Grafting

18 Grafting Pros and cons of grafting copolymers by reactive extrusion in comparison to other methods are [1,2]: + essentially no solvents, − intimate mixing of reactants compulsory, − the high reaction temperatures needed, + fast preparation, − side reactions, e.g., degradation, crosslinking or discoloration,

(LDPE) increases as the concentration of the peroxide increases. Further, the grafting efficiency depends on the means of reactive processing. In a comparative study with varying experimental setup, the lowest efficiency was found for extrusion using a typical shaping extrusion head, a higher efficiency was found with a static mixer and the highest efficiency was found with a dynamic mixer. The dynamic mixer is a cavity transfer mixer that provides shear rates of the moving melt of about 100 s−1 .

+ simple product isolation, + extrusion is a continuous process. Grafting takes place mostly by a radical reaction mechanism [3] and is also called free radical grafting. However, there are other techniques for introducing functional groups into polymers, e.g., according to the Alder-ene reaction [4].

18.1

The Techniques in Grafting

18.1.1 Parameters that Influence Grafting

Propene Content. In a series of polyolefins with different ethene/propene, the efficiency of grafting of maleic anhydride (MA) both in the melt and in solution was studied. The maleic anhydride graft content is low for polyolefins with high propene content, increases as the propene content decreases, and reaches a plateau at propene levels below 50%. Branching and crosslinking occurs for polyolefins with low propene content, while degradation is the main side reaction for polyolefins with high propene content [5].

Efficient mixing of the individual components is of critical importance for the success of a graft process. The mixing efficiency is dependent on the screw geometry, the melt temperature, the pressure, the rheological properties of the polymer, and the solubilities of the monomer and the initiator, respectively, in the polyolefin.

Mechanochemistry. Shear stresses in the dynamic mixer cause a formation of radicals even in the absence of any peroxide. Therefore, grafting of maleic anhydride on LDPE even without the action of peroxide initiator is observed. The dynamic mixer helps to obtain a high grafting efficiency on LDPE using a small concentration of peroxide initiator. Under these conditions, grafting is not accompanied by a crosslinking reaction of the poly(ethylene) chains [6].

18.1.1.2 Grafting Efficiency

18.1.1.3 Screw Geometry

In order to obtain a high grafting efficiency together with an effective suppression of the side reactions, it is necessary to transform the macroradicals on the backbone as far as possible into graft sites. In general, within reasonable limits, higher reaction temperatures, higher initiator levels, and lower throughput rates result in higher grafting efficiency.

Reactive extruders usually have a modular construction. This allows flexible arrangements of the screw elements and barrel sections as needed.

18.1.1.1 Mixing

Peroxide Concentration. The grafting efficiency of maleic anhydride on low density poly(ethylene)

18.1.1.4 Processing Temperature The processing temperature is of critical importance. Too high processing temperatures will cause degradation reaction, and the initiator may decompose too quickly to be effective.

Reactive Polymers: Fundamentals and Applications. http://dx.doi.org/10.1016/B978-0-12-814509-8.00018-X Copyright © 2018 Elsevier Inc. All rights reserved.

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18.1.1.5 Processing Pressure In contrast to temperature, a high processing pressure can improve the solubility of the monomer to be grafted and the solubility of the initiator in the polymer.

18.1.1.6 Residence Time The residence time is governed by the overall throughput which can be adjusted by the screw speed, the screw design, and the geometry of the extruder.

18.1.1.7 Removal of Byproducts The unreacted monomers and decomposition products from the initiator, etc. are removed by the application of vacuum to the melt.

18.1.1.8 Consistency Experiments of grafting maleic anhydride onto poly (propylene) by melt extrusion with dicumyl peroxide, where the poly(propylene) was fed either as powder or in granular form, showed that consistency plays a role on the degree of grafting [7]. The grafting efficiency of powdered poly(propylene) was higher than that obtained for the granular form of poly(propylene). It is believed that the grafting of powder is more successful because a better initial mixing and less diffusional resistance during the grafting is provided.

18.1.2 Free-Radical Induced Grafting The most commonly used grafting method is free radical induced grafting. However, the efficiency of grafting cannot simply be increased by increasing only the concentration of the radical initiator. More important for the grafting efficiency are proper mixing and a sophisticated choice of proper comonomers. Grafting without radical initiator is also possible. In this case, the macroradicals are formed by a shear induced chain scission. Of course, this process is accompanied by degradation or crosslinking reactions.

18.1.3 Polymer Brushes Polymer brushes are ultrathin polymer coatings, formed by highly packed polymer chains that are tethered by one end to a surface or interface [8]. For

example, covering the surface with polymer brushes could suppress non-specific protein adsorption, i.e., protein fouling, from biological media, as well as decrease the forces of bacterial adhesion on the surface. While the focus in the past decades has been mostly on their synthetic aspects and the in-depth study of their interesting properties, meanwhile, the core area of research shifted towards specific practical applications. The issues of polymer brushes have been reviewed [9]. A critical discussion of the status of development of application-oriented research on polymer brushes, and potential uses of polymer brushes in multiple research fields have been documented. Also, it has been noted that polymer brushes are a promising grafting approach to scaffolds for tissue engineering [10].

18.1.3.1 Grafting-from The molecular weight and polydispersity of the chains in a polymer brush are critical parameters determining the properties of the brush. However, the characterization of polymer brushes is difficult due to the vanishingly small mass of polymer present in the brush layers [11]. In order to have sufficient quantities of polymer for analysis, polymer brushes were grown from high surface area fibrous nylon membranes by atom transfer radical polymerization (ATRP). The brushes were synthesized with varying surface initiator densities, polymerization times, and amounts of sacrificial initiator, then cleaved from the substrate, and analyzed by gel permeation chromatography (GPC) and NMR. It was found that the surface-grown polymer chains were more polydisperse and had lower average molecular weight in comparison to solution-grown polymers synthesized concurrently. In addition, the molecular weight distribution of the polymer brushes was found to be bimodal, with a low molecular weight population of chains representing a significant mass fraction of the polymer chains at high surface initiator densities [11]. The origin of this low molecular weight polymer fraction has been proposed to be the termination of the growing chains by recombination during the early stages of polymerization. This suggested mechanism was confirmed by molecular dynamics simulations of the brush polymerization [11].

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18.1.3.2 Grafting-through A grafting-through brush polymerization mechanism has been elaborated, where monomers are supplied through the surface on which the initiators are attached rather than from solution as in the graftingfrom technique [12]. This can be accomplished by attaching the initiator to the surface of a dialysis membrane and supplying monomers through the membrane to the growing brush. This avoids the growth of very long chains while promoting the growth of shorter chains by reversing the monomer concentration gradient found in the commonly used grafting-from technique, where monomer concentration is lowest at the substrate and highest in the surrounding solution. Reversing this monomer concentration gradient results in shorter chains experiencing a higher local monomer concentration than longer chains, thus speeding up their growth relative to the longer ones. It is shown by atomic force microscopy that brush layers made by this method are thicker and have lower roughness than brushes made by a grafting-from approach. Coarse-grained molecular dynamics simulations of brush polymerization with monomers supplied through a permeable substrate have provided some insight into the mechanism of the grafting-through brush growth process. Simulations showed that it is possible to obtain a brush layer with a chain dispersity index approaching unity for sufficiently long chains [12].

18.1.3.3 Self-Assembly Assisted Grafting-to The precise synthesis of polymer brushes to modify the surface of nanoparticles and nanodevices for targeted applications has been one of the major focuses in the community for decades. A self-assemblyassisted grafting-to approach to synthesize polymer brushes on flat substrates has been reported [13]. In this method, polymers are pre-assembled into two-dimensional polymer single crystals with functional groups on the surface. Chemically coupling the polymer single crystals onto solid substrates leads to the formation of polymer brushes. Exquisite control of the chain folding in polymer single crystals allows to obtain polymer brushes with well-defined grafting density, tethering points, and brush conformation. An extremely high grafting den-

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sity (2.12 chains per nm2 ) has been achieved in the synthesized single-tethered polymer brushes. In addition, polymer loop brushes have been successfully fabricated using oddly folded polymer single crystals from telechelic chains [13].

18.1.3.4 Atom Transfer Radical Polymerization The preparation of chemically anisotropic colloidal dumbbells of which one lobe is functionalized with chemical handles in the form of chlorine groups has been reported [14]. The chlorines can be easily converted to azides and subsequently to active initiators for ATRP by Click Chemistry. These initiators are exploited for site-specific grafting of poly(N-isopropylacrylamide) brushes on the reactive patches. The geometric ratio between the grafted and nongrafted lobe is tunable by the shape of the initial dumbbell and the polymer grafting time. Furthermore, the versatility of the synthesis method is underlined by extending it to colloids with multiple reactive patches. The partially grafted dumbbell-shaped particles are used as building blocks for finite-sized colloidal clusters. A directional interaction between the nongrafted lobes is easily introduced by dispersing the partially grafted dumbbells in a high ionic strength medium [14]. Zwitterionic Polymer Brush Grafting. A feasible processing of zwitterionic polymer-grafted anodic aluminum oxide membranes by surface-initiated atom transfer radical polymerization and the geometric effect on the polymer chain growth in the confined nanopores was investigated [15]. A zwitterionic poly(3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate) (PMAPS) brush was produced on an anodic aluminum oxide membrane via the introduction of an initiator with a phosphonate group and subsequent surfaceinitiated atom transfer radical polymerization. The PMAPS-grafted anodic aluminum oxide membranes were characterized by water droplet contact angle measurement, size exclusion chromatography, scanning electron microscopy (SEM), Xray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and laser Raman microspectroscopy [15]. The linear relationship between

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the grafting yield of the PMAPS brush and numberaverage molecular weight of the unbound PMAPS indicates that there is no significant geometric effect on the chain growth under the spatial confinement inside the nanopores with diameters of ca. 200 nm. The PMAPS brushes were diminished near the center section along the nanopores because the monomer supply was retarded. The capability of the PMAPS-grafted anodic aluminum oxide membranes for inorganic nano-particle immobilization was also demonstrated using gold nanoparticles through ionexchange and reduction processes [15].

18.1.3.5 Electrochemically-Controlled Grafting Electrochemically-mediated ATRP has emerged in recent years as an alternative controlled radical polymerization technique that utilizes low concentrations of copper-based ATRP catalysts, and that can be conducted in the presence of atmospheric oxygen [16]. Electrochemically-mediated ATRP has been adapted to perform surface-initiated electrografting directly from a conducting polymer macroinitiator that also acts as the working electrode to control the oxidation state of the catalyst, and thereby the activity of the catalyst. Aqueous electrografting of hydrophilic poly(2-hydroxyethyl methacrylate) polymer brushes from the conducting polymer macroinitiator, in the presence of an ATRP catalyst, i.e., CuBr2/TPMA, was confirmed by ATR-FT-IR ¸ , water contact angle measurements, and XPS. Optimized grafting conditions were determined whereby the polymerization kinetics approached a first order characteristics, as expected for grafting via an ATRP mechanism. The prevalence of uncontrolled grafting, due to competing grafting mechanisms, as well as effects such as chain termination and degrafting, was highly dependent on the polymerization conditions, most notably on the applied electric potential [16].

18.1.3.6 Single Electron Transfer Living Radical Polymerization Photoinduced surface-initiated single electron transfer living radical polymerization is a versatile technique for the preparation of polymer brushes [8]. The vast diversity of compatible functional groups, together with a high end-group fidelity that enables a precise control of the architecture, makes this ap-

Reactive Polymers: Fundamentals and Applications

proach an effective tool for tuning the properties of surfaces. The application of photoinduced single electron transfer living radical polymerization for the surface-initiated grafting of polymer brushes from a wide range of methacrylate monomers has been reported. The living character of the process was demonstrated by the linear evolution of the polymer brush thickness in time, the ability to reinitiate the polymerization for the preparation of well-defined block copolymers. The surface patterning with these brushes could be achieved simply by restricting the irradiated area. The ability of poly(methacrylate) brushes prepared in this way to prevent non-specific protein adsorption is also demonstrated, thus indicating the suitability of this procedure for advanced applications [8].

