SORPTION AND DIFFUSION OF ORGANIC PENETRANTS INTO DICARBOXYLIC ACIDS BASED CHAIN EXTENDED POLYURETHANES

Transcription

1 CHAPTER 7 SORPTION AND DIFFUSION OF ORGANIC PENETRANTS INTO DICARBOXYLIC ACIDS BASED CHAIN EXTENDED POLYURETHANES This chapter is divided into two sections Part - A and Part B. Part - A deals with the molecular transport of a series of n-alkanes into TDI based CEPUs (MA and CA) and Part B covers the transport behavior of substituted aromatic penetrants into HDI based CEPUs (MA and CA). Molecular transport of a series of organic probe molecules through prepared CEPU membranes have been studied in the temperature range C using sorption-gravimetric method. The Fickian diffusion equation was used to calculate the sorption (S), and diffusion (D) coefficients, which were dependent on the size of the probe molecules and temperature. Sorption data is correlated with the solubility parameter of solvents and polymer. It was found that solvents of comparable solubility parameter with CEPUs interact more and thus there is an increase in sorption. In all the liquid penetrants, the transport phenomenon was found to follow the Fickian mode of transport. From the temperature dependence of diffusion and permeation coefficients, the Arrhenius activation parameters such activation energy for diffusion (E D ) and permeation (E P ) processes have been estimated. Furthermore, the sorption results have been interpreted in terms of thermodynamic parameters such as change in enthalpy ( H) and entropy ( S). 7.1 Introduction The diffusion of small molecular liquids into polymers is a subject of intense study. This type of diffusion process plays an important role in several important areas of engineering and industry [1-2]. Membrane separation of liquids in the industry has become wide spread as more traditional methods are based on absorption, pressure-swing adsorption or cryogenic. The membrane process has certain benefits compared to the cryogenics process, for example, lower investment cost and easier operation. 196

2 The effects of interaction between polymers and small molecules are of practical interest to chemical engineers, because of the inherent sorption and transport of liquid penetrants present in most processes they encounter. Now-a-days polymer membranes are increasingly used in various barrier applications. Molecular transport of organic liquids through polymer membranes has been the subject of investigations over the past several years [3-8]. Such studies are necessary due to the production of innumerable polymer membranes of commercial importance [9-10]. The total amount of liquid sorption in polymeric materials is fundamental for applications such as pharmaceuticals, food packaging, electronic and medical components. Research studies are focused on three major areas of transport mechanism of polymeric materials. These may be designated as diffusion, sorption and permeation. Diffusion studies are concerned with transport of low molecular weight materials. The diffusion coefficient is a more fundamental quantity, which describes molecular mobility in the absence of a driving force in the same operating conditions. In packaging the transport is most commonly expressed in terms of permeability or permeation rate also known as transmission rate. These two important physical properties can greatly influence performance on the materials characteristics. Depending on polymer compositions, the structure and morphology relative to the physico-chemical nature of the penetratant materials is determined to elucidate mechanisms of transport and sorption process and molecular details of polymeric structure and morphology. Hence, it is necessary to analyze the transport behaviors, which have been widely studied by various researchers [11-16]. In all these studies, it has been pointed out that the rate of solvent transport with in polymer matrix, depends upon the nature of the functional group and its interaction with the polymer chain segments. Structural characteristics of the polymer are also important factors which leads to an increased understanding about the molecular transport phenomenon into elastomeric system. Several studies have been reported in the literature regarding the transport and sorption of liquids into polymer membranes. Kim et al [17] studied the transport of aromatic and aliphatic liquids into crosslinked polystyrene (PS). Lipscomb [18] reported the thermodynamic analysis of sorption in rubbery and glassy material. In 197

3 recent years, Vergnaud et al [19-21] and Goto et al [22] employed numerical/mathematical procedures to study diffusion of liquids into polymeric membranes. Kendaganna Swamy and Siddaramaiah [23] have investigated the transport behavior of diol based chain extended PU (CEPU) membranes. In view of the importance of PU in several areas such as biomedical applications, coating, adhesives, etc., it is important to know its transport characteristics with respect to organic solvents. The sorption of CEPUs depends very much on their chemical structure and morphology. The structure and molecular weight of the reactants of CEPU significantly influence the phase separation behavior. Polyurethane elastomers are known to exhibit unique mechanical properties, primarily as a result of two phase morphology [24]. These materials are alternating block copolymers made of hard segments from the diisocyanate/chain extender and soft segments from the polyol (ether or ester, castor oil). The hard and soft segments are chemically incompatible and microphase separation of the hard segments into domains dispersed in a matrix of soft segment can occur in varying degrees. In view of the importance of PU as a barrier material in several engineering sectors [25-26], it is important to know its transport characteristics with respect to common organic solvents. Thus, knowledge of the transport mechanisms as manifested by sorption, diffusion and permeation of organic liquid penetrants in PU matrix is helpful for establishing the structure- properties relationships under severe application conditions. Although some previous studies [27-29] have been made on solvent transport through PU membrane more experimental data are still needed for a better understanding of the thermodynamic interactions between polymer and solvent. A CEPU membrane has been chosen in this study because of its good mechanical properties and wide variety of industrial engineering and biomedical applications. However, acceptability of PUs for any specific applications depends on its performance requirements before these materials seek commercial or engineering applications. Aromatic solvents have been chosen as probe molecules as these have diverse applications in process industries and in manufacture of perfumes, dyes, bulk drug formulations etc. 198

