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PS) interpenetrating polymer network membranes
Journal of Membrane Science 201 (2002) 213–227
Transport of aromatic solvents through natural rubber/polystyrene (NR/PS) interpenetrating polymer network membranes Aji P. Mathew a , S. Packirisamy b , Ranimol Stephen a , Sabu Thomas a,∗ a
School of Chemical Sciences, Mahatma Gandhi University, P.O. Priyadarshini Hills, Kottayam, Kerala 686560, India b PSC Division, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695022, India Received 27 November 2000; accepted 8 November 2001
Abstract A series of interpenetrating polymer network membranes have been synthesised from natural rubber and polystyrene by the sequential polymerisation technique. The transport of aromatic hydrocarbons through semi- and full-interpenetrating polymer network membranes (IPNs) have been studied in detail by tracing the solvent uptake up to equilibrium. The sorption was carried out in a series of aromatic solvents viz. benzene, toluene and xylene. The effect of temperature on swelling is studied by carrying out the experiments in toluene in the temperature range of 30–75 ◦ C. The effects of blend ratio, crosslinker content and nature of initiator on the diffusion of various solvents were analysed. It was found that in all cases, the uptake value increased by about 50% as the PS content decreased from 70–30%. The diffusion, sorption and permeation coefficients were evaluated. As the crosslink density was increased, the uptake decreased by 40%. Kinetic and thermodynamic parameters were evaluated from diffusion experiments. The diffusion profiles were compared with theoretical predictions. The influence of swelling on the mechanical performance of the membranes has been investigated by conducting tensile testing of swollen specimens. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Diffusion; Interpenetrating polymer networks; Crosslink density; Swelling behaviour
1. Introduction In the broadest sense, an interpenetrating polymer networks is any material containing two polymers, each in network form [1–3]. Though, the term interpenetrating polymer networks implies some kind of interpenetration of two polymer networks, molecular interpenetration occurs in the case of mutual solubility only [4,5]. In most cases, the molecular interpenetration may be restricted and a supermolecular level of penetration is observed. ∗ Corresponding author. Tel.: +91-481-598-303; fax: +91-481-561-190. E-mail address: [email protected] (S. Thomas).
The interpenetrating polymer networks can be characterised by studying the morphology, mechanical properties, thermal behaviour, transport phenomena, etc. Sperling and co-workers [6–8] have been reviewed the industrial applications, morphological behaviour, synthesis, and properties of IPNs. The synthesis and morphology development of polychloroprene/polystyrene latex interpenetrating networks have been solely studied by Burford et al.[9]. He also synthesised latex IPNs based on high styrene resin as seed latex and PS as the second polymer in different compositional ratios [10]. Hopfenberg and Paul have pointed out that the study of diffusion, sorption and permeation in blend structure provides a valuable means for additional characterisation of the
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 7 3 8 - 4
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blends [11]. Experimental determination of sorption and permeation behaviour acts as a powerful tool for characterising the detailed nature of blends [12,13]. The transport studies are of considerable importance when we come across problems like designing a barrier material or tubes for transporting liquids and gases. The microfiltration of matters using membranes have been recently studied by Fane and co-workers [14–16]. The use of penetrants as probes to infer the detailed structure of polymer system are in practice extensively in recent times. Huang et al. [17] reported the diffusion of ethanol and water through polyurethane membranes. Baysal and co-workers [18] studied the swelling of polyacrylamide gels in polyacrylamide solution and the swelling behaviour was found to deviate from the prediction of Flory–Huggins theory. Fujita [19,20] derived a relationship between critical free volume and diffusion coefficient of the molecules for the polymer–solvent systems. The diffusion behaviour of polyethylene–polystyrene (PE–PS) semi-IPNs in toluene and chloroform was investigated by Hong and Duda [21]. The results showed that the semi-IPNs consists of a continuous PE phase containing dispersed PS phase. In the blend of PE and nylon 6, it was found that the permeability to heptane, methyl salicylate and methyl alcohol varied with blend ratio [22]. The effects of temperature on penetrant activity and blend composition of PS–PPO blends on n-hexane diffusion kinetics have been systematically examined by Hopfenberg and co-workers [23]. The diffusivity in a given polymer system, whether it is rubbery polymers, glassy polymers, polymer blend, graft or interpenetrating polymer networks, varies from one polymer system to another [24,25]. Diffusivity depends on the free volume within the material and the segmental mobility of polymer chains, crosslinking of component polymer phases, size of penetrants etc. [26,27]. In the present paper, a new set of interpenetrating polymer network membranes has been prepared from natural rubber and polystyrene. The diffusion of various aromatic solvents such as benzene, xylene and toluene through natural rubber/polystyrene interpenetrating polymer network membranes has been studied in detail. The NR phase is crosslinked using dicumyl peroxide (DCP). The PS phase is linear/crosslinked in semi/full-interpenetrating polymer networks. The
crosslinking of the PS phase is varied by controlling the amount of crosslinker, divinyl benzene used. The mechanism of sorption was determined and is compared with theoretical values. The diffusion, sorption and permeation coefficient were calculated. The effect of swelling on mechanical properties have been studied by carrying out the tensile strength measurements on dumpbell samples swollen in toluene. 2. Experimental 2.1. Materials used Natural rubber (NR) used was of ISNR-5 grade. It was supplied by Rubber Research Institute of India (RRII). The characteristics of NR are given in Table 1. Styrene monomer for the interpenetrating polymer networks synthesis was supplied by Merck, India. The monomer was made inhibitor free by washing it with 1% NaOH and was dried before use. Dicumyl peroxide (40% active) was used as the vulcanising agent for rubber and as the initiator for the polymerisation of styrene. It was procured from Kishore Rubber Products Pvt. Ltd., India. p-Benzoyl peroxide was obtained from BDH, India and used as initiator for styrene. AIBN was obtained from Sigma, India and used as the initiator. 2.2. Preparation of interpenetrating polymer networks 2.2.1. Crosslinking of NR Natural rubber was masticated in a two roll mixing mill at room temperature. Dicumyl peroxide (4 ph) was added and mixed thoroughly for 15 min. The rheograph of the mix was taken on a Monsanto Rheometer and the optimum cure time determined. The mix was cured at 160 ◦ C on a hydraulic press to get the crosslinked membranes. The crosslink density of the DCP cured samples were determined by Table 1 Characteristics of natural rubber (NR) Glass transition temperature (◦ C) Density (g/cc) Solubility parameter (J m3 )1/2 × 10−3 Volatile matter (%) max Ash (%) max
−72 0.97 16.2 0.50 0.40
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swelling experiments in toluene at 30 ◦ C and it was found to be about 3.96 × 10−5 gmol/cc. 2.2.2. Polymerization and crosslinking of PS The following different series of interpenetrating polymer network membranes were prepared using different initiators. 2.2.2.1. Series I. The cured NR membranes were weighed and kept immersed in inhibitor free styrene monomer containing 1% benzoyl peroxide as initiator and 0, 2, 4 or 6% of divinyl benzene (DVB) which acts as crosslinker for PS phase. The NR membranes were swollen to different time intervals to obtain different weight percentages of PS. The detailed mechanism of transport of styrene monomer through natural rubber has already been reported in our earlier papers [28,29]. The swollen samples were kept at 0 ◦ C for few hours to achieve equilibrium distribution of styrene monomer in the matrix. The swollen networks were heated at 80 ◦ C for 6 h and 100 ◦ C for 2 h, respectively, to carry out the polymerisation and crosslinking of styrene. 2.2.2.2. Series II. In this method, 1% DCP and 0, 2, 4 or 6% of divinyl benzene which acts as crosslinker for PS phase were added to the styrene monomer and vulcanised NR membranes were allowed to swell in it. The swollen sheets were heated at 80 ◦ C for 6 h and 100 ◦ C for 2 h, respectively, to complete the polymerisation and crosslinking reaction.
