Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Relating Post-yield Mechanical Behavior in Polyethylenes to Spatially Varying Molecular Deformation Using Infrared Spectroscopic Imaging: Homopolymers Prabuddha Mukherjee,† Ayanjeet Ghosh,† Nicolas Spegazzini,† Mark J. Lamborn,‡ Masud M. Monwar,‡ Paul J. DesLauriers,*,‡ and Rohit Bhargava*,†,§ †
Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ‡ Chevron Phillips Chemical Company LP, Bartlesville, Oklahoma 74004, United States § Departments of Bioengineering, Chemical and Biomolecular Engineering, Electrical and Computer Engineering, Mechanical Science and Engineering and Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: Stress−strain curves derived from tensile specimens are the primary characteristic of bulk polymers’ mechanical properties. Current tools, however, cannot provide molecular insights from this single bulk measurement. Hence, we use Fourier transform infrared (FT-IR) spectroscopic imaging to optically and nondestructively measure molecular structure and its spatial dependence in tensile specimens in high density polyethylene homopolymers. To overcome the limitations of FT-IR imaging, we use an emerging approach involving the use of tunable quantum cascade lasers that allows imaging through thick samples and facile polarized light imaging. Crystal structure and orientation are obtained from spatially varying measurements of molecular properties in the necking region. Local molecular (re)arrangements to characterize mechanical properties of drawn samples are deduced from spectral data. A modified Eyring model was developed to quantitatively understand spatial dependence in terms of a conformational volume. We report the strain rise in high density polyethylene homopolymers is governed by the degree of association between the crystalline domains. Together, the new measurement technology and analysis reported here can relate molecular composition, microscopic gradients, and orientation to bulk mechanical properties of semicrystalline polymers.
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INTRODUCTION The tensile stress−strain curve is a classic characteristic of polymeric materials’ mechanical properties. Molecular structure and composition underlie these properties as do environmental conditions such as temperature and time. Thus, it is desirable to understand molecular compositions as well as changes and their influence on emergent properties of materials. Infrared (IR) spectroscopy is among the most accessible techniques for molecular composition and conformation information; its extensive use for polymer characterization has been well documented.1−5 For complex processes, as for example the strain hardening of high-density polyethylene (HDPE) and its blends, IR spectra can provide a link between molecular changes and resin properties. It is well-known that there are several length scales of importance that span from molecular to millimeter in a tensile sample;3 yet, a vast majority of spectroscopic studies have been performed using bulk, nonspatially resolved spectral measurements. We show here that with emerging IR spectroscopic imaging techniques6 it is possible to probe these different length scales to bridge the information between molecular and crystalline dimensions and derive an accurate relation of local molecular properties to the material’s bulk strength. © XXXX American Chemical Society
The effect of evolving morphology and polymer microstructure on post-yield behavior is still a subject of significant interest. Among the most relevant and simplest polymers to study are high density polyethylenes. During stretching of a standard mechanical testing dog bone geometry sample (Figure 1, left), for example, HDPE passes through the yielding region at low strain values (10−15%), starts to neck, and then enters the domain where the load/stress falls to a constant value and high magnitude deformations begin to occur.7 Mechanical and molecular anisotropy is induced in the sample, as the aggregate of nearly identical crystal units that are oriented randomly in the semicrystalline polymer now rotate and align upon being stretched.8−10 Bulk spectroscopic techniques obviously would not be able to visualize these transitions, though the average molecular orientation is manifested in optical anisotropy measurements such as dichroic ratios. In particular, we seek to understand necking deformations (Figure 1, middle) that measure the instability of the sample and are an important indicator of its rigidity that we intend to characterize in detail. Received: February 16, 2018 Revised: April 25, 2018
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DOI: 10.1021/acs.macromol.8b00363 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Schematic illustration of how the spatial variation of molecular response in a polymer can be used to evaluate mechanical properties spectroscopy and imaging. (Left) The physical process of necking, demonstrating the focus of the study on the necking region. (Middle) A representative stress−strain curve for HDPE overlaid on the cartoon of a necked polymer. (Right) Drawn tensile samples exhibit optical anisotropy beyond the yield point that arises from molecular reorganization and are seen in polarized infrared spectra. Anisotropy measurements by imaging allows visualization of the spatial distribution of molecular orientation in the entire sample, with localized sensitivity.
