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NR) blends by dilute solution viscometry (DSV) and scanning electronic microscopy (SEM) methods
Journal of Molecular Liquids 190 (2014) 146–150
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Miscibility studies on polychloroprene/natural rubber (PCP/NR) blends by dilute solution viscometry (DSV) and scanning electronic microscopy (SEM) methods Adonilson R. Freitas ⁎, Luciana Gaffo, Adley F. Rubira, Edvani C. Muniz Grupo de Materiais Poliméricos e Compósitos, Departamento de Química, Universidade Estadual de Maringá, Av. Colombo 5790, 87020-900 Maringá, Paraná, Brazil
a r t i c l e
i n f o
Article history: Received 17 September 2012 Received in revised form 14 October 2013 Accepted 5 November 2013 Available online 20 November 2013 Keywords: Polychloroprene Natural rubber Miscibility Viscometry SEM
a b s t r a c t The existence or not of miscibility of PCP and NR was investigated by dilute solution viscometry (DSV) in an Ubbelohde capillary viscometer and scanning electronic microscopy (SEM) techniques. The blends of these polymers were investigated by analysis of viscosimetric parameters: Δb, α, ΔK, μ, β and correlation between the reduced viscosity mass ratios of blend composition. According to the algebraic signal of the studied parameters, it was found that there is immiscibility between PCP and NR for all tested mass ratios in this study. The sigmoid adjusting obtained when the reduced viscosity was plotted versus blend composition indicated immiscibility between PCP and NR polymers. The film morphology of blends observed by SEM showed two separated phases indicating immiscible blend. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Polymeric blends are physical mixtures of structurally different polymers or copolymers, which interact through secondary forces with no covalent bonding such as hydrogen bonding, dipole–dipole forces, and charge-transfer complexes for homopolymers mixtures [1–4]. Blending of polymers may result in a reduction in the basic cost and improved processing, and enhance some final property. However, the mechanical, thermal, rheological and other properties of a polymeric blend depend strongly on its state of miscibility [5]. The final properties of a polymeric blend will commonly depend on the properties of its polymeric components, its composition and, mainly, on the miscibility of the polymers [6]. In some cases, by synergistic effects, the blend can present better properties than the pure components [6,7]. Many experimental and theoretical methods have been used to investigate the polymer–polymer miscibility. Some of the most important techniques applied to investigate the polymer–polymer miscibility are thermal analysis [8,9], electronic microscopy [10], dynamic mechanical studies [11] and viscometric techniques [12,13]. The viscometric measurements provide an effective, quick and inexpensive technique to investigate polymer–polymer interaction. Viscometry is widely used to determine the molecular weight, molecular weight distribution, and degree of polymerization [14]. Many researchers have attempted to correlate
⁎ Corresponding author at: Av. Colombo, 5790, CEP 87020-9000, Maringá, Paraná, Brazil. E-mail address: [email protected] (A.R. Freitas). 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.11.003
viscosity with miscibility of ternary polymer solutions (polymer#1/solvent/polymer#2) [9,12,15–17]. The basis for using dilute solution viscosity as efficient tool to evaluate the miscibility among different polymers is that, when in solution, macromolecules of both types may exist in a molecularly dispersed state and undergo mutual attraction or repulsion, thereby rendering positive or negative influences on viscosity. Interactions between the components forming the polymeric blend are reflected in the solution viscosity of polymer blend, because polymer–polymer interactions usually dominate over polymer–solvent ones [18]. Christopoulou et al. [19] studying binary polymer mixtures in the solution and in the solid state have shown that there is a very close relation between their behaviours in the solution and in the solid state. The compatibility founded in solution would remain even when the solvent is absent. The phenomenon is called “memory effect” [20,21]. Neiro et al. [22] investigated the miscibility on PVC/PEO blends through thermal (DSC), microscopy (SEM) and viscometry (DSV). Viscometric and thermal results showed that the PVC/PEO blends are miscible. However, the existence of PVC rich domains, smaller than 0.1 μm in blends of PVC/PEO, was observed by microscopic analysis. The miscibility of the PVC/PEO blend was explained as a result of donor–acceptor interactions between chlorine atoms of PVC, as a weak acceptor species, and oxygen atoms of the PEO, as a donor species. In this study, the miscibility between polychloroprene (PCP) and natural rubber (NR) in solution and solid state was investigated. In this sense, a study on the viscosity behaviour of mixtures of PCP with NR in a common solvent was performed. The miscibility behaviour was evaluated through two techniques: dilute solution viscometry (DSV) and scanning electronic microscopy (SEM) methods. It was
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150
found that blends of PCP and NR are immiscible at blend mass ratios studied. Furthermore, the immiscibility verified in solutions also was found in solid state, effect attributed to “memory effect”.
