Effect of Dimethyl Terephthalate and Dimethyl Isophthalate on the

Jan 5, 2018 - Abstract Image. The effect of blending two low molecular weight diluents (LMWD), dimethyl terephthalate (DMT) and dimethyl isophthalate ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Effect of Dimethyl Terephthalate and Dimethyl Isophthalate on the Free Volume and Barrier Properties of Poly(ethylene terephthalate) (PET): Amorphous PET S. Zekriardehani,† A. S. Joshi,† S. A. Jabarin,† D. W. Gidley,‡ and M. R. Coleman*,† †

Department of Chemical Engineering, University of Toledo, Toledo, Ohio 43606, United States Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States

‡

S Supporting Information *

ABSTRACT: The effect of blending two low molecular weight diluents (LMWD), dimethyl terephthalate (DMT) and dimethyl isophthalate (DMI), on the microstructure, free volume, and the barrier properties of poly(ethylene terephthalate) (PET) was investigated. Incorporation of the additives at loadings up to 3 wt % led to a substantial improvement in the barrier properties of PET to oxygen, carbon dioxide, and helium. Although the additives have similar structures, DMI showed enhanced barrier improvement at similar loadings compared to PET/DMT samples. Positron annihilation lifetime spectroscopy as well as density measurements indicated that there was a reduction in the fractional free volume of polymer upon incorporation of the LMWDs. Dynamic mechanical analysis disclosed that there was a reduction in the activation energy for mechanical relaxation due to a decrease in chain motion of PET in the presence of DMT or DMI. There was a reduction in permeability upon increase of LMWDs that was primarily due to the reduction in diffusivity which is consistent with decrease in free volume and chain mobility.

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INTRODUCTION Poly(ethylene terephthalate) (PET) has been extensively used as a packaging material for decades. There is an interest in enhancing the barrier properties of PET films and bottles to increase the shelf life of packaged products. Copolymerization,1,2 addition of oxygen scavengers,3 incorporation of nanomaterials,4−6 use of multilayer films,7 and strain-induced crystallization8,9 are a few examples of methods that have been used to improve the barrier properties of PET. However, a more economical method is needed to avoid complicated processing conditions of producing multilayered films, high cost of the nanoparticles, and limitation of the maximum crystallinity level using strain-induced crystallization. Incorporation of small amounts of low molecular weight diluents (LMWD) can provide improved barrier properties without requiring dramatic change in processing conditions and is usually inexpensive.10−13 The presence of low molecular weight diluents (LMWDs) can enhance the workability, flexibility, and ductility of glassy polymers by a phenomenon called plasticization. However, at low concentration, such additives can make the polymer/LMWD blends more brittle and stiffer, a phenomenon termed antiplasticization.14 The presence of low molecular weight diluents in a polymer matrix can affect the free volume and chain dynamics and thus reduce the permeability of small molecules in the polymer/LMWD samples.12,13,15,32 Maeda et al.15 observed a reduction in permeability of polysulfone following addition of 4,4′-dichlorodiphenyl sulfone, tricresyl © XXXX American Chemical Society

phosphate, and N-phenyl-2-naphthylamine at loadings up to 30% and attributed this to reduction in local segmental motion that lead to antiplasticization by the low molecular weight diluents. Lee et al. investigated the effect of phenacetin and acetanilide as antiplasticizers on the barrier properties of PET and concluded that combined reduction in the free volume and chain dynamics are the main reasons for improvement of the barrier properties of PET/additives.12 Burgess et al.13 studied the effect of caffeine on the barrier properties of amorphous PET and showed that incorporation of 10 wt % caffeine can improve barrier properties for both oxygen and carbon dioxide up to around 300%. Additionally, low molecular weight additives can affect the secondary relaxation processes of poly(ethylene terephthalate)12,16,17 by reducing the localized chain mobility, lowering the activation energy for mechanical relaxation of PET segments, and suppressing the phenyl ring flipping. It has been proposed in the patent literature that incorporating up to 5% of dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), diethyl phthalate (DEP), or diphenyl phthalate (DPP) can significantly improve the shelf life of PET bottles.32 To the knowledge of the authors, there is no detailed analysis available on the impact of these types of LMWDs on the microstructure and barrier properties of PET. The DMT and DMI, shown in Table 1, were Received: October 18, 2017 Revised: December 31, 2017

A

DOI: 10.1021/acs.macromol.7b02230 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Structures and Physical Properties of PET, DMT, and DMI

