Degradation Behavior of Nanocomposite Polymer Blends

CHAPTER 12

Degradation Behavior of Nanocomposite Polymer Blends Roberto Scaffaro and Luigi Botta Department of Civil, Environmental, Aerospace, and Material Engineering, University of Palermo, Palermo, Italy

Chapter Outline 12.1 Introduction

423

12.1.1 General Aspects of Polymer Degradation 424 12.1.2 Basic Concepts of Degradation of Polymer Blends 428

12.2 Thermal Degradation of Polymer Blend Nanocomposites

429

12.2.1 Role of the Filler 429 12.2.1.1 Clay 429 12.2.1.2 Other Particles 435 12.2.2 Role of Polymer Matrix 435

12.3 Photodegradation of Polymer Blend Nanocomposites

437

12.3.1 Role of Clay in Accelerating Degradation 438 12.3.2 Role of the Polymer Matrix 439

12.4 Conclusion 442 References 443

12.1 Introduction Blending two or more polymers is a common practice in obtaining materials with new enhanced characteristics that are different from those of the parent components. It is done by using common processing techniques instead of expensive and high-environmentalimpact chemical syntheses [1 4]. Another way to improve the final characteristic of a polymeric material is to add fillers that have a reinforcing effect, to provide the material with new properties or simply to decrease the cost. Recently, there have been successful attempts to disperse fillers at a nanometer scale, both in solution and in the melt. The small dimension of the filler (i.e., high specific surface) Nanostructured Polymer Blends. DOI: http://dx.doi.org/10.1016/B978-1-4557-3159-6.00012-2 © 2014 Elsevier Inc. All rights reserved.

423

424 Chapter 12 ensures a significant improvement of several physical properties (e.g., mechanical, thermomechanical) by adding only small amounts, thus preserving other important properties such as transparency, density, and processability. Of course, it is necessary to have good dispersion and adhesion and, for this purpose, the nanofillers are properly treated or modified. In other cases, an additive is used as a third component. The nanoscale fillers, which are considered to be very important, include layered silicates (e.g., montmorillonite), nanotubes (mainly carbon nanotubes), metal oxides (e.g., TiO2, Fe2O3, Al2O3), metal nanoparticles (e.g., Au, Ag), polyhedral oligomeric silsesquioxane (POSS), carbon nanomaterials (e.g., graphene), silica nanoparticles, and so on [5 15]. Several research groups have tried to prepare nanocomposites based on polymer blends’ matrices to produce materials with further improved performances [16 33]. The efforts were mainly focused on the study of morphology property relationships. In particular, it has been observed that the uneven distribution of nanoparticles in the polymer phases may result in unexpected increases of the mechanical and transport properties. This effect was attributed to the synergism among the reinforcing action of the filler and the benefits deriving from the changes in the blend’s microstructure, such as the refinement of the morphology, the enhancement of the interfacial adhesion, and the possible formation of cocontinuous morphologies [28 33]. However, the usefulness of any material depends on its degradability and durability. This chapter reviews the research on degradation of nanocomposite polymer blends and, in particular, on clay-nanocomposite polymer blends, paying particular attention to both the role of the filler and the role of the blend morphology and composition.

12.1.1 General Aspects of Polymer Degradation Natural degradation of polymers refers to the exposure of polymers to natural outdoor conditions where direct or indirect sunlight, heat, oxygen, moisture, and other factors contribute to the degradation of material properties. Microorganisms, ozone, airborne chemical pollutants such as sulfur oxides and nitrogen oxides, and salt are some of the factors that are of significance. Exposure to the outdoor environment not only affects the polymeric material itself, but also acts on other components within the matrix, such as dyes, pigments, processing additives, absorbers, and stabilizers. Each of these components reacts to the environment individually or in combination with other components. In addition, the various factors in the natural environment, which include principally solar radiation, temperature, humidity, oxidative environment, and industrial pollutants, could act interactively in the degradation process. Thus, the overall degradation effects can be extremely complex.

