Synthesis, characterization and applications of ethylene vinylalcohol copolymers

Citation for published version (APA): Ketels, H. H. T. M. (1989). Synthesis, characterization and applications of ethylene vinylalcohol copolymers. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR316591

DOI: 10.6100/IR316591

Document status and date: Published: 01/01/1989

Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)

Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne

Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim.

Download date: 01. Oct. 2021 SYNTHESIS, CHARACTERIZATION

and APPLICATIONS of

ETHYLENE VINYLALCOHOL COPOLYMERS

H.H.T.M. Ketels SYNTHESIS, CHARACTERIZATION

and APPLICATIONS of

ETHYLENE VINYLALCOHOL COPOLYMERS SYNTHESIS, CHARACTERIZATION

and APPLICATIONS of

ETHYLENE VINYLALCOHOL COPOLYMERS

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof. ir. M. Tels, voor een commissie aangewezen door het College van Dekanen in het openbaar te verdedigen op dinsdag 12 september 1989 te 16.00 uur

door

Hendrikus Hubertus Theodoor Maria Ketels

geboren te Tegelen

druk. wrbr'O dlss~rt:atn::idrukkerq. helmand Dit proefschrift is goedgekeurd door de promotoren: Prof. Dr. P.J. Lemstra Prof. Dr. Ir. H.E.H. Meijer en de copromotor: Dr. G.P.M. van der Velden CONTENTS

CHAPTER 1 INTRODUCTION

1.1 EVOH copolymers 1.2 Permeability of polymers 2 1.3 Factors influencing permeability 5 1.4 Background of the present investigation 7 1.4.1 EVOH copolymers used as harrier resins 7 1.4.2 EVOH copolymers for fiber production 10 1.5 Aim of the present investigation 11 1.6 Survey of thesis 11 1.7 References 14

CHAPTER 2 SYNTHESIS OF EVOH COPOL YMERS

2.1 Introduetion 15 2.2 Copolymerization of ethylene and vinylacetate 16 2.3 Experimental 21 2.3.1 Principle of operation 21 2.3.2 Apparatus 21 2.3.3 Copolymerization process 24 2.3.4 Characterization techniques 25 2.4 Results and discussion 26 2.4.1 Ethylene content 26 2.4.2 Molecular weight 27 2.5 Conclusions 29 2.6 References 31

CHAPTER3 MICROSTRUCIURE OF EVOH COPOL YMERS AS STUDlED BY 1H AND 13C NMR

3.1 Introduetion 33 3.2 Experimental 34 3.2.1 Materials 34 3.2.2 NMR measurements 34 3.3 Results and discussion 35 3.3.1 1H NMR 35 3.3.2 13C NMR 43 3.4 Conclusions 51 3.5 References 53

CHAPTER4 EVOH/NYWN-6 BLENDS

4.1 Introduetion 55 4.2 Experimental 56 4.2.1 Materials 56 4.2.2 Films 56 4.2.3 Characterization techniques 56 4.3 Results and discussion 58 4.3.1 Extrusion blending of EVOH and Nylon-6 58 4.3.2 Barrier properties of EVOH/Nylon-6 films 59

ii 4.3.3 Miscibility of EVOH and Nylon-6 61 4.3.4 Mechanical properties of EVOH/Nylon-6 films 64 4.3.5 Transmission Electron Microscopy 65 4.3.6 Solid State 13C NMR 66 4.3.6.1 Chemica! shifts 66 4.3.6.2 Spin lattice relaxation times 69 4.4 Conclusions 72 4.5 References 73

CHAPTER 5 EVOH/PET AND EVOH/PE BLENDS

5.1 Introduetion 75 5.2 Experimental 80 5.2.1 Materials 80 5.2.2 Blends and films 81 5.2.3 Irradiation 83 5.2.4 Morphology 83 5.3 Results and discussion 83 5.3.1 Stratified pellets 83 5.3.2 Compatibilizers 84 5.3.3 Irradiation 86 5.3.4 Films 87 5.4 Conclusions 92 5.5 References 94

111 CHAPTER6 PREPARATION AND CHARACfERIZATION OF SOLUTION (GEL-SPUN) EVOH FIBERS

6.1 Introduetion 95 6.2 Experimental 96 6.2.1 Materials 96 6.2.2 Fiber-spinning/drawing 96 6.2.3 Characterization techniques 97 6.3 Results and discussion 97 6.3.1 Synthesis of EVOH copolymers 97 6.3.2 Properties of EVOH fibers 98 6.3.3 Development of Young's modulus as a function of draw ratio .À 99 6.4 Conclusions 102 6.5 References 103

APPENDIX

Al Introduetion 105 A2 Experimental 105 A.2.1 Materials 105 A.2.2 NMR measurements 106 A3 Results and discussion 106 A4 Conclusions 114 A5 References 115

iv SUMMARY 117

SAMENVATTING 121

CURRICULUM VITAE 125

V CHAPTER 1 INTRODUeTION

1.1 EVOH copolymers

Copolymerization represents one of the most powerfut tools in tailoring polymer systems to meet specific engineering needs and sametimes extends the range of 1 3 utility of monomers that would otherwise be of Iimited value • •

Ethylene vinylalcohol copolymers are paradigmatic in this respect. The pure homopolymer polyvinylalcohol (PVOH) possesses intrinsic limitations concerning processability and moisture sensitivity. The abundance of -OH side groups promotes the absorption of water, resulting in a deterioration of physical and mechanica! properties, and moreover degradation occurs during processing via the melt. At relative low humidities however, the pure homopolymer PVOH possesses interesting properties. The -OH side group is relatively smalt and consequently PVOH is crystallizable, despite its atactic character, into a distorted monoclinic version of the polyethylene structure. Due to its crystal­ linity, but notably due to the high glass transition temperature Tg (approx. 80 oe at 0 % relative humidity (R.H.)), PVOH possesses excellent harrier properties (low permeability) against gases like oxygen (02) and carbon dioxide

(C02), at low relative humidities.

The similarity of PVOH and polyethylene with respect to the crystal structure stimulated research efforts to produce high-strength/high-modulus fibers similar to developments based on polyethylene Iike melt-spinning and/or 4 solution-spinning/ultra-drawing •

Developments in using PVOH as harrier film and high-performance fibers have only been partly succesfull, mainly as a consequence of the moisture sensitivity and lack of processability. A salution to this problem could be the incorporation of ethylene in the PVOH chain which results in the EVOH copolymer. On a technological scale, PVOH is produced via complete hydralysis (methanolysis) of polyvinylacetate (PVA). Direct synthesis from the corresponding monoroer "vinylalcohol" is impossible since this species exists in its more stable, tautomerie, keto-form: acetaldehyde. It is well known that ethylene and vinylacelate copolymerize readily into a copolymer, ethylene vinylacelate (EVA) copolymer, in which the ethylene and vinylacetate units are arranged at random along the molecule. Hydralysis of these EVA copolymers results in ethylene vinylalcohol (EVOH). The hydrapbobic ethylene segments willlower the moisture sensitivity. EVOH copolymers are quite unique because they are co-crystallizable over the 5 whole composition range • This results in a, relatively, smalt decrease of melting temperature and crystallinity of the EVOH copolymers with changing composition. Moreover, the T 1 of EVOH copolymers decreases, in comparison with PVOH, only slightly with increasing ethylene content (e.g. Tg EVOH (44 mole% ethylene) = 65 oe at 0% R.H.). The decomposition temperature of the EVOH copolymers is, in contrast to PVOH, above the melting point and consequently melt processing is possible. A disadvantage of EVOH copolymers is their brittleness due to their relative low molar mass as will be discussed in chapter 2.

In this thesis the possibilities of EVOH copolymers for the production of fibers and fortheuse as a harrier resin will be investigated. For a better understanding of the harrier properties of polymers in general, some introductory remarks about the permeability of polymers are given in the next section.

1.2 Penneability of p«)}ymers

The process of permeation through non-porous polymers is generally explained 6 in terros of the solution-diffusion model • This model postulates that the permeation of a gas through a polymer film occurs in three stages: 1 sorption of the penetrant in the surface of the polymer, ~ diffusion through the polymer

2 and l desorption from the other face. The permeability P is a combination of the diffusivity D of the gas dissolved in the polymer and its concentration gradient, which in turn is proportional to the gas solubility Sin the polymer.

Above the glass transition temperature T1 of the polymerjpenetrant system the solubility of gases usually obeys Henry's law and the sorption isotherm shows a linear dependenee of concentration C versus pressure p:

S = C/p (I)

Since the solubility of gases in rubbery polymers is sufficiently small ( < 0.2 o/o) the concentration dependenee of D is negligible. The diffusion coefficient is aften well approximated by a constant D.

From Fick's law. the flux J of a gas diffusing through unit area of film with time (steady-state) is given by:

(2)

where C1 and C2 are the concentrations at the surface and x is the thickness of the film.

In practical systems, the surface concentration is not always known and it is convenient to express the flux per unit area (J), in terms of the concentrations of the external phases Cex~ or for gases in external pressures. Combining eqs. (l) and {2) results in:

(3)

where P is the permeability coefficient.

3 In the case of gas transport through rubbery films:

P = D·S (4)

, At or below T1 penetrant diffusion and sorption processes are considerably more complicated. Both simpte gases such as e.g. C02, but also hydrocarbon vapours in glassy polymers exhibit a pressure dependent sorption isotherm which varies as:

C C8 '·b S(p) = - + (5) p = s 1 + b·p

where C8 ' is a hole or site saturation constant and b is a hole or site affinity constant.·

Originally it was supposed that one popuiadon of the sorbed molecules is molecularly dissolved and obeys Henry's law (the first term) and the second polpulation is sorbed into "microvoids" (holes) and follows a Langmuir isotherm (second term). lt was assumed that the dissolved molecules are free to diffuse down a concentration gradient whilst the second popuiadon is bound to a fixed number of absorption sites and holes within the polymer. Bound and mobile molecules however are in equilibrium.

The mean permeability P is given by the product of the Henry's law's constant S and the diffusivity of the mobile fraction of the sorbed molecules:

(6)

Later, this model was modified to allow some limited mobility of the "immobi­ lized" penetrant molecules:

4 K·R p = S·D<~ (1 + 1 + b·p ) (7)

where K = CH'·b/S and Ris a measure of the degree of immobilization.

If R = 0, total immobilization, eq. (7) reduces to eq. (6) and for R = 1 the permeability becomes pressure dependent

The model presenled above for sorption and diffusion of gas molecules through glassy polymers is only one out of many models and theories7 which have been prepared for transport phenomena in polymer systems. Glassy polymers are of course structurally more complex than rubbery systems. However, two-phase systems such as polymer blends and semi-crystalline polymers are even more complex.

Detailed rnadelling of transport phenomena in polymer systems is beyoud the scope of the present thesis. The reader who is interested in more detailed 7 information is referred to more recent reviews6. • In the next section, the permeability coefficients of polymer systems are presented and the effects of various parameters are discussed.

1.3 Factors influencing penneability

The permeability P of a polymer system is expressed as:

(amount of gas)(film thickness) p = (8) (film area)(time)(pressure)

Huglin and Zakaria8 noted 29 different units for P which appeared in the literature.

5 Based on experimental evidence the main factors influencing permeability are:

oenetrant size An increase in size in a series of chemically similar penetrants generally leads to an increase in the solubility coefficients due to their increased boiling points, but will also lead to a decrease in their diffusion coefficients due to the increased activation energy needed for diffusion. The overall effect of these opposing trends is that the permeability generally decreases with increasing penetrant size, since for many polymerjpenetrant pairs the sorption coefficient will only increase by perhaps a factor of 10 whilst the diffusion coefficient can vary by I 0 orders of magnitude.

polymer molecular weight When the polymer molecular weight increases, the number of chain ends decreases. The chain ends represent a discontinuity and may form absorption sites for penetrant molecules in glassy polymers resulting in an increase of the permeability.

functional groups The permeability of penetrants which interact with functional groups present in the polymer can be expected to decrease as the cohesive energy of the polymer increases. For example by increasing the polarity of the substituent group on a vinyl polymer backbone, oxygen permeability is reduced by almost 50,000 times. Functional groups which have specific interactions with a penetrant act to increase its solubility in the polymer. This leads to plasticiza­ tion and hence enhanced permeability.

density and polymer structure Generally, a reduction in density in a series of polymers results in an increase in permeability.

crosslinking In non-crystalline polymers, diffusion coefficients decrease approximately

6 linearly with crosslink density at low to moderate levels of crosslinking. Generally, the solubility coefficient is relatively unaffected, except at high degrees of crosslinking or when the penetrant swells the polymer significantly. However, crosslinking reduces the mobility of the polymer segments and tencts to make the diffusivity more dependent on the size and shape of the penetrant molecules and the penetrant concentration. crystallinity In crystalline polymers, the crystalline areas act as impermeable harriers to penetrating molecules and have the same effect as inert filters, i.e. they force the penetrant molecules to diffuse along longer path lengths. filled polymers The actdition of inert filters may either increase or decrease permeahility depending upon their degree of acthesion and compatihility with the polymer. An inert filler which is compatible with the polymer matrix wil he impermeable to the penetrant molecules and so permeation will he slower due to the reduced area for transport. When the filler is incompatihle with the polymer, voids tend to occur at the interface which leads to an increase in free volume of the system and, consequently, an increase in permeahility.

In general, po lar polymers which usually possess a high T g• show the lowest permeability, see also next section.

1.4 Background of the present investigation

1.4.1 EVOH topolymers used as barrier resins

Table 1.1 clearly shows the excellent harrier properties of PVOH for 0 2 at low relative humidity, in fact the lowest permeability coefficient of all synthetic polymers (generally, the perrneability of co2 is 3-5 tirnes larger in cornparison with OJ. However, PVOH ahsorhs water resulting in a decrease of the harrier properties because water molecules plasticize the arnorphous parts (lowering of

7 ) T1 and water itself can act as a gas carrier.

Table 1.1 Oxygen permeability coefficients of various polymers at 20 "C, at 0 % R.H. (relative values basedon average literature data with reference to polyethylenet

Polyethylene (low density) 100 (high density) 46 Polypropylene 50 (oriented) 29 Polystyrene 98 Pol ycarbonate 50 Poly( vinylchloride) (rigid) 1.7 Poly( ethyleneterephthalate) 0.9 Polyamide 1.0 (oriented) 0.6 Poly(acrylonitrile) 0.17 Poly(vinylidenechloride) 0.17 PVOH 0.83 x 10-3

EVOH (29 mole% ethylene) 4.2 x to·3 EVOH (38 mole % ethylene) 17 x 10-3

The use of EVOH, which contains the hydrophobic ethylene group, could be a solution to this problem: the sensitivity towards moisture deercases with increasing ethylene content. Ho wever, with an increase of the ethylene content both intermo1ecular and intramolecular hydrogen bond strengtbs decrease. Thus, in EVOH with a higher ethylene content, segment movement becomes easier and the permeability coefficients become higher as can be observed in Table l.I.

8 In Figure 1.1 the oxygen permeability of various EVOH films at different 9 10 R.H.-values is shown ' • At R.H.-values < 70% the EVOH containing the lowest amount of ethylene possesses the best harrier properties but above 70-80 o/o R.H. the influence of ethylene becomes noticable: EVOH copolymer with the highest amount of ethylene possesses the best harrier properties.

I I I PVOH/

f EPF I I i I ) I EPH /lPE 10

0 20 40 80

RH(%)

Figure 1.1 Oxygen permeability of various EVOH films at different relative humidities

9 Also important in this context is the fact that although ethylene and vinylalco­ hol are co-crystallizable over the whole range of composition, a decrease of the 4 crystallinity in comparison with the homopoJymers can be observed • This will also result in an increase of the permeability (see section 1.2). Consequently, for practical applications it is necessary to select an EVOH copolymer which is a compromise between harrier properties and moisture sensitivity. A possibility of a further reduction of the moisture sensitivity of EVOH is 10 11 coextrusion into multilayer films, botties etc • • Typically, EVOH is coextruded between layers of high moisture harrier materials such as PE and polypropylene (PP). With this construction, the high gas-harrier qualities of EVOH can be effectively maintained. Over 80% of the EVOH resins are used nowadays in combination with PE or PP. Since the highly polar EVOH resins do not adhere well to non-polar polyolefins, an adhesive layer must be used between these two polymers. Thus a typical structure involving EVOH resins might be PP/adhesive/EVOH/adhesive/PP. A drawback of EVOH still remains: its poor mechanical properties. The 9 mechanical properties of EVOH decrease with increasing ethylene content • Generally, there are two possibilities to solve this problem: 1 the already mentioned coextrusion process; in the multilayer structures the outer layers 10 provide for the (enhanced) mechanical properties • and .2. blending; EVOH is blended with a polymer which possesses good mechanica! properties. The coextrusion process bas several disadvantages, notably the impossibility to re-process the films/containers. A much more flexible system could be a blend of EVOH with other polymers.

1.4.2 EVOH copolymers for fiber production

Using PVOH in the so-called gel-spinning process, originally developed to produce fibers of ultra-high molecular weight PE with excellent mechanical 5 properties, interesting results have been obtained • PVOH fibers have some advantages in comparison with PE fibers (higher melting point and lower creep) but their major drawback is, as was pointed out before, their sensitivity towards

10 moisture: mechanical properties decrease at higher humidities. Again, the incorporation of ethylene in PVOH could be an advantage in this respect.

1.5 Aim of the present investigation

The aim of the present investigation is:

- The synthesis of EVOH copolymers with a narrow composition and molecular weight distribution to prevent a spreading of the properties.

- A precise determination of the microstructure of EVOH copolymers to obtain more knowledge about the relationship between structure and properties.

- The preparation of blends basedon EVOH copolymers and the determination of harrier and mechanical properties of their films. Also the miscibility /mor­ phology of the blends have to be studied for a better understanding of the properties.

- The preparation and characterization of gel-spun EVOH fibers, and a study of the effect of the incorporated ethylene on the moisture sensitivity and mechanical properties of the fibers.

1.6 Survey of thesis

Chapter 2 deals with the salution copolymerization of ethylene and vinylacetate and the subsequent hydralysis to obtain the EVOH copolymer. The used apparatus and metbod are described. The results of the different characteriza­ tion methods are compared and discussed.

In chapter 3 the microstructure of EVOH copolymers is studied via salution 1H and 13C NMR. An assignment of the resonances in the spectra is given. Attention bas been paid to the sequence distribution, alkyl chain branching, anomalous linkages and tacticity of the copolymers. This chapter has been

11 The blends of EVOH with Nylon-6 are described in chapter 4. The miscibility is studied using Differential Scanning Calorimetry (DSC), Pulse Induced Critica! Scattering (PICS) and optica! microscopy. From these results it can be concluded that EVOH and Nylon-6 are miscible in the melt (at least at temperatures just above the melting point of Nylon-6 and for EVOH copolymers with lower ethylene contents) and phase separate upon cooling. By consecutive crystalliza­ tion a so-called structured blend is obtained: a blend exhibiting a specific morphology. The harrier properties of films of these blends are explained taking into account this specific morphology. The mechanica! properties of the films are studied. The morphology of blown films is investigated using Transmission Electron Microscopy (TEM} and Solid State 13C NMR. Parts of this chapter have 14 15 been published or are submitted for publication •

In contrast to the EVOH/Nylon-6 blends, the blends of EVOH with poly­ (ethyleneterephthalate) and polyolefines, which are discussed in chapter 5, are immiscible. Using the so-called Multiflux static mixer, layer structures in pellets of these blends can be obtained. Because of this specific morphology these blends also belong to the class of structured blends. Attempts have been made to preserve this specific morphology upon further processing (film blowing) because stratified films are expected to possess excellent harrier properties.