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2.2.2.3. Series III. In this series, 0.5% AIBN and 0, 2, 4 or 6% of divinyl benzene which acts as crosslinker for PS phase were added to the monomer and the natural rubber membranes were swollen in it. The swollen membranes were polymerised and crosslinked at 80 ◦ C for 6 h and 100 ◦ C for 2 h. The hardened membranes, in all cases, were then kept in a vacuum air oven to make it free of unreacted styrene. The final weight of the sample was taken and the composition of the sample was determined. In all the four series, NR/PS interpenetrating polymer networks with PS content up to 70% were prepared. The preparation of semi- and full-IPNs are schematically represented in Fig. 1. The network I was impregnated with the monomer II, initiator and crosslinker, which was thermally polymerised to get the IPNs. When the DVB content is 0%, PS phase is not crosslinked and the resultant IPN will be semi- or pseudo-IPN. By the incorporation of 2, 4 or 6% of DVB, the PS phase gets crosslinked and full-IPNs with increasing crosslink density will result. The interpenetrating polymer networks were coded based on composition, initiating system and crosslinker content. The DCP, BPO and AIBN initiated system are denoted by D, B and A series. The DVB content is varied, in order to have varying level of crosslinking. The blend composition is denoted as N30 , N50 and N70 , where the subscripts indicate the weight percent of rubber. The codings are given in Table 2.
Table 2 Nomenclature of interpenetrating polymer networks Sample code
Initiator
NR/PS ratio
A0 N30 ,A0 N50 ,A0 N70 A1 N30 ,A1 N50 ,A1 N70 A2 N30 ,A2 N50 ,A2 N70 A3 N30 ,A3 N50 ,A3 N70
0.5% 0.5% 0.5% 0.5%
30/70, 30/70, 30/70, 30/70,
50/50, 50/50, 50/50, 50/50,
70/30 70/30 70/30 70/30
0 2 4 6
B0 N30 ,B0 N50 ,B0 N70 B1 N30 ,B1 N50 ,B1 N70 B2 N30 ,B2 N50 ,B2 N70 B3 N30 ,B3 N50 ,B3 N70
1% 1% 1% 1%
(BPO) (BPO) (BPO) (BPO)
30/70, 30/70, 30/70, 30/70,
50/50, 50/50, 50/50, 50/50,
70/30 70/30 70/30 70/30
0 2 4 6
D0 N30 ,D0 N50 ,D0 N70 D1 N30 ,D1 N50 ,D1 N70 D2 N30 ,D2 N50 ,D2 N70 D3 N30 ,D3 N50 ,D3 N70
1% 1% 1% 1%
(DCP) (DCP) (DCP) (DCP)
30/70, 30/70, 30/70, 30/70,
50/50, 50/50, 50/50, 50/50,
70/30 70/30 70/30 70/30
0 2 4 6
(AIBN) (AIBN) (AIBN) (AIBN)
DVB content (%)
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Fig. 1. Schematic representation of preparation of semi- and full-interpenetrating polymer networks.
2.3. Diffusion experiments From the interpenetrating polymer network membranes with different blend ratio and crosslink density, circular samples of diameter 2 cm were punched out using a sharp edged steel die. The thickness (h) of the samples was determined accurately using a micrometer screw gauge. The initial weight of the polymer samples was determined by weighing on a highly sensitive electronic balance having an accuracy of 0.001 g. The samples were then kept immersed in penetrant taken in stoppered diffusion bottles. The swollen samples were taken out at fixed time intervals, wiped free of adhering solvents, weighed on the electronic balance and replaced in the stoppered bottles. This process was continued till equilibrium swelling which was indicated by the constancy in weight. Each weighing was
completed in 40 s, so as to keep the error due to solvent escape from the sample surface minimum. To carry out the experiment at higher temperatures (50, 65 and 75 ◦ C) a thermostatically controlled air oven was made use of. For solvent resistance studies, the weight was taken everyday upto the 7th day to monitor the uptake of solvent. From the weight gain, various parameter were calculated and useful inferences were made. 2.4. Transmission electron microscopy The morphology of IPNs were studied using JOEL-JEM 2010 model transmission electron microscope. The samples were microtomed and the NR phase was stained using OsO4 . This was viewed under the microscope.
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2.5. Tensile testing To study the effect of swelling on mechanical strength, the dumpbell specimens are allowed to undergo equilibrium swelling in toluene. The swollen samples were tested on a tensile testing machine at a cross head speed of 50 mm/min with 3 cm intergrip distance. 3. Results and discussion From the diffusion data, the mol% uptake of solvent/g of the polymer, Qt was determined using the equation [28,29]: (mass of the solvent absorbed/ mol. wt. of solvent) Qt = × 100 mass of the polymer
(1)
Qt becomes Q∞ when equilibrium is reached. The Q∞ -values of the interpenetrating polymer networks for benzene, toluene and xylene at room temperature are given in Table 3. It was found that for a given sample, the uptake increases in the order xylene < toluene < benzene. The Q∞ -values were highest in benzene, then comes toluene and then xylene. This is in the order of increasing penetrant size. As the size of the penetrant increases, the resistance offered by the polymer to the transport of the solvents through the matrix is enhanced. Therefore, the uptake was low for penetrants √ of higher size. Fig. 2 shows the plot of Qt versus t of D1 N50 and D0 N50 samples for benzene, toluene and xylene, where the effect of penetrant size on diffusion in semi- and full-IPNs is shown. The difference in transport behaviour is due to difference in polymer–solvent interaction rather than the size of the penetrant. This
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leads to the conclusion that the nature of the polymer affects the diffusion process. However, in the case of penetrants with similar nature, the molecular size decides the transport behaviour. Similar observation were reported by other researchers earlier [30]. When we examine at the effect of blend ratio on Q∞ in benzene, toluene or xylene, it was found that as the PS content increases, the uptake decreases in the case of semi-interpenetrating polymer networks and full-interpenetrating polymer networks as well (Table 3). The uptake is in the order N30 < N50 < N70 in the case of semi- and full-IPNs. Fig. 3 shows the plot of D0 N30 , D0 N50 and D0 N70 membranes (semi-IPNs) in toluene. The effect of blend ratio can be explained based on the morphology. Our earlier study on the morphology of semi- and full-IPNs showed dual phase continuity [31]. The NR phase was crosslinked in all cases. The high level of PS crosslinking makes the PS phase also continuous. In D0 N70 , the PS phase is dispersed as elongated domains in the continuous NR matrix. The TEM photographs of selected samples are shown in Fig. 4. All the systems have a nanostructured morphology and dual phase continuity. It is a known fact that the continuous phase controls the transport properties of any system. As there is no phase inversion in the whole range of blend composition, the NR:PS ratio decides the uptake value. As the rubber content decreases, the flexibility of the polymer chains decreases, whereas, increase in PS phase offers stiffness to the material. The relaxation of polymer chains is a must for solvent diffusion and sorption. The Tg of NR is below the experiment temperature and, therefore, the NR chains relax readily. The Tg of PS is above the experimental temperature and, therefore, the polymer chains have limited freedom of movement and flexibility. As the rubber content decreases, the rigidity
Table 3 Values of equilibrium uptake (Q∞ (mol%)) in benzene, toluene, xylene
Benzene Toluene
Xylene
30 ◦ C 30 ◦ C 50 ◦ C 65 ◦ C 75 ◦ C 30 ◦ C
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
3.35 3.62 3.65 3.75 3.47 3.01
4.75 4.01 4.04 4.74 4.00 3.40
5.10 4.37 4.41 4.46 4.20 3.70
4.45 3.76 3.80 3.86 3.41 3.06
2.86 2.64 2.79 3.51 2.76 2.26
4.01 3.00 3.36 3.80 3.31 2.68
4.33 3.45 3.74 3.84 3.64 2.80
3.52 2.30 2.42 2.88 2.30 1.87
3.70 3.12 3.18 3.38 3.21 2.56
4.00 3.21 3.52 4.00 3.07 2.80
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Fig. 2. Plot of Qt vs.
√
t of D1 N50 and D0 N5 samples in benzene, xylene and toluene showing the effect of penetrant size.
of the polymer network increases. This restricts the solvent uptake. The effects of crosslinker content on transport properties of D0 N50 , D1 N50 , D2 N50 and D3 N50 samples with DVB content 0, 2, 4 and 6%, respectively, are compared in Fig. 5. As the DVB content increases, the crosslinking in the PS phase increases making the material more and more rigid. The flexibility of the polymer network is decreased, as DVB content is increased and, hence, the uptake decreases. Fig. 5 shows the effect of crosslinking level on the transport behaviour of interpenetrating polymer networks in toluene for DCP initiated systems at room temperature. NR phase was crosslinked to the same extent in all cases. However, it is an accepted fact that the network initially formed will govern the morphology and properties of the system. Also, the first formed network will be continuous in all cases. So, as polymer II also gets crosslinked, both the phases tend to be continuous.
