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Effect of Interfacial Chemistry on Crystallization of Polypropylene/Multiwall Carbon Nanotube Nanocomposites.

INTRODUCTION

Polypropylene (PP) is a semicrystalline thermoplastic material that has been widely studied because of its attractive combination of good processability, mechanical properties, and chemical resistance. Over the years, PP has been reinforced with various types of fillers to improve its physical and mechanical characteristics. Among these fillers, carbon nanotubes (CNTs) have captured a great amount of attention due to their outstanding tensile modulus (270-950 GPa) [1], tensile strength (ll-63GPa) [1], thermal (200-3,000 W/m/K at 300 K) [2-4] and electrical conductivity ( [10.sup.2]-[10.sup.7] S/m at 300 K) [5], high aspect ratio [6], and role as viscosity modifier [7], CNTs also act as nucleating agent for polymer crystallization [8-14] and have the potential for templating polymer interphase including ordered graphitic structure [15-17], as well as nanohybrid shish kebab [18, 19] and transcrystalline structures [20-22].

In the semicrystalline polymer composites, crystallization can have a major influence on the structure and morphology of the derived composites and thereby on properties like thermal and gas barrier as well as the mechanical properties, for example, tensile, and impact strength. Also, from the manufacturing point of view, change of crystallization kinetics brings about the necessity of adjusting processing parameters during thermoforming, molding and fiber spinning, in order to minimize and preferably eliminate warpage and dimensional instabilities. For reasons outlined above, it is important to understand the crystallization phenomena.

In general, there are two mutually opposite effects of fillers on the crystallization behavior, namely: heterogeneous nucleation ability and crystal growth retardation, both of which are related to the filler concentration and dispersion quality. Various approaches have been adopted to modify the surface chemistry of fillers through polymer grafting or chemical functionalization in order to improve their dispersion quality [23-38]. The effect of these surface modifications on the crystallization and melting behavior as compared with their unmodified counterparts is summarized in Tables 1 and 2. Various types of nanofillers include CNT, reduced graphene oxide (rGO), cellulose nanocrystal (CNC), montmorillonite (MMT), all result in up to 97% reduction in crystallization half-time ([t.sub.1/2]) or up to 18[degrees]C increase in crystallization temperature ([T.sub.c]). However, the equilibrium melting temperature ([T.sub.m.sup.0]) and melting peak maximum ([T.sub.p]) of the resulting composites either decrease or show a small increase. Such observations demonstrate that the incorporation of the fillers in the composites, in most of the cases, increases the crystallization rate of the polymer, but has little or even negative influence on the crystalline structure. For example, the silane group grafted on MWNT improved its dispersion in the matrix but has a negative effect on the nucleation of polymers. While the reduction of [t.sub.1/2] is similar between the silane-grafted MWNT and p-MWNT containing composites, more dramatic decrease of [T.sub.m.sup.0] was found in the former which brought about 32[degrees]C of reduction with 1 wt% MWNT loading [24].

In this work, the influence of three types of polymer/MWNT interfaces on the crystallization and melting behavior of the PP/MWNT nanocomposites was investigated. Through solution processing, PP or maleic anhydride-grafted-PP (MA-g-PP) was successfully coated noncovalently onto f-MWNT. The resulting PP/f-MWNT, MA-g-PP/f-MWNT master batches, or untreated pMWNT were melt microcompounded with PP. In the first part of this article, dispersion quality, isothermal, and nonisothermal crystallization studies of the three different types of nanocomposites at various MWNT concentrations are discussed. In the second part, crystal refinement and perfection as well as crystal size growth are realized through polymer self-seeding and templated growth. The proposed mechanism of the induced columnar crystalline interphase formation is also discussed.

EXPERIMENTAL

Materials

Homopolymer PP (MFR 10 g/10 min) was supplied by SABIC. Multiwall CNT (MWNT)-grade SMW200 were purchased from Southwest Nanotechnology, Norman, OK. Average number of walls (9-10) and average diameter (12 [+ or -] 3 nm) were determined from FWHM of 20-25.8[degrees] integrated peak from wide-angle X-ray diffraction (WAXD) and SEM, respectively. CNT impurity content was 2.3 wt% based on thermogravimetric analysis (TGA) in air. MA-g-PP (Epolene E-43) was obtained from Westlake Chemical Corporation, Longview, TX. Molecular weight of MA-g-PP was 9,100 g/mol. Butanol, xylene, and nitric acid were purchased from Sigma Aldrich and were used as received.

Manufacture of Nanocomposites

PP/MWNT nanocomposites were produced using master batch approach. A detailed experimental procedure for the preparation of master batch can be found in our earlier contribution [39]. The process is briefly summarized here. As-received SMW200 MWNTs were homogenized in deionized water at a concentration of 150 mg/dL for 20 min at 7,000 rpm. Then, 70% nitric acid was added to the mixture to make a final acid concentration of 10 M (40 mg/dL). MWNT-acid slurry (40 mg/dL) was sonicated for 30 min using Branson bath sonicator 3510R-MT (100 W, 42 kHz) maintained at 25[degrees]C-30[degrees]C. The sonicated dispersion was refiuxed at 120[degrees]C for 24 h. The resulting suspension was repeatedly centrifuged using deionized water until the pH of the suspension reached in the range of 6-7. Then, the final centrifugation cycle was carried out using butanol and the suspension was filtered using repeated butanol wash. The filtered f-MWNT were dispersed at a concentration of 5 mg/dL in butanol using bath sonication for 48 h.

MA-g-PP or PP was dissolved in xylene (190 mg/dL) at 120[degrees]C. The MA-g-PP or PP solution was then added drop by drop to the f-MWNT/butanol (5 mg/dL) dispersion kept at 55[degrees]C60[degrees]C. The ratio of butanol to xylene in the final mixture was 2:1. This dispersion, maintained at 55[degrees]C-60[degrees]C and kept under continuous stirring, was dried under vacuum at -100 mbar for approximately 48 h to obtain master batch containing 5 wt% f-MWNT in PP or in MA-g-PP. These are referred to as PP/f-MWNT and MA-g-PP/f-MWNT master batch in this work.

Master batches of MA-g-PP/f-MWNT and PP/f-MWNT were diluted using homopolymer PP to prepare the corresponding nanocomposites. As-received PP powder was dried in vacuum oven at 80[degrees]C for about 4 h prior to its use in melt processing. The CNT concentrations in the nanocomposites were 0.005, 0.01, 0.1, 0.3, 0.5, and 1 wt%. Mixing of master batch powder with PP powder was carried out manually using mortar and pestle. Samples with above CNT concentrations were also prepared from pristine CNTs (referred to as p-MWNT) by manually mixing using a mortar and pestle with the as-received PP. Various physical mixtures were melt blended using 15 cc microcompounder (Xplore Instruments, Netherlands). Compounding conditions for all the samples were kept the same. Temperatures of three heating zones of the barrel were set at 185[degrees]C, 215[degrees]C, and 215[degrees]C (Fig. S2 in our earlier contribution [40]). Melt temperature was recorded by a thermocouple located after the third heating zone and before the die, and in all cases it was recorded as 200[degrees]C. Microcompounder was operated in recirculation mode for 3 min at 200 rpm.

Characterization

For the isothermal crystallization study (using TA Instrument DSC Q100), samples were heated to 220[degrees]C at a heating rate of 10[degrees]C/min and held at 220[degrees]C for 5 min. These samples were then cooled at a rate of 100[degrees]C/min to 135[degrees]C and then held at that temperature for crystallization. The crystallization half-time, tU2 is defined as the time at which the extent of crystallization is 50% of total crystallization (X,). The crystallization half-time can be determined from the total crystallinity [41].

[X.sub.t] = [[integral].sup.t.sub.0] (d[H.sub.c]/dt)dt/ [[integral].sup.[infinity].sub.0] (d[H.sub.c]/dt)dt. (1)

Here, t is the crystallization time and d[H.sub.c]/dt is the heat evolution rate during the crystallization process. For nonisothermal crystallization study, samples were heated from room temperature to 220[degrees]C at 2.5[degrees]C/min and then cooled at the same rate to room temperature and then heated again to 220[degrees]C. Crystallization temperatures ([T.sub.c]) were derived from the first cooling cycle. Melting peak maximum ([T.sub.p]) and all the melt endotherms presented in this work, if not specified, are from the second heating cycle.

