ELLIPSOIDAL PARTICLES

Abstract: The present invention relates to a process for producing ellipsoidal particles from mono-disperse latices and the particles so produced. The process involves (a) preparing a homogenous dispersion of the polymer particles in a matrix capable of affine deformation and having a higher softening temperature that the softening temperature of the dispersed polymer; (b) deforming into ellipsoids the dispersed polymer in the matrix at a temperature which is at or above the softening temperature of the polymer; (c) quenching the ellipsoids to a temperature below the softening temperature of the dispersed polymer to fix the ellipsoid into a rigid shape, and (d) recovering the rigid polymer particles.

Description: ELLIPSOIDAL PARTICLES The present invention relates to a process for producing ellipsoidal particles from mono-disperse latices and the particles so produced. Spherical particles, whether as such or in the form of a latex, have many applications. Such particles are especially useful in chromatography, immunology, filtration standards, standards for light scattering electron microscopy etc. These uses represent a vast area of contemporary research and application in colloid science. It has also been recognised that if such spherical particles could be transformed into non-spherical ellipsoids, such ellipsoids would provide scope for many more uses in addition to those listed above due to the increased axial ratio of such non-spherical ellipsoids in comparison with spheres. Some work has been reported on such non-spherical ellipsoids produced from naturally occurring materials such as discoidal bentonite crystals, tobacco mosaic virus, and human erythrocytes, and from inorganic compounds such as iron oxide, barium sulphate and titanium dioxide. The method used for producing these ellipsoids is described in J. Colloid and Interface Sci., vol 63, pp 509, (1978) by Schneier, P et al; and J. Colloid and Interface Sci., vol 73, pp 295, (1980) by Boylan, C.W. et al; Briefly, such methods use either fragmentation of larger particles, nucleation followed by growth controlled processes, or aging, which are not easy to handle. The particle size is also rather narrow. For instance, the Boylan, C.W. et al reference describes shear induced deformation followed by chemical fixation of human erythrocites. Only one particle size (ca. 14 micrometers) and one axial ratio (ca. 4.6) is given and there is no disclosure of any possible variations in particle size or axial ratios. Also, one of the limitations of these prior processes is that neither the axial ratio nor the mass of the ellipsoids can be varied or controlled at the same time to any significant extent. Moreover, the ellipsoids formed are believed to be relatively unstable both chemically and morphologically, especially when subjected to any post treatments. It would be desirable to control these parameters so that the ellipsoids can be tailored to specific end uses. It has now been found that such a control can be exercised in producing non-spherical ellipsoidal particles of organic polymers using a unique process. Accordingly, the present invention is a process for producing rigid, non-spherical ellipsoids from polymer particles, said process comprising:- (a) preparing a homogeneous dispersion of the polymer particles having a given volume distribution in a matrix capable of affine deformation, said matrix having a higher softening temperature than the softening temperature of the dispersed polymer; (b) deforming under affine conditions the matrix containing the dispersed polymer at a temperature which is at or above the softening temperature of the dispersed polymer so as to deform the dispersed polymer into ellipsoids; (c) quenching the ellipsoids so formed to a temperature below the softening temperature of the dispersed polymer in order to fix the ellipsoid into a rigid shape, and (d) separating and recovering the rigid ellipsoidal dispersed polymer particles having substantially the same volume distribution as the starting polymer particles from the matrix. The term "polymer" as used herein and throughout the specification is meant to embrace both homo- and copolymers. The expression "softening temperature" as used herein and throughout the specification is meant to signify either the glass transition temperature or the melting point of the polymer, whichever is the higher of the two. The expression "affine deformation" as used herein and throughout the specification signifies a deformation which is independent of the scale of measurement and is constant throughout the deformed region. The deformation may be elastic, plastic or produced by flow. In general, the flow of liquids constrained by solid boundaries includes a shearing deformation which depends upon distance from the boundary and would not be construed as affine deformation as herein defined. The starting polymer particles are preferably spherical prior to their dispersion into the deformable matrix. The polymer particles are suitably those which have a softening temperature from 50-200"C, preferably from 70-100'C. Specific examples of the preferred polymers which can be deformed into non-spherical ellipsoids include polystyrene and polymethyl methacrylate. The deformable matrix is a solid and is suitably a polymer. The matrix is capable of