This article was downloaded by: [University of Windsor]On: 19 November 2014, At: 00:54Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of Macromolecular Science,Part B: PhysicsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lmsb20

Molecular mobility inpoly(ethylene-2,6-naphthalenedicarboxylate) as determined bymeans of deuteron NMRHeidrun Dörlitz a & Hans Gerhard Zachmann aa Institut fur Technische und Makromolekulare ChemieUniversity of Hamburg , D-20146, Hamburg, GermanyPublished online: 19 Aug 2006.

To cite this article: Heidrun Dörlitz & Hans Gerhard Zachmann (1997) Molecular mobility inpoly(ethylene-2,6-naphthalene dicarboxylate) as determined by means of deuteron NMR,Journal of Macromolecular Science, Part B: Physics, 36:2, 205-219

To link to this article: http://dx.doi.org/10.1080/00222349708220426

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information(the “Content”) contained in the publications on our platform. However, Taylor& Francis, our agents, and our licensors make no representations or warrantieswhatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions andviews of the authors, and are not the views of or endorsed by Taylor & Francis. Theaccuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liablefor any losses, actions, claims, proceedings, demands, costs, expenses, damages,and other liabilities whatsoever or howsoever caused arising directly or indirectly inconnection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

J . MACROMOL. SC1.-PHYS., B36(2), 205-219 (1997)

Molecular Mobility in Poly(ethyIene=2,6=naphthalene dicarboxylate) as Determined by Means of Deuteron NMR

HEIDRUN DORLITZ and HANS GERHARD ZACHMANN Institut fur Technische und Makromolekulare Chemie University of Hamburg D-20146 Hamburg, Germany

ABSTRACT

Selectively deuterated poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) was synthesized, obtaining a material in which either the ethylene groups (PEN-Ed4) or the naphthalene ring (PEN-NdJ was deuterated. By means of deuteron nuclear magnetic resonance ('H-NMR), the molecular mobility of the deuterated groups was investigated as a function of temperature. The spectra were measured by the solid echo method, applying different waiting times 7,. The longitudinal relaxation curves were obtained by determination of the maximum intensity of the echo as a function of 7,. It was found that in the amorphous regions of PEN, the molecular mobility of the ethylene groups is higher than in the amorphous regions of PET. In PEN crystals in the 0-modification form, trans-gauche jumps of the ethylene groups with almost equal occupation of the trans and the gauche conformation take place already at 90°C. This is not the case in either PEN crystals in the a-modification form or in PET crystals. The rotational motion of the naph- thalene ring, which according to dynamic mechanical analysis at 20 Hz starts at 6OoC, could not be observed in the NMR spectra. This is explained by the high frequency applied in the NMR experiment.

205

Copyright 0 1997 by Marcel Dekker, Inc.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

206 DORLITZ AND ZACHMANN

INTRODUCTION

Poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) is an interesting new ma- terial which can be used at higher temperatures than poly(ethy1ene terephthalate) (PET). The glass transition temperature T, is 12OOC [1,2]. By annealing it can be crystallized. Two different crystalline modifications, designated by a and 0, can be found [3]. According to Mencik 141, the unit cell of the a modification is triclinic, containing one fully extended monomeric unit. For the 0 modification a unit cell containing four monomeric units has been suggested (51. In contrast to the (Y modifi- cation, the monomeric units seem to be not fully extended. This is indicated by a somewhat smaller value for the length of the c axis of the unit cell (1.273 instead of 1.320 nm). As was shown in previous publications [6], at crystallization tempera- tures below 200OC only the a modification is obtained. At higher crystallization temperatures P may be formed in addition to a. The fraction of the crystals obtained in the P modification depended on the temperature of the melt previous to the crystallization. For example, after melting the material at 28OoC, about half of the crystals were found to be in the modification. On the other hand, after melting at 32OOC all crystals were in the a modification, independent of the crystallization temperature. No samples were obtained in which all the crystals were in the modification.