SEM was done using ZEISS Ultra 60 FE-SEM at an accelerating voltage of 2 kV. Polarized optical microscope (Leica, DM 2500P) equipped with Linkam LTS420 heating stage was used to study the crystallization behavior. For this purpose, thin pieces of samples were heated on a microscope glass cover slip covered with another cover slip. Samples were heated to 225[degrees]C for 5 min and then cooled to 135[degrees]C at a rate of 20[degrees]C/min, and held at this temperature for monitoring crystallization behavior over a period of time. WAXD was performed using Rigaku MicroMax-002 beam generator (Cu K[alpha] [lambda] = 0.1542 nm, operating voltage and current 45 kV and 0.65 mA, respectively) equipped with R-axis IV++ detector.

RESULTS AND DISCUSSION

Figure 1 and Fig. SI provide a series of optical and scanning electron microscopy (SEM) micrographs of PP/MWNT nanocomposites prepared via two types of f-MWNT-based master batches and from p-MWNT at different MWNT concentrations. MA-g-PP/f-MWNT master batch containing samples showed the most homogeneous dispersion of MNWT through the matrix, indicating that the incorporation of MA-g-PP as a compatibilizer significantly improved the CNT dispersion quality. On the other hand, p-MWNT formed aggregates of up to tens of micrometers even at concentration as low as 0.01 wt % (Fig. SI). Aggregates of much smaller size were found in the PP/f-MWNT master batch-based samples showing an intermediate level of dispersion quality among the three types of nanocomposites.

In our previous study [40], [beta] crystals were observed in MA-g-PP containing injection molded sample, but this was not observed in the control PP, p-MWNT, and PP/f-MWNT-based samples. Even in the case of the MA-g-PP, after removing the processing history (first heating cycle in differential scanning calorimetry [DSC]), only melting corresponding to [alpha]-phase was observed which led to the conclusion that the presence of [beta] crystals was a joint effect of processing parameters (i.e. shear and temperature) and the incorporation of MA-g-PP. In this study, the sample processing history was always removed by the first heating cycle imposed on the sample in DSC and the second melting endotherm was used to study the effect of interfacial chemistry on crystallization in three different systems.

In order to understand the effect of different PP/MWNT interfaces on the crystallization behavior of the nanocomposites, isothermal crystallization studies were carried out at various temperatures. Crystallization half-time ([t.sub.1/2]) for neat PP was much longer at high temperature, for example, [t.sub.1/2] is 1.25 min at 122.5[degrees]C and 30.1 min at 132.5[degrees]C. With 0.01 wt% loading of MWNT, [t.sub.1/2] at 132.5[degrees]C decreased to 18.5, 9.6, and 4.6 min for MA-g-PP/f-MWNT, PP/f-MWNT, and p-MWNT containing samples, respectively. Further increase in crystallization rate was observed at higher MWNT loading (Fig. 2a-c and Tables S1-S3). The most pronounced improvement on [t.sub.1/2] was found for the p-MWNT-based nanocomposites despite their relatively poor dispersion, followed by PP/f-MWNT and then the MA-g-PP/f-MWNT master batch-based samples. Crystallization half-time of p-MWNT-based nanocomposites was lower than the f-MWNT-based samples suggesting a better nucleation ability of p-MWNT than f-MWNT. [t.sub.1/2] of p-MWNT containing nanocomposites then progressively decreased as MWNT concentration increased to 1 wt %. On the other hand, the nucleation effect of the MA-g-PP/f-MWNT-based sample leveled off above 0.1 wt%. Functional groups on the MWNT surface are considered obstacles that disturb polymer chain folding during crystallization [42], Also, the presence of less crystallizable MA-g-PP at the interface between f-MWNT and matrix PP hindered the crystallization of PP [39]. Among the three investigated systems, PP/f-MWNT master batch containing nanocomposites obtained the balance between dispersion quality and crystallization rate. While the preservation of pristine graphitic structure is important, the amount of MWNT surfaces available as heterogeneous nucleating sites plays an important role on the crystallization behavior. As a consequence, at the intermediate MWNT loading level of 0.1 and 0.3 wt%, [t.sub.1/2] values were comparable between PP/f-MWNT and p-MWNT-based nanocomposites, where improved MWNT dispersion compensated for the negative effect of MWNT functional groups on crystallization in PP/f-MWNT system. However, when the concentration further increased to 1 wt%, [t.sub.1/2] was 3.83 and 0.9 min for PP/f-MWNT and p-MWNT-based nanocomposite at 132.5[degrees]C, respectively. This further confirms the importance of pristine graphitic surfaces for PP crystallization nucleation and growth. Unlike p-MWNT-based nanocomposites, where continuous decrease of [t.sub.1/2] was observed with increasing MWNT loading, [t.sub.1/2] of the PP/f-MWNT-based sample reached a minimum at 0.3 wt% (crystallization rate reached its maximum) and went up at 1 wt%. This implies some degree of nucleation saturation [35]. More importantly, while the nucleation density did increase with increased f-MWNT loading, the reduction of polymer mobility became a counter force on accelerating crystallization process. The reason that the increase in tm at higher filler loading was observed in PP/f-MWNT but not in p-MWNT-based sample could be as follows. First, the geometrical confinement, or the average MWNTMWNT distance, that restrained the polymer diffusion is stronger in the former due to the better MWNT dispersion. Second, the functional groups on f-MWNT prevent them from being effective nucleating agents. As a result, the cost of introducing more fillers in the system at high f-MWNT loading exceeds the benefit leading to an increase of [t.sub.1/2].

The crystallization kinetics of the nanocomposites under isothermal conditions were analyzed by using Avrami equation assuming constant nucleation rate of nuclei that undergo free growth [26, 43]. The general form of the equation is:

1-[X.sub.t] = exp {-[kt.sup.n]) (2)

Taking the logarithm on both sides, Eq. 2 is written as:

log[-ln(1-[X.sub.t])] = n log t + log k (3)

where [X.sub.t] is the relative crystalline volume fraction at crystallization time t, and defined by Eq. 1. n is Avrami index and it is a complex exponent which is related to the dimensionality of the growing crystals and time dependence of nucleation [43, 44]; k is the Avrami rate constant or crystallization rate constant involving both nucleation and growth process. Eq. 3 is used to generate the so-called Avrami plots shown in Fig. S2. The fitting was performed not considering the deviations from linearity due to the onset of secondary crystallization at longer t that cannot be described by Avrami equation [25, 45]. The reported n and k values are summarized in S1-S3. An alternative way to calculate k from [t.sub.1/2] (designated as k' in Tables S1-S3) is given as [25]:

k' = ln2/[t.sup.n.sub.1/2] (4)

The values of k obtained by two different methods are in good agreement with each other (Tables S1-S3). The overall crystallization rate k decreased greatly with increasing crystallization temperature, and it was higher for the nanocomposite samples than for the neat PP sample which is consistent with the tU2 observations. At 0.01 wt% MWNT concentration, k showed no significant difference from neat PP for MA-g-PP/f-MWNT-based nanocomposite while it increased by roughly 5 and 20 times for PP/f-MWNT and p-MWNT-based nanocomposite, respectively, within the investigated temperature range (125[degrees]C-135[degrees]C). At higher MWNT concentration of 0.3-1 wt%, there was about five times higher k compared with the neat PP for MA-g-PP/f-MWNT-based sample and 50, and 200 times for PP/f-MWNT-based and p-MWNT-based nanocomposites, respectively.

The Avrami index n is in between 2 and 3 for both neat PP and PP/f-MWNT nanocomposites (Tables S1-S3, Fig. 2d) which is similar to the reported values of isotactic PP and their nanocomposites in the literature [23-25, 27, 29, 37, 46]. For polymers, the ideal Avrami index is expected to be 3 and 4 for heterogeneous and homogenous nucleation, respectively, for the three-dimensional growth [26, 43]. Zhao et al. [26] commented that the experimental value of n might be smaller depending on the experimental conditions. Similar reason for this discrepancy was put forward by Lorenzo et al. [43] who stressed that the insufficient cooling rate to the crystallization temperature can potentially lead to an experimental error.

