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Nonisothermal crystallization kinetics of carbon nanotubes containing segmented polyurethane elastomer.

INTRODUCTION

Segmented polyurethane (SPU) is a thermoplastic elastomer and finds wide spread usage due to its superior thermal and thermo-oxidative stability, flexibility over a broad temperature range, durability and easy processability [1]. Simultaneously, it has also attracted the attention of researchers for development of shape memory composites having excellent fixity and recovery of shape upon application of different external stimuli such as heat, light, and magnetic flux density etc. [2-4]. The high elastic deformation, large recoverable strain, low cost of fabrication, and biocompatibility and biodegradability are other characteristics of SPU making it attractive for shape memory applications [5, 6]. Reinforcement of SPU with different nanosized fillers like ferromagnetic nanoparticles, nanoceramics, metal nanopowders, nanoclays, carbon nanotubes (CNTs) for significant improvement in the mechanical and functional properties have been undertaken [7-11]. However, CNTs are promising due to high aspect ratio, scope of surface modification and retention of aspect ratio during polymer processing [12-16]. Both single wall CNTs and multiwall carbon nanotubes (MWCNTs) have been used to prepare polymer nanocomposites and it has been reported that ultimate performance of the nanocomposites depends on distribution and dispersion of the CNTs along with extent of crystallization [17]. Crystallization in SPU can be carried out under both isothermal and nonisothermal conditions, but, most of the industrial polymer processes like extrusion, injection molding and film blowing follow nonisothermal conditions. Limited articles on crystallization kinetics of polyurethane (PU) under nonisothermal conditions in presence of CNTs are available [18, 19]. However, the results reported are largely incoherent and desultory particularly on the aspect of variation of crystallization enthalpy and extent of crystallization of polymer, for example, Sahoo et al. [19] have reported that crystallization enthalpy and extent of crystallization of CNT based PU nanocomposites are higher than neat sample. However, Jana et al. [18] have observed a decrease of crystallization enthalpy and extent of crystallization in poly([epsilon]-caprolactone) based PU on additions of MWCNTs. In view of the above, further studies are needed to understand crystallization kinetics of hard segment of SPU on inclusion of MWCNTs at various concentrations under non-isothermal conditions which have critical bearing on the final properties of the nanocomposites. To the best of our knowledge, no work is available in the open literature on the nonisothermal crystallization kinetics of SPU on inclusion of MWCNTs.

In this work, crystallization behavior of SPU in presence of varied MWCNTs concentrations has been studied using modulated differential scanning calorimetry (MDSC) technique under nonisothermal mode. Incidentally, in MDSC technique, an additional sinusoidal modulation is imposed over the linearly varying heating profile and this has been found to be very effective to study of the crystallization kinetics in comparison to standard DSC, where results have been reported to vary with choice of test procedures and rate equations applied to calculate the results [20, 21], The MDSC results have been correlated with thermomechanical analysis (TMA) results.

EXPERIMENTAL

Materials Used

The thermoplastic SPU (DP 9386) derived from the polytetramethylene glycol, diphenyl methane diisocynate, and 1,4 butanediol as chain extender was procured from Bayer Material Science, AG, Germany. MWCNTs were procured from Sigma Aldrich, India. The trade literature of MWCNTs shows diameter as 110-170 nm and length varying from 5 to 9 micron. The solvent tetrahydrofuran (THF), analytical grade, was obtained from Qualigens, India. The materials were used as received without any further purification.

Preparation of the PU MWCNTs

Eight percent solution of the SPU in THF was prepared in four round bottom flasks and kept separately; 1,3, and 5 wt% of MWCNTs (with respect to SPU) were separately mixed in THF, sonicated for 15 min and added into separate SPU solutions. The mixtures were stirred for 15 min followed by sonication for another 10 min to make homogenous dispersion of MWCNTs in SPU solution. The mixtures were then poured separately in Petridishes covered with an aluminium foil for evaporation to dryness at room temperature and then kept in vacuum oven at 40[degrees]C till constant weight was obtained. The nanocomposite samples thus prepared were named by prefixing PU before different wt% of CNT, namely, PUCNT1, PUCNT3, and PUCNT5. Neat SPU in fourth flask (without CNTs) was also treated in the same way and designated as PU for the control sample.

