Physical aging and enthalpy relaxation in polypropylene

Physical aging and enthalpy relaxation in polypropylene

) O U R N A L OT ELSEVIER Journal of Non-CrystallineSolids 172-174 (1994) 592-596 Physical aging and enthalpy relaxation in polypropylene J.M. Hutc...

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) O U R N A L OT

ELSEVIER

Journal of Non-CrystallineSolids 172-174 (1994) 592-596

Physical aging and enthalpy relaxation in polypropylene J.M. Hutchinson*, U. Kriesten Department of Engineering, Fraser Noble Building, King's College, University of Aberdeen, Aberdeen AB9 2UE. UK

Abstract

The physical aging of semi-crystalline polypropylene at 20°C has been studied by low strain torsional creep, and compared with the enthalpy and density relaxation behaviours. The physical aging rate of 0.72 is much smaller than the value of unity usually found for amorphous polymers. Differential scanning calorimetry studies reveal the appearance of an endothermic peak at or above approximately 35 °C, which increases in magnitude and shifts to higher temperatures on aging. Simultaneously, the glass transition temperature, Tg, increases and the relaxation strength at Tg decreases. The kinetics of enthalpy and density relaxation at 20°C in polypropylene are shown to be inconsistent with the usual behaviour of amorphous polymers below Tr It is suggested that these observations could be rationalised if the structural changes in polypropylene on aging at 20°C involve an 'ordering' of an interfacial region between the crystalline iamellae and the bulk amorphous material.

1. Introduction

Changes in the mechanical properties of polymers as a function of time at constant temperature below the glass transition, Tg, are the manifestation of the phenomenon of physical aging [1]. In amorphous polymers, physical aging is usually presumed to originate from structural changes which occur below Tg because of the non-equilibrium state of the glass. These structural changes are often measured by either volume or enthalpy relaxation. Although this hypothesis is widely accepted, there is as yet no convincing quantitative relationship between this structural relaxation behaviour and physical aging.

*Corresponding author. Tel: + 44-224 272 791. Telefax: + 44224 272 497. Telex: 73458 uniabn g

In semi-crystalline polymers, the situation is even more complex. For example, in polypropylene, physical aging is universally observed at temperatures well above Tg ( ,,~ 0°C) (see Refs. [1-4] for typical illustrations). This effect has been explained on the basis of an 'extended' glass transition interval resulting from the constraints on the amorphous interlammellar regions imposed by the adjacent crystalline lamellae, which effectively reduce the mobility of an interfacial amorphous fraction [1]. In common with the hypothesis for physical aging in fully amorphous polymers, this explanation would require there to be concomitant structural (i.e., volume and enthalpy) relaxation effects above Tg in semi-crystalline polymers. To date, these effects have received little attention. We report here the results of some enthalpy relaxation experiments on polypropylene which has been aged at 20°C, i.e., above Tg, and examine the extent to which they confirm the above explanation.

0022-3093/94/$07.00 © 1994 ElsevierScienceB.V. All rights reserved SSDI 0022-3093 (93) E0664-T

J.M. Hutchinson, U. Kriesten / Journal of Non-Crystalline Solids 172-174 (1994) 592-596

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2. Materials and experimental The polypropylene was a commercial grade supplied as extruded rod stock. Specimens for torsional creep were machined in the form of thinwalled cylinders, with diameter 12 mm and wall thickness 0.3 mm. Samples of approximately 10 mg were taken from the wall of unused creep specimens for differential scanning calorimetry (DSC). The as-received density of polypropylene in this wall region was approximately 0.905 g/cm 3. Torsional creep experiments at small strain were performed using apparatus described elsewhere [5]. Enthalpy relaxation was measured using a Perkin-Elmer DSC-4 with controlled cooling accessory and TADS interface. The usual thermal history involved a period of 'rejuvenation' for approximately 15 min at 100°C followed by a quench to the aging temperature of 19.5°C. Creep tests were made at aging times from 0.5 to 288 h, ensuring always that the creep duration was less than approximately one tenth of the aging time. For DSC experiments, each aged sample was cooled in the DSC to - 3 0 ° C and scanned at 10 K/min up to the rejuvenation tem-