18.1.3.7 Grafting Using Stable Radicals The technique of grafting using stable radicals involves two steps [17]: 1. A stable nitroxyl radical is grafted onto a polymer, which involves the heating of a polymer and a stable nitroxyl radical. 2. The grafted polymer of the first step is then heated in the presence of a vinyl monomer or oligomer to a temperature at which cleavage of the nitroxyl–polymer bond occurs and polymerization of the vinyl monomer is initiated at the polymer radical. The temperature applied in the first reaction step depends on the polymer and is, for example, 50 °C to 150 °C above the glass transition temperature (Tg ) for amorphous polymers and 20 °C to 180 °C above the melting temperature (Tm ) for semicrystalline polymers. Typical temperatures are summarized in Table 18.1. Stable nitroxyl radicals are collected in Table 18.2. The first step of the process is performed conveniently in an extruder or a kneading apparatus. In the extruder, a reduced pressure of less than 200 mbar is applied during extrusion. Volatile byproducts may be removed thereby. Typical reaction times are from 2 to 20 min. For the monomer grafting reactions, unsaturated monomers are selected from styrene, dodecyl acrylate, and other compounds. The second reaction step may be performed immediately after the first step,

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Table 18.1 Reaction Temperatures for Coupling of Stable Radicals [17] Polymer Low-density poly(ethylene) High density poly(ethylene) Poly(propylene) Poly(styrene) Styrene-block copolymers Ethylene-propylene-diene modified Ethylene/propylene rubber

Table 18.2 Stable Nitroxyl Radicals Compound Benzoic acid 2,2,6,6-tetramethyl-piperidin-1-oxyl-4-yl ester 4-Hydroxy-2,2,6,6-tetramethyl-piperidin-1-oxyl 4-Propoxy-2,2,6,6-tetramethyl-piperidin-1-oxyl Decanedioic acid bis(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl)

however it is also possible to store the intermediate polymeric radical initiator at room temperature for some time. Because the graft polymerization is a living polymerization, it can be started and stopped practically at will. The intermediate polymeric radical initiator is stable at room temperature and no loss of activity occurs up to several months. The reaction step may also be performed in a mixer or extruder. However, it is also possible to dissolve or disperse the polymer and to add the monomer to the solution. If the second reaction step is performed in a melt, a reaction time of 2 to 20 min is adequate. The grafted polymers are useful in many applications such as compatibilizers in polymer blends or alloys, adhesion promoters between two different substrates, surface modification agents, nucleating agents, coupling agents between filler and polymer matrix, or dispersing agents. The process is particularly useful for the preparation of grafted block copolymers. Grafted block copolymers of poly(styrene) and polyacrylate are useful as adhesives or as compatibilizers for polymer blends or as polymer toughening agents. Poly(methyl methacrylate-co-acrylate) diblock graft copolymers or poly(methyl acrylate-coacrylate-co-methacrylate) triblock graft copolymers are useful as dispersing agents for coating systems, as coating additives or as resin components in coatings. Graft block copolymers of styrene, (meth)acrylates,

Abbreviation LDPE HDPE PP PS SB(S) EPDM EPR

Temperature (°C) 170–260 180–270 180–280 190–280 180–260 180–260 180–260

Table 18.3 Monomers for Grafting onto Polyolefins [1] Vinyl monomer Maleic anhydride Maleate esters Styrene Maleimide derivatives Methacrylate esters Acetoacetoxy methyl methacrylate Glycidyl methacrylate Acrylate esters Ricinoloxazoline maleate Vinylsilanes

Remarks/references Most common [18] Auxiliary monomer [19] [20] [21] [20] [22,23] [24] [25] [26]

or acrylonitrile are useful for plastics, elastomers, and adhesives.

18.2 Polyolefins The synthesis of polyolefin graft copolymers by reactive extrusion has been reviewed by Moad [1,2]. The methods of modification can be classified as: 1. Free-radical induced grafting of unsaturated monomers onto polyolefins, 2. End-functional polyolefins by the ‘ene’ reaction, 3. Hydrosilylation, 4. Carbene insertion, and 5. Transformation of pending functional groups on polyolefins, e.g., by transesterification, alcoholysis.

18.2.1 Monomers for Grafting onto Polyolefins Monomers for grafting onto polyolefins are listed in Table 18.3.

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18.2.1.1 Macromonomers Polymeric or oligomeric vinyl compounds are addressed as macromonomers in the field of reactive extrusion. Examples for macromonomers are higher molecular acrylate esters, methacrylate esters, and maleimides. Macromonomers are less likely to undergo homopolymerization than low-molecular vinyl compounds. This property arises due to steric effects. Thus they may not form longer pendent chains on the grafting sites consisting of homopolymers. A disadvantage of macromonomers is their low volatility. For this reason, an unreacted or excess compound may not be easily removed by vacuum treatment in the extrusion device.

18.2.2 Mechanism of Melt Grafting Functionalized poly(propylene) (PP) has been used extensively for compatibilization of immiscible poly (propylene)/polyamide and poly(propylene)/polyester blends. Also, the interfacial adhesion of PP with glass and carbon fibers can be improved. Further, functionalized PP is a processing aid for degradable plastics [27,28]. It is generally accepted that chain scission occurs during the peroxide-initiated functionalization of PP [29]. MA is appended to a tertiary carbon atom along the PP backbone as a single ring or as a short pendant chain due to the homopolymerization of MA [30]. On the other hand, according to the ceiling temperature, there is no possibility for the homopolymerization of MA under the melt grafting process conditions at 190 °C [31]. Chemical analysis of the low-molecular weight fraction of extrudates of poly(propylene) with maleic anhydride and dicumyl peroxide by mass spectrometry indicated the products shown in Figure 18.1. No MA oligomers or MA homopolymers are found in the low-molecular weight fraction. The MA radicals always contain double bonds after termination. Peroxide residues are attached to MA molecules. A reduction of the molecular weight occurs when the degree of grafting increases. From the inspection of the chemical structure of the low-molecular weight residue, it can be concluded that the maleic anhydride is attached as a single moiety on the tertiary carbon atoms of the poly(propylene) backbone. From these experimental findings a mechanism of grafting

Figure 18.1 Structure of low-molecular weight fraction of extrudates of poly(propylene) with maleic anhydride and dicumyl peroxide.

has been proposed [32] that is given in Figure 18.2. Furthermore, the grafting of maleic anhydride onto poly(propylene) has been studied by a Monte Carlo simulation method [33]. The results presented in this study are in agreement with the experiments. The grafting efficiency of methyl methacrylate is similar to that of maleic anhydride [21].

18.2.3 Side Reactions Side reactions accompany the grafting reaction of polyolefins. These include [1,2]: 1. Radical induced crosslinking of the polyolefin substrate, 2. Radical induced chain scission of the polyolefin substrate, 3. Shear induced degradation of the polyolefin substrate, 4. Homopolymerization of the monomer, and 5. Side reactions which lead to a coloration of the product. The extent of the side reactions depends on the type of polyolefin. Some poly(ethylene) types are sensitive to branching and crosslinking. This is due to the recombination of the macroradicals [34]. Poly(propylene) and linear low-density poly(ethylene) copolymers undergo degradation rather than crosslinking, although crosslinking may occur. Degradation is often favored to synthesize controlled rheology types.

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Table 18.4 Ceiling Temperatures for Important Monomers in Reactive Extrusion Grafting [1,36] Monomer Maleic anhydride Styrene Methacrylate esters Acrylate esters

Ceiling temperature (°C) <150 >400 ∼200 >400

carbon dioxide in fact resulted in improved grafting when high levels of maleic anhydride were used. No evidence of an improvement in the homogeneity of the product was observed. However, melt flow rate showed a reduction in the degradation of poly(propylene) during the grafting reaction when low levels of maleic anhydride were used [35].

18.2.5 Ceiling Temperature

Figure 18.2 Mechanism of grafting of maleic anhydride onto poly(propylene) [32] (abbreviated).

18.2.4 Viscosity The formation of products with higher molecular weight is indicated by an increase of the apparent viscosity. On the other hand, by the introduction of polar groups during grafting, an increase of the viscosity is observed because of physical crosslinks of the individual molecules. Maleic anhydride has been grafted onto poly (propylene) in the presence of supercritical carbon dioxide. Supercritical carbon dioxide was used in order to reduce the viscosity of the poly(propylene) melt phase. A reduced viscosity should promote a better mixing of the reactants. The characterization of the products showed that the use of supercritical

The ceiling temperature is an important parameter for the ability of polymerization itself. We are dealing here with homopolymerization. The concept of the ceiling temperature is not restricted to a polymerization mechanism, because it deals with the thermodynamic equilibrium. Ceiling temperatures for important monomers in reactive extrusion grafting onto polyolefins are given in Table 18.4. The ceiling temperatures given in Table 18.4 could be important for the grafting of maleic anhydride and maleic esters [37,38]. The ceiling temperatures depend on the pressure and on the concentration of the monomer. They are usually calculated from the heats and the entropies of polymerization that are usually given at one atmosphere. In fact, the homopolymerization of maleic anhydride was observed at a higher temperature than 150 °C, even when the ceiling temperature would not predict a polymerization reaction.

18.2.6 Effect of Initiator Solubility Experiments of grafting of itaconic acid (IA) onto an LDPE with various initiators in the course of the reactive extrusion revealed that the solubility of the peroxide initiator in the molten polymer is the most important parameter in the IA grafting onto LDPE. The kinetics of decomposition is an important parameter for the efficiency of grafting. The solubility parameters of various peroxides are collected in Ta-

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Table 18.5 Solubility Parameters of Peroxides [39] Peroxide Dicumyl peroxide 2,5-Dimethyl-2,5-di(tert butylperoxy)hexane Di-tert -butyl peroxide 2,2-Di(tert -butylperoxy)-5,5,6-trimethyl bicyclo[2.2.1]heptane 2,5-Dimethyl-2,5-di(tert butylperoxy)-3-hexyne

δ (J cm−3 )1/2a 17.4 11.3 15.3 16.1 19.1

a Calculated for 25 °C.

ble 18.5. The solubility parameters δ in Table 18.5 are calculated from group contributions [40] according to Eq. (18.1).   i Ei  (18.1) δ= Na i Vi Ei

Vi Na

Contribution of every atom and type of the intermolecular interaction in the molar cohesion energy van der Waals volume of a group constituting the molecule Avogadro number

The temperature dependence of δ can be expressed by Eq. (18.2): log δ(T ) = log δ(298 K) − αk(T − 298) (18.2) Here, α is the linear thermal expansion coefficient; and k is a coefficient which is k = 1 for the polyolefin and k = 1.25 for the peroxides and the monomer. The cohesion energy density δ calculated from Eq. (18.1) correlates well with the values obtained from the heat of vaporization of the respective substances. Substances are thermodynamically miscible in the absence of strong specific interactions between them, if their solubility parameters differ by less than 2 (J cm−3 )1/2 . The solubility parameters of an imidized acrylic polymer and LDPE are 24.6 (J cm−3 )1/2 and 16.1 (J cm−3 )1/2 , respectively. Therefore, the imidized acrylic polymer and LDPE form a heterogeneous system in the melt. On the other hand, it is expected that some of the peroxides listed in Table 18.5 would dissolve in LDPE. It is assumed that radicals formed during peroxide decomposition interact first with LDPE macromolecules, then the formed macroradicals initiate the

grafting reactions with the imidized acrylic polymer. Peroxides, which are easily dissolved in LDPE, are most efficient in initiating the grafting reactions [39]. It was found that neutralizing agents introduced into the initial reaction mixture increase the yield of graft polymer, when the carboxyl groups were neutralized partially or totally. As neutralizing agents, zinc oxides and hydroxides, as well as magnesium oxides and hydroxides, can be used [41].

18.2.7 Distribution of the Grafted Groups There is a lot of research presented in the literature, and there is still a controversy concerning the mechanism and the distribution and the structure of the grafted portions on the backbone. This is reviewed in detail by Moad [1].

18.2.8 Effect of Stabilizers on Grafting The grafting of maleic anhydride onto poly(ethylene) is fully inhibited by adding a phenolic stabilizer to the reactive blend [42]. In a system consisting of itaconic acid, linear lowdensity poly(ethylene) and 2,5-dimethyl-2,5-di(tertbutylperoxy)hexane with Irganox™ 1010 (Ciba Geigy, Switzerland), i.e. the ester of 3,5-di-tert-butyl4-hydroxyphenyl-propanoic acid and pentaerythritol, the grafting efficiency decreases slightly. However, at concentrations of the stabilizer greater than 0.3% some improvement in the grafting efficiency occurs and the melt viscosity is much lower [39]. The efficiency of stabilizers on the grafting and on the crosslinking also depends on their solubility in the polymer and the monomer. For example, 1,4dihydroxybenzene has an increased affinity toward the monomer and both reduces the yield of grafting and inhibits crosslinking [43].

