exceeded to establish the physical crosslinks responsible for the permanent shape. The switching segments, on

5236-411: The shape-memory application is irreversible deformation during memory programming due to the creep . The network polymer can be synthesized by either polymerization with multifunctional (3 or more) crosslinker or by subsequent crosslinking of a linear or branched polymer. They form insoluble materials which swell in certain solvents. This material can be made by using excess diisocyanate or by using

5313-592: The shape-memory effect, such as, block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers containing polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer made from poly(2-methyl-2-oxazoline) and polytetrahydrofuran . A linear, amorphous polynorbornene (Norsorex, developed by CdF Chemie/Nippon Zeon) or organic-inorganic hybrid polymers consisting of polynorbornene units that are partially substituted by polyhedral oligo silsesquioxane (POSS) also have shape-memory effect. [REDACTED] Another example reported in

5390-555: The shape-memory property of SMPs enables wound closure with self-adjusting optimal tension, which avoids tissue damage due to overtightened sutures and does support healing and regeneration. SMPs have also potential for use as compression garments and hands-free door openers, whereby the latter can be produced via so-called 4D printing. Further potential applications include self-repairing structural components, such as e.g. automobile fenders in which dents are repaired by application of temperature. After an undesired deformation, such as

5467-434: The strains of the sample in two successive cycles in the stress-free state before yield stress is applied. Shape-memory effect can be described briefly as the following mathematical model: where E g {\displaystyle E_{g}} is the glassy modulus, E r {\displaystyle E_{r}} is the rubbery modulus, f I R {\displaystyle f_{IR}}

5544-487: The term step-growth polymerization , and recommends use of the terms polyaddition (when the propagation steps are addition reactions and molecules are not evolved during these steps) and polycondensation (when the propagation steps are condensation reactions and molecules are evolved during these steps). Most natural polymers being employed at early stage of human society are of condensation type. The synthesis of first truly synthetic polymeric material, bakelite ,

5621-400: The transition between those is often induced by temperature change. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field , light or solution. Like polymers in general, SMPs cover a wide range of properties from stable to biodegradable , from soft to hard, and from elastic to rigid, depending on the structural units that constitute

5698-503: The use of surface-modified super-paramagnetic nanoparticles. When introduced into the polymer matrix, remote actuation of shape transitions is possible. An example of this involves the use of oligo (e-caprolactone)dimethacrylate/butyl acrylate composite with between 2 and 12% magnetite nanoparticles . Nickel and hybrid fibers have also been used with some degree of success. Shape-memory polymers differ from shape memory alloys (SMAs) by their glass transition or melting transition from

5775-738: The way for the monitoring of temperature abuses of food or pharmaceuticals. Recently, a new manufacturing process, mnemosynation, was developed at Georgia Tech to enable mass production of crosslinked SMP devices, which would otherwise be cost-prohibitive using traditional thermoset polymerization techniques. Mnemosynation was named for the Greek goddess of memory, Mnemosyne , and is the controlled imparting of memory on an amorphous thermoplastic materials utilizing radiation-induced covalent crosslinking, much like vulcanization imparts recoverable elastomeric behavior on rubbers using sulfur crosslinks. Mnemosynation combines advances in ionizing radiation and tuning

5852-458: Was announced by Leo Baekeland in 1907, through a typical step-growth polymerization fashion of phenol and formaldehyde . The pioneer of synthetic polymer science, Wallace Carothers , developed a new means of making polyesters through step-growth polymerization in 1930s as a research group leader at DuPont . It was the first reaction designed and carried out with the specific purpose of creating high molecular weight polymer molecules, as well as

5929-495: Was introduced by Wallace Carothers in 1929, and refers to the type of products, respectively: The distinction between "step-growth polymerization" and "chain-growth polymerization" was introduced by Paul Flory in 1953, and refers to the reaction mechanisms , respectively: This technique is usually compared with chain-growth polymerization to show its characteristics. Classes of step-growth polymers are: A monomer with functionality of 3 or more will introduce branching in

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