Surface Modification of EthyleneâVinyl Alcohol Copolymer Films by

Chapter 25

Surface Modification of Ethylene-Vinyl Alcohol Copolymer Films by Surface-Confined Atom Transfer Radical Polymerization

Downloaded by UNIV LAVAL on April 23, 2016 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch025

1

1,2,*

1,2

Ning Luo , Scott M. Husson , Douglas E. Hirt , and Dwight W. Schwark 3

1

Center for Advanced Engineering Fibers and Films and2Departmentof Chemical Engineering, Clemson University, 127 Earle Hall, Clemson, SC 29634 Cryovac Division of Sealed Air Corporation, Duncan, SC 29334 3

Ethylene-vinyl alcohol (EVOH) copolymer films were used as substrates to grow surface-grafted poly(acrylamide) chains. In a first step, an initiator precursor, 2-bromoisobutyryl bromide, was used to functionalize surfaces o f E V O H films. This step was performed from 2-bromoisobutyryl bromide solutions in a series of solvents to study solvent effects on initiator immobilization effectiveness and film integrity. In a second step, surface-confined atom-transfer radical polymerization (ATRP) was used to grow poly(acrylamide) from the initiatorfunctionalized E V O H films; C u C l / M e T R E N was used as a catalyst. For each step, changes in the physicochemical properties of the surface were monitored by A T R - F T I R spectroscopy and X P S . Using acetone as solvent, films of poly(acrylamide) were grown to about 8 nm thickness off of the E V O H films, with no distortion or visible changes in film transparency. 6

352

© 2003 American Chemical Society

Matyjaszewski; Advances in Controlled/Living Radical Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


353 Ethylene-vinyl alcohol copolymers (EVOH) are prepared from the hydrolysis of copolymers of ethylene and vinyl acetate. The resulting vinyl alcohol units provide hydroxyl functionality along the polymer backbone, which makes E V O H different chemically from otherwise inert polyethylene(7). The existence of hydroxyl groups imparts two important features to the copolymers: high oxygen barrier properties and enhanced chemical reactivities. For the former, E V O H is used often in multi-layer films to improve barrier properties of polyethylene packaging films. In this case, the O H groups in E V O H copolymers can be used to react with tie layers so that the multi-layer films exhibit good adhesion(2-4). Regarding enhanced chemical reactivities, the O H groups serve as grafting points to incorporate additional chemical functionalities, including those used to initiate growth of other polymer chains from the E V O H surface(57). In the literature, this type of research generally concerns blood compatibility(o) and other medicalAppl.ications(7-P).For example, Yao et al.(d) grafted poly(acrylamide) (hereafter, P A ) on E V O H surfaces initiated by cerium (IV) ion. The permeability of urea through the ΡΑ-grafted E V O H film was improved compared to that of the original film, as was the blood compatibility. Kubota et al.(5) used a series of photo-initiators and cerium (IV) to graft P A on the surface of E V O H film. One difficulty that these researchers encountered was low graft efficiencies for the system studied. As reported(J), the graft efficiencies in their systems were less than 30%, indicating that a high percentage of monomer was consumed by solution-phase polymerization. With the discovery of controlled radical polymerization in the mid1990s(70,77), grafting polymers on surfaces with high density, high efficiency(72), and controllable polymer chain length (polymer brushes) has become an active topic in polymer science. As one of the controlled radical polymerization methods, atom-transfer radical polymerization (ATRP) shows great flexibility in producing surfaces with tailored physical and chemical properties. Roughly 100 journal articles and preprints have been written that describe research using A T R P to grow polymers from gold(75,74), silica(75,7 12 hours at room temperature (24 ± 3 °C) with stirring. The film was then removed, washed with 100 m L of ethyl acetate in an Aquasonic ultrasonicator, and then washed with a copious volume of water and then acetone. After drying in air, the initiator-functionalized film was characterized by ATR-FTIR spectroscopy and, in some cases, X P S . Hereafter, surfaces functionalized with initiator are referred to as E V O H - B r .