The melting behavior shows some interesting features. Usually a melting point of 27OOC is found [1,6]. By successive annealing at increasing temperatures starting from 269OC, it was possible to increase the melting point as far as to approx. 290OC. This is true for both the a and the modification [6]. No change in the modification is observed during this annealing.

The molecular mobility in PEN has been investigated by means of both dy- namic mechanical analysis (DMA) [2] and dielectric measurements [7]. The mechan- ical loss modulus G" at 20 Hz shows three maxima, at -50°, 60°, and 120°C, which Are attributed to the motion of the CH, and the COO groups, to the motion of the naphthalene rings, and to the glass transition (statistical segmental motion), respectively [2,8]. These maxima are also observed in the curves representing the electrical loss E " as a function of temperature. The positions of these maxima move to higher temperatures with increasing frequency. It has to be noted that, in contrast to the case of phenylene rings, a rotational motion of a naphthalene ring without involving any neighboring atomic groups in inhibited by the chemical bonds. Some possible mechanisms for the rotation of the naphthalene rings involving neighboring atomic groups have been discussed by Blundell and Buckingham [8]. In the case of PEN, such a motion has to include the CO groups and a small translational motion of the rest of the chain. In the present study the molecular mobility in PEN is investigated by means of deuteron nuclear magnetic resonance (2H-NMR). By selec- tively deuterating either the naphthalene rings or the ethylene groups, the motion of each group could be studied separately. These motions are investigated as a function of temperature by means of longitudinal relaxation measurements as well as by shape analysis of the NMR line. The NMR lines were obtained by applying differ- ent, "waiting times" T ~ . Thus the lines of chains showing different mobilities, such as chains in the crystals, taut chains in the amorphous regions, more mobile chains in these amorphous regions, and so on, are obtained separately. Of special interest is a comparison of the mobility of the naphthalene ring in PEN with that of the

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHYLENE-2,6-NAPHTHALENE DICARBOXYLATE) 207

phenylene ring in PET as well as a comparison of the mobility of the ethylene groups in the two materials and in the two different crystal modification of PEN.

EXPERIMENTAL Synthesis of the Materials

Nondeuterated PEN was synthesized by transesterification of dimethylnaph- thalene dicarboxylate and ethylene glycol using manganese-11-acetate and antimony trioxide as catalysts followed by polycondensation, as described in a previous publi- cation [6] . The material in which the ethylene group was deuterated (PEN-Ed,) and the one in which the naphthalene ring was deuterated (PEN-NdJ were synthesized in the same way, starting with the corresponding deuterated monomers. However, in order to optimize the transesterification with the ethylene glycol, a temperature- time program had to be applied which was somewhat different from the one used for the nondeuterated components. This program is indicated in Table 1. The trans- esterification had to be kept going until at least 95% of the maximum amount of methanol has been obtained. The temperature for the following polycondensation process also had to be changed. In the case of the nondeuterated and of the ethylene glycol deuterated material, 28OOC seemed to be the optimum. In the case of the material containing deuterated naphthalene rings, however, the highest molecular weights were obtained at 290OC. The polycondensation time varied between 30 and 60 min.

Deuterated ethylene glycol was purchased from the company Hempel (present name: Medgenix). The deuterated dimethylnaphthalene dicarbonic acid was synthe- sized in our laboratory by deuteration of normal naphthalene-2,ddicarbonic acid by HID exchange in deuterated water. To this end, the monomer was dissolved in the deuterated water by slowly adding a 40% solution of NaOD/KOD. After adding Pt/charcoal as a catalyst (0.3 g catalyst for 5 g material), the solution was placed into a autoclave and heated to temperature between 130° and l5OoC for 5 days. This procedure was repeated for three times. After each procedure the solution was filtered in order to separate it from the catalyst and some precipitated naphthalene. The naphthalene dicarbonic acid was then precipitated by adding D,SO,.