Degradation Behavior of Nanocomposite Polymer Blends 425 Depending on the cause, different types of polymer degradation are distinguished: thermal degradation (heat), thermo-oxidative degradation (heat and oxygen), thermomechanical degradation (heat and stress), photodegradation (light), photo-oxidative degradation (light and oxygen), biodegradation (biological agents), mechanical degradation (mechanical stresses), and so on. Nevertheless, much more frequently, a polymeric material undergoes thermal, thermo-oxidative, and photo-oxidative degradation during its processing and its service life. Laboratory devices (usually referred to as artificial or accelerated weathering devices) are used to obtain information regarding the potential degradation behavior of polymeric materials. Although these devices have some shortcomings, they provide valuable information regarding material behavior. Laboratory devices usually involve controlled conditions where exposure to various factors can be standardized and compared. They also permit the isolation and control of specific environmental factors for detailed evaluation. The most important advantage of laboratory devices is that they may accelerate degradation of the materials under consideration. Acceleration is achieved either by increasing the period of exposure to variables such as light, heat, and moisture (e.g., continuous exposure to artificial light, heat, and wet/dry cycles) or increasing intensity of exposure. The service lifetime of a polymeric material depends strongly on the specifications of the application under consideration. Often, visible surface degradation, such as discoloration and loss in transparency and gloss, is apparent before significant changes in the bulk physical properties such as tensile strength, elongation, or electrical properties are observed [34 35]. For the purpose of increasing the durability of polymeric materials by protecting them from environmental factors or by reducing the degradation rate, different stabilizers can be incorporated into a polymer matrix [34 38], but the knowledge of the degradation mechanism of the polymer is absolutely necessary for improving the material stability. In Figure 12.1 the general accepted pathways of degradation and stabilization of a polymer are shown. The figure is based on an original scheme for autocatalytic oxidation of hydrocarbons [38 39]. The degradation initiation involves the loss of a hydrogen atom from the polymer chain as a result of energy input from heat or light. This creates a highly reactive and unstable polymer free radical (R•) and a hydrogen atom with an unpaired electron (H•). The degradation propagation can involve different reaction paths including the interaction between a free radical (R•) with oxygen (O2) to form a peroxy radical (ROO•), which can then remove a hydrogen atom from another polymer chain to form a hydroperoxide (ROOH), thus regenerating the free radical (R•). The hydroperoxide can then split into two new free radicals, (RO•) 1 (•OH), which will continue to propagate the reaction to other

426 Chapter 12 Hydroperoxide decomposer

H' - Donor Quencher

O2 Radiation

RH

[RH]*

hu/Δ

RH

ROO'

R'

R'

+

ROOH

RH

hu Δ

Radical scavenger

RO'

HO'

UV-Absorber

Figure 12.1 General pathways of degradation and stabilization of polymers.

polymer molecules. The process can therefore accelerate depending on how easy it is to remove the hydrogen from the polymer chain. Chemical groups that result from thermal oxidation include peroxides, hydroperoxides, aldehydes, ketones, and carboxylic acids. These groups are photolabile outdoors; for example, solar radiation below 360 nm is sufficiently energetic to break down hydroperoxides: hν

ROOH - RO 1 OH

(12.1)

Along with peroxides, hydroperoxides are a ready source of reaction intermediates, and they have a marked effect on the overall rate of thermal-oxidative or photo-oxidative breakdown as they are a source of chain branching. Carbonyl groups, such as aldehydes and ketones, common products of processing, also adsorb solar ultraviolet radiation. Their incorporation into the structure of a polymer by prolonged processing in the presence of oxygen can reduce the subsequent resistance of the polymer to photo-oxidation. Carbonyl groups can also be introduced into the structure at the polymerization stage by incorporating a suitable monomer, such as methyl vinyl ketone. This approach, for example, has been developed commercially to produce photodegradable polymers. The aliphatic ketones decompose photolytically by two primary reactions. The Norrish I reaction involves a homolytic scission of the molecule to give two free radicals, as shown in Figure 12.2(a). The Norrish II reaction gives rise to an olefin and a methyl ketone and does not appear to proceed via a radical mechanism (Figure 12.2(b)). Because of their mobility, ketone groups near chain ends or in short side chains are more likely to break down photolytically than long-chain ketones. Both hydroperoxides and carbonyl groups are recognized as chromophores, which initiate photodegradation, but hydroperoxides play the more important role in the early stages of

Degradation Behavior of Nanocomposite Polymer Blends 427

Figure 12.2 Norrish I (a) and Norrish II (b) reaction mechanisms for the degradation of aliphatic ketones.

photo-oxidation. Moreover, since hydroperoxides are intermediate products of the underlying thermal-oxidative chain mechanism, by increasing their concentration by prolonged processing, polymer photostability decreases. Generally, the degradation of polymers can be studied by monitoring the changes of their structure and of their properties. The validity of each technique obviously depends on the nature of the material and the type of degradation phenomenon. If a technique is relevant to the characterization of a polymer, it is often an appropriate means of assessing the degraded material. Nondestructive characterizations are often preferred since they limit the number of specimens required and, because the same specimen can be reexamined, they can reduce variability in test data. A sensitive technique has the advantage of allowing the early stages of degradation to be detected and so reduces the time required for assessment. For this approach to be valid, however, the relationship of the sensitive test to practical performance (e.g. carbonyl growth versus embrittlement) has to be established. The impact properties, the tensile properties, and, in particular, the elongation at break, are excellent parameters to define the degradation resistance of polymeric materials as they are very sensitive to molecular and morphological changes undergone by the polymer during degradation phenomena. Indeed, these changes strongly affect the values of the elongation at break. Measurements of viscosity and gel content are useful to indicate the extent of modifications to molecular structure. Change in the gel fraction can reveal the relative importance of scission and cross-linking. Gel permeation chromatography analysis gives detailed information on molecular weight and molecular weight distribution of degraded polymers allowing the study of the scission and cross-linking process occurring during the degradation. Spectroscopic techniques such as ultraviolet (UV) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy are widely used for detecting the degradation of polymers. In particular, FTIR spectroscopy is one of the most widely used techniques for monitoring changes in the chemical structure of polymers.