In chapter 6 the preparation and characterization of gel-spun EVOH fibers are descri bed. The moisture sensitivity decreases, in comparison with PVOH fibers, with increasing ethylene content but also the efficiency of draw and maximum obtainable draw ratio (>.max) decrease. The decrease in Àmax is due to a reduction in overall chain length with increasing ethylene content. Consequently, the mechanical properties of EVOH copolymers, obtained via the process described in this thesis, are intrinsically limited. 16 This chapter is submîtted for publication •

12 In the aooendîx the results of a Solîd State 13C NMR study of EVOH copolymers are presented. The observed splitting in the methine carbon region can be explained taking into account both sequence and tacticity effects. These results 17 18 have been published or are submitted for publication •

13 1.7 Keferences

1. Alfrey Jr., T., Bohrer, J.J., Mark, H., Copolymerization, Interscience, New York, 1952. 2. Ham, G.E., ed., Copolymerization, lnterscience, New York, 1964. 3. Ham, G.E., in "Kinetics and Mechanisms of Polymerizations", vol.l, Vinyl Polymerization, Part 1, G.E. Ham ed., Dekker, New York, 1967. 4. Tanaka, H., Suzuki, M., Ueda, F., Toray Industries, Eur. Patent 146,084 (1985). 5. Matsumoto, T., Nakamae, K., Ogoshi, N., Kawazoe, M., Oka, H., Kobunshikagaku (Polymer Chemistry), 28,, 610, 1971. 6. de V. Naylor, T., in "Comprehensive Polymer Science, vol. 2, C. Boothand C. Price eds., Pergamon Press, 1989. 7. Frisch, H.L., Polym. Eng. Sci., 2.0.(1), 2, 1980. 8. Huglin, M.B., Zakaria, M.B., Angew. Makromol. Chemie, 117, 1983. 9. Iwanami, T., Hirai, Y., Tappi Joumal, 22,(10), 85, 1983. 10. Blackwell, A.L., J. Plastic Film and Sheeting, 1. 205, 1985. 11. Culter, J.D., J.Plastic Film and Sheeting,l, 215, 1985. 12. Ketels, H., v.d. Velden, G., in "The Integration of Fundamental Polymer Science and Technology 3", L. Kieintjens and P.J. Lemstra eds., Elsevier, New York and London, 1988. 13. Ketels, H., Beulen, J., v.d. Velden, G., Macromolecules, 2.1. 2032, 1988. 14. Ketels, H., v.d. Ven, L., Aerdts, A., v.d. Velden, G., Polymer Comm., 30, 80, 1989. 15. Ketels, H., Nies, E., Lemstra, P.J., submitted for publication in Polymer. 16. Schellekens, R., Ketels, H., submitted for publication in Polymer Comm .. 17. Ketels, H., de Haan, J., Aerdts, A., v.d. Velden, G., in "The Inlegration of Fundamental Polymer Science and Technology 4", L. Kieintjens and P.J. Lemstra eds., Elsevier, New York and London, 1989. 18. Ketels, H., de Haan, J., Aerdts, A., v.d. Velden, G., submitted for pubti­ cation in Polymer.

14 CHAPTER2 SYNTHESIS OF EVOH CO POLYMERS

2.1 Introduetion

As was mentioned in chapter I, the EVOH copolymers are hydrolyzed EVA copolymers (Figure 2.1)\ (in this thesis ethylene vinylacetate and ethylene vinylalcohol copolymers will be indicated as EVA and EVOH respectively).

~ n CH + m CH 2 2 y 0 I

c""oI CH 3 1

EVOH

Figure 2.1 Synthesis of EVA and EVOH copolymer

EVA can be produced by various polymerization methods like salution-, 2 5 suspension-, bulk- and emulsion polymerization • • For the production of EVOH copolymers, the precursor EVA is generally obtained via salution polymeriza­ tion in view of a better control of copolymer composition, randomness of the copolymer, branching, degree of polymerization and distribution.

15 2.2 Copolymerization of ethylene and vinylacetate

The copolymerization of ethylene and vinylacetate under high pressure was 6 publisbed in the patent literature as early as 1938 • However, no attempts were made to describe the copolymerization behaviour and the products were prepared in a merely empirical way. In spite of the fact that in 19447.s models were reported for the description of copolymerization reactions in general, it lasted until the period 1962-1977 befare more attention was paid to the 14 copolymerization reaction of ethylene and vinylacetate9- •

The derivation of the copolymerization equation considers the four propagation reactions:

-a· +a --> -a· with rate constant k.. -a· + b --> -b· with rate constant kv, -b· + a --> -a· with rate constant kba -b· + b --> -b· with rate constant ~

Where -a· + -b· are polymer ebains ending with radicals formed of the monomers a and b. The reactions with rate constants k.. and kbb are known as self-propagation and those with constants k.., and ~are called cross-propaga­ tion. If it is assumed that the reactivity of the growing chain is independent of the chain length and dependent only upon the nature of the terminal group, then the rate of consumption of rnanamers a and b is given by :

dn. --- = k ..n.I:n 8 + kban.I:llt,· (1) dt

dnb --- = Vbi:n1,- + k..,nbi:n_. (2) dt

16 where n., nb are the number of moles a and b respectively and En.·, Enb· are the concentrations of all the active centers terminating in a and b units.

At any instant during the reaction, the ratio of the amounts of each type of monoroer being incorporated into the copolymer is found by dividing eqs. (I) and (2):

(3)

The ratio En&/Enb· can be evaluated by a steady-state approximation, the concentration of each of the two types of active center remains constant:

dEn& = 0 and = 0 dt dt

The most important reactions in which the active centers are created and destroyed are the cross-propagation reactions. If now only the a· ebains are considered, then the rate of creation and destruction is respectively, given by:

dEn.· dEn.·

dt dt

At steady-state the rate of creation equals the rate of destruction and so:

(4)

17 Substituting eq. (4) in eq. (3) and simplifying leads to7.t!:

= (5) d~ r~Jn. + 1

in which n. = number of moles a in the reactor nb = number of moles b in the reactor r. • k .. I kab rb ==~I kba r. and rb are the monomer reactivity ratios.

The reactivity ratios indicate the preferenee exhibited by a certain radical at the end of the growing chain fora monomerunit of the samekindtoa monomer unit of the other kind. The copolymerization eq. (5) can be integrated yielding an exact relationship 7 17 between the monomer feed composition and the degree of conversion ·ts- :

100- Fb =0 100 (6) in which Q; = (njnb)i qi = (njnb)i rb- 1 R= - r. (fb)i Fb= 100 {1- (fJi}

fb = 100 (I - nJ(nb)0)% and (nb)o is the initial quantity of moles in the reactor.

18 German13 proved, using eq. (6), the validity of the Alfrey model for the 2 ethylene/vinylacetate solution copolymerization at low pressure (35 kgfjcm ) and 62 oe in tertiary-butyl alcohol (TBA).

Consictering the r-values, three types of copolymerization processes can be distinguished:

1: r. = 1/rb or r. x rb = 1 (ideal):

the two types of unit are arranged at random along the polymer chain in relative amounts determined by the composition of the feed and the relative activities of the two monomers.

2: r. = rb = o (alternating):

the monomers alternate regularly along the chain.

3: r. > 1 and rb > 1 :

in the extreme case, this would result in simultaneous homopolymeriza­ tion.

In most cases copolymerization behaviour of monosubstituted vinyl monomers 18 lies in between the ideal and the alternating systems, with o < ra x rb < 1 • In the period 1962 - 1977 several experiments were performed todetermine the re- and rv- values of ethylene and vinylacetate in the copolymerization. The results and reaction conditions used are shown in Table 2.1. From Table 2.1 it can be concluded that both parameters are near unity under high pressure and/or high temperature conditions. The values obtained by Erussalimsky et al 12 seem to be quite exceptional. Ho wever, the results o btained by German13 show that the difference between re and rv increases at lower pressures, while the constant value of re x rv indicates that the copolymer remains almast ideal.

19 Table 2.1 Literature values of re and rv concerning the copo1y­ merization of ethylene and vinylacetate

reaction conditions reference r.., rv re·rv temp pressure solvent 2 (OC) (kgf/cm )

2(1963) 1.07 1.08 1.16 90 1000 toluene 10(1963) 0.77 1.02 0.79 70 400 benzene 10(1963) 0.97 1.02 0.99 130 400 benzene 6(1964) 1.01 1.00 1.01 150 840 12(1967) 0.16 1.14 0.18 60 100 12(1967) 0.70 3.70 2.59 60 1200 13(1971) 0.74 1.50 1.11 62 35 TBA 14(1971) 0.78 1.42 1.11 62 600 TBA 14(1977) 0.80 1.37 1.10 62 1200 TBA

In free-radical actdition it is often found that growing ebains are terminated in chain transfer reactions. In these reactions a hydrogen atom is normally abstracted from the transfer agent although other types of atoms can be abstracted in certain cases. The radica1 formed may initiatea monomermolecule and the active center is thus maintained. Since the growing ebains are terminated premature1y, the degree of polymerization is reduced. A wide variety of substances can act as a chain transfer agent, e.g. polymers, monomers, solvents and contaminations. Consequently, in a polymerization reaction where a high degree of polymerization bas to be obtained, a solvent with a low chain transfer constant is required.

20 2.3 Expertmental

2.3.1 Principle of operation

13 The reactor, of the type as originally used by German , is a vertically placed cylindrical vessel provided with a piston. The upper compartment serves as reaction chamber, the lower compartment to control the pressure. The liquid monomer (vinylacetate (V Ac)) and the solvent (TBA, low chain 9 transfer constane ), containing the radical initiator (a,a'-azodiisobutyronitrile (AIBN)) are introduced into the reaction chamber. The upper campartment is supplied with the gaseous monomer (ethylene) and the pressure is maintained constant during the reaction. During the copolymerization the composition of the reaction mixture is studied and controlled by means of a gas chromatograph. Using a specific sample system13 the reaction mixture can directly be introduced into the gas chromato­ graph. Copolymer present in the sample is retained by a precolumn. The peak areas of the three remaining components (ethylene, VAc and TBA) are determined by electrooie integration of the detector signal and printed out by a digital printer. Depending on the composition of the reaction mixture, new V Ac is introduced into the reactor to keep the ratio of the concentrations of the monomers constant during the copolymerization. This is of utmost importance in view of maintaining a narrow distribution with respect to composition and molecular weight of the EVA copolymers. Aftera eertaio reaction time the copolymerization is stopped using hydrochi­ non. Subsequently, the EVA's are hydrolyzed to obtain the EVOH copoly­ mers1.S.20.21.

2.3.2 Apparatus

A block diagram of the system and its components is given in Figure 2.2. The gas chromatograph used is a Hewlett-Packard model 5710A. The column is a3-meter stainless steel coiled tubing t "o.d. with cbramosorb G 60-80 mesh HP as solid support and as stationairy phase: I 0 % by weight of a mixture of

21 diglycerol and carbowax 400 (60/40 by weight). The carrier gas is helium dried over Linde molecular sieves. The column temperature is 75 °C.

A drawing of the reactor is given in Figure 2.3. The reaction takes place in the campartment above the piston. Below the piston helium can be introduced in order to pressurize the vessel. Outlet H is used to collect the reaction mixture after the copolymerization process.

r­ ' '------'0 --{_EJ ' ' [J=J[§]

A: reactor B: pressure control C: sampling device D: gas chromatograph E: integrator F: recorder G: printer H: pressure control J: vinylacetate

Figure 2.2 Scheme of the inlegral equipment

22 ~~l}--~~-c r-~T).!;'I--~-~--- 0 :;:>..----E L--1*--~-~C

A: inlet B: vent C: stirrer D: heating coil E: thermo well F: piston L--1+-~-~----- G G: helium supply L----~--- H H: drain J: sampling tube

Figure 2.3 Reactor

23 2.3.3 Copolymerizatioo process

Materials

Ethylene, supplied by DSM (Geleen, The Netherlands) and of polymerization quality was used without further purification. V Ac (AKZO, Arnhem, The Netherlands) was purified by distillation. TBA, n-hexane, AIBN and hydrochi­ non (all Merck) were used without further purification. The chemieals used in the acetoxy-hydroxide transformation, methanol and acetone (both Merck) and NaOH (Aldrich), were also used without further purification.

EVA cooolymer

The initiator AIBN was dissolved in a mixture of V Ac and TBA, and intro­ doeed into the reactor. After the liquid components were introduced, the reactor was flusbed with ethylene, closed and heated by means of a thermostat. After the reaction temperature was reached, ethylene was introduced into the reactor vessel, up to the reaction pressure, under stirring of the solution. The ethylene pressure was kept constant during the polymerization process. During the reaction, samples of the reaction mixture were directly introduced in the gas chromatograph. Using these results the ratio of the concentrations of ethylene and V Ac in the reaction mixture was kept constant by adding V Ac during the polymerization process using a pump. After a certain reaction time the polymerization was stopped by adding hydrochinon. The reaction mixture was collected and poured out in a non­ solvent (n-hexane). The solid EVA copolymer was collected and dried under vacuum at 50 oe for 24 hours.

24 EVOH copolymer

The EVA copolymer was dissolved in methanol (50 g EV A/1) at 50 oe. A NaOH solution in methanol (30 mg NAOH/g EVA) was added to the solution under stirring. The EVOH copolymer precipitated and was collected. To obtain a high degree of hydralysis the copolymer was dissolved again in a methanoljH20 mixture (l/1 mixture, 50 g EVA/1). Subsequently, a NaOH solution was added (30 mg NAOH/g EVA) and the solution was stirred and refluxed for approxi­ mately 4 hours. After cooling, acetone was added to the reaction mixture to precipitate the EVOH copolymer. The EVOH copolymer was collected and dried under vacuum at 60 oe for 24 hours. From NMR measurements (absence of acetate resonance, see chapter 3) the degree of hydralysis appeared to be higher than 99% for all EVOH copolymers.

2.3.4 Characterization techniques

Ethylene content determination

The ethylene content of the EVOH copolymers was determined by 1H and 13e NMR (for equations and detailed description, see chapter 3). 200 MHz 1H NMR spectra have been recorded with a Varian XL-200 spectrometer at 50 oe. Sample concentration was approximately 3% (w /v) using perdeuteriated DMSO (Merck) as solvent and internat locking agent. 50 MHz 13e NMR spectra of EVOH copolymers were obtained with a Varian XL-200 spectrometer at 40 oe. Sample concentration was 20% (w /v) in op or in a 80/20 (wjw) phenol/DP (10-15% (wjv)) mixture. The ethylene content was also determined by dynamic thermogravimetrie 22 24 analysis (TGA) - , using a Perkin-Elmer TGS - apparatus. The EVA copoly­ mers ( 4-6 mg) were heated ( 40 oe;min) to 320 oe under nitrogen (gasflow = 2 1/h), and kept there for 30 minutes. The loss of weight (acetic acid) was determined.

25 Molecular weight determination

The number average molecular weight (Mn) of the EVA copolymers and the reacetylated25 EVOH copolymers were determined using a Hewlett- Packard High Speed Membrane Osmometer, model 502 and toluene as solvent. The ratio Mw (weight average molecular weight)/Mn of the reacetylated EVOH copolymers was determined by gel permeation chromatography (GPC) using a Waters chromatograph connected to a differential refractometer and a ultraviolet (UV) detector (254 nm). Waters ~-Styragel GPC-columns (lOS, 10\ 10\ 102 Á) were used with tetrahydrofurane (THF) as the mobile pha8e (0.9 mi/min). Calibration was performed using polystyrene samples with a narrow molecular weight distribution. The EVOH copolymers had to be reacetylated before determining Mn and Mw/Mn because they do not dissolve in toluene (osmometry) and THF (GPC).

2.4 Results and dlscussion

2.4.1 Ethylene content

In Table 2.2 the ethylene content of the EVOH copolymers synthesized at different ethylene pressures is shown. The ethylene content is determined by NMR ctH and 13C, more details in chapter 3) and by TGA. In comparison with the values obtained via 1H NMR the values from 13C NMR. are lower (ca. 0.03), possibly due to increased error propagation for small values of the ethylene content or to the neglect of anomalous linkages (chapter 3). Therefore, the 1H NMR results are more reliable than the 13C NMR results. A linear relationship can be observed between the ethylene pressure in the polymerization process arid the ethylene content of the EVOH copolymers. The ethylene contents of the EVOH copolymers determined with TGA, via the conesponding reacetylated EVOH copolymers, appear to be higher for all samples than those obtained with NMR. 1t is not clear yet how this discrepancy arises. Possibly not all present acetate groups in the EVA copolymer are abstracted from the polymer chain or the

26 reacetylation of the EVOH copolymers is not complete resu1ting in a 1ower value for the acetate contentand consequently a higher value for the ethylene content of the copolymers.

Table 2.2 Molar ethylene content of EVOH copolymers *

FE

1 13 EVOH pethyleru: HNMR C NMR TGA

2 (kgf/cm )

A 2 0.08 0.05 0.13 B 5 0.13 0.10 0.17 c 10 0.22 0.18 0.26 D 15 0.30 0.29 0.36 E .. 0.34 0.31 0.36

* prepared under same conditions: TBA: 480 ml, V Ac: 160 ml, AIBN: 400 mg,

T IX = 55 oe, tiX = 7 h **sample E = EVOH obtained from Kuraray (= EVAL-EPF).

2.4.2 Molecular weight

In Table 2.3 the Mn and Mw values of the EVA, EVOH and reacetylated EVOH copolymers (EVOH(R)) are given. U pon comparing the Mn values of EVA copolymers and reacety1ated EVOH copo1ymers as obtained by osmometry, it can be conc1uded that with copo1ymers 2 preparedat low ethylene pressures (2 kgfjcm ) a large decrease of the Mn va1ue can be observed. This is caused by the fact that the amount of hydrolyzab1e chain branches increases with increasing conversion which is obvious because at higher conversion the polymer concentration increases and consequently the amount of chain transfer to the polymer increases.

27 Table 2.3 Mn and Mw values of EVA, EVOH and EVOH(R) copolymers in kg/mole

Osmometry GPC conv.(%)

Mn Mw/Mn Mwca~c DPw Mwc:ak: EVA EVOH(R) EVA EVOH(R) EVOH(R) EVOH

A 316 129 1.9 2.0 258 3170 135 6.1 B 119 109 1.8 1.9 207 2640 111 54 c 80 74 1.8 1.9 140 1910 77 45 D 56 48 1.8 2.0 96 1400 55 39

Using GPC, the Mw /Mn distri bution of the reacetylated EVOH copolymers is determined. For all copolymers the value is about 2. Using the Mn values obtained by osmometry, and Mw/Mn values, obtained by GPC, the Mw of the reacetylated EVOH copolymers is calculated (Mw~EVOH(R)) Table 2.3). With these values the weight average degree of polymerization can be determined using eq. (7):

Mw~EVOH(R)) DPw = (7) 28 Fa+ 86 (1 - FJ

where Fa is the molar ethylene content as obtained by 1H NMR

From the DPw results also :the Mw's of the EVOH copolymers can be calculated:

Mw~EVOH) = 28 DPw F8 + 44 DPw (1- FJ (8)

These values are also given in Table 2.3. It is obvious that the Mw values of the EVOH copolymers decrease with increasing ethylene content, i.e. higher

28 ethylene pressure during the copolymerization process. This decrease in molecular weight is caused by the higher propensity of the ultimate polymerie ethylene radical to chain transfer reactions to monomeric species. From Table 2.1 (section 2.2, ref. 13 and 14) it appeared that re< 1 and rv > 1. Furthermore, we may assume that k. << kw and that all terminalion reactions are determined by diffusion. Consequent1y, this implies that at both radical chain ends (-e· or -v·) ethylene addition takes place slower than V Ac 26 addition • At higher ethylene pressures the ethylene concentration in the reaction mixture increases which results in a slower copolymerization reaction and consequently a decrease of the conversion rate as can be observed (see Table 2.3).

2.5 Conclusions

1H NMR appears to be a more reliab1e metbod to determine the ethylene content of EVOH copolymers than 13C NMR because e.g. in the 13C NMR metbod some specific structures are neglected in the calculations (chapter 3). From the Mw/Mn ratio(= 1.8) of the EVA copolymers it can be concluded that the molecular weight distribution is narrow. The Mw/Mn ratio(= 2.0) of the reacetylated EVOH copolymers is a little larger than that of the EVA copolymers. This implies that the hydrolysis of the EVA's does not result in a significant increase of the molecular weight distribution: the molecular weight distribution of the hydrolyzable chain branches is, comparable with that of the backbone chains, also narrow. At lower ethylene pressures the conversion rate increases because the concentra­ tion of the "slow reacting" ethylene decreases. However, at higher conversion rates the concentration of the po1ymer in the reaction mixture increases which results in a higher chain transfer rate to the polymer: a high amount of (hydrolyzable) chain branches is obtained.

An observation of utmost importanceis that EVOH copolymers can be obtained with Mw > 105 gjmole with an ethylene content< 15 %. Increasing the ethylene content results in a decrease of the Mw value caused by an increased transfer

29 of the ethylenic chain radical to the monomers at higher ethylene concentrations (i.e. pressures). This change in molecular weight is very important in respect to the physical properties of e.g. EVOH fibers (chapter 6).

