Nanocellular materials from polystyrene nanocomposites by in-situ polymerization

Trabajo Fin de Máster Máster en Física Nanocellular materials from polystyrene nanocomposites by in-situ polymerization Autor: Pablo Andrés Romero Proaño Tutor/es: Dr. Miguel Ángel Rodríguez Pérez Dr. Karina Carla Nuñez Carrero Resumen El uso de nanopartículas en matrices poliméricas ofrece un gran potencial en la mejora de las propiedades mecánicas, térmicas, eléctricas, entre otras de polímeros puros y compuestos convencionales. Sin embargo, para aprovechar al máximo las ventajas de los nanocompuestos, es fundamental lograr una dispersión homogénea de las nanopartículas en la matriz polimérica. En este trabajo, se explora la preparación de nanocompuestos de poliestireno (PS)/sepiolita mediante el proceso de polimerización in situ, con el objetivo de mejorar la dispersión de la sepiolita y su interacción con el polímero. Se utilizó sepiolita sin modificar y sepiolita modificada superficialmente con viniltrietoxisilano (VTES). Durante el proceso de modificación superficial se optimizaron las condiciones de reacción para maximizar la cantidad de grupos silano injertados en la superficie de la sepiolita a traves de la metodología de diseño de experimentos (DoE). Mediante reología dinámica de cizalla se encontró la formación de una red de percolación en los compuestos al agregar 1% en peso de sepiolita sin modificar a la reacción polimerización in situ. A pesar del bajo contenido de sepiolita, la densidad de partículas fue lo suficientemente alta como para formar una red de percolación debido a la excelente dispersión lograda mediante la polimerización in situ. Finalmente, se exploró, inicialmente, la preparación de espumas a partir de estos compuestos. La estructura celular de las espumas se estudió a través de microscopía electrónica de barrido. Se determinó que la modificación superficial de la sepiolita es importante en la generación de estructuras celulares debido a la unión covalente entre el polímero y la carga. Palabras clave: Nanopartículas, polimerización in situ, nanocompuesto, dispersión, sepiolita, poliestireno, red de percolación. i Abstract The use of nanoparticles in polymer matrices offers great potential for improving the mechanical, thermal, electrical, and other properties of neat polymers and conventional composites. However, to fully exploit the benefits of nanocomposites, it is essential to achieve a homogeneous dispersion of the nanoparticles in the polymer matrix. This study explores the preparation of polystyrene (PS)/sepiolite nanocomposites through the in situ polymerization process to improve the dispersion of sepiolite and its interaction with the polymer. Both unmodified sepiolite and sepiolite surface-modified with vinyltriethoxysilane (VTES) were employed. The reaction conditions for the surface modification process were optimized to maximize the amount of silane groups grafted onto the sepiolite surface using the design of experiments (DoE) methodology. Dynamic shear rheology revealed the formation of a percolation network in the composites with the addition of 1 wt% unmodified sepiolite during in situ polymerization. Despite the low sepiolite content, the particle density was high enough to form a percolation network due to the excellent dispersion achieved through in situ polymerization. Furthermore, initial exploration was carried out on the preparation of foams from these nanocomposites. The cellular structure of the foams was examined using scanning electron microscopy (SEM). It was determined that the surface modification of sepiolite plays a crucial role in generating cellular structures through the covalent bonding between the polymer and the filler. Keywords: Nanoparticles, in situ polymerization, nanocomposite, dispersion, sepiolite, polystyrene, percolation network. ii Contents List of Figures iv List of Tables vi 1 Introduction 1 2 Objectives 7 3 Methodology 8 3.1 Chemicalsandmaterials .................................. 8 3.2 DoEsmethodology ..................................... 8 3.3 Sepiolite silanization reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.4 Nanocomposites in situ polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.4.1 Suspension polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.5 Gas dissolution foaming process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.6 Characterization....................................... 11 3.6.1 Thermogravimetric analysis (TGA) . . . . . . . . . . . . . . . . . . . . . . . . 11 3.6.2 Differential scanning calorimetry (DSC) . . . . . . . . . . . . . . . . . . . . . . 11 3.6.3 Fourier transform infrared spectroscopy (FTIR) . . . . . . . . . . . . . . . . . . 11 3.6.4 Gel permeation chromatography (GPC) . . . . . . . . . . . . . . . . . . . . . . 12 3.6.5 Rheologicaltests.................................. 12 3.6.6 X-rayradiography ................................. 13 3.6.7 Densitymeasurements ............................... 14 3.6.8 Scanning electron microscopy (SEM) . . . . . . . . . . . . . . . . . . . . . . . 14 4 Results & Discussion 15 4.1 Sepiolite surface modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.1.1 FirstDoEresults .................................. 16 4.1.2 SecondDoEresults................................. 19 4.1.3 Statistical analyses and model adequacy for grafing percentage . . . . . . . . . . 20 4.1.4 Sepiolite thermogravimetric analysis . . . . . . . . . . . . . . . . . . . . . . . . 20 4.1.5 Sepiolite FTIR analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.2 Polystyrene/sepiolite nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.2.1 X-ray radiography images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 iii 4.2.2 Molecular weight and molecular weight distribution . . . . . . . . . . . . . . . 26 4.2.3 Thermalanalysis .................................. 30 4.2.4 Rheology...................................... 31 4.3 PS/sepiolitefoams...................................... 35 5 Conclusions & Outlook 38 6 Annexes 40 Bibliography 43 iv List of Figures 1.1 TEM image of sepiolite and their corresponding structure . . . . . . . . . . . . . . . . . 2 1.2 Schematic representation of clay nanocomposite preparation methods. . . . . . . . . . . 3 1.3 Vinyltriethoxysilane (VTES) chemical structure. . . . . . . . . . . . . . . . . . . . . . . 5 3.1 Schematic representation of cellular materials formation under gas dissolution foaming process. ........................................... 10 3.2 Characteristic rheological behavior of a polymer . . . . . . . . . . . . . . . . . . . . . . 13 3.3 3D diagram of the setup employed to perform the X-ray radiography measurements. . . . 14 4.1 Surface modification reaction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.2 Pareto chart of the standardized effects for measured silane grafting percentage in DoE 1. 16 4.3 Main effect plots of the four factors studied in DoE1 for silane grafting percentage. . . . 17 4.4 Interaction plots of pH and reaction time factors for silane grafting percentage. . . . . . . 18 4.5 Contour plots of VTES grafting % vs studied factors for DoE 1 results. . . . . . . . . . 18 4.6 Pareto chart of the standardized effects for measured silane grafting percentage in DoE and main effect plot of acid amount factor in DoE 2. . . . . . . . . . . . . . . . . . . . . 19 4.7 TGA and DTG curves from pristine sepioline and VTES modified sepiolite. . . . . . . . 22 4.8 FTIR spectra of pristine sepiolite, VTES and VTES modified sepiolite. . . . . . . . . . . 