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Liquid natural rubber blends at high under cooling
Thermochimica Acta 578 (2014) 1–9
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Isothermal crystallization of Polyamide 6/Liquid natural rubber blends at high under cooling G.M. Shashidhara ∗ , K.G. Pradeepa Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore 570006, India
a r t i c l e
i n f o
Article history: Received 10 August 2013 Received in revised form 17 December 2013 Accepted 20 December 2013 Available online 8 January 2014 Keywords: Polymer blends and alloys Crystallization kinetics Avrami model Sestak–Berggren model Liquid natural rubber
a b s t r a c t This study presents isothermal crystallization kinetics of Polyamide 6/Liquid Natural Rubber (PA6/LNR) blends of different compositions, at different crystallization temperatures using the DSC technique at high under cooling (T > 80 ◦ C). The results showed that the degree of crystallinity of PA6/LNR blend is less than that of PA6. Neat PA6 exhibits a single melting peak while blends exhibit an additional diffused peak at lower temperatures. The PA6/LNR blends showed relatively lower half crystallization time than PA6. The isothermal crystallization kinetics of PA6 and blends has been described by the Avrami equation and Sestak Berggren equation. The Avrami exponent, n for PA6 is in the range of 2–3 and that of blends is in the range of 1–2, at high under cooling conditions. In general, with the addition of LNR, the shape of the nuclei tends to change from spherical to plate/disk geometry. The equilibrium melting temperatures of the blends was evaluated based on Hoffman–Weeks theory. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The crystallization process is a transition from liquid phase (melts) into a solid phase after cooling. The physical, chemical and mechanical properties of crystalline polymers depend on the morphology, the crystalline structure and degree of crystallization. In order to control the rate of the crystallization and the degree of crystallinity and to obtain the desired morphology and properties, a great deal of effort has been devoted in studying the crystallization kinetics and determining the change in material properties [1]. A semi-crystalline polymer such as Polyamide 6 (PA6), when cooled from the melt either under isothermal or non-isothermal conditions will form a crystalline structure. Two competitive effects influence the crystallization behavior; cooling rate and crystallization growth rate. The resulting microstructure has a significant effect on the ultimate properties of the product such as toughness, elasticity, transparency and permeability. In turn, the microstructure of the material is determined by the thermo chemical history that the material experiences during processing [2]. PA6 is an important engineering plastic because of its excellent mechanical properties. However, it exhibits poor impact resistance below its glass-transition temperature and in the dry state, high moisture absorption, poor dimensional stability, and unsatisfactory heat deflection temperatures. To overcome this
∗ Corresponding author. Tel.: +91 821 2548285; fax: +91 821 2548290. E-mail addresses: [email protected], [email protected] (G.M. Shashidhara). 0040-6031/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.12.017
problem, several approaches have been employed to improve the toughness of PA6, which include the addition of tougheners such as different types of rubbers. An appropriate range of rubber particle size, interparticle distance and uniform distribution of the rubber particles plays an important role to achieve the desired extent of toughening. Use of modified rubbers such as Styrene Ethylene Butylene Styrene-g-maleic anhydride (SEBS-gMAH), Polyolefin Elastomer-g-maleic anhydride (POE-g-MAH) in the preparation of binary polyamide/rubber blends has been widely studied to further endow it with advanced and balanced properties [3,4]. PA6/Acrylonitrile Butadiene Styrene (ABS) blends compatibilized using imidized acrylic polymer [5], styrene/maleic anhydride copolymer [6], styrene/acrylonitrile/maleic anhydride terpolymer [7], maleated poly (methyl methacrylate) [8] have shown improved impact properties. Ethylene propylene diene monomer (EPDM), Ethylene propylene rubber (EPM) and polyethylene functionalized with maleic anhydride can be used as impact modifiers for PA6 [9–11]. The effect of the maleic anhydride coupling agent on impact behavior of nylon – rubber blends and the brittle-tough transition has been investigated by these authors. Recently we have reported that liquid natural rubber (LNR) can be used as a potential toughening agent for PA6 [12]. In this study, the isothermal crystallization behavior of Polyamide 6/Liquid Natural Rubber (PA6/LNR) blends of different compositions (95/15, 90/10, 85/15, and 80/20) at high under cooling conditions (Crystallization temperature range 170–190 ◦ C, T = 70 to 90 ◦ C) is reported for the first time. The high under cooling conditions means that, the temperature ranges where the fastest
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crystallization rate would occur. The study of the behavior of a semi crystalline polymer crystallizing in this temperature range is of great importance from practical view points. The practical importance is that, such crystallization data may be very useful for polymer processing such as fiber spinning and injection molding, because most polymer shaping processes need a very fast crystallization rate to shorten the solidification time. The importance also arises from the effect of final crystallinity on the physical and chemical properties of polymers [13]. The isothermal crystallization kinetic studies of PA6 and PA6/LNR blends were conducted using DSC. The model most often applied to isothermal crystallization data is the Avrami model [14–16]. In this study, another kinetic model, the Sestak–Berggren model [17,18] is applied to crystallization kinetics.
2.5. Isothermal crystallization studies Isothermal crystallization from the melt was performed in the sample pan of the differential scanning calorimeter as follows: The sample (typically 5 mg) was heated at a rate of 50 ◦ C/min to 260 ◦ C (about 30 ◦ C above the melting temperature) and maintained isothermal conditions for 3 min to eliminate any residual crystals. The resulting melt was rapidly cooled at 80 ◦ C/min to the predetermined crystallization temperatures (Tc ) in the range 170–190 ◦ C, and maintained isothermal at Tc till crystallization is complete. For each material, a minimum of five crystallization experiments were performed over a range of 170–190 ◦ C. The exothermal curves of heat flow as a function of time were recorded. The kinetic analysis of the crystallization process was carried out using Thermal Analysis Advantage software supplied by TA instruments.
2. Experimental 2.1. Materials
3. Results and discussions
Polyamide 6 (PA6) (Gujlon M 28 RC) was supplied by Gujarat State Fertilizer Company Ltd., Vodadara, India. The natural rubber in the form of Ribbed Smoked Sheet (RSS) and Pale Latex Crepe (PLC) (molecular weight approx. 9, 60,000 g/mol) were supplied by Kaduthuruthi Rubber marketing society, Kerala, India. The peptizing agent, Peptizol-7, was purchased from Acme Chem. Ltd., Panoli, India. Peptizol-7 is a conventional blend of pentachlorothiophenol, organometallic complexes, organic and inorganic dispersing agents.