Characterization Techniques Used

For morphological and distribution assessment of MWCNTs inside SPU matrix, images were recorded using a low vacuum scanning electron microscope (LVSEM, EV050 of Carl Zeiss, Germany). Surfaces of samples were sputter coated with gold prior to image collection. The nonisothermal crystallization behaviors of the samples were recorded using the MDSC, Q200 of TA Instruments. Approximately 5-10 mg of the samples were heated at a heating rate of 10[degrees]C/min from ambient to 200[degrees]C and kept isothermally for 2 min to remove any thermal history. Thereafter, the samples were cooled from 200[degrees]C to ambient at different cooling rates of 5, 10, 15, and 20[degrees]C/min. The dynamic [N.sub.2] flow of 50 mL/min was maintained during all the experiments. Two different approaches namely modified Avrami and combined Ozawa and Avrami models were applied for the estimation of the different crystallization kinetic parameters. The activation energy of crystallization was calculated using Kissinger method. For heat capacity measurement modulation of [+ or -] 1.00[degrees]C for every 60 s with ramp of 5.00[degrees]C/min was applied to collect the data in MDSC.

TMA was performed in the temperature range of -100 to 40[degrees]C at a heating rate of the 10[degrees]C/min using TMA 2940 of TA Instruments. The force applied was 0.0IN and [N.sub.2] purge flow of 100 mL/min was maintained throughout the experiments. All the systems were calibrated as per standard procedures prescribed by the original equipment manufacturer.

RESULTS AND DISCUSSION

Morphology

The uniform distribution of the MWCNTs inside SPU matrix is a prerequisite for preparation of the high performance nanocomposites. The homogenous dispersion of the nanotubes along with good interaction with matrix can effectively improve the functional properties of the matrix. Figure 1a-d represents the photomicrographs of the neat as well as nanocomposite samples. The bright features in the (Fig. 1b-d) correspond to the nanotubes. It can be visualized from part (a) of the Fig. 1 that surface of the neat sample is smooth while nanotubes are uniformly distributed in nanocomposite samples (Fig. 1b-d). Some agglomerates are also present and relatively more in number in the sample PUCNT5. This is perhaps due to the tendency of the MWCNTs to agglomerate at higher concentration of it inside the matrix resulting in more agglomeration.

