Makromol. Chem., Rapid Commun. 3, 753 - 756 (1982) 753

A Possible Structural Interpretation of the Two DSC Melting Peaks of Isotactic Polypropylene in the a-Modification

Paolo Corradini*, Roberto Napolitano, Leone Olioa, Vittorio Petraccone, Beniamino Piroui

Istituto Chimico dell'universiti, Via Mezzocannone 4, 801 34 Naples, Italy

Gaetano Guerra

Istituto "G. Donegani", Via Nuova delle Brecce 150, 80147 Naples, Italy

(Date of receipt: July 2, 1982)

Samples of isotactic polypropylene, which have two slightly displaced melting peaks in DSC, showing only the crystal structure a'), have been described by various authors. The presence of two peaks is induced in some cases2s3) by high temperature (> 155 "C) conditioning, in other case^^-^) by appropriate choice of the temperature of crystallization (130°C < T, c ISOOC), and in still other cases by the particular conditions under which the differential thermal analysis is performed: at a low scanning rate') (heating rate < 5 K/min) or on drawn samples restrained during the measurement 6, @.

Longer heat-conditioning times3), higher crystallization temperatures", and lower DSC scanning rates') lead to an increase of the second peak relative to the first one, which may eventually disappear.

For not restrained samples, the first endothermic peak is located between 160 and 165 OC, whereas the second peak is located between 165 and 180°C, depending on the obtainment conditions, while drawn samples, restrained during the measurements, have corresponding peaks at slightly higher temperatures.

According to Samuels6) melting temperature data may be extrapolated to 185 and to 220°C, respectively, for infinitely big crystals.

The origin of the splitting of the melting peak for isotactic polypropylene has been deeply discussed by Samuels6). Though, the X-ray data seem to indicate the existence of the crystalline a-modification only, in the examined samples, he gives clear evidence that the hypothesis of different morphologies (folded chains or extended chains; different kinds of spherulites) must be rejected. His conclusion is, that even if the source of the multiple-peak endotherm behaviour in isotactic polypropylene in the a-form is still unresolved, obviously two crystalline species are involved, either different disordered crystals, different crystal sizes, or different crystal types.

In recent times there was evidence9. lo) that the a-modification of isotactic polypro- pylene may show various degrees of disorder in the up and down positioning of the chains. The limiting structure which is ordered (henceforth denoted az) may be built up by a double layer of helicoidal macromolecules (comprising a glide plane with periodicities a = 6,65 A, c = 6,50 A, /3 = 99,3"); each double layer being repeated into the adjacent ones by centers of symmetry (which transform up chains into down chains). The limiting disordered structure (henceforth denoted a,) may correspond

0173-2803/82/10 0753-04/$01 .oO

754 P. Corradini, R. Napolitano, L. Oliva, V. Petraccone, B. Pirozzi, G. Guerra

instead to a succession, disordered along 6, of ordered double layers of the type described above; each double layer is repeated into the adjacent one, by chance either through a translation or through a center of symmetry.

The two modifications have substantially identical X-ray spectra. However, while in the a,-modification only reflections with (h + k ) even are allowed, in the az- modification also reflections with ( h + k ) uneven may be present, though generally associated with a lower intensity.

Due to the overlapping of reflections in the powder spectra (and to a lesser extent in the fiber spectra) it is possible to determine in a clear way the degree of disorder which is present in a sample only in some regions of the X-ray spectra; the most indicative one is that comprised between 2sin 0/1 = 0,34 k' and 2sin 0/1 = 0,49 A-l (15" < 0 < 21 O for CuK, radiation). In particular, the ratio ( R ) of the intensity of the peaks centered at 2sin0/1 = 0,39 A-' (0 = 17,5", CU,~) and at 2sintV1 = 0,41 A- ' (0 =

