Magneric Resonance Printed in the USA.
Imaging, All rights
Vol. IO, pp. 793-798, reserved.
1992 Copyright
0
0730-725X/92 $5.00 + .oO 1992 Pergamon Press Ltd.’
0 Session: Contribution
PROTON SPIN LATTICE RELAXATION IN AROMATIC POLYMERS D. CAPITANI AND A .L. SEGRE CNR, Istituto di Strutturistica Chimica, Area della Ricerca di Roma, M.B. 10, Monterotondo Staz., Roma, Italy The most striking requirement for the NMR imaging of an object is that this object should be made with components having different NMR parameters. In most cases these parameters are due to the presence of a fluid and to its mobility, thus measurable parameters are proton signal intensity contrasted in T, or T2. From the glass transition temperature Tg, by decreasing the temperature, all nonaromatic polymers as well as any well degassed polymers, show a continuous increase of their proton spin-lattice relaxation, which at low temperature is usually larger than lo-20 sec. However, due to O2 molecules selectively adsorbed on aromatic rings, non-degassed aromatic polymers show a marked shortening of the proton spin-lattice relaxation. This effect is maximal at rather low temperature, where T, can be shorter than 1 msec, and in many known cases shorter than 566 msec. Since the amount of sorbed-O2 is a function of the chemical nature of the polymer, the type of crystallinity (polymorphism of semicrystalline polymers), the crystalline-amorphous ratio, and so on, a careful study of T, relaxation as a function of the temperature can define optimal conditions for T, contrast. Examples regarding polymorphism in syndiotactlc polystyrene, butadiene-styrene block copolymers and blends, and poly(phenylene) oxide, will be discussed. Keywords:
Spin-lattice relaxation; Aromatic polymers; Adsorbed 02; T, contrast agent.
degassed, show a continuous increase in their proton spin-lattice relaxation 2Y3(Fig. 1) (the presence of a pendant group, i.e., a side chain, can introduce only broad secondary minima). A typical behavior of T, versus temperature is shown in the case of poly(ethyl-
INTRODUCTION The most striking requirement for the magnetic resonance imaging (MRI) of an object is that this object should be made with components having different NMR parameters. In most cases these parameters are
due to the presence of a fluid and to its mobility; thus measurable parameters are proton signal intensity, eventually contrasted in T, or T2. Most polymers, however, have quite similar proton density and to worsen the case, rather similar T, values. The T2 contrast is efficient only when there are two components, one well above its glass transition temperature Tg and the other one very stiff. Here we introduce experimental evidence and the method leading to show how strong T, differences can be induced in the case of aromatic polymers. EXPERIMENTAL
All measurements were performed at 30 MHz on a spectrometer “Spinmaster” of Stelar, Mede (PV), Italy. Experimental details and mathematical treatment of data have been previously reported.’ 2.6 RESULTS
AND DISCUSSION
From the Tg, by decreasing the temperature, all nonaromatic polymers, and aromatic polymers if well
3
6.6 4 lwWf
4.6
6
Fig. 1. T, relaxation as a function of 1000/T for a sample of rubber. (Data taken from Ref. 2.) 193
194
Magnetic Resonance Imaging 0 Volume 10, Number 5, 1992
through geometrical modeling, that is, hypothesizing surface-to-volume ratios. We have recently shown’JO that oxygen molecules are selectively adsorbed on aromatic polymers. The formed complex is paramagnetic, in a fast equilibrium with its diamagnetic uncomplexed counterpart, according to the equation
T,' = (1 - n)T;d’ + nT;d
0.1’ 0.2
2.2
I
I
4.2
6.2
I 8.2
10.2
Fig. 2. T, and T,, relaxations as a function of 1000/T for a sample of poly(ethyleneoxide). Note the strong effect of the melting on T,,. (Data taken from Ref. 1.)
