Effect of solvent parameters on the processing of layered silicate nanocomposites

Transcription

1 Chapter 4 Effect of solvent parameters on the processing of layered silicate nanocomposites Abstract This chapter deals with the various problems encountered during the preparation of the chlorobutyl rubber nanocomposites using organically modified nanoclay as filler and the various methods adopted to tackle the defects. The traditional method of melt mixing encountered several difficulties when dealt with the layered silicates. This was later on rectified by the innovation of a new technique which is a hybrid of the two well known techniques, solution mixing and melt mixing. Part of this chapter has been communicated for publication in Macromolecules

2 4.1 Introduction Recently, nanocomposites composed of clays and polymers have been found to have improved mechanical properties as well as enhanced thermal stability. Structure and properties of these nanocomposites are guided by the compatibility of polymer matrix with layered silicates having 1 nm thickness of silicate platelets. Sodium montmorillonite, an inorganic layered material belongs to the (2:1) phyllosilicate family where positively charged sodium ions are sandwiched between the negatively silicate layers. Organic cations (organo ammonium or organo phosphonium) are intercalated in the interlayer by exertion of Na+ ions to make the inorganic materials more compatible with the organic polymer matrix. The neighboring silicate sheets are connected by polymer chains to form either intercalated nanocomposite where the regular insertion of polymer chains in the interlayer or exfoliated nanocomposite where silicate layers are distributed randomly throughout the polymer matrix. In addition, the quasi-regular distribution of silicate layers results the formation of partially intercalated nanocomposites where both the exfoliated and intercalated configuration co-exists [1-6]. The improved properties are related to the degree of dispersal and exfoliation of the clay platelets in the polymer matrix. Polymer nanocomposites can be prepared mainly by three techniques, viz, in situ polymerization, solution mixing and melt mixing. In solution mixing, fillers are added directly into the polymer matrix while solution mixing involves the dispersion of the filler in an organic solvent followed by dissolution of the polymer matrix and solvent casting. It has been observed from earlier studies that solution mixing brings about better exfoliation/delamination and dispersion of nanoparticles in a polymer [7, 8]. Thus in solution mixing, solvent plays an important role in determining the properties of polymer filler nanocomposites. Both the solvents may be either the same or different. Full dispersion of clay particle in a polymer is thus a major challenge. There are many reports describing the preparation of nanocomposites by solution mixing [7-12]. But none of these have dealt on the correlation between mechanical properties of

3 vulcanized rubber nanocomposites and nature of solvents used for solution mixing. Ho and Glinka [13] in their work have shown the effect of solvent solubility parameter on the dispersion of clay. Liu etal. [14] have done a similar work with carbon nanotubes. Understanding the interaction between organically modified clay (organoclay) platelets and organic solvent molecules as well as the corresponding structure of organoclays in a suspension is a critical step towards tailoring and characterizing nanocomposites. Hansen solubility (cohesion) parameters (HSP) are widely used for predicting compatibility between two materials. Materials with similar HSP will show physical affinities. Great interest in HSP has thus been found in many applications involving for example solvent selection in the coatings industry, characterization of additives in polymers, determination of chemical resistance, swelling properties or permeation rates [15-19] of polymer materials. The concept described by Hansen is based on the fact that the cohesive energy of a liquid, which can be directly measured by its energy of vaporization, arises from the contribution of three kinds of interactions: (i) nonpolar atomic (dispersion) interactions (D) (ii) molecular dipolar interactions (P) and (iii) molecular hydrogen bonding interactions (H). The latter are perhaps more generally called electron exchange interactions. These three major types of interaction are quantitatively described by the three Hansen parameters d D, d P and d H, respectively. The theory and application of the Hansen solubility (cohesion) parameters have been extensively discussed by authors like Barton [20]. The motivation of this work was, therefore, to explore the correlation between the degree of exfoliation of organoclays and properties of the solvent in which the clays were dispersed by utilizing the corresponding solvent solubility parameters. For this purpose CIIR organoclay nanocomposites have been prepared using five different solvents. The solvents used were chloroform, xylene, tetra hydro furan, toluene and cyclohexane. The nanocomposites were characterized by XRD, SEM, AFM, TEM etc. 4.2 Results and Discussion

4 4.2.1 Effect of processing technique on the properties of chlorobutyl rubber nanocomposites The easiest way of preparation of nanocomposites using layered silicates is to use melt mixing. Moreover in the case of rubber nanocomposites the usual procedure is to carry out the traditional melt mixing. The preparation of layered silicate filled chlorobutyl rubber nanocomposites was carried out by two different methods Preparation of nanocomposites by melt mixing The traditional method was tried and the mechanical properties of the so prepared composites were analyzed. Figure 4.1 shows the TEM images of the mill mixed nanocomposites. The images show the presence of agglomerated tactoids in the chlorobutyl rubber matrix. This shows that the clay layers were not exfoliated or intercalated and they lie in an agglomerated state. Fig 4.1. TEM images of mill mixed layered silicate nancomposites Similar observation is attained from the SEM images of the nanocomposites as shown in figure 4.2. The formation of crevices of micrometer dimensions indicates the pull out of clay tactoids from the chlorobutyl rubber matrix.

