Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-11-17
2001-12-18
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S858000, C524S849000, C525S100000, C525S106000, C525S479000, C525S903000, C523S106000
Reexamination Certificate
active
06331578
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to interpenetrating polymer networks (IPNs). More particularly, the invention relates to a method of making IPNs that aids in the control of the IPN morphology. The invention also relates to novel IPN products.
BACKGROUND OF THE INVENTION
It is known in the field of polymer science that interpenetrating polymer networks (IPNs) are blends or alloys of two or more polymer components, each of which is a crosslinked three-dimensional network. The individual polymer component networks are more or less physically entangled with, but not covalently bonded to the other polymer network(s) in the IPN. The structure of an IPN is frozen by physical interlocking between the component polymer networks [Sperling1997].
A feature of IPNs is that they permit combining advantageous properties from each of two polymers which are normally incompatible. For example, in a hydrophobic-hydrophilic system, flexibility and structural integrity might be imparted by the hydrophobic polymer and lubriciousness might be imparted by the hydrophilic polymer. An IPN may be a bicontinuous system in which each of the polymers form a continuous matrix throughout the network.
Two methods for making IPNs are the sequential polymerization method, and the simultaneous polymerization method. In a typical sequential polymerization method, a solid polymer film (host) is swollen with a monomer (guest) mixture containing an initiator and a crosslinker to form an IPN reaction mixture which is then “cast”, or placed, against a solid surface or suspended in a gaseous mixture such as air or nitrogen, and initiated, for example with heat or UV radiation, to initiate polymerization and crosslinking of the monomer(s). Due to the guest monomer's low molecular weight, it can be distributed readily throughout the host polymer. Polymerization and crosslinking of the guest monomer to form a polymer network then results in an entangled polymer network of a first polymer (the host) and a second polymer (derived from the guest).
In a typical simultaneous polymerization method of making IPNs, monomers or prepolymers are mixed along with a polymerization initiator and crosslinking agent of both networks, to form an IPN reaction mixture which is then “cast”, or placed, against a solid surface or in contact with a gaseous mixture such as air or nitrogen. Polymerization and crosslinking of the component monomers may occur simultaneously, but by non-interfering reactions, to form an IPN composed of covalently independent but physically entangled component polymeric networks of a first polymer (derived from a first monomer or prepolymer) and a second polymer (derived from a second monomer or prepolymer).
Examples of typical methods of preparation of hydrophobic-hydrophilic IPNs using sequential and simultaneous methods can be found in U.S. Pat. Nos. 5,424,375 and 4,752,624 respectively. In both cases the starting mixtures were cast against glass or solid substrates that were used as molds. Substrates were chosen for their inertness and ease of demolding.
Over the years, variations in the methods used to prepare sequential and simultaneous IPNs have produced materials of widely different properties and morphologies. These include latex IPNs, in which spherical particles having a core-shell structure are formed using emulsion polymerization techniques. The original seed latex of a crosslinked polymer is immersed in a solution of a monomer, together with crosslinker and activator. Also there are gradient IPNs, in which the overall composition within the material varies from location to location on a macroscopic level. Gradient IPNs are prepared by immersing a polymer network in a solution of monomer or prepolymer. Polymerization and crosslinking of the monomer takes place as it diffuses into the host polymer network. A further type are thermoplastic IPNs, also known as polymer blend-IPN hybrids, which are prepared with physical crosslinks rather than chemical crosslinks. These IPNs flow at elevated temperatures, but at room temperature they are crosslinked and behave like IPNs. Yet another type of IPN is the semi-IPN which can be prepared by any of the above methods, in which one or more polymers are crosslinked and one or more polymers remain linear.
U.S. Pat. No. 4,423,099 concerns the preparation of gradient IPNs in which a hydrogel is swollen with water and reactant and then immersed in a medium containing a co-reactant. As the co-reactant is diffused into the host polymer network, an interpenetrated polycondensation polymer is formed within the hydrogel network. The compositional gradient of the polycondensation polymer varies from a high concentration at the surface to zero within the bulk.
U.S. Pat. No. 5,183,859 describes a latex IPN structure formed by a consecutive mutli-stage emulsion polymerization process. A rubbery polymer formed in an earlier stage is covered with a hydrophilic polymer formed in a later stage. The resultant polymer particle comprises a rubbery polymer core and a methacrylic glassy polymer shell.
U.S. Pat. Nos. 4,423,099 and 5,183,859 describe processes carried out to achieve a gradient composition within the IPN.
The methods used in the preparation of gradient IPNs and latex IPNs involve the step of immersion of a polymer network in a solution containing a monomer or prepolymer. This step is for the transfer of monomer/prepolymer into the IPN, and gives a gradient profile ranging from 100% of one polymer component at the surface to 100% of the other polymer component in the centre of the IPN. In the preparation of latex IPNs, monomers or reactants in the immersion solution become covalently linked components of the IPN. In the preparation of a gradient IPN the chemical potentials of the polymerizable reactants in the immersion solution are not the same as the chemical potentials of the same reactants in the reaction mixture. Such IPNs do not have a bicontinuous morphology throughout the IPN.
It has been postulated that there are two distinct mechanisms of phase separation during IPN formation: spinodal decomposition (SD), which form bicontinuous structures of relatively small, interconnected nodular domains of the guest polymer in the host polymer; and nucleation and growth (NG) which forms isolated guest polymer domains (or islands) dispersed in a continuous host polymer phase (sea), also referred to as sea-island morphology. The guest monomer concentration is believed to determine which occurs. In one report of a particular IPN system, it was found that IPNs with guest monomer concentrations of greater than 20% produced a bicontinuous morphology in the bulk of the completed IPN. This very stable morphology characterized by bicontinuity and very small domains is highly desirable for applications such as impact resistance polymers, reinforced elastomers, sound and vibration damping, rubber blend and electrical insulators.
The term “Bicontinuous morphology”, generally refers to at least two regions, each of substantially uniform composition which differs from the other, and each of which forms a continuous pathway from one surface of an article to another surface of an article. Thus an IPN having a bicontinuous morphology of hydrophilic and hydrophobic polymers will have two continuous pathways or two sets of continuous pathways extending from one surface of the IPN material to the other surface.
Surprisingly, it has now been reported that even so-called bicontinuous IPNs do not have a bicontinuous morphology at the surface. Murayama [Murayama1993, Murayama1993a] et al prepared hydrophobic and hydrophilic IPNs and concluded that a thin layer of one polymer existed at the surface.
Lipatov [Lipatov1999] suggested that the structure and composition of the surface layers of an IPN which had been formed near the interface with a solid substrate were dependent on the surface energy of that solid substrate.
In other work, it was reported that polyurethane-polystyrene IPNs which had been prepared by a method that included a step of ca
Cheng Yu-Ling
Turner Josephine
Dawson Robert
Eggins D. W.
Peng Kuo-Liang
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