Gradient copolymer

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Figure 1: Example of a (a) diblock copolymer, (b) gradient copolymer and (c) random copolymer Example of gradient copolymer.png
Figure 1: Example of a (a) diblock copolymer, (b) gradient copolymer and (c) random copolymer

In polymer chemistry, gradient copolymers are copolymers in which the change in monomer composition is gradual from predominantly one species to predominantly the other, [1] unlike with block copolymers, which have an abrupt change in composition, [2] [3] and random copolymers, which have no continuous change in composition (see Figure 1). [4] [5] 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. [6]

Contents

The development of controlled radical polymerization as a synthetic methodology in the 1990s allowed for increased study of the concepts and properties of gradient copolymers because the synthesis of this group of novel polymers was now straightforward.

Due to the similar properties of gradient copolymers to that of block copolymers, they have been considered as a cost-effective alternative in applications for other preexisting copolymers. [6]

Polymer Composition

Statistical composition of gradient copolymer chains produced by coordinative chain transfer polymerisation 1-s2.0-S0009250917307546-fx1 lrg.jpg
Statistical composition of gradient copolymer chains produced by coordinative chain transfer polymerisation

In the gradient copolymer, there is a continuous change in monomer composition along the polymer chain (see Figure 2). This change in composition can be depicted in a mathematical expression. The local composition gradient fraction is described by molar fraction of monomer 1 in the copolymer and degree of polymerization and its relationship is as follows: [6]

The above equation supposes all of the local monomer composition is continuous. To make up for this assumption, another equation of ensemble average is used: [6]

The refers ensemble average of the local chain composition, refers degree of polymerization, refers number of polymer chains in the sample and refers composition of polymer chain i at position .

This second equation identifies the average composition over all present polymer chains at a given position, . [6]

Synthesis

Prior to the development of controlled radical polymerization (CRP), gradient copolymers (as distinguished from statistical copolymers) were not synthetically possible. While a "gradient" can be achieved through compositional drift due to a difference in reactivity of the two monomers, this drift will not encompass the entire possible compositional range. All of the common CRP methods [8] including atom transfer radical polymerization and Reversible addition−fragmentation chain transfer polymerization as well as other living polymerization techniques including anionic addition polymerization and ring-opening polymerization have been used to synthesize gradient copolymers. [6]

The gradient can be formed through either a spontaneous or a forced gradient. Spontaneous gradient polymerization is due to a difference in reactivity of the monomers. The resulting change in composition throughout the polymerization creates an inconsistent gradient along the polymer. Forced gradient polymerization involves varying the comonomer composition of the feed being throughout the reaction time. Because the rate of addition of the second monomer influences the polymerization and therefore properties of the formed polymer, continuous information about the polymer composition is vital. The online compositional information is often gathered through automatic continuous online monitoring of polymerization reactions, a process which provides in situ information allowing for constant composition adjustment to achieve the desired gradient composition.

Properties

The wide range of composition possible in a gradient polymer due to the variety of monomers incorporated and the change of the composition results in a large variety of properties. In general, the glass transition temperature (Tg) is broad in comparison with the homopolymers. Micelles of the gradient copolymer can form when the gradient copolymer concentration is too high in a block copolymer solution. As the micelles form, the micelle diameter actually shrinks creating a "reel in" effect. Contrast matching SANS experiments revealed that the outer part of the core of the gradient copolymer micelles has a distinctly higher density than the middle of the core. This finding was attributed to back-folding of chains resulting from hydrophilic-hydrophobic interactions, leading to a new type of micelles that was referred to as micelles with a "bitterball-core" structure. [9]

The composition can be determined by gel permeation chromatography(GPC) and nuclear magnetic resonance (NMR). Generally the composition has a narrow polydispersity index (PDI) and the molecular weight increases with time as the polymer forms.

Applications

Compatibilizing phase-separated polymer blends

Figure 3: a) random copolymer blend with annealing b) gradient copolymer blend with annealing Wikipic3 1.png
Figure 3: a) random copolymer blend with annealing b) gradient copolymer blend with annealing

For the compatiabilization of immiscible blends, the gradient copolymer can be used by improving mechanical and optical properties of immiscible polymers and decreasing its dispersed phase to droplet size. [10] The compatibilization has been tested by reduction in interfacial tension and steric hindrance against coalescence. This application is not available for block and graft copolymer because of its very low critical micelle concentration (cmc). However, the gradient copolymer, which has higher cmc and exhibits a broader interfacial coverage, can be applied to effective blend compatibilizers. [11]

A small amount of gradient copolymer (i.e.styrene/4-hydroxystyrene) is added to a polymer blend (i.e. polystyrene/polycaprolactone) during melt processing. The resulting interfacial copolymer helps to stabilize the dispersed phase due to the hydrogen-bonding effects of hydroxylstyrene with the polycaprolactone ester group.

