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Solid State Communications 115 (2000) 179–183 Poly(methyl acrylate-co-sodium methacrylate) ionomer studied by solid state 13C T1r NMR aDepartment of Physics, Jeonju University, Jeonju 560-759, South Korea bDepartment of Polymer Science and Engineering, Chosun University, Kwangju 501-759, South Korea Received 22 February 2000; accepted 6 April 2000 by A. Pinczuk Abstract
The poly(methyl acrylate) (PMA) and poly(methyl acrylate-co-sodium methacrylate) containing 6.9 mol% of ionic groups (PMANa-6.9) were studied by 13C CP/MAS NMR. The 13C spin–lattice relaxation times in a rotating frame, T1r, have beenmeasured as a function of temperature. Using these T1r spin–lattice relaxation times, we discuss the mobility, the correlationtime, and activation energy for the PMA and PMANa-6.9, respectively. The molecular motion in the PMANa-6.9 needs higheractivation energies than in PMA. It is worth noting that the motion of the 1-methyl carbons in the PMANa-6.9 ionomerdistinctly differs from that in the PMA homopolymer. The slow side of the T1r minimum associated with the 1-methyl carbonsis ascribed to stronger interactions between the polymer chains in the ionomer than in the homopolymer. ᭧ 2000 Published byElsevier Science Ltd. All rights reserved.
Keywords: A. Polymers; B. Chemical synthesis; E. Nuclear resonances 1. Introduction
glassy polymer main chains in the low-to mid-kilohertzfrequency range, which are vital in determining mechanical 13C nuclear magnetic resonance (NMR) has proven to be properties of polymers such as toughness . The main a very powerful technique for studying local dynamics of chain motions of various polycarbonates at room tempera- polymers, and 13C spin–lattice relaxation time is known to ture have been determined from the 13C spin–lattice relaxa- be an important experimental quantity for probing dynamic tion measurements employing a high-resolution technique, processes in polymers. Since the 13C nucleus is of low including magic-angle spinning [6,7].
natural abundance, the relaxation is dominated by dipole Polymers of low dielectric constants, which contain a interactions with directly bonded hydrogens. By studying relatively small amount of ionic groups, show the character- the relaxation of the nuclei in different environment within istic behavior of micro-separated materials [8,9]. This beha- the chain, it is possible to obtain detailed pictures of chain vior is known to be related to ionic aggregates within the motions occurring in different parts of the chain. Thus, the relatively nonpolar polymer matrix. The ionic aggregates, measured relaxation data can be used to obtain information called multiplets , induce the formation of a second on the dynamic processes of polymer chains in different phase, termed clusters . These ionic polymers are environment [1,2]. Recent studies have shown that more known as ionomers and have both non-ionic and ionic localized motions, i.e. mobility of the polymer backbone nature. In order to explain the morphology and physical chains, should be considered along with the conformational properties of ionomers, Eisenberg, Hird and Moore transitions in order to understand the differences in the proposed a multiplet/cluster model in 1990 . According dynamics of the C–H vector at different sites of the back- to the model, at very low ion contents, only multiplets are bone [3,4]. The 13C T1r relaxation parameter is particularly formed, and the material shows only one glass transition informative since it is directly related to the motions of (Tg), i.e. matrix Tg. The mobility of chains surrounding themultiplet is assumed to be restricted. As the ion content increases, the regions of restricted mobility start to overlap.
E-mail address: email@example.com (A.R. Lim).
Once the dimension of overlapping regions of restricted 0038-1098/00/$ - see front matter ᭧ 2000 Published by Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 1 0 9 8 ( 0 0 ) 0 0 1 6 4 - 2 A.R. Lim, J.-S. Kim / Solid State Communications 115 (2000) 179–183 polymer was filtered, and dried under vacuum at 70ЊC for1 day. To determine the acid content, the acid sample wasdissolved in a benzene/methanol (9/1 v/v) mixture andtitrated with standard methanolic NaOH to the phenolphtha-lein end point. The amount of acid groups was found to be6.9 mol%. To neutralize the acid polymer, the acid samplewas dissolved in a benzene/methanol mixture, and a prede-termined quantity of methanolic NaOH was added to givepoly(methyl acrylate-co-sodium methacrylate) (PMANa-6.9). The solution was freeze-dried and then dried undervacuum at 70ЊC for 1 day. The chemical structures of thepolymers are shown in Fig. 1.