Polymerization Procedure This reaction used an organometallic catalyst comprising Cu(I)Cl and ligand, hexamethyl tris(2-aminoethyl)amine (Me TREN), synthesized via methylation of tris(2-aminoethyl)amine(23). A l l polymerizations were carried out under the same conditions: The molar ratio of Cu(I)Cl to M e T R E N was 1 to 1, and the molar concentration of acrylamide was 3 M . Two solvents were used to perform the polymerization: water and acetone. A typical polymerization run follows: 1.725 g of M e T R E N was added to 25.0 mL of dimethylformamide (DMF). A piece of initiator-immobilized film, 0.29 g of Cu(I)Cl, and 4.27 g of acrylamide were put into a separate flask. To this flask, 20.0 m L of deionized water (18.2ΜΩ, Millipore) was added to dissolve the solid acrylamide. The solution was subjected to 3 freeze-thaw cycles with vacuum evacuation and nitrogen purging to remove oxygen. To begin polymerization, 1.0 m L of the M e T R E N / D M F solution was transferred to the flask containing a piece of the initiator-immobilized film, Cu(I)Cl, and monomer. The reaction solution volume was enough to submerge the E V O H - B r film fully. Polymerization was performed at room temperature for a prescribed time up to 7 h. After removing the film from the polymerization system, it was sonicated in 400 mL of water for 15 min, and then washed with acetone. 6

6

6

6

Characterization Methods Attenuated total reflectance (ATR)-FTIR spectra of the polymer films were obtained using a Nicolet Avatar 360 FTIR spectrometer equipped with a nitrogen-purged chamber. A T R was conducted with a horizontal multibounce attachment using a Germanium crystal and a 45-degree angle of incidence. A l l spectra were taken at 4 cm" resolution and reported as an average of 540 scans. To measure the thickness of P A grown from an E V O H film, A T R - F T I R data were Anal.yzed by a procedure similar to that described in Ref. 24; in this work, a 1 mm thick P A film was used as a reference. According to the principle of ATR-FTIR spectroscopy, theAnal.ysispenetration depth can be calculated as 1


356 λ ,0.5

2·π·η

(1)

sm

where η and η are the refractive indices of the crystal and sample, λ is the wavelength of interest, and Θ is the angle of incidence. For our A T R instrument, Θ = 45° and ni = 4.0. The average penetration depth was calculated to be 408 nm for pure P A film (n = 1.546) over the wavenumber range of 1640 ~ 1660 cm" . The thickness of a grafted layer can be estimated from: {

2

2


1

(2)

d(nm)=^eî=!-.(408) ApAAm

where A is the area of the peak whose maximum absorbance is located ~ 1655 cm" for the surface grafted sample, and A m is the corresponding peak area of the amide peak (1655 cm* ) of pure P A reference. There are several assumptions inherent to Eq. 2 as described in the Results and Discussion section. A U X P S data were obtained using a Kratos A X I S 165 X P S Spectrometer equipped with a monochromated ΑΙ Κ α (1486.6 eV) X-ray source and hemispherical Anal.yzer; this unit is housed at Cryovac. mflta

1

P A A

1

Results and Discussion Attachment of 2-Bromoisobutyryl Bromide to E V O H Surfaces Figure 1 shows the ATR-FTIR spectra of plain E V O H film (Spectrum A ) and initiator-functionalized E V O H film (Spectrum B ) . In this experiment, a mixture of toluene (60% by volume) and methylene chloride (40% by volume) was used to dissolve 2-bromoisobutyryl bromide for the initiator immobilization on E V O H . In the spectrum o f plain E V O H film (A), there are no peaks in the wavenumber region between 2000 ~ 1600 cm" , which indicates that there is no measurable (by ATR-FTIR) concentration of carbonyl ester groups in the E V O H film. This finding is reasonable and supports that conversion of acetate ester groups to O H groups was efficient in the post-polymerization hydrolysis of ethylene-vinyl acetate copolymers to form E V O H . Comparing the spectra before (A) and after (B) the initiator immobilization, one sees the appearance of a carbonyl peak located at 1732 ± 12 cm" , which indicates the existence o f the bromoester groups in the surface region of the E V O H film. 1