TABLE 1 Temperature-Time Program for the Transesterification Reaction

Nondeuterated Deuterated

Time Fraction of Time Fraction of Temp. (min) methanol (Vo)'' Temp. (min) methanol (070)"

220 30 240 90 37 260 40 98 260 90 66

270 60 88 280 Variable 96

"Percentage with respect to maximum amount corresponding to complete transes- terification.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

208 DORLITZ AND ZACHMANN

The degree of deuteration of the naphthalene dicarbonic acid was determined by means of H-NMR. After the first exchange treatment a degree of 40-50% was obtained, after the second treatment for degree of deuteration was approximately 8O%, and after the third treatment values of 90-95% were reached. It seemed important to use a mixture of NaOD/KOD because otherwise the positions 1 and 5 on the ring would exchange much more slowly than the others. The optimum frac- tion of KOD was 5 molVo.

Sample Preparation

For further purification the material was dissolved in hexafluoroisopropanol (HFIP), filtered, and precipitated in ethanol. The powder obtained was dried for 24 h in vacuo at 7OOC. The samples in which the ethylene groups are deuterated are designated by PEN-Ed,, those with deuterated naphthalene rings by PEN-Nd,, and the nondeuterated ones simply by PEN.

Amorphous samples were prepared by melt-pressing the material for 2 min at 29OOC in vacuo and then quenching in ice water.

Samples containing only a-modification crystals were obtained by melt- pressing for 2 min at 28OoC, followed by crystallization at 19OOC for 1.5 h (these samples are designated by a, 190°C) or for 2 h at 24OOC (these samples are desig- nated by a, 24OOC). It is worthwhile noting that at this higher temperature also some better p crystals were obtained if the material was not deuterated [2]. Deuteration obviously, to some extent, changes the tendency the form /3 crystals.

The preparation of samples containing only p crystals was more difficult. As described in an earlier publication [2], it was not possible to obtain such a material by simply quenching the sample from the melt to the crystallization temperature. However, we have now found a more complicated thermal treatment by which only crystals in the 0 modifications are formed: The sample is molten in the heating press at 60 bar in vacuo for 1 min at 32OOC and then gradually cooled down to 26OOC at a rate of 10°C/min. Afterwards, the temperature is reduced in steps of 5OC to 22OoC, keeping the sample at each temperature for 20 min. Finally the sample is cooled to room temperature within 2 h. The wide-angle x-ray scattering (WAXS) diagrams of a sample in the a and of one in the 0 modification are shown in Fig. 1. One can clearly recognize that no common reflections are present in the two diagrams.

The different samples investigated in this study are listed in Table 2, together with their intrinsic viscosities in HFIP, [q] their apparent molecular weights M,, their densities p, and their crystallinities x,. The methods used for the determination of these quantities are described in the following section.

METHODS

The intrinsic viscosity [q] was measured in hexafluoroisopropanol. From [+I, a molecular weight was calculated by using the equation

[,,I = 5.20 x 10-4~0;695 [dL/gl (1)

which was shown to be valid for PET [lo]. Because PEN is different from PET, the values obtained must be considered to be apparent values for M,,,.

The wide angle x-ray scattering was measured by means of a Siemens D5OO

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHYLENE-2,6-NAPHTHALENE DICARBOXYLATE) 209

0101

I 100

FIG. 1. WAXS of a sample crystallized in the a modification of a sample crystallized in the @ modification, respectively.

The wide angle x-ray scattering was measured by means of a Siemens D500 goniometer.

The density was measured in a density gradient column filled with a mixture of hexane and tetrachlorethane. The degree of crystallinity was calculated from the density p by means of the equation

P - P a x p ’ Pc - Pa P

x, = ___

The values for the density of the amorphous regions pa and the crystalline regions pc are given in Table 2. In the case of the nondeuterated samples, the values were taken from the literature as indicated in the table. The density values for the deuterated samples were calculated from those of the nondeuterated samples by taking into account the measured degree of deuteration. In the case of the amorphous samples

TABLE 2 Listing of the Samples Investigated Together with

Their Intrinsic Viscosities [ q ] , Densities p , and Crystallinities as Determined from Density (x,)

Nd6a Nd,a (24OOC) Nd6@ Ed,a Ed4a (19OOC) Ed4a (24OOC) Ed&l EdgII

0.6775 - -

0.4933 - - -

30.000 - -

20.000 - - -

1.3590 1.3921 1.3971 1.3513 1.3753 1.3881 1.3966 1.3920

0 0.45 0.51 0 0.32 0.48 0.44 0.40

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

210 DORLITZ AND ZACHMANN

the calculated values can be compared with the values measured at the deuterated samples, which are also shown in Table 2. Good agreement is obtained.

