Mechanical and thermodynamical properties of Electron Beam and. Ultra-violet cured monomer/liquid crystal (LC) systems were investigated. The pure polymer ...
Mechanical and thermodynamical properties of polymer/liquid crystal systems prepared by electron-beam and ultra-violet processing Aline Olivier1, Leïla Benkhaled2, F. Kada3, Abdelkader Berrayah3, Tadeusz Pakula1, Bernd Ewen1, and Ulrich Maschke2* 1) Max-Planck-Institut für Polymerforschung, Postfach 3148 55121 Mainz, Germany 2) Laboratoire de Chimie Macromoléculaire (UMR CNRS N° 8009), Bâtiment C6, Université des Sciences et Technologies de Lille 59655 Villeneuve d'Ascq Cedex, France 3) Laboratoire de Recherche sur les Macromolécules, Faculté des Sciences, Université Aboubakr Belkaïd 13000 Tlemcen, Algeria Mechanical and thermodynamical properties of Electron Beam and Ultra-violet cured monomer/liquid crystal (LC) systems were investigated. The pure polymer networks elaborated at high radiation doses exhibit a high modulus indicating chemical crosslinking of the networks. Dilution of the initial mixture with a low molecular weight LC like E7 leads to a significant weakening of the film mechanical strength. The decrease in mechanical moduli is due to a plasticizing effect of the LC molecules randomly dispersed in the polymer network and was confirmed by Differential Scanning Calorimetry thermograms. Introduction Radiation curing is a widely used technique for applications in thin film coatings, adhesives, paintings etc [1-3]. Although UV curing is more often used because it does not require any heavy equipment or special care, the electron beam method (EB) has great advantages. Electron beam curing has the advantage of not requiring the presence of a photoinitiator which might be detrimental to the PDLC film performances and to long term ageing. This technique has been widely used in our laboratory to produce films and composite materials of the PDLC type (Polymer Dispersed Liquid Crystals) [4-7]. These materials consist of micron sized nearly spherical droplets filled with low molecular weight liquid crystal molecules (LMWLC) dispersed in a solid polymer matrix. In this paper a special emphasis is given to the effect of EB and UV-curing on the mechanical characteristics of Tripropyleneglycoldiacrylate (TPGDA) films. Moreover, the influence of adding LC to the monomer prior to irradiation was investigated in detail. Static and dynamic mechanical measurements were carried out in a wide range of compositions from 0 (pure polymer network) to 80 wt.-% LC.
Experiment Materials and film preparation The LC E7 was purchased from Merck KGaA (Darmstadt, Germany). It is an eutectic mixture exhibiting a nematic-isotropic transition at TNI=60°C. The monomer Tripropyleneglycoldiacrylate (TPGDA) was donated by Cray Valley (France). The TPGDA/E7 blends containing x weight-percent (wt.-%) of TPGDA and (100-x) wt.-% E7 were homogenized at room temperature. The samples were applied uniformly on glass plates and irradiated either by EB- or by UVcuring. 2wt.-% (with respect to the monomer) Lucirin TPO (BASF) as photoinitiator was added to the initial mixture in the latter case. Electron Beam Curing The EB generator is an Electrocurtain Model CB 150 (Energy Sciences Inc.), delivering a voltage of 175kV. The samples prepared according to the above procedure were placed on a tray which went under the irradiation source on a conveyor belt in a nitrogen atmosphere. A dose of 105kGy was achieved using a beam current of I=7mA and a conveyor speed of 0.17m/s. Ultra Violet Curing The UV light source used was a Minicure Model MC4-300 (Primarc UV Technology) equipped with a medium pressure mercury arc lamp rated 80 W per cm. The samples were placed on a conveyor belt and irradiated by a dose of 150 mJ/cm2. Mechanical measurements Static properties were analyzed using a mechanical testing machine Instron 6022. The obtained films were characterized by a thickness near 100µm. Samples of rectangular shape were cut from these films. Measurements were performed at room temperature (20°C) at a constant rate of 1mm/min. The stress vs. draw ratio λ was recorded where λ is the ratio of the final length l to the initial length l0 prior to the application of stress. Young modulus was determined from the slope of the stress/draw ratio curves at zero strain [7]. Dynamic measurements were performed by means of a Rheometrics RMS 800 mechanic spectrometer. Rectangular samples were cut from the prepared films. Uniaxial tensile deformation was applied under the condition of a controlled deformation amplitude which was changed with temperature between ∆γ=0.0001 at low T and ∆γ= 0.05 at high T but remaining in the range of a linear viscoelastic response. A special set-up designed for the investigated films was used and the experiments were performed under dry nitrogen atmosphere. E’ was measured at a constant deformation frequency of 10rad/s and a heating rate of 2°C/min starting from T=-100°C up to the temperature where the rubbery state plateau modulus was detected. Results and Conclusion Figure 1 represents the variation of E´ versus temperature for EB- and UVcured bulk TPGDA and several mixtures with E7. For the bulk TPGDA, one notes a large drop of the curve at the glass transition temperature (Tg) of the polymer network. A higher Tg of the EB-cured TPGDA compared to the UVcured sample was observed indicating a highly crosslinked network. Figure 1 includes results from TPGDA/E7 systems at increasing dilution with E7.
