Temperature changes of poly(oxymethylene) structures studied by infrared spectroscopy

Makromol. Chem. 119, 2315-2322 (1978)

Temperature Changes of Poly(oxymethy1ene) Structures Studied by Infrared Spectroscopy

Leuon Terlemezyan* and Marin Mihailov

Central Laboratory for Polymers, Bulgarian Academy of Sciences, 11 13 Sofia, Bulgaria

Pave1 Schmidt and Bohdan Schneider

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague, Czechoslovakia

(Date of receipt: August 15, 1977)

SUMMARY: Temperature changes of infrared spectra of poly(oxymethy1ene) samples in the hexagonal crystalline

form were measured; in the nascent state, these samples contained either long (type A) or short (type B) sequences of monomeric units in the G conformation. It was found that up to the temperature of “solid crystal-plastic crystal” transition, the original structure of both types remains essentially unchanged. Only in the melt the original structure is completely destroyed. The composition of the melt is the same for both cases.

In our previous paper’) we have shown that infrared spectra indicate not only differences between hexagonal and orthorhombic crystalline poly(oxymethylene), (POM), but also finer differences in the structure of various samples of hexagonal crystalline POM formed in consequence of differences in polymerization conditions293). By polymerization with simultaneous crystallization a hexagonal crystalline polymer is formed with a characteristic infrared band at 903cm-’; this we designated as the A form and proposed’), that it consists of long sequences in regular G conformation. By successive polymerization and crystallization again a hexagonal crystalline polymer is formed, but in this case it is characterized by bands at 980 and 1 130cn- and is designated as form B. We proposed’), that this form consists of short sequences of monomeric units in the G conformation. We have found” that form B is transformed to form A by mechanical treatment.

From dielectric and optical measurements it is known that in POM at 120°C the cr-transition4) and at 160°C the “solid crystal-plastic crystal” (SC-PC) transition’) take place. In transition to the plastic state the density of the crystallites decreases so that it approaches the density of the melt6).

Temperature changes of infrared spectra of POM have been studied by Tadokoro et al.’) and also by Oleinik and E n i k o l ~ p y a n ~ ~ ~ ’ . Tadokoro studied infrared spectra of POM at 150 and 170°C; he described the spectral changes without presenting a structural interpretation, excepting for crystalline band at 1238 cm-‘. Oleinik and Enikolopyan studied the changes of infrared spectra of POM between -78 and 128°C. They interpreted the observed changes as corresponding to a changing population of long sequences of G conformation and sequences of T conformation.

2316 L. Terlemezyan, M. Mihailov, P. Schmidt, and B. Schneider

In this work we have studied the temperature changes of infrared spectra of hexagonal crystalline POM in A and B forms in the temperature range involving the a-transition, the SC-PC transition, and the melting point of the samples, and we have attempted to present a structural interpretation of the temperature changes of infrared spectra.

Experimental Part

Polymer samples 1-3 (see Tab. 1) were obtained by cationic polymerization of 1,3,5-trioxane in nitrobenzene solution with BF3. O(C2Hs)Z ascatalyst. Purification ofstartingproducts and polymerization conditions are described in detail in ref.'). The 1,3,5-trioxane/l,3-dioxolane copolymer (sample 4) was obtained,in a similar way, with 5 mol-% 1,3-dioxolane in the original monomer mixture. We used a fraction of this copolymer which was stable to alkaline hydrolysis and which was isolated after alkaline treatment with water/ethyl alcohol mixture. The studied samples contained 2% by weight of antioxidant.

Tab. 1. Experimental conditions for cationic polymerization of 1,3,5-trioxane with BF3. O(CZH5)2 as catalyst in nitrobenzene and melting temperatures of poly(oxymethy1ene)s from determinations with an optical microscope and by differential scanning calorimetry (DSC)

Sample Initial Conc. of Polym. Melting Temp. of DSC conc. of catalyst temp. range in "C melting peak

oxane opt. mic- DSC in "C in mol/l roscope

no. type") 1,3,5-tri- in mol/l in "C maximum

~

1 A 1,5 1.10-' 60 178-183 172-187 180 2 B 2,o 0,5.10-3 50 176-180 170-188 179

0,5.10-3 60 172-181 - - 3 B 3,o 4 B 0.85 1.10-' 25 163-167 150-180 166

a) Sample type A-poly(oxymethy1ene) with long sequences of monomeric units in G conformation. Samples type B-poly(oxymethy1ene)s with short sequences of monomeric units in G conformation.

