Review on the Preparation and Properties of Clay-based

Keywords: clay-based nanocomposites, grafting from, grafting to, polymer brush, tethered chains

*Corresponding Author: [email protected]

Review on the Preparation and Properties of Clay-based Nanocomposites with

Covalently-bound Polymer Architecture

Philippine Journal of Science148 (4): 813-824, December 2019ISSN 0031 - 7683Date Received: 01 Aug 2019

David P. Penaloza Jr.*

Chemistry Department, College of Science, De La Salle University, Manila 1004 Philippines

The concept of tethering polymer chains on a solid substrate has attracted considerable attention from both practical and theoretical points of view. Materials with covalently bound polymer architecture are often referred to as polymer brushes. Polymer brushes, typically prepared either through “grafting to” or “grafting from” approaches, have interesting mechanical and chemical properties and represent a class of materials that can provide a wide range of applications that include stabilization of colloids, adhesion, and polymer coatings. In this review, two general methods of preparing polymer brush nanocomposites were discussed leading to covalently end-tethered polymer architecture and the resulting properties exhibited by these hybrid materials.

REVIEW ARTICLE

INTRODUCTIONThe interest in polymer nanocomposite materials has grown in recent years because of their excellent properties. By incorporating inorganic fillers like layered silicate clays in polymers, the resulting hybrid materials have better physical properties over their neat polymers and conventional composite counterparts (Alves et al. 2019; Hasegawa et al. 1998; Javaid et al. 2018; Kouser et al. 2018; Medhat Bojnourd and Pakizeh 2018; Penaloza 2019a, 2019b; Penaloza and Seery 2019; Powell and Beall 2006; Usuki et al. 1997; Yano et al. 1993, 1997). These physical property enhancements include higher modulus and strength plus flame retardant, barrier, and optical characteristics (Arjmandi et al. 2016, Chiu et al. 2004, Cui et al. 2015, Follain et al. 2016, Kong et al. 2017, Lan and Pinnavaia 1994, Martino et al. 2017, Ogawa and Kuroda 1997, Pramanik et al. 2003, Sharma et al. 2017, Takahashi et al. 2017, Yeh et al. 2001, Yoon et al. 2003). Also, the random dispersion of individual silicate platelets

of about nanometer thickness in a polymer resin results in additional properties like flame retardancy, enhanced barrier properties and ablation resistance that are not displayed in either of the components (Alexandre and Dubois 2000, Cui et al. 2015, Das et al. 2017, Giannelis 1996, Guin et al. 2015, Liu et al. 2015).

The most commonly used layered clay as a filler for polymer nanocomposites is montmorillonite (MMT). It has a layered morphology consisting of stacks of negatively-charged silicate platelets. Each platelet is about a nanometer in thickness and has lateral dimensions from 100 nm to a micron. The platelets are held together by highly exchangeable cations that are found between the clay galleries (Bishop et al. 1994, Farmer and Russell 1964, Gieseking 1939, Okada and Usuki 1995, Penaloza 2019a, Penaloza and Seery 2018).

In using layered clays as inorganic fillers for polymers, the clay particles may exist as aggregates or may become intercalated or exfoliated. The exfoliated structure is the most desired clay morphology in a nanocomposite. In most

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clay-based polymer nanocomposites where remarkable property enhancements were shown, the clay fillers have been observed to exhibit high degree of delamination of the layered silicates. In an exfoliated form, the original stacks of clay platelets are destroyed leading to individual clay platelets. The random dispersion of the clay platelets dramatically increased the interfacial area between the clay fillers and the polymer matrix (Bee et al. 2018, Dhatarwal et al. 2017, Murima et al. 2016, Piazza et al. 2015, Prabakaran et al. 2015, Zhu et al. 2019).

One particular challenge in using layered silicates like MMT in the preparation of polymer-layered silicate nanocomposites is getting the clay nanofillers to homogeneously mix with the matrix polymer. Modifying the clay filler before its incorporation in the polymer bulk is thus important. The most common method to modify clay fillers is to substitute the highly exchangeable cations originally found in the interlayers with long alkyl ammonium salts via ion exchange reactions (Jlassi et al. 2017, Lan and Pinnavaia 1994, Messersmith and Giannelis 1995, Penaloza 2019a, Xing et al. 2019, Zeggai et al. 2015). The intercalation of the alkyl chains in the galleries of the clay fillers leads to the expansion of the spaces between platelets. The intercalating agents also give more hydrophobic character to the modified clay fillers, making them more compatible with organic polymers. Another method of clay modification is using organochlorosilanes. The use of silylation reactions to modify the surface of MMT clays is possible due to the presence of hydroxyl groups in the external and internal surfaces of MMT (Bertuoli et al. 2014, Di Gianni et al. 2008, Herrera et al. 2005, Huskić et al. 2013, Penaloza and Seery 2018, Piscitelli et al. 2010, Scarfato et al. 2016, Zha et al. 2014).

