Article pubs.acs.org/Macromolecules
Unusual Fragility and Cooperativity in Glass-Forming and Crystalline PVDF/PMMA Blends in the Presence of Multiwall Carbon Nanotubes Maya Sharma,† Giridhar Madras,‡ and Suryasarathi Bose*,§ †
Center for Nano Science and Engineering, ‡Department of Chemical Engineering, and §Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India ABSTRACT: Poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) are completely miscible below 50 wt % PVDF in the blends. In this work, an attempt was made to understand the fragility/ cooperativity relation in glass-forming and crystalline blends of PVDF/ PMMA and in the presence of a heteronucleating agent, multiwall carbon nanotubes (CNTs). Hence, three representative blends were chosen: a completely amorphous (10/90 by wt, PVDF/PMMA), on the verge of amorphous miscibility (50/50 by wt, PVDF/PMMA), and crystalline (60/40 by wt, PVDF/PMMA) blends. The intermolecular cooperativity/ coupling, fragility, and configurational entropy near the glass transition temperature (Tg) were studied using differential scanning calorimetry (DSC) and broadband dielectric relaxation spectroscopy (DRS). It was observed that the blends with higher concentration of PMMA were more “fragile” (fragility index m = 141) and those with higher concentration of PVDF were more “strong” (m = 78). Interestingly, the coupling was less in the glass-forming blends (10/90 by wt, PVDF/PMMA) than the crystalline blends as manifested from DRS. This observation was also supported by DSC measurements which reflected that the cooperative rearranging region (CRR) existed over a smaller length scales in “fragile” blends as compared to “strong” blends, possibly due to restricted amorphous mobility. This effect was more prominent in the presence of CNTs, in particular for 50/50 (by wt) and 60/40 (by wt) PVDF/PMMA blends. Further, the configurational entropy, as manifested from DRS, decreased significantly in the “strong” blends in striking contrast to the “fragile” blends, supported by DSC, which manifested in an increase in the volume of cooperativity in the “strong” blends. The higher coupling in the crystalline blends can be attributed to good packing of the amorphous regions. While this is understood for crystalline blends (60/40 by wt, PVDF/PMMA), it is envisaged that enhanced dynamic heterogeneity is accountable for increased coupling in the case of blends which are on the verge of amorphous miscibility (50/50 by wt, PVDF/PMMA). The latter is also supported by broad relaxations near the Tg in DRS. Interestingly, the intermolecular coupling in the blends in the presence of CNTs has reduced, though the potential energy barrier hindering the rearrangement of CRR is lower than the blends without CNTs. In addition, the amorphous packing is not as effective as the blends without CNTs. This is manifested from reduced volume of cooperativity in particular, for 50/50 (by wt) and 60/40 (by wt) blends.
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INTRODUCTION The molecular structure of polymeric chains influence the strength of intermolecular cooperativity.1 Intermolecular cooperativity in miscible blends is related to the relaxation of a given polymer and its concomitant local environment.2−4 The polymer structure may influence the degree of cooperativity, and the concentration fluctuation dominates the relaxation behavior.2 For instance, in the case of dynamic asymmetric blends, if the relaxing polymer has higher Tg than the surrounding chains, the dynamic constraint of the surrounding chains decreases.5,6 Similar results were observed for polymer blends where the segmental relaxation broadens and shifts toward lower frequencies.2 This shows that crystalline phase restrict the segmental motion of amorphous chains and shifts the relaxation toward lower frequency (i.e., longer time scales). Several models have been developed to investigate the intermolecular interactions and interchain cooperativity in polymeric systems.7−11 These models depend on the segmental relaxation frequency, broadness in the relaxations,12 etc. Angell et al.13 © XXXX American Chemical Society
described a parameter to define the intermolecular cooperativity, termed the fragility index (m), which is envisioned to be the steepness of the temperature dependence of segmental relaxations.14 As the intermolecular interactions play a major role in determining the molecular dynamics near Tg, fragility is the key parameter to evaluate the glass-forming property in polymers. Various different models have been employed to study these intermolecular interactions such as configurational entropy,15 cooperative volume using DSC (differential scanning calorimeter),1,16 coupling parameter (n) using Kohlrausch− Williams−Watts (KWW) equation,17,18 etc. PVDF/PMMA is a classical blend comprising of crystalline and amorphous phases and exhibit upper critical solution temperature (UCST) behavior and crystallization-induced phase separation. When the concentration of PVDF is >50 wt % in Received: February 26, 2015 Revised: April 9, 2015
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DOI: 10.1021/acs.macromol.5b00418 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
blends with and without CNTs (1 wt %) were prepared using melt mixing (Haake minilab) for 20 min at 60 rpm at 220 °C. Three representative compositions were prepared. Completely amorphous blends (10/90), on the verge of complete amorphous miscibility (50/50) and crystalline (60/40) blends. Blended samples were molded into discs and thin films using compression molding. Characterizations. Calibrated TA Q800 differential scanning calorimetry (DSC) was used for evaluating the thermal properties of blends. To prevent crystallization and to ensure miscibility in the blends, samples were heated from 25 to 200 °C and later, quenched to −60 °C at 200 K/min. This step is followed by a second heating run from −60 to 200 °C at 10 K/min and then cooled to 25 °C at 10 K/min. For characterization crystallinity, Fourier transform infrared (FTIR) spectra of blends was obtained on a Perkin- Elmer frontier spectrophotometer in the range of 4000−600 cm−1. Dielectric measurements were carried using an Alpha-N Analyzer, Novocontrol (Germany), in the frequency range of 0.01 ≤ ω ≤ 107 Hz. Linear viscoelastic properties of blends was studied using a DHR-3 rheometer with a parallel plate geometry. The compression-molded discs of 25 mm diameter was used for rheological measurements. Temperature ramp experiments were done at 1% strain and at 0.1 rad/s frequency (linear viscosity regime was obtained a priori) to study the crystallization of blends.