To obtain more knowledge about the microstructure of the EVOH copolymers a detailed NMR study was performed on these polymers (chapter 3).

30 2.6 Keferences

1. Koopmans, R.J., v.d. Linden, R., Vansant, E.F., Polym. Eng. Sci., 22(10), 645, 1982. 2. Burkhart, R.D., Zutty, N.L., J. Polym. Sci. A, 1. 1137, 1963. 3. Tsuchihara, T., Kobunshi Ronbunshu, ~ (7), 473, 1979. 4. Lohr, G., Plast. Rubber: Mater. Appl., !1(4), 141, 1979. 5. Koopmans, R.J., v.d. Linden, R., Vansant, E.F., Polym. Eng. Sci, 23(6), 306, 1983. 6. Perrin, M.W., et al., B.P. 497,643 (1938) and U.S.P. 2,200,429 (1940). 7. Mayo, F.R., Lewis, F.M., J. Am. Chem. Soc., 66, 1594, 1944. 8. Alfrey Jr., T., Go1dfinger, G., J. Chem. Phys., 12.. 205, 1944. 9. Zutty, N.L., Burkhart, R.D., Advan. Chem. Ser., 34, 52, 1962. 10. Terteryan, R.A., Dintses, A.I., Rysakow, M.V., Neftekhimiya, ,J., 719, 1963. 11. Brown, F.E., Ham, G.E., J. Polym. Sci. A, 2_, 3623, 1964. 12. Erussalimsky, B., Tumarkin, N., Duntoff, F., Lyubetzky, S., Goldenberg, A., Makromol. Chem., 104, 288, 1967. 13. German, A.L., Heikens, D., J. Polym. Sci. Al, .2. 2225, 1971. 14. van der Meer, R., German, A.L., Heikens, D., J. Polym. Sci., Po1ym. Chem. Ed., ll. 1765, 1977. 15. Behnken, D.W., J. Po1ym. Sci. A, 2_, 645, 1964. 16. Meyer, V.E., Lowry, G.G., J. Polym. Sci. A, 1. 2843, 1965. 17. Ring, W., Makromol. Chem., 101, 145, 1967. 18. Young, L.J., J. Polym. Sci., _H, 411, 1961. 19. Bautl, H., U.S.P., 2,947,735 (1957). 20. Koopmans, R.J., v.d. Linden, R., Vansant, E.F., J. Adhesion, 11. 191, 1980. 21. Iwasaki, H., Yonezu, K., Kobunshi Ronbunshu, 32_(1), 33, 1978. 22. Terteryan, R.A., Shapkina, L.N., Polym. Sci. USSR, U. 1813, 1971. 23. v.d. Meer, R., German, A.L., Angew. Makromol. Chemie., 56, 27, 1976. 24. Park, W.R.R., 7thTherm. Anal. Proc. Int. Conf., 2_, Miller Bernard ed., Wiley Chichester UK, 1057, 1982. 25. Tubbs, R.K., J. Polym. Sci, Polym. Chem. Ed., i. 623, 1966.

31 26. Odian, G., in "Principles of Polymerization", 2nd ed., Wiley-Interscience, New York, 1981.

32 CHAPTER 3* MICROSTRUCTURE OF EVOH COPOLYMERS AS STUDlED BY 1H AND 13C NMR

3.1 Introduetion

Structure-property relationships have been studied extensively exploiting arnong other techniques detailed rnicrostructural analysis via NMR rnethods. In the past, several papers have demonstraled the usefulness of high-resalution 1H and/or 13C NMR methods in determining composition, sequence distribution and cotacticity of these copolymers. Detailed microstructural analysis has been 1 5 6 7 8 carried out for the following copolymers: EVA .2, EVOH3.4, EVCI • , VA-VOH • 9 10 3 and VA-VCI • • Moreover, terpolymers like E-VA-VOH and E-VA-VCln have been studied, however, in these cases only quantitative termonoroer composi­ tions could be obtained. Concentrating on the information available for EVOH copolymers, composi­ tional sequence distribution has been obtained from either salution 1H or 13C NMR data of the compositional VOH centered methine triacts eH NMR)3 or 3 4 alternatively via studies of the compositional dyads eC NMR) • Configuration­ ally (i.e. tacticity) induced splittings have been analysed only partially via salution 1H NMR of the VOH centered hydroxyl triads. Similar to PVOH, also for EVOH copolymers anomalous linkages are possible bes i des the normal head-to-taillinkage (I ,3-diol), i.e., head-to-he ad ( 1,2-diol) and tail-to-tail ( 1,4-diol). In previous salution NMR studies it was possible to detect specific resonances of 1,2-diol and I ,4-diol structures in poly(vinylalcohol-co-crotonic acid) and PVOH using ultra high field NMR equipment, respectively 125 and 100 MHz

*) Reproduced from H. Ketels, J. Beulen and G. v.d. Velden, Macrornolecules, ll, 2032, 1988, by permission of the American Chemical Society.

33 13 13 C NMR12. . 1,4-Diol structures have also been detected in EVOH copoly­ mers4·14, whereas the corresponding 1,4-diacetates could not be observed in EVA copolymers4. Up to now, in spite of several attempts, resonances due to I ,2-diol structures or adjointed 1,2-l ,4-diol structures havenotbeen detected in EVOH copolymers4'14, possibly due to two causes: low molar ethylene ratio~ 30 mole %in the copolymers) and detection limits (22.5 MHz t~c NMRt·14. It is the aim of this chapter to show in a combined 1H and 13C salution NMR study the relative merits of both techniques, concentrating on tacticity, compositional sequence distribution, branching and anomalous linkages, because a better onderstanding of the microstructure might be useful for explaining macrostructural properties.

3.2 Experimental

3.2.1 Materials

The four samples of EVOH copolymers (A-D) as described in chapter 2 were used in this study. Two commercially available EVOH copolymers have been obtained from Kuraray coded EPF (sample E) and ECF (sample F). PVOH has been obtained from Hoechst (Mowiol 66-100) and is characterized by a viscosity of 66±4 centipoise ( cP) (4 % water salution at 20 °C).

3.2.2 NMR measurements

200 MHz and on one occasion 300 MHz 1H NMR spectra have been recorded witheither a Varian XL-200 or a Varian SC-300 spectrometer at 50 oe. Sample concentration was approximately 3% (w/v) using perdeuteriated dimethylsul­ foxide (Me~O-d6, Merck) as solventand internallocking agent. Five millimeter tubes were used. The speetral width of the 200 MHz NMR spectrometer operated in the FT mode amounted to ± 2600 Hz, the acquisition time to 1.5 s, and a putse delay of 10 s and a putse width of 5 ps (60° flip angle) were chosen. The 50 MHz 13C NMR spectra were obtained with a Varian XL-200 spectrome-

34 ter. The sample concentration was 20% (w/v) in DP or in a 80/20 (w/w) phenol/DzÜ (10-15% (w/v)) mixture. Spectra were generally obtained at 40 oe, using braadband decoupling, a pu1se delay of 5 s, accumulating 15000-45000 scans and a digital resolution of 0.69 Hz point, corresponding to a speetral width of 11000 Hz and a data length of 16 K. Monomer sequence placements were determined by camparing the relative peak areasof the proton or carbon atoms involved. In performing quantitative NMR measurements via composi­ tional or configurational sequence placements, one should take into account differences in nuclear Overhauser effects (NOE) and spin-lattice relaxation 13 times (T1). No NOE's or T1's have been determined but one additional e NMR experiment has been performed on sample D using a much long er delay (15 s) and gating off the decoupler to remave the NOE. The results were identical with those obtained via 13e NMR using the standard methods. Implicitly we have assumed that no differential spin-lattice relaxation times are present for different stereoisomerie (mm, mr and rr) triads ar compasitianal triad (VOH, VOH, VOH), (VOH, VOH, E), (E, VOH, E) or analagaus dyad sequences in the methine or hydroxyl resonances in the 1H NMR (bath) or 13e NMR (only methine) spectra. No differential 1H NOE's have been considered to occur. Withinthese limits relative peak areas are proportional to the numbers of proton and carbon atoms involved.

3.3 Results and discussion

3.3.1 1H NMR

Assignments

Figure 3.1 depiets as a typical example, the 300 MHz 1H NMR spectrum of an

EVOH copolymer (sample F) recorded in DSMO-d6 at 50 oe. Approximately the same resolution as shown in this Figure could be obtained when 200 MHz NMR equipment was used.

35 H20

methylene

(E, VOH)

(E, E)

(VOH, VOH)

hydroxyl methine ~-- DMSO-d5h t (VA) methyl methyl !

5 4 3 2 ppm....,....__

Figure 3.1 300 MHz 1H NMR spectrum of EVOH (F) recorded at 50 oe

Using the results from temperature dependent studies, double resonance experiments (cf Figure 3.2), shift additivity rules and the earlier preliminary 3 assignments of Wu , we could assign all resonances. The complete assignment is given in Table 3.1 and will be discussed insome detail now. In the following sections, the mole fractions will be denoted by FE and FvoH· The kinds of monads will be denoted by E and VOH while the three kind of dyad sequences will be given by (E, E), (E, VOH) and (VOH, VOH). A similar notation is used for the three different kind of triads, i.e. (VOH, VOH, VOH), (VOH, VOH, E) and (E, VOH, E). Configurational sequence placements (tacticity induced splittings) are denoted by m or r (dyads) or mm, mr, rr (triads).

36 Table 3.1 Speetral assignments for EVOH copo1ymers, measured with 300 MHz 1H NMR

chem. shifts, ppm protons dyads or triads

1.25 methylene EE 1.34 methylene EO 1.41, 1.44 methylene 00 3.38, 3.41 methine EOE 3.62 methine EOO,m 3.65, 3.67 methine EOO, r 3.85 methine 000, mm 3.88 methine OOO,mr 3.92 methine 000, rr 3.92, 3.95 hydroxyl EOE 4.01 hydroxyl EOO, r 4.07, 4.08 hydroxyl 000, rr 4.26 hydroxyl EOO,m 4.30, 4.31 hydroxyl 000, mr 4.50 hydroxyl OOO,mm

The methylene proton resonances, centeredat 1.25, 1.34, 1.41 and 1.44 ppm are rather broad due to a combination of spin-spin coupling and configurational splittings but have been assigned tentatively to three compositional dyads (see Fig. 3.1). Quantitative information about the methylene dyads is as a conse­ quence hard to extract and is much more easily obtained from an analysis of the 13C NMR methylene dyad and tetrad data (see section 3.3.2). Not counting resonances that are due to Me;$0-d6 and H20, the number of the remaining low field resonance patterns visible is six, in two separate groups of three lines. In an attempt to clarify the assignments in this particular region (3-5 ppm) in 3 considerable more detail than before , double resonance and temperature dependent studies have been performed on sample F. In Fig. 3.2 expanded 300 MHz 1H NMR spectra of this particular EVOH copolymer are depicted showing only the expanded methine and hydroxyl resonance region.

37 rr (000) methine + (EOE) (Hydroxyl) 70 oe r (OOE) jmm (000) + mr (000) rr (000)1 j (EOO) m(OOE) 1 t mr(ooof! -

mm (000) d-----.J

70 oe

c-----

50 oe

b---_.." ------Hydroxyl methlne mr (000) + m (OOE) f rr (000) + r (OOE) + (EOE) 50 oe t

a ____,

5 4.5 4 3.5 3 ppm-1111111...----

Figure 3.2 Expanded 300 MHz 1H NMR spectra of EVOH (F), showing only methine and hydroxyl resonances. VOH units have been truncated to 0

38 As is evident from Fig. 3.2 all these resonances appear to be appreciably broadened as a consequence of spin-spin coupling effects, tacticity induced splittings and compositionally induced chemical sequence placements. Neglecting for the moment spin-spin splitting effects, already on the basis of stereoregularity (tacticity) and chemical sequence placements alone there are as many as six different hydroxyl triads and six methine triacts to be expected (12 different peaks). As already apparent from a comparison of the spectra recorded at 50 and 70 oe (Fig. 3.2a and 3.2c) the hydroxyl resonances (the chemica! shifts being dependent on temperature) can be easily addressed: all resonances which resonate downfield from 3.9 ppm onwards (at 50oe). Hidden splittings can be resolved (both at 50 and 70 oq provided that the methylene dyads are irradiated at 1.34 ppm. No resolution enhancement occurs for the hydroxyl triacts (cf Fig. 3.2a and 3.2b or 3.2c and 3.2d respectively) due to the absence of any coupling between the methylene and hydroxyl protons. Therefore, the remaining splittings have to be assigned to a combination of tacticity induced splittings (± 15 Hz? and 3 spin-spin splittings between hydroxyl and methine protons (± 5MHz) • On the other hand resolution enhancement is observed for the methine centered triacts (cf Fig. 3.2a and 3.2b) on decoupling. The remaining splitting has to be assigned, in an analogous manner as for the hydroxyl protons, to the combined effects of tacticity induced splittings (± 15Hz) and methine-hydroxyl spin-spin 3 splittings (± 5 Hz) • Using an observation obtained by Moritani and coworkers 15 for PVOH , i.e. the observed methine-hydroxyl spin-spin coupling constants increase from mm < mr < rr and m < r, one can straightforwardly assign all observed resonances. By decoupling at 50 oe (see Fig. 3.2b) the methine (VOH, VOH, VOH) triad can be seen to be nicely split in three subpeaks, which can be assigned, using the above mentioned rule of thumb15 to the three tacticity induced mm, mr and rr (VOH, VOH, VOH) methine triacts (see Table 3.1 ). As is evident from Fig. 3.2a-d overlap of methine and hydroxyl resonances could occur (see Fig. 3.2c, at 70 oq teading to incorrect composition sequence data (concentrating on methine triads), however at the lower temperature of 50 oe the tacticity induced splittings of the hydroxyl triads, which are clearly

39 visible at 70 oç, (Fig. 3.2c) are hardly distinguishable. Therefore, the subtie balance between sample concentration, measuring temperature and the acidity of the NMR salution will render the recording of 'H NMR spectra of EVOH copolymers in this region to be a non-trivial task.

Copolymer composition

As indicated by Wu3 the copolymer composition can be calculated from the total spectrum (see Fig. 3.1) using the following equation:

FvoH = (I)

where A1 and Ah represent respectively the total peak areas at low ~ 3.3 ppm) and high field (?:_ 1.6 ppm).

The copolymer composition for samples A-F has been given in Table 3.2, expressed as the ethylene molar fraction F~~a.

Table 3.2 Molar composition and tacticity in EVOH copolymers *

EVOH F~~a FBb F& m A 0.08 0.06 0.05 0.49 B 0.13 0.08 0.10 0.51 ç -0.22 0.15 0.18 0.47 D 0.30 0.26 0.29 0.50 E 0.34 0.29 0.31 0.47 F 0.34 0.26 0.31 0.54

• F~~a, FBb and F& determined via eq. (1), (4) and (5) respectively (see text).

40

----···-·------Seguence analysis

In fortunate cases as for copolymer F (see Fig. 3.2) or in less fortunate cases by adding some dropiets of DCI to the NMR tube3 teading to a rapid exchange of all hydroxyl protons, a sequence analysis can be performed most easily on the VOH centered methine triad resonances. For all copolymers the composi­ tion-averaged propagation statistics could be described as Bernoullian for moderately conversion (± 25 %) copolymers (A-D) or presumably higher conversion copolymers (E, F) as atready has been demonstrated by Wu on similar EVA 2 and EVOH3 copolymers.

Tacticity analysis

In normal, i.e. non-decoupled 1H NMR spectra, the tacticity of these copoly­ mers can be calculated (absence of anomalous structures etc.) provided that the resonance peak at 4.50 ppm (70 oe, see Table 3.1) is clearly resolved:

t -1 m = (2 Aj A1) (FvoH) (2)

where m = the probability that a growing polymer chain will forma meso sequence, A. = the peak area of the mm (VOH, VOH, VOH) triad,

A1 = the total peak area at low field (.:5_ 3.3 ppm).

Although other alternatives exist, e.g. consirlering only the hydroxyl resonances (see Fig. 3.2a), in our opinion only eq. (2) can be used in an unambiguous manner, because depending on a subtie balance of temperature, concentration etc., complex partially overlapping resonance areas can occur (Fig. 3.2a and 3.2c). The values calculated for m have been depicted in Table 3.2 for all polymers. From this Table it can be concluded that m is approximately independent of the copolymer composition, and approximately ideally atactic.

41 Anomalous structures

Methine or hydroxyl resonances which could be assigned to anomalous linkages, i.e. signals other than due to 1,3-diols have not been observed not even in decoupled 1H NMR spectra (cf Fig. 3.2b, 3.2d). This means implicitly that in the above mentioned quantitative analysis intrinsically the amount of 1,2-diol, 1,4-diol or adjoint 1,2-1 ,4-diol structures bas been included.

Branching

No evidence bas been found for short-chain non-hydrolyzable VOH branching. The only evidence for non-hydrolyzable short and/or long chain alkyl 1 branching is the presence of a weak CH3 resonance at 0.83 ppm in the H NMR spectrum (Fig. 3.1 ). In EVA copolymers with much higher molar ethylene ratio, the amount of alkyl 1 17 chain branching was determined 6, and the type of branches cou1d be identified. The amount of chain branching can now be calculated:

2ACH3 = (3) 1000

The total amount of branching bas been plotted in Figure 3.3 and increases steadily for samples A-D with increasing ethylene contents. Since the two commercial polymers (E, F) may have been synthesized under quite different conditions from those used for the preparation of samples A to D, it is not surprising that the amountof branching in these does not follow the same trend.

Possibly, terminal CH3 groups of the main chain interfere with the analysis mentioned above. At a fieldstrengthof 4.6 T there may very well be overlap between the 1H NMR signals of terminal methyl groups of the main chain and those of some alkyl side chains.

42 No 13C NMR measurements have been performed in order to be able to assign 17 the various alkyl groups •

CH3 /1000 C (0) 6 ! • I (C) (F) 5 • x (B) 4 • (E) 3 x (A)• 2

~ mole% '---.------,.------,----.----,----.----.---- ethylene 5 10 15 20 25 30 35

Figure 3.3 Amount of non-hydrolyzable chain branching determined via 1H NMR versus the molar ethylene ratio

3.3.2 uc NMR

Assignments Figure 3.4 shows the 13C NMR spectra of a series of EVOH copolymers. The spectra can be subdivided into a low field region (65-80 ppm) which is assigned to all methine carbon resonances and a high field region (20-53 ppm) 3 which is assigned to all methylene carbon resonances • In Fig. 3.4 the nomenclature proposed for ethylene-propylene copolymers18 is used again. The assignments X, Y, Z are discussed later (tacticity).

43 asy y{xsay ~ sa/) (3asj)p+ y(3say- [3s/) -l ! mole o/o yasaf3- T ethylene - - 8 t

B. 13

70 60 50 40 30 ppm _,...___ _

Figure 3.4 50 MHz 13C NMR spectra of EVOH copolymers

44 Copolymer composition

Using eq. (4) the molar VOH ratio can be calculated:

A65-80 FvoH = A65-80 + (A2D-S2 - A65-80)/2 (4)

2A65-80 = A65-80 + A:ID-sz

where A 65 etc. denote the peak areas of the resonances indicated by their respective chemical shifts.

The molar ethylene content can also be evaluated from the relative areas of the different methylene carbon resonances, providing minor amounts of anomalous structures are neglected. Adopting for EVOH copolymers a nomenclature 18 originally proposed for ethylene-propylene copolymers , the following equation is arrived at

(5)

In Table 3.2 the ethylene molar ratio using eq. (I) (FEa) (H NMR), (4)(FEb) and (5)(FEc) (both 13C NMR) has been tabulated. A reasonably good agreement can be found between the three independent methods, i.e. 1H NMR ( eq. (I)), 13C NMR (eq. (4), utilizing only total methine and methylene carbon resonances including anomalous structures), or alternatively solely the methylene carbon resonances (eq. (5)). However in comparison to the results obtained via 1H NMR a systematic deviation occurs, the results of both 13C NMR methods being 0.02-0.05 units (in absolute values) too low, possibly due to increased error

45 propagation (for small F8 -values) (eq. (4)) or to the neglect of the anomalous structures (eq. (5) vide infra). Therefore, the 1H NMR results are used further throughout this work.