4 Siddaramaiah et al have studied [30-33], and investigated the sorption and diffusion behavior of castor oil-based PU. Its IPNs and diol based CEPU have been studied for molecular transport with several organic liquids. They found that transport behavior does not merely depend on the size of the penetrants but also on the nature of liquid molecules and membranes. These studies are extremely important for the design of new polymer materials, which would greatly benefit the development of high performance membranes. Polyether-based PU foam are being studied by many scientists for the isolation of heavy metal ions like cobalt and antimony and absorption of phenol compounds in aqueous solutions [34-36]. The penetration of the solvent into the polymer membrane depends on the length of storage and nature of the solvent. The behaviour of the solvent with the membrane for a considerable length of time has to be studied. Hence, sorption of solvents is very essential to know the diffusion and permeation characteristics of polymer membranes. The principle objective of this chapter is to investigate the transport behaviour of aromatic liquids and n-alakanes (C 6 -C 9 ) through dicarboxylic acid based CEPU (MA and CA) membranes. It is expected that a systematic change in solvent power would lead to results which could be interpreted by considering the possible interactions with soft and hard segments of the polymer. Transport properties viz., sorption (S), diffusivity (D) and permeability (P) have been studied over an interval of temperatures from 25 to 60 o C to predict the Arrhenius parameters for each of the transport processes involved. 7.2 Molecular transport Sorption Sorption in polymers is a topic of great relevance in several industrial applications. Liquid, vapour or gas sorption in a polymer matrix depends on the concentration or pressure of the sorbed species and the nature of the polymer. A number of theories have been developed to study the polymer penetrant interactions during sorption experiments and these will be summarized in the forth coming section. 199

5 7.2.2 Diffusion Diffusion is a molecular process in which molecules drift as a result of random thermal motion from the region of higher concentration to one of lower concentration. The transport of a liquid is the similar process. This section is concerned with the theories of mass transfer for polymer-penetrant systems. To describe diffusion of small molecules through rubbery polymers a number of molecular and free volume models have been proposed. The molecular models are based on the analysis of specified motions of penetrant and of polymer chains related to each other taking into account the inter- molecular forces. The free volume models originated from statistical mechanical considerations and they do not offer a detailed microscopic description of the phenomenon Molecular models All the molecular models are based on the experimental observation that the penetrant transport in polymer follows Arrhenius relation. Here, diffusion is regarded as thermally activated process (called activated diffusion) with the assumption that the micro cavities of different sizes are continuously formed and destroyed within the polymer matrix due to the random movement of polymer segments. Three of the main theoretical models have been distinguished and these exist often in more than one version as described below. First one is the molecular relaxation model, which takes into account the molecular rearrangement in the polymer necessary to accommodate a change in penetrant content. Near or below T g, such molecular relaxations are very slow on the time scale of diffusion process. Different versions of the model have been applied to systems where the penetrant is a good swelling solvent to the polymer. Perhaps the difficulty of these models lies in the use of "initial state" which poorly characterize physically and introduce a significant number of adjustable parameters. The second model is concerned with connective diffusion. When a glossy polymer is strongly swollen by the penetrant, zero order absorption kinetics is noticed. More detailed absorption reveals sharp (discontinuous) penetrant fronts which separate the highly swollen outer region from the inner glassy core and advances in a constant velocity, "V". Such processes, called non-fickian, are completely rate 200

6 controlled by the swelling stress. A more precise and detailed physical picture of the phenomenon is still lacking. A phenomenological model of non-fickian diffusion has also been proposed in which the Fickian and non-fickian mechanisms are combined additively through the equation; ( C/ t) = ( / x) [D ( C/ x) - VC] (1) Assuming D as constant can solve this equation and the treatment has proved particularly useful for the description of sorption kinetics of solvents including binary mixtures. The third model, the differential swelling stress model, is based on the consideration that uneven distribution of penetrant across the polymer film during diffusion caused a correspondingly uneven swelling tendency along the plane of the membrane. Assuming the polymer to exhibit linear viscoelasticity, later refined this model. It should be noted that each of the models above has special characteristics, which enhance its utility for certain applications. However, the molecular relaxation model has been used more extensively than the other two Free volume model A number of free volume theories have been advanced to study the diffusion in polymers. The term free volume refers to the empty space between the molecules of the substance and has been discussed at various levels of sophistication [37-44]. One of the most promising and earliest free volume models developed by Fujita [45-46] in the nearly sixties enjoyed popularity for a long time. This approach employed the William-Landel-Ferry (WLF) modification of the Doolittle equation [47-48]. Fujita [45-46] suggested the molecular transport as a result of redistribution of free volume and not the thermal activation. Based on the Cohen and Turnbull formalism [49], Fujita suggested a relation between the thermodynamic diffusion coefficient [DT = D (d lnc/d ln a)] and the fractional free volume of a penetrantpolymer system. The validity of Fujita's theory has been tested for a number of organic vapours-amorphous polymer systems wherein strong dependence of D on penetrant concentration was found [45-46, 50-52]. A brief mention may be made here of some of the earlier theories concerning free volume concepts. These include Wilkins and Long [53], who considered the diffusion of local regions of high free volume in the mixture. Peterlin [54] invoked the Hildebrand concept of fractional free 201