Impact modifiers and sound or vibration dampers

The gradient copolymer have very broad glass transition temperature (Tg) in comparison with other copolymers, at least four times bigger than that of a random copolymer. This broad glass transition is one of the important features for vibration and acoustic damping applications. The broad Tg gives wide range of mechanical properties of material. The glass transition breadth can be adjusted by selection of monomers with different degrees of reactivity in their controlled radical polymerization (CRP). The strongly segregated styrene/4-hydroxystyrene (S/HS) gradient copolymer is used to study damping properties due to its unusual broad glass transition breadth. [6]

Potential applications

There are many possible applications for gradient copolymer like pressure-sensitive adhesives, wetting agent, coating, or dispersion. However, these applications are not proved about its practical performance and stability as gradient copolymers. [12] [6] Recent studies have evaluated gradient copolymers as potential drug delivery carriers. [13] Additionally, gradient copolymers have been shown the best 19F MRI signal-to-noise ratio in comparison with the analogue block copolymer structures, making them most promising as 19F MRI contrast agents. [14]

See also

Related Research Articles

<span class="mw-page-title-main">Polymer</span> Substance composed of macromolecules with repeating structural units

A polymer is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass, relative to small molecule compounds, produces unique physical properties including toughness, high elasticity, viscoelasticity, and a tendency to form amorphous and semicrystalline structures rather than crystals.

In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.

In polymer chemistry, emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomers, and surfactants. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer are emulsified in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

<span class="mw-page-title-main">Micelle</span> Group of fatty molecules suspended in liquid by soaps and/or detergents

A micelle or micella is an aggregate of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension. A typical micelle in water forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre.

<span class="mw-page-title-main">Copolymer</span> Polymer derived from more than one species of monomer

In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are sometimes called bipolymers. Those obtained from three and four monomers are called terpolymers and quaterpolymers, respectively. Copolymers can be characterized by a variety of techniques such as NMR spectroscopy and size-exclusion chromatography to determine the molecular size, weight, properties, and composition of the material.

<span class="mw-page-title-main">Radical polymerization</span> Polymerization process involving free radicals as repeating units

In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks. Free radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.


In polymer chemistry, anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but traditionally vinyl monomers are used. Often anionic polymerization involves living polymerizations, which allows control of structure and composition.

<span class="mw-page-title-main">Reversible addition−fragmentation chain-transfer polymerization</span> Chemical reaction that produces polymers

Reversible addition−fragmentation chain-transfer or RAFT polymerization is one of several kinds of reversible-deactivation radical polymerization. It makes use of a chain-transfer agent (CTA) in the form of a thiocarbonylthio compound to afford control over the generated molecular weight and polydispersity during a free-radical polymerization. Discovered at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia in 1998, RAFT polymerization is one of several living or controlled radical polymerization techniques, others being atom transfer radical polymerization (ATRP) and nitroxide-mediated polymerization (NMP), etc. RAFT polymerization uses thiocarbonylthio compounds, such as dithioesters, thiocarbamates, and xanthates, to mediate the polymerization via a reversible chain-transfer process. As with other controlled radical polymerization techniques, RAFT polymerizations can be performed under conditions that favor low dispersity and a pre-chosen molecular weight. RAFT polymerization can be used to design polymers of complex architectures, such as linear block copolymers, comb-like, star, brush polymers, dendrimers and cross-linked networks.

Poloxamers are nonionic triblock copolymers composed of a central hydrophobic chain of polyoxypropylene flanked by two hydrophilic chains of polyoxyethylene. The word poloxamer was coined by BASF inventor, Irving Schmolka, who received the patent for these materials in 1973. Poloxamers are also known by the trade names Pluronic, Kolliphor, and Synperonic.

Supramolecular polymers are a subset of polymers where the monomeric units are connected by reversible and highly directional secondary interactions–that is, non-covalent bonds. These non-covalent interactions include van der Waals interactions, hydrogen bonding, Coulomb or ionic interactions, π-π stacking, metal coordination, halogen bonding, chalcogen bonding, and host–guest interaction. Their behavior can be described by the theories of polymer physics) in dilute and concentrated solution, as well as in the bulk.