Seguence analysis

In the high field region of the spectra at least six well resolved resonance patterns are observed for the methylene carbons (see Fig. 3.4). Configurational splittings as shown by Moritani4 apparently only contribute to the line widths and could possibly be assigned to peaks occurring within one resonance pattern. The methylene resonances have been assigned in a similar manner to that 4 18 described by Moritani , except that we have adopted the nomenclature used in eq. (5). For the different EVOH copolymers, it is possible to calculate from the area intensities of the methylene lines, expressed as percentages of the total

area, the number average methylene sequence length distribution i.e. n1, n2, n3 8 and n4/ •

%-CH2

100 t A B c D E F

1 2 3 ~4 1 2 3 ;;:;4 1 2 3 ~4 1 2 3 >4 1 2 3 2:4 1 2 3 ;;:;4

Figure 3.5 Number-average methylene sequence distribution of the EVOH copolymers

In Figure 3.5 the methylene sequence distribution has been plotted for all copolymers as a function of the methylene sequence length. Obvious conclusions

are: approximately constant level of n2, and the increase of long methylene

46 sequences with increasing ethylene content. The methylene sequence data confirm again that the distribution is Bernoullian, as has been done by others 2 in earlier work on the parent EVA copolymers •

Tacticity analysis

In Fig. 3.4 the low field region of the spectra shows five to six methine carbon resonances which have been assigned tentatively to the six VOH triad sequences: (E, VOH, E), m (E, VOH, VOH), mm (VOH, VOH, VOH), r (E, VOH, VOH), mr and rr (VOH, VOH, VOHt. This mixed configurational-compositonal VOH methine centered triad has however not been used for a tacticity analysis, although 13C NMR offers in comparison with 1H NMR the advantage that the methine resonance area is free from anomalous resonances. Under our measuring conditions, the six possible VOH centered triads betonging to normal head-to-tail sequences, lead to the following set of equations (see also Fig. 3.4):

X = (E, VOH, VOH) + m (E, VOH, VOH) + mm (VOH, VOH, VOH) Y = r (E, VOH, VOH) + 2mr (VOH, VOH, VOH) Z = rr (VOH, VOH, VOH) (6)

where X, Y, Z represent the measured areasof the VOH methine centered resonances at respectively low, central and high field.

This set of equations is exactly identical with the earlier proposed assignment in vinylchloride methine centered resonances in V A-VCI copolymers10 and the 13 8 carbonyl triple region in the C NMR spectra of V A-VOH copolymers • Eq. (6) can be put in a handier form, having already proven (via 1H NMR)2 Bernoullian statistics to hold for the compositional sequence distribution:

Y 2 2m = (Fe/(1-Fe))+ (7) Z r r

47 Eq. (6) can be solved numerically for m using the experimental values of the VOH methine centered resonances (200-300 MHz 1H NMR spectra} for each copolymer (see Fig. 3.2a) and the measured 13C NMR peak areas X, Y and Z (see Fig. 3.4). Without explicit knowledge of the VOH centered compositional triads, a graphical analysis can be used (eq. (7)), assuming the value m to be constant over the whole series of copolymers. Because it bas been proven, that m is approximately constant for this series of copolymers (see Table 3.2) a plot of Y/Z vs (F.J/(1-(Fs)) is represented in Fig. 3.6.

E o---'1 3 c ----· • e --- F B ..,._.llllflllll- ... -- -- 2 _.-A --·-

0.05 0.10 0.20 0.30 0.40 0.45

Figure 3.6 Plot of Y /Z versus (Fe)!( I -(FE)) ratio

From the slope an average value for m can be calculated, being m = 0.50. Theoretically the intercept should be equal to 2 for this particular case, the experimentally determined intercept however being 1.64. An error analysis shows that slight deviations of the average m value (e.g. m being 0.45) already lead to an intercept of 1.63, therefore the m parameter determined from the slope is a more reliable value.

Anomalous structures

The anomalous structures, i.e. 1,2-diol and 1.4-diol structures in PVOH which could not be observed via 1H NMR, can easily be detected with UC NMR, as is

48 evident in Figure 3.7, where a 50 MHz 13C NMR spectrum of PVOH is represented.

+ /kt s ~y

yasaJ3 i

70 60 50 40 30 ppm -olllllll..,.._ __

Figure 3.7 50 MHz 13C NMR spectrum of PVOH recorded in D:/) showing anomalous sequences

12 13 19 The theoretically calculated (using additivity increments • ' ) chemical shifts of the methylene carbons in the diol structures are listed in Table 3.3. The resonances 1aSaB, BaSB1+ and 1BSa1 which can be attributed toa fragment of coupled 1,2- and 1,4-diol structures (a, b, c of fragment I in Table 3.3) are clearly observed.

49 Table 3.3 Calculated 13C NMR chemica! shifts of the methylene carbons in the diol structures of EVOH copolymers

fragment type ll,ppm

1 -c-c-c-c-c-c-c-c-c• • • 1,2 + 1,4 42.2 a,"YaSatJ I I I I I 30.5 b,"Y+fjaS(J-y+ OH OHOH OH OH 34.2 C,"YfJSa-y u-c-c-c-c-c-c-c-c-c. . . 1,2 42.2 a, {JaSa-y I I I I I 47.0 b,-yaSa-y OH OH OH OH OH liJ -c-c-c-c-c-c-c-c-cb e I lt 1,4 34.2 I I I I 47.0 OH ÓH OH OH

In the 13C NMR spectra of all EVOH copolymers (see Fig. 3.4), evidence for the presence of the 1,4-diol structure can easily be derived from the presence of a resonance at 36.2 ppm. The contents of the 1,4-diol structure can be calculated using the formula for copolymers containing only 1,4-diol:

Isaa mole % 1,4-diol = (8)

where Isaa is the area intensity of the 1BSa1 resonance at 36.2 ppm and 11 is the total area intensity of the methine and methylene lines.

The factor 2 bas to be left out for copolymers containing 1,2- and 1,4-diol structures. The assumption bas been made that in an EVOH copolymer either coupled 1,2- and 1,4-diol structures are present or only 1,4-diol but not both. The calculated contentsof 1,4- and 1,2-diol structures are listed in Table 3.4. As can be concluded from Table 3.4 the contents of the 1,4-diol structure in PVOH and EVOH copolymers are in the same order. Coupled 1,2- and 1,4-units (fragment I, Table 3.3) have been observed in the 13C NMR spectrum of polymers A en B. Isolated 1,4-units are observed for the polymers C, D, E and F. No evidence for the presence of isolated 1,2-diol structures in these

50 copolymers (C-F) has been found (absence of a resonance at 42.2 ppm).

Table 3.4 Calculated contents of I ,2- and 1,4-diol structures of the EVOH copolymers

PVOH A B c D E F

1,4-diol 1.2 1.0 1.9 1.1 0.9 0.8 0.4 1,2-diol 1.2 1.0 0.5

However, in the polymers A and B where coupled 1,2- and 1,4-units are present, the presence of an additional isolated 1,2-unit can not be excluded (overlapping 13C NMR resonances). The presence of isolated 1,4-diol structures has to be attributed to the insertion of ethylene fragments (at least for the polymers C, D, E and F). For the polymers where adjointed 1,2- and 1,4-diol structures are observed (A, B) both possibilities have to be considered (experiments with 13C enriched ethylene would in principal unravel this phenomenon). Thus it is possible to detect the I ,2-diol structure in EVOH copolymers with an ethylene content to approximately 10 mole %.

Branching

No evidence for non-hydrolyzable VOH or alkyl branching has been found in the salution 13C NMR spectra.

3.4 Conclusions

Using 1H and 13C NMR an assignment could be made for the methine and methylene carbon resonances. In the 1H NMR spectra the different hydroxyl resonances could be observed. In determining the molar ethylene content in the EVOH copolymers, a reasonably good agreement was found between the 1H and 13C NMR methods,

51 as was also concluded in chapter 2. Ho wever, a systematic deviation occurs between both methods, possibly due to increased error propagation or to the neglect of anomalous structures. 1H and 1~ NMR appeared to be useful for studying the tacticity of the copolymers. With both methods it was proved that the copolymers are nearly ideally atactic. To obtain more knowledge about the sequence distribution and the anomalous structures in the EVOH copolymers, 13C NMR is more useful than 1H NMR: both 1,2-diol and 1,4-diol structures could be observed in the EVOH copoly­ mers, using ~ NMR. The only evidence for non-hydrolyzable short and/or long chain alkyl branching was found in the 1H NMR spectra. It can be concluded that a combination of 1H and 13C NMR methods is necessary to obtain profound knowledge about the microstructure of EVOH copolymers. This knowledge is important for a better understanding of the properties of e.g. EVOH fibers (chapter 6).

S2 3.5 References

I. Wu, T.K., J. Polym. Sci. A2, .8., 1167, 1970. 2. Wu, T.K., Ovenall, D.W., Reddy, G.S., J. Polym. Sci., Polym. Phys. Ed., Jl., 901, 1974. 3. Wu, T.K., J. Polym. Sci., Polym. Phys. Ed., 14, 343, 1976. 4. Moritani, T., Iwasaki, H., Macromolecules, il. 1251, 1978. 5. Keiler, F., Plaste Kautsch., ll(IO), 730, 1978. 6. Zambelli, A., Gatti, G., Macromolecules, ll, 485, 1978. 7. Moritani, T., Fujiwara, Y., Macromolecules, 10, 532, 1977. 8. Van der Velden, G ., Beulen, J ., Macromolecu1es, li. 1071, 1982. 9. Okada, T., Hashimoto, K., Ikushige, T., J. Polym. Sci., Polym. Chem. Ed., 12.. 184, 1981. 10. Van der Velden, G., Macromolecules, lQ., 1336, 1983. 11. Van der Velden, G., unpublished results, 1985. 12. Amiya, S., Uetsuki, M., Macromolecu1es, li. 166, 1982. 13. Ovenall, D.W., Macromolecules, .!I, 1458, 1984. 14. Amiya, S., Iwasaki, H., Fujiwara, Y., Nippon Kagaku Kaishi (J. Chem. Soc. Japan), il, 1698, 1977. 15. Moritani, T., Kuruma, I., Shibatani, K., Fujiwara, Y., Macromolecules, 577, 1972. 16. Wagner, T., Schlothauer, K., Schneider, H., Plaste Kautsch., 29(11), 637, 1982. 17. Grenier-Loustalot, M.F., Eur. Polymer J., 2.1. 361, 1985. 18. Carman, C.J., Wilkes, C.E., Rubber Chem. Techn., 44, 781, 1971. 19. Vercauteren, F., Donners, W.A.B., Polymer, 27, 993, 1986.

53 54 CHAPTER 4 EVOH/NYWN-6 BLENDS

4.1 Introduetion

In previous chapters, I and 2, it was described that EVOH copolymers are brittie partly due to their relàtively low molar mass which is related to chain transfer reactions during copolymerization of ethylene and vinylacetate.

In chapters 4 and 5, the possibility of bleuding EVOH withother polymers will be investigated, aiming to achieve polymer blend systems combining barrier properties (EVOH)1.z and acceptable mechanical properties. In this chapter the combination of EVOH and the polyamide Nylon-6 was chosen. Polyamides are well known fortheir toughness and ease of processing. Moreover, Nylon-6 is used for barrier film applications. However, as shown in Table 1.1 in chapter I, the barrier properties of Nylon-6 are not impressive.

Combination of polymers can result in miscible, partially miscible or immiscible 3 systems .4. A miscible system is the result of molecular mixing on a segmental level, in contrast to immiscible blends which are multipbase systems in which the major component forms the continuous phase or matrix and the other component makes up the dispersed phase. The partially miscible blends constitute of two or more phases each showing a certain level of molecular mixing. The physical properties of polymer blends (e.g. mechanica} properties, barrier properties) depend on its state of mixing. This state is determined by the thermodynamics of interactions between the blend components and this interaction is a function of their physical and chemica! structures. Miscibility inthemelt was investigated with DSC, PICS and optical microscopy. The miscibility /morphology in the solid state (films) was discussed using results obtained from TEM and Solid State 13C NMR. DSC methods could not be used because the EVOH copolymers, as used in this study, and Nylon-6 possess similar Tg's.

55 4.2 Experimental

4.2.1 Materials

1 Nylon-6, Akulon 135 C (Mn = 30 kg mole' ) was provided by AKZO. EVOH 1 copolymers coded EPL (Mn = 32 kg mote· , ethylene content= 27 mole %), EPF 1 (Mn = 21 kg mole"\ ethylene content= 32 mole %), EPG (Mn = 20 kg mole- , ethylene content= 47 mole %) were obtained from Kuraray Co.

4.2.2 Films

Blends (films) of Nylon-6 and EVOH copolymers were made using a single screw extruder with a tubular film die head. The temperature of the die was 250 oe, well above the melting point of both polymers. The residence time was about 1 minute. The winding-speed of the films was 2 m/min. The thickness of the films was about 20 pm.

Permeability measurements at 0 % R.H. were performed at TNO (Delft, the 5 Netherlands) • Measurements at 60, 70 and 80% R.H. were performed at DSM. Carbon dioxide (COJ was used as testing gas.

Impact performance was measured by an instrumented dart-test. A hemispheri­ cal dart was used. Impact energies were obtained by recording the load-time curve during penetration. From this curve the total energy could be determined.

4.2.3 Charaderization techniques

Differential Scanning Calorimetry (DSC)

DSC measurements were performed using a Perkin-Elmer DSC-7 calorimeter. Indium was used for the calibration (Tm= 156.6 oe). A standard heating and cooling rate of 5 °C/min was adopted.

56 Putse Induced Critical Scattering (PICS)

6 PICS measurements were performed using a Mark 11 instrument • The blends of EVOH-EPF and Nylon-6 (25/75 and 50/50 (wjw)), homogenized in aso­ called "centrifugal homogenizer", were heated from 150 to 240 oe with a heating rate of 2 oC/min. The capillary cell containing the polymer blend was illuminated by a low-power ~e-Ne laser beam. The PICS apparatus was used in the cloud point mode.

Optical microscopy

A Zeiss optica! microscope equiped with a Mettier FP hot stage was used. Thin films of the blends (EVOH-EPF/Nylon-6 = 25/15 and 50/50 (w/w)) were heated from 150 to 240 oe with a heating rate of 2 °C/min.

Transmission Electron Microscopy (TEM)

Films of Nylon-6 and EVOH-EPF/Nylon-6 blends were imbedded in an epoxy matrix. Before staining, the samples were trimmed in an appropriate manner for sectioning. The samples were treated with phosphotungstic acid7 (PT A (Aldrich), 10% aq. solution, 24 h at room temperature) or ruthenium tetra­ oxide8.9 (Ru04) (0.2 g RuCI3·3H..P (Aldrich) in 10 milS% aq. sodium hypochlo­ rite). Microtomy was performed at room temperature using an ultramicrotome Reichert Jung 4F and diamond knife. Sections of 50-70 nm thick were obtained. A JEOL 2000 FX TEM operating at 80 KV was used for the investigation.

Solid State 13C NMR

NMR data were acquired at room temperature on a Bruker CXP 200 spectrome­ ter operating at 50.3 KHz. Samples were loaded into an air driven two component Beams-Andrew BN-POM rotor with a polyoxymethylene (POM) cap and spun at 3.0-3.5 kHz. The chemica! shifts were referenced to the

57 external crystalline and amorphous POM-resonances (both at 88.8 ppm). Typical cross-polarization (CP) putse sequences implied 4 p.s 90° pulse, 1 ms contact time and a 4 s recycle time, collecting 2048 points in the time domaio for a 20 kHz speetral width. Depending on the sample, 1000-15000 FID's we re collected for the Dipolar-Decoupling/CP/Magic Angle Spinning (DD/CP/­ MAS)-experiments1o.t1. Spin-lattice relaxation times (of the crystalline phase

(T1J and the amorphous phase (T1J) were determined by an inversion recovery sequence using CP and MAS. Depending on the sample, 500-750 FID's were colleeled for 10 recovery delays which were 0.5 p.s to 100 s in length. The amplitudes at the peak positions have been fitted to an equation of the form:

l(t) = I(oo) + A{a·exp(-r/T1J + (l-a)·exp(-r/T1J}

where ris the delay between the two pulses and I{oo) is the equilibrium magnetization. A non-linear least square regression metbod has been used. Initial estimates of I( oo ), A, a, TJa and T te are needed.

4.3 Results and discussion

4.3.1 Extrusion blending of EVOH and Nylon-6

Melt blending of Nylon-6 and EVOH is rather complicated due to the formation of gellike particles during extrusion. From IR measurements it was concluded that these gels were not due to a degradation of the EVOH. Consequently, areaction ofEVOH with Nylon-6, or possibly present oligomers, could be the only cause forthese gels. After extraction of Nylon-6 with water {90 oe, 24 h) the concentration of oligomers (lactams) in Nylon-6 was decreased. Using the extracted Nylon-6 in the film blowing process, the formation of gels could be prevented.

58 4.3.2 Harrier properties of EVOH/Nyloo-6 films

In Figure 4.1 the C02-permeability versus blend composition is shown. The observed dependency is ooo-lioear: addition of a small amount of EVOH to Nylon-6 results in a large decrease of the permeability.

• • EPG(47moln ethJ ". EPF (32mole% eth.) o ·EPL(28mole% eth.)

1

%(Wfw)EV0H ---

Figure 4.1 C02-permeability versus blend composition of different EVOH/Nylon-6 films

59 From Fig. 4.1 it can be concluded that the harrier properties of the films increase with decreasing ethylene content of the EVOH in the blend: EVOH (27 % eth.) > EVOH (32 % eth.) > EVOH (47 % eth.). Fig. 4.1 clearly demonstrates that the harrier properties of EVOH/Nylon-6 films are mainly determined by the EVOH copolymer (concentration, ethylene content) used in the blend. The non-linear relationship between harrier properties and EVOH content is also confirmed by Figure 4.2, showing the C02-permeability of EVOH­ EPL/Nylon-6 films at different R.H.-values (Please note that the vertical scale is a logarithmic scale now).

!]~ 3 ..; 10 ~ is. 104

;C\1 g 0.. 105

40 60 80 100 RH(%)

Figure 4.2 C02-permeability versus R.H. of EVOH-EPL/Ny/on-6 films

The addition of 25 % EVOH to Nylon-6 results in a large increase of the harrier properties, whereas the difference between the films containing 25 and 50 % EVOH is rather smalt. In facta ten-fold decrease in permeability is obtained in the case of EVOH-EPL (25 %)/Nylon-6 blend in comparison with pure

60 Nylon-6. Consequently, this system falls in the same category as high barrier resins such as poly(acrylonitrile) and poly(vinylidenechloride) (see Table 1.1, chapter 1). At higher R.H.-values the barrier properties decrease. The penetrating water molecules destroy the hydrogen boncts and act as plasticizer, which will result in an increase of the polymer chain mobility and consequently the permeability.

4.3.3 Miscibility of EVOH and Nylon-6

Both EVOH and Nylon-6 are capable of crystallization, although in different 12 14 crystalline structures " . Based on thermodynamica! equilibrium considera­ tîons15, one can expect an eutectic phase-diagram, provided that the components are miscible in the melt. For polymer mixtures, thermodynamic equilibrium is difficult to reach. Therefore, it is common practice to discuss experimental melting data obtained for similar experimental conditions. In this way possible differences in morphology in the mixtures and the pure components are kept toa minimum.

coolingline

EVOH

125 150 T["CJ

Figure 4.3 DSC cooling-line (5 OC/min) of a EVOH-EPF /Nylon-6 (50/50 (wjw)) blend

61 In Figure 4.3 an example is shown of a DSC cooling-line (5 oe; min) of a 50/50 EVOH-EPF/Nylon-6 blend. It is obvious that consecutive crystallization occurs upon cooling the blend. The first crystallization peak (A) belongs to Nylon-6, the second (B) is the crystallization peak of the EVOH copolymer. In Table 4.1 the T.,- and Tm-values of EVOH-EPF and Ny1on-6 obtained from DSC measurements are shown.

Table 4.1 T"-and Tm-va1ues of EVOH-EPF and Nylon-6 for different blend compositions

Ny-6 Te Tm Ny-6 192.2 225.0 80/20 192.0 225.0 60/40 190.1 221.4 40/60 184.5 219.3 165.5 183.8 20/80 177.5 216.4 165.0 185.3 EPF 165.0 185.8

Similar differences in crystallization behaviour, as can be noticed in Tab1e 4.1, have been observed for polymer solutions and blends and are due to kinetic 15 effects • The mutua1 differences between the crystallization and melting temperatures are approximately constant indicating that the experiments were performed under similar conditions. For comparative purposes it is convenient to use the experimental crystallization and me1ting temperature instead of the equilibrium temperature. 16 According to F1ory , the Tm-depression is given by:

1 1 -RV1 + ( Tmt(O) .ö.H V { Tmt = 1 1

62 where Tm is the melting temperature in the blend, T m(O) is the equilibrium melting temperature, R is the gas constant, 8H is the heat of fusion per mole repeating unit, V is the molar volume of repeating unit,

fraction in the blend, m is the relative chain length on the lattice, X12 is the Flory-Huggins interaction parameter and V, is the molar volume of the lattice site. Subscripts I and 2 indicate component 1 and 2 in the blend.

