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Crystallinity of ethylene oxide oligomers
European Polymer Journa! Vol. 17. pp. 885 to 893, 1981
0014-3057/81/080885-09502.00/0 Copyright © 1981 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
CRYSTALLINITY OF ETHYLENE OXIDE OLIGOMERS A. MARSHALL, R. C. DOMSZV, H. H. TEO, R. H. MOBBS and C. BOOTH Department of Chemistry, University of Manchester, Manchester M13 9PU England
(Received 15 January 1981) Abstraet--Nonaethylene glycol and pentadecaethylene glycol and their dimethyl ethers have been prepared and characterized, with respect to crystallinity by wide- and small-angle X-ray scattering, Raman scattering, i.r. spectroscopy and differential scanning calorimetry. Wide-angle X-ray scattering is similar to that from high molecular weight poly(ethylene oxide). The crystal habit is lamella. The lamellae are highly crystalline and the surface layers are ordered. Comparison with crystalline poly(ethylene oxide) prepared conventionally, and having a distribution of chain lengths, shows that such samples crystallize into lamellae with disordered surface layers.
difficult to achieve with sample 400. Number-average molecular weights measured by vapour pressure osmometry (H. Knauer & Co., toluene, 60°C) were 590 (600H) and 610 (600M); 380 (400H) and 440 (400M). For all samples, the conversion of hydroxy to methoxy end-groups was better than 99~ (as determined by i.r. [11] and proton magnetic resonance spectroscopy). Halide could not be detected in the samples (by elemental analysis and by acidified methanolic AgNO3). The monodisperse samples were further purified by gel filtration through Sephadex LH-20 (2 x 80 x 2.5 cm i.d. columns, ethanol solvent). Details of this procedure will be published elsewhere. Samples purified in this way contained no detectable oligomeric impurities. They are denoted by superscript prime (e.g. 15H'). All samples were stored in desiccators (silica gel or P205) immediately after distillation (9H, 15H) or after drying whilst molten under vacuum (30-60°C, 10 -3 mm Hg, 15-30 hr).
INTRODUCTION
Low molecular weight poly(ethylene oxide) (PEO) crystallizes into stacked lamellae in which the chains are extended or are folded a small n u m b e r of times [1]. Chain folding is particularly a p p a r e n t in P E O because the polymers available for study have narrow chain length distributions (Xw/X, < 1.2, often < 1.05) and fractionation during crystallization can be minimal. F o r this reason P E O has been used as a model polymer for study of the effect of chain length, chain end-group, crystallization temperature, etc. on chain folding and also the effect of chain folding on thermodynamic and other properties (e.g. Ref [1-9]). It is likely that the crystallinity of P E O is affected by the polydispersity of the samples [3,6]. Consequently it is of interest to study monodisperse PEO. Here we report results obtained for nonaethylene glycol (M = 4 1 4 g m o l - 1 ) and pentadecaethylene glycol (M = 678 g m o l - 1 ) and for their dimethyl ethers. Subsequently we hope to report on monodisperse P E O of higher molecular weight.
Wide-angle X-ray scattering
EXPERIMENTAL
Materials The preparation of nonaethylene glycol and pentadecaethylene glycol has been discussed elsewhere [10]. We denote these materials 9H and 15H respectively, the suffix H being used to indicate hydroxy end-groups. Sample 9H was better than 99 wt% pure: sample 15H contained about 2 wt% of dodecaethylene glycol. 'Polydisperse polyeth2dene glycols were obtained from Shell Chemical Co. (M, = 400, 600, 1000 and 1500g mo1-1) and from Fluka AG (M. = 2 0 0 0 and 3000g mol 1). We denote these samples by molecular weight and suffix H. Gel permeation chromatography (GPC) was used to confirm that the molecular weight distributions of these samples were narrow: e.g. Mw/M n -~ 1.10 (400H, 600H) and Mw/Mn - 1.05 (1000H, 1500H, 2000H and 3000H). A modified Williamson reaction [11] was used to prepare the dimethyl ethers of the glycols. These are denoted by suffix M. Sample 9M was better than 99wt~,,, pure: sample 15M, prepared from a second distillate of 15H, contained about 1 wt~o dodecamer and 2Wt~o octadecareer. GPC was used to confirm that the molecular weight distributions of the methoxy-ended polydisperse samples were similar to those of the starting glycols. In fact this is
Wide-angle X-ray scattering (WAXS) photographs were obtained at room temperature by use of a Debye-Scherrer camera and fine-collimated Ni-filtered CuK~ radiation from a Philips PW 1008 generator operated at 40kV, 20mA. Exposure time was 10hr (Ilford Industrial G or Kodak Kodirex film). WAXS photographs of 9M (liquid at room temperature) were obtained by exposing the sample in a stream of cold N2 ( ~ 0°C) in a Philips PW 1080 flatplate camera (CuK~ radiation, 60 kV, 35 mA, 6 hr exposure at sample-to-film distance of 14 cm). In all cases molten samples were drawn into thin-walled glass capillaries (0.3 ram, i.d.) and crystallized at room temperature (9M at or about 0°C). This operation was carried out in a dry box, and the capillaries were sealed to prevent uptake of moisture. At least two capillaries were filled for each sample. It was clear from the WAXS photographs that the crystallization procedure led to partial orientation of the higher molecular weight materials (M, >~ 1000gmol 1). Films from the Debye Scherrer camera were scanned with a Joyce-Loebl microdensitometer for scattering angle 0 = 5-25 °.
Small-angle X-ray scattering Small-angle X-ray scattering (SAXS) photographs were obtained at room temperature by means of a RigakuDenki slit-collimated camera with Ni-filtered CuK~ radiation from a Philips PW 1130 generator operated at 40 kV, 20mA. Exposure times were usually about 12hr (Ilford Industrial G film, sample to film distance of 25 cm).
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A. MARSHALLet al.
886
salt plates and then crystallized either at room temperature (2000H, 1000H, 15H, 9H) or by cooling (600H, 400H). Spectra were recorded successively at -76°C, by means of a cell similar to that described by Wagner and Hornig [12], at a temperature just above the melting point and at room temperature ~ 20°C. Precautions were taken to exclude moisture.
Table 1. Degree of crystallinity of PEO* measured by WAXS (Xx) and by DSC (xH)
Sample
Xx
XH
2000H 1000H 600H 400H 15H 9H
0.88 0.74 --0.84 0.85
0.85 0.75 0.68 0.54 0.89 0.92
Differential scannino calorimetry
*Xx determined at 20°C; XH determined by heating from room temperature and comparing AHfus (experimental) with AH°us(Tm) calculated from Eqn (3); values of Tm can be found in Table 3. Samples were crystallized as indicated in Table 2, powdered and formed into films 1 mm thick between thin Melinex sheets. Moisture was excluded during preparation and exposure of the films. Samples of n-octadecane and sodium stearate were similarly prepared and were used to calibrate the method. SAXS from samples 15M and 9M was apparent on the photographs obtained by use of Debye-Scherrer (15M) and flat-plate (9M, ~ 0°C) camera as described in the previous section.