The deuteron NMR investigations were performed on a Bruker MSL 300 FT NMR spectrometer. The NMR spectra were measured by the solid echo method. First, a progressional saturation pulse sequence consisting of ten 90 x pulses was applied, which completely destroyed all magnetization in the sample. After a wait- ing time, T,, a solid echo pulse sequence followed, consisting of a 4 5 O x and a 4 5 O y pulse separated by a time T~ which usually was 20 psec. The duration of the 4 5 O pulse rq5 was 3 psec. The NMR spectra were obtained by Fourier transformation of the echo signal observed after the last of the pulses. In each measurement, about lo3 spectra had to be accumulated.

In order to measure the longitudinal relaxation curve, the waiting time T~ was varied and the maximum intensity of the echo, M,, was determined as a function of 7w (saturation recovery).

RESULTS

Figure 2 shows a typical curve representing the longitudinal magnetization M,(T,), divided by the magnetization after infinite time M(m), as a function of the waiting time (recovery time) T~ after a saturation pulse sequence. The curve is nonexponential. It can be separated into 4 exponential curves according to the equation

The x(') are the fractions of material contributing to each component and the TI" are the longitudinal relaxation times of the different fractions. Similar curves were obtained for the other samples except that, in some cases, only 3 components were found. The separation of the curve into components with discrete relaxation times Ti'' is considered to be a good approach for evaluation although, in reality, the spectrum may be continuous (see Discussion).

FIG. 2. Longitudinal magnetization M(7,) as a function of the waiting time (= recovery time) rw for the sample Nd,a. The values of M(7,) have been normalized by dividing by M( m), the magnetization after infinite time.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHYLENE-2,dNAPHTHALENE DICARBOXYLATE) 21 1

Table 3 shows the fractions x(" and the relaxation times T(') for the different samples investigated in this work. In the crystallized samples the fraction with the largest relaxation time according to previous investigations has to be attributed to the crystals. Therefore the fractions A!'' are supposed to arise from the crystals. The values for A!') vary between 38% and 49%. They are in fairly good agreement with the degrees of crystallinity as determined from density, x,, which are also shown in the table. The relaxation times 7':'' of the ethylene groups are smaller than those of the naphthalene rings. This indicates that, in the crystals, the mobility of the ethyl- ene groups is higher than that of the naphthalene rings. In particular, the samples crystallized in the @ modification show significantly smaller values of Ti" (- 1.5 sec) than those in the CY modification (4 to 7 sec). This indicates a high mobility of the CH, groups in the /3 crystals.

Components 2, 3, and 4 have to be attributed to chains in the amorphous regions showing different mobilities. It is interesting to note that the fraction of chains contributing to the largest value of T,, namely TiZ', is larger in the case of the Nd6 than in the case of Ed, samples. In addition, only the Ed4 samples contribute considerably to the component showing a longitudinal relaxation time less than 0.1 sec. This indicates that in the amorphous state, too, the ethylene groups have a high mobility than the naphthalene rings.

