Structure and properties of b

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STRUCTURE EVOLUTION OF - AND -POLYPROPYLENES UPON UV
IRRADIATION: A MULTI-SCALE COMPARISON
Martin Obadal1*, Roman Čermák1, Miroslav Raab2, Vincent Verney3, Sophie Commereuc3
and Frederic Fraïsse3
1
Tomas Bata University in Zlín, Faculty of Technology
TGM 275, 762 72 Zlín, Czech Republic
2
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic,
162 06 Prague, Czech Republic
3
Molecular and Macromolecular Photochemistry Laboratory, Blaise Pascal University/CNRS
63177 Aubière, Cedex, France
*
Corresponding author
[email protected]
phone: +420 57 603 1334
fax: +420 57 603 1328
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ABSTRACT
The effects of UV irradiation on neat and -nucleated isotactic polypropylenes have been
studied at molecular, morphological and microscopic levels. Commercially available isotactic
polypropylene (PP)
was
modified by a
specific
-nucleating agent
based on
N, N´-dicyclohexylnaphthalene-2, 6-dicarboxamide. Compression-moulded plates were
prepared from both starting (-PP) and nucleated (-PP) polypropylenes and exposed to UVirradiation. The exposure time varied from 0 to 240 hours. Molecular degradation and the
evolution of supermolecular structure were determined using infrared spectroscopy, wideangle X-ray scattering and differential scanning calorimetry; surface cracking was observed
by light microscopy. Lower molecular degradability was found in -PP as compared to -PP;
an increase in crystallinity upon short-time exposure followed by crystallinity decrease with
prolonged exposure was detected in both samples. The -phase content within the crystalline
portion of -PP remained stable during UV irradiation under given irradiation conditions. Remelting experiments have shown that the crystallization ability of both -PP and -PP
markedly decreased with exposure time. Lower degradability of -PP as compared to -PP
has been ascribed to higher light absorbance resulting from the specific morphology of -PP.
KEYWORDS
-Polypropylene, Photo-oxidation, Structure evolution
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1 INTRODUCTION
Isotactic polypropylene (PP) has become one of the most commonly used thermoplastics.
This polymer exhibits excellent chemical resistance, low density, relatively high tensile
strength and high melting point. From the scientific point of view, PP is interesting because of
its polymorphism (,  and  crystalline phases) [1]. The monoclinic -phase is the most
common in melt-crystallized or solution-crystallized samples [1-5]. The orthorhombic -phase
was first reported in the 1960s and can be generated either by crystallization at high pressure
of the homopolymer or by crystallization at atmospheric pressure of low molecular weight
fractions or polypropylene-derived copolymers [2, 5-7]. The trigonal β-phase is observed only
occasionally during crystallization of neat PP from a melt. The relative amount of β-phase can
be considerably increased by crystallization in shear fields, in temperature gradients, or by
adding specific nucleating agents [4, 5]. Currently, the addition of selective -nucleators is the
most reliable preparation method of -phase; in particular, high-nucleation activity of a
nucleator based on N, N´-dicyclohexylnaphthalene-2, 6-dicarboxamide has been reported in
several recent works [10-12]. It has been shown that the presence of -phase significantly
influences important properties of the material: it highly enhances its toughness and
drawability but lowers E-modulus [8-12].
As generally accepted, degradation reactions of semicrystalline polymers proceed
predominantly in amorphous regions. Nevertheless, physical factors, such as the size,
arrangement and distribution of crystalline regions, affect the degradation process as well.
Photodegradation kinetics in polymer systems basically depends on oxygen permeability
through the material [13]. The rate of oxidation drops with decreasing oxygen diffusion,
following the increase of crystallinity and molecular orientation. Thus, degradability of
semicrystalline polymers is significantly influenced by their morphology. The effect of
physical structure on the degradation of polypropylene (especially induced by UV irradiation)
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has been studied in many works [e.g. 14-19]; however, according to our knowledge, except
for the findings by Kotek et al. [20] and Obadal et al. [21, 22], virtually no comprehensive
data on the effect of -phase on polypropylene degradation have been presented. And yet, the
effect of specific structures in both amorphous and crystalline phases of -PP and -PP could
and should be expected. Furthermore, although both -PP and -PP show identical glass
transition temperature (Tg), the intensity of glass transition, as indicated by dynamic
mechanical properties and thermal behaviour, is substantially higher for -PP [23, 24].
