We investigated possible physical and chemical changes in inert polymers by means of mechanical milling. No chemical changes were observed. However, in polystyrene we found two glass temperatures after milling, in contrast to one glass temperature in the original material. Under certain conditions crystalline polyethylene transformed almost completely from the orthorhombic to the monoclinic structure

Bakker, H.

Castricum, H.L.

van Deursen, J.H.

Yang, H.

English

NARCIS 

A study of milling of pure polymers and a structural transformation of polyethylene

International Migration, Integration and Social Cohesion online publications

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)A study of milling of pure polymers and a structural transformation ofpolyethyleneCastricum, H.L.; Yang, H.; Bakker, H.; van Deursen, J.H.Publication date1997Published inMaterials Science ForumLink to publicationCitation for published version (APA):Castricum, H. L., Yang, H., Bakker, H., & van Deursen, J. H. (1997). A study of milling of purepolymers and a structural transformation of polyethylene. Materials Science Forum, 211-216.General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:10 Nov 2022Tel. No. :  (+31) 20 525 6493 Fax. No. :  (+31) 20 525 5604 e-mail     :  H.L.Castricum@ uva.nl  A STUDY OF MILLING OF PURE POLYMERS AND A STRUCTURAL TRANSFORMATION OF POLYETHYLENE  H.L. CASTRICUM1, H. YANG1, H. BAKKER1  AND J.H. VAN DEURSEN2 1Van der Waals-Zeeman Laboratory, University of Amsterdam Valckenierstraat 65,  NL 1018 XE  Amsterdam, The Netherlands 2Koninklijk Shell Laboratorium Amsterdam,  Badhuisweg 3, NL 1031 CM Amsterdam, The Netherlands  Keywords: polymer; glass temperature; shear; phase transformation; scission; polymerisation  Abstract      We investigated possible physical and chemical changes in inert polymers by means of mechanical milling. No chemical changes were observed. However, in polystyrene we found two glass temperatures after milling, in contrast to one glass temperature in the original material. Under certain conditions crystalline polyethylene transformed almost completely from the orthorhombic to the monoclinic structure.   Introduction      High-energy ball milling has been used widely during the past decade for mechanical alloying (MA) of metallic systems, i.e. starting from a mixture of elemental powders and for mechanical milling (MM) of metallic systems, i.e. starting from pre-alloyed intermetallic compounds [1,2]. MA leads to alloying on an atomic level in many cases and even it turned out to be possible to alloy immiscible metallic binaries. In intermetallic compounds MM induces atomic disorder in the early stage of milling and eventually a phase transformation may follow. Pure metals do not show a phase transformation, but become nanocrystalline (see, e.g. [1]).   In contrast to the many studies on metallic systems, ball-milling experiments on polymers seem to be scarce [3,4]. In these scarce studies, on the one hand degradation of polymers by ball milling as a result of chain scission was reported [4], on the other hand mechanical milling of ABS (Acrylonitrile-Butadiene-Styrene) rather resulted in a small increase in the weight-average molecular mass than in a decrease [3]. In the same study [3] the alloying of normally incompatible polymer mixtures by MA was claimed. The possibility of producing amorphous powders of polymer mixtures by MA was reported by [5]. It was the aim of the present study to investigate possible chemical and physical changes by mechanical milling in a number of simple polymers.   Experimental procedures      For the milling of inert polymers two types of mills were used. The first mill is an improved design of a home-built vibratory mill, consisting of a stainless steel vial with an inner diameter of 6.5 cm and one hardened steel ball, with a diameter of 6 cm, as described in [6]. The second mill consists of two stainless-steel vials, with an inner height of 4 cm and an inner diameter of 4 cm, which are mounted on the disk of the planetary machine (Fritsch Pulvereisette 7). A vertical stainless-steel cylindrical-shaped rod with a height of 3 cm and a diameter of 3.2 cm was used in each mill as a milling tool. In order to guarantee the balance of the equipment both vials should contain the same quantity of material. While working, the disk is rotating clockwise, and the mills are rotating anti-clockwise. Then, the cylinder inside the mill moves always along the inner wall and performs the milling action. Typically 2 grams of polymer were loaded in each vial. The milling is carried out in an air atmosphere, and after a certain period (maximum 15 minutes) the milling is interrupted for cooling down of the rotator and for ensuring that the temperature of the mills does never exceed 50 °C.  The analysis of the milling-products was performed by X-ray diffraction, mass spectrometry, Gel Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC).  X-ray diffraction patterns were taken at room temperature by means of a position-sensitive Enraf Nonius PDS 120 diffractometer using Cu Kα radiation. The measuring range is from 2θ=10° to 130°. Mass spectrometry was performed at the Institute for Mass Spectrometry of the University of Amsterdam by a Field Desorption Mass Spectrometer (FDMS) [7], and at the FOM institute for Atomic and Molecular Physics (AMOLF) by Matrix-Assisted Laser Desorption and Ionisation on an external ion source Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (MALDI-FTICR-MS) [8]. GPC was carried out at Shell Research in Amsterdam (KSLA). DSC scans were taken on a Perkin-Elmer DSC-7 in an argon gas atmosphere.  Results and Discussion Milling of amorphous polymers      Several pure polymers were milled, mainly by using the planetary mill. As mentioned before, bond breaking (scission) and polymerisation could occur by milling (see for example [4]): chemical bonds could be broken and free radical ends could even form a new bond. This - in turn - would give a significant change in molecular masses. These are defined as follows [9], where ni is the number of molecules with mass Mi:       number-average molecular mass      M n =  Σ ni Mi / Σ ni      weight-average molecular mass       M w =  Σ ni Mi2 / Σ ni Mi      z-average molecular mass                M z =  Σ ni Mi3 / Σ ni Mi2 These quantities define the shape of the molecular mass distribution. We investigated polystyrene (chemical formula [−CH C6H5 CH2−]n) with Mw ≅ 2500, a standard purchased from Aldrich, and polystyrene (PS 87) from Shell Research with Mw ≅ 6150, by FDMS and GPC respectively. Polyethylene glycol (HO−[C2H4O]n−H), PEG 1000 from Serva (Heidelberg, Germany) with         Mw ≅ 1000,  and PEG 4000 with Mw ≅ 4000 from Fluka Chemical (Buchs, Switzerland) were analysed by MALDI-FTICR-MS. From field-desorption mass spectrometry performed on PS 2500 no significant change was found. This is clear from Table 1, where the values for the three characteristic masses are presented which were calculated from the mass spectra.   milling time Mn Mw Mz 0  minutes 2338 2562 2765 15 minutes 2466 2671 2859 30 minutes 2463 2723 2943 45 minutes 2329 2567 2768 60 minutes 2461 2697 2888  Table 1: The molecular masses Mj of PS 2500 as a function of milling time as measured                by mass spectrometry.  The same conclusion was drawn for PS 87 using GPC for the assessment of the molecular masses (Table 2). Any bond breaking or polymerisation would undoubtly have broadened the (narrow) mass distribution, and would thus have given an experimentally significant change in the values of the various molecular masses. Fig. 1, showing the molecular mass distribution before and after milling PS 87 clearly shows that no such broadening occurs. Neither was any change found by MALDI-FTICR-MS on polyethylene glycol performed at AMOLF.   Milling time Mn Mw Mz 0 hour                        5756 6155 6511 1 hour  5727 6161 6606  Table 2: The molecular masses Mj of PS 87 as a function of milling time as measured  by GPC.  