2.2. 13C solid state NMR spectroscopy The solid state NMR experiments were performed using a Varian 300 NMR spectrometer at the Korea Basic ScienceInstitute in Taegu. The CP/MAS 13C NMR experimentswere performed at the Larmor frequency of 75.46 MHz.
The samples of powder form were placed in a CP/MASprobe of 7 mm, and the magic angle spinning rate was5 kHz to minimize spinning sideband overlap. The p=2pulse time was 7.5 ms, corresponding to spin-locking fieldstrength of 50 kHz. 13C T Fig. 1. Chemical structures of the polymers.
13C spin-locking pulse after a 0.8 ms CP preparation period.
The decay of the 13C magnetization in the spin-locking field mobility, i.e. clusters, exceeds ca. 100 A was followed for spin-locking time of up to 30 ms.
a second Tg, i.e. cluster Tg, attributed to the clusteredregions. As the ion content increases further, both the matrixand cluster Tgs of the ionomer increase.
3. Results and discussion
In this paper, we studied the structure of poly(methyl The solid state structures and dynamics of PMA and PMANa-6.9 were examined using the solid state NMR.
(PMANa-6.9) by 13C cross-polarization and magic angle Fig. 2 shows the solid state 13C CP/MAS NMR spectrum spinning (CP/MAS) NMR. The 13C spin–lattice relaxation of the PMA. The spectrum consists of three signals at time in a rotating frame, T1r, was also measured as a func- chemical shifts of d 175:54; 52.31 and 41.89 ppm at tion of temperature. From the results, the mobility of poly- room temperature, which are assigned to carbonyl, 2- mer chains, correlation time and activation energy for each methyl ϩ b-methylene, and 1-methyl carbons, respectively.
carbon in the PMA and PMANa-6.9 were measured.
The carbon resonance peaks of 2-methyl and b-methyleneshow overlapping. Spinning sidebands are marked with anasterisk. The most intense signal is due to the 2-methyl and 2. Experimental
b-methylene carbons. In the case of the PMANa-6.9, the 13CCP/MAS NMR spectrum (not shown here) was similar to that of the PMA. The chemical shifts of both the PMA andthe PMANa-6.9 were measured at various degrees of The synthesis of poly(methyl acrylate) (PMA) homopo- temperature, and were found to be nearly independent of lymer and poly(methyl acrylate-co-sodium methacrylate) The spin–lattice relaxation time in a rotating frame, T1r, described elsewhere . For convenience, only a brief for each carbon of the two polymers was taken at different summary of the procedure is given. The polymers were degrees of temperature with variable spin-locks on the prepared by solution polymerization of purified methyl carbon channel following cross-polarization. The 13C acrylate and methacrylic acid monomers using benzoyl magnetization was generated by the cross-polarization peroxide as the initiator at 60ЊC. Dried and distilled tetra- after the spin-locking of protons. Then the proton Rf field hydrofuran was used as the solvent. The conversion was less was turned off for variable time t while the 13C Rf field than ca. 10 %, and the polymer samples were recovered by remained on. Finally, under high power proton decoupling, precipitation into an excess of methanol. The precipitated the 13C free induction decay (FID) was observed and A.R. Lim, J.-S. Kim / Solid State Communications 115 (2000) 179–183 Fig. 2. Solid state 13C CP/MAS NMR spectrum of PMA at room temperature.
subsequently Fourier-transformed. The T1r values were for each carbon in both the PMA and PMANa-6.9 poly- obtained selectively by Fourier transformation of the FID, mers as a function of temperature. In the case of the following the end of spin-locking and repetition of the PMA, the relaxation time of the 2-methyl, b-methylene, experiment with variation of t. All the traces obtained in and carbonyl carbons undergo motions on the slow side the two polymers were fitted with the following single expo- of the T1r minimum, while the 1-methyl carbons undergo motions on the fast side of the T1r minimum.