1


357

ι

1

4000

π

1—

3000 2000 wavenumber em"

1000


1

Figure 1. Surface immobilization of 2-bromoisobutyryl bromide on EVOH film. A: Blank EVOH film, as received. B: Initiator-functionalized EVOH film.

X P S data further support the existence of surface-immobilized bromoester groups. Table I shows that B r exists in the E V O H - B r sample, and the carbonylgroup content is higher for the functionalized surface. Because of the reaction chemistry used and the extensive solvent washing of films prior to characterization, we conclude that the initiator groups were bonded covalently to the E V O H surfaces.

Table I. X P S Characterization of Initiator-Functionalized E V O H Films Atomic Composition (%) C

0

E V O H plain film

86.9

13.1

EVOH-Br

73.2

21.4

Br

Percent of Carbon Atoms Present in Carbonyl Groups: 0-C=0, C=0 1.7

5.4

9.7

Solvent Selection for the Initiator Immobilization Reaction Any surface modification method Appl.ied to packaging films must, first and foremost, occur without distortion of the film properties (e.g., transparency). The above results demonstrate the efficacy of using chemical reaction to anchor initiator sites on E V O H film. However, when the reaction was conducted in a toluene/methylene chloride mixture, the E V O H film became distorted and non-


358 transparent. Our hypothesis was that film distortion resulted from strong interaction between the solvents and the E V O H film. Therefore, various solvent systems were studied for the initiator immobilization reaction of 2bromoisobutyryl bromide onto E V O H films. The aim was to identify a solvent system that provides high initiator density and maintains film integrity. To test this hypothesis, we selected solvents differing in polarity and hydrogen-bonding capacity, as measured by solubility parameter data. Table II lists the pure-component solvents used, along with their solubility parameter data. While values of δ are similar for all solvents, differences exist in their abilities to form hydrogen bonds (8 ) and in their polarities (δ ). A t one end, n-hexane and η-heptane are non-polar and have no capability to form hydrogen bonds. The 6 and δ of toluene are much less than those of M i B K , methylene chloride, acetone, THF, and ethyl acetate, which are polar solvents. O f those, ethyl acetate has the highest hydrogen bonding capability. Figure 2 shows experimental A T R - F T I R results for the initiator immobilization reaction in various pure solvents. The peak of interest occurs at about 1732 cm" , and corresponds to the carbonyl group of the surfaceimmobilized initiator. Clearly, toluene, n-hexane, and η-heptane provided a high content of surface-immobilized initiator; however, they also distorted the films. Acetone, T H F , M i B K , and ethyl acetate showed lower contents of surface initiator than toluene but these solvents maintained film integrity. The increased content of surface-immobilized initiator seen with nonpolar solvents might result from higher positive deviations from ideal behavior in the liquid phase, from increased swelling of the films, or a combination of these effects. Because 2-bromoisobutyryl bromide is polar and capable of hydrogen bonding, it will have stronger intermolecular interactions with polar, hydrogen-bonding solvents than with nonpolar solvents. Thus, it should be more reactive toward the polar hydroxyl groups on the E V O H surface when placed in a nonpolar solvent. In this way, the concentrations of 2-bromoisobutyryl bromide in nonpolar solvents near a film surface are probably higher than that in polar solvents. Alternatively, high conversion of O H groups and interpénétration of 2-bromoisobutyryl bromide in nonpolar solvent systems might be attributed to increased swelling of the E V O H film and, correspondingly, improved access to hydroxyl groups. Previously inaccessible reaction sites might now be available for reaction. This might account for why the films were distorted while at the same time had high initiator content. Among the polar solvents, it is not clear how, or whether, degree of solvent polarity (as measured by δ ) affects the initiator immobilization reaction. A l l polar solvents, with the exception of ethyl acetate, gave fairly constant A T R FTIR peak height at the wavenumber characteristic of carbonyl groups; this result indicates similar initiator content in the near-surface E V O H regions. A s the exception, ethyl acetate has the highest hydrogen bonding capability and lowest peak height among these polar solvents. h