*

Solubility parameter based on Hoftyzer−Van Krevelen: δ2 = δd2 + δp2 + δh2.21 position of methyl ester groups, and they both show very high compatibility with PET based on their solubility parameters. As shown in Table 1, DMI is more compatible with PET and DMI may have a greater effect on the microstructure and barrier properties of PET than the DMT, as the values of (δPolymer − δadditive)2 are 0.37 and 0.04 J/cm3 for PET/DMT and PET/DMI, respectively. Note that both DMT and DMI are much more compatible with PET than acetanilide and phenacetin. Thermogravimetric Analysis (TGA). A Q50 TGA instrument (TA Instruments) was used to quantitatively evaluate the concentration of the additives in the PET films.12 All the measurements were carried out under nitrogen purge at a rate of 40 mL/min. Samples were cut from different parts of the films to confirm an even dispersion of the additives. Samples were heated to 100 °C at a rate of 10 °C/min and held for 30 min at 100 °C in order to remove the water in the polymer. The temperature was subsequently increased to 270 °C at a rate of 10 °C/ min and held for 60 min to evaporate the additive from the polymer. The concentration of additive was estimated for each film from the difference between the mass loss of pure PET and PET/additive samples. In a separate experiment, TGA was used to find the degradation temperature of PET and PET/additive samples from room temperature to 600 °C at a rate of 10 °C/min. The temperatures at which each sample lost 10% and 50% of its mass are reported. Nuclear Magnetic Resonance (NMR). NMR was used to confirm the presence of additives in the extruded films and to investigate possibility of chemical reaction of additives with the PET matrix. 100 mg of dried samples was dissolved in 1 mL of chloroform-d and trifloroacetic acid-d mixture (80−20%).22,23 NMR spectra were recorded using a Bruker Avance III spectrometer, operating at 600 MHz (1H) and 150 MHz (13C) at room temperature. Haze. High transparency is a critical characteristic of PET films and bottles and must be maintained upon inclusion of additives. Haze measurements were conducted on films with a Haze-Gard plus Meter 4725, BYK-Gardner, Germany, to ensure that the presence of additives does not affect the transparency. Rheometer. A parallel-plate Rheometric Scientific dynamic analyzer (RDA III) with parallel disks at 270 °C in the presence of nitrogen at 15% strain amplitude was used to determine the intrinsic viscosity (IV) of each sample. Samples were vacuum-dried overnight at 100 °C to prevent hydrolytic degradation of PET during testing. The method of conversion of melt viscosity to intrinsic viscosity was described by Tharmapuram et al.24 Melt viscosity measurements were applied to

chosen as antiplasticizers for this study because of their low price, high affinity for PET, and ease of processing. Additionally, toxicological studies report negligible to low order of acute and chronic toxicity of the phthalate esters.18,19 The goal of this study was to investigate the effect of antiplasticizers at low loading of similar structures (DMT vs DMI) on the microstructure, free volume, free volume distribution, chain mobility, and the gas transport properties of PET films.

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EXPERIMENTAL SECTION

Materials and Sample Preparation. Resin selection was critical for this project since the residual catalyst in PET resin can cause reaction between the additives and PET and lead to molecular weight loss.32 While resins containing cobalt, antimony, manganese, magnesium, and cadmium exhibit substantially reduced molecular weight and intrinsic viscosity (IV), after incorporating DMT or DMI, there was little IV loss for titanium- and aluminum-based PET resins.32 An aluminum catalyst based PET resin (Laser+7000) with an intrinsic viscosity of 0.84 ± 0.02 dL/g was generously donated by DAK Americas to minimize IV loss on incorporation of DMT or DMI. DMT and DMI, shown in Table 1 with their chemical structure, were purchased from Sigma-Aldrich and used without purification. The PET pellets were dried at 140 °C overnight in a Conair S60 dehumidifier dryer with a CH16-4 hopper dryer system, and additives were dried for 2 h at 100 °C prior to mixing. A HAAKE Rheomex Brabender single screw extruder was used to mix PET pellets with ∼1 and 3 wt % DMT and ∼1 wt % DMI and processed into films with ∼0.6 mm thickness. The films were thinned to 50−100 μm at a very low speed, 0.2 cm/s, to avoid crystallization, using a long extensional tester (LET) built by T.M Long Co., Inc.20 Differential scanning calorimetry confirmed that the resulting films were amorphous. The effect of LMWDs on the microstructure and resulting barrier properties of PET is not well understood. Lee et al.12 suggested that acetanilide is more compatible with PET compared to phenacetin as indicated by difference in the solubility parameters, (δPolymer − δadditive)2, which was 2.76 and 4.96 J/cm3 for PET−acetanilide and PET− phenacetin, respectively. As a result, they observed a more pronounced improvement in barrier properties for the PET−acetanilide blends compared to PET−phenacetin. DMT (dimethyl p-phthalate) and DMI (dimethyl m-phthalate) have the same chemical structures except for the B

DOI: 10.1021/acs.macromol.7b02230 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. 1H NMR spectra of PET/DMT blends: (I) PET, (II) PET-3.3DMT, (III) PET-1.0DMT, and (IV) DMT in chloroform-d (#) and trifluoroacetic acid-d mixture (80−20% v/v). Inset indicates increase in intensity of methoxyl peak at 4.02 ppm (e) with higher concentration of DMT.