Raman spectroscopy Low frequency Raman spectra were obtained at room temperature by use of a Cary 82 spectrometer with a Spectra-Physics 164-06 Ar ion laser (514.5 nm) operated at or about 500mW. Observations were made at 90 ° with respect to the incident beam, and at frequency shifts ranging from 5 to 120 cm-i. Molten samples were drawn into thinwalled capillaries (as described earlier: WAXS) and crystallized as described in Table 2. Samples of 15H and 15M had a fluorescent background of unknown origin which obscured all but the most intense peaks. The samples purified by gel filtration were much less fluorescent.
i.r. Spectroscopy i.r. Spectra of the hydroxy-ended materials were recorded by means of a Perkin-Elmer 237 grating spectrophotometer. Molten samples were spread between warmed Table 2. Lamella spacing and LAM frequency of PEO*
Enthalpies of fusion and melting points were measured by means ofa Perkin-Elmer DSC-1B. Samples were sealed into AI pans, of a type designed to contain volatile materials. Precautions were taken to exclude moisture. Samples were then heated to 70°C and crystallized by rapidly cooling to a temperature (T~) 50K below the melting point. Starting from T~ samples were melted at heating ~ates of 0.5-16 K min-~. Peak areas were obtained by constructing a baseline by extrapolation from temperatures at which the sample was judged to be wholly crystallized or wholly molten. The heat of fusion was calculated by comparison with the peak area generated by melting pure indium. Calibration of the temperature scale was by standard materials with melting points in the range 5-62°C. Allowance was made for thermal lag [13]. Heats of fusion were reproducible to +_5 J g- t, melting points to +IK. RESULTS
Sample purity Low molecular weight P E O is hygroscopic and will imbibe 1-2 Wt~o water (i.e. 20-30 molto) in a short time. This is particularly true of the glycols. The melting range determined by DSC is a useful indicator of sample purity. Figure 1 shows peak width at half-height (AT½) plotted against root heating rate (r ~) a n d extrapolated to zero heating rate. F o r well dried monodisperse oligomers, AT~ (r = 0) ~ 0.15 K, compared with 0.0 K for pure indium a n d 2.1 K for dry 2000H. The value for 2000H p r o b a b l y represents the effects of surface melting and variation in lamella stack composition, and it may well be that these effects contribute to AT~ of the monodisperse samples. Even if AT½ of the monodisperse oligomers is attributed entirely to adventitious moisture, its value (0.15 K) corresponds to a melting point depression of less than 0.1 K.
Wide-angle X-ray scattering Figure 2(a) shows an intensity plot obtained for
Sample
Ix (nm)
11 (nm)
12 (nm)
(cm- 1)
2000H 2000M 1500H I500M 1000H 1000M 15H 15M 9H 9M
12.4 12.8 9.8 9.8 7.0 6.8 -4.7 -2.4
12.7 12.8 9.5 9.7 6.3 6.5 4.3 4.5 2.6 2.8
16.2 16.4 12.1 12.4 8.1 8.3 5.5 5.7 3.3 3.6
9.2 8.5 13.0 11.9 18.0 16.5 31.5 23.0 49.5 --
* Samples 2000H, 2000M, 1500H, 1500M, 15H, 15M, 9H crystallized at room temperature ( ~ 20°C): samples 1000H. 1000M, 9M crystallized at low temperature (~< 0°C). Ix = lamella spacing measured by SAXS at room temperature ( ~ 20°C) (9M at 0°C); 11 = extended chain length for 7/2 helix; 12 = extended chain length for planar zig-zag; 7 = LAM frequency measured by Raman scattering at room temperature (~ 20°C).
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Fig. 1. DSC thermogram peak width at half-height (AT½) vs root heating rate (x/r) for (a) 9H, (b) 2000H and (c) pure indium.
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Fig. 2. Wide-angle X-ray scattering intensity (arbitrary scale) vs Bragg angle (0) for (a) 2000H, (b) 1000H, (c) 9H and (d) 15H.
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O, degree Fig. 3. Wide-angle X-ray scattering intensity (arbitrary scale) vs Bragg angle (0) for 15M. 2000H. This has been compared with the intensities and Bragg angles reported by Takahashi and Tadokoro (Ref. [14], supplementary material) and by Lovinger et al. [15] for high molecular weight PEO, and is practically identical. Samples 2000M and 1000M have almost identical WAXS to that of 2000H. WAXS from 1000H, shown in Fig. 2 (b), is also similar to that from 2000H, though a number of reflections are weaker (notably at 0 < 9°) and one moderately intense reflection at 0 = 19.9° is absent. It is possible to index all these reflections, within a limit of +_ 0.1 °, by the 7/2 helical structure of Takahashi and Tadokoro [14]. Intensity plots for 9H and 15H are shown in Fig. 2 (c, d). Scattering from 15M (Fig. 3) is practically identical to that from 15H, except that a broad peak at 0 = 11.5-12.0 ° is not split into components. In addition a series of lines at small angles are visible in the intensity plot of 15M (see Fig. 3). These correspond to scattering from extended-chain lamellae and will be discussed in the next section. Scattering from 9M, observed in the range 6-13 ° via the flat-plate camera, is similar to that from 15M. WAXS from the oligomers shows generally similar features to those from polydisperse PEO (Fig. 2). Peaks in the WAXS of the oligomers are shifted slightly (~ 0.1 °) to higher 0. The broad peak at 11.5-12.0 °, found for all the polydisperse samples, may either be split into components (15H or 9H) or not (15M, 9M). Reflections in the range 0 = 6-9 ° are not detected (15H, 9H) or are detected only at 0 = 6.58 and 6.83° (9M, possibly 15M but obscured by the lamella scattering). WAXS from high molecular weight PEO held under tension in a planar zig-zag crystal structure has been described by Tadokoro et al. [16]. They report very strong scattering at 0 = 10.25, 12.2 and 12.7° and strong scattering at several angles in the range 19 < 0 < 31 °. The scattering from our oligomers is inconsistent with these results. The WAXS intensity plots of the hydroxy-ended
samples were analysed in a simple way [17] to obtain a numerical indication of the crystallinity of the sample. With the aid of measurements of WAXS from empty glass capillaries and from capillaries filled with liquid PEO (400H), the total area (A) under the intensity plot was separated into scattering from glass (Ag), non-crystalline (Aa) and crystalline polymer. Thus a degree of crystallinity (X) was calculated from X x = { A - (A, + A : ) } / ( A - As).
(1)
Averages of at least two determinations are listed in Table 1. Small-angle X - r a y scattering
Two to four orders of small-angle Bragg scattering were observed for the polydisperse samples (2000H, 2000M, 1000H, 1000M), in accordance with earlier observations [-1,3]. Very much sharper peaks were observed for samples 15M and 9M and the higher orders were clearly visible at small angles in the WAXS photographs. (See, for example, Fig. 3 wherein the SAXS peaks are attributable to scattering orders 3 6 from a spacing of 4.7nm). Bragg peaks up to order 8 are visible on the photograph. Spacings, calculated by direct application of Bragg's Law, are listed in Table 2. Comparison is made with the extended chain length anticipated for a 7/2 helix [14] (0.0928 nm per chain atom) and a planar zig-zag [16] (0.1187nm per chain atom). The SAXS from 15M and the polydisperse samples is consistent with stacked lamella crystals in which extended oxyethylene chains in 7/2 helical conformation are normal to the lamella end surface. The SAXS from 9M is less easily explained: the spacing corresponds to an extended helical chain tilted some 30° to the lamella end surface normal. SAXS was not observed from samples 15H and 9H, even after 72hr exposure in the Rigaku-Denki camera. An attempt was made to swell solid 15H by crystallizing a mixture of 95Wt~o 15H and 5wtVo triethylene glycol. Triethylene glycol is known to
889
Crystallinity of ethylene oxide oligomers enter the interlamella regions of solid 2000H when a mixture of the two materials is rapidly crystallized 1-18, 19] and to enhance SAXS. In the event SAXS was not observed from the mixture.