The activation energy of the crystallization, [DELTA]E can be derived from the following equation [47]:

1/n (ln k) = [A.sub.0] - [DELTA]E/RT (5)

The negative [DELTA]E (Tables S1-S3, Fig. 2e) is because that the isothermal crystallization studies were done where the lower crystallization temperature has higher crystallization rate. Difference of [DELTA]E between the PP/f-MWNT master batch containing samples and that of neat PP was relatively moderate which reached a plateau value. This is again due to the nucleation saturation and growth retardation in the f-MWNT containing nanocomposites at high f-MWNT loading. MA-g-PP/f-MWNT master batch was the most ineffective nucleating agent among the three types of nanocomposites in the sense that [DELTA]E remained at the similar level to that of the neat PP. In p-MWNT containing nanocomposites, the absolute value of [DELTA]E decreased substantially as MWNT concentration increased. This suggests that the addition of p-MWNT into the PP matrix causes more heterogeneous nucleation and thereby results in less temperature dependence of crystallization rate [28, 47] than f-MWNT.

According to Hoffman nucleation model [48], the melting temperature of the polymer crystal is determined by its lamella thickness which is inversely proportioned to the supercooling below equilibrium melting point [T.sub.m.sup.0]. That is, the crystal formed at higher temperature (less super cooling, [DELTA]T = [T.sub.m.sup.0]-[T.sub.c]) should have larger lamella thickness and thereby higher melting point [48]. The addition of MWNTs provides interface for heterogeneous nucleation that allowed crystals to grow at higher crystallization temperature (higher [T.sub.c]) due to the reduced thermodynamic driving force needed. In Table 3, the increase in [T.sub.c] in the p-MWNT containing nanocomposite was the most prominent among the three types of nanocomposites which is consistent with the observation from the isothermal crystallization experiments. The melting peak maximum of the nanocomposite samples in most cases showed a positive correlation to [T.sub.c] as expected (Fig. 3). The exception occurred at the MA-g-PP/f-MWNT master batch containing samples at f-MWNT loading above 0.3 wt% where the influence of less crystallizable MA-g-PP at the interface became noticeable. As for the PP/f-MWNT master batch containing sample, [T.sub.c] (and [T.sub.p]) reached maximum value at 0.3 wt% f-MWNT and decreased at higher loadings. As discussed earlier, the nucleation saturation, as well as the reduced polymer mobility at high f-MWNT loading is likely to be the reason for this behavior. Moreover, the interfacial PP, that is, the polymer in the master batch, has gone through solution processing, and may have fewer entanglements than the matrix PP. Melt compounding this solution processed PP with neat PP resulted in lower [T.sub.c] as compared with neat PP (Fig. S3). As the master batch concentration increases in the nanocomposite, the influence of this interfacial solution processed PP becomes not negligible and thus the lowering of [T.sub.c] (and [T.sub.p]) was observed by several degrees of Celsius.

Unlike the apparent increase of [T.sub.c], the change of [T.sub.p] due to the incorporation of fillers is relatively moderate (Table 2) indicating that despite promoting crystallization nucleation, the fillers normally have limited influence on the morphology of PP in the perspective of lamella thickness or crystal perfection [49], Most of the literature reported about 2[degrees]C increase, and in several studies, even reduction in [T.sub.p] was reported in PP-based nanocomposites. It is worth noting that in one study, 5[degrees]C and 8[degrees]C increase in [T.sub.p] was observed for p-MWNT and SWNT loaded PP nanocomposites, respectively [35]. There are two distinct characteristics in Ref. [35] compared with the others. First, the mixing technology introduced by the author resulted in good nanotube dispersion in the polymer matrix. Second, the nanotubes used in the study had not gone through any treatments thus retaining the pristine graphitic structure. Both characteristics minimize the disturbance of fillers on the polymer crystallization process as compared to a system with aggregates of fillers or unfavorable interfaces, for example, monomer or polymer grafting on fillers, that cannot provide an effective template for crystallization. In our work, significant increase of [T.sub.p] (nearly 7[degrees]C) was observed in the PP/f-MWNT containing sample with only 0.3 wt% f-MWNT loading. As the solution processed polymers in the master batch had fewer entanglements than the raw PP, they had better opportunity to interact with f-MWNTs and formed a coherent interface that could template the crystallization of the matrix polymers with higher crystal perfection. Effect of solution processing was also reported by Xie et al. [50] where about 5[degrees]C higher [T.sub.p] was found when polyethylene (PE) was crystallized from solution than from melt in the presence of MWNT. The author attributed this to the better chain regularity and larger crystal lamella in the former.

Narrower crystallization and melting peaks represent narrower crystal size distribution [9, 51]. In the MA-g-PP/f-MWNT and PP/f-MWNT-based nanocomposites, the narrowest full width at half maximum (FWHM) of [T.sub.c] was observed at the f-MWNT concentration where the crystallization rate was the fastest, that is, at 1 and 0.3 wt% in the case of MA-g-PP/f-MWNT and PP/f-MWNT, respectively (Fig. 3). Considering a homogenous distribution of f-MWNT in the polymer matrix, it is not surprising that the crystal size distribution is inversely proportional to the crystallization rate assuming crystallization is via heterogeneous nucleation. On the other hand, FWHM of [T.sub.c] of the p-MWNT-based sample was narrower when p-MWNT concentration was between 0.005 and 0.1 wt% than at the higher p-MWNT concentration. Similar observation can also be found in the melt endotherm of p-MWNT containing nanocomposites (Fig. S4). Above 0.5 wt% p-MWNT, a shoulder peak corresponds to the melting point of neat PP around 158[degrees]C which is about 7[degrees]C lower than [T.sub.p], suggesting that some of the PP was not affected by p-MWNT during crystallization due to poor MWNT dispersion. Although the crystallization rate continuously increased with increase in p-MWNT concentration, inhomogeneous distribution of MWNT resulted in discrepancy in the crystallization behavior of matrix PP which is revealed by both broadening of melt endotherm and broadening FWHM of [T.sub.c]. It is worth noting that while about 7[degrees]C higher [T.sub.p] was observed in both p-MWNT (1 wt%) and PP/f-MWNT (0.3 wt%) containing nanocomposite, a single and sharp peak observed in the PP/f-MWNT-based sample demonstrated that the crystallization of matrix polymers was uniformly modified by well dispersed f-MWNT through a longer interphase than via p-MWNT. This refinement and perfection of the PP crystals can also be seen from the equilibrium melting temperature ([T.sub.m.sup.0]) of PP, determined by linear Hoffman-Weeks plots (Fig. S5), which increased by about 5[degrees]C in the PP/f-MWNT-based nanocomposites while exhibited no significant change in the presence of 1 wt% p-MWNT. On the other hand, at MWNT concentration as low as 0.01 wt% where the difference of MWNT dispersion quality between the three types of nanocomposite is smaller as compared to that at a higher MWNT loading, the p-MWNT-based sample has the narrowest FWHM of [T.sub.c] (Fig. 3) and the largest upshift of a sharp melting peak than those of the two f-MWNT master batch-based samples (Fig. S4). Crystallinity ([X.sub.c]) were determined from 2nd melting cycle of PP and PP/MWNT nanocomposites at different MWNT concentrations via MA-g-PP/f-MWNT master batch, PP/f-MWNT master batch, and p-MWNT through nonisothermal crystallization are shown in Fig. S6.