a draw ratio which is at least twice, preferably 4-5 times that of the dispersed starting polymer at the operating temperature range. In the solid matrices the local deformation takes place by stretching and may depend on the rate of strain. The polymer particles to be deformed into ellipsoids are dispersed in a matrix which is a non-solvent for the particles. The concentration of the polymer in the dispersion is such that the dispersion suitably has less than 50 volume %, preferably less than 10 volume X and even more preferably below 2 volume % of the polymer. If dispersing a high concentration of the polymer in the matrix within these ranges, it must be ensured that the viscosity of the softened polymer and the matrix match during the deformation step. Temperature is also an important factor in deformation. Ideally, the temperature is in the range where a polymer is transformed from a substantially rubbery state to a predominantly viscous state. Typically, in the case of polystyrene of a molecular weight of about 100,000, the latex particles are in a glassy state and hence non-deformable at room temperature. For the deformation required they need to be above Tg (which in the case of polystyrene is also its softening point), and for the deformation to remain unrelaxed, also above the rubbery state. It has been found that for polystyrene the temperature range satisfying these requirements is from 180-220*C, preferably about 200*C. The polymer particles can be surface modified before incorporation into the matrix to facilitate the formation of a homogeneous dispersion and stabilisation thereof. The deformation of the matrix containing the dispersed polymer into a non-spherical ellipsoid can be carried out, e.g. either by uniaxial elongation or by uniaxial compression. The deformation is preferably carried out above the softening temperature of the dispersed polymer but below the softening temperature of the matrix polymer. In order to fix the dispersed polymer in the matrix into a rigid shape the dispersion should be quenched rapidly to a temperature below the softening temperature of the dispersed polymer. The rigid non-spherical ellipsoids of the dispersed polymer so formed can be separated from the matrix by methods well known in the art. It is preferable to use a solvent which will selectively remove the matrix leaving behind the rigid non-spherical ellipsoids of the dispersed polymer which can then be recovered by e.g. simple filtration or centrifugation. It is also possible to modify the surface of the non-spherical ellipsoids so formed. Examples of the types of surface modifications include physical adsorption, chemical reaction or combinations thereof. The object of these types of modification is for example to control the hydrophobicity of the particles or to facilitate bonding of functional species such as enzymes, cells or metal coatings on to the surface of the particles. Thus, according to a further embodiment, the present invention is a process for producing rigid, non-spherical ellipsoids from spherical polymer particles, said process comprising:- (a) preparing a homogeneous dispersion of the spherical polymer particles having a given volume distribution in a solid matrix polymer capable of affine deformation, said matrix polymer having a higher softening temperature than the softening temperature of the dispersed polymer; (b) uniaxially deforming under affine conditions the matrix containing the dispersed polymer at a temperature which is at or above the softening temperature of the dispersed polymer but below the softening temperature of the matrix polymer so as to deform the dispersed polymer particles into ellipsoids, (c) quenching the ellipsoids so formed to a temperature below the softening temperature of the dispersed polymer in order to fix the ellipsoid into a rigid shape, and (d) separating and recovering the rigid ellipsoidal dispersed polymer particles having substantially the same volume distribution as the starting polymer particles from the matrix polymer. A feature of the present invention is that the process enables the size, mass and axial ratio of the non-spherical ellipsoids to be varied and controlled as desired. Moreover, the ellipsoids are as uniform in respect of mass or volume distribution as the starting polymer and the polydispersity in shape characterised by the standard deviation of axial ratio distribution is less than 20% of the arithmetic mean of the axial ratios. The non-spherical ellipsoids produced by the process of the present invention can have a volume in the range of 10~21 mete ^ to 10" 2 meter 3 and have shape stability. The axial ratio (defined as the ratio between the longest and shortest axes of the ellipsoidal particles) of these particles is in the range from above 1 up to 12 at a draw ratio not. greater than 5. Thus according to yet another embodiment the present invention is rigid non-spherical ellipsoid particles of a polymer of substantially uniform volume and having shape stability, said particles having a volume in the range of 10~21 meter 3 to 10~ - meter 3 , preferably from 10~ 21 to 10~ 15 m 3 /particle which corresponds to equivalent spherical radii from about 400 to 20.000A having a standard deviation of about 3X by mass or volume and an axial ratio of up to 12, preferably from 1.1 to about 10. For example, for an applied draw ratio of about 4 it is possible to achieve axial ratios of the particles which are greater than 6. The process of the present invention enables rigid non-spherical ellipsoidal polymer particles to be produced which have substantially the same volume distribution as the starting polymer particles and which have axial ratios that can be varied as desired at constant volume distribution of the particles. The present invention is further illustrated with reference to the following Examples. Examples A dispersion of polystyrene (PS) particles in solid polyvinyl alcohol (PVA, softening point above 200*C) was prepared through casting films from aqueous solutions of PVA containing the PS particles. 