Raman scattering The Raman spectra of 1500H, 9H, 15H', 1500M and 15M' are shown in Fig. 4. All spectra show a broad peak near 80cm -1, which also appears in higher molecular weight PEO (6000H, 20,000H) and is assigned [20] to a torsional mode of the PEO helix. The peaks at 12-13 cm x in 1500H and 1500M can be assigned [-21] to a longitudinal acoustic mode (LAM) of the lamella crystal on the basis of an investigation of the effect of chain length and lamella spacing (l~) on frequency [21]. (LAM frequencies for all the polydisperse PEO samples investigated in the present work are listed in Table 2.) The prominent peaks in the low frequency region of the spectra of the I
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Fig. 4. Low frequency Raman spectra, recorded at sample temperature 20C, of(a) 1500H, (b) 9H, (c} 15H', (d) 1500M and (e) 15M'. Spectra illustrate peak positions: scattering intensities and background scattering corrections are not standardized. crystalline oligomers can be assigned to the same mode on the basis of the plot of frequency against reciprocal chain length shown in Fig. 5. The end effect apparent in the data (Table 2, Fig. 5) is similar to that reported by Rabolt [22] in a comparison of the LAM frequencies of crystalline n-C19H40 and n-CxsH3vOH.
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i.r. Spectroscopy Over the frequency range 3000-800 cm-1, the low temperature ( - 76°C) spectra of the monodisperse oligomers are very similar to those of the polydisperse polymers. Discernible differences between spectra are attributable to changes in chain length. The low frequency (1200-800cm -a) spectra of 9H and 15H obtained at - 7 6 ° C are shown in Fig. 6. Matsuura and Miyazawa [23] have studied ethylene glycol oligomers (x = 1-7) and polydisperse PEO (6000H), and have described band progressions at 970-930cm 1 and 870-830cm-1 in the oligomers which converge as chain length increases towards band groups at 963, 947 and 844 cm -~. These bands are apparent in our spectra (Fig. 6). They are assigned [23] to hybridized modes (CH2 rocking, C-O stretch-
A. MARSHALLet al.
890
in the published [23] low temperature spectra of lower oligomers. The band at 3450-3390cm -1, apparent in spectra of all samples at room temperature (Fig. To) and in the spectra of 2000H and 1000H at - 7 6 ° C (Fig. 7c), is assigned to O - H stretching in a disordered surface layer. This absorption is also found in the low temperature spectrum of 15H (Fig. 7c, weak tail) and 600H, 400H (Fig. 7c, overlapping the crystal bands).
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Differential scanning calorimetry '
01.2
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01.4
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Fig. 5. LAM frequency from Raman scattering (v) vs reciprocal chain length (l1:7/2 helix) for (O) hydroxy-ended and (O) methoxy-ended PEO. ing) and are associated with a trans-gauche-trans sequence of bonds O--CH2-CH2-O. Over the frequency range 3600-3000 cm-1 (O-H stretching) the spectra of the crystalline oligomers show significant changes from those of the polydisperse polymers. Spectra corrected to equivalent film thickness and equivalent incident radiation intensity by reference to the C - H stretching band at 2890-2370 cm-1, are shown in Fig. 7. The spectra of the glycols purified by gel filtration (9H', 15H') are practically identical to those of 9H and 15H depicted in Fig. 7. Following Matsuura and Miyazawa [23] the band at 3480 to 3440 c m - 1 (Fig. 7a) is assigned to O - H stretching in the melt and the band at 3220-3180 c m - t (Fig. 7b, c) to O - H stretching in the crystal. The band at 3320--3310 c m - t in the low temperature spectrum of 9H (Fig. 7c), discernible as a shoulder in the spectrum of 15H, is also assigned to O - H stretching in the crystal. This band can be seen
Thermograms of the monodisperse nonamers are compared with those of the corresponding polydisperse samples (400H, 400M) in Fig. 8. We have already commented upon the narrower melting ranges of the monodisperse materials. Compared to 400H, the thermogram of 400M is markedly different in shape (Fig. 8). Thermograms of 600H and 600M differ in a similar way. This effect was not investigated further. We note that values of Tm based on peak position will be affected by the difference in shape. Melting points and heats of fusion are listed in Table 3. The values reported for the very pure pentadecaethylene glycol (sample 15H') are a little higher than those reported earlier [10, 24]. A crystallinity parameter XH was calculated from XH = AHf,s/AHf°s
(2)
where AH°us, the heat of fusion of perfectly crystalline PEO, can be calculated from [25] AHf0us/jg - 1 = 178.6 + 0.629 (T/°C) - 2.83 x 10-3(T/°C) 2. (3) Values of XH calculated from Anf°s(Tm) and the measured AHf, s are listed in Table 1.
(a)
(b)
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1200
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Fig. 6. i.r. Spectra (1200-800cm-~), recorded at sample temperature -76°C, of (a) 15H and (b) 9H. Transmittances (arbitrary scale) are not standardized.
891
Crystallinity of ethylene oxide oligomers (b)
(o)
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Fig. 7. i.r. Spectra (3750-3000cm-1) of glycols recorded at sample temperatures (a) above their melting points, (b) at 20°C and (c) at -76°C. From top to bottom, the spectra are in order 2000H, 1000H, 15H, 9H, 600H, 400H. The latter two samples are liquid at 20°C and their spectra are as in (a). Transmittance (arbitrary scale) is corrected to equivalent film thickness and incident radiation intensity by reference to the C-H stretching peak at 2890-2870cm-1.
2;0
2;0
2~o
2~o
The crystal structure established for PEO is the 7/2 helix with a trans-gauche-trans series of bonds O-CH2-CH2-O [14]. WAXS from the melt crystallized monodisperse oligomers differs from WAXS from higher molecular weight PEO, but can be indexed by the 7/2 helix. A planar zig-zag (all trans) structure can be ruled out. Confirmatory evidence for the assignment of an essentially 7/2 helical structure to the crystalline oligomers comes from the following results. (a) The observation of a band at 80 cm-1 in the low frequency Raman spectra. This has been assigned [20] to a torsional mode of the PEO helix and is found in all our samples. (b) The similarity of the i.r. spectra of the monodisperse oligomers (9H, 15H) and of higher molecular weight PEO and, in particular, the identification of bands in the spectra (Fig. 6) assigned [23] to modes of the 7/2 helix. The crystal habit established for low molecular weight PEO is lamella. This was established initially by SAXS [1] but has been confirmed by other techniques [4] including Raman scattering [19]. In the case of 9M and 15M, our evidence from SAXS confirms the lamella habit of these oligomers. In the case of 9H and 15H, SAXS was not observed and the assignment to a lamella habit rests immediately upon the Raman spectra, through Fig. 5, and upon i.r. spectra in the O-H stretching region, a discussion of which follows.