Let us next look at the spectra of the sample Nd, (a 240OC) in which the naphthalene ring is deuterated and which was crystallized at 24OOC in the CY modifi- cation. Figure 3 represents the spectra measured at different temperatures. The spectra on the left side were obtained by using a waiting time 7, of 0.04 sec. From the relaxation times shown in Table 3 it follows that, in the case of room tempera- ture, at this very small waiting time 3% of the spectra arise from component 1, 35% from component 2, and 62% from component 3. Thus mainly the amorphous chains contribute to this spectra. At higher temperatures the contribution of the crystals may somewhat increase because the values of TI decrease with increasing temperature, but still the amorphous chains will dominate. As a main feature one has to note that, up to 100°C, no significant deviation from a Pake diagram is observed. At 14OOC a narrow central peak appears which increases in intensity while the doublet of the Pake diagram gradually disappears. This result shows that, even in the chains showing highest mobility, up to 14OOC the naphthalene ring performs no considerable motion within the frequency window which can be observed by deuteron NMR.

TABLE 3 Densities of PEN (g ~ r n - ~ )

Deuterated ~

Ed4 Nds Nondeuterated

(from literature [3,4]) Calculated Measured Calculated Measured

Amorphous 1.3303 1.3524 1.3513 1.3592 1.3577 a crystals 1.4060 1.4294 1.4369 @ crystals 1.4348 1.4587 1.4659

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

212 DORLITZ AND ZACHMANN

FIG. 3. Spectra of the sample Nd6a (24OOC) obtained at different temperatures dur- ing heating with a waiting time T, of 0.04 sec (left column) and 6.5 sec (middle column). The right column represents the difference between the two spectra.

In the middle column of Fig. 3 the spectra obtained by applying a waiting time of 7, = 6.5 sec are shown. In this case, at room temperature, 20% of the spectrum arises from the crystals, 68% from the component 2 (the least mobile amorphous chains), and 12% from the component 3 (more mobile amorphous chains). With increasing temperature the contribution from the crystals increases. Because of the considerable contribution of the crystals the Pake-like part of the spectrum is much more pronounced than in the spectra on the left side of this figure.

Finally, in the right column the difference of the spectra obtained by using 7,

= 6.5 sec and T~ = 0.04 sec is represented. At room temperature, 22% of this spectrum arises from the crystals, 70% from the amorphous component 2, and 8% from the amorphous component 3. In this case, up to 2OOOC at Pake diagram is obtained indicating that no motion detectable by deuteron NMR takes place in the crystals in the amorphous component 2.

Similar results were obtained for the sample Nd& in which the crystals are in the P modification.

Figure 4 shows the spectra of the sample Ed,a in which the ethylene groups were deuterated. Again, as in Fig. 3, different columns are related to different

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHYLENE-2,dNAPHTHALENE DICARBOXYLATE) 213

FIG. 4. Spectra of the sample E d p (24OOC) obtained at different temperatures during heating with a waiting time T,+ of 0.1 sec (left column) and 5.5 sec (middle column). The right column represents the difference between the two spectra.

waiting times. The contributions of components 1, 2, 3, and 4 to the spectrum at room temperature in the left column are 2'70, 22'70, 24'70, and 56'70, respectively. A clear deviation from the Pake diagram can already be recognized at room tempera- ture, indicating that some sort of motion occurs in the ethylene groups. However, this motion does not consist of trans-gauche jumps. A central peak, indicating the onset of trans-gauche jumps, appears at 13OOC and becomes dominant at 170OC. The spectra in the right column of this figure mainly arise from the crystals and the least mobile chains in the amorphous regions. Here a Pake diagram dominates at lower temperatures. Some motion of the ethylene groups, namely trans-gauche jumps, starts at 15OOC and becomes more dominant at higher temperature. How- ever, even at 21OOC a considerable contribution of the Pake diagram can be recog- nized.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

214 DORLITZ AND ZACHMANN

A very interesting result is obtained for the sample Ed,P (see Fig. 5) . First of all, up to 16OoC, there is almost no difference in the spectra measured at different waiting times. This indicates that the mobility of the ethylene groups in the crystals and in the amorphous regions is quite similar. The shapes which are found are typical for trans-gauche jumps of the CH, units. At and above 17OOC the spectra measured at short waiting times become narrower than the difference spectra (right side of the figure). This indicates that the motion of the chains in the crystals is slightly more restricted than that of highly mobile amorphous chains where, gradu- ally, some isotropic motion seems to start to take place. The unprecedented high mobility of the ethylene groups in the crystals can be explained by the special conformation of the chains in the @-crystal modification (see Discussion).