18.2.9 Radical Grafting of Polyolefins with Diethyl Maleate The use of maleate esters such as diethyl maleate or dibutyl maleate has been suggested because of their lower volatility and lower toxicity in comparison to maleic anhydride. However, maleate esters are less

18: Grafting

reactive towards free radical addition than maleic anhydride. Grafting polyolefins with diethyl maleate can be carried out in solution. However, the use of extruders as reactors has several economic advantages. The extruder screw is advantageously configured with different mixing elements after an additional feed zone downstream from the initial feed port for peroxide and diethyl maleate. Further there are no mixing elements beyond the vent port. Turbine mixing elements are used for the improved blending of the lowviscosity initiator and the diethyl maleate into the high-viscosity poly(ethylene). A vacuum vent port is used to eliminate the unreacted monomer. In the extruder, dicumyl peroxide (DCP) is used as initiator [44]. The kinetics of the free radical grafting of diethyl maleate (DEM) onto linear poly(ethylene) initiated by dicumyl peroxide has been studied by differential scanning calorimetry (DSC). The activation energy Ea and the order of the reaction n depend on the conditions and vary with the feed composition. The values of Ea and n increase with increasing DCP/DEM ratio because of secondary reactions, such as chain extension and degradation. The data can be described by a mathematical model which can be used to select feed composition and process parameters to obtain the desired products [45].

18.2.10 Inhibitors for the Homopolymerization of Maleic Anhydride In a series of papers, Gaylord showed that various additives are effective in reducing both the amount of crosslinking and chain scission [46,47]. These additives include amides, such as N,N-dimethylacetamide, N,N-dimethylformamide, caprolactam, stearamide, sulfoxides such as dimethyl sulfoxide, and phosphites, such as hexamethylphosphoramide, triethyl phosphite. The action has been attributed to the electron donating properties of these compounds. It was shown that these compounds also act as inhibitors of the homopolymerization of maleic anhydride, thus reducing its grafting efficiency. However, it seems that these compounds are not effective in general at least, there was some controversy [1].

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18.2.11 Inhibitors for Crosslinking p-Benzoquinone, triphenyl phosphite, and tetrachloromethane were found to be good inhibitors for the crosslinking reaction of LDPE [48]. In the melt grafting of maleic anhydride onto an elastomeric ethylene-octene copolymer, N,N-dimethylformamide was used as an inhibitor to reduce the crosslinking reaction. Further N,N-dimethylformamide is a solvent for peroxide initiator. The melt grafting was carried out in a twin-screw extruder, in the presence of dicumyl peroxide as an initiator. However, increasing the initiator concentration increased the degree of grafting and, at the same time, increased the extent of crosslinking [49].

18.2.12 Special Initiators 18.2.12.1 Bisperoxy Compounds The decomposition of the two peroxy groups in bisperoxy compounds is not concerted. The two peroxy groups decompose independently to yield a variety of alkoxy and alkyl radicals.

18.2.12.2 Functionalized Peroxides To optimize the chemical compatibility or solubility of the peroxides in a wide variety of polymeric systems, the organic character of these peroxides may be tailored by introducing suitable groups. Functionalized peroxides may be used as crosslinking, grafting and curing agents, initiators for polymerization reactions and as monomers for condensation polymerization to form peroxy-containing polymers, which in turn can be used to prepare block and graft copolymers. Some functionalized peroxides are shown in Figure 18.3 and collected in Table 18.6. Table 18.6 Functionalized Peroxides [50] Compound 1,1-Dimethyl-3-hydroxybutyl-6-(hydroxy)peroxyhexanoate 1,1-Dimethyl-3-hydroxybutyl-2-(carboxy)perbenzoate 1,1-Dimethyl-3-hydroxybutyl-2-(carboxy) peroxycyclohexanecarboxylate 1,1-Dimethyl-3-hydroxypropyl-3-(carboxy) peroxypropanoate 1,1-Dimethyl-3-hydroxybutyl-3-(carboxy)-5-norbornene-2 ylperoxycarboxylate

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Figure 18.4 Reaction of 1,1-dimethyl-3-hydroxybutyl hydroperoxide with maleic anhydride.

Figure 18.3 Functionalized peroxides, manufactured by elf atochem North America.

The half-lives of the peroxides at 180 °C are ca. 0.27 min for LUPEROX™ PMA and LUPEROX™ TA-PMA and 0.31 min for Luperco™ 212-P75 and Lupersol™ 512. The peroxides are assumed to result in acrylic carboxyl groups and propionic carboxyl groups on the tertiary carbon atoms of poly(propylene) on recombination with the tertiary radicals formed previously. The highest acidity on the polymer backbone is obtained with LUPEROX™ PMA. With respect to the functional radicals, the peroxides which yield radicals that bear double bonds have a higher grafting efficiency. It is assumed that the alkenyl radicals have a higher reactivity with respect to alkyl radicals. Further, the increased grafting efficiency may arise since macroradicals can add across the double bond of the alkenyl groups [51]. Preparation of Functionalized Peroxides. There are several routes to preparing functionalized peroxides. 1,1-Dimethyl-3-hydroxybutyl hydroperoxide reacts with two units of glutaric anhydride or maleic anhydride in ring opening of the anhydride [52] as shown in Figure 18.4. Similarly, 1,1-dimethyl-

3-hydroxybutyl-2-(carboxy)perbenzoate can be prepared from phthalic anhydride by adding 1-(3-dimethylaminopropoxy)-2-butanol in equimolar quantities. Peroxyketals. The chemical modification of molten poly(ethylene) by thermolysis of peroxyketals involves the decomposition of three cyclic or acyclic peroxyketals. An ester function by coupling of an alkyl radical bearing such a function, arising from the peroxyketals, a polymer radical, generated from the poly(ethylene), were identified as grafting products [53].

18.2.12.3 Induced Decomposition of Peroxides Peroxides show an induced decomposition with amino-functional monomers such as diethylaminoethyl methacrylate and diethylaminoethyl acrylate. Instead of a peroxide an azo compound can be used as an radical initiator.

18.2.12.4 Grafting to Poly(ethylene) with Bicumene Bicumene, i.e., dicumyl or 2,3-dimethyl-2,3-diphenylbutane, can serve as a radical initiator as an alternative to a peroxide. Compounds of the bicumene-type also serve as synergists for flame retardants poly-

18: Grafting

Figure 18.5 Dicumyl and hexabromocyclododecane.

olefin by using them in combination with a known flame retardant for polyolefin such as hexabromocyclododecane (cf. Figure 18.5) and 2,3-tris(dibromopropylene)phosphate. When a peroxide is employed as the reaction initiator, the peroxide serves as a graft polymerization initiator, but at the same time a portion of the peroxide induces a crosslinking reaction and a decomposition reaction of the polyolefin. Because of the crosslinking reaction or the decomposition reaction, the inherent physical properties of polyolefin deteriorate and the resulting modified product is unable to maintain the properties of the polyolefin. In addition, when the peroxide decomposes as the reaction proceeds, the decomposition products (e.g., butanol or other decomposition products) stain the modified product. For example, the modified product yields odor originating from the decomposition product, or turns to yellow because of the action of the decomposition product. The graft polymerization reaction starts more moderately and proceeds more selectively, in comparison to a conventional reaction using peroxide. Also, the crosslinking reaction or decomposition reaction of polyolefin is less, and the resulting modified polyolefin has the excellent physical properties of the unmodified polyolefin. For instance, when a linear low-density poly(ethylene) is employed, the modified product thereof has a high mechanical strength at low temperatures [54]. The bicumene-initiated modification of high-density poly(ethylene) at 290 °C provides no benefits in terms of selectivity when compared to a standard peroxide-based process operating at 180 °C. However, the selectivity of linear low-density poly(ethylene) modification is influenced by chain scission, which counteracted the molecular weight effects of macroradical combination [55].

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As compared with the case of using peroxide, the variation of melt index caused by the modification is smaller, and the modified product obtained shows a melt index only slightly different from that of the polyolefin employed as a starting material. Maleic anhydride is generally employed in the amount of 10−3 to 10−5 mol/g of polyolefin. When the amount of maleic anhydride exceeds 10−3 mol/g, the graft efficiency of maleic acid sometimes decreases, and unreacted maleic anhydride remains in a large amount. This results in an unfavorable effect on the physical properties of the resulting modified product. When the amount of maleic anhydride is less than 10−6 mol/g, the modification with the maleic anhydride is unsatisfactory, and accordingly the resulting modified product does not have sufficiently improved adhesive properties [54]. The graft polymerization reaction is performed by heating a mixture of the polyolefin, maleic anhydride and the initiator under kneading.

18.2.12.5 Ultrasonic Initiation The grafting of maleic anhydride onto high-density poly(ethylene) also can be performed through ultrasonic initiation. Obviously, the ultrasonic waves can decrease the molecular weight of the grafted product and increase the amount of grafted maleic anhydride. In comparison to the initiation with peroxide, ultrasonic initiation can prevent the crosslinking reaction by adjusting the ultrasonic intensity. The mechanical properties of the improved high density poly(ethylene) (HDPE) glass fiber composite produced by ultrasonic initiatives are higher than in those produced by peroxide initiatives [56].

18.2.13 Maleic Anhydride Maleic anhydride is most frequently used for grafting and functionalization of polyolefins. Many of the features are described in the general sections, e.g., Section 18.2.2. Systematic and quantitative studies of the graft copolymerization in batch and continuous mixers and kinetic data for poly(propylene) and maleic anhydride are available [57]. In the melt grafting of maleic anhydride onto low-density poly(ethylene)/ poly(propylene) blends, in the presence of DCP, the blend had lower viscosity in comparison to exclusively pure poly(ethylene) under comparable conditions. However, the grafting degree of the MA grafted

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Table 18.7 Use of Maleic Anhydride Grafted Linear Low-Density Poly(Ethylene) as Compatibilizer System Poly(propylene)/organoclay nanocomposites Low-density poly(ethylene)/ethylenevinyl alcohol Poly(propylene)/Poly(styrene) Low-density poly(ethylene)/rice starch

Reference [60] [61] [62] [63]

Figure 18.6 Indices of hydroxy groups formed during UV irradiation [64].

LDPE/PP (90/10) blend was almost the same as or a little higher than that of the MA grafted LDPE [58]. Maleic anhydride can be grafted onto poly(propylene) using benzophenone (BP) as the photoinitiator [59]. In comparison to thermally initiated grafting with peroxide initiators, photoinitiated grafting has a higher grafting efficiency. Maleic anhydride grafted LDPE is widely used as compatibilizer for various applications, as shown in Table 18.7. The compatibility of linear low density poly (ethylene) (LLDPE) can be increased by the addition of block copolymers or modified or functionalized grafted polymers. These act as interfacial or compatibilization agents. LLDPE has been chemically modified with MA in the molten state [64]. In a first step, different doses of UV irradiation were applied to generate hydroperoxide groups, which become highly reactive at the processing temperature Then, in a second reactive extrusion step, MA was grafted to the LLDPE under different processing conditions. The effect of irradiation of the formation of hydroxy groups is shown in Figure 18.6.

18.2.14 Polyolefins Grafted with Itaconic Acid Derivatives The mechanical properties of PP/LDPE blends, which were modified by the free-radical grafting of IA after compounding were investigated with DSC [65]. The data revealed the incompatibility of PP and LDPE in the composites with respect to the crystalline phases. However, favorable interactions were notified within the amorphous phases. Due to these interactions, the temperature of crystallization of PP increases by 5–11 °C, and the temperature of crystallization of LDPE increases by 1.3–2.7 °C. A single β-relaxation peak was observed that indicates a compatibility on the level of the structural units. Variations in the ratios of the polymers (PP and LDPE) result in nonadditive and complex changes in the viscoelastic properties as well as in the mechanical characteristics [65].

18.2.14.1 Poly(ethylene) Polyamide 6 Blends Two-phase blends of polyamide 6 (PA6) and LDPE have been prepared. Here in the course of reactive extrusion, an in situ grafting of IA on the LDPE takes place. The performance of blending was tested with neutralization and without neutralization of the acid groups of itaconic acid [66]. The maximum increase with regard to the mechanical properties was achieved when magnesium hydroxide was used as a neutralizing agent.

18.2.14.2 Poly(propylene) Functionalized PP by radical melt grafting with monomethyl itaconate or dimethyl itaconate is a compatibilizer in PP/poly(ethylene terephthalate) (PET) blends. Blends with compositions 15/85 and 30/70 by weight of PP and PET, prepared in a single-screw extruder, revealed a very fine and uniform dispersion of the PP phase compared to the respective noncompatibilized blends. An improved adhesion between the two phases is shown. Dimethyl itaconate as compatibilizer derived agent exhibits only a small activity to increase the impact resistance of PET in PP/PET blend. However, monomethyl itaconate is active in this respect. This finding is attributed to the hydrophilic nature of

18: Grafting

monomethyl itaconate. The tensile strength of PET in non-compatibilized blends gradually decreases with increasing content of PP. Blends containing functionalized PP exhibit, in general, higher values [67].

