Multidimensional Spectroscopy of Polymers - American Chemical

Chapter 17

Oxygen Absorption on Aromatic Polymers 1

H NMR Relaxation Study 1

1

2

D . Capitani , A . L. Segre , and J. Blicharski Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch017

1

Istituto Strutturistica Chimica, M . B . 10, Monterotondo Stazione, Roma 00016, Italy Institute of Physics, Jagellonian University, ul. Reymonta, Kracow 30-059, Poland 2

In aromatic polymers, oxygen selectively adsorbed on aromatic rings acts as a strong relaxation contrast agent. The effect is maximal at rather low temperatures, in the 80-160K range, where a well defined minimum can be observed. The position of the minimum and the relative value of the spin-lattice relaxation time are modulated by the chemical nature of the polymer, by its packing (polymorphism), by the crystalline vs. amorphous ratio and by the maximal amount of adsorbed oxygen. A full theoretical treatment of relaxation parameters has been done, leading to a best fit treatment of relaxation data. The proton-proton dipolar term at high temperature and the proton-oxygen scalar term at lower temperatures give the major contribution to spin-lattice relaxation. From the full theoretical equation, the best fit of experimental data on polymorphous polystyrenes leads to a large number of physico-chemical parameters. The activation energy for the phenyl ring libration was obtained, different for each polymorphous form. Moreover, for each aromatic polymer, the maximal number of adsorbable oxygen molecules can be obtained, giving an index for polymers suitable to act as oxygen scavengers.

I n t r o d u c t i o n . One major impact of N M R spectroscopy in polymer science comes from its capability to distinguish different configurations in homopolymers and different monomer sequences in copolymers (7) . These N M R studies in solution had as a main result the measure o f the statistical distribution of configurations, which is extremely useful in chemistry for the study o f catalytic systems and the mechanism o f 0097-6156/95/0598-0290$12.25/0 © 1995 American Chemical Society In Multidimensional Spectroscopy of Polymers; Urban, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch017

17. CAPITANIETAK

Oxygen Absorption on Aromatic Polymers

291

polymerization reactions (2). Moreover some hint on the rheological properties can be obtained (3) . However, since polymers are mostly used in the solid state, a large effort was made in order to get spectra in the solid state as narrow as possible (4); this is today routinely performed on isotopically diluted nuclei by C P - M A S techniques (5). Relaxometric techniques have been known since the early beginning of N M R (6), while not so commonly used as spectroscopic measures, however N M R relaxometry is quite useful in polymer characterization and experimentally not very demanding (7). In this paper, the relaxometric behavior of aromatic polymers and the use of oxygen as a relaxation contrast agent will be discussed. Moreover a theory of proton relaxation in the presence of paramagnetic oxygen will be presented with some selected applications to oxygen-doped polymers. Relaxation in degassed polymers. N M R relaxation in solid degassed polymers is a well known physico-chemical method (5). In polymers without side chains, the spin lattice relaxation as a function of temperature shows a single minimum at ωτ «1; polymers with side chains may also show other secondary minima. Multiple relaxation times may be observed in semicrystalline polymers; the presence o f crystalline domains in an amorphous matrix can usually be revealed since the T2 for the crystalline domain often becomes significantly shorter at low temperature. However, in most cases only one Τ \ value is observed due to spin diffusion. In well degassed polymers the dominating mechanism of relaxation is the dipole-dipole interaction , at high temperature in the weak collision case (8) and at low temperature in the strong collision case (9). On this regard aromatic polymers behave exactly like any other polymer ,(see Figure 1) for different polycrystalline polystyrenes . In this figure the relaxation behavior as a function of the temperature is shown for two different polymorphs of s-PS( syndiotactic polystyrene, namely the β and the γ forms) and a highly isotactic polystyrene. It is clear that N M R relaxometry is a technique not sensitive to polymorphism or to different tacticity. Oxygen absorption in aromatic polymers. In the presence of oxygen, however, a dramatic shortening of the spin lattice relaxation time occurs (JO), ( see the lower trace of Figure 2). The same type of curve is shown for a fully degassed sample of syndiotactic polystyrene γ form to show the extent of the O2 effect. Since the absorption of oxygen is different in different aromatic polymers and since it must be modulated by the molecular packing, a full relaxation study was performed on s-PS having different polymorphism, (see Figure 3). s-PS is in fact a semicrystalline polymer whose polymorphous forms have been extensively studied and well characterized by X-Ray and C P - M A S (77-

In Multidimensional Spectroscopy of Polymers; Urban, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


292

MULTIDIMENSIONAL SPECTROSCOPY

OF POLYMERS

T i (s) (

7

6 5

-

A "Δ

4 3

-

2

-

Δ.