h

ρ

ρ

1

ρ


359 Table II. Solubility Parameters of the Chemicals Used in this Study" Solvents and polymers

δ (J /cm )

4 (J /cm )

δ (j" /cm )

δ„ (J km ' )

15.2

15.2

0

0

14.8 ~ 14.9 18.2 ~ 18.3 17.217.5

14.8

0

0

17.318.1

1.4

2.0

15.3

6.1

4.1

19.9

17.418.2

6.4

6.1

20.020.5

15.5

10.4

7.0

Tetrahydrofuran

19.5

16.818.9

5.7

8.0

Ethyl acetate

18.6

15.2

5.3

9.2

24.433.7 15.817.1 21.226.4

23.831.9

11.613.4

0

0

14.819.8

7.28.3

m

Heptane Hexane


Toluene Methyl isobutyl ketone Methylene chloride Acetone

Poly(vinyl alcohol)

25.829.1 15.817.1 19.622.2

Polyethylene E V O H (38/62)* a

in

1/2

3/2

ρ

2

3/2

m

3

2

b

Cited from Ref. 25. Calculated according to Ref. 25.

Nonpolar-polar solvent mixtures were investigated for achieving high initiator content while maintaining the original properties of the films. Distorted films were produced in hexane/acetone (50/50) (v/v) and toluene/acetone (50/50) solvent systems. The films i n ethyl acetate/hexane (50/50) and toluene/MiBK (50/50) were not distorted, but the initiator contents were low. Figure 3 shows ATR-FTIR results for the initiator immobilization reaction in various mixed solvents. Pure toluene and a 90/10 mixture of toluene/ethyl acetate distorted the films and turned them brown. However, in the composition range o f 80/20 ~ 60/40 toluene/ethyl acetate, the initiator contents were similar, and the films showed no distortion. Unfortunately, these conditions produced films with the same initiator content of the pure, polar solvents. Therefore, initiator immobilization was done using pure acetone for all films prepared for surfaceconfined polymerization.


360


11732 c m

1

Figure 2. Comparison of the ATR-FTIR spectra supporting the initiator immobilization reactions of 2-bromoisobutyryl bromide on EVOH films from various pure solvents. For clarity, data are shown from only the wavenumber region used to identify the initiator.

0.02 au 1

1732± 12 cmToluene/Ethyl acetate (80/20) Toluene/Ethyl acetate (70/30) Toluene/Ethyl acetate (60/40) Ethyl acetate/Hexane (50/50) Toluene/MiBK (50/50) Acetone THF MiBK Ethyl acetate Plain

Figure 3. Comparison of the ATR-FTIR spectra supporting the initiator immobilization reaction on EVOHfilmsfrom all solvent systems that produced non-distorted films.


361


Growth of Surface-Confined P A from Initiator-Functionalized E V O H F i l m Figure 4 summarizes the observations and appearances of the PA-grafted E V O H films. The plain E V O H film begins as a transparent, wrinkled film (Figure 4 a). After growth of surface-confined P A by A T R P using water as the reaction solvent, the ΡΑ-grafted E V O H film became very soft and distorted after only 1 hour of polymerization. Subsequent washing with acetone hardened the film, but did not restore the film to its original size and shape (Figure 4 b). Grafting with acetone as the reaction solvent resulted in non-distorted, transparent films. The films kept their size and shape, and displayed no visible color (Figures 4 c, d). After polymerization, the reaction solutions were poured into 400 m L of methanol to determine whether any P A had formed in solution. In all cases, there was no detectable precipitation of PA, which demonstrated that only surface polymerization had occurred (i.e., high graft efficiency was obtained). This result contrasts the work of Kubota et al. (5), who reported that when eerie salt or photo-irradiation was used to graft P A on E V O H surface, graft efficiencies were less than 30%.

