Article pubs.acs.org/Macromolecules
Characterization and Structural Analysis of Solution-Grown Polyacrylonitrile-co-Methacrylic Acid (PAN-co-MAA) Single Crystals Yiying Zhang and Marilyn L. Minus* Department of Mechanical and Industrial Engineering, College of Engineering, Northeastern University, 360 Huntington Avenue, 334 Snell Engineering Center, Boston, Massachusetts 02115-5000, United States ABSTRACT: To date studies regarding the growth and characterization of polyacrylonitrile (PAN)-based polymer single crystals have been limited. Polyacrylonitrile-co-methacrylic acid (PAN-co-MAA) was used in this work, which outlines the conditions for three major types of solution-grown crystals. Electron microscopy and diffraction are used to provide a comprehensive study for chain arrangement and growth characteristics for all crystal morphologies. The three major morphological types included polyhedral and polygonal (i.e., parallelogram and elongated rhombus) crystals. Lower crystallization temperatures (65 °C) resulted in more even growth along all facets, while crystals formed at both intermediate (75 °C) and higher (85−95 °C) temperatures exhibited preferential growth along a specific growth plane. Overall crystal size also increased with elevated crystallization temperature (Tc). On the basis of the experimental d-spacing measured by electron diffraction for the (h00) and (0k0) planes, the crystal unit cell lattice constants along a and b axes are found to be ao = 1.023 ± 0.003 nm and bo = 0.612 ± 0.003 nm. This study also examines irradiation sensitivity as it pertains to lattice imaging for these single crystals.
1. INTRODUCTION Crystallization behavior and structure variations of polyacrylonitrile (PAN) and its copolymer have been studied during 1960s and 1970s.1−5 However, the crystal chain conformation and molecular packing still remain unclear and PAN is generally considered as noncrystalline.6 On the basis of measurements of the work required to form a chain fold3 and measurements of the unperturbed chain dimension,7 it is suggested that the conformational mobility of the PAN chains is limited, due to its helical conformation and the presence of large nitrile side groups. This and its lack of stereoregularity may be responsible for the typically low crystallinity exhibited by PAN materials. Despite the aforementioned issues, PAN and its copolymers are found capable of crystallizing into spherulitic complexes consisting of small lamellae.8−11 The lamellar thickness were evaluated between 3 and 15 nm, where thickness was determined as a function of crystallization temperature1,3,12,13 by measuring the metallic coating shadow lengths and angles using microscopy. Unlike polyethylene crystals, which have a linear increase of growth rate with time,14 PAN crystals have been shown to have large growth rate acceleration only after long (i.e., 20−40 h) crystallization times and ultimately develop into multilamella crystals.13 In addition to spherulites, single crystals of PAN have also been formed in dilute solutions.4 These indicate that under appropriate conditions, PAN could crystallize in the same manner as other flexible semicrystalline polymers,1 despite possessing helical chain conformation and the presence of large nitrile side groups. This may be due to the realization that strong interchain interactions do not suffice to cancel all conformational regularity, although they can somewhat distort the energetically favorable single-chain conformations.15 In addition, PAN molecules are typically © 2014 American Chemical Society