Recently, MMT clays have been modified by controlled living polymerization techniques that include atom transfer radical polymerization, reversible addition-fragmentation chain transfer polymerization, nitroxide mediated polymerization, and ring-opening metathesis polymerization (ROMP) (Advincula 2006, Fujiki et al. 1999, Hu et al. 2017, Huang et al. 2017, Jiang et al. 2010, Shi et al. 2017, Ueda et al. 2008, Zhou et al. 2001). The concept behind this approach is to promote clay exfoliation by facilitating the intercalative polymerization of monomers to occur in the galleries of the layered structure of silicate clays. The direct attachment of polymer chains onto clay particles has the obvious advantage of producing organic-inorganic hybrid materials that are more thermodynamically stable as compared with clay composites prepared by physically blending a clay filler and a polymer matrix (Fan et al. 2002). In this type of nanocomposites, there is a strong interfacial effect between the layered silicate fillers and

matrix polymers that serves as a key contributing factor for the nanocomposites to exhibit high stiffness, high modulus, and heat resistant properties (Asgari et al. 2017, Hatamzadeh et al. 2014, Huskić et al. 2013, Karaj-Abad et al. 2016, Messersmith and Giannelis 1995, Namazi et al. 2012, Zha et al. 2014). This can be done by growing the chains directly from previously anchored initiators onto the clay. The in situ polymerization involves intercalative polymerization of monomers occurring inside the host galleries of clay particle lamellae and surfaces. This may lead to the increase in the gap spacing between platelets up to the extent of fully delaminating the clay structure and also good dispersion of individual nanometer-thick platelets in a polymer matrix. Using this approach, various degrees of clay dispersion in the polymer matrix have been achieved (Beyer et al. 2002, Chen et al. 2005, Huang and Brittain 2001, Viville et al. 2004, Wang et al. 2003). For most of these approaches, the initiators for polymerization carry a cationic group which may be ion-exchanged with the exchangeable cations of the MMT clay. The ionically bound initiators were then used to initiate the polymerization inside the clay galleries. The growing polymer chains make the layers far apart from each other, eventually resulting in a high degree of delamination of the clay structure. However, one disadvantage of using this approach is, because the polymer chains are grown from initiators that are ionically bound to the clay surfaces, the chains can be detached by ion-exchange reactions (Karesoja et al. 2009). Such issue can be addressed by having covalently bound polymer architecture to the inorganic fillers. This can be made possible through the “grafting to” and “grafting from” approaches.

“GRAFTING TO” APPROACHThe grafting of polymers to inorganic surfaces, like silicate clays, is a versatile method for fine-tuning their surface properties. One structure of interest generated by this approach is a polymer brush. In a polymer brush morphology, the polymer chains are end-grafted to the surface with a grafting density sufficiently high to force the chains to stretch away from the interface, resulting in a layer height significantly larger than the radius of gyration of the attached chains (de Gennes 1980, Jordan et al. 1999, Karaj-Abad et al. 2016, Laub and Koberstein 1994, Mansoori et al. 2012a, Milnert 1991, Minko et al. 2002, Roghani-Mamaqani et al. 2012, Rzayev et al. 2015, Zhulina and Vilgis 1995).