Crystallization Behavior

Crystallization phenomena of the SPU and its MWCNTs filled nanocomposites are primarily described by onset temperature of crystallization ([T.sub.o]), crystallization peak temperature ([T.sub.p]), and associated crystallization enthalpy ([DELTA]H) [22]. The analyzed MDSC curves for the samples at a cooling rate of 5[degrees]C/min are presented in the Supporting Information Fig. S1a-d. The overlay MDSC traces for samples PU, PUCNT1, PUCNT3, and PUCNT5 at cooling rate of 5, 10, 15, and 20[degrees]C/min are shown in Fig. 2-a-d. The curves have been shifted along ordinate for better visualization and interpretation of results. The results for [T.sub.o] and [T.sub.p], have been calculated and are detailed in the Table 1. It can clearly be noted from both the Fig. 2 and Table 1 that [T.sub.o] and [T.sub.p] shift to lower temperature with increasing cooling rate for both neat and nanocomposites samples. For instance, [T.sub.p] of neat sample PU at cooling rate of 5[degrees]C/min is about 80.79[degrees]C while for a cooling rate of 20[degrees]C/min it is almost 14[degrees]C lower. Similarly, in case of the nanocomposite sample PUCNT1, the value of [T.sub.p] is reduced by 13[degrees]C by varying the cooling rate from 5[degrees]C/min to 20[degrees]C/min. The effect of cooling rate is more remarkable for [T.sub.o] of the nanocomposite samples, for example, the [T.sub.o] of the neat sample shifts about 12[degrees]C and sample PUCNT1 observed a shift of about 41[degrees]C while varying cooling rate from 5[degrees]C/min to 20[degrees]C/min. The shifts can be attributed to decreasing of the random motion of polymer chains with decreasing temperature during cooling leading to preferred ordering of the polymer chains. Slow cooling rates offer establishment of more effective temperature equilibrium and better fluidity and diffusibility of chain segments enabling formation of crystallites at higher temperature. However, at fast cooling rates molecular chains find it difficult to follow the rapid change in temperature and take more time to form crystal leading to lowering of [T.sub.p]. The effect of inclusion of MWCNTs on the [T.sub.o] and [T.sub.p] of the SPU can be visualized from the Table 1. It is evident that values of [T.sub.o] and [T.sub.p] of nanocomposite samples are higher in comparison to neat sample at a given cooling rate. Besides, [T.sub.p] and [T.sub.p] of the nanocomposite samples show positive temperature shift with increase in MWCNTs concentration, for example, Tp of sample PUCNT5 is almost 24[degrees]C higher than sample PUCNT1. MWCNTs act as heterogeneous nucleating agent to the SPU leading to accelerated crystallization resulting in increase of [T.sub.o] and [T.sub.p]. MWCNTs allowed polymer chains to align quickly by lowering energy required to reach critical stability of crystal growth [19, 23]. Among different nanocomposite samples the positive shift in [T.sub.o] and [T.sub.p] of sample PUCNT5 is highest suggesting that higher concentration of the MWCNTs has provided more nucleating sites leading to more effective heterogeneous nucleation. Another interesting feature can be observed from Fig. 2a-d where a peak shoulder is present in MDSC cooling traces of the nanocomposite samples and appearance of this shoulder becomes more remarkable at higher MWCNTs concentration. This is perhaps due to the formation of a separate phase from SPU-MWCNT chain segments. Similar observations have been reported for poly caprolactone MWCNT system [18]. A transcrystalline phase with one lamellar thickness and a spherulitic phase with a second lamellar thickness may be the reason for the formation of the second phase. The crystallization enthalpies [DELTA]H of neat and nanocomposite samples have been calculated from Supporting Information Fig. S1a-d and results are presented in Table 2. The value of [DELTA]H is highest for PUCNT5 followed by PUCNT3, PUCNT1, and neat PU samples. This again indicates nucleating action of MWCNTs and enhancement of the crystallization magnitude with increasing MWCNTs concentration. Thus, these results are in agreement with observations of Sahoo et al. [19].

Nonisothermal Crystallization Kinetics

The relative degree of crystallinity ([X.sub.T]) as a function of the crystallization temperature T during nonisothermal cooling has been defined by Eq. 1 [24].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where [T.sub.o] and [T.sub.[infinity]] represent the onset and completion temperature of the crystallization, respectively. The numerator represents crystallization up to temperature T while denominator corresponds to total crystallization. The crystallization time t can be estimated easily by incorporating the cooling rate using the equation,

t = ([T.sub.0]-T)/[PSI] (2)

where, T stands for the temperature of crystallization at time t and [PSI] is the cooling rate.

Figure 3a-d illustrates the development of the crystallinity as a function of temperature for virgin sample PU and nanocomposites at different cooling rates. It can be noted that all these curves have the same type of the sigmoidal shape showing the lag effect of cooling rate on the crystallization. The formation of a platue in the upper part of all these curves is due to spherulite impingement and attainment of a saturation stage by progressive crystallization process.

The horizontal temperature axes in Fig. 3a-d have been converted into time domain using the Eq. 2 and resulting graphs are presented in the Fig. 4a-d. It can be inferred from the Fig. 4a-d that higher cooling rates need shorter time to reach saturation of crystallization indicating that the increase in crystallization rate is governed by an increase in the nucleating rate rather than crystal growth rate [25]. It is also evident that for a particular cooling rate, the time required to complete the crystallization is more for the nanocomposite samples than neat sample indicating slow rate of crystallization of the SPU in presence of MWCNTs. In fact, MWCNTs act as physical hindrance to the crystallization due to adsorption of the SPU chains on the MWCNTs surfaces resulting in increase of viscosity and consequently decreasing transport of the polymer chain segments to crystal growth front. Such type of adsorption due to derealization and hybridization of [PI] electrons between CNT and polymers containing aromatic rings (incidentally, SPU contains aromatic rings) has already been reported [26], In addition to above, nanofiller would reduce the diffusion rate by acting as physical barrier. Therefore, it can be stated that MWCNTs simultaneously play two inconsistent roles. In one hand, it acts as heterogeneous nucleating agent facilitating crystallization while, however, as a physical hindrance withholding the growth rate [27]. Modified Avrami analysis has, therefore, been applied to understand the dual roles of the MWCNTs on SPU crystallization.