Lamella end-surface Our interpretation of the i.r. spectra shown in Fig. 7 (0-H stretching) is that the end-groups of the monodisperse glycols are either wholly ordered (9H) or almost so (15H) at - 7 6 ° C and largely ordered
2~o
2~o
2~o
T~ K
DISCUSSION
Crystal structure and habit
2~o
T,K
Fig. 8. Thermograms by DSC for (a) 9H, (b) 9M, (c) 400H and (d) 400M. Temperature scales are uncorrected: the power scale is arbitrary. Note that the temperature scale for 9H and 9M is twice that for 400H and 400M. (9H, 15H) at 20°C. The end-groups of the lower polydisperse glycols (400H, 600H, 1000H) are partially ordered at - 7 6 and 20°C, and those of 2000H are largely disordered even at -76°C. Hence the lamellae of the monodisperse glycols can be characterized as highly crystalline with chain ends in crystallographic array, while those of the polydisperse glycols can be characterized as partially crystalline with many chain ends (practically all chain ends in the case of 2000H) in a non-crystalline surface layer. No SAXS is observed from crystalline 9H and 15H, whereas SAXS is easily observed from 1000H and 2000H. The scattering from the polydisperse PEO can be attributed to an electron density deficiency in noncrystalline lamella surface layers. The absence of scattering from 9H and 15H implies that there is no corresponding electron density deficiency in the interTable 3. Enthalpy of fusion (AHr~s)and melting point (Tin) of PEO measured by DSC
Sample 2000 1000 600 400 15' 9'
OH ended AHfus Tm (j g- x) (°C) 174 148 131 100 183 179
53.6 37.0 24.0 8.0 40.0 30.0
OMe ended AHf,s Tm (j g- 1) (°C) 168 140 122 99 171 168
53.6 36.5 21.0 6.0 28.5 16.0
A. MARSHALLet al.
892 (a)
(b)
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x, chain units
Fig. 9. (a) Chain length distribution, n(x), of PEO (~. = 42 chain units, Xw/Xn= 1.04). The values of n(x) are relative to n(70)= 1. The dashed curve represents the experimentally (GPC) determined n(x). (b) Representation of a corresponding random set of unfolded chains in a "lamella" array. lamella region of these solids, in keeping with ordered hydrogen bonding and a crystallographic correspondence of adjacent lamella surfaces. Only when methoxy end-groups are substituted for hydroxy (in 9M and 15M) is SAXS apparent. The observation that 15H cannot be swollen by triethylene glycol is consistent with the sample containing highly crystalline lamellae, since swelling is attributable [19] to dilution of a non-crystalline layer. The crystallinities (X) of the monodisperse oligomeric glycols have been estimated from WAXS and DSC (Table 1) to be about 0.85-0.90 that of perfectly crystalline PEO. Polydisperse glycols of comparable M, (400H, 600H) are liquid at 25°C, and have X in the range 0.5-0.7 at lower temperatures. The lower value of X found for the lower polydisperse glycols can bc attributed to the effect of non-crystalline lamclla-surface layers.
Effect of end-group on thermodynamic properties Compared with the corresponding glycols, the dimethyl ethers have lower values of AHfu~ (Table 3). For polydisperse samples this has been attributed [8, 25-1 to rejection of the methyl end-groups from the oxyethylene crystals. For monodisperse samples, endgroups may be crystallographically ordered at the lamella surface; an explanation of the effect must include the different heats of interaction of hydroxy and methoxy end-groups localized at the lamella surface. Compared with the corresponding glycols, the dimethyl ethers of the polydisperse samples have similar melting points (Table 3, Fig. 8) but the dimethyl ethers of the monodisperse samples have melting points reduced by 11--14K. The environment of a chain end in a disordered surface layer is similar to that in the melt and, consequently, changes in melting point due to changes in end-group would not be expected for the polydisperse samples. The environment of a chain end at a crystalline lamella surface is markedly different from that in the melt and, consequently, a change in melting point is likely. A full
analysis of the melting points and enthalpies of fusion will be possible only when results are available for monodisperse samples covering a wider molecular weight range.
Concluding remarks It is to be expected that the crystallization of short chains into lamellae will be affected by a chain length distribution in the sample even though, in conventional terms, the chain length distribution is very narrow. This point is illustrated in Fig. 9 where (a) the chain length distribution of a PEO sample I-6] (~, = 42, ~,/~, = 1.04) is transformed via a histogram of six species (as shown) into (b) a random set of unfolded chains in a 2-dimensional "lamella" array. The result of incorporating such chains randomly into a lamella crystal is a disordered lamella surface layer. It is known that fractionation can occur during the crystallization of low molecular weight PEO 1-26]. Indeed macroscopic fractionation during slow crystallization of low molecular weight PEO (M, ~< 1000g mo1-1) is difficult to avoid and crystallization is usually carried out well below Tm in order to minimize its effccts. In consideration of this and other evidence, it has been suggested 1,5] that low molecular weight PEO chains segregate during crystallization to form lamellae in which chain ends are paired at the surface. The evidence presented here, particularly the comparisons made between the crystalline solids formed from the two kinds of PEO (monodisperse and polydisperse), indicates that random incorporation of chains into lamellae is a sensible model for the rapid crystallization of low molecular weight PEO.
Acknowledgements~We thank Mr R. S. Alexander, Mr R. Beddoes, Mr D. Farnsworth, Mr D. J. Roy and Mr T. G. E. Swales for help with the experimental work. Dr C. Price and Dr J. F. Rabolt gave helpful advice. The Science Research Council provided financial support.
Crystallinity of ethylene oxide oligomers REFERENCES 1. J. P. Arlie, P. A. Spegt and A. E. Skoulios, Makromolek. Chem. 99, 160 (1966). 2. P. A. Spegt, Makromolek. Chem. 140, 167 (1970). 3. D. R. Beech, C. Booth, D. V. Dodgson, R. R. Sharpe and J. R. S. Waring, Polymer 13, 73 (1972). 4. A. J. Kovacs and A. Gonthier, Kolloid Z. Z. Polym. 250, 530 (1972t. 5. C. P. Buckley and A. J. Kovacs, Proy. Colloid Polym. Sci. 58, 44 (1975). 6. P. C. Ashman and C. Booth, Polymer 16, 829 (1975). 7. C. P. Buckley and A. J. Kovacs, Colloid Polym. Sci. 254, 695 (1976). 8. D. R. Cooper, Y.-K. Leung, F. Heatley and C. Booth, Polymer 19, 309 (1978). 9. C. Booth, R. C. Domszy and Y.-K. Leung, Makromolek. Ciwm. 180, 2765 (19791. 10. A. Marshall, R. H. Mobbs and C. Booth, Eur. Polym. J. 16, 881 (1980}. 11. D. R. Cooper and C. Booth, Polymer '18, 164 (1977). 12. E. L. Wagner and D. L. Hornig, J. chem. Phys. 18, 296 (1950}. 13. J. L. McNaughton and C. T. Mortimer, Int. Rev. Sci., Phys. Chem. Ser. 2 10, 1 (1975).