Of course the mechanical properties of polymers are different from those of intermetallic compounds. The elastic character of the polymers investigated may make it more difficult to store energy in these materials by milling than in brittle intermetallic compounds. Moreover, the atomic defects that are induced in intermetallics have typically energies of 100-200 kJ/mole of defects [2], whereas the C-C bond in the polymers represents an energy of 328 kJ/mole [9]. This could also be the reason why it is difficult to break the C-C bond by milling. It is concluded that, by our milling tools, no bond breaking takes place. Thus no chemical changes were induced by milling of PS and PEG. X-ray diffraction showed that upon milling the materials remained fully amorphous. However, changes were observed in most of the milled samples of PS 87 by heating them in the DSC. Remarkably, two glass temperatures were observed after milling, one close to the original glass temperature of about 80° C and a second one at about 30° C higher temperature. After heating the samples to 200° C and an arrest at this temperature for a few minutes, the original glass temperature was recovered. Apparently physical changes of a so-far unknown nature are induced in the amorphous polymer by milling.   Milling of crystalline polyethylene, a structural phase transformation      High-density polyethylene (HDPE), chemical formula [−CH2 CH2−]n, is partially crystalline and is known in different crystal structures among which the normal orthorhombic one, and monoclinic [11]. The content of monoclinic in as-prepared material is normally small (about 5−10 %) as compared to the orthorhombic component, and is usually explained in terms of a somewhat different packing of the folded polymer chains in a strained lattice array [12,13].  HDPE beads with an average diameter of 4 mm and with Mw ≅ 2−3 × 105 (Lupolen 5021 D, ex. Basf) were milled in the vibratory mill. During milling the beads were broken into flakes. Fig. 2 shows the X-ray diffraction pattern of pure and ball-milled HDPE beads. Besides a number of weak peaks, the orthorhombic structure (O) of the original material (labelled 0h) is characterised by a very strong (110) peak and a strong (200) peak. Careful examination shows a very weak intensity of the monoclinic (110 ) peak (M). Apparently, the as-prepared polymer contains a trace of the latter phase as was found and analysed in [12,13]. As milling proceeds, the intensity of the orthorhombic (200) peak decreases and this peak almost disappears for milling periods longer than 165 hrs. The main (110) peak broadens, probably due to strain and possibly crystallite refinement, as is found in metallic systems. On the other hand a peak, left to the main peak, appears. This  (110 ) peak of the monoclinic phase is,  together with a (110) peak in the same position as the (110) peak of the orthorhombic structure, characteristic of the monoclinic structure [12,13]. From Fig. 2 it can be concluded that after 165 hrs of milling, by far most crystalline material is monoclinic, and it may even be implied that the material has nearly completely transformed to the monoclinic structure. This is, to our best knowledge, the first time that an (almost) complete structural phase transformation was driven by mechanical milling in a pure polymer. The explanation of the transformation could be sought in shear, induced by the milling process. It should be noted that, due to the irregular shape of    Fig. 1 The molecular mass distribution of (a) as-prepared and (b) 1 hour milled PS 87 samples as measured by GPC.   15 20 25 30 35 40O(200)O/M(110)M(110)350 h165 h95 h70 h0 hIntensity (a.u.) 2θ (degrees)  Fig. 2 X-ray diffraction patterns of HDPE Lupolen 5021 D after various periods of milling.   the flake-like samples, reproducible positioning of the samples in the X-ray diffractometer gives problems. Therefore, no conclusions should be drawn from small deviations in the peak positions.  In order to examine possible influences of chain length and sample shape, we milled ultra-high molecular weight HDPE powder and beads (Hostalen GUR 412, ex. Hoechst) with Mw ≅ 4−6 × 106 and low molecular weight HDPE beads (Stamylec 2H280, ex. DSM) with Mw ≅ 6 × 104 in the vibratory mill as well as in the planetary mill. In Table 3 we compare the end product forms and extent of transformation of these materials with HDPE Lupolen, which was also milled in both mills. It turns out that the only almost full transformation occurs for beads of Lupolen, milled in the vibratory mill and forming flakes.   Material Mw starting form type of mill end product form extent of transform. Hostalen GUR 412 4−6 × 106 bead (home made) vibratory bead −    planetary powder +/−   powder vibratory powder +/−    planetary powder +/− Lupolen 5021 D 2−3 × 105 bead vibratory flakes +    planetary powder +/− Stamylec 2H280 6 × 104 bead vibratory powder +/−    planetary powder +/−  Table 3: A comparison between the end products and extent of transformation of various materials under different milling conditions.   It can be seen that as soon as any material formed powders, the transformation was only partial, which leads us to the conclusion that the flake-like form is essential for a complete transformation and that molecular weight has scarcely any influence. For all materials the final state was much faster established in the planetary mill than in the vibratory mill. Hence the planetary mill is apparently more powerful than the vibratory mill. For the planetary mill the time required to form powder out of beads is longer for a higher Mw: about 2.5 hr for HDPE Stamylec, 6 hr for HDPE Lupolen and 20 hr for HDPE Hostalen. This can be understood from the influence of chain length on the force with which one molecule is bond and with some imagination it could be suggested that the milling time is linear to the root of the molecular mass Mw. No detectable changes occurred in DSC in any of the samples and after melting, the material became orthorhombic again.   Conclusions      Bond breaking (scission) and/or polymerisation by milling different pure polymers were not found neither using a planetary nor a vibratory mill. Milling beads of crystalline high-density polyethylene with high molecular weight drove an almost complete phase transformation from the orthorhombic to the monoclinic structure. The orthorhombic phase was restored after melting. The degree of transformation appeared to depend on the shape of the end-product. When flakes were obtained, the transformation was most complete.     Acknowledgements      The Dutch Foundation for Fundamental Research on Matter (FOM) and Shell Research are gratefully acknowledged for financial support. We thank Shell Research, DSM Research and AMOLF for providing the samples we used for milling. We are thankful to Dr Fokkens and Prof. Nibbering from the Institute of Mass Spectrometry of the University of Amsterdam and Dr R.M.A. Heeren from the FOM-institute for Atomic and Molecular Physics AMOLF for analysis of our samples by mass spectrometry.  References [1]  C.C. Koch, Materials Science and Technology, R.W. Cahn, P.Haasen and E.J. Kramer eds., (VHC, Cambridge, U.K., 1991), Vol. 15, p. 194 [2]  H. Bakker, G.F. Zhou and H. Yang, Progr. Mater. Science 39, 159 (1995). [3] W.J.D. Shaw, J. Pan and M.A. Gowler, Proceedings of the 2nd International Conference on Mechanical Alloying for Structural Application, J.J. de Barbadillo, F.H. Froes and R. Schwarz eds. (ASM International, Materials Park, USA, 1993), p431 [4] A. Ikekawa, Proceedings of the 1st International Conference on Mechanochemistry, K. Tcakova ed., (Cambridge Interscience Publ., Cambridge, UK, 1993), p106 [5]  T. Ishida, Journ. of Mater. Science Letters, 13, 623 (1994) [6]  I.W. Modder and H. Bakker, this conference [7] W.D. Reynolds, Analytical Chemistry, 61, 383A (1979) [8]  R.M.A. Heeren, Ch.G. de Koster and J.J. Boon, Analytical Chemistry, 67, 3965 (1995) [9]  D.W. van Krevelen, Properties of Polymers (Elsevier Science Amsterdam, 1990) [10]  H. Yang, Ph. D. Thesis, University of Amsterdam, The Netherlands (1996) [11]  J. Brandup and E.H. Immergut, Polymer handbook, third edition (Wiley, New York, 1989) [12]  C. Gieniewski and R.S. Moore, Macromolecules, 2, 385 (1969) [13]  C. Gieniewski and R.S. Moore, Macromolecules, 3, 97 (1970)    

UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl)
UvA-DARE (Digital Academic Repository)
A study of milling of pure polymers and a structural transformation of polyethylene
Castricum, H.L.; Yang, H.; Bakker, H.; van Deursen, J.H.
Published in:
Materials Science Forum
Link to publication
Citation for published version (APA):
Castricum, H. L., Yang, H., Bakker, H., & van Deursen, J. H. (1997). A study of milling of pure polymers and a
structural transformation of polyethylene. Materials Science Forum, 211-216.
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s),
other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating
your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask
the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam,
The Netherlands. You will be contacted as soon as possible.
Download date: 04 Feb 2021
Tel. No. :  (+31) 20 525 6493 
Fax. No. :  (+31) 20 525 5604 
e-mail     :  H.L.Castricum@ uva.nl 
 
A STUDY OF MILLING OF PURE POLYMERS AND A 
STRUCTURAL TRANSFORMATION OF POLYETHYLENE 
 
H.L. CASTRICUM1, H. YANG1, H. BAKKER1  AND J.H. VAN DEURSEN2 
1Van der Waals-Zeeman Laboratory, University of Amsterdam 
Valckenierstraat 65,  NL 1018 XE  Amsterdam, The Netherlands 
2Koninklijk Shell Laboratorium Amsterdam,  
Badhuisweg 3, NL 1031 CM Amsterdam, The Netherlands 
 
Keywords: polymer; glass temperature; shear; phase transformation; scission; polymerisation 
 
Abstract 
     We investigated possible physical and chemical changes in inert polymers by means of 
mechanical milling. No chemical changes were observed. However, in polystyrene we found two 
glass temperatures after milling, in contrast to one glass temperature in the original material. Under 
certain conditions crystalline polyethylene transformed almost completely from the orthorhombic to 
the monoclinic structure.  
 