Structural models suggesting different phase continuity between -PP and -PP have been
reported [10, 11, 23], but no unique explanation of this phenomenon has been accepted yet.
The aim of the present work was to compare the degradation behaviour of - and polypropylenes. A combination of several experimental methods has been applied to describe
the structural evolutions of PP on the molecular, supermolecular and microscopic levels.
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2 EXPERIMENTAL
2.1 Materials and specimens
The basic material used throughout this study was isotactic polypropylene Mosten 58412
manufactured by Chemopetrol Litvínov a.s., Czech Republic. The material is characterized by
a melt flow index of 3 g/10 min (2.16 kg, 230 °C, ISO 1133), a weight-average molecular
weight approx. 170000 (GPC) and an isotacticity index of 98 % (ISO 9113). The material
contained a standard stabilization package based on phenol-phosphite stabilizers including
Irganox 1010, Irganox 1076 and Irgafos 168 produced by Ciba Specialty Chemicals Inc.,
Basel,
Switzerland.
A
commercial
-nucleator,
the
NJ
Star
NU-100
(N, N´-dicyclohexylnaphthalene-2, 6-dicarboxamide), supplied by Rika Int., Manchester,
Great Britain, was used. To obtain homogeneous distribution of the nucleator in
polypropylene pellets, 0.30 wt.% paraffin oil and 0.03 wt.% nucleator (related to the resulting
material) were mechanically immixed and subsequently processed in a Brabender twin-screw
extruder into PP pellets. The nucleator concentration of 0.03 wt.% has recently been described
as causing maximum effect on the mechanical properties of PP [10-12].
Plates with the thickness of approximately 100 m were compression-moulded from both
starting (-PP), and nucleated (-PP) materials. After 1-minute pressing at 210 °C, the plates
were cooled at 60 °C for 10 minutes. From the plates, rectangular specimens for IR
spectroscopy measurements and circular specimens for X-ray measurements were cut.
Microsamples for DSC were taken from larger irradiated plates.
2.2 UV exposure
The specimens were irradiated at 60 °C in a routinely used SEPAP 12.24 irradiation device
see e.g. 25-27 equipped with four medium-pressure mercury lamps. During irradiation the
back side of the specimens was covered with an aluminium foil. The following exposure
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times were applied: 0, 24, 36, 48, 60, 72, 96, 120, 144, 196 and 240 hours. The WAXS
specimens irradiated for 240 hours, i.e. for the highest exposure time, were subsequently
heated to 200 °C in a hot-air oven, then cooled freely to room temperature, and, after that
thermal history, measured by WAXS.
2.3 Infrared spectroscopy and light microscopy
A Nicolet 800 spectrometer with nominal resolution of 4 cm-1 was employed in transmission
mode with a 32-scan summation. Molecular degradation was characterized by carbonyl index.
It was calculated as the area of the carbonyl absorption bands AC (occurring in the range from
1700 to 1800 cm-1) related to the area of a reference band AR (ranging from 2700 to
2750 cm-1):
Carbonyl index = AC / AR
(1)
The shape of the carbonyl band is typically broad as it reflects several degradation products.
On the other hand, the reference absorption band associated with CH bending and CH3
stretching is narrow, and it is affected neither by photo-oxidation nor by varying crystallinity.
Moreover, it is not overlapped with other absorption bands. Therefore it is frequently used as
reference [16].
A Nicplan microscope was employed in reflection mode for the examination of irradiated
specimen surfaces.
2.4 X-ray scattering
The evolution of morphology was measured by wide-angle X-ray scattering (WAXS) as a
function of exposure time. Measurements were performed using a Philips XPert
diffractometer. CuK radiation monochromatized with a Ni filter ( = 0.154 nm) and
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diffraction angle interval 2 = 10-35 ° were used. The -phase fraction (K) in the specimens
was calculated from X-ray diffractograms according to Turner-Jones et al. [2] as follows:
K = H / (H1 + H2 + H3 + H)
(2),
where H1, H2, H3 are the intensities of -diffraction peaks corresponding to angles
2 = 14.2 °; 17 ° and 18.8 °, respectively, and H is the intensity of the  peak at 2 = 16.2 °.