eneoxide) (Fig. 2); note that at low temperature T, is usually rather long, lo-20 set, and accompanied by very short T2 values, of the order of lo-’ sec4 Thus structural information was gathered mostly by TIP variations as a function of the temperature; these, however, are rather small and difficult to use for imaging contrast, as can be seen in Fig. 2.* In fact, T,, is extremely sensitive only in a narrow temperature range, quite near the melting point, or when a transition occurs, associated with motional variations, that is, exactly when an object may lose its shape. Thus TIP contrast does not seem a suitable parameter for studying polymers with MRI. When one of the components is a rubber such as polybutadiene, polyisoprene, or a polymer well above its Tg, T2 contrast is suitable; in styrene-butadiene block copolymers the free induction decay shows two components:S the fast-decaying one is due to the polystyrene, and the slow-decaying one is due to the interface and to polybutadiene (See Fig. 3). Here each component is confined in separate domains. Many structures are possible,6 as shown in Fig. 4. Thermal, rheological, and most macroscopic properties of copolymers and blends, such as transparency and impact strength, depend on the structure. The most used method7-9 for gaining structural information is electron microscopy (EM). This approach, however, is a destructive one. Moreover, with this technique extremely precise information can be obtained, but two major drawbacks must be considered: first, the staining chemical procedure necessary to distinguish between different phases; second, unwanted effects that might originate by cutting (or fracturing) the sample in order to obtain a good surface for the investigation. Furthermore, quantitative analysis is obtainable only
where n = T,, = 7’id = T,p =
molar fraction of the complex T 1experimental = measured T, value = intrinsec T, diamagnetic value Ti diamagnetic T 1paramagnetic = T, value of the complex.
The ground state of the aromatic rings is in the complexed state; this means that the total energy is lower when O2 is complexed on aromatic rings. This also means that by lowering the temperature, the amount of adsorbed O2 increases and, as a consequence, the experimental value of T,, decreases. This behavior is unique and in perfect agreement with the newly found ground state of benzene.” Thus paramagnetic O2 effectively acts as a contrast agent, capable of differentiating chemicophysical situations otherwise indistinguishable. The amount of adsorbed O2 is modulated by the chemical nature of the polymer, by the crystalline versus amorphous ratio, by the molecular packing, and most probably by the thickness of layers and sample morphology.
1oOa
1ooi 0
100
200 ld
300
400
Fig. 3. Free induction decay of samples made by small cylinders (4 mm 4, 3 mm height) of commercial high-impact, transparent copolymers or blends.4
Proton spin-lattice relaxation in polymers 0 D. CAPITANI AND A.L. SEGRE
795
Fig. 4. Common structures found in copolymers, polymeric blends, and manufacts. A morphological description of these materials can be found in Ref. 5 and references quoted therein.
In order to prove these points, studies are in progress, but few examples of the efficiency of the method can be given yet. Syndiotactic polystyrene s-PS (Fig. 5) can crystallize in four crystalline modifications, l2 giving semicrystalline polymers. These are two zig-zag planar, (IIand 0, and two helices, y and 6. The only difference between Q!and /3 is the molecular packing. 13C CP-MAS NMR is incapable of distinguishing among crystalline forms having the same chain conformation and different packing. l3 Thus only the X-ray powder diffraction method can give the structure of these polymers; however, the obtainable information on the amorphous phase is poor. In Figs. 6-8 the T, behavior as a function of the temperature is shown for the a, 0, and y polymorphous semicrystalline forms of s-PS. Here it can be seen that a temperature can be always found at which T, differ-
ences are significant. Since a quantitative analysis of the amount of different relaxations can also be obtained, it is possible to check the method with the corresponding X-ray analysis, as shown in Table 1. From these data, a structure can be proposed for the amorphous phases; these should be partially or short-range ordered and not random at all. ‘,14~r5 Thus the presence of O2 effectively acts as a strong T,contrast agent. The combined approach of X-ray diffraction, 13C CP-MAS, and pulse ‘H 02-contrasted NMR is able to give an almost complete set of structural information. This is summarized in Table 2. Information about the dimension of the domains is completely absent and might be a challenge for MRI. Another example of T,shortening can be seen in poly(2,6-dimethyl)phenylenoxide (PPO). This is a polymer used for preparing membranes able to act as
Magnetic Resonance Imaging 0 Volume 10, Number5, 1992
796
MONOMERIC
IJNIT
SYNDIOTACTIC BACKl30NE
a
ZIG
ZAG
HELIX
PLANAR
Fig. 5. Sketch of the structures of syndiotactic polystyrene.