3. Results The creep behaviour is shown in Fig. 1. These curves can be superposed to form a reasonable master curve by means of combined horizontal and vertical shifts, while maintaining a constant relaxation strength. A double logarithmic aging rate /~ = 0.72 is obtained from the horizontal shifts, which is significantly less than the value of unity commonly found for amorphous polymers [1, 6]. The enthalpy relaxation behaviour is illustrated in Fig. 2, where DSC scans are shown for polypropylene aged at 20°C for the different times indicated. Three effects of aging can be seen. First, the change in Cp at the glass transition, AC~ is reduced by aging at 20°C. The magnitude of this reduction in ACp appears to increase linearly with log aging time [-5], although there is considerable scatter in the data because the effect is small (a reduction of approximately 0.01 J/g K per decade of aging time). Second, the 'mid-point Tg' increases with aging time, as shown in Fig. 3. The glass transition region

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J.M. Hutchinson, U. Kriesten / Journal of Non-Crystalline Solids 172-174 (1994) 592-596

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Fig. 3. Plot of 'mid-point Tg', defined as temperature at which Cp lies mid-way between glassy (CpB)and liquid (C~Ovalues, as a function of log(aging time). Arrows indicate locations of midpoint T8 for zero annealing time for five separate samples. The shaded band is drawn to guide the eye.

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is b r o a d and ACp is small, resulting in some error in the estimation of Tg; nevertheless, the trend is apparent and is indicated by the b r o a d shaded band. Third, with increasing aging time, an endothermic peak is seen to appear, initially at a b o u t 35°C, and then to grow in magnitude and shift to higher temperature. The peak temperature, Tp, is measured as the temperature at which the m a x i m u m deviation from the baseline Cp trace occurs, and its dependence on aging time is shown in Fig. 4. It is noticeable also from Fig. 2 that these peaks become increasingly b r o a d as Tp increases, but that the aged trace and the re-scan always merge at or before the rejuvenation temperature. In addition to these effects, the enthalpy loss on aging at 20°C can be estimated from the difference in area beneath the aged and re-scan D S C traces. The results can be seen in Fig. 5. In fact, because of the c o m p u t a t i o n a l algorithm used to calculate these areas, the area differences shown in Fig. 5 do not take into account the area contribution arising from the change in ACp at 20°C on aging [5]. This small correction (approximately 0.3 J/g per decade

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J.M. Hutchinson, U. Kriesten / Journal o[ Non-Crystalline Solids 172 174 (1994) 592 596

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4. Discussion The striking feature of the DSC traces in Fig. 2 is the appearance of the enthalpy recovery peaks some 40°C and more above the glass transition, occurring as a result of the enthalpy lost during aging at 20°C. Although these peaks shift to higher temperatures with increased aging time (Fig. 4), they do not display other behaviours common to enthalpy recovery peaks in fully amorphous polymers. For example, they broaden rather than narrow with increasing aging, and there is no evidence of any peak or step-change in Cp in the same temperature region for the re-scan. Thus, it would be difficult to argue that they are a manifestation of an underlying glass transition at some temperature higher than that of the bulk amorphous fraction at approximately 0°C. On the contrary, the re-scan traces show only the glass transition around 0°C, with no evidence for a broadened transition interval due to constrained amorphous regions [1]. Further, the enthalpy loss on aging at 20°C, as shown in Fig. 5, occurs at a much faster rate than would be anticipated for constrained amorphous regions (even at an aging

temperature optimally situated in the glassy region) which constitute only a minor fraction of the semi-crystalline polypropylene (crystallinity approximately 60% as determined from density using formula given in Ref. [7]). For example, atactic polystyrene aged at 85°C (approximately 15°C below Tz) loses enthalpy at a rate of approximately 0.9 J/g per decade [5], very close to the rate of enthalpy loss evident from Fig. 5 for polypropylene. Additional experimental evidence, not shown here, for structural relaxation kinetics in polypropylene being different from those in amorphous polymers is afforded by density measurements in the density gradient column constructed with a miscible solution of propanol and silicone oil [5]. In amorphous polymers, the density increases (specific volume decreases) linearly as a function of log time over wide ranges of aging time and temperature [8]. In polypropylene, on the other hand, we observe a marked non-linearity, with the rate of increase of density continuously slowing down over the whole range of aging times up to 1000 h. Further, the rate of density increase in polypropylene (about 0.03 % per decade) when compared with that in polystyrene (0.09% per decade), for example, is