23 4.9 Photographs of the dispersion of unmodified and modified sepiolite in water after different times. ............................................ 24 4.10 Photographs of the dispersion of unmodified and modified sepiolite in styrene after differenttimes. ....................................... 24 4.11 In situ radical polymerization of PS/sepiolite nanocomposites diagram. . . . . . . . . . . 25 4.12 PS and PS/sepiolite composites plates and their corresponding X-ray radiography images. 26 4.13 Schematic representation of the surface coupling reaction by a) locking effect and b) weldingeffect. ....................................... 26 4.14 GPC chromatograms of neat PS, nanocomposites with unmodified sepiolite and the deconvolution of their peaks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.15 a) Impact of the amount of unmodified sepiolite added to the reaction on the weight average molecular weight Mwof the nanocomposites. b) Schematic representation of the competing mechanisms of termination and protection during the in situ polymerization process. ........................................... 29 4.16 GPC chromatograms of neat PS and composites with VTES modified sepiolite. . . . . . 29 v 4.17 a) DTG and d) DCS curves of pure polystyrene and composites containing unmodified/unmodified sepiolites. c) Relation between sepiolite added to reaction and sepiolite final amount present in the nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . 30 4.18 Schematic representation of the response of storage and loss modulus with the increment of particle density in a polymer matrix. . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.19 Angular frequency depdendence of the storage (G’) and loss modulus (G”) in the linear viscoelasticity region for neat PS and composites with unmodified and VTES modified sepiolite............................................ 33 4.20 Schematic representation of sepiolite particles in a polystyrene matrix with different dispersiondegrees. ..................................... 34 4.21 Complex viscosity versus angular frequency curves for neat PS and nanocomposites with unmodified and VTES modified sepiolite. . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.22 SEM micrographs of the foams samples produced from a) pure PS, b) PS-SepVTES1%, c) PS-SepVTES3% and PS-SepVTES6% composites. . . . . . . . . . . . . . . . . . . . 35 5.1 SEM micrograph foam obtained from PS and allyltriethoxysilane modified sepiolite. . . 39 6.1 Residuals plots including normal probability, versus fits, histogram and versus order of DoE1response........................................ 42 6.2 Residuals plots including normal probability, versus fits, histogram and versus order of DoE2response........................................ 42 vi List of Tables 3.1 First and second DoE factors and their corresponding levels . . . . . . . . . . . . . . . 9 4.1 Coefficients and p-values for the interaction model of the responses and the general linear regressionequation..................................... 21 4.2 Thermogravimetric analysis values of sepiolite and sepiolite modified with VTES at various temperature ranges, along with the silane grafting percentage. . . . . . . . . . . 22 4.3 Number- and weight-average molar masses (Mn,Mw) and dispersities (Ð) of prepared polystyrene and polystyrene/sepiolite nanocomposites. . . . . . . . . . . . . . . . . . . 27 4.4 Peak deconvolution data including maximum relative intensity and calculated Mn,Mw and Ðvalues for each peak from the GPC chromatograms of PS and PS/sepiolite nanocompositesamples. ....................................... 28 4.5 Calculated and extracted data from TGA/ DTG and DSC thermograms for the neat polystyrene and its nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.6 Density, relative density, expansion ratio and cell size values of the cellular materials produced by the gas dissolution foaming process. . . . . . . . . . . . . . . . . . . . . . 36 6.1 Experimental runs performed in DoE 1 and their response. . . . . . . . . . . . . . . . . 40 6.2 Experimental runs performed in DoE 2 and their response. . . . . . . . . . . . . . . . . 41 vii Chapter 1 Introduction Polymer nanocomposites (PNCs) are materials composed of a polymer matrix and an inorganic dispersive phase that has at least one dimension in the nanoscale. They have attracted the interest of researchers and industry as a small amount of nanoparticles can provide new or enhanced mechanical, thermal, barrier, optical, and electric properties compared with neat polymers and conventional composites.1 Since the introduction of the first nanocomposite by Toyota in the 1990s, extensive research has been ongoing to fully exploit the capabilities of these materials.2The dispersion and adhesion of nanoparticles within the polymer matrix strongly affect the properties of PNCs. When nanoparticles are uniformly distributed within the polymer matrix, forming a percolation network, the final properties of the material will be enhanced significantly.3–5 This percolation network refers to an interconnected structure where the nanoparticles are in contact with each other throughout the entirety of the polymeric structure. Over the past years, several nanomaterials such as graphene6, carbon nanotubes (CNTs)7, metallic nanoparticles8, and nanoclays9have been used to enhance polystyrene properties. Nanoclays, in particular, have become popular due to their wide availability, low cost, unique structure, and properties. These minerals include both naturally occurring clays such as montmorillonite, bentonite, and sepiolite, as well as synthetic clays like fluorohectorite and laponite.10 There is a significant amount of research on lamellar layered silicates, such as montmorillonite and bentonite, which examines the intercalation and exfoliation of the clay on nanocomposites. However, needle-like shape particles such as sepiolite have been less studied even though they present relatively weak interactions and small contact surfaces, which makes the dispersion of fibrous silicates into polymer matrix more uniform.11 Sepiolite structure can be seen in Figure 1.1, it presents a fibrous/needle morphology with varying dimensions between 100 and 5000 nm in length, 10-30 nm in width, and 5-10 nm in thickness. It can be represented by the structural formula S i12 Mg8O30(OH)4(H2O)4·8H2O, where 4H2Oand 8H2O represent coordinated and zeolitic water molecules, respectively.12 The structure of sepiolite involves the arrangement of two tetrahedral silica sheets surrounding an octahedral sheet made of magnesium oxidehydroxide as can be seen in Figure 1.1. It allows the presence of small internal and external channels with cross-section dimensions of about 0.36 nm x 1.1 nm that are filled with zeolitic water under ambient conditions. The presence of silanol (Si-OH) groups on the surface of the particles is facilitated by the discontinuity of the silica sheets.13 The high density of silanol groups (2.2/nm2) are spaced every 0.5 nm along the side of the external channels and enables an ordered availability for coupling reactions with polymers and organic surfactants.12,14 1 Chapter 3 Methodology 3.1 Chemicals and materials Sepiolite (UNV-1) was supplied by Tolsa. For the surface modification process vinyltriethoxysilane (VTES; 97%; Mw=190.31), hydrochloric acid (HCl) 35 %, and 2-Propanol solvent ((CH3)2CHOH) ≥99.8% (GC) were used, they were provided by Sigma-Aldrich, Scharlau and Honeywell, respectively. During the in situ polymerization styrene, 2,2’-Azobis(2 methylpropionitrile) (AIBN), polyvinyl alcohol (PVOH; Mw85,000-124,000 and 87-89% hydrolysed), and methanol (MeOH) were used and provided by Sigma-Aldrich. 