atactic, but the helical chain conformation allows the existence of syndiotactic sequences, which could potentially enhance crystallization.16 Propylene carbonate (PC) has been commonly used as the solvent for the growth of PAN crystals.3,4,8,17,18 Other solvents including ethylene carbonate,19 N,N-dimethylformamide (DMF),4,12 N,N-dimethylacetamide (DMAc),4,12 dimethyl sulfoxide (DMSO)12 and sodium thiocyanate2 have also been adapted as well. Typical crystallization temperature ranges from 69 to 160 °C for PAN crystal growth.1−4,12,13,19 In this work, the PAN crystals are grown at crystallization temperatures ranging from 65 to 95 °C in DMAc. This temperature range has relevance in terms of typical solution processing parameters of PAN for the fabrication of fibers and/or films, which currently use DMAc as a solvent.20 The PAN copolymer in this work is one that is typically used for carbon fiber processing from both PAN and PAN/carbon nanotube precursors.21−23 Controlling the fiber homogeneity in these cases is particularly important. This work is aimed at better understanding the potential for PAN crystallization during dope preparation. The crystal system, lattice constants, and chain arrangement for the unit cell of PAN crystals have also been debated for years and it still remains somewhat inconclusive. A hexagonal unit cell was first suggested for the PAN crystal according to observed reflections.24 In addition, multiple orthorhombic crystal cell structures were also proposed for PAN. Work done based on annealed drawn PAN fibers proposed unit cell lattice parameters of ao = 1.02 nm, bo = 0.61 nm, and co = 0.51 Received: April 9, 2014 Revised: May 15, 2014 Published: June 10, 2014 3987
dx.doi.org/10.1021/ma500737z | Macromolecules 2014, 47, 3987−3996
Macromolecules
Article
Figure 1. (a1, b1, and c1) SEM images of three morphology types (i.e., polyhedral, parallelogram, and rhombus, respectively) of single crystals observed in this work. For these SEM images, the crystals are not coated by a metallic layer to reveal all the sample features. (a2, b2, and c2) Representative EDPs for the three crystal types, respectively. Since the electron beam is fairly weak in the EDP for part a2, a beam stop was not used in order to reveal more diffraction spots. (d) Schematic illustrating the direction of incident electron beam with respect to the polymer chain axis.
nm.5,25 Subsequently, lattice constants were also reported to be ao = 1.06 nm, bo = 0.58 nm, and co = 0.51 nm based on PAN lathe-like single crystals grown in PC solutions.4 Other works including studies on PAN single crystals grown in dilute solution,13 and crystals formed by solution polymerization,2,12 suggested PAN crystal unit cell lattice parameters of ao ∼ 2.1 nm,2,12,13 bo ∼ 1.2 nm,2,12,13 and co ∼ 0.5 nm12,13 by using electron diffraction12,13 and wide-angle X-ray diffraction (WAXD).2,12,13 To date, the most widely accepted crystal unit cell for PAN is orthorhombic with approximate lattice parameters of ao ∼ 1 nm, bo ∼ 0.6 nm, and co ∼ 0.5 nm. In this study, polyacrylonitrile-co-methacrylic acid (PAN-coMAA) was used to grow single crystals isothermally in dilute PAN-co-MAA/DMAc solutions at different temperatures, in order to understand solution crystallization conditions and the resultant crystal structures. Electron microscopy imaging was conducted to characterize the morphology and structure of the single crystals. Electron diffraction patterns (EDPs) were taken and analyzed for each representative crystal type. The findings are discussed in the subsequent sections.
length. All solutions have same concentration of PAN-co-MAA (100 mg/L) and undergo the same degree of undercooling (ΔT = 30 °C). After a 2-day (∼48 h) crystallization stage, solutions were taken and immediately dropped on polished silicon wafers and TEM grids, which were preheated to each of the solution temperatures. The solvent on silicon wafers was removed by subsequently drying the solution samples in vacuum oven at 25 °C for electron microscope observations. The solvent on TEM grids was immediately wicked away using lint-free laboratory tissues, and the grids were further dried in vacuum oven at 25 °C to remove any residual solvent that might be left on copper grids to prepare the samples for microscopy imaging. The single crystals present on both TEM grids and silicon wafers at each Tc are comparable indicating that no additional crystal growth takes place during the vacuum drying process for either substrate. 