molecular sieves for gas separation.16 The aromatic moiety is in the main chain. ‘H NMR spin-lattice relaxation was measured at 30 MHz as a function of the temperature, ranging from 77 up to 400 K; its value decreases from =5.10-’ set at 293 K to =5* low5 at 130 K. ” In this case a strong complex of O2 on aromatic rings is evident. Quantitative information about the O,-polymer complex is obtainable, and these data can be well correlated with the physicochemical properties of PPO itself.
Table 1. Percentages of crystallinity in semicrystalline polymorphous syndiotactic polystyrenes obtained by X-ray powder diffraction and by ‘H pulse low-resolution NMR
0
loo
200
600
400
606
o!
T(K) Fig. 6. Proton T, relaxation at 30 MHz as a function of temperature for a sample of semicrystalline syndiotactic polystyrene, CYpolymorphous form. At low temperature, many T1 relaxations are observable; the slowest component is due to backbone protons; fast components are due to aromatic protons in crystalline or amorphous domains.
NMR
X-ray
35-40
40
101 50
P
Y
NMR X-ray
NMR X-ray
60
60 101 3o
60
40
40
% crystallinity
40
60
60
% amorphous
Proton spin-lattice relaxation
in
polymers 0 D.
CAPITANI AND
A.L.
797
SEGRE
TU.1
’ 2.2 - -+
Tlan
3
Th Tb
Tfm Tl8
t -=+2’s, Tm
-FtTlo
-iao
-
200
400
so0
800
mo
200
T(K) Fig. 7. Proton r, relaxation at 30 MHz as a function of temperature for a sample of semicrystalline syndiotactic polystyrene, /3 polymorphous form.
CONCLUSION
From the above data and considerations a possible strategy emerges leading to a method applicable for inducing T, contrast when MR images of objects containing aromatic polymers are to be observed. In fact, from a full temperature study of spin-lattice relaxations, the “best” temperature can be found’ corresponding to maximum T, differences. Careful attention must be paid in order to operate in an 02-rich atmosphere. Aromatic polymers are abundantly used
Table 2. Methods of detecting structure of semicrystalline polymers Method
Information
sao
400
600
T(K)
obtainable
X-ray powder diffraction
Structure of the crystalline Percentage of crystallinity
domains
NMR CP-MAS infrared spectroscopies
Conformation of the chains Quantitative analysis of conformers
Pulsed ‘H NMR Oz contrasted at low temperature
Quantitative analysis of amorphous/ crystalline ratio Physical characterization of both amorphous and crystalline domains
MRI
Dimension
and shape of domains
Fig. 8. Proton T, relaxation at 30 MHz as a function of temperature for a sample of semicrystalline syndiotactic polystyrene,
y polymorphous
form.
in composite materials; thus we think that the outlined method might be useful for MRI characterization. Acknowledgments-This work was partly supported by special ad hoc program Chimica Fine II-Polymeric Materials. The technical assistance of C. Marciante and E. Rossi is acknowledged.