Introduction 
     High-energy ball milling has been used widely during the past decade for mechanical alloying 
(MA) of metallic systems, i.e. starting from a mixture of elemental powders and for mechanical 
milling (MM) of metallic systems, i.e. starting from pre-alloyed intermetallic compounds [1,2]. MA 
leads to alloying on an atomic level in many cases and even it turned out to be possible to alloy 
immiscible metallic binaries. In intermetallic compounds MM induces atomic disorder in the early 
stage of milling and eventually a phase transformation may follow. Pure metals do not show a 
phase transformation, but become nanocrystalline (see, e.g. [1]).   
In contrast to the many studies on metallic systems, ball-milling experiments on polymers seem to 
be scarce [3,4]. In these scarce studies, on the one hand degradation of polymers by ball milling as a 
result of chain scission was reported [4], on the other hand mechanical milling of ABS 
(Acrylonitrile-Butadiene-Styrene) rather resulted in a small increase in the weight-average 
molecular mass than in a decrease [3]. In the same study [3] the alloying of normally incompatible 
polymer mixtures by MA was claimed. The possibility of producing amorphous powders of 
polymer mixtures by MA was reported by [5]. 
It was the aim of the present study to investigate possible chemical and physical changes by 
mechanical milling in a number of simple polymers.  
 
Experimental procedures 
     For the milling of inert polymers two types of mills were used. The first mill is an improved 
design of a home-built vibratory mill, consisting of a stainless steel vial with an inner diameter of 
6.5 cm and one hardened steel ball, with a diameter of 6 cm, as described in [6]. The second mill 
consists of two stainless-steel vials, with an inner height of 4 cm and an inner diameter of 4 cm, 
which are mounted on the disk of the planetary machine (Fritsch Pulvereisette 7). A vertical 
stainless-steel cylindrical-shaped rod with a height of 3 cm and a diameter of 3.2 cm was used in 
each mill as a milling tool. In order to guarantee the balance of the equipment both vials should 
contain the same quantity of material. While working, the disk is rotating clockwise, and the mills 
are rotating anti-clockwise. Then, the cylinder inside the mill moves always along the inner wall 
and performs the milling action. Typically 2 grams of polymer were loaded in each vial. The 
milling is carried out in an air atmosphere, and after a certain period (maximum 15 minutes) the 
milling is interrupted for cooling down of the rotator and for ensuring that the temperature of the 
mills does never exceed 50 °C.  
The analysis of the milling-products was performed by X-ray diffraction, mass spectrometry, Gel 
Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC).  
X-ray diffraction patterns were taken at room temperature by means of a position-sensitive Enraf 
Nonius PDS 120 diffractometer using Cu Kα radiation. The measuring range is from 2θ=10° to 
130°. Mass spectrometry was performed at the Institute for Mass Spectrometry of the University of 
Amsterdam by a Field Desorption Mass Spectrometer (FDMS) [7], and at the FOM institute for 
Atomic and Molecular Physics (AMOLF) by Matrix-Assisted Laser Desorption and Ionisation on 
an external ion source Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (MALDI-
FTICR-MS) [8]. GPC was carried out at Shell Research in Amsterdam (KSLA). DSC scans were 
taken on a Perkin-Elmer DSC-7 in an argon gas atmosphere. 
 
Results and Discussion 
Milling of amorphous polymers 
     Several pure polymers were milled, mainly by using the planetary mill. As mentioned before, 
bond breaking (scission) and polymerisation could occur by milling (see for example [4]): chemical 
bonds could be broken and free radical ends could even form a new bond. This - in turn - would 
give a significant change in molecular masses. These are defined as follows [9], where ni is the 
number of molecules with mass Mi:  
     number-average molecular mass      M n =  Σ ni Mi / Σ ni 
     weight-average molecular mass       M w =  Σ ni Mi2 / Σ ni Mi 
     z-average molecular mass                M z =  Σ ni Mi3 / Σ ni Mi2 
These quantities define the shape of the molecular mass distribution. We investigated polystyrene 
(chemical formula [−CH C6H5 CH2−]n) with Mw ≅ 2500, a standard purchased from Aldrich, and 
polystyrene (PS 87) from Shell Research with Mw ≅ 6150, by FDMS and GPC respectively. 
Polyethylene glycol (HO−[C2H4O]n−H), PEG 1000 from Serva (Heidelberg, Germany) with         
Mw ≅ 1000,  and PEG 4000 with Mw ≅ 4000 from Fluka Chemical (Buchs, Switzerland) were 
analysed by MALDI-FTICR-MS. From field-desorption mass spectrometry performed on PS 2500 
no significant change was found. This is clear from Table 1, where the values for the three 
characteristic masses are presented which were calculated from the mass spectra.  
 
milling time Mn Mw Mz 
0  minutes 2338 2562 2765 
15 minutes 2466 2671 2859 
30 minutes 2463 2723 2943 
45 minutes 2329 2567 2768 
60 minutes 2461 2697 2888 
 
Table 1: The molecular masses Mj of PS 2500 as a function of milling time as measured                
by mass spectrometry. 
 
The same conclusion was drawn for PS 87 using GPC for the assessment of the molecular masses 
(Table 2). Any bond breaking or polymerisation would undoubtly have broadened the (narrow) 
mass distribution, and would thus have given an experimentally significant change in the values of 
the various molecular masses. Fig. 1, showing the molecular mass distribution before and after 
milling PS 87 clearly shows that no such broadening occurs. Neither was any change found by 
MALDI-FTICR-MS on polyethylene glycol performed at AMOLF.  
 
Milling time Mn Mw Mz 
0 hour                        5756 6155 6511 
1 hour  5727 6161 6606 
 
Table 2: The molecular masses Mj of PS 87 as a function of milling time as measured  
by GPC. 
 
