DYNAMIC MECHANICAL SPECTRA OF PET/PEN ... - Centrum Textil

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DYNAMIC MECHANICAL SPECTRA OF PET/PEN FIBERS
J. Militký, A. P. Aneja
ABSTRACT
The modification of PET fibers is still very attractive for preparation of specialty materials.
Modifying component has influence on the structure and properties of fibers but it is very
complicated to separate influence of chemical composition from influence composition
from influence of technological parameters. The main aim of this work is investigation of
PET/PEN fibers dynamical mechanical spectra. The influences of PEN on the tensile
mechanical properties are investigated as well. Results are compared with data for pure
PET
1. INTRODUCTION
The polyethylene 2,6 naphtalate (PEN) is relatively new polyester having rigid naphthalene
ring in its backbone on the market. This polyester exhibit higher glass transition Tg (about
123 0 C), higher crystallization temperature (194 0 C) and higher melting point (270 0 C) than
PET. The elastic modulus of crystalline regions of PEN in direction parallel with chain axis
is 145 GPa. This is about 40% higher than corresponding modulus for PET 108 GPa [6].
Blends of PET and PEN have been attracting increasing interest because they combine the
superior properties of PEN wit the economy of PET. Mixture of these polyesters form due to
transesterification during melts processing random copolymers. The glass transition
temperature of PET/PEN increases linearly with volume fraction of PEN [4]. It is therefore
possible to control properties connected with Tg by including of PEN to PET. The addition
of PEN improves gas barrier properties as well [3].
The main aim of this work is utilization of thermo analytical methods for investigation of
PET/PEN fibers. The thermo mechanical analysis is applied for investigation of influence of
temperature on dynamic and loss modules. The influences of PEN on the tensile mechanical
properties are investigated as well. Results are compared with data for pure PET.
2. PROBLEMS OF PET FIBERS MODIFICATIONS
Main activities in the development of synthetic fibers are in the area of the physical and
chemical modifications. In this section the general aspects of fiber modifications focused to
the polyester fibers are discussed. The term „modification“ is used to designate a deliberate
change in composition or structure leading to an improvement in some fiber properties. The
main aim of modification can be:
•To obtain new properties, such as affinity for cationic dyes,
•To enhance some positive properties, such as resistance to ultraviolet radiation,
•To suppress some negative properties, such a reducing the pilling tendency.
In spite of the great number of existing modification methods no consistent classification is
available at yet. From the general viewpoint, however, it would appear advisable to classify
the modification methods by the production steps at which they are applied. The following

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classification scheme results [2]:
1. Modification in course of polymer preparation
• Preparation of copolymers,
• Using additives,
• Reducing the molecular mass.
2. Modification in course of fiber preparation
• Drawing and setting conditions readjusting,
• Speed of spinning changing,
• Texturing,
• Cross section geometry changing,
• Fineness changing,
• Bi-component and multi-component fibers production.
3. Modification applied to commercial fibers
• Grafting,
• Plasma etching,
• Controlled surface destruction.
4. Combined modification
(e.g. hollow microporous copolyester fibers containing additives)
Details about these modifications are summarized in the book [2]. The description of
modification effect on fiber properties is complicated by the fact that modification affects
not only fiber structure and fiber properties but also conditions of fiber preparation. The
main problems can be summarized to the following points:
• It is difficult to measure structural parameters directly influencing given property (tie
chain)
• The properties are distinctly dependent on chemical composition of fibbers (chain
flexibility)
• Structural parameters are measured in static state whereas properties are usually
determined in a dynamic state
• Structure is changed during the measurement of some properties
Influence of commoner on fiber properties can divided to the following categories
A. Commoner has no effect
B. Commoner has indirect effect
C. Only the commoner amount matters (equilibrium melting point)
D. It is the type of commoner that matters (Tg, dyeability)
E. Commoner type and concentration have effect simultaneously
Modification generally affect on the other technologically important characteristics as
technology of fiber preparation, molecular mass of melt, degree of melt degradation and