REFERENCES 1. Capitani, D.; De Rosa, C.; Ferrando, A.; Grassi, A.; Segre, A.L. Macromolecules 1992, in press. 2. Connor, T.M. Magnetic relaxation in polymers. The rotating frame method. In: NMR Basic Principles and Progress, Vol. 4. Berlin: Springer-Verlag. 1971; pp. 247-270. Gutowsky, H.S.; Saika, A.; Takeda, M.; Woessner, D.E. J. Chem. Phys. 27:534; 1957. Carrington, A.; McLachlan, A.D. Introduction to Magnetic Resonance. London: Harper & Row, 1979. Segre, A.L.; Capitani, D.; Fiordiponti, P.; Cantini, P.L.; Callaioli, A.; Ferrando, A.; Nocci, R. Unpublished. Paul, D.R.; Barlow, J.W. J. Macromol. Sci. Rev. Macromol. Chem. C 8:109; 1980. Russel, T.P.; Stern, R.S. J. Polym. Sci., Polym. Phys. Ed. 21:999; 1983. Kemmy, J.C.; McBrierty, V.J.; Rigby, Z.; Douglass, D.C. Macromolecules 24:436; 1991. Alfonso, G.C.; Turturro, A.; Pizzoli, M.; Scandola, M.; Caccarelli, G.J. J. Polym. Sci. Polym. Phys. Ed. 21: 1195; 1989.
Magnetic Resonance Imaging 0 Volume 10, Number 5, 1992
798
10. Segre, A.L.; Capitani, D.; Grassi, A.; Sykora, S. Macromolecules 24:623; 1991. 11. Gooding, A.; Serak, R.; Gilby, P.R. .I. Phys. Chem. 95: 7868; 1991.
12. Guerra, G.; Vitagliano, V.M.; De Rosa, C.; Petraccone, V.; Corradini, P. Macromolecules 23~1539; 1990. 13. Grassi, A.; Longo, P.; Guerra, G. Makrmol. Chem. Rapid Comm. 2310:687;
1989.
14. Mansfield, M.L. Macromolecules 20:1384; 1987. 15. Lather, R.C.; Bryant, J.L.; Howard, L.N.; Sumners, D.W. Macromolecules 19:2639; 1986. 16. Perego, G.; Roggero, A.; Sisto, R.; Valentini, C.A. .I. Membrane Sci. 55:325; 1991. 17. Capitani, D.; Clericuzio, M.; Corno,
Segre, A.L. Unpublished.
M.; Lillo, F.;
Imaging, All rights
Vol. IO, pp. 793-798, reserved.
1992 Copyright
0
0730-725X/92 $5.00 + .oO 1992 Pergamon Press Ltd.’
0 Session: Contribution
PROTON SPIN LATTICE RELAXATION IN AROMATIC POLYMERS D. CAPITANI AND A .L. SEGRE CNR, Istituto di Strutturistica Chimica, Area della Ricerca di Roma, M.B. 10, Monterotondo Staz., Roma, Italy The most striking requirement for the NMR imaging of an object is that this object should be made with components having different NMR parameters. In most cases these parameters are due to the presence of a fluid and to its mobility, thus measurable parameters are proton signal intensity contrasted in T, or T2. From the glass transition temperature Tg, by decreasing the temperature, all nonaromatic polymers as well as any well degassed polymers, show a continuous increase of their proton spin-lattice relaxation, which at low temperature is usually larger than lo-20 sec. However, due to O2 molecules selectively adsorbed on aromatic rings, non-degassed aromatic polymers show a marked shortening of the proton spin-lattice relaxation. This effect is maximal at rather low temperature, where T, can be shorter than 1 msec, and in many known cases shorter than 566 msec. Since the amount of sorbed-O2 is a function of the chemical nature of the polymer, the type of crystallinity (polymorphism of semicrystalline polymers), the crystalline-amorphous ratio, and so on, a careful study of T, relaxation as a function of the temperature can define optimal conditions for T, contrast. Examples regarding polymorphism in syndiotactlc polystyrene, butadiene-styrene block copolymers and blends, and poly(phenylene) oxide, will be discussed. Keywords:
Spin-lattice relaxation; Aromatic polymers; Adsorbed 02; T, contrast agent.