3.2 DoEs methodology The experiments were conducted following the full factorial design approach, which involves exploring all possible combinations of different factor levels to gather information about their main effects.29 The statistical software Minitab was used to generate DoEs and analyze the collected data. The silanization reaction conditions, including pH, temperature, time, and silane concentration were considered as independent variables, while the silane grafting percentage was treated as the dependent variable or response parameter. The first DoE consisted of two factors with 2 levels and two factors with 3 levels, giving a total of 36 experimental runs. The factors and their corresponding levels for this DoE are presented in Table 3.1. The second DoE used a 32full factorial design which factors and levels are presented in Table 3.1. To eliminate bias in measurements, all experiments were conducted randomly. The resulting combination of experiments in both DoEs can be found in Annexes section. 3.3 Sepiolite silanization reaction The surface modification procedure followed in the DoEs started by suspending 2.5 g of sepiolite in 50 ml of 2-propanol/water mixture (90:10 w/w) using a round bottom flask. Then, the necessary amount of HCl was added, and the solution was stirred for 10 minutes. Afterward, the necessary amount of organosilane was added and stirred under these conditions for 4 or 24 hours. Experiments carried out at 60 oCused a silicone bath under reflux conditions. The modified sepiolite was filtered and washed thrice 8 CHAPTER 3. METHODOLOGY 9 Table 3.1: First and second DoE factors with their corresponding levels used to study the grafting reaction of VTES molecules on sepiolite surface. DoE Factors Levels Values AAcid amount 3 No acid 0.1 ml 2 ml 1BTemperature 2 25 oC 60 oC CTime 2 4 h 24h DSilane concentration 3 0.25 ml/g of sepiolite 0.8 ml/g of sepiolite 2.4 ml/g of sepiolite 2AAcid amount 3 0.1 ml 0.8 ml 1.5 ml BSilane concentration 3 1.6 ml/g of sepiolite 2.4 ml/g of sepiolite 3.2 ml/g of sepiolite with distilled water and once with 2-propanol to remove unreacted silane and HCl. Finally, the product was dried in a vacuum oven at 300 mbar and 60 oCfor 24 hours. After the DoE 1 and DoE 2 analysis, the silanization process was optimized to obtain the maximum silane grafting amount for using the modified sepiolites to prepare polystyrene/sepiolite nanocomposites. For this, a solution of 2-propanol (50 ml) and sepiolite (2.5 g) was previously sonicated for 30 minutes to enhance its dispersion in this solvent. Then, the surface modification reaction with vinyltriethoxysilane and allyltriethoxysilane was carried out following the procedure described previously with 0.8 ml HCl, 8 ml of silane, a temperature of 60 oCand a reaction time of 24 hours. 3.4 Nanocomposites in situ polymerization 3.4.1 Suspension polymerization In a 100 mL two-neck flask were placed 40 mL of distilled de-ionized (DDI) H2O, 0.4 g of PVOH (1 g PVOH / dL H2O) and sepiolite. The mixture was heated and stirred under nitrogen atmosphere until a solution was formed. In a separate flask was placed 12 mL of styrene and 0.086 g of AIBN (5x10−3mol AIBN / mol styrene). The mixture was deoxygenated with N2(g) and stirred until a solution was obtained and keeping it cold at 0 oC. This organic solution was added dropwise to the aqueous system, and it was kept under inert atmosphere at 64 oCwith constant stirring during 24 h. Completing the polymerization, the reaction mixture was kept cooled to promote stabilization of the polystyrene (PS) particles. Finally, the PS particles were separated by centrifugation (10.000 rpm) and washed repetitively with hot water CHAPTER 3. METHODOLOGY 10 and methanol. The polymer was air-dried and then dried at reduced pressure for 48 h at 80 oC. 3.5 Gas dissolution foaming process Before the foaming process, the neat PS and different composites obtained from the in situ polymerization process were thermoformed in a hot plate press to obtain circular plates with 25 mm in diameter and 2 mm in width. The compression molding process was performed at a temperature of 170 oC. The foaming of the samples was achieved using the solid-state gas dissolution foaming method. For this purpose, a high-pressure vessel (model PARR 4681) provided by Parr Instrument Company has been used. The material foaming process under the different stages can be seen in Figure 3.1. Figure 3.1: Schematic representation of cellular materials formation under gas dissolution foaming process. Adapted from40 The initial step of the gas foaming process was the saturation step. In this stage, the samples were placed in the high-pressure vessel, and carbon dioxide (CO2) gas was introduced at a saturation pressure (Psat) of 8 MPa and a saturation temperature (Tsat) of 40 oCfor 24 hours. When the material was placed in the autoclave under these conditions, gas diffusion inside the material started to take place, and CO2 filled the empty spaces between the polymer chains, resulting in reduced viscosity and a lower Tgof PS.3 Thenext part ofthe processwas thedepressurization stage, during whichthe gas pressure wasreleased. The abrupt decrease in pressure creates a thermodynamic instability that can trigger a nucleation process, which leads to the formation of small nuclei as the solid and gaseous phases separate.41 In this work, as a two-step gas dissolution foaming method was employed for the foaming stage, samples were immersed in a silicone bath at 120 oCfor a duration of 1 minute. By maintaining the foaming temperature above the effective glass transition temperature, the polymer remains in a rubbery state, allowing the polymeric chains to have sufficient mobility. This mobility enables the nucleation points to start to grow, creating the final cells of the cellular structure.3The time between the pressure release and immersion in the thermal baths was approximately 2 minutes. Once the materials expanded, they were rapidly cooled in water to stabilize the cellular structure and minimize degeneration mechanisms. CHAPTER 3. METHODOLOGY 11 3.6 Characterization 3.6.1 Thermogravimetric analysis (TGA) Thermogravimetric analysis (TGA) is an analytical method employed to assess the thermal stability of a material and determine the proportion of volatile components present by monitoring the weight variation during controlled heating.42 By subjecting the sample to a specific heating protocol, TGA allows the investigation of mass changes linked to both physical and chemical transformations. TGA was used to evaluate the thermal degradation of sepiolite and estimate the organic modifier amount on sepiolite surface from the volatilized mass between 350 and 650 oC. Also, with this method it has been possible to study the thermal stability of the synthesized nanocomposites and characterize the final amount of clay present in them. The analysis was carried out using a Mettler Toledo TGA/DSC 3+, where around 10 mg of each sample in alumina crucibles were used. The measurements of the modified sepiolites were carried out under nitrogen atmosphere from room 50 to 1000 oC, with a heating rate of 20 oC/min. In the case of polystyrene/sepiolite nanocomposites, the heating range was from 50 to 850 oC. 3.6.2 Differential scanning calorimetry (DSC) Differential scanning calorimetry (DSC) is a technique that measures the heat flow difference between a sample and a reference as a function of temperature. It allows for the quantification of the energy involved in each transition that a polymer undergoes during a heating protocol. Theglasstransitiontemperature(Tg) ofthesynthesizedpolystyreneandpolystyrene/sepiolitenanocomposites were determined using this technique. The analysis was carried out using a Mettler Toledo DSC 3+equipment. The samples were heated from 20 to 160 oCat a heating rate of 10 oC/min under nitrogen atmosphere. Once the temperature reached 160 oC, it was maintained for 3 minutes before cooling back down to 20 oC. Then, the heating cycle was repeated and only the results of the second scan were reported to avoid the influence of the thermal history of the sample. 