3.1. Melting and crystallization behavior
2.2. Preparation of liquid natural rubber The LNR was prepared by mastication of known quantities of rubber with 0.5 phr peptizol-7 in a two roll mill (roll size 15.2 cm × 33 cm, friction ratio 1:14, Sohal Engineering Works Bombay, India) for desired time. The molecular weight of the resulting LNR was 19,355 g/mol.
2.3. Preparation of PA6/LNR blend PA6 was pre dried in a hot air oven for about 24 h at 100 ◦ C. A polymer master batch containing 70 wt.% PA6 and 30 wt.% LNR, was prepared by mixing required quantities of fresh sample of masticated rubber and pre-dried PA6, in an internal mixer (Polylab Rheomix RC 300P) fitted with roller rotors. The mixing conditions were as follows: temperature 215 ◦ C, mixing time 8 min, rotor speed 40 RPM. The PA6/LNR blends of different compositions (100/0, 95/5, 90/10, 85/15 and 80/20) were prepared by melt mixing the required quantities of master batch and PA6 in the internal mixer.
DSC is a convenient method for determining thermal properties of the polymers. The typical DSC thermograms of pure PA6 and PA6/LNR blends for melting run are shown in Fig. 1. A distinct melting endothermic peak at 222.8 ◦ C was observed in the DSC thermogram for pure PA6. The maximum peak melting temperatures of PA6/LNR blends are slightly lower than the maximum peak melting temperature of the PA6 as shown in Fig. 1. The degree of crystallization (Wc ) was calculated using Eq. (1); WC =
Hm × 100 wH100
(1)
where Hm is the enthalpy of melting of sample, w is the weight fraction of PA6 in the blend and H100 is the enthalpy of melting of 100% crystalline PA6 (=230 J/g) [19]. The melting characteristics of PA6 and PA6/LNR blends are summarized in Table 1. The decrease in enthalpy of melting of PA6/LNR blends is due to decrease in the crystallinity. The decrease in degree of crystallinity may be attributed to the presence of amorphous LNR content. The crystallization kinetics of PA6 and its blends were studied using DSC, as discussed in the next section.
2.4. Differential scanning calorimetry The typical DSC runs to obtain the information about the crystallization and melting process were conducted using TA instruments model Q150 DSC under Nitrogen atmosphere. The temperature scale of the instrument was calibrated with high purity indium as a melting point standard (156.6 ◦ C), while the heat flow calibration was done with a high purity alpha aluminum disk supplied by TA instruments as a calorimetric standard. About 5 mg of sample was weighed and put in an aluminum pan and covered with a lid. The sample was heated to 260 ◦ C at a scan rate of 10 ◦ C/min and cooled to −50 ◦ C and again heated to 260 ◦ C at the same rate.
Fig. 1. DSC thermograms (second heating cycle) of PA6 and PA6/LNR blends: (1) 100/0, (2) 95/05, (3) 90/10, (4) 85/15 and (5) 80/20.
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Table 1 Characteristic melting temperature, heat of melting, degree of crystallization and glass transition temperature of PA6 and PA6/LNR blends. Blend composition (weight %) PA6
Blend designation
Tm (◦ C)
Hm (J/g)
Wc (%)
Tg (◦ C)
100/0 95/05 90/10 85/15 80/20
222.8 221 221 221 221
56.46 48.63 42.38 39.86 38.38
24.55 22.25 20.47 20.38 20.85
21.0 32.8 32.8 31.6 31.5
LNR
100 95 90 85 80
0 5 10 15 20
3.2. Isothermal crystallization kinetics An understanding of the kinetics of the crystallization process is important for the selection of processing parameters such as mold temperature and hold time during injection molding. Isothermal crystallization is based on rapidly cooling the sample from the melt to the crystallization temperature and then measuring the heat evolved while the sample is held isothermal. The isothermal crystallization curves of PA6 and PA6/LNR 90/10 blend are shown in Fig. 2. The crystallization exothermic peak becomes broad with an increase in crystallization temperature. The isothermal crystallization kinetics of a material can be analyzed by evaluating its degree of crystalline conversion as a function of time at a constant temperature. The relative crystallinity, X(t) at different crystallization time, can be obtained from the ratio of the area of the exotherm up to time t divided by the total exotherm, according to Eq. (2); X(t) =
Qt Q∞
(2)
where Qt and Q∞ are the heat generated at time t and infinite time respectively. The development of the relative crystallinity with time for the neat PA6 and PA6/LNR 90/10 blend samples are plotted in Fig. 3. As can be seen, the isotherms exhibit a sigmoid dependence with few exceptions. The characteristic isotherms are shifted to right along the time axis with increase of crystallization temperature, indicating progressively slower crystallization rate. A slow increase of crystallinity with time after most of the crystallization had taken place is observed and this was attributed to the presence to secondary crystallization [1]. The crystallization half time, t1/2 , is defined as the time at which relative crystallinity X(t) is 50%. The half crystallization time can be obtained directly from Fig. 3. In general t1/2 or 1/t1/2 is taken as a measure of the overall rate of crystallization of a polymer. Longer the t1/2 , the slower will be the crystallization rate. Fig. 4 shows a plot of t1/2 of PA6/LNR blends as a function of crystallization temperature. It can be seen that the maximum crystallization rate locates at about 180 ◦ C and all the t1/2 s are <0.23 min when the samples crystallize at very high degrees of under-cooling (T > 80 ◦ C,
Fig. 2. DSC isothermal crystallization curves of selected PA6/LNR blends at various crystallization temperatures (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
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Fig. 3. Relative crystallinity versus crystallization time of selected PA6/LNR blends at different crystallization temperatures.
Tc < 180 ◦ C). As shown in Fig. 4, above 180 ◦ C, the t1/2 increases exponentially with crystallization temperature (Tc ). The increase of the half crystallization time with temperature confirms the ordering process occurred through a nucleation mechanism [14]. The crystallization rate of pure PA6 (100/0) at all Tc s are higher than those of blends, which may be attributed to higher nucleation density at the same temperature [20].
3.3. Avrami model The isothermal crystallization kinetics of PA6 and PA6/LNR blends was analyzed in terms of well known Avrami equation, 1 − X = exp(−kt n )
which provides a convenient approach to analyze the overall crystallization kinetics. In this equation, X is the crystalline conversion at time t, k is the kinetic growth rate constant and n is the Avrami exponent, related to the type of nucleation and to the geometry of the growing crystals. In order to deal conveniently with the operation, Eq. (3) is usually rewritten as a double logarithmic form as given by Eq. (4) lpg[− ln(1 − X)] = log k + n log t
Fig. 4. The crystallization half-life times of at various Tc for PA6/LNR blends: (1) 100/0, (2) 95/05, (3) 90/10, (4) 85/15 and (5) 80/20.