150g of 4.79 %w/w aqueous solution of a fully hydrolyzed PVA sample (BDH 125) and 1.20g of PS latex (SIGMA Chemical Company, LB-5, solids content 9.0 %w/w) were weighed in a beaker and were homogenised with a magnetic stirrer for 15 minutes. The average size (diameter) of the particles was 0.46 micrometers (standard deviation 0.0048 micrometers). 75 cm 3 of the homogeneous dispersion was poured into 14 x 21 cm perspex film casting frames fixed on a horizontal labelling table and was allowed to dry for four days. In order to keep out dust and any other contamination, the film casting set up was covered with a cardboard box. The air-dried film co'uld be easily removed from the surface of perspex and the concentration of the particles in the solid dispersion was 1.50 Xw/w. The thickness of the air-dried film on average was 0.07 mm and the lateral dimension was about 15 x 20cm. Then, from the film so formed strips (ca. 12 x 60 mm) were cut and before deformation, they were immersed into a hot silicone oil bath for 10-20 seconds in order to remove residual water and dissolved air. The temperature of the oil bath was 200 ± 3*C. This treatment was necessary to avoid formation of bubbles that might have adversely affected local deformation of the particles. The heat treatment resulted in a 10-15X nearly isotropic shrinkage of the strips. The heat treated strips were fixed one by one to a stretching frame so that an approximate 11 x 40 mm area was finally subjected to deformation. Then the strips were immersed into an oil bath at a temperature 200 ± 3 β C and stretched by hand to a prefixed draw ratio. The rate of strain was estimated to be on average 3-5 cm/sec. The overall residence time of the film in the hot oil bath was about 10 seconds. After deformation, the strip was quickly cooled down to room temperature and the excess of oil from the surface thereof was removed by tissue paper. In a series of experiments, 20-30 strips were drawn to a given draw ratio. In the experiments three draw ratios, namely 1.5, 2.0 and 3.0 were used. In order to avoid the effect of the inhomogeneous deformation (around the clamps to hold the strips) on the shape of the particles, the middle part of the drawn strips deformed in a parallel way were cut out and used in further experiments. The elongated ellipsoidal particles were separated from the products as follows: The residual oil layer stuck to the surface of the strips was washed off with n-propanol and the deformed strips were put into distilled water or into a mixture of 303 * v/v n-propanol and 70S * v/v distilled water used as the swelling mixture to allow the strips to swell overnight (10 hours). lg of the stretched composite film was put into 50 cm 3 swelling mixture. After swelling, the temperature of the mixture was slowly raised up to 80-85"C when the dissolution of strips began. After complete dissolution, the mixture was cooled down to room temperature and the particles were separated by low speed (ca. 3000 rpm) centrifugation in a bench centrifuge. This kind of purification was repeated twice and as a dispersion medium either distilled water or 30% v/v n-propanol-water mixture was used. The dilute suspension of the elongated particles were stored in sealed bottles at room temperature. No sedimentation or any other sign of instability was observed during a period of time of several weeks. In order to ascertain the shape of the particles a standard transmission electron microscopic study was carried out clearly revealing their ellipsoidal shape and also enabling determination of the average axial ratios and their distribution. The usual carbon film technique and magnification of x 9200 were used in all cases. For the size and shape characterisation a further 4 x photographic magnification was applied. At the three draw ratios given above the following average axial ratios were obtained: 2.1, 3.1 and 6.0. The standard deviation expressed as a percentage of the arithmetic mean of the axial ratios was about ± 15-20%. The overall experimental error of the whole procedure was estimated to be around 10-20%. The above procedure can be extended to deform spherical polystyrene particles in the size (diameter) range of 0.2-3.0 micrometers. Instead of pure high molecular weight polyvinyl alcohol copolymers thereof can also be used in order to lower the stretching temperature. Some plasticizers such as glycerol or ethylene glycol can also be added to the polymer if necessary. Example 2 Example 1 was repeated except for using strips of about 10 x40mm which were cut from the cast film and the strips clamped in a stretching frame as previously. Stretching