Interest in tethered polymers began in the early '50s when it was found out the flocculation in colloidal particles can be prevented if the particle surface is tethered with polymer molecules (Clayfield and Lumb 1966; van

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der Waarden 1950, 1951). Polymer brushes are used as adhesive materials, protein-resistant biosurfaces, chromatographic devices, lubricants, polymer surfactants, and compatibilizers. Also, polymer brushes have found applications as "smart materials" as the layer of tethered chains reacts with stimuli such as change in pH or ion strength, temperature, solvent quality, or mechanical forces (Behling et al. 2016, Duan et al. 2011, Kalluru and Cochran 2013, Mittal 2009, Wassel et al. 2019, Wu et al. 2009, Ziadeh et al. 2014)

Polymer brushes can be prepared either through physisorption or covalent attachment of the polymer architecture on the inorganic surface (Behling et al. 2016, Boven et al. 1990, Duan et al. 2011, Guino et al. 2005, Jordi and Seery 2005, Kalluru and Cochran 2013, Laible and Hamann 1975, Mittal 2009, Prucker and Ruhe 1998c, Uyama et al. 1998, Uyama and Ikada 1994, Wassel et al. 2019, Wu et al. 2009, Ziadeh et al. 2014). Unlike covalently attached chains, physically adsorbed polymer films onto solid surfaces are not thermally and solvent stable. Covalent grafting of the polymer layers to an inorganic particle is more advantageous due to the ease and controllable introduction of polymer chains. In addition, due to the stable covalent bond of grafted chains onto a surface, the long-term chemical stability of introduced chains is assured – in contrast to the less stable, physically adsorbed organic chains (Kato et al. 2003). The covalent attachment formed between the surface and the polymer chain makes polymer brushes robust and resistant to common chemical environmental conditions (Zhao and Brittain 2000).

The stable covalently attached chains can be prepared utilizing either the "grafting to" or the "grafting from" approach (Behling et al. 2016, Duan et al. 2011, Kalluru and Cochran 2013, Wassel et al. 2019, Wu et al. 2009, Ziadeh et al. 2014). In the “grafting to” technique (Figure 1), the polymer molecule having a reactive end reacts with a complementary functional group located on the surface to form the tethered layer. To form a covalent linkage between one end of the polymer molecules and the solid surfaces, often polymers are end-functionalized to react with appropriate reactive sites located on the solid surfaces (Atai et al. 2009, Ben Ouada et al. 1988, Bridger and Vincent 1980, Mansoori et al. 2012b, Wen et al. 2011, Zha et al. 2014). The tethering of chains can happen by first introducing required functionalities both on the surface of the substrate and the polymer precursors, after which grafting of the chains onto the reactive surface follows. Where silanol groups are present, as in the case of surfaces involving silicon substrates and layered silicate clays like MMT, most often, the reaction involved is between silanols of the substrate and a chloro- or alkoxy-silane. Other ways of grafting the polymer films may include

esterification, condensation, or amidation reactions between that of the end-functionalized polymers and the silanols on the inorganic surfaces (Mansky and Liu 1997, Walters and Hirt 2007, Zdyrko et al. 2006, Zhao et al. 1992).

The preformed polymer can be synthesized in a variety of ways e.g. living, anionic, radical, or ring-opening polymerization. These polymerization methods have the advantage of utilizing well-defined end-functionalized polymers for the grafting process. As a result of exhibiting a significant degree of control on the polymer architecture, well-defined brushes can be readily obtained.

This “grafting to” approach, however, is often limited with low surface coverage (Zajac and Chakrabarti 1994, 1995). The diffusing polymer molecules can encounter increased difficulty to diffuse through a layer of existing attached chains to reach the reactive sites on the surface as the grafting process progresses. This is due to the fact that as the thickness of the bound polymer layer increases, the steric hindrance effect for the oncoming unattached chains becomes more pronounced.

Figure 1. In a “grafting to” approach, preformed polymer chains with reactive end group attached to the reactive sites on the surface. As the number of attached chains becomes more, diffusing chains find it more difficult to graft onto the surface.

“GRAFTING FROM” APPROACHThe "grafting from" approach (Figure 2), also referred to as "surface-initiated polymerization" (SIP), makes use of immobilized initiators initially placed onto solid surfaces using various surface modification techniques (Biesalski and Ruhe 2003, Boven et al. 1990, Carlier et al. 1992, Penaloza and Seery 2019, Penaloza et al. 2015, Prucker and Ruhe 1998c). The polymer layer is then grown directly from these initiators attached to the surface. A polymer brush prepared from a “grafting from” approach is typically characterized by a high grafting density and high molecular weight polymer films (Biesalski and Ruhe



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1999a, 1999b; Burkett et al. 2006; Karesoja et al. 2009; Namazi et al. 2009; Paul et al. 2005; Prucker and Ruhe 1998a, 1998b).

