Macromol. Rapid Commun. 16, 155-159 (1995) 155

Spectroscopic investigation of diacetylene cross-linking in polyamide matrices of different configurations

Genrikh N. Gerasimov; EIena L. Popova, Sergey M. Phomin, Tatyana K Kiryanova, Eduard N. Teleshov

Karpov Institute of Physical Chemistry, Obukha 10, Moscow, Russia, 103064

(Received: June 17, 1994; revised manuscript of November 1 1, 1994)

SUMMARY We compared the cross-polymerization of diacetylene units, incorporated in different ways into

polyamide (PA) matrices: as fragments of the PA main chains (PA1) and as pendant groups of the PA main chains (PA2). At room temperature the reaction proceeds only in PA1 via 1,4-addition (cross-links I). However at high temperature besides of cross-links I another type of cross-links (cross-links 11) is formed, presumably via 1,2-addition reaction. In PA2 only cross-links of type I1 are formed at high temperatures.

Introduction

Many crystalline diacetylene monomers R-CE C-CEC-R are able to poly- merize with the formation of regular polydiacetylene (PDA) chains of 1,4-structure:

This results from the specific “stack-like’’ packing of the monomer molecules in the crystalline lattice, which favours a homogeneous transition of the monomer crystal to the corresponding polymer crystal. This topochemical reaction has been thoroughly investigated I ) . However, it has been shown that some diacetylene monomers are able to polymerize also in the liquid state. Presumably, the reaction in this case proceeds via a 1 ,Zaddition mechanism 2, 3):

It is likely that the same reaction occurs in some diacetylene crystals at elevated temperatures, when the molecular mobility is increased 3,4).

As has been previously demonstrated, the reactive diacetylene groups can be incorporated into different polymeric backbones +R-C=C-C=C-R jn . Cross- polymerization proceeds thermally or under UV or y-irradiation 5 - ’ 0 ) . This reaction permits to modify the optical and mechanical properties of polymeric materials. If diacetylene groups of polymer chains crystallize in a way which is similar to those in diacetylene crystals (with the formation of stacks), crosslinks with a well defined system of 1,4-conjugated bonds arise in the polymeric material without disruption of

Macromol. Rapid Commun. 16, No. 3, March 1995

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156 G. N. Gerasimov, E. L. Popova, S. M. Phomin, T. V. Kiryanova, E. N. Teleshov

packing and order of polymer This reaction can proceed also in less ordered paracrystalline domains of a polymer matrix, but PDA crosslinks which are formed in this case are more disordered (non-planar) than in crystallites, and the ambient polymer matrix is more deformed6*’). Diacetylene groups which are located in amorphous regions of polymers react at elevated temperatures when translation and reorientation motions of polymer segments begin6). In this case the formation of other types of structures is possible. Depending on the crosslink structure, the cross- polymerization of these groups leads to more or less change of the polymer volume and polymer segments packing in amorphous regions. That is why the question about the structure of the formed crosslinks is of principal character for the thermal modification of diacetylene-containing polymers.

The packing and ordering of diacetylene groups in a polymer matrix depend on the configuration of the diacetylene-containing polymer chain segments and interchain interactions. In the work presented we compare the cross-polymerization of polyamides with diacetylene-containing segments of identical chemical structure, but of different configurations. Our first model, PA1 , where diacetylene fragments are incorporated into a rather rigid polymer backbone. In this case strong interchain interactions due to hydrogen bonds favour a highly ordered diacetylene packing, needed for the 1,4-addition reaction (Scheme 1 (a)). For PA2, with diacetylene units in the side chains, such ordering is impossible: the system of hydrogen bonds, which acts between the main chains, does not influence significantly the diacetylene packing in the polymer matrix (Scheme 1 (b)).

Scheme I: \ I

111 I

f I l l - - -- --&,I- . +-

(a) PA1 (b) PA2

Moreover, presumably, high regularity in side chain arrangement can not be achieved for PA2 along the polymer backbone: a diacetylene-containing monomer unit, being asymmetric, can join to another monomer unit in different ways.