Figure 4 and Fig. S7 show the melt endotherm of PP and PP/MWNT nanocomposites with 1 wt% MWNT loading at various cooling and heating rates. The melt endotherm of the neat PP crystallized at 2.5[degrees]C/min cooling rate (Fig. S4 and Fig. 4a-1) presented a shoulder at ~164[degrees]C, which is higher than [T.sub.p] (158[degrees]C). Shoulder at similar temperature range was also apparent in the f-MWNT-based nanocomposite samples below 0.01 wt% f-MWNT loading (Fig. S4). While the upshift of [T.sub.p] can be attributed to the larger lamella thickness or higher crystalline perfection of PP resulting from the presence of CNT during crystallization [35], the shoulder peak higher than [T.sub.p] is most likely the melting of the crystal lamellae that have gone through recrystallization process during heating. When heated slowly, less stable crystals may melt and recrystallize on the existing more refined crystalline lamellae that are still present in the sample [35]. The crystalline lamella thickness has also been demonstrated to increase upon recrystallization and thereby explaining the higher melting temperature [52, 53]. Crystals that formed through faster cooling rate have lower initial degree of crystalline perfection which may accentuate the occurrence of recrystallization. This is confirmed by the observation in Fig. 4a-2, b-2, and c-2, that is, the recrystallization of the less refined crystals resulted in apparent melting peak at about 165[degrees]C as compared with the shoulder in Fig. 4a-1 and the absence of this peak in Fig. 4b-1 and c-1. On the other hand, because the reordering of polymer chains is more difficult upon fast heating than slow heating, the recrystallization process should be suppressed as the heating rate increased from 2.5[degrees]C/min to 10[degrees]C/min. This can be seen in Fig. 4a-3 and b-3, where the peak assigned for melting crystals that formed upon recrystallization in Fig. 4a-2 and b-2 reduced its intensity and disappeared in Fig. 4c-3. The occurrence of recrystallization was greatly restricted in the p-MWNT containing nanocomposite (Fig. 4d-2 and d-3) and was relatively moderate in the PP/f-MWNT than in the MA-g-PP/f-MWNT-based sample. Increased melt viscosity in the presence of MWNT (Fig. S8), which is more pronounced in the case of p-MWNT and PP/f-MWNT, indicates impeded segmental motion and thereby suppression of recrystallization.

Recrystallization process triggers crystal refinement and perfection during melting. These more perfect crystals created during recrystallization are not retained upon complete melting. Partial melting of the sample with subsequent quenching and crystallization at a lower temperature was thus conducted to investigate the self-seeding or templating effect of the highly refined crystals that do not melt at a selective temperature before complete melting. The morphology of the resulting crystal lamellae can be studied under SEM and the melting behavior (degree of crystal perfection, lamella thickness, and size distribution) can be investigated via DSC. The detail temperature profiles used in the DSC experiment for this purpose are shown in Figs. 5 and 6 for PP and PP/f-MWNT master batch (5 wt% f-MWNT), respectively. Different isothermal crystallization temperature was chosen, that is, 130[degrees]C for neat PP and 135[degrees]C for the master batch and nanocomposites, to ensure comparable test condition, that is, similar [t.sub.1/2], since the incorporation of MWNT allows PP to crystallize at higher temperature than the neat PP and this should not be overlooked when comparing the crystallization behavior of the two. The first self-seeding temperature, a temperature at which the sample is partially melted, was determined by the peak melting temperature of the sample crystallized from 130[degrees]C (or 135[degrees]C in MWNT containing samples), is 162[degrees]C for neat PP and 165.5[degrees]C for MWNT containing samples (Figs. 5a-l and 6a-l). The second self-seeding temperature is determined from the melt endotherm of the sample after one complete self-seeding cycle (i.e., 1st heating for removing thermal history, 1st cooling for isothermal crystallization, and 2nd heating for first self-seeding followed by 2nd cooling to the isothermal crystallization temperature). From the 3rd melting endotherm (i.e., the melting of the sample after one self-seeding cycle), the second self-seeding temperature was chosen to be 165.5[degrees]C for PP and 167.8[degrees]C (between two melting peaks) for PP/f-MWNT master batch (Figs. 5a-2 and 6a-2).

After one self-seeding cycle, both the samples contain crystals that have 4[degrees]C higher melting temperature (Figs. 5a-2 and 6a-2) than the crystals from samples that have only gone through isothermal crystallization (Figs. 5a-1 and 6a-1). That is, the crystals that have higher melting point than the first self-seeding temperature serve as templates for subsequent polymer crystallization when the sample was cooled to given isothermal crystallization temperature. From Figs. 5a-2 and 6a-2, it is apparent that the resulting crystals, with a good portion, have grown from these templates and possess high regularity thereby melted at higher temperature. A schematic describing the self-seeding process is given in Fig. 7 Further refinement of the crystals can be achieved through repeatedly conducting the self-seeding process multiple times as shown in the schematic in Fig. 7. In other words, by repeated melting of the less perfect crystals below the second self-seeding temperature while keeping the more perfect ones as seeds, the crystals that grow on these pre-existing templates became larger in size and higher in ratio with respect to the total crystalline population. Figures 5a-3 and 6a-3 showed the melting of samples underwent total of five self-seeding cycles which corresponds to the seventh melting curve as annotated in these figures. The melt endotherm of both PP and PP/f-MWNT master batch exhibited about 7[degrees]C peak shift compared to their counterparts that have not gone through any self-seeding cycle, i.e. increased from 162[degrees]C to 168.7[degrees]C in the neat PP sample and from 165.5[degrees]C to 172.5[degrees]C in the PP/f-MWNT master batch-based sample. Gradual reduction of the less perfect crystalline portion can also be seen in Figs. 5a-3 and 6a-3 as the sample goes through more self-seeding cycles (from 2nd melting curve to 6th melting curve). The final melt endotherm consisted of a broad peak below the second self-seeding temperature above which a major strong sharp peak attests the occurrence of templated crystal growth. From another perspective, the incorporation of f-MWNT and the self-seeding/templating crystallization jointly increase the melting temperature of PP by 10.5[degrees]C (from 162[degrees]C to 172.5[degrees]C). Typically, the thicker lamellae with high melting point can be obtained through prolonged crystallization time at lower supercooling temperatures. For example, Maiti et al. [54] obtained PP lamellae with a high melting temperature of 180.8[degrees]C via PP crystallization at 166[degrees]C for 6 months. With the combination of shear (22.5 s~!) and pressure (200 MPa) Yang et al. [55] successfully crystallized a small portion of PP with melting point of 179. 5[degrees]C which is 16[degrees]C higher than the PP crystallized under quiescent condition. While keeping the time required for the completion of crystallization relatively short, our approach affords a new method to considerably modify the melting behavior of matrix PP. The role of MWNT in the self-seeding and templated growth scheme is to provide seeds with higher crystalline perfection to start with. This can be inferred based on a higher crystallinity (55.7% vs. 49.5%) and higher peak melting temperature (165.5[degrees]C vs. 162[degrees]C) in the PP/f-MWNT master batch than neat PP from Figs. 5a-l and 6a-l. As a consequence, the resulting crystalline lamellae that have gone through five cycles of self-seeding process also possess higher crystallinity (62.4% vs. 58%) and higher peak melting temperature (172.5[degrees]C vs. 168.7[degrees]C) in the PP/f-MWNT master batch than in the neat PP as shown in Figs. 5a-3 and 6a-3. From Fig. S9, the increase in crystal size as samples went through the self-seeding treatments verifies the hypothesis of templated crystal growth in the schematic (Fig. 7).

Larger crystal size in PP/f-MWNT master batch than in neat PP (24.6 vs. 21.2 nm) is also in agreement with higher peak melting temperature in the former [56]. The morphology of the templated polymers on the f-MWNT interface was investigated by SEM where the columnar layers of crystalline PP were found surrounding f-MWNT that resulted in noticeable change in the PP/f-MWNT diameter from 25 [+ or -] 5 to 64 [+ or -] 10 nm (Fig. 8). Such type of columnar layer of polymer crystalline lamellae growing perpendicular to the long axis of fillers has been reported in PP/CNT fiber [20, 22], PP/ramie fiber [57], PP/glass fiber [58], PP/carbon fiber [59], PP/graphene oxide fiber [21, 22], and so forth, and is often referred to as the transcrystalline interphase. However, it is the first time that this type of columnar crystalline interphase has been created through designed heat treatment and crystallization cycles.