18.2.15 Imidized Maleic Groups The chemical modification of swollen HDPE particles in near-critical propane seems to be much more effective in avoiding crosslinking than the conventional modification in the melt phase. High-density poly(ethylene) grafted with 0.17% poly(ethylene) grafted with maleic anhydride (PE-gMA) can be additionally modified with 1,4-diaminobutane. After formation of amic acid groups, the excess of diaminobutane is extracted with a near critical propane-ethanol mixture. Finally, the obtained PE-g-MA-DAB is imidized to the corresponding imide (PE-g-MI) in the melt. The obtained PE-g-MI shows no increased gel content with respect to the initial PE-g-MA. It appears that PE-g-MI samples react with the anhydride groups of a styrene/maleic anhydride copolymer (SMA) during melt blending of SMA with PEg-MI, while the PE-g-MA do not react [68].

18.2.16 Oxazoline-Modified Polyolefins The free-radical induced grafting of 2-isopropenyl-2oxazoline onto PP has been reported [69].

18.2.17 Modification of Polyolefins with Vinylsilanes Vinylsilanes, e.g., vinyltrimethoxysilane (VTMS), do not readily homopolymerize. The modification of polyolefins with vinylsilanes, such as vinyltrimethylsilane, vinyltriethylsilane, or 3-trimethoxysilylpropylmethacrylate, aims at the preparation of a moisture curable crosslinked polyolefins. For example, the silane grafting of a metallocene ethylene-octene copolymer is carried out in a twin-screw extruder, in the presence of vinyltrimethoxysilane and dicumyl peroxide [26]. These materials are used in the manufacture of electrical cables.

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18.2.17.1 Vinyltriethoxysilane Bicumene initiates the grafting of vinyltriethoxysilane (VTEOS) to poly(ethylene) efficiently over an uncommonly large range of operating temperatures. The analysis of kinetics of bicumene decomposition suggests that the initiation occurs via an autoxidation mechanism that is facilitated by the interaction of cumyl radicals with oxygen [55]. The analysis of poly(ethylene-g-vinyltrimethoxysilane) by differential scanning calorimetry-successive self-nucleation and annealing indicated that the distribution of pendant alkoxysilane grafts amongst polymer chains is not uniform. Fractionation and characterization of a graft-modified model compound, tetradecane-g-VTMS, showed that the composition distributions were influenced strongly by intramolecular hydrogen atom abstraction. It yields multiple grafts per chain as single pendant units and oligomeric grafts. The chain transfer to the methoxy substituent of VTMS grafts contributes significantly to the product distribution [70]. A silane-grafted polyethylene/montmorillonite nanocomposite was prepared by reactive extrusion from LLDPE, VTMS, an organically modified montmorillonite (MMT), and DCP [71]. The morphological and thermal properties of the graft polymer and its nanocomposite have been tested. The graft polymer chains are intercalated into the MMT layers proved and the poly(ethylene) (PE) chains are bonded to the MMT layers. The nanocomposite exhibits a higher thermooxidative stability. The increase in the thermal properties results from the formation of chemical bonds between the PE chains and the OMT layers during the silane grafting and due to a thermooxidative in situ grafting of the PE chains onto the MMT surfaces [71]. The selectivity for the ratio of grafting to crosslinking shows a considerable scope for optimization through variation of monomer and peroxide loadings in the case of VTEOS as modifier, in contrast to maleic anhydride [72].

18.2.18 Ethyl DiazoacetateModified Polyolefins Ethyl diazoacetate and chloroethyl diazoacetate are inserted by a carbene insertion mechanism at 210 °C. No radical initiator is needed, however the grafting efficiency is small [73,74].

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Figure 18.8 Synthesis of quinoneimines. Figure 18.7 3,5-Di-tert -butyl-4-hydroxybenzyl acrylate and trimethylol propane triacrylate.

18.2.19 Grafting Antioxidants Routes for grafting antioxidants onto polyolefins with high grafting yields have been reported. The antioxidant 3,5-di-tert-butyl-4-hydroxybenzyl acrylate (DBBA) reacts with the trifunctional coagent trimethylol propane triacrylate (TRIS), cf. Figure 18.7, in the presence of a small concentration of a free-radical initiator in a poly(propylene) melt during processing. The major reaction is a homopolymerization of the antioxidant in the absence of TRIS. This results in low grafting levels. However, in the presence of TRIS, more than 90% grafting efficiency of DBBA on the polymer is monitored, 6% of DBBA is used. The mechanism of the grafting reaction could be established with decalin, used as a hydrocarbon model compound [24]. The decalin adds with the hydrogen atom on the bridge to the double bond of DBBA. Quinoneimines containing an N-p-hydroxyphenyl and an N-p-aminophenyl substituent have a high antioxidant efficiency when added to isoprene rubber, styrene butadiene rubber (SBR), ethylene/propylene rubber, and ethylene propylene diene monomer rubbers, because they add to the allylic –CH of the polymer giving active adducts. The synthesis of the quinoneimines is shown in Figure 18.8. The retention of the protective activity after extraction of the material indicates the grafting of these compounds during the thermal or mechanical processing of the rubbers [75].

18.2.20 Comonomer Assisted Free-Radical Grafting The idea of using styrene as a comonomer originated from a detailed analysis of the mechanism of free-radical grafting. To obtain high graft efficiency, together with a reduced degradation of polymer, it is essential that the macroradicals in the backbone react with the grafting monomers before they undergo chain scission of the backbone. If the primary monomer is not sufficiently reactive towards the macroradicals, it is helpful to add another monomer that reacts with the macroradicals faster than primary monomer. A further requirement is that the resulting pendent free radicals of the secondary monomer copolymerize readily with the primary monomer. It was shown that the addition of styrene can improve the graft efficiency of monomers such as 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA), glycidyl methacrylate (GMA), but not vinyl acetate (VAc) and ricinoloxazoline maleate (OXA). This is due to the fact that styrene copolymerizes readily with HEMA, MMA, and GMA, but not with VAc and OXA. The ring opening of a pendant oxazoline group is shown in Figure 18.9. Ricinoloxazoline maleate is a bifunctional compatibilizer agent. It can be grafted with the vinyl function of the maleate unit onto a poly(propylene) site by usual radical grafting, thus becoming oxazoline groups attached to the poly(propylene) chain. The oxazoline group can be reacted with the carboxyl groups of poly(butylene terephthalate) [25].

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protocol. TGA allows the calculation of the grafted chain density and average interchain separation on the nanotube surface as a function of the molecular weight [78].

Figure 18.9 Ring opening of a pendant oxazoline group.

18.2.20.1 Styrene-Assisted Grafting Fibrous poly(styrene-b-glycidyl methacrylate) brushes were grafted on poly(styrene divinylbenzene) beads using surface-initiated ATRP [76]. A tetraethyldiethylenetriamine ligand was incorporated on the glycidyl methacrylate block. The ligand attached beads were used for reversible immobilization of lipase. The influences of pH, ionic strength, and initial lipase concentration on the immobilization capacities of the beads have been investigated. Lipase adsorption capacity of the beads is around 78.1 mg g−1 of the beads at pH 6.0. The Michaelis constant Km for immobilized lipase is some 2.1-fold higher than that of free enzyme. Also the thermal, and storage stability of the immobilized lipase is increased. The same support enzyme could be repeatedly used for the immobilization of lipase after regeneration without significant loss in adsorption capacity or enzyme activity. A lipase from Mucor miehei immobilized on the styrene divinylbenzene copolymer was used to catalyze the direct esterification of butyl alcohol and butyric acid [76]. Radiation grafting was performed with styrene onto hydrocarbon and fluorinated polymers [77]. The influence of physical parameters such as vacuum, pressure of air or inert gas, and temperature has been studied. Vacuum and temperature are the dominant parameters. The optimization of these parameters for a specific polymer/monomer system will result in a good performance and allows mutual radiation grafting to be an attractive technique also for commercial applications. Single-walled carbon nanotubes (CNT)s can be grafted with polystyrene chains employing a graft-to-

Maleic Anhydride. The low reactivity of MA with respect to free-radical polymerization is inherently due to its structural symmetry and the deficiency of the electron density around the double bond. It is clear that the addition of a monomer capable of donating electrons, i.e., an electron-rich comonomer, would activate an electron deficient monomer like MA by changing the electron density of the π-bond. The addition of styrene to a melt grafting system as a comonomer of maleic anhydride can significantly enhance the graft degree onto poly(propylene). The maximum graft degree is obtained when the molar ratio of maleic anhydride to styrene is approximately 1:1. Styrene improves the grafting reactivity of maleic anhydride and also reacts with maleic anhydride to form a SMA before grafting onto the poly(propylene) backbone. When the concentration of maleic anhydride is higher than that of styrene, some maleic anhydride monomer reacts with styrene to form SMA, but others can directly graft onto macroradicals on the poly(propylene) chain. When the amount of styrene added is higher than that of maleic anhydride, a part of the styrene monomer may preferentially react with the macroradicals to form macroradicals with styryl ends, while others copolymerize with maleic anhydride to yield SMA [19]. On the other hand, styrene is ineffective as comonomer for maleate esters grafting onto PP [79]. This arises from the low affinity of the styryl radical towards the maleate ester species. This could be predicted from the critical inspection of the monomer reactivity ratios of styrene and maleic esters. Glycidyl Methacrylate. The reactivity of GMA in free-radical grafting onto PP is low. However, adding styrene as a comonomer for glycidyl methacrylate increases both the rate and grafting efficiency. Further the degradation of PP is reduced. It is believed that when styrene is added to such a grafting system, styrene reacts first with PP macroradicals to form

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pendent styryl radicals. These styryl radicals are the starting point for a copolymerization with GMA to form a grafted PP [22]. Poly(propylene) functionalized with glycidyl methacrylate has been used for the compatibilization of poly(propylene) and poly(butylene terephthalate) blends [80]. Similar studies have been done for the grafting of glycidyl methacrylate onto LLDPE [23].

Reactive Polymers: Fundamentals and Applications

Table 18.8 Experimental Techniques for the Characterization of Modified Polyolefins Method Titration FT-IR spectroscopy NMR spectroscopy

13 C NMR spectroscopy

Remarks Maleic anhydride, glycidyl units Most widely used method Chemical shifts are very sensitive to the chemical environment Poor sensitivity

18.2.20.2 Increasing the Grafting Efficiency with Comonomers The mechanisms that result in higher grafting yields by the addition of comonomers can be attributed to [1]: • Longer chain grafts, • More grafting sites, • Use of polyvinyl monomers. Longer chain grafts appear to be the favored alternating copolymerization of electron donor–electron acceptor forming monomer pairs. Examples are styrene, and maleic anhydride. More grafting sites emerge by a more efficient addition of the macroradicals on the backbone by the addition of a comonomer. Polyfunctional monomers effect presumptive branching or crosslinking sites when once grafted onto the backbone. In this way a star shaped or comb shaped grafting center may emerge. An example for this concept is the use of a triacrylate monomer as comonomer [81].

18.2.21 Radiation-Induced Grafting in Solution A suitable solvent for the radiation-induced graft copolymerization of styrene and maleic anhydride (Sty/MA) binary monomers onto HDPE is acetone. Untreated and treated grafted HDPE membranes have potential applications in dialysis [82]. The hydrophilicity of the membrane, the degree of grafting, and the molecular weight and chemical structure of the metabolites, such as urea, creatinine, uric acid, glucose, and phosphate salts, have a great influence on the transport properties of the membrane. The permeability increases with the degree of grafting. Basic metabolites show higher permeation rates through the modified membrane as acidic metabolites, in particular phosphate salts. The per-

meabilities of high molecular weight compounds are low.

18.2.22 Characterization of Polyolefin Graft Copolymers The characterization of the grafted functionality in modified polyolefins is difficult because the small number of modified units are overwhelmed by the normal polyolefin repeat units. The content of modified units is typically only about one to five modified units per molecule in a polymer of typical molecular weight of 20 to 40 kDa [1,2]. Some experimental techniques to characterize modified polyolefins are summarized in Table 18.8.

18.2.23 PVC/LDPE Melt Blends In blends of an LDPE with poly(vinyl chloride) (PVC) during melt blending, chemical reactions take place [83]. This is indicated by changes in the molecular weight, Mn and Mw number-average molecular weight, the polyene and the carbonyl indices, color changes, and the changes of the glass transition and decomposition temperatures. By mixing of LDPE to PVC and melt blending, short-chain LDPE-grafted PVC (s-LDPE-g-PVC) copolymers are formed. On the other hand, the dehydrochlorination reaction of PVC was suppressed.