Modified Avrami Analysis. The basic Avrami equation is written as,

1-[X.sub.t] = exp(-[Z.sub.t] [t.sup.n]) (3)

where [X.sub.t] is the relative degree of crystallinity at a particular time t, exponent n is a constant called Avrami exponent and depends on growing mechanism and growth geometry and [Z.sub.t] is a rate constant involving growth rate. This equation was originally used to describe the crystallization behavior under isothermal situation. However, Eq. 3 can also effectively be applied to describe the initial stage of crystallization under nonisothermal conditions [25].

The double logarithm of the Eq. 3 yields,

ln(-ln(1 - [X.sub.t])) = n ln t + ln [Z.sub.t] (4)

Two adjustable parameters n and [Z.sub.t] can be estimated from the slope and intercept of the curve of ln(-ln(1 -[X.sub.t])) versus In t for each cooling rate by fitting a line to the experimental data. Since, in nonisothermal crystallization process temperature changes constantly affecting temperature dependent rates of both nuclei formation and the spherulite growth, therefore, n and [Z.sub.t] do not have the same physical significance as applicable to isothermal crystallization. Due to cooling rate dependence of nonisothermal crystallization process, the parameter [Z.sub.t] has been corrected to evaluate the corresponding rate constant at unit cooling rate ([Z.sub.c]) as follows,

ln [Z.sub.c] = In [Z.sub.t]/[phi] (5)

The logarithm graphs of ln(-ln(l -[X.sub.t])) versus In t following Eq. 4 for neat sample at different cooling rates are depicted in the Fig. 5a-d. The corresponding graphs for nanocomposite samples are presented in the Supporting Information Figs. S2a-S4d. An inspection of the Figs. 5 and Supporting Information S2-S4 reveals that curves do not exhibit a linear relationship for entire range of the crystallization and display two rate gradients. One corresponds to initial stage of crystallization while other corresponds to the secondary stage of crystallization or crystal perfection. These two velocity gradients are manifestation of spherulites expansion in outward direction with constant rate until they experience hindrance at the intersection of the crystals. Thus, modified Avrami equation does not effectively describe the nonisothermal crystallization for entire range of crystallization of SPU or CNTs based nanocomposites. It is more applicable for the primary stage of the crystallization; therefore, the data for the initial linear range have been selected for the calculation of the Avrami parameters. The results obtained from the linear regression of the plots in Figs. 5 and Supporting Information S2-S4 along with regression coefficient (R) have been listed in Table 3. The values of R indicate good fitting in the selected range of crystallization. The Avrami exponent n has values ~3 for neat and nanocomposite samples suggesting athermal nucleation process followed by three dimensional diffusion control growth. This observation is consistent with the earlier published works [28-30], The fractional values of Avrami exponent might be due to varied crystallite size and mixed growth and nucleation mechanism [18]. Moreover, as stated earlier, the crystallization peak temperature [T.sub.p] shifts to lower temperature while cooling rates are increased. Hence, the lower crystallization temperature would be accompanied by lower n value as per Table 3. Similar type of observations have also been made earlier for crystallization of poly(ethylene2, 6-naphthalate) induced by modified CNT and alumina particle filled polyether ether ketone [31, 32]. The variation of n values of nanocomposites with MWCNTs concentration is ascribed to the complicated crystallization mechanism and heterogeneous nucleation of SPU in presence of MWCNTs [33], As shown in Table 3 for all the samples the values of [Z.sub.c] increase with increasing cooling rate from 5[degrees]C/min to 20[degrees]C/min. This is in accordance with the fact that higher cooling rates lead to higher crystallization rate. Further, [Z.sub.c] value of the nanocomposites is always lower than neat PU for a particular cooling rate and this reduction is more remarkable at lower cooling rates. As concentration of MWCNTs increases from 1 to 5% the value of [Z.sub.c] goes downward (Table 3) probably due the fact that higher concentration of MWCNTs creates more physical barrier to the chain segments. This again supports the confinement effect generated by the presence of MWCNTs [18, 26].