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J.M. Hutchinson, U. Kriesten / Journal of Non-Crystalline Solids 172-174 (1994) 592-596

inconsistent with only a small fraction (constrained amorphous regions) being involved in the relaxation; it should be recalled that the total amorphous fraction is about 40%, of which the constrained interfacial region presumably forms only a small part. Similar anomalies have been reported earlier in other semicrystalline polymers [9-1, where ACp is found to be too small for the amorphous content calculated from the crystallinity. By contrast with the situation in amorphous polymers below Tg, therefore, it would appear instead that the enthalpy relaxation behaviour of polypropylene could be better understood in terms of a type of 'ordering' or 'partial crystallisation' of the amorphous fraction, in an interfacial region close to the surfaces of the crystalline lamellae. On aging, this interfacial region becomes increasingly more ordered, and the corresponding DSC heating scans show what is effectively a 'melting' endotherm. The re-scan shows no such endotherm because no aging time is allowed for this ordering to take place. The effect of this progressive ordering in the interfacial regions during aging is to restrict, to an increasing extent, the bulk amorphous material in the interlamellar regions. Thus, with increasing aging time, the glass transition moves to higher temperatures (Fig. 3). Further, since part of the original interlamellar amorphous content is transformed, on aging at 20°C, into a more ordered interfacial domain, the amount of bulk amorphous material available to participate in the glass transition at around 0°C is reduced. Thus, the step change (ACp) at T~ is reduced on aging (Fig. 2). The above rationalisation of the enthalpy relaxation behaviour of polypropylene at 20°C in terms of an effectively crystalline rather than amorphous relaxation would imply a kinetic response on aging quite different from that commonly observed for fully amorphous polymers below Tg. The physical aging behaviour shown in Fig. 1, while being qualitatively similar to that for amorphous polymers, differs quantitatively in respect of the much smaller value for the shift rate,/~, found here for polypropylene. While this is in agreement with there being different underlying kinetics for the physical aging of amorphous and semi-crystalline polymers,

further microstructural evidence is required in order to confirm the above ideas.

5. Conclusions

The structural relaxation behaviour of polypropylene on aging at 20°C, as evidenced in particular by DSC, is quantitatively inconsistent with the usual kinetics of structural recovery in amorphous polymers. The endothermic peaks which appear at or above 35°C become increasingly broader on ageing, and there is evidence only of the glass transition at around 0°C, and not of any higher or more diffuse transition. Further, the rates of enthalpy and density relaxation in polypropylene are much larger than would be expected for the amorphous content of a semicrystalline polymer. Instead, it is suggested that the structural relaxation behaviour in polypropylene is associated with an 'ordering' of the amorphous material in an interfacial region between the crystalline lamellae and the bulk interlamellar amorphous material. This is shown to be consistent with a reduced physical aging rate in the torsional creep response of polypropylene by comparison with that of fully amorphous polymers. This work was supported by the Science and Engineering Research Council (UK), reference GR/F/27284.

References

[1] L.C.E.Struik, PhysicalAgingin AmorphousPolymersand Other Materials (Elsevier,Amsterdam, 1978). [2] S. Turner, Br. Plast. 37 (1964) 682. [3] C.K. Chai and N.G. McCrum, Polymer21 (1980) 706. [4] C.P. Buckleyand M. Habibullah, J. Appl. Polym. Sci. 26 (1981) 2613. [5] U. Kriesten, PhD thesis, Universityof Aberdeen (1993). [6"] J.M. Hutchinson and U. Kriesten, in: Macromolecules 1992, ed. J. Kahovec(VSP, Zeist, The Netherlands, 1993) p. 45. I-7"] F. Danusso,G. Moraglioand G. Natta, Ind. Plast, Mod. 10 (1958) 40. [8] A.J. Kovacs, Fortschr. Hochpolym.Forsch. 3 (1963) 394. [9] J. Menczel and B. Wunderlich,J. Polym. Sci. Polym. Lett Edn. 19 (1981) 261.