The K value approximately implies the relative content of -phase in a specimen.
Furthermore, the ratio of the integral intensities diffracted by a crystalline part (Ic) and total
integral intensities (I) was used to determine crystallinity (X = 100  Ic / I).
2.5 Differential scanning calorimetry
A Mettler Toledo 822e differential scanning calorimeter was employed. Nitrogen was used as
a purge gas constantly passing (50 ml/s) through the heat sink and over the cell. Irradiated PP and -PP samples of approximately 2 mg were loaded into standard aluminium pans,
heated from 50 °C to 200 °C, then held at this temperature for 2 minutes and subsequently
cooled to 80 °C. A 20 °C/min heating rate was used for all thermal scans.
I:8
3 RESULTS AND DISCUSSION
3.1 Degradation kinetics
The evolution of IR spectra in the region of carbonyl absorption (1700 to 1800 cm -1) is
presented in Fig. 1. The absorption bands in this region are generally ascribed to carbonylated
by-products as convolutions of carboxylic acids, ketones, peresters and lactones [28, 29]. The
spectra indicate gradual formation of carboxylic acid (absorption bands centered at 1712 cm-1)
as they occur already at the beginning of the exposure period. With exposure period longer
than 60 hours absorption bands at 1720 cm-1 and 1780 cm-1 indicate the formation of other
degradation products, such as ketones and lactones. This mechanism is basically similar for
both -PP and -PP. However, the absorption bands associated with -PP are markedly more
pronounced than in the case of -PP, which indicates a more intensive molecular degradation
of -PP. For integral and quantified description of the degradation process the carbonyl index
defined by Eq. (1) was used. In Fig. 2 a monotonic rise of the carbonyl index with increasing
exposure time can be observed. This effect is substantially more pronounced for -PP and
particularly evident at higher aging times. Lower degradability of the -PP sample probably
reflects its particular morphology, which is induced by the heterogeneous nucleation rather
than by the possible minor effect of UV absorbance of the nucleator itself. Actually, visual
inspection has revealed a distinct opacity of the -PP caused by specific nucleation, while the
-PP specimen showed partial transparency. Indeed, intensive light scattering and higher light
absorbance of the -PP sample observed in the visual light are even more pronounced in the
UV region.
Different morphology of -PP and -PP specimens is also reflected in the specific surface
crack patterns after long-term UV exposure. The micrographs in Fig. 3 show deep and
relatively distant macroscopic cracks on the surface of -PP specimen while the surface of -
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PP was damaged by a dense network of fine cracks, only microscopically observable. The
correlation with smaller spherulite size in the -PP specimen is evident.
3.2 The evolution of morphology
The evolution of WAXS patterns in -PP and -PP specimens is shown in Figs 4 and 5. The
diffraction patterns of -PP show typical reflections including the (110), (040) and (130)
reflections (2 = 14.2 °; 17 ° and 18.8 °), and a less pronounced (300) reflection -peak
(2 = 16.2 °). The corresponding K value indicates only ~3 % -phase within crystalline
portion and does not change with further irradiation. Non-monotonic evolution of the
diffraction patterns, especially expressed by the (110) reflection peak, should be noted: The
intensity of individual peaks increases at shorter exposure time but from 96 hours up it
decreases again. The diffraction patterns with dominating (300) -peak shown in Fig. 5
indicate that the nucleated specimen consisted of almost pure -phase in its crystalline portion
(K ~ 0.90). Similar evolution of diffractions patterns as in the case of -PP, i.e. an increase in
the individual peaks at the beginning of irradiation course followed by a decrease at
prolonged exposure, can also be observed in -PP.
From the diffraction patterns in Figs 4 and 5 the crystallinity of both -PP and -PP and the
portion of -phase in the -PP specimen were calculated. In Fig. 6, these characteristics are
plotted as functions of exposure time. While the portion of -phase in the -PP specimens
remains virtually unchanged, the non-monotonic dependencies of the crystallinity values of PP and -PP specimens are evident again. Two other observations should be noted: First, the
relative changes of crystallinity are markedly higher for -PP specimens and second, the
maximum observed crystallinity occurs at higher exposure time for -PP specimens. In Fig. 7,
these experimental facts are presented in terms of relative values. A comparison with Fig. 2
I : 10
shows that the crystallinity maxima of both specimens correspond approximately to the same
carbonyl index value (~ 8). This experimental fact suggests that a particular degree of
molecular scission corresponds to the maximum crystallinity evolution.