18.3 Other Polymers Table 18.9 summarizes polymer types other than polyolefins that have been used for grafting other units.

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Table 18.9 Polymers used for Grafting Polymer Poly(styrene) Poly(vinyl chloride) Poly(alkylene terephthalate) Starch Starch

Grafting agent Maleic anhydride n-Butyl methacrylate

Reference [84] [85]

Nadic anhydride

[86]

Vinyl acetate

[87]

Methyl acrylate

[88]

18.3.1 Poly(styrene) Functionalized with Maleic Anhydride MA can be grafted to poly(styrene) (PS) by reactive extrusion in the presence of a free-radical initiator, namely 1,3-Bis(tert-butylperoxyisopropyl)benzene. Its half-life is about 2.5 min at 180 °C. The introduction of the maleic anhydride units in PS proved to be very effective for controlling the morphology of blends of PA6 with modified PS. The rheological properties of the blends indicate the formation of long branching between the amine end groups of PA6 and the maleic anhydride unit of maleic anhydridegrafted poly(styrene) during melt mixing [84].

18.3.2 Multifunctional Monomers for PP/PS Blends Polyolefins do not have reactive functionalities. There are two commonly used approaches for compatibilization in reactive extrusion [89]: 1.

2.

In the two-step process, polymers are functionalized selectively in the first step, and then blended in an extruder in the second step. The grafting reaction should occur between the functionalized groups during blending, and graft copolymers are formed in situ. In the one-step process, low-molecular weight compounds are added into the melted blends to initiate grafting and coupling reactions at the phase interface to form graft or block copolymers during the extrusion process.

Peroxides cause serious chain scission of the PP backbone, which affects the properties of the alloys. Multifunctional monomers, such as glycol trili-

noleate (GTL), trimethylol propane triacrylate, diethylene glycol diacrylate or tripropylene glycol diacrylate in combination with DCP can suppress the PP degradation efficiently, and promote the grafting reaction to some extent at the same time. GTL is prepared by the esterification of glycerol with linoleic acid [89].

18.3.3 Poly(methyl methacrylate) Poly(methyl methacrylate) (PMMA)/SiO2 hybrid composites were prepared via a grafting onto strategy based on UV irradiation in the presence of iron aqueous solution. Two steps were used to graft PMMA onto the surface of nanosilica [90]: 1. Anchoring 3-(methacryloxy) propyl trimethoxysilane onto the surface of nanosilica to modify it with double bonds, and 2. Grafting PMMA onto the surface of nanosilica with FeCl3 as photoinitiator. It was found that it is easy to graft PMMA onto the surface of nanosilica under UV irradiation. The hybrid particles are monodisperse and have core–shell structure with nanosilica as the core and PMMA layers as the shell. Furthermore, the products initiated by FeCl3 exhibit a higher monomer conversion, percent grafting, and better monodispersion in comparison to products initiated by a traditional photoinitiator such as 2-hydroxy-4-(2-hydroxyethoxy)-2-methyl-propiophenone [90].

18.3.4 Poly(ethylene-co-methyl acrylate) Maleic anhydride can be melt-grafted onto poly (ethylene-co-methyl acrylate). The grafting is enhanced with a comonomer, i.e., divinylbenzene or vinyl-4-tert-butylbenzoate. A suitable radical initiator is 1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane (LUPERSOL™ 231). The processing temperature of the internal batch mixer is at 140 °C. It was observed that styrene and vinyl-4-tert-butylbenzoate can significantly increase the amount of anhydride grafted. The styrene comonomer system is most efficient [91]. The use of 1-dodecene in this system showed primarily a plasticizer effect.

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Grafting of PMMA onto poly(ethylene-co-1-octene) can be achieved by an in situ radical polymerization of MMA [92]. In this process, the side reactions are difficult to characterize. To increase the understanding of both the nature and the extent of the reactions, products from a related model system were characterized. There, the polymer is replaced by squalane or pentadecane. The relative selectivity of abstraction of hydrogen from the alkyl hydrocarbon bonds was studied by the reaction of radicals generated from DCP pentadecane (PD), and squalane (SQ) as model compounds for PE. The grafting on the hydrocarbon substrate is related to the reactivity of the C–H bond. The reactivity decreases from tertiary hydrogen via secondary hydrogen to primary hydrogen [92]. The dependence of the molecular weight on the initial conditions is shown in Table 18.10.

18.3.5 n-Butyl Methacrylate Grafted onto Poly(vinyl chloride) Melt grafting of n-butyl methacrylate onto poly(vinyl chloride) was achieved by a melt mixing process with a free-radical initiator [85]. A maximum of 14% graft was obtained. The graft copolymer showed significant improvement in processability and both thermal and mechanical properties.

18.3.6 Starch Esterification Starch esters with low degrees of substitution are prepared in aqueous media by batch methods [93]. Extrusion is not used widely for modification of starch, however, it has great potential. Extruders have been

used to manufacture carboxymethylated and cationic potato starch, starch phosphates, anionic starch, and oxidized starches [94–97]. Starch esters can be synthesized by extruding 70% amylose starch with fatty anhydrides and sodium hydroxide as catalyst in a single-screw extruder. The sodium hydroxide neutralizes the organic acids formed in the course of the reaction. Acetic anhydride, propionic anhydride, heptanoic anhydride, and palmitic anhydride have been used [98]. The degrees of substitution of esterified starch can be determined by hydrolyzing substituted groups with NaOH and then titrating back with acid. The degree of substitution coincides with the expected value from the monomer feeds. Some molecular weight reduction of the amylopectin fraction was detected in the esterified products from corn starch with a 70% amylose content. Lower molecular weights and higher levels of anhydride resulted in the greatest reduction in starch molecular weight. The acid esters decrease the hydrophilic character of the starch. The introduction of heptanoic anhydride and palmitic anhydride results in a higher water absorption index. This is explained by the disruption of the crystalline structure of the starch. By disrupting the crystalline structure of the starch, the opportunity for hydrogen-bonding between starch and water is increased. Clearly, the heptanoic and palmitic acid residues provide a more significant steric hindrance for the formation of starch crystals than the smaller acetic and propionic acid residues. Another approach for the acetylation of starch is the use of vinyl acetate and sodium hydroxide [87]. The acetylation reaction is accompanied by the hydrolysis of vinyl acetate and a consecutive hydrolysis reaction of the acetylated starch. The degree of substitution could be varied from 0.05 to 0.2.

Table 18.10 Degree of Polymerization and Initial Conditions [92] PD (%) 70 70 70 70 60 55 49 –

SQ (%) – – – – – 15 21 70

MMA (%) 30 30 30 30 40 30 30 30

DCP (%) 1.5 1.5 0.5 0.5 0.5 0.5 0.5 0.5

T (°C) 150 170 150 170 170 170 170 170

DPn (–) 37 29 82 44 71 49 50 56

PD Pentadecane; SQ Squalaner; MMA Methyl methacrylate; DCP Dicumyl peroxide; T Reaction temperature DPn Degree of polymerization.

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18.3.7 Starch Grafted Acrylics

18.3.8.2 Tosylation of Cellulose

Starch graft poly(methyl acrylate) could be prepared from an aqueous corn starch slurry and methyl acrylate by the initiation with ceric ions. At the end of the reaction, an additional small amount of ceric ion solution was added. After this addition no unreacted methyl acrylate monomer remained [88]. The grafted starch is intended for the use as loose-fill foam. This type of loose-fill foam has a better moisture and water resistance than other starch-based materials. Graft copolymers of starch and poly(acrylamide) could be prepared by reactive extrusion with ammonium persulfate as initiator [99].

Benzaldehyde-functional cellulose paper sheets have been synthesized by the tosylation of cellulose, followed by addition of p-hydroxy benzaldehyde [101]. By UV-induced Paterno–Büchi [2+2] cycloaddition reactions, these aldehyde functional surfaces can be grafted with triallylcyanurate, trimethylolpropane allyl ether, and vinyl chloroacetate. Allyl-functional polymers, i.e., poly(butyl acrylate) and poly(N-isopropyl acrylamide, were synthesized via reversible addition fragmentation chain transfer polymerization. These polymers were conjugated to the cellulose surface in a UV-induced grafting-to approach. With poly(butyl acrylate) hydrophobic cellulose sheets are obtained with a water contact angle of 116 °. Grafting of poly(N-isopropyl acrylamide allows the formation of smart surfaces, which are hydrophilic at room temperature, but that become hydrophobic when heated above the characteristic lower critical solution temperature at a contact angle of 93° [101]. Thus, the Paterno–Büchi reaction has been shown to be a versatile synthetic tool that also performs well in grafting-to approaches whereby its overall performance seems to be close to that of radical thiol-ene reactions [101].

18.3.8 Cellulose 18.3.8.1 Cellulose Grafted Poly(caprolactone) The grafting of polymers from polysaccharide nanoparticles is an upcoming field. Polysaccharides constitute the largest fraction of renewable biomass on our planet. The controlled decomposition of a native semicrystalline polysaccharide, can be done by acid, enzymatic hydrolysis, or by mechanical disintegration. In this way, nanoparticles with a degree of crystallinity dependent on the method decomposition can be obtained. Monocrystalline cellulose nano-whiskers have been prepared by acid hydrolysis of cotton wool followed by a Soxhlet extraction in ethanol to remove adsorbed impurities [100]. These products were modified with poly(ε-caprolactone) (PCL) using a grafting from approach using citric acid as the catalyst. The influence of the concentration of the catalyst, monomer concentration, reaction time, and reaction temperature was studied in order to optimize the process of ring opening and the extent of grafting. Modified nanoparticles with a PCL content of up to 58% were obtained. This is much more than what can be obtained using a conventional tin(II) ethylhexanoate catalyst. Since it is virtually impossible to remove all the catalyst after the grafting, the use of a benign, naturally available catalyst in the production of such materials occurs in a more environmental friendly way [100].

18.3.8.3 Nitroxide-Mediated Polymerization Nitroxide-mediated polymerization was used for the synthesis of graft and block copolymers using cellulose as a backbone, and PS and PMMA as the branches [102]. For this purpose, cellulose was acetylated using 2-bromoisobutyryl bromide. Then the bromine group was converted to a 4-oxy-2,2,6,6tetramethylpiperidin-1-oxyl group by a substitution nucleophilic reaction to afford a macroinitiator, i.e., cellulose-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). The macroinitiator obtained was subsequently used in controlled graft and block copolymerizations of styrene and MMA monomers to yield cellulosegrafted PS and cellulose-grafted PMMA-PS. From the latter composition, also an organophilic montmorillonite nanocomposite was prepared through a solution intercalation method. The addition of small amounts of organophilic montmorillonite of 3% was enough to improve the thermal stability of the nanocomposite [102].

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18.3.8.4 Cellulose Nanocrystals The self-organization properties of sulfated cellulose nanocrystals, TEMPO-oxidized cellulose nanocrystals and polymer-decorated cellulose nanocrystals suspensions in water were investigated [103]. Polarized light optical microscopy observations showed that these three systems phase separated to form a lower anisotropic chiral-nematic phase and a upper isotropic phase following a nucleation and growth mechanism. This is proving that surfacegrafted polymer chains do not inhibit the self-organization properties of cellulose nanocrystals [103]. The phase diagrams and pitch of the suspensions were shown to strongly depend on the surface chemistry of the nanoparticles and the nature of the interacting forces. The addition of small quantities of a monovalent salt induced an increase of the critical concentrations, but the values for polymer-decorated cellulose nanocrystals were always the smallest ones due to entropic repulsion forces. Thus, the results of this study show that polymer grafting provides an enhanced tunability to the chiral-nematic phase properties of cellulose nanocrystals, including an enhanced expression of the chirality [103].

18.3.8.5 Cellulose Nanofibers Hydrophobic cellulose nanofibers were synthesized using different chemical treatments including polymer and molecular grafting [104]. For polymer grafting, immobilizing poly (butyl acrylate) and PMMA on cellulose nanofibers were implemented using the free radical method. Also, acetyl groups could be introduced directly onto the cellulose nanofibers surface by acetic anhydride for molecular grafting. TGA and X-ray photoelectron spectroscopy analysis showed the high grafting density of PMMA on the surface of cellulose nanofibers [104]. Atomic force microscopy results revealed that molecular grafting created non-uniformity on the cellulose nanofibers surface, as compared to polymer brushes. In addition, the thermodynamic work of adhesion and the work of cohesion for the modified cellulose nanofibers were reduced in water and diiodomethane solvents. The dispersion energy was reduced after modification as a result of decreasing interfacial tension and the dispersibility of modified

Reactive Polymers: Fundamentals and Applications

cellulose nanofibers was improved in diiodomethane [104].

