Reactive compatibilization

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Reactive compatibilization is the process of modifying a mixed immiscible blend of polymers to arrest phase separation and allow for the formation of a stable, long-term continuous phase. It is done via the addition of a reactive polymer, miscible with one blend component and reactive towards functional groups on the second component, which result in the "in-situ" formation of block or grafted copolymers. [1]

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A large number of commercial polymeric products are derived from the blending of two or more polymers to achieve a favorable balance of physical properties. However, since most polymer blends are immiscible, it is rare to find a pair of polymers that both are miscible and have desired characteristics. An example of such pair is the miscible resin NORYL™, a mix of poly(phenylene oxide) and polystyrene. [2] Immiscible blends will phase separate and form a dispersed phase, which may improve physical properties (figure 1). DuPont’s rubber toughened Nylon consists of small particles of poly(cis-isoprene) (natural rubber) in a Nylon matrix that toughen the material by arresting crack propagation.

The NORYL family of modified resins consists of amorphous blends of Polyphenylene Oxides (PPO) or polyphenylene ether (PPE) resins with polystyrene. They combine the inherent benefits of PPE resin, with excellent dimensional stability, good processibility and low density.

Natural rubber Polymer harvested from certain trees

Natural rubber, also called India rubber or caoutchouc, as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds, plus water. Thailand and Indonesia are two of the leading rubber producers. Forms of polyisoprene that are used as natural rubbers are classified as elastomers.


Miscibility of Polymer Blends

The Gibbs free energy of mixing, , must be negative for a blend to be miscible. According to Flory-Huggins theory, a revision of regular solution theory, the entropy change per mole of lattice sites of blending polymer 1 and polymer 2 is

Gibbs free energy gibbs energy of formation

In thermodynamics, the Gibbs free energy is a thermodynamic potential that can be used to calculate the maximum of reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a thermodynamically closed system ; this maximum can be attained only in a completely reversible process. When a system transforms reversibly from an initial state to a final state, the decrease in Gibbs free energy equals the work done by the system to its surroundings, minus the work of the pressure forces.

Flory–Huggins solution theory is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing. The result is an equation for the Helmholtz free energy change for mixing a polymer with a solvent. Although it makes simplifying assumptions, it generates useful results for interpreting experiments.

, where ΔS is the change in entropy of mixing, R is the gas constant, Φ is the volume fraction of each polymer, and x is the number of segments of each polymer. [3] x1 and x2 increase with higher degrees of polymerization and thus molecular weight. Since most useful polymers are high in molecular weight, the change in entropy experienced from the mixing of two large polymer chains is very low, and typically does not bring the Gibbs free energy low enough to constitute miscibility.

The gas constant is also known as the molar, universal, or ideal gas constant, denoted by the symbol R or R and is equivalent to the Boltzmann constant, but expressed in units of energy per temperature increment per mole, i.e. the pressure–volume product, rather than energy per temperature increment per particle. The constant is also a combination of the constants from Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law. It is a physical constant that is featured in many fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation.

Compatibilization

Most processed polymer mixes consist of a dispersed phase in a more continuous matrix of the other component. The formation, size, and concentration of this disperse phase are typically optimized for specific mechanical properties. If the morphology is not stabilized, the dispersed phase may coalesce under heat or stress from the environment or further processing. [4] This coalescence may result in diminished properties (brittleness and discoloration) due to the induced phase separation. These morphologies can be stabilized by sufficient interfacial adhesion or lowered interfacial tension between the two phases.

A common technique involves functionalizing one monomer. For example, Nylon-rubber bands are polymerized with functionalized rubber to produce graft or block copolymers. The added structures make it no longer favorable to coalesce and/or increase the steric hindrance in the interfacial area where phase separation would occur.

Related Research Articles

Dynamic mechanical analysis is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus. The temperature of the sample or the frequency of the stress are often varied, leading to variations in the complex modulus; this approach can be used to locate the glass transition temperature of the material, as well as to identify transitions corresponding to other molecular motions.

In statistical mechanics, a semi-classical derivation of the entropy that does not take into account the indistinguishability of particles, yields an expression for the entropy which is not extensive. This leads to a paradox known as the Gibbs paradox, after Josiah Willard Gibbs who proposed this thought experiment in 1874‒1875. The paradox allows for the entropy of closed systems to decrease, violating the second law of thermodynamics. A related paradox is the "mixing paradox". If one takes the perspective that the definition of entropy must be changed so as to ignore particle permutation, the paradox is averted.

Copolymer when two or more different monomers unite together to polymerize, their result is called a copolymer and its process is called copolymerization

A copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained by copolymerization of two monomer species are sometimes called bipolymers. Those obtained from three and four monomers are called terpolymers and quaterpolymers, respectively.

In thermodynamics the entropy of mixing is the increase in the total entropy when several initially separate systems of different composition, each in a thermodynamic state of internal equilibrium, are mixed without chemical reaction by the thermodynamic operation of removal of impermeable partition(s) between them, followed by a time for establishment of a new thermodynamic state of internal equilibrium in the new unpartitioned closed system.

In chemistry, a regular solution is a solution whose entropy of mixing is equal to that of an ideal solution with the same composition, but is non-ideal due to a nonzero enthalpy of mixing. Such a solution is formed by random mixing of components without strong specific interactions, and its behavior diverges from that of an ideal solution only moderately. Its entropy of mixing is equal to that of an ideal solution with the same composition, due to random mixing without strong specific interactions. For two components

A polymer blend, or polymer mixture, is a member of a class of materials analogous to metal alloys, in which at least two polymers are blended together to create a new material with different physical properties.