147
The slope of Eq. (4) by plotting ηsp,m/Cm as a function of Cm gives the experimental value of bexp m which can be defined as: exp
2
2
exp
bm ¼ b1 w2 þ b2 w2 þ 2b12 w1 w2 :
ð11Þ
2. Theoretical models to evaluate the miscibility Earlier studies indicate that the interactions in binary polymeric systems can be analysed accurately evaluating by their viscosity behaviour [23]. For this reason, an adapted theoretical approach has been formulated having as reference an ideal binary polymeric system. According to Huggins [24], the behaviour of polymeric solutions could be described by Eq. (1) ηsp C
¼ ½η þ bC
ð1Þ
where ½η ¼ lim C→0
ηsp
ð2Þ
C
and b ¼ k½η
2
ð3Þ
being ηsp as the specific viscosity, C as the mass concentration of solution (mg dL−1), and [η] as the intrinsic viscosity, reflecting the interaction between polymer and solvent. The parameter b is related to the Huggins coefficient k, which reflects the binary interactions between polymer segments. The Eq. (1) can be readily adapted to polymer#1–solvent– polymer#2 ternary system [5], and given as following: 2
ηsp;m ¼ ½ηm C m þ bm C m
ð4Þ
where the subscript m indicates the polyblend and bm is related to the Huggins parameter km by 2
bm ¼ km ½ηm :
ð5Þ
m
Cm
¼
X ηsp Ci
i
i
ð6Þ
wi
where wi = Ci / Cm is the weight fraction of polymer i(i = 1,2). Combining Eqs. (1) and (6), we get ηsp
m
Cm
¼
X
½ηi wi þ C m
i
X
!2 1=2 bi wi
ð8Þ
2
1=2 1=2
ð9Þ
Eq. (9) defines the ideal value to the global viscometric interaction parameter bid m among the chains of both the polymers in given mixture of components 1 and 2 in a common solvent. The ideal value of specific interaction parameter bid 12 is assumed to geometric mean value of parameter assigned to the polymer interaction and is given as following: 1=2 1=2
b12 ¼ b1 b2 :
2 2 K 1 w1 ½η1 þ 2K 12 w1 w2 ½η1 ½η2 K 2 w2 ½η2 : 2 w1 ½η1 þ w2 ½η2
Km ¼
ð10Þ
ð13Þ
ð14Þ
In this sense Jiang and Han [12] revised this criterion by defining another parameter, β, as 2ΔKw1 ww2 ½η1 ½η2 β¼ w1 ½η1 þ w2 ½η2 2
ð15Þ
where ð16Þ
K1 ¼
b1 ½η21
ð17Þ
K2 ¼
b2 ½η22
ð18Þ
b12 : ½η212
ð19Þ
The parameter β is a function of, w and ΔK. Similarly, values of β ≥ 0 indicate miscibility whereas values of β b 0 indicate immiscibility. Catsiff and Hewett [27] have proposed to define the ideal value of interaction parameter, bid′ 12 as the arithmetic mean. b12 ¼
2
ð12Þ
where
id′
and
id
" # pffiffiffiffiffiffiffiffiffiffiffiffi K 1 w1 ½η21 þ 2 K 1 K 2 w1 w2 ½η1 ½η2 þ K 2 w2 ½η22 2 w1 ½η1 þ w2 ½η2
α ¼ Km−
ð7Þ
and the comparison between Eqs. (4) and (7) gives:
bm ¼ b1 w1 þ b2 w2 þ 2b1 b2 w1 w2 :
id
The ideal interaction coefficient, bid 12 between two polymers can be expressed by Eq. (10) and bexp 12 obtained experimentally by substituting all terms in Eq. (11). Δb ≥ 0 indicates miscibility, whereas Δb b 0 indicates immiscibility. Sun et al. [26] proposed another parameter, α, for polymer–polymer miscibility in terms of a thermodynamic parameter. The parameter α indicates the attractive (α N 0) or repulsive (α b 0) interaction among polymer segments. The zero value of α indicates neither attraction nor repulsion.
K 12 ¼
i
½ηm ¼ ½η1 w1 þ ½η2 w2
exp
Δb ¼ b12 −b12
pffiffiffiffiffiffiffiffiffiffiffiffi ΔK ¼ K 12 − K 1 K 2
The Eq. (4)-weighted average can be expressed as: ηsp
According Krigbaum and Wall [25], the information about intermolecular interactions on binary polymer blends (polymer #1 and polymer #2) can be obtained from a comparison of experimental interaction id coefficient bexp 12 and theoretical value b12.
b1 þ b2 2
ð20Þ
and exp
id′
Δb′ ¼ b12 −b12 :
ð21Þ
Using the value Δb′ given by the Eq. (21) a more effective parameter μ is obtained to predict the polymer–polymer miscibility [28]. μ¼
Δb′ ½η1 −½η2
2
ð22Þ
148
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150
Fig. 1. Schematic structure to NR (A) and PCP (B) respectively.