Figure 4. Photographs of ΡΑ-grafted EVOHfilms,(a): EVOH plain film, (b): EVOH-PAAm, solvent water, polymerization for 1 h; (c) : EVOH-PAAm, solvent acetone, polymerization for 1 hr; (d) : EVOH-PAAm, solvent acetone, polymerization for 7 hr.

Figure 5 shows that the ATR-FTIR spectra of E V O H films following polymerization display amide character (-1655 cm" ), which suggests that P A was grown from the E V O H surfaces. Also present in the spectra are peaks that correspond to the bromoester initiator (1739 ± 4 cm" ). Figure 6 shows the time dependent thickness of P A grown from E V O H using acetone as the reaction solvent. Over the 7 hour period investigated, graft thickness, as estimated using 1

1


362 Eq. 2, increased slightly and then reached a plateau below 10 nm. Based on these estimations, the thickness appears to grow non-linearly with polymerization time; for flat, low surface area substrates, non-linear growth rate behavior indicates that growing chain concentration is non-constant(7#,2d). This result is consistent with findings from other A T R P studies that used acrylamide or methacrylamide(27,2#). It also suggests that an insufficient concentration is generated of persistent, deactivating C u species. To ensure proper control for this system, one would need to use sacrificial initiator in solution, or, preferably, add C u prior to polymerization.(2P) 2 +


2 +

τ 1 3000 2000 wavenumber cm"

4000

r 1000

1

Figure 5. ATR-FTIR spectra of EVOHfilmsas a function ofpolymerization time. Data are for growth of PA from EVOHfilmsby ATRP.

It must be noted, however, that the thickness estimations (Eq. 2) assume that the refractive index of the ATRP-grown P A layer is the same as that of the P A reference film, which may not be exactly true; but the calculation of d is relatively insensitive to n in the range 1.5 < n < 1.6. More importantly, the estimates are based on an approximation that the absorbance of the peak of interest increases linearly with the thickness of P A . This approximation is approximation is valid for bands with weak absorbances. Efforts are under way in our lab to develop a quantitative method for correlating IR peak absorbance to graft layer thickness using films of known graft thickness for calibration. These efforts will allow us to test the linear approximation between absorbance and layer thickness for this system. p

2

2

The apparent steep increase in thickness seen in Figure 6 over the first 30 minutes may also be a result of the latter approximation used in Eq. 2, which


363 assumes that the absorbance per unit thickness (β) remains constant throughout the film penetration depth. In reality, β decreases from the film surface into the bulk. For measurements on thin layers at the film surface, actual β values for these thin films will be higher than that used to calibrate Eq. 2. Thus, for a given measured absorbance, Eq. 2 provides an overestimate of thickness.


10

ι H ed W-c

Ο 0 4 0

, 1

, 2

. 3

, 4

, 5

1 6

Polymerization time (h) Figure 6. PA graft thickness on EVOH film as a function ofpolymerization time. The graft thickness was calculated using Eq. 2.

Conclusions P A was grafted on the surface of E V O H films using A T R P chemistry. A t the initiator immobilization step, non-polar solvents, toluene, hexane, and heptane, caused distortion of the films. Some polar solvents, like acetone and THF, provided non-distorted, transparent film, as did some non-polar/polar solvent mixtures, like toluene/ethyl acetate. Graft polymerization of P A can be done in acetone to produce non-distorted and transparent films.

Acknowledgments This work was supported in part by the Cryovac Division of Sealed A i r Corporation and the National Science Foundation under Grant Numbers C T S 9983737 and EEC-9731680.


364 References 1. 2. 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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