Crosslink Levels

While IPNs themselves constitute a polymer/polymer composite, many engineering applications will require greater or lesser quantities of other components, such as inorganic fillers. Several problems regarding the location of polymer II in an IPN have already been touched upon, as well as the interfacial mixing characteristics. However, the presence of a third phase introduces new complications. 

Lipatov and co-workers synthesized sequential IPNs containing a reinforcing silica filler as follows Polyurethane samples based on oligo-ethylene glycol adipate and tolylene diisocyanate were prepared with various levels of Aerosil, a silica filler. Trimethylolpropane was used as the 

The Flory-Rehner equilibrium swelling method (see Section 4.2.2) was used to evaluate the change in crosslink density of the IPNs, using dioxane as the swelling agent. The Flory interaction parameter was found to be approximately 0.30 for dioxane and either polymer, simplifying the analysis. Table 6.4 gives the results for the effective crosslink densities, vj K where V is the polymer volume in cm. Table 6.4 also shows the theoretical values of vj K calculated in accordance with the law of additivity. 

Most unexpectedly, the value for yj V)exp is lower than (Pe/ V )theor in every case. Further, the divergence between the experimental and theoretical values of vj V increases with increasing levels of crosslinking in network II, and with increasing values of W2/1V1. 

An explanation of these results has been offered by Siegfried et al. 

---

In general, coating systems are designed to achieve optimum properties at crosslinking levels short of 100% conversion in order to minimize cure time. The effect of Isocyanate to hydroxy ratio and humidity on crosslink density after a fixed cure time is shown in Figure 4. The fixed cure time has been arbitrarily defined as the time at which a coating with equal Isocyanate and hydroxy functionality reaches 85% conversion in the absence of humidity (H-0). If the initial ratio of isocyanate to hydroxy is less than or equal to 1, the crosslink density drops with increasing humidity. 

In the manufacturing of crosslinked polyolefin foam, the foam expansion and cell stabilization highly dependent on the degree of crosslinking. It has been reported that at higher crosslinking level the foam produce have higher density [1, 2]. 

The variation of the domain sizes with crosslink density was recognized by Yeo et al. [28], investigating cross-poly(n-butyl acrylate)-inter-cross-polystyrene. Figure A shows the morphology of 50/50 compositions as a function of network I crosslinking level. The cellular structures are gradually transformed to finer, and more obviously cylindrical or worm-like shapes with increasing crosslink density. 

Performance vs Crosslinker Level. Table IV presents tensile and swell Index results on a series of approximately 0 C Tg VAE emulsion copolymers. ABDA containing copolymers had fair swell Index values which deteriorated rapidly as ABDA levels declined, but showed slow declines In wet tensile performance with reduced levels. Possible reasons for this are discussed below. 

A simple bubble machine is devised and successfully applied in characterising lightly crosslinked PE resins for foam expansion. The biaxial stress-strain relationship is deduced from the air injection rate and pressure. The effects of strain rate, temperatnre and crosslinker level on the stress-strain behavionr are investigated. Uniaxial extension experiments are also performed and compared with biaxial extension data. 5 refs. 

Effects of Curing Agent Type. Epoxide-Cured Propellant. Carboxyl-terminated polybutadiene is a linear, difunctional molecule that requires the use of a polyfunctional crosslinker to achieve a gel. The crosslinkers used in most epoxide-cured propellants are summarized in Table IV and consist of Epon X-801, ERLA-0510, or Epotuf. DER-332, a high-purity diepoxide that exhibits a minimum of side reactions in the presence of the ammonium perchlorate oxidizer, can be used to provide chain extension for further modification of the mechanical properties. A typical study to adjust and optimize the crosslinker level and compensate for side reactions and achieve the best balance of uniaxial tensile properties for a CTPB propellant is shown in Table V. These results are characteristic of epoxide-cured propellants at this solids level and show the effects of curing agent type and plasticizer level on the mechanical properties of propellants. 

Figure 10.10 Correlation between the crystallizability of polycyclooctene and its crosslinking level. A mixture of polycyclooctene and dicumyl peroxide (DCP 3% w/w) was subjected to the temperature program shown in (a). The sample is heated at 130° fort minutes (10 minutes in this example) for crosslinking, (b) The...