α

·π ,

S-PS 7 ( )

a

Q

S-PS /? (b)

Δ

l-PS

c

()

X

'*

* Δ

' Δ

'·.. " * Χ

-χ

1 η 250

χ

Χχ Ό* S*Ε . _. I

300

I

ι

ι

350

400

450

500

Τ(Κ) Figure 1. 3 0 M H z . Proton Ύ\ relaxation times (in seconds) as a function o f the temperature for three well degassed polystyrenes: (a) γ syndiotactic polystyrene; (b) β syndiotactic polystyrene; (c) isotactic polystyrene.



CAPITANI ET AK

0" 200

293


1

1

300

400

1

500

T(K) Figure 2. Semilogarithmic plot of proton T i relaxation times (in milliseconds), measured at 30 M H z , as a function of 1000/T ( K " ) for a well degassed sample of syndiotactic polystyrene in γ polymorphous form (upper trace) and for the same undegassed sample (lower trace); solid line through experimental points results from the best fit procedure. 1



294

MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS

Figure 3. 3 0 M H z Proton T j relaxation times as a function of the temperature for α, β, γ and δ s-PS polymorphous forms.


17. CAPITANI ET AL.


295

12). Even without any theoretical interpretation, the relaxometric data allow some preliminary conclusions for aromatic polymers in the presence of oxygen: i)proton spin lattice relaxation is a technique able to discriminate among different polymorphs. indifferences in proton spin lattice relaxation might be used to gain improved C P - M A S C spectra (75). iii)using the observed differences in proton relaxation times, optimal temperatures may be found for obtaining C P - M A S spectra. Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch017

1 3

Multi-exponential decays: the spin diffusion process. In s-PS, at temperatures higher than «220K, after a 1 8 0 ° - τ - 9 0 ° sequence (or any equivalent sequence such as an aperiodic saturation recovery or a simple saturation recovery) a single exponential decay was observed; however by decreasing the temperature a bi-exponential decay appears (14) ( see Figure 4). The two components correspond to two spin lattice relaxation times: one of the order of 1 s the second one of the order o f 0.1 s. The relative intensity of the two components is « 3 : 5 . Since in polystyrenes there are three backbone protons (aliphatic ) and five aromatic protons, the hypothesis was forwarded that different relaxation times might correspond to backbone and aromatic protons. To verify this hypothesis a backbone deuterated s-PS was synthesized and crystallized in all possible polymorphous forms. The lower trace of F i g . 4 corresponds to a backbone deuterated s-PS in the same polymorphous phase and the same crystalline content than the upper trace. Since the slow relaxing component is missing, it is clear that indeed this component is due to backbone protons and that a full separation in T i occurs between aliphatic and aromatic protons. A similar result was obtained for an amorphous polyphenylene oxide (75) ( see Figure 5) ; in fact at temperatures lower than «250 Κ two spin lattice relaxation times can be measured, whose relative intensity is 3:1. These relaxation times have been attributed respectively to the methyls and to the aromatic protons. Similarly in an alternate copolymer between carbon monoxide and styrene, at 200 Κ , the phenyl rings relaxation time is diffèrent and much shorter than the Ί\ relaxation time of the backbone, as confirmed by the corresponding data on a backbone deuterated copolymer (Capitani, D . ; Segre, A . L . ; Barsacchi, M . ; Lilla, G . paper in preparation) . Thus it appears clear that the spin diffusion process, which equalises T\ values, is not acting when the differences in relaxation values become sufficiently large. This behaviour is by no means confined to polymeric systems since an analogous behaviour was also found in benzoic acid (16). In Table I, the temperature at which the spin diffusion process is no more active is reported.


296



M(a.u.)

100' 0

' 200

1

400

· 600

1

800

t(ms) Figure 4. Ï H N M R , 30 M H z . Semilogarithmic plot o f experimental points obtained from an IR experiment at T=200 Κ on a s-PS in γ polymorphous form: a) fully protonated b) deuterated on the backbone.