Combined Ozawa and Avrami Analysis. Since modified Avrami model could not describe the nonisothermal crystallization process of SPU and nanocomposites for complete range of crystallization, it is worthwhile to explore Ozawa extension of the Avrami equation. This treatment is based on the assumption that nonisothermal crystallization process consists of infinitesimally small isothermal crystallization steps [34].

The basic equation in Ozawa treatment is as follows,

1 - [X.sub.t] = exp (-K(T)/[[phi].sup.m]) (6)

where K(T) is a function depending on the overall crystallization rate, [phi] is the cooling rate, and m is the Ozawa exponent relating to the dimension of the crystal growth.

Double logarithmic of Eq. 6 yields,

ln (-ln(1 - [X.sub.t])) = ln K(T) - m ln [phi] (7)

The plot of ln(-ln (1 - [X.sub.t])) versus ln [phi] at a given temperature, should yield a straight line for the validation of the Ozawa treatment. The slope of the line gives the value of m while intercept is used to estimate K(T). Ozawa theory, however, does not consider the slow secondary crystallization phenomenon and dependence of folded chain length on the temperature [35]. In this regard, Lee and Cakmak [36] and Kim et al. [24] have shown that Ozawa's analysis alone is not sufficient to describe the nonisothermal crystallization of polymers. Liu et al. [37], in turn, proposed a combined Ozawa-Avrami equation for the nonisothermal crystallization process relating physical variables [X.sub.t], cooling rate [phi] and crystallization temperature T. The combined form of both Ozawa and Avrami equations can be written as,

ln [Z.sub.t] + n ln t = ln K(T) - m ln[phi] (8)

The Eq. 8 can be rearranged at a given crystallinity [X.sub.t] as,

ln [phi] = ln F(T) - a ln t (9)

where, F(T) = [[K(T)/[Z.sub.t]].sup.1/m] represents required value of cooling rate to attain certain degree of crystallinity within unit crystallization time and a = n/m is defined as ratio of Avrami exponent n to the Ozawa exponent m. In this way, the physical and practical meaning of the F(T) is similar to that of [Z.sub.t] in isothermal case. The combined Ozawa-Avrami equation relates the cooling rate [phi] to the crystallization time t under dynamic crystallization condition for a selected degree of crystallization. Accordingly, at a particular degree of crystallinity plot of ln [phi] versus ln t should yield a linear relationship for the applicability of the combined model.

The traces of the ln [phi] versus ln t are represented in Fig. 6a-d. Four different percent of crystallization, for example, 20, 40, 60, and 80% have been selected for the calculation of kinetic parameters. The kinetic parameters F(T) and a have been determined from the intercept and slope of the lines along with regression coefficient and results are summarized in Table 4. All the plots show linear relationship between ln [PHI] and ln t in the selected range of crystallinity. It is evident from the regression coefficient values that combined Ozawa-Avrami model fits well to the nonisothermal crystallization process of SPU and its MWCNTs based nanocomposites. The results in Table 4 indicate that values of F(T) systematically increase with increasing of relative crystallinity X, suggesting that at unit crystallization time, higher degree of crystallinity would be obtained if cooling rate is higher. This is in accordance with the earlier observation in modified Avrami analysis, which shows shorter time for higher cooling rates to reach the same degree of crystallinity. Considering the effect of the inclusion of the MWCNTs, for a certain relative degree of crystallinity, F{T) for nanocomposites is higher than neat sample PU and this value increases (Table 4) with concentration of the MWCNTs. This means for same relative degree of crystallinity, nanocomposites require higher cooling rate in comparison to neat sample. Therefore, the crystallization rate of nanocomposites is impeded compared with that of pristine sample and, in turn, consistent with the observations made on the [Z.sub.c] in modified Avrami analysis. Similar observations have also been made earlier on other polymers [38]. The values of parameter a and its variation with cooling rates and MWCNTs content reveal that mechanism of SPU crystallization is affected by both these variables and effect of the MWCNTs inclusion is more pronounced than cooling rate. This observation is consistent with the earlier results on [T.sub.p], where variation in both cooling rates and MWCNTs concentration affected it in the similar way as observed here. The combined Ozawa-Avrami approach, therefore, appears to be more successful in describing higher degree of crystallinity under nonisothermal crystallization process of SPU and its MWCNT nanocomposites.

