Crystallinity Determination of Nylon 66 by Density Measurement and

Laboratory Experiment pubs.acs.org/jchemeduc

Crystallinity Determination of Nylon 66 by Density Measurement and Fourier Transform Infrared (FTIR) Spectroscopy N. Vasanthan* Department of Chemistry, Long Island University, Brooklyn, New York 11201, United States S Supporting Information *

ABSTRACT: Polymer science represents an important area in industrial and research laboratories for chemists and material scientists. However, experiments involving polymers are uncommon in chemistry and material science curricula; therefore, an experiment involving polymers has been developed. This experiment has been used to teach fabrication of polymer films and to investigate the structure−property relationships of polymers using a multi-instrumental approach. It also introduces principles of Fourier transform infrared spectroscopy (FTIR) and how it can be utilized for polymer characterization. This experiment has been introduced successfully in a graduate-level instrumental analysis course in the chemistry department, but is also appropriate for an upper-level undergraduate course. It is simple to conduct in a teaching laboratory and utilizes commonly used instrumentation. Students gain hands-on experience in using multiple instruments. The experiment is designed to be completed in two to three weeks. KEYWORDS: Graduate Education/Research, Upper-Division Undergraduate, Analytical Chemistry, Laboratory Instruction, Polymer Chemistry, Hands-On Learning/Manipulatives, Crystals/Crystallography, IR Spectroscopy, Phases/Phase Transitions/Diagrams, UV−Vis Spectroscopy

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olymer and fiber science represent an important area of work for chemists and material scientists in industrial and research laboratories. However, experiments involving polymers are lacking in most chemistry and material science curricula. A few articles on polymers have been published in this Journal; however, none have been published in the area of crystallinity determination using infrared (IR) spectroscopy.1−6 An experiment is described that can be used to demonstrate the determination of crystallinity of semicrystalline polymers using density measurement and Fourier transform infrared spectroscopy (FTIR) spectroscopy. The experiments can be performed in a two to three week laboratory session, depending on the level, prior background, and participation of the student. This laboratory exercise integrates the following components: fabrication of a material, learning the structure−property relationship, and use of a density gradient column and FTIR spectrometer. Nylon 66 (N66) is among the first synthetic polymers used for film and fiber applications.7−10 N66 is usually synthesized by condensation polymerization, and the repeat units of N66 are held together by an amide linkage. The structure of N66 is shown in Figure 1, and N66 is able to crystallize well due to strong hydrogen bonding. N66 was chosen for this experiment because it is used extensively in packaging and textiles, which are familiar to most students. Knowledge of the relationship between the thermal history, processing conditions, and microstructure changes of these polymers is important to control physical and mechanical properties of N66 films and fibers.11−13 © 2012 American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Structure of nylon 66 with a view of hydrogen bonding planes.

The physical and mechanical properties of polymers depend on microstructure properties such as crystallinity, crystal perfection, crystalline orientation, and amorphous orientation. Crystallinity of semicrystalline polymers has been characterized by various physical methods, such as X-ray diffraction, differential scanning calorimetry (DSC), and density measurements.14,15 However, applications of infrared spectroscopy technique have been relatively scarce.11,12,16,17 Published: January 11, 2012 387

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In this experiment, students prepare nylon N66 films using a Carver Press, crystallize the films by annealing at various temperatures, and measure the density of isothermally crystallized N66 films using a density gradient column. Crystallinity values of annealed films are estimated using known densities of crystalline and amorphous N66. Students also obtain FTIR spectra of annealed N66 films as a function of annealing temperatures and identify the bands associated with crystalline and amorphous phases. A calibration plot may be obtained between crystallinity obtained by density measurement and crystalline and reference band ratios. The primary goal of this laboratory assignment is to use density measurement and FTIR spectroscopy as pedagogical tools to reinforce the theoretical concepts related to the structure−property relationship of polymers. The secondary goal is to introduce experiments related to polymers in upperlevel undergraduate and graduate level within a material, fiber science, or chemistry curriculum. This laboratory experiment was successfully introduced in a graduate-level instrumental analysis chemistry laboratory.