2.3. Sample Characterization. Morphology characterization was performed on Zeiss Supra 25 field emission scanning electron microscope (operating voltage 5 kV). Scanning electron microscope (SEM) samples on silicon wafers were coated by Gatan high resolution ion beam coater with a thin chromium layer (15−20 nm) for imaging purpose. Crystal samples prepared on TEM grids were also imaged using SEM without metal coating, in order to preserve the intact crystals without metal contamination, which would affect the TEM imaging and diffraction results. Electron diffraction patterns were taken by JEOL 2011 high contrast digital transmission electron microscope (operating voltage: 120 and 80 kV). Crystal lattice imaging was performed on JEOL 2010 advanced high performance transmission electron microscope (operating voltage: 200 kV). TEM samples were prepared using loop tool (Electron Microscopy Sciences, Cat. No. 70944) to place droplets of the PAN-co-MAA/DMAc crystallization solution onto lacey carbon coated copper grids (Electron Microscopy Sciences, Cat. No. LC200-Cu). D-spacing measurements from EDPs and lattice spacing measurements from bright-field images were calibrated using an evaporated aluminum standard (Electron Microscopy Sciences, Cat. No. 80044). NMR spectroscopy was used to characterize the stereoregularity of the PAN-co-MAA polymer used in this study. 13C NMR spectrum was recorded on a Varian Inova 500 MHz NMR system at room temperature (∼25 °C), using GARP decoupling with 13C nuclei resonating at 125 MHz. The NMR sample was a solution of 5 wt % PAN-co-MAA in DMSO-d6. The 13C spectrum was obtained from 34 000 scans (9.5 h) of 12° pulses separated by an acquisition time of 0.96 s (with no additional relaxation delay), and Fourier transformed with 1 Hz of exponential line broadening. Digital resolution was 64k points across a spectrum width of 34 kHz (1.02 Hz/point). Deconvolution was performed with the Varian FITSPEC subroutine. The CH peak was well-described by three Lorentzian lines with frequency, height, and width optimized
2. EXPERIMENTAL SECTION 2.1. Materials. As-received PAN-co-MAA used in this work is a poly(acrylonitrile-co-methacrylic acid) random copolymer with methacrylic acid content of 4 wt %. The copolymer molecular weight is 513 000 g/mol. The stereoregularity is reported as atactic by manufacturer (Japan Exlan Co.). Nuclear magnetic resonance (NMR) data presented in the paper also shows the copolymer to be atactic. DMAc solvent was purchased from Fisher Scientific (CAS No. 127-195). Deuterated dimethyl sulfoxide (DMSO-d6) for NMR analysis was purchased from Fisher Scientific (CAS No. 2206-27-1). All materials were used as-received. 2.2. Solution Preparation. The as-received PAN-co-MAA powder was first dissolved in DMAc to form four separate solutions with a concentration of 100 mg/L at dissolution temperatures of 95, 105, 115, and 125 °C, respectively by stirring for 1.5 h. PAN-co-MAA/ DMAc solutions were then immediately transferred to water bath preheated to the crystallization temperatures (Tc) of 65, 75, 85, and 95 °C, respectively, and left unperturbed for a two-day quiescent crystallization. Fisher Isotemp Heated Immersion Circulator 6200B (115 V, 60 Hz, catalog No. 13-874-442) was used to maintain the water bath at the desired temperature constantly for the required time 3988
dx.doi.org/10.1021/ma500737z | Macromolecules 2014, 47, 3987−3996
Macromolecules
Article
Figure 2. 13C NMR spectra for PAN-co-MAA polymer used in this work. (a) Full spectrum of PAN-co-MAA showing the nitrile (CN), methylene (CH2), and methane (CH) groups as expanded peak region insets. (b) Fitted spectra for the rr, mr/rm, and mm triad peaks of the CN and CH groups in order to determine the approximate relative intensities toward identifying the PAN extent of tacticity. independently. The finer structure of the CN peak was approximated by three pairs of lines, also with frequency, height, and width optimized independently, but with the same line width enforced for all six lines. Integrals of subpeaks are given as percent intensity for each peak (CH or CN). An implicit assumption is the rr/rm/mm (i.e., syndiotactic/heterotactic/isotactic triad) subpeaks of each peak will have similar T1 relaxation times; it is expected that CH and CN will have different relaxation times.
