Thermosetting semi IPNs

JLnterpenetrating polymer networks (IPNs) are a special class of polymer blends in which the polymers exist in networks that are formed when at least one of the polymers is synthesized or cross-linked in the presence of the other. Classical or true IPNs are based solely on thermosetting polymers that form chemical cross-links. More recently, two classes of thermoplastic IPNs have been developed. Apparent IPNs are based on combinations of physically cross-linked polymers. Semi-IPNs are based on combinations of... 

Hybrid versions of silicone-thermoplastic semi-IPNs have been developed (19). A hybrid interpenetrating network is one in which the cross-linked network is formed by the reaction of two polymers with structurally distinct backbones. Hydride-functionalized siloxanes can be reacted with organic polymers with pendant unsaturated groups such as polybutadienes (5) in the presence of platinum catalysts. Compared with the polysiloxane semi-IPNs discussed earlier, the hydride IPNs tend to maintain mechanical and morphologically derived properties, whereas properties associated with siloxanes are diminished. The probable importance of this technology is in cost-effective ways to induce thermoset characteristics in thermoplastic elastomers. 

The silicone semi-IPN structure imparts unique behavior to thermoplastic composites without affecting mechanical strength and stifihess. Apart from the direct effects associated with the incorporation of thermoset character into a thermoplastic, the structure has particular value by functioning as (1) a nonmigrating internal lubricant, (2) a shrinkage and warpage modifier, (3) a fiow modifier, and (4) a release agent. As a polymer-polymer composite,... 

Kim et al (1999) examined the morphology and cure of semi-IPN epoxy resin or dicyanate-polyimide/polysulfone-carbon-flbre Aims. Polyimide or polysulfone Aims were inserted into the curing epoxy-dicyanate monomers to form semi-IPNs with sea-island morphology at the thermoset-thermoplastic interface. The final carbon-flbre-thermoplastic-dicyanate Aims had fracture toughness three to five times higher than that of unmodified carbon-flbre-dicyanate composites. 

B) Thermoset-thermoplastic blends and semi-interpenetrating networks (Semi-IPN)... 

Although the dynamically vulcanized blends such as EPDM/PP (Santoprene ) and NBR/PP (Geolast ) have sometimes been referred in the literature as semi-IPNs, we considered them as blends of crosslinked elastomer dispersions in a thermoplastic matrix and as such treated them under the elastomer blends. There is yet another class of thermoplastic/thermoset blend system in which a minor amount of the crosslinkable monomer(s) is allowed to polymerize in the thermoplastic matrix forming a loose network. 

Other types of IPN s exist, of course. For example, Johnson and Labana (1972) recently synthesized a modified type of latex IPN as follows A crosslinked polymer network I prepared by emulsion polymerization served as a seed latex to linear polymer II. The resulting semi-IPN exhibited the usual core-shell morphology. After suitable coagulation and molding steps, polymer II was selectively crosslinked to form a macroscopic network, resulting in a thermoset material. The topology of this IPN therefore involves microscopic network islands of polymer I embedded in a continuous network of polymer II. 

The idea behind semi-interpenetrating networks (semi-lPN s) is to combine the processabiity of thermoplastics with the high temperature performance of crosslinked thermosetting materials. Such mixtures should possess the desirable features of both types of materials. The majority of semi-IPN s contain continuous phases and the components are immiscible at the molecular level. 

Semi-IPNs were prepared by the sol-gel technique through in situ polymerization of BMI in thermoplastic polyetherimide (PEI) as well as in polysulfone (PSF) (Kurdi and Kumar 2006). Structure and properties of semi-IPN membranes could be varied by controlling the thermoset BMI microdomain size interpenetrating within a thermoplastic polymer network. The size of this microdomain depended on the polymerization time of BMI and on the degree of thermoset/thermoplastic phase separation. These semi-IPN membranes showed an improved Tg but a decrease in their thermal stability and could be used for Oj-emiched-air applications where high selectivity is not required. At ambient temperature, it was possible to increase the permeance of semi-IPN membranes. 

Polymer, interpenetrating network IPN is a branch of blend technology it combines two plastics into a stable interpenetrating network. There are all types of blends, such as synergistic types, to meet all types of performance requirements. In true IPN s, each polymer is cross-linked to itself, but not to the other, and two polymer networks interpenetrate each other these become thermoset (TS) plastics. In semi-IPN, only one polymer is cross-linked the other is linear and by itself would be a thermoplastic (TP) these lend themselves to TP processing techniques. The rigidity of IPN structures increases mechanical and other properties such as chemical... 

St. Clair [36] synthesized and evaluated the properties of a semi-2-IPN comprising Thermid 600 (an acetylene terminated imide oligomer from National Starch and Chemical Company) and LaRC-TPI (a thermoplastic polyimide with a Tg of 257 °C). The composition having the ratio of 65 35 of thermosetting Thermid 600 to LaRC-TPI showed the best flexural strength at ambient temperature [36]. 

Pater also prepared a semi-2-IPN, LaRC-RP41, from a mixture having the ratio of 80 20 of thermosetting PMR-15 to LaRC-TPI polyamic acid(44). LaRC-RP41 exhibited significantly improved toughness and micro-cracking resistance, but showed somewhat poorer mechanical properties at 316 °C, compared to those of PMR 15 near resin (see Fig. 13). 

Pascal et al. [49-50] studied semi-2-IPN derived from linear thermoplastic polyimides and thermosetting bismaleimides from the same type of starting... 

The thermoplastic IPNs utihze physical cross-hnks, rather than chemical crosshnks. Usually, these materials wiU flow when heated to sufficiently high temperature (hence the terminology thermoplastic), but behave as thermosets at ambient temperature, with IPN properties, often possessing dual phase continuity. Most often, physical crosshnks are based on triblock copolymers (thermoplastic elastomers being the leading material), ionomers, or semi-crystalhne materials. 

R. A. Dickie and S. Newman, Rubber-Modified Thermosets and Processes, U.S. Pat. 3,833,682 (1974). Semi-I and IPN latexes with reactive shells. Graded composition latexes containing rubber cores. Thermoset expoxies, etc. containing reactive latexes. One of three closely related patents. See U.S. Pat. 3,833, 683 (1974) and 3,856,883 (1974). 

From a topological point of view, the IPN s are closely related to polymer blends, block and graft copolymers, AB-crosslinked copolymers " and ionomeric blends. Some interesting hybrids exist between the IPN s and other polymer materials. The thermoplastic IPN s contain physical crosslinks rather than chemical (covalent) crosslinks. Physical crosslinks can be formed from block copolymers, ionomers or semi-crystalline polymers. When the temperature is raised above the softening point of the respective components, the material flows like a thermoplastic. At service temperatures, it behaves like an IPN, with thermoset behavior. Table 1 summarizes some of the terminology used to describe IPN structure and morphology. 

---