Due to the uncertainty in the experimental data and the inherent non­ equilibrium character of the experiments the exact value of X 12 giving the best fit to the experimental data according to above equation is not relevant. One could conclude that in order to obtain the melting point depression of Nylon-

6 as noticeable in Table 4.1, the interaction parameter X 12 must have a negative value. This is an indication of favourable or specific interactions between both blend components. This also means that the components are miscible in the melt at least for temperatures just above the melting temperature of Nylon-6. For example a liquid-liquid lower critical salution temperature (LCST) miscibility curve could be present at more elevated temperatures. The conclusion that EVOH-EPF and Nylon-6 are miscible in the melt is also confirmed by the results of PICS and optical microscopy experiments. In the PICS measurements no changes in scattered intensity could be observed in the temperature range of 220-240 oe. Furthermore, optical microscopy experiments using the same blends, showed no phase separation in the same temperature 11 range. Because the calculated difference in refractive index (n0 ) of both components is about 0.02, phase separation should have been detected by these optical techniques. These results support the conclusion that EVOH-EPF and Nylon-6 are miscible in the melt. It is obvious that the miscibility is accomplished by an interaction of Nylon-6 with the hydroxyl group of EVOH. Consequently, the miscibility of these polymers will diminish with increasing ethylene content of the EVOH copolymer.

As can be noticed in Fig. 4.3, upon cooling Nylon-6 and EVOH do not crystallize simultaneously: Nylon-6 crystallizes at first and subsequently EVOH

63 follows. During the actual film blowing process the cooling rates are rather high. Assuming that small Nylon-6 spherulites are formed, the EVOH phase is forced to accumulale between crystalline areasof the Nylon-6 matrix, resulting in a specific morphology. Therefore, this blend also belongs to the class of 1 19 structured blends s. • The transport of molecules like 02 and co2 takes place through the amorphous and intercrystalline regions of Nylon-6, but in this particular blend, the amorphous matrix is enriched by EVOH. Because EVOH itself is an excellent harrier resin, also excellent harrier blends are obtained.

4.3.4 Meehan1eal properties of EVOH/Nylon-6 films

In Table 4.2 the results of the dart-test obtained for EVOH-EPL/Nylon-6 films are shown. By adding EVOH to Nylon-6 the impact energy decreases as was expected. Blends containing more than 40% (w/w) EVOH copolymer possess approximately the same impact strength as pure EVOH-EPL. But as was concluded in the previous parts only minor amounts of EVOH (appr. 25% (w/w)) are necessary in the blend to obtain blends with excellent harrier properties. From Table 4.2 it can be concluded that EVOH/Nylon-6 films containing minor amounts of the EVOH still possess good mechanica) properties, provided by Nylon-6 which forms the matrix.

Table 4.2 Impact energies (KJ/m) of different EVOH-EPL/Nylon-6 films (accuracy appr. 10 %)

Ny-6 75/25 60/40 50/50 40/60 EVOH-EPF

74.1 36.0 11.7 10.9 10.8 11.5

64 4.3.5 Transmission Electron Microscopy

Transmission Electron Microscopy is a well known technique for the charac­ terization of the structure of heterogeneaus polymer systems. However, often it is necessary to enhance image contrast for polymers using staining agents. For this purpose, Nylon-6 film and EVOH-EPF/Nylon-6 film were stained using PT A and Ru04• Figure 4.4 shows micrographs of ultra-thin sections of Nylon-6 film and a film of EVOH-EPF/Nylon-6 (25/75 (wjw)) at high

magnifications, stained with Ru04• Using PTA camparabie results were obtained.

,20nm, A B

Figure 4.4 Electron micrographof Nylon-6 (A) and EVOH/Nylon-6 (B) film

No significant structural differences can be observed between the mkrographs of pure Nylon-6 and the blend. As is known, selective staining causes the non­ crystalline regions to show a dark contrast, whereas the interiors of the crystalline sections appear as bright areas. Consequently, it can be assumed that the bright regions in Fig. 4.4 arise from crystalline Nylon-6. From Fig. 4.4 it can be concluded that the crystalline partsof Nylon-6 are very small caused by the fast cooling of the melt during the film blowing process. It was impossible to detect EVOH crystalline domains.

65 4.3.6 Solld State uc NMR

10 11 Since the pioneering workof Schaefer and Stejskal ' on the characterization of solid polymers via Solid State 13C NMR several publications have appeared 23 dealing with the application of this metbod to polymer blends:zo.. • With complete (or partial) miscibility the average environment of a given constituent molecule will certainly be different from that in the case of immiscibility. In the latter case, the majority of the molecules will be surrounded by neighbours of the same type. The environment of a molecule will affect the electron density around the atoms and/or the mobility of that molecule, which is reflected in the chemical shifts and the relaxation times, respectively. Therefore, differences in chemical shifts and NMR relaxation phenomena between pure polymers and the same polymers in a blend will be an indication of a possible miscibility of the constituents.

4.3.6.1 Chemical shifts

Figure 4.5 shows the 50 MHz 13C CP/MAS spectrum of the EVOH- EPF/Nyl­ on-6 (50/50) blend (spectrum A). The methine carbon resonances of the EVOH copolymer cao be observed (60- 80 ppm) but in the region 20-50 ppm the methylene carbon resonances of Nylon-6 and the EVOH copolymer severely overlap. By subtracting spectrum B (EVOH-EPF) from A by such a factor that the methine carbon resonances of the EVOH copolymer disappear, the difference spectrum C (Nylon-6 in the blend) cao be obtained and the chemica! shifts of Nylon-6 in the blend cao be determined.

66 POM

----B

0 11 - N-C1-C2-C3-C4-C5-C6 I

100 50 10 -I ppm

Figure 4.5 50 MHz 13C CP /MAS spectra; A= EVOH-EPF/Nylon-6 (50/50), B = EVOH-EPF; C =A-B (Nylon-6 in blend);

In Table 4.3 the 13C chemical shifts of two EVOH-EPF/Nylon-6 blends, pure EVOH-EPF and Nylon-6 are listed. For semi-crystalline Nylon-6 two different 24 crystalline phases can he observed : the thermodynamically more stabie a-phase and the 1-phase. In Table 4.3 the 13C chemical shifts of the methylene carbons are not given.

67 Table 4.3 Speetral assignments (in ppm) in two EVOH-EPF/Nylon-6 blends (in respect to the POM-resonances (88.8 ppm))

c6 1 2 3 C17 c2,3 c4 Csa Cs-y

0\ 00 EVOH-EPF/ Nylon-6 (100/0) 74.6 70.4 65.5

(50/50) 173.9 74.2 69.7 64.4 40.8 30.3 26.4 36.8 34.5

(25/75) 173.8 74.9 70.1 65.1 41.0 30.3 26.6 37.0 34.3

(0/100) 173.8 40.6 30.5 27.2 37.3 33.5 The spectra are a superposition of the spectra of crystalline and amorphous materiaL However, crystalline material is generally better defined than its amorphous counterpartand the "narrower" lines assigned bere are therefore essentially due to the crystalline fraction. No significant differences can be observed between the chemical shift of pure Nylon-6 and EVOH-EPF and the analogous polymers being present in the blend. It is known that in Nylon-6 hydrogen boncts exist between amide protons and 25 carbonyl groups , both within one chain and between different chains. These 13 26 boncts will influence C NMR chemical shifts • A comparable situation exists in the EVOH copolymer. It is very unlikely that in blends with compositions as studied bere, no changes in hydrogen bonds, and thus in 13C NMR chemical shifts, would occur upon intimate mixing of the two constituents.

4.3.6.2 Spin-lattice relaxation times

In the case of a miscible blend, the spin-lattice relaxation times (T1) of carbon atoms in the polymer ebains which are mechanically coupled (m.c.) are similar. In blends, this m.c. usually signifies close intermolecular distances. When the carbon atoms are not coupled, each would have its own characteristic T 1• Intermediate stages arealso possible: when polymers are not completely coupled, butsome interaction between groups of the polymers is present, the T 1's of the carbon atoms will change. The difference between the T 1's of the carbons which participate in those interactions will decrease upon increasing coupling (interaction).

The T1a and T 1c of the methylene carbons and the carbonyl carbon of Nylon-6 film and Nylon-6 in the blend films were determined using the equation as described in the experimentalpart with a= 0.75. The values are listed in Table 4.4. The results have to be compared with those of Nylon-6 film, because the blends underwent the same manufacturing process.

69 Table 4.4

T 1 values (ins) of the methylene carbon atoms and the carbonyl carbon atom of Nylon-6 in the amorphous

and crystalline phase in different films (accuracy = T1• (10%); T,.,J~Q%); T1 (C6) (30%))

ppm 173 43 37 30 27 41 34 Nylon-6 c6 Cla Csa c2,3 c4 c17 CS7

EVOH-EPF/ 'Nylon-6

(0/100) Tla 3.0 0.5 0.3 0.2 0.3 0.3

....:1 Tlc 5.0 10.0 5.8 6.2 4.0 5.1 0 (25/75) T,. 3.0 0.5 0.4 0.3 0.5

Tic 25.0 12.0 8.4 6.5 49.0 (40/60) T,. 8.0 0.6 0.5 0.3 0.6 Tlc 100.0 27.0 22.0 26.0 10.0 (50/50) T,. 12.0 0.6 0.5 0.8 0.9

Tlc 300.0 24.0 27.0 57.0 49.0 (60/40) T,. 0.6 0.4 0.2 0.6 0.2 Tlc 36.0 23.0 19.0 14.0 5.0 It can be noticed that the T 1c values of the methylene carbons become larger with increasing EVOH copolymer content in the blend. These differences between Nylon-6 film and blend films cannot be caused by the presence of radicals as is proved by the absence of EPR signals.

In the same way as for Nylon-6, the T1's of the methine carbon atoms in EVOH copolymer film and EVOH copolymer in the blends were compared (Table 4.5).

Table 4.5

T 1 values (ins) of the methine carbon atoms of EVOH copolymer in the amorphous and crystalline phase in different films

ppm 75 70 65

EVOH COH COH COH

EVOH-EPF/Nylon-6

(25/75) Tla 2.5 1.0 2.0 Tic 21 20 24 (40/60) Tla 1 0.8 Tlc 24 15 (50/50) Tla 2.6 1.5 1.4 Tic 17 20 16 (60/40) Tl• 1.1 1.3 0.6 Tlc 16 20 16 (100/0) Tl• 2.5 2.1 3.0 Tic 15 27 16

Because of considerable overlap, the T1 values of separate methylene carbon atoms cannot be determined accurately. The a values (0.4) in the bi-exponential fit were constant for the different EVOH copolymer samples. As can be concluded from Table 4.5, no differences can be observed between the T1's of the methine carbon atoms in the EVOH samples. Upon mechanical coupling in the blends with compositions as studied here, changes in hydragen bonds would occur and consequently in relaxation times.

From the result that only T1c of Nylon-6 changes (not T1• of Nylon-6 and T1's of EVOH), the conclusion can be made that there is no evidence for intimate interactions.

71 The fact however, that the T1.,'s of the methylene carbon atoms of Nylon-6 change slightly, points to the existence of very small crystalline domains of Nylon-6 in the blends. Because only in the case of small Nylon-6 crystallites, an efficient contact between Nylon-6 and the surrounding amorphous phase

(Nylon-6 and EVOH), which probably affects the T1c of Nylon-6, is possible.

4.4 Conclusions

Based on experimental evidence, obtained from thermal analysis (DSC), light scattering (PICS) and optical microscopy, one is tempted to conetude that EVOH copolymers are miscible, in the molten state, with Nylon-6. The miscibility, accomplished by favourable interactions of the polar groups in Nylon-6 and EVOH, will depend on the ethylene content of the EVOH copolymer and probably decreases with increasing ethylene content. Upon cooling phase separation occurs caused by consecutive crystallization: at first Nylon-6 crystallizes foliowed by the EVOH copolymer, resulting in a specific morphology. Therefore, the EVOH/Nylon-6 blends also belong to the 1 19 class of structured blends s. •

Because the amorphous phase of Nylon-6, where transport of e.g. C02 takes place, is enriched hy EVOH, films of these hlends exhihit excellent harrier properties. Because the harrier properties increase highly non-linearly upon adding EVOH, only minor amounts of EVOH are sufficient to ohtain films possessing excellent harrier properties. Consequently, the mechanical properties of the blend films remain good. In the solidified EVOH/Nylon-6 hlends no information could he ohtained concerning structural and morphological details using TEM. Ho wever, hased on Solid State 13C NMR measurements one can conclude that complete phase separation has occurred.

72 4.5 Keferences

1. Blackwell, A.L., J. Plastic Film and Sheeting, 1. 205, 1985. 2. Cutter, J.D., J. Plastic Film and Sheeting, 1. 215, 1985. 3. MacKnight, W.J., Karasz, F.E., Fried, J.R., in "Polymer Blends", Academie Press, New York, 1978. 4. Olabisi, 0., Robeson, L.M., Shaw, M.T., in "Polymer-Polymer miscibility", Academie Press, New York, 1979. 5. TNO, Delft, the Netherlands, internat report. 6. Galina, H., Gordon, M., Irvine, P., Kleintjens, L.A., Pure Appl. Chem., ll(2), 365, 1982. 7. Schaper, A., Hirte, R., Ruscher, C., Hillebrand, R., Walenta, E., Coll. Polym. Sci., 22_4, 649, 1986. 8. Montezinos, D., Wells, B.G., Burns, J.L., J. Polym. Sci., Polym. Lett. Ed., 2..3.. 421, 1985. 9. Trent, J.S., Scheinbeim, J.I., Couchman, P.R., Macromolecules, .!Q., 589, 1983. 10. Schaefer, J., Stejskal, E.O., J. Am. Chem. Soc., 98, 1031, 1976. 11. Schaefer, J., Stejskal, E.O., Buchdahl, R., Macromolecules, 10, 384, 1977. 12. Matsumoto, T., Nakamae, K., Ochiumi, T., Kawarai, S., Shioyama, T., J. Soc. Fiber Sci. Japan, .3..3.(2), 49, 1977. 13. Nakamae, K., Kameyama, M., Matsumoto, T., Polym. Eng. Sci., .!2(8), 572, 1979. 14. Kinoshita, Y., Makromol. Chem., .3..3., 1,1959. 15. Smith, P., Ph.D. Thesis, University of Groningen, The Netherlands, 1976. 16. Flory, P.J., in "Principles of Polymer Chemistry", Cornell University Press, Ithaca, NY, 1953. 17. Van Krevelen, O.W., in "Properties of Polymers, Correlation with Chemical Structure", Elsevier Publishing Company, Amsterdam, 1981. 18. Meijer, H.E.H., Lemstra, P.J., Elemans, P.H.M., Makromol. Chem., Makromol. Symp., .!Q., 113, 1988. 19. Van Gisbergen, J.G.M., Meijer, H.E.H., Lemstra, P.J., accepted for pubHeation in Polymer.

73 20. Stejskal, E.O., Schaefer, J., Sefcik, M.D., McKay, R.A., Macromolecules, H. 275, 1981. 21. Crowther, M.W., Cabasso, I., Levy, G.C., Macromolecules, 21, 2924, 1981. 22. Parmer, J.F., Dickinson, L.C., Chien. J.C.W., Porter, R.S., Macromolecules, ~. 2308, 1987. 23. Dickinson, L.C., Yang, H., Chu, C.-W., Stein, R.S., Chien, J.C.W., Macro molecules,~. 1757, 1987. 24. Weeding, T.L., Veeman, W.S., Angad Gaur, H., Huysmans, W.G.B., Macromolecules, .21.. 2028, 1988. 25. Arimoto, H., J. Polym. Sci. A,~. 2283, 1964. 26. Stothers, J.B., in "Carbon-13 NMR Spectroscopy", Academie Press, New York, chapter 11, 1972.

74 CHAPTERS EVOH/PET AND EVOH/PE BLENDS

5.1 Introduetion

In chapter 4, the miscible system EVOH/Nylon-6 was discussed. In this chapter blends of poly(ethyleneterephthalate) (PET) and polyethylene (PE) with EVOH copolymers will be studied. PET is used as a harrier resin, for example in bottles, because of its excellent clearness and toughness. However, the harrier properties are not impressive (see Table 1.1, chapter 1). EVOH/PET and EVOH/PE blends are, in contrast to the EVOH/Nylon-6 blends, immiscible. Generally the group of immiscible blends can be divided in compatible · and incompatible blends. The term compatibility is used to characterize polymer mixtures possessing desired properties and practical usefulness, like good acthesion between the constituents, improved mechanical properties or ease of blending. During the bleuding process a two-phase system is formed. Starting with a coarse dispersion the spherical domains will become extended into thread1ike structures which will finally break up into fine droplets. These droptets can be extended as long as there is no equilibrium between shearing and interfacial forces. At higher volume fractions of the dispersed phase the agglomeration process becomes dominant. A1though this process is not very well understood, the agglomeration rate depends strongly not only on the volume fractions, but also on the viscosities of both phases and the interfacial properties. Consequent­ ly, (in-)compatible polymer blends can have different, intrinsically unstable, morpbologies like spheres, cylinders and lamellae depending on rheological (viscosity, fluid elasticities) properties, interfacial (diffusion rate, solubility, interfacial mobility and -energy) behaviour and the time effects in different processing routes. Properties, like harrier and mechanical properties, are highly dependent on the morphology of the blend. Because the objective in this study is, similar to that

75 presented in chapter 4, to obtain blends with excellent harrier properties and good mechanical properties, it is important to understand the correlation between morphology and harrier properties.

A theory to describe transport in polymer blends is the Effective Medium 2 Theory (EMT) • This theory provides a simpte quantitative prediction of the overall conductivity of a multipbase material, given a minimal morphological description of the composite. Using extensions of this theory proposed by 3 4 Kirkpatrick , Davis and Barrer ilrul using the topological parameters of the 7 percoladon theory6. , the effective or macroscopie diffusion coefficient Deu• of 8 ordered morpbologies (spheres, cylinders, lamellae) cao be determined • It bas to be taken into account that normally a blend exhibits a disordered macrostruc­ ture but is built up from ordered microdomains as shown in Figure 5.1.

Lamellae Spheres Cylinders Lamellae

parallel series

Figure 5.1 Microstructured domains, subpartsof a disordered macrostructure

In Figure 5.2 the calculated Detr-values of a EVOH/PP blend consisting of these ordered structures are graphically shown. The results are representative for PET/EVOH and PE/EVOH blends. At a given volume fraction of EVOH in the blend, say 30% (w /w), the absolute upper and lower bounds of Detr in Fig. 5.2 are given by the parallel and series laws, respectively9.These bounds repcesent blends with an ordered macrostruc­ ture (parallellayers and layers in series).

76 1Ö 6 ~------,

parallel

(I) EVOH spheres -...... 10-7 -~ C\1 lamelle;---:-~ E u ...... --- ~-- EVOH cylinders ---::::::::-- -CD pp cylinders -- --:::::::-:----..-:---.-... > 10-8 --- ~ -u -CD PP spheres -CD 0 10-9

1Ö104------~------r------r------.------i 0 0.2 0.4 0.6 0.8 vol. fractlon EVOH

Figure 5.2 Effective dijfusion coefficients in cm2/s of EVOH/PP blends

From Fig. 5.2 it is obvious that layers in series of EVOH have to be present in the matrix to obtain good harrier properties. A minor amount of EVOH in the blend then can result in a significant increase of the harrier properties. This highly non-linear relationship between harrier properties and EVOH content in the blend was also observed in the study of the EVOH/Nylon-6 blends (chapter 4). In contrast to the EVOH/Nylon-6 blends where the specific crystallization process results in a structured morphology, specific production processes are necessary to obtain EVOH layers in a matrix (i.e. PET), for example:

77 1. By coextrusion multilayer constructionscan be obtained. Coextrusion is used in various processing procedures: film blowing, blow moulding, cast films and recently also injection moulding.