The Mayo–Lewis equation or copolymer equation in polymer chemistry describes the distribution of monomers in a copolymer. It was proposed by Frank R. Mayo and Frederick M. Lewis.

Small-angle X-ray scattering (SAXS) is a small-angle scattering technique by which nanoscale density differences in a sample can be quantified. This means that it can determine nanoparticle size distributions, resolve the size and shape of (monodisperse) macromolecules, determine pore sizes, characteristic distances of partially ordered materials, and much more. This is achieved by analyzing the elastic scattering behaviour of X-rays when travelling through the material, recording their scattering at small angles. It belongs to the family of small-angle scattering (SAS) techniques along with small-angle neutron scattering, and is typically done using hard X-rays with a wavelength of 0.07 – 0.2 nm. Depending on the angular range in which a clear scattering signal can be recorded, SAXS is capable of delivering structural information of dimensions between 1 and 100 nm, and of repeat distances in partially ordered systems of up to 150 nm. USAXS can resolve even larger dimensions, as the smaller the recorded angle, the larger the object dimensions that are probed.

<span class="mw-page-title-main">Styrene maleic anhydride</span> Chemical compound

Styrene maleic anhydride is a synthetic polymer that is built-up of styrene and maleic anhydride monomers. In one copolymer, the monomers can be almost perfectly alternating. but (random) copolymerisation with less than 50% maleic anhydride content is also possible. The polymer is formed by a radical polymerization, using an organic peroxide as the initiator. The main characteristics of SMA copolymer are its transparent appearance, high heat resistance, high dimensional stability, and the specific reactivity of the anhydride groups. The latter feature results in the solubility of SMA in alkaline (water-based) solutions and dispersion.

<span class="mw-page-title-main">Temperature-responsive polymer</span> Polymer showing drastic changes in physical properties with temperature

Temperature-responsive polymers or thermoresponsive polymers are polymers that exhibit drastic and discontinuous changes in 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, either an upper critical solution temperature (UCST) or a lower critical solution temperature (LCST) exists.

Catalytic chain transfer (CCT) is a process that can be incorporated into radical polymerization to obtain greater control over the resulting products.

<span class="mw-page-title-main">Reversible-deactivation radical polymerization</span> Type of chain polymerization

In polymer chemistry, reversible-deactivation radical polymerizations (RDRPs) are members of the class of reversible-deactivation polymerizations which exhibit much of the character of living polymerizations, but cannot be categorized as such as they are not without chain transfer or chain termination reactions. Several different names have been used in literature, which are:

<span class="mw-page-title-main">Graft polymer</span> Polymer with a backbone of one composite and random branches of another composite

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Composition drift occurs during the process of free radical copolymerization causing variation in the instantaneous mole fraction of a monomer added to copolymer, therefore altering the chemical composition of the copolymer over the period of conversion.

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.

<span class="mw-page-title-main">Allyl glycidyl ether</span> Chemical compound

Allyl glycidyl ether is an organic compound used in adhesives and sealants and as a monomer for polymerization reactions. It is formally the condensation product of allyl alcohol and glycidol via an ether linkage. Because it contains both an alkene and an epoxide group, either group can be reacted selectively to yield a product where the other functional group remains intact for future reactions.