Experimental part

solution polycondensation: The diacetylene-containing polyamides, used in this study, were prepared by low temperature

0 0 /I

CI-C-/ \-OCH,-C=C-C=C-CH 0 M I )

Spectroscopic investigation of diacetylene cross-linking in polyamide . . . 157

Details of preparation, purification and characterization of these polyamides are described in ref. 3,

IR spectra were recorded on a Perkin Elmer model 180 infrared spectrophotometer. Resonance Raman (RR) spectra were taken on a "Joben-Ivonve" model U-1000 laser Raman spectrometer using an Ar +-ion laser with 514,5 nm excitation. Differential scanning calorimetry (DSC) measurements were performed on a DSC-4 instrument a t a heating rate of 8 "C/min under argon.

Results and discussion

PA1 and PA2 show an exotherm in the DSC trace nearly in the same temperature region (150-320 "C). Weight losses of polymers, heated to 350 "C, are not higher than 2%. That excludes the possibility of their thermal degradation. In agreement with the spectroscopic investigation of this process we attribute the exhibited exotherms to the exothermic reaction of diacetylene cross-linking.

Diacetylene fragments of initial polymers (PA1, PA2) are characterized by two weak absorption bands: at 2 160 and 2260 cm-' in the IR spectra (Fig. 1 (A)I, (B)I), from asymmetric and symmetric vibrations of CE C bonds respectively. Symmetric C= C vibrations become visible in the IR because of the break of diacetylene group symmetry in the polymer matrix. These vibrations give a strong peak at 2260 cm- ' in the RR. In PA1 , diacetylene cross-polymerization proceeds very slowly also at room temperature: for samples, kept for three months, we can see in the IR spectrum a decrease of the intensity of the initial diacetylene bands, while a new weak band at 2 100 cm- ' develops in the C=C stretching region (Fig. 1 (A)2), which is assigend to C=C bonds in long 1,4-conjugated PDA chains ' I ) (crosslinks I). C=C absorption bands cannot be determined in the IR spectra, because they are not intensive and overlap with strong absorption bands of aromatic and amide groups. In the RR spectrum there are two bands of the formed PDA chains (Fig. 2 (A)I): at 2 100 cm- ' from triple bonds and at 1504 cm-' from C=C bonds. No changes were observed in IR and RR spectra of PA2, kept for 3 months at room temperature (Fig. 1 (B)2, Fig. 2 (B)l).

The diacetylene cross-linking reaction at high temperature (1 50- 320 "C) proceeds in another way. One can observe the gradual appearing of two new C = C absorption bands at 2100 and 2210 cm- ' in the IR spectra of PA1 (Fig. 1 (A)3-5). So, besides of the well known PDA crosslinks I, some more crosslinks with 2210 cm- ' CEC band have been formed as a result of the thermal reaction in PA1 (Fig. 1 (A)5). This type of crosslinks (crosslinks 11) is the only one which forms in the course of thermal cross-polymerization (150-320°C) in PA2 (Fig. 1 (B)3-5). In the RR spectra these crosslinks are characterized by the appearance of a new broad peak in the region of double bonds at about 1600 cm- ' for both polymers (fig. 2 (A)2, (B)2), while the

158 G. N. Gerasimov, E. L. Popova, S. M. Phomin, T. V. Kiryanova, E. N. Teleshov

C 0 ln ln

.-

._ 5 2

t-

- 2 - 2

I I I 1 I I I

- Wave number in cm-' -

L

2300 2200 2100 2000 2300 2200 2 100 2 000

c- Wave number in cm-' - Fig. 1. IR spectra of PA1 (A) and PA2 (B): 1 - initial polymers; 2 - kept for three months at room temperature; 3 - heated to 21 5 "C; 4 - heated to 230 "C and held for 20 min at this tempera- ture

(B)