The data presented in Figs 4-7 suggest that the interrelations between degradation and
crystallization include two competing effects: First, scissions of molecular chains in the
amorphous region release molecular entanglements and facilitate additional crystallization in
the solid state. These processes will likely prevail in the first stage of degradation. Second, the
UV attack can finally degrade crystalline regions and introduce heterogeneities in molecular
chains, decreasing their ability to crystallize. Such mechanism will prevail after long-term
exposure, and it will influence the re-crystallization behaviour. Indeed, oxygen diffusion into
crystallites is markedly slower than into amorphous regions. Macroscopically, the competition
of the two mentioned effects is manifested as a non-monotonic dependence of crystallinity as
a function of exposure time.
3.3 Chemi-crystallization models
The process of chemi-crystallization of PP has been discussed by several authors [e.g. 16, 3032]. However, no definite model has been accepted yet. The increase of crystallinity during
UV irradiation could be caused either by a creation of new crystallites in the bulk of the
amorphous region [32] or by incorporation of loosened molecules into pre-existing crystallites
[30]. One can imagine that shorter molecular chains with enhanced mobility can be
incorporated into already existing crystalline regions. The nucleation of new crystals cannot
be excluded completely, however, it would require several mobile molecular segments
produced by chain scission in their close vicinity; this would be indicated by a certain
induction period. However, no such induction time was observed in this study. Furthermore,
the hypothetical new crystals would be much smaller than the pre-existing crystals, and they
I : 11
would thus broaden of the WAXS peaks. This effect was not observed either. Finally, an
alternative explanation suggests that intensive degradation of the amorphous region will
convert some molecules into volatile products and remove them from the material [31]. In this
case, the degree of crystallinity would increase monotonically with the degradation process.
Again, this contradicts our observations. Consequently, additional growth of existing
crystallites on account of amorphous regions is the most likely explanation of the
experimental results of this work.
3.4 Stability of -phase
The question of the -phase stability of isotactic polypropylene in the solid state is important
from the viewpoint of practice and interesting from the theoretical point of view. The possible
- transformations under the effects of mechanical loading, thermal treatments or
photooxidative degradation have been approached by several authors [3-5, 8, 10, 20]. In the
present work, the non-nucleated specimens contained virtually only -phase (see Fig. 4),
while, on the contrary, the -iPP consisted of a high amount of -phase (see Fig. 5). No phase was observed in either case. Furthermore, a careful WAXS analysis has shown that
upon long-term UV irradiation the absolute content of -phase decreased but its relative
portion within the crystalline phase has remained virtually unchanged, as evident from Fig. 6.
3.5 Re-melting of degraded samples
Extremely degraded specimens of -PP and -PP prepared for WAXS measurements were
heated to 200 °C in an oven together with a special holder, cooled-down to room temperature
and analysed by WAXS again. The results are presented in Fig. 8. Qualitative inspection of
the diffractograms shows additional diffraction peak at 19.8 ° indicating the occurrence of the
orthorhombic -phase in -PP specimens. Similarly, -phase was detected in the recrystallized
I : 12
-PP specimen. Thus, the coexistence of the three basic crystalline modifications (i.e. a 3phase crystalline system of -- phases) in the -PP specimen is indicated. Moreover, the
(300) peak corresponding to the -phase markedly decreased upon re-melting (see Figs 5 and
8). Strong crystallization tendency of the degraded material into -phase was obvious in both
samples; in the case of the -PP, the crystallization into orthorhombic -phase even exceeded
the nucleating effect of the -specific nucleator. In fact, the efficiency of the nucleator could
also have been decreased by the effect of UV irradiation. However, an unambiguous
assessment of this possible phenomenon would need a special comprehensive study. It should
be noted that no tendency to -phase formation was observed during the chemi-crystallization
in the solid state. The formation of completely new crystallites in the solid state is therefore
excluded again.
