New aspects of yielding in semicrystalline polymers related to microstructure: Branched polyethylene

Calleja, F. J. Baltá; Kilian, H. G.

doi:10.1007/bf01451528

Cited by 47 publications

(12 citation statements)

References 14 publications

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“…On the other hand, the presence of iPP crystals does not seem to inhibit the crystallization level of PA and ~PA remains nearly constant. From SAXS results we have seen that L = const, and from calorimetric data (Tm for both PP and PA is constant with composition), we conclude that the crystal thickness, lo, is also constant (the fact that lo = constant simplifies the calculation of the microhardness, as we have shown that Ho is an increasing function of Ic) [8,9]. Then, if we take into account the crystallinity depression measured for the PP component in Equation 4, use for H Pp = 145 MPa and for H Pp = 15 MPa [16], and let H PA = 152 MPa, we are led to curve 2 in Fig.…”

Section: Discussionmentioning

confidence: 91%

“…The experimental microhardness values, Hexp, of the iPP/PA blends, the H values derived from Tabor relation (H ~ 3ay) [7,8], using yield-stress data published elsewhere for these samples [6], and the crystallinity values derived from calorimetry of the iPP and PA components within each blend', ~pp and o%, are collected as function of composition, 4~, in Table II. …”

Section: Microhardness Measurementsmentioning

confidence: 99%

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Novel aspects of the microstructure of PP/PA blends prepared by reactive compounding as studied by microhardness

Fritz

Cai

Cagiao

et al. 1995

Journal of Materials Science

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The microhardness of injection-moulded i-polypropylene/polyamide (iPP/PA) blends prepared by reactive compounding was determined. The formulation rules and processing technology for the preparation of these alloys was reported previously, iPP/PA compositions between i00/0 and 50/50 using functionalized PP with various degrees of mainchain grafting, were investigated. It is shown that the deviation of microhardness from the additivity law of the single components is mainly due to a decrease in the crystallinity of the iPP phase. The results are discussed in the light of the microstructural variations as revealed by X-ray diffraction methods.

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Section: Discussionmentioning

confidence: 91%

Section: Microhardness Measurementsmentioning

confidence: 99%

Novel aspects of the microstructure of PP/PA blends prepared by reactive compounding as studied by microhardness

Fritz

Cai

Cagiao

et al. 1995

Journal of Materials Science

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“…During the last two decades it was demonstrated [8][9][10][11][12][13][14][15], that microhardness technique as a nondestructive method, due to its simplicity and high sensitivity, makes it possible to derive relevant information about the structure of polymers. Relationships between microhardness and polymer crystallinity, crystal perfection, chain conformation and other structural parameters have been derived [8,10,15].…”

mentioning

confidence: 99%

“…Relationships between microhardness and polymer crystallinity, crystal perfection, chain conformation and other structural parameters have been derived [8,10,15]. Hardness of polymers can also be correlated with many of their mechanical properties [8][9][10][11], which makes such measurements a fast and convenient method for material characterization. The microhardness technique has also been proved to detect polymorphic phase changes in polymers [12], and especially changes in polymer blends with composition [13,14].…”

mentioning

confidence: 99%

Influence of molecular weight on the microhardness of poly(ethylene terephthalate)

Vanderdonckt

Krumova

Calleja

et al. 1998

Colloid & Polymer Science

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Poly(ethylene terephthalate) (PET) was annealed in vacuum at different temperatures (190-260°C) for different times (10 min-24 h) in order to examine the mechanical properties (microhardness) of PET samples with a wide range of molecular weights (10 000-120 000). Short annealing times result in a twofold decrease in mol. wt. due to hydrolytic decomposition. However, long annealing times give rise to a substantial molecular weight increase. It is found that microhardness (H) rises linearly with the degree of crystallinity obtained during up-grading of mol. wt. and its extrapolation leads to H-values of completely crystalline PET, H.#2 "405 MPa for samples with conventional mol. wt. and of 426 MPa for samples with mol. wt. higher than 30 000. It is shown that the increase of mol. wt. for each set of samples with a given range of degree of crystallinity also causes a slight increase of H. The influence of mol. wt. upon hardness is discussed in the light of the changes in the physical structure (crystallinity, crystal thickness) which is formed at given heat treatment conditions.

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“…To( dp )= Ahf m dT~ A~ (4) where A~ is the specific volume change upon melting for an infinitely thick crystal. iii) From eq 3, T~/dT~= Tm/dTm.…”

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confidence: 99%

A New Thermodynamic Approach to the Microindentation Process on Polymeric Crystals

Hirami

Calleja

Flores

1999

Polym J

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ABSTRACT:A new thermodynamic derivation of the microhardness dependence on crystal thickness is developed. The approach makes use of a modified Clausius-Clapeyron equation to incorporate the effect of the finite size of polymer crystals. The derived equation describes the hardness depression due to finite thickness of the lamellae on the assumption that plastic deformation involves a partial melting of the polymer crystals. The present model approach offers an alternative view to the earlier concept of plastic deformation related to the energy dissipated during mechanical destruction of the crystals.KEY WORDS Microhardness / Polymer Crystals / Crystal Hardness / Crystal Thickness / Crystal Destruction / Partial Melting / Clausius-Clapeyron Equation /In recent years microhardness has been shown to be a powerful technique to detect morphological and microstructural changes in polymers. 1 • 2 This technique has been successfully applied to the study of the crystallization process in amorphous polymers. 3 Balta Calleja and Kilian 4 developed an approach based on a heterogeneous deformation model involving the heat dissipated by the plastically deformed polymer crystals to calculate the dependence of hardness on average thickness /e of the crystalline lamellae. The crystal hardness He can then be described by:Hoowhere He 00 is the hardness of an infinitely thick crystal and b is a parameter given by:(J• being the surface free energy and Ah the heat dissipated as a consequence of the plastic deformation of the crystalline blocks. It has already been indicated the existing similarity between eq 1 and the well known Thomson-Gibbs equation. 5 The latter describes the melting point depression, T~ -Tm, for a polymeric crystal of finite lamellar thickness /e with respect to the equilibrium melting point T~:where Ah? is the equilibrium melting enthalpy. The aim of this note is to present an alternative model for the derivation of eq 1 based, as well, on thermodynamic considerations. Let us assume that the indentation process induces a partial melting of the polymer crystals during the indentation process. Thus, Ah in eq 2 should be regarded, in this case, as the heat dissipated through the partial melting of the polymer crystals near the sample surface. Let us make use of the Clausius-Clapeyron equation, which is applicable to crystalline polymer systems. This equation describes the t To whom all correspondence should be addressed. increase in melting point as a consequence of an increasing pressure p:To( dp )= Ahf m dT~ A~ (4) where A~ is the specific volume change upon melting for an infinitely thick crystal. Equation 4 may be applied to finite size polymer crystals taking into account the following considerations: i) Ahf = Ahr -2(J~ / /e, where Ahr is the enthalpy of fusion of a crystal of thickness /e and fold surface enthalpy iii) From eq 3, T~/dT~= Tm/dTm.

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New aspects of yielding in semicrystalline polymers related to microstructure: Branched polyethylene

Cited by 47 publications

References 14 publications

Novel aspects of the microstructure of PP/PA blends prepared by reactive compounding as studied by microhardness

Novel aspects of the microstructure of PP/PA blends prepared by reactive compounding as studied by microhardness

Influence of molecular weight on the microhardness of poly(ethylene terephthalate)

A New Thermodynamic Approach to the Microindentation Process on Polymeric Crystals

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