μ ≥ 0 and μ b 0 values correspond to miscibility and immiscibility, respectively. According to Kulshreshtha et al. [18], the miscibility of a polymer blend also may be studied plotting the relative viscosity versus the fraction of one of the components in the blend. The miscibility between the polymers is evaluated as function of data set adjusts. Linear plot adjust characterizes the polymer blend as miscible while an “S” type adjusting plot is an indicative of immiscible blend. 3. Experimental 3.1. Materials A commercial polychloroprene (PCP) type W sample was supplied by Proquimil (Brazil) and used without previous purification. Toluene P.A. grade was supplied by Synth (Brazil) and used as received. Natural rubber (NR) was used after purification by precipitation from a toluene solution 3% (w/v) into ethanol excess (volume ratio N 10). The solid was dried for four days at room temperature and dark ambient and for 24 h under reduced pressure. The solid NR was stocked in dark ambient at − 4 °C, to prevent some oxidative process. The average viscosimetric molar mass (Mv) of purified PCP and NR was determined as being 2.4 × 104 g mol− 1 and 1.3 × 106 g mol−1 using a Cannon Ubbelohde capillary viscometer (100/E534). 3.2. Viscosity measurements All the measurements for ternary systems (polymer#1– solvent– polymer#2) were performed using a dilution capillary Ubbelohde
viscometer type, Cannon (100/E534). The viscometer was immersed in a thermostatized bath and operating at 25 °C. Ternary solution for each system was prepared by mixing the polymers to yield mass ratios (wt1/wt2), (20/80), (50/50), and (80/20). Each blend-solution was prepared by solubilizing of the polymers in toluene yielding a final concentration of 0.62% (wt/v). After this, each solution was diluted to other four with lower polymer concentrations. The dilution was done by the addition of an appropriate amount of toluene into the viscometer. The flow times were recorded (4 or 5 measures for each concentration) with an accuracy of ± 0.01 s, and the bath temperature was constant within ±0.1 °C. The miscibility also was evaluating by relative viscosity determination using mass ratios 20/40, 40/60, 60/40 and 80/20. 3.3. Scanning electron microscopy (SEM) The SEM images were taken from films based on the polymeric blends. These films were obtained through casting methodology from mix of solid PCP and NR in toluene (as solvent). The polymeric blend solutions were prepared according to the following mass ratio (wt1/wt2): (25/75), (50/50) and (75/25). Briefly, the polymer solutions were stirred for 24 h and then poured in Petri dishes, which remain for four days in dark ambient at room temperature. The images from the films were taken in a scanning electron microscope (Shimadzu, model LEO440). The range of accelerating voltage was 0.3–30 kV. The samples were first sputter coated with a thin layer of gold and then observed at magnification of 500×. All the SEM images were taken at 23 °C. 4. Results and discussions 4.1. Viscosity measurement
Table 1 Viscometric data for PCP, NR and PCP/NR blend. PCP/NR mass blend ratio (%)
Concentration (g dL−1)
Average efflux time (s)
Reduced viscosity
Relative viscosity
Intrinsic viscosity (dL3 g−1)
0/100
0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29
492.3 332.2 254.4 210.0 468.1 312.3 236.3 197.9 356.6 244.8 194.5 166.8 216.5 166.7 142.8 129.0 176.4 143.4 126.5 116.7
5.26 3.22 2.23 1.67 4.95 2.97 2.00 1.52 3.53 2.11 1.47 1.12 1.75 1.12 0.82 0.64 1.24 0.82 0.60 0.48
8.55 7.26 6.42 5.86 8.05 6.69 5.76 5.32 5.70 4.79 4.20 3.86 2.85 2.53 2.34 2.25 2.00 1.87 1.74 1.67
3.58
20/80
50/50
80/20
100/0
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.4 0.6 0.1 0.3 0.3 0.7 0.4 0.3 1.8 1.3 0.4 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1
2.90
Fig. 1 presents the polymer structure of NR (A) and PCP (B). According to Kardam [29], PCP and NR have very similar structures and the fact of molecular “size” of the chloride atoms on PCP is nearly the same as that of methyl groups on the isoprene rubber units in NR. This makes these two polymers be very similar in molecular structure and compatible with each other. In fact, there is a wide similarity between the structures of these two polymers (see Fig. 1). On the other hand, the electron donor character of methyl groups on NR is the opposite of chlorides (electron acceptor) on PCP, and due to this the presence of attractive forces among their polymer chains leading to miscibility is expected.