5 Fig 4.2. SEM image showing the morphology of the fracture surface of mill mixed CIIR nanocomposites containing 5 phr of layered silicate. The layered silicate nanocomposites did not show much improvement in properties when the melt mixing was done. So there arose the need of a novel technique for the preparation of layered silicate nanocomposites Preparation of nanocomposites by solution mixing The layered silicate was dispersed first in an organic solvent like cyclohexane and then mixed with the rubber swollen in the same solvent with stirring. The nanocomposites so obtained are cast on a petri dish at room temperature to get a thin film. The solvent was allowed to evaporate at room temperature and dried in a vacuum oven at 60 0 C till there was no weight variation. This method of solution mixing was followed by the addition of a sulphur cure package on a two roll mill and compounded for a period of 15 minutes by carefully controlling the nip gap and temperature. The compounds were mixed in a two-roll mill of 170 mm diameter, working distance 300 mm, speed of slow roll 18 rpm, and gear ratio 1 : 4 by careful control of temperature, nip gap, and time of mixing. After complete mixing the stock was passed six times through tight nip and finally sheeted out at a fixed nip gap. The CIIR composites were left for a day before vulcanization in a dessicator. Figure 4.3 shows a comparison of the tensile properties of chlorobutyl rubber nanocomposites containing cloisite 15 A prepared by melt mixing and the new

6 Tensile strength (MPa) technique. From the graph it is clearly understood that the new method is highly beneficial for the preparation of chlorobutyl rubber nanocomposites mill mixing solution mixing Clay loading (phr) Fig 4.3 Comparison of the tensile strength of nanocomposites prepared by melt mixing and solution mixing Calculation of solubility parameter of cloisite 15 A Through the group contribution method, the solubility parameter (δ) of the organomodified clay can be calculated by equation1 δ = ρ i F i / M (1) F i = (EiVi) 1/2 (2) where Ei and Vi are the cohesive energy and the molar volume of the chemical group i being considered and F i represents the molar attraction constant [19,21] proposed by Small and is in units of cal 1/2 cm 3/2. ρ is the mass density and M is the molar mass of the material. The surfactants on the C15A clay platelet surface are a

7 mixture of dimethyl di-tallow ammonium with various carbon chain lengths [21, 22] which mainly contains -CH 2 and -CH 3 groups. Consequently, the averaged δ for the surfactant mixture on C15A was calculated to be 8.80 cal 1/2 cm 3/2 as shown in table 4.1, which falls within the range specified in the literature [23] of cal 1/2 cm 3/2. Table:4.1 Solubility parameter of the rubber and layered silicate System Solubility parameter cal 1/2 cm 3/2 CIIR 8.16 Cloisite 15 A 8.8 The solubility parameter of chlorobutyl rubber and the solvents have been taken from standard literature [24,25]. The Flory-Huggins interaction for binary polymer systems consisting of components a and b is given by χ ~ (δ a -δ b ) 2, where δ is Hansen s total solubility parameter of a species. The smaller the χ value, the better the miscibility between the components. The constants a, b, c and d stand for rubber, solvent for rubber, clay and solvent for clay respectively. For rubber solvent χ AB = V m /RT (δ a -δ b ) 2 (3) Similarly, for clay- solvent χ CD = V m /RT (δ c -δ d ) 2 (4) The difference in the interaction parameter between CIIR-solvent and clay- solvent has been calculated and the values are shown in table 4.2. Since cyclohexane is an excellent solvent for the rubber matrix (χ ABvalue) the overall value of the interaction parameter decreases for the cyclohexane system.

