Abstract

Fourier transform infrared spectroscopy (FTIR) is a widespread instrumental technique used to determine the molecular structure of organic polymers. With the goal of increasing instrumental use in the undergraduate chemistry laboratory, we have devised a quantitative FTIR experiment to be used in instrumental analysis courses. The objective was to determine the % PVOH in copolymers with PE as well as to determine the % PVAc in copolymers and blends with PVP. We report on the methodology used to determine % PVOH and %PVP, as well as the results obtained via FTIR and elemental analysis. The literature regarding PVOH-PE copolymers refers to them as either EVOH copolymers or PVOH-PE copolymers, though EVOH is more common and thus these copolymers will be referred to as EVOH. The EVOH copolymers ranged from 73 to 56 mole% PVOH and the copolymers and blends of PVP-PVAc ranged from 70 to 30 mole% PVP.

1. Introduction

Infrared spectroscopy (IR) is a routine technique taught in many undergraduate organic, analytical, and instrumental analysis courses. It is used consistently for both qualitative and quantitative analyses ^{1, 2, 3, 4, 5, 6, 7, 8, 9}. In order to increase the signal to noise ratio of the IR signal, Fourier transform infrared spectroscopy (FTIR) was developed and has become a critical tool of chemists of all kinds in determining chemical structures. The utility of this technique has been well documented for determining the composition of copolymers in general ^{10, 11, 12, 13, 14, 15} as well as for EVOH copolymers in particular ^{15, 16, 17, 18}. Mieth and coworkers, for example, utilized FTIR to determine the presence of EVOH in multilayer films used in food contact materials ¹⁹. Quality control has also been done with FTIR, Cobranchi, Rintoul, and Tani ¹⁶ utilized FTIR to design a high throughput method of quality control of plastic products. Due to the omnipresence of instrumental work in the field of chemistry, our goal was to develop an undergraduate chemistry laboratory experiment designed to highlight both the qualitative and quantitative aspects of FTIR analysis. FTIR’s functionality is based on the premise that within any polymer the atoms or groups of atoms vibrate with unique, sharply defined frequencies characteristic of the compound. These frequencies lie in the infrared region of the electromagnetic spectrum. When the sample is placed in the IR beam it absorbs energy at specific frequencies characteristic of the molecule and transmits all other frequencies. The resulting pattern is known as a molecular infrared vibrational absorption spectrum. The intensity of the absorption is a reproducible function of the amount present. The objective of our experiment is to determine the % composition of polyvinyl alcohol (PVOH) in copolymers with polyethylene (PE) as well as to find the % composition of Poly n-vinyl pyrrolidone (PVP) in copolymers and blends with Polyvinyl acetate (PVAc). The EVOH and PVP-PVAc copolymers were selected because they are all commercial copolymers and are used in many everyday products. For example, ethylene-vinyl alcohol (EVOH) copolymers have many varied uses, in the food packing industry it is used as an oxygen barrier since it is superior to other plastics at keeping air out and flavors in. EVOH copolymers also have applications in the medical field as a critical component of Onyx, an FDA approved method for treating saccular aneurisms. Copolymers of n-vinyl pyrrolidone and vinyl acetate monomers are of critical importance in the pharmaceutical and cosmetic industries. Applications include hair styling, skin care, membranes for medical devices, water treatment, and as adhesives. The ratio of PVP to PVAc and PVOH to PE can significantly affect the physical properties of the final products, thus, there is a need for a quick method of quantifying the composition of these copolymers.

1. To record and analyze FTIR data on commercially available EVOH and PVP-PVAc copolymers of various compositions.

2. To prepare calibration curves plotting the absorbance ratio of specific peaks corresponding to unique moieties found in the substituent polymers in EVOH and PVP-PVAc copolymers versus % composition determined via elemental analysis.

3. To prepare and contrast calibration curves of PVP-PVAc copolymers and blends plotting absorbance ratios of carbonyl and acetate peaks versus experimental weight % composition.

2. Experimental Section

This experiment has three parts and each part can be done by a group of 2-3 students.