planes. On the basis of all the experimental d-spacing values along {h00} and {0k0} planes, the average of lattice constants in this work was found to be ao = 1.023 ± 0.003 nm and bo = 0.612 ± 0.003 nm, very close to what has been previously reported.5 The prominent peaks observed in WAXD for PAN films and fibers are often a doublet of the (200) and (110) peaks with d-spacing of 0.51 and 0.53 nm, respectively. Weak electron diffraction spots corresponding to these spacings were also observed for some single crystals in this work. However, it is well-known that for a conventional electron diffraction mode, d-spacings greater than 0.4 nm are difficult to resolve to the central beam spread (i.e., unscattered and inelastically scattered electrons). For this reason, these observed larger d-spacings are not indexed due to the lack of sharpness, repetitiveness, and directional information for these spots. It should be noted that for some crystals grown in this work, larger d-spacings (>0.5 nm) could be indexed, but do not belong to the same crystal unit as associated with PAN WAXD observations. Instead, these measurements are consistent with d-spacing values associated with larger PAN unit cell dimensions of ao = 2.046 nm and bo = 1.224 nm, similar to previous reports.12,13 For this work, since the EDPs collected for these crystals revealed only (h00), (0k0), and (hk0) reflections, no direct evidence of any crystallographic features were observed along the c-axis direction. In addition, crystals exhibited different levels of irradiation sensitivity to electron beam as a function of both morphology and size. Details regarding all crystal morphologies observed in this work will be discussed in the following subsections. For PAN crystallization, the stereoregularity along the chain may play a role toward the perfection of the crystal that is subsequently formed during solution processing. Toward this
3. RESULTS AND DISCUSSION Polymers are found to form folded-chain structures when crystallized,26 and crystallization is suggested as a kinetic rather than thermodynamic process,3 which is to say, at a given temperature the structure tends to develop at the maximum growth rate instead of to the lowest free energy, although the driving force of this change is still a thermodynamic concept.27 In this work, the crystals were categorized into three major types based on the morphologies: (i) crystallization at 65 °C leads to the formation of polyhedral single crystals (Figure 1a1), where multiple facets grow at similar rates; (ii) crystallization at 75 °C allows the formation of parallelogram crystals (Figure 1b1), which have similar growth rates along two growing directions (i.e., a- and b-axis); (iii) crystallization ranging from 85 to 95 °C also results in asymmetrical crystals with dissimilar growth rates along each growing front, leading to the elongated rhombus shaped crystals (Figure 1c1). Parts a2, b2, and c2 of Figure 1 provide representative EDP for each crystal type, respectively. All the patterns exhibit characteristic single crystal electron diffraction, similar {h00} and {0k0} reflections, and multiple spots for higher order reflections. For the planes that fall in the same zone axis [001] (i.e., the incident electron beam is parallel to the c-axis (chain direction)) (Figure 1d), the dspacing values were measured and indexed for possible {hk0} 3989
dx.doi.org/10.1021/ma500737z | Macromolecules 2014, 47, 3987−3996
Macromolecules
Article
PAN-co-MAA polymer is nearly perfectly heterotactic/atactic or completely random (i.e., 46−47% racemo diads and 53−54% meso diads, where the range is determined from both CH and CN peak intensities) (Figure 2, Table 1). If the PAN polymer consisted of mostly syndiotactic regions the rr triads would be most predominant, while for isotactic regions the mm triads would be prevalent. The rr, mr/rm, and mm percentages observed in 13C NMR (Figure 2) suggest a nearly perfectly atactic and completely random (i.e., no isotactic or syndiotactic regions) polymer. As mentioned, the methacrylic acid (MAA) content in the polymer is only ∼4%. Several peaks associated with NMR spectra for MAA36−38 are not observed in this spectrum. Therefore, its contribution to the CH peaks (Figure 2) is not considered to be significant. Fourier-transform infrared spectroscopy was also performed on the polymer. The −COOH peaks associated with the MAA monomer is also not observed at 1650 cm−1.39 For this reason, due to the low content of MAA in the copolymer, the single crystal formation is most probably dictated by the larger PAN segments. As will be discussed in the subsequent sections, based on the electron diffraction analysis no observed peaks that are associated with MAA crystal growth either. In addition, PAN crystal growth is very similar to what is expected for PAN homopolymers.4,5 3.1. Polyhedral Single Crystals. Examples of the dominant crystal morphologies formed in 65 °C crystallizing solution are shown in Figure 3. Because of the variation of the local concentration, some crystals formed relatively large (>600 nm) individual polyhedrons (Figure 3, parts a1 to a4), while most crystals exist in clusters of small-sized (