Figure 2. 1H NMR spectra of PET/DMI blends: (I) PET, (II) PET-1.5DMI, and (III) DMI in chloroform-d (#) and trifluoroacetic acid-d mixture (80− 20% v/v). Inset highlights the peak at 4.02 ppm (e) arising from methoxyl protons in DMI. additive concentration.25 All the measurements were at 23 °C to prevent temperature-dependent density changes. Differential Scanning Calorimetry (DSC). A PerkinElmer DSC 7 (Shelton, CT) was used at a heating rate of 10 °C/min to measure the thermal characteristics of PET films as a function of additive concentration.

evaluate the intrinsic viscosity (IV) of the PET and PET/additive samples and to monitor the extent of any IV loss due to reaction between additives and PET. Density Gradient Column. Water and calcium nitrate solutions of two different densities were used to produce a density gradient column with a linear gradient to monitor the density of the films as a function of C

DOI: 10.1021/acs.macromol.7b02230 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. 13C NMR spectra of (I) physical blend of PET with 3.3 wt % of DMT and (II) extruded blend PET-3.3DMT in chloroform-d and trifluoroacetic acid-d mixture (80−20% v/v) (#). Insets highlight the carbonyl carbon region (165−170 ppm) and benzylic carbon region (125−140 ppm). Dynamic Mechanical Analyses (DMA). DMA was employed to study the chain dynamics of PET and PET/additive samples using a Q800 TA Instruments DMA. Temperatures ranging from −100 °C to room temperature were chosen to capture the β relaxation peak of PET and to study the activation energy for mechanical relaxation.16,26 The strain amplitude was adjusted to 0.06% in order to perform measurements in the linear viscoelasticity region, and each measurement was performed at frequencies of 1, 2, 5, and 10 Hz. Positron Annihilation Lifetime Spectroscopy (PALS). Sixteen sheets, each about 50−100 μm thick, were stacked on either side of the 22 Na source to ensure that positrons from the source annihilated in the PET and PET/additive films. The method and specifications of the instrument were described in a previous article by the authors.9 Permeation. The pure gas permeabilities of O2, CO2, and He were measured based on a well-established constant volume-variable pressure method.27 All the measurements were done at 35 °C. A Baratron 121AA pressure transducer which was connected to a data acquisition devise to record the pressure of the downstream as a function of time. Sorption. A pressure decay method was used to measure sorption isotherms for CO2 in PET-based films.28 A sorption system consisting of a polymer chamber, a reservoir, and two pressure transducers was placed in a water bath at 35 °C. Polymer chamber was pressurized to the chosen pressure which varied from 50 to 200 psi. The change in pressure inside the polymer chamber was recorded, and the amount of gas sorbed by the polymer as a function of final pressure was plotted. Because of the low solubility levels of other gases, sorption studies were limited to CO2.

gave rise to peaks for meta connected benzene ring (a′, b′, c′) and oxydiethylene protons (∗).22,29 The presence of the peak at 4.02 ppm (e) corresponding to the methoxyl protons of the additives indicates that the DMT and DMI were blended with PET after processing and retained their chemical structure. The intensity of the peak corresponding to methoxyl protons varies with concentration of DMT as seen from the inset of Figure 1. Assignment of the 1H peaks for PET and PET with the additives is tabulated in the Supporting Information. 13 C NMR spectra were recorded to further investigate the possibility of chemical reaction of additives with the PET matrix. Physical blends of PET with equivalent amounts of additives were analyzed, and the spectra were compared with the extruded blends. Transesterification reaction between carboxyl group of the additives and hydroxyl end group of PET should result in new carbonyl peaks corresponding to additive terminated PET chains especially in the carbonyl region of the 13C spectrum (∼165−170 ppm). No evidence of such peaks was observed in all the extruded samples, and 13C spectra of physical blends and extruded blends were identical as shown in Figure 3 and Figures S-1 to S-3 in the Supporting Information. Because of the trifluoroacetylation of hydroxyl end groups of PET22,23 and the possibility of demethylation of additives in the presence of trifloroacetic acid,30 NMR was not used for quantification of additives in the extruded blends. It should be noted that the nomenclature of the samples is based on the concentration of the additives derived from TGA experiment as discussed in the next section. For example, the sample with 1 wt % DMT is referred to as PET-1.0DMT and 1.5 wt % DMI is PET-1.5DMI. Even though different toxicology studies on rats report negligible to low order of acute and chronic toxicity of the phthalate esters,18,19 migration of these phthalate esters from