2.29 Table 2 Viscometric data: [η] and b and the perspective, bexp 12 ΔB and β parameters for pure components and binary blends. 1.70
1.41
PCN/ NR
[η] (dL g−1)
b (dL g−1)2
bexp 12 (dL g−1)2
ΔB (dL g−1)2
β
Miscibility
100:00 80:20 50:50 20:80 00:100
1.41 1.70 2.29 2.90 3.58
0.99 1.84 5.59 8.38 8.10
– 0.070 0.415 0.252 –
– −2.75 −2.41 −2.57 –
– −0.766 −0.473 −0.204 –
– Immiscible Immiscible Immiscible –
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150 Table 3 The numerical values of polymer–polymer interaction parameters for PCP/NR blends. PCP/NR
α
ΔK
μ
Miscibility
20:80 50:50 80:20
−0.149 −0.363 −0.468
−1.25 −1.16 −1.61
−0.911 −0.877 −0.949
Immiscible Immiscible Immiscible
Earlier studies indicate that the interaction in binary polymer system can be analysed accurately by studying the viscosity behaviour [23]. The data set collected from efflux time measurements to pure polymer (PCP and NR) and respective blend mass ratios is presented in the Table 1. The miscibility parameters were determined using mathematic model. The Δb [23] and β parameters values were computed using Eqs. (12) and (16) and are presented in Table 2. It was founded that the Δb and β parameter values calculated for the PCP/NR blends present negative value. Due to this, the PCP/NR blends may be considered as immiscible. The values calculated for the miscibility parameters α, ΔK and μ are presented in the Table 3. According to the data presented in the Table 3, it is possible to infer that the PCP/NR blends are immiscible for all the studied conditions. Therefore, even changing the mass ratio of PCP/NR no miscibility was verified for the blends. Furthermore, it is well known [28–30] that plots of absolute viscosity versus composition deviate from linearity according to the degree of miscibility of polymer blend. Plots for miscible systems are linear whereas for immiscible systems plots are S-typed, indicating two phase formation with reversal of phases at intermediate compositions. Fig. 2 shows viscosity-composition plots for polymeric blends based on PCP/NR. It can be seen that, in the first moment, a linear adjust was obtained when the linear regression model is applied to data 100
Sigmoid adjust Linear adjust
149
set (dot line). According to predicted, linear plot would characterize this material as a miscible blend. On the other hand, when a sigmoid model was applied a best fit was obtained. The sigmoid correlation coefficient fit value obtained was best than the linear one, indicating a formation of two phases with phase inversion in an intermediate composition, as presented in Fig. 2. According to Pingping [30], this method is empirical and it is not conclusive to state the miscibility or not of a polymer blend systems. In contrast, there are diverse researchers [31,32] that apply this methodology and obtained satisfactory results. In this study, this method was satisfactory and data collected from the viscosimetric study were confirmed after evaluating the SEM images. 4.2. Analysis through scanning electronic microscopy (SEM) The morphological aspect of the PCP/NR blends with mass ratio equal to 25/75, 50/50 and 75/25 was evaluated through SEM images, which are shown in Fig. 3. All the SEM images taken to the PCP/NR blends suggested the phase separation phenomenon because the presence of two phases was observed. It is possible to observe that the fraction of PCP form small spheres dispersed in a NR matrix, which is a strong indicative of the immiscibility between the two polymers tested. From Fig. 3C, which shows the SEM image taken from the PCP/NR mass ratio 75/25, it is possible to observe small spheres, in contrast, to that formed by the NR dispersed in a matrix based on PCP. In addition from the blend with intermediary PCP/NR mass ratio (50/50), Fig. 3B, it was possible to verify the clear immiscible of this blend, which corroborates with the viscosity data. Christopoulou et al. [19] studying binary polymer mixture in solution and solid state, have shown that there is a very close relation between their behaviour in solution and solid state. The compatibility/incompatibility found in solution would remain even when the solvent is absent. The phenomenon was called “memory effect” [20,21]. This effect was found to PCP/NR blend. 5. Conclusion
Relative viscosity
80
The present investigation clearly indicated that the interaction in the polymer blends might be studied by simple measurement based on viscometry technique, providing valuable information about the miscibility of the PCP/NR polymer blend, which generally are obtained from sophisticated techniques. By studying binary polymer mixtures in solution and solid state as a function of blend ratio, we have shown that there is a very close relation between their behaviours in solution and solid state. Our results allow us to conclude that, polychloroprene and natural rubber make immiscible blends in all composition studied.
60
40
20
Acknowledgement 0
20
40
60
80
NR fraction in PCP-NR blend (%) Fig. 2. Relative viscosity as a function of NR fraction in blend.
100
AR Freitas acknowledges CAPES for the doctorate fellowship and Proquimil by polychloroprene donation. The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq - Brazil for financial support.
Fig. 3. SEM micrographs of PCP/NR blends at different mass ratios: (A) 25/75; (B) 50/50; (C) 75/25. Scale bar is 20 μm.