The interfacial strength between the filler and the matrix in a composite is an important factor in determining their mechanical properties. A number of studies have attempted to manipulate such interface or interphase through interfacial crystallization [39, 40, 60, 61], Zhang et al. [62] reported CNT fiber-based nanocomposites prepared by PE crystallization from both solution and melt. Both hybrid shish-kebab nanostructures and transcrystalline lamellae around nanotubes were observed along with improved mechanical properties. In another work, interfacial crystallization of polyvinyl alcohol) on single wall CNT from solution resulted in extended-chain crystalline layer which appeared to increase load transfer between the polymer and the CNT [63], Based on the aforementioned studies, it is believed that the self-seeding and templated crystallization approach developed in this work has a potential to help understand the effect of interphase on the mechanical properties of nanocomposite containing well-dispersed fillers. Also, the ability to increase crystallinity, crystal size, and lamella thickness of the matrix polymer via such approach should not be limited to PP but would be applicable to other semicrystalline polymers.

The parallel experiments on self-seeding and templated crystallization were also conducted in the PP/MWNT nanocomposites prepared via PP/f-MWNT, MA-g-PP/f-MWNT master batches, and p-MWNT. In Fig. S 10a and Table 4, the area under the high melting temperature peak became larger as f-MWNT concentration in the nanocomposite increased from 0.3 to 1 wt%. The corresponding heating and cooling profiles are given in Figs. S11-S14. Comparison of the ability to induce templated crystal growth between different types of interfaces is shown in Fig. S 10b and Table 5. The MA-g-PP/f-MWNT-based nanocomposite did not result in as much elevation of melting temperature as seen in other samples. The incapability of promoting prefect crystal growth may be due to the less crystallizable MA-g-PP in the vicinity of f-MWNT which did not serve as the best template for PP crystallization. Smaller area under the major melting peak in p-MWNT containing sample than in the PP/f-MWNT also pointed to the importance of MWNT dispersion quality in providing more perfect lamellar seeds for templated polymer crystal growth.

Figure 9 provides a visualization of the polymer self-seeding process. In the polarized optical microscopy (POM) experiment, the set temperature of the heating stage for isothermal crystallization and self-seeding was chosen to be 5[degrees]C higher than the experimental condition used in DSC. This is because the sample was placed on the cover slip instead of directly contacting the heating stage so some discrepancy between the temperature experienced by the sample and the set temperature of the heating stage was expected. When the sample was heated to the self-seeding temperature after isothermal crystallization at 140[degrees]C, there were still some nuclei observable in the POM image that were not completely melted (Step b in Fig. 9). As the sample was quenched from the self-seeding temperature (Step c in Fig. 9), onset of crystallization was found shifted to a higher temperature such that more observable nuclei were presented in the POM image at 150[degrees]C upon cooling. Consequently, the crystallization of the self-seeded polymers was complete much earlier than the crystallization from unseeded melt (vs. 12 min). The resulting crystalline phase was completely melted when the set temperature of the heating stage reached 180[degrees]C. The observation from the POM experiment is in good agreement with the proposed scheme of self-seeded and templated crystal growth shown in Fig. 7.

CONCLUSIONS

The influence of three types of polymer/MWNT interfaces, namely matrix PP with PP/f-MWNT, MA-g-PP/f-MWNT, or p-MWNT, on the crystallization and melting behavior of the PP/MWNT nanocomposites was investigated. Solution-based master batches where PP or MA-g-PP was noncovalently coated onto f-MWNTs, and p-MWNTs without polymer coating were used for nanocomposite preparation. The nucleation ability was the highest for p-MWNT containing samples at all MWNT concentration despite their relatively poor dispersion compared with the f-MWNT-based nanocomposites. On the other hand, MA-g-PP/f-MWNT containing nanocomposite exhibited the best f-MWNT dispersion. However, the presence of MA-g-PP in the vicinity of f-MWNTs limited the nucleation and growth of PP. As for the PP/f-MWNT-based samples, although not being as efficient as the p-MWNT, the f-MWNTs had better dispersion in the matrix. This results in a comparable crystallization rate to the p-MWNT containing sample between 0.1 and 0.3 wt% MWNT concentrations. An increase in [T.sub.p] after nonisothermal crystallization indicated a higher degree of crystal perfection in the presence of MWNTs in all cases. Within the three types of samples, relatively sharp melt endotherm in the PP/f-MWNT master batch containing sample suggested a more homogenous crystal size distribution. This was not the case in p-MWNT-based sample due to poor dispersion and in MA-g-PP/f-MWNT-based sample due to the presence of less crystallizable MA-g-PP. Among all, nanocomposite comprised 0.3 wt% f-MWNT prepared from PP/ f-MWNT master batch showed the optimum increase of crystal perfection ([T.sub.p] increase by 6.8[degrees]C as compared to the unfilled PP) along with a narrow crystal size distribution as determined by the FWHM of [T.sub.c]. DSC study at various cooling and heating rates revealed partial and complete suppression of crystal refinement and perfection upon heating in PP/f-MWNT master batch-based and p-MWNT-based samples, respectively. The suppression can be attributed to the reduced polymer mobility in the vicinity of MWNTs. With the designed heating and cooling profile for polymer self-seeding and templated growth, a special morphology of columnar crystals surrounding f-MWNTs was observed under SEM. The span of this ordered crystalline layer was about 26 nm from the surface of f-MWNT. [T.sub.p] of such polymer interphase was about 10[degrees]C higher than the polymer that merely underwent isothermal crystallization in the unfilled PP sample. PP/f-MWNT master batch-based nanocomposite has the greatest ability to induce this kind of ordered crystalline interphase compared to MA-g-PP/f-MWNT master batch and p-MWNT-based ones. The mechanism of its formation was proposed and supported by increased crystal size and the polarization optical microscopy observation. It is expected that the accelerated crystallization and the highly ordered polymer interphase surrounding f-MWNTs will bring about a synergistic effect on the physical properties of the polymer/CNT composite.

ACKNOWLEDGMENT

Financial support and intellectual inputs for this work by SABIC, and contributions of K. Gupta are gratefully acknowledged.

REFERENCES

[1.] M.-F. Yu, O. Lourie, M.J. Dyer, K. Moloni, T.F. Kelly, and R. S. Ruoff, Science, 287(5453), 637 (2000).

[2.] Q. Zhang, G. Chen, S. Yoon, J. Ahn, S. Wang, Q. Zhou, Q. Wang, and J. Li, Phys. Rev. B, 66(16), 165440 (2002).

[3.] H.Q. Xie, A. Cai, and X.W. Wang, Phys. Lett. A, 369(1-2), 120 (2007).

[4.] M.K. Samani, N. Khosravian, G.C.K. Chen, M. Shakerzadeh, D. Baillargeat, and B.K. Tay, Int. J. Therm. Sci., 62, 40 (2012).

[5.] T. Ebbesen, H. Lezec, H. Hiura, J. Bennett, H. Ghaemi, and T. Thio, Nature, 382(6586), 54 (1996).

[6.] S. Basu-Dutt, M.L. Minus, R. Jain, D. Nepal, and S. Kumar, J. Chem. Educ., 89(2), 221 (2011).

[7.] D.-H. Xu, Z.-G. Wang, and J.F. Douglas, Macromolecules, 41 (3), 815 (2008).

[8.] S. Zhang and S. Kumar, Macromol. Rapid Commun., 29(7), 557 (2008).

[9.] A.R. Bhattacharyya, T.V. Sreekumar, L. Tao, S. Kumar, L. A. Ericson, R.H. Hauge, and R.E. Smalley, Polymer, 44(8), 2373 (2003).

[10.] K.A. Anand, U.S. Agarwal, and R. Joseph, Polymer, 47(11), 3976 (2006).

[11.] J. Baneijee, S. Parija, A.S. Panwar, K. Mukhopadhyay, A. K. Saxena, and A.R. Bhattacharyya, Polym. Eng. Sci., 57(10), 1136 (2017).

[12.] S. Abbasi, A. Derdouri, and P.J. Carreau, Polym. Eng. Sci., 51 (5), 992 (2011).

[13.] Y. Chen, Q. Fan, S. Yang, Q. Zhang, and Z. Li, Polym. Eng. Sci., 59(4), 757 (2019).