As the DVB content is increased, the crosslinking and, thereby, the continuity of phase II (PS) increase blocking the easy transport of solvent through the material. So, the uptake gets retarded with increasing crosslinker level. The uptake decreases by about 40–45% on increasing the crosslinking level from 0–6% for the DCP initiated samples swollen in toluene, xylene and benzene. Fig. 6 shows the schematic representation of effect of increase in crosslinking on solvent uptake. It could be seen that the solvent molecules have to overcome the dense barrier of polymer crosslinking and entanglements to diffuse into the material. As the number of crosslinks increases, the resistance offered to transport also increases correspondingly. The crosslink density of the material also affects the solvent uptake. The effect of crosslink density values on uptake is discussed in the coming section. Zhou et al. [32] recently reported that the degree of swelling of IPNs varied with degree of crosslinking.
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Fig. 3. Plot of Qt vs.
219
√
t showing the effect of blend ratio (D0 N30 , D0 N50 and D0 N70 samples in toluene).
The effects of temperature on diffusion and sorption behaviour was studied in toluene. It was observed that Q∞ -values increase with temperature in all cases(Table 3). However, at 75 ◦ C, a decrease in Q∞ -values is observed as compared to that at 65 ◦ C. This may be due to the fact that at 75 ◦ C desorption occurs at a rapid rate compared to sorption. This is clear from Fig. 7. In both cases, Q∞ -values are in the order 30 < 45 < 75 < 65 ◦ C. When the different initiating systems are considered it was found that Q∞ -values do not exhibit any definite trend in various solvents (Table 3). So, it may be concluded that initiating system does not affect the network characteristics appreciably. Fig. 8 compares the value of D2 N50 , B2 N50 and A2 N50 samples in benzene. The high uptake of A2 N50 sample compared to D2 N50 and B2 N50 samples is due to the low crosslink density of A2 N50 sample. 3.1. Diffusion, sorption and permeation coefficient Diffusion coefficient is a kinetic parameter which depends on the polymer segmental mobility. The
diffusion coefficient was calculated from the following equation. n=1 1 8 π2 (2n + 1)2 n=0 t × exp −D(2n + 1)2 π 2 2 h
Qt =1− Q∞
(2)
where t is the time and h is the initial sample thickness. The short time limiting expression of this equation is as follows: Qt 4 Dt 1/2 = 2 (3) Q∞ π h2 This equation is rearranged and diffusion coefficient value for the interpenetrating polymer networks was calculated using the equation [33] hθ 2 D=π (4) 4Q∞ where h is the sample thickness, θ the √ slope of initial portion of the plot of Qt versus t, and Q∞ the
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Fig. 4. Transmission electron micrographs showing the nanostructured morphology of (a) D1 N30 , (b) D1 N50 , and (c) D1 N70 .
equilibrium mole percent uptake. The ‘D’ values are given in Table 4. Diffusion coefficient is the ability of the penetrant to move among the polymers segment. The ‘D’ values decrease with increasing crosslinker level for benzene, xylene and toluene. In the case of benzene, the ‘D’ value decreases by 60% as the crosslinker level increases from D0 N50 to D3 N50 sample. The ‘D’ values follow the same order as that Q∞ . This is due to the fact that the θ -values decrease with increase of crosslinking of PS phase. As the PS content increases in a series, ‘D’ values also increase. This is
explained by the lowering of Q∞ -values with increase in PS content. The sorption coefficient was calculated using the equation [34]. S=
M∞ MP
(5)
where M∞ is the mass of solvent uptake at equilibrium and Mp is the initial polymer mass. The ‘S’ values are given in Table 5. The trend shown is the same as that of Q∞ -values in the case of benzene, xylene and
Table 4 Values of diffusion coefficient (D × 107 cm2 /s)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
6.14 13.32 7.2
8.00 14.46 9.20
12.50 15.30 15.80
6.04 7.82 –
7.24 9.16 6.80
6.70 – 7.80
10.40 11.41 11.40
– 5.81 7.60
7.40 10.10 7.27
7.00 8.21 9.30
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Fig. 5. Plot of Qt vs.
√
t showing the effect of crosslinker content (D0 N50 , D1 N50 , D2 N50 and D3 N50 samples in toluene).
toluene, and petrol, diesel and kerosene as well. Sorption is a surface phenomenon and it is an indication of the tendency of the penetrant to dissolve into the polymer. Permeation can be considered as the combined effect of sorption and diffusion process. The permeation coefficient is given by the equation [34]: P = DS
221
(6)
The P values are given in Table 6. It follows the same trend as sorption. So, it may be inferred that the sorption process controls the permeability. The volume fraction of polymer φ in the swollen sample was calculated using the equation [33]: φ=
W1 /ρ1 (W1 /ρ1 ) + (W2 + ρ2 )
Fig. 6. Schematic representation showing the effect of crosslink density on solvent uptake.
(7)
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Fig. 7. Effect of temperature on solvent uptake (Qt vs.
where W1 is weight of polymer and ρ 1 the density of polymer, W2 the weight of solvent in the swollen sample and ρ 2 the density of solvent. A high value for φ is an indication of high extent of crosslinking in the system. The φ-values are given in Table 7. For semi-interpenetrating networks the φ-value are lower than full interpenetrating polymer networks. In a given series, as the PS content increases the φ-values also increase. This is due to the increased crosslink density of the membranes. For a given initiating system, as the crosslinker level is increased φ-values also increased.
√ t in toluene for D1 N50 samples).
As crosslink density increases with crosslinker content, the uptake decreases correspondingly. From the transport data, the molecular weight between crosslinks can be calculated using the equation [35,36]: Mc =
−ρP Vr φ 1/3 − 1pt ln(1 − φ) + φ + χ φ 2
(8)
where ρ P is the density of polymer, Vr is the molar volume of solvent, φ the volume fraction of polymer and χ the interaction parameter.
Table 5 Values of sorption coefficient (S)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
2.62 3.33 3.20
3.72 3.70 3.61
4.00 3.83 4.00
3.47 3.46 3.25
2.23 2.44 2.40
3.38 2.74 2.68
3.13 3.18 3.00
2.75 2.12 2.00
3.00 3.00 2.72
2.68 2.88 3.00
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Fig. 8. Effect of initiating system on swelling (A1 N50 , B1 N50 and D1 N50 samples in toluene).