In the case of the PMANa-6.9 ionomer, the relaxation Z t M0 exp Ϫt=T1r time of all carbons undergo motions on the slow side of Z and M0 represent the loss of magnetization and the 1r minimum. It is also seen that the T1r values total nuclear magnetization of 13C in thermal equilibrium, corresponding to the three peaks in the PMANa-6.9 are longer than those in the PMA. The 2-methyl, b- For the studies of molecular motion in the experimen- methylene, and carbonyl carbons also have longer tal relaxation time, it is important to know whether the relaxation time than the 1-methyl carbons. The relaxa- relaxation time is located on the slow side of the mini- mum or on the fast side of the minimum as a function carbons similarly decreases with increasing temperature of inverse temperature. At this point, it should be for both the PMA and the PMANa-6.9 polymers. The mentioned that the slow side of the curve can be inter- most significant difference between the PMA and the preted in such a way that the decrease in a T PMANa-6.9 samples appears in the relaxation time of indicates increased molecular motion; the fast side of the 1-methyl carbons: it is in the fast motion in the the curve can be interpreted in such a way that the PMA case, while it is in the slow motion in the PMANa-6.9 case. At this point, it should be mentioned motion . Fig. 3(a) and (b) shows the T that the matrix glass transition temperatures, observed A.R. Lim, J.-S. Kim / Solid State Communications 115 (2000) 179–183 mechanically dynamic, of the PMA and PMANa-6.9polymers are ca. 23 and 51ЊC, respectively. Thismeans that the polymer chains in the matrix phase inthe PMANa-6.9 ionomer experience more restrictedmobility than those in the PMA homopolymer. Thus,it can be expected that the chains in the ionomerwould show slower motion than those in non-ionic poly-mer, and that is what we observed here.
The T1r values can be related to corresponding values of rotational correlation time, tC . The rotational correla-tion time is the length of time that a molecule remains in agiven state before the molecule reorients. As such, the tC isa direct measure of the rate of motion. For the spin–latticerelaxation time in a rotating frame, the experimental valueof T1r can be expressed in terms of isotropic correlation time tC as a very rough model for the molecular motions by thefollowing function [16,17]: Here, gC and gH are the gyromagnetic ratios for the 13C and 1H nuclei, respectively, N the number of directlybounded atoms, r the internuclear distance, ប h=2pwhere h is the Planck’s constant, vC and v H are the Larmorfrequencies of 13C and 1H, respectively, and v 50 × 2p × 103 rad=s is the spin-lock field. The types ofmotion and values of tC calculated from the Eq. (2) arelisted in Table 1. From the table, it is seen that the tC valuesof the 2-methyl and b-methylene carbons are much higherthan that of the 1-methyl carbons. Also clearly shown is thatthe 1-methyl carbons in the PMA undergo a fast motion asthe correlation time of 1:33 × 10Ϫ8 s whereas those in thePMANa-6.9 undergo a slow motion as the correlation timeof 1:93 × 10Ϫ4 s: The temperature dependence of the relaxation time is a Fig. 3. (a) Temperature dependence of the 13C spin–lattice relaxa- tion time, T1r, for the PMA. (b) Temperature dependence of the 13Cspin–lattice relaxation time, T1r, for the PMANa-6.9.
where Ea is the activation energy for the molecular motion,R is the molar gas constant, and T is the absolute tempera-ture. Thus, a plot of the natural logarithm of relaxation timeas a function of inverse temperature is linear with a slopethat is proportional to the activation energy for motion.
A.R. Lim, J.-S. Kim / Solid State Communications 115 (2000) 179–183 Table 1Activation energies (Ea), types of motion and rotational correlation time (tC) for the PMA and PMANa-6.9 polymers Listed in Table 1 are also the activation energies for all the Acknowledgements
carbons derived from the slope of the straight lines passingthrough the T1r data in the plot. From a comparison between A.R. Lim thanks the Korea Basic Science Institute for the those activation energies, the trends become clear. The acti- support, and J.-S. Kim gratefully acknowledges the inter- vation energies for all the carbons in the PMANa-6.9 are disciplinary Research Program Grant 1999-2-308-002-3.
higher than those measured in the PMA. As was mentionedbefore, the molecular motion of the chains in the PMANa-6.9 clustered ionomer is restricted by the presence of ionic References
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