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RESULTS AND DISCUSSION NMR. 1H NMR spectra were recorded to confirm the presence of additives in the films as shown in Figures 1 and 2, which illustrate the full range for PET/DMT and PET/DMI samples. The peaks at chemical shifts of 8.1 ppm (a), 4.77 ppm (b), and 4.62 and 4.15 ppm (c, d) are indicative of phenyl ring and oxyethylene protons of PET chain and end groups, respectively.22 The PET resin used for this work has a small amount of isophthalic acid and diethylene glycol (DEG) which D

DOI: 10.1021/acs.macromol.7b02230 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. TGA of dried (a) PET, (b) PET-1.0DMT, (c) PET-1.5DMI, and (d) PET-3.3DMT. Samples were heated to 100 °C at a rate of 10 °C/min and held for 30 min at 100 °C in order to remove the water in the polymer. The temperature was subsequently increased at a rate of 10 °C/min to 270 °C and held for 60 min to evaporate the additive from the polymer.

PET has not been reported. Typical migration studies involve exposure of the packaging material to different food simulating media at accelerated temperatures.31 As a preliminary study, 1H NMR was also employed to qualitatively evaluate the migration of additives from the PET matrix. Details of the experiment are included in the Supporting Information. On the basis of these preliminary migration studies, it was concluded that most of the bulk of the additives were retained in the PET matrix after 5 days at 50 °C. Traces of DMT were detected in the D2O whereas no DMI migration was detected in NMR spectra. For more precise and quantitative understanding of migrations of these additives, standard techniques for migration tests need to be employed and compared with reported total allowable drinking water concentrations of phthalate esters.19 Concentration of Additives in the Polymer Matrix. The concentration of both unreacted DMT and DMI in PET blends was determined by monitoring the weight loss with TGA. Although a known concentration of the additives was used prior to mixing, a portion of the additives could be lost during processing. Therefore, TGA was applied to evaluate the concentration of LMWDs blended in the PET. Each sample was dried overnight at 100 °C to avoid hydrolytic degradation and to precisely measure the additive concentration. The temperature profile was chosen to eliminate any moisture from the PET/additive samples. The complete drying of samples was confirmed by measuring the weight loss during the first temperature ramp (i.e., room to 110 °C). Figure 4 illustrates the weight loss of dried PET and PET/additive samples and as expected the sample weight loss was greater as the initial concentration of the additives increased. Table 2 shows the calculated concentration of DMT and DMI in PET-1.0DMT, PET-3.3DMT, and PET-1.5DMI samples based on Figure 4. It should be noted that the initial concentration of the additives prior to mixing was chosen at

Table 2. Concentration of LMWD in PET As Measured by Isothermal TGA Experiment sample

feed concn of additive during processing (wt %)

concn of additive (wt %)

PET-1.0DMT PET-3.3DMT PET-1.5DMI

1.5 3.5 2

1.0 ± 0.04 3.3 ± 0.05 1.5 ± 0.09

least 20% greater than the results concentration. Since a portion of the additives was expected to be lost during processing. Thermal Properties. Maintaining thermal properties of PET including the onset temperature of degradation in the presence of additives is critical to be unaltered at the extrusion/injection molding temperature. DSC was used to investigate the effect of DMT and DMI on the glass transition, crystallization temperature, and melting point of the PET samples. TGA was used at constant temperature ramp to evaluate the degradation temperature of PET at 10% and 50% mass loss. The thermal properties of PET and PET with the additives are summarized in Table 3. Table 3 shows that there is a slight increase in the crystallization temperature of PET in the presence of additives. However, the glass transition and melting temperature of PET were not affected by the presence of DMT and DMI. As Table 3. Thermal Properties of PET and PET/LMWD Blendsa

a

E

sample

Tg

Tc

Tm

Td(10%)

Td(50%)