428 Chapter 12 It is capable of identifying and following quantitatively the loss or increase of the functional chemical groups correlated to modifications of the chemical structure of the material during degradation. For example, the growth of carbonyl can be related to the loss of important mechanical properties such as ductility.

12.1.2 Basic Concepts of Degradation of Polymer Blends The main difference between degradation of pure homogeneous polymers and that of polymer blends arises from their heterogeneous structure. However, in homogeneous blends, degradation patterns are possible that do not typically occur in homopolymers. Indeed, interactions are possible among the different species in the blends during degradation and among the degradation products. These chemical reactions can lead either to an acceleration of the degradation rate with respect to that of the pure components or to a stabilizing effect. These interactions can occur both in the bulk of the two phases and in their interface. They can be grouped into six processes: 1. 2. 3. 4. 5. 6.

Reactions between Reactions between Reactions between Reactions between Reactions between Reactions between

macromolecules and small molecules macromolecules and small radicals macroradicals and small molecules two small molecules two macroradicals macromolecules and macroradicals

Small molecules and radicals are, of course, degradation products of the pure polymers. The last two kinds of reactions can occur only at the interface. All these reactions give rise to new chemical species, which affect not only the degradation behavior of the blends but also other physical properties. The last two kinds of chemical reactions, for example, can produce copolymers that could act as stabilizer agents of the two incompatible phases, leading to an improvement of some mechanical properties in competition with the decay of the same characteristics due to the degradation. Otherwise, reactions with small molecules or small radicals can give rise to both the fast scission of the macromolecules and to chemical structures that act as stabilizer groups. The degradation behavior of the polymer blends is hard to predict on the basis of the degradation mechanisms of the single components, and both synergistic and antagonistic effects may be observed. An intermediate behavior that obeys the mixture rule is expected only for blends in which interactions between the two phases, and especially between macromolecules and products of the incoming degradation, are negligible. The miscibility of the components is not determinant. Indeed, only in some cases does the miscibility seem to play an important role in the degradation of polymer blends. In general,

Degradation Behavior of Nanocomposite Polymer Blends 429 both compatible and incompatible blends show degradation features very different from those expected on the basis of the degradation behavior of the individual components. Generally, the resulting properties of the degraded blends are also difficult to predict because they depend not only on the extent of degradation of the two homopolymers but also on the presence of new species produced through the interactions discussed. Moreover, the processes are complicated by reactivity of compatibilizers.

12.2 Thermal Degradation of Polymer Blend Nanocomposites The incorporation of nanoscale particles into a polymer matrix can generally be done in four ways [9 10]: 1. Solution method: Involves dissolution of polymers in adequate solvent with nanoscale particles and evaporation of solvent or precipitation. 2. Melt mixing: The polymer is directly melt-mixed with the nanoparticle. 3. In-situ polymerization: The nanoparticles are first dispersed in liquid monomer or monomer solution. Polymerization is then performed in the presence of nanoscale particles. 4. Template synthesis: Using polymers as a template, the nanoscale particles are synthesized from a precursor solution. The method most widely used is melt mixing, which allows the use of common processing equipment thus making it possible to produce large amounts of materials in a single step without using solvents. However, this method involves high processing temperatures that can compromise the thermal stability of the nanofiller and consequently promote the degradation of the host polymers during processing. Moreover, it is important to evaluate the thermal stability of nanocomposite polymer blends in terms of decomposition temperatures.

12.2.1 Role of the Filler 12.2.1.1 Clay Recently, many studies focused their attention on the degradation of polymer nanocomposites under different environments, and, in particular, of polymeric materials incorporating layered clay minerals [40 58]. The layered silicates mainly used for the preparation of polymer blend nanocomposites are mica, fluoromica, hectorite, fluorohectorite, and saponite. The one most used is montmorillonite (MMT), which belongs to the structural family known as the 2:1 phyllosilicates. Their crystal structure (Figure 12.3) consists of layers made up of two silica

430 Chapter 12 Al, Fe, Mg, Li OH O

Tetrahedral

Li, Na, Rb, Cs

Octahedral

Tetrahedral Exchangeable cations

Figure 12.3 Crystal structure of 2:1 layered silicates.

