Rheology modification of precursor solutions

Details Rheology modification of precursor solutions Rheology modification of precursor solutions

1999-07-27

2000-11-28

Acquah, Samuel A.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

528488, 528502, 528503, 526 59, 526 60, 526 75, 526346, 4272071, 347 6, C08F 200

Patent

active

06153711&

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the modification of the rheology of precursor solutions, and especially to modifying the viscosity of precursor poly(p-phenylene vinylene) "PPV" solutions, and their subsequent use in the manufacture of electroluminescent devices.
Organic electroluminescent devices include an organic light emissive layer and are made so that they emit light when a suitable voltage is applied across electrodes deposited on either side of the organic emissive layer. One class of such organic materials is semiconductive conjugated polymers, which are described in our earlier patent U.S. Pat. No. 5,247,190, the entire contents of which are incorporated herein by reference. PPV is one such material, and will emit light when positive and negative charge carriers are passed through it by a voltage being applied between two suitable charge carrier injecting layers. The electroluminescent efficiency of these devices is known to depend on the balancing of the electrons and holes that are injected into the device and meet to form electron/hole pairs, as well as on the efficiency with which these electron/hole pairs combine to radiate light (i.e. the photoluminescence efficiency: see, for example, N. C. Graham and R. H. Friend, Solid State Physics, 49, 1, 1995). Therefore, to achieve an efficient device it is of critical importance to have sufficiently high photoluminescence efficiency.
There are several approaches used for the processing of conjugated polymers to form electroluminescent layers. One approach uses a precursor polymer which is soluble and can therefore be easily coated by standard solution-based processing techniques (e.g. spin-coating, blade-coating, reverse roll coating, meniscus coating, contact/transfer coating and ink-jet printing) and then converted in situ by suitable heat treatment to give the finally conjugated and insoluble polymer. Another approach uses directly soluble conjugated polymers which do not require a subsequent conversion stage.
The coating technique is usually chosen to suit the area that is to be coated. Depending on the application, one or other of these approaches might be preferred. The precursor polymer approach is useful when subsequent processing (for instance patterning with a top electrode or coating with further polymer layers such as transport layers or emitting layers of a different colour) might lead to damage of a directly soluble polymer film. Converted polymer films also tend to have better thermal stability, which is of importance both during fabrication and during storage and operation of devices at high temperatures. However, problems can arise during manufacture using the precursor approach, especially with a polyelectrolyte system, owing to the special rheology of these materials. For instance, even at relatively low concentrations precursor polyelectrolyte solutions can have high viscosities (for example 400 cps or more for a precursor solution with a solid content of only 0.4%) and can therefore be difficult to coat at the desired thicknesses, especially by low-viscosity methods such as inkjet printing. In general, there is a need to be able to tailor the viscosity of the precursor to suit the needs of subsequent processes--especially coating techniques.
According to the present invention from a first aspect there is provided a method for tailoring the viscosity of a conjugated polymer precursor, the method comprising synthesising the precursor by a polymerisation reaction and, during the polymerisation reaction, applying a shear to the reactants of the polymerisation reaction so as to define the viscosity of the precursor at a desired value.
The shear is suitably provided by relative movement of the reactants and a stirring apparatus located in the reactants. The shear is preferably provided by agitating and most preferably stirring the reactants. The reactants may include the conjugated polymer precursor itself. The shear is preferably applied so as to control and most preferably reduce the viscosity of the precursor.
In a typical reactor configu

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Hsieh, B.R., "Chlorine Precursor Route (CPR) Chemistry To Poly (P-Phenylene Vinylene)-Based Light Emitting Diodes," Advanced Materials, vol. 7, No. 1, Jan. 1, 1995, pp. 36-38, XP000486181.
Alok Kumar et al., "Synthesis Of Poly(1,4-Phenylenevinylene) By Metathesis Of P-Divinylbenzene", Makromolekulare Chemie, Rapid Communications, vol. 13, No. 6, Jun. 1, 1992, pp. 311-314, XP000282076.

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