Determination of polymer solution properties - Determination of density gradient crystallinity of the polymer

Determination of polymer crystallinity by density gradient method

Polymers due to its long chain structure and molecular weight dispersion and other reasons, to form a complete crystal is very difficult, so for crystalline polymers generally can only form part of the crystal, often using crystallinity to describe the degree of crystallization , It is an important parameter of crystalline polymer. Crystallinity has an important effect on the physical and mechanical properties of the material such as strength, hardness, transparency, heat resistance, air permeability, solubility and so on. The crystallinity of the material is not only related to its chemical structure, such as the regularity of the chain, but also has a great relationship with the process conditions of the forming process, including the cooling rate, tensile orientation, nucleating agent and so on. Therefore, in order to study the crystallization of the polymer processing technology on the performance of the crystallization of the measurement is an indispensable link. Crystallinity in the industry is also often used as an intermediate indicator of polymer products.

The method of measuring the crystallinity is X-ray diffraction, calorimetry, infrared spectroscopy and density method. Among them, the density method is one of the most commonly used methods because of its simple equipment, convenient operation and data processing and high The accuracy and has been widely used. In the density measurement using density gradient method with high precision measurement, you can simultaneously compare the measurement of multiple samples, with a good density tube can be used repeatedly, etc., especially for industrial analysis.

First, the purpose of the experiment

1, master the density gradient method for the determination of polymer density and calculate the principle of crystallinity and methods;

2, master the correct use of the proportion of bottles.

Second, the experimental principle

Crystallinity refers to the weight or volume percent of the crystalline fraction in the crystalline polymer. Since the boundary between the crystalline phase of the polymer and the amorphous phase is not clear, the physical meaning of the crystallinity is also unclear. Different measurement methods have different scales for the discrimination of the crystal phase and the amorphous phase, and the crystallinity is different The The density method assumes that the crystalline polymer consists of a crystalline region and an amorphous region, with a specific capacity (or density) additive between them. The ratio of the crystalline fraction and the amorphous part is VC and Va, respectively. Capacity V is

V = XcwVa + (1 - Xc w) Va (1)

Where X c represents the weight crystallinity (the fractional fraction of the crystalline fraction), and the above equation can be written as

            Xcw = (2)

The density of the crystal and amorphous regions is ρc and ρa, and the density of the sample is ρ, then the above equation can be written as

XCw = (3)

Similarly, the volume crystallinity Xcv is obtained

          XcV = (4)

From the formulas (3) and (4), it is known that if the density ρc of the completely crystalline polymer sample and the density ρa of the completely non-crystalline polymer sample are known, it is only necessary to determine the density ρ of the polymer sample Its crystallinity.

In this experiment, the density of the polymer sample was measured by the suspension method, that is, under the constant temperature condition, the proportion of the two liquids which could be completely miscible was adjusted in the test tube with the polymer sample. Floating, but suspended in the middle of the liquid, according to Archimedes law we know that the density of the mixed liquid at this time and the polymer sample conspiracy equal to the use of pycnometer to determine the density of the mixed solution, you can get the polymer sample density.

Third, instruments and reagents

instrument:

1 set of constant temperature sink

        25ml of the weight of the bottle

        Dropper 2 root

        Tube 50ml 1 root

        One glass stir bar

drug:

Polyethylene (PE) industrial products

Ethanol Chemistry Pure 30 ml

Fourth, the experimental steps

1, the thermostat sink to (25 ± 0.1) ℃.

2, the test tube, dropper and glass stir bar according to Figure 3.3 device.

Add about 25ml of ethanol in the tube, and then add a polymer test

Like, with dropper to add distilled water, while moving up and down stirring bar,

So that the liquid mixture evenly, the first water speed can be faster, when being

When the sample was slowed down, the sample was slowly dripping

Water. (If the drop in excess, you can use another dropper back to the drop of ethanol) Figure 3.3 Schematic diagram of the experimental device

Until the sample does not sink nor float, suspended in the middle of the mixed liquid, for several minutes, then the density of the mixed liquid is the density of the polymer sample. The density of the liquid was measured with a pycnometer. Repeat the above procedure to determine the density of the other sample.

3, the determination of mixed liquid density. First use the analytical balance called the weight of the weight of the weight of the wafer W0, and then remove the stopper, filled with the measured mixed liquid, cap stopper, into the thermostat tank, the excess liquid from the capillary overflow. With the filter paper to wipe the liquid outside the capillary, when the temperature reaches equilibrium, from the thermostat out of the pycnometer and wipe the liquid outside the bottle, when the temperature

  

 Figure 3.4 Schematic diagram of the pycnometer

1. Cork Capillary; Bottle body

After the balance, from the thermostat out of the pycnometer, and wipe the liquid outside the bottle, said the liquid after the weight of the weight of the bottle after the W1. Pour the liquid in the bottle, washed with distilled water several times and then filled with distilled water, constant temperature, dry the bottle, said the weight of the weight of the bottle after the water W water, the liquid density ρ according to the following formula:

 Ρ = (W1 - W0) ρ water / (W water - W0) (5)

Fifth, data processing

1, according to formula (5) to calculate the PE sample of the plot for capacity.

2. Calculate the crystallinity of PE according to formula (3) and (4).

    Known: PE 20 ℃ when

               VC = 1.009 cm3 / g Va = 1.233 cm3 / g

               25 ℃

               VC = 1.013 cm3 / g Va = 1.161 cm3 / g

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