2. Films of a blend with spheres of EVOH in e.g. a PP matrix can be biaxially drawn in the solid state: the EVOH spheres will be "stretched" and layer-Iike structures of EVOH in the matrix are obtained. During this process not only the barrier properties are achieved (compare Fig. 5.2) but also the 10 mechanica} and optica} properties of the matrix are considerably improved •

3. Disadvantages of above mentioned possibilities are the high production costs (I) and critical process conditions (2). Possibly this can be surpassed by the use of stratified pellets, structured blends consequently, in different subsequent production processes. When the morphology can be fixated it could be possible to obtain stratified films which possess excellent harrier properties (Fig. 5.2). Stratified blends can be obtained using mixers like 11 Kenics, Sulzer, Ross and the MultiClux static mixer •

In the studies described in this chapter, the Multiflux static mixer is used to prepare the stratified pellets. A proper choice of the polymerie materials has to be made because parameters like viscosity and elasticity ratios of the used polymers and the thickness ratio of the layers are important with respect to the 12 interfacial stability of the layers • As was pointed out, the specific morphology in the pellets will be unstable and will change upon further processing. Therefore, it is important to preserve the stratified morphology from distortion.

Generally, there are three possibilities to fixate/stabilize a eertaio morphology: 1 functionalizing the polymers in the blend with reactive groups, 2 adding (block)copolymers to the blend12 and l crosslinking of the dispersed phase, e.g. 13 14 by the use of electron beam (EB) irradiation ' • The latter two methods were tested in these studies. The added (block)copolymer should contain different segments which are

78 chemically identical or compatible to those in the layers. These (block)copoly mers alter the interface in the blend and are often referred to as "compatibili­ 15 zers" because they enhance the compatibility of the components in the blend • In stratified systems intermediate layers of compatibilizer conneet the layers. When a decrease of interfacial tension can be established the start of the deformation and breaking up process can be postporred which could result in a prevention of the distortion of the layers during subsequent processing. Using EB irradiation crosslinks can be induced in one (or both) polymer(s) present in the stratified pellets. The presency of crosslinks could also result in a delay of the start of the breaking up process because the mobility of the polymer chains is decreased. A lower mobility implies a decreased flowablility of the chains which results in a delay of the relaxation process. In the case of (partially) broken layers the presence of compatibilizers and/or crosslinks could also influence the agglomeration process. Agglomeration can 16 be delayed, because the interfaces are made more immobile , or surpressed by the existence of the intermediate layer. This implies that it could be possible to postpone the start of the breaking up-, agglomeration- and finally coalescence­ process. However, a complete prevention may he impossible. Not only the shear rate and the total deformation but also the time in (further) processing is of great importance with respect to a possible preservation of the stratified structures.

Stratified PET/EVOH and PE/EVOH pellets were made using the Multiflux static mixer. Compatibilizers and EB irradiation were used to fixate/stabilize the specific morphology. Films were made of stratified PET/EVOH and PET/EVOH pellets. The morphology (scanning electron microscopy (SEM)) and the barrier properties of these films were investigated and compared with properties of films obtained from turnbie mixtures.

79 5.2. Experimental

5.2.1 ~aterlals

PET (containing 10 mole% of the isophthalic group) was provided by AKZO. 1 EVOH copolymers coded EPF (Mn = 21 kg mote· , 32 mole% ethylene) and 1 EPG (Mn = 20 kg mole· , 47 mole% ethylene) were obtained from Kuraray Co. Lineair Low Density PE-1026 (throughout this chapter defined as PE) was supplied by DSM.

1 Tm C'e) fJ(Pas, i = 100 s· )

PET 223 700 (250 oe) EVOH-EPF 181 600 (250 oe) EVOH-EPG 156 350 (200 °C) PE 127 700 (200 oe) eompatibilizers containing carboxylic groups after reaction bleuding with maleic anhydride (MA) were obtained from DSM:

C-1 (based on EPDM) 0.9 mole% MA C-2 (based on HOPE) 1.5 mole% MA e-3 (based on PP) 3.5 mole% MA e-4 (based on VLDPE) 4.5 mole% MA C-5 (based on LDPE) 5.2 mole % MA.

Orevac 9307 (86 mole % ethy1ene, Tm= 94 oe), a modified EVA, was obtained from A TO CHIMIE, France.

80 5.2.2 Bleods and films

Blends of PET/EVOH and PE/EVOH were made on a Berstorff ZE 25 Iabaratory corotating twin screw extruder.

Stratified pellets were made using a Multiflux static mixer11 • Polymer roelts from different extruders flow tagether in the multiflux where the stream is divided, deformed and rearranged (Figures 5.3 and 5.4).

Figure 5.3 Principle of a static mixer

Depending on the number of static elements in the Multiflux, layers can be introduced in the melt following:

(I)

where N = number of elements

Z0 = number of layers present in the melt before reaching the Multiflux.

Throughout the experiments five static elements were used in the Multiflux. Consequently, stratified pellets (without compatibilizer) contained 64 layers.

Films of the blends were made on a Collin single screw Iabaratory extruder fitted with a film die head. Permeability measurements were performed at DSM.

81 direction of extrusion

Figure 5.4 Multi/lux static mixer

82 5.2.3 Irradiation lrradiation of the blends was performed at IRI (Interfacultary Reactor Institute in Delft, the Netherlands) using a 3 MeV Van de Graaff Accelerator in air at room temperature. Maximum thickness of the samples was 1.0 cm and in order to achieve a homogeneaus dose distribution, two-sided irradiation was performed. Doses up to 20 MRad were used.

5.2.4 Morphology

SEM studies were carried out with a Cambridge Stereoscan Model S200. The samples were plasma-etched with oxygen and subsequently sputter-coated with gold.

5.3 Results and discussion

5.3.1 Stratified pellets

Using the Multiflux static mixer, stratified pellets we re made of PET/EVOH­ EPF and PE/EVOH-EPG blends. In Figure 5.5, electron micrographs of the blends obtained are shown.

,30.um, I A B

Figure 5.5 Electron micrographs of stratified pellets of PETj EVOH (A) and PE/EVOH ( B) blends, respectively

83 From Fig. 5.5 it is obvious that it is possible to obtain stratified pellets for both types of blends. To obtain a first impression of the stability of the structure, these pellets were used, without additional fixation, in the film blowing process. The morphologies of the blown films were investigated via SEM (Figure 5.6).

5.um j A B

Figure 5.6 Electron micrographs of blown films of PET/ EVOH (A) and PE/EVOH blends ( B ), respectively, using stratified pellets

The layer structure as present in the pellets appears not to be stabie enough to remain during the film blowing process: no layers can be observed in the blown films. During the manufacturing process the layers break up under influence of the interfacial tension, agglomerate and subsequently coalesce resulting in a morphology as shown in Fig. 5.6. Consequently, as described in the Introduc­ tion, it is necessary to fixate the morphology before further processing.

5.3.2 Compatibilizers

For adding (block)copolymers to the blends, a third extruder was connected to the Multiflux apparatus and a special die head was used by which a layer of the (block)copolymer could be introduced between the blend layers (Figure 5.7).

84 r-1--

polymer B polymer A '----I-

polymer c

Figure 5.7 Scheme of apparatus used to introduce ( block)copolymer ( B) between the blend layers

Two kinds of compatibilizer were tested in the PET/EVOH-EPF blends: polyolefines with incorporated MA-groups which probably can react with the hydroxyl groups in EVOH and the end-groups in PET, and a EVA copolymer (86 mole% ethylene). lt appeared that the MA-containing polyolefines could nat be used as compatibilizers: at the processing temperature (250 °C) they decompose,

probably caused by an elimination reaction with C02 as teaving group. Also the addition of the EVA copolymer did not result in any stabilization or fixation of the layers: it was even difficult to obtain stratified pellets with EVA as an intermediate layer. Decomposition and differences in viscosity between the different layers at processing temperatures could be the reason for this phenomenon. However, in the PE/EVOH-EPG blends the MA-containing polyolefines could be used because of the lower processing temperatures. In the obtained stratified pellets some adhesion could be observed between the different layers, especially when compatibilizers with higher MA-content were used. Pellets, containing compatibilizer C-5 were used in the film blowing process. The obtained results (morphology and harrier properties) will be discussed later in this chapter, tagether with those of films made from other (un-)stratified pellets.

85 5.3.3 Irradiation

Depending on their chemical structure polymers either crosslink or degrade upon irradiation17• The overall effect of crosslinking is that the molecular weight of the polymer increases and a network is formed. Crosslinked polymers do not dissolve completely in an usual solvent, and by determining the swollen gel fraction of an irradiated polymer, the degree of crosslinking can be calculated. In contrast, radiation degradation is a process in which the polymer chain suffers random chain scission and consequently the molecular weight decreases with radiation dose. Polymers can be classified into two groups depending of the fact whether they mainly crosslink or degrade upon irradiation18• The polymers used in this study, EVOH copolymers, PET and PE, belong to the group of polymers which crosslink upon irradiation, be it with different rates17.18. By irradiating the stratified pellets of blends of the polymers some crosslinking in the layers could possibly be established resulting in a decrease of the chain mobility which postpones the start of the relaxation process by which the layer structure is distorted. Of course toostrong an increase in overall viscosity is not allowed because it will make subsequent processing impossible. Different blends were irradiated: PET/EVOH-EPF (10, 20 MRad); PET/PE/­ EVOH-EPF (5,10 MRad); PE/EVOH-EPG (2,5 MRad); PE/C-5/EVOH-EPG (2,5 MRad). Todetermine the degree of crosslinking of the polymers, also PET, EVOH-EPF and EPG, PE and C-5 were irradiated. Via the determination of the gel fraction, crosslinks could only be indicated in the systems containing PE and C-5. No sta hilization of the stratified structure was obtained in the PET/EVOH­ EPF blends; the layer structure is lost after compression moulding (5 min, 250

0 C}. A morphology of EVOH spheres in a PET matrix is obtained. This indicates that under influence of the interfacial tension the EVOH layers break up (see also 5.3.4 Films). In contrast to the PET blends, the blends based on PE preserve the stratified morphology during compression moulding as shown in Figure 5.8.

86 A B

c D

Figure 5.8 Electron micrographs of stratified PE/EVOH-EPG pellets. A: 0 MRad, before compression moulding ( c.m.), B: 0 MRad, after c.m., C: 2 MRad, before c.m., D: 2 MRad, a/ter c.m.

5.3.4 Films

In Figure 5.9 and 5.10 electron micrographsire shown offilms of PET/EVOH and PE/EVOH blends, respectively made from tumble-mixtures, multiflux­ blends withor without compatibilizer and irradiated multiflux-blends withor without intermediate layer.

87 PET/EVOH-EPF PET/PE/EVOH- EPF (70/30) (60/10/30)

turnbie-mixture

A E

multiflux biend

B F

irradiated

PET/EPF: I 0 MRad PET/PE/EPF: 5 MRad

c G

irradiated PET/EPF: 20 MRad PET/PE/EPF: I 0 MRad

D H S,um

Figure 5.9 Electron micrographs of PET/EVOH-EPF films

88 PE/EVOH-EPG PE/C-5/EVOH-EPG (70/30) (60/10/ 30)

turnbie-mixture

A E

multiflux blend

B F

2 MRad

c G

5 MRad

D H S.um

Figure 5.10 Electron micrographs of PE/EVOH- EPG films

89 Figure 5.9 clarifies again that no stabilization or fixation could be established in the PET/EVOH blends by irradiation. The micrographs of the irradiated samples show EVOH spheres in the PET matrix: the EVOH layers are broken up. Moreover, the films of PET/EVOH contained gellike particles: parts which do nat melt at the regular melting point. Apart from the poor optica! and harrier properties of these films (Table 5.1 ), the presence of these crosslinked particals apparently maximize the radiation dose which can be used.

In contrast however, in the PE/EVOH-EPG samples an influence of irradiation can be observed. In the films irradiated with 2 MRad layers are present. At higher doses (5 MRad) however, the layers are distorted. The problem in the blend systems used, is that bath phases might crosslink upon irradiation. A high degree of crosslinking in PE results in an enhancement of the difference in viscosity of both components in the blend, which can result in a faster distartion of the layers12•

In the blends with an interlayer of compatibilizer the layers cannot be observed clearly in the film. So the influence of the compatibilizer with respect to a possible stabilization is negligible small.

The films as shown in Figures 5.9 and 5.10 were used in permeability experiments (0% R.H.). The results are shown in Tab1e 5.1. The permeability coefficients of the PE/EVOH-EPG films are rather similar to those of PET/EVOH-EPF. Bearing in mind that PET is a much better harrier resin than PE and that EVOH-EPF possesses better harrier properties than EPG (chapter 4), the conclusion can be drawn that the influence of EVOH in the PE/EVOH b1ends is more effective. This is in agreement with the results presented in Figures 5.9 and 5.10 because in PE/EVOH-EPG films a layer structure can still be noticed. At higher irradiation doses films are obtained in which the layer structure is more difficult to abserve and consequently a decrease of the harrier properties is noticed.

90 Table 5.1 Permeability of different blend films (cm3·cm/cm2·24 h·bar) (Peru. lOs)

PET films Pco2 PE films Pc02 (Figure 5.9) (Figure 5.1 0)

A 1.2 A 1.2

B 9.7 B 1.5 c bad c 0.5

D bad D bad

E 2.7 E 1.7 F 9.6 F 3.6

G 8.0 G 4.6

H bad H bad

Upon comparing the harrier properties of films made from stratified pellets with a reference experiment with films obtained from turnbie mixtures, it can be concluded that the films from turnbie mixtures possess the best harrier properties (or in one case (PE/EVOH-EPG), similar properties). From Figures 5.9 and 5.10 it is obvious that in these films, a "layered" structure is present. - --- · -- These layers (or extended) structures are induced by the biaxially strain imposed on the blend during the film blowing procéss. This process is more efficient when the volume fraction of the EVOH increases, because then a course cocontinuous morphology is formed16, prior to the biaxially stretching process, yielding layer structures as in the PP/EVOH example10 in the beginning of this chapter. Also Dupont uses this principle in its Selar process to produce biaxially drawn film of PElPolyamide blends. However, the morphology induced with processes like these will strongly depend on the geometry and the

91 processing conditions (blowup ratio etc.) and are not as universal as the use of pellets which al ready contain the des i red morphology.

5.4 Conclusions

To obtain films with excellent harrier properties, layer structures, perpendicular to the direction of diffusion of the penetrant, have to be present in the films. Using the Multiflux static mixer stratified pellets were obtained of PET j­ EVOH-EPF and PE/EVOH-EPG blends. Using compatibilizers (e.g. MA­ containing polyolefines) and/or EB irradiation attempts were made to stabilize their specific structure during further processing. Because of the high processing temperature (250 oq of PET, the used compatibilizers decompose and consequently no stratified blends could be obtained. However, in the case of PE/EVOH-EPG blends the MA-containing polyole­ fines could be used as compatibilizer. The polyolefines possessing the highest content of MA (5.2 mole %) induced some adhesion between the layers in the blend. Upon irradiation no stabilization/fixation of the morphology could be established in the pellets of the PET/EVOH-EPF blends. Already u pon compression moutding the layers break up under influence of the interfacial tension. As could be concluded from compression moutding experiments with (un-)­ stratified and (un-)irradiated pellets some degree of stabilization is introduced in PE/EVOH-EPG blends irradiated with a low dose (2 MRad). The different pellets (stratified, (un-)irradiated, turn bie-mixtures) were used in the film blowing process. The results showed again that for PET/EVOH­ EPF blends no stabilization of the stratified morphology occurs upon irradiating and/or adding compatibilizers. Ho wever, in films made of stratified PE/EVOH­ EPG pellets irradiated with 2 MRad, layers are still noticeable indicating some degree of stabilization. From permeability experiments it revealed that films with layer (or extended) structures of EVOH, obtained from turnbie mixtures or stratified pellets, possess

92 the best harrier properties indicating the validity of the calculations as presented in Figure 5.2.

Summarizing, the aim of our investigation was to induce the desired morpho­ logy already in the pellets. When the morphology can be completely fixated, a variety of subsequent processes (like injection moulding, blow moulding, extrusion, film blowing and casting) are possible, because the morphology and consequently the harrier properties will not depend anymore on the process and processing condîtions. However, our experiments show that up to now the success of this approach was limited. At least for the blend systems chosen. It wilt be very difficult to fixate the stratified morphology in PET/EVOH-EPF blends because of the high processing temperature (decomposition of compati­ bilizer) and the fact that higher irradiation doses will result in an increase of the amount of gel like particles and consequently a loss of optical and harrier properties. In the case of PE/EVOH-EPG blends some stabilization could be established upon irradiation with low doses, as could be concluded from compression moutding experiments. However, during the subsequent film blowing process, the stratified morphology is distorted. Higher doses did not improve the results and moreover will make the matrix viscosity too high to allow for further processing. Consequently, more research will be required to unravel the possibilities of a stabilization/fixation of stratified structures.

93 5.5 References

1. Perkin, W., Polymer Bull., 12. 397, 1988. 2. Landauer, R., J. Appl. Phys., 2l, 779, 1952. 3. Kirkpatrick, S., Phys. Rev. Lett., 21., 1722, 1971. 4. Davis, H.T., J. Am. Ceramic Soc., 2Q., 499, 1977. S. Barrer, R.H., in "Diffusion in Polymers", J. Crank and G.S. Park eds., Academie Press, London, 1968. 6. Shante, V.K.S., Kirkpatrick, S., Adv. Phys., ~. 325, 1971. 7. Fisher, M.E., Essam, J.W., J. Math. Phys., z., 609, 1961. 8. Sax, J., Ottino, J.M., Polym. Eng. Sci., 2l(3), 165, 1983. 9. Hashin, Z., Shtrikman, S., J. Appl. Phys., Jl, 3125, 1962. 10. Lemstra, P.J., Kirschbaum, R., Polymer, 2.6,, 1372, 1985. 11. Sluyters, R., De Ingenieur, J., 33, 1965. 12. Kahn, A.A., Han, C.D., Trans. Soc. Rheol., ll, 101, 1977. 13. Meijer, H.E.H., Lemstra, P.J., Elemans, P.H.M., Makromol. Chem., Makromol. Symp., 12. 113, 1988. 14. van Gisbergen, J.G.M., Meijer, H.E.H., Lemstra, P.J., accepted for publication in Polymer. 15. Paul, D.R., in "Polymer Blends", Academie Press, New Vork, 1978. 16. Elemans, P.H.M., Ph.D. Thesis, University of Eindhoven, the Nethetlands, 1989. 17. Charlesby, A., Nucleonics, ll(6), 18, 1954. 18. Chapiro, A., in "Radiation Chemistry of Polymerie Systems", Interscience publishers, New Vork and London, 1962.

94 CHAPTER6 PREPARATION AND CHARACfERIZATION OF SOLUTION (GEL-SPUN) EVOH FIBERS

6.1 Introduetion

Solution-spinning of ultra-high molecular weight PE (UHMW-PE), the so­ called gel-spinning process, made it possible to produce fibers with excellent mechanica! properties. Tenacities in the range of 3-4 GPa and moduli over 100 1 GPa could be obtained .2. In the past decade various attempts have been made to apply the principles of 3 the gel-spinning technique to other flexible polymers A. In the case of PVOH interesting results have been obtained, e.g. tenacities up to 2.3 GPa and moduli 5 of about 60 GPa , even based on polymers with moderate molecular weights (Mw appr. 102 kgjmole). The PVOH fibers have some advantages in compari­ son with UHMW-PE fibers: higher melting temperature (ca 250 oe), potentially Iower creep levels and better adhesion to (polar) matrices. A major drawback however, of the PVOH fibersistheir sensitivity towards moisture: mechanica} 6 properties decrease drastically with increasing R.H. • As was explained befare in chapter 1, the incorporation of ethylene in the 7 PVOH chain could probably solve this problem • EVOH copolymers are quite 8 9 unique because they are co-crystallizable over the whole range of composition • • This results in a, relatively, small decrease of melting temperature and crystallinity of the EVOH copolymers with changing composition. In this chapter, explorative results will be presented concerning bath the synthesis and properties of EVOH fibers in comparison with PVOH and PE fibers.

95 6.2 Experimental

6.2.1 Materials

EVOH copolymers a-g were preparedas described in chapter 2. PVOH powder was obtained from Hoechst (Mowiol66-l 00) and is characterized by a viscosity of 66 ± 4 cP ( 4% water solution at 20 oe).