Free radical copolymerisation of divinylbenzene gives crosslinked resins that have been shown to often still bear many unreacted pendant vinyl groups7. These remaining pendant vinyl bonds as well as the crosslinking level can be quantified by FTIR. The value obtained for the produced support is about 3.0 mmol/g of pendant vinyl bonds. 

T curve is independent of crosslink level up to very high levels, ca. 25%. Most importantly, peaks when a natural frequency of chain motion equals the external... 

The heating of the pre-polymer between 330° C and 390° C for different periods of time between 2 h and 20 h, leads to different crosslinking levels due to the opening of the terminal groups. The flash pyrolysis at 485° C of the crosslinked materials followed by the monitoring of cyclopentadiene, cyclopentene, and norbornane allows the identification of the extent of crosslinking. 

Shortly thereafter, Yeo, et al. (7.8) showed that the domain size depended quantitatively on the crosslink density, the interfacial surface tension, and the temperature. For many compositions involving non-polar polymer pairs and moderate crosslinking levels, domain sizes of the order of 50-100 nm were predicted. While the theory was developed for spheres for simplicity, it was already known that both phases tended to be cocontinuous, especially for midrange compostions... 

Figure 2. Transmission electron micrographs of six polybutadiene/polystyrene sequential IPN s and related materials, the polybutadiene portion stained with osmium tetroxide. Upper left high-impact polystyrene, commerciaL Upper right a similar composition made quiescently. Middle left semi-I IPN, PB (only) crosslinked. Middle right semi-II IPN, PS (only) crosslinked. Lower left full IPN, both polymers crosslinked. Lower right full IPN, PB with higher crosslink level. (Reproduce from ref. 5. Copyright 1976 American Chemical Society.)...

Resin Supplier Type Crosslinking level (%) Acidity (meqH /g) Particle size (mm)... 

Two resins with the same physicochemical characteristics (crosslinking level 8 % exchange capacity 4.8 meq fT /g) were studied. However, Amberlyst 119 has a larger particle size than the K1481 polymer (Table 4). 

The crosslinking of each polymer network is simply controlled by the type and concentration of the crosslinker employed in the system. The higher the crosslinker level (especially in polymer network I), the hner the morphology. 

Figure 3 shows typical rheometer cure time curves of three Blon elastomers with different crosslinker levels at 50 phr carbon black loading. With higher crosslinker content In the polymer, the torque required to shear the rubber during vulcanization Increased while cure time decreased. 

Instead of high crosslink levels and thus polymer networks with high... 

Fig. 5. Poly(n-butyl acrylate)/polystyrene IPN s, Phase domain size as a function of network I crosslink level. ...

In Table 3, experimental values of were estimated from swelling measurements and shear modulus at 10 seconds. The data confirm the difference between divinylbenzene and aerylie acid anhydride. When DVB is used as the crosslinker, the experimental values agree well with the theoretical ones. The close agreement may be fortuitous however, since one would expect physical crosslinks to contribute to the experimentally measured crosslink level, especially when modulus was used. Apparently, network defects and physical crosslinks nearly cancel each other out in this system. 

Three urethane-crosslinked polybutadiene elastomers (TB-1, TB-2, and TB-3) of varying crosslinking levels, along with a similarly crosslinked styrene-butadiene copolymer (HTSBR) and two polybutadiene polymers randomly crosslinked with dicumyl peroxide (PB-1 and PB-2), have been investigated to determine their viscoelastic behavior. Elsewhere, TB-1, TB-2, and TB-3 have been designated as HTPB-1, HTPB-2, and HTPB-3, respectively. 

Shore [56] synthesized a pendent (64) and a crosslinked (65) CBS catalyst in bead form from monomeric precursors 59a and 60 respectively (Scheme 29) but also in monolith polymer form with a crosslinking level of 2% [57]. Monolith polymers were then cut to form CBS disks (64D and 65D) roughly 2 mm in height and 5 mm in diameter. 

---