150
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Miscibility studies on polychloroprene/natural rubber (PCP/NR) blends by dilute solution viscometry (DSV) and scanning electronic microscopy (SEM) methods Adonilson R. Freitas ⁎, Luciana Gaffo, Adley F. Rubira, Edvani C. Muniz Grupo de Materiais Poliméricos e Compósitos, Departamento de Química, Universidade Estadual de Maringá, Av. Colombo 5790, 87020-900 Maringá, Paraná, Brazil
a r t i c l e
i n f o
Article history: Received 17 September 2012 Received in revised form 14 October 2013 Accepted 5 November 2013 Available online 20 November 2013 Keywords: Polychloroprene Natural rubber Miscibility Viscometry SEM
a b s t r a c t The existence or not of miscibility of PCP and NR was investigated by dilute solution viscometry (DSV) in an Ubbelohde capillary viscometer and scanning electronic microscopy (SEM) techniques. The blends of these polymers were investigated by analysis of viscosimetric parameters: Δb, α, ΔK, μ, β and correlation between the reduced viscosity mass ratios of blend composition. According to the algebraic signal of the studied parameters, it was found that there is immiscibility between PCP and NR for all tested mass ratios in this study. The sigmoid adjusting obtained when the reduced viscosity was plotted versus blend composition indicated immiscibility between PCP and NR polymers. The film morphology of blends observed by SEM showed two separated phases indicating immiscible blend. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Polymeric blends are physical mixtures of structurally different polymers or copolymers, which interact through secondary forces with no covalent bonding such as hydrogen bonding, dipole–dipole forces, and charge-transfer complexes for homopolymers mixtures [1–4]. Blending of polymers may result in a reduction in the basic cost and improved processing, and enhance some final property. However, the mechanical, thermal, rheological and other properties of a polymeric blend depend strongly on its state of miscibility [5]. The final properties of a polymeric blend will commonly depend on the properties of its polymeric components, its composition and, mainly, on the miscibility of the polymers [6]. In some cases, by synergistic effects, the blend can present better properties than the pure components [6,7]. Many experimental and theoretical methods have been used to investigate the polymer–polymer miscibility. Some of the most important techniques applied to investigate the polymer–polymer miscibility are thermal analysis [8,9], electronic microscopy [10], dynamic mechanical studies [11] and viscometric techniques [12,13]. The viscometric measurements provide an effective, quick and inexpensive technique to investigate polymer–polymer interaction. Viscometry is widely used to determine the molecular weight, molecular weight distribution, and degree of polymerization [14]. Many researchers have attempted to correlate
⁎ Corresponding author at: Av. Colombo, 5790, CEP 87020-9000, Maringá, Paraná, Brazil. E-mail address: [email protected] (A.R. Freitas). 0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2013.11.003
viscosity with miscibility of ternary polymer solutions (polymer#1/solvent/polymer#2) [9,12,15–17]. The basis for using dilute solution viscosity as efficient tool to evaluate the miscibility among different polymers is that, when in solution, macromolecules of both types may exist in a molecularly dispersed state and undergo mutual attraction or repulsion, thereby rendering positive or negative influences on viscosity. Interactions between the components forming the polymeric blend are reflected in the solution viscosity of polymer blend, because polymer–polymer interactions usually dominate over polymer–solvent ones [18]. Christopoulou et al. [19] studying binary polymer mixtures in the solution and in the solid state have shown that there is a very close relation between their behaviours in the solution and in the solid state. The compatibility founded in solution would remain even when the solvent is absent. The phenomenon is called “memory effect” [20,21]. Neiro et al. [22] investigated the miscibility on PVC/PEO blends through thermal (DSC), microscopy (SEM) and viscometry (DSV). Viscometric and thermal results showed that the PVC/PEO blends are miscible. However, the existence of PVC rich domains, smaller than 0.1 μm in blends of PVC/PEO, was observed by microscopic analysis. The miscibility of the PVC/PEO blend was explained as a result of donor–acceptor interactions between chlorine atoms of PVC, as a weak acceptor species, and oxygen atoms of the PEO, as a donor species. In this study, the miscibility between polychloroprene (PCP) and natural rubber (NR) in solution and solid state was investigated. In this sense, a study on the viscosity behaviour of mixtures of PCP with NR in a common solvent was performed. The miscibility behaviour was evaluated through two techniques: dilute solution viscometry (DSV) and scanning electronic microscopy (SEM) methods. It was
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150
found that blends of PCP and NR are immiscible at blend mass ratios studied. Furthermore, the immiscibility verified in solutions also was found in solid state, effect attributed to “memory effect”.