tetrahedral sheets fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The stacking of the layers leads to a regular van der Waals gap between them (interlayer or gallery). Isomorphic substitution within the layers generates charge deficiency (i.e., Fe21 or Mg21 replacing Al31 in MMT, and Li1 replacing Mg21 in hectorite). The deficit charges are compensated for by cations (usually Na1 or K1) absorbed between the three-layer clay mineral sandwiches. These are held relatively loosely, giving rise to the significant cation-exchange properties. It is well established that these cations can be replaced by organic cations such as alkylammonium ions to render the clay, naturally hydrophilic, more organophilic [59,60]. These organophilic montmorillonite (OMMT) clays, which have lower surface energy and are more compatible with organic polymers, increase the possibility of polymer molecules to intercalate within the galleries under well-defined experimental conditions. When these layered silicates are incorporated in a polymer, depending on the nature of the components used (layered silicate, organic cation, and polymer matrix) and the method of preparation, different structure/morphologies of composites can be obtained [6 9]. As already mentioned, direct polymer melt intercalation is the most attractive preparation method because of its relatively low cost, high productivity, and compatibility with current polymer processing techniques. If the processing temperature is higher than the thermal stability of the organic component used for clay modification, decomposition will take place to some extent, leading to variations of the interaction between the filler and the polymer matrix. Thus, it is critical to determine at the onset temperature of degradation, the chemical nature of the degradation products and the stability of the polymer matrix in the presence of layered silicates. Of course, it is also relevant to understand the relationships between the molecular structure and the thermal stability (decomposition temperature and rate, degradation products) of the organic modifier of the layered silicate [61,62]. In

Degradation Behavior of Nanocomposite Polymer Blends 431 particular, the thermal stability was considered a key factor, playing a role in the nanocomposite structure and morphology formation [63]. It is therefore very important to consider the type of organic modifier used for the modification of the clay. Studies have shown that different methods of synthesis and types of organophilic MMT influence both the morphology and thermal stability of polymer/clay nanocomposites [41]. The commonly used organo-modification agents are long carbon-chain alkyl ammonium salts. The quaternary ammonium ion is nominally chosen to compatibilize the layered silicate with a given polymer resin. However, the molecular structure (length and number of alkyl chains and unsaturation) is also the determining factor of the thermal stability of the polymer/MMT nanocomposites. It was found by several authors [64 67] that these surfactants degrade between 200 C and 500 C. The weight loss during thermogravimetric analysis of various organoclays indicates that surfactants with multiple alkyl tails have higher thermal stability than those with a single alkyl tail. It is well established that thermal degradation of ammonium salts generally proceeds either by Hofmann elimination and/or by SN2 nucleophilic substitution reaction. During thermal degradation, according to the Hofmann degradation mechanism (Figure 12.4), the ammonium cation loses an amine and an α-olefin that eventually transforms into various carboxyl compounds, leaving an acid proton on the surface of the MMT [64,65]. The presence of these clay acidic sites catalyzes the degradation of the organic modifier that occurs in three steps: (1) degradation of the free organic modifier, (2) degradation of the physically absorbed organic modifier, and (3) degradation extended to the chemically bound modifier. The degradation of the free modifier occurs in a single step, and the bound

CH3 HT

N

+

HT

CH3 Δ

α-olefin

CH3

+

N HT

[O2]

[O2] Δ

CH3

H2O OHC

H2O

Δ

HOOC [O2] Δ CO2 ; H2O

Figure 12.4 Hofmann degradation mechanism of ammonium salts of organoclays.

432 Chapter 12 modifier degrades in two steps at temperatures respectively lower and higher if compared with the neat salt. This can be explained considering that, in the earliest phases of the reaction, the catalysis of the mineral clay accelerates the degradation. Later, the mineral clay acts like a barrier trapping the volatile degradation products and thus reducing the degradation kinetic. Studies about the degradation of an organically modified montmorillonite sample under different atmospheres (nitrogen, air, and oxygen-enriched atmosphere) and for different thermal treatment exposure times revealed that the decomposition of an organic modifier provokes a collapse of the clay mineral particles that presents interlayer spacing very similar to that measured for unmodified montmorillonite [68,69]. A higher amount of organic modifier increases the degradation kinetics independently of the atmosphere used. However, the decomposition of organic modifier causes the decrease of the interlamellar distance, as fast as the oxidative nature of the atmosphere increases. Moreover, during the thermal treatment, the organic modifier in the interlamellar spaces undergoes a rearrangement, while the external unbound modifier degrades. At long exposure times the degradation gradually extends to the clay galleries involving the internal modifier. The thermal instability of organic modifiers can affect the exfoliation of the particles, the interface interactions between the filler and the polymer matrix, and the effectiveness of some additives. In addition, the degradation products may cause undesired color change, promote the degradation of the matrix, and induce microcracks that reduce the mechanical resistance [69 76]. As is well known, an important shortcoming of blending different polymers is that, as a rule, the couples form immiscible and incompatible blends that display a poor morphology with particles of the dispersed phase that are badly adherent to the matrix and badly distributed. Consequently, appropriate compatibilizers are very often used to improve the compatibility of the polymer couple, thus improving the morphology and the properties of the blend. Unfortunately, the thermal instability of the organic modifiers and, in particular, the degradation products of the modified clay can negatively interact with the compatibilizers and reduce their effectiveness. This phenomenon was shown by high density polyethylene (HDPE) and polyamide-6 (PA6) blends containing an organically modified montmorillonite (OMMT) and different compatibilizing systems prepared by melt compounding with a corotating twin screw extruder [77,78]. Indeed, despite a good morphology achieved in the filled blends and a moderate intercalation level, the mechanical performance, especially the properties at break, are not satisfactory and surprisingly the highest increments are observed in the binary HDPE/PA6 blend. This phenomenon can be attributed to the possible interaction between the compatibilizing system and the degradation products of the modified clay caused by thermo-oxidation during processing. In particular, the products coming from the degradation

