Oriented Polymers Obtained by UV Polymerization of Oriented Low

Chapter 35 O r i e n t e d Polymers O b t a i n e d b y UV P o l y m e r i z a t i o n o f O r i e n t e d L o w M o l e c u l a r Weight Species D. J. Broer and G. N. Mol

Downloaded by FUDAN UNIV on March 15, 2017 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch035

Philips Research Laboratories, P.O.B. 80000, 5600 JA Eindhoven, Netherlands

Anisotropic polymer filaments could be produced by in-situ photopolymerization of oriented acrylate monomers. Ordering of the monomers was achieved by an elongational flow prior to the polymerization process. The produced polymers showed a high elastic modulus and a low thermal expansion coefficient in the direction of the orientation. To reduce optical transmission losses under lateral forces, silica optical fibers are provided with a soft primary coating and a hard secondary coating [1,2,3]. Both coatings are applied on-line with the fiber drawing process and are cured by UV-irradiation. However, the choice of conventional UV-curing coatings gives rise to enhanced optical attenuation at low temperatures [4,5,6]. The large difference in linear thermal expansion coefficient between the secondary coating (« 10 " °C ) and silica (5-10 °C ) causes microbending of the silica fiber within the soft primary coating due to compression stresses in axial direction. Ideally, the secondary coating should have a linear expansion coefficient equal to that of silica. This requirement prompted us to investigate new polymeric materials with a low thermal expansivity which can be processed within the boundaries as fixed by the optical fiber drawing process. The latter for instance means high extrusion rates at relatively low pressures and ultra-fast solidification. As known [7,8], the thermal expansion coefficient is reduced in the direction of the molecular orientation obtained by stretching of a thermoplastic polymer during or directly after its processing. In special cases thermotropic polyesters are applied to facilitate the process of molecular orientation [9]. However, in all these cases solidification must proceed either by cooling down from the melt or by evaporation of the solvent. These relatively slow processes are not suited for on-line opticalfibercoating. In this paper we will demonstrate that molecular orientation can also be achieved by starting with a low molecular weight species (M.W. < 2000) which is oriented in an elongational flow and subsequently cured under UV-irradiation. The orientation of the monomer is frozen-in by the ultra-fast polymerization and crosslinking. The advantages are lowpressure, low-temperature processing, fast extrusion and solidification within 0.1 seconds. Moreover a three dimensional polymer network is formed, which should maintain the molecular ordering over a wide temperature range. To get a better understanding of the material properties, the present investigations have been carried out on polymericfilaments,i.e. there are no silica fibers involved as a carrier for the investigated polymer. 4

_1

-7

_1

0097-6156/87/0346-0417$06.00/0 © 1987 American Chemical Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

418 Experimental

The present studies have been concentrated o n the molecular ordering and photopolymerization o f the polyesterurethane acrylate shown i n Figure 1, to w h i c h 4 w % 2,2-dimethyl2-hydroxyacetophenone

was added as photoinitiator. Figure 2 schematically shows the

preparation o f oriented filaments. The molten monomer is extruded through a die while temperature and pressure are controlled. The resulting liquid thread is stretched and subse quently cured by irradiation with a U V lamp (Fusion Systems, electrodeless mercury lamp, 0.3 W . c m

- 2

i n the 365 n m region). After leaving the die, the liquid filament phases passes

a room temperature zone where some cooling takes place before it enters the irradiation zone. The temperature during irradiation was difficult to measure exactly but was estimated


to be between 80 and 150°C, depending on the extrusion rate. The data given i n fig. 2 serve as an example and were varied d u r i n g our experiments.