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Figure 2. FTIR spectrum of nylon 66 film annealed at 240 °C taken in the transmission mode.

Table 1. Band Assignments for N66

EXPERIMENTAL METHODS AND RESULTS

Module 1: Fabrication and FTIR Characterization

The first module involved the fabrication of N66 films, crystallization of those films by annealing at different temperatures to prepare films with varying crystallinity, and obtainment of the IR spectra. Students work in pairs, and one five-hour laboratory session is required. N66 pellets purchased from Sigma-Aldrich Chemical Co. were provided to the students, who then prepared films by melt-pressing, using the Carver Press. Ten to twelve pellets were placed between Teflon sheets and were heated above the equilibrium melting temperature of 290 °C for approximately 5 min and subsequently quenched in ice water. The films were peeled from the Teflon sheets and saved in the desiccator for further use. Some of these prepared N66 films were later crystallized by annealing in a convection oven at 40 °C, 80 °C, 120 °C, 160 °C, 200 °C, and 240 °C for 30 min. Crystalline domains are embedded in amorphous matrix in semicrystalline polymers; the size of the crystalline domain increases with thermal treatment, which produces an opaque appearance. Manipulating the morphology and crystallinity of N66 film by annealing is an excellent way of demonstrating how polymer crystallinity changes the appearance of these films. IR spectra were collected on a Nicolet Magna 760 IR spectrometer; at least 64 scans were needed to achieve an adequate signal-to-noise ratio. The spectral resolution was 2 cm−1. The area under the selected IR bands was resolved using the peak area tool in Omnic software. N66 is a semicrystalline polymer and will crystallize well due to strong intermolecular forces such as hydrogen bonding and van der Waals interaction. Many of the IR bands are conformationally dependent. The FTIR spectrum of the semicrystalline film is shown in Figure 2. The band assignments for N66 are listed in Table 1.11,17,18 Students were expected to assign those bands associated with N−H stretching, N−H bending, CO stretching, and C−H stretching vibration and to discuss their observations in the formal lab report. Students were then asked to crystallize the same N66 films at 40 °C, 80 °C, 120 °C, 160 °C, 200 °C, and 240 °C for 30 min, and to obtain FTIR spectra each time. One of the learning

Band Position/cm−1

Assignments

3444 3300 3070 2930 2860 1640 1550 1478 1373 1200 1180 1136 936 924

N−H stretch-free N−H stretch H-bonded N−H overtone Asymmetric CH2 stretch Symmetric CH2 stretch CO stretch N−H bend CH2 scissors CH2 wagging CH2 twist-wagging CH2 wagging C−C stretching CO−NH in-plane CO−NH in-plane

objectives is to show that the absorbance of IR bands associated with crystalline phase increases, the absorbance of bands associated with amorphous phase decreases, and the absorbance of reference bands stays the same with increasing annealing temperature. Therefore students were asked to determine the absorbance of selected IR bands using the software area tool in the region between 1400 and 700 cm−1 and to identify crystalline, amorphous, and reference bands. The FTIR spectra of N66 films annealed at different temperatures are shown in Figure 3. The bands associated with the crystalline phase are usually sharper than those associated with the amorphous phase

Figure 3. FTIR spectra of nylon 66 samples annealed at different temperatures: (a) 40 °C; (b) 120 °C; and (c) 240 °C. 388