18.3.9 Thermoplastic Phenol/ Formaldehyde Polymers Phenol/formaldehyde resins with high viscosity are needed in reactive extrusion with poly(propylene) to establish a favorable viscosity ratio. Most commercially available phenol/formaldehyde resins have a molar mass of 0.5 to 1 k Da. Only thermoplastic phenol/formaldehyde polymers of the novolac type meet the requirement of avoiding crosslinking in the extruder. High molecular weight novolac-type resins can be obtained by adjusting the ratio of formaldehyde to the phenol near unity [105].

18.3.10 Polyesters and Poly(urethane)s A number of techniques for polymerizing radical polymerizable monomers with polyester resins and poly(urethane) resins to obtain graft or block reaction products have been published. The graft or block reaction products have been studied to improve, for example, the impact resistance of molding compounds by using them as a compatibilizer, the adhesiveness of paints and adhesives to substrates, the curing property of the paints and adhesives, and the dispersibility of pigments [106]. The modification of high molecular weight polyesters introduces polymerizable unsaturated double bonds into the main chain or into the molecular terminal groups. The double bonds can be polymerized with radical polymerizable monomers by graft or block polymerization. Similarly, graft or block modifications for poly(urethane) can be achieved. When a high molecular weight polyester or poly (urethane) is grafted for the modification, crosslinking between the polyester molecules or the poly(urethane) molecules is more likely.

18.3.10.1 Polyesters In the case of polyesters, the sum of polymerizable unsaturated double bonds is desirably up to 20 mol% of the total acid components and diol components. When the sum exceeds 20 mol-%, various properties of the base resin itself are largely reduced.

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18.3.10.2 Polyester Poly(urethane)s The polyester poly(urethane)s should contain up to 30 polymerizable unsaturated double bonds in one molecule.

18.3.10.3 Radical Polymerizable Monomers Radical polymerizable monomers are a mixture of an electron accepting monomer and an electron donor monomer. This combination allows controlling the gelation, even if the resin has a very large amount of unsaturated bonds. Electron donor monomers are styrene, α-methyl styrene, tert-butyl styrene, and N-vinyl pyrrolidone [107]. Electron accepting monomers are fumaric acid, monoesters, and diesters of fumaric acid. Basically gelation can be avoided by a dilution of the polymeric vinyl groups by monomeric vinyl groups that are more prone to copolymerize.

18.3.10.4 Grafting Reaction This technique is a graft polymerization of the polymerizable unsaturated double bond existing in the base resin, i.e., the main chain with the radical polymerizable monomers. The graft polymerization reaction is performed by reacting the base resin, which is dissolved in an organic solvent, with a mixture of the radical polymerizable monomers and a radical initiator. Suitable radical initiators are organic peroxides and organic azo compounds. The organic peroxides include dibenzoyl peroxide and tert-butylperoxypivalate and the organic azo compounds include 2,2 -azobis(isobutyronitrile) and 2,2 -azobis (2,4-dimethylvaleronitrile). A chain transfer agent such as octyl mercaptane, dodecyl mercaptane, 2mercaptoethanol, and α-methyl styrene dimer may be used to control the grafted chain length. The solvents that can be utilized include methylethylketone, methylisobutylketone, cyclohexanone, toluene, xylene, ethyl acetate, and butyl acetate. The solvent itself should neither decompose the radical initiator by induced decomposition nor create a combination with the initiator which causes a danger of explosion that has been reported between specific organic peroxides and specific ketones. Furthermore, it is important that the solvent has a suitably lower chain transfer constant as a reaction solvent for the radical polymerization [106].

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18.3.11 Polyacrylic Hot-Melt Pressure-Sensitive Adhesive A polyacrylic hot-melt pressure-sensitive adhesive is prepared as follows. A copolymer consisting of acrylic acid, tert-butyl acrylamide, maleic anhydride, 2-ethylhexyl acrylate, n-butyl acrylate is manufactured in acetone/isopropanol solution, with 2,2 azobis(2-ethylpropionitrile) as initiator in a batch reactor. This polymer contains anhydride groups that are useful for coupling. The polymer is then degassed from the solvent in an extruder. In the next step, the acrylic hot-melt is compounded with 2hydroxypropyl acrylate. Pendent acrylate groups are formed in this way. This offers the advantage of very gentle crosslinking methods, since crosslinking can be carried out directly by way of the installed acrylate groups. The hot-melt exhibits viscoelastic behavior at room temperature [108].

18.4 Terminal Functionalization

18.4.1 Ene Reaction with Poly(propylene) Polyolefins prepared with Ziegler–Natta processes or metallocene catalysts may carry olefinic end groups. Olefinic end groups are also introduced by melt degradation. A poly(propylene) functionalized at the end groups with anhydride can be obtained via the Alder-ene reaction from a low-molecular weight amorphous poly(propylene) by reactive extrusion. The Alder-ene reaction is a pericyclic reaction with a 6-center intermediate. It involves the reaction of an ene and a enophil. The ene moiety in the Alder-ene reaction is a double bond with an allylic hydrogen. The basic mechanism is shown in Figure 18.10.

Figure 18.10 Basic mechanism of the ene reaction.

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The ene reaction is reversible [109]. However, the reverse reaction seems to be not a simple retro-ene process. The rate of the Alder-ene reaction depends on the acidity and basicity of ene and enophile, respectively. Lewis acids, like SnCl4 , TiCl4 , and AlCl3 develop fumes of hydrochloric acid during reaction. However, a less reactive Lewis acid, SnCl2 ·2 H2 O, can also catalyze the reaction without the drawback of developing HCl. The reaction is complete at 230 °C within 5 min in the presence of a stable radical, such as TEMPO, which acts as a free radical scavenger. This prevents the maleic anhydride from being grafted onto the backbone of the poly(propylene) [4,110]. The maleation of poly(propylene) by reactive extrusion via the Alder-ene reaction produces a terminal functionality of the polymer without significant chain scission.

18.4.2 Styrene Butadiene Rubber The end capping of living anions of poly(styrene butadiene) can be done with polymeric terminator molecules. A polar functional terminator is a block copolymer of poly(ethylene glycol) and poly(dimethyl siloxane) (PEG-PDMS) containing a chlorosilyl moiety at one chain end. This polymer is synthesized by two-step hydrosilylation reaction [111]. The PEG-PDMS end groups behave as polar functional groups, showing an increase of the glass transition temperature and storage modulus in a composite of end capped SBR with silica particles.

18.4.3 Diels–Alder Reaction A benzocyclobutene (BCB) capped polymer can be used to react in a Diels–Alder reaction with another polymer bearing a dienophile [112]. 4-(3iodopropyl)benzocyclobutene was used to terminate an anionic polymerization of styrene to give a poly(styrene) end capped with benzocyclobutene. A copolymer of 1-hexene and 7-methyl-1,6-octadiene was prepared by Ziegler–Natta polymerization, with the pendant double bonds intended as the grafting sites. The reaction is illustrated in Figure 18.11.

Reactive Polymers: Fundamentals and Applications

Figure 18.11 Grafting of an isomerized benzocyclobutene unit to a polyolefin dienophile [112].

18.5 Grafting onto Surfaces The effect of surface grafting on the growth kinetics during controlled radical polymerization was investigated by comparing the growth of polymers in solution with that on a flat silicon surface [113]. The surface-grafted polymers were attached to the surface via a photo-cleavable initiator, which allowed the polymers to be detached by means of UV light with a wavelength that did not lead to polymer photolysis. The molecular weights of surface-grown and solution-grown polymers were determined by size-exclusion chromatography. It could be shown that for a series of polymers synthesized from alkyl methacrylate monomers, it was principally the grafting density that determined the ratio of the molecular weight on the surface to that in solution [113].

18.5.1 Grafting onto Poly(ethylene) 18.5.1.1 Sulfonic Acid Groups In order to introduce sulfonic acid groups on poly (ethylene), poly(ethylene) samples are irradiated with UV light in a gas atmosphere containing SO2 and air to achieve a photosulfonation of the surface. The surface modification is carried out under atmospheric pressure and is considered to be an inexpensive alternative to plasma modification techniques. The hydrophilicity of the PE surface increases considerably compared to unreacted PE. The depth of photomodification reached several μ. Because of the large depth of modification, the process may also be useful for the modification of membranes. In combination with projection lithography the process could be suitable for the manufacture of gratings in thin polymer films, as required for holographic recordings and distributed feedback lasers [114].

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18.5.1.2 Sulfate Groups Sulfate groups at the surface of poly(ethylene) are introduced by immobilizing a precoated layer of either sodium 10-undecenyl sulfate (SUS) or sodium dodecyl sulfate (SDS) on the polymeric surface by means of an argon plasma treatment. SUS is synthesized by sulfating 10-undecene-1-ol with the pyridine-SO3 complex. The presence of sulfate groups at the polymeric surfaces was confirmed by XPS. The presence of an unsaturated bond in the alkyl chain of the surfactant improved the efficiency of the immobilization process. About 25% of the initial amount of sulfate groups in the precoated layer were retained at the PE surface for SUS, but only 6% for SDS [115].

18.5.1.3 Photochemical Bromination The gas phase bromination of poly(ethylene), poly (propylene) and poly(styrene) film surfaces by a freeradical photochemical mechanism occurs with high regioselectivity. The surface bromination is accompanied by a simultaneous dehydrobromination. This results in the formation of long sequences of conjugated double bonds. Thus, the brominated polyolefin surface contains bromide moieties in different chemical environments [116]. In contrast, the gas phase free radical photochemical chlorination of polyolefin films proceeds in a rather random way and is also accompanied by simultaneous dehydrochlorination.

18.5.1.4 Poly(thiophene) Poly(thiophene) can be grafted on a PE film using three reaction steps. 1.

PE films are brominated in the gas phase, yielding PE-Br.

2.

A substitution reaction of PE-Br with 2-thiophene thiolate anion gives the thiophene-functionalized PE.

3.

PT is grafted on the PE surface using chemical oxidative polymerization to give PE-PT.

The polymerization is performed in a suspension solution of anhydrous FeCl3 in CHCl3 , yielding a reddish PE-PT film after dedoping with ethanol. Infrared spectroscopy reveals that the PT is grafted on PE in the 2,5-position. SEM imaging shows islands of PT on the PE film. The thickness of the islands is in the range of 120 to

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145 nm. The conductivity of these thin films is around 10−6 S cm−1 , which is a significant increase from the value of 10−14 S cm−1 measured for an ungrafted PE film [117].

18.5.1.5 Epoxy Resins Hyperbranched Epoxy Polymer. A glass-fiber reinforced epoxy-based composite, grafted by hyperbranched polymer with hydroxyl groups, was evaluated for its mechanical properties and compared with the neat epoxy and silanized glass-fiber [118]. The composites were studied by attenuated total internal reflectance infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, thermal gravimetric analysis, mechanical properties analysis, and field emission-scanning electron microscopy. The results showed that the incorporation of hyperbranched polymer could simultaneously enhance the mechanical properties of the epoxy composites [118]. Epoxy Polymer. An efficient method of promoting the dispersing uniformity of carbon black in epoxy polymer substrate of printed circuit board by chemical grafting [119]. The method shows a promising capability in the application of advanced printable resistor ink. The grafting reaction of epoxy polymer on carbon black particles was investigated with Fourier transform infrared (FTIR), transmission electron microscopy and TGA. The FTIR spectra evidenced the polymerization of epoxy resin with the coupling agent and transmission electron microscopy investigation directly confirmed the polymerization occurred on carbon black surface [119]. The polymerization occurred on the limited part of the carbon black surfaces to form a network-structure polymer to reside on the carbon black particles and hence greatly improved carbon black dispersion in ink as evidenced in ink-droplet spreading verification on glass and printed circuit board resin substrates. On the other hand, the polymer grafting has a limited effect on the increasing of the as-cured ink filled with the grafted carbon blacks [119].

18.5.1.6 Acrylics Ion beam-modified poly(ethylene) was exposed to the solutions of acrylic acid, acrylonitrile, and bromine [120]. The chemical and structural changes were examined using spectroscopic techniques, electronpara-

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magnetic resonance, and Rutherford back-scattering techniques. Acrylic acid, acrylonitrile, and bromine react with radicals and conjugated double bonds created by the ion irradiation in the poly(ethylene). The reactions in the ion beam-modified surface layer may lead to the creation of a grafted surface layer with a thickness of up to 150 nm. Surface photo grafting of HDPE powder can be achieved with a pretreated HDPE surface by BP. Onto such a surface, acrylic acid can be graft copolymerized by photo grafting in the vapor phase [121]. The most suitable reaction temperature is 90 °C. The grafting degree can reach a comparable high value of 10%.