Degradation Behavior of Nanocomposite Polymer Blends 433 of the organic modifier may interact with the matrix thus their solubility in it would play a vital role in the process. To be more precise, the α-olefins produced with the Hoffman reaction disperse quickly from the clay galleries toward the HDPE phase due to their affinity with the polyolefin phase. As a consequence this causes the collapse of the tactoid and the reduction of the interlayer distance. This diffusion is slower when the affinity is lower, such as for PA6. In this case the final effect would be a sort of swelling of the tactoid caused by the permanence of the degradation products between the clay layers. Of course, the interactions of the degradation products with the matrix can contribute to decrease of the mechanical properties. Indeed, the total processing time in contact with air, including the extrusion and the preparation of the compression molding sheets, can be estimated as 7 to 10 minutes and, therefore, important changes may affect the OMMT during the preparation of the blend. To prevent these phenomena, a stabilizing system can be added to the nanocomposite blends. The stabilized nanocomposites show a finer morphology (Figure 12.5) and consequently an improved mechanical performance. A similar strategy was adopted in the preparation of a compatibilized high density polyethylene (HDPE)/poly(ethylene-co-vinyl acetate) (EVA) blend containing an ammonium quaternary salts modified montmorillonite designed to prevent the thermooxidative degradation of the OMMT during the processing of the nanocomposites [79].

Figure 12.5 SEM micrographs of compatibilized PA6/HDPE (25/75) blends filled with 5% of organoclay (Cloisite 15A) with or without a stabilizing system: (a) PA6/HDPE/EAA/PBO/15A, (b) PA6/ HDPE/EAA/PBO/15A/stab, (c) PA6/HDPE/EGMA/15A, (d) PA6/HDPE/EGMA/15A/stab. Source: Reprinted from [78], Copyright (2010), with permission from Elsevier.

434 Chapter 12 Indeed, the presence of the stabilizer prevents the thermo-oxidative degradation of the organic modifier by hindering the SN2 nucleophilic substitution reactions between alkyl ammonium chains and oxygen molecules. Moreover, the protective role of stabilizers may inhibit the formation of destructive degradation products which could collapse the organoclay tactoids and also deactivate the compatibilizer. However, it is worth noting that if the processing temperature is sufficiently lower than the decomposition temperature of the OMMT used, the thermal degradation of the organic modifier during melt blending is not responsible for the observed interlayer spacing decrease as observed for the heterophasic polypropylene copolymers (PP-EP–poly(ethyleneco-vinyl acetate)) (EVA)-organoclay nanocomposites [80]. Nevertheless, many studies indicate that the introduction of layered silicates into polymer matrix results in an increase of thermal stability of the nanocomposite polymer blends [81 85]. The improved thermal stability is revealed from the results of thermogravimetry (TGA), which indicate that nanocomposite polymer blends show values of onset decomposition temperature (Tonset) and maximum degradation temperature (Tmax) higher than those of the corresponding neat polymer blends. Similar results were obtained for different polymer blend matrices: EVA-linear low density polyethylene (LLDPE) [81]; PP-EP/EVA; [82] poly(phenylene oxide) (PPO)/polystyrene (PS) [83]; polyamide 6 (PA6)/ acrylonitrile-butadiene-styrene (ABS) [84]; poly(L-lactic acid) (PLLA)/poly(ethylene oxide) (PEO) [85]. The changes of thermal stability of polymer blend clay nanocomposites can be attributed to a characteristic structure of layers within the polymer matrix, to their shape, and to their dimensions. The layers of clay silicates are impermeable for gases, that is, both intercalated and exfoliated structures get created in a labyrinth for gas penetrating the polymer bulk. Thus, the labyrinth effect limits the oxygen diffusion inside the nanocomposite sample. Similarly, the small molecules generated during the thermal decomposition process can not permeate but have to bypass the clay layers. Thus, the addition of clay slows down the release rate of the decomposed byproducts and hence enhances the thermal stability of the nanocomposites. Moreover, clay layers can reduce heat conduction. In the presence of silicate layers strongly interacting with polymer matrix, the motions of polymer chains are hindered. This effect brings additional stabilization in the case of polymer/clay nanocomposites. Moreover, nanocomposites exhibit more intensive char formation on the surface of samples exposed to heat by protecting the sample bulk thus decreasing the mass loss rate during thermal decomposition. The changes in thermal stability at high temperature can also be attributed to the portion of polymeric chains that are trapped in-between the silicate layers. This portion of chain, or