[14.] S. Parija and A.R. Bhattacharyya, Polym. Eng. Sci., 57(2), 183 (2017).

[15.] B.A. Newcomb, L.A. Giannuzzi, K.M. Lyons, P. Gulgunje, K. Gupta, Y. Liu, M. Kamath, K. McDonald, J. Moon, B. Feng, G.P. Peterson, H.G. Chae, and S. Kumar, Carbon, 93, 502 (2015).

[16.] Y. Zhang, N. Tajaddod, K. Song, and M.L. Minus, Carbon, 91, 479 (2015).

[17.] J.R. Gissinger, C. Pramanik, B. Newcomb, S. Kumar, and H. Heinz, ACS Appl. Mater. Interfaces, 10(1), 1017 (2018).

[18.] M.L. Minus, H.G. Chae, and S. Kumar, ACS Appl. Mater. Interfaces, 4(1), 326 (2012).

[19.] Y. Zhang, K. Song, J. Meng, and M. Minus, ACS Appl. Mater. Interfaces, 5(3), 807 (2013).

[20.] S. Zhang, M.L. Minus, L. Zhu, C.-P. Wong, and S. Kumar, Polymer, 49(5), 1356 (2008).

[21.] J.P. Abdou, G.A. Braggin, Y. Luo, A.R. Stevenson, D. Chun, and S. Zhang, ACS Appl. Mater. Interfaces, 7(24), 13620 (2015).

[22.] J.P. Abdou, K.J. Reynolds, M.R. Pfau, J. van Staden, G. A. Braggin, N. Tajaddod, M. Minus, V. Reguero, J.J. Vilatela, and S. Zhang, Polymer, 91, 136 (2016).

[23.] V. Causin, B.-X. Yang, C. Marega, S.H. Goh, and A. Mango, Eur. Polym. J., 45(8), 2155 (2009).

[24.] Z. Zhou, S. Wang, L. Lu, Y. Zhang, and Y. Zhang, J. Polym. Sci. Part B: Polym. Phys., 45(13), 1616 (2007).

[25.] P. Verma and V. Choudhary, J. Appl. Polym. Sci., 132(13), 41734 (2015).

[26.] S. Zhao, F. Chen, Y. Huang, J.-Y. Dong, and C.C. Han, Polymer, 55(16), 4125 (2014).

[27.] V. Khoshkava, H. Ghasemi, and M.R. Kamal, Thermochim. Acta, 608, 30 (2015).

[28.] M. Zhang, Y. Liu, X. Zhang, J. Gao, F. Huang, Z. Song, G. Wei, and J. Qiao, Polymer, 43(19), 5133 (2002).

[29.] J. Ma, S. Zhang, Z. Qi, G. Li, and Y. Hu, J. Appl. Polym. Sci., 83(9), 1978 (2002).

[30.] C.-C. Chu, K.L. White, P. Liu, X. Zhang, and H.-J. Sue, Carbon, 50(12), 4711 (2012).

[31.] B.-X. Yang, J.-H. Shi, K. Pramoda, and S.H. Goh, Compos. Sci. Technol., 68(12), 2490 (2008).

[32.] H. Deng, E. Bilotti, R. Zhang, and T. Peijs, J. Appl. Polym. Sci., 118(1), 30 (2010).

[33.] A.A. Koval'chuk, A.N. Shchegolikhin, V.G. Shevchenko, P. M. Nedorezova, A.N. Klyamkina, and A.M. Aladyshev, Macromolecules, 41(9), 3149 (2008).

[34.] P. Kalakonda, G.S. Iannacchione, M. Daly, G.Y. Georgiev, Y. Cabrera, R. Judith, and P. Cebe, J. Appl. Polym. Sci., 130(1), 587 (2013).

[35.] H.E. Miltner, C.E. Koning, B. Van Mele, N. Grossiord, K. Lu, and J. Loos, Macromolecules, 41(15), 5753 (2008).

[36.] C. Garzon, M. Wilhelm, M. Abbasi, and H. Palza, Macromol. Mater. Eng., 301(4), 429 (2016).

[37.] L. Valentini, J. Biagiotti, M. Lopez-Manchado, S. Santucci, and J. Kenny, Polym. Eng. Sci., 44(2), 303 (2004).

[38.] S. Chen, X. Wang, X. Ma, and K. Wang, J. Mater. Sci., 51(2), 708 (2016).

[39.] P.-H. Wang, S. Ghoshal, P. Gulgunje, N. Verghese, and S. Kumar, Polymer, 100, 244 (2016).

[40.] S. Ghoshal, P.-H. Wang, P. Gulgunje, N. Verghese, and S. Kumar, Polymer, 100, 259 (2016).

[41.] Y. Seo, J. Kim, K.U. Kim, and Y.C. Kim, Polymer, 41, 2639 (2000).

[42.] Y.-Y. Liang, J.-Z. Xu, X.-Y. Liu, G.-J. Zhong, and Z.-M. Li, Polymer, 54(23), 6479 (2013).

[43.] A.T. Lorenzo, M.L. Arnal, J. Albuerne, and A. Muller, J. Polym. Test., 26(2), 222 (2007).

[44.] C. Berlic, V. Barna, and B. Manolescu, Dig. J. Nanomater. Biostruct., 10(4), 1365 (2015).

[45.] J. Li, C. Zhou, G. Wang, Y. Tao, Q. Liu, and Y. Li, Polym. Test., 21(5), 583 (2002).

[46.] B.P. Grady, F. Pompeo, R.L. Shambaugh, and D.E. Resasco, J. Phys. Chem. B, 106(23), 5852 (2002).

[47.] M.-J. Chu and T.-M. Wu, "Isothermal Crystallization Kinetics of Poly(Lactic Acid)/Montmorillonite Nanocomposites," in Experimental Analysis of Nano and Engineering Materials and Structures, Springer, Dordrecht, 827 (2007).

[48.] J.D. Hoffman and J.J. Weeks, J. Res. Natl. Bur. Stand. A, 66(1), 13 (1962).

[49.] W. Leelapornpisit, M.T. Ton-That, F. Perrin-Sarazin, K.C. Cole, J. Denault, and B. Simard, J. Polym. Sci. Part B: Polym. Phys., 43(18), 2445 (2005).

[50.] X. Xie, K. Aloys, X. Zhou, and F. Zeng, J. Therm. Anal. Calorim., 74(1), 317 (2003).

[51.] G.-W. Lee, S. Jagannathan, H.G. Chae, M.L. Minus, and S. Kumar, Polymer, 49, 1831 (2008).

[52.] J. Schmidtke, G. Strobl, and T. Thurn-Albrecht, Macromolecules, 30(19), 5804 (1997).

[53.] G. Strobl, Prog. Polym. Sci., 31(4), 398 (2006).

[54.] P. Maiti, M. Hikosaka, K. Yamada, A. Toda, and F. Gu, Macromolecules, 33(24), 9069 (2000).

[55.] S.-G. Yang, Z. Zhang, D. Zhou, Y. Wang, J. Lei, L. Li, and Z.M. Li, Macromolecules, 48(16), 5834 (2015).

[56.] K. Yamada, M. Hikosaka, A. Toda, S. Yamazaki, and K. Tagashira, Macromolecules, 36(13), 4790 (2003).

[57.] M. Zhou, S. Xu, Y. Li, C. He, T. Jin, K. Wang, H. Deng, Q. Zhang, F. Chen, and Q. Fu, Polymer, 55(13), 3045 (2014).

[58.] K. Na, H.S. Park, H.Y. Won, J.K. Lee, K.H. Lee, J.Y. Nam, and B.S. Jin, Macromol. Res., 14(5), 499 (2006).

[59.] C.-M. Wu, M. Chen, and J. Karger-Kocsis, Polym. Bull., 41(2), 239 (1998).

[60.] H. Quan, Z.-M. Li, M.-B. Yang, and R. Huang, Compos. Sci. Technol., 65(7), 999 (2005).

[61.] N. Ning, S. Fu, W. Zhang, F. Chen, K. Wang, H. Deng, Q. Zhang, and Q. Fu, Prog. Polym. Sci., 37(10), 1425 (2012).