Table 6 Values of permeation coefficient (P × 107 cm2 /s)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
16.10 44.30 23.20
29.50 53.40 33.28
50.02 61.80 62.30
21.01 30.00 22.50
24.40 22.30 16.30
24.01 15.80 22.00
32.80 36.30 34.00
15.00 12.33 15.23
21.60 24.30 19.75
20.35 29.20 27.82
Table 7 Values of volume fraction (φ)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
0.245 0.210 0.206
0.188 0.187 0.188
0.181 0.180 0.178
0.200 0.196 0.205
0.278 0.255 0.256
0.221 0.236 0.235
0.201 0.213 0.223
0.220 0.285 0.300
0.230 0.223 0.236
0.211 0.227 0.221
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Table 8 Values of interaction parameter (χ )
Benzene Toluene Xylene
N30 series
N50 series
N70 series
0.400 0.341 0.343
0.434 0.355 0.348
0.487 0.384 0.372
β + Vr RT(δA − δB )2
(9)
where Vr is the molar volume of solvent, δ A and δ B are the solubility parameter of solvent and polymer respectively, R is the universal gas constant and T the absolute temperature. β the lattice constant and its value is equal to 0.34. The χ -values for different samples in various solvents are given in Table 8. From the Mc value crosslink density can be calculated using the equation [37]: ν = 21 Mc
(10)
The calculated Mc and ν-values are given in Table 9. It can be observed that ν-value increases by about 30% as PS content increases from 30–70% in a given series. For e.g. in DCP series, ν increases in the order D1 N70 < D1 N50 < D1 N30 . Also, for a given blend ratio, as the crosslinking of PS phase increases, the ν-value also increases by about 50%. This is because the chain entanglement increases as the extent of PS crosslinking increases, thereby, decreasing the solvent uptake. When we compare different initiating system (D2 N50 , B2 N50 and A2 N50 ) the crosslink density values do not follow any systematic trend. 3.2. Theoretical fitting The experimental results were compared with the theoretical equation. n=1 1 8 π2 (2n + 1)2 n=0 2 2 t × exp −D(2n + 1) π 2 h
Qt =1− Q∞
ν × 104 (g mol/cc)
Mc (g/mol)
The interaction parameter χ is given by the equation [35]: χ=
Table 9 Values of molecular weight between crosslinks (Mc ) and crosslink density (ν)
(11)
D0 D1 D2 D3 A0 A1 A2 A3 B0 B1 B2 B3
N30
N50
N70
N30
N50
N70
6728 5095 4046 2726 6025 5345 3867 2965 7078 6528 4319 3015
8231 7413 5018 3313 7125 6164 5449 4991 9015 6517 5698 4198
9835 7750 7008 4564 8369 7952 7709 5923 15342 10568 97361 6798
7.43 9.81 12.35 18.33 8.30 9.35 13.00 17.00 7.06 7.65 11.57 16.58
6.07 6.74 10.00 15.10 7.01 8.11 9.17 10.02 5.54 7.67 8.87 12.00
5.26 6.45 7.13 11.00 6.00 6.30 6.45 8.44 3.24 4.73 5.12 7.35
where Qt and Q∞ are the mole% uptake at time t and equilibrium, respectively, and h is the sample thickness. The theoretical and experimental curves of B2 N50 sample in benzene is given in Fig. 9. It can be observed that the experimental values agree well with the fickian behaviour. From the swelling studies, the structure of the interpenetrating polymer networks could be predicted using the affine and phantom network models. In the affine network proposed by Flory and Rehner, the components of each chain vector transform linearly with macroscopic deformation and the junction points are considered to be embedded in the network without fluctuation [36]. The molecular weight between crosslinks for affine unit (Mc(aff) ) was calculated using the equation [38], 2/3 1/3
Mc(aff) =
1/3
−ρP Vs ν2c ν2m (1 − (µ/ν)ν2m ) [ln(1 − ν2m ) + ν2m + χ (ν2m )2 ]
(12)
where Vs is the molar volume of solvents, µ and ν are called the number of effective chain and junctions, ν 2m is the polymer volume fraction at swelling equilibrium, ν 2c the polymer volume fraction during crosslinking and ρ P the polymer density. As the interpenetrating polymer network structure is crosslinked, the term µ/ was taken as 0, in the calculations. In the phantom model proposed by James and Guth, chains move freely through one another, the junction points fluctuates over time around their mean position without being hindered by neighbouring chains
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225
Fig. 9. Comparison of experimental data of B2 N50 sample in benzene with theoretical prediction.
and are independent of deformation [39]. According to this theory, molecular weight between crosslinks for phantom limit is given by [40], 2/3 1/3
Mc(ph) =
−(1 − 2/φ)ρVs ν2c ν2m [ln(1 − ν2m ) + ν2m + χ (ν2m )2 ]
properties compared to unswollen samples. In the fully swollen state, the polymer chains tend to have reduced chain entanglements. The polymer–polymer interaction is low in the swollen state. The elonagation
(13)
where φ is the junction functionality. In our system, φ is taken as three as crosslinked networks are present. The values of molecular weight between crosslinks calculated based on these equation are given in Table 10. It was found that the experimental values are closer to affine model showing that during swelling the polymer chains deform affine. 3.3. Effect of swelling on mechanical properties The tensile properties of dumpbell specimen swollen in toluene and the unswollen samples were measured. The tensile strength and elongation at break of swollen samples are given in Table 11. It is found that the swollen samples have drastically low mechanical
Table 10 Molecular weight between crosslinks (Mc ) calculated based on phantom model and affine model
D0 A0 B0 D1 A1 B1 D2 A2 B2 D3 A3 B3
Phantom model (M ph × 104 ) (g mol/cc)
Affine model (M aff ×104 (g mol/cc)
N30
N50
N70
N30
N50
N70
2.20 1.00 2.30 1.62 1.71 2.10 1.30 1.24 1.40 0.90 0.94 1.00
2.67 2.67 3.00 2.38 2.00 2.10 1.67 1.75 1.82 1.05 1.60 1.13
3.22 2.73 2.01 2.50 2.55 3.40 2.24 2.47 3.12 1.45 2.00 2.71
6.61 6.00 7.00 5.00 5.18 6.32 4.00 3.74 4.18 2.62 2.85 3.00
8.14 7.01 9.00 7.22 6.00 6.34 5.00 5.28 5.52 3.20 4.81 4.04
9.75 8.31 15.21 7.55 7.75 10.30 6.81 7.50 9.46 4.42 5.74 6.58
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Table 11 Effect of swelling on tensile strength Sample code
D0 N50 D1 N50 D2 N30 D2 N50 D2 N70 D3 N50
Unswollen
Swollen
Tensile strength (MPa)
Elongation at break (%)
Tensile strength (MPa)
Elongation at break (%)
16.5 13.0 16.7 12.8 9.0 14.0
378 474 134 290 350 197
0.18 0.07 0.11 0.06 0.18 0.13
55 35 19 52 58 19
at break values also decreased drastically due to swelling.
4. Conclusions Interpenetrating polymer networks are relatively, a novel type of multiphase systems, where compatibility and certain degree of phase mixing is induced by crosslinking and interpenetration of component polymer chains. In the present study, a series of IPN network membranes were developed from NR and PS and their transport behaviour has been studied as a function of blend ratio, crosslinker level, initiating system, penetrant size and temperature. It was found that as the PS content increases, the Q∞ -value decreases. This is due to the fact that the introduction of plastic phase decreases the chain flexibility of the network. As expected, the uptake of solvents decreased by about 50% as the PS content decreases from 30–70%. As the NR phase is crosslinked and continuous in all the IPN membranes and as phase inversion is absent, the blend ratio decides the transport behaviour. The gradual decrease in uptake in all the systems upon the introduction of plastic phase points to the continuity of rubber phase and absence of phase inversion. The diffusion studies using different penetrants showed that the nature and size of the penetrant molecule affect the transport behaviour. As the PS crosslinker level increases, the uptake value decreases by about 40%, again showing resistance in chain relaxation of the network by the crosslinking of the rigid, brittle plastic phase. Attempts were made to correlate the transport behaviour with the morphology and blend ratio.
The effect of temperature on the transport properties was studied and it was observed that the uptake increases with temperature upto 65 and at 75 ◦ C a decrease in uptake is observed, due to desorption. The values of sorption and permeation coefficients also show a direct dependency on sample characteristics like blend composition and crosslinker level. The experimental values of Mc was closer to the Mc obtained from affine model showing that polymer deforms affine during swelling. The diffusion profile was compared with theoretical predictions and it was found that the experimental curve deviates from the normal fickian behaviour. The tensile strength measurements of swollen membranes showed a drastic decrease in tensile strength and elongation at break values compared to unswollen one. Acknowledgements One of the authors (APM) is thankful to Council of Scientific and Industrial Research (CSIR) for Senior Research Fellowship. The authors are grateful to Third World Academy of Sciences for financial assistance. References [1] J.L. Han, Y.C. Chern, K.Y. Li, K.H. Hsieh, Interpenetrating polymer networks of bismaleimide and polyurethane crosslinked epoxy, J. Appl. Polym. Sci. 70 (1998) 529. [2] D.J. Hourston, F.U. Schafer, N.J. Walter, M.H.S. Gradwell, Mechanical and morphological study of polyurethane/ polystyrene interpenetrating polymer networks containing ionic groups, J. Appl. Polym. Sci. 67 (1998) 1973. [3] J.J. Singh, R. Pater, A. Eftekhari, Microstructural characterisation of semi-interpenetrating networks by positron life time spectroscopy, Nucl. Instrum. Meth. in Phys. Res. Section B. Beam Interaction Mater. Atoms 134 (1998) 113.