degassed, show a continuous increase in their proton spin-lattice relaxation 2Y3(Fig. 1) (the presence of a pendant group, i.e., a side chain, can introduce only broad secondary minima). A typical behavior of T, versus temperature is shown in the case of poly(ethyl-
INTRODUCTION The most striking requirement for the magnetic resonance imaging (MRI) of an object is that this object should be made with components having different NMR parameters. In most cases these parameters are
due to the presence of a fluid and to its mobility; thus measurable parameters are proton signal intensity, eventually contrasted in T, or T2. Most polymers, however, have quite similar proton density and to worsen the case, rather similar T, values. The T2 contrast is efficient only when there are two components, one well above its glass transition temperature Tg and the other one very stiff. Here we introduce experimental evidence and the method leading to show how strong T, differences can be induced in the case of aromatic polymers. EXPERIMENTAL
All measurements were performed at 30 MHz on a spectrometer “Spinmaster” of Stelar, Mede (PV), Italy. Experimental details and mathematical treatment of data have been previously reported.’ 2.6 RESULTS
AND DISCUSSION
From the Tg, by decreasing the temperature, all nonaromatic polymers, and aromatic polymers if well
3
6.6 4 lwWf
4.6
6
Fig. 1. T, relaxation as a function of 1000/T for a sample of rubber. (Data taken from Ref. 2.) 193
194
Magnetic Resonance Imaging 0 Volume 10, Number 5, 1992
through geometrical modeling, that is, hypothesizing surface-to-volume ratios. We have recently shown’JO that oxygen molecules are selectively adsorbed on aromatic polymers. The formed complex is paramagnetic, in a fast equilibrium with its diamagnetic uncomplexed counterpart, according to the equation
T,' = (1 - n)T;d’ + nT;d
0.1’ 0.2
2.2
I
I
4.2
6.2
I 8.2
10.2
Fig. 2. T, and T,, relaxations as a function of 1000/T for a sample of poly(ethyleneoxide). Note the strong effect of the melting on T,,. (Data taken from Ref. 1.)
eneoxide) (Fig. 2); note that at low temperature T, is usually rather long, lo-20 set, and accompanied by very short T2 values, of the order of lo-’ sec4 Thus structural information was gathered mostly by TIP variations as a function of the temperature; these, however, are rather small and difficult to use for imaging contrast, as can be seen in Fig. 2.* In fact, T,, is extremely sensitive only in a narrow temperature range, quite near the melting point, or when a transition occurs, associated with motional variations, that is, exactly when an object may lose its shape. Thus TIP contrast does not seem a suitable parameter for studying polymers with MRI. When one of the components is a rubber such as polybutadiene, polyisoprene, or a polymer well above its Tg, T2 contrast is suitable; in styrene-butadiene block copolymers the free induction decay shows two components:S the fast-decaying one is due to the polystyrene, and the slow-decaying one is due to the interface and to polybutadiene (See Fig. 3). Here each component is confined in separate domains. Many structures are possible,6 as shown in Fig. 4. Thermal, rheological, and most macroscopic properties of copolymers and blends, such as transparency and impact strength, depend on the structure. The most used method7-9 for gaining structural information is electron microscopy (EM). This approach, however, is a destructive one. Moreover, with this technique extremely precise information can be obtained, but two major drawbacks must be considered: first, the staining chemical procedure necessary to distinguish between different phases; second, unwanted effects that might originate by cutting (or fracturing) the sample in order to obtain a good surface for the investigation. Furthermore, quantitative analysis is obtainable only
where n = T,, = 7’id = T,p =
molar fraction of the complex T 1experimental = measured T, value = intrinsec T, diamagnetic value Ti diamagnetic T 1paramagnetic = T, value of the complex.
The ground state of the aromatic rings is in the complexed state; this means that the total energy is lower when O2 is complexed on aromatic rings. This also means that by lowering the temperature, the amount of adsorbed O2 increases and, as a consequence, the experimental value of T,, decreases. This behavior is unique and in perfect agreement with the newly found ground state of benzene.” Thus paramagnetic O2 effectively acts as a contrast agent, capable of differentiating chemicophysical situations otherwise indistinguishable. The amount of adsorbed O2 is modulated by the chemical nature of the polymer, by the crystalline versus amorphous ratio, by the molecular packing, and most probably by the thickness of layers and sample morphology.
