Solution and Solid-State NMR Spectroscopy of Nylon 6

C h a p t e r 10

Downloaded by UNIV MASSACHUSETTS AMHERST on September 1, 2012 | http://pubs.acs.org Publication Date: November 6, 2001 | doi: 10.1021/bk-2002-0804.ch010

Solution and Solid-State NMR Spectroscopy of Nylon 6-Montmorillonite Clay Nanocomposites 1

Rick D. Davis , William L. Jarrett, and L o n J. Mathias* School of Polymers and High Performance Materials, University of Southern Mississippi, Hattiesburg, M S 39406-0076 C u r r e n t address: 13420 Daventry Way, Apartment D , Germantown, MD 20874 1

Nylon 6/montmorillonite clay nanocomposites were characterized by solution and solid state N M R spectroscopy. Solution C N M R spectroscopy indicates nylon 6 molecular weight is controlled by the concentration of intercalate (ammonium salt of 12-aminolauric acid) and all nylon 6 amine end-groups are protonated and presumably bound to the surface of the clay sheets. It is believed that this interaction between the clay sheets and the polymer chains is what promotes γ crystallite formation over α crystallite. Solid state N C P / M A S N M R spectroscopy indicates that the nanocomposite thermal history dictates the ratio and type of crystallites formed. Solid state H N M R spectral and T results are preliminary but indicate the nanocomposite containing component is unlike the amorphous and crystalline domains of nylon 6. 13

15

2

1

Polymer/clay nanocomposites obtained from surface-treated montmorillonite clay have unexpectedly high property enhancements at low clay content (1-5 wt-%). The improved properties must result from interactions between high surface area clay particles and the polymer. The nature, origin and strength of these interactions are not well understood. In an attempt to better understand the interaction between exfoliated clay and nylon 6, we have applied solution C N M R spectroscopy, solid state N C P / M A S N M R and solid state H N M R spectroscopy to characterization of typical nylon 6/montmorillonite clay 1 3

1 5

© 2002 American Chemical Society

In Polymer Nanocomposites; Krishnamoorti, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

2

117

118 nanocomposites.

Preliminary work and experimental details were published

1 2

previously. '


Results and Discussion of NMR Analysis 1 3

Solution

C N M R Spectroscopy 1 3

Solution C N M R peak assignments of nylon 6/montmorillonite clay nanocomposites were made based on previous publications. A solution C N M R spectrum of a nylon 6/clay nanocomposite is provided in Figure 1 (aliphatic region) and Figure 2 (carbonyl region) with complete peak assignments listed in Table I and structure illustrations provided in Figure 3. The spectra in Figures 1 and 2 are expanded with the trans-amide conformer peaks off scale to show endgroup, e-caprolactam and cis-amide comformer (denoted with "cis") peaks. Number average molecular weight ( M ) and amounts of residual e-caprolactam, acid and amine end-groups, and cis-amide conformers were calculated using relative peak intensities from the solution C N M R spectra. ' Peaks used for calculations were the C H ' s in nylon 6 that were a to amine end-groups, a to acid end-groups, a to the nitrogen of cis-amide linkages, and the C H in residual ecaprolactam that is a to the nitrogen of the amide linkage. Calculated values were ca 5 wt-% residual caprolactam, 1.7 wt-% cis-amide conformers, 1.2 wt-% acid end-groups and 19,100 g/mol molecular weight. These values are similar to those observed for commercial nylon 6. ' Interestingly, no neutral amine end-groups were observed in the solution C N M R nanocomposite spectra, apparently because most or all of amine end-groups were protonated (discussed below). Taking into account the residual e-caprolactam, a theoretical molecular weight of 20,000 g/mol was calculated based on the assumption that every intercalate (protonated 12-aminolauric acid) bound to the clay surface was available as an initiating species; i.e., one intercalate incorporated per polymer molecule. The theoretical molecular weight was very close to the actual value estimated by N M R spectroscopy (19,100 g/mol), suggesting one intercalate is incorporated into every nylon 6 chain and most or all amine end-groups are protonated. Consistent with this, a peak at 38.6 ppm was observed representing the C H alpha to a protonated amine end-group (C0NH3"*), based on previous work. In fact, when a trace of acid was added to a commercial nylon 6 N M R solution, the amine co-CH peak moved upfield from 39.6 ppm to 38.6 ppm, confirming that the 38.6 ppm peak is due to the co-CH of protonated amine endgroups. Moreover, the addition of trace acid to the commercial nylon 6 N M R sample did not alter the chemical shift of the a - C H peak of acid end-groups, also consistent with peak position assignments observed in Figure 1. 3,4,5

1 3

n

1 3

3,4 6

2

2

3 4

1 3

2

3

2

2

2


119


1

1'

2

3 2'

°*NH3+

*C1S

v j

j

j

46

j

44

,

j

,

42

j

40

j—}—|—{—j

38

1—{—j—j—|—j—j—j—j—j—i—{—;—j

36

34

32

30

28

26

24

22

13

Figure 1. Solution C NMR spectra of a nylon 6/clay nanocomposite - expansion of aliphatic region (peak assignments as indicated in Figure 3).

T

T

7',C 0 2 H 7'

185

180

175

170

165

13

Figure 2. Solution C NMR spectra of a nylon 6/clay nanocomposite - expansion of carbonyl region (peak assignments as indicated in Figure 3).


120 T

b)

o

2 ( _ )l'r m

v

/


PC02H o d) ntm*/*

c)

t &C02H

NH2 1 3

Figure 3. Chemical structure and C N M R identifications of a) nylon 6, b) ecaprolactam, c) amine end-group and d) acid end-group. l 3

Table I. Solution C N M R chemical shift values of a nylon 6/clay nanocomposite and residual e-caprolactam. Nylon 6/Clay Nanocomposite trans cis

Residual e-Caprolactam

39.85 43.12 28.81 29.76 26.39 * 36.47 32.03 25.61 24.77 176.41 178.88

43.40 30.52 35.70 23.12 183.44

£%H2

not observed not observed not observed

* * *

G>NH3+

38.56

Solution and Solid-State NMR Spectroscopy of Nylon 6

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