The melting range was followed by optical microscopy using the "Reichert-Zetopan" microscope, with average heating speed 2"C/min. The differential scanning calorimetry (DSC) curves were recorded at a heating speed of 5"C/min using the Du Pont 900 Thermal Analyzer. Infrared spectra were recorded on the Perkin Elmer 621 spectrometer in a variable temperature cell. The samples were measured in the form of thin layers of polymer powder between two KBr discs, and also pressed in KBr pellets. Heating speed between subsequent measurements was about 2"C/min; before recording of the spectra, the sample was maintained at the corresponding temperature for 30 minutes.

Results and Discussion

Infrared spectra of the A form of hexagonal crystalline POM measured at various temperatures between two KBr discs are shown in Fig. 1. At room temperature the spectrum of this sample is characterized by a strong band at 903cm-', which was proposed') to be characteristic for the long sequences of the G conformation. Bands of the B form at 980 and 1130cm-' are not detectable in the spectrum at room temperature. Even in the range 120-166"C, i.e.

Temperature Changes of Poly(oxymethy1ene) Structures Studied by Infrared Spectroscopy 2317

Fig. 1. Temperature dependence of infrared spectrum of poly(oxymethylene), type A (sample 1) between two KBr discs (cf. Tab. 1). Curves: 1 : 35°C; 2: 80°C; 3: 120°C; 4: 140°C; 5: 160°C; 6: 166°C; 7: 172°C; 8: 180°C; 9: 190°C; 10: after cooling to 35°C

4

1

2

.- c

g 4 $ 5

10 1

700 900 1100 1300

+

Wave number in cm-'

in the range of the a-transition and beginning of crystal plastication, these bands do not appear in the spectrum. Also the crystallinity characterized by the band at 1238 cm-' ') does not change in this temperature range. The only change observed at these temperatures is the change of the ratio of intensities of the A form band at 903cm-' and of the band at 938cm-' which is common to forms A and B'). Bands of forms B begin to appear in the spectrum only around 172"C, i.e. above the SC-PC transition. At this temperature also the intensity of the crystalline band of the hexagonal form decreases considerably, together with the appearance of a band at 1215cm-'; the frequency of this band corresponds to that of the orthorhombic crystalline form of POM measured at room temperature"). In the melting range above 180°C the band of the hexagonal crystalline form at 1 238 cm- ' disappears completely and only a broad band with a maximum at 1215 cm-' can be observed. Besides this, in the 880-1050 cm-' range the spectrum of the melt exhibits a broad asymmetric band with a maximum at 910 cm- The overall shape of the spectrum of the melt in the 880-1 250 cm- I range can be described as a superposition of broadened bands of the A and B forms of hexagonal crystalline POM and of bands of orthorhombic POM. After cooling of the melt to room temperature, bands of the A and B forms appear in the spectrum, as well as the crystalline band at 1 238 cm- I indicating that the sample crystallized again in the hexagonal crystalline form.

Infrared spectra thus indicate that the nascent A form of POM is very stable and that its transition to form B only begins to take place in the plastic crystalline state. Small changes


of infrared spectra which are observed between the a-transition and the SC-PC transition can be explained by the broadening of some bands in consequence of rapid motions which set in above the a-transition; these motions, however, do not lead to changes of the structure.

Infrared spectra of sample 2, with hexagonal crystalline structure of type B, measured between KBr discs, are shown in Fig. 2. In the room temperature spectrum of this sample, the band

c 0 .- + a L 0 w 2

1 I

700 900 1100 1300 Wave number in cm-'