7 volume and used Flory-Huggins equation to develop a relation for solubility coefficient in terms of polymer-solvent interaction parameter. Later developments in this area have been attempted by Vrentas and Duda [55-56]. In order to account for the difference between the diffusion behaviour of gases and organic solvents in amorphous polymers, Vrentas and Duda have proposed a new version of the free volume theory [56]. Their theory is based on the earlier models of Cohen and Turnbull [49], Fujita [45-46] and Bearman [57] between the mutual diffusion coefficient and the friction coefficient and makes use of the thermodynamic theory of Flory [58] and the entanglement theory of Bueche [59]. Their formalism was used to calculate the concentration and temperature dependence of the mutual diffusion coefficient [60] Permeation Penetrant permeation through polymer membrane is an extremely complex phenomenon for which no satisfactory theory exists. Several models have been proposed and used to interpret the experimental results, yet only a few of them met with limited success. Some models took into account the details of the postulated mechanism transport of liquids while others describe the overall phenomenon without proper mechanism. Most of the existing models tried to answer the question as to what is the nature flow of liquid through the membrane. The immediate answer would be that the flow is either viscous or diffuse or a combination of the two; the latter seems to be more logical. The broad subject of polymer permeability has been classified into three topics representing three conceptual approaches. The first topic focuses on the actual mechanism of penetration, where heavy emphasis is given to the question related to transport kinetics and to diffusion phenomenon discussed earlier. The second topic concerns the study of dimensional response of the polymer termed hygroelasticity, where, one is confronted with problems of swelling, internal stresses, etc. The theme of the third topic deals with the environmental and ecological effects on the properties of polymers. 202

8 7.2.6 Kinetics and mechanisms of the solvent sorption in polymers In order to understand the phenomenon of small molecules in polymers, it is necessary to elucidate the mechanism of diffusion on a microscopic level. Fick's relations are the starting points for first studies. If diffusion is restricted to x-direction such as in the case of thin polymer film absorbing a liquid where, diffusion into the edges of the film can be ignored then Fick's second law of diffusion is written as [61]; C/ t = D ( 2 C/ x 2 ) (2) where, t is the sorption time, C is the liquid concentration within the membrane materials and D is the concentration independent diffusion coefficient. Equation (2) was solved to calculate the values of D by the sorption method. The solution of this equation is based on the assumption that the concentration within the membrane is initially uniform and that surface concentrations are instantaneously brought to equilibrium. The relation for Fick s second law is; M t /M = 1-8/ π 2 1/(2n+1) 2 exp (-(2n+1) 2 π 2 td)/h 2 ) (3) n=o where, M t and M referred to the cumulative masses sorbed from the polymer sample at time t and t respectively. The values of M can be obtained from the plots of (M t /M ) versus t 1/2 or t 1/2 /h. There is also a limiting case equation before 50% completion of equilibrium sorption and according to this equation (3), D can be calculated as; D= π [h θ /4M ] 2 (4) where, θ and h are the initial slope and thickness of the specimen and M is the mass obtained at equilibrium. Equation (4) is generally used to estimate D of a penetrant in a polymer from the slope of the straight line portion of the sorption curve. A method of studying diffusivity and solubility of a polymer- penetrant system is to determine the rates of sorption and desorption of penetrant by the gravimetric method [62-63]. In the absence of complicating polymer relaxation rate behaviour, plots of (M t /M ) versus t 1/2 or t 1/2 /h are generally linear from the origin upto about 55% of the total concentration. Above the linear portions, the curves are bending and then show asymptotic behaviour. When the diffusion process is Fickian, the value of t/h 2 for which M t /M = 0.5. (t/h 2 ) 0.5 = 1 (1/ π 2 D) ln [ (π 2 /16)-1/9(π 2 /16) 9 ] (5) 203

9 So that, D= / (t/h 2 ) 0.5 (6) The average values of D as calculated from equation (6) have been expected as a better approximation to the value of D than the individual values. When M t /M >0.4, the sorption rate equation can be written as; ln (1- M t /M ) = ln (K/T 2 ) - DT 2 (t/h 2 ) (7) Thus D may be computed from the limiting slope of a plot of ln (1-M t /M ) versus t or t/h 2. In order to investigate the type of diffusion mechanism the sorption data of all the penetrant polymer systems have been fitted to the following equation [64-65]; log (M t /M )= log K + n log t (8) The values of n tell us something about the type of transport mechanism, Fickian or non-fickian, k is a system parameter which depends on structural features of polymer and solvent. From least square analysis of the log (M t /M ) data verses log t the values of K and n have been calculated. The slope of the straight line gives n and y-intercept gives log K. The permeability coefficient, P is calculated by the following relation [66]; P = D x S (9) Thus, the P values are considered as estimates of the permeability coefficients. Liquid ingression into a polymeric material is a phenomenon of great technological importance. In many instances, it is necessary to know the penetration depth of liquids into polymer. In most application areas, the liquid penetration rates are calculated in terms of liquid concentration profiles. These are extremely useful to predict the selflife of the polymer while in contact with the liquids Thermodynamic and activation parameters The temperature dependence of transport coefficients (P, D and S) have been used to compute the activation parameters E D and E P for the process of the diffusion and permeation respectively from the consideration of the Arrhenius relationship; X = X 0 exp (-Ea/RT) (10) 204