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Transport of aromatic solvents through natural rubber/polystyrene (NR/PS) interpenetrating polymer network membranes Aji P. Mathew a , S. Packirisamy b , Ranimol Stephen a , Sabu Thomas a,∗ a
School of Chemical Sciences, Mahatma Gandhi University, P.O. Priyadarshini Hills, Kottayam, Kerala 686560, India b PSC Division, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala 695022, India Received 27 November 2000; accepted 8 November 2001
Abstract A series of interpenetrating polymer network membranes have been synthesised from natural rubber and polystyrene by the sequential polymerisation technique. The transport of aromatic hydrocarbons through semi- and full-interpenetrating polymer network membranes (IPNs) have been studied in detail by tracing the solvent uptake up to equilibrium. The sorption was carried out in a series of aromatic solvents viz. benzene, toluene and xylene. The effect of temperature on swelling is studied by carrying out the experiments in toluene in the temperature range of 30–75 ◦ C. The effects of blend ratio, crosslinker content and nature of initiator on the diffusion of various solvents were analysed. It was found that in all cases, the uptake value increased by about 50% as the PS content decreased from 70–30%. The diffusion, sorption and permeation coefficients were evaluated. As the crosslink density was increased, the uptake decreased by 40%. Kinetic and thermodynamic parameters were evaluated from diffusion experiments. The diffusion profiles were compared with theoretical predictions. The influence of swelling on the mechanical performance of the membranes has been investigated by conducting tensile testing of swollen specimens. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Diffusion; Interpenetrating polymer networks; Crosslink density; Swelling behaviour
1. Introduction In the broadest sense, an interpenetrating polymer networks is any material containing two polymers, each in network form [1–3]. Though, the term interpenetrating polymer networks implies some kind of interpenetration of two polymer networks, molecular interpenetration occurs in the case of mutual solubility only [4,5]. In most cases, the molecular interpenetration may be restricted and a supermolecular level of penetration is observed. ∗ Corresponding author. Tel.: +91-481-598-303; fax: +91-481-561-190. E-mail address: [email protected] (S. Thomas).
The interpenetrating polymer networks can be characterised by studying the morphology, mechanical properties, thermal behaviour, transport phenomena, etc. Sperling and co-workers [6–8] have been reviewed the industrial applications, morphological behaviour, synthesis, and properties of IPNs. The synthesis and morphology development of polychloroprene/polystyrene latex interpenetrating networks have been solely studied by Burford et al.[9]. He also synthesised latex IPNs based on high styrene resin as seed latex and PS as the second polymer in different compositional ratios [10]. Hopfenberg and Paul have pointed out that the study of diffusion, sorption and permeation in blend structure provides a valuable means for additional characterisation of the
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 7 3 8 - 4
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blends [11]. Experimental determination of sorption and permeation behaviour acts as a powerful tool for characterising the detailed nature of blends [12,13]. The transport studies are of considerable importance when we come across problems like designing a barrier material or tubes for transporting liquids and gases. The microfiltration of matters using membranes have been recently studied by Fane and co-workers [14–16]. The use of penetrants as probes to infer the detailed structure of polymer system are in practice extensively in recent times. Huang et al. [17] reported the diffusion of ethanol and water through polyurethane membranes. Baysal and co-workers [18] studied the swelling of polyacrylamide gels in polyacrylamide solution and the swelling behaviour was found to deviate from the prediction of Flory–Huggins theory. Fujita [19,20] derived a relationship between critical free volume and diffusion coefficient of the molecules for the polymer–solvent systems. The diffusion behaviour of polyethylene–polystyrene (PE–PS) semi-IPNs in toluene and chloroform was investigated by Hong and Duda [21]. The results showed that the semi-IPNs consists of a continuous PE phase containing dispersed PS phase. In the blend of PE and nylon 6, it was found that the permeability to heptane, methyl salicylate and methyl alcohol varied with blend ratio [22]. The effects of temperature on penetrant activity and blend composition of PS–PPO blends on n-hexane diffusion kinetics have been systematically examined by Hopfenberg and co-workers [23]. The diffusivity in a given polymer system, whether it is rubbery polymers, glassy polymers, polymer blend, graft or interpenetrating polymer networks, varies from one polymer system to another [24,25]. Diffusivity depends on the free volume within the material and the segmental mobility of polymer chains, crosslinking of component polymer phases, size of penetrants etc. [26,27]. In the present paper, a new set of interpenetrating polymer network membranes has been prepared from natural rubber and polystyrene. The diffusion of various aromatic solvents such as benzene, xylene and toluene through natural rubber/polystyrene interpenetrating polymer network membranes has been studied in detail. The NR phase is crosslinked using dicumyl peroxide (DCP). The PS phase is linear/crosslinked in semi/full-interpenetrating polymer networks. The
crosslinking of the PS phase is varied by controlling the amount of crosslinker, divinyl benzene used. The mechanism of sorption was determined and is compared with theoretical values. The diffusion, sorption and permeation coefficient were calculated. The effect of swelling on mechanical properties have been studied by carrying out the tensile strength measurements on dumpbell samples swollen in toluene. 2. Experimental 2.1. Materials used Natural rubber (NR) used was of ISNR-5 grade. It was supplied by Rubber Research Institute of India (RRII). The characteristics of NR are given in Table 1. Styrene monomer for the interpenetrating polymer networks synthesis was supplied by Merck, India. The monomer was made inhibitor free by washing it with 1% NaOH and was dried before use. Dicumyl peroxide (40% active) was used as the vulcanising agent for rubber and as the initiator for the polymerisation of styrene. It was procured from Kishore Rubber Products Pvt. Ltd., India. p-Benzoyl peroxide was obtained from BDH, India and used as initiator for styrene. AIBN was obtained from Sigma, India and used as the initiator. 2.2. Preparation of interpenetrating polymer networks 2.2.1. Crosslinking of NR Natural rubber was masticated in a two roll mixing mill at room temperature. Dicumyl peroxide (4 ph) was added and mixed thoroughly for 15 min. The rheograph of the mix was taken on a Monsanto Rheometer and the optimum cure time determined. The mix was cured at 160 ◦ C on a hydraulic press to get the crosslinked membranes. The crosslink density of the DCP cured samples were determined by Table 1 Characteristics of natural rubber (NR) Glass transition temperature (◦ C) Density (g/cc) Solubility parameter (J m3 )1/2 × 10−3 Volatile matter (%) max Ash (%) max
−72 0.97 16.2 0.50 0.40
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swelling experiments in toluene at 30 ◦ C and it was found to be about 3.96 × 10−5 gmol/cc. 2.2.2. Polymerization and crosslinking of PS The following different series of interpenetrating polymer network membranes were prepared using different initiators. 2.2.2.1. Series I. The cured NR membranes were weighed and kept immersed in inhibitor free styrene monomer containing 1% benzoyl peroxide as initiator and 0, 2, 4 or 6% of divinyl benzene (DVB) which acts as crosslinker for PS phase. The NR membranes were swollen to different time intervals to obtain different weight percentages of PS. The detailed mechanism of transport of styrene monomer through natural rubber has already been reported in our earlier papers [28,29]. The swollen samples were kept at 0 ◦ C for few hours to achieve equilibrium distribution of styrene monomer in the matrix. The swollen networks were heated at 80 ◦ C for 6 h and 100 ◦ C for 2 h, respectively, to carry out the polymerisation and crosslinking of styrene. 2.2.2.2. Series II. In this method, 1% DCP and 0, 2, 4 or 6% of divinyl benzene which acts as crosslinker for PS phase were added to the styrene monomer and vulcanised NR membranes were allowed to swell in it. The swollen sheets were heated at 80 ◦ C for 6 h and 100 ◦ C for 2 h, respectively, to complete the polymerisation and crosslinking reaction.
215
2.2.2.3. Series III. In this series, 0.5% AIBN and 0, 2, 4 or 6% of divinyl benzene which acts as crosslinker for PS phase were added to the monomer and the natural rubber membranes were swollen in it. The swollen membranes were polymerised and crosslinked at 80 ◦ C for 6 h and 100 ◦ C for 2 h. The hardened membranes, in all cases, were then kept in a vacuum air oven to make it free of unreacted styrene. The final weight of the sample was taken and the composition of the sample was determined. In all the four series, NR/PS interpenetrating polymer networks with PS content up to 70% were prepared. The preparation of semi- and full-IPNs are schematically represented in Fig. 1. The network I was impregnated with the monomer II, initiator and crosslinker, which was thermally polymerised to get the IPNs. When the DVB content is 0%, PS phase is not crosslinked and the resultant IPN will be semi- or pseudo-IPN. By the incorporation of 2, 4 or 6% of DVB, the PS phase gets crosslinked and full-IPNs with increasing crosslink density will result. The interpenetrating polymer networks were coded based on composition, initiating system and crosslinker content. The DCP, BPO and AIBN initiated system are denoted by D, B and A series. The DVB content is varied, in order to have varying level of crosslinking. The blend composition is denoted as N30 , N50 and N70 , where the subscripts indicate the weight percent of rubber. The codings are given in Table 2.