References

  1. ↑ Kryszewski, M (1998). "Gradient Polymers and Copolymers". Polymers for Advanced Technologies. 9 (4): 224–259. doi:10.1002/(SICI)1099-1581(199804)9:4<244::AID-PAT748>3.0.CO;2-J. ISSN   1042-7147.
  2. ↑ Muzammil, Iqbal; Li, Yupeng; Lei, Mingkai (2017). "Tunable wettability and pH-responsiveness of plasma copolymers of acrylic acid and octafluorocyclobutane". Plasma Processes and Polymers. 14 (10): 1700053. doi:10.1002/ppap.201700053. S2CID   104161308.
  3. ↑ Beginn, Uwe (2008). "Gradient Copolymer". Colloid Polym Sci. 286 (13): 1465–1474. doi:10.1007/s00396-008-1922-y. S2CID   189866561.
  4. ↑ Matyjaszewski, Krzyszytof; Michael J. Ziegler; Stephen V. Arehart; Dorota Greszta; Tadeusz Pakula (2000). "Gradient Copolymers by Atom Transfer Radical Copolymerization". J. Phys. Org. Chem. 13 (12): 775–786. doi:10.1002/1099-1395(200012)13:12<775::aid-poc314>3.0.co;2-d.
  5. ↑ Cowie, J.M.G.; Valeria Arrighi (2008). Polymers: Chemistry and Physics of Modern Materials (Third ed.). CRC Press. pp. 147–148. ISBN   9780849398131.
  6. 1 2 3 4 5 6 7 8 Mok, Michelle; Jungki Kim; John M. Torkelson (2008). "Gradient Copolymers with Broad Glass Transition Temperature Regions: Design of Purely Interphase Compositions for Damping Applications". Journal of Polymer Science. 46 (1): 48–58. Bibcode:2008JPoSB..46...48M. doi:10.1002/polb.21341.
  7. ↑ Kryven, Ivan; Zhao, Yutian R.; McAuley, Kimberley B.; Iedema, Piet (2018). "A deterministic model for positional gradients in copolymers". Chemical Engineering Science. 177: 491–500. Bibcode:2018ChEnS.177..491K. doi: 10.1016/j.ces.2017.12.017 . hdl: 11245.1/e6aa0c74-10cd-4d83-87d3-5c7bfe315364 . ISSN   0009-2509.
  8. ↑ Davis, Kelly; Krzysztof Matyjaszewski (2002). Statistical, Gradient, Block, and Graft Copolymers by Controlled/Living Radical Polymerizations. Advances in Polymer Science. Vol. 159. pp. 1–13. doi:10.1007/3-540-45806-9_1. ISBN   978-3-540-43244-9.
  9. ↑ Filippov, Sergey K.; Verbraeken, Bart; Konarev, Petr V.; Svergun, Dmitri I.; Angelov, Borislav; Vishnevetskaya, Natalya S.; Papadakis, Christine M.; Rogers, Sarah; Radulescu, Aurel; Courtin, Tim; Martins, José C.; Starovoytova, Larisa; Hruby, Martin; Stepanek, Petr; Kravchenko, Vitaly S. (2017). "Block and Gradient Copoly(2-oxazoline) Micelles: Strikingly Different on the Inside". The Journal of Physical Chemistry Letters. 8 (16): 3800–3804. doi:10.1021/acs.jpclett.7b01588.
  10. ↑ Ramic, Anthony J.; Julia C. Stehlin; Steven D. Hudson; Alexander M. Jamieson; Ica Manas-Zloczower (2000). "Influence of Block Copolymer on Droplet Brekup and Coalescence in Model Immiscible Polymer Blends". Macromolecules. 33 (2): 371–374. Bibcode:2000MaMol..33..371R. doi:10.1021/ma990420c.
  11. ↑ Kim, Jungki; Maisha K. Gray; Hongying Zhou; SonBinh T. Nguyen; John M. Torkelson (Feb 22, 2005). "Polymer Blend Compatibilization by Gradient Copolymer Addition during Melt Processing: Stabilization nof Dispersed Phase to Static Coarsening". Macromolecules. 38 (4): 1037–1040. Bibcode:2005MaMol..38.1037K. doi:10.1021/ma047549t.
  12. ↑ Muzammil, Iqbal; Li, Yupeng; Lei, Mingkai (2017). "Tunable wettability and pH-responsiveness of plasma copolymers of acrylic acid and octafluorocyclobutane". Plasma Processes and Polymers. 14 (10): 1700053. doi:10.1002/ppap.201700053. S2CID   104161308.
  13. ↑ Sedlacek, Ondrej; Bardoula, Valentin; Vuorimaa-Laukkanen, Elina; Gedda, Lars; Edwards, Katarina; Radulescu, Aurel; Mun, Grigoriy A.; Guo, Yong; Zhou, Junnian; Zhang, Hongbo; Nardello-Rataj, Véronique; Filippov, Sergey K.; Hoogenboom, Richard (2022). "Influence of Chain Length of Gradient and Block Copoly(2-oxazoline)s on Self-Assembly and Drug Encapsulation". Small. 18 (17): 2106251. doi:10.1002/smll.202106251. ISSN   1613-6829.
  14. ↑ Kaberov, Leonid I.; Kaberova, Zhansaya; Murmiliuk, Anastasiia; Trousil, Jiří; Sedláček, Ondřej; Konefal, Rafal; Zhigunov, Alexander; Pavlova, Ewa; Vít, Martin; Jirák, Daniel; Hoogenboom, Richard; Filippov, Sergey K. (2021). "Fluorine-Containing Block and Gradient Copoly(2-oxazoline)s Based on 2-(3,3,3-Trifluoropropyl)-2-oxazoline: A Quest for the Optimal Self-Assembled Structure for 19F Imaging". Biomacromolecules. 22 (7): 2963–2975. doi:10.1021/acs.biomac.1c00367.
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