10 Here, X is P, D or S, and X 0 represent the constant term. E a is activation energy, R is the molar gas constant and T is absolute temperature. From least square analysis of ln D or ln P verses l/t plot, the values of X 0 and E a are calculated. Temperature dependent thermodynamic equilibrium sorption constant K s values are used to evaluate standard enthalpy (i.e., heat of sorption) ( H) and standard entropy of sorption ( S) by using van't Hoffs relation; ln K s = ( S/R) - [( H/R) (1/T)] (11) From the least square analysis of ln K s verses l/t, H and S are calculated. The percentage weight gain Q t of the soaked polymer membrane is calculated using equation; Q t = [(M t - M i ) / M i ] x 100 (12) where, M i is initial weight of the membrane and M t is the weight at time t. The weight gain during sorption process is expressed as moles of solvent uptake by 100 g of polymer sample (C t ); C ( mol %) t Wt W0 100 = X W (13) 0 M where, W 0 is the initial mass of the sample; W t is the mass at time t, that is the immersion period; and M is the molar mass of the liquid. The transport properties such as diffusion, sorption, permeation and thermodynamic parameters are discussed in detail for interaction of aromatic organic solvent in CEPU systems Present research problem A survey of the literature reveals that the molecular transport behavior of dicarboxylic acid based CEPUs has not been studied. But the sorption and diffusion behaviour of PU membranes have been studied by many researchers [67-81]. In this research programme dicarboxylic acid based CEPUs membranes have been selected, 205

11 because of its potential applications. In all these application areas, it is likely that these membranes may come in contact with penetrants such as organic solvents, salt solutions and oils which may affect the performance of the PU membranes. The main goal of the present thesis is to achieve comprehensive understanding of the transport characteristics of organic penetrants like n-alkanes and aromatic penetrants through CEPU membranes. The sorption experiment was performed at 25, 40 and 60 o C. These results are discussed in terms of the nature of the polymersolvent interaction, molecule size and viscosity of the penetrants. 7.3 Experimental Specimen preparation The MA and CA based CEPU membranes were prepared as per procedure given in chapter 3 using two diisocyanates like, TDI and HDI. The prepared CEPU membranes have been investigated for molecular transport with n-alkanes and aromatic solvents Sorption measurements These CEPU membranes were exposed to the n-alkanes for a definite period of time and the changes in mass of the membranes are monitored. The mass uptake of the penetrants by the PU membranes depends upon the polymer network structure. In these experiments, mass gain due to sorption is accurately measured as a function of time. From these results S and D values have been calculated for the organic probe molecule through the PU membranes [23, 67, 82-83]. Sorption experiments were performed at 25, 40 and 60 o C using an electrically controlled oven maintained at the desired temperature with in the accuracy of ±0.5 o C. The CEPU samples were cut circularly (diameter =1.5 cm) using sharp edged steel die. The initial thickness of the specimens was measured at several points (Mitutoyo, Japan with precision of ± 0.001) and then dried in a dessicator for one day before the experiment. Dry weights of specimens were recorded before immersion into the penetrant. The cut specimens were immersed into the organic solvents taken in a 206

12 screw tight metal cap bottle, kept in a temperature controlled oven. At specified intervals of time, the membrane was removed from the containers; surface adhered liquid drops were removed by using soft filter paper and then weighed immediately using analytical balance. The sample was then placed back immediately into the test liquid and transferred to the temperature-controlled oven. The total time spent by the CEPU membrane outside the penetrant was within sec in order to minimize the possible experimental error. The weighing of the samples continued until the equilibrium value was reached. After the membrane attained equilibrium sorption, no more mass gain occurred and this did not change significantly by keeping the samples inside the liquids for a further period of one or two days. The time until no more liquid uptake by the polymer was observed (equilibrium sorption taken for the attainment of equilibrium for different liquids varied from 70 to 90 h. Two independent readings were taken and an average value was used in all the calculations. Sorption coefficients were expressed as wt % and mol % and are calculated using eqs. (12) and (13). PART A - Transport characteristics of carboxylic acids based chain extended PU membranes with n-alkane penetrants In this section citric acid (CA) and maleic acid (MA) based CEPU membranes have been subjected to studying the molecular transport of n-alkanes. The n-alkanes such as hexane, heptane, octane and nonane of AR grade were distilled before use. Some physical properties of solvents used as penetrants are given in Table 7.1. Table 7.1. Some physical properties of n-alkane penetrants at 25 o C Penetrants Mol. Vol. (cm 3 /mol) Density (g/cc) Sol. Parameter (cal /cm 3 ) 1/2 ε Dipole moment (debye) Polaris ability (10-24 cm -1 ) BP ( o C) Viscosity (cst) Hexane Heptane Octane Nonane