E' (Pa)
10
9
10
8
10
7
10
6
10
5
4
10 -100
0% E7(EB) 20% E7(EB) 50% E7(EB) 70% E7(EB)
Tg (LC)
-50
TN-I (LC)
0
50
100
0% E7(UV) 20% E7(UV) 50% E7(UV) 70% E7(UV)
150
Temperature (°C) Figure 1. Dynamic mechanical analysis : Storage tensile modulus versus temperature of EB- and UV-cured films for different LC concentrations. Distinction between Tg’s of the network and the LC clearly indicate a phase separation and coexistence of two distinct phases. The large drop in the glass transition temperature of the polymer shows that the low molar mass LC acts as a plasticiser. These results were confirmed by Differential Scanning Calorimetry measurements. Addition of LC in the composite material induces considerable changes in the mechanical behavior. The cross-linking density of the network decreases as the precursor mixture is diluted with the LC and the molecular weight of strands between cross-links increases. The consequence of this is a more flexible network with a lower plateau modulus. Interestingly, the results from EB- and UV-cured samples corresponding to 20wt.-% E7 overlap nicely. Further increase of LC concentration leads to lower plateau moduli and decreasing polymer Tg values. In particular, UV-cured films with E7 concentrations higher than 20wt.-% exhibit higher mechanical strength than their EB analogues. Figure 2 exhibits the stress-strain dependencies for the EB- and UV-cured TPGDA/E7 system for several LC concentrations in the range from 0 (pure polymer) to 50 wt.-% E7. The stress rise of the EB cured TPGDA film is steep at low draw ratios indicating a high Young modulus whereas for the UV-cured system, a reduction of the Young modulus was observed.
Stress (MPa)
25 20 15 10
0% E7(EB) 10% E7(EB) 50% E7(EB) 0% E7(UV) 10% E7(UV) 50% E7(UV)
5 0 1.00 1.01 1.02 1.03 1.04 1.05 1.06 Lambda
Figure 2. Stress as a function of draw ratio λ for UV- and EB-cured TPGDA/E7 systems at several LC concentrations. Increasing the LC concentration leads to a large decrease of the Young modulus at low deformations and of the tensile strength of the films accompagnied by an increase of the elongation at break. Interestingly, upon addition of only 10 wt.-% of LC, the mechanical moduli drop sharply primarily because of the decrease of the crosslinking density of the polymer network. Literature [1] [2] [3] [4] [5] [6] [7]
“Radiation Curing in Polymer Science and Technology", J. P. Fouassier, J. F. Rabek, Eds., Elsevier Applied Science, London (1993). "UV curing : Science and Technology", S. P. Pappas, Ed., Technology Marketing Corporation, Stamford, Connecticut, USA (1978). A. Chapiro, "Radiation Chemistry of Polymeric Systems", WileyInterscience, New York (1962). P. S. Drzaic, "Liquid Crystal Dispersions", World Scientific, Singapore (1995). J. W. Doane, "Polymer Dispersed Liquid Crystal Displays" in : Liquid Crystals: Their Applications and Uses, B. Bahadur, Ed., World Scientific, Singapore (1990). A. Olivier, T. Pakula, B. Ewen, X. Coqueret, M. Benmouna, U. Maschke, Macromol. Mater. Eng. 287, 650 (2002). A. Olivier, L. Benkhaled, T. Pakula, B. Ewen, A. Best, M. Benmouna, U. Maschke, Macromol. Mater. Eng. 289, 1047 (2004).
