Glass transition temperature crosslinking

The argument assumes that the forces act only at the crosslinked ends of the strands. No interactions between the strands exist in this simplistic picture. The interaction of molecular segments, well above the glass transition temperature is usually rather small. 

Fig. 4.1. Glass transition temperatures of the polymers are plotted against l/IVlc, that is the inverse molecular mass between crosslinks.

Tgoa glass transition temperature of the uncrosslinked polymer Mc average molecular mass between crosslinks tj> empirical factor... 

Apparently, annealing was not impeded by crosslinks (Fig. 5.1). The density effects observed agree with the results of the glass transition temperature measurements (Sect. 4.2). There, the Tg of the annealed (and therefore denser) sample was consistently higher by about 2 K than the Tg of the quenched polymer. 

Preparation and thermal crosslinking reactions of oc, -vinylbenzyl terminated polysulfone-b-polydimethylsiloxane, ABA type block copolymers have been discussed 282,313) However, relatively little characterization was reported. Molecular weights of polysulfone and PDMS segments in the copolymers were varied between 800-8,000 and 500-11,000 g/mole, respectively. After thermal curing, the networks obtained showed two phase morphologies as indicated by the detection of two glass transition temperatures (—123 °C and +200 °C) corresponding to PDMS and polysulfone phases, respectively. No mechanical characterization data were provided. 

The mechanical properties of polymers also depend on the extent of crosslinking. Uncrosslinked or lightly crosslinked materials tend to be soft and reasonably flexible, particularly above the glass transition temperature. 

Another example involved a batch of isocyanate crosslinker which was too tacky. Upon comparing the HPGPC trace of this sample with that of a control as shown in Figure 9, it is seen that the major difference between these two samples was the level of free caprolactam. The high content of free caprolactam in sample CX-006 depressed the glass transition temperature (Tg) of the sample to such an extent that CX-006 became too tacky. This method of analysis has proved to be a reliable and useful technique for detecting low levels of free caprolactam in this type of oligomeric crosslinker. 

Synthetic and natural rubbers are amorphous polymers, typically with glass transition temperatures well below room temperature. Physical or chemical crosslinks limit chain translation and thus prevent viscous flow. The resulting products exhibit elastic behavior, which we exploit in such diverse applications as hoses, automotive tires, and bicycle suspension units. 

Probably most of these investigators were studying poly(dichlorophosphazene) in the partially crosslinked state. Most of this was summarized by Allcock (.9). More recently, highly purified, uncrosslinked II has been examined in the solid state (21). The unstressed polymer is amorphous at room temperature, but crystallization can be induced by cooling or stretching techniques. The glass transition temperature, measured by Torsional Braid Analysis, is -66°C (22). 

PHAs containing carbon-carbon triple bonds synthesized by P. oleovorans and P. putida grown with 10-undecynoic acid (10-UND=) have been reported [64]. The amount of carbon-carbon triple bond could be controlled between 0% and 100%, but the yield of the PHA containing 100% carbon-carbon triple bond was very low. The repeating units formed from 10-UND= were 3-hydroxy-8-nonynoate (3HN=) and 3-hydroxy-6-heptynoate (3HHp=) units in the amounts of 26 mol% and 74 mol%, respectively. The glass transition temperatures of PHAs synthesized from mixtures of NA and UND= increased from -30°C to -20°C as the content of carbon-carbon triple bond increased from 0% to 100%. These polymers were crosslinked when cations such as Co2+ and Pt2+ were added. 

To convert an elastomer into ebonite, the glass transition temperature, Tg, has to be raised to above 20 °C, or above the operating temperature of the product, in order to remain rigid in use. This is achieved by crosslinking the rubber with a large amount of sulphur. Typically, 25 to 50 phr is used for natural rubber ebonites. Ebonites can be produced from NR, BR, IR, SBR and NBR. Rubbers with low unsaturation, e.g., HR and EPDM, do not form ebonites. 

Electron irradiation causes chain scission and crosslinking in polymers. Both of these phenomena directly affect the glass transition temperature (Tg) of the materials. Thermomechanical (TMA) and dynamic-mechanical analysis (DMA) provide information about the Tg region and its changes due to radiation damage. Therefore, DMA and TMA were performed on all irradiated materials. 

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