FIG. 5. Spectra of the sample Ed.@ (24OOC) obtained at different temperatures during heating with a waiting time rW of 0.1 sec (left column) and 5 . 5 sec (middle column). The right column represents the difference between the two spectra.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHY LENE-2,B-NAPHTHALENE DICARBOXYLATE) 215

DISCUSSION Separation of the Relaxation Curves into Different Components

The separation of the relaxation curves into several components with discrete relaxation times has to be discussed in more detail. It is quite reasonable that the relaxation times of the chains in the crystals are separated by a gap from the relaxation times of the chains in the amorphous regions. In these regions, however, the assumption of a continuous spectrum of relaxation times seems to be more justified than that of several discrete relaxation times. Therefore the separation of the longitudinal relaxation curve into several curves with discrete relaxation times T, should be considered to be just an approximation for sake of simplicity. A determination of a continuous relaxation times spectrum, and therewith a continu- ous spectrum of correlation times, by a fitting procedure either would involve some assumption concerning the shape of the curve (e.g., symmetric or not symmetric, Gaussian or rectangular), or one would have to introduce into the fitting procedure more parameters than could be significantly determined. Therefore, we believe that the separation of the curve into three or four distinct components is a good ap- proach to extract from the spectra important information on chain mobility.

Separation of the Molecular Motion of Chains with Different Mobilities

By varying the waiting time 7, and, in addition, calculating the difference of spectra measured at different 7, values, the spectra of chains (part of molecules) having different mobilities were obtained.

A question arises regarding attribution of different kinds of chains to the different components of the spectra. There is general agreement that, in a semicrys- talline polymer, the component with the largest relaxation time arises from the chains in the crystals. It is also quite obvious that the component with the smallest relaxation time must be attributed to the chains having the highest mobility, maybe chains in the amorphous regions with one free end and one end attached to a crystal. The other two components may be attributed to loose loops hanging out of the crystals (component 3) and taut tie chains connecting two crystals. It is clear, how- ever, that these attributions are highly hypothetical. Furthermore, as the restrictions in the mobility of loops, tie chains, and chains with one free end will vary locally, this again suggests the assumption of a continuous spectrum of relaxation times rather than of distinct relaxation times.

The spectra of the chains in the amorphous regions have been measured at all temperatures. In contrast, due to the long relaxation times T, ranging from 10 to 15 sec, no fully relaxed spectra were recorded. As a consequence, even the difference spectra shown in Figs. 3 , 4, and 5 on the right side arise only partly from the crystals. To some extent they arise from the least mobile amorphous chains. Never- theless, even under these conditions some conclusions can be drawn concerning the chain mobility in the crystals, in particular concerning the lack of such mobility as it is shown in the following sections. Furthermore, at temperatures above room temperature, the relaxation times T, decrease and thus, at the same waiting time 7,, the fraction of the spectra arising from the chains in the crystals increases.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

216 DORLITZ AND ZACHMANN

Motion of the Naphthalene Ring in the Amorphous Regions

In dynamic mechanical analysis experiments (DMA) at about 20 Hz, the rota- tional motion of the phenylene ring PET seems to start at about -5OOC (120OC below Tg), and a corresponding motion of the naphthalene ring in PEN seems to start at 6OoC (55OC below Tg). Thus, in order to start the rotational jump motion, in the case of the naphthalene ring the temperature must be closer to T, than in the case of the phenylene ring. This can be easily explained by the fact that 180° rotational jumps of the phenylene rings can be easily performed without any confor- mational changes of the rest of the chains while such a motion of the naphthalene rings involves at least some translation motion of the rest of the chain and the rotation of the adjacent CO groups [8].