(3)
(4)
In several crystallizing systems, the resulting plots of the linear form of the Avrami equation, log [−ln (1 − X)] vs. log t supposed to provide as single slope associated with the value of n and k have given inconsistent results. Fig. 5 shows the typical Avrami plots of PA6 and PA6/LNR 90/10 blend. The linear fits for all the plots were conducted for regime with the relative crystallinity in the range 35–95% for neat PA6 (100/0) and 77–99% for blends. For the highest Tc employed in this study (190 ◦ C), the linear fits were performed by considering relative crystallinity in the range 40–80% for 100/0 and 30–90% for blends. As can be seen from Fig. 5, in most of the cases, two regions of different slopes are obtained. The time dependence of the relative degree of crystallinity, X at lower conversion deviates from that at higher conversion. A similar deviation from the Avrami equation has been reported by several authors [21,22]. The X value that limits these two regions varies as a function of the crystallization temperature. The least square fitting
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Fig. 5. Avrami plots of selected PA6/LNR blends at various Tc (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
Table 2 Isothermal crystallization kinetic parameters of Avrami model and Sestak–Berggren model. Blend
Tc (◦ C)
Avrami model constants k (min−1 )
Sestak–Berggren model constants n
n
m
k (min−1 )
100/0
170 175 180 185 190
49.63 25.23 13.71 8.16 5.83
± ± ± ± ±
1.064 1.018 1.005 1.003 1.024
2.86 2.37 2.05 2.11 3.10
± ± ± ± ±
0.055 0.016 0.005 0.002 0.021
0.988 0.766 0.824 0.804 0.748
± ± ± ± ±
0.022 0.017 0.012 0.006 0.007
0.731 0.559 0.566 0.582 0.600
± ± ± ± ±
0.012 0.009 0.006 0.004 0.004
14.50 11.50 8.83 6.28 4.18
± ± ± ± ±
0.38 0.23 0.12 0.047 0.033
95/05
170 175 180 185 190
29.58 17.26 11.61 8.85 7.31
± ± ± ± ±
1.068 1.042 1.019 1.0143 1.0199
1.93 1.64 1.39 1.43 2.00
± ± ± ± ±
0.058 0.039 0.019 0.013 0.019
1.280 1.240 1.170 0.907 0.992
± ± ± ± ±
0.076 0.062 0.021 0.024 0.021
0.635 0.641 0.517 0.551 0.612
± ± ± ± ±
0.043 0.036 0.012 0.013 0.012
19.00 19.10 13.80 11.30 7.65
± ± ± ± ±
1.70 1.40 0.35 0.32 0.19
90/10
170 175 180 185 190
17.82 12.91 9.62 7.78 7.31
± ± ± ± ±
1.048 1.029 1.019 1.017 1.029
1.49 1.38 1.30 1.44 2.16
± ± ± ± ±
0.043 0.028 0.018 0.016 0.029
0.975 1.000 0.864 0.959 0.954
± ± ± ± ±
0.016 0.043 0.030 0.026 0.013
0.625 0.527 0.572 0.572 0.605
± ± ± ± ±
0.009 0.025 0.017 0.015 0.002
7.75 16.90 16.70 11.60 7.63
± ± ± ± ±
0.14 0.87 0.60 0.36 0.12
85/15
170 175 180 185 190
11.35 8.63 6.77 5.67 4.71
± ± ± ± ±
1.028 1.019 1.011 1.011 1.019
1.27 1.15 1.07 1.22 1.78
± ± ± ± ±
0.029 0.022 0.014 0.014 0.025
1.260 1.110 1.030 1.160 1.030
± ± ± ± ±
0.067 0.050 0.032 0.022 0.016
0.651 0.530 0.543 0.588 0.627
± ± ± ± ±
0.037 0.028 0.018 0.012 0.008
18.70 16.20 14.20 10.40 7.22
± ± ± ± ±
1.50 0.96 0.54 0.27 0.13
80/20
170 175 180 185 190
11.99 9.59 7.74 6.65 5.94
± ± ± ± ±
1.035 1.021 1.013 1.012 1.025
1.29 1.17 1.07 1.19 1.93
± ± ± ± ±
0.041 0.025 0.016 0.013 0.030
0.821 1.000 1.040 0.991 0.923
± ± ± ± ±
0.018 0.033 0.035 0.022 0.014
0.732 0.598 0.570 0.594 0.625
± ± ± ± ±
0.010 0.019 0.020 0.012 0.007
27.80 21.20 15.80 11.60 7.60
± ± ± ± ±
0.60 0.85 0.66 0.30 0.12
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of the secondary linear portion of these plots yielded the Avrami exponent, n and the crystallization rate constant, k and the corresponding values are presented in Table 2. The non integral values of n are generally accounted for by mixed growth and/or surface nucleation and two stage crystallization [21]. The fractional values of n suggest the presence of secondary crystallization. In polymer crystallization, a combination of heterogeneous and homogeneous nucleation along with secondary crystallization has been associated with two region crystallization behavior with different Avrami exponents. This might also be the case with PA6/LNR blend systems. Avrami exponent, n represents a parameter revealing the nucleation mechanism and growth dimension. The Avrami exponent was found to vary from 2.06 to 3.10 in case of neat PA6 and 1.07 to 2.16 in case of PA6/LNR blends. Wunderlich [23] has attributed an Avrami exponent of 3 to spherical structure resulting from instantaneous nucleation (that is, the number of nuclei reaches a steady state value rapidly after crystallization begins) and an exponent between 2 and 3 to truncated spheres resulting from instantaneous nucleation with diffusion control. Correspondingly, neat PA6 is likely to result truncated spherical crystals and PA6/LNR blends produce rod like to sphere-like crystals. As shown in Fig. 3 the Avrami model seemed to fail in predicting experimental data (symbols). The Avrami equation is usually only valid at lower conversions [21]. In such cases when Avrami equation is not valid, the empirical Sestak–Berggren (SB) model can be used for the description of the crystallization kinetics, as explained in Section 3.4. Although n may be considered as a constant with crystallization temperatures (Tc ), k depends strongly on Tc . The isothermal
rate constants, k is also shown in Table 2 as a function of Tc for all the samples. This indicated that the values of k decreased with increasing Tc . 3.4. Sestak–Berggren (SB) model Two fundamental properties which can be measured in a DSC are the crystallization rate (dX/dt) which is related to the amount of heat flow at any given time, temperature, and crystallization conversion level (X), which is measured as the amount of heat evolved from the beginning of reaction until the selected time and temperature. Kinetics of the process in the condensed phase is frequently described by the so called general rate equation representing the reaction rate dX/dt as a product of two mutually independent functions [17], according to Eq. (5); dX = k (T )f (X) dt
(5)
The temperature function k (T), in Eq. (5) depends solely on temperature T and the conversion function f(X) depends only on the crystallization conversion level of the process, X. The dependence of k is often described by the Arrhenius equation, given by Eq. (6);
k = A exp −
E RT
(6)
where A is the pre-exponential factor, E is the activation energy and R is the universal gas constant. A large number of kinetic methods have been developed which are based on different forms of f(X). Crystallization is a two step process where crystal growth takes place at nucleation sites whose appearance is controlled by both