893
14. Y. Takahashi and H. Tadokoro, Macromolecules 6, 672 (1973). 15. A. J. Lovinger, C.-M. Lau and C. C. Gryte, Polymer 17, 581 (1976). 16. Y. Takahashi, I. Sumita and H. Tadokoro, J. Polym. Sci., Polym. Phys. Ed. 11, 2113 (1973). 17. R. L. Miller, Encyclopedia in Polymer Science and Technology, Vol. 4, p. 449. Wiley, New York (1966). 18. J. Terrisse, A. Mathis and A. Skoulios, Makromolek. Chem. 119, 219 (1968). 19. A. J. Hartley, Y.-K. Leung, J. McMahon, C. Booth and I. W. Shepherd, Polymer 18, 336 (1977). 20. J. F. Rabolt, K. W. Johnson and R. N. Zitter, J. chem. Phys. 61,504 (1974). 21. A. J. Hartley, Y.-K. Leung, C. Booth and I. W. Shepherd, Polymer 17, 354 (1976). 22. J. F. Rabolt, J. Polym. Sci. Polym. Phys. Ed. 17, 1457 (1979). 23. H. Matsuura and T. Miyazawa, Spectrochim. Acta 23A, 2443 (1967). 24. C. Booth, Faraday Disc. Chem. Soc. 68, 411 (1980). 25. R. C. Domszy, R. H. Mobbs, Y.-K. Leung, F. Heatley and C. Booth, Polymer 20, 1204 (1979). 26. B. Gilg and A. E. Skoulios, Makromolek. Chem. 140, 149 (1971).
0014-3057/81/080885-09502.00/0 Copyright © 1981 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
CRYSTALLINITY OF ETHYLENE OXIDE OLIGOMERS A. MARSHALL, R. C. DOMSZV, H. H. TEO, R. H. MOBBS and C. BOOTH Department of Chemistry, University of Manchester, Manchester M13 9PU England
(Received 15 January 1981) Abstraet--Nonaethylene glycol and pentadecaethylene glycol and their dimethyl ethers have been prepared and characterized, with respect to crystallinity by wide- and small-angle X-ray scattering, Raman scattering, i.r. spectroscopy and differential scanning calorimetry. Wide-angle X-ray scattering is similar to that from high molecular weight poly(ethylene oxide). The crystal habit is lamella. The lamellae are highly crystalline and the surface layers are ordered. Comparison with crystalline poly(ethylene oxide) prepared conventionally, and having a distribution of chain lengths, shows that such samples crystallize into lamellae with disordered surface layers.
difficult to achieve with sample 400. Number-average molecular weights measured by vapour pressure osmometry (H. Knauer & Co., toluene, 60°C) were 590 (600H) and 610 (600M); 380 (400H) and 440 (400M). For all samples, the conversion of hydroxy to methoxy end-groups was better than 99~ (as determined by i.r. [11] and proton magnetic resonance spectroscopy). Halide could not be detected in the samples (by elemental analysis and by acidified methanolic AgNO3). The monodisperse samples were further purified by gel filtration through Sephadex LH-20 (2 x 80 x 2.5 cm i.d. columns, ethanol solvent). Details of this procedure will be published elsewhere. Samples purified in this way contained no detectable oligomeric impurities. They are denoted by superscript prime (e.g. 15H'). All samples were stored in desiccators (silica gel or P205) immediately after distillation (9H, 15H) or after drying whilst molten under vacuum (30-60°C, 10 -3 mm Hg, 15-30 hr).
INTRODUCTION
Low molecular weight poly(ethylene oxide) (PEO) crystallizes into stacked lamellae in which the chains are extended or are folded a small n u m b e r of times [1]. Chain folding is particularly a p p a r e n t in P E O because the polymers available for study have narrow chain length distributions (Xw/X, < 1.2, often < 1.05) and fractionation during crystallization can be minimal. F o r this reason P E O has been used as a model polymer for study of the effect of chain length, chain end-group, crystallization temperature, etc. on chain folding and also the effect of chain folding on thermodynamic and other properties (e.g. Ref [1-9]). It is likely that the crystallinity of P E O is affected by the polydispersity of the samples [3,6]. Consequently it is of interest to study monodisperse PEO. Here we report results obtained for nonaethylene glycol (M = 4 1 4 g m o l - 1 ) and pentadecaethylene glycol (M = 678 g m o l - 1 ) and for their dimethyl ethers. Subsequently we hope to report on monodisperse P E O of higher molecular weight.
Wide-angle X-ray scattering
EXPERIMENTAL
Materials The preparation of nonaethylene glycol and pentadecaethylene glycol has been discussed elsewhere [10]. We denote these materials 9H and 15H respectively, the suffix H being used to indicate hydroxy end-groups. Sample 9H was better than 99 wt% pure: sample 15H contained about 2 wt% of dodecaethylene glycol. 'Polydisperse polyeth2dene glycols were obtained from Shell Chemical Co. (M, = 400, 600, 1000 and 1500g mo1-1) and from Fluka AG (M. = 2 0 0 0 and 3000g mol 1). We denote these samples by molecular weight and suffix H. Gel permeation chromatography (GPC) was used to confirm that the molecular weight distributions of these samples were narrow: e.g. Mw/M n -~ 1.10 (400H, 600H) and Mw/Mn - 1.05 (1000H, 1500H, 2000H and 3000H). A modified Williamson reaction [11] was used to prepare the dimethyl ethers of the glycols. These are denoted by suffix M. Sample 9M was better than 99wt~,,, pure: sample 15M, prepared from a second distillate of 15H, contained about 1 wt~o dodecamer and 2Wt~o octadecareer. GPC was used to confirm that the molecular weight distributions of the methoxy-ended polydisperse samples were similar to those of the starting glycols. In fact this is
Wide-angle X-ray scattering (WAXS) photographs were obtained at room temperature by use of a Debye-Scherrer camera and fine-collimated Ni-filtered CuK~ radiation from a Philips PW 1008 generator operated at 40kV, 20mA. Exposure time was 10hr (Ilford Industrial G or Kodak Kodirex film). WAXS photographs of 9M (liquid at room temperature) were obtained by exposing the sample in a stream of cold N2 ( ~ 0°C) in a Philips PW 1080 flatplate camera (CuK~ radiation, 60 kV, 35 mA, 6 hr exposure at sample-to-film distance of 14 cm). In all cases molten samples were drawn into thin-walled glass capillaries (0.3 ram, i.d.) and crystallized at room temperature (9M at or about 0°C). This operation was carried out in a dry box, and the capillaries were sealed to prevent uptake of moisture. At least two capillaries were filled for each sample. It was clear from the WAXS photographs that the crystallization procedure led to partial orientation of the higher molecular weight materials (M, >~ 1000gmol 1). Films from the Debye Scherrer camera were scanned with a Joyce-Loebl microdensitometer for scattering angle 0 = 5-25 °.
Small-angle X-ray scattering Small-angle X-ray scattering (SAXS) photographs were obtained at room temperature by means of a RigakuDenki slit-collimated camera with Ni-filtered CuK~ radiation from a Philips PW 1130 generator operated at 40 kV, 20mA. Exposure times were usually about 12hr (Ilford Industrial G film, sample to film distance of 25 cm).
885
A. MARSHALLet al.
886
salt plates and then crystallized either at room temperature (2000H, 1000H, 15H, 9H) or by cooling (600H, 400H). Spectra were recorded successively at -76°C, by means of a cell similar to that described by Wagner and Hornig [12], at a temperature just above the melting point and at room temperature ~ 20°C. Precautions were taken to exclude moisture.