Fig. 2. Temperature dependence of infrared spectrum of poly(oxymethylene) type B (sample 2) between two KBr discs (cf. Tab. 1). Curves: 1 : 35 "C; 2: 120°C; 3: 160°C; 4: 170°C; 5 : 180°C; 6: after cooling to 35°C

at 903cm-' corresponding to A type of hexagonal POM, is hardly perceptible. In the 9 W 1300cm-' range only bands of the B form (980 and 1 130cm-'), bands common to A and B forms (938 and 1098cm-') and the hexagonal crystalline band (1 238 an-') are prominent. Up to the SC-PC transition, infrared spectra do not change significantly; only the intensity ratio of B form bands and of the bands common to A and B forms, changes slightly. About the SC-PC transition (170°C) the band of the A form at 903cm-' begins to appear, whereas the bands of the B form and the crystalline band at 1238 cm-' begin to decrease. At the same time a new band appears at 1215cm-'. The infrared spectrum of the melt, and the spectrum of the sample cooled to room temperature from the melt are completely identical with the corresponding spectra of sample 1 which was originally of type A (compare Fig. 1, curves 8 and 10, with Fig. 2, curves 5 and 6). Temperature changes of infrared spectra of nascent form B thus indicate that this form is also very stable and its transformation begins to take place only in the plastic crystalline state. Intensity changes of band maxima between the a-transition and the SC-PC transition are probably also caused by the inset of chain motions which do not lead to changes of the structure. A comparison of infrared spectra of A and B forms in the plastic crystalline state indicates that even in this temperature range the structure of the polymer is largely determined by the nascent structure (see Fig.


1, curve 7 and Fig. 2, curve 4). As indicated by spectra of the melt and of the melt-crystallized polymer measured at room temperature for samples 1 and 2, only in the melt the equilibrium state of conformational forms is reached corresponding to the temperature of the melting point, so that the structure of the nascent polymer becomes irrelevant.

Besides sample 2 (B type homopolymer) we also studied the temperature changes of infrared bands of sample 4 (1,3,5-trioxane/l,3-dioxolane copolymer) which also belongs to the B structural type. The temperature changes of infrared spectra of this sample are similar as for sample 2, except that the A form band at 903 cm- begins to appear in the spectra at temperatures lower by about 10°C. This is probably connected with the fact that also the melting range of this polymer lies at slightly lower temperatures.

In our previous papers'.") we have shown that form B is transformed to form A by the effect of pressure at room temperature, so that the structure of POM changes somewhat also by pressing into KBr pellets. Because of this effect, we have also studied the temperature changes of infrared spectra of POM pressed into KBr discs. For these studies it was not possible to obtain a pellet which would contain pure B form at room temperature because during the pressing of pellets some transformation into form A always takes place. Therefore

Fig. 3. Temperature dependence of infrared spectrum of poly(oxymethylene), type A (sample 1) pressed in a KBr pellet (cf. Tab. 1). Curves: 1 : 35°C; 2: 60°C; 3: 120°C; 4: 140°C; 5: 150°C; 6: 160°C; 7: 170°C; 8: 180°C; 9: 190°C; 10: after cooling to 35°C

700 900 1100 1300 Wave number in cm-'


in KBr pellets we compared two samples which differed in the relative contents of A and B forms after pressing.

In Fig. 3 infrared spectra of sample 1 which after pressing into a KBr pellet contained predominantly form A are shown at different temperatures. Infrared spectra of sample 3 with predominating form B are shown in Fig. 4. From Figs. 3 and 4 it is seen that the temperature changes of the bands common for both forms (including the “hexagonal-crystalline’’ band), of A form bands in sample 1 (Fig. 3) and of B form bands in sample 3 (Fig. 4) are similar as in samples 1 (Fig. 1) and 2 (Fig. 2) respectively, measured between KBr discs. Although some differences in intensity ratios can be observed below the SC-PC transition, a significant disappearance of the forms predominating in samples 1 and 3 is only observed at temperatures above the SC-PC transition; but even at these temperatures the measured structure is affected by the nascent structure of the given polymer. Also in the pellets the spectra of both samples are identical in the melt, indicating that at temperatures above the melting range the population of the forms is independent of the nascent structure. Spectra of the melt in pellets differ from the polymer melt between KBr discs by a shift in the maximum of the broad band in the 88&1080crn-’ range by 20cm-’ to higher wave numbers (compare curve 8 (Fig.