3.6.3 Fourier transform infrared spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR) technique relies on detecting the specific vibrations of functional groups in molecules when they are exposed to certain wavelengths of light. These vibrations, along with their intensity (% transmission or absorbance), are plotted against the frequency of light (cm−1) to generate an FTIR spectrum. This enables the qualitative identification of the present species by examining the characteristic bands in the sample’s spectrum, providing valuable information about the sample’s chemical composition.43 FTIR is used in this work to identify the covalent bonding between silane molecules and silanol groups on sepiolite surface. Also, it is used to study the interactions between polystyrene and modified sepiolite in the nanocomposite samples. The spectra were performed using a Bruker Tensor 27 spectrometer in the range of 4000-600 cm−1. CHAPTER 3. METHODOLOGY 12 3.6.4 Gel permeation chromatography (GPC) Gel permeation chromatography (GPC) is a widely employed method for characterizing polymers, providing information about their molecular weight distribution and polydispersity. GPC achieves separation of polymer molecules based on their hydrodynamic volume by employing columns with varying porosity. The output from the columns is then analyzed using one or more detectors that are typically calibrated with linear polymer standards.44 This technique is used in this work to determine the number- and weight-average molar mass (Mn,Mw) and dispersity (Ð) of the synthesized polystyrene and polystyrene/sepiolite nanocomposites. Also, the influence of unmodified/modified sepiolite on the resulting nanocomposites molecular weight distribution is going to be studied. The analysis was carried out using a Waters HPLC 515 pump, injector and a Waters 410 refractometer. Samples are passed through 1 precolumn and an Agilent column at 35oCusing chloroform as solvent at a flow rate of 0.5 mL/min. 3.6.5 Rheological tests Rheological characterization involves studying how materials flow and deform when subjected to external forces, which are determined by their internal structure and molecular interactions and provide valuable insights into their behavior and properties. In oscillatory shear tests, the sample is placed between two plates; while the lower plate is fixed, the upper plate oscillates with a certain frequency of oscillation (ω). From a rheological point of view, polymers are defined as viscoelastic materials, which means that they present a behavior between an ideal solid elastic and an ideal Newtonian fluid.45 The viscoelastic properties that can be studied through dynamic shear rheology include the storage modulus (G’), the loss modulus (G”), and the tangent of the phase angle (δ), which are examined in relation to variables such as time, frequency, temperature, and material deformation. G’ provides insights into the material’s elastic component, indicating its ability to recover its original shape under applied stresses.46 On the other hand, G” tells us about polymer’s ability to dissipate energy, with part of the energy being used to change the structure of the material during flow, and the rest of this energy is spent in heating the material. The phase angle (δ) also provides information about a material’s damping characteristics. In the case of a viscoelastic material (such as polymers), the phase angle varies between 0oand 90o, which means that the stress shows a certain delay period compared to the strain curve.3The equations that represent the storage modulus, loss modulus and phase angle are the following: G′=σ0 γ0 cosδ(3.1) G′′ =σ0 γ0 sinδ(3.2) tanδ=G′′ G′(3.3) Where σ0is the stress amplitude and γ0is the strain amplitude. Figure 3.2 shows the typical rheological behavior of a polymer in relation to frequency. At high frequencies, the storage modulus is higher than the loss modulus, indicating the glassy region. As the frequency decreases, the mobility of the polymer chains increases, leading to a transition to the rubbery plateau where the material exhibits predominantly elastic behavior. The region between the glassy state CHAPTER 3. METHODOLOGY 13 and the rubbery plateau is known as the transition region or Tgregion, where the loss modulus reaches its maximum value. This behavior arises from the energy required to enhance the mobility of the polymer chains. At very low frequencies, the terminal flow region is reached, characterized by a higher viscous modulus compared to the elastic modulus. In this region, the viscosity remains constant. In this work, the regions of interest that are going to be analyzed are the rubbery plateau and the terminal flow region. In this frequency range, it is possible to see that the loss modulus and storage modulus curves intersect at a specific frequency, known as the cross-over frequency (ωx).3 Figure 3.2: Rheological behavior of a polymer plots showing their characteristic regions in Log G’ and G” as a function of the frequency curves. Adapted from3. In this work, dynamic shear rheology technique was used to evaluate the dispersion degree of the neat PS and the different nanocomposites. The measurements were performed in a shear stresscontrolled rheometer (AR 2000 EX from TA Instruments) at a temperature of 220oC, under a nitrogen atmosphere, and using 25 mm diameter parallel plates. To determine the linear viscoelastic region of the nanocomposites, a strain sweep test was performed at a fixed dynamic frequency of 1 rad/s. To analyze G’, G” and complex viscosity values a frequency sweep was performed covering a range of angular frequencies from 0.2 and 100 rad/s, considering that lower temperatures led to sample degradation. 3.6.6 X-ray radiography The interaction between X-rays and matter is linked to the complex refractive index (n) of the materials involved, wherein the real and imaginary components play a significant role in phase and absorption contrast, respectively40. An X-ray radiography image obtained by exposing a material to X-rays provides a shadow of intervening structures that directly depend on the material’s density (σ) and atomic number (Z).47 Thus, the resulting image is formed based on the quantity and intensity of X-rays detected at each point. The experimental setup used to obtain the radiography images can be seen in Figure 3.3. In this study, X-ray radiography is employed to evaluate the dispersion and distribution of sepiolite in the polystyrene matrix at a high-size scale. In this specific case, sepiolite aggregates that were formed due to the high concentration of silane used during the surface modification process can be analyzed with this technique. The basic elements of the radiography setup used in this work include a low energy microfocus X-ray source (L10101, Hamamatsu. Voltage: 20-100 kV, Current: 0-200 µA) and a high sensitivity flat detector panel which forms the X-ray images (C7940DK-02, Hamamatsu. 