Table 1. Physical and Mechanical Parameters (Pre- and Post-yield) of the Three Homopolymersa
a
Sample
Mn/1000 kg/mol
Mw/1000 kg/mol
Mz/1000 kg/mol
PDI
Annealed Density g/cm3
Yield Strain (%)
NDR (%)
Max Strain* (%)
Break Strain (%)
Localized Strain (%)
mPE-4 mPE-5 mPE-7
68 63 95
241 289 400
520 589 785
3.6 4.6 4.2
0.953 0.950 0.944
17 8.9 8.4
440 380 354
1260 1125 1207
1580 1530 1490
87 85 96
The symbol * refers to the maximum strain level for tensile specimens scanned along direction of stretch from un-necked to necked regions.
polyethylenes, bulk spectroscopic measurements and models5,9,10,22−26 have long focused on analyses of its vibrational spectrum. The origin and properties of specific PE vibrational modes are well established, for example, CH2 rocking, wagging, and bending modes at 725, 1368, and 1463 cm−1 respectively.1 Hence, an opportunity to understand spatial structure in PE, even under stress, lie in imaging the material at these specific vibrational modes. Emerging discrete frequency IR (DFIR) measurements using a quantum cascade laser (QCL) source can enable such measurements over large areas.6,27,28 The laser can be tuned to specific vibrational modes indicative of specific sample properties, and the high intensity allows for thick samples to be analyzed. Further, the inherent polarization of the laser allows for facile anisotropy measurements. Figure 1 (right) conceptualizes relating local IR measurements to the mechanical strength of the tensile specimens via a spatial map. Here we use spectroscopic imaging on drawn PE samples to measure not only the spatial variation in a sample but also molecular orientation for amorphous and crystalline domains. Finally, we correlate the evolution of local strain in polyethylene samples to bulk strain hardening moduli.
The stress−strain plot reflects the mechanical properties averaged lengthwise across the specimen gauge section. Consequently, strain measurements between the yield point and the onset of strain-hardening contain contributions from both necked and non-necked regions of the specimen. While tensile measurements are good measures of bulk behavior, the concept of local strain needs to be included for a molecular level understanding of polymer strength. When the bulk average strain shows values close to yield, a small fraction of the neck-region could be under enormous strain equal to the natural draw ratio (NDR) and beyond. The resulting combination of structures due to stress relaxation, NDR, and strain hardening comprise a boundary region that broadens upon further stretching in post-yielded samples. Hence, a detailed recording of molecular state within these structures is needed for an improved understanding of post-yield behavior. In this report, we focus on employing vibrational spectroscopy and imaging to examine the local morphological changes that occur in strained PE samples using primarily a model set of metallocene catalyzed homopolymers. In the second part of this study, we will further examine applying the methods developed and understanding the homopolymer behavior to copolymer resins. IR vibrational spectroscopy is sensitive to molecular structure; its combination with imaging provides a relation between sample morphology and polymer microstructures.11−17 While the measurement of molecular orientation using IR spectroscopy is well-known,18,19 emerging spectroscopic imaging can quantify the structural anisotropy across the entire sample. Fourier transform infrared (FTIR) microscopy has been used to measure semicrystalline polymers’ morphology and dissolution behavior20,21 but has been restricted to thin (∼10 μm), prepared samples and not to samples used for mechanical tests. FTIR imaging that typically measures full spectra over a field of view (∼0.5 × 0.5 mm2) is too impractical to routinely measure typical tensile-test samples (12 × 4 mm2). While IR imaging has not been used to measure structure in
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EXPERIMENTAL METHODS AND MATERIALS