18 ,5" , CuKa) may be taken as an index of the degree of order. In fact, to the first of these two peaks mainly the reflections ( 1 7 I), (1 3 2) (with h + k even), (2 3 l ) , (0 5 2) (with h + k uneven) contribute, while to the second one mainly the reflections (2 4 1) and (1 7 1) (all with h + k even) contribute. The above defined ratio reaches nearly 2 for the most ordered samples obtained by us (that is, those samples which are closer to the limiting a,-modification), while it is 0,5 for the most disordered samples (that is, for those samples which are closer to the limiting a,-modification). Guided by the idea that the presence of two endothermic peaks in the calorimetric scan could be related to the melting of crystals having a structure closer to that of the a,- modification and to the melting of crystals having a structure closer to that of the a,- modification, respectively, we have reexamined various samples, prepared according to previous work, strictly associating DSC with X-ray analysis. In this note, in particular, we refer to two samples of isotactic polypropylene arising from the same hot pressed plate, conditioned in such a way as to have two extreme values of the above mentioned ratio. Sample A was annealed at 325°C and it had R = 0,5, while sample B was annealed at 165°C and had R = 2,O (Fig. 1).

15. 18' 17' 18' 19' 20' 21' 8 15' IS' 17' 18' 1 9 ' 20' 21' 8

Fig. 1. annealed at 125'C (A) and at 165°C (B), respectively

Line profiles of the X-ray powder spectra for two samples of isotactic polypropylene

A Possible Structural Interpretation of the Two DSC Melting Peaks. . . 755

In Fig. 2 we report the DSC curves taken at a heating rate (h.r.) of 10 K/min; at this rate, as known'), the splitting of the melting endotherm is not observed. The peak temperature is 163°C for sample A and 170°C for sample B.

T /"C 110 130 150 1 7 0

I I

T 1 % 110 130 150 170

1 I 1

I I

Fig. 2 Fig. 3

Fig. 2. DSC diagram of two samples of isotactic polypropylene annealed at 125 "C (A) and at 165°C (B), respectively. Heating rate = 10 Wmin Fig. 3. DSC diagram of two samples of isotactic polypropylene annealed at 125 "C (A) and at 165 "C (B), respectively. Heating rate = 2,s K/min

In Fig. 3 we report the DSC curves for the same samples taken at an h.r. of 2,5 K/min. The rate is sufficiently low as to allow the observation of the splitting of the peak. Indeed, such an effect is observed for sample A; its melting endotherm is characterized by a maximum at 160°C (which corresponds to the peak at 163°C observed at a higher h.r.) as well as by a second maximum at 167 "C. On the contrary, for sample B only one narrow peak is observed at 168,5 "C, that is at a temperature only slightly below the one observed at higher h.r., due to the absence of superheat- ing. The behaviour of sample A at low h.r. is explained in our opinion by the trans- formation of crystals in a,-modification into crystals having a higher degree of order (closer to the limiting a,-structure) which melt at a higher temperature. The inter- pretation is in accordance with the fact that the splitting is no more observed for sample B, which is already in the more ordered form.

Notwithstanding the presence of disorder in the modification a,, we hypothesize that modification a, has always a free energy lower than that of modification a,, this slight difference being prevailingly of enthalpic character. This explains the slightly higher melting temperature of modification a, and is also in accordance with calcula- tions of the packing energy which we are performing.

756 P. Corradini, R. Napolitano, L. Oliva, V. Petraccone, B. Pirozzi, G . Guerra

Experimental Part

The polypropylene samples (from Montepolimeri S.p.A.) are highly isotactic, approximately 97,5% insoluble in heptane; intrinsic viscosity: 2,5 dl/g. To erase the previous thermo- mechanical history of the powder, plates about 1 mm thick were obtained by compression moulding, quenched in air, and annealed at various temperatures in an oven for 24 h.

The X-ray diffraction patterns were obtained using a cylindrical camera with Ni-filtered CuK, radiation, at room temperature. Intensity measurements were carried out by a microphoto- meter.

The thermal measurements were carried out on a Perkin Elmer type differential scanning calorimeter (DSC-2), in a running nitrogen atmosphere. For all experiments the sample size was kept nearly constant at approximately 4 mg. Different pieces from the same sample were examined at different heating rates.

The authors acknowledge the financial support by Progetto Finalizzato “Chimica Fine e Secondaria” C.N.R. (Italy).

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