6.2.2 Fiber-spinning/drawing

Solutions of PVOH and EVOH in ethyleneglycol (20% (w/w)) were spun at 160 oe into a liquid cooling bath (20 oe). In the case of EVOH copolymers with an ethylene content below 26 mole %, methanol was used as cooling liquid, and with EVOH copolymers with ethylene content z.. 26 mole %, dichloromethane was used as cooling liquid. This procedure was used to prevent premature L­ 4 0 L demixing, which is detrimental for drawing .I , before gelation/crystallization in the as-spun EVOH fibers. ' After complete removal of the solvent by extraction, the filaments were dried at 20 oe under vacuum and subsequently drawnat temperatures (T 0 ) well below the melting temperature of the polymers. The draw ratio (A) was determined by measuring the displacementsof ink-marks. The initial modulus (E) of the fibers was obtained using a Zwick 1445 Tensile Tester. The length (between clamps) of the fibers, which have been conditioned 24 hours at 22 oe and 6.5 % R.H., was 50 mm and a tensite speed of 5 mm/min was used.

The moisture sensitivity of the fibers was determined using:

.6.E = (1) E6SRH

where E100RH = modulus after conditioning in waterbath for 24 hours at 100% R.H .. E6SRH = modulus after conditioning at 65 % R.H. for 24 hours.

96 6.2.3 Characterization techniques

DSC measurements were performed using a Perkin-Elmer DSC-7. A standard heating rate of 5 oc;min was adopted.

200 MHz 1H NMR spectra were recorded with a Varian XL-200 spectrometer at 50 oe. Sample concentration was approximately 3% (w/v) with perdeuteria­ ted dimethyl sulfoxide (Me~O-d 6, Merck) as solventand internallocking agent.

Using the Mw/Mn and Mn valnes of the reacetylated EVOH copolymers (EVOH(R)), obtained by GPC and osmometry respectively, the weight average degree of polymerization (DPw) was calculated (see also chapter 2):

Mw...JEVOH(R)) DPw= (2) 28 Fa + 86 (1 - Fa)

where FB is the molar ethylene content eH NMR).

The Mw valnes of the EVOH copolymers (MwC!lc(EVOH)) are obtained using equation (3):

Mw.,.JEVOH) = 28 DPw FE+ 44 DPw (I -FE) (3)

6.3 Results and discussion

6.3.1 Synthesis of EVOH copolymers

In order to investigate the intrinsic possibilities of EVOH copolymers for the production of high-strengtb/high-modulus fibers, polymerization conditions were adopted to produce copolymers of maximum attainable molecular weight. 11 TBA was used as solvent because of its low chain transfer constant • During the polymerization processes the ethylene pressure and V Ac concentration we re kept constant to ensure a narrow distribution with respect to molar composition

97 and molecular weight (see chapter 2). Table 6.1 shows the characteristics of synthesized EVOH copolymers as a function of ethylene pressure using the following reaction conditions: 480 ml TBA, 160 ml V Ac, 400 mg AIBN, T = 55 oe. Table 6.1 Properties of synthesized EVOH copolymers

peth DPw Mwca~c(EVOH)

2 (kgf/cm ) (kg/mole)

EVOH-a 2 3170 135 0.08 EVOH-b 5 2640 111 0.13 EVOH-c 10 1910 77 0.22 EVOH-d 15 1400 55 0.30

* 1H NMR

It can be concluded that the degree of polymerization decreases with increasing ethylene pressure. Changing initiator concentration, reaction time, reaction temperature and V Ac concentration did hardly result in an increase of the DPw values of the EVOH copolymers. The decrease in molecular weight is probably caused by the higher propensity of the ultimate polymerie ethylene radical for chain transfer reactions to both monomeric species (see also chapter 2). Consequently, the conclusion can be made that the degree of polymerization in the radical copolymerization of ethylene and V Ac is intrinsically limited. Another synthesis route may surpass this problem.

6.3.2 Properties of EVOH fibers

In Table 6.2 the properties of solution-spun and isothermally drawn EVOH and

PVOH fibers are shown. The drawing temperature T 0 was chosen to provide maximum efficiency of draw.

98 Table 6.2 Properties of EVOH and PVOH fibers

FE Tm DPw To ).max E65RH E CC) CC) (GPa) (-)

PVOH 0.00 237 3420 190 21.5 76 0.21 EVOH-e 0.11 224 3030 190 18.5 64 0.32 EVOH-f 0.26 196 1970 140 15.5 36 0.50 EVOH-g 0.34 182 1480 130 15 23 0.57

As can be observed the moisture sensitivity decreases with increasing ethylene content caused by the incorporation of the hydrapbobic ethylene segments. The EuJORH/E65RH (~E) ratio reduces from 0.21 for PVOH to 0.57 for EVOH containing 34 mole % ethylene. It is also clear that there is a decrease in ).max and maximum Young's modulus (E) with increasing ethylene content. Since ).max is, among others, dependent on 12 the molecular weight of the polymers , this can be attributed to a decrease in chain length with increasing ethylene content, as is obvious from the decrease of the DPw values (see Table 6.1 ).

6.3.3 Development of Young's modulus as a function of draw ratio ).

13 14 Previous studies ' on solid state drawing of flexible chain polymers, drawn under conditions of optimum efficiency, pointed out that Young's modulus depends uniquely on the absolute draw ratio and is independent of molecular weight, polymer concentration and, within eertaio limits, draw temperatures of the samples. In order to study the development of Young's modulus with draw ratio for PVOH and EVOH fibers, initial moduli of these fibers were determined at various draw ratios. The obtained results are shown in Figure 6.1. As a reference the literature data 15 for PE are also given •

99 80 t'G / -D. x (!) / w / - / / / / 60 / l / / / / / / 40 / + / '/ 7 I' I' /+ 0 20 ///0/ J/{ '/0 /

0 10 20 30

.À

Figure 6.1 E65RH as tunetion of draw ratio for PVOH and EVOH copo/ymers: X ""' 0 mole %, • ""' 11 mole %, + ""' 26 mole %, 8 • 34 mole % ethylene, ----- • PE

As can be observed, the moduli of PVOH, at comparable draw ratios, are higher than those of PE. The leveland slope of the modulus/draw ratio curve of

100 EVOH copolymers decrease with increasing ethylene content; the curve of EVOH containing 34 mole % ethylene is even below that of PE.

It is rather difficult to present a straightforward explanation for the observed E vs >. curves of the various EVOH copolymers. Comparing PVOH with PE 1 17 fibers, and assuming a simpte series modeJ 6. , the higher moduli values for PVOH can be attributed toa higher value for the amorphous part (Ea) since the glass transition temperature Tg of PVOH is above the testing temperature, approximately 40 "C at 65% R.H. compared to the T8 of PE approximately - 100 "C. The crystal moduli (Ec) of PE and PVOH are comparable since both polymers possess a planar zig-zag structure in the crystalline state. Literature data for the Ec of PE are 240 to 340 GPa and 255 to 289 GPa for PVOH 18 respectively •

In the case of EVOH copolymers two independent parameters have to be taken into account:

! The Tg will drop with increasing ethylene content but the decrease is lower than to be expected for a random copolymer due to favourable hydrogen 9 bonds between -OH side groups • In the case of fibrous EVOH copolymers we observed a slight decreasein Tg. For example, the Tg of sample EVOH-g (34

mole% ethylene) at 65% R.H. is approximately 10 to 15 "C below the T8 of PVOH (appr. 40 oe, see above).

2. The overall crystallinity X., will decrease with increasing ethylene content but 9 EVOH copolymers remain crystalline due to co-crystallization • However, compared to PE and PVOH the overal crystallinity is lower, also due to alkyl side ebains which are more abundant in EVOH copolymers. With increasing ethylene content, the concentration of alkyl side ebains increases (see chapter 3) and consequently the packing of ebains should be more difficult.

At present, it is rather impossible to discriminate between the two parameters mentioned above.

101 6.4 Conclusions

The incorporation of the hydrophobic ethylene segments in PVOH results in a decrease of the moisture sensitivity. The drawback however, is that the efficiency of draw, viz. the slope of the E->. curves, decreases with increasing ethylene contentand equally important, the maximum obtainable draw ratio >.max decreases due to a reduction in chain length. Consequently, in order to produce EVOH fibers combining a reduced moisture sensitivity and attractive mechanical properties, an increase in molecular weight is a prerequisite. Due to chain transfer reactions it is difficult to produce, on an economically and technologically acceptable scale, EVOH copolymers possessing high molecular weights.

102 6.5 References

1. Smith, P., Lemstra, P.J., Coll. Polym. Sci., ill, 7, 1980. 2. Smith, P., Lemstra, P.J., J. Polym. Sci., Polym. Phys. Ed., 19, 1007, 1981. 3. Peguy. A., St.Joho Manley, R., Polymer Comm., 25, 39, 1984. 4. Schellekens, R., Lemstra, P.J., DSM/Stamicarbon, Eur. Patent 114,983 (1984). 5. Tanaka, H., Suzuki, M., Ueda, F., Toray Industries, Eur. Patent 146,084 (1985). 6. Sakurada, 1., "Po1yvinyla1cohol Fibers", Marcel Dekker Inc., New York, 1985. 7. Iwanami, T., Hirai, Y., Tappi Journal, 66(10), 85, 1983. 8. Bunn, C.W., Peiser, H.S., Nature, 159, 161, 1947. 9. Matsumoto, T., Nakamae, K., Ogoshi, N., Kawazoe, M., Oka, H., Kobunshikagaku (Polymer Chemistry), 28, 610, 1971. 10. Schellekens, R., Rotten, H.J.J., Lemstra, P.J., DSM/Stamicarbon, Eur. Patent 212,757 (1987). 11. Bautl, H., U.S.P. 2,947,735 (1957). 12. Smith, P., Matheson Jr., R.R., Irvine, P.A., Polymer Comm., 25, 294, 1984. 13. Ward, I.M., Polym. Eng. Sci., 24( 10), 724, 1984. 14. Irvine, P.A., Smith, P., Macromo1ecules, 26, 240, 1986. 15. Smith, P., Lemstra. P.J., J. Mater. Sci., ll. 505, 1980. 16. Perepelkin, K.E., Chim. Vo1okna, ~. 3, 1966.

17. Arandakumaran, K., Roy, S.K., St.Joho Manley, R., Macromolecules, ~L 1746, 1988. 18. Ward, I.M., in "Mechanica! Properties of Solid Polymers", 2nd ed., Wiley, New York (1983).

103 104 APPENDIX SOLID STATE 13C NMR STUDY OF EVOH COPOLYMERS

A.l Introduetion

High-resalution 13C NMR spectra in solids have become readily measurable by 1 2 using the combined techniques of high-power proton decoupling , cross­ 4 polarization(CP)3 and magie angle sample spinning(MAS} • However, due to anisatrapie magnetic susceptibility and chemical shift dispersion, line broadening smears out the splitting resulting from differences in tacticity. Just recently splitting due to tacticity differences was observed for the first time in 13 5 the C Solid State NMR spectra of PVOH • This appendix is concerned with 13C NMR spectra of solid EVOH copolymers. The observed splitting in the methine carbon region could be explained by taking into account bath sequence and tacticity effects.

A.2 Experimental

A.2.1 Materials

Five samples of EVOH copolymers (A-E) were preparedas described in chapter 2. Three commercially available EVOH copolymers (pellets) have been obtained 1 from Kuraray, coded EPL (F, Mn = 32 kg mote· ), EPF (G, Mn = 21 kg mole" 1 1 ) and EPG (H, Mn = 20 kg mole. ). The EVOH copolymer (powder) with an 1 ethylene content of 90 mole % {I, Mn = 25 kg mole. ) was obtained by hydrolyzing the corresponding EVA copolymer EL V AX-260 (Dupont). PVOH powder (J) bas been obtained from Hoechst (Mowiol 66-1 00) and is characte­ rized by a viscosity of 66 ± 4 cP ( 4 % water salution at 20 °C}.

105 A.l.l NMR measurements

The NMR data were acquired at room temperature on a Bruker CXP 200 spectrometer operatingat 50.3 MHz. Samples were loaded into an air driven two component Beams-Andrew BN-POM rotor with a polyoxymethylene (POM) cap. The samples were spun at 3.0-3.5 kHz. The chemical shifts were referenced to the external chemical shift of the crystalline and amorphous POM-resonances (both at 88.8 ppm). Typical CP putse sequences used 4 ps 90° putse, 1 ms contact time and a 4 s recycle time, collecting 2048 points in the time domain fora20kHz speetral width. Depending on the sample, 1000-15000 FID's were collected for the Dipolar-Decoupling/CP/MAS experiments1.6.

A.3 Results and discussion

Assignments

Figure A.l shows the 50 MHz 13C CP/MAS NMR spectra of PVOH and the EVOH copolymers A-1. The complete assignment of the chemical shifts together with the relative areas for the methine carbon resonances are given in Table A. I and will be discussed now in some detail. The CP/MAS spectra are a superposition of the spectra of crystalline and amor­ phous material. However, crystalline material is generally better defined than its amorphous counterpartand the "narrower" lines assigned bere are therefore essentially due to the crystalline fraction. Sufficient decoupling power was used to enable observation of relatively narrow lines from crystalline samples. The spectra can be subdivided into a low field region (65-80 ppm) which is assigned to all methine carbon resonances and a high field region (25-50 ppm) for all methylene resonances. In the high field region of the spectra five resonance patterns can be observed for the methylene carbons (see Fig. A.l). In the nomenclature used in Fig. A.l each of the metbytenes (or secondary) carbon atoms in the sequences is identified with S and two Greek letters denoting the nearest methine carbon atoms (see chapter 3).

106 SAMPLE J

8 A

13 B

22 c

27 D

30 E

F

47 G

56 H

90

100 60 20 ppm

Figure A.l 50 MHz 13C CP /MAS spectra of PVOH and EVOH copolymers A-1. Peak 1-3 denote the three different methine carbon resonances. Saa etc. denote the different methylene carbon resonances

107 Table A.l UC NMR assignments for PVOH and EVOH copolymers, recorded at 50 MHz, using CP/MAS methods* % ethylene 2 3 Al A2 A3

PVOH(J) 0 77.3 71.2 65.4 0.14 0.50 0.36 A 8 76.9 71.1 65.4 0.13 0.48 0.39 B 13 76.8 71.2 65.4 0.17 0.51 0.32 c 22 15.6 70.5 65.2 0.18 0.52 0.30 D 27 74.6 70.4 65.5 0.29 0.52 0.19 E 30 74.9 70.4 65.0 0.31 0.50 0.20 F 34 74.8 70.4 65.4 0.30 0.51 0.19 G 47 74.0 69.5 65.2 0.40 0.43 0.17 H 56 73.7 69.0 65.1 0.43 0.42 0.15 I 90 73.3 69.3 0.79 0.21 0 00- Sera Sa, Sn SBa SBB

J 45.7 A 46.0 27.3 B 45.9 27.2 c 45.9 42.2 33.1 29.8 27.1 D 45.3 40.5 33.6 31.0 26.3 E 46.3 40.7 33.9 29.8 26.3 F 46.1 40.7 33.7 30.9 26.9 G 49.7 39.7 33.2 29.9 26.2 H 45.2 40.0 33.5 29.9 25.9 I 40.4 33.5 • Chemica! shifts (in ppm) are relative to the 13C chemical shift of external POM (88.8 ppm). Peak 1-3 denote the three methine carbon resonances and A 1-A3 are the conesponding area intensities. In the methylene carbon region no additional resonances can be observed in comparison with the solution NMR results, so we have focussed our attention 5 to the methine carbon region, because, according to Terao , hydragen bonding certainly has more effect on the chemical shifts of these resonances. The low field region shows three methine carbon resonances, peaks 1-3. 5 Referring to Terao , and concentrating for the moment only at PVOH (J) and excluding the minor resonances of anomalous structures, peaks 1-3 in the solid state cannot simply be assigned to the tacticity induced splitting of the mm, mr and rr triacts respectively: the relative areasof the three peaks in the solid state 5 are not consistent with the triact tacticity observed in solution • Moreover, a significant downfield shift of the resonances 1 and 2 occurs compared with the chemica! shifts for mm and mr triad resonances observed in solution, resulting in much larger chemica! shift differences between the methine carbon resonances in the solid. This downfield shift of peaks I and 2 is tentatively assigned5 to the action of intramolecular hydragen bonds. This effect is larger in solicts than in solutions because in the solid state many hydrogen bands cooperate to keep the molecule in a certain conformation as opposed to solutions where frequent transitions between conformations diminish the "time averaged" hydragen bond strength as reflected by the chemical shifts. In the case of the mm triad, the oxygen atom bonded to the central methine carbon can farm two, one or zero intramolecular hydragen bond(s) and for the mr triact one or zero intramolecular bond are possible. In the rr triact no intra­ molecular hydragen bond is possible. By assuming that the line position shifts downfield by about 6 ppm per intramolecular hydragen bond, Terao5 assigned peak 1 in PVOH to the mm triact with two intramolecular hydragen bonds, peak 2 to the mm and mr triacts with one intramolecular hydragen bond and peak 3 to the mm, mr and rr triacts with no intramolecular hydragen bonds. In the spectra of the EVOH copolymers peaks 1-3 can also be observed, see Fig. A.I. In Table A.l the chemical shifts and the relative intensities of peaks 1-3 of the EVOH copolymers are shown with increasing ethylene content of the EVOH copolymers. With increasing ethylene content an upfield shift of 4 ppm is observed for resonance I, a smaller upfield shift for resonance 2, while no

109 effect has been observed for peak 3. CP/MAS NMR measurements with varying contact times were carried out for copolymers with 8 and 47 mole% ethylene (spectra A and G, Fig. A.l).

These experiments showed that the important CP parametersTuc and T 1p (H) did not vary significantly with ethylene content for all resonances. This means that the response factors in our UC CP/MAS measurements as presented in Table A. I do notchange significantly. The areasof the three peaks also change with different ethylene contents: the areas of peak 1 increases, the areas of peaks 2 and 3 decrease with increasing ethylene content. At an ethylene content of 90 mole % peak 3 has almost disappeared. In the assignment of the methine carbon resonances in the EVOH copolymers we have to take into account both tacticity effects, as in PVOH, il.ru! sequence distribution effects. For the calculation of the 13C chemical shifts in the sequence triads of~ EVOH copolymer, the 13C chemical shift additivity rules for aliphatic alcohols, as calculated by Ovenale from solution NMR measure­ ments, have been used:

a • 40.8 ppm 8 = 7.7 ppm 1 = -3.4 ppm

Assuming no conformation dependent chemica! shift effects to occur and using the chemical shift of orthorhom bic polyethylene (33 ppmt we can now calculate the chemical shifts of the methine carbon atoms in the three triads of the solid crystalline EVOH copolymer, respectively 000 (67 ppm), OOE (70.4 ppm) and EOE (73.8 ppm) where 000, OOE, EOE are abbreviations for (VOH, VOH, VOH), (VOH, VOH, E) and (E, VOH, E) triads. The chemical shift values presenled above are only meant to yield useful assignments of the several methine carbon 13C NMR signals of EVOH copolymers. These assignments are necessary because OvenaW did not report dependable estimates for all three types of methines sustained by experimental results. We areaware of chemical shift differences between liquids and solids. Moreover, the choice of orthorhombic polyethylene as a basis for the shift

llO calculations is rather arbitrary but this will cause the same uncertainty in each of the three shifts. Of more importanceis the known sensitivity of substituent­ induced shifts towards different conformational equilibria. From results obtained by Möller9 for different poly(I ,2-dimethylbutane) polymers it can be estimated that the uncertainties in our estimations amount to ca. 2 ppm. It is, however, improbable that the order of the three methine carbon signals will be misjudged. Besides these sequence effects, also effects similar as observed by Terao5 for PVOH (i.e. hydrogen bond induced tacticity shifts), have to be taken into account. In Table A.2 the theoretically calculated chemical shifts of the methine 5 carbon atoms are presented, adopting the same assumption as made by Terao , i.e. a downfield shift of 6 ppm per intramolecular hydrogen bond. The chemical shift of 000 mm(2) can e.g. be calculated as 6a + a + 21 + 2(6) = 79 ppm.

Table A.2 Calculated 13C NMR chemica! shifts (in ppm) for the methine carbons in the EVOH copolymers*

st ces te ces st te ces st te ces

000 67 mm(2) 79 000 000

mm(I) 73 mr(l) 73 mm(O) 67 mr(O) 67 rr(O) 67

OOE 70.4 OOE OOE

m (I) 76.4 m (0) 70.4 r (0) 70.4

EOE 73.8

* st = sequence triad, ces = calculated chemica! shift, te = tacticity effect.