17. CAPITANI ET AL.


297

298


Table I. Temperature at which the efficiency of the spin diffusion process is lowered and multiexponentiality is observed with full separation between spin-lattice relaxation of aromatic and


Polymer

Temperature

s-PS α

200 Κ

s-PS β

200 Κ

s-PS γ

220 Κ

PPO

240 Κ

CO-Styrene

200 Κ

Relaxation mechanism CH, CH2 dipolar phenyl ring O2 CH, CH2 dipolar phenyl ring Ô2 CH, CH2 dipolar phenyl ring O2 CH3 rotational+ dipolar+02 phenyl ring O2 C H , C H 2 dipolar phenyl ring O2

Relaxation in backbone deuterated polystyrenes. Due to the well known complexity of solving multi-exponential decays (77), and with the purpose of quantifying oxygen absorption on aromatic polymers , s-PS was synthesized fully deuterated on the backbone. The deuterated s-PS was crystallized to give α , β , γ semicrystalline polymorphous polymers, which were characterized by X-Ray diffraction. On these polymers, proton T j relaxations were measured at 30 and 57MHz as a function of temperature, (see Figures 6 and 7) . It must be noted that at low temperatures a multiple decay is again observed for T^ relaxation times. The observed relaxation multiplicity in these samples can be attributed to the different amount of oxygen absorbed in the crystalline phase on respect to its amorphous counterpart. On this respect oxygen acts as a true relaxation contrast agent, allowing the observation of different domains. A comparison between X-Rays and N M R data is reported in Table II , showing the reliability of the N M R method. Details of these data have been reported elsewhere (14). Table II. Percentage of different phases in syndiotactic polymorphous polystyrenes α crystalline phase

amorphous phase

NMR 35-40

X-Ray 40

60

β N M R X-Ray 60 60

40

γ N M R X-Ray 40 40

60


60

CAPITANI ET AL.


Τι (ms) a

Χ 1000 :

χ

l . .1

100

. .11111


_

Χ

-

χ

-

2

6

10

14

1

1000/Τ [Κ' ] ι

Figure 6. Η N M R , 30ΜΗζ . Semilogarithmic plot of Τ ι relaxation times (in milliseconds) as a function of 1000/T ( K ' l ) respectively for: a) Backbone deuterated syndiotactic polystyrene in the α polymorphous form. b) Backbone deuterated syndiotactic polystyrene in the β polymorphous form, c) Backbone deuterated syndiotactic polystyrene in the γ polymorphous form. Relaxation times were measured at 30 M H z within the temperature range 77-400 K ; solid line through experimental points results from the best fit procedure. (Continued on next page.)


299



300


CAPITANI ET AL.

301



Τ, (ms) 10000

1000

100

6

10

14

1

1000/Τ [Κ" ] Figure

7 . * H N M R , 57MHz

Semilogarithmic plot

of

Tj A

relaxation times (in milliseconds) as a function of 1000/T ( K ) respectively for: a) Backbone deuterated syndiotactic polystyrene in the α polymorphous form. b) Backbone deuterated syndiotactic polystyrene in the β polymorphous form, c) Backbone deuterated syndiotactic polystyrene in γ polymorphous form. Relaxation times were measured at 57 M H z within the temperature range 77-298 K ; solid line through experimental points results from the best fit procedure. (Continued on next page.)




302


17. CAPITANI ET AL.

303


Theory of spin lattice relaxation in oxygen-doped polymers. Consider a system of interacting spins \ and Sj with magnetic moments x

μχ. and μ§. the spin-Hamiltnian of this system can be considered as a sum of two terms: K=Xq + 3i'(t) where #o is the unperturbed Zeeman


Hamiltonian and is X'(t)=X^(t)

S

S

+ #J (t) + iiJ (t)

a perturbation

taking into account the following time dependent interactions between spins: 1) the dipole-dipole interaction between nuclear spins I ; 2) the dipole-dipole interaction between nuclear spins I and electron spins S; 3) the scalar interaction between nuclear and electron spins. The energy of interaction between the nuclear magnetic field and an electron spin S with magnetic moment μ is : E= μ Β = δ

8

ί Ι-S Jl-rXS-r)l 3

= § μΒδΝμΝ j — ρ - +

^

β

where X = d

8

ο

μ

Β

8

Ν

v

8π.