2240 x 2344 CHAPTER 3. METHODOLOGY 14 pixels, 50 µmpixel size). Both elements are positioned at a source-to-detector distance (SDD) of 580 mm. Figure 3.3: 3D diagram of the setup employed to perform the X-ray radiography measurements showing the relative position of the sample with respect to the X-ray source and detector. Adapted from48. 3.6.7 Density measurements The density of the foamed materials (ρf) was measured using the water displacement method, which is based on Archimedes’ principle and follows the ISO 1183-1 standard. For this, a density determination kit for the balance Mettler Toledo AT261 was used. The relative density and expansion ratio were calculated using the foam density and the density of the solid PS sample (ρs). The relative density (ρr) is obtained as the reciprocal of the relative density. Similarly, the expansion ratio is defined as the ratio between the density of the cellular material and ρs.3 ρr= ρf ρs (3.4) Er =1 ρr (3.5) 3.6.8 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) is a technique that uses a electron beam directed towards the material, scanning its surface. As the beam interacts with and penetrates the material, various interactions take place, resulting in the emission of photons and electrons from the sample surface or its vicinity. To generate an image, the signals produced by the electron-sample interactions are detected using different types of detectors, which depend on the specific mode of SEM being employed.49 The cellular size of the different foams prepared by gas dissolution foaming process was estimated using SEM images. From the cell size values, the cell nucleation density of the foams was calculated by using the following formula: No=6 πϕ3(1 ρr −1) (3.6) Where, ϕis the cell size and ρris the relative density of the foams. The measurements were performed in a Flex SEM 1000 from Hitachi. Prior to being placed in the SEM, the solid samples were cooled in liquid nitrogen, fractured, and coated with a thin layer of gold to ensure electrical conductivity. Chapter 4 Results & Discussion 4.1 Sepiolite surface modification The reaction mechanism of the surface modification of sepiolite starts with the hydrolysis of the ethoxy groups from vinyltriethoxysilane (VTES) or allyltriethoxysilane (ATES). As we use sepiolite conserved in normal conditions after drying, it leads to the presence of zeolitic water. Then, the hydrolysis reaction can be favored from this water and from water added to the reaction. The resulting silanol groups may interact with the sepiolite Si-OH groups and/or condense to form siloxane bonds.23 In this sense, sepiolite surface silanols serve as nucleation sites for the favorable hydrolysis and condensation of organosilanes during the reaction period.24 The scheme of the reaction mechanism in the presence of VTES can be seen in Figure 4.1. Figure 4.1: Reaction mechanism of the surface modification of sepiolite with triethoxyvinylsilane (VTES). a) Hydrolysis reaction of the organosilane and b) grafting of VTES on sepiolite. The first part of this work aims to favor the covalent bonding or grafting of organosilanes on sepiolite surface by studying four determining reaction conditions as silane concentration, time, temperature, and 15 CHAPTER 4. RESULTS & DISCUSSION 16 acid amount in the surface modification process. A first full factorial design was performed in order to determine the influence of the factors on the chemical grafting percentage of silane molecules on sepiolite surface, which is our response variable. Then, a second DoE is performed to find the silane concentration and HCl amount values that maximize the silane grafting. It is important to mention that both DoEs were performed around vinyltriethoxysilane (VTES) and 2-propanol used as solvent. The amount of silane chemically bonded to the sepiolite surface provides information about the available reaction sites on the clay surface.50 In this case, this sites corresponds to vinyl functionalities that allow the initiation and propagation of polystyrene chains on the sepiolite surface enhancing the interaction between sepiolite and polystyrene. Furthermore, during the in situ polymerization process, it is important to avoid the hydroxyl groups on sepiolite surface, as they can act as quenchers and terminate the polymerization reaction. In this way, the silane grafting percentage is an important response variable that has to be controlled for the further nanocomposite foaming process. The explanation of the silane grafting percentage estimation through thermogravimetric analysis (TGA) can be seen in section 4.1.4 of this chapter. 4.1.1 First DoE results DoE 1 consisted on the full factorial design with 4 factors, two with 2 levels and the other two with 3 levels, giving a total of 36 experimental runs. The Pareto charts, shown in Figure 4.2, allow to detect the factors and interaction effects that are most important to the surface modification process. It displays the absolute values of the effects in bars, and the ones that cross a reference red line are considered statistically significant. This line for statistical importance depends on the level of significance (α).29 In this study a 95 % confidence level is used. As can be seen in Figure 4.2 the factors studied and the interaction between the amount of acid and time were the significant parameters in the degree of silane grafting. VTES concentration is the factor with the greatest influence on the grafting reaction. Figure 4.2: Pareto chart of the standardized effects for measured silane grafting percentage in DoE 1. The main effect and interaction plots shows the mean response values at each level of a design, CHAPTER 4. RESULTS & DISCUSSION 17 providing insights into the magnitude and direction of the effects. The sign of the slope indicates whether the average response value increases or decreases, while the inclination indicates the strength of the effect.51 Figure 4.3 shows the main effect plots for the factors of silane concentration, temperature, time and acid amount. The gradual addition of VTES resulted in an increase amount of chemically grafted molecules at the surface of sepiolite, leading to a surface more covered with organosilane molecules. Within the range of 0.25 to 2.4 ml of VTES per gram of sepiolite, there is an increase of around 161 % in the silane grafting. However, it is important to consider that an excess of silane can form polycondensations between sepiolite and silane through silica bridges and form aggregations, reducing their dispersibility.24,52 Although this effect is not considered a response variable in the design of experiments due to the complexity of quantification, it will be discussed during the preparation of the polystyrene/sepiolite nanocomposite via in-situ polymerization. Figure 4.3: Main effect plots of the four factors studied in DoE1 for silane grafting percentage. In the temperature case, it was found that the grafting percentage increases for the highest temperature used. By using a temperature of 60 oCthe VTES grafting percentage rises by approximately 57.9 % in comparison when using room temperature. Previous studies have reported that temperature can accelerate the interactions between silane derivatives and silanol groups on the inorganic surface,50 further emphasizing the significance of this factor