13 7.4 Results and Discussion Sorption kinetics Two CEPU membranes were selected in this study and they are; TMA (CO+TDI+MA) and TCA (CO+TDI+CA). Sorption and diffusion of n-alkanes into different membranes of dicarboxylic acid based CEPUs have been studied. The total amount of n-alkane molecules absorbed by polymeric materials is the fundamental to measure the sorption values. During initial sorption stages, the penetrant uptake increased linearly with t ½. Later due to equilibrium, the sorption curves for all penetrant attained plateau regions at all temperatures. All CEPU membranes have reached equilibrium almost at the same time. Sorption studies can be easily understood by interpretation in terms of mass increase per 100 g of the polymer sample verses square root of time t ½. Comparison of sorption tendencies of both TMA and TCA based CEPUs with octane at 25 o C is shown in Figure 7.1. From the figure it was noticed that MA based PU has more interaction with octane than TCA based PU. This is due to TCA (the functionality of CA is three- two dicarboxylic acid groups and one hydroxyl group) being a highly polar membrane as compared to TMA. The sorption results of all the n-alkane penetrants such as hexane, heptane, octane and nonane at room temperature (25 o C) for TMA and TCA based CEPUs are presented in Figures 7.2 (a) and (b) respectively. These membranes in all alkane penetrants showed almost identical sorption tendencies. Here, sorption (S) values increases with increase in molecular size of the penetrant. There is a competition between size of the penetrant and the degree of interaction between PU membrane and solvent [82-83]. This can also be attributed to the solubility parameter factor of membrane and probe molecules. Differences in solubility parameters overcome the molecular size and hence, influence the sorption [83]. The sorption process of all penetrants followed the sequence; nonane > octane > heptane > hexane. The molecular size, solubility parameter and dielectric constant of probe molecules also followed same trend of sorption. 208

14 Qt (%) Figure 7.1. Percentage mass uptake (Q t ) versus t 1/2 for MA and CA based CEPUs with octane at 25 ºC (a) t 1/2 (min 1/2 ) Qt (%) t 1/2 (min 1/2 ) (b) Qt (%) t 1/2 (min 1/2 ) Figure 7.2. Percentage mass uptake (Q t ) versus t 1/2 for (a) MA and (b) CA based CEPUs with different n-alkanes at room temperature 209

15 For a Fickian type behavior, the plots of Q t versus t ½ should increase linearly up to about 50 % sorption. Deviation from the Fickian sorption is associated with the time taken by the polymer segments to respond to swelling stress and rearrange them to accommodate the solvent molecules [83]. This usually results in the sigmoidal shapes of the sorption curves. Thus non-fickian diffusion involves the tension between swollen (soft segment) and unswollen (hard segments) parts of PU, as the latter tends to resist further swelling. Molecular transport of liquids through the polymeric membranes depends on temperature and thus we have studied the effect of temperature on sorption. Such dependency is typically shown in Figure 7.3 for citric acid based CEPU with n-hexane. From the figure it was noticed that as temperature increases the sorption values also increased. This effect follows the conventional theory that at higher temperature the free volume increases due to an increased movement of the chain segments of the CEPUs [84]. Sorption capacity increases with increase in temperature. However, sorption at higher temperature attains equilibrium much more quickly and uptake values are also higher than those observed at lower temperatures. Qt (%) t 1/2 (min 1/2 ) Figure 7.3. Percentage mass uptake (Q t ) versus t 1/2 for CA based CEPU in hexane at different temperatures In order to know about the type of transport mode, the estimated values of n and K were calculated [85-86]; where, parameter K is a closely related function of polymer type and nature of the solvent molecules. Further, it has been shown to be related to the diffusion parameters and polymer solvent interaction [3-5, 12, 15]. 210

16 ln Mt / M Figure 7.4. A plot of ln M t /M versus ln t for CA based CEPU with hexane at different temperatures ln t Table 7.2. Sorption data for MA and CA based CEPUs in n-alkanes Solvent n K (10 2 g/g. min n ) 25 o C 40 o C 60 o C 25 o C 40 o C 60 o C MA Hexane Heptane Octane Nonane CA Hexane Heptane Octane Nonane The magnitude of n decides the transport mode. For instance, a value of n = 0.5 suggests the Fickian mode and for n = 1, a non- Fickian diffusion mode is predicted. In order to determine K and n plots of ln (M t /M ) versus ln (t) were constructed, and it is shown in Figure 7.4. The calculated empirical parameters n and K are given in Table 7.2. From the table, it was noticed that n-values lie in the range for the investigated temperature interval o C, which indicates a 211

17 Fickian mode of transport. The results of n are not dependent on temperature. K value increases with increase in temperature and lies in the range 0.87 x x 10-2 g/g min n. The temperature dependence of K for all the penetrants suggests that it increases with increase in temperature. Furthermore, K appears to depend on the structural characteristics of the penetrant molecules. Thus, it appears that, K not only depends on the structural characteristics of polymer and penetrant molecules, but also on solvent interactions with PU chains [23] Sorption The calculated sorption values of all CEPUs are tabulated in Table 7.3. From the values of sorption data, all the CEPUs showed different sorption values. This may be due to different chemical structure and morphology of different dicarboxylic acid based CEPUs. The morphology of the CEPU membranes depends on the nature of the chain extender/crosslinker, state of compatibility and micro phase segregation between the hard and soft segments. But there is no systematic variation in S values among the PU membranes. Molecular migration of each penetrant differs depending upon their size, polarity and solubility parameter and nature of polymeric membranes, thus showing the effect on the structure and/or its morphology. From this result it is evident that the molecular transport depends on the structure and/or morphological set up of the membrane material and the polymer penetrant interactions may be a physical type rather than a chemical interaction Diffusion and permeation coefficients Diffusion coefficient (D) of the polymer-solvent systems is a key parameter in many engineering areas. The transport of small molecules through the polymer membranes generally occurs by a solution diffusion mechanism. That is the penetrant molecules are first sorbed by the polymer followed by the diffusion [12]. The diffusion through the polymer depends on the amount of the penetrant molecules between the two surfaces. Diffusion coefficient was calculated using Fick s equation (4) [87]. 212

