Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-04-18
2001-11-06
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
active
06313237
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention was made with Government support under Contract F149620-94-1-0459P0001 awarded by the Air Force Office of Scientific Research and Contract CTS-931917 awarded by the National Science Foundation. The Government has certain rights in this invention.
The present invention relates to polymeric materials and composites made by frontal polymerization More particularly, the present invention relates to functionally gradient polymers formed by frontal polymerization.
Functionally gradient or graded materials (FGMs) are materials whose composition varies spatially in a controlled manner. Many different methods have been devised for forming functionally gradient materials. In one such process, centrifugal force was used to prepare gradients in a carbon fiber reinforced epoxy composite to produce composites with spatially varying conductivity. Additionally, several researchers have done work on preparing gradient interpenetrating polymer networks (IPNs). Most of the work developed by these individuals involves producing a gradient by diffusing one component into another pregelled component followed by curing, or producing a gradient in the polymer using a gradient of illumination. The diffusing method can require as much as 280 hours to produce a gradient over 10&mgr;. Using the absorption of light to produce a gradient is limited to polymers with a thickness less than 1 mm. None of these techniques can be used to produce gradients in polymers which are several centimeters in thickness.
Graded polymeric materials, such as Graded Refractive Index (GRIN) materials have found wide use in optical applications. These materials are prepared via interfacial gel polymerization, which is a slow process limited to producing gradients less than about 1 centimeter.
Another type of gradient material with definite utility is an optical limiter based on a gradient of nonlinear optical dye dissolved in a polymer matrix. An optical limiter is a device that strongly attenuates intense optical beams but allows high transmittance at low level light. Such a device would be very useful for protecting human eyes from intense laser pulses. A discussion of the types of organic materials that exhibits such nonlinear absorption is contained in Perry et al., “Organic Optical Limiter with a Strong Nonlinear Absorptive Response.” Science, 1996, pages 1533-1536. They found that metallophthalocyanine (M-Pc) complexes containing heavy central atoms work well. These dyes are compatible with poly(methyl methacrylate) and dissolve in the monomer. This affords the great advantage of inexpensive materials.
Frontal polymerization is a method for converting monomer into polymer via a localized reaction zone that propagates through the coupling of the heat released by the polymerization reaction and thermal diffusion. Frontal polymerization was first discovered at the Institute of Chemical Physics in Chemogolovka, Russia by Chechilo and Enikolopyan in 1972. Polymerization fronts can exist with free-radical polymerization of mono- and multifunctional acrylates or epoxy curing. Frontal polymerization can be achieved in solution polymerization with monomers such as acrylamide, methacrylic acid and acrylic acid in solvents such as water and DMSO.
Frontal polymerization reactions are relatively easy to perform. In the simplest case, a test tube is filled with the reactants. The front is ignited by applying heat to one end of the tube with an electric heater. The position of the front is obvious because of the difference in the optical properties of polymer and monomer. Under most cases, a plot of the front position versus time produces a straight line whose slope is the front velocity. The velocity can be affected by the initiator type and concentration but is on the order of centimeters per minute.
The defining feature of thermal frontal polymerization is the sharp temperature gradient present in the front. The temperature can jump about 200° C. over a little as a few millimeters, which corresponds to polymerization in a few seconds at that point.
In view of the foregoing, it would be a significant advancement in the art to provide a process for forming functionally gradient polymers which had a short reaction time and which could produce polymers several centimeters in thickness. Such a process and the polymeric materials created thereby are disclosed herein.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to functionally gradient materials and a process for forming the same. In a preferred embodiment, an ascending polymerization front is created in a reaction vessel. Pumps provide monomers or resins in a controlled ratio on top of the ascending front as it propagates to maintain a nearly constant thickness of unreacted monomers. The height of unreacted monomers is maintained such that the front is not extinguished, but progresses in a controlled manner. By varying the ratio of the monomers and/or the concentration of additives to the mixture, functionally gradient materials can be formed.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to functionally gradient materials and methods for forming them.
As discussed above, frontal polymerization reactions are generally relatively easy to perform. In the simplest case, a test tube is filled with reactants and a front is ignited ii by applying heat to one end of the tube. The front then either ascends up the tube or descends down the tube, depending upon the point at which it was initiated. In a preferred embodiment of the present invention, the process is carried out in a cylindrical tube such as a test tube. However, it should be appreciated that reaction vessels having different cross-sectional areas can also be used.
In the preferred embodiments, the cross-sectional area can vary from about 15 mm
2
to about 1 m
2
. The limiting factors on the upper end of the area are dependent in part upon the ability to simultaneously add additional reactants to the entire surface area and the ability of the front to progress in a uniform manner.
According to the preferred embodiment of the present invention, the polymerization front is an ascending front in a vertical container. However, it will be appreciated by those skilled in the art that other configurations can also be used without departing from the spirit or scope of the invention. For example, the reactor may be tilted or possess a varying cross section.
Frontal polymerization is often carried out at ambient temperature with the heat of reaction being sufficient to sustain the reaction as the front progresses through the material. However, in some systems it may be necessary to preheat the reactants or to provide additional localized heat as the front progresses to maintain the polymerization reaction.
The process of the present invention can be utilized to make many different types of functionally gradient polymers. In one preferred embodiment, the functionally gradient polymer comprises a polymer matrix having an optical dye dissolved therein. The concentration of the dye varies along the length of the polymer sample. This is achieved by varying the amount of dye added to the monomer that is added to the top of the polymerization front. It will be appreciated by those skilled in the art that there are many different ways of varying the concentration of the dye. For example, the dye can be added by a separate pump as the monomer is being added such that its concentration gradually changes, by varying the relative flow rates of the two feedstreams. Alternatively, the dye solution can be premixed with the monomer in the inlet reservoir by a separate pump such that the inlet stream's dye concentration varies with time.
Many different types of dyes and additives can be used in the present invention. Examples of additives include plasticizing agents, rubber toughening agents and inert fillers. The latter can be affected to great advantage because the rapid reaction in the front prevents sedimentation of the filler. For example, diethyl phth
McCardle Timothy W.
Pojman John A.
Howrey Simon Arnold & White , LLP
Michl Paul R.
The University of Southern Mississippi
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