The first part is the analysis of EVOH copolymers with the objective of recording and analyzing FTIR data of EVOH random copolymers of differing compositions. Setting up a calibration curve by correctly identifying key peaks associated with the PVOH and PE components of the EVOH copolymers and plotting the ratio of those two quantities versus the PVOH content gained by elemental analysis.

Part 2 is similar to part 1 except PVP-PVAc copolymers will be studied in lieu of EVOH.

Part 3 is similar to part 2 except PVP-PVAc blends prepared by the instructor will be analyzed in lieu of copolymers.

The instrument used in this study was a Bruker Alpha-P®FT-IR spectrometer with a Platinum® attenuated total internal reflectance (ATR) quick snap module with a diamond crystal. The Alpha-P unit has a Michelson Interferometer with a SiC globar as the IR source, the time dependence of the IR intensity is measured with a pyroelectric DTGS detector. The data analysis was done using the Bruker OPUS® software version 7.0.122. The spectra were displayed as absorbance vs frequency in wavenumbers ranging from 500 to 4000 cm^-1 since absorbance is proportional to concentration according to Beer’s Law. All of the EVOH copolymers were obtained as solids in the form of pellets from Scientific Polymer Products (SP²), Inc., Ontario, N.Y. The EVOH copolymers were randomized copolymers ranging from 73 to 56 mol% PVOH. The PVP-PVAc copolymers were also obtained from SP². These copolymers ranged from 30 to 70 mol% PVP. The 30, 50, and 70% copolymers were obtained as 50 w/w% solutions in isopropanol. Blends of PVP and PVAc were prepared by dissolving PVP homopolymers having a Mw of 40K Daltons with homopolymers of PVAc having a Mw of 39K Daltons in methanol. These solutions were evaporated in a vacuum oven at 50°C at 20 mm of Hg leaving solid blends of homopolymers ranging from 20 to 80% PVAc.

The EVOH copolymer pellets were made into thin films manually to increase surface area for FTIR spectroscopic analysis. The PVP-PVAc samples were analyzed with no sample preparation other than being pressed between the anvil and the diamond crystal of the ATR unit in solid phase. The samples were measured using 16 scans at a resolution of 4 cm^-1. Establishing the PVAc content was done using the ratio of absorbance of the carbonyl or C-O peak from the acetate of PVAc to the carbonyl peak from the PVP. The PVOH content was determined by the ratio of absorbance of the alcohol oxygen or OH peak of PVOH and the C-H peak of PE. Elemental Analysis (EA) of the EVOH and PVP-PVAc copolymers were performed by the Galbraith Laboratories, Knoxville, TN. The %C & %H were obtained for the EVOH copolymers whereas the %C, %H, %N were obtained for the PVP-PVAc copolymers.

There are no safety hazards in this experiment; all samples are commercially available materials used in everyday products in solid form. Safety glasses are mandatory and the use of hoods is advisable. All waste materials were collected for waste disposal according to EPA as well as local guidelines.

Each sample was analyzed eight times. The data reported is the average of the integrated peak areas obtained by FTIR. The standard deviations ranged from 0.259 to 0.0068. The elemental analysis of the %C, %H, and %N were reported as averages from duplicate analyses.

3. Results and Discussion

Figure 1 through 5 are FTIR spectra of PE, PVAc, PVP, EVOH, and PVP-PVAc copolymers respectively. Figure 1 shows the FTIR spectrum of 100% PE, there are characteristic peaks at 1470cm^-1 and 715cm^-1 with the 1470 cm^-1 (C-H bending) peak being used to quantify the presence of PE in EVOH copolymers due to higher resolution than the 715 cm^-1 peak. Figure 2 shows the FTIR of PVAc which exhibits an intense ester peak at 1230 cm^-1 (C-O stretch), this would be the peak used to quantify the presence of PVAc in the PVP-PVAc copolymers due to the fact that PVP does not exhibit a strong peak at 1230cm^-1. Figure 3 includes the FTIR spectrum of PVP which has a characteristic peak at 1650 cm^-1 (C=O stretch). Figure 4 showcases an EVOH copolymer with 68 mole % PVOH with characteristic peaks at around 3300 cm^-1 (O-H stretch) owing to the OH functionality of PVOH as well as a peak at 1430 cm^-1 coming from the PE aspect of the EVOH copolymer. Figure 5 shows a 50 mole %PVP - PVP-PVAc copolymer with characteristic peaks at 1670 cm^-1 due to the carbonyl functionality of PVP as well as a peak at 1230 cm^-1 from the acetate moiety of PVAc.

