On the α relaxation of the constrained amorphous phase in poly(ethylene)
Introduction
Polymeric crystals of sufficient size for detailed study are rare and thus little is known about single-crystal behaviour as a function of defect content. However, several authors postulated correlations between the defect structure and relaxation behaviour [1], [2], [3]. Details are not worked out because of lack of sufficient experimental information. The problem of better knowledge of crystal–amorphous interphase is still under consideration. The role of the interphase was already postulated (defined) by Flory in 1962 [4]. The internal friction of poly(ethylene) of low density (PE-LD) was studied first in 1953 [5]. Later measurements exhibited, comparable to those of mechanical relaxation, three dielectric relaxations labelled, γ, β, α [1], [2], [3]. However, the relaxation at the highest temperature (α′) was not observed. For the γ and α the relaxations the values of the activation enthalpy were found as 46 kJ/mol and 105 kJ/mol, respectively. The enthalpy of the β process was determined mechanically as 159 kJ/mol, although other values (67–105 kJ/mol) were also published [1], [2], [3]. The α′ relaxation occurs at high temperature and can best be observed in creep experiments but only for the samples cast from solution. The used notation, i.e. α and α′, was introduced and explained in literature [1]. The α′ relaxation was found to be sensitive to thermal history of the specimen. The following hypothesis for the mechanism of the α relaxation was postulated: the relaxation occurred due to reorientation motion within the crystals, concluded on the basis of NMR, X-ray and mechanical relaxation experiments [1], [2], [5], [6], [7]. The α relaxation did not appear in PE-LD that had been quenched from melt to low temperature, i.e. for PE with low crystalline contents. If PE-LD was not quenched, then the α peak was observed with a magnitude comparable to the β peak. It was established that the α relaxation depends on both the degree of chain branching and the method of crystallisation [5]. Post-crystallisation annealing also played apparently a role for this relaxation [8], [9], [10]. It was shown that for poly(ethylene) the decrease in the relaxation modulus with rising temperature could not be explained only by changes in the degree of polymer crystallinity. The value of the modulus is thought to be related to the reorientation of crystallites in the amorphous matrix. According to some authors [1], [2] the α relaxation is due to the motion of CH2 units in the crystals, and the molecular mechanism is the same as the γ mechanism. It is generally accepted that the γ relaxation occurs due to the motion of CH2 units in the amorphous region. Therefore, according to the common opinion the same type of molecular movement (local mode) occurs for both α and γ relaxations. In literature [1], [2], it was proposed that the α mechanism appeared due to the melting of the crystals (premelting), however, some authors questioned the melting explanation of this relaxation. Relaxations found in polymeric systems are strictly determined by the free volume, which may be estimated by dilatometric, X-ray or positron annihilation methods. Positron annihilation lifetime spectroscopy (PALS) is widely used for investigations of different aspects of polymer structures and positron behaviour in polymers [11], [12].
It is commonly assumed that the properties of α relaxation depend on crystal morphology. In our opinion, however, this hypothesis leaves the question undecided whether the α relaxation in PE is due to either the molecular relaxation within the crystal phase or to the lamellar slip mechanism. We consider the α relaxation in PE to take place in the interphase regions of the semi-crystalline structure. These regions are strongly perturbed by the presence of the lamellae, which stretch the polymer chains. It is easy to deduce, based on the model of paracrystallinity ([13] and references therein), that the chains building the amorphous phase in the bulk and in the interphase must exhibit different properties. The supermolecular structure of polyethylene was a subject of many papers. Although the concept of constrained amorphous phase was applied with success to other polymeric systems for many years [14], [15], to our best knowledge, data concerning the α relaxation in PE have been interpreted following conventional two-phase model [16], [17]. The model assumed that the α relaxation in PE occurred only in the defected crystal phase.
Section snippets
Sample preparation
Specimens were produced form granulated PE (Lupolen 2012 D schwarz 413, BASF) as a tape with a thickness of 1 mm and a width of 300 mm. The samples were divided into three parts: the first one unprocessed (A), the second one (B) drawn (40% elongation), and the third one (C) irradiated with high-energy electrons. The PE tapes were the same as used in a previous study [18]. The samples were cut from the tape, which had been prepared using the same procedure as rigorously as possible, and were
Results and discussion
The transitions, relaxations and morphologies of branched and linear poly(ethylene) have been extensively examined by numerous investigators, and the results have been reviewed in books [1], [2], [3]. Because PE is a rather highly crystalline polymer, there is no doubt that the role of the crystal–amorphous interphase must be significant. This paper leads to a broader understanding of the relationship between processing history and morphology of the interphase in the PE products. A wide range
Conclusion
The problem of the origin of the α relaxation was investigated for PE-LD. The facts presented in this paper, i.e., the low enthalpy of activation, the Arrhenius relationship and the DSC traces, indicate that the relaxation labelled in literature as the α relaxation refers to the large-scale motion of the macromolecular segments located within the amorphous regions. However, the chain arrangement in these regions are affected by the presence of crystallites. Different processes, i.e. annealing,
Acknowledgements
The fruitful discussions with Professor Vincent B.F. Mathot (DSM, The Netherlands), on the constrained amorphous phase in polymeric systems, and his remarks, which let the authors to improve the manuscript, are greatly acknowledged. The authors are indebted to Professor Julius G. Vancso (University of Twente, The Netherlands) for his careful reviewing of the manuscript and remarkable advice concerning the text. One of the authors (A.D.) would like to acknowledge the financial support of the
References (39)
Positron annihilation spectroscopy for chemical analysis
Microchemical J.
(1990)The mechanical and physical ageing of semicrystalline polymers: parts 1,2,3,4
Polymer
(1987)- et al.
A dielectric study of molecular relaxation in linear polyethylene
Polymer
(1994) - et al.
Free volume changes in physically aged polyethylene by positron annihilation
Polymer
(2001) - et al.
The temperature dependence of positron lifetimes in solid pivalic acid
Chem. Phys.
(1981) - et al.
On the crystalline-amorphous supermolecular structure of poly(4-methyl-1-pentene) films cast from solution: experimental evidences and theoretical remarks
J. Mol. Liq.
(2000) - et al.
Anelastic and dielectric effects in polymeric solids
(1967) Mechanical properties of solid polymers
(1971)Relaxation and thermodynamics in polymers
(1992)On the morphology of the crystalline state in polymers
J. Am. Chem. Soc.
(1962)
Mechanische Relaxationserscheinungen an Hochpolymeren
Kolloid Z
Eine methode zur Messung der dynamisch-mechanische Eigenschaft von dunnen Filmen, Pulvern und Fasernaus hochpolymeren
Kolloid Z
Mechanical properties of polyethylene crystals
J Polym. Sci. Part C
The effect of molecular weight on crystallization, melting, and morphology of long-chain molecules
J Polym. Sci. Part C
Size and interfacial free energies of crystallites formed from fractionated linear polyethylene
J. Polym. Sci.
Ein Beitrag zum mechanischen Relaxationsverhalten von Polyäthylen, Polypropylen, Gemischen aus diesen und Mischpolymeriesaaten aus Propylene und Äthylen
Kolloid Z
Positron annihilation in chemistry
Structure of crystalline and paracrystalline condensed matter
J Macromol. Sci.-Phys.
The interphase in lamellar semicrystalline polymers
Macromolecules
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