147
The slope of Eq. (4) by plotting ηsp,m/Cm as a function of Cm gives the experimental value of bexp m which can be defined as: exp
2
2
exp
bm ¼ b1 w2 þ b2 w2 þ 2b12 w1 w2 :
ð11Þ
2. Theoretical models to evaluate the miscibility Earlier studies indicate that the interactions in binary polymeric systems can be analysed accurately evaluating by their viscosity behaviour [23]. For this reason, an adapted theoretical approach has been formulated having as reference an ideal binary polymeric system. According to Huggins [24], the behaviour of polymeric solutions could be described by Eq. (1) ηsp C
¼ ½η þ bC
ð1Þ
where ½η ¼ lim C→0
ηsp
ð2Þ
C
and b ¼ k½η
2
ð3Þ
being ηsp as the specific viscosity, C as the mass concentration of solution (mg dL−1), and [η] as the intrinsic viscosity, reflecting the interaction between polymer and solvent. The parameter b is related to the Huggins coefficient k, which reflects the binary interactions between polymer segments. The Eq. (1) can be readily adapted to polymer#1–solvent– polymer#2 ternary system [5], and given as following: 2
ηsp;m ¼ ½ηm C m þ bm C m
ð4Þ
where the subscript m indicates the polyblend and bm is related to the Huggins parameter km by 2
bm ¼ km ½ηm :
ð5Þ
m
Cm
¼
X ηsp Ci
i
i
ð6Þ
wi
where wi = Ci / Cm is the weight fraction of polymer i(i = 1,2). Combining Eqs. (1) and (6), we get ηsp
m
Cm
¼
X
½ηi wi þ C m
i
X
!2 1=2 bi wi
ð8Þ
2
1=2 1=2
ð9Þ
Eq. (9) defines the ideal value to the global viscometric interaction parameter bid m among the chains of both the polymers in given mixture of components 1 and 2 in a common solvent. The ideal value of specific interaction parameter bid 12 is assumed to geometric mean value of parameter assigned to the polymer interaction and is given as following: 1=2 1=2
b12 ¼ b1 b2 :
2 2 K 1 w1 ½η1 þ 2K 12 w1 w2 ½η1 ½η2 K 2 w2 ½η2 : 2 w1 ½η1 þ w2 ½η2
Km ¼
ð10Þ
ð13Þ
ð14Þ
In this sense Jiang and Han [12] revised this criterion by defining another parameter, β, as 2ΔKw1 ww2 ½η1 ½η2 β¼ w1 ½η1 þ w2 ½η2 2
ð15Þ
where ð16Þ
K1 ¼
b1 ½η21
ð17Þ
K2 ¼
b2 ½η22
ð18Þ
b12 : ½η212
ð19Þ
The parameter β is a function of, w and ΔK. Similarly, values of β ≥ 0 indicate miscibility whereas values of β b 0 indicate immiscibility. Catsiff and Hewett [27] have proposed to define the ideal value of interaction parameter, bid′ 12 as the arithmetic mean. b12 ¼
2
ð12Þ
where
id′
and
id
" # pffiffiffiffiffiffiffiffiffiffiffiffi K 1 w1 ½η21 þ 2 K 1 K 2 w1 w2 ½η1 ½η2 þ K 2 w2 ½η22 2 w1 ½η1 þ w2 ½η2
α ¼ Km−
ð7Þ
and the comparison between Eqs. (4) and (7) gives:
bm ¼ b1 w1 þ b2 w2 þ 2b1 b2 w1 w2 :
id
The ideal interaction coefficient, bid 12 between two polymers can be expressed by Eq. (10) and bexp 12 obtained experimentally by substituting all terms in Eq. (11). Δb ≥ 0 indicates miscibility, whereas Δb b 0 indicates immiscibility. Sun et al. [26] proposed another parameter, α, for polymer–polymer miscibility in terms of a thermodynamic parameter. The parameter α indicates the attractive (α N 0) or repulsive (α b 0) interaction among polymer segments. The zero value of α indicates neither attraction nor repulsion.
K 12 ¼
i
½ηm ¼ ½η1 w1 þ ½η2 w2
exp
Δb ¼ b12 −b12
pffiffiffiffiffiffiffiffiffiffiffiffi ΔK ¼ K 12 − K 1 K 2
The Eq. (4)-weighted average can be expressed as: ηsp
According Krigbaum and Wall [25], the information about intermolecular interactions on binary polymer blends (polymer #1 and polymer #2) can be obtained from a comparison of experimental interaction id coefficient bexp 12 and theoretical value b12.
b1 þ b2 2
ð20Þ
and exp
id′
Δb′ ¼ b12 −b12 :
ð21Þ
Using the value Δb′ given by the Eq. (21) a more effective parameter μ is obtained to predict the polymer–polymer miscibility [28]. μ¼
Δb′ ½η1 −½η2
2
ð22Þ
148
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150
Fig. 1. Schematic structure to NR (A) and PCP (B) respectively.