18 Table 7.3. Sorption (S), diffusion (D) and permeation (P) coefficients of MA and CA based CEPUs in n- alkanes n- Alkanes Properties MA CA 25 o C 40 o C 60 o C 25 o C 40 o C 60 o C S x10 2 (g/g) Hexane Dx10 7 (cm 2 /s) P x 10 7 (cm 2 /s) S x10 2 (g/g) Heptane Dx10 7 (cm 2 /s) P x10 7 (cm 2 /s) S x10 2 (g/g) Octane Dx10 7 (cm 2 /s) P x 10 7 (cm 2 /s) S x10 2 (g/g) Nonane Dx10 7 (cm 2 /s) P x 10 7 (cm 2 /s) ln D l/t X 10 3 (K 1 ) Figure 7.5. Plots for ln D versus 1/T for CA based CEPU for different n-alkane penetrants 213

19 Calculated values of D for all CEPUs are given in Table 7.3. The variation of D depends upon the nature of the penetrant molecules in addition to the structural characteristics of CEPUs. Diffusion coefficient decreased with increasing molecular volume of their migrating liquids. The sequence of variation of D with respect to penetrants is; hexane > heptane > octane > nonane. The D values also increases with increase in temperature. Such dependency of D on molecular volume of n-alkanes suggest that larger molecules in a related series of liquids occupy larger free volumes, leading to hindered diffusion through the polymer matrix [88]. Molecular transport of probe molecules into the polymer membranes is dependent upon several factors [89-91] such as; (i) micro voids for free diffusion (ii) the construction resulting from alternately small and large pores in the transport path (iii) the construction resulting from the very close approach of the boundaries of the limiting pore within the transport path and (iv) the tortuosity imparted by the membrane material. The diffusion of solvent molecules into the dense polymer expands the network of matrix and thereby weakens the molecular interaction between the neighboring polymer segments. A highly cross-linked and crystalline polymer inhibits diffusion of liquid molecules more than a linear uncross linked polymer. The permeation of small molecules through polymers generally occurs through a solution diffusion mechanism, i.e., the penetrant molecules are first sorbed by the polymer followed by diffusion through the polymer. The net transport through the polymer depends on the difference in the amount of penetrant molecules between the two surfaces. The permeability of a penetrant in a polymer membrane depends on the diffusivity as well as solubility or sorption of the penetrant in the polymer membrane. The calculated P [92] values also followed the same trends as those of diffusion with reference to temperature and molecular size of the penetrant molecules (Table 7.3). 214

20 7.4.4 Activation parameters The Arrhenius activation parameters, viz., E D and E P for the processes of diffusion and permeation have been computed from a consideration of the temperature variation of P and D respectively. The Arrhenius plots namely ln D verses 1/T and ln P verses 1/T are presented in Figures 7.5 and 7.6 for TCA based CEPU for different penetrants respectively. Arrhenius plots exhibit linearity and this suggests that values of activation energy are roughly constant over the investigated range of temperature. The activation energy (E D and E P ) has been calculated from the slope of Arrhenius plot (Table 7.4). The values of E D and E P are in the range kj/mol and kj/mol respectively. Activation energy will be greater for the larger liquids and for the rigid polymer chain with stronger cohesive energy. On the other hand, the heat of sorption is a composite parameter, which involves contribution from Henry s law mode with the endothermic reaction contribution and Langmuir s (hole filling) type sorption giving the exothermic heats of sorption. This is due to the degree of interaction between PU-alkane being different for different membrane - penetrant systems. Activation energy, E P will be higher as compared to E D for PU-penetrant systems, because of higher degree of cohesive energy in polymer chain. Those liquids, which exhibit lower values of D have shown higher values of activation parameter E D and vice versa. ln P l/t X 10 3 (K 1 ) Figure 7.6. Plot of ln P versus 1/T for CA based CEPU for different n-alkane penetrants 215

21 Table 7.4. Activation energy for diffusion (E D, kj/mol), permeation (E P, kj/mol), enthalpy of sorption ( H, kj/mol ± 4) and entropy of sorption ( S, J mol -1 K -1 ± 1) for MA and CA based CEPUs with n-alkane systems n-alkanes Hexane Heptane Octane Nonane Sample Property E D E P H S MA CA MA CA MA CA MA CA Thermodynamic parameters The equilibrium sorption constants K s, were calculated from the following equation; K S = Number of moles of penetrant sorbed / Unit mass of the polymer (14) The calculated K s values are given in Table 7.5. A systematic decrease in K s values with increasing molecular volume of n- alkanes was observed, suggesting an inverse dependency of K s on molecular volume of n- alkanes. This is because larger size occupies more free volume than smaller molecules. A plot of ln Ks versus 1/T for both CEPUs in octane is shown in Figure 7.7. It was noticed that the plots are linear within the temperature interval of o C. The values of H and S were calculated from the figure and they are given in Table 7.4. The enthalpy of sorption is calculated from the equation; H S = E P E D (15) H S is a composite parameter involving the contribution from, (i) Henry s law needed for the formation of a site and the dissolution of the species into that site, the formation of the site involves an endothermic contribution and (ii) Langmuir s (hole filling) type sorption mechanism, in which case the site already exists in the polymer matrix and sorption by hole filling gives exothermic heat of sorption. 216