Degradation Behavior of Nanocomposite Polymer Blends 435 trapped mass of the polymer, can remain preserved by the heat effect. Indeed, the surface of clay acts as a barrier for the heat flow. Generally, the thermal stability of polymer blend clay nanocomposites is related to its content and dispersion. In particular, the exfoliation and dispersion level in polymer blend nanocomposites has an important role in improving the thermal stability. More specifically, a high exfoliation degree leads to greater improvements in thermal stability. Agglomerated clay particles do not significantly affect the thermal stability of the polymer blend matrix [84]. 12.2.1.2 Other Particles Very few works report the effect of other nanoparticles on the thermal degradation of nanocomposite polymer blends. However, it was shown that the incorporation of multiwalled carbon nanotubes (MWNT) in a poly(lactic acid) PLA–poly(butylene adipateco-butylene terephthalate) (PBAT) polymer blend enhances its thermal property [86]. Thermal stability of compatibilized blends of polyamide 12 (PA12) and PP is improved with the boehmite alumina nanoparticles addition. This-improvement can also be attributed to the reinforcement effect of the nanoparticles [87].

12.2.2 Role of Polymer Matrix Many studies have investigated the effect of clay on thermal degradation of nanocomposite polymer blends, but so far less attention has been paid to elucidating the role of blend morphology and blend composition on the thermal/thermo-oxidative degradation mechanism. These features are very important because they take into account the role of polymer blends as a matrix in polymer nanocomposites. In particular, they highlight the main differences between the thermal degradation behavior of polymer blend based nanocomposites and that of single polymer nanocomposites. The correlations between the morphological and thermal degradation properties were investigated in compatibilized PP/EVA blends containing various amounts of OMMT [88]. The morphology of this nanocomposite system was modified by changing the compatibilizer/OMMT weight ratio (C/O). Indeed, an increase in C/O monotonically decreases the EVA droplet size (Dn) but it has a dual influence on the clay interlayer spacing; in other words, initially an increase in C/O ratio up to a certain level increases the interlayer spacing until a maximum, observing a progressive decrease for higher values of C/O ratios. The TEM analysis revealed the existence of intercalated/exfoliated clay structures in the PP/EVA nanocomposites and also showed that OMMT layers are mainly located in the EVA phase. The decrease of the EVA droplet size and interparticle distance led to an increase of the activation energies of both thermal/thermo-oxidative degradation processes thus enhancing the thermal stability. Moreover, the reduction in EVA droplet size

436 Chapter 12 significantly increases the char yielding in both N2 and O2 atmospheres. The correlation of the morphological parameters with the TGA results shows that the morphological characteristics play a significant role in the degradation process. Therefore, a model (Figure 12.6) can be proposed for thermal/thermo-oxidative degradation of polymer blend nanocomposites [88]. In the proposed model, the blend morphology (i.e., the minor phase droplet size and its size distribution) plays an important role in controlling degradation behavior. The fine droplet size and the low interparticle distance of minor phase domains containing OMMT layers lead to the formation of some sort of network structures during the burning process. This network evolves into a homogenous charred residual which can Sample with Low Dn

Sample with High Dn PP phase EVA phase OMMT Before Degradation Char

20% Conversion

Char

50% Conversion

Char Thermal Degradation

Thermo Oxidative Degradation

Figure 12.6 Proposed model for thermal/thermo-oxidative degradation of polymer blend nanocomposites. Source: Reprinted from [88], Copyright (2010), with permission from Elsevier.