Of course the mechanical properties of polymers are different from those of intermetallic 
compounds. The elastic character of the polymers investigated may make it more difficult to store 
energy in these materials by milling than in brittle intermetallic compounds. Moreover, the atomic 
defects that are induced in intermetallics have typically energies of 100-200 kJ/mole of defects [2], 
whereas the C-C bond in the polymers represents an energy of 328 kJ/mole [9]. This could also be 
the reason why it is difficult to break the C-C bond by milling. It is concluded that, by our milling 
tools, no bond breaking takes place. Thus no chemical changes were induced by milling of PS and 
PEG. X-ray diffraction showed that upon milling the materials remained fully amorphous. 
However, changes were observed in most of the milled samples of PS 87 by heating them in the 
DSC. Remarkably, two glass temperatures were observed after milling, one close to the original 
glass temperature of about 80° C and a second one at about 30° C higher temperature. After heating 
the samples to 200° C and an arrest at this temperature for a few minutes, the original glass 
temperature was recovered. Apparently physical changes of a so-far unknown nature are induced in 
the amorphous polymer by milling.  
 
Milling of crystalline polyethylene, a structural phase transformation 
     High-density polyethylene (HDPE), chemical formula [−CH2 CH2−]n, is partially crystalline and 
is known in different crystal structures among which the normal orthorhombic one, and monoclinic 
[11]. The content of monoclinic in as-prepared material is normally small (about 5−10 %) as 
compared to the orthorhombic component, and is usually explained in terms of a somewhat 
different packing of the folded polymer chains in a strained lattice array [12,13].  
HDPE beads with an average diameter of 4 mm and with Mw ≅ 2−3 × 105 (Lupolen 5021 D, ex. 
Basf) were milled in the vibratory mill. During milling the beads were broken into flakes. Fig. 2 
shows the X-ray diffraction pattern of pure and ball-milled HDPE beads. Besides a number of weak 
peaks, the orthorhombic structure (O) of the original material (labelled 0h) is characterised by a 
very strong (110) peak and a strong (200) peak. Careful examination shows a very weak intensity of 
the monoclinic (110 ) peak (M). Apparently, the as-prepared polymer contains a trace of the latter 
phase as was found and analysed in [12,13]. As milling proceeds, the intensity of the orthorhombic 
(200) peak decreases and this peak almost disappears for milling periods longer than 165 hrs. The 
main (110) peak broadens, probably due to strain and possibly crystallite refinement, as is found in 
metallic systems. On the other hand a peak, left to the main peak, appears. This  (110 ) peak of the 
monoclinic phase is,  together with a (110) peak in the same position as the (110) peak of the 
orthorhombic structure, characteristic of the monoclinic structure [12,13]. From Fig. 2 it can be 
concluded that after 165 hrs of milling, by far most crystalline material is monoclinic, and it may 
even be implied that the material has nearly completely transformed to the monoclinic structure. 
This is, to our best knowledge, the first time that an (almost) complete structural phase 
transformation was driven by mechanical milling in a pure polymer. The explanation of the 
transformation could be sought in shear, induced by the milling process. It should be noted that, due 
to the irregular shape of  
 
 
Fig. 1 The molecular mass distribution of (a) as-prepared and (b) 1 hour milled PS 87 samples as 
measured by GPC. 
 
 
15 20 25 30 35 40
O
(2
00
)
O
/M
(1
10
)
M
(1
10
)
350 h
165 h
95 h
70 h
0 h
In
te
ns
ity
 (
a.
u.
) 
2θ (degrees)
 
 
Fig. 2 X-ray diffraction patterns of HDPE Lupolen 5021 D after various periods of milling. 
 
 
the flake-like samples, reproducible positioning of the samples in the X-ray diffractometer gives 
problems. Therefore, no conclusions should be drawn from small deviations in the peak positions.  
In order to examine possible influences of chain length and sample shape, we milled ultra-high 
molecular weight HDPE powder and beads (Hostalen GUR 412, ex. Hoechst) with Mw ≅ 4−6 × 106 
and low molecular weight HDPE beads (Stamylec 2H280, ex. DSM) with Mw ≅ 6 × 104 in the 
vibratory mill as well as in the planetary mill. In Table 3 we compare the end product forms and 
extent of transformation of these materials with HDPE Lupolen, which was also milled in both 
mills. It turns out that the only almost full transformation occurs for beads of Lupolen, milled in the 
vibratory mill and forming flakes.  
 
Material Mw starting 
form 
type of mill end product 
form 
extent of 
transform. 
Hostalen 
GUR 412 
4−6 × 106 bead (home 
made) 
vibratory bead − 
   planetary powder +/− 
  powder vibratory powder +/− 
   planetary powder +/− 
Lupolen 
5021 D 
2−3 × 105 bead vibratory flakes + 
   planetary powder +/− 
Stamylec 
2H280 
6 × 104 bead vibratory powder +/− 
   planetary powder +/− 
 
Table 3: A comparison between the end products and extent of transformation of various materials 
under different milling conditions. 
  
It can be seen that as soon as any material formed powders, the transformation was only partial, 
which leads us to the conclusion that the flake-like form is essential for a complete transformation 
and that molecular weight has scarcely any influence. For all materials the final state was much 
faster established in the planetary mill than in the vibratory mill. Hence the planetary mill is 
apparently more powerful than the vibratory mill. For the planetary mill the time required to form 
powder out of beads is longer for a higher Mw: about 2.5 hr for HDPE Stamylec, 6 hr for HDPE 
Lupolen and 20 hr for HDPE Hostalen. This can be understood from the influence of chain length 
on the force with which one molecule is bond and with some imagination it could be suggested that 
the milling time is linear to the root of the molecular mass Mw. No detectable changes occurred in 
DSC in any of the samples and after melting, the material became orthorhombic again.  
 