[62.] S. Zhang, W. Lin, C.-P. Wong, D.G. Bucknall, and S. Kumar, ACS Appl. Mater. Interfaces, 2(6), 1642 (2010).

[63.] M.L. Minus, H.G. Chae, and S. Kumar, Macromol. Chem. Phys., 210(21), 1799 (2009).

Po-Hsiang Wang, (1) Prabhakar Gulgunje, (1) Sushanta Ghoshal, (1) Ihab N. Odeh, (2) Nikhil Verghese, (2) Satish Kumar (iD) (1)

(1) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332

(2) SABIC Technology Center, Sugar Land, Texas 77478

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

Correspondence to: S. Kumar; e-mail: satish.kumar@mse.gatech.edu

DOI 10.1002/pen.25155

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Optical and SEM images of PP/MWNT nanocomposites prepared via (a,d) MA-g-PP/f-MWNT master batch (0.3 wt%), (b,e) PP/f-MWNT master batch (0.3 wt%), (c,f) p-MWNT (0.3 wt%), (g,j) MA-g-PP/f-MWNT master batch (1 wt%), (h,k) PP/f-MWNT master batch (1 wt%), and (i,l) p-MWNT (1 wt%). [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 2. Crystallization half-time [t.sub.1/2], Avrami index n, and activation energy [DELTA]E of PP/MWNT nanocomposites at different MWNT concentrations via MA-g-PP/f-MWNT master batch, PP/f-MWNT master batch, and p-MWNT. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. [T.sub.c], [T.sub.p], and FWHM of [T.sub.c] of PP/MWNT nanocomposites at different MWNT concentrations via (a) MA-gPP/f-MWNT master batch, (b) PP/f-MWNT master batch, and (c) p-MWNT through nonisothermal crystallization. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Melt endotherm for the 2nd heating cycle of (a) PP and PP/MWNT nanocomposite (1 wt%) via (b) MA-g- PP/TMWNT master batch, (c) PP/f-MWNT master batch, and (d) p-MWNT. Cooling (1st cycle) and heating (2nd cycle) rate of DSC tests are indicated by the left arrow and right arrow, respectively. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Melt endotherms of PP through various self-seeding thermal cycles (a). The thermal cycles are shown in (b). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Melt endotherms of PP/f-MWNT master batch through various self-seeding thermal cycles (a). The thermal cycles are shown in (b). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Schematic of the formation process of highly perfect columnar crystals surrounding f-MWNTs. During isothermal crystallization at 135[degrees]C (a), crystals that nucleated at f-MWNT exhibited both relatively high and low [T.sub.m], due to difference in crystal perfection. After quenching to room temperature and reheating to 165.5[degrees]C, (b) crystals with lower [T.sub.m] were melted, leaving more perfect crystals that have higher [T.sub.m] as "seeds." When again isothermally crystallized at 135[degrees]C (c), templated crystal growth happened at these "seeds" while crystals with both higher and lower perfection also nucleated on f-MWNT surface. After repeatedly heating to the second self-seeding temperature, that is, 167.8[degrees]C in this example (d), and followed by isothermal crystallization (e), the highly perfect crystals surrounding f-MWNTs can be achieved (f). [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. SEM images of (a,c,e) PP/f-MWNT master batch that was first heated to 220[degrees]C for 5 min followed by isothermal crystallization at 135[degrees]C for 30 min and then quenched to room temperature. In (b,d,f), the sample first underwent the same treatment as the sample in (a,c,e) and then followed the temperature profile shown in (g). Average diameter of f-MWNTs in (a,c,e) and (b,d,f) is 25 [+ or -] 5 and 64 [+ or -] 10 nm, respectively. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Polarized optical microscopy images of PP/f-MWNT nanocomposite prepared via PP/f-MWNT master batch at 1 wt% f-MWNT concentration. The sample was first heated to 225[degrees] C for 5 min and isothermally crystallized at 140[degrees]C. After fully crystallized, the sample was quenched to room temperature and then heated to 171[degrees]C at 10[degrees]C/min heating rate, followed by immediately quenching to 140[degrees]C. Again, after being fully crystallized at 140[degrees]C, the sample was first quenched to room temperature and then heated to 180[degrees]C at 10[degrees]C/min heating rate. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Effect of fillers on the isothermal
crystallization and melting behavior.

Filler type         Filler          Method
                    concentration

MA-g-PP coated      0.01-1 wt%      Solution mixing + melt
f-MWNT                              compounding

PP coated f-MWNT

p-MWNT

MA-g-PP-grafted     1 and 2 wt%     Melt compounding
MWNT; p-MWNT

Silane-grafted      0.5 and 1 wt%   Solution mixing
MWNT

p-MWNT              0.5 and 1 wt%   Solution mixing

p-MWNT              0.5-4 wt%       Melt compounding

rGO                 0.12 to 2 wt%   In situ polymerization

CNC                 1 wt%           Melt compounding

Elastomeric         5 wt%           Melt compounding
nanoparticles

MMT                 2.5-8.1 wt%     Intercalation
                                    polymerization

                    Crystallization behavior

Filler type         Change in [t.sub.1/2] (a)

MA-g-PP coated      -39% at 0.01 wt%; -70% at 1 wt%
f-MWNT              (at 132.5[degrees]C)

PP coated f-MWNT    -68% at 0.01 wt%; -90% at 0.3 wt %;
                    -87% at 1 wt% (at 132.5[degrees]C)

p-MWNT              -85% at 0.01 wt%; -97% at 1 wt%
                    (at 132.5[degrees]C)

MA-g-PP-grafted     Not reported
MWNT; p-MWNT

Silane-grafted      -90% at 0.5 wt%; -92% at 1 wt%
MWNT                (at 130[degrees]C)

p-MWNT              -87% at 0.5 wt%; -92% at 1 wt%
                    (at 130[degrees]C)

p-MWNT              -44% at 1 wt%; -63% at 4 wt%
                    (at 130[degrees]C)

rGO                 -80% at 0.12 wt%; -89% at 0.63 and
                    2 wt% (at 130[degrees]C)

CNC                 -80% at 1 wt% (at 120[degrees]C)

Elastomeric         -74% (at 125[degrees]C)
nanoparticles

MMT                 -62% at 2.5 wt%; -70% at 8.1
                    wt% (at 130[degrees]C)

                    Crystallization behavior

Filler type         Change in                         Ref.
                    [T.sub.m.sup.0] (b)

MA-g-PP coated      Not reported (c)                  This work
f-MWNT

PP coated f-MWNT    +5[degrees]C at 0.3 and 1 wt%

p-MWNT              No substantial change at 1 wt%

MA-g-PP-grafted     -15[degrees]C at 1 wt% grafted      [23]
MWNT; p-MWNT        MWNT; -21[degrees]C at 1 wt%
                    p-MWNT

Silane-grafted      -21.4[degrees]C at 0.5 wt%;         [24]
MWNT                -32.2[degrees]C at 1 wt%

p-MWNT              -9.4[degrees]C at 0.5 wt%;          [24]
                    -26.2[degrees]C at 1 wt %

p-MWNT              Not reported                        [25]

rGO                 Not reported                        [26]

CNC                 +5.5[degrees]C                      [27]

Elastomeric         Not reported                        [28]
nanoparticles

MMT                 +4.7[degrees]C at 2.5 wt%;          [29]
                    +11.4[degrees]C at 8.1 wt%

(a) Crystallization half-time.

(b) Equilibrium melting temperature derived
from linear Hoffman-Weeks plot.

(c) Comparison of [T.sub.m.sup.0] with neat PP is not
justifiable because the composite system consists
of 0.19-19 wt% of MA-g-PP.

TABLE 2. Effect of fillers on the nonisothermal
crystallization and melting behavior.

