Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Patent
1981-07-21
1983-06-28
Van Balen, William J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
264 22, 428339, 428421, 428910, 528502, B32B 2716, B29C 1702, H01L 4116
Patent
active
043906745
DESCRIPTION:
BRIEF SUMMARY
This invention relates to vinylidene fluoride polymers and in particular to such polymers having improved pyroelectric and piezoelectric properties.
It is known that vinylidene fluoride polymers can be polarised by subjecting them to an intense electric field to yield material having permanent pyroelectric and/or piezoelectric properties which can be utilised in certain electronic applications. Such a procedure is often referred to as "poling".
It is known that vinylidene fluoride polymers can exist in at least two distinct crystalline forms, known as Form I (.beta.) and Form II (.alpha.). These phases are readily distinguished by analysis of their X-ray diffraction patterns, and such measurements also provide an indication of the proportion of Form I material within a sample. Studies of this kind have shown that for a given applied polarizing field both the piezo- and pyroelectric properties are enhanced the greater the proportion of Form I material present. It has been previously accepted that the relative proportions of Form I and Form II materials are affected by temperature and that in order to achieve a high proportion of Form I material, relatively low temperatures (e.g. below 80.degree. C.) should be maintained.
Hitherto it has only been possible to prepare materials having a pyroelectric constant typically no greater than about 3 nC cm.sup.-2 K.sup.-1 and a hydrostatic piezoelectric constant, d.sub.h, typically no greater than about 9 pCN.sup.-1.
It is therefore an object of the present invention to provide vinylidene fluoride polymers having improved pyroelectric and piezoelectric properties.
According to one aspect of the invention, we provide an elongated material of a vinylidene fluoride polymer, particularly a polyvinylidene fluoride, the material having a Young's modulus from 3 to 15, typically from 4 to 10, suitably from 5 to 8 GPa.
The Young's modulus of the elongated material is determined at 21.degree. C. by a dead-loading creep experiment, as described by Gupta & Ward in J. Macromol. Sci. Phys. B1, 373 (1967), taking the 10 second response at a strain of 0.1%.
According to a further aspect of the invention, there is provided an elongated material as hereinabove defined which, on poling, as hereinabove defined, has a pyroelectric constant of at least 4, for example, from 4 to 15, typically from 4 to 10, suitably from 5 to 8 nC cm.sup.-2 K.sup.-1.
According to another aspect of the invention, there is provided an elongated material as hereinabove defined which on poling, as hereinabove defined, has a hydrostatic piezoelectric constant greater than 10 pCN.sup.-1, suitably greater than 12 pCN.sup.-1, preferably greater than 16 pCN.sup.-1.
Poling may be carried out at an elevated temperature up to 130.degree. C., for example at 100.degree. C., but may also be carried out at lower temperatures, for example room temperature. Typically, the electric field applied will range from 100 to 500 Volt/micron, more intense fields being required at lower temperatures. The field is applied across the thickness of the polymer sample by corona discharge needle electrodes, by smooth flat conductive plates or by electrodes deposited on the sample itself.
The elongated material may exhibit an X-ray diffraction pattern which, after poling, indicates the presence of at least 70%, and preferably at least 80%, by volume of the total crystalline material of Form I material. The percentage of Form I material in the elongated material is determined as a proportion of the total crystalline material present by comparing the intensities of the Form I (310) and (020) reflections and the Form II (021) and (111) reflections. Appropriate corrections are made for the structure factors and angle factors: see Hasegawa et al: Polymer J. 3, p,600 (1972).
The crystallinity of the elongated material will depend on several factors including molecular weight and previous thermal history. However, it is generally found to be from 30% to 50%, usually from 35% to 45% by volume of the total elongated material.
The pyroelectric constant
REFERENCES:
patent: 3798473 (1974-03-01), Murayama et al.
patent: 3878274 (1975-04-01), Murayama et al.
patent: 3925339 (1975-12-01), Ishii et al.
patent: 4241128 (1980-12-01), Wang
patent: 4298719 (1981-11-01), Mizuno et al.
patent: 4308370 (1981-12-01), Fukada et al.
Chemical Abstracts, vol. 87, No. 10, Sep. 5, 1977, p. 641, abstract No. 87:77463g Niguchi et al., "Piezoelectric Polymer Films".
McGrath John C.
Ward Ian M.
National Research Development Corporation
Van Balen William J.
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