within the studied temperature range. In a similar way, the silane grafting percentage demonstrates an increase of around 67.9 % when the reaction time is changed from 4 to 24 hours. Longer reaction times are beneficial for the hydrolysis and condensation of VTES molecules on the silanol groups of the sepiolite surface. Regarding the amount of acid added to the reaction, it was found that the grafting percentage increases with the addition of HCl. When 0.1 ml of HCl is added to the almost neutral solution there is an increase of around 42.6 %. Meanwhile, when the amount of HCl increases from 0.1 ml to 2 ml there is an increase of 18.8 %. The addition of acid promotes hydrolysis and improves the efficiency of hydroxyl condensation with the sepiolite surface,53 thereby enhancing the silane grafting. The interaction between acid amount and reaction time present in Figure 4.4 shows an interesting effect. Previously, it was determined that a reaction time of 24 hours is more effective than 4 hours in increasing the amount of silane grafting. However, when using an acid amount of 2 ml, a duration of CHAPTER 4. RESULTS & DISCUSSION 24 Figure 4.9: Photographs of the dispersion of unmodified sepiolite, modified with 1.2 and 3.2 ml of VTES/g of sepiolite (from left to right) in water after standing for different times. Figure 4.10: Photographs of the dispersion of unmodified sepiolite and modified with 1.2, 2.4, and 3.2 ml of VTES/g of sepiolite (from left to right) in styrene after standing for different times. slower sedimentation rate compared to pristine sepiolite, they still lacked dispersion stability. Within around one minute, all of the VTES grafted samples also settled down, with sample modified with 2.4 ml of VTES per gram of sepiolite taking the longest to do so. The poor dispersion in styrene could be attributed to the formation of sepiolite aggregates during the surface modification process, likely due to high concentrations of silane leading to polycondensation through silica bridges.24,52 Taking this into account and that a sonication process to enhance sepiolite dispersion in styrene it is difficult to perform, as this monomer can start to react before the addition of the organic phase in the suspension polymerization, it was decided to add the sepiolite to the aqueous phase instead of the organic phase during in suspension polymerization. Figure 4.11 shows the in situ radical polymerization reaction of PS/sepiolite composites. When unmodified sepiolite is used in the reaction, the polystyrene and the filler are not chemically bonded, resulting in the presence of only free polystyrene chains that undergo initiation and growth through radical CHAPTER 4. RESULTS & DISCUSSION 25 Figure 4.11: In situ radical polymerization of PS/sepiolite nanocomposites showing a) initiation and propagation of PS free chains and attached PS chains to vinyl groups on sepiolite surface. b) Schematic representation of the PS/sepiolite nanocomposites formed using unmodified and modified sepiolite. polymerization. However, when VTES-modified sepiolites are used, in addition to free chains, polymer chains become attached to the nanofillers through the vinyl groups present on the nanofiller surface. As a result, the interaction between sepiolite and the polystyrene matrix is enhanced, which can lead to improved properties of the nanocomposite. 4.2.1 X-ray radiography images X-ray radiography is employed to evaluate the dispersion and distribution of sepiolite in the polystyrene matrix at a high size scale. Figure 4.12 shows the X-ray radiography images of the different PS and PS/sepiolite composites plates prepared in this work. The darker regions within these images correspond to places in the sample where a greater amount of energy is absorbed, which depends on the density of the material. For the nanocomposites obtained using unmodified sepiolite, it is possible to observe that when 1% sepiolite is used, there is no discernible change in contrast within the image. This suggests that the sepiolite is uniformly dispersed and distributed within the matrix on this scale. However, as the sepiolite content is increased to 3% and 6%, areas with slight contrast variations become noticeable, potentially indicating a less favorable distribution of the charge but with a good dispersion. The radiography images corresponding to the composites using VTES-modified sepiolite clearly depict the presence of sepiolite aggregates within the matrix. The presence of these aggregates primarily arise from the surface modification process, during which a high concentration of silane was employed (3.2 ml of silane per gram of sepiolite). In Figure 4.13 it is possible to observe the effects that lead to the formation of sepiolite aggregates at elevated concentrations of silane, which has been reported in the silylation of layered clays and palygorskite.52,61 The locking effect occurs when neighboring clay layers connect and become fixed through the simultaneous condensation of silane (or silane oligomer). Additionally, the welding effect takes place as sepiolite particles becomes interconnected through polysiloxane CHAPTER 4. RESULTS & DISCUSSION 26 Figure 4.12: PS and PS/sepiolite composites plates and their corresponding X-ray radiography images. bridges.52,61 Figure 4.13: Schematic representation of the surface coupling reaction of VTES and sepiolite by a) locking effect and b) welding effect. 4.2.2 Molecular weight and molecular weight distribution As the molecular weight of polymers and their distribution are closely linked to their structural and rheological properties62, it is important to study the effect of sepiolite addition on the polystyrene matrix during the in situ polymerization process. The number-average (Mn) and weight-average (Mw) molecular weights, as well as the dispersities (Ð) of the samples, were determined by GPC technique and can be seen in Table 4.3. In the nanocomposites using unmodified sepiolite it can be seen that the Mwvalue increases when the sepiolite amount is increased in the polymerization medium. Also, the dispersities of the pure polymer and nanocomposites with different amounts of sepiolite are similar, revealing a wide distribution of molecular weights in all cases. To understand the behavior behind GPC results, it is important to consider the molecular weight distribution of PS and nanocomposites using unmodified sepiolite present in Figure 4.14. Deconvolution of these curves can provide further insights into the characteristics of this broad molecular weight distribution. CHAPTER 4. RESULTS & DISCUSSION 27 Table 4.3: Number- and weight-average molar masses (Mn,Mw) and dispersities (Ð) obtained by GPC of the prepared polystyrene and polystyrene/sepiolite nanocomposites. The samples were recorded in CHCl3and calibrated with PS monodisperse standards. Sample Molar masses Mn(g/mol) Mw(g/mol) Ð PS 69800 275600 3.9 PS-Sep1% 69100 286250 4.1 PS-Sep3% 65000 288450 4.4 PS-Sep6% 77300 332100 4.3 PS-SepVTES1% 50500 213600 4.2 PS-SepVTES3% 37400 124950 3.3 PS-SepVTES6% 54150 220150 4.1 The deconvolution of the curves revealed two populations of molecular weights in the neat PS and nanocomposites with unmodified sepiolite. The deconvolution of this bimodal distribution was performed using two Gaussian peaks, as seen in Figure 4.14 part b). The obtained adj. R2values above 99% in all the cases indicate a correct fitting of the peaks. Figure 4.14: a) GPC chromatograms of neat PS and nanocomposites with unmodified sepiolite recorded in CHCl3. b) Deconvolution of chromatograms using two Gaussian functions showing a bi-modal molecular weight distribution. Two simultaneous effects could influence the differences in molecular weight of the nanocomposites using unmodified sepiolite. The first one involves a reduction in the amount of higher molecular weight chains when introducing the filler into the polystyrene matrix and increasing its concentration. This is evident from the decrease in the maximum relative intensity of the second peak in Figure 4.14 and is supported by the data in Table 4.4. The presence of unmodified -OH groups in sepiolite is thought to play a role in this effect by increasing termination reactions during polymerization, thereby limiting the growth of polystyrene chains.2Therefore, the probability of growing radicals participating in irreversible reactions is enhanced by the sepiolite content.63 Additionally, the incorporation of nanoclay, acting as an impurity in the polymerization system, increases the dispersity of the lower molecular weight peak in the CHAPTER 4. RESULTS & DISCUSSION 28 bimodal distribution from 1.7 to 2.1 with a 6 wt.