The effect of UV degradation on crystallizability from the melt is also demonstrated in Fig. 9.
The monotonic decrease of the crystallization temperature for both -PP and -PP samples
could be ascribed to the introduction of irregularities to individual molecular chains by UV
attack.
I : 13
4. CONCLUSIONS
The following conclusions can be drawn from the experimental results of this work:
(1) The -nucleation specifically affected the processes of UV degradation of isotactic
polypropylene on the molecular, supermolecular and microscopic levels.
(2) Infrared spectroscopy indicated lower molecular degradability of -PP as compared to PP, reflecting higher opacity of -PP for UV light; the degradation mechanism, however, was
similar for both materials.
(3) WAXS analysis showed a non-monotonic dependence of crystallinity on UV exposure
time for both -PP and -PP. These results reflect a competition between chemicrystallization and degradation of crystalline regions. The increase of crystallinity during the
first stage of UV exposure is likely to reflect the incorporation of released chains into already
existing crystallites rather than the formation of new crystallites.
(4) The content of -phase within the crystalline portion remained stable during UV
irradiation under given irradiation conditions.
(5) The ability to crystallize from the melt of both -PP and -PP significantly decreased with
exposure time, reflecting the incorporation of molecular irregularities.
(6) The effect of different morphologies of the -PP and -PP specimens on their UV
degradation was also manifested macroscopically by more severe surface cracking of the
degraded -PP containing larger spherulites.
I : 14
ACKNOWLEDGEMENTS
One of the authors (M.O.) acknowledges the support provided by the Blaise-Pascal
University, ENSCCF and the Tomas Bata University in Zlín during his stay in ClermontFerrand. Partial support from the Czech Science Foundation - GAČR (project 102/02/1249) is
also gratefully acknowledged. In addition, the authors would like to thank Joël Cellier for his
help in wide-angle X-ray scattering.
I : 15
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I : 16
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Injection-Molded -Polypropylene and Research Perspectives of Its Degradability. In: GFP
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I : 17
Figure 1. Absorption bands in the carbonyl area of -PP and -PP specimens
I : 18
Figure 2. Effect of UV exposure on the carbonyl index of -PP and -PP specimens
30
-PP
-PP
Carbonyl index
25
20
15
10
5
0
0
50
100
150
Ageing time (hours)
200
250
I : 19
Figure 3. Surface cracks on -PP and -PP specimens after 240 h UV exposure
I : 20
Figure 4. The evolution of WAXS patterns with increasing UV exposure time of -PP
specimens
1400
1200
800
600
400
200
10
15
20
25
2 , (°)
30
35
60 72
36 48
144192240
96 120
s)
Time
Ageing
0 24
(hour
Intensity
1000
I : 21
Figure 5. The evolution of WAXS patterns with increasing UV exposure time of -PP
specimens
6000
5000
3000
2000
1000
10
15
20
25
2 , (°)
30
35
60
36 48
72 96
192240
120144
Time
Ageing
0 24
)
(hours
Intensity
4000
I : 22
Figure 6. Effect of UV exposure on -iPP and -iPP crystallinity and K value of -iPP
100
1,00
95
90
0,95
80
75
0,90
-PP
70
-PP
65
0,85
60
55
50
0,80
0
50
100
150
Ageing time (hours)
200
250
K
Crystallinity (%)
85
I : 23
Figure 7. The evolution of relative crystallinity increments for -PP and -PP upon UV
exposure
Relative Crystallinity Difference (%)
16
14
-PP
-PP
12
10
8
6
4
2
0
0
50
100
150
Ageing Time (hours)
200
250
I : 24
Figure 8. WAXD diffractograms of -PP and -PP after 240 hours UV exposure.
(Bottom) Exposed -PP; (Middle) Exposed and recrystallized -PP; (Top) Exposed and
recrystallized -PP


Intensity (a.u.)

recr.-PP, 240 h. UV
recr.-PP, UV 240 h.
-PP, UV 240 h.
10
15
20
25
2 (°)
30
35
I : 25
Figure 9. Effect of UV exposure on crystallization temperature in the course of nonisothermal crystallization
Crystallization temperature (°C)
125
-PP
-PP
120
115
110
105
100
0
50
100
150
Ageing time (hours)
200
250
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