1oOa
1ooi 0
100
200 ld
300
400
Fig. 3. Free induction decay of samples made by small cylinders (4 mm 4, 3 mm height) of commercial high-impact, transparent copolymers or blends.4
Proton spin-lattice relaxation in polymers 0 D. CAPITANI AND A.L. SEGRE
795
Fig. 4. Common structures found in copolymers, polymeric blends, and manufacts. A morphological description of these materials can be found in Ref. 5 and references quoted therein.
In order to prove these points, studies are in progress, but few examples of the efficiency of the method can be given yet. Syndiotactic polystyrene s-PS (Fig. 5) can crystallize in four crystalline modifications, l2 giving semicrystalline polymers. These are two zig-zag planar, (IIand 0, and two helices, y and 6. The only difference between Q!and /3 is the molecular packing. 13C CP-MAS NMR is incapable of distinguishing among crystalline forms having the same chain conformation and different packing. l3 Thus only the X-ray powder diffraction method can give the structure of these polymers; however, the obtainable information on the amorphous phase is poor. In Figs. 6-8 the T, behavior as a function of the temperature is shown for the a, 0, and y polymorphous semicrystalline forms of s-PS. Here it can be seen that a temperature can be always found at which T, differ-
ences are significant. Since a quantitative analysis of the amount of different relaxations can also be obtained, it is possible to check the method with the corresponding X-ray analysis, as shown in Table 1. From these data, a structure can be proposed for the amorphous phases; these should be partially or short-range ordered and not random at all. ‘,14~r5 Thus the presence of O2 effectively acts as a strong T,contrast agent. The combined approach of X-ray diffraction, 13C CP-MAS, and pulse ‘H 02-contrasted NMR is able to give an almost complete set of structural information. This is summarized in Table 2. Information about the dimension of the domains is completely absent and might be a challenge for MRI. Another example of T,shortening can be seen in poly(2,6-dimethyl)phenylenoxide (PPO). This is a polymer used for preparing membranes able to act as
Magnetic Resonance Imaging 0 Volume 10, Number5, 1992
796
MONOMERIC
IJNIT
SYNDIOTACTIC BACKl30NE
a
ZIG
ZAG
HELIX
PLANAR
Fig. 5. Sketch of the structures of syndiotactic polystyrene.
molecular sieves for gas separation.16 The aromatic moiety is in the main chain. ‘H NMR spin-lattice relaxation was measured at 30 MHz as a function of the temperature, ranging from 77 up to 400 K; its value decreases from =5.10-’ set at 293 K to =5* low5 at 130 K. ” In this case a strong complex of O2 on aromatic rings is evident. Quantitative information about the O,-polymer complex is obtainable, and these data can be well correlated with the physicochemical properties of PPO itself.
Table 1. Percentages of crystallinity in semicrystalline polymorphous syndiotactic polystyrenes obtained by X-ray powder diffraction and by ‘H pulse low-resolution NMR
0
loo
200
600
400
606
o!
T(K) Fig. 6. Proton T, relaxation at 30 MHz as a function of temperature for a sample of semicrystalline syndiotactic polystyrene, CYpolymorphous form. At low temperature, many T1 relaxations are observable; the slowest component is due to backbone protons; fast components are due to aromatic protons in crystalline or amorphous domains.
NMR
X-ray
35-40
40
101 50
P
Y
NMR X-ray
NMR X-ray
60
60 101 3o
60
40
40
% crystallinity
40
60
60
% amorphous
Proton spin-lattice relaxation
in
polymers 0 D.
CAPITANI AND
A.L.
797
SEGRE
TU.1
’ 2.2 - -+
Tlan
3
Th Tb
Tfm Tl8
t -=+2’s, Tm
-FtTlo
-iao
-
200
400
so0
800
mo
200
T(K) Fig. 7. Proton r, relaxation at 30 MHz as a function of temperature for a sample of semicrystalline syndiotactic polystyrene, /3 polymorphous form.