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rate of crystallization. It is therefore difficult to separate effect of chemical modification
from modification of technological parameters. Effect of combined physical and chemical
modification on the basic structural characteristics and dyeability of modified fibers is
described in the [1].
3. PET/PEN FIBERS AND THEIR PREPARATION
It is known, that introduction of even low level of 2,6,naphthalene units in place of
terephtalate moieties acts to disrupt crystallinity, increase Tg, improve static chain packing,
and decrease local segmental mobility in the amorphous phase of copolymers. These results
are supported by the geometrical structure of the two units (see fig.1)
O
C
O
C
O
C
O
C
0.8 nm
0.57 nm
A
B
Fig. 1 Dimensions of 2,6 naphthalene (A) and terephtalate (B) units
The molar volume of PEN V PEN = 182.4 cm 3 /mol is higher than molar volume of PET V PET
= 144 cm 3 /mol. The amorphous density of PEN ρ aPEN = 1327 kg/m 3 is lower than
amorphous density of PET ρ aPET = 1333 kg/m 3 . The crystalline density of PEN ρ cPEN =
1407 kg/m 3 is lower than density of PET ρ cPET = 1440 kg/m 3 .
The 2,6, naphthalene unit is substantially larger and does not fit into the unit cell of PET.
The bulky kinked 2,6 naphtalene units in PEN are much less mobile than the terephtalate
units. Amorphous phase free volume and local segmental mobility are reduced due to PEN
presence. At the temperatures around 60 0 C or higher the motion of rigid naphthalene ring
occur. One possible motion is hindered rotations of the naphthalene rings about the
backbone. Another possible motion is interlayer slippage of the naphthalene rings. During
the deformation of PEN, the naphthalene rings are rapidly aligned parallel to the surface of
the fibers and also occurs at highly localized regions. The subsequent slippage can leads to
necking behavior during deformation. The naphthalene portions exhibit higher creep
compliance.The fibrous samples having various content of PEN were prepared under
comparable conditions. Basic characteristics of these samples are given in the table I.
Table I Basic characteristic of PET/PEN samples
Sample PEN content Intrinsic Boiled water Dry air-180 0 C
[%] viscosity shrinkage [%] shrinkage [%]
A 0 0.625 0.4 2.7
B 5 0.56 1.4 4.7
C 10 0.586 1.2 7.0
D 15 0.582 2.2 8.4

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E 20 0.61 2.2 14.5
It is clear than the shrinkage is increasing function of PEN content. Shrinkage is generally
associated with the relaxation of the oriented amorphous chains and the removal of residual
stresses formed during processing of fibers. Stress induced crystallization can reduce the
shrinkage as well. Because are both shrinkage temperatures above the start of naphthalene
rings motion the shrinkage processes are facilitated.
4. THERMOMECHANICAL ANALYSIS
In thermo mechanical analysis (TMA) the dimensional changes (expansion or contraction)
are measured under defined load and chosen time The TMA requires a high-resolution
measurement of the linear displacement and excellent stability of measured conditions.
Most TMA instruments on the marked are not able to be sensitive to very small
displacement. This was the main reason for construction of special device TMA CX 03RA/T
at University of Pardubice. This device was developed to provide highly sensitive tool for
reproducible measurement of subtle dimensional changes even at extremely long thermal
expositions. The sample is placed on the movable sample holder connected with
displacement sensor, which measures dimensional changes of the sample. The instrument is
fully computer controlled with programmable time - temperature profiles and loading in
static or dynamic mode. The special adapters for application of this instrument for bending
and tension deformations are under preparation Described device were used for all kind of
compressive measurements.
Chopped material compressed in the silica oxide tube has been used for compressive creep
measurement. In the table 3 are results of no isothermal creep at compressive load 100 mN
and rate of heating 3 o C/min. Dependencies of height on the temperature (time) for two
limit cases are given on the fig. 2. The upper line corresponds to the heating and lower one
corresponds to the cooling conditions. For characterization of compressive compliance the
relative height changes H c were computed
H C
l
= 100 *
20
− l
l
20
200
where l 20 and l 200 are heights at 20 0 C and 200 0 C. These values are in the table II.