Table 2 Nomenclature of interpenetrating polymer networks Sample code
Initiator
NR/PS ratio
A0 N30 ,A0 N50 ,A0 N70 A1 N30 ,A1 N50 ,A1 N70 A2 N30 ,A2 N50 ,A2 N70 A3 N30 ,A3 N50 ,A3 N70
0.5% 0.5% 0.5% 0.5%
30/70, 30/70, 30/70, 30/70,
50/50, 50/50, 50/50, 50/50,
70/30 70/30 70/30 70/30
0 2 4 6
B0 N30 ,B0 N50 ,B0 N70 B1 N30 ,B1 N50 ,B1 N70 B2 N30 ,B2 N50 ,B2 N70 B3 N30 ,B3 N50 ,B3 N70
1% 1% 1% 1%
(BPO) (BPO) (BPO) (BPO)
30/70, 30/70, 30/70, 30/70,
50/50, 50/50, 50/50, 50/50,
70/30 70/30 70/30 70/30
0 2 4 6
D0 N30 ,D0 N50 ,D0 N70 D1 N30 ,D1 N50 ,D1 N70 D2 N30 ,D2 N50 ,D2 N70 D3 N30 ,D3 N50 ,D3 N70
1% 1% 1% 1%
(DCP) (DCP) (DCP) (DCP)
30/70, 30/70, 30/70, 30/70,
50/50, 50/50, 50/50, 50/50,
70/30 70/30 70/30 70/30
0 2 4 6
(AIBN) (AIBN) (AIBN) (AIBN)
DVB content (%)
216
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Fig. 1. Schematic representation of preparation of semi- and full-interpenetrating polymer networks.
2.3. Diffusion experiments From the interpenetrating polymer network membranes with different blend ratio and crosslink density, circular samples of diameter 2 cm were punched out using a sharp edged steel die. The thickness (h) of the samples was determined accurately using a micrometer screw gauge. The initial weight of the polymer samples was determined by weighing on a highly sensitive electronic balance having an accuracy of 0.001 g. The samples were then kept immersed in penetrant taken in stoppered diffusion bottles. The swollen samples were taken out at fixed time intervals, wiped free of adhering solvents, weighed on the electronic balance and replaced in the stoppered bottles. This process was continued till equilibrium swelling which was indicated by the constancy in weight. Each weighing was
completed in 40 s, so as to keep the error due to solvent escape from the sample surface minimum. To carry out the experiment at higher temperatures (50, 65 and 75 ◦ C) a thermostatically controlled air oven was made use of. For solvent resistance studies, the weight was taken everyday upto the 7th day to monitor the uptake of solvent. From the weight gain, various parameter were calculated and useful inferences were made. 2.4. Transmission electron microscopy The morphology of IPNs were studied using JOEL-JEM 2010 model transmission electron microscope. The samples were microtomed and the NR phase was stained using OsO4 . This was viewed under the microscope.
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2.5. Tensile testing To study the effect of swelling on mechanical strength, the dumpbell specimens are allowed to undergo equilibrium swelling in toluene. The swollen samples were tested on a tensile testing machine at a cross head speed of 50 mm/min with 3 cm intergrip distance. 3. Results and discussion From the diffusion data, the mol% uptake of solvent/g of the polymer, Qt was determined using the equation [28,29]: (mass of the solvent absorbed/ mol. wt. of solvent) Qt = × 100 mass of the polymer
(1)
Qt becomes Q∞ when equilibrium is reached. The Q∞ -values of the interpenetrating polymer networks for benzene, toluene and xylene at room temperature are given in Table 3. It was found that for a given sample, the uptake increases in the order xylene < toluene < benzene. The Q∞ -values were highest in benzene, then comes toluene and then xylene. This is in the order of increasing penetrant size. As the size of the penetrant increases, the resistance offered by the polymer to the transport of the solvents through the matrix is enhanced. Therefore, the uptake was low for penetrants √ of higher size. Fig. 2 shows the plot of Qt versus t of D1 N50 and D0 N50 samples for benzene, toluene and xylene, where the effect of penetrant size on diffusion in semi- and full-IPNs is shown. The difference in transport behaviour is due to difference in polymer–solvent interaction rather than the size of the penetrant. This
217
leads to the conclusion that the nature of the polymer affects the diffusion process. However, in the case of penetrants with similar nature, the molecular size decides the transport behaviour. Similar observation were reported by other researchers earlier [30]. When we examine at the effect of blend ratio on Q∞ in benzene, toluene or xylene, it was found that as the PS content increases, the uptake decreases in the case of semi-interpenetrating polymer networks and full-interpenetrating polymer networks as well (Table 3). The uptake is in the order N30 < N50 < N70 in the case of semi- and full-IPNs. Fig. 3 shows the plot of D0 N30 , D0 N50 and D0 N70 membranes (semi-IPNs) in toluene. The effect of blend ratio can be explained based on the morphology. Our earlier study on the morphology of semi- and full-IPNs showed dual phase continuity [31]. The NR phase was crosslinked in all cases. The high level of PS crosslinking makes the PS phase also continuous. In D0 N70 , the PS phase is dispersed as elongated domains in the continuous NR matrix. The TEM photographs of selected samples are shown in Fig. 4. All the systems have a nanostructured morphology and dual phase continuity. It is a known fact that the continuous phase controls the transport properties of any system. As there is no phase inversion in the whole range of blend composition, the NR:PS ratio decides the uptake value. As the rubber content decreases, the flexibility of the polymer chains decreases, whereas, increase in PS phase offers stiffness to the material. The relaxation of polymer chains is a must for solvent diffusion and sorption. The Tg of NR is below the experiment temperature and, therefore, the NR chains relax readily. The Tg of PS is above the experimental temperature and, therefore, the polymer chains have limited freedom of movement and flexibility. As the rubber content decreases, the rigidity
Table 3 Values of equilibrium uptake (Q∞ (mol%)) in benzene, toluene, xylene
Benzene Toluene
Xylene
30 ◦ C 30 ◦ C 50 ◦ C 65 ◦ C 75 ◦ C 30 ◦ C
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
3.35 3.62 3.65 3.75 3.47 3.01
4.75 4.01 4.04 4.74 4.00 3.40
5.10 4.37 4.41 4.46 4.20 3.70
4.45 3.76 3.80 3.86 3.41 3.06
2.86 2.64 2.79 3.51 2.76 2.26
4.01 3.00 3.36 3.80 3.31 2.68
4.33 3.45 3.74 3.84 3.64 2.80
3.52 2.30 2.42 2.88 2.30 1.87
3.70 3.12 3.18 3.38 3.21 2.56
4.00 3.21 3.52 4.00 3.07 2.80
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Fig. 2. Plot of Qt vs.
√
t of D1 N50 and D0 N5 samples in benzene, xylene and toluene showing the effect of penetrant size.
of the polymer network increases. This restricts the solvent uptake. The effects of crosslinker content on transport properties of D0 N50 , D1 N50 , D2 N50 and D3 N50 samples with DVB content 0, 2, 4 and 6%, respectively, are compared in Fig. 5. As the DVB content increases, the crosslinking in the PS phase increases making the material more and more rigid. The flexibility of the polymer network is decreased, as DVB content is increased and, hence, the uptake decreases. Fig. 5 shows the effect of crosslinking level on the transport behaviour of interpenetrating polymer networks in toluene for DCP initiated systems at room temperature. NR phase was crosslinked to the same extent in all cases. However, it is an accepted fact that the network initially formed will govern the morphology and properties of the system. Also, the first formed network will be continuous in all cases. So, as polymer II also gets crosslinked, both the phases tend to be continuous.