18.5.1.7 Siloxane The dyeing properties on high-strength and high modulus poly(ethylene) fibers are improved by building up a layer of a polysiloxane network. The grafting of siloxane onto poly(ethylene) proceeds first via a treatment with peroxide. Hydrogen peroxide in oxylene emulsion is emulsified by sonication. The emulsion is effective for introduction of hydroxide groups onto the poly(ethylene) fiber surface. The treatment does not influence the tensile strength of the fiber. A polysiloxane network can be built up on the fiber surface by treating the surface with a (3aminopropyl)triethoxysilane solution [122]. In fact, this method can be used to dye a poly(ethylene) fiber surface.

18.5.1.8 Silica Nanoparticles Silica nanoparticles grafted with poly(methyl acrylate) chains whose anchor points are maleimideanthracene cycloadducts have been prepared at various grafting densities in order to demonstrate the fundamental characteristics of mechanophore activation at heterointerfaces [123]. The monotonically decreasing correlation between polymer grafting density and surface-bound maleimide-anthracene mechanophore activation was quantitatively elucidated. As a result of polymerpolymer interactions, the polymer grafting density plays a significant role in heterogeneous mechanophore activation. These findings are a valuable guide in the design of efficient force-sensitive, damagereporting polymer composites, where damage is of-

Reactive Polymers: Fundamentals and Applications

ten localized to the interface between the matrix and the reinforcing phase [123].

18.5.1.9 Silicone The surface graft copolymerization of hydrogen silicone fluid onto an LDPE film through corona discharge shows an improved hydrophobicity of the grafted LDPE films. However, the mechanical properties decrease slightly. Thus there is evidence that HSF can be graft copolymerized onto an LDPE film surface through corona discharge [124]. A robust, highly selective, and efficient method to prepare dense poly(ethylene glycol) (PEG) polymer brushes on silicon substrates has been reported [125]. The method uses the solvent-free, catalystfree, strain-promoted acetylene-azide cycloaddition reaction. First, poly(glycidyl methacrylate) is grafted to the silicon substrate as an anchoring layer to immobilize cyclopropenone-caged dibenzocyclooctyneamine via an epoxy ring-opening reaction providing protected, stable, and functionalized substrates. Then, three synthesized α-methoxy-ω-azido-PEGs of different molecular weights (5, 10, and 20 kg mol−1 ) were grafted to the modified silicon substrates from the melt after the deprotection of dibenzocyclooctyne with UV-irradiation. The highest grafting density obtained was close to 1.2 chains/nm2 and was achieved for 5 kg mol−1 PEG. The prepared PEG polymer brushes displayed efficient antifouling properties and stability in Phosphate-buffered saline aqueous media for a period of at least two months [125]. Considerable attention has been paid to the manipulation and the control of the physicochemical properties of porous silicon surfaces, because of their crucial importance to the modern microelectronics industry [126]. Hybrid structures consisting of deposited polymer on porous silicon surfaces are important to applications in microelectronics, photovoltaics and sensors. In many cases, the polymer can provide excellent mechanical and chemical protection of the substrate, changes the electrochemical interface characteristics of the substrate, and provides new ways to the functionalization of porous silicon surfaces for molecular recognition and sensing. A porous silicon surface was modified by anodic treatment in an ethynylmagnesium bromide electrolyte leading to the formation of a polymeric layer bearing some bromine substituents [126]. Subsequently, the formed polymer was functionalized with

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amine molecules containing functional groups, such as carboxylic acid or pyridine, by a substitution reaction between bromine sites and amine groups, the Hofmann reaction. The chemical composition of the modified porous silicon surfaces was investigated and the grafting of polymeric chains and functional groups on the porous silicon surface was confirmed by FTIR and XPS, which displayed the principal characteristic peaks attributed to the different functional groups [126].

18.5.1.10 Graphene Graphene oxide as a two-dimensional nanoscale material, due to its unique structure and remarkable chemical and physical properties, has attracted a great deal of attention in recent years. However, its use is limited due to its agglomeration. Graft polymerization was utilized as an effective strategy to overcome this issue. The covalent and non-covalent modification of graphene oxide through polymer grafting has detailed in a monograph [127].

18.5.1.11 Carbon Nanotubes Nitroxide Radical Coupling. Poly(butylene succinate) (PBS) was grafted on the surface of TEMPO modified multiwalled carbon nanotubes (MWCNT)s via a nitroxide radical coupling reaction [128]. The TEMPO functionalized MWCNTs were synthesized using the Cu(I)-catalyzed azide/alkyne click chemistry approach. The formation of a covalent bond of the nitroxide moieties onto the MWCNT was confirmed via electron paramagnetic resonance spectroscopy. The PBS grafting on the sidewalls of the MWCNTs was carried out in solution via peroxide-induced formation of macroradicals. The grafting improves both the quality of stress transfer across the polymernanotube interface and the degree of dispersion of the filler, which also exhibited a moderate nucleating action on the PBS [128]. Diarylcarbenes. A direct and nondestructive method to functionalize CNTs has been reported [129]. A highly reactive diarylcarbene derivative was designed and synthesized. In contrast to previous approaches, this diarylcarbene contains ATRP initiator segments, which can serve as starting points for further polymer grafting. Then, the initiator segments

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are covalently bonded to the CNTs via a one-step cycloaddition of diarylcarbene and the succeeding ATRP links PS chains to the CNTs. In order to verify the effectiveness of modification, the fabricated PS-CNTs were used as reinforcement to enhance PS films. By the addition of 0.5% of the PS functionalized CNTs, the PS composite films reveal increases of 79.3% in tensile strength and 85.2% increases in Young’s modulus [129].

18.5.1.12 Surface Crosslinking Ultra-high molecular weight poly(ethylene) (UHMWPE) can be crosslinked at the surface by irradiation with electron beams [130]. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR) infrared techniques suggest that the irradiation in air atmosphere introduced hydroperoxide groups into the polymer without formation of any other oxygen-containing groups. The generated hydroperoxides could be decomposed further by subsequent heat treatment of the irradiated polymer, resulting in crosslinking of UHMWPE chains in the region of the material near the surface. As a result of this surface modification, the surface hardness of UHMWPE substantially increases.

18.5.1.13 Grafting Using Calix[4]arene An attractive methodology based on diazonium chemistry has been developed for the surface modification of polymers, such as PP, PET, and PS [131]. The grafting procedure involves the in situ formation of diazoates in basic aqueous solution. The reactivity of calix[4]arene-tetradiazonium salts and a classical aryldiazonium salt was examined through comparative studies on gold and polymer surfaces. The surfaces were analyzed with a combination of techniques such as atomic force microscopy, XPS, and ellipsometry. The results indicated that the calix[4]arene molecules are grafted as a robust and uniform monolayer both on gold and polymer surfaces, allowing a fine control over surface modification. Furthermore, the chemical postfunctionalization of the grafted calixarene platforms equipped with carboxylic-pendant groups was successfully performed with either an amine or an alcohol. These results open real possibilities in the controlled immobilization on polymers of a wide variety of molecules

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of interest such as biomolecules or chromophores and in the tailoring of polymer properties [131].

18.5.2 Grafting onto Poly(vinylidene fluoride) Economic and easy methods to tailor the surface properties of polymers, such as poly(vinylidene fluoride), without altering the bulk properties are of interest for different applications, e.g., as biotechnological devices and medical implant devices. UV irradiation is one of the simplest, easy and safe method to modify the surface properties. In the case of self-initiated grafting, it is generally assumed that the pre-treatment of the poly(vinylidene fluoride) surface with UV irradiation can yield both alkyl and peroxy radicals that appear by the breaking of bonds. These radicals are capable of initiating a subsequent surface grafting. PVDF surface modification is generally performed with different acrylate monomers owing to many advantageous properties of the corresponding polymers: hydrophilicity, biocompatibility, antifouling or antibacterial properties. However, it is possible to achieve polymer grafting using low energetic UV-A irradiation in the range of 3.1–3.9 eV without breaking bonds in the poly(vinylidene fluoride) polymer [132]. It is known that the surface of poly(vinylidene fluoride) has chemical defects such as double bonds and oxygen-containing groups. These moieties can be activated by UV radiation for grafting.

18.5.3 Grafting onto Poly(tetrafluoroethylene) 18.5.3.1 Diazonium Salts The use of aryl diazonium salts for grafting has been reviewed [133]. Functionalization of poly(tetrafluoroethylene) (PTFE) surfaces can be achieved by diazonium salts. Reduced PTFE can be grafted by nitro and bromophenyldiazonium tetrafluoroborate salts in a manner similar to that used for carbon, except that no application of a reductive potential during grafting is required. The grafting is evidenced by cyclic voltametry, X-ray fluorescence or time-of-flight single-ion monitoring mass spectroscopy [134–136].

18.5.3.2 Epoxide-Containing Monomers A pretreated PTFE film with argon plasma can be further modified by a graft copolymerization with hydrophilic and epoxide-containing monomers. The grafting is initiated by UV light. Functional monomers for grafting include acrylic acid (AA), sodium salt of p-styrenesulfonic acid, N,N-dimethylacrylamide (DMAA), and GMA. A stratified surface microstructure with a significantly higher ratio of substrate to grafted chains in the top surface layer than in the subsurface layer is always obtained. The grafted PTFE films show a number of new issues. These include [137]: • Covalent immobilization of an enzyme, such as trypsin, for AA graft copolymerized surface, • Change transfer included coating of an electroactive polymer, such as polyaniline, for AA and styrenesulfonic acid graft copolymerized surfaces, • Adhesive-free adhesion between two PTFE surfaces, for AA, styrenesulfonic acid and DMAA graft copolymerized surfaces, • Improved adhesive bonding via interfacial crosslinking of the grafted chains, for GMA graft copolymerized surfaces.

18.5.3.3 2-Hydroxyethyl Acrylate Surface modifications of Ar plasma pretreated PTFE film via graft copolymerization improve the adhesion of copper. The PTFE film surface is initially modified by graft copolymerization with a monomer, such as 2hydroxyethyl acrylate and acrylamide. These monomers contain the functional groups for epoxide groups. The modified PTFE surface is subsequently again exposed to an Ar plasma and subjected to UVinduced graft copolymerization with glycidyl methacrylate [138].

18.5.3.4 2-Hydroxyethyl Methacrylate The grafting of biocompatible poly(hydroxyethyl methacrylate) onto titanium dioxide nanoparticles can be done in a very simple way [139]. The grafting process is based on the chemical reduction of diazonium salts by reducing agents in presence of the vinylic monomer. On flat surfaces, strongly grafted and stable polymer films are formed. The process has many advantages such as a short one-step reaction

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occurring at atmospheric pressure, ambient air, and room temperature in water. TiO2 nanoparticles were synthesized by laser pyrolysis. Nanoparticles with controlled size and composition are then obtained. It was demonstrated that a poly(hydroxyethyl methacrylate) shell was successfully chemically grafted onto the surface of the TiO2 core without any significant influence on the morphology of the nanoparticles [139].

18.5.3.5 Glycidyl Methacrylate The surface modification of a PTFE film is done by the deposition of GMA in the presence of H2 plasma activation of the PTFE substrates. The H2 plasma treatment results in an effective defluorination and hydrogenation of the PTFE surface. This enhances the adhesion of Cu vapor onto the PTFE surface. In addition, a plasma polymerization with glycidyl methacrylate is performed. High adhesion strength for the Cu on such a surface is obtained only in the presence of H2 plasma activation of the PTFE substrates prior to the plasma polymerization and deposition of GMA. In the absence of H2 plasma preactivation, the deposited pp-GMA layer on the PTFE surface can be readily removed by acetone extraction. The enhancement of the adhesion of the Cu on the surface is attributed to the covalent bonding of the pp-GMA layer with the PTFE surface, the preservation of the epoxide functional groups in the pp-GMA layer, and the strong interaction of evaporated Cu atoms with the epoxide and carboxyl groups of the GMA chains [140].

18.5.3.6 Oxygen and Ammonia Plasmas PTFE can be treated in oxygen or ammonia plasmas in order to introduce oxygen-containing or nitrogencontaining groups, respectively. These groups increase the surface free energy and allow the adsorption of polyelectrolytes via electrostatic interactions [141]. The effects of such a modification can be evaluated by means of contact angle measurements.