Fig. 6. Crystallization reaction rate plotted as a function of Sestak–Berggren conversion term. (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
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time and temperature. The Sestak–Berggren (SB) equation, represented by Eq. (7);
f (X) = X m (1 − X)n
(7)
represents a plausible alternative description for the crystallization process taking place in noncrystalline solids [17]. The exponents in the conversion function are non-integer in general and that the conversion function may not have a mechanistic interpretation. By considering Eqs. (5) and (7), we can obtain SB equation as; log
dX dt
= log k (T ) + n [log[(1 − X)X m /n ]]
(8)
The Sestak–Berggren kinetics option of the TA Instruments isothermal kinetics software package was used to analyze the resultant DSC thermal curves. Fig. 6 shows the results of the SB analysis for the crystallization of PA6 and PA6/LNR 90/10 blend at different temperatures. The symbols in Fig. 6 indicate individual data points taken across the crystallization exotherm. These results demonstrate that, the SB model may appropriately be used for PA6 as well as PA6/LNR blends under consideration, since the experimental data points fall close to the line. The values of n , m and k were obtained by TA instruments isothermal kinetics software and are presented in Table 2. The values of n , m and k of PA6 are 0.988 ± 0.022, 0.731 ± 0.012 and 14.5 ± 0.38 for 170 ◦ C and 0.748 ± 0.0068, 0.600 ± 0.0037 and 4.18 ± 0.033 for 190 ◦ C. The values of k for the blends were higher than that of PA6, which indicates that the crystallization rate of blends more sensitive to temperature than that of PA6.
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3.5. Melting behavior and equilibrium melting points Fig. 7 presents a series of DSC heating thermograms for the pure PA6 and PA6/LNR 90/10 blend. All the specimens were melt-crystallized at different Tc . As can be seen, the DSC endotherms for all the specimen except for 95/5, exhibited two melting peaks. The melting peak at low temperature (Tm -I) shifts toward to the higher temperature while the melting peak at high temperature displays almost the same with increasing crystallization temperature. Such behavior can be attributed to the re-crystallization or different thermal history [24]. The melting peak at low temperature is assumed to be due to the melting of crystals formed on cooling from melt, while the second peak (Tm -II) is mainly due to melting of crystals which have re-crystallized during the heating run (after the first melting peak). Similar results have been reported for the blends of various functionalized polyolefins and PA6 by Wei et al. [25]. To perform the quantitative analysis of crystallization behavior, especially to investigate the temperature dependence of the crystalline rate, it is necessary to determine the equilibrium melting point. According to Hoffman-weeks theo ) can be deduced by plotting ory, the equilibrium melting point (Tm the melting temperature (Tm ) versus the crystallization temperao can be obtained by extrapolation of the resulted ture (Tc ). The Tm straight line to intersect the line Tm = Tc . The lower temperature (Tm -I) as a function of Tc for all specimens are plotted in Fig. 8. The high temperature peaks (Tm -II) were at approximately the same position. The plots of Tm -I as function of Tc displayed an increasing linear trend for all examined samples, according to the polymer crystallization theory [26]. The extrapolation of Tm versus Tc to Tm = Tc showed that the melting temperature data corresponding
Fig. 7. Melting DSC thermograms obtained at a heating rate of 20 ◦ C/min after isothermal crystallization at the indicated temperatures. (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tca.2013.12.017. References
Fig. 8. Plot of observed melting temperatures as a function of crystallization temperatures.
to the lower endotherms (Tm -I) were well aligned and extrapolation of the lines gives values of equilibrium melting temperature, o of 253.6, 213.2, 225.3, 219.1 ◦ C for PA6/LNR blends 100/0, 90/10, Tm o , recorded for the 85,15 and 80/20 respectively. The decrease of Tm PA6/LNR blends suggests that the thermodynamic stability of the PA6 crystals is influenced by the chemical interactions and indicate that the crystals in blends were less perfect than those in pure PA6.
4. Conclusions The crystallization studies were conducted for PA6 and PA6/LNR blends. The degree of crystallinity was found to decrease with an increase in LNR content. Isothermal crystallization studies were carried by DSC at different crystallization temperatures in the range 170–190 ◦ C. At crystallization above 180 ◦ C, the half crystallization time, t1/2 increased exponentially with crystallization temperature confirming that the ordering process occurred through a nucleation mechanism. It was observed that, as the under cooling decrease, the crystallization rate reduces and exotherm peak becomes broader. Thus the induction time of exotherm increases. The isothermal crystallization kinetics of PA6 and PA6/LNR blends was analyzed in terms of Avrami equation and Sestak–Berggren (SB) equation. The Avrami exponent was found to vary from 2.06 to 3.10 in case of neat PA6 and 1.07 to 2.16 in case of PA6/LNR blends. The Avrami model fails to provide a good fit to experimental data obtained at Tc in the range 170 ◦ C to 190 ◦ C. The crystallization rate constant k decreases with increasing Tc . The adequacy of SB equation to explain isothermal crystallization kinetics of PA6 and PA6/LNR blends was found to be satisfactory. The isothermally crystallized samples of PA6 and PA6/LNR blends (except 95/5), exhibited two melting peaks. The melting peak at low temperature is assumed to be due to the melting of crystals formed on cooling from melt. The equilibrium o ) deduced according to Hoffman–Weeks theory melting point (Tm was found to be 253.6, 213.2, 225.3, 219.1 ◦ C for PA6/LNR blends o for 100/0, 90/10, 85,15 and 80/20 respectively. The decrease of Tm the blends suggests that the thermodynamic stability of the PA6 crystals is influenced by the chemical interactions and indicate that the crystals in blends were less perfect than those in pure PA6.