Results and Discussion The polyesterurethane acrylate monomer is a semi-crystalline solid at room temperature with a glass transition o f the amorphous phase at - 6 ° C and melting o f the crystalline phase at 52°C. W h e n cooled from the isotropic liquid phase to room temperature the monomer remains undercooled for several hours. The rate o f photopolymerization and the ultimate conversion o f the acrylate groups strongly depends on the polymerization temperature. The overall activation energy for the rate o f polymerization above the melting temperature is 10 kJ/mole w h i c h is a normal value for photoinitiated acrylate polymerization [10,11]. B e l o w this temperature the activation energy increases to 89 kJ/mole, indicating that the polymerization kinetics are strongly influenced by the mobility o f the monomer. After polymerization under isotropic conditions, the glass transition becomes very broad with a temperature range o f - 20 to + 90°C as determined by D S C and D M T A . D S C and T M A measurements show melting o f a small crystalline phase at 55 °C. Crystallinity could not be detected by polarization microscopy. T o obtain filaments the monomer was extruded from the melt at 80°C, w e l l above the melting temperature (fig. 2). F i g u r e 3 shows the extrusion flow rate as a function o f pressure for two different die geometries, indicating a linear behaviour i n both cases. It was observed that there is only a limited flow rate region where stable liquid filaments are formed. This region decreases with increasing die length. A t l o w flow rates the stability o f the filaments is controlled by the surface tension, i.e. the l i q u i d thread breaks up into droplets. A t high flow rates the stability o f the filaments is controlled by the process o f molecular orientation i n the die and relaxation after leaving the die. A t too high flow rates melt fracture phenomena can be observed i n the liquid thread. The flow rate region for stable filament production can be broadened by the addition o f small amounts o f surfactant to the reactive precursor. The ultimate molecular orientation i n the filament is induced by an elongational flow prior to polymerization. The stresses needed for stretching the liquid threads are small (0.1 to 10 M P a ) . A s orientation is reduced by fast relaxation o f the molecules, the U V irradiation was already started during stretching. Since there is a continuous equilibrium between molecular orientation and relaxation, the degree o f orientation along the fiber axis (z-axis) can be considered to be a function o f the strain rate. The strain rate at position ζ , έ ( ζ ) , is given by Equation 1 [12]:


BROER A N D M O L

Oriented

Polymers

Obtained

Ο

by UV

Polymerization

Ο

R-0-[CH -CH -0-C-(CH ) -C-0] -CH -CH -0-R 2

2

Ο Η II

I


4

3

2

2

Η Ο

/—ν ι

/—ν

R: - C - N - / ) - C H

Fig.

2

2

»

- / V N - C - 0 - C H

1. Polyesterurethane

11

2

- C H

diacrylate

2

- 0 - C - C H = : C H

reactive

2

precursor.

e

T=80 C

extrusion shear: 4

Y«10 S"

uvIrradiation

Fig. 2. Experimental

1

elongational flow: é»50S-

set up for filament extrusion

1

and curing.


420

POLYMERS FOR HIGH T E C H N O L O G Y


where F is the axial tensile force, S(z) the cross section of the filament and κ(ζ) the elongational viscosity at position z. F remains constant during the drawing process, however S(z) and K(Z) change along the z-axis. S(z) decreases due to the stretching process, whereas κ(ζ) increases due to the cooling of the fiber. As a result, e(z) decreases gradually with the distance z. On the other hand the relaxation rate of the oriented molecules also decreases due to cooling of the filament. The total effect of all these phenomena is that the die-lamp distance is not very critical. A suitable distance appeared to be 0.2 m. As soon as the polymerization starts, κ(ζ) increases greatly while F and S(z) remain constant. From the resulting large decrease of e(z) it can be concluded that elongation only takes place in the liquid, non-polymerized phase. Polymeric filaments produced in this way have a diameter of 100 to 300 μπι and show anisotropy in their properties, indicating molecular orientation. For instance the birefr ingence of this fiber is 0.0073. Table 1 compares the modulus measured both in axial and in lateral direction, with the modulus of the same material cured under isotropic condition at 80°C. The increase in modulus in axial direction is obvious. The decrease of the modulus at higher temperatures can be ascribed to both the glass transition and melting of crystalline areas.

Table 1. Modulus in GPa of polyesterurethane acrylate, respectively cured under isotropic and anisotropic conditions Temperature Isotropic (°C) -

Anisotr. axial direction

Anisotr. lateral direction

40

1.8

34.2

—

25 80

0.6 0.02

14.6 0.6

0.6

—

As already stated in the introduction the main interest of our investigation is the in fluence of the molecular orientation on the axial linear thermal expansion coefficient. Figure 4 shows the linear expansion of an oriented filament measured in the two main directions and compares these measurements with the linear expansion of an isotropically cured sam ple. Again the differences are obvious. Overall, the thermal expansivity of the oriented fiber in axial direction is one order of magnitude lower than that of the isotropic material. In addi tion, the increase of the thermal expansion coefficient at the glass transition temperature has become less pronounced. The thermal expansion coefficient measured in lateral direction is higher than that of the isotropically cured sample. This can be understood from the con sideration that the volume expansion coefficient is not subjected to large changes. Table 2 compares the volume thermal expansion coefficients as calculated from the linear thermal expansion coefficients. Above the melting temperature the values of the isotropic and the oriented samples are identical. Below this temperature there are some small differences in dicating a somewhat higher degree of crystallinity in the case of the oriented sample.