Filler type        Filler concentration   Method

MA-g-PP coated     0.005-1 wt%            Solution mixing +
f-MWNT                                    melt compounding

PP coated f-MWNT

p-MWNT

Octadecylamine-    0.1 and 0.6 wt%        Melt compounding
grafted MWNT

MA-g-PP-grafted    0.5-2 wt%              Melt compounding
MWNT

MAO (b)-grafted    0.1-7.5 wt%            Hot-pressed
MWNT

MAO (b)-grafted    0.1-3.5 wt%            In situ polymerization
MWNT

Acid-treated       0.01-5 wt%             Solution mixing
MWNT

p-MWNT             0.05-2 wt%             Latex mixing

p-MWNT             5-20 wt%               Melt compounding

p-SWNT             0.05-2 wt%             Latex mixing

p-SWNT             5-20 wt%               Melt compounding

Graphite           2-45 wt%               Melt compounding

rGO                5-20 wt%               Melt compounding

CaC[O.sub.3]       1.5-3 wt%              Melt compounding

                   Crystallization behavior

Filler type        Change in [T.sub.c]

MA-g-PP coated     -1.2[degrees]C at 0.01 wt%;
f-MWNT             +2.4[degrees]C at 1 wt%

PP coated f-MWNT   +1[degrees]C at 0.01 wt%; +8[degrees]C
                   at 0.3 wt%; +5.4[degrees]C at 1 wt%

p-MWNT             +4.5[degrees]C at 0.01 wt%;
                   +10.9[degrees]C at 1 wt%

Octadecylamine-    +2.3[degrees]C at 0.1 wt%;
grafted MWNT       +4.1[degrees]C at 0.6 wt%

MA-g-PP-grafted    +7.6[degrees]C at 0.5 wt%;
MWNT               +9[degrees]C at 1 wt%

MAO (b)-grafted    +12.3[degrees]C at 0.1 wt%;
MWNT               +11.4[degrees]C at 0.5 wt%

MAO (b)-grafted    +3.8[degrees]C at 0.1 wt%;
MWNT               +7.3[degrees]C at 0.9 wt

Acid-treated       +4.5[degrees]C at 0.01 wt%;
MWNT               +7.1[degrees]C at 1 wt%

p-MWNT             +5[degrees]C at 2 wt%

p-MWNT             +5.8[degrees]C at 5 wt%;
                   +14.2[degrees]C at 20 wt or.

p-SWNT             +15[degrees]C at 2 wt%

p-SWNT             +13.2[degrees]C at 5 wt%;
                   +18.3[degrees]C at 20 wt%

Graphite           +8.1[degrees]C at 10 wt%;
                   +15.2[degrees]C at 45 wt

rGO                +6.5[degrees]C at 5 wt%;
                   +10.7[degrees]C at 20 wt or.

CaC[O.sub.3]       +7[degrees]C at 1.5 wt%;
                   +6.3[degrees]C at 3 wt%

                   Crystallization behavior

Filler type        Change in [T.sub.c] (a)         Ref.

MA-g-PP coated     +1.3[degrees]C at 0.01 wt%;     This work
f-MWNT             +3.6[degrees]C at 1 wt%

PP coated f-MWNT   +3[degrees]C at 0.01 wt%;
                   +6.8[degrees]C at 0.3 wt%; +
                   4.9[degrees]C at 1 wt%

p-MWNT             +4[degrees]C at 0.01 wt%;
                   +6.9[degrees]C at 1 wt%

Octadecylamine-    -1[degrees]C at 0.1 wt%;          [30]
grafted MWNT       -0.8[degrees]C at 0.6 wt%

MA-g-PP-grafted    +2.1[degrees]C at 0.5 wt%;        [31]
MWNT               +2.2[degrees]C at 1 wt%

MAO (b)-grafted    -1.6[degrees]C at 0.1 wt%;        [32]
MWNT               +0.5[degrees]C at 0.5 wt%

MAO (b)-grafted    -0.3[degrees]C at 0.1 wt%;        [33]
MWNT               +1.9[degrees]C at 0.9 wt%

Acid-treated       -0.4[degrees]C at 0.01 wt%;       [34]
MWNT               +2.1[degrees]C at 2 wt%

p-MWNT             +5[degrees]C at 2 wt%             [35]

p-MWNT             +2.3[degrees]C at 5 wt%;          [36]
                   +2.2[degrees]C at 20 wt%

p-SWNT             +8[degrees]C at 2 wt%             [35]

p-SWNT             -2.8[degrees]C at 5 wt%;          [37]
                   -1[degrees]C at 20 wt%

Graphite           +3[degrees]C at 10 wt%;           [36]
                   +3.9[degrees]C at 45 wt%

rGO                -0.6[degrees]C at 5 wt%;          [36]
                   +2.2[degrees]C at 20 wt%

CaC[O.sub.3]       -0.6[degrees]C at 1.5 wt%;        [38]
                   +0.3[degrees]C at 3 wt%

(a) Melting peak maximum.

(b) MAO, methylaluminoxane.

(c) SWNT, single wall carbon nanotube.

TABLE 3. [T.sub.c], [T.sub.p], FWHM of [T.sub.c], and
crystallinity of PP/MWNT nanocomposites at different MWNT
concentrations via MA-g-PP/f-MWNT master batch, PP/f- MWNT
master batch, and p-MWNT through nonisothermal crystallization.
DSC tests were conducted with heating and cooling rate of
2.5[degrees]C/min. Crystallinity was determined from
2nd heating cycle.

Materials             Concentration    [T.sub.c]       FWHM of
                          (wt%)       ([degrees]C)    [T.sub.c]
                                                     ([degrees]C)

PP                         --           122             3.7
PP/f-MWNT via MA-g-       0.005         120             4.46
  PP/f-MWNT               0.01          120.8           4.2
  master batch            0.1           122             3.7
                          0.3           122.7           3.7
                          0.5           122.3           3.7
                          1             124.4           3.5
PP/f-MWNT via PP/f-       0.005         121.8           4.4
  MWNT master batch       0.01          123             4
                          0.1           125.7           3.5
                          0.3           130             3.3
                          0.5           127.6           3.7
                          1             127.4           3.6
PP/p-MWNT via             0.005         125.4           2.7
  p-MWNT                  0.01          126.5           2.8
                          0.1           126.6           2.6
                          0.3           127.4           3.1
                          0.5           129.5           3.5
                          1             132.9           3.5

Materials              [T.sub.p]     Crystallinity
                      ([degrees]C)        (%)

PP                       158             51
PP/f-MWNT via MA-g-      159.3           51
  PP/f-MWNT              159.3           51.7
  master batch           160.7           50.5
                         162.3           52
                         161.0           50.6
                         161.6           49.3
PP/f-MWNT via PP/f-      158.7           50
  MWNT master batch      161             53
                         163             53.3
                         164.8           54.0
                         163.9           53.8
                         162.9           53.6
PP/p-MWNT via            160.6           50.4
  p-MWNT                 162             53.4
                         162             55.8
                         162.8           53.5
                         164.1           51
                         164.9           52.6

TABLE 4. Melting peak position and melting enthalpy of PP/f-MWNT
nano-composites at different f-MWNT concentrations via PP/f-MWNT
master batch after five cycles of self-seeding and templated
crystal growth.

                                     Peak          Melting
Materials                          position     enthalpy (J/g)
                                 ([degrees]C)

PP/f-MWNT via PP/f-MWNT master      172.5            35.4
  batch (0.3 wt%)
PP/f-MWNT via PP/f-MWNT master      171.7            43.5
  batch (1 wt%)
PP/f-MWNT master batch (5 wt%)      172.5            42.5

TABLE 5. Melting peak position and melting enthalpy of
PP and PP/MWNT nanocomposite via different types of
master batch at 1 wt% MWNT concentration after five
cycles of self-seeding and templated crystal growth.

                                     Peak
                                   position        Melting
Materials                        ([degrees]C)   enthalpy (J/g)

PP                                  168.7            39.8
PP/f-MWNT via PP/f-MWNT master      171.7            43.5
  batch (1 wt%)
PP/f-MWNT via MA-g-PP/f-MWNT        165.3            79.3
  master batch (1 wt%)
PP/p-MWNT via p-MWNT fl wt%)        171.2            35.6
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Author:Wang, Po-Hsiang; Gulgunje, Prabhakar; Ghoshal, Sushanta; Odeh, Ihab N.; Verghese, Nikhil; Kumar, Sat
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2019
Words:10147
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