A' t Z!z!!?L 1700 2000 2200

1

1700 2000 2200 z1 1500 c 2- +- r( al C t 1500 LLl ln lhk2 i n 2 - -- --

1500 1700 2000 2200 1500 1700 2000 2200

- Wave number in cm" - - Wave number in cm" - Resonance Raman (RR) spectra of PA1 (A) and PA2 (B): 1 - kept for three months at Fig. 2.

room temperature; 2 - heated to 230°C and held for 20 min at this temperature

Spectroscopic investigation of diacetylene cross-linking in polyamide . . . 159

peak of triple bonds in the region 2100-2200 cm-I is not visible with the method used.

So, the same diacetylene fragments (with the same neighbour groups), incorporated in different ways into a polyamide (PA) matrix, react differently. As we expected, PDA crosslinks I have been formed only in a matrix with highly ordered diacetylene groups (PA1). The fact that this reaction proceeds also at room temperature (which is much lower than the glass transition temperature of PA1 3), i.e., the mobility of polymer chains is restricted) also confirms that the initial diacetylene packing is favourable for this reaction. In PA matrices with less ordered arrangement of diacetylene groups (with only some short-range order in their arrangement) crosslinks I1 have been formed as the result of diacetylene cross-polymerization. In such matrices (PA2 and disordered regions of PA1) this reaction requires a sufficient transfer of diacetylene units; therefore it becomes possible only at high temperatures, when some mobility of polymer chains is achieved.

In agreement with the obtained spectral data, it is reasonable to suppose that crosslinks I1 have 1,2-PDA structure (see Eq. (2)). In this case the peak at 1600 cm-’ in the RR can be ascribed to C=C bonds in conjugated PDA chains. The rather high, observed frequency may be possibly explained by the fact that this reaction occurs in a matrix with disordered arrangement of diacetylene groups; so the obtained PDA chains can be deformed and have only very short conjugation lengthI2). C=C bonds which are characterized by the band at 2210 cm-’ in the IR spectra are not visible in RR, because in this case they do not belong to the conjugated main chain and there is no resonance enhancement of the Raman line from these bonds. Thus we presume that I3C NMR spectroscopy must be used to solve the question about the nature of the formed structure. Such study is now in progress.

This work was supported by Russian Fund of Fundamental Investigations (Grant No. 2-01 -35-01).

H. Bassler, H. Sixl, V. Enkelman, “Polydiacetylenes’: Adv. Polymer Sci. 63, Springer-Verlag, Berlin 1984

2, A. A. Berlin, M. I . Cherkashin, M. G. Chauser, R. R. Shifrina, Vysokomol. Soedin., Ser. A: 9, 2219 (1967)

3, G. N. Gerasimov, S. M. Phomin, E. L. Popova, E. N. Teleshov, Vysokomol. Soedin., Ser. E: 37, 140 (1995)

4, L. A. Chramtsova, N. V. Kozlova, Z. G. Makarova, V. M. Zjulin, E. N. Teleshov, G. N. Gerasimov, Dokl. Acad. Nauk SSSR, 297, 144 (1987)

5 , G. Wegner, Makromol. Chem. 134, 219 (1970) 6, H. W. Beckman, M. F. Rubner, Macromolecules 22, 2130 (1989) ’) R.-C. Liang, W.-Y. Frank Lai, A. Reiser, Macromolecules 19, 1685 (1986) *) P. A. Lovell, J. L. Stanford, Y.-F. Wang, R. J. Young, Polym. Int. 34, 23 (1994) 9, H. W. Beckham, M. E Rubner, Macromolecules 26, 5192 (1993)

lo) R. Agrawal, M. F. Rubner, Polyrn. Muter. Sci. Eng. 56, 822 (1987) I ’ ) W. F. Lewis, D. N. Batchelder, Chem. Phys. Lett. 60, 232 (1977) ’*) M. L. Shand, R. R. Chance, M. LePostollec, M. Scott, Phys. Rev. B 25, 4431 (1982)