8 Table:4.2 The difference in the interaction parameter between CIIR-solvent and clay- solvent systems Solvent Solubility parameter cal 1/2 cm 3/2 χ AB-CD Toluene Xylene cyclohexane chloroform THF In the case of cyclohexane the solubility parameter of the solvent is nearly the same as that of the chlorobutyl rubber matrix and the difference in interaction parameter between the rubber and solvent χ AB is almost equal to zero. A solution with χ = 0 is called an athermal solution Characterization by XRD, SEM, AFM and TEM Analysis of the XRD patterns (figure 4.4) clearly reveal that the polymer chains are intercalated within the gallery spaces of the layered silicates. The pristine clay is showing a peak at 2 θ =2.75. The nanocomposite prepared using chloroform as the solvent shows a peak at a lower (2θ = 2.2) value than that shown by the layered silicate. In this case there is an occurrence of a second peak near the peak of the pristine clay showing that all the clay platelets are not in the intercalated state and some are remaining as such in agglomerated state in the CIIR matrix. This suggests that chloroform was not so effective in dispersing the clay platelets and some of the clay layers remained as tactoids in the rubber matrix. Similar is the observation shown by THF but the shift in the first peak in this case is towards a lower 2θ value (ie 2θ = 2) than that shown by chloroform suggesting better dispersion. In the case of toluene, xylene and cyclohexane the peaks remain at a slightly lower 2θ value than that of pristine clay (2θ = 2) indicating intercalation of polymer chains and there is no occurrence of a second peak near the peak for the pristine clay. This clearly indicates the absence of non dispersed clay or formation of tactoids. The

9 Intensity intensity of the peak obtained in the case of the nanocomposite formed from cyclohexane is greater showing a higher degree of intercalation. This possibly suggests that the clay layers are not fully delaminated but a couple of layers of varying thickness are exfoliated throughout the CIIR matrix. The existence of a peak with high intensity i.e. full width at half minimum in the case of cyclohexane as solvent is due to the presence of a well ordered intercalated structure. In solvents like xylene and toluene, the polymer intercalation into the clay disrupted the silicate layers and produced a disordered intercalated nanocomposite [23]. This observation is clearly substantiated from the SEM and TEM micrographs C CIIR/15C Ch CIIR/15C Xy CIIR/15C Tf CIIR/15C To CIIR/15C Cy θ Fig 4.4: XRD plot of nanocomposites containing 5 phr of cloisite 15 A (15Ccloisite 15 A, Ch chloroform, Xy- xylene,tf- tetra hydro furan,totoluene,cy- cyclohexane) From the SEM micrographs it is evident that the dispersion of clay is not uniform in THF and chloroform. In the case of chloroform (figure 4.6), the SEM images reveal that the crack is propagated along the regions where clay forms tactoids due to inadequate dispersion. The SEM images of nanocomposites prepared using THF

10 (figure4.8), reveal the pull out of particles from the surface. Many larger clay particles with size range 10 to 30 µm are distributed on the fracture surface of the nanocomposites. There are also many microvoids of about the same size as the clay particles on the fracture surface. This indicates that the microvoids originate in the breaking off of the clay particles from the rubber matrix due to poor dispersion of clay. In the case of nanocomposites prepared using xylene, toluene and cyclohexane the fracture surface is rough with frequent ridgelines indicating the direction of crack propagation and also indicate good compatibility with the chlorobutyl rubber matrix. Fig 4.5 SEM images of the fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using Cyclohexane Fig 4.6 SEM images of the fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using CHCl 3

11 Fig4.7 SEM images of the fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using Toluene Fig 4.8 SEM images of the fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using THF Fig 4.9 SEM images of the fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using Xylene The AFM and TEM micrographs (figure 4.10 and 4.11) further support the conclusions drawn from XRD. It is clear from the TEM micrographs that in the case

12 of THF and chloroform as solvents there is agglomeration and formation of clay tactoids. Neverthless in the case of chloroform prepared samples there is a small amount of exfoliation whose effect is nullified by the presence of extremely large tactoids distributed throughout the nanocomposite. In the case of samples made using cyclohexane there is an ordered intercalated structure but in the case of xylene and toluene the intercalative structure is not ordered. This is clearly a substantiative evidence for the intense peak obtained in the XRD for nanocomposite samples prepared using cyclohexane as solvent. Fig 4.10 (a) AFM of the fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using (a) Cyclohexane and (b) THF (b) (a) (b) (c)

13 (d) (e) Fig 4.11 TEM micrographs of fracture surface of the CIIR nanocomposite containing 5 phr of cloisite 15 A prepared using (a) Chloroform (b) THF (c) Cyclohexane (d) Toluene (e) Xylene Correlation between the mechanical properties and the interaction parameter The relationship between the properties of the nanocomposites and the solubility parameter is well understood by correlating the mechanical properties with the difference in the interaction parameter of the nanocomposites. The tensile strength, elongation at break and modulus of the nanocomposites increases with the decrease in the interaction parameter as shown in the figures 4.12 to An exponential decay in all the parameters is observed with the difference in the interaction parameters. All the curves are found to fit into the following second order exponential decay equation y= y 0 + A 1 e -x/t 1 + A 2 e -x/t 2 where x and y represents the properties of the CIIR nanocomposites and the difference in the interaction parameters, respectively t 1 and t 2 represents the decay constants and A 1 and A 2 are two constants while y 0 represents the offset or the initial quantity at t=0.the values of A 1, A 2, t 1, t 2,y 0 and the regression coefficient are given in table 4.3. Table 4.3 Regression coefficients for mechanical property vs difference in interaction parameter plot. Property Y 0 A 1 A 2 t 1 t 2 R