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A
B
Fig.2 Dependence of height on the temperature A..PET B..PET/20%PEN
Higher H C corresponds to the higher compressive creep compliance.
Table II Parameters of non-isothermal creep
Sample T tr T 1 T 2 A 1
A 2
H C
[ o C] [ o C] [ o C] [ppm/ o C] [ppm/ o C] [%]
A 114 73 111 -486.03 -9433.11 45.5
B 121 93 139 -402.39 -6447.01 23.29
C 122 77 146 -199.13 -6412.34 28.77
D 111 80 137 -396.09 -5809.29 16.77
E 115 92 135 -1209.1 -10607.9 53.6
T tr is transition temperature between low and high temperature sensitive regions , T 1 is
temperature of low sensitive region; T 2 is starting temperature of high sensitive response.
Parameters A 1 and A 2 are sensitivity coefficients (slopes of height vs. temperature
dependences. Excluding the sample E is H c decreasing function of PEN content. The
compressive compliance is therefore lowered by addition of small amount of PEN.
5. MECHANICAL PROPERTIES
The fiber fineness has been measured on the apparatus Vibroscope (Lenzing). On the same
samples the stress strain curves were evaluated by using the apparatus Vibrodyne (Lenzing).
The ten various fibers have been tested. For characterization of tensile mechanical behavior
the following characteristics were evaluated:
P Tenacity [cN/dtex] as load at break,
E Elongation [%] as deformation at break
IM Initial modulus [cN/dtex] as modulus at 10% deformation
SM [cN/dtex] secant modulus as 100*P/E
WT [cN/dtex] toughness as P*E/50
Py [cN/dtex] yield point stress as stress in point with maximal curvature on stress strain

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curve
εy [%]yield elongation as deformation in point with maximal curvature on stress strain
curve
εp [%] plateau elongation as deformation corresponding the plateau in post yield region
[2].
Results of fineness and mechanical measurements are given in the table III. and table IV. In
brackets are given coefficient of variation CV [%]. These coefficients show relatively high
variability of measured characteristics mainly due to variability of geometrical
characteristics of fibers (variation of titer). Very high variability has the elongation at break.
This variability partially hides the influence of PEN on the mechanical characteristics.
Table III. Stress strain curve parameters
Sample Titer
[dtex]
Tenacity
[cN/dtex]
Elongation
[%]
Modulus 10%
[cN/dtex]
Toughness
WT [cN/dtex]
A 6.09 [7.2] 26.7 [10.5] 107.8 [18.2] 96.4 [13.8] 14.39
B 6.84 [12.5] 24.6 [8.1] 93.7 [18.6] 87.4 [9.4] 11.52
C 6.74 [20.4] 30.2 [15.7] 64.2 [29.8] 98.6 [14.3] 9.69
D 6.58 [16.9] 28.6 [12.5] 52.9 [39.7] 95.7 [10.3] 7.56
E 8.54 [19.4] 24.2 [12.7] 64.1 [46.3] 77.5 [11] 7.75
Table IV. Parameters in yield point vicinity
Sample Yield point stress Yield elongation Plateau elongation Secant
Py [cN/dtex] εy [%] εp [%]
[cN/dtex]
A 7.9 4.1 0.1 24.95
B 8.2 4.87 3.1 26.64
C 8.9 5 6.2 47.04
D 9.1 5.1 12.1 54.06
E 7.5 4.5 14.2 37.75
modulus
Based on these results, the following conclusions can be formulated:
1. Stress strain curves are sensitive on the PEN content. Increasing of PEN content
leads to the marked appearance of yield point and wider post yield plateau εp
(deformation softening region). It is known (see [2]), that εp characterize
deterioration of recovery power.
2. Toughness and elongation at break of fibers are decreasing function of PEN content
3. Secant modulus, yield point stress and tenacity are increasing function of PEN
content (excluding sample E with higher titer)
The PEN presence therefore acts as reinforcing of chains, and increase ultimate mechanical
properties. On the other hand, the standard comonomers addition leads to decreasing of
mechanical properties [2]. The post yield region exhibition is in accordance with slipping
motion of naphthalene chains (necking formation).