Table 1. Degree of crystallinity of PEO* measured by WAXS (Xx) and by DSC (xH)
Sample
Xx
XH
2000H 1000H 600H 400H 15H 9H
0.88 0.74 --0.84 0.85
0.85 0.75 0.68 0.54 0.89 0.92
Differential scannino calorimetry
*Xx determined at 20°C; XH determined by heating from room temperature and comparing AHfus (experimental) with AH°us(Tm) calculated from Eqn (3); values of Tm can be found in Table 3. Samples were crystallized as indicated in Table 2, powdered and formed into films 1 mm thick between thin Melinex sheets. Moisture was excluded during preparation and exposure of the films. Samples of n-octadecane and sodium stearate were similarly prepared and were used to calibrate the method. SAXS from samples 15M and 9M was apparent on the photographs obtained by use of Debye-Scherrer (15M) and flat-plate (9M, ~ 0°C) camera as described in the previous section.
Raman spectroscopy Low frequency Raman spectra were obtained at room temperature by use of a Cary 82 spectrometer with a Spectra-Physics 164-06 Ar ion laser (514.5 nm) operated at or about 500mW. Observations were made at 90 ° with respect to the incident beam, and at frequency shifts ranging from 5 to 120 cm-i. Molten samples were drawn into thinwalled capillaries (as described earlier: WAXS) and crystallized as described in Table 2. Samples of 15H and 15M had a fluorescent background of unknown origin which obscured all but the most intense peaks. The samples purified by gel filtration were much less fluorescent.
i.r. Spectroscopy i.r. Spectra of the hydroxy-ended materials were recorded by means of a Perkin-Elmer 237 grating spectrophotometer. Molten samples were spread between warmed Table 2. Lamella spacing and LAM frequency of PEO*
Enthalpies of fusion and melting points were measured by means ofa Perkin-Elmer DSC-1B. Samples were sealed into AI pans, of a type designed to contain volatile materials. Precautions were taken to exclude moisture. Samples were then heated to 70°C and crystallized by rapidly cooling to a temperature (T~) 50K below the melting point. Starting from T~ samples were melted at heating ~ates of 0.5-16 K min-~. Peak areas were obtained by constructing a baseline by extrapolation from temperatures at which the sample was judged to be wholly crystallized or wholly molten. The heat of fusion was calculated by comparison with the peak area generated by melting pure indium. Calibration of the temperature scale was by standard materials with melting points in the range 5-62°C. Allowance was made for thermal lag [13]. Heats of fusion were reproducible to +_5 J g- t, melting points to +IK. RESULTS
Sample purity Low molecular weight P E O is hygroscopic and will imbibe 1-2 Wt~o water (i.e. 20-30 molto) in a short time. This is particularly true of the glycols. The melting range determined by DSC is a useful indicator of sample purity. Figure 1 shows peak width at half-height (AT½) plotted against root heating rate (r ~) a n d extrapolated to zero heating rate. F o r well dried monodisperse oligomers, AT~ (r = 0) ~ 0.15 K, compared with 0.0 K for pure indium a n d 2.1 K for dry 2000H. The value for 2000H p r o b a b l y represents the effects of surface melting and variation in lamella stack composition, and it may well be that these effects contribute to AT~ of the monodisperse samples. Even if AT½ of the monodisperse oligomers is attributed entirely to adventitious moisture, its value (0.15 K) corresponds to a melting point depression of less than 0.1 K.
Wide-angle X-ray scattering Figure 2(a) shows an intensity plot obtained for
Sample
Ix (nm)
11 (nm)
12 (nm)
(cm- 1)
2000H 2000M 1500H I500M 1000H 1000M 15H 15M 9H 9M
12.4 12.8 9.8 9.8 7.0 6.8 -4.7 -2.4
12.7 12.8 9.5 9.7 6.3 6.5 4.3 4.5 2.6 2.8
16.2 16.4 12.1 12.4 8.1 8.3 5.5 5.7 3.3 3.6
9.2 8.5 13.0 11.9 18.0 16.5 31.5 23.0 49.5 --
* Samples 2000H, 2000M, 1500H, 1500M, 15H, 15M, 9H crystallized at room temperature ( ~ 20°C): samples 1000H. 1000M, 9M crystallized at low temperature (~< 0°C). Ix = lamella spacing measured by SAXS at room temperature ( ~ 20°C) (9M at 0°C); 11 = extended chain length for 7/2 helix; 12 = extended chain length for planar zig-zag; 7 = LAM frequency measured by Raman scattering at room temperature (~ 20°C).
4 (b) ,..,.o,-,~
0
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2
3
V ~ ' / ~ I K rain "1)
Fig. 1. DSC thermogram peak width at half-height (AT½) vs root heating rate (x/r) for (a) 9H, (b) 2000H and (c) pure indium.
I
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I
5
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I
(d)
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Fig. 2. Wide-angle X-ray scattering intensity (arbitrary scale) vs Bragg angle (0) for (a) 2000H, (b) 1000H, (c) 9H and (d) 15H.
I
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O, degree Fig. 3. Wide-angle X-ray scattering intensity (arbitrary scale) vs Bragg angle (0) for 15M. 2000H. This has been compared with the intensities and Bragg angles reported by Takahashi and Tadokoro (Ref. [14], supplementary material) and by Lovinger et al. [15] for high molecular weight PEO, and is practically identical. Samples 2000M and 1000M have almost identical WAXS to that of 2000H. WAXS from 1000H, shown in Fig. 2 (b), is also similar to that from 2000H, though a number of reflections are weaker (notably at 0 < 9°) and one moderately intense reflection at 0 = 19.9° is absent. It is possible to index all these reflections, within a limit of +_ 0.1 °, by the 7/2 helical structure of Takahashi and Tadokoro [14]. Intensity plots for 9H and 15H are shown in Fig. 2 (c, d). Scattering from 15M (Fig. 3) is practically identical to that from 15H, except that a broad peak at 0 = 11.5-12.0 ° is not split into components. In addition a series of lines at small angles are visible in the intensity plot of 15M (see Fig. 3). These correspond to scattering from extended-chain lamellae and will be discussed in the next section. Scattering from 9M, observed in the range 6-13 ° via the flat-plate camera, is similar to that from 15M. WAXS from the oligomers shows generally similar features to those from polydisperse PEO (Fig. 2). Peaks in the WAXS of the oligomers are shifted slightly (~ 0.1 °) to higher 0. The broad peak at 11.5-12.0 °, found for all the polydisperse samples, may either be split into components (15H or 9H) or not (15M, 9M). Reflections in the range 0 = 6-9 ° are not detected (15H, 9H) or are detected only at 0 = 6.58 and 6.83° (9M, possibly 15M but obscured by the lamella scattering). WAXS from high molecular weight PEO held under tension in a planar zig-zag crystal structure has been described by Tadokoro et al. [16]. They report very strong scattering at 0 = 10.25, 12.2 and 12.7° and strong scattering at several angles in the range 19 < 0 < 31 °. The scattering from our oligomers is inconsistent with these results. The WAXS intensity plots of the hydroxy-ended
samples were analysed in a simple way [17] to obtain a numerical indication of the crystallinity of the sample. With the aid of measurements of WAXS from empty glass capillaries and from capillaries filled with liquid PEO (400H), the total area (A) under the intensity plot was separated into scattering from glass (Ag), non-crystalline (Aa) and crystalline polymer. Thus a degree of crystallinity (X) was calculated from X x = { A - (A, + A : ) } / ( A - As).