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Isothermal crystallization of Polyamide 6/Liquid natural rubber blends at high under cooling G.M. Shashidhara ∗ , K.G. Pradeepa Department of Polymer Science and Technology, Sri Jayachamarajendra College of Engineering, Mysore 570006, India
a r t i c l e
i n f o
Article history: Received 10 August 2013 Received in revised form 17 December 2013 Accepted 20 December 2013 Available online 8 January 2014 Keywords: Polymer blends and alloys Crystallization kinetics Avrami model Sestak–Berggren model Liquid natural rubber
a b s t r a c t This study presents isothermal crystallization kinetics of Polyamide 6/Liquid Natural Rubber (PA6/LNR) blends of different compositions, at different crystallization temperatures using the DSC technique at high under cooling (T > 80 ◦ C). The results showed that the degree of crystallinity of PA6/LNR blend is less than that of PA6. Neat PA6 exhibits a single melting peak while blends exhibit an additional diffused peak at lower temperatures. The PA6/LNR blends showed relatively lower half crystallization time than PA6. The isothermal crystallization kinetics of PA6 and blends has been described by the Avrami equation and Sestak Berggren equation. The Avrami exponent, n for PA6 is in the range of 2–3 and that of blends is in the range of 1–2, at high under cooling conditions. In general, with the addition of LNR, the shape of the nuclei tends to change from spherical to plate/disk geometry. The equilibrium melting temperatures of the blends was evaluated based on Hoffman–Weeks theory. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The crystallization process is a transition from liquid phase (melts) into a solid phase after cooling. The physical, chemical and mechanical properties of crystalline polymers depend on the morphology, the crystalline structure and degree of crystallization. In order to control the rate of the crystallization and the degree of crystallinity and to obtain the desired morphology and properties, a great deal of effort has been devoted in studying the crystallization kinetics and determining the change in material properties [1]. A semi-crystalline polymer such as Polyamide 6 (PA6), when cooled from the melt either under isothermal or non-isothermal conditions will form a crystalline structure. Two competitive effects influence the crystallization behavior; cooling rate and crystallization growth rate. The resulting microstructure has a significant effect on the ultimate properties of the product such as toughness, elasticity, transparency and permeability. In turn, the microstructure of the material is determined by the thermo chemical history that the material experiences during processing [2]. PA6 is an important engineering plastic because of its excellent mechanical properties. However, it exhibits poor impact resistance below its glass-transition temperature and in the dry state, high moisture absorption, poor dimensional stability, and unsatisfactory heat deflection temperatures. To overcome this
∗ Corresponding author. Tel.: +91 821 2548285; fax: +91 821 2548290. E-mail addresses: [email protected], [email protected] (G.M. Shashidhara). 0040-6031/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.12.017
problem, several approaches have been employed to improve the toughness of PA6, which include the addition of tougheners such as different types of rubbers. An appropriate range of rubber particle size, interparticle distance and uniform distribution of the rubber particles plays an important role to achieve the desired extent of toughening. Use of modified rubbers such as Styrene Ethylene Butylene Styrene-g-maleic anhydride (SEBS-gMAH), Polyolefin Elastomer-g-maleic anhydride (POE-g-MAH) in the preparation of binary polyamide/rubber blends has been widely studied to further endow it with advanced and balanced properties [3,4]. PA6/Acrylonitrile Butadiene Styrene (ABS) blends compatibilized using imidized acrylic polymer [5], styrene/maleic anhydride copolymer [6], styrene/acrylonitrile/maleic anhydride terpolymer [7], maleated poly (methyl methacrylate) [8] have shown improved impact properties. Ethylene propylene diene monomer (EPDM), Ethylene propylene rubber (EPM) and polyethylene functionalized with maleic anhydride can be used as impact modifiers for PA6 [9–11]. The effect of the maleic anhydride coupling agent on impact behavior of nylon – rubber blends and the brittle-tough transition has been investigated by these authors. Recently we have reported that liquid natural rubber (LNR) can be used as a potential toughening agent for PA6 [12]. In this study, the isothermal crystallization behavior of Polyamide 6/Liquid Natural Rubber (PA6/LNR) blends of different compositions (95/15, 90/10, 85/15, and 80/20) at high under cooling conditions (Crystallization temperature range 170–190 ◦ C, T = 70 to 90 ◦ C) is reported for the first time. The high under cooling conditions means that, the temperature ranges where the fastest
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crystallization rate would occur. The study of the behavior of a semi crystalline polymer crystallizing in this temperature range is of great importance from practical view points. The practical importance is that, such crystallization data may be very useful for polymer processing such as fiber spinning and injection molding, because most polymer shaping processes need a very fast crystallization rate to shorten the solidification time. The importance also arises from the effect of final crystallinity on the physical and chemical properties of polymers [13]. The isothermal crystallization kinetic studies of PA6 and PA6/LNR blends were conducted using DSC. The model most often applied to isothermal crystallization data is the Avrami model [14–16]. In this study, another kinetic model, the Sestak–Berggren model [17,18] is applied to crystallization kinetics.
2.5. Isothermal crystallization studies Isothermal crystallization from the melt was performed in the sample pan of the differential scanning calorimeter as follows: The sample (typically 5 mg) was heated at a rate of 50 ◦ C/min to 260 ◦ C (about 30 ◦ C above the melting temperature) and maintained isothermal conditions for 3 min to eliminate any residual crystals. The resulting melt was rapidly cooled at 80 ◦ C/min to the predetermined crystallization temperatures (Tc ) in the range 170–190 ◦ C, and maintained isothermal at Tc till crystallization is complete. For each material, a minimum of five crystallization experiments were performed over a range of 170–190 ◦ C. The exothermal curves of heat flow as a function of time were recorded. The kinetic analysis of the crystallization process was carried out using Thermal Analysis Advantage software supplied by TA instruments.
2. Experimental 2.1. Materials
3. Results and discussions
Polyamide 6 (PA6) (Gujlon M 28 RC) was supplied by Gujarat State Fertilizer Company Ltd., Vodadara, India. The natural rubber in the form of Ribbed Smoked Sheet (RSS) and Pale Latex Crepe (PLC) (molecular weight approx. 9, 60,000 g/mol) were supplied by Kaduthuruthi Rubber marketing society, Kerala, India. The peptizing agent, Peptizol-7, was purchased from Acme Chem. Ltd., Panoli, India. Peptizol-7 is a conventional blend of pentachlorothiophenol, organometallic complexes, organic and inorganic dispersing agents.
3.1. Melting and crystallization behavior
2.2. Preparation of liquid natural rubber The LNR was prepared by mastication of known quantities of rubber with 0.5 phr peptizol-7 in a two roll mill (roll size 15.2 cm × 33 cm, friction ratio 1:14, Sohal Engineering Works Bombay, India) for desired time. The molecular weight of the resulting LNR was 19,355 g/mol.