Table 1 displays the data obtained via elemental analysis of the various EVOH copolymers analyzed which ranged from 73% to 56% PVOH, included is the % C, % H, and % O as well as the calculated % PVOH. The % PVOH was calculated using the following equation:

(1)

Figure 6 shows the experimental % PVOH plotted versus the nominal % of PVOH indicated by SP² displaying a straight line relation with R² value of .9647 indicating that the experimental amount of PVOH present and that provided by SP² correspond very closely.

Table 2 is a summary of the FTIR peak area ratios from the 3300 cm^-1 band associated with the PVOH substituent and the 1430 cm^-1 band associated with the PE constituent polymer. All values are presented in association with their corresponding nominal mole% PVOH constitution. As expected, the ratio increases as PVOH content increases. Figure 7, Figure 8, and Figure 9 depict the strong linear relationship between % PVOH content and the PVOH:PE peak area ratio. All show remarkable linearity, with the strongest correlation coefficient of .9998 belonging to Figure 7 which plots nominal mole % PVOH versus the integration ratios. All applicable figures include error bars which represent one standard deviation from the mean with n = 8.

Table 3 shows the elemental analysis data for the PVP-PVAc copolymers which includes % Carbon, % Hydrogen, % Nitrogen, and % Oxygen obtained by difference. These copolymers ranged from 30-70 mol% PVP, based on the data from the elemental analysis, the experimental % PVP was determined using the following equation:

(2)

The experimental Wt% PVP was plotted versus the nominal Wt% PVP and is shown in Figure 10, a linear relationship is shown having a correlation coefficient of .9642 indicating that the experimental weight % PVP corresponds well with the nominial weight % PVP reported by SP².

Table 4 displays the FTIR integration ratio of the PVP peak (1670 cm^-1): PVAc peak (1224 cm^-1) data alongside the nominal mole % PVP, nominal weight % PVP, and experimental weight % PVP data. In all cases as the % PVP increases the ratio of the 1670/1230 cm^-1 peak also increased. Figure 11, Figure 12, Figure 13 graph the data shown in Table 4, all graphs show a strong linear relationship between the integration ratio and % PVP with the highest R² value being .9933 belonging to Figure 11 which plots nominal mole % PVP versus average PVP: PVAc peak area ratios. All applicable figures have error bars that represent one standard deviation from the mean with n = 8.

Table 5 summarizes the FTIR integration ratios of the PVP-PVAc blends. As in the case with the copolymers, the ratio strictly increased as the weight % of PVP increased, a strong linear relationship was observed with an R² value of .9987, this is showcased by Figure 14 which plots the data summarized in Table 5. Figure 14 has error bars that represent one standard deviation from the mean with n = 8.

4. Conclusions

FTIR both alone and in tandem with elemental analysis can be used to quantify the composition of both PVP-PVAc and EVOH copolymers with very accurate results. FTIR can also be used to determine the composition of blends of PVP-PVAc by using peak area ratios, corroborating what was found in previous experiments ¹⁵. The FTIR results align very closely with the results obtained via elemental analysis. The above analysis showcases the importance of integration ratios in quantitative analysis at large and supports the hypothesis that FTIR can be used quantitatively to determine the composition of binary polymer blends and copolymers. The methodology used can be quickly adopted for use in an undergraduate instrumental analysis course due to the pervasiveness of FTIR within the academic community as well as the ease of use. This experiment also enhances students’ knowledge of the FTIR methodology, giving them hands-on experiences in analyzing functional group characteristics of the copolymers studied and applying them to create calibration curves.

Acknowledgements

We acknowledge the support from Hofstra University HCLAS Faculty Research and Development Grant.

Statement of Competing Interests

The Authors have no competing interests.

List of Abbreviations

PVP – Polyvinyl pyrrolidone

PVAc – Polyvinyl acetate

PVOH – Polyvinyl alcohol

PE – Polyethylene

EVOH –Ethylene Vinyl alcohol, same as PVOH-PE

References