Calculation of the Activation Energy

Considering the dynamic temperature variation in nonisothermal crystallization process a method based on the variation of crystallization peak temperature as a function of cooling rate was proposed by Kissinger using following equation [39],

d[ln ([psi]/[T.sup.2.sub.p])]/d(1/[T.sub.p]) = -[DELTA]E/R (10)

where R is the universal gas constant and [DELTA]E is the activation energy of the crystallization with other terms having usual meanings as defined earlier. The activation energy of the crystallization process can be determined from the slope of the plot of ln ([psi] [T.sup.-2.sub.p]) versus ([T.sup.-1.sub.p]) as per Eq. 10 and the corresponding plots for all samples are shown in Fig. 7. The activation energy has been calculated and results are listed in Table 2. [DELTA]E values of nanocomposites are found to be lower than neat PU and depend on the MWCNT loading (lowest value for sample PUCNT5). Higher content of MWCNTs in nanocomposites results in more nucleating sites and higher extent of effective heterogeneous nucleation [37, 40]. Wu and Chen have also reported on extensive decrease of activation energy for poly([epsilon]-caprolactone) and CNT nanocomposites and suggested that presence of CNT into the polycaprolactone matrix induced more heterogeneous nucleation during crystallization processes leading to reduction in [DELTA]E [41]. The trend of activation energy variation reported here is also similar to observations made earlier by Jana and Cho [18].

Thermomechanical Analysis

Figure 8 shows TMA plots of dimensional change versus temperature for all the samples. The glass transition temperatures ([T.sub.g]) have been evaluated and presented in Table 2. The glass transition temperature shows positive temperature shift on inclusion of the MWCNTs in SPU and shift is about 20[degrees]C in case of sample PUCNT5 (5% MWCNTs). The enhancement in [T.sub.g] on addition of MWCNTs is owing to (1) high modulus of CNTs, (2) restraining of SPU, and (3) mixing of the hard segment into soft segment. The heterogeneous nucleation generated by the MWCNTs during crystallization also gives rise to material hardness. Since SPU has aromatic rings, therefore, the interaction between SPU and MWCNT via 11-11 stacking causes shifting of [T.sub.g] [42, 43].

Measurement of Heat Capacity

Figure 9 illustrates the temperature dependence of heat capacity in the temperature range of 30 to 180[degrees]C at a cooling rate of 10[degrees]C/min. It is evident that measured heat capacity values of nanocomposites are lower than neat PU sample and follow the trend PU > PUCNT1 > PUCNT3 > PUCNT5. The heat capacity of the MWCNTs is much lower than the PU resulting in the reduction of the heat capacity of the nanocomposite samples and this decrease depends on the content of the MWCNTs. MWCNTs facilitate easy transfer of heat due to formation of heat conductive pathways in nanocomposite samples. The peaks observed for neat SPU and SPU-MWCNT nanocomposites as shown in the Fig. 9 are in accordance to the crystallization pattern reported in Table 1.

CONCLUSIONS

Based on crystallization kinetic studies of the composites of SPU with MWCNTs, it has been observed that the onset and peak temperatures of crystallization of SPU show positive temperature shift due to heterogeneous nucleation process generated by the inclusion of MWCNTs. The extent of crystallization is increased with increasing concentration of MWCNTs. Modified Avrami model has been found to be effective in describing the nonisothermal crystallization only for the initial range of the crystallization. Combined Ozawa-Avrami method has been found to be more effective in describing the nonisothermal crystallization kinetics of SPU and its nanocomposites for the whole range of crystallization. The crystallization activation energy of the nanocomposites is reduced compared with the neat SPU due to heterogeneous nucleating action offered by CNTs. Glass transition temperature of SPU shows positive shift due to addition of the MWCNTs and magnitude of the shift depends on weight fraction of the CNTs.

ACKNOWLEDGMENTS

The authors are thankful to Director, DMSRDE, Kanpur, for kind permission to forward the article for publication. Thanks are also due to Prof. Sangeeta Kale and Prof. S. Datar of DIAT, Pune for fruitful discussions and helpful suggestions. Dr. Amit Singh of DMSRDE, Kanpur is acknowledged for assistance in sample preparation.