'>+8 μΒ§ΝμΝ

5

β

1

Κ*)

Q

·

s

ί I-S ,(IT)(ST)1 | - - — + 3 ^ | V

μ

y

Μ τ

Ν

y

is the Hamiltonian for the dipole-dipole interaction and ^Se^BSN^N y Let us introduce

^(r)l · S = A — ô ( r ) l · S is the scalar Hamiltonian. polar coordinates and the raising and lowering

operators I ,S , and time dependent random functions of the angles 0(t),ef


2

iiûift

d

Ef - Ε:

dt where COjf =

π Applying the master equation for spin population and following the Solomon treatment (19,20) it is possible to obtain the following equations for the longitudinal component of I and S: Λ

· = -(W + 2Wi + W X l -1 ) - (W - W )(S - S ) dt dS, = -(W - W X l - I ) - ( W + 2WJ +W )(S - S ) dt 0

2

2

0

z

2

0

0

2

0

0

Z

2

Z

0

0

ι where W O , W J , W J , W 2 are the transition probabilities o f a two spins system and IQJSQ are the equilibrium values respectively for I and S . Z

In the case o f a "spin-like" dipole-dipole interaction Wj = W the only observable quantity is the sum

Z

;

(I +S ) and the following z

z

equation can be obtained:

& This

^

= -2(W W )(l 1+

equation

2

represents

z +

S -I -S ) z

a

0

decay

0

with

relaxation

rate:

^ = 2(W W ). i 1+

2

A

In the presence o f "spin-unlike" dipole-dipole interaction between a nuclear spin I and an electron spin S it is possible to consider S = SQ. In this case in fact the dipolar interaction generates a relaxation process which is negligible for the electron spin on the N M R time o f observation. The following equation is obtained: Z

^=-(W at

0

+ 2W W ). 1+

2

Introducing the calculated expressions for the transition probabilities W Q , W I , W 2 the following equations can be respectively obtained in the case o f "spin-like"and "spin-unlike"dipole-dipole interaction:


17. CAPITANI ET AL.

= Gπ

τ5

4F Ο) 2

F +G)

where G j t interaction. 1

4

2

2

F +4co

is the

r

2

coupling constant for


1

ld

Ι

(Dg »

is

2

2

C +(ω +

8

proton

G j s is the coupling constant for proton-oxygen interaction;

and

T

dipole-dipole

6C

2

0 +(ω -ω )

0

the

3C

, 2

A

305


^IS

:

ld

L

where A

+ BIS

2

C +G>1

28

I S

=^ G

I

S

and B

(2) 2

2

C +Q J 12 = J^G

I S

I S

In the presence o f scalar interaction the only contribution to the relaxation rate is the transition probability WQ; the equation for the relaxation rate is: 1

2

rlS ls f*2\

=-A S(S + l)Ε

l

if E = -

and ω g »

I S

as follows:-

nlS 4s

= EIS

2

+(cot-û> ) s

ω j the above equation can be semplified

Ε Ε +ω

E

(3) 2

Correlation times. As previously described the following interactions between spins were taken into account: 1) the dipole-dipole proton interaction. 2) the dipole-dipole proton electron interaction: the interaction between nuclear spins (protons) and the unpaired electrons (S=l) belonging to O2 molecules has been taken into account. 3) the scalar proton-oxygen interaction. Average effective correlation times for molecular motions depend on the temperature; these are: a) T R , correlation time for isotropic motion which obeys to the Arrhenius law:


306

T


R

= RQ expf I VRTJ

where E u is the activation energy for motions of

the polymeric backbone. b) T L , correlation

time

for

the

phenyl

ring

libration,

where E j ^ is the activation energy for librational motion. c) T Q , the effective correlation time for the oxygen in the complex; it Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch017

2

has been considered in turn, as a sum of two terms:

1 τ

0

1

=

τ 2

Ε

1 T

S

The correlation time Tj? is an Arrhenius-like term accounting for the exchange in the complex aromatic

ring-02

:

1

=

-

(-*E

Eo^xpl

j, where

ν RT

T£

Eg is the binding energy of the complex. The correlation time τ § is a collision-like term taking into account for the random motion due to the collisions in the oxygen gas; it has the ι y form: S = — =

pT'^

If other processes occur which affect the scalar proton-oxygen interaction, their effect could be included in the collision term as 1 y additional correlation rates s; in this case S = — = pT Equation

for

the

spin-lattice

relaxation

/2

+S

rate.