% loading of sepiolite. On the other hand, there is also an increase in molecular weight, particularly at higher sepiolite concentrations. This suggests that sepiolite may offer protection against termination reactions by exerting confinement on the steric mobility of chains, leading to the formation of higher molecular weight chains.2 The addition of sepiolite in the reaction raises the medium’s viscosity and reduces macromolecule diffusion, making it challenging for the growing chains to interact with the OH groups present in rigid sepiolite and other long chains with less fluidity compared to the monomer.64 This effect can be seen in the increase of Mnand Mwvalues in Table 4.3 with the addition of 6% of sepiolite. An schematic representation of this effect can be seen in Figure 4.15 part b). Furthermore, previous studies have reported that nanoclay can accelerate the polymerization rate.65,66 The presence of pendant hydroxyl groups and oxygen-containing groups in nanoclays may induce a change in polarity in the reaction medium, promoting radical activation and reducing radical recombination rates. Table 4.4: Peak deconvolution data including maximum relative intensity and calculated Mn,Mwand Ðvalues for each peak from the GPC chromatograms of PS and PS/sepiolite nanocomposite samples. Sample Peak # Max. relative intensity (mV) Mn(g/mol) Mw(g/mol) Ð PS 1 2 346.4 166.7 124559 551377 206144 601895 1.7 1.1 PS-Sep1% 1 2 307.4 138.3 122985 609635 230336 668931 1.9 1.1 PS-Sep3% 1 2 369.0 109.3 123623 731061 250300 813748 2.0 1.1 PS-Sep6% 1 2 285.2 99.5 154744 707630 319674 787340 2.1 1.1 During the in situ polymerization process, there is a competition between the effects mentioned before. Figure 4.15 a) shows the relationship between Mwand the amount of sepiolite added in the in situ polymerization reaction. In the non-deconvoluted molecular weight distribution and the lower molecular weight peak in the deconvoluted curve, which contain smaller chains, their higher mobility facilitates interactions with the OH groups, leading to increased termination reactions. As a result, the protective effect becomes more pronounced at higher sepiolite concentrations, evident in a significant increase in Mwat 6% wt. of sepiolite. On the other hand, peak 2 of the deconvoluted curve presents chains with the highest molecular weights, so the protective effect is primarily observed at low amounts of sepiolite. In this case, when 6% wt. of sepiolite is used, the Mwdecreases as the mobility of these chains is significantly reduced, hindering their interaction with the monomers and radicals in the medium. The composites containing VTES modified sepiolite exhibit a decrease in Mnand Mwcompared to the other materials, as shown in Table 4.3. This reduction in molecular weight can be attributed to the increased chain transfer reactions between the propagating chain radicals and vinyl groups, allowing the growth of polystyrene chains on sepiolite surface.52 Another interesting effect found of using VTES modified sepiolite in the in situ polymerization was the alteration of the bimodal molecular weight distribution found in neat PS and PS/unmodified sepiolite nanocomposites into a single peak distribution CHAPTER 4. RESULTS & DISCUSSION 29 Figure 4.15: a) Impact of the amount of unmodified sepiolite added to the reaction on the weight average molecular weight Mwof the nanocomposites. b) Schematic representation of the competing mechanisms of termination and protection during the in situ polymerization process. (Figure 4.16). Additionally, an unexpected reduction in the molecular weight of the PS-SepVTES3% compound was observed. This is mainly due to the sensitivity of the in situ polymerization process to experimental parameters, which can influence the molecular weight of the polymer matrix in a polymeric compound. Figure 4.16: GPC chromatograms of neat PS and composites with VTES modified sepiolite. CHAPTER 4. RESULTS & DISCUSSION 30 4.2.3 Thermal analysis Thermogravimetric analysis (TGA) was performed on neat PS and PS/sepiolite composites to determine the effects of sepiolite on their thermal stability and to evaluate the final clay amount in the nanocomposites. The resulting differential thermogravimetric (DTG) thermograms are shown in Figure 4.17 in the temperature window of 50 to 750 oC. At temperatures below 250 oC, all of the samples present an small weight loss (0.9 to 2.7 wt.%) due to humidity and residual silane molecules that were not chemically bonded to sepiolite.63 The main degradation peak for both the synthesized PS and the different nanocomposites occurred within the temperature range of 300-500 oC. Figure 4.17: a) DTG and d) DCS curves of pure polystyrene and composites containing unmodified/unmodified sepiolites. c) Relation between sepiolite added to reaction and sepiolite final amount present in the nanocomposites. The sepiolite content in the PS nanocomposites, both with unmodified and modified sepiolite, can be found in Table 4.5. The experimental relationship between the amount of clay added at the start of in situ polymerization and the final amount of charge remaining in the nanocomposite can be seen in Figure 4.17. When VTES-modified sepiolite was employed, a noticeable reduction in clay content was observed compared to using unmodified sepiolite. This difference can be attributed to the hydrophobic nature of VTES-modified sepiolite, which hindered its dispersion in the aqueous phase during in situ polymerization. Furthermore, the presence of large aggregates resulting from the polycondensation CHAPTER 4. RESULTS & DISCUSSION 31 between sepiolite and the high concentration of silane further impeded the interaction between the monomers in the organic phase and sepiolite. However, due to the heterogeneous nature of these samples with the presence of aggregates and the fact that only one TGA measurement was conducted, there are a significant experimental error in these values. The investigation of the final charge amount in the nanocomposite and its interaction with the matrix is crucial, as it can greatly impact the mobility of the chains, thereby influencing their rheological behavior and transition temperatures.2,13 Table 4.5: Calculated and extracted data from TGA/ DTG and DSC thermograms for the neat polystyrene and its nanocomposites. Sample TGA DSC Sepiolite amount (wt%) Tonset (oC)Tg(oC) PS - 428.5 101.8 PS-Sep1% 0.88 