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Y.N. Gupta, (1) T. Bhave, (2) A. Chakraborty, (1) A.K. Pandey, (1) R.B. SHARMA, (2) D.K. Setua (1)

(1) Defence Materials and Stores Research and Development Establishment (DMSRDE) PO, Kanpur 208013, Uttar Pradesh, India

(2) Defence Institute of Advanced Technology (DIAT), Pune 411025, Maharashtra, India

Correspondence to: D.K. Setua; e-mail: dksetua@rediffmail.com

Additional Supporting Information may be found in the online version of this article.

DOI 10.1002/pen.24358

TABLE 1. Onset and peak temperatures of samples at different cooling
rates.

                                       Sample

                                         PU
Cooling rate
([degrees]C/min)   [T.sub.O] ([degrees]C)   [T.sub.P] ([degrees]C)

5                          91.93                    80.79
10                         85.15                    72.01
15                         83.16                    68.62
20                         80.55                    67.15

                                       Sample

                                       PUCNT1
Cooling rate
([degrees]C/min)   [T.sub.O] ([degrees]C)   [T.sub.P] ([degrees]C)

5                          126.16                   80.87
10                          97.04                   74.70
15                          92.71                   68.62
20                          85.08                   66.35

                                       Sample

                                       PUCNT3
Cooling rate
([degrees]C/min)   [T.sub.O] ([degrees]C)   [T.sub.P] ([degrees]C)

5                          128.19                   91.21
10                         123.36                   73.86
15                         104.14                   69.39
20                          92.69                   65.37

                                       Sample

                                       PUCNT5
Cooling rate
([degrees]C/min)   [T.sub.O] ([degrees]C)   [T.sub.P] ([degrees]C)

5                          130.02                   104.04
10                         126.81                    72.70
15                         120.38                    68.84
20                          96.97                    63.23

[T.sub.O]: Onset temperature of crystallization ([degrees]C).

[T.sub.P]: Peak temperature of crystallization ([degrees]C).

TABLE 2. Heat enthalpy, glass transition, and activation energy
values.

              Heat                 Glass               Activation
Sample   enthalpy (J/g)   transition ([degrees]C)   energy (KJ/mole)

PU            5.18                -48.56                 101.02
PUCNT1        6.34                -40.94                  94.86
PUCNT3        6.58                -30.03                  61.49
PUCNT5        6.93                -28.50                  43.10

TABLE 3. Modified Avrami crystallization kinetic parameters.

             Cooling
               rate
Sample   ([degrees]C/min)     n     [Z.sub.c]     R

PU               5          2.893     0.506     0.9945
                10          2.716     0.808     0.9972
                15          2.366     0.861     0.9980
                20          2.150     0.902     0.9983

PUCNT1           5          3.065     0.268     0.9997
                10          2.878     0.595     0.9960
                15          2.550     0.789     0.9983
                20          2.389     0.863     0.9986

PUCNT3           5          3.131     0.263     0.9975
                10          2.394     0.592     0.9997
                15          2.975     0.751     0.9991
                20          2.605     0.833     0.9978

PUCNT5           5          3.105     0.236     0.9997
                10          2.977     0.575     0.9996
                15          2.583     0.742     0.9985
                20          2.447     0.823     0.9987

TABLE 4. Combined Ozawa-Avrami crystallization kinetic parameters.

Sample   [X.sub.t] (%)     a     F (T)     R

PU            20         1.433   2.895   0.9879
              40         1.429   3.190   0.9860
              60         1.416   3.402   0.9855
              80         1.421   3.825   0.9774

PUCNT1        20         1.243   3.706   0.9944
              40         1.347   4.248   0.9960
              60         1.381   4.593   0.9951
              80         1.274   4.635   0.9986

PUCNT3        20         1.275   3.709   0.9798
              40         1.361   4.387   0.9860
              60         1.291   4.614   0.9884
              80         1.232   4.674   0.9926

PUCNT5        20         1.238   3.851   0.9947
              40         1.347   4.446   0.9972
              60         1.345   4.683   0.9965
              80         1.272   4.678   0.9948


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Article Details
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Author:Gupta, Y.N.; Bhave, T.; Chakraborty, A.; Pandey, A.K.; Sharma, R.B.; Setua, D.K.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Nov 1, 2016
Words:5678
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