Miscibility

Miscibility is the property of two substances to mix in all proportions, forming a homogeneous solution. The term is most often applied to liquids but applies also to solids and gases. Water and ethanol, for example, are miscible because they mix in all proportions.

Copolymers are polymers that are synthesized with more than one kind of repeat unit. Gradient copolymers exhibit a gradual change in monomer composition from predominantly one species to predominantly the other, unlike with block copolymers, which have an abrupt change in composition, and random copolymers, which have no continuous change in composition . In the gradient copolymer, as a result of the gradual compositional change along the length of the polymer chain less intrachain and interchain repulsion are observed.

Short Fiber Reinforced Blends are partial case of ternary composites, i.e. composites prepared of three ingredients. In particular they can be considered as a combination of an immiscible polymer blend and a short fiber reinforced composite. These blends have the potential to integrate the easy processing solutions available for short fiber reinforced composites with the high mechanical performance of continuous fiber reinforced composites. The performance of these complex, ternary systems is controlled by their morphology.

Rubber toughening is a process in which rubber nanoparticles are interspersed within a polymer matrix to increase the mechanical robustness, or toughness, of the material. By "toughening" a polymer it is meant that the ability of the polymeric substance to absorb energy and plastically deform without fracture is increased. Considering the significant advantages in mechanical properties that rubber toughening offers, most major thermoplastics are available in rubber-toughened versions; for many engineering applications, material toughness is a deciding factor in final material selection.

A phase field model is a mathematical model for solving interfacial problems. It has mainly been applied to solidification dynamics, but it has also been applied to other situations such as viscous fingering, fracture dynamics, hydrogen embrittlement, vesicle dynamics, etc.

Temperature-responsive polymer

Temperature-responsive polymers or thermoresponsive polymers are polymers that exhibit a drastic and discontinuous change of their physical properties with temperature. The term is commonly used when the property concerned is solubility in a given solvent, but it may also be used when other properties are affected. Thermoresponsive polymers belong to the class of stimuli-responsive materials, in contrast to temperature-sensitive materials, which change their properties continuously with environmental conditions. In a stricter sense, thermoresponsive polymers display a miscibility gap in their temperature-composition diagram. Depending on whether the miscibility gap is found at high or low temperatures, an upper or lower critical solution temperature exists, respectively.

The lower critical solution temperature (LCST) or lower consolute temperature is the critical temperature below which the components of a mixture are miscible for all compositions. The word lower indicates that the LCST is a lower bound to a temperature interval of partial miscibility, or miscibility for certain compositions only.

Emulsion dispersion

An emulsion dispersion is thermoplastics or elastomers suspended in a waterphase with help of emulsifiers.

In electrochemistry, ITIES is an acronym for the "interface between two immiscible electrolytesolutions". Usually, one electrolyte is an aqueous electrolyte composed of hydrophilic ions such as NaCl dissolved in water and the other electrolyte is a lipophilic salt such as tetrabutylammonium tetraphenylborate dissolved in an organic solvent immiscible with water such as nitrobenzene, or 1,2-dichloroethane.

In probability theory and directional statistics, a wrapped probability distribution is a continuous probability distribution that describes data points that lie on a unit n-sphere. In one dimension, a wrapped distribution will consist of points on the unit circle. If φ is a random variate in the interval (-∞,∞) with probability density function p(φ), then z = e i φ will be a circular variable distributed according to the wrapped distribution pzw(z) and θ=arg(z) will be an angular variable in the interval (-π,π ] distributed according to the wrapped distribution pw.

Macroemulsions are homogenous transparent thermodynamically unstable sytems(particle sizes range from 5-140 nm,which form spontaneously when mixed in the correct ratio. Macroemulsions scatter light effectively and therefore appear milky, because their droplets are greater than a wavelength of light. They are part of a larger family of emulsions along with microemulsions. As with all emulsions, one phase serves as the dispersing agent. It is often called the continuous or outer phase. The remaining phase are disperse or inner phase, because the liquid droplets are finely distributed amongst the larger continuous phase droplets. This type of emulsion is thermodynamically unstable, but can be stabilized for a period of time with applications of kinetic energy. Surfactants are used to reduce the interfacial tension between the two layers, and induce macroemulsion stability for a useful amount of time.

The surfactant’s critical micelle concentration (CMC) plays a factor in Gibbs free energy of micellization. The exact concentration of the surfactants that yield the aggregates being thermodynamically soluble is the CMC. The Krafft temperature determines the solubility of the surfactants which in turn is the temperature that CMC is achieved. There are many parameters that affect the CMC. The interaction between the hydrophilic heads and the hydrophobic tails play a part, as well as the concentration of salt within the solution and surfactants.

Compatibilization in polymer chemistry is the addition of a substance to an immiscible blend of polymers that will increase their stability. Polymer blends are typically described by coarse, unstable phase morphologies. This results in poor mechanical properties. Compatibilizing the system will make a more stable and better blended phase morphology by creating interactions between the two previously immiscible polymers. Not only does this enhance the mechanical properties of the blend, but it often yields properties that are generally not attainable in either single pure component.

References

  1. ↑ Cor, Koning; Van Duin, Martin; Pagnoulle, Christophe; Jérôme, Robert (1998). "Strategies for Compatibilization of Polymer Blends". Progress in Polymer Science. 23 (4): 707–757.
  2. ↑ "NORYL™ RESIN". Saudi Basic Industries Corporation (SABIC). Retrieved 4 February 2015.
  3. ↑ Rudin, Alfred, and Phillip Choi. The Elements of Polymer Science and Engineering. 3rd. Oxford: Academic Press, 2013. Print.
  4. ↑ Xanthos, M. (1992). Reactive Extrusion, Principles and Practice. Hanser Gardner Publications. pp. 75–199.
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