μ ≥ 0 and μ b 0 values correspond to miscibility and immiscibility, respectively. According to Kulshreshtha et al. [18], the miscibility of a polymer blend also may be studied plotting the relative viscosity versus the fraction of one of the components in the blend. The miscibility between the polymers is evaluated as function of data set adjusts. Linear plot adjust characterizes the polymer blend as miscible while an “S” type adjusting plot is an indicative of immiscible blend. 3. Experimental 3.1. Materials A commercial polychloroprene (PCP) type W sample was supplied by Proquimil (Brazil) and used without previous purification. Toluene P.A. grade was supplied by Synth (Brazil) and used as received. Natural rubber (NR) was used after purification by precipitation from a toluene solution 3% (w/v) into ethanol excess (volume ratio N 10). The solid was dried for four days at room temperature and dark ambient and for 24 h under reduced pressure. The solid NR was stocked in dark ambient at − 4 °C, to prevent some oxidative process. The average viscosimetric molar mass (Mv) of purified PCP and NR was determined as being 2.4 × 104 g mol− 1 and 1.3 × 106 g mol−1 using a Cannon Ubbelohde capillary viscometer (100/E534). 3.2. Viscosity measurements All the measurements for ternary systems (polymer#1– solvent– polymer#2) were performed using a dilution capillary Ubbelohde
viscometer type, Cannon (100/E534). The viscometer was immersed in a thermostatized bath and operating at 25 °C. Ternary solution for each system was prepared by mixing the polymers to yield mass ratios (wt1/wt2), (20/80), (50/50), and (80/20). Each blend-solution was prepared by solubilizing of the polymers in toluene yielding a final concentration of 0.62% (wt/v). After this, each solution was diluted to other four with lower polymer concentrations. The dilution was done by the addition of an appropriate amount of toluene into the viscometer. The flow times were recorded (4 or 5 measures for each concentration) with an accuracy of ± 0.01 s, and the bath temperature was constant within ±0.1 °C. The miscibility also was evaluating by relative viscosity determination using mass ratios 20/40, 40/60, 60/40 and 80/20. 3.3. Scanning electron microscopy (SEM) The SEM images were taken from films based on the polymeric blends. These films were obtained through casting methodology from mix of solid PCP and NR in toluene (as solvent). The polymeric blend solutions were prepared according to the following mass ratio (wt1/wt2): (25/75), (50/50) and (75/25). Briefly, the polymer solutions were stirred for 24 h and then poured in Petri dishes, which remain for four days in dark ambient at room temperature. The images from the films were taken in a scanning electron microscope (Shimadzu, model LEO440). The range of accelerating voltage was 0.3–30 kV. The samples were first sputter coated with a thin layer of gold and then observed at magnification of 500×. All the SEM images were taken at 23 °C. 4. Results and discussions 4.1. Viscosity measurement
Table 1 Viscometric data for PCP, NR and PCP/NR blend. PCP/NR mass blend ratio (%)
Concentration (g dL−1)
Average efflux time (s)
Reduced viscosity
Relative viscosity
Intrinsic viscosity (dL3 g−1)
0/100
0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29 0.62 0.44 0.35 0.29
492.3 332.2 254.4 210.0 468.1 312.3 236.3 197.9 356.6 244.8 194.5 166.8 216.5 166.7 142.8 129.0 176.4 143.4 126.5 116.7
5.26 3.22 2.23 1.67 4.95 2.97 2.00 1.52 3.53 2.11 1.47 1.12 1.75 1.12 0.82 0.64 1.24 0.82 0.60 0.48
8.55 7.26 6.42 5.86 8.05 6.69 5.76 5.32 5.70 4.79 4.20 3.86 2.85 2.53 2.34 2.25 2.00 1.87 1.74 1.67
3.58
20/80
50/50
80/20
100/0
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.4 0.6 0.1 0.3 0.3 0.7 0.4 0.3 1.8 1.3 0.4 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1
2.90
Fig. 1 presents the polymer structure of NR (A) and PCP (B). According to Kardam [29], PCP and NR have very similar structures and the fact of molecular “size” of the chloride atoms on PCP is nearly the same as that of methyl groups on the isoprene rubber units in NR. This makes these two polymers be very similar in molecular structure and compatible with each other. In fact, there is a wide similarity between the structures of these two polymers (see Fig. 1). On the other hand, the electron donor character of methyl groups on NR is the opposite of chlorides (electron acceptor) on PCP, and due to this the presence of attractive forces among their polymer chains leading to miscibility is expected.