111 Due to chemical shift dispersion, considerable overlap occurs, as is evident after an inspeetion of the results in Table A.l and the experimentally observed chemical shift patterns (Fig. A.l ). Therefore, only three peak regions can be discerned and assigned:

Peak 1 = 000 mm (2) + OOE m (1) +EOE

Peak 2 = 000 mm (I)+ 000 mr (I)+ OOE m (0) + OOE r (0) (I)

Peak 3 = 000 mm (0) + 000 mr (0) + 000 rr (0)

where the figure in brackets indicates the number of intramoiecular hydrogen bonds.

From eq. (1) it is apparent that this resonance assignment includes the model proposed by Terao for PVOH (one extreme, Fig. A. I: spectrum J). For ethylene rich copolymer (the other extreme) this model prediets the observation of one major resonance situated at 74 ppm. From an inspeetion of Fig. A.l (spectrum I) this turns out to be true. Additional confirmation of the chemical shift assignment can be obtained from relative area measurements, bearing in mind that peak 3 ( eq. (I)) is only influenced by tacticity effects. This is obviously not true for peaks 1 and 2 which are influenced by both tacticity Blli1. sequence distribution effects.

Because (mm(O) + mr(O) + rr(O) + mm(l) + mr(I) + mm(2)] = I anq [m(O) + m(l) + r(O)] = I. eq. (1) can be transformed into:

A3 a•ooo (2) A 1 + A2 + A3 = 000 + OOE + EOE

where a' = mm(O) + mr(O) + rr(O) and A 1 - A3 being area intensities.

112 Furthermore, random Bernoullian statistics hold for EVA 10 and EVOH11 copolymers, so:

(3) and:

[ A3 ] 14 ~ 1 + A2 + ~ = a(l - F J = aFvoH (4)

where FvoH and FE are the molar vinyl alcohol and ethylene contents 14 respectively of the EVOH copolymer and a = (a') •

The value of a reflects the fraction of 000 triacts which is not influenced by any hydragen bonding. In Figure A.2 this relation bas been plotted together with a graphof eq. (4) assuming a= 1 (no effects of hydragen bonding at all).

The experimental results used in Fig. A.2 include the outcome of spectrum I (Fig. A.l). As can be noticed, the contribution of peak 3 in this sample is very low, it can be estimated at 0 to 2 %.

From this graphit can be evaluated that a 111$ 0.7 and consequently a',.. 0.5. So over the whole range of copolymers, approximately 45 to 55 % of all 000 triacts being present in the crystalline part of these copolymers do not take part in any intramolecular hydrogen bonding.

Summarizing, the results described above demonstrate clearly that in solid EVOH copolymers with different ethylene contents, the combined effects of sequence distribution and tacticity induced hydragen bonding have to be taken 1 13 into account much in the same way as in solutions 2. • Moreover, the effect of the hydragen honds on the 13C NMR chemical shifts is much larger for solicts 5 than for solutions •

113 0.8

0.6

o~~~~_J~~~-L-L~~~~~~~~~ 0 0.2 0.4 0.6 0.8 1 o : a=1 Mole fraction (1 - Fe:> • : EVOH copolymer A - J

Figure A.2 Plot of (A3/( Al+ A2 + A3JYI2 against the molar fraction ( 1-FE) (see eq. (4)). Al etc. denote the area intensities of the methine carbons

A.4 Conclusions

Using Solid State 13C NMR an assignment of the chemical shifts of the methine and methylene carbons could be made. In the assignment of the methine carbon resonances two effects have to be taken into account: tacticity effects, as in PVOH, arul sequence distribution effects. With these effects, the splitting of the methine carbon resonances was discussed. A model was proposed which explains this splitting.

114 A.S Keferences

1. Schaefer, J., Stejskal, E.O., J. Am. Chem. Soc., 2.8_, 1031, 1976. 2. Sarles, L.R., Cotts, R.M., Phys. Rev.,ll., 853, 1958. 3. Pines, A., Gibby, M.G., Waugh, J.S., J. Chem. Phys., i2, 569, 1973. 4. Andrew, E.R., Int. Rev. Phys. Chem., 1. 195, 1981. 5. Terao, T., Maeda, S., Saika, A., Macromolecules, 1§., 1535, 1983. 6. Schaefer, J., Stejskal, E.O., Buchdahl, R., Macromolecules, l.Q, 384, 1977. 7. Ovenall, D.W., Macromolecules, l1. 1458, 1984. 8. Kitamaru, R., Horii, F., Marayama, K., Macromolecules, 12., 636, 1986. 9. Möller, M., Cantow, H.-J., Polymer Bull.,~. 119, 1981. 10. Wu, T.K., Ovenall, D.W., Reddy, G.S., J. Pol. Sci. Polym. Phys. Ed., !l, 901, 1974. 11. Wu, T.K., J. Pol. Sci. Polym. Phys. Ed., 14, 343, 1976. 12. Moritani, T., Iwasaki, H., Macromolecules, ll, 1251, 1978. 13. Ketels, H., Beulen, J., v.d. Velden, G., Macromolecules, ll, 2032, 1988.

115 116 SUMMARY

This thesis describes the synthesis and characterization of ethylene vinylalcohol (EVOH) copolymers as well as the utilization of these copolymers in high harrier systems and as starting materials for high-performance fibers.

The pure homopolymer polyvinylalcohol (PVOH) is a well established polymer used for the production of harrier materials and fibers. The -OH sidegroup in PVOH is relatively small and consequently PVOH is crystallizable, despite its atactic character, into a distorted monoclinic version of the polyethylene structure. Due to its crystallinity but notably due to the relative high glass transition temperature T, of approximately 80 oe (0 % relative humidity), PVOH possesses excellent barrier properties for gases like oxygen and carbon dioxide. The similarity in crystal structure between polyethylene (PE) and PVOH stimulated research efforts to produce high-modulus fibers. The PVOH fibers have some potential advantages in comparison with PE fibers e.g. higher melting temperature, approximately 250 oe (compared to 145 oe for PE), potentially lower creep levels and better adhesion to (polar) matrices.

The major drawback however of the pure homopolymer PVOH is its intrinsic moisture sensitivity and thermal instability. Melt-processing is virtually impossible due to thermal degradation and the abundance of -OH groups in the polymer promotes the absorption of water. The mechanical and barrier properties deteriorate with increasing relative humidity.

A possible salution to this problem could be the incorporation of hydrophobic ethylene units in the main chain. On a technological scale, PVOH is produced via complete hydralysis (methanolysis) of polyvinylacetate (PV A). Direct synthesis from the corresponding monomer "vinylalcohol" is impossible since this species exists in its more stable, tautomerie, keto-form: acetaldehyde. It is well known that ethylene and vinylacetate copolymerize readily into a copolymer, ethylene vinylacetate (EVA) copolymer, in which the ethylene and

117 vinylacelate units are arranged at random along the molecule. Hydralysis of these EVA copolymers results in ethylene vinylalcohol (EVOH).

EVOH copolymers are unique in the sense that they remain crystalline over the entire composition range. The melting point decreases to some extent with increasing ethylene content but far less than is to be expected for a random copolymer due to co-crystallization of ethylene and "vinylalcohol" units. Moreover, the Tg of EVOH copolymers, with lower ethylene contents, is camparabie with that of PVOH. A major advantage is that EVOH copolymers can be processed via the melt and due to the presence of hydrapbobic ethylene units their moisture sensitivity is strongly reduced in comparison with the homopolymer PVOH.

In order to onderstand and to exploit the unique properties of these EVOH copolymers, well-defined and characterized samples are a prerequisite. The copolymerization (in solution) of ethylene and vinylacelate was studied and the polymerization conditions were optimized in order to obtain EVOH copolymers in which both the compositional and molecular weight distributions are minimized (chapter 2).

The microstructure of EVOH copolymers was studied using both 1H NMR and 13C NMR. An assignment of the resonances in the spectra was given and the sequence distribution, alkyl chain branching, anomalous linkages and tacticity were studied as a function of the overall ethylene content of the copolymers (chapter 3).

An important observation is the decrease in overall molecular weight with increasing ethylene content. This decrease in molecular weight is caused by the higher propensity of the ultimate polymerie ethylene radical for chain transfer to both monomeric species. As a consequence, EVOH copolymers are relatively brittie due to their lower molar mass.

118 It was found however, that EVOH copolymers are miscible with polyamides (Nylon-6) inthemelt (chapter 4). U pon cooling from the melt, phase separation occurs via consecutive crystallization of the two constituents. Since Nylon-6 crystallizes first, a so-called "structured blend" is obtained with a specific morphology. This structured blend possesses excellent harrier properties due to an enhanced concentration of the EVOH copolymer in the amorphous/inter­ spherulitic boundaries of the Nylon-6 matrix. The toughness of the system is provided by the Nylon-6 matrix.

In contrast to the EVOH/Nylon-6 blends, the blends of EVOH with polyethy­ leneterephthalate (PET) and polyolefines, which are discussed in chapter 5, are immiscible. Using the so-called Multiflux static mixer, layer structures in pellets of these blends could be obtained. Attempts were made to preserve the specific morphology upon further processing (film blowing) because stratified films are expected to possess excellent barrier properties.

Furthermore, the use of EVOH copolymers for the production of high-per­ formance fibers was investigated (chapter 6). The moisture sensitivity decreases, in comparison with PVOH fibers, with increasing ethylene content. However, both the efficiency of draw and the maximum obtainable draw ratio .Àmax decrease with increasing ethylene content. The decrease in .À"""' is due to a reduction in overall chain length with increasing ethylene content as mentioned above (chapter 2). Consequently, the ultimate mechanica! properties of EVOH fibers are intrinsically limited by the microstructure of the EVOH copolymers and by chain transfer reactions occurring during the copolymerization of ethylene and vinylacetate, as described in chapter 2.

119 120 SAMENVATTING

Dit proefschrift beschrijft de synthese en karakterisering van etheenvinylalco­ hol (EVOH) copolymeren, alsmede de toepassing van deze copolymeren in barrière systemen en als uitgangsmateriaal voor hoge modulus vezels.

Het pure homopolymeer polyvinylalcohol (PVOH) wordt al gebruikt voor deze toepassingen. Omdat de -OH groep in PVOH relatief klein is, kan PVOH, ondanks zijn atactisch karakter, kristalliseren. De kristalstructuur kan gezien worden als een enigszins vervormde versie van monoklien polyetheen (PE). Door deze kristalliniteit, maar nog meer door de relatief hoge glasovergangs­

temperatuur T11 van ca. 80 "e (0 % relatieve vochtigheid), bezit PVOH zeer goede barrière eigenschappen voor gassen zoals zuurstof en kooldioxyde. De grote overeenkomst tussen de kristalstructuur van PE en PVOH heeft onderzoek gestimuleerd op het gebied van de produktie van hoge modulus vezels op basis van PVOH. Deze vezels hebben enkele voordelen in vergelijking met PE vezels zoals: hogere smelttemperatuur (ca. 250 oe t.o.v. 145 oe voor PE), mogelijk een lagere kruip en een betere adhesie aan (polaire) matrices.

Echter, de nadelen van het homopolymeer PVOH zijn de vochtgevoeligheid en de thermische instabiliteit. Verwerking via de gesmolten toestand is onmogelijk omdat degradatie optreedt. Het hoge gehalte aan -OH groepen stimuleert de absorptie van water. Mechanische én barrière eigenschappen verslechteren met toenemende relatieve vochtigheid.

Een mogelijke oplossing voor deze problemen is het inbouwen van hydrofobe etheeneenheden in de hoofdketen. Op technologische schaal wordt PVOH geproduceerd door een volledige hydrolyse (methanolyse) van polyvinylacetaat (PV A). Een directe synthese via het "vinylalcohol" monomeer is onmogelijk omdat dit monomeer alleen in zijn meer stabiele, tautomere, keto-vorm voorkomt, namelijk aceetaldehyde. Het is bekend dat etheen en vinylacetaat copolymeriseren tot het etheen­ vinylacetaat (EVA) copolymeer, waarin de etheen- en vinylacetaateenheden at

121 random verdeeld zijn over het molekuul. Hydrolyse van deze EVA copolymeren leidt tot etheenvinylalcohol (EVOH). EVOH copolymeren zijn in zoverre uniek dat ze kristallijn zijn over het gehele samenstellingsgebied. De smelttemperatuur daalt met toenemend etheengehatte, maar in veel mindere mate dan verwacht kan worden voor een random copolymeer. Dit wordt veroorzaakt door het feit dat de etheen- en "vinylalcohol" eenheden cokristalliseerbaar zijn: ze kristal­ liseren in één kristalrooster. Bovendien is de glasovergangstemperatuur T8 van EVOH copolymeren met lage etheenconcentraties ongeveer gelijk aan die van PVOH. Een groot voordeel is dat EVOH copolymeren via de gesmolten toestand verwerkt kunnen worden en dat door de aanwezigheid van de hydrofobe etheeneenheden de vochtgevoeligheid sterk verminderd is in vergelijking met het homopolymeer PVOH.

Om de unieke eigenschappen van deze EVOH copolymeren te onderzoeken en te begrijpen, zijn goed gedefiniêerde en gekarakteriseerde monsters nodig. De copolymerisatie (in oplossing) van etheen en vinylacetaat werd bestudeerd en de polymerisatie-omstandigheden werden geoptimaliseerd ten einde EVOH copolymeren te verkrijgen met een smalle verdeling wat betreft samenstelling en molekuulgewicht (hoofdstuk 2).

De microstructuur van de EVOH copolymeren werd bestudeerd met 1H NMR en 13C NMR. De resonanties in de spectra werden toegekend aan de verschil­ lende atomen. Tevens werden de sequentieverdeling, alkylketenvertakking, anomale koppeling tussen de monomeren en tacticiteil onderzocht als functie van het etheengehalte in het polymeer (hoofdstuk 3).

Een belangrijke constatering is de afname van het molekuulgewicht met toenemend etheengehalte. Deze afname wordt veroorzaakt door een grotere voorkeur van het etheenradicaal aan het polymeer voor ketenoverdrachtsreakties naar beide monomeren. Door de lagere molekuulgewichten zijn de EVOH copolymeren relatief bros.

122 EVOH copolymeren blijken in de smelt mengbaar met polyamiden (Nylon-6) (hoofdstuk 4). Tijdens het afkoelen van de smelt vindt fasenscheiding plaats door het niet gelijktijdig kristalliseren van beide componenten. Nylon-6 kristalliseert als eerste en een zogenaamde "gestructureerde blend", dit is een blend met een specifieke morfologie, wordt verkregen. Deze gestructureerde blend bezit zeer goede barrière eigenschappen door een verhoogde concentratie van het EVOH copolymeer in de amorfe/interkristallijne gebieden van de Nylon-6 matrix. De Nylon-6 matrix zorgt voor de taaiheid van dit systeem.

In tegenstelling tot de EVOH/Nylon-6 blends, zijn de blends van EVOH met polyetheentereftalaat (PET) en polyolefinen niet mengbaar (hoofdstuk 5). Met behulp van de zogenaamde Multiflux menger konden lagenstructuren in het granulaat van deze blends gerealiseerd worden. Pogingen werden ondernomen om de specifieke morfologie te behouden tijdens verder verwerken (foliebla­ zen), omdat gelaagde films goede barrière eigenschappen bezitten.

Tenslotte werden de mogelijkheden van EVOH copolymeren voor de produktie van hoge modulus vezels onderzocht (hoofdstuk 6). De vochtgevoeligheid daalt, in vergelijking met de PVOH vezels, met toenemend etheengehalte. Echter, de verstrekeffectiviteit en de maximale verstrekgraad >.max dalen ook met toenemend etheengehalte. De afname van ).mal< wordt veroorzaakt door een afname van de ketenlengte met toenemend etheengehalte zoals eerder beschreven (hoofdstuk 2). Dit heeft tot gevolg dat de mechanische eigenschappen van EVOH vezels intrinsiek beperkt worden door de microstructuur van de EVOH copolymeren en door de ketenoverdrachtsreakties die optreden tijdens de copolymerisatie van etheen en vinylacetaat, zoals beschreven in hoofdstuk 2.

123 124 CURRICULUM VITAE

Harm Ketels, geboren 23 oktober 1960 te Tegelen, behaalde het diploma Gymnasium-,8 in 1979 aan het St.-Thomascollege te Venlo.

In datzelfde jaar begon hij met de studie Scheikunde aan de Katholieke Universiteit te Nijmegen. Het kandidaatsexamen (SI) werd afgelegd in mei 1982. Het doctoraal examen met als hoofdvakken Organische Chemie (Prof. B. Zwanenburg) en Farmacochemie (Prof. J.M. van Rossum) werd in juni 1985 behaald.

Vanaf september 1985 tot en met augustus 1989 was hij als wetenschappelijk medewerker verbonden aan de Technische Universiteit te Eindhoven en werkzaam in de vakgroep Polymeerchemie en Kunststoftechnologie van de faculteit Scheikundige Technologie. In het kader van het onderzoek verbleef hij enkele maanden aan de Science University of Tokyo (Prof. M. Ito), voor het verrichten van specifieke NMR metingen.

Per 1 oktober 1989 treedt hij als wetenschappelijk medewerker in dienst bij General Electric Plastics in Bergen op Zoom.

125 STELLINGEN

De thermodynamische interactie-parameters van polymeermengsels bepaald uit smeltpuntsdepressie metingen, zijn minder nauwkeurig dan vaak wordt gesuggereerd.

Kwei, T.K., Patterson, G.D., Wang, T.T .. Macromolecules. ~. 780. 1976.

2 De verklaring die Amano et al. geven voor de afname van de sonische modulus van EVOH copolymeren in vergelijking met PVOH is onvolledig. Amano, M., Nagakawa, K .. Polymer Comm., 28. 119. 1987.

3 Dat Prevorsek et al. een memory-effect van 20 40 min meten voor poly(car­ bonaat-co-tereftalaat) in de gesmolten toestand is onwaarschijnlijk, aangezien de langste relaxatietijd (op basis van groepsbijdragen) ca. 5 sec. bedraagt. Prevorsek, D.C .. de Bona, B.T .. J. Macromol. Sci., Phys. Ed.. 4 ). 515. 1986. Prevorsek, D.C., de Bona, B.T.. J. Macromol. Sci .. Phys. Ed.. ). 605. 1981.

4 Voor het bepalen van spin-spin relaxatietijden mogen de meetsignalen verkregen met de solid-echo methode en de spin-echo methode niet gekoppeld worden. Tanaka, H., Nishi, T., Phys. Rev. B. 33( 1 ). 32. 1986.

5 Uit het feit dat Amiya et al. concluderen dat voor EVOH copolymeren, emulsie polymerisatie in het algemeen een bredere molekuulgewichtsverdeling geeft dan polymerisatie in oplossing, mag geconcludeerd worden dat onvoldoende rekening is gehouden met de conversie en de samenstellingsverdeling. Amiya, S., lwasaki, H .. Fujiwara. Y .. J. Chem. Soc. Japan. LL 1698. 1977. 6 Gezien het feit dat in korte tijd drie publicaties met vergelijkbare inhoud en conclusies verschijnen waarbij de auteurs niet naar elkaar refereren, mag geconcludeerd worden dat ook tussen wetenschappers nog Europese grenzen moeten verdwijnen. Wendisch, D., Reiff, H., Dieterich, D., Angew. Makromol. Chemie, 141, 173. 1986. Au/ der Heyde, H., Hübel, W., Boese, R., Angew. Makromol. Chemie. 153, I, 1987. Gerard, J.-F .. Le Perchec, P .. Tho Pham. Q., Makromol. Chem., 189, 1719. 1988.

7 De experimenteel vastgestelde concentratie-afhankelijkheid van de X-parameter moet reeds in de partitiefunc-tie in rekening worden gebracht.

8 Het is onrechtvaardig dat managers geprezen worden om hun vermogen tot delegeren, terwijl deze eigenschap voor wetenschappers als een nadeel wordt beschouwd.

9 Uit de recente ontwjkkelingen in Japan kan geconcludeerd worden dat de invloed van de vrouw op de samenleving groter is dan tot nu toe door de westerse wereld werd aangenomen.

10 Pas na het einde van de promotiezitting is de druk van de ketels.

H.H.T.M. Ketels Eindhoven, 12 september 1989