PET PET-1.0DMT PET-3.3DMT PET-1.5DMI

78 ± 1 79 ± 1 79 ± 1 78 ± 1

141 149 150 148

244 246 247 246

380 ± 0.75 383 ± 1.36 372 ± 0.99 378 ± 2.64

408 ± 0.57 410 ± 0.26 403 ± 0.28 408 ± 1.96

All temperatures are in °C. DOI: 10.1021/acs.macromol.7b02230 Macromolecules XXXX, XXX, XXX−XXX

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ature, and no effect on the transparency. Average free volume, free volume distribution, chain dynamics, and resulting gas transport properties were evaluated to monitor the effect of LMWDs on the microstructure of PET films. Free Volume (Density). A density gradient column was used to measure the specific volume of amorphous PET and PET/ additive films. Although density measurement is an indirect and imprecise method for measuring the average fractional free volume, it can readily provide useful information about the bulk structure of a polymer. In addition, the extent of reduction in the fractional free volume of PET can be compared with the literature for other LMWD systems.12,13 Fractional free volume (FFV) is the ratio of the free volume of the polymer, V̂ f, to the observed specific volume, V̂ g. V̂ f is calculated from eq 1

expected, there was no crystallinity detected by the DSC measurement, and all the films were amorphous. The degradation temperature of PET was not significantly altered upon incorporation of the additives at both 50% and 10% weight losses. Intrinsic Viscosity. The melt viscosity and transparency must must not affected by incorporation of additives in order to produce preforms and bottles. Figure 5 shows the melt viscosity

Vf̂ = Vĝ − V0̂

(1)

where V̂ g (cm /g) is the reciprocal of the density and V̂ 0 is the theoretical volume occupied by polymer segments that was calculated according to the method reported by Fedors.21 The theoretical volume for PET/additive samples was calculated based on weight-average for the mixtures (eq 2)33 3

Figure 5. Melt viscosity of extruded PET (square), PET-1.0DMT (triangle), PET-3.3DMT (star), and PET-1.5DMI (circle). Melt viscosity at 1 rad/s (red line) was used to calculate the intrinsic viscosity.

V0̂ = (V0̂ )d w + (V0̂ )p (1 − w)

where w is the weight fraction of the additive calculated by TGA; (V̂ 0)d and (V̂ 0)p are occupied volumes and are 0.627 and 0.8 cm3/ g for PET and the additives, respectively. As shown in Table 5, there was a slight reduction in the fractional free volume for PET-1.0DMT compared to PET, and a more pronounced reduction in FFV was observed in the case of PET-1.5DMI and PET-3.3DMT compared to PET-1.0DMT. Since the concentration of DMT is higher in PET-3.3DMT than that in PET-1.0DMT, lower FFV was expected; however, the concentrations of DMT and DMI are comparable in PET1.5DMI and PET-1.0DMT films, but the FFV of PET-1.5DMI is lower than that of PET-1.0DMT and comparable to PET3.3DMT. PALS measurements were applied to confirm the density results and evaluate the change in the free volume distribution upon incorporation of additives. Positron Annihilation Lifetime Spectroscopy (PALS). PALS is a useful technique to probe the nature of free volume in a polymeric material.8 Interpretation of PALS data for polymers is usually based on three lifetime components: parapositronium (τ1), free positron (τ2), and orthopositronium (τ3).8,34,35 The oPs has an intrinsic vacuum lifetime of τ3 ∼ 1.5−3 ns in amorphous polymers, with the population percentage of positrons forming o-Ps indicated by I3. The method to calculate the average free volume in PET from τ3 and I3, explained in detail in previous work,9 was extended to PET and PET/additive in this paper. Figures 6 and 7 illustrate the average o-Ps lifetime, τ3, and the corresponding o-Ps intensity of PET and PET/LMWD samples. The intensity is proportional to the number of holes or free

of PET and PET/additive films. Values at 1 rad/s (red line in Figure 5) were used to evaluate the intrinsic viscosity (IV) of each sample. Table 4 shows the change in ΔIV of PET/additive Table 4. IV Loss and Transparency of PET and PET/Additive Samples sample PET PET-1.0DMT PET-3.3DMT PET-1.5DMI

ΔIV (dL/g)

transparency

haze

0.01 0.04 0.04

91 91 91 90

4 3 6 8

(2)

samples relative to base PET films as well as the haze and transparency parameters. The extent of IV loss is insignificant in the presence of either DMT or DMI at loadings used for this study, and the results are in agreement with the literature.32 In the case of resins containing cobalt, antimony, manganese, and magnesium the IV loss was reported as high as 0.22 and 0.101 for PET with 3 wt % of DMT and DMI, respectively.32 This will lead to reduction of IV down to values that make PET processing impossible (