22 ln KS l/t X 10 3 (K 1 ) Figure 7.7. van t Hoff s plot of ln K S versus 1/T for both CEPUs in octane The positive H S values for CEPUs suggest a Henry s type sorption and the negative H S value suggests a Langmuir type sorption. There is no systematic variation of H S with respect to penetration size. The calculated S values from the van t Hoff plots are negative for all PU-alkane systems suggesting that solvent molecules retain their liquid-state structure even in the sorbed state. Table 7.5. Equilibrium sorption constant (K S ) of CEPUs in n-alkanes Sample MA K s x 10 2 (m mol/g) Temp. ( o C) Hexane Heptane Octane Nonane CA Conclusions The work described in this section, summarizes the molecular transport of n-alkane penetrants into two carboxylic acids (CA and MA) based CEPU membranes by the gravimetric sorption method in the temperature intervals at 25, 40 and 60 o C. The sorption and diffusion tendencies of both CEPUs are different for different 217

23 penetrants. The Fickian model has been used to estimate the diffusion coefficient transport data. The values of n lie in the range suggesting that the molecular transport is Fickian mode. It was observed that factors such as solvent type, the chemical structure and morphology of the PU seem to exert tremendous influence on the transport characteristics. It was also observed that the diffusion mechanism followed the Fickian trend and that the kinetics of sorption is of the first order. Diffusion data and related activation parameters for the process of diffusion follow the principle of Eyring s theory of activated diffusion of molecules into the PU network structures. It is also observed that, the sorption-coefficient values for both PU membranes increases with increasing molecular volume of penetrants and the sequence is; nonane > octane > heptane > hexane. The estimated H S values are positive and lie in the range kj/mol. The H S values of CEPUs suggest that the sorption process is dominated by Henry s law mode. Furthermore, such studies are useful for a preliminary screening of the polymers before their intended applications in technological and engineering sectors. PART B - Molecular transport behaviour of substituted aromatic solvents with CEPUs In this section, molecular transport characteristics of hexamethylene diisocyanate (HDI) based CEPU (HCA and HMA) membranes with substituted aromatic penetrants (benzene, chlorobenzene and nitrobenzene) have been reported in the temperature range C. Molecular migration depends on the nature of the organic solvent, membrane solvent interaction, temperature, solubility parameter, molecular volume and free volume available within the polymer matrix. Typical properties of aromatic solvents are given in Table 7.6. Table 7.6. Some of the characteristic properties of the aromatic solvents Penetrants Molecular Formula Molecular weight Molar volume (cm 3 /mol) Boiling point ( o C) Viscosity (MPa s) Solubility parameter (cal/cm 3 ) 1/2 Benzene C 6 H Chlorobenzene C 6 H 5 Cl Nitrobenzene C 6 H 5 NO

24 7.6 Results and Discussion Transport behavior Transport behaviour of substituted aromatic penetrants such as benzene, chlorobenzene and nitrobenzene into CA and MA based CEPU membranes has been studied at 25, 40 and 60 o C on an immersion weight gain method. The weight gain during sorption process is expressed as moles of solvent uptake by 100g of the polymer sample (C t, mol %) and it was calculated using equation (13). The solvent uptake was monitored until the specimens attained the equilibrium values. Some typical plots of sorption curves for CA and MA based CEPUs in nitrobenzene at 25, 40 and 60 o C are presented in Figure 7.8. The sorption behaviour is a thermodynamic parameter, which depends on the strength of the interaction in polymer-probe molecule and describes the initial penetration and diffusion of probe molecules into the polymeric membranes. The sorption (S) i.e., maximum mass uptake (obtained from the plateau region of the sorption plots) of CA and MA based CEPUs in nitrobenzene follow the order; CA>MA. A similar behavior was observed for all penetrants and temperatures. This could be due to the presence of additional polar groups like -COOH and OH groups in CA based CEPU as compared to MA based CEPU, which leads to more interaction with the polar solvent and hence higher penetrant uptake. During initial sorption stages, i.e., up to 50% of the completion of the sorption penetrant uptake increased linearly with t 1/2 values. Deviations from the Fickian sorption are associated with the time taken by the polymer segments to respond to swelling stress and to rearrange themselves to accommodate the solvent molecules [68]. This usually results in the sigmoidal shapes for the sorption curves. Thus, non-fickian diffusion involves the tension between swollen (soft segments) and the unswollen (hard segments) parts of CEPU as the latter tend to resist further swelling. However, during early stages of sorption, the samples may not reach the true equilibrium concentration of the penetrant and thus, the rate of sorption builds up slowly to produce slight curvatures as shown in Figures

25 O C 0.8 C t /100 (mol%) HMA HCA t 1/2 (min 1/2 ) C C t /100 (mol%) t 1/2 (min 1/2 ) HMA HCA C C t /100 (mol %) t 1/2 (min 1/2 ) HMA HCA Figure 7.8. The mol % uptake versus square root of time for HCA and HMA in nitrobenzene at different temperatures 220