Conclusions 
     Bond breaking (scission) and/or polymerisation by milling different pure polymers were not 
found neither using a planetary nor a vibratory mill. Milling beads of crystalline high-density 
polyethylene with high molecular weight drove an almost complete phase transformation from the 
orthorhombic to the monoclinic structure. The orthorhombic phase was restored after melting. The 
degree of transformation appeared to depend on the shape of the end-product. When flakes were 
obtained, the transformation was most complete.  
 
 
 
Acknowledgements 
     The Dutch Foundation for Fundamental Research on Matter (FOM) and Shell Research are 
gratefully acknowledged for financial support. We thank Shell Research, DSM Research and 
AMOLF for providing the samples we used for milling. We are thankful to Dr Fokkens and Prof. 
Nibbering from the Institute of Mass Spectrometry of the University of Amsterdam and Dr R.M.A. 
Heeren from the FOM-institute for Atomic and Molecular Physics AMOLF for analysis of our 
samples by mass spectrometry. 
 
References 
[1]  C.C. Koch, Materials Science and Technology, R.W. Cahn, P.Haasen and E.J. Kramer eds., 
(VHC, Cambridge, U.K., 1991), Vol. 15, p. 194 
[2]  H. Bakker, G.F. Zhou and H. Yang, Progr. Mater. Science 39, 159 (1995). 
[3] W.J.D. Shaw, J. Pan and M.A. Gowler, Proceedings of the 2nd International Conference on 
Mechanical Alloying for Structural Application, J.J. de Barbadillo, F.H. Froes and R. Schwarz eds. 
(ASM International, Materials Park, USA, 1993), p431 
[4] A. Ikekawa, Proceedings of the 1st International Conference on Mechanochemistry, K. Tcakova 
ed., (Cambridge Interscience Publ., Cambridge, UK, 1993), p106 
[5]  T. Ishida, Journ. of Mater. Science Letters, 13, 623 (1994) 
[6]  I.W. Modder and H. Bakker, this conference 
[7] W.D. Reynolds, Analytical Chemistry, 61, 383A (1979) 
[8]  R.M.A. Heeren, Ch.G. de Koster and J.J. Boon, Analytical Chemistry, 67, 3965 (1995) 
[9]  D.W. van Krevelen, Properties of Polymers (Elsevier Science Amsterdam, 1990) 
[10]  H. Yang, Ph. D. Thesis, University of Amsterdam, The Netherlands (1996) 
[11]  J. Brandup and E.H. Immergut, Polymer handbook, third edition (Wiley, New York, 1989) 
[12]  C. Gieniewski and R.S. Moore, Macromolecules, 2, 385 (1969) 
[13]  C. Gieniewski and R.S. Moore, Macromolecules, 3, 97 (1970) 
 
 
 


UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)A study of milling of pure polymers and a structural transformation ofpolyethyleneCastricum, H.L.; Yang, H.; Bakker, H.; van Deursen, J.H.Publication date1997Published inMaterials Science ForumLink to publicationCitation for published version (APA):Castricum, H. L., Yang, H., Bakker, H., & van Deursen, J. H. (1997). A study of milling of purepolymers and a structural transformation of polyethylene. Materials Science Forum, 211-216.General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:10 Mar 2023Tel. No. :  (+31) 20 525 6493 Fax. No. :  (+31) 20 525 5604 e-mail     :  H.L.Castricum@ uva.nl  A STUDY OF MILLING OF PURE POLYMERS AND A STRUCTURAL TRANSFORMATION OF POLYETHYLENE  H.L. CASTRICUM1, H. YANG1, H. BAKKER1  AND J.H. VAN DEURSEN2 1Van der Waals-Zeeman Laboratory, University of Amsterdam Valckenierstraat 65,  NL 1018 XE  Amsterdam, The Netherlands 2Koninklijk Shell Laboratorium Amsterdam,  Badhuisweg 3, NL 1031 CM Amsterdam, The Netherlands  Keywords: polymer; glass temperature; shear; phase transformation; scission; polymerisation  Abstract      We investigated possible physical and chemical changes in inert polymers by means of mechanical milling. No chemical changes were observed. However, in polystyrene we found two glass temperatures after milling, in contrast to one glass temperature in the original material. Under certain conditions crystalline polyethylene transformed almost completely from the orthorhombic to the monoclinic structure.   Introduction      High-energy ball milling has been used widely during the past decade for mechanical alloying (MA) of metallic systems, i.e. starting from a mixture of elemental powders and for mechanical milling (MM) of metallic systems, i.e. starting from pre-alloyed intermetallic compounds [1,2]. MA leads to alloying on an atomic level in many cases and even it turned out to be possible to alloy immiscible metallic binaries. In intermetallic compounds MM induces atomic disorder in the early stage of milling and eventually a phase transformation may follow. Pure metals do not show a phase transformation, but become nanocrystalline (see, e.g. [1]).   In contrast to the many studies on metallic systems, ball-milling experiments on polymers seem to be scarce [3,4]. In these scarce studies, on the one hand degradation of polymers by ball milling as a result of chain scission was reported [4], on the other hand mechanical milling of ABS (Acrylonitrile-Butadiene-Styrene) rather resulted in a small increase in the weight-average molecular mass than in a decrease [3]. In the same study [3] the alloying of normally incompatible polymer mixtures by MA was claimed. The possibility of producing amorphous powders of polymer mixtures by MA was reported by [5]. It was the aim of the present study to investigate possible chemical and physical changes by mechanical milling in a number of simple polymers.   Experimental procedures      For the milling of inert polymers two types of mills were used. The first mill is an improved design of a home-built vibratory mill, consisting of a stainless steel vial with an inner diameter of 6.5 cm and one hardened steel ball, with a diameter of 6 cm, as described in [6]. The second mill consists of two stainless-steel vials, with an inner height of 4 cm and an inner diameter of 4 cm, which are mounted on the disk of the planetary machine (Fritsch Pulvereisette 7). A vertical stainless-steel cylindrical-shaped rod with a height of 3 cm and a diameter of 3.2 cm was used in each mill as a milling tool. In order to guarantee the balance of the equipment both vials should contain the same quantity of material. While working, the disk is rotating clockwise, and the mills are rotating anti-clockwise. Then, the cylinder inside the mill moves always along the inner wall and performs the milling action. Typically 2 grams of polymer were loaded in each vial. The milling is carried out in an air atmosphere, and after a certain period (maximum 15 minutes) the milling is interrupted for cooling down of the rotator and for ensuring that the temperature of the mills does never exceed 50 °C.  The analysis of the milling-products was performed by X-ray diffraction, mass spectrometry, Gel Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC).  X-ray diffraction patterns were taken at room temperature by means of a position-sensitive Enraf Nonius PDS 120 diffractometer using Cu Kα radiation. The measuring range is from 2θ=10° to 130°. Mass spectrometry was performed at the Institute for Mass Spectrometry of the University of Amsterdam by a Field Desorption Mass Spectrometer (FDMS) [7], and at the FOM institute for Atomic and Molecular Physics (AMOLF) by Matrix-Assisted Laser Desorption and Ionisation on an external ion source Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (MALDI-FTICR-MS) [8]. GPC was carried out at Shell Research in Amsterdam (KSLA). DSC scans were taken on a Perkin-Elmer DSC-7 in an argon gas atmosphere.  Results and Discussion Milling of amorphous polymers      Several pure polymers were milled, mainly by using the planetary mill. As mentioned before, bond breaking (scission) and polymerisation could occur by milling (see for example [4]): chemical bonds could be broken and free radical ends could even form a new bond. This - in turn - would give a significant change in molecular masses. These are defined as follows [9], where ni is the number of molecules with mass Mi:       number-average molecular mass      M n =  Σ ni Mi / Σ ni      weight-average molecular mass       M w =  Σ ni Mi2 / Σ ni Mi      z-average molecular mass                M z =  Σ ni Mi3 / Σ ni Mi2 These quantities define the shape of the molecular mass distribution. We investigated polystyrene (chemical formula [−CH C6H5 CH2−]n) with Mw ≅ 2500, a standard purchased from Aldrich, and polystyrene (PS 87) from Shell Research with Mw ≅ 6150, by FDMS and GPC respectively. Polyethylene glycol (HO−[C2H4O]n−H), PEG 1000 from Serva (Heidelberg, Germany) with         Mw ≅ 1000,  and PEG 4000 with Mw ≅ 4000 from Fluka Chemical (Buchs, Switzerland) were analysed by MALDI-FTICR-MS. From field-desorption mass spectrometry performed on PS 2500 no significant change was found. This is clear from Table 1, where the values for the three characteristic masses are presented which were calculated from the mass spectra.   milling time Mn Mw Mz 0  minutes 2338 2562 2765 15 minutes 2466 2671 2859 30 minutes 2463 2723 2943 45 minutes 2329 2567 2768 60 minutes 2461 2697 2888  Table 1: The molecular masses Mj of PS 2500 as a function of milling time as measured                by mass spectrometry.  The same conclusion was drawn for PS 87 using GPC for the assessment of the molecular masses (Table 2). Any bond breaking or polymerisation would undoubtly have broadened the (narrow) mass distribution, and would thus have given an experimentally significant change in the values of the various molecular masses. Fig. 1, showing the molecular mass distribution before and after milling PS 87 clearly shows that no such broadening occurs. Neither was any change found by MALDI-FTICR-MS on polyethylene glycol performed at AMOLF.   Milling time Mn Mw Mz 0 hour                        5756 6155 6511 1 hour  5727 6161 6606  Table 2: The molecular masses Mj of PS 87 as a function of milling time as measured  by GPC.  