Sample Preparation. Polymer samples (mPE-4, mPE-5, and mPE-7) were synthesized at Chevron Phillips Chemical Company LP using supported metallocene catalysts and range in weight-average molecular weight (Mw) values from 241 to 400 kg/mol. The densities reported in Table 1 were measured on annealed untested tensile coupons (as prescribed by ISO 18488). As expected, the measured annealed density value in the initial tensile coupon decreases as the molecular weight of the sample increases. ISO Test. Tensile tests were performed following the ISO 18488 standard, with the exception that the current work required very thin (∼0.05 mm thick) tensile specimens. Rather than use a standard picture frame mold, thin specimens were produced by sandwiching 0.5 g of resin pellets between Kapton sheets and flat steel backing plates during compression molding. All other aspects of the ISO 18488 standard were followed. Primary properties measured from the tensile B
DOI: 10.1021/acs.macromol.8b00363 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. (A) Stress−strain relationships of three homopolymers drawn beyond the NDR. A representative series of FTIR spectra are shown in (B) on top of the necked polymer (mPE-5) image. Each of these spectra correspond to the specific region of the polymer on which they lie (approximately 100 μm2 area). Two different spectral regions are highlighted: one that corresponds to the low frequency crystalline peaks (720−730 cm−1 region; inset illustrates the spectral fitting as discussed in the text), and the other corresponds to a combination band at 2019 cm−1 for calibrating the thickness of the sample. (C) Correlation between the crystallinity of the polymer sample with the fractional decrease of polymer thickness due to application of stress. The solid line (black) is a guide. (D) Measured and fitted FTIR spectra of the stretched (beyond NDR) region of the polymer mPE-5 for the polarization sensitive −CH2 vibrations (see text). (E and F) QCL spectra of the same polymer region under parallel and perpendicular polarizations, respectively. Fits to the spectra are shown in light gray for crystalline and blank for amorphous modes, respectively.
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tests are the natural draw ratio (NDR) and the strain hardening modulus ⟨GP⟩; these properties appear in Table 1. Both NDR and ⟨GP⟩ have been found to correlate reasonably well with PE resin stress cracking performance.29,30 Fourier Transform Infrared (FTIR) Spectroscopy. For FTIR imaging measurements, the polyethylene necked samples were laid flat on a low emission slide (MirrIR) and were used for all subsequent IR measurements. IR spectra were recorded with a PerkinElmer Spotlight 400 (PerkinElmer, Waltham, MA, USA) equipped with a thermal source, in transflection mode using a liquid cooled MCT detector. Both the background and reflection spectra were collected at 2 cm−1 spectral resolution (with 680−4000 cm−1 saved) in the point detection mode with an IR aperture of approximately 100 μm. IR spectra were collected on a line every 30 μm for ∼9 mm. The final data resulted from an average of 32 scans, were atmosphere corrected with the PerkinElmer software, and further processed and fitted using Matlab R2016. The crystallinity values for the samples were obtained by fitting the IR spectra at the 700−750 cm−1 region. Quantum Cascade Laser Infrared QCL-IR. A QCL-based singlepoint detector system, a prototype developed by Agilent equipped with a 0.72 numerical aperture refractive objective and a room temperature bolometer detector, was used for the polarization IR measurements. Two sets of polarization, one parallel to the direction of the stretch and the other perpendicular to it, were used to collect IR spectra and hyperspectral IR data sets. The wavelength range for all the spectral scans was set to 800−1900 cm−1 (with 128 coadditions), while, for the images, a 1320 to 1500 cm−1 range was used where the dwell time on a single point for each wavenumber was 0.01 s. The spectral resolution for both the images and the spectra were 2 cm−1. The hyperspectral data were collected with apertures of 5 μm per edge of the image pixel at the sample plane. For the polarization anisotropy analysis the baseline IR intensity was set at 1406 cm−1, where the sample did not absorb any IR. All the fitting procedures, image registration, and analysis are carried out with the Matlab R2016 software. Following the ISO test method, two specimens of all the three polymers were tested. One was stretched beyond the NDR to obtain the relevant stress−strain values at strain hardening, and the other one was stretched to