435.3 100.6 PS-Sep3% 2.38 437.8 101.9 PS-Sep6% 5.57 450.6 101.9 PS-SepVTES1% 0.31 430.6 101.1 PS-SepVTES3% 1.17 431.5 100.2 PS-SepVTES6% 2.14 432.7 102.1 The nanocomposites exhibit higher thermal stabilities compared to the neat polystyrene and rises by increasing sepiolite loading, as can be seen with the onset temperatures of thermal decomposition (Tonset) in Table 4.5. This improvement can be attributed to the superior thermal stability of the nanofillers, which act as a physical barrier against decomposition products.65 Among the nanocomposites, those with unmodified sepiolite demonstrate the highest Tonset values, reaching a maximum of 450.69 oCwhen 6% wt. of sepiolite is incorporated. This may be attributed to a better dispersion67 and larger amount of sepiolite present in these nanocomposites in comparison with composites with VTES modified sepiolite. To determine the glass transition temperature (Tg) of the samples, DSC analysis was performed, and the corresponding thermograms of pure polystyrene and its various nanocomposites are shown in Figure 4.17 b). The observed endothermic process accompanying the glass transition of the polymer could indicate the rearrangement of low molecular weight fractions with relaxed segments that were initially packed together. The obtained Tgvalues, as presented in Table 4.5, do not exhibit significant changes in any of the cases. It would be expected that nanocompounds with populations of chains of lower molecular weight to have lower Tgvalues, as they can reduce viscosity.63 However, the obtained Tgvalues do not provide conclusive evidence in this regard. 4.2.4 Rheology As the rheological properties of particle-filled materials are highly responsive to their structure, rheology provides a valuable tool for evaluating the dispersion state in nanocomposites. So, shear dynamic rheology is used in this work to evaluate the dispersion of sepiolite particles in the polystyrene matrix. The incorporation of inorganic fillers, such as nanoparticles, brings about modifications in the behavior of CHAPTER 4. RESULTS & DISCUSSION 32 the polymer matrix (as an increase in viscosity at low shear rates), resulting in changes in the zero-shear viscosity, storage and loss modulus curves, cross frequency points and complex viscosity.3,68. In a pure polymer matrix, the storage modulus and loss modulus exhibits a typical relaxation behavior where G′∼ω2(slope ∼2) and G′′ ∼ω(slope ∼1). Moreover, there is only one cross-over point between these two curves at lower frequencies. However, when particles are incorporated into the polymer matrix, this behavior is altered. Increasing the particle density, achieved by either increasing the particle amount or improving filler dispersibility, leads to proportional slopes of 1 for both the storage and loss modulus in the terminal region. As the particle density increases, G’ and G” exhibit a more solid and elastic behavior, particularly at lower frequencies where G’ > G”. Also, a non-Newtonian power law behavior can also be observed at lower frequencies. As the particle density continues to increase and approaches the percolation density, the number of cross-over frequency points increases from 1 to 2. A further increase in particle density can result in a response where G’ > G” across the entire frequency range. This solid-like response indicates a percolated arrangement of the fillers, where they come into contact with each other along the polymer matrix.3,68 The described effects of increasing filler density in a polymer matrix can be seen in Figure 4.18. Figure 4.18: Schematic representation of the response of storage and loss modulus with the increment of particle density in a polymer matrix. Adapted from68 The storage modulus (G’) and loss modulus (G”) behavior as a function of angular frequency for neat PS and the composites with different sepiolite amounts are present in Figure 4.19. In the case of neat PS, the typical behavior of a pure polymer with only one cross-over point can be observed. The Newonian plateau could not be observed due to material degradation at low frequencies. This degradation is primarily attributed to the absence of stabilizers in the polymer plates. In the case of PS/unmodified sepiolite nanocomposites, G’ is higher than G” across the entire frequency range. This indicates a solid-like response in the molten state with the formation of a percolation network, evenwitha low concentrationof1 wt. %ofthe filler. Thisfindingis significant becauseachieving CHAPTER 4. RESULTS & DISCUSSION 33 Figure 4.19: Angular frequency depdendence of the storage (G’) and loss modulus (G”) in the linear viscoelasticity region for neat PS and composites with unmodified and VTES modified sepiolite. a percolation state is challenging with low particle concentrations. However, in this case, despite the low sepiolite content (1%), the particle density is high enough to form a percolation network due to the excellent dispersion achieved through in situ polymerization. Figure 4.20 illustrates a schematic representation of the percolation network, where sepiolites are interconnected and in contact with one another throughout the polymeric structure. In the work of Ballesteros et al.39 PS/sepiolite blends prepared using the melt blending technique with a monodisperse commercial PS (INEOS, Styrolution PS153F) and the same unmodified sepiolites, achieved a percolation network using 8 wt.% of particles. This result indicates that the degree of filler dispersion in the polystyrene matrix can be significantly enhanced when employing in situ polymerization instead of melt blending technique. On the contrary, in the compounds containing low amounts (1 and 3 wt.%) of VTES modified sepiolite, G’ and G” exhibit two cut-off points. This suggests that the formation of a percolation network is close. This network is finally achieved when using 6 wt.% of VTES modified sepiolite, where G’ is always greater than G”. In these materials, the dispersion is poorer due to the formation of aggregates as mentioned earlier, requiring higher filler contents to achieve a percolation network. Also, it is worth Chapter 6 Annexes Table 6.1: Experimental runs performed in DoE 1 and their response. Run Factors Response Silane concentration (ml/g) Acid amount (ml) Grafting % 12.4 1.5 1.8358 23.2 1.5 1.8738 32.4 0.1 1.552 41.6 0.8 1.6695 51.6 1.5 1.7703 61.6 0.1 1.2765 73.2 0.8 2.2672 82.4 0.8 2.0984 93.2 0.1 1.3817 40 CHAPTER 6. ANNEXES 41 Table 6.2: Experimental runs performed in DoE 2 and their response. Run Factors Response Silane concentration (ml/g) Temperature (◦C) Time (h) Acid amount (ml) Grafting % 12.4 25 4 0 0.40 20.8 25 24 2 0.61 30.25 60 4 0 0.48 40.25 60 4 0.1 0.21 50.25 25 24 2 0.25 60.8 60 4 2 1.01 70.8 25 4 2 0.74 80.8 60 24 0.1 1.07 90.25 60 4 2 0.75 10 2.4 25 4 0.1 0.45 11 0.25 60 24 2 0.79 12 0.8 60 24 2 1.52 13 2.4 60 24 0 1.50 14 0.25 25 4 2 0.99 15 0.8 60 4 0 0.64 16 2.4 60 4 0 0.61 17 0.8 25 24 0 0.30 18 0.8 25 4 0.1 0.20 19 2.4 25 24 0.1 1.94 20 0.25 25 24 0.1 0.42 21 0.8 60 24 0 1.33 22 0.25 60 24 0 0.94 23 2.4 60 4 0.1 1.06 24 2.4 25 24 0 1.03 25 2.4 60 24 0.1 2.76 26 2.4 25 24 2 1.88 27 0.25 25 24 0 0.47 28 0.25 25 4 0 0.03 29 2.4 60 4 2 2.24 30 0.25 25 4 0.1 0.31 31 0.8 25 4 0 0.12 32 2.4 25 4 2 1.05 33 0.25 60 24 0.1 0.65 34 2.4 60 24 2 1.49 35 0.8 60 4 0.1 0.78 36 0.8 25 24 0.1 1.35 CHAPTER 6. ANNEXES 42 Figure 6.1: Residuals plots including normal probability, versus fits, histogram and versus order of DoE 1 response. 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