Fujiki and co-researchers (1994) used the SIP method on several monomers that can be initiated by perchlorates previously surface-bound onto ultrafine silica. The perchlorates were initially successfully attached to the silica surface by the reactions of silver perchlorate with the pendant acyl chloride groups. Styrene molecules were then reacted with the acryloyl chloride together with silica particles previously modified to contain azo groups on the surface. The percentage of the polystyrene grafting and that of the total polymer grafted onto silica (i.e., overall grafting), reached about 80 and 120%, respectively.

Berron and co-researchers (2007) reported the preparation of a thin film of poly(n-alkylnorbornenes) using the SIP approach. In their study, the influence of the alkyl side chains on the structure, the surface properties of the nanocomposites, and the growth kinetics of surface-initiated ROMP-type poly(norbornene) films was investigated. A monolayer containing a vinyl group at the end was bound to a gold surface via exposure to a mercaptan. A catalyst was then reacted with the terminal vinyl group to immobilize the catalyst on the gold substrate. The bound catalyst was then reacted with the desired monomer through ROMP to create a surface-tethered poly(n-alkyl norbornene) film. Norbornene derivatives containing different alkyl groups were used.

RING OPENING METATHESIS POLYMERIZATIONThere are two general methods of preparing polymer brushes: polymers tethered at one end to a surface, including adsorption of functionalized polymers onto surfaces; and SIP. In a method involving SIP, an initiator is directly bound onto a surface from which a polymer chain is grown. Several techniques have been employed to grow polymer chains from various monomers utilizing the "grafting from" approach. These include radical (Fujiki et al. 1999, Mizutani et al. 2008, Velten et al. 1999, Xu et al. 2004), redox reaction, (Wang et al. 2008), and anionic and cationic (Advincula 2006, Edmondson and Armes 2009, Ueda et al. 2008) polymerization methods. However, considering the pre-requisites for polymer brushes that include high molecular weight chains (Kim et al. 2000), high grafting density (Dan and Tirrell 1993), and low polydispersity, the utilization of a living polymerization technique provides better opportunity to meet these requirements. Using a living polymerization method, it is possible to precisely control the molecular weight as well as provide an opportunity to prepare a wide selection of polymer architectures such as block copolymers, comb-shaped polymers, multi-armed polymers, ladder polymers, and cyclic polymers (Webster 1991). This control of structure, in turn, results in polymers with widely diverse physical properties.

An example of a living polymerization method is the ring-opening metathesis polymerization (ROMP). Compared with other SIP methods, ROMP is generally performed under mild conditions and short reaction times (Rutenberg et al. 2004). The use of metal alkylidene catalysts to mediate ROMP provides the ability to control the polymer architecture of the nanocomposite (Choi and Grubbs 2003, Grela 2014, Jung et al. 2011, Nuyken and Pask 2013, Schrock 2014, Teo and Xia 2015, Tuba and Grubbs 2013). Metal alkylidines like ruthenium-based initiators used for ROMP have been shown to polymerize a large variety of monomers in a living fashion. Ruthenium-based catalysts are also known for increased tolerance toward a wide range of polar functionalities and decreased sensitivity to atmospheric oxygen and water (Chang et al. 2017, Demonceau et al. 1997, Thomas et al. 2011).

ROMP is an example of a polymerization process involving a chain growth that is a result of the conversion of strained cyclic olefins to a polymer (Baughman and Wagener 2005, Schulz and Wagener 2008, Wagener et al. 1991). Based on the olefin metathesis, the ROMP mechanism involves a metal-mediated carbon-carbon double bond-exchange process (Calderon et al. 1976, Calderon 1972). Hence, the unsaturation of the monomer is conserved after the polymerization. This is one of the features of the ROMP technique that distinguishes it from

Figure 2. The “grafting from” approach, otherwise known as SIP, initiates polymerization directly from the surface. Initiators are previously attached to the surface using various surface modification techniques. Polymer brushes made via this approach are of high graft density and have large molecular weight polymer films since the active ends of the attached polymer chains are accessible to still unreacted monomers.



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typical olefin addition polymerization; for example, the conversion of ethylene to polyethylene where the double functionality of the ethylene monomer is converted to single carbon-carbon bond of the polyethylene structure.