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6. DYNAMIC MECHANICAL ANALYSIS
The dynamic mechanical analysis (DMA) is commonly used to characterize a material in
response to vibration forces. DMA enables investigation of stress (or deformation)
oscillations parameters (frequency, waveform, amplitude) and temperature influence on the
deformation (or stress) changes under selected mode of deformation (tensile, bending,
compression etc.)
The dynamic mechanical thermal spectrometer DMA DX04T developed by RMI Ltd. Czech
Republic provides highly sensitive tool for reproducible measurements of fine dimensional
changes during heating, cooling or even at extremely long isothermal measurements.
The individual fibers were investigated in the tensile mode. Due to the high plastic
deformation of fibers the process of measurement was finished after few cycles. In the
second run were fibers pre deformed to strain 50 % and after that the dynamical testing were
made. Temperature dependence of real modulus and loss tangent are give on the fig 3a,b.
A
B
Fig.3 Dependence of real modulus E and tan δ on the temperature A..PET B..PET/20%PEN
Selected characteristics of peak on real modulus (T b and E) and peak on tan δ (tan δ m and
T a ) are given in the table V.
Table V Dynamic mechanical characteristics of fibers
Sample T b [ 0 C] E [GPa] tan δ m [-] T a [ 0 C]
A 90.4 44.3 0.35 126.9
B 79.1 2.94 0.32 103
C 76.1 186.8 0.274 110.6
D 64.3 180.68 0.298 108.5
E 68.8 190.19 0.276 109.3
The content of PEN leads to decrease the temperature T b of β-relaxation process
corresponding to maximum on real modulus curve. This process is caused probably by the
mobility of naphthalene rings. The temperature corresponding to the maximum on tan δ
characterizes the α-relaxation process (dynamic glass transition). For pure PEN the value
T a =135 0 C and Tb=70 0 C have been found [8].

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7. CONCLUSIONS
The properties of PET/PEN fibers are generally dependent on the content of PEN. A lot of
properties are connected with improved thermal and mechanical characteristics due to
naphthalene rings presence. On the other hand the crystalline phase is lowered and
mechanical relaxation caused by the motion of naphthalene rings appeared. As results of
these changes the increasing of PEN content leads to increasing of shrinkage and increasing
post yield region length (strain softening region) on stress strain curves. The large loss
factor found for pure PEN [9] is not observed here in the investigated range of
concentrations.
8. REFERENCES
[1] Militký, J.: Some theoretical problems of PET fibers modifications, Proc. Int. Conf.
New Fiber, Ueda July 1999
[2] Militký, J. et all. : Modified Polyester Fibers, Elsevier, Amsterdam 1991
[3] McDowell C.C. et all.: Synthesis, physical characterization, and acetone sorption of
random PET/PEN copolymers, J. Polym. Sci. B36, 2981 (1998)
[4[ Saleh Y.S., Jabarin A.: Glass transition and melting behavior of PET/PEN Blends, J..
Appl. Polym. Sci. 81, 11 (2001)
[5] Higashioji T., Bhusdan B.: Creep and shrinkage behavior of improved ultra thin
polymeric film, J. Appl. {Polym. Sci. 84, 1477 (2002)
[6] Nakamae K. et all. : Temperature dependence of the elastic modulus of the crystalline
regions of PEN, Polymer 36, 1401, (1995)
[7] Wu G., Cucullo J.A.: Structure and property studies of PET/PEN melt blended fibers,
Polymer 40, 1011(1999)
[8] Canadas J. C. et all.: Comparative study of amorphous and partially crystalline PEN ,
Polymer 41, 2899 (2000)
ACKNOWLEDGEMENTS
This work was supported by the project LN 00 B090 of Czech Ministry of Education
RESPONDENCE ADDRESS:
Dept. of Textile Materials, Textile Faculty,
Technical University of Liberec
Halkova street No 6, 461 17 Liberec, Czech Republic
e - mail : jiri.militky@vslib.cz