(1)
Averages of at least two determinations are listed in Table 1. Small-angle X - r a y scattering
Two to four orders of small-angle Bragg scattering were observed for the polydisperse samples (2000H, 2000M, 1000H, 1000M), in accordance with earlier observations [-1,3]. Very much sharper peaks were observed for samples 15M and 9M and the higher orders were clearly visible at small angles in the WAXS photographs. (See, for example, Fig. 3 wherein the SAXS peaks are attributable to scattering orders 3 6 from a spacing of 4.7nm). Bragg peaks up to order 8 are visible on the photograph. Spacings, calculated by direct application of Bragg's Law, are listed in Table 2. Comparison is made with the extended chain length anticipated for a 7/2 helix [14] (0.0928 nm per chain atom) and a planar zig-zag [16] (0.1187nm per chain atom). The SAXS from 15M and the polydisperse samples is consistent with stacked lamella crystals in which extended oxyethylene chains in 7/2 helical conformation are normal to the lamella end surface. The SAXS from 9M is less easily explained: the spacing corresponds to an extended helical chain tilted some 30° to the lamella end surface normal. SAXS was not observed from samples 15H and 9H, even after 72hr exposure in the Rigaku-Denki camera. An attempt was made to swell solid 15H by crystallizing a mixture of 95Wt~o 15H and 5wtVo triethylene glycol. Triethylene glycol is known to
889
Crystallinity of ethylene oxide oligomers enter the interlamella regions of solid 2000H when a mixture of the two materials is rapidly crystallized 1-18, 19] and to enhance SAXS. In the event SAXS was not observed from the mixture.
Raman scattering The Raman spectra of 1500H, 9H, 15H', 1500M and 15M' are shown in Fig. 4. All spectra show a broad peak near 80cm -1, which also appears in higher molecular weight PEO (6000H, 20,000H) and is assigned [20] to a torsional mode of the PEO helix. The peaks at 12-13 cm x in 1500H and 1500M can be assigned [-21] to a longitudinal acoustic mode (LAM) of the lamella crystal on the basis of an investigation of the effect of chain length and lamella spacing (l~) on frequency [21]. (LAM frequencies for all the polydisperse PEO samples investigated in the present work are listed in Table 2.) The prominent peaks in the low frequency region of the spectra of the I
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I
I
60
I
20
cm-I
(a)
I
I00
l
I
60
I
I
I
20
cm 4 I
I
I
I00
l
i
60
20
c m -n
Fig. 4. Low frequency Raman spectra, recorded at sample temperature 20C, of(a) 1500H, (b) 9H, (c} 15H', (d) 1500M and (e) 15M'. Spectra illustrate peak positions: scattering intensities and background scattering corrections are not standardized. crystalline oligomers can be assigned to the same mode on the basis of the plot of frequency against reciprocal chain length shown in Fig. 5. The end effect apparent in the data (Table 2, Fig. 5) is similar to that reported by Rabolt [22] in a comparison of the LAM frequencies of crystalline n-C19H40 and n-CxsH3vOH.
KX)
I
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I
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I
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l
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l
i.r. Spectroscopy Over the frequency range 3000-800 cm-1, the low temperature ( - 76°C) spectra of the monodisperse oligomers are very similar to those of the polydisperse polymers. Discernible differences between spectra are attributable to changes in chain length. The low frequency (1200-800cm -a) spectra of 9H and 15H obtained at - 7 6 ° C are shown in Fig. 6. Matsuura and Miyazawa [23] have studied ethylene glycol oligomers (x = 1-7) and polydisperse PEO (6000H), and have described band progressions at 970-930cm 1 and 870-830cm-1 in the oligomers which converge as chain length increases towards band groups at 963, 947 and 844 cm -~. These bands are apparent in our spectra (Fig. 6). They are assigned [23] to hybridized modes (CH2 rocking, C-O stretch-
A. MARSHALLet al.
890
in the published [23] low temperature spectra of lower oligomers. The band at 3450-3390cm -1, apparent in spectra of all samples at room temperature (Fig. To) and in the spectra of 2000H and 1000H at - 7 6 ° C (Fig. 7c), is assigned to O - H stretching in a disordered surface layer. This absorption is also found in the low temperature spectrum of 15H (Fig. 7c, weak tail) and 600H, 400H (Fig. 7c, overlapping the crystal bands).
e S
I>
j 2C
0
Differential scanning calorimetry '
01.2
'
01.4
'
Pl~ nrn-I
Fig. 5. LAM frequency from Raman scattering (v) vs reciprocal chain length (l1:7/2 helix) for (O) hydroxy-ended and (O) methoxy-ended PEO. ing) and are associated with a trans-gauche-trans sequence of bonds O--CH2-CH2-O. Over the frequency range 3600-3000 cm-1 (O-H stretching) the spectra of the crystalline oligomers show significant changes from those of the polydisperse polymers. Spectra corrected to equivalent film thickness and equivalent incident radiation intensity by reference to the C - H stretching band at 2890-2370 cm-1, are shown in Fig. 7. The spectra of the glycols purified by gel filtration (9H', 15H') are practically identical to those of 9H and 15H depicted in Fig. 7. Following Matsuura and Miyazawa [23] the band at 3480 to 3440 c m - 1 (Fig. 7a) is assigned to O - H stretching in the melt and the band at 3220-3180 c m - t (Fig. 7b, c) to O - H stretching in the crystal. The band at 3320--3310 c m - t in the low temperature spectrum of 9H (Fig. 7c), discernible as a shoulder in the spectrum of 15H, is also assigned to O - H stretching in the crystal. This band can be seen
Thermograms of the monodisperse nonamers are compared with those of the corresponding polydisperse samples (400H, 400M) in Fig. 8. We have already commented upon the narrower melting ranges of the monodisperse materials. Compared to 400H, the thermogram of 400M is markedly different in shape (Fig. 8). Thermograms of 600H and 600M differ in a similar way. This effect was not investigated further. We note that values of Tm based on peak position will be affected by the difference in shape. Melting points and heats of fusion are listed in Table 3. The values reported for the very pure pentadecaethylene glycol (sample 15H') are a little higher than those reported earlier [10, 24]. A crystallinity parameter XH was calculated from XH = AHf,s/AHf°s
(2)
where AH°us, the heat of fusion of perfectly crystalline PEO, can be calculated from [25] AHf0us/jg - 1 = 178.6 + 0.629 (T/°C) - 2.83 x 10-3(T/°C) 2. (3) Values of XH calculated from Anf°s(Tm) and the measured AHf, s are listed in Table 1.
(a)
(b)
/
r
I
a
1200
80O ' cnl-I
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1200
I
I
800
Gnl "1
Fig. 6. i.r. Spectra (1200-800cm-~), recorded at sample temperature -76°C, of (a) 15H and (b) 9H. Transmittances (arbitrary scale) are not standardized.
891
Crystallinity of ethylene oxide oligomers (b)
(o)
(e)
\ \
/
T, K
\ / f ! !