2.29 Table 2 Viscometric data: [η] and b and the perspective, bexp 12 ΔB and β parameters for pure components and binary blends. 1.70
1.41
PCN/ NR
[η] (dL g−1)
b (dL g−1)2
bexp 12 (dL g−1)2
ΔB (dL g−1)2
β
Miscibility
100:00 80:20 50:50 20:80 00:100
1.41 1.70 2.29 2.90 3.58
0.99 1.84 5.59 8.38 8.10
– 0.070 0.415 0.252 –
– −2.75 −2.41 −2.57 –
– −0.766 −0.473 −0.204 –
– Immiscible Immiscible Immiscible –
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150 Table 3 The numerical values of polymer–polymer interaction parameters for PCP/NR blends. PCP/NR
α
ΔK
μ
Miscibility
20:80 50:50 80:20
−0.149 −0.363 −0.468
−1.25 −1.16 −1.61
−0.911 −0.877 −0.949
Immiscible Immiscible Immiscible
Earlier studies indicate that the interaction in binary polymer system can be analysed accurately by studying the viscosity behaviour [23]. The data set collected from efflux time measurements to pure polymer (PCP and NR) and respective blend mass ratios is presented in the Table 1. The miscibility parameters were determined using mathematic model. The Δb [23] and β parameters values were computed using Eqs. (12) and (16) and are presented in Table 2. It was founded that the Δb and β parameter values calculated for the PCP/NR blends present negative value. Due to this, the PCP/NR blends may be considered as immiscible. The values calculated for the miscibility parameters α, ΔK and μ are presented in the Table 3. According to the data presented in the Table 3, it is possible to infer that the PCP/NR blends are immiscible for all the studied conditions. Therefore, even changing the mass ratio of PCP/NR no miscibility was verified for the blends. Furthermore, it is well known [28–30] that plots of absolute viscosity versus composition deviate from linearity according to the degree of miscibility of polymer blend. Plots for miscible systems are linear whereas for immiscible systems plots are S-typed, indicating two phase formation with reversal of phases at intermediate compositions. Fig. 2 shows viscosity-composition plots for polymeric blends based on PCP/NR. It can be seen that, in the first moment, a linear adjust was obtained when the linear regression model is applied to data 100
Sigmoid adjust Linear adjust
149
set (dot line). According to predicted, linear plot would characterize this material as a miscible blend. On the other hand, when a sigmoid model was applied a best fit was obtained. The sigmoid correlation coefficient fit value obtained was best than the linear one, indicating a formation of two phases with phase inversion in an intermediate composition, as presented in Fig. 2. According to Pingping [30], this method is empirical and it is not conclusive to state the miscibility or not of a polymer blend systems. In contrast, there are diverse researchers [31,32] that apply this methodology and obtained satisfactory results. In this study, this method was satisfactory and data collected from the viscosimetric study were confirmed after evaluating the SEM images. 4.2. Analysis through scanning electronic microscopy (SEM) The morphological aspect of the PCP/NR blends with mass ratio equal to 25/75, 50/50 and 75/25 was evaluated through SEM images, which are shown in Fig. 3. All the SEM images taken to the PCP/NR blends suggested the phase separation phenomenon because the presence of two phases was observed. It is possible to observe that the fraction of PCP form small spheres dispersed in a NR matrix, which is a strong indicative of the immiscibility between the two polymers tested. From Fig. 3C, which shows the SEM image taken from the PCP/NR mass ratio 75/25, it is possible to observe small spheres, in contrast, to that formed by the NR dispersed in a matrix based on PCP. In addition from the blend with intermediary PCP/NR mass ratio (50/50), Fig. 3B, it was possible to verify the clear immiscible of this blend, which corroborates with the viscosity data. Christopoulou et al. [19] studying binary polymer mixture in solution and solid state, have shown that there is a very close relation between their behaviour in solution and solid state. The compatibility/incompatibility found in solution would remain even when the solvent is absent. The phenomenon was called “memory effect” [20,21]. This effect was found to PCP/NR blend. 5. Conclusion
Relative viscosity
80
The present investigation clearly indicated that the interaction in the polymer blends might be studied by simple measurement based on viscometry technique, providing valuable information about the miscibility of the PCP/NR polymer blend, which generally are obtained from sophisticated techniques. By studying binary polymer mixtures in solution and solid state as a function of blend ratio, we have shown that there is a very close relation between their behaviours in solution and solid state. Our results allow us to conclude that, polychloroprene and natural rubber make immiscible blends in all composition studied.
60
40
20
Acknowledgement 0
20
40
60
80
NR fraction in PCP-NR blend (%) Fig. 2. Relative viscosity as a function of NR fraction in blend.
100
AR Freitas acknowledges CAPES for the doctorate fellowship and Proquimil by polychloroprene donation. The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq - Brazil for financial support.
Fig. 3. SEM micrographs of PCP/NR blends at different mass ratios: (A) 25/75; (B) 50/50; (C) 75/25. Scale bar is 20 μm.
150
A.R. Freitas et al. / Journal of Molecular Liquids 190 (2014) 146–150
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