For a comparison of the motion of the naphthalene rings with that of the phenylene rings in PET, Fig. 6 shows once more some partially relaxed spectra of PEN together with some partially relaxed PET spectra taken from an earlier publi- cation [9]. The rotational jump motion of the phenylene rings in the amorphous regions of PET can be clearly detected by NMR far below the glass transition [9]. It manifests itself by the additional double peak in the Pake diagram. In contrast, as demonstrated in Fig. 3, no such motion of the naphthalene ring is visible in the NMR spectra of PEN at any temperature although the waiting 7, is even shorter than in the case of PET. Instead of that, at 180°C (6OOC above the glass transition) a narrow central peak indicates the onset of isotropic motion of the naphthalene ring.

Why does the naphthalene ring motion not become visible in the NMR experi- ment? This must be due to the high frequency involved in the NMR experiment. The frequency dependence of the onset of the rotational jump motion can be de-

PET-BdL PET-BdL PET-Bdb PET-BdL

PEN-Nd6 PEN-Nd6 PEN-Nd6 PEN-Nd6 PEN-Nd6

FIG. 6. Comparison of the partially relaxed NMR spectra of PEN-Nd, (7, = 0.01 sec) with those of PET in which the phenylene rings are deuterated (PET-Bd,, 7, = 0.2 sec).

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHYLENE-2,6-NAPHTHALENE DICARBOXYLATE) 217

scribed by an Arrhenius-type equation and is much stronger than the frequency dependence of the glass transition, which is governed by a Vogel-Fulcher equation. As a consequence, during heating of PEN, the glass transition is reached before a rotational jump motion can take place with the high frequency used in the NMR experiment. This interpretation is fully supported by dielectric measurements per- formed at different frequencies [7]. With increasing frequency the peak arising from the motion of the naphthalene ring moves closer and closer to the glass transition peak.

In the difference spectra (right side of Fig. 3), no motion of the naphthalene ring becomes visible within the investigated temperature region (up to 200OC). Though these spectra do not completely arise from the crystals, one can conclude that no rotational jump motion of the naphthalene rings occurs in the crystals. This is similar to the behavior of the phenylene rings in the PET crystals [9]. Obviously, in the crystal, the interaction with the neighboring molecules prevents such a mo- tion.

Motion of the Ethylene Groups

The spectra in Fig. 4 show that some motion of the CH2 groups, which raises the plateau between the peaks of the Pake diagram, already takes place at room temperature. The observed increase of the plateau can be explained by librations and vibrations of these groups, as has been demonstrated by Hirschinger et al. [lo, 111. A broad central peak, which indicates trans-gauche jumps of the ethylene groups, is observed already at 1OS0C, that is, 12OC below the glass transition temperature. For comparison, Fig. 7 shows once more some of the partially relaxed spectra together with corresponding spectra of PET taken from another publication

PET-EdL PET-EdL PET-EdL PET-EdL

PEN-Ed4 PEN-EdL PEN-Eq PEN-Ed4 PEN-Ed6

FIG. 7. Comparison of the partially relaxed NMR spectra of PEN-Ed4 (7, = 0.1 sec) with those of PET in which the ethylene groups are deuterated (PET-Ed4, 7, = 0.2 sec).

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

218 DORLITZ AND ZACHMANN

TABLE 4 Longitudinal Relaxation Times Tf', Mass Fractions x('>, and Crystallinities x, as Determined from Density in PEN Having Deuterated Naphthalene Rings and

Deuterated Ethylene Groups, Nd6 and Ed, Respectively, in the Amorphous State (a), Crystallized in the a Modification (a), and Crystallized in the B Modification (p)

Sample (sec) x( ' ) x, (sec) x(') (sec) x(') (sec) x ( ~ )

Ndsa - - 0 1.74 0.76 0.31 0.23 0.005 0.01 Nd6a 15.6 0.43 0.45 1.32 0.49 0.10 0.08 - - Ndsb 10.7 0.47 0.51 1.58 0.42 0.28 0.11 - -

T(1) 7'") T(3) T(4)