,2-
Z
J~
T,K
V 3750
3000 C ~ -I
3000 crn-i
cm-i
Fig. 7. i.r. Spectra (3750-3000cm-1) of glycols recorded at sample temperatures (a) above their melting points, (b) at 20°C and (c) at -76°C. From top to bottom, the spectra are in order 2000H, 1000H, 15H, 9H, 600H, 400H. The latter two samples are liquid at 20°C and their spectra are as in (a). Transmittance (arbitrary scale) is corrected to equivalent film thickness and incident radiation intensity by reference to the C-H stretching peak at 2890-2870cm-1.
2;0
2;0
2~o
2~o
The crystal structure established for PEO is the 7/2 helix with a trans-gauche-trans series of bonds O-CH2-CH2-O [14]. WAXS from the melt crystallized monodisperse oligomers differs from WAXS from higher molecular weight PEO, but can be indexed by the 7/2 helix. A planar zig-zag (all trans) structure can be ruled out. Confirmatory evidence for the assignment of an essentially 7/2 helical structure to the crystalline oligomers comes from the following results. (a) The observation of a band at 80 cm-1 in the low frequency Raman spectra. This has been assigned [20] to a torsional mode of the PEO helix and is found in all our samples. (b) The similarity of the i.r. spectra of the monodisperse oligomers (9H, 15H) and of higher molecular weight PEO and, in particular, the identification of bands in the spectra (Fig. 6) assigned [23] to modes of the 7/2 helix. The crystal habit established for low molecular weight PEO is lamella. This was established initially by SAXS [1] but has been confirmed by other techniques [4] including Raman scattering [19]. In the case of 9M and 15M, our evidence from SAXS confirms the lamella habit of these oligomers. In the case of 9H and 15H, SAXS was not observed and the assignment to a lamella habit rests immediately upon the Raman spectra, through Fig. 5, and upon i.r. spectra in the O-H stretching region, a discussion of which follows.
Lamella end-surface Our interpretation of the i.r. spectra shown in Fig. 7 (0-H stretching) is that the end-groups of the monodisperse glycols are either wholly ordered (9H) or almost so (15H) at - 7 6 ° C and largely ordered
2~o
2~o
2~o
T~ K
DISCUSSION
Crystal structure and habit
2~o
T,K
Fig. 8. Thermograms by DSC for (a) 9H, (b) 9M, (c) 400H and (d) 400M. Temperature scales are uncorrected: the power scale is arbitrary. Note that the temperature scale for 9H and 9M is twice that for 400H and 400M. (9H, 15H) at 20°C. The end-groups of the lower polydisperse glycols (400H, 600H, 1000H) are partially ordered at - 7 6 and 20°C, and those of 2000H are largely disordered even at -76°C. Hence the lamellae of the monodisperse glycols can be characterized as highly crystalline with chain ends in crystallographic array, while those of the polydisperse glycols can be characterized as partially crystalline with many chain ends (practically all chain ends in the case of 2000H) in a non-crystalline surface layer. No SAXS is observed from crystalline 9H and 15H, whereas SAXS is easily observed from 1000H and 2000H. The scattering from the polydisperse PEO can be attributed to an electron density deficiency in noncrystalline lamella surface layers. The absence of scattering from 9H and 15H implies that there is no corresponding electron density deficiency in the interTable 3. Enthalpy of fusion (AHr~s)and melting point (Tin) of PEO measured by DSC
Sample 2000 1000 600 400 15' 9'
OH ended AHfus Tm (j g- x) (°C) 174 148 131 100 183 179
53.6 37.0 24.0 8.0 40.0 30.0
OMe ended AHf,s Tm (j g- 1) (°C) 168 140 122 99 171 168
53.6 36.5 21.0 6.0 28.5 16.0
A. MARSHALLet al.
892 (a)
(b)
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1 lOG
x, chain units
Fig. 9. (a) Chain length distribution, n(x), of PEO (~. = 42 chain units, Xw/Xn= 1.04). The values of n(x) are relative to n(70)= 1. The dashed curve represents the experimentally (GPC) determined n(x). (b) Representation of a corresponding random set of unfolded chains in a "lamella" array. lamella region of these solids, in keeping with ordered hydrogen bonding and a crystallographic correspondence of adjacent lamella surfaces. Only when methoxy end-groups are substituted for hydroxy (in 9M and 15M) is SAXS apparent. The observation that 15H cannot be swollen by triethylene glycol is consistent with the sample containing highly crystalline lamellae, since swelling is attributable [19] to dilution of a non-crystalline layer. The crystallinities (X) of the monodisperse oligomeric glycols have been estimated from WAXS and DSC (Table 1) to be about 0.85-0.90 that of perfectly crystalline PEO. Polydisperse glycols of comparable M, (400H, 600H) are liquid at 25°C, and have X in the range 0.5-0.7 at lower temperatures. The lower value of X found for the lower polydisperse glycols can bc attributed to the effect of non-crystalline lamclla-surface layers.
Effect of end-group on thermodynamic properties Compared with the corresponding glycols, the dimethyl ethers have lower values of AHfu~ (Table 3). For polydisperse samples this has been attributed [8, 25-1 to rejection of the methyl end-groups from the oxyethylene crystals. For monodisperse samples, endgroups may be crystallographically ordered at the lamella surface; an explanation of the effect must include the different heats of interaction of hydroxy and methoxy end-groups localized at the lamella surface. Compared with the corresponding glycols, the dimethyl ethers of the polydisperse samples have similar melting points (Table 3, Fig. 8) but the dimethyl ethers of the monodisperse samples have melting points reduced by 11--14K. The environment of a chain end in a disordered surface layer is similar to that in the melt and, consequently, changes in melting point due to changes in end-group would not be expected for the polydisperse samples. The environment of a chain end at a crystalline lamella surface is markedly different from that in the melt and, consequently, a change in melting point is likely. A full
analysis of the melting points and enthalpies of fusion will be possible only when results are available for monodisperse samples covering a wider molecular weight range.
Concluding remarks It is to be expected that the crystallization of short chains into lamellae will be affected by a chain length distribution in the sample even though, in conventional terms, the chain length distribution is very narrow. This point is illustrated in Fig. 9 where (a) the chain length distribution of a PEO sample I-6] (~, = 42, ~,/~, = 1.04) is transformed via a histogram of six species (as shown) into (b) a random set of unfolded chains in a 2-dimensional "lamella" array. The result of incorporating such chains randomly into a lamella crystal is a disordered lamella surface layer. It is known that fractionation can occur during the crystallization of low molecular weight PEO 1-26]. Indeed macroscopic fractionation during slow crystallization of low molecular weight PEO (M, ~< 1000g mo1-1) is difficult to avoid and crystallization is usually carried out well below Tm in order to minimize its effccts. In consideration of this and other evidence, it has been suggested 1,5] that low molecular weight PEO chains segregate during crystallization to form lamellae in which chain ends are paired at the surface. The evidence presented here, particularly the comparisons made between the crystalline solids formed from the two kinds of PEO (monodisperse and polydisperse), indicates that random incorporation of chains into lamellae is a sensible model for the rapid crystallization of low molecular weight PEO.
Acknowledgements~We thank Mr R. S. Alexander, Mr R. Beddoes, Mr D. Farnsworth, Mr D. J. Roy and Mr T. G. E. Swales for help with the experimental work. Dr C. Price and Dr J. F. Rabolt gave helpful advice. The Science Research Council provided financial support.
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