the blend, it crystallizes from the melt and PMMA segregates in the interspherulitic and interlamellar regions of PVDF.19,20 However, below the critical crystallization concentration of PVDF (i.e., ca. 60 wt %), the blends are completely miscible. Interestingly, due to the presence of dynamic asymmetry, the concentration of PMMA significantly influences the segmental motion of PVDF chains. In our earlier studies,17,21−23 the different structural and molecular relaxations associated with the crystalline and amorphous domains and the crystalline− amorphous interphase were discussed using DRS (dielectric relaxation spectroscopy) for PVDF-rich blends. We also observed that the structural relaxations in PVDF/PMMA blends are contingent on the concentration of PMMA. The PVDF/PMMA blend shows different structural relaxations depending on the composition, and we observed a narrow compositional window (i.e., PMMA concentration ≥10 wt % and ≤25 wt %), where the structural relaxations are dielectrically distinguishable. We attempted to relate these molecular relaxations with the crystal−amorphous interphase and with crystalline/amorphous domains using SAXS and observed that the presence of PMMA dictates the crystallinity in the blends. In addition, the effect of carbon nanotubes CNTs, which strongly influences the structural relaxations in the blends,24 was also studied systematically. As CNTs act as heteronucleating agent25,26 for PVDF, they result in formation of β-phase in PVDF. The segmental relaxations suggests that blend composition influences the molecular miscibility27−30 and cooperativity of blends. It is now understood that the concentration of PMMA in the blends dictates the crystallizability of PVDF. In light of this, three representative compositions were chosen, i.e., a completely amorphous (10/90 PVDF/PMMA by wt), on the verge of amorphous miscibility (50/50 PVDF/PMMA by wt), and crystalline (60/40 PVDF/PMMA by wt) blends, to investigate the segmental relaxations, fragility, and the cooperativity near the Tg using differential scanning calorimetry (DSC) and broadband dielectric relaxation spectroscopy (DRS). The effect of CNTs on the molecular mobility and structural relaxation is also discussed. CNTs act as heteronucleating agent, and specifically interact with PVDF, and can alter the dynamics of PVDF. CNTs can also restrict the amorphous segmental mobility and can influence the intermolecular cooperativity and coupling. Hence, the fragility, cooperativity, and the intermolecular coupling can significantly alter in the presence of high aspect ratio nanostructured materials like CNTs. Therefore, one of the primary objectives of the study was to investigate and understand the structural parameters of polymers that control their fragility. We attempt to understand the temperature dependence of segmental relaxation and how it is influenced by the presence of PMMA and CNTs. Thus, the overall objective of the study was to understand the molecular origins of the changes observed in polymer blends and understand the relationship between glass transition, fragility, and the scale of cooperative motion in the presence of CNTs.
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RESULTS AND DISCUSSION Structure and Intermolecular Coupling in “Fragile” and “Strong” Blends. As mentioned above, three different blends were chosen to understand the fragility, cooperativity, and intermolecular coupling in different PVDF/PMMA blends. It is envisioned that below 50 wt % PVDF the blends are miscible. Hence, 10/90 and 50/50 PVDF/PMMA blends were investigated using FTIR (see Figure 1a−c). The 10/90 and 50/50 PVDF/PMMA blends show similar characteristic bands as that of PMMA, suggesting complete amorphous blends. The crystalline peaks of PVDF are absent in both 10/90 and 50/50 PVDF/PMMA blends. On the contrary, the 60/40 PVDF/ PMMA blends reflected the characteristic bands/stretch at 763 and 975 cm−1 corresponding to the crystalline α-phase of PVDF. The blends with CNTs also show similar characteristic spectra as that of the neat blends. This suggests that 10/90 and 50/50 PVDF/PMMA blends are miscible in contrast to the 60/ 40 PVDF/PMMA blends, which are crystalline. Further support comes from small-amplitude shear rheology, which shed light on the evolving structures. Figure 2 illustrates the viscoelastic properties of different PVDF/PMMA blends. Isochronal dynamic temperature ramp experiments were performed to evaluate the onset of crystallization in 60/40 PVDF/ PMMA blends. All the measurements were carried out using axial force control, and the LVR region was mapped a priori using a low frequency of 0.1 rad/s. An abrupt change in G′ at a particular temperature corresponds to the rheological phase separation in the blend.21,23 From Figure 2, this crystallization induced phase separation is only observed for 60/40 PVDF/ PMMA blends, whereas 10/90 and 50/50 PVDF/PMMA blends do not show any obvious change in G′, and the viscoelastic properties are similar to that of the homopolymer (PMMA). This further supports the fact that the flow characteristics in the amorphous blends are very different from the crystalline ones. Although some literatures argue that liquid− liquid phase separation occurs just before the crystallization induced phase separation in PVDF/PMMA blends, however, the flow characteristics in the crystalline blends are largely dominated by the crystallization-induced phase separation. Hence, from
EXPERIMENTAL SECTION
Materials and Methods. PMMA grades (Atuglass V825T, MW = 95 000 g/mol) and PVDF (Kynar-761, MW = 444 000) were procured from Arkema Inc. The amine-functionalized multiwalled nanotubes (CNTs) was procured from Nanocyl, Belgium. The average diameter of the CNTs is 9−10 nm, and the average length is 600−700 nm. The −NH2 concentration is