Ed,a - - 0 1.52 0.53 0.24 0.33 0.025 0.15 Ed4a(1900C) 3.81 0.38 0.32 0.71 0.33 0.11 0.20 0.016 0.10 Ed4a(2400C) 7.19 0.39 0.48 0.65 0.38 0.13 0.11 0.044 0.12

EdgII 1.54' 0.49 0.40 0.35 0.31 0.07 0.18 0.01 0.03 EdgI 1.21 0.49 0.44 0.27 0.31 0.05 0.20 - -

[9]. It can be seen that in PET the plateau between the peaks of the Pake diagram is lower than in PEN, and that a peak indicating trans-gauche jumps of the ethylene groups does not appear below 86OC, that is, 16OC above the glass transition. Thus the mobility of the ethylene groups in PEN is higher than in PET if it is compared at the same supercooling with respect to the glass transition temperature.

An interesting result is the high mobility of the CH, groups in the 0 crystals of PEN according to Fig. 5 . The shape of the peak in the difference spectra (right side of Fig. 5 ) clearly indicates that, above 90°C, trans and gauche conformation occur with approximately the same probability, and that rapid jumps between these two conformations are performed by the ethylene groups. This result is in agreement with the conclusion from WAXS according to which the chains are not fully ex- tended in the crystal [ 5 ] . The high mobility of these groups in the crystals is also in agreement with the comparatively low values of the longitudinal relaxation time in the 0 crystals given in Table 4 (1.2 and 1.5 sec).

CONCLUSIONS

At the high frequency applied in the NMR measurements, the onset of the rotational motion of the naphthalene ring in the amorphous regions of PEN is shifted to temperatures which would lie above the glass transition, where isotropic motion already takes place. Therefore, the rotational motion cannot be observed by NMR despite the fact that such a motion is found to start at about 6OoC by dynamic mechanical analysis at 20 Hz.

Below glass transition temperature, compared at the same temperature, the ethylene groups in the amorphous regions of PEN are more mobile than those in the amorphous regions of PET, although the glass transition temperature of PEN (12OOC) is higher than that of PET (7OOC). Furthermore, trans-gauche jumps on the ethylene groups start at lower supercooling with respect to the glass transition temperature than in PET.

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14

MOBILITY IN POLY(ETHYLENE-2,6-NAPHTHALENE DICARBOXYLATE) 219

In PEN crystals in the &modification trans-gauche jumps of the ethylene groups start already at 9OoC, with an almost equal distribution of the population on trans and gauche positions.

ACKNOWLEDGMENT

This work has been supported by the Deutsche Forschungsgemeinschaft.

REFERENCES

1. 2. 3.

4. 5. 6. 7. 8. 9.

10. 11.

S. Z. D. Cheng and B. Wunderlich, Macromolecules, 21,789 (1988). D. Chen and H. G. Zachmann, Polymer, 32, 1611 (1991). H. G. Zachmann, D. Wiswe, R. Gehrke, and C. Riekel, Macromol. Chem., Suppl., 12, 175 (1985). Z. Mencik, Z. Chem. Prum., 17(2), 78 (1976). H. Noether, S. Buchner, and H. G. Zachmann, to be published. S. Buchner, D. Wiswe, and H. G. Zachmann, Polymer, 30,480 (1989). T. A. Ezquerra, F. J. Balta-Calleja, and H. G. Zachmann, Acta Polym., 44, 18 (1993). D. J. Blundell and K . A. Buckingham, Polymer, 26, 1623 (1985). R. Gehrke, M. Golibrzuch, A. Klaue, and H. G. Zachmann, Polym. Prepr., (Am. Chem. SOC. Div. Polym. Chem.), 29(1), 64 (1988). J. Hirschinger and A. D. English, J. Magn. Reson., 85, 542 (1989). J. Hirschinger, H. Miura, K. H. Gardner, and A. D. English, Macromolecules, 23, 2153 (1990).

Received April 16, 1996 Revised June 12, 1996 Accepted June 17, 1996

Dow

nloa

ded

by [

Uni

vers

ity o

f W

inds

or]

at 0

0:54

19

Nov

embe

r 20

14