As the DVB content is increased, the crosslinking and, thereby, the continuity of phase II (PS) increase blocking the easy transport of solvent through the material. So, the uptake gets retarded with increasing crosslinker level. The uptake decreases by about 40–45% on increasing the crosslinking level from 0–6% for the DCP initiated samples swollen in toluene, xylene and benzene. Fig. 6 shows the schematic representation of effect of increase in crosslinking on solvent uptake. It could be seen that the solvent molecules have to overcome the dense barrier of polymer crosslinking and entanglements to diffuse into the material. As the number of crosslinks increases, the resistance offered to transport also increases correspondingly. The crosslink density of the material also affects the solvent uptake. The effect of crosslink density values on uptake is discussed in the coming section. Zhou et al. [32] recently reported that the degree of swelling of IPNs varied with degree of crosslinking.
A.P. Mathew et al. / Journal of Membrane Science 201 (2002) 213–227
Fig. 3. Plot of Qt vs.
219
√
t showing the effect of blend ratio (D0 N30 , D0 N50 and D0 N70 samples in toluene).
The effects of temperature on diffusion and sorption behaviour was studied in toluene. It was observed that Q∞ -values increase with temperature in all cases(Table 3). However, at 75 ◦ C, a decrease in Q∞ -values is observed as compared to that at 65 ◦ C. This may be due to the fact that at 75 ◦ C desorption occurs at a rapid rate compared to sorption. This is clear from Fig. 7. In both cases, Q∞ -values are in the order 30 < 45 < 75 < 65 ◦ C. When the different initiating systems are considered it was found that Q∞ -values do not exhibit any definite trend in various solvents (Table 3). So, it may be concluded that initiating system does not affect the network characteristics appreciably. Fig. 8 compares the value of D2 N50 , B2 N50 and A2 N50 samples in benzene. The high uptake of A2 N50 sample compared to D2 N50 and B2 N50 samples is due to the low crosslink density of A2 N50 sample. 3.1. Diffusion, sorption and permeation coefficient Diffusion coefficient is a kinetic parameter which depends on the polymer segmental mobility. The
diffusion coefficient was calculated from the following equation. n=1 1 8 π2 (2n + 1)2 n=0 t × exp −D(2n + 1)2 π 2 2 h
Qt =1− Q∞
(2)
where t is the time and h is the initial sample thickness. The short time limiting expression of this equation is as follows: Qt 4 Dt 1/2 = 2 (3) Q∞ π h2 This equation is rearranged and diffusion coefficient value for the interpenetrating polymer networks was calculated using the equation [33] hθ 2 D=π (4) 4Q∞ where h is the sample thickness, θ the √ slope of initial portion of the plot of Qt versus t, and Q∞ the
220
A.P. Mathew et al. / Journal of Membrane Science 201 (2002) 213–227
Fig. 4. Transmission electron micrographs showing the nanostructured morphology of (a) D1 N30 , (b) D1 N50 , and (c) D1 N70 .
equilibrium mole percent uptake. The ‘D’ values are given in Table 4. Diffusion coefficient is the ability of the penetrant to move among the polymers segment. The ‘D’ values decrease with increasing crosslinker level for benzene, xylene and toluene. In the case of benzene, the ‘D’ value decreases by 60% as the crosslinker level increases from D0 N50 to D3 N50 sample. The ‘D’ values follow the same order as that Q∞ . This is due to the fact that the θ -values decrease with increase of crosslinking of PS phase. As the PS content increases in a series, ‘D’ values also increase. This is
explained by the lowering of Q∞ -values with increase in PS content. The sorption coefficient was calculated using the equation [34]. S=
M∞ MP
(5)
where M∞ is the mass of solvent uptake at equilibrium and Mp is the initial polymer mass. The ‘S’ values are given in Table 5. The trend shown is the same as that of Q∞ -values in the case of benzene, xylene and
Table 4 Values of diffusion coefficient (D × 107 cm2 /s)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
6.14 13.32 7.2
8.00 14.46 9.20
12.50 15.30 15.80
6.04 7.82 –
7.24 9.16 6.80
6.70 – 7.80
10.40 11.41 11.40
– 5.81 7.60
7.40 10.10 7.27
7.00 8.21 9.30
A.P. Mathew et al. / Journal of Membrane Science 201 (2002) 213–227
Fig. 5. Plot of Qt vs.
√
t showing the effect of crosslinker content (D0 N50 , D1 N50 , D2 N50 and D3 N50 samples in toluene).
toluene, and petrol, diesel and kerosene as well. Sorption is a surface phenomenon and it is an indication of the tendency of the penetrant to dissolve into the polymer. Permeation can be considered as the combined effect of sorption and diffusion process. The permeation coefficient is given by the equation [34]: P = DS
221
(6)
The P values are given in Table 6. It follows the same trend as sorption. So, it may be inferred that the sorption process controls the permeability. The volume fraction of polymer φ in the swollen sample was calculated using the equation [33]: φ=
W1 /ρ1 (W1 /ρ1 ) + (W2 + ρ2 )
Fig. 6. Schematic representation showing the effect of crosslink density on solvent uptake.
(7)
222
A.P. Mathew et al. / Journal of Membrane Science 201 (2002) 213–227
Fig. 7. Effect of temperature on solvent uptake (Qt vs.
where W1 is weight of polymer and ρ 1 the density of polymer, W2 the weight of solvent in the swollen sample and ρ 2 the density of solvent. A high value for φ is an indication of high extent of crosslinking in the system. The φ-values are given in Table 7. For semi-interpenetrating networks the φ-value are lower than full interpenetrating polymer networks. In a given series, as the PS content increases the φ-values also increase. This is due to the increased crosslink density of the membranes. For a given initiating system, as the crosslinker level is increased φ-values also increased.
√ t in toluene for D1 N50 samples).
As crosslink density increases with crosslinker content, the uptake decreases correspondingly. From the transport data, the molecular weight between crosslinks can be calculated using the equation [35,36]: Mc =
−ρP Vr φ 1/3 − 1pt ln(1 − φ) + φ + χ φ 2
(8)
where ρ P is the density of polymer, Vr is the molar volume of solvent, φ the volume fraction of polymer and χ the interaction parameter.
Table 5 Values of sorption coefficient (S)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
2.62 3.33 3.20
3.72 3.70 3.61
4.00 3.83 4.00
3.47 3.46 3.25
2.23 2.44 2.40
3.38 2.74 2.68
3.13 3.18 3.00
2.75 2.12 2.00
3.00 3.00 2.72
2.68 2.88 3.00
A.P. Mathew et al. / Journal of Membrane Science 201 (2002) 213–227
223
Fig. 8. Effect of initiating system on swelling (A1 N50 , B1 N50 and D1 N50 samples in toluene).