2.3. Preparation of PA6/LNR blend PA6 was pre dried in a hot air oven for about 24 h at 100 ◦ C. A polymer master batch containing 70 wt.% PA6 and 30 wt.% LNR, was prepared by mixing required quantities of fresh sample of masticated rubber and pre-dried PA6, in an internal mixer (Polylab Rheomix RC 300P) fitted with roller rotors. The mixing conditions were as follows: temperature 215 ◦ C, mixing time 8 min, rotor speed 40 RPM. The PA6/LNR blends of different compositions (100/0, 95/5, 90/10, 85/15 and 80/20) were prepared by melt mixing the required quantities of master batch and PA6 in the internal mixer.
DSC is a convenient method for determining thermal properties of the polymers. The typical DSC thermograms of pure PA6 and PA6/LNR blends for melting run are shown in Fig. 1. A distinct melting endothermic peak at 222.8 ◦ C was observed in the DSC thermogram for pure PA6. The maximum peak melting temperatures of PA6/LNR blends are slightly lower than the maximum peak melting temperature of the PA6 as shown in Fig. 1. The degree of crystallization (Wc ) was calculated using Eq. (1); WC =
Hm × 100 wH100
(1)
where Hm is the enthalpy of melting of sample, w is the weight fraction of PA6 in the blend and H100 is the enthalpy of melting of 100% crystalline PA6 (=230 J/g) [19]. The melting characteristics of PA6 and PA6/LNR blends are summarized in Table 1. The decrease in enthalpy of melting of PA6/LNR blends is due to decrease in the crystallinity. The decrease in degree of crystallinity may be attributed to the presence of amorphous LNR content. The crystallization kinetics of PA6 and its blends were studied using DSC, as discussed in the next section.
2.4. Differential scanning calorimetry The typical DSC runs to obtain the information about the crystallization and melting process were conducted using TA instruments model Q150 DSC under Nitrogen atmosphere. The temperature scale of the instrument was calibrated with high purity indium as a melting point standard (156.6 ◦ C), while the heat flow calibration was done with a high purity alpha aluminum disk supplied by TA instruments as a calorimetric standard. About 5 mg of sample was weighed and put in an aluminum pan and covered with a lid. The sample was heated to 260 ◦ C at a scan rate of 10 ◦ C/min and cooled to −50 ◦ C and again heated to 260 ◦ C at the same rate.
Fig. 1. DSC thermograms (second heating cycle) of PA6 and PA6/LNR blends: (1) 100/0, (2) 95/05, (3) 90/10, (4) 85/15 and (5) 80/20.
G.M. Shashidhara, K.G. Pradeepa / Thermochimica Acta 578 (2014) 1–9
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Table 1 Characteristic melting temperature, heat of melting, degree of crystallization and glass transition temperature of PA6 and PA6/LNR blends. Blend composition (weight %) PA6
Blend designation
Tm (◦ C)
Hm (J/g)
Wc (%)
Tg (◦ C)
100/0 95/05 90/10 85/15 80/20
222.8 221 221 221 221
56.46 48.63 42.38 39.86 38.38
24.55 22.25 20.47 20.38 20.85
21.0 32.8 32.8 31.6 31.5
LNR
100 95 90 85 80
0 5 10 15 20
3.2. Isothermal crystallization kinetics An understanding of the kinetics of the crystallization process is important for the selection of processing parameters such as mold temperature and hold time during injection molding. Isothermal crystallization is based on rapidly cooling the sample from the melt to the crystallization temperature and then measuring the heat evolved while the sample is held isothermal. The isothermal crystallization curves of PA6 and PA6/LNR 90/10 blend are shown in Fig. 2. The crystallization exothermic peak becomes broad with an increase in crystallization temperature. The isothermal crystallization kinetics of a material can be analyzed by evaluating its degree of crystalline conversion as a function of time at a constant temperature. The relative crystallinity, X(t) at different crystallization time, can be obtained from the ratio of the area of the exotherm up to time t divided by the total exotherm, according to Eq. (2); X(t) =
Qt Q∞
(2)
where Qt and Q∞ are the heat generated at time t and infinite time respectively. The development of the relative crystallinity with time for the neat PA6 and PA6/LNR 90/10 blend samples are plotted in Fig. 3. As can be seen, the isotherms exhibit a sigmoid dependence with few exceptions. The characteristic isotherms are shifted to right along the time axis with increase of crystallization temperature, indicating progressively slower crystallization rate. A slow increase of crystallinity with time after most of the crystallization had taken place is observed and this was attributed to the presence to secondary crystallization [1]. The crystallization half time, t1/2 , is defined as the time at which relative crystallinity X(t) is 50%. The half crystallization time can be obtained directly from Fig. 3. In general t1/2 or 1/t1/2 is taken as a measure of the overall rate of crystallization of a polymer. Longer the t1/2 , the slower will be the crystallization rate. Fig. 4 shows a plot of t1/2 of PA6/LNR blends as a function of crystallization temperature. It can be seen that the maximum crystallization rate locates at about 180 ◦ C and all the t1/2 s are <0.23 min when the samples crystallize at very high degrees of under-cooling (T > 80 ◦ C,
Fig. 2. DSC isothermal crystallization curves of selected PA6/LNR blends at various crystallization temperatures (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
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Fig. 3. Relative crystallinity versus crystallization time of selected PA6/LNR blends at different crystallization temperatures.
Tc < 180 ◦ C). As shown in Fig. 4, above 180 ◦ C, the t1/2 increases exponentially with crystallization temperature (Tc ). The increase of the half crystallization time with temperature confirms the ordering process occurred through a nucleation mechanism [14]. The crystallization rate of pure PA6 (100/0) at all Tc s are higher than those of blends, which may be attributed to higher nucleation density at the same temperature [20].
3.3. Avrami model The isothermal crystallization kinetics of PA6 and PA6/LNR blends was analyzed in terms of well known Avrami equation, 1 − X = exp(−kt n )
which provides a convenient approach to analyze the overall crystallization kinetics. In this equation, X is the crystalline conversion at time t, k is the kinetic growth rate constant and n is the Avrami exponent, related to the type of nucleation and to the geometry of the growing crystals. In order to deal conveniently with the operation, Eq. (3) is usually rewritten as a double logarithmic form as given by Eq. (4) lpg[− ln(1 − X)] = log k + n log t
Fig. 4. The crystallization half-life times of at various Tc for PA6/LNR blends: (1) 100/0, (2) 95/05, (3) 90/10, (4) 85/15 and (5) 80/20.