Degradation Behavior of Nanocomposite Polymer Blends 437 act as a good barrier against the degradation process. It means that the charred residue is dependent on the droplet size and on the interparticle distance of the minor phase. The charred residual in the case of polymer blend nanocomposite systems with a finer size of the minor phase is coarser than that of a polymer blend nanocomposite with a larger domain size. The polymer blend composition can strongly influence the thermal degradation behavior of the polymer blend nanocomposites. Indeed, in the case of poly(styrene-co-acrylonitrile) (SAN)/ poly(methylmethacrylate) (PMMA) blends containing various amounts of OMMT, the thermal degradation stability increases with the SAN content [89]. In particular, the results of TGA analyses show that, although the variation in Tmax is not monotonic with polymer blend composition, the polymer blend nanocomposites exhibit an overall improvement of the degradation stability on increasing the SAN content (i.e., SAN content plays a major role in dictating the improvement of degradation stability in these hybrids). For each SAN/PMMA blend is a clay loading that minimizes the thermal degradation. In addition, it was observed that this optimum clay level decreases on increasing the SAN content. Also for PP-EP/EVA blend clay nanocomposites, the composition of the matrix influences the thermal stability of the nanocomposite polymer blend. The maximum degradation temperature increases on increasing the EVA content [82].

12.3 Photodegradation of Polymer Blend Nanocomposites In outdoor applications such as sheets for packaging and automotive products, it is particularly important to take care of the resistance to photo-oxidation. Exposure of polymers in their use conditions provokes photo-oxidation, which causes the decrease of their macroscopic properties due to the variation of molecular weight, chemical structure, and morphology [90 92]. As widely reported in numerous research papers [93 104], the presence of the organo-modified clay in different homopolymers leads to accelerated photodegradation. Explanations for this phenomenon are: decomposition of the ammonium ions (it can lead to the formation of the catalytic acidic sites on the layers); the catalytic effect of the iron impurities; generation of a supplement amount of radicals coming from the oxidation of the modifier alkyl chain; and some migration of the polar antioxidant onto the clay (this effect is more pronounced for unmodified silicate). The photo-oxidation of polymer blend nanocomposites is an even more complex phenomenon, depending on many additional factors such as the coexistence of radicals coming from the blend constituents (i.e., the specific degradation paths of the components and uneven reaction between them). Although different kinds of nanoparticles are used to prepare polymer blend nanocomposites, the studies on the photodegradation behavior of this materials class concern only the polymer blend clay nanocomposites.

438 Chapter 12

12.3.1 Role of Clay in Accelerating Degradation The presence of OMMT strongly influences the photodegradation behavior of the polymer matrix. Generally, the organoclay nanocomposites degrade faster than the pristine polymers because of the degradation of the alkyl-ammonium cation exchanged in MMT and the catalytic effect of iron impurities of the OMMT. Iron (Fe31) can catalyze the decomposition of the primary hydroperoxide formed by photo-oxidation of polymers [96,104,105]. Moreover, the decomposition of ammonium ion may create acidic sites on the layered silicates; the complex crystallographic structure and habit of clay minerals can also result in some active sites. Thus, the reversible photoredox reaction of transition metal cations has a catalytic effect on the degradation of the polymer matrix. All these catalytic active sites can accept single electrons from donor molecules of polymer matrix and induce the formation of free radicals on UV irradiation. The generation of free radicals leads to the oxidization and break of molecular chain. Thus, the materials suffer degradation and their mechanical strength decreases. The role of the organo modifier in accelerating the photodegradation is highlighted by the results of accelerated weathering of LDPE/EVA nanocomposites containing OMMT with different amounts of organic modifier [100]. Figure 12.7 shows the dimensionless elongation at break (Figure 12.7(a)) and the carbonyl index as a function of the exposure time of LDPE and LDPE/EVA (75/25) films with two different clays. In particular CL15A and CL20A are two OMMT modified with the same organic modifier but at different concentration, that is, 95 meq/100 g clay and 125 meq/100 Sg clay, respectively. The elongation at break is the mechanical parameter most sensitive to the structural and morphological changes achieved by the materials during the photo-oxidation exposure. The presence of organo-modified nanoparticles significantly reduces the photo-oxidation induction time (i.e., the time at which the elongation at break starts to reduce itself) and accelerates the photo-oxidation rate (evaluated from the slope of the curve at the induction time). The half time of the elongation at break, measured as the time at which the value of the elongation at break is one half of the initial one, is considered as the maximum time at which the material can be used. The values of the induction and half time and the dimensionless curves of the elongation at break highlight that the photo-oxidative degradation rate of nanocomposites containing CL15A is faster than that of those containing CL20A. The high organo-modifier concentration of the CL15A certainly eases the formation of supplementary amounts of radicals. This fact can explain the higher carbonyl formation and the faster loss of the mechanical performance of the films filled with CL15A if compared with the films filled with CL20A. Moreover, the better dispersion of CL15A (CL15A is more hydrophobic than CL20A) increases the interfacial surface, and this consequently increases the prodegradation activity. The photo-oxidation behavior of the filled films depends on many different factors, as well as the radical formation from both

Degradation Behavior of Nanocomposite Polymer Blends 439

Figure 12.7 Dimensionless elongation at break (a) and carbonyl index (b) as a function of the exposure time of LDPE and LDPE/EVA (75/25) films with two different clays. Source: Reprinted from [100], Copyright (2009), with permission from Elsevier.

organo-modifier oxidation products and matrix, photoprodegradant action of the iron ions, catalytic effect of photoproducts, and so on. The synergism of these factors at high exposure times reduces the difference between the two clay-filled films in the mechanical response, whereas some significant difference is observed in the carbonyl index (Figure 12.7(b)).