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measured by density against absorbance ratio of A1200/A1180 (Figure 4); a linear correlation was observed (r2 = 0.99). This correlation plot could be used to determine the crystallinity of

since multiple conformations are possible in the amorphous phase.11,17,18 Infrared bands at 936 cm−1 (CO−NH in-plane vibration) and 1200 cm−1 (CH2 twist-wag vibration) were attributed to crystalline phase whereas the bands at 924 cm−1 (CO−NH in-plane vibration) and 1136 cm−1 (C−C stretching) were previously assigned to the amorphous phase of N66. It was demonstrated that the absorbance of bands at 1180 cm−1 (CH2 wagging) and 1640 cm−1 (CO stretching) does not depend on crystallinity; therefore these bands can be used as reference bands to monitor crystallinity changes of N66. The instructor also provided a few IR bands associated with crystalline and amorphous phases reported in the literature and asked the students to confirm with their own findings. Module 2: Density Measurement

The second module is to obtain crystallinity by density measurements and correlating them with IR band ratios (crystalline/reference). Students work in pairs, and two fivehour laboratory session are required. The density of a solid can be evaluated in various ways, one of which is to use density gradients having values above and below the density of samples. A density gradient can be established in a vertically held cylindrical column by mixing various proportions of high and low density liquids (heptane and carbon tetrachloride for nylons) so that the density in the liquid increases linearly from the top to bottom. This method can be used for any polymer that has a minimal solubility in the two liquids used in the column. Polymer density was measured on small pieces of the N66 films, using a density gradient column at 23 °C. At least three determinations were made to obtain average values. Samples were allowed to sit at least 12 h before a reading was taken, to ensure equilibration in the gradient column. The N66 samples used must be free of pores or air bubbles to obtain density accurately. The precision of the density measurement is usually ±0.1% or less, if the temperature is carefully controlled. The density of N66 films annealed at different temperatures from 40 to 240 °C is given in Table 2. The volume fraction crystallinity, χ, of N66 was determined by15

Figure 4. Calibration curve, crystallinity measured by density measurement versus absorbance ratio of 1200 cm−1 and 1180 cm−1.

N66 films. A correlation can also be established for density crystallinity against A1200/A1640 if the sample is thin enough to get the absorbance within linearity range. This method can be applied to other semicrystalline polymers if clear crystalline and reference bands are identified.

Annealing Temp/°C

Density/(g/cm3)

Cryst Fraction

A B C D E F G

25 40 80 120 160 200 240

1.130 1.135 1.140 1.145 1.150 1.160 1.165

0.27 0.30 0.33 0.37 0.40 0.47 0.50

χ = (ρ − ρa)/(ρc − ρa)

HAZARDS

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SUMMARY

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ASSOCIATED CONTENT

Normal laboratory safety procedures should be followed. Eye protection must be worn, and gloves are recommended during preparation of samples. A laser is in use during the FTIR spectrometer portion of the experiment. Do not directly view the laser, as it can damage the eyes. Extreme care must be taken when dealing with solvents and with the Carver Press.

It is important for students in material science, polymer science, fiber science, and chemistry to be familiar with polymers and various characterization techniques. This laboratory experiment introduces students to the fabrication of polymer films and to structure−property relationships while using a multi-instrumental approach to solve a problem. The students gain openended and research-like experience. The experiment also introduces principles of FTIR spectroscopy and how it can be utilized for polymer characterization. The exercise has been successfully introduced in a graduate instrumental analysis course; it is simple to conduct and does not require expensive instruments.

Table 2. Density and Crystalline Fraction of Nylon 66 Films Prepared by Melt Pressing Sample

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(1)

where ρ is the measured density, ρa is the density of the amorphous phase, and ρc is the density of the crystalline phase.15 The crystalline density is usually obtained from X-ray diffraction measurement, and fully amorphous film can be obtained by quickly quenching from the melt. The values for the density of the amorphous, ρa, and crystalline, ρc, phases of N66 were 1.090 and 1.240 g/cm3, respectively.11 The crystallinity obtained by density (eq 1) is also given in Table 2. A calibration plot was obtained by plotting crystallinity

S Supporting Information *

Student handout. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 389

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Laboratory Experiment

ACKNOWLEDGMENTS I would like to thank Long Island University for financial support. I also thank all of my graduate students for assistance with sample preparation and data collection.

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REFERENCES

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