NMR Spectroscopy of Polymers in Solution and in the Solid State

Chapter 30

Local Segmental Dynamics of Polyacrylates in Concentrated Chloroform Solutions 1

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Downloaded by MONASH UNIV on February 25, 2016 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch030

Frank D. Blum and Raj B. Durairaj 1

Department of Chemistry and Materials Research Center, University of Missouri at Rolla, Rolla, MO 65409-0010 Indspec Chemical Corporation, 1010 William Pitt Way, Pittsburgh, PA 15238 2

The behavior of methine-labeled poly(ethyl acrylate)-d (PEA -d ),poly(iso-propyl acrylate)-d (PIPA-d ), and poly(n-butyl acrylate)-d (PNBA-d ) has been studied with deuterium N M R relaxation time measurements in concentrated solutions with chloroform. PEA-d and P N B A - d behaved similarly in terms of solution dynamics, but PIPA-d was found to reorient significantly faster at similar concentrations. The relaxation times were fitted to a log-normal distribution of correlation times and the resulting mean correlation times fit to Arrhenius behavior. The energies of activation were found to increase with increasing concentration from about 6 kJ/mol at lower concentrations to 10-20 kJ/mol from about 40 to 80 wt % polymer. 1

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© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H. N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Molecular motion in polymer solutions can have significant effects on the physical properties of the systems formed from these solutions. For example, the rates of drying polymer films can determine the film properties. We have shown that the drying of a polystyrene film from toluene solutions could be predicted with the knowledge of thermodynamic parameters, plus solvent diffusion data.(i) The ability of polymers to respond to changes in conditions is determined by the ability of the polymer and/or its segments to reorient. Solvent diffusion is also correlated to the segmental motions of the polymer chains.(2) The reason for this correlation appears to be that both molecules are coupled to the same fractional free volume. The characterization of the segmental motions of polymers is difficult, especially in concentrated solutions. One technique that is well suited to study polymer segmental motions is N M R relaxation measurement.^, 4) Studies have mostly focussed on the carbon or proton relaxation behavior at lower concentrations. Dipolar interactions among protons, or protons and C , tie the relaxation phenomena to local motions of polymer segments. Proton and C techniques have been of limited use in more concentrated solutions, which to some extent is the most important regime for the development of many polymer properties. In more concentrated solutions, the overlap of spectral features and/or the complexity of the interactions make extracting motional information difficult, even i f the relaxation measurements can be made. In our laboratory, we have found that the use of deuterium N M R relaxation times could provide a convenient probe of local segmental motions in concentrated solutions. The overlap of different moieties can be avoided through the use of specific labeling. The low natural abundance of deuterium means that only the labeled sites will give any appreciable amount of signal. The moderate size quadrupole moment of the deuteron means that the relaxation of deuterons attached to carbons can be interpreted in terms of the reorientation of the C - D bond vector. Additionally, the relaxation times of deuterons on polymers tend to be quite rapid so that the experiments can be done quickly. In the present study, we report the relaxation of three acrylate polymers in concentrated solutions with chloroform. The polymers were labeled in the methine position so that the backbone motions of the molecules were probed. In order to probe the relaxation, T i and T were used because they cover a wide range of spectra densities for the polymer. We have previously reported the behavior of poly(/so-propyl acrylate)-di (PIPA-di) in chloroform(5) where we showed that the relaxation of the polymer could be probed at very high concentrations with one technique. We previously found that at higher concentrations, the relaxation data could not be adequately fit with existing models. However, the subsequent development and correction o f models allowed us to extend the concentrations where meaningful interpretations could be made.(6) The present study extends this work so that a comparison can be made among a series of polymers in the same family. 1 3

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400

Experimental


The polyacrylates were synthesized with deuterium labels on the methine position on the backbone. The monomelic acrylates were made by the exchange of the methine proton on acrylonitrile with D2O, hydroquinone and CaO, as previously reported for wo-propyl acrylate.(5) The nitrile group was converted to the appropriate ester with the appropriate alcohol and sulfuric acid. Since our first report, a more facile synthesis of these monomers has been reported.(7) The polymerizations were carried out in toluene using azobis-wo-butyronitrile (AIBN) as an initiator. The reaction scheme is given below. D D 2

CH = CHCN 2

) CH = CDCN

RDM

2

>

AJRN

CH = CDCOOR

>-(CH CDCOOR)

2

2

n

-

In the above equation the R group designates the particular acrylate. Ethyl, isopropyl, and Η-butyl acrylates were produced yielding the polymers poly(ethyl acrylate)-di (PEA-di), poly(wopropyl acrylate)-di (PIPA-di) and poly(n-butyl acrylate)-di. For PIPA the viscosity average molecular mass was measured to be 98,000 daltons.(5) The molecular masses of the other polymers were not estimated directly because their Mark-Houwink coefficients were not known, but they had similar intrinsic viscosities. At these molecular weights, we expect the relaxation times to be independent of molecular mass.(i) Deuterium N M R experiments were performed at 13.7 M H z on a J E O L F X 90Q spectrometer. The T i measurements were performed using an inversion recovery sequence and the T * values were estimated from the line widths. Because the resonances are relatively broad for the polymers in concentrated solutions, the T2*s measured were virtually the same as those measured from the Carr-Purcell-Meiboom-Gill (CPMG) method. In this case, measuring the line widths was much faster than measuring the T2's by the C P M G method. Thus the T * measurements were taken to be equivalent to the T2's. 2

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Results and Discussion Relaxation measurements have been made for P E A - d i , P I P A - d i , and P N B A - d i . The Ti and T data were interpreted using a log-normal distribution of correlation times. The relaxation times are determined by: 2

2

2

^ = ^(e qQ/h) [J(co ) χι ζυ 0

+ 4J(2(û )] 0

and


(1)

401 1 3π *> ο ir = —(e qQ/h) [3J(0) 2

2

+ 5J(œ ) + 2 / ( 2 ω ) ] 0

(2)

0

2

where the first term (e qQ/h) is the quadrupole coupling constant, taken to be 170 k H z for an aliphatic C-D, and the J's are the spectral densities at different frequencies. We note that the spectral densities probe frequencies which are on the order of ω (13.7 MHz) for Ti, and ω and 0 (indicative of slower motions) for T2. The spectral density is the relative amount of motion at a given frequency and is the Fourier transform of the autocorrelation function,

NMR Spectroscopy of Polymers in Solution and in the Solid State

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