CONCLUSION
From the above data and considerations a possible strategy emerges leading to a method applicable for inducing T, contrast when MR images of objects containing aromatic polymers are to be observed. In fact, from a full temperature study of spin-lattice relaxations, the “best” temperature can be found’ corresponding to maximum T, differences. Careful attention must be paid in order to operate in an 02-rich atmosphere. Aromatic polymers are abundantly used
Table 2. Methods of detecting structure of semicrystalline polymers Method
Information
sao
400
600
T(K)
obtainable
X-ray powder diffraction
Structure of the crystalline Percentage of crystallinity
domains
NMR CP-MAS infrared spectroscopies
Conformation of the chains Quantitative analysis of conformers
Pulsed ‘H NMR Oz contrasted at low temperature
Quantitative analysis of amorphous/ crystalline ratio Physical characterization of both amorphous and crystalline domains
MRI
Dimension
and shape of domains
Fig. 8. Proton T, relaxation at 30 MHz as a function of temperature for a sample of semicrystalline syndiotactic polystyrene,
y polymorphous
form.
in composite materials; thus we think that the outlined method might be useful for MRI characterization. Acknowledgments-This work was partly supported by special ad hoc program Chimica Fine II-Polymeric Materials. The technical assistance of C. Marciante and E. Rossi is acknowledged.
REFERENCES 1. Capitani, D.; De Rosa, C.; Ferrando, A.; Grassi, A.; Segre, A.L. Macromolecules 1992, in press. 2. Connor, T.M. Magnetic relaxation in polymers. The rotating frame method. In: NMR Basic Principles and Progress, Vol. 4. Berlin: Springer-Verlag. 1971; pp. 247-270. Gutowsky, H.S.; Saika, A.; Takeda, M.; Woessner, D.E. J. Chem. Phys. 27:534; 1957. Carrington, A.; McLachlan, A.D. Introduction to Magnetic Resonance. London: Harper & Row, 1979. Segre, A.L.; Capitani, D.; Fiordiponti, P.; Cantini, P.L.; Callaioli, A.; Ferrando, A.; Nocci, R. Unpublished. Paul, D.R.; Barlow, J.W. J. Macromol. Sci. Rev. Macromol. Chem. C 8:109; 1980. Russel, T.P.; Stern, R.S. J. Polym. Sci., Polym. Phys. Ed. 21:999; 1983. Kemmy, J.C.; McBrierty, V.J.; Rigby, Z.; Douglass, D.C. Macromolecules 24:436; 1991. Alfonso, G.C.; Turturro, A.; Pizzoli, M.; Scandola, M.; Caccarelli, G.J. J. Polym. Sci. Polym. Phys. Ed. 21: 1195; 1989.
Magnetic Resonance Imaging 0 Volume 10, Number 5, 1992
798
10. Segre, A.L.; Capitani, D.; Grassi, A.; Sykora, S. Macromolecules 24:623; 1991. 11. Gooding, A.; Serak, R.; Gilby, P.R. .I. Phys. Chem. 95: 7868; 1991.
12. Guerra, G.; Vitagliano, V.M.; De Rosa, C.; Petraccone, V.; Corradini, P. Macromolecules 23~1539; 1990. 13. Grassi, A.; Longo, P.; Guerra, G. Makrmol. Chem. Rapid Comm. 2310:687;
1989.
14. Mansfield, M.L. Macromolecules 20:1384; 1987. 15. Lather, R.C.; Bryant, J.L.; Howard, L.N.; Sumners, D.W. Macromolecules 19:2639; 1986. 16. Perego, G.; Roggero, A.; Sisto, R.; Valentini, C.A. .I. Membrane Sci. 55:325; 1991. 17. Capitani, D.; Clericuzio, M.; Corno,
Segre, A.L. Unpublished.
M.; Lillo, F.;