Chauvin and co-authors (1976) proposed a general mechanism for ROMP (Figure 3). The initiation is started by the coordination of the metal alkylidene catalyst to a cyclic olefin monomer. As a result, a four-membered metallocyclobutane intermediate is formed. A reverse cyclic reaction leads to a generation of a new metal alkylidene, now the metal center attached to the olefinic structure. While the new metal alkylidene becomes longer (due to the incorporated monomer), its reactivity to the cyclic olefin monomer is unchanged. This will serve as a propagating polymer chain. Subsequent addition of more cyclic monomers to the growing chain results in a polymer molecule until polymerization stops due to all monomers being used up, an equilibrium is achieved, or the reaction is terminated.

Figure 3. ROMP of norbornene mediated by a metal alkylidene.

The first-generation Grubbs catalyst (Figure 4), a ruthenium-based alkylidene catalyst, has been shown to be tolerable against a wide variety of functional groups (Hillmyer et al. 1995). It will preferentially react with olefins in the presence of a polar functional group such as alcohol, aldehyde, ketone, ester, and ammonium salt. This reactivity has been exploited in several studies reported in the literature and has been found to be useful in the catalytic ring-closing metathesis of functionalized dienes and ROMP (Ashworth et al. 2011, Carrow and Nozaki 2014, Hamad et al. 2013, Kang et al. 2014, Lu and Guan 2012, Occhipinti et al. 2013).

Figure 4. bis(tricyclohexylphosphine) benzylidine ruthenium (IV) dichloride, Grubbs first generation catalyst (Cy – cyclohexyl group, Ph – phenyl group).

NANOCOMPOSITES WITH COVALENTLY-BOUND POLYMERS PREPARING VIA SURFACE-INITIATED RING OPENING METATHESIS POLYMERIZATION (SI-ROMP)In using surface-initiated ring-opening metathesis polymerization (SI-ROMP), typically, a norbornenyl-bearing silylating agent is initially reacted to an inorganic substrate. A ruthenium-based metal alkylidine, Grubbs second-generation catalyst, is then used to ring open the grafted norbornene and is used to initiate the polymerization upon the addition of the monomer (Burkett et al. 2006, Karesoja et al. 2009, Namazi et al. 2009, Paul et al. 2005).

Utilizing the SI-ROMP approach, an exfoliated clay-polymer nanocomposite was prepared. Utilizing the hydrothermal‐silylation reaction between a norbornenyl‐bearing chlorosilane agent and silanol groups of the MMT clay, a metal alkylidene catalyst was bound to the surface in order to grow poly(norbornene) chains directly from the surface using ROMP (Figure 5). This leads to the preparation of nanocomposites having poly(norbornene) chains that are covalently attached to the inorganic substrate, as opposed to most conventional polymer‐clay composites that have ionically tethered chains (via the ammonium‐based modifiers of the organoclay) or physically adsorbed polymers (Penaloza et al. 2015). In another work, they prepared a clay-based polymer nanocomposite having grown cholesterol bearing poly(norbornene) brushes end-tethered from a naturally-occurring MMT clay template. The synthesis of this hybrid material involved the treatment of the clay with first-generation Grubbs catalyst, a ruthenium alkylidene, which induced a ring-opening metathesis and subsequent SIP of the cholesterol bearing monomer. This results in an inorganic/organic hybrid functional nanomaterial where the inclusion of the clay particles in a liquid crystalline domain modifies the thermal transitions (Penaloza and Seery 2019).

Figure 5. SI-ROMP mediated by a metal alkylidene on an inorganic substrate (Penaloza et al. 2015).



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CONCLUSIONThe clay-polymer nanocomposites derived from the use of clay and other related materials have attracted a great deal of technological and scientific interest owing to the promise of greatly improved properties over those of the unfilled polymers. In incorporating this modified clay to a bulk polymer, there are three challenges involved: (1) chemical compatibility issue, clay being hydrophilic by nature while most polymers used as matrices for polymer nanocomposite preparations are hydrophobic; (2) homogeneous dispersion of the clay fillers in the polymeric domain; and (3) exfoliation of the clay fillers. As discussed, these can be addressed by covalently bound polymers onto the clay fillers, referred to as polymer brushes. Not only are these systems attractive for their obvious potential as functional materials, but tethered chains can also provide a convenient system to study fundamental issues concerning confined and attached polymers. Using them as model systems for studies of their formation, structure, and dynamics can lead to a better understanding of hybrid materials and polymers in a confined environment or at a solid interface.

ACKNOWLEDGMENTSProf. Thomas AP Seery of the University of Connecticut is acknowledged for his insightful comments during the preparation of this article.

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Review on the Preparation and Properties of Clay-based