12.3.2 Role of the Polymer Matrix The blend composition can significantly affect the photodegradative behavior of polymer blend nanocomposites. Indeed, in LDPE/EVA films containing CL15A, although the presence of the OMMT reduces the photo-oxidation induction time and half time of

440 Chapter 12 elongation at break, when the EVA content is increased the photostability of the nanocomposite blend increases as well [100]. In fact, the drop of the dimensionless elongation at break of the EVA-based nanocomposites occurs at photo-oxidation time much longer than that of the LDPE-based one (see Figure 12.8(a)). Moreover, the values of the photo-oxidation induction time and of the half time increase with increasing content of EVA in LDPE/EVA-based nanocomposites. The types of polymer matrix and, in particular, the matrix polarity play an important role in the photo-oxidation behavior of the

Figure 12.8 Dimensionless elongation at break (a) and carbonyl index (b) as a function of the exposure time of LDPE/EVA films with different blend compositions. Source: Reprinted from [100], Copyright (2009), with permission from Elsevier.

Degradation Behavior of Nanocomposite Polymer Blends 441

Figure 12.9 Dimensionless elongation at break (a) and variation of the carbonyl band area (b) as a function of the exposure time of unfilled and OMMT-filled LDPE/PA6 and HDPE/PA6 blends. Source: Reprinted from [106], Copyright (2010), with permission from Elsevier.

polyolefin/clay nanocomposites because of the different ability to solubilize the alkyl radicals coming from the degradation and/or decomposition of the organo modifier. The values of the carbonyl index as a function of the irradiation time (Figure 12.8(b)) corroborate the mechanical behavior. The increase of the carbonyl index is larger on increasing the CL15A, and on increasing the LDPE content in LDPE/EVA blend films. The kind of polyethylene (LDPE or HDPE) in PE/PA6 nanocomposite blends affects the photodegradation behavior of PE/PA6 blends containing the 5% of an OMMT [106]. Indeed, an accurate characterization before the accelerated aging reveals a drastic impact of the organoclay on the morphology of the nanocomposite blends. In particular, the formation

442 Chapter 12 of a cocontinuous morphology in the HDPE-based blend was observed, and an improvement of interfacial adhesion was observed for the LDPE-based blend. Due to the higher degree of crystallinity of the HDPE phase, the unfilled and OMMT-filled HDPE/PA6 blends show a photoresistance higher than the LDPE-based blends with respect to the accelerated UV-B aging. Mechanical tests and FTIR analysis (Figure 12.9(a) and (b)) indicate that the presence of the OMMT results in an accelerated photo-oxidation degradation for the LDPE-based system. On the contrary, the organoclay improves the photo-oxidation resistance of the HDPE/PA6 blend. This could be explained in the light of the complex microstructure exhibited by this sample, in which poor interfacial adhesion and low specific interfacial area could hinder the propagation of the organic modifier degradation products and the diffusion of polyamide radicals into the polyethylene phase.

12.4 Conclusion Generally, clay has two opposite effects on the thermal stability of nanocomposite polymer blends: (1) a promoter effect of the polymer matrix degradation, which decreases the thermal stability, and (2) a barrier effect, which improves the thermal stability. The promoter effect of degradation is mainly due to thermal instability of the organic modifiers of organoclays. Indeed, the degradation products of the organic modifiers can promote the degradation of the polymer matrix and interact with the compatibilizers and reduce their effectiveness. Also the presence of hydroxyl groups on the edges of the clay and/or of active catalytic sites can promote the polymer degradation. On the contrary, improvement of thermal stability of polymer blend clay-nanocomposites can be attributed to: (1) the contribution of the clay to a tortuosity path and to the reduction of polymer molecular mobility; (2) low permeability and a decrease in the rate of evolution of the formed volatile products; and (3) the formation of high-performance carbonaceous silicate char on the nanoparticles’ surface which insulates the underlying material and slows the escape of volatile products generated during the decomposition. The presence of organoclay reduces the photostability of nanocomposite polymer blends. Generally, clay nanocomposite polymer blends degrade faster than pristine matrices. This behavior is attributed to decomposition of the ammonium ions (it can lead to the formation of the catalytic acidic sites on the layers); the catalytic effect of the iron impurities; and the generation of a supplement amount of radicals coming from the oxidation of the modifier alkyl chain. Despite the effect of the filler, blend morphology and blend composition can significantly affect, either positively or negatively, the degradation behavior of polymer blend nanocomposites.

Degradation Behavior of Nanocomposite Polymer Blends 443

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