26 This is an indication of the departure from the Fickian mode and is further confirmed from an analysis of sorption data. At later stages of the sorption experiments, due to the saturation equilibrium, the sorption values for all penetrants at all temperatures attained plateau regions and values given in Table 7.7. From the figure, it can also be observed that, the initial portion of the sorption curves are linear after which the mechanism changes. According to Southern and Thomas [93], when a polymer interacts with solvents, the surface of the polymer membrane swells immediately, but the swelling will not take place in the underlying un swollen material. Thus, a two dimensional compressive stress is exerted on the surface. The swelling stresses are either relaxed or dissipated by further swelling and rearrangement of the segments. Diffusion of low molecular weight probe molecules were found to lead typical phenomena of membrane swelling and physical relaxation. The dynamic sorption curves of MA and CA based CEPUs with different aromatic probe molecules at 60 o C are shown in Figure 7.9 respectively. A persual of the sorption curves given in Figure 7.9 suggests a systematic trend with respect to molecular volume of penetrants in the sorption behaviour of aromatic probe molecules. The sorption curve in benzene is found to be lower compared to other penetrants. This can be attributed to nonpolar nature of benzene which leads to less interaction with polar CEPU membrane. The sorption and the diffusion process are influenced by the factors such as molecular size, interaction parameter between the polymer and solvent and the solubility parameters of the membranes and the solvents. Here sorption increases with increase in molecular size of the penetrant. There is a competition between size of the penetrants and the interaction between polymers and the solvents [67-69]. This is attributed solubility parameter factor. Difference in solubility parameters overcomes the molecular size and hence, influences the sorption process [94]. The equilibrium mol % uptake for sorption as a function of molecular volume and solubility parameter of probe molecules is shown in Figures 7.10 (a) (b) respectively. 221

27 1.6 HMA C t /100 (mol %) Benzene Chlorobenzene Nitrobenzene t 1/2 (min 1/2 ) 2.5 HCA 2.0 C t /100 (mol%) Benzene Chlorobenzene Nitrobenzene t 1/2 (min 1/2 ) Figure 7.9. Mol % uptake versus square root of time for HMA and HCA in different solvents at 60 o C As the solubility parameter of nitrobenzene is closer to the solubility parameter of CEPU the equilibrium mol uptake of nitrobenzene into the CEPU is more. Similarly benzene with low solubility parameter exhibits less sorption into the polymer network. The sequence of sorption behaviour with reference to penetrants is; benzene < chlorobenzene < nitrobenzene. In this experimental investigation significant dependence (linear behaviour) of the solubility parameter of the solvent on the solvent uptake behaviour was noticed. This indicates that there are more interactions between solvent and polymer, which lead to solvation followed by diffusion into membrane. Recently Siddaramaiah et al [95-96] observed a correlation between sorption coefficient and solubility parameters of probe molecules for IPN-aromatic penetrants. Aithal et al [97] also made a similar kind of observation for PU system. The pattern of curves in Figure 7.10 indicates that there is clear-cut interaction between CEPU and probe molecules that depends on solubility parameter and molecular size. 222

28 120 (a) 90 Sx100 (%) HCA HMA Molar volume (cm 3 /mol) Sx100(%) HCA HMA Solubility parameter of pobe molecules (cal/cm3) Figure 7.10 (a)-(b). Sorption coefficient (S) as a function of molar volume and solubility parameter of probe molecules for CEPU membranes Sorption data also serve as a guide to study the effect of temperature on the observed transport behaviour. The effect of temperature on mol % uptake of nitrobenzene for CA based CEPU is shown in Figure The rate of diffusion and permeation increases with increase in temperature [98]. The same observation was noticed for all CEPU-penetrant systems. This effect follows the conventional theory that at higher temperature an increase in free volume occurs due to an increased movement of the chain segments of the polyurethane [99-101]. Increase in temperature reduces the tortuous route of the solvent and also reduces the time required to attain equilibrium. 223

29 2.0 HCA 1.6 C t / 100 (mol%) t 1/2 (min 1/2 ) 25 o C 40 o C 60 o C Figure Mol % uptake versus square root of time for HCA in nitrobenzene at different temperatures From the slope, θ of the initial linear portion of the sorption curves i.e., Q t verses t 1/2, the diffusion coefficients have been calculated by using equation (4). The values of D determined in this manner can be regarded as independent of concentration and are thus applicable for Fickian mode of transport [67, 71, 73-74, 82]. A triple evaluation of D from sorption curves gave us D values with an error of ± units at 25 o C ± units at 40 o C and ± units at 60 o C for all polymer-penetrant systems. These uncertainty estimates regarding diffusion coefficients suggest that the half times were very reproducible (to within a few tens of seconds). The calculated values of D are tabulated in Table 7.7 for all CEPU-solvent pairs. The variation in the diffusion coefficient also depends on the nature of the penetrant molecules in addition to the structural characteristics of the CEPUs. The calculated sorption (S), diffusion (D) and permeation (P) coefficients under investigated temperatures for CEPU-aromatic penetrant systems are given in Table 7.7. The sorption and diffusion coefficients of MA based CEPU were lower as compared to CA based CEPU. The sorption values increased with increase in temperature for all solvents. The sorption and diffusion coefficient values for CEPUs followed a sequence; benzene < chlorobenzene < nitrobenzene. This can be attributed 224