These advantages and limitations of rubber reflect their physical structure at molecular level. This is a three-dimensional network of folding and kinking polymerization chains, which are dispersed with cross links and limited number of free spaces. Driven by thermal energy, the segments can revolve around some key and rotate in a random way. When rubber is stretched, these chains are gradually straightened, causing the rubber to be super ductile. At the same time, due to the transverse thermal movement of the rotating segment, there is the ability to resist straightening. This thermal movement is easy to pull the two ends of the rubber together. The tensile modulus determines the modulus of elasticity, and the result is that the modulus of elasticity increases with the increase of heat energy. The value of Young modulus increases with temperature is recognized as Joule GF effect. The strain energy of elastomers can be derived from statistical thermodynamics.
w =0.5NkT(^i+^;+^;一3) (1一1)
W =0.5NkT (^i+^; + square; 3) (1, 1)
In the formula, A 1 is the extension ratio in three directions (strain length / strain length); kT is unit thermal energy (k is the tzmann constant); IV is the number of vibrational segments per unit volume (N/2 as the cross chain density); R is the absolute temperature. The amount of 0.5 Nkr is equal to the shear modulus of the material, and the young modulus E=2 (1+p) G.
The elastic modulus of rubber is quite different from that of most engineering materials. Therefore, the resistance to stretching is directly caused by the work done to overcome the attraction between molecules, and thus decreases with the increase of temperature.
The thermal movement of the air compressor accessories rubber in Ji'nan is only possible when there is free space. This is the free volume mentioned above. However, the existence of free volume also provides the opportunity for the diffusion of fluid molecules to the interior of rubber due to rubber suction fluid, which causes expansion: the expansion trend of the elastomer in contact with a particular fluid is dominated by energy. Cohesive energy density (CED) is the energy required to completely separate the constituent molecules of a material, such as an elastomer or liquid. If the bulk density of elastomers and liquids is very similar, then expansion will occur. In practice, the quantity used is usually /CED, and the "solubility" parameter takes up. The 6 values of various liquids or elastomers can be found in references. Unfortunately, hydrocarbons and elastomers tend to have the solubility parameters of 7 -t0 (cal/em3) o's in the same range, which results in a high expansion risk: the value of water is about 23, and the value of the methanol of polar compounds is between this value and hydrocarbon; therefore, the possibility of swelling is minimal. about
The mixture of liquids, 8, is proportional to the concentration of the components.
The free volume in the elastomer composite is not static in space and time; its fast vibrating chain fills with a free volume and is kept dynamically when the volume is opened elsewhere. If the temperature decreases in turn, the dynamic process slows down first, then stops, and the material becomes very hard. This explains the existence of the glass transition temperature. When t is above the rubber, the performance of the rubber is like liquid, and its performance is like super cold liquid, namely glass, when it is lower than j. In the glassy state, the chains are locked by the Van Derwal bonds between the chains, but these chains are relatively weak, so the transition between the glassy state and the rubber state is reversible. The chemical structure of the polymer chain affects T, for example, looser monomers require larger free volume, which does not occur frequently, thus increasing t. Plasticizer tends to increase free volume, thereby reducing T. The filler often has a relatively small effect.