C 0 .- - L a

n Q

0 ul

7- 700 900 1100 1300

Wave number in cm-I

Fig. 4. Temperature dependence of infrared spectrum of poly(oxymethy- h e ) , (sample 3) pressed in a KBr pellet (cf. Tab. 1). Curves: 1: 35°C; 2: 120°C; 3: 140”C;4: 160°C; 5: 170°C; 6: 180°C; 7: after cooling to 35°C


1) and curve 5 (Fig. 2) with curves 8 (Fig. 3) and 6 (Fig. 4)). This difference is probably caused by effects of the medium. After cooling of the melt to room temperature, appearance of the band at 1238cm-’ indicates that the polymer again crystallized in the hexagonal form. In both spectra of the cooled melt we also find bands of the A and B forms. However, the population of these two forms differs in the two samples. In sample 1 which originally contained more form A in the pellet, after cooling from the melt the B form predominates more strongly than in sample 3 which originally contained mostly form B (compare curves 10 (Fig. 3) and 7 (Fig. 4)). From this it is evident that the structure of samples melted in pellets is independent of the nascent structure of the polymer, but is determined by the process of cooling of the polymer particles, depending on their size and distribution in the pellet.

It is interesting that in sample 3 (Fig. 4) the weak A form band which appeared in consequence of pellet pressing, gradually disappears during heating and reappears above the SC-PC transition. From the present measurements it cannot be decided if this is caused by a reduced stability of form A formed by pressing as compared to the nascent form A formed during polymerization or if this is due to effects of the medium.

Since for sample pressed into KBr pellets quantitative measurements are more precise than those for the powder between two discs, and since with sample 1 in a KBr pellet the maxima of all bands are sufficiently well pronounced (Fig. 3), we tried to follow the temperature changes of intensities quantitatively. For sample 1, the temperature dependence of the intensities of an A form band and of a B form band, related to the intensity of a band common to both forms, is shown in Fig. 5. From Fig. 5 it is seen that the relative intensities of the band of the A form as well as of the band of the B form do not change up to the temperature of the ?-transition; at higher temperatures the content of form B increases at the expense of form A. This effect is more pronounced at temperatures above the SC-PC transition.

Fig. 5. Temperature dependence of the absorption ratio of the bands at 903 (resp. 980) and 938cm-’ of poly(oxymethylene), type A (sample 1) pressed in a KBr pellet (cf. Tab. 1) I I I I

30 70 110 150 190

Temperature in O C

The observed temperature dependences of the changes of infrared spectra demonstrate, that above the temperature of the SC-PC transition and directly below the melting range the mobility of the chains in the hexagonal crystalline lattice of POM samples strongly increases. At the same time all the observed changes in the spectra agree with the proposal, given in our previous work”, that both the A and B forms correspond to the G conformational structure, with longer and shorter uninterrupted G sequences, respectively. In the melting range of the polymer, differences due to the nascent structure of the samples disappear.


L. Terlemezyan, M. Mihailov, P. Schmidt, B. Schneider, Makromol. Chem. 179, 807 (1978) M. Mihailov, L. Terlemezyan, Dokl. Bolg. Akad. Nauk, 28, 643 (1975); Chem. Abstr. 83, 147926d (1975)

3, M. Mihailov, L. Terlemezyan, Vysokomol. Soedin. Ser. B: 17, 225 (1975) 4, K. Arisawa, K. Tsuge, Y. Wada, Jpn. J. Appl. Phys. 4, 138 (1965) ’) M. Mihailov, E. Nedkov, J. Polym. Sci., Part C, 38, 33 (1972) 6, K. OLeary, P. H. Geil, J. Macromol. Sci., Phys. 1, 147 (1967) ’) H. Tadokoro, M. Kobayashi, Y. Kawaguchi, A. Kobayashi, S. Murahashi, J. Chem. Phys. 38,

*) E. F. Oleinik, N. S. Enikolopyan, Vysokomol. Soedin. 8, 583 (1966) ’) E. F. Oleinik, N. S. Enikolopyan, J. Polym. Sci., Part C, 16, 3677 (1968)

703 (1963)

lo ) V. Zamboni, G. Zerbi, J. Polym. Sci., Part C, 7, 153 (1964) M. Mihailov, L. Terlemezyan, P. Schmidt, Vysokomol. Soedin, Ser. B: 17, 724 (1975)

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Temperature changes of poly(oxymethylene) structures studied by infrared spectroscopy