Table 6 Values of permeation coefficient (P × 107 cm2 /s)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
16.10 44.30 23.20
29.50 53.40 33.28
50.02 61.80 62.30
21.01 30.00 22.50
24.40 22.30 16.30
24.01 15.80 22.00
32.80 36.30 34.00
15.00 12.33 15.23
21.60 24.30 19.75
20.35 29.20 27.82
Table 7 Values of volume fraction (φ)
Benzene Toluene Xylene
D0 N30
D0 N50
D0 N70
D1 N50
D2 N30
D2 N50
D2 N70
D3 N50
B2 N50
A2 N50
0.245 0.210 0.206
0.188 0.187 0.188
0.181 0.180 0.178
0.200 0.196 0.205
0.278 0.255 0.256
0.221 0.236 0.235
0.201 0.213 0.223
0.220 0.285 0.300
0.230 0.223 0.236
0.211 0.227 0.221
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Table 8 Values of interaction parameter (χ )
Benzene Toluene Xylene
N30 series
N50 series
N70 series
0.400 0.341 0.343
0.434 0.355 0.348
0.487 0.384 0.372
β + Vr RT(δA − δB )2
(9)
where Vr is the molar volume of solvent, δ A and δ B are the solubility parameter of solvent and polymer respectively, R is the universal gas constant and T the absolute temperature. β the lattice constant and its value is equal to 0.34. The χ -values for different samples in various solvents are given in Table 8. From the Mc value crosslink density can be calculated using the equation [37]: ν = 21 Mc
(10)
The calculated Mc and ν-values are given in Table 9. It can be observed that ν-value increases by about 30% as PS content increases from 30–70% in a given series. For e.g. in DCP series, ν increases in the order D1 N70 < D1 N50 < D1 N30 . Also, for a given blend ratio, as the crosslinking of PS phase increases, the ν-value also increases by about 50%. This is because the chain entanglement increases as the extent of PS crosslinking increases, thereby, decreasing the solvent uptake. When we compare different initiating system (D2 N50 , B2 N50 and A2 N50 ) the crosslink density values do not follow any systematic trend. 3.2. Theoretical fitting The experimental results were compared with the theoretical equation. n=1 1 8 π2 (2n + 1)2 n=0 2 2 t × exp −D(2n + 1) π 2 h
Qt =1− Q∞
ν × 104 (g mol/cc)
Mc (g/mol)
The interaction parameter χ is given by the equation [35]: χ=
Table 9 Values of molecular weight between crosslinks (Mc ) and crosslink density (ν)
(11)
D0 D1 D2 D3 A0 A1 A2 A3 B0 B1 B2 B3
N30
N50
N70
N30
N50
N70
6728 5095 4046 2726 6025 5345 3867 2965 7078 6528 4319 3015
8231 7413 5018 3313 7125 6164 5449 4991 9015 6517 5698 4198
9835 7750 7008 4564 8369 7952 7709 5923 15342 10568 97361 6798
7.43 9.81 12.35 18.33 8.30 9.35 13.00 17.00 7.06 7.65 11.57 16.58
6.07 6.74 10.00 15.10 7.01 8.11 9.17 10.02 5.54 7.67 8.87 12.00
5.26 6.45 7.13 11.00 6.00 6.30 6.45 8.44 3.24 4.73 5.12 7.35
where Qt and Q∞ are the mole% uptake at time t and equilibrium, respectively, and h is the sample thickness. The theoretical and experimental curves of B2 N50 sample in benzene is given in Fig. 9. It can be observed that the experimental values agree well with the fickian behaviour. From the swelling studies, the structure of the interpenetrating polymer networks could be predicted using the affine and phantom network models. In the affine network proposed by Flory and Rehner, the components of each chain vector transform linearly with macroscopic deformation and the junction points are considered to be embedded in the network without fluctuation [36]. The molecular weight between crosslinks for affine unit (Mc(aff) ) was calculated using the equation [38], 2/3 1/3
Mc(aff) =
1/3
−ρP Vs ν2c ν2m (1 − (µ/ν)ν2m ) [ln(1 − ν2m ) + ν2m + χ (ν2m )2 ]
(12)
where Vs is the molar volume of solvents, µ and ν are called the number of effective chain and junctions, ν 2m is the polymer volume fraction at swelling equilibrium, ν 2c the polymer volume fraction during crosslinking and ρ P the polymer density. As the interpenetrating polymer network structure is crosslinked, the term µ/ was taken as 0, in the calculations. In the phantom model proposed by James and Guth, chains move freely through one another, the junction points fluctuates over time around their mean position without being hindered by neighbouring chains
A.P. Mathew et al. / Journal of Membrane Science 201 (2002) 213–227
225
Fig. 9. Comparison of experimental data of B2 N50 sample in benzene with theoretical prediction.
and are independent of deformation [39]. According to this theory, molecular weight between crosslinks for phantom limit is given by [40], 2/3 1/3
Mc(ph) =
−(1 − 2/φ)ρVs ν2c ν2m [ln(1 − ν2m ) + ν2m + χ (ν2m )2 ]
properties compared to unswollen samples. In the fully swollen state, the polymer chains tend to have reduced chain entanglements. The polymer–polymer interaction is low in the swollen state. The elonagation
(13)
where φ is the junction functionality. In our system, φ is taken as three as crosslinked networks are present. The values of molecular weight between crosslinks calculated based on these equation are given in Table 10. It was found that the experimental values are closer to affine model showing that during swelling the polymer chains deform affine. 3.3. Effect of swelling on mechanical properties The tensile properties of dumpbell specimen swollen in toluene and the unswollen samples were measured. The tensile strength and elongation at break of swollen samples are given in Table 11. It is found that the swollen samples have drastically low mechanical
Table 10 Molecular weight between crosslinks (Mc ) calculated based on phantom model and affine model
D0 A0 B0 D1 A1 B1 D2 A2 B2 D3 A3 B3
Phantom model (M ph × 104 ) (g mol/cc)
Affine model (M aff ×104 (g mol/cc)
N30
N50
N70
N30
N50
N70
2.20 1.00 2.30 1.62 1.71 2.10 1.30 1.24 1.40 0.90 0.94 1.00
2.67 2.67 3.00 2.38 2.00 2.10 1.67 1.75 1.82 1.05 1.60 1.13
3.22 2.73 2.01 2.50 2.55 3.40 2.24 2.47 3.12 1.45 2.00 2.71
6.61 6.00 7.00 5.00 5.18 6.32 4.00 3.74 4.18 2.62 2.85 3.00
8.14 7.01 9.00 7.22 6.00 6.34 5.00 5.28 5.52 3.20 4.81 4.04
9.75 8.31 15.21 7.55 7.75 10.30 6.81 7.50 9.46 4.42 5.74 6.58
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Table 11 Effect of swelling on tensile strength Sample code
D0 N50 D1 N50 D2 N30 D2 N50 D2 N70 D3 N50
Unswollen
Swollen
Tensile strength (MPa)
Elongation at break (%)
Tensile strength (MPa)
Elongation at break (%)
16.5 13.0 16.7 12.8 9.0 14.0
378 474 134 290 350 197
0.18 0.07 0.11 0.06 0.18 0.13
55 35 19 52 58 19
at break values also decreased drastically due to swelling.
4. Conclusions Interpenetrating polymer networks are relatively, a novel type of multiphase systems, where compatibility and certain degree of phase mixing is induced by crosslinking and interpenetration of component polymer chains. In the present study, a series of IPN network membranes were developed from NR and PS and their transport behaviour has been studied as a function of blend ratio, crosslinker level, initiating system, penetrant size and temperature. It was found that as the PS content increases, the Q∞ -value decreases. This is due to the fact that the introduction of plastic phase decreases the chain flexibility of the network. As expected, the uptake of solvents decreased by about 50% as the PS content decreases from 30–70%. As the NR phase is crosslinked and continuous in all the IPN membranes and as phase inversion is absent, the blend ratio decides the transport behaviour. The gradual decrease in uptake in all the systems upon the introduction of plastic phase points to the continuity of rubber phase and absence of phase inversion. The diffusion studies using different penetrants showed that the nature and size of the penetrant molecule affect the transport behaviour. As the PS crosslinker level increases, the uptake value decreases by about 40%, again showing resistance in chain relaxation of the network by the crosslinking of the rigid, brittle plastic phase. Attempts were made to correlate the transport behaviour with the morphology and blend ratio.
The effect of temperature on the transport properties was studied and it was observed that the uptake increases with temperature upto 65 and at 75 ◦ C a decrease in uptake is observed, due to desorption. The values of sorption and permeation coefficients also show a direct dependency on sample characteristics like blend composition and crosslinker level. The experimental values of Mc was closer to the Mc obtained from affine model showing that polymer deforms affine during swelling. The diffusion profile was compared with theoretical predictions and it was found that the experimental curve deviates from the normal fickian behaviour. The tensile strength measurements of swollen membranes showed a drastic decrease in tensile strength and elongation at break values compared to unswollen one. Acknowledgements One of the authors (APM) is thankful to Council of Scientific and Industrial Research (CSIR) for Senior Research Fellowship. The authors are grateful to Third World Academy of Sciences for financial assistance. References [1] J.L. Han, Y.C. Chern, K.Y. Li, K.H. Hsieh, Interpenetrating polymer networks of bismaleimide and polyurethane crosslinked epoxy, J. Appl. Polym. Sci. 70 (1998) 529. [2] D.J. Hourston, F.U. Schafer, N.J. Walter, M.H.S. Gradwell, Mechanical and morphological study of polyurethane/ polystyrene interpenetrating polymer networks containing ionic groups, J. Appl. Polym. Sci. 67 (1998) 1973. [3] J.J. Singh, R. Pater, A. Eftekhari, Microstructural characterisation of semi-interpenetrating networks by positron life time spectroscopy, Nucl. Instrum. Meth. in Phys. Res. Section B. Beam Interaction Mater. Atoms 134 (1998) 113.
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