14 Elongation at break % Tensile strength (MPa) Modulus (MPa) Elongation at break (%) Difference in interaction parameter X AB-CD Fig Plot of elongation at break vs difference in interaction parameter

15 Modulus at 300% elong (MPa) Tensile strength (MPa) Difference in interaction parameter X AB-CD Fig Plot of tensile strength vs difference in interaction parameter Difference in interaction parameter X AB-CD 1 Fig Plot of modulus at 100 % elongation vs difference in interaction parameter

16 4.2.7 Thermodynamic interpretation of results Better dispersion of clay as well as good polymer filler interaction in the case of cyclohexane, toluene and xylene can be predicted from the thermodynamic point of view. Gibbs free energy of mixing of a solid with a solvent is given by G m = H m - T S m (5) H m and S m are the enthalpy and entropy of mixing and T the absolute temperature. According to Boltzman equation the number of ways the solute and solvent molecules can be arranged (ω ) in a lattice is given by S m = k ln ω (6) Owing to the large size of the polymer molecule, it is evident that the number of ways of arranging the polymer segments will be relatively small or the entropy of mixing of polymer dissolution is much less compared to that for low molecular weight solutes. Therefore the change in interactions upon mixing governs the miscibility. Considering the interactions between nearest neighbors as in the lattice fluid model the overall interaction energy changes as a result of mixing through the rearrangement of contacts as depicted in figure 4.15 where cc, sc, and ss represent the interactions for polymer-polymer, polymer- solvent and solvent- solvent respectively. cc ss pp sc sc cc cc ss pp pp ss cc ss cc pp ss cc ss sc sc Figure 4.15: CIIR solvent CIIR + solvent It has been shown that Change in the contacts between nearest neighbours when a polymer chain mixes with solvent molecules H m = N 1 φ 2 Z W (7)

17 Z = coordination number N 1 = total number of solvent molecules φ 2 = volume fraction of the clay particle. Where W is the energy increment per solid solvent contact. Inserting the expression for χ, the solid solvent interaction parameter in the above equation H m becomes H m = N 1 φ 2 χ k (8) Where χ is defined as χ = Z W/ kt. Considering the lattice model for polymer solutions, Flory and Huggins arrived at the expression fro entropy as S m = - k( N 1 lnφ 1 + N 2 lnφ 2 ) (9) Where subscripts 1 and 2 stands for solvent and polymer respectively. The volume fractions in the equation are given by φ 1 = N 1 / (N 1 +n N 2 ) (10) φ 2 = n N 2 / (N 1 +n N 2 ) (11) n represents the number of segments in the polymer molecule. Therefore the free energy becomes [26] G m = kt( N 1 φ 2 χ + N 1 lnφ 1 + N 2 lnφ 2 ) (12) This equation is applicable for both clay- solvent as well as polymer- solvent systems. It is evident that lower the value of G m, better is the rubber filler interaction. It is true from the equation for enthalpy change that as the value of χ approaches zero or becomes lower, there is a greater chance that the value of free energy change becomes negative. Moreover the mixing of either clay with solvent or rubber with solvent is accompanied by a positive entropy change since there is uncertainty regarding the spatial locations of the solids when they are dispersed in solvents. There is no difference between the polymer- solvent contact and the average energy for P-P and S-S contacts and H mix = 0 regardless of φ. So the free

18 energy change for the CIIR cyclohexane system is always negative and it is a thermodynamically feasible process. 4.3 Conclusions Interaction parameter plays a vital role in deciding the dispersion of the clay particles in the rubber matrix. The choice of the proper solvent is highly essential for the successful fabrication of a composites material by solution mixing process. Decoiling of the CIIR chains in toluene, xylene and cyclohexane as a result of similar polarity and solubility parameter values leads to exfoliation/intercalation of the organoclay, thereby providing excellent mechanical properties. By means of the solution method, better intercalation was obtained since the solvent not only affected the increase of intergallery spacing but also caused the swelling of the polymer chains. In the case of cyclohexane where difference in rubber solvent and clay solvent interaction parameter is lowest, the properties are found to be the best. A thermodynamic interpretation has also been made for the results. References 1. Krishnamoorti, R., Vaia, R. A.; Giannelis, E. P, Chem. Mater, 8: 1728, Giannelis, E. P, Krishnamoorti, R,Manias. E, Adv. Polym. Sci, 138: 107, S. Su, D. D. Jiang, and C. A. Wilkie, Polym. Degrad. Stabil., 83: 321,2004.

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