Of course the mechanical properties of polymers are different from those of intermetallic compounds. The elastic character of the polymers investigated may make it more difficult to store energy in these materials by milling than in brittle intermetallic compounds. Moreover, the atomic defects that are induced in intermetallics have typically energies of 100-200 kJ/mole of defects [2], whereas the C-C bond in the polymers represents an energy of 328 kJ/mole [9]. This could also be the reason why it is difficult to break the C-C bond by milling. It is concluded that, by our milling tools, no bond breaking takes place. Thus no chemical changes were induced by milling of PS and PEG. X-ray diffraction showed that upon milling the materials remained fully amorphous. However, changes were observed in most of the milled samples of PS 87 by heating them in the DSC. Remarkably, two glass temperatures were observed after milling, one close to the original glass temperature of about 80° C and a second one at about 30° C higher temperature. After heating the samples to 200° C and an arrest at this temperature for a few minutes, the original glass temperature was recovered. Apparently physical changes of a so-far unknown nature are induced in the amorphous polymer by milling.   Milling of crystalline polyethylene, a structural phase transformation      High-density polyethylene (HDPE), chemical formula [−CH2 CH2−]n, is partially crystalline and is known in different crystal structures among which the normal orthorhombic one, and monoclinic [11]. The content of monoclinic in as-prepared material is normally small (about 5−10 %) as compared to the orthorhombic component, and is usually explained in terms of a somewhat different packing of the folded polymer chains in a strained lattice array [12,13].  HDPE beads with an average diameter of 4 mm and with Mw ≅ 2−3 × 105 (Lupolen 5021 D, ex. Basf) were milled in the vibratory mill. During milling the beads were broken into flakes. Fig. 2 shows the X-ray diffraction pattern of pure and ball-milled HDPE beads. Besides a number of weak peaks, the orthorhombic structure (O) of the original material (labelled 0h) is characterised by a very strong (110) peak and a strong (200) peak. Careful examination shows a very weak intensity of the monoclinic (110 ) peak (M). Apparently, the as-prepared polymer contains a trace of the latter phase as was found and analysed in [12,13]. As milling proceeds, the intensity of the orthorhombic (200) peak decreases and this peak almost disappears for milling periods longer than 165 hrs. The main (110) peak broadens, probably due to strain and possibly crystallite refinement, as is found in metallic systems. On the other hand a peak, left to the main peak, appears. This  (110 ) peak of the monoclinic phase is,  together with a (110) peak in the same position as the (110) peak of the orthorhombic structure, characteristic of the monoclinic structure [12,13]. From Fig. 2 it can be concluded that after 165 hrs of milling, by far most crystalline material is monoclinic, and it may even be implied that the material has nearly completely transformed to the monoclinic structure. This is, to our best knowledge, the first time that an (almost) complete structural phase transformation was driven by mechanical milling in a pure polymer. The explanation of the transformation could be sought in shear, induced by the milling process. It should be noted that, due to the irregular shape of    Fig. 1 The molecular mass distribution of (a) as-prepared and (b) 1 hour milled PS 87 samples as measured by GPC.   15 20 25 30 35 40O(200)O/M(110)M(110)350 h165 h95 h70 h0 hIntensity (a.u.) 2θ (degrees)  Fig. 2 X-ray diffraction patterns of HDPE Lupolen 5021 D after various periods of milling.   the flake-like samples, reproducible positioning of the samples in the X-ray diffractometer gives problems. Therefore, no conclusions should be drawn from small deviations in the peak positions.  In order to examine possible influences of chain length and sample shape, we milled ultra-high molecular weight HDPE powder and beads (Hostalen GUR 412, ex. Hoechst) with Mw ≅ 4−6 × 106 and low molecular weight HDPE beads (Stamylec 2H280, ex. DSM) with Mw ≅ 6 × 104 in the vibratory mill as well as in the planetary mill. In Table 3 we compare the end product forms and extent of transformation of these materials with HDPE Lupolen, which was also milled in both mills. It turns out that the only almost full transformation occurs for beads of Lupolen, milled in the vibratory mill and forming flakes.   Material Mw starting form type of mill end product form extent of transform. Hostalen GUR 412 4−6 × 106 bead (home made) vibratory bead −    planetary powder +/−   powder vibratory powder +/−    planetary powder +/− Lupolen 5021 D 2−3 × 105 bead vibratory flakes +    planetary powder +/− Stamylec 2H280 6 × 104 bead vibratory powder +/−    planetary powder +/−  Table 3: A comparison between the end products and extent of transformation of various materials under different milling conditions.   It can be seen that as soon as any material formed powders, the transformation was only partial, which leads us to the conclusion that the flake-like form is essential for a complete transformation and that molecular weight has scarcely any influence. For all materials the final state was much faster established in the planetary mill than in the vibratory mill. Hence the planetary mill is apparently more powerful than the vibratory mill. For the planetary mill the time required to form powder out of beads is longer for a higher Mw: about 2.5 hr for HDPE Stamylec, 6 hr for HDPE Lupolen and 20 hr for HDPE Hostalen. This can be understood from the influence of chain length on the force with which one molecule is bond and with some imagination it could be suggested that the milling time is linear to the root of the molecular mass Mw. No detectable changes occurred in DSC in any of the samples and after melting, the material became orthorhombic again.   Conclusions      Bond breaking (scission) and/or polymerisation by milling different pure polymers were not found neither using a planetary nor a vibratory mill. Milling beads of crystalline high-density polyethylene with high molecular weight drove an almost complete phase transformation from the orthorhombic to the monoclinic structure. The orthorhombic phase was restored after melting. The degree of transformation appeared to depend on the shape of the end-product. When flakes were obtained, the transformation was most complete.     Acknowledgements      The Dutch Foundation for Fundamental Research on Matter (FOM) and Shell Research are gratefully acknowledged for financial support. We thank Shell Research, DSM Research and AMOLF for providing the samples we used for milling. We are thankful to Dr Fokkens and Prof. Nibbering from the Institute of Mass Spectrometry of the University of Amsterdam and Dr R.M.A. Heeren from the FOM-institute for Atomic and Molecular Physics AMOLF for analysis of our samples by mass spectrometry.  References [1]  C.C. Koch, Materials Science and Technology, R.W. Cahn, P.Haasen and E.J. Kramer eds., (VHC, Cambridge, U.K., 1991), Vol. 15, p. 194 [2]  H. Bakker, G.F. Zhou and H. Yang, Progr. Mater. Science 39, 159 (1995). [3] W.J.D. Shaw, J. Pan and M.A. Gowler, Proceedings of the 2nd International Conference on Mechanical Alloying for Structural Application, J.J. de Barbadillo, F.H. Froes and R. Schwarz eds. (ASM International, Materials Park, USA, 1993), p431 [4] A. Ikekawa, Proceedings of the 1st International Conference on Mechanochemistry, K. Tcakova ed., (Cambridge Interscience Publ., Cambridge, UK, 1993), p106 [5]  T. Ishida, Journ. of Mater. Science Letters, 13, 623 (1994) [6]  I.W. Modder and H. Bakker, this conference [7] W.D. Reynolds, Analytical Chemistry, 61, 383A (1979) [8]  R.M.A. Heeren, Ch.G. de Koster and J.J. Boon, Analytical Chemistry, 67, 3965 (1995) [9]  D.W. van Krevelen, Properties of Polymers (Elsevier Science Amsterdam, 1990) [10]  H. Yang, Ph. D. Thesis, University of Amsterdam, The Netherlands (1996) [11]  J. Brandup and E.H. Immergut, Polymer handbook, third edition (Wiley, New York, 1989) [12]  C. Gieniewski and R.S. Moore, Macromolecules, 2, 385 (1969) [13]  C. Gieniewski and R.S. Moore, Macromolecules, 3, 97 (1970)    

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