was carried out manually as before by operating the frame at an average rate of 3-5cm /sec with the portion of frame holding the strip immersed in a thermostatically controlled hot oil bath. The stretched films,' while still in the frame, were cooled rapidly to room temperature and adhering oil removed as in Example 1 above. The ellipsoidal particles produced in the above Examples were subjected to rigorous measurement of axial ratios (P) and the results analysed for the relationship between the initial and final P values for the degree of uniformity within a given preparation. The results are summarised in Table 1 below. In Table 1 both Pmax (the peak value) and Parithm (arithmetical mean) in the distribution of P relate closely affinely to the marcoscopic draw ratio (L). This can be expressed separately as a function of L /L (appropriate for uniaxial extension or compression) when little difference is seen between Pmax and Parithm. Bearing in mind the multidude of variables involved in the preparation it is believed that the approximation to pure affine deformation is very satisfactory. The remainder of Table 1 provides various measures for ■ polydispersity' of the axial ratios as expressed in terms of first and second moments (PI and P2 respectively, which in the traditional usage of molecular weight determination would correspond to number and weight average respectively). The polydispersity is very narrow indeed when compared with standard MW distributions in polymer science. As seen, the largest P2/P1 value is still only 1.04 for an elongation value close to 6, which approximates a rigid rod. Table 1 - Characteristic data of ellipsoid population prepared at three different macroscopic draw ratios (diameter of starting spheres 0.46 + 1% micrometers). L 1.50 2.0 3.0 Total number of particles Parithm Pmax Standard deviation First moment of P distr. PI Second moment of P distr. P2 P2/P1 By choosing different starting latices the preparation procedure outlined above for polystyrenes can be readily extended over an arbitrarily wide mass/polymer range. It must be noted that for any given deformation series using the same starting latex material the mass (including its distribution) remains constant for all P values of the resulting product. The ellipsoidal particles of predesigned elongation and mass should thus become available for a variety of purposes. Amongst these, the most straightforward, as relating to isolated units, should be the testing of formulae presently widely used in light, X-ray and neutron scattering throughout polymer and colloid science, and similarly for testing relationships on viscosity. In addition, the matrix-elongated particle system (before dissolution of the matrix) may be of interest in itself as a specially designed anisotropic composite.

Claims: ( I CLAIMS 1. A process for producing rigid, non-spherical ellipsoids from polymer particles, said process comprisin :- (a) preparing a homogeneous dispersion of the polymer particles having a given volume distribution in a matrix capable of affine deformation, said matrix having a higher softening temperature than the softening temperature of the dispersed polymer; (b) deforming under affine conditions the matrix containing the dispersed polymer at a temperature which is at or above the softening temperature of the dispersed polymer so as to deform the dispersed polymer into ellipsoids; (c) quenching the ellipsoids so formed to a temperature below the softening temperature of the dispersed polymer in order to fix the ellipsoid into a rigid shape, and (d) separating and recovering the rigid ellipsoidal dispersed polymer particles having substantially the same volume distribution as the starting polymer particles from the matrix. 2. A process for producing rigid, non-spherical ellipsoids from spherical polymer particles, said process comprising:- (a) preparing a homogeneous dispersion of the spherical polymer particles having a given volume distribution in a solid matrix polymer capable of affine deformation, said matrix polymer having a higher softening temperature than the softening temperature of the dispersed polymer; (b) uniaxially deforming under affine conditions the matrix containing the dispersed polymer at a temperature which is at or above the softening temperature of the dispersed polymer but below the softening temperature of the matrix polymer so as to deform the dispersed polymer particles into ellipsoids, (c) quenching the ellipsoids so formed to a temperature below the softening temperature of the dispersed polymer in order to fix the ellipsoid into a rigid shape, and (d) separating and recovering the rigid ellipsoidal dispersed polymer particles having substantially the same volume distribution as the starting polymer particles from the matrix polymer. 3. Rigid non-spherical ellipsoid particles of a polystyrene polymer of molecular weight about 100,000 having substantially uniform volume and having shape stability, said particles having a volume in the range of 10~2- meter 3 to 10"" 2 meter 3 , having a standard deviation of about 3% by mass or volume and an axial ratio of up to 12.

Inventors: NAGY MIKLOS JOSEPH, NAGY, MIKLOS, JOSEPH
Applicants: BRITISH PETROLEUM CO PLC, THE BRITISH PETROLEUM COMPANY PLC
Application year: 1989
Main classification: C08J3
IPC codes: C08J3, C08L29