18.6

Special Applications

18.6.1 Stimuli-Responsive Polymers A simple method of preparing stimuli-responsive PS/PCL nanolayered films by growing poly(N-iso-

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propylacrylamide) (PNIPAM) brush on the surface by surface-initiated polymerization has been reported [142]. An ATRP initiator with a benzophenone moiety is attached onto the surface by UV irradiation. After the ATRP polymerization, poly(Nisopropylacrylamide) brush films with varying thicknesses can be produced. XPS confirmed the successful deposition of the initiator and grafting of the polymer. The behavior of the brush film as a function of temperature could be demonstrated by contact angle experiments. Photopatterning is also achieved by using a photomask which was confirmed by FTIR imaging [142].

18.6.2 Photovoltaic Polymers Alkylthio groups have received much attention because of their molecular design applications in polymer solar cells [143]. The alkylthio substitution on the conjugated thiophene side chains in benzodithiophene-based and benzodithiophenedione-based photovoltaic polymer was used to improve the extinction coefficient. The introduction of alkylthio groups into the polymer increased its extinction coefficient while the HOMO levels, bandgaps, and absorption bands remained the same. Thus, the short circuit current density and the efficiency of the device became much better than those of conventional materials. Thus, the introduction of the alkylthio functional group in polymer is an effective method to tune the extinction coefficient of the photovoltaic polymer. In this way, the photovoltaic performance can be improved without increasing the active layer thickness, which will be very helpful to design advanced photovoltaic materials for a high photovoltaic performance [143].

18.6.3 Fuel Cells A series of triazole-grafted sulfonated poly(arylene ether ketone sulfone)s were fabricated via an amide coupling reaction between 3-amino-1,2,4-triazole and carboxylic acid group [144]. The degree of sulfonation was set to 80% for preventing an excessive swelling. The chemical structures of synthesized polymers were characterized by FTIR and 1H NMR spectroscopy. The triazole group affected the properties of membranes dramatically.

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Increasing the triazole group content was conductive to promoting the thermal property and oxidative stability. All the poly(arylene ether ketone sulfone) membranes retained above 92% of their weight after the test in Fenton’s reagent at 80 °C for 1 h. The swelling ratio of a poly(arylene ether ketone sulfone)-3 membrane was only 12.66% and this is lower than that of a C-SPAEKS-3 membrane at 100 °C. It was also found that the proton conductivity was distinctly improved by introducing triazole groups and the poly(arylene ether ketone sulfone)-4 membrane exhibited a proton conductivity of 0.166 S cm−1 at 120 °C, which was larger than that of Nafion 117. In addition, all poly(arylene ether ketone sulfone) membranes showed a higher proton conductivity when compared with C-SPAEKS-3 membranes over the range of relative humidity from 20% to 90% at 80 °C [144].

18.6.4 Electrolyte Membrane A composite membrane containing phosphoric acid for its possible application in a high temperature proton exchange membrane fuel cell was prepared by radiation-induced copolymerization of 1-vinylimidazole and 1-vinyl-2-pyrrolidone onto poly(ethylene-alt-tetrafluoroethylene) films followed by protonation through phosphoric acid doping. The preparation procedure involved three steps [145]: 1.

Irradiation of the poly(ethylene-alt-tetrafluoroethylene) films by an electron beam accelerator,

2.

Copolymerization of 1-vinylimidazole and 1vinyl-2-pyrrolidone onto the electron beam preirradiated poly(ethylene-alt-tetrafluoroethylene) films, and

3.

Acid doping of the grafted poly(ethylene-alttetrafluoroethylene) films with phosphoric acid.

The physicochemical properties of the resulted membranes were analyzed in terms of the degree of grafting, grafting compositions, ionic conductivity, thermal properties and thermal stability using ATRFT-IR, XPS, TGA, and DSC. The results showed that the physiochemical properties of the membranes are comparable to Nafion 117 especially their thermal stability. At 120 °C and 0% relative humidity, the membrane remained stable at 76% degree of grafting and 7.6 mmol repeat polymer unit−1 with ionic conductivity of 53 mS cm−1 [145].

The characterization tests indicated that the membrane displayed impressive thermos-chemical and physical properties with less water dependency. At 200 °C the membrane remained thermally stable which enhances the potential application of the membrane in high temperature proton exchange membrane fuel cell operating at 100 °C and above [145].

18.6.5 Ion-Imprinted Polymers Hydrophilic poly(2-hydroxyethyl methacrylate) brushes were modified onto the surface of ion-imprinted polymers via addition-fragmentation chain transfer polymerization [146]. Several grafting densities of ion-imprinted polymers were obtained, as revealed by analysis using GPC and Brunauer Emmett Teller. All the grafted ion-imprinted polymers showed good anti-interference properties compared to the ungrafted ionimprinted polymers, although the adsorption capacity of the ungrafted ion-imprinted polymers was higher than that of grafted ion-imprinted polymers in pure water. The grafted ion-imprinted polymer, with a grafting density of β = 1.17 chains per nm2 , exhibited superior anti-interference ability in silica and polymer flocculant simulated wastewater. Moreover, it remained steady after 10 adsorption-desorption cycles [146]. SEM-EDX and XPS data revealed anti-interference and anti-blockage mechanisms in which hydrophilic poly(2-hydroxyethyl methacrylate) brushes could effectively adhere to fine particles and flocculants through Van der Waals force interactions, which make the imprinted cavities well protected in a complex wastewater environment. Also, these grafted ion-imprinted polymers exhibit similar adsorption rate constants that are approximately 2 times greater than those of ungrafted ion-imprinted polymers [146].

18.6.6 Medical Uses 18.6.6.1 Living Cell-Polymer Hybrid Structures A cytocompatible method of surface-initiated, activator regenerated by electron transfer, ATRP has been developed for the engineering of cell surfaces with synthetic polymers [147]. Dopamine-based ATRP initiators are used for both introducing the ATRP initiator onto chemically complex cell surfaces uni-

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formly and protecting the cells from a radical attack during polymerization. Synthetic polymers were grafted onto the surface of individual yeast cells without a significant loss of cell viability. The uniform and dense grafting could be confirmed by various characterization methods, including agglutination assay and cell-division studies. Thus, a strategic approach to the generation of living cell-polymer hybrid structures has been presented which opens the door to their application in a multitude of areas, such as sensor technology, catalysis, theranostics, and cell therapy [147].

18.6.6.2 Vitamin E Blended Polyethylene Artificial knee joints are continuously loaded by higher contact stress than artificial hip joints due to a less conformity and much smaller contact area between the femoral and tibial surfaces. The higher contact stress causes severe surface damage such as pitting or delamination of PE tibial inserts [148]. To decrease the risks of these surface damages, the oxidation degradation of crosslinked PE induced by residual free radicals resulting from γ -ray irradiation for crosslinking or sterilization should be prevented. Vitamin E blended PE, as an antioxidant, has been used to solve these problems. In addition, osteolysis induced by PE wear particles, bone cement, and metallic debris is recognized as one of the important problems for total knee arthroplasty. To decrease the generation of PE wear particles, the bearing surface mimicking the articular cartilage has been developed. Grafting a biocompatible polymer, poly(2methacryloyloxyethyl phosphorylcholine), onto the PE surface exhibits a high wear resistance. The properties of such materials that can be used for artificial knee joints have been studied in detail [148]. The grafted composition is expected to be one of the great bearing materials not only preventing surface damages due to higher contact stress and oxidation degradation but also improving wear resistance, and to provide much more lifelong artificial knee joints.

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functional polymer nanorods for pH-sensitive drug release of doxorubicin. Water-soluble cylindrical polymer brushes have been produced via a straightforward one-step grafting of vinyl benzaldehyde and poly(ethylene glycol) methyl ether methacrylate comonomers, in which the vinyl benzaldehyde distributed throughout the cylindrical polymer brushes provides a cost-effective and simple functionality for the subsequent conjugation of doxorubicin using imine chemistry. Atomic force microscopy showed the rod-like conformation of the cylindrical polymer brushes prior and after drug conjugation. Fluorescence spectroscopy studies revealed faster drug release in acidic environments (pH 5.0) compared to physiological pH conditions (pH 7.4). Fluorescence lifetime imaging microscopy and in vitro cell studies further highlighted the intracellular doxorubicin release from the cylindrical polymer brushes drug carriers within MCF-7 breast cancer cells [149].

18.6.6.4 Polymeric Prodrug The design of a polymeric prodrug of the anticancer agent paclitaxel by a grafting-from-drug approach has been reported [150]. Here, a chain transfer agent for reversible addition-fragmentation chain transfer polymerization could be efficiently and regioselectively linked to the C2 position of paclitaxel, which is crucial for its bioactivity. Subsequent reversible addition-fragmentation chain transfer polymerization of a hydrophilic monomer yielded well-defined paclitaxel-polymer conjugates with a high drug loading, water solubility, and stability. Further, the versatility of this approach was demonstrated by ω-end post-functionalization with a fluorescent tracer. In vitro experiments showed that these conjugates are readily taken up into endosomes where native paclitaxel is efficiently cleaved off and then reaches its subcellular target. This was confirmed by the cytotoxicity profile of the conjugate, which matches those of commercial paclitaxel formulations based on mere physical encapsulation [150].

18.6.6.3 Triggered Drug Release

18.6.6.5 Filamentous Viruses

The use of the grafting from approach to produce inherently rod-shaped polymer nanoparticles with triggered drug release has been described [149]. Cylindrical polymer brushes can be directly used to yield

The force exerted on protein complexes like filamentous viruses by the strong interchain repulsion of polymer brushes has been investigated [151]. This force can induce subtle changes of the constituent

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subunits at a molecular scale. Such changes transform into the macroscopic rearrangement of the chiral ordering of the rodlike virus in three dimensions. For this, a straightforward grafting-to PEGylation method has been developed to densely graft a filamentous virus with PEG. The grafting density is so high that the PEG is in the polymer brush regime, resulting in straight and thick rodlike particles with a thin viral backbone. A scission of the densely PEGylated viruses into fragments was observed due to the steric repulsion of the PEG brush, as facilitated by adsorption onto a mica surface [151]. The high grafting density of PEG endows the virus with an isotropic-nematic liquid crystal phase transition that is independent of the ionic strength and the densely PEGylated viruses enter into the nematic liquid crystal phase at much lower virus concentrations. While the intact virus and the one grafted with PEG of low grafting density can form a chiral nematic liquid crystal phase, the densely PEGylated viruses only form a pure nematic liquid crystal phase. This can be traced back to the secondary to tertiary structural change of the major coat protein of the virus, driven by the steric repulsion of the PEG brush [151].

18.6.6.6 Determination of Celecoxib A method has been reported for the surface grafting of N-vinylcaprolactam as a thermosensitive agent and allylimidazole with an affinity toward celecoxib onto magnetic nanoparticles [152]. The grafted nanoparticles were characterized by FTIR spectroscopy, elemental analysis, and TGA. The surface morphology was studied using SEM. The grafted nanoparticles could be used for the determination of traces of celecoxib in biological human fluids and pharmaceutical samples. The profile of celecoxib uptake by the modified magnetic nanoparticles indicated a good accessibility of the active sites in the grafted copolymer. It was found that the adsorption behavior could be fitted by the Langmuir adsorption isotherm model. Solidphase extraction for biological fluids such as urine and serum was investigated. An urine extraction recovery of more than 95% was obtained [152].

Reactive Polymers: Fundamentals and Applications

18.6.6.7 Protein Separation A weak cation exchange liquid chromatography stationary phase, i.e., nylon modified with carboxyl groups was prepared by grafting poly(acrylic acid) on to native nylon 6 capillary-channeled polymer fibers via a microwave-assisted radical polymerization [153]. The capillary-channeled polymer fiber surfaces were characterized by attenuated total reflection infrared spectroscopy and SEM. The anticipated carbonyl peak at 1722.9 cm−1 was found on the nylon-COOH fibers, but was not found on the native fiber, indicating the presence of the polyacrylic acid on nylon fibers after grafting. The nylon-COOH phase showed a ca. 12 times increase in the lysozyme dynamic binding capacity of around 12 mg ml−1 when compared to the native fiber phase which showed only ca. 1 mg ml−1 [153]. The loading capacity of the nylon-COOH phase is nearly independent of the lysozyme loading concentration and the mobile phase linear velocity. The reproducibility of the lysozyme recovery was found to be very fine. Fast baseline separations of myoglobin, α-chymotrypsinogen A, cytochrome C and lysozyme could be achieved with a nylon-COOH column [153].

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