Assuming:

F = R exp[^)+L exp| - E i : RT +S S C = FF + 0

0

E = E exp^exp| ^ -]+S RT / z

1

0n

the spin-lattice relaxation rate is expressed by the following equation

' F VF^+ωι

4F ^ F +4oiJ 2

(4)

u



17. CAPITANI ET AL.


307

It must be outlined that dipolar and scalar proton-oxygen interactions strongly depend on the motion o f oxygen molecules and on their paramagnetic relaxation (20-22). Moreover, by lowering the temperature, the molar fraction of adsorbable oxygen increases. Thus while at higher temperatures, in the weak collision case, the proton dipole-dipole interaction is active, at low temperature the only effective term is that due to paramagnetic oxygen. Note that in the equation the coefficient u is the maximum molar fraction of absorbable oxygen: a scale of u terms for different polymer gives an evaluation of the polymer as oxygen scavenger. Equation (4) is by no means exhaustive of all possible cases, in particular a low amount of oxygen such as in badly degassed polymers requires additional terms (9). Moreover, i f polymers adsorb high quantities of oxygen it might be necessary to introduce for the oxygen a translational correlation time and the diffusion coefficient (23). The general theory, in a slightly different and more complete notation, has been reported elsewhere (Capitani, D . ; Segre, A . L . ; Blicharski, J.S. Macromolecules, in press ). Best Fit of experimental data. Equation (4) can be written in a parametric form and optimized with a fit program using a least squares procedure (Sykora, S. Program "Stefit", part of the software of spectrometer "Spinmaster"). Filled curves in Figures 6 and 7 have been obtained by the optimizing eq. 4 . Even i f a full discussion of the parameters will be reported elsewhere , some of the chemico-physical information deserves a comment. In particular two kinds of parameters can be obtained, one is due to molecular motions of the polymeric system itself, see Table III; while the second set is due to the interactions between the polymeric system and oxygen , see Table I V . The parameter E has been previously defined as the activation energy for motions of the polymeric backbone. These motions become relevant to the relaxation process at high temperature. As a consequence this energy must show little or no dependence on the polymorphism and on the presence or absence of oxygen. In fact, as shown in Table III, all E values are the same within the experimental error and are not sensitive to the presence o f oxygen. The parameter E has been defined as the activation energy for the phenyl ring libration. R

R

L

Table III. Best fit parameters £ R , E L are reported for α, β and γ polymorphous polystyrenes; last column in the table refers to a well degassed γ (deg-γ) polymorphous form. Data collected at 30 M H z . 30 MHz E ET R

α 31.310.2 5.5±0.2

β 31.3±0.2 6.7±0.1

γ 31.1±0.2 4.6±0.2

deg-γ 31.1+.0.2 4.6±0.2


308


Table IV. Mean values u (molar fraction % ) and E (KJ/mol) of best fit values of parameter u and E £ , obtained from experimental data collected at 30 and 57 MHz, are reported for crystalline ("c") and amorphous ("a") fractions of α, β and γ polymorphous polystyrenes E

u

E 15.0±1 12.0+0.6 10.4±0.5 13.5±0.5 11.7±0.3 11.2±.7


E

E (a) > E (y). In agreement with this observation the β polymorphous form is characterized by a tighter molecular packing than the α form (crystallographic density of the α form is 1.033 g/cc while it is 1.076 g/cc for the β form ; De Rosa, C ; Guerra, G . ; Corradini, P. , personal communication ). In Table I V , u has been defined as the maximal molar fraction, /oo, of L

L

L

L

adsorbable oxygen in each phase. The evaluation of the term constitutes an important chemical application of the outlined theory.

u

In fact a scale o f the term u gives an evaluation of the polymer as an oxygen scavenger. This evaluation is fundamental in the study of membranes used for gas separation. The evaluation of u in different polymorphs is shown in Table I V . The following index for oxygen adsorption can be obtained: β < β < a

Multidimensional Spectroscopy of Polymers - American Chemical

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