(3)
(4)
In several crystallizing systems, the resulting plots of the linear form of the Avrami equation, log [−ln (1 − X)] vs. log t supposed to provide as single slope associated with the value of n and k have given inconsistent results. Fig. 5 shows the typical Avrami plots of PA6 and PA6/LNR 90/10 blend. The linear fits for all the plots were conducted for regime with the relative crystallinity in the range 35–95% for neat PA6 (100/0) and 77–99% for blends. For the highest Tc employed in this study (190 ◦ C), the linear fits were performed by considering relative crystallinity in the range 40–80% for 100/0 and 30–90% for blends. As can be seen from Fig. 5, in most of the cases, two regions of different slopes are obtained. The time dependence of the relative degree of crystallinity, X at lower conversion deviates from that at higher conversion. A similar deviation from the Avrami equation has been reported by several authors [21,22]. The X value that limits these two regions varies as a function of the crystallization temperature. The least square fitting
G.M. Shashidhara, K.G. Pradeepa / Thermochimica Acta 578 (2014) 1–9
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Fig. 5. Avrami plots of selected PA6/LNR blends at various Tc (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
Table 2 Isothermal crystallization kinetic parameters of Avrami model and Sestak–Berggren model. Blend
Tc (◦ C)
Avrami model constants k (min−1 )
Sestak–Berggren model constants n
n
m
k (min−1 )
100/0
170 175 180 185 190
49.63 25.23 13.71 8.16 5.83
± ± ± ± ±
1.064 1.018 1.005 1.003 1.024
2.86 2.37 2.05 2.11 3.10
± ± ± ± ±
0.055 0.016 0.005 0.002 0.021
0.988 0.766 0.824 0.804 0.748
± ± ± ± ±
0.022 0.017 0.012 0.006 0.007
0.731 0.559 0.566 0.582 0.600
± ± ± ± ±
0.012 0.009 0.006 0.004 0.004
14.50 11.50 8.83 6.28 4.18
± ± ± ± ±
0.38 0.23 0.12 0.047 0.033
95/05
170 175 180 185 190
29.58 17.26 11.61 8.85 7.31
± ± ± ± ±
1.068 1.042 1.019 1.0143 1.0199
1.93 1.64 1.39 1.43 2.00
± ± ± ± ±
0.058 0.039 0.019 0.013 0.019
1.280 1.240 1.170 0.907 0.992
± ± ± ± ±
0.076 0.062 0.021 0.024 0.021
0.635 0.641 0.517 0.551 0.612
± ± ± ± ±
0.043 0.036 0.012 0.013 0.012
19.00 19.10 13.80 11.30 7.65
± ± ± ± ±
1.70 1.40 0.35 0.32 0.19
90/10
170 175 180 185 190
17.82 12.91 9.62 7.78 7.31
± ± ± ± ±
1.048 1.029 1.019 1.017 1.029
1.49 1.38 1.30 1.44 2.16
± ± ± ± ±
0.043 0.028 0.018 0.016 0.029
0.975 1.000 0.864 0.959 0.954
± ± ± ± ±
0.016 0.043 0.030 0.026 0.013
0.625 0.527 0.572 0.572 0.605
± ± ± ± ±
0.009 0.025 0.017 0.015 0.002
7.75 16.90 16.70 11.60 7.63
± ± ± ± ±
0.14 0.87 0.60 0.36 0.12
85/15
170 175 180 185 190
11.35 8.63 6.77 5.67 4.71
± ± ± ± ±
1.028 1.019 1.011 1.011 1.019
1.27 1.15 1.07 1.22 1.78
± ± ± ± ±
0.029 0.022 0.014 0.014 0.025
1.260 1.110 1.030 1.160 1.030
± ± ± ± ±
0.067 0.050 0.032 0.022 0.016
0.651 0.530 0.543 0.588 0.627
± ± ± ± ±
0.037 0.028 0.018 0.012 0.008
18.70 16.20 14.20 10.40 7.22
± ± ± ± ±
1.50 0.96 0.54 0.27 0.13
80/20
170 175 180 185 190
11.99 9.59 7.74 6.65 5.94
± ± ± ± ±
1.035 1.021 1.013 1.012 1.025
1.29 1.17 1.07 1.19 1.93
± ± ± ± ±
0.041 0.025 0.016 0.013 0.030
0.821 1.000 1.040 0.991 0.923
± ± ± ± ±
0.018 0.033 0.035 0.022 0.014
0.732 0.598 0.570 0.594 0.625
± ± ± ± ±
0.010 0.019 0.020 0.012 0.007
27.80 21.20 15.80 11.60 7.60
± ± ± ± ±
0.60 0.85 0.66 0.30 0.12
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of the secondary linear portion of these plots yielded the Avrami exponent, n and the crystallization rate constant, k and the corresponding values are presented in Table 2. The non integral values of n are generally accounted for by mixed growth and/or surface nucleation and two stage crystallization [21]. The fractional values of n suggest the presence of secondary crystallization. In polymer crystallization, a combination of heterogeneous and homogeneous nucleation along with secondary crystallization has been associated with two region crystallization behavior with different Avrami exponents. This might also be the case with PA6/LNR blend systems. Avrami exponent, n represents a parameter revealing the nucleation mechanism and growth dimension. The Avrami exponent was found to vary from 2.06 to 3.10 in case of neat PA6 and 1.07 to 2.16 in case of PA6/LNR blends. Wunderlich [23] has attributed an Avrami exponent of 3 to spherical structure resulting from instantaneous nucleation (that is, the number of nuclei reaches a steady state value rapidly after crystallization begins) and an exponent between 2 and 3 to truncated spheres resulting from instantaneous nucleation with diffusion control. Correspondingly, neat PA6 is likely to result truncated spherical crystals and PA6/LNR blends produce rod like to sphere-like crystals. As shown in Fig. 3 the Avrami model seemed to fail in predicting experimental data (symbols). The Avrami equation is usually only valid at lower conversions [21]. In such cases when Avrami equation is not valid, the empirical Sestak–Berggren (SB) model can be used for the description of the crystallization kinetics, as explained in Section 3.4. Although n may be considered as a constant with crystallization temperatures (Tc ), k depends strongly on Tc . The isothermal
rate constants, k is also shown in Table 2 as a function of Tc for all the samples. This indicated that the values of k decreased with increasing Tc . 3.4. Sestak–Berggren (SB) model Two fundamental properties which can be measured in a DSC are the crystallization rate (dX/dt) which is related to the amount of heat flow at any given time, temperature, and crystallization conversion level (X), which is measured as the amount of heat evolved from the beginning of reaction until the selected time and temperature. Kinetics of the process in the condensed phase is frequently described by the so called general rate equation representing the reaction rate dX/dt as a product of two mutually independent functions [17], according to Eq. (5); dX = k (T )f (X) dt
(5)
The temperature function k (T), in Eq. (5) depends solely on temperature T and the conversion function f(X) depends only on the crystallization conversion level of the process, X. The dependence of k is often described by the Arrhenius equation, given by Eq. (6);
k = A exp −
E RT
(6)
where A is the pre-exponential factor, E is the activation energy and R is the universal gas constant. A large number of kinetic methods have been developed which are based on different forms of f(X). Crystallization is a two step process where crystal growth takes place at nucleation sites whose appearance is controlled by both