35.

BROER A N D M O L

Oriented Polymers Obtained

by UV Polymerization

All


120|

Ο

10 pressure

20 (bar)

Fig. 3. Flow rate vs. pressure during extrusion of the monomer at 80 °C. The die length is 5 mm (o)

and 20 mm ( • ) resp.

- 1 2 0 - 8 0 -40

0

40

80

120

temp (°C) Fig.

4. Linear

isotropically

thermal

expansion

coefficients

as a function

of temperature:

cured at 80 C; (o) and ( Δ ) oriented polymer filament (200 μm e

measured in the axial and lateral direction,

( • )

diameter)

resp.


422

POLYMERS FOR HIGH TECHNOLOGY Table 2. Volume thermal expansion coefficients in ° C as calculated from the linear thermal expansion coefficients _1

200 μπι Oriented filament

Temperature Isotropic (°Q


- 40 25 80

4

2.1 · 10" 3.9 · 10" 5.8 · 10"

4 4

4

1.7 · 10" 3.6 · 10" 5.8 · 10"

4 4

The question may arise whether the same changes in material properties can also be ob tained by stretching the filaments after the polymerization has taken place. In our experience the anisotropy was never so great in such cases, whereas the isotropic state was almost com pletely recovered when the samples were heated above the melting temperature. At this temperature the post-polymerization drawn filaments retained the original dimensions they possessed before stretching. Apparently some strain-induced crystallization yielded a metastable anisotropy which was lost under the combined action of entropy and strained crosslinks when the crystalline areas were melted. The investigation on oriented polymeric networks obtained by the photopolymerization of oriented low molecular weight species, as presented in this paper, has been carried out with a more or less conventional acrylate monomer. Already with this material an anisotropy in properties could be demonstrated. It is to be expected that even more pro nounced effects can be obtained with monomers which have a strong tendency to alignment. Based on this idea we are now investigating liquid crystalline monomers in our laboratory. Conclusions It has been demonstrated that molecular orientation can be achieved starting with a low molecular weight species which is oriented in an elongational flow and subsequently cured under UV-irradiation. The orientation of the monomer is frozen-in by the ultra-fast process of polymerization and crosslinking. Both extrusion and stretching can be carried out at relatively low temperatures and pressures. Polymerfilamentsproduced in this way are definitely anisotropic as is evidenced by their birefringence and by a strong increase of the tensile modulus and a decrease of the thermal expansion coefficient in the axial direction. References 1. L.L. Blyler, A.C. Hart, A.C. Levy, M.R. Santana and L.L. Swift, 8th European Conf. Opt. Commun., Cannes, France, 1982. 2. D. Gloge, Bell Syst. Techn. J. 1975, 54, 245. 3. D.J. Broer and G.N. Mol, 5th Int. Conf. Int. Optics Opt. Fibre Comm. and 11th Euro pean Conf. Opt. Comm., Venezia, Italy, 1985. 4. Y. Katsuyama, Y. Mitsunaga, Y. Ishida and K. Ishihara, Applied Optics 1980, 19(24), 4200.


35. BROER AND MOL

Oriented Polymers Obtained by UV Polymerization


5. N. Yoshizawa, M. Ohnishi, O. Kawata, K. Ishihara and Y. Negishi, J. Lightwave Techn. 1985, LT-3, 779. 6. T.A. Lenahan, AT&T Techn. J. 1985, 64, 1565. 7. R.S. Porter, N.E. Weeks, N.J. Capiati and R.J. Krzewki, J. Thermal Anal. 1975, 8, 547. 8. M. Jaffe, In Thermal Characterization of Polymeric Materials, E.A. Turi, Ed.; Academic Press, New York, 1981. 9. S. Yamakawa, Y. Shuto and F. Yamamoto, Electr. Letters, 1984, 20, 199. 10. G.R. Tryson and A.R. Schultz, J. Pol. Sci., Pol. Phys. Ed., 1979, 17, 2059. 11. G. Odian, In Principles of Polymerization, 2nd ed., Wiley Interscience, New York, 1981. 12. C.D. Han, In Rheology in Polymer Processing, Academic Press, New York, 1976. RECEIVED May 18, 1987


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Oriented Polymers Obtained by UV Polymerization of Oriented Low

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