Fig. 6. Crystallization reaction rate plotted as a function of Sestak–Berggren conversion term. (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
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time and temperature. The Sestak–Berggren (SB) equation, represented by Eq. (7);
f (X) = X m (1 − X)n
(7)
represents a plausible alternative description for the crystallization process taking place in noncrystalline solids [17]. The exponents in the conversion function are non-integer in general and that the conversion function may not have a mechanistic interpretation. By considering Eqs. (5) and (7), we can obtain SB equation as; log
dX dt
= log k (T ) + n [log[(1 − X)X m /n ]]
(8)
The Sestak–Berggren kinetics option of the TA Instruments isothermal kinetics software package was used to analyze the resultant DSC thermal curves. Fig. 6 shows the results of the SB analysis for the crystallization of PA6 and PA6/LNR 90/10 blend at different temperatures. The symbols in Fig. 6 indicate individual data points taken across the crystallization exotherm. These results demonstrate that, the SB model may appropriately be used for PA6 as well as PA6/LNR blends under consideration, since the experimental data points fall close to the line. The values of n , m and k were obtained by TA instruments isothermal kinetics software and are presented in Table 2. The values of n , m and k of PA6 are 0.988 ± 0.022, 0.731 ± 0.012 and 14.5 ± 0.38 for 170 ◦ C and 0.748 ± 0.0068, 0.600 ± 0.0037 and 4.18 ± 0.033 for 190 ◦ C. The values of k for the blends were higher than that of PA6, which indicates that the crystallization rate of blends more sensitive to temperature than that of PA6.
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3.5. Melting behavior and equilibrium melting points Fig. 7 presents a series of DSC heating thermograms for the pure PA6 and PA6/LNR 90/10 blend. All the specimens were melt-crystallized at different Tc . As can be seen, the DSC endotherms for all the specimen except for 95/5, exhibited two melting peaks. The melting peak at low temperature (Tm -I) shifts toward to the higher temperature while the melting peak at high temperature displays almost the same with increasing crystallization temperature. Such behavior can be attributed to the re-crystallization or different thermal history [24]. The melting peak at low temperature is assumed to be due to the melting of crystals formed on cooling from melt, while the second peak (Tm -II) is mainly due to melting of crystals which have re-crystallized during the heating run (after the first melting peak). Similar results have been reported for the blends of various functionalized polyolefins and PA6 by Wei et al. [25]. To perform the quantitative analysis of crystallization behavior, especially to investigate the temperature dependence of the crystalline rate, it is necessary to determine the equilibrium melting point. According to Hoffman-weeks theo ) can be deduced by plotting ory, the equilibrium melting point (Tm the melting temperature (Tm ) versus the crystallization temperao can be obtained by extrapolation of the resulted ture (Tc ). The Tm straight line to intersect the line Tm = Tc . The lower temperature (Tm -I) as a function of Tc for all specimens are plotted in Fig. 8. The high temperature peaks (Tm -II) were at approximately the same position. The plots of Tm -I as function of Tc displayed an increasing linear trend for all examined samples, according to the polymer crystallization theory [26]. The extrapolation of Tm versus Tc to Tm = Tc showed that the melting temperature data corresponding
Fig. 7. Melting DSC thermograms obtained at a heating rate of 20 ◦ C/min after isothermal crystallization at the indicated temperatures. (1) 170 ◦ C, (2) 175 ◦ C, (3) 180 ◦ C, (4) 185 ◦ C and (5) 190 ◦ C.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tca.2013.12.017. References
Fig. 8. Plot of observed melting temperatures as a function of crystallization temperatures.
to the lower endotherms (Tm -I) were well aligned and extrapolation of the lines gives values of equilibrium melting temperature, o of 253.6, 213.2, 225.3, 219.1 ◦ C for PA6/LNR blends 100/0, 90/10, Tm o , recorded for the 85,15 and 80/20 respectively. The decrease of Tm PA6/LNR blends suggests that the thermodynamic stability of the PA6 crystals is influenced by the chemical interactions and indicate that the crystals in blends were less perfect than those in pure PA6.
4. Conclusions The crystallization studies were conducted for PA6 and PA6/LNR blends. The degree of crystallinity was found to decrease with an increase in LNR content. Isothermal crystallization studies were carried by DSC at different crystallization temperatures in the range 170–190 ◦ C. At crystallization above 180 ◦ C, the half crystallization time, t1/2 increased exponentially with crystallization temperature confirming that the ordering process occurred through a nucleation mechanism. It was observed that, as the under cooling decrease, the crystallization rate reduces and exotherm peak becomes broader. Thus the induction time of exotherm increases. The isothermal crystallization kinetics of PA6 and PA6/LNR blends was analyzed in terms of Avrami equation and Sestak–Berggren (SB) equation. The Avrami exponent was found to vary from 2.06 to 3.10 in case of neat PA6 and 1.07 to 2.16 in case of PA6/LNR blends. The Avrami model fails to provide a good fit to experimental data obtained at Tc in the range 170 ◦ C to 190 ◦ C. The crystallization rate constant k decreases with increasing Tc . The adequacy of SB equation to explain isothermal crystallization kinetics of PA6 and PA6/LNR blends was found to be satisfactory. The isothermally crystallized samples of PA6 and PA6/LNR blends (except 95/5), exhibited two melting peaks. The melting peak at low temperature is assumed to be due to the melting of crystals formed on cooling from melt. The equilibrium o ) deduced according to Hoffman–Weeks theory melting point (Tm was found to be 253.6, 213.2, 225.3, 219.1 ◦ C for PA6/LNR blends o for 100/0, 90/10, 85,15 and 80/20 respectively. The decrease of Tm the blends suggests that the thermodynamic stability of the PA6 crystals is influenced by the chemical interactions and indicate that the crystals in blends were less perfect than those in pure PA6.
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